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Silicones Simplified

Silicones Simplified

With a vast array of different types available, Silicone Sealants can look a confusing area to even the most experienced user. There are however important differences between many products in terms of quality, application possibilities, toolability, cure speed, flexibility and adhesion.

Adding to the confusion are the many phrases and specifications used to describe the characteristics of varying Silicone Sealants, some of which are not always relevant to selecting the right product for the job and some of which are downright confusing. To try and give the average user a better understanding of the different types of Silicone Sealants available (without getting too technical!) please read on:

Cure Types

There are two primary curing methods, and these are the phrases that everyone is perhaps most familiar with, ACETOXY denotes a product that cures by a reaction with moisture in the air that gives off an odour of ACETIC ACID (vinegar). Acetoxy cure types should not be used when they will be primarily in contact with Alkaline substrates (such as concrete).
NEUTRAL is the second cure type, this refers to products that cure by a system of crosslinkers that, when activated by exposure to moisture in the air, give off almost odourless byproducts (mainly alcohols). Neutral types generally have a much lower odour than acetoxy types and as the name implies are "neutral" in their effect on the substrate they are applied to, ie Non Corrosive. Within the Neutral cure classification there are several types of curing systems which have distinctive features that make products more suitable to certain applications.

A quick summary of the key differences between the cure types as follows: In general, acetoxy types are usually better for use on smooth/ non porous substrates and are normally quicker curing, where NEUTRAL are usually better for adhesion on porous, rough surfaces and are normally slower curing. Acetoxy types are also generally slightly lower in cost than Neutral types..

Modulus

The other phrases which most users will be familiar with are "HIGH MODULUS" and "LOW MODULUS", the "modulus" is actually a calculation of stress and strain measurements of the cured sealant, but can best be explained in terms of the "hardness" or "softness" of the sealant. High modulus types have a higher "tensile" strength and less elasticity and are harder to the touch than Low Modulus types, which have a lower tensile strength and more elasticity. Most "High Modulus" products are based on Acetoxy cure systems, whereas most "Low Modulus" products are based on Neutral systems, although there are exceptions.

Elasticity

The elastic recovery of a sealant, also referred to as Movement Accommodation refers to the percentage it will expand and then contract back to its original form. Elastic recovery varies depending upon formulations but in general High Modulus, acetoxy cure types have a lower elasticity (usually maximum 25% movement accommodation) where Low Modulus neutral cure types have a higher elasticity (up to 50% movement accommodation). When selecting the correct product for a job Elastic Recovery is a key factor, for example the movement required of a sealant around a PVCu window frame will be much greater than one used to seal around a kitchen worktop.