Polyethylene Fibres

Polyolefin fibres

Fibres made from polymers or copolymers of olefin hydrocarbons such as ethylene, propylene are called polyolefins.

Polyethylene: Of all the fibre forming polymers, polyethylene (made by addition polymerisation) Ch2==Ch2 has the simplest structure.

Manufacture: Ethylene is the principal raw material for producing polyethylene fibres. Ethylene gas is obtained by cracking petroleum.

Polymerisation: Ethylene is polymerised under severe conditions in autoclaves at 200 deg C and 1500 atmospheric pressure in the presence of traces (0.01%) of oxygen acting as a catalyst. The polymer resembles paraffin wax and is characterised by low density.

Spinning : Spinning of polyethylene is carried out by melt spinning. The polymer with a molecular weight of about 15,000 is spun from the melt at about 205 deg C and extended through a spinnerette of 0.1 mm diameter into a current of cooling gas. The filaments are cooled to 15 -60 deg C and stretched 4 to 10 times their original length. The drawn monofilaments are wound on spools.

Properties of polyethylene

a. Polyethylene fibre has a round cross section and has a smooth surface. Fibres made from low molecular weight polyethylene have a grease like handle.

b. Specific Gravity- 0.92
Tenacity - 1.0-1.5 gpd
Elongation at Break %- 45-50
Tensile Strength psi - 15000
Softening Range: deg C- 85-90

c The moisture regain of polyethylene is practically nil and hence moisture does not affect the mechanical properties of the fibres.

d. Polyethylene is insoluble in most of the common organic solvents at room temperature.

e. Polyethylene fibres have a high degree of resistance to acids and alkalies at all concentrations even at high temperature.

f. The fibre is generally inert and is resistant to wide range of chemicals at ordinary temperatures. They are attacked by oxidising agents.

How to determine Seam Strength

Seam strength is the strength of seam assembly in a garment. It is a function of the strength of the thread used for the seam, type of seam assembly in a garment and type of fabric used, among other factors.

Failure of seam assembly can occur either by breaking of sewing thread, tearing of the fabric at the seam, excessive yarn slippage adjacent to the stitches or a combination of the above mentioned conditions.

How to determine seam strength in laboratory

In a lab CRE type tensile testing machines such as INSTRON is used. Before testing the samples are prepared.The method is called Grab Test


If the specimens are from manufactured items such as garment then five speicmens are cut having a length of 270 mm on either side of the stitch line and a width of 100 mm parallel to the stitch line.

If the specimens are prepared from fabric then seam strength is measured in warp, weft and bias directions. In such cases swatches are cut in these three directions.

Also two such swatches from two different rolls are joined by the seam assembly- for which we want to test the strength- to measure the parameters.

Before testing the material is conditioned to standard temperature.

The specimen is mounted on the tensile tester. It is operated until the sewn seam or fabric ruptures. The observation is made whether the rupture is caused by Fabric yarn rupture, sewing thread rupture, sewn seam yarn slippage or a combination of two or more of the foregoing. This will give the seam strength.

Repeat this Grab Test for the fabric specimens from which seam failure test is carried out. The result will give the base fabric strength.

Seam strength ( for each direction) will be calculated by the formula:

Seam strength ( gms/cm)= ( Individual specimen seam breaking force in gms)/ Width of specimen in Jaws i.e. 2.5 cm.

Base fabric breaking strength of the individual specimens are calculated by the formula:

Base fabric strength (gms/cm)= (Base Fabric breaking force in gms of individual test specimen)/ Width of specimen in jaws i.e. 2.5 cm

Seam Efficiency= Seam Strength ( Seamed Fabric Strength) x 100)/ Base Fabric Strength

If this seam efficiency ratio falls below 80%, the fabric has been excessively damaged by the sewing operation.

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

	Staple	Filament
Tenacity ( GPD)
Dry	3.8-6.2	6.0-8.5
Wet	3.2-5.0	5.0-7.6
Elongation at Break
Dry	13-26%	9-22%
Wet	14-27%	10-26%
Elastic Recovery	65-85%	70-90%
Moisture Regain	4.5-5%	3-5%

How to Identify Constituent Fibre Percentage in a Blend-2

Blend of Acrylic with Wool, Silk, Cotton, Viscose, Polyester or Nylon

1. Dissolve the acrylic fibres with (Dimethyl Formamide - DMF). Acrylic Fibres will dissolve in DMF.

2. Filter, rinse and weigh carefully to get the ratio of Acrylic Fibres.

Blend of Protein Fibres ( Wool or Silk) with cotton, polyester, nylon or acrylic

1. Take the blended fibres ( Carefully weighed) in a conical flask.

2. Add a solution of 5% (w/w) solution of Sodium Hydroxide or Potassium Hydroxide and boil for 10 minutes. Protein fibres will dissolve in Sodium Hydroxide or Potassium Hydroxide.

3. Rinse the leftover fibres with water and neutralise with dilute Acetic Acid.

4. Weigh the fibres after drying and find the ratio of protein fibres.

Blend of Polyester with Cotton or Viscose

1. Weigh the blend and heat it with Meta cresol. Polyester fibres will dissolve.

2. Weigh the residual fibres after rinsing thoroughly and drying and find the percentage of polyester fibres.

Blend of Elastane ( Spandex or Lycra) with Cotton or Viscose

1. Treat the blend with DMF. Elastane will dissolve in DMF.

2. Filter, Rinse, dry and get the weight of residual fibres to get the percentage of elastane.