Curing, rubber history

It is also of interest to evaluate the effect of the thickness of the rubber sheet when the heating system is on the external surface of the mold in contact with the surroundings, as shown in Figure 4.10 for the temperature-time history and in Figure 4.11 for the state of cure-time history. These curves are drawn as they are calculated at the mold temperature of 180°C with the rubber compound containing 2% sulfur responsible for an enthalpy of cure of 14.3 J/g. 

The state of cure-time history at the surface of the rubber in contact with the mold is the same whatever the thickness of the rubber sheets (dotted line in Figure 4.9). 

S-shaped curves are obtained for the state of cure-time histories at the mid-plane of the rubber sheets as shown in Figure 4.9 (full lines). 

FIGURE 4.9 State of cure-time history at the surface of the rubber in contact with the mold (x = L dotted line) and at the mid-plane of the rubber (a = 0 fuU Une) for various thicknesses of the rubber sheet. Mold temperature = 180°C. Enthalpy = 14.3 J/g. Heating system on the face of the mold is in contact with the rubber. 

X. The effect of the cure enthalpy on the state of cure is shown in Figure 4.19 where the state of cure-time histories are drawn either at the surface of the rubber in contact with the mold for jc = L (dotted line) or at the midplane (full line) of sheets (2 cm thick heated at 180°C with the heating system on the mold surface in contact with the rubber). Thus the same curve is shown at the rubber surface. But, if the curves drawn at the mid-plane (x = 0) start at the same time, around 10 minutes, they diverge, and the state of cure increases more rapidly when the cure enthalpy is increased. This fact results from the higher increase in temperature attained with the higher cure enthalpy. 

The effect of the kinetic parameters on the cure is evaluated in this subsection by using the values collected in Table 4.1, which are the basis values around which they are varied. The mold-rubber is considered with the heating system placed on the mold surface in contact with the rubber sheet. The results are expressed in terms of temperature-time histories and of state of cure-time histories calculated either at the rubber surface or at the mid-plane of this rubber sheet. The rubber thickness is 2 cm, and the mold temperature is 170°C. The cure enthalpy is 14.3 J/g associated with sulfur at 2%. 

Figure 4.25, where the state of cure-time history at the rubber surface and at the mid-plane are drawn as they are calculated with the three values of the order n, with n = 1.25 (curve 1) n = 1 (curve 2) n = 0.75 (curve 3). 

Thus, the temperature-time history and the state of cure-time history are calculated either at the mid-plane or on the surface of a rubber sheet of thickness (2L) 2 cm, during the cure at 170°C, by taking the three couples of values shown above. 

Figure 4.27 with the state of cure-time history at the rubber surface and at the mid-plane obtained with the three values of the activation energy. 

The result is expressed in terms of the state of cure-time history obtained at various values of the temperature of the reservoir T. The curves drawn in Figure 5.1 show the increase in the state of cure of the rubber with time, as they are calculated under isothermal condition by using the kinetic parameters of the cure reaction collected in Table 5.1. 

The results are expressed in terms of temperature-time history obtained for the rubber at the center of the spherical sample when it is located in the mold, and in terms of state of cure-time history, either in the reservoir before injection or at the center of the sphere in the mold. Complementary results are provided with the profiles of temperature developed through the radius of the sphere and with the associated profiles of state of cure, at various times, during the cure stage in the mold. 

Figure 5.3, showing the state of cure-time history at the rubber surface of the spherical sample in contact with the mold for two values of the injection temperature I) (80 and 120°C) for various temperatures of the mold T ranging from 160 to 180°C, when the radius is 1 cm. 

Figure 5.9, where the state of cure-time histories are drawn at the center of the spherical rubber sample when the mold temperature is 170°C for various values of the injection temperature T (20, 80, 100, 120°C). The radius of the sphere is 2 cm. 

FIGURE 5.3 State of cure-time history at the rubber surface in contact with the mold, with the heating system on the mold surface in contact with the rubber, for various values of the mold temperature 160, 170, 180°C. Radius = 1 cm for = 80, 120°C. 

State of cure-time histories on the rubber surface in contact with the mold appears, when the radius of the sample is 1 cm. The rate of increase in the state of cure is very high at the beginning of the process. Let us remark that these curves are the same whatever the value of the radius of the rubber sample. 

The dynamic mechanical properties of elastomers have been extensively studied since the mid-1940s by rubber physicists [1], Elastomers appear to exhibit extremely complex behavior, having time-temperature- and strain-history-dependent hyperelastic properties [1]. As in polymer cures, DMA can estimate the point of critical entanglement or the gel point. 

Processibility is dependent on the viscosity or plastic flow of the rubber compound,i.e., resistance to flow. Plasticity or viscosity determines the energy requirement of the rubber during milling, calendering or extrusion while the time to the onset of curing, i.e., scorch time, indicates the amount of heat history which can be tolerated before the rubber is converted from the plastic to the elastic state at which time processing becomes virtually impossible. 

FGD Absorber case histories confirm that the chlorobutyl linings give trouble free service when correctly applied and cured. Chlorobutyl linings offer excellent chemical, heat, weather and ozone resistance compared to natural rubber. 

Rubber technology is a mature science with a history going back some 150 years or more. Over the years a number of scientific discoveries (e.g. curing with sulphur to increase resilience and recovery, and the use of antioxidants to lengthen service life) have contributed to the material s dominance in applications requiring elasticity/recovery upon deformation combined with durability. Additives are used in rubbers in order to ensure that they possesses the correct properties to be processed, have the physical properties appropriate for the application, and sufficient stability and resistance to ageing in service. There are three basic steps associated with the processing of rubber ... 

The temperature history at equally spaced points through the thickness of the rubber sample was calculated, giving the temperature profiles obtained at a time when the sample is heated in the mold at a constant temperature. The degree of cure was then evaluated from this temperature-time history. 

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