It is well-known that additive manufacturing (AM) has great potential but several limitations, one of which is the narrow material portfolio.
In 2012, of the tens of thousands of polymers available on the market, less than 30 were available for laser sintering (of which Polyamide 12 comprised over 90% of the market). This limited number of materials is due to the specific requirements for powder-based AM, such as precise particle size distribution (~45-90 µm) to guarantee good powder flow; relatively low viscosity to avoid porosity; absorbance in the wavelength of the specific heating element (e.g. CO2 laser); and a wide processing window (i.e. the difference between melting and crystallisation temperature) to supress dimensional warping.
Fluoropolymers are an interesting family of polymers. PTFE, better known by the trademark Teflon, is the most famous and widely-used fluoropolymer. The Carbon-Fluorine bonds present lead to many desirable properties such as biocompatibility, non-adhesiveness, wide service temperature (−260 °C – +260 °C), high chemical resistance, high resistance to sunlight, flame retardance and weathering without the addition of fillers, plasticisers or stabilisers. Any of these properties can be found in other materials but fluoropolymers are uniquely suitable when two or more of these properties are required in the same application.
The downside of all these amazing properties is that fluoropolymers are quite difficult to process. Because of its high crystallinity and viscosity (1010-1012 Pa*s), PTFE is not melt-processable in contrast to most thermoplastics polymers and requires special manufacturing processes. Their high chemical resistance makes them insoluble in most organic solvents at room temperature, while the high processing temperatures required cause degradation of the polymer chain, which generates corrosive by-products that necessitate specific alloys for handling. The number of challenges further increases in AM as the commercially-available grades are tailored to traditional processes such as injection moulding (pellets) and coating (fine powders).
The investigation of fluoropolymers in AM is still very limited and mainly focused on one particular fluoropolymer – polyvinylidene fluoride (PVDF) – which has piezoelectric properties and a relatively low melting temperature (<180 °C). 3M, a North American manufacturer, is currently working on the use of PTFE in stereolithography by using a resin binder which is then removed by sintering.
At the University of Nottingham, we have been investigating three fluoropolymers with melting temperatures of around 100 °C, 200 °C, and 300 °C. The first issue encountered was to find these fluoropolymers in a powder form with the ideal powder size. This was not possible so we worked with a particle size below the optimal values. As a result, the powders were cohesive and did not flow well. Good flowability was achieved by adding a flowing agent.
A second issue was the high melting temperature of the 200 °C and 300 °C fluoropolymers, which were too high for the laser sintering system used (EOS Formiga P100, maximum powder bed temperature of ~180 °C). Isothermal crystallisation measurements confirmed that the ideal processing temperatures were above 180 °C, which caused warping after a few tens of layers for the 200 °C polymer and warping at the first layer for the 300 °C polymer. The upside was that the polymer did not age at the processing temperature and could be recycled without issue. Different scan strategies and the use of a build platform considerably reduced warpage.
The third issue was the high molecular weight and consequent viscosity of the polymer melt. The melting and solidification of the polymers occurs too quickly and the resultant layers were porous. Higher laser powers can reduce porosity but they are limited by the onset of the decomposition of the polymer which must be avoided. A solution to this is to scan the same area multiple times with a laser power that does not cause decomposition. This is not an ideal solution as the printing times would be extended for large parts.
These initial results show that fluoropolymers have the potential to be used in AM but they require high temperature printers and the collaboration of the powder suppliers to design polymer grades with properties tailored for AM (particle size, viscosity, etc.).