Microencapsulated ammonium polyphosphate by polyurethane with segment of dipentaerythritol and its application in flame retardant polypropylene☆

Shouwu Yu,Shujuan Xiao,Zewen Zhao,Xiaowen Huo,Junfu Wei*

1 School of Material Science and Engineering,Tianjin Polytechnic University,Tianjin 300387,China

2 College of Materials Science and Engineering,North China University of Science and Technology,Tangshan 063009,China

ABSTRACT Dipentaerythritol(DPER),4,4′-diphenylmethanediisocyanate(MDI)and melamine(MEL)are used as raw materials to microencapsulate ammonium polyphosphate (MAPP) in situ polymerization.The MAPP is characterized by Fourier transform infrared (FT-IR),scanning electron microscopy (SEM),transmission electron microscopy (TEM) and thermal gravimetric analysis (TGA).The results show that the coating operation can effectively improve water resistance of ammonium polyphosphate (APP),and MAPP has higher residual rate than that of APP after combustion.The flame retardant action of MAPP and APP in polypropylene (PP) is investigated by the limited oxygen index (LOI),vertical burning test (UL-94),TGA,SEM,and cone calorimeter test(CCT).The LOI value of the PP/MAPP composite at the same loading is higher than that of PP/APP composite.UL 94 ratings of PP/MAPP composites are raised to V-0 at 20 wt%loading.The results of CCT also show that MAPP is more efficient than APP.The morphological structures observed by digital photos and SEM demonstrated that MAPP could be promoted to form the continuous and compact intumescent char layer.The flame retardant mechanism of PP/MAPP is also discussed.

Keywords:Microencapsulation Solubility Ammonium polyphosphate Pyrolysis Flame retardant Stability☆ Supported by the Natural Science Foundation of Hebei Province(B2016209059).

1.Introduction

PP is one of the five popular plastics,widely used in all aspects of production and life.PP is easy to burn and difficult to flame retardant.It is of great significance to carry out in-depth research on flame retardance of PP.Traditional halogen containing flame retardants have high flame retardance efficiency when used for flame retardant PP,but they release smoke and toxic gases during combustion,and are being replaced by new environmentally friendly flame retardants.Intumescent Flame Retardant (IFR) has attracted more and more attention because of its advantages such as halogen-free,low smoke,low toxicity and anti-droplet[1-4].At present,the widely used IFR system is mainly composed of APP,pentaerythritol (PER) and MEL.APP,acting as an acid source and gas source in IFR,has strong polarity,poor compatibility with PP matrix.APP is water-soluble and gradually migrates from the matrix to the surface when in use,which not only reduces the flame retardance,but also affects the mechanical properties [5].Microencapsulation of APP is an effective way to change its surface properties,reduce its water solubility and improve its flame retardance [6,7].

It has been reported that a variety of materials can be used in the coating of APP,such as melamine resin [8-10],poly(vinyl alcohol)-melamine-formaldehyde resin[11],epoxy resin[12],various types of polyurethane [13-16],urea formaldehyde resin [17],urea-melamine-formaldehyde resin,4,4′-oxydianiline-formalde hyde resin [18],melamine containing polyphosphazene [19],melamine-formaldehyde-tris(2-hydroxyethyl)isocyanurate resin[20],Allyl-containing benzoxazine[21],and hybrid silicon materials [22-24].It is also reported that the introduction of carbon source and blowing agent into the microcapsules can further improve the flame retardant efficiency [25].

In this article,DPER,MDI and MEL were used as raw materials to microencapsulate APP in situ polymerization.The polyurethane containing DPER was formed.Carbon source and gas source were introduced on the surface of APP particles,so that the microencapsulated APP particles formed carbon source,acid source and gas source.The “three sources in one”flame retardant not only has good compatibility and water resistance,but also shows good flame retardance when used alone.

2.Experimental

2.1.Materials

APP was provided by Hubei Liushugou Chemical Technology Co.,Ltd.Diphenylmethane 4,4′-diiso-cyanate(MDI)and MEL were purchased from Shanghai Aladdin Biochemical Technology Co.,Ltd.PP resin was manufactured by Tianjin Branch of China Petrochemical Corporation.DPER was produced by Jiangsu Kailinruiyang Chemical Co.,Ltd.Ethyl acetate was from Nanjing Chemical Reagent Co.,Ltd.

2.2.Preparation of microencapsulated APP(MAPP)

APP (100 g),MDI (0.04 mol),MEL (0.04 mol) and ethyl acetate(300 ml) were fed into a 1000 ml four-neck flask which was equipped with a stirrer,a thermometer,a dropping funnel and a reflux condenser.The reaction was kept at 55°C for 4 h.Then 60 ml ethyl acetate solution containing MDI(0.04 mol)was rapidly added to the four-neck flask.The reaction temperature was raised to 75°C and the reaction continued for 2 h.Then 100 ml ethyl acetate dispersion containing DPER (0.06 mol) was added to the flask,and the reaction continued for 4 h.The whole reaction process was carried out under the protection of N2.

Finally,the product was cooled to room temperature,filtered,cleaned with ethyl acetate,and then cleaned with deionized water.The final product was white powder.

The reaction is shown in Fig.1.

2.3.Preparation of flame retardant PP

Fig.1.Preparation of MAPP.

Fig.2.FT-IR spectra of APP and MAPP.a—APP;b—MAPP.

PP,APP and MAPP were dried in a vacuum oven at 80°C for 12 h before use.They were added to the torque rheometer in proportion.The blending temperature was 185°C,the speed was 60 r·min-1,and the blending time was 15 min.Then the mixture was loaded into the mold and hot-pressed for 10 min in the plate vulcanizer to produce various specimens for LOI,UL and CCT.

2.4.Characterization and analysis

FT-IR was recorded on a VERTEX70 (Bruker,Germany) spectrometer,which the samples were prepared with KBr pellets.

Fig.3.SEM images of APP and MAPP.

The samples were observed by an S-4800 scanning electron microscope (SEM) produced by Hitachi Instruments Company of Japan.The surface of samples was sputter-coated with a gold layer before examination.

The MAPP was tested by a JEM-2010 transmission electron microscope (TEM) of Japan Electronics Corporation.The acceleration voltage was 200 kV.

The thermogravimetric analysis(TGA)was carried out on a STA-449 thermal analyzer (Netzsch,Germany),in nitrogen flow,at a heating rate of 10°C·min-1.

The flame retardancy of the sampleswas characterized by LOI and UL-94 methods.The LOI data of all the samples were obtained at room temperature from an oxygen index instrument (JF-3,China)in accordance with the GB/T2408-2008 standard.The dimensions of all the samples were 100×10×4 mm3.The vertical burning rates of all the samples were measured by a CZF-3instrument produced by Jiangning Analysis Instrument Factory,with the sample dimensions of 125×12.5×3.2 mm3in accordance with the UL-94 standard.

The CONE data were taken from Cone Calorimeter(Fire Testing Technology,UK) at a heat flux of 35 kW·m-2in accordance with the ISO 5660 standard.The sample size is 100×100×3 mm3.

3.Results and Discussion

3.1.Characterization of MAPP

The FT-IR spectra of APP and MAPP are shown in Fig.2.In the A curve 3396 and 3175 cm-1correspond to N-H stretching vibration peak and N-H bending vibration peak at 1677 cm-1,1261 cm-1corresponds to P=O stretching vibration peak,1083 cm-1corresponds to P-O stretching vibration peak,1022 cm-1corresponds to P-O stretching vibration peak of PO2and PO3,and 887 cm-1corresponds to P-O stretching vibration peak [26].

In the B curve,3070 and 2989 cm-1correspond to -NH2and-CH2respectively.1546,1511 and 1311 cm-1are caused by the vibration absorption of triazine skeleton in the coating,which indicates that MEL is involved in the bonding in the coating molecule.The characteristic absorption peaks of -C=O occur at 1708 cm-1,which is C=O from urea or urethane groups in the coatings[27,28].This indicates that the N=C=O group in MDI reacts with-NH2in MEL and -OH in DPER to form the structures of -NH-C(=O) -NH-and -NH-C (=O) -O-,which are typical characteristic groups in polyurethane molecules.

The SEM photos of APP and MAPP are shown in Fig.3.As can be seen from Fig.3,the surface of APP is smooth,most of the large particles have cracks along the longitudinal direction,and the small particles have obvious edges and corners.The surface of MAPP particles is rough,the edges and corners disappear,and the coating has a certain thickness.The comparison between the coated and untreated APP shows that the polyurethane resins(PU) are successfully coated on the surface of APP,indicating that the preparation of MAPP is successful.

The water solubility of APP was 0.4712 g at 25°C,while the MAPP was 0.3000 g,which is 36% lower than that of APP.This is because that PU is a cross-linked macromolecule and hydrophobic,and the coating can partially block the erosion of APP by water.Compared with the hydrophilic surface of APP,the coating can reduce the difference of surface properties between APP and PP,and help to improve the compatibility between APP and PP matrix.

Fig.4.TG and DTG curves of APP and MAPP.(a) TG;(b) DTG.

Table 1 Test results of flame retardant PP limit oxygen index and flame retardant rating

Fig.4 shows the thermogravimetric test (TG) and thermogravimetric differentiation(DTG)of APP and MAPP.As can be seen from Fig.4,the initial decomposition temperature of APP is about 284°C,which is relatively stable before 284°C.The thermal decomposition can be divided into two stages:the decomposition begins at 284°C,and the decomposition rate reaches its maximum at 324°C in the first stage,and the weight loss of APP is about 20%at 494°C.With the further increase of temperature,APP entered the second stage of decomposition.The decomposition rate reached the maximum at 629°C.The residue was 23.5% at 800°C.

The decomposition of MAPP can be seen as a parallel decomposition process of PU and APP,and can be divided into three stages.The first stage is the decomposition of the coating layer.The initial decomposition temperature is lower than that of APP,starting at about 240°C.In the second stage,the core and shell decompose simultaneously.With the decomposition of APP to form phosphoric acid,the carbon-forming catalytic effect is produced on the coating.The maximum decomposition rate in this stage is similar to that of pure APP,and the temperature of weight loss 20%decreases from 494°C to 389°C.With the further increase of temperature,the decomposition may be more complex,and the decomposition rate of the third stage of MAPP reaches the maximum at 661°C.The residual amount of the system is 36% at 800°C,which is significantly higher than that of APP.

When the decomposition temperature of MAPP decreases,the generated gas can reduce the oxygen concentration on the burning surface and promote the expansion of the carbon layer on the surface,which helps to form a “honeycomb”carbon layer.It can further isolate the contact between the oxygen and the matrix,reduce the heat radiation conduction to the matrix,and play a better flame retardant role.

Table 2 TG and DTG data of flame retardant PP

3.2.Characterization of flame retardant PP

The limit oxygen index(LOI)and UL-94 test data of flame retardant PP are shown in Table 1 (including LOI and UL-94 grade after 7 days of soaking in hot water at 70°C).

Table 1 shows that the oxygen index of pure PP is only 17.5.With the addition of 30 wt% pure APP to PP,the oxygen index increased to 22.0,but there is no flame retardant grade due to serious dripping.After adding 15 wt%MAPP to PP,the sample of No.2 gradually extinguishes after leaving the fire source,without droplet phenomenon,and a slightly expanded carbon layer will be formed at the burning site.The LOI value of flame retardant PP increases with the increase of MAPP addition.When the addition of MAPP is 30 wt%,the LOI of PP/MAPP reaches 32.1%.When the content of MAPP exceeds 20 wt%,UL-94 test can reach V-0.This is because MAPP acts as “three sources in one”intumescent flame retardant in the combustion process.

After hot water immersion,the limiting oxygen index (LOI) of PP/APP (70/30) material decreases by 1.5 units,while that of PP/MAPP (70/30) decreases by 0.4 units compared with that of nonimmersion.This indicates that the coating has an obvious hindrance effect on the dissolution and migration of APP.

Fig.5 shows thermogravimetric and thermogravimetric differential curves of PP,PP/APP (70/30) and PP/MAPP (70/30).From Fig.5(a),it can be seen intuitively that after adding APP and MAPP,the thermogravimetric curves generally move towards high temperature,and the final residue rate of the system increases.Fig.5(b)shows that the thermal decomposition rate of the three components decreases in turn at the maximum weight loss stage,indicating that the addition of APP and MAPP changes the degradation process of the system.

Fig.5.TG and DTG curves of flame retardant PP.(a) TG;(b) DTG.

Fig.6.HRR curves of PP and flame retardant PP.

The main data of thermogravimetric analysis are shown in Table 2,in which the temperature at 5%weight loss of flame retardant PP is taken as the initial decomposition temperature,expressed as Tin,and the temperature corresponding to the maximum weight loss rate is expressed as Tmax.From Table 2,it can be found that the initial decomposition temperature of PP is about 300°C while that of PP/APP and PP/MAPP is 315°C and 310°C,respectively.With the addition of APP,the initial decomposition temperature of PP flame retardant material is slightly increased,and the increase of Tinis much smaller with the addition of MAPP.This is because the thermal stability of PU as coating layer is lower than that of APP and the decomposition begins earlier.

The corresponding Tmaxvalues of PP,PP/APP and PP/MAPP are 404.89,419.89 and 414.89°C,respectively.The Tmaxof PP/MAPP is slightly lower than that of PP/APP.This may be due to the superposition effect caused by the decomposition synchronization of PP,coating and APP.This superposition effect can play a synergistic role in the flame retardant process of intumescent flame retardant to achieve the effect of forming an intumescent carbon layer.The final carbon residues of the three components are 0.02%,7.2%and 10.8%respectively.It can be seen that there is almost no residual after burning pure PP,while the residual carbon of PP/MAPP reaches 10.8%.The existence of these carbon residues plays a role in flame retardant.

Fig.7.HRR curves of PP and flame retardant PP.

Fig.8.SPR curves of PP and flame retardant PP.

Fig.6 shows HRR curves of three materials.As can be seen from Fig.6,pure PP has only one sharp peak of HRR,which is the common peak shape of flammable materials.The PHRR value is 831 kW·m-2.The HRR curve of PP/APP (70/30) becomes flat relative to PP,and PHRR is 432 kW·m-2.The HRR curve of PP/MAPP(70/30) is further flattened,and its PHRR is 300 kW·m-2,which is 64%lower than that of pure PP.Compared with pure PP,the burning time of flame retardant PP can be prolonged to varying degrees while the HRR peak height decreases.The HRR peaks of PP/MAPP specimens show double peaks,because when MAPP is added to the matrix material,the surface begins to burn after the material is ignited by the incident heat flow,thus showing the first exothermic peak.With combustion,the expansive carbon layer begins to form and cover the surface,which hinders heat transfer and the escape of decomposed gases,leading to combustion hindrance.However,with the continuous irradiation of the incident heat flow,the carbon layer on the surface of the material begins to decompose partially,and then form a second exothermic peak.Generally speaking,the addition of APP and MAPP reduces the heat release rate of PP,which contributes to the flame retardance of PP,and the effect of MAPP is more prominent.

Total heat release THR can be used to evaluate the safety of materials in real fire scene.Fig.7 shows the THR curves of three samples.It can be seen that the THR of pure PP is 158 MJ·m-2,while the THR values of PP/APP (70/30) and PP/MAPP (70/30) are 114 and 100 MJ·m-2,respectively.The decrease of PP/MAPP is more obvious.

Figs.8 and 9 show smoke release rates and total smoke release curves of three different materials.

Fig.9.TSP curves of PP and flame retardant PP.

Table 3 Cone calorimeter data of PP,PP/APP and PP/MAPP

From Fig.8,it can be seen that the smoke release rate of PP is higher,with its pk-SPR being 0.0691 m2·s-1,followed by PP/APP(70/30),which is 0.0384 m2·s-1,while PP/MAPP (70/30) is the smallest,which is 0.0277 m2s-1.However,because the burning time of the three materials is different,the final total smoke emission at the end of combustion is 13.11 m2,13.64 m2and 12.16 m2,respectively.The addition of APP reduced the smoke release rate of flame retardant materials,but the total smoke production increased slightly,which indicated that APP was not helpful to the smoke suppression performance of flame retardant materials.PP/MAPP not only decreased the smoke release rate,but also decreased the total smoke release by 7% and 11% compared with PP and PP/APP,which indicated that adding MAPP in PP could improve the smoke suppression effect.

Table 3 shows the conical calorimetric data of PP and flame retardant PP.

As can be seen from Table 3,pure PP has no residue after cone measurement.The residual rate of PP/APP (70/30) is 20.1%,while that of PP/MAPP(70/30)is 26.7%,higher than that of PP/APP,which indicates that MAPP has better charring effect.

The ignition time (TTI) refers to the time taken from the radiation of heat flux on the surface of material to the start of combustion at a preset incident heat flux.The TTI data of the three samples are shown in Table 3.PP is 44 s.The TTI values of PP/APP (70/30)and PP/MAPP (70/30) are shortened to 30 s and 27 s,respectively.This is because the heat of PP is easily transferred to the matrix after heating,and the time required for the surface heat and the aggregation of combustibles is longer,so the TTI value is the highest.When APP is added,the APP will partially decompose to produce gas,which will generate bubbles and phosphoruscontaining decomposition products on the surface of the matrix,which hinders the heat transfer to the matrix,so that the surface temperature rises rapidly and reaches the combustion threshold.It shows that the ignition time is ahead of schedule.The addition of MAPP has similar effect,and MAPP has charring property.The early decomposition of MAPP makes the foaming carbon layer on the surface of PP form earlier,and the effect of inhibiting heat conduction to the interior is stronger than that of APP,which further shortens the ignition time of the sample.

The ratio of TTI to PHRR is called Fire Performance Index (FPI).The larger the ratio,the easier it is to escape in case of fire.The FPI values of PP,PP/APP (70/30) and PP/MAPP (70/30) were 0.0529,0.0694 and 0.09,respectively,which indicated that the safety of PP/MAPP was the best among the three materials.

The ratio of PHRR to TPHRR is defined as FIGRA.The bigger the value of FIGRA,the stronger the material’s bombasticity.Among the three materials,the FIGRA index of PP/MAPP is the lowest,indicating that the fire spread is the slowest when burning.

Fig.10.Residue photos of pure PP and flame retardant PP.a.a′ is the overlook and side view of the residual of PP/APP.b.b′ is the overlook and side view of the residual of PP/MAPP.

Fig.11.SEM images of PP composites after combustion.(a) Surface of PP/APP residue;(b) surface of PP/MAPP residue;(c) cross section of PP/MAPP residue.

Photographs of char residues after cone calorimetry are shown in Fig.10.As can be seen from Fig.10,the residual carbon height of PP/APP(70/30)after combustion is about 6 mm.After combustion of PP/MAPP (70/30),the expanded porous black carbon layer is formed.Its surface is dense,its interior is fluffy and porous,and the residue height is about 30 mm.The good flame retardant effect of PP/MAPP is closely related to the formation of intumescent carbon layer.

Fig.11 shows the SEM photos of PP/APP and PP/MAPP carbon residues.It can be seen that charcoal residue in Fig.11 (a) is compacted with porous holes and does not have the basic characteristics of intumescent carbon layer.This is due to the lack of carbon source in the PP/APP system,so the intumescent carbon layer cannot be formed,so the flame retardant effect is also poor.Fig.11(b)shows the outer surface of the PP/MAPP charcoal residue.It can be seen that the surface of the charcoal layer is continuous and compact,with few and small stomata.Fig.11 (c) is the cross-sectional morphology of the expansive carbon layer of PP/MAPP.It can be seen that there are bubbles in the carbon layer,which is also a typical feature of the expansive carbon layer.In the PP/MAPP system,the coating layer in the MAPP not only has the function of gas source,but also has the function of carbon source.It has all the necessary components of the intumescent flame retardant,and forms the intumescent carbon layer during combustion,which plays the role of intumescent flame retardant.Therefore,the flame retardant effect of PP/MAPP is obviously better than that of PP/APP.

Fig.12.FT-IR spectra of char residue of PP/MAPP composites after combustion.

3.3.Analysis of flame retardant mechanism of PP/MAPP

The residual char after PP/MAPP combustion was tested by infrared spectroscopy.The results are shown in Fig.12.As can be seen from Fig.12,the absorption peaks of N-H at 3390 cm-1,P-OH at 1622 cm-1,P=O at 1220 cm-1,C-O-C at 1136 cm-1and P-O-P at 958 cm-1correspond to the characteristic absorption peaks of P-O-P group,which correspond to the polyphosphoric acid(P2O5or P4O10)formed by the shrinkage of APP in the core.The peak of 792 cm-1is caused by the symmetrical stretching vibration of P-O bond in P-O-C [25,27].

Combining with TG test and infrared test for residual carbon,the possible flame retardant mechanism of MAPP was speculated:when the coating of MAPP was heated,PU containing urea group would decompose and release NH3and other non-combustible gases.With the heat transfer,the APP in the core began to decompose and release NH3.The non-flammable gases released by the cladding layer and core can dilute the concentration of flammable gases in the combustion range and absorb part of the heat.At this time,the flame retardant effect is mainly embodied in the dilution of the gas phase and the endothermic effect of the condensed phase.When the temperature is further increased,the esterification reaction between APP and DPER in the coating produces a P-O-C bond,and the carbon layer is formed by simultaneous dehydration.At the same time,the system produces a large amount of gas,which makes the carbon layer foamy and fluffy.Expanded carbon layer covers the surface of the material,hinders the entry of oxygen and volatilization of combustibles,and forms an insulating layer.At this time,the flame retardant mechanism of condensed phase is dominant.At this time,the carbon layer with dense surface and porous interior plays an important role in the flame retardant effect.The possible flame retardant process of MAPP is shown in Fig.13.

4.Conclusions

Microencapsulated APP (MAPP) with PU as coating layer and APP as core is successfully prepared.Compared with APP,the water solubility of MAPP decreases by 36%,the initial thermal decomposition temperature decreases,and the residual carbon content increases by 12.5% at 800°C.MAPP has a better residual carbon rate.

When adding 30 wt% MAPP or APP to PP,the flame retardant grade of the former can reach UL-94 V-0,while the latter has no grade.The limiting oxygen index (LOI) of the former is 11.6 units higher than that of the latter.After hot water immersion,the former has higher LOI.After combustion,the former has a dense surface,a fluffy interior and a higher residual carbon rate.The cone calorimetric test shows that the former has lower heat release rate and total heat release rate and total smoke release rate than the latter,and the former increases more slowly than the latter in case of fire.

When PP/MAPP combusts,MAPP can act as flame retardant in gas phase and condensed phase respectively.At the beginning of combustion,the cladding layer and APP as core begin to decompose and release non-flammable gases,which can dilute oxygen concentration.The flame retardant effect of MAPP is mainly reflected in the dilution of gas phase and the endothermic effect of condensed phase.When combustion proceeds further,APP can esterify with the cladding layer to form an expanded carbon layer,which plays the role of flame retardant of condensed phase.