The development of laboratory coating technology has not kept up with the significant improvements that have been made in the manufacturing coating process.

The methods used in the laboratory have been virtually unchanged for many decades. Typically, sheets are coated in the laboratory using some form of doctoring device and are dried with ambient air. The coating quality and reproducibility are poor. The results are very sensitive to operator technique and significant reformulation and optimization is needed for scale-up.

There is technology available to correct these deficiencies and to provide a laboratory process with improved reproducibility and quality along with lower costs. The key technology elements needed to obtain these improvements are:

• Automated sheet coaters and dryers
• Benchtop continuous coaters
• Characterization of coating solution and supports
• Quality control procedures for coating/drying process.

The first key technology, automated laboratory sheet coaters and dryers, automate the entire coating-drying process and remove the variability introduced by the manual process. There are a variety of coating heads—knife, roll, gravure, and multiple-roll configurations available. These applicators provide a range of thickness, improve quality and they more closely simulate the full-scale process.

The automated features improve the reproducibility of the entire process and reduce the dependence on operator technique. The coating apparatus can be coupled with a dryer so that the sheet is fed directly into the dryer for controlled drying. This permits a variety of drying experiments to be conducted, and data can be obtained to help optimize the commercial-unit drying conditions. Productivity is good— 5-10 min/coating, and there is much less operator sensitivity.

Regarding economics, the cost of the machines ($40,000-$60,000) is higher than the doctor devices now in use. However the improvement in quality, productivity and reproducibility will result in a lower overall cost. A reliable reproducible source of coatings will also result in faster product development times.

Laboratory continuous benchtop coater dryers are the second key technology. These are narrow, miniature coaters that contain an unwind, applicator, dryer and rewind. They coat a narrow web, 3-12 in. and run continuously at low speed, 1-10 fpm. Because the final manufacturing process is continuous, these machines are a good step in the conversion from a laboratory sheet-coating process to a continuous web process. The continuous machines cost $60,000-$100,000.

There are several advantages for the continuous machines. They can coat and dry a continuous web using standard coating methods. The quality and reproducibility is good and material can be used for development programs and customer evaluation if the width is acceptable. Some basic data for use in scale-up to commercial process can be obtained.

The third key technology is to consistently characterize the physical properties of the coating solution and the substrate. Because this is only a laboratory coating, these properties are often not consistently measured and recorded or they are measured at the start of a new product development and ignored during the subsequent laboratory coatings. Measurement and control of the following properties will add to improved quality and reproducibility:

• Viscosity of the coating solution and the shear rate sensitivity
• Surface tension of the coating solution
• Surface tension of the support
• Physical quality of the support

Surface tension is particularly important because the surface tension of the coating solution needs to be at least 5 dynes/cm² lower than that of the support. Any differential lower than that and the coating will tend to bead instead of spreading uniformly. The quality of the support also needs to be monitored. A wrinkled support with coating defect will not lead to a good product.

Using QC and statistical techniques to monitor and control the laboratory process is the final key technology. To conduct this analysis on an ongoing basis, the characterization data obtained should be stored in a database and should be widely available so that all can benefit from the data. In addition, variables which influence the final results such as drying conditions, coating conditions, ambient air temperature and relative humidity, need to be measured and recorded. The observed defect level, quality and coatweight need to measured and included in the database.

Various QC charting and statistical techniques can then be used to monitor trends and develop correlations of variables with the quality of the final product. This data should then be used to upgrade the process.

Based in Ft. Mill, S.C., the Assn. of Industrial Metallizers, Coaters & Laminators provides a broad range of services to the converting industry. If you have inquiries concerning products, equipment or material sourcing, converting capabilities or technical issues, the group's "Ask AIMCAL" service is available at its Web site: www.aimcal.org. For more information, contact AIMCAL at 803/802-7820, fax: 803/802-7821.