Composites: Part B 43 (2012) 150–158

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Composites: Part B

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Effect of flame retardants on mechanical properties, flammability and foamabilityof PP/wood–fiber composites

Zhen Xiu Zhang a,b, Jin Zhang b, Bing-Xue Lu b, Zhen Xiang Xin b, Chang Ki Kang c, Jin Kuk Kim b,⇑a Laboratory of Rubber–Plastics, Ministry of Education, Shandong Provincial Key Laboratory of Rubber–Plastics, Qingdao University of Science and Technology, Qingdao 266042, Chinab School of Nano and Advanced Materials Engineering, Gyeongsang National University, Gyeongnam, Jinju 660-701, South Koreac R&D Center, Hwaseung R&A Co. Ltd., Yangsan, Kyoungnam, South Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 March 2011Received in revised form 22 May 2011Accepted 16 June 2011Available online 21 July 2011

Keywords:A. FoamsA. Polymer–matrix composites (PMCs)A. WoodB. Mechanical propertiesPolypropylene

1359-8368/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compositesb.2011.06.020

⇑ Corresponding author. Tel.: +82 (0)55 751 5299;E-mail address: rubber@gnu.ac.kr (J.K. Kim).

The mechanical properties, flame retardancy, thermal degradation and foaming properties of wood–fiber/PP composites have been investigated. Ammonium polyphosphate (APP) and silica were used as flameretardants. The limiting oxygen index (LOI), thermal gravimetric analysis (TGA) and cone colorimeter(CONE) were employed for the study of fire retardance. At the same time, wood–fiber/PP compositefoams were produced with the batch foaming technique using CO2 as blowing agent. The effects ofAPP and silica content, pressure and temperature on the final cell structure were investigated. Accordingto LOI, TGA and cone calorimeter results obtained from the experiments, APP and silica are effective flameretardants for wood–fiber/PP composites, and silica was shown to have a flame retardant synergisticeffect with APP in wood–fiber/PP composite. The mechanical properties of the composites decreased withaddition of flame retardants, except for the tensile strength of small amount of silica filled wood–fiber/PPcomposite. The results also revealed that the cellular morphologies of the foamed wood–fiber/PP com-posites are a strong function of the content of APP and silica as well as foaming conditions.

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1. Introduction

Wood fiber reinforced plastic composites represent an emerg-ing class of materials that combine the favorable performanceand cost advantage attributes to both wood and thermoplastics[1]. By comparison with other fillers, the natural wood fiber rein-forced polymer composites are more environmentally friendly,and are widely used in transportation, military applications, build-ing and construction industries, packaging, consumer products, etc[2]. Polypropylene (PP) has been widely used for production of nat-ural fiber/polymer composites because of its low density, highwater and chemical resistance, good processability, and highcost-performance ratio [3–5]. Due to the poor compatibility be-tween natural fibers and PP matrix, so a compatibilizer should beadded to improve adhesion between matrix and fibers which leadsto enhancement of mechanical properties of composites, a promi-nent method that represents the addition of maleic anhydridepolymers as compatibilizers (such as maleic anhydride-graftedpoly(propylene) (PP-g-MA) and poly(styrene)-blockpoly (ethene-co-1-butene)-block-poly(styrene) triblock copolymer (SEBS-g-MA)) has been widely used by some research workers [4–8].

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Another drawback of wood–fiber/plastic composites (WPCs) aretheir high flammability. As organic materials, the polymers and thewood fibers are very sensitive to flame; improvement of flameretardancy of the composite materials have become more andmore important in order to comply with the safety requirementsof the wood fiber-composite products [9]. There is little researchon the flammability of nature fiber and wood fiber composites inthe literature. Yap et al. [10] investigated the effects of phospho-nates on the flame retarding properties of tropical wood–polymercomposites. Anna et al. [11] studied surface treated cellulose fibersin flame retarded PP composites by constituting a high-perfor-mance intumescent FR system in the PP matrix, and one of their re-sults showed that the addition of ammonium polyphosphate (APP)to the cellulose fiber containing composite would result in an FRcompound. Sain and Kokta [12] investigated the properties of thecomposites of PP and chemithermo mechanical pulp reactivelytreated with bismaleimide-modified PP or premodified pulp, theresults indicated that in situ addition of sodium borate, boric acid,or phenolic resin during processing of the composite decreased therate of burning of PP. Li and He [13] investigated the flame retar-dancy and thermal degradation of linear low-density polyethylene(LLDPE)–WF composites. In their study, APP and the mixtures ofAPP, melamine phosphate (MP) or pentaerythritol (PER) were usedas FRs, and experimental results demonstrated that APP is an effec-tive FR for LLDPE–wood–fiber composites by promoting char

Z.X. Zhang et al. / Composites: Part B 43 (2012) 150–158 151

formation of the composites, however, the addition of APP(30–40 phr) reduced the Izod impact strength and hardly affectedthe tensile strength of the composites. Sain et al. [14] found thatmagnesium hydroxide can effectively reduce the flammability(almost 50%) of natural fiber filled polypropylene composites. Nosynergetic effect was observed when magnesium hydroxide wasused in combination with boric acid and zinc borate, but marginalreduction in the mechanical properties of the composites wasfound with addition of flame-retardants. Zhao et al. [15] reportedthe mechanical properties, fire retardancy and smoke suppressionof the silane-modified WF/PVC composites filled by modifiedmontmorillonite (OMMT), and observed that the fire flame retar-dancy and smoke suppression of composites were strongly im-proved with the addition of OMMT. Guo et al. [16] investigatedthe effects of nanoclay particles on the flame retarding characteris-tics of wood–fiber/plastic composites (WPC), the result indicatesthat using a small amount of nanoclay can significantly improvethe flame retarding properties of HDPE/WF nanocomposites.

Ammonium polyphosphate (APP) is an effective intumescentfire retardant for several kinds of polymer-based materials[17–19]. It is a sort of chain phosphate with high molecular weight.Its efficiency is generally attributed to increase of the char forma-tion through a condensed phase reaction. Silica is usually used asenhancing agent in thermoplastic polymers to increase themechanical properties, such as tensile strength and toughness.And it has also been recognized as inert diluents and shows someflame retardant effect. Kashiwagi et al. [20] have reported theflame retardant mechanism of silica in polypropylene blends. Fuand Qu [21] reported the synergistic flame retardant mechanismof fumed silica in ethylene–vinyl acetate/magnesium hydroxideblends, the results indicated that the addition of a given amountof fumed silica apparently increased the LOI value and decreasedthe loading of MH in EVA blends. This study mainly devoted to re-port the influence of APP and silica on the flammability and ther-mal decomposition behavior of wood–fiber/PP composite.

Recently, foaming technology has penetrated into the researchand development of wood–fiber plastic composite products [22–32]. As a result, their drawbacks such as higher density, lower duc-tility, and poor impact resistance compared with neat plastics and/or solid wood could be overcomed with the presence of cellularstructure within the composites. Foaming of plastic/wood–fibercomposites can be produced by utilizing either a chemical or phys-ical blowing agent. A pressure-quench method described by Goeland Beckman was widely used for making microcellular polymersvia supercritical carbon dioxide (scCO2) [33]. They found that themicrocellular structure could be achieved by rapid depressuriza-tion to allow the cells nucleation and growth as in the batch pro-cess after saturating polymers with scCO2. In this study, theinfluence of APP and silica on the foamability of wood–fiber/PPcomposite was also investigated.

Table 1Formulation of wood–fiber/PP composites and LOI results.

Samples* APPa Silica LOIb

a 0 0 21.4b 10 0 24.5c 20 0 26.5d 30 0 27.4e 40 0 27.9f 20 2 27.1g 20 6 28.4h 20 10 28.9

* Base material is PP 65 phr; wood–fiber 30 phr; PP-g-MA 5 phr; SEBS-g-MA 5 phr.a Ammonium polyphosphate.b Limiting oxygen index.

2. Experimental

2.1. Materials

Polypropylene (R520Y) supplied by SK Corporation, which has amelt flow index (MFI) of 1.8 g/10 min (ASTM D1238) and a densityof 0.9 g/cm3, was used as matrix in this experiment. Maleic anhy-dride-grafted styrene–ethylene–butylene–styrene (SEBS-g-MA,Kraton FG-1901X) was supplied by Shell Chemical Co. Ltd., USA.Maleic anhydride-grafted polypropylene (PP-g-MA) (CM-1120)was supplied by Honam Petrochemical Co., Korea. Ammoniumpolyphosphate (Eflam APP 201) was supplied by Well Chem., Chi-na. The wood fiber (LIGNOCEL C120) was supplied by Rettenmeier& Sohne (Ellwangen, Germany), which had an aspect ratio between

5 and 10, a particle size between 70 and 150 lm, and density was1.45 g/cm3. The precipitated silica (Z132) supplied by Rhodia SilicaKorea Co., Ltd. Commercial grade CO2, an environmentally friendlyphysical blowing agent, with a purity of 99.95% was supplied byHyundai Gas Inc. The polymers were used as received. Thewood–fiber was dried at 100 �C in an air-circulating oven for24 h prior to use. The moisture content of the wood was less than1 wt.%.

2.2. Preparation of wood–fiber/PP composites by twin screw extruder

All samples showed in Table 1 were prepared by co-rotatingintermeshing twin-screw extruder (Bau-Tech, Korea). It has ascrew diameter of 19 mm and the distance between screw axesis 18.4 mm with L/D ratio of 40. It is fitted with a modular screwconfiguration, which has different combinations of right-handedand left-handed screws and neutral kneading disk elements withone reverse-pumping screw elements as shown in Fig. 1. The screwspeed was fixed at 150 rpm while the cylinder temperature wasmaintained at 150, 165, 175, and 180 �C from the hopper to thedie. The extrudate was pelletized and dried under vacuum at80 �C for 24 h to remove any residual water. At last the sampleswere molded at temperature profile of 150/170/170/180 �C byinjection for mechanical testing.

2.3. Preparation and analysis of wood–fiber/PP composite foams

Microcellular foaming experiments were performed in a batchprocess. A schematic of the batch-foaming process is shown inFig. 2. The plate samples that were 2.0 mm thick, 60 mm long,and 4.0 mm wide were enclosed in the high-pressure vessel. Thevessel was flushed with low-pressure CO2 for about 3 min andpressurized to the saturated vapor pressure CO2 at room tempera-ture and preheated to desired temperature. Afterward, the pres-sure was increased to the desired pressure by a syringe pump(ISCO260D) and maintained at this pressure for 2 h to ensure equi-librium absorption of CO2 by the samples. After saturation, thepressure was quenched to atmospheric pressure within 3 s andthe samples were taken out. Then foam structure was allowed tofull growth during rapid depressurization.

The density of foam and unfoamed samples was determinedfrom the sample weight in air and water respectively, accordingto ASTM D 792 method A. Then the density of the foamed sampleis divided by that of the unfoamed one to obtain the relative den-sity (qr).

qr ¼qf

qmð1Þ

where qm and qf are the densities of the unfoamed polymer andfoamed polymer respectively.

Fig. 1. Screw configuration used for the PP/WGRT blending experiments.

Fig. 2. The schematics of batch-foaming process.

Fig. 3. The effect of APP content on tensile strength and elongation at break ofwood–fiber/PP composite.

Fig. 4. Effect of silica content on the tensile strength and elongation at break ofwood–fiber/PP/APP composites.

152 Z.X. Zhang et al. / Composites: Part B 43 (2012) 150–158

2.4. Measurements

2.4.1. Limiting oxygen index (LOI)A limiting oxygen index (LOI) is defined as the minimum oxy-

gen concentration requires to maintain the downward flame com-bustion of the materials. Limiting oxygen index (LOI) is measuredaccording to ASTM D 2863. The apparatus used was an HC-2 oxy-gen index meter (Jiangning Analysis Instrument Company, China).The specimens used for the test have dimensions of125 mm � 12.5 mm � 3 mm. LOI is a numerical measure of poly-mer flammability and this numerical value was calculated quanti-tatively as follows:

LOI;% ¼ ½volume of oxygen=ðvolume of nitrogen

þ volume of oxygenÞ� � 100

The LOI values obtained by this test are the average of five testsfor each sample.

2.4.2. Cone calorimeterThe cone calorimeter (Stanton Redcroft, UK) tests are performed

according to ISO 5660 standard procedures. Each specimen ofdimensions 100 � 100 � 3 mm3 is wrapped in aluminum foil andexposed horizontally to an external heat flux of 50 kW/m2. Severalparameters are obtained from a cone calorimeter test, such as timeto ignite (TTI, s), average and peak heat release (av-HRR and pk-KRR, kW/m2), average mass loss rate (av-MLR).

2.4.3. Thermogravimetry analysis (TGA) testAll thermogravimetry analysis (TGA) experiments are per-

formed on TGAQ50 analyzer in nitrogen atmosphere at a heatingrate of 20 �C/min ranging from ambient to 700 �C.

2.4.4. Mechanical properties testThe tensile properties of the dumbbell wood–fiber/PP compos-

ite samples are measured using a Tensometer 2000 (Bong Shin)tensile testing machine, with crosshead speed of 50 mm/min, andthe average value of mechanical properties was calculated usingat least five samples. Izod impact of all samples was performedby an Izod impact instrument.

2.4.5. Morphological propertiesSEM micrographs of fracture surface of all samples after tensile

measurement were obtained using a model Philips XL-30S scan-ning electron microscope and the surfaces of all samples werecoated by a thin gold layer.

3. Results and discussion

3.1. Mechanical properties

The mechanical properties of the wood–fiber/PP composites areshown in Figs. 3 and 4. Despite the presence of the compatibilizer,the addition of APP as flame-retardant shows the decreasing trendof tensile strength and elongation at break. This could be attributedto the poor compatibility of the added flame retardant with poly-mer. The next important point, causing such a decrease, is the exis-tence of the cavities within the samples, formed via thermaldecomposition of fillers and release of steam during the process.Deterioration of the mechanical properties of the filled and unfilledplastics with the addition of flame-retardants has been reported bysome researchers [34,35].

Silica is usually used as an enhancing agent in thermoplasticpolymers to increase the mechanical properties, such as tensileand toughness. But a higher loading level of inorganic fillers orflame retardants would lead to earlier breaking of the composites.Thus, the tensile strength of the composites increases with theaddition of silica, however, obvious decrease can be observed withmore than 4 phr silica. This attributes to the larger size of silicaagglomerates at higher amount of silica. These agglomerates canact as stress concentrators and mechanical failure points whichcan initiate the fracture of the specimens [36]. The elongation atbreak decreases with increasing of silica content.

Fig. 5. The effect of APP content on impact strength of wood–fiber/PP composite.

Fig. 6. Effect of silica content on the impact strength of wood–fiber/PP/APPcomposites.

Fig. 7. TG (a) and DTG (b) curves of the wood–fiber/PP composites with differentloadings of APP.

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The increase in amount of fillers (APP or silica) caused a de-crease in impact strength (Figs. 5 and 6), it can be correlated tofragile surface adhesiveness, among fillers, matrix and cavitiesformed within the sample. Typically, a polymer matrix with highloading of fillers has less ability to absorb impact energy, becausethe filler disturbs matrix continuity and each particle is a site ofstress concentration, which can act as microcrack initiator [37].

3.2. Flame retardancy

Limited oxygen index (LOI), a widely used method as a simpleand precise method for the determination of fire self-extinguish-ment, was adopted to evaluate the flame retardant properties ofwood–fiber/PP/APP/silica composites. Table 1 gives limited oxygenindex (LOI) data of all wood–fiber/PP composites samples. Fromthe experimental results shown in Table 1, wood–fiber/PP compos-ite is easily flammable and its LOI is only 21 because PP and woodare both easily flammable materials. APP is an important flameretardant for wood cellulose and an important component of intu-mescent flame retardants, which obviously enhances flame retar-dancy of wood–fiber/PP composite, whose LOI reaches 27.9 whenaddition of APP is 40 phr. So it can be concluded that APP is a veryeffective flame retardant for wood–fiber/PP composite, because

APP can effectively catalyse wood or natural fiber to form char.From Table 1, it also can be seen that when the APP content is fixedat 20 phr, the LOI increases with increasing of silica content, forexample, the LOI value of sample (d) with 30 phr APP is only27.4%, whereas for the sample (h), APP and silica is 20 and10 phr in the composite respectively, increases to 28.9%. Appar-ently, there is synergistic effect between silica and APP on enhanc-ing the LOI of wood–fiber/PP composite.

3.3. Thermal degradation

The results of the thermal analysis are shown in Figs. 7 and 8.All materials decomposed in two decomposition steps. Table 2summarizes the characteristics for the two decomposition steps.Addition of APP to the wood composite accelerates the first stepof decomposition. For example, adding 10 phr APP, the tempera-ture for the maximum mass loss is around 81 �C below the temper-ature without APP. Hence, the temperature interval for the firstdecomposition step does not only correspond to the decompositionof the wood–fiber, but is also typical for the release of NH3 and H2Ofrom the APP [38]. The char residual from the wood–fiber is in part,and another part is composed of the APP-based additive. The latteris expected to show heat insulation properties, so the seconddecomposition is shifted to higher temperature as expected. There-fore, the thermal degradation of wood–fiber/PP composite maytake place as follows: at the first stage, APP interacts with woodcomposite, leading to generation of volatile compounds and a

Fig. 8. TG (a) and DTG (b) curves of the wood–fiber/PP composites with differentloadings of silica.

Table 2TG results of samples.

Sample TDTGmax

1st step(�C)

Mass loss1st step(%)

TDTGmax

2nd step(�C)

Mass loss2nd step(%)

Charresidue600 �C (%)

a 383.39 81.81 462.34 34.64 2.96b 302.06 89.99 474.13 36.86 15.72c 317.75 90.85 475.31 38.03 18.16d 322.65 91.25 480.03 39.70 20.35e 322.65 91.48 478.85 43.99 21.69f 314.06 91.81 483.57 38.89 21.05g 308.17 91.31 488.51 39.69 24.23h 298.78 91.13 484.16 41.87 26.46

Fig. 9. HRRs versus burning time for different wood–fiber/PP composites.

Table 3Cone data of some wood–fiber/PP composite samples.

Sample APP (0 phr)sample a

APP (20 phr)sample c

APP (30 phr)sample d

APP + silica(20 + 10 phr)sample h

TTI (s) 12 18 18 32Pk-HRR (kW/m2) 701 568 505 428Av-HRR (kW/m2) 301 215 199 156Pk-MLR (gs�1) 0.175 0.145 0.124 0.11

154 Z.X. Zhang et al. / Composites: Part B 43 (2012) 150–158

phosphorus rich layer, which could protect the polymer matrixagainst heat, and then the protective layer would decompose toyield a compact char on the surface of the materials to protectthe polymer matrix effectively at the second stage. Similar effectwas observed for APP on other materials such as polyamide/ethyl-ene–vinyl-acetate, polyurethane, and PP/flax blends [39–41].

From Fig. 8, it can be observed that the addition of silica alsoaccelerates the first step of decomposition, and generates morechar residual. The temperature of the second decomposition isshifted to higher temperature. These shifts are due to the silicaincorporates with APP to form a charred layer and inhibits the heatand mass transfer between surface and melting polymer, resultingin the increase of fire resistance of the composites.

3.4. Cone calorimeter study

The cone calorimeter based on the oxygen consumption princi-ple has been widely used to evaluate the flammability characteris-tics of materials. The HRR measured by cone calorimeter is a veryimportant parameter as it expresses the intensity of a fire. A highflame retardant system normally shows a low av-HRR value. Thepk-HRR value is used to express the intensity of the fire. Thechanges of HRR as a function of burning time for different samplesa, c, d, and h are shown in Fig. 9.

It can be found from Fig. 9 that PP/WF composite (sample (a))burns fast after ignition, and has a sharp HRR curve at range of40–80 s, whereas sample c and d with 20 phr and 30 phr APPrespectively shows a decline of the HRR curve, the combustion ofsample (c) and (d) is prolonged to 350 s and 470 s respectivelyfrom 250 s of the control sample a. Silica is usually considered tobe an inert additive in flame retardant polymers. It can be foundthat sample h containing 20 phr APP and 10 phr silica shows a de-cline of the HRR curve compared with the sample (d) which con-tains 30 phr APP, the burning was also prolonged to 670 s.Table 3 lists the data of TTI, HRR, and MLR obtained from the conecalorimeter tests of sample (a), (c), (d), and (h). It can be found thatthe values HRR and MLR decrease with addition of APP and silica,which also prolonged the TTI. Sample (h) with 10 phr silica and20 phr APP shows the lower HRR and MLR than sample d with30 phr APP. This result further gives the evidence that the 10 phrsilica has the synergist effect with APP in the wood–fiber/PPcomposites.

The above data indicate that the fire-resistance performance ofwood–fiber/PP composite is enhanced partly by substitution silicafor APP. This is mainly due to the silica tends to accumulate on thesurface in fire and consequently forms a charred layer by combin-ing with APP. Another reason is that the silica can prevent thecracking of the char layer. Wrinkles and cracks are found on the

Z.X. Zhang et al. / Composites: Part B 43 (2012) 150–158 155

char layer formed during the combustion of wood–fiber/PP com-posites as shown in Fig. 10a and b, however, it is observed thatsmoother and more compact char is formed with addition of silicaas shown in Fig. 10c. This charred layer prevented heat transfer andtransportation of degraded products between melting polymer andsurface, thus reduced the HRR and MLR.

Fig. 10. Char formation for some different wood–fiber/PP com

Fig. 11. SEM microphotographs of the dispersion of flame retardants (A) and microcellu

3.5. SEM morphological observation

The mechanical properties are influenced by the shape, dimen-sion, and dimension distribution of the filler particles, the fillercontent, and the physical properties of the filler and polymer.The interfacial interaction between the polymer and the filler is

posite samples, (a) sample c; (b) sample d; (c) sample h.

lar foam (B) (24 MPa, 150 �C) of sample (a), (c), (e) and (h) which are list in Table 1.

Fig. 12. Effect of pressure on the relative density of wood–fiber/PP composites.

Fig. 13. Effect of temperature on the relative density of composites.

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one of the most important factors, because it cannot only changethe local deformation and micromechanism of the local deforma-tion and the breaking process, but also influence the crystallinebehavior of the polymer, both of which influence the mechanicalproperties of the composites [42]. Morphologies of the APP and sil-ica filled wood–fiber/PP composites are explored by SEM. Fig. 11Ais SEM micrographs of impact fracture sections of composite (a),(c), (e) and (h). With addition of APP and silica, as shown inFig. 11A, in some domains, larger particles of agglomerated APP fil-ler can be observed; these larger particles lead to a deterioration ofthe mechanical properties.

Fig. 14. Typical cell structures of sample (e) compos

3.6. Batch physical foaming

3.6.1. Effect of APP and silicaFig. 11B shows the SEM micrographs of the microcellular wood–

fiber/PP composite, APP filled and APP–silica filled microcellularwood–fiber/PP composites. As can be seen in Fig. 11B, the cellshape of composite (a) is closed cellular polyhedron; no collapseor collision in the cell system is observed. The composite (c) and(e) are wood–fiber/PP composites filled with 20 phr, 40 phr APPrespectively. From the microphotographs of microcellular compos-ite (c) and (e), it can be observed that with addition of APP, somecell size become larger and irregular shape compared with purewood–fiber/PP composite, the cells become non-spherical andnon-uniform. This is because the addition of APP results in higherviscosity, so it is more difficult to disperse the APP particles inwood–fiber/PP composites and this leads to APP agglomerateswhich destroy the closed regular cellular polyhedron.

From the Fig. 11B, the micrograph of composite (h) is quite dif-ferent from the only APP filled with wood–fiber/PP composite, thenumber of big cells decreased, and less cell collapse or collision inthe cell system was observed, the cell structure become more uni-form. This result can be explained as follows: the silica substitutefor a part of APP particle, thus reduces the breakage of closed reg-ular cell, the silica isolates the APP particle at a certain extent,which can also reduce the breakage of cell.

Figs. 12 and 13 illustrate the effect of the APP and silica loadingon the relative density of wood–fiber/PP composite. It can be ob-served that the relative density slightly decreased first withincreasing the content of fillers, and then increased. As aforemen-tioned, because of the APP agglomerates and the comparatively lar-ger particles caused some large cell size and thinner cell wall,which resulted to the lower relative density. However, the additionof too much content of fillers to polymer matrix leads to a poor sur-face adhesion between filler and polymer matrix, and the poor sur-face adhesion provides a channel through which gas can quicklyescape from the composites. In addition, the presence of filler in-creases the viscosity of wood–fiber/PP composites, which obstructscell nucleation and growth, so the relative density increased.

3.6.2. Effect of saturation pressureThe effect of pressure on the final cells structure is studied at

constant temperature (150 �C) and depressurization time of 3 s,while pressure ranged between 8 MPa and 24 MPa. Typical cellsstructures of sample (e) obtained are presented in Fig. 14. Asshown in Figs. 12 and 14, the cell size increases, while relative den-sity decreases by increasing the pressure. Increasing saturationpressure, the extent of the CO2 induced-melting temperaturedepression increases, which means the amount of CO2 dissolvedin samples increases. As a result, the melt strength of the matrixis weaker, so the sample is relatively soft and its deformability islarge. On the other hand, at the low saturation pressure, the

ite foams at 150 �C (a) 12 MPa and (b) 24 MPa.

Fig. 15. Typical cell structures of composite (e) foams at 20 MPa (a) 145 �C and (b) 155 �C.

Z.X. Zhang et al. / Composites: Part B 43 (2012) 150–158 157

depressurization rate is low, the level of supersaturation is low andthe driving force for the cell nucleation is weak, so the cell size issmaller. So the result is that increasing saturation pressure be-comes more favorable for sample to foam and for the cells to growbigger in size, so the larger cell sizes and the thinner cell wallsbring on the reduced densities and relative densities [43].

3.6.3. Effect of saturation temperatureThe effect of temperature on the final cell structure was studied

at constant pressure of 20 MPa. Temperature varied between135 �C and 155 �C. Typical cell structures are presented in Fig. 15.As shown in Figs. 13 and 15, an increase of temperature leads toa significant increase in the average cell size, while it leads to a de-crease in the relative density. This behavior is typical for the foam-ing of many polymers with CO2 [44,45]. The cell size reflects theeffect of many factors such as the number of the formed nuclei,the amount of the dissolved gas, diffusivity, and the viscosity ofthe polymer matrix. At higher temperatures the energy barrier tonucleation decreases as predicted from nucleation theory [44].The generation of nuclei becomes more difficult and, as conse-quence, fewer cells are observed in the final cell structure (celldensity reduced). Simultaneously, the CO2 solubility in the poly-mer matrix decreases and, consequently, there is less fluid avail-able for nucleation and growth of pores. Furthermore, astemperature increases, the viscosity of the polymer matrix de-creases which facilitates the growing as well as the coalescenceof neighboring cells. Also, the diffusivity of the fluid increases re-sulted in the faster growth of cells. Facilitated and faster cellgrowth lead to formation of lager cells and foams with reduced rel-ative density.

4. Conclusions

The effects of APP and silica on the flammability, mechanicaland foaming properties of the wood–fiber/PP composites werestudied. The results showed that marginal reduction in themechanical properties of the composites was found with additionof flame retardants, except for the tensile strength of small amountof silica filled wood–fiber/PP composite. APP and silica showedeffective flame retardancy for wood–fiber/PP composites basedon LOI value and CONE data, which decreased initial temperatureof thermal degradation and promoted char formation of the com-posite, and silica has been shown to have a flame retardant syner-gistic effect with APP in wood–fiber/PP composite.

Microcellular wood–fiber/PP composite foams were success-fully prepared with batch foaming method using supercritical car-bon dioxide as blowing agent. In microcellular composites, the cellsize and relative density were a strong function of APP and silicacontent. With increasing of APP content, the relative density de-creased and then increased, the cell size increased, with the addi-tion of silica leads to smaller cell size and higher relative density.

As the saturation temperature and pressure increased, the cell sizeincreased and the relative density decreased. Therefore, it may beconcluded that the cellular morphologies of foamed WPC compos-ites are strong function the content of APP and silica, as well as thefoaming conditions.

Acknowledgement

We are grateful for financial support from the Korea Ministry ofEnvironment.

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