EP0773472A1

EP0773472A1 - Method for increasing the coating speed

Info

Publication number: EP0773472A1
Application number: EP96203091A
Authority: EP
Inventors: Andrew Michael Howe; William James Harrison; Terence Desmond Blake
Original assignee: Kodak Ltd; Eastman Kodak Co
Current assignee: Kodak Ltd; Eastman Kodak Co
Priority date: 1995-11-11
Filing date: 1996-11-06
Publication date: 1997-05-14
Also published as: GB9523138D0; JPH09173975A

Abstract

Described herein is an improved method of coating a liquid onto a moving support, the liquid comprising at least one layer. The layer lying adjacent the moving support when coated comprises a dispersion of solid colloidal particles in a hydrophilic colloid, such as aqueous gelatin, which enlarges the coating window obtainable.

Description

Field of the Invention

The present invention relates to improvements in or relating to coating processes, and is more particularly concerned with the use of colloidal particles to optimise the rheology of solutions used in such coating processes.

Background of the Invention

Coating processes, such as, extrusion, bead and curtain coating, are well-known and widely used for the application of one or more liquid layers on to the surface of a moving support. In particular, such coating processes may be used for manufacturing photographic products.
In US-A-3 632 374, a support is coated by forming a freely-falling vertical curtain of liquid so that it impinges on to the support. The curtain is stable and has a uniform flow rate across its width. A controlled relationship is maintained between the flow rate of the liquid and the speed at which the support is moved so that a thin layer, of specified thickness, of the coating liquid is formed on the support. Apparatus for forming the curtain include a hopper having a downwardly inclined slide surface over which the coating liquid flows by gravity until it reaches a lip. The lip is spaced vertically above the moving support and the coating liquid flows downwards in a freely-falling curtain from the lip.
US-A-3 867 901 describes a method in which single layers are coated on to a support. US-A-3 508 947 discloses a method for coating multiple layers on to a support.
As preferred manufacturing speeds increase, the speeds achievable in curtain coating, as described in US-A-3 867 901 and US-A-3 508 947, may become limiting. The major limitation is the entrainment of air between the coating and support, which occurs when support speed is sufficiently increased. This practical limitation is also common to other coating processes where at least one liquid layer is applied to a moving support.
In coating processes, a uniform layer or layers are only obtained if the operational variables are held within fairly precise limits. These limits define the so-called "coating window". It is to be noted that the "coating window" obtained depends on the nature of the liquid material which is to be coated onto the support. It is convenient to define the "coating window" in terms of the variables of coating speed and flow rate per unit width. One of the boundaries of the coating window is formed due to the occurrence of air-entrainment.
Moreover, it is known in curtain coating processes that air-entrainment may exhibit a "hysteresis" effect. As coating speed is increased at fixed layer flow rates, or at fixed layer wet thicknesses, air-entrainment eventually begins. If coating speed is then decreased, it is found that the speed at which the air-entrainment ceases can be substantially below that at which it starts. A speed difference of 200cms^-1 or more between the onset of air-entrainment and when it ceases, is not unusual. Thus, in a curtain coating process, there may be states where, depending upon the history of the process, air-entrainment may or may not occur. These states define a metastable region where it is not possible to predict whether there will be air-entrainment or not. In this metastable region, the passage of a splice can provide a sufficient disturbance to precipitate air-entrainment when none had previously existed. Imperfections in the support can similarly precipitate air-entrainment, as can transient disturbances encountered at the start of a coating. Good practice dictates that curtain coating within the metastable region is to be avoided. Thus coating speed may be undesirably limited.
US-A-4 569 863 discloses the use of a thin, low-viscosity bottom layer to increase speeds in curtain coating. A bottom layer with a viscosity ranging from 1 to 20mPas, and a wet thickness of 2 to 30mm, is disclosed. There are several possible disadvantages to this method. Such a thin layer would not necessarily be a functional layer in a product, and so a separate delivery system, together with a hopper with an additional slot would usually be necessary.
Disadvantageously, a low-viscosity bottom layer also promotes "puddling" at the point where the freely-falling liquid curtain impinges on the moving support. A "heel" appears at the foot of the curtain. If the heel is sufficiently large, it may contain an eddy in which air bubbles and debris may become trapped, thereby generating a line or streak in the coating. A large heel can also oscillate, producing non-uniformities in the coating along and across the direction of support motion. To prevent puddling, the low-viscosity bottom layer may have to be kept thin, even though a functional bottom layer may not be thin, and the curtain height low, though this adversely affects curtain stability and restricts the room beneath the hopper for other equipment, such as a start pan.
There are still other problems which can be encountered with a low-viscosity bottom layer in liquid coating processes. Such a layer promotes flow after the coating point, due to support which may not be perfectly uniform in thickness, or due to air flows impinging on the coating before it dries. Low-viscosity liquids are also more difficult to deliver in that they are generally poor for purging lines and auxiliary equipment such as mixers, pumps and de-bubbling devices of previously resident liquid (such as cleaning solution), air bubbles, and other slugs and debris as are encountered in practice. Such poor purging has been associated with an increased probability of coating non-uniformities, most notably lines and streaks.
EP-A-0 563 086 describes an improved curtain coating process in which the coating window is enlarged. This is achieved by inserting a coating material adjacent the support onto which the liquid material is to be coated which readily shear-thins. At least the layer of liquid material adjacent the support is a pseudoplastic liquid having a viscosity greater than 20mPas at shear rates less than 500s^-1, and a viscosity of less than 10mPas at shear rates greater than 10⁶s^-1. The viscosity of the pseudoplastic liquid approaches a substantially constant value at a shear rate which lies in a range between 10⁴ and 10⁸s^-1.
The use of pseudo-plastic (shear-thinning) fluids to improve bead coating performance has been described in US-A-4 113 903. In this case, bead stability is improved by increasing coating solution viscosity at low rates of shear whilst high coating speeds are maintained, via low viscosities at high rates of shear. This behaviour is obtained by the use of a "shear-thinning thickening agent" in the bottom layer of a bead coating pack. The thickening agent is generally a polymeric material which is soluble in the chosen solvent and imparts a strongly shear-thinning property to the solution. Viscosities between 20 and 200mPas are disclosed at a shear rate of 100s^-1 and below 10mPas, preferably less than 5mPas, at a shear rate of 10⁵s^-1 are disclosed.
US-A-3 767 410 and US-A-3 811 897 describe the use of water-soluble polymers as thickening agents for photographic coating applications.
DD-A-0 286 436 describes the use of a combination of two polymer dispersions to increase the viscosity of gelatin-containing coating solutions for the preparation of photographic materials. The dispersions are formulated such that one is alkali-soluble at a pH greater than 7, the other insoluble under the same pH conditions.
It is well known to use silicon dioxide (silica) as a matting agent for bead and curtain coating processes, as described for example, in US-A-4 572 849, US-A-4 569 863, US-A-4 384 015 and US-A-4 308 344. Colloidal silicon dioxide (silica) is also known for use in a subbing layer as described in US-A-4 048 357.
US-A-3 776 726 describes the use of colloidal silica to increase the viscosity of a film-forming polymer in diffusion transfer photographic development processes.
It is also well known to use colloidal silica in current photographic products to improve layer toughness and to change matt-gloss characteristics, but not to enhance coatability.
US-A-3 359 108 discloses silver halide photographic emulsions in which colloidal silica is used in a synthetic resin latex hydrophobic binder to provide improved permability of the emulsion to processing solutions. Facilitation of the coating of photographic emulsions dispersed in said hydrophobic binder is also disclosed wherein materials are added as thickeners.

Problem to be solved by the Invention

There is still a need to maximise the coating windows obtained during coating, and in particular, to increase coating speeds without encountering the disadvantages associated with air-entrainment.
Moreover, it is desirable to eliminate or substantially reduce the effects of hysteresis in curtain coating.
Although the above mentioned problems can be reduced using suitable polymeric materials as discussed above, in some coating formulations, the use of such polymers may not be appropriate.

Summary of the Invention

It is therefore an object of the present invention to provide a method of coating which overcomes the problems mentioned above.
In accordance with one aspect of the present invention, there is provided a method of increasing the maximum coating speed of a coating process and substantially eliminating wetting failure hysteresis, wherein a material is coated onto a substrate, the material comprising at least a first layer which lies adjacent the support after coating, characterized in that the first layer includes a dispersion of colloidal particles in a hydrophilic colloid.
Preferably, the hydrophilic colloid comprises aqueous gelatin.
Advantageously, the particles comprise colloidal silica. It is preferred that the particles are negatively charged, and have a size in the range of 1nm to 10mm, preferably, in the range of 5 to 50nm.
In accordance with a second aspect of the present invention, there is provided a coating formulation which is applied directly to a moving support, characterized in that the formulation comprises a dispersion of colloidal particles in a hydrophilic colloid.

Advantageous Effect of the Invention

Advantageously, an aqueous dispersion of small, solid, negatively charged colloidal silica particles can be used as a thickener and rheology modifier for aqueous gelatin solutions for coating applications. The colloidal silica particles have sizes in a range of 5 to 50nm.
The solid particle dispersion can be added directly to the bottom layer of an existing product or, alternatively, can be coated in aqueous gelatin as a bottom layer of a multilayer pack.
The colloidal particles have been found to synergistically boost the viscosity of an alkali-processed gelatin at low rates of shear due to the strong interactions between the polymer and the solid particle. High viscosities at low shear rates improve widthwise uniformity on the hopper slide and coated material or web.
Furthermore, the shear-thinning nature of these gelatin-based dispersions is much more pronounced and commences at a lower, more desirable rate of shear than observed in the absence of such colloidal particles.
In general, the viscosity of dispersions in accordance with the present invention at shear rates of the order of ∼10⁵s^-1 is substantially lower than that for pure gelatin solutions possessing the same low-shear viscosity. Low viscosities at high shear rates favour high coating speeds without air-entrainment.
The overall rheological profiles (viscosity as a function of shear rate) of the gelatin-based colloidal silica dispersions also appear to be superior to pseudoplastic systems based on water-soluble polymeric additives (for example, in EP-A-0 563 086 discussed above and incorporated herein by reference). Specifically, the flow characteristics of the colloidal dispersion in accordance with the present invention more closely resemble the preferred rheological profiles described therein.
When the composition and rheology of the coating liquid are optimised in accordance with the present invention, the following advantages are obtained:-

a) improved longitudinal and widthwise uniformity of the coated layer;
b) increased coating speeds without air-entrainment and consequent wetting failure;
c) enlarged and more robust coating windows (that is, a substantial increase in the range of conditions under which stable, uniform coating can be maintained without the risk of catastrophic wetting failure);
d) reduced dryer load;
e) reduced dry layer thickness; and
f) reduction or elimination of wetting failure hysteresis during curtain coating.

Brief Description of the Drawings

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which:-

Figure 1 is a graph illustrating the rheological profile (viscosity-shear rate curve) for 3% w/w aqueous decalcified gelatin + 6% w/w colloidal silica (Ludox SM) in accordance with the present invention;
Figure 2 is a graph illustrating the rheological profile (viscosity-shear rate curve) for 3% w/w aqueous regular gelatin + 6% w/w colloidal silica (Ludox AM) in accordance with the present invention;
Figure 3 illustrates various possible solution rheological profiles attainable during coating processes;
Figure 4 illustrates a curtain coating map for 15% w/w aqueous gelatin on a gelatin-subbed Estar support (Estar is a trade mark of Eastman Kodak Company and comprises polyethylene terephthalate) at a curtain height of 10.2cm and an application angle of 0°;
Figure 5 is a graph illustrating the rheological profile (viscosity-shear rate curve) for 15% w/w aqueous gelatin;
Figure 6 illustrates a curtain coating map for 3% w/w aqueous regular gelatin + 6% w/w colloidal silica (Ludox AM) on a gelatin-subbed Estar support at a curtain height of 10.2cm and an application angle of 0°;
Figure 7 illustrates a curtain coating map for 15% w/w aqueous regular gelatin on a gelatin-subbed Estar support at a curtain height of 3cm and an application angle of 0°; and
Figure 8 illustrates a curtain coating map for 3% w/w aqueous regular gelatin + 6% w/w colloidal silica (Ludox AM) on a gelatin-subbed Estar support at a curtain height of 3cm and an application angle of 0°.

Detailed Description of the Invention

It has been found that the use of small colloidal particles in combination with aqueous gelatin allows a desired rheological profile to be attained for improved coating performance. Colloidal particles are defined as particles having sizes in the range of 1nm to 10mm, and in particular, for small particles in the range of 5 to 50nm.
The present invention relates specifically to the use of colloidal particles which interact attractively with an aqueous hydrophilic colloid comprising the continuous phase of a coating formulation, such as gelatin, to give the desired rheological properties for coating, the colloidal particles being ones to which the hydrophilic colloid, e.g. gelatin, adsorbs, and in particular, silica particles which have negatively charged surfaces. Although the present invention describes colloidal particles with negatively charged surfaces, it will be readily understood that it is possible to use other colloidal particles having other charges.
Rheological studies of solid colloidal particle based systems have been carried out with silica particle-gelatin dispersions. The viscosity-shear rate data shown in Figure 1 were obtained for a 3% w/w aqueous decalcified gelatin + 6% w/w colloidal silica dispersion. The colloidal silica dispersion used was Ludox SM supplied by DuPont containing spherical particles of colloidal silica having an average diameter of 7nm.
Other silicas are available with different sizes and surface properties which show rheology-modifying properties, for example, Ludox AM also supplied by DuPont, a silica-aluminate of 12nm average diameter. The rheological profile for a 3% w/w aqueous regular gelatin, 6% w/w Ludox AM colloidal silica dispersion is shown in Figure 2. The specific rheological profiles of these Ludox-containing aqueous gelatin melts may be manipulated simply by changing the concentrations and ratios of the solution components.
In Figure 3, a preferred rheological profile for coating is shown by the solid line. This profile is consistent with that defined as the optimum rheological profile in EP-A-0 563 086. The dashed lines illustrate non-optimal profiles given by simple gelatin melts of different concentrations and hence varying low shear viscosities and rheological profiles. The solid line illustrates a preferred profile achieved by adding an appropriate thickening agent to a dilute gelatin. The stippled area on the graph indicates the range of shear rates expected at the wetting line in coating processes. Low values of viscosities in this region promote high coating speeds as will be described in detail below.
The rheological behaviour depicted in Figures 1 and 2 is in good agreement with that previously defined in EP-A-0 563 086 for optimum curtain coating performance and depicted in Figure 3. It also appears more favourable than that obtained previously using water-soluble polymer additives such as PVP (polyvinylpyrrolidone), AWna (a copolymer of acrylamide and sodium 2-acrylamide-2-methylpropane sulphonate) or Kelzan-D (a xanthan gum (polysaccharide) produced by Kelco).
In curtain coating, the operational coating window is conveniently represented by plotting a map of coating speed, S (cms^-1), versus flow rate per unit width of the coating hopper, Q (cm²s^-1). A line drawn through the origin then connects all points having a constant wet thickness or laydown, Q/S (cm).
An example of a curtain coating map for a moderately shear-thinning aqueous gelatin solution (15% w/w aqueous gelatin, 67mPas) coated at a curtain height of 10.2cm, at an application angle of 0° (top dead centre on the coating roller), is shown in Figure 4. The corresponding rheological profile is shown in Figure 5.
In Figure 4, the curve ABCD defines the overall wetting failure boundary beyond which gross air-entrainment occurs and coating uniformity is destroyed. In general, the coating speed at the onset of wetting failure is inversely dependent on the solution viscosity expressed close to the wetting line, that is, the lower the viscosity, the higher the coating speed.
For a Newtonian (non-shear thinning) liquid, for example, aqueous glycerol, of comparable low-shear viscosity, a dramatic seven-fold decrease in coating speed results. Thus the shear-thinning nature of coating solutions is an important benefit.
However, for certain pseudoplastic liquids, such as the aqueous gelatin melts used in the curtain coating of photographic products, the wetting failure boundary bifurcates above a certain critical flowrate (corresponding to point B) to produce wetting failure hysteresis as discussed above and in detail in EP-A-0 563 086.
Within the region between the two boundaries defined by BCD and BE, coating is metastable with respect to wetting failure. Evidently, this metastable region may seriously restrict the useful coating window which is now limited practically to the shaded area delineated by the boundary ABE.
For a product laydown of 1.5VA (70.9mm), the maximum practical coating speed is thus reduced from ∼635cms^-1 to ∼390cms^-1. (VA is a term used to define wet layer thickness and 1VA = 47.25mm.) Overall, it has been shown that wetting failure hysteresis is due to the specific shear-thinning characteristics of the liquid that contacts the moving web. By careful modification and control of solution rheology to realise a preferred rheological profile, as shown by the solid line in Figure 3, higher coating speeds and larger more robust coating windows may be realised by eliminating or substantially reducing the metastable coating region.
The corresponding coating window for an aqueous gelatin + colloidal silica dispersion (Ludox AM), formulated to conform with the defined preferred rheological profile is shown in Figure 6 where points A, B and E correspond to the same points in Figure 4, that is, they define the practical coating window. It can be seen that the metastable region shown in Figure 4 has been eliminated in Figure 6. The rheological flow curve is depicted in Figure 2.
The main features to note in comparison to Figure 4 are:-

i) the maximum overall coating speed (represented by point C in Figure 4 and point B in Figure 6), has been increased from ∼667cms^-1 to ∼940cms^-1 (despite an almost three-fold increase in the low-shear Newtonian viscosity of the Ludox-based system);
ii) the maximum practical coating speed, point B in Figures 4 and 6, for a 1.5VA laydown, the intersect of the VA curve with boundary BE, has increased from ∼440cms^-1 to ∼875cms^-1;
iii) the metastable coating regime has been practically eliminated.

It has also been recognised by persons skilled in the art of coating processes using slide hoppers that the viscosity of the coating liquid has a strong influence on the uniformity of the final coated layer. Liquids that possess high viscosities on the hopper slide and on the web are less prone to instabilities and disturbances.
Furthermore, if the coating liquid is Newtonian over the range of shear rates that occur inside the hopper (typically less than 10³s^-1), then widthwise uniformity may be improved. Current practice indicates that the preferred viscosity range is from ∼30mPas to ∼200mPas for curtain coating applications. The Ludox-based aqueous gelatin dispersions discussed above conform to these requirements.
In EP-A-0 563 086, it was shown that layers of aqueous polymer solutions, which are micron-thin, and exhibiting the desired rheological profile, may be used as a carrier layer beneath a multilayer pack to promote wetting during curtain coating. Similar benefits may be realised with carrier layers composed of an aqueous Ludox-gelatin dispersion exhibiting the desired rheology (for example, a 3% w/w aqueous regular gelatin + 6% w/w colloidal silica, Ludox AM). The rheological profile and coating map for this formulation are as shown in Figures 2 and 6. The results of using this formulation for carrier layers are summarised in Table 1 below.

The data summarised in Table 1 were obtained with a 10.2cm curtain, coating at an application angle of 0° (top dead centre) onto a gelatin-subbed Estar support at 42°C. Where no carrier layer was employed, the maximum coating speed for 15% aqueous gelatin was taken as the point where the line defining the VA laydown intersects with the lower speed wetting failure hysteresis boundary (represented by the line BE in Figure 4).

Table 1

Total wet thickness VA (mm)	Carrier layer thickness (mm)	15% gelatin layer thickness (mm)	Maximum coating speed (cm ^-1 )
1.2 (56.7)	2.2	50.9	837
1.2 (56.7)	none	55.3	459
0.9 (42.5)	2.2	36.6	864
	7.3	26.4	952
	none	41.0	590
0.8 (37.8)	5.6	26.7	942
0.8 (37.8)	none	37.9	603
0.7 (33.1)	1.7	28.9	860
	2.2	27.9	908
	none	32.3	633
0.6 (28.4)	0.8	28.7	878
0.6 (28.4)	none	30.3	645

Evidently, the high coating speeds and negligible wetting failure hysteresis exhibited by the gelatin-Ludox dispersion alone (Figure 6), are retained to a significant extent when the material is used as a carrier layer for a liquid (15% aqueous gelatin) which, by itself, would exhibit a lower coating speed and serious hysteresis (Figure 4). This is particularly so for the thicker carrier layers (for example, 5.6mm and 7.3mm). Thus substantial improvements in practical coating speeds may be realised especially for high laydowns, for example, for laydowns greater than 1.2VA (56.7mm).
For a lower fixed carrier layer thickness (that is, 2.2mm), the maximum effective coating speed decreases gradually and minor wetting failure hysteresis (<100cms^-1) becomes apparent as the total laydown is progressively increased beyond approximately 33mm to the maximum value of 55.3mm. The improvement in coating performance, however, remains substantial.
Figure 7 shows a coating window for curtain coating of 15% w/w aqueous regular gelatin coated onto a gelatin-subbed Estar support using a curtain height of 3cm and an application angle of 0°. The practical coating window is confined within the area defined by the boundary ABE. Here, the maximum practical coating speed achieved, defined by point B, is ∼380cms^-1. For identical curtain heights, application angle and support, but for a 3% w/w aqueous regular gelatin + 6% w/w Ludox AM dispersion, the maximum coating speed is ∼817cms^-1, as shown in Figure 8, and the overall practical coating window has been significantly enlarged.
Thus, bottom layers having high viscosities at low shear rates containing Ludox AM may be coated rapidly at low curtain heights at a wide range of laydowns if so desired. As shown by the comparative examples illustrated by Figures 4 and 6, Figures 7 and 8, and given in Table 1, coating speed increases in excess of 30% are readily available. In some cases, coating speed increases of almost 100% are obtained.
Although the present invention has been described with reference to solid colloidal silica particles in aqueous gelatin, it will be readily appreciated that other solid colloidal particle types may also be useful. For example, other inorganic or organic charged particles (for example, clays) may be used.
The present invention may also be extended to include liquid colloidal particles, such as, surfactant micelles and liquid crystal droplets, and oil-in-water emulsions, such as, photographic colour coupler dispersions.
The colloidal particles may have hydrophobic surfaces instead of charged surfaces as described above, or have surfaces stabilised by charged surfactants or polymers. However, in each case, the colloidal particles must interact with the hydrophilic colloid to produce the desired rheological profile.
It will be readily appreciated that in curtain coating applications, other application angles may be used.
Although the specific examples described above relate to curtain coating processes, it will be readily apparent that the present invention may be extended to other coating processes.

Claims

A method of increasing the maximum coating speed of a coating process and substantially eliminating wetting failure hysteresis, wherein a material is coated onto a substrate, the material comprising at least a first layer which lies adjacent the support after coating, characterized in that the first layer includes a dispersion of colloidal particles in a hydrophilic colloid.
A method according to claim 1, wherein the hydrophilic colloid comprises aqueous gelatin.
A method according to claim 1 or 2, wherein the particles comprise colloidal silica.
A method according to claim 3, wherein the particles are negatively charged.
A method according to any one of the preceding claims, wherein the colloidal particles are of a size in the range of 1nm to 10mm.
A method according to claim 5, wherein the particles are of a size in the range of 5 to 50nm.
A coating formulation which is applied directly to a moving support, characterized in that the formulation comprises a dispersion of colloidal particles in a hydrophilic colloid.