A comprehensive review on surface modification of UHMWPE fiber and interfacial properties

Abstract

Ultrahigh molecular weight polyethylene (UHMWPE) fiber is considered as an ideal reinforcing component due to its high strength-to-weight ratio, good toughness and high chemical and wear resistance. However, poor interfacial adhesion with polymer matrices has hindered the development of UHMWPE-based composites with high performance. This review is intended to understand how physical and chemical changes of fiber surface promote the adhesion strengthening mechanisms at the interface and to guide future developments using the presented modifications’ techniques. The review summarized the present state of the art research on surface modification of UHMWPE fiber and UHMWPE/polymer interfacial properties. Various surface modifications of UHMWPE fiber categorized as ‘wet’ chemical and ‘dry’ techniques have been detailed with appropriate examples. Also, the relationship between fiber/matrix adhesion and mechanical properties of the composites has been reviewed. Lastly, an overview of the potential and challenges of each modification method has been discussed.

Introduction

An escalating demand of lightweight and high-performance materials have widened the prospect and significance of composite materials in particular fiber reinforced composites. Fiber reinforced composites have already substituted the state-of-the-art metal alloys as structural components in aerospace and automotive sectors. It is palpable that the onus on contemporary technology to implement lightweight electric vehicles would further reinvigorate the importance of fiber-reinforced composites in days to come. Among all the reinforcing fibers, ultra high molecular weight polyethylene (UHMWPE) has attracted tremendous attention from researchers and industrial engineers on account of its startling mechanical properties, lower density, high chemical resistance and impact strength, low moisture absorption, and high resistance abrasion [1], [2], [3], [4]. UHMWPE fiber witnessed a surge of research activities right after its commercialization in the late 1970s for a range of applications, including ballistic protection, aerospace, automotive, defense, and increasingly in medical device [5], [6], [7], [8], [9]. Despite of this wide acceptance in the realm of composites fabrication, its low surface energy, non-polar and inert nature has proven to be detrimental in the endeavour to design next generation high-performance materials [10], [11]. In fiber reinforced composite formulation, fibers are entrusted to carry the applied loads, while the surrounding polymer matrix acts as a means of transfer [12]. The performance of the composites relies mainly upon the intrinsic properties of the participating components, extent of the impregnation of fiber with polymer and more importantly the degree of adhesion between fiber and matrix at interface. Adhesion can be promoted through either physico-chemical or mechanical interlocking or both. The physico-chemical contribution towards adhesion includes chemical bonding, intermolecular interactions, and physical adhesion. While mechanical interlocking describes the adhesion when a polymer matrix penetrates surface defects, such as micropits, valleys, crevices or other surface irregularities of fiber and mechanically locks into it [13], [14], [15], [16]. The chemical bonding and intermolecular interaction can be altered by introducing appropriate chemical functional groups on the fiber surface, while enhanced surface roughness and augmented reactive sites promote superior mechanical interlocking. In many cases, both physico-chemical and mechanical interlocking work simultaneously in enhancing the extent of interfacial adhesion between the fibers and the matrix. Thus, to execute load-carrying task satisfactorily; the fibers must bridge itself firmly with host polymer matrix either through chemical bonding/interactions or mechanical interlocking, thereby delaying the inevitable fracture and failure of the composites. Surface modification or functionalization of fiber has been widely employed to improve the adhesion by introducing polar functional groups and promoting suitable surface roughness [2], [3], [17]. It is a prudent and deliberate approach to alter the surface chemical environment and surface topography of the fiber in order to invoke the adhesion-promoting mechanisms.

The appealing physical and mechanical properties of UHMWPE fibers have been attributed to its highly oriented crystalline microstructure polyethylene chains [18]. Secondary bonding (i.e., van der Waals interactions) is responsible for holding the crystalline chains with each other, and as the chains are long and oriented in same direction the large overlap aids the fibers to endure high tensile loadings. Nowadays, gel-spinning processes are commonly used to produce UHMWPE fibers. An oxygen-rich thin boundary layer is generated during spinning of UHMWPE fibers, which is believed to reduce the adhesion properties of fibers [19]. Thus, to optimize the adhesion of the fibers, elimination of oxygen-rich boundary is indispensable, which necessities the surface modification of UHMWPE fiber. These modification processes can be classified into two categories: ‘wet’ chemical and ‘dry’ methods. ‘Dry’ methods include plasma treatment [20], [21], [12], corona discharge [12], electron beam (EB) irradiation [22], gamma irradiation [23]; while ‘wet’ chemical methods include oxidative acid etching [19], coating treatment [24] and chemical grafting [25]. At the molecular level, most of the modification procedures involve the introduction of oxygen-rich functional groups on UHMWPE fiber surface, which render effective sites for chemical bonding. Further, the surface treatment would introduce irregularities or surface roughening such as micro-pits, which act as mechanical anchor points promoting mechanical interlocking of polymer matrix to fibers. While the frictional contributions to adhesion may be prominent in some ceramic matrix composites [26], regarding polymer-matrix composites, physico-chemical contribution is more prominent [27]. Thus, to ensure a robust interface, an appropriate scale of physico-chemical interactions is imperative, which could be promoted by van der Waals interactions, hydrogen and chemical bonding between fiber and polymer matrix. Several surface modification techniques have been successful in enhancing the adhesion at the interface via mechanical interlocking and chemical interactions/bonding [24], [28], [29]. However, to achieve superior interfacial adhesion, a combination of modification methods has also been employed [30], [31]. Of late, nano-reinforcement such as carbon nanotubes (CNTs), nanoclay, and graphene has been used to modify the polymer matrix to be used along with fiber to achieve the highest possible interfacial interaction through synergy [29], [32], [33].

In the present context, where conventional modification techniques are extensively used to modulate fibers viable for potential industrial applications; there is no consensus among the researchers and engineers as which modification techniques imparts maximum benefits while retaining the integrity of the fibers. Thus, it is necessary to highlight the pros and cons of existing UHMWPE fiber modification methods and envisage tangible routes for further developments. In response to the increasing publications, this review intends to cover the latest development on surface modification of UHMWPE fiber and the current position of modified UHMWPE fiber reinforced composites. Different modification techniques have been described with appropriate examples to highlight the influence of the surface modification to the interfacial properties. Further, based on the available literature, strength and limitation of each modification technique have been identified.