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SEMI-CRYSTALLINE STRUCTURE
amorphous region
crystallites
(regular folding)
Typical crystallinity values
HDPE about 95%
LLDPE about 60-85%
LDPE about 60-75%
PET between % and 30 %
As crystallinity and density increase
Heat-softening point
increases
Gas and water permeation
decreases
Stiffness increases
Stress crack resistance
decreases
Low temperature toughness
decreases
As molecular weight increases:
Toughness increases
Tensile strength increases
Environmental stress crack
resistance increases
Processing becomes more
difficult