Polyvinyl alcohol powder is a type of water-soluble film-forming adhesive substance. Its products are now widely used in some dry-mixed mortar products. It provides a relatively cheap adhesive material for powdery and paste putties.
Adding Polyvinyl alcohol powder to dry-mixed mortar products like redispersible polymer powder increases its bonding strength, but its comprehensive performance is far from comparable to that of redispersible polymer powder. Polyvinyl alcohol powder only serves as a protective colloid for redispersible polymer powder. However, it is still the only one that can achieve low-temperature flexible bonding and reflect good impermeability, alkali resistance, crack resistance, water retention, and other properties—completed by copolymers such as ethylene vinyl acetate (EVA). Therefore, using polyvinyl alcohol powder can produce some relatively low-priced dry-mixed mortar products, but the additional amount should be appropriately increased. Since polyvinyl alcohol powder is highly hydrophilic but with poor water resistance.
The alcoholysis degree of polyvinyl alcohol powder is about 88% and can dissolve well in water. Its water-soluble viscosity is also relatively large. Therefore, polyvinyl alcohol powder can be easily formulated into 801 and 802 glue as a paste. Mix interior wall putty with white cement and talcum powder and can use it as an adhesive for exterior wall putty and interface agent.
The main applications of polyvinyl alcohol in the construction industry include as a film-forming substance in water-soluble coatings, as an emulsifier in vinyl acetate emulsion, as a protective colloid in redispersible polymer powder, as the primary raw material for the production of 802 glue. The adhesion and flexibility can be improved to prevent the plaster from cracking and falling off in dry-mixed mortar products.
Water retaining agent and thickener
The water-retaining thickeners used in traditional building mortar are lime paste and microfoaming agents, and the water-retaining agents and thickeners used in dry-mixed powder mortar are cellulose ether and starch ether.
1. Cellulose ether
Cellulose ether is the general name for a series of products produced by alkali cellulose and etherifying agent reactions under certain conditions. In dry mortar products, the additional amount of cellulose ether is meager, but it can significantly improve the performance of wet mortar and is a significant additive that affects the construction performance of mortar.
(1) Classification of cellulose ethers
It is very complicated to produce cellulose ether. First extracts cellulose from cotton or wood, then adds sodium hydroxide and converts it into alkaline cellulose through a chemical reaction (alkali dissolution). The alkaline cellulose undergoes etherification. Under the action of the agent (etherification reaction), cellulose ether is generated through water washing, drying, grinding, and other processes.
Natural fibers such as cotton fiber, fir, or beech wood fiber are the primary raw materials for producing cellulose ether. Due to their different degrees of polymerization, this will inevitably affect the final viscosity of their products. Currently, all major cellulose manufacturers use cotton. Fiber (a by-product of nitrocellulose) is the primary raw material. Different etherifying agents can etherify alkaline cellulose into various types of cellulose ethers. The cellulose molecular structure is composed of bonds of dehydrated glucose units. Each glucose unit contains 3 hydroxyl groups. When under certain conditions, the hydroxyl groups are replaced by methyl, hydroxyethyl, hydroxypropyl, and other groups; various types of cellulose can be produced, such as those substituted by methyl groups called methyl cellulose, those substituted by hydroxyethyl groups called hydroxyethyl cellulose, and those substituted by hydroxypropyl groups called hydroxypropyl cellulose. Since methylcellulose is a mixed ether generated through etherification reaction, mainly methyl, but containing a small amount of hydroxyethyl or hydroxypropyl, it is called methyl hydroxyethyl cellulose ether or methyl cellulose. Hydroxypropyl cellulose ether. Due to the different substituents (such as methyl, hydroxyethyl, and hydroxypropyl) and the different degrees of substitution (the number of substituted substances for each active hydroxyl group on cellulose), various types of cellulose ethers can be generated. Variety and different brands can be widely used in fields such as construction engineering, food and pharmaceutical, daily chemical, and petroleum.
- Commonly used Cellulose ethers in dry-mixed mortar and their properties
Since ionic cellulose (hydroxymethyl cellulose salt) is unstable in the presence of calcium ions, it is rarely used in dry-mixed mortar products that use cement and slaked lime as cementing materials. Hydroxyethyl cellulose is also used in some dry-mixed mortar products, but its market share is tiny. The cellulose ethers currently used are mainly methyl hydroxyethyl cellulose ether (MHEC) and methyl hydroxyethyl cellulose. Propylcellulose ether (MHPC) ‘s market share has exceeded 90%, and the proportion of real methylcellulose ether is deficient. Methyl hydroxyethyl cellulose ether and methyl hydroxypropyl cellulose used in the construction industry are referred to as MHEC or MHPC, which play a significant role in the field of dry-mixed mortar. It is a crucial modified material such as plastering mortar, stucco gypsum, tile adhesive, putty, self-leveling mortar, spray mortar, wallpaper glue, and caulking materials. It controls mortar consistency in various dry-mixed mortars—the role of working performance, bonding performance, and water retention.
The cellulose ether used in dry-mixed mortar are mainly Methyl hydroxyethyl cellulose ether (MHEC) and Methyl hydroxypropyl cellulose ether (MHPC), so the cellulose ethers mentioned here primarily refer to MHEC and MHPC, and the properties of them.
1. Water retention
Water retention is an essential property of cellulose ether. Factors that affect the water retention effect of dry mortar include
- the amount of cellulose ether added,
- the viscosity of cellulose ether,
- the fineness of cellulose ether,
- the temperature of the use environment.
- The added amount of cellulose ether affects water retention. When the added amount of cellulose ether is within the range of 0.05% to 0.4%, the water retention rate increases with the additional amount. Increases, then the increasing trend of water retention rate begins to slow down, as shown in Figure 1.
The dosage of cellulose ether is also various according to the different uses of mortar. In masonry mortar or self-leveling mortar, the dosage is 0.02%; in plastering mortar, the dosage is 0.1%; and in ceramic tiles, it is about 0.3%~0.7%.
water-retention rate%
The addition rate of cellulose ether%
Figure 1: Relationship between water retention and addition rate
(2) Effect of viscosity of cellulose ether on water retention
The viscosity of cellulose ether has a similar relationship with the water retention rate. When the viscosity of cellulose ether increases, the water retention rate also increases. When the viscosity reaches a certain height, the increase in water retention rate also tends to be gentle. See Figure 2.
water-retention rate%
Viscosity(2% water solution, 20℃,D=2.5s¹)/(MPa · s)
Figure 2: Relationship between water retention and viscosity
Generally speaking, the higher the viscosity, the better the water retention effect; the higher the viscosity, the higher the relative molecular mass of the cellulose ether.
Its solubility will also be reduced accordingly, which will hurt the strength and construction performance of the mortar. The higher the viscosity, the more pronounced the thickening effect on the mortar, but it is not directly proportional. The higher the viscosity, the greater the viscosity of the wet mortar. During construction, it is manifested as sticking to the scraper and high adhesion to the substrate. However, it does not help much to increase the structural strength of the wet mortar itself, and the effect of improving anti-sag is not apparent. On the contrary, some modified methyl cellulose ether with medium and low viscosity have excellent performance in improving the structural strength of wet mortar.
(3) Effect of fineness of cellulose ether on water retention
The cellulose ether used in dry-mixed mortar should be in powder form, with a particle size of 20% to 60% less than 63 μm. The fineness can affect the solubility of cellulose ether. Coarse cellulose ether is usually granular and can be easily dispersed and dissolved in water without clumping. However, the dissolution speed is prolonged and unsuitable for dry-mixed mortar use. Cellulose ether is distributed among aggregates, fine fillers cement, and other cementitious materials in dry-mixed mortar. Only sufficiently fine powder can avoid cellulose ether agglomeration when water is added to stir. When cellulose ether is dissolved in water, it will not be easy to disperse and dissolve again if agglomeration occurs. Cellulose ethers with coarser fineness will reduce the local strength of the mortar. When such mortar is constructed over a large area, the curing speed of the local mortar will be significantly reduced, and cracking will occur due to different curing times. For gunite, the requirements for fineness are higher due to the shorter mixing time.
The effect of the fineness of cellulose ether on water retention. Generally speaking, for cellulose ether with the same viscosity but different fineness, at the same addition amount, the finer the cellulose ether, the better the water retention. See Figure 3
Water retention rate%
Absorption time/min
Figure 3: Effect of fineness on water retention
(4) Effect of operating temperature on water retention
The water retention of cellulose ether is also related to the operating temperature. The water retention decreases as the operating temperature increases.
Water retention rate%
operating temperature%
Figure 4: Effect of temperature on water retention
In the actual application of materials, construction in high-temperature environments is often encountered, such as applying plastering on exterior walls in the sun in summer, which will inevitably accelerate the setting and hardening of cement mortar. The decrease in water retention rate will lead to a reduction in workability and crack resistance. In this case, reducing the influence of temperature factors becomes particularly critical. Experiments have shown that increasing the etherification degree of cellulose ether can make its water retention effect at the use temperature. Better results can still be maintained under higher conditions.
2. Thickening
Another critical characteristic of cellulose ether is its excellent thickening effect. Adding cellulose ether to dry-mixed mortar products can increase the viscosity thousands of times, giving it better-sagging resistance and adhesion. In practical applications, cellulose ether can precisely control the consistency of mortar products, and these consistencies are adjusted by adding different types of cellulose ether and other amounts.
The thickening effect of cellulose ether is closely related to its unique rheology. In particular, its viscosity will affect the material’s bonding strength, fluidity, structural stability, and workability.
3. Solubility
Since the surface particles of high-quality cellulose ether have been specially treated, they have excellent solubility, whether in cement mortar, gypsum, or coating systems and are not easy to agglomerate and dissolve quickly. Especially in mortar systems with a considerable pH value, it significantly promotes rapid dissolution. It usually dissolves entirely within a few minutes.
(3) Selection of cellulose ethers
In dry-mixed mortar, cellulose ether plays a role in water retention, thickening, and improving construction performance. Good water retention properties can ensure that the mortar does not cause sanding, powdering, and strength reduction due to lack of water and incomplete cement hydration. The thickening effect dramatically enhances the structural strength of wet mortar. For example, ceramic tile adhesive has good anti-sag ability. Adding cellulose ether can significantly improve the watery viscosity of wet mortar and has suitable viscosity to various substrates, thereby improving the wall-mounting performance of wet mortar.
The functions of cellulose ether in different products are also various. For example, cellulose ether can increase the opening time and adjust the time in ceramic tile adhesives; mechanical spray mortars can improve the structural strength of wet mortar; Self-leveling mortar can prevent settlement and segregation. Cellulose ether is widely used in dry-mixed mortar products as an essential additive. However, the brand and dosage of cellulose ether are diverse in different dry-mixed mortar products. Since various dry-mixed mortars have additional technical requirements for cellulose ether, manufacturers will modify cellulose ether of the same viscosity to adapt to the different technical needs of other dry-mixed powder mortar products. It is convenient for dry-mixed mortar formula designers to choose.
