METHOCEL™ - High Performance Thickeners for Coatings
Thicken Latex Paints with METHOCEL™
The market for architectural paints and coatings has seen substantial growth over the last decade due to growing populations, a growing housing market, and increased infrastructure spending. Increased demand for high performance eco-friendly coatings has driven the market toward water-based paints, and while latex technology has come a long way over the last 60 years formulations still need to be optimized to meet performance criteria of consumers. The precise control of viscosity and rheology provided by METHOCEL™ is critical to maximizing the performance of latex paints.
What is METHOCEL™?
METHOCEL™ cellulose ethers are a family of water soluble methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) polymers that are used to effectively thicken latex paints. Latex paints thickened with METHOCEL™ offer a number of benefits.
Expiration of latex paints is typically determined by a loss of viscosity and loss of pigment suspension. The shelf life of latex paints can be extended when METHOCEL™ is used as a thickener and rheology modifier. METHOCEL™ is more resistant to enzymes released by microbes than other cellulose thickeners. This and pH stability from pH 3 to 11 allow for METHOCEL™ thickened latex paints to maintain uniform viscosity regardless of age or pH. Additionally, latex paints that have lost viscosity due to spoilage can be rethickened with METHOCEL™ to achieve a proper and stable viscosity.
Due to the pseudoplastic nature of METHOCEL™, latex paints thickened with the polymers exhibit exceptional application properties. METHOCEL™ improves the loading of the paint onto brushes and rollers. In addition, flow, leveling, and spreadibility are improved due to the rheology modifier.
Water based architectural coatings are subjected to harsh conditions on a regular basis. They need to be able to withstand cleaning chemicals, scrubbing, rain, and condensation. The addition of METHOCEL™ to water based paints and coatings helps to improve the durability and resistance to chemicals and moisture. Additionally, pin-hole formation is reduced and film uniformity is improved over surfaces with varying porosity.
Selecting the Proper Grade of METHOCEL™
Selection of METHOCEL™ grades is determined by two main criteria: viscosity (molecular weight) and degree of substitution. These criteria can be identified by the nomenclature of the grade. For example:
METHOCEL™ are cellulose-based polymers that have had hydroxyl functional groups replaced with methoxyl and hydroxypropyl functional groups. The degree and nature of the substitution vary and can be identified by the first letter of the name of the degree.
The amount and type of substitution lend to differences in organosolubility and thermal gelation temperatures of the polymers. In general, organosolubility of METHOCEL™ is increased as hydroxypropyl substitution is increased, and the thermal gelation temperature of METHOCEL™ decreases with increasing methoxyl substitution. In latex paint formulations, the "J" and "K" chemistry is typically recommended as they are compatible with most cosolvents used to improve film formation.
The viscosity of solutions produced by METHOCEL™ in water varies from grade to grade based on the molecular weight of the polymer. The mean viscosity of a solution of 2% METHOCEL™ in water is reported in each grade by a number and a "multiplier" letter with C being 100 and M being 1000. For example the grade listed above, J75MS, would have a viscosity of 75,000 mPa*s at 2% in water. The viscosity of METHOCEL™ solutions will also vary based on the concentration in which they are added and the solvent selection. The graph below shows viscosity of common METHOCEL™ grades in DI water at varying concentrations.
Another important factor to consider in regards to the rheology of solutions containing METHOCEL™ is that pseudoplasticity will increase as the molecular weight of the polymer and concentration increases. The graphs below highlight this relationship.
The final attribute that needs to be considered when selecting a METHOCEL™ grade is surface treatment. Grades of METHOCEL™ that are denoted with an "S" after the viscosity are surface treated for delayed hydration. These will not hydrate and begin to thicken until a pH adjustment of the solution has occurred to raise the pH to 9. This allows for the METHOCEL™ to be adequately dispersed in the solvent prior to hydration and prevents any clumping.
Preparing Solutions of METHOCEL™
Untreated METHOCEL™ powders are soluble in cold water and some organic solvents, however, they must first be thoroughly dispersed in the water to prevent lumping. METHOCEL™ powders are not soluble in hot water. Dispersion of untreated powders can be performed by mixing METHOCEL™ with 1/3 of the required volume of water heated to ~90°C. Once the METHOCEL™ is evenly dispersed the remaining volume of water can be added as ice or cold water. This lowers the temperature of the dispersion to the temperature at which the METHOCEL™ becomes water soluble and the powder begins to hydrate and viscosity increases.
Surface Treated Powders
Surface treated grades of METHOCEL™ have been treated with glyoxal and can be added directly to pH neutral aqueous systems and dispersed rather easily with mild agitation. Once the surface treated METHOCEL™ is evenly dispersed hydration can be triggered by a pH adjustment with a small amount of base to 8.5-9.0. Once hydration is complete the pH of the system can be adjusted and should be stable from 3-11. Due to the ease of use surface treated grades of METHOCEL™ are typically recommended for use in latex paints and coatings.
METHOCEL™ cellulose ethers are a useful tool for formulators of water based architectural paints and coatings to optimize and control the viscosity and rheology of formulations. When used to thicken paints and coatings METHOCEL™ offers improved stability versus other cellulose based polymers, increased dry film durability, easier brush loading, improved spreadability,a nd decreased splattering. Contact us to discuss your upcoming architectural coating formulations or click the link below to view all commercially available grades of METHOCEL™.