Лако-красочные материалы — производство

The basic scheme for the autooxidation of polymers is detailed in Table 1 [2]. The scheme can be broken down into several distinct reactions: initiation, propagation, and termina­tion. The first step in the oxidation mechanism, initiation, occurs when a polymeric free radical (R*) is formed by exposure to heat, light, shear, or impurities. The propagation reactions involve the very rapid reaction of the polymer free radicals with oxygen-forming peroxy radicals (ROO*). The peroxy radicals can then react with the polymer to produce hydroperoxides (ROOH), which can decompose further to form two new free-radical species (RO* and HO*), which, in turn, can participate in other propagation reactions. The mechanism of autooxidation is cyclic in nature and leads to an exponential growth of free radicals. The termination mechanisms of these free-radical reactions are cross-linking and chain scission. The ramifications of degradation via cross-linking include hardening, skinning, gel formation, a decrease in tack, and an increase in viscosity. Degradation via chain scission results in softening, a viscosity decrease, an increase in tack, and a loss of cohesive strength. In addition to the change in physical properties as a result of degrada­tion, discoloration is also possible. Although sample discoloration, in itself, may have a

Initiation

ROOH —RO* +HO*

2ROOH -! RO* +ROO + H2O

heat/light

RH —!

R0* + O2 -! R’OO*

RO*+R0H -! ROH + R0*

Propagation

ROO*+R0H -! ROOH + R0*

RO*+R0H -! ROH + R0*

HO*+R0H -! H2O + R0*

R0* +O2 -! R0OO*

2R0OO*+R0H -! R0* +2ROH + O2

Termination and cross-linking R0*+R0* -! R0 — R0 R0O* +R0* -! R0OR0 R0OO* +R0* -! R0OOR0

R0OO*+R0O* — R0OO*+R0OO*

Chain scission

CH, CH,

-CHj-C — CH^^-HO* + ~ CH —C—CH,

ООН O’

0

I

-CH;-C—CH, + ~ ‘CH,

ROOH=peroxide (impurities) R0H=polymer

minimal effect on the physical properties of an adhesive, the color formation results in an undesirable appearance which may be interpreted as being that of an inferior product.

The tactics used to inhibit the autooxidation process involve the obstruction of one or more of the degradation pathways. To inhibit or prevent the undesirable effects of degradation as a result of thermal oxidation, antioxidants are used in adhesive formulations [3,4]. Antioxidants are generally added to each component of an adhesive formulation and, in most cases, are also added to the final formulation.