1.5 INTRODUCTION TO POLYETHYLENE & ITS APPLICATION

1.5 POLYETHYLENE:

It has the simplest structure of any polymer. The main attractive features of polyethylene, in addition to its low price, are excellent electrical insulation properties over a wide range of frequencies, very good chemical resistance, good process ability, toughness flexibility and, in thin films of certain grades, transparency.

Although polyethylene is virtually defined by its very name as a polymer of ethylene produced by addition polymerization, linear polymers with the formula (CH₂)n have also been prepared by condensation reactions. It can also be prepared by the high-pressure process. The other processes that were available for PE preparation are Metal oxide catalysis in which the polymer can be prepared at low pressure with slight modifications in the structure[1,9].

1.5.1 PREPARATION OF MONOMER:

At one time ethylene for polymerization process is largely obtained from molasses, a by-product of the sugar industry. From molasses, we may get ethylene alcohol and this may be dehydrated to get ethylene. Today bulk of the ethylene is obtained from petroleum sources. When supplies of natural or petroleum gas are available, the monomer is produced at high yield by high temperature cracking of ethane and propane. Good yields of ethylene may also be obtained from gasoline fraction from primary distillation of oil is cracked. The gaseous product of the ethylene includes a number of lower alkanes and olefins and the mixture may be separated by low temperature fractional distillation and by selective absorption. Olefins, in lower yield are also obtained from cracking gas oil. At normal pressure, ethylene is a gas boiling at -103.71 degree Celsius and it has a very high heat of polymerization. In polymerization reactions, the heat of polymerization must be carefully controlled, particularly since the decomposition reaction take place at elevated temperatures are also exothermic and explosion may take place at elevated temperatures.

Since impurities can effect both the polymerization reaction and the purity of the final product, they must be rigorously removed. In particular, carbon monoxide, acetylene, oxygen and moisture must be at a very low temperature. A number of patents require that the carbon monoxide content is less than .02%.

1.5.2 POLYMERIZATION PROCESSES:

There are five distinct routes to the preparation of ethylene.

1. High pressure process

2. Ziegler process

3. The Philips process

4. The standard oil process

5. Metallaocene process

HIGH PRESSURE POLYMERISATION:

It may be said that commercial polymer is generally produced under the high-pressure process (1000-3000atm) and at a temperature of (80 – 300 degree Celsius). A free radical initiator such as benzoyl peroxide, azoide- isobutylronitirile or oxygen is commonly used. The process may be operated continuously by passing the reactants through the narrow bore tubes or through stirred reactor or by the batch reaction process in an auto clave. Because of high heat of polymerization, care must be taken to prevent run away reaction. This can be done by having a high cooling surface-volume ratio in the appropriate part of the continuous reactor and in addition by running water or a somewhat inert liquid such as benzene through the tubes to dilute the exothermal. Local run away reaction can be prevented by operating at a higher flow velocity. In a typical process, 10-30% of the monomer is converted to the polymer. After a polymer - gas separation the polymer is extruded in to a ribbon and then granulated.

ZIEGLER PROCESS:

The type of polymerization involved in this is sometimes called as co-ordination polymerization since the mechanism involves a catalyst – monomer co-ordination complex or some other directing force that controls the way in which the monomer attacks the growing chain. The co-ordination catalysts are generally formed by interaction of the alkyls. In a typical process, the catalyst is prepared from the titanium tetrachloride and aluminum triethyl or some related material.

In a typical process, ethylene is fed under low pressure in to the reactor, which contains liquid hydrocarbon to act as diluents. The catalyst complex may be first prepared and fed in to the reactor or may be prepared in the situ by feeding the components directly in to the reactor. The reaction is carried at some temperature less than 100 degree Celsius in the absence of oxygen and water both of which affect the effectiveness of the catalyst. The catalyst remain suspended and the polymer, as it is formed, becomes precipitated from the solution and the slurry is formed which progressively thickens as the reaction proceeds. Before the slurry viscosity becomes high enough to interfere seriously in removing the heat of the reaction, the reactants are discharged in to the catalyst decomposition vessel. Here the catalyst is decomposed by the reaction with the ethanol, water or caustic alkali. In order to reduce the amount of metallic catalyst fragment to the lowest possible value, the process of catalyst decomposition and subsequent purification are all-important, particularly where the polymer is intended for use in high frequency electrical application.

1.5.3 STRUCTURE AND PROPERTIES OF POLYETHYLENE:

The polymer is essentially a long chain aliphatic hydrocarbon of the type

---CH₂---CH₂---CH₂---CH₂---

in addition, would thus be thermoplastic. The flexibility of the C --- C bonds would be expected to lead to low values for the glass transition temperature. The T_g, however, is associated with the motion of comparatively long segments in amorphous matter and since in a crystalline polymer there is, only a small number of such segments the T_g have little physical significance. Far more important is the crystalline melting point T_m, which is usually in the range 108-132 ^oC for commercial polymers, the exact value depending on the detailed molecular structure. Such low values are to be expected of a structure with a flexible backbone and no strong intermolecular forces. There are no strong intermolecular forces and most of the strength of the polymer is due to the fact that crystallization allows close molecular packing. The high crystallinity also leads to opaque structures except in case of rapidly chilled film where the development of large crystalline structures is prevented.

Polyethylene, in essence a high molecular weight alkane (paraffin), would be expected to have a good resistance to chemical attack and this is found to be the case.

The polymer has a low cohesive energy density (the solubility parameter δ is about 16.1 MP_a^1/2) and would be expected to be resistant to solvents of solubility parameter greater than 18.2 MP_a^1/2. Because it is a crystalline material and does not enter into specific interaction with any liquids, there is no solvent at room temperature. At elevated temperatures, the thermodynamics are more favorable to solution and the polymer dissolves in a number of hydrocarbons of similar solubility parameter.

Now there are available many hundreds of grades of polyethylene, most of which differ in their properties in one way or another. Such differences arise from the following variables:

(1) Variation in the degree of short chain branching in the polymer.

(2) Variation in the degree of long chain branching.

(3) Variation in the average molecular weight.

(4) Variation in the molecular weight distribution (which may in part depend on the long chain branching).

(5) The presence of a small amount of co monomer residues.

(6) The presence of impurities or polymerization residues, some of which may be combined with the polymer.

Possibilities of branching in high-pressure polyethylene were first expressed when investigation using infrared spectroscopy indicated that there were about 20-30 methyl groups per 1000 carbon atoms. Therefore, in a polymer molecule of molecular weight 26000 there would be about 40-60 methyl groups, which is of course far in excess of the one or two methyl groups to be expected from normal chain ends. More studies that are refined have indicated that the methyl groups are probably part of ethyl and butyl groups. The most common explanation is that these groups arise owing to a ‘back-biting’ mechanism during polymerization.

Polymerization could proceed from the radical in the normal way or alternatively chain transfer may occur by a second backbiting stage either to the butyl group or to the main chain.

Short chain branching is negligible with Ziegler and Phillips homopolymers although it is possible to introduce deliberately up to about seven ethyl side chains per 1000 carbon atoms in the Ziegler polymers.

The presence of these branch points is bound to interfere with the case of crystallization and this is clearly shown in differences between the polymers. The branched high-pressure polymers have the lowest density (since close packing due to crystallization is reduced), the least opacity (since the growth of large crystalline structure is impeded) and a lower melting point, yield point, surface hardness and Young’s modulus in tension (these properties being dependant on the degree of crystallinity). In addition the more the branching and the lower the crystallinity, the greater will be the permeability to gases and vapours. For general technological purposes the density of the polyethylene (as prepared from the melt under standard conditions) is taken as a measure of short chain branching.

Differences in molecular weight will also give rise to differences in properties. The higher the molecular weight, the greater is the number of points of attraction and entanglement between molecules. Whereas differences in short chain branching and hence degree of crystallinity largely affect properties characterized by small solid displacement, molecular weight differences will affect properties that involve large deformations such as ultimate tensile strength, elongation at break, melt viscosity and low-temperature brittle point. There is also an improvement in resistance to environmental stress cracking with increase in molecular weight.

With the availability of the higher density polymers the value of the melt flow index as a measure of molecular weight diminishes. For example, it has been found that with two polymers of the same weight average molecular weight (4.2 x 10⁵), the branched polymer (density=0.92g/cm³) had only 1/50 the viscosity of the more or less unbranched polymer (density=0.96g/cm³). This is due to long chain branches as explained above.

Commercial polyethylenes also vary in their molecular weight distribution (MWD). Whilst for some purposes a full description of the distribution is required, the ratio of weight average molecular weight to number average molecular weight provides a useful parameter. Its main deficiency is that it provides no information about any unusual high or low molecular weight tail which might have profound significance. For polymethylenes, the ratio is about 2 whilst with low-density polymers values varying from 1.9 to 100 have been reported with values of 20-50 being said to be typical. High-density polymers have values 4-15.

The final variable to be mentioned here is the presence of impurities. These may be metallic fragments residual from Ziegler-type process or they can be trace materials incorporated into polymer chain. Such impurities as catalyst fragments and carbonyl groups incorporated into the chain can have a serious adverse influence on the power factor of the polymer, whilst in other instances impurities can have effect on aging behavior.

1.5.4 PROPERTIES OF POLYETHYLENE:

Polyethylene is a wax like thermoplastic softening at about 80-130 ^oC with a density less than that of water. It is tough but has moderate tensile strength, is an excellent electrical insulator and has very good chemical resistance. In the mass, it is translucent or opaque but thin films may be transparent.

Mechanical properties:

The mechanical properties are very dependent on the molecular weight and on the degree of branching of the polymer. As with other polymers these properties are also dependent on the rate of testing, the temperature of test, the method of specimen preparation, the size and shape of the specimen and, to only a small degree with polyethylene, the conditioning of samples before testing. The working of the sample, however, causes ‘strain softening’ by, for example, spherulite breakdown or in some cases by crystal melting so that the polymer extends at constant stress. This cold drawing, however, causes molecular orientation and induces crystallization so that there is a stiffening of the sample and an upward sweep of the stress-strain curve.

It is seen that in one case an increase in rate of strain is accompanied by increase in tensile strength and in the other case, reduction. The elongation at break of polyethylene is strongly 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 (‘creep’). Knowledge of creep behavior is important when considering load-bearing applications, water-piping being a case in point with polyethylene. In general, there will be an increase in creep with increased load, increased temperature and decreased density.

Thermal Properties:

Tough at room temperature, the polymers become brittle on cooling but some specimens do not appear to become brittle until temperature as low as -70 ^oC have been reached. In general, the higher the molecular weight and the more the branching the lower the brittle point. Measured brittle points also depend on the method of sample preparation, thus indicating that the polymer is notch sensitive. i.e. sensitive to surface imperfections.

The specific heat of polyethylene is higher than for most thermoplastics and is strongly dependent on temperature. Low-density materials have a value of 2.3 J/g at room temperature and a value of 2.9 J/g at 120-140 ^oC. T_m is seen to vary with density.

Flow properties of polyethylene have been widely studied because of the wide range of average molecular weights amongst commercial polymers the viscosities vary widely. The most commonly used materials, however, have viscosities lower than for unplasticised PVC and poly (methyl methacrylate) and higher than for the nylons.

It is interesting to note that so-called linear low-density polyethylene are said to be less pseudo plastic than conventional low-density polyethylene. Thus on comparing the two materials at the same melt flow index the ‘linear’ polymer will be found to be more viscous at higher shear rates usually encountered during processing.

As usual, an increase in temperature reduces melt viscosity. Melt processing is usually carried out in the range of 150-210 ^oC but temperatures as high as 300 ^oC may be used in some paper-coating applications. In an inert atmosphere, the polymer is stable at temperatures up to 300 ^oC so that the high processing temperatures do not lead to severe problems due to degradation, providing contact of the melt with oxygen is reduced to a minimum.

The phenomenon of elastic turbulence (waviness, bambooing, melt fracture) is also observed in low-temperature processes (e.g. bottle blowing) and when extruding at very high rates (wire covering). This situation is generally aggravated by high molecular weights and low temperature but reduced by long chain branching and increasing the molecular weights distribution.

In addition to elastic turbulence (characterized by helical deformation), another phenomenon known as ‘Sharkskin’ may be observed. This consists of a number of ridges transverse to the extrusion direction, which are often just barely discernable to the naked eye. These often appear at lower shear rates than the critical shear rates for elastic turbulence and seem more to the linear extrudate output rate, suggesting that the phenomenon may be due to some form of slip-sick at the die exit. It appears to be temperature dependent (in a complex manner) and is worse with polymers of narrow molecular weight distribution.

Chemical properties:

The chemical resistance of polyethylene is, to a large measure that expected of an alkane. It is not chemically attacked by non-oxidizing acids, alkalis and many aqueous solutions. Nitric acid oxidizes the polymer, leading to a rise in power factor and to deterioration in mechanical properties. As with the simple alkanes, halogens combine with the hydrocarbon by means of substitution mechanisms. 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 (Hypalon).

Oxidation of polyethylene, which leads to structural changes, can occur to a measurable extent at temperatures as low as 50 ^oC. Under the influence of ultraviolet light, the reaction can occur at room temperature. The oxidation reactions can occur during processing and may initially cause a reduction in melt viscosity. Further the oxidation can cause discoloration and streaking and in the case of polymers rolled for 1-2 hours on a two-roll mill at about 150 ^oC the product becomes ropey and incapable of flow. It is rare that such drastic operating conditions occur but it is found that at a much earlier stage in the oxidation of the polymer, there is a serious deterioration in power factor and for electrical insulation applications; in particular it is necessary to incorporate antioxidants. It is to be expected that the less branched high-density polyethylene, because of the smaller number of tertiary carbon atoms, would be more resistant to oxidation. That this is not always the case has been attributed to residual metallic impurities. Since purer samples of high-density polymers are somewhat superior to the low-density materials.

Since polyethylene is a crystalline hydrocarbon polymer incapable of specific interaction and with a melting point of about 100 ^oC, there are no solvents at room temperature.

Exposure of polyethylene to ultraviolet light causes eventual embrittlement of the polymer. This is believed to be due to the absorption of energy by carbonyl groups introduced into the chain during polymerization and/or processing. The carbonyl groups absorb energy from wavelengths in the range 220-320 nm. Fortunately, very little energy from wavelengths below 300 nm strikes the earth’s surface and so the atmosphere offers some protection. However, in different climates and in different seasons there is some variation in the screening effect of the atmosphere and this can give rise to considerable variation in the outdoor weathering behavior of the polymer.

When polyethylene is subjected to high-energy irradiation, gases such as hydrogen or some lower hydrocarbons are evolved, there is an increase in unsaturation and most important, cross-linking occurs by the formation of C—C bond between molecules. The formation of cross-link points interfere with crystallization and progressive radiation will eventually yield an amorphous but cross-linked polymer. Extensive exposure may lead to colour formation and in the presence of air surface oxidation will occur. Oxygen will cause polymer degradation during irradiation and this offsets the effects of cross-linking. Long exposure to low radiation doses on thin film in the presence of oxygen may lead to serious degradation but with short exposure, high radiation doses and thicker specimens the degradation effects become less significant. Since cross-linking is accompanied by a loss of crystallization, irradiation does not necessarily mean an increased tensile strength at room temperature. However, at temperatures about 130 ^oC irradiated polymer still has some strength (it is quite rubbery), whereas the untreated material will have negligible tenacity. It is found that incorporation of carbon black into polyethylene, which is subsequently irradiated, can give substantial reinforcement whereas corresponding quantities in the untreated product lead to brittleness.

If polyethylene is exposed to mechanical stress in certain environments, fracture of the sample occurs at stresses much lower than in the absence of the environment. As a corollary, if a fixed stress, or alternatively fixed strain, is imposed on a sample the time for fracture is much less in the ‘active environment’ than its absence. This placement is referred as environmental stress cracking.

Different polyethylenes vary considerably in the environmental stress cracking resistance. It has been found that with low-density polymers the Bell Test generally shows that the higher the molecular weight the greater the resistance, low-density polymers with a melt flow index of 0.4 being immune to the common detergents. Narrow molecular weight distribution distributions considerably improve resistance of a polymer of given density and average molecular weight. Large crystalline structures and molecular orientations appear to aggravate the problem. The effect of polymer density is somewhat complicated. The Bell Test is performed at constant strain and hence much higher stresses will be involved in the high-density polymers. It is thus not surprising that these materials often appear to be inferior by this test but in constant stress tests different results may be expected. Paradoxically, Phillips-type homopolymers have often been less satisfactory in service than indicated by the Bell Test.

It may seem surprising that low-density, comparatively low molecular weight (MF 120) materials have been successively used for detergent bottles in view of the stress-cracking phenomenon. (Nevertheless higher molecular weight materials are usually used here, i.e. with an MFI<0.7).>

Electrical Properties:

The insulating properties of polyethylene compare favorably with those of any other dielectric material. As it is a non-polar material, properties such as power factor and dielectric constant are almost independent of temperature 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 can lead to a serious increase in power factor. Antioxidants are incorporated into compounds for electrical applications in order to reduce the effect.

1.5.6 APPLICATION:

The characteristic of polyethylene which leads to its wide spread use may be summarized as follows.

1. Low cost
2. Easy process ability
3. Excellent electrical insulation properties.
4. Excellent chemical resistance.
5. Toughness and flexibility even at low temperatures.
6. Reasonable clarity of thin films.
7. Freedom from odor and toxicity.
8. Sufficiently low water vapor permeability for many packaging, building and agricultural applications.

Limitations of polymer:

1. Low softening point.
2. The susceptibility of low molecular weight grades to environmental stress cracking.
3. The susceptibility to oxidation (however, polyethylene is better in this respect than many other polymers)
4. The capacity of the material in bulk.
5. The wax like appearance.
6. Poor scratch resistance.
7. Lack of rigidity(limitations some applications but a virtue in others)
8. Low tensile stress.
9. High gas permeability.

There are two main classes of HDPE now on the market

1. High molecular weight, broad weight average molecular weight polymer- used primarily for blow molding and pipe.

2. Low molecular weight, narrow weight average weight polymer- used widely for injection and rotational molding.

Each type is available in varying densities. By modal M_WD polymers have become established in some markets. The high molecular weight component confers impact strength, toughness, stiffness and good environmental cracking resistance. The lower molecular weight peek improves flow behavior.

High-density polyethylene film is being increasingly used for carrier bags. It is also finding rapid acceptance as a wrapping material instead of paper for food products, probably largely a consequence of its crisp feel and crease proof nature. HDPE film is also of some interest in some pseudo-fiber applications. If extruded sheet is embossed with regular pattern and the embossed sheet biaxial stretched under controlled condition of temperature, the sheet breaks up in to a net like form. Such a process, developed by Smith and nephew in England, is now used to make nets varying in appearance from a fine web to coarse open structures. HDPE film is also used in mono axially stretched form. When it is slit into tape, and to some extent in the manufacture of fibrillated tape.

Blow molding is widely used for HDPE. One large application area is that of bottles for milk and other foodstuffs, household foodstuffs, personal toiletry and drug package. These take about 70% of blow molding market but other important areas include drums, pails and toys. In some cases the selection between blow molding and rotational molding requires careful consideration but now the tonnage rotationally molded is only about 1/30 ^th of that blow molded.

High molecular weight HDPE (HMV-HDPE) is used for blown film and for demanding molding and structural uses. Examples of blow molded drums for packing dangerous chemicals and pressure piping.

Very high(ultra- high) molecular weight polyethylene’s (VHMWPE or UHMWPE) of molecular weights in the range (3 to 6 *(10^6)), with the outstanding abrasion resistance, low temperature toughness and low coefficient of friction, are used in severe operating conditions such as ore handling and food processing. Such materials are some times cross-linked with peroxide to improve the abrasion resistance further.

The electrical insulation property of poltehylene has led extensive use in cable

Each type is available in varying densities. By modal M_WD polymers have become established in some markets. The high molecular weight component confers impact strength, toughness, stiffness and good environmental cracking resistance. The lower molecular weight peek improves flow behavior.

High-density polyethylene film is being increasingly used for carrier bags. It is also finding rapid acceptance as a wrapping material instead of paper for food products, probably largely a consequence of its crisp feel and crease proof nature. HDPE film is also of some interest in some pseudo-fiber applications. If extruded sheet is embossed with regular pattern and the embossed sheet biaxial stretched under controlled condition of temperature, the sheet breaks up in to a net like form. Such a process, developed by Smith and nephew in England, is now used to make nets varying in appearance from a fine web to coarse open structures. HDPE film is also used in mono axially stretched form. When it is slit into tape, and to some extent in the manufacture of fibrillated tape.

Blow molding is widely used for HDPE. One large application area is that of bottles for milk and other foodstuffs, household foodstuffs, personal toiletry and drug package. These take about 70% of blow molding market but other important areas include drums, pails and toys. In some cases the selection between blow molding and rotational molding requires careful consideration but now the tonnage rotationally molded is only about 1/30 ^th of that blow molded.

High molecular weight HDPE (HMV-HDPE) is used for blown film and for demanding molding and structural uses. Examples of blow molded drums for packing dangerous chemicals and pressure piping.

Very high(ultra- high) molecular weight polyethylene’s (VHMWPE or UHMWPE) of molecular weights in the range (3 to 6 *(10^6)), with the outstanding abrasion resistance, low temperature toughness and low coefficient of friction, are used in severe operating conditions such as ore handling and food processing. Such materials are some times cross-linked with peroxide to improve the abrasion resistance further.

The electrical insulation property of poltehylene has led extensive use in cable and other wire covering applications. Spectacular early uses included under sea cables and airborne radar and the materials continue to be used in substantial quantities.