Viscosity characteristics of hydroxypropyl methylcellulose aqueous solution

Abstract: The fluid type and viscosity test method of hydroxypropyl methylcellulose aqueous solution are introduced, and the viscosity test is carried out. According to the test results, the influence of the degree of polymerization, concentration and shear rate of the cellulose on the viscosity of its aqueous solution is obtained.

Key words:hydroxypropyl methylcellulose; viscosity characteristics

Hydroxypropylmethylcellulose (HPMC) is a nonionic, water-soluble mixed ether of cellulose. Appearance is white to yellowish powder or granule, tasteless, odorless, non-toxic, chemically stable, and forms a smooth, transparent and viscous solution after dissolving in water. Hydroxypropyl methylcellulose can be used as a propellant additive for the coating or bonding of gunpowder, and can also be used as a thickener, emulsifier, film-forming agent, adhesive, dispersant, protective colloid, etc. Building materials, coatings, papermaking, printing, synthetic resin, ceramics, textiles, agriculture, medicine, food, cosmetics and other industries. One of the most important properties of hydroxypropyl methylcellulose in application is that it increases the viscosity of liquids. The thickening effect depends on the degree of polymerization (DP), the concentration of cellulose ether in the aqueous solution, the shear rate, the solution temperature and other factors.

1. Fluid type of HPMC aqueous solution

In general, as long as the stress of a fluid in a shear flow has no time dependence, it can be expressed as a function f(y) of the shear rate only. According to the different forms of the function f(y), fluids can be divided into different types: Newtonian fluids, dilatant fluids, pseudoplastic fluids and Bingham plastic fluids.

Cellulose ether is divided into two categories: one is non-ionic cellulose ether, and the other is ionic cellulose ether. For the rheological properties of these two types of cellulose ethers, sc Naik et al. did a comprehensive and systematic comparative study on hydroxyethyl cellulose and sodium carboxymethyl cellulose solutions. The research results show that both the non-ionic cellulose ether solution and the ionic cellulose ether solution are pseudoplastic flow, that is, non-Newtonian flow. Only when the concentration is very low, it is close to Newtonian liquid. The pseudoplasticity of hydroxypropyl methylcellulose solution plays an important role in the application. If it is used in paint, due to the shear thinning characteristics of aqueous solution, the viscosity of the solution decreases with the increase of shear rate, which is conducive to the uniform dispersion of pigment particles and increases the fluidity of the paint. The effect is great. At rest, the solution is more viscous. Effectively prevent the deposition of pigment particles in the paint.

2. HPMC Viscosity Test Method

An important indicator to measure the thickening effect of hydroxypropyl methylcellulose is the apparent viscosity of the aqueous solution. The methods for measuring apparent viscosity usually include capillary viscosity method, rotational viscosity method and falling ball viscosity method. ASDM: D2363-79 (reviewed in 1995) stipulates that the viscosity determination method is Ubbelohde viscometer at 20±0.1℃. Determination of the viscosity of 2% aqueous solution of hydroxypropyl methylcellulose.

However, the method of measuring with capillary viscometer is more troublesome. The viscosity of many cellulose ethers is difficult to analyze using a capillary viscometer because of the presence of trace amounts of insolubles in these solutions. These insolubles are only detected when the capillary viscometer is plugged. Therefore, most manufacturers use rotary viscometers to control the quality of hydroxypropyl methylcellulose. The Brookield type viscometer is generally used abroad, and the NDJ type viscometer is used domestically.

3. Influencing factors of HPMC viscosity

3.1 Relationship with degree of polymerization

Under the condition that other parameters remain unchanged, the viscosity of hydroxypropyl methylcellulose solution is proportional to the polymer love (DP) or molecular weight or molecular chain length, and this effect increases with the increase of the degree of polymerization. It is more obvious in the case of high degree of polymerization than in the case of high degree of polymerization.

3.2 Relationship between viscosity and concentration

The viscosity of hydroxypropyl methylcellulose increases with the concentration of the product in the aqueous solution. Even a small change in the concentration will cause a large change in the viscosity. There is a nonlinear relationship between the two. With the increase of the viscosity of hydroxypropyl methylcellulose, the influence of the change of solution concentration on the solution viscosity becomes more and more obvious.

3.3 Relationship between stickiness and shear rate

The aqueous solution of hydroxypropyl methylcellulose has the characteristic of shear thinning. Hydroxypropyl methylcellulose with different nominal viscosity was prepared into 2% aqueous solution, and its viscosity at different shear rates was measured respectively. Low shear Under the shear rate, the viscosity of hydroxypropyl methylcellulose solution did not change significantly. With the increase of shear rate, the viscosity of hydroxypropyl methylcellulose solution with higher nominal viscosity decreased more obviously, while the viscosity of hydroxypropyl methylcellulose solution with low viscosity The solution did not drop significantly.

3.4 Relationship between viscosity and temperature

The viscosity of hydroxypropyl methylcellulose solution is greatly affected by temperature. As the temperature rises, the viscosity of the solution decreases. Select hydroxypropyl methylcellulose with a nominal viscosity of 4000mPa.s, 20000mPa.g, and 40000mPa.s. Base cellulose was prepared into a 2% aqueous solution, and the change of viscosity with the increase of temperature was measured.

3.5 Other influencing factors

The viscosity of the aqueous solution of hydroxypropyl methylcellulose is also affected by the additives in the solution, the pH value of the solution, and microbial degradation. Usually, in order to obtain better viscosity performance or reduce the cost of use, it is necessary to add hydroxypropyl methylcellulose to water Add rheology regulators to the solution, such as clay, modified clay, polymer powder, starch ether and aliphatic copolymer, etc., and also add electrolytes to the aqueous solution, such as chloride, bromide, phosphate, nitrate etc. These additives will not only affect the viscosity properties of the aqueous solution, but also affect other application properties of hydroxypropyl methylcellulose such as water retention, anti-sagging, etc.

The viscosity of hydroxypropyl methylcellulose aqueous solution is hardly affected by acid and alkali, generally it is relatively stable in the range of pH 3 ~ 11, and can withstand a certain amount of weak acid, such as formic acid, acetate phosphoric acid, boric acid, citric acid, etc., but Concentrated acid will reduce the viscosity. But caustic soda, potassium hydroxide, lime water, etc. have little effect on it.

Compared with other cellulose ethers, hydroxypropyl methylcellulose aqueous solution has good antimicrobial stability. The main reason is that hydroxypropyl methylcellulose has hydrophobic groups with a high degree of substitution and the steric hindrance of the groups The effect prevents the erosion of microorganisms on the cellulose chain. However, due to the substitution reaction, it is usually not uniform, and the unsubstituted anhydroglucose units are most susceptible to the erosion of microorganisms, which leads to the degradation of cellulose ether molecules and broken chains, the most direct The performance is that the apparent viscosity of the aqueous solution decreases. If you need to store the hydroxypropyl methylcellulose aqueous solution for a long time, it is recommended to add a small amount of antifungal agent so that the viscosity does not change significantly. Choose antifungal agent, preservative or bactericidal Pay attention to safety when using antiseptics, etc., and choose products that are non-toxic to the human body, stable in nature, and odorless. Products such as DOW Chern's AMICALTM fungicide, CANCUARDM64 preservative, and FUELSAVERTM antibacterial agent can all play a corresponding role.

4 Conclusion

The viscosity of the aqueous solution of hydroxypropyl methylcellulose is mainly affected by the concentration of the product in the aqueous solution, the shear rate and the temperature of the product. The viscosity of the aqueous solution increases with the increase of the product's degree of polymerization and concentration. According to actual needs, rheology modifiers, salts or safe fungicides can also be added to obtain better viscosity characteristics and application performance. In practical applications, users should consider the actual situation and economy To select the appropriate viscosity specification and dosage of hydroxypropyl methylcellulose, and to add additives to obtain the ideal viscosity performance.