Soft-tipped blades can significantly boost coating color performance
A comprehensive study by chemical engineers at Finland’s Åbo Akademi University shows that soft-tipped blades can out-perform their ceramic counterparts when it comes to delivering smooth, even coating.
Soft-tipped blades have earned a reputation for yielding improved coverage compared with conventional steel blades and ceramic blades, combining the benefits of “contour coating” with blade metering. This means coverage can be improved while preserving surface gloss and smoothness. But would those benefits be achieved regardless of the type of coating used?
To answer this question, engineers at Åbo Akademi University’s Laboratory of Paper Coating and Converting undertook an extensive series of controlled tests, using eight different coating colors with different water retention and rheological characteristics, and comparing the coating quality achieved with ceramic versus soft-tip blades.
Results showed that while the rheology and dewatering characteristics of each particular coating color had an effect on the final result, the soft-tipped blade appeared to reduce the differences in surface properties of the samples treated and yielded noticeably improved coating coverage with all colors tested.
Under the test conditions, colors with inferior rheological and water retention characteristics benefited most from the soft-tipped blade, and close examination confirmed that the soft-tip did not affect either the optical properties or porosity of the coating layer.
Smooth operator
As papermakers know, the main purpose of the coating layer is to improve the smoothness, whiteness and brightness of paper and increase its printability. Good coating coverage generally means contour coating, where the blade adapts to the shape of the paper to maintain consistent coverage. The trade-off, however, has usually been a loss of smoothness, obliging manufacturers to juggle production processes to achieve an acceptable compromise between good coating coverage and a smooth finish.
Incorporating a special elastomeric layer at the tip, soft-tipped blades can help solve that problem, adapting their shape to the surface topography of the paper to improve coating without substantially increasing surface roughness.
The nitty-gritty
FIGURE 2
The coating color hits the blade and is forced through a wedge-like channel
creating a force against the blade. The dynamic force (Fd) is a sum
of the impulse force (FI) and the hydrodynamic force (FH) and is equal
to the total mechanical load (Fm) on the blade.
FIGURE 1
The
coating color on the left immobilizes faster than the coating color on
the right despite having better water retention, because of lower immobilization
solids.
While soft-tipped blades may offer advantages over other coating blade types, top quality results depend on more than just blade performance. To determine the role played by the choice of coating color, the Finnish engineers mixed 26 different coating color formulations with different clay/GCC ratios, different types of thickener and different solids contents, and selected eight formulations for the trials.
Two factors in the coating color affect the dewatering rate and hence the immobilization of the coating layer: immobilization solids, and water retention (see Figure 1). Fast immobilization reduces dewatering and water penetration, preventing the paper swelling which negatively impacts coating uniformity and smoothness. The use of high application solids and thickeners both reduce dewatering.
At the physical level, blade coating can be performed in two different ways: using stiff blades or bent blades. Bent blades usually have tip angles of less than 15°, and are used to produce the top layer of coated board. In bent mode, the hydrodynamic force is believed to be the dominant force acting on the blade (see Figure 2), with the strength of this force dependent on the viscosity of the coating color and the thickness of the wet coating layer under the blade tip.
All relatively straightforward – except, as the research team noted, the exact viscosity of the coating color under the blade tip is unknown, because of the process of dewatering. That means that colors which immobilize fast increase blade forces and coat weight because of the color’s higher viscosity and the reduced liquid layer under the blade tip.
In addition, the rate of immobilization of the color and the wedge formed by the coating blade both play an important role in the final surface properties of the coated board. Colors that immobilize too fast can cause streaks, while slow immobilization may increase water penetration, causing swelling and reduced coverage and smoothness.
Coating colors
The eight color formulations were varied using different clay/GCC ratios, thickeners and solids contents. The clay was American and two GCC pigments were used: pigment A, with a particle size of 90% < 2µm, and pigment B, with a particle size of 60% < 2µm. The different thickeners were a synthetic alkali soluble thickener (ASE) and two CMC thickeners: Finnfix 10 and Finnfix 15W5.
Putting the colors to the test
The engineers in the Laboratory of Paper Coating and Converting measured each of the eight chosen coating colors for rheology from very low to very high shear rates. The trials involved 16 reels of 242g/m² multi-ply board with 14g/m² pre-coating, with an applied topcoat of 12g/m² at a machine speed of 400m/min.
Two types of blade were used: a standard ceramic blade, and a soft-tip blade. Each was used with all eight color formulations to allow for direct back-to-back comparison. Once the topcoat had been applied, each sample board was tested for optical properties and surface roughness using a range of specialized instruments. The coated board was also tested for porosity to see whether any changes had been induced by the different blade types.
Tests showed that all eight coating colors behaved more uniformly under the soft-tip blade. Coverage was also improved, with low high-shear viscosity and water retention colors showing the most benefit. While surface roughness increased slightly under the soft-tipped blade when colors with fast immobilization were used, neither the optical characteristics of the board nor its porosity were affected by the choice of blade.
Overall, the test results show that soft-tipped blades tend to yield more consistent coating quality, and could, under certain circumstances, be used to compensate for coating colors with poor water retention and unfavorable rheological characteristics and immobilization rates, to achieve superior finish and printability.