High temperature plastics-Advance enginnering plastics

Excerpt: High temperature Plastics are those plastics which withstand temperature higher than 150°C on their continuous service. Plastics which can be operated at temperance range of .........

There are many synonyms for the term high-performance plastics, such as: high temperature plastics, high-performance polymers, high performance thermoplastics or high-tech plastics. The name high temperature plastics is in use due to their continuous service temperature (CST), which is always higher than 150 °C. Plastics which can be operated at temperance range of 240o C to 350o C re called advanced engineering plastics. Fig.1 Shows the classifications of plastics based on temperature resistance properties and crystalline structure of polymers.

The name high temperature plastics is in use due to their continuous service temperature (CST), which is always higher than 150 °C. Plastics which can be operated at temperance range of 240o C to 350o C re called advanced engineering plastics.

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Plastics such as PBI, PEI, PAI are advance engineering plastics. PPSU, PSU, PEEK, PTFE etc. are high temperature plastics. High temperature plastics are further classified as amorphous and crystalline polymers.

Amorphous plastics include PBI, PI, PPSU, PSU, polycarbonate, and ABS etc. Most amorphous plastics are clear. These polymers have a molecular structure that is randomly ordered, which causes them to have a range of temperatures over which they melt. This also makes them relatively easy to thermoform because the molecules become mobile as heat is. The more heat applied, the more mobile the molecules will become. These thermoplastics are isotropic in flow because of their randomized molecular arrangement applied. This causes them to possess better dimensional stability and have a lesser tendency to warp. They also typically possess superior impact strength. However, amorphous polymers are reserved for structural applications because they don’t perform well as bearings or wear components. They are more prone to stress cracking than semi-crystalline polymers, and have very poor fatigue resistance. They also tend to have lower chemical resistance and higher friction than their semi-crystalline counterparts. PI, PBI, PPSU, PSU are few examples which are High Temperature amorphous plastics.

Polyimides (PI)

Polyimide is obtained by reaction of dianhydride and diamines or dianhydride or isocyanate. Polyimide can be thermoplastics or thermoset.

• High strength over a wide temperature range from -270°C to + 300°C

• Extremely good long-term thermal stability

• Glass transition temperature up to 400°C

• Good cryogenic properties

• Inherently flame retardant (UL 94 V0)

• Minimal thermal expansion

• Outstanding durability

• Excellent resistance to wear under high surface area pressure, high sliding speeds

• High compression strength pressure and creep resistance

• High radiation resistance

• High purity, low outgassing in vacuum

• Good chemical resistance to acids, fats and solvents

• Excellent electrical insulation properties

• Minimal thermal conductivity

Poly sulphones are a family of thermoplastic polymers.

Poly aryl sulfones

Poly sulphones are a family of thermoplastic polymers. These polymers are known for their toughness and stability at high temperatures. They contain the subunit aryl-SO2-aryl, the defining feature of which is the sulfone group. Polyphenylsulfone (PPSF or PPSU) Poly aryl sulphone is a type of high performance polymer. The commercially important polysulfones are prepared by condensation of 4,4'-bis(chlorophenyl)sulfone with various bisphenols. Two bisphenols for this application are bisphenol A (the polymer being called PSF) and 4,4'-bis(4-hydroxyphenyl)sulfone (the polymer being called PES). Polyarylsulphones (PSU, PPSU) are a family of thermoplastic, amorphous and polar polymers. These materials have high strength and stability, even at elevated temperatures. Polyetherimide (PEI) is very similar to polysulphone and therefore belongs to the same product category.

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Chrematistics:

• Poly sulphones are rigid

• With stand high strength

• They are amorphous and hence transparent (yellowish)

• Polysulphones are dimensionally stable

• They can be sterilized by superheated steam, resistant to hot water

• Poly sulphones are good chemical resistance

Polybenzimidazole (PBI)

The preparation of PBI can be achieved by condensation reaction of diphenyl isophthalate and 3,3’,4,4’-tetraaminodiphenyl . The spontaneous cyclization of the intermediately formed animo-amide to PBI provided a much more stable amide linkage.

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Characteristics

• PBI are usually yellow to brown solid infusible up to 400 °C or higher. Most of the linear PBI are partly or entirely dissolved in strong protonic acids (for instance, sulfuric acid or methanesulfonic acid.

• It has exceptional thermal and chemical stability and does not readily ignite.

• This material with superior stability, retention of stiffness, toughness at elevated temperature.

Due to its high stability, polybenzimidazole is used to fabricate high-performance protective apparel such as firefighter’s gear, astronaut space suits, high temperature protective gloves, welders’ apparel and aircraft wall fabrics. In recent years, polybenzimidazole found its application as a membrane in fuel cells.

Semi-crystalline high temperature thermoplastics include PEEK, PAEK, UHMW-PE, PEEK, fluoropolymers

Semi-crystalline high temperature thermoplastics include PEEK, PAEK, UHMW-PE, PEEK, fluoropolymers, and nylon. These polymers tend to be opaque, unlike amorphous thermoplastics. Also unlike the amorphous polymers, semi-crystalline polymers have a very ordered molecular structure that results in a sharply defined melting point. This sharply defined melting point makes it very difficult to thermoform them because the molecules will only become mobile above the material’s melting point. Semi-crystalline thermoplastics are anisotropic in flow. They perform very well in applications involving wear, bearings, and structural loads. Unlike amorphous thermoplastics, they have a lower coefficient of friction and good chemical resistance. Their impact resistance, however, is not as good as that of amorphous plastics. High performance semi-crystalline thermoplastics provide very good stiffness and strength, good toughness, and a very low coefficient of friction. This includes material such as PTFE and PPS.

Poly aryl ether ketone (PAEK)

Poly aryl ether ketone is (PAEK) is a family of semi-crystalline thermoplastics with high-temperature stability and high mechanical strength. PAEK is condensation product of 1,1-bis(hydroxy phenyl)-1-(phenoxy)-1-(pheny) methane and 4,4’-Difluoro benzophenone in presence of potassium carbonate in sulfonal as solvent at 200 degree C.Fig.5

PAEK has a continuous operating temperature of 250 °C (482 °F) and under short-term loads can function up to 350 °C (662 °F). When burned, it has the least toxic and corrosive fumes. It also has a low heat output when burned, so it qualifies for use in interior aviation applications. It also has good overall chemical resistance. In addition, PAEK materials show very good impact strength at low temperatures, high mechanical fatigue strength, a very low tendency to creep as well as good sliding and wear properties. Due to their unusual characteristics, poly aryl ether ketones are used for particularly demanding applications.

Characteristics:

• High mechanical strength, even at high temperatures

• Very good impact strength

• Low tendency to creep

• Good chemical resistance

• Good sliding and wear properties

• Low moisture absorption

• Good radiation resistance

• Poor resistance to weathering

Polyphenylenesulphide (PPS)

Polyphenylene sulfide is an engineering plastic, commonly used today as a high-performance thermoplastic (Fig.6).

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It is formed by reaction of sodium sulfide with p-dichlorobenzene:

ClC6H4Cl + Na2S → 1/n [C6H4S]n + 2 NaCl

It is semi crystalline polymer. In its pure solid form, it may be opaque white to light tan in colour. Maximum service temperature is 218 °C (424 °F). Due to its chemical structure, PPS is a very resistant polymer with very good mechanical strength, even at temperatures above 200°C. In addition to low water absorption, PPS also has good dimensional stability and excellent electrical properties. Furthermore, PPS is chemically very stable even at high temperatures.

Characteristics:

• High thermal stability

• Very good strength and rigidity

• Low tendency to creep

• Abrasion resistant

• Very good chemical resistance

• Excellent electrical properties

• Low water absorption

• Sensitive to weathering