3.1 Foaming procmization
The effects of fiber concentration, pH value, and surfactant type on the Lyocell fiber foam system's foam distribution and stability were investigated by orthogonal tests. It is determined that the factor levels of the main influencing factors are Fiber concentration(1%, 2%, and 3%), pH(6, 7, and 8), and surfactants(PVA, SDBS, and CTAB). The orthogonal test is according to table L9 (34), as shown in Table 1. By analyzing the half-life of the foam and the half-life of drainage, the results show that when the fiber concentration is 3%, choosing SDBS as the foaming agent, pH=7 is stable in the Lyocell fiber system.
Table 1 The half-life of foam and drainage volume at different factors
Serial number
|
Surfactant type
|
Fiber concentration,
%
|
pH
|
Half-life of the foam,
min
|
Half-life of drainage,
s
|
1
|
PVA
|
3
|
6
|
5
|
8
|
2
|
PVA
|
2
|
7
|
3
|
4
|
3
|
PVA
|
1
|
8
|
1
|
1
|
4
|
SDBS
|
2
|
8
|
>60
|
24
|
5
|
SDBS
|
1
|
6
|
>60
|
21
|
6
|
SDBS
|
3
|
7
|
>60
|
40
|
7
|
CTAB
|
1
|
7
|
>60
|
31
|
8
|
CTAB
|
3
|
8
|
>60
|
24
|
9
|
CTAB
|
2
|
6
|
>60
|
27
|
Analysis
number
|
Surfactant type
|
—
|
Fiber concentration
|
pH
|
K1
|
13
|
63
|
72
|
56
|
K2
|
85
|
49
|
55
|
75
|
K3
|
82
|
68
|
53
|
49
|
k1
|
4.33
|
21.00
|
24.00
|
18.67
|
k2
|
28.33
|
16.33
|
18.33
|
25.00
|
k3
|
27.33
|
22.67
|
17.67
|
16.33
|
R
|
72
|
19
|
19
|
26
|
Through the orthogonal experiment, the K-value and R-value are calculated by the half-life of discharge. It is known that the larger the R-value is, the more influential the factor is. If the empty column R is large, there is a non-negligible interaction between the factors. The types of surfactants have the most significant impact on the stability of the Lyocell fiber foaming system, followed by the pH value, and the fiber concentration is the smallest. There is a non-negligible interaction between the factors. Among the surfactants, SDBS and CTAB perform well, but PVA has a poor foaming effect. The reason may be that PVA is a non-ionic surfactant, and its hydroxyl functional groups in the solution will undergo hydrolysis changes with changes in pH. It also shows that foam drainage is slowed down with increasing fiber content. An increased fiber concentration results in the formation of smaller bubbles containing more liquid in equilibrium under gravity.(Haffner et al. 2017) The reduction of foam size is due to the increase of fiber concentration, which not only destroys the formation of large-sized foam in the foam system, but also causes the foam space in the slurry to be compressed. The fiber surface itself has a hydrophilic hydroxyl group, weakening the liquid film drainage and delaying the disproportionation of foam.
3.2 Analysis of mechanical properties of Lyocell fiber/styrene-acrylic emulsion reinforced buffer materials
The buffer performance is tested by a static compression test. When the Lyocell fiber pulp concentration is 3%, as shown in Fig. 4 and 5, the density of the Lyocell fiber/styrene acrylic emulsion reinforced buffer material (SA@LF) increases with the increase of styrene acrylic emulsion concentration. The reason is that the styrene-acrylic emulsion has a high density and cohesive force so that the fibers after the foam formation will be combined to a certain extent.
For further analysis, the compression performance parameters of the material are measured, as shown in Fig. 6. Fig. 7 shows the stress-strain curves of SA@LF under different concentrations of styrene-acrylic emulsion. As can be seen from Fig. 7, the composite material can be roughly divided into three deformation stages (i.e., linear elastic stage, yield stage, and nonlinear strengthening stage). (Song et al. 2021) When the strain is less than 10%, the stress and strain are basically linear, conforming to Hooke's law and having good buffering capacity. When the strain is between 10% and 30%, the stress increases with the increase of the strain. The deformation increases, showing inelasticity. When the strain increases above 30%, the material enters the compact section, the cell collapses and breaks, the stress rises sharply, and the material loses its cushioning performance.
Comparing pure Lyocell buffer materials (styrene-acrylic emulsion concentration 0%) with pure Lyocell fiber buffer materials with styrene-acrylic emulsion concentration 2%, 3%, 4%, 5% and pulp consistency of 3%, the density and elastic modulus of the composites increased. The epoxy group in styrene-acrylic emulsion could react with the reactive hydrogen group in the cellulose to form a crosslinking network. The hydroxyl energy in the emulsion forms hydrogen bonds with hydroxyl groups in the cellulose, strengthening the mechanical properties. At the same time, when the concentration of styrene acrylic emulsion reaches 3%, the effect of pure Lyocell buffer material on mechanical properties is no longer noticeable. Because the styrene-acrylic emulsion can improve the mechanical properties of the fibers after being combined with the fibers at the proper amount, the excessive styrene-acrylic emulsion may agglomerate inside the material and generate surface stress during the drying, which causes the brittleness of the material to increase. The mechanical properties were reduced. The elastic modulus of 3% styrene acrylic reinforced Lyocell buffer material is 116.12 kPa, and the density is 0.044 g/cm3. The resilience of Lyocell buffer material reinforced with 3% styrene acrylic is 92%, lower than that of pure Lyocell buffer material. When the strain is less than 42%, the static compressive stress range is 0-17 kPa, according to the load range set in the Specification for Elastic Buffer Materials for Packaging. GJB 2271-95, belonging to class 6 extra heavy load range, 10.3 ~ 27.6 kPa.
3.3 Analysis of mechanical properties of Lyocell fiber /PET fiber/ styrene-acrylic emulsion reinforced buffer material
In 2013, Finland VTT Technology Co., Ltd. foamed foam technology from laboratory to practical application. VTT used 6mm Lyocell fibers in 2015 to investigate the effect of fiber length on the formation of final products. It is concluded that products made of wood fibers and natural or artificial long fibers will become a potential new application field.(Koponen et al. 2016) Therefore, 12 mm PET fibers were selected in this study to explore its effect on buffer materials.
A single-factor experiment method was used to prepare Lyocell fiber /PET fiber/styrene acrylic emulsion reinforced buffer material (SA@LF/PET) with different fiber ratios. The results show that when pulp concentration is 3%, the ratio of Lyocell fiber /PET fiber (6dtex) / styrene acrylate emulsion (SA@LF/PET(6 dtex)) is 5:5, and the ratio of Lyocell fiber/PET fiber (17 dtex) / styrene acrylate emulsion (SA@LF/PET(17 dtex)) is 6:4, the buffer effect is well behaved. Besides, the compression performance of buffer materials is degraded if the ratio of PET fibers is too high.
To improve the elastic modulus of the SA@LF/PET. The experiments increased the concentration of styrene-acrylic emulsion. Sizing gradients of 3%, 6%, and 9% .
Table 2 Compression performance parameters of SA@LF/PET(6 dtex)
Styrene-acrylic emulsion concentration,
%
|
Elastic modulus,
kPa
|
Final
deformation,
%
|
Density,
g/cm3
|
3
|
15.38
|
0
|
0.024
|
6
|
16.66
|
0
|
0.027
|
9
|
40.41
|
0
|
0.037
|
Table 3 Compression performance parameters of SA@LF/PET(17 dtex)
Styrene-acrylic emulsion concentration,
%
|
Elastic modulus,
kPa
|
Final
deformation,
%
|
Density,
g/cm3
|
3
|
37.98
|
8
|
0.028
|
6
|
40.07
|
5
|
0.022
|
9
|
46.99
|
6
|
0.030
|
Compared with pure SA@LF, PET fibers reduce the elastic modulus. However, the density decreases, and the resilience dramatically improves, as shown in Tables 2 and 3. It is known that PET fiber (12 mm, 6 dtex) is a kind of long fiber. Because of its larger combined area with the styrene-acrylic reinforcing agent, it can improve the strength of the composite material to a certain extent. However, the elastic modulus has dropped significantly. It is speculated that the adsorption capacity of the entire PET fiber is much lower than that of the Lyocell fibers. The styrene-acrylic solvent is mostly adsorbed into the Lyocell fibers. There is no reasonable connection point formed in the battery, resulting in a decrease in strength. Therefore, adding PET fibers to the cushioning material will reduce the mechanical properties. At the same time, long fibers can play a supporting role in the three-dimensional structure and improve the resilience of the material.
Increasing the concentration of Styrene-acrylic emulsion enhanced the elastic modulus of the material to a certain extent. Under the same Lyocell fiber and PET fiber (6 dtex) ratio, the maximum elastic modulus of styrene-acrylic reinforcement with 9% concentration is 40.41 kPa. The density is 0.037 g/cm3. The resilience is 100%, and the static compressive stress range is 0-8 kPa, as shown in Fig. 8. According to the load range set in the GJB 2271-95 specification for elastic buffer materials for packaging, it belongs to the level 5 hefty load range, 6.9 ~ 10.3 kPa.
Under the same ratio of Lyocell fibers and PET fibers (17 dtex), the maximum elastic modulus of styrene-acrylic reinforcement 9% is 46.99 kPa.The density is 0.030g/cm3. The resilience is 94%. The static compressive stress range is 0-12 kPa, as shown in Fig. 9. According to the load range set in the Specification for Elastic Buffer Materials for Packaging. GJB 2271-95, belongs to the level 6 extra heavy load range, 10.3 ~ 27.6 kPa. Therefore, we can choose SA@LF/PET according to product requirements when considering the light weight.
3.4 Microstructure characterization of buffer materials
Fig. 10 shows the surface micromorphology of pure Lyocell buffer material before and after impregnation and drying with a styrene-acrylic solution. Fig. 11 shows the surface micrograph of SA@LF/PET(6 dtex).
Fig. 10 (a) and (b), the styrene-acrylic emulsion is deposited on the fiber surface. It is evenly distributed on the surface of the fibers, forming a smooth film. The microstructure of the fibers is affected by the styrene-acrylic emulsion and plays an essential role in improving the elastic modulus and buffer properties of the materials.
The comparison of Fig. 10 (c) and (d) shows that the pure Lyocell buffer material reinforced by styrene and acrylic has a three-dimensional structure inside, and the fibers interweave. Pure Lyocell buffer materials only exist through physical interleaving and hydrogen bonding between fibers at the junction of fibers. While the styrene-acrylic emulsion is bonded to the fiber buffer material when the styrene-acrylic resin is reinforced, making the fibers interweave more closely. It is conducive to improving the mechanical properties of the material.
Comparing Fig. 11 (a) with (b) of the electron microscope, Lyocell fibers adsorbed more benzene acrylic solvent. As shown in Fig. 10 (b), benzene acrylic does not form a good connection point between Lyocell fibers and PET fibers, which may decrease modulus.
3.5 Compost disintegration experiments
A biodegradability test is used to determine the duration of biodegradable foam degradation in the environment. The standard biodegradability value in the study applies to the European standards listed in EN 13432.(Arrieta et al. 2015) The reinforcement effect of styrene-acrylic solution and PET fibers on buffer material was characterized by a static compression test. Select the fiber-based buffer material with the best buffer performance to compare biodegradability. The result of biodegradability analysis in this study is presented in Fig. 12.
Buffer materials were disintegrated under composting conditions. After 10 days of incubation, LF, 3%SA@LF, and 9%SA@LF/PET(6 dtex) all showed signs of disintegration. 9%SA@LF/PET(17 dtex) became breakable after 40 days of incubation in compost. In the initial stage of degradation, the degradation rate of 3%SA@LF and LF was the fastest, and the more SA was added, the slower it became. In the whole process, the degradation rate of LF is relatively regular and gentle. The mass loss rate of Lyocell fibers was 8.31% in 20 days. 20 to 90 days was an approximately uniform mass loss process. Those aged 90 to 210 days presented an accelerated mass loss stage.
The styrene-acrylic reinforced material degrades rapidly and then slowly in the degradation process. It is speculated that the coated Lyocell fibers began to decompose after the styrene-acrylic was decomposed. PET fibers, as a kind of refractory fiber, slowed down the degradation rate. With the wrapping of styrene-acrylic emulsion, the degradation effect is not ideal.
All buffer materials changed color and became more opaque after 10 days, due to a change in the refraction index of the materials as a result of water absorption and/or the presence of products formed by the hydrolytic degradation process. In the late stage of degradation, the appearance of the material begins to disintegrate into large pieces.