Page 357 - Handbook of Adhesives and Sealants

312 Chapter Eight

systems cure by ultraviolet, anaerobic, or some other catalytic reaction
mechanism. One-part solventless adhesives are popular in many pro-
duction operations because there is no need for mixing, and waste
from mixing more adhesive than required is eliminated.
Most one-part thermosetting systems contain a latent hardener that
activates and cures the adhesive at elevated temperatures. The time
required for cure can be a few seconds to several hours for heat curing
systems. Usually, one-part systems have a limited shelf life, and re-
frigerated storage may be required. When stored under refrigerated
conditions, it is important to warm the adhesive to room temperature
before it is applied to the substrate. If a cold adhesive is applied to a
warmer substrate, such as aluminum, water from humid air may con-
dense on the substrate causing a weak boundary layer.
Certain one part adhesive systems cure by reacting with the mois-
ture in the air. Polyurethanes and silicones react in this manner. When
using these systems on non-porous substrates, it is important that the
bond area not be too large or else the moisture in the air cannot get
to the center of the bondline.
Anaerobic adhesives are single component adhesives that are
unique in that they are shelf-stable, solventless, ready-to-use adhe-
sives that cure at room temperatures. Their cure is inhibited by the
presence of air (oxygen) in the package and during application. Once
the joint is assembled and air is excluded from the liquid adhesive,
curing begins. Certain anaerobic adhesives are catalyzed by the metal
substrate on which they cure.
Some adhesive systems can cure by exposure to radiation. They can
be cured by ultraviolet light, visible light, or electron beam radiation.
Radiation cured adhesives are limited to applications where the bond-
line is relatively thin and the substrates are transparent to the energy
source. Thus, coating applications are more common than adhesive
applications. Radiation cured adhesives commonly use acrylate or un-
saturated polyester base resins.

8.2.4.2 Solvent-based adhesives. Solvent-based adhesives use organic
solvent to reduce viscosity for easier application. The solvent is simply
a carrier for the ‘‘solids’’ part of the adhesive. Because of concern for
health, safety, and the environment, solvent adhesives must be care-
fully considered before their introduction into an assembly operation.
Many water-based adhesives and solventless adhesive systems have
recently been developed to replace conventional solvent systems.
Solvent adhesives are applied by spraying, dipping, or brushing, but
the solvent must be driven off before the joint is assembled. Solvent
containing adhesive systems must have a way of letting the solvent
escape from the joint. Either the solvent is allowed to evaporate before