Liquid silicone rubber gasketing materials.

Liquid silicone elastomers have been used in injection molding since the early 1970s with the earliest applications being primarily for aerospace connectors, o-rings, mechanical seals and encapsulation of electronic components (ref. 1). During the 70s and 80s the low process costs and the high quality inherent with liquid silicone injection molding have resulted in expanded demand for liquid silicones in many industries (ref. 2). This article describes two new types of liquid silicone rubber gasketing materials, which offer fast molding speed, improved fuel resistance, low compression set and oil resistance, at a low cost.

The name silicone denotes a synthetic polymer ([RnSiO.sub.4-n/2])m, where n = 1 to 3 and m is greater than or equal to 2. R is an organic group consisting of methyl, longer chain alkyl, fluoroalkyl, phenyl and other groups for specific purposes. R can also consist of hydrogen, alkoxy, acyloxy or alkylamino or other functional groups.

Silicones have an array of properties that make them an interesting polymer. The more important of these are listed as follows:

* service temperatures from -60[degrees]C to 250[degrees]C;

* excellent resistance to environmental degradation from oxygen, ozone and sunlight (UV radiation);

* chemical inertness/resistance;

* nontoxic;

* excellent surface release properties;

* can be made optically transparent;

* excellent electrical properties;

* high compressibility.

Silicone rubber gaskets Silicone rubber has been used in gasketing applications for some time now with great success. High temperature resistance, low compression set and oil resistance allow this silicone material to seal in automotive applications even when stack tolerances are high. The function of a gasket is to seal the motor oil in the engine under a wide range of temperatures (-40[degrees]C to 200[degrees]C). M. Toub's paper (ref. 3) "Silicones gaskets versatility and reliability" discussed sealability and explained why silicones were needed to achieve a ten year/10,000 mile leak free engine. In this work the stress relaxation and its logarithmic decay with time was derived for silicone gasketing materials at different temperatures and pressures. It was concluded that silicone rubber is stable when exposed to a wide range of variable conditions and that compression set, high tear resistance and excellent thermal stability are important factors in a sealing material.

Further work was reported in a paper entitled, "Retention of sealing force by elastomeric gaskets" (ref. 4). The sealing force retention of silicone rubber versus organic rubbers was studied using a Lucas compression stress relaxometer. In this work silicone rubber was compared to fluorocarbon, EPDM, polyacrylate and nitrile and the data presented show that silicone maintains a higher sealing force over the largest temperature range, (-30[degrees]C to 150[degrees]C). Both reports concur that silicone rubber is an excellent choice for automotive gasket material.

Silicone heat-cured rubber compositions are one component materials that have a viscosity of 10 x [10.sup.4] to 20 x 104 pascal-second. The polymers in these rubber compounds are called gums and are mixed with filler, process-aids and peroxides in sigma-blade mixers, mills and internal mixers. The heat-cured rubber is usually cured by heating free-radical generators, which are, in most cases, organic peroxides. The cure is dependent on the type of peroxide used and typically widely used peroxides decompose in 1-30 minutes at temperatures of 125[degrees]C to 200[degrees]C. In some cases the silicone rubber compound will require a post-bake to reach complete cure.

Liquid silicone rubber composition Two-component liquid silicone rubber compositions have an impressive range of initial viscosities, from a low viscosity, easily-pourable material of one pascal second to a high viscosity, paste-like material of 10,000 pascal-second at 25[degrees]C.

In most cases the first component is comprised of a vinyl containing liquid polymer of 50 to 10,000 pascal-second at 25[degrees]C, an optional filler and a catalyst. The catalyst is a solubilized platinum complex of chloroplatinic acid or alternatively a peroxide.

The second component is comprised of a vinyl containing liquid polymer, a hydrogen containing silicone fluid and in some cases a filler.

The fillers used in these systems are fumed silica, precipitated silica, clays, glass fibers and metal oxides, just to name a few.

The curing of these two-component systems is via a hydrosilation reaction shown in figure 1.

The hydride functional siloxane serves as a cross-linking agent to the vinyl functional polymer. This cure is achieved without liberating any by-products. The cure will take place at room temperature in a few hours although when heated can cure in a matter of seconds. The platinum catalyzed cure system yields products with the following characteristic properties:

* no reaction by products - low shrinkage;

* deep and thin section cure;

* fast thermally accelerated cure;

* limited number of crosslink site (controlled physical properties);

* low (ppm) catalyst levels (high purity, low toxicity, reversion resistance);

* excellent shelf-life stability (all inputs are stable, nonhydrolyzable);

* variable A/B ratios;

* excellent release properties (nonpolar ethylene bond formed);

* clear high strength products possible;

* inherent flame retardant property.

Liquid injection molding process

The liquid injection molding process combines the processing advantage of thermoplastic-type molding with the product performance of thermoset silicone elastomers. Major benefits of this unique process are:

* High quality of the molded parts. Liquid injection molding is a one step, automatic, totally enclosed process, which eliminates human error, reduces process variances and contamination, and assures precise dimensional tolerances of the finished parts.

* Productivity of the molding operation. Liquid injection molding eliminates the milling, preforming and deflashing operations of compression molding, achieving significant savings in labor, capital, equipment, inventory and floor space.

The molding process is 20 to 40 times faster than compression molding, which translates into large reductions in operating costs and the ability to serve large volume applications from a smaller facility.

The low injection pressure required results in flashless molding and less wear on the equipment and molds.

Liquid injection molding system

Liquid injection molding is a completely automated operation. The liquid silicone rubber is pumped from its pails or drums through meter-mix equipment to properly proportion the components, mixes them in static mixers and delivers the two component mix blend to the molding machine. The molding machine injects or shoots into a preheated multiple cavity mold. The cure is virtually instantaneous and can be ejected from the mold after a total cure cycle of 20 seconds. The parts are ready for final use as soon as they are ejected from the mold. In the case of a formed in place gasket type application, the mixed material can be injected onto a preheated primed part, cured instantly and the gasket will release from the mold but adhere to the primed part.

New liquid silicone rubber gasketing systems

Thus far, this article has attempted to describe generally the advantages of silicone elastomers in gasketing applications, with some emphasis on the benefits of liquid silicone rubber molding. However, the liquid silicone rubber products available on the market today are not suitable for gasketing applications for the following reasons:

* None achieve low compression set as molded. Although a fast cure is possible, low compression set usually requires a post bake.

* Lack of oil resistance.

* Lack of fuel resistance.

For these properties, table 1 shows two new liquid silicone rubber systems that have been developed.

Table 2 compare the physical properties of these systems with a high consistency compression molded silicone elastomer which is designed for oil contact applications.

Monsanto rheometer data comparison

Cure characteristics of the liquid silicone rubber were compared to that of the heat cured rubber gasketing material using a Monsanto moving die rheometer (MDR 2000E) (tel. 5). A five gram sample was placed in a sealed cavity and heated to 177[degrees]C. The torque measurement in this rotorless moving die system (1[degrees] arc) is obtained by introducing a 1.66 Hz movement in the lower die and measuring the resultant torque as a function of time in the upper die. This measurement correlates to the degree of vulcanization of the rubber sample. The t-2 value is the time required for the torque to reach 2% of its maximum torque value and is a good indication of the work life or injection time constraints of rubber material. The t-90 value is the time required to reach 90% of its maximum torque value. This value is a good indication of the cure time required for the small cross-sections seen in gasketing.

Torque (final) is the total or final torque achieved in the curing process. The other values listed are peak rate and time of peak rate. The peak rate describes the changing torque in the reaction and peak time indicates a point at which the torque is changing most rapidly. When all these data are put together, the curing process can better be understood. This fingerprint is also a helpful QC tool to insure the fabricator that continued improvements in production rates will not adversely affect the quality of the gasket. Table 3 shows the speed of cure of the new systems. Using the HCR material as a benchmark, System #1 is about nine times faster and System #2 is about three times faster.

Cure rate vs. compression set

The new liquid silicone rubber systems cure in seconds with no post bake required to achieve ultimate physical properties. System #1 attains 26% compression after only being molded for 20 seconds at 177[degrees]C. The fuel resistant #2 system requires 60 seconds to reach 27% compression set. Data are listed in table 4.

Fuel resistant System #2

In addition to having good oil resistance, liquid silicone rubber System #2 has fuel resistance superior to that of standard silicone rubber compositions at very low cost as compared to fluorosilicone. In table 5, silicone HCR material, System #2 and 100 mole % fluorosilicone were tested in References Fuel C and M25 (75% Reference Fuel C and 25% methanol) per ASTM D471 (i.e. 24 hrs. at 23[degrees]C). These materials were molded under the following conditions:

* HCR was cured 15 min. at 177[degrees]C in a compression mold with no post bake.

* System #2 was injection molded 60 seconds at 177[degrees]C with no post bake.

* Fluorosilicone was compression molded 15 min. at 177[degrees]C with a post bakeof four hours at 200[degrees]C.

Gasoline and gasohol resistance are becoming increasingly important in automotive gasketing because a small amount of these fluids can contaminate motor oil during low temperature start ups. The tests were run per ASTM D 471 specification. This would represent the worst case, that of a total immersion in the reference fluid. The low tensile, elongation and Shore A losses of System #2 in Fuel C and M25 show that this material may be useful in applications involving casual contact with gasoline and gasohol, where previously the more expensive fluorosilicone was the only alternative.

Conclusions

Two new liquid silicone rubber gasketing materials have been developed that have the following features: pumpable; high strength; low compression set with no post bake; rapid molding; oil resistance; moderate fuel resistance at lower cost. These materials are designed specifically to meet gasketing application needs.

References

1. Laghi, A.A. Paper presented ACS Rubber Division Meeting, Detroit, MI, October 8, 1980.

2. Laghi, A.A. Paper presented ACS Rubber Division Meeting Cleveland, OH, October 6, 1987.

3. Toub, M. and Christie, G. Paper presented ACS Rubber Division Meeting Cleveland, OH, October4, 1982.

4. Pala, J., Buch, V, and McDowell, D. SAE paper #870002 "Retention of sealing force by elastomeric gaskets."

5. Monsanto Instruments and Equipment Division, 2689 Wingate Avenue. Akron, OH 44314.