Exploring Adhesives Used in Pressure-Sensitive Adhesive Tapes

 

 

By Christoph Nagel Director, Research & Development – tesa tape North America When considering the components that affect the performance of pressure-sensitive adhesive (PSA) tapes, it is vital to glean an understanding of the adhesive systems that are most often incorporated in the tape production process. In this article, we will explore varying types of adhesive systems and the unique characteristics of each. For champions that work inside the pressure-sensitive adhesive (PSA) tape industry or those that promote PSA products, it is likely that the following comments have been presented to you: “The tape did not work in the application, it failed”; “What kind of adhesive did you use?”; “It was just a hot-melt adhesive.” The comment that “it was just a hot-melt adhesive” did not lend any information regarding the type of adhesive, which is important knowledge to have when trying to discern the reason the tape failed to perform as desired. A Closer Look at Adhesives for PSAs

There are three primary groups of adhesives that encompass the lionʼs share of adhesives used in

PSA production. These three groups are: natural rubber adhesives, synthetic rubber adhesives, and acrylic adhesives. Within these groups, there are countless variations and distinctive features. As such, it is not possible to predict the exact performance of an adhesive simply by knowing the type of adhesive. There are, however, specific characteristics that are typical of every adhesive type, and being aware of them can prevent making the wrong tape decision for the specific application.

____________________________________________

Natural Rubber Adhesives ____________________________________________ Natural rubber (NR) is the first known adhesive technology used with PSAs. The biggest and perhaps most well-known use of natural rubber is in the production of car tires. NR by its very nature is not tacky, nor is it an adhesive. Tackifier resins are always added to NR to create a PSA. A key advantage of NR is a bi-product of one of its characteristics: very long polymer chains that are entangled with each other. This feature allows the natural rubber to be very flexible,

even at lower temperatures, due to the length of the polymer. Additionally, the entanglement of the polymers creates an inner strength of the NR. These characteristics allow for a wide-application window for NR adhesives. The type of rubber used, the compounded resins, and other ingredients determine the performance of the NR adhesive. As a general rule, natural rubber PSAs offer the following advantages: • High initial tack or “grab” • Excellent peel adhesion on non-polar and polar surfaces • Short dwell time to final adhesion level • Clean removal after application (e.g. masking tapes) NR adhesives have some disadvantages, which are important to know when determining whether or not the product is suitable for the specific application: • Poor cohesion at higher temperatures (T>70°C) • Poor aging resistance • Low environmental resistance (needs stabilizers) to protect against UV, ozone, etc. • Poor chemical and solvent resistance For many non-permanent, especially indoor applications, NR adhesives are an excellent choice. ____________________________________________

Synthetic Rubber Adhesives ____________________________________________ There are several synonyms for synthetic rubber (SR) adhe-sives, such as SBS, SIS, or SEBS. The SR polymer for adhesives shows a very unique molecular structure. There are two hard-end segments (Styrene) and a softer, rubbery middle segment (Butadiene), so the structure is: Styrene – Butadiene – Styrene = SBS.

The styrene-end segment determines the cohesiveness (internal strength) of the adhesive; the rubbery middle segment determines the adhesiveness. By itself, SR does not harbor any adhesive properties. To create a PSA, tackifier resins need to be added. In contradiction to natural rubber polymers, the SR polymer has a short length and a low molecular weight. The advantages of SR adhesives are: • High initial tack or “grab” (like natural rubber) • Very high initial bond to substrate possible (like natural rubber)

• Excellent peel adhesion on polar and non-polar surfaces • Transparent coatings possible (in contrast to natural rubber) • Low cost (due to high speed compounding + coating process); this point is often the reason why SR adhesives are very often called hot-melt adhesives. The disadvantages of SR adhesives are: • Low resistance to elevated temperatures • Poor aging resistance, but better than natural rubber • Low environmental resistance (needs stabilizers) to protect against UV, ozone, etc. • Moderate solvent resistance ____________________________________________

Acrylic Adhesives ____________________________________________ Acrylic adhesives are industrially produced by polymerization, otherwise known as a customized formulation. Adhesive properties of acrylic can be adjusted and controlled by manipulating the selection of monomers used in the formulation, as well as the polymerization process. In contradiction to NR and SR adhesives, acrylic adhesives are tacky by nature. Acrylic adhesives have long polymer chains that are cross-linked. This cross-linking feature catapults acrylic adhesives into a much higher performance category than both natural rubber and synthetic rubber adhesives. The advantages of acrylic PSAs are: • High transparency • High-aging resistance (do not yellow) • High-temperature resistance • High resistance to UV, ozone, and moisture • High resistance to solvents and plasticizers • Very good peel adhesion on polar surfaces Disadvantages are: • Low immediate peel adhesion • Longer dwell time • Lower adhesion on non-polar substrates To create acrylic PSAs with increased immediate peel adhesion or reduced dwell time, tackifier resins are often added to the acrylic polymer. The addition of resins, however, will change the cohesiveness of the acrylic adhesive. Due to the fact that the acrylic adhesive is cross-linked and shows excellent resistance to aging and temperature, acrylics are the best adhesive solution for long-term, outdoor applications.

 

tesa tape, inc. • 5825 Carnegie Boulevard • Charlotte, NC 28209 • Phone: 704-554-0707 • Toll Free: 800-426-2181 Email: customercare@tesatape.com • Web site: www.tesatape.com

ISO 9001:2008 Certified • ISO/TS 16949:2009 Certified • ISO 14001:2004 Certified

 

Aside from the three major types of adhesives used in tape production, there are a few lesser used and lesser known types of adhesives on the market, such as: • Silicone adhesive (high-temperature resistance, chemically inert) • Butyl adhesives (corrosion protection, roof sealing) • Ethylene-vinyl acetate adhesives EVA (surface protection) Your tesa representative can offer additional information regarding these adhesives should they come into question. In conclusion, it is always wise to determine the specific needs of the application in order to recommend the best tape / adhesive combination.