Friday, 5 June 2009

Manufacturing Process and Properties of PVA



Polyvinyl Alcohol Fibres

Polyvinyl alcohol (water soluble compound) can be described as a polyhydric, having secondary alcoholic groups on alternate carbon atoms of an aliphatic macromolecule.

Because of the presence of a large number of hydroxy groups, in its molecular structure, it is soluble in water. This is solublised in water by a treatment with formaldehyde.

Manufacture of Polyvinyl Alcohol



1. Production of acetic acid from acetylene

For this purpose, limestone is calcinated to give quicklime (CaO) which is treated with coke at elevated temperature to form calciium carbide. Acetylene is generated by treating calcium carbide with water. A part of acetylene is converted into acetic acidby combined hydration and oxidation.

Synthesis of Vinyl Acetate

The acetic acid formed in the above step is reacted with acetylene in the presence of zinc acetate catalyst when vinyl acetate is formed.

Polymerisation of Vinyl Acetate

A solution of vinyl acetate in methanol is used for the polymerisation of vinyl acetate in the presence of a peroxide or azo compound as a catalyst.

Conversion of PVAcetate into PVA

NaOH is added in PV Acetate solution in methanol, when alcoholysis of the acetate groups takes place.

Spinning

The precipitated PVA as obtained in the preceding step is pressed and dried. It is then dissolved in water to give a 15% solution of the polymer. This solution is extruded into a spinning bath containing sulphuric acid ( 20%), Glauber's Salt ( 25%), formaldehyde (5%) and water (50%)

Properties

Shrinkage Properties: 10% at 220-230 deg C.

At 220 deg c, It begins to turn yellow and shrinks.

The fiber is inert to animal, vegetable and mineral oils and to most common organic solvents.

It has good resistance to acids under normal conditions, Hot or concentrated mineral acids cause swelling and shrinkage. Its resistance to alkali is generally good. Strong alkalies cause yellowing without affecting the tenacity.

Fabrics made from this fibre do not get solied easily. They are easy to wash and quick to dry. They have good crease retention.


Specific Gravity:
1.28


 StapleFilament
Tenacity ( GPD)  
Dry3.8-6.26.0-8.5
Wet3.2-5.05.0-7.6
Elongation at Break  
Dry13-26%9-22%
Wet14-27%10-26%
Elastic Recovery65-85%70-90%
Moisture Regain4.5-5%3-5%





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