164 Vol.31 No.1 WU Gaihong et al: Preparation and Properties of Heat Resistant Polylactic...

Preparation and Properties of Heat Resistant Polylactic Acid (PLA)/Nano-SiO2 Composite Filament

WU Gaihong1,2, LIU Shuqiang1*, JIA Husheng3, DAI Jinming1

(1. College of Textile Engineering, Taiyuan University of Technology, Taiyuan 030024, China; 2. Fashion & Art Design Institute, DongHua University, Shanghai 200051, China; 3. College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024,

China)

Abstract: In order to improve the thermal properties of polylactic acid (PLA) filament, nano-SiO2 was applied to mix with PLA, then they were spun as composite filament by melt-spinning. The dispersion of nano-SiO2 and the fracture surfaces of filaments were studied by scanning electron microscopy (SEM). The properties of composite filament, such as orientation degree, mechanical properties, and surface friction properties, were analyzed. The thermal performances of composite filament were analyzed by differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA). The results showed that the nano-SiO2 modified by 5% KH-550 could disperse evenly and loosely in nano-scale, and 1 wt% and 3 wt% nano-SiO2 dispersed throughout PLA evenly. As the quantity of nano-SiO2 increased, the properties of composite filament, such as orientation degree, friction coefficient, thermal decomposition temperature, and glass transition temperature, increased more or less. The breaking tenacity increased when 1 wt% SiO2 was added in PLA, but declined when 3 wt% SiO2 was added.

Key words: polylactic acid; PLA; composite filament; nano-silicon dioxide (nano-SiO2); thermal property

©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2016(Received: Jan. 18, 2014; Accepted: Nov. 18, 2015)

WU Gaihong (吴改红): Ph D; E-mail: gaigai2003@126.com*Corresponding author: LIU Shuqiang(刘淑强): Assoc. Prof.;

Ph D; E-mail: liushuqiang8866@126.comFunded by the Shanxi Province Science Foundation for

Youths of China [Nos.2014021020-2 and 2015021076], the Shanxi Province Higher School Science and Technology Innovation Project [No. 2015125] and the Project of Taiyuan University of Technology [Nos. 2013T020, 2013T021, 2013T022]

DOI 10.1007/s11595-016-1347-2

1 Introduction

Polylactic acid (PLA) possesses some excell-ent performances, including biocompatibility, biodegradability, biological absorbability and so on. The raw material sources of PLA fiber are renewable grains, such as corn, wheat and rice[1-3]. Therefore PLA fiber is different from other traditional synthetic fibers, such as polyester (PET), nylon and spandex, which strongly depend on petroleum, natural gas and natural gas liquids as sources of raw materials. In addition, in water or soil, the PLA can be broken down into water

(H2O) and carbon dioxide (CO2), which would not create any environmental pollution[4-7]. Hence PLA fiber is an environment-friendly material, and different from other traditional synthetic fibers which are degraded difficultly and cause many environmental pollutions. The PLA fiber (filament) is widely used in medical, textile, clothing, decoration, agriculture, forestry, etc[8,9].

The PLA filament and its products are able to be used at room temperature, however, once the temperature rises over the PLA's glass transition temperature (about 60 ℃), the PLA filament and its products would become easy to distort, wrinkle or tear, and the mechanical properties would fall sharply[10-12]. Hence the PLA filament has poor heat resistance, and the most important purpose of this article is to improve PLA’s thermal properties or heat resistance.

In order to improve the heat resistance of PLA filament, some scholars mixed PLA with other heat-resistant polymers. For instance, Chen H[13] mixed PET, which grafted long-chain carboxylic acid and had good heat-resistance, with PLA to spin composite filament with higher heat-resistance. Mamun A A[14] applied PP

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to enhance the thermal properties of PLA. However, the other polymers which were mixed with PLA are usually non-degradable, so that they would affect the degradation of PLA. Some other scholars improved the thermal stability of PLA filament through optimizing spinning process, such as improving the spinning speed and increasing the drawing ratio. For instance, Tsuji[15] optimized the spinning process, and increased the orientation and crystallization of PLA filament to improve the heat-resistance. But using this method, the extent of improving PLA heat-resistance is very small and rather limited. Some other scholars applied some heat-resistant inorganic materials to improve heat resistance of PLA filament, such as montmorillonite (Che J[16]), hydroxyapatite (Dai YF[17]), attapulgite (Zhang W[18]), clay (Najafi N[19]) and so on. This method is usually effective on improving heat resistance of PLA, but the inorganic material is easy to block up the fine holes of spinneret when melt-spinning, and the inorganic material usually worsens the mechanical properties of PLA filaments more or less.

The inorganic nano-SiO2, which has excellent heat-resistance and 1750 ℃ melting point, can be applied to improve the heat resistance of PLA filament. Almost all related references reported that the inorganic SiO2 was mixed with PLA to make into block or film or sheet shape[20,21], not fiber or filament form, so the study of PLA/SiO2 composite filament is vitally necessary. In this article the nano-SiO2 was pre-treated by silane coupling agents, so it was of nano-particle size and could be easy to extrude with melted-PLA through holes of spinneret. Meanwhile, the nano-SiO2 possesses good functions of environmental protection, harmless and nonpoisonous[22], so the PLA/nano-SiO2 composite filament maintains environment-friendly feature. And the nano-SiO2 has very low price, so it can reduce the cost of composite filament. In addition, the color of nano-SiO2 appears white, so the composite filament appears light and can dye almost all colors.

2 Experimental

2.1 MaterialsPLA (6202D) was kindly supplied by Nature

Works Industry (U.S.).The number average molecular weight (Mn) of PLA was 51 000.

Nano-SiO2 with particle size of 20-180 nm was provided by Fengcheng Chemical Industry, Tianjin, China.

The γ- Aminopropyltriethoxysilane (NH2(CH2)3

Si(OC2H5)3, trademark KH-550 in China) as a kind of silane coupling agent was produced by Nanjin Liangui Ltd, China. 2.2 Surface treatment of nano-SiO2

The nano-SiO2 particles have high specific surface area and tiny granule diameter, so they have high surface energy and are easy to get together with each other to form big-size particles. In addition, there are a lot of hydroxyl groups (-OH) on the surface of nano-SiO2, shown in Fig.1. A large number of hydroxyl groups (-OH) between nano-SiO2 can form hydrogen bond, then form bigger SiO2 particles. When the bigger SiO2 particles were mixed with polymers, the SiO2 particles were difficult to distribute evenly throughout the polymer in nanometer size, so that the SiO2 particles could not show their perfect perspective which was caused by their nano-size. Therefore, the nano-SiO2

should be modified by some surface active agents (for example KH-550), in order to reduce its surface energy and improve the compatibility between SiO2 and PLA.

In this paper, the nano-SiO2 was modified by the γ-Aminopropyltriethoxysilane coupling agent under trademark KH-550 in China. The KH-550 coupling agent can improve the dispersion of nano-SiO2 in PLA and increase their compatibility. The KH-550 coupling agent has two different groups, in which one group can link with the hydroxyl (-OH) on surface of SiO2, another can link with PLA. So the SiO2 and PLA were connected through KH-550. Fig.2 is a schematic drawing showing connecting of SiO2 with PLA through KH-550. The detailed reaction process will be discussed in the section of “results and discussion”.

166 Vol.31 No.1 WU Gaihong et al: Preparation and Properties of Heat Resistant Polylactic...

The process of modifying was as follows: 0.04-0.2 g KH-550 and 15 mL anhydrous ethanol were put into a beaker together, and 0.1 mol/L HCl was added into the beaker to adjust pH value to 5-6. After that, the beaker was heated in water bath at 70 ℃ for 30 min, and 2 g nano-SiO2 was added to the solution. The mixtures were shocked by using ultrasonic wave at 30 ℃ and 30 kHz for 30 min. After that, the mixed solution was dried at 120 ℃ for 8 h by using the drying oven (101-2A, Beijing ZhongXing WeiYe Instrument Co., Ltd). Then the dried mass was crushed and ground into fine particles by using ball mill (QM-1SP, Nanjing Daxue Instrument Company, China)[23].2.3 PLA/nano-SiO2 masterbatch processing

The pure PLA was dried at 80 ℃ for 20 h under vacuum (SEJ-50, Changzhou FuYi Drying Equipment Co., Ltd, China).

Then the dried pure PLA and 5 wt% treated nano-SiO2 were feed into the double screw extruder (CET35-40D, Coperion (Nanjing) Machine Co., Ltd, China) during main feed opening (Fig.3(a)) and auxiliary feed opening (Fig.3(b)). The temperatures of ten heating zones (Fig.3(d)) in extruder are set in Table 1. The melt was extruded from extruder into a cool water bath (Fig.3(f)) to be solidified. At last, the solid band (Fig.3(g)) was cut as small chips (f: 3.175 mm, long: 3.175 mm) by a chip cutter (Fig.3(h), LQ, Coperion (Nanjing) Machine Co., Ltd, China). The cut chips were PLA/5 wt%SiO2 masterbatch.

2.4 Melt-spinning of PLA/nano-SiO2 com-posite filamentThe masterbatch was dried firstly at 80 ℃ for 20 h

under vacuum. Then the dried masterbatch and pure PLA were pre-mixed. After that, they were fed into the melt spinning machine (LHFJ030, WuXi LanHua Textile Machinery Co., Ltd, China) during feed opening (Fig.4(a)).

The processing parameters of the melt spinning machine were set as follows: 180-183 ℃ for the first heating zone (Fig.4(d)), 187-190 ℃ for the second heating zone (Fig.4(e)), 190-194 ℃ for the third heating zone (Fig.4(f)), 195-198 ℃ for the fourth heating zone (Fig.4(g)), 197-202 ℃ for the bend (Fig.4(h)), 199-204 ℃ for the enclosure (Fig.4(i)), 10 Mpa melt-pressure and 184 ℃ melt-temperature. The melt was then extruded through the hole of spinneret into a cool air bath to form PLA/SiO2 composite filaments. The cool air bath, provided by the cross air blasting equipment (Fig.4(l)), should be set at 25 ℃±2 ℃ air temperature and 0.2-0.5 m/s air speed. Meanwhile, the composite filaments were pulled away from the hole of spinneret at 1 000 m/min spinning speed. After that, the composite filaments were oiled on the surface by preparation device (Fig.4(m)), then stretched by three godets (Fig.4(o, p, q)). The temperatures of three godets were 72±1 ℃, 80±1 ℃ and 82±1 ℃. The godets-

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stretching ratios were 1.02-1.25 pre-stretching and 1.5-3.5 main-stretching. Finally, the stretched filaments were winded on the bobbin by the winding mechanism (Fig.4(r)).

During preparing of PLA/nano-SiO2 composite filament, the proportion of SiO2 and PLA was not over 3 wt%, or else the extruder screen pack of melt-spinning machine would be blocked by too much agglomerations of SiO2. Therefore the proportions of SiO2 and PLA in these filaments were 1 wt% and 3 wt%. Finally, the filaments’ structure and perfor-mances were tested.2.5 Characterization methods

The dispersion of nano-SiO2 and the fracture surfaces of pure PLA filament and PLA/SiO2 composite filament were studied with a JEOL JSM-6700F scanning electron microscope (SEM) under an acceleration voltage of 10 kV. Prior to the SEM examination, samples were submerged in liquid nitrogen and broken to expose the internal structure for SEM studies, and all the surfaces were sputtered with gold.

The orientation of filament was measured by a sound-velocity-orientation instrument (SCY-Ⅲ, DongHua University, China) at 25 ℃ temperature, 60% relative humidity and 0.1 g/dtex pre-tension. The speed of sound traveling in the filament was tested. Then the orientation factor (fs) was calculated as[24]:

(1)

where fs is the orientation factor of the filament, Cu is the speed of sound traveling in random orientation filament, whose value is 1.75 here, C is the measured speed of sound traveling in filament sample. The orientation factor (fs) is directly related to the degree of orientation, which means that the bigger orientation factor (fs), the higher orientation degree.

The tensile properties of filaments were measured by electronic chemical-fiber strength machine (YG(B)021H, Wenzhou Darong Instruments Co. Ltd, China) at 500 mm/min test speed and 200 mm test length.

Filament surface friction coefficient tester (Y151, Changzhou Second Textile Instrument Co., Ltd, China)was applied to measure the friction coefficient between filaments and iron metal.

Thermogravimetric analysis (TGA, Germany Netzsch TG 209 F3) was performed on neat PLA filament and PLA/SiO 2 composite filament as follows: 2.5 mg samples, nitrogen flow (600 mL/

min), temperature range from 40 to 700 ℃, 10 ℃/min heating rate were used in the tests.

The thermal properties of PLA samples (6-9 mg) were measured by a Q100 V9.4 Build 287 DSC using aluminium oxide as the standard. The melting point(Tm) and glass-transition temperature (Tg) of each sample were measured from 10 to 210 ℃ under nitrogen at a heating rate of 10 ℃/min.

3 Results and discussion

3.1 Morphological characterization of nano-SiO2

The morphological characterization of unmodified and modified nano-SiO2 was performed by a JEOL JSM-6700F scanning electron microscope (SEM). The SEM images of nano-SiO2 are shown in Fig.5.

Fig.5(a), image of unmodified nano-SiO2, showed that the nano-SiO2 was agglomerated as big mass. Fig.5(b), image of nano-SiO2 modified by 5% KH-550, showed that the nano-SiO2 was dispersed evenly and loosely in nano-scale. Fig.5(c), image of nano-SiO2

modified by 10% KH-550, showed that the nano-SiO2

168 Vol.31 No.1 WU Gaihong et al: Preparation and Properties of Heat Resistant Polylactic...

was agglomerated as big mass, even bigger than that of unmodified nano-SiO2 (Fig.5(a)).

The unmodified SiO2 was covered with many hydroxyl groups (-OH) as shown in Fig.1. The hydroxyl groups (-OH) between SiO2 nano-particles could form hydrogen bonds. Then the SiO2 nano-particles were linked by hydrogen bonds. So the unmodified nano-SiO2 was easy to be agglomerated as big mass. However, if the SiO2 nanoparticles were modified by 5% coupling agent KH-550, some hydroxyl groups on its surface would be replaced by organic chain of coupling agent KH-550 as shown in Fig.6, and the number of hydroxyl groups (-OH) on surface of SiO2

would decrease. So the agglomerating of nano-SiO2 caused by hydrogen bonds (-OH) would be weakened. Therefore, the nano-SiO2 modified by 5% KH-550 could disperse evenly and loosely in nano-scale. On the other hand, if the nano-SiO2 was modified by excessive KH-550 coupling agent (i e, 10%), there was too much KH-550 between SiO2 nano-particles, and the too much KH-550 would link SiO2 particles like some bridges between SiO2 particles. So the excessive KH-550 (i e 10%) would lead to agglomerate mass of SiO2.

3.2 Morphological characterization of PLA/ nano-SiO2 composite filamentScanning electron microscopic investigations

of PLA/1 wt% SiO2 and PLA/3 wt% SiO2 composite filaments are performed to evaluate the sample morphology and analyze the dispersion of SiO2 nanoparticles in the PLA matrix (Fig.7). The PLA/1 wt% SiO2 fracture surface (Fig.7(a)) showed a uniform dispersion of SiO2 in the PLA matrix. Moreover, SiO2 still remained nano-scale in the polymer and was not separated from PLA matrix. It was possible that the nano-SiO2 was kept in nano-scale by covering coupling agent KH-550, and the nano-SiO2 was linked with PLA through coupling agent KH-550, as shown in Fig.8.

The SEM micrograph of PLA/3 wt% SiO2 (Fig.7(b)) showed that SiO2 (circles) was agglomerated as bigger particles over 1.0 micron embedded in PLA. The images revealed that more SiO2 (i e, 3 wt%), in

fact, was able to form bigger agglomerations compared to less SiO2 (i e, 1wt%).

3.3 Orientation of PLA filamentsThe orientations of pure PLA filament, PLA/

1 wt% SiO2 filament and PLA/3 wt% SiO2 filament are measured and shown in Table 2. The orientation-factors of PLA/SiO2 composite filaments were higher than that of pure PLA filament by 11.8% or 14.1%. Moreover, the more SiO2 the composite filament contained, the higher orientation the composite filament possessed.

These results indicated the fact that the nano-SiO2

was dispersed in the PLA matrix in nano-scale, and it was able to decline the cohesion among the crystals of PLA. When the filament was drawn, the crystals were

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easy to be oriented along the direction of drawing (or in the filament direction). Meanwhile, the orientation extent of the crystals was enhanced with increase of SiO2 embedded in PLA.3.4 Tensile properties of PLA filaments

The tensile properties of pure PLA filament and composite filaments, including breaking tenacity, CV of breaking tenacity and elongation at break, are measured and shown in Table 3.

The breaking tenacity of PLA/1 wt% SiO2 filament increased by 6.8%, but that of PLA/3 wt% SiO2 filament declined by 9.3%, compared to that of pure PLA filament. The results indicated that the moderate amount of nano-SiO2 (i e, 1 wt%) could increase the breaking tenacity of filament. It was due to the good combining power between nano-SiO2 and PLA matrix, and the heterogeneous nucleation action of nano-SiO2 which could increase the crystallinity of PLA filament. However, adding too much nano-SiO2 (i e, 3 wt%) decreased the breaking tenacity of filament. Too much nano-SiO2 formed bigger agglomerates in PLA matrix (Fig.7(b)), which led to some defects or flaws inside the PLA filament. When the filament was drawn, the defects would grow into the weakest link for the filament to break.

Comparison of the CV of breaking tenacity of filaments revealed that the CV of breaking tenacity of PLA/1 wt% SiO2 filament was the smallest, since proper amount of nano-SiO2 (i e, 1 wt%) was helpful for forming crystals and regular structures inside PLA matrix. However, adding too much SiO2 (i e, 3 wt%) led to the tenacity-uneven and the structure-defects in PLA filament, which was consistent with the above analysis of breaking tenacity, so the CV of breaking tenacity declined.

In addition, the breaking elongations of two composite filaments were both smaller than that of pure PLA filament, moreover, the breaking elongations decreased with increase of nano-SiO2 in PLA filament. Since the nano-SiO2 itself did not have any elongation,

and its heterogeneous nucleation action led to crystalline particles, which were poor extensible and discontinuous with each other. This kind of structure would affect extending of the macromolecular chains in the non-crystalline region of PLA filament, so the breaking elongation declined with increase of nano-SiO2.3.5 Surface frictional properties of PLA

filamentsThe surface friction coefficients of pure PLA

filament and composite filaments are measured and shown in Table 4. The surface friction coefficients mean static friction coefficient and kinetic friction coefficient between filament and iron metal.

The results showed that the friction coefficients of two composite filaments were both higher than that of pure PLA filament, and the friction coefficient rose with increase of nano-SiO2 added in PLA filament. Because some nano-SiO2 particles were shown up on the surface of composite filament (Fig.9), they increase the roughness of filament’s surface. Meanwhile, the more nano-SiO2 added in PLA filament, the more nano-SiO2 shown up on the surface of filament, therefore, the friction coefficient of composite filament rose obviously with increase of nano-SiO2 added in PLA filament.

3.6 Thermal properties of PLA filamentsPLA filament is easy to be thermal-decomposed

at high temperature, so its thermal stability is poor. The thermo-decomposing temperature can accurately reflect the thermal stability of the filament, so it is an important parameter to evaluate material’s thermal stability. Fig.10 shows the TGA thermogravimetric curves of pure PLA filament and composite filaments.

170 Vol.31 No.1 WU Gaihong et al: Preparation and Properties of Heat Resistant Polylactic...

The thermo-decomposing temperatures of a, b and c curves (Fig.10(a, b, c)) were measured as 318.5, 326.7 and 332.1 ℃. The thermo-decomposing temperatures of two composite filaments were higher than that of pure PLA filament by 8.2 or 13.6 ℃, and the thermo-decomposing temperature increased with increase of nano-SiO2 added in PLA filament. The SiO2 itself was a kind of heat resistant material and its melting point was as high as 1 750 ℃, and its heterogeneous- nucleation-effect increased the crystallinity degree of PLA filament. All the above were helpful to filament’s thermal stability. Therefore, the thermo-decomposing temperature increased with increase of SiO2 in PLA.

The DSC heating curves of pure PLA filament and composite filaments are shown in Fig.11.

According to DSC curves in Fig.11, the glass transition temperature (Tg) and melting temperature (Tm) of filaments were measured in Table 5. The results indicated that the glass transition temperatures (Tg) of two composite filaments were higher than that of pure filament by 7.66 or 10.10 ℃, and the melting temperatures (Tm) of two composite filaments were bigger than that of pure filament by just 0.51 or 1.43 ℃. It was due to the increase of crystallinity-degree of PLA filament caused by nano-SiO2, therefore, the glass

transition temperature (Tg) increased obviously and the melting temperature (Tm) increased a little.

4 Conclusions

The nano-SiO2 modified by 5% KH-550 could disperse evenly and loosely in nano-scale. The nano-SiO2 unmodified or modified by 10% KH-550, was agglomerated as big mass. 1 wt% nano-SiO2 was distributed evenly throughout PLA matrix in nano-particle state and combined with PLA matrix closely. However, 3 wt% SiO2 was agglomerated as bigger particles over 1.0 micron embedded in PLA. Adding nano-SiO2 was helpful to the orientation of filament. 1 wt% nano-SiO2 could improve the mechanical properties (including breaking tenacity and CV of breaking tenacity) of PLA filament, but too much (i e, 3 wt%) nano-SiO2 could worsen that. The friction coefficient of composite filament rose obviously with increase of nano-SiO2 added in PLA filament. Comparing to the thermal characterization of pure PLA filament, the thermo-decomposing temperature of composite filaments mixed with 1 wt% or 3 wt% SiO2 increased by 8.2 or 13.6 ℃, the glass transition temperature (Tg) increased by 7.66 ℃ or 10.10 ℃, and the melting temperature (Tm) increased by just 0.51 or 1.43 ℃.

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