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2013 26: 16 originally published online 1Journal of Thermoplastic Composite MaterialsXiuju Zhang, Huajun Yang, Zhidan Lin and Shaozao Tan

Effect of compatibilizer on structure and propertiesPolypropylene hybrid composites filled by wood flour and short glass fiber:

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Article

Polypropylene hybrid

composites filled by woodflour and short glassfiber: Effect of compatibi-lizer on structure andproperties

Xiuju Zhang, Huajun Yang, Zhidan Lin andShaozao Tan

Abstract

Polypropylene (PP) hybrid composites filled with wood flour (WF) and short glass fiberwere prepared by melt blending and injection molding. Maleic anhydride-graftedPP (PP-g-MA), glycidyl methacrylate-grafted PP (PP-g-GMA), maleic anhydride-graftedethylene-octene copolymer (POE-g-MA), and maleic anhydride-grafted hydrogenated

styrene-butadiene-styrene (SEBS-g-MA) were used as the compatibilizers to enhancethe interfacial adhesion between PP and the fillers. The effect of the types and contentsof graft polymer on the crystallization and melting behavior, micromorphology, mechan-ical property, moisture resistance, and thermal stability of PP hybrid composites wasobserved. The result showed that PP-g-MA and PP-g-GMA had strong heterogeneousnucleation and promotion effect on PP crystallization. PP-g-MA was superior toenhance the tensile, flexural, and impact properties of composites. Hybrid compositeshad excellent moisture resistance and low water absorption. WF and PP macromolec-ular compatibilizer could improve the thermal stability of PP composites, in which the

most obvious effect was obtained in PP-g-MA-modified WF and short glass fiber hybridsystem. As a result, PP/WF composite could achieve outstanding improvement effect bybeing modified with macromolecular compatibilizer.

Keywords

Compatibilizer, wood flour, polypropylene, mechanical property, thermal behavior,micromorphology, moisture resistance

College of Science and Engineering, Jinan University, Guangzhou, PR China

Corresponding author:

Xiuju Zhang, College of Science and Engineering, Jinan University, Guangzhou 510632, PR China.

Email: [email protected]

Journal of Thermoplastic Composite

Materials

26(1) 1629

! The Author(s) 2011

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were achieved at 30% WF and 10% GSF in our previous research. For convenient

comparison, the compatibilizer content was 3% or 6%. The effects of the compa-

tibilizer type and combination on mechanical property, morphology, and thermal

behavior were indicated.

Experimental

Materials

A commercial grade isotactic PP (HP500N, MFR = 12 g/10 min at 230C) used in

this study was supplied by Reliance Industries Limited. PP-g-MA (with the MA

grafting ratio of 1.0%), PP-g-GMA (with the GMA grafting ratio of 1.0%), POE-

g-MA (with the MA grafting ratio of 1.0%), and SEBS-g-MA (with the MAgrafting ratio of 1.0%) were supplied by China Guangzhou Lushan Chemical

Materials Co., Ltd. The WF was supplied by a wood processing factory in

South China and passed through 100 mesh screen. GSFs (E-glass chopped strands)

were supplied by Dongguang tiansheng glass fiber Co., Ltd. The diameter of single

glass fiber was 13mm and the length was 4.5 mm.

Preparation of composites and test specimens

PP, WF, PP-g-MA, PP-g-GMA, POE-g-MA, and SEBS-g-MA were dried at 80

Cfor 24 h and mixed according to the composition in Table 1. The mixtures were

Table 1. Formulation of wood fiber polypropylene composites.

Sample

PP

(%)

Wood

flour (%)

Glass

fiber (%)

PP-g-MA

(%)

PP-g-GMA

(%)

POE-g-MA

(%)

SEBS-g-MA

(%)

PP 100 - - - - - -

W40 60 40 - - - - -

W30F10 60 30 10 - - - -W40M3 57 40 3 - - -

W30F10M3 57 30 10 3 - - -

W30F10M6 54 30 10 6 - - -

W30F10G3 57 30 10 - 3 - -

W30F10G6 54 30 10 - 6 - -

W30F10M3O3 54 30 10 3 - 3 -

W30F10M3S3 54 30 10 3 - - 3

W30F10O6 54 30 10 - - 6 -

W30F10S6 54 30 10 - - - 6

PP: polypropylene, PP-g-MA: maleic anhydride-grafted PP, PP-g-GMA: glycidyl methacrylate-grafted PP, SEBS-g-

MA: maleic anhydride-grafted hydrogenated styrene-butadiene-styrene, POE-g-MA: maleic anhydride-grafted

ethylene-octene copolymer.

18 Journal of Thermoplastic Composite Materials 26(1)



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blended in a twin-screw extruder at the temperature of 200C. The extruded com-

posites were cooled at room temperature and crushed into small granules in a

plastic breaker. All extruded composites were molded according to ASTM D638,

ASTM D790, and ASTM D 256 using the injection molding machine at the tem-perature of 200C to prepare tensile, flexural, and Izod impact test specimens.

Mechanical testing

Tensile, flexural, and Izod impact tests were carried out according to ASTM

Standard. For each test and type of composite, five specimens were tested and

the average values are reported. Tensile tests were conducted according to

ASTM D 638 using a Universal Testing Machine (Zwick/Roell Z005,

Zwick Roell Testing Machines Pvt Ltd) at a crosshead speed of 50 mm/min.Static flexural tests were carried out according to ASTM D 790 using the same test-

ing machine mentioned above at a crosshead speed of 2 mm/min. Izod

Notch impact tests were conducted according to ASTM D 256 using a

Universal Impact Testing Machine (ZBC-50, China Shenzhen SANS Testing

Machine Co., Ltd).

Microstructure analysis

The impact specimens were frozen in liquid nitrogen for 3 h, and then quicklysmashed. The fracture surfaces of the specimens were sputter-coated with gold

before scanning electron microscope (SEM) analysis. The fracture surface mor-

phology of the composites was observed on a Philips XL-30 ESEM SEM with

an acceleration voltage of 15 kV.

Characterization of non-isothermal crystallization and melting behavior

A TA Instruments Q200 differential scanning calorimeter (DSC) was used to study

the nonisothermal crystallization and melting behavior of pure PP and the PP/WF

composites and was calibrated using the melting temperature and enthalpy of a

pure indium standard. About 89 mg of the sample was accurately weighted for

DSC testing, and all measurements were performed in nitrogen atmosphere. A

composite sample was rapidly heated to 220C and held for 5 min to eliminate the

heat history. Subsequently, it was cooled to 60C at the cooling rate of 20C /min

for crystallization behavior study. And then, it was reheated to 220C at 20C/min

for melting behavior study.

Thermogravimetric Analysis (TGA)The thermal decomposition behavior of the composites was studied by a thermo-

gravimetry (model Q500, TA Instruments) in nitrogen atmosphere with the heating

rate of 10C/min.

Zhang et al. 19



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Water absorption testing

Water absorption test was carried out according to ASTM D 570-1998 Standard.

The sample with the size of 10 mm 10mm 4 mm was put into the drying oven at50C for 24 h, and then soaked into the beaker filled with distilled water for 48 h at

room temperature. Subsequently, the water on the surface of the sample was wiped

off with a filter quickly. The sample was weighed immediately after water immer-

sion. The ratio of the weight gained after being soaked is the water absorbing

capacity.

Results and discussionMechanical properties

The composition of PP/WF composites modified by different compatibilizers is

indicated in Table 1. Table 2 shows the mechanical properties of pure PP and

PP/WF composites modified by different compatibilizers. It could be seen that

the tensile strength of unmodified PP/WF composite lowered slightly with the

addition of WF, and elongation at break dropped significantly. However, the flex-

ural strength and flexural modulus of unmodified PP/WF composites increased to

Table 2. Mechanical properties of composites.

Sample

Tensile

strength (MPa)

Elongation

at break (%)

Flexual

strength

(MPa)

Flexual

modulus

(MPa)

Notch impact

strength (kJ/m2)

PP 31.8 0.7 15.0 27.3 0.5 61471 3.0 0.1

W40 29.1 0.3 4.9 44.8 0.8 2883114 2.3 0.1

W40M3 40.6 0.2 5.5 55.3 1.1 347365 2.4 0.1

W30F10 31.2 0.6 4.5 46.0 1.5 330725 2.4 0.1

W30F10M3 42.2 0.9 5.4 60.4 0.8 342793 2.5 0.3

W30F10M6 45.4 0.5 6.1 62.8 0.7 343340 3.2 0.4

W30F10G3 39.1 0.4 5.3 56.3 0.4 349761 2.3 0.1

W30F10G6 43.2 0.4 5.9 61.7 0.7 340066 2.9 0.4

W30F10M3O3 41.1 0.6 6.2 55.7 0.6 3193128 3.5 0.3

W30F10M3S3 33.8

0.3 5.2 48.2

0.4 3057

68 3.2

0.1W30F10O6 35.7 0.7 6.4 46.4 0.5 2763136 3.9 0.3

W30F10S6 28.0 0.7 5.2 39.7 0.4 268085 3.6 0.2

PP: polypropylene.




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164% and 470% of those of pure PP, respectively. These fully indicated that with-

out modification, WF still had strengthened and toughened effect to PP. For the

same species and content of fillers system (30% WF and 10% GSF) modified by

different compatibilizers, the improvement of tensile strength ranked as: PP-g-MA>PP-g-GMA>POE-g-MA>SEBS-g-MA. For example, when the content

of PP-g-MA was 6%, the tensile strength, flexural strength, and flexural modulus

were increased by 42.8%, 130.0%, 459.1%, respectively. Meanwhile, compared

with unmodified PP/WF composite, the elongation at break and impact strength

were also improved, which was the result of the formation of ester bonds between

polar group maleic anhydride in PP-g-MA and hydroxyl in the wood fibers.18

Therefore, better combination among wood fibers, PP, and glass fibers was

obtained. Nevertheless, the impact strength is much larger for PP-g-GMA than

for unmodied PP. It is considered that the entanglements of the grafted chain in PPshould play an important role in these results.19 Elastomer POE-g-MA, SEBS-

g-MA was chosen as the compatibilizer for sample W30F10M3O3,

W30F10M3S3, W30F10O6, and W30F10S6. As is seen from Table 2, with the

addition of elastomer, the tensile strength, flexural strength, and flexural modulus

lowered compared with the enhancement effect of PP-g-MA and PP-g-GMA but

was still higher than that of pure PP. The yield stress of the blends was decreased

with the introduction of elastomer with a low yield stress, and with the increase of

elastomer compatibilizer content there was more obvious drop of tensile strength.

When the content of SEBS-g-MA was 6%, its tensile strength was even lower thanpure PP, but the addition of elastomer made significant improvement in impact

strength. The impact strength was increased by 30% under the condition that the

content of POE-g-MA was 6%. The chemical cross-linking reaction took place

between the anhydride in SEBS-g-MA and POE-g-MA and OH in WF. Thus,

PP, compatibilizer, and filler particles got entangled with each other, which made

the mechanical properties of composite materials improved greatly. In addition,

SEBS and POE elastomer make more contribution in improving the toughness of

the composite. In general, elastomer particles were used as stress concentration

points, craze and shear band in the PP matrix were induced when larger external

impact forces was exerted. Along with the development of shear band and craze, a

large amount of energy was absorbed; thereby the impact strength of composite

materials increased.20 The modification effect of POE-g-MA on the impact strength

was superior to that of SEBS-g-MA. The incorporation of POE-g-MA not only

reduces the PP particle size in the blend but produces a more uniform particle size

distribution as well. Interfacial adhesion also seems to be improved with addition

of POE-g-MA. In order to be used as a commercial plastic, property balance is

important. Thus, 3% of PP-g-MA and 3% POE-g-MA are regarded to be appro-

priate compatibilizers for PP hybrid composites filled by WF and GSF. The ten-

sile strength, flexural strength, flexural modulus, and impact strength wereincreased by 29.3%, 104.0%, 420.0%, and 16.7% compared with those of pure

PP, respectively.

Zhang et al. 21



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SEM photographs

SEM photos of fracture surface of NCFRP composites and its modified composites

with different compatibilizer are shown in Figure 1. As is seen from the SEMphotos of unmodified NCFRP composites (Figure 1(a) and (c)), a coarse surface

with lots of exposed WF rod-shaped particles was observed. Fracture was uneven,

and the interface between WF particles and PP resin matrix was clear. Compared

with unmodified NCFRP composites, the composite modified by PP-g-MA (Figure

1(b)) had a relatively smooth surface, and no rod-shaped particle and cavity were

observed. Only a few WF particles embedded in the PP resin, and the interface

between them was tight. These indicated that PP-g-MA modified the interface

adhesion between WF and PP very well. The ester bond was generated through

reaction between the hydroxyl group in wood and anhydride in PP-g-MA, whichresulted in the formation of the interface layer and reduced the surface-free energy

of wood.21 The SEM photographs of composites modified by 6% of PP-g-MA, PP-

g-GMA, POE-g-MA, and SEBS-g-MA were shown in Figure 1(d), (e), (h), and (i),

respectively. Little wood fiber was observed in Figure 1(d), meanwhile, wood fiber

(a)

W40

(b)

W40M3

(c)

W30F10

(d)

W30F10M6

(e)

W30F10G6

(f)

W30F10M3O3

(g)

W30F10M3S3

(h)

W30F10O6

(i)

W30F10S6

Figure 1. Scanning electron microscope (SEM) photographs of polypropylene (PP)/ wood

flour (WF) composites modified with different compatibilizer.




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and PP matrix was linked closely together while the glass fiber and wood fiber were

also close to each other. A small amount of traces of glass fiber left from the PP

matrix was seen in Figure 1(e), which maybe the reason of poorer modification

effect of PP-g-GMA than that of PP-g-MA. Comparing the SEM photo of Figure1(f) and (h) with those of Figure 1(g) and (i), the only difference was the content

and type of compatibilizer. However, with the addition of POE-g-MA, it can be

seen from the SEM micrograph of the W30F10M3O3 and W30F10O6 blends that

the dispersed particles are adhered to the matrix by the formation of bridges

between the dispersed phase and the matrix (Figure 1(f) and (h)). These results

suggest that the formation of these bridges could indicate the existence of interac-

tion between the components. Otherwise, more clusters of WF and glass fiber were

seen in Figure 1(g) and (i) with the introduction of SEBS-g-MA. In the melt phase,

the POE-g-MA may interact with the evolving PP droplets through van der Waalsbonding between the PP chain.22 So the better improvement of mechanical prop-

erties were presented than those of SEBS-g-MA.

Nonisothermal cystallization and melting behavior

Some researches21,23,24 have shown that the addition of fillers and compatibilizers

have different effects on the crystallization and melting behavior of the PP.

Therefore, this article studied the effect of WF, GSF, and compatibilizer on the

crystallization and melting behavior of the PP firstly. Table 3 shows the nonisother-mal crystallization and subsequent melting parameters of the PP/WF composites

Table 3. Nonisothermal crystallization and melting parameters of PP/WF composites and its

compatibilized composites.

Sample Tpc (C) Tonsetc (

C) Hc(Jg1) Tpm (

C) Hm(Jg1)

PP 111.7 116.9 148.4 165.6 131.10

W40 113.4 119.8 99.8 166.8 96.0

W40M3 116.9 122.7 98.4 165.3 96.7

W30F10 114.6 121.0 88.3 165.9 87.5

W30F10M3 114.6 121.7 89.8 167.4 88.0

W30F10M6 116.0 122.1 97.6 165.5 98.5

W30F10G3 115.0 121.4 95.9 166.3 93.8

W30F10G6 115.8 121.5 96.2 164.8 96.9

W30F10M3O3 114.3 119.7 92.6 164.8 92.3

W30F10M3S3 112.4 118.7 86.0 166.3 84.9W30F10O6 109.1 116.0 83.5 167.0 82.5

W30F10S6 109.0 116.2 83.8 166.9 80.6

PP: polypropylene, WF: wood flour.

Zhang et al. 23



10/15

modified by different compatibilizer, in which the crystallization enthalpy (Hc)

and melting enthalpy (Hm) were normalized by the weight percent of PP. Hcand Hmof PP/WF composites were all lower than those of pure PP. It can be also

seen that the peak crystallization temperature (Tpc ), initial crystallization tempera-ture (Tonsetc ), and melting temperature (T

pm) of PP in the W40 composite increased

with the addition of WF, which indicated that WF had heterogeneous nucleation

effect on PP crystallization. When PP/WF composites were modified by PP-g-MA,

the Tpc and Tonsetc further increased, which showed PP-g-MA can also induce the

crystallization of PP and improve the crystallization rate. Also, there is a similar

effect of PP-g-GMA on PP/WF composites. But when the elastomer SEBS-g-MA

and POE-g-MA were introduced into PP/WF composites, the heterogeneous nucle-

ation effect was weakened. TheTpc andTonsetc were even lower than pure PP when

the content of SEBS-g-MA and POE-g-MA was 6%. The chemical structure of PPis close to that of the midblock of SEBS and POE, and strong interfacial bonding

exists between PP and SEBS and POE. However, grafting the MA functional group

to SEBS and POE increases the polarity of the central block. The compatibility

between nonpolar PP, SEBS and PP, POE becomes poorer after maleation.

Therefore, the functional MA group is ineffective in promoting the formation of

crystallites in PP, as evidenced by a shift to a lower value ofTpc .25

Thermal stability

Figure 2 shows TG and differential thermal gravimetric analysis (DTG) curves of

pure PP, PP/WF composites, and its composites modified by different compatibi-

lizers. From these figures, it is clear that the degradation temperature is shifted to a

slightly higher region in the case of treated composites than that of untreated

composites. This is due to the improved fiber/matrix adhesion during chemical

treatment as a result of the formation of bonds existing between fiber and matrix

provided by the compatibilizer. It could be seen that only one decomposition stage

occurred on the DTG curves of PP. However, three decomposition stages appeared

on that of WF/PP composites and PP hybrid composites filled by WF / GSF. The

three decomposition stages were in turn corresponding to the early thermal decom-

position of WF, the decomposition of PP and pyrolysis of tar substances formed by

early thermal decomposition of WF.26 However, in WF, GSF and graft polymer-

filled PP composites, the early stage of thermal decomposition of WF is no longer

evident, which is mainly the result of the reaction of hydroxyl groups in the surface

of WF particle surface with maleic anhydride groups or glycidyl methacrylate,

thereby slowing the release of WF thermal decomposition. The maximum thermal

decomposition rate temperature of PP composites filled with GSF and graft poly-

mer was improved compared with that of pure PP and PP composites only filled

with WF. The sample W30F10M6 and W30F10G6 displayed the maximumimprovement, which raised from 345C of pure PP to 360C and 363C, respec-

tively. On the one hand, graft polymer promoted the dispersion of WF in PP

matrix. On the other hand, more difficult tar substance was formed by early




11/15

Figure 2. TG and DTG curves of polypropylene (PP), wood flour (WF) filled PP and PP/WF

modified with different compatibilizer.

Zhang et al. 25



12/15

thermal decomposition of WF, and these substances were dispersed uniformly in

the PP matrix, which effectively improved the thermal stability of PP. Therefore,

the addition of SGF and grafted polymer was effective means to improve the

thermal stability of PP plastic materials.

Water absorption

The experimental result of water absorptivity is presented in Table 4. As can beseen, the water absorption rate of pure PP was very low. However, after adding

WF, water absorption rate of composites significantly increased because of the

wood fiber containing a large number of hydroxyl groups, and the hydrophilic

ability of PP/WF composites was improved. The water absorption rate of

sample W40 and W30F10 without adding compatibilizers were 0.50% and

0.52%, respectively, and the water absorption rate obviously decreased when

they were modified by compatibilizer. The reason lays in that on the one hand

the reaction of compatibilizer and the hydroxyl in wood fiber have reduced the

chance for water molecules to meet hydroxyl groups. On the other hand, wood

fibers in the matrix were more evenly distributed with the accession of compatibi-

lizer, wood fibers were tightly wrapped by continuous phase PP, and the interface

bonding was increased. Thus, the water absorption rate of the composites was

reduced due to fewer holes and cracks.27

Conclusions

In this article, PP/WF composites, PP-g-MA, PP-g-GMA, POE-g-MA, and SEBS-

g-MA modified PP/WF composites and PP/WF hybridized by GSF were prepared.

Meanwhile, mechanical property, thermal behavior, micromorphology, and mois-ture resistance of composites were studied. The compatibilizer exhibited hetero-

geneous nucleation and promotion effect on the PP crystallization. The interface

adhesion between WF and PP was poor. The addition of PP-g-MA modified the

Table 4. Water absorption data of PP/WF composites.

Sample PP W40 W40M3 W30F10 W30F10M3 W30F10M6

Waterabsorption

rate (%)

0.08 0.50 0.45 0.52 0.22 0.36

Sample W30F10G3 W30F10G6 W30F10M3O3 W30F10M3S3 W30F10O6 W30F10S6

Water

absorption

rate (%)

0.22 0.19 0.34 0.39 0.40 0.33




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dispersion and the interface adhesion of WF in PP/WF composites, and further

upgraded the mechanical properties, especially the tensile strength and toughness.

PP-g-MA and PP-g-GMA were beneficial for improving the tensile strength and

flexural strength, while POE-g-MA and SEBS-g-MA were beneficial for increasingthe impact strength. Inorganic fibers and WF were blended to prepare inorganic-

organic hybrid materials. The mechanical property and thermal stabilization of

composites were further improved. PP-g-MA had the best effect on the mechanical

property improvement, and it simultaneously improved the tensile, flexural, and

impact properties of the composites filled with WF and some GSFs. Both WF and

compatibilizer exert the positive effect on the improvement of thermal stabilization.

Comparatively speaking, the optimal improvement in mechanical property was

obtained for PP-g-MA-modified composites filled with WF and GSFs. As a

result, PP/WF composite could achieve outstanding improvement effect by beingmodified with macromolecular compatibilizer.

Acknowledgments

This work was supported by China Guangdong scientific and technological project

(No2010B080701060) and Guangdong natural science fund (No8451063201000041).

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