mechanical properties and morphology of intumescent flame retardant filled polypropylene composites

Mechanical properties and morphology ofintumescent flame retardant filledpolypropylene compositesJi-Zhao Liang*, Feng-Jiao Li and Jin-Qing Feng

The intumescent flame retardant (IFR) filled polypropylene (PP) composites were prepared using a twin-screw extruder.The tensile and impact fracture behavior of the composites were measured at room temperature. It was found that theYoung’s modulus increased roughly, while the tensile strength decreased slightly with increasing the IFR weightfraction; the toughening effect of the filler on the PP resin was significant. Both the V-notched Izod impact strengthand the V-notched Charpy impact strength of the PP/IFR composites showed a nonlinear increase with increasing thefiller weight fraction (ϕf) as ϕf was less than 20%, then it decreased. The limited oxygen index of the compositesincreases nonlinearly with increasing ϕf. The relationship between them obeyed a quadratic equation. The impactfracture surface was observed by means of a scanning electronic microscope to understand the toughening mechanismsfor the composite systems. Copyright © 2014 John Wiley & Sons, Ltd.

Keywords: polymer-matrix composite; tensile property; impact toughness; morphology

INTRODUCTION

Polypropylene (PP) is a general resin with good insulation prop-erties, small dielectric constant, good stress crack resistance, andchemical resistance.[1,2] However, PP resin has a poor flammabil-ity resistance and can continue to burn and cause flaming dripswhen ignited. In order to widen the applications of PP, flameretardants are usually added into this material for enhancing itsflame-retarding ability. Recently, the flame retardant for PP havereceived increasing attention.[3–6] Intumescent flame retardant(IFR), the kind of halogen-free flame retardants, has been widelyused in various polymers such as PP, because of its triple func-tions: filler, flame retardant, and smoke suppressant.[7–10] Thefocus of the studies for the PP/IFR composites was on the syner-gistic effect of the IFR and other retardants, such as ferric pyro-phosphate,[7] phosphorus-containing nanosponges,[8] La2O3.

[9]

Qian and his coworkers[10] researched the synthesis of a novelhybrid synergistic flame retardant and its application in PP/IFRand found that the fire resistance of PP/IFR composites couldbe improved with the combination of hybrid synergistic flameretardant. Hu and his colleagues[11] studied the synthesis andcharacterization of a novel nitrogen-containing flame retardantof polyethylene/IFR composites. Bras et al.[12] discussed thenew intumescent formulations of fire-retardant PP of the freeradical mechanism of the formation of carbonaceous protectivematerial during the thermo-oxidative treatment of the additivesfor polyphosphate–pentaerythritol system. Moreover, the rela-tionship between flame retardancy and thermal degradation forpolymer/IFR systems has been paid extensively attention duringrecent decade, such as poly(lactic acid)/starch/IFR biocomposites,[13]

PP/IFR composites.[14]

Tensile properties and impact strength are important parame-ters for characterizing the impact fracture toughness propertiesof materials. Toughening is one of the major modifications

for polymeric materials, including organic particular toughening(e.g. rubber toughening) and inorganic particular toughening.[1,2]

Since 2009, Liang and his coworkers[15–19] have investigated theeffects of inorganic particulate content and size on the tensileproperties and impact strength of the PP composites andobserved some interesting findings. They also have studied theimpact fracture toughness and mechanisms of inorganic particu-late-filled polymer composites, such as investigated the effectsof the diatomite particle size and content on the impact fracturestrength for the filled PP composites,[19] and the influence ofthe surface treatment of nanometer calcium carbonate on theimpact fracture strength for the filled PP composites,[16] and an-alyzed the interfacial stress in impact for the inorganic particu-late-filled PP composites.[20] Furthermore, they studied thequantitative description of the reinforcing and toughening forpolymer composites.[21,22]

However, there have been a few of the study reports on themechanical properties of PP/IFR composites in the past two de-cades, while the most focuses are on the compatibilization be-tween the retardant and the matrix,[23] flammability,[24] flameretardant mechanism,[25] and synthesis.[26,27] The objectives ofthis paper are to investigate the effects of the IFR content onthe tensile properties and impact fracture toughness of the PP/IFR composites.

* Correspondence to: Ji-Zhao Liang, Research Division of Green Function Materialsand Equipment, School of Mechanical and Automotive Engineering, South ChinaUniversity of Technology, Guangzhou 510640, P.R. China.E-mail: [email protected]

J.-Z. Liang, F.-J. Li, J.-Q. FengResearch Division of Green Function Materials and Equipment, School ofMechanical and Automotive Engineering, South China University of Technology,Guangzhou 510640, P.R. China

Research article

Received: 10 December 2013, Revised: 9 January 2014, Accepted: 15 January 2014, Published online in Wiley Online Library: 13 February 2014

(wileyonlinelibrary.com) DOI: 10.1002/pat.3262

Polym. Adv. Technol. 2014, 25 638–643 Copyright © 2014 John Wiley & Sons, Ltd.

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EXPERIMENTAL

Raw materials

The PP with a trademark of CJS-700G was used as a matrix resinin the present work. This resin was supplied by GuangzhouPetrochemical Works in Guangdong province (Guangzhou, China),and its density in a solid form and melt flow rate were 0.91 g/cm3

and 10g/10min, respectively.The IFR with a trademark of HFR-041, produced by Jinge fire-

fighting materials Co., Ltd. (Foshan City, China), was used as thefiller. The mean diameter and density of the IFR were about10μm and 1.68 g/cm3, respectively.

Preparation

After the PP resin was simply mixed with the IFR, they wereblended in their molten state in a twin-screw extruder at thetemperature of 165 ~ 180°C and screw speed of 200 rpm. Theextrudates of the PP/IFR composites were then water granulated.The IFR weight fraction (ϕf) were 10%, 20%, 25%, and 30%. Thediameter and length to diameter ratio of the screw were24.5mm and 40, respectively. The prepared PP/IFR compositeparticles were dried for 5 h at 80°C before the specimen injectionmolding.The specimens for impact test and tensile test were molded by

using a plastic injection machine with model UN120A suppliedby Yizumi Precision Mechanism Ltd. (Foshan City, China). Thetemperature was varying from 170°C to 210°C, and the moldtemperature was from 40°C to 50°C. The specimens for tensiletest were standard dumbbell sheets, and the size for measure-ment part was 60 × 10× 4mm. The sizes of the specimens forimpact test were 80 × 10× 3 and 80 × 10 × 6mm, respectively.

Apparatus and methodology

The impact properties of the PP/IFR composites were measuredat room temperature by means of an LCD-type plastic pendulumimpact testing machine with model of PIT501B-2 supplied byWance test equipment limited company (Shenzhen, China), boththe Izod impact test and Charpy impact test were conducted.Each group specimens contained five pieces, and the averagevalues of the measured mechanical properties were used fromthe measured data.The tensile tests of the PP/IFR composites were conducted

at room temperature by means of a universal materials testingmachine (model tensiTECH) supplied by Tech-Pro Inc. (Woodstock,USA) at room temperature, and the cross-head descending speedwas 50mm/min. Similarly, each group specimens contained fivepieces, and the average values of the measured tensile propertieswere used from the measured data.The limited oxygen index (LOI) apparatus (model JF-3) supplied

by Jianglin analysis instrument company (Nanjing, China) wasused in this work. The experiments of the flame retardant proper-ties such as LOI for the composites were carried out according torelevant international testing standards.The specimen fracture surfaces from the impact tests were

examined by means of a scanning electron microscope (SEM)(model S-3700N) supplied by Hitachi Co. Ltd. (Tokyo, Japan) toobserve the impact fracture surface, interfacial debonding,interlayer structure morphology, and the dispersion or distribu-tion of the IFR particles in the PP resin. The specimens were goldcoated before SEM examination.

RESULTS AND DISCUSSION

Tensile properties

Correlation between tensile stress and strain

Figure 1 displays the relationship between the tensile stress andstrain for the PP/IFR composites. When the IFR weight fraction is10%, the tensile strain at break is much smaller than that of theunfilled PP, and then it increases with increasing the IFR content.Moreover, the maximum tensile stress decreases with the increaseof the IFR weight fraction.

Dependence of Young’s modulus on IFR content

Figure 2 shows the dependence of the Young’s modulus on theIFR weight fraction for the filled PP composites. It can be seen thatthe Young’s modulus of the composites increases nonlinearly withan increase of the IFR weight fraction. When the IFR weight fractionis less than 25%, the Young’s modulus increases nonlinearly withincreasing the IFR weight fraction, while it decreases slightly asthe IFR weight fraction is more than 25%. This indicates that thereis certain stiffening effect of the IFR on the PP resin. This is becausethat the movement of the macromolecular chains of the matrixresin is blocked by the IFR particles, and the skeleton action ofthe inclusions will be produced in the matrix,[28,29] leading to theimprovement of the stiffness of the filled PP composites.

Figure 1. Correlation between tensile stress and strain.

Figure 2. Dependence of Young’s modulus on intumescent flame retar-dant weight fraction.

MECHANICAL PROPERTIES AND MORPHOLOGY OF PP/IFR COMPOSITES

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Dependence of tensile strength on IFR content

Figure 3 presents the dependence of the tensile yield strengthon the IFR weight fraction for the filled PP composites. It canbe observed that the tensile yield strength decreases slightlywith an addition of the IFR weight fraction. In the previous work,Liang and his coworkers[28–30] studied the interfacial adhesionbetween the inorganic particles and resin matrix and found thatthe tensile strength increases with improvement of the interfa-cial adhesion. Therefore, it might be illustrated from the resultsshown in Fig. 3 that the interfacial adhesion between the IFRand the PP matrix is good in this case.

Tensile fracture strength is an important characterization oftensile fracture toughness of materials. Figure 4 displays the de-pendence of the tensile fracture strength on the IFR weight frac-tion for the filled PP composites. When the IFR weight fraction isless than 10%, the tensile fracture strength increases, and itreaches up to the maximum at the IFR weight fraction of 10%;and then it decreases nonlinearly with an increase of the IFRweight fraction. This indicates that there is somewhat improve-ment of the tensile fracture toughness of the PP/IFR compositesin the case of low filler concentration. The main reason should bethat the matrix around the inclusions will generate plastic defor-mation owing to stress concentration in the case of low fillercontent to absorb the tensile deformation energy, leading tothe improvement of the tensile fracture toughness. The defectsin the composite system will increase with increasing filler

content because of the interaction between the filler particles,and the strength of the composites will be weakened corre-spondingly. As a result, the tensile fracture strength of the com-posites decreased slightly.

Dependence of tensile elongation at break on IFR content

Tensile elongation at break is an important characterization oftensile fracture ductility of materials. Figure 5 illustrates the rela-tionship between the tensile elongation at break and the IFRweight fraction of the filled PP composites. When the IFR weightfraction is less than 10%, the tensile elongation at breakdecreases, and it reaches up to the minimum at the IFR weightfraction of 10%; and then it increases nonlinearly with an in-crease of the IFR weight fraction. This indicates that there is cer-tain improvement of the tensile fracture ductility of the PP/IFRcomposites in the case of high filler concentration. It is generallybelieved that the tensile fracture ductility of matrix resin will beweakened usually because of the filling of filler particles. Onthe other hand, as discussed earlier, the matrix around the inclu-sions will generate plastic deformation owing to stress concen-tration; the tensile fracture ductility of the matrix resin will beimproved accordingly, resulting in increasing slightly the tensileelongation at break of the PP/IFR composites with addition ofthe filler content.

Impact strength

Izod impact strength

Figure 6 shows the relationship between the V-notched Izodimpact strength (σVII) and the filler particle weight fraction (ϕf)for the PP/IFR composites. The thicknesses of the specimen are3 and 6mm, respectively. It can be seen that the value of theV-notched Izod impact strength increases nonlinearly with in-creasing ϕf for the specimen thickness of 3mm as ϕf is less than20%, and then it decreases. When the filler particle weight frac-tion is 20%, the V-notched Izod impact strength for the compos-ites is the maximum, and the increase amplification is up toabout 77.54% comparing with the unfilled PP resin.In the case of the specimen thickness of 6mm, the value of the

V-notched Izod impact strength for the PP/IFR compositesincreases nonlinearly with increasing the filler weight fractionexcept for individual data point. When the filler particle weightfraction is 30%, the V-notched Izod impact strength for the

Figure 4. Dependence of tensile fracture strength on intumescentflame retardant weight fraction.

Figure 3. Dependence of tensile yield strength on intumescent flameretardant weight fraction.

Figure 5. Dependence of tensile elongation at break on intumescentflame retardant weight fraction.

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composites is the maximum, and the increase amplification is upto about 21% comparing with the unfilled PP resin. It may alsobe observed that the V-notched Izod impact strength for thespecimen thickness of 6mm is slightly higher than that for thespecimen thickness of 3mm under the same filler content.

Charpy impact strength

Figure 7 displays the dependence of the V-notched Charpy im-pact strength (σVCI) on the filler particle weight fraction for thePP/IFR composites. The thicknesses of the specimen are also 3and 6mm, respectively. Similarly, in the case of the specimenthickness of 3mm, the value of the V-notched Charpy impactstrength increases nonlinearly with increasing the filler weightfraction when the filler weight fraction is less than 20%, and thenit reduces slightly. As the filler particle weight fraction is 20%, theV-notched Charpy impact strength for the composites is themaximum, and the increase amplification is up to about 50%comparing with the unfilled PP resin.In the case of the specimen thickness of 6mm, the value of the

V-notched Charpy impact strength for the PP/IFR composites in-creases with increasing ϕf as the filler particle weight fraction isless than 10%, and then it decreases slightly with an increase ofϕf. When the filler particle weight fraction is 10%, the V-notchedCharpy impact strength for the composites is the maximum, andthe increase amplification is up to about 21% comparing with

the unfilled PP resin. In addition, the V-notched Charpy impactstrength for the specimen thickness of 6mm is slightly higherthan that for the specimen thickness of 3mm when the filler par-ticle weight fraction is less than 20%. As the filler particle weightfraction is more than 20%, the difference in the V-notchedCharpy impact strength between them is not obvious. It indi-cates that the effect of the specimen thickness on the impactfracture toughness of the PP/IFR composites is insignificant.

Flammability resistance

Limited oxygen index is one of the important parameters forcharacterizing flame retardant properties of polymer materials.Figure 8 shows the relationship between the LOI and the IFRweight fraction for the PP/IFR composites. It may be observed thatthe LOI of the composites increases nonlinearly with increasing theIFR weight fraction. The relationship between LOI and ϕf for thecomposite systems might be expressed by the following equationin this case:

LOI ¼ α0 þ α1ϕf þ α2ϕ2f (1)

Where α0, α1, and α2 are the coefficients related to flame retar-dant property. The values of the α0, α1, and α2 of the compositesystems may be determined by using a regressive analysismethod under test conditions. The values of the α0, α1, and α2of the composite systems are listed in Table 1. It can be seen thatthe correlation coefficient is more than 0.99.

Morphology

In general, the smoother of the impact fracture surface is, thepoorer the impact fracture toughness is the material. Figure 9is the SEM photograph of the impact fracture surface of theunfilled PP. It can be seen that the impact fracture surface ofthe specimen is relatively smooth. It means that the micro-cracks

Figure 6. Relationship between V-notched Izod impact strength andintumescent flame retardant weight fraction.

Figure 7. Relationship between V-notched Charpy impact strength andintumescent flame retardant weight fraction.

Table 1. The values of the α0, α1, and α2 of the composites

Composites α0 α1 α2 R2

Polypropylene/intumescentflame retardant

17.448 0.778 �0.007 0.999

Figure 8. Correlation between limited oxygen index and the intumes-cent flame retardant weight fraction.



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will extend directly in the specimen under impact load, leadingto smooth fracture surface and low impact fracture strength ofthe unfilled PP resin, as shown in Figs. 6 and 7.

It is generally believed that the impact properties for polymercomposites depend, to much extent, upon the status of the dis-persion or distribution of the filler particles in the matrix resin.Figure 10 is the SEM photograph of the impact fracture surfaceof the PP/IFR composite with the filler weight fraction of 10%.It may be observed that the impact fracture surface is rougherthan that of the unfilled PP resin shown in Fig. 9. This becausethat the filler particles will block more or less the developmentof the micro-cracks in the specimen under impact load, resultingin rougher fracture surface and improvement of the impactfracture toughness of the composite systems (Figs. 6 and 7).

Figure 11 is the SEM photograph of the impact fracture surfaceof the PP/IFR composite with the filler weight fraction of 20%.The dispersion or distribution of the IFR particles in the matrixis still uniform. In addition, the impact fracture surface is roughercomparing to the impact fracture surface of the composite withfiller weight fraction of 10% shown in Fig. 10. In general, the resinaround the inclusions will generate relevant deformation owing to the stress concentration under impact load, and this deforma-

tion will absorb certain impact deformation energy. As a result,the fracture surface is rougher, and the impact fracture strengthis higher, as shown in Figs. 6 and 7.Figure 12 is the SEM photograph of the impact fracture surface

of the PP/IFR composite with the filler weight fraction of 25%.It may be seen that the dispersion or distribution of the IFRparticles in the matrix is roughly uniform, and the fracture sur-face is somewhat smoother than that of the PP/IFR compositewith the filler weight fraction of 20% as shown in Fig. 11. Asdiscussed earlier, the smoother of the impact fracture surfaceis, the poorer the impact fracture toughness is the material. Inthis case, the impact toughness of the composites is slightlydecreased (Figs. 6 and 7).Figure 13 is the SEM photograph of the impact fracture surface

of the PP/IFR composite with the filler weight fraction of 30%.Similarly, the dispersion or distribution of the IFR particles inthe matrix is also roughly uniform. The impact fracture surfaceis slightly rougher than those of the PP/IFR composite with thefiller weight fraction of 25% as shown in Fig. 12 but is smootherthan that for the other composite systems. Consequently, the

Figure 10. Scanning electron microscope photograph of impact frac-ture surface of polypropylene/intumescent flame retardant composite(ϕf=10%).

Figure 9. Scanning electron microscope photograph of impact fracturesurface of unfilled polypropylene.



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impact fracture strength of the composites is slightly higher thanthat of the PP/IFR composite with the filler weight fraction of25%, as shown in Figs. 6 and 7.In short, when the IFR weight fraction is more than 25%, the

dispersion or distribution of the filler particles in the PP matrixwill be relatively poor, and the impact fracture surface presentsrelatively smooth comparing to the case of low inclusion con-tent, this should be the main reason that the impact strengthof the PP/IFR composites slightly decreases.

CONCLUSIONS

There were certain stiffening and reinforcing effects of the IFRparticles on the PP resin. The Young’s modulus of the filled PPcomposites increased roughly, while the tensile yield strengthdecreased slightly with increasing the IFR weight fraction; thetensile fracture strength reached up to the maximum at the IFRweight fraction of 10%; the tensile elongation at break increasedwith an addition of the IFR weight fraction when the IFR weightfraction was more than 10%.The effect of the IFR content on the toughness of PP resin was

significant. The maximum increase of both the V-notched Izodimpact strength and the V-notched Charpy impact strength forthe composites was more than 50% comparing with the unfilledPP resin. The V-notched Izod impact strength of the compositesshowed a nonlinear increased with increasing the filler weight

fraction (ϕf) as ϕf was less than 20%, then it decreased. Whenϕf was less than 20%, the V-notched Charpy impact strength ofthe composite increased nonlinearly with an increase of ϕf, thenit slightly reduced. Moreover, the dispersion or distribution of theIFR particles in the matrix was roughly uniform from the observa-tion by means of a scanning electronic microscope. Furthermore,the effect of the specimen thickness on the impact fracturetoughness of the PP/IFR composites is insignificant.

The LOI of the composites increases nonlinearly with increasingϕf. The relationship between them obeyed a quadratic equation.

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mechanical properties and morphology of intumescent flame retardant filled polypropylene composites

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