Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2000-05-15
2001-10-23
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S175000, C162S177000, C162S179000, C162S183000
Reexamination Certificate
active
06306255
ABSTRACT:
BACKGROUND
Cellulose-reacive sizing agents, such as those based on alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA), 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 with the aid of a dispersant system consisting of an anionic compound, e.g. sodium lignosulfonate, in combination with a high molecular weight cabonic or amphoteric polymer, e.g. cationic starch, polyamine, polyamideamine or a vinyl addition polymer. Depending on the overall charge of the compounds of the dispersant system, the size dispersions will be cationic or anionic in nature.
Cellulose-reactive sizing agents generally provide good sizing with low dosages of the sizing agent However, it has been experienced that the efficiency of conventional cellulose-reactive sizing agents is deteriorated when they are used with stocks having a high cationic demand and containing a substantial amount of lipophilic wood extractives, such as, for example, resin acids, fatty acids, fatty esters, triglycerides, etc. Due to the anionic character of lipophilic substances containing carboxylate or carboxylic acid groups, stocks containing a substantial amount of lipophilic extractives usually have a rather high cationic demand. It has been found that the lipophilic substances can be detrimental to the adsorption of sizing agents onto the fibers which may cause the poor sizing results. In order to improve sizing with such stocks the papermaker has had to increase be dosage of sizing agent, which of course is less favorable economically and can increase the accumulation of sizing agent in the while water recirculating in the papermaking process. These problems are even more pronounced in paper mills where white water is extensively recirculated with the introduction of only low amounts of fresh water into the process, thereby further increasing the cationic demand and the accumulation of lipophilic extractives and non-retained sizing agent in the white water and the stock to be dewatered.
It is accordingly an object of this invention to provide improved dispersions of cellulose-reactive sizing agent and processes resulting in improved sizing where dispersions of cellulose-reactive sizing agent are used with cellulosic stocks having a high cationic demand and/or a high content of lipophilic extrusive and/or in processes with extensive while water recirulation.
THE INVENTION
According to the present invention it has been found that improved sizing can be obtained by using aqueous dispersions of cellulose-reactive sizing agent having a hydrophobe-modified dispersing agent in the internal sizing of stocks having a high cationic demand and/or containing high levels of lipophilic substances. It has also been found that improved sizing can be obtained by using such dispersions in papermaking processes where white water is extensively recirculated, thereby creating conditions of high cationic demand and/or high contents of lipophilic extractives. Accordingly, the invention generally relates to the use of an aqueous sizing dispersion containing a cellulose-reactive sizing agent and a hydrophobically modified dispersing agent in the internal sizing of difficult-tosize stocks by adding the sizing dispersion to a stock containing cellulosic fibers, and optional fillers, and dewatering the stock on a wire to obtain a web containing cellulosic fibers, or paper. In a first aspect of the invention, the stock has a cationic demand of at least 50 &mgr;eq/litre stock filtrate. In a second aspect of the inverntion, the stock has a content of lipophilic substances of at least 10 ppm. A third aspect of the invention comprises dewatering the stock on a wire to obtain white water and a web containing cellulosic fiber3, recirculating the white water, and optionally introducing fresh water, to form a stock containing cellulosic fibers to be dewatered, wherein the amount of fresh water introduced is less than 30 tons per ton of dry cellulosic product produced. The invention thus relates to the use of an aqueous sizing dispersion containing a cellulose-reactive sizing agent and a hydrophobically modified dispersing agent in a process for sizing cellulosic fibers, as further defined in the claims.
In a preferred embodiment of this invention, the aqueous dispersion contains a cellulose-reactive sizing agent and an anionic hydrophobe-modified dispersing agent, notably an anionic hydrophobically modified cellulose-derivative, and this invention further relates to such a dispersion and its preparation, as further defined in the claims. The dispersion provides improved stability and sizing characteristics and it is particularty useful with papermaking stocks having a high cationic demand and containing lipophilic substances, notably with a high degree of white water closure.
The invention makes it possible to produce paper with improved sizing over conventional sizing dispersions at a corresponding dosage of cellulose-reactive sizing agent and to use a lower dosage of cellulose-reactive sizing agent to attain a corresponding level of sizing. The possibility of using lower amounts of sizing agent to attain in-specification sizing reduces the risk of accumulation of non-adsorbed sizing agent in the white water recirculating in the process thereby reducing the risk of aggregation and deposition of the sizing agent on the paper machine. The invention thus offer substantial economic and technical benefits.
The cellulose-reactive sizing agent according to the invention can be selected from any of the cellulose-reactive sizing agents known in the art Suitably the sizing agent is selected from the group consisting of hydrophobic ketene dimers, ketene multimers, acid anhydrides, organic isocyanates, carbamoyl chlorides and mixtures thereof, preferably ketene dimers and acid anhydrides, most preferably ketene dimers. Suitable ketene dimers have the general formula (I) below, wherein R
1
and R
2
represent saturated or unsaturated hydrocarbon groups, usually saturated hydrocarbons, the hydrocarbon groups suitably having from 8 to 36 carbon atoms, usually being straight or branched chain alkyl groups having 12 to 20 carbon atoms, such as hexadecyl and octadecyl groups. Suitable acid anhydrides can be characterized by the general formula (II) below, wherein R
3
and R
4
can be identical or different and represent saturated or unsaturated hydrocarbon groups suitably containing from 8 to 30 carbon atoms, or R
3
and R
4
together with the —C—O—C— moiety can form a 5 to 6 membered ring, optionally being further substituted with hydrocarbon groups containing up to 30 carbon atoms. Examples of acid anhydrides which are used commercially include alkyl and alkenyl succinic anhydrides and particularly isooctadecenyl succinic anhydride.
Suitable ketene dimers, acid anhydrides and organic isocyanates include the compounds disclosed in U.S. Pat. No. 4,522,686, which is hereby incorporated herein by reference. Examples of suitable carbamoyl chlorides include those disclosed in U.S. Pat. No. 3,887,427 which is also incorporated herein by reference.
In addition to the cellulose-reacive sizing agent, the size dispersions may also contain a non-cellulose-reactive sizing agent Examples of suitable sizing agents of this type include rosins, e.g. fortified and/or esterified rosin, waxes, fatty acid and resin add derivatives, e.g fatty amides and fatty esters, e.g. glycerol triesters of natural fatty acids.
The dispersion used according to the invention contains a hydrophobically modified dispersing agent, i.e. a dispersing agent having one or more hydrophobic groups. Preferably the hydrophobic group is pendent, i.e. being attached to the dispersing agent in the form of a hydrophobic side-chain. E
Frolich Sten
Lindgren Erik
Sikkar Rein
Akzo Nobel nv
Chin Peter
Parker Lainie E.
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