Introduction

The use of petrochemical-based polymers has already attracted increasing interest due to the relative environmental concerns and sustainability issues [1, 2]. Many efforts have been taken to replace the application of traditional petro-based polymers by biodegradable polymers which are derived from sustainable resources [3,4,5]. In the process of seeking alternatives, biodegradable polyester poly(lactic acid) (PLA) derived from the fermentation of renewable agricultural crops has attracted amount of interest because it possesses many satisfactory performance [6, 7]. PLA has excellent biocompatibility, high mechanical strength, easy processability and superior transparence compared with most of biodegradable polymers. Generally speaking, PLA has been used as container, textiles, films, and other products. However, the native fragility, slower crystallization rate and relative lower gas barrier properties have limited the widely application of PLA. A lot of research works have been done to make PLA become a perfect substitution of traditional polymers to satisfy much more demands. Apparently, crystallization and toughening modification of PLA are imperative.

Generally, two main stages are experienced during the crystallization process of polymers, that is, nucleation and crystal growth. A primary nucleation process may be either homogeneous or heterogeneous. The relative low homogeneous crystallization rate of PLA decelerates the whole crystallization process [8]. The addition of appropriate additives into PLA matrix can transfer the homogeneous crystallization process into heterogeneous one, as a result, the crystallization rate of matrix can be accelerated accordingly. The additives, which is helpful in nucleation of polymer and it can accelerate crystallization process, are called “nucleating agents.” Nucleating agents served as nucleation sites will help shorten the nucleation induction period and accelerate crystallization rate. With the addition of appropriate nucleating agent, the mechanical strength, thermal stability, and optical properties of semicrystalline polymers can be improved [9]. Inorganic filler talc is a commonly used physical nucleating agent for PLA and the isothermal crystallization half-times of PLA decreased nearly by 65-fold with incorporation of only 2% talc [10, 11]. Poly(d-lactic acid) (PDLA) as a enantiomer of poly(l-lactic acid) (PLLA) is also an effect nucleating agent which can form stereocomplex crystallites at higher temperature upon cooling [12, 13]. These stereocomplexation can simultaneously enhance the crystallization rate and heat resistance of PLA so as to meet the various requirements in many industrial application field [14,15,16]. Metal phosphonate is another effective kind of nucleating agent for PLA with a layered structure that is similar to montmorillonite clay. This kind of synthetic inorganic/organic hybrid material can accelerate both isothermal and nonisothermal melt and cold crystallization rates of PLA [17,18,19]. Bio-based calcium phosphonate can be prepared by mixing phenylphosphonic acid and bio-based calcium carbonate which comes from aviculture byproducts eggshells. This kind of bio-based nucleating agent has been prepared in our previous study and it reveals very significant nucleating effect for PLA materials. Besides, this kind of bio-based additive with characteristic of sustainable resources, favorable price and environmental friendly, has been proven to be an efficient nucleating agent and reinforcing filler for PLA [20,21,22].

Plasticizer can increase chain mobility of PLA and increase its flexibility. There are numerous reports on the use of monomers and oligomers as potential plasticizers for PLA. Lactide-plasticized PLA showed a significant increase in elongation at break (> 200%) [23]. Martin used glycerol, citrate ester, polyethylene glycol (PEG), PEG monolaurate, and oligomeric lactic acid to plasticize PLA and found that oligomeric lactic acid and low molecular weight PEG gave the best results while glycerol was found to be the least efficient plasticizer [24]. Plasticizer also have outstanding effects on the crystallization behavior of PLA. The increased chain mobility will facilitate the movement of chains from the amorphous phase onto the existing crystal surface, especially at lower temperatures. At present, the effect of a plasticizer, such as PEG and poly(propylene glycol) (PPG) on the crystallization behavior of PLA has been widely investigated [25, 26]. Epoxidized Soybean Oil (ESO) is an epoxidized derivative of soybean oil which is a mixture of glyceride with various saturated and unsaturated fatty acids. ESO shows excellent promise as they are derived from renewable, biodegradable, environmental friendly and easily available raw materials [27, 28]. It finds interesting uses in the polymer plasticization industry, as well as in the thermosetting industry as base resins for green composites.

This work deals with the preparation of fully biodegradable PLA composites filled with different content of bio-based fillers. Surface modified eggshell (FES) functioned as nucleating agent and ESO was used as plasticizer. The corresponding mechanical properties of the PLA composites were investigated by static tensile measurement and dynamic mechanical analysis. As well as the cold crystallization behavior including nonisothermal and isothermal crystallization was studied in detail.

Experimental

Materials

PLA 2003D with a weight-average molecular weight of 112,000 g/mol is a commercial available polyester purchased from NatureWorks LLC, it has a polydispersity index of 1.69. FES was prepared according to our previous work [20]. In brief, ball mill (GN-2, SYKY) was used to pulverize the cleansed chicken eggshells to obtain uniform size eggshell powder. After milling, the eggshell powder with average particle size of 2 μm (from SEM observation) was dried in a vacuum oven at 110 °C. Before using, the dried eggshell powder was kept in a desiccator. Mixing the eggshell powder with PPOA at a weight ratio at 5:1, then, the mixture was heated to 190 °C and reacted at this temperature for 5 min to form PPCa on the surface of eggshell. ESO with epoxide content of 6.0 wt% was procured from Shandong, China. The plasticizer was used as received commercially without any further purification and processing.

Preparation of PLA/ESO/FES composites

Before using, all of materials were dried under vacuum at 80 °C, PLA pellets, FES powder, and ESO was weighed according to the compositions defined in Table 1, where the corresponding abbreviations are used consistently hereafter in the paper. Pure PLA and PLA composites were fabricated using a torque rheometer (XSS-300, Shanghai Kechuang Rubber Plastic Mechanical Equipment Co., Ltd., China) at 170 °C with a screw speed of 60 rpm. Then, the samples sheets with an average thickness of 1 mm were hot-compressed at 170 °C under 10 MPa pressure. To study the cold crystallization behavior of neat PLA and PLA composites, the samples were quenched to 20 ± 2 °C in cold water.

Table 1 Composition of PLA composites and labeling
Full size table

Characterization

The field-emission scanning electron microscopy (SEM; FEI Co., Netherlands) was used to observe the micromorphology of these samples at a constant accelerating voltage of 10 kV. Cryogenically fractured surfaces of the composites were prepared to investigate the bulk morphology of the composites. After tensile testing, the morphology of fracture surface was also observed by SEM. In order to increase the conductivity of the samples and acquire high-definition images, all surfaces to be observed were coated with a thin layer of gold.

An Instron 1211 tensile testing machine was used to study the static mechanical properties of neat PLA and PLA composites at a crosshead speed of 10 mm/min. All dumbbell-shaped specimens cut from the quenched sheets with a total length of 50 mm and a valid testing area of 20 × 4 mm2 were subjected to tensile tests at 25 °C after storing for over 24 h under the test conditions, and at least five tests were under taken for each specimen to calculate average values.

Dynamic mechanical analysis (DMA), in tensile mode, was carried out with a DMA+450 instrument (MetraviB, France). Samples with gauge dimensions of 40 × 10 × 1 mm3 were subjected to a dynamic temperature program. The storage modulus (E′) and the loss modulus (E″) were determined at a frequency of 1 Hz and a heating rate of 3 °C/min as a function of temperature from 0 to 130 °C.

The isothermal and nonisothermal cold crystallization behavior of pure PLA and PLA composites were systematically investigated under N2 atmosphere with a differential scanning calorimeter (DSC, TA Instruments Q20) under different crystallization conditions. The instrument was calibrated with a standard indium before measurement. For the nonisothermal cold crystallization process of all the samples, all specimens quenched from melt state (about 7 mg) were firstly sealed in aluminum pans and heated to 180 °C at a rate of 10 °C/min. The isothermal cold crystallization behavior of neat PLA and PLA composites was also investigated by DSC. The quenched samples were held at desired cold crystallization temperature (Tcc) after heating at 50 °C/min. Enough time was kept until the isothermal crystallization was completed. The isothermal cold crystallization kinetics was analyzed from the recorded exothermal traces.

Results and discussions

Effect of FES content on morphology of PLA composites

The morphologies of pure PLA and PLA composites were observed by SEM. Figure 1 show the influence of ESO plasticizer on the degree of dispersion and miscibility of FES powder in the PLA matrix. As expected, pure PLA shows a fairly regular and smooth fracture surface. Empty voids with regular shape are observed in cryo-fracture surface of pPLA sample, indicating that excess ESO as disperse phase presented in the form of droplets evenly distribute within the continuous PLA matrix. In other works, similar morphologies of PLA/plasticizer blends with phase separated are observed [24, 29]. Figure 1c–e, corresponding to the incompatible plasticized PLA/FES composites, clearly illustrates rough fracture surface with homogeneous dispersed FES particles and some small voids caused by excess ESO. With regard to the morphology of PLA/FES composites, the particle size of FES shows slightly increase along with the content increase and some of the voids are filled by aggregated particles. These voids accumulated by ESO still exist in the bulk of plasticized PLA/FES composites which will make contribution to energy dissipation during tensile testing. It is worth noting that the increase in FES content obviously decreases the sizes of voids following with the number of voids increases. That may be result from the relative higher content ESO in PLA composites decrease the viscosity of PLA matrix and the organic component in FES increase the compatibility between PLA and ESO, both of these make ESO more likely to disperse homogeneously. Besides, there are no macroscopic gaps can be observed between FES particles and PLA matrix with the increase in FES content.

Figure 1
figure 1

SEM images of PLA and PLA composites: a PLA, b pPLA, c p10FES, d p20FES and e p30FES

Full size image

Mechanical properties

The tensile stress–strain curves of pure PLA and PLA/FES composites plasticized with ESO are shown in Fig. 2, and the variations in tensile properties as a function of FES content are shown in Table 2. Unmodified PLA with elongation at break (εb) of 6% is a fairly brittle polymer, it has relative high yield strength (61.1 MPa) and modulus (1.24 GPa). Tensile fracture energy (ETF) is a characterization of material tensile toughness which shows the energy absorbed before tensile fracture. ETF listed in Table 2 is calculated as area enclosed by stress–strain curve. The pPLA containing with 10 wt% ESO shows an obvious toughness with εb increased to 159% and ETF increased to 3.0 J (0.2 J for pure PLA). This increment is substantially higher than the reported values for the same ESO content [30,31,32]. The ductile fracture behaviors may follow the mechanism of internal cavitation induced extensive matrix shear yielding, which is a very efficient way of energy dissipation [33]. This may also derive from the strong plasticization effect provided by ESO on the PLA matrix, where the chain mobility is enhanced by these molecules attached to the biopolymer chains acting as lubricant. However, the contained plasticizer has a negative effect for enhancing the yield strength (σ) of PLA (31.5 MPa for pPLA) and modulus (Et) (1.19 GPa). The yield strength and modulus for pPLA have decreased by 49 and 4% compared with those of pure PLA. For PLA composites, the yield strength and elongation at break reduce continuously with increasing FES loading. For example, 30 wt% of FES decreases the yield strength of pPLA from 31.5 to 18.6 MPa, and the elongation at break from 159.2 to 70.4%. The obvious decrease in yield strength is due to the relative content of ESO in polymer matrix increase with FES content increase for PLA composites, and the plasticization effect of ESO makes the yield strength of composites decrease continuously. At the same time, with the content of FES increase, the FES particles that tend to aggregate make the compatibility of the composites worse. As a result, the elongation at break decreases due to the weak stress transfer capacity. For Young’s modulus of the PLA composite shows an uptrend with increasing FES content, a significant increase from 1.19 to 1.79 GPa occurs when pPLA is blended with 30 wt% of FES. Although the tensile toughness of PLA composites is somewhat poorer than that of pPLA, the elongation at break of PLA composites is still higher than 70%, indicating highly toughening efficiency in tensile toughness.

Figure 2
figure 2

Stress–strain curves of pure PLA and PLA composites

Full size image
Table 2 The mechanical properties of PLA and PLA composites
Full size table

Fracture morphology of PLA composites

To explore the fracture behavior of PLA composites in tensile tests, the morphology of the fracture surfaces of the specimens was observed using SEM. Figure 3 shows the tensile fractured surface micrographs of the composites. For comparison, the fractured surface images of pure PLA, pPLA and PLA composites are shown in Fig. 3. As expected, pure PLA shows a fairly regular and smooth fracture surface as before which presents typical brittle fracture. For pPLA, the fracture surface showed a large amount of cavitation and large-scale plastic deformation or fibrillation. This indicates that this sample had higher elongation during stretching and absorbed more energy, which consistent with the results from mechanical testing. As for PLA composites, the fracture surface also exhibits a rougher surface and also shows some voids. With the content of FES increasing, the plastic deformation of PLA composites is weakened. The morphology of the tensile fracture surfaces confirms the mechanical natures of the composites.

Figure 3
figure 3

Fractured surface images of PLA and PLA composites: a PLA, b pPLA, c p10FES, d p20FES and e p30FES

Full size image

Dynamic mechanical properties

The dynamic mechanical properties of pure PLA and its composites were also investigated. Figure 4 shows the relationship between dynamic mechanical parameters and temperature for pure PLA and PLA composites. As shown in Fig. 4a, the storage modulus of pPLA is slightly lower than that of pure PLA at low temperature in the glass state due to the plasticization effect of ESO. To state quantitatively, the values of E′ at room temperature (25 °C) are 3160 and 2820 MPa for pure PLA and pPLA, respectively. For PLA composites, the value of E′ increase slightly compared with that of pPLA with the increase in FES content in the glass transition region, which is consistent with the variation of tensile modulus. However, a considerable increase in E′ is observed for all samples at temperature above 70 due to the crystallization of PLA during heating program. It is clear from Fig. 4a that the modulus enhancement after glass transition for PLA composites takes place at a lower temperature than that of pure PLA, indicating the superior cold crystallization capacity of PLA composites due to the existence of nucleating agent on the surface of the FES. In addition, with temperature increasing from 80 to 110 °C, both pPLA and PLA composites have a higher E′ than pure PLA. When the temperature is over 110 °C, the storage modulus of PLA increases again to be better than that of other composites. The phenomenon may be because the small and/or imperfect crystals tend to form at relative low temperature in PLA composites, which will tend to take place phase transition at lower temperature than pure PLA.

Figure 4
figure 4

a Storage modulus and b loss modulus as a function of temperature for pure PLA and PLA composites

Full size image

Loss modulus (E″) for pure PLA and PLA composites is presented in Fig. 4b. The glass transition is identified as the energy dissipation possibilities across the free amorphous phase. And at the temperature where E″ is maximum indicating the maximum heat dissipation is also taken as Tg of the system [34]. The Tg of pPLA and all the PLA composites with plasticizer are lower than that of pure PLA due to the plasticization of ESO. Plots of E″ also reveal that the transition region of the PLA composites has been broadened with the incorporation of FES. The variation of E″ values is similar with that of E′, it presents a significant drop during glass transition region followed by an increase according to the cold crystallization of PLA. From these results, it can be concluded that FES particles can reinforce PLA and the higher cold crystallization property make PLA composites have a greater temperature resistance.

Nonisothermal cold crystallization behavior

The nonisothermal cold crystallization behavior of all samples was investigated according to the obtained DSC thermograms. Representative DSC heating thermograms of the quenched PLA and composites samples are presented in Fig. 5. Each thermogram contains the normal glass transition and a typical cold crystallization peak followed by the melting endotherm phenomenon. The influence of FES content on the nonisothermal cold crystallization behavior of PLA and PLA composites is investigated according these thermograms, and the corresponding parameter is listed in Table 3. The degree of cold crystallinity (Xcc) is estimated by comparing the cold crystallization enthalpy (∆Hcc) with the value of an infinitely large crystal (∆H 0m ), taken as ∆H 0m  = 93 J/g. It appears that the Tg values decrease from 57.2 to 48.3 °C as a consequence of the plasticization of ESO. A slightly increase is observed upon the addition of FES. It is also seen that the nonisothermal cold crystallization peak temperature (Tcc) of pPLA is clearly lower than that of PLA, Tcc decreases considerably from 118.7 °C (for pure PLA) to 102.3 °C (for pPLA), suggesting that the cold crystallization ability of PLA is enhanced with the addition of ESO. With further addition of FES, the Tcc gradually decreases to 97.5, 92.8 and 90.2 °C for p10FES, p20FES and p30FES, respectively. The decrease in Tcc indicates that incorporation of well-dispersed FES could enhance cold crystallization rate of PLA due to the nucleation activity of the PPCa on the surface of eggshell. The increased nucleation sites provided by FES also benefit to improve the cold crystallization ability of PLA. The degree of crystallinity shows a slightly increase with the addition of ESO and FES, but then leveled off, or even slightly decreases. It may be because large number of filler particles can hinder the growth of polymer crystals [35]. For the melting behavior of PLA and PLA composites, there are double melting peaks for pPLA and PLA composites compared with a single melting peak for pure PLA at 149.1 °C. All of the major peak for pPLA and PLA composites are similar with pure PLA at about 149 °C, and the other minor peak at about 142 °C is independent with the content of FES. The double melting peaks could also be explained by the melt-recrystallization mechanism [1]. The small and/or imperfect crystals tend to form at relative low temperature in PLA formulations, which have high nucleating and crystallization ability. During melting-recrystallization, the small and/or imperfect crystals successively translate to more perfect crystals [17].

Figure 5
figure 5

DSC first heating thermograms of quenched samples

Full size image
Table 3 Nonisothermal cold crystallization of quenched PLA and PLA composites
Full size table

Isothermal cold crystallization behavior

The isothermal cold crystallization kinetic studies of PLA composites were investigated at different Tcc values of 75 and 80 °C. Figure 6a, b shows the DSC thermograms of pure PLA and PLA composites crystallized at 75 and 80 °C, respectively. The corresponding plots of relative crystallinity (Xt) versus time are shown in Fig. 7a, b, and sigmoidal shapes plots are present for all samples. The overall crystallization kinetics of all the samples are characterized by a key parameter of crystallization half-time, t0.5, obtained from the curves of Xt at Xt = 50%. A long time is demanded to complete crystallization process for pure PLA because of the inferior nucleating ability. It exhibits a t0.5 of about 165 min at 75 °C and 105 min at 80 °C for pure PLA. The chain mobility of PLA macromolecules is enhanced by the addition of plasticizer ESO, giving rise to an obvious decrease in crystallization time for PLA. The t0.5 of pPLA decreases sharply with value of 16.2 and 14.4 min at 75 and 80 °C, respectively. For the effect of FES content, the time required for the composites to finish crystallization decreases further with increasing FES content. For p30FES sample, t0.5 reduces to less than 6 min at 75 °C. And with the crystallization temperature increases to 80 °C, t0.5 of p30FES further drops to 2.4 min. The decrease in t0.5 indicates the addition of ESO and FES is efficient in accelerating cold crystallization rate of PLA.

Figure 6
figure 6

DSC traces of pure PLA and PLA composites crystallized at 75 and 80 °C, respectively

Full size image
Figure 7
figure 7

Relative crystallization plots versus time of pure PLA and PLA composites crystallized at a 75 and b 80 °C, respectively. The illustration in a is the relative crystallization plots versus time of pure PLA

Full size image

The isothermal cold crystallization kinetics of pure PLA and PLA composites was analyzed by the well-known Avrami method. This semi-empirical formulation supposes that the relationship between relative crystallinity (Xt) and crystallization time (t) follows the equation:

$$ 1 \, - X_{t} = \, \exp \, \left( { - kt^{n} } \right) $$
(1)

where the Avrami index n dependents on the form of nucleation and crystal growth, and the growth rate constant k associates with both nucleation and growth contributions. The crystallization process of polymers usually contains two stages, the primary crystallization stage (linear portion) and the secondary crystallization stage (nonlinear portion). However, the semi-empirical Avrami equation is not a first-principles formulation, and it is generally considered to be applicable only for linear portion [36,37,38]. The primary crystallization stage with a reasonable conversion range (3–20%) has been chosen to obtain the corresponding Avrami plots.

Figure 8a, b shows the corresponding Avrami plots of neat PLA and PLA composites at 75 and 80 °C, respectively. The gradients of the plots are taken as Avrami index n, and k is obtained from the gradients of the curves, respectively. All of related Avrami parameters are summarized in Table 4. As revealed in Table 4, the obtained values for n are very close to one another at range from 2.0 to 2.9 at 75 °C and 2.0 to 2.7 at 80 °C. The value of k is representative of the crystallization rate and it continuously increases with the incorporation of ESO and FES. Except for the influence of the additives, the crystallization temperature also has a significant effect on the crystallization rate. The value of k presents a remarkable increase with the increase in crystallization temperature. The increment in k indicates that the crystallization rate of PLA is accelerated from the heterogeneous nucleation since surface functional FES improves higher nucleation density and plasticizer ESO enhances chain mobility of PLA.

Figure 8
figure 8

Avrami plots of pure PLA and PLA composites crystallized at 75 and 80 °C, respectively

Full size image
Table 4 Kinetic parameters of pure PLA and PLA composites isothermally cold-crystallizing at 75 and 80 °C
Full size table

Conclusions

Fully sustainable PLA/FES composites plasticized with 10 wt% ESO were fabricated by melt blending method with different content of FES. The morphology, mechanical and crystallization properties of the PLA/FES composites were investigated in detail. The phase morphology changed from a smooth surface to a porous one with the addition of 10 wt% ESO. FES-filled PLA composites obviously decreased the sizes of voids accompanied with increasing the voids number. ESO as an efficient plasticizer enhanced the toughness of PLA significantly, and the elongation at break increased up to 159% compared to 6% of pure PLA. Besides, FES showed good reinforcing effects on the PLA matrix, both the tensile and storage moduli of composites improved with increasing FES content. The elongation at break of PLA composites decreased with the content of FES increasing, even so, the elongation at break was still higher than 70%. Regarding crystallization behavior, FES with surface modified by PPCa as a good nucleating agent provided the nucleation site for PLA. The rates of nonisothermal and isothermal cold crystallization of PLA composites increased with the addition of FES. Due to the synergistic effect of plasticization and nucleation, the t0.5 of p30FES was less than 6 min compared with 165 min for pure PLA at 75 °C. In addition, t0.5 of all samples decreased with the cold crystallization temperature increasing from 75 to 80 °C. In general terms, ESO and FES as effective fillers exhibit great practicability to prepare a kind of fully sustainable and biodegradable PLA composites with superior tensile toughness and crystallization properties. These results would bring benefit to the polymer materials industrial community, which is working to develop new sustainable blend materials and technology completely derived from renewable resources.