Polyvinyl Alcohol: Structure,Preparations, Physical and Chemical properties, Uses & Hazards

Polyvinyl alcohol is an organic compound. It is a man-made polymer made up of vinyl and alcohol groups. It has adhesive properties, which make it highly useful. Polyvinyl alcohol is a water-soluble, flammable, and crystalline polymer in nature. Because of the alcohol group present, it becomes flammable. Usually, it is non-corrosive but can cause mucous irritation. We also call it PVA, PVAL or PVOH.

It is used to manufacture drugs, acetic acid, perfumes, dyes, flavoring agents, etc. A Swedish chemist named Carl Wilhelm Scheele observed polyvinyl alcohol in 1774.

The word ‘poly’ means ‘many’ while the word ‘vinyl’ is an ‘ethenyl’ functional group. The chemical formula of vinyl is -CH=CH₂. Upon removing a hydrogen atom from ethylene or an ethene molecule, we obtain a vinyl group. Polyvinyl alcohol is resistant to oils, solvents, and grease.

Table of content:

• Structure of polyvinyl alcohol
• Preparation of polyvinyl alcohol
• Physical properties of polyvinyl alcohol
• Chemical properties of polyvinyl alcohol
• Uses of polyvinyl alcohol
• Hazards of polyvinyl alcohol
• Practice problems
• Frequently asked questions

Structure of polyvinyl alcohol:

Polyvinyl alcohol is a polymer, so it comprises small monomer units bound. The monomeric unit of polyvinyl alcohol is vinyl acetate. The polymerization of vinyl acetate results in the polymer's formation of polyvinyl alcohol. The direct polymerization of vinyl acetate is not possible. Hence, we prepare polyvinyl alcohol by the hydrolysis of polyvinyl acetate in the presence of an alcohol (mainly methanol) and an alkaline catalyst.

Preparations of polyvinyl alcohol:

Unlike most vinyl polymers, PVA is not made by polymerizing the corresponding monomer, vinyl alcohol, because it is thermodynamically unstable and tautomerized to acetaldehyde. PVA is instead made by hydrolyzing polyvinyl acetate or other vinyl ester-derived polymers with formate or chloroacetate groups instead of acetate. The polyvinyl esters are typically converted via base-catalyzed transesterification with ethanol:

Physical properties of polyvinyl alcohol:

• Polyvinyl alcohol has high flexibility and tensile strength.
• Polyvinyl alcohol is miscible in water and is odorless.
• Its molecular weight ranges from about 26,000 to 30,000 g mol-1.
• Polyvinyl alcohol has a melting point of about 185°C.
• Polyvinyl alcohol is immiscible in organic solvents except for ethanol which is slightly soluble.
• Polyvinyl alcohol is ductile but also strong and flexible.
• It also functions as an aroma and high oxygen barrier.

Chemical properties of polyvinyl alcohol:

• Polyvinyl alcohol reacts with formaldehyde and butyraldehyde.

• It is resistant to grease and oil.
• Polyvinyl alcohol is an atactic type of material.

Uses of polyvinyl alcohol:

Polyvinyl alcohol has multiple uses in various fields. It is the major component in the production of many drugs. Its adhesive property is the main reason for its wide range of applications.

Use of Polyvinyl alcohol as a glue or adhesive:

• Polyvinyl alcohol is used to manufacture synthetic adhesives that find their use in various fields.
• It is soluble in water and can be used as an emulsion as well.
• Its major function is for the fixing up of a porous material.
• It is usually non-toxic, but research is finding out whether prolonged exposure to PVA glue can have harmful effects or not.
• The oral use of polyvinyl alcohol is harmless. Hence, the oral use of PVA glue should also be harmless.

Use of polyvinyl alcohol as a Polyvinyl Fabric:

• Polyvinyl alcohol is an amazing absorbent which makes it highly useful in the textile industry.
• Hence, it is used to make polyvinyl fabric.
• Another property of polyvinyl alcohol is that it dries well.

Use of polyvinyl alcohol as a polyvinyl alcohol film:

• Polyvinyl alcohol films are used in the packaging of various materials like chemicals, detergents, disinfectants, etc., in a large number of industries.
• The packaging material of the polyvinyl alcohol film is also biodegradable, which makes its use safe for the environment.
• Polyvinyl alcohol film is prepared by polyvinyl alcohol solution, which is made by dissolving polyvinyl alcohol in water.

Hazards of Polyvinyl Alcohol:

• A few polyvinyl alcohol drugs can cause a few allergic reactions to the skin and in the eyes.
• Other than that, polyvinyl alcohol is non-hazardous.
• Polyvinyl alcohol is biodegradable as well as environment friendly.
• But the researchers are still finding out whether the long-term use of polyvinyl glue is harmful or not.

Practice problems:

Q 1.  What is the most common polymerization technique for producing polyvinyl acetate?

(A) Bulk polymerization                                                        (B) Solution polymerization

(C) Emulsion polymerization                                                 (D) None of the above

Answer: (C)
Because polyvinyl acetate is commonly used as an emulsion, emulsion polymerization is widely used in the production of polyvinyl acetate. Because of its high exothermic nature, bulk polymerization of vinyl acetate presents challenges at high conversions.

Q 2. Why isn't vinyl alcohol polymerized to produce polyvinyl alcohol?

(A) Tautomerism of vinyl alcohol                                           (B) Free state existence

(C) infeasible reaction                                                            (D) Water insoluble

Answer: (A)
Vinyl alcohol does not exist in its free state because it is unstable, it forms a stable tautomer, acetaldehyde. As a result, polyvinyl acetate is used as the starting material for the manufacture of polyvinyl alcohol.

Q 3. Which of the following pairs of reactions results in the formation of polyvinyl acetals?

(A) Polyvinyl alcohol + aldehydes                                              (B) Polyvinyl alcohol + esters

(C)  Polyvinyl chloride + aldehydes                                            (D) Polyvinyl acetate +  ester

Answer: (A)
The reaction of polyvinyl alcohol with different aldehydes produces polyvinyl acetals. Condensation of 1,3 -hydroxyl groups and aldehyde groups are generally involved in this reaction.

Q 4. In which of the following solvents, is polyvinyl acetate immiscible?

(A) Benzene                                                                            (B) Ethanol

(C) Methanol                                                                           (D) Acetone

Answer: (B)
Polyvinyl acetate is soluble in benzene and acetone, and methanol. It is immiscible in ethanol.

Frequently asked questions:

Q 1. Why has the reaction between polyvinyl alcohol and butyraldehyde to make polyvinyl butyral, which will be used to make safety glass, come to a halt at roughly 75% conversion?
Answer: For safety glass applications, the reaction is stopped at approximately 75 % conversion, leaving nearly 25 % monomer unreacted to provide the required strength and adhesion to the glass.

Q 2. Why is PVA water-soluble?
Answer: PVA is very soluble in water because it has a significant number of hydroxyl groups that form hydrogen bonds with water molecules. The degree of hydrolysis and molecular weight of PVA influence its solubility. PVA solubility is inversely related to its degree of hydrolysis.

Q 3. Is PVA amorphous or crystalline?
Answer: PVA is a semi-crystalline polymer with a crystalline index that varies according to the synthesis process and physical aging. PVA is a water-soluble polyhydroxy polymer that is one of the few non-halogenated linear aliphatic polymers available.

Q 4. Is PVA  a biodegradable substance?

Answer: PVA is a water soluble synthetic polymer with a backbone made up entirely of carbon atoms that is biodegradable in both aerobic and anaerobic environments.

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