PVC windows should always be sealed with Neutral Cure Low Modulus Sealants.
Silicone Sealants could be of two kinds namely Neutral Cure Sealants and Acetoxy Cure Sealants and could be further segregated into high modulus and low modulus sealants. These terms are defined as
Neutral Cure Sealants– Neutral silicone sealants release alcohol as they cure, and has almost no smell. They have much better adhesive properties for a greater number of materials including PVC-U, most other plastics, glass, aluminium, lead, stone and masonry. It cures with atmospheric moisture and skins over in about 30 minutes, and leaves a shiny finish.
Acetoxy Cure Sealants– Acetoxy silicone sealants release acetic acid (which smells a little like vinegar) as they cure. It is more rigid and the full cure is quick. It generally has poor adhesion qualities.
High Modulus Sealant–This kind of sealant is rigid and less flexible.
Low Modulus Sealant– A low modulus sealant will require low force to stretch it so it has better elasticity and flexibility for movement. In the building industry is usually considered that linear contraction and expansion is high so low modulus sealants are more acceptable.
Hence low Modulus Neutral Cure Silicone is best for PVC windows as it is flexible, allows high movement and has excellent adhesion.