CA1036300A - Bodies of expandable synthetic resins and method of preparation - Google Patents

Abstract

ABSTRACT
Expandable synthetic resinous bodies which are capable of being expanded to thermal collapse--resistant foams are prepared by impregnating a thermo-plastic synthetic resinous body preferably an alkenyl aromatic resin body, with a blowing or expanding agent, for example, dichlorodifluoromethane, and a crosslinking agent, for example, boron trifluoride. Such impreg-nation can readily be carried out without the aid of an aqueous suspension.

Description

Expandable synthetic resinous compositions in general are being prepared by admixing an expanding agent such as a volatile fluid with a heat plastified resin in an extruder and extruding below the foaming tem-perature to provide expandable granules, or by impreg-nating granules in an aqueous suspension with a foaming agent at elevated temperatures. A particularly desirable variety of expandable synthetic resinous material are those which are slightly cross-linked. For example:
if styrene is the principle monomer, cross-linking is accomplished employing from about 0.01 to 0.25 weight percent divinylbenzene. Such foamable materials are particularly advantageous in that they are expandable into thermoplastic, thermocollapse-resistant foams. By "thermocollapse-resistant" is meant a foam that will remain stable; that is, will not shrink or collapse when heated for 10 minutes at a temperature of at least 40 above the glass temperature of the non-crosslinked or linear polymer. Generally in preparing such polymers, it is necessary to polymerize the monomer in the presence of a suitable amount of cross-linking agent which neces- ~ -sitates separate polymerization facilities for the pre- ~ -paration of such foams.
This invention provides an improved process for the manufacture of thermocollapse-resistant foams directly from linear polymers which are not cross-linked, thus permitting great flexibility in the source of synthetic resin employed to make the polymer.

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The invention also provides a process for the -preparation of thermocollapse-resistant foams which does not require the use of an aqueous suspension or the extrusion of a foamable composition.
These benefits and other advantages in accor-dance with the present invention are achieved in a method for the preparation of thermocollapse-resistant synthetic resinous foamable bodies, the steps of the method comprising subjecting a crosslinkable linear uncrosslinked thermoplastic synthetic resinous body to an atmosphere of a volatile fluid expanding agent at a tem-perature and pressure which swells but does not dissolve the body and sufficient to cause impregnation of the body by the expanding agent while introducing cross-linking means into the body to provide a desired degree of cross--linking within the resinous body.
The method of the invention may be practiced -with any synthetic thermoplastic resin which can be used for the preparation of expandable synthetic resinous particles. Such resins are well known in the art. A
simi'arly wide variety of foaming agents may be employed, including, for example, volatile hydrocarbons and halo-hydrocarbons. Alkenyl aromatic resins are employed in the practice of the invention with particular benefit.
By the term "alkenyl aromatic resin" is meant a solid polymer of one or more polymerizable alkenyl aromatic compounds. The polymer or copolymer comprises, in chem-ically combined form, at least 50 percent by weight of at least one alkenyl aromatic compound having the general formula:

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H
Ar-C_CH
wherein "Ar" represents an aromatic hydrocarbon radical, or an aromatic halohydrocarbon radical of the benzene series. Examples of such alkenyl aromatic resins are the solid homopolymers of styrene, ortho-methylstyrene, meta-methyl 8 tyrene, para-methylctyrene, ar-ethylstyrene, ar-vinylxylene, ar-chlorostyrene, or ar-bromostyrene;
the solid copolymers of two or more of such alkenyl aromatic compounds with minor amounts of other readily polymerizable oléfinic compounds such as, for example, methylmethacrylate and acrylonitrile.
Cross-linking of the polymer is most advantage-ously introduced during impregnation with the blowing agent and may be induced by well known cross-linking means for lS preformed polymers; that is, methods and means which are capable of introducing into a mass or body of linear polymer cros8-linkages. ~ross-linking material8 for various polymer systems are well known in the art and are discussed at great length in The Encyclopedia of Polymer Science & Technology, Volume 4, pages 331-414, John Wiley & Sons, Inc., 1966. For example: employing polystyrene, suitable multi-functional cross-linking compounds are dipropylene glycol, bis(chloromethyl)biphenyl oxide epoxy resins such as the polyglycidyl ether of Bisphenol A;
whereas suitable comonomers to provide reactive sites for cross-linking are, for example, vinylbenzyl chloride and glycidyl methacrylate.
By way of further illustration, the alkenyl aromatic resinous polymers are readily cross-linked by 15,768 F ~3~

.
, introducing a Friedel Crafts catalyst such as boron trifluoride into the polymer with the introduction of the -blowing agent, thereby providing simultaneous impregna-tion and cross-linking. If desired, a reactive agent is incorporated within the molecules of the polymer body by either copolymerization during formation of the polymer or by addition prior to impregnation with the blowing agent and a suitable catalyst or similar material to promote cross-linking. For example: alkenyl aromatic resins such as polystyrene are mixed in a melt or heat plastified condition with an agent which can be caused to effect cross-linking, or a chemically reactive monomer or polymer may be incorporated during or after polymeri- ~ ~
zation. Utilizing polystyrene, compounds such as ~ ~ -dipropylene glycol, bis(chloromethyl)biphenyl oxide are employed. Suitable comonomers which provide active sites for cross-linking when polymerized with alkenyl aromatic monomers such as styrene, include vinylbenzyl chloride, glycidyl methacrylate and the like. Other reactive com-pounds include the so-called epoxy resins beneficially of low molecular weight which, on the introduction of a -suitable catalyst such as borontrifluoride, will provide the desired cross-linking function. Chlorides such as vinylbenzyl chloride provide active cross-linking sites which are useful with basic catalysts such as organic amines, typically diethyl amine. Epoxy resins which are particularly suitable for use with styrene polymer systems include liquid epoxy resins such as the diglycidyl ether of bisphenol A which readily respond with a Friedel Crafts catalyst to provide cross-linking which may occur either 15,768-F -4-. . ~.

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before or during impregnation of the blowing agent.
Beneficially, cross-linking of the polymer is carried out at a temperature of from about -20C to +40C of the glass temperature of the polymer being treated. Such cross-linking may be carried out employing a suspending medium which is non-reactive with the cross-linking means and i~ saturated with the blowing agent being impregnated.
However, it is particularly advantageous to conduct the cross-linking and impregnation simultaneously under gener-ally anhydrous conditions and in the absence of a liquid suspending medium, and preferably under conditions which prevent the adhesion of particles of the polymer together.
For example, employing polystyrene and styrene polymers having polymerized therein minor amounts of a comonomer, temperatures of from about 60C to 120C are beneficial, and preferably from about 70C to 100C. The particles or bodies being impregnated may be individually supported during impregnation such as on a screen or tumbled in the presence of an anti-sticking agent such as finely divided talc or sodium chloride which is readily removed by water washing after the impregnation is complete. Generally it is desirable that such washing be done at a temperature below the temperature at which the particles foam.
Beneficially, the particles which are treated in accordance with the present invention may be of any desired shape. However, for most applications it is beneficial to employ generally spherical particles. Such particles should have a minimum dimension of about 0.05 millimeter and usually it is undesirable to employ particles larger than about 5 millimeters. For most applications, 15,768-F ~5~
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:: , ~ , ~036300 particles from about 0.5 to about 3 millimeters in di-ameter are most advantageous.
The suitability of any particular cross-linking means for any particular polymer at a desired operating temperature is readily determined by dissolving a base ~ -polymer such as polystyrene in a suitable solvent; e.g., -toluene, and depositing a film on a glass or other rigid substrate which is unaffected by the solvent, removing the solvent to leave a continuous film of about 10 to 15 microns in thickness, subjecting the supported film to blowing agent and cross-linking agent for a period of time and subjecting the treated film to a solvent for the uncrosslinked polymer and observing the increase in weight or thickness of the solvent swollen film. If the `
film increases in weight or volume from 5 to 50 times, the cross-linking agent and conditions are suitable for the practice of the invention. The amount of cross--linking introduced under conditions which cause the polymer film to become insoluble, that is, to exhibit a swelling or weight increase of about 40 to 50 in a good solvent rather than form a clear solution, is twice the amount required for the practice of the invention. There-fore the amount of cross-linking means suitable is from about one-half that required to provide a weight or volume increase of 40 to 50 volumes to the amount required to provide a swollen volume of about 5. When the density of the solvent and polymer are about the same swelling volumes can be used instead of weight.

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1036300, The following Examples serve to illustrate the invention.

A plurality of styrene polymer discs are pre-pared by polymerizing a monomeric mixture in polytetra-fluoroethylene molds at 80C for 24 hours and an addi-tional 4 hours at 100C. The discs have a diameter of 10 millimeters and a thickness of 1 millimeter. The monomeric mixture contains 0.5 weight percent benzoyl peroxide, based on the weight of the total monomer. The ` `
monomeric compounds are set forth in Table I which follows:

Run ~ Styrene % Vinylbenzyl chloride ~
100 0 ' .

2 99.95 0-05

3 99.9 0.1 Polymerization

4 99.8 0.2 initiator in all 99.6 0.4 samples 6 99.4 0.6 0.5% benzoyl 7 99.2 0.8 peroxide based 8 99.0 l.0 upon total 9 98.5 1.5 monomer.
97.5 2.0 ll 96.0 4.0 In each of the discs two small holes are drilled which are holes diametrically opposed and the discs are strung on fine wires employing 2 millimeter glass beads as separators. Six discs of each composition are employed and the wires twisted together between each group of 6 discs. An 86 cubic centimeter glass ampoule having a . :

.... . ... ~, . , 103~300 ' length of about 12 inches and 3/4 inch inside diameter is charged with 8 cubic centimeters of dichlorodifluoro-methane at a temperature of -50C. Boron trifluoride gas -is bubbled into the dichlorodifluoromethane for a period . . . .
of about two minutes, a length of time sufficient to ; ~
observe a fog at the mouth of the ampoule. The assem- ,J" ' .' blies of polymer discs are then placed in the ampoule and the ampoule sealed. The ampoule is then attached to the rim of a 20 inch diameter wheel rotating at about 10 revolutions per minute in a 70C water bath for a period of 24 hours. On removal of the ampoule from the water bath, the discs of Sample l; that is, 100 percent polystyrene, and of Sample 2, styrene containing 0.05 percent vinylbenzyl chloride, are distorted and the discs of Sample 1 have fused together. Samples 3, 4 and 5 containing from 0.1 to 0.4 percent vinylbenzyl chloride have distorted slightly but have not fused together.
With the remaining discs there is little or no distortion and no evidence of flow. The impregnated 9amples are te9ted for blowing agent concentration by determining volatile loss by heating under a pressure of 1 milli-meter mercury for 3 hours at 175C. The solubility in toluene is expressed .~s either total solubility or the swelling ratio; that is, the weight of the toluene swollen gel to the original polymer weight, and the foaming --behavior in an air oven at 125C is expressed as the ratio of the foam volume to the initial solid body (Vf/Vs).
These results are set forth in Table II which follows:

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~036300 TABLE II

Run Solubility Foaming at 125C
Polym. No. Wt. ~ in toluene (Vf/Vs) (from Table I) Volatile *(Wg/Ws) O1 2 5 10 20 min~
1 13.9 soluble 126 10 5 4 3 2 14.1 soluble 122 11 5 4 4 3 13.2 partly sol. No foaming data 4 12.9partly sol. 122 29 35 43 54 13.4partly sol. 118 21 30 40 47 6 13.6partly sol.No foaming data `
7 13.9partly sol. 1 27 33 43 53 43 8 12.917.6 1 31 37 45 55 67 9 13.213.9 1 19 22 28 32 36 12.69.9 1 13 16 20 22 25 11 14.05.3 1 11 14 17 18 18 ~ Weight of solvent swollen gel/weight of expandable particle A plurality o discs are prepared as in Example 1 (Table I) and impregnated with a mixture of ;;
99 parts of dichlorodifluoromethane plus one part by weight of dimethylamine. On impregnation, distortion -occurs with samples containing up to 1.5 percent vinyl- -benzyl chloride. No change is observed for the discs containing greater than 2.5 percent vinylbenzyl chloride.
Percent volatiles, solubility in toluene and foaming ~-behavior are determined in the manner of Example 1 and are set forth in Table III which follows. ~ -~

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TABLE: III
Run Solubility~ Foaming at 125C
Polym. No. Wt. % in toluene (Vf/Vs) (from Table I) Volatile (Wg/Ws) 01 2 5 10 20 mins.
1 14.3 soluble 118 8 4 4 3 8 11.2 soluble 116 8 5 4 4 9 14.2 soluble 124 17 8 5 5 12.2partly sol.l31 34 40 35 17 11 12.512.7 113 26 46 61 80 For purposes of comparison, styrene polymer discs are impregnated with dichlorodifluoromethane without the aid of the cross-linking agent such as boron trifluoride or dimethyl amine. All samples soften, distort and fuse together during pressurization. Percent volatiles, solubility in toluene and foaming characteristics are determined and are set forth in Table IV below.
TABLE IV

Run Foaming at 125C
Polym. No. Wt. ~ Solubility(Vf/Vs) (from Table I) Volatile in toluene 0 1 2 5 10 20 m ins.
1 13.7 soluble 1 24 9 5 4 3 8 12.9 soluble 1 22 8 4 4 3 9 13.2 soluble 1 27 7 5 4 3 11.6 soluble 1 23 9 4 4 3 11 14.0 soluble 1 20 7 4 3 3 A 25 gallon glass-lined reactor equipped with a loop agitator is charged with 75 pounds of an aqeuous phase consisting of 1 percent hydroxymethylcellulose as a suspending agent; 0. 3 weight percent potassium dichromate as a water phase polymerization inhibitor and 98.7 weight percent water; 75 pounds of a monomer phase consisting .. . ,, . :
~, , , 1036300 of 99.5 weight percent of a mixture which is 99 weight percent styrene and 1 weight percent vinylbenzyl chloride and 0.5 percent benzoyl peroxide. The mixture is heated for 24 hours at 80C and a further 10 hours at 100C
with the agitator rotating at 80 revolutions per minute.
The reactor is cooled and the contents discharged to pro-vide a plurality of clear, hard beads having a particle size ranging from about 0.8 to 1.3 millimeters. Four portions of the beads, each weighing 29.6 grams, are placed in 86 cubic centimeter glass ampoules, the first portion containing 8 cubic centimeters of a mixture of dichlorodifluoromethane and boron trifluoride; the second portion containing 8 cubic centimeters of a mixture of isobutane plus boron trifluoride; the third being a duplicate of the second, and the fourth having only 8 cubic centimeters of dichlorodifluoromethane. After sealing the ampoule, impregnation of the beads is carried out by rotating the ampoules for 16 hours in a 70C , water bath. The ampoules are cooled to room temperature, opened and 0.5 gram samples heated to 125C in an air oven and the relative foam volume after varying periods of time observed. The results are set forth in the following Table.
TABLE V
. ,- -Foaming volume at 125C after Cell No. 0 1 3 5 7.5 10 20 30 min. Size 1 1 36 56 71 80 88 102 99 fine ~ 0.1 mm 2 1 40 52 68 83 100 146 162 fine ~ 0.1 mm 3 1 28 44 55 66 74 102 120 coarse ~l mm 4 1 50 10 9 3 3 fine 0.1 mm 15,768-F -11--- :. .. .
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~036300 A portion of the impregnated beads prepared in Example 3 is melt extruded employing a Brabender extruder having a barrel temperature of 205C and a die temperature of 170C. The product is continuous strands ~-of polymer having diameters ranging from about 0.047 to about 0.060 inch. Two portions of the strands are placed in glass ampoules and impregnated with blowing agent for 72 hours at 70C. The first ampoule contains 8 cubic centimeters of dichlorodifluoromethane and boron tri-fluoride; the second ampoule contains only 8 cubic centi-meters of dichlorodifluoromethane. After cooling, the ampoules are opened and portions of the strand heated at 125C and the foaming characteristics observed. These characteristics are set forth in Table VI below.
TABLE VI
(Vf/Vs) Foam volume at 125C after No. 0 1 2 5 10 20 30 min.
1 1 31 31 47 55 54 72 (fine cells) 2 1 1.6 1.3 1.1 1.1 1.4 1.0 ~fine cells) A portion of the unimpregnated beads of Example 3 are compression molded to form a 1 millimeter thick sheet employing a pressure of 5000 pounds per square inch and a temperature of 205C. Samples of the molded sheet are impregnated with (1) a mixture of dichloro-difluoromethane plus boron trifluoride, and (2) dichloro-difluoromethane, impregnation being done at 70C for 72 hours. The sheet is then foamed in hot air at 125C
and the foaming characteristics observed. The foaming characteristics are set forth in Table VII which follows:

15~768-F -12-, . . .- . , 103630~ . . ..
TABLE VII `~
(Vf/Vs) Foam Volume at 125C after No. 01 2 3 5 10 20 30 min. ~ ~-1 111.6 34.8 41.6 49.2 67.2 101.2 140.0 : :

2 12.5 14.3 25.1 13.1 5.6 5.0 4.4 .

A plurality of styrene polymers are prepared :
by polymerizing monomer compositions containing 0.5 weight percent ben~oyl peroxide in 2 millimeter inside diameter glass ampoules for a period of 24 hours at a temperature of 80C followed by 5 hours at 100C. The :~
monomer compositions are set forth in Table VIII below. ;
TABLE VIII : :

Weight % ~ .
Sample Styrene Weight % glycidyl methacrylate (GMA) : :
1 100 0 samples contained :~
2 99.9 0.1 0.5% peroxide 3 99.5 0.5 initiator based i 4 99.0 1.0 upon total 98.5 1.5 monomer weight.
6 98.0 2.0 -7 97.0 3.0 - :
8 96.0 4.0 The resultant polymer rods are clear, hard and dissolve completely in toluene. A first portion of the rods are ~;~
treated with a mixture of dichlorodifluoromethane and :~
boron trifluoride prepared by bubbling boron trifluoride into 8 cubic centimeters of dichlorodifluoromethane for 3 minutes at a temperature of -50C and a second portion of the rods are treated with 8 cubic centimeters of dichlorodifluoromethane in 86 cubic centimeter glass 15~768-F -13-, ., . . ,. , : , . . . . . . .
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la36300 . ~ -ampoules at 70C for 24 hours. The ampoules are cooled, ~.
opened and the rods removed. Portions of the rod are evaluated for toluene solubility and foam volume ater 3 minutes at 125C. The results are set forth in Table ;
IX following:
TABLE IX
Sample No. Foam Volume Compo9ition Solubility after 3 min.
from Impregnating in toluene at 125C
Table VIII composition (Wg/Ws) (Vf/V~) 1 Pure CF2C12 Soluble 7 2 " " 7 3 " " 8 6 " " 6 7 " " 5 8 " " 7 1 CF2C12 + BF3 Soluble 8 ; ., 2 " " 17 3 " Very Highly 88 Swollen gel 4 " Highly 43 Swollen gel " 18 25 6 " 9.7 13 7 " 6.4 10 8 " 5.8 7 .5,768-F -14-.: . . . .: . . ~ ; . .. : ..
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. . , 1.036300 In a manner similar to Example 6, a plurality of rods are prepared employing the compositions set forth in Table X below.
TABLE X
Sample Weight %
No. Styrene Weight % Dipropylene glycol (DPG) 100 0 , 2 99.95 0.05 Samples contained 3 99.9 0.1 0.5% benzoyl 4 99.8 0.2 peroxide initiator 99.6 0.4 ,: .
6 99.4 0.6 `
7 99.2 0.8 Samples of each of the rods are completely soluble in toluene. Each of the rods is a clear glassy solid. In the manner of Example 6, a first portion of the rods is impregnated with dichlorodifluoromethane and boron tri- `
fluoride and a second portion impregnated with pure dichlorodifluoromethane. Impregnation is accomplished in glass ampoules at 70C for 24 hours. The ampouies are cooled, opened and portions of the rods evaluated for solubility in toluene and foaming characteristics.
The results are set forth in Table XI which follows.

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10363~0 TABLE XI

Foam volume ~ .
Solubility after 3 min.
Sample Impregnating in toluene at 125C .:
No. composition (Wg/Ws) (Vf/V~
1Pure CF2C12 Soluble 7 2 " " 7 , 3 " " 8 4 " " 6 6 " " 5 7 " " 6 ~ :

1CF2C12 + BF3Soluble 8 ~
2 " " 11 ~. ., 3 " " 14 4 " " 30 " Highly 75 swollen gel 6 " " 60 7 " 28.5 58 Similar results are obtained by hot mixing dipropylene glycol into the molten polystyrene and extruding to give strands prior to impregnation. Similar results are also obtained when bis(chloromethyl)biphenyl oxide in poly-styrene is hot mixed and extruded into strands prior to impregnation.

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15,768-F -16-~: ~ - . . - : - . .
: : . ., A plurality of ternary polymers of styrene -vinyl benzyl chloride and beta-hydroxyethyl acrylate are prepared employing 0.5 weight percent benzoyl peroxide ;
as initiator and polymerizing in one inch diameter glass ampoules at 80C for a period of 24 hours and further polymerization at 100C for an additional 5 hours. The polymers are hard and transparent and are compression molded into 1 millimeter thick sheets at a temperature of 200C and under a pressure of 5000 pounds per square inch. A plurality of one centimeter diameter discs are punched from the sheets for impregnation and foaming.
The monomer compositions are set forth in Table XII below.
TABLE XII
Styrene VBC ~ OH ethyl acrylate No. (mole fract.) (mole fract.) (mole fract.) 1.00 0.00 0.00 2 0.99 0.005 0.005 3 0.98 0.01 0.01 4 0.96 0.02 0.02 0.94 0.03 0.03 , The discs are strung on wires in the manner described in Example 1 and impregnated in 86 cubic centimeter glass ampoules containing 8 cubic centimeters of a mixture of 98 weight percent dichlorodifluoromethane and 2 percent ammonia. The ampoule is sealed and impregnation accom-plished using the apparatus of Example 1 at 70C for a period of 64 hours. At the end of this time, the ampoule is cooled and opened. Samples 1, 2 and 3 are badly dis-torted. Samples 4 and 5 show no distortion. Discs are then placed in an air oven at 125C and the foaming 15,768-F -17-, . . . .

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` 1036300 characteristics observed. These characteristics are set forth in Table XIII below.
TABLE XIII
Vf/Vo after No. 0 1 2 535 min.
_ .~Y; .~
S l 1 20 16 3 3 S 1 9 -- 17 30 , .
Co~mercial polystyrene molding granules are tumbled in a cone blender with a polydiglycidyl ether of bisphenol A having an epoxy equivalent weight of about 175, a viscosity at 25C of about S000 centipoise and commercially available under the trade designation of DER 332. After several hours the polystyrene particles are covered with a thin layer of the liquid epoxy. These compositions are ed to a Brabender extruder mixer to homogeneously mix the two materials at a temperature of 200C. The extruded mixture is cut into approximately spherical particles by a die face cutter as it emerges -from a die opening. The particles pass through the holes of a 2.38 millimeter screen and are retained on a 2.00 millimeter screen. The composition of the samples ~5 used in this Exemple is given in the following T~hle.

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1036300 . ; -TABLE XIV : :
- Run Weight % Weight % Diglycidyl Number PolystyrenP ether of bis~henol A
~ .
100 0.0 '~' 2 99.8 0.2 3 99.7 0.3 ~;
4 99 5 0.5 99.3 0.7 Particles from Runs 1-5 are heated to 70C
for 64 hours in sealed glass ampoules rotating slowly in a water bath. Each ampoule contains 29.4 grams of the polymer composition and 8 cubic centimeters of isobutane into which boron trifluoride gas has been bubbled for about 3 minutes at -50C. After cooling to about -50C
the impregnated polymer samples are removed from the ampoules and the following data obtained. ~ :
TABLE XV :
Volatile Content and Toluene Solubility of Isobutane + BF3 Pre89urized Samples Run No.* Percent Volatile Solubility in Toluene 1 10.7 Soluble .;
2 10.7 Soluble 3 11.5 Swollen gel Wg/Ws = 26.7 4 10.9 Swollen gel 19.2 10.7 Swollen gel . 12.7 * Determined by weight loss at 3 hours at 175C
Samples of the impregnated particles were foamed in a hot air oven at 125, 150 and 175C and the results :
are set forth in Table XVI. ..

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TABLE XVI
125C Foaming Temperature VFoam/vinitial solid or VF/vG
Run Percent Minutes in Oven No. Epoxy 1 3 5 10 20 30 . 1 0 41 54 39C20 2 0.2 44 6156 ~28 3 0.3 30 5060 84 119 142 4 0.5 38 5463 86 117 147 0.7 20 3539 46 57 61 150C Foaming Temperature 7 0.2 24 4 8 0.3 54 7592 114 121 123 9 0.5 61 7895 136 146 158 0.7 39 4650 56 56 46 175C Foamin~ Temperature 12 0.2 2 13 0.3 64 7885 88 85 78 14 0.5 63 8299 93 96 92 0.7 45 4949 36 24 15 In a manner similar to the foregoing examples, employing the swellability and solubility test herein- ~
before delineated, a wide variety of thermocollapse -resistant bodies are readily prepared.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for the preparation of thermo-collapse resistant synthetic resinous foamable bodies, the steps of the method comprising subjecting a cross-linkable linear uncrosslinked thermoplastic synthetic resinous body to an atmosphere of a volatile fluid expanding agent at a temperature and pressure which swells but does not dissolve the body, the pressure and temperature being sufficient to cause impregnation of the body by the ex-panding agent, while introducing crosslinking means into the body to provide a desired degree of crosslinking within the resinous body.

2. The method of Claim 1 wherein the body has a minimum dimension of 50 microns.

3. The method of Claim 1 wherein the synthetic resinous foamable body is an alkenyl aromatic resin.

4. The method of Claim 1 wherein the impreg-nation and introduction of crosslinking means into the body are conducted under conditions which, if a 10 to 15 micron thick film of the alkenyl aromatic resin is sub-jected to a like treatment and subjected to a solvent for the polymer, the polymer forms a swollen gel having a weight at least 5 times the original weight of the film and the amount of treatment is equal to at least one half the treatment required to obtain a film which on exposure to the solvent increases in weight about 50 times.

5. The method of Claim 3 where the alkenyl aromatic resin is a copolymer of styrene and up to 4 per cent vinyl benzyl chloride and the volatile fluid expand-ing agent is dichlorodifluoromethane and the crosslinking means introduced is boron trifluoride.

6. The method of Claim 3 wherein the synthetic resinous body is an alkenyl aromatic resin in admixture with a minor proportion of an epoxy resin.

7. The method of Claim 3 wherein the impreg-nation is generally anhydrous.

8. The method of Claim 1 wherein the impreg-nation and introduction of crosslinking means are carried out at a temperature of from about -20 to +40 degrees of the glass temperature of the uncrosslinked alkenyl aromatic resinous body.

9. The method of Claim 3 wherein the cross-linking and impregnation are carried out at a temperature of from about 60°C to 120°C.

10. The method of Claim 1 wherein a plurality of synthetic resinous bodies are employed in admixture with a particulate antisticking agent.