additives | Surface modification
Right: Lehman & Voss
introduced its Luvocom XTF
family of PTFE modified
compounds for extreme wear applications such as slip rings earlier this year
the difficulty in applying and controlling these products, explains Blackmore. Part designers would instead prefer to choose materials, possibly formulated with internal lubricants, that will slide smoothly. Identifying tests that can predict behavior is more efficient than trial-and-error methods. Key tribology measurements include the static ), which describes the force
coefficient of friction (µs
required to initiate motion of one surface past another, and the kinetic coefficient of friction (µk
), which
describes the force needed to sustain motion. RTP developed a new measurement—the “Glide Factor”. It defines this as the difference between µs
and µk
measured with a modified ASTM D3702 thrust washer test measuring oscillating friction. The researchers found that pressure or load can dra-
matically change the Glide Factor. After experimenting with several loads, the studies were performed at 50psi to simulate the force found in a typical drug-delivery device. “Materials with a low µs
another at start-up, and materials with a low Glide Factor are less likely to experience stick-slip. The Glide Factor concept can be used to establish thresholds for performance that will aid formulation development,” says Gerjets. Friction is a complex phenomenon that is affected
not only by the materials in contact with each other and the applied force, but also by environmental factors (such as temperature) and use factors (such as human-skin oil on the surface or changes to the material surface over time), says Gerjets. Part design- ers want to choose a material pair that will work consistently, no matter what the external factors. “We identified thresholds for both static coefficient of friction and Glide Factor using field data, and we worked with customers to verify our findings. A good formula- tion will be well below the thresholds so that the parts can tolerate variation in temperature or surface defects without problem,” says Blackmore. For example, a
will slide easily past one
typical medical-device combination of PC-POM without internal lubrication meets the static friction threshold, but not the Glide Factor threshold. When an internal lubricant is added to either material, however, the Glide Factor drops below the threshold. With lubricant in both materials, the Glide Factor is significantly below the threshold. A manufacturer of insulin auto-injector pens, for example, can choose lubricant in both materials to have greater assurance that the device will work well, and could even use this information to redesign the device with a smaller spring, says Blackmore. RTP is also developing correlations of their friction data with other critical factors for medical device design, such as the force needed to start an injection or to inject a drug at the correct rate.
Dealing with wear While friction is a concern for short-term use, wear is the concern for long-term applications such as gears or seals. Some sealing applications, such as automotive transmission seals, pump seals, and offshore oil and gas gaskets, must resist wear under high temperatures and high pressure and velocity (PV) ratios. RTP
Table 3: Bevel gear load comparison of polyketone against POM and PA66 alternative and combinations showing loading period and failure type
Source: Akro-Plastic 46 COMPOUNDING WORLD | November 2015
www.compoundingworld.com
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