Acrylate esters in general, which include butyl acrylate (BA), 2-ethylhexylacrylate (2-EHA), methyl methacrylate (MMA), butyl methacrylate (BMA), and others, represent a versatile family of building blocks for thousands of copolymer compositions. Acrylic resins based on these monomers exhibit excellent weather resistance, high gloss and color retention, and durability. For these reasons, they are the preferred compositions for architectural and industrial coatings, automotive finishes and a wide variety of other applications.
The Importance of Acrylic Ester Copolymerization
Acrylic ester copolymerization is an important technique to achieve systematic tailoring of properties required in a broad range of end-use applications. Glacial acrylic acid (GAA) and glacial methacrylic acid (GMMA) are acrylate monomers used to functionalize acrylic copolymers.
The short-chain acrylic monomers like methyl methacrylate and other monomers like styrene produce harder, more brittle polymers, with high cohesion and strength characteristics. The long-chain monomers like butyl acrylate and 2-ethylhexylacrylate enable soft, flexible, tacky polymers with lower strength characteristics. Monomers like ethyl acrylate, butyl methacrylate, and vinyl acetate contribute more intermediate glass transition and hardness values. Co-monomers such as acrylonitrile and (meth)acrylamide, can improve solvent and oil resistance.
By managing the comonomer ratios and the glass transition temperatures, chemists can balance hardness and softness, tackiness and block resistance, adhesive and cohesive properties, low-temperature flexibility, strength and durability, and other key properties to facilitate end-use goals.
Advancements in film mechanical properties; chemical, water, and abrasion resistance; durability; adhesive properties; and solvent resistance have driven the growth of acrylic copolymers, especially in water-borne technologies. A major contributor to these performance enhancements has been new polymer crosslinking chemistries. Exemplary of this trend is the use of diacetone acrylamide functional monomer, which can be incorporated in acrylic systems to afford controlled crosslinkability.
Let’s take a look at some key facts on acrylate monomers used in CASE applications.
Categories of Acrylic Copolymers
Acrylic-based coatings and adhesives can be classified into all-acrylic formulations in which the building blocks are exclusively acrylic and methacrylic ester types; acrylic-styrene formulations, which also contain styrene; and vinyl-acrylic formulations which also contain vinyl acetate monomer (VAM). The various monomers used in the copolymers can differ widely in glass transition temperature (Tg); the copolymer hydrophobic-hydrophilic balance; hardness and flexibility; and weathering/sunlight resistance. Even with a fixed Tg, copolymers with different monomer combinations vary significantly in the properties of the final paint and coatings. The most common formulations are copolymers of MMA, BA, 2-EHA and GAA, and also VAM in vinyl acrylic polymers.
Functionalization of Acrylic Polymers
Glacial acrylic acid monomer (GAA) and glacial methacrylic acid (GMMA) are unsaturated carboxylic acid co-monomers used to produced acid functional and crosslinked acrylic copolymers and polyacrylic acids. GAA and GMMA readily copolymerize with acrylic and methacrylic esters, ethylene, vinyl acetate, styrene, butadiene, acrylonitrile, maleic esters, vinyl chloride, and vinylidene chloride. Copolymers which contain GAA or GMMA can be solubilized or exhibit improved dispersions in water; the carboxylic acid moiety can be used for coupling or crosslinking reactions, and improved adhesion. Chemists use GAA and GMMA copolymers in the form of their free acid, ammonium salts or alkali salts. Copolymerization accounts for approximately 45 percent of the consumption of acid monomer (the manufacture of acrylate esters is the other major use).
Estimation of Glass Transition Temperature of Acrylic Copolymers
Acrylic copolymer formulations often contain four or more different monomers. We can estimate the glass transition temperature of a random copolymer by using the weight fraction of the different monomers and their Tg values for the homopolymer. This method assumes that the repeat unit of the copolymer can be divided into weighted additive contributions to the Tg that are independent of their neighbors. Reference Tg values for several key monomers are shown below.
Glass transition temperatures, Tg (◦C), of various monomers used in acrylic copolymers:
|Vinyl Acetate Monomer
|Glacial Methacrylic Acid
|Glacial Acrylic Acid
Advanced Crosslinking Technology
Crosslinking chemistry based on diacetone acrylamide (DAAM) and adipic acid dihydrazide (ADH), known as keto-hydrazide crosslinking, represents the most advanced technology for controlled crosslinking of acrylic latex polymers. It involves the direct reaction of the pendant ketone moiety on the DAAM segment with the hydrazide moiety of the ADH.
Self-crosslinking chemistry between diacetone acrylamide and adipic acid dihydrazide begins with copolymerizing DAAM into an acrylic copolymer, using DAAM concentration at ~1-5 wt. % of the monomer mixture. This makes the emulsion copolymer crosslinkable via a pendant ketone carbonyl moiety. Afterwards, the following steps will complete the process:
- The emulsion is neutralized with ammonia, and adipic acid dihydrazide (ADH) is then added to the emulsion as an aqueous solution. The ratio of DAAM to ADH is ~ 2.1 to 1.0.
- On drying off the water and evaporation of the ammonia, coalescence of the film occurs, and the pH becomes acidic. As the pH decreases, the crosslinking reaction rate begins to increase.
- The crosslinking process then takes place (acid catalyzed) with the formation of a chemical bond between the DAAM and the ADH.
Applications of Acrylic Copolymers
Primary applications that take advantage of the characteristics afforded by acrylic copolymers include multiple adhesives, especially pressure-sensitive adhesives(PSA); paint & coatings; caulks & sealants; textile & paper finishes; and printing inks. Because acrylic monomers contribute clarity, toughness, light & weather resistance, and chemical & moisture resistance, manufacturers use acrylic copolymers in interior, exterior, basecoat, and topcoat paint & coating formulations. Paint & coatings, adhesives & sealants, cast & extruded sheet and glazing, and printing inks are among the largest and highest growth global applications for acrylic and methacrylic ester monomers.
Processors use hydroxyfunctional (HEA, HEMA) and carboxyfunctional (GAA, GMAA) acrylic systems in applications like powder coatings were crosslinking is accomplished through isocyanates or melamine crosslinking agents.
Safe Handling of Acrylic Monomers
In addition to being flammable, direct contact with acrylic monomers can cause irritation of the eyes, skin, nose and throat, and are often considered to be skin sensitizers.
Acrylates monomers will readily self-polymerize if not properly inhibited, stored, and handled. Polymerization can be rapid and violent, generating large amounts of heat and pressure. An intercompany committee prepared an excellent reference guide to essential information on the safe handling and storage of inhibited (usually MEHQ) acrylic monomers. In order for the inhibitor to function effectively, it is important to store stabilized acrylate monomers under air and to replenish dissolved oxygen over time. See this brochure for more information: http://www2.basf.us/acrylicmonomers/pdfs/AE_Brochure_3rd.pdf
Glacial acrylic acid requires special attention. The freezing point of GAA is 13°C (55°F); storage temperatures should be maintained at 15 to 25°C (59 to 77°F) at all times. Users should avoid Freezing (or partial freezing) of GAA, because the crystallized GAA excludes the MEHQ and the solid GAA will contain a deficiency of inhibitor and oxygen. The temperature of the medium used to thaw acrylic acid should never be greater than 35-45°C (95-113°F). During thawing, processors should mix the GAA to redistribute the inhibitor and resupply dissolved oxygen. Further, GAA slowly dimerizes upon standing to form diacrylic acid. While this dimer-forming reaction has a slow rate and is not hazardous, diacrylic acid can affect the performance of the GAA at high concentrations by interfering with the free radical polymerization process.
At Gantrade, we encourage our customers to have a comprehensive understanding of the EH&S information and safe product handling procedures when working with acrylate monomers.
How to Purchase High-Purity Acrylic Acid and Ester Monomers
If you’re looking to purchase high-purity acrylate acid and ester monomers, Gantrade Corporation provides butyl acrylate, 2-ethylethyl acrylate, methyl methacrylate, glacial methacrylic acid, butyl methacrylate, and other specialty acrylic monomers for industrial use. Our packaging sizes can be 20 MT (44,080 lbs.) tank trucks, rail cars and drums, depending on the specific monomer and location.
For storage and transportation, we’ve added an inhibitor, usually MEHQ, but also 15-18 ppm of Topanol A in the case of MMA, where FDA compliance and nonstaining characteristics are required. Please contact Gantrade for more information including our Sales Specifications.