Crystalline Polymers
Crystalline Polymers
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.
Pro’s
- Good Strength
- Good Flexibility
- Good High Temperature Performance
- Good Chemical Resistance
Con’s
- High Shrinkage. Warp
- Smaller Processing Window
- Poor Impact
- Good Memory (For Living Hinges)
- Good Creep Resistance
- Not Usually Solvent Bondable or Paintable
Specifications
| 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 |
