RPA flex

Rigid Testing Platform

The RPA flex is built with an ultra-stiff testing frame and crosshead which eliminate the effects of instrument compliance on test data. Instrument compliance, or instrument deformation, produces erroneously low values of measured properties such as modulus and torque, irregular strain and torque waveform signals, and other errors.

Large diameter steel rods and a thick crosshead brace in the H-shaped load frame of the RPA flex provide unmatched rigidity to resist instrument deflection while the motor deforms the sample. This special design ensures that the commanded strain is achieved with each cycle of deformation, even for highly filled, fully cured rubbers. Additionally, a non-compliant system allows for truly sinusoidal strain profiles under all conditions. This can be verified by continuous Fourier Transform analysis of the deformation and measured torque signals which is available in the Scarabaeus Software. The superior design also guarantees smooth travel, proper alignment, and precise application of vertical load.

Direct Drive Motor

A powerful direct drive motor applies precise deformation in the RPA flex. A high quality rheological or dynamic measurement relies on the precise application of a constant rate, step, or periodic deformation. In a direct drive system the start-up delays, compliance, and translational losses seen in clutch or belt driven configurations are eliminated. The superior motor design of the RPA flex ensures that the most accurate and repeatable deformations are always applied to the sample.

The RPA flex applies the highest combination of continuously variable frequency and amplitude in any rubber rheometer. This provides important material information such as:

• The linear viscoelastic response of highly filled rubbers at low strains
• Behavior at extreme processing and use conditions characterized by high strains
• Terminal material behavior exhibited at low frequencies
• Response to high speed deformations measured at high frequencies

Advanced Data Processing

The complex deformations and stress-strain response common to rubber testing demand the most advanced data processing techniques. The RPA flex from TA Instruments utilizes a state-of-the-art 20 bit encoder and advanced data sampling technique to perform calculations based on a Fast Fourier Transform (FFT) analysis using 90 data points for each cycle of oscillation.

The RPA flex measures and reports non-linearities in torque and displacement. Higher harmonics indicating non-linearity in the applied displacement or measured torque, are reported for each data point, alert the operator with a simple indicator if test conditions are not ideal and storing this information for subsequent data validation.

High Stiffness Torque Transducer

The RPA flex benefits from a proprietary wide range ultra-stiff torque transducer. This rugged, non-compliant device measures the widest range of torques accurately and precisely. This greatly improves the accuracy and precision of measured torque, modulus, and viscosity values.

Dies & Rotors

The MDR one, RPA flex, and RPA elite rotorless shear rheometers employ the industry-standard sealed cavity biconical die design. The dies are constructed from durable, high-stiffness, low thermal expansion stainless steel to minimize system compliance and prevent gap changes with temperature. The test fixtures are connected directly in line with the motor below for precision deformation control and the torque transducer above for accurate measurement.

Direct contact electric heaters mounted within the dies provide exceptional temperature control and stability under isothermal, step and temperature ramp conditions. This highly responsive system returns rapidly to the programmed test temperature upon the addition of a cold sample, providing the most representative values for scorch time and other cure characteristics. Extremely durable user-replaceable seals provide absolute sample containment at all temperatures and conditions.

Die Surfaces

The RPA flex die surfaces feature an optimized arrangement of radial serrations to guarantee constant sample contact at even the highest strain values. Polyester or polyamide films may be used to facilitate sample release and avoid the need to clean dies between experiments.

High Pressure Pneumatic System

The RPA flex employs a high pressure pneumatic system to seal the sample properly and reproducibly. The high capacity pneumatic system applies up to 8 bar nominal pressure to the during gap closure. Proper alignment and the use of mechanical bearings ensure efficient transfer of load from the system to the sample without load frame losses. Actual sealing pressure is measured directly and recorded. This high pressure automated sample containment removes operator dependence and tightly contains the test specimen. This sealing process is particularly important for materials that undergo positive or negative volumetric changes with curing and highly stiff materials such as carbon-filled fluoroelastomers.

Torque Calibration

Torque calibration is made simple with a certified torque calibration device. This feature allows the user to calibrate the instrument directly, increasing data confidence and operation time, and reducing the reliance of service engineers for calibration.

Rheometer Automation System

All TA Instruments rotorless curemeters and rheometers are compatible with the highly reliable rubber automation system. This carousel-based autosampler allows for the unattended testing of rubber samples. Coupled with the Scarabaeus Software automated analysis, statistics, and control chart generation, the automated rheometer becomes a highly integrated part of the manufacturing control process.This improved data throughput is also invaluable for screening multiple formulations in a research or product development environment.

Suction Transfer System

Samples are moved from the autosampler tray to the test position using a suction transfer system. This system is highly tolerant of non-ideal sample geometries, so uncured rubber from many sources can be used. Samples are loaded on a carousel and more specimens may be added while another test is in progress, allowing for uninterrupted productivity.

Film Transfer System

An automated film transfer system removes the previously tested specimen and advances the film roll to prepare for the next experiment. An integrated sensor alerts the operator at the end of the roll so it can be replaced in a timely manner.

Isothermal Cure

Isothermal cure experiments are critical for rubber and elastomer processing. The TA Instruments rubber rheometers provide high precision data that is simple to analyze. All the important characteristics, such as minimum and maximum viscosity, scorch time, and conversion time can be calculated easily and automatically. The data can also be handled in its complete graphical form for comparison or alternative analyses.

Suitable Instruments: MDR one, RPA flex, RPA elite

Non-isothermal Cure

In addition to the industry-standard isothermal cure methods, the RPA and MDR can perform non-isothermal cure experiments. These experiments can be programmed to follow virtually any temperature profile and are especially valuable when simulating manufacturing processes that are not isothermal. Non-isothermal curing experiments may also be coupled with isothermal tests such as strain and frequency sweeps before or after cure to provide a more complete material data set before, through, and after cure.

Suitable Instruments: MDR one, RPA flex, RPA elite

Isothermal Curing at Variable Strain

While standard test methods often call for a single strain and frequency value to be used for all materials (0.5°, 1.67 Hz), these are not always the ideal conditions for every material. In the present example, the sample material is tested by isothermal cure at three deformation amplitudes, five times each. At the standard of 0.5° and 0.4° the experimental variability is extremely broad. This is because these experiments are performed at strains beyond the linear viscoelastic limit for this material. Testing at a smaller amplitude (0.3°) produces valid data with greatly improved reproducibility.

Suitable Instruments: RPA flex, MDR one, RPA elite

Isothermal Frequency Sweep

Measuring the frequency-dependent viscoelastic properties of a material is a powerful way to understand its molecular structure. A frequency sweep as shown can reveal information about the average molecular weight (crossover frequency) and molecular weight distribution (crossover modulus).

Suitable Instruments: RPA flex, RPA elite

Strain Sweep for Filler Loading

The strain-dependent modulus is particularly important as an indicator of the amount and type of rubber filler dispersion and interaction. in the present example, the impact of carbon black addition at five different levels is seen in the low strain region. High strain behavior is generally insensitive to filler addition, as it is less sensitive to filler-filler interaction and more dependent on polymer molecular weight, and polymer-filler interactions.

Suitable Instruments: RPA flex, RPA elite

High Strain Non-Linear Behavior

The viscoelastic response of a material at very high strains differs not only in magnitude from its linear counterpart, but also in type. The careful examination of a polymer’s stress- strain response at high strains reveals features associated with filler content and structure, as well as polymer architecture. in the present example, qualitatively different features are observed at large strains for a linear polymer, branched polymer, and a blend of the two. Both the uncorrelated data, and the FT analysis of periodic data are available through the scarabaeus software to allow in-depth analysis of this new type of data.

Suitable Instruments: RPA flex, RPA elite

Time Dependent Structure Breakdown

The van der Waals interactions that lead to increased modulus in carbon black filled rubber are highly sensitive to processing. In this example, identical samples are subjected to different lengths of milling after being removed from the mixer. Carbon network structure is reduced with each increase in milling time up to 8 minutes, after which the modulus was unchanged with increased milling. This provides critical information about the amount of milling needed to create a consistent workable material.

Suitable Instruments: RPA flex, RPA elite

Cure with Blowing Reaction

Final product density and mechanical performance are often enhanced through the use of blowing agents to create a cellular architecture. These blowing agents generate gas during decomposition in parallel with the curing reaction. Monitoring sample pressure through the curing reaction is an effective way to quantify the blowing reaction, allowing for the characterization of curing and blowing in a single experiment. These two processes must be balanced in order to form the desired cell architecture in the finished product.

Suitable Instruments: MDR one, RPA flex, RPA elite