Molecular weight resin

Low molecular weight PET and PBT resins are made by melt processes. For higher molecular weight resins, both melt processes or soHd-state polymerization are used. Although terephthaHc acid can be directly esterified, the most common process involves transesterification of dimethyl terephthalate with ethylene glycol or 1,4-butanediol in the presence of trace amounts of metal ion catalysts (67,68). 

In practice, the taffy process is generally employed for only medium molecular-weight resins (1) (n = 1-4). The polymerization reaction results in a highly viscous product (emulsion of water and resin) and the condensation reaction becomes dependent on agitation. At the completion of the reaction, the heterogeneous mixture consists of an alkaline brine solution and a water—resin emulsion and recovery of the product is accompHshed by separation of phases, washing of the taffy resin with water, and removal of water under vacuum. 

These resins are referred to as cationic resins . Paper with improved wet strength may be obtained by adding an ionic resin at the beater stage of a papermaking operation. For the best results a high molecular weight resin is required. 

Small quantities of higher molecular weight resin in powder form are also manufactured. They are used in solution or emulsion form as binders for glass-fibre preforms and also for the manufacture of preimpregnated cloths. 

It is important that care should be taken to remove residual caustic soda and other contaminates when preparing the higher molecular weight resins and in order to avoid the difficulty of washing highly viscous materials these resins may be prepared by a two-stage process. 

Addition of low molecular weight resins with narrow molecular weight distribution produces compatible resin-elastomer blends, while incompatible blends are obtained with resins having a wide molecular weight distribution. In a recent study... 

Epoxy ester Epoxy esters are a type of alkyd where a high molecular weight resin is reacted with alkyd resin. The curing mechanism remains primarily through the oil-oxidation reaction and their properties are in no way similar to the chemically reacted epoxies. They have similar properties to alkyds although with improved chemical resistance but inferior appearance. They form a reasonably hard, oil-resistant coating, which can sometimes be suitable for machinery enamels, but are primarily for interior use, since they tend to chalk rapidly on exteriors. Their best use is for chemical or water resistance where circumstances dictate that finishes that are more superior cannot be used. 

Generally, systems developed in the USA favour a combination of polyethylene with either butyl-rubber or hot-applied mastic adhesives, the latter consisting of a blend of rubber, asphalt and high molecular weight resins. In European and Far East coating plants, epoxy type primers and hard ethylene copolymer adhesives have been successfully employed. 

Reaction media play an important role in m-crcsol-paraformaldehyde reactions.22 Higher molecular weight resins, especially those formed from near-equi-molar m -cresol - formaldehyde ratios, can be obtained by introducing a water miscible solvent such as ethanol, methanol, or dioxane to the reaction. Small amounts of solvent (0.5 mol solvent/mol cresol) increased reaction rates by reducing the viscosity and improving homogeneity. Further increases in solvent, however, diluted the reagent concentrations to an extent that decreased the rates of reaction. 

However, in the presence of acid, 1,3-dihydroxymethylurea (di-methylolurea) condenses with itself to form low molecular weight resins (Figure 7.24). These are more effective, but are water insolu-... 

During drying of the paper, the relatively low molecular weight resins further polymerise to give a three-dimensional network which can protect existing bonds by homocross-linking (Figure 7.25). 

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