Method for manufacturing expandable styrene type polymer particles

A method for manufacturing expandable styrene type polymer particles is disclosed, which comprises the steps of adding a styrene type monomer to styrene type polymer seed particles suspended in an aqueous dispersing medium, continuously or intermittently to be polymerized while being absorbed thereby, and impregnating the resulting polymer particles with an easily volatile blowing agent to obtain expandable styrene type polymer particles, the improvement wherein styrene type polymer particles whose weight-average molecular weight (Mw.sub.1) is not more than 2/3 of the weight-average molecular weight (Mw.sub.2) of the resulting polymer particles are used as said styrene type polymer particles. The present invention provides expandable styrene type polymer particles excellent in moldability and foamed articles high in strength and fine in external appearance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for manufacturing expandable 
styrene type polymer particles uniform in particle size by adding a 
styrene type monomer to styrene type polymer seed particles uniform in 
particle size and suspended in an aqueous dispersing medium to thus cause 
the styrene type monomer to polymerize in said seed particles, and 
impregnating the resulting polymer particles with an easily volatile 
blowing agent. It provides expandable styrene type polymer particles which 
are not only excellent in moldability, but give foamed articles high in 
strength and fine in external appearance. 
2. Description of the Prior Art 
Expandable styrene type polymer particles release an easily volatile 
blowing agent (hereinafter referred to "blowing agent") contained therein 
to readily lose its expanding capability, and the time required therefor 
becomes shorter with particles smaller in size. Hence, when a mixture of 
particles different in size is expanded, a difference in expansion ratio 
results according to the particle size and the strength of foamed articles 
identical in density is lowered with increasing proportion of large-sized 
particles. When large-sized pre-expanded particles are included, it gives 
rise to such problems as deterioration of the charging property into a 
mold and difficulty to obtain a good foamed article. Furthermore, a foamed 
article obtained from particles broad in particle size distribution is 
different in the size of expanded particles, hence, not favorable in 
external appearance. From such a viewpoint, it is preferred to use those 
narrow in particle size as expandable styrene type polymer particles for 
producing a foamed article. 
For obtaining expandable styrene type polymer particles, there have 
hitherto been known methods roughly classified into the following 
categories; 
(1) A method for obtaining particles of a given size by suspension 
polymerization of a polymerizable monomer, then impregnating the resulting 
polymer with a blowing agent and finally sifting it. 
(2) A method for suspension polymerizing a polymerizable monomer, then 
sifting the resulting polymer and finally impregnating only the particles 
of a given range of size with a blowing agent. 
(3) A method for pelletizing a polymer to obtain pellets of a given size 
and then impregnating them with a blowing agent. 
(4) The so-called seed polymerization method in which a polymerizable 
monomer is suspension polymerized, then only particles of a given size are 
taken out and suspended in water, subsequently a polymerizable monomer is 
added either continuously or intermittently for further polymerization in 
seeds, then the grown particles are impregnated with a blowing agent. 
As to the method (1), however, all particles are impregnated with the 
blowing agent, and hence, the particles containing the blowing agent but 
not suited for intended uses because of their particle size being outside 
the intended range have to be treated for some other use, which becomes a 
heavier burden with increasing the amount of production. 
As to the method (2), since the polymer particles are used after sifting, 
the process of producing polymer particles and that of impregnating them 
with the blowing agent are necessarily separated, which results in 
complication of the process and also an increase in cost. Further, by this 
method, too, like the method (1), the burden of treating the particles off 
the desired range of the particle size for some other suitable use cannot 
be avoided, either. 
In the method (3), too, the pelletizing step is required in addition to the 
polymer producing step and the step of impregnation with the blowing 
agent, the overall process being thus highly complicated. The cells formed 
by expanding the expandable particles are extremely small and it is 
difficult to obtain a good foamed article. 
In order to eliminate the aforementioned defects there has been proposed 
the method (4) in Japanese Examined Patent Publication No. 49-2994, i.e., 
the seed polymerization method. According to this method, it is possible 
to manufacture styrene type polymer particles having the desired narrow 
particle size distribution according to the uniformity in size of the seed 
particles suspended in the medium by suspending polymer particles uniform 
in particle size in water and adding monomers containing a polymerization 
initiator. 
Expandable styrene type polymer particles can be obtained by impregnating 
in an aqueous suspension the styrene type polymer particles so obtained 
with a blowing agent, i.e., an easily volatile aliphatic hydrocarbon such 
as n-pentane, which slightly swells the polymer particles, or by 
impregnating in an aqueous suspension a blowing agent such as butane and 
propane which are gaseous at the normal temperature together with a small 
amount of solvent such as toluene and cyclohexane capable of dissolving 
the polymer particles. 
The expandable styrene type polymer particles so prepared are used as a 
material for manufacture of styrene type foamed articles. For industrial 
manufacture of foamed articles, the most common method is to first expand 
by steam heating or the like the expandable styrene type polymer particles 
to prepare pre-expanded particles, then charge the pre-expanded particles 
into a closed mold in the desired shape having many small holes in its 
wall, heat the pre-expanded particles to a temperature higher than the 
softening point of a polymer of the pre-expanded particles by letting a 
heating medium such as steam out through the small holes in the mold to 
thereby have the expanded particles fused together and then remove a 
foamed article from the mold after cooling. 
The foamed article obtained by the aforementioned method is formed through 
further expansion of the pre-expanded particles inside the mold to fill 
the gaps among the particles, but it is extremely difficult to obtain a 
foamed article completely free of inter-particle gaps. In the vicinity of 
the wall of the mold, in particular, the charging rate for the 
pre-expanded particles is lower than elsewhere, hence it is quite 
difficult to completely fill the inter-particle gaps, and in consequence, 
in the foamed article obtained there exist inter-particle gaps as 
superficial hollows. The existence of such inter-particle gaps on the 
surface of the foamed article not only deteriorates the external 
appearance of the foamed article, but also causes a decrease in the 
strength thereof. 
For solving this problem, many attempts have been made for improving the 
expandability by incorporating a plasticizer which has a plasticizing 
potential to styrene type polymer particles. Incorporation of the 
plasticizer, however, causes lowering of the softening point of styrene 
type polymer particles, that of the surface layer thereof, in particular, 
which causes aggregation of particles in the course of pre-expansion or 
melting of the surface in the course of expansion molding. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide a method for 
manufacturing expandable styrene type polymer particles giving a foamed 
article with less inter-particle gaps through nearly perfect fusion in the 
boundary plane among the individual expanded particles. 
Other objects and advantages of the present invention will become apparent 
to those skilled in the art from reading of the following description. 
The present inventors have found out that the aforementioned objects can be 
accomplished through the employment of styrene type polymer seed particles 
whose weight-average molecular weight (Mw.sub.1) is not more than 2/3 of 
the weight-average molecular weight (Mw.sub.2) of the resulting polymer 
particles. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to a method for manufacturing expandable 
styrene type polymer particles comprising the steps of adding a styrene 
type monomer to styrene type polymer seed particles suspended in an 
aqueous dispersing medium, continuously or intermittently to be 
polymerized while being absorbed thereby, and impregnating the resulting 
polymer particles with a volatile blowing agent to obtain expandable 
styrene type polymer particles, the improvement wherein styrene type 
polymer particles whose weight-average molecular weight (Mw.sub.1) is not 
more than 2/3 of the weight-average molecular weight (Mw.sub.2) of the 
resulting polymer particles are used as said styrene type polymer seed 
particles. 
The seed particles of a styrene type polymer used in the present invention 
are granules of a generally known styrene type polymer. As such particles, 
there are included homopolymer particles or copolymer particles of styrene 
and styrene derivatives such as .alpha.-methylstyrene, paramethylstyrene, 
t-butylstyrene and chlorostyrene. 
The size of the aforementioned particles is preferred to be as uniform as 
possible, because it is reflected on the particle size distribution of the 
resultant expandable styrene type polymer particles. As such styrene type 
polymer particles, preferred are a group of particles substantially 
uniform in particle size with not less than 90% by weight, preferably not 
less than 99% by weight, of the particles being 0.9-1.1 times of the 
volume-average particle size of the whole particles suspended in water. 
For easily obtaining such particles uniform in particle size, there is 
known a method in which a polymerizable monomer is dispersed in an aqueous 
medium as a group of liquid droplets uniform in size by passing them 
through a nozzle under a condition of regular vibration and such droplets 
are polymerized without aggregation or further dispersion. Alternatively, 
it is also possible to obtain such particles by classifying polymer 
particles obtained by an ordinary suspension polymerization method. As to 
the amount of such particles used, it is preferred to be 5-60% by weight, 
more preferably 5-30% by weight, of the whole amount of polymer formed 
upon completion of polymerization. If the amount is less than 5% by 
weight, the proportion of the added monomer which polymerizes into powder 
irrelevantly to the seed polymerization, not being used for growth of seed 
particles, increases. If, on the other hand, the amount is too much, the 
ratio of the size of the grown particles to the size of the seed particles 
becomes small, which is disadvantageous economically. 
The polymerizable monomer added for polymerization in the present invention 
is any monomer used in an conventional suspension polymerization and as 
such monomer, there are included styrene and styrene derivatives such as 
.alpha.-methylstyrene, paramethylstyrene, t-butylstyrene and 
chlorostyrene, these monomers being usable either alone or in combination 
of two or more. Also, bifunctional monomers such as divinyl benzene and 
alkylene glycol dimethacrylate can be used in combination with the above. 
As polymerization initiators for the aforementioned monomers in the present 
invention, radical generation-type polymerization initiators generally 
used in the production of thermoplastic polymers are usable. Typical 
examples are organic oxides such as benzoyl peroxide, lauroyl peroxide, 
t-butylperbenzoate, t-butylperpivarate, t-butylperoxy isopropylcarbonate, 
t-butylperoxy acetate, 2,2-di-t-butylperoxy butane, t-butylperoxy 
3,3,5-trimethylhexanoate, di-t-butylperoxy hexahydroterephthalate and 
3,3,5-trimethyl cyclohexane, and azo compounds such as 
azobisisobutylonitrile and azobisdimethylvaleronitrile. 
Such polymerization initiators are usable either alone or in combination of 
two or more, but, for proper adjustment of the molecular weight and 
reduction of the residual amount of monomer, it is effective to use two or 
more of polymerization initators different in decomposition temperature. 
The suspending agent used in the present invention is an ordinary one 
generally used, for example, water-soluble high molecular compounds such 
as polyvinyl alcohol, methyl cellulose, polyacryl amide and polyvinyl 
pyrrolidone or water-scarcely-soluble inorganic substances such as 
tribasic calcium phosphate and magnesium pyrophosphate. When the 
water-scarcely-soluble inorganic substance is used, the suspension 
stabilizing effect can be improved by additional use of an anionic 
surfactant such as sodium dodecyl benzene sulfonate. The joint use of the 
water-soluble high molecular compound and the scarcely-soluble-inorganic 
substance is also effective. 
The blowing agent used in the present invention is an easily volatile 
organic compound which is normally liquid or normally gaseous but liquid 
under pressure and of which boiling point is not higher than the softening 
point of the polymer. As such blowing agents, there are included aliphatic 
hydrocarbons such as propane, butane, pentane and hexane, cycloaliphatic 
hydrocarbons such as cyclobutane, cyclopentane and cyclohexane, and 
halogenated hydrocarbons such as methyl chloride, dichlorodifluoromethane 
and dichlorotetrafluoroethane. Such blowing agents can be used either 
alone or in combination of two or more. 
According to the present invention, it is by no means prohibited to use, 
besides the aforementioned materials, any substance generally used in the 
production of expandable styrene type polymer particles such as nucleating 
agents, plasticizers, solvents and flame retarding agents. 
The manufacturing method of the present invention comprises steps of 
suspending styrene type polymer seed particles in an aqueous medium and 
adding styrene type monomers thereto for polymerization to proceed and is 
characterized in that as styrene type polymer seed particles are used 
those whose weight-average molecular weight is not more than 2/3 of the 
weight-average mlecular weight of the resulting polymer particles. 
When styrene type polymer seed particles used are as low in molecular 
weight as mentioned above, the polymer particles finally obtained are wide 
in molecular weight distribution between the higher and the lower 
molecular weight, thus resulting in expandability in a wide range of 
temperature. The expandable styrene type polymer particles thus obtained 
are excellent in expandability, fused together almost perfectly in the 
boundary plane to form a foamed article with less inter-particle gaps. 
Moreover, since expandable styrene type polymer particles manufactured by 
the method of the present invention have low molecular substances existing 
near the center of the particles, unlike those containing the plasticizer 
uniformly as seen in the conventional art, there is no risk of the 
softening point of the surface layer of the particles getting lower and 
there is no problem of particles being aggregated during pre-expansion or 
of the surface of the particles being melted during expansion molding. 
Examples and comparative examples of the present invention will be given 
below for description of the invention in greater detail but, needless to 
say, the present invention is in no way limited thereby.

EXAMPLES 1-3 
1.8 liters of pure water, 7.2 g of tricalcium phosphate, 20 ml of 1% by 
weight aqueous solution of sodium .alpha.-olefin sulfonate, 360 g of seed 
particles whose average particle size range is between 0.59 and 0.50 mm, 
whose average particle size is 0.53 mm, and whose weight-average molecular 
weight (Mw.sub.1) is as shown in Table 3, and 8.1 g of ethylene 
bis-stearic acid amide whose particle size is 5-15 .mu.m were added to a 
5-liter reactor provided with a stirrer, and the temperature of the 
dispersion in the reactor was raised to 90.degree. C. under stirring. 
Then, a solution prepared by dissolving 12.6 g of coconut oil in 1390 g of 
styrene monomer and an emulsion prepared by dissolving 5 g of benzoyl 
peroxide and 1.8 g of 1, 1-di-t-butylperoxy-3, 3,5,-trimethylcyclohexane 
in 50 g of styrene monomer and then emulsifying them in 50 g of a 0.2% by 
weight aqueous solution of hydroxyethyl cellulose were continuously 
charged into the reactor at the rates shown in Table 1, respectively, and 
polymerization was conducted at 90.degree. C. 
TABLE 1 
______________________________________ 
Time elapsed (Hr) 
Condition 0-2 2-4 4-5 
______________________________________ 
Charging rate of styrene 
160 303 465 
monomer solution (g/Hr) 
Charging rate of emulsion of 
13 25 31 
polymerization initiator 
(g/Hr) 
______________________________________ 
Immediately after completion of charging of the styrene monomer solution 
and the emulsion of polymerization initiator, 32 g of cyclohexane and 153 
g of butane were added as a blowing agent, and the temperature of the 
reaction mixture was raised to 115.degree. C. and polymerization and 
impregnation with the blowing agent were continued for 3 hours. After 
cooling, the obtained expandable styrene polymer particles are taken out 
and after dehydration and subsequent drying, the expandable styrene 
polymer particles were heated with steam by means of a batch type 
pre-expander and pre-expanded particles with an apparent volume of 60-fold 
were obtained. 
The aforementioned pre-expanded particles were aged and dried for 24 hours 
in the atmosphere and then expansion molded by Pearlstar 90 Automatic 
Molding Machine (Toyo Kikai Kinzoku K.K.) with cavity dimensions of 300 
mm.times.450 mm.times.20 mm. The inner fusion rate, inner condition and 
suface condition of the foamed article obtained are shown in Table 3. 
The inner condition in Table 3 means the condition of inter-particle gaps 
of a flat plate of 300 mm.times.450 mm.times.5 mm cut out from the central 
part of the foamed article. 
The surface condition shows the smoothness and the inter-particle gaps in 
the surface of the foamed article. 
The inner fusion rate means the inter-particle fusion rate appearing in the 
fracture of the foamed article, and it is represented by percentage, 
against the whole number of particles in the plane of the fracture, of the 
number of particles cleaved in their insides and not parted in the 
boundary plane between particles in the plane of the fracture. 
COMATIVE EXAMPLES 1, 2 
Pre-expanded particles were obtained in the same manner as Examples 1-3 
except that seed particles used were in a particle size range of 0.59-0.50 
mm and the weight-average molecular weight (Mw.sub.1) thereof was as shown 
in Table 3. Then, foamed articles were obtained using the aforementioned 
pre-expanded particles and measurement and evaluation were made in the 
same way as Examples 1-3. The results are shown in Table 3. 
COMATIVE EXAMPLE 3 
Pre-expanded particles were obtained in the same manner as Examples 1-3 
except that seed particles used were in a particle size range of 0.59-0.50 
mm and the weight-average molecular weight (Mw.sub.1) thereof was as shown 
in Table 3, 36 g of toluene was further added to the monomer solution and 
the resulting monomer solution and the emulsion were charged at the rates 
shown in Table 2, respectively. Then, foamed articles were obtained using 
the aforementioned pre-expanded particles and measurement and evaluation 
were made in the same way as Examples 1-3. The results are shown in Tabled 
3. 
TABLE 2 
______________________________________ 
Time elapsed (Hr) 
Condition 0-2 2-4 4-5 
______________________________________ 
Charging rate of styrene 
164 311 477 
monomer solution (g/Hr) 
Charging rate of emulsion of 
13 25 31 
polymerization initiator 
(g/Hr) 
______________________________________ 
TABLE 3 
__________________________________________________________________________ 
Weight average- 
Inner 
Weight-average 
molecular weight of 
fusion 
molecular 
polymer particles 
rate of 
Inner Surface 
weight of 
after completion of 
Solvent 
foamed 
condition 
condition 
seed particles 
seed polymerization 
(toluene) 
article 
of foamed 
of foamed 
(-- Mw.sub.1) 
(-- Mw.sub.2) 
(Parts*) 
(%) article 
article 
__________________________________________________________________________ 
Example 1 
127000 259000 -- 100 No gaps 
Excellent 
Example 2 
162000 272000 -- 100 No gaps 
Excellent 
Example 3 
185000 281000 -- 100 No gaps 
Excellent 
Comparative 
223000 285000 -- 90 Some gaps 
Rather bad 
Example 1 
Comparative 
274000 292000 -- 80 Many gaps 
Bad 
Example 2 
Comparative 
274000 287000 2.0 90 Some gaps 
Rather bad 
Example 3 
__________________________________________________________________________ 
*Parts by weight to 100 parts by weight of expandable styrene polymer 
particles 
Expandable styrene type polymer particles wide in molecular weight 
distribution from low molecular weight to high molecular weight and broad 
in a temperature range for expansion are obtainable by using styrene type 
polymer seed particles with their weight-average molecular weight not more 
than 2/3 of that of the obtained polymer particles. Such polymer particles 
are excelled in expandability and provide foamed articles fused toghether 
nearly perfectly in the boundary plane with almost no inter-particle gaps. 
Moreover, expandable styrene type polymer particles manufactured by the 
method of the present invention have low molecular weight near the center 
of the particles, hence, there is no risk of the softening point of the 
surface layer thereof getting lower, thus precluding aggregation of 
particles in the course of pre-expantion or melting of the surface in the 
course of expansion molding. 
As described above, the present invention is capable of providing 
expandable styrene type polymer particles excellent in moldability and 
foamed articles high in strength and fine in external appearance.