Fabrication of polytetrafluoroethylene (PTFE) and some copolymers of tetrafluoroethylene, including those containing particulate carbon, is often difficult, due to the fact that even above their melting points they flow little if at all. Thus typical plastics melt processing techniques are often useless with such polymers. Previous attempts have been made by others to make these polymers more processible, in order to make usable parts. It is well known in the art to use particulate carbon to improve the physical properties of polymers and polymer parts, and to impart electrical conductivity to such polymers and parts.
Thus U.S. Pat. No.2,685,707 and J. F. Lontz, et. al., Ind. Eng. Chem., vol. 44. pp. 1805-1810 report that many low molecular weight organic compounds such as hydrocarbon and esters can be used as "lubricants" to aid in the forming of parts from particulate PTFE. The lubricants are removed typically by volatilization and/or sintering, and a solid part is produced by sintering the remaining PTFE.
U.S. Pat. Nos. 3,407,249 and 3,679,614 report that porous PTFE parts are produced by mixing particulate PTFE with poly(methyl methacrylate) (PMMA) and other compounds. The PMMA and other compounds can be removed by use of solvents for the PMMA such as acetone.
U.S. Pat. Nos. 2,985,918 and 3,054,761 describe a process for making and using a composition comprising PTFE and PMMA. This mixture may be sintered to volatilize and decompose the PMMA, and to sinter the PTFE, so that a porous PTFE article is produced.
British Patent 1,390,100 describes a mixture of a polyolefin such as polyethylene, PTFE and for example carbon black or bronze powder. The polyolefin is believed to act as a processing aid.
Japanese Patent Application 49/039642 discloses PTFE containing powdered graphite
Japanese Patent Application 49/11912 discloses a dispersion in water of a mixture of PTFE and conductive carbon, which can be used to make a conductive coating.
Japanese Patent Application 58/25368 describes a paste containing, among other items, PTFE aqueous dispersion and an electrically conductive powder such as graphite or carbon black. This too can be used to make electrically conductive coatings.
Among the desired properties a material should impart to PTFE in order to help form PTFE parts containing particulate carbon, are, easy mixing of the components, good green strength of the mixture so parts of the mixture will not deform readily, especially under sintering conditions, and clean sintering of the processing aid (lubricant) so that the remaining PTFE part is free of impurities (e.g., ash and char) and has a minimum amount of porosity. It is the object of this invention to provide such a composition and a process for making and using it. None of the above references teaches the use of poly(2,2-dialkylpropiolactones) as lubricants or processing aids for PTFE.
USPTO Backgrounds
USPTO Backgrounds