Nanotechnology applied to cellulosic fibers has quickly become an interdisciplinary field with great interest in the application as reinforcement in polymer composites, mainly due to the abundance of these raw materials, and to their mechanical properties and multifunctionality. However, one of the critical points to obtain individualized cellulose nanofibers is the drying technique (dehydration), since most of the nanofiber processes are performed in the liquid phase. According to the methodology applied to the cellulose water dehydration process, various morphologies and properties can be obtained in the cellulose fibers. This review study aims to discuss the main processes used to obtain nanocellulose (chemical and mechanical) and the drying techniques applied to nanocellulose structures, such as conventional oven drying, freeze drying (lyophilization), supercritical extraction, and spray drying.

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O objetivo deste trabalho é utilizar fibras de algodão como material de reforço em materiais compósitos poliméricos utilizando o poliestireno como matriz e o poli(estireno-co-anidrido maleico) como agente compatibilizante. Os compósitos foram desenvolvidos em uma extrusora dupla-rosca co-rotacional, precedidos de uma pré-mistura em extrusora mono-rosca e moldados por injeção. Os compósitos foram avaliados mediante ensaios mecânicos, térmicos, termo dinâmico-mecânico e de morfologia dos compósitos produzidos. Os ensaios de flexão e tração mostram que a adição de 20% de fibra de algodão faz com que essas propriedades aumentem, sendo esse efeito intensificado em presença de compatibilizante. Observou-se aumento da resistência ao impacto com adição de carga; porém, os compósitos com compatibilizante apresentaram resultados inferiores. A HDT para os compósitos com 20% de fibra de algodão foi de aproximadamente 7 ºC. Por meio do TGA, observa-se que a adição de fibras de algodão desloca o início da perda de massa para temperaturas próximas a 200 ºC. Na análise de DMTA, observa-se que, com a adição da fibra de algodão, ocorre aumento na rigidez e no módulo de armazenamento. As micrografias mostram redução no pull-out das fibras, devido a uma maior adesão fibra/matriz, com a utilização do agente compatibilizante.

The thermal degradation behavior of different types of cellulose before and after mechanical defibrillation and lyophilization was studied using isothermal and nonisothermal thermogravimetric analyses, followed by other characterization techniques, such as X-ray diffraction, Fourier transform infrared spectroscopy, degree of polymerization and scanning electron microscopy with field emission analysis. The thermogravimetric experiments were carried out in a nitrogen atmosphere at four different heating rates (5, 10, 20 and 40°C min -1 ) in a nonisothermal condition. Distinct thermal degradation behaviors were observed when the two types of cellulose were compared after defibrillation: (1) cellulose nanofibers tend to lose thermal stability and (2) cellulose nanowhiskers tend to gain thermal stability. The Flynn-Wall-Ozawa method results indicate that the apparent activation energies calculated for the cellulose fiber sample has higher values requiring more energy for the thermal decomposition. Criado curves indicated a degradation mechanism for the cellulose: one-dimensional diffusion.

This study evaluated the mechanical, thermal, rheological, and morphological properties of virgin and recycled matrices and their composites with 20 wt % of curaua fiber. The recycling process of postconsumer polystyrene was carried out by grinding and extrusion. It was found that the recycling of expanded polystyrene did not have a major influence on the mechanical properties; however, the thermal stability was increased. The addition of curaua fibers led to increases in the tensile strength, modulus of elasticity, rigidity, thermal stability and melt viscosity of the composites. The composites made with the recycled matrix revealed higher thermal stability and melt viscosity than those made with the virgin matrix. Scanning electron microscopy characterization showed empty spaces where the curaua fibers had pulled out of the matrices in the fractured regions, indicating poor interfacial adhesion without the use of a coupling agent.

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