Rubbers thermal analysis

The logical approach to problem solving for rubber analysis at Polysar Ltd was described by Chu [73] (cf. Schemes 2.4 and 2.5). Systematic analysis involves sampling, elimination of interference and measurement. Methods employed include chromatography (GC, HS-GC, HPLC, SEC, IC), spectroscopy (AAS, UV/VIS, IR, NMR), MS, microscopy and thermal analysis. The specific role of each of these techniques for the analysis of rubber compounds with or without... 

Alternative approaches consist in heat extraction by means of thermal analysis, thermal volatilisation and (laser) desorption techniques, or pyrolysis. In most cases mass spectrometric detection modes are used. Early MS work has focused on thermal desorption of the additives from the bulk polymer, followed by electron impact ionisation (El) [98,100], Cl [100,107] and field ionisation (FI) [100]. These methods are limited in that the polymer additives must be both stable and volatile at the higher temperatures, which is not always the case since many additives are thermally labile. More recently, soft ionisation methods have been applied to the analysis of additives from bulk polymeric material. These ionisation methods include FAB [100] and LD [97,108], which may provide qualitative information with minimal sample pretreatment. A comparison with FAB [97] has shown that LD Fourier transform ion cyclotron resonance (LD-FTTCR) is superior for polymer additive identification by giving less molecular ion fragmentation. While PyGC-MS is a much-used tool for the analysis of rubber compounds (both for the characterisation of the polymer and additives), as shown in Section 2.2, its usefulness for the in situ in-polymer additive analysis is equally acknowledged. 

Differential Thermal Analysis (DTA). One of the characteristics of a rubber useful in tire rubber compounds is that it is amorphous at room temperature but readily undergoes strain induced crystallization. For this reason, copolymers were prepared in order to appropriately adjust the crystalline melt temperature. 

For materials generally, change in expansion (or density) by dilatometry was traditionally the most often used method for measuring Tg. Thermal properties, for example specific heat, are also widely used, particularly the methods of differential thermal analysis". A method for rubbers using DSC is being developed in ISO TC 45 as ISO 22768, but is not yet published. The inflection point on the heat input - temperature curve is usually obtained automatically by the analyser s software but, if obtained manually, is best found from the derivative of the curve. 

Thermal analysis is a useful tool in the quality control of many incoming routine materials, which can be tested against a reference standard developed internally by analysing a large number of samples of known performance criteria to ensure that the quality of supplies is maintained. Solid elastomers can be identified by glass transition temperature (Tg) [70]. The rubber industry uses thousands of different raw materials, and this number is steadily increasing. These materials are listed in [172]. 

Whereas Redfern [57] has pointed out the advantages of simultaneous thermal analysis techniques (particularly TG-DSC and TG-DTA) over techniques conducted singly, an even more complete thermal profile is provided when a thermal analyser is coupled to some form of gas analyser (MS or FTIR). Mohler and co-workers [51] have reported TG-DSC-MS of the thermal decomposition of the vulcanisation accelerator tetramethyl thiuram disulphide (TMTD) in rubber degradation of TMTD starts at about 155 °C, as evidenced by m/z 76 (CS2) and 44 (radical of the secondary dimethylamine). 

Rubber industries produce various types of complicated products like tyre, cable, belt, seal, bearings, engine mounts, etc. The products are composed of rubber, plastics, fibre, metal, fillers and many other additives like antioxidants, accelerators, etc. Reverse engineering is a technique by which a rubber technologist can reconstruct the composition of the products based on thermal analysis. Dormagen [178] and Baranwal [179] reconstructed the formulation of a tyre, based on analyses of FTIR, spectra, thermal analysis and high performance liquid chromatography. 

HSBR HTNR HXSBR ICTA Hydrogenated styrene butadiene rubber Hydroxyterminated liquid NR Hydrogenated carboxylated styrene butadiene rubber International Confederation for Thermal Analysis... 

Antioxidants have been shown to improve oxidative stability substantially (36,37). The use of rubber-bound stabilizers to permit concentration of the additive in the rubber phase has been reported (38—40). The partitioning behavior of various conventional stabilizers between the rubber and thermoplastic phases in model ABS systems has been described and shown to correlate with solubility parameter values (41). Pigments can adversely affect oxidative stability (32). Test methods for assessing thermal oxidative stability include oxygen absorption (31,32,42), thermal analysis (43,44), oven aging (34,45,46), and chemiluminescence (47,48). 

Many relatively slow or static methods have been used to measure Tg. These include techniques for determining the density or specific volume of the polymer as a function of temperature (cf. Fig. 11-1) as well as measurements of refractive index, elastic modulus, and other properties. Differential thermal analysis and differential scanning calorimetry are widely used for this purpose at present, with simple extrapolative eorrections for the effects of heating or cording rates on the observed values of Tg. These two methods reflect the changes in specific heat of the polymer at the glass-to-rubber transition. Dynamic mechanical measurements, which are described in Section 11.5, are also widely employed for locating Tg. 

Zeyen, R.L. Thermal analysis practical applications for chemical reconstruction of rubber compounds. Kautschuk Gummi Kunststoffe 1988, 41 (10), 974-982. 

Dielectric measurements combined with thermal analysis revealed two transitions a, a glass-rubber transition between 295-340 K (10 Hz), and B, a sub-glass transition exhibiting Arrhenius behaviour. 

Figure 10.31 TG and DTG curves of a natural rubber-butadiene rubber blend. (Reproduced with permission from T. Hatakeyama and F.X. Quinn, Thermal Analysis Fundamentals and Applications to Polymer Science, 2nd ed., John Wiley Sons Ltd, Chichester. 1999 John Wiley Sons Ltd.)...

FIGURE 10.19 Dynamic mechanical thermal analysis of a block copolymer of SBR rubber and polystyrene. This illustrates two phases however, owing to the broad loss modulus peak, it may be assumed that some mixing has occurred in the polystyrene phase. (Reprinted from Wetton, R.E. et al., Thermochim. Acta 175, 1, 1991. With permission.)... 

Thermal analysis provides information that may be used to characterize plastics and rubbers. Identification tests that involve combustion of a test sample are also used, but may produce hazardous vapors. 

Semi-empirical rules, which correlate the static glass transition temperature Ty from differential thermal analysis or dilatometry with the dynamic T(J taken from the tan 8 or E" peak, may be used with caution in analyzing two-phase systems with a dispersed rubbery phase. The dynamic Tg depends on the rubber phase volume, and it may be shifted further toward lower temperature for effectively crosslinked and grafted rubber particles because of dilatation. 

Carbon black filled, vulcanised rubber is still difficult to analyse with standard analysis techniques like FTIR and NMR. Application of the thermal analysis techniques TGA and DSC offers the possibility to obtain a reasonable impression of the composition of such a rubber sample. This is illustrated below by the results of TGA and DSC measurements on a rubber sample from a Michelin MXT 185/65-R14 cartyre. 

Erom the practical point of view, fundamental information on the processability of polymers is usually obtained through thermal analysis, which provides knowledge of the main polymer transitions (melting and glass-to-rubber transition to the crystalline and amorphous phases, respectively). In addition to the well-established calorimetric techniques, experimental methods capable of revealing the motional phenomena occurring in the solid state have attracted increasing attention. 

Of interest in studies of polymer degradation are two standard tests, the ASTM D 3850 (1994) test method for rapid thermal degradation of solid electrical insulating materials by thermogravimetric method, and the ASTM D 6370 (1999) standard test method for rubber compositional analysis by thermogravimetry. 

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