Project Details Structure, Processing and Performance of Ultra-High Molecular Weight Polyethylene

Project No.:
2010-019-1-400
Start Date:
01 May 2010
End Date:
Division Name:
Polymer Division
Division No.:
400

Objective

To develop improved methods for characterizing UHMWPE mouldings, and hence improve quality assurance procedures for hip and knee prostheses. Team members will address problems of monitoring changes in molecular weight, crystallinity, crystal morphology, interfacial reptation and entanglement during moulding, and the effects of these changes on strength and wear resistance.

Description

Compression mouldings made from UHMWPE (Mw > 1 MDa) are widely used in replacement hip and knee joints. In a minority of cases, their service lifetimes are drastically shortened by micro-cracking and wear. Problems are exacerbated by osteoclast cells, which remove bone debris, but not PE. Instead, they summon reinforcements, which attack living bone and cause arthritis. Good quality control is necessary to avoid these problems, but is difficult to achieve. Reactor powder has high crystallinity (~70%) and is intrinsically rigid and strong, but adverse changes occur during moulding. Ideally, processing should produce strong, void-free interfaces, with minimum loss of stiffness and strength. However, ultra-high molecular weights limit reptation and the formation of entanglement across interfaces, and heat treatment reduces crystallinity, thereby impairing mechanical properties. There are major problems in characterizing UHMWPE at different stages of processing, and hence in relating structure to performance, because standard methods cannot be used for measuring MWt or observing flow behaviour.

To address these problems, IUPAC Subcommittee on Structure and Properties of Commercial Polymers has formed a well-balanced project team consisting of industrial and academic members with expertise ranging from molecular weight determination and electron microscopy to micro-cracking and wear of joints. Several team members already have strong backgrounds in research on UHMWPE, and the participation of a major manufacturer is an added strength. Ticona will supply three HMWPE grades, with approximate MWts of 0.4, 5 and 9 MDa, as both powder and standardized mouldings. Other participants will develop methods for observing and quantifying the changes that occur during processing, especially those affecting fracture and wear resistance, which will also be evaluated.

Progress

July 2010 – project announcement published in Chem. Int. Jul-Aug, p. 20

January 2015 update – All milestones initially proposed have been reached, including the preparation of four draft manuscripts with the following titles:

  • Structure, Processing and Performance of Ultra-High Molecular Weight Polyethylene, Part 1: Characterising Molecular Weight
  • ibid. Part 2: Crystallinity, entanglements and super-molecular structure
  • ibid. Part 3: Mechanical properties
  • Limits on formation of entanglements in Ultra-High Molecular Weight Polyethylene

The project is notable in two respects: (a) for the first time, East Asian members were fully integrated into the project team, and (b) there was equal participation by rheologists and mechanical properties specialists, so neither side felt their efforts made only a minor contribution to the study. Interest in the project has been high, and a large number of researchers have contributed to the research.

The plan is to submit three reports for publication in Pure and Applied Chemistry, giving a full account of the experimental work undertaken and the conclusions reached, and to write a separate article for submission to another leading journal, which will concentrate on the most significant new developments.

December 2016 update –
This is a wide-ranging project, covering molecular weight measurements, melt rheology, crystallinity, observations of fusion defects, and various methods for characterizing mechanical properties.  Seventeen laboratories, mainly based in Europe, but also including three in China, have contributed to the experimental program.
A large amount of data was gathered by 30th April 2014, the official completion date. However, the project was extended in order to investigate some striking and original results obtained near the end of the allotted period by Professor Altstädt’s group at the University of Bayreuth. They found that fatigue crack growth rates, in UHMWPE mouldings prepared exclusively for this project, were highly irregular and difficult to interpret. Only in the last few months has any progress been made towards understanding this exceptional behaviour. A possible explanation is that it is caused by a combination of plane strain loading, the purity of the mouldings (no antioxidants or other additives) and chemical ageing over more than 4 years, which is exacerbated by the absence of antioxidants.
Most of the fatigue data from Bayreuth were obtained under plane strain loading, using 10 mm thick compact tension specimens. To test the hypothesis that plane strain loading is one of the factors responsible for erratic crack growth, a single test was carried out recently on a 4 mm thick CT specimen. This showed conventional steady crack growth of the type reported regularly in the extensive literature on fatigue in UHMWPE. Before the observation of conventional crack propagation can be published, it needs to be confirmed. Accordingly, Professor Altstädt is planning to carry out three further fatigue tests on thin CT specimens. Those tests will complete the experimental work for the project.
Three reports on the project, covering molecular weight, structure, and mechanical properties, have already been prepared in draft form. Once the planned fatigue tests have been carried out, these drafts can be updated and circulated for comment to participating laboratories.  The manuscripts will then be submitted to Pure and Applied Chemistry. To draw attention more generally to the work of IUPAC SC 4.2.1, it is also planned to publish a paper in another journal, e.g. Biomaterials. This additional paper would focus on the erratic fatigue crack growth data, but incorporate other relevant data obtained during the course of this project.

May 2018 update –
Experimental work on the project has now been completed, and four draft reports have been prepared. These will be discussed at the annual meeting of IUPAC SC4.2.1 in Mainz on 7-9 May 2018.

The first report is entitled “Sporadic fatigue crack propagation in UHMWPE”. This covers the most important results obtained during the project, which show evidence of inadequate bonding between powder particles in plaques that were compression moulded specially for this project. The draft has been circulated to members of SC4.2.1, who will decide on the final wording of the article to be submitted to a leading journal. Polymer has been proposed as an appropriate journal. Once the article has been accepted for publication, the other three articles will be submitted as IUPAC Technical Reports to Pure and Applied Chemistry:

  • Structure, Processing and Performance of Ultra-High Molecular Weight Polyethylene, Part 1: Characterizing molecular weight
  • ibid. Part 2: Crystallinity, entanglement and supra-molecular structure
  • ibid. Part 3: Deformation, wear and fracture

Parts 1 and 2 have been circulated to members of SC4.2.1, and are essentially in final form. Part 3 has not yet been circulated. It will be revised to accommodate any comments received from committee members and the journal’s reviewers.

Page last updated 8 May 2018