Incorporating Silica Nanoparticles into Fiber Sizing

Incorporating silica nanoparticles into fiber sizing to improve the properties of corn fiber reinforced polylactide composites

Abstract

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Nanofiller-modified interfaces in man-made carbon and glass fibers reinforced composites have been investigated, while no such investigation was reported for natural fibers. In this work, we report a novel strategy of improving the interfacial nature in corn fiber/polylactide (CRF/PLA) composites by directly applying a sizing containing silica nanoparticles on the surface of corn fibers. This results in enhanced mechanical properties of the CRF/PLA composites. These improvements are mainly due to the presence of silica nanoparticles on corn fiber-PLA interfaces, which act to resist the crack propagation. The increased surface roughness of corn fibers from incorporated silica nanoparticles may also contribute to the enhanced mechanical properties. This simple methodology can be easily scaled up and thus shows great promise in industrial applications.

1. Introduction

It is well accepted that the fiber/matrix interface in composites controls not only the mechanical properties, but also physical properties [1-4]. Generally, a strong interfacial bonding helps to achieve high stiffness and strength, while a relatively weak interfacial bonding improves the energy absorbing performance under impact conditions [1, 2]. The manipulation of fiber/matrix interface is mainly achieved through surface treatment of fibers. In fact, all reinforcing fibers, particularly carbon and glass fibers, used as the reinforcements of polymer composites are surface treated [2]. After surface treatment, a polymer coating (typically epoxy), referred to as a “sizing”, is applied to protect the carbon and glass fibers during handling and improve the processability of fiber yarns [5, 6].

In recent years, apart from glass and carbon fibers, the development of green composites made from natural fibers has been a hot topic all over the world. The major motivation of using natural fibers is their lower cost (priced at one-third or less of the cost of glass fibers), weight reduction (half the weight of glass fibers), and easy recycling. There are thousands of different natural fibers in the world but only few of these fibers have been studied. Among the most popular natural fibers, flax, jute, hemp, sisal, ramie, and kenaf fibers are extensively researched and employed in different applications. Besides those natural fibers, corn fibers are also gaining interest due to their lower cost and more abundance than other natural fibers as they are waste of farms. It is believed that the use of fibers from agricultural waste as reinforcements in green composites offers a low cost and environmentally friendly solution to waste disposal and a possibility for farmers to gain a profit from waste [7-12].

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Unlike glass and carbon fibers, natural fibers possess very different properties. They are composed of bundles of elementary fibers, contain voids and defects, their cross-section is irregular, and their surface nature is hydrophilic. The hydrophilicity of natural fibers translates to incompatibility with hydrophobic polymer matrices. Similar to man-made glass and carbon fibers, to improve the fiber/matrix adhesion in natural fibers reinforced composites, surface treatments are also mandatory. Up to date, many methods have been reported [2, 13]. In general, surface treatments can be classified into two categories, physical and chemical methods [14-17]. Compared to physical methods, chemical methods are more effective and stable [18, 19]. A vast range of chemical treatments (alkaline, bleaching, coupling with silanes, acylation, grafting by monomers etc.) have been developed to improve the interfacial adhesion [13]. Unlike the alkaline treatment which is the most widely applied, surface sizings of natural fibers are still in their infancy [2]. However, the advantages of surface sizing such as improving handling properties of fibers, enhancing fiber wetting by the matrix and in particular improving fiber/matrix adhesion [20-22] make it a very promising route [2]. Interestingly, recent studies have demonstrated that adding nanofillers into sizing formulations can improve interfacial bonding and is thus gaining more and more attention [2, 6].

The reported nanofillers in sizing formulations include halloysite nanotubes [23], carbon nanotube (CNT) [24-26], graphene [27], graphene oxide [28-30], and silica (SiO₂) nanoparticles [6, 31-34]. For example, to improve the interfacial properties in carbon fiber/epoxy composites, Zhang et al. [28] directly introduced graphene oxide (GO) sheets dispersed in the fiber sizing onto the surface of individual carbon fibers and significant enhancements of interfacial shear strength, interlaminar shear strength, and tensile properties were achieved in the composites when 5 wt. % of GO nanosheets were introduced in the fiber sizing. Godara and co-workers [35] directly dispersed CNTs in a fiber sizing formulation of E-glass fibers. They compared the fiber sizing method to other two ways of incorporating CNTs to the matrix and incorporating CNTs in the fiber sizing and matrix simultaneously. They declared the improvements of interfacial shear strength in all three cases while the maximum improvement is achieved in the composite where CNTs are introduced solely in the fiber sizing. Yang et al. [31] applied a sizing containing pre-treated silica nanoparticles on the surface of carbon fibers, and significantly improved interfacial shear strength was obtained. Unfortunately, there is no report in literature regarding the nanofillers-contained sizing for the surface treatment of natural fibers, not to mention corn fibers.

In this work, a facile method to adjust the interface properties between corn fibers and polylactide (PLA) has been developed by adding nanofillers to the sizing solutions of corn fibers. Silica nanoparticles were selected as the model nanofillers since they are much cheaper than CNT, graphene, and /graphene oxide. The silica nanoparticles were dispersed in the fiber sizing and then directly coated on the surface of corn fibers. The surface-treated corn fibers (denoted as SiO₂@CRF) were used to reinforce PLA. The purpose of this work was to prepare PLA composites reinforced by SiO₂@CRF and to analyze the effect of incorporating silica nanoparticles in sizing solutions on the mechanical properties of the as-prepared SiO₂@CRF/PLA composites.

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2. Experimental

2.1. Materials

The silica nanoparticles (model W-200, average diameter = 60 nm, purity ≥99.9%) used in this work were purchased from Shouguang Baote Chemical Co., Ltd., Weifang, China. The matrix material (provided by Shenzhen Guanghua Weiye Industrial Co. Ltd., China) was an AI-0001 PLA with the basic parameters of Mn = (8-10) × 10⁵ and melt index = 512 g/10 min. The corn stalks were obtained from a farm in Hebei province, China. NaOH, analytical grade, was provided by Tianjin Chemical Reagent Co. Ltd, Tianjin, China. The KH550 silane coupling agent (g-aminopropyl triethoxysilane, molecular formula H₂NCH₂CH₂CH₂Si(OC₂H₅)₃, a colorless to slightly yellowish low-viscosity liquid) was purchased from Nanjing Chemical Industry Group, China.

2.2. Preparation of corn fibers

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The preparation process of corn fibers was similar to that reported in our previous work [7, 8]. Briefly, corn stalks were thoroughly cleaned and dried, followed by separation of straw skins using a skin separator (PRFII-0.2, Jilin Fenghe Botanical Development, China). The resultant skins were smashed to pieces using a JWF-250 Cutting Mill (Hongle Machinery Plant, Xingyang, Henan, China). Finally, a 40-mesh sieve was used to remove corn husks and debris and corn fibers with a length of ca. 3 mm were obtained.

2.3. Preparation of sizing solutions containing silica nanoparticles

First, silica dispersion was prepared by adding silica nanoparticles to the solution of sodium dodecyl sulfate (SDS, dispersant) under ultrasonication for 2 h. The ratio of silica nanoparticles to SDS was 1 : 2. Meanwhile, a sizing solution was prepared by hydrolyzing silane coupling agent KH550 in an ethanol aqueous solution (deionized water to ethanol ratio = 3 : 1 by weight) for 20 min at room temperature. The sizing material was added to the solution of KH550 with stirring at a rate of 250 rpm until a homogeneous solution was formed. The concentration of sizing agent was 12 wt.%, in the final sizing solution. The above-mentioned silica dispersion was mixed with the sizing solution, leading to four silica modified sizing solutions containing 0.5, 1.0, 1.5, and 2.0 wt.% silica nanoparticles, respectively. As a control, the unmodified blank sizing solution without silica nanoparticles was also prepared.

2.4. Surface treatment of corn fibers with silica nanoparticles-contained sizing solutions

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Prior to sizing treatment, corn fibers were treated with alkali. The process of alkali treatment was described in our previous work [7]. The alkali-treated corn fibers were dried at 95 °C in an oven followed by immersion in various sizing solutions containing silica nanoparticles under vigorous stirring for 30 min. Subsequently, the sizing-treated corn fibers were taken out of the solutions and dried in an oven at 80 °C for 12 h.

2.5. Preparation of SiO₂@CRF/PLA composites

PLA and corn fibers with varying sizings were blended at ambient temperature for 8 min at 60 rpm using a SLH-0.01 internal mixer (Shanghai Dongqiu Mixing Machine Co. Ltd., Shanghai, China). The mixed materials were then dried at 100 °C for 4 h. Finally, test specimens were injection molded at a temperature of 180 °C and an injection pressure of 70 MPa. The mass content of fiber in composite samples prepared in this work varied from 5 to 20%, unless otherwise indicated. The designation of all composite samples prepared in this work is listed in Table 1.

Table 1 The designation of the composites prepared in this work.

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