Silicone elastomers are extremely versatile materials that offer a range of attractive performance characteristics, often outperforming their TPE cousins. They work well in extreme cold and high heat temperatures, are chemical resistant, and can be customized with a variety of unique properties. Combine this with tactile characteristics that range from soft to tacky to hard, and designers are given a plethora of options. Various grades of purified silicone materials can also be used in regulated applications such as foodservice appliances and medical devices.

So, if silicones are so great, why isn’t everyone using them? A simple answer is cost. The cost of silicone elastomers can be several times more expensive than conventional TPEs. The material, with the consistency of taffy, has been difficult to work with, especially as compared to fabricating with other TPEs.

A medical device that has been overmolded with ELASTOSIL® silicone rubber from Wacker, which gives the tool a soft touch and improved ergonomics.

When considering whether to use silicone elastomer, designers have always faced a cost versus performance tradeoff. Does that gasket or seal, pushbutton or knob, need to be made from silicone elastomer?

But, things are changing. Cost is still an issue – some silicone elastomers are still four times the cost of less expensive TPEs – but new production techniques are being introduced that are helping to reduce cost. And, at the same time, a new range of silicone elastomers are hitting the market that are less costly and have even better performance characteristics than their predecessors.

The future, as they say, looks bright for silicone elastomers. Literally.

One of the newest applications for silicone elastomers is in the arena of LEDs. Mike DiPino, Senior Specialist in Silicone Elastomers, Dow Corning, Midland, Mich., cited LED covers as one new application for silicone elastomers. That thought was echoed by David Wolgemuth, a multi-shot liquid silicone rubber engineer with Phillips Plastics Corp. of Prescott, Wis., who pointed to LEDs as a new application area. “The silicone has great heat resistance and also light pipes really well, and so it is good in the use of LEDs. It can typically be mounted right onto a circuit board,” he says.

Colored silicone from Dow Corning that can be made to enhance aesthetics in appliance applications.

Phillips Plastics is one of a handful of molding companies around the country who are using silicone elastomer in a multi-shot production environment. The company molds thermoplastic materials in one process, and then in the second shot, molds that part with a silicone elastomer to get the best performance characteristic for a particular application. For instance, a gasket may need to be soft for sealing, but may have to endure mechanical stresses that would require a particular hardness level. With the multishot process, a gasket could be made of a hard thermoplastic material, and then have a soft silicone elastomer overmolded over the core gasket, providing something that is stiff on the inside, soft on the outside.

To date, Phillips has used silicone elastomers in conjunction with high heat resistant grades of PC, PBT and PA. The thermoplastics that can be used in a multishot process with the silicone elastomer must have heat resistance of at least 300 DegF to 350 DegF.

Wolgemuth says that using the material in a multi-shot environment cuts down on part costs and assembly issues. Multi-shot parts reduce assembly operations, as one multi-material unit eliminates the need for fastening separate parts made of different materials. It can also provide a stronger bond and better seal between the two materials. For parts that are difficult to produce in a one-shot process such as an complex gasket or diaphragm or a “fish hooked shaped” part, the two-shot method can often provide a solution.

The NORMAQUICK® V2 quick connector for fuel lines was  developed by Norma-Group using two-component molding and Momentive’s new FFSL.  The molded-in silicone  eliminates the need for O-rings or seals. Photo: Momentive Performance Materials

In addition, the silicone material allows for “violations” of accepted molding rules. Variations of thick to thin wall sections are not an issue as silicone cures and expands to fill, as opposed to thermoplastics that will shrink and cause sink marks. Radii, drafts and undercut designs can have a greater amount of flexibility, typically not requiring a separate movement in the tool design. Blemishes caused by gating are also eliminated, he says. “With a thermoplastic you usually have to use a gate large enough that it would leave blemishes that could be visible,” he says. “With silicone, we can make the gate so extremely tiny that it usually cannot be seen with a naked eye.”

The liquid silicone rubber is supplied in either a 5-gallon pail kit or 55-gallon drum kit. (The kit refers to having two matched containers, an A component and a B component). “The consistency of material varies based on its durometer from a fairly running toothpaste to pretty stiff (texture).”

The difference between liquid silicone rubber and traditional thermoplastics is that liquid silicone rubber requires specialized pumping system to supply to injection barrel. It can be messy if not properly handled, Wolgemuth says. “It will stick to everything and because of its great chemical resistance, there are not a lot of things that will clean it or break it down.” he says. “On the whole, it does not significantly complicate the operation; it is just an additional set of variables that have to be taken into account.”

LED covers developed by Dow Corning represent one of the growing applications for silicone elastomers.

Fabrication techniques such as Phillips’ multi-shot capability have expanded silicone elastomers use, but that is not the only reason for its growth. Wolgemuth points to a more standardized product that is on the market today. “The suppliers have really cleaned up the materials over the last 10 years,” he says.

The new materials in use at Phillips and other molders cure faster, too. Wolgemuth says that the material undergoes cross-linking during the curing process, creating the elastomer matrix that gives the material its properties. There are four main components of the provided base material, the SiO polymer, a cross linker, temperature sensitive inhibitor, and a catalyst (usually platinum for newer materials). The system is activated the moment the two parts, A and B, are mixed. However, for some fast cures, heat must be applied. “Thermoplastic is just a matter of going back and forth between liquid and crystalline or solid state, whereas the liquid silicon rubber is actually changing or finishing its chemical structure,” Wolgemuth says. “Once the matrix has been formed for liquid silicon rubber, it can not be remelted or reprocessed.”

Silicone elastomers such as Elastosil® from Wacker Chemical Corp., Adrian, Mich., is one example of a liquid silicone rubber that has been designed to cure faster than ever before. Ed Laperriere, sales manager for Wacker Chemical Corp., points to a very tiny part such as a single-wire seal that might weigh less than a gram. Every year, billions of them are made. At that volume, and their profit margin, speed of cure is very important. “These products in an 128-cavity mold would cycle in 15 seconds or less, clamp to clamp, he says.

Gaskets produced by Phillips Plastics using Wacker Chemical Corp. silicone elastomer materials. Photo: Phillips Plastics

Some grades of Elastosil are self-lubricating, or oil-bleeding, types of liquid silicone rubber. The bleeding can aid in assembly operations. In addition, the lubricity of Elastosil and other elastomers also helps with mold release, which can reduce blemishes and speed mold times by filling the gap that might exist between the wire and silicone.

The materials can be used in very utilitarian ways as well as in aesthetic and tactile ways. Today’s new materials have a large range of hardness levels. In terms of hardness, the hardness levels vary by product and application, but the hardness range could go from 10 to 70 shoreA. Momentive Performance Materials, Wilton, Conn., recently contracted with Willat Design Works, a company that designs with silicone, to produce Elexium, a reformable “ergo-morphic” product that conforms to the pressure of the human touch. Momentive will provide the silicone elastomer and silicone gel materials that will be used by Willat. The silicone elastomer will be overmolded onto a silicone gel interior. This creates a custom fit, as well as form memory that will allow the finished good to return to its original shape. The Los Angeles-based Willat will create products for the medical and dental industries.

In those industries, the materials often need to be high heat resistant so that the devices can be sterilized in an autoclave, which makes silicone elastomers a perfect choice, says Sharon Shatto, Americas marketing director for health care for the elastomers division for Momentive.

Silicone rubber from Dow Corning that can be modified to get the required properties for the application.

In terms of heat resistance, silicone elastomers work well at both the extreme low and high end of the temperature range. While, typically thought of in terms of high heat resistance, silicone elastomers can also be used at very low temperatures such as those that might be applicable to aerospace applications. Some materials have a range from –50 DegF to up to 400 DegF.  Momentive has tested some of its silicone elastomers out to 1,000 cycles in an autoclave with good results, Shatto says.

Momentive recently introduced a new fully fluorinated silicone liquid elastomer that is offered in a hardness range of  40 to 70 shoreA. The 40-shoreA oil-bleeding FFSL could be used for connectors, the 50 to 60 shoreA can be used for membranes, seals and gaskets and 70 shoreA can be used for O-rings.

The fluorine atom makes the silicone elastomer even more resistant to harsh chemicals. The previous generation of fluorinated LSR, known as FSLs, withstood exposure to diesel/RME and hot oils, and the new FFSL has the same capabilities. With increased levels of fluorine, additional  chemical resistance is achieved that is comparable to those offered by commercial peroxide curable fluorosilicone heat-cured rubber (FVMQ). According to Tim Angle, Momentive’s global marketing manager for consumer goods, appliances, the fluorine atom gives the material the extra chemical resistance.

Tubing made from NuSil’s silicone elastomer.

This resistance is measured as a rate of swell, and the rate of improvement has been dramatic. In the past, some silicone elastomers exposed to toluene would swell to twice their size. Now, that swelling rate is about 35 percent, says Angle.

Another strength of the material is its customizability, says Steve Bruner, marketing director with NuSil Technology, Carpinteria, Calif. As mentioned above, silicone chemistry can be modified to impart chemical resistance or enhance thermal performance. Silicones can also incorporate large volumes of fillers such as colors, electrically and thermally conductive additives, without changes to the elastomeric properties. The material can come in an array of material forms:  fluids, adhesives, gels and elastomers for lubrication, bonding, dampening, and the formation of molded and extruded parts. Color can be added to the translucent material with little changes to the elastomeric properties, Bruner says. The dielectric materials can be made electrically conductive or can be made to be EMI resistant. “The silicone materials are versatile and can be used in damping applications, bonding, optically clear systems, tubing. Pretty much anything can be made with it and can be made tough enough for just about any environment,” Bruner says.

NuSil also has a new product out that is geared for medical devices requiring color. The MED-4900-X series of color masterbatches has been formulated to use with liquid silicone rubber products, It can pumped in via a third line in injection molding processes for medical devices.

A range of products produced at Phillips Plastics using liquid silicone elastomers with a two-shot method.

Versatility is also a key benefit of silicone products from Multibase, a subsidiary of Dow Corning. The company offers TPSiV™, a ThermoPlastic Silicone Vulcanizate. The materials are a family of TPV products based on cross-linked silicone rubber dispersed into various engineering thermoplastics. The dispersion of the silicone internal phase is produced by dynamic “vulcanization” or cross-linking of silicone polymers within the thermoplastic organic phase. TPSiV combines the elastomeric performance of silicone rubber with the melt re-processability and thermal and chemical resistance of engineering thermoplastics and can be processed on standard thermoplastic equipment through a variety of methods, including extrusion, injection molding, co-molding, over-molding and co-extrusion.  In addition, selective grades can be bonded to PC, ABS, PU, PVC and PA substrates without adhesives or primers, says Gifford Shearer, Innovation & Portfolio manager for Multibase. Various grades are offered with a range of mechanical properties and degrees of tactile response.

The use of silicone elastomers is traditionally thought of as materials for use in specific high-end applications, but, that is changing. For instance, DiPino points to an espresso machine application in Europe. “The strong espresso is pretty corrosive,” he said, “It wants to attack the material, so we had to modify and improve the elastomer’s oil resistance and make it so it has high temperature resistance. Those units operate at 80 to 90 DegC (176 DegF to 194 DegF).

Dow Corning’s clear, uncured liquid silicone rubber being poured from a container.

The versatility of silicone elastomers has extended their use to a growing number of applications. The medical device and the espresso machine are just two examples of uses for this versatile material. As new materials and new fabrication methods continue to be developed, more design engineers are expected to calculate their cost versus performance equation in favor of silicones.

For more information, email:
Dow Corning:  chip.reeves@dowcorning.com
NuSil: SteveB@nusil.com
Momentive: Nancy.Pitts@momentive.com
Phillips Plastics Corp: David.Wolgemuth@phillipsplastics.com
Wacker Chemical Corp: info.usa@wacker.com

Sidebar: Silicone: A Universe of Possibilities

“Imagine what can be done with silicone material.” That is the theme of a Web site operated by Dow Corning that asks designers to “expand, explore and inspire” the use of silicones.

Dow Corning’s Design Studio Web site, at www.dowcorning.com/design, has been established to link designers with the resources and expertise that they need. “Designers should understand that silicones are not just elastomers,” says Chip Reeves, director, Design Programs, Dow Corning.  “The amazing design options found in silicone elastomers are also available in many different forms including foams, films, composite textiles and coatings additives. The broad family of silicone science has many potential uses in appliance design.  If designers only know silicones as elastomers, they are missing the opportunities in design.”

On the site, designers can utilize a materials database that offers designers information on available materials. Choices can be narrowed by the required properties, and other information to help designers come up with the materials that are needed for a particular application.

Other features of the web site include a concept sketchbook, design gallery, and an area to help companies working on green design. Here, designers can find information on environmentally friendly manufacturing practices, manufacturing by-product recycling, waste reduction, energy efficiency improvements and renewable energy and carbon sequestration efforts.

“We still have most of our information and resources trapped deeply in engineering-oriented web sites and brochures,” says Reeves, in a story on the site that he co-authored with Chris Lefteri, a product designer and author. “Our challenge is to unlock this information and make it accessible.”