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United Nations Industrial Development Organization

Distr.LIMITEDID/MG.335/1O/Rev.1 25 August 198ÌENGLISH

Seminar on Wood Based Panels and Furniture IndustriesBeijing, China, 23 March - 7 April 1981

SURFACE TREATMENT OF WOOD BASED PANELS *

by

Robert Vansteenkiste **

I

r '• • / :

* The views expressed in this paper are those of the author and do notnecessarily reflect the views of the secretariat of UNIDO. This document has been reproduced without formal editing.

** Chemical Engineer, Technologist, Production Manager, TIVAPAN Co., Kuume Belgium.

V .8 1 - 2 9 0 4 5

TABLE OF CONTENTS

Page

INTRODUCTION 1SURFACE QUALITY AND ITS IMPORTANCE FOR TREATED PRODUCTS 1

(1) Surface profile (PV) 2(2) Surface porosity 8(3) Surface soundness 9

IMPROVING THE SURFACE QUALITY OF PARTICLE BOARD BY HIGH- TEMPERATURE PRESSING 11

(1) Principle 11(2) Results and discussion 11

WET FINISHING 13(a) Coating equipment 14(b) Drying equipment 21(c) Auxiliary equipment 24

LAMINATING 24(a) Laminating with adhesive-bonded products 261. Wood veneer 262. High pressure decorative laminates 273. Polyvinyl chloride 27(b) Laminating with self-bonding products 30

Postforming process 32Different types of plants equlped to overlay with melamine 33I. Multi-opening press with cooling 33II. Short-cycle process with a single-opening horizontal

press without cooling 35III. Short-cycle process with a single opening vertical press

without cooling 38IV. Continuous manufacture of decorative laminates 41

Table I: Suitability of the various types of finishesfor the different types of boards 44

Table II: Investment costs, economics of . cale, value added 45Table III: Level of skills needed, power requirements 46Table IV: Data on equipment interchangeability for different

processes 47REFERENCES 48

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INTRODUCTION

The application of particle board involves specific qualities that the crude particle board does not have. Therefore, the boards have to be treated to meet the specific requirements of their final use. Most treatments are surface treatments, the impregnation of the board being seldom considered.

We may classify the treatments in groups according to the end-uses:

(a) Application of barriers by overlaying or coating. The purpose of the barrier will be:

- to suppress the absorption of humidity;- to suppress the absorption of water;- to eliminate the release of nucuois gas, like formal­

dehyde, pesticides, etc.

Generally, the barrier has no aesthetic function, but it may do so. The barriers mo3t used are foils, films, impregna­ted papers, veneers, special coatings and paints, panels.

(b) Decoration of particle boards for furniture or panelling: painting, printing, priming, varnishing, application of deco­rative thin veneers, laminating, Impregnated papers, etc.

SURFACE QUALITY AHD ITS IMPORTANCE FOR TREATED PRODUCTS

All boards are not suitable for all applications, especially boards destined for the furniture industry. The methods of decora­ting particle board panels and producing furniture have increased the need for realistic specifications of the quality of the surface layer.

The surface quality of a product is of paramount importance for its subsequent use. The individual properties influence appea­rance, strength, durability, usefulness, functionality, etc. to a

considerable extent, and according to the field of application, different combinations of properties are of relevance. Methods of refinement may be classified in three categories, namely, liquid coating, surface glueing and coating vlth decorative papers.

They determine the kind of substrate to be used. As regards the suitability of the substrate, density, resin contents, fineness of particles and surface moisture during the coating operation are relevant items. However, the properties of deeper layers may also show effects. Today, an almost comprehensive knowledge of the requirements to be met by the substrate in order to obtain an optimal refinement exists.

This subject has been widely documented, and again, the emphasis is on the particle board manufacturing aspects. A considerable amount of original research work, particularly or. the assessment of surface profile was carried out by particle board manufacturers. Subjects to be discussed have been restricted to:

(1) Surface profile (wet and dry) of Peak to Valley (PV) values;(2) Surface porosity;(3) Surface soundness.

(1) Surface profile (PV)

Early applications of particle board In furniture generally involved the use of fairly thick wood-veneer surface skins, for which the demands made on the surface quality of particle boards were not too great. In certain cases, however, the use of water- based adhesives did lead to irregular swelling of surface particles, which, in turn, caused "telegraphing" of the irregularities being visible to the eye. A method of surface measurement.known as the PV method, was developed and is now a generally accepted pro­cedure .

In this, a sensing probe traverses the surface to be measured and the profile is recorded. Figure 1 shows typical PV traces

3

obtained by the method of BS.1811 for a normal and line surface particle board, in tie dry and wet conditiona. The differences are quite apparent, aa shown also in the photographs of Figures 2.It was very soon established that telegraphing is associated with the presence of thick particles in the surface layerr, and the correlation between PV values and what may ne termed "rogue" chips in the surface layers is shown in '»figure 3.

I

t'^V'<'VUv

iiI

/ / j '

Ory Fine S';ri*.red P- jr«i id

f i^- . 1 : FV p r o f i l e r :

4

jr" :

Dry Standard Board Wet

fig«,! Photographs of a normal surface particleboard and a fine surface particleboard in the dry and wet condition (ref. fig. 1)

- 5 -

fig. 5 •' Effect of thickness of "rogue" chips on PV values

- 6 -

In order to provide particle boards w ith h igh q u a l i ty sui races in this respect it is necessary for the particle board producer to eliminate thick particles from the surface layers. In ligure 4 it vili be seen that there is a close correlation between pirticle thickness and particle size, as might be expected, so by su (.table milling of material for surface layers, and/or by appropriate mechanical sieving, the particle board producer can provide a fine surfaced board. Perhaps not unexpectedly, scure less desirable features also arise - Increased dust problems, Increased resin requirements, reduced stiffness and stability.

Pig. 4: Mechanical sieving-correlation between thickness/screen size.

While mechanical sieving provides an approximate grading of particles according to thickness, a more positive grading is obtained by air separation, separation being affected by particles falling

- 7 -

vertically through a rising stream of air. In very simplified terms, the terminal velocity V^, may be expressed as follows:

V t - K\Jptwhere: V ■ terminal velocity in m/sec.

K “ constantp ■ density of wood particles in kg/m^ t ■ thickness of wood particles in mm

This formula shows that, for the same material, separation or grading is proportional to the square root of the particle thickness and most modern particle board plants make use of this air-grading process. It should be noted that the presence of density in the formula indicates that perfect grading according to thickness is not possible under the following conditions, which unfortunately do occur in practice: If particles are of different wood species, ifparticles are of different moisture contents, if particles even of the same specie and moisture content, come from earlywood and latevood. However, while pointing out these possible limitations, the appre­ciation by the particle board producer of the factors involved in determining surface quality and the ability to deal with them has led to the availability on the market of boards well suited to most types of furniture finishing.

Typical PV values are quoted in Table I hereunder for several types of comnerclal particle boards. The PV values quoted were obtained using the BS.1811 procedure.Table I: PV values for commercial particle boards:

PV (dry)if PV (wet)*/

Constructional particle boards 15 - 30 50 - 250Standard particle boards 10 - 15 15 - 50Finer-surfaced particle boards 5 - 1 0 10 - 30Fine-surfaced particle boards 3 - 5 10 - 20

Many workers have tried to establish quality levels of PV values for various applications, and Table II lists tentative values obtained

- 8 -

from several sources (including the FESYP Working Group, F.I.R.À., Weyroc Ltd.)» With information such as this, the particle board producer can assess the suitability of his product for any proposed application, and^if appropriate,may modify his production processes accordingly.

TABLE II; Tentative PV requirements for various applications:r PV (dryV PV (wet)/,

Core Board RequirementsTop quality, gloss, decorative paper 10 - 15 30 - 50 jGood quality, matt, decorative paper 20 - 30 4 0 - 6 0 i

Fair quality, matt, decorative paper 30 - 50 80 - 120

Wood Veneers, thin 20 - 50 50 - 80Wood veneers, thick 50 -100 1 100 - 200

PVC - thin, plain 1 - 5 20 - 40PVC - thick, plain 5 - 1 0 40 - 60PVC - embossed/engralned __ 30_-_50___ __50_-_150___

Paint/lacquer As for decorative papers

Finished board requirementsTop quality, gloss, decorative paper 1 - 5To£ ^ualit^, semi-matt, decorative paper ___ 5_-_10___ —

PVC - plain,matt 5 - 1 0PVC - voodgrain, embossed 10 - 30PVC - woodgraln, matt 5 - 1 5

(2) Surface porosity:

When applied to liquid finishes, the surface porosity is a combination of absorbency and the presence of "pin-holes", while with decorative film application the latter effect is predominant. The best measurement of surface porosity would still probably be a practical test, eg. paint application.

- 9 -

Certain test procedures have been investigated and considered by the F.E.S.Y.P. Working Group on Surface Qualities, and with the growth of painted surface finishes, this property Is of consi­derable importance. For many purposes porosity may be measured by the PV method already discussed, using a stylus of sufficiently small diameter.

For the particle board manufacturer a number of fa-tors Influencing porosity have been established as Important, including the size of particles in the surface layers, the resin content used in the surface layers, the densificatlon of the surface layers, moisture content and press cycle programme. As a general rule, the finer the surface material used, provided it is adequately bonded - which implies a high resin content, the better the paintability of the surface.

(3) Surface soundness:

When used as a core for the application of surface materials such as wood veneers, high pressure laminates, PVC foils, etc. particle board surfaces must be capable of withstanding the stresses, usually peeling stresses, which may be exerted by these materials.When used with paint or lacquer finishes the particle board surfaces oust be sufficiently strong to prevent scuffing and scratching. Appropriate test procedures apply and form part of the control pro­cedures in most particle board plants. To restrict discussion to the manufacturing aspects, it is interesting to refer to Figure 5 showing the densificatlon patterns that may occur in production, and in particular to the profile for a slow-closing press. In this pattern it will be noted that the outer surface layers are of rather low density, and even after the sanding process, intended particularly to provide boards of uniform thickness, there remains outer surface layers of much lower density than in Figure 5 (c) for example.

10 -

Fig.5: Typical densification patterns.

The low density zones illustrated are caused by the well-known problem of "pre-cure" in which the high platen temperature of the press causes premature curing of the outer resinated chips to occur before adequate consolidation has been effected. Such pre-cured layers are loose and porous, and unless they are ramoved by adequate sanding, boards of poor surface soundness will result. But here again, the particle board manufacturer is well aware of this parti­cular problem and of the many factors involved in addition to the press closing time used in the example, and following the maxim that forewarned i.i forearmed, can take the measures necessary to produce particle boards with good densification and with surface soundness as required for the various applications discussed in this section.

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IMPROVING me. SukFAue QUALITY 0? PRESSING

ntiupt/nv B/uon tV UTPU-.TillPff© 1TIPPrAMJLViib lA/XUW wa u lv u 1 i r ^ UI .

1, Principle:

The development of superfine surfaces increases certainly the suitability for printing directly on particle board. However, super­fine particle board is expensive.

The smoothness of conventional particle board is generally adequate for products in whicl the oi cne particle boardis finished by applying wood veneers or plastic sheets. For appli­cations requiring painting or grain printing, the surface is still too rough and absorbent and usually must be filled, sanded, or over­laid with a film such as parchment. As a potential alternative to this, it was found that the surface could be greatly improved by re-pressing the .¿oard at very high temperatures (200 to 400°C) and pressures (2.5 to 28 kg/cm^) in a platen press for a few seconds (1 to 5 seconds). Under these conditions, the wood in the surface layer became highly plasticized, and fused to form a smooth, non­absorbent skin.

2. Results and discussion:

In general, the fusing temperature, pressure, and time required for unsanded boards appeared to be slightly lower than those for sanded boards, because the surface layers of unsarded boards were invariably low in density and loose. Temperature appeared to be the most sensitive factor. Higher temperature tended to produce a better surface, but increased the risk of damaging the board.

Surface smoothness was greatly improved on the treated boards. Figure 6 shows the surface profiles of treated and untreated particle board and some hardboarls. In general, the surface of the treated boards (both sanded and unsanded) was as smooth as that of the hard- board surface. The treated surface of the unsanded, dry boaru appeared

12 -

to be smoother than that of the sanded one. This was attributed to the much finer - and therefore more easily fused - particles on the surface of the unsanded board.

CONTROL SURFACE FUSED

Tempered Hardboard

v •i.

*

Standard Hardboard

Unsanded Particleboard

Sanded Particleboard

f '.u'" EUnsanded Particleboard After Being Soaked In Water For 3 Hours

F

Sanded Particleboard After Being Soaked In Water For 3 Hours

Fig. 6: Surface proflles of hardboard and partldeboard before and after surface-fuslng treatment.

Surface porosity and palntablllty

The Improvement of the surface quality reduces the number of coatings necessary to obtain a uniform coverage from 3 to 1.

13 -

Surface densification, water resistance and £ire-ret«rd«ncy

Fusion treatment increased the density of the surface skin fn about 0.8 to about 1.2 g/ca?. Because of the densification, the surface abrasion resistance of the treated board was increased.The »ost striking change caused by the treatment was the increased water resistance of the fused surface skin.

The surface flamabllity of treated board was appreciably ’owered.

WET FINISHING

The basic requirements for wet finishing composition board consist of three elements (see figure 7).

(a) Coating equipment ;(b) Drying equipment;(c) auxiliary equipment.

(a) Single color print

(b) Multicolor print

(c) A complete line layout Including auxiliary equipment

fig» ~l : Typical particleboard grain printing

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(a) Coating equipment:

The coating equipment can apply the finish by:- spray coating,- curtain coating,- direct-roll coating,- knife coating,- reverse-roll coating.

Face finish material new is far more sophisticated than that of just a few years ago. It ranges over solvent-free liquids, che­mically cross-linked resins, water-base finishes, and irradiation- cured polymer systems. It is anticipated that greater advances are to come through research and development by the suppliers.

High density hardboard can be finished directly with wet finishes, because of its dense, smooth surface. However, particle board and medium-density fibreboard will normally need some type of preparation for final finishing, because of the voids jetween particles on the surfaces. Recently, more very fine material has been used on the faces of the boards that will receive high-quality finishes. This reduces the need for filling. However, with the wet finishes, some type of filling will usually have to be done. Boards over 45 lbs/ft^ density (0.72 sp gr) are recommended.

A typical wet system for treating particle board or fibreboard is:

- first fill the surface;- then apply a base coat;- print on the highlights of a pattern with a gravure press;- print on the final design;- and apply a top coat.

Filler

The filler is the foundation of the coating system. In addition to uniforming the surface, it must have excellent adhesion, be

15

hard and tough, and hold out the subsequent coats so that a deep, full appearance can be obtained.

Solvent-based fillers used include lacquer, vinyl, water emulsion, alKyd, polyurethane, and urea-alkyd. Lacquer or vinyl- based fillers have good adhesion to the substrate, excellent sanding, very fast drying by conventional oven systems, excellent recoating characteristics, and they may be pigmented to give excellent hiding properties. They have a lower volume solids, however, resulting in lower filling action, higher cost, and only fair chemical resistance.

Water-emulsion fillers have high-volune solids, good adhesion to most substrates, excellent sanding, relatively fast drying with conventional oven beating, good chemical resistance, and they may be pigmented for hiding. They are usually sanded to smooth the surface before the next finishing coats are applied. They should receive at least one coat of a solvent-based material (eg. basecoat, sealer, or topcoat) which v/ill act as a moisture-vapor barrier. This will control flake swelling during the service life of the panel. Water-based fillers are a satisfactory means of deducing the air- polluting effluents on existing finishing lines.

Polyurethane and alkyd-based fillers have a high-volume solids level, good adhesion to most substrates, good sanding after curing, and excellent recoating characteristics. They have the disadvantage of relatively slow drying speeds.

Urea-alkyd fillers have high-volume solids, good adhesion, good sanding and recoating characteristics, and fast drying. How­ever, they have limited pot life.

In the past few years, new fillers have been developed, based on photochemically reactive resins, primarily polyesters and acrylates. In place of a volatile solvent, they use reactive monomers (eg. styrene, vinyl tolueve, ethyl acrylate, methyl methacrylate) which become a part of the filler during curing.

16 -

There are tvo types of filling machines in use. They are the bottom'knife coater and the reverse-roller coater (figures 8 and 9). The coating is applied to the bottom of the board in knife coatings. The filler is contained in a reservoir and is transferred to the board by a roller. The excess of coating on the surface of the board is removed by the knife, and this material flovs back into tho reservoir.

Fig. 8: Schematic drawing of a knife coater

Costing Style A

fig« 9 : Schematic drawings of two styles ofreverse coaters

17 -

Normally, a knife coater is operated at comparatively low through-put speeds and with a high knife pressure. Because the knife is flexible enough to follow small variations in the board surface, there is very little surface build-up of the coating, while complete filling of the voids is achieved.

The upside-down operation makes it a little difficult for in­line Inspection of filling operation. Also, the possibility of damage to the filled surface is much greater than with the top side coating; so, it is essential to carefully clean the conveyors and transfers. Nevertheless, knife coaters are very effective where good filling with minimal surface build of the coating is required.

The reverse-roller coater is more commonly used. The first section is essentially a direct-roller coater, followed by a doctor or metering roll. Support rolls are below the upper rolls (see figure 9). A controlled excess of coating is applied to the board by the application roll. The wiping roll is adjusted so that there is a snug fit between it and the pressure roll. As the board passes under the wiping roll, excess material is wiped from the surface and is removed from the roll by the scraper blade.The amount of material left on the surface can be controlled by varying the relative speeds of the application and reverse-wiping rolls. The faster the wiping roll speed, the less the amount of material left on the surface.

In newer machines, the two rolls are closely coupled, so that the excess material is transferred directly from the wiping roll back to the application roll. There is essentially no change in the coating composition from the beginning of the run to the end of the run, and there is no concentration of contaminants.

After the filling and curing, the fillers are often sanded with a fine-grit belt or a flap wheel unit, which is a slotted aluminium hub into which coated abrasive segments are loaded.

- 18 -

These flaps of sandpaper can be used as is, or they can be par­tially cut into narrow strips which provide extra flexibility.These flaps or strips then whip over the panel during sanding and are particularly effective in obtaining abrasive contact with out-of-level surfaces. The panels are then brushed or clown clean and proceed to the base-coating step. Soae board plants ship filled panels for final finishing by the consider.

Basecoat:

The basecoat or ground coat is then applied over the filled surface with a direct-roller coater, precision coater, or curtain coater. A curtain coater Is shown in figure 10. Precision coaters are similar to direct-roller coaters, but they can also control the texture of the coating.

Fig. 10: Double-head curtain coater (Feed speed: 60 to500 ft/min (18.3 to 152.A m/min) Application:10 to 60 less/ 1.000 sq.ft. (A.88 to 29.3 kg/lOOm^))

Basecoats are high-solids-p ..gmented materials that form an opaque film. These are usually lacquer- or vinyl-based, though water- reduced basecoats are finding increased use. It should be mentioned that such coating can also be done on paper-overlaid board, veneer-overlayed particle board, low-pressure-laminated particle board, or composition board that does not require filling. Several

- 19 -

coats of the Lisecoat may be applied. The basecoat is cured in an oven before passing through a buffer which can be a sander or a nylon mesh roll impregnated with abrasive grains. This scuff sanding can improve the quality of the print by removing high spots. All dust must be removed from the panel after buffing. The basecoat supplies the initial undertone colour for the final finish.

Printing;

A grain print is then applied by engraved rollers. The prin­ting can be done in several colours with each print keyed in register to introduce an Interplay of colour and create the illusion of depth, wood texture, and figure highlighting. The surface can be embossed to accentuate the grain design. An embossing roll is shown in figure 11. Such a roll is also used to apply ink at the bottom of the grain impressions simultaneously with the embossing. This is followed by the normal printing. Figure 12 shows a one-roll printer.

Figure 11: An embossing roll.

Fig. 12: One-rcll printing machineFeed ¿peed: 30-150 ft/min (9.14-45.72 m/min)Application: Cl lb/1.000 sq.ft.

(<0.49 kg/100 a2)

Top coat:

The sealer or topcoat is then applied by roller coater, pre­cision coater, or curtain coater. A sealer is used to protect the print of panels for furniture manufacture, where additional finishing steps are to be applied. Sealers are usually nitro­cellulose- or vinyl based. If the completed composition board panel is used as is, generally a heavier coating of a more abrasion- and stain-resistant topcoat is applied as the last step. Alkyd-ureas are most frequently used, combining high build and abrasion resistance, rapid cure response, good chemical resistance, and low cost. Nitrocellulose, catalyzed nitrocellulose, vinyl, and cellulose acetate butyrate (CAB) are also used.

Fairly rectntly, UV-curable polyesters and acrylics have been Introduced, utilizing similar technology and curing systems to the UV-curable fillers. Another recent addition is the water- reducible topcoats based on acrylic or polyester resins, cross- linked with urea- or melamine-formaldehyde resins. Both UV- curable and water-reducible topcoats will see increased use in the future as the Industry moves to improve its environmental Impact.

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(b) Drying equipment:

The drying ovens can operate as:- convection ovens using hot air,- infrared ovens that are gas fired or electric (IR),- ultraviolet radiation-type drying equlpnent (UV),- and the newly developed electron-bean curing (EBC).

The drying or hardening section is a very important phase of the operation. It is not only a problem of the quality of the coating, but also of economical importance as well as the cost of energy as the Investment costs.

The classic equipment is a kind of long chamber heated and ventilated on programme, where the coated boards are going through on conveyors. This system is not really specific and selective for the coats applicated on the boards. Therefore more specific systems have been developed.

The infra-red waves (IR) give a good surface heating, more or less specific for the compounds mixed in the coating.

Another development is the ultra-violet waves (UV). There is not any more heating for inducing the chemical reaction but an energetic and free radical inducing syscem. The deepness of penetration is much better, compared to an IR-system.

The latest developement is an accelerated electron-beam inducer (EBI). It has many features which will give a bright future for the system, especially concerning the reduction of the amount of energy, the reduction of the surface occupied and a better and faster chemical reaction of the components. The quality of paints to be hardened by electron-beams must be superior to that of other paints.

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fig. 13 : Principle of the electron-beam induce: (EBI)

- 23 -

Fig. 14: Schematic drawing of a complete EBI-plant.Legend: 1. Acceleration of electrons.

2. Vacui* pump3. Inter-gas generator4. High tension device5. Curing tunnel with lead protection6. Conveyor

Fig. 15: View of an electron-beam Inducer.

- 24 -

(c) Auxiliary equipment:

The auxiliary equipment consists of panel feeder, brushes, sanders, burnishers, stackers, and conveying means for trans­porting the board panels through the finishing process. The importance of these auxiliary elements, particularly sanders and burnishers, for proper substrate preparation prior to coating cannot be Ignored.

LAMINATING

When it comes to laminates for overlaying board substrates, there are two types: self-bonding and adhesive bonded.

For successful lamination with thin ¿.xlrns, it is Important to have very fine particles on the faces of the board panel. Very fine material, previously thought to be a resin hog in board manufacture, is being used more and more for achieving smooth surfaces. The smooth surfaces, as mentioned, are needed for pre­venting show-through of irregularities through the thi: surface in papers. It appears that more attention must also be paid to the particle size and geometry in the board core to ensure a hiph quality end product. If a high gloss on a film is used, irregu­larities are highlighted. However, most of the thin films are matt, satin or "furniture" finished, which tends to diminish this problem. Particle board with large particles on the faces needs some type of "barrier", "cushio", or underxayment sheet underneath the film surfaces, such as phelolic- or polyester- impregnated paper (kraft). The use of panels without faces of fine particles is highly unusual.

A very low moisture content of the board can cause diffi­culties when the board and surface film are combined. Too high a moisture content causes stains due to the increased vaporization and also runs the risk of a poorly closed surface. Moreover, such a condition can cause overlay "blisters" and an undue shrinkage in thickness. The optimum moisture content for good bonding ranges between 6 and 7.5 per cent.

- 25 -

Dense panel faces can give probleas with adhesion Co the films. Poorly cured adhesive joints can let go because of high tensile and shearing forces developed in the laminate. Because most particle board has a low surface density on the outermost faces, problems can occur because of the lover strength associated vlth lover density. It has been found that 0.8 to 0.9 sp gr on the panel surface appears satisfactory. Thus, lov-density faces must be sanded off, particularly If this has not been accomplished on the regular production line. These faces were mentioned in the discussion of density profile. The density of board for conventional laminating normally exceeds 45 lbs/ft (0.72 sp gr). Boards of lover densities have reportedly been used in short- cycle laminating,but this Is unusual.

Variations in specific gravity with thin boards are super­imposed upon stresses generated by the laminating pressure.These stresses can become of such magnitude as the baord shrinks and swells, due to changes in humidity, that stress cracks form in the laminates. Thi board surfaced with melamine-resin overlays, ultraviolet-cured polyester underlays, and hard urea resin coatings are particularly susceptible.

It is also possible to laminate some overlays onto particle board during the actual pressing of the particle board. This is becoming more important with the thinner boards that are now being produced, particularly the continuous type where the surface layers can be rolled upon the formed mat as it enters the hot press. In this case, almost perfect forming of the mat is neces­sary to keep a uniform density over the entire area of the final board. Let us now turn to the major types of overlays and their applications.

- 26

(a) Laminating with adhesive-bonded product":

Adhesive-bonded laminates include:- wood veneer,- the familiar high-'pressure plastic laminate used for

many years,- paper with pre-condensed amino and phenolic resins,- films underlays with phenolic resin^or amino resin

impregnation and coatings,- vulcanized fibre with amino resin coatings,- thermal plastic films such as polyvinyl chloride.

Let us now turn to the major types of overlays and their applications;

1. Wood veneer:

High-quality wood veneer overlays, of course, are a premium product as far as quality is considered in the market place.Particle board and medium density fibreboard have become the primary substrates for use with quality wood veneer. In fact, core­type hardwood plywood, which is a major product, consists of the hardwood veneer over the particle board or fibreboard substrate.Host of the volume of the material is composition board, not hard­wood plywood.

Wood veneer is also used for crossbands over the composition board substrate before high-quality veneer is applied to reduce telegraphing problems. Wood veneers such as tupelo, maple and birch are laminated onto board eo provide a base for subsequent printing or film overlaying. Veneers are applied to the substrate by conventional hot or cold pressing means developed over many years, and much experience has been gained in this type of laminating. Care must be taken to equilibrate the veneer and substrate to about 7 percent moisture content before glueing to minimize checking problems. Telegraphing is a problem with veneer laminating, con­sequently, exposing the substrate to high humidities before glueing and the use of glues with high water content should be avoided.

- 27 -

2. High pressure decorative laminates:

High-pressure decorative laminates have been around for many years. They consist of printed papers which have been satu­rated with thermosetting resin, generally of the melamine-for­maldehyde type and bonded to multiple layers of phenolic-impregnated kraft paper under high heat and pressure. This is a high-cost product with excellent impact and abrasion resistance, good chemical resistance, and durability. It should be used where the high* performance properties can be best utilized. These uses include table tops and counter tops and other horizontal surfaces on furniture, kitchen cabinets, and on bathroom walls.

3. Polyvinyl chloride:

One of the best known overlay materials is polyvinyl cMoridc (PVC) film. There are two types: plasticized film and rigid.When applied in a plant, the film is usually in a pure form and is bonded to the substrate with an adhesive in a roll-laminating process. This film can be printed and embossed. Prints on the outside of the film can wear away in areas receiving severe abrasion. This has been overcome by printing the pattern on the reverse side of the film, which is placed next to the substrate. The back print is then covered with a ground coat or colour coat which masks the substrate and adhesive. With wood grain printed pattern, the back printing technique has a degree of realism very difficult to attain otherwise. It is also possible to put a clear protective film over any of the films where face printing is desired, and this is a common practice.

Adhesives are necessary for bonding vinyl film to wood-based panels and generally are of two classes: water-based (poly­vinyl acetate) and solvent (polyester or polyurethane) which are applied to the substrate. Epoxy is being used more and more and

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has the advantage of being 100 percent solids, thus, there is not any solvent to evaporate. The solvent-based systems are two-part materials and are considered to produce the maximum overlay bond strength. The water systems are usually thermoplastic and are the types most coomonly used. Whichever system is used, the adhesive must be resistant to the plasticizers that can migrate from the PVC film. The bond of the film to the panel should be sufficiently strong to resist film creep or shrinking away from the panel edges. Application equipment for this system can be relatively simple (see Fig. 16).

Fig. 16: A typical direct-roll coating system forapplication of vinyl film to particle board.

The essential parts consist of rolls to tension the vinyl sheet, a heating unit to soften the film and make it easier to handle,

29 -

and adhesive applicator roll to apply the adhesive to the fils, a heater tc flash off the water and also soften the film, and nip or pressure rolls to provide intimate contact between the film and the panel substrate. In some cases, the adhesives nav be applied to the panel substrate rather than to the film. Folding and tucking rolls may be used for the edge wrapping of the panel, as pointed out earlier. An embossing roll is normally used to emboss a realistic wood grain effect.

V- grooving process

It is reported that polyvinyl fluoride overlaid PVC film is nearly equal to ceramic tile in stain-resistance and cleanness.This two-sheet laminated overlay is valuable for the manufacturer of vinyl-clad panels which are folded for cabinet or furniture parts. This system is used for making finished products of square, rectangular, or polyhedral shape. The first step consists of lamination of the board with the flexible plastic sheet. Then accurate mitre-grooves are cut through the substrate in the desired location« Adhesive is then metered into the grooves and the panel Is folded into the desired shape, using the flexible plastic sheet as hinges. Figure 17 shows some typical configurations that are possible. When back-printed film is used, cutting into the film results in an undesirable and distinct white line. But by using face-printed film (usually with a protective film on top), overcutting to a considerable depth is possible before it is visible from the face. The back- or reverse-printed film is normally about 3 nan thick.

30 -

Fig. 17: V-grooving process permits many simple as wellas complex forms to be accomplished in un­complicated grooving, folding, and glueing proce­dures.

(b) Laminating with self-bonding products:

The self bonding laminates are newer and Include:- melamine-lmpiegnated paper;- polyester resin-impregnated paper;- diallyl-phtalate-(DAP)impregnated paper.

The production of the overlays based on melamlne-resln-saturated alpha-cellulose paper is rapidly growing in Importance. The melamine-formaldehyde resin used has modifiers added to it to enhance the elasticity of the hardened melamine resins. Other materials are added to the resin solution, which later on improves the impregnation of the paper and helps prevent films from sticking to the platens.

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The base papers tor Che decorative films are alpha-cellulose papers, which are used almost exclusively. For Impregnation, these papers must have a V ^ h vet strength and the right porosity to accept the proper amount of resin. The board laminate consists of one to three papers: an overlay sheet, a decorative sheet and a barrier (or base) sheet. Barrier sheets are not known to be used in the United States of America and Europe. Most boards have very smooth surfaces, eliminating the need for such sheets, which are used to cover porous surfaces. It should be mentioned that balancing papers are also normally used on the back of the board so that, after it is laminated, warping will not occur. In some cases, the same type of melamine overlay is used on both sides of the panel. In this way, if some minor defect happens to occur on one side, the other side is still perfectly acceptable for a top- grade face, and warping problems are minimized with identical papers on both sides.

The melamine-saturated papers usually contain 50 to 55 percent resin. The overlay or cap paper is an unfilled alpha-cellulose paper which, upon pressing, becomes transparent and protects the decorative pa^er lying underneath, particularly against wear and tear. The use of a cap sheet is popular in the United States on the wear side of panels. To Improve abrasion resistance such cap sheets can also be used on polyesters. It has already been mentioned that the decorative papers are either pigmented solid colours or pigmented sheets with a wood grain or random design in one colour or multi-colour prints. It should be mentioned that these colours must be face-resistant and compatible with polyester resins and solvents used in polyester and melamine re­sins.

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Postforming process:A new development In particle board finishing is the so-

called "post-forming", which is much utilised in the manufacture of shelving, table-tops and kitchen furniture. The method is explained in figure 18.

Fig. 18: Post-forming process.

Before laminating, the edges of the particle board can be profiled to certain simple profiles (fig. 18b), using conventional routers and shapers.

The edges and the surface are covered with one sheet of laminating material (fig. 18c). A backing sheet is necessary.In post-forming, the laminate is moulded to the shape of the substrate.

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If 1C concerns a rigid plastic laminate, heat Is first applied until the plastic becomes pliable. This must be controlled precisely, otherwise, cracking will occur if the plastic laainate is too cool or blistering will happen if the laminate is too hot. The pliable part is then held by the post-forslng machine againat the substrate until the adhesive has cured. This is the popular method for applying a laminate to a countertop where a curved or rounded front edge is desired.

Different types of plants equiped to overlay with melamine:

1 • Multi-opening press with cooling:

Not much melamine overlay is applied in multi-opening presses. Special handling is necessary to prevent precure of the resin when loading this type of press. In the instances where such laminating is done, press times between two and eight minutes are reported at temperatures ranging from 280° to 330°F (138o-166°C). Pressure« are higher than used for polyester, being about 350 psi (2,41 MPa). Release papers can be used to give smooth or textured surfaces.

Melamines shrink slightly during curing, consequently they must be handled carefully if laminated to thin substrates below 1/2 in.(12.7am) in thickness. Warp problems must be handled. In comparison to polyesters, the melamine surfaces have better scratch resistance at the same resin level.

Formerly, relatively long times were always necessary for applying these types of decorative surface laminates. However, there has been a clear move to high temperatures at relatively short pressing times, made possible by the development of better resin systems.

Fig. 19: Multi-opening press for laminating with melamine paper,equipped with a heating and a cooling system.

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II. Short-cycle process with a single-opening horizontal presswithout cooling:

A great step forward in this type of laminating is the pre­viously mentioned short-cycle process with single-opening presses where lamination is accomplished in about one minute. High-gloss finishes so far have not been possible with this type of finishing. For most applications high-gloss finishes are not desirable because any slight imperfection in the overlay or substrate is shown up. Resins with very short cure times (from 20 to 30 seconds or less) are now available, thus, very fast press times are possible.These fast-curing resins require, however, that the time between initial contact of the film with a hot heating platen or plate to the time it takes to reach a specific pressure of approximately 120 psl (0.83 MPa) must not exceed a value of 15 to 20 percent of the condensation time. This means that in the case of a con­densation time of 30 seconds, loading of the press, closing of the press, and generating of sufficient pressure must not exceed a time of about 6 seconds. Therefore, once the board and overlays are inserted into the press, maximum pressure must be applied quickly or the fast-curing resin in the overlays will polymerize before good bonding to the substrate takes place (referred to as pre-cure).

Polyester laminating is accomplished in about 55 seconds (25 seconds for resin cure and 30 seconds for unloading and loading the press). Pressure range from about 175 to 220 psl (1,21 to 1.52 MPa). Release papers are usually necessary.

Melamine laminating takes about 60 seconds (30 seconds for resin cure and 30 seconds for press unloading and loading).Pressing pressures range as high as 350 psl (2.41 MPa), and

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resin curing temperatures range from 310° to 350°F (154° to 177°C). Release papers aay or nay not be used. The use of polished chrone plated pressing plates and melamine containing a release agent often eliminates the need for release papers.

It must be kept in mind that there are many variations of the low-pressure laminates available and that the values given here are only intended as a guideline. Figure 20 shove a short-cycle press line for overlaying. The four photos show assembly of resin- treated papers with board substrates and press loading:

a. Laydown and positioning of bottom surface papers,b. The top papers are positioned over the substrate,c. The assembly is then clamped prior to being loaded into

the press;d. The finished board is ready for unloading.

Fig. 20 a: Laydown and positioning of bottom surface papers.

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Fig. 20 b: The top papers are positioned over the substrate

Fig. 20 c: the assembly is then clamped prior to beingloaded into the press.

Fig. 20 d: The finished board Is ready for unloading.

III. Short-c^cle process with a single opentog_verticaljjress without cooling :

Another type of short-cycle laminating which has been developed Is a vertical laminator as Illustrated In figures 21 and 22. The substrate Is cleaned and transferred to the vertical position by a special vacuum lift. The overlay papers are placed in position and "tacked" there by means of a glueing process.The sandwich Is then fed Into the vertical press, and heat ard

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pressure are applied simultaneously tc both face» of the panel. In both systems the finished panel is then edge trimmed with con ventional equipment.

- ,\

|B fl

P-ress

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IV. Continuous manufacture of decorative laminates:

The heart of the production line for continuous manufacture of decorative laminates Is the so-called double band press. A pressure being uniform all over the width of the product and being moreover temporarily constant Is applied onto the product breadth moving continuously through the press.

This is done by transmission of pressure onto endless steel belts moving synchronously with the product through the press and serving at the same time as finish transmitters. The essential point is now the pressure transmission from the press frame onto the steel belts. There are three basic principles:

- sliding press sliding friction- fixed revolving rolls rolling friction- pressure cushion liquid or air friction

Between the fixed press frame and the moving steel belts the pressure transmission is a source of friction which is accor­ding to the pressure system sliding friction, rolling friction or air friction.

In the case of sliding friction the pressure is applied directly onto the steel belt by means of a pressure plate covered with a sliding material. The disadvantages of such a construction are tremendous: friction power between the pressure plate and thesteel belt and thus the belt tension as well as wear of the sliding plate material and the steel belt increase proportionally to specific pressure and press length. The driving power for the drums for moving the steel belts also quickly raises to un-economic dimensions. Therefore, this pressure principle could not succeed for the manufacture of decorative laminates.

In the case of pressure transmission by fixed or revolving rolls, the too high sliding friction from the direct contact between sliding plate and steel belt is compensated by means of single pressure cylinders, pressure rolls, roller chains or similar machine elements. The sliding friction is thus replaced to a great extent by roller friction. The tension of the steel belt is lowered. But there is another great disadvantage: a bandpress with a roller system does not apply a constant power.

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It rather shows a line pressure swelling up and down all the time which has negative influences on the curing process of many pro­ducts. Therefore, this procedure is only applicable for such products where a uniform pressure is less important.

Fig. 23: Bene principle of the continuous double-beltpress.

In the case of the double band press with pressure cushion the pressure in form of an air cushion is uniformly applied to the steel belt. On account of the minimum friction coefficient between solid bodies and air, the friction between steel belt and pressure cushion is negllgeably small so that the disadvantages mentioned above are omitted.

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On the contrary: the high uniformity of the pressure all overthe width and the length of the pressure zone as well as the uni­formity of the temperature transmission onto the lnlnate are decisive advantages of this pressure system.

Therefore, the double band press with the pressure cushion principle built now is especially suitable for the manufacture of decorative laminates.

fig. 24- : Continuous press with pressure cushion (air friction)

TABLE I: Suitability of the various types of finiahes for the different types of boards.

Plyvood ParticleBoard

Flbreboard M D F Cement BoardWet dry Low

densityHighdensity

WET FINISHING- Spray coating + + + + + + +- Curtain coating + + + + + + +- Direct-roll coating + + + + - -

. - Knife coating + + + + + - -- Reverse-roll coating + + + + + - -LAMINATINGLAMINATING WITH ADHESIVE-BONDED PRODUCTS- Wood veneer + + + + + - -- High pressure decorative laminates + + + + + -

- Polyvinyl chloride + + + + - -LAMINATING WITH SELF-BONDING PRODUCTS- Melamine impregnated paper + + + + + - -- Polyester resin impregnated paper + + + + + - -

- Diallyl-phtalate impregnated paper + + + + + - -

TABLE II: Investment costs, economics of scale, value added.

Investment costs (in US$)

Economics of scale (sq.raetres/year)

Value added (US$/sq.metre board)

WET FINISHING - Spray coating 14.200 - 86.000 9.000 - 40.000 | 0.3 - 3.0 depending

1 on: -type of product -quantity applied

- Curtain coating 20.000 - 45.700 12.700 - 21.300

- Direct-roll coating 18.500 11.700 0.S5 - 2.5 depending ̂on: -type of product 1 -quantity applied1

- Knife coating 28.500 18.200

Reverse-roll coating 28.500 - 57.000 18.200 - 26.600

LAMINATINGLAMINATING WITH ADHESIVE-BONDED PRODUCTS - Wood veneer 300.000 - 1.500.000 200.000 - 600.000 2 - 8 . 6 depending on type

of wood

- High-pressure decorative laminates 1.000.000 - 3.100.000 450.000 - 1.000.000 3-8 depending on type of lamlante

- Polyvinyl chloride 200.000 - 1.250.000 ¿130.000 - 500.000 1-5 depending on type of foil

LAMINATING WITH SELF-BONDING PRODUCTS - Melamine inpregnated paper | 950.000 - 3.900.000

depending on:-type of press system -capacity

7 600.000 - 1.500.000

)\

1 2 - 5 depending ) on type of ) laminate

- Polyester resin Impregnated paper- Diallyl-phtalate Impregnated paper

TABLE III: Level of skills needed, power requirements.

LEVEL OF SKILLS NEEDED POWER REQUIREMENTS:- electricity: KWh/m2 board- heavy fuel: kg/m2 board

WET FINISHING- Spray coating * 0,15 kWh/m2 electricity

/ 0,30 kg/m^ fuel- Curtain coating *

- Direct-roll coating * ̂ 0.12 kWh/m2 electricity v 0.25 kg/m2 fueli- Knife coating *

- Reverse-roll coating *

LAMINATINGLAMINATING WITH ADHESIVE-BONDED PRODUCTS

- Wood veneer ** 0.20 kWh/m2 electricity 0.18 kg/m2 fuel

- High-pressure decorative laminates A A* 0.30-0.46 kWh/m2 electricity 0.23-0.29 kg/m2 fuel

- Polyvinyl chloride ** 0.10-0.24 kWh/m2 electricity 0.19-0.27 kg/m2 fuel

LAMINATING WITH SELF-BONDING PRODUCTS - Melamine impregnated paper ■kirk 1 0.21 - 0.33 kWh/m2 electricity j 0.16-0.20 kg/m2 fuel- Polyester resin impregnated paper kkk

- Diallyl-phtalate impregnated paper kkkk

TABLE IV: Data on equipment interchangeability for different processes:

EQUIPMENT INTERCHANGEABILITY FOR EIFFERENT PROCESSES

WET FINISHING - Spray coating only spray coating possible

VWith application of the adapted chemical products, each type of ^drying equipment is possible:1. convection oven (hot air)2. infrared oven3. ultraviolet radiation4. electron-beam inducer/

- Curtain coating only curtain coating possible- Direct-roll coating only direct-roll coating possible- Knife coating application as direct-roll coater

possible- Reverse-roll coating application as direct-roll coater

possible

LAMINATINGLAMINATING WITH ADHESIVE-BONDED PRODUCTS - Wood veneer Other applications for laminating with adhesive-bonded products possible:

wood veneer, high-pressure plastic laminate (press for high pressure needed), paper with precondensed amino and phenolic resins, film under­lays with phenolic or amino resin coatings, thermal plastic films such as polyvinyl chloride,...

- High-pressure decorative laminates- Polyvinyl chloride

LAMINATING WITH SELF-BONDING PRODICTS- Melamine Impregnated paper- Polyester resin Impregnated paper

The equipment is interchangeable for the different self-bonding products: melamine Impregnated paper, phenolic resin Impregnated paper, modified melamine Impregnated paper, polyester resin Impregnated paper, diallyl- phtalate impregnated paper, ...- Diallyl-phtalate Impregnated paper

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REFERENCES

1. Preliminary Publication of FESYP Investigations into Elasto- mechanical Properties of Particle Boards of Importance In Construction, FESYP, Griessen, February 1979.

2. FESYP Recommendations for the Determination of the Surface Quality of Flat-Pressed Boards, FESYP, Griessen, December 1978.

3. Richter, J. A., Printing and Finishing of Particle Board,Furniture Prod. Mag. 34 (237): 36-37, 1970.

4. Shen, K. C., Improving the Surface Quality of Particle Board, Forest Products Journal, Voi. 24, no. 10, October 1974.

5. Allen, D., Manufacturing Considerations of Particle Board REquirements for Furniture Application, FESYP documentation 224, 1979.

6. Neusser, H., Surface Quality of Particle Boards and its Importance for Manufactured Products, FESYP, 1979.

7. Maloney, Th. M., Modem Particle Board and Dry-Process Fibreboard Manufacturing, California, 1977.

8. Kollaann, Fr., Holzspanwerkstoffe, Springer-Verlin Berlin/ Heidelberg, 1966.

9. Knapp, H., Development, Principles, Problems and TEchnlques of Finishing Particle Board. Proceedings of the Washington State University Particle Board Symposiim, no. 2 Pullman, Washington: WSU, 1968.

10. Plath, L., Requirements of Particle Boards for Resin Surfacing. Holz als Roh- und Werkstoff, Vol. 29, no. 10, 1971.

11. Dürr Otto, Stuttgart (Federal Republic of Germany), Lack- hRrtung durch Elektronenstrahlen.

12. Dieffenbacher J., Epplngen (Federal Republic of Germany) Presslines for veneering and lamination of boards.

13. Slempelkamp Corp., Krefeld (Federal Republic of Germany) Short-Cycle Presslines for Decorative Laminating without Recooling.

14. Becker und Van Hullen, Krefeld (Federal Republic of Germany), Short-Cycle process with a single-opening vertical press.

15. Hyven International, Federal Republic of Germany, Production Line for Continuous Manufacture of Decorative Laminates.

16. Verbestel, Jean B., Lie. sc. chlm. U. L. Br., Consultant, Kortrijk-Belgii*.