Materials Processing

Commercial Tetra Pak^® containers with dimensions 4.6 mm L x 3.6 mm W x 11.85 mm H were collected. The containers are composed of several layers of low-density polyethylene (LDPE) 20%wt., Aluminum (Al) 5%wt. and cellulose 75%wt., according to the manufacturer. Three different samples were prepared to develop this experiment. The samples were prepared in order to obtain composites without additives or reinforcements, and by using a scalable process to obtain sheets. Initially, the containers were disassembled and stacked manually. Fifteen as-received containers were stacked in 5 layers to obtain a final composite without cutting the containers and create Type I samples; Type II samples were prepared by cutting the containers into slices of 10 mm x 15 mm; and type III samples, by cutting containers into pieces of 5 mm x 5 mm. After positioning the layers in a mold of 250 mm x 120 mm x 7 mm and applying several cycles of pressure and temperature in a manual hydraulic press with hot plates, the composites were obtained. The processing parameters used in this work were selected according to the literature (^{Sun & Zhang, 2013}), and the limitations of the equipment available in the polymers laboratory of the Metropolitan Technological Institute (ITM, in Spanish). After cutting, samples were hot-pressed without any additional binder to form the composite, so the final composite is obtained using 100% recycled materials.

The procedure C recommended in ASTM D4703-03 was used: a suitable working method for thermoforming polyethylene sheets with a working pressure higher than 8 MPa. Figure 1 outlines the procedure, as well as the pressure, temperature, and time during the sheets manufacturing.

Source: Authors.

Figure 1 (a) Pressure, temperature, and time during the sheets manufacturing. (b) Details about the mold and the slices used during the manufacturing. 

Mechanical properties

The composite sheet (200 mm x 120 mm x 7 mm) was cut manually to obtain the samples, which were prepared according to the ASTM D3039 standard (Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials) by aligning the test specimens by the sheets' longest dimension (200 mm). The mechanical tests were done in a Shimadzu AGX-100 KN Universal testing machine available in polymers laboratory of the ITM. All samples were tested at a testing speed of 5 mm/min at 25 °C. In all cases, for every type of material, three samples of each type were tested, and the results were averaged.

Analysis of fractured surfaces

After performing mechanical tests, the samples were analyzed to understand the main failure mechanism and to determine how the manufacturing process could be improved. The samples were observed in a LEICA EZ4D stereomicroscope and in a FE-SEM 7100F available in the microscopy laboratory of ITM. The samples were covered with a thin layer of gold to avoid surface charging.

Water absorption tests

Water absorption tests were done according to the ASTM D570-98 (2005) standard. The samples weight was recorded before and after soaking them in water for 24 hours. Both weights were compared to obtain the water absorption. The dimensions of samples were 76 mm x 24.5 mm x 7 mm (length, width, and thickness, respectively). The procedure described in numeral 7.1 of the ASTM D570 standard was used: immersing the specimen in distilled water during 24 h at 23 ° C ± 1 ° C.

Mechanical properties

Figure 2 shows the average and standard deviation of tensile strength of the manufactured samples. The lowest resistance was found in Type I samples and the highest was found in Type III samples. Type I samples had even lower resistance than that of the LDPE matrix (13.3 - 26.4 MPa) (Barcelona, n.d.). In this case the low resistance of the composites can be explained by the lack of preparation of Type I samples since the samples were hot pressed without cutting the container.

Source: Authors.

Figure 2 Results of tensile tests. 

Type II samples showed an intermediate resistance (24.1 MPa) and the standard deviation of the results shows that the manufacturing process is uniform. Conversely, Type III samples (chopped material) report the highest tensile strength, but they also have the highest standard deviation, so the processing conditions did not provide uniform materials.

This behavior can be explained by the different orientations of the chops used for the manufactured composites. Since the orientation of the chops was not controlled during the manufacture, the fibers randomly oriented can affect the tensile strength measured in just one direction. Table 1 lists different materials investigated by other authors or commercial materials and shows the maximum tensile strength possible. It is observed that the resistance of the materials investigated in this work is similar to that observed by other authors. Moreover, the resistance of the prepared recycled Tetra Pak® composites without any further modification is similar even though those reported in the Table 1 were modified with resins and polymers. Also, some authors have studied the mechanical properties of materials for non-structural purposes in the construction industry. The values reported in this work are comparable with those found in literature for Short Pineapple-Leaf-Fiber (PALF)-LDPE Composites (10.2 MPa-13.0 MPa, and 16.3 MPa-22.5 MP, for randomly and longitudinally oriented fiber, respectively) (^{George et al., 1995}), and for Wood-plastic board (WPB), sugarcane bagasse particleboard (SBpb), Plywood board (PB), and Sugarcane bagasse fiberboard (SBfb), whose values are 28.20 MPa, 14.35 MPa, 15.20 MPa, and 31.45 MPa, respectively (^{Martínez-López et al., 2014}) for non structural purposes.

Table 1 Materials reported in the literature with similar properties. 

Source: Authors.

^{Hitchon & Phillips (1979)} reported that the fiber strength decreases as the length of the fiber increases; and ^{Pamungkas et al. (2018)} showed that the fiber length increases the mechanical properties of a composite of Zalacca Midrib Fiber (ZMF)/HDPE. This article does not include tests for the effect of the fiber length, but the size of the chops can be interpreted as an indirect evaluation of the fiber length. The results of this article show that the shorter the chops, the tensile strength increases. However, some authors have found ambivalent results, so there is no clear trend about the effect of fiber length on tensile strength (^{Amuthakkannan et al., 2013}).

On the other hand, when the fibers are perpendicular to the direction of the applied stresses, they act as stress raisers, but it is worth noticing that the fibers in the fragments of the samples had a random orientation and their effect was not studied. When the fibers are cut into small pieces, there is a higher probability of finding fibers perpendicular to the load application direction, which would imply a lower resistance of the compounds in this order: Type III < Type II < Type I. However, the behavior of the tests for this research was contrary leading to the phenomenon associated with the direction of the fibers being negligible. A similar behavior has been reported in the literature by other authors (^{Xia et al., 2002}).

Analysis of fractured surfaces

Figure 3 shows the SEM images of fractured Type I sample. They indicate that there is a fiber pull-out after the mechanical tests. Some fibers were found to be coated with LDPE and adhesive layers, as indicated by the rows in Figures 3a and 3 b.

Source: Authors.

Figure 3 SEM images of failed Type I sample.  

Also, it is possible to observe that LDPE fibrils stretched resulting in a ductile failure of the material. Most of the fibers did not show a separation from the matrix, which mean that the processing parameters can melt the polymer around the fibers. Also, uncoated fibers were observed and most of them were released from the matrix (Figures 3a and 3b). Figures 3c and 3d show that the delamination of some layers and the empty spaces between them (see arrows). Thus, it can be concluded from Figure 3 that pull-out and delamination are the common failure mechanisms in Type I sample.

The surface analysis of Type III samples showed that the composite did not fracture in a transverse plane form (Figure 4). These failures are characterized by being staggered and are caused by separation of layers. Figure 4 also shows the detail of the failure observed in the SEM. Additionally, it was observed that the fracture has been produced by a sequence of intralaminar fracture in cellulose fiber layers. ^{Greenhalgh (2009)} reported that the intralaminar fracture is intrinsically linked to fiber-dominated failure. This work also identified fiber failures since there is a reduction in the cross-section area of the fibers.

Source: Authors.

Figure 4 SEM image of the failed Type III sample. 

For Type II, sheets of cellulose are separated by low density polyethylene and polypropylene. Since those materials have different thermoforming temperatures, the inner layers were not melted during the process, leading to interlaminar failure around the polypropylene layers. Although there are some differences in the fusion temperature for the polymer components of the Tetra Pak^® containers (polypropylene and polyethylene) and these differences can decrease the mechanical anchorage, the same differences can be found for all the configurations studied in this work.

Figure 5 shows the fractured surfaces of Types II and III samples observed through a stereomicros-cope. Type II evidenced interlaminar failures (see arrows in Figure 5a) while Type III showed intra-laminar failures inside the cellulose layers instead of polypropylene layers (see arrows in Figure 5b).

Source: Authors.

Figure 5 Fractured surfaces of Type II (a) and Type III (b)  

Water absorption tests

Table 2 shows the results of water absorption tests for all samples. Chopped material favors water absorption. When the material is cut into small pieces the exposed area increases and so does the absorption of water (^{Zawadiak et al., 2017}). Inks and the polymers (PP and PE) in the samples are hydrophobic and can act as water barriers. On the other hand, the fibers (cellulose) and porous clays boos the water absorption properties (^{Yilgor et al., 2014}).

Table 2 Results from water absorption tests. 

Source: Authors.

At the end of the tests (after 24 hours), the water infiltrated the entire transverse area of the samples since the polymer did not completely cover the fibers. Also, for Type III the polymer did not cover all the chopped material, so the water absorption caused by cellulose layers was higher.