Rate of crosslinking

Delayed crosslinking is desirable because the fluid can be pumped down more easily. A delay is a retarded reaction rate of crosslinking. This can be achieved with the methods explained in the following section. 

Delayed Crosslinking Additives. Glyoxal [458,460,461] is effective as a delay additive within a certain pH range. It bonds chemically with both boric acid and the borate ions to limit the number of borate ions initially available in solution for subsequent crosslinking of a hydratable polysaccharide (e.g., galactomannan). The subsequent rate of crosslinking of the polysaccharide can be controlled by adjusting the pH of the solution. 

Why is it necessary to have a balance between the rates of crosslinking and expansion when manufacturing a foamed polyurethane What variables affect these rates ... 

The resolution capability of a resist is directly related to resist contrast (7) which, for a negative resist, is related to the rate of crosslinked network formation at a constant input dose. It is somewhat more complicated for a positive resist being related to the rate of chain scission and the rate of change of solubility with molecular weight with the latter being markedly solvent dependent. Contrast, like sensitivity, is governed by the type of chemical reactions that occur in the polymeric resist and is affected by molecular parameters such as molecular weight distribution and chemical composition. 

Using TGA, a correlation was established between the rates of crosslinking and decomposition of azodicarbonamide blowing agent in expanded EVA preparation. The relationship between processing conditions, properties and structure of cured EVA foams was also examined. 11 refs. 

The action of zinc in increasing the efficiency and rate of crosslinking is thought to involve chelation of zinc with the accelerator as well as species XVIII and XIX. Zinc polysulfide compounds such as XX are also likely intermediates. Zinc chelated to sulfur or as zinc sulfide bonds probably facilitate cleavage of sulfur-sulfur bonds in the concerted reactions described by Eqs. 9-20 and 9-21. 

Sol-Gel Measurements. The relative rates of crosslinking and scission may be estimated from soluble-fraction measurements. For an initial random molecular weight distribution and random chain scission, extrapolating a curve of S + S vs. 1/D, where S = sol fraction and D —... 

To summarize the sol-gel and ESR data, the accelerated creep during irradiation is not caused by reactions which would significantly change the rate of crosslinking, scission, concentration, or type of free radicals. 

Figure 21. Rate of crosslink development vs. rate of fiber-matrix adhesion development for 12-volume fiber glass composite...

Figure 3 Effect of external pH on erosion rate of crosslinked poly(ortho ester) prepared from 3,9-bis(ethylidene 2,4,8,10-tetraoxaspiro [5,5] undecane), triethylene glycol, and 1,2,6-hexanetriol. Disks 5.5 x 0.75 mm p-nitroacetanilide loading 2 wt. ...

Assuming that the yield for gel fraction is proportional to the rate of crosslinking, it seemed that the observation on possibility of resolving the experimental results into terms proportional to I and Ia /2, points out the dual mechanism of photoinduced crosslinking of poly (vinyl butyral). The over all character of the equation (l) suggests that the main role in the discussed reaction plays the relation gel content connected possibly with direct coupling of... 

According to Eq. 4, it is also to be expected that the photoinitiator used influences the rate of crosslinking via Oh+, V and [8]. On the other hand, from Eqs. 3 and 4 it follows that only Rj determines R p and this proportion this relation depends on the initiator used. Moreover, the initiator used also influences the inhibition time (see Eq. 5),... 

The extent and rate of crosslinking is expected to depend on both the doping level and the concentration of polymer in solution. 

Fig. 17. Dependence of rates of crosslinking and scission on the composition of methyl methacrylate copolymers irradiated in the form of films and A chain scission, crosslinking [reproduced from Ref. 89].

Despite these limitations, one can qualitatively conclude from the results depicted in Figure 1 that crosslinking increases with conversion. Furthermore, the relative rate of crosslinking, i.e., formation of crosslink sites as a function of styrene conversion, probably also increases with conversion. However, the second statement has yet to be proved in special experiments in which the formation of network sites is measured directly. 

Analysis and Correlation of Studies The photooxidation of PnBA is apparently a first order reaction with an initial rate (up to 2000 hours) that is higher than the subsequent rate. The rate constant for crosslink formation is greater than that for COC cleavage, Table II. However, the calculated number of crosslinks per chain as weathering progresses is very much lower than the number of cleaved COC groups. The rate of -COOH formation is the same order of magnitude as the rate of crosslink formation. 

Gonsideiing the reactions in formulas (1) and (2) and the foregoing discussion of the possible reaction mechanisms, it seems likely that the photoinitiated crosslinking is a second order reaction, i.e. the rate should be a linear function of the UV intensity in square. Assuming that the rate of crosslinking (which is difficult to measure) is proportional to the rate of gel formation (which is easy to measure), the data in Fig. 6 support thds conclusion at low UV intensities, i.e. at... 

Fig. 6 A log-log plot of the rate of crosslinking measured as gel content versus li t intensity for HD polyethylene.

The effect of the addition of small concentrations of the premature vulcanization inhibitor (PVI), N-(cyclohexylthio)phthalimide, is also given by Figure 7.11. This retarder (Coran and Kerwood, 1970) is frequently used to independently control scorch resistance with little effect on the rate of crosslinking (Coran, 1978). Before the development of N-(cyclohexylthio)phthalimide as a PVI, acidic retarders like salicylic acid, acetylsaUcylic acid, phthaUc anhydride, and benzoic acid were used. These additives improved scorch resistance but also gave greatly reduced rates of crosslink formation after the delay. Another retarder of the past was N-nitrosodiphenylamine, which is less active and not now used because of toxicological concerns. 

An increase in the concentration of fatty acid and hence increases in the concentration of available Zn++ causes an increased overall rate in the early reactions (during the delay period), which lead to the formation of rubber-Sjc-Ac. However, it gives rise to a decrease in the rate of crosslink formation but an increase in the extent of crosslinking (Coran, 1965). The increase in the rates of the early reactions has been explained by the interaction ... 

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