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Las bandas extensométricas para analizar el estrés de polimerización: Strain guage analysis
Jiménez de Miguel,O.F.;
Avances en Odontoestomatología , 2004, DOI: 10.4321/S0213-12852004000600002
Abstract: the aim of this study is to analyse the use of extensometry as a research method in restorative dentistry. we made class 11cavities in premolars previously selected, in which we set strain gauges 0,5 cm away from the margin cavity and parallel to the cavity' s floor. all phases in process were registered with strain gauges. the advantages and disadvantages of this technician dental materials are taking into account.
Polymerisation shrinkage versus layer thickness of a dentine bonding resin: Method development
Jafarzadeh T,?Erfan M,Watts DC,ash A
Journal of Dental Medicine , 2002,
Abstract: Dentine bonding systems are usually unfilled, and so their shrinkage may be significant. High"nshrinkage may cause internal stress at the interface between resin-composite restoration and the dentine"nsubstrate. Failure of the adhesive interface may be observed due to the interna! stress. The aims of this"nstudy were:"nA) To obtain a suitable method for measuring the kinetics of polymerisation shrinkage in unfilled resm at different thicknesses, particularly for thin films."nB) Consideraing the effect of thickness on shrinkage."nScotchbond Multipurpose (3M) adhesive bond resin was used. To overcome the particular challenges presented by thin films, a filled-ring measurement procedure was used. Also, a non-contact laser analogue displacement sensor system was developed and applied to measure polymerisation shrinkage. Regression analysis was performed on a complete data set. Non-linear regression analysis established a logarithmic relationship between polymerisation shrinkage and layer thickness. The method applied in this study was found to be sensitive and accurate procedure for determining photo-polymerisation shrinkage of thin films. Polymerisation shrinkage increased with logarithmic of the adhesive thickness.
CURING OF POLYMERIC COMPOSITES USING MICROWAVE RESIN TRANSFER MOULDING (RTM)  [PDF]
R. YUSOFF,M. K. AROUA,A. NESBITT,R. J. DAY
Journal of Engineering Science and Technology , 2007,
Abstract: The main objective of this work is to compare the difference between microwave heating and conventional thermal heating in fabricating carbon/epoxy composites. Two types of epoxy resin systems were used as matrices, LY5052-HY5052 and DGEBA-HY917-DY073. All composite samples were fabricated using resin transfer moulding (RTM) technique. The curing of the LY5052-HY5052-carbon and the DGEBA-HY917-DY073-carbon composite systems, were carried out at 100 °C and 120 °C, respectively. Microwave heating showed better temperature control than conventional heating, however, the heating rate of the microwave cured samples were slower than the conventionally cured samples. This was attributed to the lower power (250 W) used when heating with microwaves compared to 2000 W used in conventional heating. Study of thermal characteristics as curing progressed showed that the polymerisation reaction occurred at a faster rate during microwave curing than in conventional curing for both the DGEBA and the LY/HY5052 carbon composite systems. The actual cure cycle was reduced from 60 minutes to 40 minutes when using microwaves for curing DGEBA-carbon composites. As for LY/HY5052-carbon composites, the actual cure cycle was reduced from 3 hours to 40 minutes. Both conventional and microwave heating yielded similar glass transition temperatures (120 °C for DGEBA systems and 130 °C for LY/HY5052 systems). Microwave cured composites had higher void contents than conventionally cured composites (2.2-2.8% and 1.8-2.4% for DGEBA and LY/HY5052 microwave cured composites, respectively, compared to 0.2-0.4% for both DGEBA and LY/HY5052 thermally cured composites). C-scan traces showed that all composites, regardless of methods of curing, had minimal defects.
Symmetry-breaking in chiral polymerisation  [PDF]
Jonathan AD Wattis,Peter V. Coveney
Physics , 2004, DOI: 10.1007/s11084-005-0658-7
Abstract: We propose a model for chiral polymerisation and investigate its symmetric and asymmetric solutions. The model has a source species which decays into left- and right-handed types of monomer, each of which can polymerise to form homochiral chains; these chains are susceptible to `poisoning' by the opposite handed monomer. Homochiral polymers are assumed to influence the proportion of each type of monomer formed from the precursor. We show that for certain parameter values a positive feedback mechanism makes the symmetric steady-state solution unstable. The kinetics of polymer formation are then analysed in the case where the system starts from zero concentrations of monomer and chains. We show that following a long induction time, extremely large concentrations of polymers are formed for a short time, during this time an asymmetry introduced into the system by a random external perturbation may be massively amplified. The system then approaches one of the steady-state solutions described above.
Bandas de viento colombianas  [cached]
Luis Omar Montoya Arias
Boletín de Antropología , 2011,
Abstract: Ni os y jóvenes colombianos son formados a través de programas distritales con la finalidad de cambiar la mentalidad y reducir la violencia. Con esto, se les invita a promover las artes, y en específico a cultivar la música. No es casual que se hayan elegido las bandas de viento para lograr este propósito, pues éstas son una organización instrumental colectiva que conlleva el trabajo en equipo, la unidad, la disciplina y la constancia. Las bandas de viento colombianas han sido definidas como "Música por la paz de Colombia", y se han convertido también en uno de los ejes de las políticas culturales del Estado colombiano.
Indirect resin composites  [cached]
Nandini Suresh
Journal of Conservative Dentistry , 2010,
Abstract: Aesthetic dentistry continues to evolve through innovations in bonding agents, restorative materials, and conservative preparation techniques. The use of direct composite restoration in posterior teeth is limited to relatively small cavities due to polymerization stresses. Indirect composites offer an esthetic alternative to ceramics for posterior teeth. This review article focuses on the material aspect of the newer generation of composites. This review was based on a PubMed database search which we limited to peer-reviewed articles in English that were published between 1990 and 2010 in dental journals. The key words used were ′indirect resin composites,′ composite inlays,′ and ′fiber-reinforced composites.′
Nucleated Polymerisation in the Presence of Pre-Formed Seed Filaments  [PDF]
Samuel I. A. Cohen,Michele Vendruscolo,Christopher M. Dobson,Tuomas P. J. Knowles
International Journal of Molecular Sciences , 2011, DOI: 10.3390/ijms12095844
Abstract: We revisit the classical problem of nucleated polymerisation and derive a range of exact results describing polymerisation in systems intermediate between the well-known limiting cases of a reaction starting from purely soluble material and for a reaction where no new growth nuclei are formed.
Nucleated polymerisation in the presence of pre-formed seed filaments  [PDF]
Samuel I. A. Cohen,Michele Vendruscolo,Christopher M. Dobson,Tuomas P. J. Knowles
Quantitative Biology , 2011,
Abstract: We revisit the classical problem of nucleated polymerisation and derive a range of exact results describing polymerisation in systems intermediate between the well-known limiting cases of a reaction starting from purely soluble material and for a reaction where no new growth nuclei are formed.
Fiber reinforced silicon-containing arylacetylene resin composites
eXPRESS Polymer Letters , 2007, DOI: 10.3144/expresspolymlett.2007.115
Abstract: A silicon-containing arylacetylene resin (SAR), a poly(dimethylsilyleneethynylene phenyleneethynylene) (PMSEPE), was synthesized. The PMSEPE is a solid resin at ambient temperature with a softening temperature about 60°C and soluble in some solvents like tetrahydrofuran. The melt viscosity of the PMSEPE resin is less than 1 Pa s. The resin could cure at the temperature of lower than 200°C. Fiber reinforced PMSEPE composites were prepared from prepregs which were made by the impregnation of fibers in PMSEPE resin solution. The composites exhibit good mechanical properties at room temperature and 250°C. The observation on fracture surfaces of the composites reinforced by glass fibers and carbon fibers demonstrates that the adhesion between the fibers and resin is good. The results from an oxyacetylene flame test show that the composites have good ablation performance and XRD analyses indicate that SiC forms in the residues during the ablation of the composites.
Adios a las bandas  [cached]
álvaro Rojas M.,Iván Coydan T.,Jaime Olavarría A.
Panorama Socioeconómico , 2003,
Abstract: En la actualidad, el sector agrícola se encuentra inmerso en un proceso global de cambios, en que el libre mercado y los instrumentos de la globalización son los elementos que guían la apertura económica. Esta situación ha provocado una fuerte competencia por conquistar los mercados internacionales, y también a generado mecanismos de protección de la competencia extranjera. En el caso de Chile, uno de estos mecanismos ha sido las bandas de precios. En este trabajo se presenta la evolución que ha experimentado este sistema en nuestro país a través de un análisis de tendencia, considerando su origen, el modo en que operan y su impacto en la producción de trigo, azúcar y aceites. Además se sometió a un análisis las limitaciones de este sistema, el por qué de su cuestionamiento internacional, los efectos e implicancias de una posible eliminación y las alternativas que se proponen al sustituir este mecanismo. En el caso del trigo, la banda esta siendo utilizada como un instrumento de toma de decisiones por parte de los agricultores. En el caso del azúcar, la utilización de este sistema regula la venta del producto y no el ingreso monetario de los agricultores. Finalmente, para los aceites la aplicación de la banda no ha sido un mecanismo que regule el ingreso de mezclas de aceites al país. El cuestionamiento que hace la comunidad internacional al sistema no es su nivel de protección sino su carácter de arancel variable. Sin embargo, la postura de Chile es que las bandas de precio son, en esencia, un mecanismo dise ado para que el país se proteja de la variabilidad de los precios internacionales.
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