General properties of thermoplastic adhesives
When employed as adhesives, thermoplastic resins are used in the form of solutions, dispersions in water, or solids. They usually set by purely physical means. When applied as solutions of dispersions, adhesion follows evaporations or absorption of the liquid phase, a process which is often included in the expression “solvent activation.” When applied by melting and cooling the solid the terms “Hot-melt” or “Melt-freeze” are used to describe the method of use. Hot melt applications is the fastest practicable way of sticking parts together and gives higher joint strengths than application of the adhesive in the form of a solution. Although the term “setting” and “curing” are generally used synonymously in connecting with both thermoplastic and thermosetting adhesives, the term “setting” is the more common with thermoplastic adhesives unless a chemical process such as polymerization is involved, when the term “curing” is perhaps more appropriate.
Thermoplastic adhesives fall into many chemical classes but they all comprise predominantly linear macromolecules. Because of this common feature their properties are in many cases similarly influenced by factors such as molecular weight and distribution, concentrations of plasticizers and other additives and , in copolymers, the monomer ratio.
A change in one or more of these factors will in general bring about a predictable change in properties of the polymer regardless of its chemical class, and consequently the effect of these factors will not be discussed for each adhesive but will be mentioned only where special reasons make it desirable.
Most thermoplastic adhesives are capable of bonding a wide variety of materials such as paper, wood and leather. A smaller number are capable also of bonding rubbers, metals and some plastics, the most notable exceptions being the silicone and fluorocarbon plastic.
The five most important chemical classes of thermoplastic adhesive are:
- The cellulosic plastics
- The vinyl polymers, include of polystyrene
- The acrylic polymers
- The synthetic rubbers
- The Polyamides