## 3 Results
### Compostability of Paper Multilayers  
According to ISO 17088 [58], composting is the aerobic treatment of the biodegradable plastic parts of packaging waste that consumes oxygen and produces, under controlled conditions and using microorganisms, biomass, inorganic compounds, CO\({}_{2}\), and water, without leaving visible distinguishable or toxic residues. Therefore, the evaluation of compostability includes three main tests, namely disintegration, biodegradation, and ecotoxicity [54; 59; 60]. Although recycling should be more economically and energetically favorable than composoting for paper packaging, it cannot be practical in some situations because of excessive sorting and cleaning requirements [61]. Therefore, compostability of paper multilayers can be relevant from a sustainable point of view of waste management. This is particularly relevant when recycling is not feasible, for instance, in the case of disposable containers contaminated with food residues. The ripe compost used as inoculum in both the biodegradation and disintegration tests showed an initial organic matter content (VS) of 90% \(\pm\) 0.9%, expressed as volatile solids with respect to dried solids. The content of total dry solids (DS) was 79% \(\pm\) 1.3%, whereas the pH value was 8.1, measured according to ISO 14855. Figure 7 shows the results of the aerobic biodegradation test of the films and paper sheets under controlled competing conditions (58 \(\pm\) 2 \({}^{\circ}\)C for 45 days) following the method adapted from the ISO 14855. This test is based on the measurement of the CO\({}_{2}\) generated in the biodegradation process, which is considered proportional to the percentage of biodegradation in organic samples. MCC was also tested and used as positive reference. Moreover, the theoretical maximum quantity of CO\({}_{2}\) that can be produced by the biodegradation of the samples was calculated from their theoretical carbon content. One can observe that both MCC and the PHBV film completely biodegraded after 28 days and 40 days, respectively. Moreover, these samples exhibited the characteristic sigmoid profiles of respirometric tests, showing three different phases [60; 62]. In the case of the PHBV film, it presented an initial log period of 2-15 days, followed by a biodegradation phase that was prolonged up to day 40, and finished with a plateau. PHBV biodegradation is essentially an enzymatic process, where exoenzymes from bacteria and fungi or membrane-bound enzymes, such as proteases, lipases, and esterases, which are present in the composting soil, hydrolyze the biopolyester chains into their corresponding monomers (hydroxy acids) [63]. In this process, the microorganisms first colonize the surface of the biopolymer and then secret depolymerases that hydrolyze the ester bonds. This process yields to low-M\({}_{W}\) chains able to pass through the semipermeable external bacterial membranes and are metabolized. MCC, after 45 days, reached a biodegradation percentage of \(\sim\)115%, which is attributed to the \"priming\" effect. The latter is an overestimation of the CO\({}_{2}\) released that occurs when the compost inoculum in the test reactor containing the samples generates more CO\({}_{2}\) than the compost inoculum in the blank reactors [64]. This effect has been previously ascribed to the stimulation of organic matter mineralization that takes place after the addition of easily-decomposable organic matter [65]. As expected, the commercial multilayer film showed no biodegradation since this is based on polymers that  
Figure 7: Sample biodegradation as a function of time microcrystalline cellulose (MCC), the paper sheet, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and polyethylene terephthalate (PET) films, and PHBV/paper/PHBV and PET/paper/PET multilayer sheets. are not biodegradable. The uncoated paper reached a value of nearly 33%, whereas the PHBV- and PET-double coated papers showed lower biodegradation values, of 23 and 11%, respectively.