Luísa Carvalho

Scavengers for achieving zero formaldehyde emission of wood-based panels

This work examines the performance of three formaldehyde scavengers in wood-based panels. Sodium metabisulfite, ammonium bisulfite and urea were applied in different physical forms during particleboard production, and the resulting physico-mechanical properties (internal bond strength, thickness swelling, density and moisture content) and formaldehyde emission levels were compared. Formaldehyde content was measured using the perforator method, and formaldehyde emission was evaluated both by desiccator and gas analysis methods. The chemical reactions involved in each formaldehyde scavenging process are proposed and discussed. The tested scavengers showed distinct performances under the different emission testing conditions, which were interpreted in terms of the stability of the chemical compounds formed upon formaldehyde capture. Sodium metabisulfite proved to be an excellent scavenger for all formaldehyde methods allowing the production of particleboard panels with zero formaldehyde emission.

Modeling and simulation of the hot-pressing process in the production of medium density fiberboard (MDF)

Abstract A mathematical model is presented that describes heat and moisture transfer during the hot-pressing of MDF. The model includes conduction of heat between the press platens and the central layer of the mattress, phase change of water from the adsorbed to the vapor state and the convective heat and mass transfer through the edges of the mattress. This approach results in a three-dimensional unsteady-state problem in which the properties of the wood fibers, water and the wood-water system are dependent on temperature, mattress moisture content and steam pressure. For solving this problem, a finite difference scheme is used for the discretization of-spatial variables and the resulting set of ordinary differential equations is solved using an appropriated ODE solver. This theoretical model is used to predict the evolution of temperature, moisture content, steam pressure and relative humidity with time at a given position in the mattress under typical operating conditions. Several plots are shown displ...

A global model for the hot-pressing of MDF

The hot-pressing operation is the final stage in medium-density fiberboard (MDF) manufacture, where the mattress of fibers is compressed and heated to promote the cure of the resin. In MDF hot-pressing, many physical, chemical and mechanical processes are involved; the complexity of this operation arises from the fact that they are coupled. A global model is presented for this operation that integrates all mechanisms involved in the panel formation (heat and mass transfer, chemical reaction and mechanical behavior). This approach results in a two-dimensional unsteady state problem, which involves the knowledge of the polymerization kinetics of the resin, the transport properties and material properties, which are position and time dependent. This dynamic model was used to predict the evolution of the variables relating to heat and mass transfer (temperature, moisture content, gas pressure and relative humidity), as well as the variables relating to mechanical behavior (pressing pressure, strain, modulus of elasticity and density). The model performance was analyzed using the typical operating conditions for the hot-pressing of MDF and the results were compared to the experimental data from an industrial MDF press. We concluded that the model could predict in an acceptable way the behavior of the key variables for the control of the pressing cycle, as well as some physico–mechanical properties of the final product. The improvement of this model will permit the scheduling of the press cycle to fulfill objectives of minimization of energy consumption, better quality of the board and increased process flexibility.

Method for evaluating the influence of wood machining conditions on the objective characterization and subjective perception of a finished surface

A method for evaluating the influence of the operating parameters of wood machining (planing, sanding) on the quality of a finished surface was established. The influence of each of the operations involved was studied using different strategies. For the planing operation, three levels were established (Level 1, 2 and 3) by determining different values for each of the major process parameters (tool rotation speed, feed speed, depth of cut). For both, surface preparation and polishing, two levels were established with a fixed setup. Finally, as finishing products, two different transparent coatings were selected (water-borne and solvent-borne). The quantitative evaluation of the state of the surface was assessed by means of an artificial vision system for the determination of the pseudo-roughness of wood surfaces. A filtering method based on fast Fourier transforms was applied and it was possible to derive three criteria for evaluating the resulting profile.

Cognitive Functioning in Behcet's Disease

The impact of Behçets disease on higher cognitive functions is still poorly understood. We proposed (1) to characterize the neuropsychological profile of Behçets disease patients with (Neuro‐BD) and without (BD) neurological manifestations; (2) to identify which clinical, psychopathological, and genetic variables are related to neuropsychological performance; and (3) to explore the association between cognitive functioning and neuroimaging findings in BD patients. Fifteen Neuro‐BD and 35 BD patients in the nonactive phase of their illness underwent a neurological examination, performed a comprehensive battery of neuropsychological tests, and answered the hospital anxiety and depression scale. Human leukocyte antigen (HLA)‐B*51 genotyping was also performed. Patients’ neuropsychological performances were compared to those of healthy demographically matched subjects. Within one month from the testing date, a subset of 20‐BD patients underwent a magnetic resonance imaging (MRI) scan. Fifty‐three percent of Neuro‐BD and 40% of BD patients were impaired at least on one neuropsychological measure (i.e., digit span–forward). Poorer cognitive functioning in Neuro‐BD was associated with parenchymal involvement, whereas in BD it was related to presence of white matter changes in the frontal lobes, history of headache complaints, or higher levels of anxiety and depression. Current prednisone intake had a positive impact on neuropsychological performance. Disease duration, time since onset of neurological manifestations, or presence of HLA‐B*51 allele had no significant influence. Our results indicate that Behçets disease may affect cognitive abilities in the absence of overt neurological symptoms. These findings point to an insidious course of neurological involvement.

Characterization of urea-formaldehyde resins by GPC/SEC and HPLC techniques: Effect of ageing

During the last 40 years, several analytical techniques have been developed/adapted to characterize urea-formaldehyde (UF) resins. However, a great part of the research about this kind of wood adhesives has been performed by industrial producers and, thus, the main part of the existing knowledge is retained within those companies. This work describes a methodology for determining the molecular weight distribution (MWD) of UF resins using Gel Permeation Chromatography (GPC)/Size Exclusion Chromatography (SEC) with 2 detectors (differential refractive index (RI) and differential viscosity). This method permitted to characterize/distinguish commercial UF resins produced with different F/U molar ratios and to monitor the molecular weight and MWD with ageing. An HPLC method was additionally used to evaluate the fraction of unreacted urea, monomethylolurea and dimethylolurea present in commercial UF resins and measure the evolution of these three compounds with ageing.

Optimization of the Synthesis of Urea-Formaldehyde Resins using Response Surface Methodology

In the near future, companies will face the need to produce low formaldehyde emission resins, i.e., not above the emission level of natural wood. However, for producing this new generation of urea-formaldehyde resins (UF), it is necessary to optimize the synthesis process. This work describes an optimization procedure for UF resin synthesis, following an alkaline–acid process, focusing on the conditions of the condensation step. A design of experiments methodology was employed to optimize the 3 selected factors (number of urea additions, time span between urea additions, and condensation pH), in order to produce particleboards with maximum internal bond strength and minimum formaldehyde release. The condensation pH played a significant role in increasing the Internal Bond (IB) strength and reducing the Formaldehyde Emission (FE). The sequential addition of urea also has a noticeable influence on resin performance. Optimum conditions for production of UF resins have been proposed and tested by the response surface methodology using the desirability function.

Modeling the continuous hot-pressing of MDF

The continuous pressing became the most important press-drying process in the manufacture of wood-based panels and a great number of manufacturers have been substituting the conventional batch process by a continuous line. One of the most important and innovative aspects is the understanding of the combined effects of physical–chemical and mechanical phenomena involved. Due to the multiplicity and interdependency of these phenomena, the development of predictive models will permit the simulation of this operation and finally a better understanding, optimization and control of the press cycle. Based on a model previously developed for batch pressing, a three-dimensional model for medium density fiberboard (MDF) continuous pressing is presented by integrating all mechanisms involved in panel formation: heat and mass transfer, chemical reaction and mechanical behavior. In this case, the description of these phenomena corresponds to the modeling of a porous and heterogeneous media in movement. The main difficulty associated to this type of problems is the choice of the reference system for the numerical solution of the equations of conservation of mass, energy and momentum. This model was used to predict the evolution of the variables related with heat and mass transfer (temperature, moisture content, gas pressure and relative humidity), as well as the variables related with mechanical behavior (pressing pressure, strain, elasticity module and density). The model performance was analyzed using the typical operating conditions for the continuous pressing of MDF by comparison of its predictions with experimental data from the literature.

The role of sucrose in amino polymers synthesized by the strongly acid process

This article studies the incorporation of sucrose in amino polymers produced by the strongly acid process and its role on the physico-mechanical properties and aldehyde emission of the resulting particleboards. The incorporation of sucrose at different pH environments was studied and differences on molecular weight of resins were analyzed by gel permeation chromatography/size exclusion chromatography (GPC/SEC) and characteristic chemical bands by Raman spectroscopy. A reaction mechanism was proposed to explain the observed differences in GPC/SEC chromatograms and was supported by Raman spectra. It was observed that small amounts of sucrose incorporated in polymeric matrix are sufficient to improve the physico-mechanical properties of particleboards; the sucrose addition protocol plays a key role on these improvements.

Study of influence of synthesis conditions on properties of melamine–urea formaldehyde resins

Abstract The aim of this work is to assess the differences in the polymeric structure and performance of melamine–urea formaldehyde (MUF) resins, when changing relevant synthesis variables: formaldehyde/amine groups [F/(NH2)2] molar ratio (both in the methylolation and the condensation steps) and the feedrate of urea during the condensation step. This synthesis process differs from the traditional alkaline acid process, since the F/(NH2)2 molar ratio is different for the methylolation and condensation steps. It was found that the F/(NH2)2 molar ratio and urea feedrate in the condensation step are the most influential variables on the product characteristics. A relationship was established between polymeric structure of the resin and the physicomechanical properties, as well as the levels of formaldehyde. A resin formulation was obtained that exhibits a formaldehyde content, evaluated both by perforator and desiccator methods, within the Japanese F**** requirements. This resin presents an overall performance better than the one obtained by two representative commercial resins.