Overview of Silicone Elastomeric Materials

Silicone rubber is perhaps the most versatile elastomer in current use.  It is commonly used in industrial, medical, and consumer applications where it has a number of unique attributes that set it apart from other materials.  In medical applications it is favored because, depending on the formulation, it can be inert and biocompatible.  It is favored in consumer products for the same reasons, as well as its potential to be manufactured at very high rates and low cost using injection molding processes.  It is used in aerospace because it can maintain its flexibility at ultra-low temperatures, and is also resilient in very high temperature environments.

The term silicone rubber is used for elastomeric polymers made from polymethyl siloxane, polyvinyl siloxane, polyphenyl siloxane, and sometimes fluorovinyl methyl polysiloxane or a combination of several of these types.  Silicone comes in a variety of forms and can be cured with several different cure systems.  Not all silicones are polymerized elastomers.  Silicone can also be provided as an “oil” and is used as a processing aid or additive in other polymers, a grease/lubricant, or release coating.

General Types of Silicone Rubber

Silicone comes in a broad variety of formats that are optimized for different applications.  There are several different ways to group these materials:

Cure Temperature: HTV vs RTV.  High Temperature Vulcanization (HTV) silicone rubber cures at elevated temperature.  It can often be stored for weeks or months (or even years) in its mixed format prior to curing.  Room Temperature Vulcanization (RTV) silicone cures at room temperature.  It is either stored in two parts unmixed, or it is dispersed in a solvent that inhibits the reaction until it dries out, or it is stored in a moisture-proof container that inhibits certain types of cure reactions.

Viscosity: HCR vs LSR.  “High consistency rubber” (HCR) is generally used in compression molding, extrusion, transfer molding, and a type of injection molding.  This type of silicone has a clay-like consistency.  Liquid silicones are sometimes called LSR for Liquid Silicone Rubber.  Most liquid silicone rubber is platinum-cure two-part silicones that are used in a high-volume injection molding process, but there are also two-part casting RTVs, and a variety of coatings used for adhesives or release surfaces.  There are also a number of adhesive pastes that are typically used in bonding.  These are mostly moisture-cure RTV silicones.

Catalyst System: Silicone is generally cured with a platinum catalyst (also called “addition cure”), peroxide catalyst, or moisture-cure system, which refers to several types of cure chemistries that require the presence of moisture for the reaction to begin.  There are several other less common types as well, like two-part tin-cure RTV silicones.

Common Cure Systems for Silicone Rubber

Peroxide Cure- Peroxide-based curatives are commonly used for high-temperature vulcanization (HTV), high-consistency silicone rubber.   These materials must be mixed on industrial mixing equipment and can be stored after mixing for a year or more at room temperature, depending on the peroxide type and storage conditions.  Peroxide-cure silicones are used in compression molding, transfer molding, extrusion, and vacuum bag/autoclave curing.  Besides storage life, peroxide-cured silicones can offer some of the best high-temperature, low-temperature, and mechanical properties among silicone types.  They are also relatively easy to modify for specific applications.  This type is heavily used in aerospace sealing applications.

Platinum Cure- Platinum cure silicones are used in a broad variety of applications.  By volume, two-part high-temperature vulcanizing liquid silicone rubbers (LSRs) are perhaps the most common type.  These can be injection molded using conventional plastic injection molding machines.  Instead of being introduced in pelletized format, the two parts mix as they are injected and kept cold until they enter the mold cavity.  This is the most economical way to make complex silicone rubber components in high volume.  These materials can have properties that match or even exceed peroxide-cure silicones, especially in tear strength.  They can have ultra-high clarity that can match or even exceed the clarity of glass.  This is the “cleanest” type because there are no cure byproducts left after the crosslinking reaction, and for this reason it is the preferred type for medical or food-grade applications.  Platinum can also be used for two-part RTV liquid silicones, which typically have much lower mechanical properties than HTV silicones.  Platinum silicones can also be dispersed in a solvent and used in ultra-low viscosity applications like release coatings on paper liners for tapes and other adhesives. There are also platinum-cure HTV types that can be used in compression, transfer molding, or extrusion.

Moisture Cure- Also known as “condensation cure”, this type is used in industrial caulks and adhesives.  It sometimes has a paste-like consistency, but also can be a liquid or can be dispersed in a solvent for coating applications.  Moisture-cure paste-adhesive has a consistency of around 500,000 CPS, which is somewhere between cookie-dough and table syrup.  When it is supplied in a solvent-free format, like an adhesive paste, it is usually in a plastic tube.  When it leaves the tube, atmospheric moisture penetrates uncured silicone and sets off a crosslinking reaction.  There are a number of moisture-cure processes: alkoxy, acetoxy, oxime, amide, and amine cure systems.  From a user standpoint, they all function similarly.  The crosslinking process can take up to a week at room temperature, depending on the cure system, humidity, temperature, air circulation, and how exposed/deep the cure section is.  They are typically not used in rapid or high-volume manufacturing applications because of the slow cure speed and other disadvantages.  They have substantially lower mechanical properties than HTV silicones, whether peroxide or platinum cure, but they can have excellent adhesion.

PD3 Uncured B-Stage, Peroxide Cure Silicone Sheet

PD3 Composites sells uncured B-stage sheet silicone.  It has a clay-like consistency and can be stored at room temperature for a year prior to expiration.  These materials are intended to be cured under a vacuum bag, for making cauls and reusable vacuum bags.  They can also be used for compression molding.   These materials all use peroxide curatives- either 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, known as DBPH, or Bis(1-methyl-1-phenylethyl) peroxide, known as dicumyl peroxide or dicup.  These curatives are air inhibited and must be cured in a mostly oxygen-free environment.  We will be providing an additional publication on best practices when using these materials.

Silicone Contamination

Silicones have long been used in high temperature composite applications as tooling materials, release liners, adhesives (including on high temperature tapes), release coatings, gaskets, and reusable vacuum bag membranes, and there are a broad variety of silicone products that have been used.  One issue that has come up for decades is silicone contamination of composite bonding surfaces and surfaces that will be painted.  Silicone oils can be difficult to detect and remove, and can contaminate surfaces and inhibit bonding or paint adhesion.  For this reason, some aerospace composite manufacturers have taken the extreme approach of banning all silicone products from their cleanrooms.  Other companies, including large OEMs with very stringent process requirements, use silicones quite freely in composite applications.  Some of the most commonly used release agents for aerospace composites applications have been silicone-based, including Frekote 700 NC.  It is used in nearly all tape release liners.  It is used for high-temperature clean-release pressure-sensitive adhesives, including the ubiquitous blue flashbreaker tapes, Kapton/polyimide tape, and grey PTFE (Teflon) tape, although non-silicone versions of these also exist.  It is also commonly used on gaskets for through-bag connectors.

PD3 is not an expert on silicone contamination, but it is our opinion that this contamination from silicone rubber can come from one of several sources.

1. "Reversion” – certain low-temperature silicones, and particularly non-platinum RTV types, can depolymerize and break down the polymer links when exposed to high temperature for long periods. This becomes evident with the softening and increased tack of the silicone elastomer. We have not witnessed this with platinum or peroxide cure silicone material.
   
   
2. Incomplete cure due to chemical inhibition – This is most typical in platinum cure silicone materials. Sulfur compounds, amines, nitriles, tin, lead, silver, and certain hydrocarbons can cause inhibition. Inhibition is apparent because there are soft gooey spots where the rubber does not cure. A common source of contamination for platinum-cure silicones is the use of latex rubber gloves. Natural rubber latex (the tan color gloves) can have excess sulfur, which acts as a catalyst poison to platinum cure systems. Blue nitrile gloves are typically sulfur-cure as well, but we have not noticed these causing cure-inhibiting contamination, even though it seems possible. We are only aware of chemical cure-inhibition in platinum-cure silicone rubber material.
   
   
3. Incomplete cure due to poor mixing – This typically pertains to two-part liquid RTV-type silicones. This is very uncommon in HCR grades, whether peroxide or platinum cure, and it is also very uncommon for commercially mixed one-part RTV silicones, such as RTV silicone adhesive pastes.
   
   
4. Incomplete cure due to inadequate cure time or temperature – This is not especially common for heat cure systems, since they are often cured at temperatures where it only takes several minutes to fully crosslink. It is most common for deep sections of moisture-cure RTV silicone.
   
   
5. Unreacted silicone oils – Some silicone rubbers use silicone oils as either a processing aid, an internal release agent, or a functional additive that enhances ultra-high-temperature performance and endurance. When used, they are on the order of a ~1% ingredient by weight in the rubber compound. Some of these oils are intended to remain unreacted after the rubber cures. As such, they can be left on surfaces contacted by the silicone.
   
   
6. Incomplete cure due to oxygen inhibition – This only applies to peroxide-cure HCR silicones. It is typically caused by large air entrapments or total loss of vacuum bag integrity during the early stages of a cure when fabricating the silicone product. While not uncommon, it tends to be very obvious. We have not observed subsequent contamination issues in the subsequent use of silicone rubber tooling products that have had this issue.

PD3 peroxide cure silicone sheet products are designed to minimize the risk of these types of contamination or incomplete cure.