Screen printing viscosity

Similar to screen printing, the spray coating method [95] is widely used for catalyst fabrication, especially in labs. The major difference between the two is that the viscosity of the ink for spray coating is much lower than that for screen printing. The application apparatus can be a manual spray gun or an auto-spraying system with programmed X-Y axes, movable robotic arm, an ink reservoir and supply loop, ink atomization, and a spray nozzle with adjustable flux and pressure. The catalyst ink can be coated on the gas diffusion layer or cast directly on the membrane. To prevent distortion and swelling of the membrane, either it is converted into Na+ form or a vacuum table is used to fix the membrane. The catalyst layer is dried in situ or put into an oven to remove the solvent. 

In the past, electron beam radiation was applied to produce PSA exclusively however, recent improvements in UV curing technology (precise UV dose control, suitable photoinitiators) permit UV to be used to produce pressure-sensitive adhesives. PSA formulations can vary in consistency from low-viscosity liquids up to solids melting at 80°C (176°F). Therefore, applications may vary from screen printing to roll coating to melt extrusion. Coat weights for most PSA materials vary from 1 to 10 g/m. ... 

Prepare an ink containing 11.2 pi glutaraldehyde, 2 mg BSA and 0.2 mg BSE per gram of 3% (w/v) hydroxyethyl cellulose. This mixture will not have the consistency required of a screen-printing ink. The viscosity should be adjusted by the addition of graphite powder (about 300 mg graphite powder/1 g ink). 

This metallic print binder is designed solely for screen printing, and is supplied at a particularly high viscosity which is required in particular operations. The fastness of these prints is exceptionally good to drycleaning and laundering the prints show excellent brightness. 

When the shear stress of a liquid is directly proportional to the strain rate, as in Fig. 2.4a, the liquid is said to exhibit ideal viscous flow or Newtonian behavior. Most unfilled and capillary underfill adhesives are Newtonian fluids. Materials whose viscosity decreases with increasing shear rate are said to display non-Newtonian behavior or shear thinning (Fig. 2.4b). Non-Newtonian fluids are also referred to as pseudoplastic or thixotropic. For these materials, the shear rate increases faster than the shear stress. Most fllled adhesives that can be screen printed or automatically dispensed for surface-mounting components are thixotropic and non-Newtonian. A second deviation from Newtonian behavior is shear thickening in which viscosity increases with increasing shear rate. This type of non-Newtonian behavior, however, rarely occurs with polymers. ... 

The flow properties of an adhesive are largely controlled by its inherent viscosity and by the alteration of its viscosity through the addition of diluents and fillers. How properties are important for screen-printing, automatic dispensing, and capillary flow, and can be tailored by the type of filler, amount of filler, and the particle-size distribution. 

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