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Chapter 8 – Polymer Families

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1 Chapter 8 – Polymer Families
Recall polymer categories: Plastics: Thermoplastics Engineered and commodity Thermosets Elastomers: Thermosets and thermoplastics

2 Most plastics are thermoplastics. Most thermoplastics are commodity
Most commodity thermoplastics are PE, PP, PVC and PS – This accounts for 80% of all thermoplastics!!! 80% of all plastics are thermo plastics. 80% of all thermoplastics are either PE, PP, PVC and PS

3 Commodity vs. Engineered Thermoplastics
Commodity thermoplastics < $1.50/lb (’06) Engineered thermoplastics $1.50 to $6.00 range but some can be north of $100 for special plastics. See: Engineered plastics used for durable goods, have better mechanical properties or “special” property. Commodity plastics are used for consumer goods (i.e. food storage), are more readily available and generally have lower mechanical properties. 80% - 85% of all thermoplastics are commodity, balance are engineered. Bob, explain your experience here with PEEK – go to the website, note carbon reinforced peek has high compressive strength and low coefficient of friction but costs $100/lb. UHMW, low coef of friction but low compression strength. Solution: cast nylon!! 80% commodity = lower price by shear volume. Also, where are engineered plastics used??

4 Commodity Thermoplastics:
Polyethylene (PE) (includes LDPE, HDPE, UHMW, PETE) Polypropylene (PP) Polyvinyl Chloride (PVC or vinyl) Polystyrene (PS) These are broad families – many variations exist Density, blends, additives, fillers, etc… These are all ethenic polymers (based on the ethylene molecule) Account for 80% of all thermoplastics!!

5 More on Commodity Thermoplastics:
Most are linear in structure with minimal branching and cross-linking. Most are low strength to strength comparable to that of engineering plastics. Polyethylene (PE) and polypropylene (PP) both have a translucent, waxy texture, and are the only non-foam plastics that float in water (i.e. density < 1.0 g/cm3) Can be amorphous (PS, PVC) or semi-crystalline (PE, PP) See chapter 8 summary for more!

6 Ethenic Polymers (all based on the ethylene molecule)
A series of polymers can be created by modifying the ethylene monomer – essentially replacing a hydrogen atom with something else:

7 These “ethylene” polymers (with polyethylene) make up about 80% of the tonnage of polymers in use.

8 Arrangement + Effects Chain Flexibility
How easily the main chain of the polymer can move, is a key factor in determining the properties of the material. There are several factors that will determine the chain flexibility: Temperature Types of bonds Side groups Branching Additives 8

9 Arrangement + Effects Chain Flexibility – Branching
Although branching can increase the entanglement of the polymer chains, branching increases the chain flexibility. Larger branches hold the molecules further apart, increasing the free volume (and decreasing density, giving the molecules more room to move. Additives also greatly reduce chain flexibility (i.e. glass filled). 9

10 Polyethylene (PE) Surpasses all plastics in quantity produced.
Simplest of molecules CH2-CH2 Qualities: Stiffness, strength/toughness, low cost, ease of forming, resistance to chemicals, permeability to gas, ease of processing.

11 Polyethylene (PE) Crystalline – Yes only C-H bonds, flexible –no side groups Hygroscopic – No (not O or N) Glass Transition – Low (-125 C) Flammability – Yes only C-H bonds 11

12 Polyethylene (PE) Uses:
LDPE: Bread bags, frozen food bags, grocery bags. HDPE: Milk, water and juice containers, grocery bags, toys, liquid detergent bottles.

13 Arrangement + Effects Polyethylene is probably the most chemically and heat resistant thermoplastic material. It contains only carbon and hydrogen bonds. Polypropylene is also very chemically and heat resistant, it comes close to Polyethylene, but falls a little short. Polyethylene Polypropylene 13

14 Polyethylene (PE) Many options available: Most Common

15 Density Options Low Density ( g/cm3) Trash bags, grocery bags,etc Medium Density ( g/cm3) Bowls, lids, gaskets, containers High Density ( g/cm3) Bottles, piping Very high Density ( higher g/cm3) Toys UHMWPE ( g/cm3) Toys, wear items, tough, low coefficient of friction, excellent abrasion resistance Density increases – so does strength and toughness, also linearity of chain orientation increases. High density grades tend to be more crystalline. Low density grades have significant degree of branching and hence, lower melting point UHMWPE does not melt like lower molecular weight grades, at 150C still behaves like rubber. Difficult to injection mold!!

16 30% glass filled, high density, strength can approach 7 ksi
UHWPE – strength approx. 6 ksi Conclusion: can approach strength of engineered polymers w/ special polyethylene options!!

17 Polyethylene Terephthalate (PET) (aka Polyester)
Crystalline – Yes, flexible enough Hygroscopic – Yes (O) Glass Transition = (69 C) Flammability – Yes (only C-H and C=O bonds) 17

18 Polypropylene (PP) Monomer of PP contains methyl group (CH3) in place of one H: What is the chemical formula?? Show chemical formla

19 Polypropylene (PP) Crystalline – Yes only C-H bonds, flexible – side groups every other C Hygroscopic – No (not O or N) Glass Transition – Low (- 20 C) Flammability – Yes only C-H bonds 19

20 Polypropylene (PP) Strengths similar to HDPE, but easier to injection mold. Good fatigue properties. Excellent chemical resistance – no solvent for PP at room temperature! Low density (.9 to .915 g/cm3) means lighter than water (i.e. it floats). Qualities: Strength/toughness, resistance to chemicals, resistance to heat, barrier to moisture, low cost, versatility, ease of processing, resistance to grease/oil.

21 Polypropylene (PP) Uses:
Gasoline tanks, chemical tanks, luggage, battery cases, ropes, fibers or filaments. Consumer products: Ketchup bottles, cups yogurt containers and margarine tubs, medicine bottles.

22 Polypropylene (PP) Options: Su = 10 ksi Su = 5 ksi
See page 113 and 114 for definition of copolymer and homopolymer – bob define this better next year in chapter 4 Su = 5 ksi

23 Arrangement + Effects Chain Flexibility – Side Groups
Side groups restrict chain movement. The larger the side group, the more rigid the molecule Having a Methyl (CH3) group attached to one side of the main chain will add some stiffness. Polypropylene is relatively flexible even at room temperature. Having one attached to both sides of the main chain will add a lot of stiffness. PMMA is very rigid. 23

24 Polyvinyl Chloride (PVC aka Vinyl)
Monomer of PVC contains one chlorine atom in place of one H:

25 Polyvinyl Chloride (PVC aka Vinyl)
Crystalline – No, rigid (Cl to big to allow) Hygroscopic – No (not O or N) Glass Transition – High (185 F) Flammability – No (Cl puts out) 25

26 Polyvinyl Chloride (PVC aka Vinyl)
Two types Plasticized (vinyl) – low strength mostly used for decorative coatings (wallpaper), wire coating, imitation leather, etc. Rigid (no plasticizer) – much stronger!!! PVC has excellent transparency, chemical resistance, long-term stability, flammability resistance, good weatherability, flow and insulatory electrical properties. Qualities: Versatility, ease of blending, strength/toughness, resistance to grease/oil, resistance to chemicals, clarity, low cost. Low fracture toughness (brittle) Glass Transition = 81 C

27 Polyvinyl Chloride (PVC aka Vinyl)
Uses: Plumbing products/ hardware, outdoor signs Clear food packaging, shampoo bottles

28 Polyvinyl Chloride (PVC aka Vinyl)
Can be copolymerized to get property modifications. Options:

29 Polystyrene (PS) H atom substituted with a large benzene ring:
Or, simplified: Recall chain stiffening

30 Polystyrene (PS) Accounts for 20% of all thermoplastics in commercial use. Very versatile plastic that can be rigid or foamed. PS is brittle – poor impact strength. Its mechanical properties can be improved by blending with polybutadiene. Qualities: Versatility, insulation, ease of processing, low cost, clarity Horrible weatherability, does not have chemical resistance like PE and PP. Glass Transition = 100 C

31 Polystyrene (PS) Uses: Foamed Rigid
Insulation, beverage cups, fast-food sandwich containers Rigid Videocassette cases, compact disc jackets, knives, spoons and forks, cafeteria trays, grocery store meat trays

32 Polystyrene (PS) Arrangement + Effects Chain Flexibility – Side Groups
Having a Benzene ring attached to one side of the chain will greatly affect the stiffness. Polystyrene is very stiff to the point of being brittle (CD cases) 32

33 Polystyrene (PS) Crystalline – No (Benzene ring makes it too rigid)
Hygroscopic – No (not O or N) Glass Transition – High (210 F) Flammability – Yes only C-H bonds 33

34 Polystyrene (PS) Options:

35 Approximate tensile strength – note PP, PVC approach engineered thermoplastics!!

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