3.1 Dissolution Curves
The methodology propose in this work is two steps process the dissolution of WEP (Fig.1) and its recovery by precipitation. The curves of dissolution time show how long it takes for WEP to dissolve, starting from an initial concentration of 10% to a concentration of 100% WEP (%w/w) using 1g of esther (Fig.2). The results indicate that glyceryl tributyrate takes 130 min to dissolve 100% by weight of WEP (1g of WEP), while omega-3 and ethyl butyrate take 31 min and 7 min, respectively, indicating that the best solvent is ethyl butyrate, since it dissolves WEP in a shorter time.
In his research, Noguchi in the process of dissolving expanded polystyrene, what calls "WEP shrinkage" found that to dissolve 30% wt of WEP it took him 53 min using d-limonene essential oil [Noguchi et al., 1998]. On the other hand, in a more recent work, Gil used essential oils for the dissolution of WEP, reporting that the essential oil of star anise is the most efficient in its work and for 30% wt of WEP it requires 5 min, respectively [11]. In this work, the use of omega-3 for the same percentage by weight, there is a dissolution time of 6 min, for glyceryl tributyrate of 11 min and for ethyl butyrate 2 min, approximately, so the use of the proposed esters is competitive when dissolving the expanded polystyrene.
In all three cases, the curves of the times required to dissolve a certain amount of expanded polystyrene increase exponentially as the concentration of expanded polystyrene Co (%w/w) in relation to the ester increases.
The recovery of the expanded polystyrene consisted of washing with sufficient methanol and stirring until a white solid was obtained, then rinsing with isopropyl alcohol to remove any remaining residue. The WEP was allowed to dry at room temperature and characterized by SEM.
3.2 SEM
The WEP before treatment and after treatment was analyzed by Scanning Electron Microscopy. Fig.3a shows the WEP prior to treatment, in which a closed cell structure filled with air is observed [6], with cells or holes characteristic of the material [4], this is due to its manufacturing process. After treatment, this feature is no longer present; micrographs show a rough, airless surface with irregular stacked layers or sheets (Fig.3b and Fig.3c), indicating reduced WEP volume.
3.3 FTIR
The characterization of the esters used for the dissolution of WEP by the IR technique is shown in Fig.4 all the characteristic bands of polystyrene are present in each of the samples indicating a good recovery of the WEP. However, in spectrum (b) a band around 1740 cm-1 is observed, this band is due to the ester still present in the sample, so the result suggests an extra treatment to be able to eliminate traces of the ester used.
3.4 TGA
In Fig.5 the results of the Thermogravimetric Analysis belong to untreated WEP shown only a 99.29% mass loss, with a degradation temperature of approximately 360 °C [20,27]. For WEP recovered with omega-3, two mass losses are observed; the first of 3.97% in the temperature range 110-285 °C, which corresponds to the omega-3 content still present in the sample. The degradation temperature is close to 414 °C, which is where the second mass loss of 94.6% is observed, typical of the amount of WEP in the recovered material.
In the case of WEP treated in glyceryl tributyrate, two slopes are similarly observed; the first mass loss of 19.29% correspond it is at the temperature of 120 °C is due to the presence of the glyceryl tributyrate remaining in the sample. This can be corroborated since the boiling point of glyceryl tributyrate is approximately 174 °C. The degradation temperature of the recovered material is 408 °C, at which point the second slope corresponding to the loss of mass of 68.39% is shown, which belongs to the amount of recovery of the expanded polystyrene in the sample.
Finally, for the WEP treated in ethyl butyrate, two slopes are observed: the first mass loss of 2.66% in the temperature range 110-225 °C, due to the presence of residual ethyl butyrate in the sample (bp = 120 °C). The degradation temperature is 364 °C, in which the second mass loss of 96.89% is observed, a mass percentage that is related to the amount of WEP recovered.
3.5 DSC y Tg
In Fig.6 the curves of the samples analyzed by Differential Scanning Calorimetry are shown. An exothermic transition of the four materials tested is represented: WEP untreated and treated WEP esters, respectively. Through the thermogram obtained from the DSC analysis, the glass transition temperatures Tg of the four materials were calculated (see Table 1), the value obtained from the Tg of the WEP untreated is 106.7 °C, a temperature similar to that already reported (101-102 °C) [1]. Having a Tg indicates that the amorphous property of the recovered polymer was preserved.
Table 1. Glass Transition.
Sample
|
Glass Transition
(°C)
|
WEP untreated
|
106.7
|
RE-OME3
|
75.8
|
RE-GT
|
56
|
RE-EB
|
94.3
|
3.6 Apparent activation energy of degradation Ea
Activation energy was calculated with Friedman's differential method. This method compares the rates of weight loss (dα/dT) for a fractional weight loss with a certain rate of heating β (K/min) [Muñoz et al., 2015].
In this work, the analysis was done with a heating rate β of 20 °C/min (293.15 K/min) for the four samples. According to the results, the value Ea of the WEP without treatment was 268.5 JK/mol, this value enters the range of 240-275 KJ/mol that is reported in other work for experimental conditions under flow nitrogen, with an average of 245 KJ/mol [14]. WEP values recovered show variations to the approximate value of the WEP untreated, this could be due to the remnants of esters employed for each test, as these are still present in the polymer. The results are shown in Table 2.
On the other hand, different ranges and values of the WEP Ea have been reported, such as values of 125-147 KJ/mol with an average of 138.39 KJ/mol [32], 100-107 KJ/mol with an average of 104.31 KJ/mol and 126.52 KJ/mol [Jun et al., 2006], 46-170 KJ/mol with an average of 89.62 KJ/mol [17] and 60-100 KJ/mol [Azimi et al., 2008], these variations are due to the experimental conditions, that is, if the work was done with a nitrogen or air flow and the gas flow, the temperature and the heating rate, as well as the kinetic model with which the values were obtained of the Ea. However, studies show that the degradation reaction of WEP in a nitrogen or air environment is carried out in a single reaction step [14,32].
Table 2. Results of activation energy of WEP and WEP recovered by Friedman's method.
Kinetic model
|
Sample
|
Apparent activation energy (KJ/mol)
|
Friedman
|
WEP untreated
|
268.5
|
RE-OME3
|
286.3
|
RE-GT
|
220.2
|
RE-EB
|
294.3
|