Bamboo-like ultra-high molecular weight polyethylene fibers and their epoxy composites

https://doi.org/10.1016/j.compscitech.2019.107716Get rights and content

Abstract

It is important for most structural material to achieve both good strength and toughness. However, they are generally mutually exclusive, which is the lower-strength, and hence higher-toughness. Ultra-high molecular weight polyethylene (UHMWPE) fibers have excellent mechanical properties, unfortunately, the interfacial adhesion of UHMWPE fiber reinforced polymer composites is extremely low. Inspired by the shape of bamboo in nature, we herein designed and prepared UHMWPE fiber with new structure. UHMWPE fibers were periodically decorated with polymer lamellar crystals using a polymer solution crystallization method, resulting in bamboo-like structure. It was showed that the strength and toughness of bamboo-like UHMWPE fibers reinforced epoxy (EP) composites were improved simultaneously, which were attributed to the special interface structure between UHMWPE fibers and EP. The strengthening mechanism of the composites is mainly due to the bamboo-like crystals on the fiber surface, which plays a role of pinning and increases the crack propagation path.

Introduction

Fiber reinforced polymer composites are widely used in many fields [1]. Several kinds of fibers have been applied as reinforcing materials, such as carbon fibers [2,3], glass fibers [4,5], aramid fibers [6], and ultra-high molecular weight polyethylene (UHMWPE) fibers [7,8]. UHMWPE fibers possess many advantageous properties, including light weight, high tensile strength and modulus, and good resistance toward chemicals and wear, but also several drawbacks, such as inert surface and brittleness [9]. Therefore, the main disadvantage of UHMWPE fibers composites is their interface strength, which leads to difficulties in effective transfer of load between UHMWPE fibers and polymer matrix, and restricts their wider applications [10]. Efforts to improve the interfacial adhesion of UHMWPE fibers and polymer matrix have been mainly devoted to surface modification of UHMWPE fiber through plasma treatment [11,12], corona discharge [13], UV induced grafting [14,15], chemical oxidation or coating treatment [16]. Strong interfacial bonding increases strength but often leads to a decrease in toughness, that is why in most materials, the properties of strength and toughness are mutually exclusive.

People are more pursuing the improvement of material strength [17], however, applications continually strive for higher efficiency and better performance, with increasing demand for materials with high strength and toughness. Therefore, it may be worthwhile to consider approaches that use microstructural designs to offer reasonable strength and toughness [18]. At present, some progress has been made in this field. For example, Peng et al. [19] prepared HDPE/UHMWPE blends, forming shish-kebab structure at the molecular level, with superior strength and toughness performance by using loop oscillatory push-pull molding. A few studies present novel concepts where fiber structures are tailored to enhance strength or toughness. He at al [20]. prepared UHMWPE/HDPE blend fibers during a hot-stretching process and the shish-kebab structures were induced by drawing flow, which was PE lamellas in nanoscale grew around UHMWPE fiber. The mechanical performance of the HDPE/UHMWPE blend fibers was significantly improved, unfortunately there is no data on composite properties. Zhu et al. [21,22] prepared polyethylene bone-shaped-short fiber reinforced polyester composites and proved that fiber morphology, instead of interfacial strength, could solve the problem of increasing strength and toughness at the same time. These works have showed that tractions across bridged cracks are significantly improved by shaping the ends of short fibers to be like dog-bone heads [18]. However, because of the long flat part in the middle of the bone-shaped fibers, the middle part is the weak point for failure of such materials.

On the basis of above results and inspired by the morphology of bamboo in nature, in this paper, we propose to design and prepare a kind of bamboo-like fiber, the characteristic of which is a number of protrusions grow on the surface of UHMWPE fiber, so as to solve the problem of bone-shaped fibers. Referencing the method for decorating UHMWPE fiber, which will achieve the crystalline polymers to “wrap” fiber, we plan to decorate UHMWPE fiber surface using water-soluble polymer. Li et al. [23] used this method to grow polyethylene lamellas on the surface of carbon nanotubes. The advantage of this method is that the integrity of UHMWPE fiber structure is not disrupted and the properties of the UHMWPE fibers are therefore retained. The specific chain registry of the polymer upon UHMWPE fiber may occur due to the epitaxial growth of polymer crystals [24] on UHMWPE fibers and a stronger adhesion between UHMWPE fiber and polymers will be achieved.

In this study, bamboo-like UHMWPE fibers were prepared using polymer crystallization method. At present to our knowledge, there is limited literature about UHMWPE fiber morphology design and the mechanical behaviors of its composites with special interface. We hypothesize that the weak interface bonding between UHMWPE fibers and resin matrix can result in improved strength and toughness in the prepared composites. To achieve this goal, we added the bamboo-like UHMWPE fibers to epoxy (EP) matrix and both the strength and toughness of the composites were evaluated. The strengthening mechanism of bamboo-like UHMWPE fibers/EP composites were also explained.

Section snippets

Materials

The UHMWPE fibers were supplied by Dacheng Advanced Material Co., Ltd., China, and the diameter and length are about 40 μm and 3 mm, respectively. Epoxy and other chemical reagents (analytical grade) were purchased from Kunshan South Asia epoxy resin Co., Ltd., Jiangsu, China and Sinopharm Chemical Reagent Co., Ltd..

Preparation of bamboo-like UHMWPE fiber

Before preparing the bamboo-like UHMWPE fiber, the surface treatment of as-received fibers was needed. UHMWPE fibers were pretreated by chromic acid and the treatment process was

Formation of bamboo-like UHMWPE fiber

To prepare the bamboo-like UHMWPE fiber, a polymer solution crystallization technique was employed. PE was used as the model polymer. p-Xylene was used as the solvent for controlled solution crystallization. Fig. 1 shows SEM and optical microscope images of PE-decorated UHMWPE fiber after solution crystallization. It is evident that UHMWPE fibers are decorated with bamboo-like objects, which are PE crystal. The similar structure has been reported by Li et al. [24] They found that PE lamellae

Conclusions

In this work, we have obtained a novel bamboo-like UHMWPE fiber through controlling polymer solution crystallization. Multiple lamellar crystals were formed, periodically spaced along entire UHMWPE fiber. Bamboo-like UHMWPE fibers reinforced epoxy composites were prepared and in the case of weak interface, it was realized the improvement of both strength and toughness. The strengthening mechanism of the composites was attributed to the special fiber structure, which played a pinning effect and

Acknowledgements

This work was supported by the National Science Foundation for Young Scientists of China [grant numbers 51203080]; and K. C. Wong Magna Fund in Ningbo University.

References (31)

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