How it works:
The pendulum hardness tester is a commonly used instrument to evaluate the scratch resistance of coating materials, especially varnish-like materials. Its core principle is based on damping oscillations. The instrument is in contact with the coating surface by means of a hemispherical contact connecting a pendulum rod and allows it to swing freely. The softer the coating surface, the greater the resistance to swing, resulting in faster swing attenuation and shorter swing period. On the contrary, the harder the surface, the slower the swing decay and the longer the period. The hardness value of the coating is calculated by measuring the time it takes for the pendulum to swing from a certain starting angle to another angle and comparing it to the time it swings on a standard glass plate. The physical significance of this process is clear, and it provides a reliable basis for quantifying the mechanical properties of the coating.
Varnish scratch resistance
The scratch resistance of varnish is not a single attribute, but a comprehensive embodiment of various mechanical properties such as hardness, elasticity, cohesion and adhesion to the substrate. The hardness value measured by the pendulum rod hardness tester is mainly related to the material's ability to resist plastic deformation, which is the basis of scratch resistance. However, a varnish coating with only high hardness but high brittleness may chip under scratch from sharp objects; Coatings with a certain degree of elasticity and toughness can absorb part of the energy through deformation, so as to show better scratch resistance. Therefore, swing rod hardness testing is a key method to define the "bottom line" of scratch resistance, which establishes the basic mechanical strength threshold that materials must have.
Test methodology
To ensure the reliability and comparability of test results, the operation must strictly follow relevant technical standards. Before testing, the coating must be fully cured under standard conditions and the ambient temperature and humidity must be stable. The instrument is calibrated on a standard glass plate to ensure accurate reference swing times. During testing, swing rod contacts should be placed lightly on the surface of the coating to avoid applying additional pressure. The average of multiple swing cycles is usually recorded for accuracy. The main technical standards at home and abroad clearly stipulate the details of the test, and these specifications jointly ensure the status of hardness value as an effective comparative data.
| Core test parameters | Typical requirements or influences |
| Ambient temperature | 23±2°C |
| Ambient relative humidity | 50±5% |
| Coating curing state | Fully cured, for as long as the product specifies |
| Calibration substrate | Standard glass plate, hardness value defined as 100 seconds |
| Swing period measurement | It is usually calculated by the time it takes to attenuate to a specific angle |
Data interpretation
The test results of the pendulum rod hardness tester are usually expressed in seconds, and their values directly reflect the relative hardness of the varnish coating. In R&D and quality control, this value is used to set the minimum limit of qualification. For example, a certain type of wood varnish may require a pendulum bar hardness of no less than a certain value to ensure that it is resistant to common friction and scratches in use. However, as mentioned earlier, this hardness value needs to be judged in combination with the results of other tests (e.g. scratch test, impact test). Formula Hr = (t / t0) × 100% It is often used to express relative hardness, where: t For the swing time on the coating,t0 is the swing time on a standard glass plate. The closer this value is to 100%, the closer the coating hardness is to the glass.
Conclusion
The pendulum bar hardness tester, as a classic instrument, defines the basic hardness of varnish coatings by accurately measuring damping oscillations. It sets a quantifiable basic threshold for evaluating the scratch resistance of varnishes and is an indispensable tool in product development and quality control. Understanding the principles, performing tests in a standardized manner, and interpreting the data together helps to accurately grasp the boundaries of the mechanical properties of varnish materials, laying the foundation for a more comprehensive durability assessment.
References
ISO 1522: Colored paints and varnishes — Pendulum damping tests.
GB/T 1730: Colored paints and varnishes - Swing bar damping tests.
ASTM D4366: Standard Test Method for Determining the Hardness of Organic Coatings Using the Swing Bar Damping Test.