Why Hot Melt Pressure Sensitive Adhesive are Tacky
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.
Apart from offering aggressive bonds at room temperature, a hot melt pressure sensitive adhesive’s most unique and defining feature is that it is capable of cold flow under a light finger pressure at room temperature. 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 hot melt pressure sensitive adhesive formulations through trial and error tests. All successfully formulated hot melt pressure sensitive adhesives adhere to the following theory without exception.
An amorphous (non-crystalline) polymeric material possesses a glass transition temperature, 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 the transition temperature appears in the vicinity of room temperature and the value of Tangent delta is greater than one, the material cold flows and is pressure sensitive. And this then, will be made into hot melt pressure sensitive adhesive.
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