Preparation and Evaluation of an Elastic Cushion with Waste Bamboo Fiber Based on Sitting Pressure Distribution of the Human Body

Abstract

Waste bamboo fibers are mostly in a spiral coil state and exhibit a certain degree of elasticity, which has the potential to be used as elastic cushion filler. However, there are few studies on its application to elastic cushions. In order to efficiently use waste bamboo fibers, a bamboo-based elastic cushion (BEC) was prepared and evaluated. The BEC units were prepared by mixing bamboo waste fibers with ES fibers and dividing them into five grades according to elastic modulus. The BECs were arranged with BEC units based on the sitting pressure distribution of the human body and evaluated using objective and subjective methods. The appropriate process of BEC units was a heating temperature of 150 °C, heating time of 50 min, and bamboo proportion of 50~75%.The elastic modulus of units had a clear influence on the sitting pressure distribution of the BEC. With the increasing elastic modulus of BEC units, the maximum pressure and average pressure of the BEC first decreased and then increased, while the contact area showed an opposite trend. Additionally, the comfort rating of the BEC with higher elastic modulus units was higher, and the appropriate elastic modulus range was 0.25~0.40 MPa. The BECs made with units with different elastic moduli increased the comfort rating by 12.8% compared with that with the same units, and the sciatic node was the most sensitive part for humans when sitting on the BEC. The results could provide data support for the application of waste bamboo fibers in elastic cushions.

1. Introduction

Bamboo is widely used as an alternative raw material to wood because of its rapid growth and good mechanical properties [1,2,3]. However, during the utilization of bamboo, nearly 60% of bamboo waste is produced, and its general treatment is used as common fuel, which is quite low value-added [4,5]. Thus, the development of high-value utilization for waste bamboo is a hot and important direction of sustainable research.

Producing bamboo-based artificial board is an effective way to process bamboo waste into high-value products [6,7,8]. Pang et al. [9] proposed a viable strategy combining bamboo waste and rice straw into bio-composites to displace the traditional wood-particle boards. The new bio-composites had a 0.57 MPa internal bonding strength and a 7.00% (24 h) thickness swelling rate, which reached the ISO requirements of particleboard. Guan et al. [10] prepared a new type of binderless particleboard with bamboo residues processed via biological fermentation and found that its physical and mechanical properties were better than those prepared using the coarse and mixed groups with IB, MOR, and MOE values of approximately 0.56, 12.83, and 2055.30 MPa, respectively. Iswanto et al. [11] also found that adding bamboo as a reinforcing material in particleboards could greatly improve the mechanical properties of boards, increasing the elasticity modulus and bonding strength by 16 and 3 times.

Some researchers discovered that bamboo fibers have a certain degree of elasticity, leading to the development of waste bamboo fibers into elastic materials [12,13,14]. Cruz-Riano et al. [15] used bamboo fibers to reinforce polymeric composite material and found that its elasticity modulus increased by 322% in comparison with that of the polymer matrix. Wei et al. [16] fabricated natural bamboo coil springs using bamboo sliver, which had a better structural stability than that of plant-fiber-reinforced composite springs, with a stiffness retention rate of 86.3% after undergoing 8000 large-deformation compression cycles. Additionally, they put forward the structure–function–compositions relationships of bamboo cells, which could help the design and manufacturing of curved bamboo furniture and structural bamboo coil springs for cushion application [16]. Thus, waste bamboo fibers have excellent application potential in the field of elastic cushions.

Comfort is one of the key evaluation indicators of elastic cushions, which is influenced by shape, material, process, etc. [17,18,19]. Additionally, comfort is also closely related to the sitting pressure distribution of the human body [20,21,22]. Tang et al. [23] used the sitting pressure distribution as an indicator to design the distribution of foams in the cushion, and found that the cushion with a bulging front had a more uniform pressure distribution. Bao et al. [24] tested the sitting comfort of four cushions by using subjective rating and sitting pressure distribution analysis and showed that the slow-recovery cushions that were too soft or too hard would reduce the comfort. Carrigan et al. [25] developed a cushion system based on the sensor air unit and showed that adjusting the pressure of inflating cushion units with pressure mapping could obtain a uniform pressure distribution. Therefore, the match between the elasticity of cushion materials and the sitting pressure distribution of the human body is the primary determinant of comfort.

In order to improve the utilization rate and explore the relationship between the elastic modulus and sitting pressure distribution, bamboo-based elastic cushion (BEC) units were prepared and their interconnection condition and elastic modulus were measured. Additionally, the BECs were arranged with BEC units and the variations of static sitting pressure distribution and subjective comfort evaluation of the BECs with different arrangements of BEC units were studied. The results could broaden the utilization of waste bamboo fibers and provide the data to support applying waste bamboo fibers in elastic cushions.

2. Method

2.1. Preparation and Characterization of BEC Units

Waste bamboo fibers with a width of 3~5 mm and an unfolded length of 180~200 mm were selected as raw materials. After 24 h drying at 108 ± 2 °C, the waste bamboo fibers were mixed with ethylene-propylene fibers (ES fibers) and accounted for 40~80 wt.% of the total. Then, 20 g mixture was put into a cubic metal mold (60 × 60 × 60 mm³) and heated at 150 °C for 40~60 min to obtain the BEC units. The waste bamboo fibers with a moisture content of 24%, a water absorption of 469%, and a specific breaking strength of 5.3 cN/dtex were provided by Anji Chengfeng Bamboo Product Co., Ltd., Anji, China, and the ES fibers were provided by Jiangsu Yufang New Material Technology Co., Ltd., Suqian, China. The BEC units were cut down to the middle to observe their interconnection condition (Figure 1). The interconnection condition is mainly affected by the thermal transmission from outside to inside of BEC units. An elastic modulus test was conducted according to the standard (EN 1957–2012) using a universal mechanical testing machine (Shanghai Yiheng Co., LTD., Shanghai, China), where the compression speed was 1 mm/min and the maximum load was set to 250 N. The number of test samples was 6.

2.2. Preparetion of the BEC

The BEC was composed of 36 units arranged in a 6 × 6 square matrix, and the BEC units were selected by matching the sitting pressure distribution and elastic modulus (Figure 2). According to human physiology and anatomy, the comfortable sitting pressure distribution of cushions needs to meet the following characteristics: (1) the sitting pressure of the sciatic nodes is the highest, while the contact position between the thighs and the front seat is the lowest. (2) The sitting pressure tends to decrease gradually in the posterior femoral region. (3) The sitting pressure distribution should be symmetrical and the peak sitting pressure should not make people feel foreign [26]. Based on these, the elastic modulus of BEC units was divided into five classes (Figure 3). In order to find the appropriate elastic modulus decreasing trend, the experimental Groups A, B, and C and the control Group F were set. As shown in Figure 3, the units of experimental Groups A, B, and C were arranged in descending order according to the elastic modulus, while the elastic modulus of BEC units in the control Group F was set to the value closest to that of common sponge cushions (0.20~0.25 MPa). The experimental Groups B, D, and E were set to find the suitable elastic modulus of BEC units in the sciatic node.

2.3. Participants

Sitting pressure distribution is closely related to weight status, which is commonly assessed using the body mass index (BMI) [19]. The data in the ideal sitting pressure distribution (Figure 2a) were mainly obtained from medium-sized people with 18~24 BMI values. Therefore, 20 students from Zhejiang Sci-Tech University with BMI values of 18 to 24, including 10 males and 10 females, aged between 20 and 26 years and in good health, were selected as participants. To avoid the influence of other factors, the participants were asked not to have strenuous physical activity in the 24 h before the experiment. Meanwhile, informed consent about the experiment was obtained from each participant.

2.4. Objective Analysis

The static sitting pressure distribution of the BEC was measured using the Tekscan human pressure distribution measurement system with a No.5315 pressure film sensor and software BMPS Research 7.60 (Figure 4a). The size of the wooden test bench was 400 mm × 400 mm × 415 mm. According to the arrangement diagram, the BEC units were placed at the corresponding positions (Figure 4b), and then the pressure film sensor was laid on the BEC units. Before the test, the dress code of participants (cotton sweatpants without back pockets, buttons, and hard lines were required) was checked and the correct sitting posture (Figure 4c) was explained. After the participants were seated with good posture and the sitting pressure distribution data were stable, the contact area, average pressure, and maximum pressure were recorded for 5 min with a sampling rate of 8 f/min, and the pressure distribution cloud map was automatically calculated by the test system.

2.5. Subjective Assessment

It is difficult for users to evaluate the structure and parameters of a cushion, but easy to give clear feedback on the comfort of body parts [24,26]. Therefore, the subjective evaluation of BEC comfort was focused on the comfort rating of some key body parts, including buttocks, sciatic nodes, thighs, and thigh roots. Additionally, the comfort rating was obtained with a seven-level scale with ‘extremely uncomfortable’ and ‘extremely comfortable’ marked at its left and right-hand ends, respectively. In this test, the participants sat with good posture for 5 min and then evaluated the comfort of different body parts with the seven-level scale. The test of different cushions by the same participant should be at an interval of 15 min.

3. Results and Discussion

3.1. Characterization of BEC Units

Figure 5a shows the interconnection condition of BEC units. As heating time increased, the interconnection degree between the waste bamboo fibers and ES fibers clearly increased. The BEC units heated for 30 min showed as fluffy, while being tight after heating for 60 min. Incomplete interconnection might cause the BEC units to collapse when pressing, but overly tight interconnection would decrease the elasticity. Therefore, a suitable heating time of 50 min was chosen for the BEC units.

The effect of bamboo proportion on the elastic modulus of BEC units is shown in Figure 5b. The effect of bamboo proportion on the elastic modulus of BEC units was significant. With the increase in bamboo proportion, the elastic modulus of BEC units increased first and then decreased, obtaining the maximum value when the bamboo proportion was 50%. In the BEC units, the bamboo fibers played a supporting role, while the ES fibers played a bonding role. When at the small bamboo proportion, the supporting effect of bamboo fibers was weak, resulting in a low elastic modulus of BEC units. However, when the bamboo proportion was large, the enhanced supporting effect increased the stiffness of BEC units, leading to a decrease in the elastic modulus. Considering the economy and elastic modulus distribution, the bamboo proportion of BEC units was chosen to be 50~75%.

3.2. Objective Analysis of the BEC

Figure 6a shows the cloud image of sitting pressure distribution of the BEC, and the average pressure, maximum pressure, and average area of the BEC are displayed in Figure 6b. Comparing Groups A, B, and C with the increasing elastic modulus of BEC units, the maximum pressure first decreased and then flattened out, while the contact area showed an opposite trend. According to the ideal body pressure distribution, most of the weight of the human body was borne by the sciatic node, which indicated that the maximum pressure mainly appeared at the sciatic node [25]. Although the elastic modulus of BEC units in the sciatic node of groups A, B, and C was the same, the maximum pressure values were different, especially that of Group A. The possible reason was that the difference in elastic modulus between the sciatic node and its neighborhood was too large, which caused shear force around the sciatic node, leading to a higher maximum pressure. The average pressure was positively correlated with the maximum pressure and negatively correlated with contact area. Additionally, the contact area was negatively correlated with the elastic modulus of cushions [27]. This was the reason for the small contact area of Group C.

In comparison with Groups B, D, and E, with the increasing elastic modulus of the sciatic node of BEC units, the maximum pressure decreased first and then raised substantially, the average pressure increased, and the contact area decreased. The BEC units at the sciatic node provided most of the support and was where the maximum pressure was located. Thus, the maximum pressure of Group E was the largest. The elastic modulus of BEC units at the sciatic node of Group D was the lowest, but its maximum pressure was higher than that of Group B. This was due to the fact that the too small elastic modulus of BEC units could not effectively support the sciatic node, leading to a larger contact area but producing more shear force, which increased the pressure.

Compared to those of Group F, the average pressure and maximum pressure of Group B were nearly the same or lower. Additionally, the contact area of Groups A, B, and D were larger than that of Group F. These indicated that the arrangement of Group B was the best among Groups A–E and its comfort was better than the common sponge cushions (Group F). The possible reason was that (1) the cushion with BEC units with different elastic moduli could be more comfortable than that with the same elastic modulus; (2) the difference on the elastic modulus of the sciatic node and its neighborhood of Group B was smaller, leading to a bigger contact area and lower maximum pressure.

3.3. Subjective Assessment

The comfort rating on the key parts of the human body is shown in

Table 1. Comfort rating on the key parts of the human body.

Group	Comfort Rating
	Thigh	Thigh Root	Buttock	Sciatic Node	Overall Evaluation
A	4.5 ± 1.2	4.8 ± 0.7	5.0 ± 1.0	4.2 ± 1.2	4.7 ± 0.8
B	5.1 ± 1.4	5.4 ± 0.8	5.4 ± 1.3	4.9 ± 1.6	5.3 ± 0.9
C	5.3 ± 0.6	5.3 ± 0.5	5.1 ± 1.2	4.7 ± 1.0	5.2 ± 0.7
D	5.4 ± 0.8	5.1 ± 0.5	4.9 ± 0.8	4.6 ± 1.3	5.0 ± 1.0
E	5.6 ± 0.9	5.2 ± 0.6	5.2 ± 0.9	4.3 ± 1.2	5.1 ± 0.9
F	4.8 ± 0.7	5.1 ± 0.5	5.2 ± 0.7	4.3 ± 0.8	4.7 ± 0.9

. The comfort rating of key parts was similar to the overall comfort rating, which suggested that the comfort of key parts could be used to evaluate the overall comfort of the BEC indirectly. Moreover, the comfort rating of the sciatic node was the lowest and was significantly different from that of other key parts, which meant that the human was sensitive to the comfort of the sciatic node. The comfort rating of the sciatic node from Group B was the highest, because the suitable elastic modulus of BEC units gave good support for the participants and a stable sitting position, so that the participants and cushion could be fully contacted to form a better sitting pressure distribution. Additionally, the highest comfort rating of thigh root and buttock were both from Group B; this was due to the fact that the nice support of the sciatic node could share the pressure between the thigh root and buttock, and the gentle change of elastic modulus between the adjacent BEC units flattened the pressure distribution. The comfort rating of the thigh from Group E was 5.6 ± 0.9, which was the greatest value. The possible reason was that the BEC units at the sciatic node of Group E had the biggest elastic modulus and provided the most support, which decreased the pressure on the thighs. Additionally, the comfort rating of all key parts from Group A was lower than that from other groups, which indicated that a too small elastic modulus would lead to a large difference in the deformation of BEC units between the sciatic node and its neighborhood, resulting in a bad overall subjective feeling.

In comparison with Groups A, B, and C, the overall comfort rating of Groups B and C were close, and both were greater than that of Group A. The difference in elastic modulus of Groups B and C between the sciatic node and its neighborhood was smaller than that of Group A, leading to a smoother deformation. Compared with Groups B, D, and E, the overall comfort rating of Group B was higher than those of Groups D and E. The suitable arrangement of the elastic modulus on Group B made the BEC have a smaller maximum pressure and a moderate contact area (Figure 6), which improved the overall subjective comfort of the cushion. Compared to control Group F, the comfort ratings of the experimental groups were all higher, except Group A. This suggests that the BEC made by units with different elastic moduli could be better than that with the same elastic modulus. The possible reason is that suitable elastic modulus distribution could form a better fit to the human body’s pressure distribution, giving people a comfortable experience [23,25].

4. Conclusions

The elastic modulus of BEC units had a clear influence on the sitting pressure distribution of the BEC. With the increasing elastic modulus of BEC units, the maximum pressure and average pressure first decreased and then increased lightly, while the contact area showed an opposite trend. Additionally, with the increase in elastic modulus at the sciatic node, the maximum pressure decreased first and then raised substantially, the average pressure increased, and the contact area decreased.

The use of modular design methods increased the level of personalization of the cushion, allowing for the arrangement of the cushion to match the pressure of the human body. The BECs made by units with different elastic moduli were more comfortable than those with the same units. Additionally, the comfort rating of the BEC with higher elastic modulus units was higher. The sciatic node was the most sensitive part for humans when sitting on the BEC, and the comfort rating of the sciatic node from the BEC with lower elastic modulus was the highest.

These results expanded the range of potential applications for bamboo waste, and clearly increased its utilization value and rate. The innovative approach of preparing BEC units and the BEC, especially the arrangement method of the BEC, achieved customized and comfortable designs that meet the individual needs of users. These methods not only provided a reference for the application of other natural waste fibers, but also provided a new approach to enhance the user experience by offering greater customization and comfort.