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Polyoxymethylene composites with natural and cellulose fibres: Toughness and heat deflection temperature
Composites Science and Technology 72 (2012) 1870–1874
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Polyoxymethylene composites with natural and cellulose fibres: Toughness and heat deflection temperature Andrzej K. Bledzki a,b, Abdullah A. Mamun a,⇑, Maik Feldmann a a b
Institute of Material Engineering, Polymer Engineering, University of Kassel, Mönchebergstrasse 3, 34125 Kassel, Germany Institute of Materials Science and Engineering, West Pomeranian University of Technology, 19 Piastow Ave., 70310 Szczecin, Poland
a r t i c l e
i n f o
Article history: Received 29 December 2011 Received in revised form 8 June 2012 Accepted 15 August 2012 Available online 25 August 2012 Keywords: A. Short-fibre composites B. Impact behaviour B. Mechanical properties B. Thermal properties D. Dynamic mechanical thermal analysis (DMTA)
a b s t r a c t Cellulose and abaca fibre reinforced polyoxymethylene (POM) composites were fabricated using an extrusion coating (double screw) compounding followed by injection moulding. The long cellulose or abaca fibres were dried online with an infrared dryer and impregnated fibre in matrix material by using a special extrusion die. The fibre loading in composites was 30 wt.%. The tensile properties, flexural properties, Charpy impact strength, falling weight impact strength, heat deflection temperature and dynamic mechanical properties were investigated for those composites. The fibre pull-outs, fibre matrix adhesion and cracks in composites were investigated by using scanning electron microscopy. It was observed that the tensile strength of composites was found to reduce by 18% for abaca fibre and increase by 90% for cellulose fibre in comparison to control POM. The flexural strength of composites was found to increase by 39% for abaca fibre and by 144% for cellulose fibre. Due to addition of abaca or cellulose fibre both modulus properties were found to increase 2-fold. The notched Charpy impact strength of cellulose fibre composites was 6-fold higher than that of control POM. The maximum impact resistance force was shorted out for cellulose fibre composites. The heat deflection temperature of abaca and cellulose fibre composites was observed to be 50 °C and 63 °C higher than for control POM respectively. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The importance of the response to resources and environmental issues has been recognised in our society and as a consequence, there is a strong push to minimise the environmental impact at every stage of the life cycle [1]. The environmental impact of polymer and polymer composite products will be an important branch for minimising the carbon footprint. It was also observed that the demand for composites, made from versatile plastics and natural fibres, linearly increased during the past years. Natural fibres offer numerous benefits, such as reductions in weight (as compared to synthetic fibre), CO2 neutrality, and less reliance on petrochemical sources [2]. Polyoxymethylene (POM), also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts that require high stiffness, low friction and excellent dimensional stability [3]. It was introduced to industrial application in 1956 as a potential replacement for die-cast metals and is widely used in automotive, electrical, electronics, and many industrial fields [4]. This is due to its outstanding and well-balanced properties and because no other products can be substituted for ⇑ Corresponding author. E-mail address: [email protected] (A.A. Mamun). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.08.004
POM in some application fields. POM occupies an important position in industry as well as in society. It shows excellent physical and mechanical properties are mainly based on its high crystallinity. It is expected that the development of high-value-added materials will result in the requirement to distinguish them from existing POM materials [5,6]. POM, however, has a poor impact resistance, which limits its range of applications. Generally, toughening such engineering resins is accomplished by blending them with small quantities of low modulus rubbers [7]. In practice since most polymers are immiscible, it is necessary to introduce a third component during mixing called a compatibiliser. If it is located at the interface between the two polymers it anchors the component phases together and effectively increases their interfacial adhesion. This results in a blend with an improved impact property. When highly dispersed, the rubbery phase acts as an effective stress concentrator and enhances both crazing and shear yielding in the matrix. Since both processes can absorb large amounts of energy, such materials typically exhibit superior resistance to crack propagation under impact conditions [8]. The compatibiliser usually is a block or graft copolymer. It can be added separately or formed in situ during mixing. In the latter case, the component polymers must contain complementary functional groups capable of reacting together during mixing so that a covalent bond may form to stabilise the
A.K. Bledzki et al. / Composites Science and Technology 72 (2012) 1870–1874
dispersion of the rubber in the rigid matrix. The modification of Impact strength of POM via such methods has not been discussed often in published literature [9–11]. This is due to the fact that a conventional impact modifier or compatibiliser could improve the impact properties of thermoplastic polymeric materials while reducing other mechanical and structural properties [12,13]. Adding fibre or filler with polymeric materials could be an option to improve impact properties. POM polymer also suffers from limited processing temperature and low heat deflection temperature. Several authors reported that due to addition of glass fibre the heat deflection temperature of POM was found to increase about 20–40 °C [1,14,15]. It is expected that due to the addition of natural fibres (abaca) and man-made cellulose fibres, the heat deflection temperature of POM could be improved. Abaca fibre reinforced composites are gaining in interest due to the innovative application of abaca fibres in under floor protection for passenger cars by DaimlerChrysler. It was described that abaca fibre has a high tensile strength, is resistant to rotting and its specific flexural strength is nearly similar to glass fibre. Abaca is the first natural fibre to meet the stringent quality requirements for components used on the exterior of road vehicles, especially those concerning resistance to influences such as stone strike, exposure to the elements and dampness [16,17]. Apart from naturally occurring plant and plant residue fibres, cellulose can be obtained, industrially in form of man-made fibres by means of through the viscose or the NMMO process. Yarns are used in textile applications (e.g. as viscose, modal, tencel) or for reinforcing such as tires or hoses (e.g. rayon tire cord yarn) [18]. The potential of these fibres as thermoplastic reinforcement was recognised couple of years ago [19,20]. The properties of man-made cellulose fibres depend on methods of spun fibre processing. For example, Cordenka (RT/super series) is a rayon yarn spun by a special variant of viscose process and produced by Cordenka GmbH Obernburg, Germany. This standard yarn was produced as 1350 filaments together and cross sectional diameter is in the range 10–20 lm. Some approximate mechanical and structural properties of this yarn are as elasticity modulus is 20 GPa, tensile strength is 830 MPa, crystalinity 28%, crystalline orientation factor is 0.963 [18,21]. These fibres are less abrasive to the processing equipment, high strength and stiffness in the fibre direction and moderate properties in cross direction [22]. Moreover this fibre provides improved recycling properties because fibre breakage at repeated processing is much less severe than conventional synthetic (glass) fibre. Additionally, the energy recovery/generating (incineration) from the end of the life time of composites made from cellulose fibre is greater than for conventional fibres and there will be no toxic residue [23,24]. As the raw material costs of polymer rise, the use of fillers/fibres in polymer compounds becomes increasingly attractive. In addition, many useful physical and processing properties can be imparted to plastics by the use of fillers or fibres [25]. The development of high-value-added materials will be sturdily required in order to distinguish them from existing POM materials. Several research groups have investigated the preparation and properties of filled POM from a theoretical and experimental point of view, regarding aspects such as processing, fibre orientation and composite properties [26–29]. In contrast, there have been few reports on natural fibre filled POM composites [30], despite the relatively lower melt temperature (Tm) and processing temperature of POM compared to other engineering plastics. The melt temperature (Tm) of Ticona Hostaform C 27021 acetal copolymer is about 166 °C which is nearly the same as for the commodity polymer polypropylene.
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In this study, we investigated POM composites composed of abaca and man-made cellulose fibres used for developing value added engineering materials. Hereby, we focused on the mechanical, dynamic mechanical and thermal properties of the composites. Moreover, we discussed the dependence of the fibre type on the mechanical and thermal properties especially concerning the toughness and heat deflection temperature properties. 2. Experimental 2.1. Materials Abaca fibres were obtained from RIETER Automotive Heatshields AG, Sevelen, Switzerland. The fibre bundle diameter equalled 150 ± 50 lm, the elementary fibre diameter 10–30 lm, and the tensile strength of the bundle fibres was 900 MPa. Cellulose fibres were provided by Cordenka GmbH, Obernburg, Germany. Fibre type was Cordenka 700 Super3 with a, linear mass density of 2440, number of mono filament of 1350 and a breaking force of 128 N and an elementary fibre diameter of 12 lm. POM was supplied by Ticona Engineering Polymers. The trade name of POM was Hostaform C 27021 acetal copolymer; MFR was 33.84 g/10 min (2.16 kg, 190 °C), density 1.41 g/cc and the melt temperature was 166 °C. 2.2. Composites preparation POM was compounded with fibres via a coating extrusion (double screw) process. The endless cellulose fibres/filaments were dried online with a special dryer and impregnated using a special extrusion die. Afterwards the impregnated fibre was pelletized with a granulator without using a water bath. The pellets were dried at 100 °C in an air circulating oven for 24 h. The specimen for different composites were prepared by means of an injection moulding process with a temperature profile in the range 160– 180 °C and the mould temperature was kept at 80 °C. Prior to processing, the polymers and fibres were dried at 100 °C in a drying chamber till the remaining moisture content equalled <0.1%. 2.3. Characterisation 2.3.1. Morphology The morphology of fibre reinforced POM composites was investigated using the scanning electron microscope (SEM) MV2300, by CamScan Electron Optics. Flexural samples were fractured after being submerged in liquid nitrogen and test specimens were prepared sputter coated with gold. 2.3.2. Mechanical analysis Tensile and flexural tests were performed at a test speed of 2 mm/min according to EN ISO 527 and EN ISO 178 using a Zwick UPM 1446 machine. All tests were performed at room temperature (23 °C) and at a relative humidity of 50%. Instrumented notched Charpy impact test was carried out using 10 notched samples according to EN ISO 179 using Zwick Charpy impact machine. Impact tests were carried out using a low velocity falling weight impact tester at room temperature in penetration mode according to EN ISO 6603-2. The impactor’s mass was 3.65 kg and the impact velocity was 4.4 m/s. 2.3.3. Dynamic mechanical analysis DMA analyses were conducted on a dynamical mechanical analyser DMA 2980/Q800 supplied by TA Instruments. The samples
A.K. Bledzki et al. / Composites Science and Technology 72 (2012) 1870–1874
2.3.4. Heat deflection temperature analysis Heat deflection temperature (HDT) analysis was conducted on a DMA analyser according to DIN EN ISO 75. The samples (80 10 4 mm3) were analysed with 1.8 MPa bending force, heating rate 2 °K/min and HDT was measured at a fixed elongation of 2 mm. 3. Results and discussion 3.1. Mechanical properties The fractured micro-mechanisms that occurred in the materials under flexural loading were similar to those observed in tension. In a simple tensile test the stress distribution in a specimen was fairly homogeneous whereas in the case of bending tests, both tensile and compressive stresses were present. Tensile and flexural moduli of abaca and cellulose fibre composites are shown in Fig. 1. The tensile modulus of control POM was found 2.9 GPa and the property was found to increase by 90% due to the addition of abaca fibre and 105% due to addition of cellulose fibre. The flexural modulus of control POM equalled 2.8 GPa and the property was found to increase by 120% for abaca fibre composites and by 90% for cellulose fibre composites. The flexural modulus of abaca fibre composites found to better 15% in comparison with cellulose fibres composites. It is because of abaca fibre showed better compression stress resistance than cellulose fibre for the certain range of strain which is provably depend on fibre structural properties, the fibre orientation [1]. The POM polymer itself also showed negative compression stress resistance. The tensile and flexural strengths of abaca and cellulose fibre composites are presented in Fig. 2. The result shows that the tensile strength of abaca fibre composites decreased by 18% in comparison to that of control POM. The negative reinforcing effect is caused by the reduction of matrix material in the test specimens and fibre performance. Abaca fibres consist of single fibre packages, so called fibre bundles where the cellulose is bundle with hemicellulose and lignin is embedded in between cellulose fibrils, functioning as a kind of matrix. Therefore, the fibre bundle can be considered as a naturally occurring composite. Attributable to the orientation of the cellulose fibrils along the fibre axis, the fibre strength can differ. Furthermore, the fibre properties differ depending on the fibre geometry (i.e. specific surface area). Moreover the interface or interphase is most importance for composite properties [17]. On the other hand, the tensile strength of cellulose
Tensile
Flexural
Modulus [GPa]
6
4
2
Tensile
200
Flexural
160
Strength [MPa]
(30 10 4 mm3) were analysed in single cantilever clamped bending mode. The measurements were performed in different temperature ranges (50–150 °C) while controlling the strain. Tests were conducted over a wide frequency range (1, 3, 10 Hz) with a constant heating rate (3 K/min) and constant oscillating amplitude of 20 lm.
120 80 40 0 Control
Abaca
Cellulose
Fig. 2. Strength of abaca and cellulose fibre composites.
fibre composites was found to be increased by 90% in comparison to control POM. This increase results from the fibre geometry (diameter, length and specific surface area) and relatively better interface (it could be seen in SEM) between the cellulose fibre and POM matrix. The flexural strengths of composites was found to differ from the tensile strengths which were 39% higher for abaca fibre composites and 144% for cellulose fibre composites than for control POM. The notched and unnotched Charpy impact strengths of abaca and cellulose fibre composites are seen in Fig. 3. It was observed that the notched Charpy impact of abaca fibre composites was lower than that of control POM. It is because of the brittleness of material increased due to the addition of abaca fibre. The notched Charpy impact of cellulose fibre composites was found to be 560% higher than for control POM. The fibre length of man-made cellulose in composites is evidently longer than abaca fibre [20]. Fibre–fibre interaction could be occurred and intertwined each other during compounding and injection moulding. As a result, longer fibres exist in composites. That is why there is an enormous amount of fibre pull-out took place consequencely huge amount of energy absorbed during the experiment. The lower diameter (higher aspect ratio) and smooth surface of man-made cellulose fibre compared to the abaca fibre determines the composite performance under impact stress. It may affect the fibre and matrix interaction. In this context, pull-outs appear more often with man-made cellulose than with abaca. Apparently the reason could be the path length of the propagated crack which is enlarged and increases the energy amount needed to fracture the sample [4]. The thinner the fibre is, the larger the specific surface consequence, the higher the total energy necessary to pull-out the fibre [30]. In addition, man-made cellulose fibre shows much higher elongation at break and lower stiffness as, results of its increased ductility and flexibility of material. The impact energy needed more to break the sample with increasing fibre formability, and consequently, the impact strength of the composites. The falling weight impact test describes the force–displacement relation needed to assess the multidimensional load-mechanical
Charpy impact strength [kJ/m2]
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Notch A
80
Without notch
70 60 50 40 30 20 10 0
0 Control
Abaca
Cellulose
Fig. 1. Modulus of abaca and cellulose fibre composites.
Control
Abaca
Cellulose
Fig. 3. Charpy impact strength of abaca and cellulose fibre composites.
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Control Cellulose
Absorbed energy [J]
5 4 3 2 1 0 0
2
4
6
8
10
12
14
Deformation [mm] Fig. 4. Impact (energy–deflection curve) of cellulose fibre composites.
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slope of the energy absorption curve for abaca fibre composites. This could be explained as the cellulose fibre is flexible and on the other hand abaca fibre is hard and brittle. That is why a lot of fibres pull-out observed in cellulose fibre composites and a few fibres pull out observed in abaca fibre composites. The maximum withstand force was found at about 400 N for control POM and about 900 N for cellulose fibre composites (in Fig. 5). The maximum withstand force for abaca fibre composites was found at about 500 N (not show in figure). The deformation at maximum force was shorted out at 2 mm for control POM and at 10.7 mm and at 7 mm for abaca and cellulose fibre composites respectively. The withstand force depends on the stress shearing ability and the stress shearing ability in composites. This is also depend on fibre aspect ratio, fibre geometry and interface [17]. 3.2. Morphology
1000
400
The scanning electronic micrograms of abaca and cellulose fibre composites are seen in Fig. 6. In the case of the abaca fibre composites, the fibres are not well embedded in the matrix and there is a bad adhesion on other hand. There are also many fibre fracture observed. In the case of man-made cellulose fibre composites, pullouts occur more often than abaca fibre composites and a very few fibre fractures were observed.
200
3.3. DMA
Control
Cellulose
Maximum force [N]
800
600
0 0
2
4
6
8
10
12
14
Fig. 5. Impact (force–deflection curve) of cellulose fibre composites.
properties of material. The storage energy describes the energy given back to the impactor. In this case, 35.77 J impact energy was chosen and the energy was high enough to puncture the material. Therefore o no storage energy was left. The impact behaviour (maximum force, absorbed energy and deformation) of control POM fibre and cellulose fibre composites are represented in Figs. 4 and 5. The detailed analysis of the data obtained from the hysteresis impact cycles showed that the absorbed energy for cellulose fibre composites was found to linearly increase till a deformation 6.6 mm occured (in Fig. 4). This deformation could be explained as elastic deformation. The elastic deformation was shorted out 5 mm for control POM and 6.4 mm for abaca fibre composites (did not show in figure). The slope of the energy absorption curve for cellulose fibre composites was found to be 150% higher than the
The variation of the storage modulus as a function of temperature is graphically enumerated in Fig. 7. It is evident that there was a notable increase in the modulus of the virgin matrix with the incorporation of abaca and cellulose fibres. This is probably due to an increase in the stiffness of the matrix with the reinforcing effect imparted by the fibres that allowed a greater (in compare with the matrix material) degree of stress transfer at the interface. In this cases, fibres act as a stress transfer bridge with a define strain and fibres are stiffer than the matrix material. There were no changes observed for the storage modulus between abaca fibre and cellulose fibre composites. These results attributed by the compromising of fibres stiffness and the adhesion between fibre and matrix. As previously explained, the abaca fibres are stiffer than the cellulose fibres. Furthermore, the adhesion between the cellulose and POM is relatively better than the adhesion between the abaca and POM. The area under the tan @ curve gives an indication of the total amount of energy that can be absorbed by the material during an experiment. A large area under the tan @ curve indicates a great degree of molecular mobility, which translates into better damping properties, meaning that the material can absorb and dissipate en-
Fibres break
Fibres pull-out
(a)
(b)
Fig. 6. SEM of (a) abaca fibre composites and (b) cellulose fibre composites.
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were found to improve enormously addition of cheap natural/ man-made cellulose fibres without increasing density of materials.
0.08
References
Control Abaca Cellulose
3000
2000
0.06
1000
0.04
0.02 150
0 -50
0
50
100
Temperature (°C)
Tan Delta
Storage Modulus (MPa)
4000
Universal V4.1D TA Instruments
Heat deflection temperature [°C]
Fig. 7. DMA of abaca and cellulose fibre composites.
180 150 120 90 60 30 0 Control
Abaca
Cellulose
Fig. 8. HDT of abaca and cellulose fibre composites.
ergy better. The area under the tan @ curve is found to be dependent on the nature of fibre. The area under the tan @ curve of cellulose fibre was larger than that of abaca fibre composites. It is because of both fibres exhibit different aspect ratio and structure [20]. 3.4. Heat deflection temperature Fig. 8 shows the HDT values of abaca and cellulose fibre composites. The HDT values of abaca and cellulose fibre composites found to be 156 °C and 169 °C which are 50 °C and 63 °C higher than for control POM respectively. The cellulose fibres composites showed about 13 °C higher HDT than that of abaca fibres composites. The fibre size distribution, aspect ratio and fibre volume fraction could be a reason. Moreover, The HDT value of a material depends on the compactness (stiffness) and the glass transition temperature. The changes of hardness with temperature play a critical role. In cease of cellulose fibre composites, the change of hardness value is slower than that of abaca fibre composites. It is because of relatively better interface between cellulose fibres and POM. 4. Conclusion This study inspected the effect of cellulose and abaca fibre on properties of POM composites. The incorporation of both fibres showed considerable improvement in the properties of POM such as modulus, flexural strength, Charpy impact strength, energy absorption, storage modulus and heat deflection temperature (HDT). But in contrast, the tensile strength of abaca fibre composites was found to reduce. However, it can be concluded that the mechanical and the thermo-mechanical properties of composites
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Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Polyoxymethylene composites with natural and cellulose fibres: Toughness and heat deflection temperature Andrzej K. Bledzki a,b, Abdullah A. Mamun a,⇑, Maik Feldmann a a b
Institute of Material Engineering, Polymer Engineering, University of Kassel, Mönchebergstrasse 3, 34125 Kassel, Germany Institute of Materials Science and Engineering, West Pomeranian University of Technology, 19 Piastow Ave., 70310 Szczecin, Poland
a r t i c l e
i n f o
Article history: Received 29 December 2011 Received in revised form 8 June 2012 Accepted 15 August 2012 Available online 25 August 2012 Keywords: A. Short-fibre composites B. Impact behaviour B. Mechanical properties B. Thermal properties D. Dynamic mechanical thermal analysis (DMTA)
a b s t r a c t Cellulose and abaca fibre reinforced polyoxymethylene (POM) composites were fabricated using an extrusion coating (double screw) compounding followed by injection moulding. The long cellulose or abaca fibres were dried online with an infrared dryer and impregnated fibre in matrix material by using a special extrusion die. The fibre loading in composites was 30 wt.%. The tensile properties, flexural properties, Charpy impact strength, falling weight impact strength, heat deflection temperature and dynamic mechanical properties were investigated for those composites. The fibre pull-outs, fibre matrix adhesion and cracks in composites were investigated by using scanning electron microscopy. It was observed that the tensile strength of composites was found to reduce by 18% for abaca fibre and increase by 90% for cellulose fibre in comparison to control POM. The flexural strength of composites was found to increase by 39% for abaca fibre and by 144% for cellulose fibre. Due to addition of abaca or cellulose fibre both modulus properties were found to increase 2-fold. The notched Charpy impact strength of cellulose fibre composites was 6-fold higher than that of control POM. The maximum impact resistance force was shorted out for cellulose fibre composites. The heat deflection temperature of abaca and cellulose fibre composites was observed to be 50 °C and 63 °C higher than for control POM respectively. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The importance of the response to resources and environmental issues has been recognised in our society and as a consequence, there is a strong push to minimise the environmental impact at every stage of the life cycle [1]. The environmental impact of polymer and polymer composite products will be an important branch for minimising the carbon footprint. It was also observed that the demand for composites, made from versatile plastics and natural fibres, linearly increased during the past years. Natural fibres offer numerous benefits, such as reductions in weight (as compared to synthetic fibre), CO2 neutrality, and less reliance on petrochemical sources [2]. Polyoxymethylene (POM), also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts that require high stiffness, low friction and excellent dimensional stability [3]. It was introduced to industrial application in 1956 as a potential replacement for die-cast metals and is widely used in automotive, electrical, electronics, and many industrial fields [4]. This is due to its outstanding and well-balanced properties and because no other products can be substituted for ⇑ Corresponding author. E-mail address: [email protected] (A.A. Mamun). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.08.004
POM in some application fields. POM occupies an important position in industry as well as in society. It shows excellent physical and mechanical properties are mainly based on its high crystallinity. It is expected that the development of high-value-added materials will result in the requirement to distinguish them from existing POM materials [5,6]. POM, however, has a poor impact resistance, which limits its range of applications. Generally, toughening such engineering resins is accomplished by blending them with small quantities of low modulus rubbers [7]. In practice since most polymers are immiscible, it is necessary to introduce a third component during mixing called a compatibiliser. If it is located at the interface between the two polymers it anchors the component phases together and effectively increases their interfacial adhesion. This results in a blend with an improved impact property. When highly dispersed, the rubbery phase acts as an effective stress concentrator and enhances both crazing and shear yielding in the matrix. Since both processes can absorb large amounts of energy, such materials typically exhibit superior resistance to crack propagation under impact conditions [8]. The compatibiliser usually is a block or graft copolymer. It can be added separately or formed in situ during mixing. In the latter case, the component polymers must contain complementary functional groups capable of reacting together during mixing so that a covalent bond may form to stabilise the
A.K. Bledzki et al. / Composites Science and Technology 72 (2012) 1870–1874
dispersion of the rubber in the rigid matrix. The modification of Impact strength of POM via such methods has not been discussed often in published literature [9–11]. This is due to the fact that a conventional impact modifier or compatibiliser could improve the impact properties of thermoplastic polymeric materials while reducing other mechanical and structural properties [12,13]. Adding fibre or filler with polymeric materials could be an option to improve impact properties. POM polymer also suffers from limited processing temperature and low heat deflection temperature. Several authors reported that due to addition of glass fibre the heat deflection temperature of POM was found to increase about 20–40 °C [1,14,15]. It is expected that due to the addition of natural fibres (abaca) and man-made cellulose fibres, the heat deflection temperature of POM could be improved. Abaca fibre reinforced composites are gaining in interest due to the innovative application of abaca fibres in under floor protection for passenger cars by DaimlerChrysler. It was described that abaca fibre has a high tensile strength, is resistant to rotting and its specific flexural strength is nearly similar to glass fibre. Abaca is the first natural fibre to meet the stringent quality requirements for components used on the exterior of road vehicles, especially those concerning resistance to influences such as stone strike, exposure to the elements and dampness [16,17]. Apart from naturally occurring plant and plant residue fibres, cellulose can be obtained, industrially in form of man-made fibres by means of through the viscose or the NMMO process. Yarns are used in textile applications (e.g. as viscose, modal, tencel) or for reinforcing such as tires or hoses (e.g. rayon tire cord yarn) [18]. The potential of these fibres as thermoplastic reinforcement was recognised couple of years ago [19,20]. The properties of man-made cellulose fibres depend on methods of spun fibre processing. For example, Cordenka (RT/super series) is a rayon yarn spun by a special variant of viscose process and produced by Cordenka GmbH Obernburg, Germany. This standard yarn was produced as 1350 filaments together and cross sectional diameter is in the range 10–20 lm. Some approximate mechanical and structural properties of this yarn are as elasticity modulus is 20 GPa, tensile strength is 830 MPa, crystalinity 28%, crystalline orientation factor is 0.963 [18,21]. These fibres are less abrasive to the processing equipment, high strength and stiffness in the fibre direction and moderate properties in cross direction [22]. Moreover this fibre provides improved recycling properties because fibre breakage at repeated processing is much less severe than conventional synthetic (glass) fibre. Additionally, the energy recovery/generating (incineration) from the end of the life time of composites made from cellulose fibre is greater than for conventional fibres and there will be no toxic residue [23,24]. As the raw material costs of polymer rise, the use of fillers/fibres in polymer compounds becomes increasingly attractive. In addition, many useful physical and processing properties can be imparted to plastics by the use of fillers or fibres [25]. The development of high-value-added materials will be sturdily required in order to distinguish them from existing POM materials. Several research groups have investigated the preparation and properties of filled POM from a theoretical and experimental point of view, regarding aspects such as processing, fibre orientation and composite properties [26–29]. In contrast, there have been few reports on natural fibre filled POM composites [30], despite the relatively lower melt temperature (Tm) and processing temperature of POM compared to other engineering plastics. The melt temperature (Tm) of Ticona Hostaform C 27021 acetal copolymer is about 166 °C which is nearly the same as for the commodity polymer polypropylene.
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In this study, we investigated POM composites composed of abaca and man-made cellulose fibres used for developing value added engineering materials. Hereby, we focused on the mechanical, dynamic mechanical and thermal properties of the composites. Moreover, we discussed the dependence of the fibre type on the mechanical and thermal properties especially concerning the toughness and heat deflection temperature properties. 2. Experimental 2.1. Materials Abaca fibres were obtained from RIETER Automotive Heatshields AG, Sevelen, Switzerland. The fibre bundle diameter equalled 150 ± 50 lm, the elementary fibre diameter 10–30 lm, and the tensile strength of the bundle fibres was 900 MPa. Cellulose fibres were provided by Cordenka GmbH, Obernburg, Germany. Fibre type was Cordenka 700 Super3 with a, linear mass density of 2440, number of mono filament of 1350 and a breaking force of 128 N and an elementary fibre diameter of 12 lm. POM was supplied by Ticona Engineering Polymers. The trade name of POM was Hostaform C 27021 acetal copolymer; MFR was 33.84 g/10 min (2.16 kg, 190 °C), density 1.41 g/cc and the melt temperature was 166 °C. 2.2. Composites preparation POM was compounded with fibres via a coating extrusion (double screw) process. The endless cellulose fibres/filaments were dried online with a special dryer and impregnated using a special extrusion die. Afterwards the impregnated fibre was pelletized with a granulator without using a water bath. The pellets were dried at 100 °C in an air circulating oven for 24 h. The specimen for different composites were prepared by means of an injection moulding process with a temperature profile in the range 160– 180 °C and the mould temperature was kept at 80 °C. Prior to processing, the polymers and fibres were dried at 100 °C in a drying chamber till the remaining moisture content equalled <0.1%. 2.3. Characterisation 2.3.1. Morphology The morphology of fibre reinforced POM composites was investigated using the scanning electron microscope (SEM) MV2300, by CamScan Electron Optics. Flexural samples were fractured after being submerged in liquid nitrogen and test specimens were prepared sputter coated with gold. 2.3.2. Mechanical analysis Tensile and flexural tests were performed at a test speed of 2 mm/min according to EN ISO 527 and EN ISO 178 using a Zwick UPM 1446 machine. All tests were performed at room temperature (23 °C) and at a relative humidity of 50%. Instrumented notched Charpy impact test was carried out using 10 notched samples according to EN ISO 179 using Zwick Charpy impact machine. Impact tests were carried out using a low velocity falling weight impact tester at room temperature in penetration mode according to EN ISO 6603-2. The impactor’s mass was 3.65 kg and the impact velocity was 4.4 m/s. 2.3.3. Dynamic mechanical analysis DMA analyses were conducted on a dynamical mechanical analyser DMA 2980/Q800 supplied by TA Instruments. The samples
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2.3.4. Heat deflection temperature analysis Heat deflection temperature (HDT) analysis was conducted on a DMA analyser according to DIN EN ISO 75. The samples (80 10 4 mm3) were analysed with 1.8 MPa bending force, heating rate 2 °K/min and HDT was measured at a fixed elongation of 2 mm. 3. Results and discussion 3.1. Mechanical properties The fractured micro-mechanisms that occurred in the materials under flexural loading were similar to those observed in tension. In a simple tensile test the stress distribution in a specimen was fairly homogeneous whereas in the case of bending tests, both tensile and compressive stresses were present. Tensile and flexural moduli of abaca and cellulose fibre composites are shown in Fig. 1. The tensile modulus of control POM was found 2.9 GPa and the property was found to increase by 90% due to the addition of abaca fibre and 105% due to addition of cellulose fibre. The flexural modulus of control POM equalled 2.8 GPa and the property was found to increase by 120% for abaca fibre composites and by 90% for cellulose fibre composites. The flexural modulus of abaca fibre composites found to better 15% in comparison with cellulose fibres composites. It is because of abaca fibre showed better compression stress resistance than cellulose fibre for the certain range of strain which is provably depend on fibre structural properties, the fibre orientation [1]. The POM polymer itself also showed negative compression stress resistance. The tensile and flexural strengths of abaca and cellulose fibre composites are presented in Fig. 2. The result shows that the tensile strength of abaca fibre composites decreased by 18% in comparison to that of control POM. The negative reinforcing effect is caused by the reduction of matrix material in the test specimens and fibre performance. Abaca fibres consist of single fibre packages, so called fibre bundles where the cellulose is bundle with hemicellulose and lignin is embedded in between cellulose fibrils, functioning as a kind of matrix. Therefore, the fibre bundle can be considered as a naturally occurring composite. Attributable to the orientation of the cellulose fibrils along the fibre axis, the fibre strength can differ. Furthermore, the fibre properties differ depending on the fibre geometry (i.e. specific surface area). Moreover the interface or interphase is most importance for composite properties [17]. On the other hand, the tensile strength of cellulose
Tensile
Flexural
Modulus [GPa]
6
4
2
Tensile
200
Flexural
160
Strength [MPa]
(30 10 4 mm3) were analysed in single cantilever clamped bending mode. The measurements were performed in different temperature ranges (50–150 °C) while controlling the strain. Tests were conducted over a wide frequency range (1, 3, 10 Hz) with a constant heating rate (3 K/min) and constant oscillating amplitude of 20 lm.
120 80 40 0 Control
Abaca
Cellulose
Fig. 2. Strength of abaca and cellulose fibre composites.
fibre composites was found to be increased by 90% in comparison to control POM. This increase results from the fibre geometry (diameter, length and specific surface area) and relatively better interface (it could be seen in SEM) between the cellulose fibre and POM matrix. The flexural strengths of composites was found to differ from the tensile strengths which were 39% higher for abaca fibre composites and 144% for cellulose fibre composites than for control POM. The notched and unnotched Charpy impact strengths of abaca and cellulose fibre composites are seen in Fig. 3. It was observed that the notched Charpy impact of abaca fibre composites was lower than that of control POM. It is because of the brittleness of material increased due to the addition of abaca fibre. The notched Charpy impact of cellulose fibre composites was found to be 560% higher than for control POM. The fibre length of man-made cellulose in composites is evidently longer than abaca fibre [20]. Fibre–fibre interaction could be occurred and intertwined each other during compounding and injection moulding. As a result, longer fibres exist in composites. That is why there is an enormous amount of fibre pull-out took place consequencely huge amount of energy absorbed during the experiment. The lower diameter (higher aspect ratio) and smooth surface of man-made cellulose fibre compared to the abaca fibre determines the composite performance under impact stress. It may affect the fibre and matrix interaction. In this context, pull-outs appear more often with man-made cellulose than with abaca. Apparently the reason could be the path length of the propagated crack which is enlarged and increases the energy amount needed to fracture the sample [4]. The thinner the fibre is, the larger the specific surface consequence, the higher the total energy necessary to pull-out the fibre [30]. In addition, man-made cellulose fibre shows much higher elongation at break and lower stiffness as, results of its increased ductility and flexibility of material. The impact energy needed more to break the sample with increasing fibre formability, and consequently, the impact strength of the composites. The falling weight impact test describes the force–displacement relation needed to assess the multidimensional load-mechanical
Charpy impact strength [kJ/m2]
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Notch A
80
Without notch
70 60 50 40 30 20 10 0
0 Control
Abaca
Cellulose
Fig. 1. Modulus of abaca and cellulose fibre composites.
Control
Abaca
Cellulose
Fig. 3. Charpy impact strength of abaca and cellulose fibre composites.
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Control Cellulose
Absorbed energy [J]
5 4 3 2 1 0 0
2
4
6
8
10
12
14
Deformation [mm] Fig. 4. Impact (energy–deflection curve) of cellulose fibre composites.
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slope of the energy absorption curve for abaca fibre composites. This could be explained as the cellulose fibre is flexible and on the other hand abaca fibre is hard and brittle. That is why a lot of fibres pull-out observed in cellulose fibre composites and a few fibres pull out observed in abaca fibre composites. The maximum withstand force was found at about 400 N for control POM and about 900 N for cellulose fibre composites (in Fig. 5). The maximum withstand force for abaca fibre composites was found at about 500 N (not show in figure). The deformation at maximum force was shorted out at 2 mm for control POM and at 10.7 mm and at 7 mm for abaca and cellulose fibre composites respectively. The withstand force depends on the stress shearing ability and the stress shearing ability in composites. This is also depend on fibre aspect ratio, fibre geometry and interface [17]. 3.2. Morphology
1000
400
The scanning electronic micrograms of abaca and cellulose fibre composites are seen in Fig. 6. In the case of the abaca fibre composites, the fibres are not well embedded in the matrix and there is a bad adhesion on other hand. There are also many fibre fracture observed. In the case of man-made cellulose fibre composites, pullouts occur more often than abaca fibre composites and a very few fibre fractures were observed.
200
3.3. DMA
Control
Cellulose
Maximum force [N]
800
600
0 0
2
4
6
8
10
12
14
Fig. 5. Impact (force–deflection curve) of cellulose fibre composites.
properties of material. The storage energy describes the energy given back to the impactor. In this case, 35.77 J impact energy was chosen and the energy was high enough to puncture the material. Therefore o no storage energy was left. The impact behaviour (maximum force, absorbed energy and deformation) of control POM fibre and cellulose fibre composites are represented in Figs. 4 and 5. The detailed analysis of the data obtained from the hysteresis impact cycles showed that the absorbed energy for cellulose fibre composites was found to linearly increase till a deformation 6.6 mm occured (in Fig. 4). This deformation could be explained as elastic deformation. The elastic deformation was shorted out 5 mm for control POM and 6.4 mm for abaca fibre composites (did not show in figure). The slope of the energy absorption curve for cellulose fibre composites was found to be 150% higher than the
The variation of the storage modulus as a function of temperature is graphically enumerated in Fig. 7. It is evident that there was a notable increase in the modulus of the virgin matrix with the incorporation of abaca and cellulose fibres. This is probably due to an increase in the stiffness of the matrix with the reinforcing effect imparted by the fibres that allowed a greater (in compare with the matrix material) degree of stress transfer at the interface. In this cases, fibres act as a stress transfer bridge with a define strain and fibres are stiffer than the matrix material. There were no changes observed for the storage modulus between abaca fibre and cellulose fibre composites. These results attributed by the compromising of fibres stiffness and the adhesion between fibre and matrix. As previously explained, the abaca fibres are stiffer than the cellulose fibres. Furthermore, the adhesion between the cellulose and POM is relatively better than the adhesion between the abaca and POM. The area under the tan @ curve gives an indication of the total amount of energy that can be absorbed by the material during an experiment. A large area under the tan @ curve indicates a great degree of molecular mobility, which translates into better damping properties, meaning that the material can absorb and dissipate en-
Fibres break
Fibres pull-out
(a)
(b)
Fig. 6. SEM of (a) abaca fibre composites and (b) cellulose fibre composites.
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were found to improve enormously addition of cheap natural/ man-made cellulose fibres without increasing density of materials.
0.08
References
Control Abaca Cellulose
3000
2000
0.06
1000
0.04
0.02 150
0 -50
0
50
100
Temperature (°C)
Tan Delta
Storage Modulus (MPa)
4000
Universal V4.1D TA Instruments
Heat deflection temperature [°C]
Fig. 7. DMA of abaca and cellulose fibre composites.
180 150 120 90 60 30 0 Control
Abaca
Cellulose
Fig. 8. HDT of abaca and cellulose fibre composites.
ergy better. The area under the tan @ curve is found to be dependent on the nature of fibre. The area under the tan @ curve of cellulose fibre was larger than that of abaca fibre composites. It is because of both fibres exhibit different aspect ratio and structure [20]. 3.4. Heat deflection temperature Fig. 8 shows the HDT values of abaca and cellulose fibre composites. The HDT values of abaca and cellulose fibre composites found to be 156 °C and 169 °C which are 50 °C and 63 °C higher than for control POM respectively. The cellulose fibres composites showed about 13 °C higher HDT than that of abaca fibres composites. The fibre size distribution, aspect ratio and fibre volume fraction could be a reason. Moreover, The HDT value of a material depends on the compactness (stiffness) and the glass transition temperature. The changes of hardness with temperature play a critical role. In cease of cellulose fibre composites, the change of hardness value is slower than that of abaca fibre composites. It is because of relatively better interface between cellulose fibres and POM. 4. Conclusion This study inspected the effect of cellulose and abaca fibre on properties of POM composites. The incorporation of both fibres showed considerable improvement in the properties of POM such as modulus, flexural strength, Charpy impact strength, energy absorption, storage modulus and heat deflection temperature (HDT). But in contrast, the tensile strength of abaca fibre composites was found to reduce. However, it can be concluded that the mechanical and the thermo-mechanical properties of composites
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