Paper containing partially cured amino/aldehyde fibres and process for making it

Description

[0001] This invention relates to paper in which at least a proportion of the fibrous component consists of fibres formed from an amino/aldehyde resin.

[0002] In DE-A-2810299 and EP-A-9322 we describe papers made from blends of such amino/aldehyde, particular urea/formaldehyde, resin fibres, and certain cellulose pulps.

[0003] We have found that the properties of the paper may be improved if certain amino/aldehyde resin fibres are employed. In particular the tensile and burst strengths may be improved.

[0004] It has also been disclosed in DE-A-2754525 to incorporate certain inorganic oxyacid radicals, e.g. sulphite radicals, into the fibres by reacting urea and formaldehyde in the presence of source of the inorganic oxyacid radicals, e.g. sodium metabisulphite, followed by conversion of the resin so produced into fibres. When made into paper, e.g. in admixture with cellulose pumps, it is disclosed that these modified resin fibres give improvements in the paper strength compared to those made with unmodified amino-aldehyde fibres.

[0005] However we have found that these modified resin fibres, because of their ionic character, may cause problems of fibre flocculation in the paper making process, especially if the modified fibres are used in large quantities and/or with a long fibre pulp.

[0006] In our DE-A-2819461 we indicate that the incorporation of compounds such as carbohydrates, e.g. formose or glucose, into the resin composition gives fibres which donate improved strength to paper. However in the production of paper using such fibres there is a risk of build up of carbohydrate residues in the backwater which can give rise to effluent and slime problems.

[0007] We have now found that an increase in strength can be obtained using amino/formaldehyde resins and fibres that have been cured only to a certain degree.

[0008] It has been proposed in FR-A-2365001 to make paper from partially cured urea-formaldehyde resin fragments, e.g. foam fragments or fibres, and cellulose pulp and to effect further curing of the resin by heating the resultant paper. It is emphasised that the resin fragments should not be heated to above 60°C prior to forming the paper.

[0009] In the present invention paper is made from amino/aldehyde resin fibres that have been cured to 50-90% by heating at above 80°C prior to paper formation. Such paper has different properties from that described in aforementioned FR-A-2365001.

[0010] According to the present invention we provide paper in which at least 1 % by weight of the fibrous content consists of amino/aldehyde resin fibres that are free of carbohydrate and bound inorganic oxyacid radicals and that have been cured, prior to incorporation into the paper, to a degree of cure of between 30 and 90% by heating at above 80°C.

[0011] Such partially cured amino/aldehyde resin fibres have a controlled solubility in water: the degree of water solubility is such that the fibres are stable enough to be dispersed in water and incorporated into paper with unacceptable weight loss, and have adequate strength and resistance to damage by the paper making process but have the ability to bond with themselves and with cellulose fibres in paper structures.

[0012] The degree of cure is assessed by determining the proportion of fibre dissolved in water under specified conditions. The test procedure is as follows: A sample (approx 5 g) of the fibre that has previously been dried at 100°C is accurately weighed and then digested with 200 ml of water for 2 hours at 50°C. The undissolved fibre remaining is recovered by filtration and dried at 100°C in air for 2 hours and then reweighed. The (%) degree of cure is defined as

[0013] Amino/aldehyde resin fibres that are free of bound inorganic oxyacid radicals and that have been described, for example in aforesaid DE-A-2810299 and EP-A-9322, for use in paper compositions, have generally been cured using a catalyst such as ammonium sulphate for 3 or more hours at 120°C. Such fibres have a degree of cure of 90-92% or more.

[0014] The fibres of use in the present invention have a degree of cure in the range 50 to 90%, preferably 60 to 85%.

[0015] The amino/aidehyde resins are made, as is well known in the art, by reacting a polyfunctional amine, such as urea or melamine, with an aldehyde, particularly formaldehyde. The reaction is generally performed in aqueous solution using a molar excess of the aldehyde.

[0016] With area, the aldehyde/urea molar ratio is preferably in excess of 1.2:1 and is preferably in the range 1.5:1 to 2.5:1.

[0017] Some of the amine may be replaced by phenol. Preferred amino resins are made by reacting urea, optionally phenol and/or melamine in an amount of up to 1 mol of phenol and/or melamine per mol of urea, with formaldehyde, the amount of formaldehyde being such that the molar ratio of formaldehyde to urea (plus any phenol and/or melamine) is in the range 1.5:1 to 2:1.

[0018] As indicated hereinbefore, to avoid the production of fibres which gives rise to undesired additional stiffness to the paper, the reaction between the amine and aldehyde should be conducted in the absence of any significant amount of inorganic oxyacid radicals that can become bound into the resin. In particular the total amount of any sulphite, phosphite, phosphate, or borate should be less than 0.5 mols per 100 mols of aldehyde.

[0019] After reaction of the amino and aldehyde, a curing catalyst is added and the resin is spun into fibres. The nature and amount of catalyst, together with the spinning and any post spinning heat treatment conditions, will determine the degree of cure for any given resin.

[0020] Examples of weak catalysts include di(ammonium) hydrogen phosphate and ammonium formate: moderate catalysts include formic acid, ammonium sulphate, ammonium chloride and dihydrogen ammonium phosphate; while strong catalysts include phosphoric, sulphuric sulphamic and hydrochloric acids. The amount of catalyst employed with generally be within the range 0.05 to I % by weight of the resin solids.

[0021] The time necessary to effect the required degree of curing will naturally depend on the nature and quantity of catalyst employed, but for any given catalyst and concentration will depend on the curing temperature: at low temperatures longer times are required than at high temperatures. The curing will generally be effected at temperatures of 80 to 200°C: at temperatures below 100°C a time of several hours may be necessary while at above 180°C the curing time will generally be less than 5 minutes.

[0022] It will be appreciated that the curing conditions necessary to obtain the desired degree of cure can be determined by simple experimentation. In determining the curing time it should be appreciated that some curing may take place during the spinning process used to convert the resin to the fibrous form.

[0023] The resin may be converted into fibres by conventional spinning of a viscous resin syrup into hot air ("dry spinning") or into an acid bath ("wet spinning"). Alternatively the fibres can be formed by passing a fine stream or series of drops into a flowing resin-gelling liquid or by gas fibrillation (in particular air fibrillation) by means of a co-current or transverse gas stream. This process is described in the aforesaid DE-A-2754525. As a further alternative the resin may be spun by tack spinning, by pulling the resin between two surfaces to which it adheres and subsequently severing the fibres from the surfaces. For example, as described in GB-A-1141207 the resin may be moved into contact with a pair of belt surfaces so as to deposit it thereon, whereafter the surfaces of the belt are moved apart to form fibres and stretch them, and the fibres are detached and collected.

[0024] Another, and preferred, method of forming the fibres is by centrifugal spinning, for example by the process described in DE-A-2810535.

[0025] Preferably a spinning aid, such as a water soluble high molecular weight polymer, eg polyvinyl alcohol or polyethylene oxide, is added to the resin prior to spinning.

[0026] Fillers, pigments, optical brighteners and other additives may be added to the fibres provided they do not interfere with the attainment of the desired level of cure.

[0027] For use in paper making the fibres are preferably unbranched and may be either straight or crimped. For application involving use with cellulose fibres, it is desirable, for maximum strength, that only minor amounts of crimping be present. The fibres may be of circular or irregular cross section. Advantageously fibres of elliptical cross section are employed as these facilitate lay down of the paper sheet. The aforesaid centrifugal spinning process can give such fibres.

[0028] The fibres should have a mean diameter of 1 pm to 30 pm (for irregular fibres, average diameters are taken). More particularly the average is between 2 and 20 p, particularly between 5 and 15 µm. There may be present, advantageously, a range of fibre diameters from 1 pm to 30 pm to enable the formation of paper of uniform density.

[0029] When particularly smooth papers are required, it is preferred that there is an insignificant proportion of fibres of diameter above 25 pm.

[0030] The fibres used in the present invention, whether straight or crimped, characteristically have an average length of at least 1 mm. Long fibres (more than 2 mm) can be incorporated into papers without causing problems of premature flocculation in the paper making process and hence uneven formation of the sheet. It may therefore be desirable to use fibres that are as long as the papermaking process can accommodate. The practical upper limit to the length may therefore be, for this reason, in the range 5 to 10 mm. A minor degree of branching of the fibres may be present (due to fusion during production of the fibres) but preferably the fibres are essentially unbranched.

[0031] In the case of straight fibres, their linearity is preferably such that they can be compacted to a reasonably dense paper form. Crimped fibres tend to be bulky, and characteristically their bulk density is low.

[0032] Where necessary, the fibres produced by the spinning process may be reduced in length to that required for papermaking. This can be achieved by cutting, passing through rollers or milling, or by wet disintegration as is well known in the paper industry. The fibres should be cured to the desired degree of cure prior to any such disintegration process.

[0033] The amino aldehyde resin fibres preferably have an average strength of at least 50 MNm-² when measured on a tensile test.

[0034] Paper may be made from the amino/aldehyde resin fibres alone, or in admixture with other fibrous material, such as other synthetic resin fibres, or, preferably, with cellulose pulp. The paper should contain at least 1% by weight of the partially cured amino/aldehyde resin fibres and preferably contains at least 5% by weight of such fibres. Amino/aldehyde resin fibres having a higher degree of cure may be incorporated, if desired, in addition to the partially cured fibres. Preferably the fibrous component of the paper comprises 5-95% by weight of the partially cured amino/aldehyde resin fibres in admixture with cellulose pulp. The cellulose pulp may be a mechanical pulp or a chemical pulp. As is well known in the art the properties of the paper will depend on the nature of the pulp and its degree of beating.

[0035] The invention is illustrated by the following Example.

Preparation of fibres

[0036] A commercially available aqueous urea/formaldehyde resin having a U:F molar ratio of 1:2 of solids content 67% by weight was diluted with water to a viscosity of 30 poise (3 Pa's). 10% by weight, based on the weight of resin solids of an aqueous solution containing 1.6% by weight poly(ethylene oxide) and 6.7% by weight ammonium sulphate was mixed continuously with the resin solution as it was fed to a spinning cup of a centrifugal spinning apparatus. The resin was spun by the process described in aforesaid DE-A-2810535 using a spinning cup of 12.7 cm. diameter having 24 rectangular holes and rotating at 10000 rpm.

[0037] Air at 165°C was blown into the spinning chamber to transport the fibres from the spinning cup and to effect some curing. The resin was spun at a rate of 170 g min-'. The fibres were continuously removed from the spinning apparatus and cured by heating in air at 200°C for 30 minutes.

[0038] The resultant fibres which had an average diameter of 12 pm, had a degree of curing of 98.4%.

Preparation of paper

[0039] The fibers were distintegrated in a standard laboratory pulp disintegrator in water (consistency 0.3% by weight) to a length of about 2 mm.

[0040] Paper handsheets were made on standard pulp evaluation equipment from a mixture of equal weights of the UF fibres and a beaten birch sulphate pulp. The Burst Index (burst pressure in kNm-² divided by the substance in gm-²) was determined.

[0041] The above procedure was repeated using different resin spinning rates, air inlet temperatures (in the spinning apparatus), curing times and temperatures to achieve varying degrees of cure. The results are shown in the table.

Claims

1. Paper comprising amino/aldehyde resin fibres that are free of carbohydrate and bound inorganic oxyacid radicals, and optionally other fibrous material, in which the amino/aldehyde resin fibres form at least 1% by weight of the combined weights of said amino/aldehyde resin fibres and said other fibrous material, said amino/aidehyde resin fibres being partially cured prior to incorporation into the paper, characterised in that said amino/aldehyde resin fibres have been cured to a degree of cure of between 50 and 90% by heating at above 80°C prior to incorporation into the paper.

2. Paper according to claim 1 wherein its fibrous content comprises 5 to 95% by weight of said amino/aidehyde resin fibres having a degree of cure of between 50 and 90%, correspondingly, 95 to 5% by weight of cellulose pulp.

3. Paper according to claim 1 or claim 2 in which the amino/aldehyde resin fibres have a degree of cure of between 60 and 85%.

4. Paper according to any one of claims 1 to 3 in which the amino/aldehyde resin is a reaction product of urea, and, optionally, phenol and/or melamine in an amount of up to 1 mol of phenol and/or melamine per mol of urea, with formaldehyde, the amount of formaldehyde being such that the molar ratio of formaldehyde to urea, plus any phenol and/or melamine, is in the range 1.5:1 to 2:1.

5. Paper according to any one of the preceding claims in which the amino/aldehyde resin fibres have a mean diameter of 1 pm to 30 µm.

6. Paper according to any one of the preceding claims in which the amino/aldehyde resin fibres have an average length in the range 1 to 10 mm.

7. A process for making paper comprising (a) forming a carbohydrate free solution of (i) an amino/aldehyde resin that is free of bound inorganic oxyacid radicals and (ii) a curing agent into partially cured fibre, (b) forming an aqueous pulp of the fibres optionally together with other fibrous material, so that said amino/aldehyde resin fibres form at least 1 % by weight of the total weight of said fibrous material and said amino/aldehyde resin fibres in the pulp so formed, and (c) forming said pulp into paper, characterised in that said partially cured amino/aldehyde fibres have been heated, prior to incorporation into said pulp, at a temperature above 80°C until their degree of cure is between 50 and 90%.

Ansprüche

1. Papier, enthaltend kohlenhydratfreie Aminoaldehydharzfasern bzw. Aminoplastfasern, die frei von gebundenen Resten anorganischer Oxosäuren sind, und gegebenenfalls anderes Fasermaterial, wobei die Aminoaldehydharzfasern mindestens 1 Gew.-% des Gesamtgewichts der Aminoaldharzfasern und des anderen Fasermaterials bilden und vor dem Einmischen in das Papier teilweise gehärtet worden sind, dadurch gekennzeichnet, daß die Aminoaldehydharzfasern vor dem Einmischen in das Papier durch Erhitzen bei mehr als 80°C bis zur Erzielung eines Härtungsgrades zwischen 50 und 90% gehärtet worden sind.

2. Papier nach Anspruch 1, wobei der Faseranteil des Papiers aus 5 bis 95 Gew.-% der Aminoaldehydharzfasern mit einem Härtungsgrad zwischen 50 und 90% und dementsprechend aus 95 bis 5 Gew.-% Zellstoff besteht.

3. Papier nach Anspruch 1 oder 2, worin die Aminoaldehydharzfasern einen Härtungsgrad zwischen 60 und 85% haben.

4. Papier nach einem der Ansprüche 1 bis 3, worin das Aminoaldehydharz ein Reaktionsprodukt von Harnstoff und gegebenenfalls Phenol und/oder Melamin in einer Menge von bis zu 1 mol Phenol und/oder Melamin pro Mol Harnstoff mit Formaldehyd ist, wobei der Formaldehyd in einer solchen Menge eingesetzt wird, daß das Molverhältnis von Formaldehyd zu Harnstoff plus gegebenenfalls eingesetztem Phenol und/oder Melamin in dem Bereich von 1,5:1 bis 2:1 liegt.

5. Papier nach einem der vorhergehenden Ansprüche, worin die Aminoaldehydharzfasern einen mittleren Durchmesser von 1 ,um bis 3µ µm haben.

6. Papier nach einem der vorhergehenden Ansprüche, worin die Aminoaldehydharzfasern eine durchschnittliche Länge in dem Bereich von 1 bis 10 mm haben.

7. Verfahren zur Herstellung von Papier, bei dem (a) aus einer kohlenhydratfreien Lösung (i) eines Aminoaldehydharzes, das frei von gebunden Resten anorganischer Oxosäuren ist, und (ii) eines Härtungsmittels teilweise gehärtete Fasern gebildet werden.

(b) aus den Fasern gegebenenfalls zusammen mit anderem Fasermaterial ein wäßriger Faserbrei so gebildet wird, daß die Aminoaldehydharzfasern mindestens 1 Gew.-% des Gesamtgewichts des Fasermaterials und der Aminoaldehydharzfasern in dem so gebildeten Faserbrei bilden, und

(c) aus dem Faserbrei Papier gebildet wird, dadurch gekennzeichnet, daß die teilwise gehärteten Aminoaldehydharzfasern von dem Einmischen in den Faserbrei bei einer Temperatur von mehr als 80°C erhitzt worden sind, bis ihr Härtungsgrad zwischen 50 und 90% liegt.

Revendications

1. Papier comportant des fibres de résine amino/aldéhyde ne renfermant pas d'hydrate de carbone et de radicaux d'oxacides inorganiques fixés, et éventuellement une autre manière fibreuse, dans lequel les fibres de résine amino/aldéhyd représentent au moins 1 % en poids des poids combinés de ces fibres de résine amino/aldéhyde et de cette autre matière fibreuse, lesdites fibres de résine amino/aldéhyde étant partiellement durcies ou réticulées avant l'incorporation au papier, caractérisé en ce que ces fibres de résine amino/afdéhyde ont été réticulées à un degré de réticulation compris entre 50 et 90% par chauffage à une température supérieure à 80°C avant l'incorporation au papier.

2. Papier suivant la revendication 1, caractérisé en ce que sa teneur en fibres représente de 5 à 95% en poids de fibres de résine amino/aldéhyde ayant un degré de réticulation compris entre 50 et 90%, et de façon correspondante entre 95 et 5% en poids de pâte de cellulose.

3. Papier suivant la revendication 1 ou 2, caractérisé en ce que les fibres de résine amino/aldéhyde ont un degré de réticulation compris entre 60 et 85%.

4. Papier suivant l'une quelconque des revendications 1 à 3, caractérisé en ce que la résir.e amino/aldéhyde est un produit résultant de la réaction d'urée et éventuellement de phénol et/ou de métamine suivant une quantité allant jusqu'à 1 mole de phénol et/ou de mélamine par mole d'urée, avec du formaldéhyde, la quantité de formaldéhyde étant telle que le rapport molaire entre le formaldéhyde et l'urée, plus le phénol et/ou la mélamine éventuellement présent, soit compris dans une gamme allant de 1,5:1 à 2 1.

5. Papier suivant l'une quelconque des revendications précédentes, caractérisé en ce que les fibres de résine amino/aldéhyde ont un diamètre moyen compris entre 1 et 30 microns.

6. Papier suivant l'une quelconque des revendications précédentes, caractérisé en ce que les fibres de résine amino/aldéhyde ont une longueur moyenne comprise entre 1 et 10 mm.

7. Procédé pour la fabrication de paper consistant (a) à traiter une solution ne contenant pas d'hydrate de carbone de (i) une résine amino/aldéhyde qui ne renferme pas de radicaux d'oxacides inorganiques fixés et (ii) un agent de réticulation pour donner des fibres partiellement réticulées, (b) à former une pâte aqueuse de fibres éventuellement conjointement à une autre matière fibreuse, de telle sorte que ces fibres de résine amino/aldéhyde représentent au moins 1% en poids du poids total de cette matière fibreuse et de ces fibres amino/aldéhyde dans la pâte ainsi formée, et (c) à convertir cette pâte en paper, caractérisé en ce que ces fibres amino/aldéhyde partiellement réticulées ont été chauffées avant leur incorporation à la pâte à une température supérieure à 80°C jusqu'à ce que leur degré de réticulation soit compris entre 50 et 90%.