Secagem por pulverização de suspensões de celulose microfibrilada: rendimento, caracterização e redispersão para produção de filmes

Abstract

The high amount of water present in cellulose micro/nanofibril (MFC/CNF) suspensions (>95%) is one of the main obstacles that limits its industrial application and large-scale commercialization. Thus, spray drying may be an alternative to overcome the problems caused by low solids concentration. However, producing powderedMFC/CNF with the ability to re-disperse into suspension and obtaining higher spray drying yields are challenges that still need to be overcome.The general objective of this study was to investigate the effect of adding cationic surfactant cetyltrimethylammonium bromide (CTAB) to MFC/CNF suspensions on the spray drying efficiency, its effects on the dried MFC/CNF, and the redispersibility of the produced dried materials. Untreated MFC/CNF were produced and pretreated with sodium silicate at 5% and 10% (w/w) concentration and oxidized with 2,2,6,6,-tetramethylpiperidinyl-1-oxyl (TEMPO). Then, the MFC/NFC were treated with the cationic surfactant CTAB and subsequently spray dried. In a second stage, the redispersity by the ultrasonic method of the spray-dried MFC/CNF aggregateswas investigated for the production of cellulosic films by casting method. This thesis was divided into two articles.The first article discusses the yield of the spray-drying process of MFC/CNF suspensions, as well as on the microstructural, thermal, chemical and water stability characterization of the original MFC/CNF suspensions and the spray-dried cellulosic microparticles.The second article discusses the microstructural, physicomechanical, optical, water vapor barrier and surface properties of the films produced with the original suspensions and with the ultrasonically redispersed cellulosic microparticles. The highest drying yield was obtained for the MFC/CNF suspensions pretreated with sodium silicate and CTAB surfactant, S5-CTAB (48%) and S10-CTAB (51%), with lower viscosities. The dimensions of the MFC/CNF were not preserved after the spray drying process, however the crystalline fraction of all particles produced was higher than that of the original MFC/CNF. All materials, original and spray-dried MFC/CNF, were thermally stable at elevated temperatures (>200 °C).The water stability test revealed that the oxidation reaction was essential to obtain moderately stable suspensions both before and after spray drying (Article 1). Redispersion of the aggregated particles was favored for the MFC/CNF with addition of CTAB surfactant. Specifically, the results proved that for TEMPO-oxidized MFC/CNF the aggregates were completely broken, providing films with high tensile strength (87.8 MPa), excellent water vapor barrier property (water vapor permeability 4.2 g mm/m² day kPa; water vapor permeation rate 353.7 g/m² day) and higher optical transmittance. It is concluded that theyield of MFC/CNF powder from spray drying process can be maximized by employing less viscous MFC/CNF suspensions. Furthermore, the addition of CTAB surfactant in the MFC/CNF suspensions proved to be an effective strategy capable of minimizing undesirable structural changes resulting in irreversible aggregation, facilitating the redispersion of the produced dried particles.