In this research, expandable graphite (EG) and ammonium polyphosphate (APP) were incorporated into water-blow semirigid polyurethane foam (SPUF) as flame retardants. The synergistic flame retardant effects of EG with APP in SPUF had been studied by thermogravimetric analysis (TGA), limiting oxygen index (LOI), horizontal burning test, polarizing microscope, and scanning electron microscopy (SEM). The results indicated that APP and EG used together in SPUF could effectively improve the flame retardancy of SPUF. When the EG to APP ratio reached 2 : 1 under the total content of flame retardants was kept constant as 20%, the LOI value was increased by about 51% compared to pure SPUF, 2.9% to SPUF/EG and 16.3% to SPUF/APP. Besides, the residual char was increased up to 27.7% displayed by TG test results. SEM shows that burned residues of EG/APP/SPUF present more compact char and worm-like structure. Furthermore, EG shows negative effect on the mechanical property of SPUF with 21.7% decrease in compression modulus, but the mechanical property can be improved by the addition of appropriate content of APP.
Water-blown polyurethane foam (WPUF) has been attracting more attention especially in the application closely related to human beings because the foaming agent, water, is cheap, safe, nontoxic, and free from any pollution [
Typically, the main flame retardants of polyurethane (PU) foam are compounds containing halogen, alone or in conjunction with antimony trioxide [
Expandable graphite (EG) is a “particular” intumescent additive known to impart fire retardant to various materials and in particular to PU foam [
The aim of this study is to increase the flame retardant property of water-blown SPUF without sacrifice of mechanical properties. Because of the high flame retardant efficiency of EG and the reinforcement effect of APP, semirigid WPUF with EG/APP of different content was prepared. Not only the synergistic flame retardant effect of EG and APP on SPUF is studied but also the most important is that the effects of EG and APP on the pore formation and the cell structure of semirigid WPUF are also studied by polarizing microscope (POM) and scanning electron microscopy (SEM). The mechanism for synergistic flame retardancy and mechanical properties enhancement of APP was proposed in detail.
Polyether polyol (330 N, average functionality 3.0, OH content 33∼37 mg KOH/g; 3630, average functionality 3.0, OH content 25∼29 mg KOH/g) was obtained from Jv Yi Polymeric Materials Co., China. Isocyanate (MDI, NCO wt%, 26∼27%) was obtained from Jv Yi Polymeric Materials Co., China. Triethanolamine (TEA) was supplied by Beijing Chemical Plant, China. Silicone oil (8631), amine catalysts, and organic tin catalyst were supplied by Jv Yi Polymeric Materials Co., China. Ammonium Polyphosphate (APP) with a polymerization degree<20 was supplied by Zhenjiang Star Flame Retardants Co.Ltd., China. Expansible graphite (EG) (E300) was bought from Yanhai Graphite Co. (Qingdao, China); its main property, i.e., expansive rate>270 ml/g. Distilled water was obtained in laboratory. All reagents and chemicals were used without further purification.
Polyol (330 N, 3630) was mixed with distilled water, catalysts (TEA, amine catalysts, and organic tin catalyst), foam stabilizer (silicone oil 8631), and flame retardant (EG, APP or EG/APP) and then fully stirred for 60 s in a 1 L plastic beaker. MDI was then added into the beaker with vigorous stirring for 5 s. MDI reacted with distilled water to generate carbon dioxide, of which carbon dioxide inflated the reactants. After that, the mixture was immediately poured into a laboratory-made foaming mold (15 × 15 × 6 cm3) to produce foam for 15 min. The foam was cured for 24 h under 50°C to increase the cross-link density and to make the reaction more sufficient. Total content of flame retardants used in SPUF was kept constant as 20%; the proportion of EG and APP in SPUF is shown in Table
Parameters of flame retardant SPUF.
Materials | Parts by weight (pbw) |
---|---|
Polyether polyol (330 N) | 50 |
Polyether polyol (3630) | 50 |
Triethanolamine | 2 |
Amine catalysts | 0.5 |
Organic tin catalyst | 0.1 |
Silicone oil | 1.2 |
Distilled water | 3.5 |
Isocyanate (MDI) | 79 |
Fire retardant | 20 |
The LOI values of the WPUF samples were measured according to ISO4598, using a flammability testing technology instrument (JF-3, Jiangsu, China). The dimensions of the specimen were 100 mm × 10 mm × 10 mm (
The horizontal burning tests were performed with a burning test instrument (H1011D, Changchun, China) according to UL-94. The specimens for measurement were machined into sheets of 130 × 70 × 10 mm3.
The thermal stability of WPUF samples were studied using a NETZSCH STA TG instrument (449F3, Germany) and a sample mass of 2−5 mg in a nitrogen environment at a heating rate of 10°C/min from 25°C to 800°C.
The morphology of the samples, including the original and the burned samples, were examined under a ZEISS EVO18 (Germany) scanning electron microscope (SEM) with an accelerating voltage of 20 kV.
The foam cross section specimen was observed by 59XC optical microscope of Xiamen McEdio Manufacturing Co., LTD.
The hardness of bulks was tested according to QC/T 29089-1992 with Shore A-type durometer, and the results were the average of ten tests. The compressive strength and the compressive modulus were measured with universal electronic tensile machine (WSM-5 KN, Changchun Intelligent Instrument Equipment Co., LTD, China) with compression rate of 25%, compression speed 10 mm/min according to ISO 604. The apparent density was measured according to GB/T6343-1995. The sample size was 50 mm × 50 mm × 50 mm, and six samples were tested and averaged.
The LOI value and UL-94 horizontal combustion test results of SPUF, APP/SPUF, EG/SPUF, and EG/APP/SPUF are shown in Table
Flammability of SPUF containing various concentrations of flame retardants.
Samples | EG (wt.%) | APP (wt.%) | Ratio of EG/APP | LOI (%) | UL-94 HBF | Combustion rate (mm/min)/self-extinguish time (s) |
---|---|---|---|---|---|---|
SPUF | 0 | 0 | — | 18.9 | — | 49.01 mm/min |
EG/SPUF | 20 | 0 | — | 27.7 | HF-1 | 14.825 s |
APP/SPUF | 0 | 20 | — | 24.5 | HF-1 | 27.25 s |
EG/APP/SPUF-1 | 3.33 | 16.67 | 1 : 5 | 25.3 | HF-1 | 15.80 s |
EG/APP/SPUF-2 | 4 | 16 | 1 : 4 | 27.7 | HF-1 | 12.98 s |
EG/APP/SPUF-3 | 5 | 15 | 1 : 3 | 27.6 | HF-1 | 12.42 s |
EG/APP/SPUF-4 | 6.67 | 13.33 | 1 : 2 | 27.8 | HF-1 | 11.42 s |
EG/APP/SPUF-5 | 8 | 12 | 2 : 3 | 27.2 | HF-1 | 11.26 s |
EG/APP/SPUF-6 | 10 | 10 | 1 : 1 | 27.4 | HF-1 | 11.02 s |
EG/APP/SPUF-7 | 12 | 8 | 3 : 2 | 28.1 | HF-1 | 10.79 s |
EG/APP/SPUF-8 | 13.33 | 6.67 | 2 : 1 | 28.5 | HF-1 | 10.00 s |
EG/APP/SPUF-9 | 15 | 5 | 3 : 1 | 28.3 | HF-1 | Non-ignitable |
EG/APP/SPUF-10 | 16 | 4 | 4 : 1 | 28.2 | HF-1 | Non-ignitable |
EG/APP/SPUF-11 | 16.67 | 3.33 | 5 : 1 | 28.6 | HF-1 | Non-ignitable |
Thermogravimetric analysis (TGA) is widely used to evaluate the thermal stability of polyurethane [
TG and DTG curves of EG, APP, SPUF, and SPUF composites.
Thermal properties of flame retardants and SPUF composites.
samples |
|
|
|
|
|
|
Char residue (%) |
---|---|---|---|---|---|---|---|
SPUF | 215 | 388, 1.16 | 280∼450 | 450∼600 | — | — | 13.1 |
EG | 200 | — | 200∼380 | — | — | — | 78.7 |
APP | 110 | — | 110∼215 | 220∼280 | 280∼358 | 360∼437 | 27.8 |
EG/SPUF | 280 | 392, 0.81 | 215∼450 | 450∼600 | — | — | 27.9 |
APP/SPUF | 110 | 358, 1.05 | 110∼230 | 260∼400 | 400∼488 | — | 19.2 |
EG/APP/SPUF-8 | 120 | 385, 0.79 | 120∼215 | 270∼450 | — | — | 27.7 |
However, the decomposition of APP/EG/SPUF takes place at two independent stages: 120−215°C and 270−450°C. The degradation of SPUF with flame retardant of both APP and EG at 120−215°C was attributed to the initial weight loss of APP. The second degradation step at 270–450°C was caused by the decomposition of EG and the depolymerization of polyurethane to form isocyanate, polyol, primary or secondary amine, olefin, and carbon dioxide. Due to the decomposition of APP and carbonization of EG under lower temperature, the decomposition rate of APP/EG/SPUF is decreased to 0.79%/°C, but the
The char layers’ micrographs of SPUF, APP/SPUF, EG/SPUF, and EG/APP/SPUF are shown in Figures
SEM micrograph of SPUF composites’ char residue obtained from LOI test.
The mechanical properties of SPUF composites are listed in Table
Influence of EG/APP proportion on the performances of SPUF.
Samples | Ratio of EG/APP | Apparent density (kg/m³) | Shore’s hardness (HA) | Compression strength (MPa) | Modulus of compression (MPa) |
---|---|---|---|---|---|
SPUF | \ | 46.60 ± 0.76 | 55.64 ± 1.32 | 0.056 ± 0.002 | 0.75 ± 0.04 |
APP/SPUF | \ | 61.56 ± 0.97 | 64.77 ± 2.10 | 0.064 ± 0.004 | 1.25 ± 0.06 |
EG/SPUF | \ | 68.06 ± 1.43 | 58.82 ± 1.33 | 0.051 ± 0.001 | 0.59 ± 0.05 |
EG/APP/SPUF-2 | 1 : 4 | 59.60 ± 0.36 | 56.63 ± 1.12 | 0.053 ± 0.003 | 0.70 ± 0.04 |
EG/APP/SPUF-4 | 1 : 2 | 66.52 ± 1.02 | 59.54 ± 0.65 | 0.047 ± 0.002 | 0.45 ± 0.02 |
EG/APP/SPUF-8 | 2 : 1 | 61.88 ± 0.63 | 58.08 ± 0.53 | 0.044 ± 0.002 | 0.40 ± 0.02 |
POM morphology of SPUF composites.
The compressive strength and modulus are increased by 13.4% and 66.7%, respectively, in APP/SPUF sample, which can be explained by its high apparent density. For the sample with EG, although the apparent density is also high, the compressive strength and modulus are decreased, and the values are lower with the content of EG increasing. The sample of EG/APP/SPUF-2 shows better compressive strength and modulus than EG/SPUF. This is caused by the damage of the cell structure by EG, as we discussed in the apparent density paragraph. Besides the apparent density, the cell morphology is another important factor that affects the mechanical properties of foam [
SEM micrograph of SPUF composites before burning.
Compared with neat SPUF, Shore’s hardness of SPUF containing flame retardants is increased. The surface of the foam became harder because SPUF foam is flexible, while the additives are rigid. All SPUF containing flame retardants in this research with hardness within the range of 55 to 75 HA can be used as the interior parts of automobile.
This research investigated the synergistic effect between EG and APP on flammability and thermal behavior of water-blow SPUF. The study shows that the addition of APP and EG improved the flame retardancy and thermal stability of SPUF effectively. SPUF added EG/APP obtained higher LOI value and carbon residue than that with the same loading of EG or APP. TG results show that the addition of APP accelerates the decomposition of SPUF but slows down the decomposition rate of SPUF; the combination of EG with APP improved the thermal stability significantly. SEM morphology of char residue shows the “worm-like” structure developed by EG form the more compact char layer, which proved that EG acts as the main protective role in the improvement of thermal stability. The presence of EG destroys the completeness of the hole in the foam; as a result, the compression property of SPUF composites decreased. While, APP has positive effect on the mechanical properties of SPUF, and the addition of APP with small amount to the EG/SPUF can improve the compression property to a certain extent.
The data (limiting oxygen index (LOI); UL-94 horizontal burning test; mechanical properties) used to support the findings of this study are included within the article. The data (thermogravimetric analysis (TG); scanning electron microscopy (SEM); polarizing microscope) used to support the findings of this study have been deposited in the figshare repository (
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the Science and technology development project of Jilin Province, China (no. 20150301002GX).