1. Introduction
PLA resin is a biodegradable polymer material prepared by polycondensation reaction using lactic acid as the main raw material, which can be extracted from natural renewable resources such as corn and wheat [
1,
2,
3]. The production process has no pollution and the production energy consumption is only equivalent to 20% to 50% of the traditional petrochemical products, while the carbon dioxide gas produced is only 50%, which can get rid of the dependence on oil resources. After being discarded, PLA products can be decomposed in the natural environment and eventually degraded into water and carbon dioxide [
4], without any pollution to the environment and realizing its circulation in the natural world. The molecular chain structure of PLA is shown in
Figure 1. It is considered to be the most widely used type of biodegradable polymer material and has received wide attention in recent years.
PLA resin is a thermoplastic aliphatic polyester with a glass transition temperature between 55 and 65 °C, whose melting point is about 175 °C and whose processing temperature is generally controlled between 170 °C and 230 °C, which is suitable for extrusion, blow molding, stretching and other processing processes [
5].
PLA film has high crystal transparency, stable physical properties such as good solvent resistance and insolubility in alcohols, fats, hydrocarbons and edible oils. Moreover, there is a lower temperature heat sealing ability, better printability than polyolen films and good ink retention, which can retain the flavor and package aesthetics of foods to a greater extent. It makes PLA resin a good choice for replacing fossil-based plastics generally used in food packaging industry [
1,
6,
7,
8,
9].
PLA resin also inevitably has certain drawbacks, including its low gas barrier properties compared to high-performing conventional plastics, its low thermal stability and its high price still restrict its use in the market [
1,
9,
10,
11,
12], and these deficiencies needs to be overcome if PLA is to be successfully used in packaging or other applications requiring good barrier performance.
A barrier is an important standard to measure packaging materials such as moisture permeability (WVTR) and oxygen permeability (OTR). In recent decades, many researchers have proposed a variety of feasible methods to improve the barrier performance of polylactic acid. There are usually three physical methods: polymer blending modification, polymer filling modification and surface coating of film.
For polymer blending modification, Sun et al. [
13] prepared the films by blending poly(ε-caprolactone) (PCL) and PLA, and the barrier performance of the films were improved by controlling the PCL content. Yu et al. [
14] improved the barrier properties of PLA by incorporating of nanocelluloses (NCs) and focused on the influence of the differences in NC morphology and dimensions on the PLA properties. This comparative study was very beneficial for selecting reasonable nanocelluloses as nucleation/reinforcing agents in robust-barrier packaging biomaterials with outstanding mechanical and thermal performance. Le Gars et al. [
6] also filled NCs into PLA; in contrast, the nanocellulose acts as an inner layer between the two PLA films to enhance the oxygen barrier in their research.
The blending of PLA with immiscible organic compounds such as polyethylene (PE) and polypropylene (PP) has also become a common way to improve the properties of polylactic acid but requires compatibilizers to achieve stable morphologies and superior mechanical properties. Thurber et al. [
15] showed that catalyst localized at the interface can compatibilize polyethylene (PE) and polylactide (PLA) blends. In this work, compatibilized blends of HDPE (high-density PE) and PLA are created using modest amounts of hydroxyl functional PE. The molecular chain structure of PE is shown in
Figure 2. This branching of PE would give an impact on the entanglement of PE with PLA, which would change the mechanical strength of blending film.
In Xu et al. [
16], polylactide (PLA) was melt-blended with either polypropylene (PP). An ethylene−glycidyl methacrylate−methyl acrylate terpolymer (Lotader) was utilized as compatibilizer through coupling to the end groups of PLA. The remarkable efficacy of PEGMMA as a reactive compatibilizing agent allows the bridging of two immiscible but important classes of thermoplastics, polylactide and polypropylene, and the production of ductile PLA/PP blend materials.
PP, PE and other auxiliaries are added to the raw materials to achieve biodegradation. However, this view is doubtful; some people call it “pseudo-degradation”, adding auxiliaries can only allow the plastic to be degraded into particles invisible to the naked eye, and its harm to the environment still exists and cannot be completely biodegraded. At present, the main purpose is to reduce the cost.
For polymer filling modification, Jong et al. [
17] fabricated PLA-based composite films with different types of nano-clays as strengthening agent by a solvent casting method. They found that the water resistance of PLA films improved to different degrees. In order to keep the film’s flexibility, the selection of plasticizer is also important. Giuseppe Mele et al. [
18] combined cardanol oil (CA) with PLA and prepared PLA/CA films by means of hot melt extrusion processes. The presence of CA increased the oxygen transmission through the PLA/CA films; consequently, the permeability values were always appreciably higher for plasticized films. Nevertheless, the CA-plasticized PLA films showed good barrier properties similar to packaging materials commonly used in the food industry today.
For surface coating of film, Satam et al. [
19] found that bioavailable cellulose nanocrystals (CNC), chitin nanofibers (ChNF), are readily dispersed in water, enabling spray-coated films to be deposited at high rates onto uneven or delicate surfaces. They sprayed coating of cationic chitin nanofibers and anionic CNC suspensions onto PLA films layer-by-layer to improve the oxygen barrier. ChNF/CNC multilayers were found to lead to a reduction in the O2 permeability of the final composite film by as much as 73% with the largest effects seen in composites with three alternating layers (ChNF-CNC-ChNF). Multilayer ChNF/CNC coatings were found to have lower O
2 permeability and lower haze than those coated with ChNF or CNCs alone (72% and 86% lower haze, respectively), pointing to a synergistic effect. The composites had a water vapour transmission rate similar to the PLA substrate. Mericer et al. [
20] prepared multilayer composite films by coating incompatible microfibrillated cellulose on amorphous polylactic acid and semicrystalline polylactic acid, respectively. The results showed that the oxygen barrier of multilayer films was improved by more than one order of magnitude. Mattioli et al. [
21] deposited hydrogenated amorphous carbon on the surface of PLA thin films by CVD to improve the barrier properties of PLA thin films. They found that the moisture and oxygen permeability of hydrogenated amorphous carbon/PLA films decreased significantly when the treatment time was 5 min. Hirvikorpi et al. [
22] deposited Al
2O
3 with different thickness onto 40 μm thick PLA film with a roll-to-roll process. The results show that the PLA/Al
2O
3 structure significantly improves the gas barrier of the film.
Our team’s research starts from three aspects. Firstly, a small amount of synthetic resin used for packaging film (LLDPE and HDPE) are selected to modify PLA ‘s brittleness without reducing its rigidity. Due to the different polarity between PLA and PE, there is a compatibility problem; therefore, GMA-grafted ethylene propylene copolymer (EPDM-g-GMA) is selected to improve the toughening effect. Secondly, PLA was modified by filling with nano calcium carbonate, zeolite and plasticizing with Epoxidized soybean oil (ESO). Finally, for the anisotropy and non-uniformity of surface thickness of blown film, BOPLA for tape casting was selected for depositing Al2O3 on it. The mechanical, surface and water vapour barrier properties of PLA film by three modification methods were summarized. The experimental results have reference value for the production requirements of food and drug packaging film.
2. Experimental
2.1. Materials
PLA (Ingeo 4032D, Degree of polymerization:200) was purchased from Nature Works Company (Blair, NE, USA). At a temperature of 190 °C, loading pressure of 2.16 kg and cutting time interval of 30 s, the MFR of Ingeo 4032D PLA was 5.872 g/10 min.
BOPLA film (40 μm) was purchased from Shandong Shenghe Plastic Development Company (Weifang, China).
LLDPE (7042, w: 2.71 × 104) was purchased from China Petrochemical Corp. (Beijing, China).
HDPE (DMDA8007, w: 7.36 × 104) was purchased from Shenhua Energy Company (Beijing, China).
EPDM-g-GMA (E533, grafting rate: 1%,) was purchased from Ningbo New Material Corp. (Ningbo, China). The molecular chain structure of EPDM-g-GMA and GMA (Glycidyl methacrylate) is shown in
Figure 3.
Nano-CaCO3 (Nanometer scale calcium carbonate, with an average particle size of 10–100 nm) was purchased from Haofu Chemical Co., Ltd. (Shanghai, China).
Zeolite (3A molecular sieve, with an average particle size of 2–5 µm) was purchased from Yuanli Chemical Co., Ltd. (Tianjin, China).
Epoxidized soybean oil (ESO) was purchased from Xinjinlong Plastic Additives Co., Ltd. (Guangzhou, China).
Trimethyl aluminum (TMA, purity: 99.99%) was purchased from Sigma-Aldrich Trading Co., Ltd. (Shanghai, China).
2.2. Film Manufacting and Depositing Methods
Pretreatment: PLA resin was dried in vacuum drying oven at 80 °C for 4 h.
Mixed granulation: Raw materials were mixed in different proportions, and the granular resin for blowing film was produced by the extruding part of torque rheometer.
Film sample preparation: The granular resin was then added to Blown-Film part of torque rheometer. The processing temperatures of each heating area are 155 °C, 170 °C, 175 °C, and 185 °C, the rotating speed is 35 rpm, the blow-up ratio is 2.6 and the draw ratio is 5.0. May vary slightly according to different materials.
Surface deposition: Plasma-assisted atomic layer deposition is carried out by input TMA using a self-designed equipment under atmospheric pressure (
Figure 4). The deposition temperature is 80 °C.
2.3. Performance Testing and Characterization Methods
2.3.1. Mechanical Property Test
According to the plastic tensile test method of GB/T1040.2-2006, the longitudinal tensile strength of each sample was tested by intelligent electronic tension tester (XLW (EC)), which made by Labthink Electromechanical Technology company of China. The size of sample is 100 mm × 15 mm, and the tensile speed is 200 mm/min. Each sample was tested more than 5 times and the average value was taken.
2.3.2. Friction Coefficient Test
According to the method of measuring the friction coefficient of plastic film and sheet by GB/T10006-1988, Friction coefficient meter (MXD-01) made by Labthink Electromechanical Technology company of China was used to measure the friction property. Two samples with specifications of 200 mm × 80 mm and 63 mm × 63 mm were cut and moved relatively at a consistent speed. The force values were recorded and calculated. Each sample was tested more than 5 times, and the average value is taken. For the calculation method of friction coefficient, see Formulas (1) and (2):
where
μs is the coefficient of static friction,
Fs is the static friction with N as units,
Fp is the normal force with N as units.
where
μd is the dynamic friction coefficient,
Fd is the dynamic friction with N as units,
Fp is the normal force with N as units.
2.3.3. Contact Angle Test
The standard Deionization Water Drops were added to the surface of the tested sample with a micro sampler to form drops. We waited for 3 min until the drops were stable and then measured the contact angle between the sample and water. Static drop contact angle measuring instrument (DSA100) made by Kruss company of Germany was used to measure the contact angle of pure water on PLA film. Each sample was tested more than 5 times and the average value was taken.
2.3.4. Moisture Barrier Property Test
A: According to the test method for water vapour transmission of plastic film by GB/T 30412-2013, moisture permeability instrument (PERMATRAN-W 1/50G) made by MOCON company of USA was used to measure the moisture permeability on modified PLA film. The working principle is shown in
Figure 5. The size of sample is 50 cm
2 and the experimental temperature is 38 °C. Each sample was tested more than 5 times and the average value was taken.
B: According to the test method for water vapour transmission of plastic film (Cup method) by GB/T 1037-1988, constant temperature and humidity testing machine (GT-7005-T) made by the Gotech testing instruments (Dongguan) company of China was used to measure the moisture permeability on PLA film. The sample is a disc with a diameter of about 40 mm. The experimental temperature and relative humidity were 38 °C and 90%, respectively. The moisture permeable cup needed for the experiment is shown in
Figure 6. Each sample was tested more than 5 times and the average value was taken. The permeability is calculated using Equation (3) below,
where
Q is the Water Vapour Transmission Rate (WVTR), the unit is g/(m
2·24 h), ∆m is the amount of vapour passing through the film per unit time, the unit is g, t is per unit time, and A is the effective area of sample; the unit is m
2.
4. Conclusions
Blending, filling, both filling and plasticizing, and depositing Al2O3 all can keep or increase the longitudinal tensile strength of modified film, but the improvement of elongation is not obvious. The surface properties characterized by the friction coefficient show that most of the film meets the requirements of high-speed filling. Although filling reinforcement can improve the performance of packaging materials, food contact materials containing nano materials and plasticizers may cause safety problems, such as the inherent toxicity of materials and the risk of migration from materials to food and possible ingestion by consumers. Stretching oriented technology and plasma-assisted atomic layer deposition of Al2O3 is an effective tool to enhance the barrier performance of PLA and expand its application in the future. Surface deposition can significantly reduce WVTR of PLA, with a reduction range from 79.57% to 99.36%, while other methods can reduce it to a small extent, with a reduction range from 37.11% to 74.30%.
However, the preparation of ultra-thin solid Al2O3 barrier layer on PLA surface is still challenged by the very low glass transition temperature of PLA. In addition, whether the deposition of Al2O3 will affect the optical and degradation properties of PLA films is also one of the problems to be considered.