Page 379 - Handbook of Adhesives and Sealants

Adhesive Composition and Formulation 335

Figure 9.5 The effect of mineral extenders on the tensile shear
strength of epoxy adhesives (the substrate is aluminum). 14

TABLE 9.4 Effect of Fillers on Block Shear Strength of Adhesive Formulations 15
(In pounds per square inch)
Alumimum
Phenolic linen laminates tested at blocks tested at
Fillers at 100 phr 23 C 75 C 90 C 105 C 23 C 105 C
Aluminum powder ..... 2,790 . . . . 1,470 1,390
Ignited Al 2 O 3 .......... 4,600 1,360 1,195 530 1,650 1,150
Short-fiber asbestos, 1,740 1,270 580 510 2,180 3,910
24 phr ..............
Carbon black .......... 2,000 555 980 910 2,000 1,170
Silica ................. 2,840 1,600 1,250 830 1,530 600
Zinc dust ............. 2,510 600 300 225 4,080 3,865

9.3.6 Increasing tack
In adhesive formulations, it is sometimes necessary to achieve some
degree of initial tack in order for the substrates to remain in position
until the adhesive hardens. Tack can be achieved by proper selection
of adhesive composition ingredients, usually by the use of higher mo-
lecular weight resins.
A number of liquid and solid tackifiers are possible for use in ad-
hesive formulations. The role of tackifiers in uncured adhesives is well
understood. 17,18 Pressure sensitive adhesive performance, for example,
depends heavily on tackifiers and the proper balance of viscoelastic
properties. The tackifier provides the viscous nature to the adhesive