7. Synthetic fibers

7.10 Polyethylene

Polyolefin

Fiber made from polymer or copolymers of olefin hydrocarbon such as ethylene, propylene etc is called polyolefin fiber. Polyethylene and polypropylene are the important fibers in this group of fibers.

Introduction

Polyethylene has the simplest basic structure of any polymer. It has excellent electrical insulation properties over a wide range of frequencies, very good chemical resistance, good processability, toughness and flexibility and transparency. It is a polymer of ethylene produced by addition polymerization. Polyethylene (PE) is a wax-like thermoplastic softening at about 80-130ºC with a density less than that of water.

There are four quite distinct routes to the preparation of high polymers of ethylene: (a) High pressure process (b) Ziegler process (c) The Phillips process (c) The Standard Oil (Indiana) process.

Structure of polyethylene

The polyethylene is a long chain aliphatic hydrocarbon and it is thermoplastic. The Tg is approximately -120ºC. Tm depends upon the structure which ranges from 108-132ºC. It has high molecular weight alkane and has a good resistance to chemical attack. Because it is a crystalline material and does not interact with any liquids, there is no solvent at room temperature. There are many grades of polyethylene and differences arise from the following variables:

  • Variation in the degree of short chain branching in the polymer.
  • Variation in the degree of long chain branching.
  • Variation in the average molecular weight.
  • Variation in the molecular weight distribution
  • The presence of small amount of co-monomer residues
  • The presence of impurities or polymerization residues some of which may be combining with polymer.

Classification of polyethylene

1) Polyethylene of high molecular weight e.g. high density polyethylene (HDPE). HDPE has density of greater or equal to 0.941 g/cm3. HDPE has a low degree of branching and thus low intermolecular forces and tensile strength.

2) Polyethylene with low molecular weight e.g. low density polyethylene (LDPE). LDPE has density range of 0.910–0.940 g/cm 3. It has a high degree of short and long chain branching, which results in loose packing of the chains . Another variety is linear low density polyethylene ( LLDPE) which has a density range of 0.915–0.925 g/cm3 and it is a substantially linear polymer with significant numbers of short branches . It has higher tensile strength than LDPE and show higher impact and   puncture resistance than LDPE.

3) Cross linked polyethylene there are three main approaches for Cross linked polyethylene are a) Radiation cross- linking b) Peroxide cross-linking c) Vinyl silane cross-linking 4) Chlorinated cross-linking.

Mechanical Properties

The mechanical properties are very dependent on the molecular weight, on the degree of branching of the polymer and on the rate of testing, the temperature of test, the method of specimen preparation, the size and shape of the specimen and the condition of samples.

The elongation at break of polyethylene is dependent on density the more highly crystalline high density materials being less ductile. This lack of ductility results in high-density polymers tending to be brittle, particularly with low molecular weight materials. Under load polyethylene will deform continuously with time. There will be increase in creep with increased load, increased temperature and decreased density.

Thermal Properties

Polyethylene is tough at room temperature and become brittle on cooling but some specimens do not appear to become brittle until temperatures as low as -70ºC have been reached. In general the higher the molecular weight and the more the branching the lower the brittle point. The specific heat of polyethylene is higher than for most thermoplastics and is strongly dependent on temperature.

Chemical Properties

Polyethylene is resistant to non-oxidising acids, alkalis and many aqueous solutions. Nitric acid oxidises the polymer leading to the rise in power factor and to deterioration in mechanical properties. When polyethylene is chlorinated in the presence of sulphur dioxide, sulphonyl chloride as well as chlorine groups may be incorporated into the polymer this reaction is used to produce a useful elastomer. Its oxidation leads to structural changes that can occur to a measurable extent at temperature as low as 50ºC. Under ultraviolet light the oxidation reaction can occur at room temperature. The oxidation reactions results in processing and may initially cause a reduction in melt viscosity. Further oxidation can cause discoloration and streaking. When polymers rolled for one to two hours on a two roll mill at about 150º C the product becomes inadequate and incapable of flow.

Electrical Properties

The insulating properties of polyethylene evaluate favourably with any other dielectric material. It is a non-polar material property such as power factor and dielectric constant are independent of temp and frequency. Dielectric constant is linearly dependent on density and a reduction of density on heating leads to a small reduction in dielectric constant. Oxidation of polyethylene with the formation of carbonyl groups results in serious increase in power factor. Antioxidants are incorporated into compounds for electrical applications in order to reduce the effect.

Applications

Polyethylene was introduced initially for high frequency insulation. Due to its good chemical resistance it is used for chemical plant and to a small extent, for water piping.

LDPE: Major applications for LDPE film include heavy-duty sacks, carrier bags and for general packaging. The LDPE film market has now been eroded by LLDPE. The films from LLDPE have a higher impact strength, tensile strength and extensibility. Such properties allow the possibility of making film of lower gauge but with same mechanical performance.

HDPE: High Density polyethylene film is used for carrier bags. It is also used as a wrapping material instead of paper in food products and also in some pseudo-fibre application. It is also used in making various nets from a fine web to coarse open structure and in manufacturing of fibrillated tape. Filament for ropes, fishing nets and fabrics are an important outlet for the high-density polyethylene.

HDPE & LDPE mixture: They are used in toys, chemical plant, electrical fittings, bushes & many industrial items. It is used for the gas piping, water piping, and agricultural pipe. Due to its excellent electrical insulation properties, polyethylene has led to extensive use in cable and other wire covering application.

 
 
 
 
 
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