October 18, 2023 By Rheometer Spares Off

Unravelling the Secrets of the Rubber Process Analyser

Are you tired of dealing with inconsistencies in rubber manufacturing? Wish you could uncover the secrets to enhancing quality and efficiency? Look no further than the Rubber Process Analyser (RPA). This innovative technology is revolutionizing the rubber industry by providing a deeper understanding of the manufacturing process.

Read on to discover the inner working of this advanced rubber testing technology and how an RPA could improve your rubber manufacturing processes.


With the Rubber Process Analyser (RPA), every stage of rubber production becomes more accessible. From the mixing and compounding of raw materials to the evaluation of vulcanisation and curing characteristics, the RPA offers unprecedented insights into the behaviour of rubber compounds. By analysing the rheological properties of rubber, the RPA enables manufacturers to optimize their processes, reduce waste, and improve product quality.

But what makes the RPA truly extraordinary is its ability to uncover the secrets that lie within the rubber itself. By examining the molecular structure and viscoelastic behaviour of rubber, the RPA can identify the factors that contribute to its strength, durability, and performance. Armed with this knowledge, manufacturers can make informed decisions about formulation and process adjustments, leading to more consistent and superior rubber products.

Say goodbye to guesswork and hello to precision with the Rubber Process Analyser. Unravel the secrets of rubber manufacturing and take your quality and efficiency to new heights.

So, what exactly is a Rubber Process Analyser?

A Rubber Process Analyser (RPA) is a scientific instrument used to measure the rheological properties of rubber, particularly its viscoelastic behaviour. It applies controlled shear stress and measures the resulting deformation or flow of the rubber. A Rubber Process Analyser is a type of dynamic shear rheometer specific to the rubber industry. Alongside rubber and polymer manufacturing, other dynamic shear rheometers are also used in industries such as food processing and cosmetics. 

Rubber Process Analysers are used where understanding the flow and deformation characteristics of rubber is crucial for product development and quality control. By analysing the data obtained from an RPA, scientists and engineers can gain valuable insights into the behaviour and performance of rubber under different conditions, helping them make informed decisions in their respective fields.

As a rheometer, an RPA can also be used to study the vulcanisation process of rubber in the same manner as a Moving Die Rheometer. Regarded as a more advanced instrument than a Moving Die Rheometer, due to its increased functionality and experimental scope, a Rubber Process Analyser is most likely to be found in R&D and product development environments, particularly in applications that are technically demanding or highly regulated. 

How does a Rubber Process Analyser work?

A Rubber Process Analyser (RPA) works by applying controlled shear stress to a rubber sample and measuring the resulting deformation or flow. The instrument consists of two dies, with the sample placed between them. When closed, the dies create a bi-conical shape around the sample. One die remains stationary while the other die oscillates back and forth, creating a shearing motion. The force required to maintain this motion is measured, and the resulting data is used to calculate the viscoelastic properties of the rubber, such as its viscosity, elasticity, and complex modulus. 

This information is crucial for understanding how the rubber will behave under different conditions, such as temperature, pressure, or shear rate. By analysing the data obtained from an RPA, scientists and engineers can make informed decisions about rubber selection, formulation, and processing, leading to improved product performance and quality.

What are the key components of a Rubber Process Analyser?

The key components of a Rubber Process Analyser (RPA) include two dies, a motor or actuator to create the oscillating motion, a force sensor to measure the applied force, and a control system to regulate the shear stress and deformation. The dies are precisely engineered to make a bi-conical shape with grooves that reduce the slippage between the sample and the dies. It is also recommended to use a thin polyester or nylon film, either Melinex or Dartek, on both sides of the rubber sample to prevent residue and improve contact without interfering with the test measurement. 

The motor or actuator generates the oscillating motion, which can be controlled in terms of frequency, amplitude, and time. The transducer measures the force required to maintain the shearing motion, allowing for the calculation of the viscoelastic properties. The control system ensures that the shear stress and deformation are maintained within the desired range, providing accurate and reliable data. Additionally, an RPA may also include temperature control systems to study the effect of temperature on the rubber behaviour. While the dies are heated directly, cooling is either ambient or controlled using compressed air or even liquid nitrogen. 

For a rubber compound, the torque required to oscillate the sample through the set strain can vary hugely depending on the hardness of the rubber material and the test conditions. Therefore, the machine frame itself must be solid and sturdy enough to withstand these repetitive torsional movements. For this reason, most RPAs are significantly larger than the samples they test, to accommodate both the large steel cylinder and a motor with enough power to oscillate even the hardest rubber compounds. 

In addition, as an RPA has a variable strain range, often up to a full 360° oscillation, any part of the machine that is involved in the oscillatory motion will also be subjected to large shear stresses. This is a particularly important consideration for cavity seals, which also expand and contract in line with temperature control. These seals are often made from high-grade polymers that are capable of withstanding the large range of test variables available on a Rubber Process Analyser. 

Lastly, the film used during testing on either side of the rubber sample must also be strong enough to withstand these same large shear stresses. While the most popular films, Melinex and Dartek, are supplied in standard thicknesses, it is possible to experiment with thicker films and double layers when testing at higher strains. Often there is a self-limiting maximum strain at the point at which the film tears. In cases where an automatic sample feeder system is used, the choice of film is particularly important as the mechanism is reliant on a single roll of film to move, load and test rubber samples without tearing. 

What test methods are included in a Rubber Process Analyser?

In general, Rubber Process Analysers (RPAs) are used by rubber technologists who require a greater level of detail on the dynamic properties of a rubber sample in order to optimise the formulation, processing or performance of a rubber product. An RPA not only has a greater scope of functionality but also produces far more data channels than both a Moving Die Rheometer and Mooney Viscometer. Therefore, the applications of an RPA are both wider and more thorough than alternative rubber testing instruments. 

Below are some of the most commonly used test methods, according to test variable:

  • Strain: an RPA can be used to test at both high and low strains, which can be used to replicate rubber processing conditions, such as extrusion or injection moulding. A strain sweep can be used to investigate the Payne effect and also to understand Large Amplitude Oscillatory Shear (LOAS) effects at high strain rates. 
  • Frequency: testing at frequencies that mimic real-world conditions can be used to characterise the frequency-dependent material response, most often in terms of the elastic and viscous components. Typically, most RPAs can operate between 0 and 50 Hz. However, testing at frequencies outside this range can be inferred using a concept known as time-temperature superposition (TTPS). 
  • Temperature: using an RPA, temperature control can either be constant (isothermal) or variable (non-isothermal). An isothermal test can be used to measure the cure properties of raw rubber. Non-isothermal testing can be used to explore more complicated curing characteristics or understand the processability of a raw polymer. 
  • Time: accounting for time is a key consideration when assessing the processability of a raw material. A Stress Relaxation test is a defined test method for measuring the decay in stress over time as a material is allowed to relax. 


An RPA performs these test methods using the following data channels:


Elastic Torque, S′

The peak amplitude torque component which is in phase with a sinusoidal applied strain.


Viscous Torque, S′′

The peak amplitude torque component which is 90° out of phase with a sinusoidal applied strain.


Complex Torque, S*

The peak amplitude torque response measured by a reaction torque transducer for a sinusoidal applied strain. Mathematically, S* = (S′² + S′′²)^1/2


Loss Angle, delta

The phase angle by which the complex torque (S*) leads a sinusoidal applied strain.


Storage Shear Modulus, G′

The ratio of (elastic) peak amplitude shear stress to peak amplitude shear strain for the torque component in phase with a sinusoidal applied strain. Mathematically, G′ = [(S′/Area)/Peak Strain]


Loss Shear Modulus, G′′

The ratio of (viscous) peak amplitude shear stress to peak amplitude shear strain for the torque component 90° out of phase with a sinusoidal applied strain. Mathematically, G′′ = [(S′′/Area)/Peak Strain]


Complex Shear Modulus, G*

The ratio of peak amplitude shear stress to peak amplitude shear strain. Mathematically, G* = [(S*/Area)/Peak Strain] = (G′² + G′′²)^1/2


Loss Factor, Tan delta

The ratio of loss modulus to storage modulus, or the ratio of viscous torque to elastic torque. Mathematically, tan delta = G′′/G′ = S′′/S′


Dynamic Complex Viscosity, n*

The ratio of the complex shear modulus, G*, to the oscillation frequency in rads/sec.


Real Dynamic Viscosity, n′

The ratio of the loss shear modulus, G′′, to the oscillation frequency in rads/sec.


As an RPA is designed to carry out a large variety of test methods, it is often more effective to combine tests into a single linked or combination test method. Typically, this would consist of a strain or frequency sweep on uncured or natural rubber at ambient temperature, followed by an isothermal cure test (standard MDR test), followed by a second strain or frequency sweep on the cured rubber. Together, these test methods can uncover a host of information on the dynamic properties of an elastomer. To a trained eye, this also includes information on the underlying molecular structure and the interaction with any fillers, such as carbon black. 

How can a Rubber Process Analyser help in material characterisation?

Rubber Process Analysers are commonly used to study the dynamic properties of polymers, such as the determination of the viscoelastic properties before, during and after the vulcanisation process. This information is crucial for understanding the flow behaviour and processing characteristics of elastomers in industries such as automotive, aerospace and defence. 

The insights provided by an RPA can be used for quality control, formula optimisation and research purposes. These rheological properties far exceed those calculated using a Moving Die Rheometer, due to the greater control of more test variables at any one point. This means that the viscoelastic properties of a rubber sample, such as viscosity and elasticity, can be determined as a function of strain, frequency, temperature and time. 

The properties calculated using an RPA are essential for understanding the behaviour and performance of polymer materials in various applications. This is because the test conditions can be set to replicate real-life conditions, such as high-strain extrusion in a manufacturing process or frequency-dependent vibration control in an insulator. Outside of the working range of the instrument, an RPA can also be used to infer material behaviour using material science techniques like time-temperature superposition. This helps polymer testing professionals deduce important material characteristics such as structural integrity and performance under extreme conditions. 


A Rubber Process Analyser (RPA) plays a crucial role in rubber characterisation by providing valuable insights into the viscoelastic properties of rubber. It can measure parameters such as viscosity, elasticity, and flow behaviour, which are essential for understanding the behaviour and performance of elastomer materials in various applications. For example, in the study of polymers, an RPA can determine curing characteristics for a variety of different input conditions, allowing researchers to optimise processing conditions and predict the material’s behaviour during manufacturing. 

Outside of the rubber industry, other dynamic shear rheometers are also used in the characterisation of food and cosmetic products, as well as in the development of pharmaceutical formulations. Working in the same way as an RPA, they can provide valuable insights into the rheological properties of these materials, such as viscosity, elasticity, and flow behaviour. In the field of food and cosmetics, a dynamic shear rheometer can assess the texture and consistency of products, ensuring quality control and product development.

Additionally, dynamic shear rheometers are used in the field of materials science to study the mechanical properties of various materials, including metals, ceramics, and composites. By analysing the viscoelastic behaviour of these materials, researchers can gain a better understanding of their structural integrity and performance under different conditions.

Overall, a Rubber Process Analyser (RPA) is a valuable tool for characterising materials and optimising their properties for specific applications. 

To find out more about the parts and spares available for rheometers, view the rheometer category in the Rheometer Spares shop. 


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