Direct Extrusion Freeforming of Ceramic Pastes

Microextrusion freeforming of ceramic lattices from high solids ceramic pastes provides multi-scale hierarchical void structures with the advantages of low shrinkage stress and high sintered density. Alumina lattices were directly fabricated using 80-500 Pm diameter filaments. We report here on the implementation of design and fabrication of these scaffolds for band gap materials and micro fluidic devices.Mechanical Engineerin

Crack patterns in drying protein solution drops

A deposited drop of bovine serum albumin salt solution experiences both gelation and fracturation during evaporation. The cracks appearing at the edge of the gelling drop are regularly spaced, due to the competition between the evaporation-induced and relaxation-induced stress evolution. Subsequently, the mean crack spacing evolves in an unexpected way, being inversely proportional instead of proportional to the deposit thickness. This evolution has been ascribed to the change with time of the average shrinkage stress, the crack patterning being purely elastic instead of evaporation-controlled

Interfaces and interfacial effects in glass reinforced thermoplastics - Keynote Presentation

Optimization of the fibre-matrix interphase region is critical to achieving the required performance level in thermoplastic matrix composites. Due to its initial location on the fibre surface, the sizing layer is an important component in the formation and properties of the composite interphase. Consequently, any attempt to understand the science of the composite interphase must encompass an understanding of the science of sizing. In this paper the role of sizings from fibre manufacture through to performance of composite parts is reviewed. In particular the role of organosilane coupling agents and how the formation of a polysiloxane interphase is influenced by the surface properties of the fibre is examined. The influence of the sizing film former in terms of its level of interaction with the silane coupling agent is also examined. The importance of residual stresses in thermoplastic composites in the values obtained for the apparent adhesion levels in these systems is highlighted. These residual stresses are shown to play a significant role in determining the level of interfacial strength in thermoplastic composites and in particular in polyolefin matrices. By applying some of the available models for this phenomenon this analysis is extended to explore the effect of the anisotropic fibre microstructure of carbon, aramid and natural fibres on the apparent interfacial strength in thermoplastic composites

Polymerization shrinkage stress of composites photoactivated by different light sources

The purpose of this study was to compare the polymerization shrinkage stress of composite resins (microfilled, microhybrid and hybrid) photoactivated by quartz-tungsten halogen light (QTH) and light-emitting diode (LED). Glass rods (5.0 mm x 5.0 cm) were fabricated and had one of the surfaces air-abraded with aluminum oxide and coated with a layer of an adhesive system, which was photoactivated with the QTH unit. The glass rods were vertically assembled, in pairs, to a universal testing machine and the composites were applied to the lower rod. The upper rod was placed closer, at 2 mm, and an extensometer was attached to the rods. The 20 composites were polymerized by either QTH (n=10) or LED (n=10) curing units. Polymerization was carried out using 2 devices positioned in opposite sides, which were simultaneously activated for 40 s. Shrinkage stress was analyzed twice: shortly after polymerization (t40s) and 10 min later (t10min). Data were analyzed statistically by 2-way ANOVA and Tukey's test (a=5%). The shrinkage stress for all composites was higher at t10min than at t40s, regardless of the activation source. Microfilled composite resins showed lower shrinkage stress values compared to the other composite resins. For the hybrid and microhybrid composite resins, the light source had no influence on the shrinkage stress, except for microfilled composite at t10min. It may be concluded that the composition of composite resins is the factor with the strongest influence on shrinkage stress.Este estudo comparou a contração de polimerização de resinas compostas fotoativadas por luz halógena (QTH) e diodo emissor de luz (LED). Foram confeccionados bastões de vidro (5,0 mm x 5,0 cm), e uma de suas extremidades sofreu jateamento com óxido de alumínio, sobre a qual foi aplicado um adesivo e fotoativado com luz halógena. Os bastões de vidro foram acoplados verticalmente, em pares, em uma máquina universal de ensaios (EMIC DL-2000) e as resinas compostas aplicadas no bastão inferior. A distância entre os bastões foi padronizada em 2 mm e um extensômetro foi acoplado a eles. As resinas foram fotoativadas (n=20), sendo 10 por QTH e 10 por LED utilizando dois aparelhos posicionados em lados opostos, acionados simultaneamente por 40 s. A tensão de contração foi analisada em dois momentos: logo após a polimerização (t40s) e 10 min após (t10min). A tensão de contração apresentada por todas as resinas foi maior em t10min do que em t40s, independente da fonte ativadora. A resina de micropartículas apresentou menores valores de tensão de contração com valores estatisticamente significantes em relação às demais resinas. Para as resinas híbrida e microhíbrida não houve influência da unidade ativadora sobre a tensão de contração, com exceção para a resina de micropartículas em t10min. Concluiu-se que a composição da resina composta foi o fator que mais interferiu na tensão de contração da resina composta

Polymerization shrinkage stress: effect of composite type and placement technique

Efst á síðunni er hægt að nálgast greinina í heild sinni með því að smella á hlekkinnInngangur: Tilgangur rannsóknarinnar var að kanna samdráttarálag hefðbundinna og magnfyllingar- (e. bulk-fill) plastblendifyllingarefna á tannvef með því að mæla kúspaspennu. Efniviður og aðferðir: Fimmtíu efri góms forjaxlar voru steyptir niður í plasthringi og þeim skipt niður í fimm hópa (n=10). Staðlaður MOD tannskurður var skorinn í hverja tönn. Tveggja þátta sjálfætandi bindiefni (OptiBond XTR) var borið á tannskurðinn og fyllingar gerðar með aðstoð sílíkon matrixu sem hér segir: Filtek Supreme Ultra í tveggja millimetra þykkum lögum (FSUI); Filtek Supreme Ultra með magnfyllingaraðferð (FSUB); SonicFill með magnfyllingaraðferð (SF); SureFil SDR flow með magnfyllingaraðferð, hulið tveggja millimetra þykku lagi af Filtek Supreme Ultra (SDR/FSU); Tetric EvoCeram Bulk Fill með magnfyllingaraðferð (TEBF). Spennunemar (e. strain gages) festir á kinn- og gómafleti mældu kúspaspennu (e. microstrain, µε) í rauntíma á meðan plastblendifyllingarefnum var komið fyrir og þau fjölliðuð. Gögn voru greind með one-way ANOVA prófi og hópar bornir saman með Least-Squares Means prófi. Niðurstöður: Meðal kúspaspenna hópanna (µε±SD) var sem hér segir: FSUI: 730.6±104.8, FSUB: 1264.2±1418.8, SF: 539±75.9, SDR-FSU: 506.3±69.3, TEBF: 624.1±147.4. Hópur FSUI mældist með marktækt meiri kúspaspennu en allir magnfyllingarplastblendihóparnir. Einnig mældist hópur TEBF með marktækt meiri kúspaspennu en hópur SDR/FSU. Nauðsynlegt var að útiloka hóp FSUB frá tölfræðilegri úrvinnslu sökum þess hversu frábrugðin meðaltal og stalaðfrávik hans voru miðað við aðra hópa. Ályktun: Ísetning allra magnfyllingarplastblenda leiddi til marktækt minni kúspaspennu en ísetning hefðbundins plastblendis í tveggja millimetra lögum, þó svo að nokkur breytileiki hafi mælst milli hópa magnfyllingarplastblenda. Notkun hefðbundins plastblendis í magnfyllingaraðferð er varasöm og getur leitt til kúspabrota.Objective: : To compare the polymerization shrinkage stress exerted on tooth structure by bulk-fill and conventional composite resin materials, by measuring cuspal strain. Materials and methods: Fifty extracted maxillary premolars were mounted into ring molds and divided into five groups (n=10). Strain gages were attached to the buccal and lingual cusps. Custom proximal matrices were made using poly vinylsiloxane. Large MOD cavity preparations were performed with the following dimensions: the width of the occlusal isthmus was 40% of the maximum buccopalatal width of the specimen, the pulpal floor was 3.5 mm below the palatal cusp tip, the axial wall was 2 mm high, and the axial wall depth was 25% of the maximum mesiodistal width of the specimen. A 2-step universal self-etching adhesive (Optibond XTR) was applied. Two high-viscosity (SonicFill and Tetric EvoCeram Bulk Fill) and one low-viscosity (SureFil SDR flow) were used for the experimental groups. As a control, a conventional nanofilled composite resin (Filtek Supreme Ultra) was used. The specimens were restored as follows: SonicFill in bulk (SF); SureFil SDR flow in bulk, covered with a 2-mm occlusal layer of Filtek Supreme Ultra (SDR-FSU); Tetric EvoCeram Bulk Fill in bulk (TEBF). As positive and negative control groups, Filtek Supreme Ultra was placed in 2mm increments (FSUI), and in bulk (FSUB), respectively. Strain gages recorded cuspal strain in real-time as the restorations were performed, and mean maximum strain values for buccal and lingual cusps were calculated for each group. Data were subjected to one-way ANOVA and pairwise comparisons using LS Means. Results: Mean maximum strain values and standard deviations (in µε) were: SF: 539±75.9, SDR-FSU: 506.3±69.3, TEBF: 624.1±147.4, FSUI: 730.6±104.8, FSUB: 1264.2±1418.8. A significant difference was found between group FSUI and groups SF, SDR-FSU and TEBF, as well as between groups SDR-FSU and TEBF. The FSUB group was excluded from the statistical analysis due to excessively high mean and standard deviation strain values resulting from cuspal fractures. Conclusions: All tested bulk-fill composites exerted less contraction stress on tooth structure than the incrementally placed conventional composite resin. Shrinkage stress generated by bulk-fill materials seems to be product-dependent. Bulk-filling with conventional composite resins is unpredictable and contraindicated and can have detrimental effects on both the tooth structure and the adhesive interface

Class I Gap-formation in Highly-viscous Glass-ionomer Restorations: Delayed vs Immediate Polishing

This in vitro study evaluated the effects of delayed versus immediate polishing to permit maturation of interfacial gap-formation around highly viscous conventional glass-ionomer cement (HV-GIC) in Class I restorations, together with determining the associated mechanical properties. Cavity preparations were made on the occlusal surfaces of premolars. Three HV-GICs (Fuji IX GP, GlasIonomer FX-II and Ketac Molar) and one conventional glass-ionomer cement (C-GIC, Fuji II, as a control) were studied, with specimen subgroups (n=10) for each property measured. After polishing, either immediately (six minutes) after setting or after 24 hours storage, the restored teeth were sectioned in a mesiodistal direction through the center of the model Class I restorations. The presence or absence of interfacial-gaps was measured at 1000× magnification at 14 points (each 0.5-mm apart) along the cavity restoration interface (n=10; total points measured per group = 140). Marginal gaps were similarly measured in Teflon molds as swelling data, together with shear-bond-strength to enamel and dentin, flexural strength and moduli. For three HV-GICs and one C-GIC, significant differences (p<0.05) in gap-incidence were observed between polishing immediately and after one-day storage. In the former case, 80–100 gaps were found. In the latter case, only 9–21 gaps were observed. For all materials, their shear-bond-strengths, flexural strength and moduli increased significantly after 24-hour storage.</p

Interfacial strength in thermoplastic composites - at last an industry friendly measurement method?

Many elegant techniques have been developed for the quantification of composite micro-mechanical parameters in recent years. Unfortunately, most of these techniques have found little enthusiastic support in the industrial product development environment, where they are viewed as time consuming, complex, inefficient, labour intensive, and in many cases unproven or inapplicable in 'real' systems. Despite this reaction, there is a real need for a 'user-friendly' micro-mechanics to aid the composites industry to move to the next level of development. A method for deriving values for τ (the interfacial shear strength) and ηo (a fibre orientation factor) from a simple combination of the composite tensile stress-strain curve and the fibre length distribution has been available for some time. Despite the recent wealth of activity in the development of micro-mechanical test techniques, there has been little follow-up on this older technique. In this paper we explore this analysis by its application to injection moulded glass-fibre-reinforced thermoplastic composites produced using three matrices (polypropylene, polyamide 6,6 and polybutyleneterephthalate) and containing different levels of glass-fibre. We furthermore show how the analysis can be extended to obtain another important micro-mechanics parameter, σuf, the fibre stress at composite failure. Values of τ and ηo obtained using this improved version of the original model are presented and discussed

Effect of light-curing units and activation mode on polymerization shrinkage and shrinkage stress of composite resins

The aim of this study was to evaluate the polymerization shrinkage and shrinkage stress of composites polymerized with a LED and a quartz tungsten halogen (QTH) light sources. The LED was used in a conventional mode (CM) and the QTH was used in both conventional and pulse-delay modes (PD). The composite resins used were Z100, A110, SureFil and Bisfil 2B (chemical-cured). Composite deformation upon polymerization was measured by the strain gauge method. The shrinkage stress was measured by photoelastic analysis. The polymerization shrinkage data were analyzed statistically using two-way ANOVA and Tukey test (p<0.05), and the stress data were analyzed by one-way ANOVA and Tukey's test (p<0.05). Shrinkage and stress means of Bisfil 2B were statistically significant lower than those of Z100, A110 and SureFil. In general, the PD mode reduced the contraction and the stress values when compared to CM. LED generated the same stress as QTH in conventional mode. Regardless of the activation mode, SureFil produced lower contraction and stress values than the other light-cured resins. Conversely, Z100 and A110 produced the greatest contraction and stress values. As expected, the chemically cured resin generated lower shrinkage and stress than the light-cured resins. In conclusion, The PD mode effectively decreased contraction stress for Z100 and A110. Development of stress in light-cured resins depended on the shrinkage value

Comparative analysis of the shrinkage stress of composite resins

The aim of this study was to compare the shrinkage stress of composite resins by three methods. In the first method, composites were inserted between two stainless steel plates. One of the plates was connected to a 20 kgf load cell of a universal testing machine (EMIC-DL-500). In the second method, disk-shaped cavities were prepared in 2-mm-thick Teflon molds and filled with the different composites. Gaps between the composites and molds formed after polymerization were evaluated microscopically. In the third method, the wall-to-wall shrinkage stress of the resins that were placed in bovine dentin cavities was evaluated. The gaps were measured microscopically. Data were analyzed by one-way ANOVA and Tukey's test (alpha=0.05). The obtained contraction forces were: Grandio = 12.18 &plusmn; 0.428N; Filtek Z 250 = 11.80 &plusmn; 0.760N; Filtek Supreme = 11.80 &plusmn; 0.707 N; and Admira = 11.89 &plusmn; 0.647 N. The gaps obtained between composites and Teflon molds were: Filtek Z 250 = 0.51 &plusmn; 0.0357%; Filtek Supreme = 0.36 &plusmn; 0.0438%; Admira = 0.25 &plusmn; 0.0346% and Grandio = 0.16 &plusmn; 0.008%. The gaps obtained in wall-to-wall contraction were: Filtek Z 250 = 11.33 &plusmn; 2.160 &micro;m; Filtek Supreme = 10.66 &plusmn; 1.211&micro;m; Admira = 11.16 &plusmn; 2.041 &micro;m and Grandio = 10.50 &plusmn; 1.224 &micro;m. There were no significant differences among the composite resins obtained with the first (shrinkage stress generated during polymerization) and third method (wall-to-wall shrinkage). The composite resins obtained with the second method (Teflon method) differed significantly regarding gap formation.Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES

Development of high shrinkage Polyethylene Terephthalate (PET) shape memory polymer tendons for concrete crack closure

YesThe shrinkage force exerted by restrained shape memory polymers can potentially be used to close cracks in structural concrete. This paper describes the physical processing and experimental work undertaken to develop high shrinkage die-drawn Polyethylene Terephthalate (PET) shape memory polymer tendons for use within a crack closure system. The extrusion and die-drawing procedure used to manufacture a series of PET tendon samples is described. The results from a set of restrained shrinkage tests, undertaken at differing activation temperatures, are also presented along with the mechanical properties of the most promising samples. The stress developed within the tendons is found to be related to the activation temperature, the cross-sectional area and to the draw rate used during manufacture. Comparisons with commercially-available PET strip samples used in previous research are made, demonstrating an increase in restrained shrinkage stress by a factor of two for manufactured PET filament samples.Thanks must go to the EPSRC for their funding of the Materials for Life (M4L) project (EP/K026631/1) and to Costain Group PLC. for their industrial sponsorship of the project and author