The Effect of Creep and other Time Related Factors on Plastics and Elastomers

Laurence W. McKeen

• Publisher: William Andrew
• Year: 2014
• eBook ISBN: 9780323354073
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This reference guide brings together a wide range of critical data on the effect of creep and other long term effects on plastics and elastomers, enabling engineers to make optimal material choices and design decisions. The data are supported by explanations of how to make use of the data in real world engineering contexts and provides the long-term properties data that designers need to create a product that will stand the test of time.This new edition represents a full update of the data, removing all obsolete data, adding new data, and updating the list of plastics manufacturers. Additional plastics have also been included for polyesters, polyamides and others where available, including polyolefins, elastomers and fluoropolymers. Entirely new sections on biodegradable polymers and thermosets have been added to the book.The level of data included – along with the large number of graphs and tables for easy comparison – saves readers the need to contact suppliers, and the selection guide has been fully updated, giving assistance on the questions which engineers should be asking when specifying materials for any given application.- Trustworthy, current data on creep, stress-strain and environmental stress cracking, enabling easier and more effective material selection and product design.- Includes expert guidance to help practitioners make best use of the data.- Entirely new sections added on sustainable and biodegradable polymers, and thermosets.

1

Introduction to Creep, Polymers, Plastics and Elastomers

This chapter is in several parts, starting with an introduction to stress and the various types of stress. The next section covers the concept of creep. Categories or stages of creep are covered along with the various measures and ways to present creep data. Following this section is a section on plastics and polymers. It includes polymerization chemistry and the different types of polymers and how they differ from each other. Discussed are the subjects of copolymers, branching, cross-linking, steric hindrance, isomerism, crystallinity, and other factors that affect the molecular structures of the polymers used to make plastics. The next sections cover plastics compositions since plastics are rarely “neat,” reinforcement, fillers and additives are reviewed. Mechanisms of creep on the molecular levels are discussed next, followed by Poisson’s ratio and how creep data are used in plastic product design.

Keywords

additives; amorphous; atactic; Creep; crystallinity; creep modulus; compression set; charpy; degree of unsaturation; environmental stress cracking; flexural; glass transition temperature; geometric isomer; hoop stress; isochronous; isomers; impact; isotactic; Izod; modulus; molecular weight; melt point; melt index; molecular attractions; polymerization; polydispersity; Poisson’s ratio; stereoisomer; syndiotactic; steric hindrance; shear; thermoplastic; thermoset; tensile properties; tensile; thermal stability; toughness; Van der Waals forces

1.1 Introduction

In an earlier book of this series, The Effect of Temperature and Other Factors on Plastics and Elastomers [1], the general mechanical properties of plastics were discussed. The mechanical properties as a function of temperature, humidity, and other factors were presented in graphs or tables. That work includes hundreds of graphs of stress versus strain, modulus versus temperature, and impact strength versus temperature. However, when one starts designing products made of plastic, these graphs do not supply all the necessary information. This is because these graphs show the results of relatively short-term tests. The value in design is in the initial selection of materials in terms of stiffness, strength, etc. Designs based on that short-term data obtained from a short-term test would not predict accurately the long-term behavior of plastics. This is partly because plastics are viscoelastic materials. Viscoelastic by definition means possessing properties that are both solid-like and liquid-like. More precisely in reference to plastics, viscoelastic means that measurements such as modulus, impact strength, and coefficient of friction (COF) are not only sensitive to straining rate, temperature, humidity, etc. but also to elapsed time and loading history. The manufacturing method used for the plastic product can also create changes in the structure of the material which have a pronounced effect on properties.

The rest of this chapter first deals with the types of stress and a short introduction to creep. Then the chemistry of plastics is discussed and because plastics are polymeric materials the focus is more on polymer chemistry. The discussion includes polymerization chemistry and the different types of polymers and how they can differ from each other. Since plastics are rarely “neat,” reinforcement, fillers, and additives are reviewed. This is followed by a detailed look at creep, including creep-specific tests and creep graphs. The discussion takes a look at what happens at the microscopic level when plastics exhibit creep.

1.2 Types of Stress

Creep is the time-dependent change in the dimensions of a plastic article when subjected to a constant stress. Stress can be applied in a number of ways. Normal stress (σ) is the ratio of the applied force (F) over the cross-sectional area (A) as shown in Eq. (1.1).

$\sigma =\frac{F}{A}$ (1.1)

(1.1)

1.2.1 Tensile and Compressive Stress

When the applied force is applied directed away from the part, as shown in Figure 1.1, it is a tensile force inducing a tensile stress. When the force is applied toward the part it is a compressive force inducing a compressive stress.

Figure 1.1 Illustration of tensile stress and compressive stress.

1.2.2 Shear Stress

Shear stress (τ) is also expressed as force per unit area as in Eq. (1.2).

The shear force is applied parallel to the cross-sectional area “A” as shown in Figure 1.2.

Figure 1.2 Illustration of shear stress.

1.2.3 Torsional Stress

Torsional stress (τ) occurs when a part such as a rod for shaft is twisted as in Figure 1.3. This is also a shear stress, but the stress is variable and depends how far the point of interest is from the center of the shaft. The equation describing this is shown in Eq. (1.2).

$\tau =\frac{Tc}{K}$ (1.2)

(1.2)

Figure 1.3 Illustration of torsional stress.

In this equation, T is the torque and c is the distance from the center of the shaft or rod. K is a torsional constant that is dependent on the geometry of the shaft, rod, or beam. The torque (T) is further defined by Eq. (1.3), in which θ is the angle of twist, G is the modulus of rigidity (material dependent), and L is the length.

$T=\frac{\theta KG}{L}$ (1.3)

(1.3)

The torsional constant (K) is dependent upon geometry and the formulas for several geometries are shown in Figure 1.4. Additional formulas for torsional constant are published [2, pp. 63–76].

Figure 1.4 Torsional constants for rods or beams of common geometries.

1.2.4 Flexural or Bending Stress

Bending stress or flexural stress commonly occurs in two instances, shown in Figure 1.5. One is called a simply supported structural beam bending and the other is called cant...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Foreword
• Acknowledgments
• 1. Introduction to Creep, Polymers, Plastics and Elastomers
• 2. Styrenic Plastics
• 3. Polyether Plastics
• 4. Polyesters
• 5. Polyimides
• 6. Polyamides (Nylons)
• 7. Polyolefins and Acrylics
• 8. Thermoplastic Elastomers
• 9. Fluoropolymers
• 10. High-Temperature Polymers
• Appendix 1. Abbreviations
• Appendix 2. Unit Conversion Tables
• Index