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Improved properties of thermoplastic polyurethane bio-composites

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Improved properties of thermoplastic polyurethane bio-composites

Country/Region china

Company SPE limited

Categories Carbon Fiber Composite Plate

Update 2017-03-03 16:34:42

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26 February 2016
Umit Tayfun, Erdal Bayramli, and Mehmet Dogan
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An isocyanate surface treatment of flax fibers produces improved interfacial interactions of the fibers with the polymer matrix, and thus stronger eco-composites.

The interest in biomass sources for filled polymer composites continually increases because of their advantageous properties (e.g., biodegradability, recyclability, low density, and cost-effective characteristics). These advantages mean that natural fibers have become commercially competitive with man-made fibers. There is thus an opportunity to potentially replace man-made fibers with natural fibers in several applications, including household objects, packaging, as well as the manufacture of automobiles and furniture.1, 2 Incompatibility between natural fibers and polymeric matrices, however, presents a major challenge to the development of bio-composites with the desired properties.

The restriction to the development of bio-composites is generally overcome via the application of different chemical treatment methods to the natural fiber surfaces. Such modifications include alkaline (Na), silane, and isocyanate (MD) treatments. These treatments improve the interfacial adhesion of the fiber surfaces to the polymer matrix.

In this work,3 we applied Na and MD treatments to flax fiber (FF) to enhance its compatibility with thermoplastic polyurethane (TPU). Additionally, we applied a curing step to MD-treated FFs by subjecting them to heat (CM treatment). We chose FF as the natural fiber for our study because FF-reinforced polymer composites have become popular in textile, transport, and construction markets. TPU and its composites also have many modern applications, and are used effectively in the manufacture of footwear, packaging, protective coatings, cables, wires, and tubes (mainly for the automotive and construction industries). In our study we used eco-grade TPU (Pearlthane ECO D12T85), which is composed of renewably sourced polyols at a content of 46%. We thus developed recyclable TPU eco-composites by incorporating surface-treated FF. In a previous study, we carried out alkali, silane, peroxide, and permanganate treatments on FF and examined the resultant effects on TPU/FF bio-composites.4 In this study, however, we investigated the influence of diphenyl diisocyanate (pMDI) modification, followed by an alkali treatment of the FF surfaces, on the properties of TPU/FF composites.3 We prepared our TPU/FF eco-composites with the use of melt compounding in a laboratory-scale twin-screw extruder (DSM Xplore, 15ml microcompounder). We kept the FF (partially retted) loading constant, at 30% by weight, in all the composites.

Photographs and scanning electron microscope (SEM) images of the FF surfaces are shown in Figure 1. These images reveal that the FF surfaces become rougher and cleaner after the chemical treatments have been conducted. We also observe from the SEM images that fibrillation of FF to individual fibers occurs in the surface-treated FF samples.


Figure 1.

Photographs and scanning electron microscope (SEM) images of pristine and surface-treated flax fiber (FF) samples. Images are shown for FF that has been subject to alkaline (Na), isocyanate (MD), and cured isocyanate (CM) treatments.

We have also investigated the basic tensile strength characteristics of our TPU/FF composites (see Figure 2). The stressstrain curves indicate that all our treatment methods caused an improvement in tensile strength (compared with the untreated FF-filled composite). We observe the greatest increase in tensile strength for the CM-FF-loaded composite. We believe that this is because urethane and uretidione groups that are formed during the curing process cause the increased compatibility of the CM-FF with TPU.5 We also find that the composite containing the Na-treated FF has the highest strain value, whereas the isocyanate treatments caused a reduction in elongation compared with the untreated-FF composite.


Figure 2.

Stressstrain curves for the thermoplastic polyurethane (TPU)/FF (treated and untreated) composites.

The Shore A hardness parameter is commonly used to characterize elastomers and their composites. We have measured Shore A hardness values for our samples of 85.9±0.2 (TPU), 90.1±0.1 (TPU/FF), 89.9±0.2 (TPU/Na-FF), 89.0±0.2 (TPU/MD-FF), and 88.8±0.1 (TPU/CM-FF). These results thus show that the Shore A hardness of TPU increases by up to 4.2 points after untreated FF has been incorporated. The Na-treated FF also causes an improvement to the hardness of the TPU, but the composite has a slightly lower hardness value than the untreated-FF sample. Our hardness values for the TPU/MD-FF and TPU/CM-FF samples are almost identical and onl

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