Curing crosslink density

The conversion of strain mismatch into stress is a function of the stress relaxation modulus exhibited by the polymer. A predictive stress model must incorporate the complex dependencies of the modulus and stress relaxation behavior on temperature, glass transition temperature, degree of cure, crosslink density, solvent-plasticization, and reaction kinetics. 

Monomers. As shown in Table H monomers are used in UV curable systems to provide final film properties and viscosity control of the resin. They are also important in determining the speed of cure, crosslink density, and final surface properties of the cured film. Properties which dictate the selection of one monomer over another are listed below ... 

Primary accelerators can commonly be used alone as the sole accelerator in the rubber compound to impart sufficient cure rate and state of cure (crosslink density). 

It gives a neoprene compound good scorch safety and a good cured crosslink density, which is sometimes hard to duplicate with other rubber accelerators. 

Fluorosilicones consist of PDMS backbones with some degree of fluoro-aliphatic side chains. The fluorinated group can be trifluoropropyl, nonafluorohexylmethyl, or fluorinated ether side group [78,28,79]. These polymers differ not only in substituent group, but also in the amount of fluoro-substitution relative to PDMS, the overall molecular weight and crosslink density, and the amount of branching. In most commercially available cases, these polymers are addition cure systems and the reactions are those discussed previously for silicone networks. 

The pre-gel model calculates the weight average molecular weight of the reaction mixture, while the post-gel model calculates the weight of the sol fraction and the effective crosslink density. A simple computer program using the derived expressions has been written in BASIC and runs on IBM-PC compatible computers. The importance of secondary reactions on cure in typical coatings is discussed. 

It is possible to calculate a number of different kinds of "effective" crosslink densities. Bauer et al have used a quantity they termed the "elastically effective crosslink density " (Cel) correlate cure with solvent resistance and other physical properties of coatings (7-10). The correlation was basically empirical. Formally, the is a calculation of the number of functional groups attached to the infinite network for which there are at least two other paths out to the network on the given polymer or crosslinker. Thus, chains with only one or two paths to the infinite network are excluded. The following expression can be written for... 

In general, coating systems are designed to achieve optimum properties at crosslinking levels short of 100% conversion in order to minimize cure time. The effect of Isocyanate to hydroxy ratio and humidity on crosslink density after a fixed cure time is shown in Figure 4. The fixed cure time has been arbitrarily defined as the time at which a coating with equal Isocyanate and hydroxy functionality reaches 85% conversion in the absence of humidity (H-0). If the initial ratio of isocyanate to hydroxy is less than or equal to 1, the crosslink density drops with increasing humidity. 

The effective crosslink density is used to predict state of cure in thermoset coatings. Side reactions have been found to play an important role in cure of typical coating mixtures. 

In order to answer these questions, the kinetic and network structure models were used in conjunction with a nonlinear least squares optimization program (SIMPLEX) to determine cure response in "optimized ovens ". Ovens were optimized in two different ways. In the first the bake time was fixed and oven air temperatures were adjusted so that the crosslink densities were as close as possible to the optimum value. In the second, oven air temperatures were varied to minimize the bake time subject to the constraint that all parts of the car be acceptably cured. Air temperatures were optimized for each of the different paints as a function of different sets of minimum and maximum heating rate constants. 

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 equivalent weight increased from 210 for UPE A to 316 for UPE E with one third of the maleic anhydride was replaced with succinic anhydride and 634 for UPE F with two thirds of the maleic anhydride is replaced with succinic anhydride. This change will effectively decrease the crosslink density of the final cured film. The Brookfield viscosities of the two new polyesters (E and F) are less than those from the first series. Since the molecular weights should be about the same (1760), it is not surprising that the Brookfield viscosities were relatively constant for all three unsaturated polyesters. The slight decreasing trend in viscosity with increasing succinic anhydride in the unsaturated polyester may be due to the increased free volume obtained when succinic anhydride is substituted for maleic anhydride. 

The influence of ZnCFO concentration (3,0 5,0 7,0 phr) on formation of properties complex of the unfilled rubber mixes and their vulcanizates on the basis of isoprene rubber of the following recipe, phr isoprene rubber - 100,0 sulfur - 1,0 di - (2-benzothiazolyl) -disulfide - 0,6 N, N -diphenylguanidine - 3,0 stearic acid - 1,0, was carried out in comparison with the known activator - zinc oxide (5,0 phr). The analysis of Rheometer data of sulfur vulcanization process of elastomeric compositions at 155°C (fig. 5) shows, that on crosslink density and cure rate, about what the constants of speed in the main period (k2) testify, they surpass the control composition with 5,0 phr of zinc oxide. Improvement of the complex of elastic - strong parameters of rubbers with ZnCFO as at normal test conditions, and after thermal air aging (tab. 1), probably, is caused by influence of the new activator on vulcanization network character. So, the percent part of polysulfide bonds (C-Sx-C) and amount of sulfur atoms appropriating to one crosslink (S atoms/crosslink) in vulcanizates with ZnCFO are decreased, the percent part of disulfide bonds (C-S2-C) is increased (fig. 62). 

Although this change in crosslinking chemistry holds promise for increased use temperatures, a tougher product is desirable. Experiments were therefore designed to decrease the crosslink density by the addition of a monofunctional reactant (Structure IV in Table I). For these experiments, an aromatic diamine co-reactant was used to accelerate the cure (8). 

If we accept the model proposed for these mixed monofunctional/ difunctional systems, we can draw some conclusions about the network structure in polymers based on I alone. For example, Fig. 7 shows how the Tg varied with the relative crosslink density in the mixed systems. The abcissa represents the probability that a monomer chosen at random is linked to the network at both ends. At moderate degrees of crosslinking, the expected relationship between Tg and crosslink density is linear, so the data were approximated by a straight line (10). From the extrapolation in Fig. 7, one concludes that a typical bis-phthalonitrile cured to a Tg of 280 0 has a relative crosslink density of 0.5, or about 70% reaction of nitrile groups. 

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

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