Process for preparing graft styrene copolymers

A process for preparing graft styrene copolymers is effected by a two-stage polymerization of styrene in the presence of an EPDM elastomer and styrene block copolymer. The resultant graft styrene copolymer has a fine microstructure, good impact resistance and good resistance to aging.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for preparing graft copolymers 
based on styrene and an EPDM elastomer. 
Polystyrene is a thermoplastic which is widely used in the plastics 
industry by virtue of the ease with which it can be obtained and worked. 
Unfortunately, its use is limited as a result of its lack of heat 
stability and of its mediocre impact strength. 
In order to improve these properties, it has been proposed to use 
copolymers of styrene with other monomers such as acrylonitrile or 
acrylates. It has also been proposed to use graft polymers obtained by 
polymerizing styrene with an elastomer, e.g., a predominantly saturated 
elastomer such as an ethylene/propylene elastomer or an 
ethylene/propylene/polyene monomer elastomer (EPDM rubber). 
However, the use of such graft copolymers presents numerous difficulties. 
In particular, the predominantly saturated rubbers, which have a low 
proportion of double bonds, do not favor the grafting of styrene onto the 
elastomer. (The terms "rubber" and "elastomer" are used interchangeably 
herein.) Accordingly, the compositions obtained have a very low grafting 
ratio, i.e., a very low proportion of polystyrene grafted onto the 
elastomer, which is at most of the order of 0.5% by weight. 
Furthermore, microscopic observation makes it possible to establish that 
the resultant compositions contain large globules of rubber which are 
dispersed in the composition without any cohesion, and the size of which 
can be as large as 10 to 15 microns. 
The composition obtained with a dispersion of this type does not have a 
fine microstructure, resulting in products which are not very shiny and 
whose surface appearance is unsatisfactory. Apart from the problems 
described above, which relate to the heat stability and resistance to 
aging, the impact strength, the grafting ratio of polystyrene and also the 
microstructure, it is necessary, in order to obtain compositions 
possessing advantageous properties, to increase the degree of crosslinking 
of the elastomer. 
When styrene is polymerized in the presence of elastomers of the 
ethylene/propylene/polyene monomer type, e.g., EPDM rubbers, crude graft 
products are obtained which comprise, in particular, a graft copolymer 
formed by reaction of the rubber with the styrene monomer, and rubbery 
units consisting of "bridged" units, i.e., rubber units joined to one 
another by chemical bonds. The crosslinking density of the "bridged" 
rubber is expressed as the "swelling index", which is inversely 
proportional to the degree of crosslinking, i.e., the lower the swelling 
index the higher the crosslinking density and the more advantageous the 
properties of the copolymer. 
To solve these problems, processes have been proposed which include 
sophisticated reaction steps of the type involving the use of a mixture of 
special solvents, partial oxidation of the rubber before grafting, or 
multiple-stage polymerization. Not all these processes are satisfactory 
and they lead, in particular, to graft polymers which frequently still 
have a low grafting ratio together with a mediocre degree of crosslinking 
and a coarse microstructure. Examples of such processes are found, e.g., 
in French Pat. Nos. 2,320,950; 2,263,260; and 2,164,832. 
The process of the present invention makes it possible to obtain graft 
copolymers not having these disadvantages. 
SUMMARY OF THE INVENTION 
The present invention provides a process for preparing styrene-EPDM graft 
copolymers, comprising the steps of: 
(a) effecting a prepolymerization of styrene in the presence of an 
elastomeric EPDM terpolymer, and styrene block copolymer, until 
polymerization is about 20-30% complete; wherein the amount of EPDM 
terpolymer is at most 20% by weight relative to the total weight of the 
organic phase, and the amount of styrene block copolymer is 5-30% by 
weight relative to the weight of the EPDM terpolymer; and wherein the 
viscosity of the EPDM terpolymer is 30-100 centipoises, expressed as the 
viscosity of a solution of 5 weight parts of the EPDM terpolymer in 95 
weight parts of styrene; and 
(b) effecting a suspension polymerization of the resultant mixture from 
step (a), in the further presence of an amount of water such that the 
weight ratio organic phase:water phase is from 0.8:1 to 1.3:1, and 
recovering the resultant graft copolymer. 
The invention further provides a graft copolymer prepared according to the 
foregoing process. 
Upon further study of the specification and appended claims, further 
objects and advantages of this invention will become apparent to those 
skilled in the art. 
DETAILED DISCUSSION 
By carrying out the process according to the invention, graft copolymers 
can be obtained which have a high degree of gelling of from 25 to 40%, and 
also a good swelling index equal to at most 15, most frequently below 10, 
this index denoting good crosslinking of the elastomer. Furthermore, 
microscopic observation of the graft copolymers obtained by the process of 
the invention confirms that they have good cohesion and a fine 
microstructure such that at least 90% of the dispersed elastomer particles 
have a diameter of less than 5 microns, or an average diameter less than 4 
microns. Furthermore, by carrying out a process of this type, graft 
copolymers are obtained which have good mechanical characteristics, good 
resistance to aging and good heat resistance. 
To carry out the process of the invention, the polymerization is effected 
in two stages. The first stage, in which styrene, elastomer and styrene 
block copolymer are reacted, is called "preliminary polymerization" or 
"prepolymerization". It is effected at a temperature of at most about 
100.degree. C. under conventional mass polymerization conditions. The 
reaction mixture obtained in this first stage is then subjected to a final 
polymerization in suspension, in the presence of suspending agents and at 
a temperature of 90.degree.-150.degree. C. 
Suitable elastomers for use in the present process include terpolymers 
based on ethylene, propylene and a polyene monomer, e.g., 1,4-hexadiene, 
dicyclopentadiene, tricyclopentadiene, 5-vinyl-2-norbornene, 
5-ethylidene-2-norbornene, 5-methylene-2-norbornene, 
5-(2-propenyl)-2-norbornene, 5-(5-hexenyl)-2-norbornene, 
4,7,8,9-tetrahydroindene and isopropylidene-tetrahydroindene. Other 
suitable polyene monomer components of EPDM rubbers can be found in, e.g., 
Saltman, "The Stereo Rubbers", pp. 378-381 (John Wiley and Sons, N.Y. and 
London, 1977). Preferably, the elastomeric terpolymer has an ethylene 
content of 10-73% by weight, a propylene content of 23-77% by weight and a 
polyene content of 4-20% by weight (the foregoing refers to the contents 
of the respective monomer-derived units). 
The elastomeric EPDM terpolymer is used in an amount at most 20% by weight, 
relative to the total weight of the organic phase. 
One essential characteristic of the process of the invention is the use of 
styrene block copolymers in addition to styrene itself. Suitable such 
styrene block copolymers include, e.g., styrene/butadiene copolymers and 
mixtures of styrene/butadiene and styrene/ethylene copolymers. Typical 
commercially available block copolymers have molecular weights of 60,000 
to 300,000 preferably of the order of 200,000 and contain about 38% by 
weight of styrene, relative to the total weight of the copolymer. 
Molecular weights herein are expressed as weight average molecular 
weights, and are measured by gel permeability chromotography. Suitable 
such styrene/butadiene blocks copolymers are described in U.S. Pat. Nos. 
3,265,765 and 3,639,521. 
Typical commercial available styrene/ethylene block copolymers have 
molecular weights of the order of 100,000. 
Where mixtures of styrene/butadiene and styrene/ethylene block copolymers 
are used, the weight ratio of the styrene/butadiene copolymer to the 
styrene/ethylene copolymer is preferably at least about 1:1. 
According to the invention, the styrene block copolymers are used in 
amounts of 5 to 30% by weight, preferably 15-25%, relative to the amount 
of elastomer. The use of an amount of more than about 30% by weight is to 
no advantage, because the mechanical properties, in particular the impact 
strength, remain unchanged. Moreover, the use of such a large amount has 
the disadvantage of being uneconomical. The use of an amount of less than 
5% by weight, relative to the elastomer, leads to products having nodules 
of large particle size. 
Another essential characteristic of the process according to the invention 
is the use of elastomeric terpolymers having a low viscosity of between 30 
and 100 centipoises at 20.degree. C. The use of an elastomeric terpolymer 
having a high viscosity, i.e., a viscosity of more than 100 centipoises, 
causes perturbations in the suspension, which limits the amount used. The 
viscosity of the elastomeric terpolymer as used herein is defined as the 
viscosity at 20.degree. C. of a solution of 5 parts by weight of 
elastomeric terpolymer in 95 parts by weight of styrene. Whenever the 
viscosity of the elastomeric terpolymer is mentioned herein, it always 
corresponds to this definition. 
The prepolymerization step is most preferably effected substantially in the 
absence of water, which makes it possible to obtain graft copolymers 
having a highly uniform morphology. Nevertheless, it is possible to effect 
the prepolymerization step in the presence of as much as about 45% by 
weight of water, relative to the weight of the organic phase, without 
seriously impairing the desirable properties of the resulting graft 
copolymer. 
The process of the invention is carried out in the following manner. The 
styrene, the elastomeric terpolymer, the styrene block copolymer, and 
optionally a small amount of water if the prepolymerization is carried out 
by a slightly wet method, are introduced into an autoclave fitted with a 
heating device, a stirrer and a cooling device. From about 0.5 to about 2% 
by weight of conventional polymerization catalysts, relative to the weight 
of the organic products, are added. Suitable catalysts include, e.g., 
peroxides such as benzoyl peroxide, dicumyl peroxide, ditert.-butyl 
peroxide, tert.-butyl perbenzoate, 
2,5-dimethyl-2,5-bis-(tert.-butylperoxy)-3-hexyne and tert.-butyl 
perhexanoate. Chain-transfer agents such as mercaptans, e.g., 
n-dodecylmercaptan, can be added during the prepolymerization: these 
compounds are added in amounts of at most about 0.2%, relative to the 
organic phase. It is also possible to use known additives such as triallyl 
cyanurate: these are used in amounts of the order of at most 0.2%. 
The prepolymerization is effected at a temperature which should not exceed 
100.degree. C., and the temperature should not exceed about 95.degree. C. 
at the end of this step. Phase inversion occurs during the 
prepolymerization step. For about 20 to 30 minutes after the phase 
inversion, the medium is preferably subjected to the action of shear 
forces, for example with the aid of a turbine stirrer. The 
prepolymerization step is ended when polymerization is about 20-30% 
complete. 
After the prepolymerization, the polymerization is completed in suspension. 
Additional water, or the whole of the water of the suspension if the 
prepolymerization has been carried out in the dry phase, is added 
preferably in an amount such that the weight ratio organic phase:water 
phase is from 0.8:1 to 1.3:1. After the addition of water, it is 
advantageous to increase the temperature stepwise up to a final level of 
120.degree.-150.degree. C. This phase of the polymerization is effected 
for about 5-8 hours. 
During this second polymerization step, it may be useful to add 
conventional surface-active agents and suspending agents, such as 
carbonates, phosphates or chlorides, in amounts of at most about 0.5% by 
weight of each. The catalysts used for this step are conventional 
polymerization catalysts such as peroxides, e.g., 
2,5-dimethyl-2,5-bis-(tert.-butylperoxy)-3-hexyne or ditert.-butyl 
peroxide; the peroxides can be used singly or in mixtures, in amounts of 
between 0.5 and 2% by weight. The amounts of the above-mentioned additives 
and of the catalysts are expressed by weight, relative to the total weight 
of the organic phase. 
The graft copolymer is recovered by filtration and drying. 
The graft copolymers obtained by the process of the invention have good 
mechanical properties, together with good heat resistance and good 
resistance to aging. With this combination of properties, together with 
their fine morphology, these graft copolymers are preferred in fields of 
application where a good stability to aging is required, such as garden 
suites, caravans. 
Without further elaboration, it is believed that one skilled in the art 
can, using the preceding description, utilize the present invention to its 
fullest extent. The following preferred specific embodiments are, 
therefore, to be construed as merely illustrative, and not limitative of 
the remainder of the disclosure in any way whatsoever. In the following 
examples, all temperatures are set forth uncorrected in degrees Celsius; 
unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1 
The following are introduced into an autoclave fitted with a heating 
device, a stirrer and a cooling device: 
88 parts by weight of styrene 
9.6 parts by weight of an EPDM rubber having a monomer content of 44% by 
weight of ethylene, 45% by weight of propylene and 11% by weight of 
5-ethylidene-2-norbornene, 5 parts by weight of the elastomer dissolved in 
95 parts by weight of styrene having a viscosity of 60 centipoises at 
20.degree. C. 
2.4 parts by weight of a styrene/butadiene block copolymer containing 28% 
of styrene block and having a number average molecular weight of 200,000. 
The following are then added: 
0.13 part by weight of benzoyl peroxide 
0.1 part by weight of triallyl cyanurate 
0.125 part by weight of n-dodecylmercaptan. 
The reaction mixture is subjected to a prepolymerization step. This step is 
effected by initially subjecting the reaction mixture to stepwise heating 
so that its temperature reaches 92.degree. C. after 60 minutes, and then 
stirring it for 1.5 hours with the aid of a turbine stirrer. 
When this prepolymerization stage is complete, the following are added: 
120 parts by weight of water 
0.25 part by weight of t-butyl perbenzoate 
0.6 part by weight of ditert.-butyl peroxide 
0.5 part of 2,5-dimethyl-2,5-bis-(tert.-butylperoxy)-3-hexyne. 
The reaction mixture is then heated stepwise so that its final temperature 
reaches 150.degree. C. after 6.5 hours. This gives an organic phase, which 
is filtered off and dried. 
The rubbery phase of the polymer obtained has a particle size distribution 
such that 90% of the particles have a diameter of less than 4.5 microns 
and an average diameter of less than 2 microns. The particle size 
distribution is determined by electron microscopy, according to the 
procedure described in Kato, in J. Electro. Micros., 14, 1965 (20), using 
osmium tetroxide as the contrast agent. Its characteristics are summarized 
in Table 1. 
The degree of gelling, which is a measure of the proportion of crosslinked 
phase, i.e., the amount of polystyrene grafted onto the elastomer, is 
determined by stirring a 1 g sample of the graft copolymer in toluene at 
room temperature, e.g., 20.degree.-25.degree. C., centrifuging the whole, 
and recovering the gel insoluble in toluene. The degree of gelling is 
expressed as the weight percentage of toluene-insoluble gel relative to 
the test sample. 
The dry weight of the insoluble polymer is determined by treating the 
recovered insoluble gel in vacuo. The swelling index, which is a measure 
of the cross-linking density, is equal to the ratio of the weight of the 
recovered, toluene-insoluble gel to the dry weight thereof. 
The Izod impact strength is determined according to ASTM Standard D-256. 
The modulus of rigidity is determined according to ASTM Standard D-63.872, 
and breaking load is determined according to ASTM Standard D-63872. 
TABLE 1 
______________________________________ 
Degree of Izod impact 
Modulus of 
Breaking 
gelling Swelling strength rigidity load 
% index kg-cm/cm kg/cm.sup.2 
kg/cm.sup.2 
______________________________________ 
29.61 7.78 3.3 18,200 285 
______________________________________ 
EXAMPLE 2 
Example 1 is repeated, but the prepolymerization step is carried out in the 
presence of water and not in the dry phase. The same reactants and the 
same amounts are used as in Example 1, but 30 parts by weight of water are 
also added. 
When the prepolymerization step is complete, the process is continued as in 
Example 1 with the polymerization step, except that 90 parts by weight of 
water are added instead of 120 parts. 
After treatment of the polymer as in Example 1, the product obtained has a 
particle size distribution such that 90% of the particles have a diameter 
of less than 2 microns and an average diameter of 1.5 microns. The other 
characteristics of the polymer are summarized in Table 2. 
TABLE 2 
______________________________________ 
Degree of Izod impact 
Modulus of 
Breaking 
gelling Swelling strength rigidity load 
% index kg-cm/cm kg/cm.sup.2 
kg/cm.sup.2 
______________________________________ 
38.69 7.17 3.2 16,300 250 
______________________________________ 
EXAMPLE 3 
Example 2 is repeated, but the styrene/butadiene block copolymer is 
replaced by a mixture of the styrene/butadiene block copolymer of Example 
1 and a styrene/ethylene block copolymer which contains 28% of styrene 
block and has a molecular weight of 100,000. 1.2 parts by weight of each 
of these copolymers are used. After treatment as in Example 2, the product 
obtained has particles with an average diameter of 3.8 microns. The other 
characteristics of the polymer are summarized in Table 3. 
TABLE 3 
______________________________________ 
Degree of Izod impact 
Modulus of 
Breaking 
gelling Swelling strength rigidity load 
% index kg-cm/cm kg/cm.sup.2 
kg/cm.sup.2 
______________________________________ 
30.35 8.51 4.1 16,000 260 
______________________________________ 
COMATIVE EXAMPLE 4 
By way of comparison, Example 2 is repeated, but without the 2.4 parts by 
weight of the styrene/butadiene block copolymer. In this case, 12 parts by 
weight of the same terpolymer of Example 2 are used instead of 9.6 parts. 
After treatment, the product obtained has a very disperse particle size 
distribution, the maximum particle diameter reaching 15 microns and the 
average diameter being 7 microns. 
The properties of the polymer are summarized in Table 4. 
TABLE 4 
______________________________________ 
Degree of Modulus of 
Breaking 
gelling Swelling rigidity load 
% index kg/cm.sup.2 
kg/cm.sup.2 
______________________________________ 
21.65 14.34 15,200 220 
______________________________________ 
COMATIVE EXAMPLE 5 
Example 2 is repeated, but the elastomeric terpolymer of Example 2, which 
has a viscosity of 60 centipoises at 20.degree. C., is replaced by a 
terpolymer which has the same chemical composition as that of Example 1, 
but has a viscosity of 150 centipoises at 20.degree. C. 
After treatment, the polymer obtained has particles with an average 
diameter of 5.2 microns, 90% having a diameter of less than 10 microns. 
Its characteristics are summarized in Table 5. 
TABLE 5 
______________________________________ 
Degree of Izod impact 
Modulus of 
Breaking 
gelling Swelling strength rigidity load 
% index kg-cm/cm kg/cm.sup.2 
kg/cm.sup.2 
______________________________________ 
36.73 7.15 2.9 16,700 250 
______________________________________ 
COMATIVE EXAMPLE 6 
Example 2 is repeated, but an elastomeric terpolymer having a viscosity of 
210 centipoises is used. 
After treatment, the polymer obtained has a particle size distribution such 
that the particles have an average diameter of 7 microns, 90% having a 
diameter of less than 40 microns. Its characteristics are summarized in 
Table 6. 
TABLE 6 
______________________________________ 
Degree of Izod impact 
Modulus of 
Breaking 
gelling Swelling strength rigidity load 
% index kg-cm/cm kg/cm.sup.2 
kg/cm.sup.2 
______________________________________ 
34.82 7.46 3.1 17,300 250 
______________________________________ 
The comparative examples show that both the use of a styrene block 
copolymer and the viscosity of the terpolymer are critical factors in the 
present process in order to produce graft copolymers with the desired 
properties, especially the fine microstructure which characterizes the 
copolymer produced thereby. 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art can easily 
ascertain the essential characteristics of this invention and, without 
departing from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.