Лако-красочные материалы — производство

Some thermosetting binders, such as resins based on drying oil modified resins, like alkyds and epoxy esters, are cured by a cross­linking reaction that is triggered by the diradical and atmospheric oxygen; this is commonly known as oxidative or autoxidative curing. The first step in the cross-linking reaction is oxygen uptake by the film and autoxidation of an active methylene group of an unsatura­ted fatty acid by oxygen to form hydroperoxide. Subsequent steps in the autoxidation process involve hydroperoxide decomposition to a free radical, which in turn initiates the curing reaction (polyme­

rization). The autoxidation process proceeds at a very slow rate that is practically not acceptable; therefore, catalysts are used to speed up drying of such systems.

Driers, also referred as siccatives, are compounds used to catalyze the autoxidation process in drying or semidrying oil based resins at ambient or elevated temperatures. They are typically organometallic compounds, most commonly metal soaps of long chain monocarboxy­lic acids, supplied in a suitable solvent (carrier). These metal soaps are synthesized from a variety of metals and acids. Although the metal, being the active part of the compound, effects the drying reaction, the monocarboxylic acid component confers solubility and compatibility of the drier in solvents and resin.

The first driers were based on fatty acids or rosin, which were sub­sequently replaced by naphthenic acid, a material obtained from crude petroleum. Due to scarcity of naphthenic acid, in modern driers it has been replaced by branched chain synthetic acids such as 2-ethylhexanoic acid and neodecanoic acid.

Driers are supplied commercially as solutions in hydrocarbon sol­vents such as white spirit and a low amount of alcohols to improve stability against precipitation. Driers are generally specified by their metal content in the supplied form.

Based on their catalytic effects, driers can be classified into two groups: active driers (primary) and auxiliary driers. Active driers are soaps of metals that have more than one oxidation state and the­refore act as redox catalysts. They promote the uptake of oxygen and decomposition of hydroperoxides, which leads to cross-linking of resins that produce tack-free films. They are also known as surface or top driers. Another group of metals with a single oxidation state have a weak catalytic effect when used alone but are effective in combination with active driers. Some that promote the polymeriza­tion reaction for curing of resins are termed through-driers, while others that enhance the effect of redox catalysts are known as auxi­liary driers. Certain driers are ineffective at ambient conditions but they are catalytically active at elevated temperatures. Common exa­mples of active driers (redox type) are cobalt, manganese, cerium, vanadium and iron, while examples of through-driers are lead, zir­conium, lanthanum, neodymium, aluminum, bismuth, barium and strontium. Some widely used auxiliary driers are calcium, lithium, potassium and zinc. Some pure organic compounds are also known to act as driers. Such driers function by their interaction with carboxyl and hydroxyl groups on the alkyd, producing coordination linkages. In effect, they act as through-driers and are also called coordination driers. Generally they are used along with a primary drier, such as cobalt, to give optimum drying performance, provided that the resin has sufficient acid and hydroxyl groups.

Cobalt is the most important and most widely used active drier metal that acts as a surface drier. If used alone or excessively, it may cause surface wrinkling and poor through-drying. In recent days, use of cobalt is becoming questionable due to its possible toxicological effects. Other cobalt-free alternatives are available on the market. They include coordination complexes of metals such as iron, acting as an effective substitute for conventional cobalt-based driers. Man­ganese and vanadium are other alternatives to cobalt. Manganese is an effective active drier, but it is not suitable for white paints, as it can cause considerable discoloration.

Iron and cerium driers effectively catalyze polymerization and through — drying at elevated temperatures and therefore are used in baking ena­mels. Iron driers have the limitation of dark color, and hence cerium based driers are often preferred where color retention is important.

Lead was used for years as a very effective through-drier, but for ecological and toxicity reasons, is rarely used in recent years. Zir­conium driers are by far the most widely accepted replacements for lead. They improve through-drying mainly by the formation of coordination bonds with hydroxyl and carboxylic acid groups of the resin, and also by increasing the catalytic effect of primary driers.

Zinc driers have the capability to keep the film open, thus permit­ting curing of the film through the entire thickness and preventing surface wrinkling. Zinc driers are also used as effective wetting and pigment dispersing agents. Calcium driers are very useful and effective when used along with cobalt and zirconium to promote drying under adverse weather conditions, such as low temperatures and high humidity.

A combination of primary and auxiliary driers is often used to achieve optimum curing with the minimum drier dose. Some opti­mized combinations are sold as mixed driers. Most driers are added in the let-down stage of paint manufacturing, except for auxiliary driers such as zinc and calcium that are generally added to the mill base due to their effectiveness as wetting and dispersing agents.