ThermoPlastic Olefin
Encyclopedia
Thermoplastic PolyOlefin (TPO) is a trade name that refers to polymer
/filler blends usually consisting of some fraction of PP (polypropylene
), PE (polyethylene
), BCPP (block copolymer polypropylene
), rubber, and a reinforcing filler. Common fillers include, though are not restricted to talc, fiberglass, carbon fiber, wollastonite
, and MOS (Metal Oxy Sulfate). Common rubbers include EPR (Ethylene propylene rubber
), EPDM (EP-diene rubber), EO (ethylene-octene), EB (ethylene-butadiene), SEBS (Styrene-ethylene-butadiene-styrene). Currently there is a great variety of commercially available rubbers and BCPP's. They are produced using regioselective and stereoselective catalysts known as metallocene
s. The metallocene catalyst becomes embedded in the polymer and cannot be recovered.
The geometry of the metallocene catalyst will determine the sequence of chirality
in the chain, as in, atactic, syndiotactic, isotactic, as well as average block length, molecular weight and distribution. These characteristics will in turn govern the microstructure of the blend.
The components are blended together at 210 - 270 °C under high shear. A twin screw extruder or a continuous mixer may be employed to achieve a continuous stream, or a Banbury compounder
may be employed for batch production. A higher degree of mixing and dispersion is achieved in the batch process, but the superheat batch must immediately be processed through an extruder to be pelletized into a transportable intermediate. Thus batch production essentially adds an additional cost step.
As in metal alloys the properties of a TPO product depend greatly upon controlling the size and distribution of the microstructure
. PP and PE form a vaguely crystalline structure known as a spherulite
. Unlike metals, a spherulite cannot be described in terms of a lattice
or unit cell, but rather as a set of polymer chains that pack down closely next to one another and form a dense core. The PP and PE components of a blend constitute the "crystalline phase", and the rubber gives the "amorphous phase".
If PP and PE are the dominant component of a TPO blend then the rubber fraction will be dispersed into a continuous matrix of "crystalline" polypropylene. If the fraction of rubber is greater than 40% phase inversion may be possible when the blend cools, resulting in an amorphous continuous phase, and a crystalline dispersed phase. This type of material is non-rigid, and is sometimes called TPR for ThermoPlastic Rubber.
To increase the rigidity of a TPO blend, fillers exploit a surface tension phenomena. By selecting a filler with a higher surface area per weight, we can achieve a higher flexural modulus.
TPO blends have densities 0.92 - 1.1
TPO is frequently found in outdoor applications such as roofing because it does not degradehttp://mrca.org/i4a/headlines/headlinedetails.cfm?id=1060 under solar UV radiation, a common problem with nylons. TPO is used extensively in the automotive industry.
TPO is easily processed by injection molding, profile extrusion, and thermoforming. TPO cannot be blown, or sustain a film thickness less than 1/4 mil (about 6 micrometres), where 1 mil is equal to 0.001 inch.
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
/filler blends usually consisting of some fraction of PP (polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
), PE (polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
), BCPP (block copolymer polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
), rubber, and a reinforcing filler. Common fillers include, though are not restricted to talc, fiberglass, carbon fiber, wollastonite
Wollastonite
Wollastonite is a calcium inosilicate mineral that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids...
, and MOS (Metal Oxy Sulfate). Common rubbers include EPR (Ethylene propylene rubber
Ethylene propylene rubber
Ethylene propylene rubber is an insulation used for high voltage cables. It has improved thermal characteristics over more traditional cables, such as cross-linked polyethylene, enabling a smaller cross sectional area for the same load carrying capacity...
), EPDM (EP-diene rubber), EO (ethylene-octene), EB (ethylene-butadiene), SEBS (Styrene-ethylene-butadiene-styrene). Currently there is a great variety of commercially available rubbers and BCPP's. They are produced using regioselective and stereoselective catalysts known as metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
s. The metallocene catalyst becomes embedded in the polymer and cannot be recovered.
The geometry of the metallocene catalyst will determine the sequence of chirality
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
in the chain, as in, atactic, syndiotactic, isotactic, as well as average block length, molecular weight and distribution. These characteristics will in turn govern the microstructure of the blend.
The components are blended together at 210 - 270 °C under high shear. A twin screw extruder or a continuous mixer may be employed to achieve a continuous stream, or a Banbury compounder
Banbury mixer
The Banbury Mixer is a brand of internal batch mixer. The "Banbury" trademark is owned by Farrel Corporation. Internal batch mixers such as the Banbury mixer are used for mixing or compounding rubber and plastics. The original design dates back to 1916. The mixer consists of two rotating spiral...
may be employed for batch production. A higher degree of mixing and dispersion is achieved in the batch process, but the superheat batch must immediately be processed through an extruder to be pelletized into a transportable intermediate. Thus batch production essentially adds an additional cost step.
As in metal alloys the properties of a TPO product depend greatly upon controlling the size and distribution of the microstructure
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
. PP and PE form a vaguely crystalline structure known as a spherulite
Spherulite
In petrology, spherulites are small, rounded bodies that commonly occur in vitreous igneous rocks. They are often visible in specimens of obsidian, pitchstone and rhyolite as globules about the size of millet seed or rice grain, with a duller luster than the surrounding glassy base of the rock,...
. Unlike metals, a spherulite cannot be described in terms of a lattice
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
or unit cell, but rather as a set of polymer chains that pack down closely next to one another and form a dense core. The PP and PE components of a blend constitute the "crystalline phase", and the rubber gives the "amorphous phase".
If PP and PE are the dominant component of a TPO blend then the rubber fraction will be dispersed into a continuous matrix of "crystalline" polypropylene. If the fraction of rubber is greater than 40% phase inversion may be possible when the blend cools, resulting in an amorphous continuous phase, and a crystalline dispersed phase. This type of material is non-rigid, and is sometimes called TPR for ThermoPlastic Rubber.
To increase the rigidity of a TPO blend, fillers exploit a surface tension phenomena. By selecting a filler with a higher surface area per weight, we can achieve a higher flexural modulus.
TPO blends have densities 0.92 - 1.1
TPO is frequently found in outdoor applications such as roofing because it does not degradehttp://mrca.org/i4a/headlines/headlinedetails.cfm?id=1060 under solar UV radiation, a common problem with nylons. TPO is used extensively in the automotive industry.
TPO is easily processed by injection molding, profile extrusion, and thermoforming. TPO cannot be blown, or sustain a film thickness less than 1/4 mil (about 6 micrometres), where 1 mil is equal to 0.001 inch.