HPMC Viscosity Test Method

An important index to measure the thickening effect of hydroxypropyl methyl cellulose is the apparent viscosity of the aqueous solution. Apparent viscosity is generally measured by capillary viscosity method, rotational viscosity method, falling ball viscosity method and so on. American Society for Testing and Materials ASDM: 132363-79 (1995 review) provides the United States viscometer at 20 ℃ ± 0.1 ℃ under the viscosity measurement method, the viscosity of hydroxypropyl methylcellulose 2% aqueous solution, the viscosity of which is calculated as 1 ):

V = Kdt Where: for the apparent viscosity, mPa-s; K is the viscosity of the viscometer; d is the density of the solution sample at 20/20 ℃; t is the viscometer from the upper to lower time, s; K through the known viscosity Standard oil flow through the viscometer to determine the time.

However, measurement with a capillary viscometer is cumbersome. The viscosities of many cellulose ethers are difficult to analyze using a capillary viscometer because of the presence of trace amounts of insoluble matter in these solutions, which is only detected when the capillary viscosity is impeded. Therefore, most manufacturers use rotational viscometers to control the quality of hydroxypropyl methyl cellulose. Brookfield type viscometer is commonly used in foreign countries, and NDJ type viscometer is used in China.The test result of NDJ type viscometer is 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 the viscosity of HPMC

1 Relationship with degree of polymerization

When other parameters are constant, the viscosity of hydroxypropyl methylcellulose solutions is proportional to the degree of polymerization (DP) or molecular weight or molecular chain length and increases with increasing degree of polymerization. The effect is more pronounced at lower degrees of polymerization than at higher degrees of polymerization.

2 Viscosity versus concentration

The viscosity of hydroxypropylmethylcellulose increases with increasing concentration of the product in aqueous solution. Even small changes in concentration can cause large changes in viscosity, as exemplified by the nominal viscosity of hydroxypropyl methylcellulose. The effect of increasing solution concentration on solution viscosity becomes more and more obvious.

3 Relationship between viscosity and shear rate

Hydroxypropyl methylcellulose aqueous solution has shear thinning characteristics, different nominal viscosity of hydroxypropyl methylcellulose formulated into a 2% aqueous solution, respectively, to determine the viscosity at different shear rates. As shown in Fig. At low shear rate, the viscosity of hydroxypropyl methylcellulose solution did not change significantly. With the increase of shear rate, the viscosity of hydroxypropylmethylcellulose solutions with higher nominal viscosity decreased more significantly, while the viscosity of low-viscosity solutions decreased insignificantly.

4 Viscosity and temperature

The viscosity of hydroxypropylmethylcellulose solution is greatly affected by temperature, and the viscosity of the solution decreases with increasing temperature. As shown in the figure, a 2% aqueous solution was prepared to determine the change in viscosity with temperature.

5 Other influencing factors

The viscosity of aqueous solutions of hydroxypropyl methylcellulose is also affected by the additives in the solution, the pH value of the solution and the degradation of microorganisms. 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, aliphatic copolymer, etc. to the aqueous solution of hydroxypropyl methyl cellulose. Electrolytes such as chlorides, bromides, phosphates, nitrates, etc. can also be added to the aqueous solution. These additives affect not only the viscosity properties of the aqueous solution, but also other properties of the hydroxypropyl methylcellulose, such as water retention and anti-sagging.

The viscosity of hydroxypropyl methyl cellulose aqueous solution is almost unaffected by acid and alkali, generally stable in the range of 3 to 11, can withstand a certain amount of formic acid, acetic acid, phosphoric acid, boric acid and other weak acids, citric acid and so on. But concentrated acid will reduce the viscosity. However, caustic soda, potassium hydroxide, lime water, etc. have little effect on it. Hydroxypropyl methyl cellulose aqueous solution has good antibacterial stability compared with other cellulose ethers. The main reason is that hydroxypropyl methyl cellulose has a hydrophobic group with a high degree of substitution and the spatial site resistance of this group. This action prevents microbial attack on the cellulose chain. However, since the substitution reaction is usually inhomogeneous, the unsubstituted dehydrated glucose anhydride unit is most susceptible to microbial attack, leading to degradation of the cellulose ether molecule, which is most directly manifested by the apparent viscosity of the aqueous solution. If aqueous solutions of hydroxypropyl methylcellulose need to be stored for long periods of time, it is recommended that trace amounts of antifungal agents be added so that the viscosity does not change significantly. When using mold inhibitors, preservatives or fungicides, pay attention to safety.

The viscosity of the aqueous solution of hydroxypropyl methylcellulose is mainly affected by the degree of polymerization of the product, the concentration of the product in the aqueous solution, the shear rate and the temperature of the solution. With shear, the viscosity of the aqueous solution increases with the degree of polymerization and product concentration. Decrease the rate and temperature of the solution and add rheology modifiers, salts or safe biocides according to the actual needs to obtain better viscosity characteristics and application performance. In practical applications, users should consider the actual situation and economy to choose the appropriate viscosity specification and addition amount of hydroxypropyl methylcellulose, and add additives to obtain the desired viscosity properties.