Before spending money on high-performance adhesives, converters would do well to perform rheology testing to ensure a proper application and smooth running of converting equipment.
Kishan Puwar, project supervisor at National Adhesives, studies the screen of the ARES Fluid Rheometer, which shows a waterborne adhesive's viscosity as a function of shear rate.
Senior technician Dhiru Patel uses the Bohlin Control Stress Rheometer, which shows a hot-melt, pressure-sensitive adhesive's viscosity as a function of stress. This material will be used for tape converting.
In the final analysis, rheometric analysis eliminates guessing about manufacturing.
In converting applications, a well-equipped rheology laboratory enables an adhesive user and an adhesive supplier to test an adhesive's elasticity, flow properties and other characteristics, minimizing the risk of development or switching to a different adhesive. As a result, converters can maximize productivity as they improve quality.
Rheometric analysis becomes a valuable service for both converters and their end-user customers. Adhesive suppliers who invest in sensitive rheometers can also streamline their own manufacturing and maximize batch-to-batch consistency and adhesive performance.
By studying flow properties, laboratory staff learn how a specific adhesive will perform on, and penetrate into, various substrates. For example, some converters need adhesives that will provide reliable performance in very hot or very cold environments, explains Kishan Puwar, project supervisor at National Adhesives, a unit of National Starch & Chemical Co., Bridgewater, NJ. Highly sensitive rheometers can simulate production techniques, coating properties and adhesive performance under a wide range of manufacturing processes.
Both extremes
"Our RDA III, two ARES and other three rheometers enable us to test elasticity from -150 deg C to 600 deg C (-238 deg F to 1,112 deg F)," says Puwar. "A well-known computer printer manufacturer wanted to know if the use of one of our pressure-sensitive adhesives would provide a strong reliable bond in an environment of 200-300 deg C (392-572 deg F). Our most advanced rheometers that have been installed during the last several years were able to provide the data needed to answer this customer's question."
Several rheometers are linked, enabling the recording of the rotation rate and the adhesive's ability to resist the force through time. All of the rheometers work with multi-functional, custom software. "We have a very high degree of control with some of our rheometers," says Puwar.
He notes that the rate of rotation can be so slow that it would take 30 years to one complete rotation. "Our rheometers sense and record very small forces," he adds. The National Bureau of Standards is just now installing rheometers as advanced as the ones National Adhesives has used for nearly a year, Puwar says.
A complete solution
Though advanced rheometers are essential, they're not the complete solution. In addition to a fully equipped lab, the people staffing it must know now to mimic an application, material science and adhesive and polymer chemistry. When teams of scientists at National Adhesives accept a customer project, they bring a wealth of technical know-how to bear on solving a problem or developing an improved adhesive.
Some of the scientists use spectroscopy, scanning electron microscopes and other devices that permit investigation at the molecular and atomic levels. Puwar notes that with robust and advanced tools, it's possible to determine what really adds to strength and flexibility in an adhesive and what doesn't, regardless of the temperatures that most converter applications require.
Experimentation procedures conform to ISO 9000 series requirements. These are important for some American customers and all European patrons. National Adhesives' rheology lab included only one rheometer 15 years ago. Its usefulness has been so instrumental in developing adhesives and problem-solving that the company quickly realized that rheological investigations are key to satisfying converters and maintaining a leadership role in providing advanced products.
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