SCHOLZ "ATI" (Application Technology Information) Paper industry Application Technology Information for the Paper Industry Basics With regard to the colouring of paper the paper industry distinguishes between decor paper, coloured paper, coated paper and shaded paper. Decor paper To manufacture decor paper the pigments or pigment preparations are directly added to the stock preparation (mash) which is also known as mass colouring. Afterwards the coloured papers are processed with the aid of melamine formaldehyde resins by pressing them into flake boards to obtain decorative layer press plates or decorative coatings. These boards / laminates are mainly used in the furniture industry. Coated paper Paper which shall subsequently be used for printing (e. g. for brochures, calendars etc.) is often coated or, as people say, it is "furnished with a coat". Judging from their composition, coating colours can be compared to dispersion paint. Components of such coating colours are binding agents, fillers and titanium dioxide. In case the coating colour shall be used for carton or cardboard, the proportion of titanium dioxide is higher than for other applications. This is due to the use of unbleached pulp and a high proportion of recovered paper in cardboard production resulting in a brown or grey inherent colour. To achieve good opacity (covering power) of coating colours in this field of application, the proportion of titanium dioxide should be approx. 15% by weight. For normal paper, however, it is often sufficient to apply coating colour formulated with extender to achieve the required opacity. If pigment preparations are applied these must be compatible with coating colours (no rubout / flocculation effects). Coloured paper Coated paper is also used for the production of coloured paper. Shaded paper Paper manufactures often strive to increase the whiteness of paper. The white effect is composed by the following factors: inherent white of the raw materials, reduction and shading. 1/7
Apart from the use of optical brighteners which convert invisible ultraviolet light into visible blue light shading can also influence the wave length spectrum to that effect. Thus the undesired yellow wave lengths (yellowish shades) are partly absorbed and shifted towards a bluish shade. Bluish paper suggests higher whiteness. For shading either dyestuffs or organic pigment preparations can be used. For this purpose approx. 10 100 ppm in relation to the total mixture have to be applied. To achieve an optimum increase of whiteness, shading is frequently done with a combination of blue and violet colouring matters. Potential colouring matters - Pigments - Pigment preparations - Dyestuffs As a matter of principle the following parameters have to be considered when selecting possible colourants: - Optical properties - Dispersibility - Retention behaviour, especially with regard to pigment preparations - Migration resistance - Temperature stability - Light fastness - Authorisations for use Optical properties Optical properties like colour scheme, tinting strength (lightening power in case of white pigments) and opacity (covering power) define the suitability of a colourant. Thus the actual colouration is determined. Colouring can be categorised by means of the light-absorption (K) ratio and light scattering power (S) where the following applies: - K is larger than S especially deep shade - S is larger than K light shades - S is much larger than K white pigment - K is much larger than S organic pigments and dyestuffs which again do not show any scattering power - K=S inorganic pigments 2/7
Dispersability To achieve optimal and continuous colouring in pulp production, the pigments applied should be easily dispersible. Only if the pigments are optimally disaggregated / dispersed during incorporation and are thus available in the form of primary particles they can show their full performance and will be equally dispersed in the pulp. This can also be realised by producing an aqueous pigment suspension a so-called slurry - first. For such a method only inorganic pigments are suitable, ideally micronised pigments, which are mostly already available in the form of primary particles. Organic pigments cannot be completely disaggregated / dispersed with this method and therefore they are unsuitable for this procedure. In pigment preparations, especially in those based on organic pigments, the pigment particles are already very close to their primary particles due to intensive grinding. Hence pigment preparations can be used. In case of dye there is no problem of dispersion as dyestuffs are soluble. Retention behaviour During the production process in the pulper retention agents are added to the pulp mass. On the one hand these retention agents serve to retain as many fillers and fines on the sieve as possible, on the other hand they serve to reduce the fill factor and wastewater contamination. Furthermore, the colourants added are thus attached to the cellulose fibre, this especially applies for pigment preparations. In addition to the application of retention agents, the agglomerations of colourants are also significantly steered by the charge of the pigment preparation. The charge of the pigment preparation results from the wetting agents and dispersing agents used in the preparation. In this connection a distinction is made between anionic-active, non-ionogenic and cationicactive. Purely cationic-active wetting- and dispersing agents do not exist, if at all, they show a slight tendency towards cationic activity. Direct dyestuffs are anionic-active whereas basic dyestuffs are cationic-active. 3/7
Migration resistance Migration resistance is influenced - among others - by the water solubility of the colourant applied. Inorganic pigments are absolutely insoluble in water and therefore problems like bleeding, efflorescence and insufficient solvent resistance are not to be expected. If organic pigments and dyestuffs shall be used their suitability has to be checked in each individual case. The situation is the same for corresponding pigment preparations. Temperature stability The necessary temperature stability of the pigments has to be observed especially during production of decor paper. Especially inorganic yellow pigments can shift into a reddish tinge due to the temperature of approx. 150 C required f or pressing with melamine formaldehyde resins. This change of shade is caused by reduction of trivalent iron. Especially the Colortherm shades produced by Lanxess provide the required temperature stability due to appropriate subsequent treatment. Light fastness Depending on the field of application of the manufactured papers, light fastness of the colourants must also be taken into account. Decor papers, which are pressed to decorative layer press plates, are used in furniture industry and for this reason adequate light fastness is required. It is well known that light fastness of inorganic pigments is approx. 8, whereas this value may be considerably lower in case of organic pigments. Authorisations for use Papers, cardboard boxes and paperboards for consumer goods which either have food contact or affect foodstuffs have to be approved according to BfR XXXVI. The admittance examination is directly effected on the dyed paper, cardboard or paperboard. Such examinations should ideally be implemented with the customer's support. For authorisations according to the Tobacco Product Regulation especially the "Z" types produced by Lanxess have been examined and approved. In the scope of regulation EN 71 (toy safety standard) no general statements can be made as this standard prescribes an examination of the finished product (toy). 4/7
Colouring matters provided by Scholz Pigments - titanium dioxide - Bayferrox shades - lightfast pigments - carbon black Pigment preparations - based on the above mentioned pigments - organic pigment preparations As mentioned before, it is essential during selection of pigment preparations to consider the charge of the respective preparation. The selection can be made as follows: Euronox / Euronyl LF 2 = non-ionogenic Euronox / Euronyl PI Euronox AR Euronyl CB Euronox TiO2 = anionic-active = anionic-active = both anionic-active and non-ionogenic = both anionic-active and non-ionogenic Anionic-active preparations show very good retention properties especially during bulk colouration and therefore they are well suited for this field of application. For the colouring of dispersion paint intended for coating, non-ionogenic LF 2 types have proven to be effective. Nonetheless it is recommendable to carry out corresponding compatibility tests in advance. 5/7
Glossary Decor paper: Special papers for surface finishing of wood-based materials with high requirements regarding dimensional stability, smoothness, high stability of the paper web and consistent penetration behaviour towards resins. Coloured paper: Papers with a surface subsequently coloured, refined or embellished by different techniques such as coating, spraying or others. Retention: Retention = holding back; this means that the filtration performance of the pulp is deliberately deteriorated to the effect that the ingredients of the entire paper mixture and the colourants attach to the cellulose fibre in a better way. In this connection high retention means large proportion of additives in the paper. Dispersion: In order to obtain the optimal shade and tinting strength, the existing coarse pigment particles are split into primary particles by means of sufficient mixing- / dispersion energy. Migration: Bleeding / efflorescence, this describes the migration of dissolved pigment particles from the application medium to the surface of the surrounding medium. Light fastness: Resistance of the entire system against daylight. Assessment is often done according to the blue scale specified in the textile industry which consists of eight light units of graded colours on wool. Light fastness is specified by ratings between 1 (very poor) and 8 (outstanding) light fastness. This assessment is directly compared to a second one to be carried out according to the wool scale. Examinations are continued until a clear difference in shade becomes visible, at least one of grade 3. Temperature resistance: Specifies the resistance / stability of colourants against temperature exposure. Temperature influences may cause partially substantial variations in the field of colouring. Not only the temperature prevailing but also the time of exposure influences the change of shade to a great extent. 6/7
To provide further insights on all topics related to the colouring of paper please also consider the following publications: - Anorganische Pigmente Papier from Lanxess (inorganic pigments for paper), edition January 2006 - Pulp & Paper Weiße-Nuancierkonzepte (pulp & paper - white shading concepts) from Kemira, edition May 2008 - Faserstoff-Prüfungen im Naßlabor (fibre tests in the wet laboratory), volume 15, edition December 1995 - Industrielle Organische Pigmente (industrial organic pigments), second edition, published by VCH Our experts will be glad to help you with any questions regarding the colouring of paper please do not hesitate to contact us! This Scholz ATI (application technology information) is continuously updated. The latest version of this ATI (please see the version number indicated in the footnote) can be downloaded from the internet at www.harold-scholz.de. Harold Scholz & Co. GmbH Central phone number of our Recklinghausen branch: +49 (0) 2361-9888-0 Contact for inquiries by e-mail: info@harold-scholz.de To contact other branch offices please have a look at Scholz information contact dates 7/7