Furan resin refers to

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

s produced from various

furan Furan is a Heterocyclic compound, heterocyclic organic compound, consisting of a five-membered aromatic Ring (chemistry), ring with four carbon Atom, atoms and one oxygen atom. Chemical compounds containing such rings are also referred to as f ...

compounds, of which the most common starting materials are

furfuryl alcohol Furfuryl alcohol is an organic compound containing a furan substituted with a hydroxymethyl group. It is a colorless liquid, but aged samples appear amber. It possesses a faint odor of burning and a bitter taste. It is miscible with but unstabl ...

and

furfural Furfural is an organic compound with the formula C4H3OCHO. It is a colorless liquid, although commercial samples are often brown. It has an aldehyde group attached to the 2-position of furan. It is a product of the dehydration of sugars, as occu ...

. In the resin and in the cured polyfurfurol, the furan rings are not connected by conjugation. The resins are generally used as binders for sand castings. The furan monomer is typically converted to a free-flowing

resin A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...

with mild acid catalysis. Curing is achieved using strong acid.

Types

The term furan resins is not used uniformly in the literature. For example, some authors refer to resins based on furfuryl alcohol and furan as furan resins, while othersMichael Biron: '' Thermosets and composites : material selection, applications, manufacturing, and cost analysis.'' Elsevier, Amsterdam, 2014, , S. 257–259. use the term only for resins based on furfuryl alcohol. In addition to homopolymers of the two starting materials, also copolymers comprising for example methanal,

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

phenol Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire. The molecule consists of a phenyl group () ...

are counted as furan resins.Arne Gardziella: '' Furanharze (FF).'' In: Wübrand Woebcken (Hrsg.): ''Kunststoffhandbuch'' (=''Duroplaste.'' Band 10). 2. vollständig neu bearbeitete Auflage, Hanser Verlag, München, 1988, , S. 70–84. Furfurylated wood is made by impregnating wood and then forming a furan resin.

Production

Furan resins based on

are produced by

polycondensation In polymer chemistry, condensation polymers are any kind of polymers whose process of polymerization involves a condensation reaction (i.e. a small molecule, such as water or methanol, is produced as a byproduct). Natural proteins as well as s ...

under the presence of weak acids. The polycondensation leads to various linear oligomers that differ on chainlength and linking between the furan units. The linking of the furan units via methylenbridge (–CH₂–) is predominating but the rings can be linked via dimethylenetherbridges (–CH₂–O–CH₂–) The etherbridges are unstable especially in very acidic environment. They can be converted to methylenbridges by release of

formaldehyde Formaldehyde ( , ) (systematic name methanal) is an organic compound with the chemical formula and structure , more precisely . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde. It is stored as ...

. To produce a storable resin, the reaction is interrupted by the addition of

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula . It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly corrosive base (chemistry), ...

. The products are brownish in colour and have a low to medium viscosity. They are stable at 40 °C for approximately 6 months. In a second step, the resins can be cured to a

thermoset In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...

either at room temperature by adding acids (e.g. ''p''-toluenesulfonic acid,

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...

) or at higher temperatures by adding latent curing agents such as

ammonium nitrate Ammonium nitrate is a chemical compound with the formula . It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, but does not form hydrates. It is predominantly us ...

. The reactions involved are complex and have been studied for a long time. Segments with conjugated double bonds are proposed to lead to

cross-link In chemistry and biology, a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...

ing.Mekki Choura, Naceur M. Belgacem & Alessandro Gandini: ''Acid-Catalyzed Polycondensation of Furfuryl Alcohol: ''Mechanisms of Chromophore Formation and Cross-Linking.'' In: ''Macromolecules.'' 29 (11), 1996, S. 3839–3850, doi:10.1021/ma951522f. Konjugation Polyfurfurylalkohol

Properties

Before curing, the properties of furan resins are similar to those of other curable resins. They can be used as

binders Ring binders (loose leaf binders, looseleaf binders, or sometimes called files in Britain) are large folders that contain file folders or hole punched papers (called loose leaves). These binders come in various sizes and can accommodate an arra ...

, are reactive to acids, thermally reactive and cross-linkable. Cured furan resins are resistant to attack by strong acids, bases, and halogenated hydrocarbons. They are attacked by oxidizing agents. Furan resins exhibit good thermal stability. Continuous use at 100-120 °C is routine. Some furan resins can even be used at up to 150 °C. Some grades are characterized by their low flammability and low

smoke emission Smoke is an aerosol (a Suspension (chemistry), suspension of airborne particulates and gases) emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrainment (engineering), entrained or otherwise ...

.Michael Biron: '' Thermosets and composites : material selection, applications, manufacturing, and cost analysis.'' Elsevier, Amsterdam, 2014, {{ISBN, 1-4557-3125-0, S. 86. They also have a high

strength Strength may refer to: Personal trait *Physical strength, as in people or animals *Character strengths like those listed in the Values in Action Inventory *The exercise of willpower Physics * Mechanical strength, the ability to withstand ...

References

Organic polymers Adhesives Synthetic resins Thermosetting plastics