Basic chemistry of Phenol Formaldehyde Resins : Part 1

 At present, there are many kinds of plastics used for myriad number of applications starting from FMCG to aerospace industries which has a significant impact on our daily lives. Generally, Plastics are classified into two major types according to their thermal behaviour.

Thermoplastics: Plastics materials which can melt upon heating and forms the required shape. If the material is heated , it can melt and form into different shapes again.

Thermosetting plastics: Plastics materials which can melt upon heating and forms the required shape. If the material is heated again, it cannot melt again since the material is crosslinked / cured.

The topic of interest for us is thermosetting plastics and there are various types of thermosetting plastics used commercially because of its unique properties.

Phenol Formaldehyde Resins (PFR) is the oldest plastic with the history of more than a century. After World War II, many kinds of plastics were developed and commercialized successfully. Phenolic resins have been growing steadily in the applications like moulding compound, foundry, refractory, abrasives, friction materials, composites, coatings etc. Refer below for the sources of differentiation in properties and their corresponding applications of PFR.

Sources of differentiation in properties	Application
Binding property	Binders in glass wool, wood, sand, metals and composites
Flammability Smoke and Toxicity (FST) property	Mines and Construction
Rich carbon content	Refractories and electrodes
Crosslinker	Crosslinking agent for epoxy resin, elastomers, printed circuit boards and foundry
Higher thermal resistance	Friction materials in brakes, bonded and coated abrasives
Chemical resistance	Filters in automobiles, photoresists and printers.

 

Chemical structure of Phenol Formaldehyde Resins

 

 Reference: Phenolic Resins: A Century of Progress by Louis Pilato

1.    Phenol reacts with formaldehyde under either acidic or basic conditions to form a versatile array of phenolic resins. Resoles are formed by the reaction of excess molar amount of formaldehyde (F/P>1) with phenol in the presence of base catalysts like alkali and alkaline metal hydroxides. Novolaks are formed by the reaction of excess molar amount of phenol with formaldehyde (F/P<1) in the presence of acid catalysts like Oxalic acid, HCL, Sulfuric acid etc.

2.    Before getting into the details on the chemistry of resoles and Novolaks, it is important to understand the reaction and hardening process of PFR.

 

Stage	Process	Application
A stage	Phenol and formaldehyde react to become resinous and at this stage resin is soluble in solvents	Resin manufacturers mostly supply the material in this stage
B stage	This is a middle stage between A and C, known as state of gel	Intermediate stage in the process in applications like impregnation, composites etc.
C stage	The state at which the resin is completely cured by heat or acid or alkaline. The resin becomes insoluble and thermally infusible in this stage.	End Product

Refer the Basic chemistry of Phenol Formaldehyde Resins : Part 2 for further information.

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