Use of lignin as a compatibiliser in hemp/epoxy composites
Benjamin M.Wood,Stuart R.Coles ⇑,Steven Maggs,James Meredith,Kerry Kirwan
WMG,University of Warwick,Gibbet Hill Road,Coventry,CV47AL,UK
a r t i c l e i n f o Article history:
Received 7December 2010Accepted 13June 2011
Available online 24August 2011Keywords:
A:Polymer-matrix composites (PMCs)B:Fibre/matrix bond B:Impact behaviour
D:Scanning electron microscopy (SEM)E:Resin transfer moulding (RTM)
dtmfa b s t r a c t
This study was designed to ascertain if the addition of lignin to hemp-epoxy composites was beneficial to their mechanical properties.Composites were made using a VARTM method with a two-part epoxy resin and a non-woven hemp fibre mat.Lignin was added to the resin before infusion at concentrations varying between 0and 10%w/w.Samples were then tested according to the relevant ISO standards.There was an increase in impact properties of the fabricated composites with the energy absorbed by the composite containing 5%w/w lignin being 145%higher than the composite with no lignin added.Both flexural and tensile modulus showed an increase when lignin was added up to 2.5%w/w,although there was a drop in both when the lignin was increased to 5%w/w,attributed to poor mixing and infusion due to the increased viscosity of the resin.In all cases,the addition of lignin increased the structural properties of the composites to some degree when compared with composites with no additional lignin.
Ó2011Elsevier Ltd.All rights reserved.
1.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 rac-quets 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 thermoplas-tic or thermoset polymer,with reinforcements such as glass,car-bon,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 [1–6].
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学院学报Two common types of matrix are ester-based (typically polyes-ter and vinyl ester)and epoxy-based resins.The former are nor-mally 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 al-loys,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 gen-erally 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 fi-bres include hemp,jute (hessian),kenaf,flax,coir,wood and pine-apple [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 atmo-sphere.This energy recovery process is not possible with glass fi-bres 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 avail-ability 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 car-bon.However they are generally cheap,widely available and bio-degradable.One of the main problems with using natural fibres as reinforcement is the poor interface between the hydrophobic fi-bres and the hydrophilic resins [9].
When composites were beginning to be developed it was as-sumed 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
0266-3538/$-see front matter Ó2011Elsevier Ltd.All rights reserved.Corresponding author.Tel.:+44(0)2476523387.
E-mail les@warwick.ac.uk (S.R.Coles).
low applied stresses,the shear stresses at thefibre ends of a rein-forced composite could exceed the interfacial shear strength,lead-ing to failure by debonding andfibre pull-out.
Fig.1shows the deformation that occurs in the area of matrix surrounding a singlefibre that is subjected to tensile loading.More recently it has been established that thefinal mechanical proper-tie
s of a composite are dependent on the magnitude of the strength of the bond between thefibre and matrix[12–14].This interface is the limiting factor offibre-reinforced composite performance as it ultimately defines the amount of load that can be transferred from onefibre to the next by the matrix.A review of some of the re-search carried out in thisfield was conducted by Herrera-Franco and Drzal[15].Work done to model the interfacial bond suggests that there is not a distinct interface betweenfibre and matrix, but rather an interphase region resulting from the complex chem-ical interactions between the resin,sizing agents,and the mechan-ical surface of thefibres[16].As such,the poor adhesion that exists between thefibres and resins prevents NFRCs from having com-mercially-useful structural properties.
There are a wide variety of treatments that can be used to im-prove thefibre–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[19–21] and acetylation with acetic anhydride[9,22–24].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 en-ergy 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 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 natu-ralfibre reinforcement and impair the structural properties of the composite.
In this work,it was proposed that even in the solid state,the lig-nin would improvefibre-to-matrix adherence and structural prop-erties of the resulting composite whilst keeping the number of steps and chemical treatments to a minimum.Hempfibres 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 appli-cations,and therefore has industrially relevant properties.
2.Materials and methods
2.1.General considerations
Kraft lignin was generously supplied by Warwick HRI and dried under vacuum to constant weight bef
ore use to remove volatile material.Chopstrand hemp mat was purchased from Hemcore and EP-522resin and H-522hardener were purchased from Alchemie.
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2.2.Lignin characterisation
Particle Size Analysis was carried out using a Carl Zeiss Axio-Scope A1upright optical microscope with fully automated stage.
A small sample of the lignin powder was placed between glass slides and then mounted in the microscope.Darkfield microscopy was used to produce an image with a black background and illumi-nated particles.The microscope was connected to a PC running Carl Zeiss AxioVision Release4.8software and automatic measurement software plug-ins‘MosaiX’and‘Extended Focus’which were used to calculate the effective diameter of each observed particle.
2.3.Composite fabrication
Composite materials were obtained by a VARTM process[32]. An aluminium sheet was prepared beforehand by applying a re-leasefilm to the surface and coating with silicon release spray to ensure facile removal of the composite after fabrication.A plastic sheet,including a vacuum inlet port and a r
esin inlet port,was then used to cover the part and sealed with tack tape to make it airtight. The resin mixture was made immediately before use,composed of 100parts EP-522and22parts H-522.Lignin was then added if de-sired at the appropriate level(1–10%w/w)and mixed throughout the resin to achieve homogeneity.The mixture was then infused through thefibre mat using vacuum ensuring no air entered the system.Once infusion was complete,both ends were sealed off and the composite was allowed to stand at room temperature for 24h to cure.The composite part was then post-cured at120°C for2h and then cut into samples with the desired dimensions for testing.
2.4.Materials testing
Tensile strength and modulus were determined according to EN ISO527-4:1997[33]andflexural strength and modulus were determined according to EN ISO14125:1998[34].Testing was per-formed on an Instron5800universal testing machine equipped with extensometer,with a cross-head speed of2mm/min.Charpy impact tests were carried out using a Ray-Ran Advanced Universal
of application of tensile loading on the matrix surrounding
B.M.Wood et al./Composites Science and Technology71(2011)1804–18101805
Results were normalized for sample size and ambient temperature.A hammer with a mass of 1.198kg,a velocity of 2.9m s À1and im-pact energy of 5J was used for all materials.In all cases,ten sam-ples were used for each test and the results averaged.
2.5.Optical microscopy
In order to investigate the presence of porosity,optical micros-copy was used to evaluate images of the structure of the compos-ites.Samples of each composite were prepared by first mounting Buehler Epocolor red-tinted epoxy resin to give a contrast with the 0.3micron Alumina paste.Images of the mounted samples were captured using a Carl Zeiss AxioScope A1upright optical micro-scope with fully automated stage.The microscope was connected to a PC running Carl Zeiss AxioVision Release 4.8software.Zeiss Image Analysis software was used to estimate porosity of the pol-ished samples by calculating the percentage of the image taken up by the red Epocolor resin used to mount the samples.2.6.Scanning electron microscopy
热解SEM images of the fracture surface of the tensile test samples were taken and the surface was gold coated by sputtering for 120s using a current of 25mA and an approximate coating thick-ness of 30nm.The SEM used was a Zeiss Sigma FXM microscope with a field emission gun as the electron source.The acceleration voltage was 15kV.3.Results and discussion 3.1.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 re-moved from the lignin before use by drying to constant weight;usi
ng ‘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 infuse in a VARTM process.The viscosity of the standard resin is 700MPa Ás at 25°C,however this was observed to increase with increasing addition of lignin.
Fibre content (by mass)of all samples was between 20.5%and 22.5%;exact values for each lignin content are shown in Table 1.Fig.2.Distribution of particle size analysis of lignin added to composites.
1806 B.M.Wood et al./Composites Science and Technology 71(2011)1804–1810
micron,with 65%of the particles having a 5and 10microns.The perceived wisdom is particles are more beneficial to the process as they effectively penetrate the non-woven hemp fibre mat. 3.3.Tensile testing
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The results for the tensile testing are shown in Figs.3and 4and Table 2.In all cases,it would appear that 2.5%w/w lignin is the optimum amount for improving the properties of the final part.The Young’s modulus increased from 3.96GPa at 0%w/w to 5.11GPa at 2.5%w/w and the Ultimate Tensile Strength (UTS)in-creased from 22.6MPa to 31.1MPa for the same addition of lignin.A further increase to 5%w/w lignin content caused a 7.4%reduc-tion in Young’s modulus and a 25.0%reduction in UTS,most likely caused by the large volume of lignin particles wetting out of the fibre reinforcement [28]the brittle nature of the composite,given that on average 27%higher for composites containing than 5%w/w lignin.
Fig.4.Tensile modulus vs.lignin additive content for composites.
Table 2
Tensile modulus and UTS for composites.Lignin content (%w/w)Modulus (GPa)UTS (MPa)0.0 3.96±0.1822.63±2.111.0 2.91±0.1620.57±1.152.5 5.11±0.3931.15±2.715.0
4.74±0.31
23.37±2.24
that level;increasedflexibility but reduced This can be attributed to a combination of between the lignin and the hempfibres and 3.4.Flexural testing
An increase in strength was also observed when the composites were tested for theirflexural stress
properties(Figs.5and6).The maximumflexure stress was observed at2.5%w/w with a value of86.16MPa,which was a31.0%increase
with no lignin content.There is an increase
ductility for both the1%w/w and the2.5%
cating good mixing and infusion.The composites
w/w lignin have retained the ductility of
smaller amounts of added lignin,but the
below that of the0%w/w composite;60.0
65.7MPa(Table3).Poor mixing due to
would also account for this drop in strength
Fig.6.Flexural modulus vs.lignin additive content for composites.
Table3
Flexural results for composites.
Lignin content(%w/w)Modulus(GPa)Max.stress(MPa)
3.66±0.3965.77±6.65
4.58±0.3378.07±1.99
4.18±0.2586.16±
5.11
3.11±0.2760.00±
4.16
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