The present invention relates to an aqueous sizing dispersion comprising a ketene dimer based sizing agent and a process for production of paper comprising adding the aqueous sizing dispersion.
Cellulose-reactive sizing agents, such as those based on ketene dimers, also referred to as 2-oxetanone sizing agents, are widely used in papermaking at neutral or slightly alkaline stock pH's in order to give paper and paper board some degree of resistance to wetting and penetration by aqueous liquids. Paper sizes based on cellulose-reactive sizing agents are generally provided in the form of dispersions containing an aqueous phase and finely divided particles or droplets of the sizing agent dispersed therein. The dispersions are usually prepared by homogenising the hydrophobic, water insoluble material in an aqueous phase in the presence of a dispersant using shear forces and fairly high temperatures. The ketene dimers are made from different fatty acid mixtures which results in sizing agents with varying melting temperature ranges. Fatty acid mixtures containing fatty acids having carbon chains with irregularities like double bonds and/or chain branching usually have lower melting temperatures than regular ketene dimer sizing agent produced from fatty acid mixtures containing unsaturated straight chain fatty acids having mainly 16 and 18 carbon atoms. Ketene dimer sizing agents made from fatty acid mixtures rendering lower melting points have show improved properties compared to regular ketene dimer sizing agents. However, paper sized with dispersions containing ketene dimer sizing agents having lower melting ranges have impaired toner adhesion properties.
EP 0666368 A2 refers to the use of paper in high speed converting or reprographic operations where the paper is sized with a 2-oxetanone sizing agent which is not a solid at 35░ C. The sizing agent is made from a fatty acid selected from the group consisting of oleic, linoleic, linolenic or palmitoleic fatty acid chlorides or a mixture of them.
EP 0629741 A1 relates to paper sized with 2-oxetanone sizing agents having one and two 2-oxetanone rings. Exemplified saturated monocarboxylic acids are inter alia stearic, myristic, palmitic, decanoic, etc. However, the document is silent about molar ratios between specific saturated carboxylic acids.
EP 0742315 A1 discloses a sizing composition which is not a solid at 35░ C. comprising a mixture of 2-oxetanone compounds containing saturated and unsaturated fatty acids. Saturated fatty acids which are mentioned are stearic, palmitic, myristic and lauric, however, the document does not disclose specific molar ratios between specific saturated carboxylic acids.
WO 98141686 A1 refers to a method of manufacturing a paper grade whereby the paper is sized with a 2-oxetanone based size whereby at least one of the fatty acids used has a branched carbon chain with no double bonds. The only exemplified fatty acids are those containing 16 and 18 carbon atoms.
It is accordingly an objective of the present invention to provide an aqueous dispersion comprising a ketene dimer sizing agent which results in paper having improved toner adhesion properties.
A further objective of the present invention is to provide an aqueous dispersion which improves toner adhesion properties while maintaining desirable non-impact printing (NIP) properties of paper.
Yet a further objective of the present invention is to provide an aqueous sizing dispersion comprising a sizing agent having a lower melting temperature range than regular sizing agents rendering paper with improved toner adhesion properties.
Another objective of the present invention is that the paper obtained will improve performance in printers, e.g. OcÚ printers, such as less ketone transfer to the belt. Still further objects of the invention will appear hereinafter.
In accordance with the present invention it has surprisingly been found that paper with improved toner adhesion properties is obtained when an aqueous dispersion according to the claims is used for stock sizing or surface sizing of paper. More specifically, the invention relates to an aqueous sizing dispersion comprising a ketene dimer based sizing agent, wherein the sizing agent is obtained by
(i) providing a mixture of fatty acid halides comprising saturated fatty acid halides having 12 and 16 carbon atoms, the amount of fatty acid halides having 16 carbon atoms being from 20 up to 500 molar % based on total amount of fatty acids having 12 carbon atoms, and
(ii) removal of hydrogen halide from the saturated fatty acid halides thereby forming ketene dimer. The invention further relates to a process of producing paper comprising adding the aqueous sizing dispersion to a cellulosic suspension, forming and dewatering the suspension on a wire.
According to the present invention the ketene dimer based sizing agent is made from a mixture of fatty acids, suitably monocarboxylic fatty acids, comprising saturated fatty acids having 12 and 16 carbon atoms, whereby the amount of fatty acids having 16 carbon atoms is from 20 molar % up to 500 molar % based on total amount of fatty acids having 12 carbon atoms. Suitably, the ketene dimer based sizing agent is made from a mixture of fatty acids comprising saturated fatty acids having 12 and 16 carbon atoms, whereby the amount of fatty acids having 16 carbon atoms is from 25 molar % up to 400 molar % and most preferably the amount of fatty acids having 16 carbon atoms is from 30 molar % up to 300 molar % based on total amount of fatty acids having 12 carbon atoms. The amount of fatty acids having 14 carbon atoms present in the mixture may range from 20 molar % up to 500 molar % based on total fatty acids having 12 carbon atoms, suitably, from 30 molar % up to 400 molar % and more preferably from 45 molar % up to 400 molar %. By fatty acid is meant any carboxylic acid residue which can be used for the production of ketene dimer based sizing agents such as carboxylic (fatty) acid halides, suitably fatty acid chlorides. The amount of fatty acids in the mixture having 12 carbon atoms can be up to 45 weight % based on total fatty acids in the mixture, suitably, up to 40 weight % and more preferably up to 35 weight % based on total fatty acids in the mixture. Preferably, the fatty acid mixture comprises fatty acids containing 12, 14, 16 and 18 carbon atoms in a more or less equimolar amount.
Suitably, the fatty acids contained in the mixture have from 6 up to 26 carbon atoms and more preferably from 10 up to 20 carbon atoms which may contain irregularities such as chain branching and/or double bonds. The fatty acids contained in the mixture can be monocarboxylic acids and/or dicarboxylic acids. If dicarboxylic acids are present, multimers are formed. Preferably, the mixture is essentially free, or even more preferably, free from dicarboxylic fatty acids, thus, the only ketene dimer sizing agents formed are ketene dimers. Suitable saturated fatty acids present in the mixture include caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid and lignoceric acid and mixtures thereof. Examples of branched fatty acids are isostearic acid and isopalmitic acid.
According to one preferred embodiment of the present invention the ketene dimer based sizing agent is made from a mixture of fatty acids which is essentially free from branched fatty acids, suitably, the mixture of fatty acids is free from branched fatty acids. According to another preferred embodiment of the present invention the fatty acid mixture is essentially free from unsaturated and branched fatty acids. By essentially free is meant that the amount of branched and/or unsaturated fatty acids in the mixture is suitably less than 5 molar % based on fatty acid in the mixture, more preferably less than 2 molar %.
The ketene dimer sizing agent can be prepared using methods disclosed in EP 612739. The method suitably comprises providing fatty acid chlorides formed from a reaction mixture of fatty acids comprising saturated fatty acids having 12, e.g. lauric acid, and 16 carbon atoms, e.g. palmitic acid, whereby the amount of fatty acids having 16 carbon atoms is from 20 molar % up to 500% based on total amount of fatty acids having 12 carbon atoms by using phosphorous trichloride or other conventional chlorinating agents. The fatty acid chlorides are then dehydrochlorinated in the presence of a tertiary amine which functions both as a reactant and a solvent/diluent, whereby ketene dimers are formed. The ketene dimers are obtained by stripping off the tertiary amine followed by separation of formed crystals of tertiary amine hydrogen halide by acid extraction.
Preferably, the ketene dimer sizing agent comprised in the dispersion according to the invention is liquid in a temperature range of from −30 up to 70░ C., suitably from 10 up to 50░ C., most preferably from 30 up to 45░ C. By liquid is meant that the sizing agent per se flows without heat of fusion.
The mixture of the ketene dimer based sizing agent may be prepared by using methods for the preparation of ketene dimers or multimers. In the first step suitably acid chlorides are formed from a mixture of fatty acids using a chlorination agent. The fatty acid chlorides are then dimerised in the presence of tertiary amines such as triethyleamine to form ketene dimers or multimers.
The aqueous dispersion comprising a ketene dimer based sizing agent suitably contains agents which facilitate the formation of emulsions or dispersions such as polyelectrolytes. The polyelectrolytes present in the aqueous dispersion can be non-ionic, anionic, cationic or amphoteric. The cationic or anionic polyelectrolytes may contain groups of the opposite charge as long as the overall charge is anionic or cationic, respectively. The charged groups may be native or can be introduced synthetically. The polyelectrolytes, often referred to as dispersing agents, can be derived from natural or synthetic sources. Whereas suitable natural sources are polysaccharides such as starch originating from potato, corn, wheat, tapioca, rice, waxy maize or other starch containing natural sources, suitable synthetic polyelectrolytes include condensation polymers and vinyl addition polymers. Suitable condensation polymers include epihalohydrin polymers, polyamidamine polymers, polyamideamine-epichlorohydrin copolymers, polyamines, dimethylamine-epichlorohydrin coploymers, dimethylamine-ethylenediamine-epichlorohydrin copolymers, polyethyleneimine polymers, polyurethanes, naphtalene sulphonates. Suitable vinyl addition polymers include polyacrylamide and polyethyleneoxid.
Anionic polyelectrolytes may have a degree of substitution varying over a wide range; the degree of anionic substitution (DSA) can be from 0.01 to 1.4, suitably from 0.1 to 1.2 and preferably from 0.2 to 1.0. Cationic polyelectrolytes can have a degree of substitution (DSC) from 0.01 to 1.0, suitably from 0.1 to 0.8 and preferably from 0.2 to 0.6.
By dispersion is meant any aqueous medium comprising finely divided particles in solid and/or liquid state and/or particles in any state therebetween.
According to one preferred embodiment the aqueous sizing dispersion comprises a ketene dimer sizing agent and a cationic starch. The cationic substitution (DSC) of the starch may vary from 0.01 to 0.2, suitably from 0.1 to 0.15. Suitably, a further anionic compound is present in the dispersion such as naphthalene sulphonate condensation polymer in an amount from about 0.1 up to about 50% by weight, suitably from 5 to 40% by weight, based on the sizing agent.
According to yet another preferred embodiment the aqueous dispersion comprising a ketene dimer sizing agent is essentially free from starch which is liquid at a starch concentration of 30% by weight at room temperature or slightly above, i.e. a viscosity of less than 10000 cps.
Other suitable compounds can also be incorporated such as fungicides, alum, aluminium containing compounds and polyaluminium compounds.
The aqueous dispersion according to the invention can be anionic or cationic depending on the amount of anionic and/or cationic compounds such as polyelectrolytes/dispersing agents present in the dispersions.
The aqueous dispersion according to the invention can be produced by mixing an aqueous phase with the dispersing agent and optionally any other compound or compounds facilitating the formation of a dispersion and the ketene dimer based sizing agent, preferably at a temperature where the sizing agent is liquid and homogenising the composition so obtained suitably under pressure.
The aqueous dispersion containing ketene dimer sizing agents according to the invention can have ketene dimer contents within the range of from 5 up to 50% by weight and preferably from 10 up to 35% by weight, based on total dispersion.
The amount of dispersing agent/polyelectrolyte or any other charged components present in the dispersion according to the invention can be varied over a broad range depending on, among other things, type and charge density of the dispersing agent, type of sizing agent, desired anionicity/cationicity and solids content of the resulting dispersion. In the dispersion, the dispersing agent such a cationic polysaccharide, e.g. starch, can be present in an amount of up to 100% by weight, usually from 0.1 to 50% by weight, suitably from 5 to 40% by weight, based on the sizing agent.
The aqueous dispersion comprising a ketene dimer based sizing agent according to the invention can be used in a process for the production of paper comprising adding an aqueous dispersion of a ketene dimer sizing agent as described above to a stock containing cellulosic fibres and optional fillers, dewatering the stock on a wire to obtain paper. The aqueous dispersions may be used both for surface sizing and internal or stock sizing. Furthermore, the dispersions according to the invention are suitably added to a cellulosic wet stock comprised in a process for production of paper having runnability problems created by wet end and press section deposits. The term “paper”, as used herein, is meant to include not only paper but all types of cellulose-based products in sheet and web form, including, for example, board, cardboard and paperboard. The stock contains cellulosic fibres, optionally in combination with mineral fillers, and usually the content of cellulosic fibres is at least 50% by weight, based on dry stock. Examples of mineral fillers of conventional types include kaolin, china clay, titanium dioxide, gypsum, talc and natural and synthetic calcium carbonates such as chalk, ground marble and precipitated calcium carbonate. The present invention preferably relates to a method for the production of paper in which an aqueous dispersion, as defined above, is used as a surface or stock size. Suitably the amount of sizing agent either added to the stock containing cellulosic fibres, and optional fillers, to be drained on a wire to form paper, or applied on the paper surface as a surface size, usually at the size press, is from 0.01 to 1.0% by weight, based on the dry weight of cellulosic fibres and optional fillers, preferably from 0.05 to 0.5% by weight, where the dosage is mainly dependent on the quality of the pulp or paper to be sized, the cellulose-reactive sizing agent used and the level of sizing desired.
The dispersions according to the present invention are preferably used in a process for the production of paper where the stock comprises calcium carbonate such as chalk, preferably precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC). Whereas chalk and other ground carbonates are produced by the mechanical treatment, such as crushing and grinding, of naturally occurring materials, precipitated calcium carbonate is a different material produced synthetically in a chemical precipitation process, usually the carbonation process, where calcium hydroxide is converted to calcium carbonate by reaction with carbon dioxide. The stock can contain calcium carbonate like PCC in an amount up to about 35% by weight based on dry weight of cellulosic fibres, suitably from 5 to 30% by weight and preferably from 10 to 25% by weight.
Further additives which are conventional in papermaking may also be comprised in the stock whereto the aqueous dispersion is added such as dry drainage and retention agents, strength agents, wet strength agents, optical brightening agents, dyes, etc.