Paper making and fiber liberation – Processes and products – With coating after drying
Reexamination Certificate
1998-10-09
2001-03-06
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
With coating after drying
C162S158000, C162S181100, C162S181800, C162S175000, C162S173000, C428S340000, C428S341000
Reexamination Certificate
active
06197155
ABSTRACT:
The invention relates to a coated web printing paper suitable for printing with cold-set offset printing ink.
Such a paper is already known from EP-A 0 785 307. As regards the need for enhanced cold-set paper qualities and the related problems, the introductory statements in this older document are referred to.
The web printing paper described in EP-A 0 785 307 is a so-called mat quality. It is explained in detail in this older document that to achieve the pressability and printability of a coated web printing paper in the cold-set offset process, special demands must be made on the paper regarding its wetting/water penetration behavior and its ink absorption speed. These are properties which at times are in turn considerably disadvantaged by a glazing on smoothnesses of 1,000 to 1,600 sec. Bekk, as is necessary for producing typical smooth papers. For this reason, it was first managed to provide a coated mat quality for the cold-set process.
Now that a high degree of advertizing effectiveness is to be achieved via newspaper supplements, only glossy paper can be considered for certain advertizing orders. In the absence of cold-set-suitable glossy LWC papers, these brochures are still printed via the conventional heat-set process as before. To enable the cold-set printer to take on such printing orders as well, the main point was to develop a glossy paper quality that closes this quality gap and can be pressed and printed without problems via the printing machine configurations that are typical in the newspaper printing domain (eight-tower and satellite printing machines). After such a quality has established itself in the domain of mass-produced printing paper, economic aspects are also to be taken into consideration.
The invention is therefore based on the technical problem of providing a coated web printing paper for the cold-set process that has sufficient gloss for more demanding printing products, in particular advertizing supplements and the like, and can be manufactured economically.
This technical problem is solved by the features of patent claim
1
.
The mat paper quality described in EP-A-0 785 307 for the cold-set process is first of all characterized by its water absorbency, measured on the wetting angle of contact of a water drop or by determining the penetration behavior. A further important property of the paper, which may be dependent on the printing machine to a certain extent, however, is the ink absorption behavior of the paper. It was already indicated in EP-A-0 785 307, namely on page 12, lines 19-23, that the two aforementioned properties reciprocally determine each other to a certain extent, in such a way that a paper is also suitable for printing in the cold-set process if the values for the water absorption capacity and the ink absorption range in their opposite limit ranges, in such a way that in this case a certain compensation of the properties seems to take place. The reasons for this are not yet known.
It has also been shown that a glossy printing paper with cold-set suitability cannot be obtained by glazing to a high smoothness value as is otherwise typical for LWC papers.
Not only the combination of the two aforementioned properties, which appear essential for suitability in the cold-set process, is affected by this. In addition, the paper's pick-resistance generally required for the offset process is also reduced; this cannot be compensated to a corresponding degree by additional binder in the coat because the latter in turn influences the two basic values considered decisive for the cold-set process.
It has been shown that a glazing to smoothness values of more than 1,000 sec. Bekk. of a basically cold-set-suitable printing paper destroys the cold-set suitability. Smoothnesses in the range of approx. 500 sec. Bekk still seem feasible. Acceptable gloss values can be attained according to the invention with smoothnesses as of 250 sec. Bekk. A preferred range is 300 to 400 sec. Bekk. The finishing of the paper with such a smoothness may already have reactions on the pressability and printability in the cold-set process, however.
But even a glazing to smoothnesses in the 500 sec. Bekk. range does not yet necessarily lead to a paper with a strived-for gloss which, measured according to Lehmann, should be at least roughly 25% at a 75° angle. The gloss for the indicated limited smoothness can be increased by the selection of suitable coating pigments. This selection is relatively stratified, however, such that it is practically impossible to individually indicate the coating pigment compositions leading to the success strived for. This is made even more difficult by the fact that different coating pigment compositions as a rule also require qualitatively and quantitatively different binder compositions / proportions, which in turn have a reaction on the basic values required for the cold-set suitability. The paper according to the invention is therefore defined by minimum gloss values in addition to the ranges for water absorbency, ink absorption and smoothness. Within the framework of the tests taken as a basis for the invention, basic selection criteria were determined that lead to the strived-for success and provide the expert a sufficient lesson as to how he shall proceed. These selection criteria are included in subclaims. In addition, the added examples of execution contain concrete details as to how a paper according to the invention can be produced.
The possible and preferred limits for the smoothness values to be adhered to were already indicated. Water absorption according to the Emco test should be situated in the 85-25% range after one second, preferably in the 70-30% range. The ink absorption test should yield a value of 1.1 to 0.25, preferably a value of 0.8 to 0.3. Gloss measured according to Lehmann at 75° should be at least 25%, but preferably between 30 and 55%, to yield a glossy appearance of the paper that is commonly considered sufficient.
The testing methods used, in particular the method of gloss measuring, are explained in more detail further below. The penetration test according to Emco and the ink absorption test are defined as already described in EP-A-0 785 307.
The paper described in this case as well must have a pick-resistance sufficient for the offset process; this pick-resistance is determined and qualitatively assessed in typical manner. In this regard as well, the statements in EP-A-0 785 307 are referred to.
Fine-particle pigments in the coat composition generally accelerate printing ink drying (shortening of the ink absorption time, expressed by a lower densitometer value) and water absorption. With the selection and/or mixture of the pigment grading the expert therefore has the ability, according to the invention, to influence both values. If it is possible with a specific printing machine arrangement to press a paper that has very rapid ink absorption times, highly active synthetic binder is preferably chosen as binder for the coater, in connection with polyvinyl alcohol to the extent possible. The binder for such a coater can thus consist of 6-12% synthetic binder and from 1% to 4% PVA in relation to coating pigment. If a longer ink absorption time is required with the same water absorbency, this can be achieved by additional binders in the coat recipe, for example by adding 0.5 to 1.5% carboxyl methyl cellulose (CMC), depending on the composition of the coating pigment. If the binder is given additional starch, in the range of roughly 6-10 weight percent, for delaying the ink absorption time, this may also have a reducing influence on water absorbency.
Beyond the binder content and mixture also influenced by the fineness of the gloss-developing pigments used, the necessarily high water absorbency, the desired printing ink drying time and a good coat setting should be taken into account. In addition, it should be taken into consideration that the paper gloss values decrease as the binder content increases. Altogether, the binder content in the coater should not exceed 18 weight percent in relation to coat
Hofmann Hans-Peter
Wurster Hartmut
Chin Peter
Fortuna Jose′A.
Haindl Papier GmbH
Smith , Gambrell & Russell, LLP
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