The Synergic Effect of Primary and Secondary Flame Retardants on the Improvement in the Flame Retardant and Mechanical Properties of Thermoplastic Polyurethane Nanocomposites

Abstract

Recently, nanocomposites of polymers have attracted attention due to their advanced features compared to their complement polymer microcomposites. In this study, thermoplastic polyurethane (TPU) was used as a matrix; antimony trioxide (primary flame retardant) and montmorillonite organo-clay (secondary flame retardant), along with benzoflex (plasticizer), were used as fillers to examine their synergistic effect. Nanocomposites of various compositions (TPU-1 to TPU-6) were prepared via the melt-mixing method and compressed to form sheets of the desired dimensions with a compression molding hydraulic press machine. Characterization of the samples was conducted with Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). A tensile test was performed through a universal testing machine (UTM) which showed that the Young’s Modulus improved from 147.348 MPa for the pure sample (TPU-1) to 244.568 MPa for TPU-6. A UL-94 test was executed to observe flame retardance. The sample of interest (TPU-6) achieved V-0 classification in UL-94. All these results confirmed the synergistic effect of primary and secondary flame retardants. An optimum increase in fire resistance and mechanical strength was observed for TPU-6.

1. Introduction

Nanocomposites exhibit exceptional features due to their higher surface-to-volume rate, thus providing maximum interaction between matrix and nanofiller. Nanocomposites are widely applied in the plastics, optics, coatings, electronics, and rubber industries [1]. Polymers and their nanocomposites are used in our daily lives due to their multifunctional properties such as their chemical stability, light weight, and low cost. Polymer nanocomposites have attracted consideration, since a nanofiller is added to the matrix in a very minute quantity [2,3].

Polyurethanes have attained the highest growth rate in the market segment because of their distinct properties such as high elasticity, melt processability, and higher abrasion resistance [4]. Among them, thermoplastic polyurethane (TPU) has been widely used for many applications such as automotive interior parts, textile coatings, garments, sports equipment, leisure items, etc., because of its excellent chemical resistance, weldability, outdoor durability, and mechanical properties. However, like most polymeric materials, the high flammability of TPU materials can lead to fire hazards in practical applications. Furthermore, the degraded products and toxic smoke may give rise to environmental pollution and health risks. Although several efforts have been made to address this issue, the flame-retardant efficiency of TPU still needs to be improved [3].

TPUs are linear block copolymers with alternating soft and hard segments. The soft segments are comprised of long-chain diols (commonly polyesters, polyether, silicon, poly-caprolactone, or hydrocarbon) with 1000–4000 gmol⁻¹ molecular mass, while the hard segments typically consist of di-isocyanates, and the sequences are short-chain extenders. The hydrogen bonds break, and the linear chains are disintegrated at melting temperature. In the meantime, the urethane linkage (-NHCOO-) of the hard segment becomes unstable and is decomposed reversibly to alcohol and free isocyanates (Figure 1). The TPU properties can easily be adjusted by fluctuating the components and ratios of the hard and soft fragments to achieve an extensive range of PUs (polyurethanes) varying from thermosetting foams to thermoplastic elastomers [5,6,7]. Different processing techniques have been used to make polymer nanocomposites from polymer beads and nano additives. These include melt blending, the solution method, and in situ polymerization. From an industrial standpoint, melt blending is the most economical mass-production method as it is fast, and a solvent is not involved [8]. In recent years, montmorillonite (MMT) has attracted the most interest because of its catalytic and barrier effects [9]. It is reported that in the case of well-dispersed nanocomposites, the burning process of the polymer is slowed down, while charring is promoted due to the physical barrier effect enhanced by the ablative reassembling of the silicate [10]. The research results, however, pointed out that clays are not sufficient for commercial applications, since they fail to act as stand-alone flame retardants in important regulatory fire tests, such as UL [11]. Antimony trioxide Sb₂O₃ (an inorganic compound) has heat stability at higher temperatures up to 570 °C and has application in polymeric matrices as a flame retardant [12]. Almost 20,000 metric tons of Sb₂O₃ were consumed in the USA in 1990 for the flame retardancy of polymers. Although it is found in nature, it is difficult to use due to its impurities. To manufacture flame retardant grade Sb₂O_3, other oxides of antimony, antimony metal or sulfide, and ore (b oxidation in air at 600–800 °C) are needed [13].

Many researchers have reported the flame retardant and synergistic properties of antimony trioxide and montmorillonite. Riyazuddin et al. [14] reported the influence of antimony oxide on an epoxy-based intumescent flame retardation coating system; they reported an LOI value up to 31 with a reduction in the flammability to the V-0 level with the addition of 4% Sb₂O₃ in the epoxy matrix. In another study, Liu et al. [15] examined the synergistic flame-retardant effect of organic montmorillonite (OMMT) on intumescent flame-retardant PP/CA/APP systems. They reported that the LOI value of the flame-retardant polypropylene containing 1.0 wt.% OMMT increased from 30.8 to 33.0, and the UL-94 rating was lowered to V-0 from V-1. Their study highlighted the significance of the synergistic effect on the flame retardancy of IFR-PP with a low content of OMMT. The synergistic effect of OMMT on the intumescent flame-retardant thermoplastic polyurethane composites was reported by Li et al. [16]. They reported an increase in the LOI of the TPU composites from 17.2 to 28 and a UL-94 ranking of V-0 with the addition of 22% intumescent flame retardant and 3% OMMT fillers.

In this work, TPU nanocomposites were prepared via the melt-mixing technique using a primary flame retardant (antimony trioxide Sb₂O₃) and a secondary flame retardant (nanoclay–montmorillonite). Moreover, the synergic effect of the two additives on the efficiency and improved properties of the fire resistance was observed.

2. Experiments

2.1. Materials

The chemicals used for this study were acquired from different sources. All the chemicals were of laboratory grade and were used as procured. Thermoplastic polyurethane (TPU) was supplied by Gansu Jinchuan Hengxin Polymer Technology Co., Ltd. (Jinchang, China). Antimony trioxide (Sb₂O₃) and montmorillonite surface-modified organo-clay were purchased from Sigma Aldrich (St. Louis, MO, USA). Benzoflex, a common plasticizer, was purchased from Eastman plasticizers.

2.2. Synthesis of Nanocomposites

All samples were prepared by the melt-mixing method; thermoplastic polyurethane (TPU) was melted and stirred in an internal mixture at 60 rpm for a period of 10 min at a temperature of 190 °C for pure TPU (TPU-1). The samples (TPU-2 to TPU-6) with different filler quantities were also prepared, as described in

Table 1. The components and their ratios used for the experiment.

Raw Material (wt. %)	TPU-1	TPU-2	TPU-3	TPU-4	TPU-5	TPU-6
TPU	100.00	97.00	97.00	94.00	93.80	93.50
Benzoflex		3.00		3.00	3.00	3.00
Sb2O3			3.00	3.00	3.00	3.00
MMT nanoclay					0.20	0.50
Total wt. %	100.00	100.00	100.00	100.00	100.00	100.00

. All the samples were prepared under similar operating parameters (190 °C temperature, 60 rpm, and 10 min mixing time). This ensured that the additives were completely dispersed in the composites to obtain excellent properties [17]. Excellent dispersion is necessary to obtain the optimized features of the nanocomposites. Agglomeration decreases the performance of the nanocomposites, because of the formation of voids that tend to become the sites of cracks [18,19,20]. All these nanocomposites were pressed using a hydraulic press at 180 °C for 10 min to make sheets of 4 × 7 inches.

2.3. Characterization

Functional groups present in the components as well as in the nanocomposites were inspected by using Fourier transform infrared spectroscopy (FTIR), using model Nicolet 6700 in nitrogen gas. The wavelength ranged from 4000 to 400 cm⁻¹, the number of scans was 128, and the resolution was 8. SEM (SEM FEI brand model S50) was performed to examine the dispersion of fillers in the matrix. The Young’s modulus and yield stress were examined according to ASTM D3039 using rectangular sample strips (7 × 0.5 inches) of 1 mm thickness. The flammability of the standard specimens was tested by Underwriters Laboratories (UL-94) in a vertical configuration. It showed the tendency of the polymer composite to permit the spread of flame. Finally, the samples were tested to find the limiting oxygen index (LOI) value according to the ASTM standard (ASTM D 2863).

3. Results and Discussion

3.1. Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR spectrum of organo-modified montmorillonite (O-MMT) was recorded in the 4000–500 cm⁻¹ range as shown in Figure 2a. There were several peaks corresponding to the respective functional groups in O-MMT. The stretching band at 3618 cm⁻¹ and 3420 cm⁻¹ indicated the presence of a hydroxyl group (-OH). The peak at 1640 cm⁻¹ confirmed the bending in-plane vibrations of the OH group. The peaks at 2923 cm⁻¹ and 2843 cm⁻¹ presented the symmetric and nonsymmetric stretching vibrations of a methylene group (-CH₂), while that at 1479 cm⁻¹ was for the methyl group (-CH₃). A small peak at 911 cm⁻¹ showed the in-plane stretching vibrations of the Si-O bonds (Tetrahedral silica layers). All the peaks were confirmed by the literature [21,22,23].

Figure 2b represents the FTIR of pure TPU (TPU-1) and its composites with MMT (TPU-6). Both spectra showed the characteristic peaks of polyurethane. The peak at 3320 cm⁻¹ was associated with the hydrogen-bonded –NH group. The peaks at 2923 cm⁻¹ and 2853 cm⁻¹ belonged to the nonsymmetric and symmetric stretching vibrations of the –CH groups, respectively. C=O was confirmed from the peak position at 1730 cm⁻¹ (stretching vibrations of free C=O) and 1702 cm⁻¹ (hydrogen bonded C=O). The peaks at 1126 cm⁻¹ and 1050 cm⁻¹ were due to the stretching vibrations of the O-Si-O group. All these peaks were confirmed by the literature [24,25]. In the above composites, the polymer chains were well incorporated and intercalated into the OMMT galleries; however, the peak positions for the neat polymer were identical to those of the composites, indicating that the chemical structure of the polymer was not affected by adding filler [26].

3.2. Scanning Electron Microscopy (SEM)

SEM analysis was performed for three samples to examine the microstructure of the TPU and the effect of the addition of antimony trioxide and organo-clay on the dispersion of fillers inside the polymer matrix. A continuous matrix of polymer (TPU-1) is seen in Figure 3a. Figure 3b,c shows the morphology of the composite with different magnifications in which the antimony trioxide was uniformly dispersed in the polymer matrix (TPU-3). The dispersion of the filler (white particles) can be seen from the SEM images. In Figure 3d,e, uniform dispersion of the organo-clay along with antimony trioxide at different magnifications can be observed in the TPU-6 samples. This uniform dispersion is the key reason behind the improvement of the mechanical properties of the composites. These results show that there was a synergistic effect between organo-clay and antimony trioxide, which resulted in the improvement of the flame retardancy and mechanical strength of the TPU composites.

3.3. Tensile Test

The mechanical features of all the specimens were examined with tensile testing. Figure 4 shows the stress/strain curves of all the polymer–filler composites. These curves were found to be unique for each sample and were established by recording the extension or deformation (strain) at different intervals of standard force (tensile) loading.

The detailed tensile properties of the TPU composites at different compositions are represented in

Table 2. Mechanical features of the TPU-filler composites.

Sample	Young’s Modulus (MPa)	Tensile Strength (MPa)	Stress at Break (MPa)	Strain at Break (%)
TPU-1	147.348	4.234	422.04	4.40
TPU-2	161.811	6.105	612.56	6.99
TPU-3	147.641	5.194	517.56	6.24
TPU-4	195.053	6.848	647.86	8.12
TPU-5	244.568	6.488	684.72	8.35
TPU-6	180.989	6.408	638.52	7.86

. The tensile strength and Young’s Modulus of the pure TPU were 4.234 MPa and 147.348 MPa, which increased to 6.105 MPa and 161.811 MPa, respectively, with the addition of 3% Benzoflex (TPU-2). This plasticizer has nothing to do with flame retardance, but it greatly improves the mechanical strength. The next nanocomposite sample (TPU-3) containing 3% antimony trioxide also showed improved tensile strength as compared to the pure TPU (TPU-1). A low quantity of inorganic filler (such as antimony trioxide) improves the strength of composites because of its strong mechanical features [27,28]. However, this improvement was less than that of the TPU-2 because this sample lacked the plasticizer, the main cause of the improvement in the mechanical strength of the polymer. With the addition of both the plasticizer and the primary flame retardant Sb₂O₃ (3% of both), the mechanical strength further improved. The Young’s Modulus of the sample TPU-5 with the previous nanofillers plus 0.2% of the secondary flame retardant organo-clay o-MMT was exceptionally improved to 244.568 MPa. This improvement was credited to the polymer–clay tethering and hydrogen bonding among the composite [29]. However, the mechanical strength of the sample of interest (TPU-6) with 0.5% o-MMT decreased slightly. With the increase in nanofillers, agglomeration occurred resulting in poor compatibility between the nanofillers and the TPU molecular chains [3,30]. However, based on the mechanical properties, it can be concluded that the mechanical strength of the sample of interest (TPU-6) was high enough to meet the needs of the commercial standards.

3.4. Limiting Oxygen Index (LOI)

The flame performance of the nanocomposites was determined generally by the Limiting Oxygen Index (LOI). The LOI is the least concentration of oxygen, found in a flowing oxygen-nitrogen mixture, that supports a material’s combustion. It signifies the capacity of the polymer to endure the fire. A higher LOI value means that it has a higher ability for fire resistance, i.e., it is difficult to ignite [31]. The following equation was used to calculate the LOI values.

n% = O₂ (O₂ + N₂)⁻¹ × 100,

(1)

where n% = LOI;

O₂ = Volumetric speed of flow of O₂ gas (mL s⁻¹);

N₂ = Volumetric speed of flow of N₂ gas (mL s⁻¹) [12].

The flame-retardant behavior of the nanocomposites is described in

Table 3. Flame retardant performance of composites.

Sample	LOI	UL-Ranking	UL-Dripping	t1/t2 (s) a	N b
TPU-1	19	V-2	yes	>50	>28
TPU-2	19	V-2	yes	42/15	>18
TPU-3	21	V-1	yes	20/10	>12
TPU-4	21	V-1	yes	18/8	>10
TPU-5	26	V-1	yes	9/7	0/3
TPU-6	29	V-0	no	0/2	0

^a Total combustion time after first/second application of flame. ^b The average number of drops after first/second application.

. The LOI value of the TPU was enhanced when the flame retardants were added. The LOI value was the lowest (19) in TPU-1, where no flame retardant was added. In TPU-2, Benzoflex was added to the TPU matrix, which is a plasticizer and not a flame retardant; so, the LOI remained the same, i.e., 19. The LOI increased to 21 with the addition of the antimony trioxide flame retardant in TPU-3. The inorganic Sb₂O₃ improved the flame retardancy of the composites because it is noncombustible and decreased the thermal conductivity and delayed the heat transfer to protected blends. It melted as the combustion started and formed a protective layer on the surface of the polymer nanocomposite; hence, it kept the polymer from contacting the oxygen. At elevated temperatures, antimony trioxide vaporizes and dilutes the oxygen present in the vicinity, and in this way, it acts as a flame retardant [27].

Further enhancement in the LOI up to 26% was noticed when as little as 0.2% of MMT nanoclay was added (TPU-4), keeping the above fillers the same. The maximum value of the LOI, 29%, was obtained when the quantity of MMT nanoclay was increased to 0.5%. This is because organo-clay forms a thermally insulating layer of carbonaceous (inorganic) residues while burning, which shields the heat and retards the burning of the underlying polymeric material [20]. It can be concluded that a large quantity of flame retardants is needed to achieve the self-extinguishing features of polymers [32]. The above results indicated good synergic effects of the antimony trioxide and MMT nanoclay on the flame retardance of the TPU polymer.

3.5. UL-94 Flammability

The results of the UL-94 flammability of all samples are summarized in Table 3. The pure polymer (TPU-1) showed high UL-94 flammability results. It caught fire easily, and the fire propagated considerably along with the melting of the polymer. Upon adding 3% plasticizer (TPU-2), the polymer’s ability to catch fire and its propagation decreased to some extent. In TPU-3, the antimony trioxide (primary flame retardant) presented good flammability behavior. In TPU-4, the ability to catch fire, as well as the propagation, was reduced, but dripping still occurred. In TPU-5, the dripping of the polymer was very little, and the flame resistance of the polymer was greatly enhanced. The lowest flammability properties out of all the samples were shown by TPU-6. It did not catch fire, and neither did it melt or drip.

4. Conclusions

In this study, different loadings of a primary flame retardant, antimony trioxide, and a secondary flame retardant, montmorillonite, were optimized to design the best combination of flame-retardant additives for the TPU polymer matrix. The FTIR results confirmed the presence of the characteristic functional groups peaks of polyurethane in the nanocomposites. SEM analysis showed the uniform dispersion of fillers in the polymer matrix. The tensile test showed that the mechanical strength of the samples increased with the addition of fillers. The UL 94 flammability and LOI results showed that an improvement in flame performance and tensile strength was achieved, and it was optimum for TPU-6. This confirms the synergistic effect of primary and secondary flame-retardant fillers and the reduced flammability to the V-0 level.