Direct fluorination of UHMWPE fiber and preparation of fluorinated and non-fluorinated fiber composites with LDPE matrix

Abstract

In this study, the surface of ultra-high-molecular-weight polyethylene (UHMWPE) fiber used as reinforcement in low-density polyethylene (LDPE)/UHMWPE fiber composites was modified by direct fluorination using elemental fluorine, (10% F₂+90% He) mixture, at 25 °C. A polymer composite based on LDPE/UHMWPE fibers was prepared. We studied the influence of surface treatment of the fibers on properties of the composite. The fibers were first used as received, and then were used after fluorine (direct fluorination) treatment of their surface. Stress–strain behavior of the prepared samples was measured. Thermal properties and the trans-crystallization of the matrix over the fibers were observed. The crystallization of the matrix on the fiber surface was followed by scanning electron microscopy and by differential scanning calorimeter (DSC) analysis. The structure of the interface was then observed by X-ray diffraction. Surface energy was determined by contact angle measurement and it indicates that the wetting properties of the fluorinated fiber composites were improved.

Introduction

High-performance fiber composites with a polymeric matrix have been developed as innovative materials for future air and spacecraft technologies as well as conventional machinery design and construction. These materials increasingly replace classic construction materials such as steel and aluminum, particularly when a high ratio of mechanical properties to density is required. For technical purposes, thermosets are mainly used as matrix materials. However, the application of thermoplastics promises a number of advantages because manufacturing (and technology) is simple. Thermoplastic composites are superior with respect to impact strength, surface quality, and storage capability [1]. Nowadays, due to ecological considerations and limitations on recycling of used polymeric materials, it is necessary either to produce composites that can be biologically degraded by micro-organisms or to develop fiber-reinforced materials consisting of fibers and matrix with identical chemical structure (one-polymer-composite). Such composites can be easily recycled by a melting process, e.g. by extrusion or injection molding, and can be used in a second application.

Ultra-high-molecular-weight polyethylene (UHMWPE) fibers currently represent synthetic fibers whose mechanical performance (specific modulus and strength) is among the most interesting on the market. They are used successfully in many fields because of their properties, such as biocompatibility, hydrophobic nature, chemical resistance and electric insulation. However, low melting point and poor fire resistance restricts their field of use. Moreover, use of these fibers as reinforcing material in composite applications is basically impossible because of their non-polar chemical nature which makes strong interactions with most polymeric matrices impossible. So, insertion of polyethylene fibers (PEFs) does not allow optimal transmission of interfacial stresses of one fiber on another, and this limits the application. Nevertheless, some tests were carried out to use these high-performance fibers as reinforcement in composite materials, mainly in thermosetting matrices of the epoxy type, by treating the fiber surface by plasma or a suitable chemical treatment [2], [3]. Some studies were also undertaken by Devaux et al. [4], [5], where the chemical similarity of structure between fiber and surrounding matrix is exploited, i.e. the extreme surface of the UHMWPE fibers whose melting point is higher than that of the matrix (polyethylene) melts, and co-crystallization occurs. This co-crystallization results in the appearance of a transcrystalline interphase, which improves the adhesion between the two materials present.

Reinforcing a common polyethylene with PEFs leads to a strong and stiff single polymer composite [6]. Lacroix et al. [7] studied the TC of UHMWPE fibers/HDPE composites, and found that the surface crystallization UHMWPE fibers act as nucleation centers for the high-density PE matrix, which might result from epitaxial crystallization. After crystallization from the melt, a TC layer was found having lamellar crystals grown perpendicular to the fiber axis, which was independent of air-cooling, or isothermal crystallization conditions. Vaisman et al. [8] reported that UHMWPE fibers treated by photochemical bromination, resulting in a higher concentration of crystallization nuclei, generated a denser TC layer with higher specific radial orientation with respect to the fiber axis compared with the untreated fiber. HDPE [9] is often used as the matrix, while the fiber reinforcements are usually of the same type of polymer [10], [11], [12], i.e. PE fibers. He and Porter [13] reported that, aided by the similarity between the HDPE matrix and high-modulus PE fiber, a higher PE fiber fraction in the composites increased the nucleation density. The isothermal crystallization of a single PE composite with 50 wt% fiber fraction showed dual-step crystallization. The ultra-high-modulus PE fiber exhibited very good nucleation ability toward linear high-density polyethylene (LHDPE) [14]. Also, when recycled short GF was used with new PE matrix, the residual matrix could recrystallize [15].

However, contact of this kind of fiber with a matrix of a similar chemical nature may be improved by modifying the interfacial physico-chemical interactions. We have established [16] that a PE matrix, whose melting temperature is clearly less than that of the fiber, can be processed without causing significant damage to the fiber. Surface modification is a well-established method for enhancing the performance of polymeric materials in a number of applications. A particularly effective approach to surface modification is direct fluorination, i.e. the treatment of a polymer surface with gaseous fluorine or a fluorine gas mixture. Due to the relatively weak F–F and strong C–F bonds, fluorination can proceed spontaneously at practically acceptable rates at ambient temperatures without the need for any initiation [17]. Moreover, a suitable surface treatment of the reinforcing material may have a positive effect on adhesion by eliminating the existing weak boundary layer and by increasing the surface contact between the two components. In addition, such a chemical treatment increases the surface energy of the fiber and, thus, favors wetting by the liquid matrix. Kazanci et al. [18] studied extensively single-polymer composites based on UHMWPE fibers in a PE matrix with different types of PE and a range of length and orientation of the fibers.

The present study has been driven by the idea that a specifically selected surface treatment of the PE fiber can generate a double effect, in which the polarity of the fiber surface is increased. The identical chemical nature and crystalline morphology of the constituents in this composite system result in their mutual compatibility at the interface. This generates, in addition to a relatively good interfacial adhesion, a unique fiber/matrix interface, in which the matrix crystallizes preferentially on the fiber surface. It has been hypothesized that surface fluorination of the PE fiber can implant polar moieties on the fiber surface which serves to enhance matrix nucleation and transcrystallization, and improve the mechanical properties of the system.