Vulcanized stress/strain curves

Figure 9 Stress-strain curves for EPDM, vulcanized EPDM, Zn-SEPDM ionomer, and 50/50 blend of Zn-SEPDM and ZnSt2-...

Fig. 83.—Stress-strain curve for gum-vulcanized natural rubber. The tensile force given on the ordinate axis is referred to the initial cross section.

Fig. 96.—Theoretical and experimental stress-strain curves for simple elongation of gum-vulcanized rubber. (Treloar. )...

Anthony, Caston, and Guth obtained considerably better agreement between the experimental stress-strain curve for natural rubber similarly vulcanized and the theoretical equation over the range a = 1 to 4. KinelP found that the retractive force for vulcanized poly-chloroprene increased linearly with a — l/a up to a = 3.5. 

Attempts were made to remove the third assumption above, and it was shown that correct considerations of the limited extensibility of the chain adequately explain the S-shaped feature of stress-strain curves observed in uniaxial extension of vulcanized rubbers. However, the improved theory still gave zero for bW/bI2. Up to now there is no molecular theory available which predicts bW/bI2 that varies with//. 

Fig. 1-2. Stress-strain curves, (a) Synthetic fiber, like nylon 66. (b) Rigid, britile plastic, like polystyrene, (c) Tough plastic, like nylon 66. (d) Elastomer, like vulcanized natural rubber.

This discovery culminated in the commercial production and the announcement (41) in 1965 of thermoplastic elastomers from block polymers of styrene and butadiene (S-B-S) and of styrene and isoprene (S-I-S). To rubber scientists and technologists the most outstanding property of S-B-S and S-I-S was the unvulcanized tensile strength compared to that of vulcanized NR and vulcanized SBR carbon black stocks. Stress-strain curves, to break, of these latter materials are compared to that of S-B-S in Figure 2. It was pointed out that the high strength of S-B-S must be due to physical crosslinks. 

Figure 1.14. Theoretical (—) and experimental (-0-) stress-strain curves for simple elongation of gum-vulcanized rubber. The increase in modulus beyond a = 4 or 5 is caused by the onset of crystallinity, and also by the finite extensibility of the chains (Shen et al, 1968.)...

Figure 10.1. Tensile stress-strain curves for four natural rubber compounds of different hardnesses 73 IRHD contains 50 parts of a reinforcing black, and different vulcanizing systems account for the different curves of the two gum compounds (47 and 33 IRHD). (Lindley, 1964.)...

This initial microcrack formation is reflected in a stress-strain curve by the deviation from the linear range of the elastic constants. In fact, the failure is analogous to the microcracks that form between spherulites when a semi-crystalline polymer is deformed. (Source Osswald, T.A. and G. Menges, Material Science of Polymers for Engineers, Hanser Publishers, New York, 1996). Refer also to Vulcanization, Peroxides, Peroxide Cross-Linking, Sulfur Vulcanization, and Vulcanizing Agents. 

Figure 15.9 (a) Stress/strain curves and (b) evolution of the parameters stress (UTS) and deformation (e ) at break to vulcanized NR and the magnetic nanocomposite (NR/NF). 

Figure 22.1 Tensile stress-strain curve of the vulcanized NR sample stretched up to X = 7.5. Insets correspond to WAXS patterns of NR upon stretching at room temperature at X = (a) 0, (b) 2.5, (c) 3, (d) 4, (e) 5, (f) 7 and (g) 7.5. (Reprinted with permission from ref 18.)...

Tensile tests were carried out in order to evaluate the influence of the addition of NR to the TPS matrix and the vulcanization process. The test was carried out to 50 mm/min and the extensometer length was 25 mm. Average stress-strain curves of the blends produced are shown in Figure 26.13. 

In the vulcanized rubber, the elastic force / is proportional to the number of chemical cross-link-1 and that of the pseudo-cross-link-2. For rapid deformation, the force is elastic and proportional to the sum of link-1 and link-2 and the stress-strain curve in extension coincides with that of retraction, but slow deformation induces slip of link-2 and is accompanied by hysteresis H. For filled rubber, H is very large. Elastic force in extension,... 

The above picture of the network structure of vulcanized rubber is supported by -the success of the kinetic theory of rubberlike elasticity (see part 4, page 14) calculations based on this model agree well with experimental measurements of stress-strain curves and other properties (James and Guth, 1943 Flory, 1944). Excellent evidence that the swollen gel contains the same network as the unswollen rubber has been presented by Flory (1944, 1946), based on studies of butyl rubber. Using the network model, the number of cross-links in the structure can be calculated in three ways (o) from measurements of the proportions of insoluble (network) and soluble (unattached) material in samples of different initial molecular lengths (b) from the elastic modulus of the unswollen rubber (c) from the maximum amount of liquid imbibed by the gel when swollen in equilibrium with pure solvent. The results of these three calculations for butyl rubber samples were in good agreement. 

At concentrations of 20-30%w, the stress-strain curve resembles that of a vulcanized rub-... 

Radiation-vulcanized NR latex, where the hydrocarbon chains were linked directly by single bonds using y-radiation, was mixed with layered silicates. The stress-strain curves of the composites containing different types of layered silicates are depicted in Figure 7. It can be seen that the gum compound (serving as reference)... 

Figure 7. Stress-strain curves of radiation-vulcanized NR latex films as a function of their composition.

Elastomers are polymers with rubberlike properties. The word elastic refers to the ability of a material to return to its original dimensions when unloaded, and the term mer refers to the polymeric molecular make up in the word elastomer. Vulcanized rubber materials typically have more than 200% elongation in a tensile test and are capable of returning rapidly and forcibly to their original dimensions when load is removed. This elastic response is due to the three-dimensional cross-linked network molecular structure they have. An elastomer, on the other hand, typically has elongation rates of 100% or more and a significant amount of resilience. Resilience is represented by the area under the elastic portion of the stress-strain curve, and therefore, refers to a material s ability to undergo elastic deformations. 

Figure 15.16 Stress-strain curves to 600% elongation for unvulcanized and vulcanized natural rubber.

Stress-strain curves for vulcanized and unvulcanized natural rubber are presented in Figure 15.16. To produce a rubber that is capable of large extensions without rupture of the primary chain bonds, there must be relatively few crosslinks, and these must be widely separated. Useful rubbers result when about 1 to 5 parts (by weight) of sulfur are added to 100 parts of rubber. This corresponds to about one crosslink for every 10 to 20 repeat units. Increasing the sulfur content further hardens the rubber and also reduces its extensibility. Also, because they are crosslinked, elastomeric materials are thermosetting in nature. 

Young s modulus E is the ratio of nominal stress to strain, as shown in equation (70). However, vulcanized rubbers do not obey Hooke s law (as is shown in Figure 5), so E is not a constant. The stress-strain relationship is generally assumed to be linear over small tensile or compressive strains, and Young s modulus is usually defined as the slope of the stress-strain curve in this range of deformation. " Hardness measurement is another way of determining values of this modulus. It is noteworthy that the slope of the stress-strain curve in the tensile and... 

---