Improvement of coating durability, interfacial adhesion and compressive strength of UHMWPE fiber/epoxy composites through plasma pre-treatment and polypyrrole coating

https://doi.org/10.1016/j.compscitech.2016.03.026Get rights and content

Abstract

Ultra-high-molecular-weight polyethylene (UHMWPE) fibers have exceptionally higher specific strength and stiffness compared with other high-performance fibers. However, the interfacial adhesion and compressive performance of UHMWPE fiber-reinforced polymer composites (FPCs) are extremely low. The challenges are to achieve load transfer at the interface between the fiber and matrix at a molecular level. Here, we show that plasma pre-treatment of UHMWPE fibers followed by coating with polypyrrole (PPy) results in an 848% improvement in the interfacial adhesion and 54% enhancement in compressive performance. This method takes advantage of a toughening mechanism observed in spider silk and collagen, which the hydrogen bond power the load transfer. The results showed that these improvements of interfacial adhesion and compressive strength were attributed to hydrogen-bonding interactions between the plasma pre-treated UHMWPE and PPy, which improves the fiber–matrix–fiber load transfer process. In addition, the hydrogen-bonded PPy coatings also endowed durability electrical conductivity properties of the UHMWPE fiber.

Introduction

High-performance ultra-high-molecular-weight polyethylene (UHMWPE) fibers possess remarkable strength, stiffness and a low density (0.97 g/cm3). They have received much attention in the field of fiber-reinforced polymer composites (FPCs) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. The main disadvantage of UHMWPE FPCs is their lower compressive strength compared with other FPCs (e.g., carbon and Kevlar FPCs), which restricts their wide applications. The main reason for the low compressive strength is interfacial delamination (e.g., de-bonding), which leads to difficulties in effective transfer of the load between the UHMWPE fibers and the polymer matrix.

Efforts to improve the interfacial adhesion of UHMWPE FPCs have been mainly devoted to modification of the UHMWPE fiber surface through plasma treatment [7], [8], corona discharge [9], UV-induced grafting [10], [11], chemical oxidation or coating treatment [12], [13]. In particular, the coating treatment is distinct from the other modification techniques in that it is easy to operate, does not require special equipment and can effectively improve the load transfer [12], [13], [14], [15], [16]. Graphene oxide and carbon nanotubes have been reported as coating materials to improve the interfacial strength of FPCs [15], [16]. In a previous study, we reported the use of polypyrrole (PPy) to enhance the adhesion between UHMWPE fibers and epoxy [12]. The improvement was attributed to mechanical force of roughening surface and ionic bonding between the UHMWPE fiber and PPy coating [12], [13]. However, the coating–fiber interactions at a molecular level remain insufficient.

More recently, Kotov et al. [17] reported the use of hydrogen bonding to strengthen the nanoparticle–matrix interaction in nanoparticle-reinforced polymer composites (NPCs) and indicated that composites with interfacial hydrogen bonds were even stronger than those with interfacial ionic bonds. Hydrogen-bond-assisted load transfer has also been reported in spider silk and collagen [17], [18]. To form hydrogen bonds on the interface, a polymer matrix (poly(vinyl alcohol), PVA) containing hydroxyl groups plays an important role [17], [18], [19], [20]. However, using interfacial hydrogen bonds to improve the compressive performance of UHMWPE FPCs has not been reported in the research literature.

In this work, the hydrogen bond power load transfer mechanisms of natural materials have been closely reproduced in PPy-coated UHMWPE fiber/epoxy composites to enhance the interfacial and compressive performance. A two-step method was employed for the coating treatment, i.e., plasma pre-treatment of UHMWPE followed by PPy deposition. We demonstrate that the presence of hydrogen bond interactions effectively enhances the interfacial adhesion and compressive performance of the UHMWPE FPCs.

Section snippets

Materials

Analytical-grade pyrrole was purchased from Aldrich. Iron (III) chloride hexahydrate was obtained from Tianjin Chemical Reagent Institute. Both chemicals were used as received. UHMWPE fibers (115.6 Tex) were provided by Yizheng Chemical Fiber Company, China. Bisphenol-A with an epoxy value of 0.45–0.50 was obtained from Jingdong Chemical Factory (Tianjin, China).

PPy coating process

UHMWPE fibers were placed in a plasma chamber (Europlasma, CD1200PC, Belgium) under an oxygen atmosphere. The discharge power was

Preparation of PPy-coated UHMWPE fibers

Fig. 1A shows the two-step process of plasma pre-treatment and PPy deposition. UHMWPE fibers were pre-treated by low-temperature plasma to obtain P-UHMWPE fibers. After depositing FeCl3 on the pre-treated fiber surface, pyrrole was polymerized on the P-UHMWPE fiber surface to obtain PPy-P-UHMWPE fibers. This method is simple, low cost and continuous, which offers potential for commercial exploitation [21].

Fig. 1B and C show the photos of UHMWPE and PPy-P-UHMWPE fibers. The PPy-P-UHMWPE fibers

Conclusions

Plasma pre-treatment effectively improves the interfacial adhesion between the PPy coating and UHMWPE fibers. Such an improvement results from hydrogen bond interactions. In comparison with previous methods, this plasma pre-treatment method has two advantages: I) stiffening the UHMWPE molecular chain associated with the hydrogen bond on the interface leads to a highly effective load transfer between the epoxy and UHMWPE fiber and therefore enhanced interfacial and compressive performance and

Acknowledgements

The authors gratefully acknowledge the financial support from the National Nature Science Foundation of China (#51573136 and #51103101), Natural Science Foundation of Tianjin City, China (#15JCYBJC17800), China Postdoctoral Science Foundation (#20110490785 and #2011M500525), The Science and Technology Plans of Tianjin (No. 15PTSYJC00230, No. 15PTSYJC00250) and Funding from “State key laboratory of separation membranes and membrane processes” of Tianjin Polytechnic University, China (#M3-201504,

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Xin Jin and Wenyu Wang contributed equally to this study and share first authorship.

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