The effect of fibre content on the thermal and fire performance of polypropylene–glass composites

Thermoplastic composites demand constant improvements in fire retardant and mechanical properties to fulfil their full market potential, especially in demanding sectors such as rail, aerospace and infrastructure, where fire performance is critical. The aim of this work is to understand the effect of reinforcing fibres on the flammability of polypropylene–glass (P‐G) composites and the means of improving their fire performance in a cost‐effective manner. A number of P‐G composites with 0%, 10% and 20% (w/w) glass fibres were prepared using short length glass fibres. The effect of fibre content on the thermal stability, flammability and mechanical performance of the P‐G composites without and in the presence of conventional fire retardants was studied. It was observed that while the presence of glass fibre lowered down the limiting oxygen index value of the composite, the rate of flame spread in a UL‐94 equivalent test was also lowered. The reduction in limiting oxygen index is due to the fact that glass fibre reduces the melt dripping behaviour of polypropylene and does not let the polymer (polypropylene) move away from flame, which then burns. Cone calorimetric study indicated that the presence of glass fibre reduces the overall flammability of the composite laminate. Copyright © 2011 John Wiley & Sons, Ltd.     

A review on the characterisation of natural fibres and their composites after alkali treatment and water absorption

Natural fibre-reinforced polymer matrix composites are gaining increased attention among the researchers due to their low density, biodegradability, abundance, good mechanical properties, etc. Significant amount of research works can be found on the material characterisation of natural fibres like hemp, flax, sisal, kenaf, coir and jute and their composites based on the polymer matrices. Natural fibres are hydrophilic in nature and exhibit poor interfacial adhesion between fibre and matrix. Modification of the fibre surface by chemical methods, such as alkalisation, benzoylation and acetylation, has been used by researchers to improve the above-mentioned shortcomings. This review paper focuses on the effect of alkali treatment on the material properties of various natural fibres and their composites along with their water absorption behaviour.     

Flax fibre physical structure and its effect on composite properties: Impact strength and thermo-mechanical properties

Earlier investigations by the authors showed that the tensile modulus of flax fibre mat polypropylene composites (NMT) could surpass the values of glass mat reinforced thermoplastic (GMT) on fibre weight basis. The tensile and flexural strength could reach values of up to 65% of the GMT strength values, however, very much dependent on the fibre physical structure. This study deals with the Charpy impact and the thermo-mechanical properties of flax NMT materials. The trend is that the Charpy impact strength decreases with increasing fibre internal bonding and enhanced fibre-matrix adhesion, which is opposite to the trend for the tensile and flexural properties. The impact strength of the NMT materials is lower than generally reported for GMT materials. Dynamic mechanical thermal analysis reveals that with increasing temperature the storage modulus of the NMT materials reduces more slowly when the fibre internal bonding and the fibre-matrix adhesion are improved. In order to approach the tensile, flexural and impact strength of GMT materials, composites should be based on the strong elementary flax fibres. The axial tensile strength of elementary fibres approaches the strength of glass fibres and the lateral strength of the elementary fibres is higher than the technical flax fibres lateral strength. The thermo-mechanical properties can probably be improved when non-cellulosic material can be removed from the flax fibre surface without damaging the fibre.     

Evaluating the heat resistance of thermal insulated sandwich composites subjected to a turbulent fire

The fire structural response of sandwich composite laminates incorporating bio‐derived constituents subjected to a turbulent flaming fire was investigated. Fire structural tests were conducted on thermal insulated sandwich composites incorporating a thin surface‐bonded non‐woven glass fibre tissue impregnated with char‐forming fire retardant, ammonium polyphosphate. The sandwich composite laminates were loaded in compression at 10%, 15% or 20% of the ultimate compressive strength while simultaneously subjected to turbulent flames imposing an incident heat flux of 35 kW/m². Generally, the failure time increased with the reduced applied compressive load. The thermal insulated sandwich composite laminates had considerably improved fire resistance in comparison to their unmodified counterparts. The unmodified composites failed 96 s earlier than the thermal insulated specimens when the compression load was 10% of the ultimate compressive strength. The presence of ammonium polyphosphate at the heat‐exposed surface promoted the formation of a consolidated char layer, which slowed down heat conduction into composite laminate substrate. The fire reaction parameters measured via the cone calorimeter provided insights into the thermal response hence fire structural survivability of sandwich composite laminates. Copyright © 2015 John Wiley & Sons, Ltd.     

The use of fire‐retardant intumescent mats for fire and heat protection of glass fibre‐reinforced polyester composites: Thermal barrier properties

This study investigates the use of integral, hybrid intumescent thermal barriers (mats) to provide surface protection to the core fibre‐reinforced polyester composite structural integrity when exposed to a fire or heat source. Glass fibre‐reinforced composites protected by intumescent mats/fabrics containing silicate fibres, expandable graphite and in some cases borosilicate glass bounded together by an organic matrix have been evaluated for fire performance under a constant heat flux of 50kW/m². The effect of insulative fabric thickness as well as chemical composition on the flammability of the resultant hybrid composites is evaluated. Glass fibre‐reinforced polyester (GRP) composites without any surface protection have a relatively higher time‐to‐ignition and peak heat release rate values when compared with core composites protected by insulative fabrics. Thermograms representing the variation of temperature on the reverse side of the hybrid composites with time when exposed to a constant heat flux show that the inclusion of intumescent surface barriers results in retarded temperature increments within the core GRP composites. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of fire‐retardant additives and a surface insulative fabric on fire performance and mechanical property retention of polyester composites

This study investigates the simultaneous use of conventional fire‐retardant additives and an insulative intumescent thermal barrier/mat to improve the fire performance and mechanical property retention of glass‐fibre‐reinforced polyester (GRP) composites. Significant reductions in the peak heat release rate (PHRR) and total heat release (THR) were observed from measured cone calorimetric data following the addition of nitrogen, phosphorous, halogen containing and hydroxylated fire‐retardant additives. Some fire‐retarded glass‐fibre‐reinforced composites further protected by an intumescent mat containing silicate fibres, expandable graphite and borosilicate glass bound together by an organic matrix show further reductions in PHRR. Despite improving the fire retardancy of the composites, the presence of fire‐retardant additives alone does not improve flexural modulus retention following exposure to a heat source. However, the introduction of a ‘passive’ fire proofing insulative fabric enhances fire performance while preserving the mechanical properties of composites exposed to high heat fluxes or fires. Copyright © 2010 John Wiley & Sons, Ltd.     

Preparation,morphology and properties of natural rubber composites filled with untreated short jute fibres

Green composites were obtained by incorporation of short jute fibres in natural rubber matrix using a laboratory two-roll mill. The influence of untreated fibre content (1, 2.5, 5, 7.5 and 10 phr) on the mechanical properties, dynamic mechanical properties, swelling properties was examined. The behaviour of prepared green composites under cyclic compression was also investigated. Fibre dispersion in rubber matrix was studied by scanning electron microscopy. The highest tensile strength (21.1 MPa) and highest tear strength (39.9 N/mm) were found for composites containing 2.5 and 5 phr of short jute fibres, respectively. The results also suggested that increasing fibrous filler content resulted in increasing of tensile moduli 100, 200 and 300 % of elongation and hardness, and decreasing of rebound resilience and abrasion resistance of prepared jute/natural rubber composites. The cyclic compression test showed that increasing the amount of short jute fibres in the rubber matrix is related to increase of the energy dissipated in the composite. The incorporation of short jute fibres into the rubber matrix improves the stiffness of the composites, and it is related to the interaction between fibre surface and rubber matrix. The application of short fibres in higher amounts leads to formation of fibre agglomerates reducing the mobility of the rubber polymer chains. The mentioned agglomerates act as defects in rubber matrix, which caused decreasing of some properties, e.g. relative elongation at break.     

Experimental assessment of the improved properties during aging of flax/glass hybrid composite laminates for marine applications

The investigation for natural fibers composites in terms of performance, durability, and environmental impact for structural applications in marine environments is a relevant challenge in scientific and industrial field. On this context, the aim of this work is to assess the durability and mechanical stability in severe environment of epoxy/glass–flax hybrid composites. For the sake of comparison, also full flax and glass epoxy composites were investigated. All samples were exposed to salt–fog environmental conditions up to 60 aging days. Wettability behavior during time was compared with water uptake evolution to assess water sensitivity of hybrid composite configurations. Moreover, quasi-static flexural and dynamic mechanical analysis were carried to evaluate as aging conditions, laminate configuration influence the surface and mechanical performances stability of the hybrid composites. The addition of glass fibers on flax laminate allows to enhance both flexural strength by 90%, and modulus by 128%, even if these properties are lower than those of full glass laminates. The results evidenced that the hybridization of flax fibers with glass ones is a practical approach to enhance the aging durability of epoxy/flax composite laminates in marine environmental conditions, obtaining a suitable compromise among environmental impact, mechanical properties, aging resistance, and costs. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47203.     

The effect of chemical treatment on the properties of hemp,sisal, jute and kapok for composite reinforcement

Two chemical treatments were applied to hemp, sisal, jute and kapok natural fibres to create better fibre to resin bonding in natural composite materials. The natural fibres have been treated with varying concentrations of caustic soda with the objective of removing surface impurities and developing fine structure modifications in the process of alkalisation. The same fibres were also acetylated with and without an acid catalyst to graft acetyl groups onto the cellulose structure, in order to reduce the hydrophilic tendency of the fibres and enhance weather resistance. Four characterisation techniques, namely XRD, DSC, FT-IR and SEM, were used to elucidate the effect of the chemical treatment on the fibres. After treatment the surface topography of hemp, sisal and jute fibres is clean and rough. The surface of kapok fibres is apparently not affected by the chemical treatments. X-ray diffraction shows a slight initial improvement in the crystallinity index of the fibres at low sodium hydroxide concentration. However, high caustic soda concentrations lower the fibre crystallinity index. Thermal analysis of the fibres also indicates reductions in crystallinity index with increased caustic soda concentrations and that grafting of the acetyl groups is optimised at elevated temperatures. Alkalisation and acetylation have successfully modified the structure of natural fibres and these modifications will most likely improved the performance of natural fibre composites by promoting better fibre to resin bonding.     

Unidirectional hemp and flax EP‐ and PP‐composites: Influence of defined fiber treatments

In some technical areas, mainly in the automotive industry, glass fiber reinforced polymers are intended to be replaced by natural fiber reinforced polymer systems. Therefore, higher requirements will be imposed to the physical fiber properties, fiber‐matrix adhesion, and the quality assurance. To improve the properties of epoxy resins (EP) and polypropylene (PP) composites, flax and hemp fibers were modified by mercerization and MAH‐PP coupling agent was used for preparing the PP composites. The effects of different mercerization parameters such as concentration of alkali (NaOH), temperature, and duration time along with tensile stress applied to the fibers on the structure and properties of hemp fibers were studied and judged via the cellulose I–II lattice conversion. It was observed that the mechanical properties of the fibers can be controlled in a broad range by using appropriate mercerization parameters. Unidirectional EP composites were manufactured by the filament winding technique; at the PP matrix material, a combination with a film‐stacking technique was used. The influence of mercerization parameters on the properties of EP composites was studied with hemp yarn as an example. Different macromechanical effects are shown at hemp‐ and flax‐PP model composites with mercerized, MAH‐PP‐treated, or MAH‐PP‐treated mercerized yarns. The composites' properties were verified by tensile and flexural tests. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2150–2156, 2004     

Hemp fibre as alternative to glass fibre in sheet moulding compound Part 1 – influence of fibre content and surface treatment on mechanical properties

Abstract

Hemp fibre mat reinforced unsaturated polyester composites were fabricated using a conventional sheet moulding compound process. The influence of fibre and CaCO₃ filler content on strength and stiffness of these hemp fibre reinforced sheet moulding compounds is reported and compared with data for chopped glass fibre reinforced sheet moulding compounds. In addition the influence of alkaline and silane treatments of the hemp fibres is evaluated. The experimental data are compared to modified versions of the Cox–Krenchel and Kelly–Tyson models, supplemented with parameters of composite porosity to improve the prediction of composite tensile properties. A good agreement was found between the modified models and experimental data for strength and stiffness. The results indicate that hemp fibre reinforced sheet moulding compounds are of interest for low cost engineering applications that require high stiffness to weight ratios.     

Injection‐molded short hemp fiber/glass fiber‐reinforced polypropylene hybrid composites—Mechanical,water absorption and thermal properties

Natural fiber‐based thermoplastic composites are generally lower in strength performance compared to thermoset composites. However, they have the advantage of design flexibility and recycling possibilities. Hybridization with small amounts of synthetic fibers makes these natural fiber composites more suitable for technical applications such as automotive interior parts. Hemp fiber is one of the important lignocellulosic bast fiber and has been used as reinforcement for industrial applications. This study focused on the performance of injection‐molded short hemp fiber and hemp/glass fiber hybrid polypropylene composites. Results showed that hybridization with glass fiber enhanced the performance properties. A value of 101 MPa for flexural strength and 5.5 GPa for the flexural modulus is achieved from a hybrid composite containing 25 wt % of hemp and 15 wt % of glass. Notched Izod impact strength of the hybrid composites exhibited great enhancement (34%). Analysis of fiber length distribution in the composite and fracture surface was performed to study the fiber breakage and fracture mechanism. Thermal properties and resistance to water absorption properties of the hemp fiber composites were improved by hybridization with glass fibers. Overall studies indicated that the short hemp/glass fiber hybrid polypropylene composites are promising candidates for structural applications where high stiffness and thermal resistance is required. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2432–2441, 2007     

Experimental and numerical investigations on the thermal response of multilayer glass fibre/unsaturated polyester/organoclay composite

Organoclay glass fibre reinforced polymer (GFRP) nanocomposites are fabricated using the vacuum assisted resin transfer moulding. The unsaturated polyester resin is prepared with and without organoclay involving mechanical mixing, sonication, dilution solvent and heat treatment. Three levels of organophilic clay content are added, and its influences on the fire performance of composite samples are investigated. A novel numerical procedure combining pyrolysis analysis of the organoclay‐composites and the fire dynamic simulation of the combustion process are developed to validate the thermal responses obtained from the cone calorimetry experiments. Kinetic parameters obtained from the TGA tests and pyrolysis analyses are used as inputs for the models measuring the fire growth index and total heat release. To account for multilayer composite structure and organoclay distribution, three numerical models are proposed including composite (CPS), component (CPN) and CPN‐layer models. While CPS model assumes the homogeneity of the composite, later models consider multilayer effects with uniform (CPN model) or concentrated (CPN‐layer model) distribution of organoclay. Numerical results are compared with experimental ones in terms of total heat release, fire growth index. Finally, the fire resistance and total smoke release of the polyester/glass composites with the addition of organoclay will be evaluated taking into account influences of the fabrication processes.     

High Stiffness Natural Fiber‐Reinforced Hybrid Polypropylene Composites

Abstract

Natural fibers are potentially a high‐performance non‐abrasive reinforcing fiber source. In this study, pulp fibers [including bleached Kraft pulp (BKP) and thermomechanical pulp (TMP)], hemp, flax, and wood flour were used for reinforcing in polypropylene (PP) composite. The results show that pulp fibers, in particular, TMP‐reinforced PP has the highest tensile strength, possibly because pulp fibers were subjected to less severe shortening during compounding, compared to hemp and flax fiber bundles. Maleic‐anhydride grafted PP (MAPP) with high maleic anhydride groups and high molecular weight was more effective in improving strength properties of PP composite as a compatiblizer. Coupled with 10% glass fiber, 40% TMP reinforced PP had a tensile strength of 70 MPa and a specific tensile strength comparable to glass fiber reinforced PP. Thermomechanical pulp was more effective in reinforcing than BKP. X‐ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM) were used to aid in the analysis. Polypropylene with high impact strength was also used in compounding to improve the low‐impact strength prevalent in natural fiber‐reinforced PP from injection molding.     

The effect of traditional flame retardants,nanoclays and carbon nanotubes in the fire performance of epoxy resin composites

The effectiveness of distinct fillers, from micro to nano‐size scaled, on the fire behaviour of an epoxy resin and its carbon fibre reinforced composites was assessed by cone calorimetry. The performance was compared not only regarding the reaction to fire performance, but also in terms of thermal stability, glass transition temperature and microstructure. Regarding the fire reaction behaviour of nanofilled epoxy resin, anionic nanoclays and thermally oxidized carbon nanotubes showed the best results, in agreement with more compact chars formed on the surface of the burning polymer. For carbon fibre reinforced composite plates, the cone calorimeter results of modified resin samples did not show significant improvements on the heat release rate curves. Poorly dispersed fillers in the resin additionally caused reductions on the glass transition temperature of the composite materials. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of fibre length and chemical modifications on the tensile properties of intimately mixed short sisal/glass hybrid fibre reinforced low density polyethylene composites

Hybrid composites prepared by the incorporation of two or more different types of fibres into a single polymer matrix deserve much attention. This method of hybridisation of composites offers a profitable procedure for the fabrication of products while the resulting materials are noted for their high specific strength, modulus and thermal stability. The influence of the relative composition of short sisal/glass fibres, their length and distribution on the tensile properties of short sisal/glass intimately mixed polyethylene composites (SGRP) was examined. Different compositions of sisal and glass such as 70/30, 50/50 and 30/70 have been prepared with varying fibre lengths in the range of 1–10 mm. Emphasis has also been given to the variation of fibre–matrix adhesion with several fibre chemical modifications. Chemical surface modifications such as alkali, acetic anhydride, stearic acid, permanganate, maleic anhydride, silane and peroxides given to the fibres and matrix were found to be successful in improving the interfacial adhesion and compatibility between the fibre and matrix. The nature and extent of chemical modifications were analysed by infrared spectroscopy while improvement in fibre–matrix adhesion was checked by studying the fractography of composite samples using a scanning electron microscope. Assessment of water retention values has been found to be a successful tool to characterize the surface of the stearic acid modified fibres. It was found that the extent of improvement in tensile properties of SGRP varied with respect to the nature of chemical modifications between fibre and matrix. Improved mechanical anchoring and physical and chemical bonding between fibre and polyethylene matrix are supposed to be the reasons for superior tensile strength and Young's modulus in treated composites. Several secondary reasons such as high degree of fibre dispersion and reduced hydrophilicity in chemically modified fibres also are believed to play a role. Among the various chemical modifications, the best tensile strength and modulus was exhibited by the SGRP with benzoyl peroxide treated fibres. This is attributed to the peroxide‐initiated grafting of polyethylene on to the fibres. Copyright © 2004 Society of Chemical Industry     

Mechanical and thermal characterization of compression moulded polylactic acid natural fiber composites reinforced with hemp and lyocell fibers

This research evaluates the effects of PLA/PP blend ratio and Lyocell/hemp mixture ratio on the morphology, water absorption, mechanical and thermal properties of PLA‐based composites. The composites were fabricated with 30 mass % hemp using compression moulding. As a reference composites made from PP were also studied. Combining of hemp and Lyocell in PLA composite leads to the reduction of moisture absorption and can improve the impact, tensile, flexural properties when compared with PLA/hemp. Composite based on the PLA/PP blend‐matrix could not improve the tensile and flexural properties compared with PLA/hemp, however; the lighter composite with better impact properties was obtained. The crystallization temperature of the PLA‐PP/hemp increased compared with pure PLA. This result was also confirmed by the SEM micrographs. The moisture absorption of PLA‐PP/hemp was higher than PLA/hemp. Based on theoretical analysis of DMTA data, there was favorable adhesion in all composites. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40534.     

Functional interphases with multi-walled carbon nanotubes in glass fibre/epoxy composites

The interphase between reinforcing fibre and matrix is a controlling element in composite performance. We deposited multi-walled carbon nanotubes (MWCNTs) onto electrically insulating glass fibre surfaces leading to the formation of semiconductive MWCNT-glass fibres and in turn multifunctional fibre/polymer interphases. The deposition process of MWCNTs onto glass fibre surfaces involved both electrophoretic deposition (EPD) and conventional dip coating methods. The EPD coating method produces a more homogeneous and continuous nanotube distribution on the glass fibre surface compared with the dip coating. According to fragmentation test results, the interphase with a small number of heterogeneous MWCNTs in the EPD fibre/epoxy composites, mimicking a biological bone structure, can remarkably improve the interfacial shear strength. We found that the semiconductive interphase results in a high sensitivity of the electrical resistance to the tensile strain of single glass fibre model composites. This material provides a possible in situ mechanical load sensor and early warning of fibre composite damage.     

Life cycle studies on hemp fibre reinforced components and ABS for automotive parts

With the increasing importance of environmental interactions, several innovations of the environmental performance are introduced in automotive industry. One aspect of innovation is an environmental material selection including renewable raw materials. Products of renewable raw materials are generally regarded as environmentally friendly, including products from hemp, jute and flax. The ecological preferences of products of natural fibres can be investigated and described by means of ecological balances. However, no general principles for the ecological advantages or disadvantages of fibre plants can be deducted from their life cycle assessment, since the ecological compatibility of the different products strongly depends on the circumstances of the journey of life of the product. The objective of this study is a decision support of automotive engineers by giving an ecological balance of the benefits of substituting ABS by hemp fibres for covering applications. A Life Cycle Assessment (LCA) of hemp fibre reinforced components is introduced. This study contains the agricultural cultivation of fibre plants, the method of harvesting and the processing of the harvested crops. Finally, the analysis includes the further processing of the fibre, starting from the manufacturing of the fibre composite matrix, on which the production of form press components for the automotive industry bases. The differences of energy demand and emissions amount during the use phase of a passenger car as well as different recycling scenarios are assessed.     

Dangers relating to fires in carbon‐fibre based composite material

Inhalable carbon fibres have been suspected to pose similar threats to human health as asbestos fibres. It is well‐known that fibres having a diameter of less than 3 µm might be inhaled and transported deep into the human respiratory system. Some composite materials use carbon fibres as structural reinforcement. These fibres do not pose any risks as such as they are firmly connected to the laminate and surrounded by a polymer matrix. Also, these fibres typically have diameters >6 µm and thus, are not inhalable. However, if the material is exposed to a fire, the carbon material might be oxidized and fractionated and thereby, inhalable fibres might be generated into the fire smoke. The capability of carbon fibre‐based composite material to produce dangerous inhalable fibres from different combustion scenarios has been investigated. It was found that the risk of fires generating inhalable carbon fibres is related to the surface temperature, the oxygen level and the airflow field close to the material surface. The temperatures necessary for oxidation of the carbon fibre is so high that it is possible that only a flashover situation will pose any real danger. Other possible danger scenarios are highly intense fires (e.g. a liquid fuel fire), or situations where structural damage is part of the fire scenario. Copyright © 2005 John Wiley & Sons, Ltd.