Orthoplastics - Science making a material difference

The Effect of Manufacturing Conditions on the Properties of UHMWPE

N T Hubbard. P Allanson.

Introduction

UHMWPE has been used as the bearing surface in orthopaedic implants for about 30 years. During this time several polymers and manufacturing techniques along with a multitude of process conditions have been used to convert the polymer powder to a solid. This has led to a wide spread of material properties1,2 and consequently to variation in service life3 and even physical failure.
Due to the extreme molecular weight, UHMWPE has almost no melt flow and is therefore processed by sintering techniques. This work demonstrates the variations in physical properties that occur when the manufacturing conditions are changed.

Experimental

Due to the increasing popularity of compression moulding both in the bulk form and as semi-finished parts, the experimental work has been limited to this method but is also compared with established ram extruded data. The 5" X 4" X 2" thick test mouldings were produced in a purpose built laboratory press with ramp programmable fluid circulating heating (including mould) and ramp programmable pressure control. Both control systems utilise feedback PID controls for accuracy and repeatability. This replicates the capability of the production press. The conditions studied were temperature and time with an established production pressure cycle as the constant. All processing conditions used produced visually good mouldings. The maximum temperature was the known safe processing temperature. The two most popular polymers, Ticona GUR 1020 and GUR 1050 were used. All mouldings were tested to the requirements of ISO 5834/2 and the swell ratio to ASTM D2765. Morphological assessment was also carried out.

Results

In all cases there was a combined efffect of temperature and time.
Density. Temperature had the major effect. GUR 1020 was slightly affected but GUR 1050 failed the test at lower temperatures. Increasing temperature increased density.
Yield Strength. Increasing time and temperature both had a beneficial effect making a 5% improvement.
Ultimate Tensile Strength and Elongation. While the results had a wider spread, the trend was improvement with increasing time and temperature.

Swell Ratio. Lower temperature and shorter time gave higher swell ratio results.

Morphology

100 �m thick samples were examined under darkfield lighting and 150 times magnification to assess the internal structure. All of the processing conditions used had eliminated sharp grain boundaries and gross fusion defects. The degree of inter-particle bonding and optical uniformity varied with conditions. Samples at low temperatures and short cycle times were very mottled while samples at high temperature and long cycle times were clear. Ram extruded samples were similar to the short cycle compression mouldings.

Comparison with Ram Extrusion

Typical ram extrusions from the same polymer batch have lower mechanical strength than current optimised compression mouldings. The greatest difference is in the Swell Ratio which is halved by the longer residence time of compression moulding.

Discussion

Throughout this series of mouldings and the comparison with ram extrusion it is evident that process conditions have a predictable effect on the material properties. Ram extrusion with it's short residence time at temperature produces a material much like a fast compression moulding. With a longer processing time the polymer can reach a stable temperature through the thickness and the individual grains have time to knit together to form a homogeneous mass. Controlled slow cooling increases crystallinity and decreases stresses. The effect is most obvious in the reduction of swell ratio due to the additional cross-linking and chain entanglement for a specific polymer.

Conclusions

The properties of UHMWPE can be altered by variations in processing conditions.
Increasing the processing temperature to the safe limit is not sufficient to maximise the physical properties.
Long processing time is essential4 and improves inter particle bonding.
Ram extrusions have a low residence time at the processing temperature and therefore are similar to short time processed compression mouldings.

References

1. ASTM F648 - 96 X1.9
2. Duus et al. Trans. 42nd ORS p479
3. Bankson et al. Clinical Orthopaedics Number 317.
4. Bellare and Spector. Trans. 24th Soc.for Biomaterials.