The influence of a highly complex modified commercial rosin resin (Unik Print, UP) on the thermomechanical performance of four commercial grades of polylactic acid (PLA) has been evaluated and compared. Comparative experiments were carried out with polylactides of different molecular weights and phase structures. The melt-extruded formulations were prepared by considering 3 parts per hundred resins (phr) of modified rosin resin, which was previously verified to be the suitable amount of UP resin effective to enhance PLA performance. Several analytical characterization techniques were used for comparison purposes. Among them, the thermogravimetric analysis allowed to determine that UP resin does not influence PLA's thermal decomposition behavior, regardless of PLA molecular weight and crystallinity degree. Differential Scanning Calorimetric (DSC) evaluation showed that UP resin eliminated both exothermal and endothermic peaks of amorphous PLA. At the same time, it was proved that the formation and growth of different types of crystal can be promoted in semi-crystalline PLA. Moreover, a toughness improvement was observed in all formulations. Besides, the rotational rheometer allowed to measure the viscosity of the final materials, finding that in amorphous PLA with low molecular weight, the UP resin did not cause apparent changes. However, the complex viscosity was increased for both semi-crystalline PLA (low and high molecular weight).
Research Article
Influence of phenolic free modified rosin resin on the thermomechanical behavior of poly (lactic acid) having different crystallinities and molecular weights
https://doi.org/10.21203/rs.3.rs-4325240/v1
This work is licensed under a CC BY 4.0 License
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UP resin
Unik Print
Poly (lactic acid)
molecular weight
modified rosin resin
phenolic free
Plastics are materials with countless applications nowadays; however, due to constantly increasing demands and market growth, the disposal of conventional plastics represents a high risk of pollution [1]. Biopolymers are naturally derived or biodegradable materials with recently growing applications on the market. They are advantageous due to their easier recyclability, reduction of fossil fuel usage, renewability, and lower energy consumption in production processes. Proper modifications of biopolymers allow them to be applied for food packaging, agriculture, or domestic applications as an alternative to conventional, petroleum-based polymers. Polylactic acid (PLA) is one of the most abundant bio-based and biodegradable plastic materials with favorable properties and various applications [2–4]. The material is obtained from lactic acid through catalytic reaction and different reaction parameters, such as temperature, pressure, pH, or polymerization time, which also influence the properties of the resulting PLA. Additionally, different degrees of the crystalline structure of PLA are obtained with different enantiomer content during the polymerization reaction [5]. The variety of PLA types with a broad range of molecular weights and the presence of crystalline structure makes it often challenging to choose the appropriate PLA type for specific applications since different PLA may affect final properties after polymer matrix modification [1].
Gum rosin is a naturally derived resin obtained from pines and conifers, with growing attention for its applications due to its low cost, favorable availability, and biodegradability. Rosin, or colophony, is a non-volatile fraction of pine tree-derived fluid oleoresins. Rosin composition consists mainly of resin acids, monocarboxylic diterpenoid acids, and neutral components, like diterpenoid alcohols or aldehydes [6, 7]. To enhance rosin resin properties, they are often modified with alkyl phenols or formaldehyde, among others. However, these modifications are not desired due to human health risks and increasing environmental awareness. Therefore, new alternative approaches of phenol-free modifications are currently being studied, such as ones based on acrylic acid or maleic anhydride. A phenol-free rosin resin is known to have weaker permeability than phenol-modified resin. For instance, phenol-free resin showed satisfactory results for coating applications. Gum rosin can be applied to paints, coatings, bioplastics, or soldering [6, 8].
Previous works have been conducted to investigate gum rosin resin and its derivatives as additives for biopolymers. Pavon et al. [9] created bilayer films based on poly(ε-caprolactone) coated with electrospray gum rosin microspheres. Aldas et al. [10] used rosin resin and pentaerythritol esters of gum rosin to modify thermoplastic starch based MaterBi biopolymer. Adding gum rosin to the multiphase biopolymer, besides improving material performance, was beneficial to increasing miscibility between biopolymer matrices and enhancing compatibility between biopolymeric phases. Furthermore, previous research showed that gum rosin and modified pentaerythritol ester of gum rosin tend to lubricate PLA molecular chains, resulting in favorable processing properties [2][11].
In this work, the research aim is focused on developing PLA and phenol-free rosin resin formulations, unveiling the potential of the rosin derivative in improving the PLA thermomechanical performance. To this purpose, different types of PLA and the rosin resin derivative (UP) were mixed by extrusion method in concentrations of 3 parts per hundred resins (phr). In addition, characterization techniques, such as infrared spectroscopy, electron scanning microscopy, thermogravimetric analysis, mechanical test, and differential scanning calorimetry, were used to identify the most relevant changes for PLA matrices after modification.
2.1 Materials
Four commercial grades of biodegradable poly (lactic acid) with molecular weight between 245 Kg mol− 1 and 59 Kg mol− 1 [12–15] in pellet form were used as polymeric matrices to run the experiments: two grades of PLA Luminy® (LX-175 and L130) purchased from Corbion Purac (Amsterdam, Netherlands), and two grades of PLA Ingeo™ (2003D and 6201D) obtained from NatureWorks LLC (Minnetonka, USA).
The main physical properties and molecular weights of selected PLAs are shown in Table 1. A complex phenolic-free modified rosin resin under the trade name Unik Print™ 3340 (UP), kindly supplied by United Resins - Produção de Resinas S. A (Figueira da Foz, Portugal), was used to produce the blends. UP is a maleic anhydride and fumaric acid modified-rosin resin with a softening point of 135°C, acid value < 35 mL KOH/g, and viscosity in the 15–30 Pa·s (23°C, 25 s− 1).
| Physical properties | Commercial Name | |||
|---|---|---|---|---|
| Luminy® LX-175 | Luminy® L130 | Ingeo™ 2003D | Ingeo™ 6201D | |
| Molecular weight (Kg mol− 1) | 245b | 170b | 120b | 59b |
| Density (g/m3) | 1.24a, b | 1.24b | 1.24a, b | 1.24b |
| Melting temperature Tm DSC (°C) | 155a | 175a | 145-160a | 155-170a |
| Glass transition temperature Tg DSC (°C) | 60a | 60a | 55-60a | 55-60a |
| MFI (210°C/2.16Kg) (g/10 min) | 8b | 23a | 6a, b | 15-30b |
| MFI (190°C/2.16Kg) (g/10 min) | 3b | 10b | Not detailed | Not detailed |
| Stereochemical purity (% L-isomer) | 96a | min. 99a | 95.7b | 98.6b |
a provided by the supplier in the product technical data sheet.
b literature information [15, 16]
2.2 Blends compounding and sample preparation.
Binary blends of PLA and UP resin were produced by melt compounding. UP resin derivative was cracked into small fragments (average size 2 mm in diameter). The UP resin grains were blended with the different PLA grades in a concentration of 3 parts per hundred resins (phr), which was previously found to be the maximum quantity of UP resin efficient to enhance PLA performance before saturation [17]. Formulation labeling is indicated in Table 2. Several steps occurred, from the blend's preparation to obtaining the standard specimens for characterization. The first step was drying all materials at 50°C for 24 h in a dehumidifier oven D-82152 from MMM-Medcenter GmbH (München, Germany) to reduce the moisture and avoid the hydrolysis of PLA during processing. In the second step, materials were manually premixed in plastic bags. Subsequently, they were dosed by a KQ-X2 single-screw gravimetric feeder from K-Tron/GmbH (Niederlenz, Switzerland) into a twin-screw compounder extruder (24:1 L/D ratio) from Haake Rheocord (Karlsruhe, Germany) with a temperature profile of 180/180/175/170/160/60°C (from the material outlet nozzle to the feed hopper) at a screw speed of 20 rpm. In a third step, the extruded formulations were shaped into standard test specimens (dumbbell and rectangular shape) by injection molding, following the guidelines of ISO 527-2 [18] and ISO 179-1 [19], respectively. The injection process was carried out in a Sprinter 11t injection-molding machine from Erinca-S. L (Barcelona, Spain), with a temperature profile of 175-180-185°C, setting the injection time at 2s and the cooling time at 40s.
| UP content (phr) | 0 | 3 |
|---|---|---|
| Samples label | PLA (LX-175) | PLA (LX-175)-UP(3phr) |
| PLA (L130) | PLA (L130)-UP(3phr) | |
| PLA (2003D) | PLA (2003D)-UP(3phr) | |
| PLA (6201D) | PLA (6201D)-UP(3phr) |
2.3 Techniques for characterization
2.3.1 Thermal characterization of PLA's blends with UP resin
The thermal stability of the processed samples was evaluated by thermogravimetric analysis using a Seiko Exstar 6300 TGA analyzer (Tokyo, Japan). The weight changes of samples (13–15 mg), placed in standard alumina crucibles, were monitored and recorded by performing a dynamic heating cycle from 30°C to 700°C at a constant heating rate of 10°C/min in a nitrogen atmosphere (30 mL min− 1). The onset degradation temperatures (T5%) of the samples were determined at a 5% loss of their initial mass, whereas the temperatures of the maximum degradation rate (Tmax) were determined from the corresponding peak of the first derivative of the TGA curves (DTG).
DSC tests were conducted in a Q200 calorimeter from TA Instruments (New Castle, USA). 8 mg average weight samples sealed in standard aluminum pans of 40 µl were subjected to three cycles of dynamic thermal analysis program: (1) heating cycle from 30°C to 190°C, (2) a cooling cycle from 190°C to -30°C, and (3) a second heating cycle from − 30°C to 200°C. Tests were made at a heating/cooling rate of 10°C min− 1 under an inert N2 atmosphere (30 mL min− 1). The crystallinity degree (Xc) of PLA's and PLA's blends was reported and calculated using Eq. 1:
$${X}_{C}= \left[\frac{\varDelta {H}_{m}-\varDelta {H}_{cc}}{\varDelta {H}_{m}^{0}\bullet (1-w)}x 100 \right]$$
1
Where\(\varDelta {H}_{m}\) ∆Hm is the thermodynamic melting enthalpy (Jg− 1) of each sample taken from the thermal curves of the second heating cycle, ∆Hcc is the cold crystallization enthalpy (Jg− 1), and ∆H0m is considered as the theoretical melting enthalpy of a 100% crystalline PLA, i. e., 93.0 (Jg− 1) [20][21], and \((1-w\)(1- w) corresponds to the weight fraction of PLA in the sample blends.
2.3.2 Spectroscopic Analysis
The interaction of the different PLA matrices with the UP resin was examined by Fourier Transform Infrared Spectroscopy Analysis (FTIR) using a JASCO 615 plus spectrometer (Easton, MD, USA). The sample spectra were recorded with 118 consecutive scans at 4 cm− 1 resolution in the wavelength between 4000 and 600 cm− 1 overwritten in a background spectrum, previously registered to compensate for the humidity effect and presence of carbon-dioxide in the air.
2.3.3 Mechanical properties
To evaluate the influence of UP resin incorporation on the mechanical properties of the different PLAs as a function of their molecular weight, tensile tests were performed using a universal electronic tensile-tester ELIB 30 from S-A-E.-Iberstest (Madrid, Spain) by setting a cross-head speed of 10 mm min− 1, with a load cell of 5 kN according to ISO 527-2 guidelines [18]. Tests were conducted in five standard testing specimens (dumbbell-shape "1BA") obtained by injection molding. The stress-strain curves obtained in the uniaxial tensile tests are reported as test results. In addition, the toughness values calculated from the area under the stress-strain curves are reported.
2.3.4 Rotational Rheology
The rheological characterization of different PLAs and their corresponding formulations with UP resin was obtained by using an Ares N2 rheometer from Rheometric Scientific (Reichelsheim, Germany), with parallel plates geometry (25 mm diameter) at 1.5 mm gap. The dynamic temperature ramp was performed by heating from 160°C to 250°C at a rate of 3°C min− 1, a frequency of 1 Hz, and a maximum strain (γ) maintained at 3%, which was previously verified to be in the linear regime of the viscoelastic response of the materials. The verification of the linear regime was done by a dynamic strain sweep test, performed at 185°C covering the strain range of 0.5-8% with a strain increment of 0.5%. The complex viscosity (η*) as a function of the temperature is reported.
2.3.5 Morphological evaluation
The morphology of the cross-section surfaces of the different PLAs and their corresponding formulations with UP resin was observed and characterized by Field Emission-Scanning Electron Microscope (FESEM), using a microscope ZEISS SUPRA 25 (Germany) operated at 2 kV. Prior observation, all samples were coated with gold to increase their surface conductivity, on an automatic Sputter Coater Agar-B7341. Images were evaluated using secondary electrons.
3.1 Evaluation of thermomechanical behaviour
It is well known that the thermal degradation of PLA is triggered due to the susceptibility of the ester groups to temperature, so a random backbone scission reaction occurs [22, 23]. When analyzing the thermal degradation behavior of the different types of PLA, no variation in the thermal decomposition patterns was observed (see Fig. 1) in accordance with that previously reported by Atalay et al. [24], who studied and compared the thermal behavior of polylactides with different structure type and molecular weight. As a result, they stated that thermal degradation behavior is independent of the D-L enantiomers content and molar mass. On the other hand, after studying the thermal degradation of the different types of PLAs when the modified rosin resin is incorporated in low concentration (3phr), it was observed that regardless of the PLA grade used in the formulations, the modified rosin resin (UP) did not influence the thermal degradation kinetics of PLA, as shown in Fig. 1, Fig. 2, and thermal parameters shown in Table 3. These parameters show that both the onset and maximum degradation values remain constant without apparent changes. A similar result was reported by Wan et. al [25], who studied the thermal stability of a PLA grade with an amorphous structure by producing binary mixtures of PLA with amorphous cellulose and PLA with crystalline micro-cellulose. In both cases, it is stated that the added reinforcements do not significantly influence the thermal degradation of the PLA under study. To this fact, it is assumed that the above-mentioned components and the quantity they have been used do not influence the thermal behavior of polylactide polymers.
When comparing the TG curves of the different PLA types without resin with the respective TG curves of the PLA formulations with UP resin, the same pattern of thermal decomposition was identified. Therefore, it is assumed that UP does not alter the thermal stability of PLA, regardless of polymer crystallinity degree or molecular weight. Even more, it was observed that the percentage of remaining residue, at 600°C, is similar for each material and comparable to the residual material of different unmodified PLAs.
In addition, the developed PLA-UP formulations showed low interfacial adhesion and the absence of new chemical bonds (as later reported in the spectroscopic and microscopic analysis). Therefore, no significant changes at the chemical level that could generate an improvement in the thermal stability of PLA were found. On the other hand, other components, such as coupling and crosslinking agents or chain extenders, are able to improve the thermal stability of PLA, by preventing chain scission and promoting branching of the polymer structure, according to what was previously reported by Hung et. al [26].
Several factors can influence polylactic acid response to temperature conditions, among them the content of enantiomers (D-lactic acid and L-lactic acid), due to the polymerization process [27]. To this effect, the DSC curves that characterize each type of selected PLA were registered, and the influence of UP resin on the main thermal transitions of the different PLAs matrices has been evaluated, as shown in Fig. 3.
Here in, characteristic curves of amorphous PLAs are observed for PLA (LX-175) and PLA (2003D), with broad exothermic peaks between 110°C to 145°C and 120°C to 140°C, respectively, that reveal the limited ability to form crystalline structure of these two PLA grades, due to their high content of D-lactic acid [28]. In the same manner, the reduced endothermic melting peak of each type of PLA is identified, suggesting a limited or non-crystalline structure for the polymer. On the other hand, in the case of semi-crystalline PLA grades, as reported in Fig. 3a, the exothermic peaks of the cold crystallization process of both PLAs are clearly identified, respectively, between 90°C and 110°C for PLA (L130) and between 105°C and 125°C for PLA (6201D), followed by the respective endothermic peaks. When UP resin is incorporated in the different PLA matrices, it was observed that, in the case of amorphous PLA, the broad exothermic peaks were practically eliminated, together with the reduction of the endothermic peaks. This effect was more noticeable for the amorphous PLA with lower molecular weight (PLA (2003D)-UP(3phr)). Meanwhile, for the amorphous PLA with high molecular weight (PLA (LX-175)-UP(3phr)), the exothermic and endothermic peaks are scarcely visible. Based on these results, it can be supposed that UP resin has a hindering effect on the disorganized polymer chains of amorphous PLA, limiting their mobility even more.
Meanwhile, for PLAs with semi-crystalline structure, it was observed that the incorporation of UP resin had a more significant impact on semi-crystalline PLA with lower molecular weight, PLA (6201D)-UP(3phr). This effect is noted in the displacement of the cold crystallization event at lower temperatures (shift of the crystallization peak temperature from 125°C to 111°C). To this fact, it can be supposed that UP resin contributed to the growth and formation of a new type of crystals in the formulations with semi-crystalline PLA with lower molecular weight (Fig. 3b), evidenced by the increased intensity and displacement of the Tcc peak at a lower temperature, together with the appearance of a small shoulder before main melting peak [29]. Regarding the semi-crystalline PLA with higher molecular weight, PLA (L130)-UP(3phr), it was observed a slight increase of the cold crystallization event at about 3°C, with no changes in the melting peak. In other published works, Piekarska et. al [30] reported an increase in the exothermic peaks of amorphous PLA and, consequently, an improvement in cold crystallization when incorporating calcium carbonate as a filler, a contrary effect to what was observed in the behavior of the amorphous PLA of this study. Perinović et. al [31] describe the influence of magnesium hydroxide as a filler in the semi-crystalline phase PLA matrix. As a result, they state that this filler prevents the crystallization of PLA by reducing the formation of crystals, evidenced by the decrease in the enthalpy of cold crystallization. It is worth noting the difference that exists between the additives used as fillers in the aforementioned case of studies (calcium carbonate and magnesium hydroxide) and the rosin derivative additive used in this work. Thus, depending on the additives' nature, composition, and chemical structure, the effect induced on the polylactic acid polymeric matrix must vary.
Finally, in the degree of crystallinity (Xc), as expected, the amorphous PLA presented values of 0.4% and 1.6% for low molecular weight PLA and high molecular weight PLA, respectively. On the other hand, the PLA with a semi-crystalline structure presented values of 14.5% and 10% for PLA of low molecular weight and for PLA of high molecular weight, respectively. Due to the UP resin incorporation into PLA matrices, the degree of crystallinity for both amorphous PLAs was eliminated entirely, resulting in zero percent in both cases. While for PLAs with semi-crystalline structures, the effect of UP resin was utterly different. In the case of the low molecular weight (PLA (6201D)-UP(3phr)), the degree of crystallinity was reduced by more than 75% (promoting the reduction of the rigidity of the formulation, in accordance with the increase in the deformation of this formulation observed in the mechanical characterization). On the other hand, in the case of high molecular weight PLA(L130)-UP(3phr)), the degree of crystallinity increased by 30%.
3.2 Spectroscopic characterization
The analysis of FTIR resulting spectra for all the studied PLA grades and their respective formulations with the UP resin revealed similarities between all the samples. As it is shown in Fig. 4, peaks corresponding to the C-H stretching vibrations present in both the molecular structure of unmodified and modified PLA were observed, approximately at 2950 cm− 1. The C-H bond was confirmed by secondary peaks observed for bending vibrations at around 1450 cm− 1 and 870 cm− 1. Therefore, a slight change in intensity could be noticed between unmodified and modified PLA formulations in the bandwidth. Subsequently, the strong and narrow peak observed at 1750 cm− 1 was related to the presence of C = O stretch bonds. This peak was supported by a secondary peak at 1180 cm− 1, which indicated the presence of ester groups in the formulations. The peak at approximately 1115 cm− 1 indicates C-O-C stretching ester vibration, confirming the structure of the PLA matrix. At around 1500 cm− 1, a peak associated with the presence of phenolic structures was observed. However, the spectra of the PLA-UP- formulations did not show clear evidence of this group, and only a slight variation in bandwidth was noted after the modification of PLA 2003D with the resin. Therefore, FTIR spectra confirmed the limited effect of UP resin on spectroscopic curves of PLA-based formulations and, accordingly, limited, or absent interaction between the modified resin and the different PLA matrices [32].
3.3 Mechanical properties
As shown in Fig. 5, the results from tensile tests confirmed a comparable behavior in terms of Young's modulus, elongation at break, and maximum tensile strength for the different studied PLA matrices. Regarding the maximum tensile strength, the results obtained corroborate with the typical values of amorphous [33] and semi-crystalline structures [34] of polymers based on lactic acid. For the amorphous matrices (PLA LX-175 and PLA 2003D), a maximum resistance of around 58 MPa was obtained, while for the semi-crystalline matrices (PLA L130 and PLA 6201D) a maximum resistance was obtained between 63–65 MPa (approximately 12% higher than the maximum resistance values obtained in amorphous matrices).
After UP resin incorporation, some differences were observed in the tensile strength and the elongation at break, see Fig. 5b. These differences were more evident in the PLA formulations with amorphous structures.
The unmodified PLA with an amorphous structure and high molecular weight (PLA LX-175) did not show significant changes after the incorporation of UP in terms of tensile modulus and tensile strength. At the same time, an improvement of the elongation at break by 2.2% was detected (Fig. 5b). In contrast, the amorphous structure and low molecular weight PLA (PLA (2003D)-UP(3phr)), the resistance and tensile modulus were reduced by more than 6.5% and 52%, respectively, while the elongation at break improved by 80%.
On the other hand, PLA grades of high molecular weight and semi-crystalline structure (PLA L130) not only improved the elongation at break by 52% but also, the presence of the UP resin decreased the tensile strength of the formulation by about 15%. Regarding the formulation with PLA of low molecular weight and semi-crystalline structure (PLA (6201D)-UP(3phr)), the tensile strength and modulus decreased by approximately 8% and 42%, respectively, while the elongation at break improved by 50%.
The aforementioned allows us to confirm that the effect of adding UP resin to PLA can differ depending on the molecular weight or the type of structure. According to Thiyagu et al. [35], Comyn [36], and Fong et al. [37], the addition of plasticizers or substances of lower molecular weight may promote the mobility of the polymeric chains by increasing the free volumes between them and, therefore, reducing their interaction. In addition, the higher the molecular weight in a polymer, the greater the length of the polymer chains and, therefore, the lower the number of chain ends. On the contrary, the lower the molecular weight, the shorter the chain length and, therefore, the greater the number of chain ends [38]. In our case, the greater free volume between the polymer chains allowed for the free mobility of the polymer chains. Hence, a notable increase in toughness in the formulation of PLA with amorphous structure and low molecular weight (PLA (2003D)-UP (3phr)) was observed.
In contrast to these results, the study of Aldas et al. [39], which considered formulations of starch and rosin derivatives, showed a decrease in tensile modulus and an increase in elongation at break, even though the amount of rosin additive used was significantly higher (15 wt%). It is well known that, in the crystalline PLA, the amorphous and semi-crystalline regions have different mobility. Amorphous regions have greater mobility and displacement capacity due to their lack of order, while semi-crystalline regions have limited mobility capacity. Therefore, amorphous regions can suffer more significant deformation than crystalline regions [40]. For this reason, the slight increase in the elongation at break of the formulations of the different PLA grades with semi-crystalline structure is attributed to the effect of the UP resin by preventing the rearrangement of the polymeric chains, consequently reducing the crystallinity of the material, and allowing a greater deformation.
The toughness of the different types of PLA and their respective formulations with UP resin was also estimated. Table 4 shows the toughness values calculated from the area under the stress-strain curves. In general, it was observed that the modification of the different PLA with UP resin increased the toughness of the materials. These effects are ascribed to an increase in the free volume between the polymer chains induced by the presence of the UP resin, resulting in a greater mobility of the polymer chains and, consequently, a greater capacity to undergo plastic deformation [41]. However, when comparing the influence of the UP resin on the toughness of PLA having different molecular weights and crystallinity degree, no clear correlation was found, since very similar values were obtained in all cases (in correlation with the increment of elongation at break in all samples after addition of UP resin), except for the formulation with PLA of amorphous structure and low molecular weight (PLA (2003D)-UP(3phr), also presenting higher value of elongation at break.
Compared to the present work, Pawlak et al. [103] reported the modification of a PLA with a semi-crystalline phase and low molecular weight (PLA 6201D) by incorporating maleinized linseed oil. As a result, they described the increase in the toughness of PLA at values of approximately 3000 kJ/m3, a value lower than that obtained in the present study after incorporation of UP resin in the same type of matrix (PLA (6201D)-UP(3phr), 3537 kJ/m3.
|
Unmodified PLAs |
Toughness (kJ/m3) |
Modified PLAs |
Toughness (kJ/m3) |
Toughness increment (%) |
|
|---|---|---|---|---|---|
|
PLA (LX-175) |
2585.2 ± 30.4 |
PLA (LX-175)-UP(3phr) |
3313.9 ± 32.8 |
28.2 |
|
|
PLA (L130) |
2411.5 ± 37.8 |
PLA (L130)-UP(3phr) |
3415.0 ± 41.5 |
41.6 |
|
|
PLA (2003D) |
2372.9 ± 35.6 |
PLA (2003D)-UP(3phr) |
3939.5 ± 52.5 |
66.0 |
|
|
PLA (6201D) |
2654.2 ± 46.7 |
PLA (6201D)-UP(3phr) |
3537.9 ± 37.2 |
33.3 |
3.4 Dynamic rheological analysis
The analysis of dynamic viscosity by parallel plates was carried out in order to evaluate the influence of the UP resin incorporation on the complex viscosity of the different PLA based on molecular weight and crystallinity degree. The analyzes were carried out in a temperature range from 160°C to 250°C, as previously reported in a study [42]. As reported by Domenek et al. [41], it is important to understand the viscoelastic behavior of PLA, to know its processing and flow capacity. Figure 6 shows the tendency of viscosity as a function of temperature for the different types of PLA and their respective formulations with UP resin.
As expected, the dynamic viscosity decreases with the increasing temperature, making the PLA easier to flow. Among the different types of studied PLA, the most viscous (in the entire temperature range evaluated) was the PLA of amorphous structure with low molecular weight (PLA 2003D), see Fig. 6a. Curiously, this result is contrary to that reported by Garlotta [43], who observed higher viscosity in crystalline structure PLA compared to amorphous PLA. Moreover, Naser et. al [44], state that the viscosity level of a crystalline structure PLA is higher compared to an amorphous PLA due to the stronger intermolecular forces produced by the organization of the polymeric chains, which leads to relatively high resistance to flow. Meanwhile, the intermolecular forces of the disorganized polymeric chains in an amorphous PLA are weaker. Therefore, they tend to present low resistance to flow. However, different explanations could be given based on the results obtained in the present study. From one side, it should bear in mind that the different types of PLA evaluated here have different molecular weights. This factor also influences the rheological behavior of polylactide polymers. On the other hand, in most of the studies in which amorphous and semi-crystalline PLA are evaluated in terms of rheological behavior, similar molecular weights are compared [24].
After the incorporation of UP resin (softening point of 135°C), it was observed that each type of PLA behaved differently. As shown in Fig. 6b, the lowest value for resistance to flow was obtained in the formulation of semi-crystalline PLA with low molecular weight (PLA (6201D)-UP(3phr)), but only from 175°C (temperature in which the polymer is completely melted, in accordance with the DSC value). In addition, it was observed that for amorphous phase PLA with low molecular weight (PLA 2003D), the addition of UP resin did not cause significant changes in viscosity.
On the other hand, adding UP resin to the PLA with a semi-crystalline structure increased viscosity, specifically at the beginning of the evaluation temperatures (170°C). For the formulation of semi-crystalline PLA with high molecular weight (PLA(L130)-UP(3phr)), the effect of UP resin was the increase of the viscosity in almost the entire temperature range evaluated, while for the formulation of semi-crystalline PLA with low molecular weight (PLA (6201D)-UP(3phr)), initially, the viscosity increased and then decreased with an inflection point at around 210°C. This increment in viscosity could be associated with the effect of UP resin under temperature conditions, which increases the free volume between polymer chains, allowing less friction between the chains and, therefore, greater mobility. It should be noted that UP is a complex modified rosin resin with amorphous behavior. In contrast, a similar behavior was reported in a previous study where the influence of rosin resin in the PLA matrix was evaluated. As a result, they explain that the increase in viscosity may be ascribed to a mobility change in the PLA chains [45]. Due to the inflection point observed in the formulation PLA (6201D)-UP(3phr), around 210°C, it is important to note that from the perspective of material processing and functionality, the most crucial range is the low-temperature range, since at high temperatures there is a risk of degradation of the material and changes in its properties and structure. Finally, when evaluating each type of PLA individually with their respective formulations with UP resin, it was observed that only the formulation with PLA of amorphous structure and high molecular weight (PLA (LX-175)-UP(3phr)) showed a slight decrease of viscosity in comparison with same matrix sample without resin (PLA LX175).
3.5 Microstructural evaluation
Field emission scanning electron microscopy (FESEM) allowed to evaluate the changes occurring in the microstructural configuration of the different types of PLA after its modification with the UP resin. Figure 7 shows the FESEM images of the samples studied, taken at 500x and 1000x magnification. From the microstructural perspective, slight changes were observed in the microstructure of the different types of PLA and the formulations. A rough surface with some grooves was observed for the unmodified PLA matrices (Fig. 7a, c, e, and g). In contrast, the microstructure of the PLA matrices with UP resin showed fine threads of torn material (Fig. 7b, d, and f), and the presence of possible UP resin microdomains together with little slits in the resulting material Fig. 7f and h). On the one hand, the small tear threads are associated with a ductile detachment of the material because of the increased plastic deformation caused by the UP resin, in accordance with the increase of toughness that occurred in all the formulations. In addition, the microdomains could be associated with small particles of UP resin that have not been fully incorporated into the PLA matrix. According to Lu et al. [213], the microstructural changes in the development of mixtures depend on the intrinsic morphological characteristics of each material and the mixing conditions under which it has been made. Therefore, it is not ruled out that the mixing conditions used gave rise to a low dispersion of the UP resin in the PLA. In addition, the possibility of saturation of UP resin is ruled out since, in a previous study, it was verified that below 3phr, there is no saturation in the PLA matrix [2]. Moreover, there is a possibility that these microdomains have given rise to the phenomenon described as cavitation. This mechanism leads to increased toughness [46], in accordance with the tensile results and the increased toughness values presented by the formulations where the microdomains were observed, see Fig. 7f and h, for PLA (2003D)-UP(3phr) and PLA (6201D)-UP(3phr), respectively.
When evaluating the influence of the UP resin in the different types of PLA matrices, it was observed that the thermal degradation of PLA did not suffer any apparent changes, regardless of its molecular weight and type of structure, leading to assume that molecular weight and crystallinity degree does not influence in the thermal stability of PLA when combining with UP resin. On the other hand, DSC analyses showed that the UP resin eliminated both the endothermic and exothermic peaks of PLAs with an amorphous structure. Meanwhile, for PLA with a semi-crystalline structure and a lower molecular weight, it was observed that the incorporation of UP resin shifted the cold crystallization process to a lower temperature, leading to assume that molecular weight and crystallinity degree of PLA highly influence the thermal transition when combining with UP resin. Moreover, the crystallinity degree of semi-crystalline PLA was slightly increased, whereas the amorphous PLA structure presented a crystallinity degree of zero percent. In addition, UP resin led to a noticeable increment of the toughness in all types of PLA matrices by more than 25%, and especially in the PLA matrix with amorphous phase and lower molecular weight, where the increment was greater than 60%. A slight increase in the viscosity of the PLAs with a semi-crystalline phase was observed despite the differences in molecular weight and crystallinity degree of the studied PLA.
Author contributions
Conceptualization HDR. MA; Formal analysis. HDR. CP. FD; Funding acquisition JLM. MDS; Investigation HRD. CP. FD; Methodology HDR. DP. FD; Project administration JLM. MDS; Resources JLM; Supervision MDS; Validation LT. DP. MR; Visualization HDR. MA. CP; Roles/Writing - original draft HDR. FD. CP; Writing - review & editing HDR. MDS. DP.
Funding
This research was funded by MCIN/AEI/ 10.13039/501100011033 through PID-AEI Project (grant PID2021‐123753NA‐C33 and PID2020-116496RB-C22) and TED-AEI Project (grants TED2021-129920A-C43), and, as appropriate, by "ERDF A way of making Europe", by the "European Union" or by the "European Union NextGenerationEU/PRTR".
Acknowledgements
H. de la Rosa thanks UPV for the grant received through the (FPI-2018-S2-31946) program and the UPV doctoral school for the interchange mobility grant (Resolution. 16/12/21). UPV authors thank United Resins—Produção de Resinas S.A. (Figueira da Foz, Portugal) for kindly supplying the UP resin and for the collaboration in Project nº E! 114728 "Development and demonstration of innovative bio-resin-based polymers for industrial applications" - DDIBIORESIN (Project EUREKA – EUROSTARS 2).
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No competing interests reported.