Reinforcement Mechanisms of the Fibres or Tubes in the Scaffold

Engineering materials composed of two or more constitutes are called composites and mainly consist of a matrix and reinforcement [93]. Both these materials have different physical and chemical properties, but as a composite they are connected and distinct on a microscopic level [94, 95]. In polymer composite materials, the polymer acts as a matrix and the fibres acts as a reinforcement material. The properties of the scaffold depend on bond between matrix and fibres [96]. The main advantage of composite materials is the matrix holds/transfers the load along the reinforcement. In general, polymer materials are used as matrix and natural/syn- thetic fibres as the reinforcement. Polymer composites are classified into different types, fibre reinforced polymer composites where fibres are used as reinforcement either natural or synthetic and particle reinforced polymer composite, the reinforcement is converted into particle form of different dimensions [93, 97].

Based on the nature of polymer composites (renewable/non-renewable), they are categorised namely as renewable composites where both the matrix and reinforcement are based on 100% renewable materials. In partly renewable composites either the matrix or reinforcement material are obtained from renewable source. Both materials for non-renewable composites are obtained from non-renewable source [93].

Due to economic and environmental reasons, natural fibres are gaining much attention. Natural fibres have higher stiffness/strength compared with synthetic fibres. Although natural fibres show some disadvantages such as being hydrophilic in nature and less thermally stable [97]. However, to overcome these disadvantages, researchers have developed various techniques such as surface modification and pretreatment [98]. Many reviews have been published reporting detailed techniques and analysis for the chemical surface modifications of natural fibres [97, 99-102]. Natural fibres/polymer composites have been frequently used for many applications due to their mechanical properties [103, 104]. Some commercially important thermoset polymer composites are phenolic, epoxy, polyester, unsaturated polyester; vinyl ester composites, which are discussed herein.

In phenolic composites, the phenol-formaldehyde (PF) resin system is the most common. Using compression-moulding technique - natural fibre reinforced with PF resin system has been developed [105]. In this study, the authors analysed various mechanical properties such as tensile strength, flexural strength, and compressive strength and wear resistance with respect to fibre loading. The mechanical properties increased up to 30 wt.% of fibre loading and decreased beyond this loading. These composites demonstrated high mechanical strength. PF matrix grafted with grewia optiva fibres as the reinforcement showed better mechanical properties compared to the un-grafted composites. To improve the adhesion of the sisal fibres with the PF polymer matrix, the sisal fibres were modified via hydroxymethylation. These modified fibres showed highed impact strength and strong bond between the fibres and matrix [106]. PF polymer composites have been prepared using short sisal fibres using two different methods: (1) direct mixing and (2) polymerisation under com3 The Mechanical Properties of the Scaffolds Reinforced by Fibres or Tubes...

95

Fig. 3.7 Water absorption curves of RCF/epoxy composites [119] © by Elsevier - reprinted with permission

pression moulding conditions. It was observed that the polymer composites prepared via polymerisation showed better mechanical properties. Reinforcement of the phenol-furfural polymer matrix with sisal fibres - the polymer composite exhibited high mechanical properties, which is an alternative method to avoid the use of formaldehyde [107-109].

Different natural fibres like pine needles and banana fibres have been used as reinforcement in PF polymer composite with and without surface modification [110, 111]. Banana fibres were made into micro- and nano-sized fibres using a steam explosion process and reinforced with PF polymer matrix. Enhanced mechanical properties were observed for the nanofibre reinforced composite when compare with the microfibre reinforced system [112].

Different types of natural fibres based epoxy polymer composites have been developed [113-115]. The mechanical properties of composites prepared from palm tree fibre as a reinforcement and epoxy as a matrix have been studied using resin transfer molding (RTM). It was observed that chemical modification of fibres helps the bonding between fibre and matrix and increases the mechanical properties [116]. To improve interfacial adhesion between fibres and the matrix, short palm fibres were reinforced with polyester and epoxy matrices. Surface modification improved the bond between the fibre and matrix via esterification. It was concluded that epoxy based composites exhibited higher mechanical properties due to better interfacial adhesion when compared to unsaturated polyester matrix [117]. To understand the real potential of natural fibre-reinforced composites, [118] studied effect of surface modification on mechanical properties of composites. Natural fibres were aged in distilled water and alkali medium and used to reinforce epoxy and polyester matrices. Alkali treated fibres showed better adhesion and increased the mechanical properties with low water absorption [118]. The effect of fibre loading (19, 28, 40 and 46 wt.%) on mechanical properties such as flexural strength, flexural modulus, fracture toughness and impact strength was studied on recycled cellulose fibre (RCF) reinforced epoxy composites. The mechanical properties and water uptake were directly proportional to the fibre loading (Fig. 3.7) [119].

Polyester is another polymer matrix widely used for preparation of natural fibre reinforced polymer composites [93, 97]. To improve the fibre-matrix interface, the outer surface of Luffa fibres were completely removed and treated with 5% sodium hydroxide. Due to the high surface area and roughness of the fibres, high flexural mechanical properties were observed [120]. Bamboo derived fibres have been treated with various chemicals such as maleic anhydride, permanganate, benzoyl chloride, benzyl chloride. These fibres have been used as reinforcement of polyester based matrices and to improve the tensile, flexural, impact and water absorption properties. Irrespective of the type of chemical treatment, the mechanical properties improved significantly for the resultant composites [121]. To improve the adhesion between bacterial cellulose and unsaturated polyesters composites, surface functionalisation has been fibres used. The mechanical properties of silane functionalised fibre composites have been shown to exhibit higher fibre-resin adhesion strength when compares with untreated fibres [122]. Different types of fibres such as hardwood and softwood high yield pulp (HYP), Kraft pulp and whatman cellulose pulp have been used to reinforce hunsaturated polyester (UPE) and vinyl ester (VE) to evaluate interfacial bonding. HYP fibres were shown to be more compatible with UPE resin due to residual lignin content, which acted as a natural coupling agent and helped improve the fibre-matrix interfacial bond, whereas Kraft and whatman pulp were more compatible with the VE based resin [123].

 
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