Article published in the Finnish Trade Journal: Paperi ja Puu - Paper and Timber Nr. 7, October 2001
by Directors  R.H. Donnelly, Ph.D., and  M. Y. O. Kangas, M.Sc., of Dryad Technology, Inc., USA.

Paper surface treatment, either sizing or coating, is a demanding process in paper manufacturing. This process stage has an adverse effect on the production efficiency and limits the machine operating speed.

Further, the complexity and the size of required equipment is a limiting factor in paper product development. Applications of several layers of coating on the paper surface in order to achieve desired paper properties is today a costly process.

The limitations encountered in the most advanced coating systems with blades or applicator rolls have shown that further development is needed. There is a growing consensus that the next step forward involves spraying. This technology is in principle simple: a set of nozzles in an applicator box spraying the size or the coating.

It is apparent that spraying technology, widely used in other industrial applications, will offer significant advantages for the paper or board manufacturers and their customers.

The use of spraying technology for paper applications was tested in the early seventies. Some efforts were made periodically later on to apply this technology, but were largely unsuccessfully.

There has been a further effort to develop spray coating based on high pressure, small-opening nozzles operating at 100+ bar pressure level. For the industry segment working in paper surface treatment, complicated systems combined with nozzles that either plug or quickly wear out are not very attractive.

Safety is another consideration. High pressure applications are normally avoided.

Dryad Technology, offers an alternative in low pressure spray sizing and coating concept designed to retrofit existing coating stations and sizing applicators, as well as providing the basis for new paper machine designs.

The DT concept is a developed version of spraying technology, designed for industrial applications with safety, reliability and simplicity in mind.

The low pressure spraying is aided by low pressure gas (under two bars). The DT low pressure dispersion works with equal results as the high pressure gradient over the nozzle tip in a high pressure environment (Fig 1).

Fig 1 Nozzle design

Dryad Technology, Inc. (US) verified, based on a major paper company pilot scale testing, that coated paper manufactured with its patented process can result in equal product quality as is achievable with present blade coating technology.

The opening of a single nozzle can as large as 5+ mm offers the following advantages in day-to-day operations:

• Elimination of plugging caused by raw material impurities

• Avoids rapid wear abrasives

Currently, there is further development. Dryad Technology, Inc. (US) is completing – with a major nozzle manufacturer -- a series of tests regarding a new type of nozzle design. To summarise the test status:

• Achieving full control over the low pressure nozzle behaviour while spraying coating colors or sizing liquids using a proprietary nozzle design

• Eliminating commonly experienced coating build-ups on the nozzles

Another characteristic of the patented Dryad Technology is the possible use of "a smoothener" after the spray application, both in coating and sizing. The "smoothener" is a low-pressure blade or air-brush, or a combined blade/air brush. The "smoothener" appears to be capable of creating first class rotogravure or offset surface for any grade. Also a pair of rolls after the surface treatment could serve as a "smoothner" depending on the printing application.

We want to emphasize the meaning of the "smoothner" and the potential use of the low-pressure blade. The design of the "smoothner" blade is different from a conventional doctor blade and can be applied at a much lower angle against the paper web.

The Dryad Technology concept will provide a new arena for the paper industry in developing high efficiency operations at a new level and in tailoring new paper grades for specific clients. To summarize:

• Speed -- no speed limits in paper surface treatment, combined with a major runnability improvement

• Cost Savings -- a positive effect on the environment as bleaching of the base paper could be significantly reduced or even replaced with controlled application of pigment layers on the paper surface. The use of secondary fibre will become more practical for all surface treated applications.

Specific considerations for sizing and coating include the following:

• Sizing --in sizing, spraying of starch at high solid contents, 15 % and above, thus reducing the need for after-dryer capacity

• Coating flexibility -- flexibility in selecting pigments with a desired property -- such as whiteness -- as the pond between paper and "the smoothener" is eliminated and the rheological properties of the slurry do not play a major role ( Fig 2 )

• New multi-layer coating approach -- significant cost savings, especially in providing for a flexible multi-layer coating. The final paper product can be constructed starting with a lower brightness and lower strength base paper -- first coating layer applies low-cost pigment, and only the last layer of high-bright, small-particle size is high-cost pigment. This appears possible without significantly increasing the equipment size, and there is the flexibility to add layers of pigment with desired properties (Fig 3 )