Page 368 - Handbook of Adhesives and Sealants

324 Chapter Nine

formulation, plasticizers separate the polymer chains and enable their
deformation to be more easily accomplished. Plasticizers generally af-
fect the viscoelastic properties of the base resin, whereas diluents sim-
ply reduce the viscosity of the system. Whereas diluents result in brit-
tle, hard adhesive systems, plasticizers result in increased flexibility
and lower modulus. The temperature at which polymers exhibit rub-
bery properties, the glass transition temperature, can be reduced with
plasticizers.
Certain resinous materials which act as plasticizers are also well
noted for increasing the tack of the formulation. Traditional tackifiers
were based on naturally occurring resins such as pine tar. Tackifiers
used in modern adhesive formulations include aliphatic and aromatic
hydrocarbons, terpenes, and rosin esters. Tackifying resins that are
commonly used in adhesive formulations are identified in Table 9.2.
Tackifiers are useful in pressure sensitive adhesives or adhesives
which require aggressive tack or ‘‘green strength’’ to assist in assembly
of the product. In addition to increasing tack, increased peel strength
and decreased shear strength also result from the addition of tackifiers
in the adhesive formulation. Figure 9.2 shows the dependence of tack,
peel, and shear on an adhesive formulation’s resin/tackifier ratio.
Various solvents, soluble agents, resinous powders, or inert fillers
can be used to control the viscosity of the adhesive. Viscosity of ad-
hesive systems may need to be increased or decreased depending on
the application. Viscosity control is one method commonly used to
maintain a consistent product and bondline thickness. Thickeners and
thixotropic agents are used to maintain reasonable thickness of the
glue line through viscosity adjustment. Thixotropic fillers are mate-

TABLE 9.2 Incomplete Listing of Commercially Available Tackifying Resins and
Illustrative Properties 3
Glass
transition Softening
temperature temperature
Commercial resin Type ( C) C( F)
Piccolyte HM-85 (Hercules) Styrenated terpene 35 181 (358)
Nirez K-105 (Reichhold) Polyterpene 54 192 (378)
Regalrez 1094 (Hercules) Hydrogenated aromatic 37 186 (367)
Piccovar AP-25 (Hercules) Aromatic 25 148 (298)
Wingtack 10 (Goodyear) C-5 28 139 (283)
Escorez 1310 (Exxon) C-5 40 186 (367)
Foral 85 (Hercules) Glycerine rosin ester 40 181 (358)
Foral 105 (Hercules) Hydrogenated pentaerythritol 57 192 (378)
rosin ester
Zonarez A-100 (Arizona) Terpene 55 193 (379)
Escorez 2101 (Exxon) Mixed aliphatic/aromatic 36 185 (365)