What is a Hot Melt Pressure Sensitive Adhesive and Why are PSAs Tacky?

Formulating hot-melt glues that flow in a cool state is as much an art as a science. September 2, 2006

This article was provided by Pierce Covert of Glue Machinery Corporation.

Pressure sensitive is a term commonly used to designate a distinct category of adhesive tapes and adhesives which in dry form (solvent/water free) are aggressively and permanently tacky at room temperature. This adhesive firmly adheres to a variety of dissimilar surfaces with mere contact and without the need for much pressure. These products require no activation by water, solvent or heat in order to exert a strong adhesive holding force with substrates such as paper, plastic, glass, wood, cement and metal. They have sufficient cohesive holding power and elastic nature so that, despite their aggressive tackiness, they can be handled with the fingers and removed from smooth surfaces without leaving a residue.

How can material offer such unique performance without water, solvent, or heat? Though pressure sensitive adhesives are made and applied daily worldwide, very few people really understand why they are so sticky.

While offering aggressive bonds at room temperature a pressure sensitive hot melts most unique and defining feature is that it is capable of cold flow under a light finger pressure at room temperature. This means that it is moveable or flowable without heat. Hot melt adhesives that do not cold flow under the same condition are not pressure sensitive.

How do adhesive manufacturers design a material to offer cold flow characteristics at room temperature? This cold flow phenomenon requires a basic understanding of viscoelasticity or rheology.

Unfortunately, most adhesive formulators do not understand rheology and are able to develop HMPSA formulations through trial and error tests. All successfully formulated HMPSAs adhere to the following theory without exception:

An amorphous (non-crystalline) polymeric material possesses a glass transition temperature (Tg), at which the molecular chain exhibits the greatest free volume or maximum flow character. When an amorphous material is heated from its glassy state to either a rubbery or a fluid state; if Tg appears in the vicinity of room temperature and the value of Tangent delta (damping factor, the ratio of loss modulus-G to storage modulus-G) is greater than one (1); the material cold flows and is pressure sensitive!

How to utilize rheological concepts and parameters to formulate or design a material to be pressure sensitive will be addressed in future articles.

This article was provided by Pierce Covert of Glue Machinery Corporation.