DOI: 10.3303/CET24108011 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 19 January 2024 ; Revised: 29 February 2024 ; Accepted: 21 March 2024  
Please cite this article as: Abosnina A.E., Mohamad Z.B., Majid R.A., 2024, Thermal Stability and Flammability Properties of Filled Rigid 
Polyurethane/ Aluminium Hydroxide Composite Foam, Chemical Engineering Transactions, 108, 61-66  DOI:10.3303/CET24108011 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 108, 2024 

A publication of 

 
The Italian Association 

of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Carlo Pirola, Antonio Espuña 
Copyright © 2024, AIDIC Servizi S.r.l. 
ISBN 979-12-81206-08-3; ISSN 2283-9216 

Thermal Stability and Flammability Properties of Filled Rigid 
Polyurethane/ Aluminium Hydroxide Composite Foam  

Aisha Elhadi Abosnina*, Zurina binti Mohamad, Rohah Abdul Majid 
School of Chemical and Energy Engineering, Faculty of Engineering Universiti Teknologi Malaysia, 81310 Johor Bahru, 
Malaysia. 
aisha.abosnina@gmail.com 

Rigid polyurethane foam (RPUF) has been prepared at various amounts of aluminium hydroxide (ATH) flame 
retardants (10,20,30,40,50 wt. %). This study aims to examine the thermal, and fire behaviour of RPUF by 
exploring the ATH as a flame retardant. The effect of different contents of ATH constituents on RPUF was 
investigated. Results from thermogravimetric analysis TGA showed that the thermal stabilities of composite 
foam were higher than that of pure RPUF. The results of the limiting oxygen index test showed that 50 wt. % 
aluminium hydroxide (ATH) enhanced the limiting oxygen index (LOI) value of the composite by 23 % compared 
to pure RPUF.  All aluminium hydroxide (ATH) contents were observed to have contributed to the boosted flame 
retardancy of RPUF. The RPUF composite showed great potential for being employed as insulation material.  

1. Introduction  
Polymeric foams consist of both solid and gaseous phases. They are classified into two categories: 
thermoplastics and thermosetting foams (Lee and Ramesh et al., 2004).  They can be rigid, flexible foam, or 
elastomeric and are produced from a wide range of polymers such as polyurethane (PU). Foam production 
worldwide has been dominated by polyurethane foams (PUFs), followed by polystyrene and polyvinyl chloride 
foams (Titow, 2001). Polyurethane (PU) faces limitations in structural, electronic, and transportation applications 
due to ignition, high flammability, and limited thermal stability. Additionally, its low dimensional stability poses 
challenges in insulation applications (Liu et al., 2016; Srihanum et al., 2022). Rigid polyurethane foam (RPUF) 
is categorized as a flammable polymer, releasing substantial heat and toxic gases like carbon monoxide and 
hydrogen cyanide during a fire outbreak (Chen et al., 2018). This has necessitated the incorporation of flame 
retardants (FR) in these sectors. PU's primary market is in PU foam, which encompasses various foaming 
systems. Rigid polyurethane foam (RPUF) is a polymer-based foam featuring carbamate groups in its main 
chain. Consequently, in the event of a fire, rigid polyurethane foam (RPUF) undergoes rapid combustion, with 
the flames spreading quickly due to its porous structure, low thermal inertia, and extensive surface area. Inhaling 
these emitted gases presents a significant health risk to humans during combustion (Zhou et al., 2020). 
Therefore, it is essential to enhance the fire retardancy and smoke-suppressing characteristics of rigid 
polyurethane foam (RPUF) by incorporating flame retardants (FR) (Cao et al., 2017). Like other PU foams, rigid 
polyurethane foam properties can be easily modified by incorporating inorganic hydroxide fillers into the polymer 
to produce a composite with superior flammability and thermal performance (Peng et al., 2019; Sałasińska et 
al., 2021). Commonly employed inorganic hydroxide fillers, such as magnesium hydroxide, aluminium hydroxide 
(ATH), and layered double hydroxides, are recognized for their non-toxic and environmentally friendly attributes. 
They are routinely integrated into polymer matrices to augment thermal and flammability properties (Pang et al., 
2019). Among these, aluminium hydroxide (ATH), stands out due to its cost-effectiveness, lack of odor, ease of 
handling, non-toxicity, chemical inertness, and non-volatility. In applications where aluminium hydroxide (ATH) 
filled composites are subjected to heat and subsequent combustion, aluminium hydroxide (ATH), serves to 
absorb and uniformly disperse the applied heat within its particles, thereby mitigating the heating rate of the 
composite (Modesti et al., 2002). The effective dispersion of fillers within the matrix significantly influences the 
reinforcing impact.  

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Therefore, this study's goals are to investigate and compare the influence of pure rigid polyurethane foam 
(RPUF) and different content of aluminium hydroxide (ATH) filler on the thermal, and flammability properties of 
rigid PU foam-based composites.   

2. Experimental 
2.1 materials  
Polyol (Component A) is one of the materials used to produce rigid PU foam, it is a liquid mixture that contains 
polyester polyol, flameproofing agent, benzyl-dimethylamine content 1 % as a catalyst, 1,1-dichloro-1-
fluroethane content 20 % as blowing agent, and N, N, N', N' – tetramethyl hexamethylenediamine content 0.1 
% as crosslinker. In contrast, isocyanate (Component B) was diphenylmethane-4,4 diisocyanate (PMDI). Wee 
Tee Tong Chemicals Pte Ltd., Malaysia, supplied both ready mixed components A and B. Aluminium hydroxide 
(ATH) was purchased from Merck Malaysia Sdn.Bhd. All materials were utilized without any additional 
processing. 

2.2 Preparation of rigid PU Foam Composite 

The rigid polyurethane foam (RPUF) was synthesized via a one-step mixing procedure utilizing components A 
and B, as outlined in Table 1. Initially, component A (polyol) underwent stirring with a high-speed mechanical 
stirrer and underwent a 2-minute degassing period. Subsequently, the flame retardant was introduced into a 
beaker containing the polyol. The resulting pre-mixture was stirred at 1000 rpm for two minutes using a 
mechanical stirrer to achieve a homogeneous solution. Once the flame retardant was thoroughly wetted and 
dispersed within the mixture, component B (diisocyanate) was added to the pre-mixture solution, undergoing 
continuous stirring for a brief duration. The resulting mixture was then transferred into a close-type stainless-
steel mould to facilitate creaming and self-rising. Following this, the mould was left at room temperature for 24 
hours before being cut into specimens to ensure complete curing.  

Table 1: Formulation of rigid PU foams with different content of aluminium hydroxide (ATH) ATH used in this          
study. 

Samples Polyol PMDI        ATH wt. %  
Pure RPUF 
RPUF/ATH 
RPUF/ATH 
RPUF/ATH 
RPUF/ATH 
RPUF/ATH 

50 
50 
50 
50 
50 
50 

50 
50 
50 
50 
50 
50 

0 
10 
20 
30 
40 
50 

 

2.3 Characterization of RPUF/ATH 
 
2.3.1 Thermal properties of RPUF/ATH 
Thermal gravimetric analysis (TGA) was performed using (TGA Q500 V20.13 build 39) instrument to investigate 
the thermal degradation of the RPUF/ATH composite. The weight changes of the sample were monitored with 
respect to temperature. The sample was placed in a platinum pan and subjected to heating from 28 °C to 700 
°C at a rate of 10 °C/min, within a nitrogen gas atmosphere flowing at 100 mL/min. 

2.3.2 Limiting Oxygen Index (LOI) test 
The limiting oxygen index (LOI)  test was conducted following the ASTM D2863-97 standard, The analysis 
involved preparing five samples from each formulation, each with dimensions of 100 × 12.5 × 12.5 mm³ (length, 
width, and thickness). The limiting oxygen index (LOI) values were recorded by burning five samples for each 
formulation. According to Alongi et al. (2015), polymers with limiting oxygen index (LOI) values above 27 % are 
considered self-quenching, those below 21 % are highly combustible in the air, and those falling between 21-
27 % are classified as slow-burning. 

3. Results and discussion 
3.1 Thermal Gravimetric Analysis (TGA) of RPUF/ATH Composites 
As shown in Figure 1 (TGA Curve) T5 (the beginning of degradation) for pure rigid polyurethane foam (RPUF) 
of 231 oC was recorded. All the samples produced major two-stage thermal degradation. Release of volatile 
molecules namely water molecules was detected due to the high moisture uptake of the porous RPUF below 
250 oC. The initial stage of the breakdown of rigid polyurethane foam (RPUF) arose around 231- 424 oC, 

62



attributed to the break in urethane linkages. The second stage occurred at about 512-712 oC, ascribed to the 
cleavage of strong bonds like aromatic groups, aliphatic carbon-carbon, and flame retardants (Kairytė et al., 
2020). The RPUF/ATH composites at the highest content of aluminium hydroxide (ATH) 50 wt. % displayed a 
major peak at approximately 316 oC, mainly ascribed to the superior heat absorbing capabilities of aluminium 
hydroxide (ATH) at higher loading, resulting in increased degradation temperature (Akdogan et al., 2020). While 
Tmax for pure rigid polyurethane foam (RPUF) was observed to be 296 oC, there was a considerable increase in 
Tmax for modified rigid polyurethane foam (RPUF), ranging from 304-316 oC, increasing with an increase in 
aluminium hydroxide (ATH) content. This can be interpreted as an enhancement in thermal stability after adding 
aluminium hydroxide (ATH). These increases could be credited to the water molecules released from aluminium 
hydroxide (ATH); the water molecules tend to cool the hot polymer hence the treated rigid foam requires higher 
heat energy to break down ((Thirumal, M et al., 2010). Polymer's degradation temperature ought to rise generally 
as aluminium hydroxide (ATH) content increases. That's because aluminium hydroxide (ATH) undergoes 
endothermic breakdown, lowering the temperature in the area around the components. Also, the combustible 
gases are reduced by water dilution and the development of a protective layer of aluminium oxide, which serves 
as a barrier for the passage of oxygen and fuel into polymers. On the other hand, Figure 1 (DTG curve) shows 
the DTG curve for pure rigid polyurethane foam and ATH-modified rigid polyurethane foam at different loading. 
As observed, rigid polyurethane foam/aluminium hydroxide (RPUF/ATH) composites displayed a shoulder peak; 
this may be because of the removal of surface-active complexes used in ATH to enhance its dispersal in the 
polymer matrix. The DTG curves indicate that the thermal stabilities of rigid polyurethane foam/aluminium 
hydroxide (RPUF/ATH) composites are higher compared to pure rigid polyurethane foam (RPUF). The unfilled 
foam thermally breaks down with a greater decomposition rate than the filled foam. This can be associated with 
the formation of different thermally stable constituents namely phosphorous-based compounds and aluminium 
oxide during the degradation. The aluminium hydroxide (ATH) as a flame retardant in rigid polyurethane foam 
(RPUF) has huge capacities to form a compact char hence shielding the inner bulk materials. Shown in Table 
2 are the char residue values of the filled and unfilled rigid polyurethane foam composites. The weight residue 
reveals that the highest volume of ash was recorded with (50 wt. %) ATH loading. More so, the mass residual 
of RPUF/ATH (50 wt. %) and RPUF/ATH (40 wt. %) were 29.16 and 25.85 wt. %. These findings demonstrate 
that the aluminium hydroxide (ATH) boosted the thermal stability of the composite by reducing weight loss, 
moving it to a higher decomposition temperature, and significantly increasing the remaining char concentration 
compared to pure rigid polyurethane foam (RPUF). 
 

 
 
Figure 1: TGA and DTG curves of RPUF and RPUF/ATH composites incorporated with 10-50 wt. % ATH. 
 
 
 
 
 

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Table 2:  Thermogravimetric data for pure RPUF and RPUF/ATH composites 

Samples T5 (oC) T50 (oC) Tmax (oC) Char residue (%) at 600 oC 
Pure RPUF 
RPUF/ATH10 wt.% 
RPUF/ATH20 wt. % 
RPUF/ATH30 wt. % 
RPUF/ATH40 wt. % 
RPUF/ATH50 wt. % 

231 
252 
256 
278 
266 
280 

318 
326 
333 
334 
346 
360 

296 
304 
300 
306 
314 
316 

15.18 
16.42 
18.01 
21.20 
25.85 
29.16 

3.2 Limiting Oxygen Index (LOI) 
Table 3 depicts the limiting oxygen index (LOI) values for the aluminium hydroxide (ATH) incorporated 
composites blending.  According to the specimen observation, all the composites containing aluminium 
hydroxide (ATH) show higher limiting oxygen index (LOI) values as the aluminium hydroxide (ATH)  loading 
increases compared with pristine rigid polyurethane foam (RPUF).  But RPUF/ATH (50 wt. %) have the best 
improved limiting oxygen index (LOI) of 23 %. Comparing the limiting oxygen index (LOI) of three composites, 
RPUF/ATH (20 wt. %), RPUF/ATH (30 wt. %), and RPUF/ATH (40 wt. %), present a relative increase in their 
limiting oxygen index (LOI). These outcomes suggest they are combustion-resistant with slow-burning attributes.  
It was documented in a previous study that composites with a limiting oxygen index (LOI) range of 21-27 % are 
classed as slow-burning. The major reason responsible for the increase in RPUF/ATH composite limiting oxygen 
index (LOI) aluminium hydroxide (ATH) is an organic filler with crystal water in its chemical structure.  Hence, 
during the endothermic reaction at (220 oC – 450 oC) it releases about 34 wt.% steam.  This lowers the composite 
temperature during combustion and the released steam dilutes the combustible gases, resulting in increased 
limiting oxygen index (LOI).  Additionally, aluminium hydroxide (ATH) exhibits a dehydration reaction to generate 
Al2O3 and carbonize the composites giving rise to flammable volatiles and preventing the spread of flame (Zhu 
et al., 2014). Some previous studies have reported improved limiting oxygen index (LOI) values when some 
amount of aluminium hydroxide (ATH) flame retardant additives were incorporated into a polymer matrix such 
as polyurethane foam filled with a mixture of aluminium hydroxide and triphenyl phosphate (PUF/ATH/TPP) 
(Thirumal et al., 2008).  The presence of aluminium hydroxide (ATH) in the composites contributed to the 
reduction in flame-spread rate and increased limiting oxygen index (LOI).   Similarly, aluminium hydroxide (ATH) 
played a synergistic role in the rigid polyurethane foam/aluminium hydroxide/Dimethyl methyl phosphonate 
(RPUF/ATH/DMMP) hybrid composite by substantially enhancing the composite flame retardancy (Zhu et al., 
2022).  Like another additive-type flame retardant (such as magnesium hydroxide), aluminium hydroxide (ATH) 
retard polymer heat capacity, its high volume when present in the composite function as a solid phase diluter, 
and their addition into any resins, rubber, and plastics can reduce the concentration of flammable substances 
in composite materials (Ahmad Ramazani et al., 2008; Peng et al., 2019). 

Table 3:   Limiting Oxygen Index (LOI) of pristine RPUF and RPUF/ATH composites. 

Samples ATH Loading LOI 
S1 
S2 
S3 
S4 
S5 
S6 

0 
10 
20 
30 
40 
50 

18 
19 
20 
21 
22 
23 

*S1 stands for sample 1 
 

         As illustrated in Figure 2, (a) is the limiting oxygen index (LOI) residue of pristine rigid foam, (b-f) for rigid 
polyurethane foam/aluminium hydroxide (RPUF/ATH) (10-50 wt. %) respectively. From the digital pictures, it 
was established that pristine rigid polyurethane foam (RPUF) char residue is small and light. Within a short 
period, this sample burns profusely, leaving behind a small residue insufficient to protect underneath the 
polymer. An indication of their poor flame resistance on exposure. However, RPUF/ATH 30 wt. % and 
RPUF/ATH 40 wt. % had bulkier char residue, an indication of superior fire resistance compared to pristine rigid 
polyurethane foam (RPUF). A recent study reported that the amount and quality of char are responsible for 
excess non-flammable residues, acting as a physical shield and enhancing composite fire retardancy efficiency 
(Xiao et al., 2021). For RPUF/ATH 50 wt. %, the higher concentration of aluminium hydroxide (ATH) in the 
composite was responsible for their higher limiting oxygen index (LOI) value. Aluminium hydroxide (ATH) clogs 
the entire surface of the rigid foam thereby disrupting any flame propagation hence no major burning took place. 

64



The presence of 50 phr aluminium hydroxide (ATH) in rigid polyisocyanurate-PU foam composite substantially 
boosted their resistance to flame during combustion and the marginal burning residue was noticed. 

 

Figure 2:   Limiting oxygen index (LOI) test residue images of RPUF composites at (a) 0 wt. % ATH, (b) 10 wt. 
% ATH, (c) 20 wt. % ATH, (d) 30 wt. % ATH, (e) 50 wt. % ATH. 

4. Conclusions 
Rigid polyurethane foam (RPUF) composites were satisfactorily prepared and modified with increasing 
incorporation of 10-50 wt. % ATH as a standalone filler using a step one-shot system. The RPUF/ATH 
composites were characterized in thermal stability using TGA, and flammability properties via the limiting oxygen 
index (LOI) test. The measurement of RPUF/ATH thermal stability under a nitrogen gas environment showed 
that the composites modified with 50 wt. % ATH displayed the highest improvements. The addition of ATH also 
confers superior heat-absorbing capacities and higher degradation temperature on the composites. While 
burning, ATH releases water molecules that cool the molten polymer, resulting in a lower temperature around 
the composites. Both ATH 50 wt. % and ATH 40 wt. % modified composites showed the highest char residue, 
a confirmation of the improved thermal stability of the composites.  Data obtained from the limiting oxygen index 
(LOI) test confirmed a boost in the fire retardancy of RPUF/ATH composite specimens due to the addition of 
ATHs. The limiting oxygen index (LOI) was greatly improved and excellent at 50 wt. % ATH loading.   The 
mechanism by which fire retardancy of RPUF/ATH composites improved is via the creation of an unbroken solid 
char layer in the condensed phase during the decomposition of the polymer matrix. The char structure efficiently 
hinders the inside thermal decomposition products into the flame zone and that of the oxygen the underneath 
polymer matrix. In summary, this study presents a novel technique for the fabrication of filled rigid polyurethane 
(RPUF) composite foam using aluminium hydroxide (ATH) as a flame retardant. The main contribution of this 
work lies in the optimization of ATH content, implementation of efficient mixing and dispersion methods, 
utilization of a mould-based creaming and self-rising process, and a strong emphasis on reproducibility and 
scalability. By employing this unique approach, this study successfully produces foam samples exhibiting 
enhanced thermal stability, improved flame retardancy, and significant potential for various applications as 
insulation materials. The study offers valuable insights into the thermal stability and flammability characteristics 
of these composites, highlighting their practical utility in industries where fire safety and thermal performance 
are of utmost importance. 

Acknowledgments 

The authors are grateful to the Malaysian Ministry of Education (MOE) and University Technology Malaysia 
(UTM) for financial support under the FRGS Grant (FRGS/1/2019/TK05/UTM/02/17 and UTMFR21H44). 

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	Thermal Stability and Flammability Properties of Filled Rigid Polyurethane/ Aluminium Hydroxide Composite Foam