WO2011098656A1

WO2011098656A1 - Coating

Info

Publication number: WO2011098656A1
Application number: PCT/FI2010/050092
Authority: WO
Inventors: Harri Repo
Original assignee: Dinoto Oy
Priority date: 2010-02-11
Filing date: 2010-02-11
Publication date: 2011-08-18

Abstract

A coating comprises: aqueous film-forming elastic acryl copolymer as binding agent, and electrically conducting particles mixed with the binding agent in such an amount that the surface resistivity of the dried coating is at the level of 1x10E3 - 1x10E11 ohm/square, the mixture of the binding agent and the electrically conducting particles forming a transparent coating.

Description

COATING

The present invention relates to a coating which is of the type set forth in the preamble portion of appended claim 1.

Coatings have been developed for various purposes with desired electrical conductivity levels, that is, they differ from pure insulating materials. Conductive particles mixed with a polymeric binding agent are commonly used in such coatings to provide electrical conductivity. Depending on the level of conductivity one can speak about static dissipative or ESD (electrostatic discharge) shielding.

In addition to electrical conductivity, mechanical resistance is required of a coating. This is important in particular when the coating is subjected to attrition, or material on which it is coated will be folded. Resistance to folding is particularly important in packaging application.

At the moment, xylene-soluble acryl copolymers (of PMMA type) aiming principally at hardness are known. Organic solvents like xylene that are used in manufacture of the coatings cause problems of toxicity and fire safety, which are generally recognized as problems that should be eliminated. Further, coatings based on an intrinsically conducting polymer, aniline, are known, the main problem of which is poor folding resistance.

It is the purpose of the present invention to provide a new coating fulfilling the above-mentioned criteria. To achieve this aim, the coating is primarily characterized in what is presented in the characterizing portion of appended claim 1.

The coating comprises the following components essential for its functionality:

aqueous dispersion based film-forming elastic acrylic copolymer as binding agent, and electrically conducting particles mixed with the binding agent in such an amount that the surface resistivity of the coating is at the level of 1x10E3 - 1x10E1 1 ohm/square (10³ - 10¹¹ ohm/square),

the mixture of the particles and the acrylic copolymer binding agent forming a transparent coating.

Preferably the surface resistivity of the coating is 1 x10E6 - 1x10E8 ohm/square fulfilling ESD demands set on packages. The water-based acrylic copolymer provides a certain degree of elasticity to the coating, which has clear advatages when flexible and foldable materials are coated especially in view of packaging applications, because the coating persisists without cracking and thus retains the desired level of electric conductivity. Especially the coating must withhold creasing, when a folding boxboard coated with it is made to a package. Creasing is a well-known operation where a surface pattern is made in a folding boxboard blank to facilitate later folding and it interrupts the smooth surface of the carboard material and therefore easily creates points of discontimuity in the coating.

One of preferable binding agents is a binding agent under tradename "PRIMAL™ ECO-16" (Rohm and Haas), which is a nonionic, self- crosslinking acrylic copolymer (methyl methacrylate - ethyl acrylate copolymer), which can be applied on a substrate as an aqueous emulsion. Depending on mechanical requirements, grades "PRIMAL™ ECO-36" (self-crosslinking, anionic) and "PRIMAL™ ECO-8" (sel- crosslinking, nonionic) may also be used. As MMA/EA copolymers, corresponding polymer producs from other manufacturers may also be used.

Another possible water-based acrylic copolymer may be ethylene acrylic acid copolymer (EAA). These dispersion polymers are also known under various tradenames. For example TECSEAL E-799/45 (Trub Emulsions Chemie), an ethylene-acrylic acid dispersion (solids content 45%, Tg 4°C), may be used. It must be understood that the invention is not limited to the use of substances identified by certain tradenames, but other substance with similar properties for the purpose and effects of the invention may be used.

Acrylic copolymers of different types with differing mechanical properties may be blended with each other to achieve the desired final properties of the coating. The advantages of the acryl copolymers can be summarized as

- heat resistance

- transparency

- elasticity, and

- adhesivity,

- heat-sealability, and

- blocking resistance.

Styrene-butadiene latexes and polyurethane latexes can also be used in the coating compositions as aqueous film-forming polymer binding agents in mixture with the above-mentioned acrylic copolymers. However, the proportion of these additional polymers should be kept moderate. They can be present in an amount not exceeding 50 wt-%, preferably at the most 20 wt-% of the total dry weight of the film- forming polymers in the mixture.

Styrene-acrylates, still one group of acryl copolymers, are useful alone or especially as blends where a "hard" and "soft" polymer is combined to provide a creasable and at the same time block resistant surface for different packaging materials. The elasticity can be increased through the addition of a compatible polurethane dispersion. For example NeoCryl XK-188 (DSM), an acryl ic-styrene emulsion, blended with NeoRez-1007 (DSM), a polyurethane dispersion, is a sufficiently stable mixture for production scale. The binding agent of the coating should not occlude the particles so that the electric conductivity is maintained. Because of this requirement, the binding agent of the coating is not crosslinked in coating conditions. Even if the binding agent is crosslinkable, it is dried, after being applied onto the substrate in admixture with the conductive particles, in such a temperature - time profile where crosslinking has no time to take place. However, the drying after spreading of the coating may be conducted in such a way that crosslinking occurs at the interface of the fibrous substrate and the coating while the outer layer of the coating remains unaffected. The elasticity of the above-mentioned aqueous dispersion based acrylic copolymer binding agent or an aqueous dispersion based binding agent composition containing the acrylic copolymer as main constituent (over 50 wt-%) is preferably at such a level that a dry film made of it has minimum elongation at break above 300 %, and optimally above 600 %.

The amount of electrically conductive particles in the coating is preferably 5-60 wt-% of the total mass of the coating (on dry weight), more preferably 10-40 wt-%.

In principle any electrically conductive particles can be used that have sufficiently small size (average diameter) to be able to disperse uniformly in the liquid coating composition and to form, upon drying of the coating, a uniform network of interconnecting particles at least in the immediate vicinity of the top face of the coating, preferably in the interface of the coating, even in cases where the coating is relatively thin, only of the order of 1 ,5 to 15 g/m² as expressed in square weight of the coating when applied wet on the substrate in one or several steps. Expressed in dry matter, the amount of coating may be in the range of 1 - 10 g/m². However, one important characteristic of the conductive particles is that they do not increase the opacity of the coating, a property which is quite contrary to the requirements of the common coating compositios of paper and cardboard products. Electrically conductive particles whose colour is close to white (i.e. they do nor absorb light) and which have a small particle size should be used. In the following, types of particles that are particularly suitable for the coating are discussed.

Especially in packaging applications printability despite functional coatings applied on the same substrate is important. Therefore, to make any impression printed on the substrate visible underneath the coating, the conductive particles are transparent particles. Transparent particles are meant to be particles that do not raise, in concentrations required for conductivity, the light absorbency of the clear elastic coating based on acrylic copolymer to such an extent that they would block the impression or the original substrate surface from being visible through the thickness of the coating once the coating has dried, that is, they are light-coloured and "colour-neutral". Alternatively, the original non-printed packaging material surface may be visible through the transparent coating, which in turn is provided with impression (printed) on its top surface.

Particularly suitable particles with optical properties for achieving a transparent coating in combination with the polymeric binder are for example mineral particles that are coated to be electrically conductive. These are typically mineral particles having a metal oxide coated or metal coated (metallized) surface. One example is conductive pigment sold under trade name "Minatec" by Merck. This pigment consists of mica particles coated with (Sn/Sb)O₂, tin oxide doped with antimony. Suitable grades are for example "Minatec® 40 CM" and "Minatec® 31 CM Transparent". Both grades provide for transparency of the coating, if the binding agent is transparent. The particle size of both grades is under 15 m as determined by laser diffraction measurement. By transparency is meant that the coating may not necessarily be clear but it may have some color. However, the transparency of the coating retains the original colour of the dye and does not distort it or change its hue, thus, the coating has "clear" colour. An important component in the coating composition is dispersion aid (dispersant) which helps to distribute the particles uniformly in a coating composition that upon application and drying results even in a relatively thin coating. For example isopropanol optionally supplemented with propylene glycol may be used. The amount of the dispersion aid is preferably 1 -5 wt-% in the aqueous coating composition.

In general, the coating composition based on the above-mentioned acryl copoymers in aqueous dispersions and conductive particles is an improvement over resin technology. Aqueous acryl copolymer dispersions and their best compatibility with various printing machines and coating machines can be utilized directly without any intermediary resin step or solvent based premixture of the pigment. The aqueous polymer dispersion can be used directly in preparing the coating composition to be applied to the substrate, by mixing desired components into it. Further, the coating process is environmentally friendly resulting only in release of water vapour and negligent amounts of alcohol vapours (dispersion aids) when the coating is dried.

The invention will be described in more detail with the help of following examples that do not restrict the scope of the invention.

Coating compositions, that on drying (evaporation of the water) form the coating, can be applied onto a substrate by many different methods, such as flexographic, gravure coating, silk screen printing, roll coating, size press coating and blade coating methods. The substrate may be cardboard, paper or plastic. Especially packaging materials are suitable for substrates. The coating can be provided with glue (for example to make glue lines on the surfaces of package blanks), printed (in which case the impression is on top of the coating) and it can be coated with a topcoat, according to need.

Paper and carboard are preferred substrates and the coating can be applied with methods generally used for coating these substrates. The paper or cardboard surface to be coated may be previously coated with a coating improving the printability, provided that this coating is compatible with the coating composition of the invention. However, the substrate can also be a plastic film, where the elasticity properties of the coating are important, and materials that can be coated with the coating composition of the invention include various polyolefins, such as LDPE, HDPE, PP, OPP (oriented polypropylene), MDPE and LLDPE. The substrate can also be a layered substrate. Thus, plastic films consisting of two or more polymer layers and paper or carboard laminates coated with one or more polymer layers may be used. The surface of such a layered substrate to be coated with the composition may be a polyolefin, for example some of the above-mentioned polyolefins. To enhance the adhesion of the coating to the substrate the polymer surface may be pretreated by known methods, for example by corona treatment.

The substrate can also be a laminate where the coating composition is applied on the paper or carton surface and one or several additional layers are on the reverse side in relation to the surface where the coating composition is applied.

One aspect of the invention is thus to provide a coating composition that can be spread on the substrate as an aqueos composition containing all components that are part of the final solid elastic coating permanently attached to the surface of the substrate. The carrier of the coating composition, water, evaporates, as do solvents used as dispersants, leaving behind a finished coating consisting mainly of the acrylic copolymer and the particles. The film-forming elastic acrylic copolymer binder is in form of dispersion (emulsion) in the aqueous carrier that contains the other components of the coating composition such as the electrically conductive particles and the dispersant. It is possible to coat the substrate in form of a continuous web when it runs past a coating station that is arranged to apply the coating composition in one of the above-mentioned methods. The coating can be applied on a paper or board substrate in a paper or board machine that manufactures paper or board from water-based fibrous pulp, or in a separate paper or board coater (finishing machine) spreading a coating on a paper or board previously manufactured. In both alternatives the coating on the web is dried in the same machine subsequently to the coating station.

The paper or cardboard substrates suitable to be coated with the compositions described above are preferably uncoated paper and cardboard (i.e. uncoated on the surface to be coated with the coating composition), such as uncoated liner, board, and paper, including products made of recycled fibre. These substrates may be printed or non-printed.

Example 1 (amounts expressed as weight parts)

Folding boxboard coating with flexo

- polymer PRIMAL ECO 16 77.8 (wet, d.m. 45.5.%; pH 2-4)

- isopropanol 2.0

- propylene glycol 1.5

- pigment Minatec 40 CM 18.5

- antifoam agent 0.2

The board was one-side blade-coated Tambrite, which was coated on both sides twice with the composition, top side (coated side of the board): 1.6 + 1.4 g/m2 wet, and back side: 1.8 + 1.8 g/m2 (wet weight), and dried. The coating was transparent due to the transparent nature of the pigment used as the conducting particles. The surface resisitivity of the dry coating was measured with Perell ohm/square measuring device, with the following results: top side 1x10E6 ohm/sq, back side 1x10E7 ohm/sq.

Example 2 (amounts expressed as weight parts)

Folding boxboard coating with flexo

- polymer PRIMAL ECO 16 65.3 (wet, d.m. 45.5.%; pH 2-4)

- polymer PRIMAL ECO 8 4.0 (wet, d.m. 45.5.%; pH 3)

- isopropanol 2.0

- propylene glycol 2.0

- pigment Minatec 31 CM 26.5

- antifoam agent 0.2 The board was one-side blade-coated Tambrite, which was coated on top side once and back side twice with the composition, top side: 1.7 g/m2 wet, and back side: 1.7 + 1.7 g/m2 (wet weight), and dried. The surface resisitivity of the dry coating was measured with Perell ohm/square measuring device, with the following results: top side 1x10E3 ohm/sq, back side 1x10E5 ohm/sq.

Example 2a (amounts expressed as weight parts)

Ready made coloured version of liner coating performed on a flexo machine

- polymer TECSEAL E-799/45 66.3

- isopropanol 1.0

- propylene glycol 1.5

- pigment Minatec 40 CM 20.5

- ammonium 25 % 0.4

- antifoam agent 0.2

- Siegwerk blue paste 141 -944534 10.5

Substrate 125 SCA Kraft liner, Flexo application 4.9+4.9 g/m² (wet weight), and drying. Surface resistivity 1x10E4 ohm/sq.

When the composition of Example 2a was used together with PRIMAL ECO 16 in a ratio 50/50 in a Flexo application to form a single layer of 1.8 g/m² (wet weight), the resulting value for the dry coating was 1x10E10 ohm/sq.

Example 3 (amounts expressed as weight parts)

Printed board coating with gravure

- polymer PRIMAL ECO 16 76.8 (wet, d.m. 45.5.%; pH 2-4) - isopropanol 1.0

- propylene glycol 1.5

- pigment Minatec 40 CM 20.5

- antifoam agent 0.2

The board was printed board, which was coated using one gravure unit on top side once the composition, 1.5 g/m2 wet, to obtain maximal transparency, and dried. The impression remained visible through the coating due to the transparent nature of the pigment. The surface resisitivity of the dry coating was measured with Perell ohm/square measuring device, with the result of 1 x10E8 ohm/sq.

Example 4 (amounts expressed as weight parts)

blade coating of paper or board

- polymer PRIMAL ECO 16 40.4 (wet, d.m. 45.5.%; pH 2-4)

- polymer PRIMAL ECO 36 13.4 (wet, d.m. 46.5-47.5.%; pH 6-7)

- isopropanol 1.0

- propylene glycol 1.0

- pigment Minatec 40 CM 12.0

- pigment Minatec 31 CM 12.0

- filler talc 20.0

- antifoam agent 0.2

The substrate coated was machine-glazed paper, which was blade- coated on one side with the composition in an amount of 12 g/m2 wet, and dried. The surface resisitivity of the dry coating was measured with Perell ohm/square measuring device, with the result of 1 x10E5 ohm/sq. The surface of the coating had sufficient conductivity despite the talc used as nonconductive filler. The transparency was not affected by the minor amout of talc used as assistant filler which gave the surface a matt appearance and a greiyish hue for the coating, but it did not hide the background surface colouration. This coated product is suitable for printing on it, because it is optically similar to conventional printing papers while having surface conductivity. Thus, the coating composition may contain conventional non-conductive fillers as one fraction of particle material to adjust the optical properties for printing on the coating.

Assistant fillers that can be used in addition to the transparent conductive fillers can be talc (for example Talc de Luzenac with average particle diameter of 15 μιτι) or silica (for example Degussa Aerosil 200 silica). The assintant fillers, whose amount is preferably lower that of the conductive pigments in the composition, can be used to adjust the surface mattness of the coating and increase the abrasion resistance of the coating surface.

In the above examples, all coating compositions were adjusted to pH 8.5-9.0 before coating to optimize the coalescing properties. All compositions were based on water in which other components were dispersed or dissolved.

The mechanical properties of the coating are influenced also by the glass transition temperature, Tg of the polymer used as binding agent alone or in mixture with other polymers. The polymers in the above compositions have the following Tgs: Primal ECO 8 -14°C, Primal ECO 16 +33°C, and Primal ECO 36 -12°C. The preferable packaging materials where the invention can be used are those that are made to packages of electronic industry, for example by folding (preferably preceded by creasing of the package blank) or deep-drawing, where the coating continues uninterrupted over the folds of folded material or bends of the deep-drawn article.

When the coating is applied to packaging materials, the heat sealing properties of the dried coating can be used in sealing the package.

The invention is not imited to the above examples. The coating that is formed of the composition need not exist as uniform uninterrupted coating on the substrate, but it can also form areas, between which the original substrate remains exposed after the coating operation. The coating can thus form patterns, which may have functionality because of the electrical properties of the coating.

Claims

Claims:

1. Coating comprising polymeric binding agent and conductive particles in mixture, characterized in that the coating comprises:

aqueous dispersion-based film-forming elastic acryl copolymer as primary constituent in the elastic binding agent, and

electrically conducting particles mixed with the binding agent in such an amount that the surface resistivity of the dried coating is at the level of 1x10E3 - 1x10E1 1 ohm/square,

the mixture of the electrically conducting particles and the binding agent being substantially transparent.

2. Coating according to claim 1 , characterized in that the conducting particles are mineral particles that are coated to be electrically conductive

3. Coating according to claim 1 or 2, characterized in that conductive particles are in the coating in an amount of 5-60 wt-%, preferably 10 to 40 wt-%, calculated on the dry mass of the coating.

4. Coating according to claim 1 , 2 or 3, characterized in that conductive particles are present in at least two fractions with different average particle sizes. 5. Coating according to any of the preceding claims, characterized in that the basis weight of the coating is 1 ,

5 - 15 g/m2 calculated as wet coating.

6. Coating according to any of the preceding claims, characterized in that it exists as coating on top of packaging material.

7. Coating according to claim 6, characterized in that the transparent coating is on the surface of packaging material on top of impression.

8. Coating according to claim 6 or 7, characterized in that the packaging material comprises creases, folds or deep-drawn shapes.

9. Coating according to any of the preceding claims, characterized in that the surface resistivity of the coating is 1x10E6 - 1x10E8 ohm/square fulfilling ESD demands set on packages.

10. A package comprising packaging material coated with a coating according to any of the claims 1 to 9.