3.1 Differential Scanning Calorimetry
The DSC analysis performed showed important results for understanding and understanding the interaction of the polymeric matrix, composed of LDPE and TPS with hybrids MMT-C and MMT-E. Table 2 shows the thermal properties with respect to the second heating ramp: melting temperatures (Tm), melting enthalpy (ΔH), and degree of crystallization (Xc) for the five formulations previously defined.
Table 2
Enthalpy, melting temperature and degree of crystallization of the matrix samples with and without the hybrid.
SAMPLES | ∆H (J/g) | Tm (°C) | Xc (%) |
LDPE/TPS | 5,555 | 113,75 | 3,97 |
LDPE/MMT-C | 7,765 | 114,26 | 5,54 |
LDPE/MMT-C/TPS | 9,707 | 114,32 | 6,93 |
LDPE/MMT-E | 17,160 | 114,03 | 12,25 |
LDPE/MMT-E/TPS | 13,510 | 113,64 | 9,65 |
The samples with the MMT-C hybrid showed an increase of approximately 0.5 ºC in the thermal event Tm, for the samples with the MMT-E the variation in the value of Tm was less pronounced.
On the other hand, the fusion enthalpy was higher for samples with the MMT-E hybrid compared to MMT-C, which may be related to greater chemical affinity of the MMT-E hybrid with the polymer matrix, greater cohesion and, therefore, increase in the heat of fusion. This is in agreement with previous work published by the present research group [2], which shows MMT by its potential as a carrier for both essential oils, in addition to providing an increase in the thermal stability of the formed composite.
Graphics 1 and 2 - LDPE/TPS DSC Heating and Cooling Curves and their compositions with the MMT-E and MMT-C hybrids, respectively.
The structural and morphological differentials found were attributed to the molecular structure of each essential oil, eugenol is possibly partially encapsulated between the lamellae of the MMT, so that the allyl group is outside the structure of the MMT, and therefore available to interact with polyethylene, mainly due to PE double bonds.
The crystallization process is not only of theoretical interest to understand the morphology of the polymer in question, but of great importance to define practical processing operations.
The crystallinity of the polymers was determined by DSC, where the relation between the fusion enthalpies was used, as described in Eq. 1, where Δ𝐻𝑓 is the experimental enthalpy of fusion eΔ𝐻𝑓 is the theoretical enthalpy of pure 100% crystalline LDPE which is 140 J/g and for LDPE/TPS polymer matrix 98.5 J/g.
\(Xc \left(\%\right)= \frac{{{\Delta }\text{H}}_{\text{m}}}{{{\Delta }\text{H}}_{\text{M}{\infty }}}\) x 100 (1)
It is observed in Table 2, that there is a significant increase in crystallinity, referring to the relationships in which the thermoplastic starch and the hybrid meet. Due to the previously described affinity of OEE with the polymeric matrix, it causes greater efficiency, in terms of increased crystallinity and interaction. For carvacrol, the increases are almost insignificant. In addition, it is possible to identify the miscibility property of the polymeric matrix by not observing oscillations in the melting peak, in order to suggest a possible amorphous characteristic, thus forming a homogeneous mixture [17].
3.2 Fourier Transform Infrared Spectroscopy
FTIR analysis was performed to observe the behavior of molecules in the presence of radiation interference and their vibrations. The energy of each peak presented by the interferogram, in an absorption spectrum, corresponds to the frequency of vibration of part of the sample molecule.
Graphis 3 and 4 - Full infrared spectra of the LDPE/TPS blend and their interactions with the MMT-E and MMT-C hybrid and the TPS bands and their compositions, respectively.
There are regions related to the angular stretches of CH2 and CH3, they are at the peak of 1463 cm− 1 this stretch can be presented around 1375 to 1460 cm− 1. Thus, angular stretching regions of CH2 that occur between 700 to 730 cm− 1 [18] [19] were also detected.
The spectra, graphic 3, showed bands at 3600 cm− 1 and at 1650 cm− 1, in the presence or not of the hybrid, which refers to the stretching and angular deformation of the -OH group of the starch and the water absorbed by the compound. Near 3400 cm− 1, there is an increase in intensity, still referring to -OH bonds, this time caused by essential oils in possible interaction [20].
In 850 cm− 1 relating to the functional group C-O-C (glycopyranosis ring), which presents the vibrational mode of asymmetric stretching. In Graph 4, there is a characteristic vibration in samples containing ATP, related to α conformation of the D-glucose unit, in addition to axial deformations of the C-O-C group and C-O bonds in alcohols, which are also characteristic of essential oils, which are visible from 900 to 1160 cm− 1 [21][22].
It should also be noted that in 3800 cm− 1 to 2500 cm− 1 there is a polymer-water-glycerol interaction, which together, water and glycerol, change the hydrogen bonding patterns in the system, given that after deconstructing the starch structure, a divergent H2 bond pattern appears, mainly in the processes of amylose crystallization and/or amylopectin recrystallization [23].
The most accentuated stretches in the spectrum are related to LDPE and its CH Methylene (2848.9 cm− 1) and CH Methyl (2914.8 cm− 1) connections, attributed to the vibrational model of asymmetric stretching of the links of the functional group CH (groups CH2).
The clay, in turn, does not have active peaks in the infrared, which concerns the organophilic deformation of its structure are masked by pre-existing peaks [19]. In this case, therefore, clay presents itself as a carrier principle for essential oils, which in turn, interacts strongly with the polymeric actress, where starch carries very strong bonds to enhance this interaction.
3.3 Migration Test
Graph 5 - Migration analysis of LDPE/TPS blends and their incorporation with the MMT-C and MMT-E hybrids.
Visually, graph 5 shows the formulations described previously, and their migration. It is observed that the graph shows the most accentuated migration in the sample LDPE/TPS/MMT-C, although in the beginning there was a drop in the sample with eugenol oil, at the end of the analysis there was greater exudation of the essential oil carvacrol, than the migration presented by eugenol essential oil.
It is also possible to understand the percentage of migration of the two composites, LDPE/TPS/MMT-C and LDPE/TPS/MMT-E, in which carvacrol exudes 9% more than eugenol, in 0.23%, present in the Table 3.
Table 3
Results of the migration analysis of compounds with and without Carvacrol and Eugenol oils.
| LDPE/TPS | LDPE/MMT-C | LDPE/MMT-E | LDPE/MMT-C/TPS | LDPE/MMT-E/TPS |
1st Day | 8,2814 g | 6,4745 g | 6,3948 g | 3,3324 g | 6,7188 g |
21st Day | 8,1376 g | 6,4217 g | 6,3250 g | 3,0247 g | 6,7030 g |
Migration | 1,76 (%) | 0,81% | 1,09% | 9,23% | 0,23% |
The molecular structure of carvacrol presents favorable conditions for a complete encapsulation between the lamellae of the MMT, since in its molecular structure it does not present an implantation nor a functional group, which interacts directly with the polymeric matrix. Thus, the interaction of this essential oil occurs mainly with MMT and does not make a resistant bond to the matrix, being more prone to exudate. Favorable for application in active packaging technology since the objective is periodically controlled exudation to thereby increase the shelf life of the food.
3.4 Microbiological assays
Microbiological analyzes were carried out in the laboratory, using the ABNT NBR ISO 6887-1 standard, for the bacteria Pseudomonas Aeruginosa and E.Coli, which are important for the food application of polymer films. As a result, in the samples of the final compounds (LDPE/TPS/MMT-E; LDPE/TPS/MMT-C), by the qualitative determination technique, the absence of CFU (Colony Forming Unit), thus proving the antibacterial action by the organic agents, of the developed polymeric film, as showed in Fig. 1.