Cellulose (2020) 27:6961–6976
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Foaming agents
of 1.08% and a charge value of approximately 1 mmol/g dry pulp. The apparent viscosity at the shear rate 10 1/s was 2.48 Pas. Optical micrographs of the different fine components are shown in Fig. 2 and scanning electron microscope (SEM) micrographs in Fig. 3. Figure 4 shows an estimated size scale of the employed fibre and fine components based on microscopy. The fibre width was spread over 5 and the fibre length over 4 orders of magnitude. The foam- aided material formation was able to handle the large size differences among the raw materials.
Two different foaming agents were applied: anionic sodium dodecyl sulphate, NaC 12 H 25 SO 4 , SDS (Sigma-Aldrich) with 90% purity, and polyvinyl alcohol PVA (Poval Kuraray). PVA is a linear water-soluble synthetic polymer that inherently cre- ates a suitable type of foam for the foam-forming process. On the other hand, PVA is a strong binder: it is 3–4 times stronger than starch and 2–3 times stronger than acrylic or PVAc-based latexes (Miller et al. 1998; Finch 1992). The molecular weight of PVA usually varies between 15,000 and 250,000 g/mol. This weight is much higher than that of small anionic surfactants (e.g. the molecular weight of SDS is about 288 g/mol) but smaller than the molecular weight of LBG. PVA grades 6–88 (partially hydrolysed, M w * 37,000 g/mol) and 28–99 (fully hydrolysed, M w * 145,000 g/mol) were applied. Since the foaming of grade 28–99 was fairly poor, a blend with better foaming 6–88 was employed to optimize foaming efficiency and binding effect. Both SDS and PVA foaming agents are readily biodegradable.
Hydrocolloidal and water-soluble polymers
Locust bean gum (LBG), a galactomannan obtained from the seed endosperm of the carob tree (i.e. Ceratonia silique), was obtained from Sigma-Aldrich. LBG is widely utilized as an additive in various industries such as food, pharmaceuticals, paper, tex- tile, oil well drilling, and cosmetics (Barak and Mudgil 2014). It is a straight chain polymer of mannose with one galactose branch on every fourth mannose and has an average molecular weight of approx. 310 000 g/mol (Barak and Mudgil 2014). Industrial appli- cations of LBG are based on its ability to form hydrogen bonding with water molecules. LBG was added as a powder into the foam at room temperature. According to Barak and Mudgil (2014), LBG is partially soluble in cold water. The conventional cationic wet end starch Raisamyl 50021 (DS = 0.035) was employed mainly for ensur- ing high retention of the CMF and the strongly negatively charged TCNF in particular.
Foam generation and forming
Water, a foaming agent (SDS or PVA blend), and the furnish components were mixed in a cylindrical vessel. CMF and TCNF were mixed with the fibre suspension before the surfactant was added and before foaming. Cationic starch, when applied, was mixed with the pulp slurry approximately 30 s before the foaming agent was added. Foam generation was continued until the air content of the fibre foam
Fig. 2 Optical micrograph images of a V-fines, b CMF, and c TCNF. V-fines and CMF were stained with Congo red, and TCNF was stained with toluidine blue. The measure bar shows the length of 200 l m
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