# Biodegradable and Compostable Shopping Bags under Investigation by FTIR Spectroscopy
## 3 Results and Discussion
### The Predictive Linear Regression Model (PLRM)  
The scope of this part of our study was to build a linear regression model by least squares approach, using selected spectral data and the relative PLA concentrations computable as explained later, to predict information on PLA content of a sample. The model was built to provide MecPlast with a tool for estimating the minimum PLA content in a newly extruded film, prepared with the correct content of PLA, but that for any unforeseen events occurred during the production process, could not respect the expectation. The model was not created to highlight any fraud and identify non-compliant bags, which is the task of the bodies which grant the conformity mark. According to the FTIR spectrum of PLA (Figure 3b), we decided to calculate the ratio between the areas of bands (A/A%) in the range 1800-1600 cm\({}^{-1}\) and 3037-2800 cm\({}^{-1}\). The measurements were made using FTIR spectra in the absorbance scale automatically, by using the Turbo chrome workstation (version 6.1.1.) equipment software. The band at 1800-1600 cm\({}^{-1}\) (C=O group of PLA) was considered indicative of PLA% content on the basis of the loading plots obtained by the PCAs, and therefore it was selected as significant spectral data, to construct the model. Moreover, in order to minimize the error related to the difference in the optical path of the sample pieces deposited between the KBr dishes, the values of the area ratios (A/A%) between the A% of the band in the range 1800-1600 cm\({}^{-1}\) and that of the bands in the range 3037-2800 cm\({}^{-1}\), also evidenced as significant bands by loading plots, were considered as more reliable spectral data. The relevance of these bands was confirmed by the loadings profiles obtained by the PCAs (Figure 6b, Figure 7b, and Figure 8b). Taking as reference the Vincotte-S732 sample, having a minimum declared content of PLA of 40% and an A/A% value of 1.52, the minimum PLA percentage content of all the film samples was calculated through the use of mathematic proportions. The data obtained were reported in Table 5.  
Curiously, the minimum PLA content of TUV-S0092(1) was the same of that of Vincotte-1 (50%), which, since in PCA shown in Figure 6a, was out of the group of TUV-S0092(1) and very closer to starch, should have a major content in renewable material. This apparent inconsistency derives from the fact, that not only the PLA content determined the separation of the samples in the PCAs, as already confirmed by the loading plots reported in Figure 6b, Figure 7b, and Figure 8b. The PLA% content calculated for each sample was reported on a graph (y axis) as a function of A/A% values (x axis), obtaining the LPRM (R = 0.999), using Microsoft Excel software (Figure 9). Its equation was Equation (1), and the very high values of the Pearson product-moment correlation coefficient (R = 0.999) and of the coefficient of determination (R\({}^{2}\) = 0.998) suggested a condition of linearity. Obviously, since the value taken as reference (PLA \(\geq\) 40%) is only a declared and not verified value, the PLRM must be considered a self-validated model. \[y=26.155\mathrm{x} \tag{1}\]  
Equation (1) will allow us to estimate the minimum PLA% content in a freshly extruded film, simply by acquiring its FTIR spectrum and A/A% value, which represents the variable x of the model. The model in Figure 9 confirmed that, in addition to the irregular PE-based sample, four other samples also have an insignificant PLA content, even if they are present on the market as regular bags. Furthermore, it was found that in the population of films deemed compliant with the regulations on biodegradable and compostable films [23], only some samples had the requirements to be used as primary packaging for bulk foods or to be supplied for hygienic purposes, while the others were eligible to be used as carrying bags, only.