A new generation of addition curing silicone heat vulcanizing rubbers.

The overall silicone elastomers market (heat cured and liquid silicone rubber) is about 150,000 metric tons and it's still growing at about 10%. LSR (liquid silicone rubber) takes more and more part of this market (about 25,000 mt). Silicone elastomers have a lot of unique properties, e.g., they don't change their mechanical properties significantly over a wide range of temperatures (-50 [degrees]-200 [degrees] C), show very good heat aging values, high flame retardancy, good chemical resistance, high transparency and biological compatibility. These elastomers are used in several industrial, automotive, household and health care applications because of their unique properties (ref. 1). There are two different ways of crosslinking silicone elastomers: Via peroxides by a radical reaction (HCR: heat cured rubber) and via platinum catalyzed hydrosilation reaction (LSR and HCR).

The common HCRs are crosslinked radically, which results in peroxide by-products. HCRs can also be crosslinked by an addition curing reaction like LSR; this is still the minor part of the market, but there is a trend seen that more customers ask for addition curing HCRs. This article will focus on a new class of heat cured rubbers, cured by a platinum catalyzed hydrosilation reaction. These HCRs utilize a new catalyst-inhibitor system which allows a one-part HCR that provide customers ease of processing and productivity.

Why is there a trend to addition curing silicone rubbers?

The addition curing reaction is very fast, which results in high productivity. Cycle times, are in general, shorter than with peroxide curing silicones. As mentioned before, during the peroxide initiated crosslinking reaction, by-products are built. Depending on the kind of manufacturing, different problems occur.

In the molding process, the built by-products often remain in the molding form, which results in problems like stickiness and bad demolding. Yellowing is often seen when dicumylperoxide is used for molding of transparent articles.

For the extrusion of silicone tubes, profiles and cables, 2,4-dichlorobenzoylperoxide is used. This may generate polychlorinated biphenyls (PCBs) during the curing process. Post curing is needed to avoid blooming of the peroxide by-products. Yellowing is also seen by using this peroxide.

Figure 1 shows a comparison of cure speed from two molding and three extrusion HCR grades at 175 [degrees] C. The curves show that the molding grades with platinum-vinyl-siloxane complex (ref. 2) as catalyst and ethinylcyclohexanole (ECH) as inhibitor (addition curing grade) and the DCP curing grade show significant differences in the cure speed. Using an addition curing grade re-sults in lower cycle times and higher productivity, besides other problems that can occur with peroxides as discussed before.

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The curves from the extrusion grades do not show such significant differences. Addisil (addition curing one-part HCR with the new catalyst-inhibitor system) and a platinum-vinyl-siloxane complex/ECH-inhibited two-part extrusion HCR are not as fast as rubber cured with 2,4-dichlorobenzoylperoxide. The cure speed is not dramatically different; all systems are suitable for fast extrusion processes.

Addition curing HCRs show very good mechanical properties compared to peroxide cured HCRs, with platinum cured material tear values of up to 50 N/mm and elongation values up to 1,000% achievable.

HCR extrusion market

HCR is used in any kind of extrusion where silicones give advantages to the user. Cables, profiles and tubes are the most extruded articles made out of silicone.

Especially in the health care and appliance industries, an increasing number of profiles and tubes is made out of addition curing silicone. As mentioned before, no by-products are produced during the cure process. Less post curing is required and there are only some remaining siloxanes which can migrate in the transported liquids (ref. 3). Especially by-products from 2,4-dichlorobenzoyl-peroxide are not welcome in tubes for medical applications (e.g., pumps for blood transportation). Producers of vendor machines, coffee machines and other appliances tend more and more to use addition cured tubes and profiles, because it is often seen that the taste of the transported liquids changes when they are transported through a peroxide cured tube.

A new addition curing HCR

Addition curing HCRs suitable for extrusion are available on the market as two-part systems. The manufacturer of tubes or profiles has to mill the

crosslinker (siloxane hydride, SiH) and the catalyst (platinum complex) batch together before use. The shelf-life of the ready-milled product is about 2-4 days, depending on the material.

We developed a one-part addition curing HCR suitable for extrusion with a shelf-life of up to six months, which makes the product ready-to-use for the manufacturer. This one-part system gives a lot of productivity and ease of processing to the user. The main advantage is that the milling step is avoided, which means less labor, and the milling can cause failures and inaccuracies. Manufacturing Addisil as a ready-to-use product in a clean room (healthcare) gives a big benefit to the manufacturer because a mill causes a lot of dirt in such an environment.

Kinetic studies of Addisil using the novel catalyst-inhibitor system are presented in table 3 and compared to the common platinum-vinyl-siloxane catalyst/ECH inhibited addition curing products (in the following text these products will be named as ECH inhibited systems/grades). Besides the platinum catalyst and the inhibitor, another important ingredient for the addition curing reaction is the SiH-crosslinker (hydride) and the ratio Si-H to Si-vinyl (SiVi). There are some differences seen from the common ethinylcyclohexanole (ECH) inhibited system to the Addisil grades.

Table 1. - comparison of 40 durometer peroxide and
platinum cure product

                                          Peroxide     Addisil

Durom. A hardness     DIN 53 505             40           40
Tensile strength      DIN 53 504 S2       10.0 N/      10.0 N/
                                         [mm.sup.2]   [mm.sup.2]
Elongation at break   DIN 53 505 S2         700%        1000%
Tear strength         ASTM D 624 Die B    20 N/mm      40 N/mm
Table 2 - comparison of 60 durometer peroxide and
platinum cure product

                                          Peroxide     Addisil

Durom. A hardness     DIN 53 505             60           60
Tensile strength      DIN 53 504 S2       11.0 N/      11.0 N/
                                         [mm.sup.2]   [mm.sup.2]
Elongation at break   DIN 53 504 S2         450%         600%
Tear strength         ASTM D 624 Die B    20 N/mm      42 N/mm
Table 3 - mechanical data from ECH inhibited HCR grades

                                           SiH/SiVi      SiH/SiVi
Addisil                                   ratio = 1.5   ratio = 3
Durometer A hardness   DIN 53 505             60            63
Tensile strength       DIN 53 504 S2        10.3 N/      10.0 N/
                                          [mm.sup.2]    [mm.sup.2]
Elongation at break    DIN 53 504 S2         630%          520%
Tear strength          ASTM D 624 Die B     41 N/mm      32 N/mm

Regarding the mechanical properties of the tested sheets with different Sill to SiVi ratios, there is the trend that more hydride leads to higher network density, which shows a slightly higher hardness, lower elongation and tear for the ECH-inhibited addition curing system. The effect regarding Addisil is not very significant.

Comparison of figures 2 and 3 shows that the SiH/SiVi ratio has a higher impact at lower temperatures for the Addisil grades than for the ECH-inhibited grades. For the higher temperatures, the cure speed for all SiH/SiVi ratios is almost the same.

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Table 4 - mechanical data from Addisil grades

                                           SiH/SiVi     SiH/SiVi
Addisil                                   ratio = 1.5   ratio = 3
Durometer A hardness   DIN 53 505             60            61
Tensile strength       DIN 53 504 S2        9.5 N/       10.0 N/
                                          [mm.sup.2]    [mm.sup.2]
Elongation at break    DIN 53 504 S2         550%          560%
Tear strength          ASTM D 624 Die B     37 N/mm      32 N/mm

The most common behavior to describe the rate of a chemical reaction affected by temperature is described with the empirical expression observed from Arrhenius:

k = A [multiplied by] exp([-E.sub.a]/R [multiplied by] T)

k is the constant rate to the absolute temperature T (in Kelvin).

However, transferring the measured cure speeds from figures 2 and 3 into an Arrhenius chart, where In k (in our studies In t[60]) is plotted against 1/absolute temperature (1/K), gives more information about the temperature dependence (figure 4). These data can be extrapolated to the reaction rate at room temperature for getting some information about the shelf-life of the one-part rubber.

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The ECH inhibited grades, which are used for the studies, are not the commercially available extrusion grades with the shelf-life of 2-4 days. The ones here have a higher amount of ECH, otherwise the studies are not precise enough because of scorching.

Discussion of results

Extrapolation of In t(60) to 1/K value for room temperature (1/298 = 0.0034) gives a hint for the shelf-life of the product. In general, the extrapolated data result in higher shelf-lives as they are. The problem is that the Arrhenius plot cannot simulate temperature differences, volatiling of ECH or other reactions which occur when a product is stored.

Comparing figures 5 and figure 6, there is a huge difference in the extrapolated shelf-life time for the Addisil grades (more than two years) and the ECH inhibited grades (1-3 months). These data correlate with the data what we obtained when we stored the material. Studies on Addisil grades showed a good extrudability without detecting failures at the article up to six months. The stored ECH inhibited grades showed shelf-lives up to two months.

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We are still evaluating stored samples and comparing them to the extrapolated data. The method needs to be further developed to get closer to the real shelf-life. We will continue our development on the method because it is very helpful to predict as precise as possible the shelf-life of our ready-to-use material.

Conclusions

Shelf-life of the one-part HCR grades with the novel addition curing catalyst-inhibitor system (Addisil) by storing at room temperature can be up to six months. The kinetic plots show that the temperature dependency is steeper than for the common (ECH) inhibited addition curing systems. The new system allows production of one part addition curing HCRs with a high reactivity at processing conditions (temperature 175 [degrees] C and higher) suitable for extrusion applications.

Detection of shelf-lives by using the Arrhenius equation to extrapolate measured cure times correlates with the observed shelf lives.

References

(1.) Hans-Heinrich Moretto, Manfred Schulze and Gebhard Wagner, Ullmann's Encyclopedia of Industrial Chemistry, VCH, 1993, vol. A24, 69-80.

(2.) Larry N. Lewis, Robert E. Colborn, Hans Grade, Garold L. Bryant, Chris A. Sumpter and Robert A. Scott, Organo-metallics, 1995. 14, 2,202-2,213.

(3.) Mel Toub, 156th ACS Rubber Division Meeting, 1999, Paper No. 74A.