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.

We have also investigated the basic tensile strength characteristics of our TPU/FF composites (see Figure 2). The stress–strain 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.

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 only one unit lower than the hardness of the TPU/FF composite. These results are consistent with those obtained in our previous study,⁶ in which some of the chemical treatments caused a reduction in hardness of the untreated fiber composite.

To investigate the damping properties of our samples, we also performed dynamic mechanical analyses on the TPU and TPU/FF composites. The results shown in Figure 3 illustrate that all our surface treatments caused significant increases in tan delta (ratio of loss modulus to storage modulus) values. The addition of surface-treated FF also caused the tan delta peak (T_g) of TPU to shift to higher temperatures. This is because TPU chain motions are hindered after the surface treatments.⁷

Water aging of bio-composites is necessary to observe their outdoor performance. We thus measured the water uptake tendencies of the TPU and TPU/FF composites over a 30-day period (see Figure 4). Our results clearly show that the TPU sample rapidly absorbed about 1% water over a few days and then reached its maximum capacity. The greatest water uptake value we observe is for the untreated FF-loaded composite. This characteristic arises from the formation of microvoids that exist between the fiber surfaces and TPU. We also find that water absorption exhibits a decreasing trend with increasing addition of surface-treated FF.

Finally, we studied the morphology of the composites' fracture surfaces from SEM images (see Figure 5). We observe large gaps and debonding between the fiber and TPU matrix, which indicates poor adhesion between these two phases. In contrast, we see that the surfaces of the fibers are covered by TPU and gaps are not present. We also find that the Na- and MD-treated FFs are dispersed more homogeneously within the TPU matrix than the untreated FFs. These observations are consistent with similar previous work⁷ and predictions,⁸ and they indicate that our applied treatments enhanced the interfacial adhesion between FF and TPU.

In summary, we have applied alkaline, isocyanate, and cured isocyanate treatments to FF to improve its properties as a filler for TPU composites. We find that these treatments result in improved tensile strength compared with a composite that contains untreated FF, as well as an increase in hardness of TPU. According to our thermo-mechanical analysis, the T_g of TPU shifts to a higher temperature after the addition of surface-modified FF. In addition, all the composites containing surface-treated FF exhibited reduced water uptake capacities compared with the untreated-FF sample (all the composites, however, had higher capacities than neat TPU). Our SEM images indicate that the surface-modified fibers have a homogeneous dispersion, whereas untreated FF exhibits a lack of adhesion to TPU. Overall, we conclude that the CM modification generally gives the best results among all the treatments because of the formation of TPU-compatibile uretidione moieties on the fiber surfaces during the curing process. In our future studies we will focus on new eco-friendly approaches to surface treatments for biocomposite applications.