Use of lignin as a compatibiliser in hemp/epoxy composites
Highlights
► Lignin was used as a compatibiliser in hemp-epoxy composite samples ► Impact strength of samples are increased as amount of lignin is increased ► Tensile and flexural strength of samples show optimal amount of lignin is required ► Addition of lignin gives improved structural properties of composites
Introduction
Fibre-reinforced composites (FRCs) are ubiquitous in today’s society, in areas such as construction of components for boats, cars and aeroplanes, as well as sports equipment such as tennis racquets and golf clubs. Their benefits include high specific modulus and high specific strength, making them ideal for applications requiring good material properties with low weight. They are made from a combination of a matrix, which can be either a thermoplastic or thermoset polymer, with reinforcements such as glass, carbon, aramid or natural fibres. There has been a wide range of work looking at the properties of these types of composites, either by changing the type of matrix or the type of fibre [1], [2], [3], [4], [5], [6].
Two common types of matrix are ester-based (typically polyester and vinyl ester) and epoxy-based resins. The former are normally used in low-performance applications such as bathtubs, shower trays and piping. They do not adhere well to the surface of carbon fibres and are therefore tend to be used in conjunction with glass fibres. For more high-performance composites, epoxy resin is used. When reinforced with boron, carbon and glass fibres its strength is comparable with titanium, steel and aluminium alloys, while showing a significant reduction in weight [7]. Epoxies are commonly used on commercial aircraft and in the manufacture of sporting equipment.
The majority of energy production for industry comes from non-renewable resources, increasing concerns over the volume of fossil fuels used for energy generation. The use of natural materials which require little energy is therefore desirable. Plant fibres grow naturally, and therefore require minimal man-made energy input. Natural fibres suitable for reinforcement of polymer materials generally contain large amounts of ligno-cellulosic matter. Lignin and cellulose are stringy, tough, wood and plant fibres which help to maintain the structure of plants. Plants high in ligno-cellulosic fibres include hemp, jute (hessian), kenaf, flax, coir, wood and pineapple [8]. Apart from the low process energy required during their manufacture, natural fibres are attractive to create natural fibre-reinforced composites (NFRCs) because of their renewable and sometimes biodegradable characteristics. Even if composting of a particular fibre is not possible it can be burned to recover energy while producing no net increase in carbon dioxide in the atmosphere. This energy recovery process is not possible with glass fibres due to the high temperatures necessary and because of their tendency to cause soiling of the furnace.
However, a disadvantage of natural fibres is their lack of availability as a woven, engineering material. Most natural fibres used currently in the manufacture of biocomposites are made from chop strand mat, and therefore have correspondingly low mechanical properties when compared to woven synthetic fibres such as carbon. However they are generally cheap, widely available and biodegradable. One of the main problems with using natural fibres as reinforcement is the poor interface between the hydrophobic fibres and the hydrophilic resins [9].
When composites were beginning to be developed it was assumed that this interface was able to transmit stresses between the fibre and the matrix perfectly [10], however as photoelastic techniques were developed to allow a visual analysis of stress in a fibre, these theoretical models were found to underestimate the stresses encountered close to the fibre ends [11]. At relatively low applied stresses, the shear stresses at the fibre ends of a reinforced composite could exceed the interfacial shear strength, leading to failure by debonding and fibre pull-out.
Fig. 1 shows the deformation that occurs in the area of matrix surrounding a single fibre that is subjected to tensile loading. More recently it has been established that the final mechanical properties of a composite are dependent on the magnitude of the strength of the bond between the fibre and matrix [12], [13], [14]. This interface is the limiting factor of fibre-reinforced composite performance as it ultimately defines the amount of load that can be transferred from one fibre to the next by the matrix. A review of some of the research carried out in this field was conducted by Herrera-Franco and Drzal [15]. Work done to model the interfacial bond suggests that there is not a distinct interface between fibre and matrix, but rather an interphase region resulting from the complex chemical interactions between the resin, sizing agents, and the mechanical surface of the fibres [16]. As such, the poor adhesion that exists between the fibres and resins prevents NFRCs from having commercially-useful structural properties.
There are a wide variety of treatments that can be used to improve the fibre–matrix adhesion in composites [4], [5], [17], [18]. The vast majority of these involve some form of chemical processing such as mercerisation with sodium hydroxide solution [19], [20], [21] and acetylation with acetic anhydride [9], [22], [23], [24]. However, it is desirable to reduce the chemical input and associated wastes with the process. There are some alternative treatments e.g. steam explosion [25], [26] that avoid some of the chemical input to the composite manufacture; however there is a large associated energy cost with generating the steam required.
An alternative to chemical treatments are the use of natural materials as compatibilisers in the composite structure. Lignin is particularly interesting as it is a waste product from the paper industry [27]. Previous work has shown that lignin can be used as an additive in composite fabrication by RTM; Wool et al. showed that lignin can impart beneficial properties to the structure of a composite by either dissolving the lignin in aqueous sodium hydroxide [28] or chemical modification of the lignin with butyric anhydride to solubilise it in an epoxy resin [29]. However both of these methods still require some additional chemical processing to the lignin before the composite is manufactured. Lignin has also been utilised in compression moulding techniques to make natural fibre–polypropylene composites [30], [31] although this method uses high temperatures which can be potentially damaging to the natural fibre reinforcement and impair the structural properties of the composite.
In this work, it was proposed that even in the solid state, the lignin would improve fibre-to-matrix adherence and structural properties of the resulting composite whilst keeping the number of steps and chemical treatments to a minimum. Hemp fibres were chosen for the reinforcement because of the availability and cost effectiveness of this material in the UK. Epoxy resin was selected as the matrix as this is used in a variety of high performance applications, and therefore has industrially relevant properties.
Section snippets
General considerations
Kraft lignin was generously supplied by Warwick HRI and dried under vacuum to constant weight before use to remove volatile material. Chopstrand hemp mat was purchased from Hemcore and EP-522 resin and H-522 hardener were purchased from Alchemie.
Lignin characterisation
Particle Size Analysis was carried out using a Carl Zeiss AxioScope A1 upright optical microscope with fully automated stage. A small sample of the lignin powder was placed between glass slides and then mounted in the microscope. Dark field microscopy
Resin infusion
Hemp–epoxy composites were prepared where the resin had a range of lignin contents (0, 1, 2.5 & 5% w/w). Moisture was removed from the lignin before use by drying to constant weight; using ‘wet’ lignin caused an uncontrollable exotherm in the setting stage and prevented successful infusion. An attempt at a composite with the resin containing 10% w/w lignin was also made but was unsuccessful; addition of lignin to the resin increases the viscosity and at 10% w/w the resin becomes too viscous to
Conclusions
Composites made from an epoxy resin and natural hemp fibre reinforcement with varying amounts of added kraft lignin were fabricated. The addition of lignin was shown to be beneficial towards improving the impact, tensile and flexural strength, although the latter two also showed a decrease when excessive lignin was added. This is attributed to the lignin particles preventing complete resin infusion across the hemp mat which subsequently reduces the physical properties. This method of adding
Acknowledgments
We would like to thank the Warwick Innovative Manufacturing Research Centre (WIMRC) for funding this research and Dr. Vannessa Goodship for her helpful discussions regarding data interpretation.
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2022, Materials and DesignCitation Excerpt :Consequently, quite a few attempts have been made to apply lignin as a coupling agent in various polymer/lignocellulosic fiber composites [62–74]. Although the lack of a coupling effect has been reported occasionally in related papers [62,67], very often coupling and definite improvements in properties have been claimed for at least at a specific concentration range [63–66,68–74]. Unfortunately, these claims of coupling are again based on visual observations of primary results, as well as on the belief and conviction of the authors, without any reliable quantitative analysis.