Polyethylene

Abstract

Polyethylene (PE), despite having the simplest basic structure of any polymer (a repetition of CH₂ units), is the largest tonnage plastics material. The main attractive features of PE are its low price, excellent electrical insulation over a wide range of frequencies, very good chemical resistance, good processability, toughness, flexibility, and—in thin films of certain grades—transparency. Until the mid-1950s, all commercial PE was produced by high-pressure processes: these processes result in branched materials of moderate number average molar mass, M_w < 50 kg mol⁻¹, classified as low-density PE. The discovery that some metal compounds, based on Cr (Phillips catalyst) or Ti (Ziegler–Natta), are able to catalyze the polymerization of ethylene in less extreme conditions has allowed the controlled synthesis of a large varieties of PE architectures with very diverse properties. PE grades ranging from high-density to linear low-density, as well as ultra-high molar mass and other PE copolymers of commercial significance are discussed in this chapter.

10.5.6 Properties of Metallocene-catalysed Polyethylenes

Metallocene-catalysed polyethylenes exhibit the general characteristics of polyethylene as noted in the introductory paragraph of Section 10.5. Furthermore they are more like low density polyethylenes (LDPE and LLDPE) than HDPE. As with LLDPE they are usually copolymers containing small quantities of a low molecular weight α-olefin such as but-1-ene, hex-1-ene and oct-1-ene. The property differences largely arise from the narrow molecular weight distribution, the more uniform incorporation of the α-olefin and the low level of polymerisation residues (about one-tenth that of Ziegler–Natta catalysed LLDPE).

It is generally claimed that metallocene polyethylenes (often abbreviated to m-PE) exhibit superior mechanical and optical properties as well as better organoleptic properties (resulting from the lower residue levels). As an example m-LLDPE is particularly favoured as a stretch film for wrapping because of the good prestretchability, high puncture resistance and tear strength, all of which are claimed to be better than with conventional LLDPE.

As previously mentioned, narrow molecular weight distribution polymers such as m-PE are less pseudoplastic in their melt flow behaviour than conventional polyethylenes so that given an m-LLDPE and a conventional LLDPE of similar melt index (measured at low shear rates), the m-LLDPE will have a much higher melt viscosity at the high shear rates involved in film processing. The polymers are also more susceptible to melt fracture and sharkskin. This difference requires that such steps be taken as to use more highly powered extruders, to use special processing aids such as fluoroelastomers or to make compromises in the polymer structure which may, however, reduce the advantages of m-PE materials. One obvious approach would be to produce bi-, tri- or other polymodal blends (see the Appendix to Chapter 2 for explanations) to overcome the inherent disadvantages of narrow molecular weight distribution polymers. It is of interest that ‘bimodal’ polymers produced by a two-reactor system have become available which have enhanced resistance to cracking and are rapidly finding use in pipe applications.

Metallocene-catalysed very low density polyethylene (m-VLDPE) has become available with densities of as low as 0.903. This is of use for sealing layers of multi-layer films since sealing can commence at lower temperatures than with conventional materials such as LLDPE and EVA (see Section 11.6) with the polymer seal exhibiting both cold strength and hot tack strength.

16.2.1.3.1 Polyethylene

Oxygen barrier

Here the PE layers in the coextruded barrier film may be HDPE for maximum moisture barrier and to add stiffness to the laminate (see next section). Dow’s RecycleReady technology involves blending RETAIN functional polymer into the PE layers of the coextruded film (see Fig. 17.17). This ensures that the film is recyclable in the PE recycle stream.

Polyethylene

Polyethylene or polythene (PE), IUPAC name polyethene or poly(methylene), is the most common plastic. As of 2017, over 100 million tons of PE resins are produced annually, accounting for 34% of the total plastics market (Geyer et al, 2017). Many kinds of PE are known, with most having the chemical formula (C₂H₄)n. PE is usually a mixture of similar polymers of ethylene with various values of n.

PE is a thermoplastic; however, it can become a thermoset plastic when modified (such as cross-linked PE) (Plastics Europe, 2017). PE is classified by its density and branching. Its mechanical properties depend significantly on variables such as the extent and type of branching, the crystal structure, and the molecular weight.

There are several types of PE: Ultra high molecular weight polyethylene, Ultra low molecular weight polyethylene, High molecular weight polyethylene, High density polyethylene (HDPE), High density cross-linked polyethylene, Cross-linked polyethylene, Medium-density polyethylene, Linear low density polyethylene (LLDPE), Low density polyethylene (LDPE), Very low density polyethylene, Chlorinated polyethylene. As for sold volumes, the most important PE grades are HDPE, LLDPE, and LDPE. The degree of branching of the different types of PE can be schematically represented as in Fig. 1 (Kaiser, 2011; Panayotov et al., 2016).

Fig. 1. The figure shows polyethylene backbones, short-chain branches, and side chain branches. The polymer chains are represented linearly.

Reproduced from Kaiser, W., 2011. Kunststoffchemie für Ingenieure von der Synthese bis zur Anwendung, third ed. München: Hanser.

Medium-density polyethylene (MDPE)

MDPE is produced, as HDPE and LLDPE, by low-pressure polymerization techniques using Ziegler-Natta catalysts or metallocene catalysts. It is less opaque than HDPE but not as clear as LDPE, and it has better impact and environmental crack resistance than HDPE but is less rigid and hard. Like other polyolefins, it has excellent chemical resistance, is easy to process, and is low in cost.

It is less notch sensitive than HDPE and has better stress-cracking resistance. MDPE is produced on a much smaller scale than HDPE. It is typically used in gas pipes and fittings where enhanced resistance to environmental stress cracking is needed, garbage sacks, shrink film, packaging film, and carrier bags.

11.2.1 Polyethylene

1.3 What is Polyethylene?

Polyethylene is a polymer formed from ethylene (C₂H₄), which is a gas having a molecular weight of 28. The generic chemical formula for polyethylene is –(C₂H₄)_n–, where n is the degree of polymerization. A schematic of the chemical structures for ethylene and polyethylene are shown in Figure 1.4.

Figure 1.4. Schematic of the chemical structures of ethylene and polyethylene.

For an ultra-high molecular weight polyethylene, the molecular chain can consist of as many as 200,000 ethylene repeat units. Put another way, the molecular chain of UHMWPE contains up to 400,000 carbon atoms.

There are several kinds of polyethylene (LDPE, LLDPE, HDPE, UHMWPE) that are synthesized with different molecular weights and chain architectures. LDPE and LLDPE refer to low density polyethylene and linear low density polyethylene, respectively. These polyethylenes generally have branched and linear chain architectures, respectively, each with a molecular weight of typically less than 50,000 g/mol.

High Density Polyethylene (HDPE) is a linear polymer with a molecular weight of up to 200,000 g/mol. UHMWPE, in comparison, has a viscosity average molecular weight of 6,000,000 g/mol. In fact, the molecular weight is so “ultrahigh” that it cannot be measured directly by conventional means and must instead be inferred by its intrinsic viscosity (IV).

Perhaps more relevant from a clinical perspective, UHMWPE is significantly more abrasion- and wear-resistant than HDPE. The following wear data for UHMWPE and HDPE was collected using a contemporary, multidirectional hip simulator [3]. Based on hip simulator data, shown in Figure 1.5, the volumetric wear rate for HDPE is 4.3 times greater than that of UHMWPE.

Figure 1.5. Comparison of wear rates of HDPE and UHMWPE in a multidirectional hip simulator [3].

In the early 1960’s, UHMWPE was classified as a form of high-density polyethylene (HDPE) among members of the polymer industry [4]. Thus, Charnley’s earlier references to UHMWPE as HDPE are technically accurate for his time [5], but have contributed to some confusion over the years as to exactly what kinds of polyethylenes have been used clinically. By a close reading of Charnley’s works, it is clear that HDPE is used synonymously with RCH-1000, the trade name for UHMWPE produced by Hoechst in Germany [6]. With the exception of a small series of 22 patients who were implanted with silane-cross-linked HDPE at Wrighington [7], there is no evidence in literature that lower molecular weight polyethylenes have been used clinically.

5.3.6 Polyethylene Joining and Welding

Polyethylene can be welded by various techniques like vibration, ultrasonic, friction, hot gas, and hot plate welding. LDPE is easier to weld than HDPE. Most adhesives can be used with polyethylene. It is important to clean the surfaces well before applying the adhesives.

Ultrasonic welding of polyethylene is restricted to the near energy field. Radiofrequency welding is not suitable for polyethylenes as the energy generated does not adequately heat the polymer to the required temperatures. Thin sheets of polyethylene (<0.2 mm thick) can be welded with lower power CO₂ lasers. Nd:YAG lasers with lower absorption capabilities of the shorter wavelength can join materials with a thickness range between 0.2 to 2.0 mm. In order to adhesive bond polyethylene, the surface must be primed and cleaned before applying the adhesive. This is because polyethylene has a very low surface energy (30 dynes/cm) and has low “wettable” surface properties. Epoxies, polyurethanes and polyacrylates are the most effective adhesives for polyethylene.

23.18 Conclusions

The UHMWPE–HA biomaterials were designed to take advantage of natural synovial lubrication mechanisms. UHMWPE–HA is UHMWPE with a small amount of HA in the bulk that extends HA “roots” at the surface of the UHMWPE to which an optional HA surface coating can be applied. The HA-rich material is hydrophilic, lubricious, and well-hydrated. The UHMWPE–HA family of materials includes UHMWPE–HA with nonintentionally cross-linked UHMWPE, cross-linked UHMWPE–HA, and cross-linked compatibilized UHMWPE–HA. UHMWPE–HA wears considerably less than plain UHMWPE and cross-linked compatibilized UHMWPE–HA has a wear resistance on par with or better than UHWMPE having a similar level of cross-linking. Furthermore, after two million cycles of wear cross-linked compatibilized UHMWPE–HA surfaces look similar to unworn surfaces, indicating little to no UHMWPE wear. Preclinical studies indicate UHMWPE–HA is noncytotoxic and well-tolerated in the knee. Human clinical experience and follow-up has exceeded three years as of the writing of this chapter with excellent results. Publications of these data are planned in the future.