PET: Polyethylene terephthalate

General Properties

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PET

Polyethylene terephthalate


Polyethylene terephthalate is a semi-crystalline polymer; its semi-crystalline state is sometimes designated PET-C or C-PET and its amorphous state is designated PET-A or A-PET. Amorphous PET is mainly employed for beverage bottles, since it features a high transparency and resistance to breaking. For the use as a construction material, a high degree of Crystallinity / Degree of CrystallinityCrystallinity refers to the degree of structural order of a solid. In a crystal, the arrangement of atoms or molecules is consistent and repetitive. Many materials such as glass ceramics and some polymers can be prepared in such a way as to produce a mixture of crystalline and amorphous regions.crystallinity can be advanta- geous since this prevents shrinkage of the components due to post-CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization.*

Structural Formula


Properties

Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.Glass Transition Temperature70 to 85°C
Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).Melting Temperature245 to 260°C
Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).Melting Enthalpy140 J/g
Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. Decomposition Temperature425 to 445°C
Young's Modulus2100 to 3100 MPa
Coefficient of Linear Thermal Expansion (CLTE/CTE)The coefficient of linear thermal expansion (CLTE) describes the length change of a material as a function of the temperature.Coefficient of Linear Thermal Expansion80 to 100 *10-6/K
Specific Heat Capacity1.04 to 1.17 J/(g*K)
Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.Thermal Conductivity0.24 W/(m*K)
DensityThe mass density is defined as the ratio between mass and volume. Density1.33 to 1.45 g/cm³
MorphologySemi-crystalline thermoplastic
General propertiesHigh stability and stiffness. Good abrasion resistance. Good sliding properties. Resistant to diluted acids, aliphatic and aromatic hydrocarbons, oils, fats and alcohols. Tear and weather resistant. Good electrical insulating properties
ProcessingInjection blow molding, stretch blow molding, injection molding
ApplicationsFibers (polyesters), e.g., for sportswear. Packing (e.g., beverage bottles). Instrument and apparatus engineering. Medical engineering

NETZSCH Measurement

InstrumentDSC 204 F1 Phoenix®
Sample Mass8.43 mg
IsothermalTests at controlled and constant temperature are called isothermal.Isothermal Phase5 min
Heating/Colling Rates10 K/min
CrucibleAl, pierced lid
AtmosphereN2 (50 ml/min)

Evaluation

Polyethylene terephthalate (PET) exemplifies how the ratio of amorphous and crystalline phases within a sample can be affected by different cooling rates. During production, the material undergoes very fast cooling, resulting in a high amorphous content. This is evident in the 1st heating (blue) from the large Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition step (Δcp of 0.34 J/(g.K)) and subsequent cold or post-CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization at 137°C (peak temperature). Post-CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization is generally associated with a volume change (shrinkage). At 251°C (1st heating, blue), all crystalline phases melt. 
After a controlled cooling at 10 K/min, the amorphous content of the polymer was considerably lower than before. For this reason, the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition step height in the 2nd heating (red) was lowered and post-CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization was almost completely eliminated. The Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting temperature in the 2nd heating (peak temperature) occurred at 249°C. The difference between peak temperatures of the 1st and 2nd heatings is due to the better contact between the sample and crucible bottom after the first melting.