WO2008149128A1

WO2008149128A1 - Base paper, coated paper, and method of making a base paper

Info

Publication number: WO2008149128A1
Application number: PCT/GB2008/001987
Authority: WO
Inventors: Christopher Cusick; Nicholas John Kite; Andrew Ward-Askey; Jean-Marie Baumlin
Original assignee: Arjowiggins Licensing
Priority date: 2007-06-08
Filing date: 2008-06-06
Publication date: 2008-12-11
Also published as: GB2449922A; EP2152971A1; GB0711059D0

Abstract

A base paper for use as a substrate in a curtain coated paper comprises a wood-free paper having an even fibre formation as represented by an Ambertec value of four or less. The paper has a smooth surface topography as represented by an Altisurf Sa value of 2.5μm or less, and a high hydrophobicity as represented by a DPM transmission value after 5 seconds of 30% or greater.

Description

Base paper, coated paper, and method of making a base paper

The present invention relates to a base paper for use as a substrate for a coated paper, a method of making a base paper, and a coated paper. In particular, the invention relates to a curtain coated paper and a base paper for use as a substrate for a curtain coated paper.

The invention is concerned particularly, but not exclusively, with coated wood-free papers that are suitable for use in the coated graphic market. Coated wood-free papers are high quality printing papers that are typically used in the coated graphic market for printing company reports and high-end catalogues, magazines and promotional materials. Such papers generally have a paper substrate with a pigment coating that provides a uniform surface with very good printing characteristics. They can be supplied in gloss, satin or matt finishes.

The term "wood-free" is here used in its conventional sense to mean papers derived from wood pulp, in which the pulp is made using a chemical pulping process. Such papers are distinguished from so-called "mechanical" papers, in which the pulp is made using a mechanical pulping process. According to established practice in the paper industry, wood- free papers may comprise up to 20 % of pulp from mechanical pulping process. Such wood-free papers are generally of higher quality than mechanical papers and are used for high-end applications as indicated above, as well as for general reprographic purposes. Both coated and uncoated wood-free papers are available, coated wood-free papers being of the highest quality while uncoated wood-free papers are most commonly used as general office papers (for printing and photocopying). Coated wood-free paper is normally coated using a blade or roll coating technique. The coat applied to the paper substrate usually contains mineral pigments and a binder such as latex, plus various other additives. The coating provides very good printing characteristics and a very smooth top surface that masks any surface roughness (or "topography") in the underlying paper substrate.

One disadvantage of the blade coating process is that because the surface topography of the underlying base paper is never entirely flat, the coating has an uneven thickness. When the paper is printed, the printing ink is absorbed by the coating and, because the coating is not of uniform thickness, this can produce visible variations (mottle) in the print density, particularly in areas of block colour. Ink dry time is another feature that is improved by using curtain coating over that of blade coating. This was interpreted by Dr Tietz as a result of finer pore structure obtained with curtain coating. ["Curtain coating of pigment coats", Dr. Martin Tietz, 22^nd Coating symposium, 20-23 Sept 2005, Baden-Baden.]

Another inherent disadvantage of blade coating is that during coating the contact of the blade with the moving paper substrate can cause the paper to break, halting operation of the coater. This is a particular problem when coating lightweight papers or papers with a high proportion of recycled fibre.

Alternatively, it is possible to coat a paper substrate using a curtain coating process, in which a continuously falling curtain of liquid coating materials is poured onto the surface of the paper as it moves beneath the coater. Such a process is described for example in EP1411168A1. The coating produced by a curtain coating process has a uniform thickness and follows the topography of the underlying paper substrate. It does not therefore experience the problem of mottle found in papers coated by a blade coater. Curtain coated papers are also recognised to provide reduced mottling as compared to blade coated papers ("Curtain Coating of Pigment Coats" by O. Birket et al, Professional Papermaking 2-2006). Also, because curtain coating is a non-contact process, the problem of paper breakage is significantly reduced.

Another advantage of using curtain coating is the capacity to apply many different layers by one coating station, some of these layers being thin with a coat weight as small as lg/m². Thus the coating of product having a thin top layer designed for the specific printing application onto a thick bottom layer containing coarse and cheap pigments is possible with curtain coating. It leads to material savings in comparison with the case of a single layer product and also in comparison with the case of a two layer product where the layers are applied with two coaters.

However, another consequence of the fact that the coating follows the surface topography of the underlying paper substrate is that the surface of the coated paper is not very smooth. This is caused both by the inherent roughness of the paper substrate, and also by the factthat the fibres at the surface of the paper substrate tend to absorb water and swell / roughen upon contact with the coating liquid. The coated paper therefore tends to have a rather poor surface aspect with a certain amount of graininess that is visible by reflected light. This graininess remains even after calendering the paper. Curtain coated papers have not therefore found general acceptance with printers in the coated graphic market, who require a high quality product with very good printability and surface aspect.

Various attempts have been made to overcome the inherent disadvantages of curtain coated papers, for example by modifying the formulation of the coating composition to produce a coating that masks some of the surface roughness of the underlying paper substrate. However, such methods result in an uneven coating thickness and variations in print density similar to those found in blade coated products.

Attempts have also been made to improve the smoothness of the underlying base paper by pre-calendering the paper, prior to applying the coating. However, this provides only a marginal improvement, owing to re-wetting and swelling of surface fibres in the paper substrate upon contact with the coating composition.

Another method involves the use of a blade downstream of the curtain coater, which reduces unevenness in the surface topography of the coat. However, this again results in uneven coating thickness and variations in print density, as with blade coated products.

It is also known to calender curtain coated papers to improve its surface smoothness. However, this only has a marginal affect on the surface aspect of the paper and such an approach is incompatible with matt papers.

One objective of the invention is to provide a curtain coated paper having very good printability, which also has a surface aspect and smoothness that is as good as or better than that of available blade coated papers.

A further object is to provide a base paper for use as a substrate for a curtain coated paper, and a process for manufacturing such a paper.

According to one aspect of the invention there is provided a base paper for use as a substrate in a curtain coated paper, the base paper comprising a wood-free paper having an even fibre formation as represented by an Ambertec value of four or less, a smooth surface topography as represented by an Altisurf Sa value of 2.5μm or less, and a high hydrophobicity as represented by a DPM transmission value after 5 seconds of 30% or greater.

The base paper according to the invention may be coated to provide a curtain coated paper that is as good as a blade coated paper, but which avoids the inherent disadvantages of the blade coating process. The paper may also provide the added benefit of improved print performance over blade coated papers, notably in relation to dry time and mottle. The invention overcomes the base paper topography issue and significantly improves the finished product with respect to aspect, thus overcoming a problem that has not been solved to date with curtain coated papers.

Advantageously, the base paper has hydrophobicity as represented by a DPM transmission value after 5 seconds of 50% or greater.

Advantageously, the base paper has a surface topography as represented by an Altisurf Sa value of 2.1μm or less, preferably 2.0μm or less.

Advantageously, the base paper has a fibre formation as represented by an Ambertec value of three or less.

Advantageously, the base paper is made from cellulose fibres comprising mainly short fibres. Preferably, the base paper is made from cellulosic fibres comprising fibres derived mainly from hardwoods. For example, the cellulosic fibre composition preferably comprises at least 90% eucalyptus fibres, more preferably approximately 100% eucalyptus fibres.

The base paper is preferably a wood-free paper. The wood-free paper may comprise up to 20 % of pulp from a mechanical pulping process. The wood-free paper preferably has a weight in the range 60-200g/m².

The base paper may include a hard internal sizing agent to provide hydrophobic properties and prevent wetting of the fibres by the water-based coating composition. Suitable hydrophobic sizing agents include for example Alkyl Ketene Dimer (AKD), Alkenyl Succinic Anhydride (ASA) and rosin-based chemicals. Additionally, the base paper may include a surface sizing agent, for example starch.

Alternatively or in addition, the base paper may include a hydrophobic surface barrier layer. This may consist for example of a binder such as a latex polymer (e.g. Latexia RTM from Ciba RTM) and one or more pigments, for example clay or a mixture of clay and calcium carbonate. Typically, the surface barrier layer may have a coat weight of about 7.5g/m² per side.

According to another aspect of the invention there is provided a method of making base paper for use as a substrate in a curtain coated paper, the method comprising forming a wet wood-free paper stock and manufacturing a base paper from the stock while controlling the manufacturing processes so that the manufactured base paper has an even fibre formation as represented by an Ambertec value of four or less, a smooth surface topography as represented by an Altisurf Sa value of 2.5μm or less, and a high hydrophobicity as represented by a DPM transmission value after 5 seconds of 30% or greater.

A hydrophobic sizing agent may be added to the wet stock. A hydrophobic sizing agent may be applied to the surface of the manufactured paper base. Alternatively or in addition, a hydrophobic barrier coat may be applied to the surface of the manufactured paper base.

The manufactured coated paper base may be calendered to a surface smoothness represented by a Sa value of 1.5μm or less, preferably 0.75μm or less.

According to another aspect of the invention there is provided a curtain coated paper comprising a substrate and a coating applied by a curtain coating process to one or both sides of the substrate, wherein the substrate comprises a wood-free base paper having an even fibre formation as represented by an Ambertec value of four or less, and wherein the coated paper has a surface smoothness represented by an Altisurf Sa value of 1.5μm or less.

If a gloss finish is required, the coated paper may have a calendered surface with a smoothness represented by an Altisurf Sa value of 0.75μm or less, preferably 0.5μm or less.

The coating preferably comprises a pigment and a binder.

Advantageously, the base paper has a smooth surface topography as represented by an Altisurf Sa value of 2.5μm or less, and a high hydrophobicity as represented by a DPM transmission value after 5 seconds of 30% or greater.

According to one embodiment, this invention relates to the manufacture of a curtain coated paper having equal or better print performance properties to those of blade coated products; the surface topography of the curtain coated product being substantially free from the noticeable topographically derived defects (surface aspect, orange peel, and graininess) normally associated with a contour coating process such as curtain coating, and having a surface roughness Sa value of 1.5μm or less (preferably 0.75μm or less) as measured on the Altisurf, said paper is further categorised as having a fibre formation value of 4 Ambertec and a water resistance of >50% at 5 seconds as measured by DPM using a water test solution.

To achieve the required quality for the end user, this invention identifies three main criteria that need to be specified, namely: the surface smoothness of the base paper used as the substrate for the coated paper, the hydrophobicity of the base paper, and the fibre formation of the base paper. Information on the formation and hydrophobicity test equipment:

The Emco DPM27 (Dynamic Penetration Meter) uses ultrasound transmission to measure the speed of interaction between a planar material and a penetrant. In all of our testing the planar material was paper, and the penetrant, water. The target diameter through which the measurement was averaged was 35mm.

The speed of ultrasound transmission is proportional to the material's rigidity — since water causes paper to lose rigidity, this method is an excellent technique for assessing sizing and water holdout of paper products. It is a widely used method within the coating industry.

Test procedure:

The paper is secured to a plastic backing material by double-sided tape - a proprietary grade made for and supplied by the manufacturer (TYP3). The paper and backing plate was then secured to the apparatus, and plunged into a pool of deionised water at room temperature. The speed with which the water enters the paper via the exposed paper surface, causing the sheet to soften and lose rigidity, is measured by changes in the intensity of ultrasound transmission. These changes are shown graphically on a computer, which gives a continuous reading of change in normalised transmission against time from 34ms to the end point of the test (normally 60s).

Water holdout properties of samples can be compared by visual comparison of the graphs. Alternately, set points along the graph of change in normalised ultrasound transmission against time can taken from the computer and used for comparison. At coating speeds of near 1200m/min DPM a reading at 5 seconds represents approximately the time from the coating impacting the paper surface to the drying section on the coating machine.

The Ambertec Beta Formation Tester (BFT-I) uses transmission of beta particles (electrons) to assess formation of paper. It houses a Pm 147 source of 5mCi activity.

Transmission of these particles is proportional to the area basis weight density of the material being tested. This means that, after calibration with samples of known basis weight, the instrument gives a measurement of basis weight for each small area of a material placed between the source and sensor.

Test procedure:

The default values for the instrument were used in all testing - the aperture was set to 3.5 x 3.5mm, and the area under test was 20 x 20 measurements in both cross and machine directions, giving a total of 400 readings per sample. The value output by the tester is the standard deviation of the 400 individual grammage points.

This test has never been ratified by an international standards body, but is widely used within the industry to give an objective measurement of formation.

Currently two side coated products are mainly manufactured by blade coating base papers in the weight range 60 - 200g/m². Generally speaking, the surface roughness of these base papers is high, with Sa values of about 3.7μm as measured by Altisurf 500.

Surface roughness from the paper surfaces was measured with an Altisurf 500^® (Altimet France), using a non-contact 300 μm white light chromatic probe with 2μm lateral stepping. Waviness was filtered out from the altitude maps thus acquired and spurious points were thresholded out, yielding micro-roughness digital maps of the paper surfaces. A variety of standardised parameters can be computed from digital altitude maps such as 'average roughness' S₃ (in μm). This was done according to the guidelines presented in the EUR 15178 EN report and the future ISO 25178 standard: S_a is to be computed as the arithmetic mean of the deviation in both directions from the mean altitude of the surface.

The smoothness required to give the desired visual appeal and printing performance is developed partly by the blade coating process itself, and partly by additional calendering. Normally Parker Print Surf (PPS) is a common measure of surface roughness in the coated paper market. Altisurf Sa measurements were found in this instance to correlate better to surface aspect than PPS. A general finding in our analysis is that 1 unit measured on the Altisurf is equal to 1.5-2 units on the PPS instrument. As the Altisurf method measures 'true' surface topography, unlike PPS that measures air leakage over the surface to be measured, it is feasible to equate "what the eye sees" as surface topography to that of a light sensor device measuring this parameter. The Sa measurement correlated to an R squared value of 0.762 verses that of 0.627 for PPS when plotted against visual topography assessment. This is shown diagrammatically in Figs. 1 & 2.

Also, the rate of liquid penetration into the base papers currently used for blade coating is high: indeed, they are often almost sponge like, allowing nearly 100% absorption of the coating solution within 5 seconds.

Fibre distribution or sheet formation is another aspect of paper quality not normally considered of prime importance for blade coating. Indeed, base papers for blade coating often have very poor fibre distribution and unevenness when compared to the requirements of this invention. Again, the process of blade coating is itself beneficial to papers not having good formation.

Fibre distribution can be measured directly by instruments such as the Ambertec that measure beta particle transmission, and it has been noted that base papers for blade coating typically show formation values of 6 units and higher on this instrument (Table 1C). After blade coating and calendering the coated product can have an Ambertec value of around 3 units (Table 4). This formation improvement does not occur in curtain coating as no force is applied to the coating to redistribute it onto and indeed into the base sheet. Therefore it has been found that formation values measured on the ingoing base paper remain largely unchanged after curtain coating. (Comparison example 2 is of a paper whose Ambertec value is 6 units before coating and 5.3 units after curtain coating and calendering). This is a disadvantage of the curtain process compared to blade; it is necessary to ensure, by fibre choice and process optimisation, that the base sheet formation of a curtain coated base is of adequate quality to compensate for the inability of the curtain coating process to overcome deficiencies in the base sheet. One aim of the present invention is to show how this can be satisfactorily done.

According to one embodiment, this invention describes the method of manufacturing a base paper (optionally with a barrier coat) with overall properties of surface smoothness, hydrophobicity and fibre formation that make it possible to obtain the process advantages of curtain coating while at the same time maintaining product quality. According to an embodiment of the present invention, a base sheet for curtain coating two side coated papers would have the following properties:

• Sa smoothness ~ 2.5 μm or less, preferably 2μm or less.

• Internal or surface size or both to give a DPM transmission values of 30% or greater (preferably 50% or greater) after 5 seconds immersion in water solution.

• Formation of ~4 units or less as measured on the Ambertec device.

A typical base paper for blade coating is relatively simple in construction (Comparison example 1) in that it comprises a base sheet in weight terms 60-200g/m².

In comparison example 1, the fibre composition was as follows: Comparison example 1

Long fibre 1 : Pine 21%

Short fibre 1 : 20% Bctmp (mechanical pulp) and 59% Mix blend of Birch

/ Eucalyptus

Chalk/Mineral filler Pigment: up to 30 % of dry weight.

Wet strength additive: Yes

Fibre sizing / treatment (i.e. hydrophobic sizing agent):None. Fibre refining: Pulp wetness after refiners, Schopper-Riegler 25°. Surface sizing: Up to ~2g/m² of starch and calcium carbonate or other low cost filler. Application would be by either flooded or metered size press, with the twin HSM or Speed Sizer as examples of the latter.

The recipe for the surface size in Comp Ex 1 was as follows.

The base sheet for blade coating is designed to have high mechanical strength so as to resist the stresses at the point of blade coating; and to achieve rapid absorption of the coating into the base sheet under the pressure of the blade. Fibres such as Pine and Birch are selected for strength and absorbency, and the sheet structure is maintained as open as possible, with low refining and minimal surface treatment. Such a base sheet would normally be calendered in a single nip on line on the paper machine. The calender nip would be likely to be a single nip of either soft (rubber / steel) or hard (steel/steel) rollers.

Using a base as described above for curtain coating results in poor product quality, primarily due to the poor 'visual appeal' caused by the contour coating nature of the falling curtain, which follows the inherent roughness of the base sheet. As discussed above, curtain coating also cannot influence sheet formation in the same way as blade coating. The results for a base designed for blade coating but coated by curtain process (Comparison example 2) are shown in tables 4 and 5.

Additionally, the need for the base paper to show resistance to fibre rewetting, which induces additional roughening in the coating process, is a further important feature of this application. In contrast to curtain coating , high fibre rewetting is advantageous to the blade coating process in that it helps promote coating penetration into the base layer, giving the coating both adhesion and uniformity. Measurement of the speed at which a coating penetrates a paper surface can be determined by taking a dynamic penetration measurement (DPM) using, for example, an ultrasonic transmission instrument. This expresses results in terms of a DPM transmission value (T) that varies with time (t). A conventional base for blade coating shows very high initial penetration speed (as shown in figure 3 by a rapidly falling DPM transmission value T), leading to rapid absorption of the coating solution. Typically, the base has a DPM transmission value of less than 10% after 5 seconds, indicating that over 90% of the applied liquid is absorbed into the base paper support in that time.

Consequently, a base paper designed for curtain coating must not only be much smoother than is required for blade coating, having good fibre formation, but must also be more hydrophobic. The hydrophobicity requirement has been found to be provided by either of two methods. The first uses a high degree of internal sizing onto the base paper fibres and a simple surface size of starch; while the second uses a base support (which may be either highly sized or not) that incorporates a barrier layer or pre-coat. In this invention a barrier layer or pre-coat is a coating layer applied to the base paper to slow down penetration of the aqueous coating into the fibrous network of the base support. Both options of a "highly sized sheet" and that of a "precoated barrier support" of this invention have been found to slow down the coating penetration into the support base paper and as such reduce re- wetting induced surface roughness.

The examples in the present invention show that curtain coated papers whose bases exhibit good surface smoothness, good formation characteristics and a slow rate of liquid penetration can meet the final visual appeal and print quality demands of the market to the same degree as traditional blade-coated papers, while avoiding the problems associated with blade-coated papers as described above.

Various embodiments of the invention will now be described by way of the following illustrative examples.

Patent invention Ex 1

The base paper for Invention Ex 1 was made according to the following specification:

Long fibre 1 : 0 %

Short fibre 1 : 100% Eucalyptus fibre. Chalk/Mineral filler <1 % of dry weight. Wet strength additive: Yes, 0.5% Kymene from Hercules company.

Fibre sizing / treatment

(i.e. hydrophobic sizing agent): Yes, 0.5% Aquapel from Hercules company.

Fibre refining: Schopper-Riegler 45°.

Surface sizing: Starch 100% dry solids applied by size press.

The surface size had a coat weight <1.0g/m² per side, the recipe being as follows:

The wet strength additive (Kymene) is based on a polyamide epichlorohydrin polymer.

Patent invention Ex 2/3/4

The base paper for Invention Examples 2-4 was made according to the following specification:

Long fibre 1 : 13% Pine fibre Short fibre 1: 87% Eucalyptus fibre. Chalk/Mineral filler <20 % of dry weight. Wet strength additive: Yes, 0.5% Kymene from Hercules company.

Fibre sizing / treatment

(i.e. hydrophobic sizing agent): Yes, 0.1% Aquapel from Hercules company.

Fibre refining: Schopper-Riegler 35°.

Surface sizing: None.

In each of Invention Ex 2-4 a barrier coat was applied to the base paper as follows:

Barrier coat 2 Latex / Clay / Coat weight = 7.5g/m² per side Barrier coat 3 Latex / Clay - 40 / Calcium Carbonate - 60 Coat weight = 7.5g/m² per side.

Barrier coat 4 Latex / Clay - 10 / Calcium Carbonate -90 Coat weight = 7.5g/m² per side.

The barrier coatings 2, 3 and 4 were applied by via a twin metered size press in-line on the paper machine. The mix recipes are shown in table IA. Table IB highlights the mix test properties. Table IA

Recipe Barrier layer 2

COMPONENT COMPONENT WET WEIGHT kg SOLIDS %

WATER 15.70 0

PIGMENT CLAY Kl 1020 SLURRY 100.00 66

BINDER LATEX LATEXIA 301 18.48 50

DEFOAMER FOAMASTER B 0.13 100

RHEOLOGY AID RHEOVIS CTA 161 0.40 17

TOTALS 134.71 56.0

Recipe Barrier layer 3

COMPONENT COMPONENT WET WEIGHT kg SOLIDS %

PIGMENT CACO₃ CARBITAL 90 (Imerys) 76.92 78

PIGMENT CLAY Kl 1020 SLURRY 60.61 66

BINDER LATEXIA 301 28.00 50

RHEOLOGY AID VISCOLAM 30 0.52 29

WATER 17.4 0

TOTALS 183.45 62.2

Recipe Barrier layer 4

COMPONENT COMPONENT WET WEIGHT kg SOLIDS %

PIGMENT CACO₃ CARBITAL 90 (Imerys) 115.39 78

PIGMENT CLAY Kl 1020 SLURRY 15.15 66

BINDER LATEXIA 301 28.00 50

RHEOLOGY AID VISCOLAM 30 0.52 29

WATER 24.60 0

TOTALS 183.65 62.2

Table IB

Patent invention Ex 5

The base paper for Invention Ex 5 was made according to the following specification:

Long fibre 1 : 0 %

Short fibre 1 : 100% Eucalyptus fibre.

Chalk/Mineral filler <3 % of dry weight. Wet strength additive: Yes, 0.75% Kymene from Hercules company.

Fibre sizing / treatment

(i.e. hydrophobic sizing agent): Yes, 0.75% Aquapel from Hercules company.

Fibre refining: Schopper-Riegler 40°. Surface sizing: Starch 100% dry solids applied by size press. Coat weight <1.Og/m² per side

The base papers had the following properties, Table 1C: Table 1C

Comparative examples 1 and 2 use the same bases, comparative example 1 being blade coated whilst comparative example 2 is curtain coated as described in the text.

Paper made from Invention Ex 1 was made extremely smooth for a coated paper, this smoothness was designed from the fibre choice, refining as well as other known papermaking techniques. It has high density due the smoothing effect of a multi nip 'breaker stack' applied during the papermaking process. The nip pressures and other on- machine principle parameters for making a smooth base were sufficient to generate the Sa smoothness of 2.09μm (PPS smoothness value of 3.4μm). The paper base for this example was made with little filler pigment and had an ash content of approximately 1%. A high level of refining was applied to the pulp in the wet state prior to running on the paper machine. The paper was also wet pressed to give good surface smoothness along the papermaking process. Final calendering on machine was made with a multi-nip steel calender.

The fibre treatment (internal sizing) in Invention Ex 1 was by addition of a hydrophobic sizing agent in the wet stock stage of the papermaking process. The sizing material (Aquapel) was AKD (Alkyl Ketene Dimer) . Other chemicals that may be used to add hydrophobicity could be selected from ASA (Alkenyl Succinic Anhydride) or rosin based chemicals. The function of the internal size is to allow the fibres to develop hydrophobic properties during the papermaking drying process.

In Invention Ex 5 the base paper was made similar to Invention Ex 1 and was similar in smoothness with a Sa value of 2.04μm. Fibre refining was lower than invention 1 with the fibre refined to a Schopper-Riegler value of 40° wetness. Sizing was made with higher levels of AKD, higher levels of wet strength additive (Aquapel) was also used in comparison to Invention Ex 1. The formation of Invention Ex 5 was 3.99 as measured on the Ambertec formation tester, which was worse than Invention Ex 1.

In each of Inventions Ex 2-4 the base paper was made similar to Invention 1 in regard to its fibre furnish consisting of 100% Eucalyptus fibres. Refining of the fibre furnish was 35° wetness. The base paper of Invention Ex 2-4 was made with lower internal sizing levels than Invention Ex 1 but higher than that made for blade coated papers. Coating was with a HSM metered size press coater. The coating from the HSM coater was final calendered on machine with a single steel / steel nip calender. Three formulations were applied by the HSM coater, barrier coatings 2 (Invention Ex 2), 3 (Invention Ex 3) and 4 (Invention Ex 4).

The base papers made as Invention Examples 2-4 had Sa smoothness values of around 2.0μm before calendering and as such they were similar to that of invention Ex 1. The fibre formation of the barrier coated base paper was near 4 units whereas in invention Ex 1 it was 3.1 Ambertec units.

DPM data charts for each of the invention examples are shown in figures 4-8, multiple lines representing measurements taken from opposite sides or multiple samples of the papers. These charts show that all of the examples slow down the rate of water/coating ingress into the surface of the base sheet. At 5 seconds the ultrasound transmission values on the DPM curves were 85% (Invention 1), 35% (Invention 2), 62% (Invention 3), 46% (Invention 4) and 95% (Invention 5), showing that the test solution has not fully penetrated the paper base. The time of 5 seconds is regarded as being important as this represents a typical or order of magnitude dwell time of the coating in the wet phase between the coating head and the drying tunnel on the coating machine.

Further improvements to reduce the hydrophilic nature of the aqueous coatings could be envisaged by adding hydrophobic ingredients to the barrier layer thus increasing the coating hold out time. This is a further important feature of this invention.

The coating compositions for coated publication grades made by blade coating or curtain coating generally have high solids in the region of 60-70%. Formulations are generally binders - synthetic lattices such as (styrene-butadienes) or (styrene-acrylate) polymers plus ground mineral pigments such as calcium carbonates, clays and talcs. For coated papers these coating recipes would normally be tinted to the desired whiteness/ brightness with optical brightening agents (OBAs) and colorants. Suitable rheology modifiers would also be included in the coating colour plus de-watering components to aid production processing.

The coating mixtures (curtain or blade) were very similar, except that processing aids appropriate to each process were added as shown in table 2. The recipes are designed for glossy coated two side products rather than matt or satin.

Invention and comparative examples were coated on a pilot line at a line speed of 450m/min. Samples were coated using a slide curtain die, where the coating liquid is allowed to flow from a cavity onto a polished surface before forming a free-falling curtain and impinging onto the surface to be coated. The curtain height was typically in the range of 100 to 150 mm above the surface, and a vacuum system was used to remove the air layer which entrained with the moving sheet. Prior to the coating operation the coating mix is degassed in order to remove air bubbles, which are detrimental to the finished product quality. This is done using a commercial degassing apparatus utilising the principle of a thin film formed under a vacuum. After the coating point, the substrate passes into a bank of driers which blow hot air onto the coating. Coating solids were 67.5% for the curtain coating recipe and 70% solids for the blade recipe. Other mix test data is shown in table 3. The coating weight applied by both blade and curtain was 20g/m². For curtain coating the coating colour was run through a slide curtain coating as a single layer. Optionally a multi-layer curtain could be formed with a lower cost recipe as a sub layer.

Table 2

Table 3

Conventionally papers for glossy C2S (coated two sides) grades are highly calendered to give a gloss value in the mid 70s as measured at an angle of 75 degrees with a gloss meter. All of the examples were calendered on a pilot calender to achieve a gloss value of at least 70 degrees at an angle of 75 degrees and assessed for visual appearance and measured roughness values. The results are tabulated in table 5.

The pilot line calendering conditions used to generate the gloss target were:

Nip Pressure: 1200ρsi

Speed: 50m/min

Temperature: Top = 80⁰C

Bottom = 70⁰C

Top Roll: Steel

Middle Roll: Black Cotton

Lower roll: Chrome

Nips: 2

Surprisingly, the measured Sa surface smoothness values of all the samples after pilot plant calendering are similar but there is a huge difference visually between the curtain coated sample using a base made for blade coating (Comp Ex 2) and a base according to the invention. The invention examples are considered visually equal to or better than the blade coated product for surface aspect (appearance). This surface aspect can be described as micro roughness or imperfections seen by diffusion or shallow angle reflection. The scale of this roughness/imperfection is less than lmm and mainly in the range of 100-500μm. With curtain coating these micro roughnesses / imperfections are of such fine scale that final heavy calendering can reduce but not totally alleviate them hence the need to improve the base or substrate quality prior to coating.

Using multivariate software analysis tools to evaluate data inputs on surface aspect and the important levers to give a base paper support satisfying the market's needs regarding acceptable surface appeal /quality we have found that surface smoothness, fibre formation and water resistance are all equally important and perhaps inter-related and interdependent on each other. A highly water resistant base showing positive influences on surface aspect and low fibre formation / surface smoothness results in an improved surface aspect. According to an embodiment of the invention, the base or support (base plus a barrier layer) of a curtain coated product that matches the visual appeal of a blade coated product has the following characteristics:

1. Has a surface smoothness as measured by Sa of < 2.5μm. Upon coating via curtain coating the base paper has a smoothness of 1.5μm or less. After calendering the resultant coated and finished product has a Sa value of 0.75μm or less, preferably 0.5μm or less.

2. The DPM transmission value of the base paper or support at 5 seconds is equal to or greater than 30% (preferably equal to or greater than 50%) using a water penetration test.

3. The fibre formation of the sheet or coated sheet is controlled by fibre choice / refining and papermaking processes to give an Ambertec formation value of equal to or less than 4 units, and preferably equal to or less than 3 units.

Table 4

Print properties were derived from test data (table 5) following a four colour print on each sample using a Roland press. The data shows that curtain coated paper according to the invention is as good as that from a blade coater with the added benefit of improved print performance, notably in relation to dry time and mottle. This invention also overcomes the base paper topography issue and significantly improves the finished product with respect to aspect, thus overcoming a problem that has not been solved to date with curtain coating.

Grammage: Measured

Humidity: Measured

Temperature: Measured

Trapping: Densitometer data that reflects the ability of the Red to repel on the blue:

100 excellent 60 not very good.

Ink Density: Density of the black: the higher the best

Image Contrast: The higher the better, relative difference between 100% black and 75 %.

Dry time: Ink dry time of a 4 colours full ink loading.

Mottling: Data generated from Keops tool. Usually 11 (good) to 18 (bad).

Dusting: This is related to white dust that appears on the blanket. It comes from a poor layer adhesion.

Gloss: Measure at angle of 75° gloss

Ink off set: From a rating of the dry time 0: good, 5 bad. Printed sheet pressed against backside to assess ink offset/drytime

Claims

1. A base paper for use as a substrate in a curtain coated paper, the base paper comprising a wood-free paper having an even fibre formation as represented by an Ambertec value of four or less, a smooth surface topography as represented by an Altisurf Sa value of 2.5μm or less, and a high hydrophobicity as represented by a DPM transmission value after 5 seconds of 30% or greater.

2. A base paper according to claim 1 , wherein the base paper has a hydrophobicity as represented by a DPM transmission value after 5 seconds of 50% or greater.

3. A base paper according to claim 1 or claim 2, wherein the base paper has a surface topography as represented by an Altisurf Sa value of 2.1 μm or less.

4. A base paper according to claim 1 or claim 2, wherein the base paper has a surface topography as represented by an Altisurf Sa value of 2.0μm or less.

5. A base paper according to any one of the preceding claims, wherein the base paper has a fibre formation as represented by an Ambertec value of three or less.

6. A base paper according to any one of the preceding claims, wherein the base paper is made from a furnish comprising mainly short fibres.

7. A base paper according to any one of the preceding claims, wherein the base paper is a wood-free paper.

8. A base paper according to any one of the preceding claims, wherein the base paper includes a hard internal sizing agent.

9. A base paper according to any one of the preceding claims, wherein the base paper includes a surface sizing agent.

10. A base paper according to any one of the preceding claims, wherein the base paper includes a hydrophobic surface barrier layer.

11. A method of making base paper for use as a substrate in a curtain coated paper, the method comprising forming a wet wood-free paper stock and manufacturing a base paper from the stock while controlling the manufacturing processes so that the manufactured base paper has an even fibre formation as represented by an Ambertec value of four or less, a smooth surface topography as represented by an Altisurf Sa value of 2.5μm or less, and a high hydrophobicity as represented by a DPM transmission value after 5 seconds of 30% or greater.

12. A method according to claim 11 , wherein a hydrophobic sizing agent is added to the wet stock.

13. A method according to claim 11 or claim 12, wherein a hydrophobic sizing agent is applied to the surface of the manufactured paper base.

14. A method according to any one of claims 11 to 13, wherein a hydrophobic barrier coat is applied to the surface of the manufactured paper base.

15. A method according to any one of claims 11 to 14, wherein the manufactured paper base is calendered to a surface smoothness represented by an Sa value of 1.5μm or less.

16. A curtain coated paper comprising a substrate and a coating applied by a curtain coating process to one or both sides of the substrate, wherein the substrate comprises a wood-free base paper having an even fibre formation as represented by an Ambertec value of four or less, and wherein the coated paper has a surface smoothness represented by an Altisurf Sa value of 1.5μm or less.

17. A curtain coated paper according to claim 16, wherein the coated paper has a calendered surface with a smoothness represented by an Altisurf Sa value of 0.75μm or less.

18. A curtain coated paper according to claim 16 or claim 17, wherein the coating comprises a pigment and a binder.

19. A curtain coated paper according to claim 16 or claim 17, wherein the coating includes an upper layer and a lower layer.

20. A curtain coated paper according to claim 19, wherein the upper layer has a coat weight lower than the half the coat weight of the lower layer.

21. A curtain coated paper according to any of claims 16 to 20, wherein the base paper has a smooth surface topography as represented by an Altisurf Sa value of 2.5μm or less, and a high hydrophobicity as represented by a DPM transmission value after 5 seconds of 30% or greater.

22. A curtain coated paper according to any of claims 16 to 21, wherein the paper is coated on both sides.

23. A curtain coated paper according to any of claims 16 to 22, wherein the paper is a coated graphic arts paper.