Paper impregnated and coated with thermosetting resins is commonly used as a decorative and protective laminate for wood composites. The paper laminates are manufactured in a paper treater by impregnating décor paper with a mixture of melamine (MF) and urea formaldehyde (UF) resin or UF resin. This resin is then cured, and the paper is coated with a protective layer of MF resin. There is ongoing pressure to reduce the production cost of the laminates by increasing paper treatment speeds and minimizing the consumption of the expensive MF resin. Attempts to implement such strategies often result in poor quality laminates that contain defects (unfilled pores) in the MF coating. Examination of the relationship between UF resin treatment and paper type on the surface quality and distribution of MF resin in treated and pressed laminates revealed that the level of UF resin treatment strongly influenced the quality of the MF coating in laminates. Low levels of UF resin treatment (<25.0 g m−2) are associated with high levels of coating defects, and certain paper types in particular are prone to develop such defects. Scanning electron microscopy and Raman microscopy reveal that defects in MF coatings occur when MF resin flows from the surface of paper into voids that have not been filled by UF resin. The implications of these findings for the manufacture of protective laminates for use in low-pressure melamine panels are discussed briefly.
Introduction
Resin impregnated paper is often used as a decorative and protective laminate for composite wood panels (particleboard, [1] and medium density fibreboard (MDF). The manufacture of such laminates commonly involves impregnation of décor paper in a paper treater with a mixture of thermosetting melamine formaldehyde (MF) and urea formaldehyde (UF) resins [2] or less commonly UF resin. The resin is then cured and the impregnated paper is coated with an MF resin. The paper laminate is then pressed onto composite wood panels where heat and pressure cause the MF resin to flow to produce a hard surface finish. Fig. 1 shows a schematic diagram of the first stage of the treatment process when décor paper is impregnated with UF resin. The paper runs against a pre-wetting roller that applies UF resin to one side of the paper and drives air out of the paper. The paper then passes through a bath of UF resin and excess resin is removed by additional rollers (Fig. 1). The UF resin impregnated paper passes through an oven at temperatures of 160–190 °C which cures resin at the surface of the paper, and a coat of MF resin is applied to both sides of the paper using gravure rollers (not shown). The coated paper is then cured in second stage ovens at temperatures of up to 200 °C. The impregnating resin, which is significantly cheaper than MF resin minimizes the quantity of MF resin required to coat the paper, whereas the MF resin imparts the desired stain, abrasion and impact resistance to the laminated panel. The production of decorative laminates, which is dominated by resin impregnated paper laminates, is growing at 6% per annum and is projected to reach 16 billion square feet by 2005 [3]. Despite their importance there is little published information on the manufacture of resin impregnated paper laminates.
Décor papers for the production of resin impregnated laminates are generally selected on the basis of their appearance, particularly colour and decorative pattern, however, there is pressure to reduce the cost of laminates by increasing the production speed of paper treaters or reducing the consumption of the expensive MF resin. Attempts to achieve these aims, particularly when UF resin is used in the impregnation stage, often result in laminates with poor surface quality that are not suitable for the market. During the commercial production of laminates in a paper treater we have observed significant variation in the speed with which different décor papers can be treated, and the quantity of MF resin needed to satisfactorily coat papers. The causes of this variation, and the propensity of certain paper types to develop higher levels of coating defects are not known. We suggest, however, that they arise as a result of incomplete filling of voids in décor paper with UF resin (during the first stage of the treatment process) allowing migration of the MF resin away from the paper surface into the core of the paper. We examine and report on the relationship between UF resin treatment and paper type on the surface quality and distribution of MF resin in treated and pressed laminates here. From a practical point of view, knowledge of the influence of paper type and UF resin treatment on the quality of resin impregnated laminates is important because it could allow the treatment process to be tailored to the characteristics of décor paper, which could permit increased treatment speeds and allow lower amounts of MF resin to be used for certain papers, or reduce levels of defects in others.
