What is PVA powder used for?

Not to be confused with polyvinyl acetate

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Chemical compound

Polyvinyl alcohol (PVOH, PVA, or PVAl) is a water-soluble synthetic polymer. It has the idealized formula [CH2CH(OH)]n. It is used in papermaking, textile warp sizing, as a thickener and emulsion stabilizer in polyvinyl acetate (PVAc) adhesive formulations, in a variety of coatings, and 3D printing. It is colourless (white) and odorless. It is commonly supplied as beads or as solutions in water.[3][4] Without an externally added crosslinking agent, PVA solution can be gelled through repeated freezing-thawing, yielding highly strong, ultrapure, biocompatible hydrogels which have been used for a variety of applications such as vascular stents, cartilages, contact lenses, etc.[5]

Although polyvinyl alcohol is often referred to by the acronym PVA, more generally PVA refers to polyvinyl acetate, which is commonly used as a wood adhesive and sealer.

Uses

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PVA is used in a variety of medical applications because of its biocompatibility, low tendency for protein adhesion, and low toxicity. Specific uses include cartilage replacements, contact lenses, and eye drops.[6] Polyvinyl alcohol is used as an aid in suspension polymerizations. Its largest application in China is its use as a protective colloid to make PVAc dispersions. In Japan its major use is the production of Vinylon fiber.[7] This fiber is also manufactured in North Korea for self-sufficiency reasons, because no oil is required to produce it. Another application is photographic film.[8]

PVA-based polymers are used widely in additive manufacturing. For example, 3D printed oral dosage forms demonstrate great potential in the pharmaceutical industry. It is possible to create drug-loaded tablets with modified drug-release characteristics where PVA is used as a binder substance.[9]

Medically, PVA-based microparticles have received FDA 510(k) approval to be used as embolisation particles to be used for peripheral hypervascular tumors.[10] It may also used as the embolic agent in a Uterine Fibroid Embolectomy (UFE).[11] In biomedical engineering research, PVA has also been studied for cartilage, orthopaedic applications,[12] and potential materials for vascular graft.[13]

PVA is commonly used in household sponges that absorb more water than polyurethane sponges.[citation needed]

PVA may be used as an adhesive during preparation of stool samples for microscopic examination in pathology.[14]

Polyvinyl acetals

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Polyvinyl acetals are prepared by treating PVA with aldehydes. Butyraldehyde and formaldehyde afford polyvinyl butyral (PVB) and polyvinyl formal (PVF), respectively. Preparation of polyvinyl butyral is the largest use for polyvinyl alcohol in the US and Western Europe.

Preparation

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Unlike most vinyl polymers, PVA is not prepared by polymerization of the corresponding monomer, since the monomer, vinyl alcohol, is thermodynamically unstable with respect to its tautomerization to acetaldehyde. Instead, PVA is prepared by hydrolysis of polyvinyl acetate,[3] or sometimes other vinyl ester-derived polymers with formate or chloroacetate groups instead of acetate. The conversion of the polyvinyl esters is usually conducted by base-catalysed transesterification with ethanol:

[CH2CH(OAc)]n + C2H5OH &#; [CH2CH(OH)]n + C2H5OAc

The properties of the polymer are affected by the degree of transesterification.

Worldwide consumption of polyvinyl alcohol was over one million metric tons in .[7] Large producers include Kuraray and Sekisui Specialty Chemicals, while mainland China has installed a number of very large production facilities in the past decade[clarification needed] and currently accounts for 45% of world capacity.

Structure and properties

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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.[3]

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

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Polyvinyl alcohol is widely used, thus its toxicity and biodegradation are of interest. Tests showed that fish (guppies) are not harmed, even at a poly(vinyl alcohol) concentration of 500 mg/L of water.[3]

There are several different grades of PVA depending on the degrees of polymerization and hydrolysis, which will affect their physical and chemical properties as well as their biodegradability.[3] Aqueous solutions of PVA degrade faster, which is why PVA grades that are highly water-soluble tend to have a faster biodegradation.[15] Not all PVA grades are readily biodegradable, but studies show that high water-soluble PVA grades such as the ones used in detergents can be readily biodegradable according to OECD screening test conditions.[16]

Orally administered PVA is relatively harmless.[17] The safety of polyvinyl alcohol is based on some of the following observations:[17]

• The acute oral toxicity of polyvinyl alcohol is very low, with LD(50)s in the range of 15-20 g/kg;
• Orally administered PVA is very poorly absorbed from the gastrointestinal tract;
• PVA does not accumulate in the body when administered orally;
• Polyvinyl alcohol is not mutagenic or clastogenic

See also

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References

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Polyvinyl alcohol (PVA for short) is a white powder in appearance. It is a water-soluble polymer with a wide range of uses. Its performance is between plastic and rubber. Its uses can be divided into two major uses: fiber and non-fiber.

Because PVA has unique strong adhesion, film flexibility, smoothness, oil resistance, solvent resistance, protective colloid, gas barrier, abrasion resistance and water resistance after special treatment, in addition to being used as a fiber raw material, It is also widely used in the production of coatings, adhesives, paper processing agents, emulsifiers, dispersants, films and other products, which are used in textiles, food, medicine, construction, wood processing, paper, printing, agriculture, polymer chemical industry and other industries.

Product performance: The polyvinyl alcohol resin series products are white solid, and the appearance is divided into three types: flocculent, granular and powdery; non-toxic, tasteless, non-polluting, and can be dissolved in water at 80-90 °C. 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.

Product use: mainly used for warp yarn size, fabric finishing agent, vinylon fiber raw material in the textile industry; 107 glue, interior and exterior wall coatings, adhesives in the building and decoration industry; used as a polymer emulsifier, dispersant and polyvinyl alcohol condensation agent in the chemical industry formaldehyde, acetal, butyral resin; used as paper adhesive in paper industry; used as soil conditioner, pesticide adhesion synergist and polyvinyl alcohol film in agriculture; also used in daily cosmetics and high frequency quenching agent, etc.

How to use: The polyvinyl alcohol resin series products can be dissolved in hot water below 95&#;, but due to the difference in the degree of polymerization, alcoholysis degree, and alcoholysis methods, there are certain differences in dissolution time and temperature. When using different brands of polyvinyl alcohol resin, the dissolution method and time need to be explored. When dissolving, this product can be slowly added into cold water at about 20&#; while stirring to fully swell, disperse and escape of volatile substances (do not add this product to water above 40&#; for direct dissolution to avoid the occurrence of clumps.) Then the temperature is raised to about 95 °C to accelerate the dissolution, and the temperature is maintained for 2 to 2.5 hours until the solution no longer contains tiny particles. After filtering impurities, it can be used for later use.

The stirring speed is 70~100 rpm. When heating up, indirect heating methods such as water bath can be used, or direct heating with water vapor can be used. However, direct heating cannot be used with an open flame to avoid overheating and decomposition. If there is no mixer, it can be dissolved by blowing steam in a tangential direction.

The aqueous solution concentration of polyvinyl alcohol resin series products is generally below 12~14%; the aqueous solution concentration of low alcoholysis degree polyvinyl alcohol resin products can generally be around 20%.

The method to check whether this product is completely dissolved: take out a small amount of solution, add 1~2 drops of iodine solution, if there is a blue mass transparent body, it means that it has not been completely dissolved, if the color can spread evenly, it means that it has been completely dissolved.

Storage of the solution:

Preservative: If stored for a long time, the water in the aqueous solution will be corrupt, but it will not affect the performance of this product. At this time, 0.01-0.05% (based on PVA) formaldehyde, salicylic acid or other preservatives should be added.

Anti-rust: When storing with iron, a small amount of weak alkali should be added, and when using copper, 0.02-0.05% (based on PVA) sodium nitrite should be added, preferably stainless steel or plastic containers.

Addition of antifoaming agent: When preparing an aqueous solution, this product is not easy to foam, but when the solution concentration is high and the rotation speed is fast, a small amount of foam will also be generated. based on octanol, tributyl phosphate or 0.2-0.5% (based on PVA) silicone emulsion.

Storage and transportation: Store in a ventilated, cool and dry place, away from fire. It should be handled with care during transportation to prevent damage to the package.

*Information comes from the Internet. If in doubt, please contact the author.

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