In-vulcanization bonding

In hot PV bonding, the bonding agents react with the rubber and no further preparation of the rubber is required. The bonding agents used are often suitable for in-vulcanization bonding, but the reverse is not true. 

For components made by in-vulcanization bonding, the quality of the bond formed (the adhesion - see Adhesion - fundamental and practical) depends not just on the nature... 

The double bonds in isoprene and chloroprene allow these compounds to be vulcanized. In vulcanization, sulfur attaches to the doubly bonded carbon to produce cross-linked chains ... 

In the rubbery region, which is just above (in terms of temperature) the leathery region, polymer chains have high mobility and may assume many different conformations, such as compact coils, by bond rotation and without much disentanglement. When these rubbery polymers are elongated rapidly, they snap back in a reversible process when the tension is removed. This elasticity can be preserved over long periods of time if occasional cross-links are present, as in vulcanized soft rubber, but the process is not reversible for linear polymers when the stress is applied over long periods of time. 

Figure 3.19 Sulfur cross-links obtained in vulcanization processes. S (C—S — C), monosulfur cross-link. S2 (C—S — S — C), disulfur cross-link. S [(C—S — C), n = 3-6] polysulfur cross-link, (a, b) Intrachain cyclic. (c,d) Double-bond conjugated.

Symmetrical bonds such as C-C, C=C, C C, N=N, 0-0, S-S, manifest themselves by giving the most intensive bands in Raman spectra, and especially structures with the latter heavy atoms, while they are inactive in the infrared. Among spectral methods Raman, is exceptional in showing the structure of natural S-S cross-linkages in biomolecules, artificial ones in vulcanized... 

Chem. Descrip. Disp. of polymers, org. resins, and fillers in org. soivs. Uses Cover coat and post vulcanization bonding agent adhesive for bonding rubbers to metal and other substrates during vulcanization and post-vulcanization... 

In vulcanized rubber the molecular chains are cross linked by chemical bonds which inhibit macro-flow. That is why vulcanized stocks are more perfectly elastic than raw rubber (cf. the models having piston and spring in series.) The chemical cross links, randomly distributed over the mass of rubber, act as permanent junction points. A netlike structure is obviously present in vulcanized rubber. 

Pure natural rubber is soft and tacky. To be useful, natural rubber has to be vulcanized. In vulcanization, natural rubber is heated with sulfur. A reaction takes place that produces cross-links between the dr-polyisoprene chains and makes the rubber much harder. Sulfur reacts both at the double bonds and at allylic hydrogen atoms ... 

The environment in which an article is used may influence bond durability (see also Durability fundamentals). Atmospheric ozone can cause time-dependent crack growth in vulcanized elastomers in addition, ozone can induce failure at a bond with certain bonding agents. Although water is only slightly soluble, it can permeate elastomers by an osmotic mechanism induced by salt-Uke impurities. As a result, the uptake in salt water is generally less than that in pure water. Rubber to metal bond failure has been found to occur in a time-dependent manner under salt water in the presence of electrochemical activity but much more slowly, if at all, in its absence (see also Cathodic disbondment). In the absence of imposed electrochemical activity, effects are likely to depend particularly on the metal used and its corrosion resistance. Provision of a bonded rubber cover layer over all metal surfaces subject to immersion is likely to enhance bond durability. 

Rubber to metal bonding - basic techniques P LEWIS In- and post-vulcanization bonding... 

Since the selection mles for Raman and infrared spectra are different, Raman spectra yield information complementary to the infrared spectra. For example, the S—S linkages in vulcanized mbber and the C=C bonds yield strong Raman spectra but are very weak or unobservable in infrared spectra. 

Early theoretical approaches to the gel-formation [1-4] as the Flory-Stockmayer theory do not take into account several aspects which naturally occur as the individual molecules grow to form the gel, such as cyclic bond formation, excluded volume effects and steric hinderance. The Flory-Stockmayer theory assumes that in the gelation process each bond between two individual monomeric, oligomeric or polymeric molecules is formed randomly. Thus this theory assumes point-like monomers. This apparently is not the case when already existing macromolecules are crosslinked, i.e., in vulcanization reactions as well as in copolymerization reactions of macromolecules with the functionality /> 3 with bifunctional monomers. 

---