Isothermal cure experiments are the most common type of test for quality control in rubber and elastomer processing. MonTech Moving Die Rheometers provide high precision data as well as a simple operation of the instruments. All the important characteristics, such as minimum / maximum elastic torque, scorch times, cure times and reaction rates are precisely calculated, with over 3500 different datapoints. All data is available in numerical as well as graphical form; limits, control gates and tolerance graphs can easily be set, and Pass / Fail status is automatically evaluated after each test.

Especially for sealing applications, blowing agents form a vital part of compound recipes in order to produce a cellular structure via a foaming process that runs in parallel to the cure reaction. The cellular matrix structure which is created during the foaming process reduces density, increases thermal and acoustic insulation, and affects the relative stiffness of the mix. Therefore, MonTech Rheometers can be fitted with a precision normal force measurement transducer in the die cavity in order to calculate cavity pressure simultaneously during the curing and reaction in a single test, and revealing interrelations between the two reactions.

In addition to isothermal static cure testing, MonTech MDRs and RPAs can perform tests at variable temperatures. These non-isothermal sequences can be programmed in order to follow virtually any temperature profile, making them especially valuable for the simulation of manufacturing processes which are usually not isothermal. Typical processes that can be simulated are mixing, milling, extrusion, compression moulding, injection moulding, and storage conditions. Of course, non-isothermal test sequences can be executed in a single test with any other static or dynamic sequence, such as strain and frequency sweeps, providing the most accurate data of the material‘s behavior at any production stage and material state.

Test data from similar static or dynamic test sequences executed at different temperatures can automatically be evaluated and modelled for an advanced cure kinetics analysis, providing information about Reaction Rate, Order of Reaction (n), Rate Constant (k), Activation Energy (E) and Incubation Time (ti).

In general, the mechanical properties of materials depend on frequency. A good under-standing of the influence of frequency on a material is therefore very important for its practical use. For example, a material appears stiff under the action of a force at high frequency, but soft when the force is applied slowly. Isothermal frequency sweeps provide information about the weight distribution MWD (crossover modulus) as well as average molecular weight AWM (crossover frequency). But the behavior of viscoelastic materials like polymers not only depends on frequency, it also depends on temperature. MonTech has incorporated further advanced testing capabilities such as the Time-Temperature Superposition principle (TTS), which is based on the equivalence between frequency and temperature behavior during transition processes, forming the basis of WLF master-curve modelling available on MonTech dynamic Rheometers, even for predicting material performance at frequencies outside the range that can be measured with a dynamic mechanical analyzer.

The rheological properties of rubbers are related to their structural characteristics and will influence the behavior of the rubber during processing and the performance of the final product. While Mooney testing does not provide sufficient information to clearly differentiate branching and Molecular weight distribution the Rubber Process Analyzer can easily be used as a tool for solving production problems. Using frequency sweeps to scan the material over the whole shear rate range can reveal substantial material
differences and variations e.g. causing a particular material to be very sticky and therefore difficult to process while others can be perfectly processed.

These test can be performed in the linear and also non-linear viscoelastic range to cover all different processing methods and material states. ISO 13145 suggests a simple and quick test procedure utilising a rotorless sealed shear rheometer (RPA) for rheological evaluations as an alternative to traditional Mooney Viscometer testing.

Dynamic oscillatory shear tests are common in rubber rheology - more specifically, small-amplitude oscillatory shear (SAOS) tests are the most common test method for measuring linear viscoelastic properties of rubber compounds and polymers.

But in processing operations, the shear rates can be large and rapid; non-linear material properties form an even more important part in understanding material response. Therefore, MonTech Rheometers provide Fourier transformation analysis capabilities of periodic data, along with full raw-data access, for in-depth analysis to investigate and quantify the nonlinear viscoelastic behavior by using large-amplitude oscillatory shear (LAOS) testing in order to characterize and quantify material stress response which is no longer purely sinusoidal (linear), allowing a better understanding of filler content and structure, as well as the polymer architecture.

Typically, cure experiments on rubber compounds - especially for quality control purposes - are performed with a fixed oscillation angle of +/- 0.5° and a frequency of 1.67 Hz. However, for specific rubber compounds or challenging materials such as silicones or epoxy resins, this might not be ideal as either reaction torque readings are too low, providing only a limited ability to distinguish between different batches, or might be too high causing high result variability as the material is damaged as strain already exceeds the linear viscoelastic range. MonTech Rheometers provide the possibility of testing with variable oscillation angles to allow measurements within the ideal strain amplitude for optimal signal-to-noise ratio and the most precise test results, while avoiding any structural breakdown or slippage of the sample in the die cavity.

Rubber compounds are extremely sensitive to processing operations such as milling. Increasing strain causes the carbon black network - which is held together by Van der Waals-London attraction forces to break, causing a decrease in shear modulus of filled rubber vulcanizates. Therefore, MonTech Rheometers provide simulation capabilities for almost any possible production process, providing irreplaceable data for developing rubber compounds, as well as understanding and simulating manufacturing processes and environments.

The Payne effect is a particular feature of the stress-strain behavior of rubber, especially rubber compounds containing fillers such as carbon black and silica. Physically, the Payne effect can be attributed to deformation-induced changes in the material‘s microstructure, i.e. to breakage and recovery of weak physical bonds linking adjacent filler clusters.

Measurement of modulus vs. strain is therefore essential to understanding and quantifying filler loading, filler dispersion and filler-filler interaction in the low strain region, and polymer-filler interaction at higher strain. The resulting characterizations of material structure are essential as they directly impact dynamic stiffness and damping behavior of final products such as rubber bushings, automotive tyres and all other rubber goods. Similar to the Payne
effect under small deformations is the Mullins effect, which is observed under larger deformations in the non-linear viscoelastic range.

Good processing performance is influenced by three main criteria: throughput flow, die swell and surface finish. The rubber is required to flow through the extruder. The flow will be controlled by the viscosity of the rubber. The shear rate from an extruder and extrusion die can easily be calculated and used as the specific test parameters in a Rubber Process Analyzer test setup. The shear rate in MonTech Rubber Process Analyzer is proportional to the frequency multiplied by the oscillation angle.

A low viscosity will mean than rubber will easily flow through the extruder with low die pressure. Once the rubber is extruded it is required to be in the correct size – however as the rubber is extruded through the die it is in compression across the direction of flow and extension in the direction of flow. When leaving the die, the elastic nature of the compound will cause the rubber to expand, resulting in die swell. MonTech Rubber Process Analyzers can obtain the Storage shear Modulus G' at high strains (typically 100%) allowing an excellent prediction of die swell.

The surface finish of the extrudate is required to be smooth, and not rough. Roughness tends to occur when a stick-slip resonance is set up between the speed of the extruder and the elastic response of the compound. Testing at variable shear rates using a frequency sweep allows the comparison of compounds that extrude with smooth and rough finishes revealing processing differences in the storage shear modulus G'.

In filled rubber compounds carbon-black particles form a network of mutually interactive agglomerates. These effects can me measured and quantified using a simple D-RPA 3000 Matrix test. Storage shear modulus (G’) results at low strains (e.g. +/- 1%) are typically high and get reduced after a larger strain amplitude (e.g. +/-50%) is applied for a short period of time. With lower strain amplitudes applied over time, the reduced Storage shear modulus (G’) will partially recover. This effect relates to the Van der Waals forces linking the agglomerates, getting broken at higher strain amplitudes and the partially recovering over time.

The extent of recovery of the Storage shear modulus (G’) directly relates to the Dispersion Rating (DR) of the rubber compound. If the carbon black is poorly dispersed, the recovery of the Storage shear modulus (G’) will be much lower indicating a much weaker filler structure and therefore reduced mechanical performance properties.

A simple CBDI performed by a Rubber Process Analyzer allows consistent testing and quality control on the Carbon Black Dispersion rating: CBDI = Initial storage shear modulus G’ (50°C, 1Hz, 1%) / Final storage shear modulus G’ (50°C, 1Hz, 1%). The higher the CBDI value, the better the carbon black dispersion.