Over the years, Albright has developed strong partnerships with major silicone suppliers. Albright stocks a large inventory of silicones with a range of physical properties intended for many different applications. Commercial silicone compounders have developed silicone product lines that either maximize or optimize desirable material properties.
As in other industries, the naming to differentiate silicone materials can be confusing. The names applied to silicone types may refer to its uncured properties (LSR and HCR), its chemical structure (FSR) or its curing process (RTV and HCR).
- Liquid Silicone Rubber (LSR) – Most present-day silicones are named “LSRs” with the reference that this form flows easily into a mold, hence the word “liquid”.
- Fluorosilicone (FSR) – A silicone product with fluorocarbon rings rather than methyl or ethyl rings. Fluorosilicone has better solvent resistance than other silicone compounds.
- High Consistency Rubber (HCR) – A legacy product form – also called “gum rubber”. Uncured HCRs retain their form with a clay-like consistency.
- Room Temperature Vulcanization Silicone (RTV) – Commercial over-the-counter silicones for sealing and adhesives are this type.
- High Temperature Vulcanization Silicone (HTV) – Silicones that have additives to require higher temperature cure.
Silicone Catalysts and Curing
Curing in silicone is accomplished when the individual silicone polymer structures attach to adjacent polymers, also called cross-linking. Most LSRs and HCRs that are processed today use a platinum-based catalyst cross-linking chemistry. The cross-linking or curing is accelerated to completion by raising the silicone temperature to a range between 200 to 350oF (95 to 175oC) . The cure time for a platinum-catalyzed material is typically between 30 seconds and 2 minutes, depending on the material type and the part volume/thickness.
RTV’s cure or crosslink when exposed to water, usually from atmospheric moisture. As the moisture must penetrate the full depth of the silicone part, RTVs typically require hours to days to cure, again depending on the RTV type and the part volume/thickness.
Silicone Types by Cured Material Properties
RTV’s cure by reaction after mixing components. The reaction rate depends on the cure system used such as peroxide, platinum, tin. The reaction occurs at around room temperature and may require hours to days to complete depending on the RTV type and the part volume/thickness.
General Purpose Silicone
The first choice for most products. Expect mid-characteristics with respect to temperature resistance, percent elongation, tear strength, solvent resistance and other. General purpose silicones are the most common solution for seals, gaskets, o-rings, custom parts and encapsulation in the aerospace, defense, automotive, industrial, and consumer markets.
Silicones for Medical, Pharmaceutical and Food Applications
Silicone can be compounded to create materials that are essentially bio-inert and fully compatible with biological systems. Besides the chemistry or compounding to obtain the correct basic material, a post-process to extract unwanted residuals is performed. Post-processing can obtain the purest and least–reactive silicones.
Silicone suppliers have created guidance for the use of these materials by creating in-house certifications for bio-contact applications. These certifications are based on the positive results to a series of USP test challenges. There are six USP test challenges, identified as Level I through Level VI. See Table 1. Classification of Plastics. Materials that pass each of the six test challenges may be considered the most bio-compatible and are sometimes referred to as “Grade 6” materials.
- Food Grade Silicones – Food applications can be more difficult to screen for compatibility than human body compatibility testing. In addition to FDA criteria and USP testing, the Department of Agriculture (USDA) has guidelines and requirements for food processing and acceptable material contact.
- Medical Non-Implantable – Silicones of this type are used in medical applications that are not used inside the body. Some of the typical applications that would use this form of silicone include medical equipment, hoses, feeding tubes, and medical disposables.
- Medical Short Term Implantable – This type of silicone can be implanted into the body for 29 days or less. Some typical applications using short term medical silicone include punctal plugs, surgical tools, shunts, certain types of catheters.
- Medical Long Term Implantable – This form of silicone can be implanted for longer than 29 days. Knuckle, toe, ocular and cardiovascular implants are just a few applications that use long term implantable silicone.
- Pharmaceutical – Materials in pharmaceutical applications, such as for an indwelling combination device, must pass the six USP biocompatibility test challenges and also be compatible with any drug that may contact the part.
Optically Clear Silicones
From contact lenses to sterile barriers for optical medical applications, silicones with broad spectral clarity and appropriate dielectric constants have been commercially developed. Check out Albright’s optically molded silicone products here and rate the engineering WOW! factor.
Albright will mold your silicone part in any of the commercially available colors. Color additives are easily mixed into the silicone of your choice. The color degree can be adjusted from a light tint through deep, solid shades. Colored silicone may be procured directly from a major supplier or through mixing in a silicone colorant from a tint and dye manufacturer. To learn more about silicone dispersions and colorants visit our liquid silicone colorants page.
Additives to Modify Silicone Characteristics
Finished product properties of silicone such as improved surface lubricity, increased thermal conductivity, increased electrical conductivity, improved mold release, improved bonding to dissimilar materials and cure–time/temperature are a few that may be modified with additives.