1. Introduction
The production of wood-based boards encompasses the utilization of wood of lower quality classes [
1,
2,
3,
4] and obtaining suitable materials with improved physical and mechanical properties [
5,
6,
7,
8,
9,
10]. Properties of particleboards (PB) are described in detail in the work of [
11,
12]. The oriented strand boards (OSB) belong to this product group, but they are also considered an input material in the furniture and construction industries [
9,
13,
14,
15]. A description of OSB in terms of their preparation and properties is defined in the work of [
16,
17]. These materials are also analysed within the scope of insulation materials [
18,
19,
20,
21]. They are a part of sandwich panels in low-energy houses [
22,
23,
24,
25]. They are typically used as an interior sheathing material [
26] or furniture [
27,
28,
29,
30]. Research on the fire resistance of the mentioned materials is also rich [
31,
32,
33,
34,
35].
Large-size wood-based materials form the largest percentage of wood material in timber houses [
36,
37,
38]. These materials can be directly exposed to fire [
39,
40,
41] or the effect of radiant heat [
42,
43]. Thermal degradation and potentially even ignition of wood-based boards are caused by the action of the ignition source [
41,
42,
43,
44,
45,
46,
47,
48]. These processes are affected by both the combustible material and the environment in which it is located [
49,
50]. The ignition process cannot be characterized by a single property [
51]. Rantuch et al. [
52] used ignition parameters to define the term ignition. Two of these ignition parameters (critical heat flux and ignition temperature) are used here to compare OSB and PB with thicknesses of 12, 15 and 18 mm. This article presents the differences in the results of the research between PB and OSB due to the influence of external heat flux.
Ignition is the ability of a sample to ignite under the action of an external thermal initiator and under defined test conditions, according to [
53]. According to ISO 3261 [
54], it is the ability of a material to ignite. The process of ignition is characterized by the time-to-ignition of a sample, which depends on the ignition temperature, thermal properties of materials, sample conditions (size, humidity, orientation) and critical heat flux [
55]. Definition of “ignition temperature” can be interpreted as the minimum temperature to which the air must be heated so that the sample placed in the heated air environment ignites, or the surface temperature of the sample just before the ignition point [
56,
57,
58].
Separate attention is paid to the issue of simulating the ignition of wood under external heat flux from calculations of ignition parameters [
59,
60]. A prediction model presented in Chen et al.’s paper [
61] studies the pyrolysis and ignition time of wood under external heat flux. The solution of the model provides the temperature at each point of the solid and the local solid conversion, and the time-to-ignition of the wood is predicted with the solution of surface temperature [
62]. Chen et al. [
61] obtained good agreement between experimental and theoretical results.
The aim of this paper is to evaluate the behaviour of uncoated particleboards and OSB exposed to radiant heat. The significant influence of board density and thickness on time-to-ignition and mass loss of PB and OSB samples is monitored and observed. At the same time, the difference in the thermal degradation of PB and OSB samples is sought by comparing the results between time-to ignition and mass loss of PB and OSB samples.
3. Results and Discussion
The minimum value of radiant heat flux for particleboards and OSB was approximately 43 kW.m
−2. This value represented the critical heat flux for the selected samples. The maximum value of the radiant heat flux, to which the selected sheet board materials were exposed, was 50 kW.m
−2. The heat flux was gradually increased by 1 kW.m
−2 (
Table 3).
The sample was placed horizontally under the cone calorimeter and exposed to selected heat fluxes which led to gradual thermal degradation and generation of flammable gases. Thermal degradation (
Figure 3) is manifested by mass loss (
Table 3). Ignition occurs when the critical temperature is reached [
69]. Time-to-ignition was recorded, while considering only the permanent ignition of the surface of the analysed sample when exposed to a selected level of heat flux density. The carbonized residue (
Figure 4) remained on the surface which has been exposed to radiant heat [
70,
71,
72,
73], which proves the thermal insulation properties of the particleboard and OSB [
25].
Time-to-ignition of particleboard and OSB samples of the same thickness (
Figure 4) differed in experiments with lower heat flux values, i.e., at 43 to 46 kW.m
−2. Particleboards and OSB with thicknesses of 12 and 15 mm had the same time-to-ignition values starting from 47 kW.m
−2 (
Figure 4 and
Table 1). Samples of particleboard and OSB with a thickness of 18 mm showed the same time stamps starting from 48 kW.m
−2 (
Figure 5).
In comparison with OSB, particleboards generally showed lower time-to-ignition values. The cause can be found in the board structure. OSB consist of larger wood chips compared to particleboards.
The box plot graph for time-to-ignition OSB and PB samples shows the dispersion of the obtained data (
Figure 5c). PB samples, in all thicknesses, have comparable results (in
Figure 5c), marked with the numbers 2 as PB 12, 4 as PB 15 and 6 as PB 18. The above matrix presents the data obtained from heat flux 43 to 50 kW.m
−2. It confirms the fact that the thickness of the sample does not have a significant influence on time-to-ignition for PB samples. OSB samples show a significant dispersion of the obtained data and confirm the ratio with increasing heat flux; the ignition time is shortened (see also in
Figure 5a).
The values of time-to-ignition and mass loss of OSB have a greater dispersion of results, as evidenced by the created box graphs (
Figure 5). The variability results from the nature of the board, which is composed of large-area wood particles from pressed flat chips that are pressed under the influence of high pressure and temperature (
Figure 6). The binder is a formaldehyde-based resin [
74]. Osvald et al. [
75] do not assume the influence of the bonding material (glue as well as other additives) on the thermal degradation of the OSB surface.
When comparing the mass loss of particleboards and OSB, lower mass loss values are observed in particleboards of all thicknesses. This difference decreases with increasing sample thickness. Mass loss values of particleboard and OSB samples with a thickness of 18 mm are the same (
Figure 5b). A detailed analysis of time-to-ignition and mass loss results for individual sample thicknesses exposed to selected heat flux values is shown in
Figure 6. The comparison of time-to-ignition values of particleboards and OSB showed interesting results, apart from the results with the heat flux of 43 kW.m
−2 (
Figure 6a).
Figure 6 shows the linear dependences of time-to-ignition increase on the sample thickness. At the same time, the graphs are supplemented with quantitative analysis through box graphs. The presented graphs confirm the description of the behaviour of OSB and PB due to the action of radiant heat. Particleboards record lower time-to-ignition values than OSB up to the heat flux of 47 kW.m
−2 (
Figure 6b–e). Subsequently, the particleboard and OSB time stamps become identical (
Figure 6f–h). All linear dependences maintain an increasing tendency (
Figure 6a–h), i.e., the time-to-ignition increases with increasing sample thickness. The given increasing tendency was, however, no longer found at heat flux of 49 and 50 kW.m
−2 (
Figure 6g,h).
Naturally, mass loss (
Δm) results show the opposite tendency:
Δm decreases with increasing sample thickness (
Figure 6i–p), while the
Δm of OSB is generally greater than the
Δm of particleboards. Interesting results can be seen at the heat fluxes of 43 (
Figure 6i), 44 (
Figure 6j) and 46 (
Figure 6l) kW.m
−2, where there is a change in
Δm occurring in samples with a thickness of 18 mm. These cases show higher
Δm values of particleboard samples compared to OSB.
The results confirm relatively similar behaviour of particleboard and OSB samples. OSB have generally higher time-to-ignition values, i.e., they withstand the effect of radiant heat longer than particleboards. On the other hand, OSB have a higher Δm value compared to particleboards during thermal degradation and subsequent combustion.
Our results show that as the thickness of samples increases, the differences in the behaviour of the samples disappear under action radiant heat, which can be seen in
Figure 6. Practice should take into account the importance of thickness when applying these materials in building structures or elements.
For the purpose of this analysis, another parameter evaluating the behaviour of solids in the event of a fire was calculated, namely the burning rate of OSB (
Figure 7a) and particleboards (
Figure 7b). The process of thermal degradation of wood-based materials is associated with the charring of the surface, hence some authors [
49] call this parameter the charring rate. Once again, dependence between the increase in the rate of burning and the increase in heat flux was confirmed. The burning rate (g.m
−2.s
−1) is calculated as the ratio of mass loss
Δm to the time of thermal degradation. The results show a decrease in the rate of burning with increasing thickness of the sample (
Figure 8), which is also stated by Richter et al. [
49]. This fact confirms that particleboards act as thermal insulators.
The box plots added to
Figure 7 show the same tendency for the burning rate to increase. The values of 43,44, 45 and 46 kW.m
−2 have exactly the same burning rate values, and significant changes occur at heat flows of 48-50 kW.m
−2.
Despite the previous linear dependences, it is not possible to draw a clear conclusion. This fact is also confirmed in
Figure 7. The results show a relationship between the thickness of the samples and the burning rate, which is again linear, but the lines differ (
Figure 7).
Richter et al. [
49] addressed the effect of oxygen concentration and heat flux on the ignition and burning of particleboards. The experiments were performed on samples of particleboards with different oxygen concentrations (0%–21%), heat fluxes (10–70 kW.m
−2), sample densities (600–800 kg.m
−2) and sample thicknesses (6–25 mm). The results of Richter et al. [
49] showed the effect of heat flux and oxygen concentration on the rate of burning, ignition time and combustion type (pyrolysis, smouldering, combustion).
Maciulaitis et al. [
70] watched, among other things, the influence of 30, 35, 40, 45 and 50 kW.m
−2 heat flows in accordance with LST ISO 5657: 1999 [
65] with 6 mm, 10 mm, 15 mm and 18 mm thick oriented strand boards (OSBs).
Statistical evaluation of measurement data
The assessment of the impact of the kind samples (PB, OSB) and the impact of thickness (12,15 and 18 mm) on time-to-ignition and mass loss was carried out by statistical analysis. We used the multifactor analysis of variance (ANOVA) using LSD (95% level of provability) of the test (software Statistica 10).
Table 4 confirms significant differences for thickness. The OSB 18 mm has the highest time-to-ignition value.
The mass loss for all samples was 15% of the original weight of the samples. The obtained statistical data did not confirm the significance of the influence of the kind of sample and its thickness on mass loss (
Table 5).
Thermal analysis is another method which uses constant heating to analyse the sample. The results confirm thermal decomposition of samples in two stages [
49], as is the case with other cellulosic materials (
Table 6). Individual stages of thermal decomposition of particleboard and OSB samples were defined with the use of thermogravimetric analysis in an atmosphere of air.
Thermal decomposition of the OSB sample (
Figure 8) took place in two stages. The first stage of thermal decomposition, the main decomposition of the sample, occurred at a temperature of 179 °C. The highest mass loss (65.07%) was recorded at 325.7 °C within the first stage of decomposition, which ranged between the temperature of 179 °C and 381 °C. The second stage of thermal decomposition began at 381 °C. At this stage, the second maximum rate of mass loss was recorded at 443 °C, with a mass loss of 39.34% and a resistant residue of 0.61% after decomposition.
A similar course of thermal degradation was observed in particleboards. The main decomposition of the particleboard sample occurred at a temperature of 146 °C within the temperature range of up to 378 °C. At the same time, the highest mass loss of 64.65% was recorded at the temperature of 320.3 °C. In the second stage of thermal decomposition, which took place at the temperature range of 378 °C to 525 °C, the second maximum rate of mass loss was recorded at 445.7 °C. At this stage, there was a mass loss of 29.53% and the resistant residue after decomposition amounted to 0.64%.
Given values show the behaviour of boards subjected to thermal stress, where the OSB with a thickness of 12 mm begins to thermally degrade at 179 °C and its ignition time is 107 s at a heat flux of 43 kW.m−2.
Particleboard with the thickness of 12 mm begins to degrade at 146 °C and its ignition time is 89 s. The reported results are consistent in all sample thicknesses and heat flux values.
Sinha et al. [
76] studied the effect of exposure time on the flexural strength of OSB and plywood at elevated temperatures. They reached a critical temperature of 190 °C at which the strength decreased and thermal degradation occurred. Very interesting research on time-to-ignition on Ancient Wood was conducted by Wang et al. [
77].