Grafted nitrile rubber--plasticized PVC blends as thermoplastic elastomers

Compositions comprising grafted nitride rubber, PVC and plasticizer are good thermoplastic elastomers without vulcanization. These compositions exhibit good solvent and abrasion resistance and usually good resistance to compression set.

BACKGROUND OF THE INVENTION 
This invention relates to thermoplastic elastomer compositions comprising 
blends of graft copolymers, polyvinyl chloride (PVC) and plasticizers. The 
compositions require no curing or vulcanization to develop elastomeric 
properties. The compositions thus remain thermoplastic and can be 
repeatedly remolded or extruded. 
The term "thermoplastic elastomer" has generally been applied to elastomers 
that can be readily processed and reprocessed by conventional melt 
processing equipment by virtue of the fact that such elastomers are not 
cured or vulcanized. The reprocessability of these elastomers compared 
with conventional cured or thermoset rubbers results in a great reduction 
in loss due to scrap, with consequent economic benefits for the processor. 
A variety of such materials have been introduced in recent years such as 
thermoplastic polyesters, styrene block copolymers, and thermoplastic 
olefin-rubber blends. Typical of such materials are the 
styrene-butadiene-styrene block copolymers sold as Kraton brand elastomers 
by the Shell Chemicals Co. and the Hytrel brand polyester elastomers sold 
by DuPont. Many of these elastomers have found wide application in 
consumer goods such as in shoe soling formulations and the like, as well 
as in such industrial applications as wire coating, hose and tubing, 
electrical connectors and automotive parts. 
Currently available thermoplastic elastomers suffer some disadvantages in 
use. In particular, formulations based on olefinic resins including SBS 
block copolymers exhibit poor resistance to hydrocarbon solvents and low 
abrasion resistance which may limit their use in particular environments. 
Additionally, adhesion to dissimilar materials is poor, and a surface 
chlorination or other primer treatment is often needed to increase 
adhesive bonding between, for example, a molded shoe sole formed of such 
materials and a synthetic shoe upper. Primer treatment of the surfaces of 
molded goods is also needed where the part is to be painted, which further 
increases the production cost of such goods. 
Nitrile rubbers or elastomers are essentially random, non-crystalline 
copolymers of 1,3-dienes and acrylonitrile containing from 15 to 50% 
acrylonitrile. These rubbers are widely available commercially and have 
long been used in the manufacture of oil-resistant gasketing, hoses and 
the like. As produced, nitrile rubbers are generally soft, low-strength 
thermoplastic gums that are soluble in or swelled by a variety of 
solvents. When compounded with reinforcing fillers and vulcanized, nitrile 
rubbers are tough useful elastomers with excellent oil and solvent 
resistance. The oil and fuel resistance of cured nitrile rubbers generally 
increases with increasing acrylonitrile level. However, the improved oil 
resistance is gained at some sacrifice in resilience and low temperature 
flexibility. Further, the vulcanizing or curing process results in highly 
cross-linked materials which are insoluble and intractable. The cured 
nitrile elastomers thus become thermoset and are no longer thermoplastic 
and readily reprocessable. 
The modification of nitrile rubber stocks by adding PVC together with a 
conventional PVC plasticizer has long been practiced in the rubber 
compounding art. A minor proportion of PVC, usually less than 33 wt %, is 
used to impart increased sunlight and ozone resistance to nitrile rubber, 
together with improved abrasion and tear properties. Such formulations 
find use in wire and cable coverings and in the production of hose and 
tubing, as well as in shoe sole formulations. These blends are, for most 
applications, normally vulcanized to provide elastomeric character and 
therefore are not considered to be thermoplastic elastomers. 
Oil-resistant thermoplastic elastomers comprising graft copolymers prepared 
by graft copolymerizing mixtures of monovinyl aromatic monomers and vinyl 
nitrile monomers in the presence of a nitrile rubber substrate have 
recently been disclosed. These compositions are thermoplastic elastomers 
without being vulcanized, and exhibit a high degree of oil resistance 
while retaining low temperature properties. For some applications, 
however, these materials exhibit an undesirably high level of shrinkage in 
molding and further formulation is required. 
Thermoplastic elastomer formulations based on nitrile rubber graft 
copolymers would be a useful advance in the art. These graft copolymers 
are readily produced by a variety of well known and economical processes. 
As is well known, graft copolymers may be readily modified by varying the 
type and proportion of monomers used in their preparation to selectively 
improve such characteristics as abrasion and solvent resistance, adhesion, 
weatherability and the like. Elastomer formulations based on these graft 
copolyemrs could thus find application in the production of molded and 
extruded goods to meet a wide variety of environmental requirements 
including shoe soling, extruded hose and tubing, wire and cable 
insulations, the production of flexible cord, automotive parts and the 
like. 
SUMMARY OF THE INVENTION 
Compositions comprising blends of thermoplastic nitrile rubber graft 
copolymers, PVC and plasticizer are useful as thermoplastic elastomers. 
The compositions do not require vulcanization, exhibit a rubbery feel and 
appearance, have good resistance to compression set and may be 
melt-processed in convention molding and extrusion equipment. 
DETAILED DESCRIPTION OF THE INVENTION 
The thermoplastic graft copolymers useful in the practice of this invention 
are graft copolymers with a rubber substrate and a graft phase. The 
substrate consists essentially of one hundred parts by weight of a 
conventional nitrile rubber, i.e., a rubbery copolymer of a 1,3-diene and 
a vinyl nitrile, and the graft phase consists essentially of from 10 to 
100, preferably from 30 to 70 parts by weight of a copolymer of a 
monovinyl aromatic monomer and a vinyl nitrile monomer. The graft 
copolymer also may be characterized as a high-rubber graft copolymer 
having from 60 to 80 wt % rubber substrate, and, correspondingly from 40 
to 20 wt % grafted superstrate copolymer. 
The nitrile rubber component useful as the substrate in preparing the 
nitrile rubber graft copolymers may be any of the conventional random, 
non-crystalline rubbery copolymers of a 1,3-diene such as butadiene or 
isoprene with acrylonitrile or methacrylonitrile. Typically, such 
copolymers will comprise from 85 to 50 wt % diene and from 15 to 50 wt % 
acrylonitrile. The preferred nitrile rubbers are those prepared in 
emulsion polymerization processes and containing greater than 40 wt % 
1,3-butadiene and, correspondingly, less than about 40 wt % acrylonitrile, 
and most preferred are those containing less than about 30 wt % 
acrylonitrile. The toluene-insoluble gel content of these nitrile rubber 
laticies is low, typically less than 60%. 
The graft phase or superstrate component will comprise a monomer mixture of 
vinyl aromatic monomers such as styrene, vinyl toluene, alpha 
methylstyrene and the like and mixtures thereof, and vinyl nitrile 
monomers such as acrylonitrile, methacrylonitrile and mixtures thereof. 
The graft polymerization may be carried out by a conventional emulsion or 
suspension free radical graft polymerization process. However, the 
preferred method for preparing oil-resistant thermoplastic elastomers will 
be the emulsion graft polymerization of the graft phase monomer mixture in 
the presence of a preformed latex of the substrate nitrile rubber, 
employing conventional free radical or redox polymerization catalysts such 
as cumene hydroperoxide, dicumyl peroxide or the like together with 
activators, or alkali metal persulfate initiator systems. 
PVC resins useful in the practice of this invention include homopolymer is 
of vinyl chloride and copolymers of vinyl chloride containing up to 20 wt 
% of copolymerizable monomers such as vinylidene chloride and the like. 
Methods for preparing the PVC polymers are also well known and such resins 
are widely available commercially in a variety of molding and extrusion 
grades. 
The plasticizers useful in the practice of this invention include any of 
the plasticizers commonly used in the art with vinyl chloride resins. 
Typical plasticizers include ester plasticizers such a dialkyl phthalates 
and the like and the phosphate plasticizers such as 
tri(alkylphenyl)phosphates and the like, as well as a wide variety of 
plasticizers based on vegetable oils such as expoxidized soyabean oil and 
the like. 
The composition of this invention comprises 100 pbw PVC resin, from 40 to 
250, preferably from 50 to 200 parts by weight of nitrile rubber graft 
copolymer, and from 80 to 120, preferably from 95 to 110 parts by weight 
of a PVC plasticizer. The compositions may be prepared by any of the 
conventional rubber compounding techniques including dry blending the 
components then melt processing in a compounding extruder or on a two-roll 
mill. Alternatively the components may be mixed in batch melt mixing 
equipment such as a Banbury mixer or the like. The compositions may then 
be further extruded or injection molded in conventional ram- or 
screw-injection molding machines to form extruded or molded goods. 
The compositions of this invention may further include antioxidants, 
stabilizers, fillers, pigments, extenders, secondary plasticizers, flame 
retardants, dyes and the like as is commonly practiced in the rubber 
compounding art.

The practice of this invention may be better understood through 
consideration of the following examples, which are provided by way of 
illustration of the invention and not in limitation thereof. 
In the examples, the following terms are employed: 
Tensile Str=tensile strength at room temperature, ASTM D638 
E=elongation at break, ASTM D638 
Hardness=shore hardness, ASTM D2240, A range 
Taber Abr=Taber abrasion, loss in grams/1000 cycles 
Vol Swell=volume swell on exposure to indicated solvents at the indicated 
temperature for 24 hours 
Comp Set=compression set at indicated temperature 22 hours, ASTM D395 
Elast Rec=Elastic Recovery, determined by extension at 100% for 1 min, 
releasing and measuring after 60 seconds 
EXAMPLE 1 
Preparation of Nitrile Rubber Substrate 
Potassium soap of tallow fatty acid (2780 g of a 10%-wt aqueous solution), 
potassium persulfate (27.8 g), t-dodecyl mercaptan (69.4 g), water (16000 
g), acrylonitrile (2780 g), and butadiene (6936 g), were changed to a 10 
gal pressure reactor. The mixture was heated to 40.degree. C. and held at 
this temperature with stirring until a monomer conversion of 95% or higher 
was reached (approximately 8.5 hr.). After cooling to 25.degree. C., the 
NBR latex was drained from the reactor. The NBR obtained contained 21.5% 
acrylonitrile. The solids content of the latex was 31%. 
EXAMPLE 2 
Preparation of Nitrile Rubber Graft Copolymer 
A portion of the NBR latex (6000 g) of Example 1 was diluted with water 
(3750 ml). To this latex were added cerelose (7.41 g), 
tetrasodiumpyrophosphate (0.741 g), ferrous sulfate heptahydrate (0.0988 
g) and t-dodecylmercaptan (11.5 ml). The latex mixture was then charged to 
a 5 gal flask equipped with stirrer and heated at 60.degree. C. under 
nitrogen flow. A mixture of styrene-acrylonitrile (1235 g, wt ratio 
styrene/acrylonitrile=7.33/1) and cumene hydroperoxide (7.4 ml, 83% 
active) was pumped into the flask over a period of 1/2 hr. After 10 min 
pumping time, the temperature was increased from 60.degree. to 70.degree. 
C. over a period of 20 min. After completing the addition, the reaction 
mixture was heated an additional 2.5 hours at 74.degree. C. After addition 
of 17 ml N,N-diethyl-hydroxylamine and 165 ml of an antioxidant emulsion 
(25% active), the latex was cooled to ambient temperature and coagulated 
with aqueous alum solution. The graft copolymer obtained, which had a 
rubber content of 60 wt %, was isolated by centrifugation and dried in a 
vacuum oven at 70.degree. C. overnight. Test specimens were injection 
molded on a Battenfeld 1 oz. injection molding machine, at a stock 
temperature of 350.degree. F. using a ram pressure of 1000-1300 psi and a 
mold temperature of 70.degree.-100.degree. F. The test specimens exhibited 
the following properties: tensile strength, 930 psi; elongation, 280%, 
permanent set at break, 25%; volume increase after 24 hr immersion; 1% in 
ASTM #3 oil; 4% in Ref. Fuel A, 68% in Ref. Fuel B. 
EXAMPLES 3-6 
In the following examples 3-6, compositions were prepared by compounding 
the formulations on a two-roll mill at 350.degree. F., sheeting out the 
compositions and cooling to room temperature. The compositions were then 
cut into strips and injection molded on a 1.0 oz Battenfeld reciprocating 
screw machine, using barrel temperatures in the range 
280.degree.-300.degree. F. and a mold temperature of 100.degree. F. to 
form test specimens. The formulations and the physical properties are 
summarized in Table I. 
TABLE I 
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Example No.: 3 4 5 6 
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Composition: 
Graft Polymer, pbw 
0 50 200 100 
PVC pbw 100 100 100 0 
Plasticizer pbw 
83 100 118 19 
Stabilizer pbw 
3.3 3.75 2.5 0 
ODPP pbq 3.3 3.75 2.5 0 
Properties: 
Tensile Str, psi 
1200 1010 740 560 
E % 180 200 255 250 
Hardness, Shore A 
62 62 65 57 
Taber Abr, g/Kc 
0.21 0.24 0.33 0.32 
Vol Swell, 70 hr 
ASTM #3, 212.degree. F. 
-19 -11 3.8 22 
Fuel B, RT 6.2 18 38 56 
Comp Set, %/22 hr 
Rt 22 24 34 51 
158.degree. F. 
72 63 45 65 
212.degree. F. 
87 74 55 54 
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Notes: 
Graft Copolymer = 40 wt % SAN grafted onto nitrile rubber substrate; see 
Example 2 
PVC = Polyvinylchloride, obtained as Geon 103 EP F76 from B. F. Goodrich 
Co. 
Plasticizer = Dioctylphthalate 
Stabilizer = Mark 1900 bariumlead soap from Argus Chemicals Co. 
ODPP = Diphenylisoctylphosphite from BorgWarner Chemicals, Inc. 
For testing definitions, see text. 
It will be apparent that the compositions of this invention, Examples 4 and 
5, exhibit good tensile strength and abrasion resistance, as well as 
useful solvent resistance. Surprisingly, the compression set 
characteristics of these compositions, particularly at elevated 
temperatures, is considerably better than either of the control blends, 
Examples 3 and 6. The compositions of this invention are thus tough, 
durable thermoplastic elastomers. 
It will be recognized by those skilled in the art that further 
modifications may be made in the graft copolymer component and in the base 
formulation to emphasize and enhance particular physical properties such 
as solvent resistance, hardness and abrasion resistance through use of a 
different proportion of the grafting monomers, rubbery substrates having 
different levels of nitrile monomer, different proportions of PVC and 
plasticizer, and selection of plasticizers as may be needed for a 
particular end-use. The compositions of this invention thus provide the 
compounder with a great degree of flexibility in meeting the requirements 
for use in a variety of widely differing applications. 
It will be seen that this invention is a composition comprising 100 pbw 
PVC, 40 to 250, preferably from 50 to 200 parts by weight of a nitrile 
rubber graft copolymer, and from 80 to 120, preferably from 95 to 110 
parts by weight PVC plasticizer. The preferred nitrile rubber graft 
copolymers may further be described as comprising from 30 to 70 parts by 
weight of a graft phase formed of a mixture of monomers comprising vinyl 
aromatic monomers such as styrene and alpha methylstyrene, acrylic 
monomers such as methylmethacrylate and vinyl cyanide monomers such as 
acrylonitrile, and 100 parts by weight of a nitrile rubber substrate, 
defined as a rubbery copolymer of from 85 to 50 wt % 1,3-diene monomer and 
from 15 to 50 wt % acrylonitrile, preferably less than 40 wt % 
acrylonitrile. 
The compositions are useful as thermoplastic elastomers in formulating 
molded and extruded consumer goods such as in shoe soling compositions, 
adhesives, hose and tubing and the like. As will be recognized by those 
familiar with the rubber compounding art, these compositions may further 
comprise additional thermoplastic resins, as well as antioxidants, 
stabilizers, dyes, pigments, flame retardants, fillers, processing aids, 
extenders, secondary plasticizers and the like as needed for particular 
uses, and such further additions and modifications are thus contemplated 
as within the scope of the invention.