Handbook of Solvents - George Wypych - ChemTech - Ventech!

850 George Wypych
Peel strength, N/25 mm
150
100
50
0
1min
7day
A B C D
Adhesive type
Figure 14.1.1. Green strength and adhesion of several
adhesives. Symbols are explained in the text. [Data
from B Archer, International J. Adhesion & Adhesives,
18, No.1, 15-8 (1998).]
Peel strength, N/25 mm
but lacks green strength and therefore does not meet the performance requirements of shoe
manufacturers and other industries. 11 Adhesive (C) is a blend of polychloroprene with polyurethane
in a water-based system. This modification gives both green strength and peel
strength but Figure 14.1.2 shows that peel strength of a freshly applied adhesive is lower
than that of solvent based polychloroprene which may cause problems in holding both adhering
surfaces together. The adhesive (D) was developed in an interesting new process
which involves the emulsification of a solvent based adhesive obtained from styrene-isoprene-styrene
rubber. 11 After emulsification, the solvent is stripped under vacuum
to produce a solvent-free adhesive. The reason for emulsification of the complete adhesive
as opposed to emulsification of the rubber alone is to produce a homogeneous adhesive system
which would otherwise suffer from separation of polymer particles surrounded by a
layer of emulsifying agents. Figures 14.1.1 and 14.1.2 show that this system is superior to
solvent-based adhesives. Adhesives can be further improved by polymer blending and by
adhesive foaming. A foamed adhesive layer requires less material (approximately 4 times
less than a conventional adhesive), it requires less drying time (less water and faster evaporation),
and provides an improved bond strength. A foamed adhesive has a larger surface
area which increases the surface area of contact with the substrate. The compressed rubber
foam has a higher tear strength than unfoamed film of the same thickness. 11
Also, regulations are helping to reduce the solvent content of adhesives. 12,13 The use of
chlorinated solvents, frequently used in primers and for cleaning, has been discontinued
based on the Montreal Protocol. From June 1998, the production of a pair of shoes in Europe
should not involve the use of more than 20 g of solvent. This is only partially successful solution
since shoe production is expected to move out of developed countries to less restrictive
jurisdictions. Solvent Emission Directive will continue to restrict solvent use in Europe.
Many changes have occurred and more are expected in adhesives based on thermoplastic
polyurethanes (TPU). 3,14,15 In the last 30 years, TPU based adhesives were manufac-
60
50
40
30
20
10
0
A B C D
Adhesive type
Figure 14.1.2. Contactability of various adhesives.
Symbols are explained in the text. [Data from B Archer,
International J. Adhesion & Adhesives, 18, No.1,
15-8 (1998).