Secrets of the foundry binder system: the importance of resins and catalysts

The substances used for cohesion of components are resins, which can be of phenolic, polyurethane, epoxy, acrylic, alkaline or urethane type.

These resins are chosen based on production requirements and specific requirements for the cores or moulds of the foundry.

The mechanical strength of the sand castings depends on the type of binder used, which can be of organic or inorganic origin.

Among the organic binders we find synthetic resins, such as phenolic, urea or furan, while among the inorganic ones we can mention synthetic binders, such as sodium and potassium silicate, cement or gypsum, or natural binders, such as bentonite.

The hardening process takes place through the mixing of components, and varies depending on the type of binder used.

The binder system must have specific chemical and physical properties to ensure quality molding:

• Rapid polymerization reaction
• Regulation of hardening speed
• Mechanical strength at low temperatures
• Minimum gas development during casting
• High thermal stability
• Compatibility with subsequent use of sand
• Limited environmental impact both in molding and casting
• Mechanical or thermal recycling of sand
• Contained costs

The most commonly used are organic binders, as they allow for rapid production of molds and easy de-sanding through the combustion and thermal cracking of the polymer.

There are two different types of these systems:

• Self-hardening, composed of resin mixed with a liquid hardener. The hardening rate can be adjusted to allow processing of the mixture and obtain the desired shape.
• Hardening by gassing: the mixture is gassed with a gaseous catalyst after being inserted into the mold box.
• Thermal treatment hardening: a catalyst is introduced into the mixture that is activated with heating.

For the various chemical molding processes, the following technical terms are used:

• Work life or time: it is the period of time during which it is possible to mix, transport and compact the sand without compromising the final properties;
• De-molding time: time interval between pouring the mixture into the mold box and removing the mold. It is inversely proportional to the amount of catalyst used, which in turn reduces the work time. In the Hot-Box system, this effect can be balanced by increasing the temperature of the mixture, which has a more moderate catalytic effect;
• Maximum strength time: time required to reach the maximum mechanical strength of the aggregate from the time of mixing.

Phenolic resins

Phenolic resins are among the most widely used chemical binders, both for combustion curing systems and for gasification and thermal treatment curing systems. They are obtained through the condensation of phenol with formaldehyde. Depending on the type of catalyst, reaction temperature, and the molar ratio between phenol and formaldehyde, different types of phenolic resin can be obtained. Among the most common types used in foundry, there are resole, novolac, and benzyl types.

Phenolic resole resins are obtained through a condensation with molar ratios between phenol and formaldehyde between 1:2 and 1:3. They are produced under strongly basic conditions and are heat-curing liquid resins, soluble in alcohol and partially in water. They can be polymerized into Bakelite at the final bakelite stage with an acid catalyst at room temperature or heated at high temperature while kept in a basic environment.

Resoles are used in both the Hot-Box process and the Cold-Box process (even for self-curing systems with sulfonic acid).

Novolac resins are a type of resin obtained through a condensation in a strongly acidic environment, with a proportion of phenol and formaldehyde close to 1:0.8. The condensation reaction occurs rapidly, but due to the low proportion of formaldehyde used, a solid thermoplastic polymer is obtained with a melting point between 80°C and 120°C.

If a formaldehyde activator (urea) is added at high temperature (> 150°C), a heat-curing resin with high thermal stability is obtained. These resins are mainly used for the shell moulding process, which involves creating hollow cores by removing the non-hardened mixture “shell”.

Using less material allows for lower costs and improved sterrability.

Phenolic benzyl ether chemical binders are obtained through the condensation of phenol with formaldehyde in the presence of metalorganic catalysts such as lead, copper, nickel, cobalt, and zinc. 

The phenol to formaldehyde ratio is similar to that of novolaks, around 1:1. 

The condensation reaction is very slow and only occurs at high temperatures. These resins are used exclusively in Cold-Box polyurethane systems, where they polymerize in the presence of a second component, the isocyanate, through an addition reaction. 

Gas amines such as TEA and DMEA (Triethylamine and Dimethylethylamine) are used as catalysts. During moulding, the phenolic system releases phenol, formaldehyde, and any additives, so it is important to use phenolic resins with low levels of free monomers, i.e. free phenol less than 3.5% and free formaldehyde less than 0.5%.

In casting, phenolic resins generate high levels of sulfur dioxide emissions due to the use of highly acidic catalysts with high sulfur concentrations. The main advantages of this system are its long-lasting life, low cost, and thermal stability during casting, due to the presence of numerous aromatic rings. Therefore, they are used for heavy, high-thickness castings. The mechanical resistance of phenolic cores is not very high, with a sand of 55 AFS granularity and a dosage of 1.0% of resin, the strength reaches 240-260 N/cm2 within 24 hours. Additionally, sand agglomerated with phenolic resins has difficulties in terms of mechanical regeneration, as the resin does not easily detach from the sand. Furthermore, using a large amount of resin in molding results in a sand P.A.C. higher than the limits imposed for regenerated sand, and the use of strong acids causes higher residual acidity in regenerated sand, compromising its workability.

The phenolic resin solution (Part A) has a viscosity of 180 mPa.s at 20°C and a density of 1.05 kg/dm3. It is not soluble in water. The polyisocyanate solution (Part B) has a viscosity of 25 mPa.s at 20°C and a density of 1.10 kg/dm3. It is poorly miscible in water. It is recommended to use these components for the production of light castings in shell molding, using fine silica sand with an AFS 50-55 fineness, with a ratio of 100 parts by weight of sand to 0.50 parts by weight of resin part A and 0.50 parts by weight of resin part B. Depending on the needs of the core to be produced and the characteristics of the sand used, the components are selected, and there are different variants available.

The impact of the type of resin used on the lifespan of the mixture

The lifespan of the mixture is the maximum period within which the sand-resin combination must be used without significant changes in the properties of the castings produced; this is influenced by high temperatures, prolonged times before use, the acid requirements of the sands, and the type of resin used.

In the Cold-Box phenolic-urethane process, it is crucial to maintain a constant temperature of the sand-resin mixture throughout the year, as:

• Too low temperatures of the mixture (below 10°C) cause an increase in the viscosity of the resins (especially resin A) and poor homogeneity of the mixture itself, which can cause insufficient fluidity and increased use of the amine.
• Elevated mixture temperatures above 35°C cause significant problems with reducing the lifespan of the mixture and produce castings with lower and weaker mechanical strengths, leading to defects that cause casting rejections.

Furan resins

Furan resins are composed of the chemical reaction between furfuryl alcohol and formaldehyde, which is activated in an acidic environment or with the use of acidic catalysts and heat. The most commonly used catalyst is sulfonic acid. The condensation reaction takes place with furfuryl alcohol/formaldehyde ratios between 5:1 and 2:1. These resins have low viscosity and high storage stability.

The advantages of furan resins are high cold mechanical resistance, favorable demolding times, high stiffness of the aggregate, good sterrability, low odor in molding, and the use of weaker acids compared to phenolic resins, which allows good mechanical regeneration.

However, being highly reactive, the furan system requires special attention for the bench life of regenerated and hot sands.

Therefore, it is suitable for the production of medium-small size castings.

To catalyze the Hot-Box process, strong acidic ammonium compounds are used, such as ammonium sulfate, ammonium nitrate, etc.

Taking ammonium nitrate as an example, its acidic behavior is determined by the equilibrium: NH4NO3 -> NH4+ -> NO3-.

By heating the salt solution, the equilibrium shifts towards the formation of nitric acid: NH4NO3 + H2O -> NH4OH + HNO3. This occurs because ammonia (NH3), being a gas, easily leaves the reaction system, especially if the temperature is high (200 ÷250 °C).

The nitric acid that remains in solution is the true catalyst of polymerization and can then proceed in very short times.

The ammonium salt solution must be kept at room temperature in a closed container to avoid reducing bench life.

During the summer season, basic additives are added to block polymerization, these reach boiling temperature and are removed from the mixture when they come into contact with the hot mold.

During molding, furanic resins generate emissions of furfuryl alcohol and formaldehyde, so it is important to use resins with low levels of formaldehyde residual monomers.

Additionally, during the casting process, emissions of sulfur dioxide are generated due to the use of acidic catalysts that contain sulfur.

The furanic resin polymer is characterized by high rigidity, which makes it easy to detach from the sand during mechanical regeneration.

Compared to the phenolic system, it allows the use of a lower amount of resin and less aggressive acids, which results in a lower residual acidity of mechanically regenerated sand.

Urea resins

Resins obtained from the combination of urea and formaldehyde are called urea resins.

Their polymerization takes place through a condensation process in an acidic environment or with the use of organic catalysts and heat.

The proportion of formaldehyde and urea used varies between 2:1 and 4:1, determining a water-soluble liquid resin with viscosity varying depending on the proportion used.

Such resins are often used in binder blends to reduce costs.

Every binder presents a specific environmental impact during formation, mainly due to the presence of unreacted monomers (such as formaldehyde, phenol, etc.) and any solvents or additives.

Typically, polymers have limited toxicity and a contained odor due to the large size of the molecules, which reduces volatility.

Phenolic polyurethane resins contain free phenol and organic solvents used to dilute them due to high viscosity.

The isocyanic part using MDI (methylene diphenyl isocyanate) is odorless, so the odor is caused solely by the phenolic resin solvent.

Commonly used solvents contain polycyclic aromatic and aliphatic hydrocarbons, polar carboxylic acid esters, plasticizers, fatty acid esters, and organic silicates.

Recently, efforts have been made to reduce the emission of odorous compounds or Benzene-Toluene-Xylene (BTX) produced by solvents. These have often been banned due to the potential emission of BTX. However, in this context, it must be kept in mind that most sources of BTX are made up of phenol-formaldehyde and isocyanate resins, which represent about 70% of a Cold-Box mixture.

The production of BTX substances is therefore inevitable due to the basic chemical structure of the system, regardless of the solvent formulation. In addition, high temperatures and the reducing effect of a cast iron jet could cause the recombination of apparently harmless organic substances into benzene derivatives.

To improve environmental impact and economic efficiency, an effective strategy is optimizing binder performance by increasing its reactivity while simultaneously reducing the amount used. By using new resin synthesis methods and specific solvent combinations, binders with higher efficiency compared to traditional systems can be designed. The mechanical properties of cores produced with these systems are similar to those of standard products, despite the reduction in binder quantity (up to 25%). However, the increase in reactivity can cause higher immediate strength but may limit formability.

Organic binders also have limitations during casting as they are subject to solvent evaporation, combustion and thermal cracking, causing the production of gases and fumes that must be extracted and treated. These environmental and high-temperature stability problems can cause defects in the castings, such as porosity, blowholes and geometric deformations. Additionally, with the number of castings made, the mould must be cleaned manually due to the build-up of pitch on its surface, which otherwise prevents proper cores 

The transporter must have an ADR license for transport and have qualified vehicles and drivers.

To avoid errors in resin or catalyst transfer systems, it is important to use different shape couplings.

Products supplied in small tanks or drums must be clearly identified (for example with different colours).

In the storage warehouse, which must be well ventilated, it is necessary to separate the different resin qualities and place them in tanks to avoid contaminations or leaks into the environment. In the case of drums positioned outside, it is advisable to position them with the cap down to prevent water infiltration (the resin does not leak out due to its density).

Each product must be accompanied by a safety data sheet that provides information on how to act in case of emergencies.

Particular attention must be paid to catalysts, as some are regulated by laws and can be corrosive or flammable. The warehouse must be well protected, ventilated and equipped with antiexplosive electrical systems. The catalysts must also be accompanied by safety data sheets.

Regarding quantities and storage standards, reference must be made to local regulations.

The products must be stored at the temperature indicated on the safety data sheet and check the expiration date, always finishing the previous supply before moving on to the newly arrived one.

The handling of materials and the responsibility for storage must be managed by qualified and assigned personnel.

Granular materials can be new or recycled, angular or rounded in shape, with different grain sizes, but must be absolutely dry and free of dust.

• The rounded grain shape provides better mechanical properties compared to the angular shape with the same resin percentage.
• Angular grain materials require a larger amount of resin as their surface area is greater compared to rounded grain.
• The resin percentage must be increased if the grain size of the granular materials is finer or if there is dust present.
• Moisture is a determining negative factor. It can be present in the granular materials, in the blast air, in the washing air, or in the storage of the cores. Absence of moisture is crucial not only to obtain cores with good mechanical properties and an acceptable storage time, but also to avoid defects in the fusion of the cores where they are used.
• The optimal temperature range to use the sand is between 20 and 25°C, however, with adequate precautions in production, it can be tolerated between 10 and 30°C.
• Low sand temperature slows down the reactivity of the mixture and makes it less flowable.
• On the other hand, with hot sand during the mixing phase, solvent evaporation from the resins can occur, causing product deterioration and poor mechanical resistance of the core.
• The sand-resin mixture must be used as quickly as possible, avoiding leaving it in the mixers or on the conveyor belts as it can harden, accumulate moisture, be contaminated by dust and compromise bench life.
• Once the sand-resin mixture has reached the end of its bench life, it is no longer usable and the produced cores will not have the required characteristics.
• It is important that the mixer can be programmed for the amount and type of mixture required for each single core shot in the case it feeds more machines.

There are various types of additives that can be used in the Cold-Box polyurethane process:

• Mineral and wood flour blends: these are powder additives used to eliminate fins in iron castings. Used at 1.0-1.5% on sand, they can improve heat resistance.
• Red iron oxide: used at 0.25-3% on sand, it reduces porosity and erosion defects. A purity grade of over 80% Fe2O3 is recommended.
• Black iron oxide: used at 0.25-3% on sand, it prevents gas defects and, due to its low fines content, it does not require an increase in aggregate. The purity grade of Fe3O4 is also important.
• Powders of different nature, used for the same purposes as above: used at 3-8% on sand, they improve the compactness of the core reducing sintering and penetration. In some cases, they can replace the coating.
• Inhibitors: these are additives necessary with some aluminium and magnesium alloys to inhibit the reaction of air with magnesium. The additions to the sand are generally between 0.1 and 1% in the case of potassium fluoborate and between 0.25 and 0.50% in the case of sulfur.