Silicone types in common use in our industry include:
•Neutral cure - Acetoxi cure
•High Modulus - Low modulus
Is there much difference, and does it really matter?
The answers to these two questions are Yes, and Yes.
The cost difference between Acetoxi cure and Neutral cure can be quite a few pence a tube, and as many in the industry seem to think that 'it is all the same', some are often repeatedly using the wrong type of silicone for the job.
Silicone sealants are classified for use in either a) glazing joints, or b) for building joints other than glazing. There are further classifications to do with how well the movement capability is accommodated. The movement accommodation of the higher % ones (20%+) are then classed into low modulus and high modulus.
The meaning of high and low can be explained simply: a low modulus will need just a low force to stretch it and will have better elasticity and movement accommodation, a high modulus is more rigid. In deciding which modulus is best for a given application, desired movement accommodation is the main thing to take into consideration. In our industry it is generally accepted that linear expansion and contraction of the plastics we use, joint movement itself can be reckoned to be high, therefore a low modulus silicone should be used to accommodate such expected movement. Some of the better silicones boast being able to accommodate movement of up to 50% of the nominal joint width.
The words 'Acetoxi' and 'Neutral' cure give no clues as to which is right for what job. In a nutshell the type used will determine how well it will adhere to the materials is used on. If you have seen silicone that almost peels off on it's own but at other times it seems hopeless trying shift the stuff without a silicone remover chemical, then here is why:
These (meaningless for many) names derive from the curing system, and from the by-products that are emitted as the silicone is curing, i.e.
•Acetoxi silicone sealants release acetic acid (which smells a little like vinegar) as they cure.
•Neutral silicone sealants release alcohol as they cure, and has almost no smell.
This is the most commonly used, it is more rigid and the full cure is quick. On the downside it generally has poor adhesion and leaves much to be desired for in how well it 'sticks' to PVC-U, most other plastics, glass, aluminium and Polycarbonate. Shrinkage can be acceptable if it does not contain added solvent. I have heard that the chemical released with Acetoxi cure attacks and degrades polycarbonate in some way.
Much better adhesive properties for a greater number of materials including PVC-U, most other plastics, glass, aluminium, lead, stone and masonry, and Polycarbonate. It cures with atmospheric moisture and skins over in about 30 minutes, and leaves a shinier finish. On the downside it is more expensive and is slower to cure at one to five days depending on thickness, temperature and humidity conditions.
The benefit of using a good Low Modulus Neutral Cure Silicone can be summed up thus:
It offers high movement accommodation and excellent adhesion to almost all building and glazing materials, without any of the unwanted 'side effects' Acetoxi could possibly cause.
If you want to use a silicone sealant that will accommodate the movement of plastics to other materials as it should, a silicone that is going to 'stick like billy-o', then it is Low modulus, Neutral cure for you. The worst you can use would be High Modulus Acetic cure, because although this would seem to initially adhere, it will not be long before it quickly becomes unstuck. Just to complicate matters Acetoxi cure in low modulus is available.
Suppliers will often say "how much are you paying for silicone" when they are trying to sell to you, but just make sure that they are quoting for neutral cure. If you are in our industry and have always used Acetoxi cure, 'because it's all the same, stop, and start using the right stuff.