Polytetrafluoroethylene as a suitable filler for poly(lactic acid) composites
In recent years, biobased and biodegradable polymers have attracted an increasing amount of attention in efforts to reduce the dependency on petroleum-based polymers, lessen the amount of plastic waste, and reduce carbon dioxide emissions.1–3 One of the most commonly used of such polymers is poly(lactic acid)—PLA—which can be produced from renewable resources. Although this commercially available material is used extensively in packaging, biomedical, transportation, and structural applications,4–7 its poor melt viscosity, brittleness, and low thermal resistance mean that it cannot fully satisfy the requirements of industrial and medical applications.8–10
To try and overcome these problems with PLA, and to thus broaden its potential applications, various techniques have been investigated to improve its properties and processability. For example, an established strategy for tuning the material properties of polymers is to add fillers, plasticizers, or a second polymer. The filler approach is one of the most practical and cost-effective ways to potentially improve PLA's properties. Typical fillers include talc, clay, nanotubes, and graphene. In addition, polytetrafluoroethylene (PTFE) is thought to be a good option as a filler material for improving the material properties of PLA. For instance, during blending, PTFE tends to deform into fibrillar structures that have large aspect ratios and develops topological interactions (physical entanglements).11 The PTFE filler can therefore reinforce the PLA matrix and improve its mechanical and foaming response. Furthermore, the formation of fibrils from PTFE during shearing has been observed in several studies.11–14 It has also been reported that PTFE particles can form in situ fibrillated networks within a polypropylene polymer matrix, under high shear-stress conditions, during twin-screw extrusion.15–18
With the aim of addressing the problems with PLA, we have therefore used PTFE as the filler material in a set of PLA melt-blending experiments.19 To prepare the PLA/PTFE composites via twin-screw extrusion, we first melt-processed the PLA melt with 0.5, 1, or 3wt% PTFE. We then used microcellular injection molding (with supercritical nitrogen as the physical blowing agent) to foam the composites. Subsequently, we performed a detailed study of the morphological, mechanical, rheological, and foaming properties of our composites.
The morphologies of our ‘as-extruded’ PLA-0 and PLA-P1 solid samples (i.e., containing 0 and 1wt% PTFE) that were manually fractured, without the use of liquid nitrogen, are shown in Figure 1. The scanning electron microscope images show that the surface of the pure PLA (PLA-0) is smooth, whereas there are a number of fibrils in the PLA-P1 sample. Furthermore, it is apparent that because of the non-brittle fracturing, the fibrils have been pulled out from the PLA matrix. The fibrils are also obviously self-assembled and entangled. These results thus confirm that the PTFE underwent fibrillation and deformed into a nanofibrillar structure (with a large aspect ratio). We also illustrate the morphologies of the core layers of our microcellular injection-molded PLA and PLA/PTFE tensile bar specimens in Figure 2. We find that all the PLA/PTFE composites have smaller cell sizes than the pure PLA and that the 1wt% PTFE sample has the most uniform and smallest cells.
A selection of the results from our tensile tests are shown in Figure 3 and Table 1. We observe that the Young's modulus of the samples increases linearly with the PTFE content (i.e., increasing from 3380.16MPa for the pure PLA to 3586.60MPa for the sample containing 3wt% PTFE). We believe that this linear relationship is caused by the formation of numerous fibrils on the PTFE particles that give rise to a lubricating effect.17 Our results therefore indicate the inclusion of PTFE in PLA composites has a reinforcing and toughening influence on the PLA matrix.
Figure 3.Table 1.
|Sample||Young's modulus (MPa)||Elongation at break (%)||Tensile strength (MPa)||Toughness (×106J/m3)|
The measured storage modulus (G′) values for our PLA and PLA/PTFE samples are given in Figure 4. Except for the PLA-P3 composite (i.e., containing 3wt% PTFE), our results show that G′ did not vary much over the range of measured frequencies. The difference in the linear viscoelastic response of the PLA and PLA-P3 composites, however, indicates that the PTFE fibrils can have a substantial effect (especially at low frequencies) and that the G′ was greatly enhanced by the addition of 3wt% PTFE. We can explain the observed low-frequency region by the existence of a microstructure (composed of a percolated, physical network of fibrils in the molten PLA) that stores the deformation energy over long timescales.17, 20
In summary, we have investigated the use of PTFE as a filler material to enhance the material properties of PLA. The results of our melt-blending experiments indicate that the PTFE has a reinforcing effect on the Young's modulus of our PLA/PTFE composites and that the elongation at break of the samples can be reduced by 72% (with the incorporation of 3wt% PTFE). Furthermore, the inclusion of the PTFE significantly improves the storage modulus of the samples at low frequencies. The presence of only a small amount of PTFE therefore substantially improves the characteristics of a PLA matrix. In the next stage of our work, we plan to study whether PTFE can be used to similarly reinforce other biopolymers.
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