Views: 2 Author: Site Editor Publish Time: 2022-06-09 Origin: Site
Hydroxypropyl methyl cellulose (HPMC) is a mixed ionic, water-soluble cellulose ether. Appearance is white to slightly yellow powder or granular, tasteless, odorless, non-toxic, chemical properties of the product is stable, dissolved in water after forming a smooth viscous solution. One of the most important functions of hydroxypropyl methyl cellulose is to add liquid viscosity. The thickening effect depends on the polymerization degree (DP) of the product, the concentration of cellulose ether in aqueous solution, shear rate, solution temperature and other factors.
01. Fluid type of HPMC aqueous solution
In general, the stress of a fluid in shear activity can be shown as a function of the shear rate alone, as long as there is no time dependence. Gamma, as a function. ? (γ) of the way, the fluid can be divided into different types, separated by: Newtonian fluid, swelling fluid, pseudoplastic fluid and bingham plastic fluid.
Cellulose ether is divided into two categories: a kind of wrong and wrong ionic cellulose ether, one is ionic cellulose ether. About the rheological properties of these two cellulose ethers. S C Naik et al for hydroxyethyl cellulose and sodium carboxymethyl cellulose solution made relatively comprehensive system for the target of the discussion, the research results show that the non-ionic cellulose ether solution and ionic cellulose ether solution are pseudoplastic activities, namely the non-newtonian activities, as long as the concentration is very low, only close to the Newtonian liquid. The pseudoplasticity of hydroxypropyl methyl cellulose solution has an important effect in application. If used in the paint guess, because of the shear thinning characteristics of aqueous solution, with the increase of shear rate, the viscosity of solution drops, is conducive to the uniform lax pigment particles, also make the paint active to add, the effect of latex paint brush flat, brush light is very big.
HPMC viscosity test method
The apparent viscosity of aqueous solution is an important objective to measure the thickening effect of hydroxypropyl methyl cellulose. The apparent viscosity can be measured by capillary viscosity, rotary viscosity and falling ball viscosity. According to ASDM:132363 -- 79(1995 review) rule, the viscosity measurement method is to measure the viscosity of 2% aqueous solution of hydroxypropyl methyl cellulose with Uxler viscometer at 20℃±0.1℃. The viscosity is calculated according to Formula (1) :
Where, is the apparent viscosity, mPa·s; K is the viscometer constant; D is the density of solution sample at 20/20℃; T is the moment when the solution passes through the bottom mark of the upper part of the viscosimeter, s; K is measured at a time when a gauge oil flow with known viscosity passes through the viscometer.
However, the method of capillary viscometer determination is more difficult. The viscosity of many cellulose ethers is difficult to analyze with a capillary viscosimeter because of the presence of trace insoluble substances in these solutions, which are not detected until the capillary viscosity is measured. Therefore, the vast majority of manufacturers choose rotating viscosimeter to control the quality of hydroxypropyl methyl cellulose, Brookfield viscosimeter is commonly used in foreign countries, and NDJ viscosimeter is used in China. The results of NDJ viscometer test show that:
Eta = K alpha
Where, η is the definite viscosity, mPa·s; K is the coefficient; Alpha is the deflection point, or pointer reading.
03. Factors influencing the viscosity of HPMC
3.1 Relation with degree of polymerization
The viscosity of hydroxypropyl methyl cellulose solution is proportional to polymerization degree (DP) or molecular weight or molecular chain length when other parameters remain unchanged, and increases with the improvement of polymerization degree. This effect is more significant in the case of low polymerization degree than in the case of high polymerization degree.
3.2 Relation between viscosity and concentration
The viscosity of hydroxypropyl methyl cellulose increases with the increase of the concentration of the product in aqueous solution, and even a small concentration change will form a large change in viscosity. With the increase of the nominal viscosity of hydroxypropyl methyl cellulose, the influence of solution concentration change on the viscosity of the solution is more and more significant.
3.3 Relation between viscosity and shear rate
Hydroxypropyl methyl cellulose aqueous solution has the property of shear thinning. Hydroxypropyl methyl cellulose aqueous solution with different nominal viscosity is prepared into 2%, and its viscosity at different shear rates is measured separately. The results are shown in the figure. At low shear rate, the viscosity of hydroxypropyl methyl cellulose solution did not change significantly. With the improvement of shear rate, the viscosity of hydroxypropyl methyl cellulose solution with higher nominal viscosity decreased more significantly, while the solution with low viscosity did not decrease significantly.
3.4 Relation between viscosity and temperature
The viscosity of hydroxypropyl methyl cellulose solution is greatly affected by temperature. As the temperature increases, the viscosity of the solution decreases. As shown in the figure, an aqueous solution with 2% concentration is prepared to measure the change of viscosity with the temperature rise.
The viscosity of hydroxypropyl methyl cellulose aqueous solution is simply not affected by acid and base, generally in the range of 3 ~ 11 are relatively stable, can withstand a certain amount of weak acid, such as formic acid, acetic acid, phosphoric acid, boric acid, citric acid, etc., but concentrated acid will make the viscosity attack decline. But caustic soda, potassium hydroxide and lime water have little effect on it. Hydroxypropyl methyl cellulose aqueous solution, compared with other cellulose ether, has the very good antimicrobial safe sex, the primary cause is the hydroxypropyl methyl cellulose with a high degree of substitution of space block effects of hydrophobic groups and groups, hindering the microbial corrosion effect of the cellulose chain, however, because the alternative response is not uniform, The unsubstituted dehydrated gluconic anhydride unit is most simply corroded by microorganisms, which then leads to cellulose ether molecular degradation and chain breakage. The most direct embodiment is the decrease of apparent viscosity of aqueous solution. If the need for a long time to store hydroxypropyl methyl cellulose aqueous solution, advocate to participate in trace mildew inhibitor, so that the viscosity does not attack significant changes. Attention should be paid to safety when choosing anti-mildew agent, preservative or fungicide. It is necessary to choose products that are not toxic to human body, stable and odorless in nature, such as DOW Chem's AMICAL fungicide, CANGUARD64 preservative, FUELSAVER agent and other products can play corresponding effects.
A small summary
The aqueous viscosity of hydroxypropyl methyl cellulose is mainly affected by the degree of polymerization of the product, the concentration of the product in aqueous solution, shear rate and solution temperature. The aqueous viscosity increases with the increase of the degree of polymerization and concentration of the product, and decreases with the increase of the shear rate and solution temperature. Rheological modulators, salts or safe fungicides can also be used to obtain better viscosity properties and service function according to practical requirements. In practical use, users should consider the practical situation and economy to select the appropriate viscosity standard and dosage of hydroxypropyl methyl cellulose, as well as additives to achieve desired viscosity function.