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Investigation of pilot trials of the production of PVC resin from hydroxypropyl methylcellulose

Views: 0     Author: Site Editor     Publish Time: 2023-03-17      Origin: Site

Abstract: The production process of domestic HPMC was introduced, and the main role of domestic HPMC in the production process of PVC and its influence on the quality of PVC resin were studied in the pilot test. The results show that: ①The performance of domestic HPMC is excellent, and the performance of the PVC resin produced is equivalent to the quality of PVC resin produced by imported HPMC products; ②When domestic HPMC is used in PVC production, PVC can be improved and fine-tuned by adjusting the type and amount of HPMC The performance of resin products; ③ Domestic HPMC is suitable for the production of various loose PVC resins. The PVC resin particles produced have thin film and light sticking to the kettle; ④ Domestic HPMC products can replace imported HPMC products.

Key words: PVC; dispersant; hydroxypropyl methylcellulose

The production of HPMC with refined cotton in foreign countries began in 1960, and my country began to develop HPMC in early 1970. Due to the constraints of equipment, technology and other factors, the quality could not be stable, and the appearance was fibrous. For this reason, the HPMC required by the PVC resin industry, pharmaceutical industry, high-end building materials, cosmetics, steel, food and other industries all rely on imports, mainly from the United States and Japan, and HPMC is subject to foreign monopoly. In 1990, the Ministry of Chemical Industry organized relevant units to jointly tackle key problems, and produced products that met the industrial quality requirements of PVC, realizing the localization of HPMC. In recent years, excellent domestic HPMC manufacturers have firmly established the development concept of innovation, coordination, green, openness, and sharing, insisted on innovation-driven development, and successfully achieved high-quality development through independent innovation, scientific development, and accelerated conversion of old and new kinetic energy. Proposed by the China Petroleum and Chemical Industry Federation, the GB/T 34263-2017 "Hydroxypropyl Methyl Fiber for Industrial Use", which was designated by the China Chemical Standardization Technical Committee and approved by the drafting unit, was promulgated in 2017, and it was released nationwide on April 1, 2018. officially implemented. Since then, there are standards for PVC enterprises to purchase and use HPMC products.

1. Refined cotton quality

30# refined cotton is in the shape of fine fibers under the microscope. A mature cotton fiber has hundreds of crystallized basic element fibers in its cross section, and the basic element fibers are aggregated into hundreds of bundled fibers. These fibril bundles A cotton fiber is helically coiled in concentric layers. This is conducive to the formation of alkalized cellulose and the uniformity of etherification degree, and is conducive to improving the glue retention ability of HPMC during PVC polymerization.

30# refined cotton uses cotton linters with high maturity and low polymerization degree as raw material, the production process is complicated, it needs to be purified, and the production cost is high. 1000# refined cotton uses cotton linters with high maturity and high degree of polymerization as raw material, the production process is not complicated, and the production cost is low. Therefore, 30# refined cotton is used to produce high-end products such as PVC resin/medicine/food, and 1000# refined cotton is used to produce building materials grade or other application fields.

2. The nature, model and production process of HPMC products

2.1 Properties of HPMC products

HPMC is a non-toxic, odorless, tasteless white or off-white fibrous or granular powder made of natural refined cotton as the main raw material. It is a semi-synthetic, inactive, viscoelastic polymer, non-ionic type compounds. The Chinese aliases are hydroxymethyl propyl cellulose, cellulose hydroxypropyl methyl ether, and hypromellose, and the molecular formula is [C6H7O2(OH)2COOR]n.

The melting point of HPMC is 225-230 °C, the density is 1.26-1.31 g/cm³, the relative molecular mass is about 22,000, the carbonization temperature is 280-300 °C, and the surface tension is 42-56 mN/m (2% aqueous solution ).

The physical and chemical properties of HPMC mainly include the following points.

(1) Particle size index: The HPMC particle size index for PVC resin has high requirements. The pass rate of 150 μm is greater than 98.5%, and the pass rate of 187 μm is 100%. The general requirement of special specifications is between 250 and 425 μm.

(2) Solubility: soluble in some solvents such as water and alcohols, water-soluble and has surface activity. High transparency, stable performance of the solution, different specifications of products have different gel temperatures, solubility changes with viscosity, the lower the viscosity, the greater the solubility, different specifications of HPMC have certain differences in performance, and the solubility in water is not affected by the pH value.

The solubility in cold water and hot water is different. Products with high methoxyl content are insoluble in hot water above 85°C, products with medium methoxyl content are insoluble in hot water above 65°C, and products with low methoxyl content are insoluble in hot water above 65°C. Hot water above 60°C. Ordinary HPMC is insoluble in organic solvents such as ethanol, ether, and chloroform, but soluble in 10% to 80% ethanol aqueous solution or a mixture of methanol and dichloromethane. HPMC has a certain hygroscopicity. At 25°C/80%RH, the equilibrium moisture absorption is 13%, and it is very stable in a dry environment and a pH value of 3.0-11.0.

(3) HPMC has excellent characteristics of being soluble in cold water but insoluble in hot water. Putting HPMC in cold water and stirring it can completely dissolve and turn into a transparent liquid. Some brand products are basically insoluble in hot water above 60°C, and can only swell. This property can be used for washing and purification, which can reduce costs, reduce pollution, and increase production safety. With the decrease of methoxyl content, the gel point of HPMC increased, the water solubility decreased, and the surface activity also decreased.

(4) HPMC is used as a suspension stabilizer and dispersant in the polymerization of vinyl chloride and vinylidene. It can be used together with polyvinyl alcohol (PVA) or independently, and can control the particle shape and particle distribution.

(5) HPMC also has strong enzyme resistance, thermal gel properties (hot water above 60°C does not dissolve, but only swells), excellent film-forming properties, pH value stability (3.0-11.0), Water retention and many other characteristics.

Based on the above excellent characteristics, HPMC is widely used in industrial fields such as medicine, petrochemical industry, construction, ceramics, textile, food, daily chemical, synthetic resin, coating and electronics.

2.2 HPMC product model

The ratio of methoxyl content and hydroxypropyl content in HPMC products is different, the viscosity is different, and the product performance is different.

2.3 Production process of HPMC products

HPMC uses refined cotton cellulose as the main raw material, and forms cotton powder through crushing treatment. Put the cotton powder into a vertical polymerization kettle, disperse it in about 10 times the solvent (toluene, isopropanol as a mixed solvent), and add in sequence Lye (food-grade caustic soda is dissolved in hot water first), propylene oxide, methyl chloride etherification agent, etherification reaction is carried out at a certain temperature and pressure, and the reaction product is neutralized with acid, iron removed, washed and dried , and finally obtain HPMC.

3. Application of HPMC in PVC production

3.1 Principle of action

The application of HPMC as a dispersant in PVC industrial production is determined by its molecular structure. It can be seen from the molecular structure of HPMC that the structural formula of HPMC has both a hydrophilic hydroxypropyl (-OCH-CHOHCH3) functional group and an lipophilic methoxyl (-OCH,) functional group. In vinyl chloride suspension polymerization, the dispersant is mainly concentrated in the interface layer of the monomer droplet-water phase, and arranged in such a way that the hydrophilic segment of the dispersant extends to the water phase, and the lipophilic segment extends to the monomer droplet. In HPMC, the hydroxypropyl-based segment is a hydrophilic segment, which is mainly distributed in the water phase; the methoxy-based segment is a lipophilic segment, which is mainly distributed in the monomer phase. The amount of lipophilic segment distributed in the monomer phase affects the primary particle size, degree of aggregation, and porosity of the resin. The higher the content of the lipophilic segment, the stronger the protective effect on primary particles, the smaller the degree of primary particle aggregation, and the resin The porosity of the resin increases and the apparent density decreases; the higher the content of the hydrophilic segment, the weaker the protective effect on the primary particles, the greater the degree of aggregation of the primary particles, the lower the porosity of the resin, and the higher the apparent density. In addition, the protective effect of the dispersant is too strong. With the increase of the viscosity of the polymerization reaction system, at a higher conversion rate, the bonding between the resin particles is prone to occur, making the particle shape irregular; the protective effect of the dispersant is too weak, and the primary particles It is easy to coalesce at the stage of low conversion rate in the early stage of polymerization, thus forming resin with irregular particle shape.

It has been proved by practice that adding HPMC and other dispersants to the suspension polymerization of vinyl chloride can reduce the interfacial tension between vinyl chloride and water at the initial stage of polymerization. Stable dispersion in the water medium, this effect is called the dispersion ability of the dispersant; on the other hand, the lipophilic functional group of the dispersant adsorbed on the surface of the vinyl chloride droplet forms a protective layer to prevent the aggregation of the vinyl chloride droplet. The droplet plays a role of stabilization and protection, which is called the colloid retention ability of the dispersant. That is, in the suspension polymerization system, the dispersant plays a dual role of dispersing and protecting the colloidal stability.

3.2 Application performance analysis

PVC resin is a solid particle powder. Its particle characteristics (including its particle shape, particle size and distribution, microstructure and pore size and distribution, etc.) largely affect the processing performance of plastics and product performance, and determine PVC. There are two factors that have the greatest influence on the characteristics of resin particles: ① The stirring of the polymerization tank, the equipment is relatively fixed, and the stirring characteristics are basically unchanged; ② The dispersant system of the monomer in the polymerization process, that is, how to choose the type, grade and Dosage is the most critical variable controlling the properties of PVC resin pellets.

From the resin granulation mechanism in the suspension polymerization process, it is known that adding a dispersant before the reaction mainly serves to stabilize the monomer oil droplets formed by stirring and prevent the mutual polymerization and merging of oil droplets. Therefore, the dispersion effect of the dispersant will affect the main properties of the polymer resin.

The colloid retention ability of the dispersant has a positive relationship with the viscosity or molecular weight. The greater the viscosity of the aqueous solution, the higher the molecular weight, and the higher the strength of the protective film adsorbed on the vinyl chloride-water phase interface, the less prone to film rupture and grain coarsening.

The aqueous solution of the dispersant has interfacial activity, the smaller the surface tension, the higher the surface activity, the finer the monomer oil droplets formed, the smaller the apparent density of the obtained resin particles, and the looser and more porous.

It has been confirmed through experimental research that the interfacial tension of HPMC is relatively small in the aqueous dispersant solutions of gelatin, PVA, and HPMC at the same concentration, that is, the smaller the surface tension, the higher the surface activity of HPMC in the vinyl chloride suspension polymerization system, which indicates that The stronger the dispersing ability of HPMC dispersant. Compared with medium and high viscosity PVA dispersants, the average relative molecular weight of HPMC (about 22 000) is much smaller than that of PVA (about 150 000), that is, the adhesive retention performance of HPMC dispersants is not as good as that of PVA.

The above theoretical and practical analysis shows that HPMC can be used to produce various types of suspension PVC resins. Compared with PVA with a degree of alcoholysis of 80%, it has weaker glue retention ability and stronger dispersion ability; .Compared with 5% PVA, the glue retention ability and dispersion ability are equivalent. HPMC is used as a dispersant, and the resin particles produced by HPMC have less "film" content, poor regularity of resin particles, finer particle size, high absorption of resin processing plasticizers, and actually less sticky to the kettle, because it is non-toxic and easy Produces medical-grade resins with high clarity.

According to the above theoretical and practical production analysis, HPMC and PVA, as the main dispersants for suspension polymerization, can basically meet the quality requirements of resin products, but it is very difficult to meet the requirements of adhesive retention ability and interfacial activity in polymerization production. Because the two have their own characteristics, in order to produce high-quality resin products, most manufacturers use composite systems with different adhesive retention capabilities and interfacial activities, that is, PVA and HPMC composite dispersant systems, to achieve the effect of learning from each other.

3.3 Quality comparison of HPMC at home and abroad

The gel temperature test process is to prepare an aqueous solution with a mass fraction of 0.15%, add it to a colorimetric tube, insert a thermometer, slowly heat up and stir gently, when the solution appears milky white filamentous gel is the lower limit of the gel temperature, continue to heat up and stir , when the solution completely turns milky white is the upper limit of the gel temperature.

3.4 State of different models of HPMC at home and abroad under microscope

The photos of different types of HPMC under the microscope can be seen: ①Foreign E50 and domestic 60YT50 HPMC both present an aggregated structure under the microscope, the molecular structure of domestic 60YT50HPMC is compact and uniform, and the molecular structure of foreign E50 is dispersed; ②The aggregated state of domestic 60YT50 HPMC The structure can theoretically reduce the interfacial tension between vinyl chloride and water, and assist vinyl chloride to disperse uniformly and stably in the water medium, that is, because the hydroxypropyl content of 60YT50 HPMC is slightly higher, it makes it more hydrophilic, while ES0 Due to the high content of methoxyl groups, theoretically, it has stronger rubber retention performance; ③prevents the merging of vinyl chloride droplets in the early stage of the polymerization process; ④prevents the merging of polymer particles in the middle and later stages of the polymerization process. The aggregate structure mainly studies the mutual arrangement of cellulose molecules (crystalline and amorphous regions, the size and form of the unit cell, the packing form of molecular chains in the unit cell, the size of crystallites, etc.), the orientation structure (molecular chain and Orientation of microcrystals), etc., are conducive to the full grafting reaction of refined cotton during etherification, and improve the intrinsic quality and stability of HPMC.

3.5 State of HPMC aqueous solution at home and abroad

The domestic and foreign HPMC was prepared into 1% aqueous solution, and the light transmittance of domestic 60YT50 HPMC was 93%, and that of foreign E50 HPMC was 94%, and there was basically no difference in light transmittance between the two.

The domestic and foreign HPMC products were formulated into 0.5% aqueous solution, and the solution after HPMC cellulose was dissolved was observed. It can be seen from the naked eye that the transparency of both is very good, clear and transparent, and there is no large amount of insoluble fiber, which shows that the quality of imported HPMC and domestic HPMC is better. The high light transmittance of the solution shows that HPMC reacts fully in the process of alkalization and etherification, without a large amount of impurities and insoluble fibers. First, it can easily identify the quality of HPMC. White liquid and air bubbles.

4. HPMC dispersant application pilot test

In order to further confirm the dispersion performance of domestic HPMC in the polymerization process and its influence on the quality of PVC resin, the R&D team of Shandong Yiteng New Materials Co., Ltd. used domestic and foreign HPMC products as dispersants, and domestic HPMC and imported PVA as dispersants. The quality of the resins prepared by different brands of HPMC as dispersants in China was tested and compared, and the application effect of HPMC in PVC resin was analyzed and discussed.

4.1 Pilot test process

The polymerization reaction was carried out in a 6 m3 polymerization kettle. In order to eliminate the influence of the monomer quality on the quality of PVC resin, the pilot plant used the calcium carbide method to produce vinyl chloride monomer, and the water content of the monomer was less than 50×10-6. After the vacuum of the polymerization kettle is qualified, add the measured vinyl chloride and ion-free water to the polymerization kettle in sequence, and then add the dispersant and other additives required by the formula into the kettle at the same time after weighing. After pre-stirring for 15 minutes, hot water at 90°C was introduced into the jacket, heated to the polymerization temperature to start the polymerization reaction, and chilled water was introduced into the jacket at the same time, and the reaction temperature was controlled by DCS. When the pressure of the polymerization kettle drops to 0.15 MPa, the polymerization conversion rate reaches 85% to 90%, adding a terminator to terminate the reaction, recovering vinyl chloride, separating and drying to obtain PVC resin.

4.2 Pilot test of domestic 60YT50 and foreign E50 HPMC resin production

From the quality comparison data of domestic 60YT50 and foreign E50 HPMC to produce PVC resin, it can be seen that the viscosity and plasticizer absorption of domestic 60YT50 HPMC PVC resin are similar to those of similar foreign HPMC products, with low volatile matter, good self-sufficiency, The qualified rate is 100%, and the two are basically close in terms of resin quality. The methoxyl content of foreign E50 is slightly higher than that of domestic 60YT50 HPMC, and its rubber retention performance is strong. The obtained PVC resin is slightly better than domestic HPMC dispersants in terms of plasticizer absorption and apparent density.

4.3 Domestic 60YT50 HPMC and imported PVA used as dispersant to produce resin pilot test

4.3.1 Quality of PVC resin produced

PVC resin is produced by domestic 60YT50 HPMC and imported PVA dispersant. The quality comparison data can be seen: using the same quality 60YT50HPMC and imported PVA dispersant system to produce PVC resin respectively, because theoretically 60YTS0 HPMC dispersant has strong dispersion ability and good rubber retention performance. It is not as good as PVA dispersion system. The apparent density of PVC resin produced by 60YTS0 HPMC dispersion system is slightly lower than that of PVA dispersant, the plasticizer absorption is better, and the average particle size of resin is finer. The test results can basically reflect the various characteristics of 60YT50 HPMC and imported PVA dispersant systems, and also reflect the advantages and disadvantages of the two dispersants from the performance of PVC resin. In terms of microstructure, the surface film of HPMC dispersant resin Thin, the resin is easier to plasticize during processing.

4.3.2 Film condition of PVC resin particles under electron microscope

Observing the microstructure of the resin particles, the resin particles produced by HPMC dispersant have a thinner microscopic "film" thickness; the resin particles produced by PVA dispersant have thicker microscopic "film". In addition, for calcium carbide resin manufacturers with high content of vinyl chloride monomer impurities, in order to ensure the stability of the formula system, they have to increase the amount of dispersant, which results in an increase in the surface deposits of the resin particles and thickening of the "film". Downstream processing plasticizing performance is unfavorable.

4.4 Pilot test of different grades of HPMC to produce PVC resin

4.4.1 Quality of PVC resin produced

Using various domestic grades of HPMC (with different viscosities and hydroxypropyl content) as a single dispersant, the amount of dispersant is 0.060% of the vinyl chloride monomer, and the suspension polymerization of vinyl chloride is carried out at 56.5 ° C to obtain The average particle size, apparent density, and plasticizer absorption of PVC resin.

It can be seen from this that: ① Compared with 65YT50 HPMC dispersion system, 75YT100 has a viscosity of 65YT50 HPMC less than 75YT100HPMC, and the hydroxypropyl content is also less than 75YT100HPMC, while the methoxyl content is higher than 75YT100 HPMC. According to the theoretical analysis of dispersants, viscosity and hydroxypropyl The decrease of the base content will inevitably lead to the decrease of the dispersing ability of HPMC, and the increase of the methoxy content will promote the enhancement of the adhesive retention ability of the dispersant, that is, the 65YT50 HPMC dispersion system will cause the average particle size of PVC resin to increase (coarse particle size),

The apparent density increases and the plasticizer absorption increases; ②Compared with 60YT50 HPMC dispersion system, the hydroxypropyl content of 60YT50 HPMC is greater than that of 65YT50 HPMC, and the methoxy content of the two is close and higher. According to the dispersant theory, the higher the hydroxypropyl content, the stronger the dispersing ability of the dispersant, so the dispersing ability of 60YT50 HPMC is enhanced; at the same time, the two methoxyl content is close and the content is higher, the glue retention ability is also stronger, In the 60YT50 HPMC and 65YT50 HPMC dispersion systems of the same quality, the PVC resin produced by 60YT50HPMC than the 65YT50 HPMC dispersion system must have a smaller average particle size (fine particle size) and lower apparent density, because the methoxyl content in the dispersion system is close to ( rubber retention performance), resulting in similar plasticizer absorption. This is also the reason why 60YT50 HPMC is generally used in the PVC resin industry when selecting PVA and HPMC composite dispersants. Of course, whether 65YT50 HPMC is reasonably used in the composite dispersion system formula should also be determined according to specific resin quality indicators.

4.4.2 Particle morphology of PVC resin particles under microscope

The particle morphology of PVC resin produced by 2 kinds of 60YT50 HPMC dispersants with different hydroxypropyl and methoxyl content under the microscope can be seen: with the increase of hydroxypropyl and methoxyl content, the dispersion ability of HPMC, retention The glue ability is enhanced. Compared with 60YT50 HPMC (8.7% hydroxypropyl mass fraction, 28.5% methoxyl mass fraction), the PVC resin particles produced are regular, without tailing, and the particles are loose.

4.5 Effect of 60YT50 HPMC dosage on the quality of PVC resin

The pilot test uses 60YT50 HPMC as a single dispersant with a mass fraction of methoxyl group of 28.5% and a mass fraction of hydroxypropyl group of 8.5%. The average particle size, apparent density, and plasticizer absorption of PVC resin obtained by carrying out suspension polymerization of vinyl chloride at 5°C.

It can be seen that as the amount of dispersant increases, the thickness of the dispersant layer adsorbed on the droplet surface increases, which enhances the dispersant performance and adhesive retention capacity of the dispersant, resulting in a decrease in the average particle size of the PVC resin and a decrease in the surface area. The apparent density increases and the plasticizer absorption decreases.

5 Conclusion

(1) The application performance of PVC resin prepared from domestic HPMC products has reached the level of similar imported products.

(2) When HPMC is used as a single dispersant, it can also produce PVC resin products with better indicators.

(3) Compared with PVA dispersant, HPMC and PVA dispersant, the two kinds of additives are only used as dispersant to produce resin, and the resin indicators produced have their own advantages and disadvantages. HPMC dispersant has high surface activity and strong monomer oil droplet dispersing performance. It has the same performance as PVA 72 .5% alcoholysis degree similar performance.

(4) Under the same quality conditions, different grades of HPMC have different methoxyl and hydroxypropyl content, which have different uses for adjusting the quality index of PVC resin. 60YT50 HPMC dispersant has better dispersion performance than 65YT50 HPMC due to its high hydroxypropyl content; 65YT50 HPMC

Due to the high methoxy content of the dispersant, the rubber retention performance is stronger than that of 60YT50HPMC.

(5) Usually in the production of PVC resin, the amount of 60YT50HPMC dispersant used is different, and the adjustment of the quality and performance of PVC resin also has obvious changes. When the dosage of 60YT50 HPMC dispersant increases, the average particle size of PVC resin decreases, the apparent density increases, and plasticization

The absorption rate of the agent decreases, and vice versa.

In addition, compared with PVA dispersant, HPMC is used to produce resin series products, which shows great elasticity and stability to parameters such as polymerization kettle type, volume, stirring, etc., and can reduce the phenomenon of equipment kettle wall sticking to the kettle, and reduce the resin surface film Thickness, non-toxic resin, high thermal stability, enhance the transparency of resin downstream processing products, etc. In addition, domestic HPMC will help PVC manufacturers reduce production costs, improve market competitiveness, and bring good economic benefits.