Rubber rheometer measurements

Rheometer measurements measure cure and cure rate characteristics of the rubber. The component manufacturer performs this test on unvulcanized rubber. The rheometer measures the viscosity of the rubber as a function of time at a constant temperature. As time increases, the degree of cure or cross-linking increases and thus the viscosity increases. 

Dick. John S., The optimal measurement and use of dynamic properties from the moving die rheometer for rubber compound analysis. Rubber World (January 1994). 

Continuous low level current measurements are used to investigate the vulcanisation of natural rubber with sulphur, 2-mercaptobenzothiazole and zinc oxide in the presence and absence of stearic acid. At elevated temperatures, the mixtures showed increased conductivity, which was attributed to the presence of ions. Current curve maxima, characteristic of transitory ionic species are detected. Possible ions are discussed in coimection with references in the literature. For the mixture NR/S/MBT/ ZnO/StAc, the comparison with the rheometer measurements demonstrates a relationship between the formation of ionic intermediates and crosslinking of the polymer. The suitability of the electrochemical method for the investigation of ionic species in crosslinking reactions is demonstrated. 20 refs. 

In order to test this concept a series of compounds was prepared in a 5 L Shaw Intermix (rubber internal mixer, Mark IV, Kl) with EPDM (Keltan 720 ex-DSM elastomers an amorphous EPDM containing 4.5 wt% of dicyclopentadiene and having a Mooney viscosity ML(1 +4) 125°C of 64 MU 100 phr), N550 carbon black (50 phr), diisododecyl phthalate (10 phr), stearic acid (2 phr), and l,3-bis(tert-butylperoxy-isopropyl)benzene (Perkadox 14/40 MB ex Akzo Nobel 40% active material 6 or 10 phr). A polar co-agent (15 phr) was admixed to the masterbatch on an open mill and compounds were cured for 20 min at 180°C in a rheometer (MDR2000, Alpha Technologies). The maximum torque difference obtained in the rheometer experiments was used as a measure of... 

The specimen was prepared by the following method. After mixing HAF carbon black (50 phr) with natural rubber (NR) in a laboratory mixer, carbon gel was extracted from unvulcanized mixture as an insoluble material for toluene for 48 h at room temperamre and dried in a vacuum oven for 24 h at 70°C. We made the specimen as a thin sheet of the carbon gel (including carbon black) by pressing the extracted carbon gel at 90°C. The cured specimen was given by adding sulfur (1.5 phr) to the unvulcanized mixture and vulcanized for 30 min at 145°C. The dynamic viscoelastic measurement was performed with Rheometer under the condition of 0.1% strain and 15 Hz over temperatures. 

The degree of vulcanisation of a rubber compound is assessed technically by the indefinite terms of undercure, correct cure, optimum cure and overcure. It may be given precision by (a) measurement of stress-strain relationship of a range of cures, (b) measurement of the modulus at 100% elongation, (c) measurement of the volume swelling in benzene, or (d) by the use of instruments such as the oscillating disc rheometer and the moving die rheometer. 

In the extrusion or capillary rheometer, rubber is forced through a small cylindrical die under a known pressure and the volume extruded in a given time measured (or at a constant rate and the pressure measured). It is, therefore, rather similar in action to extruders used in the factory. 

If a fully compounded thermosetting rubber is subjected to a plasticity measurement at a high enough temperature and for long enough, it will cure and, consequently, there is not always a clear distinction between a plasticity test and a test for scorch or rate of cure. For example, the Mooney viscometer is used to measure scorch, i.e. the onset of vulcanisation, and an oscillating disc rheometer will measure the plasticity of the compound before the onset of cure as well as the increase in stiffness as curing takes place. 

The effect of the experimental agents on natural rubber hysteresis was determined by measuring tan 5 at 5% strain using an ARES-A Rheometer at 50°C and 15 Hz. Testing results are provided in Table 2. 

Vulcanization rate was determined by a Monsanto Rheometer. The Monsanto Oscillating Dii Rheometer is designed to measure the ccanplete curing characteristics of a single rubber specimen, heated and maintained utder continuous pressure during vulcanization. Sinusoidal oscillation of a conical dirir, embedded in the vul-canizable mix confined in a heated square carity, exerte a riiear strain on the speci-... 

An interesting, indirect application of the statistical theory to filled rubbers has been published by Wolff (/33). In recent years increasing use has been made of oscillating disc rheometers to follow the course of vulcanization. These instruments operate at low frequencies and at vulcanization temperatures, so that torque recorded at full cure is a good measure of an equilibrium shear modulus and hence proportional to v. Wolff finds for polymers filled with furnace blacks and cross-linked with dieumyl peroxide ... 

The Plasticorder, described as a "torque rheometer" by the supplier, is one version of a practical tool well known and used in the plastics and rubber industries. We believe that our use of the Plasticorder as a polymerization reactor may be unique, and that the instrument is probably not well known in polymer science laboratories, so we felt it appropriate to picture it. Additional detail of the mixing chamber/measuring head and the removable sigma blade rotors is also pictured (see Photographs A, B, and C). 

Measurements were taken on two natural rubber compounds rubber A with a very short scorch time, and rubber B with a very long one. For both rubbers, experiments were conducted at 120 C, 140°C, 160°C, 180°C, and 200 C. A Monsanto rheometer and a Wallace-Shawbury curometer were used, as well as isothermal apparatuses (see Table 1.1). 

Laboratory measur ent of curing characteristics have been somewhat revolutionized by the introduction of the cure meters. There were two types of cure meter the reciprocating paddle type as in the Wallace-Shawbury curometer, and the oscillating disc type such as the Monsanto rheometer. In the oscillating disc type (ODR) a biconical disc is embedded in the rubber in a closed cavity. The disc is oscillating through constant angular displacement, and the torque required monitored. 

Rubber viscosity can be measured by a rotational viscometer, a capillary rheometer, or an oscillating rheometer. These are discussed later in the chapter. 

Virtually all rubber compounds are non-Newtonian. This means that their measured viscosities will decrease with increasing shear rates. The slope of this viscosity drop is compound dependent. It is quite possible that a compound 2 could have a lower viscosity than a compound 1" at low shear rates, but compound 2 could cross over at higher shear rates where compound 1 has the lower viscosity [96], This is illustrated in Fig. 25 [97]. The Mooney viscometer, when run at the standard rotational speed of 2 r min, has a maximum shear rate of about 1 s (the shear rate changes across the rotor radius) [98]. The capillary rheometer is used because it can measure viscosity at much higher shear rates (at shear rates as high as 1000s ). 

Some capillary rheometers (such as the Monsanto Processability Tester") are equipped to directly measure the die swell of the rubber compound after it exits the die. This is done with special optical die swell detectors. Running die swell is measured at... 

Decker. G. E., Wise, R. W., and Guerry. D An oscillating disk rheometer for measuring dynamic properties during vulcanization. Rubber Client. Tech... 36. 451 (1963) and Rubber World, 147, No. 3,68 (1963). Also presented at the Cleveland Rubber Division. ACS. October 17 19. 1962. 

After 9 minutes of mixing in the torque rheometer at 230°C the polybutadiene phase as measured by Cp at the glass transition is reduced by 40%, the SAN phase by 20% and the PVC phase by approximately 9%. Clearly the PBD phase is degraded more rapidly under these conditions than either the SAN or PVC phase. The SAN and PVC Cp at Tg for the blend is more difficult to measure since the transitions partially overlap. Nevertheless an unmistakable trend can be seen form the data which shows that mixing of PVC with ABS at 230 C leads to accelerated deterioration of both ABS phases but most significantly to the rubber component of the blend. 

Cure of rubber vulcanizates is commonly measured using a Monsanto cure rheometer, which plots the change in torque as a function of time as is shown in Figure 1. From the percentage rise of the graph, the percent cure can be calculated. 

The concepts of the cone-plate and biconical rheometers developed in the 1940s (Fig. 13). The cone-plate instrument is due to Freeman and Weissenherg [FIO] and intended for modest-viscosity fluids. It has the basis of his rheogo-niometer which also measured normal stresses. The biconical rheometer was developed in the same period by Piper and Scott [P12] of the BRMRA and was from the beginning intended for rubber. Similar instruments are discussed by Turner and Moore [T12] and Montes et al. [M37, M38]. In the latter instruments, the pressure is controlled by charging the rubber into the rheometer by an attached pressure-driven device. 

The capillary rheometer, one the oldest and most widely applied experimental tools for measuring the viscosity of fluids (Fig. 15), was used extensively on almost all classes of complex fluids by the founders of modern rheology during the 1920s and 1930s. Its application to rubber dates to the work of Marzetti [M8-11] and Dillon and coworkers [D7-9], and it has been widely used through the years. 

Popovic, RS Plavsic, M Popovic, RG Die N. Correlation between Viscosity Measured by Capillary Rheometer and Mooney Viscometer on Testing Rubber and Rubber Mixes. Kaut. Gummi Kunstst., 1991, vol.44, o7, 702 - 709. 

In bromobutyl/chlorobutyl rubber blends, both elastomers have the polyisobutylene backbone and halogen reactive functionality. These polymers, being molecularly miscible, constitute an ideal system for co-vulcanization. Bromobutyl and chloro-butyl can be used interchangeably without significant effect on state of cure as measured by extension modulus, tensile strength, and cure rheometer torque development. Bromobutyl will increase the cure rate of a blend with chlorobutyl. However, where bromobutyl is the major part of the blends, chlorobutyl does not reduce scorch tendencies because the more reactive halogen unit can dominate. 

Rheometer re- a-m9-t9r [ISV] (ca. 1859) (plastometer) n. An instrument for measuring the flow behavior of high-viscosity materials such as molten thermoplastics, rubbers, pastes, and cements. The most widely used principle is that of the capillary rheometer of which a variety of makes and models are in daily use. Instruments for measuring the flow properties of less viscous fluids, e.g., dilute polymer solutions, are called viscometers but the terms rheometer and viscometer are often used interchangeably. Currently, computerized, on-hne capillary rheometers linked to a single control station can simultaneously monitor melt viscosity in ten or more extruders in a resin-finishing plant. 

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