Дисертації з теми "Low-density polyethylene films"

The growing requirement of reliability for an insulation system gives researchers greater responsibility to investigate new techniques for monitoring and diagnosing dielectrics subjected to an electric field. It is well known that the presence of space charge is one of the important factors causing premature failure of polymeric high voltage cables. Space charge surveillance is becoming the most general applied skill to evaluate polymeric materials, particularly high voltage cables. The well-known pulsed electroacoustic method (PEA), a reliable non-destructive method, gives a reasonable resolution to the concentration of the space charge in the insulation material. My work contributes to the measuring of space charge in low density polyethylene using PEA. From the experiments to study space charge formation and distribution at the interface on multi-layer sample under DC and AC applied voltage, electrode materials and frequency are determined as two important factors in measuring the charge injection and distribution; the interface between films acts as a trap for charge carriers, especially for electrons; and positive charge has a high mobility compared to negative charge. Surface potential decay was studied to explain the crossover phenomenon and to find physical mechanism on charge decay of the corona charged film sample. Charge mapping technique (PEA) was successfully introduced to the potential study and it provides an alternative way to investigate charge decay process and allows monitoring charge migration through the bulk of corona charged film. One essential phenomenon, bipolar charge injection, has been first derived from the results of space charge distribution. The advanced PEA measurement system with high rate test and excellent phase resolving capability was designed in the last part of the study. Compared with the old system the new system can provide the enhanced experiment result for fast change situation, which can achieve high-quality diagnosis for the virtual industry situation such as polarity reversal and transient voltage failure.

CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
Materiais plásticos são largamente utilizados em nosso dia-a-dia em embalagens, sacos e outros produtos. Este tipo de material é utilizado devido a suas propriedades como baixo custo, fácil processabilidade, baixa densidade, resistência a microorganismos e água, estabilidade química e durabilidade. Devido às duas últimas propriedades, os polímeros apresentam baixa degradabilidade, causando problemas ambientais. Como óxido de titânio (TiO2) tem se apresentado eficiente como fotocatalisador, reforçar plástico com partículas deste material tem sido uma nova maneira de decompor polímeros a céu aberto. Nanotubos de trititanato (TTNT) podem ser tratados para produzir nanomateriais à base de TiO2 com alta atividade fotocatalítica para a degradação de gases poluentes. Desta forma, o presente trabalho tem como objetivo produzir e caracterizar filmes de polietileno reforçados com quatro tipos de nanomateriais à base de TiO2: TTNT sem pós-tratamento (A1), TTNT pós-tratado termicamente a 550 graus Celsius (A5), TTNT pós-tratado com ácido (A11) e, como referência, partículas de óxido de titânio comercial fornecido pela Degussa (P-25). Os filmes foram expostos à luz UV em uma caixa fechada por 350 horas em temperatura ambiente. A degradação foi avaliada por meio da perda de peso do filme ao longo do tempo. Os filmes virgens e fotodegradados foram caracterizados por Difração de Raios-X (DRX), Calorimetria diferencial de Varredura (DSC), Termogravimetria (TGA) e Microscopia Eletrônica de Varredura (MEV). Os filmes com TTNT pós-tratado fotodegradaram mais do que os com TTNT não tratado, mas menos que os que continham TiO2. Este resultado foi parcialmente atribuído à dificuldade de dispersão dos nanomateriais.
Plastic materials are widely used in our daily lives in bags, food packaging and other products and applications. This type of material is used because of properties such as low-cost, easy processability, low density, resistance to water and microorganisms, and chemical stability and durability. Due to the last two properties, polymers show low biodegradability causing enviro nmental pollution. As titanium dioxide (TiO2) has been shown to be an efficient photocatalyst, the mixture of plastic with this material has been proven to be a new and useful way to decompose solid polymers in open air. Trititanate nanotubes (TTNT) can also be used as a route for developing TiO2-based nanomaterials with high photocatalytic activity for degradation of gas pollutants. Thus, the present research aims to produce degradable polyethylene polymer (PE) films composed with four types of TiO2-based nanomaterials: TTNT as synthesized (A1), TTNT with thermal post-treatment at 550 Celsius degrees (A5), TTNT with acid post-treatment (A11), and, as a reference, commercial TiO2 nanoparticles from Degussa Company (P25).The main characterization tool was the weight reduction measurement during the degradation process. The films were exposed to artificial UV light under ambient air for 350 hours. Virgin and degraded filmes where characterized by X-ray Diffraction, UV-Vis absorption, Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and Scanning Electron Microscopy (SEM). Films with post-treated TTNT showed stronger degradation than films with non-treated loads, but weaker than films containing TiO2. This result was partially assigned to the poor dispersion of the nanomaterials.

The effect of changing processing conditions on the mechanical properties of LDPE, LLDPE and blends of LDPE and LLDPE blown films was studied. The results were analysed by relating the change in mechanical properties with the change in the residual strain of manufactured film. The residual strain was measured by using a shrinkage method.

The course of polymer film functioning has been a crucial concern in the advent of packaging technology. The thesis project aims towards obtaining an understanding of mechanical properties for a class of these materials, namely LDPE and PET. A constitutive understanding of this behavior in the case of LDPE is acquired through incorporating a plastic stress strain relationship in an iterative approach with focus put on the sensitivity of a few parameters by following a simple linear curve-fit technique in a way that the global as well as the local response are predictable. FE-models also developed in this way are validated with experimental data. An inverse analysis testing validity or usefulness of DIC technique in identifying a material model is done and some discussions are drawn towards this area. A relative numerical study with respect to experimentally obtained global response for tearing of these polymers is done through use of a similar material model developed from tensile tests and the challenges faced in this area have been addressed.

As poliolefinas, em especial os polietilenos (PE) são materiais poliméricos muito utilizados para a produção de filmes tubulares, sendo um dos materiais mais amplamente empregado na indústria de embalagens flexíveis. Dentre os diversos PE industriais, o polietileno de baixa densidade (PEBD) apresenta propriedades reológicas únicas em comparação aos PE lineares e os de alta densidade. A alta viscosidade e as numerosas ramificações longas encontradas neste polímero influenciam na redução da produtividade quando processados. Aditivos das mais variadas composições são adicionados ao polietileno durante o processo de extrusão tubular a fim de melhorar suas propriedades. Um dos aditivos mais empregados para facilitar o fluxo do fundido para processamento de filmes são as ceras sintéticas, sendo a mais usada a de polietileno oxidado (CP). Na procura de alternativas de cera de fonte orgânica e/ou de fonte natural, o objetivo deste trabalho é avaliar a influência do tipo e teor de cera na processabilidade e propriedades finais de filmes tubulares de PEBD. Foram usados 3 tipos de ceras: CP (cera de polietileno), a carnaúba (CC) e o monoestearato de glicerol (CM), sendo processadas 4 formulações de PEBD/cera nas proporções mássicas de 99,5/0,5; 99/1; 98/2 e 96/4 m/m com os três tipos de cera e comparados com o PEBD sem cera. Os filmes foram caracterizados através de ensaios físicos, ópticos, químicos, térmicos, reológicos e mecânicos. Propriedades ópticas como o brilho e opacidade foram alteradas pela adição das ceras pois um aumento na concentração das ceras aumenta o grau de cristalinidade dos filmes. A cera de carnaúba apresentou amarelamento nos filmes produzidos com maiores concentrações. Resultados deste estudo mostraram que o uso da cera sintética, CP e da natural de carnaúba, aumentam a produtividade do filme tubular de PEBD, sendo o teor ótimo de 1% de cera, sem ter influência significativa na espessura e largura do filme tubular. O uso de agentes de fluxo alternativos de natureza orgânica é viável pois obtiveram resultados similares e superiores ao padrão nos filmes testados.
Polyolefins, especially polyethylenes (PE) are widely used polymeric materials for the production of tubular films, being one of the most widely used materials in the flexible packaging industry. Among the various industrial PE, low density polyethylene (LDPE) has unique rheological properties compared to linear and high density PE. The high viscosity and the numerous long branches found in this polymer influence the reduction of productivity when processed. Additives of the most varied compositions are added to the polyethylene during the tubular extrusion process in order to improve their properties. One of the most used additives to facilitate the flux of melt for film processing is synthetic waxes, the most used being oxidized polyethylene (CP). In the search for organic and / or natural source wax alternatives, the objective of this work is to evaluate the influence of the type and content of wax on the processability and final properties of tubular films of LDPE. Three types of waxes were used: CP (polyethylene wax), carnauba (CC) and glycerol monostearate (CM), and 4 formulations of LDPE / wax were processed in mass proportions of 99.5 / 0.5; 99/1; 98/2 and 96/4 m / m with the three types of wax and compared with LDPE without wax. The films were characterized by physical, optical, chemical, thermal, rheological and mechanical tests. Optical properties such as brightness and opacity are altered by the addition of waxes as an increase in the concentration of the waxes increases the degree of crystallinity of the films. Carnauba wax shows a yellowing in the films produced with higher concentrations. Results of this study showed that the use of synthetic wax, CP and natural carnauba, increase the productivity of the tubular film of LDPE, being the optimal content of 1% of wax, without having a significant influence on the thickness and width of the tubular film. The use of alternative flow agents of organic nature is feasible because they obtained similar and superior results to the standard in the films tested.

The overall theme of the research discussed in this dissertation has been to explore processing-structure-property relationships for submicron polymeric fibers produced by electrospinning (Part I) and to ascertain whether or not the length of the short chain branch has any effect on the physical properties of films of linear low-density polyethylenes (LLDPEs) (Part II). Electrospinning is a unique process to produce submicron fibers (as thin as 100 nm) that have a diameter at least two orders of magnitude smaller than the conventional fiber spinning processes based on melt and solution spinning. As a result, the electrospun fibers have a very high specific surface. The research efforts discussed in Part I of this dissertation relate to some fundamental as well as more applied investigations involving electrospinning. These include investigating the effects of solution rheology on fiber formation and developing novel methodologies to fabricate polymeric mats comprising of high specific surface submicron fibers of more than one polymer, high chemical resistant substrates produced by in situ photo crosslinking during electrospinning, superparamagnetic flexible substrates by electrospinning a solution of an elastomeric polymer containing ferrite nanoparticles of Mn-Zn-Ni and substrates for filtration applications. More specifically, it was found that the solution rheological parameters like concentration and viscosity, in addition to molecular weight play an important role in governing the fiber formation during electrospinning of polymer solutions. Furthermore, it was found that fiber formation depends strongly on the solution concentration regime, i.e., at low and dilute concentrations, droplets and beaded fibers were formed whereas uniform fibers were observed to form at a solution concentration greater than at least six times than that of the critical chain overlap concentration, c*, for linear homopolymers of poly(methyl methacrylate) that had molecular weight distributions ranging from 1.03-1.35 (Mw/Mn). In contrast, uniform fibers were observed at ten times the value of c* for the relatively broader molecular weight polymers (Mw/Mn~1.6-2.1). Novel methodologies were developed to in situ photocrosslink the electrospun jet to produce a crosslinked network in the form of a submicron fiber that could potentially be utilized for applications where a high resistance to chemical environments is required. In addition, flexible superparamagnetic substrates were developed by electrospinning a solution of an elastomeric polymer containing magnetic nanoparticles based on "mixed" ferrites of Mn-Zn-Ni where the specific saturation magnetization and the magnetic permeability of these substrates were found to increase linearly with the wt% loading of the nanoparticles. The methodology to simultaneously electrospin two polymer solutions in a side-by-side fashion was developed to produce bicomponent fibers with the rationale that the resulting electrospun mat will have properties from a combination from each of the polymer components. Bicomponent electrospinning of poly(vinyl chloride)- polyurethane and poly(vinylidiene fluoride)-polyurethane was successfully performed. In addition, filtration properties of single and bicomponent electrospun mats of polyacrylonitrile and polystyrene were investigated. Results indicated lower aerosol penetration or higher filtration efficiencies of the filters based on submicron electrospun fibers in comparison to the conventional filter materials. In addition, Part II of this dissertation explores whether or not the length of the short chain branch affects the physical properties of blown and compression molded films of LLDPEs that were synthesized by a single site metallocene catalyst. Here, three resins based on copolymers of ethylene/1-butene, ethylene/1-hexene, and ethylene/1-octene were utilized that were very similar in terms of their molecular weight and distribution, melt rheology, density, crystallinity and short chain branching content and its distribution. Interestingly, at higher deformation rates (ca. 1m/s), the breaking, tear and impact strengths of films based on ethylene/1-hexene and ethylene/1-octene were found to be superior than those based on ethylene/1-butene. While the origin of these differences in mechanical properties with increasing short chain branch length was not fully understood, the present investigation did confirm this effect to be pronounced only at high deformation rates for both the blown and compression molded LLDPE films.
Ph. D.

Les polymères sont utilisés dans de très nombreux secteurs d’activités tels que l’emballage, l’automobile, … car sont dotés de bonnes propriétés mécaniques, thermiques et barrières. Les enjeux économiques et environnementaux poussent le développement vers des matériaux plus performants et allégés. L’objectif de ce travail de thèse visait l’élaboration de films composites multicouches à base de polyéthylène (PE) et de polyamide (PA6) offrant de hautes propriétés barrières aux gaz et à l’eau. Un procédé de coextrusion équipé d’éléments multiplicateurs a permis de réaliser des multicouches de 100 μm d’épaisseur contenant jusqu’à 2049 couches. Deux séries de films PE-liant-PA6 de compositions différentes ont été étudiées. Des argiles (montmorillonites modifiées) ont été incorporées (à 0,5 et 5 wt%) dans les couches alternées de PE et PA6. Les propriétés structurales, thermiques, mécaniques de ces multicouches ont été corrélées aux propriétés de transport. Nous avons observé des effets de confinements sur la nanostratification des films et la cristallinité des polymères et l’impact sur les propriétés barrières. Nous avons montré la complexité des structures multinanocouches impliquant les interphases ainsi que la complexité des mécanismes de transfert. Le modèle en série de prédiction de perméabilité a mis en évidence des améliorations significatives des propriétés barrières aux gaz des couches confinées de PE mais pas à l’eau. L’effet barrière sur l’ensemble des multicouches a été toutefois limité en raison des orientations « on edge » des phases cristallines et des défauts de structure. Le confinement des nanocharges (à 1% v/v) dans les couches de PA6 a permis d’accroître les propriétés barrières des multicouches
Polymers are used in many fields such as packaging, automotive, etc. as they have good mechanical, thermal and barrier properties. Economic and environmental challenges are driving development towards more efficient and lighter materials. The aim of this thesis work was to develop multilayer composite films based on polyethylene (PE) and polyamide (PA6) with high gas and water barrier properties. A coextrusion process with multiplier elements made it possible to carry out 100 μm thick multilayers containing up to 2049 layers. Two series of PE-binder-PA6 films of different compositions were studied. Clays (organo-modified montmorillonites) were incorporated (at 0.5 and 5 wt%) into the alternating layers of PE and PA6. The structural, thermal and mechanical properties of these multilayers have been correlated with the transport properties. We observed confinement effects on nanostratification of films and crystallinity of polymers and the impact on barrier properties. We showed the complexity of the multinanolayer structures involving interphases as well as the complexity of the transfer mechanisms. The serial model for predicting permeability highlighted significant improvements in the gas barrier properties of confined PE layers but in water. The barrier effect on all of the multilayers was, however, limited due to the “on edge” orientation of the crystalline phases and structural defects. The confinement of nanofillers (at 1% v/v) in PA6 layers has made it possible to increase the barrier properties of multilayers

There is a need in industry for a commodity polyethylene film with controllable degradation properties that will degrade in an environmentally neutral way, for applications such as shopping bags and packaging film. Additives such as starch have been shown to accelerate the degradation of plastic films, however control of degradation is required so that the film will retain its mechanical properties during storage and use, and then degrade when no longer required. By the addition of a photocatalyst it is hoped that polymer film will breakdown with exposure to sunlight. Furthermore, it is desired that the polymer film will degrade in the dark, after a short initial exposure to sunlight. Research has been undertaken into the photo- and thermo-oxidative degradation processes of 25 ìm thick LLDPE (linear low density polyethylene) film containing titania from different manufacturers. Films were aged in a suntest or in an oven at 50 °C, and the oxidation product formation was followed using IR spectroscopy. Degussa P25, Kronos 1002, and various organic-modified and doped titanias of the types Satchleben Hombitan and Hunstsman Tioxide incorporated into LLDPE films were assessed for photoactivity. Degussa P25 was found to be the most photoactive with UVA and UVC exposure. Surface modification of titania was found to reduce photoactivity. Crystal phase is thought to be among the most important factors when assessing the photoactivity of titania as a photocatalyst for degradation. Pre-irradiation with UVA or UVC for 24 hours of the film containing 3% Degussa P25 titania prior to aging in an oven resulted in embrittlement in ca. 200 days. The multivariate data analysis technique PCA (principal component analysis) was used as an exploratory tool to investigate the IR spectral data. Oxidation products formed in similar relative concentrations across all samples, confirming that titania was catalysing the oxidation of the LLDPE film without changing the oxidation pathway. PCA was also employed to compare rates of degradation in different films. PCA enabled the discovery of water vapour trapped inside cavities formed by oxidation by titania particles. Imaging ATR/FTIR spectroscopy with high lateral resolution was used in a novel experiment to examine the heterogeneous nature of oxidation of a model polymer compound caused by the presence of titania particles. A model polymer containing Degussa P25 titania was solvent cast onto the internal reflection element of the imaging ATR/FTIR and the oxidation under UVC was examined over time. Sensitisation of 5 ìm domains by titania resulted in areas of relatively high oxidation product concentration. The suitability of transmission IR with a synchrotron light source to the study of polymer film oxidation was assessed as the Australian Synchrotron in Melbourne, Australia. Challenges such as interference fringes and poor signal-to-noise ratio need to be addressed before this can become a routine technique.

A reciclagem de resíduos sólidos tem se tornado cada vez mais relevante devido a alguns fatores como os impactos ambientais ocasionados com o seu descarte inadequado, ao rigor crescente de legislação específica e ao custo elevado para o gerenciamento apropriado destes resíduos. Na área de materiais poliméricos, os resíduos poliméricos podem ser de origem pósconsumo ou pós-industrial. Alguns rejeitos industriais poliméricos possuem características estruturais ou químicas que impossibilitam o seu reaproveitamento direto no processo que o gerou, como é o caso filmes multicamadas de polietileno de baixa densidade (PEBD) com poliamida 6 (PA6), usados para a produção de embalagens. O presente trabalho teve como objetivo central o reaproveitamento de rejeitos de filmes multicamadas de PEBD com PA6 na forma de blendas recicladas destes polímeros, fazendo uso de agente compatibilizante e variação na composição das blendas com a incorporação de PA6 virgem, a fim de produzir materiais com propriedades adequadas e custo compatível para a aplicação comercial. O desenvolvimento experimental foi dividido em três etapas que consistiram na avaliação do teor mais adequado de agente compatibilizante de fases da blenda (etapa 1), na determinação do comportamento das propriedades das blendas não compatibilizadas em diferentes composições de rejeito do filme multicamada e PA6 virgem (etapa 2) e na associação do uso de compatibilizante com variação das composições das blendas (etapa 3). A partir da execução experimental das atividades das etapas 1 e 2 do trabalho foi possível determinar que o teor de 2,5% do agente compatibilizante polietileno grafitizado com anidrido maleico (PEg- AM) é o mais adequado para compatibilizar as blendas poliméricas e que a maioria das suas propriedades das blendas apresentam um comportamento próximo ao aditivo linear, que leva em consideração somente as propriedades iniciais dos componentes poliméricos PEBD e PA6 isolados. Os diversos materiais gerados no desenvolvimento desta pesquisa apresentam desempenho de propriedades e custo de produção variáveis em função do teor de PA6 nas blendas, que podem viabilizar o uso deste material reciclado em aplicações comerciais específicas.
Recycling of solid waste has been an important issue due to some factors such as environmental damage caused by inappropriate discards, increasing rigor of specific legislation and high cost to the appropriate management of these waste. Polymeric waste can be originated from post-consume or post-industrial sources. Some polymeric waste have chemical or structural characteristics that difficult their direct reuse in the same productive process such as multilayer film of low density polyethylene (LDPE) and polyamide 6 (PA6), which are used for packing. The aim of this work has been the study the mechanical recycling of multilayer films of PEBD and PA6 as PEBD/PA6 blends with adequate properties and compatible cost for commercial applications. The experimental development has been performed in 3 steps, which correspond to evaluation of more adequate content of compatibilizer agent for interface of blends (step 1), determination of behavior of the properties of blends without compatibilizer agent on different compositions of waste form multilayer films (step 2) and production of blends with compatibilizer agent graphitized polyethylene with maleic anhydride (PE-g-MA) (step 3). Has been verified that the content of 2.5% of PE-g-MA is more adequate to be used in the compatibilization of recycled PEBD/PA blends and that the performance of the blends dependents of the content of PA6 in the material. The recycled materials have potential for commercial applications.

Orientador: Jose de Assis Fonseca Faria
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia de Alimentos
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Resumo: O objetivo deste trabalho consistiu no processamento e avaliação de desempenho de um sistema asséptico piloto para filme de polietileno de baixa densidade (PEBD). Através da metodologia de planejamento experimental fatorial foram feitas variações de tempo de contato, temperatura do banho e concentração da solução esterilizante, bem como a variação do tempo de exposição à radiação ultravioleta (UVC). Estas variações tiveram por finalidade promover de 4 a 6 reduções decimais na população inicial de esporos de Bacillus subtilis var. globigii ATCC9372, em um curto espaço de tempo, gerando o menor residual do esterilizante em água envasada. Foram inoculados uniformemente 100µL do microrganismo teste em uma superfície de 16cm2. Testou-se como agente de esterilização química, o peróxido de hidrogênio (H2O2) e, como agente físico, a radiação UVC, sendo o primeiro aplicado na forma de banho de imersão e o segundo, por uma exposição a 2,5cm de distância do filme de PEBD logo após o banho de imersão. O sistema utilizado constituiu em uma envasadora automática e um protótipo com um tanque em aço inoxidável, desenvolvido especialmente para este tipo de teste. O H2O2 e a radiação UVC apresentaram ação esporicida significativa. O H2O2 demonstrou ação esporicida entre as concentrações de 18 e 35%, em um intervalo de temperatura de 46 a 70ºC, promovendo de 3 a 7 reduções decimais de Bacillus subtilis. A concentração de 20% de H2O2 não conferiu sabor estranho à água mineral envasada em embalagens de PEBD sanificadas, não diferindo, significativamente (p<0,05), do padrão. O residual de H2O2 das superfícies-teste sanificadas com soluções contendo de 18 a 35% de H2O2 foi superior a 0,5mg/L, nível máximo permitido pela FDA presente em água envasada sob as condições de produção. Os dados obtidos forneceram dois modelos matemáticos quadráticos, representativos da ação do H2O2 e radiação UVC, em função do tempo e da temperatura, sobre esporos de B. subtilis. Tais modelos, com R2 de 0,9349 para os tratamentos sem radiação UVC, e R2 de 0,9763 para os tratamentos com radiação UVC, limitaram-se a uma concentração de 35% de H2O2 e tempo máximo de exposição à radiação UVC de 8s. Superfícies-teste sanificadas com soluções de 28% de H2O2 a 60ºC, por 8s sem exposição à radiação UVC apresentaram 4 ciclos de redução decimal. Para as mesmas condições, porém com exposição à radiação UVC por 8s, este valor elevou-se para 7, demonstrando a existência do sinergismo entre a associação destes dois agentes. O sistema de esterilização testado apresentou desempenho satisfatório, podendo ser perfeitamente utilizado por indústrias de pequeno e médio porte na esterilização de filmes plásticos, sendo capaz de promover até 7 reduções decimais na população de esporos bacterianos
Abstract: The aim of this research consisted in the processing and performance evaluation of an aseptic pilot system for Low Density Polyethylene film (LDPE). Through the screening design methodology, variations of contact time, bath temperature, sanificant concentration and exposition time variation of ultraviolet radiation (UVC) was made. Such variations had as purpose to promote from 4 to 6 decimal reductions in the Bacillus subtilis var. globigii ATCC9372 spores, when applied to the LDPE film surface, in a short time, generating the minor residual of the sanificant in the plastic surface. Was evaluated as chemical agent, the hydrogen peroxide (H2O2) and as physical agent, UVC radiation, being the first one applied as immersion bath and the second one as an exposition after the immersion bath, with 2,5cm of distance of the PEBD film. The system used was constituted in an automatic filling machine and an equipment with a stainless steel tank, especially developed for this type of test. H2O2 showed sporicidal action in from 18 to 35%, in temperatures of 46 to 70ºC, causing from 3 to 7 decimal reductions. The H2O2 20% did not confer off-taste in mineral water used as filling product, as well no significant difference from the standard was observed (p<0,05). The hydrogen peroxide residual surface-test after sanitation with 18 to 35% H2O2 was higher than 0,5mg/L. However, the use of hot air knives or roller pressers after the immersion bath will be able to diminish this residual. The quadratic model, with R2 of 0,9349 for the treatments without UVC radiation and R2 of 0,9763 for the treatments with UVC radiation are restricted to 35% H2O2 and UVC maximum exposition time of 8s. Surface-test treated with 28% H2O2 / 60ºC / 8s without UVC radiation exposition had presented 4 decimal reduction. In the same conditions, however with UVC radiation exposition (8s), this value was raised for 7, demonstrating the existence of synergism between these two agents. The sanification system tested showed satisfactory performance, effective to be used in small and medium industries, being capable to promote up to 7 decimal reductions in spores population
Mestrado
Mestre em Tecnologia de Alimentos

Antimicrobial (AM) films containing naturally-derived AM agents, thymol or carvacrol, were developed. These agents were incorporated into a low-density polyethylene (LDPE) substrate using the techniques of both compression moulding and extrusion film blowing. Different film formulations containing additive polymers, ethylene vinyl acetate (EVA) and polyethylene glycol (PEG), were investigated for their potential to enhance the retention and to control the release of the AM agents from the films. The physical and mechanical properties of the films were evaluated in order to assess the effect that the addition of AM agent has on the ultimate properties of the film. The extruded films were also studied for AM agent release and retention during storage. Films were tested for AM activity in-vitro enabling the optimum concentrations for AM activity to be obtained and the effect of AM films on the bacterial inhibition in liquid media to be successfully modelled. The effects of combined AM systems in LDPE/EVA-based films were also studied. Having identified the AM activity in laboratory media, the films were then used to package Cheddar cheese in order to assess the usefulness of AM films in the enhancement of food preservation. The AM films had a positive effect on the microbial and physio-chemical attributes of Cheddar cheese under actual storage conditions. The addition of higher concentrations of AM agents clearly imparted a noticeable odour to Cheddar cheese during storage. The shelf life extension of Cheddar cheese by AM films was affected by the type and concentration of the AM agent.

The integration of antimicrobial (AM) agents into packaging materials is aimed at killing or inhibiting the spoilage and pathogenic microorganisms that may contaminate packaged food products. Over the years there has been an increased emphasis on naturally derived AM agents and polymer films containing AM agents derived from basil, for example, exhibit an AM effect against a wide spectrum of microorganisms. Due to the relatively high temperatures involved in manufacturing such films, however, there is a considerable evaporation loss of AM agent during the film blowing process. The present study aims at developing effective AM films and subsequently reducing the loss of active AM agents. The effect of polyethylene glycol (PEG) and ethylene vinyl acetate (EVA) in minimising the loss of active AM agent during the manufacturing of low-density polyethylene (LDPE) film is explored by measuring the release of AM agent into food simulants. The release of AM agents from the film is satisfactorily and consistently described by short-term and long-term migration equations and by first order kinetics. Furthermore, the polymer additives PEG and EVA play a role in controlling the release of the AM agents. The incorporation of AM agent does not adversely affect the mechanical or optical properties of the extruded LDPE/EVA film and the films retain ca. 75% of AM agents after extrusion.

The inherent volatility and/or heat sensitivity of many natural antimicrobial (AM) additives can be detrimental to their widespread use in commodity polymer packaging film formulations. In this study, beta-cyclodextrin (β-CD) inclusion complexes with naturally-derived AM agents: thymol, carvacrol, and linalool were prepared using a co-precipitation technique. The complexes were optimised and then characterised by techniques including thermal analysis. They were then incorporated into low-density polyethylene (LDPE) films with AM agents added directly for comparison. The subsequent release of the AM agents into food simulants was studied followed by an investigation of the efficacy of the films in vitro against selected bacteria. The films were later tested on real foods to assess their potential for controlling microbial growth and lipid oxidation. In the initial experiments, conditions for synthesising the β-CD/AM agent complex including solvent composition, temperature, reaction time, and total solvent volume were investigated to optimise the inclusion efficiency (IE) and yield. Electrospray ionization mass spectrometry and gas chromatography were used to confirm the formation and quantify the amount AM agents that were encapsulated, absorbed onto the surface, or remaining in the filtered solvent. The systematic optimisation of the conditions improved both the yield of the complex and the IE of the AM agents. Using a 1:1 mole ratio of the AM agent to β-CD, the optimised parameters resulted in maximum yields of 87, 84 and 86% (w/w) for thymol, carvacrol and linalool respectively with IE’s close to 100% (w/w) for each agent. The kinetics of the thermal decomposition of the optimised β-CD and complexes of the three AM agents were then investigated using thermogravimetric analysis. Under a linear temperature ramp and in the degree of conversion, α, domain: 0.1 ≤ α ≤ 0.8, the major decomposition steps of the β-CD, and complexes with carvacrol, linalool and thymol occurred at ca. 300°C and followed Avrami-Erofeev kinetics with apparent activation energies, Ea, of: 156 ± 6, 107 ± 7, 96 ± 3 and 110 ± 3 kJ mol-1 respectively. Below ca. 300°C there were staged mass losses from each of the complexes that were not observed for the neat β-CD. These were attributed to lower energy binding interactions and accounted for a little over half of the available guest species in the complex in each case. Lower temperature mass losses for β-CD complexes with carvacrol (ca. 140 to 230°C) and linalool (ca. 95 to 150°C) were analysed and found to be adequately fitted by second-order kinetics with apparent Ea values of: 37 ± 1 and 69 ± 6 kJ mol-1 respectively. The results suggest the optimized complexes are generally thermally stable and would potentially be suitable for high-temperature extrusion processes with acceptably low losses. The next experiments involved the incorporation of the AM agents into LDPE films either directly or encapsulated in β-CD. Quantification of the AM agents was performed immediately following thermal processing, then six and thirty days after the film samples were stored in an open atmosphere. After six days, no AM agent was detected in the films where the agent was added directly to the film whereas the films containing encapsulated agents showed only small decreases in the concentrations of the agents up to 30 days. The migration of AM agents from LDPE films into 95% (v/v) ethanol/water mixtures food simulants at 4℃ was adequately described using first-order kinetics and Fick’s second law of diffusion. For the AM agents added directly to the film, the initial release rates were between four and eight times greater than those of the encapsulated agents. Similarly, the diffusion coefficients of the free agents were ca. four to five times greater than the encapsulated agents. The free and encapsulated natural AM agents incorporated into LDPE film were tested against Escherichia coli (ATCC 25922) in order to assess the potential of the AM inclusion complexes for use in food packaging films. The direct incorporation of the complexes in the film formulations resulted in little inhibition of the target bacterium as assessed by the agar diffusion method even with AM levels as high as 5% (w/w). In comparison, levels of 2% (w/w) of free thymol and carvacrol added directly to the film demonstrated inhibition. The addition of glycerol to the film formulations was investigated as a means of facilitating the AM agent from the complex. A concentration of 1% (w/w) of glycerol in the film formulation was found to result in microbial inhibition which increased with additional glycerol. The use of 2% (w/w) glycerol resulted in a more pronounced inhibition of targeted microorganism. Upon the addition of glycerol, all of the films showed AM activity against the target bacterium with the exception of those containing linalool in either the free or encapsulated forms. Upon the addition of 2% (w/w) of glycerol to the film formulation, encapsulated thymol at a concentration of 2% (w/w) was more effective than encapsulated carvacrol at a concentration of 3% (w/w) against E. coli with zones of inhibition of 30.70 ± 0.72 and 29.61 ± 0.86 mm respectively. In the final experiments, the LDPE films containing encapsulated thymol were tested on real food systems. The level of thymol was 1 to 3% (w/w) relative to the LDPE and glycerol was added in order to obtain the optimum controlled release. In the case of packaged minced beef inoculated with E. coli, no inhibition was observed when the concentration of encapsulated thymol was 3% (w/w) with 2% (w/w) glycerol, however, the same film reduced E. coli growth by 0.7 log10 CFU g-1 on chicken breast fillets compared with the control during storage at 4℃ for 12 days. The growth of E. coli was found to be affected by the temperature at 4℃ whereby the bacterial counts remained relatively low with slow growth over the test period under refrigeration. However, when the temperature increased to 10℃ it was also found that the presence of coliforms interfered with growth of E. coli and, in general, the films containing encapsulated thymol effectively reduced coliform growth. Analysis of the antioxidant (AO) activity of films using the diphenyl-picrylhydrazyl (DPPH) radical assay showed a 71% reduction in DPPH concentration for the LDPE/thymol/β-CD films containing 3% (w/w) thymol. Furthermore, a film comprised of 1% (w/w) thymol with glycerol stored at room temperature for 20 months showed a reduction by 23% in DPPH concentration confirming that the films are suitable for extended storage. Analysis of the formation of thiobarbituric acid reactive substances on packaged minced beef showed decreases in lipid oxidation of 60 and 75% for films containing 1% and 3% (w/w) thymol in the film respectively. The films therefore show promise for the dual purpose of AO and AM activity in order to prolong the shelf-life of selected food products.

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Pressure variation during the coextrusion of two low density polyethylene melts was investigated. Melt streams were delivered to a die from two separate extruders to converge in a 30 degrees degrees geometry to form a two layer extrudate. Melt flow in the confluent region and die land to the die exit was observed through side windows of a visualisation cell. Stream velocity ratio was varied by control of extruder screw speeds. Layer thickness ratios producing wave type interfacial instability were quantified for each melt coextruded on itself and for the combined melts. Stream pressures and screw speeds were monitored and analysed. Wave type interfacial instability was present during the processing of the melts at specific, repeatable, stream layer ratios. Increased melt elasticity appeared to promote this type of instability. Analysis of process data indicates little correlation between perturbations in extruder screw speeds and stream pressures. The analysis did however show covariance between the individual stream pressure perturbations. Interestingly there was significant correlation even when interfacial instability was not present. We conclude that naturally occurring variation in extruder screw speeds do not perturb stream pressures and, more importantly, natural perturbations in stream pressures do not promote interfacial instability.

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The effect of side stream channel height on flow stability in 30 degrees coextrusion geometries was investigated. The studies were conducted on a Dow LD150R low density polyethylene melt using a single extruder to feed a flow cell in which the delivered melt stream was split before, and rejoined after, a divider plate in a slit die. Wave type interfacial instability occurred at critical stream thickness ratios. Reducing the side stream channel height broadened the layer ratio operating range before the onset of interfacial instability, therefore improving process stability. Stress fields were quantified and used to validate principal stress differences of numerically modelled flow. Stress field features promoting interfacial instability in each of the die geometries were identified. Interfacial instability resulted when the stress gradient across the interface was asymmetric and accompanied by a non-monotonic decay in the stress along the interface from its inception.

Defects in polyethylene film are often caused by contaminant particles in the polymer melt. In this research, particle properties obtainable from in-line melt monitoring, combined with processing information, are used to predict film defect properties. “Model” particles (solid and hollow glass microspheres, aluminum powder, ceramic microspheres, glass fibers, wood particles, and cross-linked polyethylene) were injected into low-density polyethylene extruder feed. Defects resulted when the polyethylene containing particles was extruded through a film die and stretched by a take-up roller as it cooled to form films 57 to 241mm in thickness. Two off-line analysis methods were further developed and applied to the defects: polarized light imaging and interferometric imaging. Polarized light showed residual stresses in the film caused by the particle as well as properties of the embedded particle. Interferometry enabled measures of the film distortion, notably defect volume. From the images, only three attributes were required for mathematical modeling: particle area, defect area, and defect volume. These attributes yielded two ”primary defect properties”: average defect height and magnification (of particle area). For all spherical particles, empirical correlations of these properties were obtained for each of the two major types of defects that emerged: high average height and low average height defects. Analysis of data for non-spherical particles was limited to showing how, in some cases, their data differed from the spherical particle correlations. To help explain empirical correlations of the primary defect properties with film thickness, a simple model was proposed and found to be supported by the high average height defect data: the “constant defect volume per unit particle area” model. It assumes that the product of average defect height and magnification is a constant for all film thicknesses. A numerical example illustrates how the methodology developed in this work can be used as a starting point for predicting film defect properties in industrial systems. A limitation is that each prediction yields two pairs of primary defect property values, one pair for each defect type. If it is necessary to identify the dominant type, then measurement of a length dimension of sufficient defects in the film is required.