Frame Materials:
PVCu

Correct sealants 

Silicone is silicone; it's all more or less the same - innit?
This page examines the properties of silicone sealants commonly used in the installation of windows, doors and conservatories. 
There is much confusion about different silicone types, which is not surprising, as their descriptions give few clues as to the correct grade and type for a given application, and there is no one single type of silicone sealant that should be used for everything.

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
Silicone sealants are classified for use in either a) glazing joints or b) building joints other than glazing. There are further classifications to do with how well the movement capability is accommodated. Those that accommodate a higher degree of movement (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, the 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, so 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.

Acetoxy cure
This is the most commonly used (maybe out of ignorance or because it is cheaper); it is more rigid and the full cure is quick. On the down side, it generally has poor adhesion and leaves much to be desired in its ability to 'stick' 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
This has 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, leaving a shinier finish. On the down side, it is more expensive and is slower to cure - 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 use a silicone sealant that will accommodate the movement of plastics to other materials as it should AND 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 acetoxy cure, because although this seems to adhere initially, it will not be long before it (and maybe you) 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. 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. Tubes don’t seem to display the movement capability as often as I remember they used to, but you could always seek out a manufacturer’s data sheet. The down side of using a silicone that has been 'bulked out' is that as well as a higher than desired shrinkage when it has cured, you could get discoloration and poorer adhesion.