A. Valea

Mechanical behavior of wood/polypropylene composites: Effects of fibre treatments and ageing processes

Practical applications of wood based composites are limited by the hydrophilic nature of cellulose, which causes dispersion and moisture absorption problems. This work rests on the effects of wood treatment on the mechanical properties of composites made with wood flour and polypropylene (WF/PP). Two different fiber treatments (mercerization with a 10% NaOH solution and functionalization with Epolene E-43 MAPP copolymer), their combined action, and two compounding methods have been studied. In addition, the investigation includes the analysis of the mechanical behavior of composites after exposure to variable conditions of temperature and moisture.

Dynamic and mechanical properties of DGEBA/poly(propylene oxide) amine based epoxy resins as a function of stoichiometry

An investigation was carried out into the effect of stoichiometry on the dynamic and mechanical properties of epoxy resins cured with two different poly(propylene oxide) based amines. Glass transition temperatures as well as crosslink densities of the networks have been related to the stoichiometry of both mixtures as well as to the structure of the curing agent employed. Mechanical properties have been analyzed as a function of stoichiometry and chain structure of the components used. With the exception of mixtures with an amine/epoxy ratio of 0.6, the stiffness was a minimum around the stoichiometric composition for both systems. A good correlation between the toughness of these materials and their strength and ductility has been found. Toughness behaviour is explained as a function of the network homogeneity and the extensibility of the segments constituting the crosslinked structure.

Strain rate and temperature effects on the mechanical behaviour of epoxy mixtures with different crosslink densities

Abstract The microstructure and the flexural behaviour of two highly crosslinked mixtures have been investigated. Besides the usual α and β relaxations appearing for any epoxy mixture, an additional ω relaxation, located slightly above room temperature, was evident for the mixture with higher crosslink density. This relaxation, which seems to be related to microstructural variations originating in a free volume increase, permits us to explain the lower mechanical properties of the larger higher crosslinked mixtures in the temperature range between the β and α relaxations. Whilst at room temperature the flexural modulus did not show any significant variation upon testing rate and the strength slightly increased for both epoxy mixtures.

Vinyl ester and unsaturated polyester resins in contact with different chemicals: Dynamic mechanical behavior

A prior study has shown for a vinyl ester resin that the adequate choice of the curing conditions becomes completely necessary to reach the maximum glass transition temperature. In the same study, the variations produced in the dynamic mechanical properties because of exposure to different solvents were related to the chemical structure of both the resin and solvent. This investigation was undertaken in an attempt to analyze the effects of solvent exposure on the viscoelastic properties of resins currently used for applications in which the resin is kept in contact with the solvents. Several vinyl ester resins as well as various unsaturated polyester resins immersed in different liquids were investigated. The influence of exposure time to the solvent as well as that for the temperature were characterized using dynamic mechanical analysis. The chemical structure of the resin was found to be determinant in the changes produced after solvent exposure.

Influence of cure schedule and solvent exposure on the dynamic mechanical behavior of a vinyl ester resin containing glass fibers

This study deals with the dynamic mechanical properties of a glass-reinforced vinyl ester resin. The viscoelastic parameters of the loss factor and the storage modulus as a function of the cure temperature were used as a criterion to determine the optimum cure conditions to be employed. It is shown that the cure temperature usually used to cure these resins is not enough to reach the maximum glass transition temperature, and, therefore, an additional postcure should be used. The influence of several solvents on the dynamic mechanical behavior of these resins cured following the cure pathway proposed by the supplier was also analyzed. This behavior was compared with the solvent uptake at various exposure times, and the changes observed were related to the crosslink density as well as to the chemical structure of both the resin and the solvent. Finally, the effects of varying the exposure temperature were also investigated for the resin exposed to a liquid which simulates petroleum fluid.

Off-axis Flexure Test: A New Method for Obtaining In-plane Shear Properties

Off-axis three-point flexure test for unidirectional composites is proposed as a new method for obtaining in-plane shear modulus and in-plane shear strength. The method of determination of the in-plane shear modulus GLT has been explained in a previous work where the displacement field of an off-axis flexure test was analyzed. In the present work, besides summarizing the calculation equations of GLT, the condition of small displacements is analyzed. Otherwise, an error analysis is carried out in order to study the influence of the calculated value of GLT on the other elastic parameters. Normal and shear stresses in the fiber–matrix interface change from point to point in the specimen depending on the fiber orientation angle and specimen geometry. A critical point of failure that depends on the fiber orientation angle is determined and optimum conditions for obtaining in-plane shear strength are discussed. Experiments for different fiber orientations and geometric conditions have been carried out for IM7/8552 epoxy matrix based carbon fiber reinforced unidirectional composite material.

Correlations between rheological and thermal behaviour of TGDDM/m-PDA epoxy systems

Abstract Isothermal variation of viscosity has been investigated at various temperatures for an epoxy-aromatic amine system (TGDDM/m-PDA system). The variation of the extent of reaction during the curing process has been also determined by means of differential scanning calorimetry. Two different models were used to predict the viscosity during cure: a model based on a modified Williams-Landel-Ferry equation, and a percolation-gelation analogy. This analysis reflects that the reaction mechanism does not change with temperature in the studied range.

Improvement of adhesive bonding of polypropylene and maleic anhydride grafted polypropylene blends to aluminium by means of addition of cyclic butylene terephthalate

ABSTRACT In this work the adhesive bonding of polymeric blends based on polypropylene (PP), cyclic butylene terephthalate (CBT) and maleic anhydride grafted polypropylene (PP-g-MA) to aluminium was studied, with the aim of developing metal/polymer hybrid laminates for a multitude of applications among which the automotive and aeronautic sectors stand out. Different films with homogeneous compositions have been prepared and joined to aluminium (with any previous surface treatment) and the adhesive bonding has been characterized under the standard UNE-EN 1465. For binary blends, there is a critical PP-g-MA and CBT composition from which the shear strength declines. The combined presence of the PP-g-MA and CBT in the blend prevents this critical composition due to the possible Van der Waals forces and/or esterification/transesterification reactions created between the reactive chains. In addition, if the transformation from CBT to pCBT takes place at the same time as the joining process, the shear resistance of the laminated increases between 10% and 25%. The adhesive bonding between layers improves in two ways: the lower viscosity of the developed blends makes an easier substrate impregnation, and the active sites created during the ring opening polymerization (ROP) are capable of forming chemical bonds.