Injection molded liquid silicone rubber (LSR) can be used to produce a wide range of products, from medical devices to cookware to electronics. Many LSR applications require bonding to plastic substrates. But LSR's ultra-low surface energy and chemical resistance have long made it difficult to bond to other materials.

There are three common methods for bonding LSR to thermoplastic substrates. Substrates have long been used to bond LSR to substrates. Self-adhesive LSR was introduced to the market more than 20 years ago and can save a lot of time compared to base adhesives. More recently, a new technology has emerged that allows standard LSRS to acquire self-adhesive properties, making the bonding process safer and more economical. This new adhesive additive can be easily injected into ordinary non-self-adhesive LSRS to achieve self-adhesive capability. This additive technology uses standard additive injection systems commonly used in colorants. The recommended adhesive additive injection level for non-self-adhesive LSRS is 1wt%. Additives do not affect the physical properties of the LSR. Thermoplastics can be bonded without a substrate, including polyamide (PA), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), polyphthalamide (PPA), and so on.

Regardless of the method used -- adhesives, self-adhesive LSRS, or standard LSRS with adhesive additives -- understanding their properties can help with manufacturing process design, material selection, and cost savings.

1. The glue

Substrates have long been used to bond LSR to substrates. If a base adhesive is used, the plastic component to be bonded must first be injection molded. After molding, remove and clean the parts to ensure that the surface is free of organic or inorganic contaminants (such as grease, dirt, rust, oil and oxide) and apply primer. Surface treatment is one of the most important factors in bonding process.

Proper primer application is also critical for best results, including spraying, dipping or brushing. Each coating method requires different equipment. If only a specific part of the substrate needs to be coated, it may be difficult to operate. Masking may be required to avoid discoloration or flash in non-LSR coated molded areas. Spraying requires a spray room equipped with professional spraying and ventilation equipment to prevent the diffusion of volatile organic compounds from solvent-based substrates. Brushing the bottom glue needs to be applied manually on each component, which is a labor-intensive process and will increase the operation time. Impregnation involves immerging the component in one tank/barrel of bottom glue, which is very economical, but difficult to control coating thickness and avoid fisheye defects and contamination. If wet sensitive adhesives are used, impregnation should not be chosen as it will affect the service life of the adhesives.

Either way, most adhesives need to be dried to allow the solvent to evaporate. At room temperature, this takes roughly 30 minutes, but at higher temperatures (say 65 degrees Celsius) it can take just a few minutes. After the bottom glue is applied and dried, the plastic parts can be loaded into the LSR mold for LSR injection molding. The period between the application of the substrate and the forming of the LSR is known as the intermediate stabilization period, which varies depending on the chemical properties of the substrate used, but is usually very short (typically no more than 5 hours) for conventional adhesives. Base adhesives are the preferred solution for bonding LSR to complex substrates. However, the adhesive will greatly increase the production cost and cycle time.

2. Self-adhesive LSR and LSR adhesive additive

If a self-adhesive LSR or standard LSR with adhesive additives is used, the LSR can be injected directly into the plastic parts by cladding or duplex injection molding without mold removal and primer application process. With dual injection molding, there is no need to clean the thermoplastic substrate before the LSR is formed because the thermoplastic components will not come out of the mold. Depending on the thermoplastic used and the desired bonding strength, the thermoplastic component can be treated with flame or plasma prior to forming the LSR to increase bonding strength.

Self-adhesive LSRS and standard LSRS with adhesive additives provide similar production improvements over traditional adhesive systems, but there are other advantages and disadvantages to consider. Self-adhesive LSR can bond specific substrates to a limited range of hardness. According to the manufacturer, the viscosity of most self-adhesive LSRS fails or decreases significantly after 6 months out of the factory.

Bonding additives can be used for LSRS of any hardness as well as a variety of plastic substrates, and this compatibility is important considering the potential combination of materials. Adhesive additives usually have a shelf life of one year. Non-self-adhesive LSRS generally have twice the shelf life of self-adhesive products. Therefore, the shelf life is longer if adhesive additives and non-self-adhesive LSR are used.