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carbon nanofibre composite by laser sintering
Polymer Testing 30 (2011) 94–100
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Processing of a Polyamide-12/carbon nanofibre composite by laser sintering R.D. Goodridge a, * , M.L. Shofner b, R.J.M. Hague a, M. McClelland a, M.R. Schlea b, R.B. Johnson b, C.J. Tuck a a b
Additive Manufacturing Research Group, Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, LE11 3TU, UK School of Polymer, Textile & Fiber Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0295, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2010 Accepted 29 October 2010
Additive layer techniques, such as laser sintering, are increasingly being considered for the production of fully functioning end-use parts due to the significant advantages they hold in the design and implementation of products. However, one of the main obstacles to widespread adoption of this technology is the limited range of materials that can currently be processed using additive techniques. This paper presents initial research into the reinforcement of laser sintered polyamides with carbon nanofibres (CNFs). The effects of CNF addition on the processing parameters and mechanical properties of laser sintered parts have been investigated. A 3wt % carbon nanofibre-polyamide 12 composite (CNFPA12) powder was prepared using melt mixing and cryogenic milling. Following laser sintering, characterisation of the polymer nanocomposite parts by SEM and dynamic mechanical testing showed that the nanofibres were well dispersed within the polymer matrix and gave a 22% increase in the storage modulus compared to the base material. However, the cryogenic fracturing method used in this research did not produce powder with suitable morphology for laser sintering. If improved powder production can be achieved, the use of CNF reinforcements to improve mechanical properties in laser sintering holds promise. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Laser sintering Additive manufacturing Polymer nanocomposites
1. Introduction Additive layer techniques, such as laser sintering [1,2], have historically been used for “Rapid Prototyping”, exploiting their increased freedom of design in a range of areas such as concept modelling, pattern building, assembly verification and functional testing [3,4]. The fabricated parts have, for these applications, only been required to possess sufficient mechanical integrity and surface quality for handling and demonstration purposes. Increasingly however, additive techniques are being considered for the production of actual end-use parts, a concept that has recently been defined as Additive Manufacturing (AM) [5,6]. * Corresponding author. Tel.: þ44(0) 1509 227567. E-mail address: [email protected] (R.D. Goodridge). 0142-9418/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2010.10.011
The principal advantage of AM is the ability to manufacture parts with significantly greater complexity of geometry than traditional techniques entirely without the need for mould tooling. As mould tooling is not required for production, the design constraints normally associated with the subtractive and formative methods of manufacture are reduced, providing significant advantages in the design and implementation of products [7]. In addition, as tooling is no longer required the economics of production are changed dramatically, enabling low volume production and even completely customised products [6]. However, one of the most widely acknowledged stumbling blocks in the progression of this technology into manufacturing is the very limited range of materials, particularly polymeric materials, that can currently be processed using additive techniques [8]. Current additive
R.D. Goodridge et al. / Polymer Testing 30 (2011) 94–100
materials do not meet the requirements of the majority of commercial products, so there is a need to develop and to be able to process a much greater variety of materials than is currently possible. Although in theory any polymer available in powder form can be processed by laser sintering, the complex thermal chemistries and consolidation phenomena of polymers have to date limited the choice of laser sintering polymers to mainly polyamide based powders. Polyamide 11 and 12 (PA11 and PA12) are certainly the most widely used laser sintering materials at the present time. However, the move towards AM has necessitated increasing research into the processing of alternative materials with new and improved properties. This paper presents initial research into the reinforcement of laser sintered polyamides with carbon nanofibres in an attempt to improve their mechanical properties and enable their use in wider AM applications. 2. Literature review Previous attempts to improve the mechanical or physical properties of polymeric laser sintered parts have involved reinforcing them with micron-sized inorganic fillers, such as glass beads [4,9], silicon carbide [10,11], hydroxyapatite [12,13], aluminium powder [14,15], or nano-sized fillers such as clay [16–19], nanosilica [20] and nano-Al2O3 [21]. Commercial examples of polymer-filled materials include glass filled polyamide (e.g. DuraFormÒ GF, PA 3200 GF), and aluminium filled polyamide (e.g. Alumide) [22,23]. Polymer nanocomposites are of particular interest for LS as a relatively small volume of nanofiller can have a considerable effect on the properties of the composite, due to the high surface to volume ratio of the nanofiller, as well as in some cases their extremely high aspect ratio [24]. In addition to improving mechanical properties such as strength and stiffness, nanofillers can be used to enhance other properties such as optical properties, thermal conductivity, heat resistance and flame retardancy or to accelerate biodegradability or increase bioactivity [25,33]. Positive effects specific to the laser sintering process have also been observed. In particular, the addition of certain nanoparticles has been found to decrease the energy required from the laser to consolidate the powder due to more efficient absorption of laser power by the fillers. Wang et al. [17] reported this effect when looking at a PA12-rectorite composite; the more rectorite that was added, the less laser power was needed for optimum tensile strength. It is thought that this is due to the fillers absorbing the laser radiation more efficiently than the base polymer. Ho et al. [26] looked at a number of common plastic fillers, namely quartz, silica, graphite and talc, in polycarbonate on the basis that laser radiation absorbing additives could reduce the amount of laser energy required for sintering [27]. Of the fillers that Ho et al. [26] examined, graphite powder was found to have the largest effect on improving the absorptance of the laser sintering powder. It was proposed that the addition of graphite powder may go some way to minimising “bonus-z” (growth of the part in the z-axis) and other thermal-related problems experienced in laser sintered parts as, the greater the absorptance of a powder, the less
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laser energy is required for sintering and less energy is transmitted through the graphite powder. Analogous to powder production for conventional processing techniques, uniform particle dispersion and good interfacial adhesion between filler and polymer matrix are key to successful production of a nanocomposite material with improved properties. If this is not achieved, the resulting mechanical properties of the filled material can be lower than the original base polymer [18]. Therefore, preparation of the starting material is a prime factor. Many reinforced polymers for laser sintering (including the current commercially available materials) are prepared by simple mechanical mixing of the filler with the neat base polymer [10,14,17,20,28,29]. However, uniformly mixing two powders with different particle sizes (particularly when one powder is very small, e.g. nanofillers), or with significantly varied densities (e.g. nylon and metal powders), using this method is problematic. Kruth et al. [30] reported frequent coagulation and, thus, segregation during layering of both aluminium and copper filled polyamide 12 for this reason. Another method has been to coat the fillers with the base polymer. This is the technique used to produce metal parts by indirect laser sintering processes (i.e. using a sacrificial polymer binder that is burnt off in a post-processing step) due to the improved sintering compared to simple mechanical mixing [31]. Yan et al. [32] applied this method to polyamide 12-coated aluminium powders produced using dissolution-precipitation and ball-milling (cAlPA), and compared the results to those of a neat polyamide 12 powder prepared by an identical dissolution-precipitation process but without addition of the aluminium particles (NPA), and to a polyamide 12-aluminium composite prepared by simple mixing [mAlPA]. Despite both being prepared using the same method, the NPA had an irregular particle morphology whereas the cAlPA had a more spherical morphology and uniform size. It was proposed by Yan et al that this was due to the aluminium powder and the conditions of the preparation technique determining the characteristics of the cAlPA and the NPA, respectively. Increasing the mass fraction of Al filler improved the strength of the cAlPA samples whereas it decreased the strength of the mAlPA relative to the NPA samples. SEM showed aggregates of Al particles in the mAlPA samples which created mechanical defects, confirming previous findings reported above. Two explanations were offered for the increase in strength of the cAlPA samples. The first rationalised that by coating the aluminium particles in the preparation stage, the interfacial adhesion between the two materials occurs before the laser sintering process, requiring the laser energy to simply fuse the polyamide coatings together. In contrast, for mAlPA powders, bonding between the polyamide and aluminium particles, as well as between different polyamide particles, must occur during the extremely short scanning period of the laser sintering process, limiting the interfacial adhesion that can occur. The second explanation offered relates to the lower laser radiation absorption of metals as compared to polymers [27]. The aluminium particles in the mAlPA powder are exposed to the laser during sintering, whereas those in the cAlPA are covered by the polymer, resulting in much lower absorption
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R.D. Goodridge et al. / Polymer Testing 30 (2011) 94–100 Table 1 Laser Sintering parameters used to process each material.
Chamber Temperature ( C) Bed Temperature ( C) Laser Power (W)
Fig. 1. SEM images of CNF-PA12 prior to laser sintering at 20 k. The bright objects, some of which have been circled for identification, are the carbon nanofibres.
by mAlPA compared to cAlPA. Increasing the mass fraction of aluminium filler in the cAlPA samples was found to improve the dimensional accuracy of the laser sintered parts. In addition, the tensile strength, elongation at break, and impact strength of the samples were found to increase with decreasing average Al particle size. Other authors have adopted similar techniques. Zheng et al. [21] used nano-Al2O3 particles coated with polystyrene (PS) by emulsion polymerisation as fillers to reinforce PS based composites for laser sintering. No agglomeration occurred and the nanoparticles were found to have dispersed homogeneously. Again, this was attributed to the polymer forming a boundary layer between the individual nanoparticles, preventing agglomeration and, thereby, improving dispersion. Yan et al. [15] used a dissolution-precipitation process to produce a 3wt% nanosilica/polyamide 12 composite powder. The nanosilica was dispersed uniformly in the polymer matrix and, following laser sintering, a 20.9% increase in tensile strength, 39.4% increase in tensile modulus and 9.54% increase in impact strength was reported for the nano composite material compared to the neat PA12 that had also been prepared using an identical dissolution-precipitation technique. The nanofillers chosen for this study were carbon nanofibres (CNFs). CNFs consist of graphene planes oriented in a tree ring structure around a hollow core. Their diameters are of the order of 100 nm, and their properties are roughly one order of magnitude below those for carbon
CNF-PA12
N-PA12
AS-PA12
164 151 20
166 151 17
170 150 21
nanotubes. With the high cost of carbon nanotubes, CNFs offer a more practical solution allowing the enhancement of mechanical properties without significant increase in the cost of the material. A study by Koo et al. [33] demonstrated the ability of CNF’s to increase the flame retardant properties of a polyamide 11 (PA11) material commonly used in laser sintering, but for their study the parts were produced by injection moulding. The effect carbon nanofibres have on the mechanical properties of actual laser sintered parts has not yet been investigated and reported. This paper therefore examines the influence that the addition of carbon nanofibres has on the processing and mechanical properties of laser sintered PA12 samples. 3. Method 3.1. Preparation of the nanofibres The CNFs were obtained from Pyrograf Products, Inc. The product number for the CNFs used was PR-24-PS. Literature from the manufacturer indicated that the CNFs had diameters between 60 and 150 nm with lengths ranging from 30 to 100 mm. The CNFs were purified from the as-received condition to remove other carbon species and catalyst using a five day reflux in methylene chloride, followed by a washing step and another reflux in water for one day. This procedure has been shown to open fibre bundles while preserving fibre length [34].
3.2. Production of the polymer-nanocomposites A 3wt % carbon nanofibre-polyamide 12 composite (CNF-PA12) was prepared by melt mixing. DuraForm PA12Ò, supplied by 3D Systems Inc, was chosen as the base polyamide due to its proven history in laser sintering. The melt mixing equipment used was a Brabender Intelli-Torque polymer processing system with an internal batch mixer attachment. The mixer had a 60 mL capacity and was
Fig. 2. SEM micrographs of AS-PA12 (left), N-PA12 (centre), and CNF-PA12 (right) at 500.
R.D. Goodridge et al. / Polymer Testing 30 (2011) 94–100
used with roller blades. Prior to mixing, the polymer powder was dried in a vacuum oven to prevent hydrolytic degradation during processing. The materials were added to the mixer and mixed at 190 C for 10 min. During the first 9 min of mixing, the mixer speed was 60 rpm. The last minute of mixing was performed at 90 rpm. After mixing, the CNF-PA12 was compression moulded into a sheet. A neat PA12 powder (DuraForm PA12Ò without addition of CNF, from here on known as N-PA12) was prepared similarly for comparison (mixed and pressed). Both materials were cryogenically fractured to produce powders, with an average particle size of 50 mm, for laser sintering. 3.3. Characterisation of the polymer nanocomposite The powdered nanocomposite material was imaged by SEM to observe the dispersion of the nanofibres in the polymer matrix. The material was sputter coated with gold prior to imaging to prevent charging. 3.4. Laser sintering of the polymer nanocomposite The PA12-CNF composite was laser sintered along with two control materials – the N-PA12 prepared as described above, and DuraFormÒ PA12, as supplied by the manufacturer (AS-PA12). Laser sintering was carried out on an EOS P100 Formiga system. A variety of processing conditions, including laser power, scan speed, and bed and chamber temperatures, were optimised for each material, determined by processing ability and maximum mechanical properties. 3.5. Characterisation of the laser sintered polymer nanocomposite The laser sintered nanocomposite was imaged using SEM to observe the degree of sintering and to confirm that the nanofibres remained individually dispersed within the matrix after processing. The material was sputter coated with gold prior to imaging to prevent charging. The mechanical properties of the N-PA12 and CNF-PA12 composites were measured using dynamic mechanical analysis (DMA)
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according to ISO 6721[35]. The testing was performed in tension at a frequency of 1 Hz and a heating rate of 2 C/min. All tests were performed in the linear viscoelastic strain range determined by strain sweep measurements. 4. Results and discussion 4.1. Characterisation of the polymer nanocomposite prior to laser sintering SEM micrographs of the nanocomposite powder prior to laser sintering showed uniform dispersion of the nanofibres in the polymer matrix (Fig. 1). The CNFs, seen on the micrograph as the brighter objects, were observed to be isolated from one another and surrounded by the polymer matrix. The fracture surface showed exposed CNFs and holes in the matrix, indicating that the interfacial interactions were not strong. In spite of a weak interphase, reinforcement of the modulus was achieved in tensile testing. Low magnification SEM micrographs showed, however, that the two materials produced by melt-mixing (CNFPA12 and N-PA12) had a non-uniform, angular particle morphology compared to the regular, near-spherical particles of the AS-PA12 material. These morphologies had an impact on the sintering behaviour of the melt mixed materials, as described in the following section (Fig. 2). 4.2. Laser sintering of the polymer nanocomposite The laser sintering process conditions used for each material are shown in Table 1. The laser scan speed was kept constant at 2500 mm/s. All other parameters used were the manufacturers default parameters for PA12 on a P100 Formiga system [36]. From this data, it can be seen that both CNF-PA12 and N-PA12 powders necessitated a drop in the process chamber temperatures, and the N-PA12 powder required a lower laser power. As both meltmixed materials required lower processing conditions it is proposed that it is due to the preparation of the powder, more specifically the powder morphology, rather than the more efficient absorption of the laser by the nanofillers. In fact, the CNF-PA12 required an increase in laser power,
Fig. 3. Photographs of the powder bed (top) and produced part (bottom) of the three materials.
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compared to N-PA12, implying in this case, that the nanofibres have an opposite effect. Photographs of the powder bed and produced parts can be seen in Fig. 3. The prepared materials (CNF-PA12 and N-PA12) had a rough powder surface compared to the AS-PA12, due to the irregular morphology of the powder particles. Nevertheless, they spread similarly to commercial materials. White light emission and fumes were observed when laser sintering the CNF-PA12 material, similar to that observed by Koo [37]. Future study should address the exact cause of these observations.
1800 AS-PA12 CNF-PA12 N-PA12
Storage Modulus (MPa)
1600 1400 1200 1000 800 600 400 200 0 20
30
40
50
60
70
80
90
100
110
Temperature (°C) 140 AS-PA12 CNF-PA12 N-PA12
Loss Modulus (MPa)
120 100 80 60 40 20 0 20
30
40
50
60
70
80
90
100
110
Temperature (°C)
0.14 AS-PA12 CNF-PA12 N-PA12
0.12
Tan Delta
0.1 0.08 0.06 0.04 0.02 0 20
30
40
50
60
70
80
90
100
110
Temperature (°C) Fig. 4. DMA results for all three materials processed by laser sintering.
4.3. Characterisation of laser sintered parts Fig. 4 records the results of the dynamic mechanical testing for all three materials. The quantities measured were storage modulus, loss modulus, and tan delta. The first and second quantities represent the elastic and viscous components of the dynamic response, respectively, based on the ability of the materials to store and dissipate energy. Tan delta is the ratio of loss modulus to storage modulus, which may be taken as an indirect measure of the materials’ toughness at low strains. The CNF-PA12 samples showed improvement over the N-PA12 samples, with the storage modulus of the former being approximately 22% higher than that of the latter. Loss modulus was also increased with respect to the N-PA12 by 15% at their respective peak temperatures. However, the AS-PA12 samples had the highest storage and loss moduli of the three samples, attributed to more uniform particle size and morphology. It may also have been affected by the reduced thermal history of the starting powder, as the AS-PA12 powder was “virgin” laser sintering powder, i.e. powder that had not previously been used in a laser sintering build and had, therefore, not been subjected to the thermal processes within the LS equipment. It is commonplace for “used” powder, i.e. that consisting of polymer that has previously been used in the LS process, but has not been consolidated by the laser (e.g. the supporting powder cake or overflow), to be used in the LS process, although repeated use increases the molecular weight of the polymer and eventually leads to a decrease in strength of the parts [38]. In this work, the N-PA12 and CNF-PA12 powders, whilst they also had not previously been through a laser sintering process, had been subjected to a thermal process
Fig. 5. SEM Micrographs of cross sections of laser sintered N-PA12 150.
R.D. Goodridge et al. / Polymer Testing 30 (2011) 94–100
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Fig. 6. SEM Micrographs of cross sections of laser sintered CNF-PA12 150.
Fig. 7. SEM Micrographs of cross sections of laser sintered AS-PA12 150.
through the melt-mixing method used to prepare them, which may have had a similar effect. Changes in tan delta and glass transition temperature were independent of the powder quality. Tan delta of the CNF-PA12 samples was decreased with respect to the values for unfilled PA12 due to the higher reinforcement of storage modulus. The glass transition temperature (as measured from the peak of the loss modulus curve) was 3–4 C higher than the N-PA12 and AS-PA12 materials indicating decreased polymer mobility in the nanocomposite. SEM micrographs of the laser sintered parts can be seen in Figs. 5–8. In Figs. 5 and 6, definition of the layers can still be seen for the CNF and N-PA12 samples, suggesting that complete consolidation has not occurred. This is likely to be
due to sub-optimum processing conditions. However, higher magnification images of the CNF-PA12 samples revealed that the CNF’s remained well dispersed within the polymer matrix after laser sintering and remained predominantly in one plane (Fig. 8). It is clear from these results that CNF’s remain well dispersed following processing and are able to increase the mechanical properties of laser sintered parts. However, the inability to produce a nanocomposite powder with suitable morphology and size for laser sintering is preventing this improvement from being seen when compared to the original PA12 material supplied by the manufacturer. An alternative method for producing the nanocomposite power is, therefore, required for the full potential of CNF addition to be realised.
Fig. 8. SEM micrographs of CNF-PA12 before and after laser sintering.
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5. Conclusions This work has demonstrated that CNF’s can increase the strength of a base polyamide 12 laser sintering polymer prepared using a melt-mixing technique. Following laser sintering, characterisation of the polymer nanocomposite parts by SEM and dynamic mechanical testing showed that the nanofibres were well dispersed within the polymer matrix and gave a 22% increase in the storage modulus compared to the base material. However, the cryogenic fracturing method used in this research did not produce powder with suitable morphology for laser sintering. Improvements to the production of the nano-composite starting powder are required to use these materials effectively with laser sintering. References [1] Deckard CR. Selective Laser Sintering. PhD dissertation, Department of Mechanical Engineering, University of Texas at Austin; 1988. [2] US Patents 4247508/4863538. [3] C.K. Chua, K.F. Leong, C.S. Lin, Rapid Prototyping: Principles and Applications, third ed. (Jan.2010) 540pp. [4] T.H.C. Childs, A.E. Tontowi, Selective laser sintering of a crystalline and glass-filled crystalline polymer: experiments and simulations, Proc IMechE Part B, J Engineering Manufacture 215/11 (2001) 1481–1495. [5] ASTM Committee F42 on Additive Manufacturing Technologies (May 2009). [6] N. Hopkinson, R. Hague, P. Dickens, Rapid Manufacturing: An Industrial Revolution for a Digital Age: An Industrial Revolution for the Digital Age. Wiley-Blackwell, 2005. [7] R. Hague, S. Mansour, N. Saleh, Material and design considerations for rapid manufacturing, Int J Prod Res 42/22 (2004) 4691–4708. [8] R.S. Evans, D.L. Bourell, J.J. Beaman, M.I. Campbell, SLS Materials Development Method for Rapid Manufacturing. Sixteenth Solid Freeform Fabrication Symposium. The University of Texas, August 2005, 184–196. [9] H. Chung, S. Das, Processing and properties of glass bead particulatefilled functionally graded nylon-11 composites produced by selective laser sintering, Materials Science and Engineering A 437 (2006) 226–234. [10] T.J. Gill, K.K.B. Hon, Experimental investigation into the selective laser sintering of silicon carbide polyamide composites, Proceedings of the Institution of Mechanical Engineers Part B Journal of Engineering 218 (2004) 1249–1256. [11] K.K.B. Hon, T.J. Gil, Selective laser sintering of SiC/polyamide composites, CIRP Annals 52/1 (2003) 173–176. [12] F.E. Wiria, K.F. Leong, C.K. Chua, Y. Liu, Poly-e-capralactone/ hydroxyapatite for tissue engineering scaffold fabrication by selective laser sintering, Acta Biomaterialia 3/1 (2007) 1–12. [13] K.H. Tan, C.K. Chua, K.F. Leong, C.M. Cheah, W.S. Gui, W.S. Tan, F.E. Wiria, SLS of biocompatible polymers for applications in tissue engineering, Biomedical Materials and Engineering 15 (2005) 113–124. [14] A. Mazzoli, A. Moriconi, M.G. Pauri, Characterisation of an aluminium-filled polyamide powder for applications in selective laser sintering, Materials and Design 28/3 (2007) 993–1000. [15] C.-Z. Yan, Y.-S. Shi, J.-S. Yang, J. Liu, Nanosilica/Nylon-12 composite powder for selective laser sintering, Journal of Reinforced Plastics and Composites 28/23 (2009) 2889–2902.
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Processing of a Polyamide-12/carbon nanofibre composite by laser sintering R.D. Goodridge a, * , M.L. Shofner b, R.J.M. Hague a, M. McClelland a, M.R. Schlea b, R.B. Johnson b, C.J. Tuck a a b
Additive Manufacturing Research Group, Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, LE11 3TU, UK School of Polymer, Textile & Fiber Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0295, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2010 Accepted 29 October 2010
Additive layer techniques, such as laser sintering, are increasingly being considered for the production of fully functioning end-use parts due to the significant advantages they hold in the design and implementation of products. However, one of the main obstacles to widespread adoption of this technology is the limited range of materials that can currently be processed using additive techniques. This paper presents initial research into the reinforcement of laser sintered polyamides with carbon nanofibres (CNFs). The effects of CNF addition on the processing parameters and mechanical properties of laser sintered parts have been investigated. A 3wt % carbon nanofibre-polyamide 12 composite (CNFPA12) powder was prepared using melt mixing and cryogenic milling. Following laser sintering, characterisation of the polymer nanocomposite parts by SEM and dynamic mechanical testing showed that the nanofibres were well dispersed within the polymer matrix and gave a 22% increase in the storage modulus compared to the base material. However, the cryogenic fracturing method used in this research did not produce powder with suitable morphology for laser sintering. If improved powder production can be achieved, the use of CNF reinforcements to improve mechanical properties in laser sintering holds promise. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Laser sintering Additive manufacturing Polymer nanocomposites
1. Introduction Additive layer techniques, such as laser sintering [1,2], have historically been used for “Rapid Prototyping”, exploiting their increased freedom of design in a range of areas such as concept modelling, pattern building, assembly verification and functional testing [3,4]. The fabricated parts have, for these applications, only been required to possess sufficient mechanical integrity and surface quality for handling and demonstration purposes. Increasingly however, additive techniques are being considered for the production of actual end-use parts, a concept that has recently been defined as Additive Manufacturing (AM) [5,6]. * Corresponding author. Tel.: þ44(0) 1509 227567. E-mail address: [email protected] (R.D. Goodridge). 0142-9418/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2010.10.011
The principal advantage of AM is the ability to manufacture parts with significantly greater complexity of geometry than traditional techniques entirely without the need for mould tooling. As mould tooling is not required for production, the design constraints normally associated with the subtractive and formative methods of manufacture are reduced, providing significant advantages in the design and implementation of products [7]. In addition, as tooling is no longer required the economics of production are changed dramatically, enabling low volume production and even completely customised products [6]. However, one of the most widely acknowledged stumbling blocks in the progression of this technology into manufacturing is the very limited range of materials, particularly polymeric materials, that can currently be processed using additive techniques [8]. Current additive
R.D. Goodridge et al. / Polymer Testing 30 (2011) 94–100
materials do not meet the requirements of the majority of commercial products, so there is a need to develop and to be able to process a much greater variety of materials than is currently possible. Although in theory any polymer available in powder form can be processed by laser sintering, the complex thermal chemistries and consolidation phenomena of polymers have to date limited the choice of laser sintering polymers to mainly polyamide based powders. Polyamide 11 and 12 (PA11 and PA12) are certainly the most widely used laser sintering materials at the present time. However, the move towards AM has necessitated increasing research into the processing of alternative materials with new and improved properties. This paper presents initial research into the reinforcement of laser sintered polyamides with carbon nanofibres in an attempt to improve their mechanical properties and enable their use in wider AM applications. 2. Literature review Previous attempts to improve the mechanical or physical properties of polymeric laser sintered parts have involved reinforcing them with micron-sized inorganic fillers, such as glass beads [4,9], silicon carbide [10,11], hydroxyapatite [12,13], aluminium powder [14,15], or nano-sized fillers such as clay [16–19], nanosilica [20] and nano-Al2O3 [21]. Commercial examples of polymer-filled materials include glass filled polyamide (e.g. DuraFormÒ GF, PA 3200 GF), and aluminium filled polyamide (e.g. Alumide) [22,23]. Polymer nanocomposites are of particular interest for LS as a relatively small volume of nanofiller can have a considerable effect on the properties of the composite, due to the high surface to volume ratio of the nanofiller, as well as in some cases their extremely high aspect ratio [24]. In addition to improving mechanical properties such as strength and stiffness, nanofillers can be used to enhance other properties such as optical properties, thermal conductivity, heat resistance and flame retardancy or to accelerate biodegradability or increase bioactivity [25,33]. Positive effects specific to the laser sintering process have also been observed. In particular, the addition of certain nanoparticles has been found to decrease the energy required from the laser to consolidate the powder due to more efficient absorption of laser power by the fillers. Wang et al. [17] reported this effect when looking at a PA12-rectorite composite; the more rectorite that was added, the less laser power was needed for optimum tensile strength. It is thought that this is due to the fillers absorbing the laser radiation more efficiently than the base polymer. Ho et al. [26] looked at a number of common plastic fillers, namely quartz, silica, graphite and talc, in polycarbonate on the basis that laser radiation absorbing additives could reduce the amount of laser energy required for sintering [27]. Of the fillers that Ho et al. [26] examined, graphite powder was found to have the largest effect on improving the absorptance of the laser sintering powder. It was proposed that the addition of graphite powder may go some way to minimising “bonus-z” (growth of the part in the z-axis) and other thermal-related problems experienced in laser sintered parts as, the greater the absorptance of a powder, the less
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laser energy is required for sintering and less energy is transmitted through the graphite powder. Analogous to powder production for conventional processing techniques, uniform particle dispersion and good interfacial adhesion between filler and polymer matrix are key to successful production of a nanocomposite material with improved properties. If this is not achieved, the resulting mechanical properties of the filled material can be lower than the original base polymer [18]. Therefore, preparation of the starting material is a prime factor. Many reinforced polymers for laser sintering (including the current commercially available materials) are prepared by simple mechanical mixing of the filler with the neat base polymer [10,14,17,20,28,29]. However, uniformly mixing two powders with different particle sizes (particularly when one powder is very small, e.g. nanofillers), or with significantly varied densities (e.g. nylon and metal powders), using this method is problematic. Kruth et al. [30] reported frequent coagulation and, thus, segregation during layering of both aluminium and copper filled polyamide 12 for this reason. Another method has been to coat the fillers with the base polymer. This is the technique used to produce metal parts by indirect laser sintering processes (i.e. using a sacrificial polymer binder that is burnt off in a post-processing step) due to the improved sintering compared to simple mechanical mixing [31]. Yan et al. [32] applied this method to polyamide 12-coated aluminium powders produced using dissolution-precipitation and ball-milling (cAlPA), and compared the results to those of a neat polyamide 12 powder prepared by an identical dissolution-precipitation process but without addition of the aluminium particles (NPA), and to a polyamide 12-aluminium composite prepared by simple mixing [mAlPA]. Despite both being prepared using the same method, the NPA had an irregular particle morphology whereas the cAlPA had a more spherical morphology and uniform size. It was proposed by Yan et al that this was due to the aluminium powder and the conditions of the preparation technique determining the characteristics of the cAlPA and the NPA, respectively. Increasing the mass fraction of Al filler improved the strength of the cAlPA samples whereas it decreased the strength of the mAlPA relative to the NPA samples. SEM showed aggregates of Al particles in the mAlPA samples which created mechanical defects, confirming previous findings reported above. Two explanations were offered for the increase in strength of the cAlPA samples. The first rationalised that by coating the aluminium particles in the preparation stage, the interfacial adhesion between the two materials occurs before the laser sintering process, requiring the laser energy to simply fuse the polyamide coatings together. In contrast, for mAlPA powders, bonding between the polyamide and aluminium particles, as well as between different polyamide particles, must occur during the extremely short scanning period of the laser sintering process, limiting the interfacial adhesion that can occur. The second explanation offered relates to the lower laser radiation absorption of metals as compared to polymers [27]. The aluminium particles in the mAlPA powder are exposed to the laser during sintering, whereas those in the cAlPA are covered by the polymer, resulting in much lower absorption
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R.D. Goodridge et al. / Polymer Testing 30 (2011) 94–100 Table 1 Laser Sintering parameters used to process each material.
Chamber Temperature ( C) Bed Temperature ( C) Laser Power (W)
Fig. 1. SEM images of CNF-PA12 prior to laser sintering at 20 k. The bright objects, some of which have been circled for identification, are the carbon nanofibres.
by mAlPA compared to cAlPA. Increasing the mass fraction of aluminium filler in the cAlPA samples was found to improve the dimensional accuracy of the laser sintered parts. In addition, the tensile strength, elongation at break, and impact strength of the samples were found to increase with decreasing average Al particle size. Other authors have adopted similar techniques. Zheng et al. [21] used nano-Al2O3 particles coated with polystyrene (PS) by emulsion polymerisation as fillers to reinforce PS based composites for laser sintering. No agglomeration occurred and the nanoparticles were found to have dispersed homogeneously. Again, this was attributed to the polymer forming a boundary layer between the individual nanoparticles, preventing agglomeration and, thereby, improving dispersion. Yan et al. [15] used a dissolution-precipitation process to produce a 3wt% nanosilica/polyamide 12 composite powder. The nanosilica was dispersed uniformly in the polymer matrix and, following laser sintering, a 20.9% increase in tensile strength, 39.4% increase in tensile modulus and 9.54% increase in impact strength was reported for the nano composite material compared to the neat PA12 that had also been prepared using an identical dissolution-precipitation technique. The nanofillers chosen for this study were carbon nanofibres (CNFs). CNFs consist of graphene planes oriented in a tree ring structure around a hollow core. Their diameters are of the order of 100 nm, and their properties are roughly one order of magnitude below those for carbon
CNF-PA12
N-PA12
AS-PA12
164 151 20
166 151 17
170 150 21
nanotubes. With the high cost of carbon nanotubes, CNFs offer a more practical solution allowing the enhancement of mechanical properties without significant increase in the cost of the material. A study by Koo et al. [33] demonstrated the ability of CNF’s to increase the flame retardant properties of a polyamide 11 (PA11) material commonly used in laser sintering, but for their study the parts were produced by injection moulding. The effect carbon nanofibres have on the mechanical properties of actual laser sintered parts has not yet been investigated and reported. This paper therefore examines the influence that the addition of carbon nanofibres has on the processing and mechanical properties of laser sintered PA12 samples. 3. Method 3.1. Preparation of the nanofibres The CNFs were obtained from Pyrograf Products, Inc. The product number for the CNFs used was PR-24-PS. Literature from the manufacturer indicated that the CNFs had diameters between 60 and 150 nm with lengths ranging from 30 to 100 mm. The CNFs were purified from the as-received condition to remove other carbon species and catalyst using a five day reflux in methylene chloride, followed by a washing step and another reflux in water for one day. This procedure has been shown to open fibre bundles while preserving fibre length [34].
3.2. Production of the polymer-nanocomposites A 3wt % carbon nanofibre-polyamide 12 composite (CNF-PA12) was prepared by melt mixing. DuraForm PA12Ò, supplied by 3D Systems Inc, was chosen as the base polyamide due to its proven history in laser sintering. The melt mixing equipment used was a Brabender Intelli-Torque polymer processing system with an internal batch mixer attachment. The mixer had a 60 mL capacity and was
Fig. 2. SEM micrographs of AS-PA12 (left), N-PA12 (centre), and CNF-PA12 (right) at 500.
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used with roller blades. Prior to mixing, the polymer powder was dried in a vacuum oven to prevent hydrolytic degradation during processing. The materials were added to the mixer and mixed at 190 C for 10 min. During the first 9 min of mixing, the mixer speed was 60 rpm. The last minute of mixing was performed at 90 rpm. After mixing, the CNF-PA12 was compression moulded into a sheet. A neat PA12 powder (DuraForm PA12Ò without addition of CNF, from here on known as N-PA12) was prepared similarly for comparison (mixed and pressed). Both materials were cryogenically fractured to produce powders, with an average particle size of 50 mm, for laser sintering. 3.3. Characterisation of the polymer nanocomposite The powdered nanocomposite material was imaged by SEM to observe the dispersion of the nanofibres in the polymer matrix. The material was sputter coated with gold prior to imaging to prevent charging. 3.4. Laser sintering of the polymer nanocomposite The PA12-CNF composite was laser sintered along with two control materials – the N-PA12 prepared as described above, and DuraFormÒ PA12, as supplied by the manufacturer (AS-PA12). Laser sintering was carried out on an EOS P100 Formiga system. A variety of processing conditions, including laser power, scan speed, and bed and chamber temperatures, were optimised for each material, determined by processing ability and maximum mechanical properties. 3.5. Characterisation of the laser sintered polymer nanocomposite The laser sintered nanocomposite was imaged using SEM to observe the degree of sintering and to confirm that the nanofibres remained individually dispersed within the matrix after processing. The material was sputter coated with gold prior to imaging to prevent charging. The mechanical properties of the N-PA12 and CNF-PA12 composites were measured using dynamic mechanical analysis (DMA)
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according to ISO 6721[35]. The testing was performed in tension at a frequency of 1 Hz and a heating rate of 2 C/min. All tests were performed in the linear viscoelastic strain range determined by strain sweep measurements. 4. Results and discussion 4.1. Characterisation of the polymer nanocomposite prior to laser sintering SEM micrographs of the nanocomposite powder prior to laser sintering showed uniform dispersion of the nanofibres in the polymer matrix (Fig. 1). The CNFs, seen on the micrograph as the brighter objects, were observed to be isolated from one another and surrounded by the polymer matrix. The fracture surface showed exposed CNFs and holes in the matrix, indicating that the interfacial interactions were not strong. In spite of a weak interphase, reinforcement of the modulus was achieved in tensile testing. Low magnification SEM micrographs showed, however, that the two materials produced by melt-mixing (CNFPA12 and N-PA12) had a non-uniform, angular particle morphology compared to the regular, near-spherical particles of the AS-PA12 material. These morphologies had an impact on the sintering behaviour of the melt mixed materials, as described in the following section (Fig. 2). 4.2. Laser sintering of the polymer nanocomposite The laser sintering process conditions used for each material are shown in Table 1. The laser scan speed was kept constant at 2500 mm/s. All other parameters used were the manufacturers default parameters for PA12 on a P100 Formiga system [36]. From this data, it can be seen that both CNF-PA12 and N-PA12 powders necessitated a drop in the process chamber temperatures, and the N-PA12 powder required a lower laser power. As both meltmixed materials required lower processing conditions it is proposed that it is due to the preparation of the powder, more specifically the powder morphology, rather than the more efficient absorption of the laser by the nanofillers. In fact, the CNF-PA12 required an increase in laser power,
Fig. 3. Photographs of the powder bed (top) and produced part (bottom) of the three materials.
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compared to N-PA12, implying in this case, that the nanofibres have an opposite effect. Photographs of the powder bed and produced parts can be seen in Fig. 3. The prepared materials (CNF-PA12 and N-PA12) had a rough powder surface compared to the AS-PA12, due to the irregular morphology of the powder particles. Nevertheless, they spread similarly to commercial materials. White light emission and fumes were observed when laser sintering the CNF-PA12 material, similar to that observed by Koo [37]. Future study should address the exact cause of these observations.
1800 AS-PA12 CNF-PA12 N-PA12
Storage Modulus (MPa)
1600 1400 1200 1000 800 600 400 200 0 20
30
40
50
60
70
80
90
100
110
Temperature (°C) 140 AS-PA12 CNF-PA12 N-PA12
Loss Modulus (MPa)
120 100 80 60 40 20 0 20
30
40
50
60
70
80
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110
Temperature (°C)
0.14 AS-PA12 CNF-PA12 N-PA12
0.12
Tan Delta
0.1 0.08 0.06 0.04 0.02 0 20
30
40
50
60
70
80
90
100
110
Temperature (°C) Fig. 4. DMA results for all three materials processed by laser sintering.
4.3. Characterisation of laser sintered parts Fig. 4 records the results of the dynamic mechanical testing for all three materials. The quantities measured were storage modulus, loss modulus, and tan delta. The first and second quantities represent the elastic and viscous components of the dynamic response, respectively, based on the ability of the materials to store and dissipate energy. Tan delta is the ratio of loss modulus to storage modulus, which may be taken as an indirect measure of the materials’ toughness at low strains. The CNF-PA12 samples showed improvement over the N-PA12 samples, with the storage modulus of the former being approximately 22% higher than that of the latter. Loss modulus was also increased with respect to the N-PA12 by 15% at their respective peak temperatures. However, the AS-PA12 samples had the highest storage and loss moduli of the three samples, attributed to more uniform particle size and morphology. It may also have been affected by the reduced thermal history of the starting powder, as the AS-PA12 powder was “virgin” laser sintering powder, i.e. powder that had not previously been used in a laser sintering build and had, therefore, not been subjected to the thermal processes within the LS equipment. It is commonplace for “used” powder, i.e. that consisting of polymer that has previously been used in the LS process, but has not been consolidated by the laser (e.g. the supporting powder cake or overflow), to be used in the LS process, although repeated use increases the molecular weight of the polymer and eventually leads to a decrease in strength of the parts [38]. In this work, the N-PA12 and CNF-PA12 powders, whilst they also had not previously been through a laser sintering process, had been subjected to a thermal process
Fig. 5. SEM Micrographs of cross sections of laser sintered N-PA12 150.
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Fig. 6. SEM Micrographs of cross sections of laser sintered CNF-PA12 150.
Fig. 7. SEM Micrographs of cross sections of laser sintered AS-PA12 150.
through the melt-mixing method used to prepare them, which may have had a similar effect. Changes in tan delta and glass transition temperature were independent of the powder quality. Tan delta of the CNF-PA12 samples was decreased with respect to the values for unfilled PA12 due to the higher reinforcement of storage modulus. The glass transition temperature (as measured from the peak of the loss modulus curve) was 3–4 C higher than the N-PA12 and AS-PA12 materials indicating decreased polymer mobility in the nanocomposite. SEM micrographs of the laser sintered parts can be seen in Figs. 5–8. In Figs. 5 and 6, definition of the layers can still be seen for the CNF and N-PA12 samples, suggesting that complete consolidation has not occurred. This is likely to be
due to sub-optimum processing conditions. However, higher magnification images of the CNF-PA12 samples revealed that the CNF’s remained well dispersed within the polymer matrix after laser sintering and remained predominantly in one plane (Fig. 8). It is clear from these results that CNF’s remain well dispersed following processing and are able to increase the mechanical properties of laser sintered parts. However, the inability to produce a nanocomposite powder with suitable morphology and size for laser sintering is preventing this improvement from being seen when compared to the original PA12 material supplied by the manufacturer. An alternative method for producing the nanocomposite power is, therefore, required for the full potential of CNF addition to be realised.
Fig. 8. SEM micrographs of CNF-PA12 before and after laser sintering.
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5. Conclusions This work has demonstrated that CNF’s can increase the strength of a base polyamide 12 laser sintering polymer prepared using a melt-mixing technique. Following laser sintering, characterisation of the polymer nanocomposite parts by SEM and dynamic mechanical testing showed that the nanofibres were well dispersed within the polymer matrix and gave a 22% increase in the storage modulus compared to the base material. However, the cryogenic fracturing method used in this research did not produce powder with suitable morphology for laser sintering. Improvements to the production of the nano-composite starting powder are required to use these materials effectively with laser sintering. References [1] Deckard CR. Selective Laser Sintering. PhD dissertation, Department of Mechanical Engineering, University of Texas at Austin; 1988. [2] US Patents 4247508/4863538. [3] C.K. Chua, K.F. Leong, C.S. Lin, Rapid Prototyping: Principles and Applications, third ed. (Jan.2010) 540pp. [4] T.H.C. Childs, A.E. Tontowi, Selective laser sintering of a crystalline and glass-filled crystalline polymer: experiments and simulations, Proc IMechE Part B, J Engineering Manufacture 215/11 (2001) 1481–1495. [5] ASTM Committee F42 on Additive Manufacturing Technologies (May 2009). [6] N. Hopkinson, R. Hague, P. Dickens, Rapid Manufacturing: An Industrial Revolution for a Digital Age: An Industrial Revolution for the Digital Age. Wiley-Blackwell, 2005. [7] R. Hague, S. Mansour, N. Saleh, Material and design considerations for rapid manufacturing, Int J Prod Res 42/22 (2004) 4691–4708. [8] R.S. Evans, D.L. Bourell, J.J. Beaman, M.I. Campbell, SLS Materials Development Method for Rapid Manufacturing. Sixteenth Solid Freeform Fabrication Symposium. The University of Texas, August 2005, 184–196. [9] H. Chung, S. Das, Processing and properties of glass bead particulatefilled functionally graded nylon-11 composites produced by selective laser sintering, Materials Science and Engineering A 437 (2006) 226–234. [10] T.J. Gill, K.K.B. Hon, Experimental investigation into the selective laser sintering of silicon carbide polyamide composites, Proceedings of the Institution of Mechanical Engineers Part B Journal of Engineering 218 (2004) 1249–1256. [11] K.K.B. Hon, T.J. Gil, Selective laser sintering of SiC/polyamide composites, CIRP Annals 52/1 (2003) 173–176. [12] F.E. Wiria, K.F. Leong, C.K. Chua, Y. Liu, Poly-e-capralactone/ hydroxyapatite for tissue engineering scaffold fabrication by selective laser sintering, Acta Biomaterialia 3/1 (2007) 1–12. [13] K.H. Tan, C.K. Chua, K.F. Leong, C.M. Cheah, W.S. Gui, W.S. Tan, F.E. Wiria, SLS of biocompatible polymers for applications in tissue engineering, Biomedical Materials and Engineering 15 (2005) 113–124. [14] A. Mazzoli, A. Moriconi, M.G. Pauri, Characterisation of an aluminium-filled polyamide powder for applications in selective laser sintering, Materials and Design 28/3 (2007) 993–1000. [15] C.-Z. Yan, Y.-S. Shi, J.-S. Yang, J. Liu, Nanosilica/Nylon-12 composite powder for selective laser sintering, Journal of Reinforced Plastics and Composites 28/23 (2009) 2889–2902.
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