Study of the Effect of Different Electron Irradiation Doses on the Decomposition Temperature of Azodicarbonamide.
Cellular polymers also called polymer foams are two-phase materials in which a gas is dispersed in a continuous macromolecular phase [1,2]. Blowing agents are introduced in a polymer matrix to generate the gas phase necessary to produce polymer foams . They can be classified as either physical or chemical blowing agents (CBAs). Physical blowing agents produce cells by a phase change in the case of liquids or the diffusion of gas (mainly [N.sub.2] and C[O.sub.2]) in the polymer matrix [4,5]. CBAs generate gas by thermal decomposition of a chemical compound. To be used with a specific polymer, the decomposition temperature of the CBA must occur in the temperature range of processing. Furthermore, the residues should be nontoxic and odorless. The blowing agent must not decompose spontaneously, and the gas yield should be as high as possible. In addition, the CBA should easily be incorporated and dispersed in the polymer matrix .
Azodicarbonamide (ADCA) is the most extensively used chemical blowing agent for polymer foams. ADCA is an orange-yellow powder which decomposes exothermically in a range of temperatures from 195 to 235[degrees]C. The gas yield of the ADCA is much higher than that of other typical blowing agents, such as endothermic sodium bicarbonate or exothermic 4,4'-oxydibenzenesulfonyl hydrazide . The decomposition reaction of ADCA produces several residues, such as biurea and urazol, which decompose at higher temperatures in an endothermic process generating additional gas .
Cram et al.  reported the main chemical compounds generated during ADCA decomposition. Biurea, urazole and cyanuric acid were the main residues produced and [N.sub.2] and CO were the main gases released. C[O.sub.2] and N[H.sub.3] were also formed but to a lesser extent. Particle size is a crucial factor in the production of ADCA. The release of gas depends on the size and distribution of the particles. Finer particle sizes shift the decomposition temperature of ADCA to lower temperatures. When the particle size decreases the surface area increases, and as a consequence, the particles can readily react with activators particles or even with unreacted ADCA particles. There are other parameters, such as the heating cycle (temperature and time), which has a critical effect on the decomposition rate. The decomposition process is accelerated at higher temperatures .
The decomposition temperature of ADCA can be decreased by using activators or promoters. Lally and Alter  proved that the decomposition temperature of ADCA could be lowered by the use of some salts of lead and tin. Wright  analyzed the effect of silica, a lead salt, and a tin salt, using HDPE and PVC as polymer matrices. Activators not only decreased the decomposition temperature of the ADCA but also increased the gas yield and induced changes in the composition of the gases produced, increasing the amount of C[O.sub.2] and N[H.sub.3], which is generated during the decomposition reaction. Wright proposed a mechanism in which catalytic hydrolysis of one amide of the ADCA was involved releasing N[H.sub.3] and a final decarboxylation produced C[O.sub.2] and [N.sub.2]. Marshall studied the effect of activators and inhibitors in plastisol formulations . By using an activator, the decomposition temperature of the ADCA was as low as 150[degrees]C. Bhatti et al.  studied the effect of some activators dispersed in ADCA. They concluded that the presence of an electron deficient surface favored the catalytic reactivity.
Crosslinked polyolefin foams are an essential group inside the polymeric foams market [15-17]. Crosslinking is a process in which the polymer chains are bonded to provide stability by increasing the viscosity of the polymer matrix . Crosslinking can be achieved by using chemical crosslinkers, such as peroxides or silanes [19-23] or by irradiation with high energy sources [24,25]. Electron beam irradiation is a process in which high energy electrons are used to promote the crosslinking reaction of the polymer. The energy of the electrons is absorbed by the polymer producing radicals in the polymer chains. This unstable energetic situation is solved by multiple chain bonding [26,27].
The advantage of using electron irradiation for crosslinking over the use of organic peroxides is that it is possible to decouple the crosslinking and foaming process. Furthermore, no chemical residues are generated during the crosslinking process when high energy sources are employed.
In order to have a better understanding of the foaming process of irradiated polymers when ADCA is used as a chemical blowing agent, it is essential not only to understand the crosslinking reaction of the polymer matrix when electron beam irradiation is employed but also to analyze if there is an effect of the electron irradiation on the other components of the formulation and among them the impact on the decomposition kinetics of ADCA.
There are several works in the literature analyzing the effect of irradiation on different organic compounds. Tolbert and Lemmon  published a review about the different chemical changes produced by irradiation on different organic groups such as saturated hydrocarbons, unsaturated aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, organic halides, alcohols, amino acids, carboxylic acids and quaternary ammonium salts.
Concerning azo groups, Wojnarovits and Takacs  analyzed the chemical reactions of different azo compounds in aqueous solution (e.g., wastewaters) with hydrated electrons ([e-.sub.aq]), hydroxyl radicals (*OH) and hydrogen atoms (*H) by using a pulse radiolysis technique. They presented results for compounds for azo groups such as azobenzenes and derivatives. Neta and Whillans  performed a study based on the effect of irradiation (2.8 MeV electrons) on ADCA but in aqueous solution. However, as far as we know, no studies related to the effect of electron irradiation on solid ADCA have been reported.
Taking these ideas in mind, this article is mainly focused on analyzing the effect of electron irradiation on the decomposition kinetics of ADCA and on determining which changes are produced in the ADCA particles.
ADCA Porofor ADCA/M-C1 with an average particle size of 3.9 [+ or -] 0.6 microns was supplied as a powder by Lanxess AG (Leverkusen, Germany). The density of the ADCA was 1.73 g/[cm.sup.3]. The gas yield was 228 mL/g, and the decomposition temperature starts at 210[degrees]C.
Irradiation of the ADCA was performed at room temperature in air in Mevion Technology (Soria, Spain). The energy employed was 10 MeV. The power and the intensities were 40 kW and 4 mA, respectively. Irradiation dose ranged from 25 to 150 kGy, in steps of 25 kGy per pass. The conveyor speed was 19 m/min. An electron paramagnetic resonance dosimetry and an alkaline type dosimetry reader were employed to control the real doses received by the samples.
Differential Scanning Calorimetry
The thermal behavior and the decomposition kinetics of both non-irradiated and irradiated ADCA were studied using a Mettler DSC [822.sup.e] differential scanning calorimeter (DSC) previously calibrated with indium, zinc, and n-octane. The average weight of the samples used for these experiments was 1.22 [+ or -] 0.07 mg.
For the purpose of studying the thermal decomposition of the ADCA, the following heating program was chosen. Samples were heated from 25 to 300 [degrees]C at a heating rate of 10[degrees]C/min under nitrogen atmosphere.
The temperature of the maximum and the width at mid-height of the exothermic peak that characterizes ADCA decomposition were calculated as an average of three measurements. The maximum standard deviation obtained for the temperature of the maximum was [+ or -] 0.80[degrees]C.
The thermal decomposition of irradiated ADCA was also studied by thermogravimetric analysis (TGA). A TGA/STDA 861 thermogravimetric analyzer model from Mettler Toledo previously calibrated was used. The weights of the samples were 2.25 [+ or -] 0.17 mg, and the experiments were performed in a temperature range between 50 and 650[degrees]C with a heating rate of 20[degrees]C/min under [N.sub.2] atmosphere. The onset of the decomposition step was calculated as the average of three measurements. The maximum SD for this onset was [+ or -] 0.74[degrees]C.
Fourier Transform Infrared Spectroscopy
Fourier transform infrared (FTIR) spectra in attenuated total reflectance mode of the samples were collected using a Bruker Tensor 27 spectrometer. The spectra were obtained under a [N.sub.2] purge after 32 scans with a resolution of 4 [cm.sup.-1] over a wavenumber range of 4,000 to 500 [cm.sup.-1]. Furthermore, a baseline correction was conducted to correct the shifts from temperature changes in each experiment.
Scanning Electron Microscopy
Scanning electron microscopy (SEM) micrographs of the ADCA particles were taken to observe if the particle size changed when the ADCA was irradiated. For the preparation of the samples, ADCA particles were spread over an adhesive tape. After that, the sample was blown with compressed air to remove the excess of particles, which were not stuck on the tape. Finally, the adhesive tape with the adhesive bonded particles was vacuum coated with a thin layer of gold to make them conductive. A Quanta 200 FEG microscope was used to observe the ADCA particles.
Particles from the SEM images were drawn on tracing paper. Then, the images were scanned and binarized to estimate the particle size. Particle sizes were measured by using a software tool based on Image J .
The density of the non-irradiated and the irradiated ADCA was measured using gas pycnometry. A Micromeritics AccuPyc II 1340 pycnometer was used to perform the measurements. The measurements were conducted according to ASTM D1895 standard method.
The X-ray diffraction (XRD) experiments were performed in a Bruker D8 Discover A25 diffractometer using Cu K[alpha] radiation. Measurements were performed in a 2[theta] range from 10 to 65[degrees], the pitch angle employed was 0.02[degrees] and the time of testing was 25 min. The interplanar spacing (d) of the crystalline planes was calculated using the Bragg's equation:
d = [lambda]/2sin([theta]) (1)
where [lambda] is the wavelength and 2[theta] is the diffraction angle.
EXPERIMENTAL RESULTS AND DISCUSSION
Figure 1 shows the DSC results for the non-irradiated ADCA and the ADCA irradiated with different doses. Three different signals appear in the thermograms. The two initial ones ([T.sub.1] and [T.sub.2]) at around 210-216 and 232-234[degrees]C, respectively are exothermic and are connected to the exothermic decomposition of the ADCA. The endothermic peak at approximately 252[degrees]C ([T.sub.3]) corresponds to the endothermic decomposition of the solid residues generated during the exothermic decomposition of the ADCA . In this work, the analysis is focused on the exothermic signals related to the generation of the gas phase typically employed in foaming processes.
The collected data from the thermograms are summarized in Table 1. As can it be observed, in the exothermic region of the thermogram, when the ADCA is irradiated a shoulder appears below 215[degrees]C ([T.sub.1]). This signal is almost negligible for the non-irradiated ADCA. When the dose increases, the intensity of this shoulder also increases. Furthermore, the temperature of this first maximum tends to decrease when the irradiation dose increases. The difference in the decomposition temperature of this first peak between the ADCA irradiated with 150 kGy, and the non-irradiated is 5.70[degrees]C. The decomposition temperature of the second exothermic peak ([T.sub.2]) is not affected by the irradiation process, being this temperature similar for irradiated and non-irradiated samples. Table 1 also shows the width at mid-height of the exothermic signal. It can be inferred that the width at mid-height increases when the dose increases. The difference between the non-irradiated and the 150 kGy irradiated ADCA is 5.63[degrees]C. When the ADCA is irradiated the first peak in the decomposition range tends to be more intense and is shifted to lower temperatures widening the exothermic peak. Furthermore, no significant changes are observed in the endothermic peak ([T.sub.3]). The temperature of this endothermic signal is almost constant for the non irradiated and the irradiated ADCAs.
The results of the TGA experiments are presented in Fig. 2. Figure 2a shows the thermograms of the non-irradiated and the irradiated samples. Figure 2b shows the first derivative of the thermograms. As can it be observed in Fig. 2a, the decomposition drop has two stages. The first one corresponds to the exothermic decomposition of the ADCA (from 220 to 245[degrees]C). The second one is a small shoulder which is related to the endothermic process (from 245 to 260[degrees] C) associated with the thermal decomposition of the residues of ADCA. Figure 2b shows that when the irradiation dose increases, a shoulder appears in the weight loss rate at around 210[degrees]C. This behavior is similar to what it was observed in the DSC curves.
The onset of the decomposition reaction for all materials was measured (Table 2). It is observed that there is a reduction in the onset of decomposition when the irradiation dose increases. The difference between the non-irradiated ADCA and the material irradiated with 150 kGy is 4[degrees]C. This result is agreement with the effect observed in the DSC curves (Table 1).
Furthermore, the percentage of mass decomposed at several temperatures (210, 215, and 220[degrees]C) has also been estimated as it is shown in Table 3.
It is observed that more amount of gas is released for the samples irradiated with higher doses (125 and 150 kGy) independently of the chosen temperature. The difference between the non-irradiated ADCA and the one irradiated with 150 kGy is around 50% at 220[degrees]C, and as a consequence, more gas is released under these conditions. This result is connected to the previous results shown in Tables 1 and 2, respectively.
From the thermal analysis, it can be concluded that the irradiation produces a modification of the decomposition kinetics of the ADCA. Furthermore, this effect is more evident when the irradiation dose increases.
SEM images of two representative samples (non-irradiated ADCA and 150 kGy irradiated ADCA) are shown in Fig. 3. ADCA particles are observed in the images. Particle sizes were estimated by binarizing the images. The average particle sizes of both samples were calculated to analyze if there were differences in the particle size when the ADCA was irradiated.
An average value of 4.66 [+ or -] 2.19 microns for the non-irradiated ADCA and 4.72 [+ or -] 1.98 microns for the 150 kGy EB irradiated ADCA were obtained. Figure 3 shows the particle size distributions. As it can be observed, the average particle size and particle size distributions were in both cases very similar. It seems that the irradiation does not affect the particle size. From this analysis, it can be concluded that the decomposition of the ADCA is not a result of changes in the average size of the ADCA particles.
Non-irradiated and irradiated ADCA were characterized by FTIR analysis to check possible modifications in the chemical nature of the ADCA induced by the irradiation process. Figure 4 shows the FTIR spectrum of these samples. Lee et al.  evaluated the infrared spectra of an ADCA pentamer cluster characterizing the different peaks. The frequency of the different peaks has been assigned to each vibration according to the characterization of Lee. The collected data are shown in Table 4.
ADCA possesses a strong molecular symmetry. For this reason, some elements of the structure are symmetrically equivalent. The hydrogen of an amide group is equivalent to another hydrogen from the other amide group. However, the two hydrogens from the same amide group are not equivalent. This result explains why there are two stretching N-H peaks above 3,000 [cm.sup.-1] . The two carbonyls are bonded to the same molecular moiety. Because of this, there is the only one peak in the carbonyl region which overlaps the contribution of both carbonyls (1,723 [cm.sup.-1]).
As it is observed in Fig. 4a and b, there is no difference between the spectrum of the non-irradiated ADCA and the spectra of the irradiated ADCA. The position of the peaks is the same, and the absorbance does not follow any trend with the irradiation dose. It seems that the chemical structure does not vary with the irradiation.
To corroborate the previous statement, the ADCA was heated using an isothermal program was performed at 200[degrees]C. Nonirradiated and 150 kGy irradiated ADCA powders were introduced in an oven in several crucibles at 200[degrees]C. A crucible of each type of ADCA was taken out from the oven in steps of 5 min in a range of time of 25 min.
The isotherm study is shown in Fig. 4c. The region of N-H stretch vibration (3,300 [cm.sup.-1]) was selected because it illustrates well the decomposition process. When time increases from 5 to 25 min the N-H stretch peak at 3,326 [cm.sup.-1] is reduced because of the decomposition of the ADCA. In addition, when the time increases a peak near 3,400 [cm.sup.-1] grows, and the intensity of this peak becomes higher when the time is increased. This effect can be explained by the formation of new subproducts in the process of the ADCA decomposition.
Focusing on the possible differences between non-irradiated and irradiated ADCA, it is observed that the same peak is formed once the ADCA decomposition takes place.
From the FTIR it is not possible to explain why the irradiated ADCA decomposes faster than the non-irradiated one. No changes in the chemical nature of the particle or changes in its surface have been detected.
Figure 5 shows the XRD characterization of the non-irradiated ADCA and ADCA irradiated with different irradiation doses. The correspondent hkl index for each peak has also been included.
In general, the non-irradiated ADCA and ADCA irradiated with different doses present the same peaks in the analyzed range of angles. However, some diffraction lines present a slight shift which changes depending on the irradiation dose employed. One example of this shift is presented in Fig. 6 [plane (110)] in which it is possible to appreciate in more detail how the maximum of the peak is reduced (i.e., the correspondent distance between planes is increased) when the irradiation dose increases.
These displacements of the diffraction lines are due to changes in the lattice parameters. With the aim of appreciating the shifts of these diffraction lines, the interplanar spacing was calculated for some hkl planes according to Eq. 1 (Fig. 7).
Figure 7 shows that the increment (in percentage) of the irradiation dose produces variations of the interplanar spacing in most of the hkl planes studied. In general, when the irradiation dose increases the interplanar spacing also increases. However, it is possible to observe that in some hkl planes the shift is more pronounced than in other planes: (110) or (100). The most affected plane is (110). These results could be the explanation of the ones obtained by thermal analysis in which the decomposition temperature of ADCA decreased with the irradiation dose (see Table 1). As it was reported in the work of Walker et al. , there is a dependency of the interlayer distance with the attractive energy associated with the Van der Waals forces. The attractive energy is reduced when the interlayer distance increases. Taking into account this effect, the changes observed in the lattice parameters are connected to variations in the attractive energy. As the irradiation dose increases, the interlayer distance also does and therefore the attractive energy between atoms of ADCA crystal drops. As a result, the decomposition temperatures are reduced.
Furthermore, the full width at half maximum (FWHM) for the more intense peak (110) was also calculated to determine differences between samples (Table 5).
As it is observed in Table 5, the width increases when the irradiation dose increases indicating a ciystalline structure with more defects.
As it has been proved in the previous section, irradiating ADCA promotes an increment of the interlayer distance. This should be detected macroscopically in a density reduction of the particles. The values of the densities for each sample as a function of the irradiation dose are plotted in Fig. 8.
As expected the density of the ADCA decreases as the irradiation dose increases. A difference of around 2.5% is observed in the extreme cases (the non-irradiated powder and the one irradiated with 150 kGy) which is of the same order of magnitude than the changes of interplanar spacings detected previously (Fig. 7). This result confirms that the changes in the decomposition kinetics are due to changes in the crystalline structure of the ADCA.
ADCA powder samples were irradiated with different doses ranging from 25 to 150 kGy. A modification of the decomposition kinetics of the ADCA was observed in the DSC and TGA thermograms when the powder is irradiated. A significant reduction of 5[degrees]C in the decomposition temperature has been detected.
To explain this effect, several studies were carried out to determine if modifications on the average particle size or in the chemical structure of the irradiated samples contributed to modify the decomposition pattern of the ADCA. However, both parameters were not affected by the irradiation.
The results obtained using XRD showed that the increment in the irradiation dose produced variations in the interplanar spacing for several hkl planes. These variations in the interplanar spacing seem to be the origin of the reduced decomposition temperatures of ADCA when this material is irradiated. There is an increase in the distance between atoms that is translated into a reduction in the binding energy. This result is confirmed by a clear density reduction of the particles when they are irradiated.
This work performed with the financial support from DI grant DI15-07952 (E. Lopez-Gonzalez) from the Spanish Ministry of Economy, Industry, and Competitiveness, CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico--Brasil), and FPI grant Ref: BES-2010-038746 (Alberto Lopez-Gil). Financial assistance from MEMECO, FEDER, UE (MAT2015-69234-R) and the Junta de Castile and Leon (VA01 IUI6) are gratefully acknowledged.
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Eduardo Lopez-Gonzalez [iD], (1,2) Leandra Oliveira Salmazo, (2) Alberto Lopez-Gil, (1) Miguel A. Rodriguez-Perez (2)
(1) CellMat Technologies S.L. Paseo de Belen 9A, 47011, Valladolid, Spain
(2) Cellular Materials Laboratory (CellMat), Condensed Matter Physics Department, University of Valladolid, Paseo Belen 7, 47011, Valladolid, Spain
Correspondence to: E. Lopez-Gonzalez; e-mail: firstname.lastname@example.org Contract grant sponsor: Consejeria de Educacion, Junta de Castilla y Leon; contract grant number: VA011U16. contract grant sponsor: Conselho Nacional de Desenvolvimento Cientifico e Tecnologico, contract grant sponsor: Ministerio de Economia y Competitividad. contract grant sponsor: Spanish Ministry of Economy, Industry and Competitiveness; contract grant number: DI-15-07952. FPI GRANT: BES-2010-038746. contract grant sponsor: Junta de Castile and Leon; contract grant number: VA011U16. contract grant sponsor: MINECO, FEDER, UE; contract grant number: MAT2015-69234-R. contract grant sponsor: Conselho Nacional de Desenvolvimento Cientifico e Tecnologico--Brasil; contract grant number: BES-2010-038746. contract grant sponsor: Ministry of Economy, contract grant sponsor: DI; contract grant number: DI-15-07952.
Published online in Wiley Online Library (wileyonlinelibrary.com).
Caption: FIG. 1. Experimental DSC curves of non-irradiated ADCA and irradiated ADCA.
Caption: FIG. 2. (a) TGA thermograms and (b) first derivative of the decomposition curve for the different ADCAs under study.
Caption: FIG. 3. SEM images of two samples of ADCA: (a) non-irradiated ADCA. (c) ADCA EB irradiated with 150 kGy. Particle size distributions: (b) non-irradiated ADCA. (d) ADCA EB irradiated with 150 kGy.
Caption: FIG. 4. (a) FTIR analysis for the non-irradiated ADCA and ADCA EB irradiated with different doses, (b) zoom between 1,800 and 1,000 [cm.sup.-1], and (c) comparative between non-irradiated ADCA and the ADCA irradiated with 150 kGy.
Caption: FIG. 5. X-ray pattern of the non-irradiated ADCA and ADCA irradiated with different doses and the assignation of the hkl planes.
Caption: FIG. 6. Examples of diffraction lines (plane 110) which are shifted with the irradiation dose.
Caption: FIG. 7. The increment (in percentage) of the interplanar spacing (d) for different diffraction planes.
Caption: FIG. 8. Density of the ADCA powder versus the irradiation dose.
TABLE 1. Parameters obtained from the DSC thermogram. Temperature Temperature Temperature first second third Maximum Maximum Minimum Sample ([degrees]C) ([degrees]C) ([degrees]C) FWHM non-irradiated 216.3 232.9 251.67 23.46 ADCA ADCA 215.3 233.9 252.00 26.66 25 kGy ADCA 213.6 233.4 251.33 26.12 50 kGy ADCA 212.3 234.2 251.67 28.80 75 kGy ADCA 211.8 233.7 251.67 28.71 100 kGy ADCA 210.9 234.2 251.67 29.86 125 kGy ADCA 210.6 233.5 251.33 29.09 150 kGy TABLE 2. Onset of the decomposition step from the ADCA. Sample ADCA onset ([degrees]C) non-irradiated ADCA 217.7 ADCA 25 kGy 217.4 ADCA 50 kGy 216.7 ADCA 75 kGy 216.2 ADCA 100 kGy 215.6 ADCA 125 kGy 216.6 ADCA 150 kGy 213.7 TABLE 3. Percentage of mass decomposed at 210, 215, and 220[degrees]C. Sample % mass % mass % mass (210[degrees]C) (215[degrees]C) (220[degrees]C) Non-Irradiated 0.77 1.73 4.42 ADCA ADCA 25 kGy 0.58 1.36 4.03 ADCA 50 kGy 0.77 1.82 5.26 ADCA 75 kGy 0.78 2.75 7.64 ADCA 100 kGy 1.02 2.99 7.52 ADCA 125 kGy 1.20 4.41 9.56 ADCA 150 kGy 1.15 4.13 8.59 TABLE 4. Assignments of ADCA vibration frequencies. Wavenumber Vibration ([cm.sup.-1]) 3,326 N-H stretch 3,156 N-H stretch 1,723 C=O stretch 1,627 C=O stretch + N-H scissor 1,362 N-C stretch + NC=O bend + N-H rock 1,330 N-C stretch + NC=O bend + N-H rock 1,115 N-H rock + C=O stretch + N-C stretch 855 N-H torsions 692 N-C=O bend + other couples TABLE 5. FWHM values for the diffraction peak (110). Sample FWHM non-irradiated ADCA 0.290 ADCA 25 kGy 0.303 ADCA 50 kGy 0.292 ADCA 75 kGy 0.316 ADCA 100 kGy 0.359 ADCA 125 kGy 0.357 ADCA 150 kGy 0.401
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|Author:||Lopez-Gonzalez, Eduardo; Salmazo, Leandra Oliveira; Lopez-Gil, Alberto; Rodriguez-Perez, Miguel A.|
|Publication:||Polymer Engineering and Science|
|Article Type:||Case study|
|Date:||Apr 1, 2019|
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