Polyvinyl acetal is prepared by treating polyvinyl alcohol with aldehydes. Butyraldehyde and formaldehyde provide polyvinyl butyral (PVB) and polyvinyl formal (PVF) respectively. In the United States and Western Europe, the preparation of polyvinyl butyral is the largest use of polyvinyl alcohol. Polyvinyl alcohol is widely used, how much do you know about PVA? How is polyvinyl alcohol made?
Polyvinyl alcohol for food is tasteless, tasteless, translucent, white or milky white granular powder.
Poly(vinyl alcohol) (PVOH, PVA, or PVAL) is a water-soluble synthetic polymer. It has the ideal formula [CH 2 CH(OH)] n. It is soluble in water, slightly soluble in ethanol, but insoluble in other organic solvents. Generally, a 5% solution of polyvinyl alcohol exhibits a pH of 5.0 to 6.5. The melting point of polyvinyl alcohol is 180 to 190°C. Its molecular weight is between 26,300 and 30,000, and its degree of hydrolysis is 86.5 to 89%.
The main raw material used to make polyvinyl alcohol is vinyl acetate monomer. It is made by the polymerization of vinyl acetate and then partial hydrolysis. The hydrolysis process is based on partial substitution of ester groups in vinyl acetate by hydroxyl groups and is completed in the presence of aqueous sodium hydroxide. After gradually adding the water-based saponifier. The polyvinyl alcohol is precipitated, washed and dried. The degree of hydrolysis is determined by the point in time when the saponification reaction stops.
Unlike most vinyl polymers, PVA is not prepared by polymerization of corresponding monomers, because vinyl alcohol is a thermodynamically unstable monomer relative to its tautomer, acetaldehyde. Instead, PVA is a polyvinyl acetate prepared by hydrolysis, with a formate or chloroacetic acid group instead of acetate or other sometimes vinyl ester-derived polymers. The conversion of polyvinyl esters is usually carried out by alkali-catalyzed ethanol transesterification:
[CH 2 CH(OAc)] n + C 2 H 5 OH→[CH 2 CH(OH)] n + C 2 H 5 OAc
The performance of the polymer is affected by the degree of transesterification.
In 2006, the world's consumption of polyvinyl alcohol exceeded 1 million tons. Large-scale producers include Kuraray (Japan, Europe, and the United States) and Sekisui Specialty Chemicals (United States), while China has installed many super-large production facilities. In the past ten years, it currently accounts for 45% of the world's production capacity.
PVA is widely used, so its toxicity and biodegradation are of concern. Solutions containing up to 5% PVA are non-toxic to fish. But biodegradation is slower.
The performance of polyvinyl acetate depends on its degree or degree of hydrolysis, especially its whole or part, which divides it into two categories, namely (a) partial hydrolysis and (b) complete hydrolysis.
The molecular weight obtained by the PVA product may vary (20,000-400,000), depending on the length of the initial vinyl acetate polymer, the degree of hydrolysis to eliminate the acetate groups, and whether it occurs under alkaline or acidic conditions.
The level of hydrolysis varies from a typical value of 80% to over 99%. The almost completely hydrolyzed form forms a PVA hydrogel with adjustable properties through cross-linking of linear polymers, and then forms a polymer (gel)-fluid (sol) species. The polymer content affects the physical state of the resulting material: a low polymer content causes the material to soften because the fluid will flow freely through the matrix, while a higher polymer content causes the material matrix to become significantly harder and stronger.
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