Cure time, measurement

Fig. 9.3.14 Applications of the NMR-MOUSE to soft matter analysis, (a) Comparison of normalized transverse relaxation times of a carbon-black filled NR for different curing times measured at 300MHz (DMX 300) and with the NMR-MOUSE at 17.5 MHz [Blu3].

Processing of rigid foams from two part formulations involves combining measured quantities of the polyisocyanate with a polyhydroxyl such that there are no or limited reactive isocyanate functional groups. Moisture is not required to complete the cure. Once the reactants are combined the mixture is poured into a form where expansion and polymerization take place simultaneously. Cure times are usually very fast, on the order of minutes. 

Measuring Vulcanization. The formation of a three-dimensional structure during vulcanization increases the stiffness (modulus) of the compound. Therefore, following the modulus increase versus cure time provides a continuous picture of the vulcanization process. Oscillating disk rheometers provide a useful method to do this (17). In this test, a preweighed sample of uncured mbber is placed into a preheated cavity containing a conical rotor. The cavity is closed and the rotor is set to oscillate within the mbber sample. As vulcanization proceeds, the compound s resistance to rotor movement increases and this resistance is followed as a function of time, thereby generating a continuous profile of cure behavior. These cure curves,... 

The FDEMS sensor-controlled run significantly reduced the time lag and viscosity difference between the center ply and the tool surface ply. The amount of flow as measured by the magnitude of the viscosity minimum was greater in the FDEMS sensor-controlled run. The approach of ds"/dt to zero was used to determine cure completion. The total cure time of 200 min in this FDEMS controlled run is 40 min less than the conventional cure cycle. 

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). 

It should be noted that AM is only a measure of an apparent crosslink density of compounds. It is beyond the scope of the present work to investigate in detail the effective crosslinking (physical and chemical). However, for a qualitative assessment it can be concluded that the apparent crosslink density decreases or is influenced by the E-beam irradiation of PTFE powder. PTFE500kGy-EPDM composites show much lower AM and hence lower apparent crosslink densities. It can be inferred that the state of cure and crosslinking efficiency are strongly dependent on irradiation dose. Table 3 shows the optimum curing time (f90, time required to reach 90% of the AM) as a function of PTFE loading and irradiation dose for different PTFE-filled EPDM composites. 

In all the above methods, it is necessary to cure specimens of test samples for each of a series of curing times and then perform the desired test on the vulcanizate. However, in the test for continuous measurement of vulcanization complete information could be obtained with saving in time. The mooney viscometer test approaches this objective. However a weakness of the mooney viscometer test is that the test is completed before a measurable modulus value after the scorch point has been obtained. This is because the test sample is destroyed after the induction period is passed due to tearing by continuous rotation of the rotor whether small or large. To overcome this deficiency and to provide a total cure curve for the entire vulcanization cycle, a series of instruments called cure meters was developed. In each of these instruments the stiffness or modulus of the compound was chosen as parameters for vulcanization continuously. The Vulkameter developed by Bayers, Germany was the first of the cure meters developed. 

Fig. 14. Dependence of the relaxation times T2. and the fractions of protons with different mobility (f.) for unsaturated polyester on the curing time, as measured from broad line NMR ( ), Hahn spin-echo ( ) and Carr-Purcell pulse sequence (O)- Symbol x indicates the initial distribution of styrene and unsaturated polyester protons (adapted from Ref. S5))...

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. 

Tests for scorch and rate of cure should be distinguished from tests for degree of cure or optimum cure measured on the vulcanised material. The latter type of test estimates degree of cure by measuring the physical properties of test pieces vulcanised for various times, tensile properties, swelling and set measurements being the parameters most commonly used. 

In the past, parallel plate compression plastimeters have been quite widely used for measuring rate of cure and methods have been standardised. The test pieces are heated for various times and then tested in the plastimeter. The change in plasticity or recovery or some combination of these, is then plotted against time of heating to give a scorch curve. An even more time consuming procedure was to measure tensile properties as a function of cure time. 

Nevertheless, it was decided to make the measurements of density and of refractive index for specimens after different times of curing. Since the molecular weight of the polymerized silane was not known, the molecular refractivity could not be calculated but only the ratio of this to the molecular weight. Nevertheless, interesting results were obtained with a linear relationship between the cure time and refractive index up to 18 h. The relationship between the cure time and the ratio of molecular refractivity to molecular weight is shown in Fig. 3 and indicates a steady increase in molecular complexity up to 24 h, when it approaches a constant value suggesting a final state of crosslinking. 

Here three constants appear Go is the equilibrium modulus of elasticity 0p is the characteristic relaxation time, and AG is the relaxation part of elastic modulus. There are six measured quantities (components of the dynamic modulus for three frequencies) for any curing time. It is essential that the relaxation characteristics are related to actual physical mechanisms the Go value reflects the existence of a three-dimensional network of permanent (chemical) bonds 0p and AG are related to the relaxation process due to the segmental flexibility of the polymer chains. According to the model, in-termolecular interactions are modelled by assuming the existence of a network of temporary bonds, which are sometimes interpreted as physical (or geometrical) long-chain entanglements. 

The largest volume of fluoroelastomers (about 60% of total) is processed by compression molding. A blank (preform) is placed into a preheated mold, compressed, and cured at the appropriate temperature (see above) for a time established empirically. A good estimate for the curing time in the mold is the value of Co from the measurements by oscillating disk rheometer. In the mold design, it is necessary... 

At low cure times, only Ale and A2c polysulfidic structures (50 ppm) are observed. At longer cure times, Ale and A2c polysulfidic structures reduce in sulfur rank to monosulfide (45 ppm), and Blc (58 ppm). Bit (64 ppm) and Clc (45 ppm) polysulfidic structures are observed. A small amount of ds-to-trans isomerisation was observed, which increased with sulfur content. The reversion reactions of TBSI-accelerated systems result in a lower degree of sulfurisation as opposed to TBBS-accelerated samples. Based on the equilibrium swelling measurements, TBSI is found to be a less efficient accelerator than TBBS. 

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