Paper making and fiber liberation – Processes and products – Synthetic fiber
Reexamination Certificate
1998-01-05
2001-01-23
Silverman, Stanley S. (Department: 1731)
Paper making and fiber liberation
Processes and products
Synthetic fiber
C162S009000, C162S182000, C162S184000, C008S116100, C008S116400, C008S120000, C428S364000, C428S375000, C428S368000, C604S375000
Reexamination Certificate
active
06176973
ABSTRACT:
TECHNICAL FIELD
The invention relates to an absorbent material, substantially consisting of cellulose, in particular fluff pulp, and to a method for preparing the material, comprising defibering, i.e. individualizing the cellulose fibres and crosslinking them. The invention also relates to an absorbent body in which the material is included as the main component. A method for producing material which is used as a crosslinking agent for cellulose is also dealt with.
1. State of the Art
There has been an interest in chemically crosslinked fluff pulp for many years and it has been proposed that this pulp should be used in absorbent hygiene products. This proposal has first and foremost been based on the satisfactory properties of the crosslinked cellulose, especially with regard to absorption capacity, wet specific volume and absorption rate, properties which are particularly important when producing absorbent hygiene products such as nappies, sanitary towels, etc.
Several authors have proposed using di-, tri- or poly-functional carboxylic acids as the crosslinking reagent and performing the crosslinking reaction in the dry state.
WO 95/00703 (Norlander) describes an absorbent cellulose product which is characterized in that it possesses a fibre structure having improved compressibility under the influence of heat and pressure, which structure can be obtained by crosslinking cellulose fibres, which are impregnated with an effective quantity of crosslinking agent and at least one di-, tri- or poly-functional alcohol, in the dry state, with the fibres being individualized, i.e. defibered, prior to carrying out the crosslinking reaction. Di-, tri- or poly-functional organic acids, and also di-, tri- or poly-functional aldehydes, are mentioned, inter alia, as suitable crosslinking substances. The compressibility of the defibered and crosslinked cellulose product is influenced by adding a di-, tri- or poly-functional alcohol, expediently together with the crosslinking reagent. A suggested hypothesis regarding a probable chemical mechanism involves the hydroxyl groups of the polyfunctional alcohols reacting with the crosslinking reagent in the same manner as the hydroxyl groups of the cellulose. In this way, the alcohols are bound by covalent bonds to the cellulose molecule, whose swelling state is altered, with the resilience decreasing in the dry state. The cellulose fibres which are chemically crosslinked in this manner can be compressed to a substantially higher density, at greatly reduced pressure, as compared with cellulose fibre which is crosslinked in the dry state without any of the said alcohols being added. According to the said WO 95/00703, the cellulose fibres are crosslinked at a temperature which is expediently between 140 and 210° C. Despite the fact that it is suggested that alcohols having boiling points which exceed the temperature of the crosslinking reaction should be used in combination with the crosslinking reagent, it has nevertheless been found that there is a substantial emission of polyol (di-, tri- or poly-functional alcohol) in exhaust air from the reaction chamber for the crosslinking reaction. This implies environmental pollution with organic compounds and/or demands the installation and maintenance of purification equipment for the exhaust air from the plant. In addition, the emission involves a substantial loss of active compound.
2. Brief Disclosure of the Invention
The object of the present invention, like that of the invention which is described in WO 95/00703, is to offer a fibred cellulose product which is of the type specified in the preamble, which possesses a fibre structure having, as compared with the prior art, an improved, preferably controllable, compressibility, and which can be used in absorbent products, especially in absorption bodies which are intended to collect body fluids, for example those products which include baby nappies, sanitary towels and incontinence products, and especially to be able to offer thinner and more comfortable absorbent products—but nevertheless possessing good absorption properties—but with a substantially reduced emission of organic compounds when carrying out the crosslinking reaction (the setting).
These and other objects can be achieved by crosslinking the cellulose fibre by means of reacting the cellulose with an effective quantity of one or more water-soluble polymers, which have been prepared in advance, which have an average molecular weight, Mw, of between 350 and 70,000 g/mol, preferably between 350 and 25,000 g/mol, and expediently between 450 and 10,000 g/mol, and which possess free acid or aldehyde groups. Good results have been achieved using water-soluble polymers which have been produced by means of reacting at least one first di-, tri- or poly-functional compound, which is not a polyol, with at least one polyol, so that the said polymers are obtained which possess free acid or aldehyde groups. Typically, in these cases, the crosslinking substance consists of a mixture of water-soluble polymers which comprises reaction products of the type FP, FP
2
, F
2
P, F
2
P
2
, F
2
P
3
, etc., where F is that part of the polymer molecule which derives from the said first compound and P is that part of the polymer molecule which derives from the said polyol (di-, tri- or poly-functional alcohol). The polymers/oligomers possess functional groups, such as carboxyl or aldehyde, which are able to react with the hydroxyl groups of the cellulose on heating and, where appropriate, in the presence of a catalyst. If, for example, citric acid is reacted with glycerol, a series of polymers, especially oligomers, is formed: FP, FP
2
, F
2
P
2
, etc., where FP has a molecular weight Mw=266 g/mol. By means of using this product, which has been prepared in advance, as the crosslinking agent, the emission of glycerol from the crosslinking reaction oven can be reduced to less than 10% of the emission which has been measured when citric acid and glycerol are added separately, in accordance with the said WO 95/00703, and only brought to react with the cellulose fibres, and with each other, in connection with carrying out the crosslinking reaction.
In order further to reduce the emission of polyol, and also in order to offer a cellulose product which is improved still further, a di-, tri- or poly-functional alcohol having a higher molecular weight than glycerol, for example trimethylolpropane (TMP), can be used as the polyol. By means of polymerizing the citric acid and TMP using an esterification reaction, the quantity of free TMP in the fibre is reduced when the crosslinking reaction is carried out, which is analogous to the results obtained from the system citric acid and glycerol and which results in the emission of TMP in exhaust air from the crosslinking plant decreasing to an even greater extent than in the case of the system citric acid and glycerol.
The invention can be said to be a result of efforts to offer an absorbent cellulose product which, from the point of view of the user, possesses properties which are essentially at least as good as those of the product which is described in the said WO 95/00703 at the same time as the undesirable emission of chemicals during the setting reaction is essentially eliminated. These aims can be achieved by, for example, those chemicals which are added to the pulp separately, or at least unreacted, in accordance with WO 95/00703, being reacted with each other in advance so that a water-soluble polymer mixture is obtained which possesses functional acid or aldehyde groups. Experiments have demonstrated that the cellulose can be crosslinked with this polymer which has been prepared in advance, with the crosslinked pulp having the same satisfactory properties as were obtained when the preparations were added separately to the pulp. These results are surprising on the basis of the theories propounded in WO 95/00703. Without binding the invention to any novel or special theory, it is the inventor's judgement that that which is important for achieving the desired results
Fortuna Jos'e A.
Nixon & Vanderhye
Silverman Stanley S.
Stora Kopparbergs Bergslags Aktiebolag (publ)
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