Abstract

MUF/APP preparation

The MUF3 resin was prepared according to the method used by Tohmura et al. (2001). The final molar ratio was 1.5 and 1.0 for F/(M+U) and M/U, respectively. Melamine content to MUF resin was 46 percent. The MUF3 was mixed with APP with a mass ratio of 100:60.

Preservative preparation

The preservative copper azole type C (CA-C) was used in this test. The active ingredients of the preservatives and the treating solution concentrations are referenced in AWPA P5‐10 (American Wood Protection Association 2011) and Ma et al. (2013a). The treating solution concentration was 0.4 percent (wt/wt).

Wood species preparation

P. radiata and P. deltoides were dried and sawed.

Inhibition zone test

The Oxford cup method was used to assess the antifungal activity of the test chemicals via modified filter paper discs (Ma et al. 2009). Four Oxford cups, with inner diameters of 6 mm and heights of 10 mm, were placed on the surface of potato dextrose agar petri dishes that had been inoculated with a fungal mycelial suspension created from actively growing cultures. The fungi used in the inhibition test were the white rot Trametes versicolor (L. ex Fr.) Pilat and the brown rot Gloeophyllum trabeum (Pers. ex Fr.). Three hundred microliters of the chemicals was added to each of three Oxford cups as treatment. An equal amount of water was added to a remaining cup as a control. The petri dishes were cultivated at 26°C to 28°C for 3 to 7 days until hyphae were visible, and then the diameters of the inhibition zones were measured with a Vernier caliper. Three replicates (petri dishes) were measured for each formulation.

Decay resistance tests

The size of the specimens for decay testing was 20 by 20 by 10 mm (radial by tangential by longitudinal). Thirty-six replicates of each wood species were used. Twelve blocks were only surface treated with CA-C according to Ma et al. (2013b). Surface retention of P. radiata was approximately 3.3 g/m² and that of P. deltoides was approximately 3.8 g/m². Another 12 blocks were only brush treated with fire retardant (MUF3/APP) with a glue spread amount of 400 g/m² (Ma et al. 2013b). The resin was allowed to slowly solidify on the samples at room temperature. The remaining 12 blocks were nontreated controls. Samples were then prepared for decay resistance tests.

Decay testing was conducted according to Chinese standard LY/T 1283 (Standardization Administration of the People's Republic of China 2011) and Ma et al. (2013a). Two decay fungi, the white rot T. versicolor and the brown rot G. trabeum, were tested. Each specimen was weighed and recorded to an accuracy of 0.01 g before and after the decay test, and the percent mass loss was calculated. Decay resistance was rated on a scale of I to IV defined as mass losses: (I) highly resistant, mass loss from 0 to 10 percent; (II) resistant, mass loss from 11 to 24 percent; (III) moderately resistant, mass loss from 25 to 44 percent; (IV) slightly or nonresistant, mass loss more than 45 percent.

Fire retardant properties of the MUF3/APP test

The size of the specimens for cone calorimeter testing was 100 by 100 by 15 mm (radial by tangential by longitudinal). Three replicates were prepared for MUF3/APP surface treatment and the fire retardancy test. The fire retardancy test was conducted according to International Organization for Standardization (ISO) standard 5660‐1 by using cone calorimeter (ISO 2002) and Ma et al. (2013b). All the tests were conducted at an irradiance level of 50 kW/m². The duration time of each individual test was 600 seconds. The average mass loss rate was calculated by followed formula: Average mass loss rate = (mass of the treated specimen before burning − mass of the treated specimen after burning for 600 s)/(600 s·0.1 m × 0.1 m).

Antifungal activity

The MUF3, APP, and MUF3/APP mixture systems were tested via inhibition activity to decay fungi (Fig. 1). Table 1 shows that the control was no inhibition and was recorded as 6 mm. APP had slight antifungal activity: only a 13.3-mm inhibition zone diameter was observed for G. trabeum, and an 11.6-mm zone diameter was observed for T. versicolor. The MUF3 showed strong activity against decay fungi, with a 28.2-mm inhibition zone for G. trabeum and a 22.5-mm inhibition zone for T. versicolor. The MUF3/APP mixture system showed strong activity against fungal growth, with inhibition zone diameters of 25.3 and 16.6 mm for G. trabeum and T. versicolor, respectively.

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Table 1 Comparison of the inhibition zone between chemicals.

^a Values are means with standard deviations in parentheses.

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Decay resistance

Mass losses of wood specimens exposed to the two decay fungi are shown in Table 2. The mass losses of untreated P. deltoides specimens were 65.5 and 62.3 percent when exposed to G. trabeum and T. versicolor, respectively. Untreated P. deltoides were nonresistant against decay fungi (class IV). The CA-C–treated P. deltoides had no mass loss, showing high decay resistance (class I).

Table 2 Mass losses of surface-treated wood after being exposed to decay fungi.

^a Values in parentheses are standard deviations.

^b MUF3 = melamine-modified urea-formaldehyde resin type 3; APP = ammonium polyphosphate.

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The mass losses of untreated P. radiata exposed to G. trabeum and T. versicolor were 31.0 and 42.2 percent, respectively. Untreated P. radiata were moderately resistant against decay fungi (class III). The mass losses of CA-C–treated P. radiata were 1.7 percent in tests with the white-rot fungus, and no mass loss was observed with the brown-rot fungus, showing high decay resistance (class I).

Mass losses of MUF3/APP-treated P. deltoides and P. radiata exposed to G. trabeum were 2.1 and 0.9 percent, respectively. Mass losses for these two wood species exposed to T. versicolor were 7.5 and 5.5 percent, respectively. All mass loses were less than 10 percent, indicating high decay resistance (class I; Table 2). The results showed that surface treatment with MUF3/APP significantly enhanced the decay resistance of wood species.

Fire retardant properties of MUF3/APP-treated wood

Table 3 shows the fire retardant properties of P. deltoides and P. radiata surface treated with MUF3/APP. The ignition times of the untreated P. deltoides and P. radiata were 15.7 and 14.9 seconds; however, the specimens surface treated with MUF3/APP prolonged the ignition time significantly, up to 337.9 and 325.1 seconds, respectively. Delay of ignition time of 322.2 seconds for P. deltoides and 310.2 seconds for P. radiata indicated reduced flammability of treated specimens. The total heat released in 600 seconds from the untreated P. deltoides and P. radiata specimens were 54.3 and 70.4 MJ/m², respectively, but the total heat released from the MUF3/APP-treated wood decreased significantly, to 5.5 and 6.5 MJ/m², respectively. The average mass loss rate of the treated P. deltoides specimens decreased from 7.4 to 3.3 g/s·m². The average mass loss rate of the treated P. radiata specimens decreased from 8.2 to 4.1 g/s·m². Delay of ignition time, reduction of the total heat released, and the average mass loss rate means that the MUFs/APP-treated wood is more effective in retarding fire. It showed that treatment with MUF3/APP significantly enhanced the fire resistance of wood species.

Table 3 Fire retardant properties of wood surface treated with melamine-modified urea-formaldehyde resin type 3 mixed with ammonium polyphosphate.

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