Table 4.
Flame retardancy of Betula platyphylla perforated plate treated with difference impregnation time.

Sample	Remaining flame time (sec)	Smoldering time (sec)	Carbonized area (mm2)	Carbonized length (mm)
	less than 10 sec	less than 30 sec	less than 5000 mm2	less than 200 mm
	N-10min	0	0	3576	100.5
N-30min	0	0	3341	90.6
N-60min	0	0	2673	81.8
N-90min	0	0	2604	78.5
N-120min	0	0	2781	80.1
N-240min	0	0	2730	83.3