METHOCEL™ - High Performance Thickeners for Coatings

What Is METHOCEL™? 

METHOCEL™ products are cellulose-based thickeners made of water-soluble methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC), which compose a family of polymers called cellulose ethers. These water-soluble polymers are derived from cellulose that comes from natural wood or cotton linters. While these additives find use in many different applications, they are used to effectively thicken latex paints. METHOCEL™ thickeners impart a number of benefits to paints, detailed below. 

Stability

The expiration of latex paints is typically determined by a loss of viscosity and pigment suspension. The shelf life of latex paints can be extended when METHOCEL™ is used as a thickener and pigment stabilization additive. Although METHOCEL™ is a cellulose-based thickener, it is more resistant to enzymes released by microbes than other cellulose-based thickeners. In addition to pH stability from pH 3 to 11, paints containing METHOCEL™ thickeners maintain a uniform viscosity regardless of age or pH. Additionally, METHOCEL™ thickeners can be used to boost the viscosity of latex paints that have thinned due to spoilage. 

Easy Application

Due to their pseudoplastic nature, METHOCEL™ cellulose ethers provide more than just thickening. They impart a pseudoplastic rheology that enables the exceptional application of paints. When paints use METHOCEL™ thickeners, viscosity increases at rest, improving the loading of the paint onto brushes and rollers when they are dipped in. When the paint is brushed or rolled onto surfaces, shear forces decrease viscosity, allowing for easier application. After the paint is applied, viscosity decreases to prevent dripping or sagging. 

Film Durability

Water-based architectural coatings are regularly subjected to harsh conditions. They need to withstand cleaning chemicals, scrubbing, rain, and condensation. METHOCEL™ additives in water-based paints and coatings help improve paint film durability and resistance to chemicals and moisture. Addition, flow, leveling, and spreadability are also improved, reducing pinhole formation and increasing film uniformity over surfaces with varying porosity. 

Sustainability

Being derived from cellulose, METHOCEL™ imparts bio content in formulations and provides a future-focused advantage in terms of driving product sustainability. Many other thickeners and additives that provide the functionality detailed so far are derived from nonrenewable raw materials, such as petroleum. 

Selecting the Proper Grade of METHOCEL™

The selection of METHOCEL™ thickeners is governed by two main criteria: viscosity, which is associated with molecular weight, and degree of substitution. These criteria can be identified by the grade nomenclature. An example is presented below. 
 

Substitution

Chemically speaking, METHOCEL™ cellulose ethers are cellulose polymers with hydroxyl functional groups substituted for methoxyl and hydroxypropyl functional groups. The degree and nature of cellulose functional group substitution varies and can be identified by the first letter in the name of the grade. 
 

Chemistry	A	E	F	J	K
Methyoxyl substitution, %	27.5-31.5	28.0-30.0	27.0-30.0	16.5-20.0	19.0-24.0
Hydroxypropyl substitution, %	0	7.0-12.0	4.0-7.5	23.0-32.0	25.0-32.0

The amount and type of cellulose substitution lend to differences in the cellulose ethers' organosolubility and thermal gelation temperatures. In general, the organosolubility of METHOCEL™ thickeners 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” thickeners are typically recommended as they are compatible with most cosolvents used to improve film formation. 

Viscosity

The viscosity of solutions produced by METHOCEL™ thickeners in water varies from grade to grade based on the molecular weight of the cellulose polymer. Many options exist to provide the paint formulator with the right level of thickening needed per application. The mean viscosity of a 2% solution of 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 cPs at 2 wt% in water. The viscosity of solutions using METHOCEL™ thickeners 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 polymer’s molecular weight and concentration increase. This means that the viscosity of solutions with higher–molecular weight METHOCEL™ thickeners will become lower as shear increases. The graphs below highlight this relationship. 

 

 

Surface Treatment

The final attribute to consider when selecting a METHOCEL™ grade is the surface treatment. METHOCEL™ cellulose ethers denoted with an “S” after the “multiplier” letter for viscosity have a glyoxal surface treatment applied to the cellulose polymer for delayed hydration. Here, the cellulose polymer will only begin to hydrate and provide thickening after the pH is adjusted to at least 9. This allows for METHOCEL™ to be adequately dispersed in the solvent prior to hydration, preventing clumping and insufficient thickening of paint and coating formulations. 

Preparing Solutions of METHOCEL™

Untreated Powders

Untreated METHOCEL™ powders are only soluble in cold water and some organic solvents. For water-based solutions, they must first be thoroughly dispersed to prevent clumping. METHOCEL™ powders are not soluble in hot water. Dispersion of untreated powders can, therefore, be achieved by mixing METHOCEL™ with one-third of the required volume of water heated to approximately 90°C. Once METHOCEL™ is evenly dispersed, the remaining volume of water can be added as ice or cold water. This lowers the temperature to a level at which the METHOCEL™ thickener will be water-soluble, thus allowing it to begin to hydrate and thicken the solution. 

Surface Treated Powders

Surface-treated METHOCEL™ thickeners 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 adjusting the pH to 8.5–9.0 with a small amount of alkali. Once hydration is complete, the pH of the system can be readjusted. Solutions with METHOCEL™ are stable from pH 3 to 11. Due to the relative ease of preparation, surface-treated grades of METHOCEL™ are typically recommended for use in latex paints and coatings. 

Conclusion

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 as thickening additives in paints and coatings, METHOCEL™ offers improved viscosity and pigment stability, increased dry film durability, easier brush loading, improved spreadability, and decreased dripping and sagging. Being cellulose-based polymers, METHOCEL™ also increases the bio content in paint and coating formulations. Click here to view all the available METHOCEL™ grades. Alternatively, contact us below to discuss the METHOCEL™ rheology modifiers for your architectural paint formulation.