Acrylic and Alkyd Resins

Acrylic compounds is the general name given to plastics, resins and oils obtained by synthesis. Depending on the acrylic monomer used, the type of compound and the process conditions, the result can be a hard, transparent solid, a soft, flexible solid or a sticky, viscous liquid. Acrylic resins are the raw material for many products, including moulded building materials, optical components, jewellery, adhesives, coatings and textile fibres. Orlon and Acrilan, for example, are trade names of acrylic fibres, and Plexiglas is the trade name of a glass-like acrylic sheet. In this article we cover acrylic resins and also the alkyd resins widely used in the paint and coatings industry.

What Are Acrylic Resins?

The main monomers of the polymer family known as polyacrylics are acrylic acid, methacrylic acid and their esters. The methyl esters of these acids polymerise readily in the presence of peroxide initiators.

Acrylic resins are made by polymerising or copolymerising monomers based on acrylic acid, methacrylic acid, acrylonitrile and their esters, and are also known as acrylate resins. Their colour and gloss retention, resistance to alkalis and oxidation, hardness, adhesion and binding strength, and long film life give them an important place among coating formulations.

Acrylic Acid

Acrylic acid is an organic acid with the formula CH2=CHCOOH and is also called propenoic acid. Today it is produced industrially mainly by the two-stage catalytic oxidation of propylene (first to acrolein, then to acrylic acid). The Reppe process, based on the reaction of acetylene, carbon monoxide and water over a nickel catalyst, and the hydrolysis of acrylonitrile are older routes that have largely been abandoned. Acrylic acid is the starting material for acrylic polymers.

Acrylic-Based Paint

Acrylic-based paints are water-thinned synthetic paints that use an acrylic resin as the binder. Other binders such as styrene, epoxy or polyvinyl acetate can be added to the formulation to give specific properties or to reduce cost. An acrylic binder dries quickly, flows smoothly, cleans up easily with water and mixes readily with pigments. It can give both the transparent brilliance of watercolour and the body of oil paint, and it withstands heat and other damaging influences better than oil paint.

The latex on which acrylic-based paints are built is defined as follows:

1. Essentially, a stable dispersion of a polymeric substance in an aqueous medium.
2. A dispersion of a plastic or resin in water; latex may be natural or synthetic, and synthetic latex is made by emulsion polymerisation. Because the product obtained after polymerisation is a solid dispersed in water, it is technically not an emulsion; in the paint industry, however, the terms latex and emulsion are used interchangeably.

High-performance acrylic-based paints contain only acrylic resin as the binder.

100% acrylic paint: a water-thinned paint that uses acrylic resin as its only binder. Paints of this quality are used in many different architectural coatings and offer strong adhesion, long-term flexibility, breathability, alkali resistance, hardness and gloss retention.

Alkyd Resins

Alkyd resins are polyester-type polymers produced by the condensation of oils or fatty acids with polyacids and polyols, and they are mainly used as binders in topcoats.

In 1927 it was found that alkyds made with the fatty acids of drying oils dry in air and form durable films. Industrial use developed steadily from then on, and alkyd resins have taken the important place they hold today.

Classification of Alkyd Resins

Because oil or fatty acid makes up a large part of their structure, alkyd resins are classified by oil content (oil length). For example, alkyds with 35–45% oil can be classed as short oil, 46–55% as medium oil, 56–70% as long oil and above 70% as very long oil.

Alkyd Production Processes

1. Fatty acid method: the whole charge of fatty acids, polyhydric alcohol and dibasic acid is heated to 210–250 °C and held at this temperature until the required properties are reached. If the polyhydric alcohol and dibasic acid are heated before the fatty acid is added, viscosity rises sharply and the molecular weight distribution becomes very broad. Stability after the addition of driers is also poor, and the film skins over prematurely.
2. Fatty acid–oil method: fatty acids, oil, polyol and dibasic acid are again heated to 210–250 °C and held there until the required properties are reached. Compared with the monoglyceride method, the films dry quickly at the surface and slowly underneath. There is a risk of partial gelation during the reaction.
3. Oil dilution method: oil is added at high temperature to an alkyd prepared by one of the other methods. This improves brushability, but the films are soft and weak and the colour deteriorates quickly.
4. Alcoholysis (monoglyceride) method: the oil is first reacted with a polyol, usually glycerine and sometimes another polyol, to form partial esters (monoglycerides). The remaining hydroxyl groups are then reacted with the dibasic acid. The first stage is carried out at around 280 °C without a catalyst, or at around 230–240 °C with one. Oil does not dissolve in methanol, whereas the monoglyceride formed dissolves in twice its volume of methanol; this test shows that the first stage is complete. The temperature is then lowered to around 200 °C, the dibasic acid and, if needed, some more polyol are added, and the temperature is raised to 230–250 °C. The product is held at this temperature until it reaches the required properties.

Factors Affecting Alkyd Manufacture

The main factors affecting alkyd manufacture are the raw materials, the main reactions, the order of addition, the agitator type and speed, the reaction temperature and medium, and the catalysts.

The raw materials are mainly polybasic acids, polyhydric alcohols and fatty acids. Among the polybasic, or more precisely dibasic, acids the most widely used are the aromatic phthalic acid isomers, above all phthalic anhydride. Unsaturated maleic anhydride and fumaric acid are also important. Aliphatic dibasic acids give softer products with glycerine as the carbon chain between their carboxyl groups gets longer. Maleic acid, which has no such chain, and the phthalic acid isomers, whose carboxyl groups are fixed to the benzene ring and cannot change position, give hard and brittle resins.

The most important monobasic components of alkyd resins are fatty acids. Almost all of them are obtained from vegetable oils, that is, from glycerides. The most important are stearic, oleic, linoleic, linolenic, eleostearic, ricinoleic and dehydrated ricinoleic acids.

The driers added to alkyds are oil-soluble salts of cobalt (Co), manganese (Mn), zirconium (Zr), calcium (Ca) and zinc (Zn), normally used at 0.1–1%. Lead (Pb) driers, common in the past, are now restricted by health and environmental regulations.

Acrylic and alkyd resin production requires temperature-controlled reactors and process equipment such as condensers and decanters. Makfen designs and manufactures this equipment for resin production plants.