Method of making uniformly-sized expandable polymeric particles

A method of making expandable polymeric particles having a uniform size comprising the size classification of partially polymerized particles and subsequent full polymerization and impregnation with blowing agent of only those particles of a desired size.

This invention relates to a method of making expandable polymeric particles 
having a uniform size, which method involves the formation of polymeric 
beads via suspension polymerization and includes the step of classifying 
the particles by size prior to fully polymerizing the particles. In a 
preferred embodiment of the invention, partially polymerized particles of 
an undesired size are recycled to the feed monomer(s) and blowing agent is 
incorporated only into beads falling within the desired range, preferably 
during the completion of the polymerization of the particles. 
The formation of expandable polymeric particles, such as expandable 
polystyrene beads for use in expandable molding processes, has 
historically presented two major difficulties: how to best incorporate the 
blowing agent into the particle in order to render it expandable and how 
to achieve expandable particles of a relatively uniform size. 
Discussions of problems attending the impregnation of polymeric particles 
with blowing agent may be found in U.S. Pat. Nos. 3,192,169 to Doak; 
3,265,643 to Hatano et al.; 3,359,220 to Wright; 3,657,162 to Finestone et 
al.; 3,696,060 to Burt; and 4,153,763 to Bracke; and in British Pat. Nos. 
807,237 to Styrene Products Ltd.; 998,289 to Dow Chemical Co.; 994,074 to 
Koppers Co., Inc.; and 1,364,758 to Monsanto Ltd. Where, for example, the 
polymeric particles or beads are formed by suspension polymerization, if 
the blowing agent is added to the initial polymerization mixture the 
following problems may be encountered: reduction in the rate of 
polymerization; formation of deformed rather than spherical particles; 
formation of lower molecular weight polymer than would be obtained in the 
absence of blowing agent; and coagulation of the beads in the suspension. 
If, on the other hand, the practitioner awaits the formation of finished 
beads and then incorporates blowing agent via a post-polymerization 
steeping process, difficulties of a different sort are presented: 
relatively long steeping periods are required because of the low 
absorption rate of blowing agent into the finished polymer; the 
incorporated blowing agent is not uniformly distributed in the beads, 
usually resulting in particles having hard cores; an excess of blowing 
agent must be introduced to the impregnation vessel, necessitating the 
subsequent separation of the excess blowing agent from the impregnated 
beads; and additional additives are usually required. The preceding 
art-recognized drawbacks have led to suggestions of impregnation methods 
involving the addition of some or all of the blowing agent at a point 
intermediate the initiation of the suspension polymerization and the 
formation of fully polymerized beads. See, for example, U.S. Pat. Nos. 
3,192,169; 3,265,643; 3,359,220; and 3,657,162. 
The difficulties of achieving expandable particles of a relatively uniform 
size are described in U.S. Pat. Nos. 4,085,169 to Saito et al.; 4,153,763 
to Bracke; and 4,174,425 to Saito et al; and in British Pat. Nos. 994,074 
to Koppers Co., Inc.; 1,255,237 to Sekisui Kagaku Kogyo Kabushiki Kaisha; 
and 1,364,758 to Monsanto Ltd. Uniformly-sized expandable beads are 
desired because accurate metering of particles for molding or other 
processing is facilitated, and thus more uniform quality of finished goods 
is attainable; and because non-uniformity in size leads to non-uniformity 
in expansion in the production of finished molded goods. Moreover, beads 
respectively sized below and above certain minima and maxima may not be 
useful as expandable particles: for instance, particles that are too small 
will not retain blowing agent and therefore have no storage or shelf life 
as expandable particles, generally have a very low degree of 
polymerization, and present recovery and waste water contamination 
problems; and particles that are too large result in unduly long 
impregnation times for the incorporation of blowing agent and a 
non-uniform distribution of blowing agent within the beads, leading to the 
production of inferior molded articles. 
The inherent difficulty of producing uniformly-sized particles by 
suspension polymerization, which has been attributed to the necessary 
presence of a dispersing agent and the need for agitation of the 
polymerization mixture, has led to a variety of proposed methods for 
achieving a relatively uniform expandable bead size, most of which include 
sieving the particles to segregate beads of suitable size. 
One method has been to form polystyrene beads by suspension polymerization, 
screen the finished beads to arrive at a desirable bead size population, 
and then render the selected beads expandable by impregnation with blowing 
agent. Although this method avoids the incorporation of blowing agent into 
beads of a size unsuitable for useful expandable particles and permits 
rejected beads to be used in non-expanded articles, the screening and 
re-suspension of the beads for impregnation with blowing agent according 
to this method necessarily includes the previously described drawbacks 
associated with impregnating already-formed beads with blowing agent. 
If, on the other hand, the re-suspension procedure is avoided by 
incorporating blowing agent into all of the particles yielded by 
suspension polymerization, blowing agent is wastefully added to particles 
that are so small as to be practically useless as expandable particles, 
due to the lack of storage life, and too large to be uniformly impregnated 
with blowing agent. And as to both the afore-mentioned types of particles 
and those particles useful as expandable beads per se but of a size not 
suitable for the intended application of the sought-for product expandable 
beads, the utility of the undesired beads is limited by the presence of 
the blowing agent. Moreover, if the blowing agent is added with the 
initial polymerization mixture, the unsatisfactory ramifications discussed 
above are realized. 
One proposed method for obtaining uniformly-sized expandable particles 
essentially utilizes uniformly-sized seed particles of polymer to form 
uniformly-sized larger particles by accretion and polymerization of 
monomer. Since no source of seed or nucleating particles is described, 
other than via screening the product of a conventional suspension 
polymerization, and since no mention is made of the presence of blowing 
agent in the seed particles, this method apparently has utility only as an 
alternative use for undersized particles formed in a conventional process 
utilizing a post-bead formation blowing agent impregnation system. 
Among the objects of the present invention is the provision of a method of 
making uniformly-sized expandable polymeric particles whereby only beads 
of a desired size are fully formed and whereby the problems of disposing 
of beads that are either larger or smaller than desired may be obviated by 
separating same from beads of a desired size prior to the full 
polymerization of said undesirable beads and recycling the 
undesirably-sized beads to the feed monomer(s) for the suspension 
polymerization. Also provided is a method whereby expandable beads are 
formed in such a way as to avoid the impregnation of undesirably-sized 
beads with blowing agent.

According to the invention, a suspension polymerization is conducted in 
accordance with known procedures for forming polymeric particles until the 
particles have attained sufficient physical integrity to undergo size 
classification, for example by screening the particles after cooling and 
removal of same from the polymerization vessel, but prior to full 
polymerization of the particles. Because the polymerization is 
discontinued substantially prior to completion, the segregated particles 
contain substantial amounts of unpolymerized monomer. The blowing agent 
and other additives are readily absorbed by this monomer and are uniformly 
distributed throughout the bead. The optimum degree of polymerization may 
readily be determined by those skilled in the art. If the polymerization 
is allowed to proceed to close to completion, the time to absorb the 
blowing agent becomes unduly long and uniform distribution difficult to 
obtain. On the other hand, if the polymerization does not proceed 
adequately for the particles cannot be classified. 
The polymerization should be at least 58% complete, preferably from 60 to 
80%. These ranges are useful particularly in the case of the preparation 
of polystyrene beads. The partially polymerized particles are then 
classified by size in order to segregate those particles falling within 
the size range desired for the final product expandable beads, with the 
so-segregated particles preferably being simultaneously impregnated with 
blowing agent and fully polymerized in a subsequent polymerization 
reaction. Optionally and preferably, the partially polymerized beads of 
undesired size are recycled to the feed monomer(s) for dissolution. 
The polymerizable material employed in the practice of the invention is 
preferably styrene, but it can also be a mixture of styrene with one or 
more copolymerizable monomers, preferably a vinylidene monomer such as a 
different monovinylidene aromatic monomer, such as alphamethylstyrene, 
vinyl toluene, p-ethylstyrene, 2,4-dimethylstyrene, o-chlorostyrene, 
2,5-dichlorostyrene, and the like. When a comonomer is employed with 
styrene in the practice of the invention, its concentration should be such 
that the styrene constitutes at least 50 percent, preferably at least 75 
percent, of the weight of the polymerizable material. 
The polymerizable material can also contain a rubber, such as natural 
rubber, EPDM, polybutadiene, polyisoprene, copolymers of butadiene and/or 
isoprene with lesser amounts of comonomers such as styrene, acrylonitrile, 
methyl methacrylate, and the like, dissolved in the styrene and any other 
monomers included in the polymerizable material. When employed, the rubber 
ordinarily constitutes from about 1 to about 10 percent by weight of the 
polymerizable material. 
The blowing agent employed in the practice of the invention is preferably a 
volatile organic compound which has an atmospheric boiling point of less 
than about 80.degree. C., preferably in the range of about -10.degree. C. 
to about 80.degree. C. It should be non-reactive and have at most a slight 
solvent action on the end-product polymer. Exemplary of suitable blowing 
agents for thermoplastic styrene polymers are the aliphatic hydrocarbons 
such as butane, isobutane and the pentanes. The preferred blowing agent 
for use in the present invention is pentane. 
When desired, the blowing agent can comprise a mixture of a major amount, 
for example 70-99 percent by weight, of a blowing agent of the type 
described above and a minor amount, such as 30-1 percent by weight, of an 
organic solvent having a solvent action on the product polymer(s). 
Exemplary of suitable solvents for thermoplastic styrene polymers are 
acetone, methylene chloride, styrene monomer, benzene, xylene, carbon 
tetrachloride, chloroform, and the like. 
The total amount of blowing agent employed in the process of the invention 
is from about 3 to about 10 percent, preferably from about 5 to about 8 
percent, based on the weight of the polymerizable material. 
In one preferred manner of utilizing the invention in the production of 
uniformly-sized, expandable polystyrene beads, an 8000 liter suspension 
polymerization vessel is charged with 3400 liters water, 165 grams 
Naccanol 90F, 10 grams potassium persulfate, 15 kilograms tricalcium 
phosphate, 3750 liters styrene monomer, 16.8 kilograms benzyl peroxide 
(75% solution) and 3.5 kilograms T-butyl perbenzoate. The reactor 
temperature is raised to about 90.degree. C. over a period of about one 
hour and the suspension polymerization conducted at that temperature for 
about another five hours. At about that time an exotherm is noted by the 
reaction vessel temperature climbing to about 92.degree. C. At that point, 
about 70 percent of the styrene has been converted to polystyrene and the 
beads are hard and discrete and still contain styrene monomer. 1.7 
kilograms tricalcium phosphate are added, the reaction vessel is cooled to 
about 82.degree. C., and the partially polymerized polystyrene beads are 
dumped, washed, rinsed, dewatered and dried. The dried polystyrene 
particles are screened and the particles of desired size are segregated. 
Beads of a size larger or smaller than the desired expandable bead size 
are preferably recycled to the styrene monomer holding tank for 
dissolution and reuse in the suspension polymerization vessel. 
Once sufficient beads in the desired size range have been accumulated, the 
polymerization vessel is charged with 3400 liters demineralized water, 
3400 kilogram beads, 1650 grams tricalcium phosphate and 10 grams 
potassium persulfate. 34 kilograms BCL 46L tetrabromovinyl cyclohexane and 
8.5 kilograms dicumyl peroxide dissolved in 50 kilograms styrene monomer 
are added to the reactor, the temperature of which is then raised to 
100.degree. C. During temperature build-up in the reactor, the reactor is 
pressurized with nitrogen to 10 psi for one-half hour. Then 405 liters 
pentane are slowly added over a one and one-half hour period. The reactor 
is then held at 110.degree. C. for 6 hours and then cooled to 35.degree. 
C., at which point antifoam is added and the reactor dumped. The beads are 
washed with water containing nitric acid, rinsed, dewatered in a 
centrifuge, dried in a fluidized bed at 90.degree. F., and lubricated with 
0.0075% zinc stearate and 0.01% HISIL to yield uniformly sized, expandable 
polymeric beads of polystyrene suitable for use in expansion molding 
processes.