Homogenisation of Natural Fibres in Production of Composite Reinforced Scaffolds

Natural fibres including flax and cellulose/biomass are attractive as possible alternatives to reinforcement of polymer matrix composites due to their high mechanical and biodegradable property. Also, these fibres are environmentally friendly, economical and biocompatible. However, major limitations include identification of natural raw materials and nanosizing. To overcome these problems, researchers have developed a number of techniques such as pre-treatment and surface modification of natural fibres. Furthermore, mechanical processes such as homogenisation have been employed to produce nano-sized fibres - however, it is a high-energy consumption process, which is uneconomical and therefore not commercially viable [134]. Alternatively, cryocrushing [135], enzyme pre-treatments [136] and chemical pre-treatments [137] are employed to reduce size of the fibres prior to homogenisation, which helps to reduce the energy consumption and makes the process more economical.

Cellulose is a natural biopolymer composed of р-D-glucopyranose units linked together by p-1-4-linkages and present in animals, plants and some bacteria [138]. Introduction of nano-sized cellulose fibres into other polymers helps to improve the mechanical properties of the reinforced scaffold. These nano-reinforced scaffolds showed low thermal expansion, high aspect ratio and good mechanical/optical properties. One of the widely used techniques to produce nano-sized cellulose fibre is high-pressure homogenisation (HPH). The process involves the pumping of cellulose slurry through a very small nozzle at high speed and the pressure into a vessel generates shear rate in the stream, which converts micro-sized cellulose fibres to nano-sized fibres. For example, [139] first produced nano-sized cellulose fibres from wood pulp using HPH [139]. They reported that the main drawback of HPH process was the clogging of the orifice due to is small size. To avoid this problem, the size of the fibres needs to be reduced prior to the HPH using one of the many mechanical pre-treatments available. Nanofibrillated cellulose from wheat straw and wood fibres has been pre-treated by milling and homogenised at 150 MPa [140]. Refining can also be used as a pre-treatment method, whereby the diluted fibre slurry is pumped between stator and rotor disks. The grooves of the rotor help to create sequential stress, which causes irreversible alterations and increases the bonding potentials [141].

To fibrillate nano-sized fibres from hard wood and soft wood, [142] used a refining and homogenisation technique at 50 MPa (Fig. 3.8) [142]. They reported that

Effect of mechanical refining on hard- and softwood cellulose fibres, before and after 5, 25, and 75 passes through the refiner

Fig. 3.8 Effect of mechanical refining on hard- and softwood cellulose fibres, before and after 5, 25, and 75 passes through the refiner. The presented images are regarded to be representative of the material after the applied number of processing steps [142] © by ACS - reprinted with permission

the diameter of nano-sized fibres was around 10-25 nm and increasing the number of cycles exhibited a decrease in the strain to failure and strength of softwood. Also they demonstrated that refining was more efficient and faster for softwood when compare to hard wood pulp. Due to internal and external fibrillation phenomenon of the cellulose, softwood broke after 25 passes, whereas even after 75 passes hardwood fibres remained intact.

Alternatively, microfluidisers are regularly used to produce nano-sized fibres. The principle behind a microfluidiser is like HPH. The fibres collide under high pressure to defibrillate the fibres using shear and impact forces against colliding streams and channel walls [143, 144]. Increasing the number of cycles (< 20) has been shown to cause agglomeration due to high surface area. Microfluidisers gener?ate nano-sized fibres with a more homogeneous size distribution. Also, adoption of the microfluidising methodology does not significantly change the kappa number of nano-sized cellulose fibre compared with original cellulose. To reduce the cellulose to nano-size fibres, grinding is another potential method. The pulp slurry passes through static and rotating grinding stones, which helps to break cell wall structure using shear forces. Different pulp materials such as bleached eucalyptus pulp, bleached rice straw and bagasse pulps have been tested using this technology to reliably and efficiently produce nano-sized fibres [145, 146]. When the input energy increased from 5 to 30 W/kg, the fibres formed mainly in two forms: (1) highly kinked and untwisted fibrils and (2) entangled and twisted nanofibres. Extended fibrillation of untwisted nano-sized fibres could form nano-sized whiskers with high crystallinity. Also, refining was the main process for isolation of the nano-sized cellulose fibre, while HPH helped to form smaller and uniform size nanofibres. These processes do not affect the degree of polymerisation of the fibres, however the number of cycles demonstrated during HPH influenced the characterisation of nanosized cellulose fibres.

Examples of other mechanical fibrillation methods include cryocrushing and high intensity ultrasonication (HIUS). Cryocrusing involves soaking cellulosic fibres in water and then immersing the fibres in liquid nitrogen - followed by crushing using a mortar and pestle. The cell walls of the frozen fibres rupture due to high impact force thus forming nano-sized fibres. For HIUS, the hydrodynamic force of ultrasound isolates the cellulose fibrils.

The aforementioned mechanical fibrillation methods need high consumption of energy in order to operate; therefore such techniques are not economical for producing nano-sized fibres of good yield and appropriate length. Consequently either pre-treatment and/or combination of multiple methods have been to nano-sized natural nanofibres. For example, refining and microfluidisation results in uniform nano-sized cellulose fibres with high fibrillation when compared to individual methods [147], combination of HIUS and HPH produced uniform nano-sized fibres [148]. Overall, homogenisation produces nano-sized cellulose fibres demonstrating a high surface area. However, grinding and microfluidiser showed nano-sized fibres exhibiting good mechanical, optical and physical properties with low energy consumption.

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