COATING COMPOSITION

TECHNICAL FIELD

The present invention relates to a coating composition and a substrate coated with said coating composition.

BACKGROUND OF THE INVENTION

A substrate derived from cellulosic material or paper for using in packaging application typically need a coating to improve its properties such as moisture and gas barrier, enhance heat seal ability and strengthen seal strength , print quality and printability on the substrate.

Currently, the cellulosic based substrate is typically coated with polyethylene (PE) by extrusion coating or lamination process. Polyethylene can be easily softened and bonded - together once heat seal. Alternatively, the coating can be performed by using waxes such as paraffin wax. However, polyethylene or wax coating cannot be recycled using existing process or machinery. The recycling of the coated substrate requires an investment on separating unit or special machinery or adding some chemical substance to separate the substrate components from said polyethylene or wax. Besides, the separated polyethylene and wax will not naturally degrade if the coated substrate is composted, thus causes negative impact on environment. Consequently, there is an effort to develop an aqueous coating composition for a substrate in place of the polyethylene or wax coating.

The following references disclose substrates coated with aqueous coating compositions.

US 8,202,623 B2 discloses a heat seal coated substrate which can be sealed at a temperature range of 40-90°C and exhibits smooth surface with good anti-blocking characteristics. Said coating composition comprises an aqueous dispersion of non-polar copolymer of polyethylene and alpha-olefin as main components. The coating composition is suitable for coating on polyolefin film or paper.

US 7,364,774 B2 discloses a method of producing multilayer coated substrate to improve water/oil resistance and/gas barrier properties, etc. The substrate is coated with various coating formulations to form multilayer coatings using a curtain coating. US 2011/0027601 Al discloses a coating composition which improves substrate's barrier properties and coating composition's stability. The coating composition comprises a polymeric binder, an amine stabilizer and fillers or nanoparticle fillers to enhance barrier properties. Further, the coating composition includes ethylene vinyl acetate (EVA) and ethyl methacrylate (EMA) or other polymers to form sealing layers for various sealing methods, such as heat sealing or ultrasonic sealing.

EP 1266997 Al discloses a heat seal paper with improved gas barrier property by a lamination of heat seal layer. Said heat seal layer comprises polyolefins synthetic pulp and synthetic fibers from polyethylene, polypropylene or polyester. Said composition is formed into a laminated film on the paper.

US 2011/0262745 Al discloses a coated paper or paperboard and a method for their production. The coating composition comprises polymer emulsion about 70-90 dry weight % of the coating composition and pigment about 10-30 dry weight % of the coating composition. The coating process is performed at an operating point located apart from the paper or paperboard machinery.

From the exemplary inventions described above, environmental problems of polyethylene or wax coatings seem to be minimized. However, the substrate coating according to those inventions require multilayer coating to achieve desired properties. Besides, the coating compositions and the coated substrates have some limitations that need improvement, such as water resistance, seal strength of heat-sealed substrate, printing quality of the coated substrate and recyclability of substrate components, etc.

SUMMARY OF THE INVENTION

This invention relates particularly to a coating composition comprising a binder in an amount of 50-90 dry weight % of the coating composition, a pigment in an amount of 10-50 dry weight % of the coating composition, and an additive in an amount of 0.2-7 dry weight % of the coating composition characterized in that the binder comprising ethylene vinyl alcohol (EVOH) polymer or polyvinyl alcohol (PVOH) polymer or a mixture thereof in an amount of 10-30 dry weight % of the coating composition, and styrene-butadiene emulsion in an amount of 40-70 dry weight % of the coating composition.

In addition, this invention relates to a substrate coated with the coating composition as described above where at least one side of the substrate is coated by the coating composition. The objective of this invention is to develop a coating composition comprising aqueous substances without addition of volatile organic solvents. The aqueous coating can be easily handled in an existing process and generates less pollution during the production. The coating composition of this invention, when coated on a substrate, can improve water resistance, seal strength and printing quality thereof even applying only one layer of the coating composition.

In addition, the substrate coated with the coating composition according to this invention has several advantages, for example:

- the coated substrate can be recycled to recover substrate components, e.g. pulp, for reusing in a substrate production without an investment on additional machines. Hence, it can reduce production cost and create value for a business resulting in environmental sustainability;

- the coated substrate can be used in various applications, e.g. heat-sealable packaging which provides good seal strength at low sealing temperature; and

- the coated substrate has good printability and printing quality. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph showing heat seal force values of samples: (la) a paper coated with the coating composition comprising a binder containing styrene butadiene emulsion with glass transition temperature (T _g ) in a range of 1-10°C; and (lb) a paper coated with the coating composition comprising a binder containing styrene butadiene emulsion with T _g in a range of 20-30°C.

Figure 2 is a graph showing heat seal force values of samples: (2a) a paper coated with the coating composition comprising a binder containing EVOH 1; (2b) a paper coated with the coating composition comprising a binder containing EVOH 2; and (2c) a paper coated with the coating composition comprising a binder containing PVOH 2.

Figure 3 is a graph showing heat seal force values of samples (3a) and (3b) according to this invention.

Figure 4 is a graph showing heat seal force values of samples: (4a) a commercial packaging paper; (4b) a paper coated with coating-S; and (4c) a paper coated with coating-S on one side and coating-P on the other side. Figure 5 is a graph showing heat seal force values of papers coated with coating-S on one side and coating-P on the other side at sealing time of 0.5 second and 1 second.

Figure 6 is photographs showing the printing quality of samples: (6a) a calendered commercial bag paper; (6b) a non-calendered paper coated with coating-P; (6c) a non- calendered paper coated with coating-S; (6d) a calendered paper coated with coating-P; and (6e) a calendered paper coated with coating-S.

Figure 7 is a graph showing printing density of the printed samples (6a) - (6e).

Figure 8 is photographs showing: (8a) an outer side of the coated paper according to this invention; (8b) an inner side of the coated paper according to this invention; and (8c) an enlarged photograph of inner side of the uncoated paper, when tested for water resistance.

Figure 9 is photographs showing: (9a) an uncoated paper as a comparative sample; and (9b) a paper coated with the coating composition according to this invention, when tested for water resistance.

DETAILED DESCRIPTION OF THE INVENTION

The following will disclose the details of the invention without any intention to limit the scope of the invention. Any aspects disclosed herein means applying to other aspects of this invention, except otherwise specified.

Definitions of technical or scientific terminology used herein are understood by those skilled in the art of this field, except otherwise specified.

All compositions and/or processes disclosed and claimed for this invention are intended to cover aspects of the invention, whether derived from the act, operation, modification or alteration of any factors, without any experiments substantially differed from this invention, and providing similar characteristics, benefits, and properties to the one(s) in this invention according to the opinion of the person skilled in the art, although not specified in specific claims. Hence, any substitution or similar thing(s) to the aspect of this invention including any minor modifications or changes clearly visible to persons skilled in the art should be considered under the intent, scope, and concept of this invention.

This invention discloses a coating composition comprising

- a binder in an amount of 50-90 dry weight % of the coating composition - a pigment in an amount of 10-50 dry weight % of the coating composition, and

- an additive in an amount of 0.2-7 dry weight % of the coating composition.

According to a preferred embodiment of the invention, the binder has the amount in a range of 60-80 dry weight % of the coating composition, the pigment has the amount in a range of 20-30 dry weight % of the coating composition, and the additive has the amount in a range of 0.5-5 dry weight % of the coating composition.

Preferably, the pigment is selected from clay, talcum, calcium carbonate, titanium dioxide, or a mixture of at least two of said pigments, more preferably, clay or calcium carbonate or a mixture thereof.

Preferably, the additive is selected from thickener, anti-blocking agent, dispersant, surfactant, or a mixture of at least two of said additives.

According to this invention, the binder preferably comprises ethylene vinyl alcohol (EVOH) polymer or polyvinyl alcohol (PVOH) polymer or a mixture thereof in an amount of 10-30 dry weight % of the coating composition, and styrene-butadiene emulsion in an amount of 40-70 dry weight % of the coating composition.

According to a preferred embodiment of this invention, the ethylene vinyl alcohol polymer or polyvinyl alcohol polymer has a degree of hydrolysis at least 85 mole %, more preferably at least 95 mole %, and the ethylene vinyl alcohol polymer has an ethylene content in a range of 1-30 weight %, more preferably 5-20 weight %.

A preferred ethylene vinyl alcohol polymer or polyvinyl alcohol polymer of this invention has a viscosity in a range of 5-30 centipoise at 4% polymer solution at 20°C, more preferably 10-25 centipoise.

A preferred styrene butadiene emulsion of this invention has a glass transition temperature (Tg) in a range of 1-30°C, more preferably 1-10°C. In another aspect, this invention discloses a coated substrate comprising a substrate and a coating composition coated on at least one side of said substrate where the coating composition has the characteristics as described above.

According to this invention, a preferred substrate is a cellulosic material, e.g. materials derived from unbleached chemical pulp, bleached chemical pulp, pulp from waste paper or recycled pulp. Preferably, the substrate has a weight in a range of 30-300 g m ² , more preferably 60-150 g/m ² .

According to the preferred embodiment, the coating composition has a solid content in a range of 15-60 dry weight % of the coating composition, and the coated substrate has a dry weight of the coating composition in a range of 1 -20 g/m ² .

Preferably, the substrate is coated with the coating composition at a temperature in a range of 25-70°C, more preferably 35-60°C.

The coated substrate according to this invention can be used for producing various products, especially for packaging (e.g. box or bag), where the coated side can be used for printing or heat seal. In addition, the substrate's components of disposal product can be recycled up to 80-90%.

The following is a more detailed description of the invention, including appropriate samples for this invention. It is not intended to limit the scope of the invention.

Preparation of coated substrates

A sample substrate is a paper with weight in a range of 30-150 g m ² , and Cobb's water resistance value at 1 min in a range of 15-35 g m . The paper was coated with the coating compositions according to various samples as described below. The coating is performed by using existing techniques (e.g. film press, rod coating, curtain coating) at temperature below 80°C, preferably in a range of 35-60°C. Then, the coated paper was dried by hot air. The preferred viscosity and solid content of the coating composition are in a range of 80-800 centipoise, and 20-50 wt%, respectively. However, the preferred viscosity and solid content of said coating composition are depended on many factors such as substrate properties, coating technique, coating composition properties and substrate applications, etc.

According to the sample(s) of the invention, the paper is coated on one side or two sides with at least one coating layer on each side. The coating on each side may be for various purposes according to the following definitions.

"Coating-P" according to this invention means a coating for printing purpose that a paper is coated on its printing side. The paper can, for example, be used for producing packaging. The aim of this coating-P is to enhance print quality, retard water absorption, and improve seal strength. According to this invention, the preferred coating composition for the coating-P is a coating composition comprising preferred compositions and characteristics according to this invention as mentioned above. A dry weight of the coating composition for coating-P is preferably in a range of 1-7 g m ² , more preferably 2-5 g m ² .

"Coating-S" according to this invention means a coating that a paper is coated on its heat seal side. The aim of this coating-S is to improve seal strength and retard water absorption. According to this invention, the preferred coating composition for the coating-S is a coating composition comprising compositions and characteristics according to this invention as mentioned above. A dry weight of the coating composition for coating-S is preferably in a range of 4-20 g m ² , more preferably 8-15 g m ²

According to this invention, the paper may be coated on one side with coating-P or coating-S, or both sides where one side coated with coating-P and the other coated with coating- S. Alternatively, the paper may be coated with coating-P or coating-S and further coated with other coating compositions aside from this invention to improve paper properties suitable for various applications. In case of coating-P, the paper surface may be calendered before and after coating to control an absorption of the composition into the paper providing uniform coating thickness and smooth coated surface suitable for printing.

Effect of styrene butadiene type on coating

The seal strength of the samples was conducted for the following:

(la) a paper coated with the coating composition comprising a binder containing styrene butadiene emulsion with T _g in a range of 1-10°C; and

(lb) a paper coated with the coating composition comprising a binder containing styrene butadiene emulsion with T _g in a range of 20-30°C.

The result of the seal strength was measured according to ASTM F88 standard as shown in Figure 1. The sample (la) has a higher seal strength than the sample (lb).

A folding endurance test was carried out according to ISO 5626 standard of samples (la) and (lb). The results are shown in Table 1.

Table 1 : Folding endurance of samples (la) and (lb)

Number Sample (la) Sample (lb)

1 3551 2608

2 3046 1877 3 3062 2109

4 3113 2376 mean (fold number) 3193 2243

log (number) 3.50 3.35

Table 1 shows that the sample (la) provides higher folding endurance value than the sample (lb).

Effect of the type of polyvinyl alcohol (PVOH) and ethylene vinyl alcohol (EVOH) on the coating

The effect of PVOH and EVOH with different properties was tested when applied as a binder of the coating composition. The properties of PVOH and EVOH samples are presented in Table 2.

Table 2 Properties of PVOH and EVOH

viscosity of 4% polymer solution at 20°C

The seal strength of the paper coated with the coating compositions having EVOH and PVOH samples in Table 3 was tested. Each specimen of the coated paper was heat sealed by an impulse sealer with a capacity of 2800 watts at a certain pressure which determined by the device. The heating level used for sealing can be adjusted from low to high (numbers 1-10). This experiment was tested at number of heating level for sealing number of 1-5 where the results for each sample are shown in Table 3.

Table 3 Heat seal ability of samples

Paper with coating Coating Number of heating level for sealing composition composition 1 2 3 4 5 weight (g m ² ) low temperature high temperature

Low density polyethylene film - SB MB MB MB

Paper coated with coating 5 - - SB SB MB composition comprising (low

styrene butadiene emulsion force)

(70%) and pigment (30%) SB

10 - (low SB MB MB force)

SB

15 - (low MB MB MB force)

Paper coated with coating SB composition comprising 5 - - - - (low styrene butadiene emulsion force) (50%), pigment (30%) and SB

PVOH 1 (20%) 10 - - - (low SB force)

SB

15 - - (low SB MB force)

Paper coated with coating SB

composition comprising 5 - - - (low SB styrene butadiene emulsion force)

(50%), pigment (30%) and 10 - - SB MB MB EVOH 1 (20%) SB

15 - (low SB MB MB force)

Remarks:

MB = Material break or failure at the substrate material

SB = Seal break or failure at sealing interface

From Table 3, it was found that the coating compositions comprising EVOH 1 provides better heat seal ability than the one comprising PVOH 1 as it can be sealed at lower temperatures and shows higher seal strength.

The seal strength test was conducted on the following samples: (2a) a paper coated with the coating composition comprising the binder containing EVOH 1;

(2b) a paper coated with the coating composition comprising the binder containing EVOH 2; and

(2c) a paper coated with the coating composition comprising the binder containing PVOH 2.

The seal strength of each sample is shown in Figure 2.

From Figure 2, it was found that the coating compositions comprising EVOH 1 and EVOH 2 (samples 2a and 2b) provide similar heat seal strength. In addition, the use of coating composition comprising PVOH 2, which has an equal viscosity to EVOH 1 (samples 2a and 2c), as a component of the binder provides similar heat seal strength.

Effect of percentage of styrene butadiene emulsion and ethylene vinyl alcohol polymer (EVOH) on coating

The experiment was conducted to study an effect of percentage of styrene butadiene emulsion and EVOH by measuring the seal strength of paper coated with the coating composition as shown in Table 4.

Table 4 Binder composition

The heat seal strength of samples (3a) and (3b) was measured as shown in Figure 3. From Figure 3, the ratio between EVOH and styrene butadiene emulsion within a range of samples (3a) and (3b) provides good heat seal strength. In addition, it was found that the EVOH ratio higher than 30% is not suitable for coating as it affects the flow properties of the coating composition.

Test results of coated substrates

1. Heat seal ability The heat seal ability was conducted according to the -ASTM F88 standard. The testing samples include: (4a) a commercial packaging paper, (4b) a paper coated with coating-S and (4c) a paper coated with coating-S on one side and coating-P on the other side.

From Figure 4, we can summarize as follows;

- The coated papers according to this invention, i.e. samples (4b) and (4c), provide higher seal strength than the comparative sample (4a), an average of 3-5 N/15 mm or 50 -100% at sealing temperatures of 130-210°C.

- Sample (4c) provides a maximum seal strength of 9.4 N/15 mm at a sealing temperature of 190°C.

- Sample (4c) provides a maximum seal strength of 9.8 N/15 mm at a sealing temperature of 170°C.

The heat seal ability of coated paper was also investigated.

According to the invention, the paper coated with coating-S on one side and coating-P on the other side was tested to obtain the heat seal strength at various temperatures as shown in Figure 5. The results show that the paper coated with the coating composition according to this invention can be heat sealed at relatively low temperature. That is, the paper coated with the composition according to this invention has clearly improved seal ability comparing with the comparative sample, although paper only coated by coating-S. Besides, paper coated with coating-S and coating-P provides higher seal strength, although using relatively low sealing temperature.

2. Printing quality

The printing quality of coated paper according to the invention was studied comparing between (6a) a calendered commercial bag paper, (6b) a non-calendered paper coated with coating-P, (6c) a non-calendered paper coated with coating-S, (6d) a calendered paper coated with coating-P and (6e) a calendered paper coated with coating-S.

Figure 6 is photographs showing printing quality of the samples (6a) - (6e) when printed with gravure solid printing. Figure 7 shows a printing density of the samples (6a) - (6e). Figures 6 and 7 shows that the coated paper according to the invention, both coating-P and coating-S, provide good printing quality with uniform ink receptivity and greater printing density than the comparative one. In addition, the calendering of the coated paper prior to printing improves the printing quality.

3. Water resistance

The water resistance of coated paper according to this invention was investigated by evaluating water absorption using distilled water with dye. The samples were soaked for 2 hours using Cobb's tester. The results are shown in Figure 8.

Figure 8 shows an enlarged image of a paper coated with the coating composition according to this invention where (8a) an outer image of the coated paper according to this invention, (8b) an inner image of the coated paper according to this invention and (8c) an inner image of the uncoated paper as a comparative sample. The coated paper according to the invention shows no water permeation to inside while non-coated paper, i.e. comparative sample, shows water permeation to inside.

In addition, Figure 9 shows additional experiments to assess the water resistance through sample (9a) an uncoated paper as a comparative sample and (9b) a paper coated with the coating composition according to this invention. After 3 hours, the image shows that the coated paper according to the invention has a significantly better water resistance than the comparative one.

4. Recyclability

The recyclability of paper pulp was studied comparing between the comparative sample (i.e. uncoated paper) and the coated paper according to this invention at 45°C. The results are shown in Table 5.

Table 5 Recyclability of samples

Sample Disintegration % Reject % Pulp accept % Loss

(revolution)

Comparative 25000 0 92 8 sample

50000 0 92 8

(Uncoated paper)

Coated paper 25000 14 84 2 according to the

50000 12 86 2 invention Table 5 shows that the coated paper according to this invention is capable to be recycled up to 80-90%.

Application of coated substrate

The coated substrates according to this invention can be used for both primary and secondary packaging. For example, the packaging may be bags with different designs according to their applications. Packaging can be formed by heat sealing, where the parameters for sealing depend on the characteristics of the packaging, production machinery and production process.

BEST MODE OF THE INVENTION

Best mode of the invention is as disclosed in the detailed description of the invention.