Appendix 1. 1 Our referencr. 15 March 1994 Page number. Currency Policy Department. ocv/hdh/vr PAPER ROUGHNESS AND PRINTING QUALITY

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1 De Nederlandsche Bank 15 March Our referencr Appendix 1. ocv/hdh/vr PAPER ROUGHNESS AND PRINTING QUALITY H.A.M. de Heij De Nederlandsche Bank NV with thanks to: Mr N.M. Bronold (VHP Security Papermill) Mr J.J.A.M. Lemmers (Joh. Enschedé Security Printing) 1 INTRODUCTION The smoother the paper surface, the better the printing quality. Intaglio prints on coated paper are beautiful, very rich in detail, while the Same prints on cotton banknote paper are not that bright. In 1992, during our meeting in Burgos, Mr Schertenleib from Orell Füssli showed US very convincing examples underlining this statement [i]. One year later, Mr Müllner from the Austrian National Bank also reported much better intaglio printing on a 20 Schilling banknote printed on machine coated banknote paper [2]. Recent new production techniques such as hot stamped foils also require a smoother paper surface for optima1 gloss [3]. And finally Mr Aitchison.MacNab reported that it seems that a rougher paper surface dirties more easily than a smoother surface [4]. Why are banknotes not printed on smoother paper? The answer to this question is two-fold, related to the two techniques that can be used to make smoother paper: 1) machine-calandering, 2) machine-coating (MC) followed by machine-calandering. The more paper with a watermark is calandered the worse the quality of the watermark wil1 be. A good quality watermark in heavy calandered

2 De Nederlandsche Bank 15 March 1994 Page numher 2 Our reterence ocv/hdh/vr paper is not possible. Also other typical mechanica1 banknote paper specifications like the double fould value and the tensile strength will deteriorate when heavy machine-calandered paper is used. Problems with MC banknote paper were reported by Orell Füssli and the Austrian National Bank [2,3]. For their new series, the Swiss decided again to use standard banknote paper, because banknote inks printed on coated paper had a poor adhesive strength. That same year Austria reported opposite results. They were quite satisfied with a circulation trial with the 20 Schilling note printed on MC banknote paper. The Swiss reported a higher resistance of the kinegram on standard and rough paper than on coated banknote paper. With great interest we follow the Austrian experiments with MC banknote paper, but in the following I will stick to the standard (smooth) calandered banknote papers. It is clear that we have here a problem of optimizing. On the one hand the quality of the watermark which implicates a fairly rough paper surface and on the other hand the print quality which is best on a smooth paper surface. Last year we have also done some research in this field. The spots in the printed bar code were our specific reason. According to the printer these spots were caused by the paper roughness (see Figure 1). Of course for optimum bar code reading and also for the physical quality of the banknote we do not want spots in the bar code.

3 ~ 15 Dc Nederlandsche Bmk DJte March 1994 Pqe number 3 Our rererencc - ocvlhdhlvr A B C FIGURE 1 Microscope pictures of a (part of a) bar code printed on A) banknote paper, B) wood-free writing paper and C) MC paper. 2 DEFINITIONS Paper roughness and paper smoothness are not exactly the same. The first is measured usually with the Bendtsen method [ml/min], the second with the Bekk method [SI. In principle a low Bendtsen-value means a high Bekk-value and vice versa. Both measure the quality of the paper surf ace. Another paper specification is the porosity, which can also be determined by another Bendtsen- or Bekk-method. Table 1 gives an overview of the different measurements. Banknote paper usually has a rough surface and a closed structure. The IS0 standards advise the Bendtsen-method (IS , TAPPI 538) for measurements on rougher paper surfaces, while the Bekk-method (IS , TAPPI 479) is more appropiate for the more smoother papers, with a maximum thickness of 0.3 mm. In spite of-this advise we use, like some other BPC-countries, the Bekk-method for the Dutch paper specifications. Other countries, like Belgium, Finland, Norway, Sweden, Finland and Turkey measure the Bendtsen-values, as we can read in our last 'Survey of Paper Specifications' held in 1992 [SI.

4 De Nederlandsche Bank - 15 March Our reierencc - ocv/hdh/vr TABLE 1 Overview methods Bekk and Bendtsen to determinate paper specifications. -> is advised in case of banknote paper. There is no ISO, DIN or TAPPI for porosity Bekk. For most papers, the Bendtsen-method is appropriate for porosity measurements (IS0 5636/3, TAPPI 547). But for banknote paper the value found wil1 often be around O ml/min. This is probably the reason that many BPC countries do not specify the porosity. Some specify that it should be less than 25 ml/min Bendtsen or only specify the porosity after crumpling 151. For the more closed papers, with an air permeability lower than 30 ml/min, the Bendtsen-method is therefore, we find, not very useful. This is the reason that we use the air permeability Bekk for the porosity. There is, as far as we know, no corresponding ISO, DIN or TAPPI for our Dutch norm NEN 2014, which describes the air permeability Bekk. Maybe this measurement by Mr Bekk is only used in the Netherlands because the German-born Mr Julius Bekk worked for many years in the paper industry in the Netherlands. We specify the porosity of the paper as > 20 s Bekk. To be complete, there are also other methods to measure the roughness and porosity. The roughness can also be measured by actually measuring the 'valleys and hills', for example with PPS (Parker Print Surface). According to Kurz the differences between the paper tops and bottoms in banknote paper can be as much as 20 pm.

5 ~ ocv/hdh/vr De Ncderlandschc Bank - 15 March Gur rerereiict The porosity can als0 be measured by means of the Sheffield-method (IS0 5636/4) or Gurley-method (IS0 5636/5). During our research we tried to find a relationship between the paper surface measurements Bendtsen and Bekk. The result is presented in Figure 2. As you can see we could find a good correlation for the 15 different paper samples we examined on both paper sides, in total 30 samples. The smoothness Bendtsen of al1 these samples was between mì/min Io(K m o O smoothness Bekk (s.) I FIGURE 2 Relationships found between 30 paper surface measurements according to Bendtsen and Bekk. In Table 1 the roughness Bendtsen and the more-or-less corresponding smoothness Bekk are given for different types of paper.

6 ~ ocv/hdh/vr De Nederlandsche Bank - 15 March Page nurnber Our reierencc Bend t sen Bekk Paper [ ml /min] [SI... not calandered paper ordinary offset banknote (gelatine) banknote (PVOH) stocks stamps with phosphor coating stamps without phosphor coating MC art paper supercalandered MC paper O TABLE 2 Different types of paper with some typical roughness Bendtsen and smoothness Bekk values. 3 BENDTSEN AND BEKK VALUES WITHIN THE BPC From our 1992 survey we can read that the BPC-countries use the following ranges for Bendtsen and Bekk (minima and maxima specified) 151: TABLE 3 Minimum,and maximum values for Bendtsen and Bekk used for banknote paper within the BPC, for both mould and felt side. Some countries als0 measure the porosity after crumpling according to the Bendtsen-method. In this case only maxima are specified between ml/min.

7 ~~ ocv/hdh/vr De Nederlandsche Bank - 15 March Our reterence The roughness Bekk for Dutch banknote paper is now specified between s Bekk for both the mould and felt side (corresponding with a future specification of ml/min smoothness Bendtsen). 4 PRINTING ON 15 TYPES OF PAPER In order to find an optimum for paper roughness and print quality of the bar code we overprinted the already mentioned 15 different papers on both the mould and felt side, which gives a total of 30 samples. The samples were 'printed' with the IGT-in ink apparatus, with the ink of the bar code. This IGT-apparatus with a rubber-in-ink rol1 cannot easily be compared with the steel-to-steel letter press printing of the bar code, but it was the best we could do. Our conclusion was that there is no difference in the chance of 'wholes' between ml/min Bendt sen. Another reason for our research on paper surface was the printing of 'SAM'. SAM is a security feature developed by Joh. Enschedé aimed especially at protection against colour copy machines. The principle of SAM is based on fine lines of pm, printed with an ink of a very high density. Between these lines SAM requires intermitted narrow spaces of 25 and 35 pm. These not-printed spaces are best on a smooth surface and on a closed paper. What is the best (available) printing technique for SAM? To answer this question we did for three different printing techniques some additional research on the 'transfer resolution lose' from printing plate to paper. The results are shown in Table 4. In our banknote NLG 100/Little OW1 SAM is printed in intaglio. In our next new notes we wil1 use litho offset. But as you can see from Table 4 the print resolution of both printing techniques of our banknote paper is not sufficient for an optimum SAM. Our conclusion here is that the paper surface is here dominant for the printing solution.

8 ~~ 8 De Nederlandsche Bank - 15 March 1994 Our rekerencc ocv/hdh/vr Printing lines smaller than 30 pm. 2 lp/mm) are not possible on banknote paper with a paper roughness of about 300 ml/min Bendtsen. Table 4 The influence of the banknote paper on the printing resolution for different printing techniques. Further research showed a correlation between the air permeability of the paper according to Bekk and the ink absorbtion: the higher the air permeability Bekk, the better the ink absorbtion (Figure 3 ). Not such a strong correlation was found between the air permeability of the paper according to Bekk and the quality of the printing. Here we found the opposite relationship: the higher the air permeability Bekk, the worse the printing quality (Figure 4). An optimum for daily practice for both a good ink absorbtion and an good printing quality seems a value of around 60 s for the air permeability Bekk.

9 De Nederlmdsche Bmk DJte 15 March 1994 rjrc nuflioe: 9 ih: :c:ei-nli ocvlhdhlvr 94 O a o 50 1 O Ar Permeability 'Bekk'(S.) FIGURE 3 Relationshíps found between the air permeability Bekk [SI absorbtion 'KM'. and the ínk.. FIGURE 4 O 50 1 O0 I Oû Ar Permeability Bek (S.) Relationships found between the air permeability Bekk [SI quality IK'. and the print

10 ~ 15 ~ 10 De Nederlandsche Bank March 1994 Our reterenct. ocv/hdh/vr CONCLUSIONS 1 The banknote paper surface should be specified in roughness Bendtsen [SI. 2 The experimental optimum paper surface value for (not MC) banknote paper within the BPC countries is between ml/min Bendtsen. This means that the paper can have a good watermark and a reasonable printing resolution of 12 lines/mm (40 pm). For our Dutch banknotes this means that the bar code wil1 have some wholes and that SAM can not be printed according to al1 the specifications. 3 If priority is given to solid printing above the watermark quality, a paper smoothness of less than 200 ml/min Bendtsen (more than 50 s Bekk) is necessary. 4 If the porosity is specified for banknote paper this can best be done with air permeability Bekk (NEN 2014). 5 There is a correlation between the air permeability of the paper according to Bekk and the ink absorbtion: the higher the air permeability Bekk, the better the ink absorbtion. 6 There is a (not so strong) correlation between the air permeability of the paper according to Bekk and the quality of the printing: the higher the air permeability Bekk, the worse the printing quality. 7 An optimum for the air permeability Bekk in respect to both a good ink absorbtion and a good print quality seems a value of around 60 s Bekk.

11 ~ 15 ~ ocv/hdh/vr De Nederlandsche Bank D3tc March Our reterence REFERENCES 1 Schertenleib, B.; "What makes the paper surface influence the printing quality in intaglio printing" Orell FUssli Graphic Arts Ltd, Banknote and Securities, BPC/Paper Committee, Burgos Spain, March MUllner, R.; "Tests with coated paper" Austrian National Bank, BPC/Paper Committee, ZUrich Switzerland, March Schumacher, A. and M. Urech; "Kinegram application tests on different paper surface qualities" Orell Ftissli, Banknote and Securities, BPC/Paper Committee, ZUrich Switzerland, March Aitchison MacNab, N.; "Dirty money" Bank of England Printing Works BPC/Paper Committee, ZUrich Switzerland, March Survey of Paper Specifications; BPC/Paper Committee, Burgos Spain, March 1992, To be presented at the BPC/Paper Committee meeting in Helsinki; Finland, April 1994.