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 paints and coatings has driven the market toward water-based options, such as latex paint. While latex technology has come a long way over the last 60 years, paint formulations still need to be optimized to meet the performance criteria of consumers. The precise control of viscosity and rheology provided by METHOCEL™ is critical to maximizing the performance of interior-and exterior-applied latex paints.
What is METHOCEL™?
METHOCEL™ cellulose ethers are a family of water-soluble methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) polymers used as rheology modifier additives to thicken latex paints effectively. Latex paints thickened with METHOCEL™ offer several advantages, as discussed below.
Stability of Paints
The expiration of latex paints is typically determined by a loss of viscosity and pigment suspension. METHOCEL™ is resistant to enzymes released by microbes that can destabilize viscosity, and thus when added to latex paints as a thickener or rheology modifier during initial formulation, it can also extend shelf life. The broad stability range of METHOCEL™ in pH 3 to 11 allows thickened latex paints to maintain uniform viscosities in various solution conditions. If a latex paint has lost its viscosity due to spoilage, it can even be re-thickened with METHOCEL™ to achieve a proper and stable viscosity.
Easy Application of Paints
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.
Paint Film Durability
Architectural latex paints are subjected to harsh conditions regularly. Therefore, they need to withstand cleaning chemicals, scrubbing, rain, condensation, and exposure to ultraviolet light. The addition of METHOCEL™ to paints and coatings helps to improve dry paint film durability and resistance to abrasion, moisture, and chemicals, such as oils, greases, and petroleum hydrocarbons. METHOCEL™ also has excellent ultraviolet light stability, making it ideal for use in both interior and exterior latex paints.
Selecting the Proper Grade of METHOCEL™
The selection of METHOCEL™ grades is determined by two main criteria based on the needs of each application: viscosity and degree of substitution. These criteria can be identified by the nomenclature of the grade. For example:
METHOCEL™ grades are cellulose-based polymers that have had hydroxyl functional groups replaced with methoxyl and hydroxypropyl functional groups. The degree and nature of substitution vary and can be identified by the first letter of the name of the degree, as seen below.
The amount and type of substitution lend to differences in the degree of organic solvent solubility and thermal gelation temperatures of solutions when using these in formulations. In general, the organic-solvent solubility of METHOCEL™ increases as hydroxypropyl substitution increases, and the thermal gelation temperature of METHOCEL™ decreases with increasing methoxyl substitution.
Organic solvents may be used in latex paint formulations to increase film-formation properties. Therefore, the "J" and "K" grades in these latex paint formulations are typically recommended as they are compatible with most organic 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. Many options exist to provide the paint formulator with the right level of thickening needed per application. The mean viscosity of a solution of 2 wt% METHOCEL™ in water is reported for each grade by a number and a "multiplier" letter, with C being 100 and M being 1000. For example, the grade J75MS would have a viscosity of 75,000 mPa*s at 2 wt% in water. The viscosity of solutions thickened with METHOCEL™ will also vary based on the concentration in which they are added and the solvent(s) used. The graph below shows the viscosity imparted by common METHOCEL™ grades in DI water at varying concentrations.
Another important factor to consider regarding the rheology of solutions containing METHOCEL™ is that pseudoplasticity will increase as the molecular weight of the polymer and concentration increase. This means that the viscosity of solutions thickened by higher-molecular weight METHOCEL™ grades will become lower as shear increases. The graphs below highlight this relationship.
The final attribute that needs to be considered when selecting a METHOCEL™ grade is the surface treatment. Grades of METHOCEL™ that are denoted with an "S" after the "multiplier letter" for viscosity are surface treated with glyoxal for delayed hydration. These will only begin to hydrate and thicken after the pH adjustment of the solution to raise the pH to 9. This allows for METHOCEL™ to be adequately dispersed in the solvent prior to hydration, preventing clumping.
Preparing Solutions of METHOCEL™
Untreated METHOCEL™ powders are soluble in cold water and some organic solvents. For water-based solutions, they must first be thoroughly dispersed to prevent lumping. METHOCEL™ powders are not soluble in hot water. Dispersion of untreated powders can therefore be performed by mixing METHOCEL™ with 1/3 of the required volume of water heated to ~90°C. Once METHOCEL™ is evenly dispersed, the remaining volume of water can be added as ice or cold water. This lowers the dispersion temperature to a level at which the METHOCEL™ will be water-soluble, thus allowing it to begin to hydrate and thicken solutions.
Surface-treated grades of METHOCEL™ 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 alkali to 8.5-9.0. Once hydration is complete, the pH of the system can be re-adjusted. Solutions with METHOCEL™ are stable from pH 3-11. Due to the relative ease of preparation, 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 viscosity and pigment stability, increased dry film durability, easier brush loading, improved spreadability, and decreased splattering. Click here to view all the available METHOCEL™ grades. Alternatively, contact us below to discuss the METHOCEL™ rheology modifiers for your architectural paint formulation.