Introduction to Fracture Interfacial Fracture Soft Material Fracture Cavitation
Polymer Introduction Common Polymers Rheology Entanglements in polymers
Pressure Sensitive Adhesives Classes of Pressure Sensitive Adhesives Bonding of Pressure Sensitive Adhesives Chang Visco-elastic Windows
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Contents

Classes of PSAs

PSAs encompass a diverse range of materials that can be broadly categorized based on their chemical composition, namely: acrylics, natural rubber, styrenic block copolymers, and silicones. The choice of chemical composition depends upon the chemical compatibility of the substrate, the mechanical demands of the bond, and cost. Further considerations are necessary for functional adhesives, such as those used in transdermal drug delivery or heat dissipation.

Acrylic-based PSAs

Acrylic-based PSAs are the most widely used class of PSAs, primarily due to their suitability for various applications and the simplicity of formulation. They are synthesized through the copolymerization of soft (low $T_g$), hard (high $T_g$), and functional acrylic monomers 1, typically in the range of 90–97 wt.% soft and acrylic acid (hard and functional) 3–10 wt.% monomers 2. The large choice of comonomers enables precise tailoring of mechanical and adhesive properties 1. Finally, acrylic monomers can undergo controlled reversible deactivation reversible polymerizations (RDRP) reactions, enabling more fine-tuned control of their molecular architecture (i.e., bottle brush structures).

Acrylates

Natural Rubber-based PSAs

Natural rubber-based PSAs are composed of cis-1,4 polyisoprene and tackifier additives, which soften the PSA 3‧4‧5. These are easily mixed (rather than copolymerized). Natural rubber-based PSAs exhibit high cohesive strength due to strain-induced crystallization, and in contrast to acrylics, they can adhere to low surface energy substrates such as plastics 4‧5. Natural rubber-based PSAs were the earliest PSAs in production and are still very common as they are the simplest and cheapest to produce 6.

natural rubber

Styrenic Block Copolymer-based PSAs

Styrenic block copolymer (SBC) based PSAs are typically formulated as blends of styrene-isoprene-styrene triblocks and styrene-isoprene diblocks combined with tackifier resin. These PSAs exhibit nanophase separation, with styrene domains dispersed within an isoprene matrix, where the styrene domains act as physical cross-links 6. The adhesive properties are tailored by controlling the characteristics of the nanophase domains 7‧8. The physically cross-linked nature of SBC-based PSAs makes them thermoplastic elastomers, which can be processed above the order-disorder temperature of the block copolymer 7.

Styrene Isoprene

Silicone-based PSAs

Silicone-based PSAs are considered high-performance due to their excellent stability over a much wider range of temperatures than acrylic and natural rubber-based PSAs 1. Similarly to natural rubber, they can adhere to low surface energy substrates. Typically a silicone gum polydimethyl siloxane (PDMS) is copolymerized with a silanol-functionalized siloxane resin (MQ resin). The PDMS gum provides elasticity, whereas the resin behaves as a tackifier 9.

PDMS

References

  1. Czech, Z. & Milker, R. Development trends in pressure-sensitive adhesive systems. Materials Science- Poland 23, (2005). ↩ ↩2 ↩3

  2. Mapari, S., Mestry, S. & Mhaske, S. T. Developments in pressure-sensitive adhesives: a review. Polymer Bulletin (2020). ↩

  3. Martin-Martinez, J. Natural rubber-based adhesives. Handbook of Adhesion 292–294 (2005). ↩

  4. Deng, X. Progress on rubber-based pressure-sensitive adhesives. The Journal of Adhesion 94, 77–96 (2018). ↩ ↩2

  5. Khan, I. & Poh, B. T. Natural Rubber-Based Pressure-Sensitive Adhesives: A Review. J Polym Environ 19, 793–811 (2011). ↩ ↩2

  6. Creton, C. Pressure-Sensitive Adhesives: An Introductory Course. MRS Bulletin 28, 434–439 (2003). ↩ ↩2

  7. Brown, K., Hooker, J. C. & Creton, C. Micromechanisms of Tack of Soft Adhesives Based on Styrenic Block Copolymers. Macromolecular Materials and Engineering 287, 163–179 (2002). ↩ ↩2

  8. Creton, C., Hu, G., Deplace, F., Morgret, L. & Shull, K. R. Large-Strain Mechanical Behavior of ↩

  9. Lin, S. B. New silicone pressure-sensitive adhesive technology. International Journal of Adhesion and Adhesives 14, 185–191 (1994). ↩