Chapter 8 – Polymer Families • Recall polymer categories: • Plastics: • Thermoplastics • Engineered and commodity • Thermosets • Engineered and commodity • Elastomers: • Thermosets and thermoplastics
Most plastics are thermoplastics. Most thermoplastics are commodity Most commodity thermoplastics are PE, PP, PVC and PS – This accounts for 80% of all thermoplastics!!!
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: http://www.curbellplastics.com/catalog.asp#1 • 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.
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!!
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!
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:
These “ethylene” polymers (with polyethylene) make up about 80% of the tonnage of polymers in use.
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
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).
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.
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
Polyethylene (PE) • Uses: • LDPE: Bread bags, frozen food bags, grocery bags. • HDPE: Milk, water and juice containers, grocery bags, toys, liquid detergent bottles.
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
Polyethylene (PE) • Many options available: Most Common
Density Options • 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
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!!
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)
Polypropylene (PP) • Monomer of PP contains methyl group (CH3) in place of one H: What is the chemical formula??
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
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.
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.
Polypropylene (PP) Options: Su = 10 ksi Su = 5 ksi
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.
Polyvinyl Chloride (PVC aka Vinyl) • Monomer of PVC contains one chlorine atom in place of one H:
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)
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
Polyvinyl Chloride (PVC aka Vinyl) • Uses: • Plumbing products/ hardware, outdoor signs • Clear food packaging, shampoo bottles
Polyvinyl Chloride (PVC aka Vinyl) • Can be copolymerized to get property modifications. Options:
Polystyrene (PS) • H atom substituted with a large benzene ring: Or, simplified: Recall chain stiffening
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
Polystyrene (PS) • Uses: • Foamed • Insulation, beverage cups, fast-food sandwich containers • Rigid • Videocassette cases, compact disc jackets, knives, spoons and forks, cafeteria trays, grocery store meat trays
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) Polystyrene (PS)
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
Polystyrene (PS) Options:
Approximate tensile strength – note PP, PVC approach engineered thermoplastics!!