Injection molded liquid silicone rubber (LSR) can be used to make many products, from medical devices to cookware to electronics. Many LSRS need to be bonded to plastic substrates, but LSR’s ultra-low surface energy and chemical resistance have historically made it difficult to bond with other materials.
Three common methods of bonding LSR to thermoplastic substrates are currently being used. Primers have long been used to bind LSR to the substrate. Self-binding LSR has been on the market for more than 20 years and offers significant time savings compared to primers. Recently, a new technology has been introduced that makes the bonding process faster and more cost-effective by imbuing a standard LSR with self-bonding properties. This new bonding additive technology can be easily added to ordinary non-self-bonding LSRS for self-bonding capabilities. This adhesive additive technique is usually used with colorants using typical additive metering systems. The recommended bonding additive dose is 1wt % relative to non-self-bonding LSR. The additive can work without affecting the physical properties of the LSR and enables brimless bonding to thermoplastics, including polyamide (PA), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), and polyphthalamide (PPA).
Regardless of which method is used, primer, self-bonding LSR or standard LSR used with bonding additives, understanding the capabilities of each method can help design manufacturability, material selection, and cost savings.
Primers have long been used to bind LSR to the substrate. If a primer is used, the plastic parts to be bonded must first be injected. After forming, the part is ejected and cleaned to ensure that the surface is free of organic and inorganic materials (i.e. grease, dirt, rust, oil, and oxide layers), then a primer is applied. Surface treatment is one of the most important factors affecting adhesion in the bonding process.