Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2000-12-14
2003-12-30
Nguyen, Dean T. (Department: 3629)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S168300, C162S181500, C162S181700, C162S183000
Reexamination Certificate
active
06669816
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the manufacture of coated liquid packaging board, of the type using a sizing dispersion comprising an aqueous dispersion of rosin material, a synthetic sizing agent and an aluminum compound.
PRIOR ART REFERENCE
One of the main characteristics of paper and similar products is water-repellency or resistance to the penetration of water or other liquids, such as lactic acid, hydrogen peroxide solutions, etc. The main processes to confer hydrophobic properties on paper are stock sizing which affects the whole structure of the paper, surface sizing, more or less limited to the surface thereof, and combinations of both methods. The more common hydrophobic agents in stock sizing are rosins, synthetic sizing agents, such as alkyl ketene dimers, isocyanates, acid anhydrides and carbamoyl chloride and combinations of both components.
It has been observed recently, however, that in liquid packaging board the initial resistance to the edge penetration of solutions of lactic acid and hot hydrogen peroxide obtained with the traditional methods or products, is not maintained after a process of coating or conversion of the cardboard, a fact that causes financial and handling problems, and problems for the end user.
It should be borne in mind that coating is a rather complex treatment. The size of the fibers (0.1 to 3 mm) forming the paper leads to an irregular surface causing differences, throughout the sheet, in porosity, absorption, compressibility and printability. To make the surface uniform and improve the afore named properties, the paper or board is subjected to a coating process. Coating means applying to the surface a suspension composed of a binder or binders (latex, carboxy methyl cellulose, alginates, starches), pigments (inorganic compounds of the calcium carbonate type, kaolin, titanium dioxide), as well as other ancillary reagents to confer specific properties on the paper or board (whiteners, starches, sizing agents, etc). After the coated paper or board is dried and calendered, a uniform surface containing only micropores is obtained (unlike the macropores of the non coated paper or board).
The purpose of the coating is to confer specific properties, unlike those observed in the non coated paper or board: better appearance, less dimensional variance, improvement of optical properties (greater whiteness, opacity, gloss), improvement of the printing properties (greater clarity, less transparency on the back, deeper and more contrasted colors, less ink consumption), increase in hydrophobic properties, improvement of the mechanical properties (smoothness, strength), etc. It also involves a financial improvement on increasing the weight of the paper or board, with a surface layer, not formed by cellulose and cheaper. In general, where the properties of coated papers are concerned, further to the features in common with other papers, the properties derived from the coating have to be considered. The features of the coating layer depend on:
Composition of the coating liquid mixture or formula used (type of components and mutual proportions.
Amount of layer applied.
Application process (type of coating machine and drying system).
Mechanical finish or enhancement (calendering, brushing)
Some of the specific properties of the coated paper or board, already hinted at above, are:
Good tearing-off strength.
Water resistance
Low absorbency.
Very smooth surface.
Elasticity.
Opacity to avoid transparency.
Alkaline pH to avoid delay in ink drying.
Water insoluble components.
The support paper on which the coating is applied represents 70% of the weight (90% of the volume), while the coating layer represents the remaining 30% (25% pigments and 5% binders).
The applications of coated and non coated papers and boards are also different. The latter are directed to products having less added value, whereas the coated papers and boards have a better quality, higher value and are directed to a market segment also of higher quality. In this sense, the printing requirements are more demanding (definition of print, gloss, intensity and ink drying) or resistance to certain chemical agents (lactic acid and peroxides, such as is the case of coated board for packaging liquids, water, fats, etc) is needed.
It has been indicated that the stock sizing with rosin cationic sizes, such as those described in the documents ES-8900750. GB-2159153, EP-0 200 002, U.S. Pat. No. 3,966,654 and U.S. Pat. No. 4,199,369, or with a conventional anionic rosin size (emulsion, pastes or soap) gives the finished paper a good resistance to water penetration. They have, however, disadvantages such as certain pH and temperature limitations, the difficulty of conferring a good resistance to acid liquid penetration on the paper or the need to be used in comparatively high amounts to obtain a satisfactory degree of sizing. On the other hand, rosin dispersions are widely used and cannot be replaced by synthetic sizing agents in certain circumstances. Thus, for example, the rosin dispersions confer a good adhesion to the rolling cylinder, providing a better gloss of the paper.
The synthetic sizing agents react with the cellulose to give an irreversible bond. Although said sizing agents generally confer a good resistance both to water and to other liquids, they also suffer from certain drawbacks. For example, the sizing process should be carried out in a neutral or lightly alkaline medium (pH between 7 and 8.5) to be effective, hydrolysis reactions and loss of effectiveness can take place in water and it is not possible to achieve a good resistance to edge penetration of hot peroxide solutions. Also, some printing properties are relatively poor.
It is known that the combination of rosin and some synthetic sizing agents allows more generally applicable sizing agents to be obtained, a fact that allows to some of the above mentioned drawbacks to be overcome, when both components are used individually. For example, EP-A-0 074 544 describes a sizing method using cationic dispersions that contain as disperse phase particles of fortified rosin as well as particles of synthetic sizing agent. EP-0 275 851 describes a sizing method using of the above described anionic and cationic dispersions which also contain a polyaluminum compound. EP-0 693 589 describes a sizing method for paper and similar cellulose products containing precipitated calcium carbonate as filler, with the use of the above named dispersions. In WO 96/35841, a water-soluble inorganic alkali metal salt is added to improve the stability of cationic dispersions of agents based on rosin sizes or synthetic sizing agents. U.S. Pat. No. 4,522,686 describes a sizing dispersion formed by a synthetic sizing agent, fortified rosin and a water-soluble dispersing agent with nitrogen in its composition, the latter two being the components of a cationic rosin size. EP-A-0 292 975 describes a method to be used in the production of liquid packaging board. SU 1795994 describes a process for increasing the resistance to attack by hot hydrogen peroxide that comprises the successive addition of an alkylketenedimer emulsion and a rosin glue with an Al compound. In the above mentioned publications, nevertheless, at no time is there suggested the existence of any problem caused by a decrease in the resistance to edge penetration by lactic acid or hot hydrogen peroxide solutions after a coating or conversion process, that is to say, the board is within specifications in the reel but, after the said coating or conversion process, said resistance diminishes until reaching a level at which it remains constant.
Nevertheless, it has been observed that this drawback does really exist in coated boards. A negative interaction appears between the coating layer and the cellulose in this type of board when sized with the methods or products described up to now. This interaction involves a loss of the board's strength to lactic acid and peroxide over time. These parameters are very important in the end product quality, since they allow it to
Bisbal Tudela Josep Lluis
Poch Juan José Costas
Erplip S.A.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Nguyen Dean T.
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