Natural rubber Compressibility

FIGURE 9.16 Resilience values for chlorobutyl rubber (CIIR), butadiene rubber (BR), unfilled natural rubber (NR), filled natural rubber (SRB), and polyurethane (PU) samples tested using a Shore rebound resibometer, an Instron compression tester and a scanning probe microscope (SPM). (From Huson, M.G. and Maxweb, J.M.,... 

Neoprene is the generic name for polychloroprene rubber. It has been produced commercially since 1931 and had rapid and wide acceptance because it is much superior to natural rubber for heat and oil resistance. Heat resistance is far better than NR, BR or SBR. but less than EPDM. When heated in the absence of air, neoprene withstands degradation better than other elastomers which are normally considered more heat resistant, and retains its properties fifteen times longer than in the presence of air. Compression set at higher temperature is better than natural rubber and 100°C is typically the test temperature rather than 70°C. Abrasion resistance is not as good as natural rubber but generally better than most heat resistant and oil resistant rubbers. This is also true for tear strength and flex resistance. 

Many workers have studied and published correlations between various types of plastimeter, often to show that they do not agree and to illustrate the superiority of the instrument which supposedly agrees best with processing behaviour. Several comparisons are included in the literature already noted and other examples are shown in Figure 6.6. Figure 6.6a shows the relatively close correlation obtained between two compression instruments, the Wallace rapid and Williams plastimeters, for materials of similar flow characteristics (plasticised natural rubber). When such rubbers are compared on two basically different instruments (compression and extrusion) the... 

An attempt was made to derive the extent of crosslinking by measuring the compression modulus of specimens which had been allowed to swell in ethanol. While this technique has been successfully used for natural rubbers [10], no adequately constant results were obtained to make this calculation possible for this material. 

Compression molding is an old and common method of molding thermoset (TS). It now processes TS plastics as well as other plastics such as thermoplastics (TP), elastomers (TS and TP), and natural rubbers (TS). By this method, plastic raw materials are converted into finished products by simply compressing them into the desired shapes... 

Code designation, for gasket materials a = asbestos, white (compressed or woven) b = asbestos, blue (compressed or woven) c = asbestos (compressed and rubber-bonded) d = asbestos (woven and rubber-frictioned) e = CR-S or natural rubber 1 = Teflon... 

Fig. 1 a,b. Strain amplitude dependence of the complex dynamic modulus E E l i E" in the uniaxial compression mode for natural rubber samples filled with 50 phr carbon black of different grades a storage modulus E b loss modulus E". The N numbers denote various commercial blacks, EB denotes non-commercial experimental blacks. The different blacks vary in specific surface and structure. The strain sweeps were performed with a dynamical testing device EPLEXOR at temperature T = 25 °C, frequency f = 1 Hz, and static pre-deformation of -10 %. The x-axis is the double strain amplitude 2eo... 

Figure 3.24 Compressive stress-strain curves for natural rubber vulcanizate showing the effect of shape factor S. (From Ref. 24.)...

Solvents produce different effects than do corrosive chemicals. Both silica and carbon black filled natural rubbers were more resistant to solvents than unfilled rubber. Also, the cure time was important, indicating that the bound rubber plays a role in the reduction of a solvent sorption. The diffusion coefficient of solvents into rubbers decreases with longer cure times and higher fillers loadings. Polychloroprene rubber swollen with solvent has a lower compression set when it is filled with carbon black. 

Thus, as an elastomer is compressed in, say, the Z-direction (as in an isolator on a rubber grommet, engine mount, or transmission mount), the mount will deform in the X and Y directions. This value is nearly 0.5 for natural rubbers (typically used for mounts in automotive systems). For steel, Poisson ratios are around 0.3. The Poisson ratio has no units. 

As natural rubber is vulcanized, the disulfide bonds shown in Figure 8.11 shorten the chains of the rubber and increase the rate at which the chain will contract. The greater the number of disulfide bonds, the greater is the hardness of the natural rubber. Hardness affects the seal s ability to compress as well as its performance in thermal cycling events. 

The concept of traditional thermoset elastomers was pioneered by Goodyear s discovery in 1839 that heating natural rubber with some sulfur converted the material from one that was tacky when warm and brittle when cold into a vulcanized rubber that was conveniently useful over a wide temperature range. Crosslinking of the macromolecules of rubber with sulfur bonds endowed the naturally occurring material with some elastic memory and caused it to behave as we have come to expect elastomers to behave. Excessive sulfur crosslinking converts the stretchable, compressible, bouncy rubber into hard rubber such as the material found in the heads of mallets used in machine shops to pound sheet metal into desired shapes. A small dose of crosslinking prevents the macromolecules of natural rubber to crystallize at low temperatures and turn into a brittle solid and to become a tacky, sticky semifluid at elevated temperatures. 

In addition to Mooney viscosities, the rheological properties of raw natural rubber samples were determined at 100 C using the Monsanto Processability Tester (MPT) with a die of 2,01 mm diameter (L/D ratio = 16). Elongational properties were simply estimated at 22°C, using a tensile tester and dumbbell samples die-cut from compression moulded sheets. 

The original crosslinking process for natural rubber, called vulcanisation, involved mixing in 2-3% of sulphur plus an accelerator. On heating to 140 °C the sulphur reacts with C=C bonds on neighbouring polyisoprene chains to form sulphur crosslinks C—(S) —C. Typically, 15% of the crosslinks are monosulphide [n = 1), 15% are disulphide and the rest are polysulphide with n > 2. The polysulphide crosslinks are partially labile, which means that they can break and reform with other broken crosslinks when the applied stresses are high. This leads to permanent creep in compressed rubber blocks. To avoid such permanent set, efficient vulcanisation systems have been developed that produce only monosulphide crosslinks. 

Compressive Young s modulus of a rubber spring vs. shape factor curves labelled with the rubber shear modulus in MPa (from Lindley, R B., Engineering Design with Natural Rubber, 4th Ed, Malayan Natural Rubber Producers Association, 1974). 

FIGURE 6.16 Optical birefringence for 1,4-polybutadiene and natural rubber networks under tension and compression the stress optical coefficient is given by the slopes = 3.6 and 2.0 GPa, respectively (Mott and Roland, 1996). 

The example chosen here to illustrate this type of composite involves a polymeric phase that exhibits rubberlike elasticity. This application is of considerable practical importance since elastomers, particularly those which cannot undergo strain-induced crystallization, are generally compounded with a reinforcing filler. The two most important examples are the addition of carbon black to natural rubber and to some synthetic elastomers and silica to polysiloxane elastomers. The advantages obtained include improved abrasion resistance, tear strength, and tensile strength. Disadvantages include increases in hysteresis (and thus heat buUd-up) and compression set (permanent deformation). 

Figure 11-5. Relationship between the stress an and the draw ratio a — Lf Im for cross-linked natural rubber — O—OExperimental —, calculated according to Equation (11-39). Measurements by elongation (at a > 1) or compression (a < 1). (After L. R. G. Treloar.)...

The measured dependence of a on A is shown in Figure 3.7 for natural rubber. The fit is good up to A = 1.2. The fit for compression (A < 1.0) is excellent. The lack of fit above A = 1.2 is due to several factors which include (i) rather simple assumptions in the model (ii) that the chains cannot be Gaussian at high extensions since they cannot extend further than their own contour length (see (3.N.6)) and (iii) at high extensions vulcanized natural rubber crystallizes. 

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