Poly-lactic acid self-reinforcing composites with improved toughness utilising solid-state drawn oriented tape

With the increasing white pollution problem and the enhancement of people’s awareness of environmental protection, the bio-based and biodegradable polymers have received unprecedented attention. Poly-lactic acid (PLA) is considered as one of the most potential alternatives to petroleum-based polymer materials. However, Due to its inherent brittleness and the difficulties of post-disposal and recycling caused by traditional processing methods, PLA has not been fully accepted by the market. To overcome the issues, self-reinforcing PLA composites (SRPLA) were prepared with different PLA grades as the matrix and the reinforcement respectively. High toughness SRPLA were prepared by hot pressing using PLA3052 casting films as the matrix and PLA6202 oriented tapes fabricated by solid state drawing as the reinforcement. In addition, the toughening mechanism was analyzed. Mechanical tests showed that the strength, modulus and elongation at break were increased by 124.3%, 114% and 145.2% respectively compared with pure PLA hot pressed film. Particularly, the tensile toughness of SRPLA is more than 200% that of pure PLA. The interface debonding and tape pullout were the main toughening mechanisms.


Introduction
Plastic waste is mainly composed of polymer blends, polymer-based composites containing fillers and fibers, which have caused serious white pollution worldwide due to their non-degradable and difficult separation and extraction of each component.Therefore, poly-lactic acid self-reinforced composites (SRPLA) derived from biological materials and composed of only a single component have received extensive attention in recent years.For example, the elongation at break of SRPLA prepared by Gall et.al. [1] with mixed yarns at different melting points was about 2 times higher than that of pure PLA while its strength did not improved.In addition, it was completely degraded after being kept in 40℃ seawater for 12 months.With oriented tape as reinforcement, the SRPLA prepared by Mai et.al. [2] achieved 12 times the impact toughness of pure PLA.But its tensile toughness had not been revealed.In this work, two different grades of PLA were used as matrix and reinforcement to prepare SRPLA.The tensile properties were studied, and its toughening mechanisms were discussed.

Materials
Poly-lactic acid (Nature Works® Ingeo™ 3052D and 6202D) were derived from Nature Works, USA, and have a molecular weight (Mw) of about 50,000.PLA3052D were used as the matrix and PLA6202D were used as the reinforcement.

Methods
As shown in Fig. 1, PLA3052 and PLA6202 tapes were prepared by twin-screw extruder (Xplore MC 15 HT).The PLA6202 tapes were then drawn to different drawing ratios (DR=2, 4, 8) at different temperatures (70°C, 80°C, 90 °C) using a universal testing machine with a hot chamber (SANS, CMT5105), where Equation 1 l: The total length of the specimen after drawing; l 0 : The original length of the specimen.SRPLA was prepared by stacking 4 layers of matrix and 3 layers of reinforcement layer in a hot press.The hot-press temperature was 145℃, the pressure was 6MPa, and the processing time was 1 min, 2 min, 3 min respectively.A reference was obtained by stacking 7 layers of matrix and pressing at 145℃ for 2 min.

Results and discussion
The stress-strain curves of PLA tape and PLA oriented tape with different drawing ratios at different temperatures were shown in Fig. 2. In general, solid-state drawing is an effective method to improve the mechanical properties of PLA tape.This is consistent with the literature [3] .Drawing temperature and DR significantly affect the improvement of mechanical properties.In this work, the tensile strength and modulus decreased with the increase of the tensile temperature.On the other hand, tensile strength and modulus increased with the increasing DR.This is due to crystallization and molecular alignment caused by drawing.The elongation at break increased first and then decreased with the increase of temperature and DR.This is also the result of the joint influence of crystallization and orientation [4] .Tensile toughness refers to the area between the stress-strain curve and the X-axis, Fig. 3 shows the tensile properties of SRPLA composites and reference.Compared with pure PLA, the strength, modulus and elongation of the composite have been improved.In particular, the toughness of the composite (1.34MJ/m3 ) after hot pressing for 2 min is 2.35 times that of the reference (0.57MJ/m 3 ).In addition, with the increase of hot-pressing time, the strength and modulus of the composite increased, but the toughness increased first and then decreased.This is related to the interfacial adhesion between matrix and reinforcement.Better interfacial adhesion corresponds to better stress transfer and thus better strength and modulus [5] .However, too much bonding leads to difficult or even impossible layer separation, which absorbs less energy and thus less toughness [6][7] .In order to discuss its toughening mechanisms, the tensile and brittle fracture sections of the composite were observed by SEM, as shown in Fig. 4. Different from the smooth interface after brittle fracture, the interface debonding and tape pullout can be observed at the tensile fracture, which absorbed energy and led to the improvement of tensile toughness of the composite.

Conclusion
In this study, in order to overcome the brittle nature of PLA, oriented tape was prepared as reinforcement, and the influence of drawing conditions on the tensile properties of oriented tape was discussed.Subsequently, SRPLA with tensile toughness 2.35 times that of pure PLA was prepared.In addition, in terms of toughening mechanism, the SEM showed that interface debonding and tape pullout were the main toughening mechanisms.

Fig. 1 .
Fig. 1. Preparation process of SRPLA.Note the blue short line is to represent the state of the molecules inside the tape, changing from a random state to a more ordered state due to the drawing.2.3.Characterization 2.3.1.Morphology.The fracture surface morphology of SRPLA was characterized by scanning electron microscopy (Phenom ProX).2.3.2.Mechanical properties.The tensile properties of the oriented tape and SRPLA were measured through a universal testing machine (INSTRON, 68SC-2).