2. Principal Chemical and Analytical Methods Used in Reverse Engineering
2.3 Thermal Analysis
2.3.5 Application of DSC
2.3.5.1 Glass Transition Temperature, Tg
Polymers have different applications depending on their structure. Though they have dif- ferent thermal characteristics, overall features could be described by a hypothetical DSC trace (Figure 2.13). The main thermal transitions are given in Table 2.10.
A transition at low temperature appears as a shift in the baseline. The temperature at which it occurs is called the glass transition temperature (Tg) in polymer science. This is due to a change in the specific heat of the polymer as a result of the onset of chain segment motion in amorphous regions of polymers.
Temperature Glass
transition Crystallization Cross-linking (cure)
Oxidation decompositionor
Melting
Heat Flow -> Exothermic
FIGURE 2.13
Typical DSC thermogram of rubber compound.
Next, an exotherm appears for the crystallization of polymers followed by an endotherm which is due to the melting of the polymer crystal. These peaks are used for the charac- terization of crystalline polymers. This also depends on the nature of the thermal history of the samples. The melting peak position and shape are both significant factors. A sharp peak indicates a highly crystalline and highly oriented material, while a broad peak indi- cates an imperfectly crystalline material.
Oxidation is indicated next by an exotherm, and finally polymers degrade at a high tem- perature. Both of these portions of the trace give an idea about the stability of polymers or reactions occurring due to the presence of oxygen.
A polymer having Tg lower than the room temperature and having an amorphous nature behaves as rubber. The Tg of fibers and plastics is above room temperature. There is a strong effect of structure on the values of Tg. Tg increases with the increase in bulkiness of the group on the backbone. Side chain also affects the value of Tg. All of these results could be related fundamentally to the free volume of a polymer. Factors that favor an increase in Tg are main chain rigidity, increased polarity, bulky or rigid side chain, increased molecu- lar weight, increased cohesive energy density, and crosslinking. Factors that decrease Tg
are main chain flexibility, the addition of diluent or plasticizer, increased tacticity, branch- ing, etc.
An approximate idea about the molecular weight can also be obtained if Tg is known.
This is given by the following equation:
Tg = Tg α − K/Mn (2.37)
where K is a constant of a given polymer, Tg α is the glass transition temperature for a poly- mer of infinite molecular weight, and Mn is the average molecular weight.
The molecular weight of polystyrene (Mn) can be found out by using this method.
The compatibility of two polymers could also be determined from Tg. A single Tg indi- cates that the polymers are compatible.
The transition from a glassy state to a rubbery state, the glass transition, is accompa- nied by a change in heat capacity, but there is no change in enthalpy. Thus, the transition appears on the DSC curve as a discontinuity in the baseline at the glass transition tem- perature, Tg.
TABLE 2.10
Main Thermal Transition in DSC for Polymeric Compound Temperature Rise Transition or Reaction Heat Effect
Glassy transition, Tg —
Crystallization Exo
Melting, Tm Endo
Viscous flow Endo
Modification Exo
Crosslinking Exo
Oxidation Exo
Degradation Endo
Depolymerization Endo
Destruction Endo
Tg determination can be used as a tool for:
1. Polymer identification 2. Structure and uniformity 3. Product uniformity 4. Plasticizer efficiency 5. Degree of curing 6. Quality control
In the case of a random co-polymer, a single Tg, which is intermediate between the values of two homopolymers, is obtained, whereas two Tg values, characteristics of component homopolymers, would be expected for the block co-polymer. Compatibility of two poly- mers in a blend can be judged by measurement of Tg.
2.3.5.2 Melting and Crystallization
Crystallization and melting are important for semi-crystalline and crystalline polymers (Figure 2.13). DSC has been used for studying the crystallization kinetics of polyvinylidine fluoride. The time dependence of crystallization could be obtained at higher rates with sufficient accuracy. Crystallization of natural rubber has also been studied. The rate of crystallization increases as the temperature drops below Tm. At some temperature, a maxi- mum rate will occur which may or may not be realizable experimentally. The rate again decreases at lower temperature and reaches zero at Tg.
The degree of crystallinity also could be measured from DSC trace, after dividing the measured heat of fusion ΔHf of the sample by ΔHfo, the heat of fusion for 100% crystal- line polymer. However, for polymers that have more than one crystalline structure, this method is a little complex.
Crystallinities of Nylon-6, polyethylene terephthalate (PET), etc., have been determined by this method. The amount of crystallinity and Tm depend on processing conditions and heat history.
Peaks due to crystallinity and melting are important for fiber characterization. Nylon-6 and PET tire cord are characterized by these peaks.
As the temperature is slowly increased, an exotherm for crystallization of polymer appears, followed by an endotherm, due to the melting of polycrystallites (i.e., melting point, Tm) of crystalline/semi-crystalline polymers. Tg,Tm,and crystallinity of representa- tive polymers are given in Table 2.11.
TABLE 2.11
Phase Transition Data for Polymers by DSC
Polymer Tg (°C) Tm (°C) Crystallinity (%)
Polybutadiene –105 –30 —
Nylon-6 60 215 40
Nylon-6,6 65 255 42
Polyethylene –120 122 32
DSC can also be used for determination of the degree of crystallization and crystalliza- tion kinetics. Crystallization and melting are important for semi-crystalline and crystal- line polymers (e.g., fibers). Polyethylene, polypropylene, and polyethylene terephthalate are well-known polymers for crystallization study. The degree of crystallization can also be measured from DSC trace for fusion by the following equation:
Degree of crystallinity = Enthalpy change foor the sample 100 Enthalpy change for 100%
×
ccrystalline polymer (2.38) The crystallinity of Nylon-6 and PET has also been determined by this method. Peaks due to crystallinity and melting are important for fiber characterization. The purity of the polymer additives can also be judged from Tm and the degree of crystallinity.
2.3.5.3 Curing or Vulcanization
The state of curing or vulcanization of rubber can be found out by TG/DSC. Many cur- ing reactions also could be studied using DSC. Common examples are rubber or epoxy resins, which are cured with different chemical species. Cure exotherm could be used for this purpose.
The activity of catalysts and the effect of fillers or other additives on curing can also be measured by thermal analysis.
Curing/crosslinking/vulcanization can be studied by DSC because of its exothermic nature of reaction. The appearance of peak strongly depends on the polymer to be cured and curing agents, including accelerators/activators.
The curing of phenolic, epoxy resins, polyurethane, and different rubbers can be stud- ied by isothermal or non-isothermal DSC in a N2 atmosphere simulating the exact curing conditions (i.e., following a specific time-temperature program). Typical curing data for natural rubber (NR) vulcanization and the effect of the type of crosslinking system are illustrated in Table 2.12.
The extent of curing of semi-cured materials can also be estimated by the following equation:
Extent of curing (%) = Enthalpy of curing foor semi-cured material 100 Enthalpy of curi
×
nng for 100% uncured material (2.39)
2.3.5.4 Oxidation and Degradation
The oxidation or degradation of a polymer indicates its thermal stability, when a polymer is heated in air/oxygen. It combines with oxygen and gives oxidative degradation products
TABLE 2.12
DSC Study of Compounded NR in Nitrogen Atmosphere
Sample Vulcanization System Onset (°C) Peak (°C) ΔH (J/g)
NR-compound Conventional (CV) 178 199 26
NR-compound Semi-efficient (SEV) 186 190 14
NR-compound Efficient (EV) 192 195 06
(noted by exothermic peak). Sometimes it breaks into its components (noted by endothermic peak). DSC has been used to determine the thermal stability of polyvinyl chloride (PVC).
The evaluation of an antioxidant in rubber can be correlated by the amount of heat change or enthalpy during degradation. The smaller the heat change, the more effective is the antioxidant. Relative effectiveness of different antioxidants can also be measured by estimating the shifting in oxidation exotherm to a higher temperature.
Wire and cables for electrical applications are coated with many polymeric materials including PVC, polyethylene, and different rubbers. Degradation of polymer sheathing on cable is a major concern in the choice of suitable polymers. An inhibitor is always added to suppress degradation. The effect of the inhibitor on polymer properties could be moni- tored by DSC using elevated temperature and oxygen atmosphere.
At higher temperatures, a polymer decomposes (degrade) or oxidizes, depending upon the surrounding atmosphere. In the presence of inert atmosphere, energy is required to rupture the chemical bonds, and the overall pyrolytic process is endothermic. However, in some cases, an exothermic transition prior to final decomposition is obtained which indicates bond-forming reactions like crosslinking, cyclization, etc. The relative thermal stability of different polymers can be obtained from the position of the peak in DSC curve.
Oxidative degradation of polymers occurs in oxygen atmosphere by chain scission involving peroxide formation followed by distinct gelation, which is characterized by an exothermic peak in the DSC curve with no or marginal weight loss. This ultimately leads to autocatalytic oxidation, which is very fast and is characterized by a very strong and erratic exotherm in the DSC curve. Degradation data by DSC are listed in Table 2.13. For polymers, the following can be useful:
• To assess thermal and thermoxidative stability
• To assess the effect of additives like antioxidants
• To elucidate the mechanism and kinetics of oxidation
• To predict lifetime in service 2.3.5.5 Specific Heat or Heat Capacity
Thermal properties like specific heat of polymers can be measured from a DSC run at any temperature (where no physical/chemical transition occurs) by the following equation:
Cp =M1 dH/dt
dT/dt (2.40)
where dH/dt is the energy difference between the sample and blank at temperature T; dT/dt is the heating rate of the DSC run; and M is the mass of the polymer.
TABLE 2.13
Degradation Data for Polymers by DSC Polymer
In Nitrogen In Oxygen
Peak (°C) ΔH (J/g) Peak (°C) ΔH (J/g)
Natural rubber — — 218 1.45
Styrene butadiene rubber 387 1.05 214 0.60
Polybutadiene rubber 377 1.95 200 0.91