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

Abstract

 An amino-aldehyde, eg urea-formaldehyde, resin is converted into fibres and used to form at least 1% by weight of the fibrous component of paper. The resin fibres, which are free of bound inorganic oxyacid radicals and carbohydrate, have a degree of cure of between 50 and 90%. The use of such partially cured resin fibres gives an improvement in the strength of the paper.

Description

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

[0002] In our German OLS Specification 2810299 (corresponding to UK Patent Application 10404/77) and European Patent Application 79.301669) 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 our German OLS Specification 2754525 (corresponding to UK Patent Application 55199/76) to incorporate certain inorganic ox^yacid radicals, eg sulphite radicals, into the fibres by reacting urea and formaldehyde in the presence of source of the inorganic oxyacid radicals, eg sodium metabisulphite, followed by conversion of the resin so produced into fibres. When made into paper, eg in admixture with cellulose pulps, 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 quantaties and/or with a long fibre pulp.

[0006] In our German OLS Specification 2819461 corresponding to UK Patent Application 19081/77 we indicate that the incorporation of compounds such as carbohydrates, eg 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] 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 free of carbohydrate and bound inorganic oxyacid radicals and having a degree of cure of between 50 and 90%. 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 without 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.

[0009] 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 5g) 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

[0010] Amino/aldehyde resin fibres that are free of bound inorganic oxyacid radicals and that have been described, for example in aforesaid German OLS 2810299 and European Application 79.301669, for use in paper compositions, have generally been cured using a catalyst such as ammonium sulphate for 3 or more hours at 120^oC. Such fibres have a degree of cure of 90 - 92% or more.

[0011] The fibres of use in the present invention have a degree of cure in the range 50 to 90%, preferably 60 to 85%.

[0012] The amino/aldehyde 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.

[0013] With urea, 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.

[0014] Some of the amine may be replaced by phenol. Preferred amino resins are made by reacting urea, optionally phenol and/or m,elemine 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.

[0015] As indicated hereinbefore, to avoid the production of fibres which give 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 radicals present during the reaction of the amine and aldehyde should be less than 0.5 mols per 100 mols of aldehyde.

[0016] After reaction of the amine 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 detmine the degree of cure for any given resin.

[0017] 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 will generally be within the range 0.05 to 1% by weight of the resin solids.

[0018] 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.

[0019] 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.

[0020] 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 German OLS Specification 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 UK patent 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 from fibres and stretch them, and the fibres are detached and collected.

[0021] Another, and preferred, method of forming the fibres is by centrifugal spinning, for example by the process described in our German OLS Specification 2810535 corresponding to UK Patent Application 10405/77.

[0022] 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.

[0023] 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.

[0024] For use in paper making the fibres are preferably unbranched and may be either straight or crimped. For applications 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.

[0025] 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 µm, 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.

[0026] When particularly smooth papers are required, it is preferred that there is an insignificant proportion of fibres of diameter above 25 µm.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] The amino aldehyde resin fibres preferably have an average strength of at least 50 MNm^-2 when measured on a tensile test.

[0031] 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 weigh 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.

[0032] The invention is illustrated by the following Example. Preparation of fibres.

[0033] 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. 10% by weight, based on the weight of resin solids of an aqeuous 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 German OLS Specification 2810535 using a spinning cup of 12.7 cm. diameter having 24 rectangular holes and rotating at 10000 rpm.

[0034] 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^-1. The fibres were continuously removed from the spinning apparatus and cured by heating in air at 200°C for 30 minutes.

[0035] The resultant fibres, which had an average diameter of 12 µm, had a degree of curing of 98.4%

Preparation of paper.

[0036] The fibres were disintegrated in a standard laboratory pulp disintegrator in water (consistency 0.3% by weight) to a length of about 2 mm.

[0037] 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.

[0038] 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 in which at least 1% by weight of its fibrous content consists of amino/aldehyde resin fibres free of carbohydrate and bound inorganic oxyacid radicals and having a degree of cure of between 50 and 90%.

2. Paper according to claim 1 wherein the fibrous content comprises 5 to 95% by weight of said amino/aldehyde resin fibres having a degree of cure of between 50 and 90% and, 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 µm to 30 um.

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.