Polymeric material Flashcards
Polymer - structure
- Natrual gas or crued oil are degraded into small molecules which are polymerized into long chains
- Polymerization:
1. Addition: fast with no bi-products
2. Condensation: slower but stops automatically
Plastics - structure
- Polymer + additives
- Bigger and heavier molecules results in higher melt viscosity
- Additives modify properties:
1. Softeners - increase flexibility
2. Blowing agents - polymer foams
3. Reinforcements - to make carbon and glass fibers
4. Flame retardants - prevent degration at high temp.
Plastics - aging
- Physical aging: due to slow movement of molecular chains over time and causes shrinkage and increased stiffness
- Chemical aging: due to chemical exposure like UV causing mechanical properties to worsen
Plastic - proerties and usage
+Low density
+Large designe freedom
-Low stiffness
-Low strength
- Packaging
- Electronics
- Medical products like orthoses and prosthetics
(used for about 100 years)
Thermoplastic - structure
- Amorphous: lack of shape (cooked spaghetti) with no sharp melt point
- Semi-crystalline: more aranged shape with sharp melt point
- The longer chains used the greater the impact resistance but more difficult injection molding
Amorphous thermoplastic - properties
+Low shrinkage
+Good creep resistance (does not elongate during load)
-Low chemical resistance
-Too soft above Tg (glass transition temperature where material goes from stiff to soft)
Semi-crytsalline thermoplastics - properties
+Can be used above and below Tg
+Good chemical resistance
-High shrinkage
-Poor creep resistance
Commodities - types
- Polyethylene (PE)
- Polypropylene (PP)
- Polyvinylchloride (PVC)
- Ethylene-vinyl acetate (EVA)
Polyethylene
- Most common plastic
- Semi-crystalline consisting of carbon and hydrogen
- Used for: bottles, packaging, toys
+Low density and price
+Chemical resistant
+No moisture absorption
-Low stiffness and strength
-Low creep resistance
Polypropylene
- Second most common plastic
- Semi-crystalline consisting of C and H
- Used for: bottles, containers, foam
+Low density and price
+Chemical resistant
+No moisture absorption
-Brittle at low temp.
-Poor UV-resistance
Polyvinylchloride
- Amorphous plastic of C, H, and Cl
- Softeners (additive) to make softer
- Used for: bottles, rain wears, shoe soles
+Low price and density
+Chemical resistant
+No mositure absorption
-Poor UV-resistance
-Low temp. tolerance
Ethylene-vinyl acetate
- Rubber-like qualities and is a co-polymer
- VA-modified PE (4% of VA), thermoplastic EVA (4-30%), EVA-rubber (over 60%)
- Used for: padding and shock absorber to sport equipment, handles and fishing rods
+Stress-crack resistant
+Uv-radiation resistant
-Low stiffness and strength
-Low temp. tolerance
Engineering plastics - types
- Polyamide (PA)
- Polyester (PET and PBT) - PET is fourth most common plastic, ex. bottles
Polyamide
- Most common engineering plastic
- Semi-crystalline whith subtypes where PA6 and PA66 are most common
- Used for: fan blades, gas pedals, cog wheels
+Good electrical properties
+Can be made flame-retardant
-Absorb moisture from air, which alters properties
-Brittle at low temp. (if not modified)
Bioplastics
- Bio-based from ex. tree fibers, sugar canes
- Just some parts have to be bio-based
+Lower emission of greenhouse gases
+Bio-degradable in som cases
-Poor thermal resistance due to biodegradable
-Expensive
