Manufacture of an Impregnated Decor Paper or Non-woven

Field of the Invention

The invention relates to the manufacture of impregnated decor papers and non- wovens, and is especially applicable to the formation of impregnated decor papers for decorative laminates. The invention also extends to decorative laminate products manufactured using the impregnated decor paper according to the invention, wherein final pigmentation and the desired opacity are achieved during the impregnation process. In the specification the term "paper" is understood to encompass "non-woven".

Background of the Invention

A disadvantage inherent in the current method of manufacturing unicolour decor base papers is the high basis weight (grams per square meter = gsm) required to obtain sufficient opacity with good retention of the pigments. In order to produce a decor base paper an aqueous pulp mass of approximately 98% water and a 2% mixture of cellulose, pigments and other modifying agents is delivered to the wire section at the front of the paper machine, also called the wet-end. After formation on the wire the paper is then dewatered, pressed and dried to form the ready to use unicolour decor paper or unicolour decor print base paper. The desired colour and opacity are achieved by use of various pigments, such as Tiθ ₂ and Fe2O ₃ , in quantities up to or occasionally in excess of 40% measured as a % of the final paper weight (in gsm). To achieve the desired uniform colour and opacity the pigments have to be uniformly distributed and held within the cellulosic fibre matrix.

The need to avoid loss of expensive raw materials for economic as well as environmental reasons limits the papermaker in the choice of technical parameters of the process and the raw materials. One important parameter is the particle size of TiO ₂ which, due to its high specific gravity of about 4, needs to be kept within a size range of 0.2 - 0.3 μm to promote efficient retention and avoid losing pigment particles into the waste water stream. The particle size of 0.2 μm is also known to be the optimum size for light scattering and refraction. A second important parameter is the wet strength of

the decor paper which decreases as the ash content of the paper increases. This conundrum of pigment retention and opacity has also been observed in international patent publication WO 2005/028750 A1 , however the inventors in that instance have tried to overcome the problem on the paper machine in a manner that causes resin impregnation in a later process to be increasingly difficult.

The fact that both the cellulose and the thermosetting resin used to impregnate the decor paper, whether melamine formaldehyde (MF) resin or urea formaldehyde (UF) resin or a combination of these, are clear when pressed under heat and pressure is a factor in calculating the amount of pigmentation to obtain the desired opacity. Therefore a laminate manufacturing method which could be substantially carried out using a lightweight lightly filled or unfilled paper as the carrier material for the resin and pigments would result in a significant economic advantage.

It is known that to improve the opacity of a white decor paper additional Tiθ ₂ is added in small amounts to the coating resin, applied by gravure rolls or reverse rolls after the initial saturation and drying step. The amount of TiO ₂ is usually in the range 2 - 10 gsm contained in 30 - 50 gsm resin, usually MF resin. Others have also pigmented the first saturating resin bath. Both of these methods for applying incremental amounts of pigmentation could be used in addition to the method of the invention, but only in the case of unicoloured, and not printed, decors.

Any reference herein to a known or existing process or to a specific publication is not to be read as an admission that the process or publication or any part thereof is common general knowledge.

An object of the invention is to provide an improved method for forming an impregnated and pigmented decor paper that at least alleviates one or more of the aforementioned disadvantages or constraints in conventional processes.

Summary of the Invention

In accordance with the invention, it has been realised that the above object can be met by arranging for the primary application of pigment to take place during the

impregnation process, thus avoiding use of highly loaded pigmented pulp mass in the papermaking process. It is found that this approach allows the desired colour and opacity to be obtained with a reduction of total raw materials, and with a higher pigment to fibres/resin ratio.

The invention provides a method for forming an impregnated and pigmented decor paper or non-woven, including:-

applying a liquid to a paper or non-woven web suitable for forming decor paper to obtain a first impregnated paper or non-woven,

and applying to the first impregnated paper or non-woven while it is still wet a coating containing a selected pigment, whereby said first impregnated paper is converted to an impregnated and pigmented decor paper or non-woven.

The invention also provides an impregnated decor paper or non-woven formed by a method according to the invention, and a decorative laminate product containing an impregnated and pigmented paper or non-woven according to the invention.

Embodiments of the Invention

Preferably, said coating contains the pigment as a dispersed particulate material.

In a particularly useful application of the invention, the coating containing the selected pigment is a thermosetting resin containing the pigment as a dispersed particulate material.

The liquid applied to the paper web to obtain the first impregnated paper or non- woven is then also preferably a thermosetting resin compatible with the resin of the coating. Both may be MF, or the resin applied to the web may be UF and the resin of the coating MF.

The coating containing the selected pigment is preferably applied under pressure, for example via a slotted die across which the first impregnated paper or non-

woven is relatively drawn. The coating containing the selected pigment may be continuously agitated and/or circulated in order to maintain the pigment dispersed in the coating. A suitable mode of applying the coating is found to be an ARP (Abrasion Resistant Process) applicator conventionally employed to apply alumina or other abrasion resistant particles to an impregnated paper web.

In a particular application of the invention, after initial saturation of the paper or non-woven in a first resin bath, and usually before any drying step, a highly pigmented coating is delivered under pressure by being forced through a slotted die, or similar pressure application unit, and contacts the paper that has already been through the first resin bath.

Preferably, the coating is a liquid and so application of the coating to the first impregnated paper and non-woven is a wet-on-wet contact. This ensures that sufficient pigment is captured on the paper or non-woven to achieve the desired colour and opacity.

It will be apparent to those skilled in the art that a reduction in resin solids in the first bath to reduce the amount of solid resin pick-up may result in a high water content and that a minor adjustment to the water content by a short intermediate drying step may be required while still permitting a wet-on-wet application of the pigmented coating.

It will also be understood by those skilled in the art that in the case of an already printed paper the pigmented coating is typically applied to the non-printed side of a lightly filled decor paper, or to either side of a clear overlay paper.

The paper or non-woven with resin from the first resin bath and the pigment coating is then preferably partially dried to a volatiles level in a predetermined range. If MF resin is used in both the first bath and the coating resin then no further coating is necessary other than if a higher resin loading is desired, and as such the impregnated and coated paper is dried to a b-stage, usually 4 - 8% volatiles. However, if it is economically advantageous to use UF resins in the first resin bath and possibly as a component of the pigment coating, then a coating of MF resin must be applied to

provide the final MF surface qualities as well as to close the b-stage film on both sides to avoid sticking of the b-stage pre-pregs due to the hygroscopic nature of UF resin. When necessary the final MF resin coating is performed after partial drying of the first stage impregnated and coated paper to an optimal volatile level of approximately 10 - 15%, then passing the web between gravure rollers or reverse coating rollers where a desired amount of MF resin is applied to one or both sides of the web. It is obvious to those skilled in the art that if the first resin bath is UF resin and the pigment coating uses MF resin it is then practical to use the gravure rollers to only coat the side of the web with the exposed UF resin from the first bath. The paper is then further dried to the desired final volatile level of around 4 - 8% and is ready for use in the manufacture of a decorative laminate.

The paper or non-woven web to which the liquid is applied may be a clear overlay paper whereby said first impregnated paper is substantially free of the predetermined pigment. Such a clear overlay paper is preferably of a weight not exceeding 50gsm, preferably in the range 18-30gsm. Alternatively, the paper or non- woven web to which the liquid is applied may be a low ash content, or lightly filled or pigmented paper. Such a low ash content or lightly filled paper is preferably of a weight not exceeding 60gsm, preferably in the range 30-45gsm. By "lightly filled" or "lightly pigmented" is meant that the pigment content is substantially less than half that of the impregnated and pigmented decor paper formed by the method of the invention.

Where the first impregnated paper is a printed paper, the coating containing the pigment is usually applied to the non-printed side of the paper.

The invention as such allows for clear overlay papers or lightly filled decor papers to be used as the medium for the resin or resins, and pigment. Overlay papers are suitably in a range from 10 - 50gsm, more suitably in a range from 12 - 30gsm, or most preferably in a range from 18 - 30gsm. Lightly filled decor papers are suitable in a range from 20 - θOgsm, more suitably in a range from 25 - δOgsm, or most preferably in a range from 30 - 45gsm. Notwithstanding the economic objectives in using low grammage papers, the method according to the invention would also be suitable to replace standard high grammage highly filled papers for intensely coloured high opacity

decors, generally above 100gsm, with papers of a significantly lower grammage and filler content being approximately 70gsm.

The pigment may be ^" TϊO2, preferably in rutile form, but may alternatively or additionally be Fe ₂ O ₃ , Al (OH) ₃ , BaCo ₃ , BaSO ₄ , CaSO ₄ , or any mixture of one or more of these.

The invention as such also allows for a reduction in the quantity of pigmentation needed to achieve the desired opacity. The current state of the art shows that a 80gsm white decor paper will contain approximately 35% rutile type TiO ₂ , rutile having a higher covering power than anatase T1O ₂ , to achieve the desired depth of colour and opacity, equating to approximately 28gsm of TiO ₂ . The invention shows that by reducing the amount of cellulosic fibres and resin, the pigmentation can also be reduced while achieving the desired level of opacity.

A further economic advantage is that the costs associated with transport of the base paper are significantly reduced when the base paper is nearer 20gsm as when it is 80gsm. Even a 45gsm base paper reduces the transportation costs per square meter by approximately 50%.

Still another economic advantage is that the costs of pigmentation can be significantly reduced by substituting expensive TiO ₂ with alternatives generally not used in traditional decor paper making such as PCC (precipitated calcium carbonate) or kaolin, or other low cost alternatives well known in the art, such as but not limited to

AI(OH) ₃ , BaCO ₃ , BaSO ₄ , CaSO ₄ , or any mixture of these.

Example 1

A 25gsm clear overlay paper web was saturated in a first resin bath with MF resin and the resin pick-up controlled by passing the web through a pair of metering rollers. Then in a second step before any drying step the impregnated web substantially free of TiO ₂ pigment was coated wet-on-wet with a combined pigment and resin in which the ratio of TiO ₂ pigment to solid resin was approximately 1 :1. The pigmented resin was forced through a slotted die by pumping the resin, and the overflow was captured and

recycled within the process. The resulting impregnated and pigmented decor paper was dried to the desired volatiles content of approximately 6%. The final b-stage pre-preg weighed approximately 95gsm made up of 25gsm paper, 20gsm TiO ₂ , 45gsm MF resin and 5gsm (6%) remaining volatiles.

Example 2

A 45gsm lightly filled (approx 8gsm TiO ₂ ) white decor paper web was saturated in a first resin bath with MF resin and the resin pick-up controlled by passing the web through a pair of metering rollers. Then in a second step before any drying step the web was coated wet-on-wet with a combined pigment and resin in which the ratio of TiO ₂ pigment to solid resin was approximately 1 :1. The pigmented resin was forced through a slotted die by pumping the resin, and the overflow was captured and recycled within the process. The resulting impregnated and pigmented decor paper was dried to the desired volatiles content of approximately 6%. The final b-stage pre-preg weighed approximately 125gsm made up of 35gsm fibrous paper, 23gsm TiO ₂ , 60gsm MF resin and 7gsm (6%) remaining volatiles.

Comparative Example 3 [prior art]

A standard white decor paper of 80gsm containing 35% (28gsm) TiO ₂ was first saturated in UF resin, passed through a pair of metering rolls, dried to the desired interim volatile content (approximately 13%), then further coated with MF resin on both sides by means of a pair of gravure rollers before further drying to the desired final volatiles level of 6%. The final b-stage pre-preg weighed 180gsm made up of 52gsm paper, 28gsm TiO ₂ , 45gsm UF resin, 45gsm MF resin and 10gsm (6%) remaining volatiles.

Economic Comparison (weight/weight)

Example 1 Exam Die 2 Example 3

(Prior Art)

Cellulose 25gsm 35gsm 52gsm

TiO ₂ 20gsm 23gsm 28gsm

UF resin 45gsm

MF resin 45gsm 60gsm 45gsm volatiles 5gsm 7gsm 10gsm

95gsm 125gsm 180gsm

Pigment percentage of pre-preg excluding volatile content is set out as follows:

Example 1 ; pigment = 20/90 = 22.2% Example 2; pigment = 23/118 = 19.5% Example 3; pigment = 28/170 = 16.5%

These weight/weight comparisons shows that a clear economic advantage is gained when producing an impregnated decor paper according to the invention.