Condensation Polymeriation Product

Phenolic resin designate a group of synthetic resins that are probably the most varied and versatile that we know. That may be made from almost any phenolic body and an aldehyde. Phenol Formaldehyde resins constitute by far the greatest proportion, but phenol furfural, resorcinol formaldehyde, and similar resins are also included in this group. The product obtained depends primarily on the concentration and chemical nature of the reactants, the nature and concentration of the catalyst used, the temperature and reaction time, and the modifying agents, fillers, and extenders. The initial reaction between the phenol and a mixture of cresols with formaldehyde, using an alkaline catalyst, produces benzyl alcohols.

C₆H₅OH + CH₂O  ------► o- or p- C₆H₄OH . CH₂OH

Simultaneously, additional formaldehyde may react to produce both di-and trimethylol phenol. These alcohol continue to condense and polymerize and polymerize with each other rapidly and almost violently.

These resins, when classified according to the nature of the reaction during their production, are one of two fundamental types.

1. One-step resins. In these, all the necessary reactants (phenol, formaldehyde, catalyst) required to produce a thermosetting resin are charged into the resin kettle in the proper proportions and react together. An alkaline catalyst is used. The resin, as discharge from the kettle, is termosetting or heat reactive and required only further heating to complete the reaction to an infusible, insoluble state.
2. Two step resins. Only part of the necessary formaldehyde is added in the kettle is making these resins, and an acid catalyst is used. They are permanently fusible or thermoplastic when discharged from the kettle but react with aditional formaldehyde to produce a thermosetting resin. This additional formaldehyde is furnished by "hexa" (hexamethylenetramine). Both one and two step resin are used separately or in combination in commercial molding materials. Both types are believed to polymerize to similar end products.

The phenol and formaldehyde are placed in the reaction kettle with the catalyst (sulfuric acid) and heated 3 or 4 h at a temperature of 140 to 163oC. During condensation, reaction water is eliminated and forms the upper of two layers. This water of reaction is removed under vacuum without the addition of heat.

The warm, dehydrated, viscose resin is run out of the kettle into shallow trays and allowed to cool and harden. The cooled, brittle resin is crushed and finely ground and becomes the resin binder for molding phenolic resins.

Phenolic molding compounds are molded primarily in compression and transfer molds. The powder, mixed with filler, lubricant and plasticizer, is further reacted on steam heated, rools, cooled and ground. In compression molding the powder is placed in hardened steel molds at temperature of 132 to 182oC and at pressure from 13.8 to 35 MPa.

In transfer molding the thermosetting material is subjected to heat and pressure in an outside chamber, from which it is forced by means of a plunger into a closed mold where curing takes place.