1, cross-linked structure
The polymer chains of rubber are connected by branched chains to form three-dimensional network macromolecules, forming a cross-linked structure. Cross-linking bond type and cross-linking density are the most important parameters in cross-linking structure, which respectively represent the structure of cross-linking bond and the density of cross-linking points distributed between rubber molecular chains. After crosslinking, the physical properties of rubber have changed greatly, among which modulus and hardness are the most significant properties affected by crosslinking density. Because the cross-linking points between chains produced by cross-linking inhibit the sliding between polymer chains, the modulus and hardness increase with the increase of cross-linking density. The relationship between crosslinking density and tensile strength and tear strength is complicated, and its properties have a peak in a certain range of crosslinking degree. The fatigue resistance and thermal oxidation aging resistance are greatly influenced by the types of cross-linking bonds. Due to the fracture rearrangement of polysulfide bonds, the vulcanizate network with more polysulfide bonds has better fatigue resistance, and the carbon-carbon cross-linking bonds with higher bond energy are beneficial to improve the thermal oxygen aging resistance of vulcanizate.
2. Cross-linking bond type
Blackman et al. found that the following crosslinking bonds existed in vulcanized rubber: polysulfide crosslinking bonds, disulfide crosslinking bonds, monosulfide crosslinking bonds and carbon-carbon crosslinking bonds. The study of natural rubber vulcanizate network also confirmed this point.
The type of crosslinking bond varies according to the type of curing system used. Taking natural rubber as an example, vulcanizates with different structures are obtained by different levels of vulcanization systems, and the vulcanizate network obtained by CV (common sulfur vulcanization system) contains more polysulfide bonds. The main sulfur bond obtained by EV (effective sulfur vulcanization system) is monosulfide bond. However, the proportion of cross-linking bond types obtained by semi EV (semi-effective sulfur vulcanization system) is between the first two; Carbon-carbon bonds with high bond energy were obtained by peroxide vulcanization system.
3. Characterization method
Qualitative analysis: According to the different absorption peaks corresponding to different cross-linking bond types, the cross-linking bond types of rubber can be qualitatively analyzed by infrared spectrum and ultraviolet spectrum. Infrared spectrum and ultraviolet spectrum can effectively characterize the types of rubber crosslinking bonds. According to the absorption principle of infrared spectrum, the intensity of absorption peak also reflects the number of corresponding chemical bonds to some extent. However, due to the characteristics of infrared and ultraviolet spectrum analysis technology and the limitations of current analysis technology, quantitative analysis using the intensity of spectral absorption peak will be affected by many uncertain factors, and it is difficult to make accurate analysis.