of transesterification. Worldwide consumption of polyvinyl alcohol was over one million metric tons in 2006. Large producers include Kuraray and Sekisui Specialty Chemicals, while mainland China has installed a number of very large production facilities in the past decade and currently accounts for 45% of world capacity. Structure and properties PVA is an atactic material that exhibits crystallinity. In terms of microstructure, it is composed mainly of 1,3-diol linkages [−CH2−CH(OH)−CH2−CH(OH)−], but a few percent of 1,2-diols [−CH2−CH(OH)−CH(OH)−CH2−] occur, depending on the conditions for the polymerization of the vinyl ester precursor. Polyvinyl alcohol has excellent film-forming, emulsifying and adhesive properties. It is also resistant to oil, grease and solvents. It has high tensile strength and flexibility, as well as high oxygen and aroma barrier properties. However, these properties are dependent on humidity: water absorbed at higher humidity levels acts as a plasticiser, which reduces the polymer's tensile strength, but increases its elongation and tear strength. Safety and environmental considerations PVA is widely used, thus its toxicity and biodegradation are of interest. Solutions containing more than 5% PVA are toxic to fish. It biodegrades slowly. See also Vinyl acetate Polyvinyl nitrate References External links MSDS "Slime" recipe Forming PVA layers in PET bottles Vinyl polymers Biodegradable plastics Polymers E-number additives