History
Polyester was largely developed by American chemist Wallace Carothers, who led an effort at the Du Pont Company during the 1920s and
1930s to develop synthetic fiber. His research produced neoprene, which is a synthetic rubber, and nylon. He experimented with polyester, but could not
produce a variety with the necessary strength and high melting point. After World War II, British scientists Jon Whinfield and J. T. Dickson improved on
Carothers's experiments, making a stronger polyester that produced usable fibers. In 1946, E.I. du Pont de Nemuours co. (DuPont) purchased the
registered trademark for the material and began to produce the fibers commercially under the brand name Dacron in 1952.
Formula
As defined by the Federal Trade Commission of USA PET is chemically composed of "at least 85% by weight of an ester of a substituted
aromatic carboxylic acid, including but not restricted to substituted terephthalic units, p(-R-O-CO- C6H4-CO-O-)x and para substituted hydroxybenzoate
units p(-R-O-CO- C6H4-O-)x."The para substituted hydroxybenzoate units in PET are ethylene group (CH2-- CH2).
PET is synthesized by reacting ethylene glycol (C2H6O2) and either terephthalic acid or it's metyl ester with the aid of the an antimony oxide catylyst. The reaction is carried out in a vaccum at high temperatures to achieve high molecular weight. The clear colorless molten polyester flows onto the casting wheel and forms into very small chips.
Properties
Polyethylene Terephthalate (PET):
STRUCTURE: Crystalline
SPECIFIC DENSITY: 1.37
WATER ABSORBTION RATE (%): 0.15
ELONGATION (%): 70
TENSILE STRENGTH (psi): 6600
COMPRESSION STRENGTH (psi): 14000
FLEXURAL STRENGTH (psi): 16000
FLEXURAL MODULUS (psi): 400000
IMPACT (IZOD ft. lbs/in): 0.8
HARDNESS: R120
FABRICATION
- BONDING: G
- ULTRASONIC WELDING: G
- MACHINING: E
DEFLECTION TEMPERATURE (deg. F)
- @ 66 psi: 330
- @ 264 psi: 212
UTILZATION TEMPERATURE (deg. F)
- min: -4
- max:212
MELTING POINT (deg. F): 480
COEFFICIENT OF EXPANSION: 0.000039
ARC RESISTANCE: 80
DIELECTRIC STRENGTH (kV/mm): 20
TRANSPARENCY:
UV RESISTANCE: Good
CHEMICAL RESISTANCE
- ACIDS: Good
- ALKALIS: Good
- SOLVENTS: Good
The ester groups in the polyester chain are polar, with the carbonyl oxygen atom having a somewhat negative charge and the carbonyl
carbon atom having a somewhat positive charge. The positive and negative charges of different ester groups are attracted to each other. This allows the ester
groups of nearby chains to line up with each other in crystal form, which is why they can form strong fibers. This is also the cause of PET's high glass
transition and melting temperature and modulus. Its chemical resistance is similar to that of Nylon. PET does crystallize, but in many applications the
crystallinity is minimized by a rapid quenching. In others, such as fiber production, maximum crystallinity is desirable. PET has high moisture resistance,
and when made into a fiber, the resulting clothes are crease resistant.
Material Uses & Manufacture Techniques
The three main uses of PET are as fibers in fabric or cord (polyester fabric), films for packaging (Mylar) and as liquid containers (bottles).
Melt spinning technique is followed for the production of polyester filament. In this technique molten viscous polymer melt is extruded through a spinneret
and then cool air is passed over the extruded stream to help solidification. PET bottles are blow molded: a bubble of hot PET is dropped into a cast and
blown to the outer edges, conforming to the shape. Mylar film is cast also in thickness from .002" to .014" thick.
The most unique use I've seen for PET is it's exclusive use as Mylar in combination with aluminum foil to make those fancy shiny,
unbreakable helium balloons that they banned from school.
Bibliography
PET: http://arvind.coe.drexel.edu/MATE200/PET.html
Polyester Fiber: http://afma.org/f-tutor/polyester.htm
Polyesters:http://www.psrc.usm.edu/macrog/pet.htm
Mylar (Polyester: Polyethylene Terephtalate): http://www.indplastic.com/polyeste.htm
PET-Polyethylene Terephtalate:http://www.plasticsusa.com/pet.html