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. 1. Introduction Creep is the deformation of solid material or viscous liquids under the influence of stress. In polymers, creep occurs by chains untangling and slipping relative to one another, because a polymer consists of long chain-like molecules in a tangled and coiled arrangement. Polymer deformation occurs progressively at a decreasing strain rate until it is constant or zero. Polymeric materials exhibit viscoelastic behavior; they do not exhibit purely elastic (ideal solids) or purely viscous (ideal liquid) behavior but a combination of both viscous and elastic properties in varying amounts. The rate of viscoelastic deformation depends on nature of the material, duration and temperature of exposure, and the magnitude of applied structural load. A typical creep test is performed by applying constant stress and monitoring the strain levels with time. Mechanical analogs are used to describe viscoelastic behavior of polymers. The simplest of them all is the Hookean spring while the Burgers model offers the best qualitative description of both creeps and strain relaxation behavior of viscoelastic materials [1]. According to this model the response to a constant stress is given by [2] where is the applied stress, the strain, the viscosity of the dashpot, and the spring modulus for the springs in a Burgers model, and the relaxation time. There are two ways of predicting long time behavior of