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ZE SCIENCE & TECHNOLOGY | DAS PAPIER R IE DAS P A P LL CHEMIN G viewed peer re Rein for cem ents Non-Woven and Paper Based Epoxy Composites By Henri Kröling, Johann Fleckenstein, Narmin Nubbo, Angelika Endres, Dr. Frank Miletzky & Prof. Dr.-Ing. Samuel Schabel Composite materials are increasingly used for various applications. The reasons for the use of composite materials are their high stiffness and strength values at a low weight. A composite material consists of at least two materials, in the present case one material is paper and the other material is the polymeric thermoset matrix. The task of the fibres is to take tensile and flexural loads, while the matrix is keeping the fibres in their place and transfers the loads between the fibres. Usually artificial fibres like glass or carbon fibres are used if the highest strength and stiffness values are required. However, composites with natural fibres (NF) are used more often for example in car parts, recently. Among these natural fibres hemp and flax are the most common ones in Europe. Despite that the availability of wood pulp fibres is much higher and the strength values are comparable to other NFs 1, 2, wood pulp fibres are used only in a few materials in combination with a thermoplastic matrix (for example UPM ForMi, Södra DuraPulp, Mondi FIBROMER). Besides the use in thermoplastic matrixes, paper as reinforcement for thermoset is known to achieve high strength and stiffness values (for example 3-8). It was studied as reinforcement material already in the 1940’s 3, 4 with a focus on an optimum matrix content and a minimum void content in the later composite. It was found that the composite strength is increasing with a decreasing resin content. If the resin content was lower than 30 %, the tensile strength dropped because of the void volume. The authors concluded that a thoroughly impregnation with enough resin is crucial. In newer studies a number of different fibres (e. g. pulp grades) have been examined as reinforcement 5-9. During mechanical pulping fibres are damaged and consequently achieve lower The previously conducted studies show that paper can reinforce a thermoset very efficiently, but unfortunately there is no direct comparison to other natural fibre mats that are already in industrial use. The aim of this paper is to show that paper composites can compete with and even outperform other natural fibre composites. Materials & Methods Fibre mats For the first trials, two different commercially available papers were used: Speciality paper made from chemical long fibre pulp and teabag paper made from chemical pulp from wood and abaca. In addition to that, RapidKöthen lab sheets were made from a eucalyptus pulp. Table 1 comprises an overview over the used commercially available fibre mats, paper and the lab sheets. With a grammage of 50 g/m2 respectively 150 g/m2 the spun laces are in the range of paper and board products, while the apparent density is comparatively low. The teabag paper has an outstanding low grammage and apparent density. The flax spun lace is designed for the application in composites. Typical fibres are 25 mm to 50 mm long. Typical fibre lengths in the viscose spun lace are between 15 mm and 40 mm. Grammage in g/m2 Thickness in μm Apparent density in g/m3 Orientation Viscose spun lace 50 500 0.10 Yes Flax spun lace 150 800 0.19 Yes Teabag paper 13 52.6 0.24 Yes Speciality paper 60 115 0.52 Yes Lab sheets 160 277 0.58 No Sample Tab. 1: Overview over the different fibre mats 2 strength than chemical pulp fibres 6. The published data show also that hardwood reinforced composites are on the same strength and YM level as soft wood reinforced composites 7. Papermaking processes have been used in a very limited range to optimize paper properties for such applications. A few oriented papers have been used as reinforcements resulting in a composite anisotropy 5,7. Refining has been recently proven to be a powerful process to increase the tensile strength of paper-thermoset composites 10. 6-7/2014 www.ipwonline.de CHEMIN P G view peer re ed R IE DAS P A ZE LL SCIENCE & TECHNOLOGY | DAS PAPIER Laminating The laminating process is illustrated in Figure 1. The thickness of the composites should be around 2 mm to match the requirements of the standard DIN EN ISO 527-4 “Test conditions for isotropic and orthotropic fibre-reinforced plastic composites”. In order to achieve this thickness the number of sheets was adjusted to a thickness of around 1.8 mm so that the later composite had a thickness of roughly 2 mm. The number of sheets varied from 2 for the flax spun lace to 42 for the teabag paper. Fig. 1: laminating process For the laminating, a cold curing system (Epoxy resin EP, Larit L-285, hardener Larit 287 – blau, both Lange + Ritter GmbH) was used. Both components were mixed and applied to a single fibre mat with a brush. After that, the next fibre mat was laid upon the already impregnated sheet and pressed onto it with a roller. The new sheet was then also impregnated with the epoxy resin as described before. This procedure was repeated until all the fibre mats were impregnated. The impregnated stack of fibre mats was then put into a press, where it was pressed for 5 h at 50 °C and a pressing force of 50 kN to cure the resin. After the curing, the samples were tempered at 60 °C for 10 h and then cut into the specimen for the tensile test with the dimensions of 150 mm x 20 mm. Testing The composite testing was conducted according to DIN EN ISO 527-4. All specimens were plain and provide no force transmission elements. The material was characterised by its YM, tensile strength and fibre volume fraction Vƒ. The fibre volume fraction was calculated with following equation: (1) Where mA is the basis weight and n is the number of corresponding fibre mats used for the composite fabrication. The fibre wall density ρƒ was estimated to be 1.5 g/ cm³. d is the thickness of the composite. 1 Reinforcement 2 Fibre Volume Fraction in % Speciality paper 36.8 Tea bag paper 18.4 Viscose spun lace 13.6 Flax spun lace 15.6 Lab sheet 39.1 Tab. 2: Fibre volume fractions of the different composites The tensile strength of the composites is shown in Figure 2. If the fibre mat has an orientation, the test is conducted in the direction of the fibre orientation. Surprisingly, the lab sheet reinforced composite achieved a tensile strength almost as high as the oriented speciality paper, followed by the tea bag paper reinforced composite. The tensile strengths of the spun lace reinforced composites are lower than these paper composites. The speciality paper and the lab sheets have a high fibre volume fraction that contributes directly to the composite strength. On the other hand the short fibre length in all of the papers should reduce the composite strength 8. However, the actual single fibre strength, the shear strength of the matrix-fibre interface and fibre orientation are influencing the tensile strength 8 of the composite and are not known for these fibre samples. The most important outcome of these results is that the comparatively short fibre length of paper making fibres compared to other natural fibres is no disadvantage if it comes to strength. Result & Discussion 120 100 Tensile strength in MPa The differences in the apparent density are reflected in the fibre volume fractions of the composites (Table 2). The speciality paper and the lab sheets achieve a fibre volume fraction of almost 40 %, while low density teabag paper and spun laces achieve a fibre volume fraction of below 20 %. A high fibre volume fraction is beneficial because the mechanical properties of the composite are directly linked to the fibre volume fraction. Besides the better performance, the pulp fibres are a renewable resource and therefore a high fibre volume fraction is equivalent with a high share of renewable resources that can make such a material desirable from an ecological point of view. 80 60 40 20 0 Speciality paper MD Tea bag MD Viscose MD Flax MD Labsheet Fig. 2: Tensile strengths 6-7/2014 3 ($"# ($"# ($"# R IE ZE (B"# SCIENCE & TECHNOLOGY | DAS PAPIER (B"# (B"# P DAS P A C:D*:9-0+#902+?70>#-?#3@.# CHEMIN G (("# (("# (("# ):JK::L!+.2/1K::L# ):JK::L!+.2/1K::L# ):JK::L!+.2/1K::L# ):JK::L!+.2/1K::L# ):JK::L# ):JK::L# It was clearly shown that paper yields better com):JK::L# 160 ):JK::L# (""# (""# ):JK::L!/.0+K::L# ):JK::L!/.0+K::L# posite properties than commercially available natural (""# ):JK::L!/.0+K::L# 150 ):JK::L!/.0+K::L# M.2LK::L!+.2/1K::L# fibre reinforcements. Furthermore the different paper M.2LK::L!+.2/1K::L# 140 M.2LK::L!+.2/1K::L# M.2LK::L!+.2/1K::L# A"# A"# 130 reinforcements contribute differently to theM.2LK::L# composM.2LK::L# A"# M.2LK::L# M.2LK::L#120 M.2LK::L!/.0+K::L# ite properties. In a recent publication 10, the authors M.2LK::L!/.0+K::L# M.2LK::L!/.0+K::L# 110 M.2LK::L!/.0+K::L# '"# showed that the reinforcement effect of paper can be '"# '"# "# $# '# refining (Figure %# 3). With these&# "# '#with $# ("#$# %# &#processed '# &# increased "# %# &# %# LL viewed peer re pulps, the advantage of paper as reinforcement for therE("""#@;F#2+G:/HI:?9# E("""#@;F#2+G:/HI:?9# ""#@;F#2+G:/HI:?9# mosets is even increasing. E("""#@;F#2+G:/HI:?9# Modelled strength in MPa C:D*:9-0+#902+?70>#-?#3@.# C:D*:9-0+#902+?70>#-?#3@.# C:D*:9-0+#902+?70>#-?#3@.# (%"# (%"# (%"# 100 90 80 70 60 50 40 '# ("# ("# ("# Own Data Du et al. [8] @BU: Fig. Effect of refining (in revolutions) on n PFI revolutions) on 3: composite [10] @BU:@BU: Fig. 3: Effect ofstrength refining (in PFI PFI on composite composite strength strength [10] [10] Fig. 3: Effect of refining (inrevolutions) PFIthe revolutions) Furthermore it is possible to predict compositeon composite strength [10] 50 60 70 80 90 100 110 120 130 140 Furthermore is predict the composite tensile ""40cc of based tensileit paper based very accu-strength ofto paper based ct the composite tensile strength "c of Furthermore itstrength is possible possible to predict thecomposites composite tensile strength of paper paper based Composite strength in MPa Furthermore it is possible to predict the composite tensile strength "c of properties paper based rately without any fitting constant from the mechanical ut any fitting constant from the mechanical properties of the composites very accurately without any fitting constant from the mechanical of composites very accurately without any fitting constant from the mechanical properties of the the the composites very accurately without any fitting constant from the mechanical properties Fig. 4: Modelling of the composite tensile of strength 10 properties of the paper and the thermoset (Figure 4, ig. 4, Equation 2) [10]. paper and the thermoset (@BU: Fig. 4, Equation 2) [10]. paperpaper and the thermoset (@BU: Fig. 4, Equation 2) [10]. and the10thermoset (@BU: Fig. 4, Equation 2) [10]. 150 160 Equation 2) . "m ""c=ZSTI*c =ZSTI*cf+(1-V +(1-Vf)(# )(#c/# /#m)*" )*"m (2) (2) (2) (2) Youngs modulus in MPa c f f c m m 10000 (2) "c=ZSTI*c f+(1-Vf)(#c/#m)*"m 9000 ndex of the paperZSTI as zero span tensile index of the paper With: 8000 With: ZSTI as zero span tensile index of the paper With: ZSTI as zero span tensile index of the paper of the fibres in the With: ZSTI as zero span tensile index ofinthe paper ccff composite as weight concentration of the fibres the composite 7000 as weight concentration of the fibres in the composite as weight concentration of the fibres in the composite n at break cf as weight concentration of the fibres in the composite ##cc as composite elongation at break 6000 as composite elongation at break as composite elongation at break break #matrix elongation at break c as composite # as elongation at break 5000 a s matrix elongation at break m #m as #matrix elongation at break th elongation at break m as matrix 4000 matrix tensile strength ""mm aas s matrix tensile strength as matrix tensile strength "m as matrix tensile strength 3000 e to achieve high fibre volume fractions with a paper These data show, that it is possible to achieve high fibre 2000 These data that it is possible to volume fractions with paper These data show, show, thatZero itthat is Span possible to achieve achieve high fibre fibre volume fractions with aawith paper bre strength, represented byshow, the Tensile Index ishigh These data it is possible to achieve high fibre volume fractions a paper volume fractions with a paper reinforcement and also, 1000 reinforcement and also, that the fibre strength, represented by the Zero Span Tensile s. reinforcement and also, that the fibre strength, represented by the Zero Span Tensile Index Index is is that the fibreand strength, represented the Zerorepresented Span reinforcement also, that the fibrebystrength, by0 the Zero Span Tensile Index is fully in such composites. fully exploited exploited in such composites. Speciality Tea bag MD Viscose MD Flax MD Tensile Index is fully exploited in such composites. fully exploited in such composites. hmen paper MD Grafik bitte aus übernehmen Grafik The bitteYM ausofOriginal Original übernehmen the different composites are shown in Fig- bitte Original übernehmen omposite tensile Grafik strength [10]aus ure 5. Here again the specialty paper has the highest Labsheet Fig. 5: Young’s moduli of the different fibre mat composites @BU: Fig. 4: Modelling of the composite tensile strength [10] values. The of the lab composite sheet is lesstensile than the half of[10] during the spun lace process. The comparatively low YM @BU: Fig. 4:Fig. Modelling of the strength es are shown in@BU: @BU: Fig. 5.YM Here again the specialty 4: Modelling of the composite tensile strength [10] the specialty paper and in the range of the natural fibre of the lab sheet can be partly explained by the random YM of theThe lab YM sheetofisthe lessdifferent than the half of the specialty composites are shown in @BU: Fig. 5. Here again the The YM ofYM theof different composites are shown in @BU: Fig. in-plain 5.Fig. Here again the specialty specialty reinforcements. orientation. On the other hand the high fibre The the different composites are shown in @BU: 5. Here again the specialty ral fibre reinforcements. paper has the highest values. The YM of the lab sheet is less than the half of the specialty paper has the highest values. The YM of the lab sheet is less than the fraction half of the specialty volume should more than offset the orientapaper has the highest values. The YM ofGPa the1lab sheet istion. less The thanYM the of half ofspecialty the specialty paper and in the range of the natural fibre reinforcements. As the YM for flax fibres with about 70 is much the paper in CD (6694 MPa, paper and inand the range ofthat theof natural fibrepulp reinforcements. out 70 GPa [1] is much higher than softwood paper in the range the natural fibre reinforcements. higher than that of softwood pulp fibres (17 GPa 2 to 45 not displayed) is higher than the YM of the isotropic lab one would expectGPa that7),the composite YM is also higher, one would expect that the composite YMmuch is alsohigher sheet at that a similar fibre volume This indicates As YM for flax fibres with about 70 GPa [1] is than of softwood pulp As the theAs YM for flax fibres with about 70 GPa [1] is much higher than that ofthat softwood pulp fraction. ion. An explanation for the low YM of the flax spun lace the YM for flax fibres with about 70 GPa [1] is much higher than of softwood pulp higher, even at a lower fibre volume fraction. An explathat the YM of the fibres in the speciality paper is higher (17 GPa [2] to 45 GPa [7]), one would expect that YM fibres (17lower GPa [2] to [2] 45 GPa [7]), paper. one would expectexpect that the the composite composite YM is is also alsoishigher, higher, e fractionfibres is much than theYM specialty Also, fibres (17 GPa to 45 [7]), one would the composite also higher, nation for theinlow of GPa the flax spun lace composites isthatthan that of theYM Eucalyptus fibres or the production even aa lower fibre fraction. An for low YM of the flax spun lace even at at lower fibre volume volume fraction. An explanation explanation for the the method low YM ofthe thepaper flax spun lace thatat the fibre volume fraction fraction. is much lower than in the for of YM influences YM of the composeven a lower fibre volume An explanation the low of the flax spunthe lace composites is that the fibre volume fraction is much lower than in the specialty paper. Also, composites is that the fibre volume fraction is much lower than in the specialty paper. Also, specialty paper. Also, the flax fibres might be oriented ite (Besides the orientation). Unfortunately there are no composites is that the fibre volume fraction is much lower than in the specialty paper. Also, in thickness direction or the fibres might get damaged Conclusion Composite strength in MPa 140 130 120 110 Softwood-earlywood 100 Softwood-latewood Softwood Hardwood-earlywood 90 Hardwood Hardwood-latewood 80 0 2 4 6 *1000 PFI revolutions 8 10 Fig. 3: Effect of refining (in PFI revolutions) on composite strength 10 4 6-7/2014 values for the YM of hard wood fibres published and only very few for some selected soft wood fibres. www.ipwonline.de The main goal of this investigation was to compare paper reinforcements with a commercially available flax fibre spun lace reinforcement and a viscose spun lace reinforcement. Both paper and natural fibre spun laces reinforce the resin significantly. It was shown that paper can outperform both spun laces in terms of tensile strength and Young’s modulus. Even paper composites made from untreated eucalyptus pulp achieve higher tensile strengths than the spun laces. Furthermore the tensile strength of paper composites can be strongly increased by refining and fibre orientation. The fibre orientation of the paper leads to a corresponding anisotropy in the composite properties. CHEMIN P G view peer re ed R IE DAS P A ZE LL The Young’s moduli of paper composites are not necessarily higher than that of natural fibre spun lace reinforced composites. The reason why the Young’s moduli of some papers composites are lower than that of the spun lace reinforced composites is yet unclear and needs further investigations. If only tensile strength and Young’s moduli are regarded paper-thermoset composites match most of the glass fibre reinforced Sheet Moulding Compounds (SMC) parts presented in the brochure “Design for Success”, issued by the European Alliance for SMC/BMC 11. The presented parts cover many areas from the automotive industry, transport to building & construction materials to name a few. Of course, there are other properties that are important for specific applications as well that have not been measured here. Furthermore, the paper-thermoset composites are produced in a small scale laboratory process far away from an industrial fabrication process. Nonetheless this comparison shows the good and interesting potential of such materials. ■ SCIENCE & TECHNOLOGY | DAS PAPIER Literature [1] H. Schürmann, Konstruieren mit Faser-Kunststoff-Verbunden. Berlin/Heidelberg: Springer-Verlag, 2007. [2] L. Groom, S. Shaler, and L. Mott, “Mechanical Properties of Individual Southern Pine Fibers. Part III: Global Relationships Between Fiber Properties and Fiber Location Within an Individual Tree,” Wood and Fiber Science, vol. 34, no. 2, pp. 238-250, 2002. [3] H. Cox and K. Pepper, “Paper-Base Plastics. Part I. The Preparation of Phenolic Laminated Boards,” J Soc Chem Ind, vol. 63, no. 11, pp. 150-154, 1944. [4] K. Pepper and F. Barwell, “Paper-Base Plastics. Part II. Production at Low Pressure,” Journal of the Society of Chemical Industry, vol. 63, no. 11, pp. 321-329, 1944. [5] E. K. Gamstedt, E. Sjöholm, C. Neagu, F. Berthold, and M. Lindström, “Effects of fibre bleaching and earlywoodlatewood fractions on tensile properties of wood-fibre reinforced vinyl ester,” in Proceedings of the 23rd Risø International Symposium on Materials Science, 2002, pp. 185-196. [6] E. Sjöholm, F. Berthold, E. K. Gamstedt, C. Neagu, and M. Lindström, “The use of conventional pulped wood fibres as reinforcement in composites,” in Proceedings of the 23rd Risø International Symposium on Materials Science, 2002, pp. 307–314. [7] R. C. Neagu, E. K. Gamstedt, and F. Berthold, “Stiffness Contribution of Various Wood Fibers to Composite Materials,” Journal of Composite Materials, vol. 40, no. 8, pp. 663699, Jul. 2005. [8] Y. Du, N. Yan, and M. Kortschot, “Investigation of unsaturated polyester composites reinforced by aspen highyield pulp fibers,” Polymer Composites, vol. 33, no. 2, pp. 169-177, 2012. [9] Y. Du, T. Wu, N. Yan, M. Kortschot, and R. Farnood, “Pulp fiber-reinforced thermoset polymer composites: effects of the pulp fibers and polymer,” Composites Part B: Engineering, vol. 48, no. 1, pp. 10-17, Dec. 2013. [10] H. Kroeling, S. Mehlhase, J. Fleckenstein, N. Nubbo, A. Endres, S. Schabel, and F. Miletzky, “Engineering and Modeling of Tensile Strength of Paper-Thermoset Composites,” in 19th International Conference on Composite Materials, 2013, no. 1, pp. 5280-5292. [11] N. N., “Design for Success.” [Online]. Available: http:// www.smc-alliance.com/downloads/. MSc. Henri Kröling Chair of Paper Technology and Mechanical Process Engineering PMV TU Darmstadt Darmstadt, Germany M. Eng. Dipl.-Ing. (FH) Johanna Fleckenstein Fraunhofer Institute for Structural Durability and System Reliability LBF Darmstadt, Germany kroeling@papier. tu-darmstadt.de johanna.fleckenstein@ lbf.fraunhofer.de Dipl.-Wirtsch.-Ing. (FH) Narmin Nubbo Fraunhofer Institute for Structural Durability and System Reliability LBF Dipl.-Ing. Chemie (FH) Angelika Endres Papiertechnische Stiftung PTS München, Germany Dr rer. nat. Frank Miletzky Papiertechnische Stiftung PTS München Prof. Dr-Ing. Samuel Schabel Chair of Paper Technology and Mechanical Process Engineering PMV TU Darmstadt narmin.nubbo@ lbf.fraunhofer.de angelika.endres@ ptspaper.de frank.miletzky@ ptspaper.de schabel@papier. tu-darmstadt.de 6-7/2014 5