Polymers thermal stability

This increase in polymer thermal stability translates to improved thermal stability of the adhesive, as shown in Fig. 10 for the steel lapshear adhesive strength after thermal aging at 121 °C for 48 h. 

Each sample was evaluated by thermogravimetry to determine if the thermal stability could be enhanced by removing some residual cobalt chloride. The BTDA-ODA polyimide film thermal stability is reduced about 50 C due to the cobalt chloride dopant. Soa)cing or extraction with water has no positive effect on the thermal stability whereas soxhlett extraction with DMAc severely degrades the polymer stability. For the BDSDA-ODA polyimide films the incorporation of cobalt chloride also reduces the bul)c polymer thermal stability. Soa)cing this film in water for 24 hours, however, increased the bul)c thermal stability slightly from 512 to 532 C. 

The definition of polymer thermal stability is not simple owing to the number of measurement techniques, desired properties, and factors that affect each (time, heating rate, atmosphere, etc). The easiest evaluation of thermal stability is by the temperature at which a certain weight loss occurs as observed by thermogravimetric analysis (tga). Early work assigned a 7% loss as the point of stability, more recently a 10% value or the extrapolated break in the tga curve has been used. A more realistic view is to compare weight loss vs time at constant temperature, and better yet is to evaluate property retention time at temperature one set of criteria has been 177°C for 30,000 h, or 240°C for 1000 h, or 538°C for 1 h, or 816°C for 5 min (1). 

The advantage of these polymers over poly(vinylferrocenes) or related species with pendent organometallic units is that the condensation polymers have ferrocene units in the main chain where they can exert their maximum influence on polymer thermal stability. The disadvantage of the condensation products is that, except in the last example, the molecular weights are too low to favor fiber or flexible film formation. Nevertheless, this work indicated the potential usefulness of polymers with metallocene units in the main chain. 

Introducing organomodified layered silicates into polymer matrix, it brings to polymer thermal stability change. 

Effects of Hydroxy Double Salts and Related Nanodimensional-Layered Metal Hydroxides on Polymer Thermal Stability... 

Tan s experiments showed that the polymer would be more stable if it is under thermal degradation when the temperature is gradually increased so that the oxygen is consumed at low temperatures. However, the initial polymer viscosities were different in his experiments (57.8 mPa s at 75°C constant temperature compared with 77.5 mPa s under the temperature gradient). The water TDS was 362.6 mg/L, and sand was used in the tests. Tan tried to imitate the actual thermal degradation conditions. He also observed that oil did not affect the polymer thermal stability. 

Tan (1998) also investigated the effect of oil sand on HPAM thermal stability. He observed that oil sand improved the polymer thermal stability. 

The effect of temperature gradient on polymer thermal stability. Source Data from... 

FIGURE 5.26 A plot of polymer thermal stability tests (750 mg/L HPAM S525, 0.5 mg/L oxygen). Source Data from Tan (1998). 

Thus, all used additives may be devided into two groups 1) increasing polymer resistance to thermal and thermooxidative destruction 2) decreasing polymer thermal stability. Moreover, effect of additive on thermo- and thermooxidative stability will depend on the length of conjugation chain in modifier s molecule. 

EGA techniques have been widely used in polymer thermal stability and degradation studies, the analysis of trace impurities and additives, and in the elucidation of polymer structures (12). They are also used in vapor-pressure measurements and toxicity studies of constituents in polymer systems. Langer (128) has reviewed the applications of EGA techniques to polymers as well as numerous other compounds. Only a few illustrative examples will be discussed here. 

The estimated Td,o-temperature giving an indication of the polymers thermal stability, holds only for polymers with the more or less simple thermal degradation processes of chain depolymerisation and/or random decomposition. S. Shkolnik and E.D. Weil [30] reported, however, that for non-stabilised PK copolymer a process of furan ring formation can start at temperatures of about 250 °C. Water, one of the reaction products, is coming free during this intramolecular... 

Styrenic Polymers Stabilization of polystyrene and its copolymers is necessary for articles expected to be exposed to solar radiation or indoor fluorescent lighting. Because of the signiflcant role played by thermal oxidation products in the effect of these sources on the polymers, thermal stabilization at the processing stage is required to reduce their sensitivity to light. The use of a phenolic antioxidant was shown to increase the retention of mechanical properties and. 

Stability can be said as the protection of polymeric materials from which lead to deterioration of properties [9]. In literature, there are different and sometimes contradictory reported papers concerning the effect of the nanoparticles on polymer thermal stability. There are papers suggesting that nanoparticles have no obviously effect on thermal stability, some of them suggested a small to substantial enhancement and some others suggested acceleration of thermal decomposition. In a study performed by Ollier et al. [10], the author incorporated 5 % weight of bentonite in unsaturated polyester (UP) matrix. They noted that the addition of bentonite... 

Organoclays have also similar effects on polymers thermal stability and in this case the achieved clay dispersion (intercalated-exfoliated) as well as the used modifier can also alter thermal decomposition of polymers. Cationic compounds used for organomodification of MMT have a negative effect in thermal degradation while the exfoliated stmcture due to the finer dispersion of the clay nanoparticles can lead to thermal stabilization than intercalated structure. 

Thermogravimetric analysis (TGA) is one of the most commonly used techniques to study the primary reactions of the decomposition of polymers and other materials. TGA is also useful for the characterization and evaluation of polymer thermal stability. Although synthetic mbber nano composites have excellent mechanical properties, these properties may interfere with its low thermal stability. This may cause the polymer chain to be more susceptible to degradation. Degradation usually starts from a head-to-head stmcture, a site of unsaturation or a tertiary carbon atom [77]. 

Thermogravametric analysis (tga) provides a measnre of polymer thermal stability. In this test, polymer samples are heated at a programmed rate and sample weight is recorded as a function of temperatnre. Two studies (15,16) made tga measurements and reported the temperature for 10% weight loss (see Table 1). The thermal stability of these polymers is lower in air than in nitrogen. 

In separate work aimed at elucidating filler effects on surface, oxidation Vesely and coworkers [39] have examined the oxidation of polyethylene and polypropylene containing high levels of calcium carbonate. The grade of filler used was found to decrease polymer thermal stability, an effect they associated with transition metal impurities. They also believe that, in the presence of fillers, heteregeneous oxidation reactions become more important. They observed black polyenes to be formed above about 320 °C and postulated that these could be acting as photo-sensitisers. 

The principal effects observed in cone calorimeter tests are a marked reduction in peak and average rate of mass loss and in heat release. There appears to be little reduction in total heat of combustion or in smoke levels. The workers at NIST report that there seems to be little difference between intercalated and delaminated forms of nano-clay, despite evidence that polymer thermal stability can be more improved by the intercalated structure. 

Over the years many fluoroelastomers have been prepared in addition to the materials described earlier in this chapter. These include fluorinated polyurethanes, fluorinated polyepoxides, hexafluoro-acetone/propylene oxide copolymers and polyfluorals. Many of these materials are thermally unstable, a fact which stresses the point that the presence of C—F bonds with their high bond strength is no guarantee of polymer thermal stability. One particular type of fluoroelastomer which is of technical importance, the fluorosilicone rubber family, are however of good thermal stability and are considered together with the silicone rubbers in a later chapter. 

The increase in the IPA fragments content shifts the melting interval to lower temperatures and reduces the polymer thermal stability. 

As known, the polymer thermal stability depends on many factors including the structure, the molecular weight characteristics, the concentration of defects and labile end groups (hydroxyl-groups in this case) in macromolecules as well as on low-molecular weight admixtures (for example, monomers, catalysts, metal ions fi om raw materials and equipment). 

We measured the concentration of inorganic admixtures in the LCP and appropriate monomers by ICP emission spectrometry. Taking LCP-2 as an example, we tried then to compare the metal content with the polymer thermal stability. To clarify the effect of metal impurities we have increased their concentrations artificially by introduction of appropriate inorganic salts. The data obtained for the effect of Cu, Fe, Ni, Ca and A1 on the decomposition onset temperature Tj according to TGA are listed in Table 2. 

The total amount of organic moieties in samples is not more than (1.0-2.0) 10 % (w/w) that practically does not affect the polymer thermal stability. 

---