Hydroxypropyl methyl cellulose (E464) (also HPMC) is an emulsifier, thickening and suspending agent, and an alternative to animal gelatin. It is a solid, and is a slightly off-white to beige powder in appearance and may be formed into granules. The compound forms colloids when dissolved in water. Although non-toxic, it is combustible and can react vigorously with oxidising agents. Hydroxypropyl methyl cellulose in an aqueous solution, like methylcellulose, exhibits a thermal gelation property. That is, when the solution heats up to a critical temperature, the solution congeals into a non-flowable but semi-flexible mass. Typically, this critical (congealing) temperature is inversely related to both the solution concentration of HPMC and the concentration of the methoxy group within the HPMC molecule (which in turn depends on both the degree of substitution of the methoxy group and the molar substitution. That is, the higher is the concentration of the methoxy group, the lower is the critical temperature. The inflexibility/viscosity of the resulting mass, however, is directly related to the concentration of the methoxy group (the higher is the concentration, the more viscous or less flexible is the resulting mass).
The CAS registry number of hydroxypropyl methyl cellulose is 9004-65-3.
See molecule here:
Usages and Applications
There are many fields of application for hydroxypropyl methyl cellulose. These are not limited to the following
- Construction materials
- Paints & Coatings
- Detergents & cleaners
Of the listed fields of application, construction material industry uses a vast majority of the supply of methylcellulose and its derivatives.
Functional Roles in Construction Materials
Controlling solutions and air content
Water is one of the most important ingredients in building products. Too much water can lead to cracking, reduced strength and unacceptable performance while not enough water can make proper application of the product virtually impossible. hydroxypropyl methyl cellulose are designed to retain water in the system so that the right balance of properties is achieved and performance of the building product is optimized. hydroxypropyl methyl cellulose are cellulose ether based chemistry and provide variety of benefits in a very diverse set of cementious, gypsum or latex-based building products. In many building products such as masonry cement and mortars, plaster, render & stucco systems, the incorporation of small, evenly distributed air bubbles is required. Other applications such as self-leveling compounds require the absence of such air bubbles. Various hydroxypropyl methyl cellulose products are used to regulate such entrained air in the various mortar systems.
Improving workability and pumpability
By retaining water, hydroxypropyl methyl cellulose products enable the development of structure and body in the building product. This leads to a creamy texture, superb consistency that provides easy application of the building product. By being a lubricant which exhibits cohesive yet shear-thinning properties, hydroxypropyl methyl cellulose enables the production of excellent plasters, renders and shotcretes which can be applied by standard pumping and spraying machinery. The variety of pump-able building products span from self-leveling low viscosity compounds, to higher viscous building materials which will build in viscosity once the pumping action subsides. This allows for minimal pump effort or water loss in application of the building material, as well as excellent placement properties due to the rapid viscosity build-up upon contact with surface.
Providing stabilization and reducing sag
The ability to suspend particles and to retain water by hydroxypropyl methyl cellulose products gives such stability to the wet mortar, that over a long period of time, the wet mortar can remain virtually unchanged in consistency, texture, body, and air content. This in turn means that wet mortar will not suffer phase separation or segregation. With the right hydroxypropyl methyl cellulose, it is easy to create a building material which is easy to apply but still have the body and cohesiveness to maintain their position on a vertical or overhead surface. Such building materials may include tile adhesives, thick coat plaster or renders, EIFS elements, and others. Application may be manual or machine based.
Controlling water retention and extending working time
With the high temperature environments or other adverse, it is often difficult to retain the water in a building material long enough for proper application and proper setting. hydroxypropyl methyl cellulose effectively eliminates this problem by its ability to increase the water retention of the wet mortar and thereby giving more working time for the proper application and setting of the building material.
Increasing strength and improving appearance
By improving the water retention, hydroxypropyl methyl cellulose improves the time required for proper cement hydration and proper crystallization. This in turn increases the strength of the cement and improves performance. By retaining enough water to allow the cementious or gypsum binders to fully hydrate, and by slowing the drying/curing process, hydroxypropyl methyl cellulose products very effectively reduces the shrinkage related to such building materials, and thus eliminates cracking associated with shrinkage. By improving the workability and consistency, hydroxypropyl methyl cellulose makes creating a smooth surface an easy goal. All these characteristics which hydroxypropyl methyl cellulose provides contribute toward achieving a more perfect look.
Various benchmark tests are used to qualify Hydroxypropyl Methyl Cellulose:
- Degree of Substitution (D.S.)
- Molar Substitution (M.S.)
- Salt Content
Because Hydroxypropyl Methyl Cellulose solution is a non-newtonian solution and exhibits pseudoplastic, more specifically, thixotropic behavior, various test methods are available, and the results of different methods and viscosmeters do not necessarily correspond to each other. Also, due to viscometer acceptable ranges of error, viscosity is typically given as a mean, or as a range. Typical viscosity test will specify the following:
- Solution concentration (1%, 2%, 1.9% bone dry, etc.)
- Viscometer (Brookfield LV or RV, Höppler falling ball, Haake Rotovisco, etc.)
- Viscometer spindle number (1 ~ 4 for Brookfield LV, 1 ~ 7 for Brookfield RV, etc.)
- Solution Temperature (20°C, 25°C, etc.)
Degree of Substitution (D.S.)
Degree of substitution is the average level of methoxy substitution on the cellulose chain. Since there are maximum three (3) possible sites of substitution with each cellulose molecule, this average value is a real number between 0 and 3. However, Degree of Substitution is often expressed in percentages.
Molar Substitution (M.S.)
Molar substitution is the average level of hydroxypropoxy substitution on the cellulose chain. Since hydroxypropoxy base can be attached to each other on side chains and does not each require a base substitution site on the cellulose molecule, this number can be higher than 3. However, Molar Substitution is also often expressed in percentages.
Content of NaCl in weight percentage of dry powder weight.
Since all cellulose ethers are hygroscopic, they will absorb moisture from surrounding if left exposed from original packaging. Thus, moisture must be tested and weight corrected to ensure adequate amount of dry active material are aportioned for usage. Moisture is tested by taking a weighing a sample of X grams on an analytic scale, and drying the sample in an oven at 105°C for 2 hours, then weigh the sample again on the same scale.