HK1108588A - Curtain coating process using a high solids content composition, and coated product - Google Patents

Description

Curtain coating process and coated article using high solids content composition

Technical Field

The present invention relates to a curtain coating process for coating a substrate web (web) using a high solids content composition and to a coated article.

Background

In the manufacture of printing papers, the pigmented coating compositions are usually applied at high speed by, for example, knife coating, bar-wound knife coating, air-knife coating or reverse-roll type coating methods. However, the coating process has no profile on a rough substrate (other than an air knife coating process), which means that any irregular substrate surface will result in an uneven coating thickness, which may lead to irregularities during the printing process.

Curtain coating processes are developed today in the paper industry for coating paper webs to achieve uniformity in surface coating, which is a necessary condition for obtaining excellent final print reproduction (rendering).

Curtain coating processes are well known and widely used in the photographic industry to coat one or more liquid layers onto a surface of a moving support. In fact, this technique was developed for photographic films that require the deposition of many different coatings, typically 8-10, with severe limitations on the surface conditions and the coating thickness of the coating.

Curtain coating is a pre-metered coating process, which means that only the required amount of coating liquid on the web is pumped through the coating head. Curtain coating processes are based on the free flow of liquid over a surface from a coating head located above the surface to be coated. The support is coated by forming a free-falling vertical curtain of liquid so that it impinges on the support. A controlled relationship is maintained between the flow rate of the liquid and the speed of movement of the support so that the curtain is stable and has a uniform flow rate across its width to obtain a layer coated on the substrate. The properties of the coating fluid are used to determine the coating head to obtain the most uniform possible coating film thickness in the running or cross direction of the machine.

One of the advantages of curtain coating is that excellent quality of the substrate (i.e. the paper web) and a more uniform surface can be obtained.

Another advantage is that there is no contact between the coating head and the support, unlike contact-type coating methods such as knife coating and bar coating. This provides a method of eliminating the forces exerted on the support during coating that would in particular cause web breaks, and may have a non-negligible effect on the increase in machine speed and therefore may reduce production costs.

Curtain coating has the further advantage that two or more coatings can be applied simultaneously.

Despite many attempts to coat a substrate web with one or more coating layers using curtain coating methods, the main limiting factors of curtain coating are curtain stability and air entrainment-which allows air to be trapped between the coating composition and the web, resulting in a large number of bubbles and irregularities in the coating. Puddling of the curtain (puddling) can also be a problem and result in heel formation at the curtain impingement zone, which is often due to too low a viscosity of the coating fluid. This phenomenon may lead to coating non-uniformity and may also induce air entrainment at relatively low web speeds. Highly viscous and elastic curtains tend to 'stretch' along the web, which can also lead to uneven coating due to fluctuations in the curtain impingement area. In curtain coating, a uniform layer(s) can only be obtained if the manipulated variables are kept within fairly accurate ranges. These limitations define the coating 'window'. One example is the air entrainment described in U.S. patent No.5,391,401(Blake et al). This patent describes a method of mitigating the air entrainment problem. An optimal relationship between viscosity and shear rate for curtain coating is described. The desired rheological profile promotes low viscosity at expected shear rates near the dynamic wetting line where the coating wets the substrate, and high viscosity at expected lower shear rates in other flow regions. Can be prepared by adding and bondingThe agent-interacting thickener increases the viscosity of the coating composition, which has the effect of increasing the viscosity of the mixture at low shear rates without significantly increasing its viscosity at high shear rates, which means that high viscosity at high shear rates is a drawback. Many of this work have been described for formulations containing binders (particularly gelatin) and silver halide particles for photographic applications. U.S. Pat. No.5,393,571 (Suga et al) describes a method for producing a polymer by mixing at low shear rates (10 s)^-1) Mixtures with lower viscosities above 90mPa.s were used on rough surfaces (0.3 μm) to reduce air entrainment and puddling of the curtain coating. U.S. Pat. No. 6,099,913(Clarke et al) describes formation in the vicinity of the substrate surface at 10000s^-1A viscosity of 90mpa.s to 220mpa.s at a shear rate, which allows the formation of a free-falling curtain and maintains a higher coating speed without air entrainment. These applications are clearly applicable to photographic preparations which typically contain gelatin and silver halide particles, typically with low solids content.

A second example of a limiting factor is curtain stability, which is related to the ratio of inertial force to surface tension (Weber number). This means that higher flow rates and lower surface tensions are beneficial for curtain stability. However, high flow rates are undesirable in some cases, especially when high solids mixtures are used and lower coat weights are required.

Many typical paper coating compositions are highly colored, have a high solids content, and are inherently shear thinning (pseudoplastic) in nature. Such rheological properties may be useful for applications employing knife coating, wire bar coating, reverse roll type coating, slot coating, or slip coating techniques. However, it was observed that many pigmented paper coating formulations failed to form a stable curtain at low flow rates (for aqueous systems, especially when the flow rate of Q-per unit of die length was equal to or lower than 10)^-4m³At/(s.m).

If the curtain coating process is to be used for coating paper formulations with a high solids content at low coat weights, this can currently only be achieved by using faster web speeds. However, air entrainment becomes a practical problem at faster web speeds, especially when coating on smooth, less porous substrates.

Diluting the mixture to run at a slower web speed is not an option. High solids content mixtures are preferred in the coating process because of their lower drying capacity requirements and because they use lower grammage of the base stock (< 80 g/m)²) The binder may easily break at high wet coating weights during the process. Higher solids content mixtures impart improved properties such as higher gloss to the coating medium. Diluting the mixture also reduces the viscosity of the mixture, which if it becomes too low may result in heel formation at the curtain impingement zone.

The invention

Therefore, there is a need to find coating formulations that can produce a stable curtain at low flow rates.

An improved coating method for pigmented coatings that allows the coating 'window' (including curtain stability and coating uniformity) to be extended is described herein. This means that the coating process can be carried out over a wide range of web speeds, reduced flow rates, and therefore lower corresponding wet layups and dry coat weights, without destabilizing the curtain. Another option allowed is the ability to coat onto smoother coated substrates, which tend to start to air entrainment more easily as the web speed increases than coarser substrates. This provides greater flexibility with the overall coating process and makes it more successful in producing high quality coated substrates such as paper, wood board (board) and plastic webs.

This is achieved by adding a small amount of a polymeric additive (rheology modifier) and a nonionic surfactant in combination to the aqueous coating composition. The former (i.e., rheology modifier) increases the viscosity of the coating composition at moderate to high shear rates, and the latter (i.e., nonionic surfactant) reduces the surface tension of the coating composition. The extensional viscosity is influenced by the choice of rheology modifier and its level.

Rheology modifier additives include anionic polyacrylamide/acrylate polymers and ionic hydrophobic polyethers. The advantage of these polymeric additives is that they can be incorporated into the formulation in small amounts (< 2% dry weight) with regard to the product properties (gloss, opacity, color coordinate L)^*a^*b^*Stiffness, smoothness) or printing performance (image quality, optical density, drying time). The surfactant is preferably selected from nonionic surfactants.

The combination of surfactant and rheology modifier appears to have a significant impact on the curtain coating 'window' for extended operation. There was no indication of this: the increase in viscosity at medium to high shear rates due to the presence of the rheology modifier has any effect on the occurrence of air entrainment or puddling at the paint impingement zone.

The invention provides a method for producing a coating on a moving web at or below 1 x 10^-4m³A free-falling curtain of an aqueous colouring composition having a high solids content at a flow rate value (Q) per unit mould length of (s.m).

The composition comprises a surfactant (which reduces the surface tension of the composition) and a polymeric rheology modifier.

More particularly, the present invention provides a curtain coating process for coating a substrate having a stable curtain and a uniform coating at low flow rates, wherein the curtain coating is carried out at or below 1X 10^-4m³At a flow rate per unit die length value (Q) of/(s.m) a free-falling curtain comprising a high solids content aqueous composition comprising a polymeric rheology modifier and a nonionic surfactant is coated on a moving substrate.

The rheology modifier is selected from the group consisting of an aqueous phase thickener and an associative thickener, or a combination of both.

The surfactant is selected from the group consisting of alkyl aryl ethoxylates, alkoxylated acetylenics, alkyl acetylenic diols, non-alkoxylated acetylenics, secondary alcohol alkoxylates, and mixtures thereof.

The composition comprises a binder and a coating pigment. In particular, the binder is selected from the group consisting of copolymers of styrene, in particular styrene-butadiene or styrene-acrylate, styrene-maleic anhydride, polyvinyl alcohol, polyvinyl pyrrolidone, carboxymethyl cellulose, starch, protein, polyvinyl acetate, polyurethane, polyester, and mixtures thereof.

Preferably, the pigment is selected from the group consisting of calcium carbonate, kaolin, talc, titanium dioxide, silica, alumina, boehmite alumina, barium sulfate, zinc oxide, conductive pigments, aluminum silicate, and mixtures thereof.

The aqueous composition has a high solids content of greater than or equal to 50%, preferably greater than 60%, by dry weight.

The concentration of the rheology modifier in the composition is less than 5% dry weight, preferably less than 1% dry weight, more preferably less than 0.5% dry weight of the total composition dry weight.

The concentration of said surfactant in the composition is less than 1% dry weight, preferably less than 0.5% dry weight, more preferably less than 0.3% dry weight of the total composition dry weight.

Preferably, the viscosity of the aqueous composition is in the range of 1000s^-1At a shear rate of 50-200mPa.s at 10000s^-1Is 25 to 90mPa.s at a shear rate of 100000s^-1Is 20-75mpa.s, all data sets being recorded at 25 ℃.

Preferably, the static surface tension of the aqueous composition is less than 45 mN/m.

Preferably, the dry coating weight applied to the substrate is less than or equal to 12g/m²Preferably less than or equal to 10g/m²。

According to the invention, the substrate is a fibrous substrate, such as a paper or wood board, or a plastic web.

Preferably, the curtain is coated onto a continuous web substrate that is a) uncoated or unprimed, b) pre-coated or pre-primed, c) pre-coated and subsequently calendered.

In particular, when the substrate is a paper substrate, the grammage of the paper substrate prior to coating is less than or equal to 150g/m²More particularly less than or equal to 80g/m²。

The free-falling curtain is composed of one or more layers.

According to a particular embodiment of the invention, the free-falling curtain consists of two layers of aqueous composition having a high solids content as described above.

In accordance with another aspect of the present invention, there is provided a coated article comprising a substrate and a substantially uniform coating on the substrate, the coating comprising a high solids content composition comprising a polymeric rheology modifier and a nonionic surfactant.

The invention provides a paper or plastic support (web or sheet) obtained by the process, in particular a high gloss paper.

Example (b):

examples of rheology modifiers used are summarized in table 1 and examples of surfactants used are summarized in table 2.

Table 1.

Rheology modifier	Company(s)	Class of polymers	Active mode	Is mainly characterized in that
					SterocollBL	BASF	Anionic water-in-oil emulsions of acrylamide-acrylic acid copolymers	Aqueous phase thickener	Increase of medium shear rate viscosity, increase of extensional viscosity
Rheolate212	Elementis	Hydrophobic ethoxylated polyurethanes	Associative thickeners	Increasing high shear rate viscosity
					Rheolate350	Elementis	Polyether polyols	Associative thickeners	Increasing high shear rate viscosity
Rheolate425	Elementis	Hydrophobically modified alkali swellable polyacrylates	Aqueous phase/associative thickeners	Increasing low/medium shear rate viscosity
					Rheovis802	CIBA	Anionic polyacrylamide/acrylate	Aqueous phase thickener	Increasing high/medium shear rate viscosity
Mowiol40-88	Kuraray	Polyvinyl alcohol	Aqueous phase thickener	Increase of high/medium shear rate viscosity, increase of extensional viscosity

Table 2.

Surface active agent	Company(s)	Type of molecule
			Surfynol CT211	Air Products	Alkyl acetylenic diols
Surfynol 420	Air Products	Ethoxylated acetylenic materials
			Surfynol 2502	Air Products	Ethoxylated/propoxylated acetylenic materials
Surfynol 485	Air Products	Ethoxylated acetylenic materials
			Dynol 604	Air Products	Non-ethoxylated acetylenic materials
Tergitol 15-S-7	DOW	Secondary alcohol ethoxylates
			Tergitol 15-S-9	DOW	Secondary alcohol ethoxylates
Tergitol TMN6	DOW	Branched secondary alcohol ethoxylates
			Triton X100	DOW	Octylphenol ethoxylate
Dapro W77	Elementis Specialities	Ethoxylated fatty acid esters

Base material

Steel roll calender set using 2-zone (nip) at 600 m/min at 10⁵Will be at 130g/m at N/m²Base stock of virgin paper and 20g/m²150g/m of clay paint composition²The clay coated substrate was calendered. The physical properties of the coated paper substrate are shown in table 3.

Table 3.

Parameter(s)	Value of
		Total surface energy (dyne/cm)	41.4
Bromonaphthalene with contact angle (°) water contact angle (°)	81.729.8
		Bekk smoothness (seconds)	4135
Bendtsen air permeability (mL/min)	0
		Gloss (75 degree (%)	24.7

Examples

Example 1 (comparative)

Calcium carbonate pigment (85 parts) was dispersed in water. Latex binder (15 parts) was added to the formulation and the mixture was stirred for 0.5 hours. The solids content of the formulation was recorded as 64.7%.

Example 2 (comparative)

Calcium carbonate pigment (84.8 parts) was dispersed in water. Latex binder (14.97 parts) was added to the formulation and the mixture was stirred for 0.5 hours. Surfynol CT211(0.23 parts) was added to the mixture and stirred for a further 0.5 hours. The solids content of the formulation was recorded as 65.5%.

Example 3 (inventive)

Calcium carbonate pigment (84.62 parts) was dispersed in water. Latex binder (14.94 parts) was added to the formulation and the mixture was stirred for 0.5 hours. Surfynol CT211(air products) (0.23 parts) was added to the mixture and stirred for a further 0.5 hours. At the end of the formulation, 0.21 part Sterocoll BL (BASF) was added. The mixture was stirred for a further 0.5 h. The solids content of the formulation was recorded as 65.08%.

Example 4 (inventive)

Calcium carbonate pigment (84.44 parts) was dispersed in water. Latex binder (14.90 parts) was added to the formulation and the mixture was stirred for 0.5 hours. Surfynol CT211(0.23 parts) was added to the mixture and stirred for a further 0.5 hours. At the end of the formulation, 0.42 part of Rheovis 802(CIBA) was added. The mixture was stirred for a further 0.5 h. The solids content of the formulation was recorded as 65.0%.

Example 5 (inventive)

Calcium carbonate pigment (84.44 parts) was dispersed in water. Latex binder (14.9 parts) was added to the formulation and the mixture was stirred for 0.5 hours. Surfynol CT211(0.23 parts) was added to the mixture and stirred for a further 0.5 hours. At the end of the formulation, 0.42 part of Mowiol40-88(Kuraray) was added. The mixture was stirred for a further 0.5 h. The solids content of the formulation was recorded as 63.5%.

Example 6 (inventive)

Calcium carbonate pigment (90.52 parts) was dispersed in water. Latex binder (8.42 parts) was added to the formulation and the mixture was stirred. Surfynol CT211(0.27 parts) was added to the mixture. 0.03 part of a defoamer was added to the mixture, followed by 0.41 part of Mowiol 4-98 (as rheology modifier) (Kuraray) and 0.05 part of Sterocoll BL (BASF), with stirring between the addition of each component. The mixture pH was adjusted to 10.3 with sodium hydroxide.

The solids content of the formulation was recorded as 65.5%.

Results and discussion

Example 1 contained no surfactant and no rheology modifier and was only at 1.8X 10^-4m³A curtain can be formed at a value of Q (flow rate per unit of mode length) of/(s.m). The static surface tension was measured to be 45 mN/m. At a web speed of 400 m/min, this corresponds to 26.6g/m²Is much greater than < 10g/m for coated papers of good quality (image quality and drying time) for offset printing²Coating weight requirement of (2). However, the coating is unstable at the impingement zone, possibly due to air entrainment, and poor coating uniformity is obtained. The viscosity of the mixture is 100s^-1At a shear rate of 125mPa.s at 1000s^-1At a shear rate of 37mPa.s at 10000s^-1Is 14mpa.s at a shear rate of 100000s and^-1is 13mpa.s at shear rate.

Example 2 relates to the addition of a non-ionic surfactant to example 1, which results in a curtain flow rate per unit of mode length that is much reduced compared to example 1 (Q9.26 × 10)^-5m³V (s.m)). This is due to the reduction of the surface tension of the mixture to 35 mN/m. This resulted in 14.0g/m²Is much lower than the dry coating weight in the absence of surfactant, but is higher than the target value of 10g/m². The viscosity of the mixture is 100s^-1At a shear rate of 142mPa.s at 1000s^-1At a shear rate of 43mPa.s at 10000s^-1Is 18mpa.s at a shear rate of 100000s and^-1is 18mpa.s at shear rate.

Example 3Involves the addition of surfactant and rheology modifier (Sterocoll BL) to example 1 at 6.73X 10^-5m³A stable curtain is obtained at a flow rate per unit of mode length Q (s.m). This gave 9.4g/m²This is within the desired target. The static surface tension increases beyond example 1 (rheology modifiers tend to increase the surface tension) to 40 mN/m. However, the flow rate required for curtain stability was lower than in example 1. Until a speed of 600 m/min was reached, at which air entrainment began to occur, a uniform coating distribution was obtained. The viscosity of the mixture is 100s^-1At a shear rate of 438mPa.s at 1000s^-1At a shear rate of 107mPa.s at 10000s^-1Is 50mpa.s at a shear rate of 100000s^-1At a shear rate of 48 mpa.s.

Example 4 involves the addition of surfactant and rheology modifier (Rheovis 802) to example 1 at 6.17X 10^-5m³A stable curtain is obtained at a flow rate per unit of mode length Q (s.m). This gave 8.5g/m²This is within the desired target. The static surface tension increased over example 1 (rheology modifiers tend to increase the surface tension) to 37 mN/m. However, the flow rate required for curtain stability was again lower than in example 1. Until a speed of 600 m/min was reached, at which air entrainment began to occur, a uniform coating distribution was obtained. The viscosity of the mixture is 100s^-1At a shear rate of 355mPa.s at 1000s^-1At a shear rate of 80mPa.s at 10000s^-1Is 28mPa.s at a shear rate of 100000s^-1Is 24mpa.s at shear rate.

Example 5 involves the addition of surfactant and Mowiol40-88 polyvinyl alcohol to example 1 at 7.86X 10^-5m³A stable curtain is obtained at a flow rate per unit of mode length Q (s.m). This gave 10.0g/m²This is within the desired target. The static surface tension increased over example 1 to 42 mN/m. However, curtain stability stationThe required flow rate was lower than in example 1. Until a speed of 600 m/min was reached, at which air entrainment began to occur, a uniform coating distribution was obtained. The viscosity of the mixture is 100s^-1At a shear rate of 161mPa.s at 1000s^-1At a shear rate of 124mPa.s at 10000s^-1Is 77mPa.s at a shear rate of 100000s^-1Is 34mpa.s at shear rate.

Example 6 relates to the addition of a surfactant and dual rheology modifier system (SterocollBL + Mowiol 4-98) to a coating mixture of latex binder and calcium carbonate. At 9.45X 10^-5m³A stable curtain is formed at a flow rate per unit of die length Q value,/(s.m), and a static surface tension value of 35.5 mN/m. A uniform coating distribution without air entrainment was obtained at a line speed of 600 m/min. The coated paper obtained exhibits excellent printing properties. The dry coating weight was 10.0g/m². The viscosity of the mixture is 100s^-1At a shear rate of 255mPa.s at 1000s^-1At a shear rate of 78mPa.s at 10000s^-1Is 37mPa.s at a shear rate of 100000s^-1At a shear rate of 29 mpa.s.

The viscosity results for each mixture at various shear rates are summarized in table 4.

TABLE 4

Example No. 2	Presence or absence of surfactant	Rheology modifier/% dry parts	Shear rate(s)-1)
								10	100	1000	10000	100000
			1	Is free of	Is free of	442	125						37	14	13
2	Is provided with	Is free of						544	142	43	18	18
			3	Is provided with	0.21％Sterocoll BL	2411	438						107	50	48
4	Is provided with	0.42％Rheovis 802						1914	355	80	28	24
			5	Is provided with	0.42％Mowiol 40-88	2252	427						94	37	34
6	Is provided with	0.05％Sterocoll BL0.41％Mowiol 4-98						1350	255	78	37	29

Effective extensional viscosities do not appear to have an effect on curtain stability (see results in table 5). Example 3 at 100000s^-1The lower one shows a high elongational viscosity of 174mpa.s and example 4 a low elongational viscosity of 4mpa.s at the same shear rate, whereas the minimum flow rate obtained for curtain stability is slightly lower for the latter.

TABLE 5

Example No. 2	At 100000s-1Effective extensional viscosity (mPa.s) at shear rate of
		1	0
2	0
		3	174
4	4
		5	1
6	150

TABLE 6

Example No. 2	Qmin (curtain self-forming) (m)3/(s.m))	Speed of the web (m/min)	Solid content of mixture (%)	Dry coating weight (g/m)2)	Density of mixture (g/cm)3)	Static surface tension of the mixture (mN/m)	Air entrainment
								1	1.80×10-4	400	64.7	26.6	1.526	47	Comprising (more than or equal to 200 m/min)
2	9.26×10-5	400	65.5	14.0	1.538	35	Is free of
								3	6.73×10-5	400	65.1	9.4	1.423	40	Is free of
4	6.17×10-5	400	65.0	8.5	1.410	37	Is free of
								5	7.74×10-5	400	63.5	10.0	1.354	42	Is free of
6	9.45×10-5	600	65.5	10.0	1.61	35.5	Is free of

Test method

Viscosity-measured using a Brookfield RVT viscometer. The spindle speed was selected to be 100 rpm. Spindle size was either sp2 or sp 3. The temperature of the mixture was recorded during the viscosity measurement.

Density-was measured using a 100mL densitometer. The temperature was recorded during the density measurement.

pH-was measured using a HI 9024 microcomputer pH meter (Hanna Instruments). The temperature was recorded during the pH measurement.

Solids (%) -was measured using a CEM Labwave 9000 microwave moisture/solids analyzer.

Contact angle-measured using FibroDAT 1100.

Surface tension-measured using a DCA 132 apparatus with a platinum plate.

Paper gloss-measured with a gloss meter (BYK Gardner GmbH) set at an angle of 75 °.

Paper smoothness-measured using a Bekk smoothness tester (Messmer Instruments Ltd).

Air permeability-was measured using a Bendtsen tester (Lorentzen & Wettre).

Rheology-flow data were measured at 25+/-0.1 ℃ with parallel plates at a gap of 40 μm by means of a CV0120 high resolution rheometer (Bohlin Instruments). The shear rate is in the range of 10-100000s^-1。

Effective extensional viscosity-measured on a Paar automatic high shear viscometer HVA6 with capillary lengths of 10mm and 5mm and a capillary diameter of 0.6 mm.

Mixture preparation and coating method

All parts are expressed as dry weight based on 100 parts dry weight of the total composition.

Thus, the coating formulations of examples 1-6 are expressed in% dry weight (parts) of the total composition.

All formulations were mixed using a Greaves GM dispenser. Agitation is optimized to ensure good mixing but avoid excessive air entrainment. The curtain head used was of the slide type with a width of 0.49m and a die gap of 300 μm. The curtain applicator head is equipped with edge guides-running water flows down on each side, there being vacuum suction to remove the water at the bottom of the edge guides. The collector also acts as a baffle-a mechanical barrier to limit air entrainment at the impingement zone. A suction vacuum (0.3 bar) may optionally be applied to reduce the web movement across the curtain in the impingement zone and further limit the onset of air entrainment. The curtain height was 100 mm.

The coating weight of each coated sample was determined by the known volumetric flow rate of the pump used to deliver the mixture to the curtain head, web speed, density and% solids of the mixture, and curtain width. By mixing 100cm²The coated and uncoated substrate samples of (a) were placed in an oven at 150 ℃ for 10 minutes and the weight difference between the two samples was measured to verify the coating weight.

Materials for use in formulations

Calcium carbonate pigment: 95% by weight of ground calcium carbonate having an average particle size of less than 2.5 μm. The solids content was 78%.

Latex adhesive: aqueous dispersions of styrene-butadiene copolymers. The solids content was 50%.

Mowiol 40-88(Kuraray): the polyvinyl alcohol was 88% hydrolyzed. The 4% solution has a viscosity of 40cps (manufacturer's data) at 25 ℃ as measured on a Brookfield RVT viscometer. As rheology modifiers (aqueous phase thickeners) or as binders. Polyvinyl alcohol was used as a 10% solution obtained by heating polyvinyl alcohol particles and water at 95 ℃ for 0.5 hour.

Rheovis 802: anionic water-in-oil emulsions of polyacrylamide/acrylate. As rheology modifier (aqueous phase thickener).

Mowiol 4-98(Kuraray): the polyvinyl alcohol was 98% hydrolyzed. The 4% solution has a viscosity of 4.5cps (manufacturer's data) at 25 deg.C as measured on a Brookfield RVT viscometer. As rheology modifiers (thickeners) or as binders. Mowiol 4-98 was used as a 25% solution obtained by heating polyvinyl alcohol particles and water at 95 ℃ for 0.5 hour.

Claims (29)

1. Curtain coating process for coating a substrate with a stable curtain and a uniform coating at low flow rates, wherein the curtain coating is carried out at 10 or below^-4m³At a flow rate per unit die length value (Q) of/(s.m) a free-falling curtain comprising a high solids content aqueous composition comprising a polymeric rheology modifier and a nonionic surfactant is coated on a moving substrate.

2. Curtain coating process as claimed in claim 1, wherein the rheology modifier is selected from aqueous phase thickeners and associative thickeners and mixtures thereof.

3. Curtain coating process as claimed in claim 2, wherein the aqueous phase thickener is selected from anionic polyacrylamide/acrylate polymers, ionic hydrophobic polyethers and polyvinyl alcohols, carboxymethyl cellulose, hydroxyethyl cellulose, starch, protein derivatives and alkali swellable polyacrylates.

4. Curtain coating process as claimed in claim 2, wherein the associative thickener is selected from hydrophobic ethoxylated polyurethanes and polyether polyols.

5. Curtain coating process as claimed in claims 1-4, wherein the surfactant is selected from alkyl aryl ethoxylates, alkoxylated acetylenics, alkyl acetylenic diols, non-alkoxylated acetylenics, secondary alcohol alkoxylates, and mixtures thereof.

6. Curtain coating process as claimed in any one of claims 1 to 5, wherein the aqueous composition has a high solids content of greater than or equal to 50% dry weight, preferably greater than or equal to 60%.

7. Curtain coating process as claimed in any one of claims 1 to 6, wherein the aqueous composition has a viscosity of 1000s^-1At a shear rate of 50-200mPa.s at 10000s^-1Is 25 to 90mPa.s at a shear rate of 100000s^-1Is 20-75mpa.s, all data sets being recorded at 25 ℃.

8. Curtain coating process as claimed in any one of claims 1 to 6, wherein the static surface tension of the aqueous composition is less than 45 mN/m.

9. Curtain coating process as claimed in any one of claims 1 to 8, wherein the aqueous composition comprises a binder and a pigment.

10. Curtain coating process as claimed in claim 9, wherein the binder is selected from copolymers of styrene, in particular styrene-butadiene or styrene-acrylate, styrene-maleic anhydride, polyvinyl alcohol, polyvinyl pyrrolidone, carboxymethyl cellulose, starch, protein, polyvinyl acetate, polyurethane, polyester, acrylic acid, and mixtures thereof.

11. Curtain coating process as claimed in claim 9 or claim 10, wherein the pigment is selected from calcium carbonate, aluminium silicate, kaolin, talc, titanium dioxide, silica, alumina, boehmite alumina, barium sulphate, zinc oxide, plastic pigments, conductive pigments, and mixtures thereof.

12. Curtain coating process as claimed in any one of claims 1 to 9, wherein the concentration of rheology modifier in the aqueous composition is less than 5% dry weight, preferably less than 1% dry weight of the total composition dry weight.

13. Curtain coating process as claimed in any one of claims 1 to 10, wherein the concentration of surfactant in the composition is less than 1% dry weight, preferably less than 0.5% dry weight of the total composition dry weight.

14. Curtain coating process as claimed in any one of claims 1 to 13, wherein the dry coating weight applied to the substrate is less than or equal to 12g/m²Preferably less than or equal to 10g/m²。

15. Curtain coating process as claimed in any one of claims 1 to 14, wherein the moving substrate is a fibrous substrate or a plastic substrate.

16. Curtain coating process as claimed in claim 15, wherein the fibrous substrate is a paper or a wood board.

17. Curtain coating process as claimed in claim 16, wherein the grammage of the paper substrate before coating is less than or equal to 150g/m²More particularly less than or equal to 80g/m²。

18. Curtain coating process as claimed in claims 16 to 17, wherein the curtain is applied to a continuous web of a) uncoated or unprimed, b) precoated or primed, c) precoated and subsequently calendered.

19. Curtain coating process as claimed in claim 15, wherein the moving substrate is a plastic web or film.

20. Curtain coating process as claimed in any one of claims 1 to 19, wherein the free-falling curtain consists of one or more layers.

21. Curtain coating process as claimed in claim 20, wherein the free-falling curtain comprises two layers of aqueous composition with a high solids content.

22. A coated article comprising a substrate and a substantially uniform coating on the substrate, the coating comprising a high solids composition comprising a polymeric rheology modifier and a nonionic surfactant.

23. The coated article of claim 22, wherein the rheology modifier is selected from the group consisting of aqueous phase thickeners and associative thickeners, and mixtures thereof.

24. A coated article according to claim 22 or claim 23, wherein the surfactant is selected from the group consisting of alkylaryl ethoxylates, alkoxylated acetylenics, alkyl acetylenic diols, non-alkoxylated acetylenics, secondary alcohol alkoxylates, and mixtures thereof.

25. The coated article of any of claims 22-24, wherein the coating composition comprises a binder and a pigment.

26. A coated article as claimed in any one of claims 22 to 25, wherein the concentration of rheology modifier in the coating composition is less than 5% dry weight, preferably less than 1% dry weight of the total composition dry weight.

27. A coated article as claimed in any one of claims 22 to 26, wherein the dry coating weight applied to the substrate is less than or equal to 12g/m²Preferably less than or equal to 10g/m²。

28. A coated article as claimed in any one of claims 22 to 27, wherein the substrate is a fibrous substrate or a plastics substrate.

29. The coated article of claim 28, wherein the substrate is a grammage of less than or equal to 150g/m exclusive of the coating²More particularly less than or equal to 80g/m²The paper substrate of (1).