Injection-molded liquid silicone rubber (LSR) is used in the manufacture of many goods, from medical devices to cookware to electronics. Many LSR applications require that the silicone be bonded to plastic substrates. The ultra-low surface energy and chemical resistance of LSR, however, has historically made it difficult to bond to other materials.
Three common methods to bond LSR to thermoplastic substrates are presently in use. Primers have long been used to enable bonding of LSR to substrates. Self-bonding LSRs, when introduced to the market more than two decades ago, offered significant time savings over primers. Recently, a new technology has been introduced that makes the bonding process even faster and more cost effective by imparting self-bonding properties to standard LSRs. This new adhesion additive technology can be easily dosed into common non-self-bonding LSR, imparting self-bonding capability. This adhesion additive technology uses typical additive dosing systems, commonly used with colorants. The recommended adhesion additive dosing level is 1wt% to non-self-bonding LSR. The additive works without impacting the physical properties of the LSR and can achieve primerless adhesion to thermoplastics, including polyamide (PA), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), polyphthalamide (PPA), and more.
Whatever method is being used—primer, self-bonding LSR, or standard LSR used with an adhesion additive—knowing the capabilities of each can aid in designing for manufacturability, material selection, and cost savings.
Primers have long been used to enable bonding of LSR to substrates. If using a primer, the plastic component being bonded must first be injection molded. After molding, the part will be ejected and cleaned to ensure the surface is free of organic and inorganic containments (i.e., grease, dirt, rust, oils, and oxide layers) and then coated with the primer. Surface preparation is one of the most important factors influencing adhesion in the bonding process.
Proper primer application is also essential to achieve optimal results. Primers may be applied by spraying, dipping, or brushing. Each application method requires different equipment and can be challenging if the primer only needs to be applied to a specific part of the substrate. Masking may be required to avoid potential discoloration or sheen in areas not over molded with LSR. Spray application of primers requires spray booths with specialized spray equipment and ventilation to manage the volatile organic compounds associated with solvent primers. Applying primer by brush requires coating every part by hand, a very labor-intensive process that increases cycle times. Dip methods, where parts are submerged in a bath/tank of primer, are economical but challenges exist controlling film thickness and avoiding fish-eye defects and bath contamination. If using a moisture-sensitive primer, avoid using a dipping method as it will limit the life of the primer.
Regardless of application method, most primers require a drying step to evaporate the solvent. At room temperature, this typically takes 30 minutes or less; if drying at a slightly elevated temperature (e.g., 65 degrees C), it may only take a few minutes. After the primer has been applied and dried, the plastic part can be loaded into an LSR mold for LSR injection. The time between the primer application and LSR molding, known as layover stability, varies depending on primer chemistry used; however, the layover stability is typically quite short for traditional primers (usually less than 5 hours).
Use of a primer has been the go-to method for bonding LSR to difficult substrates. However, primers drastically increase manufacturing costs and cycle times.
Self-Bonding LSRs and LSR Adhesion Additives
When using a self-bonding LSR, or a standard LSR coupled with an adhesion additive, the LSR can be injected directly onto the plastic part by over-molding or two-shot injection molding, eliminating the part removal and primer application steps. With two-shot injection molding, it is not necessary to clean the thermoplastic substrate prior to molding the LSR as the thermoplastic component will not be ejected from the mold.
Depending on the thermoplastic being used, and desired bond strength, bonding strength can be increased by employing a flame or plasma treatment to the thermoplastic component prior to LSR molding.
Self-bonding LSR and standard LSR with an additive offer similar manufacturing improvements over a traditional primer system, but there are other advantages and disadvantages to consider between the two. Self-bonding LSRs provide adhesion to specific substrates and are usually available in limited durometers. Depending on the manufacturer, most self-bonding LSRs expire, or degrade to negligible bond strength, 6 months from the date of manufacture.
Adhesion additives can be used with any durometer of LSR and various plastic substrates, but characterizing its compatibility is recommended given the potential material combinations. Adhesion additives generally have a shelf-life of one year. The shelf-life of non-self-bonding LSR is generally two times longer than their self-bonding counterparts. As a result, when using adhesion additives and non-self-bonding LSRs, shelf-life is typically longer.
Since adhesion additives are used with standard LSRs, which are more common and readily available, the standard LSR coupled with an adhesion additive results in a significant raw material cost savings generally making it a more cost-effective solution than using primers or self-bonding LSRs.
Lead times associated with self-bonding LSRs are also important considerations. Self-bonding LSRs are typically available only in specific durometers targeted at specific substrates, with lead times varying based on regional availability. Some manufacturers have reported lead times of 10-18 weeks. Standard LSR has shorter lead times, and when coupled with the adhesion additive, can cost up to 50 percent less than self-bonding LSRs.
An expanding range of products on the market today call for LSR, from the soft-touch buttons on electronics to valves and seals in critical medical equipment. With such a variety of applications, manufacturers should carefully review and select the best bonding agents for their individual products, considering initial cost, process steps, availability, and more. Once a bonding agent has been selected, understanding the associated best practices will not only ensure a durable finished product but will improve the company’s return on investment.