Polyethylene (PE)

        The polymer unit for polyethylene (PE) is shown in Figure 7.2. The functional group (X in Figure 7.1) is simply hydrogen. PE is the simplest of all polymers, with just two carbons and four hydrogens in the basic polymer repeating unit.

Figure 7.2 Polymeric representation of polyethylene (PE).

        Many properties of PE can be predicted from its basic polymer representation. For instance, PE consists of only carbons and hydrogens, usually with high molecular weights, and so it is relatively insensitive to most solvents. This is an advantage when PE is used for applications such as chemical reaction vessels or pipes, where inertness of the container is critical. However, the solvent insensitivity is a problem when inks, paints, or other solvent based materials are used to mark or decorate PE. The inks and paints will generally not adhere. This problem can be overcome by treating the surface of PE where the ink or paint is to be placed with a flame or electric spark, thus changing the chemical nature of the surface. The problem with this solution is that it requires an extra step in the manufacturing process and, furthermore, the adherence of the ink or paint is still not very good. 

        Even though polyethylene is resistant to solvents, inks, and paints, it will stick to other polyethylene, especially when hot. Hence, some labels for polyethylene products that have had good success are made of polyethylene itself and then applied to the product after heating or by placing the label in the mold and then filling the mold with the molten or semi molten polyethylene. The good adhesion of polyethylene to itself when heated is also used to advantage when marking an extruded polyethylene product using coextrusion. In this process, just before the product exits the extrusion die, a second stream of molten polyethylene is introduced into the die using a small extruder. This second stream is usually different in color, thus giving a stripe to the final product. Such a product is shown in Photo 7.1. 

        Joining or bonding PE is another process that is made difficult because of the inherent resistance of polyethylene to solvents and other materials. Many adhesives for plastics depend upon the ability of their solvent base to soften or partially dissolve the surface of the plastic material to form a good bond. This will not work well with PE. This problem is overcome by joining PE to itself by using bonding techniques that melt the surfaces of the PE parts to be joined and then pressing them together. Processes of this type are more difficult than solvent welding and require special joining equipment. They will be discussed in further detail in Chapter 21, on finishing and assembly.

Photo 7.1 Polyethylene hose showing co-extruded blue stripe marking.

        High electrical resistance is another property that results from the basic chemical nature of PE. The carbons and hydrogens have approximately the same electronegativity, resulting in little polarity. As a result, electrical charge is not easily transferred and PE is, therefore, an excellent insulator, used extensively for insulating wires and cables and in many electrical devices. For many of the same reasons, PE is also a good thermal insulator. However, the melting point of PE is quite low and so its use is limited to applications where the temperature is not excessive (typically, below the boiling point of water). 

        Perhaps the most important applications for PE are based upon its low cost and ease of manufacture. PE is polymerized from ethylene gas that is easily and inexpensively obtained from either natural gas (methane) or from crude oil. Furthermore, the processes used to make PE are easily scaled to manufacture the polymer in very large quantities. To further reduce its cost, the temperatures required for processing PE into final shapes are also the lowest of any of the common high-use thermoplastic materials. This means that comparatively little energy is required in the molding operations. The molding operations are further simplified because PE is stable during processing and poor-quality parts can be re-ground and reprocessed with very little difficulty. PE applications that benefit from this low cost and ease of processing include trash bags, packaging and other films, containers (such as milk bottles), many children's toys, and various housewares. 

        Some properties of PE depend more on the way the PE molecules interact with each other; these interactions are most apparent in the micro and macro views of the polymer that were discussed in previous chapters. The interactions of the PE molecules are strongly dependent on the shape (steric effects) of the molecules. The differences in shapes, which could not be reasonably predicted from the simplified view of addition polymerization presented earlier showing the basic chain extension mechanism, can result in changes in PE properties that are often very important in choosing the type of PE for a particular application. However, it should be remembered that the basic properties of PE arise from its basic nature and are largely unaffected by the changes in shape. In other words, the property differences between PE types arising from shape are relatively minor when compared to the differences between any of the PE types and other non-polyethylene polymers. 

        The major differences in the shape of PE molecules arise from changes in the conditions in the polymerization reactor during the polymerization reaction. Reactor conditions such as temperature, pressure, and catalyst type can have a major effect on the shape by either creating or suppressing the formation of molecular branching. Branching is the formation of side chains off the basic polymer backbone. These side chains can form when a hydrogen-carbon bond is broken during the polymerization reaction. (In the basic view of addition polymerization presented in a previous chapter, only carbon-carbon double bonds were assumed to be broken.) However, when polymerization is carried out at high temperatures, there is often sufficient energy in the molecules that some carbon-hydrogen bonds break, thus creating a free radical on the carbon. (The hydrogen takes with it one electron from the bond and leaves the other electron localized on the carbon, thus forming a carbon free radical.) This carbon free radical can then serve as a site for chain growth to begin. When this occurs, the chain can grow at two locations simultaneously the normal end of the chain and the newly created free radical location. The net result is a branch off the main carbon backbone. The mechanism for the breaking of carbon-hydrogen bonds and the formation of side chains (branches) is illustrated in Figure 7.3. 

        Changes in the amount of branching (that is, the number of side chains and the length of the side chains) result in major differences in the inter-actions between PE molecules.

Figure 7.3 Branching mechanism for polyethylene.

Table 7.1 The Effect of Branching on Several Polymer Properties.

Figure 7.4 Different types of polyethylene showing the effects of branching. 

        Some of the properties affected by branching are given in Table 7.1. Branching causes strong steric interference between molecules and thus forces an open, non-crystalline structure, which has many effects on properties. The melt temperature of highly branched PE material is lower than that of close-packed, crystalline materials because fewer inter-molecular attractions exist in the open structure and, therefore, the energy that allows the molecules to move independently is lower. Lower creep resistance, lower tensile strength, lower stiffness, and lower hardness (scratch resistance) also result from the lower inter-molecular forces in the open polymer structure of the branched material. Impact toughness is higher because the open structure can move more readily and absorb the energy of the impact. Transparency is higher because of the decrease in crystal structures, which often cause light to diffract. The oxidative resistance, UV stability, and solvent resistance are all lower because the oxygen, UV light, or solvents can more easily penetrate the structure. This ease of penetration also increases the permeability. There is also a decrease in shrinkage because the open structure is less likely to contract to a highly packed structure when it is cooled. 

        Three general types of commercially made PE differ chiefly in the way the molecules interact, caused by the amount and type of branching, illustrated in Figure 7.4. The three materials are distinguished on the basis of density because density is a property that is easily measured and is directly dependent on the amount and type of branching. The differences in polymer shapes represented in Figure 7.4 are idealized. Actually, there is some overlap in the nature of the materials and so the densities are normally given in ranges, which are listed in Table 7.2. Each of the three major types of PEs will be discussed separately.

Table 7.2 Densities of Polyethylene Types.

7.2.1. Low-Density Polyethylene (LDPE)

The type of PE formed under high-temperature and high-pressure polymerization conditions is called low-density polyethylene (LDPE). The density is low because these polymerization conditions give rise to the formation of many branches, which are often quite long and prevent the molecules from packing close together to form crystal structures. Hence, LDPE has low crys-tallinity (typically below 40%) and the structure is predominantly amor-phous. 

The low density and highly amorphous nature of the structure affects the physical properties of LDPE, as reflected in Table 7.1. LDPE is therefore used in applications that re- quire its flexibility, im-pact toughness, and stress crack resistance-in films and flexible tubing, for example. (See Photo 7.2.) Furthermore, LDPE is the lowest melting and easi-est to process of the PE types so it is used exten-sively in high-volume applications, such as packag-ing films, toys, and squeeze bottles for food and other household applications, especially when strength and other mechanical properties are not critical.

7.2.2. High-Density Polyethylene (HDPE)

        Under polymerization conditions that result in limited branching (that is, low temperature and pressure), a PE that is more linear, with only a few, short branches, is created. This type of PE is called high-density polyethylene (HDPE). As the name implies, the polymer chains in HDPE can easily pack tightly and form crystalline structures, thus increasing the density. The properties of HDPE relative to LDPE can be determined from Table 7.1, when examined with the realization that branching is much lower in HDPE than in LDPE. In general, HDPE is stiffer, stronger, and more abrasion resistant than LDPE. 

         In order to get long polymer chains under the HDPE conditions, a process that requires much lower temperatures and lower pressures than the process used to make LDPE, a catalyst is required. The first catalyst was developed by Karl Ziegler in 1952 and was then applied to polymerizations of other monomers by Giulio Natta. The catalyst is called a Ziegller-Natta catalyst, which is a general name applied to all similar catalysts even though some more recent types were developed and patented by different inventors.

Photo 7.2 Examples of low density polyethylene (LOPE) and high density polyethylene (HOPE) products. (a) Trash bags. (b) Milk jugs.

         HDPE is used in preference to LDPE when greater stiffness or strength is required. For instance, milk, water, detergent, and bleach bottles are HDPE because they are usually made with very thin walls to save material and cost, yet still must retain their shape. (See Photo 7.2.) HDPE gives sufficient stiffness to accomplish this, whereas LDPE would tend to sag. The improved stiffness and strength are even more important as the size of the container increases. Therefore, barrels, trash carts, and chemical storage tanks are usually made of HDPE, in part because of its superb chemical resistance. HDPE is used for auto motive fuel tanks because of its strength, chemical resistance, and low permeability. 

         The HDPE molecules are essentially linear with little entanglement in the melt, at least compared to LDPE. Therefore, when processed in the melt, HDPE molecules tend to be aligned in the direction of flow, especially when the flow path is highly restricted. This orientation also leads to rapid crystallization and high shrinkage upon cooling. Hence, the cooling rate of HDPE is faster than that of LDPE, which can be an advantage in very high-volume processes such as the manufacture of margarine tubs. This orientation in the melt also adds to the strength of the melt, which is useful in blow molding very large parts, an advantage explained in more detail in Chapter 13. 

         Both LDPE and HDPE are used in extruded pipe, but the HDPE pipe is generally used in higher-value, more critical applications such as pipe for high-pressure delivery of natural gas. In any pipe application using PE, the joining of pipe to fittings must be accomplished by some method other than with solvent adhesives. The natural gas application requires special joining techniques and equipment that melt and press the pipe and fittings together. These special techniques ensure that no leaks occur. For low-pressure, noncritical water pipe and tubing applications where LDPE is used, mechanical joints can be used, although small leaks are common. (The pipe used in sprinkler systems, where pressures are relatively high but leaks cannot be tolerated, is made of PVC. The pipe used for drain, waste, and vents in houses where stiffness and high impact strength are needed are made of ABS. Both types are discussed later in this chapter.) 

         The optical properties of HDPE reflect the increased crystallinity. HDPE is less optically clear than LDPE, all other factors being equal. Hence, HDPE cannot be used when optical clarity is an important consideration. HDPE is used for packaging but is most often used for applications such as grocery bags, where visual clarity is unimportant and strength is at a premium. 

         HDPE has the disadvantage of increased brittleness compared to LDPE. For applications where the high strength of HDPE and high impact toughness are required, a very high molecular weight grade of HDPE has been produced. This material, called ultra high molecular weight polyethylene (UHMWPE), is a subgroup of HDPE, since it is made by a similar process. UHMWPE will typically have molecular weights in the range of 3 million to 6 million versus typical HDPE molecular weights of 50,000 to 300,000. UHMWPE is less widely used than the other types of PE even though it has higher impact toughness and abrasion resistance. Because of these properties UHMWPE is used for liners in coal cars, guides in mechanical equipment where rubbing is expected, such as in gears, and for prosthetic devices. The density of UHMWPE is generally slightly higher than conventional HDPE. Therefore, the material is even higher in solvent resistance and lower in permeability than HDPE and this has led to some unique applications in the chemical industry. 

         The major problem with UHMWPE is the difficulty of melting the material. The molecular weight is so high that decomposition will often occur before melting. Hence, the material cannot be processed in traditional plastic molding equipment. It is generally sintered (a process in which a powdered material is packed into a mold, heated to just below the melt temperature, and held for an extended period under pressure; the powder particles fuse together and take the shape of the mold). Sintering is not practical for producing complicated parts and so the shapes obtainable in UHMWPE are limited.

7.2.3. Linear Low-Density Polyethylene (LLDPE)

        A third type of PE, linear low-density polyethylene (LLDPE), is made by a low-pressure catalyst process similar to the HDPE process, but which produces longer and more branches. LLDPE typically has 16 to 35 branches per 1000 backbone carbons, whereas HDPE typically has 1 to 2 branches per 1000 backbone carbons. This branching in LLDPE is sufficient to prevent close packing of the molecules. Therefore, LLOPE has a low density like LDPE but a linear structure much like HDPE. 

        The side chains are made by adding another monomer (called a comonomer) to the ethylene monomer during the polymerization process, with an appropriate catalyst. The comonomer must contain a carbon-carbon double bond and then a few (two, four, or six) additional carbons. (Organic molecules of this type are called olefins, where the a indicates that the double bond is between the first and second carbons.) The additional carbons become the side chains and are two, four, or six carbons long, depending on the comonomer used (butene, hexene, or octene). Longer side chains generally give improved physical properties because of increased chain entanglement and stronger secondary bonding. The number of side chains is determined by the concentration of the comonomer relative to the amount of ethylene and is typically 8% to 10% in most commercial grades. 

        Although technically a random copolymer (because ethylene and the comonomer are polymerized together), LLDPE is conventionally referred to as a homopolymer because its chemical and physical properties are so similar to the other PE homopolymers (LOPE and HDPE). Commonly, the term copolymer, when associated with PE, refers to copolymers with significantly different properties from homopolymer PE. These are discussed later in this chapter. 

        The LLDPE process has proven to be less expensive than that used to make conventional LDPE. Since the introduction of the LLOPE process in the late 1970s, all new plants constructed to make low-density polyethylene have used this technology rather than the high-pressure/high-temperature process used to make LDPE. The advantages of the LLDPE process are shown in Table 7.3. 

Table 7.3 Comparison of High-Pressure and Low-Pressure Processes for Making Low-Density Polyethylene.

        The molecular interactions between LLDPE molecules are different from those of either LDPE or HDPE and yet are related to both. The effects of lack of crystallinity and density are obviously similar in LLDPE and LDPE, and the linear shape of the LLDPE molecule is similar to the shape of HDPE molecules. These similarities and differences result in LLOPE properties that are generally between the properties of LDPE and HDPE. For instance, the strength of LLDPE is about 15% higher than LDPE. The stiffness of LLDPE can be as much as 25% greater than LDPE, and impact toughness is about 10% higher in LLDPE over LDPE. These property differences can often result in about a 25% reduction in the weight of plastic used when changing an application from LDPE to LLDPE. The liabilities of LLDPE compared to LDPE include the following: melt processing temperature about 20°F higher, shrinkage about 8% greater, less clarity (optically), less flexibility, and higher densities caused by its higher melt index (in LDPE the melt index and density are more independent of each other). 

        An LLDPE material that is occasionally identified as a separate product is ultra low-density polyethylene (ULDPE). This material has a density range of 0.880 to 0.915 g/cm³ and is made by using only the longer comonomers, such as 1-octene, and adjusting the polymerization conditions so that crystallinity is very low. These materials are very flexible yet have good tear strength. Their heat sealability is excellent. Applications would include food packaging, shrink wrap, heavy-duty film, and heat-seal layers. ULDPE can also be blended with other polymers, such as polypropylene and HDPE, to improve tear and impact toughness.

7.2.4. Relationship Between Density and Molecular Weight in Polyethylene

        When purchasing or designing with a PE material, both the density and the molecular weight (or the melt index) are generally specified. The relationship between molecular weight and density is complex. Their effects are interrelated for some polymer properties and are nearly independent of each other for other properties. These relationships were previously discussed in Chapter 3, but a review here is appropriate because molecular weight and density are so important in understanding the properties of PE. 

        A graph illustrating the relationships between crystallinity and molecular weight and density is presented in Figure 7.5, which shows the entire range of molecular weights for PE-like materials, from liquids (oils), through greases, waxes, and finally polymers. Each type of material has a range of variation of crystallinity with molecular weight. Above molecular weights of 10,000, the materials would be considered polymers. In that region, the soft materials are identified as having relatively low crystallinity (LDPE). The linear materials (HDPE) are also shown. Note that the combination of high crystallinity and low molecular weight results in brittle materials. If the molecular weight is increased, the materials become stiff and tougher. Increasing density will increase hardness and abrasion resistance. The increase in crystallinity with increases in density is, of course, expected. The overall trend in the polymer region of Figure 7.5 is that increases in molecular weight result in increases in crystallinity. Hence, in a general sense, crystallinity and density increase with molecular weight. These properties are therefore not totally independent, although, as the graph shows, there is some variation in this relationship.

Figure 7.5 Relationships between crystallinity and molecular weight and density for polyethylene. 

        Another important and related characteristic of PE that affects properties is molecular weight distribution (MWD). The major effect of MWD is in processing. When the MWO is narrow, melting occurs over only a few degrees (called a sharp melting point), and the resulting viscosity is generally lower than for a material with an equivalent molecular weight and a broad MWD. This sharp melting point and low viscosity is ideal for injection molding. In contrast, broad MWD materials melt over a wide range of temperatures. The low molecular weight materials melt first and act as lubricants for the higher molecular weight molecules, facilitating their flow. Wide MWD materials will have higher viscosity in the melt because of the presence of high molecular weight molecules that are only partially melted. These high molecular weight molecules are still entangled and they give strength to the melted polymer mass. Hence, broad MWD polymers have high melt strength, a property of value in processes where the melt must retain its shape, such as extrusion, blow molding, and thermoforming. As the MWO broadens, the toughness of the material decreases, the ESCR increases, part shrinkage decreases, and the tendency to warp decreases. All of these changes in properties result from the ability of the low molecular weight polymers to move into the spaces (amorphous regions) of the longer materials and reduce the effects of having only long polymers.

7.2.5. Crosslinked Polyethylene

        Some applications have taken advantage of the crosslinking capability of PE. This crosslinking can be done either by electron irradiation or by chemical means. When done by electron irradiation, the bonds are formed by passing the molded material through a chamber where high-energy electrons are created and accelerated through the material. When the high-energy electrons hit the PE molecules, some of the carbon-hydrogen and carbon-carbon bonds are broken. The breaks create free radicals in much the same way as high temperatures created free radicals in the branching of LDPE. In electron irradiation, the free radicals are more likely to react with another free radical in the vicinity, because the polymer chains are already fully formed and somewhat restricted in their movement (in contrast to the branching phenomena in LDPE, where the free radicals are created during the polymerization process and the molecules are still highly mobile). This tendency to react locally will often result in a rearrangement of the bonds between atoms, which can cause crosslinks to form between nearby polymer molecules. As a result, the PE is converted from a thermoplastic to a thermoset material. Electron irradiation is more effective in amorphous regions than in crystalline regions, because in crystalline materials the structures are so rigid that electron penetration is more difficult and when a free radical is formed, less rearrangement is likely to occur. Hence, highly amorphous LDPE is the material that is most commonly crosslinked by irradiation. (Further discussion of this process is given in Chapter 20.) 

        In a chemical crosslinking process, a peroxide and a bridge molecule are used to initiate the addition polymerization reaction. This bridge molecule, or chemical crosslinker, has the capability of forming multiple free radicals on the ends of a large organic molecule. (Triallylcyanurate is one of the most common of these chemical crosslinkers.) The peroxides begin several polymerization chains that are all joined together by the multiended bridge molecule and are, therefore, crosslinked. This method is effective in both LDPE and HDPE. 

        The crosslinking of PE by either electron beam bombardment or by chemical methods results in a polymeric material that cannot be melted prior to its decomposition. It becomes, therefore, a thermoset. However, the number of crosslinks formed is usually not sufficient to change the fundamental nature of the material, except in some selected properties. In most cases, crosslinked and non-crosslinked PE are difficult to distinguish without subjecting the material to testing for those specifically modified properties (discussed later), which often requires the use of sophisticated equipment. However, a simple method of distinguishing crosslinked from non-crosslinked PE is to place a sample of the material in question on a hot plate. Both materials will soften and become tacky as they heat. As the temperature continues to increase, the non-crosslinked material will eventually melt into a liquid. The crosslinked material will continue to soften but will never completely liquify because in the crosslinked sample, although some of the polymer chains may not have been linked to other chains and will liquify, the bulk of the material will not liquify. Eventually the bulk of the non-crosslinked material will char. 

        Some properties are, of course, strongly affected by crosslinking. Both electron and chemical crosslinking can increase significantly impact toughness and the environmental stress crack resistance of PE. Applications in which impact toughness is important include trash containers and storage tanks, especially those where cold weather is expected or very harsh chemicals are stored. Wire and cable coating is an application in which the improved environmental stress crack resistance of crosslinked PE is important. To be effective, the crosslinking should usually link over 80% of the molecules (this level is determined by solvent extraction). 

        Another application for crosslinked PE is shrink tubing. This application relies upon the memory of the material when the crosslinks are formed. The memory of the material reflects the fact that when the crosslinks are formed they are in a stable energy state. If the material is subsequently distorted, the internal energy of the system is raised and the system becomes less stable. The material has a strong internal energy that will favor a return to the original, lower-energy shape. This contributes to elasticity in plastics. Therefore, if the material is first formed into a tube and then crosslinked, the energy of the crosslinked material will be set at a level that favors returning to the original form. If the material is subsequently heated, expanded, and then cooled in the expanded form, the internal energy of the system will be higher. (This is like an elastic band that is stretched and then not allowed to spring back.) If the material is later heated sufficiently to allow the molecules to move, they will return to their shape when first crosslinked. In other words, the tubing that is crosslinked in one size and then expanded will shrink to its original, crosslinked size when heated. Additional information about radiation processing of polymers will be discussed in Chapter 20.