1,4-Polybutadiene failure

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

Haley (Ref 86) reported on the effect of gamma radiation in the chemical and physical properties of TP-H-8009 propint which consisted of AP/polybutadiene-acrylic acid/Al. This Minuteman propint is stable up to 2.5 x 107 R. In this dose range, no significant losses were observed. However, beyond this range, the proplnt was unable to survive the initial pressure buildup in a rocket engine consequently ballistic failure occurred... 

The comparative effect of the polystyrene and poly-2,6-dichlorosty-rene fillers on the tensile strength of a polybutadiene vulcanizate is shown in Figure 6. Despite the large difference in Tg values for these fillers, there is no difference in their effect on the vulcanizate. This is illustrated further by the failure envelope plot shown in Figure 7, where the data points for the two fillers, at equal volume fraction, appear to coincide quite well. The fact that all the points fall on the same envelope is a good indication of the constant crosslink density for these vulcanizates. Thus, the similarity in effect of these two fillers appears to be more related to their similar modulus values. 

CA 63,17781 (1965) Proplnt failure characteristics were measured in uniaxial and biaxial stress states for polybutadiene acrylic acid and Nitroplastisol proplnts, and failure conditions were examined over a wide range of temps. The observed failure conditions were compared for various failure criteria, and it was found that a... 

In order to understand the behavior of composite propellants during motor ignition, we conducted a study of the mechanical and ultimate properties of a propellant filled with hydroxy-terminated polybutadiene under imposed hydrostatic pressure. The mechanical response of the propellant was examined by uniaxial tensile and simple shear tests at various temperatures, strain rates, and superimposed pressures from atmospheric pressure to 15 MPa. The experimentally observed ultimate properties were strongly pressure-sensitive. The data were formalized in a specific stress-failure criterion. 

Although elastomers are usually amorphous, strain-induced crystallization occurs in rubbers such as cA-l,4-polybutadiene, butyl rubber, and NR. Crystallization under stress, discovered 200 years ago [239], increases the modulus and most failure properties of rubber and is essential to performance in many... 

Fig. 14. SEM photograph of the fracture surface of polybutadiene filled with glass beads (30-95/im in diameter) showing the high degree of interfacial failure.

Hanson, D. E. Martin, R. L., How Ear Can a Rubber Molecule Stretch Before Breaking Ah Initio Study of Tensile Elasticity and Failure in Single-Molecule Polyisoprene and Polybutadiene. J. Chem. Phys. 2009,130, 064903. 

The cross-link density of the polymer network, as well as its properties, depend on the functionality, the length and the chemical nature of the polyene (R ) and thiol (R) prepolymer chains, and it can thus be tailored as desired. Low-modulus polymers suitable for adhesive applications were obtained by using aliphatic prepolymer chains, in particular with polybutadiene-based elastomers which were cross-linked very efficiently by UV-irradiation in the presence of a tri- or tetrathiol [45-48]. As only a few cross-links need to be formed between the polymer chains to make the rubber insoluble, low concentrations of thiol (2 wt%) proved to be sufficient to achieve an effective and fast cross-finking. Hardening was found to hardly occur upon UV-curing (increase of the Persoz hardness from 40 to 55 s), which is essential to ensure outstanding adhesion. At the same time, the shear adhesion failure temperature (SAFT) increased from 80 to 160°C, due to the formation of the chemical network (Fig. 4). 

Cohesive failure was found to be the predominant mode of failure for each rubber compound containing Saret 633 (Figure 8.7). Therefore, it would be expected that as the Saret 633 concentration is increased, the rubber compound would become stronger due to additional crosslinking, which would result in an increase in adhesive strength at the interface between rubber and substrate. This proved to be the case and is shown in Figure 8.8 for EPDM bonded to untreated steel. As the Saret 633 concentration was increased from 0 to 20 phr, the shear adhesion increased from approximately 0.55 MPa for the control to over 11.0 MPa. Cohesive failure was the predominant mode of failure at each concentration. Similar performance was observed for other rubbers, such as nitrile, natural, polybutadiene, silicone and hydrogenated nitrile. 

A base polymer particularly interesting for studying the effect of the aforementioned mechanisms on rubber failure is polybutadiene. In fact, polybutadiene microstructure can be changed in an extremely wide range, by making use of the host of catalyst systems. developed by the ingenuity of chemists, just starting with the same monomer. 

Polybutadienes with vinyl contents of 10 7 and 72.4% have been selected for further fatigue-to-failure analysis. Fatigue life measurements have been performed on a large number of specimens by means of the Fatigue-to-Failure Tester (Monsanto). Carbon-black-filled compounds with 50 phr N330 carbon black were cured with conventional curing system at 145°C. Curative levels have been chosen to obtain vulcaniza-tes at several levels of 300% modulus. Fatigue tests were performed on un-notched specimens at room temperature and at several deformation amplitudes. Data discussed here were obtained at 136% strain amplitude. 

Fig. 7. Failure probability density as a function of number of cycles in polybutadienes with different vinyl content.

The question now arises which fracture processes, if any, are strongly affected by the local chemical structure Two examples are considered below tearing and crack growth, and abrasive wear. Under certain conditions these failure processes are found to depend upon particular features of the elastomer molecule and they are therefore distinctly different, even for closely-related chemical structures. Natural rubber can usefully be compared with cis 1, 4-polybutadiene in this respect, because, although their chemical structures are superficially similar, large differences are observed in their resistance to tearing and in the mechanism of wear. 

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