Abstract
The growing demand for sustainable materials is driving interest in cellulose fibres as eco-friendly alternatives to traditional insulation and cushioning materials, including expanded polystyrene (EPS), Extruded Polystyrene (XPS), and mineral wool, which pose environmental challenges. Foam forming has been extensively studied as a method for producing lightweight structures from cellulose fibres, but air-laying—a common nonwoven method—has been less explored. This study examines how wood fibre type and fibrillation level affect the structure, insulation, and strength of foam-formed and air-laid materials. A novel binding method is introduced for air-laying, involving post-laying water spraying to enhance bonding. Foam-formed materials had an average pore size of 300 to 600 μm with a wide distribution, including millimetre-scale pores; while air-laid materials had a smaller, more uniform pore size of 100 to 150 μm. Mechanical refining increased the pore size in foam-forming. Thermal conductivity decreased with decreasing fibre length, pore size, and increasing tortuosity of the fibre phase. The highest compression stress was achieved with refined chemi-thermomechanical pulp (CTMP), and the best recovery with unrefined bleached softwood kraft pulp (BSKP) and mixtures of acacia and BSKP. The findings suggest that mixing short hardwood fibres with longer softwood fibres in foam-forming could enhance performance in thermal insulation applications.
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Effect of Fibre Properties on the Structure, Strength, and Thermal Conductivity of Foam-Formed and Air-Laid Cellulosic Lightweight Fibrous Materials
Antti Koponen 
,* Titta Kiiskinen , and Elina Pääkkönen
The growing demand for sustainable materials is driving interest in cellulose fibres as eco-friendly alternatives to traditional insulation and cushioning materials, including expanded polystyrene (EPS), Extruded Polystyrene (XPS), and mineral wool, which pose environmental challenges. Foam forming has been extensively studied as a method for producing lightweight structures from cellulose fibres, but air-laying—a common nonwoven method—has been less explored. This study examines how wood fibre type and fibrillation level affect the structure, insulation, and strength of foam-formed and air-laid materials. A novel binding method is introduced for air-laying, involving post-laying water spraying to enhance bonding. Foam-formed materials had an average pore size of 300 to 600 μm with a wide distribution, including millimetre-scale pores; while air-laid materials had a smaller, more uniform pore size of 100 to 150 μm. Mechanical refining increased the pore size in foam-forming. Thermal conductivity decreased with decreasing fibre length, pore size, and increasing tortuosity of the fibre phase. The highest compression stress was achieved with refined chemi-thermomechanical pulp (CTMP), and the best recovery with unrefined bleached softwood kraft pulp (BSKP) and mixtures of acacia and BSKP. The findings suggest that mixing short hardwood fibres with longer softwood fibres in foam-forming could enhance performance in thermal insulation applications.
DOI: 10.15376/biores.20.4.10922-10958
Keywords: Cellulose fibres; Refining; Foam forming; Air-laying; X-ray microtomography; 3D structure; Pore size; Thermal conductivity; Strength properties
Contact information: VTT Technical Research Centre of Finland Ltd, Koivurannantie 1, 40400 Jyväskylä, Finland; *Corresponding author: antti.koponen@vtt.fi
Graphical Abstract
INTRODUCTION
There is an increasing demand for sustainable alternatives to fossil-based materials across various applications and industries. In the construction industry, traditional insulation materials, such as expanded polystyrene (EPS), extruded polystyrene (XPS) and glass or mineral wool, are difficult to recycle and cause a significant environmental impact. The packaging industry is using increasingly cardboard-based solutions; however, cushioning is dominantly solved with various fossil-based materials, such as EPS, expanded polyethylene (EPE), and polyurethane. Replacing these materials with sustainable alternatives is challenging due to their excellent performance and properties: they are lightweight, durable, resistant to moisture and impact, and provide excellent thermal insulation.
Cellulose is an abundant, biodegradable, and renewable material, making it a natural candidate for sustainable insulation and cushioning materials (Hurtado et al. 2016; Fereira et al. 2021). While alternative sources for cellulose, such as agricultural side streams, exist and are intensively studied, wood fibres are still the most important source of cellulose for industrial applications. Wood fibres have well-established industrial processing methods and value chains. Depending on the application, chemical processing can produce relatively uniform, longer fibres, while mechanical processing results in shorter fibres with a broader length distribution. Additionally, refining can further modify fibre length and enhance fibrillation on fibre surfaces, influencing their bonding properties and overall performance.
One advantage of cellulose fibres is their ability to bond together by means of hydrogen bonds; no binders or glues are required to create strong and cohesive structures. This natural bonding ability enhances the sustainability of cellulose-based materials by eliminating the need for synthetic adhesives. This principle is widely utilised in the pulp and paper industry, where fibres are processed as an aqueous fibre suspension, and the fibres bond strongly during the drying phase. Dry fibres cannot create these bonds on their own, as hydrogen bonding relies on the presence of moisture.
For insulation and cushioning applications, lightweight, porous structures are required. In this case, wood fibres cannot be processed as aqueous suspensions, as the structures tend to collapse during dewatering and drying, even without a pressing phase (Zhu et al. 2022). More complex methods, such as freeze-drying (Ngyuen et al. 2014) or supercritical drying (Lopes et al. 2017), exist, but they are hardly practical for industrial-scale production.
Foam-forming (Hjelt et al. 2022; Nechita and Năstac 2022) is an alternative method for processing, e.g., wood fibres. In the foam-forming process, fibres, water, and a foaming agent are mixed under high shear, creating fibre foam with an air content of approximately 60 to 70%. Once the fibre foam has been generated, it is placed onto a mesh screen, where part of the water drains away due to gravity, or by using a moderate vacuum, to prevent the collapse of the structure. Foam forming has some advantages over water forming: less water is needed during processing, bubbles prevent early fibre flocculation, which often leads to uniform structures, and it enables the use of materials that would otherwise settle in the water, forming non-uniform structures. Additionally, the bubbles support the fibre-foam structure during drying, resulting in the formation of lightweight fibrous materials with a density as low as 10 kg/m3. This makes foam forming well-suited for making insulation (Pöhler 2017; Lecourt et al. 2018; Lohtander et al. 2022) and cushioning materials (Ottenhall et al. 2018; Pääkkönen et. al 2024). Other applications are also promising, such as sound insulation (Pöhler 2016; Nechita and Năstac 2018; Seciureanu et al. 2023), internal decoration (Härkäsalmi 2017; Siljander 2019) and filtration (Jahangiri et al. 2014).
In commercial cellulose fibre-based insulation products, the thermal conductivity typically ranges from 38 to 42 mW/m·K (Berge and Johansson 2012; Pöhler et al. 2017). For comparison, the thermal conductivity of air is 26 mW/m·K at 20 °C. In the review (Dominguez-Muñoz 2010), the thermal conductivity of insulation materials made of cellulosic fibres ranged between 35 and 45 mW/m·K in a set of 282 samples having a density between 17 and 115 kg/m3. The best-performing cellulose fibre-based materials exhibit thermal conductivities comparable to those of glass wool, XPS, and EPS.
Several papers have reported thermal conductivities for lightweight foam-formed cellulosic fibre networks between 28 to 35 mW/m·K (Ngyen 2014; Seantier et al. 2016; Pöhler et al. 2017; Zhu et al. 2022; El Hajam et al. 2024; Hou et al. 2025). The lowest values have usually been achieved through the use of additives, such as microfibrillated cellulose or non-organic particles, and by more complex drying methods.
The mechanical properties of lightweight cellulose fibre materials are inferior compared to traditional polymer foams. Understanding the mechanical properties of foam-formed lightweight fibrous materials is thus important for their practical application (Alimadadi and Uesaka 2016; Ketoja et al. 2019; Burke et al. 2021; Wagner et al. 2025). To keep the materials biobased, reinforcement has been studied by adding fibrils, fines, cellulose micro/nanofibrils or biopolymers to the structure (Korehei et al. 2016; Li et al. 2016; Pöhler et al. 2020; Alonso et al. 2024).
An interesting research area is the 3D structure of the foam-formed fibrous materials (Al-Qararah et al. 2015; Burke et al. 2020; Pääkkönen et al. 2023). The ability to control the pore size distribution would enable fine-tuning the material properties according to the application. For example, a wide pore size distribution – particularly the presence of exceptionally large pores – can weaken the material’s strength. Additionally, pore size distribution may affect the material’s thermal insulation (Walle and Janssen 2019) and filtration properties (Chen et al. 2018). While many researchers have studied the 3D structures of their foam-formed materials, few authors have studied systematically its dependence on process parameters, fibre properties or additives.
Air-laying is a widely used process for creating nonwoven materials (Russel 2022; Paunonen et al. 2024). In this method, fibres are suspended in air and gradually collected onto a forming fabric. Because no water is involved, the drying phase is eliminated, resulting in significant energy savings. This makes air-laying an attractive option for processing wood fibres into lightweight, porous structures. However, the need for binders to hold the fibres together can limit its potential, as these binders may impact the material’s recyclability and biodegradability.
In this study, the effect of wood fibre type and fibrillation level on the structure, thermal insulation properties, and strength properties of foam-formed and air-laid lightweight fibrous materials was examined. The dry air-laid fibre networks were bound by a novel procedure that involves spraying water into the dry structures after they have been air-laid.
EXPERIMENTAL
Furnishes
Table 1 presents the properties of furnishes and fibres used in this study. The Schopper Riegler (SR) degree was determined according to the EN ISO 5267-1 (2000) standard. The Canadian Standard Freeness (CSF) was measured according to the ISO 5267-2 (2001) standard. The conversions between the CSF and SR values were performed using the AFT freeness conversion chart (AFT 2018). The fibre length, fibre width, gravimetric coarseness, and the percentages of the A-type and B-type fines (flakes and fibrils type elements, See Appendix 1) were measured with the FS5 Fibre Image Analyzer (Valmet Automation, Kajaani, Finland).
Bleached softwood kraft pulp (BSKP) from Metsä Fibre (Äänekoski mill, Finland) was used at three SR levels. Here, traditional refining was performed at KCL (KCL Laboratory, Espoo, Finland). Bleached chemi-thermomechanical pulp (CTMP) from Metsä Fibre (Kaskinen mill, Finland) included 70 to 100% hardwood and 0 to 30% softwood and had the brightness (ISO 2470-1) of 82%, according to the manufacturer. Metsä Fibre CTMP was highly refined at VTT using a Masuko supermasscolloider (Masuko Sangyo Co., Ltd., Japan). Acacia furnish from April Asia was used both as such and as a 50% – 50% mixture (w/w) of acacia and unrefined BSKP. Refined CTMP was used both as is and in combination with a 4% (w/w) addition of TEMPO-oxidized cellulose nanofibrils (CNF), referred to subsequently as TEMPO. The TEMPO was obtained from Nippon Paper Industries Co. Ltd. as a 3% suspension. BSKP fluff pulp (BioBright untreated) from UPM Raumacell, obtained in a defibrated state, is primarily used for air-laid and hygiene products.
Microscopic images of the fibres are shown in Figs. 1 to 3. Note that the different fibre properties were not independent. The correlation between fibre length and fibre width was 0.40 (positive correlation), fibre width and coarseness 0.75 (positive correlation), and fibre length and coarseness 0.63 (positive correlation). This should be kept in mind when analysing the properties of the produced materials. In Table 1, the fibre properties were measured with the Valmet Fibre Image Analyzer (Valmet FS5). The values for the acacia-BSKP mixture were calculated with the formulae shown in Appendix 2. The SR values in parentheses were obtained from the AFT freeness conversion chart (AFT 2018).
Table 1. Used Furnishes, Freeness, Length Weighted Fibre Length, Fibre Width, Gravimetric Coarseness, and Fines A and B Percentages
Fig. 1. Microscopic images of a) unrefined BSKP (SR16), b) refined BSKP (SR23), c) highly refined BSKP (SR89); The scale bar is 200 µm long.
Fig. 2. Microscopic images of a) CTMP, b) highly refined CTMP; The scale bar is 200 µm long
Fig. 3. Microscopic images of a) acacia b) BSKP fluff; The scale bar is 200 µm long
Foam Forming of Samples
The target sample density and thickness were 30 kg/m3 and 30 mm, respectively, resulting in a target basis weight of 900 g/m2. Three parallel samples were made for each trial point. The samples were made at room temperature (21 oC).
Fig. 4. a) Experimental set-up for the preparation of the fibre foams; b) Foam-forming sheet mould (Keränen et al. 2023)
The furnish was placed in a vessel with tap water and surfactants, and the fibre foam was generated using a laboratory mixer (Netzsch, Denmark, see Fig. 4a). The tap water at Jyväskylä, where the experiments were performed, is soft; salinity was 20 to 50 mg/L. As the surfactant, a mixture of sodium dodecyl sulphate (SDS) 0.6 g/L and polysorbate 20 (Tween 20) 0.6 g/L was used. Through using a co-surfactant with the SDS, the foaming agent residue in the foam-formed materials can be minimised (Viitala et al. 2020). The average foam bubble diameter was not measured but was likely of the order of 100 μm (Mira 2014; Koponen et al. 2020). Forming consistency was, in most cases, 3.5%. For acacia, the forming consistency was 4.4%, as retention was too low with 3.5% consistency (the basis weight of the final sample was only 80% of the target value). For refined CTMP, the forming consistency was 2.5% due to the high viscosity of the refined CTMP suspension. The water removal was also slow from the refined CTMP samples.
The target air content of the fibre-laden foam was 60%. In practice, the air content of wet foams varied between 57 to 67%. The foam generation time was kept constant at 8 min, with a rotational speed varying between 2000 to 4900 rpm, depending on the furnish type. The generated foam was poured along a tilted plane into a 33 cm × 24 cm handsheet mould (see Fig. 4b). The foam was left to drain in ambient conditions, while slightly pressing. The samples were dried overnight at 70 °C in an oven. The dried sheets were rewetted to reach a solid content of 50% by spraying water on the top and bottom surfaces of the sheet. The samples were placed in plastic bags, and the moisture content was allowed to balance for 4 h, turning the samples after 2 h. The samples were then compressed between metal plates to the target thickness and dried overnight at 70 °C in the oven. Before thermal conductivity measurements, the samples were cut to the size of 15 cm × 15 cm. Thermal conductivity was measured for two samples with densities that were closest to the target density and/or having the least variation in their thickness. For the compression stress measurement, five parallel samples were evaluated. The samples were cut from one sheet and had the size of 50 mm × 50 mm (25 cm2) and the thickness of 27 to 30 mm.
Air Laying of Samples
The air-laid samples were prepared by using an air-laid laboratory drum former (Walkisoft, Anpap), as shown in Fig. 5. The relative humidity in the laboratory was 32 to 45% and the temperature was T=23 °C. Fluff fibres were pre-separated using a kitchen blender for 8 s. These fibres were then placed into a rotating cylindrical drum with rectangular gaps, which sieved the fibres onto a forming wire using airflow. A rotational speed of 60 rpm was used. The forming time was 5 min while changing the rotation direction three times, starting with a clockwise direction. The airflow speed through the device was controlled by a flow limiter valve, with setting values ranging from two to eight (lower values correspond to higher airflow). In this work, a setting of two was used. Towards the end of the forming time, the remaining fibres in the drum were forced through the rectangular gaps with compressed air while rotating to achieve the desired grammage of 450 g/m2 for the sheets. The thickness of the obtained sheets was approximately 45 mm.
In contrast to foam forming, the final samples of air laying were obtained by overlapping two or three sheets and bonding them into a single sheet. The 30 kg/m³ sample was composed of two sheets, combined to reach a total target basis weight of 900 g/m². The resulting sample was moistened to a 50% moisture content by spraying water on both sides. It was subsequently compressed to a thickness of 30 mm and dried overnight at 70 °C between forming mesh screens, plastic plates, and metal plates. The 45 kg/m³ sample was composed of three sheets, resulting in a total target basis weight of 1350 g/m². After combining and moistening the sample, it was sealed in a plastic bag for 4 h to allow the moisture to equilibrate. The sample was compressed and dried following the same procedure as the 30 kg/m³ sample.
Notice that instead of using water spray to moisten the airlaid sheets, fibres or airlaid sheets could potentially be moistened with gas or steam, as suggested in the patent (Kiiskinen et al. 2023).
Fig. 5. a) The air-laying laboratory device; b) The chamber of the air-laying device with fibres in the drum and at the bottom a formed sheet on the wire; c) A close-up image of the rectangular gap drum into which the fibres were fed at the start of the operation.
X-ray Microtomography
Three-dimensional (3D) images of the fine-scale structure of the studied materials were obtained using X-ray tomography with an Rx Solutions Desktom 130 scanner (Chavanod, France). Samples of 2 × 2 × 3 cm3 (these are x, y, and z-dimensions) were cut from regions with the most homogeneous structure. The imaging voxel size was 11.8 µm, but the images were scaled down to approximately 24 µm for the calculations due to the large file size of the volume data. From the 3D images, it was possible to calculate the fibre orientation distributions, porosity profiles, pore size distributions, and geometric tortuosities for the materials. Details of the X-ray tomographic measurements and analyses are provided in Appendix 3.
Thermal Conductivity Measurements
Conduction of heat in a certain direction, through a material due to temperature differences, is described by Fourier’s law,
(1)
where Q is heat flux (unit W), A is the area under inspection (m2), L is the thickness of the sample (m), ΔT (K) is the temperature difference over the sample, and λ (W/m·K) is the thermal conductivity of the material.
The thermal conductivity of the studied materials was measured with the heat flow meter HFM Fox314, see Fig. 6a. In this device, the studied material is placed between a hot and cold plate. The upper cold plate is moved downwards until the plate has good contact with the studied material. In the authors’ measurements, the temperatures of the cold and hot plates were set to 10 °C and 30 °C, respectively. Appendix 3 includes a discussion of the accuracy of the HFM method and the reasons for preferring it over other existing measurement methods.
The chamber size of the HFM Fox314 device is 30 cm × 30 cm, but the actual measurement area is 10 cm × 10 cm, and one can measure thermal conductivity for rather small samples. In such measurements, the rest of the chamber is filled with insulation material (see Fig. 6b). Foam rubber was used for this purpose, with a measured thermal conductivity of 38.6 mW/m·K. Consequently, the thermal conductivities measured for the 15 cm × 15 cm samples in this study were likely somewhat higher than the true values (see Appendix 3).
Fig. 6. a) Heat flow meter HFM Fox314; b) The chamber size of HFM Fox314 is 30 cm × 30 cm, but the actual measurement area is 10 cm × 10 cm. When measuring small samples (here 15 cm × 15 cm), the rest of the chamber was filled with foam rubber.
Measurement of Mechanical Properties
Compression stress was measured using a Lloyd R10K universal tester (Lloyd Instruments Ltd., Bognor Regis, West Sussex, UK). During the measurement, the sample was compressed to 50% of its initial thickness at a rate equal to its original thickness in mm per minute (speed mm/min). For the recovery measurement, the thickness of the sample was determined under a pressure of 250 Pa after it had recovered from compression for 1 min.
RESULTS AND DISCUSSION
Foam-Formed and Air-Laid Samples
Figure 7 shows cross-sections of various foam-formed and air-laid samples. Visual differences were observed in the uniformity and pore size of the samples, which varied depending on the furnishes used and/or the forming methods applied. Table 2 shows the density, pore size, and tortuosity in the solid phase, fibre orientation angle in the z-direction (vertical direction), and the thermal conductivity of the samples. The final densities were adjusted by rewetting and compressing the samples, but this was found challenging; refining and the addition of TEMPO usually resulted in a higher final density during drying due to shrinking. Tortuosity in the void phase was less than 1.01 for all samples and is, therefore, not included in Table 2. In the following, tortuosity in the solid phase is referred to simply as ‘tortuosity’.
Fig. 7. Cross-sections of various samples. Those containing BSKP fluff were prepared by air-laying, while all the others were foam-formed. There are five samples in each stack. The target shape of the samples was a rectangular prism.
Table 2. Density, Pore Size, and Tortuosity in the Solid Phase, Fibre Orientation Angle in the z-direction (Relative to the xy-plane), and Thermal Conductivity of the Samples
Structural Analysis of Foam-Formed and Air-Laid Samples
Figures 8 to 10 show structural images in the mid-vertical plane for different furnishes (more structural images are presented in Appendix 4).
The average pore size, pore size inside the sample, and pore size at sample boundaries are shown in Fig. 11. Pore size inside the sample was calculated by excluding the first 7 mm from both ends of the z-directional pore size profile (see the inset image in Fig. 11). Pore size at the sample boundaries was determined by averaging the z-directional porosity profile over the first and last 5 mm. Notice that the pore sizes of BSKP and CTMP were close to the values predicted by the formulae presented in Keränen et al. (2023), that give the average pore size as the function of density for lightweight foam-formed BSKP and CTMP materials.
It can be seen from Figs. 8 to 10 that the foam-formed structures were not homogeneous. There were many large voids, primarily caused by the merging of foam bubbles and fibres during forming and drying, as well as the non-optimal dispersion of the fibres. Moreover, both foam-formed and air-laid samples had a higher density (or lower porosity) on the upper and lower (wire side) boundaries. This can also be seen from the nominal porosity profiles shown in Fig. 12. Densification of the samples at their boundaries can occur both during forming (Lecourt et al. 2018; Burke et al. 2019; Keränen et al. 2023) and during compression after rewetting. The densification observed here was likely a combination of these two effects. Figure 13 shows z-directional pore size distributions for BSKP, CTMP, acacia, and BSKP fluff. It can be seen that, in all cases, pore size was smaller at the sample boundaries. Imaged samples were rather small when compared to the highest pore sizes, which explains the rather wide variation of the pore size in the distributions.
Figure 10c shows a structural image for the air-laid BSKP fluff-30 sample. The BSKP fluff-40 sample was similar (see Appendix 4). Compared to foam-formed samples, the structure of the air-laid samples was much more homogeneous, and the pore size was substantially smaller (see Fig. 11).