Selection Guide for Hydroxypropyl Methylcellulose in Dry-Mix Mortar
Jun. 18, 2021
HPMC Cellulose is a non-ionic hydroxypropyl methyl cellulose. It is commonly used as a water retaining agent in cement/mortar applications and is the most commonly used water retaining material.
The production process of HPMC cellulose is the alkalization and etherification of cotton fibers to form a polysaccharide ether product. It is not charged by itself and the gel-like material in the charged ions does not participate in any reaction and has stable properties. hpmc cellulose is less expensive than other types of cellulose ethers and is widely used in dry mortar.
The role of hydroxypropyl methyl cellulose
1. Thickening effect: HPMC cellulose can make the fresh mortar thicken and have a certain wet viscosity to prevent segregation.
2. Water retention effect: Water retention is also one of the most important properties, which helps to keep the free water in the mortar. It gives more time for the cementing material to hydrate after mortar construction.
3.Outgassing effect: HPMC cellulose has outgassing effect, which can introduce uniform fine air bubbles into mortar and improve mortar construction.
For hydroxypropyl methyl cellulose, the greater the viscosity, the better the water retention performance. Viscosity is an important parameter of HPMC cellulose performance. Different HPMC Cellulose Manufacturers use different methods and instruments to determine the viscosity of HPMC cellulose.
Comparison of viscosity must be done under the same test methods
Regarding particle size, finer particles have better water retention. When large particles of cellulose ether come in contact with water, the surface immediately dissolves and forms a gel-coated material that prevents further penetration of water molecules. Sometimes long time stirring does not make the HPMC cellulose evenly dispersed and dissolved, and a cloudy flocculent solution or lumps will be formed. This will greatly affect the water retention of cellulose ether.
Solubility is one of the elements for choosing cellulose
HPMC cellulose requires good quality of dry mortar, low water content, 20% to 60% particle size should be less than 63um. fineness affects the solubility of hydroxypropyl methyl cellulose ether. Coarse HPMC cellulose is usually granular, dissolved in water easily without clumping, but the dissolution speed is very slow, not suitable for dry mortar. In dry mortar, HPMC cellulose is dispersed in aggregate, fine filler and cement and other cementitious materials. Only enough fine powder can avoid the clumping of methyl cellulose ether after adding water.
If HPMC cellulose is difficult to disperse and dissolve in water. the fineness of MC will not only waste but also reduce the local strength of mortar. In such a large-scale dry mortar construction, if the maintenance performance of local dry mortar is significantly reduced, cracking will occur due to different maintenance time. The use of spray mortar in mechanical construction requires finer powder due to the shorter mixing time.
In general, the higher the viscosity, the better the water retention effect. However, the molecular weight of MC increases with increasing viscosity, which can reduce the solubility of MC and negatively affect the strength and construction properties of the mortar.
The higher the viscosity, the more obvious the thickening effect on the mortar, but not proportional. As the viscosity increases, the wet mortar becomes more viscous. It shows high adhesion of the viscous scraper to the substrate during construction. However, it has little effect on improving the structural strength of mortar, which shows that the anti-sagging performance is not obvious. On the contrary, some low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.
The water retention of HPMC cellulose is also related to the ambient temperature
The water retention effect of methyl cellulose ethers decreases with increasing temperature. However, in practice, dry crusts are often exposed to hot substrates at high temperatures in many cases, such as summer exterior putty plasters, which tend to accelerate cement curing and dry mortar hardening. The reduction in water retention can affect construction and crack resistance. Under such conditions, the effect of temperature reduction becomes particularly critical. Methyl hydroxypropyl cellulose ether additives are considered to be at the forefront of current technological developments. Despite the increased amount of methyl hydroxypropyl cellulose, the construction and crack resistance effects are still not satisfactory.
HPMC cellulose has a gel temperature and is generally divided into 60, 65 and 75 types. 75 type will be the best choice for companies using river sand in ready-mixed mortar. The content of HPMC cellulose in mortar should not be too high, otherwise it will increase the water requirement, sticky trowel, prolong the setting time, and finally affect the construction. Different mortar products use different viscosity of HPMC cellulose, please do not use high viscosity HPMC cellulose at will.
Therefore, although Hydroxypropyl Methylcellulose is good, it cannot be called successful without excellent user experience.