Physical Properties of Polymer Compounds

1. Physical Properties of Polymer Compounds The materials selection component of a part design demands careful consideration of all required properties. Consider the following case studies: • electric drill casing • automobile bumper • aircraft tire What properties must a given material provide for each of these components? As engineers, you must be able to translate qualitative terms (strong, flexible) into engineering terms for which quantitative data is available. • We will survey various physical testing methods that are used industrially, and highlight important behavioural characteristics of polymers • Throughout the course we will refer to these testing methods as we examine adhesive, elastomer, plastic, fiber and coatings applications. CHEE 490

2. Polymer Material Selection - Key Questions When developing a polymer compound for a given application, you may ask yourself the following questions: • What are the maximum and minimum temperatures the compound will experience throughout its lifetime? • includes manufacturing as well as product use • To what loads will the material be subjected, and what is the frequency of load application? • engine mounts, fishing line • Is the part transparent, translucent or opaque? Colouring? • Is flame resistance necessary, and to what environmental conditions will the product be exposed? • solvent resistance, oxidative degradation CHEE 490

3. Static Testing of Polymers and Polymer Compounds Stress-strain analysis is the most widely used mechanical test. However, it is only a rough guide as to how a material will behave in a given application. Test specimens are prepared in the form of “dog bones” whose dimensions are known accurately: A static test involves deformation of the sample at a steady rate, usually with one end fixed and the other pulled at a constant rate of elongation (tensile testing). The retractive force of the material is recorded as a function of the elongation, and the engineering stress, s, is calculated as a function of the engineering strain, e. CHEE 490

4. A A B A Static Testing of Polymers and Polymer Compounds We will soon see that observed polymer properties are strongly dependent on temperature and the applied rate of deformation. Under some conditions, an elastomer can behave like a brittle plastic, and vice-versa. Three typical behaviours are illustrated here. Often cited sample properties: A: Ultimate tensile stress (Pa) and elongation at break (%) B: Yield tensile stress, Pa Toughness: Area under s - e curve. CHEE 490

5. Compression and Shear vs. Tensile Tests Stress-strain curves are very dependent on the test method. A modulus determined under compression is generally higher than one derived from a tensile experiment, as shown below for polystyrene. Tensile testing is most sensitive to material flaws and microscopic cracks. Compression tests tend to be characteristic of the polymer, while tension tests are more characteristic of sample flaws. Note also that flexural and shear test modes are commonly employed. CHEE 490

6. Static Testing of Polymers and Polymer Compounds Shown is a representative stress-strain curve for a polymer undergoing brittle failure. An often quoted material property is the tensile (or Young’s) modulus, E: which relates strain to retractive stress over the linear region. E (Pa) Copper 1.2*1011 Polystyrene 3.0*109 Soft Rubber 2.0*106 CHEE 490

7. Mechanical Properties of Representative Polymers Elastic Yield Ultimate Elongation Modulus Strength Strength to Break (GPa) (MPa) (MPa) (%) Poly(propylene) 1.0-1.6 23 24-38 200-600 Poly(styrene) 2.8-3.5 --- 38-55 1-2.5 Poly(tetrafluoroethylene) 0.41 10-14 14-28 100-350 Poly(methylmethacrylate) 2.4-2.8 48-62 48-69 2-10 Nylon 3.8 800 25 Poly(ethylene):low-density 0.1-0.3 6.9-14 10-17 400-700 Note that these values depend on temperature and strain rate. • We will see that behaviour is highly influenced by temperature when we examine factors such as degree of crystallinity, glass transitions and melt viscosity. CHEE 490

8. Temperature Sensitivity of Polymer Properties Crystallinity and crosslinking of polymer chains influence the modulus of a polymer as shown below. • At room temperature, poly(ethylene) is above Tg but below Tm, while nylon-6,6 is below both Tg and Tm. CHEE 490

9. Heat Distortion Temperature The maximum temperature at which a polymer can be used in rigid material applications is called the softening or heat distortion temperature (HDT). A typical test (plastic sheeting) involves application of a static load, and heating at a rate of 2oC per min. The HDT is defined as the temperature at which the elongation becomes 2%. A: Rigid poly(vinyl chloride) 50 psi load. B: Low-density poly(ethylene) 50 psi load. C: Poly(styrene-co-acrylonitrile) 25 psi load. D: Cellulose acetate (Plasticized) 25 psi load. CHEE 490

10. Transient Testing: Creep Tests Creep tests can be made under all load conditions, and provide data needed to design products that sustain loads for long periods • A constant stress, so, is applied, with the strain, e, varying with time. Creep behaviour arises from the viscoelastic properties of polymers and their compounds. Above are illustrated the response of different idealized materials to step changes in applied stress. A: Elastic B: Viscous C: Viscoelastic CHEE 490

11. Transient Testing: Impact Resistance Impact tests are high-speed fracture tests that measure the energy required to break a specimen. Izod and Charpy (shown to right) impact tests use a weighted pendulum to measure the loss of kinetic energy associated with specimen fracture. Agreement between different methods can be poor, and results are not material constants, but dependent on sample geometry, notching and size. Impact strength units vary, but notched tests are defined in terms of energy per unit length of notch: kJ/m. CHEE 490

12. Transient Testing: Resilience of Cured Elastomers Resilience tests reflect the ability of an elastomeric compound to store and return energy at a given frequency and temperature. Change of rebound resilience (h/ho) with temperature T for: 1. cis-poly(isoprene); 2. poly(isobutylene); 3. poly(chloroprene); 4. poly(methyl methacrylate). CHEE 490

13. Flow Characteristics – Rheology of Polymer Melts Polymer melts and solutions are pseudoplastic, meaning that they exhibit shear thinning behavior CHEE 490

14. . hE+(t,e) Flow Characteristics – Rheology of Polymer Melts Extensional thickening effects are observed when tracking the extensional viscosity as a function of time. If a sample, initially at its rest state, is subjected to steady simple extension at a rate e starting at a time=0, the tensile stress growth coefficient, defined as: shows the onset of strain hardening effects depend on the applied shear rate. . CHEE 490

15. Thermal Expansion If a part is to be produced within a close dimensional tolerance, careful consideration of thermal expansion/contraction must be made. Parts are produced in the melt state, and solidify to amorphous or semi-crystalline states. Changes in density must be taken into account when designing the mold. CHEE 490