Natural fibres as reinforcement in polylactic acid (PLA)composites
K.Oksman a,*,M.Skrifvars b ,J.-F.Selin c
a
Department of Machine Design and Materials Technology,NTNU,Norwegian University of Science and Technology,Rich.Birkelands vei 2,
N-7491Trondheim,Norway
b SICOMP AB,POB 271,SE-94126Pitea ˚,Sweden c
Fortum Oyj,POB 310,FIN-06101Porvoo,Finland
Accepted 21February 2003
Abstract
The focus in this work has been to study if natural fibres can be ud as reinforcement in polymers bad on renewable raw materials.The materials have been flax fibres and polylactic acid (PLA).PLA is
a thermoplastic polymer made from lactic acid and has mainly been ud for biodegradable products,such as plastic bags and planting cups,but in principle PLA can also be ud as a matrix material in composites.Becau of the brittle nature of PLA triacetin was tested as plasticizer for PLA and PLA/flax com-posites in order to improve the impact properties.The studied composite materials were manufactured with a twin-screw extruder having a flax fibre content of 30and 40wt.%.The extruded compound was compression moulded to test samples.The processing and material properties have been studied and compared to the more commonly ud polypropylene flax fibre composites (PP/flax).Preliminary results show that the mechanical properties of PLA and flax fibre composites are promising.The composite strength is about 50%better compared to similar PP/flax fibre composites,which are ud today in many automotive panels.The addition of plasticizer does not show any positive effect on the impact strength of the composites.The study of interfacial adhesion shows that adhesion needs to be improved to optimi the mechanical properties of the PLA/flax composites.The PLA/flax composites did not show any difficulties in the extrusion and compression moulding process and they can be procesd in a similar way as PP bad composites.
#2003Elvier Science Ltd.All rights rerved.
Keywords:PLA;Flax;Renewable raw materials;Composites;Extrusion;B.Mechanical properties;B.Microstructure;DMTA
校园秀1.Introduction
The growing environmental awareness and new rules and regulations are forcing the industries to ek more ecologically friendly materials for their products.For example automotive applications bad on natural fibres with polypropylene as matrix material are very common today.Less work has been done to study composites with matrices,which originate from renew-able raw materials.There are many different polymers of renewable materials:for example polylactic acid,cel-lulo esters,poly hydroxyl butyrates,starch and lignin bad plastics.The problems with the polymers have been poor commercial availability,poor processability,low toughness,high price and low moisture stability.
The long-term properties of renewable materials are also very important especially if the products are not single u applications.
Natural fibres have many advantages compared to synthetic fibres,for example low weight,and they are recyclable and biodegradable.They are also renewable and have relatively high strength and stiffness and cau no skin irritations [1–8].On the other hand there are also some disadvantages:m
oisture uptake,quality var-iations and low thermal stability.Many investigations have been made on the potential of the natural fibres as reinforcements for composites and in veral cas the results have shown that the natural fibre composites own good stiffness but the composites do not reach the same level of strength as glass fibre composites [1–8].The manufacturing methods of natural fibre thermo-plastic composites have been modified lay-up/press moulding (film stacking method),pultrusion,extrusion and injection moulding [4–8].
0266-3538/03/$-e front matter #2003Elvier Science Ltd.All rights rerved.
doi:10.1016/S0266-3538(03)00103-9
Composites Science and Technology 63(2003)1317–1324
/locate/compscitech
*Corresponding author.Tel.:+47-73-59-38-26;fax:+47-73-59-41-29.
E-mail address:kristiina.u.no (K.Oksman).
While many studies have been made on the potential of naturalfibres just a few investigations are made on the possibilities to u renewable polymers as matrix for suchfibres[9–13].The studied biopolymers have been soy-oil bad epoxy,starch,polycaprolactone (PCL),polyhydroxybutyrate(PHB),modified cellulo, acetic acid,polylactic acid(PLA)and polyester amide [9–13].
Polylactic acid polymers or polylactides are polyesters of lactic acid,and the polymers have recently been introduced commercially for products where biode-gradability is wanted.Polylactic acid is a versatile poly-mer made from renewable agricultural raw materials, which are fermented to lactic acid.The lactic acid is then via a cyclic dilactone,lactide,ring opening poly-merid to the wanted poly
lactic acid.The polymer is modified by certain means,which enhance the tempera-ture stability of the polymer and reduce the residual monomer content.The resulting polylactic acid can be procesd similarly as polyolefines and other thermo-plastics although the thermal stability could be better. Reinforcing withfibres is one possibility to enhance thermal stability.Polylactide polymers are stiffand brittle materials,and it is therefore necessary to u plasticizers to improve the elongation and impact properties.The polylactide is fully biodegradable.The degradation occurs by hydrolysis to lactic acid,which is metabolid by micro-organisms to water and car-bon monoxide.By composting together with other biomass the biodegradation occurs within two weeks, and the material has fully disappeared within3–4 weeks[14].
There are veral promising markets for biode-gradable polymers such as polylactide.Plastic bags for houhold bio waste,barriers for sanitary pro-ducts and diapers,planting cups,disposable cups and plates are some typical applications.To date no commercial large-scale production of polylactide exists,but this is likely to change in the near future. The starting material,lactic acid,will also need new capacity.Commercial markets for biodegradable polymers are expected to increa substantially in the coming years.
The aim of this study was to make an initial investigation how PLA will act as matrix material for natu
ralfibre composites.As PLA can be procesd in nearly the same way as polypropylene,it should be possible to prepareflax reinforced composites by extrusion.The mechanical properties of the compo-sites were studied according to the tensile testing. Further,the thermal properties were studied with dynamic mechanical thermal analysis(DMTA),the morphology was studied with scanning electron micro-scopy(SEM)and the possible degradation of PLA dur-ing extrusion was determined by gel permeation chromatography(GPC).2.Materials and methods
2.1.Materials
Matrix:PolyL-lactic acid(PLA),POLLAIT from Fortum was ud.The MFI for PLA is between1
and2g/10min(190 C,2.16kg),which is lower
than for the ud PP.
Matrix(ref):Polypropylene(PP),from Adstiff770ADXP,Montell polyolefins.This is a PP
with specific high MFI,45g/10min(230 C,2.16
kg).It is a suitable matrix for extrusion of com-
posite materials.
Additive:Triacetin,glycerol triacetate ester,from Sigma-Aldrich,was ud as plasticizer for PLA.
It has good compatibility with the polymer,and
increas the elongation of the plastic from less
than10%to more than250%.
Reinforcements:Enzyme rettedflaxfibres in the form of long heckledfibres were connected
together by hand,called‘‘hand made roving’’.
The appearance of udfibres can be en in
Fig.1.
Table1shows the properties of ud materials.PLA has better mechanical properties compared to PP while PP has higher MFI and lower density.Theflaxfibres have superior mechanical properties compared to PP and PLA,which can be expected to result in good rein-forcing effect for both polyme
rs.
Various formulations of studied materials are shown in Table2,totally12different formulations were studied. 2.2.Methods
2.2.1.Compounding of composite materials
The composite materials were manufactured using a twin-screw extruder(Coperion Werner&
Pfleiderer Fig.1.Hand madeflaxfibre roving.
超市的一天
1318K.Oksman et al./Composites Science and Technology63(2003)1317–1324
ZSK25WLE).Theflaxfibre content was30and40 wt.%.Thefibres were fed into the side extruder and the fibre content in the composite was calculated according
to feeding speed and the weight of the roving per meter. The processing parameters are shown in Table3.The liquid triacetin plasticizer was pumped into the extru-der.
2.2.2.Compression moulding
Test samples for mechanical testing were compression moulded with a conventional compression moulding press(Fjellman Press Mariestad AB)with a maximum press capacity of3100tons.The mould temperature was 50 C and the pressure was about70MPa.
2.2.
3.Mechanical testing
The tensile testing was performed according to ASTM3039standard for tensile testing on an Instron. Impact testing was performed according to ISO179 unnotched Charpy standard forfibre reinforced com-posite materials.At least10specimens were tested for every material.
2.2.4.Electron microscopy
Fractured surfaces of the materials were studied with a CamScan scanning electron microscope(SEM) with an acceleration voltage of30kV.The sample surfaces were sputter coated with gold to avoid charging.2.2.5.GPC
Number and weight average molecular weights before and after the extrusion of the PLA were determined by gel permeation chromatography on a Waters GPC sys-tem with Styragel columns(105,104,103and500A)and a refractive index detector.Chloroform was ud as effluent,and the analysis was done at room temperature.
A universal calibration was ud for the calculation of the molecular weights,using Mark–Houwink constants for PLA and polystyrene,which was ud as standard.
2.2.6.DMTA
Dynamical mechanical thermal analysis(DMTA, Rheometric Scientific Mk III)was performed to initially investigate if the addition of theflaxfibres will improve the thermal properties,such as maximum u tempera-ture of PLA.Four materials,PLA,and PLA/flax,PLA/ triacetin and PLA/flax/tria
cetin,were tested tofind the maximum u temperature for the u and also to e possible interaction effects between PLA matrix andflax fibres.DMTA was run in the dual cantilever bending mode and the typical sample dimensions were:thickness 1–2mm,length25mm and width4mm.The tempera-ture interval was from room temperature,about25 C, to180 C with a heating rate of1.5 C/min and using a frequency of1Hz.
Table1
Material properties of ud raw materials
结对子Materials Flexural-modulus
(GPa)Tensile strength
(MPa)
Elongation to break
(%)
MFI(g/10min)
(2.16kg,190 C)
Density
(g/cm3)
PP 1.630–450.9 PLA a 3.46081–2 1.26 Flaxfibres70–80600–10001–2– 1.5 a Data from the manufacturer.
Table2
Compositions of different materials
Materials Matrix
(wt.%)Flaxfibres
(wt.%)
Triacetin
(wt.%)
努洛伊曼皇宫
PP100––PP/flax7030–PP/flax6040–PLA100––PLA/flax7030–PLA/flax6040–PLA/triacetin95–5 PLA/triacetin90–10 PLA/triacetin85–15 PLA/triacetin/flax55405 PLA/triacetin/flax504010 PLA/triacetin/flax454015Table3
Processing ttings for extrusion
Material PP/flax PLA/flax Speed(rpm)250250 Torque(%)31–41
执法如什么
Vacuum vent.Yes Yes Temp.profile(C )
Zone1180180 (PP,PLA,plastizer)
Zone2––Zone3180180 Zone4(Flaxfibres)170170 Zone5175175 Zone6180180 Zone7––Zone8185185 Zone9190190 Zone10––Zone11200200 Total output(kg/h)16
K.Oksman et al./Composites Science and Technology63(2003)1317–13241319
3.Results
3.1.Mechanical testing
The mechanical properties of PLA/flax composites were compared to PP/flax.Table 4shows the summary of the results.Generally,the pure PLA has better mechanical properties than pure PP.
Fig.2shows the tensile stress and Fig.3the stiffness of the tested materials.The pure PLA has a tensile strength of 50MPa and a modulus of 3.4GPa com-pared to 28MPa and 1.3GPa of pure PP.The addition of flax fibres will not improve the tensile strength,which is an indication of poor adhesion between the flax fibres and the matrix.The stress is not transferred from the matrix to the stronger fibres.
The addition of flax will increa the modulus but the higher fibre content will not improve the modulus in the PLA composites as it will for PP composites.A possible explanation of this can be the fibre orientation.The test samples are compression moulded and the fibres can be orientated differently from one sample to another.
Cyras et al.[10]have studied starch/PCL/sisal fibre composites and reported a tensile modulus of 0.7GPa and a maximum strength of 14.4MPa with a 30wt.%sisal fibre content.The values are very low compared to our PLA/flax modulus 8.3GPa and the strength 53MPa.A rearch group at DLR in Germany lead by Riedel [11,12]have studied different biocomposites and reported very high mechanical properties of composites they studied.They have ud Bioceta,Sconacell and PLA as m
atrix and unspecified natural fibre mats as reinforcements and reached mechanical properties near by glass fibre mat reinforced plastics.
搬家贺词
Becau of the brittle nature of PLA,triacetin was ud to plasticize the pure PLA and for the PLA/flax composites.Triacetin has previously been ud for plasticizing of pure PLA with good results.Usually the triacetin content is 12–15%,lower amounts do not give effects.In this work with addition of fibres we also wanted to test lower amounts.The fibre content was held constant during this test,40wt.%.Table 5shows a summary of the mechanical properties of plasticized PLA and PLA/flax composites.
The results show that the tensile stress is decread with incread triacetin content and this trend was even more visible in PLA/flax composites.The addition of triacetin showed a positive effect on the elongation to break for pure PLA and PLA/flax composites,which was expected becau of the softening effect.The highest triacetin addition (15%)clearly shows a negative effect for PLA/flax composites,both the stress and stiffness are strongly decread (e also Figs.4and 5).
Fig.5shows how the stiffness of PLA and PLA/flax composites is affected by addition of triacetin.The stiffness of PLA/flax composites is strongly decread
Table 4
The mechanical properties of PP/flax and PLA/flax composites Materials Elongation to break (%)S.D.Max.Stress (MPa)S.D.E-modulus (GPa)S.D.PP
––30– 1.6–PP/30%flax 2.7 1.529 4.250.4PP/40%flax 1.50.829 3.17.60.9PLA
20.250 2.4 3.40.1PLA/30%flax 1.00.253 3.18.30.6PLA/40%flax
0.9
0.2
44
7.2
7.3
0.5
Fig.2.Tensile stress of PLA/flax composite compared to
手工制作钟表PP/flax.
Fig.3.Tensile modulus of PLA/flax composites compared with PP/flax.
1320K.Oksman et al./Composites Science and Technology 63(2003)1317–1324
with the triacetin content while the triacetin did not affect the stiffness of pure PLA in the same level.
Fig.6shows how the impact properties of PLA and PLA/flax composites are affected by the addition of triacetin.It can be en that the addition of triacetin did not affect the impact properties of the PLA/flax com-posites at all as expected.The addition of5%triacetin in PLA shows the best impact strength.Higher triacetin content does not show any positive effect on Charpy impact strength of the composites.Notice that the standard deviation was incread with incread triace-tin content.The residual moisture content in theflax fibres had most likely a negative effect on the triacetin/ PLA/fibre system.
3.2.Electron microscopy
Fig.7shows the fracture surface of the PLA/flax composite.It is possible to e that there are manyfibre pull-outs and that thefibre surfaces are clean which indicates poor adhesion between thefi
bres and the PLA matrix.Thefibres are also orientated(which can be en in Fig.7b).
Further,Fig.7shows that theflaxfibres are in the form of singlefibres and that indicates that thefibres have been parated during the extrusion process.The fibres are also very well disperd in the PLA matrix. Good dispersion of singlefibres andfibre orientation should result in very high mechanical properties.
3.3.GPC
The GPC analysis showed that the weight average molecular weight was97000g/mol for the pure PLA.
Table5
Mechanical testing of plasticized PLA with and withoutflaxfibres
Materials Elongation to break(%)S.D.Max.stress(MPa)S.D.E-modulus(GPa)S.D. PLA 2.00.250.3 2.4 3.40.1 PLA/5%Tri 2.20.241.7 3.4 3.00.2 PLA/10%Tri 1.80.243.6 1.6 3.40.2 PLA/15%Tri 2.60.137.2 1.6 2.60.2 PLA/40%flax0.90.244.17.27.30.5 PLA/5%Tri/40%flax 1.40.343.2 2.67.30.4 PLA/10%Tri/40%flax 1.10.529.5 4.0 5.4 1.0
PLA/15%Tri/40%flax 2.3 1.816.6 4.7 2.4
0.4
Fig.4.The tensile stress of PLA with5%,10%and15%triacetin content and40%flax
fibres.
Fig. 5.The effect of triacetin and40%flaxfibres on the tensile modulus of
PLA.
浪花朵朵Fig.6.The effect of triacetin and40%flaxfibres on the unnotched
Charpy impact strength of PLA.
K.Oksman et al./Composites Science and Technology63(2003)1317–13241321
