Silicone types in common use in our industry include:
Neutral cure - Acetoxy 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 low modulus and high modulus is not much, but the cost difference between Acetoxy cure and Neutral cure (dearer) 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.
Modulus explained:
Silicon 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, and a high modulus is more rigid. In deciding which modulus is best for a given application, then 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, as well as 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.
Cure explained:
The words 'Acetoxy' 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 is used on. If you have seen silicone that almost peels off on it's own, but then 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.
Acetoxy silicone sealants release acetic acid (which smells a little like vinegar) as they cure.
Neutral (meaning neutral alcoxy) silicone sealants release alcohol as they cure, and has almost no smell.
Acetoxy cure.
This is the most commonly used (maybe out of ignorance, or because is cheaper?), it is more rigid and the full cure is quick. On the downside it generally has poor adhesion and leaves much to be desired in how well it 'sticks' to PVC-U, most other plastics, and glass, aluminium and Polycarbonate. Shrinkage can be acceptable if it does not contain added solvent. I have heard that the chemical released with Acetoxy cure attacks and degrades polycarbonate in some way.
Neutral cure.
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.
Conclusion:
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' Acetoxy could possibly cause.
If you want to be using a silicone sealant that will accommodate the movement of plastics to other materials as it should, AND a silicone that is going to 'stick like billy-o', then it is Low modulus, Neutral cure for you from now on. 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 (and maybe you) quickly becomes unstuck. Just to complicate matters Acetoxy 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, and if you are in our industry and have always used Acetoxy cure, 'because it's all the same, innit' - then stop, and start using the right stuff, Neutral cure - and low mod.
Tip: When you compare makes of silicone/prices, have a look to see if the movement capabilities listed on the tubes are better or worse, expressed as a %, because some sealants are 'bulked out' with fine chalk powder etc., to make a cheaper but inferior product. Not so many tubes seem to display the movement capability as often as I remember, but you could always seek out a manufacturers data sheet. The downside of using a silicone which has been 'bulked out' is that as well as a higher than desired shrinkage when it has cured, you could get discolouration, and poorer adhesion.
