Printing papers for decorative laminates

Layers of papers in melamine-faced boards.

By T.K. UNNIKRISHNAN

Impregnation is a process where the filler materials like paper, fabric, glass, asbestos, etc., are populated with required levels of resins and is also called treating. The process allows the filler material to come in contact with the resin. This resin-dipped paper or fabric may be wet and is dried by passing through a hot chamber. The material is then either rolled into reels or cut to the size of the laminate.

There are two types of impregnation techniques: Using a vertical drying chamber for drying the wet materials; and using a horizontal drying chamber for drying the impregnated, wet materials. To increase drying efficiency and production output, more drying chamber sections can be added.

The horizontal type of impregnation and drying is done in air float-type dryers. Air is pumped from the top and bottom jets and the quantity is adjusted so that the wet materials passing are kept afloat, without touching the metal parts of the hot air jets.

If the wet paper comes in contact with sharp metallic parts it will snap, resulting in paper breakage. While designing the machine, engineers ensured that the air pressure from top and bottom are balanced to keep heavy wet material afloat.

The machine, with a working width of 54 inches and three drying sections of 5 metres each, can produce 4-foot-wide laminated sheets. Multiple-section drying chambers are in vogue for medium-sized industrial units. An impregnating machine will have the following sections and areas:

•        Dual unwinding station with automatic roll splicing (jointing) arrangement at the starting (dry end)

•        Automatic device to control the tension of the paper while it is being run

•        Four guide rolls and a dancing roll for paper to travel before entering the resin tray. The dancing is provided to auto-adjust the tension if some variation occurs due to voltage fluctuations, frequency variations, etc.

•        A dip roll is provided at the entrance to allow the paper to remain dipped and in position

•        Three dip roll positions are provided to allow for paper with a long, medium or short dip

•        A pair of sky rolls on top for delayed movement of paper after the dip in resin and to re-enter the tray

•        Scraper roll-blade assembly to remove excess resin that the paper or fabric carries before it enters the next stage

•        A pair of doctor rolls or squeezing rolls to remove excess resin absorbed

•        Smoothing roll assembly of three rolls fitted with a variable speed drive and a forward-backwards drive for smoothness to the top surface of the impregnated material without resin lines, marks, spots, etc.

•        Guide roll to guide paper into the hot air dryer

•        Chain-cum-cross bar (horse) arrangement to convey the clip-attached material to the end of the dryer, known as the threading arrangement. This system is disengaged as soon as the paper reaches the dry end

•        Air cooling section fitted with an exhaust fan, hood and duct line to throw the hot and fumy air out

•        Cooling rolls assembly of three or four rolls provided with water cooling to cool the impregnated and hot paper before cutting or rolling into a reel, thus avoiding pre-mature curing of impregnated resin during storage

•        Pneumatically-operated sheer to cut the paper to size. Rotary cutters are found to be less efficient

•        Collection table with a scissor lift arrangement to collect the cut sheets of impregnated materials, preferably on a wooden pallet

•        Each section has a blower and exhaust fan to pump hot air into the dryer and remove fumes of formaldehyde and unwanted matter through the exhaust outlets

•        The machine is fitted with valves to control the entry of heating medium- steam or thermic fluid into pipelines and radiators for proper distribution of temperature inside the dryer

•        The machine is fitted with speed-control devices, speedometers, temperature indicators, counters for the number of sheets produced and pressure gauges for air, water, steam, etc.

•        The drying chamber is insulated with glass wool in the outer walls to ensure proper thermal insulation.

Kraft papers

Multiple drying chambers are used to impregnate natural Kraft papers, while   two or three chamber units are deployed to impregnate design papers and overlay tissue papers.

All guide rolls are made from stainless steel rolls. Where the rolls do not come in contact with the resin, rubber or ebonite rolls are also used.

The doctor roll or the squeezing rolls are of chilled, cast-steel construction. These rolls are further ground to very fine accuracy (3 to 5 microns) and hard chrome plated to a coating thickness of 5-6 thou.

Natural Kraft paper is mounted on the machine and treated with phenol- formaldehyde resin to produce impregnated Kraft called decorative core, which forms the base for the laminate. The decorative core numbers are increased or decreased depending on the thickness of the laminate to be pressed.

During impregnation of the decorative core, three important parameters for producing good-quality laminates are tracked:

•        Percentage of resin picked up by the paper or fabric

•        Percentage of volatile matter that is retained by the paper after impregnation

•        Flow property of the impregnated material, calculated either in percentage terms (European standard) or terms of measurement (BritishAmerican standard).

Equipment for testing these properties are substance indicator; punching press or templates to cut samples for testing; flow press to test the flow; and laboratory oven 0-300 degree C range.

Natural Kraft comes in different substance weights measured as grams per square metre (GSM). Papers having a GSM ranging from 100 to 200 are used in the manufacture of decorative laminates depending on the thickness and desired properties. These papers are specially known as absorbent Kraft papers.

The absorbency and penetration properties are tested along with a few more parameters and paper is manufactured to these specifications by the paper mills.

These tests are conducted in the main quality control laboratory, while other data like resin percentage, volatile content and flow property are tested in the process control laboratory near the machines, to make corrections if necessary.

Decorative core is the phenol-formaldehyde resin treated or impregnated natural absorbent Kraft paper, used as the body for producing decorative laminates. This is manufactured from absorbent Kraft papers of varying substance weights starting from about 80 GSM to 200 GSM.

Low resin content core

Low resin Kraft papers have low volatile content and low flow content. An increase or decrease of resin content is done by manipulating the squeezing rolls, either by opening and increasing the gap between the roll or decreasing the gap, whereby excess resin absorbed is squeezed out.

While a certain amount of resin content gives a certain volatile and flow content, it is not a thumb rule as other factors like gel time, cure time, solid content, the water content of resins etc., are important.

Medium resin content decorative core material will have slightly higher resin content, volatile matter and flow content compared to low-resin content core materials, but lesser than that of high-resin content cores.

These grades are generally known as OK core and form the middle portion of the body of the laminated sheet. The resin flows from a higher level to a lower level and enables the laminated sheet to remain flat without creating any dimensional instability or distortion.

High resin content core has higher volatile and flow properties, within specified and standard limits and is also called the base core. This level of high resin, along with high levels of volatile matter and flow contents, compensates for the pull of the laminate towards the surface due to comparatively high shrinkage property of melamine formaldehyde to that of phenol-formaldehyde resins, the lower layers being heavy.

This upward pull forces the laminate to bend upwards leading to front warping of the pressed sheet. The heavy, high-resin content core material takes care of this problem and keeps the laminate flat on its back.

Colour & designs

Solid colours and printed design papers form the decorative part of the laminate and are different from absorbent Kraft papers. While natural absorbent Kraft papers are brown in colour and unbleached, absorbent base papers are made from bleached pulp.

The coloured and pigmentation are as per requirements. These papers vary from 40 GSM to 80 GSM, while plain or solid colours vary from 80 GSM to 200 GSM in substance weights.

The papers are impregnated with melamine formaldehyde resins as they are clear in colour and will not interfere with the plain or printed colours. These designs and plain colours are treated using sky rolls for better resin absorption and penetration into the paper due to the long distance and time available to induce this.

Wetting agents can be added to improve the penetration properties of the paper. Moreover, these papers form the top layer as melamine resins can impart better surface properties like temperature, weather and stain resistance, etc.

These designs are directly pressed over the press moulds or with an overlay tissue paper over the design paper, depending upon the grade of laminate. Never use overlay tissue paper or plain or solid colours as there are no printed design to protect from abrasion.

Plain colours are accordingly impregnated to a higher degree of resin content to incorporate better surface resistance through higher melamine content.

Overlay tissue papers are manufactured from very fine and refined alfa cellulose pulp and used in a substance weight range of 14 GSM to 40 GSM. Thinner the paper and lower the substance weight is a better option in terms of clarity of printed designs and in terms of cost inputs.

Barrier paper is a white base paper that has more or less the same properties, including titanium dioxide loading, but to a lesser level. This layer of paper, impregnated in melamine formaldehyde, melamine urea formaldehyde or urea formaldehyde resin, is used as a substratum below light plain or solid colours to avoid phenol formaldehyde resin seepage from the decorative core below the plain colour.

Barriers under the design papers give a natural look, avoiding any shades of the phenolic Kraft cores. Barriers are sometimes used below certain light wood grains, marbles and flowers etc., to retain their original shades.

(Concluded)

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