F. Henry

Coating of fertilizers by degradable polymers

The conventional agriculture leads to some important pollution of ground water (particularly, by nitrates). The solution is the coating of fertilizers by degradable polymers. In this work, we have studied the water vapour and liquid diffusion through polymer films detached from their support. Therefore, we may classify polymers as a function of their properties like water vapour and liquid barrier. We may choose the best polymer(s) for coating.coated fertilizers by chosen polymer(s) with mechanical techniques such as fluidised bed and pan coating. Moreover, the electron microscopy used to see the quality of the wall has showed the presence of pores due to the rapid evaporation of solvent. A drying in air current and an annealing could be done to avoid this problem.followed the ions release of fertilizers immersed in distilled water by conductimetry. The more interesting result was obtained with fertilizers coated by polylactic acid. In effect, the total release reached three weeks.

Free and/or bound water by dielectric measurements

Abstract In the past few years, the research on new methods (FTIR, NMR, microwaves) to measure the quantity and quality of water has been largely improved, particularly in the agricultural industry. Numerous studies have been made in order to establish a relationship (BET, GAB, …) between parameters such as temperature, the water content in the matrix and the partial pressure of water vapour. The dielectric spectroscopy in the microwave domain shows a strong discrimination between bound and free water. The measurements in a resonant cavity allow calculation of the complex permittivity. This methodology has been used to quantify the grade of a seeds viability, and by extension, to discriminate between living seeds, containing mostly bound water, and dead seeds, containing particularly free water. This was explored with the society CLAUSE, in tomato, pimento and melon seeds. In a resonant cavity, the dielectric response of each seed is dependent on the weight, the heterogeneity of the shape (depolarisation factor of the electric field), and the anisotropy of density distribution of water in the matrix. Consequently, in order to take into account these parameters, the rotation of the seed in the electric field gives the total dielectric response or the dielectric signature of each seed. In this first approach, using only four perpendicular positions of the seed in the cavity, the mathematical envelope of the dielectric response is usually greater when the seed is dead.

Dielectric Characterisation of Plastics for Microwave Oven Applications

The materials used in microwave oven cavities must have specific dielectric properties in order to maintain the efficiency of the food heating. Plastics, by their mechanical and chemical properties and low cost, are one of those potential materials. In this study, we present the results of the measurements of complex dielectric constant, ´´ ´ * e e e í − = , in the microwave frequency region, on different plastics: polyoxymethylene (POM), polypropylene (PP) and polybutylene terephtalate (PBT), using the cavity resonant method. We measure the shift in the resonant frequency of the cavity, Df, caused by the insertion of the sample, which can be related to the real part of the complex permitivitty, e´, while the change in the inverse of the quality factor of the cavity, D(1/Q), gives the imaginary part, e´´. The relations are simple when we consider only the first order perturbation in the electric field caused by the sample.

The Impact of Blue Inorganic Pigments on the Microwave Electrical Properties of Polymer Composites

We present the results of the measurement of complex dielectric permittivity, in the microwave frequency region, on glass reinforced polybutylene terephthalate (PBT) with blue inorganic pigments. The cavity resonant method had been used in order to measure the shift in the resonant frequency of the cavity, caused by the insertion of a sample, which can be related to the real part of the complex permittivity. Also, the quality factor of the cavity decreases with the insertion of a sample. The changes in the inverse of this quality factor give the imaginary part. In order to predict the dielectric behavior of this composite, we had developed a program of numerical simulation to calculate the complex permittivity of the inclusion. By using some of dielectric mixture laws (Maxwell-Wagner-Sillars, Hanai, Looyenga, inverse and direct Wiener, and Bruggemann), we can predict the dielectric behavior of the composite in a large range of volume fraction of inclusions.

PTCR effect in polymer composites

Some materials display an abrupt increase in resistivity when the temperature changes only over a few degrees. This phenomenon, known as PTCR effect (positive temperature coefficient in resistivity), has been largely studied [1, 2], due to its potential applications in industry. Particularly, it can be used in auto-controlled heaters and in security systems for electronic circuits. If there is a short circuit in an electric circuit, the increase of the temperature by the Joule effect induces an increase of the resistivity and consequently a much lower electrical current. Ceramic materials are largely used to obtain that effect [3–6], but in some polymer composites the phenomenon with a rapid thermal rate is also reported [7]. The addition of conducting particles to an insulating polymer is expected to produce a material which displays good conductivity. The conductivity, rather than being a linear function of loading, typically shows the behavior depicted in Fig. 1. Initially the conductivity is insensitive to loading, although it rises abruptly as the so-called percolation threshold is reached [8]. At this point the particles contact with each other, leading to a formation of a continuous conductive network. If it is possible to interrupt that network thermally, there will be a significant increase in the resistivity. This is the basic principle behind the PTCR effect in these materials. Two different insulating matrixes were studied, polystyrene and SBR, doped with popypyrol or carbon black particles. In Fig. 2 we can observe the results obtained with dc electrical conductivity measurements,

Structural, morphological and microwave dielectric properties of (Bi 1-xEu x)NbO 4 ceramics prepared by the sol-gel method

Since the microwave dielectric properties of BiNbO4 were first reported, various attempts have been undertaken to improve the properties of this ceramics, such as the addition of oxides or the substitution of bismuth by lanthanides. In this work, (Bi1-xEux)NbO4 (x = 0.00, 0.05, 0.10, 0.20, 0.50) samples were prepared using the sol-gel method and treated at different temperatures. The structure was studied by X-ray diffraction and Raman spectroscopy and the morphology by scanning electron microscopy. The measurement of the complex permittivity was made using the small perturbation theory, at resonant frequency of 2.7 GHz. In order to study the effect of the treatment temperature and x value in the dielectric properties, the two-way ANOVA method was used. It is clear that the most significant parameter is the treatment temperature. The highest Q × f value was obtained for the sample treated at 1,150°C due to the densification of the obtained ceramics.

Biodegradable Polymer Studied by Physical Properties Measurements

Polylactid acid (PLA), as an aliphatic polyester, is one of the most important and studied biodegradable material. The structure, the morphology, the molar mass regulate that biodegrability. During degradation, the process of chain scission occurs through hydrolysis. In this work we have investigated the water penetration through industrial and purified PLA films, in order to understand that degradability. Using different physical measurements, particularly the activation energy of b relaxation, the crystallinity, the water vapour permeability, the glass transition and the ion selective permeability. The complementary of the techniques permits to obtain results that support the physical model presented.

Dielectric method for the determination of aw

Abstract The measurement of a specific surface area with water vapour by the classical methods of sorption always proceeds by the conditioning of the material under constant P / P 0 of the given vapour. This makes the method very slow, typically lasting a few days. Dielectric spectroscopy in the microwave domain, in which the relaxation of free water is strongly discriminated from that of bound water, allows the quantification of the adsorption and desorption of water vapour on the surface of solid materials. In this paper we present a new dielectric methodology, using a special resonant cavity, for simultaneously obtaining the dielectric parameters and the weight losses of hydrated material under test. A theoretical approach was developed to calculate (during desorption or adsorption), the value of water activity a w . For that, a resonant cavity was projected and tested to allow calculation of the specific surface area with water vapour of a paper sheet, using the BET model.

Dielectric Properties of Polymers at Low Temperatures

Dielectric spectroscopy is a powerful method that allows the study of the dynamics of polymers in a wide frequency range. The different regimes of the dielectric function can be observed and the dynamics of the primary and secondary relaxations can be found. In fact, to obtain a complete characterization, a large range of frequencies and temperatures must be used. In this work, the investigation was focused in poly(lactid acid), PLA, in two forms, industrial and purified. This polymer is an aliphatic polyester, and one of the most important biocompatible and biodegradable material that has received increasing attention in the last 10 years. The β relaxation was observed between −150 and −30 °C, in frequency domain measurements between 1 Hz and 100 kHz, and was assigned to the secondary relaxation in the glassy state. The changes in the structure, which are connected with the water penetration in the polymer, directly affect that relaxation process. Water molecules confined by the polymer chains and in the polymer networks itself play an important role in the degradation of the material. We studied the evolution of that degradation during 4 weeks, in a controlled humidity environment. It is accepted that water preferentially enters in the amorphous zones, but also affects the crystalline regions. It is observed a clear evolution of the relaxation activation energy during the degradation of the polymer. The dielectric relaxation studies are complemented with water permeability measurements during the degradation process with time.

A 5 GHz Resonant Cavity for Complex Permittivity Measurements: Design, Test Performances and Application

The theoretical treatment of a cavity resonator consists of solving the Maxwell equations in that cavity, respecting the boundary conditions. The resonance frequencies appear as conditions in the solutions of the differential equation involved and are not significantly affected by the fact that the cavity walls have a finite conductivity. Solutions for rectangular cavities and for the lowest resonant mode, where the probability of mistaking one mode from another is slight, are readily obtained. The measurement of the complex permittivity, ε* = ε´-iε´´, can be made using the small perturbation theory. In this method, the resonance peak frequency and the quality factor of the cavity, with and without a sample, can be used to obtain the complex dielectric permittivity of the material. We measure the shift in the resonant frequency of the cavity, f, caused by the insertion of the sample, which can be related to the real part of the complex permitivitty, ε´, while the change in the inverse of the quality factor of the cavity, (1/Q), gives the imaginary part, ε´´. In this work we report the construction details, the performance tests of the cavity to confirm the possibility of the use of the small perturbation theory, and the application of the technique to measure the complex permittivity of a reinforced plastic.