Natural rubber crosslink density

Time-crosslink density superposition. Work of Plazek (6) and Chasset and Thirion (3, 4) on cured rubbers suggests that there is one universal relaxation function in the terminal region, independent of the crosslink density. Their results indicate that the molar mass between crosslinks might be considered as a reducing variable. However, these findings were obtained from compliance measurements on natural rubber vulcanizates,... 

The nature of the matrix The nature of the rubber The type of curing system The crosslinking density... 

Fig. 23. Mooney-Rivlin plots of extended natural rubber vulcanizates. The upswing at small A 1 values, i.e. large strains, is due to finite extensibility [Mullins (72S)]. At high crosslinking densities (upper curves) the upswing occurs at a smaller strain...

The importance of crosslink density has already been encountered in terms of the vulcanisation (i.e. sulphur-crosslinking) of natural rubber. With low crosslink densities (i.e. low levels of sulphur) the product is a flexible elastomer, whereas it is a rigid material when the crosslink density is high. 

Polyurethane is a very common crosslinked polymer and many materials produced from it end up as waste. Given the quantity of material, recycling is a major problem. Cryogenic pulverization systems have been developed which can process PU foam to particles smaller than 1 mm (preferably <100 pm)/ " These particles are homogenized with polyol and then reacted with isocyanates to produce foam. This foam with 5% pulverized PU foam has a density equivalent to a similar foam produced without the recycled material. A further increase in filler content causes a density increase. Pulverized PU foam particles were also tried as a filler in natural rubber vulcanizates with good results. Figure 12.10 shows the effect of PU... 

Figure 15.33 shows benzene uptake by natural rubber samples. Filled samples absorb less solvent (lower swelling). The carbon black containing sample had a lower benzene uptake than the silica filled sample. The lower swelling of the carbon black containing sample is due to high bound rubber content, the crosslink density of the black filled vulcanizate, and a strong rubber-filler interaction. 

Most or all of the monomer units in the chains are capable of participating in the crosslinking (vulcanisation or curing) reaction. This class includes natural rubber, butadine rubbers, SBR and nitrile rubbers. The crosslink density in this class is controlled by the amount of curing agents in the compound. 

Natural rubber of Mn = 2.23x10 was vulcanized with 2% sulfur. Calculate an estimate of the extent of swelling of the vulcanizate in benzene at equilibrium, given that the interaction parameter of rubber and benzene is 0.41. Assume that all sulfur is used in crosslinldng and that there is, on the average, one crosslink for every eight sulfur atoms. [Data Rubber density = 0.91 g/cm molar volume of benzene = 89.4 cm- /moL]... 

Increases in sulfur and accelerator concentrations give higher crosslink densities and, therefore higher moduli, stiffness, hardness, and so on. However, as the ratio of the concentration of accelerator to the concentration of sulfur increases, the proportion of monosulfldic crosslinks increases in natural rubber... 

Over the years, much of the research on accelerated-sulfur vulcanization was done by using natural rubber as a model substrate. Natural rubber was the first elastomer and therefore the search for the understanding of vulcanization originated with work on natural rubber. Most of the work cited in the previous sections is related to natural rubber. However, some rather early studies have been directed to the vulcanization of butadiene 1,4-polymers (Skinner and Watson, 1969 Wolfe et al, 1329 Gregg and Katrenick, 1970). More recent is the work of Pellicioli and coworkers. Early basic work on the vulcanization of ethylene-propylene-diene-monomer rubber (EPDM) has been carried out (van den Berg et al., 1984a,b). Recently, Kuno and coworkers did basic work on EPDM networks. They found that, essentially, the vulcanizate properties depend only on the crosslink density, not on the type of curing system (Dijkhuis et al., 2009). 

Figure 37-1. Network density ft in amount of network chains per unit volume vulcanizate as a function of the vulcanization time for the vulcanization of natural rubber with 2 phr sulfur, 5 phr zinc oxide, 1 phr 2>mercaptobenzthiazole, and 1 phr stearic acid at 140 C. Cl, main cross-linking reaction Cii, long-term crosslinking A chain degradation. (After D. A. Smith.)...

It is known that sulfur-vulcanized Natural Rubber (NR) can be completely recycled at 200 to 225°C by using diphenyldisulphide [41]. Recently, the effi-ciacy of various disulphides as recycling agents for NR and EPDM vulcan-izates were reported [42]. While complete de vulcanization was observed on sulfur-cured NR at 20b°C, a decrease in crosslink density of 90% was found when EPDM sulfur vulcanizates with diphenyldisulphide were heated to 275°C in a closed mold for 2 hours. At the same time, EPDM cured by peroxide showed a decrease in crosslink density of about 40% under the same conditions. 

---