Halogen-Free Flame Retardant Impact on Rigid Polyisocyanurate Foam Properties

Abstract

This study evaluates the impact of different flame retardants on the mechanical and thermal properties of rigid polyisocyanurate (PIR) foams, focusing on formulations with isocyanate indexes of 335 and 400. The flame retardants tested include triethyl phosphate (TEP), ammonium polyphosphate (APP), aluminium hydroxide (Al(OH)₃), and a combination of APP and Al(OH)₃. FOAMAT^® was used to analyse the foaming kinetics, while further tests assessed density, thermal conductivity, and compression strength. TEP, a liquid flame retardant, was found to reduce peak heat release rate (pHRR) and total heat release significantly, outperforming solid flame retardants. Although solid flame retardants like APP slightly increase start times and gel times due to their non-reactive, filler role, they increase the foam’s density and somewhat limit the effectiveness in reducing flammability. The uneven dispersion and lower compatibility of solid additives may lead to suboptimal improvements in fire resistance. APP displayed dual-phase decomposition, aiding char formation to a degree. Overall, TEP proved most effective in enhancing PIR foam’s fire resistance, demonstrating the advantage of liquid over solid flame retardants in achieving uniform distribution and better integration with the foam matrix, thus optimising thermal insulation and mechanical performance.

1. Introduction

Fire resistance is a crucial property of construction materials. All building materials must meet minimum fire safety standards. Some materials, such as concrete and metal, are non-combustible, while others, like natural materials and most polymers, can ignite. In buildings that utilise a mix of materials, heat can also harm non-combustible options during a fire. To enhance fire safety for combustible materials, flame retardants are incorporated.

Tris(chloropropyl)phosphate (TCPP) is a commonly used flame retardant in building materials, including polyurethane (PUR) and polyisocyanurate (PIR) foams. However, it is also linked to human health hazards [1,2,3,4,5,6,7]. Due to this, it is included in the European Commission REACH restrictions roadmap [8] and potentially will be phased out. In the market, there are other alternatives like triethyl phosphate (TEP), ammonium polyphosphate (APP), and aluminium hydroxide (Al(OH)₃). However, introducing new flame retardants may alter the behaviour of rigid PUR or PIR foam materials. Therefore, it is crucial to evaluate their suitability in industrial applications to ensure quick acceptance and seamless integration.

These alternatives are considered more environmentally friendly and pose fewer health risks than TCPP. For instance, TEP is known for its effectiveness as a flame retardant while exhibiting a lower toxicity profile [2]. APP, on the other hand, serves as a dual-function compound that not only imparts flame retardancy but also enhances the char formation during combustion, thereby improving overall fire resistance [9,10]. Al(OH)₃ works by releasing water vapour when heated [11,12], which can help cool the surrounding area and dilute flammable gases. However, solid flame retardants like Al(OH)₃ and APP introduce new challenges to production processes. Solid flame retardants need different dosage equipment than liquid flame retardants, which could lead to the necessity of installing new equipment in a PIR factory. Additionally, Al(OH)₃ is abrasive to metal.

Flame retardants play a crucial role in enhancing the fire safety of construction materials, but their selection and application require careful consideration to balance efficiency, safety, and environmental impact. As the construction industry shifts toward more sustainable practices, ongoing research and development efforts are focused on identifying new alternatives that maintain high fire resistance while minimising health risks.

While traditional flame retardants like TCPP have served their purpose in enhancing fire safety, the trend towards sustainable and safer alternatives is shaping the future of flame retardant technology in construction materials. Ongoing improvements and re-studies will play a vital role in ensuring that buildings not only resist fire but do so while protecting human health and the environment. This article focuses on three flame retardants, TEP, APP, and Al(OH)₃, as well as a mixture of APP and Al(OH)₃. This study gives a broader perspective on halogen-free flame retardants and their concentration’s influence on PIR foam properties. We studied flame retardants and their concentration’s effects on not only thermal properties but also PIR foaming parameters, apparent density, thermal conductivity coefficient, and compressive strength.

2. Materials and Methods

2.1. Materials

The following reagents were used to develop rigid PIR foam. PIR systems without flame retardants were supplied by TENAX PANEL Ltd. (Dobele, Latvia). Flame retardant triethyl phosphate (TEP) was supplied by TENAX PANEL Ltd., (Dobele, Latvia), ammonium polyphosphate (APP) was supplied by Clariant (Hurth-Knapsack, Germany) under brand name EXOLIT AP 422, and aluminium hydroxide (Al(OH)₃) was supplied by WTH GMBH (Stade, Germany) under brand name Addforce FR B415 (grounded and surface-treated, average grain size 10–15 µm).

2.2. Preparation of PIR Foams and Characterisation of Foaming Parameters

PIR foams were obtained in a one-step method by mixing the isocyanate component—polymeric 4,4-methylene diphenyl isocyanate (pMDI)—with a mixture of the polyols, catalysts, surfactants, water, and physical blowing agents. Flame retardants were introduced in polyol systems in 6 different concentrations—5, 11, 13, 15, 20, and 25 pbw. For each concentration, PIR foams with 2 isocyanate indexes (the ratio of OH groups to NCO groups multiplied by 100) were prepared—335 and 400. Initial PIR foams without any flame retardant and isocyanate indexes of 335 and 400 were prepared as well. The selection of two PIR foam formulations with isocyanate indices of 335 and 400 was made to evaluate the influence of crosslinking density on the thermal, mechanical, and flame-retardant performance of the foams. These indices represent typical industrial formulations.

After mixing for 5 s with a mechanical stirrer, the reaction mixture was quickly poured into the universal foam qualification system FOAMAT^® (Format Messtechnik GmbH, Karlsruhe, Germany) advanced test container (ATC), allowing the free foam to rise in the vertical foam direction. The temperature in the ATC was 60 °C.

The parameters of the foaming process, such as the dielectric polarisation of the reaction mixture, the temperature in the foam core, the height of the foam, and the foam rise pressure, were analysed using the FOAMAT^® device. These parameters illustrate the reactivity of the PIR system.

Larger samples were prepared in an open-top mould using the same formulations and mixing time as for FOAMAT^® testing. Foams were conditioned for two hours at 60 °C and an additional 22 h at room temperature.

2.3. Characterisation of PIR Foams

The apparent density of the obtained PIR foams was measured according to ISO 845 [13].

The thermal conductivity coefficient of PIR foams was analysed using the A FOX 200 (TA Instruments-Water LLC, New Castle, DE, USA) according to ISO 8301 [14], using samples with dimensions of 200 × 200 × 50 mm after 24 h from foam preparation. During the thermal conductivity measurement, a one-way heat flow between the hot (20 °C) and cold (0 °C) plates was established at an average temperature of 10 °C between the two plates.

Compressive strength tests were carried out using a Zwick/Roell Z010 (10 kN) static materials testing device (Zwick Roell, Ulm, Germany) coupled with a 1 kN force cell, according to EN ISO 844:2021 [15], in the directions parallel and perpendicular to the direction of foam rise. Instead of standard samples, cylindrical samples with a 20 mm diameter and a 22 mm height have been used.

Limited oxygen index (LOI) was tested according to ISO 4589-2 standard [16]. The sample size was 100 × 10 × 10 mm (length × width × thickness).

Small-scale reaction to flame was carried out as a single-flame-source test according to ISO 11925-2:2020 standard [17]. Samples with dimensions 200 × 98 × 40 mm (length × width × thickness) with a marked line at 150 mm were cut out of rigid PU foam samples, which were then tested from the front and the backside.

Thermogravimetric analysis (TGA) was performed using the TA instrument Discovery TGA (New Castle, DE, USA). The PIR foam samples were heated at a rate of 10 °C per minute in a nitrogen atmosphere, covering a temperature range of 30 °C to 700 °C on platinum scale pans.

The cone calorimeter test was carried out according to guidance ISO/TR 5660-1:2015 [18] “Reaction-to-fire tests—Heat release, smoke production, and mass loss rate—Part 1: Heat release rate (cone calorimeter method) and smoke production rate (dynamic measurement)”. A cone calorimeter that operates on the “oxygen consumption” principle was made by Fire Testing Technology Limited (Sussex, UK) and was used to test the samples. Foams were tested at 50 kW/m² of heat flux. Fire testing parameters such as time of ignition and flameout, fuel load, total heat release (THR) and the peak of total smoke release (HRP), total smoke release (TSR), the maximum average rate of heat emission (MAHRE), and consistent burning were tested. The sample size was 25 × 100 × 100 mm (length × width × thickness).

3. Results and Discussion

For any heat insulation material, the most important characteristics are mechanical, heat insulation properties, and reaction to fire. For material production, it is important to know process kinetic parameters, i.e., foaming start time, foaming gel time, foaming rise time, foaming curing time. As this study focuses on rigid PIR foams, we put the emphasis on PIR foam technological parameters determined by a universal foam qualification system FOAMAT^®, as well as PIR foam density, coefficient of thermal conductivity, compression strength, as well as TGA and cone calorimeter analysis. This research examines the impact of four different flame retardants systems—triethyl phosphate (TEP), ammonium polyphosphate (APP), aluminium hydroxide (Al(OH)₃), and a combination of APP and Al(OH)₃—on the properties of rigid polyisocyanurate (PIR) foams.

3.1. Rigid PIR Foam Foaming Parameters

Liquid flame retardant—TEP—had no effect on reaction start time to start of foaming (start time) compared to a sample with no flame retardant (Figure 1). Solid flame retardants increased start time in some cases—PIR foams with an isocyanate index of 400 and APP as a flame retardant and PIR foams with an isocyanate index of 335 and APP + Al(OH)₃ as a flame retardant. In both cases, there was about a 5 s jump in start time, and after the initial jump the start time remained the same. As this effect is minimal and non-consistent, the most likely explanation is increased viscosity of the reacting mass and mechanical restrictions to the foam bubble growth.

Flame retardant content has a quite significant impact on rise time (Figure 2). While an increase in liquid flame retardant TEP content slightly decreases the PIR foam rise time, the opposite was observed for solid flame retardants (APP, Al(OH)₃, and APP + Al(OH)₃). As solid flame retardants do not react with isocyanate or polyols, during the reaction they have only a non-reactive filler function, resulting in diluted -OH and -NCO group concentration and a physical barrier between reactive groups, as well as an increased viscosity of the reaction mixture. Any filler introduced in rigid PU foams can slow down the reaction [19,20]. The mentioned effects result in slower rise times for PIR foams containing solid flame retardants. Although all three solid flame retardants increased the rise time, for the APP-containing formulations this effect was more pronounced.

Similarly to rise time, PIR foam gel time also increased with an increase in solid flame retardant content for similar reasons as for rise time increases (Figure 3). Also, liquid flame retardant slightly decreased the PIR foam gel time. The presence of solid fillers leads to prolonged gel time. Additionally, the higher viscosity introduced by solid flame retardants further impeded the mobility of reactive components, delaying gel time. Similarly to rise time, the APP-containing PIR foams had longer gel times than Al(OH)₃-containing ones. The incorporation of the liquid flame retardant TEP facilitated a more uniform mixture, resulting in slightly decreased gel time as well as rise time.

Curing time characterises the last process in the PIR reaction, the formation of isocyanate trimerisation products and the final crosslinking of the PIR polymer matrix. While the liquid flame retardant TEP and Al(OH)₃ had minimal impact on curing time, APP in high concentrations (25 pbw) increased curing time (Figure 4). Curing time is another PIR foaming parameter that was higher for foams containing APP compared to Al(OH)₃.

The polyol -OH group reaction with isocyanate -NCO is exothermic. The temperature peak describes the maximal temperature in PIR foams during foaming reaction. With increasing flame retardant content, the maximal temperature decreased in PIR foams as the OH and -NCO group concentration decreased in the reaction mixture (Figure 5). This effect is more pronounced for PIR foams with an isocyanate index of 335, as -NCO group content in the reaction mixture is lower compared to an isocyanate index of 400. The same trend was observed by Kuranska et al. [21]. This is an undesirable effect as it changes the curing conditions of the PIR foams. For the formation of the isocyanate trimerisation products, a temperature above 140 °C is essential.

3.2. Mechanical Properties and Thermal Conductivity

Apparent density (Figure 6) is one of the key parameters for any construction material, as other properties like compression strength and thermal conductivity are linked to the apparent density of the foamed polymer. Density was in the range of 40–46 kg/m³ for all tested samples. Generally, all PIR foams containing TEP as a flame retardant have a lower apparent density than those containing the rest of the tested flame retardants. This is logical as TEP is the only liquid-form flame retardant. PIR foams containing Al(OH)₃ as a flame retardant have a lower apparent density than the ones containing APP, which has the highest density of all the tested PIR foams. This tendency is especially evident for PIR foams with an isocyanate index of 335.

Thermal conductivity is one of the most important properties of any thermal insulation material. Thermal conductivity is characterised by the thermal conductivity coefficient and the lower the thermal conductivity coefficient, the better the insulation material. All tested PIR foams have a coefficient of thermal conductivity in the range of 23.2–24.5 mW/(m·K) (Figure 7). These results are consistent with typical rigid PIR foams with a coefficient of thermal conductivity in the range of 18–28 mW/(m·K) [22]. The impact of flame retardant content on the coefficient of thermal conductivity is minimal for PIR foams with an isocyanate index of 400. On the other hand, the coefficient of thermal conductivity has a more pronounced effect on the coefficient of thermal conductivity for PIR foams with an isocyanate index of 335, especially for concentrations above 15 pbw.

Compression strength is a key parameter for building materials, including PIR foams, as it characterises a materials’ ability to maintain its shape and effectiveness under mechanical strain. As compression strength depends on density, in order to exclude density influence all tested PIR foams were normalised to 40 kg/m³ [23].

$${\sigma }_{normal}=\sigma {\left({\displaystyle \frac{40}{\rho }}\right)}^{2.1}$$

(1)

where

• σ_normal—normalised compressive strength, MPa;
• σ—compressive strength, MPa;
• ρ—density, kg/m³.

As expected, compression strength was higher in the parallel than the perpendicular growth direction because the foam cells are elongated in the rise direction [24,25,26]. PIR foam’s compressive strength decreased with an increase in flame retardant content (Figure 8 and Figure 9). Different types of flame retardants had different reasons for lowering compression strength. The liquid flame retardant TEP can act as a plasticiser in PIR foams [27]. As a result, increased TEP content reduced PIR foam rigidity and led to lower compression strength. Solid flame retardants—APP, Al(OH)₃ and APP + Al(OH)₃—act like a filler. Fillers are harder to distribute evenly in the PIR foam system; the presence of fillers might modify the microstructure of PIR foam cells and that might lead to decreased compression strength.

3.3. Thermal Stability and Fire Behaviour

The thermal stability of rigid PIR foams depends on two main factors: the isocyanate index and the flame retardant type and concentration. Rigid PIR foams with a higher isocyanate index have a higher proportion of isocyanurate linkages that are more thermally stable than urethane linkages [28]. This was confirmed in our study as well, as the maximal degradation temperature for PIR foams without any flame retardants shifted from 310 °C to 328 °C for isocyanate indices of 335 and 400, respectively (Figure 10). Similar results were obtained by other authors for PIR foams [29,30].

The addition of flame retardants naturally increased the thermal stability of PIR foams. It is less evident for PIR foams with an isocyanate index of 400, as those foams have a higher maximal degradation temperature to start with. Rigid PIR foams containing APP as a flame retardant have two distinct decomposition phases (340 °C and 525 °C) and have higher residue content than PIR foams without APP. In the first APP degradation process, ammonia is released, and the formation of branched structures and stable free radicals occurs [31,32]. The next phase is water release and decomposition to more simple structures, such as P₂O₅ [33]. The formation of free radicals during APP decomposition helps with char formation in PIR foams, leading to a higher residue at 700 °C than PIR foams with other flame retardants.

Aluminium hydroxide has a more simple degradation process—it decomposes into water and A_l2O₃. Decomposition starts around 230 °C [11], reaching maximal degradation at 275 °C (Figure A1). As decomposition releases water and is an endothermic process, it slows down PIR foam decomposition, leading to a slight shift in maximal degradation peak and lower peak intensity. Moreover, the evaporation of water also consumes energy that otherwise would be spent on PIR polymer matrix degradation. Rigid PIR foams with an isocyanate index of 400 continue to decompose similarly to foams with no flame retardant.

Similarly to APP, one of the TEP decomposition products is phosphoric acid. This can promote the char formation of PIR foams. This leads to a slight shift in the maximal degradation temperature peak as well as a lower peak intensity compared to PIR foams without flame retardants.

The influence of flame retardants plays a crucial role in the reaction-to-fire performance of PIR foams. In this study, the fire behaviour of the developed rigid PIR foams was evaluated using three tests: the small flame reaction test (ISO 11925-2:2020), the limiting oxygen index test, and the cone calorimeter test.

The results of the small flame reaction test and LOI measurements are summarised in

Table 1. Results of the small-flame-reaction test and oxygen index of the rigid PIR foam with an isocyanate index of 335 and 400 filled with different amount of TEP.

TEP Content, pbw	Length of Damaged Area for PIR Foams Containing TEP, cm		Oxygen Index
			TEP		APP + Al(OH)3
	Isocyanate Index 335	Isocyanate Index 400	Isocyanate Index 335	Isocyanate Index 400	Isocyanate Index 335	Isocyanate Index 400
0	12.8	9.4	22.4	22.7	22.4	22.7
5	10.0	8.0	24.2	24.3	23.9	24.0
11	7.8	8.0	25.7	25.7	24.8	25.1
13	8.9	8.3	25.1	26.3	25.1	24.6
15	9.3	8.0	26.0	27.2	25.5	25.7
20	7.6	7.3	27.5	27.7	25.8	26.3
25		8.1	28.6	28.3	26.3	27.4

. The data indicate that the burned area of the rigid PIR foam samples increases proportionally with the TEP flame retardant content for both isocyanate indices (335 and 400). In the small-flame test, a single flame source was applied for 30 s, followed by an additional 30 s of observation. All samples self-extinguished before the 30 s mark, suggesting that the developed material could achieve a Class B reaction-to-fire rating for construction products according to ISO 11925-2:2020 standard.

The LOI values (Table 1) show a clear trend: as the flame retardant (both TEP and APP + Al(OH)₃) content increases, so does the LOI. Notably, all samples exhibited LOI values above 22, indicating that they do not burn easily—even those without added flame retardants. These LOI results are consistent with other halogen-free flame retardant systems, reinforcing the material’s fire-resistant properties [34].

During the cone calorimeter test, the PIR foam samples were subjected to a heat flux of 50 kW/m². The cone calorimeter test simulates an open-fire scenario and serves as an effective method to compare flame retardants, helping to identify formulations suitable for further evaluation, such as in a single burning item test.

A critical parameter in fire safety and combustion science is the peak of heat release rate (pHRR). The pHRR represents the maximum rate at which heat is released during the combustion of a material or product. It is typically measured in kilowatts per square metre (kW/m²) for surface area-specific rates, or in kilowatts (kW) for total heat release. The pHRR is a key indicator of fire intensity—higher pHRR values signify that a material can release energy more rapidly, potentially accelerating fire spread.

Neat PUR foams with isocyanate indices of 110–150 exhibit pHRR values of 250–320 kW/m² [33]. By increasing the content of polyisocyanurate groups in PUR-PIR foams (isocyanate indices of 150–250), the pHRR decreases to approximately 200 kW/m² [35]. PIR foams with isocyanate indices exceeding 320 exhibit even lower pHRR values of below 185 kW/m² [36]. The initial PIR foams developed in this study demonstrated comparable pHRR values, as shown in Figure 11.

The incorporation of a liquid flame retardant—TEP—reduced the pHRR from 185 kW/m² to 150 kW/m² at a TEP loading of 5 pbw for PIR foams with isocyanate indices of both 335 and 400. Solid flame retardant systems, such as APP and APP combined with Al(OH)₃, also reduced the pHRR of PIR foams with an isocyanate index of 400; however, their performance was less effective compared to TEP.

For PIR foams with an isocyanate index of 335, the solid flame retardants did not significantly reduce pHRR values. On the contrary, the pHRR increased from 185 kW/m² to approximately 200 kW/m². This undesirable increase in pHRR could be attributed to changes in PIR foam morphology and the uneven dispersion of solid flame retardant particles within the foam structure.

A similar effect was observed in the total heat release, depicted in Figure 12, as was seen for the peak heat release rate (pHRR) in Figure 11. Among the flame retardants tested, TEP was the only one that effectively reduced the total heat release of PIR foams for both isocyanate indices. For PIR foam with an isocyanate index of 400, the total heat release was reduced by both TEP and APP.

The released smoke and maximum average rate of heat emission (MARHE) of the developed rigid PIR foams are shown in Figure A2 and Figure A3, respectively, demonstrating similar flammability reduction trends for TEP and APP. However, the solid flame retardants were not effective in reducing the flammability characteristics of the rigid PIR foams.

The limited effectiveness of solid flame retardants can be attributed to several factors. Solid flame retardant particles may not disperse uniformly within the PIR foam matrix during the mixing and foaming processes. This uneven distribution can result in localised areas with inadequate flame retardant protection, reducing their overall effectiveness. Additionally, solid flame retardants often exhibit limited compatibility with the polymer matrix, particularly in rigid PIR foams. This incompatibility can lead to weak interactions between the flame retardant and the foam, diminishing the flame retardant’s efficiency in inhibiting combustion or enhancing char formation.

Effective flame retardants typically promote the formation of a cohesive and thermally stable char layer, which acts as a barrier to heat and oxygen [9,27,29]. If solid flame retardants fail to adequately support char formation, their ability to reduce heat release is diminished. Furthermore, solid flame retardant particles may agglomerate during processing, forming larger clusters instead of being finely dispersed. These agglomerates can negatively impact the structural integrity of the foam and limit the effectiveness of the flame retardant in reducing flammability.

In summary, the reduced effectiveness of solid flame retardants in PIR foams is likely due to their poor dispersion, adverse effects on foam morphology, and limited chemical interactions with the polymer matrix. Liquid flame retardants, such as TEP, often perform better because they integrate more uniformly and can chemically bond or interact with the foam structure, thereby enhancing fire resistance properties.

4. Conclusions

This study investigated the influence of different flame retardants (TEP, APP, Al(OH)₃, and APP + Al(OH)₃) on the properties of rigid polyisocyanurate (PIR) foams with varying isocyanate indexes (335 and 400).

A distinct contrast is observed between the effects of liquid and solid flame retardants on reaction kinetics and thermal properties. Triethyl phosphate, a liquid flame retardant, does not alter or lower the reaction parameters, whereas solid flame retardants like APP and Al(OH)₃ contribute to a marginal increase in reaction parameters due to the non-reactive filler effect they cause.

The incorporation of solid flame retardants impacts thermal conductivity, particularly with lower isocyanate indexes. While both types of flame retardants enhance thermal stability, ammonium polyphosphate displays a unique dual-phase decomposition, promoting more substantial char formation.

TEP demonstrated superior performance in reducing flammability compared to solid flame retardants with lower density and higher compression properties, as well as lower total heat release than solid flame retardants.