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Non-postcure fluoroelastomers.

The consumption of fluoroelastomers, which have excellent thermal resistance, oil resistance and chemical resistance, is increasing each year; as a result, the fluoroelastomer field is also facing the task of productivity improvement. However, for such heat-resistant rubbers as fluoroelastomers intended for use at high temperatures, a postcure is an essential step of their preparation process. This postcure is carried out for the purposes of:

* Completing the curing of the unreacted crosslinking groups that are still remaining after the primary cure (press cure); and

* eliminating the crosslinking-reaction residue generated during the primary cure.

It has, for example, the following effects on the properties of the resulting formed products:

* The effect of improving the mechanical properties and compression set property; and

* the effect of reducing gas generated at the time of use, that is, inhibiting the shrinkage of formed products.

Therefore, eliminating a postcure affects the physical properties of final formed products substantially, and it has not been realized in the manufacturing process of fluoroelastomers.

Among the curing systems of fluoroelastomers, peroxide curing, which uses an organic peroxide and a multifunctional unsaturated compound in combination, was developed originally using bromine groups as the cure sites (ref. 1). The characteristic of this curing system is that, unlike other curing systems of fluoroelastomers, that is, amine curing, which uses a polyamine compound as the crosslinking agent, and polyol curing, which uses a combination of a polyhydroxy compound and an onium salt, it exhibits its effects especially in the curing of polymers with a high fluorine content. The reason for this is that, because the cure sites in peroxide curing are already present in raw material rubber, it is not necessary to form olefins as the cure sites from a polymer by dehydrofluorination, unlike the aforesaid two curing systems; as a consequence, the concentration of vinylidene fluoride copolymerization units has almost no effect on curability.

Peroxide-curable fluoroelastomers (ref. 2) developed by Daikin that use iodine groups as the cure sites have outstanding curability because iodine groups act upon radicals far more actively than bromine groups and also because the cure sites are introduced to the polymer terminals, which are the most effective positions, at a very high probability. Thus, they are greatly different from fluoroelastomers cured by the aforesaid bromine-type peroxide curing. This is evident in that the curing temperature of our fluoroelastomers is 10 [degrees] C lower under standard conditions and in that the addition of metal oxide, which is essential in the bromine-type peroxide curing, is not necessary.

With the use of this type of fluoroelastomer having a high cure-reactivity, molded products can be produced by what is called a non-postcure process, which does not require postcure. However, further improvements are required in respect to the physical properties and shrinkage of these molded products.

A non-postcure process can be realized by the use of this type of highly curable fluoroelastomer, but doing so has posed some problems for physical properties, shrinkage problems at the time of use, and the like.

The non-postcure compounds presented in this article attain their objective by combining a polymer design having excellent crosslinking efficiency with novel cure compositions that yield excellent properties after the primary cure and that reduce volatile matter in the resulting formed products.

Polymer design

Daikin's iodine-containing, peroxide-curable fluoroelastomers are formulated by "iodine-transfer polymerization" and have iodine groups, which are the cure sites, at both ends of a straight-chain polymer, as described in the foregoing. Because the cure sites are located at polymer terminals in an approximately constant quantity (linear-type), these fluoroelastomers have good crosslinking efficiency, and very little extract comes out of their cured products. With respect to their physical properties, in spite of their low modulus and high ductility, they exhibit high strength and, furthermore, an excellent compression set property.

However, to meet a more difficult demand for a sealing property, it is necessary to increase the crosslinking density, and, as a way to realize this, a high-sealing (branchtype) has been developed, in which iodine groups are also introduced to the polymer side chains in order to increase the cure-site concentration.

The effects of the introduction of iodine-containing cure site monomers (IM) to polymer side chains have already been reported (ref. 3). That is, if the molecular weight is the same, the molecular weight between crosslinking points decreases because the number of iodine groups that is present in one polymer chain increases. As a consequence, although the strength of the obtained cured product does not change, the modulus increases and the elongation decreases, as shown in figure 1. In other words, the crosslinking density increases. Due to this increase of the crosslinking density, cured products with what is called good heat resistance, that is to say, with strength that does change much by heat aging, and with an improved compression set property can be obtained, as shown in figures 2 and 3.

[Figures 1-3 ILLUSTRATION OMITTED]

To realize a non-postcure fluoroelastomer, that is, to attain good physical properties from the primary cure, we conducted the present study to optimize formulation, using the aforesaid high-sealing type fluoroelastomers.

Search for optimal crosslinking agent

The primary cause of shrinkage that occurs in actual use, which is a problem in a non-postcure method, is the volatilization of the decomposition residue of an organic peroxide, which is a crosslinking agent. Therefore. the following are required of the crosslinking agent used in a non-postcure compound:

* It yields excellent physical properties after a primary cure; and

* it can impart excellent properties to cured products even if it is used in a small amount in order to reduce volatile matter.

Table 1 shows the organic peroxides examined in this study. These peroxides are dialkyl peroxides that can cure fluoroelastomers whose decomposition temperatures at which their half life becomes one minute are in the range of 173 to 186 [degrees] C.
Table 1 - organic peroxide (ref. 4)

Commercial Decomposition
name Chemical name temperature ([degrees] C)

Perhexa 25B 2,5-dimethyl-2,5-bis 179.8
 (t-butylperoxy)hexane

Percumyl D Dicumyl peroxide 175.2

Perbutyl P [Alpha],[Alpha]'-bis 175.4
 (t-butylperoxy)
 diisopropyl benzene

Perbutyl C t-butyl cumyl peroxide 173.3

Perbutyl D Di-t-butyl peroxide 185.9


When a postcure is a part of the manufacturing process, 2,5-dimethyl-2,5-bis-tert-butyl-peroxy-hexane is often used as the crosslinking agent because the properties of the obtained cured products are excellent. Although this peroxide yields excellent properties after a postcure, the improvement of the properties takes place largely in the postcure, and the properties after the primary cure are not quite satisfactory. Furthermore, because the volatile matter measured from a cured sheet is relatively large, it is conjectured that shrinkage at the time of use is large; thus, this agent is not suitable for "non-postcure," the objective of this study.

Through our study, we learned that percumyl D and perbutyl P were the crosslinking agents that were ideal for non-postcure formulation, that is to say, those that satisfied the aforesaid requirements, as shown in table 2. As shown in figure 4, percumyl D in particular exhibits a better compression set property when it is incorporated in a small amount if only a primary cure is carried out. Thus, it can be said that it is the ideal type of crosslinking agent for the objective of the present study, "non-postcure."

[Figure 4 ILLUSTRATION OMITTED]
Table 2 - effect of organic peroxide on some properties

Formulation 1 2 3 4 5

DAI-EL G-912(*1) 100 100 100 100 100
MT carbon black(*2) 20 20 20 20 20
TAIC M-60(*3) 6.7 6.7 6.7 6.7 6.7
Perhexa 25B 0.5 1.0 1.5
Percumyl D 0.1 0.5
Perbutyl P
Perbutyl C
Perbutyl D

Curing properties JSR curelastometer Type 2
Test temperature ([degrees] C) 160 160 160 170 160
Minimum torque (kgf) 0.29 0.30 0.29 0.22 0.32
Maximum torque (kgf) 4.82 5.10 4.93 3.29 4.88
Induction time (min.) 1.0 0.7 0.7 1.2 0.7
Optimum cure time (min.) 3.2 1.9 1.8 3.8 2.3

Curing conditions
Press cure 10 min. at same temperature as
Oven cure that of curing properties shown
 below each column

Mechanical properties Press cure only
100% modulus (MPa) 11.0 11.7 11.2 6.4 11.7
Tensile strength (MPa) 20.1 21.2 18.2 18.6 20.7
Elongation (%) 175 170 170 235 180
Hardness Shore A 74 75 73 75 75

Compression set
70 h. at 200 [degrees] C (%) 29.5 24.9 25.0 40.7 21.8

Mechanical properties Oven cure 4 h. at 180 [degrees] C
100% modulus (MPa) 14.9 14.4 15.1 7.9 14.5
Tensile strength (MPa) 26.7 24.7 27.8 22.5 24.8
Elongation (%) 175 165 160 230 165
Hardness Shore A 77 77 77 75 77

Compression set
70 h. at 200 [degrees] C (%) 20.4 17.3 17.1 35.1 19.0

Weight loss
After oven cure (%) 0.75 0.85 1.20 0.31 0.51

Formulation 6 7 8 9

DAI-EL G-912(*1) 100 100 100 100
MT carbon black(*2) 20 20 20 20
TAIC M-60(*3) 6.7 6.7 6.7 6.7
Perhexa 25B
Percumyl D 1.0 1.5
Perbutyl P 0.5 1.0
Perbutyl C
Perbutyl D

Curing properties JSR curelastometer Type 2
Test temperature ([degrees] C) 160 160 160 160
Minimum torque (kgf) 0.34 0.36 0.28 0.32
Maximum torque (kgf) 5.52 4.79 4.88 5.39
Induction time (min.) 0.5 0.5 0.8 0.5
Optimum cure time (min.) 1.4 1.4 2.8 1.5

Curing conditions
Press cure 10 min. at same temperature as
Oven cure that of curing properties shown
 below each column

Mechanical properties Press cure only
100% modulus (MPa) 9.8 11.3 11.9 13.3
Tensile strength (MPa) 18.4 19.2 21.0 21.8
Elongation (%) 195 170 175 165
Hardness Shore A 74 73 75 74

Compression set
70 h. at 200 [degrees] C (%) 21.2 24.0 23.1 22.8

Mechanical properties Oven cure 4 h. at 180 [degrees] C
100% modulus (MPa) 13.2 14.8 15.4 16.0
Tensile strength (MPa) 25.9 24.3 25.6 26.9
Elongation (%) 180 160 160 160
Hardness Shore A 76 77 77 78

Compression set
70 h. at 200 [degrees] C (%) 16.9 17.3 18.7 17.6

Weight loss
After oven cure (%) 0.42 1.13 0.50 0.83

Formulation 10 11 12 13

DAI-EL G-912(*1) 100 100 100 100
MT carbon black(*2) 20 20 20 20
TAIC M-60(*3) 6.7 6.7 6.7 6.7
Perhexa 25B
Percumyl D
Perbutyl P
Perbutyl C 0.5 1.0
Perbutyl D 0.5 1.0

Curing properties JSR curelastometer Type 2
Test temperature ([degrees] C) 160 160 170 170
Minimum torque (kgf) 0.29 0.28 0.23 0.26
Maximum torque (kgf) 4.80 5.02 4.50 4.70
Induction time (min.) 1.1 0.7 1.0 0.7
Optimum cure time (min.) 4.0 1.9 4.5 2.1

Curing conditions
Press cure 10 min. at same temperature as
Oven cure that of curing properties shown
 below each column

Mechanical properties Press cure only
100% modulus (MPa) 11.1 12.5 10.3 12.8
Tensile strength (MPa) 19.2 22.1 21.3 22.3
Elongation (%) 190 175 200 165
Hardness Shore A 74 74 75 75

Compression set
70 h. at 200 [degrees] C (%) 27.0 23.9 27.7 23.3

Mechanical properties Oven cure 4 h. at 180 [degrees] C
100% modulus (MPa) 14.4 16.1 12.9 15.0
Tensile strength (MPa) 24.6 27.0 24.4 27.1
Elongation (%) 160 165 170 165
Hardness Shore A 77 77 76 77

Compression set
70 h. at 200 [degrees] C (%) 17.5 19.1 20.8 18.0

Weight loss
After oven cure (%) 0.43 0.76 0.25 0.37


(Note) (*1) A terpolymer with 71% fluorine content prepared by a copolymerization with a curesite monomer IM

(*2) Thermax N-990 Cancarb Ltd.

(*3) Triallylisocyanurate (60% activity) Nippon Kasei Chemical Co., Ltd.

Although it will not be discussed in detail, the use of multi-functional unsaturated compounds other than triallylisocyanurate (TAIC) as the curing aids was considered, but the TAIC was eventually selected because it can achieve a high degree of completion of the crosslinking in the first cure.

Characteristics of non-postcure compounds

Using DAI-EL LT302, which has excellent low-temperature resistance, as the fluoroelastomer with a high-sealing property, non-postcure compounds were designed. The properties of these compounds are shown in table 3.
Table 3 - properties of non-post-cure compounds of DAI-EL LT-302

Formulation Non-post-cure Post-cure

DAI-EL LT-302(*1) 100 100
Viton GLT - -
MT carbon black 30 30
ZnO(*2) 10 10
Ca(OH)2(*3) - -
Percumyl D 0.75 -
Perhexa 25B - 1.5
TAIC M-60 6.7 6.7

Curing properties at 160 [degrees] C
 (320 [degrees] F)
Minimum torque (kgf) 0.35 0.21
Maximum torque (kgf) 4.80 5.05
Induction time (min.) 0.6 0.5
Optimum cure time (min.) 2.1 1.5

Curing conditions
Press cure 10 min. at 160 [degrees] C
 (320 [degrees] F)
Oven cure Non post cure 4 h. at 180 C
 [degrees] (356 [degrees] F)
Mechanical properties
100% modulus (MPa) 6.7 8.8
Tensile strength (MPa) 15.8 20.7
Elongation (%) 210 220
Hardness Shore A 72 74

Compression set
70 h. at 200 [degrees] C
 (392 [degrees] F) (%) 20 20

Low temp. flexibility
TR10 ([degrees] C) -32 -32

Shrinkage(*4)
After press cure 1.90 1.91
After aging 4h at 180 [degrees] C
 (356 [degrees] F) 2.15 2.37

Weight loss(*5)
After aging 4 h. at
 180 [degrees] C (%) 0.50 0.70(*6)

Formulation Competitive grade

DAI-EL LT-302(*1)
Viton GLT 100
MT carbon black 25
ZnO(*2) -
Ca(OH)2(*3) 3
Percumyl D -
Perhexa 25B 1.5
TAIC M-60 6.7

Curing properties at 170 [degrees] C
 (338 [degrees] F)
Minimum torque (kgf) 0.80
Maximum torque (kgf) 3.64
Induction time (min.) 0.8
Optimum cure time (min.) 5.4

Curing conditions
Press cure 10 min. at 170 [degrees] C
 (338 [degrees] F)
Oven cure 24 h. at 230 [degrees] C)
 (446 [degrees] F)

Mechanical properties
100% modulus (MPa) 7.5
Tensile strength (MPa) 20.4
Elongation (%) 200
Hardness Shore A 71

Compression set
70 h. at 200 [degrees] C
 (392 [degrees] F) (%) 30

Low temp. flexibility
TR10 ([degrees] C) -31

Shrinkage(*4)
After press cure -
After aging 4h at 180 [degrees] C
 (356 [degrees] F) -

Weight loss(*5)
After aging 4 h. at
 180 [degrees] C (%) -


(*1) A fluoroelastomer with excellent low-temperature resistance prepared by a copolymerization with a cure sit monomer IM

(*2) Zinc oxide Sakai Chemical Ind. Co., Ltd.

(*3) Caldic 2000 Ohmi Chemical Industry, Ltd.

(*4) Shrinkage of the inner diameter of an o-ring having an outer diameter of 115 mm and a width of 3.5 mm in relation to tile die size

(*5) Measured using the same o-ring as (*4)

(*6) This data was weight loss after oven cure

These compounds can realize excellent properties, especially an excellent compression set property, without a postcure, and the shrinkage of products formed from them that can occur in actual use can be reduced to about half that of prior compounds that were subjected to no postcure.

Conclusion

Good properties and small volatile matter (product shrinkage) can be attained by the combination of a fluoroelastomer having a high sealing property and dicumyl peroxide. This makes a non-postcure process possible, which fact is expected to lead to the improvement of the productivity of highly low-temperature-resistant and chemical-resistant products formed from fluoroelastomers.

[Figure 5 ILLUSTRATION OMITTED]

References

(1.) U.S. patent no. 4,035,565.

(2.) M. Oka, M. Tatemoto "Contemporary topics in polymer science," Plenum Press, New York, 763 (1984).

(3.) IRC '95 Kobe, Full Texts', The Society of Rubber Industry, 271-274 (1995).

(4.) Technical data of organic peroxide NO. OP-1, Nippon Oil and bats Co., Ltd. (1995).3
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Author:Kishine, Mitsuru
Publication:Rubber World
Geographic Code:1USA
Date:Nov 1, 1999
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