Cellulose-thickened paints, properties

The introduction of these new rheology modifiers provides paints manufacturers with a very attractive alternative to cellulosic thickeners. These rheology modifiers are the first acrylic associative thickeners that offer a similar rheology, in-can feeling, appearance, and application properties as cellulosic thickeners. They also show some key advantages compared to cellulosic thickeners. Supplied as liquids, they are easier to handle and because of their polymeric chemistry, they are less sensitive to microbial attack In application trials, professional painters found that new rheology modifier based paint performs similarly to cellulose thickened paint but with a better spattering resistance. 

Table II. Properties of Cellulosic- and HEUR-Thickened Paints...

The use of rheology additives such as clays, plant exudates and natural polymers to formulate paints dates back to ancient times. These materials are used to thicken the fluid, suspend dispersions of additives in the fluid and improve the stability of the ensuing dispersion as a function of temperature and shear history. This paper classifies cellulosic rheology modifiers with respect to their influence on paint properties related to rheological profiles. 

The goal is to offer to paint formulators an easy way to convert from cellulosic thickeners to designed rheology modifiers maintaining rheology profile and overall properties. 

Chem. Desctip /tmide with enzyme blocking agents Uses Preservative for paints/coatings, prods, containing cellulosic thickeners, and dry systems such as mortars, joint cements Features In-can preservation stable to 100 C act. against bacteria, molds, and yeasts go for hot climates Properties Crystalline fine powd. 

Cellulosic thickeners are among the most commonly used thickeners for decorative latex-based paints. They are produced by chemical modifications of cellulose such as carboxymethylation or etherification. The most widely used derivatives are hydroxyethyl cellulose, ethylhydroxyethyl cellulose, methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, and sodium carboxymethyl cellulose. The important characteristics that control the properties of different grades of individual derivatives are degree of polymerization, degree of substitution and molar substitution. 

ATER-SOLUBLE POLYMERS (WSPs) are an important class of industrial polymers. They have many applications in solution and in the solid state. In solution, they are widely used as thickeners to control the rheology of various water-based formulations, such as latex paints, drilling muds, foods, cosmetics, and building materials. Chemically modified natural polysaccharides such as starch, cellulose, and guar are a large class of commercial water-soluble polymers. The appropriate chemical modification of these polysaccharides can lead to the modified solution properties needed for specific applications. 

Table I compares typical test results for low-, mid-, and high-effective molecular weight thickeners in a vinyl-acrylic latex paint. Many of the same trends are present with cellulosic and HE UR thickeners when the effective molecular weight designation for HEUR thickeners is used. As the effective molecular weight of an HEUR increases, Stormer thickening efficiency, roller spatter, and water resistance increase, whereas ICI viscosity decreases. The major difference between these two thickener types is in their low-shear behavior. As effective molecular weight increases, cellulosic paints improve in leveling and decrease in sag resistance. HEUR paints decrease in leveling and improve in sag resistance. Other property differences between these two thickener types are highlighted in Table II.

Coatings formulations based on water borne polymers usually depend on specific ingredients for rheological profile adjustment and overall properties optimization. There is a large variety of additives which can be used to modify the rheology of a system. Natural organic derivatives (e.g. cellulose ethers) have been traditionally used in the paint industry and are well established. Synthetic thickeners, on the other hand, have appeared later but quickly became very popular, mostly because of their ease of use (liquid versus solid)... 

Thickeners, mainly cellulose derivatives (e.g., methyl cellulose, ethylhydroxy-propyl cellulose) or polyacrylates, are generally used in emulsion paints. Recently polyurethane thickeners (associative thickeners) with more favorable leveling properties are also increasingly used. 

Chem. Descrip. Bentonite, cellulose gum Uses Thixotrope, thickener for aq. cosmetic/toiletry creams and lotions, putties, caulks, cleaning compds., waxes, polishes, matte and semigloss emulsion and water-reducible paints, adhesives Properties Lt. cream powd. particle size 85% <125 pm sp.gr. 2.41 g/cc ... 

Most ethers are water soluble and are used as thickeners in foods, cosmetics, pharmaceuticals, paints, etc. Critical properties of ethers include solubility, water-binding capacity, nontoxicity, and chemical stabihty. Degree of substitution (DS) determines properties of ethers that is the average number of -OHs substituted on anhydroglucose unit. Table 21.3 shows some of cellulose ether derivatives. 

Methyl cellulose n. A cellulose ether in which some of the cellulosic -OH groups have been replaced by -OCH3. The degree of substitution determines properties and uses as thickeners and emulsifiers. Commercially, a granular, white, flakey material, which acts as a water-soluble thickener and stabilizer used in water-based paints. 

The methyl and ethyl ethers of cellulose have properties that are similar to those of the ester derivatives, and they have been used for related applications. Ethyl cellulose is injected-molded into parts with good low-temperature toughness and used as an additive in coatings and adhesives to improve flexibility. The methyl derivative is used as a thickener and emulsifier for paints, adhesives, food, and pharmaceutical products. 

Water-borne paint. Mainly nonionic cellulose ethers are used as rheology modifiers for water-borne paint. The rheology control of the paint influences such properties as paint consistency, brush load, levelling, sagging and hiding power. Besides the thickening, the polymer takes an active part in the particle stabilization in the paint. This is by far the most important application for a hydrophobically modified cellulose derivative (HM-CD). 

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