The development of new materials derived from renewable sources is an objective of high priority on a technological and environmental point of view to reduce dependence on non-renewable sources (oil). Currently, the global consumption of plastics is more than 200 million tons, with an annual increase of approximately 5% [1
]. Although the use of oil for plastics is assessed at about 4% of global consumption, it is anyway important to try to replace them with alternative raw materials. Until now petrochemical-based plastic have been used as packaging materials. This kind of petrochemical plastics are preferred because they show good mechanical performances, heat stability, good barrier properties, high availability, and a relative low cost. Nevertheless, their end-life management is often not easy, because only some plastic items, e.g., those made of only one material, can be selectively collected and recycled [2
]. Plastic films can be recycled but they have a low apparent density. The packages consisting of different materials, e.g., multilayer packages, cannot be easily recycled, but they usually can neither be composted.
The production of flexible films by using compostable plastics can be particularly important because they can be used for preparing low apparent-density packaging, such as pouches or bags, or multilayer packaging by coupling them with paper or board, thus producing fully compostable packaging. Anyway, for addressing this market, it is necessary to overcome the issue of low tear resistance of this kind of compostable films, and allow their production with conventional industrial processing methods, such as blown film extrusion.
PLA is an environmentally friendly polymer because it is compostable and it derives from renewable resources. It shows a high elastic modulus and a high transparency. PLA has also strong limitations: low glass transition temperature, low thermal stability, high brittleness, and low crystallization rate.
The production of PLA presents numerous advantages, as it is renewable [3
], its production consumes high quantities of carbon dioxide [4
], it provides significant energy savings [5
], it is recyclable and compostable [6
], and the physical and mechanical properties can be partially tailored by properly selecting the polymer architecture [8
]. PLA is currently used in rigid containers for water, juice and yogurt. In particular PLA based packaging is used in Europe, Japan and North America [10
]. Currently it represents one of the cheapest bio-based and biodegradable alternative to petrochemical plastics.
To increase the flexibility of PLA it is possible to add a low molecular weight plasticizer [11
] or blending it with a flexible polymer, such as poly(butylene adipate-co
-terephthalate) (PBAT) [13
]. In the latter case, the use of a proper compatibilizer was demonstrated to be important for a better modulation of properties thanks to the achievement of a phase morphology characterized by a lower dimension of the dispersed PBAT phase and an increased adhesion [15
]. The addition to PLA of poly(butylene succinate) (PBS) was also reported for increasing the ductility of PLA based blends [17
]. Recently, the addition of nano-calcium carbonate to PLA/PBS blends allowed for improving their properties maintaining an improved impact resistance, but combined with an improved flexural strength [20
]. Interestingly, PLA/PBS blends with a PLA matrix were found to show a partial shape memory effect [21
] that makes them interesting for producing films for secondary packaging. Furthermore, tricontinuous blends consisting in PLA, PBAT, and PBS (each at 33.3% by weight) were demonstrated to be promising for preparing biobased blends with properties similar to poly(ethylene) [22
]. However, the phase distribution is usually not easy to be controlled and stabilized in ordinary industrial processing. Moreover, PLA based blends are more promising in terms of cost and environmentally friendly, as PLA is fully from renewable sources, whereas PBAT and PBS are partially synthesized, at least up to now, from non-renewable resources. PBS will soon be produced completely from renewable sources as plants for producing succinic acid from biomass have been developed [22
]. In the future, also PBAT may be renewable, as the industrial synthesis of its monomers from renewable resources has been almost fully achieved [23
An important issue in extrusion is the control of crystallization rate, usually low in PLA, as it influences the possibility to rapidly and efficiently cut the extruded strand, thus easily producing pellets. Moreover, in blown films extrusion, it much influences the process stability and the achievement of a stiff film. In order to increase the crystallization rate usually nucleating agents are added [24
] and interesting synergistic effects were noticed by using both a plasticizer and a nucleating agent in PLA [25
]. In particular, Fehri et al. [27
] noticed that the use of oligomeric lactic acid as plasticizer and LAK301 (an aromatic sulfonate derivative) as nucleating agent, were much more efficient in decreasing the half crystallization time of PLA, and, by applying a design of experiment approach, they identified the best composition range where the half crystallization time was minimized.
In applications such as flat die extrusion or blow extrusion, it is important the fine tuning of melt viscosity and melt strength. In these cases, the use of chain extenders, much used in the field of polyester processing [28
], can be fundamental. In some cases, the use of plasticizers with reactive groups having a low number of reactive groups per molecule (two or three) can allow better modulating PLA based compounds viscosity, thus also limiting the migration of the plasticizer from the final film thanks to its partial grafting to PLA [29
]. The use of a compatibilizer bearing epoxide groups that decreases the melt flow rate of PLA based blends was also demonstrated to be effective for improving the processability by blown film extrusion [30
]. On the other hand, the necessity of increasing the melt strength of pure PLA was yet investigated by several authors often using modifiers with epoxide moieties [31
]. The crystallization of PLA is hindered by cross-linking [34
] because of the decreased mobility of polymer chains. As the use of chain extenders having a low number of reactive groups per molecule do not result in extensive cross-linking, the capability of crystallizing of PLA is generally not widely affected [36
] because a moderate branching also results in an increased crystallization velocity. In good agreement, Mallet et al. [37
] combined an epoxydic chain extender with a nucleating agent consisting in talc and N
′-ethylenbis (stearamide). They demonstrated that a higher kinetic of crystallization can also be achieved for chain extended and branched PLA formulated with adequate amounts of nucleating agents and plasticizers. Crystallization induced during process was also observed. Mechanical properties of the optimized PLA-based films obtained by blown extrusion revealed it to be a more flexible and ductile than pure PLA.
The blown film extrusion parameters were also demonstrated to influence the biaxially oriented PLA (BOPLA) film properties. In particular, Jariyasakoolroj et al. [38
] evidenced that when the stretching rate and the draw ratio in biaxial-stretching technique, increased above a certain level, small δ-crystallites of about 10 nm size with isotropic orientation are mainly formed, thus improving the toughness of BOPLA film, which is about four times higher than the PLA film that is produced by conventional stretching.
Although many papers evidence the high importance of producing PLA based films with improved resistance by blown extrusion, there is not a systematic work about the specific improvement of tear resistance as resulting from the modulation of composition in terms of reactive plasticizer and nucleating agents. In the present paper PLA based blends containing PBAT or PBS were employed for producing films by blown film extrusion. A reactive plasticizer was used in extrusion laboratory trials to both increasing flexibility and improve compatibility, then the process was scaled up in a semi-industrial extruder to prepare pellets adding also LAK 301 as nucleating agent. Films were then prepared by blown extrusion. The reactive plasticizer and the nucleating agent were used to achieve a good control of both processability and final mechanical properties of flexible films, with a focus on the tear resistance of films. The results are discussed by considering the morphological and mechanical properties of films that were prepared by blown extrusion.
2. Experimental Section
Poly(lactic acid) (PLA) containing 4,5% of D-lactic acid units, was PLA INGEOTM 2003D from Natureworks LLC, (Minnetonka, MN, USA) having a nominal average molecular weight Mw = 199,590 and a density of 1.24 g/cm3.
Polybutylene succinate (PBS) was PBS GS PLA®
FD92 WD from Mitsubishi Chemical Co., Tokyo, Japan. It consists of poly(butylenesuccinate-co
Poly(butylene adipate-co-terephthalate) (PBAT) was the product F Blend C1200 purchased from BASF, Ludwigshafen (FRG). Polypropyleneglycol diglygidyl ether (EJ) used as reactive plasticizer was EJ-400 Glyether® Resin purchased from Jsi Co., Ltd., Pyeongtaek, Gyeonggi, Korea.
Dimethyl 5-sulfoisophthalate, potassium salt (LAK) used as nucleating agent was the product LAK 301, purchased from Takemoto Oil & Fat Co., Ltd., Gamagoori, Aichi, Japan.
2.2. Blends Preparation
Preliminary extrusion trials in a laboratory twin screw extruder were carried out to evaluate the capability of liquid plasticizer to improve PLA/PBAT or PLA/PBS flexibility and also compatibility (blends prepared: PLA/PBAT 67/33, PLA/EJ400/PBAT 67/10/23, PLA/PBS 67/33, PLA/EJ400/PBS 67/10/23). Then, a successive series of trials was carried out in semi-industrial extruder where the effect of nucleating agent onto mechanical and thermal properties was investigated (blends prepared: PLA/EJ400/PBAT/1%lak; PLA/EJ400/PBAT/2%lak; PLA EJ400/PBAT/3%lak; PLA/EJ400/PBAT/4%lak; PLA/EJ400/PBS/0%lak; PLA/PBS/EJ400/1%lak; PLA/PBS/EJ400/3%lak; PLA/PBS/EJ400/5%lak).
Regarding the blends prepared in miniextruder, all polymers (PLA, PBS, PBAT) in pellets were dried at 60 °C for 48 h, and they were mechanically mixed with a spatula in a glass Becker with liquid plasticizer at room temperature for 10 min. Then, the LAK was added and mixed again for 10 min using the same equipment. The mixtures were then processed with a MiniLab II HaakeTM Rheomex CTW 5 conical twin-screw extruder (Haake, Vreden, Germany). The extrusion was carried out at 190 °C with a screw speed of 100 rpm for a time of 1 min. The molten materials were transferred through a preheated cylinder to the Haake TM Minijet II mini injection moulder (Thermo Scientific, Waltham, MA, USA). The injection moulding of Haake type III specimens (Haake, Vreden, Germany).was carried out at 180 °C, with a holding time of 20 s and a pressure of 700 bar.
Before the extrusion in a semi-industrial twin screw extruder, after drying materials in pellets in an oven for 24 h, they were weighted and mixed mechanically at room temperature. Then, the extrusion was carried out in a Comac EBC 25 HT twin-screw extruder (Comac, Milan, Italy) to obtain granules. The temperatures in the zones 1 to 11 were: 190/175/180/170/185/185/185/185/180/160/165 °C, with the die zone at 140 °C. The screw rate was 200 rpm. The extruded strand is cooled in a water bath at room temperature and reduced in pellets by an automatic knife cutter. The granules, after drying, were filmed in an extrusion film blowing plant. The blown extrusion was carried out in a MAM40 equipment (MAM2, Varese, Italy) at 150 °C with a screw speed of 30 rpm, melt pressure 150 bar. The nip roll was running at the speed of 9 m/min.
The processing parameters investigated in the study are the blow-up ratio (BUR), the draw down ratio (DDR) and the forming ratio (FR). The BUR is the ratio of bubble diameter (
) to die diameter (
), defined by Equation (1).
where the bubble diameter can be evaluated from the lay flat width (LF), i.e.,
. The DDR and BUR are related as follows:
is the die gap and
is the film thickness. The FR ratio provides an indication of the balance of the stretching between main direction, along the long axis of the bubble, (MD) and transverse (cross) direction, around the hoop direction of the bubble, (CD). FR is evaluated as follows
The blowing parameters are shown in Table 1
, along with the processing parameters that are obtained from them.
Differential scanning calorimetry (DSC) was performed by a Q200 differential scanning calorimeter from TA Instruments—Waters LLC, New Castle, DE (USA), equipped with a RSC cooling system and a nitrogen gas purge set at 50 mL/min flow rate was used for all of the calorimetric measurements. Aluminum pans with samples were sealed before measurement. The sample masses of the analyzed blown extruded films varied from 11 to 15 mg. Films were tested after 12 months from their production. They were heated from −50 to 250 °C at 10 °C/min. The specific melting enthalpy of the blown extruded films was calculated for PLA by summing the negative crystallization enthalpy ΔHc
and the positive melting enthalpy ΔHm
related to PLA transition. The ΔH values related to PLA or PBS were obtained by dividing per the weight fraction of PLA or PBS in the blend and multiplying per 100.The resulting value can be divided per the ΔH0PLA
of PLA with 100% crystallinity to obtain crystallinity value (111 J/g from the literature [40
]). For PBS the crystallinity can be estimated by the specific melting enthalpy based on the melt blend baseline and dividing per 110.5 J/g as ΔH0PBS
Dynamical-mechanical analyses (DMA) were carried out using a Gabo Eplexor 100N (Netzsch Gabo Instruments GmbH, Ahleden, Germany) by heating at 2 °C/min from −100 °C to 200 °C. The tests were performed at a frequency of 1 Hz.
Tensile mechanical properties were measured using a 5500R tensometer (Instron, Norwood, MA, USA) with a load cell of 10 kN with software TestWorks 4.0 (MTS Systems Corporation, Eden Prairie, MN, USA) onto Haake type III specimens following the ASTM D638 indications. Samples were tested at two different temperatures. Dog bone samples were tested at room temperature at a crosshead speed of 10 mm/min. Samples in the form of thin strips with uniform width of 10 mm, total length of 100 mm and 0.05 mm thick, were tested at 25 °C, following the ASTM D882-12.
Trouser tear tests (ASTM D1938-02) were performed onto specimens of 2.5 cm width and 9 cm length with two legs having a width of 1.25 cm. The two legs were inserted in the grips at a distance of 5 cm. The test was performed at 250 mm/min.
The morphology of the blends was investigated by scanning electron microscopy (SEM) using a JEOL JSM-5600LV (Tokyo, Japan), by analysing cryofractured sample surfaces, previously sputtered with gold.