EP0072801B1

EP0072801B1 - Paper product and process for its preparation

Info

Publication number: EP0072801B1
Application number: EP81902942A
Authority: EP
Inventors: Kaj Otto Holger Fuxelius
Original assignee: BILLINGSFORS PAPPERSBRUK AB
Current assignee: BILLINGSFORS PAPPERSBRUK AB
Priority date: 1980-11-04
Filing date: 1981-10-16
Publication date: 1985-07-03
Also published as: BR8108997A; SE8007737L; EP0072801A1; NO822321L; WO1982001516A1

Abstract

Carbon paper comprising a base paper based on cellulose and an ink-releasing layer applied thereto, the base paper containing a hygroscopic substance essentially evenly distributed therein which has for a function to increase the equilibrium moisture content of the base paper, and a process for its manufacture.

Description

Technical Field

The present invention relates to paper products, more particularly carbon paper comprising a base paper based on cellulose and a ink-releasing layer applied thereon, and a process for its manufacture.
The manufacture of carbon paper, particularly the one-time type, so-called OTC-paper (one time carbon) offers multi-faceted practical problems. The requirements on such carbon paper are high in regard to its function in connection with the intended use, which often resides in utilization in multi-layered products, for example preprinted debit notes, consignment notes and the like. Thus, there is required for the manufacture of carbon paper a thin and flexible base paper having the ability of providing for good copying sharpness also when typing several copies. Moreover, the base paper must possess sufficient strength to avoid web failure in connection with the manufacture of the carbon paper. It must show good release properties in regard to the ink-releasing layer and must not give raise to so-called "curl", i.e. the base paper must maintain its planarity after manufacture to facilitate handling of the finished carbon paper in connection with automized production. In this handling there are often used so-called vacuum heads for transporting the product between the different stations, and such transport techniques require inter alia planarity of the base paper and the multi-layered product, i.e. freeness from curl.
The conventional technique has not been able in a satisfactory and practical manner to solve the problems associated with the manufacture of carbon paper, which has been indicated by the technical problems illustrated above, and, therefore, the present invention has for its purpose to provide a product and a process for its manufacture, wherein the deficiencies of the conventional technique are eliminated or at any rate substantially reduced while advising a technique which well meets the practical requirements.

Summary of the Invention

In connection with the creation of the present invention it has been surprisingly found that by adding to the base paper a hygroscopic substance which is evenly distributed in the base paper there is obtained a significant increase of the moisture content of the base paper at equilibrium, and that thereby essential advantages are obtained which will be further discussed below. The invention thus provides for a carbon paper consisting of a base paper based on cellulose and a ink-releasing layer applied thereon. The carbon paper according to the invention is characterized thereby that the base paper contains a hygroscopic substance essentially evenly distributed therein having the function of increasing the moisture content of the base paper at equilibrium.
The invention also provides for a process for the manufacture of such carbon paper, and in this manufacture the base paper at some stage before its final drying, for example on a so-called glazing calender, is supplied with a suitable hygroscopic substance, said hydroscopic substance may be supplied already in the pulp or at some later stage after taking up the pulp on the wire or immediately before the transfer of the paper web to the cylinder intended for the final drying. It is particularly suitable to supply on the paper web across its full width, for example by shower or spraying, an aqueous solution of the hygroscopic substance, for example an inorganic salt, the base paper being supplied with a content of evenly distributed material resulting in an increase of the equilibrium moisture content of the paper.
By the expression "hygroscopic substance" there is meant in the present disclosure a substance which by its presence in the base paper due to its inherent properties results in an increased equilibrium moisture content in the base paper. This hygroscopic substance preferably consists of an inorganic, water-soluble salt, the salts of the alkali and alkaline earth metals being preferred. As counterions chloride, sulphate, nitrate and carbonate ions are conceivable,?and particularly preferred salts are the magnesium and calcium salts, particularly calcium or magnesium chloride.
As previously indicated the base paper or the starting material therefor is suitably supplied with the hygroscopic substance in the form of an aqueous solution. Thus, it is particularly preferred to utilize an aqueous solution of calcium chloride or of magnesium chloride. The quantity of the hygroscopic substance supplied to the base paper may vary within relatively wide ranges, but a suitable upper limit is about 8 percent by weight based on the dry paper weight. This corresponds to an increase of the equilibrium moisture content of the paper of up to about 15 percent by weight of water. A particularly suitable range in regard to the hygroscopic substance is about 1-7 percent by weight, particularly 3-6 percent by weight.
The base paper suitable for manufacturing the carbon paper in accordance with the technique of this invention is preferably a so-called low weight paper, i.e. a paper having a specific area weight of the order of 15-30 g/m². By using the technique of the present invention one may use as starting material for the manufacture of the base paper sulphite pulps and mechanical pulps as starting materials, for example unbleached sulphate pulps, while obtaining a product possessing quite favourable properties, as further described below.

Examples

The invention will now be further illustrated by non-limiting examples. In the examples percentages refer to weight if not otherwise indicated.

Example 1

a) Manufacture of base paper

Starting from conventional unbleached sulphate pulp having an SR° of about 70-80 in the form of an aqueous slurry containing about 0.10% dry solids there is prepared in a conventional paper machine (Yankee) a base paper of the MG-type (machine glazed). The web speed in the machine is about 400 m/min.
Before transfer onto the Yankee cylinder for final drying and glazing the paper web is sprayed by means of a lateral ramp with nozzles with an aqueous solution containing about 40-50% of calcium chloride in such a quantity adjusted to the feeding rate of the paper web that the product obtained will contain about 5% of calcium chloride based on the paper dry weight.
From the paper machine there is obtained a base paper having a specific area weight of about 20 g/m² and an equilibrium moisture content of about 10-13%. The normal equilibrium moisture content for the low weight paper of this type but without hygroscopic substance added thereto is about 6%.

b) Manufacture of carbon paper.

A so-called carbon mass or paste of conventional type is prepared consisting of carbon black (about 50%), paraffins and waxes (about 25%) and mineral oils (about 25%), said carbon mass being intended to constitute ink-releasing layer in the carbon paper made.
The prepared carbon mass heated to about 100°C for suitable consistency is supplied to the non-glazed face of the base paper prepared according to the above by means of a coating roller to the desired quantity, suitably about 4-7 g/m², and the coated paper web is transferred to cooling by being passed over a cylinder cooled with cold water, the glazed face of the base paper facing the cooling cylinder. After cooling the carbon paper obtained is ready for the intended use. In view of the increased equilibrium moisture content of the paper the product obtains very good properties, which is further explained below.

Example 2

The procedure of Example 1 is repeated but in this instance while preparing a base paper containing about 7% calcium chloride. This increased calcium chloride content results in an increase of the equilibrium moisture content to about 14-15%. The carbon paper obtained has properties similar to that prepared in Example 1.

Example 3

The procedure in Example 1 is repeated but for making a base paper containing about 3.5% calcium chloride. This modification of the base paper results in an increase of the equilibrium moisture content to about 9-10%.

Example 4

The procedure of Example 1 is repeated but in this instance while using an aqueous solution of magnesium chloride. The magnesium chloride is added to the base paper to a content of about 7%, which results in an equilibrium moisture content. of the base paper of about 10-11 %. From this it can be deducted that the magnesium chloride has a somewhat lower effect than calcium chloride at a corresponding quantity in regard to the increase in equilibrium moisture content. However, the base paper obtained has properties comparative to those of that obtained in Example 1.

Example 5

Example 1 is repeated but using an aqueous solution of magnesium sulphate in a corresponding quantity on a molar basis. A base paper is obtained having essentially same good properties as those of the base paper obtained in Example 1.
The improvement in the properties of the carbon paper provided by using the technique of this invention seems in principal to be a result of the increase in equilibrium moisture content caused by the supply of hygroscopic substance. As mentioned above the equilibrium moisture content for the types of base papers relevant in the context is about 6%, this at a relative humidity of about 50%. Among the advantages resulting from the technique of the instant invention the following may be mentioned.

a. In connection with the manufacture of the carbon paper, i.e. the application of the ink-releasing layer onto the base paper, a reduced web failure frequency can be observed, which seems to be a result of the fact that the base paper shows reduced brittleness in turn causing increased stretchability.
b. The carbon paper according to the present invention shows improved release properties, i.e. improved release of the copying ink. Even if the invention is not delimited to any theory it seems, at least when using the traditional type of hydro- phobous carbon paste, that this results from reduced penetration of the paste into the base paper. This in turn results in improved economy in the form of reduced consumption of paste.
c. When using multi-layered products there is obtained with the carbon paper according to the present invention improved copy sharpness also in regard to the bottom copies, and this seems to be due to the fact that the paper in view of the increased equilibrium moisture content is more flexible.
d. The carbon paper according to the present invention shows improved planarity, i.e. eliminated curl, which is essential in order to facilitate handling in connection with automized production. The alternative to avoid curl is remoisturing of manufactured paper and renewed drying or the use of a pulp of higher quality (for example sulphite pulp) but in this case, however, a paper is obtained which possesses lower strength.
e. The manufacture of the base paper involves lower energy consumption in connection with drying, alternatively higher speed of production at the same energy requirements, this in view of the fact that the increased equilibrium moisture content means that a smaller quantity of water need to be removed by evaporization.
f. In view of the higher final moisture content of the base paper the glazing of the paper will be less pronounced, which results in the advantage of increased friction between the layers, which in turn facilitates the manufacture of multi-layered products.

Claims

1. Carbon paper comprising a base paper based on cellulose and an ink-releasing layer applied thereto, characterized thereby that the base paper contains a hygroscopic substance essentially evenly distributed therein which has for a function to increase the equilibrium moisture content of the base paper.

2. Carbon paper according to claim claim 1, characterized thereby that the hygroscopic substance consists of an inorganic water-soluble salt.

3. Carbon paper according to claim 2, characterized thereby that said salt is selected from salts of alkali and alkaline earth metals.

4. Carbon paper according to claim 3, characterized thereby that the salt as an anion contains chloride, sulphate, nitrate or carbonate ions.

5. Carbon paper according to claim 4, characterized thereby that the salt is calcium chloride or magnesium chloride.

6. Carbon paper according to any of the preceding claims, characterized thereby that said substance is present in the base paper in an amount of up to about 10 percent by weight based on the base paper dry weight.

7. Carbon paper according to claim 6 in its dependence on any of claims 2-5, characterized thereby that said amount is about 1-8 percent by weight, particularly about 3-6 percent by weight.

8. Carbon paper according to any of the preceding claims, characterized thereby that the ink-releasing layer consists of a hydrophobic carbon mass, for example containing carbon black in a matrix of paraffin, wax and mineral oil.

9. A process for the preparation of a carbon paper comprising a base paper based on cellulose and an ink-releasing layer applied thereto, characterized thereby that the base paper, before its final drying, is supplied with an aqueous solution of a hygroscopic substance having for a function to increase the equilibrium moisture content of the base paper.

10. A process according to claim 9, characterized thereby that the base paper web across its whole width is sprayed with an aqueous solution of an inorganic salt, preferably calcium or magnesium chloride.