App in Pp and Pu - Chen m Et Al - 2014

Original Article

Thermal stability and combustionbehavior of flame-retardantpolypropylene with thermoplasticpolyurethane-microencapsulatedammonium polyphosphate

Man Chen1, Yang Xu1, Xiaolang Chen1, Yonghong Ma1,Weidi He1, Jie Yu2 and Zhibin Zhang3

AbstractIn this article, thermoplastic polyurethane-microencapsulated ammonium polyphosphate (MTAPP) is prepared and wellcharacterized by Fourier transform infrared spectroscopy, scanning electron microscopy, and thermogravimetric analysis(TGA). MTAPP and APP are added onto polypropylene (PP) as a novel intumescent flame-retardant system to improvethe flame retardancy of PP. The flammability, thermal stability, and mechanical properties of the flame-retardant PP com-posites are investigated by limiting oxygen index (LOI), UL-94 vertical burning test, cone calorimeter test (CCT), TGA,and mechanical properties tests. The results show that MTAPP exhibits better flame retardancy and thermal stability thanthat of the APP in the flame-retardant PP composites. The LOI value of the PP/MTAPP composite at the same loading levelis higher than that of PP/APP composite. The dripping of MTAPP system disappears compared with APP system from UL-94 test. The results of the CCT also indicate that MTAPP is an effective flame retardant in PP. The improvement may beattributed to the better charring capacity of MTAPP from TGA. Additionally, the mechanical properties of MTAPP arebetter than those of APP in PP.

KeywordsPolypropylene, ammonium polyphosphate, thermoplastic polyurethane, flame retardancy, thermal stability

Introduction

Polypropylene (PP) is an important commodity plastic and

has been widely used in many fields because of its out-

standing mechanical performances, low cost, and ease of

processing.1–3 However, the use of PP in electrical and

electronic applications, architectural materials, interior

decorations, or automobiles is severely limited due to its

flammability and dripping tendency.4–6 Therefore, improv-

ing the fire-retardant behavior of PP is still a major chal-

lenge for extending its use to most applications.

One of the most effective methods for improving the

flame retardancy is to incorporate an intumescent flame

retardant (IFR) onto PP matrix, which has attracted more

and more attention in recent years, because they are not only

more environmentally friendly than the traditional halogen-

containing flame retardant but also have higher flame-

retardant efficiency than inorganic flame retardants, for

example, metal hydroxides.7,8 A typical IFR system usually

involves three components: an acid source, a char-forming

agent, and a blowing agent.9–11 When heated beyond a crit-

ical temperature, the IFR system can undergo intensive

expansion to form a protective foam char layer, which effec-

tively holds back the transfer process of the heat and fuel

1 Key Laboratory of Advanced Materials Technology, Ministry of Education,

School of Materials Science and Engineering, Southwest Jiaotong University,

Chengdu, China2 National Engineering Research Center for Compounding and Modification

of Polymer Materials, Guiyang, China3 School of Life Science and Engineering, Southwest Jiaotong University,

Chengdu, China

Corresponding author:

Xiaolang Chen, Key Laboratory of Advanced Materials Technology,

Ministry of Education, School of Materials Science and Engineering,

Southwest Jiaotong University, Chengdu 610031, China.

Email: [email protected]

High Performance Polymers2014, Vol. 26(4) 445–454ª The Author(s) 2014Reprints and permission:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0954008313517910hip.sagepub.com

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during combustion, subsequently leading to a quick self-

extinguishment of the burning materials.12 But in the mixed

system of IFRs, there still are some shortcomings, such as

hydrophilicity, nonuniform mixing, and so on, which has a

bad impact on the physical and mechanical properties of

flame-retarded products.13,14

Ammonium polyphosphate (APP) can be used as an acid

source and a blowing agent (ammonia (NH3) evolved from

APP during its degradation can play this role) at the same

time in an IFR system. Also, in comparison with ternary

flame-retardancy system, APP, which is used as the only

filler, showed good flame-retardant performance for PP.15

APP is helpful for composite to form foamed char. This

will not only improve the utilization of flame retardants but

also enhance the economic efficiency and achieve the opti-

mization of process control.16

The flame-retardant additives also affect mechanical

properties of composites.17,18 The interfaces between the

additives and polymer matrix play a key role in the struc-

ture and property of composites.19,20 For this purpose,

some modifiers were employed to improve the interfacial

compatibility. These modifiers combine the functions of

dispersing and PP matrix by chemical bonds. Hence, the

flame-retardant performance could be improved with better

mechanical properties. Compared with the small inorganic

polar APP, the thermoplastic polyurethane (TPU) is more

compatible with the polymer matrix. On the other hand,

TPU as a char-forming polymer possesses better stability

in the PP/IFR system.21,22 Therefore, the introduction of

TPU as a modifier can improve the flame retardancy and

mechanical properties with the single APP added to the PP.

In this work, APP was microencapsulated with TPU

(MTAPP) and then added into PP as an IFR system to

enhance the flame retardancy of PP. For comparison, the

flame-retardant PP with APP was also investigated. The

combustion behaviors and thermal stability of the pure and

the flame-retardant PP composites were studied using

Fourier transform infrared spectroscopy (FTIR), limiting

oxygen index (LOI) test, UL-94 test, cone calorimeter test

(CCT), thermogravimetric analysis (TGA), and mechanical

properties tests.

Experimental

Materials

PP (B4808, melt flow rate ¼ 10 g 10 min�1) was pur-

chased from Yanshan Petrochemical (Beijing, China).

TPU (481, polyester grade) was obtained from Bayer

Company (Germany). Ammonium polyphosphate (APP,

Z201, the degree of polymerization: n > 1000) was offered

by Shifang Taifeng New Flame Retardant Co. Ltd

(Sichuan, China). All materials were dried in vacuum

oven at 80�C for 12 h before use.

Preparation of MTAPP

A calculated amount of APP and TPU (TPU:APP ¼ 1:9)

were compounded on a two-roll mill (SJK-160, Wuhan,

China) at the melting temperature 160�C of TPU for 10

min. The obtained sheets were pulverized by a high-

speed pulverizer (FW-400A, Beijing Kewei Yongxing

Instrument Co., Ltd, Beijing, China) into a fine powder at

room temperature.

Preparation of the flame-retardant PP composites

The flame-retardant PP composites were compounded in a

twin-screw extruder (Type HFB-150/3300, Nanjing Rubber

Plastic Machinery Factory Co., Ltd, Nanjing, China) at 140–

190�C, and the screw speed was 150 r min�1. The extruded

pellets were dried at 80�C for 8 h prior to being injection

molded into standard text samples of various sizes using a

J80M (Technovel Corp., Osaka, Japan) injection molding

machine. The temperature profiles of the injection molding

Table 1. The formulations of flame-retardant PP and its composites.

Samples MTAPP (wt%) APP (wt%) PP (wt%)

UL-94 test

Rating Dripping

PP 0 0 100 Fail YesPTA05 5 0 95 Fail NoPTA10 10 0 90 Fail NoPTA15 15 0 85 Fail NoPTA20 20 0 80 Fail NoPTA25 25 0 75 Fail NoPTA30 30 0 70 Fail NoPA30 0 30 70 Fail Yes

PP: polypropylene; MTAPP: microencapsulated ammonium polyphosphate; APP: ammonium polyphosphate; PTA05: polypropylene/microencapsulatedammonium polyphosphate with 5 wt%; PTA10: polypropylene/microencapsulated ammonium polyphosphate with 10 wt%; PTA15: polypropylene/microencapsulated ammonium polyphosphate with 15 wt%; PTA20: polypropylene/microencapsulated ammonium polyphosphate with 20 wt%; PTA25:polypropylene/microencapsulated ammonium polyphosphate with 25 wt%; PTA30: polypropylene/microencapsulated ammonium polyphosphate with30 wt%; PA30: polypropylene/ammonium polyphosphate with 30 wt%.

446 High Performance Polymers 26(4)



machine were 175, 185, and 195�C from hopper to die. The

formulations of PP samples are presented in Table 1.

Measurements and characterization

FTIR spectroscopy. FTIR spectra were recorded with a Nico-

let 560 spectrophotometer (Nicolet Instrument Corp.,

Madison, Wisconsin, USA). Samples were mixed with a

potassium bromide powders, and the mixture was pressed

into a tablet.

Scanning electron microscopy. Scanning electron microscopic

(images of APP and MTAPP particles were investigated

using a scanning electron microscopy (model (JSM-7500F,

JEOL, Japan). The studied surfaces were first sputter coated

with a thin layer of gold before the measurement. The accel-

erated voltage was 20 kV.

Thermogravimetric analysis. TGA of samples was examined

under nitrogen (N2) flow with a flow rate of 60 mL min�1

in a temperature range from ambient to 700�C at a heating

rate of 10�C min�1 by a TG 209F1 thermogravimetric ana-

lyzer (Netzsch, Germany). About 8–10 mg specimens were

used in this text.

Limiting oxygen index. The LOI value was measured using a

JF-4 type instrument (Jiangning Analysis Instrument Fac-

tory, Nanjing, China) on sheets 120� 6.5� 3 mm3 accord-

ing to the standard oxygen index test (ASTM D2863-77).

UL-94 testing. The vertical test was carried out on a CZF-1

type instrument (Suzhou QILE Electronic Technology Co.,

Ltd, China) on sheets 127 � 12.7 � 2.7 mm3 according to

ASTM D635-77 standards.

Cone calorimeter test. CCTs (Stanton Redcroft, UK) were

carried out according to ISO 5660 standard procedures.

Each specimen of dimensions 100 � 100 � 3 mm3 was

wrapped in aluminium foil and exposed horizontally to

an external heat flux of 35 kW m�2.

Mechanical properties tests. The tensile tests of samples

were carried out on a tensile tester (AGS-J, SHIMADZU

International Trading Co., Ltd, Shanghai, China) with a

crosshead speed of 100 mm min�1. Notched Izod impact

strength was measured using a pendulum impact testing

machine. The radius of notch used in the specimens was 2

mm (GB 1043-79). All the tests were performed at 23 +2�C. The results were the average values of at least five

specimens.

Results and discussion

Characterization of MTAPP

The FTIR spectra of APP, MTAPP, and TPU are shown in

Figure 1. It is clearly observed that the typical absorption

peaks of APP include 3200 (N–H stretching of amide

group), 1256 (P¼O stretching vibration), 1075 (P–O sym-

metric stretching vibration), 880 (P–O asymmetric stretch-

ing vibration in P–O–P), 1020 (symmetric vibration of PO2

in H2PO�4 and PO3 in HPO2�4 ), and 800 cm�1 (P–O–P

stretching vibration).23 For TPU, the main absorption

peaks appear at 3324 (N–H stretching of amide group),

2985 (CH2 stretching vibration), 2936 (CH3 stretching

vibration), 1735 (C¼O nonbonded urethane and C¼O

associated urethane stretching), 1600 (N–H deformation

vibration), 1530 (H–N–C¼O amide II combined motion),

1230 (N–H deformation vibration and C–N stretching

vibration amide III combined motion), and 1100 cm�1

(C–O–C symmetric and asymmetric vibration).24,25 At the

same time, it is obviously found that for MTAPP, the main

absorption peaks appear at 3200, 1735, 1600, 1530, 1256,

1075, 1020, and 880 cm�1. The absorption peaks of

1256, 1075, 1020, and 880 cm�1 are due to the typical

absorption peaks of APP, and the absorption peaks of

1735, 1600, and 1530 cm�1 are due to the typical absorp-

tion peaks of TPU. The peaks at 2600–3700 cm�1 may

be due to the combination of the both. The spectrum of

MTAPP reveals not only well-defined absorption peaks

of TPU but also the characteristic bands of APP, which

indicates that TPU exists in the MTAPP.

The surface morphologies of the APP and MTAPP par-

ticles are shown in Figure 2. It is clearly observed that the

surface of APP particles is very smooth, as shown in Figure

2(a). After encapsulation, MTAPP presents a comparably

rough surface. This also indicates that APP has been suc-

cessfully encapsulated by TPU.

Figure 3 presents the TGA curves of APP and MTAPP

samples under N2 flow at a heating rate of 10�C min�1.

It is clearly seen from Figure 3 that the curve of APP

4000 3500 3000 2500 2000 1500 1000 500

TPU

MTAPP

APPTra

nsm

itta

nce

(%)

Wavenumber (cm–1)

3200

1256 1075 8801020

1735 1600 15303324 2985 29361230

1100

800

Figure 1. FTIR spectra of APP, MTAPP, and TPU. FTIR: Fouriertransform infrared; MTAPP: microencapsulated ammonium poly-phosphate; APP: ammonium polyphosphate; TPU: thermoplasticpolyurethane.

Chen et al. 447



exhibits two-step degradation. At the first decomposition

process, its initial temperature is at about 270�C and the

decomposition ends at about 500�C with the weight loss

of 24%. Its evolution products at this step are mainly

NH3 and water. In addition, the cross-linked polyphospho-

ric acids are formed simultaneously.23 The second step is

main decomposition process of APP occurring at above

540�C. The temperatures of maximum mass loss rate (Tmax)

for the two steps are 310 and 585�C, respectively. The left

out residue of APP at 700�C is only about 20.4%. However,

MTAPP shows different degradation compared with APP

at the whole process. MTAPP presents one main decompo-

sition process and initially decomposes at about 230�C,

which is lower than that of APP. The reason is that the ther-

mal stability of TPU matrix is so weak. What’s more, APP

will release polyphosphoric acids that might react with

functional group of TPU with increasing the degradation

temperature further. This is why MTAPP decomposes

faster than APP. However, when the decomposition tem-

perature is above 600�C, MTAPP is more stable than APP

and the degradation ends, which suggests that a charred

layer with better thermal stability is formed because of the

reaction between TPU and APP. The value of Tmax of

MTAPP is 325�C. In addition, MTAPP after the decompo-

sition at 700�C left about 37.2% residue, whereas the resi-

due for APP at this temperature is 20.4%, indicating that

APP encapsulated by TPU indeed greatly improves the

residual left.

Combustion of the PP composites: LOI and UL-94 test

The LOI and UL-94 tests are usually used to evaluate

the fire resistances of polymer materials, especially for

screening the flame-retardant formulations of the materi-

als.26 Figure 4 shows the changes of LOI values of the

flame-retardant PP composites with different MTAPP con-

tent. Pure PP is easily flammable, and its LOI value is only

18.2. Clearly, the LOI increases linearly with increasing the

Figure 2. SEM images of (a) APP and (b) MTAPP particles. MTAPP: microencapsulated ammonium polyphosphate; APP: ammoniumpolyphosphate; SEM: scanning electron microscopy.

100 200 300 400 500 600 700

20

40

60

80

100

Wei

ght

(%)

Temperature (°C)

MTAPP

APP

Figure 3. TGA curves of APP and MTAPP. MTAPP: microen-capsulated ammonium polyphosphate; APP: ammonium polypho-sphate; TGA: thermogravimetric analysis.

0 5 10 15 20 25 3017

18

19

20

21

22

23

24

LO

I (%

)

Flame retardant contens (%)

PP/MTAPP PP/APP

Figure 4. Effect of flame retardant on the LOI of the PP com-posites. LOI: limiting oxygen index; PP: polypropylene.




content of MTAPP. When 30 wt% MTAPP (PTA30) is

added, the LOI value of the PP composites increases to

22.9. However, when loading APP (PA30) at the same

weight, the LOI value is only 20.1. This indicates that

MTAPP can enhance the flame retardancy of the PP/

MTAPP composites.

The UL-94 testing data are summarized in Table 1, and

the optical images shown in Figure 5 depict the burning

process of the pure and the flame-retardant PP composites.

As also listed in Table 1, pure PP and the flame-retardant

PP composites failed in UL-94 test; however, the burning

process is significantly different. The pure PP burns sharply

with dripping severely, which is dangerous and easy to

cause ‘‘secondary combustion.’’ When MTAPP is added

into PP, the dripping disappears, and the burning gradually

becomes slow with increasing the content of MTAPP, as

shown in Figure 5 (b) to (d). However, compared with

MTAPP, when the same weight APP is added (i.e.

PA30), the dripping still exists and the sample burns much

better (Figure 5(e)). The above results further confirm that

the TPU encapsulated APP can enhance the flame-retardant

performances of PP due to char formation generated by

TPU, which eliminates dripping during burning process.

Combustion of the PP composites: CCT test

CCT is a bench-scale test to simulate real fire conditions.27–29

It provides parameters such as heat release rate (HRR),

total heat release (THR), time to ignition (TTI), mass loss,

carbon monoxide (CO), and carbon dioxide (CO2) production

rate. Thus, the cone calorimeter is a useful tool for the evalua-

tion of fire-retardant materials.

HRR, especially the peak HRR (PHRR), has been found

to be one of the most important parameters to evaluate fire

safety.30 Figure 6 shows the HRR curves of the flame-

retarded PP composites with different MTAPP content.

The detailed data obtained of the aforementioned series

of samples are listed in Table 2. It can be clearly seen that

the pure PP burns very fast after ignition and a sharp HRR

peak appears at the range of 100–400 s with a PHRR value

of 577.5 kW m�2. It is clear that the addition of MTAPP

has considerable effect on the values of HRR. The values

of HRR of PP/MTAPP composites decrease rapidly with

the addition of MTAPP and decrease gradually with

Figure 5. Digital photographs taken during burning of pure PP and its composites: (a) PP; (b) PTA10; (c) PTA20; (d) PTA30; (e) PA30.PP: polypropylene; PTA10: polypropylene/microencapsulated ammonium polyphosphate with 10 wt%; PTA20: polypropylene/micro-encapsulated ammonium polyphosphate with 20 wt%; PTA30: polypropylene/microencapsulated ammonium polyphosphate with30 wt%; PA30: polypropylene/ammonium polyphosphate with 30 wt%.

0

100

200

300

400

500

600

PA30PTA30

PTA25PTA20

PTA15

PTA10

PTA05

Hea

t re

leas

e ra

te (

kWm

–2)

Time (s)

pure PP

1000 200 300 400 500 600 700 800

Figure 6. HRR curves of the pure and the flame-retardant PPcomposites. HRR: heat release rate; PP: polypropylene.

Chen et al. 449



increasing the content of MTAPP. Interestingly, the HRR

curve of PP had only a single peak, whereas, there were two

peaks for PP/MTAPP and PP/APP composites. This can

delay the decomposition and combustion of polymer

matrix. As a result, the flame-retardant performance of

PP composites is improved. The presence of one peak is

easy to understand because the pure PP matrix burns fier-

cely. For the flame-retarded PP composites, the first peak

is assigned to the development of the intumescent protec-

tive char. The remaining one is attributed to the destruc-

tion of the protective charred layer with a lot of

combustible gases when the flame-retarded PP compo-

sites are continuously exposed to the heat and the forma-

tion of new char. Similar results have also been reported

by Chen and Jiao.31

In addition, it can be observed from Figure 6 that the PP/

MTAPP sample shows lower HRR value than APP/PP sam-

ple at the same loading level of the flame retardant. When

the 30 wt% flame retardant is loaded, the PHRR value of

PP/MTAPP is 140.6 kW m�2; however, the PHRR value

of PP/APP is 201.1 kW m�2. The lower HRR value indicates

the higher flame retardancy for PP/MTAPP composites. As a

result, the LOI value of PP/MTAPP is higher than that of PP/

APP. The higher flame retardancy of the PP/MTAPP com-

posites should be attributed to the better charring perfor-

mance of MTAPP mentioned previously (TGA curves).

As also summarized in Table 2, the TTI of the flame-

retardant PP composites is lower than pure PP. This is a

typical character of IFR system. This possible reason is that

introduction of the flame retardancy into PP decreases the

apparent stability of polymer matrix, which leads to the

ease of ignition.

THR is commonly used to evaluate the fire safety of the

materials in a real fire. The THR values versus time for PP,

PP/MTAPP, and PP/APP are shown in Figure 7. It can be

seen that the change of THR curves are similar to that of

HRR for the above-mentioned samples. The THR values sig-

nificantly decrease for the flame-retardant PP composites

compared with pure PP and decrease further with increasing

the content of MTAPP. This phenomenon can be explained

due to the formation of expanded intumescent shield covers

on the surface of the matrix while burning, which makes a

thermal insulation, provokes the extinguishment of the

flame, prevents combustible gases from feeding the flame,

and separates oxygen from burning materials. At the same

time, it is clear that the THR value of PP/MTAPP is lower

than that of PP/APP at the same loading level of flame retar-

dant. As shown in Figure 7 and Table 2, for PTA30 with 30

wt% MTAPP, the THR value is 41.8 MJ m�2, whereas the

THR value of PA30 with 30 wt% APP is 44.5 MJ m�2. This

illustrates that the MTAPP has better flame-retardant perfor-

mance than APP in the flame-retardant PP composites.

Polymer materials can release a large amount of toxic

gases and smoke during the combustion process. A great

number of people died in the fire because of toxic gases

and smoke.32 The CO and CO2 curves can effectively

evaluate the release of toxic gas in combustion process.

Combustion of CO and CO2 in polymer materials strongly

depends on fire and material performances.33 Figures 8

Table 2. Cone calorimeter data of the pure and the flame-retardant PP composites.

Samples TTI (s) PHRR (kW m�2) THR (MJ m�2) Residue (%)

PP 68 577.5 82.7 0.7PTA05 57 395.4 67.2 7.2PTA10 42 282.5 63.7 9.2PTA15 40 214.9 59.9 16.8PTA20 32 193.6 57.3 20.3PTA25 30 145.4 64.1 25.1PTA30 31 140.6 41.8 27.6PA30 41 201.1 44.5 21.4

PP: polypropylene; PTA05: polypropylene/microencapsulated ammonium polyphosphate with 5 wt%; PTA10: polypropylene/microencapsulatedammonium polyphosphate with 10 wt%; PTA15: polypropylene/microencapsulated ammonium polyphosphate with 15 wt%; PTA20: polypropylene/microencapsulated ammonium polyphosphate with 20 wt%; PTA25: polypropylene/microencapsulated ammonium polyphosphate with 25 wt%; PTA30:polypropylene/microencapsulated ammonium polyphosphate with 30 wt%; PA30: polypropylene/ammonium polyphosphate with 30 wt%; TTI: time toignition; THR: total heat release; PHRR: peak heat release rate.

0

10

20

30

40

50

60

70

80

90

PA30

PTA30PTA25PTA20

PTA15

PTA10

Tot

al h

eat

rele

ase

(MJm

–2) pure PP PTA05

Time (s)1000 200 300 400 500 600 700 800

Figure 7. THR curves of the pure and the flame-retardant PPcomposites. THR: total heat release; PP: polypropylene.




and 9 show the CO and CO2 curves of the pure PP and its

flame-retardant composites, respectively. It is clear that

the curves of CO and CO2 emission are very similar to that

of HRR curves. The peaks of CO and CO2 for pure PP are

very sharp and narrow, indicating the PP material burns

acutely. However, the peaks of CO and CO2 become wide

and flat due to the addition of MTAPP. Additionally, the

CO and CO2 values of PP/MTAPP composites decrease dra-

matically compared with that of PP. And the values of CO and

CO2 of PP/MTAPP composites decrease gradually with

increasing the MTAPP content. At the same time, it is clear

that the CO and CO2 values of PP/MTAPP are lower than that

of PP/APP at the same loading level of flame retardant, as

shown in the curves PTA30 and PA30. This confirms further

that the MTAPP has better performance than APP in the

flame-retardant PP composites because of high charring

capacity of MTAPP.

Thermal stability of the PP composites

In order to clearly understand the flame retardancy, the

thermal degradation of the PP composites is investigated

using TGA. TGA and derivative thermogravimetric curves

for the above samples under N2 atmosphere at a heating

rate of 10�C min�1 are shown in Figures 10 and 11, respec-

tively. Pure PP starts to decompose around 390�C, almost

decomposes completely at 480�C, and leaves about

0.16% of char residues at the temperature of 700�C. It is

clearly seen from Figure 10 that the initial decomposition

temperatures of all the PP/MTAPP composites are lower

than that of pure PP due to the low thermal stability of TPU

in PP/MTAPP composites and the decomposition of APP

with release of NH3 and water at this stage. At higher tem-

peratures (above 420�C), however, the thermal stabilities of

all the flame-retardant composites are better than that of

pure PP, as shown in PTA10–30 of Figure 10. And, the char

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

0.0035

0.0040

0.0045

0.0050

PA30PTA30

PTA20PTA25

PTA15

PTA10

CO

pro

duct

ion

rate

(gs

–1) pure PP

PTA05

Time (s)1000 200 300 400 500 600 700

Figure 8. The CO emission curves of the pure and the flame-retardant PP composites. CO: carbon monoxide; PP:polypropylene.

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

0.32

0.36

PTA30PA30

PTA25

PTA20

PTA15

PTA10

PTA05

CO

2 pro

duct

ion

rate

(gs

–1) pure PP

Time (s)1000 200 300 400 500 600 700

Figure 9. The CO2 emission curves of the pure and the flame-retardant PP composites. CO2: carbon dioxide; PP: polypropylene.

0

20

40

60

80

100

420 430 440 450 460 470 480

0

20

40

60

80

100

PA30 PTA30

PTA20 PTA10

pure PP

Wei

ght

(%)

Temperature (°C)

100 200 300 400 500 600 700

Figure 10. TGA curves of the pure and the flame-retardant PPcomposites. TGA: thermogravimetric analysis; PP: polypropylene.

–3.0

–2.5

–2.0

–1.5

–1.0

–0.5

0.0

400 420 440 460 480 500

–3

–2

–1

0

PA30

PTA30PTA20

PTA10pure PP

PA30

PTA30PTA20

PTA10pure PP

Der

iv.w

eigh

t (%

min

–1)

Temperature (°C)100 200 300 400 500 600 700

Figure 11. DTG curves of the pure and the flame-retardant PPcomposites. DTG: derivative thermogravimetric; PP:polypropylene.

Chen et al. 451



residues of the PP/MTAPP composites are greater than that

of pure PP at 700�C. At the same time, the thermal stability

and char residues of PP/MTAPP composites are enhanced

with increasing the MTAPP content.

As shown in Figure 10, although the decomposition

processes of TGA curves of PP/MTAPP composite

(PTA30) and PP/APP composite (PA30) are similar, there

are still some different thermal decompositions. The ini-

tial decomposition temperature of PTA30 is higher than

that of PA30, and at high temperatures, PTA30 is more

thermally stable than PA30. Additionally, at the same

loading of flame retardant, the char residues left at

700�C for PTA30 and PA30 are about 19.9 and 12.7%,

respectively, suggesting better thermal stability at high

temperatures for the composite containing MTAPP. The

increase in the amount of char residue of PTA30 may be

attributed to the formation of more thermally stable carbo-

naceous char. From the above results, it can be concluded

that PP/MTAPP is better than PP/APP in improving the

thermal stability of PP composites.

Mechanical properties of the PP composites

Table 3 provides the mechanical properties of the pure and

the flame-retardant PP composites. As listed in Table 3, the

tensile strength, impact strength, and elongation at break of

pure PP are 28.9 MPa, 11.1 kJ m�2, and 204%, respec-

tively. It is found that the introduction of the flame retar-

dant to the PP material affects the mechanical properties.

The addition of flame retardant leads to a decrease in the

mechanical properties. And, the tensile strength, impact

strength, and elongation at break of the composites

decrease gradually with increasing the MTAPP content.

When the MTAPP content changes from 5 to 30 wt%, the

tensile strength of the PP/MTAPP composites is reduced by

2.8–32.5%; the impact strength is reduced by 23.4–25.2%,

the decrease of which is the smallest; and the elongation at

break is reduced by 12.1–52.2%, the decrease of which is

the largest. Interestingly, it is noted that the introduction

of TPU into APP can improve the mechanical properties.

As shown in Table 3, when the loading of flame retardant

is 30 wt%, the tensile strength, impact strength, and elonga-

tion at break of the PP/MTAPP composites are 19.5

MPa, 8.5 kJ m�2, and 111%, respectively; however, the

tensile strength, impact strength, and elongation at break

of the PP/APP composites are 18.9 MPa, 8.0 kJ m�2, and

103%, respectively.

Conclusions

In this work, APP is microencapsulated with TPU and char-

acterized by FTIR and TGA. MTAPP as an intumescent

flame-retardant system shows high efficiency in enhancing

flame-retardant performances of PP. The LOI values of

PP/MTAPP composites increase with increasing the

MTAPP content and are higher than that of PP/APP compo-

sites at the same loading level. It has been observed that

both pure PP and PP/APP composites have serious drip-

ping during burning process, whereas the dripping disap-

pears for PP/MTAPP composites. The results indicate that

the MTAPP has better flame retardancy compared with

APP in the flame-retardant PP composites. In addition,

HRR, RHR, CO, and CO2 values of PP/MTAPP compo-

sites obtained from CCT significantly decrease compared

with those of pure PP and PP/APP composites. The ther-

mal stability obtained from TGA also shows that MTAPP

can form a stable charred layer and higher charred resi-

dues compared with APP. Additionally, the mechanical

properties of the flame-retardant PP composites are worse

than those of pure PP. However, compared with APP,

MTAPP presents better mechanical properties in the

flame-retardant PP composites.

Funding

The present work was financially supported by the National

Natural Science Foundation of China (51003088, 51373139),

Table 3. Mechanical properties of the pure and the flame-retardant PP composites.

SamplesTensile strength

(MPa)Notched impactstrength (kJ m�2)

Elongation atbreak (%)

PP 28.9 + 0.3 11.1 + 0.4 232 + 1PTA05 28.1 + 0.3 8.5 + 0.4 204 + 6PTA10 27.0 + 0.3 8.8 + 0.5 191 + 3PTA15 26.1 + 0.2 8.5 + 0.2 169 + 8PTA20 24.0 + 0.2 8.3 + 0.3 137 + 7PTA25 22.1 + 0.4 8.7 + 0.2 121 + 6PTA30 19.5 + 0.1 8.5 + 0.4 111 + 4PA30 18.9 + 0.1 8.0 + 0.3 103 + 3

PP: polypropylene; PTA05: polypropylene/microencapsulated ammonium polyphosphate with 5 wt%; PTA10: polypropylene/microencapsulatedammonium polyphosphate with 10 wt%; PTA15: polypropylene/microencapsulated ammonium polyphosphate with 15 wt%; PTA20: polypropylene/microencapsulated ammonium polyphosphate with 20 wt%; PTA25: polypropylene/microencapsulated ammonium polyphosphate with 25 wt%; PTA30:polypropylene/microencapsulated ammonium polyphosphate with 30 wt%; PA30: polypropylene/ammonium polyphosphate with 30 wt%.




National Science and Technology Supporting Project Founda-

tion of China (2007BAB08B05), Fundamental Research Funds

for the Central Universities (SWJTU12CX009), and Sishi Star

Foundations of Southwest Jiaotong University (2011).

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App in Pp and Pu - Chen m Et Al - 2014