(819d) Effective Blending of Ultrahigh Molecular Weight Polyethylene With High Density Polyethylene Achieved Via Solid-State Shear Pulverization

Authors: 
Diop, M. F., Northwestern University
Torkelson, J. M., Northwestern University



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.