Enhanced hydrophilicity of low melting point polylactic acid by butenediol vinyl alcohol copolymer via melt blending

In order to solve the poor hydrophilicity of low melting point polylactic acid (LMPLA) limiting its application in absorbent sanitary products. LMPLA/butenediol vinyl alcohol copolymer (BVOH) blends with different ratios were manufactured by simple melt blending. The BVOH exhibited good compatibility and dispersity in LMPLA matrix without chemical reaction, and LMPLA/BVOH blends showed sea-island structure. The introduction of BVOH could promote the crystallization of LMPLA and improve the crystallinity, whereas the crystallization of BVOH were limited. Furthermore, the introduction of BVOH could also decrease the thermal stability of LMPLA without affecting its application, but the tensile stress of LMPLA could be significantly increased. The tensile stress of LMPLA/BVOH blends could reach 78.59 MPa (increased by 10.9%) when he BVOH content was 3 wt%. Most importantly, the introduction of BVOH could significantly improve the hydrophilicity of LMPLA. The initial water contact angle decreased from 68.5° to 51.4° with the increase of BVOH content, and the contact angle decreased from 63.7° to 44.6° at 60 s. Moreover, the contact angle change rate of LMPLA/BVOH blends increased with the increase of BVOH content at different contact time, which also indicated adding BVOH could sharply improve the hydrophilicity of LMPLA.


Introduction
Low melting point fiber is a fiber with a lower melting point than conventional fiber, which can melt at a lower temperature, and has strong bonding performance.Compared with adhesives, low melting point fiber exhibits many excellent properties, such as fast bonding, stable performance, good mechanical properties, non-toxic, pollution-free and low energy consumption [1][2][3].Based on the number of components, low melting point fiber can be mainly divided into one component and two components.Low melting point fiber with one component is mainly composed of polyolefin, copolyamide, and copolyester [4,5].Moreover, it also shows the advantages of easy preparation, high bonding strength and stable performance.However, there is a problem that it is easy to be resinification and lose the fiber shape during hot melt bonding, which has a great impact on the performance, and has certain limitations in application [5][6][7].
Low melting point fiber with two components is usually prepared by using the low melting point component and the conventional component in the form of sheath core structure.The sheath of fiber plays the role of bonding due to the low melting point component, while the core of fiber is used to maintain the fiber shape because of its high melting point and good mechanical properties during hot melt bonding.Therefore, the combination of two components can solve the problem that the low melting point fiber is prone to be resinification during hot melt bonding [6,8,9].In current, sheath core fibers with two components, such as polyethylene (PE)/ polypropylene (PP) fibers (ES fibers), PE/polyethylene terephthalate (PET) fibers, and low melting point PET (LMPET)/ PET fibers, have been widely applied in the field of thermal bonding [8][9][10][11][12][13].These sheath-core fibers have been also successfully used in the production of hot-air cotton and absorbent sanitary products by nonwoven hot air technology.In particular, it needs to point out that the ES fibers currently occupy a dominant position in the field of absorbent sanitary products [10,[13][14][15].
In countries with seriously aging populations, such as China, Japan and South Korea, the demand for adult absorbent sanitary products has increased significantly.Moreover, the sizes of adult absorbent sanitary products are generally larger than that of children's absorbent sanitary product, indicating the raw material consumption of a single adult absorbent sanitary product is significantly higher than that of children's absorbent sanitary product.Besides, the daily consumption of adults is also significantly higher than that of children.Therefore, the consumption of ES fibers exhibits explosive growth due to the rapid growth of adult absorbent sanitary products market.However, the raw materials of ES fibers are all derived from petroleum, and serious white pollution can be caused due to the nonbiodegradability and non-renewability of ES fibers [8,11].Moreover, the oil crisis and environmental pollution can be also aggravated due to the huge use of absorbent sanitary products.
Polylactic acid (PLA) is an aliphatic polyester and a degradable polymer material, which is derived from plant starch, such as corn, sugarcane and cassava [16,17].PLA has been widely deemed as a potential substitute to replace conventional fossil-based polymers owing to the excellent degradability, applicable mechanical properties, and good processibility.Therefore, PLA has been successfully made into different products, such as industrial packing, hygienic products, drug delivery vehicles and tissue engineering scaffolds [16][17][18][19][20][21][22][23][24].In general, the wide application of bicomponent PLA fibers in the field of absorbent sanitary products can significantly reduce the oil crisis and environmental pollution caused by ES fiber.However, it is quite rare to find the mature application of bicomponent PLA fibers in absorbent sanitary products.The reasons are complicated and can be roughly divided into two aspects.On the one hand, the resinification of bicomponent PLA fibers cannot be completely solved due to the narrow processing temperature window and its own structural characteristics, resulting in poor product comfort and making them difficult to be recognized and accepted by the market [25][26][27][28].On the other hand, the surface layer (made of ES fibers) of conventional absorbent sanitary products needs to be hydrophilic modified in order to rapidly export the liquid, whereas PLA fibers exhibit poor hydrophilicity, which also seriously limits the application.Therefore, the hydrophilic modification of PLA fibers can further improve the application in absorbent sanitary products.
Several researches have been done to improve the hydrophilicity of PLA due to the requirements of various application fields.Zhu et al [29] developed a simple approach to prepare hemocompatible PLA membranes, and zwitterionic poly (sulfobetaine methacrylate) (PSBMA) was successfully grafted on the PLA membranes via atom transfer radical polymerization (ATRP).It was found that the hydrophilicity of modified PLA membranes significantly increased due to a hydration layer formed by the grafted sulfonic groups and tertiary amine groups via electrostatic interaction and hydrogen bonds.Yue et al [30] used CF 4 microwave plasma treatment to improve the hydrophilicity of electrospun poly (L-lactide) (PLLA) microfibrous membrane, and the water contact angle sharply decreased from 116 ± 3.0°to 0°under a relatively low power of plasma treatment.Hendrick E and Frey M [31] introduced poly (ethylene glycol) (PEG) homopolymer and PLA-b-PEG copolymers with different block lengths into PLA electrospinning solution to prepare hydrophilic nanofibers.The results showed that the addition of PLA-b-PEG co-polymers could improve the hydrophilicity of PLA nanofibers to a greater extent than PEG homopolymer at similar overall PEG content.Yu et al [32] prepared PLA/butenediol vinyl alcohol copolymer (BVOH) blends by simple melt blending, and the water contact angle results showed the hydrophilicity of PLA obviously increased by introducing BVOH.However, these studies mainly focus on the membranes or nanofibers of conventional PLA (melting point 170 ∼ 190 °C).And organic solvents and complex physical or chemical process are also used to improve the hydrophilicity in the previous research, which could affect the industrial scale production.Moreover, there are few studies on the modification of low melting point polylactic acid (LMPLA) to improve the processibility and hydrophilicity.
As the sheath of bicomponent PLA fibers, the processability and hydrophilicity of LMPLA determine whether the bicomponent PLA fibers can overcome the problem of resinification and meet the hydrophilic requirement of the surface layer of absorbent sanitary products in the process of nonwoven hot air technology.Therefore, the hydrophilic modification of LMPLA can promote the mature application of bicomponent PLA fibers in the field of absorbent sanitary products, which has great social value and research significance.Meanwhile, we ardently anticipate that the results of this work could provide reference significance for developing the degradable absorbent sanitary products.As a multifunctional new copolymer, BVOH exhibits good melt processibility, biodegradability and hydrophilicity [33,34].In our previous research [35], BVOH has also been used to modify the mechanical property and hydrophilicity of conventional PLA via melt blending, indicating that BVOH shows good compatibility with conventional PLA and decreases the water contact angle.Therefore, the hydrophilicity of LMPLA is also improved by melt blending with BVOH in this work.A series of LMPLA/BVOH blends are prepared by twin-screw extruder via simple melt blending, and a detailed study on the morphological structure, thermal behaviors, mechanical properties and hydrophilicity of blends are also carried out.

Preparation of PLA/BVOH blends
The LMPLA and BVOH resin particles were firstly mechanical mixed with different composition (100/0, 99/ 01,97/03, 95/05, 90/10).The LMPLA/BVOH blends were then prepared by melt blending via an SISZ-10A corotate twin-screw extruder (RuiMing Co., Ltd, Wuhan, China).The diameter and length of conical screw were 10/25 mm and 190 mm, respectively.The length-diameter ratio of conical screw could reach 19:1.All resin particles were vacuum dried before melt blending at 60 °C for 8 h to remove residual moisture.The melt blending process lasted 3 min with a screw speed of 30 rpm.The temperature from the initial part to the final part was 185, 190, 190, and 190 °C, respectively.An SZS-20 injection molding machine (RuiMing Co. Ltd, China) was then used to manufacture the specimens for the tensile test.The injection and mold temperature were 200 °C and 65 °C, respectively.The injection time was 5 s, and the cooling holding time was 30 s under the injection pressure of 25 MPa.In order to distinguish the samples, the LMPLA/BVOH blends with various BVOH contents were named as LMAB x , where x is the BVOH content.The preparation process flow diagram of LMPLA/BVOH blends is shown in figure 1.

Characterization
The chemical structures of LMAB x blends were examined using a Nicolet-10 Fourier transform infrared spectrometer (Thermo Fisher Scientific Co. Ltd, MA, America) with the test range from 4000 cm −1 to 400 cm −1 with a resolution of 4 cm −1 .A Miniflex 300 x-ray diffractometer (Rigaka Co. Ltd, Japan) was used to perform the crystallization behavior of LMAB x blends with Cuka radiation (λ = 0.154 nm) at a generator voltage of 40 kV and a current of 40 mA.The scattering angles varied from 3 to 60°with a scanning speed of 3°min -1 .A Gemini SEM 300 scanning electron microscope (Carl Zeiss AG, Jena, Germany) was used to observe the morphological structure of LMAB x blends at 3 kV accelerating voltage with a cryo-fracture surface coating with a thin gold layer.The crystallization and melting behavior of LMAB x blends were measured using a DSC-3 differential scanning calorimetric (Mettler Toledo Co., Ltd, Zurich, Switzerland).The equipment was firstly calibrated by In and Pb.Then all samples (6 ∼ 10 mg) were vacuum dried at 60 °C for 8 h before the test.The test procedure was divided into two steps.Firstly, samples were heated to 210 °C with a 10 °C min -1 heating rate and kept at 210 °C for 3 min to erase the heat history.Secondly, samples were also examined from 30 °C to 210 °C with a heating and cooling rate of 20 °C min -1 .Moreover, the test was carried out in a nitrogen atmosphere with 50 ml min -1 flow rate.The thermal stability of LMAB x blends was carried out using a TGA/DSC 3+ thermogravimetric analyzer (TGA) (Mettler Toledo Co. Ltd, Zurich, Switzerland) from 30 °C to 800 °C with a 10 °C min -1 heating rate under nitrogen atmosphere (50 ml min −1 ).The tensile properties of LMAB x blends were measured by an EZ-SX tensile tester (Shimadu Co., Ltd, Nagoya, Japan) based on the ASTM D638.The loading speed was 5 mm min −1 , and each sample was tested least five times to calculate the average value.The hydrophilicity of LMAB x blends was characterized by an OSA 60 video contact angle analyzer.Each sample was tested at least three different locations under 25 °C and 60% relative humidity to calculate the average value of water contact angle.The volume of deionized water drop is 5 μl.

Morphology and chemical structure of LMAB x blends
The Fourier transform infrared spectrum (FT-IR) of PLA, BVOH and LMAB x blends is shown in figure 2. It is found that a small band of LMPLA at about 3356 cm −1 represents the hydroxyl groups.The sharp band at 1748 cm −1 belongs to the stretching vibration of symmetric C=O.The small bands at 1452 cm −1 and 1358 cm −1 are assigned to the asymmetric bending deformation of -CH 3 , and stretching vibration of -CH and asymmetric deformation of crystalline -CH 3 , respectively.The bands at 1180 cm −1 and 1081 cm −1 are related to the stretching vibration of C-O-C.Moreover, the small bands at 2996 cm −1 and 2948 cm −1 belong to the asymmetric and symmetric stretching vibration of -CH 3 , respectively [28,[34][35][36].For original BVOH, the broad band at 3310 cm −1 belongs to the O-H stretching vibrations of the hydroxyl groups.The bands at 2918 cm −1 and 2851 cm −1 belong to the asymmetric and symmetric stretching of -CH 2 .The bands at 1452 cm −1 and 1088 cm −1 are associated with C-OH stretching vibration, and at 1249 cm −1 to the C-C stretching vibration [32,33,35].Furthermore, the band at 1735 cm −1 is attributed to the C=O stretching vibration which indicates the existence of byproducts during the synthesis of BVOH, because there is no C=O bond in the molecular structure of BVOH.
The x-ray diffractometer (XRD) patterns of LMPLA, BVOH and LMAB x blends are shown in figure 3. The XRD curve of pure BVOH has two obvious diffraction peaks at 19.7°and 42.6°, whereas there are almost no diffraction peaks on the XRD curve of pure LMPLA.Only a small diffraction band at about 16.8°corresponding to (110)/(200) plane can be observed for LMPLA, which indicates that LMPLA exhibits almost an amorphous structure with only a small amount of crystalline structure [37].However, with the introduction of BVOH, the intensity of diffraction band of (110)/ (200) plane increased, which indicates that the introduction of BVOH can promote the crystallization of LMPLA.Moreover, the basal diffraction peaks at 19.7°and 42.6°of BVOH are not visible for the LMAB x blends, indicating a uniformly dispersed morphology without obvious agglomerates.
The scanning electron microscope (SEM) images of cryo-fracture cross-section of LMAB x blends with different BVOH content are shown in figure 4.These images clearly indicate that LMAB x blends possess a clear two-phase structure (sea-island structure).It is obvious that LMPLA (major component) forms the continuous phase (sea phase), and BVOH (minor component) forms the dispersed phase (island phase) [35].It can be observed that LMAB x blends all exhibit a homogenous morphology with no visible aggregates, confirming a good dispersion of BVOH in a granular state.This phenomenon also supports the previous XRD analysis.Moreover, it can be also found that the BVOH is uniformly dispersed in LMPLA matrix in an irregular form with similar size when the BVOH content is not more than 3 wt% as seen in figures 4(a), (b).Moreover, the size of island phase is about 0.2 ∼ 0.6 μm.The size of island phase and the difference in island phase size both significantly increase by increasing the BVOH content as shown in figures 4(c), (d).Moreover, the morphology of island phase changes from irregular form to mainly spherical form when the BVOH content is more than 3 wt%, which can be explained by the change of surface tension caused by the dimensional variations.The size of island phase ranges from 0.2 μm to 4.5 μm when the BVOH content is more than 3 wt%.
Furthermore, the obvious boundary cannot be observed at the interface of the sea phase (LMPLA) and island phase (BVOH) for LMAB x blends with different BVOH content as shown by the red scissors.This phenomenon indicates the good compatibility between BVOH and LMPLA, which can be due to the similar polarity due to the polar group of alcoholic hydroxyl group in BVOH and ester carbonyl in LMPLA molecular.Moreover, some spherical holes throughout the cryo-fracture cross-section of LMAB x blends can be also observed from figure 4. The edges of these spherical holes are also smooth, indicating the interfacial adhesion between LMPLA and BVOH is not strong enough [38,39].This also results in BVOH phase being easily pulled out during the cryofracture.The size and number of spherical holes also increase by increasing the BVOH content.

Tensile properties of LMAB x blends
The stress-strain curves and elongation at break of LMPLA, BVOH and LMAB x blends are shown in figure 5. From figure 5(a), it can be observed the stress yield phenomenon for pure LMPLA, and the elongation at break can reach 10.27%.This indicates that pure LMPLA exhibits partial ductile fracture characteristics due to the  amorphous structure.However, pure BVOH shows typical ductile fracture characteristics, and the elongation at break is 22.89%.The tensile properties of LMPLA can be significantly affected due to the introduction of BVOH.The tensile stress of LMAB x blends firstly increases and then decreases by increasing the BVOH content.The tensile stress of LMAB 3 blends can reach 78.59 MPa, which is 10.9% higher than that of LMPLA (70.84 MPa).This improvement can ascribe to the increase of crystallinity by adding BVOH as confirmed in XRD and DSC curves.Moreover, the LMAB x blends exhibit brittle fractures when the BVOH content is not higher than 5 wt%.Furthermore, the tensile stress of LMAB 10 blends is only 64.26 MPa, even lower than pure LMPLA, and LMAB 10 blends also exhibit partial ductile fracture characteristics.This phenomenon may be explained by the stress concentration due to the increase of difference in the BVOH particle size.
Therefore, the effect of BVOH on tensile stress of LMAB x blends may be divided into two aspects.Firstly, the BVOH component with small particle size can promote the crystallization of LMPLA to increase the tensile stress of LMAB x blends when the BVOH content is not higher than 5 wt%.Secondly, the increased difference in BVOH particle size can cause stress concentration, leading to the decrease of tensile stress when the BVOH content is higher than 5 wt%.In contrast, the elongation at the break of the LMAB x blends exhibits a decreasing trend followed by an upward trend by increasing the BVOH content, and that of LMAB x blends is always lower than that of pure LMPLA.The elongation at break of LMAB 3 blends is only 6.58% with a decrease of 35.9%, which can be also attributed to the improvement of crystallinity.However, the increase of elongation at the break may be due to the good dispersion and compatibility of BVOH in LMPLA matrix [39].With the increase of BVOH content, the ductile fracture begins to dominate during the stretching process.

Thermal stability of LMAB x blends
The thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) curves of LMPLA, BVOH, and LMAB x blends are shown in figure 6.Table 1 also lists the degradation temperature corresponding to a weight loss of 5% (T 5% ), the maximum rate of degradation in first stage (T max1 ), the maximum rate of degradation in second stage (T max2 ), and the end of degradation (T end ).From figure 6, it can be found that LMPLA exhibits a one-stage thermal degradation within 800 °C.When the temperature is lower than 350 °C, the TG curve of LMPLA is straight, indicating good thermal stability.The slope of TG curve changes sharply from 350 °C to 410 °C, indicating a rapid thermal degradation of LMPLA.As the temperature continues to increase, the thermal decomposition slows down.When the temperature reaches above 420 °C, the thermal decomposition basically ends, and the maximum weight loss rate can reach above 99%.The results show that LMPLA macromolecules are basically completely decomposed.
However, BVOH exhibits a three-stage thermal degradation within the range of test temperature.The slight weight loss in the first stage of degradation below 125 °C is caused by the volatilization of adsorbed water due to the rich hydroxyl bonds in BVOH macromolecules.The high weight loss in the second stage (225-380 °C) can be attributed to the rapid thermal degradation of BVOH macromolecules.The final stage of degradation (380-460 °C) can be attributed to the further decomposition of BVOH causing by the combustion of the pyrolysis residue [38,40].As shown in figure 6 and table 1, it is obvious that the T 5% of the LMAB x blends decreases because of the introduction of BVOH, and the T 5% of the LMAB x blends reduces by increasing the BVOH content.Furthermore, the T max1 of LMAB x blends also exhibits the same varying tendency, indicating the thermal stability of LMPLA can be reduced by the introduction of BVOH.The T max2 of LMAB x blends increases by increasing the BVOH content, but is still lower than that of pure LMPLA.It is worth noting that the T 5% of LMAB x blends is much higher than the melting point of LMPLA, indicating that the introduction of BVOH cannot affect the application of LMPLA in the field of absorbent sanitary products by nonwoven hot air technology.

Crystallization behavior of LMAB x blends
Figure 7 shows the differential scanning calorimetry (DSC) curves of LMPLA, BVOH, and LMAB x blends.Table 2 lists the parameters of crystallization and melting behaviors derived from DSC analysis curves.The degree of crystallinity (X c ) is calculated according to the following equation: where ΔH m is the melting enthalpy of samples, ΔH f is the melting enthalpy of 100% crystalline PLA (93 J g −1 ) [36,37], and x is the BVOH content.
As seen in figure 7, the glass transition temperature (T g ) can be both detected in the reheating and cooling curves of BVOH.Moreover, the crystallization peak and melting peak can be also observed, indicating that   BVOH belongs to the crystalline polymer.For LMPLA, the T g in the reheating and cooling curves can be also observed, whereas only weak melting endothermic peak can be detected, indicating that LMPLA has poor crystallization capacity.This phenomenon also confirms the previous XRD analysis.A glass transition temperature (62.3 °C), and an obvious crystallization peak (129.4 °C) can be observed from the cooling curves of BVOH.However, with the addition of BVOH, the crystallization exothermic peaks of LMAB x blends are almost undetectable with different BVOH content like pure LMPLA.Therefore, the addition of BVOH cannot change the crystallization properties of LMPLA, and the PLA macromolecules can hinder the crystallization of BVOH.Moreover, the glass transition temperature in the cooling process (T g-c ) of LMAB x blends tends to move towards low temperature by adding BVOH, indicating that the motility of LMPLA molecules can be improved in the cooling process.From figure 7(a) and table 2, it can be also found a glass transition temperature (73.1 °C) and an obvious melting peak (179.8 °C) from the reheating curves of BVOH.However, only a glass transition temperature (62.9 °C) and very weak melting endothermic peak (154.6 °C) can be observed for LMPLA.The absence of cold crystallization peaks indicates that LMPLA is difficult to recrystallize during the endothermic process.With the addition of BVOH, there is no obvious change in the melting curves of LMAB x blends, only the melting endothermic peaks become apparent.The melting enthalpy and crystallinity of LMAB x blends increase with the increase of BVOH content as seen from figure 7 and table 2.Moreover, the melting temperature (T m ) of LMAB x blends tends to move towards high temperature by adding BVOH, indicating LMAB x blends possess a much more thermostable structure than LMPLA.Furthermore, only one melting peak can be detected in the reheating curves of LMAB x blends, indicating the good compatibility between LMPLA and BVOH, and the hindering effect of LMPLA macromolecules on the crystallization of BVOH [39].Therefore, the introduction of BVOH cannot affect the low melting point characteristic of LMPLA, which cannot affect the application in in the field of absorbent sanitary products by nonwoven hot air technology.

Hydrophilicity of LMAB x blends
The hydrophilicity of LMAB x blends is characterized by the water contact angles (WCAs) as seen in figure 8.
From figure 8, it can be found that LMPLA possesses weak hydrophilicity, and the WCAs are 68.5°when the test droplet first comes into contact with the sample.As the contact time increases, the WCAs of LMPLA slightly decrease.The WCAs of LMPLA can reach 63.7°when the contact time is 60 s, because of the hydrophilicity of terminal hydroxyl groups in LMPLA macromolecules.Furthermore, it can be also found that the introduction of BVOH can improve the hydrophilicity of LMPLA.The WCAs of LMAB x blends decrease by increasing the BVOH content.The WCAs of LMAB 10 blends are 51.4°when the contact time is 0 s, which can be explained by the excellent hydrophilicity of BVOH owing to large amount of hydroxyl groups existing in macromolecules.Moreover, the WCAs of LMAB x blends also decrease by increasing contact time, but the decline degree is limited.The WCAs of LMAB 10 blends are only 44.6°when the contact time is 60 s, indicating much better hydrophilicity than LMPLA.
Moreover, the change rate of the water contact angle can further reflect the effect of BVOH on improving the hydrophilicity of LMPLA because the initial contact angle is various for LMPLA and LMAB x blends.The water contact angle at different times of each sample was all measured three times to calculate the average value.Therefore, the contact angle change rate (CACR) of LMPLA and LMAB x blends at different contact time is shown in figure 9.The CACR (δ a ) can be calculated by , where θ 0 and θ f represent the original contact angle and final contact angle at different time, respectively [41].It can be found that the CACR of LMAB x blends at different contact time are all higher than that of pure LMPLA, indicating that the addition of BVOH can significantly decrease the water contact angle and improve the hydrophilicity at the same time.It can be also observed that the CACR increases with the increase of contact time, but the growth rate decreases.

Conclusions
The LMPLA/BVOH blends were prepared by simple melt blending.The BVOH exhibited good compatibility and dispersity with LMPLA matrix to form sea-island structure in blends without chemical reaction.The introduction of BVOH could improve the tensile strength of LMPLA with lower content, but reduce the elongation at break.The fracture mode of LMPLA also changed from partial ductile fracture to brittle fracture during the tensile test when the BVOH content was low.The introduction of BVOH could improve the crystallity and promote the motility of LMPLA molecules, but limited its own crystallization.Moreover, the thermal stability of LMPLA was remarkably reduced due to the low thermal stability of BVOH.Furthermore, the introduction of BVOH could significantly improve the hydrophilicity of LMPLA by decreasing the water contact angle and increasing the contact angle change rate.The interesting results established that hydrophilic modification of LMPLA by introducing BVOH would provide a promising development strategy for the disposable absorbent sanitary products.The research results could also provide reference significance for  developing the bicomponent PLA fibers, which could replace the ES fibers to reduce the excessive consumption of fossil resources and serious environmental pollution.

Figure 8 .
Figure 8. Water contact angles of LMPLA and LMAB x blends at different contact times.

Figure 9 .
Figure 9.The contact angle change rate (CACR) of LMPLA and LMAB x blends at different contact times.