A Further Research on the Properties of PLA/TiO2 Nanocomposites

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Abstract. Organic/inorganic nanocomposites have been widely focused because of their special properties. By modifying nano TiO2 and polylactic acid (PLA) with lactic acid, PLA/TiO2 nanocomposites was prepared, and the test on the dynamic mechanical property, thermal property, and thermal deformation temperature showed that adding nano TiO2 helped improve PLA’s mechanics property, thermal decomposition temperature, and thermal deformation temperature to certain extent.

Keywords: PLA; TiO2; Composite

1 Introduction

In recent years, the researches on PLA nano composites are remarkable. It has been reported that the composite of PLA and inorganic nanometer particles such as montmorillonite [1-5], carbon nanotubes [6-8], hydroxyapatite [9-13], and SiO2 [14-17] can play a role in improving heat resistance, gas barrier, mechanics, crystallization, and degradation, etc. Also, a small number of researches are made on the related properties of PLA/TiO2 nanocomposites [18-21].

The preliminary research results showed that [22] that the nanocomposites of TiO2 modified via lactic acid surface grafting and prepared via the melt blending in twin-screw extruder possessed good dispersity, making PLA‘s elongation at break and impact property largely increased, and also PLA‘s crystallization improved well. In this paper, the dynamic mechanical properties, thermal properties, and thermal deformation temperature of PLA/TiO2 nanocomposites will be further discussed.

2 The experiment

2.1 Experimental materials

(1) PLA: 4032D, Natureworks?; dried in vacuum oven at 65℃before it was used.

(2) G-TiO2: self-made in the laboratory (anatase nanometer TiO2 obtained by modifying the surface grafting via the solution without catalyst, and the grafting rate was about 7 wt.%); dried in vacuum oven at 50℃ until constant weight, and then set aside in dry device.

2.2 The preparation of PLA/TiO2 nanocomposites

According to the formula (see table 1), PLA and g-TiO2 were weighted first and then blended evenly in plastic bag, and then extruded out and cut into granules via twin-screw extruder (screw’s diameter was Ф=20.5mm and draw ratio was L/D=44; produced by Nanjing Coperion Machinery Co., Ltd.). The temperature from the head to the extrusion section of the twin-screw extruder was set to be 170℃~195℃. The screw rotation speed was 90rpm/min. The obtained granules were dried in vacuum at 60℃ for 24h, and the residual water and small molecular substances were removed, and then set aside in dry device. Part of the above mentioned sample extruded via melt extrusion was injected and molded into dumbbell-shaped tensile splines and rectangular, shocking, and dynamic test splines in injection molding machine (Germany K-TEC 40; screw diameter: 25 mm; draw ratio: 20/1; maximum injection molding quantity: 41.7g; MIMACRON Co.). Injection molding experiment conditions were as follows: temperature 170~190℃; pressure: 80Mpa; injection molding time: 10s. All specimens were first aged at room temperature for 48h and then could be used in related tests.

Part of the above mentioned granules extruded via melt extrusion was suppressed into pieces in plate vulcanizing machine (Qingdao’s Third Rubber Machinery Factory XK13-624-0036). Test conditions were as follows: temperature: 185℃; preheating for 5 min; pressurization: 10MPa; pressure maintaining: 3 min; pieces were taken out after pressure maintaining was naturally cooled to room temperature.

2.3 Material properties test and characterization

(1) DMA on the properties

Dynamic mechanical analysis (DMA) was made for PLA/TiO2 nanocomposites of different g-TiO2 content using thermal analyzer DMA Q800 (TA Company). The test conditions were as follows: test temperature: -20~150℃; heating rate: 3℃/min; the vibration frequency: 1Hz; the spline specification: 4×2×45mm3; vibration type: double cantilever beam model.

(2) Thermal property (TG) test

The thermal properties of PLA and PLA/TiO2 nanocomposites were analyzed using TG 209 F1(NETZSCH). The test conditions were as follows: N2; air flow rate: 50ml/min; the heating rate 10℃/min from room temperature to 600℃.

(3) Heat deformation temperature (HDT) test

The heat deformation temperatures (HDT) of PLA and PLA/TiO2 nanocomposites were tested using ALLAS HDV2 Vicat distortion temperature tester. The test conditions were as follows: the length, height, and width of the sample: 120mm, 4mm, and 10mm; stress: 455KPa.

3 Result and discussion

3.1 The mechanical property of PLA/TiO2 nanocomposites

The relationship between PLA and PLA/TiO2’s energy storage modulus (E) and temperature is as shown in figure 1. It is seen that the change trends of pure PLA and PLA/TiO2 nanocomposites were the same basically: before the glass-transition temperature, chain faulting was frozen, polymer was in the glassy state, and energy storage modulus was in a constant basic range basically. Compared with pure PLA, however, the addition of TiO2 particles before the glass-transition temperature significantly helped improve the energy storage modulus of PLA composites: the energy storage modulus of PLA/TiO2-0.5, PLA/TiO2-1, PLA/TiO2-2, and PLA/TiO2-5 nanocomposites increased 1.1, 1.5, 1.4, and 1.6 times respectively at 40℃compared with pure PLA, while the effect of nanoparticles on PLA’s energy storage modulus was insignificant after the glass-transition temperature.

In the classical theory [23] of inorganic particles filling high polymer materials, it is thought that energy storage modulus increases along with the increase of the filling amount if the particles are well compatible with the substrate and the effect from the interface can be ignored. Combined with DMA and DSC analyses, it was seen that the increase of the energy storage modulus of the nanocomposites prepared in the experiment might be related to a lot of graft polymers existing on the surface of TiO2 particles. After nanometer TiO2 was modified via surface grafting, the existence of these graft polymers not only enhanced the lipophilicity on the surface of TiO2 particles, and also the molecules via melt blending and substrate were entangled physically. Thus, E increased along with nanoparticles’ dispersion in the substrate as well as the increasing area of the interface with the substrate, and the increasing friction effect between substrate molecules. From the previous electron microscope observation result, it was seen that the dispersion of the nanoparticles in PLA/TiO2-1 nanocomposites was better than other nanocomposites with more anoparticles in the substrate, so they make a greater contribution to the energy storage modulus. However, the interaction or physical entanglement between nanoparticles and PLA molecular chain because of intensified PLA molecular chain motion after the glass-transition temperature, so that the effect of nanoparticles on PLA energy storage modulus was not very obvious.

3.2 The thermal stability of PLA/TiO2 nanocomposites

Thermal stability is one of the most important indexes to decide if the material is of application value, which plays a decisive role in the material processing temperature range and the application range to large extent. PLA’s stability is poorer and serious decomposition will be produced if PLA is in high temperature or sustained at high temperature for a while. Therefore, the thermal stability of PLA is of great significance to its processing and application.

The thermo-gravimetric curve of PLA/TiO2 nanocomposites is as shown in figure 2. It is seen that the decomposition process of PLA and its nanocomposites was relatively simple, and the decomposition started at more than 300℃ and the decomposition range was in 346℃~375℃, and also all samples were decomposed in one step.

In table 2, the initial decomposition temperature (Ti) of PLA and PLA/TiO2 nanocomposites, the temperature (T50%) in the decomposition of 50% material, and the maximum decomposition rate temperature (Tmax) are shown. Seen from the experimental result, the initial thermal decomposition temperature of the nanocomposites was higher than that of pure PLA, suggesting the addition of nanometer TiO2 helped increase thermal stability of PLA to a certain extent, and this was very useful for materials processing and production.

3.3 The thermal deformation temperature of PLA/TiO2 nanocomposites

In figure 3, the thermal deformation temperature curve of PLA/TiO2 nanocomposites with different nano content is shown. It is seen that the thermal deformation temperature of the nanocomposites increased slightly compared with pure PLA as the content of nano TiO2 increased, but the effect was not very big.

Conclusion

The preparation of PLA/TiO2 nanocomposites is a subject of very scientific significance and practical value to study the fully biodegradable materials of new biological bases. The following conclusion is made from this paper:

First, the dynamic mechanical properties of PLA are affected by nanometer TiO2 particles. The addition of nanometer TiO2 helps improve its energy storage modulus and this should be related to a lot of graft polymers on the surface of nanometer TiO2 particles.

Second, the addition of nanometer TiO2 helps increase the thermal decomposition temperature of PLA, but the effect is not very big.

Third, the addition of nanometer TiO2 can improve the thermal deformation temperature of PLA to certain extent.

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