Research engineers from the Multilayer Modular Flooring Association set out to investigate the criteria of joint locking strengths.

The goal of the Multilayer Modular Flooring Association (MMFA) is to ensure long-term certainty in handling the attractive MMF floorings for both floor layers as well as users. The association’s engineers have investigated methods of measurement and criteria for the locking strength of flooring elements.

The mechanical joint strength of so-called click joints plays a decisive role in their ability to float lay flooring elements in an easy and glue-free way. The joint strength also affects how easy it will be for the interlocking elements to detach.

For many years, different qualities of LVT (luxury vinyl tiles) have been entering the market. The European MMFA has started examining the binding quality requirements of these planks because concerns regarding the locking strength between the individual models have arisen.

While the click joints originally developed for laminate floorings have proven their durability on billions of square metres of flooring installations, a significant portion of MMF flooring is made from materials other than the original wood-based panel substrate. These materials include resilient thermoplastics (plastics that are formed under the action of heat) and these substrates can have a different type of elasticity. Floating flooring can receive a significant amount of wear and tear in both residential and commercial applications and the module joints of these flooring systems need to stay in place so that the planks to not come apart.

Together with the Institute of Wood Technology in Dresden, the MMFA gathered the necessary basic knowledge of these flooring systems to assess reliability and further develop the quality of the joints in these semi-resilient floorings. The research engineers set out to create laboratory tests that would simulate everyday encounters that the floors could experience. The research engineers based their tests on comparable tests for laminate floorings in accordance with ISO 24334 (Laminate floor coverings – Determination of locking strength for mechanically assembled panels). A test machine for determining the locking strength was set so that the claws or jaws pulled apart the defined installed test pieces at a speed of 5 mm/min.

The optimum measurement test speed was determined to be 5 mm/min because a parallel test series showed that for semi-rigid click joints, one test speed was too low while the other was too high. If the test speed is too high, it is impossible to determine the joint opening and test accuracy diminishes. The research engineers also measured the tensile force applied to joints twice, because click joints can open under tensile forces, but well-designed joints close again after the tensile force reduces. The team measured the tensile force once at a joint opening of 0.2 mm and then on reaching the maximum force under which the joint tears apart. This is the only way to determine whether the joint of the modules is stable under normal use and holds for a long time.

Four different materials were subjected to the jointly defined test procedure in eight separate laboratories. These materials included (1) 5 mm thick elements with fibreglass reinforcement in the vinyl middle layer, (2) 4 mm thick elements with a vinyl middle layer without fibreglass reinforcement, (3) 4.5 mm thick elements with a thermoplastic non-vinyl middle layer and (4) 4.5 mm thick elements with a composite core.

The materials were cut to the same length and width and all of the test pieces were fixed together so that they had an identical initial position before they were conditioned in a room climate of 22 to 23°C with 50% relative humidity. The research team then carried out the test procedures on the joints of the long sides as well as the end faces.

MMFA results of Robin test Jnl 7 16

Based on the test results, the researchers were able to conclude the following:

•    It is technically possible to determine the respective force applied for a 0.2 mm joint opening.
•    If the joint between the elements has a large clearance fit, relatively large standard deviations result in the test series, especially for the forces applied to achieve the opening F0.2 mm and at increased test speeds.
•    MMF (LVT) products should be sensibly tested with a test speed of 5 mm/min at measuring point F0.2 mm or F0.3 mm.

A range of proposals for future test methods was derived from this exercise. The testing methods as well as the results are contributing to the CEN standardisation work (CEN/TC134/WG10/PG2) by the MMFA experts. Dr Theo Smet, Chairman of the “Locking strength” project group and technical consultant of the MMFA, comments:

“The objective of our project group is to develop a uniform test method for determining the locking strength, with suitable test parameters for each material type laid floating. These should enable objective and inter-comparable assessment and provide clarity for the user of the floorings regarding what they can and cannot do.”

For more information contact the Multilayer Modular Flooring Association on +49 521 965 3339 or via www.mmfa.eu.