Cellulose is a polysaccharide that forms a variety of water-soluble ethers. Cellulose thickeners are nonionic water-soluble polymers. Its use history is very long, more than 30 years, and there are many varieties. They are still used in almost all latex paints and are the mainstream of thickeners. Cellulosic thickeners are very effective in aqueous systems because they thicken the water themselves. In the paint industry, the most commonly used cellulose thickeners are: methyl cellulose (MC), hydroxyethyl cellulose (HEC), ethyl hydroxyethyl cellulose (EHEC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC) and hydrophobically modified hydroxyethyl cellulose (HMHEC). HEC is a water-soluble polysaccharide widely used in the thickening of matt and semi-gloss architectural latex paints. Thickeners are available in different viscosity grades and thickeners with this cellulose have excellent color compatibility and storage stability.
The leveling, anti-splash, film-forming and anti-sagging properties of the coating film depend on the relative molecular weight of HEC. HEC and other non-associated water-soluble polymers thicken the aqueous phase of the coating. Cellulose thickeners can be used alone or in combination with other thickeners to obtain special rheology. Cellulose ethers can have different relative molecular weights and different viscosity grades, ranging from a low molecular weight 2% aqueous solution with a viscosity of about 10 mP s to a high relative molecular weight viscosity of 100 000 mP s. Low molecular weight grades are usually used as protective colloids in latex paint emulsion polymerization, and most commonly used grades (viscosity 4 800–50 000 mP·s) are used as thickeners. The mechanism of this type of thickener is due to the high hydration of hydrogen bonds and the entanglement between molecular chains.
Traditional cellulose is a high molecular weight polymer that thickens mainly through the entanglement between molecular chains. Due to the high viscosity at low shear rate, the leveling property is poor, and it affects the gloss of the coating film. At high shear rate, the viscosity is low, the splash resistance of the coating film is poor, and the fullness of the coating film is not good. The application characteristics of HEC, such as brush resistance, filming and roller spatter, are directly related to the choice of thickener. Also its flow properties such as leveling and sag resistance are largely affected by thickeners.
Hydrophobically modified cellulose (HMHEC) is a cellulose thickener that has hydrophobic modification on some branched chains (several long-chain alkyl groups are introduced along the main chain of the structure). This coating has a higher viscosity at high shear rates and therefore better film formation. Such as Natrosol Plus Grade 330, 331, Cellosize SG-100, Bermocoll EHM-100. Its thickening effect is comparable to that of cellulose ether thickeners with much larger relative molecular mass. It improves the viscosity and leveling of ICI, and reduces the surface tension. For example, the surface tension of HEC is about 67 mN/m, and the surface tension of HMHEC is 55~65 mN/m.
HMHEC has excellent sprayability, anti-sagging, leveling properties, good gloss and anti-pigment caking. It is widely used and has no negative effect on the film formation of fine particle size latex paints. Good film-forming performance and anti-corrosion performance. This particular associative thickener works better with vinyl acetate copolymer systems and has properties similar to other associative thickeners, but with simpler formulations.
Post time: Apr-25-2024