Nádia Paiva

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.

Determination of Resin and Moisture Content in Melamine Formaldehyde Paper using Near Infrared Spectroscopy

This paper describes the use of near infrared spectroscopy as a tool for the determination of moisture and resin content on papers impregnated with melamine-formaldehyde resins for high-pressure laminate production. The papers had different colours and grammages. The near infrared analysis range comprised wavelengths between 12,000 cm−1 and 4000 cm−1. Several multivariate calibration procedures and pre-processing techniques were tested for selection of the best spectral interval, including interval partial least-square, forward interval partial least-square and synergy interval partial least-square. The performance of calibration models was evaluated computing the root mean-squared error of cross-validations and the coefficient of determination (R2). An external validation procedure was done using different decorative papers (red, pearl, yellow, violet and pale green). The performances of the best models were compared using the statistical criterion root mean square error of prediction. It was shown that the developed models can be applied in the determination of resin content independently of the grammage and colour of the papers. However, regarding the volatile content, the models seemed to be affected by external factors, such as the presence of dyes and pigments, and were only applicable to papers having spectra similar to those used in the calibration model.

Biosourced Binder for Wood Particleboards Based on Spent Sulfite Liquor and Wheat Flour

Currently, the majority of binders used in wood particleboard (PB) manufacturing are formaldehyde-based synthetic resins. Because of the toxicity of formaldehyde, there is a strong demand for eco-friendly alternatives with similar performances and economic viability. In this work, thick spent sulfite liquor (TSSL), an industrial byproduct from sulfite pulp mills, is proposed as a binder for fully bio-based PBs. The results showed that PBs bound with TSSL present appropriate mechanical performance, which was further improved when TSSL was combined with wheat flour at an 84:16 dry weight ratio and preheated to 94 °C prior to application. For 13.2% binder content per dry wood weight, the PB internal bond strength was 0.46 N mm−2, which is above the standard requirements for PB type P2 (0.35 N mm−2). Optical microscopy showed that TSSL hinders the gelatinization of starch granules during preheating, allowing the binder mixture to maintain a low viscosity suitable for combination with wood particles and PB production.

Copolymerization of UF Resins with Dimethylurea for Improving Storage Stability without Impairing Adhesive Performance

Urea-formaldehyde (UF) resins are the most used resins in the wood industry due to high reactivity and low price. However, their reduced stability during storage is a drawback, imposing strict limits in terms of allowable shipping distances and storage times. This instability, manifested by viscosity increase that renders the resin unusable, occurs due to the progress of condensation reactions between the polymeric species present in the liquid medium. In order to achieve a stable resin formulation, dimethylurea (DMeU) was selected for being less reactive than urea. Dimethylurea is shown to co-polymerize with the UF polymer during the acidic synthesis condensation step. However, during storage it behaves like an end group blocker, due to its lower reactivity at basic pH. By adding 1.25% DMeU, it was possible to obtain a formulation that remained with stable viscosity during two-month storage at 40 °C. The reference UF resin remained stable only for eight days in these conditions. Wood particleboards produced with modified resins showed internal bond strengths of about 0.5 N·mm−2, similar to the fresh reference UF resin, even when the resins were used after the two-month storage period. Formaldehyde content values were below the limit for E1 class, ≤8 mg/100 g oven dry board (EN 13986).

Utilization and characterization of amino resins for the production of wood-based panels with emphasis on particleboards (PB) and medium density fibreboards (MDF). A review

Abstract Particleboard (PB), medium density fibreboard (MDF), oriented strand board (OSB) and plywood (PW), the most common products of wood panel industry, are produced by means of synthetic adhesives (resins). From the wide range of adhesives employed, amino resins as combination of formaldehyde (F) and urea (U), and melamine (M) are the most important ones, which include the most popular UF and MUF resins. This review is an introduction to the UF and MUF synthesis processes, and also addresses the key parameters for the performance of these adhesives and gives an overview about the final characteristics of PB and MDF prepared with amino resins. The characterization methods for amino resins and panels are outlined. The strategies for the optimization of resin properties and pressing conditions are discussed in terms of their good performance also under conditions of low formaldehyde emission.

Improving hydrophobic and oleophobic performances of high-pressure laminates

High-pressure laminates are decorative materials that are widely used in furnishing and building industries. One of their major handicaps, especially in the case of high gloss laminates, is the ease of staining by fingerprints. This problem can a priori be minimized by increasing the hydrophobicity and oleophobicity of the surface. To improve hydrophobicity, the chemical composition of the resin used for impregnation of the laminate’s top layer decorative paper was modified. A melamine-benzoguanamine-formaldehyde resin was used as starting point, and caprinoguanamine was added as a comonomer at different stages of the synthesis. Addition at the end of the synthesis process showed to be the most promising approach for achieving a water contact angle of 91.2°, while the contact angle for a commercial laminate, impregnated with a standard melamine–formaldehyde resin, is 58°. Regarding oleophobicity, several commercial additives were tested with the previously formulated resin. A fluorocarbon additive showed the best result, leading to contact angles of 114° for water and 76° for an artificial fingerprint liquid. In addition, it was shown that fingerprints were easier to clean from the modified laminate surface.

Development of phenol-formaldehyde resin with low formaldehyde emissions that respects LEED certification

Abstract In the last years, production of particleboards with good overall performance and very low formaldehyde emission has been a challenge to wood based panels (WBP) industry, mainly since the re-classification of formaldehyde by the IARC (International Agency for Research on Cancer) as ‘carcinogenic to humans (Group 1)’. Moreover, a new important limitation to the use of formaldehyde-based resins has been recently imposed by the LEED (Leadership in Energy and Environmental Design) certification for ‘Green Building’ construction: ‘wood composites must contain no added urea-formaldehyde resins’. In this context, the main purpose of this study is to develop a PF resin for particleboard production that fulfils formaldehyde emission restrictions and LEED criteria, while presenting appropriate reactivity and bond strength. The mechanical performance and formaldehyde emissions of particleboards were optimized, changing both the resin synthesis and board production procedures. The synthesis process of these resins was carried out under an alkaline environment, and with an excess of formaldehyde towards phenol, in order to produce resol-type PF resins. The effect of changing the amount of added sodium hydroxide was studied. The particleboard production parameters were also changed, both in terms of blending conditions (amount of hardener and resin) and hot-pressing conditions (pressing time). A PF resin with very good internal bond strength, low formaldehyde strength and reasonable board pressing times was obtained using the following conditions: sodium hydroxide amount of 9% during the synthesis process, and 10% hardener (based on oven-dry weight of resin) together with gluing factor between 4·5 and 5% on the core layer during particleboard production. The best performing resin obtained demonstrated to be appropriate for use in the so called ‘Green Building’ construction.