Highly crystalline polymers are rigid, high melting, and less affected by solvent penetration. Crystallinity makes a polymers strong, but also lowers their impact resistance. As an example, samples of polyethylene prepared under high pressure (5000 atm) have high crystallinities (95 – 99%) but are extremely brittle.
Small molecules and ions form a three-dimensional lattice with an extended regular structure that makes large crystals possible.
We usually describe such lattices with a unit cell – the smallest repeating unit in the lattice . In the case of NaCl, the unit cell is said to be face-centered cubic.
Polymer molecules are very large so it might seem that they could not pack together regularly and form a crystal. It now is known that regular polymers may form lamellar (plate-like) crystals with a thickness of 10 to 20 nm in which the parallel chains are perpendicular to the face of the crystals.
- Good Strength
- Good Flexibility
- Good High Temperature Performance
- Good Chemical Resistance
- High Shrinkage. Warp
- Smaller Processing Window
- Poor Impact
- Good Memory (For Living Hinges)
- Good Creep Resistance
- Not Usually Solvent Bondable or Paintable
|Polymer||Advantages||Drawbacks||Some Typical Applications|
|PEEK||Extreme chemical resistance, Wear, Stength||High cost||Wear plates, Semiconductor|
|LCP||Very fast flowing||Hot Water||Electronic/Electric connectors|
|PPS||Inherently FR, High Temperature||Nitric acid||Electronics, Reflectors|
|PPA||Low water absorption, Good strength||Phenols, Some acids||Electronics, Industrial, Cooling systems|
|PET||Appearance, Stiffness, Creep resistance||Sterilization, Hot water||Engine components, Appliance housings|
|PBT||Appearance, Moldability, Chemical resistance||Sterilization, Hot water||Connectors, Relays, Housings|
|NYLON||Fatigue resistance, Scuff resistance||Sodium Hydroxide, Salts||Cable ties, Velcro, Auto|
|POM||Snap-fits, Wear, Critical tolerances||Fumes, Tool corrosion||Clips, Knobs, Gears|
|PMP||Transparency, Chemical resistance||UV exposure, Chlorine||Labware, Cookware|
|PP||Living hinges, Flexible, Chemical resistant||Nitric acid, Warp, Low temperature||Consumer, Disposables|
|HDPE||Semi-rigid, Low temperature impact, Chemical resistance||Nitric acid, Acetone||Lids, Handles, Containers|
|LLDPE||Flexible, Low temperature impact, Chemical resistance||Nitric acid, Acetone||Lids, Handles, Containers|
|LDPE||Very Flexible, Low temperature impact, Chemical resistance||Nitric acid, Acetone||Lids, Handles, Containers|