Heat stabilized polymers

The tendency to crosslink of polymer compositions comprising polymer units derived from acrylonitrile and maleic anhydride is controlled by the use of a sulfur acid.

BACKGROUND OF THE INVENTION 
This invention relates to polymers and polyblends, comprising, as 
polymerized components, an unsaturated dicarboxylic acid anhydride and an 
unsaturated nitrile. More specifically, it relates to such polymers and 
polyblends that have been stabilized against certain effects of high 
temperatures. 
It has been found that when maleic anhydride and acrylonitrile are present 
in the same polymeric environment, there is an apparent tendency for a 
reaction to occur that results in crosslinking and sometimes the evolution 
of carbon dioxide and/or water. This reaction occurs at elevated 
temperatures of the kind that can be reached during extrusion and molding 
operations. It would appear that the crosslinking occurs whether the 
acrylonitrile and maleic anhydride components are in the same or different 
polymer molecules. 
The effect increases in significance with the amounts of the components and 
generally above about 15% of each component in a polymer composition is 
enough to generate the effect to a noticeable extent if the temperature is 
sufficiently elevated, that is above about 265.degree. C. 
While it is usually possible to extrude and mold at lower temperatures 
where no problem is encountered, it is desirable to provide accommodation 
for the wide variation of conditions that occur as a matter of course in a 
commercial operation. The present invention provides a means of reducing 
the significance of the crosslinking effect thus permitting the use of a 
wider range of molding and extrusion conditions with such polymers. 
DISCUSSION OF THE PRIOR ART 
The crosslinking effect referred to above has been identified, for example, 
in U.S. Pat. No. 4,223,096 which describes the preparation of rubber 
modified terpolymers of styrene, maleic anhydride and acrylonitrile. In 
that patent, the crosslinking tendency is controlled by the use of chain 
transfer agents such that up to about 20% acrylonitrile can be 
incorporated in a polymer containing from 15% to 30% of maleic anhydride 
before the crosslinking tendency renders the polymer non-thermoformable. 
Other polymers in which such a tendency might be encountered in at least 
part of the ranges described include those described in U.S. Pat. Nos. 
3,642,949, 4,141,934, 4,167,543, 4,197,263, 4,197,376 and 4,205,140. 
The present invention provides a means of controlling this tendency and 
makes it possible either to extend the composition range so as to achieve 
even more advantageous properties, or to broaden the range of permissible 
processing conditions for the polymers. 
DESCRIPTION OF THE INVENTION 
The present invention provides a polymer composition with a reduced 
tendency to generate crosslinks comprising polymer units derived from an 
unsaturated nitrile and an unsaturated dicarboxylic acid anhydride, which 
composition comprises an effective amount between from 0.001 to 0.005 mole 
of a sulfur acid per 100 gm of the polymer. 
The polymeric composition can be provided by a single polymer or it may be 
a blend of polymers. Thus, the polymeric composition can be, for example, 
a styrene maleic anhydride/acrylonitrile terpolymer (or its 
rubber-modified equivalent) or a blend of a styrene/maleic anhydride 
copolymer (or its rubber-modified equivalent) with a styrene/acrylonitrile 
copolymer or an acrylonitrile/butadiene/styrene (ABS) copolymer or a 
nitrile rubber. 
In general, the crosslinking effect begins even where the proportions of 
the nitrile and anhydride components are quite low but as might be 
expected, the significance of the effect increases proportionately with 
the amounts. Thus, the present invention has particular utility when the 
proportions of the nitrile and anhydride components are each above about 
5% by weight of the composition weight. The process of the invention is 
particularly useful when the proportion of anhydride in the composition is 
between about 5 and 30% and the nitrile proportion is between about 5 and 
20% of the total polymer composition. 
The term sulfur acid is herein used to indicate a compound containing 
sulfur that yields a hydrogen ion when dissolved in water. The term "acid" 
therefore indicates a conventional Bronsted acid and the acid selected is 
preferably an organic acid though inorganic sulfur acids such as sulfuric 
acid may be used. Aliphatic sulfonic acids such as methane sulfonic acid 
and halogen substituted alkyl sulfonic acids can be used but the preferred 
organic sulfur acids are aromatic acids such as benzene sulfonic acid, 
dodecyl benzene sulfonic acid, toluene sulphonic acid, xylene sulfonic 
acid and naphthalene sulphonic acid. Excellent results have been obtained 
using para-toluene sulphonic acid. 
The amount added can be from about 0.001 up to 0.005 mole acid per 100 gm 
of the polymer composition weight. With some acids, however, increasing 
the amount added leads to an increase in the degree of crosslinking 
probably by a different mechanism to that initially suppressed by the use 
of the acid. Thus, with above about 0.005 mole per 100 gm of polymer of 
toluene sulfonic acid the crosslinking effect seems to increase in 
significance and the optimum level appears to lie in the range from about 
0.002 to 0.004 mole per 100 gm of polymer. The optimum effective range 
will differ with each additive, but in practice, the best properties are 
obtained within the above-stated range for all sulfur acids. 
The acid can be added in any convenient manner but it is found that, when 
the anhydride-containing component and the nitrile containing component 
are in different polymers, there is advantage in blending a portion of the 
acid with each polymer component before they are blended together or 
alternatively with the nitrile containing component alone. The reason for 
this effect is not known for certain but it may simply be a reflection of 
the better dispersion of the additive thereby obtained. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To illustrate the invention further a number of sulfur acids were blended 
with a polyblend comprising 37% by weight of a styrene/maleic 
anhydride/methyl methacrylate terpolymer comprising about 25.5% by weight 
of maleic anhydride and 63% of ABS (SAN-grafted polybutadiene particles 
dispersed in a matrix of an SAN copolymer) comprising about 13.7% by 
weight of acrylonitrile. 
Except where otherwise indicated the terpolymer and a first ABS component 
were blended together with a plasticizer (where one was used), and with 
the acid. This was then blended with a further ABS component to give the 
above blend. The blend was dried at 80.degree. C. in a circulating air 
oven and extruded in a one inch Killion extruder using a two stage screw 
with vent or compounded in a Banbury mixer. Blending temperatures of about 
200.degree. C. were used. 
Pellets of the blend were vacuum dried 16 hours at 80.degree. C. and molded 
in a one ounce Arburg molding machine using 800 psi pressure. Izod bars 
(1.27 cm.times.1.27 cm.times.12.7 cm) were molded directly from the Arburg 
at a stock temperature of 260.degree. C., and also after "Dwell Times" 
(i.e. length of time in Arburg at stock temperature) of 5, 10 and 15 
minutes. At each time, three bars were molded and inspected closely to 
determine the extent of any blistering (indicating decomposition) that had 
occurred. The bars were then ground in a Thomas mill. The ground bars were 
vacuum dried 16 hours and used to measure apparent viscosity at 100 
sec.sup.-1 at 246.degree. C. using a Monsanto Automatic Capillary 
Rheometer. (Both the rheometer and the method for obtaining apparent 
viscosity are described in the Instruction Manual for Model 3501-H 
Automatic Capillary Rheometer designed and developed by Dr. Samuel 
Steingiser, Monsanto Research Corporation, Dayton, Ohio, April 1972 
Edition.) The change in apparent viscosity gives an excellent indication 
of the changes that occur after exposure to the elevated temperature for 
prolonged periods. A higher viscosity indicates an increase in molecular 
weight due to crosslinking.

EXAMPLE 1 
This Example illustrates the use of sulfuric acid to control crosslinking 
in the above composition. A comparative run (no acid) and runs 
incorporating 0.125% and 0.25% of sulfuric acid were performed. The acid 
was incorporated as a 98% solution. The results are shown in Table 1 
below. 
TABLE 1 
______________________________________ 
SULFURIC ACID ADDITIVE 
Apparent Viscosity 
Dwell Time 
(Kp) Blistering 
(Min.) No Acid 0.125% 0.25% No Acid 
0.25% Acid 
______________________________________ 
0 14.0 13.2 13.0 None None 
5 15.0 14.9 13.4 Few None 
Blisters 
10 17.3 17.0 14.9 Blisters 
Blisters 
______________________________________ 
As can be seen the sulfuric acid is not very effective at a level of 0.125% 
(0.0013 mole/100 gm polymer) but at 0.25% (0.0026 mole/100 gm polymer) the 
crosslinking as evidenced by the increase in apparent viscosity has been 
significantly reduced. In addition, it would appear that evolution of gas 
as evidenced by the blistering of the bar has likewise been delayed by the 
presence of the acid. 
EXAMPLE 2 
This Example uses the same formulation as Example 1 except that 0.79% 
(0.0024 mole/100 gm polymer) of dodecyl benzene sulfonic acid was blended 
into the formulation. The results are set forth in Table 2 below. 
TABLE 2 
______________________________________ 
DODECYL BENZENE SULFONIC ACID ADDITIVE 
Amount of 
Dwell Time Apparent Viscosity 
Acid (Min.) (Kp) Blistering 
______________________________________ 
0% 0 14.0 None 
5 15.0 Few Blisters 
10 17.3 Blisters 
0,79% 0 12.4 None 
5 12.9 Few Blisters 
10 14.8 Few Blisters 
______________________________________ 
As will be appreciated from the above the dodecyl benzene sulfonic acid 
significantly reduces the crosslinking and also appears to moderate the 
severity of blistering even at 0.79% addition. 
EXAMPLE 3 
This Example illustrates the use of para-toluene sulfonic acid monohydrate 
to achieve the stabilization of a polyblend similar to that used in 
Example 1 except that 50% of the polyblend weight is provided by each of 
the terpolymer and the ABS. The results are set forth in Table 3 below. 
TABLE 3 
______________________________________ 
Para-Toluene Sulfonic Acid Additive 
Apparent 
Amount of 
Dwell Time Viscosity 
Acid (Min.) (Kp) Blistering 
______________________________________ 
% 0 20.5 None 
Acid 5 20.1 Few Blisters on surface 
(Control) 
10 26.7 Blisters 
15 32.0 Blisters and swelling 
0.50 0 18.6 None 
5 19.4 One or two small blisters 
10 20.6 Surface blisters 
15 21.6 Surface blisters 
1.0% 0 21.5 None 
5 23.6 Surface blisters 
10 25.1 Surface blisters and 
swelling 
15 28.4 Surface blisters and 
swelling 
2.0% 0 30.6 None 
5 35.2 Surface blisters 
10 38.1 Severe blisters 
______________________________________ 
This set of runs illustrates the danger of incorporating an excessive 
amount of the acid. Here, for example, at 1% and 2% of the acid (0.005 and 
0.011 mole/100 gm polymer respectively) the effect was significantly 
detrimental whereas at 0.5% (0.003 mole/100 gm polymer), the acid was 
effective to control crosslinking and blistering. 
EXAMPLE 4 
The following Example describes the results obtained using para-toluene 
sulfonic acid monohydrate in the formulation used in Examples 1 and 2. The 
ABS and the sulfonic acid additive (0.265, 0.371, 0.435, 0.477, 0.583% by 
weight) were blended together in a Banbury mixer and this blend was 
extruded with the terpolymer. The results obtained are set forth in Table 
4 below. 
TABLE 4 
______________________________________ 
Para-Toluene Sulfonic Acid Additive 
Para toluene Apparent 
sulfonic acid, % 
Dwell Time Viscosity 
(mole/100 gm. polymer) 
(Min.) (Kp) Blistering 
______________________________________ 
None 0 14.4 None 
5 15.7 Blisters 
Blisters 
10 18.6 plus 
slight 
swelling 
0.265 0 12.4 None 
(0.0014) 5 12.8 Blisters 
10 15.5 Blister 
0.371 0 12.0 None 
(0.0020) 5 12.3 None to few 
10 14.6 Blisters 
0.477 0 12.1 None 
(0.0025) 5 13.0 None 
10 14.4 Blisters 
0.583 0 12.3 None 
(0.0031) 5 12.5 None 
10 14.4 Blisters 
______________________________________ 
Clearly, the crosslinking problem and blistering are significantly 
controlled at para toluene sulfonic acid monohydrate levels between 0.265 
and 0.583%. 
From the above Examples it will be seen that a wide range of sulfur acids 
is effective in controlling crosslinking and delaying the onset of 
blistering normally encountered in thermoforming a polyblend comprising 
units derived from an unsaturated anhydride monomer and an unsaturated 
nitrile monomer. 
The polymer composition whose processability is improved by the use of the 
sulfur acid compounds can be single polymers such as terpolymer of 
styrene, maleic anhydride and acrylonitrile or a rubber-modified version 
of such a polymer as described, for example, in U.S. Pat. No. 4,262,096. 
Preferred terpolymers of this type comprise from 15 to 30% by weight of 
maleic anhydride and from 5 to 20% by weight of acrylonitrile. 
Particularly advantageous terpolymers containing from 10 to 30% by weight 
of a rubber having a glass transition temperature below 0.degree. C. and 
preferably below -30.degree. C. Suitable rubbers include polybutadiene, a 
rubbery copolymer of styrene or acrylonitrile and butadiene, polyisoprene, 
polychloroprene, EPDM rubbers, ethylene/vinyl acetate rubbers, acrylate 
rubbers and polypentenamer. 
Alternatively, and often preferably some or all of the unsaturated nitrile 
can be provided by a different polymer from that providing the unsaturated 
dicarboxylic acid anhydride. Thus, the polymer composition can comprise an 
anhydride-containing polymer such as a styrene/maleic anhydride copolymer 
preferably one containing at least 10% by weight, and more preferably at 
least 20% by weight such as from 20 to 35% by weight of maleic anhydride, 
or a terpolymer with part of the styrene replaced by a nonnitrile 
copolymerizable monomer such as an acrylate or methacrylate ester so as to 
provide from 2 to 20% of the copolymer weight. The anhydride-containing 
polymer can, of course, be rubber-modified. 
The nitrile-containing component of such a polymer composition can be 
provided by polymers such as styrene/acrylonitrile comprising from 20 to 
70% by weight of acrylonitrile and acrylonitrile/butadiene/styrene (ABS) 
wherein the acrylonitrile content is preferably at least 8% and more 
preferably at least 15% of the ABS polymer weight. 
The above exemplification is in terms of maleic anhydride and acrylonitrile 
but it is understood that some or all of these monomers can be replaced by 
their well known homologues such as (respectively) itaconic anhydride, 
aconitic anhydride and citraconic anhydride and methacrylonitrile. 
In addition to the sulfur acid the polymer composition can contain other 
conventional additives such as antioxidants, plasticizers, chain-transfer 
agents, flame retardants, flow aids, pigments, antistatic additives, 
fibrous or particulate fillers and the like, to improve specific aspects 
of their physical or chemical properties. It is intended that all such and 
related modifications be considered within the purview of the invention.