Page 74 - Handbook of Adhesives and Sealants

42 Chapter One

1.7 General Materials and Processes
This section will describe the most common materials and processes
used for adhesive bonding and sealing. They are described here in the
most general and functional of terms and then will be addressed again
in more detail in later chapters.

1.7.1 Materials used for adhesives and
sealants
Solid surfaces brought into intimate contact rarely stick to one an-
other, but any liquid placed between them will cause some degree of
adhesion. Therefore, almost anything could be used as an adhesive or
sealant. However, the nature of the substrate, its surface chemistry,
the processing method, and the type of the load and service environ-
ment will dictate what material is best to use. The material scientists
have developed many substances of high molecular weight which give
good adhesion and sealing ability to a variety of substrates.
Although adhesives and sealants are often formulated from the
same types of base materials, they are usually ‘‘engineered’’ to have
different properties. Adhesives and sealants are highly formulated
materials with many components. They generally have an organic
base, although there are some mineral-based adhesives that perform
very well in certain applications. Table 1.12 shows the origin (natural
or synthetic), basic type, and chemical families of common types of
adhesive materials. The modern organic materials are all high poly-
mers: long chains of carbon atoms that can form three-dimensional
networks. The oldest adhesives and sealants, many that are still used
today, are of natural origin. The most common of these are animal
glues, starches, and tar or pitch. Naturally occurring adhesives and
sealants also include dextrin, asphalt, vegetable proteins, natural
rubber, and shellac.
For a material to be a potential adhesive or sealant, it must meet
three basic criteria. First, the material has, at some stage, to be in a
liquid form so that it can readily spread over and make intimate con-
tact with the substrate. Second, the material must be capable of hard-
ening into a solid to withstand and distribute stress. Lastly, the ma-
terial must resist the environments that it will see during processing
and during service.
Modern polymers are ideal materials to use as adhesives and seal-
ants because they can be applied as low viscosity liquids and, then by
various means, hardened into a strong material with relatively good
resistance to stress and various environments. This hardening may
occur through loss of water or solvent, cooling from the molten state,