Acrylic resins represent the largest category of polymers used in the coatings, adhesives, inks and finishes industries. Based on acrylate and methacrylate monomers, they provide excellent durability and weather resistance, gloss retention, and excellent adhesion, abrasion and thermal resistance characteristics. They are widely used in water-borne, solvent based, powder and radiation-cured technologies.
Acrylate Copolymerization Reactivity Ratios
The “reactivity ratio” of acrylic monomer pairs is the most important parameter in determining the compositional architecture in standard free-radical copolymerizations. Published data provides reactivity ratios for common monomers used in copolymerization. Using published reactivity ratios and feed compositions of the contributing monomers allows calculation of the final copolymer compositional distribution. When reactivity ratios differ, heterogeneity exists in the copolymer compositions.
Acrylic adhesives come in multiple forms and associated chemistries. High polymer resins in a solvent or an aqueous emulsion can be applied to a substrate and the solvent or water then evaporated. Two component and thermosetting acrylic adhesives are formulated from crosslinkable polymer systems and monomers, and are cured in place using, for example, peroxide initiators and activators. Polymerizable mixtures of polymers dissolved in MMA monomer are popular structural adhesives supplied as two component systems containing a metal salt or amine that reacts with a peroxide component upon mixing to initiate polymerization of the MMA monomer. Other modes of application for acrylic adhesives include UV cure and hot melts formulated with tackifiers and plasticizers.
Acrylic elastomers fall into two categories of thermoplastics, ethylene acrylate copolymers (AEMs) like EEA copolymers and all acrylic copolymers (polyacrylates), often based on ethyl acrylate copolymers. Tg’s are usually well below room temperature. They are widely used in the automotive industry, seals & gaskets, construction and roof coatings. The acrylic elastomers exhibit good oil and fluid resistance, weatherability and high temperature resistance.
Acrylic emulsions and latex are water borne, environmentally friendly systems with total resin solids levels of ~50-65 %. Emulsion resins are prepared by polymerization of acrylic monomer stabilized in water with surfactants or protective colloids. The emulsion/latex are easy to handle, apply and clean-up, easy to formulate with low VOC contents and are in low hazard. Modern emulsion systems exhibit enhanced durability, high gloss, abrasion resistance, adhesion and excellent weathering properties. They are used in industrial and architectural coatings, adhesives, inks, textile finishes and a myriad of other applications.
Acrylic Ink Binders
Acrylic resins are used as binder components in flexographic, gravure and screen printing inks, and as overprint coatings. Acrylic resins contribute to good pigment dispersion, excellent adhesion to a wide range of substrates, UV, temperature and abrasion resistance, and resistance to greases, oil and water. Anionic, acrylic and styrene acrylic ink vehicles are used in both solvent and water based liquid inks. Functionalized acrylic oligomers are also used in UV and EB cured inks.
Acrylic Powder Coating Resins
Acrylic polymers occupy a significant niche’ in powder coatings and have shown high growth rates. Acrylics used in powder coatings are multifunctional resins cured with crosslinking additives. The crosslinkers are dependent on the functionality of the acrylic – hydroxy, epoxy, carboxy and combinations of all. Crosslinking agents include triglycidyl isocyanurates (TGIC), blocked isocyanates and dicarboxylic acids. Powder coating applications range from automotive appearance and trim parts, clear coats, wheel coatings, window frames, appliance coatings and others. These coatings cure to hard, conformal surfaces with scratch and chip resistance, external durability, chemical resistance and abrasion and mar resistance.
Acrylic Radiation Curable Coatings
The most common resins for Rad Cure coatings, which includes UV cure and electron beam, are acrylate macromolecules, di-ann tri-acrylate monomers, urethane acrylics, epoxy acrylates and silicone acrylates. The acrylic coating formulations consist of acrylic functional oligomers, multifunctional acrylate monomer diluents and photo-initiators in the case of UV cure. The primary application areas are in wood & industrial metal coatings, adhesives, electronics, packaging and inks. The driver for growth of Rad Cure systems is non-VOC requirements.
Crosslinkable Acrylic Resins
Acrylic resins can be copolymerized with a wide range of functional monomers to provide systems that can be crosslinked or self-crosslinked. As a class, crosslinkable acrylic resins exhibit lower molecular weights and viscosities, and thus allow higher application solids. Functionalization improves substrate adhesion, pigment wetting and a host of other properties. After crosslinking, coatings and films demonstrate excellent solvent and moisture resistance, durability and abrasion (scrub) resistance and better thermal properties. Crosslinking can be achieved through functionalized acrylic resins and keto-hydrazide technologies (DAAM & ADH pairs), isocyanates, melamines, epoxy resins and moisture curable silane functionality.
Functionalized Acrylic Resins
Functional comonomers for acrylic resins can be classified as follows. Carboxy-functional (acrylic acid & methacrylic acid), hydroxy-functional (hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate), epoxy-functional (glycidyl methacrylate), silane-functional (methacryloxypropyl trimethoxysilane, vinyl trimethoxysilane), amino-functional (dimethylaminoethyl methacrylate) and keto-functional (diacetone acrylamide). Suitable crosslinking agents include amino-formaldehyde adducts like melamine-formaldehyde resins, diamine such as adipic dihydrazine, epoxy resins such as TGIC, and diisocyantes. Silyl-functional acrylic resins can be self crosslinked and moisture cured.
Glass Transition Temperature, Tg
For polymers, the Tg is the temperature at which the polymer transitions from a hard and glassy state to an elastomeric, soft and viscous state, when increasing the temperature. This transition is reversible, in that the material will return to its hard and glassy state when cooled below the Tg. Depending on the application, polymers are used at temperatures below the Tg (e.g. in paints and coatings) or above the Tg (e.g. in pressure sensitive adhesives). The Tg of an acrylic resin can be predicted from the Tg contributions of the individual acrylate monomers. The Tg influences many properties including surface properties, flexibility, hardness, Minimum Film-forming Temperatures, etc.
Minimum Film-forming Temperature
The Minimum Film-forming Temperature of an acrylic latex is the lowest temperature at which the emulsion system uniformly coalesce and form a continuous film. For architectural paints, the MFT is generally formulated to be below 5 °C. In industrial applications and inks where high temperatures are experienced, the MFR can be higher. MFT’s are influence by the Tg of the polymer, the emulsion particle size and formulation, such as the incorporation of coalescent agents.
Solvent Based Acrylic Resins
Solvent based acrylic resins are used in coatings, adhesives and ink. They can be basic acrylic copolymers or crosslinkable systems based on moisture cure, diisocyanates, melamine, epoxy and other types of crosslinking agents. Solvents include butyl acetate, butanol, isopropanol, toluene and xylene. Solvent based acrylic systems are known for excellent durability, weatherability and good appearance characteristics.
|Butyl Acrylate (BA)||Methacrylic Acid (MAA)|
|Butyl Methacrylate (BMA)||Methyl Acrylate (MA)|
|Diacetone acrylamide (DAAM)||Methyl Methacrylate (MMA)|
|Ethyl Acrylate (EA)||Vinyl Acetate Monomer (VAM)|
|2-Ethylhexyl Acrylate (2-EHA)||Vinyl Neodecanoate (Shivena 10)|
|Glacial Acrylic Acid (GAA)||Adipic Dihydrazide for crosslinking|