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BACKGROUND OF THE INVENTION
This invention pertains to a process for the preparation of porous or
cellular resinous bodies.
Previously, U.S. Pat. No. 2,450,436 issued Oct. 5, 1948 to O. R. McIntire
disclosed the method of producing a plastic cellular article by charging a
vessel with a solid thermoplastic polymer, e.g. polystyrene, together with
a normally gaseous agent capable of being dissolved by the polymer,
subjecting the mixture to superatmospheric pressure--to form a gel of the
polymer--and opening an outlet--so as to cause the gel to flow from the
vessel. The invention is restricted to release of the pressure on the
solution at a time when the latter is at a temperature preferably between
50.degree. and 125.degree.C.
The use of certain specific nucleating agents in controlling pore size of
polystyrene foamed articles is claimed in U.S. Pat. No. 3,188,295 issued
June 8, 1965 to D. E. Ballast et al, wherein the organic pigments
indigotin and blue copper phthalocyanine are shown. Furthermore, in U.S.
Pat. No. 3,224,984 a relatively low molecular weight (1000-4000)
polyethylene wax is used as nucleating agent.
A combination of nucleating agents, wetting agents and a foaming agent was
taught in U.S. Pat. No. 3,413,387 to E. O. Ohsol, but his nucleating
agents were made up of two materials which react to form carbon dioxide
and water, and the foaming agent was absorbed on an absorbent. The use of
polystyrene as a nucleating agent for foamed polyethylene was taught in
U.S. Pat. No. 3,065,190 to D. S. Chisholm et al.
In Cellular Plastics, 4, No. 9, September 1968, "Evaluation of Four Foaming
Techniques for Inflating a PK-Life Raft", by I. O. Salyer, J. L.
Schwendeman and C. E. McClung, compositions are described similar to those
of the invention, but without the critical feature of the particular
nucleating agents required for certain polymers to produce foams of small
and uniform cell size.
Despite the advances made in the prior art, we have found that the prior
art methods yielded foams of widely varying cell size, usually with cells
larger than desired, and consequently, of relatively low compressive
strength. Additionally, the foams frequently shrink, or even collapse, or
the blowing agent volatilizes before the foam hardens sufficiently to
support its own weight.
Furthermore, the use of organic pigments as nucleating agents yielded
colored products which are limited in utility.
Last, but not least, the products of this invention can be prepared at room
temperature, and do not require the elevated temperature required by the
prior art. Consequently, these foams can be generated in areas remote from
complex processing equipment, and require only a valved pressure vessel
for containment until ready for use. The foam can even be discharged
incrementally from the pressure vessel with prolonged periods of time
between each release.
The products of the present invention are useful for flotation devices, for
packaging of delicate equipment, for emergency splints for personal
injuries, for thermal insulation, for closing openings, for decorative
purposes, etc. The foam is generally low-density with predominately
closed-cell structure having high compressive strength and dimensional
stability, with small uniform cells. Its whiteness makes it attractive for
aesthetic reasons. However, it may be dyed readily, as by adding a soluble
dye such as Congo Red or Brom Cresol Green in suitable concentration to
the foamable mixture, e.g. 0.2-2.0% by weight of the solid foam.
SUMMARY OF THE INVENTION
An object of the invention is to produce a uniform low-density cellular
structure from a styrene homopolymer or copolymer, vinyl chloride/vinyl
acetate copolymers, and polyvinyl acetate. A further object is to provide
a predominately closed-cell foam having high compressive strength and
dimensional stability, and containing small uniform cells. Still a further
object is to provide a white foam matrix to which organic and inorganic
colorants can be added if desired.
These and other objects hereinafter defined are met by a process or method
of producing a substantially uniform, solid, cellular thermoplastic
article wherein the steps comprise charging a vessel with (1) a solid
thermoplastic polymer, (2) a solvent mixture the amount of which in the
range of from about 25-50% by weight of said polymer boils at or below
0.degree.C. and to prevent shrinkage the minor amount of which in the
range of from about 3-30% by weight of said polymer boils at or near
20.degree.C., said solvent mixture being capable of forming a gel with the
polymer when subjected to super-atmospheric pressure, (3) water in
sufficient amount to lubricate the flow of the gel from the vessel as
specified in step (c) below, and (4) for homogeneous polymer solutions a
nucleating agent selected from the group consisting of spherical
polyethylene powder, glass hollow spheres, phenolic hollow spheres and
mixtures thereof; (b) subjecting the contents of the vessel to
superatmospheric pressure at a temperature in the range of room
temperature to below the critical temperature of said solvent mixture for
a time sufficient to form a flowable gel; and, (c) thereafter rapidly
opening an outlet at the bottom of the vessel to cause the gel to flow
from the vessel and form the cellular article. Foamable compositions
usable in the process to make the cellular articles are described. Major
means more than 50% by weight of the solvent mixture and minor means less
than 50% by weight of the solvent mixture as the terms imply.
In its narrower aspects, the compositions of the invention are as follows:
The solution of the amorphous polymer has a solubility parameter (defined
in J. Appl. Chem., 3, February 1953, page 73) between 8.4 and 10.4, and a
molecular weight as measured by intrinsic viscosity between 0.5 and 5 and
preferably about 1; dissolved at 40 to 80% solids, preferably at 60 to 70%
solids, in a solvent system consisting of a major amount of a low boiling
solvent such as dimethyl ether and methyl chloride, and a minor portion of
a solvent which boils near room temperature such as Freon 11, Freon 21,
chloroform, or ethylene oxide; plus 0.5 to 25 parts, and preferably 5 to
10 parts of an annionic surfactant containing about 25% water; plus a
nucleating agent selected from the group consisting of glass
microballoons, phenolic microballoons and micronized polyethylene.
The process itself consists of venting or depressurizing the polymer
solution as described above through a straight-thru ball valve or similar
nonconstricting, quick-opening valve into air, into a closed vented mold,
or onto any solid or liquid surface, stationary or moving.
The requirement for the polymer is that it shall be soluble, at high solids
concentration, in a very low boiling solvent such as methyl chloride and
dimethyl ether.
To be soluble in these solvents, the polymer should have a solubility
parameter between 8.4 and 10.4.
The polymer used must be of high molecular weight having an intrinsic
viscosity value between 0.5 and 5.0 and preferably about 1.0.
The polymer must be amorphous in the sense that it cannot contain a
significant quantity of a crystalline phase which prevents solubility at
or near room temperature. Polyethylene, polypropylene, Nylon 6 or 66, and
polyformaldehyde are examples of crystalline polymers not suitable for
preparation of instant foams.
The polymers used for preparing instant foam must not contain high
percentages of gel or be crosslinked during polymerization. Although
butadiene/styrene copolymers are listed above as one of a group of
polymers suitable for the practice of this invention, not all
butadiene/styrene copolymers or polybutadienes are suitable. As is well
known in the rubber industry, many of the diene type polymers crosslink
and develop gel content if taken to high conversion during polymerization.
For many rubber applications this gel or crosslink material is not
objectionable. However, for preparing instant foams, the polymer should
preferably be completely soluble.
The range of solids content of polymer which can be used in preparing
instant foams which do not collapse, is limited to the range of 40 to 80%
solids, and preferably about 60 to 70% solids. The density of the product
foam is inversely related to percent solids and ranges from 0.6 to 6
lb/ft.sup.3.
As the major solvent we prefer either dimethyl ether or methyl chloride.
These two solvents both have boiling points around -25.degree.C. and a
vapor pressure of about 60 psig at room temperature. The low boiling point
and the 60 psig vapor pressure at room temperature which is obtained with
these two specific solvents are not matched by many other organic
compounds. Additionally, these solvents have solubility parameters of
around 9.3, and therefore will dissolve polymers having solubility
parameters from 8.4 to about 10.4. All of the soluble non-crystalline,
noncrosslinkd organic polymers listed as suitable for this process do
indeed have solubility parameters in this range.
Importantly, inert gases such as air, N.sub.2, CO.sub.2, and nitrous oxide,
can be used as auxiliary pressurizing gases in order to extrude the higher
density foam formulations at a faster rate, or at lower temperatures than
would otherwise be possible.
In order to prevent shrinkage of instant foam, whether made of polystyrene
or another polymer or copolymer, it is necessary to have a suitable
co-solvent whose boiling point is preferably at or near room temperature.
The purpose of this second minor solvent is to prevent shrinkage by
diffusing out of the polymer foam matrix into the gas space after the foam
has been blown, and thus supply the pressure (equal to ambient) required
to keep the foam inflated as the low boiling major solvent diffuses fairly
rapidly out of the foam. If the secondary solvent had a boiling point
significantly above ambient, it could not fulfill this function. If the
secondary solvent had a boiling point much lower than room temperature it
would probably escape from the blown foam at about the same rate as the
major solvent and therefore would not prevent shrinkage. Minor co-solvents
of this type, which boil near room temperature include, but are not
limited to, Freon 11, Freon 21, chloroform, ethylene oxide and diethyl
ether. Certain hydrocarbons such as pentane, cyclopentane, and butadiene
are valuable as minor co-solvents for low solubility parameter alkyl
hydrocarbon type polymers such as polyisobutylene, butyl rubber,
polybutadiene, and butadiene/styrene copolymers.
In the absence of a suitable surfactant and/or water, the foam produced
from an otherwise good formulation of polymer and solvents is of very poor
quality having large and uneven cells. Furthermore, the foam does not
extrude well from a bottle or other container. The surfactant and/or water
has an equal or more important function of lubricating the container walls
and discharge valves and pipes and thus facilitating flow of the foam
solution. The preferred surfactant at this time is Triton X-200, an
aqueous solution of the sodium salt of an aryl alkyl polyether sulfonate.
It is very necessary for the water to be present in the surfactant.
Experiments in which Triton X-200 was vacuum dried in order to remove the
water produced foam of very poor quality. Adding the water alone, without
any Triton X-200, produced better quality foam than when the water was
omitted but not as good as with the combination of anionic surfactant and
water. We do not believe that suitable surfactants should be limited to
Triton X-200 since other similar anionic surfactants to which a comparable
amount of water is added should work equally well. It is likely that
cationic and nonionic surfactants to which water is added will also make
suitable surfactants.
In addition to a suitable surfactant, a separate nucleating agent is
required in order to obtain high quality solvent blown foam having a small
and uniform cell size. In homogeneous polymer solutions, such as
polystyrene, styrene acrylonitrile copolymers, or polyvinyl acetate, the
use of a finely divided particulate nucleating agent such as glass
microballoons, phenolic microballoons, or finely divided polyethylene is
very beneficial to the production of smaller and more uniform celled foams
than obtained with surfactant alone. Of the nucleating agents tested, the
glass microballoons appear to be the most generally useful.
However, in non-homogeneous polymer systems, that is polyblend and graft
copolymers, such as high impact polystyrene and ABS, the use of glass
microballoon or other nucleating agents is not necessary, and in some
instances may be disadvantageous to obtaining small cell size uniform
foams.
For extruding the instant foam solution to form product of uniformly small
cell size, the design of the orifice and valve is critical. For example, a
needle-type valve which is opened gradually will literally tear up the
foam and disrupt its structure as it is flowing around the needle valve.
Preferably a straight-thru ball valve in which the opening is the same
diameter as the orifice immediately prior to and after the valve is the
most desirable arrangement. A gradual narrowing down from the pressure
cylinder to the orifice is also highly desirable. A pressure cylinder
which has the general configuration of an "S" curve with gradual
transition from straight to curved sections and a gradual transition
between the curved surfaces.
The important considerations then are that there should be a smooth and
gradual reduction from the diameter of the pressure cylinder to the
configuration of the orifice. However, it is not necessary that only round
orifices be used. Square, triangular, or other shaped orifices can also be
used with the same limitations as already stated for the orifice and
valve.
The pressurized container in which the final solution is packaged should
preferably be fitted with either a dip tube or a bottom entry port in
order to permit the use of separate extra pressurizing gases such as
nitrogen and the others already listed. This modification to the
pressurized dispensing cylinder is necessary or desirable only in order to
obtain faster rates of extrusion of the higher density foams, or fast
rates of extrusion at low temperatures.
Instant foam can be extruded through a multiplicity of orifices in order to
form small diameter strands of foam, or spagetti which could conveniently
be generated on site for packaging applications.
Instant foam can be extruded as a round or other shaped log of foam onto
the surface of water, in order to form a floating dam, a bridge, or
sections of foam for pontoons, life rafts, etc.
Conceivably, the instant foam could also be extruded as a thick sheet and
laminated immediately after exit from the orifice with paper, cloth, foil,
or polymeric films, on one side or both sides.
Instant foam can also be extruded into a suitably shaped vented container
in order to make instant life jackets or an instant life raft, all of
which would be non-sinkable.
Instant foam can be extruded into a closed, vented mold of any size or
shape. The foam will knit well to itself and take the size and shape of
the container with little or no shrinkage.
Instant foam being of high porosity is especially useful for absorbing oil,
and a preferred type of instant foam for absorbing oil, and a preferred
type of instant foam for this use is made from styrene/acrylonitrile
copolymer, e.g. about 70% by weight styrene and 30% by weight
acrylonitrile copolymer. Such a copolymer is less swelled or dissolved by
oil, especially the aromatic constituents of oil, than is polystyrene and
so better reusable after squeezing out oil or otherwise removing absorbed
oil.
Instant foam, especially at higher gas pressures, can be generated under
water and used for refloating sunken ships or as a flotation device to
carry objects from underwater to the surface.
Instant foams at about 2 lb/ft.sup.3 density can be generated on-site and
used as energy absorbing foams for air drop deceleration.
The quantitative release of water/surfactant instant foam is best achieved
from glass containers, glass coated containers, or containers with other
hydrophylic coatings which are wetted with water.
The thermoplastic polymers to which this invention pertains covers a wide
range of compositions, and includes, but is not limited to
acrylonitrile-butadienestyrene, the acetal resins such as
polyoxymethylene, the acrylics such as poly(methacrylate), cellulose
acetate, cellulose acetate butyrate, cellulose propionate, the
polycarbonates, the soluble polyolefins, polyisobutylene, polybutadiene,
butyl rubber, styrene-butadiene, styrene polymers and copolymers, soluble
urethanes, and the vinyl polymers and copolymers including polyvinyl
acetate and ethylene/vinyl acetate. The present invention has been found
to be particularly useful for foams of homogeneous polymers such as
polyvinyl acetate, vinyl chloride/vinyl acetate copolymers, polystyrene,
styrene, .alpha.-methyl styrene styrenebutadiene copolymers,
styrene-acrylonitrile copolymers and, to nonhomogeneous polymer systems,
i.e. polyblends and graft copolymers, such as high impact polystyrene,
acrylonitrilebutadiene-styrene.
It has been found that any number of solvents can be used as the major
solvent. It is preferable that the major solvent form a gel with the
polymer to be foamed and that it boils at or near 0.degree.C. It was found
that methyl ether (b.p. -24.8.degree.C.) and chloromethane (b.p.
-24.1.degree.C.) were excellent solvents and gelling agents. These two
solvents alone in a range of from 25 to 50% by weight of the polymer, and
in combination with minor amounts of other low-boiling solvents, can be
used to foam amorphous polymers, such as styrene, styrene-butadiene
copolymer rubbers, or the highly polar styrene-acrylonitrile copolymers or
polyvinyl acetate.
In order to prevent shrinkage of the foams produced by this invention,
whether made of polystyrene or another polymer or copolymer, it is
necessary to have a suitable co-solvent whose boiling point is preferably
at or near room temperature. The function of this second solvent is to
prevent shrinkage by diffusing out of the polymer matrix into the gas
space after the foam has been blown and thus supply the pressure required
to keep the foam inflated as the low boiling major solvent diffuses fairly
rapidly out of the foam. Minor co-solvents of this type include, but are
not limited to pentane (including isopentane, cyclopentane, etc.),
1,1-difluoro-1-chloroethane, dichlorofluoromethane,
trichlorofluoromethane, etc.
It was found that the proportion of minor solvent could vary from about 3
to about 30% by weight of the polymer. Although the minor solvent in the
above range was satisfactory, it was found that from about 10 to about 20%
by weight of the polymer produced foams having greatly enhanced
properties.
The critical role of a surfactant in the formulation was clearly shown. In
the absence of a suitable surfactant and/or water the foam was of very
poor quality, and had very large and uneven cells. Furthermore, the foam
did not extrude well from the pressure vessel. It was found that a sodium
salt of an alkyl aryl polyether sulfonate was quite satisfactory, although
other suitable surfactants could be used in the present process.
In homogeneous polymer solutions, such as polystyrene,
styrene/acrylonitrile copolymers, or polyvinyl acetate, the use of a
finely divided particulate nucleating agent, such as glass microballoons,
phenolic microballoons, or finely divided polyethylene, is very beneficial
to the production of smaller and more uniform celled foams than obtained
with surfactant alone. Of the nucleating agents tested, the glass
microballoons appear to be the most generally useful. However, in
non-homogeneous polymer systems, i.e. polyblends and graft copolymers such
as acrylonitrile-butadiene-styrene, the use of a nucleating agent is not
beneficial and may, in many instances, be disadvantageous.
The nucleating agents are solids that are readily dispersed in the gelled
polymer mixture and apparently serve as centers for bubble formation
resulting in uniform fine cells. Preferred are spherical polyethylene
powder having a particle size range of from 4 to 40.mu., an average
particle size of less than 20.mu., and a melt index of less than 40; glass
hollow spheres having a particle size range of from 10 to 270.mu., and an
average particle size of approximately 65 microns; and phenolic hollow
spheres having a maximum bulk density of 0.105 g./cc. and a particle size
range of 2-60 microns. The nucleating agents are used in a range of from
1.5 to 6% by weight based on the solid foam. Somewhat better foams may be
obtained at levels of about 3% by weight.
The melt index is a well-established basis for classifying polyethylene and
is measured by a simple capillary rheometer as described in ASTM D 1238.
See also "Melt Flow of Polyethylene", J. P. Tordella and R. E. Joley,
Modern Plastics, Vol. 31, No. 2, page 146 (1953).
Additives may be incorporated in the foamable mixture in small proportions
without adverse effects, e.g. plasticizers, flame retardants, dyes,
fillers, etc.
In certain applications it may be desirable to supplement the vapor
pressure of the foamable composition with a gas such as nitrogen or carbon
dioxide. This is conveniently done by adding the supplemental gas to the
already charged container, pressurizing it to any desired pressure, e.g.
100 or 400 p.s.i.
The shape and size of the foam is determined to a great extent by the
discharge outlet in the pressure vessel. Thus, a round outlet produces a
cylindrical foam of generally larger diameter than the opening, e.g. a 1
inch diameter outlet produces about a 5 inch diameter foam. Shapes of
various cross-sections are obtained by various shaped outlets, e.g.
square, rectangular, etc. Foams varying from pudgy cylinders to thin
board-like structures, short or long in length, are readily fabricated.
Although the foam is usually discharged into air, it may also be
discharged into a non-solvent liquid, e.g. water. The foam may be
discharged incrementally (batch wise) from the pressurized vessel in which
it is contained with prolonged periods of time between each release.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention is further illustrated by, but not limited to, the following
examples.
EXAMPLE 1
Various formulations of foamable compositions were made as follows. Each
was prepared in a separate glass pressure bottle (Coke bottle), either of
6.5, 16 or 26 fl. oz. capacity. The polymer (e.g. Lustrex HH-101), the
surfactant, and the nucleating agent were added to the tared bottle in
weighed amounts. The bottle was chilled in a suitable bath, e.g.
acetone-dry ice, to about 0.degree.C. and to it was added the cold,
liquefied methyl ether and the cold, liquefied secondary solvent (e.g.
Genetron 21 or Freon 11) to the desired weight. Thereupon the bottle was
quickly capped and allowed to warm up to ambient temperature. Finally the
bottle and its contents were rotated gently until the polymer dissolved.
At 25.degree.C. the pressure of the contents was about 60 psig.
Foamable compositions containing the quantities of polymer, surfactant,
solvents and nucleating agent, if any, shown below as Samples A-P were
prepared.
Foams were prepared by quickly removing the bottle cap from the bottle,
with the bottle inverted so that the contents discharged quickly through
an opening approximately 0.62 inch in diameter. The properties of the
foams are reported below.
FOAMABLE COMPOSITION
__________________________________________________________________________
Genetron 21 Properties of Foam
Dichloro- Closed
Lustrex Triton
Methyl
fluoro- Foam Cell Cell Comp.
HH-101 X-200
Ether
methane Nucleating Agent
Density
Content
Size Strength
Sample
(grams)
(grams)
(grams)
(grams) Name (g.) (g./cc.)
(%) (p.p.i.)
(p.s.i.g.)
__________________________________________________________________________
A 157.5 12 69 21 -- -- 0.014
100 5-7 1.0
B 157.5 12 69 21 FN-500**
2.0 0.024
20 10-20 14.5
C 157.5 12 69 21 FN-500 5.0 0.031
20 15-20 25.3
D 157.5 12 69 21 FN-500 10.0 0.038
20 20-30 55.7
E 52.5 4 23 7 -- -- 0.043
90 4-6 2.1
F 52.5 4 23 7 FN-500 2.0 0.032
85 50-60 26.0
G 52.5 2 15 15* -- -- 0.035
100 1-2 --
H 52.5 2 15 15* FN-500 0.5 0.033
85 15-20 --
I 52.5 2 15 15* FN-500 1.5 0.034
85 10-20 --
J 52.5 4 23 4 -- -- 0.021
85 15-20 6.3
K 52.5 4 23 4 Glass 1.5 0.022
100 100-150
12.2
Microballoons
L 157.5 6 69 12 " 3.0 0.030
70 30-50 14.5
M 157.5 6 69 -- " 3.0 0.030
70 30-40 --
N 157.5 6 69 12 " 2.0 0.023
-- 30-40 15.0
O 157.5 6 69 12 -- -- 0.021
70 5-10 2.6
P 157.5 6 69 12 Phenolic
2.0 0.031
60 20-30 16.0
Microballoons
__________________________________________________________________________
*Genetron 142b (1,1-difluoro-1-chloroethane) substituted.
**Polyethylene, micronized, low-density type, marketed by U.S. Industria
Chemical Company (USI).
Sample N was compared with commercial foamed polystyrene as follows:
Commercial
"N" Product
______________________________________
Compressive strength (p.s.i.)
21 25-30
Tensile strength (p.s.i.)
53 50-55
Flexural strength (p.s.i.)
42 55-75
Dielectric constant (1000 kc)
1.04 1.02-1.24
______________________________________
Lustrex HH-101 is a heat resistant polystyrene molding compound having a
tensile strength of 8100 p.s.i. and a deflection temperature under load of
205.degree.F. at 264 p.s.i fiber stress. It is available from Monsanto
Company, St. Louis, Mo., and described in their Data Sheet No. 5063C.
Genetron 21 is dichlorofluoromethane and is available commercially from
Allied Chemical Company, Morristown, N.J.
Triton X-200 is an aqueous dispersion containing 28% of the sodium salt of
an alkyl aryl polyether sulfonate described in the trade bulletin "Triton
Surface-Active Agents", 1951, of the Rohm and Haas Company, Philadelphia,
Pa.
Microthene FN-500 is a microfine polyethylene powder having spherical
particles ranging from 4 to 40.mu., with average particle size less than
20.mu.. It is available from U.S. Industrial Chemicals Co., New York,
N.Y., and described in their trade bulletin "Microthene F microfine
polyolefins" PTD-40-265: low-density polyethylene, melt index = 22;
density = 0.915 g./cc.; bulk density = 17-20 lb./cu. ft. (0.27-0.32
g./cc.), spherical particles with average size <20.mu..
Glass Microballoon Spheres IG-101 are hollow bubbles of glass ranging in
size from 10 to 270 microns in diameter, with typical average particle
size of approximately 65 microns. They are available from the Vistron
Corporation, Cleveland, Ohio and described in their product bulletin
"Glass Microballoon Spheres, Industrial Grade": sodium borosilicate glass,
bulk density | | |
