Paper making and fiber liberation – Processes and products – With coating after drying
Patent
1998-03-24
2000-11-28
Chin, Peter
Paper making and fiber liberation
Processes and products
With coating after drying
1621644, 1621814, 1621815, D21H 1938
Patent
active
061530549
DESCRIPTION:
BRIEF SUMMARY
Conventional pigment-coated raw release papers exhibit a layer of pigment/binder mixtures on one or both sides, clay (kaolin), talc or calcium carbonate being used as pigments, separately or in combination, and chiefly polymer dispersions as binders, frequently in a mixture with modified starch products. The superior smoothability and thus higher surface density permit lamelliform pigments such as clay or even, to a limited extent, talc.
In general, these pigment-coated raw release papers are designated as "clay-coated papers", which is already an indication of the coating pigment primarily used (Coating, 1987, 10:366-372, and 11:396-398).
Compared with unpigmented paper coatings, these paper qualities exhibit economic and qualitative advantages, such as largely closed silicone film with greater dehesive action.
More recent developments in the field of coating technology, which are based upon the direct or indirect film-transfer technique, already permit the application of thin pigment coatings of less than 5 g/m.sup.2 (solid) to the raw paper, within the paper machine. Primarily used for this on-line pigmentation are roller application plants with volumetric premetering (gate-roll and blade-metering film presses or blade-application aggregates with a premetering unit: High special-metering dosage technique, such as Billblade HSM, LAS, HSM and twin-HSM) (see Das Papier [Paper], 1991, 10A:120-124, Wochenblatt fur Papierfabrikation [Paper Production Weekly] 1993, 10:390-393, and 1994, 17:671-676).
The purpose of these coatings is primarily to improve the printability of the paper, especially when offset printing is used.
This new application technology, also frequently termed thin-coating technology, is therefore likewise used for the production of pigment-coated raw release papers with thin coatings.
In contrast to the fields of application known thus far, in which the emphasis is generally upon printability by controlled adjustment of the coating porosity and the capacity of the paper to absorb coatings, the focal point is now the achievement of a largely closed paper surface with the thinnest possible coating application. Only thus, as in the case of the already mentioned classical "clay-coated" raw release papers with a frequently higher application, can the penetration tendency of silicone resins during subsequent coating to form release papers be kept within limits.
Pigment-coated raw release papers, produced using thin-coating technology, with applied coatings of approximately 5 g/m.sup.2 (solid) have been produced since 1994. Used as pigments are primarily special clay mixtures with a defined particle-size distribution and the most pronounced lamelliform structure possible. There has also been no lack of attempts to employ the likewise lamelliform talc or calcium carbonate as a coating pigment. The last-named pigment does not however adequately satisfy the imposed requirements with regard to surface density and transparency due to its bead structure and is for that reason used mostly only in combination with clay or talc.
During the coating of raw release papers with silicone resins to produce release papers, the highest demands are imposed upon the silicone coating, because otherwise unacceptably high deviations in the releasing behavior of the silicone-treated papers results, and thus for example disturbances during the labeling process. The uniformity of the applied silicon coating is ordinarily determined by x-ray fluorescence measurement of the silicon as the principal component of a silicone resin, the penetration of the x-rays into the thickness of the paper being limited to approximately 5 .mu.m.
Clay (natural aluminum silicate) or talc (natural magnesium silicate), significantly disturbs however the exact determination of the application weight of the silicone due to their silicon content or, in the case of classic "clay-coated" raw release papers, make pigment-coating applications in excess of 5 g/m.sup.2 (solid) impossible.
In the latter case, only the volumetric measurement of the use of
REFERENCES:
patent: 5635291 (1997-06-01), Yoshino et al.
Chin Peter
Kammerer GmbH
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