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Hybrid composites manufactured by resin infusion with a fully recyclable bioepoxy resin
Composites Part B 132 (2018) 69e76
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Hybrid composites manufactured by resin infusion with a fully recyclable bioepoxy resin G. Cicala a, *, E. Pergolizzi a, F. Piscopo a, D. Carbone b, G. Recca b a b
University of Catania, DICAR, Viale Andrea Doria 6, 95125 Catania, Italy CNR-ICTP, Via Paolo Gaifami 17, 95100 Catania, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2017 Received in revised form 18 June 2017 Accepted 28 August 2017 Available online 31 August 2017
Bioepoxy based monomers were formulated with a cure inhibitor and a cleavable amine to obtain a recyclable epoxy system suitable for resin infusion at room temperature. Hybrid flax/carbon fiber layup were used. Tensile, flexural and dynamo-mechanical properties for the composites were studied. The cured laminates were chemically recycled obtaining from the epoxy matrix a thermoplastic. The recycled was processed by fused deposition modelling (FDM) and injection molding after mixing with short kenaf fibers. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Flax Kenaf Recycling Resin infusion Epoxy Fused deposition modelling
1. Introduction The use of epoxy based composites is increasingly accepted in many fields. In the automotive sector, electric-mobility, CO2 emission limits, gasoline and energy prices are some of the driving factors guiding the lightweight automotive design. The requirement for lightweight structures, combined with the need for high volume production rates, pushed the development of mass production techniques like High Pressure Resin Transfer Molding (HPRTM) [1]. In the aerospace sector, liquid resin infusion of thermosets is increasingly accepted because of its low cost and suitability for producing of large structures [2e4]. Liquid resin infusion is well established in transportation and naval sectors too [4,5]. In the civil fields the use of epoxy composites is widely accepted too [6,7]. The widespread use of thermoset composites is raising environmental concerns about the recycling options and because of the use of petroleum based raw materials. To address the latter point, natural fibers as reinforcements [8e15] and bioepoxy resins are becoming a focal point of interest for industry [16,17]. In the last years a lot of attention has been drawn by the use of hybrid natural fiber layup to optimize composite's mechanical properties [18e20].
* Corresponding author. Tel.: þ39 095 7382760. E-mail address: [email protected] (G. Cicala). http://dx.doi.org/10.1016/j.compositesb.2017.08.015 1359-8368/© 2017 Elsevier Ltd. All rights reserved.
The use of hybrid fabric layup add complexity to the recycling operation if fiber recovery is desired because, in some cases, the high value of some natural fabrics oblige not to destroy them. The status of the recycling techniques for fiber reinforced composites was recently reviewed by Oliveux et al. [21] and many differences compared to traditional plastic recycling of solid waste can be observed [22]. Rybicka et al. [23] analyzed the technology readiness level for many composite recycling techniques: incineration and landfilling were classified as TRL 9 while, pyrolysis for carbon fiber and mechanical grinding for glass fiber applications resulted on a TRL 8. Pyrolysis for glass fiber and mechanical grinding for carbon fiber achieved TRL 7. Finally, fluidized bed pyrolysis and solvolysis process achieved a median TRL of 4. The main limitations of thermal and mechanical recycling processes are fiber's properties degradation and that matrices are destroyed or only partially recovered in useful forms. Chemical recycling is an interesting alternative approach because it allows to obtain clean reclaimed fibers and valuable monomers from the epoxy matrix [24]. Similar methods have been presented by several authors [25,26]. Researchers from Lamborghini showed their interest in the use of chemical recycled fibers [27]. However, most of the known chemical recycling approaches rely on the use of solvent mixtures which inhibit their application to natural fiber reinforced composites. A sustainable solution to
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overcome these limits is offered by the cleavable amines developed by Connora Technology. These amines allow to synthesize epoxy thermosets which can be recycled yielding thermoplastics and clean fibers in mild aqueous solutions at low temperatures (80e120 C) [28]. In a previous paper, we discussed the environmental benefits of this approach showing the avoided environmental impacts associated with the use of recyclable epoxy resins [29]. We also presented some data about the development of a recyclable epoxy formulation for HP-RTM with some preliminary characterization of the recycled thermoplastic [30]. In the present paper, we extended the use of the Connora recycling approach by developing a recyclable biobased epoxy formulation suitable for resin infusion. Hybrid flax/carbon fibers layup were used and the effect of the stacking sequence evaluated. The composites panels obtained were then recycled and the potential uses of the recycled thermoplastic addressed. 2. Experimental 2.1. Materials and method 2.1.1. Materials SuperSap epoxy monomers CLX(S) and the cure inhibitor INH by Entropy Resins were mixed with the Recyclamine® 301 by Connora Technologies. The inhibitor was added in the percentage of 25% to the epoxy monomer to have a pot life of 90 min at 25 C. The CLX(S) monomer is composed of epoxidized pine oils, bisphenol A/F type epoxy resin, benzyl alcohol, and proprietary reactive epoxy diluents. Similar resin systems are reported as a green system for resin transfer molding [31]. The Recyclamine® 301 is a cleavable polyamine ether patented by Connora Technology. The carbon fabric used was a twill fabric of T300 carbon fibers with 200 gsm (grams per square meter) areal weight purchased from Prochima, Italy. Flax fabrics (400 gsm, twill) were purchased by Composites Evolution (UK). Short kenaf fibers were supplied by Sachsenleinen, Germany. 2.1.2. Composite preparation Composite panels were prepared by resin infusion. Different stacking sequences were tested as reported in Table 1. The dry fabrics were stacked on a steel plate. During mold layup of the natural fibers fabrics no relevant fabric deformation was observed with this type of woven fabrics in contrast to what reported for non-woven fabrics [32]. An adhesive silicone tape was placed around the perimeter of the layered stack to provide a proper seal and a flexible vacuum bag was placed on top. An inlet tube and an outlet tube were placed inside the vacuum bag. The inlet tube was connected by a valve to a pot filled with unmodified epoxy resin while the outlet tube was connected to a vacuum pump. The vacuum was applied while the inlet valve was closed in order to compact the layers and to remove excess air. The premixed epoxy resin was vacuum infused into the stacked layers, which was maintained at 25 C under a constant vacuum (75 cm Hg). The laminates were kept at 25 C for 6 h before demolding them. 2.1.3. Composites recycling A sample (200 gr) of composite was treated with 3 L of acetic solution (25 vol % of acetic acid) at 80 C for 1.5 h. The mixture was then filtered and the fibers separated from the liquid phase. The reinforcing fibers were allowed to dry and weighed. The acetic solution was neutralized with a NaOH (pH ¼ 10) until a solid precipitate appeared. The mixture was cooled and filtered. The solid was washed in distilled water at about 40 C. The solid dissolved again. Few drops of NaOH solution were added and a white solid
precipitated from the solution, an epoxy thermoplastic polymer. 2.1.4. Recycled thermoplastic processing The recovered thermoplastic was blended with 5 wt% and 10 wt % of short kenaf fiber at 190 C for 10 min with a speed of 30 rpm using a batch mixer (Brabender 50 EHT) controlled by a Lab-Station. The thermoplastic and the kenaf fibers were dried at 50 C under vacuum for 48 h before mixing. Tensile specimens with dimensions according to ASTM D638 were fabricated from the blends using a 12 cc microinjection molder (DSM Xplorer) at 190 C melt temperature and 50 C mold temperature with injection and holding pressure of 16 bar. The specimens were allowed to cool down in the mold for 5 min before extraction. Thermoplastic filaments with 1.75 mm diameter were also produced from the recovered thermoplastic. Thermoplastic filaments were prepared in a single-screw extruder with screw diameter (D) of 20 mm and screw length of 25 D (model E 20 TH; Collins). Thermoplastic pellets were dried before use at 50 C under vacuum for 48 h; after that pellets were loaded in the extruder hood by using a volumetric feeder. The temperature pattern of the extruder was 55 Ce160 C-170 C-190 C-190 C from input to output zones, the screw speed was set at 30 rpm, and the melt pressure, checked during all of the extrusion process, was 40 bar. An extruder head with a circular die (diameter ¼ 3 mm) was used. The melted thermoplastic was drawn, cooling it with an air knife before collecting it on a rotating spool. The melt extruded filament, due to the drawing action of a rotating spool, varied in its diameter from 3 mm, at the exit of the extrusion head, to 1.75 ± 0.05 mm on the spool. The extruded filaments were processed by Fused Deposition Modelling (FDM) using a prototypal FDM machine Roboze one 400þ (Roboze, Bari, Italy). 2.2. Characterization 2.2.1. Mechanical testing Composite specimens were cut from the panel and tested in tensile and flexural mode accordingly to ASTM D3039 and ASTM D790. A universal testing machine Instron 5982 operating at a constant speed of 1 mm/min with a load cell of 100 kN was used for testing. Differences in mechanical results, for the composites samples, were statistically analyzed by one-way analysis of variance (ANOVA) using Minitab 17 software. To identify which groups were significantly different from other groups, means comparison was done using the Tukey's test with a 95% confidence level. Tensile properties of the injection molded specimens were measured by using an Instron 5982 universal testing machine, equipped with a load cell of 10 kN in accordance to ASTM D638 type 1. 2.2.2. Dynamic mechanical analysis (DMA) Composite specimens (mm 15 10 2) were tested by dynamic mechanical analysis (DMA). Storage modulus and Tand were measured using a dynamic mechanical analyzer (Tritech by Triton Ltd, UK). Experiments were carried out in single cantilever mode by
Table 1 Designation of the non-hybrid and hybrid laminate composites. Sample Code
Stacking sequence C: Carbon F: Flax
NF CF FCF CFC
FFFF CCCCCCCC FCCCCCCF CCCFFCCC
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Table 2 Data from tensile (st and Et) and flexural (sf and Ef) testing for non-hybrid and hybrid laminates. The standard deviations are given between brackets (Std). Sample Code
st
NF
82.20 (6.04) 518.91 (23.02) 309.69 (26.54) 300.76 (34.86)
CF FCF CFC
[MPa]
Et [GPa]
sf [MPa]
Ef [GPa]
9.97 (0.69)
77.48 (10.57)
6.47 (1.10)
23.71 (2.87) 16.31 (0.51) 25.56 (3.84)
193.14 (31.94) 90.20 (10.50) 213.70 (16.82)
31.20 (4.19) 6.26 (0.99) 35.24 (2.46)
heating the specimens from 25 C to 150 C at a constant heating rate of 2 C/min and 1 Hz frequency. 3. Results and discussion
Fig. 1. Tensile testing results for composite panels.
3.1. Mechanical testing All the results from mechanical testing are summarized in Table 2. The graph in Fig. 1 shows the effect of the stacking sequence on the tensile properties. The laminates with carbon fibers displayed the highest tensile properties (518.91 MPa, 23.71 GPa) while, the laminates reinforced with flax fibers showed the lowest tensile stress and modulus (82.20 MPa, 9.97 GPa). The hybrid laminates with natural fibers on the outer layers (FCF) showed tensile strength and modulus of 309.69 MPa and 16.31 GPa while, the hybrid laminates with natural fibers placed internally (CFC) showed tensile strength and modulus of 300.76 MPa and 25.56 GPa, respectively. The results of the Tukey analysis are summarized in Fig. 2. The tensile strength means (Fig. 2a) were all significantly different (p < 0.05) with the exception of the pairwise FCF-CFC (p ¼ 0.925). The tensile modulus means were not significantly different for the laminates CFC and FCF compared to the laminates CF (p-value of 0.876 and 0.047, respectively). If the ANOVA was carried out comparing only on the three samples with carbon fibers (CF, CFC, FCF) a p-value of 0.016 is obtained which means that at least one mean is similar with a statistical significance of 99%. The means for the samples NF and FCF, if compared directly, gives a p-value of 0.000 and thus are significantly different. Similar results were presented by other authors [33e35]. Zhang et al. [33] showed that the tensile modulus for all the hybrid composites were almost the same regardless to the stacking sequence. This finding can be ascribed to the fact that the tensile modulus refers to the elastic stage of the tensile stress strain curve and thus, the interfaces did not play a significant effect on the modulus. On the opposite, the tensile strength is affected by the stacking sequence because of the improved stress transfer efficiency on the hybrid interface between flax and carbon fibers. These conclusions are supported in literature by the fracture mode of natural fibers composites [36]. In a recent review, Pickering et al. [37] reported that the tensile strength and modulus for thermoset composites reinforced with natural fibers can span from a minimum of (80 MPa; 5 GPa) for multiaxial fabrics to a maximum of (150 MPa; 43 GPa) for unidirectional thermosets. Therefore, combining natural fibers with high performance carbon fibers, as it was experimented here, always outperform composites with natural fibers only. Fig. 3 displays the results from flexural testing. As expected the CF laminates outperformed the NF laminates. The stacking sequences affected significantly the flexural behavior. The FCF laminate showed lower properties than CFC laminate (p-value ¼ 0.000). Tukey analysis showed that the means are all significantly different
for the flexural strength (p < 0.05) and for the flexural modulus with the exception of the FCF-NF and CFC-CF pairwises (pvalue > 0.05) (Fig. 4). The observed flexural behavior depended on the fact that flexural strength and modulus are controlled by extreme layers of reinforcement [38]. From the mechanical point of view, FCF stacking sequence is not optimal because carbon fiber layers are placed internally where no longitudinal stresses or strains occurs.
3.2. Dynamic mechanical analysis The evolution of the storage modulus, loss modulus and tand with temperature is shown in Fig. 5. The storage modulus showed a single decay in the range of 40e80 C which correspond to the glass transition temperature (Tg) of the matrix. The glass transition temperatures are summarized in Table 3. From storage modulus curves the data below (30 C) and above (100 C) the glass transition temperature were extracted (Fig. 6). As expected, the storage modulus of the sample with natural fiber only (NF) was lower than CF. The hybrid samples (FCF and CFC) displayed an intermediate behavior between the sample with carbon fibers and flax fibers only. The sample with carbon fibers placed externally showed an higher storage modulus in all the temperature range confirming the trend observed from flexural testing. The loss factor (tan d) showed a different trend depending on the measurement temperature. At 30 C the NF sample displayed higher loss factor compared to CF while, at the glass transition temperature, the opposite trend was found (Fig. 7). The hybrid fabrics displayed an intermediate behavior. The same trend was reported by Duc et al. [39] when comparing glass to flax fibers. The influence of flax fibers on the damping of hybrid carbon/flax composites is relevant for applications. Recently, in the CARBIO project (http://carbioproject.com/technology/), focused on the development of novel car parts, the company Composite Evolution showed a layup configuration similar to the one reported here with 50et% carbon distributed on the external layers and 50 wt% of flax fibers concentrated in the middle. This layup allowed to reach an improvement in damping of 58% compared to full the carbon layup. Other relevant applications which benefit of the damping improvements were reported for sport equipments (i.e. http://www. lineo.eu/applications). In addition to that, some authors recently showed that natural fibers composites obtained from bioepoxy similar to those used here can perform better in terms of impact resistance [40].
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Fig. 2. Tukey analysis graphs for tensile data: a) tensile strength; b) tensile modulus.
3.3. Recycling of the laminates Samples cut from the cured laminates were treated as described in 2.1.3 to recover the fibers and the matrix. The recycling process was performed both on the CF and NF samples. The chemical process allows to obtain clean carbon fibers [29] and natural fibers. In addition to obtaining clean reinforced fibers the process allowed to obtain a reusable thermoplastic. The recovered polymer was melt mixed with short kenaf fibers. The compounds obtained were processed by injection molding and the resulting mechanical properties are reported in Table 4. The unmodified thermoplastic displayed a tensile strength of 55.43 MPa with a modulus of 2.21 GPa. This values are different than other values found recycling HP-RTM samples [30] but, the epoxy monomer used in this study is also different. The addition of 5 wt% and 10 wt% of kenaf resulted in the increase of both tensile strength and modulus. This result Fig. 3. Flexural testing results for composite panels.
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Fig. 4. Tukey analysis graphs for flexural data: a) flexural strength; b) flexural modulus.
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Fig. 6. Storage modulus values extracted from Fig. 5a at 30 C and 100 C.
Fig. 7. Loss factor values extracted from Fig. 5b at 25 C and at Tg.
Fig. 5. Dynamic mechanical curves for composites specimens: a) Storage modulus versus temperature; Tand versus temperature.
Table 3 Glass transition temperatures calculated from the tand peak at 1 Hz. Sample Code
Tg [ C]
NF CF FCF CFC
56.3 51.5 55.6 59.8
confirmed that the recovered thermoplastic is suitable for injection molding and that it can also be effectively reinforced with natural fibers for application in green design. The recovered thermoplastic was processed in a pilot line for producing filaments for FDM printers [41]. The thermoplastic was transformed in a filament with average diameter 1.75 mm ± 0.05 mm and then it was tested for FDM printing. The result of the FDM printing test are reported in Fig. 8. The printing test showed that the filament obtained for the recycled thermoplastic was easy to print and yield a good detail resolution. Thermal analysis reported before [29] confirmed the suitability of this thermoplastic for FDM [42].
Table 4 Tensile properties of recovered thermoplastic reinforced with short kenaf fibers (standard deviation between bracket). W% Kenaf
Tensile Strength (MPa)
Tensile Modulus (GPa)
0 5 10
55.43 (3.82) 58.57 (1.53) 58.87 (1.84)
2.21 (0.22) 2.60 (0.11) 2.84 (0.07)
4. Conclusions Hybrid flax/carbon fiber specimens were obtained using a biobased epoxy monomer and a cleavable ammine. The laminates were processed by resin infusion at room temperature using a cure inhibitor. The mechanical characterization confirmed the higher properties for the laminates with carbon fibers over those with natural fibers only. The trend was reversed for the loss factor and thus in terms of damping. The characterization of the two hybrid laminates revealed that the best tradeoff is obtained with all the carbon fibers placed in the external layers. In addition to that, recent works reported in the literature led to concluded that having on the external layer mineral fibers can be beneficial for durability too [20]. The cured laminates were also chemically recycled in mild acetic acid aqueous solutions obtaining clean fibers and a reusable
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Fig. 8. Samples manufactured by FDM with the filament obtained by the extrusion of the recycled thermoplastic.
thermoplastic. The recycled polymer obtained showed interesting mechanical properties when processed alone and even better properties if reinforced with kenaf fibers. In addition to injection molding processing, the recycled thermoplastic demonstrated its efficiency for FDM processing. Acknowledgments The authors thanks University of Catania for the support with the Grant NanoCult. Mr Davide Giordano is kindly acknowledged for his invaluable help to conduct the FDM printing tests. References [1] Fais C. Lightweight automotive design with HP-RTM. Reinf Plast 2011;55: 29e31. http://dx.doi.org/10.1016/S0034-3617(11)70142-4. [2] Lunn P. Cost-effective resin infusion. Reinf Plast 2009;53:38e9. http:// dx.doi.org/10.1016/S0034-3617(09)70047-5. [3] Marsh G. Resin film infusion - composites cost reducer. Reinf Plast 2002;46: 44e9. http://dx.doi.org/10.1016/S0034-3617(02)80124-2. [4] Williams C, Summerscales J, Grove S. Resin infusion under flexible tooling (RIFT): a review. Compos Part A Appl Sci Manuf 1996;27:517e24. http:// dx.doi.org/10.1016/1359-835X(96)00008-5. [5] Scalici T, Pitarresi G, Badagliacco D, Fiore V, Valenza A. Mechanical properties of basalt fiber reinforced composites manufactured with different vacuum assisted impregnation techniques. Compos Part B Eng 2016;104:35e43. http://dx.doi.org/10.1016/j.compositesb.2016.08.021. [6] Colangelo F, Russo P, Cimino F, Cioffi R, Farina I, Fraternali F, et al. Epoxy/glass fibres composites for civil applications: comparison between thermal and microwave crosslinking routes. Compos Part B Eng 2017. http://dx.doi.org/ 10.1016/j.compositesb.2017.06.003. [7] Yan L, Kasal B, Huang L. A review of recent research on the use of cellulosic fibres, their fibre fabric reinforced cementitious, geo-polymer and polymer composites in civil engineering. Compos Part B Eng 2016;92:94e132. http:// dx.doi.org/10.1016/j.compositesb.2016.02.002. [8] Dunne R, Desai D, Sadiku R, Jayaramudu J. A review of natural fibres, their sustainability and automotive applications. J Reinf Plast Compos 2016;35: 1041e50. http://dx.doi.org/10.1177/0731684416633898. [9] Faruk O, Bledzki AK, Fink HP, Sain M. Biocomposites reinforced with natural fibers: 2000-2010. Prog Polym Sci 2012;37:1552e96. http://dx.doi.org/ 10.1016/j.progpolymsci.2012.04.003.
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Hybrid composites manufactured by resin infusion with a fully recyclable bioepoxy resin G. Cicala a, *, E. Pergolizzi a, F. Piscopo a, D. Carbone b, G. Recca b a b
University of Catania, DICAR, Viale Andrea Doria 6, 95125 Catania, Italy CNR-ICTP, Via Paolo Gaifami 17, 95100 Catania, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2017 Received in revised form 18 June 2017 Accepted 28 August 2017 Available online 31 August 2017
Bioepoxy based monomers were formulated with a cure inhibitor and a cleavable amine to obtain a recyclable epoxy system suitable for resin infusion at room temperature. Hybrid flax/carbon fiber layup were used. Tensile, flexural and dynamo-mechanical properties for the composites were studied. The cured laminates were chemically recycled obtaining from the epoxy matrix a thermoplastic. The recycled was processed by fused deposition modelling (FDM) and injection molding after mixing with short kenaf fibers. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Flax Kenaf Recycling Resin infusion Epoxy Fused deposition modelling
1. Introduction The use of epoxy based composites is increasingly accepted in many fields. In the automotive sector, electric-mobility, CO2 emission limits, gasoline and energy prices are some of the driving factors guiding the lightweight automotive design. The requirement for lightweight structures, combined with the need for high volume production rates, pushed the development of mass production techniques like High Pressure Resin Transfer Molding (HPRTM) [1]. In the aerospace sector, liquid resin infusion of thermosets is increasingly accepted because of its low cost and suitability for producing of large structures [2e4]. Liquid resin infusion is well established in transportation and naval sectors too [4,5]. In the civil fields the use of epoxy composites is widely accepted too [6,7]. The widespread use of thermoset composites is raising environmental concerns about the recycling options and because of the use of petroleum based raw materials. To address the latter point, natural fibers as reinforcements [8e15] and bioepoxy resins are becoming a focal point of interest for industry [16,17]. In the last years a lot of attention has been drawn by the use of hybrid natural fiber layup to optimize composite's mechanical properties [18e20].
* Corresponding author. Tel.: þ39 095 7382760. E-mail address: [email protected] (G. Cicala). http://dx.doi.org/10.1016/j.compositesb.2017.08.015 1359-8368/© 2017 Elsevier Ltd. All rights reserved.
The use of hybrid fabric layup add complexity to the recycling operation if fiber recovery is desired because, in some cases, the high value of some natural fabrics oblige not to destroy them. The status of the recycling techniques for fiber reinforced composites was recently reviewed by Oliveux et al. [21] and many differences compared to traditional plastic recycling of solid waste can be observed [22]. Rybicka et al. [23] analyzed the technology readiness level for many composite recycling techniques: incineration and landfilling were classified as TRL 9 while, pyrolysis for carbon fiber and mechanical grinding for glass fiber applications resulted on a TRL 8. Pyrolysis for glass fiber and mechanical grinding for carbon fiber achieved TRL 7. Finally, fluidized bed pyrolysis and solvolysis process achieved a median TRL of 4. The main limitations of thermal and mechanical recycling processes are fiber's properties degradation and that matrices are destroyed or only partially recovered in useful forms. Chemical recycling is an interesting alternative approach because it allows to obtain clean reclaimed fibers and valuable monomers from the epoxy matrix [24]. Similar methods have been presented by several authors [25,26]. Researchers from Lamborghini showed their interest in the use of chemical recycled fibers [27]. However, most of the known chemical recycling approaches rely on the use of solvent mixtures which inhibit their application to natural fiber reinforced composites. A sustainable solution to
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overcome these limits is offered by the cleavable amines developed by Connora Technology. These amines allow to synthesize epoxy thermosets which can be recycled yielding thermoplastics and clean fibers in mild aqueous solutions at low temperatures (80e120 C) [28]. In a previous paper, we discussed the environmental benefits of this approach showing the avoided environmental impacts associated with the use of recyclable epoxy resins [29]. We also presented some data about the development of a recyclable epoxy formulation for HP-RTM with some preliminary characterization of the recycled thermoplastic [30]. In the present paper, we extended the use of the Connora recycling approach by developing a recyclable biobased epoxy formulation suitable for resin infusion. Hybrid flax/carbon fibers layup were used and the effect of the stacking sequence evaluated. The composites panels obtained were then recycled and the potential uses of the recycled thermoplastic addressed. 2. Experimental 2.1. Materials and method 2.1.1. Materials SuperSap epoxy monomers CLX(S) and the cure inhibitor INH by Entropy Resins were mixed with the Recyclamine® 301 by Connora Technologies. The inhibitor was added in the percentage of 25% to the epoxy monomer to have a pot life of 90 min at 25 C. The CLX(S) monomer is composed of epoxidized pine oils, bisphenol A/F type epoxy resin, benzyl alcohol, and proprietary reactive epoxy diluents. Similar resin systems are reported as a green system for resin transfer molding [31]. The Recyclamine® 301 is a cleavable polyamine ether patented by Connora Technology. The carbon fabric used was a twill fabric of T300 carbon fibers with 200 gsm (grams per square meter) areal weight purchased from Prochima, Italy. Flax fabrics (400 gsm, twill) were purchased by Composites Evolution (UK). Short kenaf fibers were supplied by Sachsenleinen, Germany. 2.1.2. Composite preparation Composite panels were prepared by resin infusion. Different stacking sequences were tested as reported in Table 1. The dry fabrics were stacked on a steel plate. During mold layup of the natural fibers fabrics no relevant fabric deformation was observed with this type of woven fabrics in contrast to what reported for non-woven fabrics [32]. An adhesive silicone tape was placed around the perimeter of the layered stack to provide a proper seal and a flexible vacuum bag was placed on top. An inlet tube and an outlet tube were placed inside the vacuum bag. The inlet tube was connected by a valve to a pot filled with unmodified epoxy resin while the outlet tube was connected to a vacuum pump. The vacuum was applied while the inlet valve was closed in order to compact the layers and to remove excess air. The premixed epoxy resin was vacuum infused into the stacked layers, which was maintained at 25 C under a constant vacuum (75 cm Hg). The laminates were kept at 25 C for 6 h before demolding them. 2.1.3. Composites recycling A sample (200 gr) of composite was treated with 3 L of acetic solution (25 vol % of acetic acid) at 80 C for 1.5 h. The mixture was then filtered and the fibers separated from the liquid phase. The reinforcing fibers were allowed to dry and weighed. The acetic solution was neutralized with a NaOH (pH ¼ 10) until a solid precipitate appeared. The mixture was cooled and filtered. The solid was washed in distilled water at about 40 C. The solid dissolved again. Few drops of NaOH solution were added and a white solid
precipitated from the solution, an epoxy thermoplastic polymer. 2.1.4. Recycled thermoplastic processing The recovered thermoplastic was blended with 5 wt% and 10 wt % of short kenaf fiber at 190 C for 10 min with a speed of 30 rpm using a batch mixer (Brabender 50 EHT) controlled by a Lab-Station. The thermoplastic and the kenaf fibers were dried at 50 C under vacuum for 48 h before mixing. Tensile specimens with dimensions according to ASTM D638 were fabricated from the blends using a 12 cc microinjection molder (DSM Xplorer) at 190 C melt temperature and 50 C mold temperature with injection and holding pressure of 16 bar. The specimens were allowed to cool down in the mold for 5 min before extraction. Thermoplastic filaments with 1.75 mm diameter were also produced from the recovered thermoplastic. Thermoplastic filaments were prepared in a single-screw extruder with screw diameter (D) of 20 mm and screw length of 25 D (model E 20 TH; Collins). Thermoplastic pellets were dried before use at 50 C under vacuum for 48 h; after that pellets were loaded in the extruder hood by using a volumetric feeder. The temperature pattern of the extruder was 55 Ce160 C-170 C-190 C-190 C from input to output zones, the screw speed was set at 30 rpm, and the melt pressure, checked during all of the extrusion process, was 40 bar. An extruder head with a circular die (diameter ¼ 3 mm) was used. The melted thermoplastic was drawn, cooling it with an air knife before collecting it on a rotating spool. The melt extruded filament, due to the drawing action of a rotating spool, varied in its diameter from 3 mm, at the exit of the extrusion head, to 1.75 ± 0.05 mm on the spool. The extruded filaments were processed by Fused Deposition Modelling (FDM) using a prototypal FDM machine Roboze one 400þ (Roboze, Bari, Italy). 2.2. Characterization 2.2.1. Mechanical testing Composite specimens were cut from the panel and tested in tensile and flexural mode accordingly to ASTM D3039 and ASTM D790. A universal testing machine Instron 5982 operating at a constant speed of 1 mm/min with a load cell of 100 kN was used for testing. Differences in mechanical results, for the composites samples, were statistically analyzed by one-way analysis of variance (ANOVA) using Minitab 17 software. To identify which groups were significantly different from other groups, means comparison was done using the Tukey's test with a 95% confidence level. Tensile properties of the injection molded specimens were measured by using an Instron 5982 universal testing machine, equipped with a load cell of 10 kN in accordance to ASTM D638 type 1. 2.2.2. Dynamic mechanical analysis (DMA) Composite specimens (mm 15 10 2) were tested by dynamic mechanical analysis (DMA). Storage modulus and Tand were measured using a dynamic mechanical analyzer (Tritech by Triton Ltd, UK). Experiments were carried out in single cantilever mode by
Table 1 Designation of the non-hybrid and hybrid laminate composites. Sample Code
Stacking sequence C: Carbon F: Flax
NF CF FCF CFC
FFFF CCCCCCCC FCCCCCCF CCCFFCCC
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Table 2 Data from tensile (st and Et) and flexural (sf and Ef) testing for non-hybrid and hybrid laminates. The standard deviations are given between brackets (Std). Sample Code
st
NF
82.20 (6.04) 518.91 (23.02) 309.69 (26.54) 300.76 (34.86)
CF FCF CFC
[MPa]
Et [GPa]
sf [MPa]
Ef [GPa]
9.97 (0.69)
77.48 (10.57)
6.47 (1.10)
23.71 (2.87) 16.31 (0.51) 25.56 (3.84)
193.14 (31.94) 90.20 (10.50) 213.70 (16.82)
31.20 (4.19) 6.26 (0.99) 35.24 (2.46)
heating the specimens from 25 C to 150 C at a constant heating rate of 2 C/min and 1 Hz frequency. 3. Results and discussion
Fig. 1. Tensile testing results for composite panels.
3.1. Mechanical testing All the results from mechanical testing are summarized in Table 2. The graph in Fig. 1 shows the effect of the stacking sequence on the tensile properties. The laminates with carbon fibers displayed the highest tensile properties (518.91 MPa, 23.71 GPa) while, the laminates reinforced with flax fibers showed the lowest tensile stress and modulus (82.20 MPa, 9.97 GPa). The hybrid laminates with natural fibers on the outer layers (FCF) showed tensile strength and modulus of 309.69 MPa and 16.31 GPa while, the hybrid laminates with natural fibers placed internally (CFC) showed tensile strength and modulus of 300.76 MPa and 25.56 GPa, respectively. The results of the Tukey analysis are summarized in Fig. 2. The tensile strength means (Fig. 2a) were all significantly different (p < 0.05) with the exception of the pairwise FCF-CFC (p ¼ 0.925). The tensile modulus means were not significantly different for the laminates CFC and FCF compared to the laminates CF (p-value of 0.876 and 0.047, respectively). If the ANOVA was carried out comparing only on the three samples with carbon fibers (CF, CFC, FCF) a p-value of 0.016 is obtained which means that at least one mean is similar with a statistical significance of 99%. The means for the samples NF and FCF, if compared directly, gives a p-value of 0.000 and thus are significantly different. Similar results were presented by other authors [33e35]. Zhang et al. [33] showed that the tensile modulus for all the hybrid composites were almost the same regardless to the stacking sequence. This finding can be ascribed to the fact that the tensile modulus refers to the elastic stage of the tensile stress strain curve and thus, the interfaces did not play a significant effect on the modulus. On the opposite, the tensile strength is affected by the stacking sequence because of the improved stress transfer efficiency on the hybrid interface between flax and carbon fibers. These conclusions are supported in literature by the fracture mode of natural fibers composites [36]. In a recent review, Pickering et al. [37] reported that the tensile strength and modulus for thermoset composites reinforced with natural fibers can span from a minimum of (80 MPa; 5 GPa) for multiaxial fabrics to a maximum of (150 MPa; 43 GPa) for unidirectional thermosets. Therefore, combining natural fibers with high performance carbon fibers, as it was experimented here, always outperform composites with natural fibers only. Fig. 3 displays the results from flexural testing. As expected the CF laminates outperformed the NF laminates. The stacking sequences affected significantly the flexural behavior. The FCF laminate showed lower properties than CFC laminate (p-value ¼ 0.000). Tukey analysis showed that the means are all significantly different
for the flexural strength (p < 0.05) and for the flexural modulus with the exception of the FCF-NF and CFC-CF pairwises (pvalue > 0.05) (Fig. 4). The observed flexural behavior depended on the fact that flexural strength and modulus are controlled by extreme layers of reinforcement [38]. From the mechanical point of view, FCF stacking sequence is not optimal because carbon fiber layers are placed internally where no longitudinal stresses or strains occurs.
3.2. Dynamic mechanical analysis The evolution of the storage modulus, loss modulus and tand with temperature is shown in Fig. 5. The storage modulus showed a single decay in the range of 40e80 C which correspond to the glass transition temperature (Tg) of the matrix. The glass transition temperatures are summarized in Table 3. From storage modulus curves the data below (30 C) and above (100 C) the glass transition temperature were extracted (Fig. 6). As expected, the storage modulus of the sample with natural fiber only (NF) was lower than CF. The hybrid samples (FCF and CFC) displayed an intermediate behavior between the sample with carbon fibers and flax fibers only. The sample with carbon fibers placed externally showed an higher storage modulus in all the temperature range confirming the trend observed from flexural testing. The loss factor (tan d) showed a different trend depending on the measurement temperature. At 30 C the NF sample displayed higher loss factor compared to CF while, at the glass transition temperature, the opposite trend was found (Fig. 7). The hybrid fabrics displayed an intermediate behavior. The same trend was reported by Duc et al. [39] when comparing glass to flax fibers. The influence of flax fibers on the damping of hybrid carbon/flax composites is relevant for applications. Recently, in the CARBIO project (http://carbioproject.com/technology/), focused on the development of novel car parts, the company Composite Evolution showed a layup configuration similar to the one reported here with 50et% carbon distributed on the external layers and 50 wt% of flax fibers concentrated in the middle. This layup allowed to reach an improvement in damping of 58% compared to full the carbon layup. Other relevant applications which benefit of the damping improvements were reported for sport equipments (i.e. http://www. lineo.eu/applications). In addition to that, some authors recently showed that natural fibers composites obtained from bioepoxy similar to those used here can perform better in terms of impact resistance [40].
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Fig. 2. Tukey analysis graphs for tensile data: a) tensile strength; b) tensile modulus.
3.3. Recycling of the laminates Samples cut from the cured laminates were treated as described in 2.1.3 to recover the fibers and the matrix. The recycling process was performed both on the CF and NF samples. The chemical process allows to obtain clean carbon fibers [29] and natural fibers. In addition to obtaining clean reinforced fibers the process allowed to obtain a reusable thermoplastic. The recovered polymer was melt mixed with short kenaf fibers. The compounds obtained were processed by injection molding and the resulting mechanical properties are reported in Table 4. The unmodified thermoplastic displayed a tensile strength of 55.43 MPa with a modulus of 2.21 GPa. This values are different than other values found recycling HP-RTM samples [30] but, the epoxy monomer used in this study is also different. The addition of 5 wt% and 10 wt% of kenaf resulted in the increase of both tensile strength and modulus. This result Fig. 3. Flexural testing results for composite panels.
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Fig. 4. Tukey analysis graphs for flexural data: a) flexural strength; b) flexural modulus.
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Fig. 6. Storage modulus values extracted from Fig. 5a at 30 C and 100 C.
Fig. 7. Loss factor values extracted from Fig. 5b at 25 C and at Tg.
Fig. 5. Dynamic mechanical curves for composites specimens: a) Storage modulus versus temperature; Tand versus temperature.
Table 3 Glass transition temperatures calculated from the tand peak at 1 Hz. Sample Code
Tg [ C]
NF CF FCF CFC
56.3 51.5 55.6 59.8
confirmed that the recovered thermoplastic is suitable for injection molding and that it can also be effectively reinforced with natural fibers for application in green design. The recovered thermoplastic was processed in a pilot line for producing filaments for FDM printers [41]. The thermoplastic was transformed in a filament with average diameter 1.75 mm ± 0.05 mm and then it was tested for FDM printing. The result of the FDM printing test are reported in Fig. 8. The printing test showed that the filament obtained for the recycled thermoplastic was easy to print and yield a good detail resolution. Thermal analysis reported before [29] confirmed the suitability of this thermoplastic for FDM [42].
Table 4 Tensile properties of recovered thermoplastic reinforced with short kenaf fibers (standard deviation between bracket). W% Kenaf
Tensile Strength (MPa)
Tensile Modulus (GPa)
0 5 10
55.43 (3.82) 58.57 (1.53) 58.87 (1.84)
2.21 (0.22) 2.60 (0.11) 2.84 (0.07)
4. Conclusions Hybrid flax/carbon fiber specimens were obtained using a biobased epoxy monomer and a cleavable ammine. The laminates were processed by resin infusion at room temperature using a cure inhibitor. The mechanical characterization confirmed the higher properties for the laminates with carbon fibers over those with natural fibers only. The trend was reversed for the loss factor and thus in terms of damping. The characterization of the two hybrid laminates revealed that the best tradeoff is obtained with all the carbon fibers placed in the external layers. In addition to that, recent works reported in the literature led to concluded that having on the external layer mineral fibers can be beneficial for durability too [20]. The cured laminates were also chemically recycled in mild acetic acid aqueous solutions obtaining clean fibers and a reusable
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Fig. 8. Samples manufactured by FDM with the filament obtained by the extrusion of the recycled thermoplastic.
thermoplastic. The recycled polymer obtained showed interesting mechanical properties when processed alone and even better properties if reinforced with kenaf fibers. In addition to injection molding processing, the recycled thermoplastic demonstrated its efficiency for FDM processing. Acknowledgments The authors thanks University of Catania for the support with the Grant NanoCult. Mr Davide Giordano is kindly acknowledged for his invaluable help to conduct the FDM printing tests. References [1] Fais C. Lightweight automotive design with HP-RTM. Reinf Plast 2011;55: 29e31. http://dx.doi.org/10.1016/S0034-3617(11)70142-4. [2] Lunn P. Cost-effective resin infusion. Reinf Plast 2009;53:38e9. http:// dx.doi.org/10.1016/S0034-3617(09)70047-5. [3] Marsh G. Resin film infusion - composites cost reducer. Reinf Plast 2002;46: 44e9. http://dx.doi.org/10.1016/S0034-3617(02)80124-2. [4] Williams C, Summerscales J, Grove S. Resin infusion under flexible tooling (RIFT): a review. Compos Part A Appl Sci Manuf 1996;27:517e24. http:// dx.doi.org/10.1016/1359-835X(96)00008-5. [5] Scalici T, Pitarresi G, Badagliacco D, Fiore V, Valenza A. Mechanical properties of basalt fiber reinforced composites manufactured with different vacuum assisted impregnation techniques. Compos Part B Eng 2016;104:35e43. http://dx.doi.org/10.1016/j.compositesb.2016.08.021. [6] Colangelo F, Russo P, Cimino F, Cioffi R, Farina I, Fraternali F, et al. Epoxy/glass fibres composites for civil applications: comparison between thermal and microwave crosslinking routes. Compos Part B Eng 2017. http://dx.doi.org/ 10.1016/j.compositesb.2017.06.003. [7] Yan L, Kasal B, Huang L. A review of recent research on the use of cellulosic fibres, their fibre fabric reinforced cementitious, geo-polymer and polymer composites in civil engineering. Compos Part B Eng 2016;92:94e132. http:// dx.doi.org/10.1016/j.compositesb.2016.02.002. [8] Dunne R, Desai D, Sadiku R, Jayaramudu J. A review of natural fibres, their sustainability and automotive applications. J Reinf Plast Compos 2016;35: 1041e50. http://dx.doi.org/10.1177/0731684416633898. [9] Faruk O, Bledzki AK, Fink HP, Sain M. Biocomposites reinforced with natural fibers: 2000-2010. Prog Polym Sci 2012;37:1552e96. http://dx.doi.org/ 10.1016/j.progpolymsci.2012.04.003.
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