Lifetime Prediction Methods for Degradable Polymeric Materials—A Short Review

The determination of the secure working life of polymeric materials is essential for their successful application in the packaging, medicine, engineering and consumer goods industries. An understanding of the chemical and physical changes in the structure of different polymers when exposed to long-term external factors (e.g., heat, ozone, oxygen, UV radiation, light radiation, chemical substances, water vapour) has provided a model for examining their ultimate lifetime by not only stabilization of the polymer, but also accelerating the degradation reactions. This paper presents an overview of the latest accounts on the impact of the most common environmental factors on the degradation processes of polymeric materials, and some examples of shelf life of rubber products are given. Additionally, the methods of lifetime prediction of degradable polymers using accelerated ageing tests and methods for extrapolation of data from induced thermal degradation are described: the Arrhenius model, time–temperature superposition (TTSP), the Williams–Landel–Ferry (WLF) model and 5 isoconversional approaches: Friedman’s, Ozawa–Flynn–Wall (OFW), the OFW method corrected by N. Sbirrazzuoli et al., the Kissinger–Akahira–Sunose (KAS) algorithm, and the advanced isoconversional method by S. Vyazovkin. Examples of applications in recent years are given.

Frequency-dependent viscoelastic properties of Chinese fir (Cunninghamia lanceolata) under hygrothermal conditions. Part 1: moisture adsorption

To elucidate the frequency-dependent viscoelasticity of wood under a moisture non-equilibrium state, changes in stiffness and damping as a function of frequency were investigated during the moisture adsorption process. The moisture adsorption processes were carried out at six temperatures (30–80°C) and three relative humidity levels (30, 60 and 90% RH). During the moisture adsorption process, the wood stiffness decreased, and damping increased with the increment of moisture content (MC). Regardless of the moisture adsorption time, the wood stiffness increased, and damping decreased with the increasing testing frequency. Based on the re-organized Williams-Landel-Ferry (WLF) model, the time-moisture superposition (TMS) relation was assumed to be applicable for developing a master curve of wood stiffness during the moisture adsorption process. The frequency ranges of the stiffness master curves spanned from 16 to 23 orders of magnitude at temperatures ranging from 30 to 80°C. However, the TMS relation was not able to predict the wood damping properties during the moisture adsorption process due to the multi-relaxation system of the wood and the non-proportional relationship between free volume and MC at transient moisture conditions.

Creep and Recovery Behavior of Compression Molded Low Density Polyethylene/Cellulose Composites

Low density polyethylene (LDPE) is an important industrial material because it is durable, light-weight, easily processed and characteristically inert, but its everyday use is hazardous to the environment. The solution to this seems to consist of incorporation of biopolymers in the structure of LDPE to form composites. Compression molded composites at different cellulose loading were subjected to creep tests at 30, 40, 50, and 60°C. The samples were displaced for 12 minutes and allowed to recover for 12 minutes. Creep behavior of the polymer composites was governed by temperature, time, and cellulose loading. Creep performance decreased with increase in temperature and improved with cellulose loading while creep modulus decreased with increase in time and temperature. Time temperature superposition was used to predict the long time (up to 106 s) creep behavior of the samples. William-Landel-Ferry (WLF) model offered a better description of the shift factors based on the short term data that was used to predict the long time behavior of the polymer composites by shifting the curves along the logarithmic time axis. The deformation was dependent on free volume.

Simulation of the gas-assisted injection moulding process using a viscoelastic extension to the Cross-WLF viscosity model

An approximation to the viscoelastic Maxwell model is developed and combined with a Cross-WLF shear- and temperature-dependent model as a means of introducing aspects of viscoelasticity into the Cross-WLF model at a low computational cost. The main objective of the model is to simulate the gas-assisted injection moulding (GAIM) process with the aspect of a material's strain history included. It is shown that the model gives a transient and steady response comparable to the Doi–Edwards viscoelastic model in constant rate shear and uniaxial deformations, and follows WLF temperature dependence. The model is implemented in a three-dimensional finite element code using the ‘pseudo-concentration’ method to model the polymer and gas phases. The ordinary Cross-WLF model had demonstrated a consistent under-prediction of residual wall thickness (RWT) measurements in comparison to experimental results. It is shown that the viscoelastic extension to the Cross-WLF model gives a marked increase in RWT and exhibits aspects of stress relaxation and history dependence. The model is tested against variations of other process control parameters. It is shown that the simulation gives the correct qualitative response for all control parameters assessed, with quantitative prediction within a factor of 2.

Dynamic Mechanical Analysis of Multi-Walled Carbon Nanotube/HDPE Composites

Since the discovery of carbon nanotubes (CNTs), their remarkable properties make them ideal candidates to reinforce in advanced composites. In this attempt, an enhancement of mechanical properties of high density polyethylene (HDPE) by adding 1 wt% of CNTs is studied using Dynamic mechanical and Thermal analyzer (DMTA). The chemically treated and functionalized CNTs were homogeneously dispersed with HDPE and the test samples were made using injection molding machine. Using DMTA, storage modulus (E′), loss modulus (E″) and damping factor (tan δ) of the sample under oscillating load were studied as a function of frequency of oscillation and temperatures. The storage modulus decreases with an increase of temperature and increases by adding CNTs in the composites where the reinforcing effect of CNT is confirmed. It is concluded that the large scale polymer relaxations in the composites are effectively restrained by the presence of CNTs and thus the mechanical properties of nanocomposites increase. The transition frequency of loss modulus is observed at 1 Hz. The loss modulus decreases with an increase of temperature at below 1 Hz but opposite trend was observed at above 1 Hz. The shift factor could be predicted from Williams-Landel-Ferry (WLF) model which has good agreement with experimental results.