Md Obaidur Rahman 朱斐超 杨潇东 黄瑞杰,汪伦合 刘蕊 徐晓天 于斌

摘要： 聚乳酸（PLA）生物可降解熔喷非织造材料因其超细纤维结构和环境友好性而体现出有力的市场竞争力，但PLA由于其本身力学韧性不足，限制了拓展应用。文章以高流动性热塑性聚氨酯（TPU）为PLA的增韧材料，采用熔融共混法制备熔喷非织造用PLA/TPU复合母粒，对其相结构形态、热-结晶性能、热稳定性和晶型结构变化进行研究，进一步制备了PLA/TPU熔喷非织造材料。结果表明：PLA与TPU为不相容体系，TPU对PLA的结晶过程和晶型结构几乎无影响，但使PLA的热稳定性有所下降。TPU共混质量比在20%以内，PLA/TPU的熔喷加工性较佳，PLA/TPU熔喷非织造材料相比单一PLA熔喷材料体现出更好的强度和拉伸延展性。

关键词：

聚乳酸;热塑性聚氨酯;增韧;熔喷;非织造

中图分类号： TQ323.8

文献标志码： A

文章编号： 1001-7003（2021）10-0028-08

引用页码： 101106

DOI： 10.3969/j.issn.1001-7003.2021.10.006（篇序）

Study on toughened polylactic acid and its meltblown nonwovens by thermoplastic polyurethane

RAHMAN Md Obaidur1a， ZHU Feichao1a，b， YANG Xiaodong1a， HUANG Ruijie2， WANG Lunhe3， LIU Rui1a， XU Xiaotian1a， YU Bin1a，b

（1a.College of Textile Science and Engineering; 1b.Zhejiang Provincial Key Laboratory of Industrial Textile Materials and Manufacturing Technology，Zhejiang Sci-Tech University， Hangzhou 310018， China; 2.China General Nuclear（CGN） Juner New Materials Co.， Ltd.， Wenzhou325000， China;3. Zhejiang Honor Biomaterials Co.， Ltd.， Taizhou 318000， China）

Abstract：

Polylactic acid（PLA） biodegradable meltblown nonwovens show superior market competitiveness due to its ultrafine fiber structure and environmental friendliness. However， the lack of mechanical toughness of PLA has limited its application. In this paper， high-fluidity thermoplastic polyurethane（TPU） was used as the toughening material of PLA to fabricate the PLA/TPU composite masterbatch for the use of melt-blown nonwovens through melt blending method， and its phase structure， thermal-crystallization performance， thermal stability， and the changes in the crystal structure changes were studied， and PLA/TPU meltblown nonwovenswere further prepared. The results show that PLA and TPU are incompatible systems， and although TPU almost has no effect on the crystallization process and crystal structure of PLA， it can reducethe thermal stability of PLA. The mass ratio of TPU blending was less than 20%， and PLA/TPU showed a good the meltblown fabricability. PLA/TPU meltblown nonwovensexhibited superior strength and tensile ductility than single PLA meltblown material.

Key words：

polylactic acid; thermoplastic polyurethane; toughening; meltblown; nonwoven

收稿日期： 2021-01-24;

修回日期： 2021-09-17

基金項目： 浙江省自然科学基金项目（LQ21E030013）;浙江理工大学科研启动基金项目（20202293-Y）

作者简介： Md Obaidur Rahman（1994），男，硕士研究生，研究方向为生物可降解熔喷非织造材料。通信作者：于斌，教授，yubin7712@163.com。

At the end of 2019， the prevailing coronavirus（COVID-19） all over the world has increased the use of protective meltblown masks at an alarming rate day by day. But as the most common raw material of of the mask， polypropylene（PP）， despite its advantages such as inexpensive， high flexural strength， and chemical resistance[1]， is a petroleum-based polymer derived from the olefin monomer propylene[2]， yet not fully environment friendly， resulting in a huge amount of energy/gas combustion， high environmental pollution[3]. This research introduces a biodegradable and flexible meltblown nonwoven fabric that can greatly benift human health and environmental safety and can replace the use of a large number of PP masks in everyday use. Meltblown is an industrialized simple， unique，and one-step process for the production of superfine diameter fibers in the form of a micrometer or Nanoscale（0.5-5 um）[4]. As Poly Lactic Acid（PLA） is a natural biodegradable polymer with high strength and stiffness， excellent transparency， and biodegradability，as well as an excellent antibacterial property， it is used for various purposes such as biomedical， filtration， food packaging， etc[5-6]. Its a compostable and bio-based polyester， while PLA is aliphatic polyester generally derived from natural sources such as corn， cassava， sugar， starch， potatoes or， other biomass[7]. PLA has some shortcomings such as inherent brittleness， thermal stability， high processing costs， and relatively poor mechanical properties，limiting its use in some specific areas[8]. Blending two or more polymers with the same or different physical and chemical properties is a common method for reducing cost and enhancing the overall properties of a product by taking advantage of the properties of each component[9]. A lot of polymers have been used in various researches for PLA touthening purposes[10] such as PA11[11]，Polycaprolactone（PCL）[12]， Poly Butylene Succinate（PBS）[13]， etc.

Thermoplastic polyurethane（TPU） with high toughness properties， durability， biocompatibility and flexibility is one of the most potential alternatives for enhancing the strength of Polylactic acid（PLA）[14-22]. Thermoplastic polyurethane（TPU） is also extensively applied to many fields such as automotive instrument panels， power tools， sporting goods， medical devices， footwear， inflatable rafts， and a variety of extruded films， sheets and profile applications[15]. Despite a few existing studies on PLA/TPU melt blend technic for PLA toughening[16-18]， most of which are focused on toughening film or injection modeling， while studies on superfine fiber or MB nonwovens are rare. Thus in this study， Thermoplastic Polyurethane was used for improvingthe overall performance of PLAsuch as reducing inherent brittleness， improving mechanical properties（increasing strength）， flexibility and also enhancing the surface of the composite meltblwn nonwovens. The current study is aimed to investigate the morphology， crystallization， and thermal behaviors of PLA/TPU meltblown fabric， which is hopefully a good candidate for replacing petroleum-based polymer in the meltblown applications. In view of some miscibility found in the blends， it is recommended that some facile or additive can be used in the blend to obtain a uniform high-quality melt-blown blend.

1 实 验

1 Experiment

1.1 原料与仪器

1.1 Materials and instruments

Materials： PLA（grade 6252D especially for MB processing） was supplied by NatureWorks LLC（USA），and the flexible Thermoplastic Polyurethane（grade 1080A） was purchased from BASF. All materials were used as received to produce biodegradable melt blown nonwovens without any further purification. The materials used in the experiment all were AR grade and used as received.

Instruments： Twin screw extruder（TSE-30A; Nanjing Ruiya Extrusion System Limited， China. field emission scanning electron microscopy（FE-SEM）-（Ultra 55， Carl Zeiss， AG， Germany）， Differential scanning calorimetry（DSC） Perkin-Elmer DSC 8000（USA）， TGA-Mettler Toledo TGA/DSC1/1600（Switzerland）， Wide-angle X-ray diffractometer（WAXD）-Bruker AXS D8 Discovery（Germany）， Fourier transform infrared spectroscopy Perkin-Elmer Spectrum 100 ATR-（FTIR） machine， Mini meltblown machine（China）.

1.2 實验方法

1.2 Methods

Melt compounding was performed using a twin-screw extruder（TSE-30A; Nanjing Ruiya Extrusion System Limited， China; length/diameter [L/D]=40） with a screw diameter of 18 mm And an L/D ratio of 40. Prior to this， the PLA & TPU were dried in a vacuum oven at 80 ℃ for 24 hours to remove the moisture. PLA/TPU blend was mixed in a mixer at 60 ℃ under 50 r/min at a certain weight ratio of 100/0， 95/5， 90/10， 85/15， 82/20. The 7-chamber temperature of the melt blended extruder was set to 175 ℃， 185 ℃， 190 ℃， 200 ℃， 180 ℃， 175 ℃， and 170 ℃ respectively. Setting the temperature according to the characteristics and melting point of the polymers is very important to obtainthe superior quality of the blend. Too high or too low temperature may result in bottleneck operation during the whole extrusion process. The materials were extruded automatically using a semi-automatic machine according to the material pouring & machine speed. Lower temperature resulted in improper blending and higher temperature resulted in the jamming of extruder chamber or ununiform melt flow of the blend.

Before the meltblown process， the materialswere heated in the vacuum oven dryer at 80 ℃ non-disturbed for 16 hours to remove the moisture. As shown in Fig.1， PLA/TPU MB nonwovens were manufactured by a laboratory-scale MB nonwoven machine（Jiaxing Longman Measurement and Control Technology Co.， Ltd.， China） with a single orifice die. All the diameterswere set based on our previous work experience for the purpose of obtaining a uniform melt flow. The distance between the die and collector played a crucial role during the meltblown experiment for getting the superior quality of superfine meltblown fabric. The（DCD） was set to about 14 cm. The diameter of the orifice was 0.3 mm， and the L/D was 40. PLA and PLA/TPU were set at the temperature of 220 ℃ to 230 ℃. The hot air temperature was set to 260-270 ℃， the material temperature was 220 ℃ and the bucket temperature was 230 ℃. The platform

speed and itinerary was set to 0.5 mm/s and 200 mm. The meltblown nanofibers were cooled at room temperature and collected in a stainless steel roller（collector） with a suction set; the collector speed and the die-to-collectordistance（DCD） were optimized according to the practical adjustment， in order to get uniform MB nonwovens.

1.3 测试与表征

1.3 Testing and characterization

1.3.1 表面形貌

1.3.1 Surface morphology characterization（SEM）

The surface morphology and structure of PLA-TPU melt blended polymer were observed by using a field emission scanning electron microscopy（FE-SEM）（Ultra 55， Carl Zeiss， AG， Germany） at an accelerating voltage of 2-3 kV. The specimens were firstly dispersed in liquid nitrogen for about two to three minutes and the sample was cut with two stainless steel handle into small parts， and then attachedon the gold plate for testing. A clean and smooth surface played a crucial role in getting good quality images. The MB nonwovens（1 cm×1 cm） were gold-coated（2-3 nm） before observation. Fiber diameters of MB nonwovens were measured using image analysis software（ImageJ 1.48 V; National Institutes of Health， the USA） by following the method of Ellison et al[19].

1.3.2 熱-结晶性能

1.3.2 Differential scanning calorimetry（DSC）

Differential scanning calorimetry（DSC） tests were carried out using Perkin-Elmer DSC 8000（USA） under N2 atmosphere（20 mL/min）. Samples of（5±0.5） mg were weighed and sealed in aluminum crucibles. Samples were heated from 25 ℃ to 230 ℃ at a rate of 10 ℃/min（first heating）， equilibrated at 230 ℃ for 3 min to eliminate the thermal history， and then cooled to 25 ℃ at a rate of 10 ℃/min， and then heated again from 25 ℃ to 230 ℃ at a rate of 10 ℃/min（second heating）. The crystal weight fraction（Xc） of PLA was calculated according to Equations（1）.

Xc（PLA）=ΔHm（PLA）-ΔHcc（PLA）ΔH0（PLA）×100Wt（1）

Here， the measured melting enthalpy is expressed as ΔHm， the measured cold crystallization enthalpy is expressed as ΔHcc， is the melting enthalpy for 100% crystalline PLA（93.1 J/g[20]） is expressed as ΔH0， and the weight fraction in PLA/TPU blends is denoted as Wt.

1.3.3 热重分析（TGA）

1.3.3 Thermogravimetric analysis（TGA）

TGA test was performed using a Mettler Toledo TGA/DSC1/1600（Switzerland） under N2 atmosphere（20 mL/min）. Samples of （5±0.5） mg were weighed and put into a ceramic crucible; samples were heated from room temperature to 550 ℃ at a rate of 20 ℃/min.

1.3.4 熔融共混PLA/TPU（XRD）的晶型结构

1.3.4 Crystal forms of melt blended PLA/TPU（XRD）

A wide-angle X-ray diffractometer（WAXD） was obtained by using Bruker AXS D8 Discovery（Germany） at room temperature. The samples of PLA， TPU， and MBs with a size of 10 mm×10 mm were tested. The CuKa radiation source was operated at a power of 40 kV and a current of 40 mA. The scanning angle ranged from 10° to 40° with increments of 0.02°.

1.3.5 拉伸性能

1.3.5 Tensile performance

Mechanical behaviors of PLA and PLA/TPU MB nonwovens were investigated using a universal tensile machine（Instron-3369， USA） according to the method of ISO 9073-3—1989. All samples were conditioned under standard laboratory conditions（25 ℃±2 ℃ and 65% RH） for 24 h. Samples with a size of 5 cm×2 cm were selected in a rolling direction， at a tensile speed of 50 mm/min. All the results were calculated as the average of five samples.

2 結果与分析

2 Results and analysis

2.1 共混物的表面形态

2.1 Surface morphology of blends

The morphology of blendswas observed by scanning electron microscopy（SEM）. The SEM images of the PLA/TPU of 100/0， 95/5， 90/10， 85/15， 80/20 are shown in Fig.2， respectively. As shown in figure A， 100% melt blended PLA had a very smooth surface compared to other ratios ofsurface. The phase morphology in binary PLA/TPU polymer blends was generally affected by the weight and viscosity ratios of blending components. Obviously， PLA/TPU blends still exhibited a "sea-islands" structure shown by thePLA/TPU blends. TPU phases were uniformly dispersed in PLA（Polylactic Acid） matrix in a globule shape with a diameter of roughly 8-9 mm， and some dispersed phases began to contact and even compress with each other as the blending weight ratio of TPU increased. It should be noted that there still existed some voids and de-bonding. It was observed that， as the amount of TPU increased in the composites， the thermal stability was improved.

The glass transition temperatures of the composites increased with the addition of thermoplastic polyurethane， which could be attributed to the decrease in the segmental motion of the polymer chains. However， as the PLA/TPU blend with the same compositionwas in good condition， and exhbited stable performance， indicating a uniform TPU distribution of sea-island type composite fibers.

2.2 热-结晶性能

2.2 Melting and crystallization

DSC heating curves of PLA/TPU blends are shown in Fig.3， and the corresponding thermal parameters are listed in Tab.1. In the trace of the PLA heating curve， it is observed that the glass transition temperature（Tg） is 57.66 ℃， and the cold crystallization temperature（Tcc） is 112.11 ℃ due to the mobility and rearrangement of PLA macromolecules.

PLA is a semicrystalline polymer， which determines its mechanical properties basically dependent on its crystallization behavior. As shown in the graph， after adding the TPU for increasing the flexibility of the blend， the structure of the blend did not change signfiicantly. The melting point and the crystallization rate of blended polymer exhibited an excellent relation between the corresponding polymers.

It can be seen that the ratios of the glass transition temperature of PLA did not change much after gradually adding the TPU in the blend， indicatingthe fundamental stability of PLA in the blend. All the temperatures were nearly 57 degrees celsius and on the other hand， the cold crystalization temperature range started basically from 111 degrees to 112， also indicating a stable temperature of the total blend.

2.3 熱稳定性分析

2.3 Thermal stablility of blends

Biodegradable polymers sometimes exhibited poor thermal stability， which may affect the processing and properties of the final product. Polymeric materials properties such as glass transition temperature（Tg） and thermal stability are very important parameters in the applications of composite materials. Fig.4 shows the weight as a function of temperature for Poly Lactic Acid and Thermoplastic Polyurethane blends. A single thermal degradation stage was observed in both PLA and TPU. The initial thermal decomposition temperature（Tinitial） of PLA was about 282.4 ℃. In comparison with PLA， TPU exhibited btter thermal stability at 332.3 ℃.

The thermal degradation of Tinitial PLA and TPU occurred through different mechanisms，because two distinct decomposition stages could be observed in PLA/TPU blends. Both non-free and free radical theories were adopted to explain the thermal degradation of PLA.

2.4 晶型结构

2.4 Crystal forms

To investigate the effect of TPU on the crystal structure of PLA， the WAXD patterns of TPU with and without modification and PLA/TPU MBs were determined， as shown in Fig.5. The XRD graph shows that all the baselines of the melt blended pure PLA and PLA/TPU were nearly the same. Only two major peaks were observed in the XRD graph.

The main two peaks of PLA blend were located at 2θ（theta） angle of 16.64° and 18.94°， corresponding to（110） lattice plane and（203） lattice plane， respectively. The othertwo peaks were found at 14.75° & 22.31°， corresponding to（010） lattice plane and（105） lattice plane， respectively. After adding TPU in the PLA， no significant differences were found in the peak values， indicating that TPU did not affect the basic structure of PLA.

2.5 PLA/TPU熔喷材料的形态和纤维直径分布

2.5 Morphology and fiber diameter distribution of PLA/TPU MB nonwovens

Fig.6 and Tab.2 show the morphologies and fiber diameter distributions of PLA/TPU MB nonwovens. It was found that the surface of PLA MB fibers was smooth， diameters ranging from 0.5-7 μm. The turbulent drawing airflow resulted in the irregular drawing force of hot air on the extruded polymer melt. Therefore， the diameters of MB fibers varied and exhibited a normal distribution with the a peak valueat 2-3 μm. It could be seen that 100% PLA meltblown exhibited a very clear surface compared to other ratios of surface， which was the same as the blending process shown in figure 2A. As the amount of TPU increased in the blend， the image smoothness of the meltblown fabric decreased， because a higher amount of TPU increased the miscibility between the two polymers. The poor miscibility between PLA and TPU was another non-negligible factor that increased the fiber distribution， and the defects of PLA/TPU meltblown nonwoven were further aggravated due to the phase separation， especially at a TPU content of exceeding 15%.

The relationship between the average mean flow pore diameter and melt blowing pressure， hot air， or other parametersof the membrane is unique. The mean stream pore distance acrossis characterized as an incentive in which the stream in the film is reduced significantly when performing a fractional stream test in a hairlike stream boundary instrument. Despite a bigger fiber width and increments with the expanding pressure at lower melt blowing pressure， as indicated before， it isthe fiber thickness that determines the mean stream pore distance across.

2.6 PLA/TPU熔噴材料力学拉伸测试

2.6 Mechanical properties of PLA/TPU MB nonwovens

The tensile mechanical properties of PLA/TPU meltblown nonwovens are shown in Fig.7. Basically， in the advanced process of meltblownsconsisting of many superfine fibers and cohesive points that provide mechanical strength to the fabric， high-quality nonwovens fabric with porous and fluffy structures（nano-fiber） can be produced. When the meltblown nonwovens started stretching， it is generally that the nonwoven fabric is firstlynarrowed bythe tensile force，followed by a grdual drop of the fiber strength at an equilibrium stage. Adding TPU at 15% and 20% could significantly improve the toughness of the polylactic acid， as shown in the figure above in accordance with enhanced elongation at break， especially at 15% content. As shown in above Tensile strength-elongation figure， PLA/TPU 85/15 exhibitsan excellent

tensile strength compared to pure PLA or other blends. The 80/20 the tensile strength ishigher， but it drops quickly; it is unstable like 85/15， because the compatibility between two polymers is not so good in this mixture， or it may be a chemical reaction. Some other studies have also revealed the same result that adding too much TPU in the blend can also lead to a decrease in the tensile strength.

3 結 论

3 Conclusions

In order to toughen PLA and reduce its brittleness， one of the most popular and cost-saving ways is to melt it with PP， TPU or other materials with excellent flexibility， ductility， and strength. In the near future， the use of biodegradable PLA products will continuoulsy and rapidly increase the market share， and it has great potential to penetrate new markets， including some portions of the apparel market and textile industry. Poly Lactic Acid and Thermoplastic Polyurethane blends can be fabricated by melt-blown or melt-spinning to prepare a biodegradable and biocompatible polymer blend material with improved toughness， which can further be used in disposable filtration， adsorbing， health care， and medicals such as gloves， mask， etc. Only a few studies on PLA/TPU melt blown composite nonwovens have been carried out. In this research work， the fabric we made is very suitable for the production offlexible， soft and high-quality melt blown ffabrics and masks， which canprevent the spread of COVID-19. Asit is made of biodegradable polymer， it can alleviate environmental pollution. In addition，we have also foundsome miscibility in the composite blend between the two polymers. However， its very difficult to get a uniform blend from a natural polymer like PLA and to reduce the miscibility between the polymers blend according to our experience. A compatibilizer or an additive can be used in the blend to get more uniform and good quality of PLA/TPU biodegradable meltblown nonwoven fabric. However， to obtain the PLA/TPU MB nonwovens with better performance， the miscibility between PLA and TPU should be further improved using a simple and effective method.

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