This study explores the physical ageing of rubber, traditionally associated with temperature changes, with a focus on room temperature effects, particularly in carbon black-filled rubber. Contrary to previous beliefs, experiments reveal that it takes up to four days for the shear modulus to equilibrate at room temperature. The findings have significant implications for the mechanical behavior of systems containing such rubber components during temperature variations. The paper concentrates on modeling torsional energy flow in filled rubber isolator subjected to a temperature shift at room temperature. The constitutive model incorporates a novel approach considering contributions from free volume and configurational changes to represent the impact of physical ageing on the shear modulus. The resulting torsional energy flow exhibits shifts in levels, peak frequencies and trough frequencies across the audible frequency range, persisting even after temperature stabilization. This highlights the crucial role of physical ageing in the design of vibration isolation systems.

Filler reinforced rubber is widely used for engineering applications; therefore, a sound characterization of the effects of physical aging is crucial for accurately predicting its viscoelastic properties within its operational temperature range. Here, the torsion pendulum is used to monitor the evolution of the storage and loss modulus of carbon black filled samples for four days after a temperature drop to 30 degrees C. The storage modulus presents a continuous increase, while the loss modulus generally displays a steady decrease throughout the four days that each test was conducted. The relationship of the recovery rates with the carbon black properties is also studied, analysing its dependency on the particle size and aggregate structure. The evolution of the recovery rate seems to depend linearly on the surface area while the carbon black structure appears to have a much weaker influence on the physical aging behavior for the set of compounds tested. The obtained results corroborate the presence of physical aging at room temperature for filler rubber materials and the ability of the torsion pendulum to monitor the storage and loss modulus change, providing pivotal data on the influence of physical aging on the viscoelastic properties of the material.

The viscoelastic properties of filler reinforced rubber are measured at small strains with the torsion pendulum after a temperature change and the evolution of the storage and loss modulus is measured against the physical ageing time. Physical ageing refers to the state in which the molecules are out of thermodynamic equilibrium after a temperature change. The time needed to achieve equilibrium is the physical ageing time, during which the material displays shifting mechanical properties. For unfilled rubber, physical ageing occurs only after temperature changes below the glass transition temperature, T-g. However, filler reinforced rubber presents physical ageing after a temperature change at temperatures significantly above T-g. Therefore, the existing models of physical ageing are unsuitable for filler reinforced rubber and further characterization of the evolution of their viscoelastic properties after a temperature change is needed. The torsion pendulum is utilised to measure the material properties since it allows measurements in the rheological simple linear viscoelastic small strains and during long periods of time. The impulse response function obtained from the torsion pendulum measurements is used to calculate the storage and loss modulus at different times after a rapid temperature decrease at room temperatures. The resulting evolution shows a progressive stiffening of the material with an increase of storage modulus and decrease of loss modulus with physical ageing time. This investigation on the evolution of the viscoelastic properties after a temperature change sets the path for a proper characterization of the physical ageing phenomena for filler reinforced rubber and this allows a more reliable constitutive model to be derived.

A comprehensive set of guidelines on the use of the torsion pendulum for the viscoelastic characterisation of carbon black reinforced rubber is presented, including the set up selection, the post processing steps and the final evaluation of the method using temperature sweep results at small strains. The torsion pendulum is based on the free vibration principle, in which the sample is instantaneously perturbed to initiate a torsional movement and then left to freely vibrate. From the frequency and damping of this impulse response function, the properties of the material can be obtained against temperature and time. However, despite the extensive use for many years of the torsion pendulum, the difficulty of activating only the desired torsional mode hinders the acquisition of accurate results. The method presented aims to improve the torsion pendulum performance and can be divided in two stages. The pre-testing phase covers the influence of the test set up and sample geometry on the measured response. The test set up will determine the frequency at which the dynamic properties are obtained, while the sample geometry will influence the torsional strain and bending deflection. The post-testing phase includes the steps to post-process the curve and the criteria to identify and select the most reliable data sets from all of the measured data. The quality of the curve is assessed in the frequency domain by evaluating the frequency components of its Fast Fourier Transform and in the time domain by evaluating its fit to an ideal exponentially decreasing sinusoidal curve. The final selection of the response functions that can be used to measure the viscoelastic properties of the rubber samples is made based on the maximum strain level and the minimum variation of the dynamic properties within one curve. The validity of the method is then tested by comparing temperature sweep results before and after the application of the presented guidelines, where a clear decrease on the results deviation can be observed. Finally, the results obtained with this method are compared to Dynamic Mechanical Analysis results over the same temperature range, where a good fit is obtained. Thus, this investigation presents a comprehensive method, useful to all torsional pendulum users aiming to measure and characterise the mechanical behaviour of viscoelastic materials.

Change in the free volume of an elastomer is modelled to predict the physical ageing of filler reinforced rubber by a nonlinear, fractional differential equation as a function of physical ageing temperature and physical ageing time. Elastomers undergo physical ageing after a temperature alteration when the molecules, out of thermodynamic equilibrium, tend to recover their equilibrium state resulting in the evolution of free volume towards its equilibrium value. The effects of physical ageing were thought to be restricted to temperature changes at low temperatures. Conversely, it has been discovered that for the case of filled elastomers, physical ageing is also associated to temperature alterations at normal temperatures. In this paper, a novel free volume differential equation for physical ageing of filled rubbers is developed. The developed free volume equation can then be used in the constitutive equations to predict the material properties for filler reinforced rubber more accurately.