Abstract:

Objective: The hygroscopicity and swelling behavior of Catalpa bungei earlywood, latewood and the growth ring was real-timely and synchronously documented. This work aimed to reveal the coupling and interaction between the swelling behavior of the earlywood and latewood in individual growth ring. Method: Dynamic vapor sorption analysis with Dino X Lite Digital Microscope(DVS Resolution)was used to determine the separated earlywood(EW), separated latewood(LW), combined earlywood(ELW-E), combined latewood(ELW-L)and the growth ring(ELW)of C. bungei heartwood in individual growth ring. The measurements were taken at a constant temperature of (25±0.1)℃, starting at 0% relative humidity(RH)and increasing in increments of 10% RH up to 90% RH. The each RH process was divided into the water vapor sorption period and equilibrium moisture content(EMC)constant period. During the water vapor sorption period, radial and tangential swelling strain and the sorption isotherm were synchronously measured. During the EMC constant period, whether there was hysteresis between "the swelling behavior" and "EMC" or not was investigated. Result: 1) LW exhibited a slightly higher EMC than EW at the range of 0%-90% RH. As the RH increased, the difference between EW and LW increased at first and then decreased. The reproducibility of latewood samples was better than that of the earlywood samples. 2) In the radial direction, the LW shown a higher swelling strain than EW, and ELW-L was also higher than ELW-E. The growth ring was between the earlywood and latewood. The difference of earlywood and latewood radial swelling strain increased with the increasing of RH. 3) In the tangential direction, the EW showed obviously the lowest swelling strain. However, ELW-E exhibited a similar situation as the ELW, ELW-L and LW. 4) The ratio of tangential to radial swelling for ELW, ELW-L and LW were all about 1.30. At RH above 40%, ratio of tangential to radial swelling for ELW-E and EW suddenly increased, and finally reached 2.28 and 1.83, respectively. 5) Compared with those at 'the water vapor sorption period', the radial and tangential swelling strain of ELW, EW and LW at 'the EMC constant period' were less four orders in magnitude. Conclusion: At any RH during the moisture sorption, the EMC of C. bungei earlywood was less than that of the latewood. The radial swelling of C. bungei growth ring was determined by the net effect of earlywood and latewood, and that of the latewood was dominant. Moreover, the tangential swelling of C. bungei growth ring might be also dominated by the latewood. The ratio of tangential to radial swelling for earlywood was higher than that of the latewood. 'EMC' and 'the equilibrium of radial and tangential swelling strain' of the growth ring, earlywood and latewood were changed synchronously, indicating that there may be no hysteresis between 'the swelling behavior' and 'EMC'.

Key words: Catalpa bungei, growth ring, earlywood, latewood, radial swelling, tangential swelling, anisotropy swelling