Viscoelasticity and Pressure Sensitive Adhesive
No pun intended, but it is hard to separate the relationship between viscoelasticity and pressure sensitive adhesive (PSA). What exactly is viscoelasticity and why is it important in a PSA? Let's try to take a potentially complex issue and break it down.
What is Viscoelasticity?
Viscoelasticity is a blend of the words viscosity and elasticity.
- Viscosity refers to the resistance a liquid has to flow, generally related to the thickness of the liquid.
- Elasticity is the ability for a solid to return to its original shape once strain is removed.
Examples of elastic materials include viscoelastic foam, rubber bands, stress balls and bungee cords. High viscous materials are generally thick liquids or gels. Honey, motor oil, and syrups are considered high viscosity liquids. Low viscosity liquids flow easily, like water.
The Affect of Heat on Viscosity
Heat reduces viscosity which you can see in daily life. Oil loses viscosity as it heats in an engine. Maple syrup will flow easier when warmed. Gels will even lose their form when subjected to heat. This is an important fact to keep in mind when discussing the relationship between viscoelasticity and PSA. Heat has an effect on both.
The Affect of Strain on Elastic and Viscous Material
Elastic materials strain when stretched, and then return to their original shape when that strain is released. Since highly viscous materials resist flow, they are strained with pressure and over time.
Viscoelastic materials display the characteristics of both highly viscous and elastic material. They react to a time-dependent strain.
Viscoelasticity is a molecular rearrangement. When stress is applied to a polymer, for example, portions of the long polymer chain actually change positions. This movement is referred to as creep. Polymers remain solid even through this rearrangement with back stress allowing them to return to their original shape once stress is removed. This is demonstrated in many of the materials used in foam fabrication. Polymers can be designed to create varying lags in strain, providing differing qualities of the foam. This can increase or decrease insulation values, sound proofing effectiveness or levels of comfort.
Characteristics of Pressure Sensitive Adhesives
While characteristics of viscoelastic materials will be described using factors like creep, stress and strain, Properties of a pressure sensitive adhesive are described in shear resistance, tack and peel strength.
- Tack: The ability to form a bond with another material with light pressure quickly. It defines how sticky a substance is.
- Shear Resistance and Peel Strength: These describe the varying increases in bonding due usually to increased contact time and pressure. Temperature can also play a role in this bonding.
Since most adhesives have viscoelastic-like behaviors, the relationship is a close one. In fact, tack, shear and peel are used to describe the role viscoelastic properties play in the creation of a PSA. These properties and the performance of a PSA by using a somewhat complex Rheological concept of the four-quadrant window can be used in the development of a PSA with varying characteristics. Rheology is a branch of physics that deals with the flow of matter in liquids and soft solids. It is used extensively in the creation of Polymers and PSA as well as in material used for flexible foam fabrication processes.
The point is, polymers used in foam and the production of PSA materials are a science. It allows us as a company to deliver the flexible foam products that will work best for your specific application. There will be far different material for memory foam used in a hospital bed to that of foam packaging. The foam used for soundproofing a music studio is manufactured and fabricated far differently from that used in sports equipment. Through science, experience, and a true concern for the needs of our clients, we'll continue to provide exacting products.
Have Questions about Viscoelasticity or Pressure Sensitive Adhesive?
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