Note: Descriptions are shown in the official language in which they were submitted.
CA 02405376 2005-05-27
ANTI-LUMPING COMPOUNDS FOR USE WITH
EXPANDABLE POLYSTYRENE BEADS
FIELD OF THE INVENTION
This invention relates generally to expandable polystyrene beads
and, more particularly, to an anti-lumping agent useful for the formation of
expandable polystyrene beads and low density foams formed therefrom.
BACKGROUND OF THE INVENTION
Methods for the production of expandable polystyrene beads are
well known. Such expandable beads generally comprise a mixture of an
expandable styrene or styrene derivative polymer and a blowing agent. The
expandable polystyrene beads are generally initially formed into beads of
relatively small size having a diameter of from about 0.2 to 4 millimeters.
The
beads are generally formed in a suspension polymerization reaction. The
blowing agent is introduced into the beads either during or after the
polymerization reaction. The blowing agent is generally homogeneously
dispersed within the polymer and the blowing agent may be, in general, a
hydrocarbon which is gaseous or liquid under normal conditions and which does
not dissolve the polymer. In addition, the boiling point of the blowing agent
must
be below that of the softening point of the polymer.
Typically, the expandable polystyrene beads go through at least a
two-step process prior to molding. In the first step, the expandable
polystyrene
beads are expanded to form
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CA 02405376 2002-09-26
what are ymown as "grills" or as a "pre-puff', This step comprises heating the
beads to a
tc;mperaturc above their softening temperature and thereby above the boiling
point of the
blowinc agent resulting in vaporization of the blowinC ajent and expansion of
the beads to
form individual particles of foam grills. These grills are quite fragile and
prior_to molding in
the second step it is generally necessary to allow the grills to age for a
period of time. To age
the grills they are maintained under a normal atmosphere. Auring the wing
process blowing
agent within the grills diffuses out and air from outside diffuses into the
grills. Once the grills
have aged they can be placed in a mold and heated main such that the grills
expand to fill the
molding space and form a molded objet-t. During this period the grills expand
and fuse ox
to adhere to each other to form the molded object. The molded objects can
either be large
blocks 'which are subsequently cut u'ith a. hot wire cutter into sheets or the
grills can be
molded directly into a particular shape.
Increasingly manufacturers of molded expanded polystyrene are attempting to
shorten
the expansion time frame and to mold increasingly lower density foams, thereby
leading to an
i5 increase in profitability. One ncaative result of expanding foams to lower
densities is that as
the density of the foam decreases there is a significant increase in "lumping"
of the pulls.
This problem is especially setrere for expandable polystyrene beads that
include low levels of
bJawin~ agents. Lumpintr is defined as the expandable polystyrene beads fusing
together
during the initial expansion step, which results in forn~ation of large lumps
of grills. These
20 )arge lumps create at least tyro problems. First, the expansion process
becomes difficult to
control becau~c of the formation of these Iarge lumps. The second problem is
that if the
Lumps are sufficiently latae enough the material becomes onus able for the
subsequent
molding steps. In an attempt to control this lumping phenomenon, a number of
external
CA 02405376 2002-09-26
coatings have been utilized. Some examples of commonly used external coatings
include zinc
ste~irate, glycerol mono stearate, ethylene-bis stearamide (acrawaxj, and
silica. Unfortunately,
these cotrm~only used lubricants are not successful in preventing lumping when
the
expandable polystyrene beads an expanded to very low densities.
Thus, it would b~ very advantageous to provide an anti-lumping compound that
could
be utilized with expandable polysytrene beads containing low levels of a
blowing agent that
would permit these beads to be expanded to very low densities previously not
routinely
obtainable.
io SUM1MARY Ol~ TF~E INVLNTION
In general terms, t)ris in~~ention provides expandable polystyrene beads
having a
reduced lumping tendency. Tn particular, this invention provides a class of
anti-lumping
agents for use with expandable polystyrene beads which significantly reduces
lumpino- when
expandin; the polystyrene beads into low-d4nsity foams of from about 1.5 to
0.75 pounds per
cubic foot (pcf), and more preferably from about 1.2 to 0.85 pcf. This
invention also finds
special utilization when preparin' expandable polyst5~rene beads having low
ievels of blo«~ing
agents.
In a fit~st embodiment, the present invention is an expandable polystyrene
bead
comprising an expandable styrene or styrene derivative polymer and from about
2. to about
?0 5% by weight, based on the total weiCht of the bead, of a C; to C~
hydrocarbon blowing agent,
wherein the bead is coated with an anti-lumping agent comprising a block
copolyn~.cr of
propylene oxide and ethylene oxide blocks, said block copolymer. comprising
from about 800
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CA 02405376 2005-05-27
to 65% ethylene oxide and from about 20% to 35% propylene oxide, and having
a hydrophilic-lipophilic balance {HLB) value of from about 16 to 29.
In a second embodiment, the present invention is a method for the
formation of an expandable polystyrene bead comprising the steps of: forming
an expandable polystyrene bead containing from about 2% to about 5% by
weight, based on the total weight of the bead, of a C3 to Cg hydrocarbon
blowing agent; and coating the expandable polystyrene bead with an anti-
lumping agent comprising a block copolymer comprising blocks of ethylene
oxide and propylene oxide, wherein the block copolymer comprises from 80% to
65% ethylene oxide and from 20% to 35% propylene oxide, and has an HLB
value of from about 19 to 29, said anti-lumping agent present in an amount of
from 250 to 2500 ppm based on the weight of the bead.
These and other features and advantages of this invention will
become more apparent to those skilled in the art from the detailed description
of
a preferred embodiment.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
In general terms, this invention provides expandable polystyrene
beads having a reduced lumping tendency. In particular, this invention
provides
a class of anti-lumping agents for use with expandable polystyrene beads that
significantly reduces lumping when expanding the polystyrene beads into low
density foams of from about 1.5 to 0.75 pcf. This invention also finds special
utilization when preparing expandable polymer beads having low levels of
blowing agents.
The expandable polystyrene beads are generally formed via the
well known aqueous suspension method wherein the monomers are
polymerized and combined with a blowing agent and other additives to form the
beads. The beads are then separated from the aqueous
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portion of the suspension, washed and dried. In at least a first expansion
step the beads are
heated, usually via steam, to a temperature above the boiling point of the
blowing agent. This
heating causes vaporization of the biowinc anent and expansion of the beads
into a form
known as a grill or prepuff, The prepuff is then allowed to age for a.
sufficient period of time,
usually suspended in mesh bags. During the aging process external air
equilibrates across the
prepuff and some residual blowing agent leaves the prepuffs. The prepuff is
then placed into
a closed mold and heated again to form a shaped object, be it a block or a
more complex
shape. Zn some procedures the prepuffs are expanded 2 to 4 times prior to
their use in the
closed mold.
1o In the aqueous suspension polymerization reaction. the monomers used to
forth the
polymer comprise styrene or styrene derivati~e.s_ The styrene derivatives
include: alpha-
methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, ar-
ethylstyrene, ar-
vinylxylene, ar-chlorostyrene; and ar-bron~ostyrene. The derivatives may
include minor
amounts of divinylbenaene, methylmethacrylate, or acrylonitrile. The preferred
monomer is
styrene. The monomer or mixture of monomers is suspended in an aqueous
solution and
polymerised. The blowing agent is ceneraliy added during the suspension
polymerization, bur
can be added durinG Inter processing steps. The suspension polymerization also
generally
inelud,es: chain transfer agents, suspension stabilizers, and polymerization
catalysts. Typical
chain transfer agents include: dimeric oc-methylstyrene. After polymerization,
the polymer is
2o generally present in an amount of from about 93 to 98 weight percent based
on the weight of
the bead.
Blowing agents suitable for the present in~~ention generally comprise C;-C6
hydrocarbons and nurtures thereof, with pentane isomers being preferred. The
blowing agent
IIRH 65,205.063
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CA 02405376 2002-09-26
must have a boiling point below that of the melting point of the polymer so
that upon healing
of the expandable polystyrene beads the blowing agent will vaporize anal leave
the beads
thereby expanding the bead. Generally a boiling point below 70° C is
preferred. Suitable
blowing agents include, for example_ propane; butane; isobutane; n-pentane;
isopentane;
neopentane; cycJopentane; methylcyclopentane; 2-methyl pentane; 3-methyl
pentane; 2,2-
dimethylbutane; 2,3-dimethylbutane; pentane petroleum distillate fractions;
hexane;
cyclohexane; methylcyclohexane; and hexane isomers. Blowing agents are
generally used at
levels of from about 2 to about 5 weight percent, more preferably from about
2.5 to about 4
weight percent based on the total weight of the bead.
1o As discussed above, the suspension polymerization usually is carried out in
the
presence of suspension stabilizers and polymerization catalysts. Common
suspension
stabilizers include: molecular colloids, such as, polyvinyl alcohol (PVA) and
polyvinyl
pyrrolidone (1'VP); the pickering salts such as Ca;(,PO,~)~ in combination
with an extender
such as dodeeylbenzol sulfate. Typical polymerization catalysts include
radical catalysts such
~5 as dibenzoylperoxide, tent-butyl perbenzoate, and dicumyl peroxide. Other
suitable catalysts
are well lcno~m to one of ordinary skill in the art.
The expandable polystyrene beads can also include other additives like flame
retardants based on orcanie bromo ox c.hloro compounds, such as,
rris(dibromopropyl)
phosphate, hexabromocyclododecane, chloroparaffin. Other additives include
antistatic
2o a~cnts, stabilizers, dyes, lubricants, and fillers. To aid in demolding the
object formed from
the expandable polystyrene beads one may also include compounds like glycerine
ester,
hydroxycarboxylic acrid ester.
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CA 02405376 2002-09-26
As discussed above, typical anti-lumping agents include zinc stearate,
melamine
fotrnaldehyde condensates of silica eel, glycerol mono stearate, ethylene-bis
steararnide
(acrawa~;j, and silica. The typical agents are not ef..fective when expanding
beads to Iower
densities or when using the indicated love levels of blowing agents. The
present invention
discloses a new class of anti-lumping agents that are. block copolymers.
By the term block copolymers it is meant a polymer having blocks formed of a
plurality of a repeating unit, such as ethylene oxide (E0), that are attached
to additional
blocks of a plurality of another repeating unit, such as, propylene oxide
(~'O). A given anti-
lumping copolymer may comprise a series of these blocks in an arrangement such
as: EOX-
to POy; EO;~-POy-EO,.; POx-EO~ PO,; etc. The particular arrangement of the
blocks along with
the overall HLB value for the polym~:r are important characteristics. rt is
preferable to match
the arrangement of the bloclrs to the overall hydrophobicity of the expandable
bead. In other
words, when the bead is more hydrophilic, then an anangernent of EOr-POYEOZ is
beneficial.
When the bead is more lipophilic, then an arranUement of POr EOYPOz is more
beneficial.
t5 in general, it is preferable that the amount of EO, which is generally
hydrophilic, in the block
copolymer range between about 65 and ~0% by weight based on the total weight
of the
copolymer. The amount of P0, which is generally lipophilic. is preferably from
about 2.0 to
35~o based on the total weight of the copolymer. Tt is preferable that the
number average
molecular weight of the copolymer be from appro~cimately 4,000 to 15,000,
preferably from
2o about 6,000 to 11,000, most preferably from about 7,000 to 10,000_ In
Qeneral, the viscosity
of these copolymers is from about x.50 to 2.500 cps at 77°C, provided
they are in a solid form
at ?0°C. The melt point of the copolymers ranbes from approximately 45
to 60°C.
CA 02405376 2002-09-26
1n gencra.l, it is preferable that the HLB value of the copolymer range from
about 1G to
29, depending on thz surface characteristics of the bead as discussed above,
when it is utilized
to coat an expandable polystyrene bead prepared by an aqueous suspension
reaction. The
HI..,B value is a semi-empirical number determined from the weight percentage
of hydrophilic
stoups to lipophilic groups within a molecule or mixture. The value is a
general indication of
the polarity of the molecules and ranges from 1 to 40. The HLB value increases
with
increasing hydrophilicity. Preferred HI.B values of the block copolymers are
from about 19-
29, and most preferably fzom about ~~ to 29.
Methods for formation of the block copolymers described above an: well known
in the
to art and will only briefly be described. Typically, one utilizes an
initiator molecule such as a
lower molecular polyhydroxyl or polyamine initiator molecule. Ey way of
example, the
initiator can be a diol, triol, diamine or triamine. Examples include
propylene glycol,
ethylene Glycol, glycerol, and ethylenediamine. The initiator is utilized in
an alkali-catalyzed
polymerization reaction v~ith a.Ikylene oxide compounds such as ethylene:
oxide or propylene
oxide to generate a polyctherol. When one caxries out the alkali-catalyzed
polymerization
sequentially using only a Single alkylene oxide at a time, one can form block
copolymer
polyetherols that are useful as the anti-lumping agents of the presont
invention. One can also
utilize amines such as ethylenediamine as the initiator in these reactions.
Suitable block
copolymers include the Pluronic'" or Pluronic R~' copolymer series produced by
BASK
2o Corporation. Preferred block copolymers include the PL.URONiC F68 and F108
products.
After formation of the polystyrene beads they are separated from the aqueous
phase,
washed and dried. One method of drying the beads is to pass them into a screw
conveyor that
feeds a heated airveyor for flash drying. One method for application of the
ne~~~ anti-lumping
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aCcnts is to add i.t to the beads in the screw conveyor. Some mixing occurs in
the screw
conveyor and the rest occurs in the flash dryer. The block copolymer anti-
lumpinG agent can
be added alone or with other anti-lumping agents. The ant-lumping agent is
Generally
incorporated into the polymer bead in an amount of from about 2~0 to 2500 ppm,
based on
the we.i~ht of the polymer. Preferably, the anti-lumping agent is present in
an amount of from
about 300 to 1250 ppm.
Once dried the beads are sized into fraction<. The formed beads generally have
a
diameter of from about 0.2 to 4.0 mm. Another method for adding the new block
copolymer
anti-lumping agents to the. beads comprises adding the agent to a powder
blending system
to after sizing of the beads. These blending systems typically include a plow
blade mixer such
as the Littleford FKIvI mixer'. The anti-lumping agent and other additives
a:re combined with
the beads and mixed usinC a plow blade mixer.
The initial expansion of the beads is generally carried out in a closed vessel
batch
expander with a. steam injection. process. Typical eaam.ples of such expanders
include:
Weiser VIrT400, Kurtz 1~V 10U0, and Dingledein VA2000. The beads are passed
throuQ.h the
expander and are heated such that the blowing agent vapoi~zus thereby
expanding the beads.
The flow rate of the beads through the expander deterniines the amount of
expansion and it is
generally reported as pounds per hour per cubic foot of expander volume, or
for a given
expander as pounds per hour. During the initial expansion stage lumping of the
expanding
2o polystyn;ne beads can. occur. To achieve low density polystyrene beads the
expansion time
must be increased, but in the past increasinC the expansion time has also
increased the arn.ount
of lumping. With tho anti-lumping went of the present in~~ention it is
possible to achieve low
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density polystyrene beads ~~ithout an inczease in the amount of lumping. As
explained above,
when the beads lump, they can be difficult to use in a subsequent molding
process.
.E~~,Z~PLE 1
To demonstrate the effectiveness of the block copolymer anti-lumping agents of
the
present invention, a series of expandable polystyrene beads containing from
about 2~o to
about 5%a by weight, based on the total weight of the bead, of a C~ to C6
hydrocarbon blowing
agent are prepared in the presence or absence of PLURONIC F68 (F68). PLURONIC
F68 is
formed by initially adding propylene oxide (PO) to the hydroxyl groups of
propylene glycol
1U initiator molecule to form a PO block, then ethylene oxide (E0) is added to
the reactive ends
of the PO block and polymerisation continues to form a structure of EO~-
POYEOZ. This
block copolymer has 80~u by weight EO and 20% by weight PO, a number average
molecular
weight of 8400, an HLB of L9, a melt point of 5?° C and a viscosity of
1000 at 77° C. The
F68 was present in an amount of 1000 ppmbased on the total weight oFthe
unexpanded
beads. The aqueous suspension z~action for formation of the expandable beads
is carried out
using a typical aqueous suspension polymerization. The pLUIZONrC F68 is added
to the
beads by adding it to a powder blending system after suing of the beads. The
blending
system included a ploy: blade Littlcford FkM mixer.
The expandable beads foamed utilizing the above-referenced aqueous suspensions
are
then tasted for lumping, the minimal density that could be formed during the
initial expansion
step (prepuffs) and the ma:ximaI rate at v~~hich the beads could be expanded
during the initial
step. Tn addition, each sample is tested for a number of parameters durinc a
molding step
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CA 02405376 2002-09-26
including the averace cycle time for a block mold and the average cycle tine
for a shaped
mold. The results of these experiments are presented in Tabl-c z below:
Table 1
Measured X'ro _X 2 ~3
ert ' ~
Minimal density 0.90 0.88 1.05
in pci;
w/out F68. '
Maxin.xl flow 300 3~0 244
rate in
lbsmr v~lout
FGS
Minimal density 0.86 0.90 1.00
in pcf
with F6S
Maximal flow 315 3d'? 315
rate in
1bslhr with FG3 ___
Avera?e cycle 7-10 rains. 7-10 rains. Not determined
time. blook
mold wlaut F68
Average cycle 100 sees. SO-100 sets. 55 5ecs.
time
sha ed mold w/out
F6S
Cycle time shapedsees. 92 sees. 65-110 sets.
mald
with FGR ~
Lumping w/out 5.0-10.0 10.0-15.0 10.0-20.0
F6S
~fo by wt;ight
of the
ex anded buds
Lumping with 0.0 0.0 0.0
FG8
Vii, by weight
of the
ex anded beads
The data in the Table I demonstrate that PL~fJRONIC FGfi utilized in an amount
of
1000 ppm enables one to f()IZlI a prepuff havity a similar or lower density
than that obtainable
without FG8 and to foam the prepuff at a much hither rate. Thus, inclusion of
the
PLURONIC F68 anti-lumping agent increases the rate of formation of prepuff. In
addition,
inclusion of the F68 permitted a faster cycle time during the molding step
when molding a
shape. In addition, the data clearly show the complete prevention of lumpinC
by inclusion of
the PL:UROI~TIC F68. The amount of lumping in the absence of the anti-lumping
agent of the
present invention was significant.
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CA 02405376 2005-05-27
Thus, use of these anti-lumping agents will enable one to create
prepuffs with no lumping, at a faster rate and to substantially reduce the
average
cycle time for the molding step, thereby leading to faster production of
molded
parts.
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