Hydroxypropyl methylcellulose (HPMC) is a non-ionic, water-soluble cellulose mixed ether. Appearance is white to slightly yellow powder or granular material, tasteless, odorless, non-toxic, chemically stable, and dissolves in water to form a smooth, transparent and viscous solution. One of the most important properties of hydroxypropyl methylcellulose in application is that it increases the viscosity of the liquid. The thickening effect depends on the degree of polymerization (DP) of the product, the concentration of cellulose ether in the aqueous solution, the shear rate, and the solution temperature. And other factors.
01
Fluid type of HPMC aqueous solution
In general, the stress of a fluid in shear flow can be expressed as a function of only the shear rate ƒ(γ), as long as it is not time-dependent. Depending on the form of ƒ(γ), fluids can be divided into different types, namely: Newtonian fluids, dilatant fluids, pseudoplastic fluids and Bingham plastic fluids.
Cellulose ethers are divided into two categories: one is non-ionic cellulose ether and the other is ionic cellulose ether. For the rheology of these two types of cellulose ethers. S C Naik et al. conducted a comprehensive and systematic comparative study on hydroxyethyl cellulose and sodium carboxymethyl cellulose solutions. The results showed that both non-ionic cellulose ether solutions and ionic cellulose ether solutions were pseudoplastic. Flows, i.e. non-Newtonian flows, approach Newtonian liquids only at very low concentrations. The pseudoplasticity of hydroxypropyl methylcellulose solution plays an important role in application. For example, when applied in coatings, due to the shear thinning characteristics of aqueous solutions, the viscosity of the solution decreases with the increase of shear rate, which is conducive to the uniform dispersion of pigment particles, and also increases the fluidity of the coating. The effect is very large; while at rest, the viscosity of the solution is relatively large, which effectively prevents the deposition of pigment particles in the coating.
02
HPMC Viscosity Test Method
An important indicator to measure the thickening effect of hydroxypropyl methylcellulose is the apparent viscosity of the aqueous solution. The measurement methods of apparent viscosity usually include capillary viscosity method, rotational viscosity method and falling ball viscosity method.
where: is the apparent viscosity, mPa s; K is the viscometer constant; d is the density of the solution sample at 20/20°C; t is the time for the solution to pass through the upper part of the viscometer to the bottom mark, s; The time that the standard oil flows through the viscometer is measured.
However, the method of measuring by capillary viscometer is more troublesome. The viscosities of many cellulose ethers are difficult to analyze using a capillary viscometer because these solutions contain trace amounts of insoluble matter that are only detected when the capillary viscometer is blocked. Therefore, most manufacturers use rotational viscometers to control the quality of hydroxypropyl methylcellulose. Brookfield viscometers are commonly used in foreign countries, and NDJ viscometers are used in China.
03
Influencing factors of HPMC viscosity
3.1 Relationship with the degree of aggregation
When other parameters remain unchanged, the viscosity of hydroxypropyl methylcellulose solution is proportional to the degree of polymerization (DP) or molecular weight or molecular chain length, and increases with the increase of the degree of polymerization. This effect is more pronounced in the case of low 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 increase of the concentration of the product in the aqueous solution. Even a small concentration change will cause a large change in viscosity. With the nominal viscosity of hydroxypropyl methylcellulose The effect of the change of solution concentration on the viscosity of the solution is more and more obvious.
3.3 Relationship between viscosity and shear rate
Hydroxypropyl methylcellulose aqueous solution has the property of shear thinning. Hydroxypropyl methylcellulose of different nominal viscosity is prepared into 2% aqueous solution, and its viscosity at different shear rates is measured respectively. The results are as follows As shown in the figure. At low 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 solution with low viscosity did not decrease obviously.
3.4 Relationship between viscosity and temperature
The viscosity of the hydroxypropyl methylcellulose solution is greatly affected by the temperature. As the temperature increases, the viscosity of the solution decreases. As shown in the figure, it is prepared into an aqueous solution with a concentration of 2%, and the change of the viscosity with the increase of temperature is measured.
3.5 Other influencing factors
The viscosity of the aqueous solution of hydroxypropyl methylcellulose is also affected by additives in the solution, 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 rheology modifiers, such as clay, modified clay, polymer powder, starch ether and aliphatic copolymer, to the aqueous solution of hydroxypropyl methylcellulose. , and electrolytes such as chloride, bromide, phosphate, nitrate, etc. can also be added to the aqueous solution. 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. , sag resistance, etc.
The viscosity of the aqueous solution of hydroxypropyl methylcellulose is almost not affected by acid and alkali, and is generally stable in the range of 3 to 11. It can withstand a certain amount of weak acids, such as formic acid, acetic acid, phosphoric acid, boric acid, citric acid, etc. However, 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 high degree of substitution and steric hindrance of groups However, since the substitution reaction is usually not uniform, the unsubstituted anhydroglucose unit is most easily eroded by microorganisms, resulting in the degradation of cellulose ether molecules and chain scission. The performance is that the apparent viscosity of the aqueous solution decreases. If it is necessary to store the aqueous solution of hydroxypropyl methylcellulose for a long time, it is recommended to add a trace amount of antifungal agent so that the viscosity does not change significantly. When choosing anti-fungal agents, preservatives or fungicides, you should pay attention to safety, and choose products that are not toxic to the human body, have stable properties and are odorless, such as DOW Chem’s AMICAL fungicides, CANGUARD64 preservatives, FUELSAVER bacteria agents and other products. can play a corresponding role.
Post time: Oct-20-2022