The starch used for the synthesis of bioplastic was extracted from the pulp of sweet potato Ipomea batata [6] procured on the TOTAL market place in Brazzaville.

After receiving the material (sweet potato), it is cleaned and peeled before being grated. The grated pulp thus recovered is placed in a container to which drinking water is added. This mixture was then filtered through a sieve. The filtrate obtained is submitted to a decantation and 2 hours later the supernatant is removed with a pipette; the starch obtained is air-dried for two days.

Prepare in a pan, 300 mL of distilled water, 10mL of glycerol and 15 g of starch. Mix and bring to boil until it becomes thick and translucent. Continue to heat for 10 min without stopping to stir with a wooden spatula. Pour the still hot and liquid mixture onto a glass plate. Allow to cool for one hour to allow bubbles to escape then place in the oven at 100˚C for 24 hours.

The sampling is carried out on the obtained bioplastics. We have two types of samples (Table 1).

‒ Type A; bioplastic of circular shape 3.5 cm of radius not having undergone the photonic stress.

‒ Type B; bioplastic of circular shape 3.5 cm of radius having undergone the photonic stress.

The main physico-chemical analyses were determined with reference to international and French standards. Two types of bioplastic subjects were targeted in these analyses, those which underwent the different light stresses and those which did not undergo any stress at all.

We have thus realized,

• The water content (AOAC 950.0, 1990),

• The ash content (NF T 76-110 sept. 1981),

• Hygroscopy: Measure a mass m in grams of the bioplastic, expose to air and measure every 24 hours for 1 week. The hydroscopy is given by the difference of the initial mass and the final mass

Follow-up of hygroscopy during photonic stress by measuring the mass of the bioplastic every hour for 7 hours.

Biodegradation test: This test is carried out by placing 1 g of bioplastic for two months in glasses containing different types of earth: black, yellow and white to which we assign respectively the symbols T₁, T₂, and T₃. The black soil was taken from around the plantations; the yellow soil is of the clay type and the white soil of the marine type. Then, the mass of the bioplastic is measured again [7].

T D ( % ) = M 0 − M 1 M 0 × 100

M₀: mass of film before degradation; M₁: mass of film after degradation

Light stress: Cut the bioplastic into a circle of 3.5 cm radius. Place the film in the chamber and measure the diameter every 10 minutes, reproduce the experiment 10 times. The aging chamber is a device specifically designed to simulate

solar aging on our bioplastic samples. It is a wooden box of 15.6 cm³ of volume whose interior is covered with a black plastic sheet. On the upper face is placed an incandescent lamp type Edison (15 cm from the sample) of power 100 W and capacity 210 lux, connected to an electric cable connected to a voltage regulator but also of a thermometer which indicates the temperature within the enclosure. The internal temperature T_i is noted immediately after closing the chamber. The internal temperature T_f is recorded again before opening the chamber.

Our variety of sweet potato, with a brown skin, is likely to provide consumers with 440 KJ or 105 Kcal per 100 g of potato [3]. More often used as a complementary food in Congo, it gave us an extraction yield of 9.5%, which confirms that it is a root rich in starch, which is also easy to extract. However, this yield is rather low compared to the one given by Tshizembe and all on cassava tubers.

Like most of the products resulting from a retrogradation of starch in the presence of a plasticizer: glycerol, the films obtained have a rubbery aspect [8]. The resistance to touch seemed to us to be suitable. The thickness chosen by flattening two glasses reflects that changes have occurred within the matrix of the macromolecule.

The glycerol used as plasticizer does not interact chemically with the matrix in which it is dispersed. It simply increases the free volume between two polymer chains to reduce the interactions and thus promote the movement of one with respect to the other [9]. The presence of this plasticizer reduces heating because the plasticizer has already introduced free volume between the chains. The film is thus transformed from a rigid material into a plastic. The film obtained is thus more resistant to tension and bending. In addition, the addition of glycerol makes the film transparent, which is very useful for packaging purposes.

Heating serves to destructure the starch. Once destructured, the starch mixes easily with the glycerol. A destructured starch is a starch that is no longer in its original form of grain: the polymers constituting it (amylose and amylopectin) are dispersed. We notice a phase that gels quickly above another whose fluidity is greater. The first one should correspond to the behavior of amylose whose fusion occurs quickly and the other part to the behavior of amylopectin whose gelling is slower and limited because of its branched structure. This is what is actually observed, so we have indeed obtained a plastic film by retrogradation. Indeed, retrogradation refers to the structural reorganization (or recrystallization) that takes place during the cooling of a destructured starch suspension [10].

The subjects who did not undergo the light stress show an average water content of 36.28% while those who underwent the light stress give an average content of 33.78%. This result proves that the action of the lamp has a real effect on the water content. Indeed, an incandescent lamp produces light whose spectrum is close to that of the sun, but the efficiency between the luminosity of this type of lamp and the energy consumed is 10%; this explains why these lamps produce a lot of heat, moreover the temperature within the box reaches on average 90˚C. There is therefore an increase in temperature during stress, which explains the large difference in the average water content returned during this work. It is also important to note that a new texture for our plastics, with a more rigid solid appearance and a more sustained elasticity, is observed. These observations show that real structural modifications occur within the matrix of our bioplastics [11]. Indeed, these modifications can be due to a reorganization of the structure by the development of a much more cross-linked network [12]. The number of cross-links in the matrix increases, making the structure stronger and therefore more able to resist certain external factors such as humidity.

The ability of our bioplastics to resist moisture is shown by hygroscopic analysis. We found that the light-stressed bioplastic showed the lowest values.

We can thus say that the structure of the bioplastic contains sites of various forms where water molecules are inserted. During stress, the bioplastic matrix undergoes a reorganization and modifies these different sites. These sites become less available to accommodate water molecules, resulting in a weaker hydroscopy and thus a better resistance to alteration (Figure 1). This result is very encouraging because one of the points limiting the impact of starch-based plastics is precisely their capacity to absorb the water vapor contained in the ambient air. Indeed, starch is very hydrophilic, which is a real challenge for professionals in the field.

Light stress is a significant alternative to this limitation, and further confirmatory analyses are required [13].

The light-stressed bioplastic matrix has formed a crystal lattice. The cross- linking caused by the effects of light considerably removes the sites that could receive water molecules from the air. Thus, a more compact and better organized structure was formed [14].

However, it was noted that exposure to light stress did not change the intrinsic composition of our bioplastic, only the bioplastic matrix was the site of a change. Indeed, the ash content remained almost the same for both types of bioplastics analyzed (Table 2).

The observation of the intrinsic composition which remains unchanged leads us to think that the behavior of our bioplastics in front of the natural degradation should also remain unchanged. Thus, three tests were carried out and revealed that light stress considerably strengthens the resistance to biodegradation. The longevity of our bioplastics is thus improved, this result may seem anecdotal but is in agreement with the new structuring of the bioplastic thus obtained.

Bioplastics not exposed to light stress generally underwent greater biodegradation. These results are consistent with the findings of previous analyses.

The degradation kinetics of bioplastics is accelerated by water activity. A bioplastic that is poor in binding sites for water molecules offers a better resistance profile to microorganisms. The newly formed cross-link network is also a major contributor to biodegradation resistance [14]. However, the latter remains (Table 3), otherwise the interest in producing bioplastics would no longer exist. These results show moreover, that the soil (marine soil) T₃ is the one in which this biodegradability is the most attenuated, this is in agreement with the behavior of this type of soil with regard to the retention of water. Indeed, the marine type soil does not retain water and in the absence of water the bioplastic undergoing stress is in ideal conditions to preserve its state for a longer period of time, moreover this type of soil contains few microorganisms. Contrary to the black soil which is very rich in humus and therefore in microorganisms and retains considerably the water runoff.

When the bioplastic is left in the enclosure for 7 hours, it shows a behavior reminiscent of the Gaussian curve, part of the water vapor contained in the enclosure is initially integrated into the matrix of the bioplastic, hence the increase of the mass. After two hours, we notice that the mass of the sample decreases, this decrease continues until a plateau is established around 2.79 g (see Figure 2). This behavior confirms that the bioplastic matrix undergoes a considerable modification which prevents the fixation of the water contained in the air although at each opening of the enclosure this air is renewed.