EP1280961A1

EP1280961A1 - Improved lubricant for coating webs

Info

Publication number: EP1280961A1
Application number: EP01942391A
Authority: EP
Inventors: Craig J. White
Original assignee: Akzo Nobel NV
Current assignee: Akzo Nobel NV
Priority date: 2000-01-13
Filing date: 2001-01-11
Publication date: 2003-02-05
Also published as: AU2001229389A1; WO2001051706A1; JP2003519731A

Abstract

Improved lubricants for addition to coating formulations for coating moving webs are provided. The lubricants are in a blend including lecithin and at least one fatty ester. They may have up to 5 % of a surfactant. The lubricant components are blended without the need for elevation of the temperature to 100 degrees F. or more. The fatty ester or fatty ester combination used is selected on the basis of the effect the fatty ester or fatty ester combination has on the high shear reaction of the mixture. These improved lubricant blends have improved storage stability, viscosity maintenance, coating homogeneity and structure maintenance. They provide improved performance over related prior art compositions as well as improved anti-weeping; high shear, and rheological characteristics. As a result coatings incorporating a lubricant blend of this invention have improve appearance, durability, and printability.

Description

IMPROVED LUBRICANT FOR COATING WEBS

BACKGROUND

The present invention relates to improved lubricants for coating moving webs. Of particular interest is that these lubricants have improved performance in high-speed applications without significant viscosity changes.

Paper and paperboard webs are often coated to improve properties such as printability and appearance. The coating mixtures used are applied to webs that are moving through the coating apparatus at high speeds. Typically these coating mixtures are composed of pigments, adhesive binders, and other additives which improve or modify the coating properties and characteristics of interest. Pigments used may include, among others, Kaolin and other clays, titanium dioxide, calcium carbonates or silicates, zinc oxide, talc, diatomaceous earth, synthetic polymers, and aluminum powders. Styrene butadiene latex, starches, and polyvinyl acetates are some of the binders that can be used. When the coating compositions have poor stability under processing conditions and/or storage conditions, the color distribution becomes non-uniform and undesirable. This can occur both from changes in the concentration distribution of the pigments used and from variations in the coating layer thickness.

Shear force and other chemical phenomena and physical forces can cause adverse effects such as in the viscosity, integrity and uniformity of the coating mixture and the coatings as they are applied to the moving webs. This can result in costly problems in both the coating application process and in the finish and quality of the coated paper products, problems that are well known among the manufacturers of coated paper products. As the speed of the web moving through the coating apparatus is increased, shear forces are increased on the coating mixture being applied to the web This increase in shear force can make any existing problems in the process and paper quality even worse Problems related to increased speed include ones in viscosity control, weeping, calcification, streaking, whiskeπng, and generally poor performance (or "runnabihty") To avoid the poor quality coated sheets resulting from these problems, improved uniformity of the coating mixture at these high shear levels is critical

Typical additives to improve performance, overall mtegπty, and uniformity of the coating and coating mixture are lubricant additives such as calcium stearate dispersions and polyethylene emulsions These lubricant additives in coating mixtures function primarily to increase the lubricity of coatings However there are many other effects from these lubπcants that are known to occur including, for example, effects on rheological properties, plasticity, smoothness, coating gloss, anti-dusting, and printing quality Not all of these effects are desirable Hence it would be advantageous to reduce the number of different additives to a coating mixture as much as possible without loss of the benefits of the given composition

Orthoefer in U S Patent No 4,200,551 describes the use of blends of crude lecithins and surfactants to form concentrated water dispersible lecithin additives The surfactants are polyoxyalkylene monoglyceπdes or diglyceπdes, polyoxyalkylene derivatives of partial fatty acid esters and hexitol anhydride, or combinations thereof Preparation of these blends is conducted at elevated temperature of between 40 and 100 degrees C (104-212 degrees F ) These '551 additives with improved dispersion in water are disclosed to be particularly suited for the food industry Kronstein et al. in U. S. Patent No. 4,520,153, No. 4,056,494 and No. 4,126,591 disclose the use of lecithin and lecithin derivatives for modification of pigments in paint coating compositions in order to get uniform pigment concentration throughout the subject coatings upon mixing even after extended storage. Of particular interest to the authors is the maintenance of uniform coloring when more than one pigment is used. The former two patents disclose the direct application to solid pigment particles of as little as 0.5% (based on the amount of pigment) lecithin in a fluid mixture in order to get improved stability and uniformity. The ' 153 patent is also interested in the provision of non-corrosive paints. Although a wax may be used with the lecithin, the use of fatty acid esters is neither taught nor required. The '591 patent further discloses that the use of at least 4% lecithin in the paint coating composition will provide similar improvements. For aqueous based paints, the use of lactic acid or its esters, or propylene glycol or alkyl esters of condensed ethylene glycol is taught. However, these dispersants are specified for use with the modified lecithins disclosed as needed for the aqueous systems. Use for highspeed web coating is not suggested.

In U.S. Patent No. 5,277,992 (Shinohara et al.), the use of two slip agents in a coating for a thermal transfer ink sheet is taught. One of the slip agents used can be a lecithin and/or a higher fatty acid alkyl ester. However, the second slip agent is taught to be selected from polyoxyalkylenealkyl (or aryl) phosphates.

Nikoloff et al. in U.S. Patent No. 4,766,015 tried to address the problems of high speed paper coating with additive mixtures of fatty acid (vehicle), surfactant (emulsifier), and crude lecithin components. The surfactant is required in these mixtures so that they are sufficiently dispersible and lubricating in order to perform as paper coatings. Further, it is stated that the surfactant is needed to keep the lubricant in the coating formulation since lecithin is not water dispersible. Optionally, these mixtures can contain a plasticizer such as a fatty ester. Examples describe preparation of these mixtures requiring agitation with heat at 120 degrees F. However these mixtures do not perform well in the more modern and higher speed coating processes such as with modem, high-speed printing equipment that have speeds greater than 6000 fpm, more than those speeds intended for use with the '015 coating mixtures.

Nikoloff tries to address these problems in U.S. Patent No. 5,858,933 with additive mixtures of chemically modified lecithins and (C 8-20) fatty esters. The preferred esters are methyl ester, butylene glycol ester, ethylene glycol ester, pentaerythritol ester, polyethylene glycol ester, polypropylene glycol ester, sorbitol ester, and trimethylol ethane ester. The chemical modifications of the lecithin are said to function as self-emulsifiers thereby permitting the omission of surfactants from the mixtures. Of particular focus in '933 is the preparation of stable clay coatings and printings at the very high speeds of modern coating and printing equipment. As in '015, mixtures of '933 are prepared under agitation with heat at about 130 degrees F.

Similarly, Mizobuchi et al. describe in U.S. Patent No. 5,196,080 the use of lecithins and/or fatty acid esters as release agents, emulsifiers, and surfactants in roll coatings, gravure coatings, and hot melt coatings. The coating mixtures are either melted or they are kneaded at 120 degrees C. The authors do not teach the use of unmodified lecithins with fatty acid esters as being of any particular benefit.

Williams et al. disclose the use of modified and/or unmodified lecithins in an aqueous release coating composition in U. S. Patent No. 5,284,690. The lecithin is in a mixture with a water-soluble polymer and/or a latex polymer. These polymers are said to be nonionically stabilized One group of stabilizers possible includes ethylene oxide derivatives of long chain fatty acids It is stated that the release coatings could be scrape-coated and applied by knife coating, roller coating, or pπnt coating All examples given contain less than 44% solids It is neither suggested nor taught that it is more beneficial to use both lecithin and fatty acid esters in the coating mixtures than using lecithin alone

SUMMARY OF THE INVENTION

The present invention relates to improved lubricant additive mixtures which do not require the addition of surfactants nor the chemical modification of lecithin in order to make coating mixtures with improved high stability at the very high speeds shear effects of modern coating and printing equipment This high stability is maintained even with a high solids content Surfactants and other additives frequently have multiple effects on lubπcant mixtures At times some of these effects are undesirable Hence it is preferable to have lubπcant mixtures with as few components as possible in order to get the best results uncomplicated by undesirable side effects of other additives In particular, the present inventive mixtures require neither ethanolamme nor dipropylene glycol as additives Further, the preparation of these inventive mixtures does not require heating to at least 104 degrees F as disclosed m the '551 , '080, '933, and '015 patents The components are slowly blended directly until the blend is completely homogeneous In addition, storage stability is improved over the related prior art additives such as those described in the '015 patent

Unlike the '015 mixtures which include fatty acids as mixture vehicles, the inventive mixtures herein described do not In the present inventive mixtures, fatty esters are combined with unmodified lecithin, unlike the required combination with modified lecithins in the '591 and '933 patents. High shear reaction level has been found to be controllable by the choice of fatty ester for use.

The improved lubricants of the present invention therefore provide for uniform coating structure with improved rheology, particularly with respect to viscosity reactions, when applied to webs in very high-speed processes The resultant coatings have superior uniformity, flow properties, and brightness

Further, the improved lubricants of the present invention provide a stable coating structure that effectively alleviates or minimizes problems such as non-uniform color appearance, component migration or bleeding, streaking, dusting, whiskeπng, blade scratching, and weeping Not only are these lubπcants more stable than related prior art lubricants heretofore used during the coating process, but also they are more stable during storage than the related prior art lubricant mixtures

Additionally, the improved lubricants for coating moving webs of the present invention maintain the uniform and stable coating structure even under the variations in pressure and the physical forces encountered during the coating process In other words, they are more resistant to the adverse effects of high shear and maintain stable viscosity within desired ranges irrespective of the high shear forces.

DETAILED DESCRIPTION OF THE INVENTION

Lecithin is a naturally occurring mixture of phospholipids found in animal and vegetable cells The lecithin mixture varies from source to source and includes phosphatidylcholine, phosphatidyhnositol, and phosphatidylethanolamines Chemically described, lecithin is a glycerol reacted with two fatty acids and with an amino group carrying phosphoric acid reacted with the remaining hydroxyl.

As described above, lecithin and its derivatives are preferred lubricants for use in the paper coating industry, particularly for moving web applications. These lubricants may be coated directly onto solid pigments, mixed into coating formulations for subsequent application to webs, or applied separately from the coating formulation such as by spraying onto the webs. However, given the limitations of the prior art lubricant mixtures and the continuing improvements in equipment speed, new and improved lubricants are constantly being sought.

The lecithin used with the present inventive mixtures is unmodified. It can be from plant and/or animal sources. Further, selected lecithin materials can be used such as a phosphatidylcholine or a phosphatidylcephalin alone.

The fatty acid esters for use include straight and branched alkyl esters as well as alkylene glycol esters and diesters of the (C₅-C₂₂) fatty acids. They may be used in combinations with each other and may optionally be substituted with (poly) oxyalkylene groups and/or other neutrally acting substituents. The fatty moieties in diesters may be the same or different. Typical fatty acid esters which can be used in the present inventive mixtures includes: methyl oleate; propylene glycol dioleate; butyl stearate; isopropyl myristate; ethyl oleate; octyldodecyl myristate; ethylene and propylene oxide derivatives of long chain fatty acids such as stearic, lauric, capric, oleic, myristic, palmitic, and caproic acids; sorbitan monostearate; pentaerythritol oleate; alkyl and mixed alkyl esters and diesters of ethylene, butylene or propylene glycols; alkyl esters of Tall Oil or of Red Oil; and combinations thereof. The present invention provides improvements in lecithin lubricant mixtures through the preparation of lecithin blends with fatty esters without the need for elevated temperature duπng the blending process. Clear, stable blends can be obtained without further additives to prevent cloudiness or precipitates. Reaction of the blends to high shear can be controlled by the choice of the fatty ester for use. For instance, use of methyl oleate stabilizes the lubπcant mixtures against viscosity shifts under high shear. Other fatty esters and combinations of fatty esters can raise or lower the stable high shear viscosity of the lubricant mixtures.

In general, the mixtures of the present invention contain from 50-85% lecithin, 15-50% fatty esters, and 0-5% surfactant. Exemplary of the surface active agents, whether used alone or in admixture, are the traditional amonic agents such as the alkali metal salts of fatty acids, alkyl sulfates, alkylsulfonates, alkylarylsulfonates, sulfosuccmates, alkyl phosphates, abietic acid salts, whether or not hydrogenated, nonionic agents such as polyethoxylated fatty alcohols, polyethoxylated and optionally sulfated alkylphenols, polyethoxylated fatty acids, etc.

Pigments can be used with these lubricant mixtures. The preferred pigments for use include calcium carbonate, titanium dioxide, precipitated calcium carbonate, delaminated clay, calcined clay, aluminum trihydrate, and combinations thereof. However, other pigments known in the industry can be used with the present inventive lubricant mixtures.

In the make down process, the components of the lubricant mixture are mixed in a reactor with blades on low in order to prevent the introduction of large volumes of air If the mixing becomes labored, slight heat can be applied just enough to reduce the viscosity sufficiently to allow for unlabored mixing. Build up of coating particles m a blade coater or excessive break down of coating structure can cause weeping in the coating process. In order to evaluate the anti-weeping properties of the product mixture, the product being tested is put into a sample coating formulation with 1 5 wet parts per hundred of pigment The initial test formulation contains diglyceπde. This initial test coating formulation is then allowed to sit for 2-4 hours m order to determine if the diglyceπde will begin to separate out

If this initial test is passed without separation of the diglyceπde, a coating formulation of the product being tested without diglyceπde is tested for viscosity response through Brookfield viscosity evaluation A dramatic change in viscosity upward or downward is considered a negative effect

If the product being tested does not show a negative effect in the viscosity response test, it is then tested for high shear and rheological effects using a Hercules high shear viscometer

First, dramatic shifts in high shear viscosity of the coating are looked for If they are absent, then slight shifts are looked for Depending upon the intended application, some slight shifts may be acceptable in the high shear viscosity of the coating

EXAMPLE 1

A first charge of 800 lbs of lecithin is put into a reactor with blades on low mixing speed A second charge of 200 lbs of methyl oleate is then mixed slowly into the lecithin until there is no separation of the two liquids The resultant product was stable upon storage for a day and provided resistance to high shear viscosity changes when added to coating formulations tested with a Hercules high shear viscometer EXAMPLE 2

A charge of 830 lbs. of lecithin is first placed in a reactor with the blade speed on low.

Then a charge of 150 lbs. of propylene glycol dioleate is mixed in until the blend is homogeneous. Lastly, a charge of 20 lbs. of Tergitol Np-9 is mixed in until the product is completely homogeneous. The resultant product was stable upon storage for a day and the stable high shear viscosity was raised.

COMPARATIVE EXAMPLE 1

Lecithin, Red Oil, and Trydet 2692A (12:5:3) were mixed under agitation at 120 degrees F. until homogeneous (according to the method in the '015 patent). The product was cloudy and after storage for a day presented with a brown precipitate.

COMPARATIVE EXAMPLE 2

The same combination of components used in the first comparative example was mixed similarly in a ratio of (21 : 14:5). Similar results were obtained as in the first comparative example.

COMPARATIVE EXAMPLE 3

Mixtures similar to comparative examples one and two were prepared in a similar manner with the addition of triethanolamine and dipropylene glycol. Products were clear and very stable, showing no precipitate after a day of storage.

The embodiments of the present invention described above and in the claims that follow are illustrative of the novel features of this invention. Various changes and modifications of the preferred embodiments will be apparent to those skilled in the art. Such changes and modifications are to be understood as within the scope of the present invention as defined by the appending claims, unless they depart therefrom.

Claims

What is claimed is:

1. An improved lubricant additive mixture adapted for combination with an aqueous- based paper or paperboard coating mixture, said lubricant additive mixture comprising:

(a) 50-85% by weight of lecithin;

(b) 15-50% by weight of at least one fatty ester, each of said at least one fatty ester having a corresponding high shear reaction; and

(c) 0-5% by weight of surfactant.

2. An improved lubricant additive mixture adapted for combination with an aqueous- based paper or paperboard coating mixture, said lubricant additive mixture comprising:

(a) 50-85% by weight of lecithin;

(c) 0-5% by weight of surfactant;

wherein said mixture is blended without elevation of the temperature except for slight heating, if needed, to reduce the mixture viscosity just enough to allow the mixing blades unlabored movement.

3. A lubricant additive mixture as described in claim 1 wherein said at least one fatty ester is selected from the group of propylene glycol dioleate, dipropylene glycol dioleate, methyl oleate and combinations thereof.

4. A lubricant additive mixture as described in claim 2 wherein said at least one fatty ester is selected from the group of propylene glycol dioleate, dipropylene glycol dioleate, methyl oleate and combinations thereof.

5. A lubricant additive mixture as described in claim 1 consisting of 80% lecithin and 20% methyl oleate.

6. A lubricant additive mixture as described in claim 2 consisting of 80% lecithin and 20% methyl oleate.

7. A lubricant additive mixture as described in claim 2 consisting of 83% lecithin and

15% propylene glycol dioleate, and further comprising 2% emulsifier, wherein said emulsifier is selected from the group of Tergitol Np-9, other emulsifiers and combinations thereof.

8. A lubricant additive mixture as described in claim 1 consisting of 83% lecithin and 15% propylene glycol dioleate, and further comprising 2% emulsifier, wherein said emulsifier is selected from the group of Tergitol Np-9, other emulsifiers and combinations thereof.

9. A method of preparing a lubricant additive mixture comprising lecithin and at least one fatty ester, said method comprising the step of blending said lecithin and said at least one fatty ester without elevation of the temperature to 104 degrees F. or greater.

10. A method as described in claim 9 which further includes the step of blending in a reactor with at least two speed settings, wherein one of said settings is a low setting, and with at least one blade wherein the speed of said at least one blade is kept on said low setting.

11. A method as described in claim 10 wherein said blending occurs without elevation of the temperature except for slight heating, if needed, to reduce the viscosity of said mixture just enough to allow unlabored movement of said at least one blade.

12. A method as described in claim 9 which further includes the steps of (a.) adding said lecithin first to a reactor; (b.) turning on the mixing mechanism prior to addition of said at least one fatty acid; and (c.) adding said at least one fatty ester second to said reactor.

13. A method as described in claim 1 1 which further includes the steps of (a.) adding said lecithin first to a reactor; (b.) turning on the mixing mechanism prior to addition of said at least one fatty acid; and (c.) adding said at least one fatty ester second to said reactor.

14. A lubricant additive prepared by the method described in claim 11.

15. A lubricant additive prepared by the method described in claim 9.

16. A lubricant additive mixture as described in claim 1 wherein said at least one fatty ester is selected according to the high shear reaction desired for said mixture which corresponds to the presence of said at least one fatty ester.

17. A lubricant additive mixture as described in claim 2 wherein said at least one fatty ester is selected according to the high shear reaction desired for said mixture which corresponds to the presence of said at least one fatty ester.

18. A lubricant additive as described in claim 15 wherein said at least one fatty ester is selected according to the high shear reaction desired for said mixture which corresponds to the presence of said at least one fatty ester.

19. A lubricant additive mixture as described in claim 1 wherein said lubricant additive mixture is a component of a web coating formulation containing at least one pigment, said at least one pigment selected from the group of Kaolin and other clays, titanium dioxide, calcium carbonates or silicates, precipitated calcium carbonate, zinc oxide, talc, diatomaceous earth, delaminated clay, calcined clay, aluminum trihydrate, synthetic polymers, aluminum powders, and combinations thereof.

20. A lubricant additive mixture as described in claim 2 wherein said lubricant additive mixture is a component of a web coating formulation containing at least one pigment, said at least one pigment selected from the group of Kaolin and other clays, titanium dioxide, calcium carbonates or silicates, precipitated calcium carbonate, zinc oxide, talc, diatomaceous earth, delaminated clay, calcined clay, aluminum trihydrate, synthetic polymers, aluminum powders, and combinations thereof.