Overview of Silicone Adhesives
Although PD3 Composites does not sell silicone adhesives, our customers use them to bond our silicone sheet materials regularly. Cured silicone sheet can be bonded with uncured peroxide-cure high temperature vulcanization silicone, such as PD3’s S753, B-stage silicone sheet, but this is often not convenient to do, since it has to be done using a vacuum bag and autoclave, or alternatively, in a compression mold. The most common type of silicone adhesive is a moisture cure (also known as condensation cure) 1-part adhesive. These are also called “RTV-1” adhesives, especially in Europe. There are also 2-part RTV adhesives (RTV-2), but these are typically very fast curing adhesives used in automated-dispensing applications for industrial bonding. We will touch on these, but they are much less common for occasional users. There are also one-part and two part high temperature vulcanizing adhesives, which we will also briefly discuss.
Who are the primary manufacturers of Silicone Adhesives?
In the United States, Dow Silicones and Momentive were historically the primary silicone manufacturers and have overlapping product lines. Dow is currently by far the largest silicone monomer manufacturer in the USA, and is the only remaining company that has fully integrated production within the continental US. GE Silicones was formerly a division of General Electric, but it was spun off under the name Momentive in 2006, and had been a fully integrated producer until it shut down its silicone monomer plant in Waterford, NY in 2022. Since 2024, Momentive has been owned by the Korean silicone manufacturer KCC, and uses silicone monomer from a number of sources, including KCC and Dow. The GE Silicones brand, which is still used for some products, is owned by Momentive.
There are a number of other manufacturers such as Henkel/Loctite, Permatex, Sika, 3M, CRC Industries, Silicone Solutions, Masterbond, and others, that do not manufacture the silicone monomer, but mix or convert silicone synthesized at one of the integrated companies or a specialty chemical producer, and sell it under their own adhesive brand. These are typically for more niche industrial applications or for the construction industry.
In Europe, both Elchem and Wacker have competing lines of silicone adhesives. In Japan Shin-Etsu is a major supplier. In Korea there is KCC/Momentive. All three of these companies are vertically integrated suppliers similar to Dow. Although they have a significant US presence for other silicone products, their branded adhesives are not widely available in the USA. There are also a number of Chinese suppliers, but their products are also not commonly sold in the USA, and none of them have been evaluated by PD3 Composites.
RTV-1, Moisture Cure Adhesive
One part condensation cure adhesives, also known as RTV-1 adhesives, and moisture-cure silicone adhesives, are extremely versatile and widely used. “RTV” stands for “room temperature vulcanization”, and the -1 indicates that it is a one-part adhesive. These tend to have terrific adhesion to cured silicone and metal. Adhesion to plastics can be more difficult, especially for low surface-energy thermoplastics like fluoropolymers or polyolefins. In those cases special surface treatments and/or primers may be required. Thermosetting polymers, such epoxies and polyesters used in composite manufacturing, tend to be relatively high surface energy, and easier to bond to. Condensation cure silicone adhesives. can be relatively cheap at $10 per 10oz tube, or relatively expensive, at as much as $200 or more for a 3oz tube for specialized types. Condensation cure RTV-1 adhesives require moisture to activate and then requires modest ventilation to evacuate cure bi-products. Very thick sections, or areas that are trapped between impermeable materials, can take a very long time to come to full strength. The other disadvantage is that because it comes pre-catalyzed from the manufacturer, the cure speed cannot be easily adjusted. In some cases, these adhesives can take a full week or longer to reach full strength.
Figure 1. General characteristics of various condensation cure RTV-1 types.
Acetoxy-cure is the most common type of RTV-1 moisture cure adhesive, and typically the cheapest. Low-cost construction sealants sold at hardware stores are often made of this type. The RTV-100 series from Momentive (RTV102, RTV103 etc) and Dow 732 are the low-cost, general-purpose adhesive type in this category. These types cost about $15 for a 10oz tube. Their mechanical performance and bond strength are not as good as some higher strength types, but they certainly are suitable for many applications. There are some very high strength acetoxy-cure such as RTV157 and RTV159, which are sold in 3oz tubes that cost $30-80 each. Acetoxy cure RTV adhesive produces acetic acid as a byproduct, which has a distinctive smell vinegar-like smell. This can cause corrosion with some substrates.
Alkoxy-cure is the other common type, used with higher cost RTV-1 moisture cure adhesives. These are also sometimes described as “alcohol cure” or “neutral cure”, to indicate it does not produce acidic byproduct. PD3 Composites’ preferred RTV adhesive, in most cases, is Dow 3145. This is an alkoxy-cure RTV-1 with exceptional bond strength to most substrates, and very high elongation and tensile strength. It has a long working time and is easier to handle than some of the lower cost Acetoxy types. Momentive RTV167 is a comparable product to Dow 3145, but we do not have as much experience with it. These usually cost about $90-150 for a 10 oz tube, depending on quantity and where it is purchased from.
Oxime-cure is not commonly used type in aerospace or composite-manufacturing related applications, it is a neutral cure type that costs more than the cheapest acetoxy types, but less than the high-performance alkoxy types. Its mechanical performance is similar to the lower cost acetoxy-cure RTV, at least for those types with readily available technical data sheet. Hybrid-cure types, with acrylic or urethane chemistry, are also not typically used in composites and aerospace applications. These are generally for construction applications and general-purpose bonding. They can have excellent adhesion and mechanical performance, but much worse low and high temp exposure limits.
Condensation-cure RTV-1 adhesive have a two-step reaction. In the below example we use alkoxy type RTV-1, but acetoxy is similar. Initially, the material is stored in a container (generally a plastic tube) that is impermeable to water vapor. When it leaves the container, water vapor from the atmosphere permeates into the uncured silicone paste, which causes crosslinking to commence. Atmospheric moisture combines with alkoxy silane groups to form silanol groups and alcohol (either ethanol or methanol, depending on the type). Then in a secondary step the silanol reacts with alkoxy or hydrolyzed alkoxysilane to form a polymer chain.
Step 1: Hydrolysis of Alkoxy Groups
Step 2: Condensation (Crosslinking)
Two silanol groups (or a silanol and a hydrolyzed alkoxysilane) react to form a siloxane bond:
Or, silanol reacts with unhydrolyzed alkoxy:
Various silane additives are included that are intended to promote adhesion to different substrates. In most cases these silane additives will degrade with long term heat exposure. Many types of silane additives, that are the typical adhesion promoter used in RTV-1 adhesives, degrade with time at temperatures above about 150°C. There are specialized types that have very good long-term performance above this temperature, but in critical high temperature applications it is important to conduct heat-aging studies with the exact substrate that will be used, and the user should not rely on the maximum temperature stated on the data sheet. The silicone polymer itself may remain relatively undegraded when the adhesion to the substrate has failed.
RTV-2 Condensation Cure and Addition Cure
For the most part, these are used in applications where very rapid cure time is required. For example, they are commonly used in automotive bonding, where the two parts are mixed in a dispensing gun as it is applied. They often cure in seconds or minutes. Addition cure, also known as platinum cure, have no chemical byproducts produced in the reaction. Condensation Cure RTV-2 has a similar reaction chemistry to RTV-1 but because the moisture is already incorporated into one of the two parts, it typically does not require atmospheric moisture to cure. This silicone type is also sometimes called “tin cure” RTV silicone, because of the tin based catalyst that is often used (there are other catalysts available as well). Both tin cure and addition cure silicones are used for 2-part room temperature cure silicone adhesives, and also for 2-part room temperature cure casting silicones that can be used for mold making.
Other types of Silicone Bonding
Peroxide-cure silicones are a very common type used for bonding in compression molding applications (along with addition-cure). They often are used to bond to metal or plastic inserts, or to fabrics, during the cure process. These are “high consistency silicones”, such as S753, from PD3 Composites, which have a clay-like consistency in the uncured condition. These can be used to bond cured silicone to itself or to other materials (usually with an adhesive primer). They can either be used under a vacuum bag in an autoclave, or they should be used in a compression mold application. It is very common to use this process when splicing extruded silicone o-ring cord, for very long, o-rings, in a splice-mold. This is type of silicone requires heat to cure. Many peroxide curatives are oxygen inhibited, meaning they cannot be exposed to oxygen concentrations above a certain threshold during the cure process. Adhesion promoting ingredients, such as silanes, can be incorporated into the silicone during the mixing process, if desired.
There are also a few 1-part addition-cure adhesives that have to be stored in a freezer to inhibit polymerization. These can either cure at room temperature, or at elevated temperature. These are relatively uncommon, but they have the advantage of being easy to handle, and can be extremely fast to cure. Unlike RTV-2 fast-curing adhesives which have only a few minutes of working time, they can have a room-temperature out-time of several days, before being cured for a few minutes in a high temperature oven.
Please keep an eye out for additional blog posts at https://www.pd3composites.com/blogs/technical-content . In subsequent posts, we will discuss best-practices for bonding using silicone adhesives, including cleaning, etching, and priming.