Estudo de mecanismos de degradação do polietileno em reciclagem primária

Tese de Doutoramento em Ciência e Engenharia de PolímerosEste trabalho teve como objectivo final estudar, com base em diferentes técnicas experimentais, as alterações de propriedades que ocorrem durante o reprocessamento de peças moldadas em polietilenos, relacionando-as com as alterações químicas e microestruturais sofridas pelos respectivos polímeros. Para o efeito, foram utilizadas misturas de material virgem e reciclado de 5 variedades de polietileno: 4 de alta densidade (PEAD) e 1 de baixa densidade (PEBD). A gama de materiais utilizados permitiu considerar no estudo o efeito da massa molecular e do grau de ramificação nos mecanismos de degradação associados ao reprocessamento sucessivo em moldação por injecção. Os resultados obtidos baseiam-se em conjuntos de moldações correspondentes a 10 ciclos de reprocessamento com diferentes graus de incorporação de material virgem. Os provetes obtidos (ou amostras de material retiradas dos mesmos) foram posteriormente a um conjunto alargado de ensaios de avaliação comportamental (mecânicos, reológicos e térmicos), bem como caracterização química e microestrutural (incluindo espectroscopia de infra-vermelhos e ressonância magnética nuclear). As análises efectuadas permitiram comprovar a coexistência de dois mecanismos de degradação da estrutura polimérica, em resultado das reacções termooxidativas associadas ao ambiente termo-mecânico imposto durante o processo de moldação por injecção. De facto, os resultados obtidos evidenciam o desenvolvimento de reticulações e cisão de cadeias moleculares. O peso relativo destes dois mecanismos é dependente da estrutura do material e das condições de processamento utilizadas. Foi também possível verificar que os polietilenos de massa molecular mais elevada apresentam uma maior sensibilidade à degradação durante o processamento. Analogamente, verificou-se que o polímero mais ramificado, o PEBD é menos susceptível aos fenómenos termo-degradativos. O trabalho permitiu ainda concluir que a reciclagem primária de polietileno, quando efectuado de acordo com protocolos adequados, traduz -se numa diminuição pouco significativa das propriedades deste material.This thesis envisaged the study of the dependence of the material properties on the continous reprocessing of injection moulded polyethylene. It is based on the use of a wide range of exprimental techniques and relates the observed changes in the material behaviour with the chemical and microstructural modifications developed in selected polymers. Five different grades were used for this purpose, including four types of high density polyethylene (HDPE) and one variety of low density polyethylene (LDPE), in the form of mixtures between virgin and recycled material. This range of polymers enabled the study of importance og the material molecular weight and degree of branching on the degradation mechanisms associated the reprocesing actions in injection moulding. The obtained results were based on mouldings collected from ten reprocessing cycles, with different levels of incorporation of virgem material. The respective moulded specimens (or the material samples got from them) were submited to an extensive list of tests in order to assess the the macriscopical behaviour (mechanical, rheological and thermal) and to evaluate chemical and microstructural modifications (among other techniques, infra-red sepctroscopy and nuclear magnetic ressonance were used). The developed studies allowed to prove the coexistence of two distinct degradation mechanisms, as a result of thermo-oxidative reactions associated to the thermo-mechanical environment resulting from the injection moulding process. It is evident from the obtained results, the development of both crosslinks and chain scissions in the polymer chains. The relative importance of these two mechanisms is dependent on the material structure and on the processing conditions used. It was also possible to confirm that the polyethylenes with higher melecular mass evidence a lower sensitivity to degradation during processing. Furthermore, higher branched polymers (as the LDPE) proved to be less sensitive to the studied thermo-degradative phenomena. This work also concluded that primary recycling of polyethylene, if performed under adequate procedures, leads to minor material property loss.PRODEP III - Formação Avançada de Docentes do Ensino Superior - Bolsa de Doutoramentos - Projecto 5.3/C/1065.001/0

Alginate-Based Rapid Prototyping System

Alginate hydrogels are an import class of ionic biopolymers for medical and biotechnological applications, produced by the combination of alginate with a proper cross-linking agent. During the gel formation, cross-links can be formed between the alginate chains and cationic species, changing the elastic behaviour of the material that controls the volume change phenomena of the gels. This paper proposes a new rapid prototyping system to produce three-dimensional alginate structures by extruding, layer-by-layer, a previously prepared solution of sodium alginate in water mixed with a solution of calcium chloride, both of known concentration. The building process to obtain these gel structures is described, from a chemical point of view, and some obtained structures are shown. The alginate concentration over both the kinetics and accuracy effects of the process is investigated. The preliminary findings of this research work promise to open an exciting new area for medical applications.Mechanical Engineerin

Alginate/Aloe Vera Hydrogel Films for Biomedical Applications

AbstractThis paper describes a methodology to produce hydrogel films, composed of alginate and Aloe vera, for wound healing and drug delivery applications. The films were prepared through the solvent-casting method and subsequently submitted to an additional cross-linking step to improve their properties. Alginate films with different Aloe vera contents (5, 15 and 25%) were prepared and its properties evaluated in terms of thickness, transparency, swelling behavior and in vitro degradation. Results show a positive influence of Aloe vera on the transparency of the films, in both dry and wet state. Films were immersed in acetate buffer at pH 5.5 simulating the value of the skin, to evaluate its water absorption capacity. It was found that water absorption increases as the Aloe vera content increases, suggesting that Aloe vera enhances the hydrophilic properties of the films. The in vitro degradation tests were performed through the incubation of the films, for 10 weeks, in a simulated body fluid at 37°C. After this period, films kept its structure integrity exhibiting a weight loss in the range of 14.1-18.6%

Biopolymer-based manufacturing system for medical applications

Comunicação em pósterinfo:eu-repo/semantics/publishedVersio

Monitoring the modifications of Aloe vera by high performance liquid chromatography

Aloe vera is a widely known and studied plant due to its therapeutic properties. The therapeutic properties exhibited by the Aloe vera exudates have been associated to the presence of certain compounds, such as, chromones, anthrones and anthraquinones. In this study, we have identified, and monitored by reversed phase high performance liquid chromatography (HPLC), six major compounds present in the Aloe vera exudates. The chromatographic profiles were followed in the course of time and at different wavelengths. This monitoring allowed us to verify the constitution and modifications of the samples, in order to identify the main changes responsible for the chemical degradation and loss of proprieties exhibited by the exudates over time.info:eu-repo/semantics/publishedVersio

Influence of Aloe vera on water absorption and enzymatic in vitro degradation of alginate hydrogel films

This study investigates the influence of Aloe vera on water absorption and the in vitro degradation rate of Aloe vera-Ca-alginate hydrogel films, for wound healing and drug delivery applications. The influence of A. vera content (5%, 15% and 25%, v/v) on water absorption was evaluated by the incubation of the films into a 0.1 M HCl solution (pH 1.0), acetate buffer (pH 5.5) and simulated body fluid solution (pH 7.4) during 24 h. Results show that the water absorption is significantly higher for films containing high A. vera contents (15% and 25%), while no significant differences are observed between the alginate neat film and the film with 5% of A. vera. The in vitro enzymatic degradation tests indicate that an increase in the A. vera content significantly enhances the degradation rate of the films. Control films, incubated in a simulated body fluid solution without enzymes, are resistant to the hydrolytic degradation, exhibiting reduced weight loss and maintaining its structural integrity. Results also show that the water absorption and the in vitro degradation rate of the films can be tailored by changing the A. vera content

Development of novel alginate based hydrogel films for wound healing applications

Alginate and Aloe vera are natural materials widely investigated and used in the biomedical field. In this research work, thin hydrogel films composed by alginate and Aloe vera gel in different proportions (95:5, 85:15 and 75:25, v/v) were prepared and characterized. The films were evaluated regarding the light transmission behavior, contact angle measurements, and chemical, thermal and mechanical properties. These thin hydrogel films, prepared by crosslinking reaction using 5% calcium chloride solution, were also investigated relatively to their water solubility and swelling behavior. Results showed that Aloe vera improved the transparency of the films, as well their thermal stability. The developed films present adequate mechanical properties for skin applications, while the solubility studies demonstrated the insolubility of the films after 24 h of immersion in distilled water. The water absorption and swelling behavior of these films were greatly improved by the increase in Aloe vera proportion.info:eu-repo/semantics/publishedVersio