According to the Pressure Sensitive Tape Council, the definition for a Pressure Sensitive Adhesive (PSA) requires two things:
1. Aggressive and permanent tack
2. Adhesion with finger pressure
When considering industrial bonding applications using pressure sensitive adhesives (bonding tapes), there are three main chemistries used to provide durable assemblies. Tapes in these chemistries have aggressive, permanent tack and the ability to adhere with finger pressure alone. (But please note that when actually creating an assembly, you should use a roller for quality and consistency!) As with liquid adhesives, there are many other chemistries that can be used to create a bonding tape, and as with anything, there are many exceptions to the information provided. Our goal is to provide a high level overview of each chemistry to allow comparison. There are many other factors to consider when choosing the proper tape or adhesive for use in an assembly.
The three common chemistries for tapes are:
Development of acrylics for pressure sensitive adhesives began because of the tendency of natural rubber to yellow and degrade when exposed to UV. Acrylic chemistry also tends to be naturally tacky and doesn’t require tackifiers to the extent that rubber chemistry does.
Acrylic tapes are the workhorse of industrial bonding. The ever-changing capabilities of acrylic chemistry allow formulations to provide high or low temperature performance, adhesion to LSE materials, varying thickness, or even unique properties such as flame retardancy. These tapes are found in nearly every industry.
Unlike “acrylics” used for liquid adhesives, the acrylic chemistry on a tape is provided fully cured so there is no additional curing that takes place in order to form a bond. The bond is created by using primarily mechanical interlocking in addition to some Van der Waals interactions and hydrogen bonding at the interface.
While silicone-containing polymers are used to formulate the very sticky adhesive itself, different silicone chemistry is used to create a non-stick polymer used in tape release liners!
Silicone tapes are best known for their ability tolerate a very wide range of temperatures – from -40°F (-40°C) to over 500°F (260°C). They bond very well to silicone (as expected) as well as fluorinated materials such as PTFE and oleophobic (oil resistant) coatings. As more electronics become resistant to fingerprints, more silicone-based tapes are used to bond those materials.
“Crosslinking” is the common term for forming a polymer from a lower-molecular weight monomer. However, when that is done with rubber it is called “vulcanization”, a term derived from the Roman god of fire, Vulcan. In prehistoric times, rubber was crosslinked with intense heat alone.
While many of the rubbers used for adhesives are naturally derived (such as from the Hevea rubber plant), “rubber” may also refer to synthetic materials such as polychloroprene (Neoprene) or various block co-polymers (e.g. SBR). Their ability to be “tackified” makes them attractive, low-cost solutions for large surface lamination bonds or bonds requiring high tack or clean removability.
Natural rubber (poly cis-isoprene) is mechanically worked to provide lower molecular weight polymers that can be readily dissolved or dispersed in a solvent. Synthetic polymers (such as styrene-isoprene block copolymers) may also be used. Tackifiers such as pinene (from pine sap, among other sources) are added to give the adhesive additional tackiness allowing it to be used as a PSA.
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