Polyimides (sometimes abbreviated PI) is a class of polymers containing imide cycles in the main chain [1] , usually condensed with aromatic or other cycles. Heat-resistant aromatic polyimides, derivatives of tetracarboxylic acids with five-membered imide cycles in the main chain, have found the greatest use.
Content
Classification
According to the composition of their main chain, polyimides can be:
- Aliphatic (linear polyimides),
- Semi-aromatic,
Aromatic: R 'and R "are two carbon atoms of the aromatic ring. These are the most commonly used polyimides because of their thermal stability.
Depending on the type of interaction between the main circuits:
- Thermoplastic
- Thermoset
Synthesis
Polyimides are usually obtained by polycondensation of tetracarboxylic acid dianhydrides — pyromellitic (benzene-1,2,4,5-tetracarboxylic), 3,3 ', 4,4'-tetracarboxydiphenyl, etc., with diamines; in the case of synthesis of heat-resistant polyimides, commonly used aromatic diamines (4,4'-diaminodiphenyl oxide, m-phenylenediamine, etc.). In the case of fusible polyamides, polycondensation can be carried out in the melt in one stage, however, in the case of heat-resistant and insoluble polyimides, a two-stage process is used. The first stage of polycondensation is carried out in solution, and the interaction of dianhydride (for example, pyromellitic acid dianhydride) with diamine synthesizes polyamic acid B , aliphatic amides (dimethylformamide, N, N'-dimethylacetamide, N-methylpyrrolidone) or The polyamic acid obtained in the first stage then cyclizes to polyimide C :
The cyclization of the amic acid into polyimide (imidization) can be carried out both thermally and chemically.
During thermal imidization, films, fibers or powder formed from polyamic acid are heated in a vacuum or inert atmosphere to 300–350 ° C, and water is split off and the imide cycle is closed; the degree of polymerization in this process usually decreases.
In chemical imidization, the polyamic acid solution is treated with a mixture of carboxylic anhydride (most often with acetic anhydride) and a tertiary amine (triethylamine, pyridine, etc.); the process proceeds at relatively low temperatures (20-100 ° C) and with preservation of the degree of polymerization of the initial polyamic acid .
Polyimides are also synthesized by the interaction of tetracarboxylic anhydrides with diisocyanates, the reaction proceeds with the release of carbon dioxide:
Properties
Thermosetting polyimides are known for their thermal stability, good chemical resistance, excellent mechanical properties, and have an orange / yellow color. Polyimides are reinforced with carbon or fiberglass to form composite materials. Thermosetting polyimides exhibit high tensile strength. These properties are maintained at a temperature of 232 ° C. Most polyimides do not affect commonly used solvents and oils, including hydrocarbons, esters, esters, alcohols, and freons. They are also resistant to weak acids, but their use in alkaline and inorganic acid environments, as well as hot water and steam is not recommended.
Application
- Thin (less than micron thick) polyimide films are used as optical windows in synchrotron radiation sources [2] .
- Self-adhesive tape in the form of rolls, for high-voltage and high-temperature insulation of electrical circuits. "Amber Scotch" "Kapton" is used for fixing moving conductors in electronic devices.
- Manufacturing of flexible printed circuit boards
- Inserts grippers for contact with hot glass, on glass production , instead of traditional bronze and graphite inserts. Modifications with a high content (40%) of graphite are used.
- High-speed sliding bearings operating under heavy loads and high temperatures up to +350 ° C. Modifications with the addition of graphite (15-40%) and fluoroplast-4 (PTFE) are used . PV factor can reach 12 MPa x m / s with dry friction on steel
- Piston rings (compression, guides) compressors operating without lubrication. Modification with the addition of 15% graphite
Notes
- ↑ Maréchal, E .; ES Wilks. Generic source-based nomenclature for polymers (IUPAC Recommendations 2001) (English) // Pure and Applied Chemistry : journal. - 2001. - Vol. 73 , no. 9 - P. 1511-1519 . - ISSN 1365-3075 0033-4545, 1365-3075 . - DOI : 10.1351 / pac200173091511 .
- ↑ Y. Tamenori, J. Synchrotron Rad. (2010), 17, 243–249