Silicone Sealant & Adhesive Basic Overview
Navigating the world of Silicone sealants and adhesives can be challenging. There are a lot different variations that can solve various applications. Weather you're trying to figure out the difference between 1 and 2 part silicone products or working to determine which chemical cure system is right for your application This silicone sealant and adhesive basic overview is built to help our provide a basis understanding of all the different options that go into selecting the correct silicone product.
The basics of understanding and selecting the right silicone sealant and adhesive line in three key areas. These include:
Cure System: What chemical reaction is changing the product to its final state
Parts or Components: is it a 1 or 2 part system
Viscosity: what is the consistency of the product. (Paste-like to a liquid)
The Cure Systems: Three Main Systems
The curing system is the key chemical reaction initiates the silicone product conversion to it's final state. Both 1 and 2 part systems can utilize all the common curing systems with the one exception being Platinum addition curing systems. This system is only used in a 2-part silicone product. Each curing systems offers unique advantages and disadvantages.
What is Addition Curing (Platinum Cure Silicone) - 2-Part Silicone System
Addition cured silicone systems are commonly referred to as platinum catalyzed silicone's and are generally manufactured in two-part systems. This curing system use a platinum catalyst to initiate the cure andwill not produce any by-products during the cure process. Generally, the Part A component contains silicone's and the platinum catalyst, while component B contains polymer, crosslinker, and cure inhibitor. Cure inhibitors are additives used to control the the cure rate of the system. Once catalyzed or mixing has begun, they begin curing even in a sealed enclosure and do not need to be open to the atmosphere. Cure conditions vary with product mass. For example a thin sheet of silicone will cure much quicker than a thick section. The curing of a addition cured silicone can be heat accelerated. Depending on the specific formulation of a product, addition cured silicone's can be fully cured within minutes, but require high heat temperatures above 120 degrees C.
Mixing Notes: These systems also require a precise chemical balance to produce the correct physical properties in the cured product. It is therefore, important that the A&B parts are thoroughly mixed prior to weighing out and the correct mix ratio is carefully adhered to.
Contamination Risk: Special care must be taken to eliminate the presence of contaminants that might have a negative impact on the catalyst. The platinum catalyst is susceptible to attack from certain chemical compounds which in turn will lead to inhibition of cure, resulting in a partially cured product. Bringing the uncured material into contact with the following chemical compounds should be avoided during the manufacturing process; nitrogen, Sulphur, phosphorus, arsenic, organotin catalysts, PVC stabilizers; epoxy resin catalysts, Sulphur vulcanized rubbers, and condensation cure silicone rubbers.
What is Moisture Curing (RTV - Room Temperature Vulcanization Silicone) - 1-Part & 2-Part Systems
Moisture Cure Silicone is one of the most commonly manufactured versions of silicone sealants and adhesives. Moisture Cure RTV Silicone adhesives and sealants utilize water and a curing agent to form an adhesive bond or seal. When these materials combine during cross-link, a chemical byproduct is released. Depending on what material curing agent you are using, different byproduct will be created including acidic, basic, or neutral. These material curing agents also have a large impact on the final properties that will be displayed by the silicone product.
Moisture cure - RTV silicones can be utilized in many forms including the use of handheld tubes and cartridge guns to fully automated dispensing systems utilizing pails or drums. Silicone Adhesives are an incredibly diverse product. Many key characteristics include high flexibility, wide operating temperature range, resistance to harsh weather conditions, humidity, mold & mildew, and also carry excellent electrical properties. RTV silicones have a 30+ year life span and a high degree of elongation; they are easy to dispense, even in cold temperatures, and are VOC compliant, with excellent UV and thermal stability properties.
Three Main Moisture (RTV) Curing Systems
Acetoxy Cure System
Acetoxy functional systems are typically used in the formulation of one-part dispersions, sealants and adhesives. These materials are very effective when cured in thin section and provide good adhesion to most substrates. The cure system consists of hydroxyl-terminated polymers, alkyltriacetoxysilane crosslinkers and a tin catalyst.
Acetoxy cure is likely the more commonly used type of RTV silicone. It is used across the building materials industry in applications such as such as window installations, expansion joints, and residential bathrooms and kitchens. This is due to its ability to bond to common substrates like wood and tile, and because of its fast tack-free time and resistance to high temperatures.
Alkoxy Cure System
Alkoxy or alcohol cure systems are typically used in low consistency or pourable elastomers and foams. These materials can be cured in thicker section than acetoxy crosslinked materials and are generally supplied as two component products. The cure system consists of a hydroxyl-terminated polymer, a tetraalkoxysilane crosslinker and a tin catalyst. Like the acetoxy cure system, water vapor (moisture) is required for vulcanization. Unlike the acetoxy system, the leaving group is an alcohol rather than acetic acid and as such it is non-corrosive. These materials also require 7 days or more to reach complete cure depending on humidity and thickness of the silicone section.
Oxime Cure System
Oxime cure systems are typically one-part, thin section, moisture cure adhesives. The cure mechanism is again the hydroxy end-blocked silicone reacted with an oxime functional crosslinker in the presence of tin catalyst and ambient humidity. The leaving group, methyl ethyl ketoxime, is non-corrosive.
Oxime can adhere to concrete among many other substrate surfaces, and is non-corrosive on most metals. It also releases a non-pungent odor during cure, has a long tack-free time, and is resistant to oil and high temperatures. Unlike acetoxy, however, it is not food grade and can discolor copper in confined spaces.
What is Dual System / UV Curing and Heat Curing
Dual Cure Systems
Heat Cure Systems
Heat cured silicone adhesive sealants are particularly useful where production methods demand very fast cure times or when there is a need to apply the material and have a delay before curing, perhaps to carry out other assembly procedures The chemistry used is based upon a platinum catalyst which is, in effect, retarded and only starts to work when heat is applied. Most 1-part addition cures require temperatures above 80ºC to cure the material, by elevating the temperature the cure speed will increase to a maximum temperature of approx. 150ºC.
Adhesion is normally a little harder to achieve using these materials when compared with RTV’s. Adhesion promoters are added to improve adhesion, but these normally require the use of higher temperatures for slightly longer periods of time. For example, a typical adhesive may cure after 30 minutes at 100ºC while elevating the temperature to 150ºC for 30 minutes will ensure adequate adhesion to the substrate.
UV Cure Systems
UV Cure Systems
The Difference between 1-Part & 2-Part Silicone Systems
Silicone sealants and adhesives can break down at the simplest level into two product categories, a 1 or 2 part system. This distinguishes weather the silicone product will require 2 different formulations to be mixed (2-part) or the silicone product comes in a single formulation ready to use (1-part). Once broken down into these two categories 1 and 2 part silicone products can branch out into many different viscosity / consistencies (flowable and non-flowable), different cure systems, and other unique characteristics.
Silicone adhesive and sealants are most commonly manufactured in one-part systems, and encompass the most basic form of Silicone Sealants and Adhesives. They require no mixing and when formulated as a RTV cure(Room Temperature Vulcanization) you can simply apply the Silicone material to your application and wait for the curing process to complete. The largest benefit to 1-part silicone is that there is no mixing required. They come in a wide range of different viscosity / consistencies, which a provide everything from a low viscosity (flowable self-leveling liquid) to a higher viscosity (non-flowable paste).
Two-part silicone's are created through the mixing of 2 components. Component 1 is a rubber base compound that is mixed with component 2 that is a curing agent. A more in depth look at each component shows that Component 1 is made up of a polymer and sometimes additional fillers and additives, while component 2 contains chemical crosslinkers and catalyst. When component 1 and 2 are mixed, the mixture immediately starts curing to form the silicone into its final form.