: “Proteína de soja entrecruzada para preparación de films biodegradables: síntesis, caracterización y biodegradación”

A. GONZÁLEZ, M. STRUMIA Y C. I. ALVAREZ IGARZABAL

Simposio; IX simposio Argentino de Polímeros; 2011

The preparation of SPI films using different concentrations of Gen (0%; 0.1%; 1%; 2.5%; 5%; 7.5% and 10% w/w of the mass of SPI) and glycerol (Gly) as a plasticizer was carried out by “casting”. The films were flexible and both color intensity and opacity linearly increased with the increase of Gen added. The results of opacity obtained expressed in UA.nm/µm were: SPI-Gly: 1.023; SPI-Gly-Gen 0.1%: 1.317, SPI-Gly-Gen 1%: 3.482; SPI-Gly-Gen 2.5%: 8.342; SPI-Gly-Gen 5%: 10.819; SPI-Gly-Gen 7.5%: 17.527; SPI-Gly-Gen 10%: 18.633. Analysis of FTIR was carried out confirming the cross-linking reaction. The cross-linking degrees were calculated taking into account the percentage of amino groups unreacted after the cross-linking reaction with respect to the total amount of amino groups contained in the protein matrix. The CL% linearly increases with the amount of Gen added.. To evaluate the effect of the cross-linking, different film properties such as MC, TSM, WVP, TS and E were evaluated and the results are shown in table 1. It was observed that MC values did not vary significantly. However, TSM values showed a marked variation since the sample with only 1% of Gen revealed a decrease of TSM of approximately 45% with respect to the control film without cross-linker. When the films were kept in contact with water, they initially showed softening and a subsequent swollen state. It was observed the reduction in swelling ratio values of cross-linked SPI films. High cross-linking degree (CL%) allows the formation of three-dimensional, rigid and less expandable structures with a smaller capacity of softening and swellability. Mechanical properties were evaluated by TS and E from the stress-strain curves of each film. TS values showed an increase until they reach a limit value at 1% of Gen (w/w of SPI). Larger additions of Gen do not vary significantly the TS values. In cross-linked films, E values show an increase when Gen additions are fewer than or equal to 1%. An increase in Gen concentration leads to a diminution in E values. Table 1.  Moisture content (MC), total soluble matter (TSM), water vapor permeability (WVP), tensile strength (TS) and elongation at break (E) of SPI films with 0; 0.1; 1; 2.5; 5; 7.5 and 10 % (w/w of SPI) of Gen added. Film MC (%) TSM (%) WVP × 10-10 (g m Pa-1 s-1 m-2) TS (MPa) E (%) SPI-Gly 27.07 ± 4.19a 64.66 ± 3.47c 2.41 ± 0.04b 3.22 ± 0.10a 22.53 ± 5.02bc SPI-Gly-Gen 0.1% 28.37 ± 2.29a 51.58 ± 2.36b 2.22 ± 0.09ab 3.28 ± 0.16a 26.71 ± 4.77cd SPI-Gly-Gen 1% 26.03 ± 2.79a 35.14 ± 4.42a 1.88 ± 0.10ab 4.16 ± 0.38b 45.84 ± 0.25e SPI-Gly-Gen 2.5% 24.74 ± 3.41a 33.94 ± 3.81a 1.72 ± 0.11a 4.46 ± 0.04b 36.86 ± 0.46de SPI-Gly-Gen 5% 25.60 ± 3.22a 34.08 ± 3.12a 1.81 ± 0.16ab 4.60 ± 0.11b 12.14 ± 4.46ab SPI-Gly-Gen 7.5% 25.43 ± 2.28a 35.20 ± 2.16a 1.80 ± 0.20ab 4.52 ± 0.13b 3.22 ± 0.89a SPI-Gly-Gen 10% 26.14 ± 3.17a 34.19 ± 3.43a 1.89 ± 0.21ab 4.58 ± 0.09b 2.79 ± 1.48a Any two means in the same column followed by the same letter are not significantly (P ≥ 0.05) different according to Turkey test.   WVP was evaluated for each sample, showing relatively low permeability values. The effect of Gen amount is inversely proportional to WVP values with additions up to 1% (w/w of SPI) of Gen, remaining constant with larger additions. The superficial structures of the films were visualized by SEM micrographs. Biodegradation tests were carried out by burying the films into the soil under indoor conditions for 33 days. Pictures of the samples were recorded and weight loss was measured at different times. The macroscopic examination revealed that the degradation of the different films in soil strongly depends on the cross-linking degree. The film with no modification almost completely degraded in 14 days, whereas the SPI-Gly-Gen 10% remained unaltered after 33 days. This dependence can be attributed to the effect of the chemical networks on the enzymatic degradation (Martucci, J.F. and Ruseckaite, R.A, 2009). As demonstrated in the swelling assays, the water absorption of most cross-linked films is lower than that of the less cross-linked. This effect produces a decrease in the bio-availability of water inside the matrices and could explain the fact that in most cross-linked films, microbial attack, proteolytic enzyme action, and hydrolysis occur to a lesser extent.  It was found that all materials absorb water (in a larger o lesser extent), losing their initial shape and structural integrity afterwards. After the 23rd incubation day a characteristic decomposition smell appeared. The best global properties for the SPI-Gly films were obtained with the aggregate of 1% (w/w of SPI) of Gen since TS and E were maxima while WVP and TSM were minimal. As projections of this work it raises the utilization as a material for controlled drug release