Feeding steam into a twin-screw extruder during fabrication improves the microstructure of PET nanocomposites while subsequent solid-state polymerization enhances their mechanical properties.
Polyethylene terephthalate (PET), a low-cost engineering polymer, is employed in a range of applications due to its excellent transparency and good barrier properties.1 Recent studies have shown that the presence of organoclay platelets in PET can lower its permeability to oxygen,2–4 carbon dioxide, and water vapor.5, 6 These properties would be particularly useful in the food and beverage packaging industry. Improved compatibility between the organoclay and PET is also sought to improve the mechanical properties of the material. Like many other polymer nanocomposites, PET/ organoclay can be prepared by in situ or melt-mixing methods, with the latter proving more attractive due to its flexibility and cost-effectiveness. However, melt processing at high temperatures can cause degradation of both the organoclay modifier and the PET itself.Combining the benefits of solution and conventional melt-mixing methods, water-assisted melt mixing is a practical approach to the preparation of nanocomposites.7,8 Various studies have reported the success of injecting water into the extruder to enhance the microstructure of nanocomposites based on polyamides (PA6 and PA11) and polypropylene (PP).9–12 In addition, solid-state polymerization (SSP: a method to increase the molecular weight after processing) is widely used to compensate for the polymer hydrolysis that occurs during the melt-mixing process, which can cause material degradation.13, 14 We have studied the rheological, mechanical, thermal, morphological, and barrier properties of processed PET and PET nanocomposites prepared by water-assisted melt mixing and SSP.To investigate the effect of feeding rate and water on the microstructure of the PET nanocomposite, we prepared our samples via twin-screw extrusion. The PET and PET/organoclay powders were fed into the extruder at two different rates— 0.6 and 3.3kg/h— with a nominal content (2wt%) of the nanoclays Cloisite 30B (C30B) and Nanomer I.28E (I28E), while steam was fed at a rate of 0.3 liters per hour. After the extrusion process, the samples were ground and dried. We subsequently performed SSP at 215°C for 8h. This process takes place under the flow of dry nitrogen, to facilitate the removal of by-products.
Transmission electron microscopy (TEM) images of our PET nanocomposites are shown in Figure 1. Figure 1(a, b) shows a better dispersion of C30B in PET compared to I28E: most of the C30B particles are broken down to single layers, while I28E particles are in the form of clay platelets (tactoids). PET-C30B nanocomposites prepared at the lower feeding rate— Figure 1(a, c, d)— show a better dispersion and distribution of C30B nanoparticles in the PET matrix compared with samples processed at the high feeding rate: see Figure 1(e, f). Moreover, in nanocomposites processed with water, a better dispersion and distribution of C30B is observed: see Figure 1(c, f). In these cases, the lower feeding rate and longer residence time in the extruder led to a greater delamination of tactoid stacks. The images also show that SSP does not change the microstructure of the nanocomposites. Using TEM image analysis, we were able to determine that the aspect ratio and the number of silicate layers per tactoid are the same before and after SSP.Figure 1.Transmission electron microscopy images of nanocomposites fabricated with polyethylene terephthalate (PET), Cloisite 20B (C30B), and Nanomer I.28E (I28E). The PET/ organoclay powders were fed into the extruder at rates of 0.6kg/h (L) and 3.3kg/h (H). (a) PET-C30B-L, (b) PET-I28E-L, (c) W-PET-C30B-L, (d) SSP-W-PET-C30B-L, (e) PET-C30B-H, (f) W-PET-C30B-H. In these images, the thin lines with black color show the organoclays and the gray area is PET. SSP: Solid-state polymerization. W: Water.The presence of 2wt% (nominal) organoclay increases the tensile modulus of PET nanocomposites, compared to the neat PET: see Figure 2(a). The PET-C30B nanocomposite processed at the low feeding rate has a slightly smaller tensile modulus, although the morphology suggests a better dispersion and distribution of C30B nanoparticles. This may be due to a more severe degradation of the PET matrix as a result of the low feeding rate. On the other hand, PET-C30B prepared by conventional melt mixing exhibits a smaller modulus compared to the nanocomposites prepared by water-assisted melt mixing and SSP: the tensile modulus is improved by 15 and 20% for PET-C30B-H and SSP-W-PET-C30B-H, respectively. This improvement in the tensile modulus is probably due to better dispersion and distribution of C30B, in addition to the larger molecular weight of the PET matrix as a result of SSP.
Figure 2. Mechanical properties of PET and PET nanocomposites. (a) Tensile modulus. (b) Elongation at break.The effects of C30B and different processing conditions on the elongation at break of PET nanocomposites are presented in Figure 2(b). The elongation at break significantly reduces in the presence of C30B for conventional PET nanocomposites. In addition, nanocomposites processed at a low feeding rate have lower elongation at break. Interestingly, the nanocomposites prepared via water-assisted extrusion and SSP provide superior elongation values at break compared to those prepared by conventional processing. For example, the values are 130 and 180% for SSP-W-PET-C30B-L and SSP-W-PET-C30B-H, respectively, compared to 3 and 6% for PET-C30B-L and PET-C30B-H. These results demonstrate the strong potential of water-assisted extrusion and SSP for improving the ductility of PET nanocomposites.In summary, we have shown that water-assisted melt mixing improves the microstructure of PET nanocomposites by increasing the number of single and double layers of C30B, and increasing the aspect ratio of C30B in the PET matrix. Results of small amplitude oscillatory rheology and intrinsic viscosity show that the weight-average molecular weight of PET increased significantly after SSP, with no noticeable change in the structure of PET molecular chains, as confirmed by proton and carbon NMR (nuclear magnetic resonance) analysis. The PET nanocomposites prepared by water-assisted extrusion followed by SSP show enhanced mechanical properties compared with those prepared by conventional melt mixing. In particular, the elongation at break for SSP-W-PET-C30B is significantly improved compared to conventional nanocomposites (PET-C30B). In the future, we hope to prepare a variety of polymer nanocomposites using water-assisted melt mixing, and to develop this method for use in the commercial development of PET.