2. Principal Chemical and Analytical Methods Used in Reverse Engineering
2.3 Thermal Analysis
2.3.6 Principle of Thermogravimetric Application
2.3.6.1 Interference
Several factors can contribute to interference with a sample TG-DTG procedure. One fac- tor is the overlap of the process oil, resin, etc., with a polymer decomposition region. The second and by far the greatest complication is observed for elastomers with a heteroatom in the monomer, like acrylonitrile butadiene rubber (NBR), chloroprene rubber (CR), polyvinyl chloride (PVC), chlorosulfonated polyethylene rubber (CSM) (e.g., DuPont™
Hypalon®), polyacrylate (ACM), fluoroelastomer (FKM), epichlorohydrine rubber (CO), epichlorohydrine-ethylene oxide copolymer (ECO), etc. These polymers leave a char (car- bon residue) after degradation in nitrogen (Table 2.14), making it difficult to estimate the elastomer or carbon black included in the recipe.
The literature contains several references that clearly illustrate that adjusting (slowing) the heating rate during weight change improves resolution. Polymers containing hetero- atom leave a carbonaceous residue (char) after degradation in nitrogen, which is oxidized along with the carbon black included in the recipe, when the environment is changed to air or oxygen. This gives a higher estimation of carbon black and lower polymer content value than are actually present. It may be observed that carbonaceous residue amounts estimated by different workers for the same polymers are somewhat different. This is attributed to the following factors: different chlorine content of different poly(chloroprene rubber), temperature of determination, run time (prolonged heating), and rate of heating.
It has been observed that there are at least three stages of the degradation process for chloroprene rubber. Sulfur curing does not increase the carbon residue but gives rise to an additional DTG peak at 190°C. The presence of calcium carbonate increases carbon residue. The amount of carbon residue increases with acrylonitrile content of NBR. So when acrylonitrile content is plotted against carbon residue, a correction factor can be determined. In order to determine the acrylonitrile content, estimation of Tg (glass transi- tion temperature) is required.
The most promising approach to separate polymer, char, and carbon black oxidation peaks in a TG-DTG experiment is in the use of lean oxygen gas. Further work needs to be carried out with different lean gas compositions (different nitrogen/oxygen or helium/oxygen ratio)
–40 –35 –30 –25 –20 –15 –10 –5 0 5
81.3 125 169 213 257 301 345 389 433 477 521 565 609 653 697 741 785 829 Temp, °C
Derivative Wt %
FIGURE 2.15
Typical DTGA thermogram of rubber compound.
on various formulations. The imposition of lower heating rate, isothermal oxidation, and/
or use of reduced pressure, along with the slow feed of lean oxygen would help improve resolution.
To overcome the problem of overlap of process oils and polymer, the following exercise can be practiced:
1. Extract the sample to remove oil, excess curatives, etc., prior to TG analysis. This provides a reasonable estimate of oil/plasticizer content if corrections are made for various low molecular weight polymeric and non-polymeric materials (excess curatives, antioxidant fragment, etc.) removed with oil.
2. Establish a correction curve based on a reference temperature for a given polymer compound. This procedure is somewhat lengthy and requires knowledge of the polymer and oil types as well as curatives.
3. Analyze isothermal curves below the polymer decomposition temperature.
4. Use reduced pressure to aid in removing oil at a lower temperature where poly- mer decomposition is not significant.
2.3.6.2 Application of Thermogravimetric Analysis in Polymer The TGA method is used mainly:
• To determine percent volatiles
• For polymer stability
• For analysis of additives in polymers
• For kinetics of degradation TABLE 2.14
Carbonaceous Residues for Miscellaneous Elastomers Type Elastomer % Carbonaceous Residue (550°C)
CR Neoprene W 21.0
Neoprene GT 22.0
Neoprene AJ 23.0
CSM Hypalon 20 2.0
Hypalon 40 3.5
Hypalon 45 2.0
FKM Viton A 4.0
Viton C-10 7.0
Viton E-60 3.7
Viton E-60C 4.0
Fluorel 2140 5.5
Fluorel 2160 8.0
CO Hydrine 100 13.0
ECO Hydrine 200 8.0
ACM Hycar 4041 7.5
Hycar 4042 6.0
Hycar 4043 5.0
The useful life of a polymer could be predicted from the dynamic TG using the Arrhenius equation and activation energy. Since there are myriad of complications, lifetime predic- tion is never accurate, but it gives approximate data.
TGA is the oldest among all of the thermal procedures. It monitors continuous weight loss of sample as a function of temperature with the help of thermobalance. This is a combina- tion of suitable electronic microbalance with a furnace associated temperature programmer.
Typical TG and DTG curves for polymers are shown in Figures 2.14 and 2.15. As the temperature is gradually increased from ambient to a higher temperature at a fixed heat- ing rate, the low molecular weight additives such as monomer, stabilizer, plasticizer/
extender, moisture, etc., volatilize away giving rise to weight loss at 300°C. On further heating, polymer chain starts to degrade, involving rupture of chemical bonds. The nature of TG curve depends upon the surrounding atmosphere of the sample under testing. In an inert atmosphere, degradation due to chain cleavage occurs, whereas in the presence of oxygen, oxidative degradation takes place through peroxide (free radical) formation by oxygen absorption.
2.3.6.3 Decomposition
The DTG curve in nitrogen atmosphere allows fingerprinting of elastomer type in a single polymer/blend of polymers/polymer composites, since each polymer has a characteristic temperature Tmax.It depends upon the nature of the polymer as well as the heating rate and surrounding atmosphere. Tmax values of some typical elastomers in different atmospheres are shown in Table 2.15.
Along with Tmax values, the onset of decomposition and decomposition range (the tem- perature range in which major decomposition occurs in between 250 and 500°C in nitro- gen atmosphere) is found to be very useful to characterize the polymeric material. In order to avoid the effect of heating rate on these measurable parameters, most often two parameters, To and Tmax (onset temperature and Tmax) are calculated from the data obtained from at least three TG runs at different rates.
In general, the decomposition of polymer in oxygen (oxidation) occurs at a compara- tively lower temperature (Table 2.15). This process, compared to that in nitrogen atmo- sphere is fast, erratic, and generally affected by sample size and additives. Most often, multiple oxidation steps (i.e., Tmax) are observed in the TG/DTG curve. These often lead to inconclusive results in polymer characterization. But with sophisticated instruments having greater control over the sample and experimental parameters, one can get useful information regarding the characterization of polymers.
TABLE 2.15
Typical Tmax Values of Different Polymers (at 40°C/min Heating Rate) Elastomer Tmax (°C) in N2 Tmax (°C) in O2
Natural rubber 411 380
Polybutadiene rubber 500 440
Styrene-butadiene rubber 488 443
Isobutylene isoprene rubber 430 389
Ethylene propylene diene rubber 498 353
Nitrile rubber 491 442
Polychloroprene rubber 403 396
Acrylic rubber 441 384
2.3.6.4 Stability and Kinetics
TGA method is the quickest method in estimating the thermal stability of polymeric mate- rial under inert as well as thermo-oxidation environment. TGA provides the mechanism of degradation at lower and higher temperatures. The kinetic parameters as well as the estimation of lifetime in service or polymer degradation can also be obtained from a TGA decomposition thermogram. The effect of curing agent, filler, and other additives on a polymer can be studied using this method.
2.3.6.5 Composition
TGA is also used for quantitative estimation of different polymers and additives in a poly- meric composite. The oil amount in oil extended rubbers like OE-SBR and OE-EPDM can be determined as oil is volatile and such rubbers are thermally stable in the lower tem- perature zone (<350°C). With the help of an established calibration curve and knowing the DTG peak temperature, identification of oil type is also possible.
TGA can be used to identify carbon blacks since particle sizes of blacks affect oxidative TG trace. TGA is a good tool to investigate the reinforcing cord’s material in a cord rubber composite (e.g., tires, conveyor belts, etc.).
Composition in four major fractions, namely polymer, volatile, black filler, and non-black filler/mineral oxide additives, can be determined by TG curve from ambient temperature in nitrogen atmosphere, followed by changeover of atmosphere to oxygen in between 600 and 650°C (Figure 2.16). Under nitrogen atmosphere, low temperature volatilizates like oil, plasticizers, etc., volatilize at somewhat lower temperature, followed by decomposition of the polymer at higher temperature. The carbon black content can be estimated by burn- ing the sample after changeover in air or oxygen atmosphere. The ash left over contains mineral fillers and ZnO.
900 800
700 600
500 400
Temperature (°C) 300
200 –15 100
–10 0 10 20 30 40 50 60 70 80 90 100
110115 X1 = 27.748°C Y1 = 100,0012%
Y2 = 87.3705%
X2 = 325,000°C
X1 = 325,000°C Y1 = 87,3705%
X1 = 550,000°C Y1 = 32,7298%
Y2 = 1,7396%
X2 = 794,355°C Y2 = 32,7298%
X2 = 550,000°C Delta Y = 54,6407%
Delta Y = 30,9902%
Weight (%)
Delta Y = 12.6307%
FIGURE 2.16
Four major fractions in a TGA curve.