1. Introduction
Petrochemical-based plastics (polypropylene (PP), polypropylene, polystyrene, etc.) are widely used in the automotive industry [
1], the aviation industry [
2], electronics [
3], building materials [
4], agriculture [
5], and other areas due to their advantages of light weight, easy processing, and corrosion resistance. However, recycling waste plastics is difficult and uneconomical. By 2025, 11 billion metric tonnes of plastics are expected to be thrown directly into the natural environment [
6,
7]. Unfortunately, plastics usually have poor degradability due to their strong covalent bonds, such as C-C bonds [
8]. The incomplete decomposition products of microplastics (i.e., plastic fragments or particles with a diameter of less than
) accumulate in the soil, causing serious long-term environmental pollution [
9⇓-
11].
Biodegradable materials have recently been developed to replace conventional petrochemical plastics [
12⇓-
14]. Compared with petrochemical plastics, biodegradable materials can not only reduce the use of oil but also effectively reduce greenhouse gas emissions, making them one of the new research hotspots [
15⇓-
17]. Melt-blown nonwovens are widely used in various areas such as healthcare, the automotive industry, filtration materials, and environmental protection [
18⇓-
20]. However, commonly used melt-blown nonwovens are mostly made of non-degradable PP [
19,
21]. Due to its unique balance between degradability and durability, polylactic acid (PLA) is widely used in medical treatment, pharmacy, agriculture, packaging, clothing, and other fields [
22⇓⇓⇓⇓-
27]. However, melt-blown nonwovens made of PLA usually exhibit low strength and poor toughness. To improve the mechanical performance of PLA, degradable polymers such as poly(butylene adipate-co-terephthalate) [
28], polyethylene glycol [
29], poly(butylene succinate) [
30], poly(
-caprolactone) [
31], and nylon-11 [
32] have been added to PLA. However, most reported systems still have limitations in terms of strength and elongation enhancement. Studies [
33⇓-
35] have shown that adding poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) to PLA can improve the mechanical performance of PLA; moreover, the resultant PLA/PHBV blend is completely biodegradable. The above studies [
33⇓-
35] have shown that the addition of PHBV can effectively improve the crystallinity of PLA. Nevertheless, the mechanical properties of the blends remain limited due to poor compatibility. To solve this problem, polyethylene glycol [
36] and zinc acetate [
37] have been added to improve the compatibility of PLA and PHBV. In addition, a study of PLA/PHBV fiber by Liu et al. [
38] showed that reducing the amount of PHBV benefited the crystallization nucleation of PLA, improving the crystallization rate. The above reports on PLA/PHBV mechanical enhancement have mainly focused on composite materials. However, the preparation of biodegradable PLA/PHBV melt-blown nonwovens with sufficient mechanical properties is still extremely challenging.
Here, we present an efficient approach that can fabricate biodegradable PLA/PHBV melt-blown nonwovens with high strength and excellent toughness on a large scale (
Fig. 1(a)). First, starch, as the original raw material, is obtained from corn or other plants. Next, PLA and PHBV are prepared from the starch by means of fermentation. Finally, PLA and PHBV are simply mixed into melt-blown equipment, and PLA/PHBV melt-blown nonwovens are directly prepared after melting extrusion (
Fig. 1(b)). It is worth mentioning that the PLA/PHBV melt-blown nonwovens are simple to prepare and can be prepared on a large scale (
Figs. 1(c) and
(d)). A tensile test showed that the stress of the PLA/PHBV (5 wt% PHBV) melt-blown nonwovens reached
and the elongation reached
. In addition, the biodegradable melt-blown nonwo-vens can be prepared into surgical and facial masks (
Fig. 1(e)) to meet application requirements (Figs. S1 and S2 in Appendix A). More importantly, melt-blown nonwovens are biodegradable at their end of life. The decomposition products-carbon dioxide and water-can be reused by plants in nature to form a closed loop. The tensile process of PLA/PHBV melt-blown nonwovens is characterized and analyzed in this article. We also describe a general tensile-failure model of melt-blown nonwovens, which can assist in valuable mechanical performance enhancement for nonwovens.
2. Materials and methods
2.1. Materials
PLA (Ingeo biopolymer, melt flow rate ) was provided by Nature Works LLC. (USA). PHBV (Enmat Y1000) was provided by Ningbo Tianan Biologic Materials Co., Ltd. (China). PP melt-blown nonwovens were prepared by our laboratory. Effective microorganisms (EM) composting bacteria was supplied by Shandong Junde Biotechnology Co., Ltd. (China).
2.2. Preparation of PLA/PHBV melt-blown nonwovens
First, the PLA and PHBV were dried in a vacuum drum drying oven (JM-500ZGX; Shanghai Jinma Institute of Electro-optical Technology, China) at
for
to remove moisture. Then, the dried PLA and PHBV were mixed directly with various PHBV concentrations of0,2.5,5.0, and
. Finally, the samples were fabricated using a melt-blown device (FCN-2; Zibo Fangchen Masterbatch Factory, China), as shown in
Fig. 1(b). The melt-blown parameters are provided in Tables S1 and S2 in Appendix A.
2.3. Mechanical tensile test
The tensile properties of the PLA/PHBV melt-blown nonwovens were tested using a multifunctional fabric-strength machine (YG026G-III; Wenzhou Fangyuan Instrument Co., Ltd., China). Samples with an effective tensile size of were stretched until fracture at a tensile rate of . Each mechanical tensile test was repeated five times. It is worth noting that only the breaking strength could be obtained at this time. The stresses of the PLA/PHBV melt-blown nonwovens were calculated using the following formula: stress , where , and are the breaking strength, width, and thickness of the samples. The sample information is shown in Table S3 in Appendix A.
2.4. Degradation performance test
To investigate the outdoor biodegradability of the melt-blown nonwovens, PP, PLA, and PLA/PHBV (5.0 wt% PHBV) melt-blown nonwovens were cut to a size of . They were then buried together in soil at a depth of . To speed up biodegradation, we added EM composting bacteria to the soil. The samples were taken out, cleaned, and dried monthly to characterize their morphologies and structural changes.
2.5. Characterizations
The microstructures were characterized by means of VEGA III scanning electron microscopy (SEM; TESCAN Ltd., the Czech Republic). The diameters of 100 melt-blown nonwoven fibers were randomly measured and counted using Acrobat Reader software (Adobe Acrobat Pro DC 2020; Adobe Systems Inc., USA). Pore size distribution was investigated using a CFP-1100AI capillary flow porometer (Porous Materials Inc., USA). Porosities were calculated using the following formula: porosity , where and are the weight and volume, respectively, and is the density of the PLA/PHBV fiber. The Fourier-transform infrared (FTIR) spectra of PLA, PHBV, and PLA/PHBV (5.0 wt% PHBV) were characterized via a Nicolet iS10 Fourier-transform infrared spectrometer (Thermo Fisher Scientific Inc., USA). The fiber orientation distribution was analyzed by means of ImageJ (National Institutes of Health, USA). Crystal structures were measured via X-ray diffraction (XRD; XRD-600, Bruker, Germany). A moisturizing performance test was carried out in which a wet facial mask was placed in a constant environment at a temperature of and relative humidity of for ; the weight was tested every . The water absorption was calculated using the following formula: water absorption , where and are the weights of the wet and evaporated facial mask, respectively. The filtration performance of the masks before and after a corona electret was tested using a TSI 8130A automated filter tester (TSI Inc., USA). The flow rates of sodium chloride (NaCl; Sinopharm chemical reagent, Co., Ltd., China; diameter ) and aerosol were 32 and .
3. Results and discussion
3.1. Morphology and structure of PLA/PHBV melt-blown nonwovens
PLA/PHBV melt-blown nonwovens with various PHBV concentrations
were fabricated, and the morphology and structure of the PLA/PHBV melt-blown nonwovens were investigated.
Figs. 2(a)-(d) shows the surface morphologies of melt-blown nonwovens fabricated with different PHBV concentrations. The SEM images indicate that the melt-blown nonwovens consist of randomly stacked fibers, which are fluffy and porous.
Figs. 2(e)-(h) describes the fiber diameter distribution. With an increase in PHBV content from 0 to
, the average diameter decreases significantly from 3.7 to
. Moreover, the proportion of fibers with a diameter greater than
decreases from
at 0 PHBV to
, and
at2.5,5.0, and
PHBV, respectively. With an increase in PHBV concentration, the fiber diameter decreases, while the distribution becomes wider. Strikingly, when the PHBV concentration increases from 0 to
, the proportion of nanofibers is almost five times higher (from
to
. The insets in
Figs. 2(g)-(h) shows a diameter distribution histogram of the nanofibers. The thinnest fibers are as small as
in diameter, accounting for
. We attribute this result to the fact that the presence of PHBV reduces the spinning stability. Due to the difference in the fluidity of PLA and PHBV, the stability of the jet flow of the PLA/PHBV melt is reduced by the high-temperature and high-speed airflow. Thus, the average fiber diameter of the melt-blown nonwovens decreases, although some fibers are not stretched enough to increase their diameter, and other fibers are thinned into nanofibers by excessive stretching. Compared with microfibers, nanofibers have greater molecular-chain orientation, resulting in higher strength. It should be noted that reducing the fiber diameter (i.e., average diameter and distribution) of melt-blown nonwovens to the nanometer scale will significantly affect the mechanical performance [
39,
40].
To further explore the influence of the fiber stack structure on the mechanical performance of the PLA/PHBV melt-blown nonwo-vens, two other key factors-namely, pore size distribution and porosity-were characterized. As illustrated in
Fig. 2(i), the pore size of the PLA/PHBV melt-blown nonwovens varies between 5 and
. Moreover, it was found that, with an increase in PHBV concentration, the pore size decreases first and then increases. Furthermore, compared with pure PLA, the pore size of the PLA/PHBV decreases from 19 to
as the PHBV concentration increases from 0 to 5 wt%. At the same time, the PLA/PHBV melt-blown non-wovens become increasingly fluffy, and the porosity increases from
at 0 PHBV to
, and
at2.5,5.0, and
PHBV, respectively (
Fig. 2(j)). In general, pore size and porosity have a strong negative correlation with fiber diameter [
18,
20]. Obviously, as the fiber diameter decreases, the pore size decreases significantly. However, the porosity of the melt-blown nonwovens increases with a greater proportion of thinner fibers, perhaps because the proportion of thinner fibers increases in number but decreases in volume. Therefore, the melt-blown non-wovens are fluffier due to the support of coarse fibers.
To explore the influence of PHBV addition on mechanical performance, the chemical structure and crystallization properties of the samples were characterized.
Fig. 2(k) provides the FTIR spectra. In the FTIR spectra of PLA, there is a
stretching mode at
, a C-O-C stretching mode at
, and the wagging absorption of
at
. In the FTIR spectra of PHBV, the peak at
is assigned to the
stretching mode, and the peaks at 1248 and
are assigned to the
stretching mode. In the FTIR spectra of PLA/PHBV, the peaks at 1749 and
correspond to the
stretching mode, the peak at
is assigned to the C-O-C stretching mode, and the peak at
corresponds to
[
41]. No new absorption peak appears in the FTIR spectra of PLA/PHBV; there are only intensity changes in the absorption peaks and a slight shift in peak position. Thus, no chemical reaction occurs between the PLA and PHBV.
Fig. 2(1) shows the XRD patterns of the PLA/PHBV melt-blown nonwovens; in the figure, the increase in crystal peak height can be seen directly from the increase in crystallinity. PLA has two crystal phases: the
’-phase
and the
-phase
[
42]. With an increase in PHBV concentration, the XRD peaks shift from
at0PHBV to
, and
at2.5,5.0, and
PHBV, respectively. The results indicate that the PLA/PHBV melt-blown nonwovens contain both the
-phase and the
-phase. Moreover, the addition of PHBV increases the proportion of
-phase in PLA; the PLA macromolecular chains become more orderly, and the van der Waals forces between the chains become stronger. Considering that the formation temperature of the
’-phase is less than 100
and the formation temperature of the more stable
-phase is
, the increase in the
-phase is mainly due to the acceleration of the crystallization rate.
3.2. Mechanical performance of PLA/PHBV melt-blown nonwovens
Previous researches [
43,
44] have shown that melt-blown non-wovens have relatively thin fibers and poor mechanical performance. In addition, the slow crystallization rate of PLA causes PLA melt-blown nonwovens to have poor strength and toughness, which limits their application performance. In order to determine the effect of PHBV addition on the mechanical performance of PLA melt-blown nonwovens, the tensile mechanical performance was tested. As shown in
Fig. 3(a), the addition of appropriate PHBV content improves the stress and strain, while the addition of excessive PHBV reduces the stress and strain. To reveal the effect of PHBV addition on melt-blown nonwovens, the PHBV phase in a PLA/PHBV composite was observed (Fig. S3 in Appendix A). It was found that, when a small amount of PHBV
is added, the PHBV is evenly dispersed in the PLA as small particles. As expected, the mechanical properties of the PLA benefit from the presence of PHBV. However, excess PHBV
is dispersed in the PLA in the form of large particles. Due to the thermodynamic incompatibility of PLA and PHBV, PHBV tends to unite and separate from PLA as the content of PHBV increases, resulting in a decrease in the mechanical properties of the PLA/PHBV melt-blown fibers.
In addition, when the PHBV supplemental level reached
, some fibers became thinner and other fibers became coarser, leading to an increase in the coefficient of variation of the fiber diameter (
Figs. 2(e)-(h)). During the tensile process of melt-blown nonwovens, an increase in the coefficient of variation of the mechanical properties of the fibers results in fiber fracture at different times. In conclusion, at a PHBV content of
, the lower mechanical properties of a single fiber and the increase in the coefficient of variation of the mechanical properties of the fibers lead to a decrease in the mechanical properties of the melt-blown nonwovens.
As illustrated in
Fig. 3(b), Young’s modulus and toughness of the PLA/PHBV melt-blown nonwovens were calculated. As the concentration of PHBV increases from 0 to 5%, Young’s modulus increases slightly, mainly because the presence of a small amount of PHBV promotes the crystallization process of the melt-blown nonwovens (Table S3 in Appendix A). When the concentration of PHBV increases from 5 to
, Young’s modulus of the melt-blown nonwovens decreases sharply from 105.0 to
. This result may be ascribed to the fact that the presence of excess PHBV reduces the stability of the melt-blown process, resulting in insufficient stretching of the melt-blown fibers. As expected, when the PHBV concentration is 5%, the toughness of the melt-blown non-wovens reaches
, which is about 1.9 times greater than that of pure PLA melt-blown nonwovens. We attribute this result to the synchronous improvement in tensile stress (up to 2.5 MPa) and strain (up to
). Such superior mechanical properties have rarely been achieved to date. A comparison of the tensile performance of the PLA/PHBV melt-blown nonwovens and nonwo-vens reported in other studies is shown in
Fig. 3(c) [
45⇓⇓⇓⇓⇓-
51].
As presented in
Fig. 3(a), the tensile stress-strain curves of the melt-blown nonwovens can be divided into three stages: ① the Hookean elastic region, ② the strain region, and ③ the breaking region. To provide insight into the mechanical performance of the melt-blown nonwovens at three different stages, we recorded the whole stretching process of melt-blown nonwovens with a PHBV concentration of 5% (Supplementary Video in Appendix A).
Fig. 3(d) displays the stretching photographs under different strains. Fig. S4 in Appendix A and
Figs. 3(e)-(g) describe the fiber orientation distribution under strains of 0,
, and
, respectively. In this work, in the Hookean elastic region (strain
), the morphology of the melt-blown nonwovens is almost unchanged, and the fibers in the melt-blown nonwovens are anisotropic. This finding indicates that the distribution state of the fibers does not obviously change. In contrast, the stress of the melt-blown nonwovens increases rapidly due to the straightening of the bending fibers. In the strain region (2.5% < strain < 45%), the melt-blown nonwovens shrink in the middle under tension, and the fibers in the melt-blown nonwovens gradually orient along the tensile direction. This finding indicates that the increase in the stress in the melt-blown nonwovens is due to fiber orientation, and the rapid increase in strain is related to fiber slippage. In the breaking region (strain
), the melt-blown nonwovens cannot withstand the tensile force and break, resulting in zero strength within a short time. In conclusion, as the elongation increases, the fiber direction gradually tends to align with the tensile direction
and
). This change in fiber orientation causes the fibers to slip against each other, and this constant change in position is the reason for the significant increase in material elongation. In addition, the closer the fiber orientation is to the tensile direction, the more strength the fiber will contribute to the material, resulting in an improvement in strength. According to our results, elongation is more significantly affected by fiber orientation adjustment than strength.
Fig. 3(h) depicts the tensile fracture process of the PLA/PHBV melt-blown nonwovens in the tensile test according to their microstructure. The minimum unit of melt-blown nonwovens is equivalent to a triangle formed by the overlap of fiber 1, fiber 2, and fiber 3. Based on a simple analysis of fibers 1 and 2, fiber 1 is mainly affected by the drawing force
and friction
between fiber 2 and fiber 1. More specifically,
gives the fiber a tendency to move, whereas
keeps it in its original state.
in the Hookean elastic region; the fibers are straightened by
, but there is no slip between the fibers. In the strain region,
does not change significantly. In contrast,
increases rapidly due to the contraction of the melt-blown nonwoven. When
, the fibers slide over each other to orient along the stretch direction, resulting in a rapid increase in tensile strain. At the same time,
increases due to the continuous contraction of the melt-blown nonwoven.
increases to accommodate the larger
, and their constant competition leads to a gradual increase in the tensile stress. It is clear that the tensile strain increases rapidly while the stress increases slowly, which is opposite to what occurs in the Hookean elastic region. It is worth pointing out that although
continues to increase,
(the breaking strength of a single fiber). When
in the breaking region, the fiber will break. The breaking of multiple fibers causes the web to be weaker than the tensile force, resulting in the fracture of the melt-blown nonwoven. In addition, the inset shows that the mechanical properties of the PLA/PHBV single fibers are enhanced by PHBV. A small amount of PHBV is evenly dispersed in the fiber, creating a two-phase system. The PHBV (acting as a nucleating agent) improves the crystallinity (
Fig. 2(l)). As a result, the arrangement of the PLA macromolecular chain is more orderly, and the van der Waals forces between molecular chains are enhanced. When the fiber is stretched and becomes thinner, the molecular chains of PLA and PHBV are stretched, and the van der Waals forces between the PLA and PHBV phases are enhanced (Fig. S3 in Appendix A). When the PLA molecular chain breaks, the PHBV molecular chain prevents crack propagation, thereby strengthening the single fiber [
37]. It should also be mentioned that when the strength of a single fiber increases, the strain of the melt-blown nonwoven increases to provide enough friction to break the fiber. Therefore, the reinforcement of a single fiber not only enhances the stress of the melt-blown nonwoven but also improves its strain.
3.3. Biodegradation performance of PLA/PHBV melt-blown nonwovens
Biodegradable materials should maintain good mechanical properties during service and then degrade quickly at their end of life to reduce environmental damage. For comparison, PP, PLA, and PLA/PHBV melt-blown nonwovens were respectively buried in the same soil environment for biodegradation experiments. Visually, the PLA and PLA/PHBV melt-blown nonwovens broke down after being buried in the soil for two months (
Fig. 4(a)). We attribute this result to the rupture of
bonds in the PLA and PHBV due to water [
14,
52]. Then, microorganisms (e.g., bacteria and fungi) in the soil attacked and digested the PLA and PHBV macromolecules by secreting enzymes. Finally, the PLA and PLA/ PHBV melt-blown nonwovens were completely biodegraded after four months of burial. To demonstrate the important role of soil microorganisms in the degradation, melt-blown nonwovens were immersed in pure water for comparison. The results show that the morphology of the PLA and PLA/PHBV melt-blown nonwovens placed in pure water did not change significantly after four months (Fig. S5 in Appendix A). In contrast, the PP melt-blown nonwovens showed no change in either condition, suggesting that nonbiodegradable plastics will continue to affect the environment for a long time.
Fig. 4(b) describes the weight change of the melt-blown nonwo-vens. The weight of the PLA and PLA/PHBV melt-blown nonwovens decrease rapidly over time, while the weight of the PP remains constant. In addition, the weight loss rate of PLA/PHBV is faster than that of PLA. SEM images of the melt-blown nonwovens after two months of biodegradation indicate that the fibers in the PP melt-blown nonwovens have not changed significantly, whereas a large number of the fibers in the PLA and PHBV melt-blown non-wovens have cracked and fractured. On a macroscopic level, the PLA and PHBV melt-blown nonwovens have become defective, and their mechanical properties have decreased significantly (
Figs. 4(c)-(e)).
Fig. 4(f) describes the two-step degradation process of the PLA/PHBV melt-blown nonwovens. First, water molecules penetrate the amorphous region of the fiber, and some of the
bonds in the PLA and PHBV are hydrolyzed. At the microscopic level, the long molecular chains of the PLA and PHBV break into short molecular chains; at the macroscopic level, the fibers and melt-blown nonwovens fracture. Subsequently, the short molecular chains of the PLA and PHBV are completely degraded by microorganisms. Compared with the PLA composites, the PLA and PLA/PHBV melt-blown nonwoven fibers have smaller diameters
and higher porosity
. The ultra-high specific surface area enables the melt-blown nonwoven fibers to be hydrolyzed and completely degraded at the fastest speed. The PLA and PLA/PHBV melt-blown nonwovens have excellent mechanical properties while in service, but they are easily hydrolyzed and degraded by microorganisms in the soil. Compared with non-degradable materials, this good balance between mechanical and biodegradable properties is extremely advantageous.
4. Conclusions
In this work, a simple strategy was demonstrated for the large-scale preparation of biodegradable PLA/PHBV melt-blown nonwo-vens with high strength and excellent toughness. Without additional treatment, dried PLA and PHBV are added into a melt-blown machine together, where they are fully melted in a screw extruder. The high-temperature melt is then rapidly stretched into fibers in a high-temperature and high-speed airflow field. Finally, the partially cooled fibers are bonded to form PLA/PHBV melt-blown nonwovens under the action of the airflow. It is worth mentioning that the addition of a small amount of PHBV improved the crystallization rate and crystallinity of the melt-blown nonwoven. As the PHBV concentration increased from 0 to , the proportion of nanofibers increased significantly from 7.7% to 42.9%. The PLA/PHBV melt-blown nonwovens exhibited enhanced stress (2.5 MPa, an increase of ), higher elongation (45%, an increase of ), and excellent toughness , an increase of ) with the appropriate addition of PHBV (5 wt%). However, excessive PHBV content increased the coefficient of variation in the diameter and the mechanical properties of the melt-blown fibers, leading to a rapid decrease in the mechanical properties of the materials. Importantly, a general tensile-failure model of the PLA/ PHBV melt-blown nonwovens was proposed based on the tensile curve and tensile process. In addition, the mechanically high-performance PLA/PHBV melt-blown nonwovens are environmentally friendly and can be completely degraded to carbon dioxide and water after four months in soil. We envision that biodegradable PLA/PHBV melt-blown nonwovens could replace the existing PP melt-blown nonwovens, with excellent application prospects in medical, filtration, environmental protection, and other fields.
Acknowledgments
This work is supported by the National Key Research and Development Program of China (2022YFB3804903 and 2022YFB3804900), the National Natural Science Foundation of China (52273052), the Program of Shanghai Academic/Technology Research Leader (21XD1420100), and the International Cooperation Fund of Science and Technology Commission of Shanghai Municipality (21130750100).
Compliance with ethics guidelines
Gaohui Liu, Jie Guan, Xianfeng Wang, Jianyong Yu, and Bin Ding declare that they have no conflict of interest or financial conflicts to disclose.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.eng.2023.02.021.
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