Technology and Application of Engineering i Polymers

Transcription

1 EBB 427 Technology and Application of Engineering i Polymers EBB 427 Course Synopsis : This course covers topics on technology and applications of various polymers in engineering applications. The course covers the properties and the processing techniques for three types of polymeric materials such as thermoset, thermoplastics and elastomer. It also covers the examples of new polymeric materials and commercially available polymeric materials, for instance thermoplastic and thermoset for general and engineering applications 1

2 References R J Young and P A Lovell, Introduction to Polymers, Chapman & Hall, R J Crawford, Plastics Engineering, Pergamon Press, D H Morton-Jones, Polymer Processing, Chapman & Hall, N G McCrum, C P Buckley, C B Bucknall, Principles of Polymer Engineering, Oxford/ University Press, An Introduction o to Rubber Technology, ogy, Andrew Ciesielski, e s Rapra a Technology Ltd,1999. Rubber Technology Handbook, Werner Hofmann, Hanser Publisher, Revision What is the difference between polymers, plastics and resins??? 2

3 Revision Molecular Arrangement of Polymers Think of how spaghetti noodles look on a plate - Amorphous organization. An Amorphous polymers are generally transparent. This is an important characteristic for many applications such as food wrap, plastic windows, headlights and contact lenses. Revision Molecular Arrangement of Polymers The translucent and opaque polymers - crystalline arrangement. By definition a crystalline arrangement has atoms, ions, or in this case, molecules in a distinct pattern. You generally think of crystalline structures in salt and gemstones, but not in plastics. Just as quenching can produce amorphous arrangements, processing can control the degree of crystallinity. The higher the degree of crystallinity, the less light can pass through the polymer. Therefore, the degree of translucence or opaqueness of the polymer is directly affected by its crystallinity. 3

4 Classification of Polymer Linear chain molecules Branched chain molecules Weakly cross-linked chain molecules Highly cross-linked molecules - Thermoplastics - Thermoplastics - Elastomers - Thermosets Thermoplastic - meaning that once the polymer is formed it can be heated and reformed over and over again (facilitates recycling). Thermosets & Elastomers - can not be remelted. 4

5 Revision Characteristics of plastics when compared to Metals and Ceramics Applications of Plastics; 1. Packaging 2. Medical 3. Recreational 4. Textiles 5. Furniture & Housewares 6. Transportation 7. Construction, etc. Some Important Terminology Polymer Plastics Resin Synthetic polymer Natural polymer 5

6 Polymer- long molecules made up of smaller molecules-joined together by chemical bonds Plastics- Large molecules (synthetically made or naturally occuring), are highly modified Resin- Polymer that has not been formed into its final useful shape Synthetic polymer- polymer that do not occur naturally, they are manufactured Natural polymer- polymer that occur in nature Some Important Terminology Backbone atom Branching Copolymer Homopolymer Monomer 6

7 Backbone atom- atom along the main chain of a polymer Branching- side chain of a polymer main chain Copolymer- a polymer formed from more than the minimum number of monomer, i.e. ABS Homopolymer- polymer made from the minimum number of monomer type Monomer- a single unit that can be combined with others to form a polymer Some Important Terminology Crosslink Curing Thermoplastic Thermoset Catalyst t 7

8 Crosslink- covalent bond between polymer chain Curing- process of hardening a polymer by the formation of crosslink Thermoplastic- a polymer solid at room temp, that can be melted and cooled to solidify in the desired shape Thermoset- a polymer that may be either liquid or solid at room temp., p, when heated it will harden and cure Catalyst- a molecule or material that facilitates a chemical reaction, but does not become part of the reaction Some Important Terminology Amorphous Crystallinity Degree of crystallinity Melting point (Tm) Steric effect 8

9 Amorphous- no regular structural pattern occur in a area of polymer Crystallinity- area within a polymer in which a polymer molecules fold into a tight, regular structure Degree of crystallinity- the amount of structure that is crystalline as opposed to the amount that is amorphous Melting point (Tm)- the temperature at which material changes from solid to liquid (vice versa) Steric effect- The influence of molecule shapes on the properties of a material Some Important Terminology Glass transition temperature (Tg) Virgin material Aging Degradation decomposition 9

10 Glass transition temperature (Tg)- Virgin material- Resin that has not been previously processed Aging- long term, low temperature degradation Degradation- the decomposition of a material Decomposition- the breaking of primary bond in a molecule Flow properties of polymer melts Rheology- science of the deformation and flow of Rheology science of the deformation and flow of bodies Rheometry- the technology of measuring the flow behavior In plastic processing, the materials to be processed must be in flowable condition- through increase in temperature caused melting, dissolving the materials in solvent, etc. During such processing, the viscosity is important 10

11 Traditional materials; 2 categories Elastic solid (iron, concrete, copper, etc)- materials completely recover their shape & restore energy Viscous fluid (water, oil, etc)- materials that flow when exposed to an imposed shear force, do not return to original shape Polymer- do not follow the trend of traditional materials.why??? Viscosity Viscosity is a measure of the resistance of a Viscosity is a measure of the resistance of a fluid to deform under shear stress Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction (water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity) During flow process in plastic processing machinery, the melt is subjected to shear This can be illustrated by 2 plate model (next slide) 11

12 Viscosity D θ S A F 1. Consider 2 plates (A= area of the plate), separated dby distance, D 1. The space between them is occupied by the liquid 3. One plate moves relatively to the other with velocity U 4. The movement is resisted by the viscous reaction in the fluid 5. Since the movement is in shear, the Reaction is the shear viscosity Shear stress, ζ = Shear force/area of the shear face = F/A Nm -2 Shear strain,γ = Amount of shear displacement, S/Distance between shearing surfaces (D) = Tan θ Viscosity, η = Shear stress/rate of shear strain = ζ / (d γ/dt) = ζ / γ Viscosity The unit of viscositiy i i was poise, P, or centipoise, i cp. 1 mpa s = 1 cp. η rapidly decreases as temperature increases. Ideal fluids are called Newtonian. The viscosity is independent of the rate of shear Shear rate is a measure of the rate of shear deformation Rheogram for Newtonian liquids. A - high viscosity, B - low viscosity. 12

13 Newtonian Liquid Newtonian liquid, where shear stress is proportional to shear rate, with the proportionality constant being the viscosity A Newtonian fluid (named for Isaac Newton) is a fluid that flows like water For example, water is Newtonian, because it continues to exemplify fluid properties no matter how fast it is stirred or mixed. If the liquid id is not Newtonian, a plot of shear vs. the rate of shear is not a straight line but a curve Dilatant A dilatant material is one in which viscosity increases with the A dilatant material is one in which viscosity increases with the rate of shear (also termed shear thickening). The dilatant effect can be seen more readily with a mixture of corn starch and water 13

14 Pseudoplastic Pseudoplastic, or shear-thinning fluids have a lower apparent viscosity at higher shear rates. Pseudo-plastic substance. Pseudo-plastic substance with yield value Viscosity - Most polymer melts & rubber compound behave in pseudoplastic. How can we relate the pseudoplastic behavior to the morphology of the polymer (long chain & coiled in complex structure)??? Newtonian and non-newtonian bahavior -Dilatant behavior can cause processing difficulties Variation of apparent viscosity with shear rate 14

15 Viscosity Thixotropy Thixotropy is the property of some non-newtonian pseudoplastic fluids to show a time-dependent change in viscosity. Viscosity decreases as the material is stirred until some minimum value is reached. It increases again when the substance is no longer agitated. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated Thixotropic substance at different shear rates. Viscosity When the curve is nonlinear, the viscosity May be defined in two ways; 1. Calculating apparent viscosity, η a 2. Calculating consistency viscosity, η c η o viscosity at a very low shear Rate, which behave like Newtonian behavior η c η a is the slope of the secant line from the origin to the shear stress at the given value of shear rate η o η c the slope of the line at the chosen value of Rate of shear η a The η a is greater than η c 15

16 Viscometers are employed to measure viscosity. Capillary viscometer Rotational rheometer Simple shear viscometer Cone & plate rheometer Parallel plate viscometer Tensile & extensional viscometer Schematic diagram of a rotational viscometer Schematic diagram of a cone and plate viscometer. Melt Flow Index (MFI) The Melt Flow Index is a measure of the ease of flow of the melt of a thermoplastic polymer or a measure of the ability of the material's melt to flow under pressure. It is defined as the weight of polymer in grams flowing in 10 minutes through a capillary of specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures. The melt flow rate is an indirect measure of molecular weight, high melt flow rate corresponding to low molecular weight The melt flow rate is inversely proportional to the viscosity of the melt at the conditions of the test 16

17 MFI Apparatus How to relate MFI with molecular weight??? Comprises a cylinder containing polymer melt which loaded from above by a piston carrying a weight. There is a capillary die at the bottom of the cylinder The procedure is to measure the output t by cutting off sections of extrudate at known time intervals and weighing them Melting of Thermoplastic Originally solid, must be heated to above its melting or softening point The heat comes from 2 sources; 1. The external heat supplied-i.e. by heater on the barrel of extruder, etc 2. Heat generated when a highly viscous fluid being sheared at high shear rate 17

18 Latent heat the amount of energy in the form of heat that is required for a material to undergo a change of phase (also known as "change of state"). Two latent heats are typically described. One is the latent heat of fusion (melting), and the other is the latent heat of vaporization (evaporation). They are so named as to describe the direction of heat flow from one phase to the next: solid liquid gas. The energy change is endothermic when going from solid to liquid to gas, but exothermic when going in the opposite direction. Specific heat capacity The typical unit for specific heat capacity is the kilojoule per kilogram kelvin, kj kg- 1 C-1 the amount of energy required to raise the temperature of one kilogram of the substance by one Celcius. Heat capacity can be measured by using calorimetry. The SI unit would be joule per kilogram celsius 18

19 Freezing of Melts The reverse of the melting process The molding must be removed from a mould without danger of its distortion. To estimate cooling rate, need to find thermal diffusivity, 19