Viscoelastic behavior, effect plasticizers

Papers Viscoelastic Behavior of Plasticized Polyvinyl Chloride at Large Deformation. III. The Effect of Filler, J. Potym. Scu., Part A2, 1909-192U (196I+) co-author R. Sabia. 

The mechanical behavior of plastics on time-dependent applied loading can cause different important effects on materials viscoelasticity. Loads applied for short times and at normal rates (Chapter 2) causes material response that is essentially elastic in character. However, under sustained load plastics, particularly TPs, tend to creep, a factor that is included in the design analysis. 

The behavior observed in the stress-strain curve corresponds to viscoelastic behavior that is typical of polymeric materials. The viscoelastic behavior is highly dependent on the temperature at which the test is performed and its relationship to the Tg of the sample. It is also dependent on the rate of deformation, as mentioned previously. In general, very rapid deformation does not allow time for molecular rearrangement to occur and results in behavior characteristic of a more brittle material. The effects of temperature and rate of testing on plastic materials are illustrated in Figs. 3.54 and 3.55, respectively. 

A fluid, which although exhibits predominandy viscous flow behavior, also exhibits some elastic recovery of the deformation on release of the stress. The term viscoelastic is reserved for solids showing both elastic and viscous behavior. Most plastic systems, both melts and solutions, are viscoelastic due to the molecules becoming oriented due to the shear action of the fluid, but regaining their equilibrium randomly coiled configuration on release of the stress. Elastic effects are developed during processing such as in die swell, melt fracture, and frozen-in orientation. 

When a plastics (polymer) is subjected to stress, the structure can react in a number of ways. In the first reaction the bonds are stressed by stretching or bending which is the elastic response. Unlike the more ordered structures, adjustment of the strain in the materials is hindered by the interference between molecules so that all but the very initial response is hindered by frictional effects and the material shows a delay between the application of the stress and the resulting strain. This behavior is referred to as viscoelastic behavior. From Fig. 1-9 it can be seen how the molecules slide past each other to increase spacings and reduce the elastic load on the bonds. Sustained stress causes actual displacement of the molecular chains with extensive movement of the chains past each other and results in flow-like behavior which is referred to as creep or cold flow. At constant initial strain the slippage of the molecules and the adjustment of position lead to another condition which is called stress relaxation. The level of the resistance of the structure to applied deformation drops and the material assumes a lower energy configuration. 

There are sharp changes in these effects at certain critical temperatures such as the glass transition temperature and the crystalline melting temperature. It has been found that by using these reference temperatures, it is possible to predict the effects of the temperature on the elastic and viscoelastic behavior of the plastics materials. 

They reported that Eq. 23.7 could be used successfully to describe uniaxial tension and compression behavior of various metal alloys. Equation 23.7 was later modified by McLellan (1969) to accommodate strain rate effects. McLellan interpreted the terms E, K, and ii of Eq. 23.7 as material functions with the function E representing viscoelastic behavior and functions if and W representing work-hardening characteristics. The terms E, K, and t/ were all described as functions of the strain rate (ds/dt) so that rigidity, stress, and plastic flow, respectively, were aU affected by variations in the strain rate. 

The resistance to plastic flow can be schematically illustrated by dashpots with characteristic viscosities. The resistance to deformations within the elastic regions can be characterized by elastic springs and spring force constants. In real fibers, in contrast to ideal fibers, the mechanical behavior is best characterized by simultaneous elastic and plastic deformations. Materials that undergo simultaneous elastic and plastic effects are said to be viscoelastic. Several models describing viscoelasticity in terms of springs and dashpots in various series and parallel combinations have been proposed. The concepts of elasticity, plasticity, and viscoelasticity have been the subjects of several excellent reviews (21,22). 

In the preparation and processing of ionomers, plasticizers may be added to reduce viscosity at elevated temperatures and to permit easier processing. These plasticizers have an effect, as well, on the mechanical properties, both in the rubbery state and in the glassy state these effects depend on the composition of the ionomer, the polar or nonpolar nature of the plasticizer and on the concentration. Many studies have been carried out on plasticized ionomers and on the influence of plasticizer on viscoelastic and relaxation behavior and a review of this subject has been given 119]. However, there is still relatively little information on effects of plasticizer type and concentration on specific mechanical properties of ionomers in the glassy state or solid state. 

With plastics there are two types of deformation or flow viscous, in which the energy causing the deformation is dissipated, and elastic, in which that energy is stored. The combination produces viscoelastic plastics. See Chapter 2, MATERIAL BEHAVIOR, Rheology and Viscoelasticity, regarding their effects on fabricated products. 

The time and temperature dependent properties of crosslinked polymers including epoxy resins (1-3) and rubber networks (4-7) have been studied in the past. Crosslinking has a strong effect on the glass transition temperature (Tg), on viscoelastic response, and on plastic deformation. Although experimental observations and empirical expressions have been made and proposed, respectively, progress has been slow in understanding the nonequilibrium mechanisms responsible for the time dependent behavior. 

The observation that an increase in temperature or a decrease in rate both result in the same fracture response points toward a viscoelastic influence on thermoset fracture behavior, especially crack initiation. This characteristic behavior of epoxies has been explained qualitatively by consideration of the temperature and strain rate effects on the plasticity of the material at the crack tip . In effect, test conditions which promote the formation of a so-called crack tip plastic zone, or blunt the crack by a ductile process, promote unstable crack propagation. This aspect of unstable fracture is subsequently discussed in more detail. 

With the long term objective of treating the effects of moisture and other plasticizers on the mechanical properties of materials, a new scheme that yields a complete constitutive model of viscoelastic materials has been developed. The time-temperature principle is an integral part of this modeling with a quantitative description of the glass transition behavior of pol3nmers. 

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