Polyvinyl alcohol

Thermoplastic

Crystalline, white to pale-yellow, water-soluble plastic

Fixed

1.19 to 1.31 g/cm3 (depending on hydrolysis and degree of polymerization )

200-228 ° C (depending on the hydrolysis and degree of polymerization )

About 85 ° C ( completely hydrolyzed ) ( a function of hydrolysis and degree of polymerization )

Polyvinyl alcohol ( PVA or PVOH abbreviation ) is an artificial, thermoplastic polymer. The production of the water-soluble polymer is effected by hydrolysis of polyvinyl esters, mostly polyvinyl acetate, which is accessible. The direct synthetic route is not possible.

• 5.1 Toxicology
• 5.2 wastewater

History

Polyvinyl alcohol was first manufactured in 1924 by Willi Herrmann and tungsten Haehnel by saponification of polyvinyl esters with stoichiometric amounts of sodium hydroxide solution. Another advance in the manufacture of polyvinyl succeeded Herrmann, Haehnel and Berg in 1932. They found that polyvinyl alcohol can also be obtained by transesterification of polyvinyl acetate with absolute alcohols in the presence of catalytic amounts of alkalis produce. This process is also called polymer-analogous reaction.

Production and recovery

In contrast to most of the vinyl polymers, polyvinyl alcohol can not be prepared by simple polymerization of the corresponding monomer. The required monomer Ethenol exists only in its tautomeric form as acetaldehyde. The synthesis by Polyaldolkondensation from acetaldehyde is not managed successfully, since only low molecular weight polymers are formed. Polyvinyl alcohols are obtained by transesterification or by alkaline hydrolysis of polyvinyl acetate. The hydrolysis is well controlled. Besides are some Polyvinylalkoholcopolymerisate of importance; they are derived from polyvinyl acetate copolymers.

Structure and Properties

Chemistry

Similar to polyvinyl acetate, polyvinyl alcohol predominates in the head -to-tail arrangement of the monomers. The content of blocks in head -to-head is less than 1 to 2%. The proportion of these shares has a great influence on the physical properties of the polymer, so the solubility in water. Polyvinyl alcohol is lightly branched in the control, caused by chain transfer in the synthesis of polyvinyl acetate. The degree of polymerization is about 500 to 2500.

The degree of hydrolysis of the technically relevant types varies depending on the intended use between 70 to 100 mol%. Was only partially hydrolyzed the acetyl groups can be distributed depending on the method random or block in the polymer. The distribution of these important properties such as acetyl influences the melting point, the surface tension of aqueous solutions or protective colloid properties.

Polyvinyl alcohol which has been derived from polyvinyl acetate, is an atactic plastic. But he still has on the hydroxy -crystalline regions. Influence on these crystallinity of the polymer has the structure and history, so branching, degree of hydrolysis, distribution of acetyl groups. The higher the degree of hydrolysis, the better is the ability to crystallize. Heat treatment of hydrolyzed products, the crystallinity can still increase, thereby in turn reduces the water solubility. The higher the proportion of acetyl groups, the weaker the formation of crystalline zones.

Physical Properties

Polyvinyl alcohol is an excellent film-forming, emulsifying and adhesive. It has a high tensile strength and flexibility. These properties are dependent on humidity, as the plastic absorbs water. Water acts as a plasticizer, the PVOH (PVAL ) loses in tensile strength under high humidity conditions, however, gains in resiliency. The melting point is 230 ° C, the glass transition temperature at 85 ° C for fully hydrolyzed forms. The ceiling temperature is 200 ° C. The temperature specifications vary with the molecular mass.

Chemical Properties

It is resistant to oils, fats and organic solvents.

Use, processing

• Adhesion and thickeners in latex paints, hair sprays, shampoos, and adhesives.
• Part of the sheet of water-soluble individual packages of automatic dishwashing detergent tabs
• Barrier layer for carbon dioxide in PET bottles.
• Part of play dough and so-called Slime.
• Part to the seal of hygiene products such as feminine hygiene and incontinence products.
• Mold release agents in the manufacture of fiber-plastic composite materials.
• Sizing agents in paper manufacture
• As water-soluble film for the production of packaging bags.
• Spray carrier for handmade products.
• Selective embolization of tumor vessels, such as in the preoperative treatment of juvenile Nasenrachenfibroms
• Wetting and adhesion agents in artificial tear
• One-day - hydrogel contact lenses with low oxygen permeability
• Entrapment immobilization of cells and enzymes in Microbiology
• Artificial casings for sausages.
• Sizing agent in textile industry
• Textile fiber

Trade names of polyvinyl alcohol are Kuraray Poval, Mowiflex, Alcotex, Elvanol, Gelvatol, Gohsenol, Lemol, Mowiol, Nelfilcon A, Polyviol and Rhodoviol.

Environmental aspects & Toxicology

Toxicology

Studies on skin and mucous membrane tolerance in animals (LD50, oral, and dermal application ) showed no adverse effects. Both dietary intake as well as on the skin no health hazards are expected.

Under certain conditions, it may generate dust. When heated above 200 ° C, flue gases, which can irritate eyes, nose and throat. It can also lacrimation, burns, red eyes, or burning occurs in the nose and throat.

About other health problems for humans, there is no data. Animal studies have shown a decrease in hemoglobin and red blood cell number with occasional full anticoagulation. Some animal studies also suggest the possibility of carcinogenicity.

Sewage

Polyvinyl acetate as a precursor can be easily used in the form of dispersions into drains, but is not toxic. However, it is in an aqueous environment only poorly degraded.

The preparation dispersionshaltiger wastewater in sewage treatment plants is usually not a problem, they are easy to precipitate and accumulate in the sludge, which they are then disposed of.