tailieunhanh - Rubber Compounding - Chemistry and Applications Part 11

Tham khảo tài liệu 'rubber compounding - chemistry and applications part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 10 Antioxidants and Other Protectant Systems Sung W. Hong Crompton Corporation Uniroyal Chemical Naugatuck Connecticut . I. INTRODUCTION To extend the service life of vulcanized rubber goods it is very important to protect them from oxygen ozone light heat and flex fatigue. Most natural and synthetic rubbers containing unsaturated backbones natural rubber NR styrene butadiene rubber SBR polybutadiene rubber BR and nitrile rubber NBR for example must be protected against oxygen and ozone. Usually internal components that are not exposed to atmosphere require only antioxidants. However external components that are exposed to the environment require both antiozonants and antioxidants. Antioxidants react with oxygen to prevent oxidation of vulcanized rubber and react with free radicals that degrade vulcanized rubber. There are four principal theories on the mechanisms of antiozonant protection of vulcanized rubber. The first is the scavenger theory which postulates that the antiozonant competes with the rubber for ozone. The second theory is that the ozonized antiozonant forms a protective film on the surface of the vulcanized rubber preventing further attack. The third mechanism postulated is that the antiozonants react with elastomer ozonide fragments relinking them and essentially restoring the polymer chain. The fourth theorized mechanism suggests that Criege zwitterions are formed from the ozonide produced. Paraffinic and microcrystalline waxes are often added to the rubber as protective agents. Usually waxes have poor solubility in rubbers so they Copyright 2004 by Taylor Francis migrate to the surface of the vulcanized rubber and form a protective film or barrier that prevents ozone attack. During dynamic flexing these barriers can be broken exposing the rubber to ozone attack. Therefore waxes can protect against ozone only under static conditions. Antiozonants alone do not prevent ozone cracking at the initial stage because their migration rate is much .