Development and Application of Ultra-High Molecular Weight Polyethylene 

Development and Application of Ultra-High Molecular Weight Polyethylene (UHMWPE)

1. Introduction

Ultra-high molecular weight polyethylene (UHMWPE) is a linear-structured thermoplastic engineering plastic with excellent comprehensive performance. The world's first industrialization was achieved by Allied Chemical in the United States in 1957, followed by Hoechst in Germany, Hercules in the United States, and Mitsui Petrochemical in Japan. China successfully developed and industrialized UHMWPE in 1964. 

The development of UHMWPE has been rapid. Before the 1980s, the global average annual growth rate was 8.5%, but after the 1980s, the growth rate increased to 15% to 20%, and in China, it exceeded 30%. In 1978, the global consumption was between 12,000 to 12,500 tons, but by 1990, the global demand reached approximately 50,000 tons, with the United States accounting for 70%.

UHMWPE, with an average molecular weight of about 3.5 to 8 million, exhibits superior properties such as impact resistance, wear resistance, self-lubrication, and chemical corrosion resistance due to its high molecular weight. Additionally, UHMWPE has excellent low-temperature performance, retaining high impact strength at -40°C and remaining usable at -269°C.

Due to its excellent physical and mechanical properties, UHMWPE is widely used in machinery, transportation, textiles, paper-making, mining, agriculture, chemical industry, and sports equipment, with applications in large packaging containers and pipelines being the most prevalent. Furthermore, its excellent physiological inertness allows its use in clinical medicine as heart valves, orthopedic parts, and artificial joints.

2. Molding and Processing of UHMWPE

Due to the high viscosity (108 Pa·s) of UHMWPE in its molten state and its poor fluidity, making it difficult to process using conventional mechanical methods. Recently, the self-developed UHMWPE processing and molding technology by Shandong Beckwell Advanced Materials Co., Ltd.  has advanced rapidly, evolving from the initial press-sintering-processing production process to direct continuous molding methods.

2.1 General Processing Techniques

(1) Press Sintering: This is the original method of processing UHMWPE, which has low production efficiency and a tendency for oxidation and degradation. Direct electric heating methods can improve efficiency. Werner and Pfleiderer developed a high-speed fusion processing method using a blade mixer, where the blades' maximum speed of 150 m/s heats the material to processing temperature within seconds.

(2) Extrusion Molding: The main equipment includes plunger extruders, single-screw extruders, and twin-screw extruders. Initially, plunger extruders were used in the 1960s, but by the mid-1970s, single-screw extrusion processes were developed in Japan, the US, and Germany. Mitsui Petrochemical succeeded in rod extrusion technology in 1974, and China developed the Φ45-type single-screw extruder for UHMWPE by the end of 1994 and achieved industrial production with the Φ65-type single-screw extrusion line by 1997.

(3) Injection Molding: Mitsui Petrochemical developed injection molding technology in 1974, commercialized it in 1976, and later developed reciprocating screw injection molding technology. In 1985, Hoechst also realized screw injection molding for UHMWPE. In 1983, China modified the XS-ZY-125A injection machine to successfully produce UHMWPE support rollers for beer packaging lines and shaft sleeves for water pumps, achieving medical artificial joints in 1985.

(4) Blow Molding: UHMWPE's property of retracting after being extruded from the die without sagging makes it suitable for blow molding hollow containers, particularly large containers like fuel tanks and barrels. Blow molding UHMWPE can result in films with balanced strength in both directions, addressing the long-standing issue of strength disparity in HDPE films.

2.2 Special Processing Techniques

2.2.1 Gel Spinning: The gel spinning-ultradrawing technique emerged in the late 1970s to produce high-strength, high-modulus polyethylene fibers. DSM first applied for a patent in 1979, followed by industrial production in the US and Japan. China Textile University began researching this project in 1985, developing high-performance UHMWPE fibers.

2.2.2 Lubricated Extrusion (Injection): This technique forms a lubricating layer between the material and the mold wall to reduce shear rate differences and deformation while enhancing extrusion (injection) speed at low temperatures and energy consumption. There are self-lubricating and co-lubricating methods.

(1) Self-Lubricating Extrusion (Injection): Adding external lubricants like fatty acids, silicones, waxes, and low molecular weight resins to UHMWPE can reduce friction and improve flow uniformity and mold release.

(2) Co-Lubricating Extrusion (Injection): Methods include injecting lubricants into the mold or blending with low viscosity resins to improve processing flow.

2.2.3 Roll Molding: A solid-state processing method applying high pressure below the melting point of UHMWPE, effectively fusing particles through deformation. The main equipment includes a rotating wheel with a spiral groove and a sliding block with a tongue groove.

2.2.4 Post-Heat Treatment Compression Molding: Short-term heating of UHMWPE powder at 140-275°C for 1-30 minutes can significantly improve physical properties, resulting in products with better transparency, surface smoothness, and low-temperature mechanical performance compared to untreated UHMWPE.

2.2.5 Radio Frequency Processing: This new method mixes UHMWPE powder with high dielectric loss carbon black, then irradiates with radio frequency to generate heat, softening the UHMWPE surface for consolidation under pressure. This method can efficiently mold thick, large parts.

2.2.6 Gel Extrusion for Porous Membranes: Dissolving UHMWPE in a volatile solvent, then extruding and cooling to form a gel membrane, can produce porous membranes with applications in waterproof fabrics, filtration, and battery separators.

3. Modification of UHMWPE

3.1 Improvement of Physical and Mechanical Properties: UHMWPE has drawbacks like low surface hardness, thermal deformation temperature, flexural strength, and creep resistance compared to other engineering plastics, which can be improved through filling and crosslinking.

3.1.1 Filling Modification: Adding fillers like glass beads, glass fibers, mica, talc, silicon dioxide, aluminum oxide, molybdenum disulfide, and carbon black can enhance hardness, stiffness, creep resistance, and thermal deformation temperature. Surface treatment with coupling agents can further improve results.

3.2.1 Crosslinking

Crosslinking aims to enhance morphological stability, creep resistance, and environmental stress cracking resistance. By crosslinking, the crystallinity of ultra-high molecular weight polyethylene (UHMWPE) decreases, revealing its hidden toughness. Crosslinking can be categorized into chemical crosslinking and radiation crosslinking. Chemical crosslinking occurs when an appropriate crosslinking agent is added to UHMWPE, causing crosslinking during the melting process. Radiation crosslinking uses electron beams or gamma rays to irradiate UHMWPE products, inducing molecular crosslinking. Chemical crosslinking of UHMWPE can be further divided into peroxide crosslinking and coupling agent crosslinking.

(1) Peroxide Crosslinking

The peroxide crosslinking process involves three steps: blending, molding, and crosslinking. During blending, UHMWPE is melt-mixed with peroxide, generating free radicals under the action of peroxide, which couple to form crosslinks. The temperature must be controlled to prevent complete crosslinking of the resin. The blended UHMWPE with a low degree of crosslinking is then molded at a higher temperature and further crosslinked.

After peroxide crosslinking, UHMWPE has a unique structure, distinct from thermoplastics, thermosets, and vulcanized rubber. It possesses a three-dimensional network structure without complete crosslinking, combining the characteristics of all three, such as thermoplasticity, excellent hardness, toughness, and stress-cracking resistance.

While foreign literature reports the use of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane-3 as a crosslinking agent, it is scarce domestically. Tsinghua University researched using the inexpensive and readily available dicumyl peroxide (DCP) as a crosslinking agent. They found that a DCP content of less than 1% could increase the impact strength of pure UHMWPE by 15-20%, with an optimal content of 0.25% raising the impact strength by 48%. Increased DCP content also improved the heat deformation temperature, making it suitable for heat-resistant piping in heating systems.

(2) Coupling Agent Crosslinking

Two main types of silane coupling agents are used for UHMWPE: vinyl siloxane and allyl siloxane, with vinyl trimethoxysilane and vinyl triethoxysilane being common. Peroxides, usually DCP, initiate the coupling process, with organic tin derivatives serving as catalysts.

The silane crosslinking of UHMWPE involves the thermal decomposition of peroxide into highly active free radicals, which abstract hydrogen atoms from polymer molecules, turning the polymer backbone into reactive free radicals. These radicals then graft with silane. The grafted UHMWPE undergoes hydrolysis condensation under the action of water and silanol condensation catalysts, forming crosslinked bonds, resulting in silane-crosslinked UHMWPE.

(3) Radiation Crosslinking

Under specific doses of electron beams or gamma rays, the main or side chains of UHMWPE may be cleaved, generating a certain amount of free radicals. These free radicals combine to form crosslinked chains, transforming UHMWPE from a linear molecular structure into a network macromolecular structure. Radiation improves UHMWPE's physical properties, such as creep resistance, oil resistance, and hardness.

Gamma ray irradiation of artificial UHMWPE joints sterilizes while crosslinking them, enhancing the hardness, hydrophilicity, and creep resistance, thereby extending their service life. Studies have shown that combining irradiation with PTFE grafting can also improve UHMWPE's wear and creep behavior, making it suitable for in vivo implants due to its tissue compatibility.

3.2 Improvements in Processing Performance

The long molecular chains of UHMWPE make them prone to breaking under shear force or degrading under heat. Thus, lower processing temperatures, shorter processing times, and reduced shear are essential.

To address UHMWPE's processing challenges, besides specially designing conventional molding machinery, resin formulations can be improved by blending with other resins or adding flow modifiers, enabling processing on standard extruders and injection molding machines. This method is known as lubricated extrusion (injection) described in 2.2.2.

3.2.1 Blending Modification

Blending is the most effective, simple, and practical method to improve the melt flow of UHMWPE. Patents commonly cover these techniques, involving blending UHMWPE with low-melting, low-viscosity resins like LDPE, HDPE, PP, and polyester. Medium molecular weight PE (molecular weight 400,000-600,000) and low molecular weight PE (molecular weight < 400,000) are frequently used. When the blend system is heated above its melting point, UHMWPE resin suspends in the liquid phase of the second resin component, forming an extrudable and injectable suspension.

(1) Blending with Low and Medium Molecular Weight PE

Blending UHMWPE with low molecular weight LDPE (molecular weight 1,000-20,000, optimally 5,000-12,000) significantly improves processability but reduces mechanical properties like tensile strength and flexural modulus. HDPE also enhances UHMWPE’s processing flow but lowers impact strength and wear resistance. Adding nucleating agents like benzoic acid, benzoates, stearates, and adipates to the blend can maintain mechanical properties by increasing PE crystallinity and homogenizing spherulite size. Patents indicate that adding minute amounts of fine nucleating agents like wollastonite (particle size 5nm-50nm, surface area 100m2/g-400m2/g) to UHMWPE/HDPE blends compensates for mechanical performance loss effectively.

(2) Blending Morphology

Although chemically similar to other PE types, UHMWPE blends exhibit non-uniform morphology under typical melt-mixing conditions due to significant viscosity differences between components. UHMWPE/LDPE blends processed with standard single-screw extruders show separate crystallization, with UHMWPE acting as a filler in the LDPE matrix. Prolonged melt treatment and mixing in a two-roll mill slightly enhance inter-component interactions and properties but still fail to achieve co-crystallization.

Vadhar found that a two-step blending method, melting UHMWPE at high temperature first, then adding LLDPE at a lower temperature, could produce co-crystallized blends. Solution blending also achieved co-crystallized UHMWPE/LLDPE blends.

(3) Mechanical Strength of Blends

For UHMWPE/PE systems without nucleating agents, large spherulites with distinct interfaces form during cooling, introducing internal stress due to molecular chain misalignment, leading to cracks and lower tensile strength. Under impact, cracks propagate quickly along spherulite interfaces, reducing impact strength.

3.2.2 Flow Modifiers

Flow modifiers enhance polymer flow by promoting disentanglement of long chains and lubricating between macromolecules, improving polymer processability.

Flow modifiers for UHMWPE primarily include aliphatic hydrocarbons and derivatives, such as n-alkanes (carbon atoms > 22), paraffin from petroleum, and derivatives with terminal aliphatic hydrocarbon groups, internal functional groups (carboxyl, hydroxyl, ester, carbonyl, carbamoyl, mercapto), carbon atoms > 8 (optimal 12-50), and molecular weight 130-2000 (optimal 200-800). Examples include fatty acids like capric, lauric, myristic, palmitic, stearic, and oleic acids.

China has developed an effective flow modifier (MS2), which significantly improves UHMWPE’s flowability with minimal addition (0.6%-0.8%), lowering the melting point by 10℃, enabling injection molding on standard machines with only slight reductions in tensile strength.

Additionally, modifying UHMWPE with styrene and derivatives enhances processability, extrusion ease, and maintains excellent wear resistance and chemical corrosion resistance. Compounds like 1,1-diphenylethylene, styrene derivatives, and tetrahydronaphthalene confer superior processing performance, high impact strength, and wear resistance.

3.2.3 In Situ Composite Liquid Crystal Polymers

In situ composite liquid crystal polymers (TLCP) blended with thermoplastic resins enhance UHMWPE's processability. TLCP’s rigid molecular structure aligns under force, producing significant shear thinning and forming an oriented reinforcement phase in the base resin. This local fiber formation enhances thermoplastic resins and improves flowability.

Tsinghua University's Zhao Anchi et al. achieved significant improvements in UHMWPE processing performance using in situ composite technology, maintaining high tensile and impact strengths and increasing wear resistance.

3.3 Polymer-Filled Composites

Polymer-filling involves treating fillers to create active sites on particle surfaces, where ethylene, propylene, and other olefins polymerize during filling, forming resin-encased particles with unique properties. This method creates composite materials with superior dispersion and interface adhesion, retaining filler shapes and providing enhanced mechanical properties compared to traditional blends, particularly in high-filler content scenarios. These composites exhibit increased hardness, flexural strength, and modulus, suitable for bearings and load-bearing components, and improved thermodynamic properties, making them ideal for high-temperature applications.

Polymer-filled composites allow controlling UHMWPE molecular weight using hydrogen or chain transfer agents, facilitating processing. U.S. patents describe high-modulus composites made from UHMWPE with neutral surface fillers like hydrated alumina, silica, insoluble silicates, calcium carbonate, basic aluminum sodium carbonate, hydroxyapatite, and calcium phosphate. Additionally, Qilu Petrochemical Research Institute synthesized UHMWPE composites using diatomaceous earth and kaolin as fillers.

3.4 Self-Reinforcement of Ultra-High Molecular Weight Polyethylene (UHMWPE) [27, 28]

Incorporating UHMWPE fibers into a UHMWPE matrix creates a composite material with excellent mechanical properties due to the strong interfacial bonding and chemical compatibility between the matrix and fibers. Adding UHMWPE fibers significantly enhances the tensile strength, modulus, impact strength, and creep resistance of the composite. Compared to pure UHMWPE, incorporating UHMWPE fibers at a volume fraction of 60% increases the maximum stress and modulus by 160% and 60%, respectively. This self-reinforced UHMWPE material is particularly suitable for load-bearing biomedical applications, such as total joint replacements, which have gained significant attention in recent years. The low volumetric wear rate of self-reinforced UHMWPE materials can extend the lifespan of artificial joints.

4. Alloying of Ultra-High Molecular Weight Polyethylene (UHMWPE)

Besides forming alloys with other plastics to improve processing performance (see sections 3.2.1 and 3.2.3), UHMWPE can also achieve other enhanced properties through alloying, with PP/UHMWPE alloys being the most prominent.

Toughening polymers typically involve introducing flexible chain segments into the resin to form composites (e.g., rubber-plastic blends), with the toughening mechanism based on "multiple crazing." However, the toughening effect of UHMWPE on PP cannot be explained by the "multiple crazing" theory. In 1993, it was first reported in China that UHMWPE effectively toughened PP, increasing the notched impact strength of the blend by more than twice that of pure PP when UHMWPE content was 15% [29]. Recent reports indicate that blending UHMWPE with copolymer PP containing ethylene segments increases impact strength by more than double when UHMWPE content is 25% [30]. This phenomenon is explained by the "network toughening mechanism" [31].

In the PP/UHMWPE blend system, the submicroscopic phase state is a bicontinuous phase, with UHMWPE and PP long-chain molecules forming an interpenetrating network. The blend network serves as a skeleton, providing mechanical strength, while the PP network interweaves to form a "linear interpenetrating network." The blend network deforms under external impact to absorb energy, thereby toughening the material. The more complete and denser the network, the better the toughening effect.

To ensure the formation of a "linear interpenetrating network" structure, UHMWPE must disperse at a quasi-molecular level in the PP matrix, requiring advanced blending techniques. Research at Beijing University of Chemical Technology found that a quad-screw extruder can uniformly disperse UHMWPE in the PP matrix, while a twin-screw extruder performs poorly.

EPDM can compatibilize PP/UHMWPE alloys due to its main chain segments' affinity with both PP and UHMWPE, facilitating its dispersion at the interface. EPDM plays a role in insertion, segmentation, and refinement during co-crystallization, significantly improving toughness and notched impact strength.

Additionally, UHMWPE can form alloys with rubber, resulting in superior mechanical properties compared to pure rubber, such as wear resistance, tensile strength, and elongation at break. Rubber is vulcanized above the softening point of UHMWPE during blending.

5. Composites of Ultra-High Molecular Weight Polyethylene (UHMWPE)

UHMWPE can be vulcanized with various rubbers (or rubber-plastic alloys) to produce modified PE sheets, which can further be combined with metal sheets to form composites. UHMWPE can also be coated onto plastic surfaces to improve impact resistance.

Pressing uncured rubber sheets containing vulcanizing agents together with UHMWPE sheets above its softening point yields laminated products with significantly higher peel strength compared to those without vulcanizing agents. This method also effectively bonds uncured rubber-plastic alloys (e.g., EPDM/PA6, EPDM/PP, SBR/PE) with UHMWPE sheets.