Effect of additives and mixing tank geometry on flocculation of long-fiber foam suspensions

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Kemian tekniikan korkeakoulu | Master's thesis
Biosystems and Biomaterials Engineering
Degree programme
Master’s Programme in Life Science Technologies
Natural fiber composites are becoming an effective option in replacing the petroleum-based materials and the network structure created by the fibers increases the mechanical strength of the products. First process in manufacturing of fiber composites involves proper dispersion of the fibers into a carrier phase. If during mixing, the fibers flocculate, it will hinder the further processing and decrease the quality of the final product. Utilization of foam forming, which is a type of wet-Laying process, allows flexible design of materials with variety of properties suitable for the application of interest. Foam forming limits the flocculation of fibers and increases the uniformity of the product. Fibers with long length are desirable due to their ability to increase the mechanical strength of the fiber composite. However, these longer fibers are more prone to flocculation and foam forming alone isn’t sufficient in preventing the formation of fiber flocs. In this master’s thesis, the ability of different chemical additives or modification of mixing tank geometry to reduce the flocculation during mixing of long-fiber foams was studied. Lyocell fibers of 10mm / 15 mm length and consistencies 1.5 % - 6 % were used in this study. To evaluate the potential of chemical additives, two varieties of carboxymethyl cellulose (CMC), two varieties of guar gum (GG) and Chia seed mucilage (CM) were studied. To evaluate the potential of modification of mixing tank geometry, rectangular baffles and different impeller diameter-to tank diameter ratio (D/T) were studied. The determination of flocculation in the samples was done via X-ray tomography and analyzing the images with image processing software Fiji. To evaluate the role of viscosity on the flocculation, the viscosity of additive suspensions was measured. Also, the air content of the fiber foams was determined. The results showed that at optimal concentration CMC (Finnfix 5), Guar gum (Polygum 240/06), GG (Polygum 240/40) were effective in reducing the flocculation of 15 mm fibers at 1.5 % fiber consistency, compared to no additive. CM showed promising results at 0.2 % additive concentration in reducing flocculation at all fiber lengths and fiber consistencies compared to no additive. The results provided indication that there could be an effective viscosity range in which the chemical additives were able to reduce the flocculation compared to no additive. When the additive suspension viscosity values were in between 1.7 mPa·s – 32 mPa·s, the flocculation was significantly reduced. Decreasing or increasing the viscosity outside of this range drastically increased the flocculation. When considering all fiber lengths and fiber consistencies, 21/22 data points that fell outside of the speculated viscosity range showed a significant flocculation (0.5 % <). Sucrose at 27 % concentrations showed viscosity values within the effective range, but showed significant flocculation, indicating that likely other factors in addition to suspension viscosity played a role in flocculation. Utilization of baffles increased the flocculation significantly for the D/T ratio of 0.67 and each addition of extra baffle up to three further increased the flocculation. For the D/T ratios of 0.44 and 0.53, the negative effect of baffles on flocculation was insignificant. D/T ratio 0.67 produced significantly higher flocculation than the lower D/T ratios indicating that in our study the reduction of mixing tank diameter provided undesired effect on floc formation. Increasing the concentration of the additives, addition of baffles and increase of D/T ratio all consistently reduced the air content of the foam. To evaluate the underlying mechanisms of the flocculation, for example the role of drag reduction or baffles, it would be beneficial to study the flow patterns during the mixing. Also, study of the role of inter-fiber friction in fiber flocculation could allow more efficient screening of chemical additives.
Kostiainen, Mauri
Thesis advisor
Koponen, Antti
Prakash, Baranivignesh
flocculation, additives, foam forming, carboxymethyl cellulose, guar gum, fiber foams
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