Abstract
Foam technology enables the preparation of new fiber-based materials with reduced density and improved mechanical performances. By utilizing multi-scale structural features of the formed fiber network, it is possible to enhance the elasticity of lightweight cellulose materials under compressive loads. Sufficient strength is achieved by optimally combining fibers and fines of different length-scales. Elasticity is improved by adding polymers that accumulate at fiber joints, which help the network structure to recover after compression. This concept was demonstrated using natural rubber as the polymer additive. For a model network of viscose fibers and wood fines, the immediate elastic recovery after 70% compression varied from 60% to 80% from the initial thickness. This was followed by creep recovery, which reached 86% to 88% recovery within a few seconds in cross-linked samples. After 18 h, the creep recovery in those samples was almost complete at up to 97%. A similar improvement was seen for low-density materials formed with chemi-thermomechanical fibers. The formed structure and elastic properties were sensitive not only to the raw materials, but also to the elastomer stiffness and foam properties. The improved strain recovery makes the developed cellulose materials suitable for various applications, such as padding for furniture, panels, mattresses, and insulation materials.
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Improving Compression Recovery of Foam-formed Fiber Materials
Sara Paunonen,* Oleg Timofeev, Katariina Torvinen, Tuomas Turpeinen, and Jukka A. Ketoja
Foam technology enables the preparation of new fiber-based materials with reduced density and improved mechanical performances. By utilizing multi-scale structural features of the formed fiber network, it is possible to enhance the elasticity of lightweight cellulose materials under compressive loads. Sufficient strength is achieved by optimally combining fibers and fines of different length-scales. Elasticity is improved by adding polymers that accumulate at fiber joints, which help the network structure to recover after compression. This concept was demonstrated using natural rubber as the polymer additive. For a model network of viscose fibers and wood fines, the immediate elastic recovery after 70% compression varied from 60% to 80% from the initial thickness. This was followed by creep recovery, which reached 86% to 88% recovery within a few seconds in cross-linked samples. After 18 h, the creep recovery in those samples was almost complete at up to 97%. A similar improvement was seen for low-density materials formed with chemi-thermomechanical fibers. The formed structure and elastic properties were sensitive not only to the raw materials, but also to the elastomer stiffness and foam properties. The improved strain recovery makes the developed cellulose materials suitable for various applications, such as padding for furniture, panels, mattresses, and insulation materials.
Keywords: Fibrous materials; Cellulose; Elasticity; Compression; Recovery; Foam forming
Contact information: VTT Technical Research Centre of Finland Ltd, Solutions for Natural Resources and Environment, Tekniikankatu 1, Tampere, P.O. Box 1300, FI-33101 Tampere, Finland;
* Corresponding author: sara.paunonen@vtt.fi
INTRODUCTION
Foam forming is a technique that uses wet foam instead of water to deposit fibers and bond them into networks. In simple terms, it combines the ability of water to act as a strong cellulose binder with the ability of bubbles to tailor a porous structure and prevent fiber flocking. Lately, the technology has been intensely studied in the preparation of sheet structures (Radvan and Gatward 1972; Lehmonen et al. 2013) and bulk materials (Madani et al. 2014; Alimadadi and Uesaka 2016) made from wood fibers, especially for filtering (Heydarifard et al. 2016) and insulating applications (Poranen et al. 2013; Pöhler et al. 2016). Forming and subsequent water removal and drying methods have a profound effect on the sheet structure (Timofeev et al. 2016; Haffner et al. 2017). Foam-formed fiber networks can have a much lower density (Madani et al.2014), improved and tailored pore size distribution (Al-Qararah et al.2015a), and more diverse fiber orientation (Alimadadi and Uesaka 2016) compared with similar water-formed materials.
Foam-formed three-dimensional wood fiber networks (3DFNs) have structural similarities with sheet materials, such as paper. The load-bearing elements in both are the same: fibers, joints, and a connected network. Paper exhibits three types of deformation: linear and non-linear elastic strain, visco-elastic time-dependent recoverable creep, and plastic non-recoverable deformation (Brezinski 1956; Alava and Niskanen 2006). Alimadadi and Uesaka (2016) reported the same phases for foam-formed 3DFNs made from pure thermomechanical (TMP) reject pulp.
However, the fiber network structure of 3DFNs is very different from that of sheets. The number of bonds a fiber makes with neighboring fibers at low densities is different from that for thin sheets. 3DFNs also retain their integrity more easily than sheets (Alimadadi and Uesaka 2016). Moreover, 3DFNs show a unique and much higher deformation recovery compared with paper and some nonwoven sheets. This is because of the wider 3D fiber orientation distribution and the large proportion of very large open pores in the fiber network. Individual fibers can bend without local geometric restrictions; thus, they avoid stress buildup that drive plastic deformation. In denser networks of natural fibers, only a small proportion of the fibers contributes to the load bearing (Kulachenko and Uesaka 2012), which leads to a highly concentrated stress buildup and plastic deformation. In addition to the number of bonds (determined by the material density) (Borodulina et al. 2012; Borodulina et al. 2016), the stress and strain distribution in a material is affected by several other factors, such as the fiber length and stiffness, and bond compliance and strength.
The goal of this study was to find methods to improve the elastic and viscoelastic creep recovery of foam-formed 3DFNs and extend their use to application areas where highly flexible and elastic materials are preferred. Examples of such applications are padding for furniture, panels, shoes, pillows, mattresses, and insulation materials. Ideally, these materials would exhibit a sponge-like spring-back behavior after repeated compression cycles. The hypothesis of the authors was that this can be achieved by optimally combining: 1) a low-density structure containing large, open pores obtained by foam forming, 2) a multi-scale fiber network with structural elements of different length-scales (fibers and fines), and 3) an elastic polymer component that accumulates at the fiber joints during drying and helps the network structure to recover after deformation. Figure 1 summarizes the postulated behavior of a 3DFN from foamed furnish to dry material. When water is removed during drying, the suspended polymer component accumulates at the fiber joints, which causes an increase in the network recovery after deformation.
Fig. 1. Schematic illustration of the hypothesis. Left: Wet foam with air bubbles (circles), fibers (bars), and an added polymer component (short lines) during foam forming; Right: Accumulation of the polymer at fiber joints during drying
The foam properties have a considerable effect on the properties of a material. The material density can be reduced by increasing the air content of the wet foam (Madani et al. 2014), which also improves the foam stability. Additionally, the mean pore size can be controlled by the bubble size and fiber stiffness (Al-Qararah et al. 2015a). Fine particles and polymers are carried in bubble vertices and interfaces. The surface tension drives the free water towards fiber crossings during drying, which also causes the accumulation of the elastic polymer at these regions. In the fully dried material, the polymer helps the network recover after deformation.
In this study, the hypothetical behavior was studied via two model systems in which the above ingredients were present. In the first system, non-bonding flexible viscose fibers were combined with a new type of lignin-containing wood fines material (Saharinen et al.2016). This allowed separate control of the long-fiber density and bonding ability. The second system was based on a more typical stiff fiber material, called chemi-thermomechanical pulp (CTMP).
Natural rubber was used as a model polymer, as it is known to bind to the surface of cellulose (Flink et al. 1988). Binding can be further improved through chemical surface modification (Kato et al. 2015). Natural rubber has been previously used as a matrix polymer in cellulose fiber-reinforced composites (Kato et al. 2015; Zhou et al.2015). The approach of this study was similar to that of earlier studies, but the target material densities were much lower. The density of vulcanized rubber (920 kg/m3 to 1200 kg/m3) (McPherson 1927) is lower than that of cellulose crystals (1500 kg/m3) (Alava and Niskanen 2006), which makes natural rubber an appealing polymer for lightweight fiber materials. Moreover, aqueous foam provides a natural carrier for hydrophobic and non-polar rubber polymers to adhere to the air bubbles in the foam during structure forming. Strong binding of the polymers with lignocellulosic fibers and fines was achieved in the final structure without any chemical modification.
The objective of this study was to assess the effects of the furnish composition, polymer content and crosslinking, and foam properties on the compression recovery of foam-formed 3DFNs.
EXPERIMENTAL
Materials
Two types of long-fiber material were used to prepare the samples (Fig. 2). The staple viscose fiber material (Danufil, Kelheim Fibres GmbH, Kelheim, Germany) had a fiber length of 6 mm and a linear mass density of 1.7 dtex (mass in g/10000 m). The fiber cross-section was roughly elliptical with a major axis diameter of 17.5 µm. The material had a moisture content of 50%. The spruce CTMP, which had an average (length-weighted) fiber length of 1.6 mm and a freeness of 570 mL, was obtained from a Finnish paper mill. The CTMP included a large amount (approx. 30 w/w%) of sub-micron fine particles, i.e. fines.
A new type of lignocellulosic fines material prepared by grinding wood (Saharinen et al. 2016), called v-fines (Fig. 2), was used at a consistency of 2.5% with the viscose fibers. The special V-patterned surface of the grinding stone enables a 90% conversion of the wood directly into fines in one process step. Consequently, in addition to cellulose, the fines also contain hemicellulose and lignin. The average (length-weighted) length of the v-fines was 0.18 mm, and 71% of the particles were less than 0.2 mm in length.
The aqueous natural rubber latex (Liquid Latex Direct, Burton-upon-Stather, UK) had a total solids content of 60.3% (of which 0.38% was ammonia) and a pH of 10.6. Natural rubber is a polymeric hydrocarbon produced by some tropical plants (Kohjiya and Ikeda 2014) and is collected from trees as an aqueous latex. Vulcanization cross-links polymers and leads to an elastic and less-sticky rubber (Kohjiya and Ikeda 2014; Kato et al. 2015). In this study, the pre-vulcanized latex contained a dye to aid in the analysis of the macroscopic latex distribution in the samples after their production. The preliminary vulcanization was further enhanced by adding sulfur (Sigma-Aldrich, St. Louis, MO, USA), which was either in colloidal or powder form, and employing a 10-min heat treatment at 155 °C.
The samples were prepared using two different types of foaming agent: 10 w/w% solution of anionic sodium dodecyl sulfate (SDS; C12H25SO4Na) (Sigma-Aldrich) with a purity of 90% and a non-ionic surfactant (TWEEN 20, Sigma-Aldrich).
