Synergistic Flame Retardant Effect of Ammonium Polyphosphate and Aluminum Hydroxide on Polyurethane

ZENG Lijuan ,YANG Liu ,AI Lianghui ,YE Zhibin ,LIU Ping＊

(1.State Key Laboratory of Luminescent Materials and Devices,Research Institute of Materials Science,South China University of Technology,Guangzhou 510640,China;2.Innova Electronic Materials Co.Ltd.,Foshan 528399,China)

Abstract: The flame-retardant properties of polyurethane (PU) containing ammonium polyphosphate(APP) and aluminum hydroxide (ATH) were investigated.Moreover,the flame retardant performance was investigated through thermogravimetric analysis,limiting oxygen index (LOI),vertical combustion (UL 94),and cone calorimeter.When 15 wt% APP and 5 wt% ATH were added together,the PU/15%APP/5%ATH sample shows better thermal stability and flame-retardant properties.When 15 wt% APP and 5 wt% ATH were added together,the LOI value of the PU/15%APP/5%ATH sample was 30.5%,and UL 94 V-0 rating was attained.Compared with PU,the peak heat release rate (PHRR),total heat release (THR),and average effective heat combustion (av-EHC) of the PU/15%APP/5%ATH sample decreased by 43.1%,21.0%,and 29.4%,respectively.In addition,the flame-retardant mechanism was investigated through cone calorimeter.The APP/ATH addition simultaneously exerted condensed phase and gas phase flame retardant effects.APP and ATH have synergistic flame retardant properties.

Key words: polyurethane;ammonium polyphosphate;aluminum hydroxide;synergistic flame retardance;flame retardancy mechanism

1 Introduction

Polyurethane (PU) has many excellent properties,such as heat insulation,sound insulation,elasticity,wear resistance,oil resistance and water resistance.Based on the molecular design,PU products with different properties can be prepared by different raw materials.Therefore,PU is widely applicable in construction,furniture making,packaging,automobile,and aerospace,

etc

.

PU is a flammable material that is easily ignitable even in small flames.This is due to the high oxygen,carbon,and hydrogen content in the chain.Therefore,it is very important to improve the flame retardancy of PU.In the early years,the flame retardants used in PU were halogen compounds.Traditional halogencontaining flame retardants are widely used due to their excellent compatibility with other materials and high flame retardancy.However,halogen-containing flame retardants release many toxic substances and corrosive smoke during combustion.

Flame retardants containing phosphorus and nitrogen have attracted significant attention because of their good flame retardancy,low flue gas toxicity,and less additive.As a phosphorus-nitrogen flame retardant,ammonium polyphosphate (APP) is characterized by its thermal expansion (carbonization),isolation (air),heat insulation (reduces the ignition point),and flame retardancy mechanism of the gasphase flame retardant.

As an environmentally friendly inorganic flame retardant,aluminum hydroxide (ATH) is widely investigated due to its flame retardancy mechanism,which is that of the condensed-phase flame retardant.ATH decomposes to form alumina.A synergistic effect could be achieved by adding ATH a as synergist to other flame retardants through physical blending.

In our previous study,we prepared series of organic boron derivatives,including 2,4,6-tris(4-boronic-2-thiophene)-1,3,5-triazine (3TT-3BA),2,4,6-tris-(4-boron phenoxy)-(1,3,5)-triazine (TNB),(4-boronic acid-phenoxy)-cyclophosphazene (CP-6B) and organic/inorganic synergistic flame retardants(TNB/APP),and investigated their flame retardant properties.The results obtained indicated that the organic boron derivatives gave good flame-retardant properties for epoxy resin (EP) and polyethylene (PE).Herein,to develop flame-retardant PU,a composite material consisting of ammonium polyphosphate (APP)and aluminum hydroxide (ATH) was prepared.The synergistic flame-retardant effect of APP/ATH on PU was investigated,and the flame retardancy mechanism was also discussed.

2 Experimental

2.1.Materials

Ammonium polyphosphate (APP,99% +),Aluminum hydroxide (Al(OH),99.8%,10 μm)were purchased from Shanghai Mclean Biochemical Technology Co.,Ltd.Polypropylene glycol (PPG),toluene diisocyanate (TDI) were obtained from Wanhua Chemical Group Co.,Ltd.Trimethylolpropane (TMP)was supplied by Tianjin Kemio Chemical Reagent Co.,Ltd.

2.2 Synthesis of PU

Polypropylene glycol (PPG) was added into three flasks at a controlled temperature of 70-90 ℃.Next,toluene diisocyanate (TDI) was added in drops with continuous stirring at 100-120 r/min.The reaction was carried out for 3-4 h,and the reaction temperature was controlled at 70-80 ℃.Then the crosslinking agent,TMP,was added at 60 ℃ for 0.5 h.Afterward,the system was vacuumed to remove the bubbles and PU was obtained.

2.3 Characterization

Fourier transform infrared (FTIR) spectra were recorded using a Nicolet 6700 FTIR spectrometer(Madison,WI,USA).Thermogravimetric analysis(TG) was performed using a Netzsch 209 F3 thermal analyzer (Selb,Germany) at a heating rate of 10 ℃/min under nitrogen atmosphere.The limiting oxygen index (LOI) was tested using a Fire Testing Technology(FTT,East Grinstead,UK) instrument,according to ASTM D2863.The dimensions of each sample were 80 mm ×10 mm × 4 mm.Vertical burning (UL 94)tests were performed with a FTT UL 94 instrument by using samples with dimensions of 125 mm×12.7 mm×3.2 mm using an FTT UL 94 instrument,according to ASTM 3801 guidelines.The burning grades were classified as either V-0,V-1,V-2,or no rating (NR),depending on the self-extinguishing time and dripping effect.Cone calorimeter (cone) tests were performed using an FTT instrument according to ISO-5660 guidelines,with an incident flux of 50 kW/m.The dimensions of the samples were 100 mm ×100 mm× 3 mm.

2.4 Preparation of flame-retardant samples

Table 1 shows the weight ratios of the PU samples.Different weights ratio of APP and ATH were added to the PU.The resulting mixture was heated to 80 ℃ and stirred to homogeneity.The mixture was poured into molds and cured at 60 ℃ for 2 h,80 ℃for 2 h,100 ℃ for 2 h,and 120 ℃ for 2 h.Finally,the samples were slowly cooled to room temperature.

3 Results and discussion

3.1 The rmogravimetric (T G) and differential thermal gravity (DTG)analysis

Fig.1 shows the TG (left) and DTG (right)curves of the PU,PU/APP,PU/ATH,and PU/APP/ATH samples.Their thermal decomposition data are summarized in Table 2.The TG curve of the PU sample showed two weight-loss stages under a nitrogen atmosphere.The weight loss at 240-360 ℃mainly involved the degradation of the hard segment carbamate,and the decomposition at 360-480 ℃mainly involved the degradation of soft segment polyether polyols.The PU sample had poor flameretardant properties and a residual char yield of 1.5%at 800 ℃.When APP was added,the PU/APP sample exhibited more obvious two weight-loss stages and two differential DTG peaks,correspondingly.The first stage was in the temperature range of 240-330 ℃,corresponding to a DTG peak (

T

).Subsequently,the second stage was in the temperature range of 330-410 ℃,corresponding to a strong DTG peak (

T

).Compared with the PU sample,

T

and

T

of the PU/APP sample appeared earlier due to the addition of APP,which promoted the decomposition of PU.When a 20 wt% APP was added,the residual char yield at 800 ℃ reached 9.9%.It was also observed that the carbon residue of the PU/APP sample at 800 ℃ was higher than that of the PU sample.The carbon residue increased with an increase in APP content.These results indicate that the addition of APP promoted the formation of a protective char and acted as a flame retardant.Through heating,APP was decomposed into acidic substances (phosphoric acid,metaphosphoric acid,

etc

),which enabled the dehydration of PU to form a carbon layer.Besides,the APP released noncombustible gases (ammonia (NH),nitrogen (N),nitrogen dioxide (NO),

etc

),prevented combustion,and acted as the flame retardant.

Fig.1 (a) TG and (b) DTG curves of PU,PU/APP,PU/ATH,and PU/APP/ATH samples under a nitrogen atmosphere

Initial temperature at 5% mass loss; Temperature at the maximum mass loss rate; Residual char yield at 800 ℃

The PU/20 wt%ATH sample exhibited three weight-loss stages and three differential DTG peaks,correspondingly.The three stages were in the temperature ranges of 240-310 ℃,310-360 ℃,and 360-440 ℃,respectively,corresponding to DTG peaks of

T

T

，

T

.A residual char yield of 10.1%was attained for the PU/20 wt%ATH sample at 800 ℃.In the first weight-loss stage (240-310 ℃),aluminum hydroxide was dehydrated,and the residual char containing aluminum oxide was very stable,which acts as a barrier layer to cover the surface of the substrate and thus plays a role in the flame retardancy.When APP and ATH were added together,the PU/APP/ATH sample exhibited two weight-loss peaks.In the first weight-loss stage,ATH was dehydrated and degraded to aluminum oxide (AlO),which catalyzed the release of NHand water from APP to produce aluminum polyphosphate and aluminum metaphosphate.At the same time,APP promoted the degradation of urethane.In the second weightloss stage,the PU soft segment degradation and the evaporation of aluminum metaphosphate and phosphorus pentoxide occurred.The PU/APP/ATH sample simultaneously exerted the gas phase flame retardant effect of APP and the condensed phase flame retardant effect of ATH.

3.2 Limiting oxygen index (LOI) and vertical burning

Fig.2 (a) HRR and (b) THR curves of PU,PU/APP,PU/ATH,and PU/APP/ATH

LOI and UL 94 tests were conducted to study the flame-retardant properties of the materials.Table 3 shows the LOI values and vertical burning ratings of PU,PU/APP,PU/ATH,and PU/APP/ATH samples.Once the PU was ignited,it was easy to burn,accompanied by dripping.The LOI value of PU was determined as 18.4%.Thus,PU could not pass the UL 94 test.When APP was added to PU,the LOI value of the PU/APP sample increased.The LOI values of PU/5 wt%APP,PU/10 wt%APP,and PU/15 wt%APP samples were recorded as 24.1,24.4,and 24.6%,respectively.The dripping phenomenon could be improved by increasing the APP content in PU.No dripping effect was observed for the PU/20 wt%APP sample,and the UL-94 level reached the V-0 level.As an acid source,APP promoted the dehydration of the system to form carbon,and released phosphorusoxygen radicals (PO·),which can quench the active radicals of the combustion reaction in the gas phase and terminate the free radical reaction.At the same time,the non-combustible gas produced during combustion diluted the surrounding oxygen concentration,preventing further combustion.With the increase of the APP content,the flame retardant effects are more obvious.However,when more than 20 wt% APP was added,the flame retardancy decreased as a result of the compatibility with PU.When 20 wt% ATH was added to PU,the LOI and dripping effect did not improve significantly.This is because when the carbon residue of PU/20%ATH at 80 ℃ was 10.06%,the alumina powder didn’t promote the formation of the carbon layer to protect the PU.

When APP and ATH were added at the same time,the ATH decomposed and released a lot of water vapor,which took away a lot of heat brought about by the thermal decomposition through evaporation.Through evaporation,the water vapor content reduced from ATH,and the non-flammable gas formed from APP diluted the concentration of air and combustible gas,which improved the combustion difficulty.Also,the phosphoric acid generated by the decomposition of APP reacted with the aluminum hydroxide to form aluminum phosphate.The carbon layer covered with phosphate was more compact and stable,and its heat insulation,oxygen isolation,and smoke suppression properties were also better.When 15 wt% APP and 5 wt% ATH were added together,the LOI value of the PU/15 wt%APP/5 wt%ATH sample reached 30.5%and no dripping effect was observed.Also,the UL 94 reached a V-0 rating.The PU/15 wt%APP/5 wt%ATH sample simultaneously exerted the flame-retardant mechanism of APP and ATH.The APP/ATH combination achieved higher flame-retardant efficiency.

3.3 Cone calorimeter tests

Cone calorimeter test is a useful method for evaluating the fire behavior of materials.In general,the combustion parameters used in this method include time to ignition (TTI),heat release rate (HRR),peak heat release rate (PHRR),total heat release (THR),average effective heat combustion (av-EHC),fire performance index (FPI),fire growth index (FGI),and average mass loss rate (av-MLR).The relevant combustion data for PU,PU/APP,PU/ATH,and PU/APP/ATH samples are shown in Tables 4 and 5.

Fig.2 shows the HRR and THR curves of PU,PU/APP,PU/ATH,and PU/APP/ATH samples.The HRR of PU peaked at 95 s,and the PHRR of the PU sample was 950 kW/m.When 15 wt%APP/5 wt%ATH was added to PU,the HRR of the PU/15 wt%APP/5 wt%ATH sample peaked at 200 s,and the PHRR of the PU/15%APP/5%ATH sample was obtained as 540.6 kW/m.The THR of the PU sample was the highest,and its value was determined as 86.54 MJ/m.With an increase in the APP mass fraction,the THR decreased gradually.When 20 wt%APP was loaded,the THR decreased to 66.03 MJ/m.When 15%APP/5 wt%ATH was loaded,the THR decreased to 68.39 MJ/m,which is higher than that of the PU/20 wt%APP sample.This could be attributed to the increased THR brought about by the addition of ATH.However,the addition of 15%APP/5%ATH can still effectively reduce the THR value.The synergistic effect of the APP and ATH enhanced the dehydration and carbonization of PU and also reduced the condensed phase temperature.These effects suppressed the decomposition of PU and reduced its HRR and THR values.

Tables 4 and 5 show the typical cone test parameters of the PU,PU/APP,PU/ATH,and PU/APP/ATH samples.The effective heat combustion (EHC)reflects the degree of combustion of the volatile gas in the gas phase,which is useful for the analysis of the flame-retardation mechanism.With the increase of the APP content from 5% to 20%,the av-EHC value of samples decreased.The av-EHC value of the PU/APP/ATH sample was lower than that of the PU sample.The APP/ATH enabled the formation of a dense carbon from PU and inhibited the internal resin decomposition.At the same time,the decomposition of APP and ATH produced non-flammable gases and water vapor，which could dilute the oxygen and combustible gases produced through the decomposition of PU.

The FPI and FGI are important parameters for evaluating fire hazard.Table 5 lists the FPI values of PU,PU/APP,PU/ATH,and PU/APP/ATH samples.The FPI values of the PU/APP and PU/ATH samples were higher than that of the PU sample,while the FGI value of PU/APP and PU/ATH samples was lower than that of the PU sample.This indicates that the addition of APP or ATH could reduce the risk of fire.The FPI value of the PU/15%APP/5%ATH sample was almost 8 times higher than that of the PU sample,which implies that APP/ATH could increase the time available for people to escape a full-scale fire.Moreover,the FGI value of the PU/15 wt%APP/5 wt%ATH sample was only about 23.3% of the corresponding value of PU,which indicates that APP/ATH could effectively reduce the fire risk and the combustion intensity of PU.

Fig.3 Mass loss curves of PU,PU/APP,PU/ATH,and PU/APP/ATH samples

The mass-loss rate (MLR) expresses the rate of mass loss during the combustion of materials and can be used to clarify the flame retardation mechanism in the condensed phase.Fig.3 shows the mass loss curves of PU,PU/APP,PU/ATH,and PU/APP/ATH samples.When 15%APP/5%ATH was added,a minimum av-MLR was obtained and the residual char yield reached 16.9%.This indicates that the char layer and the nonflammable gases have a protective effect on PU.

Fig.4 Digital photographs of residual chars of (1) PU,(2) PU/5%APP,(3) PU/10%APP,(4) PU/15%APP,(5) PU/20%APP,(6) PU/20%ATH,and (7) PU/15%APP/5%ATH;A,top view;B,front view

Fig.4 shows the digital photographs of the residual chars after the cone calorimeter tests.The PU burned completely,with almost no carbon residue.An increase in the APP content increased the carbon layer of the PU/APP sample.When 15% APP was added to the PU sample,the residual carbon content of PU/15%APP could cover the base of the combustion chamber.These results are consistent with the TG and LOI data.The APP promotes the formation of the carbon layer and plays a role in the condensed phase.After further addition of ATH,a more complete carbon layer of PU/15%APP/5%ATH was observed,which indicates that ATH increases the action of APP in promoting a more robust carbon layer.Hence,this is the reason for the observation of the highest LOI value after adding 15%APP/5%ATH.

3.4 Morphologies of PU,PU/APP,PU/ATH,and PU/APP/ATH at different temperatures

Fig.5 shows the digital photographs of PU,PU/APP,PU/ATH,and PU/APP/ATH samples maintained at various temperatures for 15 min in a muffle furnace.The PU began decomposing on the surface at 250℃ and began expanding afterward at 300 ℃.The complete combustion then occurred at 650 ℃.A little amount of residual char was left behind in the crucible.By contrast,the PU/15%APP/5%ATH sample began to decompose and expand at 350 ℃.The decomposition of 15 wt%APP/5 wt%ATH was better than that of 20 wt% APP or 20 wt% ATH at 350 ℃.When maintained at 650 ℃ for 15 min,the amount of residual char from the PU/15 wt%APP/5 wt%ATH sample was still as high as 14.08 wt%.These results indicate the improvement of the thermal stability when APP/ATH was added to the PU sample.

3.5 FTIR spectra of PU composites

Fig.6 shows the FTIR spectra of PU,PU/20%APP,PU/20%ATH,and PU/15%APP/ 5%ATH residual chars.The residual char of PU/20%ATH displayed only a small amount of IR absorption peaks.The residual char of PU/20%APP and PU/15%APP/5%ATH displayed more IR absorption peaks,indicating the presence of several structures.The IR absorption peak at 839 and 1 004 cmcorresponds to the stretching and vibration absorption peaks of P-O-P,respectively.The IR absorption peaks at 1 176 and 1 251 cmcorrespond to the bending and stretching vibration absorption peaks of P=O,respectively.The disappearance of the O=POH and the P-OH absorption peaks were visible at 493 and 1 633 cm,respectively.The phosphoric acid and metaphosphoric acid from the pyrolysis of APP might have reacted with AlOfrom ATH to produce aluminum phosphate or aluminum polyphosphate,which played a condensed phase flame retardant effect.

4 Conclusions

Fig.5 Digital photographs of pure PU,PU/20%APP,PU/20%ATH and PU/15%APP/5%ATH after being maintained at various temperatures for 15 min in a muffle furnace

Fig.6 FTIR spectra of the residual char of PU,PU/20%APP,PU/20%ATH,and PU/15%APP/5%ATH

When the ratio of APP to ATH was 3:1 with 20 wt% content,the flame retardant effect of the PU sample was the best.In comparison with pure PU,PU/20%APP,and PU/20%ATH samples,the PU/15%APP/ 5%ATH sample had higher thermal stability,more residual carbon,and more complete and dense carbon layer.When 15 wt%APP and 5 wt%ATH were added to the PU sample,an LOI value of 30.5%and UL 94 V-0 level were obtained.In comparison with the PU sample,the PHRR of PU/15%APP/5%ATH decreased from 950.0 to 540.6 kW/m,THR decreased from 86.54 to 68.39 MJ/m,and av-EHC decreased from 20.1 to 14.2 MJ/kg.The PHRR,THR,and av-EHC of the PU/15%APP/5%ATH sample decreased by 43%,21.0%,and 29.4% respectively.APP and ATH exerted gas phase and condensed phase flame retardant effects when added to PU.Thus,APP and ATH can be used as organic/inorganic flame retardants of PU.