26 September 2013

Five methods of gluing silicone

Silicone rubber can be difficult to glue, due to the flame the surface tension which means that glue cannot normally wet it and provide adhesion. But there are some good options:

1. Primer + Cyanoacrylate (quick glue). Brush the primer onto the silicone surface, wait a few seconds for the solvent to evaporate, then glue with the instant glue. Imaginable products are Loxeal Primer 7 and Loxeal 34 quick glue. This technique works decently for small details. CA glue is hard and brittle and does not fit when you think the glue joint should be elastic, which quickly becomes the case if you glue slightly larger details of silicone rubber. The joint also does not withstand the outdoor environment particularly well.

2. Room temperature curing silicone adhesive. There are many varieties of silicone glue, but not all of them are suitable for gluing silicone rubber with. We have good experience with, for example TSE397C and RTV108. They are able to bond to all kinds of silicone rubber, often completely without a primer (provided that the rubber is clean of release agent residues and catalyst residues, see also below). TSE397C is very fast curing, but RTV108 is FDA rated. However, you get the best gluing results with IS5628E, which is specially developed for gluing silicone rubber with. IS5628E is a very strong (tensile strength 7.5 MPa), viscous and room temperature curing adhesive, which bites superbly and provides a very durable and elastic joint. An excellent choice for joining o-rings and gaskets, silicone cloth or for repairing cracked silicone molds.

3. Heat-curing silicone adhesive. The advantage of a heat-curing product is that you get a very short curing time and can design the joint better than with moisture-curing glue (which requires that air/moisture can access the joint). The choice of the right heat-curing product, however, requires knowing which silicone rubber you are dealing with – whether it is condensation-curing or addition-curing. If you don't know it, you have to try it yourself. We generally have good experience with Addisil 6101 and TSE3221S.

4. PSA glue. This is contact adhesive based on silicone, and PSA = Pressure Sensitive Adhesive. You dispense the mixed glue on one side of the thing to be glued, join and then you have to press together. The glue provides high strength in the joint already within a few seconds, and then continues to cure for 2-3 days. The process can be accelerated with heat. The PSA glue provides good flexibility and works great even for larger details. The tear strength is relatively high, and therefore these systems are excellent for gluing gaskets together. But laminations can also be made with PSA glue as there is no limit to how wide the joints can be and the gap can be very thin. PSA529 + catalyst SRC18 is a very good alternative to 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. Pretreatment. Within this category are several methods. Larger details of silicone rubber or silicone gaskets are often manufactured with peroxide as a hardener. Peroxide is generally added in excess to get a quick cure in the mold, but unfortunately the residue acts as a release agent afterwards. Post-vulcanization solves the problem; for example 4 hours at 150 degrees C causes peroxide residues to be driven out of the rubber. Make sure that all residues can be vented out, i.e. the details must lie without overlaps. If that doesn't help and you can't find any glue that works (which is very rare), you may end up having to raise the surface tension of the silicone by e.g. corona treatment. There are companies that can treat corona as contract work; you don't need to invest in your own equipment. Corona treatment is very effective and after such treatment you can glue with most types of glue. However, perhaps silicone glue is still the best option, because silicone rubber is soft and silicone glue provides an elastic joint.

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