GUR® UHMWPE

Figure 9.20 Dynamic coefficient of friction versus pressure at a velocity of 10 m/min of Celanese GUR UHMWPE [9],...

Fig. 28 FCP rates as function of stress intensity factor range for unaged UHMWPE (GUR 4150HP) following different sterilization methods...

Under typical compaction pressures, the bulk density of UHMWPE powder is relatively high and the void content is low. Much of the final molded density can be reached by application of pressure alone. For example (13), UHMWPE powder cold pressed at 9 MPa attained a bulk density of 0.80 g/cm while the fused polymer density is 0.935 g/cm. Under typical commercial compaction pressures of 3-5 MPa, UHMWPE powder (e.g. Ticona GUR 4000 series) attains a bulk density of 0.60-0.65 g/cm. Once compacted, very little material melt flow is required to sinter the powder. The numerous particle to particle contact points of the compacted powder exhibit sufficient flow to achieve fusion after melting. Typical sintering pressures and temperatures for UHMWPE are 3-5 MPa and 180-220 °C respectively. 

The orthopedics literature contains numerous references to different trade names for UHMWPE, which fall into two categories 1) GUR resins currently produced by Ticona and 2) the 1900 resins, previously produced by Basell. The UHMWPE grades currently used in the orthopedic industry are summarized in Table 2.2. 

Today, compression-molded sheets of GUR 1020 and 1050 are produced commercially by two companies (Perplas Medical and Poly Hi Solidur Meditech). Ticona stopped producing compression-molded UHMWPE in 1994. Perplas s molding facility is in England, whereas Poly Hi Solidur produces medical grade UHMWPE sheets in the United States and in Germany. 

Today only a few converters supply medical grade, GUR 1020 and 1050 ram extruded UHMWPE to the orthopedic industry. Medical grade extrusion facilities for Poly Hi Solidur and Westlake Plastics are based in the United States, whereas Perplas Medical s medical grade extrusion is in England. 

An example of machining marks in an as-machined (never implanted) GUR 1050 UHMWPE component. 

Effect of radiation and thermal processing on uniaxial tensile behavior for GUR 1050. For the highly crosslinked UHMWPE, the annealed material has an average degree of crystallinity of 60%, whereas the remelted material has a crystallinity of 43%. 

Representative load-displacement curve developed by testing of a small punch Sf>ecimen in equib-laxial tension curve here reflects typical behavior of GUR 1020. Primary metrics including initial stiffness, peak load, ultimate load, and ultimate displacement. WTF shown in grey. Unirradiated UHMWPE exhibits a bend and a stretch region as shown. 

Comparison of GUR 1050 in three conditions as described by the respective load-displacement curves. Longevity and DURASUL are commercially available formulations of highly crosslinked UHMWPE, which exhibit the characteristic geometric strain hardening in the drawing phase of the small punch test. Free radical quenching by remelting UHMWPE above its f>eak melt transition typically reduces the observed peak load as exhibited here (Edidin and Kurtz 2001). 

Spiegelberg and associates (2001), who noted a greater crosslink density was observed in UHMWPEs with lower polydispersion indices, with GUR 1050 having the lowest polydispersion index of the materials tested. 

Load-displacement curves observed by testing virgin and crosslinked UHMWPE in shear punch apparatus. Curves are quite similar, suggesting that changes in shear resistance are unlikely to explain the increased resistance to wear exhibited by crosslinked UHMWPEs. Materials tested were virgin GUR 1050 and GUR 1050 irradiated to 75 kGy and annealed at tempreratures below the melt point. (Adapted from Kurtz et al. 2002.)... 

Conventional and highly crosslinked UHMWPE materials exhibit similar qualitative behavior as a result of the underlying similarities of the material microstructures. In this section we examine the behavior of four different UHMWPE materials to illustrate the characteristic material response of UHMWPE. This data will also be used as the basis for the development of material models in the rest of the review. Two of the four materials were conventional UHMWPE and two were highly crosslinked UHMWPE. All materials were created from the same lot of ram-extruded GUR 1050 (Eigure 14.1). 

Comparison between experimental data for UHMWPE (GUR 1050, 30 kGy, Y-N2) and predictions from linear elasticity. 

Co-Cr-Mo (ASTM F799) + UHMWPE (GUR 415) 0.060-0.093 Reciprocating motion friction bench (line/flat) Maximum stress = 6 MPa Lub 86% Glycerine Frequency = 1 Hz Duration 500 000 cycles Sliding Dist. = 100 mm/cycle Ruger (1995)... 

Cast Co-Cr-Mo pins (ASTM F-75) + UHMWPE (GUR 415 plate) PE thickness change 64 13 ijum NA Reciprocating pin-on-flat Sterilized with 2.5 Mrad Lub deionized water 36 MPa Line contact stress Frequency = 2.1 Hz Stroke length = 15 mm Duration = 2 x 10 cycles Final friction = 0.079 0.001 Abrasive wear of PE. Transfer of PE on Co-Cr pins. Oxidative wear of Co-Cr pins. Poggie et al (1992)... 

Figure 7.11 Stress-strain curves for Celanese GUR 4120 UHMWPE measured at different temperatures [1],...

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