Conclusion

The hypothesis proposed in the introduction to this Chapter that defects in the surface of LPM panels after pressing are caused by inadequate saturation of papers by UF resin resulting in the flow of MF resin from the surface into the unfilled voids in the paper needs to be slightly modified. In the majority of cases effective saturation was necessary to significantly reduce the level of defects in MF coatings on decor paper. However it was demonstrated in one paper, Beech, that even though UF resin saturation was less than complete, it was sufficient to prevent significant migration of MF resin from the surface thereby reducing the level of surface defects. Thus an effective barrier of UF resin just below the surface could prevent MF migration, however, this might be difficult to achieve in practice. Thus complete saturation of

decor paper with UF resin is still required in the majority of cases to minimize surface defects in the MF coating on LPM panels and work in the rest of the thesis is based on this premise.

The hypothesis that the type of paper influences the quality of the MF coating can be supported by the results in this Chapter although in some cases there were interactions of level of resin treatment and paper type on defects in the MF coating.

Relationship between the physical

properties of decor papers and surface

defects in LPM overlays

An effect of paper type on level of surface defects in resin impregnated paper on MDF was demonstrated in the previous chapter. The purpose of this chapter is to determine whether this effect can be related to any of the standard measures of paper properties and also to the imbibition performance of the papers.

4.1

Introduction

One of the main aims of this thesis is to better understand the factors that affect the impreg- nation of thermosetting resins into decor papers in order to improve the treatment process and performance of the resin impregnated papers when they are pressed onto wood based compos- ites.

A strong effect of paper type on the level of defects in MF coatings on decor paper was observed in Chapter 3. The generation of such defects may be related to certain physical characteristics of paper that influence the flow of resin into the paper. Accordingly the rate of liquid imbibition of different decor papers should be related to generation of defects and in turn the physical properties of decor papers.

There are many methods of measuring the physical and imbibition characteristics of decor paper. They can be described in terms of their Gurley porosity (measure of air permeability), Klemm wicking (rate of water movement in the machine and cross directions i.e. in plane), total resin uptake (g), bulk density (g/cm3 which includes void volume, Mark et al. (2002)), paper weight (g/m2) and thickness (µm).

There is very little information in the literature, however, about the effect of these charac- teristics on the performance of decor papers. Arledter (1957) showed how the capillary rise of a solution in a paper (Klemm test) might give supporting evidence for the treatability of an absorbent paper with aqueous solutions, and Verhoeff et al. (1963) used the Klemm test to measure the velocity of capillary rise in papers subjected to different levels of beating. Seiler (1957) looking at penetrability examined the saturation of kraft papers for resin impregnation and concluded that rate of penetration on its own does not define the overall impregnating and saturating qualities of paper. Neither he or others have looked at the relationship between the saturating of paper and the quality of the finished products, especially the relationship between resin saturation, subsequent resin flow and defects. Cussons (1997) stated that the Klemm test, although widely used did not relate well to resin penetration. A detailed review of work on measuring fluid flow into paper was given in Chapter 2.

It was therefore important to develop an accurate method of determining the uptake of liquid into decor papers. The majority of applications of paper eg. printing require liquid penetration resistance rather than enhanced penetration properties. Thus most tests of liquid penetration into paper have been developed to assess degree of sizing; or water resistance of paper (Biermann, 1996). As such these test methods are designed to measure penetration of liq- uids into paper over periods of many minutes rather than fractions of a second. However, resin penetration into decor paper occurs rapidly (<2 s) (Cussons, 1997) and hence test methods described in the literature and reviewed in Chapter 2 may not accurately assess the suitability of decor paper for resin treatment. Arledter (1957) in his study of the penetration of resins and oils into absorbent papers stated “for most of their history the paper makers were concerned

with trials to reduce the unwanted natural absorbency of paper. The invention of blotting paper probably started the paper industry to devote time to improve the natural ability of cellulose fibers and the paper structure to absorb water or ink, and the industry is still working on this problem”. The development of test methods for measuring resin uptake in decor papers has re- ceived little attention and better test methods are required to measure rates of liquid penetration into decor paper over short time scales.

The aim of the work described in this chapter is to determine whether standard properties measured by paper manufacturers to describe decor papers could be related to the level of defects that were found to occur in treated papers (Chapter 3). Secondly, to develop a new method for quantifying liquid imbibition into decor papers and determine whether it would be a better guide to the performance of decor papers than those currently used.