- Email: [email protected]
Manufacturing sisal–polypropylene composites with minimum fibre degradation
Composites Science and Technology 63 (2003) 367–374 www.elsevier.com/locate/compscitech
Manufacturing sisal–polypropylene composites with minimum fibre degradation Krishnan Jayaraman* Department of Mechanical Engineering, University of Auckland, Private Bag 92019, Auckland 1001, New Zealand Received 25 June 2001; accepted 8 April 2002
Abstract Natural fibres, such as sisal, flax and jute, possess good reinforcing capability when properly compounded with polymers. These fibres are relatively inexpensive, originate from renewable resources and possess favourable values of specific strength and specific modulus. Thermoplastic polymers have a shorter cycle time as well as reprocessability despite problems with high viscosities and poor fibre wetting. The renewability of natural fibres and the recyclability of thermoplastic polymers provide an attractive ecofriendly quality to the resulting natural fibre-reinforced thermoplastic composite materials. Common methods for manufacturing natural fibre-reinforced thermoplastic composites, injection moulding and extrusion, tend to degrade the fibres during processing. Development of a simple manufacturing technique for sisal fibre-reinforced polypropylene composites, that minimises fibre degradation and can be used in developing countries, is the main objective of this study. Composite sheets with a fibre length greater than 10 mm and a fibre mass fraction in the range 15% to 35% exhibited good mechanical properties. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Recycling; B. Fibre/matrix bond; B. Mechanical properties; B. Strength; Natural fibre-reinforced composites; Sisal–polypropylene sheets
1. Introduction The growth in environmental awareness has led to the use of natural fibre-reinforced composite materials in automobile interior panels [1–3] and as building materials [4]. Panels made of natural fibre-reinforced composite materials are reasonably strong and light. Moreover, natural fibre-reinforced composites come from renewable materials and can be mechanically recycled. Common methods for manufacturing natural fibrereinforced thermoplastic composites are injection moulding and extrusion [5]. These techniques require expensive machinery and tend to degrade the fibres during processing. Hence, the development of a simple manufacturing technique for natural fibre-reinforced thermoplastic composites, that minimises fibre degradation and can be used in developing countries, is the
* Corresponding author. Tel.: +64-9-373-7599; fax: +64-9-3737479. E-mail address: [email protected] (K. Jayaraman).
main aim of this study. This article examines the viability of a simple manufacturing technique for producing sisal fibre-reinforced polypropylene composites and explores the possibility of recycling these composites. 1.1. Fibres Fibres provide the strength and stiffness and act as reinforcement in a fibre-reinforced composite material. Synthetic fibres, such as glass, carbon and aramid, reinforce the majority of polymer composite materials currently produced. However, environmental concerns associated with synthetic fibres and uncertainties in oil prices have led to intensive research on the suitability of natural fibres as reinforcement. Natural fibres are composites of hollow cellulose fibrils held together by a lignin and hemicellulose matrix. Each fibril has a complex, layered structure, consisting of a thin primary wall encircling a thick secondary wall. The secondary wall is made up of three layers and the thick middle layer determines the mechanical properties of the fibre. The middle layer consists of a series of helically wound cellular micro-
0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(02)00217-8
368
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
fibrils formed from long chain cellulose molecules; the angle between the fibre axis and the microfibrils is called the microfibril angle. The mechanical properties of these fibres are dependent on the cellulose content in the fibre, the degree of polymerisation of the cellulose and the microfibril angle [6,7]. Fibres with higher cellulose content, higher degree of polymerisation and a lower microfibrillar angle exhibit higher tensile strength and modulus. These fibres exhibit variations in mechanical properties both along the length of an individual fibre and between fibres. All natural fibres are strongly hydrophilic due to the presence of hydroxyl groups in the cellulose molecules. Sisal fibres were selected for this study because of their relatively high specific strength and modulus [7].
1.4. Scope of this study The objectives of this study were: development of a novel method to from sisal fibre mats, establishment of a simple technique to produce sisal fibre-reinforced polypropylene sheets, estimation of the strengths of the composite sheets, assessment of the effect of a fibre surface treatment on the tensile strength of the composite sheets and evaluation of the potential for recycling of the composite sheets.
1.2. Matrix The matrix in a fibre-reinforced composite material binds the fibres together, transfers applied load to these fibres and protects them from harmful environmental effects. When a composite material is formed into a shape or a profile, the matrix protects the fibres from damage. The properties of available polymers, thermosets and thermoplastics, make them ideal matrices. The requirements for the matrix in a sisal fibre-reinforced thermoplastic composite are: Melting point should be lower than the degradation temperature of the fibres. Fibre wettability must be good. Polypropylene was chosen as the matrix material in this study because of its good property range, low processing temperature and fairly good fibre wettability. 1.3. Fibre–matrix interface The fibres and matrix must cooperate for a composite to be an effective load bearing system. This cooperation between the fibres and the matrix will not exist without the presence of the interface. The interface acts as a ‘binder’ and transfers load between the matrix and the reinforcing fibres. Further, because each fibre forms an individual interface with the matrix, the interfacial area is very large. The interface, therefore, plays a key role in controlling the mechanical properties of a composite. Interfacial bonding is a result of good wetting of the fibres by the matrix as well as the formation of a chemical bond between the fibre surface and the matrix. A thin layer of waxes and fatty acids present on the surfaces of sisal fibres may adversely affect interfacial bonding. The effect of fibre surface treatment with an alkaline solution [8] on the mechanical properties of sisal fibre-reinforced polypropylene composite sheets was also examined in this study.
2. Production of composite sheets 2.1. Fibre preparation Sisal fibre bundles and sisal twine obtained from local sources were used in this study. The bundles were used in the production of composite sheets of fibre lengths greater than 9 mm (10, 20 and 30 mm) and to study the effect of fibre surface treatment in composite sheets of random fibre lengths. The twine was used in the production of composite sheets of fibre lengths less than 9 mm (1, 3, 5 and 8 mm). The sisal fibre bundles were cut to the required lengths using a paper guillotine and this method worked well for random lengths and lengths greater than 9 mm. But, this cutting method was inappropriate for fibre lengths less than 9 mm because, the lengths of the cut fibres below 9 mm showed wide variations. Hence, for fibre lengths less than 9 mm, the sisal twine was cut in a pelletiser, situated at the end of an extruder, to the required lengths with the help of a guide block. The fibres were then laid out in an oven tray, dried in the oven at 120 C and stored in airtight plastic bags to prevent them from re-absorbing any moisture. The fibre length distributions were checked with the help of a image processing and analysis program called Table 1 Statistical data on fibre lengths Target fibre length (mm)
Mean of fibre lengths (mm)
Standard deviation of fibre lengths (mm)
1 3 5 8 10 20 30
1.17 2.99 4.94 7.44 10.93 21.02 30.36
0.27 0.62 0.81 0.99 0.97 1.43 2.14
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
UTHSCSA ImageTool [9]. The fibres from each chopped fibre sample were separately spread thinly on a black cloth and a digital photo was taken of the samples with a ruler in the frame for calibration. The images were then loaded into the software package and analysed. The length of 50 fibres from each sample were measured and recorded. The distribution of the lengths of the fibres, for a target fibre length of 1 mm, shown in Fig. 1 were not quite as uniform as expected. The averages of the fibre lengths produced were relatively close to the target lengths as shown in Table 1 but the distributions were not standard normal distributions in all cases.
369
2.2. Polymer preparation Polypropylene powder (Cotene 9800 rotational moulding grade supplied by JR Courtenay Ltd., Auckland) and polypropylene sheets (in the form of a roll of 1.8 mm thickness supplied by Plastech International Ltd., Auckland) were used in this study. While the polypropylene powder did not require any preparation, the polypropylene sheets had to be flattened before use. Square panels (140 mm140 mm) of the same size as the final prepregs were cut from the polypropylene roll using a metal working guillotine. The curved panels
Fig. 1. (a) A sample of 1 mm fibres chopped by the pelletiser; (b) distribution of fibre lengths for the 1 mm sample.
370
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
were heated with a heat gun until they became pliable and then placed between two flat aluminium plates to cool. The panels were flat when removed from the aluminium plates. 2.3. Fibre mat formation using a drop feed tower A thick cardboard sheet was cut out, folded into shape and hot-glued together to form a drop-feed tower of 1 m height and a square cross-section (140 mm140 mm). A small cardboard hopper was constructed and glued onto the top to guide stray fibres into the dropfeed-tower. Fibres were sprinkled down the drop-feedtower onto a substrate to form loose fibre mats. The
fibre mats would disintegrate if disturbed, loosing their structure and uniformity. The fibre orientation distributions were checked with the help of the program UTHSCSA ImageTool [9]. A digital photo was taken of the mat and then the angles of the fibres relative to a datum were measured and recorded. The frequency chart of the fibre angles shown in Fig. 2 displays a relatively uniform distribution indicating that the fibre mat possesses random orientation. 2.4. Polymer incorporation Polymer incorporation was achieved by interleaving sisal fibre mats with polypropylene powder or sheet to
Fig. 2. (a) A thin sample layer of fibres dropped down the drop feed tower; (b) distribution of fibre orientations for the sample layer.
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
produce a three layer sandwich (polypropylene powder/ sisal mat/polypropylene powder) or a five layer sandwich (polypropylene sheet/sisal mat/polypropylene sheet/sisal mat/polypropylene sheet). The sandwiches were prepared for each target fibre content by adding the required mass of polypropylene to a known mass of the fibre mat. 2.5. Consolidation 2.5.1. Vacuum forming The polypropylene/sisal mat sandwiches were consolidated into composite sheets in a locally designed and built vacuum former shown in Fig. 3. The vacuum former consisted of a teflon-coated consolidation chamber of 1300 mm length, 180 mm width and 8 mm depth. The consolidation chamber was heated by an electrical heater element and cooled by compressed air blown through an aluminium pipe with tiny air holes. The sandwiches were placed in the consolidation chamber along with aluminium stoppers of various thicknesses to control the thickness of the composite sheets. A thermocouple wire was inserted into one of the sandwiches, in order to monitor the temperature. The sandwiches were then covered with a diaphragm and the top platen was clamped into place. The sandwiches were heated to a temperature of 180 C and held at this temperature for 45 min. A vacuum pressure of 80 kPa was applied to remove water vapour and volatiles resulting from the heating process and force the molten polypropylene into the fibre mats. The heater was switched off at the end of the consolidation time and cooling air from a compressor was blown into the chamber for 90 min. The clamps were then loosened, the top platen and diaphragm were removed and the composite sheets were taken out. 2.5.2. Hot pressing The polypropylene/sisal mat sandwiches were consolidated in a platen press at a temperature of 180 C and a pressure of about 0.7 MPa. The temperature and pressure were held steady for five minutes and the pre-
371
preg was removed and cooled between two thick aluminium plates.
3. Strength of composite sheets The tensile, three-point flexural and Charpy impact tests were performed according to the standard BS 2782: Part 10 which describes test methods for reinforced plastics. The tensile and flexural tests were carried out in an Instron universal testing machine (Model 5567) and the impact tests were carried out in a Ceast impact tester (Resil 25). The specimens were cut from the composite sheets of thicknesses ranging from 1.5 to 3.7 mm using a paper guillotine. The cut edges were made smooth with the help of sandpapers to have a close control on the specimen dimensions. All the specimens were conditioned at room temperature for 24 h prior to tests. Overall, the type of consolidation (vacuum forming or hot pressing) and the type of sandwich (three layers with polypropylene powder or five layers with polypropylene sheets) resulted only in small variations in the tensile strengths of the composites. Quantitative results have not been provided since the differences in tensile strengths in the above cases were not statistically significant. In general, sisal fibres were well encapsulated by the polypropylene matrix within the composite sheets. 3.1. Effect of fibre mass fraction The effect of fibre mass fraction on the mechanical properties of the composite sheet was studied by consolidating and testing sheets of various fibre mass fractions (9, 16.5, 28.5 and 37.5%) and fibre lengths greater than 9 mm (10, 20 and 30 mm). Tensile and flexural strengths showed an initial decrease followed by a modest increase with increasing fibre content as shown in Fig. 4. The fibres may act as flaws or fillers at a fibre mass fraction of 9% leading to a reduction in the strengths of the composite. Manikandan Nair et al. [10]
Fig. 3. Schematic of the vacuum former.
372
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
have reported a similar drop in the tensile strength of randomly oriented sisal-polystyrene composites at a fibre loading of 10% followed by an increase. Impact strength mostly showed a steady increase with increasing fibre content. Composite sheets of fibre mass fraction ranging from 15 to 35% exhibit tensile and flexural strengths greater than the strengths of polypropylene. 3.2. Effect of fibre length The fibre mass fraction of 25% was selected to study the effect of fibre length on the mechanical properties of the composite sheet based on the results shown in Fig. 4. Composite sheets of fibre lengths less than 9 mm (1, 3, 5 and 8 mm) and a fibre mass fraction of 25% were consolidated and tested. A range of fibre lengths between 8 mm and 30 mm improved the tensile strength of the composite sheet above that of polypropylene as shown in Fig. 5. Composite sheets of fibre lengths greater than 10 mm exhibited flexural strengths greater than that of polypropylene as shown in Fig. 5. Impact strength mostly showed a steady increase with increasing fibre length.
4. Interfacial bonding Alkali treatment is known to remove the waxy layer of fatty acids from the surface of sisal fibres and produce a rough surface topography [5]. Hence, such a surface treatment was tried for the improvement of fibre–matrix adhesion in sisal–polypropylene sheets. A bundle of the long fibres was soaked in 0.5 N (molar) Sodium Hydroxide (NaOH) solution for approximately 2 1/2 days as mentioned by Bisanda and Ansell [8]. Half of the fibres were rinsed with water at the end of this period and the other half were not. They were left to air dry on a plastic tray and were turned over every day for 5 days. The fibres were then chopped up into random lengths using the paper guillotine, laid out in an oven tray, dried in the oven at 120 C and stored in airtight plastic bags to prevent them from re-absorbing any moisture. Composite sheets of treated fibres having a fibre mass fraction of 25% were consolidated and tested. The results of tensile tests showed (Fig. 6) that fibre surface treatment did not improve the tensile strength of the composite. Prepregs made from the treated fibres were
Fig. 4. Tensile strength, flexural strength and impact strength of sisal fibre-reinforced polypropylene composite sheets as a function of fibre length (above 9 mm) and fibre mass fraction. The dashed lines represent the strengths of polypropylene.
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
373
Fig. 5. Tensile strength, flexural strength and impact strength of sisal fibre-reinforced polypropylene composite sheets of 25% fibre mass fraction as a function of fibre length. The dashed lines represent the strengths of polypropylene.
Fig. 6. Tensile strength of sisal fibre-reinforced polypropylene composite sheets as a function of fibre surface treatment and recycling.
quite porous and this could have affected the results. Fibre surface topography and fibre surface chemistry have to be investigated before any firm conclusion on Sodium Hydroxide treatment can be reached.
5. Recycling A granulator cuts materials composed of polymers into small pieces that could be fed directly into the screw
374
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
of a injection moulder. Most of the tested specimens from the various prepregs, except the ones containing the treated fibres, were chopped into smaller pieces in a granulator. The composite sheets granulated easily into pieces that were approximately 2–3 mm in size and regular in shape. The granulated material was moulded into tensile specimens in a BOY injection moulder (capacity 50 tonnes) with the barrel temperature set at 185 C. Fibres in the prepregs would have been cut during granulation and moulding making it difficult to estimate the fibre lengths in the recycled specimens. Further, the fibres appeared to be somewhat browned after passing through the barrel of the injection moulder. The fibres tended to be oriented in the direction of the flow of the polymer and hence were aligned in the longitudinal direction of the specimens. This would be expected to increase the tensile strength of the specimens. The tensile strength of the recycled material improved in comparison to that of the initial material (Fig. 6). Flexural and impact strengths of the recycled material have to be studied before any firm conclusion on recyclability can be reached.
6. Conclusions Sisal fibre mats were successfully formed by sprinkling the fibres down a drop feed tower . This innovative fibre mat production method allowed the fibres to avoid much of the processing degradation that is often encountered during conventional shear mixing processes. Composite sheets were manufactured from these
sisal fibre mats interleaved with polypropylene by vacuum forming or hot-pressing. The best possible mechanical properties for the sisal fibre-reinforced polypropylene composites were achieved when the fibre length was greater than 10 mm and the fibre mass fraction was in the range 15–35%.
References [1] Anon. Natural fiber-reinforced composites are light and strong. Modern Plastics International 1994; May: 69. [2] Anon. Natural born fillers. In: Breuer R, editor. Daimler-Benz high tech report 2. Stuttgart: Daimler-Benz AG; 1995. [3] Mapleston P. Natural-fibre composites rev-up role in interior panels. Modern Plastics International 1997: May; 39–40. [4] Swift DG. Sisal-cement composites and their potential for rural Africa. In: Marshall IH, editor. Composite structures 3. New York: Elsevier; 1992. [5] Li Y, Mai Y-W, Ye L. Sisal fibre and its composites: a review of recent developments. Composites Science and Technology 2000; 60:2037–55. [6] Prasad SV. Natural-fiber-based composites. In: Kelly A, editor. Concise encyclopaedia of composite materials. New York: Pergamon; 1989. [7] Ruys D, Crosky A, Evans WJ. Ecologically sustainable composite materials for low energy city vehicles. In: Waste and Byproducts as Secondary Resources for Building Materials Conference Proceedings, vol. 1, New Delhi, India, 1999. p. 328–34. [8] Bisanda ETN, Ansell MP. Properties of sisal-CNSL composites. Journal of Materials Science 1992;27:1690–700. [9] UTHSCSA ImageTool version 3.0. Available from: http:// ddsdx.uthscsa.edu/dig/itdesc.html [last revised 22 February 2002]. [10] Manikandan Nair KC, Diwan SC, Thomas S. Tensile properties of short sisal fiber reinforced polystyrene composites. Journal of Applied Polymer Science 1996;60:1483–97.
Manufacturing sisal–polypropylene composites with minimum fibre degradation Krishnan Jayaraman* Department of Mechanical Engineering, University of Auckland, Private Bag 92019, Auckland 1001, New Zealand Received 25 June 2001; accepted 8 April 2002
Abstract Natural fibres, such as sisal, flax and jute, possess good reinforcing capability when properly compounded with polymers. These fibres are relatively inexpensive, originate from renewable resources and possess favourable values of specific strength and specific modulus. Thermoplastic polymers have a shorter cycle time as well as reprocessability despite problems with high viscosities and poor fibre wetting. The renewability of natural fibres and the recyclability of thermoplastic polymers provide an attractive ecofriendly quality to the resulting natural fibre-reinforced thermoplastic composite materials. Common methods for manufacturing natural fibre-reinforced thermoplastic composites, injection moulding and extrusion, tend to degrade the fibres during processing. Development of a simple manufacturing technique for sisal fibre-reinforced polypropylene composites, that minimises fibre degradation and can be used in developing countries, is the main objective of this study. Composite sheets with a fibre length greater than 10 mm and a fibre mass fraction in the range 15% to 35% exhibited good mechanical properties. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Recycling; B. Fibre/matrix bond; B. Mechanical properties; B. Strength; Natural fibre-reinforced composites; Sisal–polypropylene sheets
1. Introduction The growth in environmental awareness has led to the use of natural fibre-reinforced composite materials in automobile interior panels [1–3] and as building materials [4]. Panels made of natural fibre-reinforced composite materials are reasonably strong and light. Moreover, natural fibre-reinforced composites come from renewable materials and can be mechanically recycled. Common methods for manufacturing natural fibrereinforced thermoplastic composites are injection moulding and extrusion [5]. These techniques require expensive machinery and tend to degrade the fibres during processing. Hence, the development of a simple manufacturing technique for natural fibre-reinforced thermoplastic composites, that minimises fibre degradation and can be used in developing countries, is the
* Corresponding author. Tel.: +64-9-373-7599; fax: +64-9-3737479. E-mail address: [email protected] (K. Jayaraman).
main aim of this study. This article examines the viability of a simple manufacturing technique for producing sisal fibre-reinforced polypropylene composites and explores the possibility of recycling these composites. 1.1. Fibres Fibres provide the strength and stiffness and act as reinforcement in a fibre-reinforced composite material. Synthetic fibres, such as glass, carbon and aramid, reinforce the majority of polymer composite materials currently produced. However, environmental concerns associated with synthetic fibres and uncertainties in oil prices have led to intensive research on the suitability of natural fibres as reinforcement. Natural fibres are composites of hollow cellulose fibrils held together by a lignin and hemicellulose matrix. Each fibril has a complex, layered structure, consisting of a thin primary wall encircling a thick secondary wall. The secondary wall is made up of three layers and the thick middle layer determines the mechanical properties of the fibre. The middle layer consists of a series of helically wound cellular micro-
0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(02)00217-8
368
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
fibrils formed from long chain cellulose molecules; the angle between the fibre axis and the microfibrils is called the microfibril angle. The mechanical properties of these fibres are dependent on the cellulose content in the fibre, the degree of polymerisation of the cellulose and the microfibril angle [6,7]. Fibres with higher cellulose content, higher degree of polymerisation and a lower microfibrillar angle exhibit higher tensile strength and modulus. These fibres exhibit variations in mechanical properties both along the length of an individual fibre and between fibres. All natural fibres are strongly hydrophilic due to the presence of hydroxyl groups in the cellulose molecules. Sisal fibres were selected for this study because of their relatively high specific strength and modulus [7].
1.4. Scope of this study The objectives of this study were: development of a novel method to from sisal fibre mats, establishment of a simple technique to produce sisal fibre-reinforced polypropylene sheets, estimation of the strengths of the composite sheets, assessment of the effect of a fibre surface treatment on the tensile strength of the composite sheets and evaluation of the potential for recycling of the composite sheets.
1.2. Matrix The matrix in a fibre-reinforced composite material binds the fibres together, transfers applied load to these fibres and protects them from harmful environmental effects. When a composite material is formed into a shape or a profile, the matrix protects the fibres from damage. The properties of available polymers, thermosets and thermoplastics, make them ideal matrices. The requirements for the matrix in a sisal fibre-reinforced thermoplastic composite are: Melting point should be lower than the degradation temperature of the fibres. Fibre wettability must be good. Polypropylene was chosen as the matrix material in this study because of its good property range, low processing temperature and fairly good fibre wettability. 1.3. Fibre–matrix interface The fibres and matrix must cooperate for a composite to be an effective load bearing system. This cooperation between the fibres and the matrix will not exist without the presence of the interface. The interface acts as a ‘binder’ and transfers load between the matrix and the reinforcing fibres. Further, because each fibre forms an individual interface with the matrix, the interfacial area is very large. The interface, therefore, plays a key role in controlling the mechanical properties of a composite. Interfacial bonding is a result of good wetting of the fibres by the matrix as well as the formation of a chemical bond between the fibre surface and the matrix. A thin layer of waxes and fatty acids present on the surfaces of sisal fibres may adversely affect interfacial bonding. The effect of fibre surface treatment with an alkaline solution [8] on the mechanical properties of sisal fibre-reinforced polypropylene composite sheets was also examined in this study.
2. Production of composite sheets 2.1. Fibre preparation Sisal fibre bundles and sisal twine obtained from local sources were used in this study. The bundles were used in the production of composite sheets of fibre lengths greater than 9 mm (10, 20 and 30 mm) and to study the effect of fibre surface treatment in composite sheets of random fibre lengths. The twine was used in the production of composite sheets of fibre lengths less than 9 mm (1, 3, 5 and 8 mm). The sisal fibre bundles were cut to the required lengths using a paper guillotine and this method worked well for random lengths and lengths greater than 9 mm. But, this cutting method was inappropriate for fibre lengths less than 9 mm because, the lengths of the cut fibres below 9 mm showed wide variations. Hence, for fibre lengths less than 9 mm, the sisal twine was cut in a pelletiser, situated at the end of an extruder, to the required lengths with the help of a guide block. The fibres were then laid out in an oven tray, dried in the oven at 120 C and stored in airtight plastic bags to prevent them from re-absorbing any moisture. The fibre length distributions were checked with the help of a image processing and analysis program called Table 1 Statistical data on fibre lengths Target fibre length (mm)
Mean of fibre lengths (mm)
Standard deviation of fibre lengths (mm)
1 3 5 8 10 20 30
1.17 2.99 4.94 7.44 10.93 21.02 30.36
0.27 0.62 0.81 0.99 0.97 1.43 2.14
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
UTHSCSA ImageTool [9]. The fibres from each chopped fibre sample were separately spread thinly on a black cloth and a digital photo was taken of the samples with a ruler in the frame for calibration. The images were then loaded into the software package and analysed. The length of 50 fibres from each sample were measured and recorded. The distribution of the lengths of the fibres, for a target fibre length of 1 mm, shown in Fig. 1 were not quite as uniform as expected. The averages of the fibre lengths produced were relatively close to the target lengths as shown in Table 1 but the distributions were not standard normal distributions in all cases.
369
2.2. Polymer preparation Polypropylene powder (Cotene 9800 rotational moulding grade supplied by JR Courtenay Ltd., Auckland) and polypropylene sheets (in the form of a roll of 1.8 mm thickness supplied by Plastech International Ltd., Auckland) were used in this study. While the polypropylene powder did not require any preparation, the polypropylene sheets had to be flattened before use. Square panels (140 mm140 mm) of the same size as the final prepregs were cut from the polypropylene roll using a metal working guillotine. The curved panels
Fig. 1. (a) A sample of 1 mm fibres chopped by the pelletiser; (b) distribution of fibre lengths for the 1 mm sample.
370
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
were heated with a heat gun until they became pliable and then placed between two flat aluminium plates to cool. The panels were flat when removed from the aluminium plates. 2.3. Fibre mat formation using a drop feed tower A thick cardboard sheet was cut out, folded into shape and hot-glued together to form a drop-feed tower of 1 m height and a square cross-section (140 mm140 mm). A small cardboard hopper was constructed and glued onto the top to guide stray fibres into the dropfeed-tower. Fibres were sprinkled down the drop-feedtower onto a substrate to form loose fibre mats. The
fibre mats would disintegrate if disturbed, loosing their structure and uniformity. The fibre orientation distributions were checked with the help of the program UTHSCSA ImageTool [9]. A digital photo was taken of the mat and then the angles of the fibres relative to a datum were measured and recorded. The frequency chart of the fibre angles shown in Fig. 2 displays a relatively uniform distribution indicating that the fibre mat possesses random orientation. 2.4. Polymer incorporation Polymer incorporation was achieved by interleaving sisal fibre mats with polypropylene powder or sheet to
Fig. 2. (a) A thin sample layer of fibres dropped down the drop feed tower; (b) distribution of fibre orientations for the sample layer.
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
produce a three layer sandwich (polypropylene powder/ sisal mat/polypropylene powder) or a five layer sandwich (polypropylene sheet/sisal mat/polypropylene sheet/sisal mat/polypropylene sheet). The sandwiches were prepared for each target fibre content by adding the required mass of polypropylene to a known mass of the fibre mat. 2.5. Consolidation 2.5.1. Vacuum forming The polypropylene/sisal mat sandwiches were consolidated into composite sheets in a locally designed and built vacuum former shown in Fig. 3. The vacuum former consisted of a teflon-coated consolidation chamber of 1300 mm length, 180 mm width and 8 mm depth. The consolidation chamber was heated by an electrical heater element and cooled by compressed air blown through an aluminium pipe with tiny air holes. The sandwiches were placed in the consolidation chamber along with aluminium stoppers of various thicknesses to control the thickness of the composite sheets. A thermocouple wire was inserted into one of the sandwiches, in order to monitor the temperature. The sandwiches were then covered with a diaphragm and the top platen was clamped into place. The sandwiches were heated to a temperature of 180 C and held at this temperature for 45 min. A vacuum pressure of 80 kPa was applied to remove water vapour and volatiles resulting from the heating process and force the molten polypropylene into the fibre mats. The heater was switched off at the end of the consolidation time and cooling air from a compressor was blown into the chamber for 90 min. The clamps were then loosened, the top platen and diaphragm were removed and the composite sheets were taken out. 2.5.2. Hot pressing The polypropylene/sisal mat sandwiches were consolidated in a platen press at a temperature of 180 C and a pressure of about 0.7 MPa. The temperature and pressure were held steady for five minutes and the pre-
371
preg was removed and cooled between two thick aluminium plates.
3. Strength of composite sheets The tensile, three-point flexural and Charpy impact tests were performed according to the standard BS 2782: Part 10 which describes test methods for reinforced plastics. The tensile and flexural tests were carried out in an Instron universal testing machine (Model 5567) and the impact tests were carried out in a Ceast impact tester (Resil 25). The specimens were cut from the composite sheets of thicknesses ranging from 1.5 to 3.7 mm using a paper guillotine. The cut edges were made smooth with the help of sandpapers to have a close control on the specimen dimensions. All the specimens were conditioned at room temperature for 24 h prior to tests. Overall, the type of consolidation (vacuum forming or hot pressing) and the type of sandwich (three layers with polypropylene powder or five layers with polypropylene sheets) resulted only in small variations in the tensile strengths of the composites. Quantitative results have not been provided since the differences in tensile strengths in the above cases were not statistically significant. In general, sisal fibres were well encapsulated by the polypropylene matrix within the composite sheets. 3.1. Effect of fibre mass fraction The effect of fibre mass fraction on the mechanical properties of the composite sheet was studied by consolidating and testing sheets of various fibre mass fractions (9, 16.5, 28.5 and 37.5%) and fibre lengths greater than 9 mm (10, 20 and 30 mm). Tensile and flexural strengths showed an initial decrease followed by a modest increase with increasing fibre content as shown in Fig. 4. The fibres may act as flaws or fillers at a fibre mass fraction of 9% leading to a reduction in the strengths of the composite. Manikandan Nair et al. [10]
Fig. 3. Schematic of the vacuum former.
372
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
have reported a similar drop in the tensile strength of randomly oriented sisal-polystyrene composites at a fibre loading of 10% followed by an increase. Impact strength mostly showed a steady increase with increasing fibre content. Composite sheets of fibre mass fraction ranging from 15 to 35% exhibit tensile and flexural strengths greater than the strengths of polypropylene. 3.2. Effect of fibre length The fibre mass fraction of 25% was selected to study the effect of fibre length on the mechanical properties of the composite sheet based on the results shown in Fig. 4. Composite sheets of fibre lengths less than 9 mm (1, 3, 5 and 8 mm) and a fibre mass fraction of 25% were consolidated and tested. A range of fibre lengths between 8 mm and 30 mm improved the tensile strength of the composite sheet above that of polypropylene as shown in Fig. 5. Composite sheets of fibre lengths greater than 10 mm exhibited flexural strengths greater than that of polypropylene as shown in Fig. 5. Impact strength mostly showed a steady increase with increasing fibre length.
4. Interfacial bonding Alkali treatment is known to remove the waxy layer of fatty acids from the surface of sisal fibres and produce a rough surface topography [5]. Hence, such a surface treatment was tried for the improvement of fibre–matrix adhesion in sisal–polypropylene sheets. A bundle of the long fibres was soaked in 0.5 N (molar) Sodium Hydroxide (NaOH) solution for approximately 2 1/2 days as mentioned by Bisanda and Ansell [8]. Half of the fibres were rinsed with water at the end of this period and the other half were not. They were left to air dry on a plastic tray and were turned over every day for 5 days. The fibres were then chopped up into random lengths using the paper guillotine, laid out in an oven tray, dried in the oven at 120 C and stored in airtight plastic bags to prevent them from re-absorbing any moisture. Composite sheets of treated fibres having a fibre mass fraction of 25% were consolidated and tested. The results of tensile tests showed (Fig. 6) that fibre surface treatment did not improve the tensile strength of the composite. Prepregs made from the treated fibres were
Fig. 4. Tensile strength, flexural strength and impact strength of sisal fibre-reinforced polypropylene composite sheets as a function of fibre length (above 9 mm) and fibre mass fraction. The dashed lines represent the strengths of polypropylene.
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
373
Fig. 5. Tensile strength, flexural strength and impact strength of sisal fibre-reinforced polypropylene composite sheets of 25% fibre mass fraction as a function of fibre length. The dashed lines represent the strengths of polypropylene.
Fig. 6. Tensile strength of sisal fibre-reinforced polypropylene composite sheets as a function of fibre surface treatment and recycling.
quite porous and this could have affected the results. Fibre surface topography and fibre surface chemistry have to be investigated before any firm conclusion on Sodium Hydroxide treatment can be reached.
5. Recycling A granulator cuts materials composed of polymers into small pieces that could be fed directly into the screw
374
K. Jayaraman / Composites Science and Technology 63 (2003) 367–374
of a injection moulder. Most of the tested specimens from the various prepregs, except the ones containing the treated fibres, were chopped into smaller pieces in a granulator. The composite sheets granulated easily into pieces that were approximately 2–3 mm in size and regular in shape. The granulated material was moulded into tensile specimens in a BOY injection moulder (capacity 50 tonnes) with the barrel temperature set at 185 C. Fibres in the prepregs would have been cut during granulation and moulding making it difficult to estimate the fibre lengths in the recycled specimens. Further, the fibres appeared to be somewhat browned after passing through the barrel of the injection moulder. The fibres tended to be oriented in the direction of the flow of the polymer and hence were aligned in the longitudinal direction of the specimens. This would be expected to increase the tensile strength of the specimens. The tensile strength of the recycled material improved in comparison to that of the initial material (Fig. 6). Flexural and impact strengths of the recycled material have to be studied before any firm conclusion on recyclability can be reached.
6. Conclusions Sisal fibre mats were successfully formed by sprinkling the fibres down a drop feed tower . This innovative fibre mat production method allowed the fibres to avoid much of the processing degradation that is often encountered during conventional shear mixing processes. Composite sheets were manufactured from these
sisal fibre mats interleaved with polypropylene by vacuum forming or hot-pressing. The best possible mechanical properties for the sisal fibre-reinforced polypropylene composites were achieved when the fibre length was greater than 10 mm and the fibre mass fraction was in the range 15–35%.
References [1] Anon. Natural fiber-reinforced composites are light and strong. Modern Plastics International 1994; May: 69. [2] Anon. Natural born fillers. In: Breuer R, editor. Daimler-Benz high tech report 2. Stuttgart: Daimler-Benz AG; 1995. [3] Mapleston P. Natural-fibre composites rev-up role in interior panels. Modern Plastics International 1997: May; 39–40. [4] Swift DG. Sisal-cement composites and their potential for rural Africa. In: Marshall IH, editor. Composite structures 3. New York: Elsevier; 1992. [5] Li Y, Mai Y-W, Ye L. Sisal fibre and its composites: a review of recent developments. Composites Science and Technology 2000; 60:2037–55. [6] Prasad SV. Natural-fiber-based composites. In: Kelly A, editor. Concise encyclopaedia of composite materials. New York: Pergamon; 1989. [7] Ruys D, Crosky A, Evans WJ. Ecologically sustainable composite materials for low energy city vehicles. In: Waste and Byproducts as Secondary Resources for Building Materials Conference Proceedings, vol. 1, New Delhi, India, 1999. p. 328–34. [8] Bisanda ETN, Ansell MP. Properties of sisal-CNSL composites. Journal of Materials Science 1992;27:1690–700. [9] UTHSCSA ImageTool version 3.0. Available from: http:// ddsdx.uthscsa.edu/dig/itdesc.html [last revised 22 February 2002]. [10] Manikandan Nair KC, Diwan SC, Thomas S. Tensile properties of short sisal fiber reinforced polystyrene composites. Journal of Applied Polymer Science 1996;60:1483–97.