Resins in Waterbased Paints

23rd May 2024
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What is waterbased resins?

In the formulation of water-based wood finish paint, the binder or resin is crucial for forming the paint film and determining its performance. The binder, which can be a polymer emulsion, dispersion, or hybrid, should constitute more than 80% of the formulation, ideally more than 90%. Since emulsions, especially aqueous dispersions, typically have low solid content (usually 30%-35%), the formulation should aim to maximize the binder content. This ensures a higher effective film-forming substance in the paint liquid, allowing for a thicker, more substantial paint film with a single coat application.

The choice of emulsion or aqueous dispersion depends on the intended use of the water-based wood paint. For outdoor water-based wood paints, which face harsh weathering conditions, it is recommended to use aliphatic urethane dispersions or acrylic emulsions. Indoor paints can use aromatic polyurethane dispersions or acrylic emulsions, with a high glass transition temperature or the ability to crosslink during curing to enhance water and heat resistance. The binder requirements for putty and primers can be lower to allow users to reduce material usage. In pigmented water-based wood paints, some pigments and fillers are included, thus reducing the amount of emulsion or dispersion used to around 70%-80%. In putty, which contains even more fillers, the amount of emulsion or dispersion can be as low as 50%.

4 kinds of waterbased resins

Vinyl Acetate Emulsion

Polyvinyl acetate emulsion, commonly known as white glue, is widely used as an adhesive in various applications, such as wood, furniture, paper, fiber, and glass. It is also extensively used in products like cigarette adhesives and carpet adhesives.

When used as a coating binder, although it is not of the highest grade, it can meet general requirements. Particularly when formulating coatings, it allows for a high pigment (filler) to binder ratio, enabling the production of coatings of various grades according to specific needs.

In the early days, high-quality polyvinyl acetate emulsions had a solid content of 50%. Due to several price increases of vinyl acetate and specific market demands, many products with different solid contents have been introduced to the market. Currently, well-regarded white glues on the market have a solid content of 35%-36%.

Acrylic Emulsion

Common acrylate monomers used in these resin emulsions include methyl acrylate, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, isobutyl acrylate, methyl methacrylate, ethyl methacrylate, and butyl methacrylate. In addition to producing pure acrylate emulsions through homopolymerization or copolymerization of these acrylates, other monomers are often copolymerized to impart the desired properties to the emulsion polymer. Common comonomers include vinyl acetate, styrene, acrylonitrile, and ethylene.

Functional monomers such as (meth)acrylic acid, maleic acid, fumaric acid, itaconic acid, and (meth)acrylamide, as well as crosslinking monomers such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, N-methylol acrylamide, ethylene glycol dimethacrylate, butanediol diacrylate, trimethylolpropane triacrylate, divinylbenzene, and alkyd resins modified with linseed oil and tung oil, are often added. The amount of carboxyl-containing and crosslinking monomers generally ranges from 1.5% to 5% of the total monomer content. Different monomers impart various properties to the emulsion polymer.

Polyurethane Acrylic Emulsion (PUA)

Waterborne polyurethane resin refers to a binary colloidal system in which polyurethane resin is dissolved or dispersed in water. Polyurethane resins are typically synthesized by reacting compounds containing two or more isocyanate groups (-NCO) with compounds containing two or more active hydrogens, such as hydroxyl or amino groups. As the name suggests, the molecular structure of this polymer contains a significant number of urethane (-NHCOO-) groups.

In addition, the molecular structure may also include ether bonds (-O-), ester bonds (-COOR), urea bonds (OCNCO), and urethane bonds (HNCONCONH). The presence of these bonds leads to multiple hydrogen bonds between adjacent molecular chains. The formation of multiple hydrogen bonds enables linear polymers to exhibit good performance even at relatively low molecular weights. Furthermore, polyurethane can be considered a block copolymer containing soft and hard segments. The soft segments are composed of low-molecular-weight polyols (usually polyether or polyester polyols), while the hard segments consist of polyisocyanates or their combinations with small molecule chain extenders.

Water-Soluble Resin

The water solubility of resin is primarily achieved by introducing sufficient hydrophilic groups, such as -COOH, -OH, and -NH2, into the macromolecular chain. Additionally, reducing the crystallinity of the polymer, as seen in methyl cellulose and ethyl cellulose, also contributes to water solubility. The counter-ion effect of electrolytes is another important factor in promoting resin solubility in water. Hydrogen bonding significantly enhances the water solubility of resins, as many polymers associate with water molecules through hydrogen bonds to achieve solubility.

Temperature is a critical factor affecting the water solubility of resins. For some resins, increased temperature reduces their association with water molecules due to thermal motion, thereby decreasing solubility and causing gel formation. However, for most resins, solubility increases with rising temperature. Naturally, the water solubility of resins is closely related to the structure and molecular weight of the macromolecules. Linear polymers generally have better water solubility than branched polymers. As the molecular weight of the polymer increases, both the dissolution rate and solubility decrease. This is due to the slower diffusion rate of larger macromolecules in water and the increased solution viscosity, which further hinders macromolecular movement.

As film-forming substances in water-based coatings, water-soluble resins are mostly used directly in the production process in the form of aqueous solutions. Therefore, the water solubility of these resins is a crucial property. However, after being used to manufacture water-based coatings, these water-soluble resins must lose their solubility upon application and various forms of drying and dehydration to form an insoluble and durable coating. This transition is achieved through the curing and crosslinking performance of the water-soluble resin.

 

Features of waterbased resins

Resins PA PU PUA PA PUD Water-Soluble Resin
Features Good hardness, excellent abrasion resistance, high transparency, and good chemical resistance. Thermally tacky but brittle at low temperatures. High cost, adjustable hardness, good fullness, excellent abrasion resistance, good film formation at low temperatures, moderate alcohol resistance, good weather resistance, and excellent impact resistance. Combining the properties of polyurethane and acrylic, offering a high cost-performance ratio. Good fullness, slow surface drying which can attract dust, excellent transparency, and outstanding performance. Good transparency, high cost, excellent durability, strong ability to cover sanding marks, and good flexibility. Same appearance as oil-based products, high VOC, poor water resistance, and good fullness.

 

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