Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.1 Adhesives and sealants 849
which contain methyl ethyl ketone. Most formulations of silicone sealants do not contain
solvents. In some sealants, traces of benzene and toluene can be found.
Acrylic sealants are water-based but they may also contain ethylene and propylene
glycols, mineral spirits and mineral oil. There are also solvent-based acrylic sealants which
contain substantial amounts of solvents such as mineral spirits, toluene and xylene.
Polysulfide sealants usually contain toluene but methyl ethyl ketone is also used. The group
of class B sealants contains substantially more solvents (up to 40% by volume) but there are
some exceptions. PVC sealants are based on plastisols and they can be made without solvents.
Butyl rubber based sealants usually contain hydrocarbons (C 6-C 12). Styrene-butadiene-styrene
based sealants usually have a large amount of solvents selected from a group
including toluene, heptane, hexane, methyl ethyl ketone, isobutyl isobutyrate, n-amyl acetate,
n-amyl ketone. They are usually processed in solvent mixtures. Polychloroprene is
usually dissolved in a mixture of solvents including ketones or esters, and aromatic and
aliphatic hydrocarbons. The list includes naphtha, hexane, acetone, methyl ethyl ketone,
benzene, and toluene.
The world market of sealants was estimated in 1996 at $2 billion and was expected to
grow in 2003 to $2.75 billion with an annual growth rate of 4.5% which is slightly lower
than that expected for adhesives (5.3%). 5
The changing trends are clearly visible when developments in technology are studied
but many barriers to reductions in solvent use exist such as the high investment required,
longer processing time, frequently higher material cost of adhesives, and the psychological
barrier of changing established adhesive practices. In many instances, adhesive performance
is predicted by its superficial characteristics such as strong smell which might suggests
that the material has superior properties, its initial green strength which for many
indicates good bonding properties, and high viscosity often related to good processing characteristic.
10 Since the alternative materials may not have much odor, or require of longer
time to reach strength and have a low viscosity, users are suspicious that their potential performance
may be inferior. The following information reviews some recent findings which
may contribute to future changes.
In the shoe industry, a major breakthrough occurred in 1928 when polychloroprene
was first introduced. 1,10 The first, simple formulation is still manufactured and is used
worldwide because the glue can be easily prepared by simply making a solution of the polymer.
This gives a product with good adhesion to various substrates. Many new products are
available today as potential replacements. Hot melt adhesives can be used in some applications
but they still require solvents for cleaning, degreasing, and swelling. Also, their bond
strength is frequently inadequate. Reactive systems are not yet used in the shoe industry but
reactive hot melts are finding applications. Their broader use is hampered by their sensitivity
to moisture which requires special equipment and special care. 10 Water-based adhesives
are the most likely replacement product. They also need special equipment for processing
because of the high heat of evaporation of water (although water based adhesives contain
50% polymer compared with 15-20% in solvent based adhesives). 10 Two sport shoe manufacturers,
Nike and Reebok, already use this technology.
Traditional polychloroprene adhesives can be modified in several ways to be useful in
water-based systems. Figure 14.1.1 shows peel strength of several adhesives. The solvent
based adhesive (A) has excellent properties both in terms of green strength and bond
strength. A simple emulsion of polychloroprene (B) has relatively good ultimate strength