DOI: https://doi.org/10.21203/rs.3.rs-1673735/v1
A fiber network is a common structure in both industrial (paper and nonwovens) and biological materials (plant cells and animal tissues).(Alimadadi and Uesaka 2016) The fibers are entwined with one another, forming a strong mechanical support. It does, however, make it more difficult to distribute. Aqueous foam is a great way to carry items that are in a dispersed form. The foam bubbles serve as spacer particles between the fibers, preventing flocculation.(Burke et al. 2019) Foam is used for this purpose in the nonwoven industry, for example, to carry long fibers.(Lehmonen et al. 2020) In this study, a wet-laid nonwoven foam forming method was used to produce fiber-based buffer material.
Cellulose has the advantages of being recyclable and environmentally benign as a renewable resource with vast reserves.(Edgar and Zhang 2020) Regenerated cellulose fiber has gradually seized the market in recent years as the concept of green manufacturing has been upgraded. Lyocell fiber is one of the most successful regenerated cellulose fibers developed in recent decades. It is described as "green and eco-friendly fiber."(H. Zhang et al. 2021) Without employing carbon disulfide, the physical dissolving of cellulose in N-methyl morpholine oxide (NMMO) allows for the production of regenerated cellulose fibers in a straightforward, resource-conserving, and environmentally benign way. Lyocell fibers are high-strength, biodegradable, and absorbent fibers that have been widely used in the textile industry.(S. Zhang et al. 2018) In this study, the possibility of using Lyocell fibers as a recyclable raw material to prepare buffer material was explored.
Styrene-acrylic emulsions are widely used as an adhesive because of their nontoxicity, nonflammability, reduced pollution, weathering resilience, and chemical corrosion resistance.(Dou et al. 2019) Another significant benefit is the use of water as a dispersion rather than potentially harmful chemical solvents.(Villanova et al. 2012) Poly(ethylene terephthalate) (PET) is a semicrystalline thermoplastic polyester with exceptional features such as good gas barrier for packaging films and bottles and high fiber tenacity. High tensile strength, high modulus, and minimal thermal shrinkage are expected features when PET fibers are utilized as tire cables.(X. Zhang et al. 2008)
Buffer materials have a bright future in a variety of areas, including sound absorption (Cucharero et al. 2021), automobiles(Huang et al. 2014), clothing(Li et al. 2014), aerospace(Tadano et al. 2011), and many more. {Deng, 2003 #5;Huang, 2014 #4;Li, 2013 #8}The most common foam forming methods are physical foaming and chemical foaming. The physical foaming process uses water vapor as a foaming agent, mixing fibers and starch and forming bubbles between raw materials in water vaporization to achieve foaming molding.(Lopez-Gil et al. 2015; Song et al. 2021) Chemical forming adds chemical foaming agents to the raw material, which produces gas in the heating process for foaming.(Dong and Wang 2021) Most of the raw materials in chemical forming are wastepaper, fibers, and agricultural residues. (Wu et al. 2022)
As for the wet-laid nonwoven foam forming method, it is similar to a papermaking process. With the addition of different surfactants and rapid mixing in a mechanical blender, the raw material can be well dispersed in the water-based foam. The foaming slurry is poured into the molding device for injection molding and dehydration. Finally, the mixer was dried and shaped using an oven. Random fibre networks are created using wet-laid foam forming processes, allowing for porous structural design. With a foam carrier, the size of the pore in the material can be adjusted by the foam's bubble size, which has an impact on material properties. Meanwhile, wet-laid nonwoven foam formation is a low-cost, eco-friendly, and simple approach that uses water as the carrier throughout the process.(Härkäsalmi et al. 2017)
For fiber-based materials, the foaming method can be classified into two types: the one-step molding method and the two-step molding method. The one-step molding process consists of pulverizing the fiber material, mixing it with a foaming agent and other additives, and then putting it into a dedicated mold, as shown in Fig. 1. It is shaped after foaming in a shape defined by the mold. As shown in Fig. 2, the two-step molding process is as follows: pulverizing the plant fibers into a fiber shape of fewer than 5 mm2, mixing the fibers with starch in a particular ratio, and mixing with starch for granulation. The mixed particles are fed into the extruder to make cylindrical particles. During the granulation in the extruder, the raw material is foamed by water vapor to form foamed particles. The foamed particles are sent into the hot press molding machine's particular metal mold and foamed under pressure in the metal mold. Generally speaking, buffer materials with larger volumes and heavier quality should adopt a two-step molding process. Using the one-step molding process to make a smaller volume and lighter buffer material is less complicated. The fiber porous material produced by foam forming has the advantages of being lightweight, low cost, and recyclable, but its high porosity also limits its ability to improve mechanical properties.
To strengthen the mechanical properties, this study selected styrene-acrylic acid and PET fibers as the main modification components. The effects of fiber concentration, pH value, and surfactant type on the Lyocell fiber foam system's foam distribution and stability were investigated by single factor and orthogonal tests. This study chose a one-step molding method in order to make light weight buffer materials. The fibers are made of foam slurry by foaming, dehydrating, defoamed, forming, and drying into a fibrous material with a three-dimensional mesh and a fluffy porous structure.
2.1 Materials
Lyocell fibers, 6 mm long. PET fibers, 12 mm long, 6 dtex and 17 dtex fineness. Styrene acrylic emulsion(SA), solid content of about 45%, Ji'nan Europe New Material Co., Ltd. Sodium dodecylbenzene sulfonate(SDBS), analytical purity, Shanghai Lingfeng Chemical Reagent Co., Ltd. Cetyltrimethylammonium bromide(CTAB), analytical purity,Shanghai Boao Biotechnology Co., Ltd. Aqueous solution of polyvinyl alcohol(PVA), solid content of about 3%.
2.2 Sample preparation
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 to optimize the foaming process.
The composite fiber buffer material was prepared according to the following conditions, as shown in Fig. 1. First, 200g of plant fiber foaming slurry was weighed. The pulp formulation for the foaming process consisted of 3% wt. , absolute dry fibers, 0.15% wt. SDBS, SA and water. Various dosages of SA (0%, 3%, 6%, 9%) were applied to investigate the effect of buffer material. The pH was adjusted to 7. After disintegrating for 30 seconds (HP-BJQ fiber unwinder, Jinan Hengpin Electromechanical Technology Co., Ltd.) and foaming for 10 minutes (RW20 overhead mixer, IKA company, Germany.), the foaming slurry was poured into the molding device for injection molding and dehydration. The mixing speed of the agitator was fixed at 2000 rpm to ensure effective mixing and foaming. Finally, the mold is moved into the drying box to dry for 8 h at 80 ℃ (DHG-9030A electric blast drying oven, Shanghai Qixin Scientific Instrument Co., Ltd) and solidify.
2.3 Characterization and testing
Characterization and test of static buffer performance. The static buffer performance of the sample shall be tested according to GB / T 8168-2008 static compression test method for buffer materials for packaging. The pressing plate gradually increases the load on the sample along the height direction at the speed of 12 mm/min (INSTRON 5565 tensile and compression material testing machine, Instron, USA.). The upper and lower surfaces of the cylinder are flat. At least 10 points are recorded or measured by the material testing machine to determine the compressive stress-strain curve of the material in the thickness direction.
Microstructure characterization. The material's internal structure was observed by scanning electron microscope (SEM, COXEM EM-30Plus scanning electron microscope, kusem company, Korea. NETZSCH thermogravimetric analyzer, Netzsch, Germany.) to analyze the material's mechanical properties.
Compost disintegration experiments. The disintegration in composting conditions of mats was tested at laboratory scale level according to the ISO 20200 standard. All buffer materials (cut in 15 mm × 15 mm) were buried at 4–6 cm depth in perforated plastic boxes containing a solid synthetic wet waste prepared with 10% of compost (Mantillo, Spain), 30% rabbit food, 10% starch, 5% saccharose, 1% urea, 4% corn oil and 40% sawdust and approximately 50 wt% of water content. Buffer materials samples were then incubated at aerobic conditions at 58 °C and further recovered at 10, 20, 30, 40, 50, 60, 70, 80, 90, 150 and 210 days of disintegration. After each extraction photographs were taken to all samples to have a qualitative check of the degree of physical degradation in compost as a function of time.
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.
This study used styrene-acrylic emulsion and PET fibers to enhance the Lyocell fiber buffer materials. The foam-forming method realized the light weight and high resilience of buffer materials. With the addition of different surfactants and rapid mixing in a mechanical blender, the raw material can be well dispersed in the water-based foam.
The reinforcement effect of styrene-acrylic solution and PET fibers on buffer material was characterized by a static compression test.Compared with the pure Lyocell buffer material, the material's elastic modulus increased from 37.30 kPa to 116.12 kPa when reinforced with 3% styrene-acrylic emulsion. The Lyocell buffer material's strength, density, resilience, and stress compression range are reinforced by styrene-acrylic acid increases. Compared to SA@LF, the addition of long PET fibers reduces the elastic modulus, but the density decreases, and the resilience is greatly improved. Increasing the concentration of styrene-acrylic emulsion enhances the elastic modulus of the material to a certain extent.
By optimizing the foam forming process and adjusting various influencing factors in the foam forming process, the buffer materials were prepared, which have the advantages of low density and high resilience. Among them, LF and SA@LF are degradable in a certain way. At the same time, this study gives the performance characterization of the buffer material under different process parameters and raw materials, which can be used as a reference to produce buffer materials with different densities and different performance requirements to replace styrofoam such as polyurethane with LF or SA@LF.