Page 355 - Handbook of Adhesives and Sealants

310 Chapter Eight

TABLE 8.10 Various Types of Adhesives by Form
1. Solid adhesive
a. Film adhesives (unsupported)
b. Tape adhesives (supported)
c. Solid powders and preforms
d. Solvent based adhesives and primers
e. Hot melt adhesives
2. Liquid adhesives (100% solids paste and liquid)
a. One-component: long shelf life
i. heat cured
ii. cured by surface or anaerobic catalysts
iii. cured by exposure to ultraviolet (UV) light, radiation, or some other energy
source
b. Two-component: short pot life
i. room temperature cured
ii. heat cured
3. Liquid solvent containing adhesives (similar to 100% solids systems above but
with solvent for viscosity reduction)
a. Solvent based contact adhesives
b. Water based adhesives
i. contact or pressure sensitive adhesives
ii. solutions or emulsions
c. Pressure sensitive adhesives

8.2.4.1 Pastes and liquids. The difference between paste and liquid
adhesive lies mainly in viscosity and method of application. Liquid
adhesives are free flowing fluids that can be applied in thin films. They
tend to flow, spread, and sag during cure, especially at elevated tem-
peratures when their viscosity is even further reduced. Paste adhe-
sives are heavily bodied, often thixotropic systems that must be ap-
plied with spreading equipment such as a trowel or caulking gun.
Paste adhesives are formulated for gap filling and other bonding ap-
plications where sag resistance is important during the cure of the
adhesive. The following sections will describe these various adhesive
forms in more detail.

8.2.4.1.1 Two-part solventless. Two-part solventless adhesive systems
usually require proportioning and mixing immediately prior to use.
Most can cure at low temperatures (e.g., room temperature) or in
shorter times at elevated temperatures. Once mixed, two-part adhe-
sives have a limited working life. As the mixture cures, the viscosity
increases to the extent that it can no longer be applied. If the adhesive
system has a large exotherm, the working life is also dependent on
the mass of adhesive that is mixed.
Once applied and the substrates are joined, there is a certain time
period for which the adhesive reaches a handling strength (i.e., a