Pendulum Hardness Testing of Coating Test Panels: König and Persoz Damping Test Methods

Pendulum hardness testing of coating test panels is a standardized laboratory method for quantifying the surface hardness of paints, varnishes, and other organic coatings by measuring the damping time of an oscillating pendulum resting on the film. Performed in accordance with ISO 1522, ASTM D4366, and GB/T 1730, this technique is widely used by coating manufacturers, R&D laboratories, and accredited third-party laboratories for quality control, formulation comparison, and cure-behavior studies.

Technical Overview

The pendulum damping test is one of the most widely used bench-scale methods for determining the hardness of organic coating films applied to flat test panels. A pendulum, supported on the coating surface by two hardened steel balls, is deflected to a defined starting angle and released. As the pendulum oscillates, mechanical energy is dissipated into the coating film through viscoelastic deformation beneath the contact points. Softer or incompletely cured films absorb more energy and damp the oscillation rapidly, while harder films damp the oscillation more slowly, producing a longer damping time.

The test result is expressed as the time, in seconds, required for the oscillation amplitude to decay from a specified initial angle to a specified final angle. Because the measured value depends on pendulum geometry, ball diameter, and applied load, results from different pendulum types are not directly interchangeable. The two internationally recognized pendulum designs are the König pendulum and the Persoz pendulum, both of which are covered by ISO 1522 and ASTM D4366.

• ISO 1522 — Paints and varnishes — Pendulum damping test
• ASTM D4366 — Standard Test Methods for Hardness of Organic Coatings by Pendulum Damping Tests
• GB/T 1730 — Determination of film hardness by pendulum damping test
• DIN 53157 — König pendulum damping test for paints and varnishes
• BS 3900-E6 — Methods of test for paints: Pendulum damping test (König)

Pendulum hardness is a comparative measurement rather than an absolute physical constant. Results are influenced by film thickness, substrate rigidity, temperature, relative humidity, and the degree of cure, so strict control of test conditions is essential for reproducible data.

Testing Samples

Coated Glass Panels

Float glass panels, typically approximately 100 mm × 100 mm with a thickness of about 5 mm, are the preferred substrate for pendulum hardness measurements. Glass provides a rigid, smooth, and dimensionally stable support that eliminates substrate flexing as a source of error. Polished float glass panels are also used as reference materials for instrument calibration because their damping behavior is well characterized.

Coated Metal Panels

Coated steel or aluminum panels are commonly tested when the coating is intended for metal substrates, such as coil coatings, industrial maintenance paints, and automotive finishes. Panels must be flat, rigid, and free from warpage or vibration, since any panel movement under the oscillating pendulum shortens the measured damping time and biases the result toward lower hardness.

Other Rigid Substrates

Coated rigid substrates such as anodized aluminum, lacquered plastics, and sealed wood panels may also be evaluated, provided the surface is flat, smooth, and sufficiently rigid. Flexible or thin substrates should be backed or bonded to a rigid support before testing.

Sample Preparation and Conditioning

Coatings are applied to the panels by film applicator (drawdown), spraying, or other defined methods to achieve the dry film thickness specified in the product standard or test plan. After application, coated panels are cured or dried under the conditions specified for the coating system. Prior to testing, panels are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 3270, typically for a minimum of 16 hours. The test surface must be free of dust, defects, orange peel texture, and visible damage.

Testing Items

König Pendulum Hardness

The König pendulum test measures the damping time for the oscillation amplitude to decay from 6° to 3°. The König pendulum is relatively light and produces lower contact pressure on the film. Typical König values range from a few seconds for very soft films to more than 200 seconds for fully cured, hard coatings; the reference value on polished float glass is approximately 250 seconds.

Persoz Pendulum Hardness

The Persoz pendulum test measures the damping time for the oscillation amplitude to decay from 12° to 4°. The Persoz pendulum is heavier and imposes a higher load through larger balls, and it is particularly useful for harder coating systems. Persoz values are numerically larger than König values for the same coating; the reference value on polished float glass is approximately 420 seconds.

Hardness Development and Cure Behavior

By repeating pendulum measurements at defined intervals after application, laboratories can generate hardness-development curves that characterize drying speed, through-cure behavior, and post-cure hardening. This approach is valuable for comparing binder systems, crosslinkers, drier packages, curing schedules, and forced-dry conditions.

Testing Methods

Apparatus and Calibration

Testing is performed with a pendulum damping hardness tester equipped with either a König or a Persoz pendulum assembly mounted on a rigid, vibration-isolated stand fitted with a spirit level and amplitude scales or an electronic timing device. Before use, the instrument is verified on a clean polished float glass panel at standard conditions: the damping time should conform to the reference values stated in the applicable standard. Instruments should be kept away from drafts, direct sunlight, and sources of vibration.

Test Procedure

• Verify that the instrument is level and that the ambient temperature and relative humidity are within the specified range.
• Place the coated test panel, film side up, on the instrument table and position it so the measurement area is clear of edges and application marks.
• Gently lower the pendulum so that both supporting balls rest on the coating surface without impact or sideways dragging.
• Deflect the pendulum to the specified initial angle using the deflection template or release mechanism, then release it without imparting additional motion.
• Start the timer simultaneously with release and record the time for the amplitude to decay to the specified final angle.
• Lift the pendulum, reposition the panel to an adjacent untouched area, and repeat the measurement; at least three valid determinations per panel are normally required, ensuring that ball impressions do not overlap.

Expression of Results

The result is reported as the median damping time in seconds, together with the pendulum type (König or Persoz), the test standard applied, substrate and dry film thickness, curing and conditioning conditions, and the number of determinations. When hardness-development testing is performed, results are reported as damping time versus elapsed time after application.

Factors Affecting Results

Key variables include dry film thickness (thin films may allow the substrate to influence the measurement), temperature and humidity, substrate rigidity, surface texture, and incomplete cure. Consistent panel preparation and strict environmental control are therefore critical for meaningful interlaboratory comparison.

Applications

Pendulum hardness testing of coating test panels supports a broad range of industrial and scientific applications in the coatings value chain:

• Quality control of production batches of paints, varnishes, lacquers, and clear coats against internal or customer specifications.
• Research and development of resin systems, crosslinking agents, and formulation variables affecting film hardness.
• Automotive OEM and refinish coatings, where hardness development after application directly affects polishing and handling times.
• Coil coatings, powder coatings, and industrial finishes for evaluating bake schedules and cure response.
• Wood coatings, floor coatings, and furniture lacquers where surface durability and through-dry behavior are essential.
• Aging and weathering studies, where changes in pendulum hardness indicate degradation, post-hardening, or embrittlement of exposed films.
• Independent verification and batch release testing performed by accredited third-party laboratories.

Because the method is rapid, quantitative, and applicable to virtually any flat coated surface, it remains a cornerstone physical test in coating laboratories worldwide.

FAQ

What is the difference between the König and Persoz pendulum tests?

Both tests measure how quickly an oscillating pendulum is damped by a coating, but they use different pendulum geometries, masses, ball sizes, and amplitude windows. The König test measures decay from 6° to 3°, while the Persoz test measures decay from 12° to 4° with a heavier pendulum. Persoz values are therefore numerically higher, and results from the two methods cannot be directly converted.

Why do harder coatings give longer damping times?

Hard, well-cured films deform very little under the pendulum balls, so they absorb less mechanical energy per oscillation cycle. The oscillation decays slowly, producing a longer damping time. Soft or tacky films absorb energy readily through viscoelastic deformation, damping the pendulum quickly.

What sample size and film thickness are required?

Panels are typically around 100 mm × 100 mm and must be flat and rigid, with enough clear surface for at least three non-overlapping measurements. Dry film thickness should follow the relevant product standard or specification, since films that are too thin can be influenced by the substrate, and this must be reported with the result.

How is the instrument calibrated?

The pendulum tester is verified by measuring the damping time on a clean, polished float glass panel under standard conditions. The reference values, approximately 250 seconds for the König pendulum and approximately 420 seconds for the Persoz pendulum, confirm that the instrument is functioning correctly before testing coatings.

How does pendulum hardness compare with other hardness methods?

Pendulum hardness is a damping-based, quantitative measure expressed in seconds, whereas pencil hardness (ASTM D3363) is a scratch-based grading method and Buchholz indentation (ISO 2815) measures indentation resistance. The methods probe different deformation mechanisms, so they often rank similar coatings consistently but do not produce numerically interchangeable results.

What conditions must be controlled during testing?

Testing must be carried out at 23 ± 2 °C and 50 ± 5 % relative humidity on a stable, vibration-free bench. Panels must be properly cured and conditioned, the surface must be clean and defect-free, and the pendulum must be lowered onto the film without shock to obtain valid, repeatable data.