Adhesive Bonding of Rubbers

Manufacturer: We are bonding vulcanized rubbers with adhesives and observe that with Neoprene and Nitrile rubbers there is lesser problem compared with EPDM and silicone rubber. Please explain why and how can we improve the bonding of EPDM and silicone rubber?

John Woon (Senior Latex Consultant):  How the adhesive bonding of fully cured rubbers performs will depend greatly on the rubber's surface characteristics. 

Polar rubbers with a relatively high surface energy (e.g. Neoprene/polychloroprene or Nitrile) generally require only cleaning and mechanically abrasion prior to bonding because most adhesives will easily wet the surface of these rubbers.

However, rubbers having a relatively low surface energy (e.g. EPDM or silicone rubber) must be chemically pretreated before being bonded. This is to raise the surface energy (or sometimes a compatible primer must be used). 

The following are the various pretreatments you can use:  

Chlorination - The rubber is immersed in concentrated hydrochloric acid - sodium hypochlorite solution. Improved adhesion is due to a combination of surface modifications including surface roughening, increased surface energy with the formation of polar groups.

Cyclization - Some rubbers require "cyclizing" with concentrated sulfuric acid until hairline fractures are formed on the surface. Cyclizing slightly oxidizes and hardens the surface resulting in wetting and adhesion.

Plasma - Plasma discharge can be used to clean and / or chemically modify the surface of a rubber surface. This has been found effective for silicone rubber and styrene butadiene copolymers. 

Primers - Primers are also often used on low energy rubbers such as fluoroelastomers, EPDM, and silicone rubber. The primer acts as a "sandwich" intermediate layers which provides good adhesion to the rubber and a more wettable surface for the adhesive.