Hydroxypropyl methylcellulose (HPMC) can be used as a retarder, water-retaining agent, thickener and binder. Hydroxypropyl methylcellulose (HPMC) can play an important role in ordinary dry mortar, exterior wall insulation mortar, self-leveling mortar, tile adhesive, interior and exterior wall putty, caulking agent and so on. Hydroxypropyl methylcellulose (HPMC) plays an important role in the water retention, water addition, workability, cohesiveness and retardation of the mortar system. According to these different applications, our company has developed different types of Hydroxypropyl methylcellulose (HPMC).Hydroxypropyl methylcellulose (HPMC) is a nonionic, water soluble cellulose mixed ether. Appearance is white to yellowish powder or granular, colorless, odorless, non-toxic, chemically stable, and dissolved in water to form a smooth transparent sticky solution.
One of the most important properties of hydroxypropyl methylcellulose in application is its increased liquid viscosity. The thickening depends on the degree of polymerization (DP) of the product, the concentration of cellulose ether in the aqueous solution, the shear rate, and the temperature of the solution and other factors.
Fluid Type of HPMC Aqueous Solution
In general, the stress of the fluid in the shear flow can be expressed as a function of the shear rate, ƒ(γ), as long as there is no time dependence. The form of ƒ(γ) can be divided into different types: Newtonian fluid, dilatant fluid, pseudoplastic fluid and Bingham plastic fluid.
Cellulose ethers are divided into two categories: one is nonionic cellulose ether, and the other is ionic cellulose ether. Rheological properties for both types of cellulose ethers. The results show that both the nonionic cellulose ether solution and the ionic cellulose ether solution are pseudoplastic flows, ie non-Newtonian flows, which are close to Newtonian liquids only at very low concentrations. The pseudoplasticity of the hydroxypropyl methylcellulose solution plays an important role in the application. If applied in paints, due to the shear thinning characteristics of the aqueous solution, as the shear rate increases, the viscosity of the solution decreases, which is beneficial to the uniform dispersion of the pigment particles, and also increases the fluidity of the paint, and smoothes and brushes the latex paint. The effect is great; at rest, the viscosity of the solution is large, effectively preventing the deposition of pigment particles in the coating.
HPMC Viscosity Test Method
An important measure of the thickening effect of hydroxypropyl methylcellulose is the apparent viscosity of the aqueous solution. The method for determining the apparent viscosity generally includes a capillary viscosity method, a rotational viscosity method, and a falling ball viscosity method. The American Society for Testing and Materials ASDM: 132363-79 (1995 review) specifies the viscosity measurement method of U.S. viscometer at 20 ° C ± 0.1 ° C, the viscosity of hydroxypropyl methyl cellulose 2% aqueous solution, the viscosity of which is 1) Calculation:
V= Kdt
Where: is the apparent viscosity, mPa·s; K is the viscosity of the viscosity meter; d is the density of the solution sample at 20/20 ° C; t is the time from the upper to the bottom of the viscosity meter, s; K through the known viscosity The standard oil flows through the time of the viscometer to determine.
However, the method of measuring with a capillary viscometer is more troublesome. The viscosity of many cellulose ethers is difficult to analyze using a capillary viscometer because of the presence of traces of insolubles in these solutions, which are only discovered when the capillary viscosity is blocked. Therefore, most manufacturers use a rotary viscometer to control the quality of hydroxypropyl methylcellulose. The Brookfield-type viscometer is commonly used in foreign countries, and the NDJ-type viscometer is used in China. The results of the NDJ type viscometer test are expressed as:
η=Kα
Where: η is the absolute viscosity, mPa·s; K is the coefficient; α is the deflection angle, that is, the pointer reading.
Factors Affecting HPMC Viscosity
1 Relationship with degree of polymerization
When the other parameters are unchanged, the viscosity of the hydroxypropyl methylcellulose solution is proportional to the degree of polymerization (DP) or molecular weight or molecular chain length, and increases as the degree of polymerization increases. This effect is more pronounced in the case of a low degree of polymerization than in the case of a high degree of polymerization.
2 Relationship between viscosity and concentration
The viscosity of hydroxypropyl methylcellulose increases as the concentration of the product in aqueous solution increases. Even a small change in concentration causes a large change in viscosity, with the nominal viscosity of hydroxypropyl methylcellulose. The increase of the concentration of the solution has an increasingly obvious effect on the viscosity of the solution.
3 Relationship between viscosity and shear rate
The aqueous solution of hydroxypropyl methylcellulose has shear thinning characteristics, and hydroxypropyl methylcellulose with different nominal viscosities is formulated into a 2% aqueous solution, and the viscosity at different shear rates is measured, respectively. The figure shows. At low shear rate, the viscosity of hydroxypropyl methylcellulose solution did not change significantly. As the shear rate increases, the viscosity of the hydroxypropyl methylcellulose solution with higher nominal viscosity decreases more significantly, while the low viscosity solution does not decrease significantly.
4 Viscosity and temperature
The viscosity of the hydroxypropyl methylcellulose solution is greatly affected by the temperature, the temperature rises, and the viscosity of the solution decreases. As shown in the figure, a 2% aqueous solution is prepared to determine the change in viscosity with temperature.
Factors Affecting HPMC Aqueous Solution Viscosity
The aqueous solution viscosity of hydroxypropyl methylcellulose is affected by multiple factors, including:
- Additives in the solution
- pH of the solution
- Degradation caused by microorganisms
In addition, the viscosity is mainly influenced by:
- Degree of polymerization of the product
- Concentration in the aqueous solution
- Shear rate
- Temperature
The viscosity increases with higher polymerization degree and concentration, and decreases with increasing shear rate and temperature.
1. Effect of Additives
To obtain better viscosity properties or reduce cost, additives can be introduced into the aqueous HPMC solution, such as:
- Rheology modifiers: clay, modified clay, polymer powder, starch ether, aliphatic copolymer
- Electrolytes: chlorides, bromides, phosphates, nitrates
These additives not only affect viscosity, but also influence other properties of HPMC, such as:
- Water retention
- Anti-sag performance
2. Effect of pH
The viscosity of HPMC aqueous solution is generally stable within a pH range of 3 to 11.
- It can withstand weak acids such as formic acid, acetic acid, phosphoric acid, boric acid, and citric acid
- Concentrated acids will reduce viscosity
- Alkalis such as caustic soda, potassium hydroxide, and lime water have little effect on viscosity
3. Microbial Stability and Degradation
HPMC aqueous solution has relatively good antimicrobial stability compared with other cellulose ethers.
- This is due to hydrophobic groups with a high degree of substitution and steric hindrance, which prevent microbial attack on the cellulose chain
- However, substitution is not uniform, and unsubstituted anhydroglucose units are more susceptible to microbial attack
- This can lead to degradation of the cellulose ether molecule
- The most direct result is a decrease in the apparent viscosity of the solution
4. Storage and Preservation
For long-term storage of HPMC aqueous solution:
- It is recommended to add a small amount of antifungal agent to maintain viscosity stability
- When using preservatives or fungicides, safety should be considered
- Non-toxic, stable, and odorless products are preferred
Examples include:
- DOW Chem AMICAL fungicide
- ANGUARD64 preservative
- FUELSAVER microbial agents
Practical Application Considerations
In practical applications:
- Rheology modifiers, salts, or safe bactericides can be added according to actual needs
- These help achieve desired viscosity characteristics and application performance
Users should consider:
- Actual application conditions
- Economic factors
To select:
- Appropriate viscosity grade
- Proper dosage of HPMC
- Suitable additives
Key Points Summary
- Viscosity is affected by polymerization, concentration, shear rate, temperature, additives, pH, and microorganisms
- Stable pH range: 3–11; weak acids tolerated, concentrated acids reduce viscosity
- Additives influence both viscosity and performance (water retention, anti-sag)
- Microbial degradation reduces viscosity over time
- Preservatives are recommended for long-term storage
- Practical formulation requires balancing performance, cost, and application conditions