Durability testing

The European classification standard for fire retardant products, EN 16755, was approved by vote in June 2017 and is called “Durability of reaction to fire performance - Classes of fire-retardant treated wood products in interior and exterior end use applications”. The new standard replaces the previous technical specification CEN / TS 15912 with the same name, but since the two do not differ in their requirements, the products that have been tested and approved according to the CEN / TS 15912 will continue to be approved. The aim of the durability test is to assess the hygroscopic properties of the fireproofing agent and its long-term reaction to fire properties after weather exposure, not the properties of the wood itself.

Testing according to EN 16755

The basic principle of accelerated aging is that a material is exposed to recurring cycles in a climate chamber that is meant to simulate a natural exposure to rainfall, heat, drying, UV rays, etc.. Before the testing process is initiated, samples are taken from the actual batch, which are weighed and fire tested in accordance with the ISO 5660-1 (concalorimeter) in order to determine the reaction to fire parameters. The results give indications of the durability of reaction to fire properties that can be expected under specific conditions such as normal indoor climate, increased humidity in interior installations and installation in an exterior environment.

The technical specification consists of four classes. Inbound data is compared with outbound data after testing has been performed. In order to achieve an approved result, the deviation for each value may be a maximum of 20%. Please note that this limit for permissible deviation must not be exceeded even if the classification according to the EN13501-1 is retained after the test has been completed. An example: if the input parameter is THR 5.0 and the output parameter is THR 6.5, then the deterioration is 30 % and thus cannot be approved.

The basic principle of the hygroscopics test is that it prescribes the procedure of determining the equilibrium moisture content after exposure to normal conditions with high relative humidity of (90 ± 5)% at (27 ± 2) ° C, depending on the DRF class. The method is based on the NT Build 504 [16], with changes in A.3 with a further humid climate condition included for testing. It is a modified and extended version of the ASTM D3201-94 (with the same title) [10].

The extension aims at generating more replicable and reproducible results by requiring a ratio of equilibrium in the moisture content. The standard EN 16755 includes some additional modifications, mainly testing for two internal conditions: dry and humid.

Testing methods

To test reaction to fire properties after weather exposure, method A or B should be used as mentioned below.

Method A

Put the samples through an exposure cycle of twelve one-week cycles. Each cycle should consist of 96 hours of water exposure and 72 hours of drying. Shorter time periods, e.g. four-week cycles, can be used for screening.
Spray water in a moderately fine stream, evenly distributed over the exposed sample surface with spray nozzles that provide an average of 0.30 litres per minute and m² of sample surface, at a temperature of between 2 and 16 ° C. The water is not to be reused. Water quality can in some cases affect the results, especially its pH and hardness levels, and should therefore be recorded.
Dry at a thermostatically controlled temperature of between 57 to 60 ° C in a room or chamber. The baseline should be air temperature measured at 2.5 cm above the sample surface. Thereafter, continue with the additional drying process where the air flow is directed towards the sample surface at a minimum rate of 7.6 m / min.
At the beginning of the next cycle, change the position of each sample in the exposure rig so that all samples may occupy each location for approximately the same number of cycles.

Method B

Put the samples through a 24 hour exposure cycle consisting of four hours of water exposure, four hours of drying, four hours of water exposure, four hours of drying, and finally eight hours of rest. This cycle should be repeated for a total of six weeks.
Spray water in a moderately fine stream, evenly distributed over the exposed sample surface at a rate of (12 ± 0.8) litres per minute and m² of sample surface. The temperature must not exceed 32 ° C. After each of the first three cycles, the remaining water is drained and is not to be reused. During each subsequent spraying period, a volume of at least 18 litres of fresh water is circulated through each nozzle.
Dry at a temperature of (63 ± 3) ° C which should be reached within 15 minutes starting from when the drying process is initiated. The baseline should be air temperature at 2.5 cm above the sample surface. The temperature is measured with naked thermocouples or other thermosensors that are protected from direct UV radiation from the lamps, for which the covers must not be larger than 13 cm². Thereafter, continue with the additional drying process where the air flow is directed towards the sample surface at a minimum rate of 7.6 m / min. The UV heat exposure should be continuous throughout the drying period.
At the beginning of the next cycle, change the position of each sample in the exposure rig so that all samples may occupy each location for approximately the same number of cycles.

For testing of hygroscopic properties, the following test method should be used:

Weigh each sample with an accuracy of ± 0,1 % of the mass recorded.
Expose all samples to a constant humidity condition of (90 ± 5) % at (27 ± 2) ° C until constant mass is reached. The samples should be stacked appropriately so that all surfaces are exposed.
If a sample is likely to excrete moisture, chemicals, or both during the exposure, measures should be taken to collect any droplets which will then be included in the sample mass. One tray or bowl should be used per sample. Report if excretion occurs.
Immediately weigh each sample with an accuracy of ± 0,1%, one at a time, when removed from the test chamber. Repeat the weighing periodically until constant mass is reached. Constant mass can be assumed when two consecutive readings at an interval of at least 24 hours correspond within 0.1%. The samples should be returned to the test chamber immediately after weighing. Observe and record the general appearance of the samples, e.g. if there are any salt stains on the surface.
Dry each sample in an oven at 103 ± 2 ° C until approximate constant mass is achieved and weigh again. Constant mass can be assumed when two consecutive readings spaced two hours apart differ with 0.1 % or less. Avoid drying for longer periods than necessary when trying to reach constant mass, as thermal decomposition of chemicals or of the wood may occur, which can in turn result in a faulty reading of the humidity rate.