Thermal Degradation Studies of Some Strongly Acidic Cation Exchange Resins

The thermal degradation of some sulfonic cationites namely Amberlite IR-120, Indion-223 and Indion-225 was investigated using instrumental techniques like thermal analysis (TG) and Scanning Electron Microscopy (SEM). Fourier Transform Infrared Spectroscopy (FTIR) was used to characterize the resins degradation steps. The sulfonic cationites undergo degradation through dehydration, followed by decomposition of sulfonic acid functional groups liberating SO2. The thermogravimetric analysis of above cationites at higher temperature up to 520 ̊C, show mass loss of 61.61% and 25.43% respectively for Indion-223 and Indion-225, while Amberlite IR-120 cationite get burned off completely.


Introduction
Ion-exchange resins are produced and commercialized in a wide range of formulations with different characteristics, and have now a large practical applicability in various industrial processes, such as chemical, and nuclear industry for treatment of liquid waste [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].For their versatile properties, the cationic resins are used both in the ion exchange area and in the heterogeneous catalysis field [17].These resins exhibit a high exchange capacity and an excellent osmotic shock resistance.So, the cationic resins, produced with a high degree of purity, became important as catalysts in various food technologies [18] and for purification in heavy-water moderated nuclear reactors in nuclear industries [19][20][21].In many cases their use is limited by the relatively low thermal stability [22].Hence, knowledge of the thermal behavior of cation exchange resins is necessary.Abundant data exist on the thermal degradation of anion exchange resins [23][24][25] and on carboxylic cationites with low acidity [26][27]; literature seems to offer relatively poor information on polystyrene-divinylbenzene sulfonic cationites [28][29][30][31][32][33].Therefore, in the present investigation thermal degradation of strongly acidic sulfonic cationites was performed to understand the degradation steps and to compare the relative thermal stability.

Materials
The following commercial cationites were used: • Strongly acidic gel-type resin with sulfonic acid functionality based on styrene-divinylbenzene matrix: Amberlite IR-120 (Rohm and Haas Co, USA).• Nuclear grade strongly acidic gel-type resin with sulfonic acid functionality based on styrene-divinylbenzene matrix: Indion-223 (Ion Exchange India Ltd., Mumbai).• Strongly acidic gel-type resin with sulfonic acid functionality based on styrene-divinylbenzene matrix: Indion-225 (Ion Exchange India Ltd., Mumbai).The details regarding the physical properties of cationites used are given in Table 1.
The soluble impurities of the resins were removed by repeated soxhlet extraction using water and occasionally with distilled methanol to remove non polymerized impurities.The resins were then dried over P 2 O 5 in desiccators at room temperature.

Thermal Analysis
The thermogravimetric experiments were performed on a DTG-60H, (Shimadzu, Japan) thermal analysis system between 30˚C -550˚C using aluminum cell (6 mm in diameter and 2.5 mm in depth).The measurements of resin samples were carried out in nitrogen flow (50 mL•min -1 ) at heating rate (β = 10 K•min -1 ).The mass of resin sample used was ~5 -20 mg.In order to characterize the decomposition steps of the investigated ion-exchange resins, FTIR and Scanning Electron Microscopy (SEM) were used in addition to thermal analysis.

FTIR Spectra
FTIR spectra (in 4000 -450 cm -1 range) of thermal decomposed samples, up to the characteristics mass-loss steps temperatures, were recorded in KBr pellets (2 mg cationite/200 mg KBr) using a FTIR PerkinElmer 1750 spectrophotometer.
Since in the thermal analysis, the major weight loss was observed between 200˚C -400˚C, the resin samples were heated in an oven for 3h at 10˚C higher temperatures above the maximum operating temperature, and also at 200 and 400˚C.The thermal degradation of resin was studied by comparing the spectra of fresh and heated resin samples.

Scanning Electron Microscopy (SEM)
The thermal degradation studies of ion exchange resins was also studied by examining the surface morphology of fresh resin samples and samples heated at 400˚C using JSM-6380LA Scanning Electron Microscope (Jeol Ltd., Japan).The powders were precisely fixed on an aluminum stub using double-sided graphite tape and then were made electrically conductive by coating in a vaccum with a thin layer of carbon, for 30 seconds and at 30 W. The pictures were taken at an excitation voltage of 10 -15 KV and a magnification of ×100 and ×130.

Characterization and Thermal Degradation
Study of Amberlite IR-120:

TGA Analysis
Figure 1 represents a dynamic weight loss profile of Amberlite IR-120 from room temperature to 550˚C.Thermogravimetric curve of Amberlite IR 120 shows 22% weight loss up to 200˚C due to moisture content and the weight decreased gradually till 400˚C.Above 400˚C whole compound burned off completely and weight loss measurement was not possible.

FTIR Analysis
Figure 2 shows the IR spectrum of the fresh resin sample Amberlite IR-120.The bands at 2923 and 2876 cm -1 are due to the aliphatic C-H stretching absorbance of methyl group in the main chain and in aromatic rings and of methylene group respectively.SO 2 asymmetric stretching at 1382 cm -1 .Strong band at 1652 cm -1 indicates aromatic C=C bond.The four sharp peaks at 1009 cm -1 , 1037 cm -1 , 1126 cm -1 , 1186 cm -1 are due to SO 3 symmetric stretching.The peaks at 1500 -1600 cm -1 are due to deformation and skeletal vibrations of C-H in DVB.Bands appear at 2366 cm -1 which may be assigned to O-H stretching vibration originating from the polymer.At 200˚C, the 21% weight loss was observed without any loss of peak in the IR spectra.At 400˚C IR investigation indicates that the peaks in the region 1500 -1000 cm -1 either shows a general broadening or no longer exists (Figure 3).Non-existence of the SO unit was confirmed by the absence of the peaks from 1500 -1000cm -1 , with decomposition of the functional group i.e. sulfonic portions of the ring.But slight broadening of the band is observed at 1652 cm -1 for aromatic C=C bond remained unchanged.This corresponds to the few chain scissions in the DVB matrix.

SEM Analysis
Figure 4, shows the surface morphology of Amberlite IR-120 resins at room temperature indicating plane spherical structure.At 400˚C, resin show large cracks and thread like appearance on the surface (Figure 5).

Characterization and Thermal Degradation
Study of Indion-223:

TGA Analysis
Thermogravimetric curve of Indion-223H + shown in Figure 6.The 30% weight loss up to 200˚C can be attributed to moisture content.The second major weight loss begins at 270˚C and ends at 400˚C, which might be due to slow degradation of side chain and loss of sulphonic functional group.The mass loss from 400˚C to 521˚C was gradual which might be due to degradation of styrene/DVB matrix.

FTIR Analysis
T he IR spectrum of Indion-223 in the 3700 -400 cm −1     FTIR investigation shows that that strong bands at 668 cm -1 , 1019cm -1 and 1382 cm -1 which are related to SO and 2364cm -1 stretching frequency of C-H units remained upto 200˚C, but at 400˚C both the groups disappeared (Figure 8).This indicates that at 200˚C the 29.7% weight loss is due to moisture, whereas 31.8%loss is due to the functional group i.e.SO and breaking of C-H bond.

SEM Analysis
Surfaces of resins at room temperature (Figure 9) and at 400˚C (Figure 10) were examined by a Jeol JSM-6380LA scanning electron microscope.It was found that at 400˚C resin showed crack in the spherical structure which supports breaking of polymer matrix at that temperature.

Characterization and Thermal Degradation
Study of Indion-225:

TGA Analysis
Thermogravimetric curve of Indion-225 H + is shown in Figure 11.Thermogravimetric curve shows ~13% weight loss up to 200˚C, corresponding to the moisture content.Degradation of resin between 270˚C -340˚C takes place sharply and further gradually up to 521˚C which might be due to decomposition of the sulphonic functional group with rapid evolution of SO 3 or SO 2 showing mass loss of ~12.5%.
The degradation occurs in a single step and mass loss is 25% up to 521˚C.SO 2 is the dominate product evolved at 190˚C and 380˚C while water is also present during the entire degradation pathway but in relatively small amount.Loss of SO unit can also be confirmed by IR investigation.The characteristic band for SO unit at 1009 cm -1  , 1037 cm -1 , 1126 cm -1 ,1186 cm -1 are no longer exist (Figure 13).The evolution of water occurs in the same temperature regime as SO 2 , between 185 and 400˚C, as spectrum at 200˚C shows broadened bands indicating light decrease in SO content.This is likely due to the s     formation of sulfurous acid by the process noted above.
Water is also evolved early in the degradation and this is likely due to the loss of physically combined water as has been observed for the other compounds [34].

SEM Analysis
Figure 14 shows the surface morphology of the Indion-225 H + at room temperature indicating plane spherical structure.Similar to Indion-223 H + ; Indion-225 H + also shows crack on the spherical surface when heated at 400˚C (Figure 15)

Conclusions
From the FTIR analysis of three sulfonic acid cationites, it was observed that the degradation takes place through dehydration, followed by decomposition of sulfonic acid functional groups.The thermal analysis shows that up to 200˚C, Indion-225 cationite shows mass loss of only 13%, as against mass loss of 21% and 30% shown by Amberlite IR-120 and Indion-223 respectively.The thermal analysis at a higher temperature up to 520˚C, Amberlite IR-120 cationite gets completely burned, while Indion-225 and Indion-223 shows total mass loss of 25% and 62% respectively.Hence the thermal stabilty of three cationites increases in the order of Amberlite i

Figure 4 .
Figure 4. Scanning electron micrograph of the surface of the Amberlite IR-120 at room temperature.

Figure 5 .
Figure 5. Scanning electron micrograph of the surface of the Amberlite IR-120 at 400˚C.region is shown in Figure7.FTIR spectral analysis shows SO 3 sharp symmetric stretching band at 1005 -1126 cm -1 .O-H stretching at 2364 cm -1 .S-O stretching at 668 cm -1 .SO 2 asymmetric stretching at 1382 cm -1 .C=C aromatic nucleus skeletal vibration band at 1500 -1600 cm -1 and OH hydrogen bonded broad stretching band at 3200 -3500 cm -1 .The band at 2900 cm -1 attributed to C-H stretching vibrations in the main chain and in aromatic rings; the peaks at 1500 -1600 cm -1 are due to deformation and skeletal vibrations of C-H in Poly-

Figure 9 .
Figure 9. Scanning electron micrograph of the surface of the Indion-223 at room temperature.

Figure 10 .
Figure 10.Scanning electron micrograph of the surface of the Indion-223 at 400˚C.

Figure 14 .
Figure 14.Scanning electron micrograph of the surface of the Indion-225 at room temperature.

Figure 15 .
Figure 15.Scanning electron micrograph of the surface of the Indion-225 at 400˚C.