Some options: a)a thicker adhesive b)a primer/adhesion promoter c)stiffen up the natural rubber. It's well known that a stiffer backing on tape leads to higher peel values. i.e., If you coat the same adhesive with the same thickness on a 20 lb paper and on a 80 lb paper, the latter will give you a significantly higher peel strength. There is and there isn't a certain degree of gamesmanship in this approach. I won't go into the details unless you ask for them and sign a release waiver holding me not responsible for providing you with a massive information overload.
Quoting from RUBBER IN ENGINEERING (Chemical Publishing Co., 1946), page 175:
"To obtain high-performance bonds between rubbers and metals, the bonding must be carried out during vulcanization. The most important bonding methods are those dependent upon brass-plating, upon the use of chlorinated resinous materials such as chlorinated rubber, or upon the use of cyclo-rubbers."
And later, from pages 176-177:
"The most important commercial method of bonding is undoubtedly that depending upon the adhesion of rubber to brass. Excellent adhesion is obtained and the bond is not heat sensitive. Not all types of brass will bond to rubber and the actual composition appears to depend to a large extent on the reactivity of the rubber compound. In this country, the ratio favoured appears to be usually about 70 parts of copper to 30 of zinc, but in the United States it tends to be rather higher in copper
-- 75/25. It should be understood, however, that bonds have been obtained with brasses of compositions differing widely from those just quoted. If it is possible to deposit a coherent and adherent brass layer on any metal or alloy, then it is possible to bond rubber to it. Iron, steel, stainless steel, brass, bronze, aluminum and duralumin can be successfully bonded either to natural or synthetic rubber in this way."
Try one of the popular nitrile/phenolic resin blends for an excellent heat setting adhesive. I used them for YEARS bonding friction composites to steel, iron, aluminum, copper, etc.
A small search on these resins brought me to 3M. It is unfortunate a previous call to customer service ended up by: We do not provide any adhesives for natural rubber...
The most promissing from 3M are the bonding films but have to check on heat activation temperature. Solvent activation with MEK or acetone is out of the question.
Sovereign seems to have a water activated film, this is good.
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