II. Types of Acrylic Resins

(1) Classification by Thermal Behavior (Film-forming Characteristics):

① Thermosetting Acrylic Resins: Based on acrylic monomers as their fundamental components, these resins undergo further reactions—either among their own functional groups or with active functional groups in other resin systems (such as amino resins, epoxy resins, polyurethanes, etc.)—during the heating or film-forming process. This results in curing to form a cross-linked network structure. They exhibit excellent color retention, high hardness, good solvent and weather resistance, as well as superior abrasion and scratch resistance.

② Thermoplastic Acrylic Resins: Generally linear polymers, these resins do not undergo further cross-linking reactions during heating or film formation. They can be repeatedly softened by heat and solidified by cooling. They possess excellent gloss and color retention, as well as good water and chemical resistance. Furthermore, they are characterized by ease of molding and processing, rapid film drying, and convenient application.

(2) Classification by Physical State:

① Solid Acrylic Resins: Primarily consisting of thermoplastic acrylic resins—though also including some thermosetting acrylic resins—these materials exhibit excellent mechanical and optical properties at room temperature.

② Liquid Acrylic Resins:

These can be broadly categorized into two main groups: solvent-based acrylic resins and water-based acrylic resins. Solvent-based acrylic resins typically utilize organic solvents as their medium, whereas water-based acrylic resins utilize water as their medium.

Solvent-based Acrylic Resins: These are primarily synthesized through the copolymerization of pure acrylate monomers, resulting in materials characterized by small particle sizes, multifunctionality, and outstanding performance. They typically present as viscous liquids and are widely utilized in fields such as coatings and adhesives. Preparation methods for solvent-based acrylic resins include emulsion polymerization and suspension polymerization. Emulsion polymerization involves the reaction and polymerization of monomers, initiators, and reaction solvents; typically, aromatic solvents (such as toluene or xylene) or esters (such as ethyl acetate or butyl acetate) are employed as the reaction medium. Suspension polymerization is a relatively complex manufacturing process, primarily utilized for the production of solid resins.

Water-based acrylic resins encompass three major categories: water-soluble, water-dispersible, and emulsion-based types. What is commonly referred to within the industry as "water-soluble resin" is, in fact, a dispersion of acrylic resin aggregates formed in water (typically ranging from 0.01 to 0.1 μm); technically, this falls within the colloidal category. However, because the particles within this dispersion are extremely fine—resulting in a transparent appearance—we generally designate it as "water-soluble" to distinguish it from the water-dispersible type. Water-soluble acrylic resins contain a sufficient number of polar or ionic groups within their main or side chains, thereby enabling them to dissolve in water. They can be synthesized using various methods, including solution polymerization, emulsion polymerization, inverse emulsion polymerization, and graft polymerization. Water-soluble acrylic resins feature low curing temperatures and exhibit properties such as viscosity enhancement, electrical conductivity, and ion-exchange capability. They are easily modified, allowing for the addition of various auxiliaries to create products with diverse properties and classifications (e.g., thickeners, fabric treatment agents, dispersants, scale inhibitors, flocculants, water stabilizers, cosmetic additives, etc.). Furthermore, they offer adjustable visual color, excellent gloss, superior adhesion, strong scratch resistance, and high light transmittance.

Emulsion-based acrylic resins are typically produced through the emulsion copolymerization of various acrylate monomers, resulting in a bluish-milky or bluish-white appearance. Depending on the specific ratio of acrylate monomers used, the resulting copolymers can be classified by their film-forming properties as soft, medium-hard, or hard. These resins find application in fields such as coatings, paints, inks, adhesives, rubber products, and chromatography column packing materials. Acrylic resin emulsions possess excellent weather resistance and heat resistance, as well as superior mechanical properties—characterized by high hardness and good abrasion resistance. They feature short surface-drying and through-drying times, facilitate easy application, and form transparent, glossy films with rich, full colors; moreover, they offer flexible modification options and adjustable viscosity. Water-dispersible acrylic resins are synthesized using solution polymerization techniques (specifically, the two-step method or self-emulsification). This process involves incorporating hydrophilic functional groups and subsequently neutralizing them to form salts, thereby rendering the resin hydrophilic and enabling it to disperse stably within an aqueous phase. Characterized by a relatively low molecular weight, these resins exhibit poor film-forming properties when used alone; consequently, they are typically employed within cross-linking systems. The most common variant is the hydroxyl-functionalized acrylic dispersion; when formulated with a curing agent to produce low-VOC, two-component (2K) coatings, it delivers exceptional gloss, weather resistance, and acid-alkali resistance, serving as a viable partial substitute for solvent-based hydroxyl-functional acrylic resins.

(3) Classification by Reaction Type

Reaction-crosslinkable acrylic resins possess functional groups within their prepolymer chains that lack the capacity for self-crosslinking. Therefore, they require the addition of an external crosslinking agent—containing at least two functional groups (e.g., melamine, epoxy resins, etc.)—to undergo a reaction-induced crosslinking and curing process.

Self-crosslinkable acrylic resins, conversely, feature prepolymer chains that inherently contain two or more reactive functional groups (such as hydroxyl, carboxyl, amide, or hydroxymethyl groups). When heated to a specific temperature or in the presence of a catalyst, these functional groups react with one another, enabling the resin to undergo self-crosslinking.

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