Investigating agricultural waste-reinforced polyurethane elastomer green composites

3 June 2015
Erdal Bayramli, Umit Tayfun, and Mehmet Dogan
Enhancing interfacial interactions between natural filler and polymer matrix by modifying the surface of rice straw results in stronger thermoplastic polyurethane-based eco-composites.

Green composites are gaining notice because of the increase in environmental consciousness and regulations.1, 2 Specifically, interest in lignocellulosic-filled polymer composites has increased because of their advantages, including reduced cost and improved mechanical properties. Despite these advantages, existing problems such as low thermal stability of lignocellulosics, poor interfacial adhesion between polar lignocellulosics and nonpolar matrix material, and moisture uptake are limiting their wider application.1–5 The main factors affecting the final mechanical properties of natural fiber-reinforced polymer composites are the degree of fiber dispersion and the effective stress transfer at the fiber and matrix material interface. Better filler dispersion and strong adhesion between the matrix and fiber can be achieved via several physical and chemical methods.5–12

Previous reviews have shown that alkaline (sodium or Na), silane (Si), benzoyl peroxide (BP), hot water (HW), and permanganate (PM) treatments have been used widely to improve the interfacial adhesion between lignocellulosics and polymers.5–12 We applied these treatments to rice straw (RS) in order improve its adhesion and dispersion in bio-based thermoplastic polyurethane (TPU) matrix composites.13

RS contains mainly cellulose, hemicellulose, lignin, proteins, and silica. While high silica content restricts its use as animal feed, TPU has a broad range of applications due to the variety of reactants derived from petroleum and/or renewable resources. If the polyol is produced from renewable resources, the TPUs are considered to be bio-based. They are used in a range of markets and applications, including automotive, sporting goods, and medical devices in the form of sheets, films, or profiles.1

We prepared TPU/RS composites via melt compounding in a counter-rotating, twin-screw micro-extruder. We kept RS loadings constant at 30% by weight in all composites. Scanning electron microscopy (SEM) images show that the RS surfaces became rougher and cleaner after Na and HW treatments due to the partial removal of soluble components (see Figure 1).5–12 By removing the outer surface, the ordered white dots, which are rich in Si, can be seen on the surface of treated RS.11 The additional Na-RS treatments did not alter the surface morphology.


Scanning electron microscopy (SEM) images of pristine and surface-treated rice straw (RS). Na: Alkaline. HW: Hot water. Si: Silane. BP: Benzoyl peroxide. PM: Permanganate.

We saw that the tensile strength and elastic modulus (stress-strain curves) of the composites are improved with respect to TPU/RS composites with Na, HW, and Si treatments (see Figure 2). After Na and HW treatments, the RS surface becomes rougher (see Figure 1). Also, the hemicellulose portion, which reduces the adhesion between lignocellulosics and the polymer matrix, is removed.14 Thus, both HW and Na treatments improved the wetting of RS and increased the adhesion between phases. To improve the wettability and adhesion, we further modified Na-RS fiber by Si, BP, and PM treatments. We observed the greatest increase in tensile strength and elastic modulus in Si-RS composites.


The basic tensile test characteristics of thermoplastic polyurethane/rice straw (TPU/RS) composites are shown by stress-strain curves.

Hardness is a characteristic parameter for polyurethane elastomers and their composites. Shore A hardness values of TPU and TPU/RS composites are listed in Table 1. The TPU hardness increased with the incorporation of both untreated and treated RS. A composite containing alkali-treated RS resulted in slightly higher hardness than TPU/RS. A composite with HW-RS exhibited the highest value, about a 10-unit increase in TPU Shore A hardness. Table 1 shows that adding silane-treated RS to TPU also caused a sharp increase. The hardness values of TPU/RS and TPU/BP-RS are almost identical. Composites containing PM-RS showed the lowest values among treatments, but still had higher hardness values than unmodified RS composites.

Shore A hardness values of TPU and TPU/RS composites.

TPUTPU/RSTPU/Na-RSTPU/HW-RSTPU/Si-RSTPU/BP-RSTPU/PM-RS
85.9±0.289.3±0.189.7±0.295.5±0.292.2±0.189.2±0.288.4±0.2

All surface modifications reduced the water uptake with respect to TPU/RS (see Figure 3). Water absorption was mainly related to hydrogen bonding of water molecules to the hydroxyl groups present on lignocellulosics and to the compatibility between the matrix and reinforcing material. Although Na and HW treatments make RS more hydrophilic, the water uptake nonetheless is reduced after these treatments because of the reduction in the number of capillaries (pores) in the composite structure, which creates a suction effect.


The water uptake of composites in 60-day periods.

We can clearly see that the composite containing Si-RS absorbs much less water due to the well-known hydrophobicity of the Si coupling agent and the increase in compatibility of the composite materials. The tensile tests performed after the water absorption test showed that all RS composites have lower tensile strength and elastic modulus values than TPU after water absorption. The reduction in tensile strength and elastic modulus observed in RS-containing composites is, on the average, about 20%.

We investigated the effect of surface treatments on the composites' morphology by studying SEM micrographs of their fracture surfaces (see Figure 4). We saw that gaps formed between straw particles and the TPU matrix in unmodified RS, BP-RS, and PM-RS composites, indicating poor adhesion between TPU and RS. TPU covers the surface of Na-RS, HW-RS, and Si-RS, indicating that Na, HW, and Si treatments greatly improve the interfacial adhesion between straw and polymer matrix.


Representative SEM micrographs of fracture surfaces of selected composites.

In summary, HW, Na, and Si treatments improved the tensile strength and modulus of RS-containing composites. Si-RS composites had the highest tensile strength and elastic modulus due to better dispersion and strong interfacial interactions between fiber surface and TPU matrix. The inclusion of BP-RS and PM-RS reduced the mechanical properties with respect to composites containing Na-RS. We think the reduction in mechanical properties of composites arises from RS losing mechanical strength during treatments. All surface treatments reduced the water uptake capacity with respect to RS-TPU composites. The Si-RS composite had the lowest water uptake values. We found that the Si-modified sample has the best mechanical and water uptake properties among the applied treatments in the TPU/RS composite applications investigated to date.


Authors

Erdal Bayramli
Middle East Technical University

Erdal Bayramli is a professor of physical chemistry. His research revolves around surface chemistry, material processing, and polymeric materials.

Umit Tayfun
Middle East Technical University

Umit Tayfun is a PhD candidate in polymer science and technology whose research focuses on the surface modification of fillers, preparation of nanocomposites, eco-composites, and carbon-fiber-reinforced thermoplastic polymer composites.

Mehmet Dogan
Textile Engineering, Erciyes University

Mehmet Dogan is an expert in natural fibers, green composites, and flame retardancy of polymer composites. He has published more than 20 articles in international journals that are mainly related to flammability properties of polymer composites.


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DOI:  10.2417/spepro.005729



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