Drying and heat treatment process of bamboo strips

Bamboo and its composites have been increasingly used to supplement timber-based products owing to their fast growth and superior strength attributes. To make laminated products, green bamboo strips must be dried to moisture contents below 10%. Drying determines both the productivity and quality of laminated bamboo, but is currently very inefficient due to limited understanding of many critical phenomena. This thesis investigates the fundamentals and industrial practice of bamboo strip drying, and the effects of different heat treatment processes on their morphology and properties. In the first part, green strips were subjected to different steam pretreatments called caramelization and dried in a step-wise schedule in lab. Strip deformations and flexural strength were evaluated. Mould resistance was tested using controlled inoculation experiments, and starch modification was observed by scanning electron microscopy (SEM). The results showed that the 160 °C saturated-steam caramelization reduces bamboo strength and causes longitudinal bowing, transverse deformation, and node shrinkage. The caramelization does not appear to improve mould resistance in the long term, but it delays the onset of mould colonization, which may be explained by the continued presence of modified starch. In the second part, the drying department of a typical laminated bamboo factory was investigated. Drying involves five steps in chronological order: caramelization, primary drying, ambient conditioning, steam reconditioning, and secondary drying. From monitoring in-situ environmental conditions, the step-wise drying schedule can adequately simulate the progressive tunnel kiln drying process. Statistical analysis suggests reconditioning rather than caramelization is the key process for controlling strip moisture content and dimensional changes. Bamboo cell collapse was investigated using SEM to elucidate the patterns and severity of the cellular deformation under different reconditioning regimes. Collapse occurs in parenchyma cells, and the patterns in the cross-section are complex and governed by the spatial locations relative to fiber bundles and the inner or outer edge of the culm. Bamboo requires saturated steam at higher temperatures (>100 °C) than wood for effective collapse recovery. This thesis pointed to real potential for improving industrial bamboo drying by eliminating pre-caramelization and reducing cell collapse. Further research is needed to implement the findings in bamboo industry.Forestry, Faculty ofGraduat