Gelation rate

In addition to acting as impact modifiers a number of polymeric additives may be considered as processing aids. These have similar chemical constitutions to the impact modifiers and include ABS, MBS, chlorinated polyethylene, acrylate-methacrylate copolymers and EVA-PVC grafts. Such materials are more compatible with the PVC and are primarily included to ensure more uniform flow and hence improve surface finish. They may also increase gelation rates. In the case of the compatible MBS polymers they have the special function already mentioned of balancing the refractive indices of the continuous and disperse phases of impact-modified compound. 

When the resin temperature drops below the boiling point of the reactive diluent (usually styrene) the resin is pumped into a blending tank containing suitability inhibited diluent. It is common practice to employ a mixture of inhibitors in order to obtain a balance of properties in respect of colour, storage stability and gelation rate of catalysed resin. A typical system based on the above polyester fomulation would be ... 

An interest has been developed in the use of vanadium naphthenates as accelerators. In 1956 the author found that if MEKP was added to a polyester resin containing vanadium naphthenate the resin set almost immediately, that is, while the peroxide was still being stirred in. Whereas this effect was quite reproducible with the sample of naphthenate used, subsequent workers have not always obtained the same result. It would thus appear that the curing characteristics are very dependent on the particular grade of resin and of vanadium naphthenate used. It was also observed by the author that the gelation rate did not always increase with increased temperature or accelerator concentration and in some instances there was a retardation. Subsequent workers have found that whilst the behaviour of the naphthenate varies according to such factors as the resin and catalyst used, certain vanadium systems are of value where a high productivity in hand lay-up techniques is desired. 

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

Chromium(III) is a commonly-used crosslinker for preparing profile control gels with polymers having carboxylate and amide functionalities (la,b). Cr(III) is applied in many forms. For example, it can be used in the form of simple chromic salts of chloride and sulfate, or as complexed Cr(III) used in leather tanning (2), or as in situ generated Cr(III) from the redox reaction of dichromate and bisulfite or thiourea. The gelation rate and gel quality depend on which form of Cr(III) is used. 

We have found that the Cr olates produced by hydrolysis of Cr(III) ions are the reactive crosslinking species. The different gelation rates are due to the different degrees of olation. Furthermore, by controlling the degree of hydrolysis, Cr(III) derived from various sources mentioned above can exhibit the same gelation rate. 

We noticed that the gelation of polymers by the redox method is promoted if 2-3 times the calculated molar ratio of thiosulfite is used (see half Equation 9 below). The gelation rate was very slow when x=l or x=4 (Table V). 

Table V. Gelation Rate as a Function of SgOg Concentration...

The gelation rate influences the pore structure. Fast gelation gives an open structure because the particles are quickly connected and cannot undergo further rearrangements. M... 

The significant effect of NaOH/phenol ratio on gel time is plotted in Figure 5. The fastest gelation rate occurred between NaOH/phenol molar ratios... 

The variation of TPAOH amount during the synthesis of silica-aluminas has two macroscopic effects the change of the pH and of the relative amount of TPAOH to silico-aluminate oligomers. The textural properties of the final silica-aluminas may result from the different sequence of hydrolysis and condensation reactions (and the reverse reactions esterification and alcoholic or hydrolytic depolymerization) of the Si and A1 alkoxides. Indeed an increase in TPAOH content and therefore of the pH corresponds to an increase of OH" availability that can favor the hydrolysis and depolymerization reactions, giving rise to different gelation rate and to different network formation. 

In 1969, Majors and Rogers examined in further detail the conditions for imprinted gel formation that affect reproducibility in selective adsorption [31]. In their paper, the authors found that the ageing time, drying method and gelation rate all affect the binding capacity and selectivity much as Dickey had hypothesised... 

Figure 6-7 Influence of High-Methoxyl Pectin Concentration on the Gelation Rate (inverse of the apparent gel time, tc) (A) and on the Rate of Change in G at the Gelation Threshold (B), The slope of the straight line in (A) was 3.6 = 0.981) least squares fit of data in (B) gave a slope of 4.1 and = 0.986.

Figure 6-9 Arrhenius Treatment for the Influence of Temperature on the Gelation Rate of 1% High-Methoxyl Pectin Dispersion (60% Sucrose, pH 3), Measured at Different Aging Temperatures in the Range 15-60°C.

Xantharv chromium gels 42 Temperature dependence of the gelation rate measured at different temperatures Nolle et al. (1992) ... 

Kuo, C.K. Ma, P.X. lonically crosslinked alginate hydrogels as scaffolds for tissue engineering Part 1. Structure, gelation rate and mechanical properties. Biomaterials 2001,22, 511-521. 

The gelation rate of the expanding foam is decreased, thereby preventing thermal pressure cracks and chaning of the foam in large applications. 

The preparation of SiC>2 gel is not very complicated. Either the reaction of an appropriate amount of acid and silicate or the gelation of Si02 colloidal particles can result in Si02 gel. However, the preparation of Si02 gel with various properties is not that simple. Previous studies indicated that the particle size, porosity, and hardness of the Si02 gel strongly depended on the gelation rate that was affected by many factors.1111... 

Dent Glasser and colleagues found that a correlation exists between the gelation rate and the synthesis of zeolites.[41] They found that a low gelation rate would lead to a higher framework density for the synthesized zeolites. The framework densities for some typical zeolites are summarized in Table 5.9. 

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