1. INTRODUCTION

Thermal protection materials are designed to slow the process of combustion heat transfer to the structural element, delaying the effect of temperature variation on its resistance. According to their nature, they can compose an insulating barrier, reducing the heat transfer rate to the protected element. There are also protective materials that develop chemical reactions due to rising temperatures. These reactions can absorb some of the energy that would be aimed at the structural component and/or form material with insulating features.

Heat transfer occurs by physical means via conduction (static means) and convection (moving means). When it does not take place by physical means, heat transfer occurs by radiation, where energy is transferred by electromagnetic waves. A fire protection material can act as a physical barrier, reducing the energy transfer rate. As an example, there are insulating materials such as masonry.

Another possibility is the use of a material that, when exposed to high temperatures, undergoes chemical reactions that absorb a significant portion of the energy that would be aimed directly at the structure and/or lead to forming a new thermal protection interface. For example, there is plaster, which due to its ability to maintain and chemically release water, is used as a fire protection material. When heated, the existing chemical bonds in the hydrated plaster begin to break, releasing hydration water. This reaction absorbs energy from the fire that would be conducted to the structural element, thereby forming a thermal barrier. Through this layer, the protected material remains at a constant temperature around 100ºC.

As for intumescent paint, protection materials discussed in this paper, they are chemical compounds which, when subjected to elevated temperatures, undergo various reactions that form a charred foam with high insulation performance.

The advantages of this protection are:

1. They present many of the desirable features of traditional decorative paint as different colors, good surface finish and durability;
2. They do not take up space and load increase is insignificant from a structural point of view;
3. Although they demand experience and strict quality control, their application is simple and does not require attachment to the structure, in addition to easy maintenance properties;
4. They can be used in the protection of structural connecting areas;
5. They do not modify the intrinsic properties of the substrate (as mechanical features). Therefore, they can be applied to existing structures without loss/modification of structural capacity.

2. Chemical composition

In the field of structure engineering, intumescent paint is usually associated as protection material for steel structure under fire conditions, delaying the loss of strength of structural elements. However, intumescent compounds are employed also in the protection of flammable materials and even plastic.

Regardless of their area of use, intumescent compounds basically have the same chemical composition. According to Troitzsch, intumescence is obtained by the following components:

• - acid source: usually, it is the salt of a non-volatile inorganic acid, such as boric, sulfuric or phosphoric acid. The most used are the salts of phosphoric acid, such as ammonium phosphate and ammonium polyphosphate, which release their acid at temperatures above 150ºC. The resulting acid initiates the first of a series of chemical reactions, beginning with dehydration of carbonaceous compounds and their subsequent charring.
• - carbonaceous compound: it is a compound with many hydroxyl radicals (-OH) that dehydrates when subjected to etching through a reaction of esterification and carbonized. Frequently used compounds are pentaerythritol, amide, and urea-formaldehyde or phenolic resins.
• - foaming compounds: for this purpose, compounds such as chlorinated paraffin, melamine or guanidine are used. Under the influence of heat, they release large quantities of non-flammable gases such as hydrogen chloride (HCl), ammonia gas (NH3) and carbon dioxide (CO2), generating a foam with an aspect of carbonized material on the substrate. Decomposition products of these materials (e.g., chlorinated paraffin residues) often contribute to further carbonization of the carbonaceous compound.
• - binding resins: responsible for involving the gases, preventing their dispersal. They must not harden. Instead, for better performance, they must have thermoplastic characteristics. An example of highly recommended materials are chlorine-based rubbers, which soften and melt when exposed to high temperatures and, simultaneously, aid the expansive agent forming HCl, besides contributing to carbonization.

Camino et al cites a 1948 American patent record to present chemical composition of an intumescent compound. According to the record, in the formulation of intumescence, the carbonaceous compound is the source of carbon to form carbonized foaming that involves the gases. Vandersall apud Camino et al [10] classified the chemical components of the intumescent systems into four categories:

1. Inorganic acid, free or originated due to the rise in temperature to 100 - 250ºC;
2. Polyhydric compound rich in carbon;
3. Organic amine or amide;
4. Halogen compound (usually formed by fluorine or chlorine).

For intumescence to develop, a series of chemical and physical processes must occur in the proper sequence, as the temperature is raised. The carbonaceous compound should not decompose or volatilize before the acid is available to dehydrate it. Also, gases responsible for foaming must be developed through small bubbles dispersed in the carbonized mass, resulting in an aerated compound. This process requires a suitable gas development rate and viscosity of the formed mass. The viscosity of any fluid comes from its internal friction, which originates from the attractive forces between relatively close molecules. Thus, the smaller the intermolecular forces, the lower the viscosity. If the formed mass has low viscosity, the gases will escape, resulting in a poor foam whose surface is filled with breaks. However, if the viscosity is too high, the intumescence does not develop.

In general, carbonized foam has gas bubbles with diameters between 20 and 50 micrometers and wall thickness from 6 to 8 micrometers. Sometimes inert fine aggregates are added, such as titanium dioxide and silica, to allow greater control over the size of the bubbles formed. Other elements are added to intumescent compounds to increase their mechanical and thermal performance. For example, the addition of vitrification agents, such as borates and mineral fibers, increases resistance to physical impact from air currents during fire..

3. Chemical reactions of intumescent systems

As previously mentioned, in intumescent systems, in general, carbonization occurs by the interaction between the inorganic acid and a polyhydric compound. Cellulose is the polyhydric compound whose carbonization reaction by inorganic acids have been most studied, since it is the most abundant naturally available organic material and also for being a major fuel source for fire. These studies provided information on the acidic dehydration mechanism, which can be assumed to be comparable to other polyhydric compounds with similar structure to cellulose. However, the structure of Pentaerythritol is quite different from cellulose and thus, one can't correlate the carbonization processes.

Vandersall apud Camino et al reports that, with a gradual increase in temperature, dipentaerythritol - ammonium polyphosphates - soften and melt at 215ºC, staying with clear aspect to 238ºC, when gas development starts. Then the mass darkens and hardens at 360ºC. Therefore, the following sequence is suggested:

1. Phosphate breakdown at 215ºC;
2. Subsequent esterification of alcohol, forming water;
3. Solidification of phosphorus-carbon carbonized foam at 360ºC.

Next, the chemical reactions according to Troitzsch [9] are presented, for the intumescent mechanism of ammonium dihydrogen phosphate, Pentaerythritol and chlorinated paraffin. The first step takes place at temperatures between 150 and 215ºC - decomposition of the inorganic salt producing ammonia gas and phosphoric acid, according to Equation 1.

NH₄H₂PO₄→ΔNH₃+H₃PO₄NH₄H₂PO₄→∆NH₃+H₃PO₄ (1)

At a slightly higher temperature, the acid formed reacts with the carbonaceous compound, esterifying the polyhydric compound. This reaction is shown in Equation 2 and may be catalyzed by amines and amides.

C₅H₈(OH)₄+H₃PO₄→C₅H₈(OH)₄.H₃PO₄C₅H₈(OH)₄+H₃PO₄→C₅H₈(OH)₄.H₃PO₄(2)

With temperatures ranging from those presented in the first step and during esterification, the binder partially melts.

Between 280 and 350ºC phosphoric ester breaks down, according to Equation 3 - the mixture melts and bonds are broken, forming acid, water and carbon residue.

C₅H₈(OH)₄.H₃PO₄→ΔH₃PO₄+H₂O+CC₅H₈(OH)₄.H₃PO₄→∆H₃PO₄+H₂O+C (3)

At the same time the reaction occurs in Equation 4, where the compound responsible for providing gases that will inflate the mixture decomposes, releasing gases (such as hydrogen chloride or hydrochloric acid) inflating the melted mass.

CnH(2n+1)Cl→HCl+CCnH(2n+1)Cl→HCl+C (4)

The binder resin, softened, forms a thin layer on the mixture, preventing the gases from dissipating. As temperature is elevated, the viscosity of the foam increases. Because of crosslinking and carbonization, the formed foam solidifies, forming a highly porous material. At temperatures above 600ºC occurs thermal decomposition and/or oxidation of the carbonized mass.

When compared to the original thickness, the intumescent layer is about 50 to 100 times larger, forming a thermal barrier that protects the substrate from heat influence and decomposing.

These are general information about ntumescent paint, we hope readers have more useful information for studying and research. Mega Vietnam Chemicals Co., Ltd is providing sufficient materials for this product line, if you have any questions please contact us.

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