One of the large-scale industrial applications of Moso bamboo and poplar in China is the production of standardized fiberboard. When making fiberboard, a steam blasting pretreatment without the addition of traditional adhesives has become increasingly popular because of its environmental friendliness and wide applicability. In this study, the steam explosion pretreatment of Moso bamboo and poplar was conducted. The steam explosion pressure and holding time were varied to determine the influence of these factors on fiber quality by investigating the morphology of the fiber, the mass ratio of the unexploded specimen at the end face, the chemical composition, and the tensile strength. The following conclusions were drawn: As the steam burst pressure and holding time increased, more cellulose and hemicellulose degradation occurred (the degradation of hemicellulose was greater than that of cellulose), the lignin content rose, and the fiber bundle strength decreased. The degradation of bamboo cellulose was slightly higher than that of poplar, and the degradation of poplar hemicellulose was significantly faster than that of bamboo. Furthermore, increasing the steam explosion pressure and pressure holding time could not effectively increase the lignin content. It is recommended to use a steam blasting pressure of 2.5 MPa or 3.0 MPa and a holding time of 180 s to perform steam blasting on bamboo and poplar specimens.
Most wood has a long growth cycle and slow recovery after felling. Improper felling can easily cause irreparable damage to the environment. China’s
Bamboo and poplar are often used in Chinese plantation forests as supplementary or alternative resources for natural forest timber. Bamboo has the advantage of being the fastest growing plant in the world [
The main large-scale industrial applications of Moso bamboo and poplar are the production of standardized fiberboard, recombinant bamboo [
Adhesive-free gluing utilizes a synthetic resin adhesive (e.g., urea-formaldehyde resin or phenolic resin) to achieve “self-cementing” and adhesion. The success of the adhesion process depends upon the chemical composition of the wood material (biomass) and the ambient conditions. There are many board forming technologies, including the chemical catalysis method, the enzyme activation method, and various natural substance conversion methods (such as the common hot pressing method, steaming hot pressing method, and steam explosion pretreatment method). The chemical catalysis method causes secondary pollution, the enzyme activation method is cumbersome, and the water resistance and strength of the plate pressed by the common hot-pressing method and the steaming hot-pressing method are low. In comparison, the steam explosion pretreatment method is much more desirable since it causes no pollution to the environment and can realize the conversion of raw natural materials; for this reason it has gradually become a research hotspot.
Many studies have reported on the use of steam explosion treatments in particleboard and fiberboard fabrication. For example, Lu et al. [
The above studies conducted research on the preparation of fiberboard by steam explosion, with the findings demonstrating that the performance of fiberboard is directly related to the quality of the fiber prepared, and the quality of the fiber depends on the main factors of steam explosion pressure and holding time [
In this study, steam explosion pretreatment was conducted on a large number of Moso bamboo and poplar samples sourced from China. The morphology of the fiber, the mass ratio of the specimen at the end face, the chemical composition, and the tensile strength were compared before and after steam explosion and under various steam explosion parameters to determine the effects of different steam explosion pressures and holding times on fiber quality. Finally, the best steam explosion pressure and holding time were identified to provide a reference for further research on the steam explosion pretreatment of plant fibers.
Poplar (Populus × euramericana ‘San Martino’ I-72) veneer and Moso bamboo (
First, the poplar veneer was cut into wood chips of 4–6 cm in length, 1.5–2.5 cm in width, and 0.2 cm in thickness, and bamboo branches were cut into bamboo chips of 2.5–5.5 cm in length, 0.5–1 cm in width, and 0.5–1 cm in thickness. Next, the poplar and bamboo chips were soaked in water for 1 h at 25°C. Finally, steam explosion of bamboo and poplar chips was carried out by changing the steam explosion pressure and holding time.
For our observations, we used a ZW-U500 optical microscope produced by Shenzhen Zhongwei Kechuang Technology Co., Ltd., an Austria Bauer-MCNett fiber sieving instrument, a YJ-IID constant load testing machine (loading speed: 0.5 mm/min) developed by Yantai Xintiandi Test Technology Co., Ltd., and a digital micrometer (accuracy: ± 0.003 mm) produced by Shanghai Siwei Instrument Manufacturing Co., Ltd.
The environmental parameters of the tests were temperature 20–25°C and humidity 50%–60%.
Steam ejection was carried out by a QBS-80, which is capable of the sudden release of high-density energy within 0.0875 s [
The steam explosion pressures used in the tests were 2.0 MPa, 2.5 MPa, and 3.0 MPa, and the pressure holding times were 120 s, 140 s, 160 s, 180 s, and 200 s.
Phenyl alcohol extraction in the chemical composition analysis was conducted in accordance with the provision GB10741-1989; the National Renewable Energy Laboratory (NREL) in the provision was adopted for cellulose, hemicellulose, and lignin content tests.
After steam explosion, the fibers were naturally dried and finely ground, and the sieving value of the fibers was determined by the Bauer-MCNett fiber sieving instrument according to the relevant method in TAPPI T233.
The unexploded test piece on the end face refers to a test piece where at least one end face cannot form fiber separation under steam explosion.
In the tensile strength test, the fiber bundle diameters of poplar and bamboo were about 0.23 mm and 0.15 mm, respectively. Fifty fiber bundles were selected for each working condition. The fiber bundles were subjected to a quasi-static tensile test with the YJ-IID constant load tester at a tensile distance of 20 mm, and the tensile force
The ZW-U500 optical microscope was used to observe the morphology of fiber after steam explosion with various parameters, as shown in
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The poplar fibers after steam explosion (
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The Bauer-MCNett fiber sieving instrument was used to sieve the fiber after steam explosion, and the mass ratio of the fiber was calculated with a sieve value below 100 mesh (fiber length 0.15 mm), as shown in
The higher the steam explosion pressure and the longer the pressure holding time, the higher the mass ratio of the fiber below sieve value 100 mesh. At the steam explosion pressures of 2.5 MPa and 3.0 MPa, the holding time of 200 s will result in a significant increase in the short fiber mass ratio. In the case of high steam explosion pressure, if the pressure holding time is too long, the material will be excessively degraded, which is unfavorable for the post-production of fiberboard.
The same batch of materials was selected for the test piece, and the data were relatively concentrated, so the standard deviation of the test data was relatively small, and the law obtained from the test was reasonably representative.
To further study the sufficiency of the fiber separation of the test piece under steam explosion, at least one end surface of the test piece that could not form fiber separation was selected after steam explosion, and its mass was compared with the total mass of the test piece to calculate the cross-section. The mass ratio of blasting test pieces is shown in
It can be seen from
When the steam explosion pressure was 2.5 MPa and the holding time was 180 s, fibers were not effectively blasted apart in some individual sections of bamboo specimens. Under the steam explosion pressure of 3.0 MPa and holding time of 180 s, the bamboo and poplar specimens were effectively separated.
Steam explosion technology mainly uses the sudden release of high-temperature and high-pressure water vapor to degrade wood material cellulose and hemicellulose into low molecular sugars and activate lignin. In the later hot-pressing process of preparing binderless fiberboard, the conversion of monosaccharides to furfural, the condensation of lignin and furfural, the combination of lignin and hemicellulose, and other processes can produce adhesion. The material fibers are glued together, so the binderless fiberboard has good performance [
To study the chemical composition of bamboo fiber and poplar fiber during steam explosion, the chemical composition of the fiber before and after steam blasting was measured, as shown in
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It can be seen from the standard deviation of
When the pressure holding time reached 200 s, under the steam explosion pressures of 2.5 MPa and 3.0 MPa, the degradation of cellulose and hemicellulose tended to accelerate.
It can be seen from
Under the steam explosion pressures of 2.5 MPa and 3.0 MPa, the increase of lignin content was not significant, whereas the increase of benzene alcohol extractive content was significant. Considering the standard deviation of the test data, the contents of lignin and benzene alcohol extractive at the steam explosion pressures of 2.5 MPa and 3.0 MPa were not much different. This shows that when the steam explosion pressure is high, once the pressure holding time passes a certain threshold, further increasing it does not effectively increase the lignin content.
When the integrity of the fiberboard is guaranteed, the tensile strength of the fiber has the most direct effect on the mechanical properties of the fiberboard. Therefore, the tensile strength of fiber bundles under different steam explosion conditions was studied, as shown in
It can be seen from
The test data were relatively concentrated, the standard deviation was small, and the law obtained from the test was representative.
When the pressure holding time reached 200 s, under the steam burst pressure of 2.5 MPa or 3.0 MPa, the strength reduction rate of the fiber bundle increased.
The greater the steam explosion pressure and the longer the holding time, the better the fiber separation effect. It is difficult to separate the fibers with a steam explosion pressure of 2.0 MPa. At a steam explosion pressure of 3.0 MPa and a holding time of 200 s, the material degrades strongly and causes coking.
As the steam explosion pressure and pressure holding time increase, more cellulose and hemicellulose degradation occur (the hemicellulose degrades more than cellulose), and the lignin content rises. The degradation rate of bamboo cellulose is slightly higher than that of poplar cellulose, and the degradation of poplar hemicellulose is significantly faster than that of bamboo hemicellulose. Increasing the steam explosion pressure and pressure holding time cannot effectively increase the lignin content.
The fiber bundle strength decreases with the increase of steam explosion pressure and holding time. Under the steam explosion pressures of 2.5 MPa or 3.0 MPa, when the pressure holding time reaches 200 s, the strength of the fiber bundle decreases rapidly.
It is recommended to use a steam explosion pressure of 2.5 MPa or 3.0 MPa and a holding time of 180 s to perform steam explosion on bamboo and poplar specimens.
A steam explosion pressure of 2.5 MPa and a holding time of 180 s failed to effectively blast apart fibers of individual bamboo specimens. At a steam explosion pressure of 3.0 MPa and a holding time of 180 s, some fibers had been excessively decomposed, causing coking. If the long-term safe use of the steam explosion equipment is considered, a steam explosion pressure of 2.5 MPa and a holding time of 180 s are the preferred options.
This article is based on the conclusion of low-density fast-growing broad-leaved trees with a density of about 390 kg/m3 (length 4–6 cm, thickness 0.2 cm) and perennial gramineous plants with a density of about 720 kg/m3 (length 2.5–5.5 cm, thickness 0.5–1.0 cm). As the density or thickness of the test piece decreases, the steam explosion pressure or holding time can be appropriately reduced, although this relationship needs to be further studied.
All authors contributed equally to this work. We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.