Super toughened renewable poly(lactic acid) based ternary blends system: effect of degree of hydrolysis of ethylene vinyl acetate on impact and thermal properties

Abstract

Poly(lactic acid) (PLA) is one of the most promising biopolymers due to its biodegradable nature and good tensile strength. In spite of these advantages, the low impact strength and slow crystallization rate exhibited by PLA limit their end use applications. In order to overcome these drawbacks, an attempt has been made in the present study to prepare binary and ternary blends of PLA with Ethylene Vinyl Acetate (EVA) and ethylene vinyl alcohol (EVOH). The effect of degree of hydrolysis of EVA on the thermal and mechanical properties of PLA is investigated. Fourier transform infrared spectroscopy analysis elucidated the fact that the hydrolysis process is effective with respect to the increase in hydrolysis reaction time. Differential scanning calorimetry results disclose the prominent decrement in cold crystallization temperature and display a distinct melt crystallization peak in comparison with neat PLA. This is a clear indication that EVOH acted as a nucleating agent to increase the crystallization rate of PLA. The decrement in terms of tensile properties clearly indicated the reduction in stiffness after addition of EVOH in the PLA matrix. For the ternary blends of PLA, four fold increment in terms of elongation at break (%) and impact strength properties are observed as compared to neat PLA. Morphological studies revealed the presence of good adhesion between the PLA matrix and EVOH.

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Introduction

In the recent years, there has been considerable interest in generation of biopolymers from natural resources. This is because of the depletion of fossil fuel resources and growing concerns to solve the environmental pollution problem posed by conventional plastic materials. At present, biopolymers namely poly(lactic acid) (PLA),¹ poly(caprolactone) (PCL),² poly(butylene succinate) (PBS)³ and poly(hydroxybutyrate) (PHB) are being studied extensively as eco-friendly replacements for fossil-fuel derived plastics.⁴ This is owing to the fact that these polymers are biodegradable, biocompatible and available in the market commercially. An added advantage associated with these polymers is that their bio based origin can significantly contribute towards reducing the environmental carbon footprint.⁵ This can be achieved in case of properly designing the end life of biopolymers via composting or recycling approach. It is sure that bio based polymers can be sustainable alternatives to replace the conventional plastics, if development of innovative technology is achieved at reducing the production cost for biopolymers.

Amidst of all the biopolymers, PLA is being currently explored intensively for its potential in biomedical, automobile, electronics and packaging applications.⁶ This is due to the excellent process ability of PLA as compared to other biopolymers. Despite this, PLA exhibits disadvantages such as high brittleness, slow crystallization rate and poor impact resistance, which in turn limit its wide-spread applications.⁷ In order to address these drawbacks, incorporation of several nanoscale reinforcements namely cellulose nanocrystals (CNCs),⁸ graphene (GR),⁹ carbon nanotubes (CNTs)¹⁰ and clay¹¹ is practiced in the PLA matrix. The drawback associated with the incorporation of nanofillers is that enhancement in terms of impact and crystallization properties can be expected only if there exists compatibility between nanofillers and the PLA matrix.

An alternative approach envisaged in order to overcome the drawbacks associated with PLA is blending technology in which PLA is blended with certain other polymers namely PHB, PCL and PBS.^12–14 The blending technology finds wide-spread utilization in industrial sectors due to its potential in combining the characteristics of individual polymers into a unique end use product. Blending of PLA with other polymers can help in significantly reducing the cost for the final product as compared with developing a new polymer.¹⁵ The selection of co-polymer used for blending with PLA should be circumspectly selected. In other words, the co-polymer should exhibit compatibility with the PLA matrix or otherwise can end up in phase separation or immiscible blends.¹⁶ There are reports envisaged for addition of external compatibilizers in order to enhance the miscibility of the PLA blends.¹⁷ However, the most economically attractive method to enhance the miscibility of the PLA blend is to select a co-polymer which exhibits high compatibility with PLA in devoid of external compatibilizer.

Ethylene Vinyl Acetate (EVA) is one such co-polymer which is reported to exhibit compatibility with PLA matrix. The rubbery as well as resin properties offered by EVA make it as an attractive material to be used as a co-polymer in order to improve the impact properties of PLA. Therefore, investigation on blending of EVA as a co-polymer with PLA has been carried out by several researchers.^18,19 It was found that increment in terms of vinyl acetate (VA) content in the PLA matrix helped in enhancing the compatibility between EVA and PLA.¹⁹ With 40–50 wt% of VA content, it was found that the impact strength was increased by 30%. Therefore, in the current work, EVA containing 40 wt% of VA content is considered as a co-polymer for blending with PLA. The main aim of this work is to enhance the impact strength and crystallization properties of PLA. In order to achieve this, EVA is modified into ethylene vinyl alcohol (EVOH) via hydrolysis and subsequently used for blending with PLA matrix. Even though, there are reports on direct blending of PLA with EVOH co-polymer, studies on the modification of EVA into EVOH and comparative analysis on the properties of PLA–EVA and PLA–EVA–EVOH blends are not yet attempted to the best of our knowledge. The novelty of the current work lies in examining the effect of degree of hydrolysis on compatibility, impact strength and crystallization properties of PLA–EVA–EVOH blends in comparison with PLA–EVA blend and neat PLA. In the current work, binary blend of PLA–EVA (90/10 wt%) and ternary blends of PLA–EVA–EVOH (90/5/5 wt%) are developed and studied the effect of EVA under different degree of hydroxyl content. The blend samples were prepared via melt mixing technique and are then subjected to injection molding process. The influence of degree of hydrolysis of EVA on the toughening properties as well as phase behavior of injection molded PLA–EVA–EVOH blends is studied in comparison with PLA–EVA and neat PLA samples. The toughening mechanism is also explained based on the morphological analysis and impact strength properties of the prepared blends. Thermal degradation kinetics of PLA blends was studied using Coats–Redfern method for the better understanding of degradation behavior.

Experimental section

Materials

Poly(lactic acid) – 3001 D was procured from Nature Works, USA. Ethylene vinyl acetate (EVA) with the commercial name “Levaprene 400” and specific gravity of 0.98 was obtained from Lanxess India Pvt., Ltd. Sodium hydroxide (NaOH) and tetrahydrofuran (THF) (AR grade) was obtained from Merck, India.

Modification of EVA

Hydrolyzed EVA was synthesized according to the fallowing procedure. Initially, 50 g of EVA was dissolved in 250 mL of THF. Thereafter, 80 mL of 0.5 M NaOH was added to the reaction mixture and was refluxed under nitrogen atmosphere at 50 °C for different time intervals 1 h, 2 h and 3 h respectively. The reaction mixture was neutralized by 1 N HCl, then precipitated in chilled distilled water. The precipitate was washed thoroughly with distilled water and was dried at 80 °C till constant weight was achieved. The hydrolysis reaction of EVA with NaOH and its subsequent conversion to EVOH is illustrated in Fig. 1.

Fig. 1 Hydrolysis reaction of EVA with NaOH.

Determination of hydroxyl content of hydrolysed EVA

The hydroxyl content was determined by titration method using phthalic anhydride in presence of pyridine. One gram of hydrolysed EVA was dissolved in toluene (25 mL) and 5 mL of acetic anhydride/distilled pyridine (70 : 30) was added and the mixture was refluxed for 24 h. Unreacted phthalic anhydride was determined by titrating against 0.5 N NaOH using phenolphthalein as an indicator.

Preparation of the blends

Prior to blending, virgin PLA was dried in a hot air oven at 80 °C for 6 h. Binary blend of PLA with EVA (10 wt%) composition was melt mixed in a microprocessor controlled twin screw co-rotating extruder with L/D ratio of 40 : 1. The temperature condition in the range of 165–190 °C and the screw speed with 60 rpm were optimized for processing the blend. The extrudate strands obtained after processing were pelletized by quenching in cold water. The drying process for the extruded granules was carried in a hot-air oven, which was maintained at 60 °C. The drying process lasted for 6 h and further, the granules were subjected to injection molding using Electronica Endura 90 in order to prepare the test specimen as per ASTM standard. The temperature and injection pressure used for injection molding process were maintained in the range of 175–190 °C and 110–120 bar, respectively. The ternary blends of PLA (90 wt%)/EVA (5 wt%)/EVOH (5 wt%) were also prepared using injection molding process. The temperature and injection pressure conditions for preparation of ternary blends were maintained in the range of 165–190 °C and 110–120 bar, respectively. The various blend compositions are shown in Table 1. The numerical values (1, 2, 3) followed after EVOH in the ternary blends corresponds to EVA hydrolyzed at different intervals i.e., 1 h, 2 h and 3 h, respectively.

Table 1 Material designation, composition of PLA, PLA–EVA and PLA–EVA–EVOH blends

Blend composition	PLA (wt%)	EVA (wt%)	EVOH (wt%)
PLA	100	—	—
PLA–EVA	90	10	—
PLA–EVA–EVOH-1	90	5	5
PLA–EVA–EVOH-2	90	5	5
PLA–EVA–EVOH-3	90	5	5

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Fourier transform infrared (FTIR) spectroscopy analysis

All the samples such as EVA, EVOH, PLA, PLA–EVA and PLA–EVA–EVOH blends were subjected to FTIR analysis (Model: Avatar 370 Make: Thermo Nicolet). The FTIR spectra for these samples were recorded in the wavenumber region ranging from 400 to 4000 cm⁻¹.

Thermogravimetry analysis (TGA)

The thermal analysis for PLA, PLA–EVA, PLA–EVA–EVOH blends were carried out using a TGA analyzer (Model: Q50, Make: TA instruments, Walter, USA) in the temperature range of 30 to 600 °C at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. The thermal degradation kinetics for PLA and PLA blends was carried with the data obtained at a heating rate of 10 °C min⁻¹ using Coats–Redfern method.

Differential scanning calorimetry (DSC) analysis

Glass transition temperature (T_g), cold and melt crystallization temperatures (T_cc and T_mc), melting point (T_m), cold and melt crystallization enthalpy (ΔH_cc and ΔH_mc) and melting enthalpy (ΔH_m) for PLA, PLA–EVA and PLA–EVA–EVOH blends were determined using DSC analysis (Model: Q50, Make: TA instruments, Walter, USA). The PLA, PLA–EVA and PLA–EVA–EVOH blends were subjected to double heating cycle in which the first heating cycle was performed in the temperature range of 35 to 210 °C at a heating rate of 10 °C min⁻¹. At 210 °C, the samples were kept at isothermal condition for 5 min. Followed by this; the samples were allowed to cool at a cooling rate of 10 °C min⁻¹. At 35 °C, the samples were kept at isothermal condition for 5 min. The percentage crystallinity for PLA, PLA–EVA, PLA–EVA–EVOH blends was determined using the following equation.where, ΔH_m is the melting enthalpy and ΔH_o is the crystallization enthalpy for 100% crystalline PLA (93.7 J g⁻¹).

Mechanical properties

The tensile properties for PLA, PLA–EVA, PLA–EVA–EVOH blends were determined using Shimadzu Autograph (model: AG-IS 50KN) tensile tester as per ASTM D-638. Injection molded dumb bell-shaped specimens (165 mm × 13 mm × 3.2 mm) were used with cross-head speed of 10 mm min⁻¹ for the analysis. The flexural properties for PLA, PLA–EVA, PLA–EVA–EVOH blends were also determined by three-point bending test using Universal testing machine and the analysis was performed in accordance with ASTM D-790-Type B. The span length was set at about 96 mm. The testing speed was set at 3 mm min⁻¹ and the flexural properties were determined at room temperature and specimen dimensions were 127 mm × 12.5 mm × 6.4 mm.

The Izod impact strength test for PLA, PLA–EVA and PLA–EVA–EVOH blends was carried out according to ASTM D-256 using a Tinius Olsen IT 504 impact tester, USA. The specimen dimensions were of 63.5 mm × 12.7 mm with a notch depth of 2.54 mm and notch angle of 45°.

Scanning electron microscopy analysis

The morphological analysis for tensile fractured specimens was carried out by scanning electron microscopy (Model: JEOL-6390LV, Make: Germany). Before subjecting to SEM analysis, the fractured samples were sputtered with platinum and dried for a period of 30 min.

Results and discussion

EVA hydrolysis

The hydrolysed EVA samples were designated as EVOH-1, EVOH-2 and EVOH-3 based on the degree of hydrolysis. The hydroxyl content present in the hydrolysed EVA was determined by reaction with excess acetic anhydride and successive titration.²⁰ The degree of hydrolysis (DOH) was calculated by taking theoretical value of OH into consideration. The hydroxyl value and the extent of hydrolysis with respect to reaction time are presented in Table 2. The values indicated that the hydrolysis of EVA is a time dependent reaction since both OH value and degree of hydrolysis value increased with increase in reaction time. In case of EVOH-1, the DOH was obtained to be 32% with a hydroxyl value of 163. While, the DOH for EVOH-2 and EVOH-3, was found to be 58% and 80%, respectively. The corresponding hydroxyl values for EVOH-2 and EVOH-3 were 297 and 412, respectively. From the DOH and hydroxyl values, it is clear that the maximum degree of hydrolysis is achieved for EVOH-3.

Table 2 Effect of reaction time on hydroxyl value and extent of reaction

Polymer	Reaction time (min)	Hydroxyl value (A)	DOH (%)
PLA/EVA/EVOH-1	60	163	32
PLA/EVA/EVOH-2	120	297	58
PLA/EVA/EVOH-3	180	413	81

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FTIR analysis

The FTIR spectra for EVA and EVOH samples are presented in Fig. 2. The FTIR analysis revealed the modification of EVA into EVOH via hydrolysis reaction. It is observed from FTIR spectra of EVA that the peak present at 1735 cm⁻¹ corresponds to the presence of acetate group.²¹ It can be seen from the figure that the intensity of the acetate peak decreases with respect to hydrolysis time for EVOH samples. In addition to this, peak due to OH stretching can be seen in the wave number region of 3200–3400 cm⁻¹.²² The appearance of OH peak in EVOH samples confirms the occurrence of hydrolysis process. The increase in the intensity of the hydroxyl peak with respect to hydrolysis time reveals the increase in hydroxyl content due to maximum extent of degree of hydrolysis, which in turn, confirms the modification of EVA into EVOH.

Fig. 2 FTIR spectra for EVA, EVOH with different hydroxyl reaction time, PLA and PLA–EVA–EVOH-3.

The FTIR spectra for PLA, PLA–EVA and PLA–EVA–EVOH samples are depicted in Fig. 2. The appearance of peaks at 3000 and 2950 cm⁻¹ is attributed to the asymmetric and symmetric mode of C–H groups present in PLA, respectively. The presence of absorption band in the wavenumber region ranging from 1700–1800 cm⁻¹ corresponds to the carbonyl group available in the PLA.²³ The availability of CH₃ band in the PLA is confirmed by the appearance of peak at 1450 cm⁻¹. The existence of band at 1386 cm⁻¹ is attributed to the C–H deformation. The C–O–C asymmetric stretching of ester groups is confirmed by the presence of band around 1082 cm⁻¹. The presence of band at 922 cm⁻¹ corresponds to the rocking mode of CH₃ groups. The presence of EVA in the PLA matrix is confirmed by the appearance of acetate peak position at ∼1735 cm⁻¹ in the case of PLA–EVA blend. The presence of EVOH in the PLA matrix is confirmed by the presence of hydroxyl peak in the wavenumber region ranging from 3000–3300 cm⁻¹ in the case of PLA–EVA–EVOH blends. Otherwise, the FTIR spectra for PLA and its blends remain almost unaltered due to the existence of similar functionalities in both the cases.

Thermogravimetry analysis

The thermal properties for neat PLA, PLA–EVA and PLA–EVA–EVOH blends are investigated using TG analysis. The TGA and DTG curves for PLA and its blends are presented in Fig. 3. It can be seen from Fig. 3(a) that thermal degradation of PLA starts around 347 °C. The degradation of PLA corresponds to hydrolysis, lactide reformation, oxidative main chain scission, and inter or intramolecular transesterification reactions.²⁴ Unlike PLA, PLA–EVA blend exhibits two stages of degradation. The first stage of degradation for PLA–EVA blend is found to be in the temperature range of 310–355 °C. This weight loss that occurs in this region corresponds to the removal of acetic acid from EVA. As a part of de-acetylation process, small fractions of ketone, carbon dioxide and carbon monoxide also evolve during the degradation process of EVA.²⁵ The second stage of degradation for PLA–EVA blend occurs in the temperature range of 355–450 °C. The weight loss that corresponds to this degradation step is due to the chain scission reaction. Similar to PLA–EVA blend, PLA–EVA–EVOH blends also exhibit two stage degradation process. However, the slightly earlier degradation of PLA–EVA–EVOH (300–440 °C) blend may be due to the decomposition of PLA induced by the presence of hydroxyl groups in the EVOH.

Fig. 3 (a) TGA and (b) DTG curves for PLA, PLA–EVA and PLA–EVA–EVOH blends.

The comparative investigation of thermal degradation profiles of PLA, PLA–EVA and PLA–EVA–EVOH blends was carried out at various degradation points. The degradation points include the temperature at which degradation starts (T_onset) and temperature at which 50% weight loss occurs (T_50%). Also, the temperature at which maximum rate (T_max) of degradation occurs is interpolated from the derivative weight loss curve for PLA, PLA–EVA and PLA–EVA–EVOH blends. The temperature at various degradation points (T_onset, T_50% and T_max) for PLA, PLA–EVA and PLA–EVA–EVOH blends are tabulated in Table 3. In case of PLA, the T_onset, T_50% and T_max values are found to be 347 °C, 364 °C and 371 °C, respectively. While case of PLA–EVA blend, the T_onset, T_50% and T_max values are found to be 318 °C, 336 °C and 341 °C, respectively. After addition of EVOH in the PLA–EVA blend, further reduction in the T_onset, T_50% and T_max values are observed as compared to neat PLA and PLA–EVA blend. It can be seen from Table 3 that the thermal stability of PLA–EVA–EVOH blends decreases with increase in hydrolysis time. However, the final decomposition temperature in case of PLA–EVA and PLA–EVA–EVOH blends can be noticed at 450 °C. While in case of PLA, complete decomposition occurs at 400 °C. This indicates that addition of EVA and EVOH in the PLA matrix helps to prolong the complete degradation process.

Table 3 TGA results for PLA, PLA–EVA and PLA–EVA–EVOH blends

Sample name	Tonset (°C)	T50% (°C)	Tmax (°C)
PLA	347.10	364.01	371.01
PLA–EVA	318.20	336.02	341.12
PLA–EVA–EVOH-1	314.81	334.30	338.85
PLA–EVA–EVOH-2	311.49	332.17	336.58
PLA–EVA–EVOH-3	310.23	330.57	336.58

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Thermal degradation kinetic analysis

The thermal degradation kinetic analysis for PLA, PLA–EVA and PLA–EVA–EVOH blends is carried out in order to have knowledge about degradation behavior of the blends as compared to neat PLA. Coats–Redfern method is used to perform the thermal degradation kinetic analysis and the corresponding model equations are as follows:

Plotting the left hand side term versus −1/T gives the straight line and activation energy is obtained from the slope of the straight line. The value of the pre-exponential factor, A is obtained from intercept of the straight line, by considering the expression inside the parenthesis as 1. Analysis is done using only single heating data, which is different from other kinetic models where multiple heating data are required for analysis.

The Coats–Redfern method requires TG data with only one heating rate to calculate kinetic parameters such as activation energy (E_a), reaction order (n) and pre-exponential factor (A). In this study, TGA data of PLA and PLA blends are obtained at a single heating rate (10 °C min⁻¹). For this method, a reaction order “n” is assumed and the assumed value is substituted in eqn (2) and (3). The plot of the left hand side of eqn (2) and (3) versus −1/T−1T is fitted to calculate the R² values.^26–30 This process is repeated until the best R² value is obtained.

Fig. 4 represents the linear fitted graph for neat PLA, PLA–EVA and PLA–EVA–EVOH blends for different “n” values. The reaction order which is obtained at the best R² value is regarded as the reaction order for PLA, PLA–EVA and PLA–EVA–EVOH blends (Table 4). Then the activation energy and the pre-exponential factor are obtained from the slope and intercept of the fitted straight line. The activation energy of neat PLA, PLA–EVA, PLA–EVA–EVOH-1, PLA–EVA–EVOH-2 and PLA–EVA–EVOH-3 is found to be 176, 173, 150, 146 and 145 kJ mol⁻¹, respectively. The decrement in the activation energy is an indication of reduction in thermal stability for PLA matrix upon addition of EVA and EVOH elastomers.

Fig. 4 Determination of kinetic parameters by left part in eqn (2) and (3) against using Coats–Redfern method: (a) PLA, (b) PLA–EVA, (c) PLA–EVA–EVOH-1, (d) PLA–EVA–EVOH-2 and (e) PLA–EVA–EVOH-3.

Table 4 Activation energy (E_a) and regression coefficient (R²) for PLA, PLA–EVA and PLA–EVA–EVOH blends

Sample name	Activation energy (Ea) (kJ mol^−1)	Regression coefficient (R^2)
PLA	175	0.993
PLA–EVA	173	0.995
PLA–EVA–EVOH-1	150	0.995
PLA–EVA–EVOH-2	146	0.996
PLA–EVA–EVOH-3	145	0.993

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Differential scanning calorimetry (DSC)

DSC thermographs for PLA, PLA–EVA and PLA–EVA–EVOH blends during second heating and first cooling cycles are represented in Fig. 5(a) and (b), respectively. The thermal properties for PLA, PLA–EVA and PLA–EVA–EVOH blends such as glass transition temperature (T_g), melting temperature (T_m), and cold crystallization temperature (T_cc) are considered from the DSC thermograph obtained during the second heating cycle at a heating ramp of 10 °C min⁻¹ and are reported in Table 5. While, the melt crystallization temperature (T_mc) for PLA, PLA–EVA and PLA–EVA–EVOH blends is considered from the DSC thermograph which is obtained during the first cooling cycle at a cooling ramp of 10 °C min⁻¹ and are reported in Table 5. It can be noticed from the figure that the peak that corresponds to melting temperature of PLA appears at 170 °C. The melting peak of PLA is unimodal and endothermic in nature, which in turn indicates the α-crystalline arrangement of PLA.^31–33 The stable crystal formation in PLA is confirmed by the absence of double melting peak. This may be due to the occurrence of homogeneous nucleation process in PLA.³¹ The melting peak obtained for PLA–EVA and PLA–EVA–EVOH blends is also found to be unimodal and endothermic in nature. The unimodal nature of endothermic peak confirms the formation of stable crystals with uniform thickness with the addition of EVA and EVOH in the PLA matrix.

Fig. 5 DSC curves of (a) second heating and (b) first cooling thermographs for PLA, PLA–EVA and PLA–EVA–EVOH blends.

Table 5 DSC results for PLA, PLA–EVA and PLA–EVA–EVOH blends

Sample name	Tg (°C)	Tcc (°C)	ΔHc (J g^−1)	Tm (°C)	ΔHm (J g^−1)	χ (%)
PLA	62.17	106.13	25.55	170.87	35.44	37.86
PLA–EVA	58.88	97.39	8.95	168.81	50.31	53.75
PLA–EVA–EVOH-1	59.20	84.78	16.25	169.11	48.87	51.89
PLA–EVA–EVOH-2	60.85	79.86	12.79	166.22	44.07	47.09
PLA–EVA–EVOH-3	63.70	83.32	16.12	167.24	44.53	47.57

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The non-isothermal cold crystallization temperature for PLA, PLA–EVA and PLA–EVA–EVOH blends are reported in Table 5. It can be noticed that crystallization temperature for PLA–EVA and PLA–EVA–EVOH decreases as compared to neat PLA (Table 5). With respect to increase in the degree of hydrolysis, it is observed that the prominent reduction in the crystallization temperature for PLA–EVA–EVOH blends. Due to the nucleating effect of EVOH, crystallization phenomenon is initiated at a lower T_cc which in turn resulted in the development of stable and thick crystals. The reduction in the T_cc values is an indication of requirement of less energy for achieving desired degree of crystallinity.²⁴ In case of glass transition temperature, it remains unaffected for both PLA–EVA and PLA–EVA–EVOH blends in comparison with PLA. The non-isothermal melt crystallization temperature (T_mc) for PLA, PLA–EVA and PLA–EVA–EVOH blends are reported in Table 5. The T_mc values for PLA–EVA and PLA–EVA–EVOH blends seem to increase as compared to neat PLA. With respect to increase in degree of hydrolysis, further increment in the T_mc values for PLA–EVA–EVOH blends can be seen in comparison with PLA–EVA blend. This is due to the fact that EVOH helps in enhancing the crystallization of PLA by promoting the heterogeneous nucleation process. The crystallinity (%) for PLA and its blends were shown in Table 5. The crystallinity (%) for PLA matrix is increased after blending with EVA and EVOH.

Mechanical analysis

The tensile, flexural, elongation-at-break (%) and impact strength properties obtained for neat PLA, PLA–EVA and PLA–EVA–EVOH blends are presented in Fig. 6. It can be seen from Fig. 6, that the tensile strength and tensile modulus values for PLA–EVA and PLA–EVA–EVOH decreased as compared to neat PLA. This is due to the reduction in stiffness induced by the addition of EVA and EVOH in the PLA matrix. In addition to this, it can also be seen from the figure that the flexural properties for PLA–EVA and PLA–EVA–EVOH blends are found to be reduced as compared to neat PLA. This is an excellent indication of improvement in flexibility for the PLA blends. This is further confirmed by the enhancement in terms of elongation-at-break (%) for PLA–EVA and PLA–EVA–EVOH blends when compared with neat PLA (figure). The elongation at break (%) values was found to be higher than that of neat PLA for both binary and ternary blends of PLA. In case of PLA, the elongation-at-break (%) is obtained to be 3.5. After addition of EVA in the PLA matrix, the elongation-at-break (%) is found to increase up to 27. With respect to increase in the degree of hydrolysis, the elongation-at-break (%) values were tending to show increment. For, PLA–EVA–EVOH-1, PLA–EVA–EVOH-2 and PLA–EVA–EVOH-3, the elongation-at-break (%) values are found to be 8.8, 11.8 and 12.5, respectively. This improvement in elongation-at-break (%) values for both PLA–EVA and PLA–EVA–EVOH blends is attributed to the enhancement in ductility of PLA.

Fig. 6 Mechanical properties of PLA, PLA/EVA and PLA/EVA/hydrolysed EVA with different degree of hydrolysis as a function of weight fraction (a) tensile strength (b) tensile modulus (c) % elongation (d) flexural strength (e) flexural modulus (f) impact strength.

The impact strength (Fig. 6) of neat PLA was only 3.1 kJ m⁻², and the samples fractured clearly in a brittle manner. The impact strength of PLA–EVA with 10 wt% of EVA was higher (4.65 kJ m⁻²) than that of neat PLA. It can be seen from the figure that remarkable increase in terms of impact strength was obtained for PLA–EVA–EVOH blends. Like elongation-at-break (%) values, the impact strength also tends to increase with respect to increase in degree of hydrolysis. The impact strength for ternary blend of PLA–EVA–EVOH-1 with hydroxyl value of 32 is found to be 6.58 kJ m⁻². The impact strength values for PLA–EVA–EVOH-2 (hydroxyl value: 58) and PLA–EVA–EVOH-3 (hydroxyl value: 81) are found to be 7.9 and 11.73 kJ m⁻², respectively. The results indicate that the impact strength for PLA–EVA–EVOH-3 blend is increased by ∼4 fold in comparison with neat PLA. This may be due to the strong interfacial adhesion between the PLA matrix and the EVOH resulting in brittle to ductile transition. The phase behavior of the ternary blend also played an important role in increasing the toughness of PLA. The elastomeric nature of EVOH enabled to absorb impact energy and acted as stress concentrator during impact deformation which retarded the crack initiation and propagation and subsequently led to better toughness for PLA–EVA–EVOH blends.³⁴

Toughening mechanism via surface morphology of impact fractured surface

To further study the toughening effect of PLA binary and ternary blends, the fracture surface of impact specimens was investigated using SEM analysis. The morphology images obtained for PLA, PLA–EVA and PLA–EVA–EVOH blends are shown in Fig. 7. It can be seen from the Fig. 7(a) that the neat PLA showed a smooth and featureless brittle fracture surface with no plastic deformation. The PLA–EVA blend with 10 wt% of EVA content showed a clear stress-whitening surface (Fig. 7(b)). In addition to this, presence of oval cavities around the matrix can also be seen from Fig. 7(b), which is indicative of deformation. The dispersed EVA in the PLA matrix cavitates and de-bond upon application of load. Particulate cavitation and void formation generate new stress whitening that facilitates the deformation of matrix. The plastic deformation effectively helps in energy dissipation and attaining high impact strength for PLA matrix.^35,36

Fig. 7 SEM micrographs of impact fracture surfaces of (a) neat PLA, (b) PLA–EVA-10 and (c) PLA–EVA–EVOH-3.

The Fig. 7(c) shows the morphology of ternary blend of PLA containing EVOH with a DOH value of 81%. The mechanical properties of the blends can be correlated with the morphology of the blends. It can be seen that surface of the dispersed phase contained more and longer fibril threads by the addition of EVA and EVOH. This indicates greater adhesion between PLA and toughening agent. The observation supports the increase in impact strength. The addition of EVA and EVOH helps in sharing the load. Along with this, the presence of EVA and EVOH make substantial contribution towards energy dissipation and there by prevents the formation of cracks. This is due to this reason; enhancement in terms of toughness is achieved for the PLA blends. There is no observation regarding “pulling out phenomenon” for both PLA–EVA and PLA–EVA–EVOH blends. This reveals the fact that PLA exhibits excellent compatibility with EVA and EVOH elastomers.³⁷

The miscibility of the components in the blend determines the phase behaviour and interfacial compatibility that has greater control on the mechanical properties of the system. The glass transition temperature of PLA–EVA–EVOH blends showing a single peak and it is shifted to slightly higher temperature side than the other compositions in DSC thermographs. It is an indication of miscibility between PLA, EVA and EVOH.³⁸ This may be due to the better interaction between OH groups of modified EVA and end functional group of PLA, in presence of EVA. EVOH acts as a better compatibilizer for the binary blend of PLA and EVA. Thus, modified EVA with greater degree of hydrolysis imparted greater toughness than binary blends of PLA/EVA and PLA–EVOH. The reaction between terminal –O–C=O group of PLA and hydroxyl groups of EVOH was the major possible comptabilisation at the desired processing temperature. Accordingly, in PLA–EVA–EVOH ternary blend, EVOH acted unique dual role as an effective toughening agent and as excellent compatibiliser. The improved compatibility of EVOH with EVA and PLA is clearly demonstrated in both SEM images and DSC thermographs.

Conclusions

The FTIR results elucidate the increase in intensity of the hydroxyl peak (3200–3400 cm⁻¹) with increase in degree of hydrolysis. The highest degree of hydrolysis (81%) is achieved for EVOH-3. The increase in the degree of hydrolysis has brought prominent improvement in the mechanical properties such as elongation at break (12.5%) and impact strength (11.73 (kJ m⁻²)) for ternary blends of PLA matrix. The DSC results confirm the nucleation effect of EVOH by reducing the cold crystallization temperature for PLA. The decrement in the activation energy for PLA–EVA–EVOH blends is observed when compared with neat PLA. This is an indication of reduction in thermal stability due to decomposition of hydroxyl group present in the EVOH. The SEM images depict good compatibility among PLA and its blends.

Acknowledgements

The first author thankfully acknowledge the financial support by Department of Chemicals and Petrochemicals, GREET-Project, ARSTPS, CIPET, Chennai.

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