This work investigates the surface and bulk properties of nanofibrillated cellulose (NFC) and bacterial cellulose (BC), as well as their reinforcing ability in polymer nanocomposites. BC possesses higher critical surface tension of 57 mN m-1 compared to NFC (41 mN m-1). The thermal degradation temperature in both nitrogen and air atmosphere of BC was also found to be higher than that of NFC. These results are in good agreement with the higher crystallinity of BC as determined by XRD, measured to be 71% for BC as compared to NFC of 41%. Nanocellulose papers were prepared from BC and NFC. Both papers possessed similar tensile moduli and strengths of 12 GPa and 100 MPa, respectively. Nanocomposites were manufactured by infusing the nanocellulose paper with an epoxy resin using vacuum assisted resin infusion. The cellulose reinforced epoxy nanocomposites had a stiffness and strength of approximately ~8 GPa and ~100 MPa at an equivalent fibre volume fraction of 60 vol.-%. In terms of the reinforcing ability of NFC and BC in a polymer matrix, no significant difference between NFC and BC was observed.
Aqueous foam can be used as a transfer medium to form lightweight materials from natural and man-made fibers together with other types of raw materials. This review discusses mechanisms that underlie the forming process and thus influence physical properties of formed fiber networks such as microporous structure, strength behavior, and transport properties. Homogeneous fiber materials can be formed from versatile raw materials, which makes the technology suitable for a vast range of product applications. An intriguing feature of the method is that the wet foam characteristics provide an additional tool to tailor the performance of the dried material. Understanding foam rheology and how that is affected by added fibers is important in developing the forming process. We introduce both fundamental foam properties and practical forming methods, and show how the material properties are affected by the foam-fiber interaction. The basic features of an industrial production process are also described. The potential material properties are compared against key requirements in typical product applications.
Foam forming has recently attracted increasing interest due to the paper industry’s continual efforts to find new possibilities to minimize raw material consumption, and to improve energy and water efficiency. Foam forming is also thought to be a possible solution to the industry’s need to widen its product portfolio with novel and more valuable products. In foam forming, foam properties (air content, bubble size and half-life) are obviously key process variables, but there are only a few studies in which their effect on the sheet properties have been studied in pilot conditions. Moreover, all previous studies have used foam generated in stirring tanks, and there are hitherto no studies in which in-line foam generation has been considered. In this paper both these gaps are filled with experiments performed in VTT’s pilot foam forming environment. The combination of tank and in-line generation was found to work well in foam forming, providing extra flexibility for foam generation and decreasing surfactant needs. The results show that foam forming generally improves formation, but the foam quality can have a significant effect on sheet properties.
Recent developments in making fibre materials using the foam-forming technology have raised a need to characterize the porous structure at low material density. In order to find an effective choice among all structure-characterization methods, both two-dimensional and three-dimensional techniques were used to explore the porous structure of foam-formed samples made with two different types of cellulose fibre. These techniques included X-ray microtomography, scanning electron microscopy, light microscopy, direct surface imaging using a CCD camera and mercury intrusion porosimetry. The mean pore radius for a varying type of fibre and for varying foam properties was described similarly by all imaging methods. X-ray microtomography provided the most extensive information about the sheet structure, and showed more pronounced effects of varying foam properties than the two-dimensional imaging techniques. The two-dimensional methods slightly underestimated the mean pore size of samples containing stiff CTMP fibres with void radii exceeding 100 μm, and overestimated the pore size for the samples containing flexible kraft fibres with all void radii below 100 μm. The direct rapid surface imaging with a CCD camera showed surprisingly strong agreement with the other imaging techniques. Mercury intrusion porosimetry was able to characterize pore sizes also in the submicron region and led to an increased relative volume of the pores in the range of the mean bubble size of the foam. This may be related to the penetration channels created by the foam-fibre interaction.
This paper summarizes recent developments in foam forming that were mainly carried out in pilot scale. In addition to improving the efficiency of existing processes and allowing better uniformity in material, a wide variety of raw materials can be utilized in foam forming. The focus of this paper is thin webs—papers, boards and foam-laid nonwovens, along with the pilot scale results obtained at VTT in Finland. For paper and board grades, the most direct advantage of foam forming is the potential to produce very uniform webs from longer and coarser fibers and obtain material savings through that. Another main point is increased solids content after a wet press, which may lead to significant energy savings in thermal drying. Finally, the potential to introduce “difficult” raw materials like long synthetic or manmade fibers into a papermaking process enables the manufacturing of novel products in an existing production line. This paper also briefly discusses other interesting foam-based applications, including insulation and absorbing materials, foam-laid nonwovens, and materials for replacing plastics.
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