Paint Know how

Specifications

PAINT INGREDIENTS

The main ingredients of paints are pigments and binders. Pigments are used for opacity and coloring purposes, whereas binders are used for adhesion on specific substrates, film integrity, durability etc. Addi­tionally paint also contain solvents and additives. Solvents ease the application, flow and leveling process of paints, whereas additives control specific properties like viscosity, settling, dispersion stability, skimming etc. Generally, the composition of the paints used at present is as indica­ted |n table 1.3.

Table 1.3—Composition of Paints

     Solvent based               Water based

Components                        Enamels     Primers      Oil bound         Plastic

           distemper      Emulsion Paint

In modern times economy in cost, time, labor and energy has brought about significant changes in paint formulations and their composition.

Binders: Until the development of synthetic resins, natural resins like shellac, copal gum, dammar cooked with oils or treated oils were very commonly used binders. But now alkyd resins are more commonly used because of the improved overall performance of the paints incorporating it. By their use the adhesion, chemical resistance, gloss, durability and film integrity are considerably improved at a little extra cost. Today almost 80% of the paints manufactured in India utilize alkyd resins as binders. The main raw materials used in their manufacture are vegetable and marine oils; dibasic acids like phthalic anhydride, isophthalic acid; and polyols like glycerine and pent aery thritol. These resins are compara­tively simple to synthesize. The production of alkyd resins is reserved for small scale at present. These resins are used in the manufacture of ready mixed enamels which constitute about 57% of the total production of paints in the country. These resins are soluble in relatively inexpensive solvents like mineral turpentine and give pale colored, non-yellowing, quick drying varnishes.

Other resins like chlorinated rubber, vinyl resins, epoxies, acrylics etc require exotic solvents like ethers, acetates etc. The usage of these coatings is restricted mainly for protection purposes. In automotive finishes, a change over from nitrocellulose to acrylic is now perceptible, owing to the toughness, durability and color retention properties of the latter. But polyurethane and silicones have not as yet made any signifi­cant dent in the  paint market chiefly because of their non-availa­bility and high price.

At present, the large scale production of polymers and binders of low cost depends on a small group of inexpensive monomers, produced by the petrochemical industry viz. ethylene,

propylene, isobutylene, butadiene, styrene, vinyl chloride, vinyl acetate and acrylics. It is likely that over the next decade almost 80 per cent or even more of polymeric materials will

be made from these monomers or combination of them. Nevertheless there still remains considerable scope for the production of new materials from the existing monomers by copolymerization, poly blending and 'cross-linking'.

Pigments: At present rutile titanium dioxide is perhaps the most widely consumed pigment which has already replaced white lead, zinc oxide and lithopone because of its highest hiding power amongst all the white pigments. Among other pigments the discovery of prussian blue in 1709 was by far the most important. The success in isolating various metals like cobalt, zinc, chromium and lead, eventually made it possible to discover many modern pigments. Synthesis of organic pigments like mono-azo reds, phthalocyanine blue gave bright shades, perfectly ade­quate and suitable for oil and water based paints. Lead chromes, ranging from greenish yellow to bright yellow pigments, have until today no equal in brightness of shade, opacity, gloss retention and durability at mode­rate price, though environmental pressure threatened their elimination from paint compositions.

Availability of Raw Materials: The paint industry is extremely raw material intensive industry. A company manufacturing industrial and decorative products like plastic emulsion paints, distempers, primers, air-drying and stoving enamels, putties and chemical resistant paints, has to maintain an inventory of as many as 500-600 different raw materials, although approximately 30 out of these are major and constitute approximately 80% of the total purchase billj Based on their weighted average consumption in paint formulations and assuming a total turnover of paint industry at USD 52 MILLIION, the cost of raw materials of paint is approximately USD 26 MILLION.

PAINT MANUFACTURING

Paint is made by mixing the pigment with part of the vehicle, grinding the mixture to obtain good dispersion, adding the remainder of the vehicle, and tinting the batch to the correct color, followed by straining to remove foreign particles, then packaging. The ratio of pigment to vehicle varies widely depending on specific properties of the pigment and vehicle and the requirements of the coating. The ratio may be expressed on a weight basis or on a volume basis. For paint formulating and pro­duction it is convenient to use the weight basis. However, when considering film characteristics it is better to know the concentration of pigment by volume in the dry coating. This ratio usually is expressed as PVC—pigment volume concentration in the dry coating. Since the binder is the vehicle solids of a paint, PVC is also equivalent to pigment-vehicle solids ratio by volume. The volume ratio may be calculated readily from the weight ratio and the bulking values of the various constituents. The specific gravity of pigments varies considerably; therefore, the same weight of different pigments may occupy entirely different volumes

Binders for organic coatings are available in five different conditions; (lV as 100% solids without volatile material; (2) as solutions in organic solvents; (3) as dispersions in volatile organic compounds; (4) as solutions in water; and (5) as emulsions with water as the continu­ous phase. These conditions represent two general types—organic and aqueous—and they behave differently in their wetting action on various surfaces. The organic type wets organophilic surfaces more readily than hydrophilic surfaces and vice versa. Since pigments range in their surface characteristics from organophilic to hydrophilic, care must be taken to select suitable pigment-vehicle combinations to obtain best dispersion. Certain pigments are surface treated by the manufacturer so that they are available in both organophilic and hydrophilic grades. A variety of additives also is available to facilitate wetting or dispersion and to maintain good dispersion in the paint.

The particle size of pigments ranges from 0.01 micron for fine grades of carbon black to about 25.0 microns for coarse earth pigments. Since very fine particles have much greater surface area per unit of weight than coarse particles, more work is required to wet or disperse the fine particles in paint vehicles. Pigments, like all fine particle size materials, form aggregates in the dry state. It has been quite well established that the grinding or dispersion processes employed in the paint industry break down the pigment aggregates to varying degrees but do not reduce the ultimate particle size of the pigment to a significant extent. The ultimate particle size of a pigment is determined by the manufac­turing process employed for its manufacture.

More vehicle is required to wet a unit weight of fine particles than is required for coarse particles, because of the larger surface area of the former. The amount of vehicle required to wet a given weight of pigment is known as its 'oil absorption'. The consistency of paint is affected to  /some extent by the oil absorption of the pigments. A high oil absorption A pigment produces greater consistency than a low oil absorption type, because it absorbs more vehicle on the pigment surface so that there is less free vehicle in the paint. Consistency is also effected by pigment vehicle ratio, viscosity of vehicle, and factors such as reactivity of pigment with the vehicle and presence of materials such as dispersing agents and moisture. Since consistency affects the application and flow properties of coatings, it must be maintained uniform within relatively narrow limits for specific coatings. It might appear that excessive consistency in a particular batch of paint could be reduced by simple addition of volatile solvent. This unfeasible remedy would lower the solid content of the paint, and a thinner coating would result if such paint were applied over the normal area per gallon.

It is also essential that paints be applied uniformly and at the correct film thickness required for various service conditions. Premature failure of coatings may result from improper application and/or insufficient preparation of the surfaces to which the coating are applied.

The problem of maintaining color standards and matching successive batches of paint to a standard is a difficult one. Usually, colored paints contain more than one color pigment. Although care must be taken to weigh the correct amount of each color for a paint, the procedure is not accurate enough to avoid tinting each batch to standard. 

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