Studying dimensional changes in multi-layered flooring

by Darren
MMFA studying dimensional changes in multi layered flooring Jnl 1 17

Materials used in multi-layered flooring can change shape. The MMFA studies these changes.

The MMFA (Multilayer Modular Flooring Association), together with the Wood Technology Institute in Dresden (IHD), launched a joint project named “Temperature Resistance”, where dimensional changes in multi-layered flooring from temperature fluctuations were studied.

The wear layer of multi-layer flooring generally consists of thermoplastics, as does a high proportion of the core material. Under the influence of temperature fluctuation, thermoplastics can theoretically change shape, so the leeway that is incorporated during installation for walls and doorways, for example, allows for these dimensional changes. The project aimed to uncover how temperature fluctuations cause a floor to “grow” or “shrink” regardless of its design, and how great the differences are.

According to the MMFA, the usual temperature range of underfloor heating is relatively insignificant in this respect, because the classic scenario of solar radiation through a large north-facing window on a summer’s day, can easily lead to temperatures hotter than the tar on a Formula 1 race track in Abu Dhabi or tropical Malaysia.

While the possible contractions of these floorcoverings resulting in permanent change after exposure to thermal stress have been regulated in EN ISO 23999, practical behaviour in the form of temporary expansions during exposure to thermal stress has not yet been taken into account by this standard. This led to a proposal to study the dimensional changes in multi-layer flooring during exposure to heat.

Flooring samples in three different compositions were selected for testing in six different laboratories, namely 5 mm thick elements with glass fibre reinforcement in the vinyl core, 4 mm thick elements with a vinyl core without glass fibre reinforcement and 4.5 mm thick elements with a non-vinyl thermoplastic core. Sample pieces were cut at the IHD, the institute in charge, with side measurements of 160 x 160 mm, and each laboratory received three samples of each of the three types. Each sample was subjected to two different test cycles in six stages. One cycle reached a maximum of 60°C, the other reached as high as 80°C. Each 72-hour test cycle (whether 60°C or 80°C) included the following steps:
(1) Test pieces were acclimatised for a minimum of 24 hours at 23°C and 50% relative humidity
(2) Initial measurements of each sample were taken and recorded in the X and Y dimension
(3) Samples were mounted for six hours on defined steel plates at maximum temperature for the respective cycle (60°C or 80°C), measurement was taken after six hours
(4) Samples were mounted for 18 hours at 23°C and 50% relative humidity on the same steel plate, measurement was taken after 18 hours
(5) Samples were mounted for a further six hours at the maximum temperature for the respective cycle (60°C or 80°C), measurement was taken after six hours
(6) Samples were mounted for a final 18 hours at 23°C and 50% relative humidity and dimensions were measured after 18 hours.

Two different digital measuring devices were used, namely a frame with a dial gauge and a frame with calliper gauge, both of which offer a measuring accuracy of 0.01 mm. Significant data for dimensional changes were produced from the six testing laboratories that were used for the project.

The results of the first and second heating and cooling part-cycles (steps 3/4 vs. 5/6) showed that the changes vary so little that it might be possible to consider single heating and cooling cycles and still produce reliable results, thereby shortening the testing process significantly and saving costs in practice.

The conclusion
According to the experts running the round robin tests, the following conclusions were drawn: Unlike glued vinyl flooring, when elements are laid as a floating installation, it is important to take a measurement after heating. These measurements provide important information on how the click connections are subject to stress.

The requirements for the tests need to contain a time limit for measuring directly after heating. Reproducibility of the test results at 60°C proves to be better than at 80°C. Thus, consideration should still be given as to whether a 60°C test would suffice, especially as this temperature seems more relevant to practical applications.

A further question for discussion that arose from this project is how to record convex or concave deformation in the Z direction, which arises in a similar manner through the effects of temperature, since the inter-laboratory testing described here only recorded two-dimensional changes.

According to the MMFA, dimensional behaviour under the influence of temperature fluctuations plays a crucial role and therefore has to be researched and standardised, particularly in the case of modular floating installations with click connections. Expansion and contraction create forces that will prove relevant for the MMFA “Locking Strength” project running in parallel with this.

“Through the right composition of flooring elements and the strength of the plank locking system, we must ensure that floating floors are as unaffected as possible by mechanical influences as well as temperature fluctuations,” commented Volker Kettler (MeisterWerke), MMFA vice president and convenor of the association’s Technical Working Group.

The next step is for the MMFA to develop independent testing methods for floating floors of varying compositions with thermoplastic surfaces and the click connection system, and embed these into international standards work.

For more information, contact the MMFA via www.mmfa.eu

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