CROSSLINKING RATE

K. E. Cawiezel and J. L. Elbel. A new system for controlling the crosslinking rate of borate fracturing fluids. In Proceedings Volume, pages 547-552.60th Annu SPE Calif Reg Mtg (Ventura, CA, 4/4-4/6), 1990. 

The purpose of the various Ingredients Is to provide desirable fluid properties at different times 1n the treatment. For example, the crosslinker system may be designed to have a low reaction rate while the fluid 1s at low temperatures (such as those found 1n the mixing tanks and before the fluid has traveled very far Into the fracture). Then as the fluid temperature Increases above a threshold value, the crosslinking rate accelerates and changes the rheology of the fluid to one more desirable 1n the low-shear, but high-temperature, environment of the fracture. 

The G value for crosslinking may also be estimated from the sol-gel data. In the present study the crosslinking rate of stressed samples was essentially the same as for unstressed samples, but the rate of scission in stressed samples was zero compared with 0.013 in the absence of stress. (If one were to envisage the stress plus radiation-biased creep as being caused by radiation scission followed by stress-induced flow of the scissioned chain segments, one could expect a higher rate of perma-... 

The effects of ionizing radiation on a low and a high density polyethylene and an ionomer indicate that similar networks are formed in the ionomer and in the low density polyethylene. The former crosslinks about 1.5 times as readily as the latter. In the high density resin, the network characteristics, and hence, presumably, the type and distribution of links, differ considerably from those in the other two resins. As a result, crosslinking rates in the high and low density polyethylenes cannot be directly compared. 

In view of the similarity in behavior of the networks in the ionomer and low density resin, we are able to deduce from modulus and elongation at rupture measurements the relative crosslinking rates. It appears that Surlyn A crosslinks 50 to 60% more rapidly than DFD 6040. The increased crosslinking in the ionomer may be associated with the radio-... 

Figure 5. NP normalized crosslinking rate (Rx-Unk/[NP]o) at T= 64 for CAS Samples 9, 9/1, HI 7, HI 7A, HI9, H23, and H25. E-Estane, NP=nitroplasticizer, H HMX, I=Irganox, PBX=PBX 9501 (See Table I for composition of each sample.) Open circles-rate determinedfrom weight-averaged MW with RI detection. Closed circles=rate determined from MWD with RI detection. Open diamonds =rate determined from weight-averaged MW with MALS detection.

As indicated by Eq. 1, the crosslinking rate is related directly to the light intensity. In Fig. 4 an example is given for the crosslinking of VEi using the DPA/I(i) initiator system. The plot of Rv vs, the incident light intensity demonstrates the rate of polymerization is directly proportional to the absorbed ligth intensity. 

The crosslinking of polydimethylsiloxane and polydimethyldiphenyl-siloxane was studied as a function of temperature by dynamic mechanical methods [366]. A reduction in crosslinking rate was observed in N20 and air compared with vacuum [367]. Marked discolouration of the samples occurred in the presence of air or nitrous oxide. 

Among the UV sensitizers, some of which may accelerate the crosslinking rate by 100-200 times that of ordinary free-radical initiated processes, are ethers of benzoin, benzophenones, anthrone, benzil, and Michler s ketone [81]. 

Temperature (°C) Relative Crosslinking Rate, rxtxlO ) Xw of Primary Chains at Gel Point (xIO )... 

Data taken from Saunders and Dobinson (1976). rx = kx/ kp, where k is the crosslinking rate constant and kp is the propagation rate constant. ... 

Frequently, mixtures of accelerators are used. Typically, a benzothiazole type is used with smaller amounts of a dithiocarbamate (thiuram) or an amine type. An effect of using a mixture of two different types of accelerators can be that each activates the other and better-than-expected crosslinking rates can be obtained. Mixing accelerators of the same type gives intermediate or average results. 

Different types of accelerators impart vulcanization characteristics that differ with respect to both scorch resistance and crosslinking rate. Figure 7.11 is a map of accelerator system characteristics. Within groups or types, differences can be obtained by choosing the individual accelerators. In the group of benzothiazolesulfenamides, the scorch resistance and vulcanization time increase in the order TBBS or CBS, MBS, DCBS. 

There are obvious differences between accelerated vulcanization and unaccelerated vulcanization. (Greater crosslinking efficiencies and greater crosslinking rates are obtained with accelerated vulcanization.) But there are more subtle differences. Results from model reactions with curing ingredients indicate that sulfur becomes attached to the rubber hydrocarbon almost exclusively at allylic positions (Skinner, 1972). This is not the case with unaccelerated-sulfur vulcanization, thus ... 

Shyichuk A. V. White, J. R. Scission and crosslinking rates during photooxidation of thick-sectioned polyolefins depth profiles for different exposmes. In Natural and Artificial Ageing of Polymers Reichert, T. Ed. Gesellschaft fur Umweltsimulation Pfinztal, Germany, 2007 pp 175-184. 

As a comparison of torque as a function of time (see Fig. 5) shows, addition crosslinking is around 2.5 to 5 times faster than radical polymerization. This has a significant effect, especially with thin molded articles, on the production cycle time because with thin parts the crosslinking rate is directly proportional to the quantity of catalyst. In articles with a thick wall, the effect of heat transmission, which because of the thermal insulation effect of polymers is known to be poor, is predominant. 

Fig. S. Comparison of crosslinking rates of variously catalyzed silicone rubbers.

Figure 10.52 shows that addition of the plasticizer decreased dehydrochlorination rate of PVC. Crosslinking rate was reduced with the presence of plasticizers. Also, the carbonyl group formation was slower in the presence of a plasticizer. Comparison of data presented by the same authors for the effect of radiation above and below 290 nm stresses importance of wavelength selection on the results obtained. When studies were conducted with an unfiltered mercury lamp, presence of phthalates increased the carbonyl group formation. 

When a butyl rubber, an ethylidene norbomene (ENB)-type ethylene-propylene-diene monomer (EPDM), or a mixture of desired proportions of a butyl rubber and an ENB-type EPDM is thermally crosslinked using an alkylphenol-formaldehyde resin and a triazole compound, a higher crosslinking rate is obtained. When a hydrazide compound is also used, even higher crosslinking rates are obtained. 

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