Modelling easily biodegradability of materials in liquid medium-relationship between structure and biodegradability

In the present project, twenty materials (e.g., polyhydroxybutyrate-hydroxyvalerate, polycaprolactone, cellulose acetate, polyacticacid, polyethylene), representing varied biodegradability levels were studied. An aerobic respirometric test, based on the CEN Draft, was setup. The biodegradability of each plastic film was evaluated by measuring the percentage of carbon converted into CO(2) during 35 days. The values of the CO(2), production were plotted versus days as a cumulative function. In order to reduce its number of points, the cumulative curve was modeled using a sigmoid function (Hill sigmoid). This model was compared to one found in the literature. A chi(i)(2) test showed that the biodegradation curve was more accurately fitted with the model than the previous one. Three kinetic parameters were determined by this "Hill model": one represents the maximal percentage of carbon converted into CO(2), the second the "half-life time" in days of the degrading part of the material and the third one the curve radius. In addition, the following analyses were carried out on each sample: elemental analysis, thickness, hydrophobicity and surface free energy measurements. In order to compress the information and to keep only relevant pieces, these parameters were submitted to a Principal Component Analysis. PCA found linear combinations of variables that describe major trends in the data. The two principal components which separate groups of materials were closely related to a chemical and a physical axis respectively. Materials showing a high biodegradability were related to high oxygen (and nitrogen) contents and low hydrophobicity: Material thickness did not influence the likeliness to biodegradability described by the maximum biodegradation rate. Finally, this study established the correlation between the biodegradation and the structure of biopolymers.