Effective Blending of Ultrahigh Molecular Weight Polyethylene With High Density Polyethylene Achieved Via Solid-State Shear Pulverization

Effective Blending of Ultrahigh Molecular Weight Polyethylene with High Density Polyethylene Achieved via

Solid-State Shear Pulverization

In comparison with conventional polyolefins, ultrahigh molecular weight polyethylene (UHMWPE) possesses outstanding mechanical properties, including impact strength, making it highly desirable for medical (e.g., joint prosthesis), industrial (e.g., lead acid battery separators), and defense (e.g., ballistic cloth for bullet-proof vests), applications. Unfortunately, UHMWPE comes with a downside: an ultrahigh melt viscosity that renders common melt processes useless for making products from UHMWPE. For this reason UHMWPE is often processed by ram extrusion, compression molding, or sintering. Attempts have been made to improve the processability of UHMWPE by blending with lower molecular weight polymers such as polyethylene (PE), polypropylene (PP), and polyethylene glycol. In addition to improving UHMWPE processability, technological interest in these blends is also associated with the potential to improve impact properties and crack resistance of the lower molecular weight polymers. Efforts to prepare these blends via conventional melt mixing have been unsuccessful because of the enormous viscosity mismatch and have led to the formation of poor blend morphologies (i.e., large suspensions of UHMWPE particles within the PE matrix). Scientific interest in these blends, as well as their sister-systems (i.e., ultrahigh molecular weight polypropylene (UHMWPP)/PP blends and UHMWPE/PP blends) is largely associated with the study of flow-induced crystallization; for these studies blends are prepared by solution blending which provides effective blending while utilizing copious amounts of solvent.

The primary challenge associated with blending UHMWPE/PE is the development of an industrially scalable method of effectively mixing the blend components. Conventional melt processing cannot be used to achieve intimate mixing of UHMWPE and PE (because of the vast viscosity mismatch between blend components) while solution processing is not industrially viable (because of the use of large quantities of solvent). In the present work we demonstrate our ability to achieve effective mixing of UHMWPE and PE by taking advantage of one of the major benefits of solid-state shear pulverization (SSSP), i.e., its exceptional utility in the achievement of intimate mixing free from kinetic limitations.

Solid-state shear pulverization (SSSP) is a novel processing method, which utilizes a modified twin-screw extruder that operates under near-ambient temperature conditions. During SSSP, polymers are processed in their solid state using high compressional and shear forces that cause repeated fragmentation and fusion of the material. Because polymers are processed under near-ambient temperature conditions during SSSP—keeping all material in the solid state—we avoid limitations imposed by the vast viscosity mismatch; this is particularly important and advantageous as we attempt to blend UHMWPE with HDPE. The uniqueness of SSSP has been exploited in some other systems where processing via SSSP provides an advantageous edge over conventional melt processing; the following are examples of previous SSSP studies:

1.     Polymer Blends – Using SSSP, intimate mixing and compatibilization has been achieved for immiscible blends resulting in blends with superior material properties as compared to their melt processed counterparts.

2.     Nanocomposites – Solid-state shear pulverization has been used to achieve effective dispersion and exfoliation in polymer nanocomposites; such nanocomposites prepared via SSSP possess superior material properties as compared to their melt processed counterparts.

3.     Block Copolymer Formation – As a result of mechanochemistry that occurs during SSSP, block copolymer formation, which results from interpolymer radical coupling has, been observed.

4.     Polar Functionalization of Polyolefins – By taking advantage of unique radical chemistries associated with the near-ambient temperature processing conditions utilized during SSSP polar functionalization of polyolefins (e.g., polypropylene) can be achieved with dramatically reduced molecular weight reduction.

Here, we present a novel method of achieving effective blending up to 50 wt% UHMWPE with HDPE via SSSP. Using oscillatory shear rheology, differential scanning calorimetry (DSC), and tensile testing we analyze the effect of UHMWPE on the material properties of the blends prepared via SSSP. Rheology and DSC data indicates that the degree of mixing between UHMWPE and HDPE domains can be increased dramatically with subsequent passes of SSSP and single screw extrusion. Additionally, all blends (including the 50/50 wt% UHMWPE/HDPE blend) demonstrate low viscosities at high frequencies (or shear rates), which strongly suggest that SSSP blends can be easily processed by post SSSP, melt extrusion. This is further supported by the fact that SSSP blends are easily extruded by single screw extrusion. Finally, we demonstrate that UHMWPE/HDPE blends prepared via SSSP result in dramatic increases in impact strength; for example, for the 50/50 wt% UHMWPE/HDPE blend the impact strength increases by almost 600 % (relative to the neat HDPE from which the blends were made) after the second pass of SSSP.