Application of Hydroxypropyl Methyl Cellulose (HPMC) in PVC Industry

Abstract: The production technology of hydroxypropyl methylcellulose (HPMC) and its application in PVC industrial production are introduced. Using domestic HMPC instead of imported HPMC to produce SPVC products can greatly reduce production costs, and the supply of raw materials is guaranteed.

Key words:PVC; hydroxypropyl methylcellulose; application

0. Preface

Hydroxypropyl methylcellulose (HPMC) is the most versatile and best-performing variety among all kinds of cellulose. It is widely used in various industrial fields and daily life, and is called "industrial monosodium glutamate". At present, in the suspension method of PVC synthesis, HPMC is one of the main dispersants used by most manufacturers in my country. It is a water-soluble non-ionic cellulose ether. The product obtained during substitution. HPMC is a white powder, tasteless, odorless, non-toxic, completely unchanged in the human body and excreted from the body. The product is soluble in water, and the aqueous solution is a colorless transparent viscous substance. HPMC has excellent thickening, emulsifying, film-forming, dispersing, protective colloid, adhesive, acid and alkali resistance, enzyme resistance and other properties, and is widely used in construction, coatings, medicine, food, textiles, oil fields, cosmetics, detergents, ceramics, Ink and chemical polymerization process.

At present, the total production capacity of cellulose ether in the world exceeds 500,000 t/a, of which non-ionic type accounts for about 33%. Due to the high technical requirements for the production of cellulose ether, only a few industrially developed countries and regions in the United States, Europe and Japan can produce it on a large scale. Among them, the United States and Germany are the main producers of water-soluble cellulose ether in the world, with relatively large-scale production equipment and advanced production technology. South Korea and Japan imported technology and equipment from Germany and the United States respectively, and their production capacity reached or exceeded 10,000 t/a respectively, becoming the world's major producers.

The industrial production of cellulose ether in my country began in the mid-1960s, when Wuxi Chemical Research and Design Institute, Luzhou Chemical Plant and Xi'an Hui'an Chemical Plant began to research and develop HPMC. In recent years, my country's HPMC production technology has made great progress. The main manufacturers are Shandong Ruitai Chemical (Group) Co., Ltd., Hercules Pu Chemical Co., Ltd., Shandong Heda Co., Ltd. and Shanghai Huiguang Fine Chemical Co., Ltd., producing The capacity has exceeded 5 kt/a.

1. HPMC production technology

At present, the more advanced non-ionic cellulose ether production technologies in the world mainly include Dow Chemical Co., Hercules Co., Clariant Co. production technology. Japan's Shinetsu Chemical Co. and South Korea's Samsung Fine Chemical Co. imported technology from Dow Chemical Company of the United States and Loedige Co. of Germany.

In recent years, through technological innovation and introduction of technology, Chinese enterprises have made the production quality of HPMC close to the world's advanced level, and have a certain market competitiveness. In particular, HPMC has been able to partially replace imported products in PVC production, and has achieved gratifying results.

The traditional HPMC production process generally adopts the batch method, but the technology is relatively backward, and there are defects in product quality control. There are differences in the product quality of each kettle, and the products can only be sold by reducing the index, which cannot meet the requirements in many fields, and belongs to the phase-out process. One-step HPMC production process has obvious advantages in technology, equipment, scale, environmental protection, automatic control, etc., and is the main development direction of HPMC production process.

The production process of cellulose ethers has a common feature, that is, the refined cotton or wood pulp is impregnated with liquid caustic soda, and the excess lye is removed by pressing to obtain alkali cellulose, and then solvent and etherification agent are added to carry out etherification reaction at a certain temperature and pressure. , The end point of the reaction is subject to the required degree of etherification, and then the finished product is obtained through neutralization, washing, drying, and crushing.

2. Application characteristics of HPMC in PVC industry

2.1 The role of HPMC in the polymerization of VCM

In VCM suspension polymerization, in order to prevent the coalescence of early polymerization droplets and middle and late polymer particles, it is necessary to add a dispersion protective agent to the VCM suspension polymerization system. When the stirring characteristics are fixed, the type, nature and dosage of dispersant become the key factors to control the characteristics of PVC particles. Commonly used dispersion systems are the two major composite dispersion systems of multi-component PVA system and HPMC+multi-component PVA, and the latter is mostly used in China at present.

(1) Polymerization temperature: The polymerization temperature basically determines the average molecular weight of PVC, and the dispersant has basically no effect on the molecular weight. However, in order to ensure the dispersant’s colloidal dispersion effect on the polymer, the gel temperature of the dispersant should generally be high. at the polymerization temperature.

(2) Particle characteristics: Particle diameter, shape, porosity and particle distribution are important indicators of SPVC quality, which are related to agitator/reactor design, polymerization water-oil ratio, dispersion system and final conversion rate of VCM, among which dispersion system is particularly important .

(3) Stirring: Like the dispersion system, it has a great influence on the quality of SPVC, because the final particle depends on the size of the VCM droplet in the water, the stirring speed increases, and the droplet size decreases; when the stirring speed is too high, the droplets will coalesce .

(4) Dispersion protection system: the protection system protects the VCM droplets in the early stage of the reaction to avoid merging; and once the generated PVC is precipitated in the VCM droplets, the dispersion system protects the coagulation of the controlled particles to obtain the final SPVC particles. The dispersion system is divided into the main dispersion system and the auxiliary dispersion system. The main dispersants include PVA with high degree of alcoholysis, HPMC, etc., which affect the overall performance of SPVC; the auxiliary dispersion system is used to improve some characteristics of SPVC particles. In general, large reactors can use the least amount of dispersion.

(5) Primary dispersion systems: They are water-soluble and stabilize VCM droplets by reducing the interfacial tension between VCM and water. The dispersant must be used below its gel temperature, and the viscosity of the main dispersant solution should not be too high. The ideal SPVC is spherical, with high porosity, narrow particle size distribution and high packing density. However, high porosity and high bulk density are contradictory, and the best balance can be achieved by controlling the dispersion system. Currently in the SPVC industry, the main dispersants are PVA and HPMC. HPMC has the advantages of less dosage, good thermal stability and plasticizing performance of SPVC, and is widely used despite its high price.

(6) Auxiliary dispersion system: They are used in combination with the main dispersion system to increase porosity and adjust particle distribution. They are comparable in solubility to VCM compared to the water solubility of the primary dispersant. Generally, the amount of main dispersant can be slightly reduced after adding auxiliary dispersant.

2.2 HPMC quality index and performance analysis

(1) Appearance comparison: Prepare a 5% (mass fraction) HPMC aqueous solution, take an appropriate amount and place it on a clean self-made film glass plate placed horizontally, and vacuum-dry to obtain a film. From the appearance, it can be directly judged whether the HPMC is qualified or not.

(2) Determination of interfacial tension: Measure the interfacial tension between 0.1% (mass fraction) HPMC aqueous solution (20°C) and VCM (generally replaced by 1,1-EDC or TCE) at 50°C by capillary method, which reflects The interfacial activity of HPMC plays an important role in the dispersion of VCM in water. In addition, the degree of polymerization, substituent, degree of substitution and temperature of HPMC also have a great influence on the interfacial tension.

(3) Viscosity measurement: Prepare 2% (mass fraction) HPMC aqueous solution and measure it with a rotational viscometer. The general measurement value should be 40-60mPa·S. It is related to HPMC production method, degree of polymerization, etc.

(4) Determination of cloud point and gel point: prepare 2% (mass fraction) HPMC aqueous solution, heat up slowly on a water bath, and the temperature when a large amount of turbidity appears in the solution is the cloud point temperature. 2% (mass fraction) HPMC aqueous solution is placed on a water bath and slowly heated up, and the viscosity at different temperatures is measured with a rotational viscometer. The temperature at which the viscosity suddenly drops or rises is called the gel temperature. These two indicators are the main basis for determining the HPMC model, which are mainly related to the degree of substitution of methoxy groups, and can reflect the degree of substitution of HPMC substituents to a certain extent.

(5) Determination of glue-retaining ability: prepare 0.2% (mass fraction) HPMC aqueous solution, measure the glue-retaining ability of the dispersant by the vibration layering method, and characterize it by the proportion of the volume of the dispersed layer after a certain period of time. This indicator is related to HPMC substituent group, degree of substitution, degree of polymerization, concentration and temperature. The gel retention ability of HPMC decreases with the increase of hydroxypropyl content, and increases slowly with the increase of polymerization degree (viscosity).

(6) Determination of moisture permeability: a number of vials with color-changing silica gel inside the bottle, tightly seal the free film on the bottle mouth with 502 glue, after precise weighing, put it into a container filled with saturated NaCl solution, keep the constant temperature at 37°C and relative humidity 75%, accurately weigh the mass of the vial at 12, 24, 48 and 60 h, measure the film and moisture permeability area with a micrometer and a vernier caliper, and calculate the moisture permeability coefficient. This index characterizes the penetration ability of HPMC dispersant to water, and reflects the characteristics of HPMC water solubility and hydrophilicity.

The above indicators can be used as the basic basis for determining domestic HPMC to replace imported HPMC, and can also be used as a basis for judging whether HPMC is qualified when the quality of SPVC is unstable, and as a basic analysis method for SPVC enterprises to strengthen the control of HPMC raw materials.

2.3 SPVC quality control

Using domestic HPMC instead of imported HPMC to produce SPVC products can greatly reduce production costs and improve the competitiveness of enterprises.

First analyze the basic indicators of domestic HPMC meet the requirements, and then carry out the substitution test. The alternatives are: ①The mass ratio of domestic HPMC to imported HPMC is 1:1; ②The mass ratio of domestic HPMC to imported HPMC is 3:1; ③All domestic HPMC. This plan is gradually promoted, which can minimize the risk of SPVC production and ensure that the quality of SPVC products can basically meet the requirements of processing enterprises.

Analysis and comparison of SPVC conventional indicators: viscosity, impurity particle size, volatile matter, sieve residue, number of "fish eyes", plasticizer absorption, whiteness. These indicators can directly characterize the quality of domestic HPMC and imported HPMC.

The internal analysis indicators of SPVC quality include plasticizing time, maximum torque, balance torque, and balance humidity. These indicators can directly reflect the processing performance of domestic HPMC after replacing imported HPMC.

In suspension polymerization, dispersant and stirring are interrelated. Orthogonal experiments can be used to adjust (fine-tune) the amount of dispersant added or change the stirring form to meet domestic HPMC replacement requirements, obtain the best quality SPVC, and reduce production costs.

In addition, when formulating domestic HPMC alternatives, orthogonal cold mold tests can also be used to ensure that there is nothing wrong with formal production and application.

3. Conclusion

At present, the domestic HPMC production technology is developing rapidly and the technology is very mature. Using domestic HPMC to replace imported HPMC can not only reduce the production cost of SPVC, ensure the reliability of raw material supply, but also promote the development of domestic HPMC national industry and improve the domestic HPMC industry. Competitiveness is of great significance.