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.

Pros

  • Good Strength
  • Good Flexibility
  • Good High Temperature Performance
  • Good Chemical Resistance

Cons

  • High Shrinkage. Warp
  • Smaller Processing Window
  • Poor Impact
  • Good Memory (For Living Hinges)
  • Good Creep Resistance
  • Not Usually Solvent Bondable or Paintable

Specifications

Advantages

Disadvantages

Polymer Family

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