Tape manufacturers usually use pressure sensitive adhesives or PSAs on their tapes. These adhesives require pressure to adhere to the substrate. This is why applying the right amount of pressure is so important to achieve good adhesion. PSA’s usually gain strength over time by further wetting the substrate. The adhesive quite literally flows into microscopic pores and crevices on the substrate along with removing air bubbles. To achieve high peel strength, the adhesive on the tape not only has to adhere to the substrate but also has to be viscoelastic.
Most pressure sensitive adhesives are applied to a plastic, fabric or paper backing. These tapes are either made with adhesive on one side (single-faced tape) or on both sides (double-faced tape). A major drawback of most PSAs is their relatively low bond strength and lack of creep resistance. For this reason, they are not suitable for structural bonding. However, in recent times, some high bond strength double-faced tapes have been developed for semi-structural applications. These adhesives are typically applied to a foam carrier which improves adhesion to rough surfaces, contours, and gaps in the joint area.
Depending on the application, manufacturers of tapes choose adhesives mainly from five different classes. There are many variations within each adhesive class. Below is brief description of the main types of PSA’s and some general guidelines that might be helpful to choose the right tape for the attended application.
Natural rubber (NR) was the first elastomer used in the earliest pressure sensitive adhesives and still remains one of the major elastomers used in the manufacture of tape adhesives, so other types of elastomers, such as styrene-butadiene rubbers and SIS block copolymers, have replaced them in many applications.
Rubber is often a good choice for less expensive types of tapes. It has good initial adhesion properties, but doesn't have much creep resistance unless some method like partial crosslinking (vulcanization) has been employed to increase the cohesive strength. Crosslinking will also improve the holding power, and the solvent and heat resistance of natural rubber based adhesives.
Natural rubber based PSAs are often perfectly adequate for applications where the adhesive is subjected to low shear stress. They can be used at ambient and low temperature situations. The low cost of the adhesive together with a inexpensive backing makes them ideal for short-term tape applications. Besides natural rubber, many other rubbers are widely used in the manufacture of pressure sensitive tapes. They are synthetic rubbers derived from various monomers (isoprene, butadiene) and sometimes blended with natural rubber to improve its durability, oxygen and heat resistance.
Natural rubber PSAs, unlike PSA acrylics, have very little tack and have to be formulated with tackifiers and plasticizers to achieve pressure sensitive properties. Often other ingredients such as antioxidants, pigments and fillers are added to lower cost and to tailor and optimize their properties.
Styrene-butadiene rubbers or SBRs are widely used in the manufacture of pressure sensitive tapes. They are synthetic elastomers derived from styrene and butadiene. SBRs, whether solventborne or waterborne, are found in many PSAs and are tailored for variety of applications. Their properties depend on the percent of bound styrene, the average molecular weight and its distribution, and the presence of functional groups introduced during polymerization. In general, styrene-butadiene based PSAs have better heat-aging properties then NR based PSAs, whereas their physical strength, resilience and low temperature properties are usually inferior to natural rubber based PSAs. SBRs are sometimes blended with natural rubber to achieve the optimal properties.
Styrene-block copolymers or SBCs of type A-B-A and A-B have become one of the most important elastomers for pressure sensitive adhesives and have replace natural rubber based PSAs in many applications. They consist of (thermoplastic) polystyrene endblocks, while the B block consists of an elastomer, in most cases polyisoprene or polybutadiene. The most important SBCs are polystyrene-polyisoprene-polystyrene (SIS) and polystyrene-polybutadiene-polystyrene (SBS). The two blocks are incompatible and form two distinctive phases. This class of PSAs is very versatile and is used for a broad range of applications. Like natural rubber and unlike PSA acrylics, they have very little tack, and therefore, have to be formulated with tackifiers and plasticizers. They can be tailored for a variety of applications and have excellent performance properties. For example, they can be formulated to give aggressive tack, tailored peel strength and high cohesive strength.
Acrylic adhesives are another important class of pressure sensitive adhesives. They usually have better heat and oxygen resistance than rubber based adhesives, mainly due to the absence of double bonds in the rubber. Acrylic adhesives can be made with a wide variety of adhesion properties, from low-adhesion and low tack to very aggressive tack and to bond permanently to the substrate. The temperature range of acrylic adhesive tapes is much broader than that of rubber-based PSAs. They usually have excellent heat and oxygen resistance, often allowing for tapes to withstand temperatures of 250°F to 300°F (120 - 150°C). Many acrylic-based PSA’s also have good adhesion properties at low temperatures, making them suitable for low temperature applications as well.
Silicone adhesives are usually the most expensive PSA’s, but they have some of the best properties for high-performance applications. For example, they can withstand temperatures up to 500 °F. Silicone based pressure sensitive adhesives are often applied to more expensive carriers like glass cloth, Kapton and aluminum foils to make them suitable for higher temperature applications. Silicone based adhesives can also be used for very low temperature applications, down to -100 °F. It is also the only type of PSA that will stick to low surface energy substrates like silicone or silicone coated products such as release liners. It's chemical resistance is generally excellent.
1The global pressure sensitive adhesive market created a turnover of about US$ 50 Billion in 2017, and is expected to reach US$ 68 Billion by 2022. Source: PRNewswire, Dublin, November 3, 2017