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Thermogravimetric Analysis of Polymers

Article · November 2018

DOI: 10.1002/0471440264.pst667

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THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

1. Introduction and History of Thermogravimetric Analysis

Thermal analysis of polymers typically involves a combination of various tech-niques, including well-established thermogravimetric analysis (TGA), differentialthermal analysis (DTA), differential scanning calorimetry (DSC), and evolved gasanalysis (EGA) (1).

The earliest TGA experiments were performed by heating the samples to aspecific temperature and then removing the sample from the sampling head atspecific time intervals for weighing (2). In 1912, Urbain and Boulanger (3) builta breakthrough device that comprised a conventional balance adapted for null-point electromagnetic compensation, with the sample hanging from the balancearm into an electrically heated furnace in a gas-tight environment. They also de-signed it to have an input for the supply of absorbent materials for evolved gases.In this setup, the sample could be weighed while it was continuously heated. In1915, Honda took the idea of the initial TG development and produced a more de-pendable TG (4). His equipment was designed for the heating rate to be graduallyreduced from an initial rate during the occurrence of weight losses. It could alsobe called a sample-controlled thermogravimetric system. In this period, TG wasmostly used for the analysis of inorganic compounds, and it reached its pinnaclein Duval’s monumental description of over 1000 gravimetric precipitates of nearly80 elements (5).

In modern TGA, it determines the quantity and the frequency of the weightvariation of the samples against temperature and time in a controlled atmosphere(eg, the purging of nitrogen gas) (6–8). TGA can be used primarily to investigatethe thermal stability (the strength of the material at a given temperature), oxida-tive stabilities (the oxygen absorption rate on the material), as well as the com-positional properties (eg, fillers, polymer resin, solvents) of the samples. Besides,the weight gain/loss of the samples corresponds to different factors. Generally,weight gain is attributed to the adsorption or oxidation, whereas weight loss isattributed to decomposition, desorption, dehydration, desolvation, or volatiliza-tion (9–12). TGA is especially important in applications that use polymeric ma-terials such as injection molding, coating material for electrical and electroniccomponents, paints, pharmaceutical, food industries, petroleum, adhesive, and soforth (13).

Other types of thermal analyses such as dynamic mechanical analy-sis (DMA), DSC analysis, and temperature-modulated differential scanningcalorimetry (TMDSC) could complement the TGA analysis. The DMA analysis

1

Encyclopedia of Polymer Science and Technology. Copyright c ⃝ 2018 John Wiley & Sons, Inc. All rights reserved.

DOI: 10.1002/0471440264.pst667

2 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

is useful to study the viscoelastic behavior of polymeric materials as a functionof temperature or frequency. The material is subjected to a sinusoidal mechanicalstress, and the strain of the sample is measured. DMA allows to detect thermaleffects from the variations in the complex modulus. TMDSC can complement TGAby giving more information than conventional DSC. However, the measurement ofthe glass transition temperature is limited because the short range of frequencyused. TMDSC analysis can provide a better absolute heat capacity with propercalibration. It could be used to study the reversible melting and crystallization ofpolymer (14,15). Meanwhile, DSC analysis is used to measure energy absorbed orreleased by a material as a function of temperature when the material is heatedor cooled. It could provide quantitative and qualitative data on endothermic andexothermic processes. The properties such as melting temperature, glass transi-tion temperature, crystalline phase transition temperature, and specific heat orheat capacity could be obtained from DSC analysis (16).

2. Theory

In general, TGA studies the changes in the weight of a sample while the sam-ple is being heated or cooled at a controlled temperature, T(t), and the changesare continuously monitored. The program that controls the temperature can beisothermal (T (t) is constant) or nonisothermal. Isothermal is a condition whenT(t) is constant, and nonisothermal is a condition when the heating rate is con-stant (caused by the linear change in temperature with time). The heating rate(𝛽) can be expressed as

𝛽 = dTdt

(1)

where dT is the change in temperature and dt is the change in time (17).The mechanism and kinetics of the process linked to the weight changes of

the sample can influence the shape and position of the TG curves. Three majorvariables that can influence the kinetics of the thermally simulated processes arethe temperature (T), the extent of the conversion (𝛼), and pressure (P) (18,19). Thevariable can be implicitly shown as

d𝛼dt

= 𝜅 (T) f (𝛼) h (P) (2)

Although pressure can have a large effect on the kinetics processes, h(P) is usuallyignored in the kinetic simulations (where h(P) = constant, that is, the environmentis equilibrium). This happens when the entire reactive gas that reacts with thesample is removed (either by flowing an excess amount of reactive gas in a gas–solid reaction or purging high flow rates of inert gas).

By assuming pressure has no effect on the kinetics, the process rate wouldbecome a function with only 2 variables, T and 𝛼:

d𝛼dt

= 𝜅 (T) f (𝛼) (3)

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 3

where 𝜅(T) represents the temperature dependence of the rate and the f(𝛼) is theconversion dependence by the reaction model. The extent of conversion is deter-mined from the experiment as a fraction of total weight loss:

𝛼 =mi − mmi − mf

(4)

where mi and mf are the initial and final weight of the sample, respectively. The𝛼 value can vary from 0 to 1, referring to the overall transformation from thebeginning (materials) to completion (products).

The temperature dependence of the rate constant commonly obeys the Ar-rhenius equation (20):

k (T) = A exp(−ERT

)(5)

where A and E are kinetic parameters, for example, the preexponential factor(representing the frequency of the occurrence of a condition that leads to reaction)and activation energy, respectively. The universal gas constant is denoted as R,and T is the temperature that is set by the TGA program (in kelvins). Equation (5)can be described as an isothermal TG curve:

g (𝛼) =

𝜒

∫0

d𝛼f (𝛼)

= A exp(−ERT

)t (6)

where g(𝛼) is the integral form of the reaction model. The above equation can beobtained by substituting equation (5) into equation (3), followed by an integrationas shown. In an isothermal condition, normally only 𝛼 would be changing through-out the experiment. Thus, 𝛼 versus t curve would be defined by reaction models(21). The reaction models can be separated into three major models: (1) acceler-ating, (2) decelerating, and (3) sigmoidal. It would look like Figure 1 when it isplotted on the weight loss curve.

1. Accelerating models can be shown as the weight loss rate increases contin-uously throughout the entire process, and the model can be shown as below:

g (𝛼) = 𝛼1∕n (7)

where n is a constant value.2. As for the decelerating model, it was represented by the weight loss rate

that decreases throughout the entire process, and the model can be shownas below:

g (𝛼) =1 − (1 − 𝛼)1−n

1 − n(8)

where n is a reaction order ≠1 (for n = 1, g(𝛼) = −ln(1 − 𝛼)).

4 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

1

3

2

Time (s)

Wei

ght l

oss

(%)

Fig. 1. The reaction models of accelerating (1), decelerating (2) and sigmoidal (3) when itis plotted into the weight loss curve (17).

3. The Avrami–Erofeev model is applied to the sigmoidal models as this modelshows processes when the rate reaches its maximum point at some inter-mediate values during the conversion. The Avrami–Erofeev model is shownbelow:

g (𝛼) =[−ln (1 − 𝛼)

]1∕n (9)

On the other hand, equation (5) can be substituted into equation (3), fol-lowed by integration of equation (1), producing an equation that describesa nonisothermal TG curve as explained below:

g (𝛼) = A𝛽

T

∫0

exp(−ERT

)dT (10)

However, the shape formed from this nonisothermal TG curve cannot be re-lated to the reaction model 3. For the nonisothermal condition, the heating rateremains constant, which is caused by simultaneous variation of both T and 𝛼. Thiscondition contradicts with the condition of reaction model 3 in which the sigmoidalreaction has an acceleratory phase followed by a decelerator phase.

3. Instrumentation and Operating Principles

TG instrument (Fig. 2) consists of three different components to provide theflexibility for the accurate analytical data: balance (for measuring weight of

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 5

Furnace

Sample Display Tare

GasGas

Temperature

Weighing mechanism

Wei

ght

Temperatureprogrammer

Fig. 2. Schematic diagram of TG instrument.

sample), furnace (providing linear heating), temperature programmer (for tem-perature measurement and control), and recorder (for recording unit for weightand temperature changes) (10). As the weight variations occur, a balance beamtends to deviate from its normal position. There are two types of balance can beused: null or deflection balance (22–25). A null type of balance is commonly usedin which a sensor is employed to sense the deviation of the balance beam from itsnull point position. A restoring force is then applied to the balance to restore itsnull point. Some systems measure the beam deflection by converting the beamdeflection using different techniques (photographic recording, recording electricalsignals, or using an electrochemical device) into a suitable weight.

There are several variables that can affect the sensitivity of the instrumentand alter the shape of the TG curves (i–v). By considering those variables, theinstruments are required to be calibrated before conducting the experiment.

I. Sample: The chemical description including its source, purity, and pretreat-ments, the particle size, must be clearly known.

II. Sample holder: The capacity of the holder should be large enough to holdthe whole sample to avoid loss of signal due to spurting and creeping. Then,to obtain an accurate data or signal, the heat transfer between the furnaceand sample holder needs to be really effective, which is why a metal holderis better.

III. Crucible: A recommended type of crucible is a platinum crucible, which iscylindrical and has sufficient height. Other types of the recommended cru-cible are gauze crucible, polyplate crucible, and labyrinth crucible.

IV. Atmosphere: Choice of different atmospheric environment depending on thetype of gas purged in. It could be inert or oxidative to the sample. The

6 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

chemical changes due to the oxidative environment could be detected dif-ferently by the analyzer, and it could become a very useful information re-garding the characterization of a sample.

V. Heating rate: The rate of decomposition depends on the heating rate. Thehigher the heating rate, the higher the decomposition temperature. Usually,a recommended heating rate is 3.5◦C/min for reproducible TGA.

Typically, a TGA instrument offers a temperature range from 1000 to 1600◦Cand the heating rate can be set from a range of a few decimal places to hundredsof ◦C/min. However, typical heating rate is between 1 and 20◦C/min, with veryfast (>20◦/min) or very slow (<1◦/min) rates used only for some special reactions.The resolution of the thermogram can be controlled by the programmed heat rate,and usually the slower the heating rate the higher the resolution of the curves.The modern TGA can run on a single temperature program (eg, a constant heatrate at 5◦C/min from 30 to 900◦C) or using a several combination of temperatureprograms (eg, isothermal at 100◦C for 1 h to totally remove all the adsorbed mois-ture followed by heating from 100 to 900◦C at a heating rate of 10◦C/min to studythe thermal properties of the sample).

The sample tested could be either in solid, gel, or liquid form with the weightranging from as small as 1 to 100 mg, up to 100 g. Some of the examples of the masschange processes are decompositions, vaporizations of the solvents, oxidations,and reductions of the polymer electrolytes at different temperature and differ-ent atmospheric environment. On the other hand, the TG curves data are usuallycoupled with a derivative thermogram (DTG) curve to allow better understand-ing or resolution of consecutive weight changes. The DTG curve can be extractedfrom the TG curve data by deriving the weight data as a function of tempera-ture. Figure 3 illustrates the TG curves together with its DTG curves of verticallyaligned reduced graphene oxide/polyvinyl alcohol (VArGO/PVA) composite. Fromthe figure, it can be seen that the first small peak of DTG was associated with thefirst weight loss of the composite, followed by the second and third as labeled inFigure 3.

4. Applications

Thermogravimetry provides information regarding changes in mass of the ma-terials due to change in temperature or time in a controlled manner. Therefore,this is specific to the study of decomposition and oxidation reactions and physicalprocess such as sublimation, vaporization, and desorption. It is especially usefulfor the study of polymeric materials, including thermoplastics, thermosets,elastomers, composites, films, fibers, coatings, and paints. TGA measurementsprovide valuable information that can be used to select materials for certainend-use applications, predict product performance, and improve product quality.The technique is particularly useful for the following types of measurements.

4.1. Thermal Stabilities Assessment and Moisture Content. Ther-mal stability is one of the most important properties of polymers to study their ap-plications in numerous fields. The analysis of thermal stability of polymers using

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 7

2.0

1.8

1.6

1.4

1.2

1.0

Der

ivat

ive

wei

ght (

%/°

C)

Wei

ght (

%)

0.8

0.6

0.4

0.2

0.0

900800700600500

Temperature (°C)

III

Thermogravimetric curve

Derivative thermogravimetric curve

III

400300200100–60

–40

–20

0

20

40

60

80

100

Fig. 3. TG and DTG analysis of VArGO/PVA composite (26).

thermogravimetry is a prominent application of this technique. Thermogravimet-ric curves provide decomposition mechanism of the polymers. This decompositionprofile defines the characteristics and identification of the polymer. The onset ofmass loss often defines the upper limit of thermal stability for the material. How-ever, it should be appreciable that extensive degradation of polymer may alreadytake place before the point at which detectable changes in mass occur. Polymerdegradation can be categorized through the following mechanism:

a. main-chain scissionb. side group scissionc. eliminationd. depolymerizatione. cyclization, andf. cross-linking.

The categories (a)–(d) usually result in the evolution of unstable volatileproducts, hence mass change occurs. The assessment of thermal propertiesdepends on the environment during the analysis. The polymers quantitativelydegrade into parent monomers in an inert atmosphere. However, oxidation ofpolymer into oxides of the constituent elements occurs in the presence of air. Forexample, halogen containing polymers yield hydrogen halides (H-X) and nitrogencontaining polymers form hydrogen cyanide or ammonia. Moreover, cyclizationand cross-linking rarely occur unless in conjunction with (a)–(d). These twocannot be detected easily by thermogravimetry.

Figures 4a and 4b show the thermogravimetric curves of polypropylene afterpassing through nitrogen and oxygen, respectively. The degradation behavior is

8 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

105

(a)

(b)

90

75 PP x0PP x1PP x2PP x3PP x4PP x5PP x6

PP x0PP x1PP x2PP x3PP x4PP x5PP x6

60

Mas

s (%

)45

30

15

0300 350 400 450 500

2000

15

30

45

60

Mas

s (%

)

75

90

105

250 300

Temperature (°C)

350 400

Fig. 4. Thermogravimetric curves of polypropylene in (a) nitrogen and (b) oxygen (27).

clearly different under nitrogen and oxygen atmosphere. The systematic degrada-tion is observed under a nitrogen atmosphere, which is due to the chain scission ofpolypropylene. However, the oxidation temperature dropped sharply for samplesunder an oxygen atmosphere.

Furthermore, thermogravimetric curves of polyethylene in nitrogen and oxy-gen are shown in Figures 5a and 5b. The curves for nitrogen show only slightdifferences in the degradation temperatures; however, they do not change sys-tematically, but they do decrease with the number of extrusion cycles, again, dueto the scission of the polyethylene chains, which become shorter and are prone tothermal degradation at lower temperatures. The curves for oxygen are very dif-ferent from each other and show an initial mass increase owing to oxygen uptakeand formation of peroxy radicals and hydroperoxides, which are not stable towardhigher temperatures and rapidly convert into other labile products and are sub-sequently evolved. The mass increase is higher for the degraded samples sinceshorter chains react promptly with oxygen. After reacting with oxygen, the repro-cessed samples start losing mass faster since volatile compounds are evolved (27).

Similarly, the same study reveals the systematic decrease in degradationtemperature of polypropylene under nitrogen atmosphere, which might be due to

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 9

105(a)

(b)

90

75PE x0PE x1PE x2PE x3PE x4PE x5PE x6

PE x0PE x1PE x2PE x3PE x4PE x5PE x6

60

Mas

s (%

)

45

30

15

0400 450

Temperature (°C)500

1500

15

30

45

60

Mas

s (%

)

75

90

105

300

Temperature (°C)

450 600

Fig. 5. Thermogravimetric curves of polyethylene in (a) nitrogen and (b) oxygen (27).

scission of polypropylene chains. Although it is widely accepted that polypropy-lene degrades via chain scission, polyethylene mainly undergoes branchingand cross-linking, in addition to chain scission. The results presented abovedemonstrate that polyethylene and polypropylene under the extrusion conditionsundergo chain scission, which is consistent with the results published by someauthors (28–30).

Figure 6 displays the thermal degradation curves of microcrystalline cellu-lose (MCC) and cellulose tri-stearate (CTs) under argon environment. The curveshows the series of thermal degradation such as dehydration at 100◦C (evapora-tion of moisture contents) and thermal pyrolysis of cellulose backbone at 300◦C.However, the thermal degradation of CTs shows different thermal curve as com-pared MCC. CTs did not show significant weight loss around 100◦C because ofhydrophobic nature. The temperature corresponding to maximum thermal degra-dation of MCC was 344.6◦C whereas 371◦C for CTs. The thermal stability of CTsis higher than MCC ,which is due to the substituted groups. The long-chain fattyacid groups on the cellulose chain can be in a regular arrangement and form an

10 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

1000

20

40

(1–α

)/% 60

80

100

200

MCCCTs

Temperature/°C300 400

Fig. 6. Thermal degradation curves of MCC and CTs (31).

ordered structure. Such work is useful in predicting the thermal stability of poly-mer skeleton after substitution (31).

The degradation of block and graft copolymers rarely corresponds to thesum of their parent polymers. Copolymerization may result in a material thatis thermally more stable than either of the parent polymers. TGA of purehigh-fluidity polypropylene (HF-PP), metallocene poly(ethylene-butene-hexene)copolymer (mEBHC), and their blends at a heating rate of 10◦C/min under a nitro-gen atmosphere was investigated. The thermal degradation of HF-PP shows onestage of weight loss, which exhibits the onset degradation temperature (Tonset, at5% weight loss) at 373◦C, whereas the Tonset of mEBHC is 368◦C. Incorporation ofmEBHC was found to increase the thermal stability of HF-PP with the increasein Tonset. Overall, the more mEBHC is incorporated, the higher the Tonset of theblend (32).

4.2. Filler Content in Polymers. Fillers are particles added to the ma-terial (plastics, composite material, concrete) to lower the consumption of moreexpensive binder material or to better some properties of the mixture material.Worldwide, more than 53 million tons of fillers with a total sum of approximatelyEUR16 billion are used every year in different application areas, such as paper,plastics, rubber, paints, coatings, adhesives, and sealants. As such, fillers, producedby more than 700 companies, ranked among the world’s major raw material andare contained in a variety of goods for daily consumer needs (33).

Formerly, fillers were used predominantly to cheapen end products, in whichcase they are called extenders. Among the 21 most important fillers, calcium car-bonate holds the largest market volume and is mainly used in the plastics sec-tor (34). While the plastic industry mostly consumes ground calcium carbonate,the paper industry primarily uses precipitated calcium carbonate that is derivedfrom natural minerals. Wood flour and saw dust are used as filler in thermosettingplastic.

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 11

In some cases, fillers also enhance properties of the products, for example,in composites. In such cases, a beneficial chemical interaction develops betweenthe host material and the filler. As a result, a number of optimized types of fillers,nano-fillers, or surface-treated goods have been developed.

Polymers reinforced with nanofillers are promising candidates for variousapplications requiring lightweight and high-performance, multifunctional mate-rials. The basic characteristics of nanosized materials that are responsible fortheir unique properties are the nanometer length scales and enormously large sur-face area-to-volume ratio. These features can enhance the overall material perfor-mance by synergistically producing unique properties resulting from phenomenathat occur only when the morphology and the physics coincide at the nanoscale(35). The main challenges in utilizing these materials as polymer fillers are tocontrol and optimize their dispersion within the polymer matrix and their inter-face chemistry, and maintain their nanoscale morphology during the manufac-turing process (36). In addition to the filler and manufacturing process employedto fabricate the composites, the resulting properties strongly depend on the poly-mer used as a matrix. Petroleum-based polymers such as polypropylene, nylons,polystyrene, and epoxies have commonly been used (37).

TGA is the most common technique for determining the quantity of a fillerwithin a polymer. The product is placed on a balance inside of a furnace and heatedto burn away all of the organic material until the filler content is all that remains.TGA provides a percentage of mass that remains from the initial amount of mate-rial tested. Additionally, a muffle furnace can be used to determine the ash content(filler) as well.

Rheem and co-workers prepared thermally expandable microspheres of acry-lonitrile, methyl methacrylate, and methacrylonitrile containing a blowing agentthrough suspension polymerization. These microspheres showed two-step decom-position in TGA as shown in Figure 7. The first stage represented the outburstand evaporation of the blowing agent, and the second stage was due to the decom-position of polymers (38).

Thermal analysis of the polyolefines (polypropylene and polyethylene) andfillers was performed in nitrogen and air atmosphere. From Figure 8, TGA curvesof the fillers in nitrogen as well as air atmosphere showed a slight weight lossbetween 40 and 100◦C, indicating the vaporization of water. A second weight lossfrom approximately 150–500◦C was due to the decomposition of the three ma-jor constituents of biofillers, namely cellulose (275–350◦C), hemicellulose (150–350◦C), and lignin (250–500◦C). At 700◦C, rice husk and saw dust left a higheramount of char content, decomposing only by about 65% and 80% in nitrogen at-mosphere. The ash in the rice husk was mainly composed of silica (96%). In anair atmosphere, rice husk degradation shifted to lower temperatures and splitinto two processes (320 and 438◦C). The second might be associated with ther-mal oxidation degradation of char. Therefore, the residues of rice husk filler at700◦C in the air atmosphere (20%) were lower than that in a nitrogen atmosphere(36%) (39).

4.3. Compositional Analysis of Multicomponent Polymers. Oneof the most important applications of TGA is the assessment of thecompositional analysis of polymeric blends. Among thermal analysis techniques,thermogravimetry can be used for precise and accurate analysis of composition

12 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

120

100

80

60

Wei

ght %

(%

)

40

20

00 100 200 300

Temperature (°C)

400 500 600

V-70 30 °CV-70 40 °CV-70 50 °CV-65 50 °CV-65 60 °CV-65 70 °CV-59 60 °CV-59 70 °CV-59 80 °CAIBN 60 °CAIBN 70 °CAIBN 80 °C

Fig. 7. TGA thermograms of expandable microcapsules prepared with different initiatorsand temperature (38).

100

80

60

Wei

ght (

%)

40 RH-N2

SD-N2

RH-air

SD-air20

0

0 100 200 300 400

Temperature (°C)

500 600 700

Fig. 8. TGA curves of stock materials: fillers saw dust and rice husk as well as polyolefinesPP and PE in nitrogen and air atmospheres (39).

and for the identification of polymers from their decomposition pattern and, there-fore, it can be used for quality and process control. Compositional analysis of mul-ticomponent polymers can be done using TGA–FTIR (Fourier infrared spectrom-etry), high-resolution TGA, auto stepwise TGA, and sample-controlled thermal

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 13

analysis (SCTA). The information about the composition of the polymer blend canbe developed through these numerous types of TGA, where the TGA heats up thepolymer samples and holds it under isothermal condition and instrument detectsweight loss. Fernandez-Berridi and co-workers used high-resolution TGA to mon-itor the blend composition of rubber (tire). In hi-resolution TGA, the heating rateof the sample material is dynamically and continuously modified in response tochanges in the rate of decomposition of the material so as to maximize weightchange resolution. This approach allows very high heating rates to be used in re-gions where no weight changes occur, whereas the heating rate is decreased whena weight change is detected. This method gives narrower and more intense deriva-tive weight loss peaks with a greater resolution from competing and overlappingweight losses, and even with a substantial reduction of the experimental time.This approach was used to analyze the main components of rubber under nitro-gen atmosphere. Thermogram of TGA showed different weight losses at a differenttemperature, which indicated the components of rubber. The first weight loss un-der nitrogen flow, between 200 and 300◦C, corresponds to 9% of the total weight ofthe tire sample and is attributed to the volatilization of processing oil or any otherlow boiling point components. The next mass loss of 37% observed in nitrogen flow,which has a maximum rate at 350◦C, due to the decomposition of natural rubber,whereas the weight loss of 25% at 424◦C corresponds to styrene butadiene rubber.These temperatures have been assigned according to the obtained results from thepure component TGA. Finally, when air atmosphere is activated, oxygen reactswith carbon black and leads to the fourth weight loss at 563◦C. The nonvolatileresidue (6%) corresponds mainly to inorganic fillers (40). Auto-stepwise TGA canbe used to observe the composition of polymer blends. It was used to observe thecomposition of ABS polymer alloy composed of a styrene acrylonitrile copolymerwith butadiene. The weight loss transition of the butadiene rubber component oc-curred extremely close to the decomposition of the SAN copolymer. Standard TGAcannot even separate the two events. However, auto-stepwise TGA potentially an-alyzed the quantitative compositional analysis of ABS matrix. The butadiene wasnicely separated from SAN decomposition, and this provided excellent informa-tion, which is shown in Figure 9.

Conventional TGA has limitations in the case of closely occurring eventsthat happen at a similar temperature range and the overlapping of decompositionevents occur. Attempts to improve the ability to separate closely occurring event,by changing the heating rate in conventional heating schedules, might yield betterresults but are not always sufficient for a proper discrimination.

Plastisols are composed of vinyl resins mainly polyvinyl chloride (PVC) andplasticizers that increase the flexibility. The plasticizer contents have severe sideeffects on human health and on the environment. The contents of plasticizercan be determined using special TGA. Conventional TGA cannot determine theplasticizer contents as the decomposition of PVC overlaps with the release ofplasticizer. SCTA is a type of TGA that provides much higher resolving power and,therefore, has an ability to successfully overwhelm the problems of conventionalheating rates for the separation of closely occurring decomposition events. Inthermal analysis, the resolving power concerns the extent to which adjacent, orpartially overlapping, thermally induced processes are separated. Despite thisability, the potential of SCTA for analytical studies has not been explored so far.

14 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

ABSAuto stepside TGA

Wei

ght l

oss

(%)

SANcopolymer

Butadiene

Temperature (°C)

Carbon

Fig. 9. Auto-stepwise TGA results for ABS polymer alloy (41).

In SCTA experiments what is controlled is not the temperature–time profile, asin conventional analysis, but rather the evolution of the reaction rate with thetime. Most usually, the temperature is controlled in such a way that the reactionrate is maintained at a constant value previously selected by the user along theentire process. In such a case, the technique is known as constant rate thermalanalysis (CRTA). The enhanced control over the decomposition process providedby CRTA makes it a much more effective tool than conventional methods forthe discrimination of overlapping processes. Furthermore, the temperature–timeprofile, which is not known in advance and depends on the evolution of thereaction with temperature, can provide a good deal of information about thenature of the process studied, since it can be related to the mechanism thatdrives the reaction. Sanchez-Jimenez and co-workers used the sample-controlledthermal analysis to determine the composition of plastisols containing PVC andplasticizers, a system where the individual components have very similar thermalstabilities, thereby rendering useless thermogravimetric experiments run underconventional conditions. Different SCTA procedures, such as CRTA, which hasreceived special attention, and the results obtained have been compared withthe linear heating rate technique. It has been proven that CRTA can be usedeffectively to determine the exact composition of the blend (42).

4.4. Measurement of Low Level of Volatiles. Another important ap-plication of TGA is the quantitative measurement of a low level of volatile matter(typically with the weight loss less than 1%) evolved by samples. Examples ofvolatile matter are water, residual solvent, residual monomer, and low molecularweight polymer. In the pharmaceutical sector, the existence of moisture, whichmay come from a bulk drug, inactive excipients, and environmental conditions,can greatly affect the product stability, tablet compaction, microbial growth, andwet granulation (43). Likewise in injection molding technology that use poly-mers (eg, thermoplastics, thermosets, and elastomers), the measurement of the

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 15

Low level of volatiles from PET resinat 130 °C

Delta Y = 0.219%

100

99.95

99.90

99.85

Wei

ght %

(%

)

99.80

99.750 100 200 300

Time (min)

400 500 600

Fig. 10. Low level of volatiles from a single pellet of PET under isothermal condition (41).

variation of volatiles has become increasingly critical. High sensitivity and ex-cellent stability of TGA is very useful to determine very small weight changes ofmaterials. This is due to the fact that the blow molding process can be affectedby the existence of small amounts water, solvent, or monomer. As a consequence,bubbles or imperfections may be produced during the process.

Another example for PET is shown in Figure 10 in which the measurementwas taken during the sample was kept under isothermal conditions at 130◦C. Theweight loss observed was only 0.219% at 10 h (41).

4.5. Sample-Controlled Thermogravimetry. Generally, thermo-gravimetry analysis is conducted under isothermal or linear rising temperature(linear nonisothermal) conditions. In isothermal, this practice may contributeto errors because the samples usually do not heat instantaneously when a hightemperature is applied. While for linear nonisothermal, it takes a long durationof analysis when need to adjust the temperature to limit the maximum rateof reaction. Thus, there is another alternative that enables a precise control ofthe reaction environment. That is the rate of increase of induced temperatureis monitored (either by slowing down or suspending) once some predeterminedweight loss is detected. This smart temperature control technique is calledsample-controlled thermogravimetry. Most often, the technique is implementedin the industrial sector to achieve a desired characteristic profile, high quality, orspecific sample properties.

Two types of control heating approach have been designed: stepwise isother-mal and dynamic rate (45). Stepwise isothermal (proposed by Sorenson) is usedwhen the rate of temperature is governed by alternating the induce temperaturevariation between linear rising temperature with isothermal segments. Duringanalysis, the temperature is increased when the reaction rate drops below theminimum until the rate exceeds the maximum. Then, the induced temperatureis held constant until the reaction rate again drops below the minimum. At thispoint, the temperature is again increased causing the whole steps to be performed

16 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

Fig. 11. TG curve of low-density polyethylene/nylon-6 laminate obtained at a dynamicheating rate (47).

over and over until all weight loss variations have been obtained. The main benefitof this approach is the determination of steps used for multiple step decomposi-tion from a single TGA analysis. In addition, these steps can be reused for thefuture analysis on similar samples.

While for dynamic heating rate (a novel method patented by Sauerbrunn andGill), the rate of heating is sigmoidally linked to the rate of weight loss (46). Dur-ing decomposition, the rate of temperature variation is slowly reduced from theinitial rate as the rate of weight loss increases. This approach results in high res-olution of particular thermal events. For instance, the heating rates can be set upin the range from a fraction to hundreds ◦C/min, the most commonly used range is1–20◦C/min. A TGA run can include a single temperature program, for example,heating from 25 to 600◦C at 10◦C/min, or a combination of several programs, forexample, heating from 25 to 100◦C at 100◦C/min, followed by an isothermal holdat 100◦C for 60 min, then heating from 100 to 600◦C at 10◦C/min. The measure-ments are conducted in controlled gaseous atmosphere that can be either staticor dynamic. The most commonly used controlled static atmosphere is a gas atreduced pressure that can be as low as 10−3 to 10−4 Pa. On the other hand, fordynamic atmosphere, the measurement can be accomplished by running a gasaround the sample at a certain flow rate (50–100 ml/min). The gas can be eitherinert (nitrogen, or more rarely, argon or helium) or reactive (oxygen, hydrogen,carbon dioxide). The example of measurement that employed the variable heat-ing rate method is depicted in Figure 11. The figure shows the derivative mass losscurves for a bilayer polyethylene/nylon-6 film. Although the degradation of eachcomponent in the film (ie, polyethylene and nylon-6) is not well separated, quan-titative analysis of the components still can be measured from the mass changebetween the plateau curves.

4.6. Lifetime Prediction and Degradation Kinetics. In polymer ap-plications (eg, wire insulation, scaffolds, prosthetics), the ability to estimate thelifetime of the product is very important. Lifetime prediction is useful in the

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 17

development of polymers for long-term usage in various applications. Tradition-ally, the lifetime of polymers can be estimated through oven aging analysis, whichis time consuming especially for stable polymeric materials. The test typicallytakes weeks to months of exposure at elevated temperatures until voltage break-down occurs. Then several tests need to be carried out at different oven temper-ature to obtain a semilogarithmic plot versus failure temperature. The obtaineddata sometimes do not span the temperature range of interest and require fur-ther long experiment at a lower temperature to be conducted. This method fitsfirst-order kinetics, and the lifetime can be estimated using extrapolation.

Nevertheless, the estimation of polymer lifetimes following first-order kinet-ics is unpredictable because many polymers tend to decompose. The sample usu-ally has started to decompose before being carried to the desired temperature.One strategy to overcome this problem is by adjusting a rate-controlling variableduring an experiment. This method is known as a parameter jump method, orspecifically, in this case, it is called temperature jump TG (48). From this practice,the rate of decomposition can be determined from the change in temperature (T1or T2) and the Ea can be obtained from equation (11)

Ea = R ln⎡⎢⎢⎢⎣

(dmdt

)T1(

dmdt

)T2

⎤⎥⎥⎥⎦[

1T1

− 1T2

](11)

where (dm∕dt)T1and (dm∕dt)T2

are the rate of weight loss at T1 and T2.Degradation kinetics can be analyzed using data from linear rising temper-

ature TG. Several measurements are performed at different heating rates, andthe temperatures at which weight loss happened is observed. Figure 12 shows the

110

100

90

80

70

Wei

ght %

(%

)

60

50

40200 300 400 500 600

Temperature (°C)

5°C/min

15°C/min

10°C/min

20°C/min

700 800 900

Fig. 12. TGA curve of PEEK at different heating rates (49).

18 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

1.4

1.2

1

log

(β)(°C

/min

)0.8

0.61.14 1.16 1.18 1.2

1000/T (1/K)

2.5%

10.0%

5.0%

12.5%

7.5%

15.0%

1.22 1.24

Fig. 13. Kinetic analysis of results obtained from Figure 12 to calculate Ea (49).

1.00E+11

1.00E+09

1.00E+07

1.00E+05

1.00E+03

1.00E+01 1.05 year

Life

time

(yea

rs)

0 100 200 300

Temperature (°C)

400 500 600

1.00E–01

1.00E–03

Fig. 14. A predicted lifetime of PEEK versus temperature (49).

example of TG curve for polymer poly(ether ether ketone) (PEEK) at the heatingrates of 5, 10, 15, and 20◦C/min. The temperature values are then plotted as afunction of logarithms of heating rate 𝛽, and Ea can be calculated (Fig. 13).

Using the Ea obtained, it is possible to estimate the lifetime of the sample asa function of temperature. The example as displayed in Figure 14 explains thatthe time for PEEK decomposition in 5% is about 1.05 year if it is exposed to thetemperature of 350◦C (49).

There is another method for analyzing sample degradation kinetics calledmodulated temperature TG. The temperature profile for this method superim-poses a sinusoidal temperature fluctuation with a conventional linear risingtemperature program as shown in Figure 15a. The figure illustrates a modulatedtemperature TG curves in which the experiment was done on an ethylene-co-vinylacetate copolymer. The plot exhibits the temperature modulation (solid time) and

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 19

Fig. 15. (a) Modulated temperature TG curve of poly(ethylene-co-vinyl acetate). repre-sents the temperature curve, whereas —– represents the dm/dT curve. (b) The correspond-ing plot of weight and Eaversus temperature. (47).

the effect of the modulation on the rate of weight loss (−dm/dt) (dotted line).These data allow for the determination of Ea, which can be calculated accordingto the following equation:

Ea =R[T2

av −(0.5Tamp

)2]

L

Tamp(12)

where Tav is the average thermodynamic temperature, Tamp is the amplitude ofthe temperature modulation, and L is the logarithm of the amplitude of the rateof the mass loss over one modulation. The calculated Ea is useful to be studied forplotting another type of data as shown in Figure 15b. Typically, when no mass lossis observed, the value of Ea is set to zero. However, in this case, the Ea changesthroughout the copolymer mass decomposition.

4.7. Combined Techniques.4.7.1. Thermogravimetry/Differential Scanning Calorimetry. DSC is a

thermal analysis technique, which provides quantitative and qualitativeinformation about physicochemical changes in the materials. For example, thistechnique can be used to calculate the difference in heat flow between a sampleand inert reference substance when the temperature is varied. Hence, the changesin heat capacity of the sample, the activity that consumes heat (endothermic), andrelease heat (exothermic) can also be measured (50). Other properties that aremostly measured and studied using the DSC technique are glass transition tem-perature, the degree of crystallinity, melting/crystallinity (J/g), cross-linking reac-tions, polymorphic transformation/stability, protein denaturation, decompositionbehavior, thermal/oxidation stability, boiling points, and sample purity. Figure 16displays the possible transitions in a DSC curve (51).

Figure 17 illustrates the setup of TGA–DSC to measure the heat flux.Briefly, the sample is placed inside a crucible, which is then placed inside themeasurement cell (furnace) of the DSC system along with a reference hematicaluminum pan, which is usually empty. The pans are then placed on a metal orceramic plate containing thermocouples. Thermocouples are served to detect the

20 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

Glasstransition

*Crystallization*Polymorphic conversion

*Melting*Polymorphic conversion*Denaturation

Oxidationor

decomposition

Cross-linking(Cure)

endo

Hea

tflow

Temperature

exo

Fig. 16. The possible transitions in a DSC curve (51).

Sample

Sample panCylindrical

furnaceReference pan

Thermoelectricdisk

Measurement thermocouples

Fig. 17. TGA–DSC illustrates setup.

difference in temperature between these two pans and convert this to a thermalenergy difference (50).

This approach has some advantages over conventional DSC and TG throughthe saving in the experimental time needed to acquire two sets of data (52). Thistechnique is worthwhile to be able to determine energy changes related to ther-mal decomposition reactions. Most of the chemical reactions in polymer systems,for example, curing of thermosets are condensation reactions, water, or ammoniamay be released during the process of the reaction. This will result in a changein mass of the sample in addition to the heat of reaction, which can be measuredby DSC. Normally, the physical changes in polymer structure such as those that

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 21

100

80

Wei

ght l

oss

(%)

60

DSC

40

200 100 200 300

Temperature (°C)

400 500 600

TGA

Fig. 18. TGA–DSC curve of the BaZrO3 (53).

accompany the glass transition, crystallization, or melting cannot be detected byTGA, but with the extra information provided by DSC will be useful. Figure 18shows the example of TGA–DSC graph for zinc oxide (ZnO) nanofibers. Threeweight losses can be observed at the TG curve, whereas the DSC curve exhibitsthe first endothermic peak at 105◦C and three exothermic peaks at 255, 285, and500◦C (53).

4.7.2. Thermogravimetry/Evolved Gas Analysis. TGA is capable of mea-suring the weight variation of a sample against temperature. However, in somecases, the curves can be complex and especially for multiple decompositionproducts. In addition, TGA is only capable of measuring the weight changes, whichare difficult to separate and identify the individual degradation products (off-gases). Another type of thermal analysis, called evolved gas analysis or EGA, is atype of thermal analysis in which the amount of gaseous volatile products evolvedby samples is measured as a function of temperature. By coupling EGA with TGA,the capabilities owned by EGA and TGA can be utilized in one instrument. Twotypes of analysis techniques that employ evolved gases are FTIR or mass spec-trometry (MS) (54–56).

Infrared spectroscopy measures the light absorbed by different types of vi-brations in molecules. Infrared radiation from the light sources is divided intotwo beams by the beam splitter. Each of the beams will be reflected onto a movingmirror and also onto a stationary mirror, respectively. Both beams are then re-combined and pass through the sample toward the detector. Fourier transforma-tion of the resulting interferogram yields a mid-infrared transmission spectrum.Figure 19 illustrates the schematic diagram of FTIR spectrometry.

22 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

Detector

Sample

Moving mirrorBeamsplitterLight source

Fixed mirror

Fig. 19. Schematic diagram of FTIR spectrometry.

Resonant ionDetector

Nonresonant ion

Source

DC and AC voltage

Fig. 20. Schematic diagram of MS.

Figure 20 shows the schematic diagram of MS where it characterizes sub-stances by identifying and measuring the intensity of molecular fragment ions ofdifferent mass-to-charge ratios (m/z). At first, the incoming gas molecules will beionized in the ion source. Then, it will form positive molecular and fragment ions.Both types of ions are separated corresponding to their m/z values by a combina-tion of magnetic and electrostatic fields. A mass spectrum is recorded by scanningthe field strength so that ions of increasing m/z ratio arrive at the detector.

When gases are evolved, the FTIR or MS can track their evolution profile.This is because infrared spectra and mass spectra are substance specific. Thus,the spectra can be used to characterize the substance or substance class viaspectral interpretation and comparison with database reference spectra. As aresult, the decomposition pathway can be elucidated. This combination techniqueis able to use in research and development, in quality control, and to investigatethe failure of the materials; for example, characterization of starting material

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 23

Table 1. Advantages of TGA–FTIR and TGA–MS technique

Technique Features Benefits

TGA–FTIR High chemicalspecificityand fastmeasurement

TGA–FTIR is able to characterizesubstances by determining theircompounds and functional groups.This technique is ideal for theonline measurement of substancesthat indicate medium to stronginfrared absorption

TGA–MS High sensitivitycoupled withvery fastmeasurement

TGA–MS is only able to detectextremely small amounts ofsubstances. This technique is idealfor the online characterization ofall types of volatile compounds

Purgegas

TG analyzer Transfer line

Heater

Computer

Gas analyzer

MS or FTIRBalance Furnace

Purgegas +

products

Fig. 21. Workflow of operating procedure of TGA–FTIR and TGA–MS.

and end products, detection of additives in a matrix, and investigation of thechemical reaction such as catalysis, synthesis, and polymerization.

Besides the high resolution of TGA–FTIR and TGA–MS, which can interpretqualitatively the overlapping weight losses, they also offer other specific featuresand advantages as shown in the Table 1.

Figure 21 shows the flow of operating procedure of TGA–FTIR and TGA–MS. The total volume of purge gas and gaseous decomposition products formedin the TGA can be investigated online by using the combination of TGA and MStechniques.

In TGA-FTIR technique, the gases are transferred through a heated glass-coated steel transfer capillary line into a heated gas cell in the FTIR spectrometer.Usually, nitrogen, which does not exhibit IR-absorption, is used as purge gas. TheFTIR spectrometer determines the spectra of the gases in the gas cell. Figure 22ashows TGA curves of phenoxy resin at different heating rates under N2 atmo-sphere. TGA curves show only one step of weight loss. The thermal decompositionoccurs between 325◦C and 475◦C depending on heating rate. In Figure 22b, TGAcurves for phenoxy resin in air atmosphere are shown. The TGA curves exhibitedmainly two decomposition steps located between 300◦C and 450◦C for the firststep and between 450 and 600◦C for the second step depending on the heatingrate, being their weight losses around 70 and 30%, respectively. The analysis of

24 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

(a) (b)

(c) (d)

Fig. 22. (a) TGA curves of phenoxy resin under nitrogen at indicated heating rates.Dots: selected temperatures for FTIR analysis of residue (b) TGA curves of phenoxy resinunder air atmosphere at different heating rates. Dots: Selected temperatures for FTIRanalysis of residue. (c) FTIR spectra of phenoxy resin at room temperature and solidresidues obtained in nitrogen atmosphere at selected temperatures (see dots in a). (d)FTIR spectra of phenoxy resin residue obtained in air atmosphere at selected temperatures(see dots in b) (56).

solid residues formed at various temperatures can also provide direct informationon the changes that occurs in the chemical composition of the polymeric materialduring thermal degradation process. In Figure 22c, it is shown the infrared spec-tra in the range 4000–700 cm−1 of phenoxy resin at room temperature in nitro-gen atmosphere as well as of the solid residues obtained at higher temperatureswhich were selected along the decomposition range observed on TGA curves reg-istered at 10◦C min−1. The selected temperatures are marked with dots in TGA.Figure 22d shows the FTIR spectra of solid residues obtained at various temper-atures (marked with dots in Figure 22b for phenoxy resin under air atmosphere.Observing the evolution of FTIR spectra with temperature it can be clearly seenthe appearance of a new peak at 1730 cm−1 at 404◦C this peak is attributed to thepresence of carbonyl groups that can appear as consequence of thermooxidative

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 25

10–9

44

64

96

7000600050004000

Time / s

3000200010000

48

m/z

10–10

10–11

Ion

Cur

rent

/ A

10–12

10–13

Fig. 23. TGA/MS curve of microcrystalline cellulose modified with (3mercaptopropyl)trimethoxysilane (57).

reactions undergone by the resin. This band persists in all the spectra of theresidue from 358 to 503◦C (56).

While, in TGA-MS technique, the TGA is coupled to the MS via a silica cap-illary tube heated at 200◦C to prevent condensation. Part of the gases evolvedfrom the sample in the TGA is sucked into the vacuum in the MS. The MS repeat-edly measures the whole mass spectrum of monitor m/z values characteristic forspecific structural features. Since MS generally has high sensitivity, only ∼1% ofthe effluent gas (argon or nitrogen) is required. The TGA-MS curves of hydropho-bic polymer matrix and cellulosic fibers modified with silane coupling agents ((3-mercaptopropyl) trimethoxysilane) is shown in Figure 23. Based on the figure,the sample shows the generation of furfural (m/z = 96), carbon dioxide (m/z = 44),thiols (m/z = 64) and sulfur oxide (m/z = 48). These elements demonstrated thepresence of thiol functionalities and thereby prove the formation of solixane layeronto cellulose surface (57).

5. Conclusion

TGA is a reliable technique that can be used to study the thermal stabilityand decomposition of polymeric materials. It determines the rate of change inweight of the material, whereas the material is being heated or cooled at acontrolled temperature. Specifically, this technique allows for the assessmentof the compositional analysis of the polymer blend. TGA can also be applied tomeasure the filler content in polymer systems. Furthermore, TGA can be used for

26 THERMOGRAVIMETRIC ANALYSIS OF POLYMERS

measurement of low level of volatile matter evolved in polymer systems such aswater and residual solvent. The TGA analysis can be conducted under differentheating rate approaches: isothermal or dynamic rate. This analysis is also able topredict and estimate the lifetime of the materials. The instrument manufactureshave upgraded the fundamental design of TGA instruments by combining withother techniques such as DSC and EGA in one system. The combination of thesetechniques can shorten the experiment duration as it utilizes different analysisprinciples during a single measurement.

ACKNOWLEDGMENT

This work was supported by University of Malaya Research Grant (UMRG:RG382-17AFR).

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Glossary

A Preexponential factordT/dt Heating rate–dm/dt Rate of weight lossE Activation energym WeightP PressureR Universal gas constantT Temperature𝛼 Extent of the conversion𝛽 Heating rate𝜅( T ) Temperature dependence of the rateCRTA Constant rate thermal analysisCTs Cellulose tri-stearateDSC Differential scanning calorimetryDTA Differential thermal analysisDTG Derivative thermogramEGA Evolved gas analysisFTIR Fourier infrared spectrometryGCC Ground calcium carbonateHF-PP High-fluidity polypropyleneMCC Microcrystalline celluloseMS Mass spectrometrymEBHC Metallocene poly(ethylene-butene-hexene) copolymerm/z Mass-to-charge ratioNR Natural rubberPCC Precipitated calcium carbonatePEEK Poly (ether ether ketone)PET Polyethylene terephthalatePVC Polyvinyl chlorideSBR styrene butadiene rubberSCTA Sample-controlled thermal analysis

THERMOGRAVIMETRIC ANALYSIS OF POLYMERS 29

TG ThermogravimetricTGA Thermogravimetric analysisVArGO/PVA Vertically aligned reduced graphene oxide/polyvinyl alcohol

H. M. NGNORSHAHIRAH M. SAIDIFATIN SAIHA OMARK. RAMESHS. RAMESHCenter of Ionics University of Malaya,Department of Physics, University of Malaya,Kuala Lumpur, Malaysia

SHAHID BASHIRDepartment of Chemistry, University of Malaya,Kuala Lumpur, Malaysia

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