Coating viscosity control is a significant challenge for all coating operations. While many factors impact the film's ultimate characteristics, the viscosity of the coating material is of central importance in achieving consistency. The viscosity of a coating relates directly to the concentration of coating solids in the fluid, which is the basis for the thickness and uniformity of the film.
Incorrect film thickness results in off-quality unusable end products, with unacceptable visual or other performance characteristics. Scrap reduces yield, in addition to the loss of costly coating materials and the environmental costs associated with manufacturing. Periodic manual viscosity checks increase labor costs and are subject to operator variance. Plant operational characteristics-temperature changes, blending variations in coating materials, and process-driven evaporation-frequently make off-line measurements impractical.
A typical coating system includes: supplies of base stock and make-up fluid, an agitated mix tank, a pump, heater, filter, the feed line to the applicator, the applicator itself, and the return line to the mix tank. The applicator can utilize a dip, roll, spray, sputter or other process. To accurately and reliably control coating viscosity, Cambridge Viscosity recommends the following best practices.
1. Utilize closed-loop control. Locate the viscosity sensor so it is measuring the characteristics of the material that is actually being applied to the end product. The best location is in the feed line to the applicator. Sensor information should be fed directly to the electronics that controls the addition of make-up fluid or solvent. An integrated PID controller provides excellent results. A separate level sensor for base stock addition completes the system.
2. Temperature-compensated viscosity is essential. Temperature has a significant impact on viscosity, but not necessarily on coating concentration. Unfortunately, temperatures in production environments are often not well controlled. Viscosity and thus costing solid levels must be correct despite temperature variations. Coatings have repeatable temperature-viscosity relationships that can be readily modeled mathematically, and can be directly programmed into electronics to provide temperature-compensated viscosity for accurate viscosity control despite temperature changes.
3. Offline measurements are important, but must be tightly controlled. Off-line measurements are often the standard to which coaters control viscosity. Unfortunately, they are subject to significant error from a variety of sources. These off-line measurements can be via cups in the production environment, or more sophisticated laboratory equipment. It is important that:
a) The sample is drawn from coating that is typical of that which is being applied.
b) Samples must be taken frequently enough so that variations.
c) Shear conditions in the line be duplicated in the lab.
d) The sample taken and measured such that no off-gassing of the coating is allowed.
e) All aspects of the tests must be handled consistently to negate inherent operator variability.
4) Automated in-line viscosity control removes error. Variations in sample taking and lab techniques are best controlled through automated in-line measurement and control. The sensor can be located so that it measures coating fluid that is representative of the fluid being applied. Solvent or make-up fluid can be automatically added via solenoid control to assure coating fluid consistency. Fluid shear is overwhelmingly controlled for most fluids by the way the fluid is pumped and piped. Consistent shear is achieved by mounting the sensor in the applicator feed line. In this way, operator-induced errors are minimized for accurate and repeatable results.
5) Small size sensors that are suitable for hazardous areas make installation and maintenance easier. Compact sensors are easier to install and maintain. Where sample conditioning is necessary, small sensors require smaller amounts of fluid treatment. Self-cleaning/low maintenance sensors are available in in-line or in-tank configurations.
About Cambridge Viscosity
Cambridge Viscosity is the leading supplier of viscometers for performance coatings worldwide. Cambridge lab and process viscometers use the same core technology and are fully compliant with ASTM D7483. Cambridge instruments are compact, require small amounts of sample, incorporate temperature sensors and are extremely accurate, repeatable and robust. The sensors-utilizing easy to maintain and operate oscillating piston technology-incorporate best practices and are fully proven as reliable devices for performance coating control. Cambridge's global reach provides application engineering support and service wherever and whenever needed.
To learn more visit www.cambridgeviscosity.com.