J Polym Environ DOI 10.1007/s10924-016-0813-4

ORIGINAL PAPER

Effect of Nano-SiO2 and Bark Flour Content on the Physical and Mechanical Properties of Wood–Plastic Composites Mohammad Farsi1

Ó Springer Science+Business Media New York 2016

Abstract The aim of this study is to evaluate the impact of nano-SiO2 and bark flour (BF) on the natural fiber–plastic composites engineering properties made from high density polyethylene (HDPE) and beech wood flour (WF). For this purpose, WF and BF in 60 mesh size and weight ratio of (50, 0 %), (30, 20 %), (10, 40 %) and (0, 50 %) respectively were mixed with HDPE. In order to increase the interfacial adhesion between the filler and the matrix, the maleic anhydride grafted polyethylene was constantly used at 3 wt% for all formulations as a coupling agent. The nano-SiO2 particles with weight ratio of 0, 1, 2, and 4 % were also utilized to enhance the composites properties. The materials were mixed in an internal mixer (HAAKE) and then the bark and/or wood–plastic composite samples were made utilizing an injection molding machine. The physical tests including water absorption and thickness swelling, and mechanical tests including bending characteristics and un-notched impact strength were carried out on the samples based on ASTM standard. The results indicated that as the BF content increased in the composite, mechanical and physical properties were reduced, but the given properties were increased with the addition of nanoSiO2. The addition of nano-SiO2 had a negative impact on the physical properties, but when it was up to 2 %, it increased the impact strength. Keywords Wood–plastic composites (WPCs)  BF  NanoSiO2  Mechanical properties  Physical properties

& Mohammad Farsi [email protected] 1

Department of Wood and Paper Science and Technology, Sari Branch, Islamic Azad University, 7th Kilometer Khazar Blvd., P.O. Box 48161-19318, Sari, Iran

Introduction Polymeric materials are used to modify the mechanical properties of fillers. The bark includes of thousands of inorganic compounds that are currently been used to produce thermal energy [1] and it appears to have the potential to produce polymeric composites. But it is mostly used in the manufacture of particle board and MDF [2–4]. The bark may also be utilized as a source of adhesives, other chemicals and energy, substances absorbing, soil pollutants and conditioner [5]. Trees bark makes 10–15 % of the trees volume and contains lignin, hemicellulose, and other extraneous materials e.g., phenols, lignans, fatty acids and resins which contain hydroxyl, carboxylic, ether, and phenolic functional moieties. Based on the structure, chemical composition and properties, trees bark as lignocellulosic material are significantly different from wood [5]. Such differences exert limits on its application in wood industry conventional processes and making its application problematic [6]. Every year, million tons of barks remain as waste in the wood and paper industry. There are limited sources of researches conducted on the application of this non-fibrous material as fillers in the manufacture of natural fiber–plastic composites (NFPCs). For instance, Yeme et al. [7] found that most mechanical properties such as strength, except for tensile toughness and strain at fractures, were lower for spruce bark–HDPE composites compared to neat HDPE which was also reported by other researchers [8, 9]. Besides the physical properties, the WPCs mechanical properties are different from those of bark–plastics composites. For example, the research conducted by Kazemi Najafi et al. [10] showed that among the composites of maleic anhydride grafted polypropylene (MAPP), those composites containing higher bark flour (BF) content showed lower water absorption and lower thickness swelling.

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Recently, thanks to the advent of nanotechnology in the field of materials science, polymeric composites reinforced with nano-phase have established high interest in industry and research area. In fact, nanocomposites have created new kinds of polymeric composites in which particles at nano-scale are utilized including carbon nanotubes (CNTs), nanoclay (NC) particles, nano-SiO2 etc. [11–14]. One of the most attractive nanoparticles is nano-SiO2 with various favorable advantages such as being relatively inexpensive, nontoxic, biocompatible, and high thermal resistant and especially its ability to reinforce polymer matrix’s mechanical properties [15–17]. However, the high hydrophilicity of nano structured SiO2 surface can induce the nanoparticles to be easily agglomerated and hardly dispersible in polymer matrix. Considering the nano-silica application in composites, several studies have been conducted on the utilization of WPCs. Wen et al. [18] in a study on mechanical properties of gypsum panel boards filled with nano-SiO2 concluded that the addition of 3 % nano-SiO2 improved the elasticity modulus and flexural strength of gypsum panel boards. Norbakhsh et al. [13] used nano-SiO2 as fillers in four levels of 0, 1, 2, 3 and 4 wt% in producing the polymeric composites made from polypropylene and rice husk flour. The results revealed that generally low levels of fillers could improve some mechanical properties of the composites and the water absorption faced a reduction by increasing the nano-SiO2 content to 3 %. But such increase was not significant compared with the pure NFPC samples. Deka and Maj [19] simultaneously incorporated NC and nano-SiO2 in the production of WPCs. The results showed improved mechanical properties by the addition of 3 % NC and nano-SiO2 that serves as flame retardant properties of the composites. In addition, the water absorption witnessed a reduction by increasing the NC and nano-SiO2 content in WPCs. Salari et al. [20] in a study on evaluating the characteristics of oriented strand board (OSB) made from Paulownia strand and nano-SiO2 concluded that utilizing 3 % of nano-SiO2 in the given boards resulted in improved physical and mechanical properties, and formaldehyde emission was reduced. Accordingly, this study sought to evaluate the effect of nano-SiO2 and beech BF on the physical and mechanical properties of WPCs.

Materials and Methods High density polyethylene (HDPE) supplied by Maroon Petrochemical Industries Co., Iran, which has a melt flow index (MFI) of 23 g/10 min and a density of 965 kg/m3, was utilized as a matrix in this experiment. Maleic

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anhydride grafted polyethylene (MAPE) as coupling agent was obtained from Kimia Javid Sepahan Co., Iran (MFI = 0.4 g/10 min, 0.8 % maleic anhydride). Two types of lignocellulosic materials were investigated in this study: beech wood and bark. The materials were grinded into flour to a particle size of 60 mesh using a Thomas-Wiley mill. The chemical components of the beech wood flour (WF) and BF are shown in Table 1. Nanoparticles of SiO2 (nanoSiO2) a product of Degussa Co., Germany, were used as nano-reinforcement for this work. The average diameter, density and specific surface of the silica nanoparticles were 12 nm, 0.37 g/cm3 and 200 m2/g respectively. The polymer and nano-SiO2 were used as received. The lignocellulosic materials were dried at 100 °C in an air circulating oven for 24 h before use. The moisture content of the lignocellulosic materials was less than 1 wt%. Composite Preparation HDPE, beech WF and BF, and nano-SiO2 were weighed and bagged based on the formulations given in Table 2. The mixing process was prepared by melt blending the materials using a high shear internal mixer (HBI System 90, USA) operating at 60 rpm and was then discharged at 160 °C. First the HDPE was fed into a mixing chamber. After HDPE melting, compatibilizer and nano-SiO2 were added. At the fifth minute, the WF and/or BF were fed and the total mixing time was 14 min. The compounded materials were then grinded using a pilot scale grinder (WIESER, WGLS 200/200 Model). The resulted granules were dried at 105 °C for 4 h. The test specimens were prepared using injection molding machine (Imen machine, Iran) at 175 °C and at a pressure of 10 MPa. The specimens were stored under controlled conditions (50 % relative humidity and 23 °C) for at least 40 h prior to testing. Mechanical Test Tensile strengths were measured according to ASTM D 638 (loading speed; 5 mm/min.). Impact strength test was measured based on ASTM D256 using SANATAM-SIT20D impact testing machine. Physical Tests Water absorption and thickness swelling tests of the nanocomposites were carried out based on the ASTM D7031-04 standard. Five specimens from each combination were taken and dried in an oven for 24 h at 100 ± 3 °C. The weight and thickness of dried specimens were measured at an accuracy of 0.001 g and 0.001 mm, respectively. The specimens were then immersed in distilled water for 24 h and kept at a temperature of

J Polym Environ Table 1 - Chemical Compositions of the beech WF and BF

Source

Extractives (%)

Cellulose (%)

Hemicellulose (%)

Lignin (%)

Others (%)

Wood

3

37

25

22

16

Bark

10

23

32

30

15

Table 2 - Composition of evaluated formulations NanoSio2 (wt%)

MAPE (wt%)

0

0

3

0

1

3

50

0

2

3

50

0

4

3

50

30

20

0

3

50HDPE/30WF/20BF/1NS/3M 50HDPE/30WF/20BF/2NS/3M

50 50

30 30

20 20

1 2

3 3

8

50HDPE/30WF/20BF/4NS/3M

50

30

20

4

3

9

50HDPE/10WF/40BF/0NS/3M

50

10

40

0

3

10

50HDPE/10WF/40BF/1NS/3M

50

10

40

1

3

11

50HDPE/10WF/40BF/2NS/3M

50

10

40

2

3

12

50HDPE/10WF/40BF/4NS/3M

50

10

40

4

3

13

50HDPE/0WF/50BF/0NS/3M

50

0

50

0

3

14

50HDPE/0WF/50BF/1NS/3M

50

0

50

1

3

15

50HDPE/0WF/50BF/2NS/3M

50

0

50

2

3

16

50HDPE/0WF/50BF/4NS/3M

50

0

50

4

3

Number

Composite formula

HDPE (wt%)

WF (wt%)

1

50HDPE/50WF/0BF/0NS/3M

50

50

2

50HDPE/50WF/0BF/1NS/3M

50

50

3

50HDPE/50WF/0BF/2NS/3M

50

4

50HDPE/50WF/0BF/4NS/3M

50

5

50HDPE/30WF/20BF/0NS/3M

6 7

20 ± 2 °C. Weight and thicknesses of the specimens were measured after excessive water was removed from their surface. The values of the water absorption in percentage were calculated using the following equation: WAðtÞ ¼

WðtÞ  W0  100 W0

ð1Þ

where WA(t) is the water absorption (%) at time t, WO is the oven dried weight and W(t) is the weight of the specimen at a given immersion time t. Also, the values of the thickness swelling in percentage were calculated using Eq. 2. TSðtÞ ¼

TðtÞ  T0  100 T0

ð2Þ

where TS(t) is the thickness swelling (%) at time t, To is the initial thickness of specimens, and T(t) is the thickness at time t. Statistical Analysis Completely randomized block design based on 4 9 4 factorial experiments was used for statistical analysis. The analysis of variance (ANOVA) showed the significant difference between the averages at a = 0.05 and 0.01 confidence levels.

BF (wt%)

Results and Discussion The Analysis of Variance Table 3 shows the results of analysis of variance (ANOVA) of independent and interactive impact of the lignocellulosic fillers and nano-SiO2 content on the physical and mechanical properties of WPCs. Water Absorption and Thickness Swelling Water absorption is one of the key parameters in quality assessment of NFRCs. Hydrophilicity represents a characteristic of the filler surface induced by the hydroxyl groups capable of interacting with one another to form inter- and intramolecular hydrogen bonds. In this respect, chemical compositions of fillers play a prominent role in water absorption properties of NFRCs [21]. As shown in Table 3, it can be noted that the independent and interactive impact of the WF/BF and nano-SiO2 content on the water absorption and thickness swelling of WPCs was significant at 0.05 and 0.01 confidence levels. Figures 1 and 2 show the water absorption and thickness swelling of WF/BF-HPDE nanocomposites.

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J Polym Environ Table 3 Analysis variance (ANOVA) of the lignocellulosic fillers and nano-SiO2 content on the physical and mechanical properties of WPCs

Source of variance

Degree of freedom

f value WA

TS

TE

TM

IS

WF/BF Content (A)

3

1.992*

1.868*

261.584**

49.586**

28.543**

NanoSio2 (B)

3

77.959**

13.211**

0.29ns

5.264**

6.633**

AB

9

75.432*

6.429*

33.294**

5.425**

1.405ns

WA water absorption, TS thickness swelling, TE tensile strength, TM tensile modulus, IS impact strength * Significant at 50 % level; ** significant at 90 % level; ns no significant

Fig. 1 Effect of nano-SiO2 and BF content on water absorption of WPCs

54.4 % in samples without nano-SiO2. Large amounts of extractives in BF (13 %) cause a decrease in the polarity on the surface of the filler and a decrease in the wettability, therefore they limit the performance of MAPE. Saputra et al. [23] showed that extractives form a weak boundary layer in pine flour and that the removal of this layer by extraction improves the shear strength between the PP matrix and the extracted wood filler. Also, the water absorption test results showed that in the presence and absence of BF in the filler combination, the composites containing nano-SiO2 showed higher levels of water absorption and thickness swelling than the controls (samples without nano-SiO2). By increasing the nano-SiO2 from 1 to 4 wt%, water absorption and the thickness swelling of the nanocomposites increased, accordingly. This could be possible due to the existence of a poor adhesion between the matrix and nano particle, owing to the presence of more gaps in the interfacial region and also more hydrophilic groups as hydroxyls that are available for hydrogen bonding with water. Several studies have supported the above observation [10, 24, 25]. According to Figs. 1 and 2, samples containing 4 wt% of nano-SiO2 have maximum water absorption and thickness swelling with 4.91 and 4.73 %, respectively. Tensile Strength and Modulus

Fig. 2 Effect of nano-SiO2 and BF content on thickness swelling of WPCs

As earlier mentioned, the composites water absorption is caused by the hydrogen bonding of the water molecules to the free hydroxyl groups present in the cellulosic cell wall materials and the diffusion of water molecules into the filler-matrix interface. In addition, large numbers of porous tubular structures present in fiber accelerate the penetration of water by capillary action [22]. As can be observed, composites containing 50 % of the BF as the filler have lower water absorption and thickness swelling compared with composites with 50 % of WF. Such difference is

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The average tensile elasticity modulus and strength of WPCs containing different amounts of BF and nano-SiO2 are given in Figs. 3 and 4, respectively. As can be seen, by increasing the BF content in the composite, the tensile strength and elasticity modulus dropped to 40 %. But by increasing the BF content up to 50 % (pure bark in composite), a slight increase was observed in tensile modulus of the composites. As shown in Table 3, it can be noted that the independent and interactive impact of the WF/BF and nano-SiO2 combination content on the elasticity modulus of WPCs was significant at 0.01 confidence level. The composites elasticity modulus depend on many factors such as the amount of fiber used, the fiber orientation, the fiber and matrix bonding in interfacial region and its density [26].

J Polym Environ

Fig. 3 Tensile modulus of the WPCs as a function of nano-SiO2 and BF

Fig. 4 Tensile strength of the WPCs as a function of nano-SiO2 and BF

When considering the strength properties, homogeneity of the overall composite needs to be taken into consideration [27]. Tensile strengths of composites increase with the addition of bark portion, which causes an increase in the tensile modulus of all the composites. Also, Fig. 3 shows that in samples containing 50 % of WF, the addition of nano-SiO2 increases the composites tensile modulus which is not statistically significant. As shown in Fig. 3, the highest tensile modulus among the samples containing nano-SiO2 is related to the composites containing 50 % of WF and 4 wt% of nano-SiO2 with a tensile strength of 1196.41 MPa while the minimum tensile modulus is related to the samples without nano-SiO2 and 40 % BF with a tensile strength of 783.34 MPa. The increase in elasticity modulus as nano-SiO2 increases has also been reported by other researchers [13, 28–30]. The elasticity modulus of nanocomposites containing 50 % of BF at 1, 2, and 4 wt% of nano-SiO2, were 14, 24.7 and 42.4 higher than the samples without nano-SiO2, respectively. Composites made up of 50 % BF containing 1, 2, and 4 wt% of nanoSiO2 have elasticity modulus of 16.95, 27 and 41.8 higher than their corresponding samples without nano-SiO2.

Besides, the nano-SiO2 and BF impacts on tensile strength are shown in Fig. 4. Generally, the mechanical performance of polymer nanocomposites is associated with the interface strength between the nano-fillers and the polymer matrix as well as the dispersion of nano-fillers [31]. The improved interface and more uniform nano-SiO2 dispersion consequently lead to enhanced mechanical strength. As can be seen in Fig. 4, in samples containing 50 % WF, by increasing the nano-SiO2 content up to 4 wt% the tensile strength increased by 10 %. So that the tensile strength of 24.47 MPa in control samples was changed to 26.94 MPa in nanocomposites containing 4 wt% nano-SiO2 which is not statistically significant (Table 3), but partly showing an appropriate dispersion of nano-SiO2 in the matrix. The results also revealed that by increasing bark portion in the composite, the tensile strength witnessed a decrease. It was reported by Karakus¸ et al. [32] that when the surface interaction is not strong enough in the soft fillers such as WF, the tensile strength would be reduced. The reduction in tensile strength as a result of increasing the amount of bark in the combination was also shown in researches by C¸etin et al. [33], Safdari et al. [34], Harper and Eberhardt [8], Boufif et al. [9]. But generally, the tensile strength of the sample containing WF is higher than the samples containing BF. This could be ascribed to the differences in the chemical compositions of these two fillers [34]. Generally, the results showed that the maximum elasticity modulus and tensile strength were observed in nanocomposites containing 4 wt% of nano-SiO2 and 50 % WF with 1196.41 and 26.94 MPa, respectively. Impact Strength As shown in Table 3, it can be noted that the independent impact of the WF/BF and nano-SiO2 combination content on the un-notched impact strength of WPCs was significant at 0.01 confidence level. The un-notched impact strength of nanocomposites is shown in Fig. 5. The impact strength of

Fig. 5 Impact strength of the WPCs as a function of nano-SiO2 and BF

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composites varies significantly with fiber types and nanoSiO2 particles. According to Fig. 5, for samples without nano-SiO2, composites containing 50 % of WF have higher impact strength (26.27 J/M) compared to composites containing BF (14.37 J/M) which was also observed in the study of C¸etin et al. [33]. The un-notched impact strength shows the material strength against fracture and cracks which always occur at the weakest point of composites i.e. in the interfacial area between the lignocellulosic material and the polymer. Since in all combination, the coupling agent is considered constant, the issue is therefore related to the filler type. With the addition of up to 2 wt% nanoSiO2, the composite impact strength initially increased and then plunged into a reduction trend as the nano-SiO2 increased to 4 wt%. Thus, the addition of nano-SiO2 led to different results in impact strength of composites, both negative and positive effect depend on the addition of 2 and 4 wt% of nano-SiO2, respectively. Possibly, the nano-SiO2 aggregation occurs by increasing the content to 4 wt%. The particle aggregation can create stress concentration points that are prone to fracture and crack development. The results of this research conforms with the results of the studies carried out by Farsi and Mashi Sani [35] and Hosseini et al. [36]. The nano-SiO2 content effect on un-notched impact strength of nanocomposites containing 50 % WF and 1–4 wt% of nano-SiO2 showed that the sample with 2 wt% of nano-SiO2 had the highest impact strength (44.5 J/M) and samples containing 50 % of the BF and no nano-SiO2 had the lowest impact strength (14.37 J/M).

Conclusion The study focused on evaluating the effect of nano-SiO2 and BF amount on the physical and mechanical properties of WPCs. Based on the results of the study, the following conclusions were obtained: 1.

2.

3.

The more the BF content in the composites, the lower the water absorption and thickness swelling. But with the addition of nano-SiO2 up to 4 wt%, there was an increase in water absorption and thickness swelling. The addition of nano-SiO2 up to 4 wt% improved the elasticity modulus and tensile strength of nanocomposites and as the bark portion in the combination increased, the elasticity modulus and tensile strength were reduced. The un-notched impact strength of composites increased as the nano-SiO2 content increased up to 2 wt% but it fell into a reducing trend as the nano-SiO2 content increased to 4 wt%. In general, the BF had a negative effect on impact strength of nanocomposites and reduced un-notched impact strength.

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4.

The overall results show the advantages and reinforcing effect of nano-SiO2 in WPCs. In addition, the application of BF reduces water absorption and thickness swelling of composites. Accordingly, the highest elasticity modulus, tensile strength and impact resistance belong to the samples containing 50 % of WF and 4 wt% of nano-SiO2. The highest physical properties were observed in the samples containing 50 % of BF with no nano-SiO2.

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