Film extrusion is the most used process for the production of plastic packaging. It consists of an extrusion line that can include one extruder, or several (in the case of co-extrusion), a die and equipment to stretch/blow, cool down, cut or wind the extrudate. The process starts at the extruder throat where the material, in granular form, is fed to the screw. The rotating screw forces the polymer granules ahead in the extruder barrel, which has several controlled heating zones ( ).
The material experiences progressive heating and pressure until melting. This allows the polymeric granules to slowly melt, reducing the risk of overheating, which can result in polymer degradation. After melting, the polymer passes through the die, which shapes the melt into an initial high thickness geometry (annulus, in the case of blown film). Afterwards, and in the blown film case, this thick annulus is stretched in the two main directions: in the machine or longitudinal direction, by the action of the pulling rolls; in the transversal or circumferential direction, by the effect of compressed air inflated through the die. The cooling of the resulting film bubble is carried out by forced air, blown through the air ring ( A). When films are directly produced in the flat shape, the extrusion die has a rectangular shape, and the film is only oriented in the machine direction; in this case, the cooling process is promoted by direct contact with the chill rolls, as presented in B. The conventional extruders used in these extrusion lines are usually of the single screw type ( ). Another type of extruders, having two screws, usually co-rotating and intermeshed (twin-screw extruders), are used for compounding purposes (e.g., to produce polymer blends, to incorporate additives/active agents, to prepare composites and masterbatches). This type of extruders are, therefore, used to prepare the compounds that are fed to single screw extruders, for the production of the final film.
The extrusion process allows the production of blown or flat films with one or more layers (mono and multilayer films, respectively). In the multilayer case, the process used is called co-extrusion, and involves more than one extruder (at least one for each different polymer system to be included in the structure of the film). The various polymers are extruded through a single die, constituting a single multilayer structure at its exit. The remaining downstream equipment is similar to that of a conventional extrusion line.
Another process used to produce multilayer films for flexible packaging is the lamination process, illustrated in . This process combines individual films, which can be polymeric or non-polymeric, into a multilayer structure. Polymeric adhesives (water or solvent-based) are used to bond the different layers. This process can be used in or outside ( A) an extrusion line. Another possibility is the lamination by extrusion, wherein the extruder provides the molten film that acts as adhesive ( B). In order to improve the adhesion of the substrates, a treatment on the film surface, such as corona, can be incorporated in the extrusion line.
Multilayer films have been used for increased barrier properties. Due to the multilayer structure, they can reduce the permeation of gases through the film, and thus avoid changing the headspace composition of the package over time. However, according to the type of food, this strategy is not always the best solution to increase food shelf-life (e.g., fresh and oxygen-sensitive foods). When this strategy is not enough, the incorporation of active agents in one of the polymeric layers can be a solution. This section reports works where the direct addition of active agents to the polymer matrix was employed as a solution. summarizes the details of some studies, such as materials used, the function of the developed packaging, amount of active agent added, and their main effects. The amount of the active substances migrated are also mentioned. Below, these works will be reported, emphasizing the conditions used in the extrusion processes and the parameters that can influence the activity of the active agent, such as the thickness of the films, dispersion of the active agent, and processing temperatures.
For example, Di Maio et al. [23] studied the effect of adding a polymeric oxygen scavenger (OS) (unsaturated hydrocarbon dienes—Amosorb DFC 4020) in a multilayer film using the same polymeric material, namely PET, to apply in fresh fruit. The films were produced by a co-extrusion process using a laboratory cast film extruder with a temperature profile of 285–280 °C. The OS was incorporated in the core layer and the pure PET was kept in the outer layers. Multilayer films with different thicknesses were produced. The authors reported that the active films developed showed a good oxygen scavenging capacity and a longer duration of activity time when the active internal layer presented a higher thickness. On the other hand, the oxygen scavenging rate was consistently lower when the external neat PET layers presented a higher thickness. The results are explained by the diffusion of oxygen through these layers, which needs more time for thicker samples before reacting with the active film. On the other hand, active monolayer films, also considered in this study, were saturated in a few days, which was explained by the fast reaction of the oxygen with the active compounds.
In 2013, Sängerlaub et al. [25] studied a multilayer film with different materials, namely PET, Al, and PE films, to apply in oxygen-sensitive food. The iron was used as an OS and was added to the PE inner layer. The PE active film was produced by extrusion (but the temperatures used were not specified). Afterwards, the lamination process was used to join the PET and aluminum (Al) films using an adhesive. The final structure obtained was PET/Adhesive/Al/Adhesive/PE-containing active agent without and with a sealing layer of PE. They also studied the effect of the OS layer and sealing layer thickness, showing that the thickness of these layers influenced the OS activity. In addition, they studied the effect of the addition of OS on sealing defects, such as small pinholes up to a diameter of 10 and 17 mm, and showed that the OS was able to compensate the sealing defects. Granda-Restrepo, Peralta, Troncoso-Rojas, & Soto-Valdez [52] studied the antioxidant properties of PE-HD/EVOH/PE-LD multilayer films produced by a blown film co-extrusion process (the temperatures used were not specified). Different active agents, such as BHA, BHT, and α-TOC, were added to the inner layer (PE-LD). TiO2 was added to the outer layer (PE-HD) to prevent light transmission through the films, avoiding the use of an Al layer in the film structure. They packaged whole milk powder to perform migration tests and the PE-LD layer with antioxidant agent was put in direct contact with the product. They showed that the structure of these films avoided the loss of antioxidants to the environment and favored the migration to the product. However, they also showed that during the extrusion process the concentration of BHA, BHT, and α-TOC decreased approximately 17, 41, and 23%, respectively, which was explained by the processing temperatures.
Soysal et al. [8] studied the antimicrobial activity of PE-LD/PA/PE-LD multilayer packaging film using different antimicrobial agents, such as nisin, chitosan, potassium sorbate, or silver substituted zeolite (AgZeo). In addition, the authors selected different polymers to combine different barrier properties, namely a good barrier to water vapor that comes from PE-LD and a good barrier to gases that comes from PA. The multilayer film was produced by a blown film co-extrusion process (temperatures used were not specified). The drumsticks were the product selected for the study and singly vacuum-packaged in active films developed, it means that the active agents acted by direct contact with product. The results showed that the incorporation of the antimicrobial agent was an asset to avoid the microbial growth and increase the shelf life of product. They showed that the nisin and chitosan were among all those that reduced the levels of antimicrobial activity. However, the addition of these antimicrobials increased the cost of food packaging (not more than 2%), but they mentioned that this could be compensated by the benefits of increasing the food shelf-life.
Nowadays, the use of multilayer films is common among food packaging solutions, but there are few publications about active packaging based on this type of films. This is probably related to the lack of laboratory co-extrusion lines or to the difficulty in using the high throughput industrial co-extrusion lines. On the other side, there are a lot of studies about active packaging in monolayer films; some of them are presented below and in . There are also studies where the extrusion process was only used to produce compounds, being the films produced by compression molding in a hydraulic press. Since these films are not representative of the industrial ones, these studies were not considered in the present review. As with all multilayer active films, most of these works evaluated the development of antimicrobial and antioxidant activity of films.
Beigmohammadi et al. [47] studied the antimicrobial activity of PE-LD film loaded with Ag, Cu, and ZnO. The authors selected metallic nanoparticles to study their effect on microorganisms’ growth in cheese, since these NPs have significantly reduced the microbial population in other products. PE-LD/NPs blends were produced using a twin-screw extruder, and different compounds were developed. Afterwards, the compounds were processed in a cast film extrusion line and active monolayer films were produced, using a temperature profile of 185–239 °C. Cheese samples were packaged with these films. However, the type of the packaging used (direct contact or headspace) was not specified. Of all active films developed, the one incorporating CuO was the one showing the lower coliform load of the cheese, and not showing any toxicity. Moreover using metallic nanoparticles, Li et al. [49] studied the antimicrobial capacity of PE-LD with Ag/TiO2 nanopowder against Aspergillus flavus and the mildew. First, they prepared a masterbatch incorporating Ag/TiO2 in a PE-LD matrix, using a twin-screw extruder. This masterbatch was later diluted in more PE-LD for the production of a flexible film, by the blown film extrusion process. The temperature profiles used in these extrusion processes were not provided. After, they studied the antimicrobial activity of the films and performed the migration test of Ag+ ion using rice. The type of packaging used (direct contact or headspace) was not specified. The results showed that the small amount of silver migrated from the active films inhibited the A. flavus significantly and reduced the mildew of rice during storage. Emamifar et al. [53] also studied the effect loading different particles in a PE-LD film, such as P105 powder (with TiO2 + Ag NPs) and ZnO NPs, on antimicrobial activity in packaging of fresh orange juice (packaging with direct contact with product). They produced the compounds in a twin-screw extruder, after they used a blown film extrusion line to produce a monolayer flexible film using a temperature profile of 60–175 °C. They reported that increasing the ZnO NPs concentration up to 1 wt.% caused the NPs agglomeration during the processing and this decreased the antimicrobial activity of the film. To reduce the tendency for NPs agglomeration, Emamifar & Mohammadizadeh [54] used a compatibilizer, namely polyethylene-grafted with maleic anhydride (PE-g-MA), in the preparation of the blends. They obtained a better dispersion of NPs even increasing their concentration for 3 and 5 wt.% This procedure resulted in a considerable increase in the antimicrobial activity of the film.
When the active agents are sensitive to the temperature and easily released, such as natural extracts or essential oils, some authors have tried some specific strategies. For example, Zhu, Lee, & Yam [55] incorporated α-TOC (3000 mg/kg) into the PE-LD/PP blends using a single-screw extruder, at 221 °C, and produced the PE-LD/PP blends monolayer films with antioxidant proprieties. They reported that 90% of α-TOC incorporated into the films was retained after the extrusion process. Concerning the release of α-TOC, the results showed that the higher the PP ratio in the blend the slower was the α-TOC release. The authors explained that this happened likely due to the more tightly packed structure and higher crystallinity of PP when compared to LD-PE. Graciano-Verdugo et al. [56] added 20 and 40 mg/g of α-TOC into pure PE-LD. First, they pre-mixed manually the component at room temperature, and then the blown film was produced at 165 °C, using a pilot size single-screw extruder. Even without using high temperature during mixing, it was not possible to avoid the losses of 5 and 25% α-TOC in films with 20 and 40 mg/g, respectively, after the extrusion process. However, and in both cases, the antioxidant capacity of the film was still observed in corn oil, where the active packaging acted by direct contact with the food product.
Biodegradable polymers have also been used to develop active film packaging, since many of them can already be used in conventional polymer processing technologies. For example, Llana-Ruiz-Cabello et al. [40] developed an active film with PLA containing Proallium as an active agent to produce films with antioxidant and antimicrobial properties. They made bags with the developed films and stored the iceberg salad within a modified atmosphere in some studies. Different concentrations of Proallium were incorporated into the PLA matrix and active films were obtained by extrusion using a twin-screw extruder at temperatures ranging between 200 and 205 °C. The Proallium was introduced into the extruder through a lateral barrel port where the polymer matrix was already molten to reduce its possible volatilization and degradation. They reported that Proallium alone lost around 80% of weight at temperatures up to 150 °C, but when Proallium was added to PLA, no films were formed. The results showed a great antimicrobial activity with the highest concentration of Proallium, such as 6.5 wt.%, and did not show antioxidant activity. Concerning the optical properties of the film produced, it was observed that the Proallium reduced its transparency, but no significant visual differences were observed. The authors did not mention if the films have the characteristic odor of Proallium. Manzanarez-López, Soto-Valdez, Auras, & Peralta [57] used also PLA as polymeric matrix and added 3% w/w of α-TOC to produce a film with antioxidant properties. After the production of the compounding with a twin-screw extruder, the film was produced by blown extrusion process (pilot plant size extruder) using the same temperature profile of 165–170 °C. The concentration of α-TOC decreased to 2.58 wt.% after compounding, but after the film production the authors did not observe any loss. This happened because the film was immediately cooled after blowing, while the filament of the compounding was cooled at room temperature during 10–15 min. The PLA film produced with α-TOC showed a yellowish appearance. This difference was not perceptible to the naked eye in the single film, but perceptible in the film rolls. The authors did not explain the origin of the yellow color, but it probably originated from the high concentration of α-TOC used. Cestari et al. [58] developed an active biodegradable film with the addition of oregano essential oil (OEO) and potassium sorbate into TPS and PBAT (commercial name Ecoflex®). These mixtures were made using a twin-screw extruder with five heating zones (with a temperature profile of 90 and 120 °C). Films were produced by blown film extrusion using a temperature profile of 115–120 °C. Then the antimicrobial and antioxidant effects were studied in frozen chicken steaks stored with the film developed, but the authors did not specify the type of packaging (direct contact or headspace). They reported that the films reduced the risk of pathogen contamination, delayed the oxidation process of chicken meat, and extended its shelf-life.
Studies on ethylene scavenger and moisture absorber systems, with direct incorporation into polymer matrix and applied in monolayer packages, are scarce in the literature. For example, Tas et al. [32] studied the ethylene scavenging capacity of halloysite nanotubes HNTs-loaded PE-LD films. The incorporation of different concentrations of HNTs into PE-LD was performed using a twin-screw extruder. The film was produced in a blown film extrusion line with a temperature profile of 165–185 °C. To study the effect of these films, some products, such as bananas and tomatoes, were selected and tested, but the type of packaging (direct contact or headspace) was not specified. The authors observed that HNTs had an effect on the slowdown of the ripening process of bananas and on the retention of the firmness of tomatoes. Another example was presented by Sängerlaub et al. [59] that developed an active film with moisture absorber properties. They blended NaCl crystals with PP polymer using a twin-screw extruder with a temperature profile of 180–250 °C. The blend was used as masterbatch where the concentration of NaCl was 60% in weight. Afterwards, the masterbatch was blended (diluted) with neat PP and monolayer films were produced using a single screw extruder with a temperature profile of 180–230 °C. The results showed that the NaCl crystals incorporated in the film were able to avoid water vapor condensation in areas of reduced temperature. Moreover, the films showed an absorption capacity of water vapor around 80%.
The incorporation of active compounds through the extrusion processes can bring several advantages, namely in the production of films at the industrial typical high extrusion rates. However, extrusion is not adequate for some active compounds, such as the ones based on natural compounds, since it uses relatively high temperatures during the process. These high temperatures may lead to the degradation of the active agents, resulting in a loss of activity and change of color. Therefore, different alternative strategies have been explored to incorporate the active compounds. These will be presented in the following sections.
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