Dynamic Heat Buildup

Dynamic heat buildup in applications such as wheels is important. Polyurethanes with a high resilience or a low tan 8 in the operating range are important. Polyurethane elastomers used in wheel applications can be evaluated using a test rig where the urethane can be run under load for a fixed period or until failure. 

A typical test speed is 32 kph at a load of 55 kg. The surface temperature of the wheel can be measured using an infrared thermometer. Failure will be due to hysteresis work, and the part will decompose from the inside out. 

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The essential performance properties for an engine mount are low heat buildup and low dynamic compression set. These have been determined for a typical engine mount compound based on NR, with and without Perkalink 900—Table 14.50. 

The important vulcanizate properties demanded by this application, low heat buildup, and low dynamic set have been determined in the Goodrich Flexometer test. The compound containing the antireversion agent exhibits a marked decrease in heat buildup and dynamic permanent set (Figures 14.21 and 14.22). 

Both rolling resistance and heat buildup are related to hysteresis that is the amount of energy that is converted to heat during cychc deformation. It is well known that hysteresis of tread compounds, characterized by the loss factor, tan 8, at high temperature, is a key parameter. It not only governs heat buildup of the compounds under dynamic strain but also shows a good correlation with the... 

The amount of testing and verification carried out depends largely on the importance of the part being produced and its future function. An end stop for a tube may only need the appearance, hardness, and the fact that it will correctly fit the tube checked. A flexible mounting of similar size also will need extensive testing to prove that it has the correct dynamic properties and can take the load and vibrations and not fail due to excessive heat buildup. 

Filler dispersion is a property that determines how well the filler partciles in a given rubber compound are dispersed as a result of the mixing process. This relates to carbon black dispersion as well as the dispersion of nonblack fillers such as silica, clay, calcium carbonate, titanium dioxide, etc. Also rubber curatives such as sulfur and accelerators can be poorly dispersed (commonly these ingredients are added late in the mixing cycle). Poor dispersion makes a mixed stock less uniform, and commonly the cured ultimate tensile strength will have more variability. Poor dispersion can affect other important cured physical properties such as abrasion, tear, and fatigue resistance, flexometer heat buildup, and other dynamic properties. 

Fatigue crack growth, abrasive wear, and thermal degradation resulting from heat buildup can be grouped together as dynamic failure mechanisms, although with some nonstrain... 

Flexometer tests are used to determine thermal stability under dynamic straining conditions. Measurements include temperature rise after a specified period of cycling, set and creep, and in some instances the time or number of cycles to failure in the form of thermal runaway or test piece destruction. In contrast to fatigue cracking tests, heat buildup tests... 

Tire heat buildup under loaded dynamic conditions. 

Reducing the monol content also improves the dynamic properties of cast elastomers. Figure 9.3 shows the DMTA response for the two polymers described above. The polymer derived from the ultra-low monol polyol has a flatter rubbery plateau region. The higher modulus in the rubbery plateau is consistent with the polymer s higher tensile modulus. The substantial reduction in tan delta (8) across the entire temperature range should also be noted. Lower tan delta translates into improved performance in dynamic applications due to lower heat buildup and improved rebound as noted above. 

ACN (> 42%) containing HNBR is used in maximum fuel and oil resistance as well as freon resistance. Low ACN (< 25%) containing HNBR is used for goods that require better dynamic properties (e.g., rolls, belts, and oil field parts). Very low ACN (< 17%) containing HNBR is used for improved low and high temperature/heat buildup resistance goods such as required for arctic oil and gas drilling fields and automatic uses. Typical recipes are... 

The rubber s polymer network allows elasticity and flexibility to be combined with crystallization-induced strength and toughness when stretched. The elastic nature of this network also accoimts for the exceptional resilience of cured rubber products. This resilience means less kinetic energy is lost as heat during repeated stress deformation. Products made from natural rubber are less likely than most other elastomers to fail from excessive heat buildup or fatigue when exposed to severe dynamic conditions. This has secured the place of natural rubber as the preferred sidewall elastomer in radial tires. 

Heat BuUdup and Compression Fatigue - Heat buildup in elastomers is the accumulation of thermal energy and resultant temperature increase in a rubber product due to internal friction when the product is subjected to repeated, rapid cychc deformation. The loss in properties of an elastomer due to heat buildup and other molecular effects of dynamic deformation is known as fatigue. Elastomeric compositions which show the least heat... 

A major consideration in all dynamic applications is the buildup of heat. The low conductivity of the polyurethanes does not allow for rapid removal of heat. The design of the part and the grade choice are very important. The part must not be allowed to deflect too much, and a heat sink must be provided. It is desirable to keep the cross-section of the polyurethane at a minimum. 

The above models differ with respect to timing and therefore can be tested with isotopic techniques. However, not only are the models very different in terms of timescales, they also differ with respect to the environment that would be created on Earth. In the first two cases the Earth would form with a hot dense atmosphere of nebular gas that would provide a ready source of solar noble gases in the Earth. This atmosphere would have blanketed the Earth and could have caused a dramatic buildup of heat leading to magma oceans (Sasaki, 1990). Therefore, the evidence from dynamic models can also be tested with compositional data for the Earth, which provide information on the nature of early atmospheres and melting. 

In a so called heal buildup test the prime object is to induce a temperature rise in the lest piece, to measure its magnitude, and to study its effects. The heating is a result of the viscoelastic nature of polymers some of the work done in stressing the material is dissipated by viscous forces between molecules and converted to heat energy. Compression, shear, or some combination of them is normally used with relatively bulky test pieces. The tests are almost exclusively applied to rubbers and were generally intended to be relevant to tires. The geometries used are inevitably arbitrary and involve superimposing dynamic strain or stress cycles onto a prestress or strain. 

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