Plastics

1. Plastics Just cover 4 – 15 thermoset vs. thermoplastic – rest is review

2. The word POLYMER means many ‘mers’ • A ‘mer’ is a unit • Polyethylene means many ‘ethylenes’ • The molecular weight of a polymer (length of the chain – number of ‘mers’) will effect the properties. • 10-20 ethylenes – greases or oils • 200-300 waxes • 20,000 + polyethylene H H | | C=C | | H H Ethylene Polyethylene H H H H H H H H H H H H H H | | | | | | | | | | | | | | [-C-C-C-C-C-C-C-C-C-C-C-C-C-C-] | | | | | | | | | | | | | | H H H H H H H H H H H H H H

3. Basic Molecular Structure: Polyethylene = many molecules of ethylene Ethylene molecules attach to each other through covalent bond between carbon atoms (needed to satisfy valence requirements for carbon) Many of these ethylene molecules join together producing polyethylene – See physical structure

4. 2 basic types of polymer materials • Thermoset – ‘eggs’ – Undergo a chemical process which crosslinks the polymer chains. Will not re-melt • Thermoplastic – ‘butter’ – Soften when heated and harden when cooled; repeatable process • Two basic types of thermoplastic materials • Semi-crystalline – orderly, dense polymer molecular chains – packing of molecular chains in ORDERLY array – not the same as atoms making “grains” in metals or ceramics!! • Amorphous – without form, random array of polymer chain molecules

5. Thermoplastic vs. Thermoset - Properties • Chemical Resistance • Thermoplastic – Some have good chemical resistance, but most can be dissolved or weakened by at least some chemical compounds • Thermoset – Better chemical resistance than any thermoplastic • Dimensional Stability • Thermoplastic – Because they respond to heat, their dimensional stability is related to their service temperature and their glass transition temperature. • Thermoset – Excellent dimensional stability • Creep (cold flow) • Thermoplastic – Although it can be improved by adding fillers and reinforcements, they are not as good as Thermosets • Thermoset – Much better than thermoplastics • Molded-in-stresses (warpage) • Thermoplastic – caused by uneven cooling, part design, mold design, and process parameters • Thermoset – relatively low stresses which yield less distortion.

6. Thermoplastic vs. Thermoset - Properties • Toughness • Thermoplastic – Inherently tough, although some types can be brittle • Thermoset – not tough, brittle unless reinforced – fiberglass • Coloration • Thermoplastic – Easily colorable and color is maintained • Thermoset – Limited options and colors tend to fade or discolor over time. • Clarity • Thermoplastic – Many clear polymers are available • Thermoset – Very few options for clarity • Shrinkage • Thermoplastic – Varies based on type of polymer (Semi-crystalline/Amorphous) • Thermoset – Varies by process

7. Thermoplastic vs. Thermoset - Properties • Long term properties • Thermoplastic – Need to be estimated based on short term data. • Thermoset – Can be predicted based on experimental data • Cost • Thermoplastic – Less expensive and easier to process • Thermoset – Need more skill and produce more scrap which cannot be reprocessed. • Tool Wear • Thermoplastic – Less tool wear with unreinforced polymers • Thermoset – More prevalent mold wear and greater potential for mold damage. Most Thermosets are filled.

8. Thermoplastic vs. Thermoset - Properties • Density • Thermoplastic – Lightweight, very close to the weight of water • Thermoset – Limited in some applications due to their higher weight. • Cycle time • Thermoplastic – Generally fast, dependent on wall thickness of part - cooling • Thermoset – Generally slower, dependent on wall thickness of part – heating and holding • Flammability • Thermoplastic – some burn freely, while others will self-extinguish • Thermoset – Inherently non-flammable

9. Entanglement Plastic material can be envisioned as a plate of spaghetti. You have very long molecules which get entangled with each other as they try to move. • This entanglement holds the material together along with secondary forces. • Entanglement is why plastics burn before they enter a gaseous form • Plastic molecules do not share primary bonds with the adjacent plastic molecules. If they did, they would be cross-linked and depending on the degree of cross linking, would not re-melt when re-heated • Side groups and secondary forces can greatly increase the entanglement forces and thereby increase the viscosity of the material

10. Non-Newtonian Flow • A Newtonian fluid is one which you push harder to get it to flow faster. The relationship between force (viscosity) and speed (shear rate) is linear. • With plastics, as you push them faster, they flow easier • The molecules ‘line up’ as the shear rate increases • The faster you go, the more Newtonian the material behaves

11. Hygroscopic • Some materials will absorb moisture into their matrix based on the relative humidity of the surrounding environment • These materials need to be dried prior to use or the moisture in the matrix will cause rapid degradation of the material which will reduce the mechanical and visual properties of the final parts (splay) • Nylon is the most moisture sensitive material

12. Orientation • When plastics flow quickly, the molecules align themselves in the direction of flow • If the molecules are frozen in that aligned orientation, the part will have greater mechanical properties in the direction of orientation, but weaker properties transverse to the alignment. • Shrinkage is also affected • Very prevalent in thin walled parts • Causes internal stresses

13. Crystallinity • Some polymers tend to ‘fold up’ and form densely packed regions in at least a portion of the polymer matrix. (>35% crystallized = Semi-crystalline) • These materials are referred to as semi-crystalline • Semi-crystalline materials have a much sharper melt temperature range. • Semi-crystalline materials require more energy to melt • You have to melt the crystals • Side groups, secondary branching, and cooling rate all affect the degree of crystallinity of the final product • Crystalline materials tend to be more chemically resistant

14. Amorphous (PC, PS, PVC, PMMA, ABS) Generally • Higher viscosity than semi-crystalline (s/c) materials – harder to make flow • Shrink less than s/c (0.005-0.007 in/in) • Don’t have a true melt temperature – soften more above Glass Transition Temperature – Tg • Less chemically resistant than s/c • Clearer than s/c (can be translucent/optical quality). • Better weather resistance vs. s/c • Better creep vs. s/c

15. Semi-crystalline (PE, PP, PA, PET, POM) Generally • Lower viscosity than amorphous materials – flow easier – allows them to form crystals • Wide range of shrinkage values (.008-.050 in/in) – depends on degree of crystallinity • Have a clearly defined melting point and a Glass Transition Temperature – Tg • Usually translucent to opaque • More brittle than amorphous

16. Misc. • Strength – thermoplastics have no elastic limit. Some have endurance limit (fatigue limit) but most do not. Various methods used to calculate strength and E. • Elongation – can have enormous % elongation. Key: elastomers elastic strains > 100%. % elongation can be 1,000% for polymers! • Properties highly dependent on temperature. Properties also dependent on aging, strain rate, loading speed, etc.. • Electrical – nearly all plastics good electrical insulators. • Chemical – most plastics resistant to deterioration by most chemicals but be careful! • Thermal conductivity – low thermal conductivity therefore good heat insulating materials.

17. I. Molecular Weight Molecular Weight – KEY POINTS: • Understand what molecular weight means when dealing with polymers • Understand the effect of molecular weight on material properties • Understand entanglement

18. Molecular Weight When we talk about molecular weight in terms of polymers, we are really talking about the length of the individual chains. The polymerization process is subject to variation so there is no single chain length, there is actually a wide range of lengths, so when we discuss molecular weight, we really mean the average molecular weight of the material. This average is found by measuring samples of the material as it is produced. I. Molecular Weight

19. Molecular Weight There are two different categories of molecular weight average that are commonly used: The first is the Number Average Molecular Weight ( ) The second is the Weight Average Molecular Weight ( ) I. Molecular Weight

20. Properties Increasing the molecular weight of the material increases many of the properties of the material by increasing the entanglement of the molecules. A higher molecular weight: Increases the chemical resistance - to a point It takes more damage to the main chains of the molecules before it will affect the strength of the material The big loophole to this is if you have a chemical that is very similar to the chemical makeup of the main chain, it will dissolve it much more easily Like Dissolves Like I. Molecular Weight

21. Properties A higher molecular weight: Increases how far the material can stretch before rupturing (ductility) The higher degree of entanglement allows the material to be pulled further before the chains break I. Molecular Weight

22. Properties A higher molecular weight: Increases ductility: A candle and Polyethylene (PE) have basically the same molecular structure. The chain length of the candle is just much shorter than that of the PE. If you bend a bar of PE in half – it will bend, if you bend a candle in half, it will fracture. I. Molecular Weight

23. Properties A higher molecular weight: Increases the impact resistance of the material The higher degree of entanglement means that in order to rupture, more polymer bonds need to be broken, this means that the polymer can absorb more energy before failing. I. Molecular Weight

24. Properties A higher molecular weight: Increases the weather resistance of the material Same type of reasoning behind the increase in chemical resistance, the chains are longer, so they can withstand more damage before the mechanical properties will start to diminish I. Molecular Weight

25. Properties A higher molecular weight: Increases the viscosity of the material – makes it harder to process the material using conventional methods The longer the chains, the harder it is to get them to flow More tangled I. Molecular Weight

26. Properties Processors want materials that will flow easily in order to form complex geometries, but that can affect the properties of material used to create the product. Many times it turns out to be a trade-off between the required properties and processability of the material. CD’s and DVD’s are made from the same material as most safety glasses, Polycarbonate. Safety glasses require a higher molecular weight in order to provide the necessary property of impact resistance. CD’s and DVD’s require a lower molecular weight material in order to fill out the thin walls. CD’s and DVD’s can shatter, safety glasses don’t. I. Molecular Weight

27. Properties I. Molecular Weight

28. II. Polymerization • Addition Reactions • In addition reactions, the double or triple bonds between the atoms of the molecule are broken and the chain grows longer when another molecule that has also had its bonds broken links together with it. • In Polyethylene, the double carbon bond in the ethylene molecule separates and links with another carbon bond from another ethylene molecule.

29. Condensation Reactions In condensation reactions, a portion of the ‘mer’ molecule reacts with another ‘mer’ molecule to form a new bond and gives off water, carbon dioxide, or possibly an acid. The portion of the ‘mer’ that reacts is known as the functional group. Condensation reactions usually take longer than addition reactions In addition reactions any chain end will react with any other chain end and the molecules grow at different rates depending on what size chains combine. In condensation reactions the chains typically grow at the same rate as the chemicals that make up the polymer chain are consumed, the reaction rate slows down. II. Polymerization