Sustainable composites from biodegradable poly (lactic acid) modified with isosorbide-based plasticizer: Preparation and performance

Abstract

Bio-based resin plasticizers not only overcome the toxicity and non-degradability of phthalate plasticizers, but also expand the application areas of polymers. In this study, the environmentally sustainable plasticizer isosorbide dinonanoate (ISN) is chosen to plasticize and toughen PLA for enhancing the impact strength and processability, further expanding the application in the field of flexible packaging materials. Our work primarily focuses on the impact strength, crystallinity, glass transition temperature (Tg), processability, and morphology for PLA composites. The results demonstrate that adding ISN at a concentration of 25 phr reduces the Tg by 22°C, representing a 35.4% decrease relative to neat PLA, thus confirming the effective plasticizing impact of ISN on PLA; the Izod impact strength arrives at 688 J/m, approximately 30-fold increase, demonstrating dramatically enhanced toughness of PLA; and the equilibrium torque is 1.9 Nm, involving improved processability of the composites. DSC tests show that the crystallinity of PLA composites improves when the amount of ISN increases. Notably, the crystallinity increases by 248% from 7.07% to 24.59% when the addition of ISN is 25 phr. Meanwhile, SEM reveals that ISN has good compatibility with PLA, as evidenced by the absence of phase separation on the fracture surfaces of the composites.

Keywords

Poly (lactic acid)isosorbide dinonanoate mechanical properties processability crystallization behavior

Introduction

Along with polymers utilized in all aspects of life and manufacturing, while enjoying convenience of polymers, people have become aware that polymers are threatening the environment on which we always depend.^1,2 Because of stable structure of most polymers, they are hardly degraded by natural microorganisms, and furthermore the toxic metabolites produced in the process of decomposition are harmful to the environment.^3,4 When waste polymer materials are left untreated in the nature for long periods of time, it can give rise to a number of negative consequences on ecological system such as variations in the characteristics of the soil,⁵ and interfere with normal growth and development for plants and animals.^6,7 Besides, it is the poisonous ash and gases produced by incineration that can arouse atmospheric and land pollution.⁸ More importantly, due to the shortage of non-renewable petroleum raw materials, biodegradable materials have emerged at a history moment.^9–15

To the best of our knowledge, PLA is a renewable resin with considerable potential for development that has been extensively examined for application in medical treatment,^16,17 agriculture¹⁸ and packaging materials^19,20 thanks to its excellent optical properties, mechanical durability and good machinability. Furthermore, PLA is completely biodegradable in a humid environment and at a temperature above the glass transition. However, certain drawbacks of PLA including low crystallinity, inherent brittleness and poor thermal stability hinder its further applications.^21–24 In order to improve the mechanical properties of PLA, numerous measures have been developed and implemented in recent years. Among of them,co-polymerization is a modification method using flexible monomers to copolymerize with PLA to regulate the molecular structure of the polymer and improve the mechanical properties of the polymer such as tensile and impact performance.^25–27 Nevertheless, co-polymerization modification is not in line with the concept of economical sustainability and is difficult to produce on an industrial scale. In contrast to the former, it is more cost-effective to blend PLA with other biodegradable and non-biodegradable resins, such as poly(ε-caprolactone) (PCL),²⁸ poly(butylene adipate-co-terephthalate) (PBAT),²⁹ acrylonitrile butadiene styrene (ABS)³⁰ and ethylene-co-vinyl acetate (EVA).³¹ Unfortunately, above-mentioned blends suffer from poor compatibility, which gives rise to limited improvement in the mechanical properties of the materials.

At present, many researchers also promote another modification method that PLA blends with plasticizers to expanding the application in the field of flexible packaging materials. Frequently used plasticizers include dioctyl phthalate,³² citrate esters,^33–36 polyethylene glycol (PEG),^37–39 vegetable oil⁴⁰ and epoxidized vegetable oil.^41–44 Although they can improve the brittleness of PLA, most of them are less compatible with PLA and have potential migration problems. Therefore, it is necessary to exploit green plasticizers with excellent properties.

Isosorbide esters are typically renewable plasticizers, which are obtained by esterification of isosorbide, a versatile biobased diol. Their performance can be easily tailored by employing carboxylic acids with different chain lengths. More attractively, isosorbide esters are readily biodegradable and harmless to mammals.⁴⁵ In this paper, ISN, an environmental-friendly plasticizer, is utilized to prepare PLA composite with high toughness and excellent processing performance. The brittle resistance, crystallinity, dynamic mechanical thermal behaviors, processability, corresponding compatibility and migration resistance for PLA composites are systematically analyzed, while the possible plasticizing mechanism of ISN is discussed in detail.

Experimental

Materials

ISN is homemade in the laboratory. The specific synthesis method can refer to previous work by our group.⁴⁶ PLA (4032D, MFR = 7 g/10 min at 190°C, 2.16 kg, density = 1.24 g/cm³) is obtained from Nature Works (America).

Preparation of PLA/ISN composites

The preparation of PLA/ISN composites is based on the formula in Table 1. Dry PLA under vacuum at 60°C for 12 hours before usage. Then PLA and ISN are mixed at 180°C and 60 rpm for 12 minutes in a torque rheometer (TYP557-9301, Haake, Germany) and the variations of torque during processing of PLA and PLA/ISN composites are recorded. The processing flow is shown in Figure 1.

Table 1.

Composition of PLA and PLA/ISN composites.

Sample	PLA (g)	ISN (g)	ISN (g) in 100g PLA
PLA	60	0	0
PLA/ISN5	60	3	5
PLA/ISN10	60	6	10
PLA/ISN15	60	9	15
PLA/ISN20	60	12	20
PLA/ISN25	60	15	25

Figure 1.

Preparation flowcharts of PLA/ISN composites.

Characterization

Dynamic mechanical thermal analysis (DMTA)

The samples are measured by a dynamic mechanical analyzer (Diamond PE, PerkinElmer, USA) within the temperature ranges from 0 to 120°C at a heating rate of 3°C/min. Maintain the frequency at 1 Hz and the deformation of 0.1%. The size of samples is 30 × 10 × 1 mm and Tg of the PLA and PLA/ISN composites is obtained from the tanδ-temperature variation curves.

Mechanical properties

The impact splines (length × width × thickness is 65 × 12 × 3 mm) and the tensile splines (length × width × thickness is 50 × 4 × 1 mm) are used for testing at room temperature. Notched Izod impact tests are performed using XJU-5.5 cantilever beam impact tester with a pendulum value of 5.5 J according to Chinese standard GB/T1843-2008 and the impact speed is 3.5 m/s. The tensile properties of PLA and PLA/ISN composites are tested by an Instron 1121 universal electronic stretching machine with a 1.0 kN load cell and a constant crosshead speed of 50 mm/min based on Chinese standard GB/T1040.1-2018. The final data represents the average of at least five tests.

Differential scanning calorimetry (DSC)

About 5 mg samples are thermally analyzed by a differential scanning calorimeter (Pyrussapphire, PerkinElmer, America) under a nitrogen atmosphere (nitrogen flow of 20 mL/min). In the temperature range of 25 to 180°C, the samples are heated up, cooled down and heated up again, in which the temperature change rate of is 10°C/min and a 5-min residence time at each temperature cycling node. The degree of crystallinity (X_c) is calculated from the below equation:

X_{c} = \frac{{∆ H}_{m} ‐ {∆ H}_{c c}}{δ_{P L A} \times Δ H_{m}^{0}} \times 100 %

where δ_PLA is wt% of PLA in PLA/ISN composites, ΔH_cc and ΔH_m are the enthalpies of cold crystallization and melting of PLA and PLA/ISN composites, the ΔH_m⁰ is enthalpies of melting for 100 % crystalline PLA, with a theoretical value of 93.7 J/g.⁴⁷

X-ray diffraction analysis (XRD)

PLA and PLA/ISN composites are tested and analyzed using a wide-angle X-ray diffractometer (D/MAX 2000/PC, Rigaku, Japan) with an angular resolution of 0.5°, a scanning range of 5–50° and a Cu Kα radiation wavelength of 1.54 Å.

Scanning electron Microscope (SEM)

Use an ion sputtering instrument to spray gold onto impact fracture surface, and then place the prepared sample under a SEM (JEOL Electronics Co., Ltd, Japan) to observe the impact section morphology of the composites.

Surface migration test

The tested specimens of size 50 × 50 × 1 mm were prepared using a hot press and weighed to obtain an initial mass M₀. Clamp the specimens with pure PLA sheet (60 × 60 × 2 mm), Perform surface migration test by air circulating oven, according to ISO 177:2016 standard (70°C, load of 5 kg, duration of 24 h) and finally, reweigh testes specimens to obtain mass M₁. The migration resistance of ISN in the samples is evaluated based on the mass loss ratio (V) of the samples, which is calculated according to the following formula:

V = \frac{M_{0} - M_{1}}{M_{0}}

Rheological property test

PLA and PLA/ISN composites are made into circular thin sheets (diameter × thickness is 25 mm × 1 mm) and the rheological property tests are carried out with a rotational rheometer (AR2000EX, TA Instruments, America) within the angular frequency ranges from 0.1 to 100 rad/s and a temperature of 180°C. The distance between the plates of the rotational rheometer is 1 mm and the strain amplitude is set as 0.1 % to ensure a linear viscoelastic response.³⁶

Results and discussion

Dynamic thermomechanical analysis

In the dynamic thermomechanical analysis, the maximum tanδ value corresponding to the Tg of composite materials. It can be observed from Figure 2, The DMTA spectra of all the composites showed a single peak, indicating good thermodynamic miscibility between ISN and PLA. When the addition of ISN increases, a phenomenon of the peak of tanδ moving towards the low-temperature region is found, indicating that ISN plays a good plasticizing role. When the amount of ISN added is 25 phr, Tg of PLA decreases from 62°C to 40°C, which is a decrease of 22°C. Specifically, the incorporation of ISN to PLA resin reduces the extent of entanglement between polymer chains and improves sliding properties.

Figure 2.

DMTA curves for PLA and PLA/ISN (a) Tanδ as a function of temperature, (b) storage modulus.

Subsequently, the storage modulus of PLA/ISN composites are expectedly lower than that of neat PLA, and with the increase of ISN, the storage modulus of the composites gradually decreases. When the concentration of ISN is 25 phr, the storage modulus drops to 1880 MPa, which is down by 43.86% compared to neat PLA. Besides, around the glass transition region, the free volume in the blends starts to thaw and the thermal motion of the molecules has sufficient energy, which leads to a sharp decrease of the storage modulus.

Mechanical properties

Figure 3 demonstrates the effect of the ISN concentration on the Izod impact strength and tensile properties of PLA/ISN composites, and specific data is listed in the Table 2. The results show that ISN is conspicuously beneficial to enhance the flexibility and brittleness resistance of PLA composite materials, and simultaneously, the tensile strength and elastic modulus are inversely proportional to the amount of ISN added. Nevertheless, when the ISN amount reaches 5 phr, the deformation resistance of the PLA composites increases along with the decrease in elongation at break and Izod impact strength, resulting in an “anti-plasticizing” phenomenon.^48,49 Pure PLA is rigid before yielding and fractures at low strain, with an elongation at break of only 5.11%. Adding 10 phr and 15 phr of ISN does not transform PLA from fragile to pliable, with elongation at break of 5.78% and 5.92%, respectively. However, when the quantity added of ISN reaches the value of 20 phr, the elongation at break dramatically increases from 5.11% to 319.12% for PLA, representing a significant increase of 61-fold, which marks a substantial improvement in the mechanical properties of PLA composites. An analogous variation in elongation at break has been observed in other studies.^50–52 With the addition of ISN reaches 25 phr, the Izod impact strength reaches 688 J/m, which is approximately 30 times tougher than pure PLA. Combined with the following DSC and XRD analyses, it can be surmised that the toughening effect may be due to the action of ISN nucleating agent. DSC showed that ISN significantly increased the crystallinity of PLA/ISN composites. After a large amount of ISN is introduced into the PLA resin, some polar groups in ISN interact with the polar groups in the PLA molecular chains, forming new interactions. Meanwhile, the non-polar long carbon chains of ISN insert into the PLA molecular chains, causing physical isolation and shielding. This results in a reduction of the extent of entanglement between the PLA molecular chains and an increase in free volume, allowing composite samples to effectively transfer and absorb energy under external impact.

Figure 3.

The mechanical properties of PLA and PLA/ISN composites. (a) Stress-strain curve, (b) Izod impact strength.

Table 2.

Mechanical properties of PLA and PLA composites.

Samples	Izod impact strength (J/m)	Elongation at break (%)	Tensile strength (MPa)	Tensile modulus (MPa)
PLA	24.67 ± 1.52	5.11 ± 2.24	48 ± 1.6	1900 ± 48
PLA/ISN5	22 ± 1	2.47 ± 0.02	43 ± 1.3	2058 ± 54
PLA/ISN10	35.67 ± 4.04	5.78 ± 0.32	24 ± 1.1	1811 ± 49
PLA/ISN15	104.36 ± 12.66	5.92 ± 0.44	20 ± 0.8	1570 ± 42
PLA/ISN20	120 ± 6.1	319.12 ± 10.97	18 ± 0.6	1030 ± 23
PLA/ISN25	688 ± 11.68	302.17 ± 13.75	17 ± 0.3	351 ± 11

Thermal transition

The DSC thermographs of PLA/ISN composites are exhibited in Figure 4 and the relative results are listed in Table 3. Figure 4(a) shows the cooling thermograms of the samples recorded at a cooling rate of 10°C/min. It can be seen that there is a decrease in the Tg of PLA after the incorporation of ISN, which is in agreement with the results obtained by DMTA. It is also observed that neither PLA nor PLA/ISN composites show crystalline behavior during the cooling step. Expectedly, all PLA materials exhibit cold crystalline behavior during the second heating process. As shown in Figure 4(b), the cold crystallization peaks in PLA/ISN composites decidedly shift toward the lower temperature zones with increasing of ISN concentration. With 25 phr ISN addition the cold crystallization temperature of the composite falls to 94.14°C, a marked decline of 12.28°C compared to neat PLA. Generally, the lower T_cc, the better crystallization performance of the composites. In our fabricated PLA composites, ISN can promote PLA molecular chain movement, and effectively improve the crystallization rate. Hence, the X_c of PLA/ISN composite elevates from 7.07% of PLA to 24.59%, an increase of 248%. Concurrently, due to molecular chains of PLA composites possess higher mobility, the melting temperatures of the composites have gradually decreased with increasing of ISN concentration.

Figure 4.

The DSC curve of PLA and PLA/ISN composites (a) cooling process, (b) secondary warming process.

Table 3.

Thermal transitions of PLA composites.

Sample	T_g (°C)	T_cc (°C)	T_m (°C)	ΔH_cc (J/g)	ΔH_m (J/g)	X_c (%)
PLA	60.41	106.42	170.32	31.44	38.07	7.07
PLA/ISN5	57.34	103.54	167.63	30.45	38.25	8.74
PLA/ISN10	52.84	100.38	166.48	30.73	39.52	10.32
PLA/ISN15	47.81	97.16	165.26	29.63	43.39	16.88
PLA/ISN20	44.51	95.67	164.31	27.96	44.31	20.95
PLA/ISN25	41.34	94.14	163.52	18.63	36.67	24.59

XRD

In Figure 5, for the PLA samples, 2θ = 18.5° and 32.6° belong to the characteristic diffraction peaks of PLA crystals. It is clearly seen that the characteristic peaks of PLA and PLA/ISN are not significantly sharp but appear as ‘buns'. Similar with pure PLA, the diffraction peaks of PLA composites do not exhibit a significant shift while the peak area increases to some extent after the addition of ISN. The change is a sign that ISN promotes the crystallization of PLA as evidenced in the DSC result earlies. A conclusion can be drawn that the crystallinity of PLA composites is enhanced, and simultaneously, there are a large number of amorphous zones in both the plasticizer-added composites and pure PLA.

Figure 5.

The XRD curve of the PLA and PLA/ISN.

Microtopography analysis

By impact section SEM graphs, the microstructure changes of PLA materials can be intuitively observed. From Figure 6, neat PLA features smooth and flat impact cross-section, which presents a representative characteristic of brittle fracture, consistent with its poor toughness of 24 J/m. As the amount of ISN added to the blend system increases, the impact section begins to become rougher. As the incorporation of ISN is 25 phr, the impact cross-section of sample becomes more and more rough and uneven, showing the thread-pulling phenomenon, which indicates that more energy is absorbed during the impact process and the resistance to brittleness is significantly improved.

Figure 6.

SEM images of the impact cross-section of PLA and PLA/ISN composites: (a) PLA; (b) PLA/ISN5; (c) PLA/ISN10; (d) PLA/ISN15; (e) PLA/ISN20; (f) PLA/ISN25.

Meanwhile, none of the samples show phase separation in the impact section micromorphology, suggesting that ISN has excellent compatibility with PLA matrix, which may be due to the interaction of oxygen atoms on the ester bonds and heterocycles in the ISN molecules with the PLA chains. The results further showed that different contents of ISN had good miscibility with PLA matrix, which is in agreement with the results in DMTA.

Migration test

The microtopography analysis and DMTA show that ISN is structurally compatible with PLA resin and therefore ISN has a highly plasticizing efficiency. However, the unavoidable migration of small molecule plasticizers in the matrix can negatively affect the mechanical properties and appearance of PLA materials, so surface migration tests are necessary. The migration of ISN is investigated by subjecting PLA and PLA/ISN composites to a migration temperature of 70°C (above the glass transition temperature). Figure 7 shows the weight loss ratios of plasticized PLA samples with different contents after migration tests. It can be seen that the pure PLA also shows a small weight loss after the test, which may be due to a small amount of degradation after prolonged high temperature treatment. The weight loss ratio of the PLA composites increases with the increment of ISN contents, reaching 6.18 % at 25 phr. Above the glass transition temperature, the PLA molecular chains slide more easily, and without the influence of external forces, ISN dispersed in PLA resin will gradually agglomerate together and eventually detach from the PLA matrix due to thermal movement. In addition, PLA/ISN composites with high ISN content tends to transfer ISN towards the pure PLA sheet, which ultimately leads to the result that the higher the ISN content, the higher the migration rate of ISN.

Figure 7.

Migration test of PLA/ISN composites.

Figure 8.

Rheological behavior of PLA and PLA composites: (a) processing balanced torque; (b) complex viscosity; (c) storage modulus; (d) loss modulus.

Processability analysis

The rheological performance of PLA and PLA/ISN composites is analyzed using a torque rheometer and a rotational rheometer. The torque–time curves is exhibited in Figure 8(a). As observed that the equilibrium torque of the PLA composites drops obviously when the addition of ISN climbs. As the quantity added of ISN reaches 25 phr, the equilibrium torque of the composites decreases to 1.9 N·m, which is 64.8 % lower relative to neat PLA. The phenomenon is attributable to the following reasons: the incorporation of ISN causes an impairment in the interaction of PLA molecular chains and improves the flexibility of PLA resin. It results in a decline in viscosity and an enhancement in the fluidity of the composites, thereby reducing energy consumption during processing. The rheological performance of neat PLA and PLA/ISN is sequentially analyzed using rotational rheometer to further understand the impact of ISN on the rheological behavior of the composites.⁵³ As can be seen from the viscosity change curves in Figure 8(b), the viscosity of the composites decreases with the incorporation of ISN. With the angular frequency of 10 rad/s and 25 phr ISN added, the complex viscosity of the blended system reaches 53.44 Pa·s, 94.5 % lower than that of the neat PLA (968.6 Pa·s). Specifically, ISN can impair the interaction forces between the polymer chains in PLA/ISN, while the non-polar long carbon chain in ISN increases the activity space between the molecular chains. This makes it easier for the molecular chains to undergo the relative motion, thus leading to the improvement of the fluidity and processing performance of the composites. Figure 8(c) shows the variation of storage modulus of PLA and PLA/ISN composites with increasing angular frequency. Conspicuously, the deformable resistance of the blended systems with diverse ISN concentrations suffers impairment as the ISN incorporation increases. This is due to the increasing dispersion density of ISN droplets in the PLA molecular chain with increasing ISN addition. Under the influence of ISN, the expanded space between the polymer chains makes slip easier and the elasticity of the mixture gradually decreases, ultimately resulting in a gradual reduction in the storage modulus. Meanwhile, when PLA composites are exposed to external stresses, part of the energy is expended by internal friction, which is often referred to as the loss modulus. Figure 8(d) shows the loss modulus versus angular frequency for PLA and PLA/ISN composites. From the figure, it ought to be observed that the loss modulus of PLA/ISN declines with the increment in the concentration of ISN. At an angular frequency of 10 rad/s, when the ISN content in PLA resin increases from 0 phr to 25 phr, the loss modulus drops from 9659 Pa to 534.4 Pa, a decrease of 94.4%. Specifically explained as the polar part of ISN binds to the PLA molecular chain, while the non-polar part plays an important lubricating role in composites. As a result, the interaction force and the friction between the polymer chains reduce, suppressing the energy dissipation when the material is subjected to irreversible viscous deformation, which results in a gradual reduction of loss modulus.

Plasticizing mechanism of ISN

Figure 9 shows a schematic diagram of possible plasticizing mechanism in PLA composites. Due to the induction effect, the electron cloud of carbon-oxygen bonds shifts towards oxygen atoms with stronger electronegativity, leading to a negative charge on the oxygen atom and a positive charge on the carbon atom. During the melt blending process, the oxygen atoms in the heterocycle and ester group of ISN form new interaction forces with the positively charged carbon atoms on the PLA chain, which gives a disservice to existing electrostatic interactions in the PLA chains. Simultaneously, owing to the sensitivity of the electrostatic force to the distance between atoms, when ISN inserts into the PLA chain, the non-polar long carbon chain plays a role in increasing the free volume and shielding electrostatic interactions between PLA chains, so that PLA composites can commendably transfer and absorb the energy when given the external forces. Moreover, the polymer chains still have high activity at lower temperatures, which manifests as a reduction in Tg.

Figure 9.

Possible plasticizing mechanism of ISN on PLA.

Conclusion

The rigid PLA is successfully plasticized by bio-based ISN through melt blending. A significant decline in glass transition temperature is observed in the PLA/ISN composites, indicating that ISN has a discernible plasticizing effect on PLA resin. The results demonstrate that when the content of ISN is 25 phr, the composite shows the best mechanical properties. Specifically, the impact toughness arrives at 688 J/m, and simultaneously, the elongation at break is 302%, which is 28 and 59 times higher than pure PLA, respectively. Besides, ISN can obviously improve crystallinity excluding impaired crystal structure, and at same time, the PLA composites become easier to process, as analyzed from DSC, XRD, and rheological behavior. Microscopic morphological observations show that the brittle-to-ductile transition occurs in PLA composite materials and phase separation is not discovered, which indicates that ISN and PLA are completely compatible.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work is financially supported by the Project of Jilin Provincial Department of Science and Technology (20240301030GX).

ORCID iD

Liang Ren

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