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| ECHA InfoCard | 100.105.516 |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Phenol formaldehyde resins (PF), also calledphenolic resins or phenoplasts,[1] are synthetic polymers obtained by the reaction ofphenol or substituted phenol withformaldehyde. Used as the basis forBakelite, PFs were the first commercial synthetic resins. They have been widely used for the production of molded products includingbilliard balls, laboratory countertops, and as coatings andadhesives. They were at one time the primary material used for the production ofcircuit boards but have been largely replaced withepoxy resins andfiberglass cloth, as with fire-resistantFR-4 circuit board materials.
There are two main production methods. The acid-catalyzed reaction of phenol and formaldehyde, with the phenol in excess quantity, forms a prepolymer known asnovolac which can be moulded and then cured with the addition of more formaldehyde and heat.[2][3] The other method has instead the formaldehyde in excess and uses a base as catalyst, forming a prepolymer known asresole. There are many variations in both production and input materials that are used to produce a wide variety of resins for special purposes.
Phenol-formaldehyde resins, as a group, are formed by astep-growth polymerization reaction that can be eitheracid- orbase-catalysed. Since formaldehyde exists predominantly in solution as a dynamic equilibrium of methylene glycololigomers, the concentration of thereactive form of formaldehyde depends on temperature and pH.
Phenol reacts with formaldehyde at theortho andpara sites (sites 2, 4 and 6) allowing up to 3 units of formaldehyde to attach to the ring. The initial reaction in all cases involves the formation of a hydroxymethyl phenol:
The hydroxymethyl group is capable of reacting with either another free ortho or para site, or with another hydroxymethyl group. The first reaction gives amethylene bridge, and the second forms anether bridge:
The diphenol (HOC6H4)2CH2 (sometimes called a "dimer") is calledbisphenol F, which is an important monomer in the production ofepoxy resins. Bisphenol-F can further link generating tri- and tetra-and higher phenol oligomers.
Novolaks (or novolacs) are phenol-formaldehyde resins with a formaldehyde to phenol molar ratio of less than one. In place of phenol itself, they are often produced from cresols (methylphenols). The polymerization is brought to completion using acid-catalysis such assulfuric acid,oxalic acid,hydrochloric acid and rarely,sulfonic acids.[4] The phenolic units are mainly linked by methylene and/or ether groups. The molecular weights are in the low thousands, corresponding to about 10–20 phenol units. Obtained polymer is thermoplastic and require a curing agent or hardener to form athermoset.
Hexamethylenetetramine is a hardener added to crosslink novolac. At a temperature greater than 90 °C, it forms methylene and dimethylene amino bridges. Resoles can also be used as a curing agent (hardener) for novolac resins. In either case, the curing agent is a source of formaldehyde which provides bridges between novolac chains, eventually completely crosslinking the system.[2]
Novolacs have multiple uses as tiretackifier, high temperature resin, binder for carbon bonded refractories, carbon brakes,photoresists and as a curing agent forepoxy resins.
Base-catalysed phenol-formaldehyde resins are made with a formaldehyde to phenol ratio of greater than one (usually around 1.5). These resins are called resoles. Phenol, formaldehyde, water and catalyst are mixed in the desired amount, depending on the resin to be formed, and are then heated. The first part of the reaction, at around 70 °C, forms a thick reddish-brown tacky material, which is rich in hydroxymethyl and benzylic ether groups.
The rate of the base-catalysed reaction initially increases withpH, and reaches a maximum at about pH = 10. The reactive species is the phenoxide anion (C6H5O−) formed by deprotonation of phenol. The negative charge isdelocalised over thearomatic ring, activating sites 2, 4 and 6, which then react with the formaldehyde.
Beingthermosets, hydroxymethyl phenols will crosslink on heating to around 120 °C to form methylene and methyl ether bridges through the elimination of water molecules. At this point the resin is a 3-dimensional network, which is typical of polymerised phenolic resins. The high crosslinking gives this type of phenolic resin its hardness, good thermal stability, and chemical imperviousness. Resoles are referred to as "one step" resins as they cure without a cross linker unlike novolacs, a "two step" resin.
Resoles are major polymeric resin materials widely used for gluing and bonding building materials. Exterior plywood,oriented strand boards (OSB), engineeredhigh-pressure laminate are typical applications.
When the molar ratio of formaldehyde:phenol reaches one, in theory every phenol is linked together via methylene bridges, generating one single molecule, and the system is entirely crosslinked. This is why novolacs (F:P <1) do not harden without the addition of a crosslinking agents, and why resoles with the formula F:P >1 will.
Phenolic resins are found in myriad industrial products. Phenoliclaminates are made by impregnating one or more layers of a base material such as paper,fiberglass, or cotton with phenolic resin and laminating the resin-saturated base material under heat and pressure. The resin fullypolymerizes (cures) during this process forming thethermoset polymer matrix. The base material choice depends on the intended application of the finished product. Paper phenolics are used in manufacturing electrical components such as punch-through boards, in household laminates, and inpaper composite panels. Glass phenolics are particularly well suited for use in the high speedbearing market. Phenolic micro-balloons are used for density control. The binding agent in normal (organic) brake pads, brake shoes, and clutch discs are phenolic resin.Synthetic resin bonded paper, made from phenolic resin and paper, is used to make countertops. Another use of phenolic resins is the making ofduroplast, famously used inTrabant automobiles.
Phenolic resins are also used for making exterior plywood commonly known asweather and boil proof (WBP) plywood because phenolic resins have no melting point but only a decomposing point in the temperature zone of 220 °C (428 °F) and above.
Phenolic resin is used as abinder inloudspeakerdriver suspension components which are made ofcloth.
Higher endbilliard balls are made from phenolic resins, as opposed to the polyesters used in less expensive sets.
Sometimes people select fibre reinforced phenolic resin parts because theircoefficient of thermal expansion closely matches that of the aluminium used for other parts of a system, as in early computer systems[5]andDuramold.
The Dutch painting forgerHan van Meegeren mixed phenol formaldehyde with his oil paints before baking the finished canvas, in order to fake the drying out of the paint over the centuries.[citation needed]
Atmospheric re-entry spacecraft use phenol formaldehyde resin as a key component in ablative heat shields (e.g.AVCOAT on the Apollo modules). As the heat shield skin temperature can reach 1000-2000 °C, the resinpyrolizes due to aerodynamic heating. This reaction absorbs significant thermal energy, insulating the deeper layers of the heat shield. The outgassing of pyrolysis reaction products and the removal of charred material by friction (ablation) also contribute to vehicle insulation, by mechanically carrying away the heat absorbed in those materials.
Phenol-formaldehyde is degraded by the white rot fungusPhanerochaete chrysosporium andRhodococcus phenolicus.[8][9]