The properties of fl ax fi ber reinforced wood fl our/high density polyethylene composites

Jingfa Zhang?Haigang Wang?Rongxian Ou?Qingwen Wang,

Introduction

Due to their increasing application,wood-plastic composites(WPC)have received signi fi cant attention from both applied science and industry.WPC is widely utilized in decking,trays,fencing,windows and playground equipment due to several advantages over wood or plastic alone(Jiang and Kamdem 2004;Markarian 2002).However,applications of WPC for structural construction are still restricted due to its weak mechanical performance resulting from the inherent incompatibility between hydrophilic wood and hydrophobic polyole fi n matrix(Wu et al.2014).This in turn results in low impact toughness and creep strain(Lai et al.2003).

In order to enhance the mechanical and physical performance of WPC,different physical processing methods(Rong et al.2001;Ou et al.2014;Ferreira et al.2014),as well as chemical modi fi cations(Wei et al.2013;Bledzki et al.2015;Dong et al.2014)have been utilized to treat wood fl our(Facca et al.2007;Aggarwal et al.2013).However,all current methods have shortcomings,such as complicated operation,high cost,or pollution,constraining their commercial development.Adding reinforcement was always an effective method to improve the mechanical properties of the resulting composites.Hybridization of wood fi ber with high-strength fi bers is suspected to significantly improve the mechanical properties of resulting composites.Reinforcing fi bers,such as carbon fi ber,glass fi ber,basalt fi ber,and synthetic fi ber are widely used(Rahmat and Hubert 2011;Zhao et al.2015;Yuan et al.2013).Adding glass fi ber has been reported to enhance fl exure properties and impact strength of WF/PE composites(Thwe and Liao 2002;Zolfaghari et al.2013;Jiang et al.2003).Incorporation of basalt fi ber produced similar results(Chen et al.2013).The addition of a small amount(2–3%)of Kevlar fi ber has been shown to simultaneously improve the strength and toughness of WF/PE composites(Ou et al.2010).In addition,carbon fi bers have been used in WPC,resulting in signi fi cant improvements of mechanical properties(Zhou et al.2014).However,all of these fi bers are derived from non-renewable resources and all of them are mixed with wood fl our or plastic prior to pelleting.

With increasing environmental consciousness,the use of natural cellulosic- fi ber (NC-Fiber) as reinforcement increased with a special emphasis on the use of hemp.Mechanical properties of different NC-Fibers(hemp,jute,kenaf and paper fi ber)indicate that they have the potential to substitute glass fi ber as a reinforcement in a speci fi c state(Wambua et al.2003;Yang et al.2015).Hemp fi ber has been widely used in a reinforced plastics matrix(Bledzki et al.2015;Corrales et al.2007).Recently,hemp has been added into WPC,resulting in improved tensile properties of wood fl our/kenaf fi ber/polypropylene hybrid composites compared to composites without kenaf fi bers(Mirbagheri et al.2007).

Flax fi ber(FF)is a natural plant fi ber with high strength due to high cellulose content(Zhang and Yu 2003).The global production of FF was 311,000 tons in 2014(Chyxx.com 2016).Most of the FF is usually used in the textile industry.Although the cost of FF is higher than wood- fl our,it is less expensive than the cost of synthetic materials and is increasingly used in WPC.

This study was designed to determine the reinforcing effects of fl ax fi bers( fl ax is widely planted in northeastern China)and the properties of the resulting WPC,while the biomass material content remained the same.Subsequent to mixture extrusion,the fi bers were mixed with the WF/PE particles to lessen the damage to the fi bers.Mechanical properties,dynamic mechanical properties,creep resistance,and rheological behavior of the composites were analyzed.Interfacial adhesion and fl ax features were characterized via scanning electron microscopy.

Materials and methods

Materials

Flax fi bers with an average diameter of 20 μm were commercially obtained and cut into lengths of 5–10 mm.The fl ax is grown in Heilongjiang province,China.HDPE pellets(5000 S)with a density of 0.954 g cm-3and a melt fl ow index of 0.7 g 10 min-1were purchased from Daqing Petrochemical Co.,Daqing,China.Wood fl our(WF)measuring 40–80 mesh wassupplied by the Harbin Yongxu Company.Poplar is used for the wood fl our and is grown in Heilongjiang.Maleic anhydride grafted polyethylene(MAPE)was supplied by the Shanghai Sunny New Technology Development Co.,Ltd.,with a MA grafting ratio of 0.9 wt%and a melt fl ow index of 2 g 10 min-1(190°C).

Sample preparation

In order to avoid fi ber damaging during pelletizing,FF was added into the WF/PE composite subsequent to the twinscrew granulation process.This is different from most previous preparing processes of fi ber reinforced WPC and was as follows:(1)WF and FF were dried at 103°C for 24 h and then stored in a sealed container for later use;(2)WF,HDPE and MAPE were mixed using a speci fi c ratio(Table 1)in a high-speed mixer for a total of 5 min;(3)subsequently transferred to a twin-screw extruder to produce WF/PE pellet particles.The temperature of the extruder ranged from 145 to 170°C,increasing by increments of 5°C,with a rotation speed of 50 rpm;(4)particles and FF were then blended in the high-speed mixer for 20 min and then extruded,resulting in a FF/WF/PE composite sheetwith a cross sectionaldimension of 40 mm×4 mm.The processing temperature for extrusion was 160 °C during the melting period and 170 °C during the die zone.Rotation speed of the single-screw was 20 rpm.

Mechanical tests

Specimens measuring 80 mm×13 mm×4 mm were cut from the FF/WF/PE sheet and tested under three-point bending using a universal mechanical machine with a 50 KN load cell(CMT5504,The MTS(China)Co.,Ltd.),according to ASTM D790-2004.A cross head speed of 2.0 mm min-1was used and fi ve replicates used for each formulation.

Dumbbell-shaped tensile specimens measuring 165 mm×13 mm×4 mm were tested in accordance with ASTM D638-2004 using the same universal mechanical machine.A cross head speed of 5.0 mm min-1and a span length of 50 mm were used.Five parallel samples were tested.

Unnotched charpy impact testing was conducted on standard samples with nominal dimensions of 80 mm×10 mm×4 mm using an impact instrument(CJ5,Chengde Testing Machine Co.,Ltd.China)inaccordance with ISO 179-2000.There were ten parallel samples in each group.

Table 1 Formulations of the composites for extruding

Dynamic mechanical analysis

Dynamic mechanical properties of the composites were analyzed via a dynamic mechanical analyzer(DMA Q800,TA Instruments,New Castle,USA).Tests were performed using the single cantilever strain controlled mode with oscillating amplitude of 50 μm and a frequency of 1 Hz.The temperature ranged from-40 to 130°C at increasing intervals of 3°C min-1.Three specimens with dimensions of 35 mm×12 mm×3 mm were tested.

Creep measurement

The 24 h creep test of the composite sample 100 mm×17 mm×4 mm was performed using a RD-100 electronic creep testing instrument(Changchun Ke Xin Experimental Instrument Co.,Ltd,China)at 23°C.The span was 64 mm and the loading force 30 N(approximately 15% of the maximum load).

Torque rheology

Rheological behavior was evaluated using the Haake torque rheometer(Polylab OS,Thermo Scienti fi c,Germany)equipped with two counter rotating rotors.WF/PE particles and FF were quickly forced into the mixing chamber when the rotors began to rotate.The test was run at 175°C and 50 rpm for a total of 10 min and with a constant degree of fi lling of 70%.Three parallel samples were tested.

Scanning electron microscopy(SEM)

Cryo-fractured surfaces were produced by breaking of the FF/WF/PE composites under liquid nitrogen conditions and subsequent sputter coating with gold.The fractured surfaces were analyzed with a scanning electron microscope(FEI QUNGTA200,USA)at an accelerating voltage of 10 kV.

Fig.1 Effects of FF content on fl exure and tensile properties of WF/FF/PE composites:a fl exural strength and modulus,b tensile strength and modulus.The error shows the standard deviation from the average value

Results and discussion

Mechanical properties of composites

Fig.2 The un-notched impact strength of FF/WF/PE composites.Ten specimens were tested for each FF content.The error bars show the standard deviation from the average value

Compared to WF/PE,the fl exural strength and modulus of WF/FF/PE-9 increased by 14.6 and 51.4%,respectively(Fig.1a).However,the results started to decrease for values above 9%FF content.Numerous small cracks would generate when the composite was subjected to external loads.With increasing force,these cracks extended until the material was damaged.However,FF crosses a crack and prevents further expansion.At material failure,FF bears the majority of the force.With further loading,increasing FF would be pulled out or off,consuming a large amount of energy.Therefore,adding FF into WF/PE composites increases the fl exural strength of WF/FF/PE.The modulus of FF was higher than that of WF(Cao et al.2014),resulting in the improvent of fl exual modulus.In addition,synergistic enhancement of physical interaction among WF,FF and PE was detected,limiting their respective deformation.The interaction hindered polyethylene molecular chain slippage.Consequently,the fl exual strength and modulus improved.However,FF may bunch up with increasing content.This was the reason for decreasing fl exural strength and modulus of FF/WF/PE-12.Increasing the content of fl ax fi ber resulted in an increase in both tensile strength and modulus of the resulting composites of 4.3 and 13.6%,respectively(Fig.1b).Compared to fl exural strength and modulus,the tensile strength showed no obvious changes.This is due to FF being too short to generate suf fi cient interfacial shear strength to bear the force.Most of the FF was arranged along the extrusion direction and therefore,the tensile force would easily extend along the interface.The improvement of tensile performance was suboptimal.

Fig.3 The Storage modulus(G′)and loss modulus(G′)of FF/WF/HDPE composites.The curve was an average of three parallel samples.WF wood fl our;FF fl ax fi ber;PE polyethylene

The unnotched impact strengths of the composites increased considerably when fl ax fi ber was added.The results of the impact strength study are depicted in Fig.2.With fl ax fi ber loading of 12 wt%,an increase of 26.5%in unnotched impact strength was obtained.

The unnotched impact strength of composites is affected by crack initiation and propagation energy.When fl ax fi ber was loaded,the impact strength improved due to the loading being transferred to the FF by the shear forces between FF and the matrix.Therefore,FF bore the impact force until the fi bers were either pulled off or out.At the same time,wood fl our and fl ax fi bers twined with each other.Based on the crazing cut and fi ber crack resistance theory(Jia et al.2007)WPC produces numerous small cracks during early damage due to external forces.FF stretched across the cracks,thus arresting developing cracks.Consequently,adding fl ax fi bersigni fi cantly improved the impact properties of WPC.

Dynamic mechanical analysis

Fig.4 The loss tangent(tanδ)of FF/WF/HDPE composites.WF wood fl our;FF fl ax fi ber;PE polyethylene

The storage modulus of composites increased subsequent to adding FF(Fig.3),echoing the fl exure modulus.The storage modulus of FF/WF/PE composites decreased due to an increase in temperature and converged to a narrow range at high temperatures.The reduction of storage modulus(G′)with the temperature rise was due to matrix softening,and the G′of FF/WF/PE composites initiation of the relaxation process which is the natural character of polymers(Pothan et al.2003).The G′fi rst increased,but then decreased with increasing FF content.There are two reasons for this:on the one hand,the stiffness of FF is higher than that of WF(Cao et al.2014).Therefore,adding FF improves the modulus of composites.On the other hand,FF and WF interact and form a grid-like structure,embedded in the viscoelastic matrix(Huang and Terentjev 2012).However,as FF content increased to 12%,FF reunited,leading to a drop of the G′of FF/WF/PE.The loss modulus of the material is associated with either the viscous response or the dampening effect of the material.Figure 3 shows that the change in loss modulus(G′′)was similar to that of the storage modulus and peaked in the transition region at approximately 60°C.This relaxation peak is known as α-relaxation of HDPE,and is related to a complex multi-relaxation process associated with the molecular motion of the HDPE crystalline region.The temperature of α-relaxation increased with FF loading.However,it dropped back to initial levels when the content of FF was increased to 12 wt%.FF limited the movement of the HDPE molecules due to their three dimensional network structure(Fig.7e).However,the FF bunched up,disturbing the continuity of the matrix at relatively high contents of FF(12 wt%).

Fig.5 Creep resistance behavior of WF/FF/PE composites.WF wood fl our;FF fl ax fi ber;PE polyethylene

Fig.6 Effects of FF content on the mixing torque of the composites.The curve is an average of two parallel samples.WF wood fl our;FF fl ax fi ber;PE polyethylene

The tanδ,which shows differences in the viscoelastic response of the composite,is a ratio between the loss modulus and the storage modulus.In a low temperature range,the content of FF had an obvious effect on the magnitude of tanδ(Fig.4).With increasing FF content,the FF/WF/PE showed a decreased value of tanδ as compared to WF/PE composites.This indicated that the FF/WF/PE composites had more elastic character than typical curves.

Creep measurement

Adding FF improved creep resistance(Fig.5)and with increasing content,the value gradually decreased.This indicates that a small amount of FF could improve creep resistance and a content of 9%FF was found to be optimal,resulting in effective creep reduction.Further increases of FF content may cause poor dispersion of FF within the matrix,a negative factor for properties such as creep value and tensile strength.Wang et al.(2015)reported that with increasing size of wood fi bers,the creep strain was reduced.This phenomenon was attributed to the large fi ber aspect ratio which can lead to improved creep resistance.Compared to WF,there were more friction forces between FF and plastic due to the larger surface area of a single FF fi ber compared to a WF particle.In addition,the interaction between FF,WF,and HDPE was enhanced with increasing FF content,restraining matrix deformation.

Fig.7 The SEM micrographs of the fractured surfaces of WPCs(a),FF/FF/PE-3(b and c),FF/FF/PE-6(d),FF/FF/PE-9(e),and FF/FF/PE-12(f)

Rheological properties during processing

The balance torque and temperature of the composite melts increased with increasing FF content(Fig.6).This may be attributed to the interaction among FF,WF,and HDPE inhibiting the thermal mobility of the HDPE chains.Moreover,adding FF increased the internal friction of the composites and improved shear heat.This led to a rise in melt temperature.

Micrographic analysis of fracture surface

Most wood particles in the HDPE matrix were well-bonded as a result of coupling.However,the interface was noticeable(Fig.7a).Furthermore,the interface between FF and HDPE was similar to that of the WF/PE composites(Fig.7b).Figure 7c indicates that fi ber pullout was the dominant mode of failure for the WF/FF/PE composites.In a general way,the failure modes of fi ber-reinforced polymers included interface de-bonding, fi ber fracture, fi brillation,and buckling under different test conditions(Yue and Padmanabhab 1999).

With increasing FF content,the complicated con fi guration ofFF becomesincreasingly bene fi cialto the mechanical interlocking among FF,wood- fl our,and the resin matrix forming a three-dimensional network structure(Fig.7e).This can lead to a more ef fi cient stress transfer between the FF and matrix,thereby producing a composite with superior strength and toughness as compared to that of WPCs.Figure 7f shows FF agglomeration present in the composite when the content was as high as 12%.This furtherdestroyed the continuity ofthe matrix and decreased the fl exural and tensile strength(Fig.1).

Conclusions

The incorporation of FF as a reinforcement material plays a vital role in WF/PE composites,improving mechanical properties and dynamic modulus without changing the content of biomass fi bers.This has mainly been attributed to the high strength of the fl ax fi ber and its excellent compatibility with both wood- fl our and HDPE matrix.Adding fl ax fi bers can improve toughness and creep resistance of WPC.However,the processing performance of WF/FF/PE declined.

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