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, Volume 11, Issue 1, pp 12–17 | Cite as

Single-component composites made from pure cellulose

  • Johanna M. Spörl
  • Frank Hermanutz
  • Michael R. Buchmeiser
Cover Story Innovative Composites
  • 233 Downloads

Single-component composites made from pure cellulose are sustainable, recyclable, and biodegradable. This enables them to overcome the recycling issues associated with conventional fiber-reinforced composites. The DITF Denkendorf have been looking into the material properties, thepotential and the challenges of this class of alternative materials.

Introduction

Most fiber-reinforced plastics based on glass, carbon or natural fibers are produced using petroleum-based polymer matrices. With a production volume of 2.3 million tons p.a. in Europe, glass-fiber reinforced plastics (GFRP) used in construction and structural parts account for the largest share [1]. These materials, however, preclude the possibility of proper recycling. Since there is currently no technically viable method of fully recycling GFRP end-of-life waste (currently around 300,000 tons p.a. [2]), GFRP waste is disposed of through pyrolysis of the polymer matrix, with the residual ash having to go to landfill. In contrast, an alternative process provides for the disposal of GFRP waste as an aggregate material for cement clinker following appropriate preparation and admixture [2]. The demand for recyclable and sustainable composite materials has grown significantly in recent years owing to increased environmental awareness and legal regulations. Cellulose fibers are used extensively in biocomposites and natural-fiber reinforced polymers (NFRP) thanks to their good mechanical properties and widespread availability, information box on page 16. One example is NFRPs with a polyactide matrix that nevertheless only have low thermal resistance. Owing to different polarities in these systems, adhesion at the interface between a hydrophobic polymer matrix and hydrophilic (natural) fibers is frequently very poor. The low mechanical strength, rigidity, and impact resistance of composite materials are the consequence, while the potential of cellulose as a reinforcing component is not fully exploited. High material thicknesses or bonding agents such as maleic anhydride in polypropylene (PP) have to be used in order to achieve the appropriate mechanical characteristics in the composite. However, these composite materials also lack a satisfactory recycling concept.

Single-component polymer composite materials overcome the problems of weak fiber-matrix bonding.

The mechanical properties of the recycled composites were identical to those of the original composite.

On the one hand, single-component polymer composites overcome the issues of weak fiber matrix bonding, allowing the forces acting on the component to be effectively transferred from the matrix to the fibers. On the other hand, as homogeneous composite materials, they offer the prospect of particularly straightforward recycling, as the fibers and matrix do not need to be separated from each other in order to obtain fully discrete materials.

Composite Materials Made from Pure Cellulose

Laboratory prototypes of all-cellulose composites (ACC) actually exhibit mechanical properties that are superior to those of conventional biocomposites [3]. In addition to discontinuous natural fibers, high-strength regenerated cellulose fibers — for example from the viscose process — which are available as filament yarn, are ideal as a reinforcing component. Studies using these reinforcing fibers in the cellulose composite material matrix demonstrated very good mechanical properties, depending on the processing parameters [4, 5]. For example, tensile strengths of over 140 MPa were measured with a unidirectional layer arrangement, while 75 MPa was measured in an alternating layer structure. The flexural modulus is around 7 GPa, and impact resistance according to Charpy over 70 kJ/m2, meaning that it is higher than usually seen in NFRPs. Scanning electron microscope images of cross sections of the composite material reveal that the fibers are very well embedded in the cellulose matrix, Figure 1. The information box on page 17 describes the manufacture of these composite materials.
FIGURE 1

Scanning electron microscope images of cross sections of all-cellulose composites (© DITF)

Disposal of End-of-Life Waste

From an ecological point of view, ACCs are significantly more sustainable than traditional NFRPs, since they are biodegradable[6]. However, besides biodegradability, the aim should be the complete and proper disposal of composite materials at the end of their useful life. All-cellulose composite materials can be fully recycled, as they can be fully dissolved in a corresponding solvent — preferably IL. The recycled material can subsequently form the matrix or the fiber component of a new composite material. In sample tests, the all-cellulose composites were ground up and redissolved in IL before being used as a matrix precurser [4]. In order to investigate the influence of recycling on material properties as well as on processing behavior, the composite materials were recycled three times, thus producing four generations in total. To this end, the degree of polymerization and the distribution of molecular weight of the recycled cellulose were first determined along with the flow behavior of the matrix precursor, which consisted of 100 % recycled material. The mechanical properties of the recycled composite materials were then also determined. Both the degree of polymerization and molecular weight distribution as well as the flow properties of the matrix precursors, which was characterized based on non-dynamic viscosity, initially showed a difference between the original cellulose used for the matrix (1st generation) and the recycled cellulose (2nd generation), which could be explained by a slight reduction in molecular weight as a result of dissolving in IL and subsequent regeneration. This effect did not recur between the individual recycling tests (generations 2 through 4). However, the most important characteristic for successful recycling is the mechanical properties profile of the composite materials. In order to test this, a five-layer structure with alternating layer arrangement at 90° and identical production parameters was selected for all composite materials. The mechanical properties of the recycled composites were identical to those of the original composite. It was thus possible to successfully demonstrate recycling across four generations. Figure 2 shows the first three generations of the recycling experiments.
FIGURE 2

Composite materials from recycling tests (© DITF)

The surface needs to be coated in order to protect the structures from water absorption.

A further possibility of disposal which is not expanded on here, is the softening of the composite in hot steam and then subsequent hot-press molding.

Subsequent Processing of the Composites

The composite materials can be formed into appropriate profiles using hot pressing before they are dried, Figure 3. The same applies to composite materials that have already been dried after treatment in hot steam, provided the composite material is not coated, or the surface protected in some other way. The current concept for manufacturing all-cellulose composites therefore provides for the production of fiber-matrix composites using IL technology to make semifinished products with low water content at a central location in order to concentrate the recovery of IL for renewed use in one place. It is then intended to create solutions for the relevant application based on product platforms in the form of appropriate semifinished products that can be molded using conventional tools such as hot presses.
FIGURE 3

Shaping an ACC semifinished product into a profile (© DITF)

Challenging is the ability of cellulose to absorb water from the surroundings. The surface of composite material needs to be coated in order to protect relevant structures from unintentionally absorbing water, and undergoing deformation and expansion. Here, special attention must be paid to the edges of the composite material. At the same time, appropriate surface treatment can increase scratch resistance and lower surface roughness. Initial tests to coat the surface with cellulose esters, polyurethanes and alkyd resins showed positive effects in this area, Figure 4. From an optical perspective, the aim is to achieve a smooth, flawless surface without surface defects such as roughness or rippling. Besides hydrophobic treatment of the surface, possible solutions include (bio-based) polyester films, (partly bio-based) polyurethane, PVC, modified polyamide and biopolymers such as polylactide. These can be conveniently removed at the end of the product’s useful life before the composite material is disposed of through reshaping, recycling or composting. Furthermore, varnishes — for example with biological binding agents such as cellulose acetate or traditional binding agents such as acrylic or epoxy resin — can offer a satisfactory barrier to protect the composite material. These can be removed mechanically (sandblasting) or chemically (solvents) at the end of the product’s life time. Initial tests on the weathering stability of the composites proved positive. Finally the relatively high thermal stability of up to 250 °C should be mentioned.
FIGURE 4

Surface (left) and polished section (right) of a coated all-cellulose composite (© DITF)

Potential Applications and Outlook

Focus is currently directed towards transportation and the automotive industry as one of the largest fields of application for NFRPs and GFRPs. All-cellulose composites constitute an interesting material, in particular for automotive interior trims like door panels, dashboards, or rear window shelves. The major advantages of the new cellulose-based composite material over current state-of-the-art alternatives are deemed to be the thin-walled components that can be realized thanks to good fiber-matrix adhesion with the same or improved spectrum of properties, the disposal strategy for end-of-life waste, and behavior with regard to emissions and fogging that are expected to be judged positively. Furthermore, it may be possible to use the material in other industries in applications with great added value where the sustainability of materials is a special argument in marketing — for example for sports, leisure and lifestyle products such as surfboards, goods for camping and outdoor, hard shell cases, furniture production, or plant containers. Another large-volume example are special products in the construction industry such as cladding panels or in timber construction. Since the technology is so complex, a period of between three and four years must be assumed before it can be implemented on an industrial scale. Technical implementation can be achieved earlier in particular for applications with small production batches where the sustainability of the materials constitutes a key feature. The latest investigations are primarily concentrated on upscaling the manufacture of composite materials. Special attention is being given to improving the washing step to transform it from a static into a dynamic, continuous process.

Cellulose Fibers as Reinforcing Component in Bio-composites

Cellulose is the most frequently occurring biopolymer on earth, and with an annual production of around 75 billion tons through biosynthesis constitutes an almost inexhaustible raw material. Less than 0.3 % of this volume is used by the chemical industry as raw material, with the pulp and paper industry processing a major share. Cellulose serves as a building and structural material for plants and demonstrates excellent intrinsic properties with regard to its specific strength and stiffness, Figure 5.
FIGURE 5

Specific tensile properties of cellulose (© DITF)

Cellulose fibers are used in natural-fiber reinforced polymers (NFRP) primarily in the form of bast fibers, flax, hemp, kenaf, or recycled cotton. So far, regenerated cellulose fibers, for example from the viscose process, have found little use as a reinforcing component. The matrices are petroleum-based thermosetting plastics or thermoplastics (epoxides, unsaturated polyesters, polyurethanes (PU), acrylates, and polyolefins) [7]. Furthermore, recent years also saw the development of biocomposites with biological matrices such as polylactides, biopolyethylene, biopolyesters and (partly) bio-based epoxides, or PUs as well as resins based on palm or linseed oil. NFRP applications include interior trims in automobile trunks or doors [8, 9].

Producing Single-component Composites from Pure Cellulose

Pure cellulose is infusible and hardly soluble. There are, however, various dissolution methods, of which the relatively new process based on ionic liquids (IL) constitutes an especially environmentally compatible approach that makes sparing use of resources. The ionic liquids used are nontoxic salts that are liquid at room temperature. Unlike conventional processes, they enable cellulose to be dissolved directly with efficient use of materials. What is more, they can be recovered using existing technologies and reused in the process.

In the production of all-cellulose composites, cellulose is processed from solution, the so-called matrix precursor, in order to form the composite material matrix. The cellulose matrix is then regenerated in a subsequent coagulation step — for example in water. Various production routes for ACCs with derivatizing and non-derivatizing solvent systems have been described in literature [3], with the solvent system of lithium chloride in dimethylacetamide (DMAc/LiCl) being used most frequently. In addition, N-methylmorpholine oxide (NMMO), similar to the Lyocell process, and a more environmentally compatible solvent system based on an aqueous NaOH/urea solution were also used. The production processes described are two-step, i.e. the cellulose for the fiber and matrix come from different raw materials or process steps, or one-step, if the matrix is formed by the partial dissolution of the fiber component. However, the corresponding one-step process destroys the high-quality reinforcing fiber through partial dissolution. Instead of being embedded in a defined matrix surrounding, the reinforcing fibers are, if anything, stuck together. This partially diminishes the reinforcing effect of the fiber component, and the relatively expensive fiber is degraded to an unaligned solid. In contrast, the preferred two-step process allows the fiber volume content in the composite material to be set over a broader area thanks to the concentration of the cellulose in the matrix precursor or its layer thickness. The use of ILs as solvent for the matrix precursor in the two-step process therefore permits a defined matrix environment to be created for the reinforcing fiber. Figure 6 shows a schematic diagram of the production process. The regenerated cellulose fiber is only partially dissolved at the surface, meaning that its mechanical properties remain largely unaffected, and also guaranteeing good bonding between fiber and matrix. The IL is removed in the subsequent coagulation step through rinsing and can be recycled. The composite material is finally consolidated through hot pressing. A detailed study into the process parameters, layer structure, concentration, resting time and temperature has been published elsewhere [4].
FIGURE 6

Schematic diagram of the production process for all-cellulose composites with the aid of ionic liquids (© DITF)

Notes

Thanks

The authors would like to thank the Baden-Württemberg Ministry for Economy, Labor and Housing for funding research initiative 7-4332.62-DITF/73, as well as Cordenka and BASF for providing the high-strength viscose fibers and the IL.

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Copyright information

© Springer Fachmedien Wiesbaden 2018

Authors and Affiliations

  • Johanna M. Spörl
    • 1
  • Frank Hermanutz
    • 1
  • Michael R. Buchmeiser
    • 1
  1. 1.German Institutes of Textile and Fiber Research (DITF)DenkendorfGermany

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