Chemical-physical, technological properties and related fields of application of thermosetting resins

Generally, a resin can be defined as an organic, solid or semi-solid product, of natural or synthetic origin, without a precise melting point and, generally, of high molecular weight.

Resins can be divided into:

• thermoplastics
• thermosetting

Thermoplastic resins are linear or branched polymers that can melt or soften without undergoing alterations in the chemical composition.

They can therefore be forged in any form using techniques such as injection molding and extrusion. The process of melting-solidifying the material can be repeated without making substantial changes to the performance of the resin. Generally thermoplastic polymers are amorphous and do not crystallize easily, following a cooling, since the polymer chains are very tangled.

Even those that crystallize never form perfectly crystalline, but semi-crystalline materials characterized by crystalline zones and amorphous zones. The amorphous resins, and the amorphous regions of the partially crystalline resins, show the phenomenon of the glass transition, characterized by the passage, sometimes quite abrupt, from the glassy to the rubbery state.

This transition coincides with the activation of some long-range movements of the macromolecules that make up the material. Below the glass transition temperature (Tg), the polymer chains are in blocked positions.

Both the melting temperature and the glass transition temperature increase as the rigidity of the chains making up the material increases and the intermolecular interaction forces increase.

Thermosetting resin is a very rigid material consisting of cross-linked polymers in which the motion of the polymer chains is severely limited by the high number of existing cross-links.

During heating they undergo an irreversible chemical modification. Resins of this type, under the action of heat in the initial phase, soften (become plastic) and then solidify. Unlike thermoplastic resins, therefore, they do not have the possibility of undergoing numerous forming processes during their use.

The thermosetting resins, as we have seen, are very rigid materials in which the motion of the polymer chains is strongly constrained by a high number of existing crosslinks.

In fact, during the production process they undergo irreversible chemical changes associated with the creation of transversal covalent bonds between the chains of the starting pre-polymers.

The density of the interconnections and the nature depend on the polymerization conditions and on the nature of the precursors: generally they are liquid systems, or easily liquefied by heat, consisting of organic compounds with low molecular weight, often multifunctional, chemically reactive, sometimes in the presence of initiators or catalysts.

In most cases they undergo a polymerization in situ by means of polycondensation and polyaddition reactions which transform them into thermosetting materials or into complex three-dimensional vitreous lattice structures, insoluble in the most common, infusible and degradable solvents when heated to very high temperatures.

Many formulations require the presence of a comonomer, generally defined curing agent, provided with two or more reactive functional groups, and / or heat and / or electromagnetic radiation for reticulate.

The cross-linking or cure reaction begins with the formation and linear growth of polymer chains that soon begin to branch. As the treatment proceeds, the molecular weight increases rapidly and the molecular dimensions increase because many chains begin to covalently bind to each other creating a network of infinite molecular weight.

The transformation from a viscous liquid to an elastic gel, called “gelification“, is sudden and irreversible and involves the formation of the original structure of the three-dimensional network.

Before gelation, in the absence of a cross-linking agent, the resin particles are separated from each other or interact only by virtue of weak reversible intermolecular forces, van der Waals forces.

Then the resin is soluble in appropriate solvents. As the cross-linking reaction progresses, covalent covalent bonds are formed, a covalent gel, while weak interactions remain. Unlike the secondary valence gel that can be broken without difficulty, there is no such strong solvent to cause the breakdown of covalent bonds.

Therefore the macromolecular structure created by this transformation does not completely dissolve but swells in the solvent because it still contains traces of monomer, free or aggregated, and soluble branched molecules, thus appearing in the form of a biphasic sol-gel system.

This is the original structure of the thermoset three-dimensional network.

Another phenomenon that can occur during the treatment reaction is the “vitrification“, ie the transformation of a viscous liquid or an elastic gel into a glassy solid, which marks a variation in the kinetic control of the reaction mechanism passing from a type chemical to a diffusive type.

The reaction rate decays rapidly both because the concentration of reactive monomer is decreased and because its diffusion towards the reactive sites of the polymer bulk is slowed down by the presence of cross-links between the chains.

However, the fact that there is a further increase in density shows that chemical reactions continue to occur but at much lower speeds.

Among the various types of thermosetting resins, there are epoxy resins, which are essentially polyethers, but retain this name on the basis of the starting material used to produce them and by virtue of the presence of epoxy groups in the material immediately before crosslinking. The main use of epoxy resins is in the field of coatings, as these resins combine flexibility, adhesion and chemical resistance properties.

A wide variety of resins are formulated to meet the most varied requirements taking into account the following parameters:

Reactivity: the epoxy group reacts with a wide variety of chemical reagents.

Flexibility: the distance of the epoxy groups can be varied according to the molecular weight, obtaining three-dimensional cross-linked systems with more or less wide meshes and therefore more or less flexible and elastic products.

Chemical resistance and adhesion: the predominant chemical bonds are carbon-carbon and carbon-oxygen, bonds with remarkable chemical inertia. The hydroxyls are secondary and therefore of low reactivity. The polarity of the molecules and the hydroxyls are due to the high forces of adhesion to metal substrates.

Thermal stability: closely linked to the density of cross-linking.

Applications: epoxy systems have taken on great importance in those sectors where high performance is required for thermal, mechanical, chemical and electrical stresses. They are used in the automotive, space, aeronautical, naval, electronic and plant engineering industries, as main components in paints, adhesives, waterproofing, composite materials and for printed circuits.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.