João Ferra

Mechanical study of PLA–PCL fibers during in vitro degradation

The aliphatic polyesters are widely used in biomedical applications since they are susceptible to hydrolytic and/or enzymatic chain cleavage, leading to α-hydroxyacids, generally metabolized in the human body. This is particularly useful for many biomedical applications, especially, for temporary mechanical supports in regenerative medical devices. Ideally, the degradation should be compatible with the tissue recovering. In this work, the evolution of mechanical properties during degradation is discussed based on experimental data. The decrease of tensile strength of PLA-PCL fibers follows the same trend as the decrease of molecular weight, and so it can also be modeled using a first order equation. For each degradation stage, hyperelastic models such as Neo-Hookean, Mooney-Rivlin and second reduced order, allow a reasonable approximation of the material behavior. Based on this knowledge, constitutive models that describe the mechanical behavior during degradation are proposed and experimentally validated. The proposed theoretical models and methods may be adapted and used in other biodegradable materials, and can be considered fundamental tools in the design of regenerative medical devices where strain energy is an important requirement, such as, for example, ligaments, cartilage and stents.

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.

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.

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.

Sodium metabisulphite as a scavenger of air pollutants for wood‐based building materials

Abstract This paper reports on different applications of sodium metabisulphite as a formaldehyde scavenger. The reduction in formaldehyde emission and the effect on physicomechanical properties of particleboards were studied. The scavenger was mixed with urea–formaldehyde resins with different formaldehyde to urea molar ratios and added separately in the production of particleboards. Several differences between the formaldehyde content and physicomechanical properties of boards were found. When applied to melamine–formaldehyde resins, the changes in formaldehyde emission were less significant, probably due to the incorporation of sodium bisulphite in polymeric matrix. The type of formaldehyde-based resin and the form of scavenger addition are the major factors affecting formaldehyde scavenging efficiency.

Evaluation of Bonding Performance of Amino Polymers Using ABES

In this paper, the use of Automated Bonding Evaluation System (ABES) for evaluation of bonding strength of wood adhesives is presented. A simple reactive model is proposed, which considers the phenomena involved during resin cure: formation of wood-resin-wood bonds. In the present study, experimental data of a standard urea-formaldehyde (UF) resin using different catalysts, phosphoric acid and ammonium sulphate, are presented and discussed. The proposed can be applied to wood adhesives, namely formaldehyde-based resins under different temperatures. The model fits well to experimental data, presenting high determination coefficient values.

Low formaldehyde emission MDF overlaid with wood veneer: bonding problems assessment

Abstract The aim of this work is to assess wood bonding problems in medium density fibreboard (MDF) surfacing with decorative wood veneer (beech). Three sets of tests were carried out. The first set was carried out using automatic bonding evaluation system (ABES) in order to assess the bond strength development rate of two UF resins produced according to different processes. The second set permitted the study of the behaviour of bonding between MDF and beech veneer using ABES, and for this purpose a special MDF sample was designed. These MDF samples were pressed to conventional beech test strips and pulled at any given time immediately after pressing. A third set of tests were performed in a pilot hot-press used for MDF surfacing with beech veneer. Different operating conditions were tested (pressing time and resin type). The quality of bonding between wood veneer and MDF was evaluated using the surface soundness test (EN311). The tensile load required to pull off a defined surface area of overlaid panel was recorded and it was observed that in all samples the failure mode occurred within the underlying board. An analysis of variance test was performed to evaluate the significance level of the effects of the different factors on surface soundness. The main conclusion of this work is that the MDF surfacing operation seems to induce degradation in the MDF surface layers, probably due to the presence of urea, which could be responsible for a decrease in the surface soundness, which was mostly observed for longer pressing times.

Viscosity determination of amino resins during synthesis using near-infrared spectroscopy

Abstract Fourier transform near-infrared (FT-NIR) spectroscopy has been described as a powerful technique for online monitoring and control of polymerisation reactions. In this work, the technique was used to monitor the viscosity of urea-formaldehyde resins during the condensation reaction. The main objective of this study was to develop a chemometric technique that can be used for online or offline determination of the resin’s viscosity during synthesis. The best model uses the spectral region between 5000 and 4246 cm−1 and the first derivative with vector normalisation for preprocessing.