Tailoring of polyethylene fiber surface by coating silane coupling agent for strain hardening cementitious composite

Highlights

• A novel idea of using the SCA to strengthen the properties of SHCC was developed.

• The performance of SHCCwas measured to exhibit the improvement of modification.

• The method showed great advantages based on micromechanical model analysis.

Abstract

Polyethylene (PE) fiber is a common material for preparing ultra-high-strength strain hardening cementitious composites (SHCC). However, the interfacial bonding between fiber and matrix is weak due to the hydrophobic nature of the PE fiber surface and resulting in low PE fiber utilization. In this study, a method is proposed by applying silane coupling agents (SCA) coating to the fiber surface to enhance the interfacial bonding and further improve the mechanical properties of PE fiber reinforced SHCC. The KH172 is proved to be the best one in five selected SCAs for PE fiber surface modification by surface energy analysis. The coating feasibility and hydrophilicity of SCA coated PE fiber are verified by the infrared absorption test and contact angle test, respectively. The effectiveness of this approach is evaluated by means of the single fiber pullout test and uniaxial tensile test. With the SCA coating, the interfacial fractional bond between fiber and matrix is increased by 52%, and the slip hardening response is remarkable. The ultimate tensile stress and tensile strain capacity of PE fiber-reinforced SHCC are increased by 48% and 112%, respectively. Based on the micromechanical analysis, the mechanism of composite property improvement due to interfacial enhancement by fiber surface SCA coating is demonstrated, and one of the greatest advantages of this approach is the saving on the dosage of fiber.

Introduction

Strain hardening cementitious materials (SHCC) has been used in the critical position of the structure [1], traffic engineering [2], and explosion and impact projects [3] due to its excellent toughness and durability caused by strain hardening behavior and multiple cracking failure patterns [4]. The typical polyvinyl alcohol fiber reinforced SHCC (PVA-SHCC) has achieved a tensile ductility of 3–5% and compressive strength of less 60 MPa. [5], [6], [7]. When the compressive strength of the PVA-SHCC exceeds 60 MPa the tensile strain hardening behavior is difficult to be maintained [4]. To obtain an ultra-high strength and high ductility SHCC, multiple methods such as replacing and hybrid fibers [8], [9], [10], improving matrix compactness [11], [12], using nanomaterials modification [13], [14], and changing curing methods [15] have been widely adopted in recent years. A class of high strength high ductility SHCC is designed by replacing the PVA fibers with PE fibers and meanwhile tailoring the matrix with the addition of silica fume and fine aggregate to improve the compactness. The compressive strength, tensile strength, and tensile strain of PE fiber reinforced SHCC can achieve to 110–150 MPa, 10–20 MPa, and 3–8%, respectively [11], [12], [16].

A variety of high strength and modulus fibers, including carbon fiber, steel fiber, and PE fiber have been used to generate SHCC. Obviously, the PE fiber is more successful in preparing the SHCC. However, the interfacial bonding with the cement matrix is disappointing due to the lack of functional groups and low fiber surface wettability [14], [17], [18]. Many approaches are available for fiber surface modification, including etching modification [19], [20], chemical grafting [21], [22], and surface adsorption [13], [14], [23], [24]. However, the research on PE fiber surface modification is relatively limited due to PE fiber is composed of non-polar macromolecules and has excellent chemical stability. At present, the physical modification approaches are mainly applied to the PE fiber for increasing the surface roughness and then strengthen the interfacial transition zone between fibers and matrix [13], [14]. Li et al. have indicated that PE fiber surface after plasma treatment exhibits a higher interfacial frictional bond with the cement matrix than non-treated fiber [19], [20]. However, under the exposure of high energy, high-temperature of the plasma beam, the fiber is damaged, which eventually leads to decreases of fiber strength and the mechanical properties of SHCC materials. Yang et al. have indicated that the PE fiber surface can absorb carbon nanofibers (CNFs) via the hydrophobic interactions resulting in the enhancement of interfacial frictional bonding strength between PE fibers and matrix. This phenomenon is attributed to the nano-pores filling effect and nano-cracks bridging effect in the interfacial transition zone [13]. Similarly, Leung et al. reported that the graphene oxide coated PE fibers can strengthen the interfacial transition zone, resulting in a 71.2% increase in interface frictional strength [14]. The process of fiber surface modification aforementioned is complex, and the cost of nanomaterials is high, which may limit the applications of these approaches. It is necessary to explore a simple, low cost and more effective technique to enhance the fiber surface frictional bond and further improve the efficiency of fiber utilization.

Silane coupling agent (SCA), a material containing organic and inorganic groups in the same molecule, is widely used in chemical engineering to enhance the interface interaction between matrix and filler [25]. Wang et al. have studied SCA modification on the interface bonding property between resin and cement matrix. The results show that the addition of SCA can reduce the thickness of the interfacial transition zone (ITZ). Moreover, the formation of nanoparticles from SCA and cement hydration products make the ITZ denser [26]. Huang et al. used SCA to develop a cementitious coating layer around rubber particles to strengthen rubber-modified cement concrete performance. In this way, the compressive strength and tensile strength were increased by 10–20% compared to un-modified rubber reinforced concrete [27]. SCA is also used as a medium to graft silica onto the surface of carbon fiber and PVA fiber. The pullout force of grafted PVA fiber and carbon fiber from the cementitious matrix was increased by 43% and 88.4%, respectively [18], [21]. The SCA is effective for improving the interfacial property between the cement matrix and different reinforcements [28]. It is worthy of investigating the influence of SCA on the interfacial property between PE fiber and the cementitious matrix.

The mechanisms of interface modification in enhancing the properties of composite materials are demonstrated as follow: (i) physical effect, such as the changes of infiltration, diffusion, miscibility of each component, and interpenetrating network of the interface structure; (ii) chemical effect, which leads to chemical reactions in the interface and the formation of new interfacial layer structures; (iii) mechanical effect, which affects the stress distribution on the interface [29]. Since no chemical bond exists between PE fiber and matrix, the influence of interface on the properties of PE fiber reinforced SHCC is mainly attributed to the physical and mechanical effects. It is well known that the better the hydrophilicity of fiber surface, the greater the interfacial friction bond between fiber and cement hydrates. Yu Xiang et al. [24] have proved that the SCA coating can enhance the hydrophilicity of basalt fiber due to the existence of hydroxyl groups. Based on the similar mechanism, it is achievable to employ SCA modification to reduce the contact angle and enhance the hydrophilicity of the PE fiber surface so that the interfacial frictional bond between PE fiber and matrix will be improved [29].

In this study, the CSA coating approach was applied to the PE fiber surface, and the characteristics of the SCA coated fibers were identified by contact angle analysis and attenuated total reflection infrared spectroscopy analysis. To verify the effectiveness of SCA coating on improving interface property between PE fiber and matrix, the single fiber pullout test and the uniaxial tensile test were carried out to determine the interfacial property and tensile performance. The morphology of the pullout fiber surface was observed by scanning electron microscopy (SEM) to analyze the mechanism of interface enhancement. Furthermore, the micromechanical model was adopted to elaborate the reasons for the improvement of the mechanical properties.