FIbre composites

1.             General information

Fibre reinforced polymer composites (FRP) are composed of a polymeric medium such as polyester, epoxy or polylactic acid, and a fibrous material such as carbon fibre, glass fibre, aramid fibre or natural fibre. The resultant composites are generally defined by the fibre type, e.g. Carbon Fibre Reinforced Polymer Composite (CFRP), Glass Fibre Reinforced Polymer Composite (GFRP) or Natural Fibre Reinforced Polymer Composite (NFRP). The combination of the two material types results in a ‘composite’ material that is superior in both form and function to that of the constituent materials in isolation. In general, the fibres provide the strength while the polymer, which is sometimes referred to as the matrix, provides dimensional stability and the transfer of shear stresses between the fibres. The result is a strong functional material which can be easily customised to deal with a wide variety of uses. 

Fibre reinforced polymer composites form a multi-billion dollar market internationally. 95% of this market is comprised of glass reinforced plastics (GRP) (Mohanty 2009). The construction industry accounts for one of the largest shares in GRPs, second only to the automotive industry (Figure 1). Polymers in isolation are used extensively in the construction industry with many applications including electrical fittings, light fittings and design features. GRPs are used in structural applications such as beams, self supporting structures and reinforcement in concrete or for architectural features such as external cladding.

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Figure.  Market share of composite materials.

Natural fibre reinforced polymers (NFRP) have been identified as a potential low impact alternative to GRPs. Although the replacement of glass fibre with natural fibres for reinforcement in polymer composites appears to be a modern phenomenon, NFRPs are not strictly modern by invention. With the advent of glass fibre technology in 1938, GRPs have, up until recently, been more economically viable than NFRPs (Thygesen 2006). However, with the economic viability of materials set to become increasingly influenced by their embodied impact, natural fibres that are less harmful to humans, machinery and the environment are a realistic alternative to the energy intensive production of glass fibre (Mohanty 2009).

Constituent materials: Fibres

The reinforcing fibres in a composite generally carry most of the load rendering the flexural strength and modulus of the matrix negligible (Halliwell, Reynolds 2004). In order to evaluate the reinforcing efficacy of natural fibres it is necessary to compare their mechanical properties with Glass fibre. E-glass fibre serves as a good reference point for natural fibre due to its importance in composite technology and its wide use in construction applications.

Bast, leaf and fruit fibres have all been identified as potential replacement of glass in FRPs (Mohanty 2009). However, it is bast and leaf fibres that have received the most attention. Flax, Sisal and Hemp have been identified as very important fibres for use as reinforcement in polymer composites due to their proven mechanical performance in comparison with glass fibre (Cripps, Handyside et al. 2004, Wambua, Ivens et al. 2003, Anderson, Jansz et al. 2004). Figure 2 presents a visual comparison between E-Glass, hemp and flax fibres in terms of their mechanical properties. The ‘specific E-Modulus’ is the E-modulus divided by the density of the material. The density of natural fibres is generally lower than that of Glass: typically hemp fibre is 1.5 g/cm³ while glass can be as much as 2.6 g/cm³. Hence, the specific properties of NFRPs sometimes achieve a better ratio between E-Modulus and weight than GFRPs. The weight of a material is a key consideration in application as a lower weight material can reduce costs in some areas of manufacture, fabrication, transportation and construction (Halliwell, Reynolds 2004). 

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Figure. Visual comparison of E-glass properties with hemp and flax fibre.

Technical hemp and flax fibre is classed as bast fibre which is extracted from the outer portion of the plant stem just below the outer layer of skin. Bast fibres are extracted from the plant by a microbial process known as ‘retting’ which breaks the chemical bonds that hold the stem together by decomposition of lignin and hemicelluloses (Thygesen 2006). After retting, fibres are separated from the stalk by a mechanical decortication  process. The properties of hemp fibres are dictated by the location, yield, type and processing methods (Roulac 1997).

Hemp and flax fibres are mainly composed of cellulose, hemicellulose, lignin, pectin and other extraneous materials and are characterised by a cellular structure (Figure 2.1) (Thomsen, Thygesen et al. 2006). Each cell has one external wall and three side walls containing crystalline cellulose regions called microfibrils which are interconnected via the lignin and hemicellulose fragments (Kozlowski, Wladyka-Przybylak 2008). The individual cells are known as elementary fibres (Dai, Fan 2010). The cell walls are defined by the concentration of constituent materials and the microfibrilar angle. The cellulose microfibrils are almost parallel to the elementary fibre in the inner part of the secondary cell wall (0-2°), helical in the outer part of the secondary cell wall (25-30°) and helical in the primary cell wall (70-90°)(Thygesen 2006). The lumen is hollow and allows the transport of moisture and nutrients within the plant by a capillary action which can also lead to the ready absorption and retention of internal moisture which is enhanced by the presence of lignin and hemicellulose (Pejic, Kostic et al. 2008). The primary cell wall is rich in lignin and pectin with decreasing volumes of lignin towards the inner part of the fibre (Thygesen 2006).

Biocomp_3 

Figure. Composition of technical fibre bundles and elementary fibres.

The mechanical properties of hemp and flax fibres vary greatly depending on yield, species, weather conditions, location and exposure, to name but a few. Additionally, fibres present a number of physical characteristics such as flaws and a variable cross-section which makes an accurate analysis of the mechanical properties difficult. The primary and secondary cell walls of hemp fibres present different deformation and breaking behaviours which is associated with the microfibril angle of each wall (Dai, Fan 2010). Natural fibre mechanical properties are also dependant on temperature and moisture content (Placet 2006). Given the high degree of variability, many researchers have adopted statistical analysis and probability distribution analysis to characterise hemp fibres (Fan 2010, Virk 2010, Pickering, Beckermann et al. 2007).

The stress-strain behaviour of many natural fibres is quasi-linear (Nechwatal, Mieck et al. 2003). Every fibre shows a different initial curve shape. Therefore, the calculation of modulus is uncertain. A. Nechwatal et al 2003 found an interesting correlation between the initial curve range and the total curve range resulting in a non-elongation corrected modulus En of: 


Where, LFV is the length at force Fv and ΔLFmax is the elongation at maximum force (Fmax) and A is cross-sectional area.

Fan 2010 found that the tensile stress-strain curve of hemp fibres is linear, in agreement with Hooke’s law. A relationship between tensile strength and fibre diameter was found and tensile strength was reliably predicted from the power regression (r²=0.88) (Fan 2010):

Placet et al 2012 investigated the diameter dependence of young’s modulus in hemp fibres and found that the ultrasructural parameters such as cellulose crystallinity and microfibril angles are the main influencing factors (Placet, Trivaudey et al. 2012). The strength of fibres is dictated by the arrangement of the microfibrils. The more parallel the microfibrils to the axis of the fibre, the stronger the fibre (Kozlowski, Wladyka-Przybylak 2008).

A contribution to failure of natural fibres is the presence of flaws (Silva, Chawla et al. 2008, Fan 2010). Natural fibres have naturally occurring flaws where stress concentrations can occur and lead to failure in tension. The defects have been observed as kink bands, dislocations, nodes and slip planes (Dai, Fan 2010). It has been shown in tests that as the length of a sample fibre is increased, the volume of defects present in the sample also increases leading to more mechanism for failure in tension as there is more potential for the linking of flaws and the propagation of cracks (Silva, Chawla et al. 2008)

Another major problem in determining the mechanical properties of natural fibres is cross sectional area. In order to calculate the stress, the cross sectional area is required. The determination of the cross sectional area in an individual strand of fibre is a notoriously difficult problem as the cross section is rarely circular and invariably non-uniform along the length of the fibre (Dai, Fan 2010). The cross sectional area (A) can be calculated from the density (ρ), and the fineness of the fibre ( T), and a good correlation between microscopic measurements has been found (Nechwatal, Mieck et al. 2003). Virk 2010 developed a fibre area correction factor for the use of hemp fibres as reinforcement in polymer composites. The area correction factor is applied to models for composite strength and modulus where the cross-sectional area of fibres has been assumed to be circular.

Placet 2006 investigated the thermomechanical behaviour of hemp fibres. Under cyclic stress regimes, hemp fibres increase in stiffness. The mechanical properties stabilise after a number of cycles suggesting that mechanical behaviour involves biochemical adaption and/or structural adaption such as microfibril reorientation (Placet 2006). The rigidity and endurance of fibres are highly affected by temperatures above 150°C and up to 180°C (Placet 2006).

Constituent materials: Polymers

Polymers can be classified as elastomers, plastics and fibres. Plastics can be further defined as either thermoplastic or thermoset. Thermoplastic and thermoset polymers form the matrix phase in an FRP composite. The matrix phase is required to transfer the loads to the fibres and must protect the fibres from impact, abrasion, chemical and moisture attack. It binds the fibres providing rigidity to fibre bundles that would otherwise be relatively pliable. Most commercially available thermoplastics such as polypropylene (PP) and thermosets such as epoxy are derived from a petro-chemical based origin. Biopolymers, on the other hand, are polymers derived from natural resources such as polylactic acid (PLA) from corn oil, Cashew Nut Shell Liquid Oil (CNSL) and thermoplastic and thermoset products from soy (Mohanty 2009). Biopolymers have some limitations in terms of cost versus performance. They are typically difficult to procure at the moment, but with increasing interest and continued research they may become commercially viable in the near future. Alternatively, a copolymer of petrochemical origin can be used to create a hybrid matrix which improves the performance of the biopolymer inviting wider interest in various industries. The main advantage and attraction of a biopolymer is its ability to biodegrade at the end of its useful life.

 

2.             Expertise on production technologies

The manufacture of NFRPs is based on the manufacturing processes of petrochemical based composites such as compression, extrusion and pultrusion. Injection moulding of NFRPs is considered by some to have potential in industrial applications (Fowler, Hughes et al. 2006). However, opportunities for NFRPs are hindered by regulations for existing materials, highlighting the need for further development of characterisation methodologies and theoretical analysis.

It has been noted that at temperatures over 200˚C during manufacturing processes, flax fibres can be irreversibly damaged (Bodros, Pillin et al. 2007). This is primarily due to vapour evaporation and differential expansion coefficients in the cell walls of the fibres which leads to porosity in the fibres and reduces their mechanical properties (Bodros, Pillin et al. 2007). Controlling the processes in the manufacture of NFRPs is essential to preserve the mechanical properties of the fibres. Film stacking method reduces damage to the fibres as it requires only one heating process (Bodros, Pillin et al. 2007).

Randomly oriented fibres are known to provide a good formability and are slightly cheaper to produce than highly directional fibre composites (Zampaloni, Pourboghrat et al. 2007). However, they do not display the same mechanical advantages of directional fibres. M. Zampaloni et al 2007 investigated the manufacturing problems and solutions of natural fibre reinforced composites. Squeeze flow tests showed that randomly orientated fibres display directionality due to the ‘by hand’ manufacture process (Zampaloni, Pourboghrat et al. 2007). The optimal fabrication method was discovered to be a layered sifting of micro fine polypropylene fibre and chopped fibre in a compression moulding process (Zampaloni, Pourboghrat et al. 2007). A 3% Epolene G3015 coupling agent was also found to aid fibre/ matrix adhesion (Zampaloni, Pourboghrat et al. 2007).

G. Mehta et al 2005 developed a method by which NFRPs can be manufactured by a sheet moulding process traditionally used for glass fibre reinforced polyester composites. The process can be adapted for any kind of natural fibre and repeatable results were observed. It was proposed that bioresins could be used in the future instead of a polyester matrix and that with further optimisation, natural fibre reinforced NFRPs could be produced with similar mechanical properties to glass/ polyester composites (Mehta, Mohanty et al. 2005).

Injection moulding processing technique is the most commonly used method in the plastics industry and is suitable to produce a wide variety of products (Wan Abdul Rahman, Sin et al. 2008). However, the process requires an in depth knowledge of polymer processing and rheology of the materials to minimise the cost of production (Wan Abdul Rahman, Sin et al. 2008). There are commercially available software packages that can model injection moulded products such as Moldflow and Moldex3D which help to study the flow patterns in the injection moulding process (Wan Abdul Rahman, Sin et al. 2008). A model was developed using commercially available software to analyse a natural fibre composite window frame manufactured by an injection moulding process (Wan Abdul Rahman, Sin et al. 2008). The results were encouraging but more information is needed on the thermal properties of natural fibre composite to determine their rheology and flow rate at different temperatures (Wan Abdul Rahman, Sin et al. 2008).

A thermal pressing process for manufacturing flax fibre and flax mucilage has been successfully trialled (Alix, Marais et al. 2008). Glutaraldehyde was used to crosslink the matrix solution providing moisture resistance (Alix, Marais et al. 2008). However, glutaraldehyde is not ideal as it is toxic to the environment (Alix, Marais et al. 2008). The sorption of water by the composite was reduced when the composite was prepared with a high level of protein in the mucilage (Alix, Marais et al. 2008).

 

3.             Expertise on the use of products

Research and development has shown that there is a diverse range of potential uses for NFRPs in the construction industry (Mohanty 2009, Dai, Fan 2010, Mehta, Mohanty et al. 2006, Mwaikambo, Ansell 2003) but without case studies, long term data and anecdotal evidence of performance, the mainstream construction industry will remain apprehensive of their widespread use. Many natural fibre based materials have emerged in the construction industry in recent years such as hemp/ lime concrete, hemp or wool insulation and compressed straw board materials. Such products support the use of natural fibres in the construction industry and promote the motives for developing NFRPs as construction materials. The proliferation of natural fibres in the industry will not only galvanise market confidence in their use, it will also provide further opportunities for innovations and development of NFRP technology by developing sources for raw material and business partnerships.

Architectural and functional

Perhaps the most successful area for NFRPs and other natural fibre composites is in non-structural architectural and functional applications such as doors, door frames, skirting boards and other decorative features. Architecturally they can be aesthetically pleasing and have potential in a wide range of applications as they can be easily formed into a variety of different products. Their proven durability in research (Singh, Gupta 2005) would suggest that NFRPs are well suited to exposed applications such as window frames or decking. In general, natural fibres have good thermal properties and the use of an NFRP might be beneficial in situations where thermal bridging could be an issue. Natural fibres also have good acoustic dampening properties although more research is needed to verify this.

Structural

Preconceptions about the inherent weaknesses of natural fibres can obscure their potential as a serious alternative to glass fibre. Research has shown that many of the mechanical issues associated with natural fibres can be overcome with chemical modification and that natural fibres have similar properties to glass fibres. Furthermore, NFRPs can be easily formed into structural components such as beams, reinforcing bars or board materials that could potentially be used in racking panels.

Research has shown that NFRPs and biocomposites can be used in self supporting structural applications such as a monolithic roof structure trialled in South Carolina in 2003 (Richard 2005). Structural tests proved the capacity and safety of the roof while the formability and ease of construction suggested that the composite had many potential applications including corrugated form work for concrete bridge decking.

4.             Product characteristics 

Mechanical properties

Although the mechanical properties of natural fibre composites vary greatly depending on fibre type, fibre architecture, polymer type and fibre surface treatment, it is generally accepted that tensile strength and modulus increase with an increase in Vf (Munikenche Gowda, Naidu et al. 1999, Facca A.G., Kortschot M.T. et al. 2007, Patel, Ren et al. 2010, Hargitai, Racz 2005). Many researchers have found a maximum Vf of approximately 50% before mechanical properties begin to diminish (Mwaikambo, Tucker et al. 2007, Hargitai, Racz 2005, Zampaloni, Pourboghrat et al. 2007).

The mechanical properties of an FRP can be improved by increasing the aspect ratio of the fibres (the ratio between the length and the diameter of the fibres); the higher the aspect ratio, the greater the reinforcing efficacy (Goda, Sreekala et al. 2006). However, by increasing the length of the natural fibre there is an increase in the presence of flaws (Silva, Chawla et al. 2008). Consequently, increasing the aspect ratio can have a detrimental effect on an NFRP. Various fibre treatments such as nano-cellulose treatments are being researched that can repair or fix these naturally occurring flaws and improve the tensile strength of composites with high aspect ratio (Dai, Fan et al. 2013). Conversely, natural fibres can be divided into nano-fibres which effectively eliminate the defects that lead to stress concentrations in larger fibres, improving the mechanical properties of the composite (Goda, Sreekala et al. 2006). The efficacy of either approach will depend on the end use of the composite.

Madsen et al 2007 found that composites reinforced with a unidirectional hemp yarn with a fibre volume fraction of 48% achieved a tensile modulus of 28 GPa and tensile strength of 280 MPa, demonstrating excellent properties for use in high strength applications (Madsen, Hoffmeyer et al. 2007).

Ahmed et al 2006 found that the tensile modulus of flax fibre reinforced composites was increased with the inclusion of glass fibre. However, the Poisson’s ratio was reduced by the inclusion of glass fibre (Ahmed, Vijayarangan 2006). No improvement in shear modulus was found with the inclusion of glass fibres. A close agreement was found with classical lamination theory (CLT) and rule of mixtures models with a deviation of 20% (Ahmed, Vijayarangan 2006). (Shazhad, Isaac et al. 2013) also found an improvement in tensile properties oh hemp fibre reinforced polyester with the inclusion of glass fibre.

Dhakal et al 2007 investigated the effect of moisture absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites. The percentage of moisture uptake increased with an increase in . The tensile and flexural properties were found to decrease with an increase in moisture uptake due to the degradation of the fibre matrix interface. Increased degradation was noted at elevated temperatures (100°C) (Dhakal, Zhang et al. 2007).

Facca et al 2006 investigated the agreement of natural fibre reinforced polymer composites with existing micromechanical models for tensile modulus. Nairn’s generalised shear lag model and the Halpin-Tsai equation were found to be the most accurate at predicting the modulus of hemp fibre reinforced polyester composites. The Halpin-Tsai equations were considered to be less useful as they are semi empirical, reliant on measured properties (Facca, Kortschot et al. 2006). This was also noted by (Virk 2010). Nairn’s shear lag model which is based on Cox’s shear lag theory was found to fit with experimental data using an interface parameter of 0.242 which indicates a poor interfacial bond (Facca, Kortschot et al. 2006). Facca et al 2007 found a shear strength of 16.1 MPa for hemp fibre reinforced composites and a maximum fibre volume (Vf) of 58.7% (Facca A.G., Kortschot M.T. et al. 2007).

Interfacial bond of NFRPs

There can be a lack of interfacial adhesion between hydrophilic natural fibres and hydrophobic matrix solutions such as polyester (Misra, Misra et al. 2002). Chemical modification is widely considered to be necessary to improve the adhesion between hydrophilic natural fibre reinforcement and hydrophobic polymer matrix solutions. However, it has been shown that some treatments are toxic and potentially harmful to the environment (Alix, Marais et al. 2008).

One of the most common and efficient methods of fibre modification is alkali treatment which has been used to treat a wide variety of fibres with some success (John, Anandjiwala 2008). The formation of covalent bonds between the fibre and the matrix solution is a successful way of improving the adhesion between the phases (John, Anandjiwala 2008). Maleated coupling agents and silane coupling agents have also seen a good degree of success (John, Anandjiwala 2008). More research and development is required to develop coupling agents from natural products.

The rough surface texture of natural fibres after treatment with NaOH and different alkali concentrations did not appear to improve the adhesion between the matrix and the fibre (Gassan, Bledzki 1999). However, an increase in strength properties was noted. Young’s modulus was found to have a linear relationship with the content of natural fibre in the composite. On comparing the treated fibres with untreated, there was an increase in young’s modulus. There was also deterioration in fatigue behaviour of treated fibres as a result of the improvement of specific damping capacity caused by shrinkage in the fibres during alkali treatment (Gassan, Bledzki 1999).

Oloeyl chloride was used as a coupling agent to transform the surface of natural fibres from hydrophilic to hydrophobic thereby improving the bond between the fibre and the matrix material (Rahman, Huque et al. 2008). Complementary techniques such as scanning electron microscopy (SEM) were used to prove the effectiveness of the esterification and modification reaction (Rahman, Huque et al. 2008). Pyridine as a solvent medium was found to be most effective at improving the accessibility of hydroxyl groups. The polarity and catalytic capacity of the solvent was found to affect the efficiency of the reaction (Corrales, Vilaseca et al. 2007).

G. Sebe et al 2000 treated hemp fibres with reactive vinylic grafts using methacrylic anhydride and pyridine. Composites made with untreated hemp fibres were compared with treated hemp fibre composites. They conducted flexural tests in accordance with BS 2782: part 10: method 1005: 1997 and calculated the flexural stress at rupture. The impact strength was calculated in accordance with BS 2782: part3 method 359:1984. The treatment did not increase the strength. Impact strength was not improved. They concluded that fibre pull out was one of the main contributing factors to the strength of the composites (Sebe, Cetin et al. 2000).

Urotropine post-treatment improves the hydrophilic nature of jute fibre (Rahman, Huque et al. 2008) and therefore improves the interfacial bond with a polypropylene matrix solution. However, this post-treatment method is considered to be acceptable at research and development level but is not viable for commercial exploitation (Rahman, Huque et al. 2008). Contrary to other studies, the mechanical properties of jute fibre/ polypropylene composites were found to reduce with increasing Vf (Rahman, Huque et al. 2008).

K. Goda et al 2006 investigated the effect of applying a tensile load during mercerization on the structural properties of natural fibres. It was established that fibres treated with NaOH and subjected to tensile loading saw an increase in fracture strain as well as an improvement in tensile strength  (Goda, Sreekala et al. 2006). Cyclic tests revealed that the treated fibres responded with a greater plastic deformation in tensile loading which was considered to be as a result of a decrease in the microcrystalline in the microfibrils and removal of the binding materials around the microfibrils during mercerization. The increase in strength was considered to be as a result of a change in the microfibril angle; The more parallel the microfibrils are to the fibre axis, the higher the strength of the fibre (Rahman, Huque et al. 2008).

The dynamic mechanical properties of both treated and untreated short sisal fibre reinforced composites was investigated in relation to fibre loading, fibre length, chemical treatment, frequency and temperature (Corrales, Vilaseca et al. 2007). Treated fibre composites were shown to have improved properties to untreated fibres (Joseph, Mathew et al. 2003). The storage modulus and loss modulus were found to increase with the incorporation of short sisal fibre into polypropylene matrix (Joseph, Mathew et al. 2003). The storage modulus was found to decrease with an increase in temperature. The applicability of various theoretical models was investigated and found to relate well to experimental results.

Thermophysical properties of NFRPs

In general, the transverse thermal conductivity through depth of NFRPs is lower than the in plane conductivity (Ramanaiah, Ratna Prasad et al. 2013, Behzad, Sain 2007, Liu, Takagi et al. 2011, Dhakal, Zhang et al. 2008). Transverse thermal conductivity decreases with an increase in Vf and in plane conductivity increases with an increase in Vf (Behzad, Sain 2007, Annie Paul, Boudenne et al. 2008) which is in line with the accepted theory for thermal conductivity of FRPs. Many researchers have found a good correlation with thermal conductivity models for FRPs. The simplest form is the rule of mixtures (ROM) equations for in plane and transverse thermal conductivity. In plane conductivity is given as the parallel rule of mixtures equation:

 

Where, k is the in plane fibre conductivity and km is the matrix conductivity. Behzad et al 2007 found a good correlation between unidirectional hemp fibre reinforced polyester and the parallel ROM equation. The transverse conductivity is given as the series rule of mixtures equation:

 

Where, kf is the transverse thermal conductivity of the fibre. Idicula et al 2006 found a good correlation with the series model for NFRPs. The parallel and series rule of mixture equations act as the theoretical upper and lower bounds for thermal conductivity respectively. In practice, the thermal conductivity of NFRPs generally lies between the two. Behzad et al 2007 found a good correlation between experimental values for hemp fibre reinforce polyester composites and the Springer-Tsai equation (Springer, Tsai 1967):

 

Where, kis the conductivity of the composite and θ is the fibre angle to the direction of heat transfer. When θ = 90°the equation reduces to the parallel ROM and when θ = 0° it reduces to the series ROM.

Liu et al 2011 found a good correlation between experimental results for manila hemp fibre (banana fibre) reinforced polyester and the Hasselman- Johnson model:

 

Other models which have been corroborated with natural fibre composites (Li, Tabil et al. 2008) are the Maxwell and Russell models. Maxwell developed a relationship for the conductivity of distributed non-interacting homogeneous spheres in a homogeneous medium (Maxwell 1954):

 

(Russell 1935) developed an electrical analogy assuming that the fibres are cubes of the same size dispersed in the matrix:

 

Using a transient plane source (TPS) technique thermal diffusivity, conductivity, and specific heat of natural fibre polyester composites were tested at ambient temperature (Agarwal 2006) . The thermal conductivity was found to increase with some fibre surface treatments. Treatment with 1% NaOH was found to produce the greatest increase in thermal conductivity. It was concluded that the improved interfacial bond was the mechanism for an increase in conductivity. Chemical treatment of fibres reduces the thermal contact resistance (Idicula, Boudenne et al. 2006).

The specific heat capacity of hemp fibre reinforced polyester composites has been shown to increase linearly with an increase in temperature between 20 and 100°C (Behzad, Sain 2007). Specific heat and thermal diffusivity have been shown to increase with an increase in Vf for flax fibre composites (Li, Tabil et al. 2008). The influence of temperature on the thermal conductivity of flax fibre composites was shown to be minimal in the range of 170-200°C.

Liu et al 2011, found a good correlation between experimental results and the Hasselman-Johnson model for transverse thermal conductivity of hemp fibres (0.1847 W/mK) (Liu, Takagi et al. 2011). Behzad et al 2008 found a parallel conductivity of 1.48 W/mK and a transverse conductivity of 0.115 W/mK. Theroetically, the thermal conductivity of fibre reduces with an increase in the volume of lumen (Liu, Takagi et al. 2011).

5.             Product characteristics

Durability

The durability of a composite is sometimes considered to be a function of its appearance (Halliwell 2003). The degradation of the gelcoat can lead to visible cracking, crazing and delamination. Although this process does not have a direct effect on the mechanical or structural performance of the material, delamination of the gelcoat can lead to exposure of the reinforcing fibres which can lead to structural degradation (Halliwell, Reynolds 2003). This could prove to be a critical consideration in NFRPs as a hydrophilic natural fibre would be expected to absorb more moisture and degrade more quickly than glass when exposed to the elements. The effects are likely to be irreversible on drying and reductions in strength and modulus are likely. The deterioration of an FRP can develop from the following;

·         mechanical stresses ( fatigue, impact, erosion, abrasion)

·         Chemical ( water, solvents fuels, oils, acids, cleaning liquids)

·         Radiation ( sunlight)

·         Heat ( high temperatures and large rapid fluctuation in temperature)

·         Biological attack ( fungi, insects, marine borers)

Due to their novelty, reliable data and anecdotal evidence of the durability of NFRPs is limited. Current research would suggest that NFRPs show a lot of promise in accelerated weather conditions. Doors and window frames made from a flax fibre reinforced composite have shown a good degree of durability over a period of three years exposure with little or no dimensional instability observed (Singh, Gupta 2005). One recommendation is that an extra gelcoat could be added to further protect the composite.

Fire performance

Fire is always a critical design factor, especially in light weight structures where there is no inherent fire resistance. Fire risk is defined as the potential for any material or object to catch fire in any given situation (Morgan, Gilman 2012). Fire in structures accounts for the highest fire risk to humans and it is assumed that a structure will experience one fire during the course of its design life (Bregulla 2003). Domestic fires account for two thirds of all building fires which are responsible for 88% of all casualties in fire. In the year 2011-12 there were 43,500 domestic fires which resulted in 287 fatalities in the UK (DCLG 2012).

Polymer composites are not normally employed as structural elements within a building. However, they are used as external and internal wall coverings and architectural features within buildings such as furniture , door frames or window frames. Consequently they do account for a large proportion of potential fire load within a building. Maximum temperature of FRPs is governed by two main factors: the glass transition temperature (Tg) and the temperature at which the chemical decomposition starts to become significant (Halliwell, Reynolds 2004). High temperature resins can be obtained but they are generally more expensive (Halliwell, Reynolds 2004). Initially, the design of FRPs for use in the construction industry mysteriously neglected to incorporate fire performance. After several disasters, the fire performance of FRPs is as important a design consideration as the modulus, yield stress and clarity (Halliwell, Reynolds 2004). The main emphasis on fire performance is in adding fire retardants into the resin mixture. The use of fire retardant additives has a detrimental effect on the weathering of FRPs (Halliwell, Reynolds 2004). Therefore a highly fire retardant substrate combined with a regular gelcoat is desirable.

The effect of type and volume of fibre reinforcement on the thermal properties of NFRPs has been studied (Rudnik 2007). A Hydroxypropyl starch was used for the matrix and flax and cellulose fibre for the reinforcement. In general, it was established that incorporating natural fibres into modified starch matrix leads to an increase in the glass transition temperature (Tg) and flax and cellulose reinforcement provided an increase in thermal stability. H.S. Kim 2005 found that with increasing presence of lignin, there is an increase in the thermal stability of the composite. Contrarily, Dorez et al 2013 found that the introduction of flax fibres into polybutylene succinate composite led to a reduction in thermal stability. Poor interfacial adhesion in NFRPs contributes to thermal degradation and the thermal stability of biodegradable polymers is lower than plastics (Kim, Yang et al. 2005). Natural fibres start degrading at about 150˚C. Low temperature degradation is associated with hemicelluloses, whereas high temperature degradation is associated with the presence of cellulose (Kim, Yang et al. 2005).

The efficiency of intumescent additive systems to polyefin/ flax composites has been confirmed (Le Bras, Duquesne et al. 2005). The work confirms that a Polyproylene/ flax fibre composite with added ammonium polyphosphate (APP) as an ecologically friendly fire retardant produces an optimised fire resistance performance for the composite (Le Bras, Duquesne et al. 2005). Hapuarachchi et al 2007 investigated the influence of fire retardants (calcium carbonate and aluminium hydroxide) on the fire performance of hemp fibre reinforced polyester composites. It was found that hemp fibre reinforced polyester composites which were treated with a fire retardant performed well in terms of peak heat release rates compared to other established construction products (Hapuarachchi, Ren et al. 2007). Biswal et al 2011 studied the thermal stability and flammability of banana fibre reinforced polypropylene nancomposites. The char formation with the introduction of nanoclays improved the flame resistance of the composites. The clay layer acts as a barrier to mass transport. Differential scanning calorimetry showed an increase in the melting and crystallization temperatures of the polypropylene (Biswal, Mohanty et al. 2012). Chand et al 1989 investigated the effect of a chloride based fire retardant on the thermomechanical properties of hemp fibres. It was found that the ultimate tensile strength of fibres exposed to flame for 10 seconds increased with an increase in the volume of fire retardant magnesium chloride (Chand, Verma 1989).

Dorez et al 2013 investigated the influence of natural fibres on the thermal and fire behaviour of Polybutylene succinate composites. It was found that although the thermal stability of composites decreased, pHHR decreased with an increase in the volume of natural fibres. An increase in the production of a char resdue was attributed to the lower pHHR which is in agreement with (Hapuarachchi, Ren et al. 2007). D. Hapuarachchi 2006 conducted cone calorimetry tests on a natural fibre sheet moulding compound (SMC) in comparison with other traditional building products. It was observed that natural fibre composites present similar reaction to fire as timber based products such as OSB. It was also noted that with increasing Vf in NFRPs, there was an improvement in reaction to fire. With increasing  there was more char produced which insulates the substrate and protects the product from the propagation of fire and reduces the peak heat release rate (pHRR) (Hapuarachchi 2006)

Dorez et al 2013 found that the time to ignition (TTI) was reduced with the incorporation of flax fibres which was attributed to the flammability of gas released by lignocellulosic fibres (Dorez, Taguet et al. 2013). This is contrary to the findings of Hapuarachchi et al 2007, who found that the TTI for fire-retardant and non fire retardant hemp fibre reinforced polyester composites was greater than glass fibre reinforced composites. In other studies, Hapuarachchi 2006 found that an increase in hemp fibre volume also led to a decrease in TTI. The fire resistance of polyester composites reinforced with Jute, Flax and Sisal were investigated by (Manfredi, Rodríguez et al. 2006). It was found that flax reinforced composites had a longer ignition time and less observable thermal degradation due to the low lignin content (Manfredi, Rodríguez et al. 2006).

Naughton et al 2013, investigated the residual mechanical properties of fire damaged hemp fibre polyester composites. It was found that tensile strength was diminished after exposure to relatively low temperatures (≈180°C). The loss in tensile strength was found to be associated with the thermal decomposition of lignin in the primary cell walls of elementary fibres which led to the degradation of interfacial bond. The TTI was observed to reduce with an increase in fibre volume. Although the development of a carbonatious char was reduced with an increase in fibre volume, the thermal resistance of the char layer increased with an increase in fibre volume (Naughton, Fan et al. 2014).

Most studies on the fire performance of NFRPs are based on cone calorimetry experiments and do not account for residual strength characteristics which are a prerequisite for fire resistance. More research is required in the field of fire resistance of NFRPs and more testing of resistance is required to develop an understanding of the parameters of performance in full-scale fire resistance tests.

There appears to be some disparity between different researchers on the thermal and fire performance of NFRPs. Some researchers have found improved thermal stability, while others have found diminished thermal stability with the inclusion of natural fibres. Some researchers have found ignition time to be reduced while others found ignition times increased. The disparity suggests that the fibre type, matrix type and the interaction between the two has a significant influence on the mechanisms of thermal decomposition. It also suggests that these mechanisms are not yet fully understood.

  

 

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