[Expert advice] Silicone sealant. The basics

Silicone was first obtained in 1934 by Dr. James Franklin Hyde at the glass company Corning Glass Works. It was originally intended for use as a seal between iron and glass, but the material turned out to be one of the most versatile ones available. The first curing silicone-based sealants appeared in the 1970s and have since come into wide use in almost every industry, from rocket building to home repair. In construction, silicone sealants are used to fill joints of all kinds in order to seal them and protect them from moisture and other adverse factors, and also to bond various surfaces.

Types of silicone sealant
Sealants come in one-component and two-component versions. Two-component sealants are supplied in two separate containers and are mixed directly during application. They are meant for special tasks and are far less common on the market. One-component sealants are more popular: they cost less and are easier to use, while their properties suit most jobs.
By cure system, one-component sealants divide into acid-curing (acetoxy cure) and neutral ones, which in turn divide into alcohol-based (alkoxy cure) and oxime-based (oxime cure). Acetoxy sealants cost less than neutral ones but have certain limits in use. They must not be used in contact with metals, or with cement-based substrates, gypsum and natural stone, because the acid attacks these materials. Neutral sealants are more versatile, have no restrictions in use and may be in contact with any material.
By purpose, sealants are general purpose, sanitary and special — for example, high temperature sealants, sealants for glass, for aquariums or for natural stone. Sanitary sealants are meant for use in wet rooms and are more resistant to mold and mildew thanks to their higher biocide content. Aquarium sealants, on the contrary, contain no biocide at all, because it is harmful to aquarium plants and animals. High temperature sealants withstand heating to 300°C and above. Sealants for natural stone have a neutral cure system and contain no components that leave oily marks on natural stone.

How sealants cure
All construction silicone sealants belong to the so-called RTV sealants. RTV stands for Room Temperature Vulcanising, that is, "curing at room temperature". The mechanism of curing, or vulcanization, can differ. In two-component sealants it takes place through the whole thickness of the material at once, through the reaction of the two components after they are mixed. In one-component sealants vulcanization moves from the surface inwards through the reaction with the moisture in the air. For fast curing, enough moisture matters more than a high temperature: in hot but dry air a one-component sealant vulcanizes more slowly than in cool but damp air.

Sealant specifications
Let us go through the most important properties of silicone sealants.
Most sealant tests are run under so-called standard conditions, that is, at an air temperature of 23°C and 55% humidity. This is set out in the standards and makes it possible to compare the same property of different sealants measured in different tests.
Tack-free time is the time after which a sealant applied to a plate stops sticking to a glass rod. As a rule, the faster the skin forms, the better: less dust sticks to the sealant before the skin has formed, and there is less chance of getting dirty by accidentally touching the joint.
Cure rate is the thickness of the sealant layer that has cured in 24 hours. The faster a sealant cures, the sooner the joint can be put into service, so a high cure rate is a clear advantage. Note that a sealant that skins over faster does not always cure faster as well. Often it is the other way round.
Adhesion to aluminum and concrete is the force needed to pull a standard-size block of sealant off an aluminum or a concrete substrate. In general, the higher the adhesion, the better. The exception is joints that are meant to be taken apart later, where high adhesion can create difficulties.
Tensile strength is the force needed to tear a standard sealant specimen apart. The higher the strength, the better.
Elasticity is the percentage by which a sealant can be stretched before it fails. Elasticity matters particularly in moving joints, where the parts move considerably relative to each other.
Modulus is the force needed to stretch a sealant to twice its length. Sealants are divided into low modulus, medium modulus and high modulus ones. Low modulus sealants are good in moving joints, where the main task is to keep the joint tight while the parts move relative to each other. High modulus sealants are used in structural joints, where the sealant has to carry a mechanical load as well as seal.

What a silicone sealant is made of
Manufacturers who are too fond of cheap marketing often print "100% silicone" on sealant packaging, hinting at absolute quality. A good sealant, however, cannot be made from silicone alone. It contains many components, each of which affects particular properties of the sealant and its quality as a whole. The formulator's skill lies in assembling a quality product out of many components while keeping the price competitive.
The base of a silicone sealant is the silicone polymer, and the quality of the finished product depends largely on its properties and quality.
A silicone sealant always contains a curing agent as well, which starts the curing of the sealant when it reacts with the moisture in the air. Acetic acid (acetoxy sealant), ethyl methyl ketoxime (neutral oxime cure) or methyl alcohol (neutral alkoxy cure) are precisely the by-products of that reaction. The curing agent is usually added with a margin so that, while the sealant is in its packaging, it acts as a preservative and neutralizes any moisture that gets in by accident.
Two more important components are the filler, which is responsible for strength and for thixotropy (the ability of the sealant to hold its shape and not run off vertical surfaces), and the plasticizer, which makes the sealant elastic. Sealants for natural stone and marble deserve a separate mention: they use more expensive special plasticizers that do not penetrate the pores of natural stone and leave no oily stains around the joint.
Pigment is added to give the sealant a particular color, since silicone cannot be painted after application: paint simply will not hold on it.
Biocide is added to fight mold and mildew. Sanitary sealants contain the most biocide; general purpose ones contain it too. Aquarium sealants, however, have no biocide, because it is harmful to fish and plants.

Can silicone sealants be cleaned with detergents?
All silicone sealants resist household cleaning and washing products. The only thing they should not be treated with is petroleum solvents such as gasoline, acetone and the like. Under the action of solvents silicone sealants soften, swell and lose strength.

Can automotive silicone sealants be used instead of construction ones?
Automotive sealants are made to work in harsher conditions and as a rule outperform construction ones. In most cases, therefore, an automotive sealant may perfectly well be used instead of a construction one if the higher price is acceptable. The exception is rooms with high humidity, where mold is likely. Automotive sealants are better not used there, as they contain no biocide to fight mold.

Is there anything better than silicone sealants?
Technology does not stand still, and however good silicone sealants are, hybrid ones are gradually taking their place. They have taken the best of polyurethane, silicone, acrylic and rubber adhesives and sealants. Compared with silicone ones they have higher adhesion to a range of surfaces and are more resistant to mold, mildew and various kinds of soiling. Hybrids do not shrink and can be painted. The main thing holding back the spread of hybrid sealants is their higher price compared with silicone.