News

1 1、 The Core Role of Acrylic Polymer (Acrylic lotion) in Intumescent Fire Retardant Coatings

The classic system of expansion and fire protection: the ternary expansion system of acid source (APP ammonium polyphosphate)+carbon source (pentaerythritol)+gas source (melamine)+acrylic acid lotion (binder/film forming resin) determines whether it can be foamed; Acrylic lotion determines whether the foaming process can retain gas, form a continuous carbon layer, and assume the function of coating at room temperature.

1. Dry film stage at room temperature (without ignition)

1. Continuous film formation and bonding carrier

bonding APP, melamine, carbonization agent, and filler together to form a complete and well adhered solid coating; If there is no lotion, the powder is loose and cannot form a film. It is easy to pulverize when encountering water, and the flame retardant components migrate and separate out.

2. Provide physical and chemical properties

endow the coating with water resistance, weather resistance, flexibility, crack resistance, and decorative properties; The pure ternary powder system has no physical strength.

3. Stable formula system

lotion colloid maintains the suspension of pigment and flame retardant powder, and prevents the layered sedimentation of storage.

2. Thermal expansion stage (the core role of fire determines the expansion ratio and carbon layer mass)

High temperature melting forms a continuous viscoelastic melt film (most importantly). When the temperature rises to the range of 180-280 ℃, the acrylic resin softens and melts, forming a viscous liquid phase film with appropriate viscosity. Synchronous reaction of ternary system: APP releases polyphosphoric acid to catalyze dehydration and carbonization of pentaerythritol, and melamine decomposes to release NH3/CO ₂ inert gas.

Molten acrylic resin acts as a “envelope” for bubbles, enveloping the expanding gas and preventing it from escaping quickly; Gas trapped inside the molten system drives volume expansion.

Participate in the formation of carbon and reinforce the carbon layer skeleton. Acrylic resin undergoes thermal degradation, cyclization, and aromatization at high temperatures, contributing a portion of residual carbon and interweaving with carbon generated from pentaerythritol to form a three-dimensional continuous carbon skeleton; The carbon layer will not easily break, puncture, or fall off in large areas.

Adjusting the viscosity of the melt and controlling the foaming rate, the resin melt viscosity directly determines the upper limit of expansion: the melt is too thin → gas quickly breaks through and dissipates → low expansion, porous and large pores in the carbon layer, and extremely easy to burn through; The melt is too hard and has too high viscosity → the gas cannot push the melt → it is difficult to foam, and the expansion ratio is extremely low.

The process of polymer thermal decomposition continues to absorb heat, slightly delaying the temperature rise of the substrate.

3. High temperature carbon layer maintenance stage

After the expansion is completed, the resin gradually carbonizes further, supporting the sponge like insulation carbon layer; Prevent the collapse of the carbon layer under the erosion of flame airflow.

Summary: The ternary system consists of a gas source and a char forming reactant; Acrylic lotion is a foaming “airbag coating+carbon layer bonding framework”. Without proper acrylic lotion, no matter how excellent the ternary system is, it will not be high, and the carbon layer will dissipate as soon as it is burned.

2 2、 With the same formula and different acrylic lotion, the root cause of the difference in expansion ratio

The essence of expansion ratio: the balance between the gas generation rate and the gas holding capacity of the molten resin melt in a closed manner.

Different lotion have different structural parameters → high temperature melting behavior and thermal decomposition characteristics → foam retention ability varies greatly. Below are the core influencing factors (industrial selection is the most important):

1. Monomer composition (pure propylene/styrene propylene/silicon propylene, ratio of soft and hard monomers, the first major influencing factor)

① Styrene content (styrene propylene>pure propylene)

Styrene contains benzene rings, making it easier to undergo aromatization at high temperatures and resulting in a higher residual carbon rate; The melting range is wider, and the high-temperature melt is less prone to rapid cracking and thinning. The expansion ratio of suitable styrene acrylic lotion is generally higher than that of ordinary pure acrylic lotion. Full fatty pure propylene (without benzene ring) decomposes faster, the high-temperature melt quickly becomes thinner, the bubbles are easily broken, and the expansion is relatively low.

② The ratio of soft/hard monomers directly determines the resin softening range and melt viscosity

High Tg lotion (more hard monomer MMA): softening temperature is higher. If the softening temperature of the resin is greater than 260 ℃: the ternary system has already produced a large amount of gas foaming, but the resin has not yet melted and sealed, and the gas directly escapes, resulting in a low expansion ratio.

Low Tg, large amount of soft monomers (such as 2-ethylhexyl acrylate): early melting at low temperatures, but highly susceptible to thermal degradation at high temperatures. After 280 ℃, the melt rapidly becomes thinner, and later bubbles burst, unable to expand and loosen the carbon layer.

Optimal matching window: resin softening range of 200-250 ℃, synchronized with the initial decomposition range of APP/melamine.

2. Molecular weight and molecular weight distribution

High molecular weight: The melt viscosity is too high, the melt elasticity is too strong, the gas is difficult to expand, the foaming is hindered, and the expansion is low;
Low molecular weight: viscosity drops rapidly after high-temperature melting, bubbles are easily broken, and gas retention is poor;
Wide distribution lotion: some polymers provide melt strength, small molecules improve melt fluidity, and it is easier to obtain high expansion.

3. Crosslinking characteristics of lotion (no crosslinking/self crosslinking/core-shell structure)

A large number of chemical cross-linking networks have been formed inside the dry film of lotion (a large number of NMA, GMA cross-linking monomers) with high self crosslinking at room temperature; It is difficult to fully melt and flow after being heated, and the melt is too rigid to be expanded by gas stretching → the expansion ratio decreases significantly (many finishing fire retardant coatings step on this pit! The water resistant high cross-linked lotion of exterior wall is not suitable for expansion fire protection).
Low cross-linked/non cross-linked thermoplastic acrylic lotion: when heated, it can fully melt and flow, making it easier to foam;
Soft core and hard shell core-shell lotion: the heated shell melts first to provide the coating strength, the core assists in flow, and the carbon layer has better toughness, giving consideration to both expansion and carbon layer strength.

4. Acid value (carboxyl content)

The acrylic monomer in the lotion provides – COOH carboxyl group: high acid value lotion: carboxyl group will interact with APP (weak acidity) to change the thermal decomposition behavior of the system; Meanwhile, high temperatures are more likely to promote dehydration into charcoal. But the acid value is too high: the storage stability of the system deteriorates, and the APP is prone to hydrolysis; If it is too low, the compatibility between the resin and the expansion component interface will be poor, resulting in phase separation and uneven foaming.

5. Thermal stability and residual carbon rate (key TGA indicators)

The higher the carbon residue of acrylic acid lotion in the range of 300~400 ℃, the easier it is to maintain the carbon skeleton: the thermal decomposition of some modified acrylic acid (phosphorus containing, silicone modified) is gentle, and it will not completely decompose and fail in a short time; Ordinary low-cost acrylic acid undergoes rapid and severe thermal weight loss, with an extremely narrow melting window that can only encapsulate gas for a short period of time and limited expansion rate.

6. Lotion particle size, emulsifier type (secondary but not negligible)

Small particle size lotion, more uniform mixing with flame retardant powder, better melt continuity when heated; Emulsifiers with excessive anions are prone to volatilization at low temperatures and rapid decomposition at high temperatures, disrupting the continuity of the molten film and causing bubble escape, thereby reducing expansion.

3 3、 The most intuitive explanation of the two typical phenomena (often encountered in production practice)

Phenomenon 1: A lotion expands 30 times; Replace with B lotion, and the same formula only expands 8~12 times

Priority order of investigation:

DSC tests the softening temperature Tg and melting range of two lotion: B lotion softening temperature does not match the decomposition range of the ternary system;
Judgment of crosslinking degree: B lotion is a high self crosslinking lotion, which cannot be fully melted and flow when heated;
TGA vs. carbon residue: B lotion loses mass rapidly at 300~400 ℃, and the melt disappears quickly, unable to lock bubbles.

Phenomenon 2: The expansion is very high, but the carbon layer is loose and shatters at a touch, and the fire resistance time is not long

The molecular weight of lotion is low, and the high-temperature melt is diluted prematurely. Although it expands temporarily, the carbon skeleton cannot be maintained in the later stage; High expansion ratio ≠ good fire resistance, and it is necessary to balance the expansion ratio and the compactness and integrity of the carbon layer.

4 4、 Summary of experience in selection of acrylic lotion for intumescent fire retardant coating

Low/medium self crosslinking biphenyl acrylic lotion is preferred; Carefully select high cross-linked pure acrylic and high Tg hard lotion;

The resin melting softening range is controlled between 200-255 ℃, matched with APP+melamine foaming window;

Medium molecular weight, do not use ultra-high molecular weight or very low molecular weight industrial lotion;

Pre screening methods for testing: rapid foaming comparison in a small-scale muffle furnace, simultaneous TGA thermogravimetric analysis, DSC melting curve;

Misconception correction: It is not necessarily better to expand higher. Excessive expansion often leads to more pores in the carbon layer and poor insulation continuity, requiring a balance between expansion ratio and carbon layer strength.

Post time: Jul-31-2026