chemical attack, thereby creating a problem in making fluorocarbons adhere either to themselves or other common substrates. Fluorel elastomer compounds may be bonded to various metals, other elastomers, and to themselves even in the fully cured state. Uncured Fluorel elastomer compounds may be bonded to steel by utilizing certain specialized bonding agents. Among the most successful are Fluor-0-Alloy primer and Chemlok 607. With both adhesives, a superficial bond is initiated during the press-cure cycle, and the actual bond is achieved in the course of the postcure. All metal substrates should be thoroughly sand blasted and degreased with a solvent such as trichloroethylene or methyl ethyl ketone. The bonding agents should be applied, according to the manufacturer’s instructions, as soon as the solvent has dried. Since the postcure seems to be critical in obtaining these bonds, it is advisable to step cure from a lower temperature, say 250’ F., in four to six steps u p to 400’ F. over a period of several hours in the manner sometimes required for molded parts. Table IX shows adhesions obtained between uncured compounds and steel. Fully cured Fluorel elastomer may also be bonded to steel utilizing the adhesive EC-1838. Metal preparation is the same as with press bonding, and the rubber need only be lightly abraded and wiped with solvent. The bond is achieved in 16 to 20 hours at room temperature under 3 to 5 p.s.i. pressure. Table I X lists bond strengths of these combinations. In general, adhesion to aluminum is comparable to adhesion to steel. Although good bonds to brass have been obtained, there is more inconsistency because of varieties of brass, especially if the bond is exposed to steam or hot water for any length of time. The best adhesion. to brass is obtained with low-copper alloys of nonporous nature. Uncured Fluorel elastomer may be laminated in the press to uncured acrylic-type elastomers such as Hycar 4021. NO
cements, primers, or tie coats are required. Postcure conditions should not exceed the heat resistance of the acrylate material. The laminate thus formed is homogeneous and blister-free. Some success has been achieved using a three-ply laminate, employing Fluorel, Hycar 4021, and Hycar 2202 (brominated butyl rubber). Three-part solvent solutions of these compounds can also be used to obtain adhesion between Fluorel elastomer and the general purpose brominated butyl. The bonding sequence is Fluorel elastomer to Hycar 4021 to Hycar 2202. Table IX also illustrates how cured Fluorel elastomer may also be bonded in a generally superficial manner to a variety of other cured elastomers. For the most part these bonds are poor; however, both EC-2216 and EC-1838 produce bonds of fair strength to Hycar 4021, whilc EC-1838 produces a good bond with Hycar 1072. Very good bonds have been obtained between fully cured strips of Fluorel elastomer utilizing adhesives EC-1838 and EC-2216. Both of these adhesives are two-part epoxy systems. Bonding is accomplished at room temperature in about 16 to 20 hours under 3 to 6 p.s.i. pressure. EC-2216 is more desirable as a general purpose rubber-torubber adhesive, since the bonds formed remain very flexible. The last part of Table I X shows these data. literature Cited
(1) Am. SOC. Testing Materials, Philadelphia, Pa., “ASTM Standards on Rubber Products,” 20th ed., 1961. (2) Dixon, S., Rexford, D. R., Rugg, J. S., Znd. Eng. Chem. 49, 1687 (1957). (3) Stivers, D. A,, Honn, F. J., Robb, L. E., Zbid. 51, 1465 (1959).
RECEIVED for review September 23, 1963 ACCEPTED January 10, 1964 Division of Rubber Chemistry, 145th Meeting, ACS, New York, September 1963.
MODIFIED ACRYLIC ELASTOMERS AND THEIR VULCANIZATION WITH A PEROXIDE=COAGENT SYSTEM M 0 RR IS A
.
M E N D E L S 0 H N , Westinghouse Research Laboratories, Pittsburgh 35, Pa.
The design of polyacrylic elastomers having improved low temperature flexibilities is discussed in terms of their chemical structure. Compounding characteristics were related to a structure sensitivity study. Resistance to oxidative degradation of a series of acrylic rubbers having good low temperature properties was determined. A peroxide-coagent vulcanization system was developed which imparts good resistance to oxidative degradation a t elevated temperatures.
elastomers are generally superior to natural and most synthetic rubbers in resistance to degradation by oxygen at normal and elevated temperatures and to many types of oils and lubricants, especially the sulfur-bearing types. The acrylates have found use in applications such as seals in automatic transmission, gaskets, hydraulic hose, and protective linings. Their applicability would be extended further if their flexibility a t low temperatures were improved. In some applications in the electrical industry acrylic elastomers having improved low temperature properties could be used in place of other synthetic rubbers because of their superior resistance to oxidation at elevated temperatures. A further upgrading of the thermal stability of acrylic elastomers combined with
A
CRYLIC
improved low temperatures properties would permit them to be used as partial replacement for the more expensive silicone rubbers. This led to the preparation of polyacrylic elastomers whose low temperature flexibilities exceeded those of the available modified poly(buty1 acrylates). To improve the oxidation resistance at elevated temperatures, a free radical vulcanizing system was developed to replace the amine system now used. Polymer Design
Polyacrylates which have superior flexibility at low temperatures were prepared by both internally plasticizing the chains and using a disordered chain structure. For example, coVOL. 3
NO. 1
MARCH 1964
67
200
polymers of butyl acrylate and 2-ethylhexyl acrylate give chain structures such as:
l
,
I
l
I
l
1
IW
Y
L
c=o
c=o
0
0
I
170
2 1w Y
1
4
2
z
150
E" 140 0
130
Even though the butyl groups would be considered as internal plasticizers, it is apparent that the bulkier ethylhexyl group would provide further separation of the chains and thus increase their mobility and rate of stress relaxation. In structure sensitivity studies (5, 7, 77) it has been reported that the flexibility increases and brittle points decrease with the length of the n-alkyl groups until n-octyl, after which the trend reverses. This reversal has been ascribed to crystallites which are formed from the extending n-alkyl groups rather than from the backbone of the polymer as in the case of natural rubber and polyethylene. Increases in amorphous structure result from use of mixed monomers and employment of nonsymmetrical side chains. Greater flexibility of the polymer is accompanied by lowered viscosity of uncured gum and reduced tensile strength, brittle point, and hardness in the cured vulcanizate. Copolymerization with styrene was also explored, since this results in polymers having reduced moisture absorption and improved hydrolytic stability and electrical properties (76). Copolymerization of butyl acrylate with a small quantity of styrene gave a polymer which had better low temperature flexibility than the commercial acrylonitrile-modified poly(butyl acrylates). Even though styrene generally imparts rigidity, this effect is a t least partially compensated for by the resultant disorder which interferes with a compact alignment of the chains. The low viscosity gums required to achieve low brittle points are normally too soft and tacky for compounding with many fillers on conventional rubber mills. T o facilitate their ease of handling during compounding, some of the materials were cross-linked slightly during their preparation by incorpora-
VI
5 YI $
120 110
Figure 1.
Effect of aging in air
Butyl acrylate-styrene polymer (material A)
tion of a small quantity of divinyl monomer, ethylene dimethacrylate (EDMA). This slight degree of prevulcanization facilitated the processability of the gums. Acrylonitrile was incorporated into several of the formulations to increase the strength of the gum and thus aid in processing. Experimental. The polyacrylic elastomers were prepared by emulsion polymerization according to a method similar to that previously described (3, 5, 77). In most cases the yields of polymer varied between 96 and 100%. The emulsions were coagulated with CaClt and the polymers were then washed with distilled water and dried by heating a t 110' C. at 10 to 15 mm. of Hg pressure for 16 hours. In most cases the polymers were insoluble in common organic solvents. This insolubility occurred in every case where EDMA was used and with the majority of other materials, indicating that some cross linking resulted from chain branching with subsequent termination reactions between rubber radicals. Compounding was performed by milling 100 parts of polymer with the bulk fillers and 1 part of stearic acid at about 25' C. for several minutes and then for 5 to 10 minutes a t about 150'
Table 1. 2-Ethylhexyl Acrylate 100
Polymer No 1 I
2 3 4 5
Composition of Flexible Polyacrylates Given in Weight Per Cent Ethylene Butyl AcryloniDimethylEthyl Acrylate Styrene trile acrylate Acrylate Other
89 80 67 67 50
6 7 8 9 10
50 40 40 40
11 lla 12 12a
40 40 10 10
5
13 14
33 50
8
10
0.5 0.5
100 100
15 16 17
95 95 90 90
18 19
20
0.5 0.5
5 10 10
-
-
11 20 25
50
50 90 90 95 .~
Processability@ of Gum
0.5 0.5
5
+ +-I++ +++ 4++ 4++
20 (hexyl methacrylate) 67 13 21 100 (isobutyl acrylate) a Polymer could be processed on a two-roll mill f o r compounding with 75 parts Mistron Vapor, I part stearic acid, and vulcanizing agents ( Table ZZ) per 100 parts polymer. - Too soft and tacky f o r processing. ~~~
68
~
I & E C P R O D U C T RESEARCH A N D DEVELOPMENT
C. Vulcanizing agents were then milled into the material at about 25' C. Molded sheets about 0.070 inch thick were subjected to aging by being placed in forced draft air circulating ovens. Tensile strengths were measured according to ASTM D 412 ( 7 ) , and hardness measurements were made with a Shore A durometer. Properties of Polymers
The results of a study of the processability of several gums (Table I) are consistent with the structural sensitivity concepts described above. Polyacrylates which had very low brittle points were in general too soft and tacky for easy processing on a two-roll mill. Styrene did not increase the tensile strength and firmness of the gum as much as an equivalent weight of acrylonitrile (Table I, 8 and 9 ) ; however, comparisons were not made on a mole basis. Use of EDMA considerably improved the firmness and tensile strength of the gum, thus making it much easier to process. Poly(buty1 acrylate) was barely processable; however, copolymerization
with 0.5% EDMA greatly improved the ease of milling. Vulcanizates prepared from both materials in an identical manner were indistinguishable. The advantages of a partial prevulcanization were similarly realized for materials 18 and 19. Polymers l l a and 12a were too soft for processing; however, use of EDMA rendered the corresponding materials, 11 and 12, processable. Poly(isobuty1 acrylate) gave a much firmer gum and a stiffer vulcanizate than the poly(buty1 acrylate). This was expected, since the isobutyl group has a more compact and sterically hindered structure than the butyl group. Furthermore, it is possible that the isobutyl group, having a tertiary hydrogen, entered into branching reactions, thus giving a more highly cross-linked network. A series of elastomers consisting of a butyl acrylate-styrene copolymer, butyl acrylate-2-ethylhexyl acrylate copolymer, butyl acrylate-2-ethylhexyl acrylate-acrylonitrile terpolymer, 2 10
*
200
2- 190
z -
2 180 CT
170
Y
. - 70 s a-
E 160
a
I
