Synthesis of a Phenol-Formaldehyde ~hermosettingPolymer Y. B k l a r d and B. R i d Wpattement des sciences du bois, Universite Laval, Ste-Foy, PQ, Canada G I K 7P4 Thermosetting polymers, sometimes called resins in t h e industry, can be found everywhere in our environment (shoe soles, electric appliances parts, light switches, foundry refractory brick, and adhesives). Thermosetting polymers, a s their name implies, polymerize, in a mostly irreversible manner, with t h e application of heat. One of t h e oldest thermoset known, discovered by Leo Bakeland in the early 1900'8,' is phenol-formaldehyde or PF, also known a s a novolac (acid cured) or resol (base cured). PF are widely used a s adhesives in the wood composite (particleboard or plywood) i n d u ~ t r y . ~ If one examines, for instanceaplywood sheet, it ispossibleto see the adhesiveas a thin brownline betweeneach successive veneer sheet. T h e following procedure allows for the synthesis of a 50% aqueous solution of a prepolymer, which can be used to bind wood or other materials together, with proper application of heat and pressure. There are two steps in the formation of t h e polymer. T h e first is the methylolation, where formaldehyde adds to the phenol
+
C6H50H CH,O
-
C,H,OH(CH,OH)
(1)
to form methylolphenol, i n a n exothermic reaction. T h e second sten involves condensation of phenol with a methylolphenol "nit with the production of water molecule to form a methylene bridge between the unsaturated rings:
+
CsH50H C6H,0H(CH20H)
-
(C,H,OH),CH,
+ H20
(2)
This reaction is endothermic. This last molecule may also be methylolated again b y formaldehyde and form new methylene bridges with others. T h u s a polymer is formed. Since all three orcho and para sites on the phenol molecule are susceptible t o react, pushing reaction 2 t o its limit, i.e., continuing t o heat t h e mixture for toolonga time, will result in the resin curing, t h a t is, forming a gel, and then a glass, preferably outside (!) the reaction flask, since the cure reaction is irreversible,due t o the nearly inf;nite viscosity of t h e final resin. Materials Standard 1WO-mL 3-neck flask (or, better, 4-neck) Mechanical stirrer Thermometer Addition separatory funnel Condenser Heating mantle Paraformaldehyde Phenol (technical grade, may be in solution) NaOH 50%wlv water solution NHIOH 30% w/v water solution Safety Precautions All work is to be done under a fume hood, with workers wearing rubber gloves and safety glasses. Prolonged and repeated exposition to formaldehyde or its polymeric form, paraformaldehyde, may result in throat, eye, and skin irritation. Formaldehyde is mutagenic
' Gillis, J.: Oesper, R. E. J. Chem. Educ. 1984, 41, 224.
Chiu, S.-T. U S . Patent, 4, 433, 120, 21 Feb. 1984 (filed 30 Sep. 1981.)
and a suspected carcinogen. Since the finished prepulymer aquwur rein may contain l 4 . I a c free formaldehyde, the aame preraurions also apply to it. Phenol has low vapor pressurr and RO does not constitute a serious respiratory hazard. However, it is readily absorbed through the skin, causing irritation. At high levels, such as 100 ppm in vivo, it has neurotoxic effects. Sodium hydroxide and ammonium hydroxyde are strong bases in the presences of water and are especially dangerous to the eye surface. If a spill occurs, remove contaminated clothing immediately and wash affected area with soap and large amounts of water until no evidence of chemical remains. Get medical attention immediately. Paraformaldehyde, phenol, sodium hydroxide, and ammonium hydroxide, in solid form, are stable but should be kept in sealed, air-tight containers. Setup Under a fume hood, set up a three-neck 100-mL flask, equipped with a condenser, a mechanical stirrer, and either a thermometer or addition separatory funnel, following the procedure outlined in the next paragraph. A heating mantle (bottom part only necessary) should be used to warm the solution. It is not recommended to use a magnet-type stirrer, since the solution may become too viscous for efficient stirring. Procedure The following will make 260 mL of resin, as a 30%onlutim in water.The formaldehyde phenol ratio (molar) a 2.2, giving the best thermosetting properties. In the flask add 120g of a 90%pbenollwater solution (108g phenol 12 mL water), add 176 gof a43.25 wlw suspension ofparaformaldebvde in water (76 e oaraformaldehvde and 100 mL water). A comkercial40%formaidehyde solution-maybe used as a substitute for the oaraformaldehvde. , . but we orefer to manioulate oaraformaldehyde. a polymerrzed form of furmaldehyde, a* it is safer. Add dropwise, over an approximately 10-min period, 16 g of a 5 F c w,v NaOH aqueuur solution with an addition separatory funnel. Heplace the funnel with a thermometer and stopper. Bring up the mixture to 65 "C and maintain for 65 min. It is important to monitor closely the temperature and adjust the heating accordingly, since the methylolation reaction at the beginning (when the temperature reaches annrax. 45 OC) is exothermic (the solution may actuallv .boil.. hence tb; presence ofthe condenser). After a few minutes, this readion eases up, and the polymerization, which is endothermic, carries on with gentle heating of the solution. At the stated 65 min, with the heating discontinued, and the mixture brought down to 30-40 'C, add 11mL of a 50% wlv solution of NaOH and 7 mL of a 30% wlv ammonium hydroxyde solution, with stirring. The finished resin is a translucent red liquid. It will keep for a few months in a sealed jar if kept in a refrigerator. Otherwise, its shelf life (called "pot life" in the industry) is limited to B few davs or weeks. Asanoptional prucedure, aresin with a higher viscosiry \and also faster setting) can be obtained by increasing the65 O C remperdrure level to up toB5T.The viricosity ofthe solution can be checked by withdrawing a few drops with a pasteur pipet and depositing a drop on a chilled watch glass held up vertically: a more viscous resin will stop flowing and eel rapidly. Working with a 85 'C temperature for more than 65 minis not recommended, since the resol may set in the reaction vessel.
+ ~
~
~. ~
Very few parameters generally characterizes the prepoweight percent solids, and time of cure: Ivmer: viscositv, DH, chose are, for
