Note: Descriptions are shown in the official language in which they were submitted.
CA 02222333 2006-09-05
WO 96!40821 PCTl~1S96/08087
CONCENTRATE FOR USE IN THE MELT
FABRICATION OF POLYESTER
J
Field of the Invention
The present invention relates generally to polyester extrusion in which
to polyester melt strength is enhanced by reacting the polyester with a
polyfunctional branching compound. More particularly, the invention concerns a
resin concentrate obtained by melt processing a polyester and sufficiently
high
concentrations of a branching agent to minimize the formation of crosslinked
compounds in the concentrate. Compared with processes in which a polyester
i 5 to be foamed is combined directly with neat branching compound and then
foamed, we have discovered that the pre-mixed concentrate of the present
invention can substantially improve the stability of the polyester foam
extrusion
process and reduce the amount of unreacted branching agent present in
articles fabricated from the polyester foam. Use of the concentrate of the
2o invention reduces viscosity of the polyester extrusion mass near the
loading
end of the extruder, while achieving desired melt strength enhancement toward
the end of the extrusion line. Hence, use of the concentrate substantially
enhances throughput in foam and non-foam polyester melt fabrication
processes.
Background Of The Invention
Articles prepared from foamed polymers offer several significant
advantages over those made from unexpanded materials. For example, the
insulation value per unit thickness of a foamed sheet is greater than that of
a
3o sheet of unfoamed polymer. . Also, the strength-to-weight ratio of foamed
materials is higher than unfoamed materials. These attributes have been
applied to greatest advantage in the building trades where foamed polymer
sheets are used as building insulation, and in the packaging area where foamed
materials are used to fabricate lightweight trays and other food service
products. Although polystyrene foam articles are the norm for applications
which have less demanding thermal or mechanical requirements, there is a
growing desire to successfully commercialize foamed articles obtained from
other resins such as crystalline or semi-crystalline polyesters, primarily
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polyethylene terephthalate, ("PET"). Based on its resin properties, PET can be
expected to provide better mechanical and thermal performance than
polystyrene foam and better chemical and flame resistance. However, there
are a number of well documented problems associated with the extrusion of
s polyester foam. In particular, we have observed instability and poor density
reductions in polyester foam processes described heretofore. For example,
U.S. Patent No. 4,981,631 to Cheung ~ al. discloses foamed articles such as
dual ovenable trays obtained from PET that contain 1 to about 6 wt.% of a
polyolefin (i.e. polypropylene or polyethylene). Unfortunately, the maximum
to obtainable density reductions (about 50%) are generally considered
unsatisfactory for many applications. We have also discovered that when a
branching agent is used to improve the processing of the polyester, relatively
high levels of the unreacted branching agent, for example pyromellitic
dianhydride, remain in the finished article, which may be objectionable in the
1 s context of potential food uses or in the context of the long term
stability of
structural foam products sold into the housing market.
By way of background, the foaming of a resin via extrusion typically
involves the steps of melting the polymer in the extruder, adding a foaming
agent to the molten resin, then cooling the mixture in the forward sections of
the
2o extruder and forcing it through a die. Often this process is carried out in
a pair
of single screw extruders connected in tandem. The use of a twin screw
extruder for the process is also well known. The foaming agent, usually a gas
or a low boiling compound, is mixed with the molten resin in the extruder
under
sufficiently high pressure to maintain the resin/blowing agent mixture as a
single
2s phase within the extruder. Foaming occurs when this pressurized mixture
exits
the die, travelling from the region of higher pressure within the extruder, to
a
region of lower (usually atmospheric) pressure outside the extruder. The
reduction in pressure causes the blowing agent to expand and form bubbles,
thereby imparting a foamed quality to the extruded resin.
3o Substantial difficulties encountered in the extrusion of crystalline or
semi-
crystalline polyester foams (as distinguished from polystyrene foam) are
caused
primarily by: (1) a narrow "operating range" and (2) poor melt strength. The
term "operating range" is recognized in the art as the optimum temperature for
extruding the resin to produce a stable foam. At temperatures below the
35 operating range, the molten polymer will either be too viscous to process,
or if
processable, too viscous to support foam cell growth, which means that any
extrudate will have little if any foam character. At temperatures above the
operating range the viscosity of the extrusion mass is low enough to permit
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3
expansion of the blowing agent gas, but too low to prevent the foam bubbles
from collapsing, which also produces a poorly foamed, dense product. Ideally,
the polymer to be foamed should have a very wide operating range. For
example, in the case of polystyrene the difference between the highest and
s lowest temperature at which the resin can be successfully foamed is about
fifty
degrees centigrade. Generally speaking, the more slowly a polymer changes
viscosity in response to a constant temperature gradient, the larger will be
its
operating range. In general, amorphous resins (e.g., polystyrene) have very
large operating ranges because they typically elicit a gradual change in
1 o viscosity with temperature. On the other hand, semi-crystalline polymers
such
as polyethylene terephthalate) ("PET") exhibit a relatively abrupt transition
from
a low viscosity material above the crystalline melting temperature to a high
viscosity polymer below the melting temperature. This permits a very narrow
temperature region in which the polyester can be foamed. In the case of PET,
15 the narrow operating range means that unless the extrusion melt temperature
is
very closely controlled, the foam will either form too easily upon extrusion
of the
resin from the die (and collapse on itself), or not form at all. The
additional
process cost associated with maintaining the extrusion conditions within PET's
operating range is seen to be impractical.
2o Even if measures can be taken to maintain the extrusion process
temperatures within PET's narrow operating range, a second problem noted
above, the poor melt strength of the resin, impairs the ability of PET to
support
the growth of bubbles upon extrusion of the resin from the die. A number of
patents disclose melt strength improvement of PET by reacting PET with a
2s branching component. However, most of these patents do not discuss
improvements in PET melt strength in the context of a foaming process. For
example, Leslie et ~I. U.S. Patent 4,145,466 discloses adding a polyanhydride
(e.g. PMDA) to the PET in an amount from 0.1 to 5 wt.% based on the weight of
the PET, preferably 0.2 to 1.5 %, and most preferably 0.3 to 1.0 wt.%, to
3o enhance blow molding and injection molding of PET. The manner in which the
PMDA is added to the PET is described at column 3 lines 7-12, and Example 1
of the Leslie et al patent where it is stated that the PMDA and the PET are
melt
mixed in an extruder. Although the patentee alludes to the manufacture of
foams at column 3 line 38, no teachings of any foaming processes are given in
35 the patent.
Another patent addressing the problem of PET's poor melt viscosity
characteristics is Dijkstra et ~I U.S. Patent 3,553,157. Example 4 and Figure
1
of Dijkstra ~t ~I disclose mixing PMDA and PET where the amount of PMDA
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4
ranges from .5 to 1.0 wt% PMDA based on the weight of the PET. According to
Fig. 1 of Dijkstra ~t ~L, the patentees' findings are that a maximum
improvement
in intrinsic viscosity is achieved at levels of PMDA of about 0.7 wt% based on
the weight of polyester. Dijkstra et al. disclose that introduction of the
s polyfunctional compound (e.g. PMDA) can be achieved by masterbatching in
which a mixture is prepared. containing PET and the polyfunctional compound
such that the mixture is very rich in polyfunctional compound. However, the
patent does not state whether the masterbatch is prepared via extrusion or by
simply dry mixing the PET and the polyfunctional compound. Dijkstra et al.
1 o patent does not deal with foam production.
A further patent dealing with melt strength improvement of PET is
McCracken U.S. Patent 4,933,429. Like the patents discussed above,
McCracken does not discuss foam extrusion. The patentee discloses reaction
of PET with about 0.05 to about 3.0 wt.% of a polyepoxide compound. In order
is to reduce exposure to patentee's preferred polyepoxide (triglycidyl
isocyanurate, or "TGIC"), patentee states that it is preferred to initially
prepare a
concentrate by blending a relatively large amount of the TGIC with the
polyester, where the amount of the TGIC in the polyester is in the range of
about 3-20 wt% based on the weight of the concentrate. The concentrate is
2o then blended with polyester to obtain a desired final level of TGIC in the
polyester.
Turning now to patents which discuss polyester foam production,
unexamined published Japanese patent application No. 59-210955 (1984),
having a publication date of November 19, 1984, discloses a method for
2s manufacture of a thermoplastic polyester resin foam in which polyester
resin
(including PET) is mixed with 0.01 to 2 mole % (based on the PET) of a
multifunctional carboxylic acid anhydride (including pyromellitic dianhydride
"PMDA") and 0.03 to 2.5 wt.% of a multifunctional glycidyl ester. Although the
method disclosed in the Japanese patent application concerns use of a PET
3o resin composition containing (1 ) a multifunctional carboxylic acid
anhydride and
(2) a multifunctional glycidyl ester, Figure 2 of the published Japanese
application discloses the improvement in melt viscosity obtained when PET is
combined with PMDA in the absence of the glycidyl ester.
Further patents dealing with polyester foam production are Hayashi et al.
35 U.S. Patents 5,000,991 and 5,134,028 which disclose a process for producing
a
thermoplastic polyester resin foam. The process of Hayashi et al. comprises
melting a thermoplastic polyester resin (e.g. PET), mixing the molten resin
with
a blowing agent and extruding the mixture into a low pressure zone to carry
out
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foaming. The process is characterized in that a compound having two or more
acid anhydride groups per molecule (e.g. PMDA) is added to the thermoplastic
polyester resin. Hayashi et al. '991 disclose using an amount of PMDA in the
range of from about 0.05 to 5.0 parts by weight per hundred parts by weight of
5 the thermoplastic polyester resin. The patentees teach at column 5, lines 10
to
13, that "when the amount [of PMDA) exceeds 5.0 parts by weight, the gelation
of the molten material of the thermopolastic polyester proceeds and extrusion
foaming cannot be effected". Hayashi ~ al. '991 state at column 6, lines 36 to
61, that the thermoplastic polyester resin can be mixed with the compound
io having two or more acid anhydride groups (i.e. PMDA) in any of three
possible
methods: First, by mixing (without melting) the polyester resin pellets with
PMDA powder to coat the pellets with PMDA; or, secondly, by pre melt-mixing
the PMDA with a thermoplastic resin (which can be the same or different from
the polyester), pelletizing the mixture, and adding the pelletized mixture to
the
polyester; or, thirdly, by melting the polyester in the extruder and then
adding
the PMDA to the extruder to effect mixing. The patentees equate these
methods and do not suggest or teach any different outcome in the foaming
process if one mode of combining the PET and PMDA is chosen over another.
Hayashi ,e~ al. '991 also disclose the addition of a compound of a metal of
2o Groups I, II or III elements of the periodic Table (e.g. sodium carbonate)
to the
polyester resin in an amount of 0.05 to 5 parts by weight per hundred parts of
polyester resin. The patentee states that the metal compound results in foams
having higher tensile elongation and finer cells. Hayashi et al. '991 teach
against using levels of sodium compounds in the process below about 220
ppm.
Notwithstanding the disclosure of Hayashi ~ al., we have observed
instability in the process for extruding a low density polyester foam wherein
PET, PMDA, (and optionally sodium carbonate) and a foaming nucleator, such
as talc, are fed into either a twin screw extruder or a 3/4" single screw
extruder.
3o In particular, we observed large variations in product quality during runs
of more
than an hour. Instability of the process was indicated by the fact that the
extruder torque and pressure were observed to double or halve without any
change in process settings. The inherent viscosity of samples exhibited
similar
wide fluctuations. These factors resulted in PET foams with large density and
3s microstructure inconsistencies. Moreover, we were unable to attribute these
inconsistencies to feed variations, moisture effects or equipment variations.
In
addition we have determined that the process of Hayashi et al. '991 results in
amounts of unreacted PMDA which it is desired to reduce.
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A further problem in the extrusion foaming of polyester foam is the
difficulty in using recycled PET as a feedstock for such foaming. Generally,
the
inherent viscosity of recycled PET is lower than virgin PET due to processing
performed by recycling processors to remove impurities. A problem
s encountered in attempting to use recycled PET is that there are wide
differences in the extent to which different lots or sources of PET recycle
can be
improved in melt viscosity via chain branching reaction with PMDA. Some lots
of recycled PET exhibit very good improvement in melt viscosity when reacted
with a chain branching agent such as PMDA, while others do not. We have
1 o sought to overcome this problem so that a manufacturer of foamed PET
articles
can consistently and reliably employ PET from all possible sources, including
recycle.
Still another problem to overcome in PET melt processing is that extruder
throughputs, while generally satisfactory, should be higher in order to render
s such processes more economical. The branching reaction between PMDA and
PET, while improving the processability of PET, has a tendency to cause
increases in the extruder torque near the beginning of the extrusion line
which
can hamper throughput. For example, when PET is extruded with PMDA to
obtain a melt. strength enhanced resin, a subsequent remelting of this already
zo extruded material to melt fabricate articles typically encounters high
viscosities
at the beginning of the extrusion line, hence reducing throughput. It is
desired
to minimize this problem as much as possible so that extruder throughput can
be increased.
25 Summary Of The Invention
The ability of the present invention to achieve process stabilization
using polyester concentrates in which levels of branching compound exceed
about about 1 wt.% is surprising because polyester resin compositions
containing about 1.0 wt.% of branching compound are difficult to process, and
3o because the art (see e.g. Hayashi et al., '991 discussed above) has taught
against using high levels of branching compound (i.e. greater than 5 wt.%) due
to gel formation. Quite surprisingly, we have succeeded in using typical
extrusion conditions to prepare pelletized concentrates which do not exhibit
gels, notwithstanding concentrations of branching compound therein of greater
3s than about 5 wt.%, and preferably about 8 to about 12 wt.%, based on the
weight of the concentrate. Because gel formation can cause problems in
polyester extrusion processes, it is particularly surprising that use of a
concentrate containing relatively high levels of branching material would
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actually improve, rather than degrade, the stability of a polyester extrusion
process.
In view of the properties and advantages of the branching agent
concentrate used herein, one embodiment of the present invention is a
J composition for improving the processibility of polyester. The 'composition
is
produced by melt processing a mixture comprising at least 50 weight percent
polyester resin and greater than 5 and no more than 20 weight percent of a
polyfunctional branching compound.
The term "branching compound" or "branching agent" is used herein to
to describe polyfunctional compounds which react with polyesters to produce
branching thereof.
Another embodiment of the invention is a concentrate useful for melt
processing with a thermoplastic polyester, the concentrate being produced by
melt processing a mixture comprising a polyester resin carrier and a
1 s polyfunctional branching compound. The branching compound includes at
least
3 branching agent reactive sites per molecule. The polyester resin carrier
includes highly reactive end groups, and the ratio of branching agent reactive
sites to highly reactive endgroups in the mixture prior to melt processing is
at
least 6:1.
2o As used herein, the term "branching agent reactive sites" refers to all
functional groups of a branching agent which ultimately are expected to react
with polymer chains under melt processing conditions.
The term "highly reactive end groups" means those endgroups which are
expected to readily react with branching agent under the melt processing
25 conditions used to prepare the concentrate. The term and is explained in
greater detail below.
In yet another embodiment of the invention, a concentrate useful for melt
processing with a thermoplastic polyester comprises a polyester resin, a
branching compound, less than one weight percent of crosslinked reaction
3o products of the polyester resin carrier and the branching compound, less
than
1000 parts per million of water, and optionally one or more polyester
additives
selected from the group consisting of nucleating agents, stabilizers,
plasticizers,
expansion nucleating agents, crystallization nucleating agents, pigments,
fillers,
flame retardants, rubber modifiers, colorants, antioxidants and antistatic
agents.
35 As used herein, the term "crosslinked" refers to reaction products of a
branching agent and polyester in which four or more polyester chains become
attached through two or more branching agent molecules.
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In still another embodiment of the invention, the concentrate of the
invention can further be defined in terms of the torque requirements for melt
processing the mixture used to produce the concentrate. In this embodiment,
the concentrate is prepared by melt processing a mixture of a branching agent
s amount of at least five weight percent of the mixture and a polyester
carrier
amount of at least fifty weight percent of the mixture. The composition of the
concentrate is characterized by a maximum melt-mixing torque no greater than
ten percent of the highest maximum melt-mixing torque exhibited by melt
processing mixtures containing greater polyester carrier amounts and lower
1 o branching agent amounts.
As used herein, the term "maximum melt-mixing torque" means the
maximum mixing torque of a 72 gram sample of the polyester plus branching
agent, after the sample has undergone mixing in a molten state, in a standard
60 cc Haake mixing bowl employing roller blades, at a mixing speed of 60 rpm,
1 s and a mixing temperature approximately at the midpoint of the operating
range,
which is the case of a PMDA/PET mixture is about 270°C, for a mixing
period of
at least 10 minutes. Under certain prescribed conditions, explained in greater
detail below, the concentrates of the invention exhibit melt-mixing torques of
less than about 1 Newton-meter.
2o Another embodiment of the invention is a process for producing and
using a concentrate in the melt processing of a polyester. The process
includes
the steps of melt processing a mixture of at least 50 weight percent of a
polyester resin carrier and between 5 and 20 weight percent of a
polyfunctional
branching compound to produce a solid concentrate containing less than 1
25 weight percent of crosslinked reaction products of the branching compound
and
the polyester resin carrier. The solid concentrate subsequently is melt
processed with a polyester which can be the same as or different from the
polyester resin carrier.
Polyester foam produced from a concentrate in accordance with the
3o present invention, such as that obtained from PET, has many potential uses
including those in food packaging and insulation markets. The foam's thermal
and mechanical properties are superior to polystyrene, and can find
applications in markets currently serviced by polystyrene foam, as well as
those
markets where polystyrene foam is not used due to its inferior properties (for
35 example, microwaveable containers). PET foam may also be gaining wider
public acceptance as a recyclable product, which could confer a marketing
advantage. The PET foam produced in the present invention can be used in a
wide variety of end-uses and may be laminated for some of these applications
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and/or coextruded for hot water pipe insulation or cable coating. Regardless
of
end-use, the process and product improvements afforded by the present
invention confer a substantial economic advantage.
s Detailed Description of the Invention
Generally speaking, a context in which the present invention can be
practiced is that of polyester foam extrusion wherein polyester is melted and
pressurized in an extruder; a suitable blowing agent is introduced into the
molten polyester; and the polyester is then extruded through a conventional
die
is apparatus into a region of lower pressure and temperature whereupon the
blowing agent expands to foam the polyester. The foam extrudate, upon
cooling, can then be subjected to other conventional processing steps, such as
thermoforming, to obtain finished articles. Another context in which the
invention can be practiced is that melt-fabrication of non-foam polyester
articles.
A central feature of the invention involves the manner in which a suitable
branching compound can be added to the polyester in the extruder to achieve
greater stability in a foaming process and to bring about a reduction in the
amount of residual unreacted branching compound present in foamed articles.
Instead of directly charging a neat branching compound into the foam extruder
2o concurrently with the polyester to be foamed, as is done in the prior art,
the
process of the present invention adds the branching compound to the extrusion
line in the form of a previously melt-extruded concentrate material
incorporating
a relatively high level of the branching compound.
In the case of foaming extrusion, the resin carrier used to compound the
2s concentrate of the present invention is preferably a polyester, and can be
the
same or different from the polyester which wilt be foamed in the extruder. In
the
case of foaming PET, we have found that foam quality is poor when
polyethylene resin or polypropylene resin is substituted for PET as the
carrier
resin of the concentrate. The concentrate typically comprises greater than
3o about 5 wt.% of a branching compound capable of branching the polyester. A
particularly preferred amount of branching compound in the concentrate is from
about 8 wt.% to about 12 wt%. At concentrations of branching compound
below about 2 wt%, the concentrate is difficult to process due to high
viscosity.
At concentrations above about 20 wt.%, the extrusion mass has a tendency to
35 break rendering pelletization difficult. The concentrate of the invention
is
surprising in that it can be prepared using conventional pellet extrusion with
no
appreciable gel formation.
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In a process for using the invention, a minor amount of concentrate,
conveniently prepared in the form of extruded pellets, can be loaded along
with
a major amount of pellets or powder of virgin or recycle polyester feedstocks
into the hopper of a conventional extruder, whereupon the total concentrate +
s polyester mixture is melted, intimately mixed and then extruded through a
suitable die, such as a flat die, an annular die or a nozzle-type die. The
relative
amounts of pre-extruded concentrate that can be added to a foam extruder
along with virgin or recycle polyester feedstocks are such that there is
achieved
a final branching agent concentration in the extrusion mass of from about 0.1
1 o wt% to about 1 wt% based on the total weight of the extrusion mass.
Although
it is known in the art to incorporate this level of branching agent in a
polyester
intended for foaming, the art neither discloses nor suggests the substantial
benefits in terms of foam properties and process stability which can be
achieved when a branching agent is incorporated into the polyester in the form
s of the concentrate of the present invention.
In somewhat greater detail, the polyester resins suitable for use in the
present invention include linear polyesters or polycondensates of an aromatic
dicarboxylic acid component and a diol component. Examples of dicarboxylic
acid components include terephthalic acid, isophthalic acid,
Zo naphthalenedicarboxylic acid, Biphenyl ether carboxylic acid, Biphenyl
dicarboxylic acid, Biphenyl sulfone dicarboxylic acid and
diphenoxyethanedicarboxylic acid. Examples of diol components include
ethylene glycol, trimethylene glycol, tetramethylene glycol, neopentyl glycol,
hexamethylene glycol, cyclohexanedimethanol tricyclodecanedimethanol, 2,2-
2s bis (4-b-hydroxy ethoxy phenyl) propane, 4,4-bis (b-hydroxy ethoxy)
Biphenyl
sulfone, diethylene glycol and 1,4-butanediol.
Polyesters prepared from the above components are well known in the
art, and can be prepared via the dicarboxylic acid, or suitable derivatives
such
as dimethylesters of the above acids. In many cases, polyesters suitable for
3o use in the invention are available for purchase from a variety of
suppliers.
Examples of polyesters that can be employed in the present invention include
polyethylene terephthalate, polybutylene terephthalate, polybutylene
terephthalate elastomer, amorphous polyesters, polycyclohexane terephthalate,
polyethylene naphthalate, polybutylene naphthalate and mixtures of the
35 foregoing. Specific examples of commercially available polyester resins
useful
in the present invention are Goodyear PET resins 7207 and 9506 ("C '=ET"),
Teijin Limited PET resin TR8580, and Eastman Kodak PET resin 9902. It
should be understood that the present invention contemplates the extrusion of
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recycle PET which may already contain low levels of branching agent. It should
be further understood that the present invention contemplates the extrusion of
PET which may already contain low levels of a crystallization aid such as
lower
melting temperature materials including polyolefins and liquid crystalline
polymers.
The term "branching compound" or "branching agent" as used herein is
intended to encompass polyfunctional compounds which react with polyesters
to produce branching thereof. Particularly preferred in the present invention
in
view of their surprising ability to result in gel free concentrates are
branching
to compounds having two or more acid anhydride groups per molecule.
Pyromellitic dianhydride ("PMDA") is particularly preferred because it is a
relatively inexpensive, commercially available material that reacts quickly
with
the polyester resin.
Promotion of reaction between the branching agent and the polyester
i ~ can be obtained by adding to the foaming extruder directly, or as part of
the
concentrate of the invention, an organic or inorganic Group I, II, or III
metal
compound. As disclosed in Hayashi et ~I U.S. patent 5,000,991 such
compounds when used at levels disclosed in the patent can be used to
facilitate
the reaction between the branching agent and PET and also to serve as a
2o nucleating agent for bubble formation. Examples of inorganic compounds
include potassium chloride, sodium chloride, sodium hydrogen carbonate,
sodium carbonate, potassium carbonate, zinc carbonate, magnesium
carbonate, calcium carbonate, aluminum carbonate, sodium oxide, potassium
oxide, zinc oxide, magnesium oxide, calcium oxide, aluminum oxide and the
2~ hydroxides of these metals. Examples of the organic compounds include
sodium stearate, potassium stearate, zinc stearate, magnesium stearate,
calcium stearate, aluminum stearate, sodium montanate, calcium montanate,
lithium acetate, sodium acetate, zinc acetate, magnesium acetate, calcium
acetate, sodium caprylate, zinc caprylate, magnesium caprylate, calcium
3c caprylate, aluminum caprylate, sodium myristate, zinc myristate, magnesium
myristate, calcium myristate, aluminum myristate, calcium benzoate, potassium
terephthalate, sodium terephthalate, sodium ethoxide and potassium
phenoxide. The compounds of Group I or II metals of the Periodic Table, for
example sodium carbonate, are preferred.
In addition to the optional Group I, II, or III metal compounds referred to
above, other conventional additives may be added directly to the foam
extruder,
or incorporated in the concentrate of the invention to improve the physical
properties of the thermoplastic polyester resin foams and molded articles
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thereof. Examples of such additives are stabilizers, plasticizers, expansion
nucleating agents (to aid foaming), crystallization nucleating agents (to aid
later
thermoforming steps), pigments, fillers, flame retardants, rubber modifiers,
colorants, antioxidants and antistatic agents. There is no intention to
restrict the
s types of expansion or crystallization nucleating agents that can be used in
the
process of the invention. Generally speaking, the nucleating agent can be any
material, or mixture of materials, in amounts effective to induce
crystallization,
or to enhance cell formation and cell structure. If desired, the nucleating
agent
can be another crystalline resin. For example, as disclosed in Cheung et al.
to U.S. Patent 4,981,631, foamed articles such as dual ovenable trays are
obtained from PET that contains 1 to about 6 wt.% of a polyolefin (i.e.
polypropylene or polyethylene). In the present invention, talc and sodium
carbonate are found to be excellent nucleants for foam cell formation
resulting
in foam of excellent qualities. A nucleating agent combination which produces
~ s excellent foam quality in the process of the present invention comprises
up to
about 5 wt.% talc and up to about 0.5 wt.% sodium carbonate, based on the
total composition being foamed.
The optional additives referred to above, including the Group I, II and II
metal compounds, can be added to the foam extrusion process by either (1)
2o directly placing the additives into the foaming extruder as neat materials
separate from the concentrate. of the invention; (2) compounding the additives
into resin concentrates separate from the resin concentrates of the invention,
and then adding desired amounts of the concentrates to the foaming extruder;
or (3) incorporating the additives directly into the concentrate of the
invention so
25 that a single concentrate may be used to add branching agent and any other
desired additives to the foam extrusion line. The second and third options are
particularly preferred as they reduce the amount of separate feeds that need
to
be metered into the foaming extruder. For example, concentrates which
contain the required amounts of branching agent according to the present
3o invention can be obtained by melt extruding PET resin, the branching agent,
talc, and sodium carbonate. The relative amounts of the talc, branching agent
and sodium carbonate in the concentrate can be readily adjusted so that any
given amount of concentrate capable of supplying a desired level of branching
compound to resin being foamed will also supply the correct weight
35 percentages of the other optional additives present in the concentrate.
The upper limit of the total amount of additives that can be compounded
in the concentrate of the invention is determined by the processability of the
filled carrier resin. It has been found for example, that PET could be
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13
compounded with 20 wt% branching compound, 15 wt% talc, and 6 wt%
Na2C03, for a total of 41 wt% additives in PET. This high level of additives
was difficult but possible to process. The lower limit of various other
additives in
the concentrate is a matter of choice dependent upon the final concentrations
of
s such additives which are desired in the extrusion mixture.
With respect to preparing a concentrate of polyester and branching agent
suitable for use in obtaining a foamed polyester according to the present
invention, we have found it is critical that the concentrate comprise greater
than
about 2 wt% branching agent based on the weight of the concentrate. At levels
i o of branching agent within the range of from about 0.5 wt% to less than
about 2
wt%, melt preparation of the concentrate is difficult due to high viscosity in
the
melt system caused by reaction of the branching agent and the polyester. Such
high viscosity can render extrusion of the concentrate difficult to such an
extent
that extrusion is commercially unattractive. Nevertheless, in order to obtain
the
above-mentioned advantages of the concentrate, it is critical that the
concentrate of the invention be prepared via a melt processing process such as
melt extrusion. The advantages of the invention are not obtained using a dry-
mixed concentrate containing the same relative amounts of polyester and
branching agent. Surprisingly, at levels of branching agent greater than about
2
2o wt.% of the concentrate, the melt processibility of the concentrate is
excellent,
thereby rendering extrusion pelletization of the concentrate far more
economically feasible. Hence, it is critical in the present invention to
employ
levels of branching agent in the concentrate greater than about 2 weight
percent based on the weight of the concentrate. From the standpoint of making
the most economical usage of the concentrate, the branching agent
concentration is preferably greater than about 5 wt.% of the concentrate. A
particularly preferred amount of branching agent is about 8-12 wt%. This range
effectively balances the benefits of using large amounts of branching agent in
the concentrate, against the need to avoid handling and processing
difficulties
3o which can begin to arise at increasingly higher levels of branching agent.
Reference may be had to Figures 1 and 2 which graphically depict the
relationship between the maximum torque required to melt-mix the concentrate
as a function of branching agent (PMDA) concentration. Figure 1 (concentrate
with sodium carbonate) and Figure 2 (concentrate without sodium carbonate)
35 illustrate the viscosity behavior of the molten concentrate at levels of
branching
agent (PMDA) above and below 2 wt%. The curves plotted in Figures 1 and 2
illustrate that high viscosities can be readily avoided in the melt processing
of
the concentrate by employing greater than about 2 wt % branching agent in the
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14
concentrate, with concentrations of greater than 5 wt%. branching agent
yielding surprisingly low mixing torques of less than about 2 N-m. The
viscosity
behavior of the concentrate of the invention is particularly surprising given
the
teachings of Hayashi et al U.S. Patent 5,000,991 in which the patentee states
that levels of branching agent above 5% induce gelation. In fact, for the
concentrate preparations reported in Figure 2, no gelation was observed in
concentrate samples containing greater than 2 wt.% PMDA. Although the
concentrate samples of Figure 1 were not analyzed for gel content, the
viscosity
curves set forth in Figure 1 indicate that gelation was not effecting the
~~ o processibility of these samples.
Given the low viscosities attainable in molten polyester/branching agent
concentrate preparations comprising greater than about 2 wt% branching agent,
a concentrate according to the invention can be defined as having a maximum
melt-mixing torque of not greater than about 3 Newton-meters, preferably not
15 greater than about 1 Newton-meters. As used herein, the term "maximum melt-
mixing torque" shall be understood to mean maximum mixing torque of a 72
gram sample of the polyester plus branching compound, after the sample has
undergone mixing in a molten state, in a standard 60 cc Haake mixing bowl
employing roller blades, at a mixing speed of 60 rpm, and at a mixing
2o temperature approximately at the midpoint of the operating range, which is
the
case of a PMDA/PET mixture is about 270°C, for a mixing period of at
least
about 10 minutes. The stated 10 minute mixing period should be understood as
commencing at the point when the polyester plus branching compound, after
being charged to the mixing bowl as a solid, has finished converting from a
solid
25 state to a molten state. Although there will be observed a brief spike in
the
viscosity curve (see curves in Figures 1 and 2) just prior to the 10 minute
mixing
period, this is attributable to the polymer plus branching material not yet
being
melted in the mixing bowl. It should be understood that the term. "maximum
melt-mixing torque" is not intended to refer to torque values observed prior
to
3o melting of the polymer plus branching compound. It should further be
understood that the concentrate's melt mixing torque should be determined in
the substantial absence of any catalytic materials (e.g., sodium carbonate)
which may enhance the reaction of the branching compound and the polyester.
Concentrates melt processed in accordance with the present invention
35 should be melt processed at temperatures sufficient to melt both the
polyester
and branching compound and with sufficient mixing to provide for homogeneity
of the melt processed mixture. Typically, the mixture will be melt processed
at
temperatures between 200 and 310 °C.
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The surprisingly low mixing torques exhibited by the highly branching
agent-loaded concentrates in Figures 1 and 2 is attributable to compositional
differences in the melt processed concentrates. Relatively highly branching
agent-loaded concentrates are believed to be compositionally different from
5 lower loaded concentrates in that the more highly loaded concentrates are
substantially free of cross-linked structures which are relatively insoluble
in a
polyester during subsequent melt processing steps. As used herein, the term
"crosslinked" refers to reaction products of a branching agent and polyester
in
which four or more polyester chains become attached through two or more
1 o branching agent molecules. The compositional differences are best
explained
by comparing the following Examples A and B.
Example A
In this Example, a PET resin contains the three moieties shown below as
structures P1, P2 and P3.
HOCHz- P~--CH.,OH
P~
O
HOCHz --~ -C - OH
Pz
O O
HO - C P C- OH
P3
where denotes a PET polymer chain.
P1, P2 and P3 comprise 90 weight percent of a PMDA/PET mixture to be
melt processed into a concentrate. P1, P2 and P3 are present in the mixture in
weight percents of about 40, 40 and 10 weight percent, respectively, with the
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remaining 10 weight percent of the PMDA/PET mixture comprising substantially
unreacted PMDA.
The PMDA/PET mixture is melt processed by extrusion to form
concentrate pellets useful for melt processing with PET in subsequent melt
:, processing operations.
During formation of the concentrate pellet, PMDA, P1 and P2 react to
yield the structures shown below as C1 and C2.
O O O O
II
\OCH~ p CH.,O ~ ~ O
\ II COH HOC
O
O ~~ O
C~
O O
o ~~ II
HOC- PO-CHZO~ \
O
HO I/
O
C2 O
to
As can be seen by comparing P1 and P2 to the structures of C1 and C2,
PMDA reacts with PET hydroxyl endgroups to form compounds C1 and C2
which are relatively soluble in the concentrate.
C1 and C2 are highly stable in PET, and are easily melt processed into
15 PET in a subsequent melt processing operation such as the process described
in connection with Figures 1 and 2. It is believed that the weight percent of
crosslinked structures in the foregoing concentrate is less than about 0.1
weight
percent, and possibly less than 0.01 weight percent.
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17
x m IeB
In this Example, PET resin contains moieties P1, P2 and P3 in the same
ratio as in Example A. P1-P3 comprise 99 weight percent of the mixture to be
melt processed into a concentrate, with PMDA comprising the remaining 1
weight percent of the mixture.
The melt processed concentrate contains relatively small amounts of
moieties C1 and C2_ The relatively low PMDA to hydroxyl endgroup ratio
permits further reactions of PMDA with PET to occur, resulting in the
production
of crosslinked structures in which four or more PET polymer chains have
1 o reacted with two or more PMDA molecules. The structure of one such
crosslinked molecule appears below as compound X1.
° o
i i i
HO-CI-~-CH20 ~ OCHz pO-CHz ~ O-CHZ- p ~-OH
HO-C-~-CHzO \ ~ OCHz ~-CH20 \ O-CH -
I z PO- i -OH
I
O O O O O O
x~
Compounds similar to X1 and their precursors in which multiple PET
polymer chains are bound through multiple PMDA molecules are relatively
insoluble in PET.
The presence of crosslinked compounds similar to X1 and their
2 o precursors in melt-processed concentrate explains why the melt processed
concentrates containing up to about 2 weight percent of PMDA exhibit the
relatively high mixing torques shown in Figures 1 and 2. Conversely, where the
PMDA concentration in the concentrate is greater than about 2 weight percent,
the relative low ratio of hydroxyl end groups to PMDA molecules results in a
zs concentrate in which highly PET-soluble, non-crosslinked compounds such as
C1 and C2 are present in much higher amounts than X1-like crosslinked,
compounds. The relative absence of crosslinked compounds explains the
relatively low mixing torques exhibited on Figures 1 and 2 for concentrates
containing greater than about 5 weight percent PMDA.
3o More specifically, as illustrated in Figures 1 and 2, branching agent
loadings in highly branching agent-loaded concentrates exhibit maximum
mixing torques during preparation which are a factor of about 3 (at 2 weight
percent PMDA) to 10 (at 5 weight percent PMDA) to as much as about 20 (at
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18
weight percent PMDA) or more lower than the maximum mixing torque value
obtained when melt processing a concentrate of PMDA and PET having about
1 weight percent PMDA loading.
The inventive concept illustrated by Examples A and B can be used to
5 prepare other branching agent and polyester carrier concentrates useful in
subsequent melt processing operations. The mixture of branching agent and
polymer used to prepare such a concentrate should contain a stoichiometric
excess of branching agent sufficient to limit the formation of crosslinked
structures in the concentrate to a level below which substantial increases in
o mixing torque will occur during preparation of the concentrate. Branching
agent
to polymer ratios or concentrations useful in the preparation of polymer and
branching agent concentrates may be determined in the following manner.
Mixing torque data for melt processing preparation of concentrates containing
various amounts of branching agent and polyester and any required additives
i5 are determined in accordance with the procedures used in connection with
Figures 1 and 2. The total weight percent of polyester and bonding agent
should be about equal in each case. From this data, the branching agent
weight percent producing the highest maximum melt-mixing torque for a given
amount of polyester is identified. Higher branching agent to polyester ratios
or
Zo concentrations yielding lower maximum melt-mixing torques during
concentrate
preparation are then identified and beneficially used to prepare concentrates
according to the present invention. The preferred concentrates will exhibit at
least a factor of 2 reduction in mixing torque during preparation from the
identified maximum mixing torque, although compositions characterized by
25 greater reduction factors such as the 3, 10 and 20 reduction factors
illustrated
by the PET/PMDA data discussed above are preferred. Preferred concentrates
consist essentially of polymer, branching agent, their noncrosslinked reaction
products and any additives. A concentrate can be considered to consist
essentially of the noncrosslinked reaction products of the polyester resin
carrier,
3o the branching agent any additives, and the noncrosslinked reaction products
of
the resin carrier, branching agent and additives when the mixing torque
required to prepare the concentrate is at least a ten factor less than the
maximum melt-mixing torque determined as explained above.
The concentrates in accordance with the present invention can also be
35 characterized by the weight percent of crosslinked material present in the
melt
processed concentrate. Generally, the weight percent of crosslinked material
in
the concentrate should be less than about 1.0 weight percent, with less than
0.1
weight percent being preferred, and less than 0.01 weight percent being most
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19
preferred. The amount of crosslinked material may be characterized by
dissolving the concentrate in boiling nitrobenzene and collecting and weighing
any remaining insoluble material.
Yet another way to prepare the melt processed concentrates in
accordance with the present invention is to specify the ratio of branching
agent
reactive sites to highly reactive polyester end groups. As used herein, the
term
"branching agent reactive sites" includes all functional groups of the
branching
agent which ultimately are expected to react with polymer chains under melt
processing conditions. For example, a PMDA molecule contains four branching
io agent reactive sites. The term "highly reactive end groups" includes those
endgroups which are expected to readily react with branching agent under the
melt processing conditions used to prepare the concentrate. Typically, highly
reactive endgroups are hydroxyl end groups. Conceivably, other highly reactive
endgroups could be present, such as secondary amines found in polyesters or
s introduced into polyesters, either alone or in combination with hydroxyl
endgroups. In such a case, those endgroups would be included when
determining the number of highly reactive endgroups used in the branching
agent functional site to highly reactive endgroup ratio. Where relatively non-
reactive endgroups such as carboxylic acid endgroups are present in the
Zo polyester, these endgroups are not considered highly reactive endgroups for
the purpose of determining the branching agent reactive site to highly
reactive
end group ratio.
Generally, it is preferred that the ratio of branching agent reactive units
(e.g. moles of branching agent multiplied by the functionality of the
branching
25 agent) to highly reactive endgroups be at least 6:1, more preferably
between
12:1 and 100:1, and most preferably between 20:1 and 50:1. In the case of
concentrates containing PMDA and a PET such as Goodyear 7207 containing
approximately the relative amounts of P1, P2 and P3 specified in Example A, it
is preferred that the ratio of branching agent reactive sites (e.g. moles of
PMDA
3o multiplied by 4) to free hydroxyl terminal groups be at least 6:1, more
preferably
between 12:1 and 100:1, and most preferably between 27:1 and 43:1, the latter
range corresponding approximately to a concentrate prepared from a PET and
about 8 to 12 weight percent of PMDA.
A process useful in practicing the invention comprises: (1 ) forming a
molten mixture comprising (i) a major amount of a first resin composition
comprising polyester and from 0 up to about 1 wt.% of a compound capable of
branching the polyester, and (ii) a minor amount of the branching agent
concentrate discussed above, wherein the relative amounts of (i) and (ii) are
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such that the molten mixture comprises from about 0.1 to about 1 wt.% of the
branching compound; (2) adding a blowing agent to the molten mixture and (3)
extruding the resultant mixture to obtain a foam. The polyester foam
obtainable
from this process represents density reductions of at least about 30%,
preferably at least about 60%, and most preferably at least about 80%. Lower
densities are preferred for building insulation, whereas higher densities can
be
found acceptable for food service applications.
Additional enhancements in foamability and foam ~ quality can be
achieved if either the concentrate itself, or the virgin or recycle polyester
feed to
io which the concentrate is added, or both, are first treated with an aqueous
solution of alkali or alkaline earth metal compound, preferably aqueous sodium
hydroxide, under conditions of time and temperature sufficient to incorporate
about 10 to about 175 ppm of alkali or alkaline earth metal in the total
composition intended for foaming. Suitable conditions for achieving such
i5 incorporation comprise an aqueous wash with the alkali or alkaline earth
metal
compound for about 2 to about 30 minutes at a temperature of from about 85 to
about 105°C. The beneficial effects of such treatment are surprising in
that the
prior art (see e.g., Hayashi ef al. '991 discussed above) teaches that the
presence of a group I, II, or III metal at levels under about 220 ppm does not
zo impart any processing advantages in the foaming of branched polyester.
While
these low levels of metal, preferably sodium, may be employed to significant
advantage in the present invention, this does not preclude the use of
additional
levels of sodium or other compounds of Group I, II, or II metals as disclosed
in
Hayashi et al U.S. patent 5,000,991.
25 Use of the above-described concentrates in the process of the present
invention allows a high melt strength extrusion mass to be formed during the
latter portions of the extruder residence time, while avoiding high viscosity
at
the beginning of the extrusion. Conventional melt-strength improved resins
obtained by melt processing a branching compound with resin, wherein the
3o branched resin is then remelted for use in fabricating articles, typically
suffer
from production rate limitations due to high viscosity near the loading end of
the
extruder. The present invention solves this problem not only in the context of
foaming extrusion but in other fabrication contexts where production rates are
important, (e.g. extrusion blow molding, injection molding, etc.).
Accordingly,
35 the invention is further directed to a process for melt processing
polyester
comprising: (1) formation of a molten mixture comprising (i) a major amount of
a
first resin composition comprising polyester and from 0 up to about 1 wt. % of
a
compound capable of branching the polyester, and (ii) a minor amount of a
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21
second polyester resin composition comprising at least about 50 wt.% polyester
resin and greater than about 2 wt.% of a compound capable of branching the
polyester, wherein the relative amounts of (i) and (ii) are such that said
molten
mixture comprises from about 0.1 wt.% to about 1 wt.% of said branching
J compound; (2) melt-processing of the resultant molten mixture under
conditions
of time and temperature sufficient to enhance the melt strength of the
mixture;
and (3) direct fabrication of the molten mixture into a film, sheet or
injection
molded article. Suitable melt-processing conditions are those sufficient to
produce a desirable increase in the melt strength of the resin without causing
o unacceptable amounts of degradation in the resin. Residence times of about 1
to about 20 minutes at temperatures of about 240° to about 310°C
are
acceptable. Melt processing can be performed via screw-type extrusion, but .is
not limited to that technique. Melt strength enhancement is evident when the
melt exhibits shear thinning characteristics at low shear rates, i.e., 1 Sec-
1.
m The term "direct fabrication" as used herein should be understood to mean
that
the molten mixture obtained in the process of the invention as described above
is not first converted to powder or pellets for subsequent remelting in a
later
melt fabrication of desired articles, but is instead immediately melt
fabricated
into such articles.
2o A further discovery of the present invention is that throughput of the
foaming extruder can be improved without loss of other process or product
advantages afforded by this invention by including a styrenic resin in the
final
(i.e., polyester + concentrate) composition subjected to foaming in the
invention.
Specifically, this feature of the invention involves incorporating an amount
of
25 styrenic resin, preferably polystyrene, in the polyester composition to be
foamed
in amounts effective to improve the throughput of the foaming extruder. In a
preferred embodiment this feature of the invention can approximately double
foaming extruder throughput. We have determined that an amount of styrenic
resin greater than about 1 wt. % and particularly within the range of about 5
to
3o about 20 wt% polystyrene, based on the weight of the total composition
being
foamed, is effective for providing significant throughput improvements in the
polyester foaming process of the invention. Amounts of polystyrene above or
below this range can be employed as desired. However, at increasingly high
levels of polystyrene, the sought after property advantages of polyester foam
35 tend to merge with those of polystyrene, whereas at lower levels of
polystyrene
throughput improvements may become de minimus. In addition to increasing
extruder throughput, the presence of styrenic resin also expands the operating
range of the polyester foaming composition. The styrenic resins useful for
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22
improving extruder throughput according to the invention are resins containing
repeating units having the following general formula:
R2
R~ ' C - CH2
wherein R1 represents an aromatic hydrocarbon radical, or an aromatic
halohydrocarbon radical of the benzene or substituted benzene series, and R2
is either hydrogen or the methyl radical. Resins which can be used as the
styrenic foam core resin include such alkenyl aromatic compounds as the solid
1 o homopolymer of styrene; alpha-methyl styrene; ortho-methyl styrene;
meta-methyl styrene, para-methyl styrene; the solid copolymers of one or more
of such alkenyl aromatic compounds with amounts of other polymerizable
compounds such as methylmethacrylate, acrylonitrile, malefic anhydride,
acrylic
acid, and the like; impact polystyrene, which is a polystyrene modified by or
1 s containing elastomer moieties, such as styrene butadiene or polybutadiene;
and
blends of a styrenic resin such as polystyrene/poly(2,6-dimethylphenylene
oxide). Other modified polystyrene resins which can be used in the invention
include brominated or halogenated polystyrene such as polydibromo styrene
(e.g., PDBS-10 and PDBS 80 commercially sold by Great Lakes Chemicals).
2o With respect to extrusion conditions for carrying out the foaming process
of the invention, such conditions can be adjusted in a manner known to those
of
ordinary skill in the art. Broadly speaking these conditions should be
adjusted
such as to allow the completion of the reaction between the branching
compound present in the concentrate and the polyester resin to which the
2~ concentrate has been added. In general, the foam extruder residence time
should be in the range of about 2 to about 20 minutes and barrel zone
temperatures should be in the range of about 210°C to about
310°C.
The type of extrusion equipment suitable for carrying out the process of
the invention is a matter of selection within the skill of the art. For
example, the
3o process can be performed on a single screw, twin screw, planetary gear
extruder. Often these different types can be arranged in tandem, with the
second extruder in the tandem arrangement used to cool the melt.
Any suitable physical or chemical blowing agent, or blowing agent
mixture, can be used ,e production of polyester resin foams in the present
invention, so long a~ . .ch agent or mixture thereof is easily vaporizable or
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23
thermally decomposable. Easily vaporizable blowing agents include inert
gases, such as argon, saturated aliphatic hydrocarbons, saturated alicyclic
hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, ethers and
ketones are preferred. Examples of easily vaporizable blowing agents include
carbon dioxide, nitrogen, methane, ethane, propane, butane, pentane, hexane,
methylpentane, dimethylbutane, methyl cyclopropane, cyclopentane,
cyclohexane, methylcyclopentane, ethylcyclobutane, 1,1,2-trimethyl
cyclopropane, trichloro monofluoro methane, dichloro difluoro methane,
monochloro difluoromethane, trichloro trifluoro ethane, dicloro tetrafluoro
i o ethane, dichloro trifluoro ethane, monochlorodifluoro ethane,
tetrafluoroethane,
dimethyl ether, 2-ethoxy acetone, methyl ethyl ketone, acetylacetone dichloro
tetrafluoro ethane, monochlorotetrafluoroethane, dichloro monofluoro ethane
and difluoroethane. Particularly preferred blowing agents are butane, propane,
ethane, pentane, Freon 11, Freon 22, Freon 134a, Freon 142b, carbon dioxide,
nitrogen, water and suitable mixtures thereof.
The blowing agent is injected into the molten blend of the polyester resin,
concentrate and other additives present in the extruder. The amount of the
blowing agent to be injected is from 0.05 to 50% by weight based on the
amount of the molten blend. When the amount of the blowing agent is less
2o than 0.05% by weight, the resulting foam is not sufficiently expanded,
while
when the amount is more than 50% by weight, the gas of the blowing agent is
not accommodated for foaming, but blows off and the foam cannot be formed
into a desired shape. A particularly preferred amount of the blowing agent is
0.1 to 30% by weight based on the amount of the molten blend.
25 The present invention also contemplates use of chemical blowing agents
such as mixtures of sodium bicarbonate and citric acid.
A wide variety of economical articles are made possible by the
concentrate and process of the invention. Examples of such articles are
building insulation board having R value of at least 4 per inch and preferably
at
30 least about 5 per inch, and food packaging articles such as bowls, cups and
trays which can withstand conventional and microwave oven conditions
necessary for heating pre-cooked foods to serving temperature. Examples of
other foamed articles that can be manufactured using the invention are
flotation
devices, cushioning articles, sound reduction and sound absorption materials,
35 automobile headliners, highway sound barriers, foamed bats for baseball and
softball and other toy or novelty items, and decorative molding such as
artificial
wood trim, etc.
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24
The present invention can be used to obtain foamed polyester articles
having densities in the range of about .02 to about .9 grams/cc. Within this
range, preferred densities for polyester foam building insulation are in the
range
of about .03 to about .15 grams/cc, while preferred densities for food trays
are
are in the range of about .06 to about 0.3 gramslcc.
Sirigie or double laminate articles, such as food containers or trays, can
be produced by bonding a non-foam film of a thermoplastic resin to one or both
sides of an extrusion polyester foam sheet produced according to the
invention,
followed by thermoforming the laminate to obtain a food container of desired
i c shape. If desired, the solid resin film laminated to one or both sides of
the
polyester foam core can be selected such that it will impart oxygen barrier
properties to the food tray or container. Suitable processes for producing
single
or double laminates are disclosed in Mazur, U.S. Patent No. 3,699,794 and
Whelan e~ al., U.S. Patent No. 3,616,020.
J Although the laminates described in Mazur and Whelan g~ ~I. include
laminates
in which an impact modified polystyrene layer is extrusion coated onto a
polystyrene foam sheet, the processes disclosed in these patents can also be
applied to the extrusion lamination of a resinous layer onto a polyester foam
core layer. R polyester foam core sheet produced according to the .present
2o invention can also be substituted for a polystyrene foam core layer in the
multi-
layer construction of Luetkens, Jr. ,e~ ~L, U.S. Patent 5,128,196.
. Examples of thermoplastic resins suitable for lamination as a non-foam
layer to a polyester foam sheet of the invention include liquid crystal
polyester
resins, polyolefin resins, polyamide resins, polyvinyl chloride resins,
25 polyacrylonitrile resins, poiyvinylidene chloride resins and ethytene-vinyl
alcohol
copolymers. The relative thicknesses of the non-foam layer and the polyester
foam layer can be adjusted depending upon the end uses intended for
particular laminate articles. In addition to the patents referred to above,
various
methods for laminating a non-foam layer onto a polyester foam sheet are
3o discussed in Hayashi gf al., U.S. Patent 5,000,991, for example, the foam
sheet
and the non-foam sheet can be separately prepared and separately wound up
into rolls and then, laminated onto each other while unwinding and passing
through a pair of rollers.
The laminates described above can be thermoformed into a variety of
35 articles, such as food service articles, using techniques which are well
known in
the art. For example, the thermoforming can be' carried out by using a molding
die. The die may be composed of a male mold and a female mold, but may be
comw::~sed of either one of them. When a die composed of both molds is used,
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- 25
molding can be carried out merely by putting the laminated sheet between both
molds and pressing it. However, when either one of molds is used, air present
between the sheet and mold is removed, or the sheet is pressurized from the
upper side thereof and pressed.
s With respect to the production of articles, including foamed articles, a
particularly advantageous and unexpected feature of the present invention is
that such articles can be produced with substantially reduced levels of
unreacted branching compound (e.g. pyromellitic dianhydride) as compared
with foam extrusion in which branching compound, instead of being charged to
1 o the process as an extruded concentrate, is added as a neat material (e.g.
in
powder form) directly to the foaming extruder. Accordingly, the present
invention is further directed to polyester articles comprising a branching
compound selected from compounds having two or more acid anhydride groups
per molecule, said branching compound being present in the article in the form
1 s of (i) a reaction product of said branching compound and said foamed resin
and
(ii) unreacted branching compound; and wherein the ratio of the amount of
reacted + unreacted branching compound to the amount of unreacted
branching compound, is greater than about
20 20:1 when the amount of reacted + unreacted branching compound
in the article is greater than about 5000 ppm based on the weight of
the article; or
40:1 when the amount of reacted + unreacted branching compound
2 s in the article is less than about 5000 ppm.
The reason for making a distinction between articles having greater than
or lesser than about 5000 ppm total branching compound is due to minor
limitations in the ability of the analytical technique described herein to
detect all
30 of the unreacted branching compound in a given sample. This difficulty
results
in ratios of total to unreacted branching agent which tend to be slightly
higher
than expected at lower levels of total branching compound. The analytical
methods used to measure total versus unreacted branching compound in a
polyester sample are described in Example C and D, below.
35 Foamed articles made according to the invention, wherein the branching
compound comprises pyromellitic dianhydride in a total amount of about 1500
ppm to about 5000 ppm in the article, will preferably contain less than about
100 ppm unreacted PMDA based on the weight of the article. When the total
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26
amount of PMDA is in the range of about 5000 ppm to about 6000 ppm, the
amount of unreacted branching compound in the article will be less than about
150 ppm based on the weight of the article. When the total amount of PMDA is
about 6000 ppm to about 8000 ppm in the article, the amount of unreacted
s PMDA will be less than about 375 ppm based on the weight of the article.
These amounts of unreacted PMDA represent reductions of about 50 to 75%
over the levels obtained in a polyester extrusion process which does not
employ
the concentrate addition technique of the present invention. While reductions
in
free PMDA may be particularly advantageous in end use applications involving
io food contact, the general benefit in eliminating a freely reactive
branching
compound from finished articles in any application will be readily
appreciated.
The following examples will serve to illustrate but not limit the present
invention. In the following examples, all PET resin samples including
concentrates were dried in a forced air dessicant oven at 120°C
overnight prior
1 s to use.
Examl la a C
Analysis of Total PMDA in PET Foam
1. Weigh 0.5000 gram foam sample into a 100 ml 1-neck round
bottom boiling flask.
2. Add 20 ml dimethylsulfoxide (DMSO) to flask via 20 ml volumetric
pipet.
3. Add 5 ml 5N alcoholic sodium hydroxide (NaOH) to flask via 10 ml
transfer pipet.
4. Set up flask to reflux with stirring using a heating mantle plugged
into a variac to heat and a stirring bar/stir plate to stir.
5. After sample has dissolved (~1/2 to 1 hour after start of heating),
turn off heat and replace heating mantle with a cork ring to allow
samples to cool with continued stirring.
6. After sample has cooled to room temperature, add 50 ml
deionized water to the flask via 50 ml volumetric pipet to dissolve
the sodium salts. At this point, sample should be clear.
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7. Determine PMDA concentration using high performance liquid
chromatography (HPLC) by eluting an aliquot of the neutralized
sample through a high pressure liquid chromotography (HPLC)
system using a concentration-gradient mobile phase of
acetonitrile/water. The method is calibrated using standards
containing known amounts of PMDA. The calibration curve for
these standards is not linear; compensation for this is made by
bracketing sample analyses with analyses of standards containing
PMDA in the same range as the samples.
Exam~he D
Analysis of Extractable l"Free"~ PMDA in PET Foam
1. Weigh 1.000 gram of foam sample into a 100 ml 1-neck round
1 s bottom boiling flask
2. Add 25 ml nitrobenzene to flask via topsider dispenser or 25 ml
volumetric pipet.
3. Set up flask to reflux with stirring using a heating mantle plugged
into a variac to heat and a stirring bar/stir plate to stir. Note: any
talc and/or sodium carbonate in the sample will not dissolve in the
nitrobenzene.
4. After sample has dissolved (~1/2 hour after start of heating in
most cases), turn off heat and replace heating mantle with a cork
ring. Continue stirring and allow sample to cool to room
temperature on its own (do not quench in ice or cold water bath).
Note: the PET will precipitate from the nitrobenzene ~85C and
3o form a continuous viscous liquid; however, the PMDA will remain
in solution in the nitrobenzene.
5. After sample has cooled to room temperature, add 50 ml
deionized water to the flask via 50 ml volumetric pipet and
3s continue stirring (amount of time is not critical).
6. Homogenize PETlnitrobenzene slurry with the water using a Bio-
homogenizer mixer for ~2 minutes at high speed. If PET has
formed a particularly stiff viscous liquid upon precipitation from the
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nitrobenzene, ensure that all material comes in contact with the
water by moving the homogenizer probe around the entire area of
the flask. This is a critical point in the procedure, as the water
extracts the PMDA from the nitrobenzene.
7. Perform high performance liquid chromatography (HPLC) on the
aqueous phase obtained in step 6 by eluting an aliquot of the
water portion of the sample through a high pressure liquid
chromotography system using a concentration-gradient mobile
i o phase of acetonitrile/water. The method was calibrated using
standards containing known amounts of PMDA.
In the above procedure it is found that a small residual amount of
unreacted PMDA cannot be extracted from the nitrobenzene phase, hence the
m procedure tends to slightly understate the amount of extractable, or free
PMDA
in a given sample. Notwithstanding this difficulty, the procedure is quite
suitable
for determining differences in extractable PMDA among different foam samples
as well as for determining whether a particular foam sample meets the
requirements of the present invention.
Comparative Examale 1
(unstable foam process)
A mixture comprising 97.95 wt% PET (Goodyear 7207), 1.0 wt%
pyromellitic dianhydride (PMDA) obtained from Diacell, 0.3 wt.% Na2C03
(Aldrich), and 0.75 wt% talc (Cyprus Mineral Co. Mistron Monomix) was melted
and further mixed with Freon-22 using a ZSK-30~ corotating twin screw
extruder to produce a polyester foam. The extruder (35:1 UD), 30mm screw
diameter) was supplied by Werner and Pfliederer Inc. and was operated at a
rate of 14 Ib/hr through a 2" x 0.35" slot die using a screw rotation speed of
75
3o rpm. The screw design consisted of a feed conveying section followed by a
melt seal, continuing with conveying and mixing sections to the die. Fifteen
separate temperature controllers were used to control process temperatures
from the feed hopper (zone 1 ) to the die (zone 15). Temperatures were
maintained at 245, 260, 290, 290, 295, 295, 290, 270, 265, 260, 250, 240, 240,
240, 220°C from zones 1 through 15 respectively. The PET pellets were
fed to
the extruder using a K-tron~ S-200 volumetric feeder. The PMDA, talc, and
sodium carbonate were fed using an AccuRate~ dry material feeder into the
same extruder fed throat as the PET pellets. The Freon-22 was fed into the
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extruder immediately downstream of the melt seal using an American Lewa~
diaphram liquid metering pump. Extruder torque (as a percent of designed
capacity) and die pressure were monitored continuously during foam
production. The extrudate exited the die into a region of atmospheric pressure
., and entered conventional shaping and haul off equipment. During an extended
run during which no process parameter changes were made, samples of the
foam extrudate were collected at regular (10 minute) time intervals and the
corresponding extruder torques and die pressures recorded. As shown in Table
1 below, the extruder torque and foam quality both varied significantly over
the
1 o course of this run, despite the stability of process settings. Extruder
torque
oscillated between 47% and 84% of full scale, while foam density ranged from
to 39 pounds per cubic foot, indicating instability in the process in which
PET
is co-fed with an additive package such as PMDA, sodium carbonate, and talc
to a foaming process at the concentrations identified. This instability was
not
i5 eliminated by dry blending the additives with PET and an optional small
amount
of mineral oil to allow feeding with a single feeder. Elimination of sodium
carbonate from the feed stream did not eliminate the process instability.
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TAB E I
Time (minutes) Density Torque Die Pressure
(glcc) (%of full torque(psi)
10 0.56 54 100
20 0.26 68-70 150
30 0.15 61-64 260-310
0.58 75-78 160-170
0.18 80-82 260-320
0.18 75-79 270-330
0.19 81-84 270-400
0.19 78-81 270-340
0.19 74-77 290-390
100 0.21 74 260-300
110 0.20 70 240
120 0.59 60 180
130 0.30 57 130
140 0.55 47 120
150 0.27 59 150
160 0.4.1 52 120
170 0.26 58 120
180 0.16 70-74 220-250
190 0.24 73 210
200 0.52 64_68 170
210 0.26 68-70 170
220 0.24 70 200
230 0.28 70-72 190-230
240 0.30 63 160
250 0.63 54 130-160
260 0.35 52 100
270 0.50 53 110
280 0.45 50 110
Example 1
s IStable Process Using Concentrate of the Present Invention)
In this example, large amounts of PMDA, talc, and Na2C03 are
concentrated into PET instead of being added directly to the foaming extruder
as in Comparative Example 1. These concentrates are added to additional PET
to to achieve the desired final concentration of each additive in the final
product.
Thus a concentrate according to the invention was prepared by mixing of 79.5
wt% PET, 10 wt% PMDA, 3 wt% Na2C03, and 7.5 wt% talc in a Haake System
90 Torque Rheometer inrith single screw extruder attachment. The 3/4"
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diameter screw was a standard metering type with LID of 25:1, and 3:1
compression ratio. The material was compounded at 120 RPM with a
temperature profile from feed throat to die of 260/280/280/280°C. The
extrudate polymer strand was passed through a water bath and pelletized.
.. After drying, these concentrate pellets were mixed in a 1:9 ratio with
unmodified
PET and fed using a K-tron S200 volumetric feeder into a ZSK-30 twin screw
extruder for reactive extrusion and foaming under process conditions
comparable to Comparative Example 1. In this example the resin feed rate was
measured to be 11 pounds/hour, and the temperatures were maintained 245,
Zo 260, 290, 290, 295, 295, 290, 270, 265, 260, 250, 235, 230, 225, and
215°C
from zones 1 through 15 respectively. Samples, torque and pressure readings
were collected at regular (10 minute) time intervals as in Comparative Example
1. The torque recorded during this example varied between 78% and 85% full
scale load as shown in Table 2. The foam product quality was observed to be
1 s significantly more uniform than that of Comparative Example 1. Foam
density
ranged from 7.2 to 8.8 pounds per cubit foot.
TABLE II
Time (minutes) Density Torque Die Pressure
(glcc) (%of full torque(psi)
0.13 80-83 270-290
0.14 82-84 260-300
0.14 80-83 270-290
0.13 80-82 250-270
0.13 79-83 260-280
0.12 78-81 260-270
0.14 81-84 210-330
0.14 81-83 240-310
0.13 78-80 260-310
100 0.12 80-84 270-340
110 0.12 82-85 270-410
120 0.14 81-84 270-340
130 0.13 80-83 250-300
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Comparative Example 2
(Unstable Process)
Goodyear 7207 was dry blended with 1.0 wt% PMDA, 0.3 wt% Na2C03
and 0.75 wt% talc and fed into a 3/4" single screw extruder attached to a
Haake
System 90 torque rheometer. The extruder screw had a standard metering
profile with 3:1 compression ratio and 25:1 UD ratio. The temperature profile
was 260/280/280/280°C from feed hopper to die as in Example 1. The
materials were extruded through a .06" diameter stranding die at 60 RPM while
i o the torque and final barrel pressure were recorded as a function of time.
Pressures ranged from <800 to >3600 psi, while torque oscillated mainly
between 15 and 40 Newton-meters. The process was apparently cycling as
opposed to merely exhibiting one torque maximum and then a decline. PET
processed under similar conditions without any reactive additives resulted in
an
1 J extruder torque of about 15 Newton-meters which suggests that the reaction
between PET and PMDA is not being consistently completed during processing
in the manner of this example. The variations seen in the IV's of material
collected at the torque/pressure peaks and valleys of this run supported this
conclusion. This example provides further evidence that co-feeding PET with a
2o viscosity modifying additive package such as PMDA, sodium carbonate and
talc
to the extrusion foaming process in the identified concentrations is not a
stable
process.
Example 2
(Stable Process Using Invention)_
25 A sample of the concentrate produced in Example 1 was dry blended in
a 1:9 ratio with unmodified PET and extruded through the Haake torque
rheometer as in Comparative Example 2. Pressures range between 2400 and
3200 psi after startup, while torque oscillates mainly between .26 and 32
Newton-meters. This oscillation is dramatically reduced from that observed in
3o Comparative Example 2. This example and Example 1 illustrate that the
process oscillations associated with the method described in comparative
examples 1 and 2 can be dramatically reduced by employing the method
described in this invention, i.e., concentrating the additive package into a
carrier
resin prior to adding these components to the foaming extruder.
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Exams I,~ a 3
(The Invention Using Separate Additive Concentrates)
In this experiment, the branching agent, nucleant and catalysts were
s each concentrated separately into a carrier resin. A concentrate "A"
consisted
of 90 wt% PET and 10 wt% PMDA, a concentrate "B" consisted of 97 wt% PET
and 3 wt% Na2C03, and a concentrate "C" consisted of 92.5wt% PET and
7.5wt% talc. Concentrates A, B, and C were prepared using the Haake single
screw extruder. One pound of each concentrate (A, B, and C) was dry blended
i c with seven pounds of PET and fed into the ZSK-30 twin screw extruder for
foaming. The torque was stable and the process produced a foam of excellent
quality comparable to that obtained in Example 1. This experiment
demonstrates that it is not critical for any two of the additives to be
co-concentrated for the final foam product to be produced in a stable manner.
i ~ The process has been repeated using concentrates prepared on the ZSK-30
extruder instead of the Haake system 90 rheometer. The process stability and
foam quality are excellent irrespective of which machine was used to prepare
the concentrates.
z o Example 4
Using a Haake torque rheometer with mixing bowl attachment, Goodyear
7207 PET was mixed with various amounts of PMDA powder (Chriskev) and
sodium carbonate powder (Aldrich) at 270 C. The bowl volume was 60cc and
the roller type blades were rotated at 60 RPM. The torque needed to rotate the
25 blades was monitored on a continuous basis by the software provided with
this
instrument. In each experiment, a large torque increase was noted immediately
after adding 72 grams of the pellet/powder mixture to the bowl. This peak is
commonly referred to as the loading or melting peak. The maximum melt
mixing torque reported in Tables III and IV refers to torques obtained after
the
3o pellets are completely melted. In each run reported in the tables below,
the
maximum melt-mixing torque was obtained in less than ten minutes after
completion of melting. Tables III and IV contain the torque results showing
the
criticality of utilizing amounts of branching agent greater than about 2 wt%
to
compound the concentrate of the invention. The full torque curves for the
35 concentrates of Table III are plotted in Figure 1, whereas the torque
curves for
the Table IV concentrates are plotted in Figure 2.
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Table III
(Torque Data for Concentrates Containing PET,Sodium Carbonate and PMDA)
wt% PMDA Maximum Melt Mixing Torque
(Newton-meters)
2
0.5 17
1.0 22
2.0 6
1
Table IV
(Viscosity Data for Concentrates Containing PET and PMDA Without Sodium
Carbonate)
wt% PMDA Maximum Melt Mixing Torque
(Newton-meters)
0 1
.5 g
1.0 11
1.5 6
2.0 3
10 1
1 o Example 5
(Foam Tray of the Invention)
In this experiment, the ZSK-30 described above was used to produce
Concentrates A and B as described in Example 3. Concentrate A consisted of
90 wt% Goodyear 7207 PET and 10 wt% PMDA. Concentrate B consisted of
97wt% Goodyear 7207 and 3% Na2C03. The concentrates were added to the
extruder with a PET mixture containing 25% Goodyear PET 9506 regrind and
25% virgin PET 9506. The relative amounts of the concentrates and the 9506
resin were such that the amount of PMDA in the total composition intended for
foaming was .2 wt.% and the final amount of sodium carbonate was .04 wt.%.
2o Foam production took place using an Egan 4.5 inch diameter single screw
extruder modified with a gas delivery system. The seven barrel zones and the
extrusion die were set at the following temperatures: 540, 540, 540, 520, 520,
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520 and 520, and 520°F. A flat sheet die was used to produce PET foam
sheet. A dry mixture of Concentrates A and B was fed into the extruder through
a side feed hopper separately from the above described mixture of virgin and
regrind 9506 PET resin. The PET 9506 resin and the concentrates were dried
.~ to moisture levels of under 50 ppm prior to addition to the extruder. Foam
was
produced at a nominal rate of 600 pounds/hour. C02 was injected into the 4th
barrel segment of the seven zone extruder. This resulted in a final foam
product containing 0.2% PMDA and 0.04% Na2C03 by weight. The faam
sheet had a density of approximately 30pcf (despite passing through a three
roll
1 o stack) and was successfully thermoformed into trays. The trays, when
analyzed
for extractable PMDA using the analytical techniques of Examples A and B, had
levels of unreacted PMDA in the range of about 30 ppm to about 40 ppm based
on the weight of the tray.
The procedures of this example were repeated, except that the levels of
m PMDA and sodium carbonate were increased to 0.3 wt.% and 0.06 wt.%,
respectively, based on the weight of the total composition being foamed. The
resultant extruded foam sheets had a density of 9 pcf. Trays thermoformed
from the sheet had density of about 28 pcf.
zc Example 6
(Insulation Board of the Invention)
A sheet of foam insulation according to the invention was made as
follows. A ZSK-30 extruder was used with a temperature profile (15 zones
feeder to die) as follows: 245, 260, 290, 290, 295, 295, 290, 270, 265, 250,
2 s 235, 225, 220, 220, and 220 C. Freon-22 was fed in the 7th barrel section
at a
rate of 2wt% with respect to the resin feed of about 19 Ib/hour. The resin
feed
to the extruder hopper was a dry mixture containing 91.67 wt.% Goodyear 7207
PET, 5 wt.% of Concentrate "A" of Example 3, and 3.33 wt% of Concentrate "B"
of Example 3. Upon exiting the die the foam was passed through a forming
3o table using a belt pulley device at a rate of 40 inches/minute The observed
torque was between 92 and 96% full scale. Pressure at the die was 440-540
psi while that at the point of gas injection was 260-280 psi. The screw
profile
used consisted of conveying elements in the feeding/melting zones, a melt
sealing device at the 6th barrel section, and conveying elements towards the
35 die. The resultant foam had a density 6.3 pcf, and cell size 0.56mm. The
foam
sheet had thickness of .6 inches, compressive strength of 53 psi, and
excellent
moisture and thermal dimensional stability. The process set forth in this
example results in foam insulation boards having R values of at least 4 per
inch.
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- 36
x m 1e 7
(Extractable PMDA Levels in Foam Articles)
s Foam trays and sheets were manufactured from PMDA branched PET
using the concentrate described in Example 1. The samples were obtained
from the trays or sheets obtained in Examples 5 and 6. The foam samples
were measured for total PMDA and extractable PMDA (i.e. unreacted PMDA)
using the analyses of Examples A and B. The results are reported in Table V
1 o below
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TABLE V
The Invention--Concentrate Arlrlitinn of Pnnnn
Total PMDA Extractable PMDA
Sample # (ppm) (ppm)
1 1320 (none detected)
2 1530 (none detected)
3 1860 (none detected)
4 1960 15
2000 35
6 2400 3g
7 2600 27
8 2300 2g
9 4100 43
4100 42
11 4100 61
12 4200 46
13 2900 24
14 3200 13
3000 21
16 3000 27
17 7300 361
18 7500 253
1g 5472 53
5699 55
21 5226 52
22 4840 55
23 4867 55
24 4712 55
4457 55
26 4788 55
27 4731 55
28 4322 55
29 4400 55
4329 56
31 6700 120
32 6900 200
33 7600 210
34 7300 200
6500 180
36 2900 30
37 3100 70
38 3000 47
39 6700 120
5002 55
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Comlaarative Example 7
(Extractable PMDA Levels in Foam Articles--Direct Addition of PMDA
Powder
Foam samples were manufactured from PMDA-branched PET using a
s direct addition of PMDA to the foaming extruder as generally described in
Hayashi et al 5,000,991. The foam samples were measured for total PMDA
and extractable PMDA (i.e. unreacted PMDA) using the analyses of Examples
A and B. The results are reported in Table VI below. Samples 1 and 2 were
taken from commercially available PET foam trays believed to be prepared from
1 c a polyester foaming process in which PMDA is added as a powder directly to
the foaming extruder.
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TABLE VI
LComoarative)--Powder Addition of Neat PMDA
Total PMDA Extractable PMDA
Sample # (ppm) (ppm)
1 2700 166
2 3000 183
3 4808 160
4 4820 163
4652 110
6 4759 159
7 5177 166
8 4945 166
9 5425 162
4669 163
11 4669 163
12 4775 163
13 5366 166
14 5361 166
5353 162
16 5194 166
17 7000 440
18 6500 420
19 6800 630
6700 590
21 6500 380
22 3300 190
23 3000 150
24 3000 150
3000 ~ 230
26 2800 260
27 1420 - 72
28 1460 66
29 1470 63
Example 8
Sodium Incorporation--The Invention
Goodyear 7207 PET pellets were ground to powder and slurried in a 4%
NaOH solution at a temperature of about 95°C for 10 minutes. The
powder was
then rinsed with water and dried. This process resulted in the incorporation
of
to 33.9 ppm sodium into the resin sample. Sodium analysis was carried out
using
inductively coupled plasma spectroscopy (ICP). The aforementioned PET resin
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sample, and additional commercially procured PET samples containing varying
levels of sodium, were studied as follows. A total of 7 PET samples containing
varying levels of sodium were mixed with concentrate of the invention
containing 10 wt.% PMDA. The final mixtures, all of which contained 1.0 wt.%
5 PMDA, were melt mixed in a Haake mixing bowl at 270°C and 60 RPM. The
results of the Haake mixing bowl study are shown in Table VII, below. The
table reports starting torque of the melt system prior to reaction of PMDA and
PET, the maximum torque reached in the system following reaction, the time to
reach this maximum torque, and the rate of increase in torque. These results
to show that about 30 ppm sodium affords a significant improvement in the
reactivity of PMDA and PET. The process of the invention can thus be
enhanced when sodium is incorporated into the polyester resin at relatively
low
levels using the caustic wash described above.
Ta 1e VII
Sample ppm Na Torque- Torque- Time to Rate
# minimum maximu Tq-max (meter-
(meter m (sec) grams/
grams) (meter min.)
grams)
1 5.6 259 823 561 123
2 57 352 1853 352 481
3 45 326 1496 362 353
4 5.6 223 1035 492 170
5 43 331 1952 330 540
6 9.3 186 849 580 154
7 33.9 384 1333 356 340
Example 9
This example demonstrates that PET foams of inferior quality are
obtained when PET extrusion foaming is attempted using a branching agent
Zo concentrate compounded with polystyrene or polypropylene instead of
polyester. In each of the examples (a) through (c) below, the concentrates
were prepared using a Haake System 90 torque rheometer with a 3/4" 25:1 L/D
single screw extruder attachment. A metering screw with a 3:1 compression
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ratio was used. The PMDA and sodium carbonate were dried before
compounding. The mixtures were dry blended, extruded and pelletized. The
concentrates were dried before use in foaming.
(a) (invention) A mixture was prepared containing 91.67 wt% PET, 5 wt%
of Concentrate A of Example 3, and 3.33 wt% of concentrate B of Example 3.
A ZSK-30 twin screw extruder was used. Zones 1 and 2 were off. Zones 3-15
were set to the following temperatures: 250, 280, 295, 295, 290, 280, 275,
270,
250, 240, 240, 230 and 230 °C. Maximum throughput of 122 glmin was
achieved at a typical torque reading which was 77-85% of the extruder torque
i o capacity. A foam of excellent quality was obtained.
(b) (comparison--PS used as concentrate carrier resin). the foaming
extrusion of Example 9(a) was repeated under identical conditions except that
polystyrene was substituted for PET in Concentrate A and Concentrate B. The
extruder torque readings ranged between 47-51 % of the extruder torque
15 capacity. Although extruder throughput was improved over run(a) above, the
resultant foam was of markedly inferior quality to that produced in (a) above.
(c) (comparison--PP used as concentrate carrier resin) The foaming
extrusion of Example 9(a) was repeated except that polypropylene was
substituted for PET. Under operating conditions identical to the examples
2o above, a typical extruder torque reading was 49-55% of the extruder torque
capacity. A foam was obtained which was markedly inferior to that obtained in
(a) above.
Exams la a 10
2J This example (runs (a) and (b)) demonstrates that the presence of 10
wt% polystyrene in a PET composition being foamed according to the
concentrate technique of the invention, significantly reduces the measured
torque on the foaming extruder, hence allowing higher feed rates, without
effecting the quality of the foamed product. Comparison Examples 10(c) and
30 10(d) illustrate that this advantage is not obtained using polypropylene or
high
density polyethylene.
(a) (invention--without polystyrene in foamed composition). A dry mixture
mixture was prepared containing 91.67 wt% PET, 5 wt% of Concentrate A of
Example 3, and 3.33 wt% of Concentrate B of Example 3. The mixture was put
35 into the ZSK-30 twin screw extruder for foaming. Foaming was carried out
under conditions similar to Example 1. A maximum throughput of 120 glmin
was achieved at a typical torque reading which was 91-94% of the extruder
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torque capacity. A foam of excellent quality was obtained having a density of
7
pcf and cell size .61 mm.
(b) (invention with 10 wt% polystyrene in foamed composition). In this
example, the foaming extrusion of Example 10(a) was repeated, except that 10
wt% of the total composition intended for foaming was replaced with
polystyrene. A throughput of 124 g/min was reached and a typical torque
reading was 59-62% of the extruder torque capacity. At this throughput rate,
an
excellent foam was produced. The feed rate was then increased to bring the
torque up to approximately the level of Example 10(a) above. A throughput of
io 241 g/min was reached and a typical torque reading was 93-98% of the
extruder capacity. A foam was obtained having quality and density comparable
to Example 10(a).
(c) (comparison). Example 10(a) was repeated except that 10 wt%
polypropylene was used in place of polystyrene. The torque readings obtained
i s under operating conditions identical to 10(a) above were 45-55% of
extruder
torque capacity. This run produced foam of markedly inferior quality to that
obtained in Example 10(a).
(d) (comparison). Example 10(a) was repeated except that 10 wt% high
density polyethylene was used in place of polystyrene. The readings obtained
2o under operating conditions identical to 10(a) above were throughput of 139
g/min, with a typical torque reading of 79-83% of the extruder capacity. The
resultant foam was markedly inferior to that obtained in Example 10(a).
Additional experiments have shown that it is advantageous to produce
and use concentrates in accordance with the present invention which contain
2 s relatively little water. The advantages of low water content concentrates
are
illustrated by Example 11, below.
Example 11
Concentrate pellets containing from 5 to 20 weight percent PMDA in
3o accordance with the present invention were prepared by feeding appropriate
weights of DIACEL PMDA powder obtained from Chriskev Company, Inc. of
Leawood, Kansas into the feeder of a twin screw extruder under a nitrogen
blanket. Also added to the feeder were appropriate weights of SHELL
CLEARTUF 7207 PET, which was dried for 16 hours in a forced air oven at
35 150°C. The weight ratios of PET and PMDA were selected to yield a
concentrate having the weight percents in the concentrate as specified in
Table
VIII, below. "Dry" concentrates having the PMDA concentrations listed in Table
VIII were prepared by extruding the foregoing described mixtures through a
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43
ZSK-30 twin screw extruder at 100 rpm at a melt temperature of about 243
°C.
In the case of the 20 weight percent concentrate, the concentrate was prepared
from a modified PET already containing 0.5 weight percent PMDA.
After the dry concentrates were prepared, the water content of each
batch of concentrate pellets was measured by gas chromatography. A portion
of each concentrate pellet batch was then "conditioned" by allowing the
respective pellets to sit in a 50% relative humidity environment for 120
hours,
with the water content of each conditioned batch thereafter determined by gas
chromatography.
to Both dry and conditioned pellets were subsequently extruded with
additional dried PET and a sufficient amount of Na2C03 to obtain an extruded
product containing approximately 0.1 weight Na2C03 and 0.5 weight percent
PMDA.
The die used in the final extrusion process was equipped with a pressure
i5 transducer to measure the die pressure during extrusion. The weight
percents
of PMDA and water in each sample and the measured die pressures are
summarized in Table VIII, below.
Table VIII
PMDA % H20 Die Pressure~~si)
5 (dry) 0.0221 1150-1410
5 (conditioned) 0.3214
540-550
10 (dry) 0.0367 1000-1500
10 (conditioned) 0.4766 700-800
15 (dry) 0.0259 1560-1580
15 (conditioned) 0.9078 500-800
20 (dry) 0.0520 1320-1670
20 (conditioned) 0.9957 610-670
As can be seen by comparing the data obtained by melt processing both
the "dry" and "conditioned" concentrates, die pressures for dry samples
typically
were about a factor of 2 higher for the "dry" concentrates. The decrease in
die
pressure for the conditioned samples shows that the melt viscosity of the
final
extruded product has been substantially decreased by the presence of water in
the concentrate pellet.
The data in Table VIII illustrates the importance of producing a
concentrate having a relatively low water content if the melt viscosity of a
final
CA 02222333 1997-11-26
WO 96/40821 PCT/US96/08087
44
concentrate/polyester blend is to be maintained. It is therefore preferred to
produce concentrate having low water content, as concentrate in accordance
with the present invention surprisingly cannot be dried after production under
typical PET drying conditions without causing PMDA to sublime from the
s concentrate. Concentrate having a low water content may be produced by any
means known in the art, such as production in cool, low humidity and/or inert
environments. Of course, precautions should be taken to ensure that
concentrate is not allowed to absorb water prior to subsequent melt blending
operations. Water concentration in concentrate according to the present
Io invention should not exceed 2000 ppm, preferably be less than 1000 ppm, and
most preferably be less than 600 ppm.
It should be noted that the 5 to 20 PMDA weight percent concentrates
described in Example 11 were tested for the presence of insoluble gels by
dissolving the concentrates in boiling nitrobenzene No insoluble particles
were
1~ detected by visual observation in any of the samples. This test is believed
to
have a sensitivity sufficient to detect the presence of insoluble gels at a
level of
about 0.1weight percent or more.
Concentrates in accordance with the present invention may also be
prepared from sources of recycled polyesters or mixtures of recycled and
virgin
z o polyesters. An example of one such concentrate is presented in Example 12,
below.
Example 12
In this Example, a concentrate containing 10 weight percent of PMDA was
prepared in accordance with the procedure described in Example 11, except
25 that "green flake" recycled PET was used as a PET source instead of virgin
PET. The green flake recycled PET was obtained from Martin Color Fi, Inc. of
Edgefield, South Carolina. The water content of the concentrate pellet was
measured by gas chromatography to be 0.0103 percent.
The 10 weight percent PMDA concentrate was subsequently extruded
3o with virgin PET and Na2C03 to produce the 0.5 weight percent PMDA
composition described in Example 11. The die pressure measured during
extrusion ranged from 1600-2000 psi.
Example 12 shows that recycled PET compounds may be used to
prepare concentrates in accordance with the present invention. The extrusion
35 die pressure for the final extruded product prepared from virgin PET and a
concentrate prepared from recycled green flake compares favorably to the die
pressure measured for 10 weight percent PMDA concentrate prepared from
virgin PET.
