Brief introduction about the properties of poly (PTFE) in compression

PTFE is a very useful material because it has a unique combination of properties. PTFE is chemically inert, weatherable, excellent electrical insulation, high temperature resistance, a low coefficient of friction and non-adhesive properties.

Polymers are commonly used in manufacture and engineering, however, published research describing their mechanical properties is seemingly underrepresented given their importance. Much of the data that is presented, all too commonly gives insufficient information about the exact pedigree of the polymer tested and its processing history. This is possibly because establishing the base-line material characterisation is often as difficult as performing the actual mechanical tests. Additionally, accurate computer modelling of polymer mechanical response is still in its infancy. Many empirical methods are commonly employed, but they tend to be inaccurate outside of a narrow parameter range. One reason for this, aside from the complexity of polymer response, is that often data is unavailable outside of a narrow experimental parameter range to challenge and expand the robustness of empirical or phenomenological based constitutive models. Here, we present the first results of a concerted multi-disciplinary effort aimed at understanding the mechanical response of a well characterised polymer from both an experimental view point and later, coupled with the production of a robust theoretical model capable of being implemented into computer codes.

The polymer described in this study is poly(tetrafluoroethylene) (PTFE). It was chosen for several reasons, including its use as a common engineering material for small high-performance parts and its availability from several manufacturers. While studied extensively in the past, it has received little attention in the open literature forthe last 25 years. We have chosen to revisit this material because of its structural complexity and lack of mechanical data. PTFE is a remarkable material in many ways. It exhibits useful properties over the widest temperature range of any polymer; PTFE retains some measure of ductility at 4 K and in some situations is used in applications at 540 8C. It is insoluble in all common solvents and is resistant to almost all acidic and caustic materials. PTFE has amongst the highest resistivity of any material, a very high dielectric strength and low dielectric loss. The coefficient of sliding friction between PTFE and many engineering materials is extremely low and when sintered with wear reducing compounds, an industrially important class of bearing materials are formed. Coupled to its low coefficient of friction and chemical stability, PTFE is almost impossible for other materials to adhere to. This property is often used in industrial processing technology where ease of cleaning is important. One aspect of PTFE that has held it back from more extensive industrial and engineering use is its high melt viscosity (1011 P at 380 8C). This prevents injection and blow moulding from being possible and only expensive sintering and extrusion manufacturing processes are available for part production.

This paper focuses on base-line material characterisation and the compressive response of the pedigreed PTFE materials at differing strain-rates and temperatures. Future papers will deal with the tensile and shear response, detailed effects of polymer crystallinity, ballistic and shock behaviour and the development of an applicable theoretical constitutive model.

Very little previous research on the compressive properties of PTFE has been published. Some research on the creep properties exists, but in terms of engineeringdeformation, only six references have come to the attention of the authors. In 1963, Davies published a paper on the development of a Split-Hopkinson bar system. As part of this report, a single room temperature stress/strain curve for PTFE was presented at z1700 sK1. The maximum strain imposed in this system was only 3%. Further highstrain rate data on polymer versus temperature was published by Gray and Walley. Koo published stress/strain data for an Imperial Chemical Industries PTFE product called Halon G-80 in 1965. The effects of temperature on the mechanical response were also briefly discussed.

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