Silicone elastomers.

The use of silicone elastomers has become prevalent in virtually every industry because of their wide range of unique and valuable properties. Since the physical and mechanical properties of a material limit the applications and environments in which it can be used, the versatility and reliability of silicones make them ideal candidates for numerous designs in demanding conditions.

Silicones are manufactured in several physical forms, the three primary types being fluids, resins and elastomers. They are also formulated as emulsions, greases and gels. Further, these polymers may be reactive (triggered by moisture, heat or radiation) or non-reactive.

Although silicones appear in different physical forms, they nevertheless share many common properties. Among these are superior thermal and oxidative stability, a high degree of chemical and biological inertness, excellent weather-ability and good electrical insulation properties.

It is because of silicone's unique chemistry that these materials display a striking performance and property profile, for the inherent versatility of this chemistry allows molecular structures or formulation ingredients to be changed easily, resulting in products with properties which are ofte contradictory. For example, some silicone compositions act as adhesives, while others are release agents. Silicone fluids can function as defoamers, or they can be used to create new foam materials. They can be soft, spongy elastomers or rigid, glasslike coatings. Additionally, silicones can either conduct or insulate electricity.

The chemical structure of silicone polymers is different from that of organic polymers and accounts for silicones' superior stability and inertness in demanding environments. While organic polymers have a molecular backbone of carbon atoms, silicone polymers have a backbone of alternating silicon and oxygen atoms.

The silicon-oxygen bond - found in other heat-resistant materials such as sand, ceramic and glass - is the primary reason for silicones' high-temperature resistance. Rather than a backbone of carbon atoms, which can undergo oxidation and resultantchain scission, the siloxane chain is oxidatively stable because it is already in its highest oxidation state. In addition, the siloxane linkage is immune to attack by ozone (because there is no carbon-carbon unsaturation) and UV light (because it is transparent to UV).

Attachedto the silicon atoms in the silicon-oxygen backbone are organic side groups, which modify properties and provide a reaction site for other molecules. Varition of these side groups can give the silicone polymer different characteristics, such as flexibility, and radiation, thermal and fuel resistance.

Silicone elastomers

Among the three major families of silicones - fluids, resins and elastomers - elastomers, or rubbers, are particularly important to the design engineer because they fulfill requirements not satisfied by organic elastomers.

Silicone elastomers are based on high-molecular-weight linear polymers. Through the curing process, silicone polymers of appropriate molecular weights are cross-linked to increase polymer molecular weight and to provide elastomeric properties. Catalysts control the rate of the curing process. When linked, the long-chain compounds create a flexible network that is elastomeric in character but physically weak compared to organic polymers. Reinforcing fillers are added to increase strength. To obtain other desired properties, additives such as antioxidants, adhesion promoters and pigments can also be added.

There are three types of solid silicone elastomers: room-temperature-vulcanizing (RTV) products; liquid-injection-molded products; and heat-cured-rubber products (listed in order of increasing tensile strength). Additionally, foamed silicone elastomers can be obtained from the heat-cured and room-temperature-vulcanizing processes.

RTV elastomeric adhesives and sealants

Room-temperature-vulcanizing silicones are the most flexible of the three families of elastomers. Based on intermediate-weight silicone polymers, they are typically used as adhesives, sealants and encapsulants. The cured polymer has a moderate strength compared to structural adhesives such as epoxies and methacrylates, but offers the advantage of a softer, more flexible bond.

RTV silicones are vailable in two forms: one-componet adhesive/sealants and two-component products. Most RTVs offer primerless adhesion to a variety of substrates, such as metal, glass, ceramic, some plastics and concrete, serving both a sealing and bonding purpose. They can also be used for molding and encapsulating applications.

Processing

The uncured RTV product is supplied as a liquod or paste-like mixture of polymers, fillers, additives, curing agents and catalysts. With one-component products, the mixture is packaged to protect it from moisture, since exposure to atmospheric water vapor activates the reaction that cures the polymer. Cure occurs at room temperature (hence the name), though the cure time is both moisture and temperature dependent. Generally, the higher the humidity and/or temperature, the faster the cure. The time required for full cure also depends on the mixture's composition (wheter it is an acetoxy, alkoxy or amine-cure type chemistry, for example) and the thickness of the silicone layer. For example, a 1/8-inch section of acetoxy-cure RTV adhesive/sealant will typicall cure through in about 24 hours at 77[degrees]F and 50% RH.

Two-component RTVs do not require moisture to trigger their cure. Instead, the cure is initiated upon mixing of the two components, one of which consists of or contains the catalyst. With some products, the addition of heat can accelerate cure. The product's two components, which vary in viscosity from easily pourable to stiff paste-like materials, are supplied in separate packaging. They can be used in place of one-component products when longer work times, thicker section cures or faster cure times are required. Some two-component RTVs can cure in as little as 10 minutes.

Applications

The chemical structure of RTVs gives these polymers excellent resistance to temperature extremes. In the aviation industry, for example, their high-temperature stability enables them to be used as gaskets and seals on aircraft doors and windows. RTVs are well suited as automotive transmission and engine seals, where the presence of hydraulic or lubricating fluids and high-operating temperatures causes most organic elastomers to fail.

Their thermal insulation properties allow them to adhere to and separate two substrates that must be kept at significantly different temperatures. An RTV silicone, for instance, acts as an adhesive and a sealant between the inner and outer shells of a refrigerator.

As electrical insulators, RTV silicones can joint two metal components and prevent galvanic corrosion between them. In electrical and electronic assemblies, RTVs can conveniently coat and join irregular parts or junctions; in many cases, a satisfactory bond can form without need of a primer.

The flexural and elongation properties of cured RTV silicones adhesives, sealants and gels allow them to bond to flexible substrates or those with significantly different thermal-expansion characteristics. The silicone can move with the substrates while still maintaining the bond. Also, good gap-filling properties make them useful in linking substrates of different shapes or textures.

Silicone RTVs are also biologically, chemically and environmentally inert. In medical applications, they are used to make prostheses and catheters, to encase pacemakers and to create dental molds.

In construction applications, their resistance to chemical and environmental attack is extremely important. The weatherability and solar, ozone and water resistance of silicones gives them a service life much superior to that of organic rubbers over a broader temperature range. These sealants do not require primers in order to adhere well to many standard panel materials, including glass, ceramic-coated metals and some types of aluminum and stainless steel.

Liquid-injection-molded elastomers

Liquid-injection-molded silicones intermediate in strength between RTVs and heat-cured silicone rubber and are molded in a process similar to thermoplastic injection molding. This process combines the advantages of thermoplastic molding-speed, accuracy and design flexibility - with the unique physical properties of thermoset-silicone rubbers. Consequently, liquid-injection-molded silicones are replacing products made with heat-cured silicone rubber and/or traditional organic elastomers processed by compression or injection molding.

Processing

Liquid-injection-molded silicones are based on two-component RTV compositions that are modified to become heat-curing systems. Inhibitors are added to the RTV mixture that only release the catalyst at elevated temperatures. This inhibited product yields a mixture with long pot life (72-96 hours) for easier handling and mixing, and also allows for a rapid cure.

The molding process adapts the basic principles of injection molding to the processing of thermoset elastomers. Like compression molding, heat and pressure are used. But instead of requiring several steps, it is a closed, integral, one-step process. The freshly mixed liquid silicone is pumped from its container directly into the molding equipment, where it is injected into hot, multiple-mold cavities. Cure is virtually instantenous at typical molding temperatures (150-200[degrees]C/302-392[degrees]F). Parts are ready for use as soon as they are ejected from the mold, since they normally require no post-mold curing. Because of the relatively low molding pressures, (around 1,500 psi), and rapid cure rates, no flash or excess rubber needs to be trimmed from the finished part, speeding and simplifying production.

Applications

Because of silicone's superior temperature stability, liquid-injection-molded products have replaced such materials as flexible urethanes, flexible PVC and neoprene in several industries. For example, liquid-injection-molded products are being used as engine block seals and spark plug boots in automotive applications. Also, because of their ease of processing and cost effectiveness, they replace silicone heat cured rubber as connector seals, temperature probes and tubing in appliance and electrical applications.

In te medical/health care area, high purity and product consistency are extremely important. Since the molding process takes place in a closed system, it ensures minimal product contamination. Also, silicones are biologically and environmentally inert, are non-toxic and are very durable in demanding environments.

Liquid-injection-molded products are also commonly used in construction applications, where their weatherability makes them ideal materials for a variety of products.

Heat-cured rubber compounds

Heat-cured rubber (HCR) products are the oldest and most widely used form of silicone elastomers. They offer the highest levels of strength and physical properties of any silicone elastomer and can be extruded, molded or coated on fabric. Extruded and molded parts are used as oven gaskets and wire jacketing on flat-iron cords. HCRs are also used to make catheter tubing, shunts and flexing joints, and coated electrical tapes.

Processing

Most HCRs, unlike RTVs, are based on high-molecular-weight-polymer gums. The consistency of the uncured-rubber mixtures range from a tough putty to a hard, deformable plastic. Before being used, the silicone-rubber mixture (gum, fillers and additives) is catalyzed and freshened; that is, the catalyst is added and the fresh mixture is run through a water-cooled rubber mill which transforms it into smooth continuous sheets. The sheets, which are easily worked, are now processed directly.

Freshly mixed silicone rubber compounds are usually molded at 100-180[degrees]C (212-356[degrees]F) and 800-1,500 psi, conditions which lead to complete thermal cure in a few minutes. In the manufacture of insulated wire, rods, tubing and other similar products, the silicone mixture is extruded through standard rubber-extrusion equipment.

To coat glass cloth or other fabrics, HCRs dispersed in aromatic solvents are used. The cloth is dip-coated into the dispersion and then dried and cured in heated towers. The cloth can also be calendered with a soft silicone stock and cured without being disperesed in solvent. This process results in a flexible product of sheet or tapes, which are used to wrap parts of heavy electrical machinery, large cables and connectors.

Applications

One of the heaviest usages of HCRs is in the electrical industry, where they provide insulation for wire and cable. For power cables, particularly the underground variety, HCRs dissipate heat and increase current-carrying capacity. They are also used to make silicone rubber tapes for insulation of large cables, irregular shaped conductors, and splices and terminals. Additionally, HCRs are used in the automtive industry where their excellent chemical resistance to hot ethylene glycol, diesel fuel and engine-cleaning solvents, as well as their electrical and thermal performance and flex-life, enables them to outperform organic rubber. In the aviation industry, HCRs are valued for their resistance to pressure and temperature extremes.

Foamed silicone elastomers

Foamed silicone elastomers may be based on HCR or RTV formulations. The earliest foamed product, silicone sponge, is manufactured from heat-cured rubber. Silicone sponge materials can be cast-in-place to make molded parts or laminated onto foils or fabrics. These materials have the highest mechanical properties of any foamed silicone product; however, they are more complicated to process, and are relatively heavy and expensive compared to the newer silicone foams.

Newer RTV-based silicone foams have been frequently used for thermal or electrical insulation, and also for light-weight molded parts. They can be continuously cas sheets, or used as a freeblown potting compound. Offering greater density reduction and design and manufacturing versatility, they have replaced silicone sponge in applications where high performance and weight are critical and where mechanical strength requirements are lower.

The newest generation of RTV silicone foams combines the best of previous foamed silicone technology. With mechanical strength close to that of silicone rubber sponge, the design and processing versatility of RTV foams, and excellent thermal and inherent flame resistance, these products are beginning to displace flexible organic foams such as polyurethane. Their properties are particularly important in light of increasingly stringent government standards on the flammability, smoke and toxicity of materials used in public buildings and transportation applications.

Processing

Processing of silicone rubber sponge is accomplished by first thoroughly mixing a chemical blowing agent with unvulcanized silicone rubber on the mill as a masterbatch, after the rubber has been softened. Next, the mixture is heated, which decomposes the blowing agent (causing it to form bubbles) at the same time it vulcanizes the rubber. As the rubber begins to cure around the rising bubbles, a cell structure is formed. The sponge may be formed by free-blowing (as extrusions or calendered sheets) or press blowing (for thicker parts or for parts which must maintain tighter dimensional tolerances). Blowing agents need to be selected for the grade of silicone rubber used and to best suit the particular processing method chosen. Some grades of silicone rubber are said to give better results than others. The density of the resultant sponge product will vary with molding time and temperature, the amount and type of blowing agent used, and the degree of confinement applied during the blowing and cross-linking phase.

RTV silicone foams are based on a two-component liquid system which is capable of curing at room temperature. The uncured product is supplied as a curing agent and a base compound which start to cross-link when mixed. The foaming of silicone foams occurs without any external agents. Instead, hydrogen gas is liberated as a by-product of the cure reaction, serving to foam the material. Complete foaming can occur in as little as two minutes at elevated temperatures. Processing of the foam mixture is straightforward; it can be sprayed, cast, coextruded or molded. The molding process can occur in thermoplastic injection-molding machines, with a cool barrel and a heated mold for cure. The foam can also be processed using compression molding, since it will cure rapidly at room temperature. The spray system uses standard equipment and is solvent-borne.

Using standard lamination techniques, these foams can also make excellent backings for high-performance composite fabrics. The just-catalyzed material continuously passes between rolls of fabric and release paper and then through an oven. The foams form a good, primerless adhesion to most fabrics, such as wool/nylon blends, as well as to fiberglass and foils.

Applications

In building and construction applications, silicone foams are currently being used as fire/smoke barriers, especially around wires and cables. Also, their low mechanical properties are advantageous in such applications because the foams are easily repairable. Their light weight and thermal and flame resistance make these materials excellent choices for fabric backings. Here, they can be used in areas where heat and moisture transfer are a concern and where organic foams cannot meet performance requirements.

In the automotive market, silicone foam-coated fabrics are being considered for their thermal and acoustic shielding properties. A composite of aluminum foil with silicone foam can be used to shield plastic gas tanks that are located close to hot exhaust systems. This composite can also minimize possible vibration-associated noise in body pillars. Additionally, silicone-foam padding, along with silicone-composite fabrics, can reduce the amount of heat radiated upward from catalytic converters into the passenger compartment.

Superior flame-retardant properties make silicone foams ideal materials where safety is a strong consideration, such as in buildings and public transportation. They can be used as a flame-retardant carpet backing and to increase insulation and flame-retardant properties of draperies.

Conclusions

Silicone elastomers - whether solid or foamed and whether RTV adhesive/sealants, liquid-injection-molded elastomers or heat-cured rubber products - offer designers a number of performance, design and manufacturing benefits that organic competitors cannot. Elastomeric silicones provide exceptional thermal, chemical and oxidative stability in most environments. In fact, their reliability and long-term property retention, even in hostile environments, enables them to be used in applications where organic polymers would not even be considered. Now, improvements in chemistry and processing enable these versatile materials to penetrate even more diverse applications in virtually every industry.

Dr. R. Bruce Frye is Manager, Polymer Hydrosilation Systems for GE Silicones in Waterford, NY