Hydroxypropyl methylcellulose (HPMC), as a kind of cellulose ether, has been widely used in the construction industry because of its excellent performance in the construction industry and its small amount of usage.
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.
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.
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:
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:
Where: η is the absolute viscosity, mPa·s; K is the coefficient; α is the deflection angle, that is, the pointer reading.
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.
5 Other influencing factors
The aqueous solution viscosity of hydroxypropyl methylcellulose is also affected by the additives in the solution, the pH of the solution, and the degradation of the microorganisms. In order to obtain better viscosity properties 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 aqueous hydroxypropyl methylcellulose solution. It is also possible to add electrolytes such as chlorides, bromides, phosphates, nitrates, etc. to the aqueous solution. These additives not only affect the viscosity properties of the aqueous solution, but also other properties of hydroxypropyl methylcellulose such as water retention. , anti-sag, etc.
The viscosity of the aqueous solution of hydroxypropylmethylcellulose is hardly affected by acid and alkali, and is generally stable in the range of 3 to 11, and can withstand a certain amount of weak acid such as formic acid, acetic acid, phosphoric acid, boric acid, citric acid, etc. However, concentrated acid will reduce the viscosity. However, caustic soda, potassium hydroxide, lime water, etc. have little effect on it. Hydroxypropyl methylcellulose aqueous solution has good antimicrobial stability compared with other cellulose ethers. The main reason is that hydroxypropyl methylcellulose has a hydrophobic group with high degree of substitution and a space hindrance of the group. The action prevents the microbial attack on the cellulose chain. However, since the substitution reaction is usually not uniform, the unsubstituted anhydroglucose anhydride unit is most susceptible to microbial attack, resulting in degradation of the cellulose ether molecule, the most direct The performance is the apparent viscosity of the aqueous solution. If it is necessary to store the hydroxypropyl methylcellulose aqueous solution for a long time, it is recommended to add a trace amount of anti-fungal agent so that the viscosity does not change significantly. When using anti-mold agents, preservatives or fungicides, it is necessary to pay attention to safety. Products that are not toxic to the human body and have stable and odorless properties, such as DOW Chem’s AMICAL fungicide, ANGUARD64 preservative, FUELSAVER microbial agents, etc. Can play a corresponding role.
The aqueous solution viscosity 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. The viscosity of the aqueous solution increases as the degree of polymerization and concentration of the product increases, with shearing. The rate and the temperature of the solution are lowered, and a rheology modifier, a salt or a safe bactericide can be added according to actual needs to obtain better viscosity characteristics and application properties. In practical applications, users should consider the actual conditions and economy to choose the appropriate viscosity specifications and addition amount of hydroxypropyl methylcellulose, and add additives to obtain the desired viscosity performance.