Comparing Liquid and High Consistency Silicone Rubber Elastomers: Which Is Right for You?

Low viscosity makes liquid silicone rubbers suitable for molding applications requiring complex, intricate molds. Photos: Dow Corning Corp.

These elastomers are commercially available in two types: millable high consistency silicone rubber and pumpable liquid silicone rubber. For companies entering the device manufacturing market or expanding current operations, the type of silicone rubber selected will determine the equipment, floor space, and labor requirements necessary to perform the device fabrication process.

ELASTOMER CHEMISTRY AND PROPERTIES

Silicone elastomers are proprietary compositions that contain silicone polymers, reinforcing and extending fillers, and cure ingredients.

Silicone Polymers. The polymers used in silicone elastomers are of the general structure depicted in Figure 1, where R represents -OH, -CH=CH₂, -CH₃, or another alkyl or aryl group, and the degree of polymerization (DP) is the sum of subscripts x and y. For high consistency silicone rubber elastomers, the DP is typically in the range of 5000 to 10,000. Thus, the molecular weight of the polymers--generally called gums--used in the manufacture of high consistency silicone rubber elastomers ranges from 350,000 to 750,000 or greater. In liquid silicone rubber elastomers, the DP of the polymers used typically ranges from 10 to 1000, resulting in molecular weights ranging from 750 to 75,000. The polymer systems used in the formulation of these elastomers can be either a single polymer species or a blend of polymers containing different functionalities or molecular weights. The polymers are selected to impart specific performance attributes to the resultant elastomer products.

Figure 1. Chemical structure of typical silicone elastomers.

Reinforcing Fillers. Many manufacturers use reinforcing fillers to add strength to the finished elastomer product. Typically, these fillers are amorphous fumed silicas, although the use of precipitated silicas has increased in recent years. Particle sizes of standard reinforcing fillers normally fall within the range of 5 to 20 nm in diameter.

The reinforcement that occurs is the result of interaction between the polymer and the filler, whose surface typically has a silanol functionality. Because of the hydrogen bonding initiated by this silanol functionality, the interaction between polymer and filler can actually become so significant that it results in elastomers that are extremely stiff, giving the appearance of a cross-linked system. This phenomenon--commonly referred to in the industry as crepe--can be reversed, because it is possible to break down the hydrogen bonding through the addition of shear energy in the form of mixing or milling.

To achieve a level of polymer and filler interaction that provides a stable product while maintaining the reinforcing nature of the filler in the elastomer, a treatment, or pacification, of the silanol species on the reinforcing filler is necessary. This is typically carried out either through the addition of silanol-endblocked polydimethylsiloxane oligomers or via a capping reaction using reactive silanes or silazanes.

Extending Fillers. In order to impart particular performance attributes to silicone elastomers, extending fillers are sometimes employed. Examples of some common ones in the medical device industry include barium sulfate, used to produce radiopaque products, and titanium dioxide, a pigment that serves as a whitener.

Low viscosity makes liquid silicone rubbers suitable for molding applications requiring complex, intricate molds.

CURE MECHANISMS

Two major cure mechanisms are used in the manufacture of silicone rubber elastomer products: free radical cure and addition cure.

Free Radical Cure. Free radical cure systems employ peroxide catalysts that are either vinyl specific or nonspecific in nature. Adding heat causes the peroxide to decompose into two free radical­containing components, which then react with either an alkyl or a vinyl species along the polymer backbone, transferring the free radical to the silicone polymer. The cross-linking mechanism is terminated by the reaction of the free radical on the polymer chain with an alkyl species on another polymer chain.

There is little difference between liquid and high consistency silicones in terms of physical properties.

Nonspecific peroxides--such as bis(2,4-dichlorobenzoyl) peroxide or benzoyl peroxide--do not require the presence of vinyl or other unsaturated alkyl species in the polymers making up the elastomer formulations. These peroxide catalysts are commonly used in extrusion, but may also be appropriate for certain molding applications. Vinyl specific peroxide catalysts--for example, dicumyl peroxide or 2,5-bis(tert-butyl peroxy)-2,5-dimethyl hexane--require the presence of vinyl or other alkenyl species in the polymers contained in the elastomers. These vinyl specific catalysts do not perform well in extrusion applications, and are thus limited to use in molding.

A serious drawback to the use of peroxide cure in silicone materials is that it leaves residues in the cured elastomer, including acid by-products and polychlorinated biphenyls (PCBs). If the acid residue is not removed, it can manifest itself as a powder that forms on the part surface --a phenomenon commonly referred to as bloom. The application of an oven cure or postcure cycle is generally required to remove these by-products from the cured parts.

Despite this time-consuming postcure processing needed to eliminate residues, silicone elastomers incorporating the free radical peroxide cure are still widely used. Such elastomers are supplied either with the peroxide already formulated into the material, or uncatalyzed, in which case the fabricator adds the peroxide at the time of use. Examples of medical devices and components manufactured from peroxide cured, high consistency silicone rubber elastomers include tubing, pacemaker-lead coverings, and hydrocephalic shunts. No liquid silicone rubber elastomers that use the free radical peroxide cure mechanism are commercially available. Table I lists typical properties obtained in high consistency silicone rubber elastomers cured via peroxide.

Addition Cure. The addition cure mechanism--known as hydrosilylation--involves the addition of a silicon hydride (*SiH) to an unsaturated carbon-carbon bond in the presence of a noble metal catalyst. The most commonly used of these hydrosilylation catalysts are based on platinum, although palladium and rhodium catalysts are also available. In order for the cure to occur, the silicone polymers contained in the elastomers must include a vinyl or other alkenyl functionality. Both high consistency and liquid silicone rubber products can employ the addition cure mechanism. Elastomers featuring this type of cure system are supplied as two-part kits: one part contains the catalyst species, the other a silicon hydride­functional cross-linker and an inhibitor to provide working time once the two parts have been mixed.

The major advantage of addition cure for elastomers is that the cure reaction produces no by-products. Therefore, postcuring of the elastomer is normally not necessary, although a postcure cycle is sometimes performed to stabilize or enhance the properties of the finished product. Addition cure is inhibited by contact with materials containing amines, sulfur, phosphorous, tin complexes, peroxides, and peroxide by-products, and care must be taken to avoid contamination by any of these materials.

Table I. Typical properties obtained in high consistency silicone rubber elastomers cured via peroxide.

Table II. Typical properties obtained in high consistency silicone rubber elastomers with addition cure.

Table III. Typical properties obtained in liquid silicone rubber elastomers with addition cure.

Medical devices or components made from addition cured, high consistency silicone rubber elastomers include tubing, pump diaphragms, and catheters. Among the devices made from addition cured liquid silicone rubber elastomers are pump diaphragms, external male catheters, and wound-drainage bulbs. Typical properties obtained with an addition cure system for high consistency and liquid silicone rubber elastomers are listed in Tables II and III, respectively.

MATERIAL APPLICATIONS AND PROCESSING

As the data in Tables I, II, and III indicate, there is little difference between high consistency and liquid silicone rubber elastomers in terms of physical properties, regardless of the cure chemistry used. However, because of the disparity in the material types, the processing of these materials and their fabrication into medical devices vary significantly (see Figure 2).

Figure 2. Comparison of processing steps in molding of high consistency silicones (top), extrusion of high consistency silicones (middle), and molding or extrusion of liquid silicones (bottom).

High consistency silicone rubber elastomers are ideal for use in extrusion because of the high viscosity polymers used in their formulation. The resulting products show very good green strength, which is the ability of the material to retain its extruded profile in the uncured state.

Liquid silicone rubbers, on the other hand, do not perform well in most standard extrusion applications because their viscosity is so low that they flow under little, if any, shear stress. Their utility in such applications is therefore limited to supported extrusion--that is, extrusion onto another substrate. Sleeving and membrane films are two examples of supported extrusion applications.

Both high consistency and liquid silicone rubbers are used extensively in the molding of elastomeric devices and device components. High consistency elastomers are typically molded using transfer or compression molding techniques, both of which are labor-intensive. Liquid silicone rubbers are molded in highly automated injection molding systems. Given their significantly lower viscosity, liquid silicone rubber elastomers find utility in molding applications that require highly complex and intricate molds. Their ability to be molded in automated injection molding systems also lends itself to production runs involving large numbers of molded parts.

Processing High Consistency Silicone Rubbers. The processing of high consistency silicone rubber elastomers involves five steps: mill softening and catalyzation, preparation of a preform, extrusion or molding, vulcanization, and finishing.

Mill Softening and Catalyzation. The initial step in the processing of high consistency silicone rubber elastomers for use in either extrusion or molding applications is mill softening and catalyzation. This procedure reverses any "crepe hardening" that has occurred in the elastomer during storage in inventory at the supplier or fabricator. The milling is also used by fabricators to add peroxide catalyst to a free radical cured elastomer, if necessary, or to blend the two parts of an addition cured elastomer prior to fabrication.

Milling and catalyzation are not only labor-intensive, but also require the use of a two-roll mill, which costs approximately $60,000 to purchase and install. The elastomer is passed through the mill numerous times until the compounded material is homogeneous, then removed in sheet form for further processing.

Preparation of a Preform. Once a high consistency elastomer is fully compounded, the next step in the fabrication process is the preparation of a preform. This requires no special equipment but is very labor-intensive, particularly in high volume applications that are almost continuous in nature. For extrusion and injection molding applications, the preform is prepared simply by cutting the elastomer sheet into strips, which are used to feed the extruder. For transfer molding operations, the preform is typically cut with a die-cutter into a plug that will fit in the transfer reservoir of the transfer press. For compression molding applications, the process is slightly more complicated, since the preform must be cut in the basic geometric configuration of the final part.

Extrusion. For high consistency elastomers, extrusion is accomplished using a single-screw extruder. The preformed strips are fed to the extruder from a roller feed wheel into the extruder barrel, and the elastomer is extruded through a die and mandrel assembly to form the desired profile. It is also possible to carry out supported extrusion with high consistency silicone elastomer. This is achieved by fitting a crosshead assembly onto the extruder, passing the supporting geometry through the crosshead, and extruding a layer of silicone rubber over it.

The cost of an extruder appropriate for medical applications is approximately $100,000, including installation. However, most extrusion system fabricators equip their machines with laser micrometers and feedback controllers to monitor product quality, an enhancement that adds significantly to the cost of setting up an extrusion process. In addition, extrusion can be considered labor-intensive, since it requires the presence of an operator at all times to ensure that the extruder has a constant supply of elastomer and is operating properly.

Molding. High consistency silicone applications vary with the type of molding equipment used. Transfer and injection molding processes require operational personnel to load the elastomer into the equipment and to demold the finished parts. For compression molding, operators must place preforms in all the individual cavities in each mold. Because of the relatively slow cure cycles employed with high consistency elastomers, it is possible for the molds to have large numbers of cavities. The cost of installing a transfer or injection molding press is approximately $1000 to $2000 per ton of clamp force. Compression molding presses are considerably less expensive.

Vulcanization. Vulcanization of the extruded product is typically achieved with hot-air vulcanizing ovens, or HAVs. These HAVs are available in both horizontal and vertical configurations. In the horizontal models, the extruded profile is laid on a continuous belt and passed through the oven, where hot air is forced over the extrudate to initiate the vulcanization mechanism. In the vertical oven configuration, the HAV is equipped with a variable-speed drum at the top to pull the extruded profile upward through the oven, where it is cured. The cost of a typical HAV oven is approximately $30,000. Other types of vulcanization ovens employed include radiant-heat ovens and steam autoclave ovens.

Finishing. The degree of finishing required depends on the specific application. For extrusion processes, finishing involves visual inspection and cutting of the tubing into specified lengths. If the tubing has been cured with a peroxide, the finishing process also entails an oven postcure to remove peroxide by-products. For molding applications, finishing includes the trimming or deflashing of the molded parts, often using die-cutting machinery to cut the individual parts from a larger molded sheet, and any oven postcuring that is necessary. In addition to wasting material, finishing of high consistency silicones is labor-intensive and requires additional equipment such as ovens and die-cutters. However, this equipment is relatively inexpensive to purchase.

Processing Liquid Silicone Rubbers. In contrast to high consistency materials, the processing of liquid silicone rubber elastomers requires only three steps: meter-mixing, molding, and finishing. The major advantage of the liquid silicone rubber system is that it is designed to be used in highly automated, closed systems, with very little labor required once the system has been put into operation.

Meter-Mixing. The initial step, meter-mixing, is performed using pneumatic pail or drum pumps. These pumps deliver the two parts of the liquid silicone rubber--at a 1:1 ratio--to a multielement static mixer, where the two parts are airlessly mixed until they are homogeneous. Meter-mix systems have improved tremendously over the years, and also allow for the controlled incorporation of other additives, such as pigments. The cost of a meter-mix system is approximately $15,000 to $25,000.

Molding. The molding of liquid silicone rubbers is accomplished using modified plastic injection molding machines. These machines are highly automated and, once operational, require almost no labor to operate. Such equipment costs approximately $1000 to $2000 per ton of clamp force required. The greatest expense in the molding of liquid silicone rubber elastomers is the cost associated with the design and production of the mold itself. Depending on complexity, a mold can cost from a few thousand to several hundred thousand dollars. The recent introduction by several suppliers of all-electric molding machines bodes well for the future of liquid silicone rubber injection molding in the medical device industry, since these units can be used in cleanroom environments, from which typical hydraulic machines are precluded because of potential contamination by hydraulic fluids.

Finishing. For typical applications, finishing operations are not necessary in liquid silicone rubber injection molding systems. The molds, if properly tooled, produce minimal flash, eliminating the need for trimming. Likewise, because these materials use the addition cure mechanism, a postcure cycle is not required, although it may be performed to stabilize or enhance the properties of the cured parts.

CONCLUSION

The selection of the right type of silicone rubber elastomer for a specific use is largely a matter of personal preference and availability of equipment. There is little observable difference between peroxide cured high consistency silicone rubbers, addition cured high consistency silicone rubbers, and liquid silicone rubbers in terms of physical property performance. However, the materials differ significantly in terms of the processing necessary to fabricate medical devices and components.

For extrusion applications, high consistency silicones are the material of choice. Liquid silicone rubbers do not exhibit the green strength necessary to maintain extruded profiles until they can be vulcanized. Either peroxide or addition cure systems can be used, although peroxide systems do require an additional postcure step.

For molding applications, either high consistency or liquid silicones are acceptable. For facilities already processing high consistency elastomers, continuing with the same type of material may be the most efficient and cost-effective course of action. However, new operations entering the marketplace should give serious consideration to using liquid silicone rubber, because the capital costs and labor involved are significantly lower than those associated with the processing of high consistency material. Whatever the choice, an extensive knowledge base exists to provide technical assistance, and material and equipment suppliers alike are available to share their expertise and help ensure manufacturing success.

BIBLIOGRAPHY

Clarson SJ, and Semlyen JA, Siloxane Polymers, Englewood Cliffs, NJ, PTR Prentice Hall, 1993.

Morton M (ed), Rubber Technology, 3rd ed, New York, Van Nostrand Reinhold, 1987.

Byron E. Wolf is a development specialist with Dow Corning Corp. (Midland, MI), where his responsibilities include leading the R&D program for liquid silicone rubber elastomers in the company's Healthcare Industries Science and Technology group. He is also an adjunct faculty member in chemistry and chemical engineering at the South Dakota School of Mines and Technology.

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