CUREMETER

FIGURE 28.2 Standard oscillating disc curemeter curve. (Redrawn from Andrew, C., Introduction to Rubber Technology, Knovel e-book publishers, 1999.)... 

Rheological measurements of nonvulcanized rubber compounds - Dynamic mechanical measurements were performed using a RPA 2000 dynamic curemeter... 

Cure characteristics and reinforcement parameter (aF) - The cure characteristics were determined using the RPA 2000 dynamic curemeter (Alpha Technologies). The increase in torque at a frequency of 0.833 Hz and 2.79% strain was measured. The optimal vulcanization time (f90) of the samples was determined and used for curing of the samples in the press. 

The cure characteristics for the compounds were measured according to the procedure described in Sect. 4.1.2. From the curemeter data shown in Table 13 it becomes clear that the SBR compounds containing coated sulfur show longer scorch times (ts2) and optimal curing times (t90) than the compound with uncoated sulfur. The only exception is for sample PPASg-4, which gives a similar optimal curing time to uncoated sulfur (Sg). 

This is not of course the complete story. Most often, thermoplastic elastomers are processed on plastics machinery and it will be convenient, and sensible, that test pieces are produced in the same way. The thermoplastics processability tests are also likely to be more relevant and, certainly, curemeter tests are irrelevant. 

Three types of mixer are recognized, plus a miniature mixer which would provide just enough compound for a curemeter test and one sheet. Mixers types A] and A2 and the miniature device have non-interlocking (tangential) rotors whilst type B has interlocking rotors. Dimensions of the larger mixers are specified fairly precisely, including new and worn rotor clearances, and they are required to have temperature control, a system to record power or torque and a timer. The miniature mixer is only specified in terms of capacity, rotor speed and friction ratio but is required to control of temperature, indication of power and a timer. 

Details are given for the press and mould construction and for the vulcanisation procedure. The most important parameters are the time and temperature of moulding and ISO 2393 specifies close limits, 0.5°C, on the latter. ISO 2393 requires only that the mould is loaded and unloaded as quickly as possible but the mouldings are to be cooled in water, or on metal plates if intended for electrical tests, on removal. One has to assume that cure time is derived from curemeter measurements. 

Mention was made above that oscillating die curemeter based instruments are now widely used for obtaining measures of processability. In fact, the other types of curemeter considered in Section 4, oscillating disc,... 

Laboratory measurement of curing characteristics was revolutionised by the introduction of so-called curemeters in the middle of the last century, which quite rapidly became almost universally used for the routine control of fully compounded rubbers. These instruments were so successful that the use of the Mooney to measure scorch and the need for routine measurement of physical tests on moulded test pieces has been much reduced. 

The reciprocating paddle instrument is now largely a matter of history and a third type, the rotorless curemeter, has rapidly become the most popular. The rotorless type is a curemeter in which one half of the die enclosing the test piece, rather than a paddle or disc within the test piece, oscillates or reciprocates (Figure 6.7). 

Figure 6-7. Principles of oscillating disc and oscillating die curemeters. (a) Oscillating disc ...

Despite the widespread use of curemeters, progress to international standardisation was relatively slow, partly because of patent difficulties as a... 

A similar estimate of cure time is taken from an oscillating paddle or rotorless curemeter curve. 

ASTM D 5289111 for rotorless curemeters, although rather more specific, is overall very similar to ISO 6502 in technical requirements. The British standard"2 is identical to ISO 6502. 

Norman113 gave a valuable discussion of the problems with curemeters, pointing out that there is no one level of cure which gives optimum values for all physical properties and no satisfactory procedure for dealing with a "marching modulus". He also listed problems such as non-uniform temperature distribution, possible slip of the test piece over rotor or cavity and porosity. 

Curemeters are usually run under isothermal conditions to determine cure parameters at the temperature(s) of interest. Rosea and Vergnaud123 investigated the use of a temperature ramp as being a more efficient way of obtaining kinetic parameters. 

ISO 3417, 1991. Method for the determination of vulcanisation characteristics using an oscillating disc curemeter,... 

Kinetic tests on rubbers have up to now been limited primarily to the analysis of curemeter curves. In such investigations, the formation of a three-dimensional network is determined by means of its mechanical properties. The kinetics of the crosslinking reactions involved is virtually unknown. Detailed knowledge even of the chemical processes involved is fiequently unavailable. This is, however, not the case for silicone rubbers, which cure as a result of the hydrosilylation reaction ... 

The objective of this study is to record both network structure (curemeter curves) and chemical conversion simultaneously. The influence of the chemical reaction on the network formation is determined. The kinetics with respect to different polymers and crosslinkers will be also described. 

The cure times used in 1958 for the original materials could not be derived from measurements in a curemeter as these instruments were only in the experimental stage. Hence, they were derived from a programme of curing for various times and measuring physical properties. The cure times for the new materials were derived from measurements on a Monsanto rheometer. 

Curemeters, oscillating disc, and reciprocating paddle types are forms of plastimeter that measure plasticity before the onset of cure, as well during the cure. The viscoelastic behavior of uncured rubber tested by dynamic measurement is characterized by in-phase and out-of-phase moduli and loss tangent. 

Other laboratory instruments able to measure the curing characteristics, the so-called curemeters, are widely used for the routine control of fully compounded rubbers. 

Two types of curemeters are in common use the reciprocating paddle type, such as the Wallace-Shawbury curometer, and the osaUating disc type, such as the Monsanto rheometer. 

The British standard, BS 1673, Part 10 [8] covers reciprocating paddle, as well as oscillating disc curemeters. 

ISO 667. 1975. Determination of Rate of Cure Using the Shearing Disc Rheometer. BS 1673. Part 10. 1977. Measurement of Prevulcanising and Curing Characteristics by Means of Curemeters. 

ISO 6502 and ASTM D5289 describe rotoricss curemeters u,scd in the rubber industry [116]. The ASTM standard was published in 1993. This new technology is widely accepted by the rubber industry. 

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