Polyvinyl alcohol
Polyvinyl alcohol, referred to as PVA. Molecular formula: (C2H4O) n. Saponified polymer organic compound made of polyvinyl acetate. Polyvinyl alcohol is a white powder, flake, or flocculent solid with a glass transition temperature of 60 to 85 ° C. Polyvinyl alcohol contains many alcohol groups, has polarity, and can form hydrogen bonds with water, so it can be dissolved in polar water; it can also be dissolved in hot hydroxyl-containing solvents such as glycerin, phenol, etc., not soluble in methanol, benzene, General organic solvents such as acetone and gasoline. Mainly used in the manufacture of polyvinyl acetal, gasoline-resistant pipes and vinylon fibers; also used as adhesives for temporary protection of films, fabrics, leather, etc., binding materials, sizing agents for fabrics, emulsifiers and protective colloids Wait.
Discover history
Polyvinyl alcohol was first discovered by the German chemists WOHerrmann and Dr. WWHachnel in 1924. Because it can carry out the typical polyol chemical reaction and through insoluble treatment, it denatures and has different functional effects, resulting in a A series of synthetic materials are widely used in industrial and agricultural production and medical applications. Polyvinyl alcohol achieved industrial production in 1926, and the product achieved large-scale industrialization in the 1950s. In the early 1980s, the world's production capacity was about 640kt.
The production route of polyvinyl alcohol is divided according to raw materials, there are two types of ethylene method and acetylene method.
Physical and chemical properties
Polyvinyl alcohol
Polyvinyl alcohol, referred to as PVA
Polyvinyl alcohol is a water-soluble polymer obtained by hydrolysis of polyvinyl acetate without polymerization of monomers, abbreviated. White flake, flocculent or powdery solid, odorless. The physical properties of polyvinyl alcohol are affected by the chemical structure, degree of alcoholysis, and degree of polymerization. There are two chemical structures in the polyvinyl alcohol molecule, namely 1, 3 and 1, 2 ethylene glycol structure, but the main structure is 1, 3 ethylene glycol structure, that is, "head and tail" structure.
Relative density of polyvinyl alcohol (25 ℃ / 4 ℃) 1.27 ～ 1.31 (solid), 1.02 (10% solution), melting point 230 ℃, glass transition temperature 75 ～ 85 ℃, heated in air to It slowly changes color and becomes brittle above 100 ℃. It is dehydrated and etherified when heated to 160-170 ° C, and loses its solubility. It begins to decompose when heated to 200 ° C. Beyond 250 ° C, it becomes a polymer containing conjugated double bonds. Refractive index 1.49 ～ 1.52, thermal conductivity 0.2w / (m · K), specific heat capacity 1 ～ 5J / (kg · K), resistivity (3.1 ～ 3.8) × 107Ω · cm. It is soluble in water and generally needs to be heated to 65-75 ° C for complete dissolution. Insoluble in gasoline, kerosene, vegetable oil, benzene, toluene, dichloroethane, carbon tetrachloride, acetone, ethyl acetate, methanol, ethylene glycol, etc. Slightly soluble in dimethyl sulfoxide. 120 ～ 150 ℃ soluble in glycerin. But when it is cooled to room temperature, it becomes jelly. To dissolve polyvinyl alcohol, the material should be added to room temperature water with stirring. After dispersing evenly, increase the temperature to accelerate the dissolution, which can prevent agglomeration and affect the dissolution rate. The aqueous solution of polyvinyl alcohol (5%) is very sensitive to borax and boric acid, and it is easy to cause gelation. When the borax reaches 1% of the mass of the solution, irreversible coagulation will occur. Chromate, dichromate, and permanganate can also make polyvinyl alcohol gel.
Classification introduction
Polyvinyl alcohol is a kind of water-solubility made by alcoholysis. The main chain of the molecule contains ―CH―CH (OH) ― group.
According to the degree of polymerization: it can be divided into ultra-high degree of polymerization (molecular weight of 25-30 million), high degree of polymerization (molecular weight of 17-220000), medium degree of polymerization (molecular weight of 12-150000) and low degree of polymerization (molecular weight of 2.5-3.5 Million).
According to the degree of alcoholysis: it can be divided into three types: complete alcoholysis (98-100% alcoholysis), partial alcoholysis (87-89% alcoholysis) and 78% alcoholysis.
Polyvinyl alcohol with high degree of polymerization and alcoholysis is the main raw material of polyvinyl formal fiber. As the degree of alcoholysis increases, its solubility in water decreases significantly. For polyvinyl alcohol, the thousands and hundreds of digits of the degree of polymerization are generally placed in the front, and the degree of alcoholysis is placed at the back. For example, polyvinyl alcohol 17-99 means that the degree of polymerization is 1700 and the degree of alcoholysis is 99%. Generally, the degree of polymerization increases, the viscosity of the aqueous solution increases, and the strength and solvent resistance after film formation increase, but the solubility in water and elongation after film formation decrease.
PVA 17-88 aqueous solution viscosity gradually increases with time at room temperature. However, the viscosity at 8% concentration is absolutely stable, regardless of time. The special phenomenon c polyvinyl alcohol has good film-forming properties and is highly uncomfortable for many gases except water vapor and ammonia. Good light resistance, not affected by light. It can burn when there is an open fire and has a special smell. When the aqueous solution is stored, it sometimes becomes toxic. Non-toxic, non-irritating to human skin. Used as an emulsion stabilizer for emulsion polymerization of polyvinyl acetate. Used in the manufacture of water-soluble adhesives. Used as a modifier of starch adhesives. It can also be used to prepare photosensitive adhesive and benzene solvent-resistant sealant. Also used as release agent, dispersant, etc. Store in a cool, dry warehouse. Moistureproof and fireproof.
Polyvinyl alcohol 17-92 is abbreviated as PVAl 7-92, white particles or powder. Easily soluble in water, the dissolution temperature is 75 ～ 80 ℃. Other properties are basically the same as PVA17-88. Used as an emulsion stabilizer for emulsion polymerization. Used in the manufacture of water-soluble adhesives. Stored in a cool, dry warehouse, fire and moisture.
Polyvinyl alcohol 17-99 is also called sizing resin (Sizing resin), abbreviated as PVAl7-99. White or light yellow powder or floc solid. The glass transition temperature is 85 ℃, and the saponification value is 3 ～ 12mgKOH / g. It is soluble in hot water at 90 ～ 95 ℃, almost insoluble in cold water. An aqueous solution with a concentration greater than 10% will gel at room temperature to freeze, and will dilute at high temperatures to restore fluidity. In order to stabilize the viscosity, an appropriate amount of sodium thiocyanate, calcium thiocyanate, phenol, butanol and other viscosity stabilizers can be added to the solution. The PvAl7-99 solution is more sensitive to the gel caused by the sand than PvAl7, 88. The 0.1% borax of the solution mass will gel the 5% PVAl7-99 aqueous solution and cause the same concentration of PVA 17-88 aqueous solution gel The amount of borax is 1%. For aqueous solutions of polyvinyl alcohol with the same concentration and the same degree of alcoholysis, borax is more likely to gel than boric acid. PVAl7-99 is more resistant to solvents such as benzene, chlorinated hydrocarbons, esters, ketones, ethers, hydrocarbons, etc. than PVAl7-88. It will gradually change color when heated to above 100 ℃. It will change color quickly when above 150 ℃, and it will decompose when above 200 ℃. The property of discoloration of polyvinyl alcohol when heated can be suppressed by adding 0.5% to 3% boric acid. It has good light resistance and is not affected by light. It has chemical reactivity such as esterification, etherification and acetalization of long-chain polyols. Open fire will burn with a special smell. Non-toxic, non-irritating to human skin.
Polyvinyl alcohol 17-99B is mainly used to manufacture high viscosity polyvinyl butyral. Widely used as dispersant for sizing materials. Other types of 17-99 are used as emulsion stabilizers for emulsion polymerization of polyvinyl acetate, but the effect is not as good as 17-88. Generally, 17-99 is mixed with 17-88. 17-99 is used to manufacture aqueous solution of polyvinyl formal (mainly l07 building glue). 17-99 is also used to prepare benzene solvent-resistant sealant. Store in a cool, dry warehouse, protected from moisture and fire.
Toxic effects
Non-toxic, non-irritating to the skin, will not cause skin allergies, but dust has an irritating effect on the eyes.
production method
The production route of polyvinyl alcohol is divided according to raw materials, there are two types of ethylene method and acetylene method
The production route of polyvinyl alcohol is divided according to raw materials, there are two types of ethylene method and acetylene method.
1. Direct ethylene synthesis
The direct synthesis method of petroleum cracking ethylene was successfully developed by Japan Kuraray Co., Ltd. (former Kurashiki Rayon Co., Ltd.) for the first time and used in industrial production. At present, the process route for producing PVA in the world is dominated by the ethylene method, and its quantity is 72% of the total production capacity. The United States has completed the conversion of the acetylene method to the ethylene method. Japan ’s ethylene method also accounts for more than 70%, while only two of China ’s manufacturing companies are ethylene methods.
The technological process includes: obtaining ethylene and synthesis, rectification, polymerization, polyvinyl acetate (PVAc) alcoholysis of vinyl acetate (VAc), recovery of acetic acid and methanol.
Petroleum ethylene process characteristics: the production scale is larger than the acetylene process, the product quality is good, the equipment is easy to maintain, manage and clean, the heat utilization rate is high, the energy saving is obvious, and the production cost is 30% lower than the acetylene process.
2. Direct synthesis method of natural gas cracking acetylene
Acetylene synthesis method can be divided into calcium carbide acetylene synthesis method and natural gas cracking acetylene synthesis method according to different sources of raw materials.
1. Synthesis method of calcium carbide acetylene
Calcium carbide acetylene synthesis method, the earliest industrialized production.
Characteristics of calcium carbide acetylene method: simple operation, high yield, and easy separation of by-products. Therefore, there are still 10 factories in China that still use this method for production, and most of them use the high alkali method for production. However, due to the high energy consumption, low quality, and high cost of this process route, the pollution of the environment caused by impurities produced in the production process is also more serious. The cost is higher than the PVA 800-1000 yuan / t produced by other two methods, which lacks market competitiveness and is gradually Eliminate the process. Advanced foreign countries have all used low-alkali production processes as early as the 1970s.
2. Natural gas cracking acetylene
Direct acetylene synthesis method In areas rich in natural gas, coal and electricity, the natural gas acetylene method still has vitality. Countries such as Europe and North Korea dominate the natural gas acetylene method, and China has a production facility that uses this method.
Borden method with natural gas acetylene as raw material not only has mature technology, but also produces acetylene which is beneficial to comprehensive utilization. The production cost of VAc is 50-70% lower than calcium carbide acetylene method. But natural gas acetylene investment and technical difficulties are greater.
Ethylene method, natural gas method and acetylene method have their own advantages and disadvantages. The process methods and characteristics are shown in the following table:
Raw material route Petroleum ethylene Natural gas acetylene Carbide acetylene
Reaction method
Fixed bed gas phase method Fixed bed gas phase method Boiling bed gas phase method
Process conditions
Temperature / ℃ 150 ～ 200 170 ～ 210 170 ～ 210
Pressure / MPa 0.49 ～ 0.98 Normal pressure Normal pressure
Airspeed / L · h-1 2040 ～ 2100 250 ～ 280 110 ～ 150
Raw material ratio (molar ratio) Ethylene: acetic acid: oxygen = 9: 4: 1.5 C2H2: HAc = 1: 7 ± 1 C2H2: Hac = 1: 3 ± 1
Catalyst composition Palladium, gold noble metal Zn (Ac) 2 / activated carbon Zn (Ac) 2 / activated carbon
Catalyst life 5 ～ 6 months 3 months 5 ～ 6 months
Technical effect
One-way conversion rate,% 15 ～ 20 60 ～ 70 30 ～ 35
Space-time yield / t · (m3 · d) -1 6 ～ 8 2.0 ～ 2.5 1.0 ～ 1.3
Advantages Less by-products, less corrosive equipment, high catalyst activity, good product quality, good heat energy utilization, cheap and easy catalyst, less side reactions, mature technology, less investment, easy catalyst availability
Disadvantages Precious catalyst High acetylene cost High acetylene cost
The main purpose
Polyvinyl alcohol is widely used. It can be used as an emulsion stabilizer and dispersion stabilizer in polymerization reactions. It can replace starch and bone glue as adhesives. It is widely used in papermaking, fiber processing, wood processing, medicine, leather, construction, glass, Packaging and many other industries. For example: in fiber processing, it can be used as warp sizing agent, fabric finishing agent (see textile additives), etc .; as a pigment adhesive in paper processing for the surface coating of paper, as a glue for the surface application of paper Glue; used in medicine for pharmaceutical adhesives, suspensions, coating materials, ointment base, and even as a substitute for plasma.
Medical polyvinyl alcohol
Is a hydrolysis product of polyvinyl acetate. It is a white powder containing a large amount of hydroxyl groups and is soluble in water. Has good biocompatibility and anticoagulant properties. The cross-linked hydrogel irradiated with ultraviolet light is used to inject and fill the vitreous cavity of the eye. The crystalline hydrogel can be used as an artificial cartilage material, and can also be used as a hemostatic fiber, a contraceptive film, and an eye film.
Polyvinyl alcohol resin
Polyvinyl alcohol resin series products are white solid, and the appearance is divided into three types: flocculent, granular and powdery; non-toxic, odorless and pollution-free, and can be dissolved in 80-90 ℃ water. Its aqueous solution has good adhesion and film-forming properties; it is resistant to most organic solvents such as oils, lubricants and hydrocarbons; it has chemical properties such as esterification, etherification and acetalization of long-chain polyols.
Preparation method: It is made by saponification of vinyl acetate.
Uses: mainly used for warp sizing, textile finishing agent, vinylon fiber raw materials in the textile industry; 107 adhesives, interior and exterior wall coatings, adhesives in the construction and decoration industry; used as a polymerization emulsifier, dispersant and polyvinyl formal in the chemical industry , Acetal, butyral resins; used as paper adhesives in the paper industry; used as soil improvers, pesticide adhesion synergists and polyvinyl alcohol films in agriculture; also used in daily cosmetics and high-frequency quenching Agents and other aspects.
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