Acrylic Resins

Acrylic resins are obtained through polymerisation via addition reaction on reactive double bonds (C=C) of acrylic monomers and their derivatives, such as acrylic and methacrylic acid, with or without functional groups. Monomer selection is an important parameter in acrylic resins. Acrylic monomers are vinyl monomers containing acid or ester groups (Table 1). Depending on the intended application, various monomer combinations can impart numerous properties to resins, and resins with desired characteristics can be developed (Table 2). The properties of copolymers produced using two or more monomers are superior to those of homopolymers obtained using a single monomer. Another important parameter in acrylic resins is the glass transition temperature (Tg), which is the temperature at which chain motion begins. If a monomer providing very rigid structure is copolymerised with a second monomer providing very soft structure, polymers with desired flexibility and desired Tg can be produced, and Tg values of copolymers can be determined experimentally. There is an approximate relationship between the Tg of copolymers and the Tg values of the homopolymers of monomers in their composition: 1/Tgk= (A1/Tg1+A2/Tg2). Although Tgk value is an approximate value of the copolymer, Tg1 and Tg2 represent the glass transition temperature values of homopolymers, and A1 and A2 represent the weight ratio values of both monomers (A1+A2=1). Acrylic resins are generally produced by solution polymerisation method. There are factors that affect solution viscosity and polymer molecular weight. These are: Reaction Temperature: As heat increases, the number of monomers bonded to active radicals decreases. Thus, polymer chains become shorter and more numerous, producing polymers with low molecular weight and low viscosity. Monomer Concentration: If monomer concentration is low, low molecular weight and low viscosity polymers are obtained; if monomer concentration is high, high molecular weight and high viscosity polymers are obtained. Solvent: For obtaining uniform polymer, monomers must be dissolved in appropriate solvent. Chain transfer agents show different reactivity properties in different solvents and produce polymers of different molecular weights. Solvents with the highest transfer coefficients produce polymers with the lowest molecular weight. Initiator Concentration: The amount of initiator used varies between 0.2%-4%. When initiator concentration decreases, the molecular weight of the polymer increases. When initiator concentration is increased, the molecular weight and viscosity of the resulting polymer decreases. Peroxides that initiate the reaction and generate free radicals when heated have varying radical generation times. Therefore, the decomposition temperatures and half-lives of peroxides must be considered (Table 3).

1.1. Thermoplastic Acrylic Resins

These are long-chain polymers without active functional groups that form films through solvent evaporation without internal chemical reaction, have high molecular weight (~30,000-130,000), and show softening properties when heated. They are obtained by solution polymerisation using initiators that generate free radicals from monomers without functional groups such as methyl (meta) acrylate, butyl (meta) acrylate, acrylic acid, and styrene. Products made with thermoplastic acrylic resin are single-component (1K). Rapid curing and air-drying properties provide ease of application (Figure 1). Thermoplastic acrylic paints and varnishes advantages: • Resistant to UV light. High resistance to external atmospheric conditions. • Resistant to water, lime, and abrasion. • Light-coloured products with very clear structure. • High gloss level. • Can be easily shaped after various thermal treatments.

1.2. Thermosetting Acrylic Resins

Reactive properties are imparted to the polymer using monomer mixtures containing functional groups (hydroxyl, carboxyl, epoxy, amine, amide) (Table 4). Thus, these functional groups react with polymers containing other reactive groups, forming long-chain films with high cross-link density. These are two-component (2K). The molecular weight of thermosetting acrylic resins varies between ~20,000-30,000. Polymers containing hydroxyl groups cure through cross-linking reactions with amino formaldehyde resins and isocyanates; Polymers containing carboxyl groups, through cross-linking reactions with epoxy resins and polyvalent metal salts; Polymers containing amine groups cure through cross-linking reactions with epoxy resin, isocyanate, and amino formaldehyde resins; Polymers containing amido and carbamate groups cure through cross-linking reactions with aldehydes. Thermosetting acrylic paints and varnishes advantages: • Films formed with cross-chemical bonds are resistant to physical and chemical effects. • Create highly glossy films. • High solvent and chemical resistance. • Hard structure with hard film formation. • High UV light resistance, non-yellowing. Applications of thermosetting acrylic resins: • Acrylic resins modified with acrylic amide; used in painting of general household appliances such as dishwashers and refrigerators, • Acrylic resins with carboxyl functionality; when cured with epoxy resin, used in production of household furniture and metal decorative paints, • Acrylic resins with hydroxyl functionality; when cured with amino formaldehyde resins, used in production of automotive topcoat gloss and non-yellowing paints due to their resistance to external conditions and gloss film formation, • When cured with cross-linking agent isocyanate, used in production of automotive refinish paints. References Yürekli, Ş., Resin and Paint Technology, Volume 1, 1995. Deligny, P., Tuck, N., Resins for surface coatings, Acrylics&Epoxies 2nd edition, Vol.1 (Edited by PKT Oldring), 2001. Coating Formulation 2nd Revised Edition, Vinzenz, Network 2012 Gündüz, G., Paint Science, Chamber of Chemical Engineers, Ankara, 2007. Dr. Çiğdem Yüceel Project and R&D Manager Serkim Reçine / Serkim Resin Zeynel Turna R&D Specialist Serkim Reçine / Serkim Resin