GB2241966A - Process for the manufacture of paper - Google Patents

Abstract

The process treats a vegetable fibre paper base having a high cellulose polymerisation index with a polyvinyl alcohol based paste and a paste to improve the dry strength of the paper, after which the base is hot pressed. The resulting paper is particularly suitable for the manufacture of bank notes and other valuable documents.

Description

PROCESS FOR THE MANUFACTURE OF PAPER FOR USE AS BILLS The present invention relates to a process for the manufacture of paper, specifically safety paper, for instance that used in notes and other valuable documents.

The process subject hereof is designed to provide specific dirtprocf treatment to the paper, which treatment need not use any component or product that releases formaldehyde.

It is a well-known fact that safety paper, used as a common substrate to manufacture bank notes and other valuable documents, has remarkable physical properties so far as mechanical resistance is concerned, manufacture thereof normally involving the use of raw materials such as vegetable fibres having a high alpha cellulose content, for instance cotton, linen and other annual plant fibres. These fibres are rather easy to bleach because of the low lignin content of the plants they are taken from and provide a remarkably strong fibrous web. In fact, a typical safety paper is capable of withstanding over 1 kgvmm lengthways of manufacture, or 2,000 double-foldings and a tearing force of 60 grammes under normal respective standardised test conditions.It should also be able to be simultaneausly offset, letterpressed and copper engraved.

However, deterioration of the paper is not the reason why used notes are withdrawn by the issuing bank in frost developed countries. This takes place because of the dirt they gradually acxxnmulate, stunk to their surface or cnwhined with their structure.

Dirt is therefore, in practice, the st important determinant factor when establishing the "life" of a bank note or similar bill. The mechanism whereby dirt sticks to the paper varies widely according to the type of dirt, which can in principle be classified as being liquid or solid, and the former may at the same time be subdivided into liquid dirt due to aqueous or oily and solvent substances.

It is a well-known fact that the vegetable-fibrous web making up the paper is bonded when the recently formed sheet of paper is dried in the paper making machine. This bond is therefore sensitive to water amongst other liquids which may cause the bonds between fibres to be severed, moistening and breaking the same up.

In order to overcome this problem, the paper has usually been provided with substances, generically denominated "pastes" since they originally derived from paste, rendering them waterproof after machine drying. Their function is to delay penetration of fluids such as water, oil and so on inside the paper. Other products were subsequently developed which, without modifying the speed or expeditious penetration of fluids in the paper, improved its moisture strength.

All these substances have, to a greater or lesser extent, assisted in protecting the paper from the main source of its real deterioration, namely dirt. When producing note and other safety paper the type of pasting used imitates archaic processes which involve dipping or pressing the ready-formed paper in a gelatin bath, or synthetic substitutes thereof, preferably polyvinyl alcohol based. Two main classes of moisture strong agents or resins are normally employed: melamine formaldehyde based, essentially in an acid medium, or neutral or alkaline medium based epichlorhydrin polyamine. Gelatin based pastes usually carry a second aldehyde based component such as formaldehyde for insolubilization thereof.It is therefore very usual for note or other safety paper to release formaldehyde when pasting or curing or polymerizing the m lamine resin, which gradually releases formol. These notes are therefore hazardous to the health during large-scale handling during production.

Production of pasted and moisture strong paper is technically possible without formaldehyde, but the resulting moisture strength and even dry strength, critical for the basic structure to be maintained, is usually far lower than if substances releasing formol were used.

Dip or press pasting is very significant in safety paper strength, in addition to providing the same with its partially fluid repellent properties. Paper is a highly anisotsopic material whose characteristics are strongly dependent upon the orientation in which they are measured, i.e., the machine or transverse direction. By pasting, this anisotropy is usually reduced, the cellulose fibres are protected from external attack and pores allowing access of foreign dirtenerating substances are covered, in addition to a compact mass being formed improving bond between fibres.

Each dip or press paste also has specific characteristics.

Thus, gelatin is heavily grease repellent, though waterproofing ability is rather low, and decreases in time and the paper surface guarters through handling by the public. Therefore, come a given point of tiztia, notes pasted with gelatin rapidly deteriorate.

Polyvinyl alcohol has better characteristics to be provided with waterproof properties though, on the other hand, it is not as oilrepellent as gelatin. This property can be positively used to obtain better printing properties than those of gelatin paper, besides the fact that, in the case of polyvinyl alcohol paper, deterioration due to circulation can be more gradual. However, there are rheological limitations in achieving a good impregnation of the web of paper manufactured with a strong dose of polyvinyl alcohol.

Something that could be said to be highly influential in the rapid deterioration of a material such as paper could be summed up using the term lageing". Natural oxidation of any chemical organic substance in the presence of air, accelerated by adverse environmental conditions, such as high temperature and humidity, especially in tropical and marine climates, actually degrades cellulose, gelatin and polyvinyl alcohol, as well as the other, mostly organic, components of paper. Frequently, dirt includes germs, specifically bacteria and above all fungi, which accelerate the natural ageing process since they feed off organic matter.

According to - all the foregoing constraints, an ideal safety paper should only be provided with raw materials which are as pure as possible, with the greatest chain length in the case of polymers, in order that degradation thereof be as slow as possible. The paper should have a high dry and moisture strength, a waterproof pasting effect and be slightly oilrepellent in order to expedite optimum printability thereof.

Its structure should be as stable as possible to resist normal folding, tearing, tensions, creasing and other treatment during public circulation. It should not be prone to develop spaces or surfaces to which solid dirt can stick.

Thesmost important limitation when fulfilling the above using polyvinyl alcohol is that its high molecular weight and long chain length gives it a high viscosity and therefore it becomes unwieldy above 6% at the ordinary working temperatures used in pasting, roughly 500c.

The process for the manufacture of safety paper according to the present invention fully solves the above problems. It relies on using polyvinyl alcohol's self-polymerization capacity under suitable conditions, as well as using low viscosity and low polymerization index polyvinyl alcohol, to yield a structure filling all gaps left by the cellulose, in a greater proportion than is usual in known processes in the manufacture of notes and similar papers.

More particularly, it involves starting off with a cotton or linen paper base, having a high cellulose polymerization index and manufactured with fibres bleached above 900 and refined at over 50 SR (Schopper-Riegler - ISO/TC6/SC3) measured under normal test conditions, in order to achieve elevated bond; adding mineral loads and dyestuffs according to the desired opacity and colour and treating with a neutral medium wet strength resin to give over 25% humidity strength with regard to dry strength under normal test conditions, to guarantee moisture protection thereof.

The base is then pasted with a polyvinyl alcohol and low viscosity based paste, for suitable impregnation thereof, at a bath concentration of at least 8%, so that a dose of at least 5% remains in the paper and, under such conditions, selfpolymerization thereof will be easy, preferably at a pH of less than 7. All percentages, unless otherwise stated, are given by weight.

The paste's specific water repellent may be, e.g. a cetene dimer based product, although it may carry some gelatin, at up to around 10% with regard to polyvinyl alcohol, with the complementary function of expediting polymerization, and CMC, starch or alginates as complementary pasting agent, specifically providing the same with high dry strength.

Germicides can also be added to prevent fungi-based biological degradation. Useful germicides are the so-called insoluble and scattered germicides, preferably isothiaziolone based so as not to be affected by moisture.

This bath can moreover eventually be provided with other specific oil-repellents assisting in the polyvinyl alcohol and C action, for instance fluohydrocarbons at an additive dose, without becoming a main component.

Finally, the safety paper is subjected to a hot-pressing stage, so that its specific surface decreases and thus so does the possibility of foreign particles adhering to the same. Hotpressing will similarly be aimed at improving internal bond and reducing pores, without actually damaging the basic fibre structure, wherefore such hot-pressing operation will take place at 10-60 Bekk seconds, under normal test conditions.

Industrial testing has been carried out within the theoretical constraints set out above, providing different embodiments according to each case's specific use or application. According to given technical specifications for the final product, the following conditions are preferred: 85-95% linen and comber bleached cotton fibre.

5-15% linter linen and bleached cotton fibre.

1-10% titanium dioxide on fibres.

1-10% aluminium silicate on fibres.

2-20% polyamine-epichlorhydrin.

5-15% low viscosity polyvinyl alcohol.

0.5-15% cetene dimer based product on polyvinyl alcohol.

1-5% CMC, starch, gelatin or alginates.

Hot-pressing up to 10-60 Bekk seconds (ISo 5627).

Claims (9)

1. A process for the manufacture of paper which comprises treating a vegetable fibre paper base having a high cellulose polymerisation index with: (a) A neutral medium moisture strong resin and a low viscosity poly-vinyl alcohol based paste such that the paper absorbs at least 5% W/W of said paste, (b) A paste to improve the dry strength of the paper, (c) Thereafter hot-pressing the product for between 10 and 60 Bekk seconds.

2. A process as claimed in Claim 1 wherein the vegetable fibre is cotton or linen.

3. A process as claimed in Claim 1 or Claim 2 wherein the paste used to provide dry strength is carboxymethyl cellulose-, starchor alginate-based.

4. A process as claimed in any one of Claims l to 3 wherein the paper is further treated with a water repellent.

5. A process as claimed in Claim 4 wherein the water repellent is a cetene dimer-based product.

6. A process as claimed in any one of Claims 1 to 5 wherein the paper is further treated with a germicide.

7. Paper whenever made by a process as claimed in any one of Claims 1 to 6.

8. The use of paper as claimed in Claim 7 in the manufacture of paper currency.

9. A process as claimed in Claim I and substantially as hereinbefore described.