Silicone rubber can be difficult to bond, due to the low surface tension which means that glue cannot normally wet it and provide adhesion. But there are some good alternatives:
1. primer + cyanoacrylate (quick glue). Brush the primer onto the silicone surface, wait a few seconds for the solvent to evaporate and then glue with the instant adhesive. Possible products are Loxeal Primer 7 and Loxeal 34 quick glue. This technique works reasonably well for small parts. CA glue is hard and brittle and is not suitable when you intend the glue joint to be elastic, which quickly becomes the case if you glue slightly larger details of silicone rubber. The joint also does not withstand outdoor environments very well.
2. room temperature curing silicone adhesive. There are many types of silicone adhesives, but not all are suitable for bonding silicone rubber. We have good experience with, for example TSE397C and RTV108. They can bond to all types of silicone rubber, often without the need for a primer (provided the rubber is clean of release agent and catalyst residues, see also below). TSE397C is very fast curing, but RTV108 is FDA classified. However, the best adhesive results are obtained with IS5628E, which is specially developed for bonding silicone rubber. IS5628E is a very strong (tensile strength 7.5 MPa), viscous and room temperature curing adhesive, which bites extremely well and produces a very durable and elastic joint. An excellent choice for joining o-rings and gaskets, silicone cloth or for repairing cracked silicone moulds.
3. Thermosetting silicone adhesive. The advantage of a thermosetting product is that you get a very short curing time and can design the joint better than with moisture-curing adhesives (which require that air/moisture can access the joint). However, choosing the right thermosetting product requires knowing which silicone rubber you are dealing with - whether it is condensation curing or addition curing. If you don't know, you'll have to experiment. We have generally good experience with Addisil 6101 and TSE3221S.
4. PSA adhesive. This is contact adhesive based on silicone, and PSA = Pressure Sensitive Adhesive. You dispense the mixed adhesive on one side of what is to be glued, join and then you have to press together. The adhesive provides high strength in the joint within seconds, and then continues to harden for 2-3 days. The process can be accelerated with heat. PSA adhesives offer good flexibility and work well even for larger parts. The adhesive strength is relatively high, so these systems are ideal for bonding gaskets together. However, laminations can also be made with PSA adhesives as there is no limit to how wide the joints can be and the gap can be very thin. PSA529 + catalyser SRC18 are a very good alternative for silicone rubber (as well as a variety of other materials, including stainless steel sheet). The disadvantage of PSA adhesives is that they are solvent-based and must be handled in fume cupboards.
5. pre-treatment. Within this category there are several methods. Larger parts made of silicone rubber or silicone gaskets are often manufactured using peroxide as a curing agent. peroxide is generally added in excess to get a quick cure in the mould, but the residue unfortunately acts as a release agent afterwards. Post-curing solves the problem; e.g. 4 hours at 150 degrees C will drive peroxide residues out of the rubber. Make sure that all residues can be ventilated out, i.e. the parts must lie without overlaps. If this does not help and you cannot find an adhesive that works (which is very rare), you may end up having to increase the surface tension of the silicone by e.g. corona treatment. There are companies that can do corona treatment on a subcontracting basis; you do not need to invest in your own equipment. Corona treatment is very effective and after such treatment you can glue with most types of adhesives. However, silicone glue is perhaps still the best option, as silicone rubber is soft and silicone glue provides an elastic joint.
