Recyclability of PET/WPI/PE Multilayer Films by Removal of Whey Protein Isolate-Based Coatings with Enzymatic Detergents

Abstract

Multilayer plastic films provide a range of properties, which cannot be obtained from monolayer films but, at present, their recyclability is an open issue and should be improved. Research to date has shown the possibility of using whey protein as a layer material with the property of acting as an excellent barrier against oxygen and moisture, replacing petrochemical non-recyclable materials. The innovative approach of the present research was to achieve the recyclability of the substrate films by separating them, with a simple process compatible with industrial procedures, in order to promote recycling processes leading to obtain high value products that will beneficially impact the packaging and food industries. Hence, polyethyleneterephthalate (PET)/polyethylene (PE) multi-layer film was prepared based on PET coated with a whey protein layer, and then the previous structure was laminated with PE. Whey proteins, constituting the coating, can be degraded by enzymes so that the coating films can be washed off from the plastic substrate layer. Enzyme types, dosage, time, and temperature optima, which are compatible with procedures adopted in industrial waste recycling, were determined for a highly-efficient process. The washing of samples based on PET/whey and PET/whey/PE were efficient when performed with enzymatic detergent containing protease enzymes, as an alternative to conventional detergents used in recycling facilities. Different types of enzymatic detergents tested presented positive results in removing the protein layer from the PET substrate and from the PET/whey/PE multilayer films at room temperature. These results attested to the possibility of organizing the pre-treatment of the whey-based multilayer film by washing with different available commercial enzymatic detergents in order to separate PET and PE, thus allowing a better recycling of the two different polymers. Mechanical properties of the plastic substrate, such as stress at yield, stress and elongation at break, evaluated by tensile testing on films before and after cleaning, were are not significantly affected by washing with enzymatic detergents.

Keywords: enzymatic detergents; multilayer films; polyethylene (PE); polyethylene terephthalate (PET); protease; recyclability; whey protein isolate.

Figure 1

FT-IR spectra of (a) PET and whey/sorbitol samples; (b) PET/whey sorbitol after washing, respectively, for 2 h, 4 h, and 24 h.

Figure 2

FT-IR spectra of samples with glycerol after washing, respectively, for 2 h, 4 h, and 24 h.

Figure 3

Composition of multilayer film with a whey protein-based barrier layer.

Figure 4

PE/Whey/PET films after treatment with 4% Terg-a-zyme for 1, 2, and 4 h.

Figure 5

FT-IR spectra of multi-layer samples PE, PET/Whey, and PET/Whey/PE.

Figure 6

FT-IR spectra of PET in the recovered separated films after washing with Alcalase.

Figure 7

FT-IR spectra of PET in the recovered separated films after washing with Alcalase.

Figure 8

Representative stress/strain curves of PET films before (left) and after (right) coating with whey and whey removal by washing with Alcalase at 50 °C for 2 h.