Reinforcements Non-Woven and Paper Based Epoxy … & Methods Fibre mats For the first trials, two different commercially available papers were used: Speciality paper made from chemical

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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 pro-cess to increase the tensile strength of paper-thermoset composites 10.

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 compos-ites.

Materials & MethodsFibre matsFor 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, Rapid-Köthen lab sheets were made from a eucalyptus pulp. Table 1 comprises an overview over the used commer-cially 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 com-posites. Typical fibres are 25 mm to 50 mm long. Typical fibre lengths in the viscose spun lace are between 15 mm and 40 mm.

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 thermo-plastic matrixes, paper as reinforcement for thermoset is known to achieve high strength and stiffness values (for example 3-8). It was studied as reinforcement mate-rial 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 exam-ined as reinforcement 5-9. During mechanical pulping fibres are damaged and consequently achieve lower

Reinforcements

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 ma-terials are their high stiffness and strength values at a low weight.

Sample 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

Tab. 1: Overview over the different fibre mats

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The tensile strength of the composites is shown in Fig-ure 2. If the fibre mat has an orientation, the test is conducted in the direction of the fibre orientation. Sur-prisingly, the lab sheet reinforced composite achieved a tensile strength almost as high as the oriented speciality paper, followed by the tea bag paper reinforced com-posite. 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 vol-ume 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 influ-encing 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.

LaminatingThe 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.

For the laminating, a cold curing system (Epoxy resin EP, Larit L-285, hardener Larit 287 – blau, both Lange + Rit-ter 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 impreg-nated 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 speci-men for the tensile test with the dimensions of 150 mm x 20 mm. TestingThe composite testing was conducted according to DIN EN ISO 527-4. All specimens were plain and provide no force transmission elements. The mate-rial 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 fabrica-tion. The fibre wall density ρƒ was estimated to be 1.5 g/cm³. d is the thickness of the composite.

Result & DiscussionThe 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 tea-bag 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 ecologi-cal point of view.

Fig. 1: laminating process

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

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Tea bag MD Viscose MD Flax MD Labsheet

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during the spun lace process. The comparatively low YM of the lab sheet can be partly explained by the random in-plain orientation. On the other hand the high fibre volume fraction should more than offset the orienta-tion. The YM of the specialty paper in CD (6694 MPa, not displayed) is higher than the YM of the isotropic lab sheet at a similar fibre volume fraction. This indicates that the YM of the fibres in the speciality paper is higher than that of the Eucalyptus fibres or the production method of the paper influences the YM of the compos-ite (Besides the orientation). Unfortunately there are no values for the YM of hard wood fibres published and only very few for some selected soft wood fibres.

ConclusionThe 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 ani-sotropy in the composite properties.

It was clearly shown that paper yields better com-posite properties than commercially available natural fibre reinforcements. Furthermore the different paper reinforcements contribute differently to the compos-ite properties. In a recent publication 10, the authors showed that the reinforcement effect of paper can be increased with refining (Figure 3). With these processed pulps, the advantage of paper as reinforcement for ther-mosets is even increasing.

Furthermore it is possible to predict the composite tensile strength

It was clearly shown that paper yields better composite properties than commercially available natural fibre reinforcements. Furthermore the different paper reinforcements contribute differently to the composite properties. In a recent publication [10], the authors showed that the reinforcement effect of paper can be increased with refining (@BU: Fig. 3). With these processed pulps, the advantage of paper as reinforcement for thermosets is even increasing.

@BU: Fig. 3: Effect of refining (in PFI revolutions) on composite strength [10]

Furthermore it is possible to predict the composite tensile strength "c of paper based composites very accurately without any fitting constant from the mechanical properties of the paper and the thermoset (@BU: Fig. 4, Equation 2) [10]. "c=ZSTI*cf+(1-Vf)(#c/#m)*"m (2)

With: ZSTI as zero span tensile index of the paper cf as weight concentration of the fibres in the composite #c as composite elongation at break #m as matrix elongation at break "m as matrix tensile strength These data show, that it is possible to achieve high fibre volume fractions with a paper reinforcement and also, that the fibre strength, represented by the Zero Span Tensile Index is fully exploited in such composites. Grafik bitte aus Original übernehmen @BU: Fig. 4: Modelling of the composite tensile strength [10]

The YM of the different composites are shown in @BU: Fig. 5. Here again the specialty paper has the highest values. The YM of the lab sheet is less than the half of the specialty paper and in the range of the natural fibre reinforcements. As the YM for flax fibres with about 70 GPa [1] is much higher than that of softwood pulp fibres (17 GPa [2] to 45 GPa [7]), one would expect that the composite YM is also higher, even at a lower fibre volume fraction. An explanation for the low YM of the flax spun lace composites is that the fibre volume fraction is much lower than in the specialty paper. Also,

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of paper based composites very accu-rately without any fitting constant from the mechanical properties of the paper and the thermoset (Figure 4, Equation 2) 10.

(2)

It was clearly shown that paper yields better composite properties than commercially available natural fibre reinforcements. Furthermore the different paper reinforcements contribute differently to the composite properties. In a recent publication [10], the authors showed that the reinforcement effect of paper can be increased with refining (@BU: Fig. 3). With these processed pulps, the advantage of paper as reinforcement for thermosets is even increasing.

@BU: Fig. 3: Effect of refining (in PFI revolutions) on composite strength [10]

Furthermore it is possible to predict the composite tensile strength "c of paper based composites very accurately without any fitting constant from the mechanical properties of the paper and the thermoset (@BU: Fig. 4, Equation 2) [10]. "c=ZSTI*cf+(1-Vf)(#c/#m)*"m (2)

With: ZSTI as zero span tensile index of the paper cf as weight concentration of the fibres in the composite #c as composite elongation at break #m as matrix elongation at break "m as matrix tensile strength These data show, that it is possible to achieve high fibre volume fractions with a paper reinforcement and also, that the fibre strength, represented by the Zero Span Tensile Index is fully exploited in such composites. Grafik bitte aus Original übernehmen @BU: Fig. 4: Modelling of the composite tensile strength [10]

The YM of the different composites are shown in @BU: Fig. 5. Here again the specialty paper has the highest values. The YM of the lab sheet is less than the half of the specialty paper and in the range of the natural fibre reinforcements. As the YM for flax fibres with about 70 GPa [1] is much higher than that of softwood pulp fibres (17 GPa [2] to 45 GPa [7]), one would expect that the composite YM is also higher, even at a lower fibre volume fraction. An explanation for the low YM of the flax spun lace composites is that the fibre volume fraction is much lower than in the specialty paper. Also,

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With: ZSTI as zero span tensile index of the paper

It was clearly shown that paper yields better composite properties than commercially available natural fibre reinforcements. Furthermore the different paper reinforcements contribute differently to the composite properties. In a recent publication [10], the authors showed that the reinforcement effect of paper can be increased with refining (@BU: Fig. 3). With these processed pulps, the advantage of paper as reinforcement for thermosets is even increasing.

@BU: Fig. 3: Effect of refining (in PFI revolutions) on composite strength [10]

Furthermore it is possible to predict the composite tensile strength "c of paper based composites very accurately without any fitting constant from the mechanical properties of the paper and the thermoset (@BU: Fig. 4, Equation 2) [10]. "c=ZSTI*cf+(1-Vf)(#c/#m)*"m (2)

With: ZSTI as zero span tensile index of the paper cf as weight concentration of the fibres in the composite #c as composite elongation at break #m as matrix elongation at break "m as matrix tensile strength These data show, that it is possible to achieve high fibre volume fractions with a paper reinforcement and also, that the fibre strength, represented by the Zero Span Tensile Index is fully exploited in such composites. Grafik bitte aus Original übernehmen @BU: Fig. 4: Modelling of the composite tensile strength [10]

The YM of the different composites are shown in @BU: Fig. 5. Here again the specialty paper has the highest values. The YM of the lab sheet is less than the half of the specialty paper and in the range of the natural fibre reinforcements. As the YM for flax fibres with about 70 GPa [1] is much higher than that of softwood pulp fibres (17 GPa [2] to 45 GPa [7]), one would expect that the composite YM is also higher, even at a lower fibre volume fraction. An explanation for the low YM of the flax spun lace composites is that the fibre volume fraction is much lower than in the specialty paper. Also,

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as weight concentration of the fibres in the composite

It was clearly shown that paper yields better composite properties than commercially available natural fibre reinforcements. Furthermore the different paper reinforcements contribute differently to the composite properties. In a recent publication [10], the authors showed that the reinforcement effect of paper can be increased with refining (@BU: Fig. 3). With these processed pulps, the advantage of paper as reinforcement for thermosets is even increasing.

@BU: Fig. 3: Effect of refining (in PFI revolutions) on composite strength [10]

Furthermore it is possible to predict the composite tensile strength "c of paper based composites very accurately without any fitting constant from the mechanical properties of the paper and the thermoset (@BU: Fig. 4, Equation 2) [10]. "c=ZSTI*cf+(1-Vf)(#c/#m)*"m (2)

With: ZSTI as zero span tensile index of the paper cf as weight concentration of the fibres in the composite #c as composite elongation at break #m as matrix elongation at break "m as matrix tensile strength These data show, that it is possible to achieve high fibre volume fractions with a paper reinforcement and also, that the fibre strength, represented by the Zero Span Tensile Index is fully exploited in such composites. Grafik bitte aus Original übernehmen @BU: Fig. 4: Modelling of the composite tensile strength [10]

The YM of the different composites are shown in @BU: Fig. 5. Here again the specialty paper has the highest values. The YM of the lab sheet is less than the half of the specialty paper and in the range of the natural fibre reinforcements. As the YM for flax fibres with about 70 GPa [1] is much higher than that of softwood pulp fibres (17 GPa [2] to 45 GPa [7]), one would expect that the composite YM is also higher, even at a lower fibre volume fraction. An explanation for the low YM of the flax spun lace composites is that the fibre volume fraction is much lower than in the specialty paper. Also,

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as composite elongation at break

It was clearly shown that paper yields better composite properties than commercially available natural fibre reinforcements. Furthermore the different paper reinforcements contribute differently to the composite properties. In a recent publication [10], the authors showed that the reinforcement effect of paper can be increased with refining (@BU: Fig. 3). With these processed pulps, the advantage of paper as reinforcement for thermosets is even increasing.

@BU: Fig. 3: Effect of refining (in PFI revolutions) on composite strength [10]

Furthermore it is possible to predict the composite tensile strength "c of paper based composites very accurately without any fitting constant from the mechanical properties of the paper and the thermoset (@BU: Fig. 4, Equation 2) [10]. "c=ZSTI*cf+(1-Vf)(#c/#m)*"m (2)

With: ZSTI as zero span tensile index of the paper cf as weight concentration of the fibres in the composite #c as composite elongation at break #m as matrix elongation at break "m as matrix tensile strength These data show, that it is possible to achieve high fibre volume fractions with a paper reinforcement and also, that the fibre strength, represented by the Zero Span Tensile Index is fully exploited in such composites. Grafik bitte aus Original übernehmen @BU: Fig. 4: Modelling of the composite tensile strength [10]

The YM of the different composites are shown in @BU: Fig. 5. Here again the specialty paper has the highest values. The YM of the lab sheet is less than the half of the specialty paper and in the range of the natural fibre reinforcements. As the YM for flax fibres with about 70 GPa [1] is much higher than that of softwood pulp fibres (17 GPa [2] to 45 GPa [7]), one would expect that the composite YM is also higher, even at a lower fibre volume fraction. An explanation for the low YM of the flax spun lace composites is that the fibre volume fraction is much lower than in the specialty paper. Also,

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as matrix elongation at break

It was clearly shown that paper yields better composite properties than commercially available natural fibre reinforcements. Furthermore the different paper reinforcements contribute differently to the composite properties. In a recent publication [10], the authors showed that the reinforcement effect of paper can be increased with refining (@BU: Fig. 3). With these processed pulps, the advantage of paper as reinforcement for thermosets is even increasing.

@BU: Fig. 3: Effect of refining (in PFI revolutions) on composite strength [10]

Furthermore it is possible to predict the composite tensile strength "c of paper based composites very accurately without any fitting constant from the mechanical properties of the paper and the thermoset (@BU: Fig. 4, Equation 2) [10]. "c=ZSTI*cf+(1-Vf)(#c/#m)*"m (2)

With: ZSTI as zero span tensile index of the paper cf as weight concentration of the fibres in the composite #c as composite elongation at break #m as matrix elongation at break "m as matrix tensile strength These data show, that it is possible to achieve high fibre volume fractions with a paper reinforcement and also, that the fibre strength, represented by the Zero Span Tensile Index is fully exploited in such composites. Grafik bitte aus Original übernehmen @BU: Fig. 4: Modelling of the composite tensile strength [10]

The YM of the different composites are shown in @BU: Fig. 5. Here again the specialty paper has the highest values. The YM of the lab sheet is less than the half of the specialty paper and in the range of the natural fibre reinforcements. As the YM for flax fibres with about 70 GPa [1] is much higher than that of softwood pulp fibres (17 GPa [2] to 45 GPa [7]), one would expect that the composite YM is also higher, even at a lower fibre volume fraction. An explanation for the low YM of the flax spun lace composites is that the fibre volume fraction is much lower than in the specialty paper. Also,

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as matrix tensile strength

These data show, that it is possible to achieve high fibre volume fractions with a paper reinforcement and also, that the fibre strength, represented by the Zero Span Tensile Index is fully exploited in such composites.

The YM of the different composites are shown in Fig-ure 5. Here again the specialty paper has the highest values. The YM of the lab sheet is less than the half of the specialty paper and in the range of the natural fibre reinforcements.

As the YM for flax fibres with about 70 GPa 1 is much higher than that of softwood pulp fibres (17 GPa 2 to 45 GPa 7), one would expect that the composite YM is also higher, even at a lower fibre volume fraction. An expla-nation for the low YM of the flax spun lace composites is that the fibre volume fraction is much lower than in the specialty paper. Also, the flax fibres might be oriented in thickness direction or the fibres might get damaged

80

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0 2 4 6 8 10

Com

posi

te st

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Softwood-earlywood Softwood Softwood-latewood Hardwood-earlywood Hardwood Hardwood-latewood

Fig. 3: Effect of refining (in PFI revolutions) on composite

strength 10

40 50 60 70 80 90

100 110 120 130 140 150 160

40 50 60 70 80 90 100 110 120 130 140 150 160

Mod

elle

d st

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Composite strength in MPa

Own Data

Du et al. [8]

Fig. 4: Modelling of the composite tensile strength 10

0

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Speciality paper MD

Tea bag MD Viscose MD Flax MD Labsheet

Youn

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Fig. 5: Young’s moduli of the different fibre mat composites

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Literature[1] H. Schürmann, Konstruieren mit Faser-Kunststoff-Verbun-den. Berlin/Heidelberg: Springer-Verlag, 2007.[2] L. Groom, S. Shaler, and L. Mott, “Mechanical Properties of Individual Southern Pine Fibers. Part III: Global Relation-ships 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. Pro-duction 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 earlywood-latewood fractions on tensile properties of wood-fibre reinforced vinyl ester,” in Proceedings of the 23rd Risø Inter-national 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 Materi-als,” Journal of Composite Materials, vol. 40, no. 8, pp. 663-699, Jul. 2005.[8] Y. Du, N. Yan, and M. Kortschot, “Investigation of un-saturated polyester composites reinforced by aspen high-yield 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: Engineer-ing, 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 Mod-eling 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/.

Dr rer. nat. Frank MiletzkyPapiertechnische Stiftung PTS München

frank.miletzky@ ptspaper.de

M. Eng. Dipl.-Ing. (FH) Johanna FleckensteinFraunhofer Institute for Structural Durability and System Reliability LBF Darmstadt, Germany

johanna.fleckenstein@ lbf.fraunhofer.de

Prof. Dr-Ing. Samuel SchabelChair of Paper Technology and Mechanical Process Engineering PMV TU Darmstadt

schabel@papier. tu-darmstadt.de

Dipl.-Wirtsch.-Ing. (FH) Narmin NubboFraunhofer Institute for Structural Durability and System Reliability LBF

narmin.nubbo@ lbf.fraunhofer.de

Dipl.-Ing. Chemie (FH) Angelika EndresPapiertechnische Stiftung PTS München, Germany

angelika.endres@ ptspaper.de

MSc. Henri KrölingChair of Paper Technology and Mechanical Process Engineering PMV TU Darmstadt Darmstadt, Germany

kroeling@papier. tu-darmstadt.de

The Young’s moduli of paper composites are not neces-sarily higher than that of natural fibre spun lace rein-forced 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 inter-esting potential of such materials. ■