Phenol formaldehyde resin
Phenolics ( DIN-Abbreviation: PF ) are thermoset plastics, which are made based on phenolic resins by curing. Phenol resins ( PF- resins, phenol -formaldehyde resins) are synthetic resins ( thermosetting resins ) which are prepared by polycondensation of phenols and aldehydes. Important starting materials for the preparation of the resins are phenol and formaldehyde.
History
Phenolic resins are one of the first plastics. The first produced on a large scale phenoplast is the 1907 Leo Hendrik Baekeland invented by phenol -formaldehyde condensation resin, which was marketed under the trade name Bakelite and used for decades in many areas. Phenolic resins because of their temperature resistance, surface hardness and low price today the most important thermosetting plastics.
Production
Phenolic resins consist of cured phenolic resins which are obtained by the synthesis of phenols with aldehydes. In addition to phenolic compounds such as 3 -cresol, 3,5-xylenol or resorcinol are used.
By an electrophilic substitution of phenol precursors are formed, which carry a function of the amount of formaldehyde used one to three hydroxymethyl group ( - CH2 -OH). The substitution is carried out only in the ortho or para position of the phenol:
It forms o- hydroxymethylphenol (1 ) and p- hydroxymethylphenol (2). When an excess of formaldehyde under basic conditions and the compounds with up to three hydroxymethyl groups can form. By catalyzed polycondensation of these phenol derivatives, the resins form. Depending on the desired result, the precondensates are then combined with an acidic or basic condensation agents.
Novolaks: In an acidic environment formed from the phenyl alcohols by condensation via methylene group ( -CH2-) linked oligomers, called Novolaks. Formaldehyde and phenols are converted or less formaldehyde in the ratio 4:5:
Novolaks are semi-liquid or even fusible. They are storage stable, ie not self-curing. With formaldehyde donors such as hexamethylenetetramine curing novolaks at temperatures above 120 ° C to infusible thermoset compositions of.
Resols: With basic condensing agents on the other hand form fusible and soluble in many solvents resins resole. Due to the most frequently used larger amounts of formaldehyde ( to 2.5:1 ) next to methyl groups and ether groups are formed.
Resols tend to self cure by further condensation and form the intermediate resitol. If the pre-condensates heated, obtained with further elimination of water crosslinked polymers infusible and insoluble final stage, the Resit.
After curing of a resin to form closely -linked polymers, since the phenol groups are connected to one another by up to three methyl groups.
Properties
- Density: 1.30 ... 1.45 g / cm ³
- Hard, very resistant to breakage
- Yellow to brown; darkens on exposure to light
- Only machining possible
- Brennprobe: usually flame resistant; yellowish flame; easily sprayed sparks; Tears and bursts material cracking and charred; Smell of phenol and formaldehyde
Use
Resin moldings: Fillers such as wood flour, carbon black, graphite, silica sand, glass dust and textile fibers, the phenolic resins obtained more substance and a greater strength. The resins together with the additives, molding compounds and used in the molding process for the production of stable, heat-resistant and relatively heavy plastic parts. With small amounts of additives and molding methods are possible. With fillers molded products, such as handles and housing parts heat- stressed household and electrical appliances, billiard balls, bowling balls are made .
Laminates: For the production of composite materials wood, paper or fabric sheets are superimposed in several panels, soaked in thin liquid phenolic resin and pressed. At temperatures above 150 ° C, the material hardens. Resulting products include hard paper and printed circuit boards made therefrom, hard bands for machinery parts and insulating, laminated densified wood ( treated wood ). Phenolic resin - fiber composites with carbon fibers serve as heat shields with high-temperature resistance for aircraft and rocket construction.
Pure phenolic resins are used, inter alia for the production of coatings, adhesives, fillers and foams.
Trade names
Aramith, bakelite, Erinoplast, Catalin, Pertinax, GESADUR, Plastacart, Plastaflex, Prestofol, Linax, Novotex, Resinol, Supraplast, Ruwatex, Prefere, Vyncolit, Phenodur, Uravar