Note: Descriptions are shown in the official language in which they were submitted.
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A FLEXI~LE POLYALKENYL A~O~TIC POLYMERIC FOAMED SHEET
BACKGROUND OF TH~ I~VE.~TION
.... ~
Polyalkenyl aromatic polymeric foamed sheets are
known to the ar-t. Such sheets are known to be fragile
and, hence, limit their acceptance in the art.
Heretofore, such sheets have been generally foamed
with blowing agents that have a relatively high solubility
in the polyalkenyl aromatic polymer-such 2S fluorocarbons
or petroleum hydrocarbons, e.g. pen1:ane and the like.
Such sheets retain such blowing agents during and after
manufacture. As such blowing agents migrate from the
sheet ~he physical properties change with time and the
sheet gradually becomes brittle, as the plasticizing
ef~ects of the blowing agent is lost. Since such blowing
agents have an equilibrium solubility in the polyalkenyl
aromatic polymers, varying levels of the blowing agent are
retained which caused the sheet to vary in physical prop- `
erties. Because of the solubility of such blowing agents
in the polymer small and uniform cell size distribution
has been difficult to produce without using incompatible
nucleating agents that can degrade foam sheet properties.
U. S. Patent 3,770,666 discloses such foams blown
with fluorocarbons with solid nucleating agent to produce
fine cell size, wherein about 50% of the blowing agent is
retained in ,he cells. U. S. Patent 3,231,524 discloses
such foams blown with hydrocarbons or fluorocarbons using
a styrene/maleic anhydride copolymer as nucleating agent.
~2~ C-08-12-0~03
. S. Patent 3,084,126 discloses polyalkenyl aromatic
foamed polymers using hydrocarbons with silicates as nucle-
ating agents. U. S. Patent 3,658,973 discloses a process
for extruding foamed polyalkenyl aromatic polymers using
fluorocarbons and hydrocarbons with carbon dioxide as a
nucleating agent. U. S. Patent 2,941,964 discloses the use
of hydrocarbons and a carbon dioxide liberacing agent as a
nucleating agent with data to show that carbon dioxide
liberating agents alone fail to produce low density foams.
It is the objective of the present invention to pro-
vide a low density foamed polymeric sheet, based on a poly-
alkenyl aromatic polymer, that has multiaxial flexibility.
Another objective is to provide an improved process
wherein polyalkenyl aromatic polymers can be formed into
foamed sheets tha~ have multiaxial flexibility as extruded
or after loss of blowing agent.
SUMMARY OF THE INVENTION
The present invention relates to a low density foamed
polymeric sheet comprising a polyalkenyl aromatic polymer
having dispersed therein about 0.1 to 5.0% of a polyolefin
resin, based on said polymer, said foamed sheet having
multiaxial flexibility.
The present invention also relates to a process for
preparing a foamed polymeric sheet by extruding a composi-
tion comprising:
(A~ a polyalkenyl aromatic polymer,
(B) a volatile blowing agent, and
~C) a nucleating agent, as a melt under
pressure through a sheet die into a
zone of low pressure to permit foaming
of said composition, the improvement
which comprises having present in said
composition about 0.1 to 5~ by weight of
a polyolefin resin based on said alkenyl
aromatic polymer, providing a foamed
sheet having multiaxial flexibility.
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PREFERRED EMBODIME~TS
The polyalkenyl aromatic polymers are prepared from
alkenyl aromatic monomers selected from the group consist-
ing of styrene, alpha methyl styrene, aralkyl styrene,
e.g., ortho, para and meta substituted styrenes with alkyl
groups such as methyl, ethyl, etc.; arhalostyrenes such as
chloro and bromostyrenes, aralkylhalostyrenes, e.g., para-
methyl~ortho, chloro or bromo styrene. Such polymers have
a molecular weight of about 100,000 to 600,000
The polyolefin resins are those prepared by conven-
tional polymerization having a molecular weigh~ of at least
about 20,~00, pre~erably ~0,000 to 500,000 and a melt index
(gm/10 mm) of about 0.1 to 55, preferably about 0.15 to 15
with a density of about 0.890 to .967 gms/cu.cm. Such
resins are available from Union Carbide, New York, N. Y.,
and Northern Petrochemical Inc., Des Plaines, Illinois.
Conventionally, the nucleating agents are made up of
-two materials which react to form carbon dioxide and water.
The two materials are normally used in approximately
equivalent amounts. As the carbon dioxide libera~ing
materials there can be used ammonium, alkali and alkaline
earth carbonates or bicarbonates, e.g., ammonium bicarbon-
ate, sodium bicarbonate, sodium carbonate, potassium bi-
carbonate, calcium carbonate. The other material is an
a~id or acid-reacting salt, preferably solid~ which is
sufficiently strong to liberate the carbon dioxide from
the carbonate cr bicarbonate. Generally, the acid has at
least 3.0 milliequivalents of acidic hydrogen, and prefer-
ably at least 10.0 miliequivalents, per gram. The acid
can be organic or inorganic. Suitable acidic materials
include boric acid, sodium dihydrogen phosphate, fumaric
acid, malonic acid, oxalic acid, citric acid, tartaric
acid~ potassium acid tartrate, chloro-acetic acid, maleic
acid, succinic acid and phthalic acid. In place of the
anhydrous acids or salts there can be used the solid hy-
drates, e.g., oxalic acid dihydrate and citric acid mono-
~2~6~)
-- 4 --
hydrate.
The nucleating agents can be solid inert materials
such as talc, wax or hydrated salts or silicates used in
finely divided form. The nucleating agents are used in
amounts of about 0.2 to 10% by weight based on the poly-
alkenyl aromatic polymer, preferably about 0.2 to 2% by
weight is used. The nucleating agents are conventionally
dry blended with the polymer prior to extrusion.
As the volatile blowing agent liquid, there can be
used aliphatic hydrocarbons boiling between 10 and 100C.
and preferably between 30 and 90C., e.g., petroleum
ether (containing primarily pentane or hexane or a mixture
of these hydrocarbons), pentane, hexane, isopentane, cy-
clohexane, cyclopentane, pentadiene and neopentane. Other
volatile liquids include methanol, ethanol, methyl ace-
tate, ethyl acetate, butane, acetone, methyl formate, ethyl
formate, dichloroethylene, perchloroethylene, dichloro-
tetrafluoroethane, isopropyl chloride, propionaldehyde,
diisopropyl ether, dichlorodifluoromethane, a mixture of
pentane with 5 to 30% of methylene chloride or other vola-
tile lower halogenated hydrocarbon. Trichlorofluoro-
methane, methyl chloride, ethyl chloride and other fluoro-
carbons of khe Freon type may be used. Gases such as CO2
and N2 have also been found to be suitable blowing agents.
The blowing agent is used in amounts of about 2 to 20% by
weight based on the polyalkenyl aromatic polymer.
One suitable apparatus for extrusion of said poly-
alkenyl aromatic polymers containing carbon dioxide dis-
persed therein under pressure through a die into a zone of
low pressure is disclosed in U. S. Patent 3,451,103.
The extrusion apparatus consists of an extruder
(preferably a single screw extruder) which contains three
separate functional zones or sections. The first or plas-
ticating zone of the extruder melts and delivers the
melted resin to the second zone at a high temperature and
pressure. The structure and design of the screw in the
first zone may take a wide variety of forms, but typically
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consists of a constant pitch screw which increases in root
diameter in the downstream direction. Heating means are
usually included in the first zone to assist in melting
the resin. If desired, the first zone may consist of two
S elements, as for example by having a plasticating extruder
arranged in tandem with the plasticating zone of a second
extruder and delivering melted resin thereto.
The screw in the second zone may take a wide variety
of forms, but usually is a constant pitch screw which has
a constant root diameter. In addition, the root diameter
in the second zone is usually either identical with or
slightly smaller than the root diameter at the discharge
end of the first zone. The second or injection zone is
provided with specially designed means for injecting
liquid substances into the melted resin. It is preferred
to employ a plurality of such injection means and to have
them symmetrically disposed about the chamber wall. The
injection means employed are capable of injecting the
liquid into the extruder at a pressure substantially
higher than the pressure of the mel-ted resin, e.g., at a
pressure of at least about 500 p.s.:i. higher than the
pressure of the melted resin. The injection means also
include an element adapted to seal the liquid delivery
orifice when li~uid is not being injected into the melted
resin. This feature prevents the melted resin from flow-
ing into and plugging the orifice of the injection means.
The sealing element preferably consists of ta) a discharge
orifice whose feed inlet terminates in a valve face, (b)
a cooperatively functioning valve which is adapted to seat
against and seal the valve face of the orifice, (c) a
first and fixed pressure means acting upon and urging the
valve into seated relationship with the valve face, and
(d) a second pressure means acting upon the valve and
urging it out of seated relationship with the valve face;
said second pressure means being responsive to and actu-
ated by the pressure of the liquid within the injection
means. To prevent melted resin ~rom flowing into the in-
jection means, the fixed pressure means urging the valve
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to seat against and seal the valve face should be preset
to a pressure above that developed within the resin in the
second zone of the extruder.
The third zone of the extruder performs two functions.
First, the pressure on the melted resin is increased to the
level required to express the resin through the die.
Second, the melted resin is cooled (or in some circumstances
heated) to substantially the temperature at which it will
leave the die. To properly cool (or heat) the resin,
at least the aft section of the third zone should include external
heat transfer means. Depending upon the length of the second
zone of the extrusion apparatus, it is sometimes desirable
to maintain the mixture of melted resin and liquid at a
relatively high temperature in the fore section of the third
zone. In this event, external heating means may be provided
to heat the chamber wall oE the fore section of the third zone.
In addition, the root diameter of the screw may be increased
in the fore section of the third zone so that frictional heat
will be developed within the resin. In this event, however,
the root diameter is preferably subsequently decreased in the
aft section of the third zone.
The die affixed to the extrusion apparatus may be of
any design presently used in extruding thermoplastic resins.
Scores of suitable dies are known and reported in the art.
The three zones described can be separate extruders
as practiced in the art.
The extrusion can be carried out at tempera-tures of
about 135 to 160C. for the extrusion temperature at the
die. Pressures for extrusion generally are greater than
30 1000 psi and range 1700 to 3700 psi with the blowing agent
being injected at pressures at least about 500 psi higher
than the pressure of melted resin in the extruder. About
2 to 20~ by weight of liquid blowing agent based on the
polymer is injected into the melt depending on the density
desired in the foam sheet which can range from about 0.008
to 0.16 gm/cm3. The blowing agent can be incorporated
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by dry blending the blowing agent with the polyalkenyl aro-
matic polymer prior to extrusion in amounts of about 2 to
20% by weight based on the polymer. The blowing agent be-
comes dispersed in the polymer melt under pressure. The
melt containing the blowing agent is extruded from a flat
sheet die or an annular die over an annular mandrel using
conventional equipment commercially available. The sheet
expands freely under atmospheric pressures as a lower
pressure. Conventionally the sheet extruded from the die
continuously has a longitudinal or machine direction di-
mension and a wid.h or transverse dimension along with
thickness or a gauge dimension.
The sheet can have a thickness of about 1 to 500 mils
(0.025 to 12.5 mm.) with a cell size less than about 0.5
mm. ranging from about 0.005 to 0.5 mm. The cells under
magnifaction have a unique cell structure wherein the cell
wall has a generally uniform thickness with a uniform dis-
tribution of cells. It is believed that the foamed sheet
of the present invention, having unexpectedly uniform cell
thickness, provide a foamed sheet with greater flexibility
and less brittleness than prior art foamed sheets. The
foamed sheet of the present invention has proven to have
high utility as a ~rapping sheet or interlayer sheet ~or
packaging, e.g., fruit in that it can be wrapped around the
fruit without splitting and retain its crushable foam prop-
erties to protect the fruit. Such sheet can be used
broadly as a wrapping and cushioning sheet or con~ainer for
packaging fragile items of commerce in general.
` Foamed sheets as extruded have the blowing agent con-
tained in the cells which will diffuse out of the cells on
storage. Generally, sheeting having compatible or soluble
blowing agents such as hydrocarbons or fluorocarbons, are
stored for a number o` days to allow the blowing agen. to
diffuse out the sheet before commercial use as formed
trays, egg cartons, etc., so that they will have the re-
quired rigidity.
An accelerated test has been developed wherein the
sheet is held at 70C. for 48 hours which will simulate .
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such an aging period. The sheet can then be tested.
The sheet can be easily treated for flexibility by
ASTM Test (D-2167-63T) which tests folding endurance.
Here, the sheet is folded repeatedly on itself through an
180 angle until the sheet parts, splits or cracks. It has
been found unexpectedly that the foamed sheet of the
present invention, having the polyolefin resin present, can
be folded repeatedly on itself at least about 10 times
without splitting or cracking, whereas sheets blown with
hydrocarbons or fluorocarbons alone will crack on folding
180 having essentially no folding endurance.
A very simple test to determine the flexibility of the
foam is to wad, roll or crush the sheet into a ball. If
the sheet does not split or crack~ the sheet is considered
flexible and will have the utility needed as an overwrap or
formed sheet for packaging.
The polyolefin resins are resins of an olefin-monomer
containing two to six carbon atoms. The preferred monomers
are ethylene and propylene and are unsaturated hydrocarbons
of the type CnH2n. Low de.~sity polyethylene generally has
a density of about 0.910 to 0.925 ~ms/cm3; medium density
polyethylene about 0.926 to 0.940 gms/cm3 and high density
polyethylene about 0.941 to 0.967 gms/cu3. Propylene gen-
erally has a density of about 0~890 to b . 908 gms~cm3. The
amount of polyolefin resins found effective in preparing
polyalkenyl aromatic foamed polymeric sheet is about 0.1%
to 5.0% more preferably about 0.3% to 3.0% and most prefer-
ably about 0.5% to 2.5%, based on the polyalkenyl aromatic
polymer.
The following examples zre set forth to illustrate the
best mode of the present invention to those skilled in the
art and do not limit the scope of the invention.
EXAMPLE 1
A sheet of foamed polystyrene was prepared employing an
apparatus as disclosed in U. S. Patent 3,451,103. The
barrel of the extruder was 2.5" (6.25 cm) in diameter and
120" (300 cm) long. Zone 1 is 50" (125 cm) long, Zone 2 is
28" (70 cm) long and Zone 3 is 42" (105 cm) long. The
screw helical land has a constant pitch throughout its
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entire length. In Zone 1, .he first 7.5 L/D section of the
screw has a root diameter of 1.76" (4.4 cmj; the second
5 L/D section of the screw has a root diameter which in-
creases from 1.76" to 2.16" (4.4 cm to 5.4 cm); the third
7.5 L/D section of the screw has a root diameter of 2.16"
(5.4 cm~. Zone 2 has a constant root diameter of 2.16"
~5.4 cm). In Zone 3, the first 7 L/D screw section has a
root diameter of 2.25" (5.625 cm) and a final 10 L/D screw
section having a root diameter of 2.00" (5.0 c~). A sheet
die of conventional construction was attached to the ex-
truder to form a sheet of about 60 mil (1.5 mm).
Polystyrene pellets were dry blended with low density
polyethylene (density about 0.92) pellets and a nucleating
agent (sodium bicarbonate-and citric acid mixed on a 70/30
weight ratio of bicarbonate to citric acid) to form a dry
blend containing 0.5~/O by weight nucleating agent and 0.5% by
weight polyethylene, the remaining 99% being polystyrene.
The dry blend was fed to Zone 1 of the extruder at a
rate of 54 5 kgs/hr. and is melt extruded with melt enter-
ing Zone 2 at a temperature about 450F (232C.) under apressure of about 2700 psi (1.9 x 1o6 kgs/m2). Liquid tri-
chlorofluoromethane is injected in Zone 2 into the melt
stream a~ a pressure of about 3100 psi, (2.2 x 106 kgs/m2)
at a rate of about 6.8 kgs per hour becoming dispersed in
the melt in Zone 2. The melt enters Zone 3 at the tempera-
tures and pressures of Zone 2 and is maintained at those
conditions for about 17" (42.5 cm) of travel in Zone 3, then
cooled by proper jacket temperatures to 310F. (154C.) over
the last 25" (62.5 cm) of travel in Zone 3 leaving Zone 3
at about 2500 psi (1.75 x 106 kgs/m2~. The polystyrene melt
is passed through a screen and breaker plate assembly and
enters the die at about 1500 psi (1.05 x 106kgs/m2). The
sheet is extruded as a 60 mil sheet (1.5 mm) having a dens-
ity of about 2 lbs/ft. (32 kgs/m ). The cells of the foam
sheet are less than 10 mil (.25 mm).
The sheet was tested by treating in a circulating air
oven at 70C. ~or 48 hours wherein the foamed sheet comes
into equilibrium with air, i.e., the cells become essen-
tially air filled. This test simulates air agir.g at ambient
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~ te~.peratures for a period of 60 days for commercially blown
foam sheets before used by fabricators wherein the blowing
agent is essentially replaced by air.
The sheet was found to be flexible having a fold en-
durance under ASTM D-2167-63T of greater than 10 folds when
repeatedly folded back and fourth through an angle of 180
on itself. The film was found to be crushable when wadded
into a ball without splitting or cracking. Simple trays
were formed as fruit containers having a depth of about one
inch. The trays could be folded 90 without splitting.
EXAMPLE 2
Example 1 was repeated using no polyethylene in the
dry blend. The sheet was tested as in Example 1 and found
to be brittle in that the sheet could not be folded on
itself through 180 and could not be wadded into a ball
without splitting. Trays formed from the sheet were
brittle, i.e., they could not be folded 90 without split-
ting.
EXAMPLES 3 - 8
Exmple 1 was repeated using ].. 0, 1.5, 2.0, 2.5, 3.0
and 5.0~ low density polyethylene in the dry blend. The
sheets were tested as in Example 1. All sheets were found
to be flexible in the îold test taking more than 10 folds
without splitting and were also crushable without cracking
or splitting.
Tensile strength was also run on the sheets of Ex-
amples 1-8.
.
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TABLE I
% Tensile
P.E. Strength* Flexibility
0 22.6 brittle
0.5 29.2 flexible
1.0 38.8 flexible
1.5 4~.5 flexible
2.0 45.7 flexible
2.5 43.8 flexible
3.0 38.4 flexible
5.0 36.6 flexible
* ASTM Test D-638 (kg/cm2)
The data indicates that tensile strength is optimum
at about 1.5 to 2.0% consistent with flexibility.
EXAMPLES 9 - 13
Example 1 was repeated using 0.5%, 1.0%, 2.0%,,3.0%
and 5.0% of polypropylene resin having a density of about
0.902 gm/cm in the dry blend, The sheets were tested as
in Example 1 and test data is sho~l in Table II.
20 TABLE II
%
Poly- Tensile
pro~ylene Strength Flexibility
0 22.6 brittle
0.5 28.9 flexible
1.0 35.3 flexible
2.0 48.6 flexible
3.0 45.2 flexible
5.0 44.3 flexible
EXAMPLES 14 - 18
Example 1 was repeated using 0.5%~ 1.0%, 2.0%, 3.0%
and 5.0% of high density polyethylene resin having a density
of about 0.950 gms/cm3 in the dry blends. The sheets were
tested as in Example 1 and the test data is shown in
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Table III.
TABLE III
% High
Density Tensile
Polyethylene Strength F'lexibility
0 ' 22.6 brittle
0.5 33.5 flexible
l.0 34.8 flexible
2.0 35.6 ' flexible
10 3.0 34.1 flexible
5.0 32.5 flexible
Maximum tensile strength appears at about 2.0% of
high density polyethylene in the foamed sheet.
EXAMPLES 19 - 21
Lower levels Gf polyolefin resins were also studied to
determine their effect on flexibility. The amounts of
polyolefin type and test results are shown in Table IV.
The sheet was prepared and tested as in Example 1.
TABLE IV
% by
Densi~y Weight Tensile
Polyolefin gm/cm~ Used Stren~th ' Fl'e'xibil'i'~x
Polyethylene O.g20 0.1 24.8 flexible
Polyethylene 0.950 0.1 25.7 flexible'
Polypropylene O.gO2 0.1 26.2 flexible
It is evident that amounts as low as 0.1% by weight
based on the foamed sheet can be used to induce flexibil'ity
with a positive gain in tensile strength.
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