Cellulose is the oldest and abundant natural polymer on the earth. It is inexhaustible and the most precious natural renewable resource for human beings. Cellulose has the characteristics of low price, abundant material, biodegradability, low heat, non toxicity and good biocompatibility. The basic ring of cellulose macromolecule is dehydrated glucose, and its molecular formula is (c6h1005) n. It contains 44.44% carbon, 6.17% hydrogen and 49.39% oxygen. Each glucose residue ring contains three alcohol hydroxyl groups, including two secondary alcohol hydroxyl groups and one primary alcohol hydroxyl group, which play a decisive role in the properties of cellulose. A series of cellulose derivatives can be obtained by chemical modification of cellulose. Cellulose ether can be prepared from natural cellulose by alkalization, etherification, neutralization, purification and drying.
Cellulose ether is one of the important derivatives of cellulose. It can be widely used in food, medicine, cosmetics, building materials, papermaking, coating, textile printing and dyeing, daily chemical industry, petroleum exploitation and other industries. It has the characteristics of solubility, viscosity, stability, non-toxic and biocompatibility. According to the types of substituents, ionization and solubility of cellulose ethers, there are different classifications. The substituents on cellulose ethers have great influence on their properties. According to different substituents, cellulose ethers can be classified into MC, HEC, CMC, HPMC, HEMC, etc., refer to Fig. 1. This paper mainly discusses the physical and chemical properties and application of HPMC and HEMC in the construction industry.
Hydroxypropyl methyl cellulose (HPMC) can be produced from refined cotton, wood pulp, methyl and polyhydroxypropyl ether of cellulose. It is prepared by etherification of cellulose with propylene oxide and chloroform. The methoxy group on methyl chloride replaces the hydroxyl group on the glucose ring, and the hydroxyl group is replaced by hydroxypropoxy and chain polymerization occurs. The structure is shown in Fig. 2. HPMC has the characteristics of thermal gel, its solution has no ionic charge, does not interact with metal salts or ionic compounds, has strong mould resistance, and has good dispersion, emulsification, thickening, adhesion, water retention and gel retention properties.
The production and preparation of hydroxyethyl methyl cellulose (HEMC) is slightly different from that of HPMC. After the cellulose is alkalized, propylene oxide is replaced by ethylene oxide to replace the hydroxyl group on the glucose ring. The structure is shown in Fig. 3. Compared with HPMC, the chemical structure of HEMC has more hydrophilic groups, so it is more stable at high temperature and has good thermal stability. Compared with the common HPMC cellulose ether, it has a relatively higher gel temperature and has an advantage over high temperature. Like HPMC, HEMC has good mildew resistance, dispersion, emulsification, thickening, adhesion, water and glue retention.
2. Physical and chemical properties
The physical and chemical properties of the standard include: appearance, fineness, dry weight loss, sulfate ash, pH value, solution transmittance, solution viscosity, gel temperature and group content (excluding mortar application test).
Appearance, fineness, weight loss on drying, sulphate ash, pH value and transmittance of solution, viscosity, etc. are related to the model and function of the product. The level of different manufacturers is different, so it is not discussed here.
2.1 Cellulose ether group content
Due to the different substituents of HPMC and HEMC, cellulose ether samples can be heated and reacted in a closed reactor. Under the catalysis of adipic acid, the substituted alkoxy groups are quantitatively cracked by hydroiodic acid to generate corresponding iodoanes. The reaction products are extracted with o-xylene, and the extraction solution is injected into the gas chromatograph for component separation, Hydroxypropoxy and hydroxyethoxy can be distinguished. The internal standard method was used to quantify and calculate the content of components to be tested in the sample. Fig. 5 is the GC spectrum of HPMC, and Fig. 6 is the GC spectrum of standard solution for calibration (containing methoxy, hydroxyethoxy, and hydroxypropoxy). It is not difficult to find that the separation time of hydroxyethoxy group is between methoxy group and hydroxypropoxy group. The type of group can be judged by comparing the separation time of standard solution. The group type was determined by the peak time, and the group content was calculated by the peak area. In general, the methoxyl content of HPMC ranges from 16% to 30%, the propoxy content can be 4-32%, the methoxyl content of HEMC is 22%- 30%, and the Hydroxyethoxyl content is 2% -14%.
2.2 Gel temperature
Gel temperature is an important indicator of cellulose ether. The cellulose ether aqueous solution has the characteristics of thermo gel. As the temperature rises, the viscosity decreases continuously. When the solution temperature reaches a certain value, the cellulose ether solution is no longer transparent, but forms a white colloid, which eventually loses its viscosity. The gel temperature test means that the cellulose ether sample is made up of cellulose ether solution of 0.2% concentration and heated slowly in the water bath until the solution appears white or even white gel, and the viscosity is completely lost. The temperature of the solution is the gel temperature of cellulose ether. Fig. 7 is a random selection of 8 gel temperatures of cellulose ether products at home and abroad. The result is that the overall gel temperature of HEMC is slightly higher than that of HPMC. In general, the gel temperature of HPMC is 60 degrees ~75 and eMC is at 75 C ~90 C.
The ratio of methoxy and hydroxypropyl to HPMC has certain effects on water solubility, water holding capacity, surface activity and gel temp erature of the product. Generally, the HPMC with high methoxy content and low hydroxypropyl content has good water solubility and good surface activity, but the gel temperature is low: increasing hydroxypropyl content and reducing methoxy content can increase the gel temperature, but the excessive content of hydroxypropyl group will reduce the gel temperature, and decrease the water solubility and surface activity. Therefore, the cellulose ether manufacturer must strictly control the group content to ensure the quality and stability of the products.
3. Application of construction industry
HPMC and HEMC have similar functions in building materials. It can be used as dispersant, water retaining agent, thickener and binder, etc. it is mainly used in the molding of cement mortar and gypsum products. It is used in cement mortar to increase its cohesiveness, workability, reduce flocculation, improve viscosity and shrinkage, and has the functions of water retention, reducing water loss on concrete surface, improving strength, preventing cracks and water-soluble salt weathering. It is widely used in cement, plaster, mortar and other materials. It can be used as film-forming agent, thickener, emulsifier and stabilizer in latex coatings and water-soluble resin coatings. It has good wear resistance, uniformity and adhesion, and improves surface tension, acid-base stability and compatibility with metal pigments. Due to its good viscosity storage stability, it is especially suitable for emulsion coatings as dispersant. In a word, although the amount of the system is small, it has a great effect and is widely used.
The gel temperature of cellulose ether determines its thermal stability in application. The gel temperature of HPMC is usually at 60 C ~75 C, depending on the type, group content and different production processes of different manufacturers. Due to the characteristics of HEMC group, it has a higher gel temperature, usually above 80 C, so its stability under high temperature is due to HPMC. In practical application, in the hot construction environment in summer, the water holding capacity of HEMC with the same viscosity and dosage is better than that of HPMC. Especially in the south, mortar will sometimes be constructed at high temperature. Cellulose ether with low gel temperature will lose its thickening and water retention at high temperature, thus accelerating the hardening of cement and mortar, and directly affects the construction and cracking resistance.
Because there are more hydrophilic groups in the structure of HEMC, it has better hydrophilicity. The water retention rate of HEMC in mortar is slightly higher than that of HEMC at the same dosage of products with the same viscosity. In addition, the vertical flow resistance of HEMC is also relatively good. Therefore, the application of HEMC in ceramic tile adhesive will be better.
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KEY WORDS:cellulose ether,HPMC,HEMC,physical and chemical properties,architectural application.