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.

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Applying the primer correctly is also crucial to getting the best results. Primer can be applied by spraying, dipping, or brushing. Each application method requires different equipment and may present challenges if only a specific part of the substrate needs to be applied with the primer. Masks may be needed to avoid discoloration or sheen in areas not molded with LSR. The spraying of primers requires a spray room with dedicated spraying equipment and ventilation equipment to manage the volatile organic compounds associated with solvent primers. Applying primer with a brush requires applying each part by hand, a very laborious process that increases cycle time. Immersing parts in a primer bath/tank is economical, but there are challenges in controlling film thickness and bath contamination.

Regardless of the method used, most primers require a drying step to evaporate the solvent. At room temperature, it usually takes 30 minutes or less; If drying is done at a slightly increased temperature (e.g. 65 degrees Celsius), it may only take a few minutes. After the primer is applied and dried, the plastic parts can be loaded into the LSR mold for LSR injection. The time between the application of the primer and the formation of the LSR (called coating stability) depends on the chemical properties of the primer used. However, for traditional primers, the retention time is usually very short (usually less than 5 hours).

Using a primer has become the preferred method for bonding LSR to difficult substrates. However, primers greatly increase manufacturing costs and cycle time.

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Self-bonding LSR and LSR bonding additives

When using a self-adhesive LSR or a standard LSR in combination with a bonding additive, the LSR can be injected directly into the plastic part through a wrap molding or double injection method, eliminating the need for part removal and primer construction. For double injection molding, it is not necessary to clean the thermoplastic substrate before forming the LSR because the thermoplastic components do not pop out of the mold.

Depending on the thermoplastic used and the bond strength required, the bond strength can be improved by flame or plasma treatment of the plastic assembly before forming the LSR.

Self-adhesive LSR and standard LSR and additives offer similar manufacturing improvements over conventional primer systems, but there are other advantages and disadvantages to consider between the two. Self-binding LSR provides adhesion to a specific substrate and is usually available in a limited hardness range. Depending on the silicone manufacturer, most self-bonded LSRS fail after 6 months from the date of manufacture or degrade to negligible bonding strength.

Bonding additives can be used with any hardness range of LSR and a variety of plastic substrates, but it is recommended to characterize their compatibility given the possible material combinations. Bonding additives usually have a shelf life of one year. The shelf life of non-self-bonded LSRS is typically twice as long as that of self-bonded LSRS. As a result, shelf life is usually longer when bonding additives and non-self-bonding LSRS are used.

Because bonding additives are used in conjunction with the more common and readily available standard LSRS, the combination of standard LSRS with bonding additives can result in significant savings in raw material costs, often making them more cost-effective than using primed or self-bonded LSRS.

The lead time associated with self-bonding LSRS is also an important consideration. Self-bonded LSRS is typically only available in a specific hardness range for a specific substrate, with delivery times varying according to regional availability. Some manufacturers report delivery times of 10-18 weeks. Standard LSRS have shorter lead times and, when used with bonding additives, reduce their cost by up to 50% compared to self-bonding LSRS.

From the touch of a button on electronic devices to valves and seals in critical medical devices, an increasing number of products on the market today require LSR. In a wide variety of applications, manufacturers should carefully consider and select the best adhesive for the initial product, taking into account initial costs, process steps, availability, etc. Once an adhesive has been selected, knowing the best practices involved can not only ensure a durable finished product but also improve the company’s return on investment.