CURTAIN COATING

Slot-Curtain Coating

Precision Curtain Coating

Precision curtain coating, arguably the most commercially-advantaged liquid film coating method available because the method's compelling operational features (described below), can be leveraged to achieve an unexcelled combination of high productivity and superb coating quality.

A strategically-important limitation -

the flow rate per width of coating must be greater than a certain minimum in order to stably maintain curtain integrity.

Even after optimizing coating liquid formulation and using capable curtain edge guides, this minimum flow (typically between 0.7-and-1.3 cc/sec-cm) is relatively high compared with typical low-range flow rates for slot coating and other methods.

Strategic consequences:

High-speed curtain coating is required for thinly-coated applications and this may have important implications for production volumes and capital investment in suitable overall coating facilities.

Nevertheless, curtain coatings benefits are so compelling that the question of curtain coating suitability should be definitively answered when considering a new coating application.

DCT can assist you in assessing curtain coating's suitability for your particular coating application.

Compelling Process Features

In addition to the previously-described quality and productivity benefits inherent with all die coating methods, curtain coating has the following additional advantages:

• High coating speed
  The curtain's "hydrodynamic assist" resists air entrainment [Blake et al, 1994] so that maximum coating speed is shifted to much higher speed than possible with slot-bead, extrusion. Curtain coating speeds can be 1000 M/min and even higher.
• Robust fluid dynamical process capable of coating
  • rough and uneven webs
  • most splice geometries
  • overboard web edges, and
  • unsupported webs in spans
• Not subject to the visco-capillary instabilities (e.g., ribbing, chevrons, barring, etc) which commonly limit roll and bead coating.
• Uniformity is insensitive to web thickness variation and vibration
• Large applicator-to-web clearance in which the application
  • cannot entrap particles or bubbles (a chronic problem with a nip or tight gap),
  • cannot break web (low total web stress and no pinch points), and
  • requires only modest die-to-substrate set-up precision

Slot and Multilayer Slide Formats

3-Layer Slide-Curtain Coating

3-Layer Curtain Coating at Impingement

Two general formats of curtain coating dies:

• Slot-Curtain die
  • similar to slot-bead or extrusion die
  • single-layer application normally, but 2-layer application is possible
  • simple die mounting and positioning
• Slide-Curtain die _
  • can simultaneously coat practically unlimited number of accurate, uniform, thin layers (up to 10 layers is practicable)
  • more expensive die, more sophisticated die attachments (slide and curtain edge guides) and die mounting/positioning arrangement
  • achieved great commercial success in photographic industry since 1980's

Simultaneous Multilayer Coating
allows advantageous positioning of concentrations of important (and usually expensive) functional species within a multilayer package. This can achieve:

• combined product functionality and cost optimization
• coating process latitude extension
• coating facilities simplification and cost reduction

DCT provides both slot- and multilayer slide-curtain coating dies of high precision and thoroughly capable design.

Curtain Coating Applications

Successful curtain coating applications tend to be those with large production volumes and/or those significantly benefiting from curtain coating's unique process benefits.

The successful production of photographic film and paper, for instance, requires application of multiple (8-10 is common), ultra-precise, very thin coated layers.

Precision curtain coating is now being applied in the paper industry in both single and multilayer versions where its high coating speed capability can match legacy methods
(e.g., short-dwell flexible blade and metering size press), but can additionally achieve superior "coverage", and a number of other quality and operational features such as greatly improved "runability".

The adhesives industry is similarly beginning to use curtain coating (especially for PSA emulsions) at high application speeds without experiencing shear and film-split related defects and production problems.

Current new and developing curtain coating applications include

• single and multilayer specialty coated papers _ thermal printer, carbonless
• pin-hole free barrier papers such as OGR, MVTR, and water resistant applications
• surface functional papers such as Si-release and Ink-jet receiver layers
• coated paperboard
• single and multilayer aqueous PSA coatings for labels, tapes, sheets

DCT can assist you in developing your commercially-successful curtain coating application.