High Tech Adhesive Testing

March 4, 2003
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Pressure on suppliers and users of bottle and container labels to turn a profit makes it smart business to avoid waste and production downtime by testing materials before use. Problems with adhesives or release liners are amplified by the higher speeds of modern label-application stations as bottles and plastic containers whiz by at thousands of units per hour. Setting adhesive parameters to make the marriage of surface and label is key to assuring highly efficient and effective operations.

The crack-and-peel label business has blossomed over the past few years, too. The do-it-yourself businesses and retail label makers have printers and software to create small batches and single-use labels of great innovation and graphic appeal. Label producers are eager to serve this growing and profitable market, and quality is very important. One bad performance will turn these quality-conscious users away, with a resulting loss in reputation and profits.

The Tag and Label Manufacturers Institute (TLMI) and other industry associations have outlined an easy and acceptable group of standards for adhesive manufacturers, label producers and label applicators to follow. Instrument companies produce the testing machines that can verify and document the standards. (See Figure 1.) Waste (labels and containers), equipment downtime and poor performance in desktop and limited-batch operations can be minimized by verifying conformance with the specifications. Testing by manufacturers and users can uncover the source of a problem and minimize its impact.

Figure 1. The machine can be adapted to perform either the probe tack test or the loop tack test.

Label Stock Components and Their Important Properties

Label stock has several elements that are important to printing and proper application. (See Figure 2.) The face stock provides the printing surface for text and graphic messaging to the consumer. The adhesive holds the silicone-coated release liner and face stock together, then allows the application of the face stock to the container or media. The release liner is the backing material that protects the adhesive prior to use, holds its dimensions consistent during application, and is then discarded after application. Its silicone coating is important in preserving the adhesive before application. All of these components have distinct characteristics to make them successful, and each must be tested to specification to ensure proper performance.

Many of these properties are the responsibility of their producer. The face stock properties are monitored and adjusted as needed by the paper mill or paper converter. Optical properties like brightness, whiteness, gloss and opacity are tested in the lab by instruments that measure to international standards. Dimensional stability, such as tension and tear, and printability properties, including ink holdout and ink contact angle, are also tested in these off-line procedures. The tests are very important to the “look” and “feel” of the label in its final form – providing a message to the consumer.

Sometimes a label producer or user will use a handheld device to check opacity, brightness and gloss on a spot basis. A spectrophotometer can be used to check color matches and process color application. For the very sophisticated and waste conscious, color-proofing equipment using mini-sample prep devices that simulate the flexographic, rotogravure and offset printing processes are available to check ink formulae and match customers’ exacting color requirements.

Figure 2. Label stock has several elements that are important to printing and proper application.
The release liner is tested for similar dimensional stability, though its silicone coating may have to undergo further testing to make sure it performs according to its task. The silicone coating can be checked for its coefficient of friction, thickness and sensitivity to environmental change. Paper converters, many of them on the same site as the paper mill, will conduct off-line testing to ensure silicone-coated liners hit the target for label production.

Paper mills and some converters use sophisticated data-acquisition procedures for recording quality data for Certificates of Analysis and Quality Assurance documentation. The testing labs may have data-management systems, often integrated in a total quality management system (like ISO) that can be made available to customers. In the event of a reject, producers should be able to follow a lot back through the process to identify a possible cause. In this way, disputes can be settled easily or technical service employed smoothly. Use of quality data, taken by standard testing procedures, tends to eliminate finger pointing and accelerate problem solving on both sides of the business relationship.

Figure 3. In the probe tack test, the machine brings the tip of a probe into contact with a pressure sensitive adhesive held in a fixture.

Probe Tack Testing

Having checked out the raw material components thoroughly, the most sensitive and complex part of the label needs testing – the adhesive. The chemical nature of the adhesive, its cost and application make testing imperative. There are three fundamental tests – probe tack, loop tack, and release and adhesion.

The probe tack test is important for determining adhesive properties. The test is administered with a machine that brings the tip of a probe into contact with a pressure sensitive adhesive (PSA) held in a fixture. (See Figure 3.) A well-engineered product for testing will afford the user a choice of probe material, tip shapes and sizes, fixture options, and control of the probe itself. These control parameters include speed, contact pressure, dwell time between the probe and the specimen, and the rate of separation between the two.

A variety of machines have been on the market for 35 years, including the original Polyken single-reading probe tack tester with its gauge reading and hand crank. In the 1980s, Testing Machines Inc. (TMI) developed an updated instrument for single readings that included a digital readout and RS 232 output. Today, TMI and others produce PC-controlled probe tack testers where users program control parameters, create test methods, and tag specimens and tests with individual and recognizable names, dates and lots. Such instruments deliver real-time data on each test, prompt users to initiate tests and provide confidence in the tests through high resolution and accuracy including force measurement to 0.1 gram, position measurement to 0.1 micron and 0.02% test accuracy.

Figure 4. The critical points revealed during a probe tack test include the initial peak, a second peak and the point of separation.
The real-time data generated from the probe tack test include force/position curves, with thousands of data points marked during a test, providing insight into transient properties of the separation of adhesive and substrate. The data reveal adhesive behavior during the test, giving quality managers information into commercial applications. With the peak force established, adhesive properties are considered for both their single value and their development over a curve.

A typical curve will set force against position. The critical points revealed during the test include the initial peak, a second peak and the point of separation. (See Figure 4.) The initial peak from the starting point demonstrates the point at which the adhesive’s maximum strength is achieved for bonding. The second peak is the beginning of the de-bonding process, and the test is continued until the point of separation. Typical curves for comparison on permanent adhesives (e.g., rubber hot melts and emulsion acrylic) and removable adhesives (e.g., PSAs) have been established by users and found to be useful in changing formulations and applications.

Figure 5: In the loop tack test, the specimen is "looped," fixed to the probe and brought into contact with a flat surface.

Loop Tack Testing

The loop tack test is important for determining the adhesive properties in the final product. Common product types are pressure sensitive labels, tape and seals. The test begins when a specimen is prepared by “looping” it, fixing it to the probe and bringing it into contact with a flat surface. (See Figure 5.) The test parameters, function and results are similar to the probe tack test.

Figure 6. The release and adhesion tester measures the force to separate two adhered materials at a given speed and over a specified length and a specified angle.

Peel Testing

The release and adhesion test demonstrates the peel strength of the final product. Applications for peel are widespread – pressure sensitive labels, adhesives of all types, converters, medical products, office supplies, electronics, and a variety of other packaging and adhesive products. The basic peel test measures the force to separate two adhered materials at a given speed and over a specified length and a specified angle. (See Figure 6.)

The peel test is most commonly applied on pressure sensitive labels. TMI has two machines to measure the force required to remove the liner from the face stock – a basic measuring device and a PC-driven device. The basic machine is a chain-driven sled (about three feet in length) and an arm that can be set up in three different angles (90, 135 and 180 degrees). A sample is attached to the motorized test bed with double-sided tape, adhesive or a special test fixture. The portion of the sample to be removed is attached to a clamp set at its angle. The clamp is fitted with a stationary piezoelectric force transducer. By setting the angle and speed, the user can simulate the behavior of the label under high-speed production conditions.

The transducer reading is processed with auto-range software to produce test results. Hundreds of readings are taken during a typical test to generate force/position curves. Area analysis allows the operator to select a portion of the curve for further study. Min/max and a variety of other statistical data are generated for reports. TMI’s machine is fully programmable to allow the user to create his or her own test methods – number of tests, specimen names, sample conditions, areas of analysis and test parameters (angle, speed and length) that reflect the production environment.

The real-time data and final results can be stored and recalled for later comparisons. Data can be exported in standard formats. The TMI machine comes with a set of report templates that can be printed and distributed in forms that are easy to read and evaluate.

Profit Through Testing

The adhesive properties of other materials (packages, tape, et al) are measured in the same way — a standard test surface, specified angle and specified speed — as described above for pressure sensitive labels. The test can determine the uniformity of adhesive application (coating), bonding adhesion strength to the face stock and force required to remove the face stock. Typical failure results are adhesive failure (premature peel), cohesive failure (tacky residue on both surfaces), adhesive failure along the face stock and test surface, inconsistent adhesive application, and paper tearing.

Obviously, identification of any of the problems triggers changes to manufacturing or ordering that can save thousands of dollars in waste. Manufacturers of adhesives will use the results to develop coatings and substrates to fit the demands of users and maximize profitability of existing products. Users can set parameters for label stations, predict label performance, and avoid waste and downtime by performing this simple test. The cost of a release and adhesion tester (and most other testing equipment) is recovered in reduced downtime, optimized use of materials, waste reduction and improved customer satisfaction

Adhesives manufacturers may find that adhesion is too low, the face stock releases prematurely from the liner or “tunneling” results from excess adhesive wetness (showing up as bumps on the face stock). Another common problem is excessive adhesive strength that results in tearing or inability of the face stock to release from the liner.

Manufacturers of silicone compounds and release liners need to determine the interaction between silicone and the adhesive. Establishing the “lock up” point (failure of the liner to release the face stock), avoiding the transfer of silicone to the adhesive and finding the optimal coating amount can maximize the productivity of the label, adhesive and silicone coating.

When users improve their understanding of the properties of the materials they purchase, their suppliers will be in a better position to meet their needs. Any product that requires an adhesive is a candidate for release and adhesion testing. Converters and companies that make consumer products are well advised to consider peel testing. Companies that print, cut or perform any operations on “raw” label stock can use release tests to evaluate incoming and outgoing labels. Manufacturers of medical products like bandages, transdermal patches and newer items that use an adhesive coating can be sure that adhesion is within established limits for adherence and removal. Manufacturers of ink jet and laser printers can use the device to measure peel strength of cartridges and toner. The peel test can measure forces to open pouches, delaminate adhered surfaces and separate one substrate from another.

There are a few machines on the market that can perform the peel test. Each has its own advantages and shortcomings, but all are cost-effective compared to the cost of downtime. Most TMI customers have come back to upgrade machines, knowing that more data captured during the test provides more information for analysis, resulting in profits for the manufacturer and customer alike.

For more information:

Additional information on testing equipment for label stock components is available from Testing Machines Inc., 2 Fleetwood Couårt, Ronkonkoma, NY 11779; phone: 631-439-5400 or 800-678-3221; fax 631-439-5420; or e-mail info@testingmachines.com.

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