Reactive gelation

Reactivity Gelation Chelation Sorption Light attenuation... 

The chemical reactivity of metal alkoxides toward hydrolysis and condensation mainly depends on the positive charge of the metal atom 5(M) and its ability to increase its coordination number N. For a given oxidation state Z, it increases when going down the periodic table as shown in Table 1. Silicon alkoxides are not very reactive. Gelation occurs within several days after water has been added. Cerium alkoxides are very sensitive to moisture. They must be handle with great care in a dry glove box. Precipitation occurs as soon as some water is present. 

Marti, N., Quattrini, R, Butte, A., and MorbideUi, M. 2005. Production of polymeric materials with controlled pore structure The "reactive gelation" process. 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

Addition of dialkyl fumarates to DAP accelerates polymerization maximum rates are obtained for 1 1 molar feeds (41). Methyl aUyl fumarate [74856-71-6] (MAF), CgH QO, homopolymerizes much faster than methyl aUyl maleate [51304-28-0] (MAM) and gelation occurs at low conversion more cyclization occurs with MAM. The greater reactivity of the fumarate double bond is shown in copolymerization of MAF with styrene in bulk. The maximum rate of copolymerization occurs from monomer ratios, almost 1 1 molar, but no maximum is observed from MAM and styrene. Styrene hinders cyclization of both MAF and MAM. 

Polyester Resins. Reinforced polyester resins are thermosets based on unsaturated polyesters from glycols and dibasic acids, either or both of which contain reactive double bonds. The ratio of saturated to unsaturated components controls the degree of cross-linking and thus the rigidity of the product (see Polyesters, unsaturated). Typically, the glycols and acids are esterified until a viscous Hquid results, to which an inhibitor is added to prevent premature gelation. Addition of the monomer, usually styrene, reduces the viscosity to an easily workable level. 

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 ... 

In order to increase the capacity of a production line especially by reducing the necessary press times, adhesive resins with a reactivity as high as possible should be used. This includes two parameters (1) a short gelation time and (2) a rapid bond strength increase, and this even at a low degree of chemical curing. 

Stein166 has indicated that the reactivity of the terminal double bond of the macromonomer (112) is 80% that of VAc monomer. The kinetics of incorporation of 112 have also been considered by Wolf and Burchard175 who concluded that 112 played an important role in determining the time of gelation in VAc homopolymerization in bulk. 

In considering step polymerisation with polyfunctional molecules a number of assumptions are made. They are (i) that all functional groups are equally reactive, (ii) that reactivity is independent of molar mass or solution viscosity, and (iii) that all reactions occur between functional groups on different molecules, i.e. there are no intramolecular reactions. It is found experimentally that these assumptions are not completely valid and tend to lead to an underestimate of the extent of reaction required to bring about gelation. 

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. 

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