Aqueous resins

It should be possible to form linear noncross-linked polymers of melamine—formaldehyde or phenol—formaldehyde by reaction of one mole of the patent with one mole of formaldehyde, but this is generally not the case. The melamine crystal itself is very insoluble in water and only becomes soluble as the formaldehyde molecules add on. If much less than 1.5 moles of formaldehyde pet mole of melamine ate used, the aqueous resin solution is very unstable. 

Two classes of resol are generally distinguished, water-soluble resins prepared using caustic soda as catalyst, and spirit-soluble resins which are catalysed by addition of ammonia. The water-soluble resins are usually only partially dehydrated during manufacture to give an aqueous resin solution with a solids content of about 70%. The solution viscosity can critically affect the success in a given application. Water-soluble resols are used mainly for mechanical grade paper and cloth laminates and in decorative laminates. 

Melamine can also be added to a UF-resin in the form of melamine salts like acetates, formates or oxalates [31-34]. These decompose in the aqueous resin mix at higher temperatures and enable considerable savings of melamine for the same degree of water resistance compared to traditionally prepared MUF-resins. These salts can act additionally as hardeners. 

The generally poorer mechanical properties exhibited by acetylated lignocellulosic material in composites bonded using aqueous resin systems was considered by Korai etal. (2001). Fibres of yellow cedar (Chamaecyparis nootkatensis) were acetylated to a WPG of 24.8 % and then ozonated to different extents to increase the hydrophilicity of the fibre surface. Boards were fabricated from the fibres using an aqueous MF resin. Ozonation improved IBS of boards fabricated from acetylated fibres, proportional to level of ozone charge, and resulted in IBS values comparable to those of nonacetylated controls at higher levels of ozonation. However, although ozonation also improved MOR, the values obtained for acetylated fibres were always less than those obtained with unmodified fibres. 

Water-dispersible polyisocyanate compositions containing polyethylene oxide were previously prepared by the authors (2) and used as aqueous resin adhesives. 

Similar reports have appeared in the technology of polymerizing acrylic monomers such as acrylonitrile (753-759) or acrylamide (760-763), Korolev et al. (764) have used ascorbic acid as a reducing agent in the mixture to increase the polymerization rate. The kinetics of polymerization in the presence of oxygen has been studied in systems containing ascorbic acid (765), Recent patents (766,767) have been issued with ascorbic acid in the dispersion of acrylic aqueous resins. 

Solution stability of the aqueous resin system is also a key factor with regard to co-solvents for electrodeposition paints. It was determined at both room temperature and 40°C. The criteria used were pH, conductivity, and haze. Haze is a function of light transmission at 5560 A and was measured using a Cary model 14 spectrometer. All tests were run in sealed containers. 

A second approach to postembedding electron microscopic immunocytochemistry is the use of special aqueous resins, while these resins are still hydrophobic, they tolerate some aqueous material in the tissue. Examples of these resins are LR White or Lowicryl K4M. Another approach with aqueous embedding is to freeze tissue in buffer very rapidly, and to section with a cryo-ultramicrotome. Cryo-ultramicrotomy is a technique requiring considerable training and cannot be used by the novice. 

In the method outlined here, aqueous resin-soil suspensions are equilibrated for 18 hours, the soil is washed from the resin, the sorbed Mo is displaced from the resin with 2-M NaN03, and the Mo in the leachate is measured. The extraction procedure is a modification of that of Jarrell and Dawson (1978), with the procedure for organic-matter destruction as described by Jackson and Meglen (1975). Measurement of the Mo obtained after extraction is carried out using the automated KI-H2O2 procedure (Chapter 6, this volume). 

The superprimer formulation is usually based on an ionic or non-ionic aqueous resin dispersion. The solid content of the water-borne resin is often around 55wt% and it may contain small amounts of organic solvents. Typical resins used in the primers are epoxies, acrylates and polyurethanes. The minor binder is incorporated in order to improve a particular coating property such as cold formability. Table 1 gives an overview of the family of superprimers based on different resin-cross-linker-silane-pigment combinations invented and investigated in our laboratory [7-11]. 

Melamine can also be added in the form of melamine salts, such as acetates, formiates, or oxalates (47-53), which decompose in the aqueous resin mix at higher temperatures. The melamine is then incorporated into the UF resin, forming an MUF resin and generating acids as latent hardeners. Furthermore it has been reported that using this procedure with melamine salts, the amount of melamine needed is much lower than in other MUF resins (47-52). MUF resins can also contain various other compounds that can react with formaldehyde such as urea derivatives, guanamines, or amides (54,55). 

Thermosetting aqueous resins Electrophotographic printing plate resins... 

Wiersma et al. at DSM Research, Geleen, The Netherlands [284] described an interesting way of coating textile fibers with CP/polyurethane and CP/alkyd blends from aqueous colloidal dispersions made in a commercially available aqueous resin dispersions. The dispersions included a polyurethane-in-water dispersion (DSM Resins URAFLEX XP 401 UZ, 40% solids content). A variant of the template-type in-situ polymerization was used. After polymerization, the dispersed CP sol could be applied as a coating onto the textile fibers. CPs used included P(Py) and P(ANi). 

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