Analysis of Processing Characteristics of Ultra-High Molecular Weight Polyethylene
          Ultra-High Molecular Weight Polyethylene (UHMWPE), with its excellent performance and wide range of uses, has garnered significant attention both domestically and internationally, becoming a hotspot for research and application. This article focuses on discussing the unique processing properties and phenomena encountered during the processing of UHMWPE.
1 Characteristics of Powder Raw Material
    Currently, the UHMWPE resin raw materials available on the market are all in powder form. For example, the UHMWPE powder produced by the Beijing Auxiliary Agent No.2 Factory has a particle size of approximately 60 mesh. Due to the material's low coefficient of friction, the powder is very loose and extremely fluid. At room temperature, the powder has virtually no elasticity. When the powder is added to a pressing mold and compressed at room temperature, there is no significant change in volume, indicating that the powder is essentially incompressible at room temperature. These characteristics can cause feeding issues during extrusion and injection, but they have special significance in processes such as pressing, pressing sintering, and transfer molding, especially when molding thick or deep-cavity products. The looseness and fluidity of the powder are crucial for the material to fill the cavity uniformly, forming a uniform high pressure in the mold, thereby ensuring uniform product density and reducing melt flow and orientation in the mold to minimize internal stress and shrinkage deformation in the product.
    UHMWPE melts rapidly at temperatures above 180°C, but within the temperature range of 140°C to 180°C, the material takes a long time to melt. In normal production, material melting is not observable in this temperature range. However, within this temperature range, the powder exhibits elasticity, and its looseness and fluidity decrease. Therefore, it is necessary to apply pressure early after feeding during pressing to ensure that the material quickly fills the cavity at low temperatures and establishes a uniform high pressure inside the cavity, ultimately reducing internal stress and ensuring uniform product density. Based on the above considerations, we believe that the raw materials should not be preheated before pressing.
2. The "Material Plugging" Problem
Due to the design of the feeding section of screws in traditional single-screw extruders and injection molding machines being based on solid conveying mechanisms, solid materials are propelled forward by the friction between the solid plug and the inner surface of the barrel. When such equipment is used to process UHMWPE, the friction coefficient is too small to generate sufficient friction to propel the solid material. As a result, the material can only cling to the screw, rotating with it but not advancing forward. Therefore, there are currently two main types of extruders and injection molding machines used for processing UHMWPE both domestically and internationally: one is the reciprocating plunger-type extruder and injection molding machine, and the other is a special screw-type extruder and injection molding machine dedicated to UHMWPE. The screw of this extruder and injection molding machine is specially designed to accommodate the characteristics of UHMWPE, and the barrel has also undergone necessary modifications.
UHMWPE remains in an elastic solid particle state similar to high-hardness powder rubber at temperatures between 130°C and 180°C for a short period. Based on this characteristic, we propose a "rubber plug conveying" mechanism, where the material in the screw feeding section's flight is compressed into an elastic rubber plug rather than a compacted solid plug. By adding a small amount of liquid additive, UHMWPE exhibits significant elastic behavior (or rubber-like properties) within a certain temperature range, significantly increasing the friction coefficient between the rubber plug and the barrel. This solves the problem of difficulty in solid conveying due to the low friction coefficient between the solid plug and the barrel. Based on this mechanism, we have achieved extrusion of UHMWPE using a standard single-screw extruder and injection molding using a standard injection molding machine.
3. Melt Characteristics
Above 180°C, UHMWPE (Ultra-High Molecular Weight Polyethylene) melt exhibits extremely high viscosity, as well as pronounced viscous and elastic behavior. Within the temperature range of 260°C to 340°C, as the temperature rises, the melt viscosity decreases more rapidly, enhancing flowability, while the elasticity of the melt significantly diminishes. This change is primarily due to macromolecular degradation and should not be considered a normal variation of melt viscosity with temperature. When UHMWPE melts, it does so according to the melting of its powder particles, forming melt particles that, without sufficient external force, will not flow or move. However, due to the high surface tackiness of these melt particles, they will adhere to each other. The properties exploited in pressing and sintering processes rely on this characteristic. Owing to the high viscosity and elasticity of the melt particles, they typically do not deform significantly without the application of external force. Once force is applied, the amount of deformation directly correlates with the magnitude of the applied pressure; that is, the level of pressure determines the density of the product. Therefore, pressing typically requires high forming pressures to achieve dense products. When sufficient pressure is applied, the melt particles deform and slide, eliminating the gaps between them, and all particles completely adhere together to form a cohesive whole, resulting in a product that is essentially transparent. It should be noted, however, that even then, the product may still exhibit flashing, with thicknesses as small as 0.01mm.
The described properties of melt particles are practically valuable in the pressing of filter plates with microporous (through-hole) structures. By applying varying forming pressures to control the extent of melt particle deformation, UHMWPE microporous filter plates with different pore sizes can be produced.
The high elasticity of the melt is also manifested in the following ways: during pressing, if the pressure is released before the product is fully cured, the mold will automatically spring open by a certain gap; in injection molding, if the product is not sufficiently cooled, it will show noticeable expansion in the thickness direction after ejection. Utilizing the elasticity of the melt, or its compressibility, higher forming pressures can be used during pressing or injection molding to reduce the shrinkage of the product.
The volume of UHMWPE melt particles significantly increases compared to the volume of the powder particles at room temperature. This phenomenon is closely related to the entanglement of macromolecular chains and is the reason for the high elasticity and compressibility of the melt. Consequently, this inevitably leads to a larger shrinkage rate for UHMWPE, which can be as high as 5%.
When UHMWPE is pressed or extruded using a plunger extruder, since the formation of the product involves the deformation and adhesion of UHMWPE melt particles, it can be assumed that there is no or virtually no disentanglement of macromolecules. At this time, there is no molecular orientation within the product, resulting in an isotropic structure. However, due to the high shrinkage rate, after the product is ejected from the mold, structural asymmetry, uneven wall thickness, or non-uniform cooling can lead to warping and internal stress within the product. Practical experience shows that under these circumstances, the internal stresses in the plastic part are not significant and can be eliminated in a relatively short time, without affecting the long-term use of the product. During cooling, by clamping and cooling the product, its deformation can be corrected. Due to the clamping force, the UHMWPE particles in the stressed regions of the product undergo inhomogeneous elastic recovery and shrinkage that correspond to the stress. Once the product has cooled, the inhomogeneous elastic recovery and shrinkage of the UHMWPE particles in the original stress regions are "frozen," essentially eliminating the internal stresses.
4. Macromolecule Untangling
Under low pressure, low speed, and low shear conditions, the flow of UHMWPE melt primarily manifests as the translational movement of melt particles, macroscopically appearing as plug flow. As flow pressure and velocity increase, shear forces are generated within and on the surface of the melt particles due to their deformation, adhesion between particle surfaces, and the dragging effect created by the sliding of melt particles at their interfaces. This leads to the elongation of melt particles, and the tangled macromolecules of UHMWPE stretch and elongate under shear, exhibiting slight untangling and orientation characteristics. With further increases in flow pressure and velocity, the shear effect intensifies, significantly untangling the macromolecules of the melt particles under strong shear. This results in shear flow characteristics for the melt, with pronounced macromolecular orientation. However, it's important to note that the untangling of macromolecules at this stage is only partial, both in terms of degree and quantity. This can be evidenced by slow tensile tests conducted on pressed and injected dumbbell splines, where the elongation at break can reach up to 1700%. The untangled macromolecules of UHMWPE exhibit enhanced slipperiness. Despite the low melt flow rate of UHMWPE, it can still be extruded and injection molded, enabling the production of large and complex products through injection molding. The preparation of UHMWPE fibers demands even higher slipperiness of the macromolecules, necessitating maximum untangling. The good slipperiness of the untangled UHMWPE macromolecules can be demonstrated through high-speed shearing of UHMWPE using a torque rheometer or co-rotating twin-screw extruder, evident in changes in torque or current.
The melt strength of UHMWPE is closely related to the untangling of macromolecules. When there is no untangling or only slight untangling, the UHMWPE melt is formed by the adhesion of melt particles. Due to the relatively small adhesive force between the melt particles and the limited deformation of the melt particles, the melt strength is very low, making melt fracture extremely likely. As the degree of macromolecule untangling increases, the adhesion between melt particles gradually transforms into intermolecular entanglement and the stretching and straightening of macromolecules, significantly increasing the melt strength. When the untangling reaches a certain level, UHMWPE possesses sufficient melt strength to meet the processing requirements of the product without experiencing melt fracture.
The UHMWPE melt without macromolecule untangling exhibits no memory effect during flow. However, as macromolecule untangling occurs and intensifies, molecular orientation and the degree of orientation increase, making the memory effect of melt flow gradually apparent. Simultaneously, there is a tendency for the internal stress of the final product to gradually increase. During injection molding, the untangled macromolecules become irregularly oriented within the product due to the flow of the melt filling the mold. The large shrinkage rate of UHMWPE and the irregular orientation of the macromolecules result in a complex internal stress situation within the molded part. By studying the stress cracks in the injection-molded parts, it was found that there are many irregularly oriented cracks on the front surface of the product, regular stratification in the thickness direction of the product, and irregular lamellar layers separated from the main body of the product in the area near the gate during packing.
When UHMWPE macromolecules exhibit significant untangling and orientation, their impact on the internal stress, post-shrinkage, and corresponding dimensional stability of the product is long-lasting. In some injection-molded products, large-scale cracking may suddenly appear after being stored for up to a year. We believe this is due to a gradual increase in internal stress that eventually exceeds the material's strength, leading to sudden fracture. However, the question arises as to why internal stress would gradually increase over time, given that in most plastic products, internal stress tends to decrease until it is eliminated. The reason may be related to the crystallinity of UHMWPE. In injection-molded products, it is impossible to have completely untangled and straightened UHMWPE macromolecules. Instead, the untangling of UHMWPE macromolecules in the product is likely to be partial and localized, with varying degrees of untangling and orientation among molecules. After the product cools, some stretched segments of the macromolecular chain tend to return to a crystalline state, leading to post-crystallization and post-shrinkage issues. This phenomenon is similar to the post-shrinkage observed in HDPE products, which are generally considered to stabilize in size within 48 hours or, at most, a week. In this context, the partial untangling of UHMWPE macromolecules and the presence of numerous entangled macromolecular chains determine that the post-shrinkage of UHMWPE products is a long-term process. Therefore, internal stress may gradually increase over time. On the other hand, this phenomenon also indicates that the untangling and orientation of UHMWPE macromolecules significantly improve the stress relaxation and creep resistance of UHMWPE materials.
The untangling and orientation of UHMWPE macromolecules, if properly utilized, can produce special effects on improving certain product properties. As the degree of UHMWPE molecular untangling increases, its fibrous characteristics gradually emerge. We know that UHMWPE fibers exhibit high tensile strength and cut resistance. For example, we once mixed UHMWPE with a molecular weight of 2 million and PP (T30S) in a 1:1 ratio at 200℃ for a long time. The resulting blend was a rigid material with a hardness comparable to that of MC nylon.
5.Oxidation and Degradation
UHMWPE is highly susceptible to oxidation and degradation during the molding process. When exposed to air at temperatures between 180℃ and 200℃, the melt quickly oxidizes and turns red. At temperatures above 240℃, oxidation occurs along with degradation, causing the color to darken or turn black. Within the temperature range of 260℃ to 340℃, UHMWPE can rapidly degrade even without exposure to air, and the degradation accelerates with increasing temperature. Degraded UHMWPE, even with a melt flow index comparable to LDPE, can still retain some of its original properties such as a low coefficient of friction and non-stickiness. Therefore, we believe that the degradation of UHMWPE may primarily involve the breakage of the macromolecular backbone, while the long branch chains remain largely unaffected. Although existing olefin polymer antioxidants and heat stabilizers provide some benefits, their effectiveness is limited. Coating UHMWPE particles with certain high-boiling liquid additives can effectively inhibit oxidation but has no effect on macromolecular degradation at high temperatures.
6. Conclusion
Due to the unique processing properties of UHMWPE, special attention should be paid when processing and modifying it.
It is essential to strengthen research on the microscopic structure and characteristics of UHMWPE molecules and develop specialized processing aids, especially stabilizers.