DUVIOL T Tadondzo，XUE Hao，CHENG Wanli，HAN Guangping?，WANG Ge，CHENG Haitao

（1.Key Laboratory of Bio?based Material Science and Technology（Northeast Forestry University），Ministry of Education，Harbin 150040，China；2.International Bamboo and Rattan Center，Beijing 100102，China）

Abstract：Textile reinforced polymers（TRP）are rapid developed due to their inestimable performances such as light weight and high mechanical properties.TRP has been used as an important raw material in automotive vehicles，build?ings，and electronic devices.To be engineering products，it is desirable to have high?class mechanical properties.Thus，glass fibers and carbon fibers are used in the conventional TRP?reinforced composites，however，their high costs and nonbiodegradability limited the development and application of the composites.The objective of this work was to fabricate vinyl ester composites using naturally renewable and biodegradable jute woven fabrics as reinforcing material via mold pressing to increase the composite mechanical performances.The effects of yarn orientation on the density，void content，and mechanical properties of the composites were investigated.In order to study the effects of warp and weft yarn angle on the mechanical behaviors of the composites，one of the bilayers of the woven jute fabrics was arranged at five different angles，i.e.90°，60°，45°，30° and 0°，respectively.The mold pressing condition of the composites was set with a pressure of 7 MPa at 80 ℃for 150 min.In order to examine the effect of pressing tem?perature on the composite，another composite of 30° was prepared at lower temperature condition with a pressure of 7 MPa at 40 ℃for 150 min.The results showed that，compared with the control，the jute woven fabrics can enhance the mechanical properties of the composites.The flexural strength increased by about 24.77% and the impact strength also increased by up to 39.83% compared with the control.This was obtained due to the space created by the yarn width.The gap between the two warp yarns and the weft yarns of the fabrics was also contributed to the improved properties.The orientation of the fabric（warp and weft yarns）demonstrated its capacity to influence the impact strength of the composites significantly.It was found that the density and the void content of the composite had signifi?cant impact on the mechanical properties of the composite.The voids created by the warp and weft yarns could be a great indicator of the density of the composite and an important place for the resin to be stacked.Within the research scope，the condition of warp yarn angle at 60° and pressing temperature at 40 ℃was the most suitable condition to fabricate woven jute fabric reinforced vinyl ester composites.

Keywords：jute fiber；woven fabric；vinyl ester resin；mold pressing；yarn angle；mechanical property

1 Introduction

Natural fibers used as reinforcement in polymer composites have created much interest for producing low cost engineering materials［1］.Due to their biode?gradable，sustainable，and recyclable nature，using natural fibers as reinforcement in composite materials has been developed as part of a high tech revolution in recent years［2］.People have utilized composite ma?terials for thousands of years，which are more and more used for primary structures in aerospace，marine，automotive，and recreational industries［3-4］.One of such examples is to enhance surface properties and interfacial adhesion of a laminated composite ma?terial using jute fiber mixed epoxy resin by alkali treatment.The interfacial shear strength of jute/epoxy improved significantly by up to 40% and the adhesion strength also enhanced［5］.

Due to their excellent properties，thermoset resins such as epoxy，polyester，vinyl ester（VER）are increasingly used as a matrix material in fiber?re?inforced composites［6-7］.In commercial and biomedical applications，VER resin is one of the most important thermosetting polymers used for achieving high performance composites［8］.The mechanical properties of VER resin reinforced with jute fibers were investigated［9］.The flexural strength was improved from 199.1 MPa to 238.9 MPa（about 20%）and the laminar shear strength was increased from 0.238 MPa to 0.283 MPa（approximately 19%）.VER resin is less expensive and easier to process than epoxy polymers and has superior properties compared with other thermosetting polymers，such as unsaturated polyester resins［10］.By mixing with natural fibers，the overall properties of VER composites can be successfully enhanced.The natural fiber reinforcement has gained an attraction as a re?placement to synthetic fiber in various applications［11］.Numerous research projects have been carried out on natural fiber composites［12-14］.Natural fibers such as jute，sisal，flax，hemp，coir and bamboo have been investigated as reinforcing ma?terials to enhance the properties of polymer compos?ites［15-16］.

Textile composites encompass a large number of textile structures such as braids，weaves，knitting nonwoven and woven fabrics［17-19］.Its attraction comes from the interlacing of the yarn，which enhances the structural stability and performance of the composite material.Unidirectional laminas request more balance compared with the woven fabric compos?ite.The weaving of the fiber gives an interlacing of a set of yarns（warp and weft）perpendicular to each other，which can achieve the strength that is higher than the fiber?matrix binding strength［20］.Sapuan et al.［21］manufactured the household telephone stands using woven banana fabric reinforced epoxy，in which，the tensile stress showed greater in X?direction（warp）than Y?direction（weft）.Rajesh et al.［22］found that plain type weaving patterns improved the mechanical properties of banana?epoxy composites when compared with the other patterns such as twill and basket.A number of researches on woven jute fiber composites have been carried out so far，but they did not explore the effect of different angles of the layout of the woven jute fabric layers on the per?formance of the VER composites.Charlebois et al.［23］investigated the permeability of 2D braids at different angles，and the results showed that the braid angles affected the fiber volume in the composite and thus they found that the braiding angle was from ±45° and± 50°，the permeability decreased as the volume fraction of the fiber was increased.In most cases，the jute fiber was used either in multilayered armor to re?place the aramid fabric，and the jute non?woven pro?vided a weight reduction of 5.4% and a cost reduction of 474%［24］.In order to improve the performance of the fibers，researchers have been worked out to find new solutions.Stocchi et al.［25］worked on an alkali treatment superimposed to biaxial tensile stress applied to woven jute fabric/vinyl ester laminates.Results revealed that the properties of the composites with fabrics treated with alkali were significantly im?proved compared with the control.This was due to the lowest tensile properties of the material in the loading direction（the warp or weft direction of the jute fabric）compared with the alkali?tension treated com?posite.The impact properties of oil?palm?woven jute fiber based epoxy was investigated［26］.Results showed that the hybridization of the fibers enhanced the impact strength of the composite.

Therefore，our work focused on the preparation of the VER composites reinforced with woven jute fab?rics using mold pressing for the purpose of increasing the mechanical properties of the composite material.The objective of this study was to investigate the effect of yarn angle of woven jute fabric on the mechanical properties of the composites.The scanning electron microscope was used to examine the morphological structure and the interfacial bonding between fibers and resin matrix.

2 Materials and methods

2.1 Materials

The R?804 vinyl ester resin（45% styrene with a viscosity of 2.5 Pa·s/25 ℃）and the hardener of tert?butyl peroxybenzoate（TBPO）were provided by Showa Highpolymer Co.Ltd.，Shanghai，China.The demolding agent was provided by Aorui Composite Materials Co.Ltd.，Shenyang，China.Jute woven fab?ric was provided by Meixi Xinyao Hemp Textile Fac?tory，Anji，Zhejiang，China.The fabric type used in this work was the diamond braid（（1/1），single o?verlap），the Z?twist was used for the weaving yarns.The areal density was 278 g/m2with the single twist yarn of 20.84 turns per 10 cm，and the elongation at break was 1.6%.The tenacity of yarns was 1 850.8 MPa with the linear density（tex）of 357.1，and the density of the fabric was 50 tows/10 cm，whether it was warp or weft.

2.2 Design and preparation of the composites

In order to investigate the effect of the yarn angle on the mechanical properties of the composites，five orientations selected were inspired by the trigonometric circle as shown in Fig.1.Also，based on the previous work of lamina and laminate，the mi?cromechanics of the composite were inspired by the research described in the work of Cheung et al.［27］.The bottom layer of the five woven fabrics was fixed，with the warp yarn oriented at 90° and the weft yarn oriented at 0°.The top layer of fabric was superim?posed on the bottom layer and the warp yarn was rota?ted at 90°，60°，45°，30° and 0°，respectively.

Fig.1 Schematic design of the five woven fabrics with different orientation angles

The preparation process of the composites by mold pressing is shown in Fig.2.In order to obtain good interfacial bonding between the woven jute fabric and vinyl ester resin，1.5% TBPO hardener of the a?mount of vinyl ester weight was mixed together and applied by impregnating the woven fabric into the mixed resin matrix.The trapped air in the laminates was removed by compressing the fabric using a steel roller and making only an alternating movement to a?void crimping and unevenness in the fabric.In prepa?ration for the hot press，the mold was preheated at 40℃and the impregnated woven fabric was attached to a metallic frame as shown in Fig.2c.Following this，the bilayer structural jute woven fabric was placed in the mold.In order to avoid any bubbles，the resin was poured into the mold gently and continuously so that the rebound effect was low before the mold was cov?ered，as shown in Fig.2d［28］.The bilayer woven jute fabric?reinforced vinyl ester composites were pressed at 60 ℃for 10 min and 80 ℃for 150 min，and the average rate of the mold cooling was 5 ℃/min.The sample prepared using the same procedure as above without fabric（clear cast）was used as unreinforced controls.In order to examine the effect of temperature on the composite，another sample of 30° was prepared at low temperature（LT）condition with a pressure of 7 MPa at 40 ℃for 150 min.The dimension of the prepared composite material was 300 mm×300 mm×3 mm，and the actual thickness was determined by the gauge of the mold.

Fig.2 Preparation of the composites by mold pressing

2.3 Calculation of density and void contents

According to the ASTM?D792?08“Standard test methods for density and specific gravity（relative den?sity）of plastics by displacement”，the composite den?sity was measured.The rectangle samples were prepared with a dimension of 10 mm×10 mm and the ratio of each thickness was taken into consideration.At room temperature，distilled water was used as the im?mersion fluid in this study and the mass of the compos?ite specimens were measured using a digital balance with a 10-3gram resolution.The theoretical density(ρ1) and experimental density(ρ2) of the composite laminate was obtained based on equations 1 and 2：

where，mis the mass of the 10 mm×10 mm sample in gram，vis the volume of the 10 mm×10 mm sample in cm3，Mairrepresents the mass of the samples meas?ured in the air andMwateris the mass of the composite measured in distilled water.Five specimens were measured，and the average value was reported.Void percentage(V) in the composite laminates was calcu?lated using the method according to the ASTM D2734?09“Standard test methods for void content of rein?forced plastics”and based on equation 3：

As per ASTM D638?14“Standard test method for tensile properties of plastics”，the tensile test was carried out using the electromechanical universal testing machine（MTS SYSTEMS Co.Ltd.，CMT 5504，China）with a testing speed of 2 mm/min.The specimens for the tensile test were individually cut into rectangular shape of 165 mm×19 mm，where the warp fiber axis of the bottom layer（0°-90°）fabric was maintained along the loading direction.The repor?ted tensile strength and modulus values were the aver?ages of the five samples in this study.

2.4 Flexural test

The flexural strength and modulus of the composite specimens were examined through the three?point bending test according to the ASTM D790?03“Standard test methods for flexural properties of unreinforced and reinforced plastics and electrical in?sulating materials”.The composite specimens were sized into rectangular shape of 127 mm×12.7 mm.Tests were conducted using an electromechanical uni?versal testing machine（MTS Systems Co.Ltd.，CMT 5504，China）；the crosshead speed applied was 1.0 mm/min.Each sample was loaded until the core broke and the average value of five samples was re?ported.

2.5 Impact resistance

Following the ASTM D256?04“Standard test methods for determining the izod pendulum impact re?sistance of plastics”，the impact strength was tested using an XJC?15W izod machine（Chengde Precision Testing Machine Co.Ltd.，Hebei，China）and the samples were prepared without a radius.The speed of the pendulum was 3.5 m/s and the distance between the striking edge radius and the moveable vise jaw was 22 mm.The specimens were cut into a rectangular shape with a dimension of 63.7 mm×12.7 mm.Five replicates were used in the test.

2.6 Morphological analysis

The morphology of the composites，showing the interfacial bonding between fiber and matrix，was ex?amined from fractured or delaminated surface of the specimens for tensile，flexural，impact using the scanning electron microscope（SEM）（Japan elec?tronics JMS?7500S）at 20 kV accelerating voltage.Before the examination，the samples were coated with gold by means of DC sputtering.

3 Results and discussion

3.1 Density and void content

In order to evaluate and clearly demonstrate the results of the manufactured composite material，the densities and void percentage of the different compos?ites are given in Table 1.It was observed that the the?oretical and the experimental density were approxi?mately the same，indicating that the nature of the re?inforced material had little impact on the density of the composite.It can be observed that the ［30°/0°-90°］presented the lower density and the highest void content compared with other samples.This can be explained by the fact that the yarn orientation angle reduced the covering factor area where the resin tacked.The significant percentage of void content was present in the composite materials，with the exception of the clear cast which showed lower void content.This was due to the fact that no reinforced material was add?ed，so no bubbles were produced by the fibers in the composite when pressure was applied to the mold.The composites prepared at low temperature also showed anincrease of the density.The results also showed that the void content in the composites increased as the densities of the composites were reduced.Pressure ap?plied to the mold during the consolidation of the com?posite and the challenges with flushing out the matrix through the mold vent was also a reason that may pro?duce the bubbles in the composite.

Table 1 Density and void content of the composites with different yarn angles

3.2 Tensile properties

The influence of the yarn angles of jute fabric on the tensile strength of the composite is shown in Fig.3.The composites of ［45°/0°-90°］and ［90°/0°-90°］showed decreased tensile strengths of 32.76 MPa and 32.07 MPa，respectively，compared with the clear cast sample which was 46.16 MPa.It was represented decreases of 29.02% and 30.52% com?pared with the composites without fabric reinforcement.This was due to the density of the vol?ume amount of the fibers presenting in the compos?ite［29］.The composites of ［0°/0°-90°］，［30°/0°-90°］and ［60°/0°-90°］LT had quite similar de?crease in tensile strengths of 21.94%，21.20% and 20.62%，respectively；indicating the tensile strength of ［60°/0°-90°］LT composite was slightly higher.The high covering factor between the yarns of warp and the weft direction of the woven fabric was also a critical factor in the decrease of the tensile strength and the elastic modulus of the composites.This was the reason that the composites with the jute woven re?inforcement fabric had lower tensile strength.The ten?sile modulus of the different composites with fabrics was quite similar to the variation of the tensile strength compared with the unreinforced sample which showed a remarkable modulus decrease.This was be?cause of the influence of the woven jute fabric orienta?tion.

Fig.3 Influence of different yarn angles on tensile properties of composites

3.3 Flexural properties

The effect of the reoriented fabric reinforcement on the flexural properties of the composite is shown in Fig.4.The composites of ［45°/0°-90°］and ［60°/0°-90°］LT showed an increase in flexural strength of 24.77% and 18.56%，respectively，compared with the unreinforced composites（clear cast）.The ［45°/0°-90°］jute fabric orientation gave great effect on the composite.This demonstrated that the linear density of the yarns in the composite was added by the fabric orientation［29］.That was why its void content was slightly higher than other composites.The ［60°/0°-90°］LT composite also showed good flexural strength.Here，the lower temperature used for its manufacturing was an advantage and main reason of the increase in the flexural strength.The shift or ori?entation of the fibers in the fabric influenced the me?chanical strength of the composite［30］.Duc et al.［31］confirmed that the alignment of the fibers along the axial load caused an additional elongation，and the bending strength and the stiffness of braided composite materials increased.This was related to the space created by the yarn width and the space created between the two warp and two weft yarns（called matrix only region of the fabric as shown in Fig.1）.

Fig.4 Influence of different yarn angles on flexural properties of composites

3.4 Impact properties

The variation in the impact strength of the com?posites with different yarn angles is given in Fig.5.The composite of ［90°/0°-90°］showed an increase in impact strength of 34.94 J/m，reflecting 39.83% of increase compared with the unreinforced samples（clear cast）.The significant influence of the warp and the weft yarn angles on the impact properties of the composites was clearly demonstrated.This was due to the same alignment of the weft and the warp yarn orientation of the lamina.The composite of［30°/0°-90°］showed lower impact strength.This was confirmed by its density and its void content pres?ented in Table 1.The shift of the jute woven fabric in?creased the impact strength of the composites signifi?cantly compared with the composite without reinforce?ment fibers.Apart from the ［30°/0° -90°］composite，all the composites with jute fiber rein?forcement showed the significant increase in the impact strength.It seemed that the composite of［60°/0°-90°］LT had higher impact strength than other conditions，indicating that the influence of tem?perature applied to the composite.It was found in this study that the addition of the reinforcing material can influence the mechanical properties of the composite and yarn angles can also add significant value in in?creasing the impact strength of the composite.This may be a good solution to problems related to the composite strength enhancement.

Fig.5 Influence of different yarn angles on impact properties of composites

3.5 Morphological analysis

The morphology analysis on the fractured surface of flexural，tensile and impact test specimens are vetted out and observed using SEM images in order to understand the collapse mechanism as shown in Fig.6.Fig.6a shows a void on the matrix which occurred during molding process，which was probably due to the pressure loading on the mold and the difficulties in flushing out the resin during the compression of the mold.However，it can be seen that the matrix and fi?ber had good adhesion.Fig.6b shows a matrix crack，which was due to the unsupported charge released during the fiber breakage that was carried out during the tensile test.Fig.6c shows the warp and the weft breakage，which occurred at the interlaced surface of jute fabric.Fig.6d shows favorable matrix interaction between the warp and the weft yarn joint.This was due to the good impregnation of the matrix into the yarn of the jute woven fabric before it was attached to the mold.

Fig.6 The fractured surface of different tensile specimens used in the experiment

The fractured surface of flexural test specimens is shown in Fig.7.The delamination between the matrix and the jute woven fabric layers can be observed in Fig.7a and b.This was due to the uniform stress dis?tribution during the flexural loading charge and the unsupported load carrying capacity of the jute woven yarns.Fig.7c and d show better adhesion in matrix and good impregnation of the fiber respectively.This was due to the viscosity of the matrix and proper han?dling during the manufacturing process of the compos?ites.

Fig.7 The fractured surface of different flexural specimens used in the experiment

The fractured surface of impact specimens used in the experiment is illustrated in Fig.8.In Fig.8a，the half collapse of the fiber bundle could be attributed to higher cohesive forces between the two reoriented woven fabrics and the matrix as well as the minimum unit cell and matrix region between warp and weft fiber yarn direction.As the image showed in Fig.8b，the poor adhesion between the fiber and ma?trix occurred due to the crimping of the fabric and the displacement of the fabric during the closing of the mold.Fig.8c demonstrates that the yarn breakage was due to the non?uniform stress distribution along the warp and weft directions，as well as the angle of twist influences the impact properties of composites.The voids showed in Fig.8d were created due to the same reasons of Fig.6a and during the incorporation of fibers and matrix into the mold.Another reason may be the temperature and the time applied to the com?posite，making the matrix slight brittle，which may significantly affect the mechanical properties of the composites.The matrix crack was caused by the high resistance of bilayer fabric to breakage.

Fig.8 The fractured surface of different impact specimens used in the experiment

4 Conclusions

In this work，the effect of warp and weft yarn an?gles on the mechanical properties of jute woven fabric reinforced vinyl ester composites was studied.The re?sults revealed that the reinforcement with jute woven fabric can enhance the mechanical properties of the composites.The use of changing angle of the woven jute fabric in vinyl ester matrix improved the flexural strength by up to 24.77% and the impact strength by up to 39.83% compared with the composites without reinforcement.Controlling yarn angle of fabric was a feasible approach for improving the mechanical prop?erties of a composite except for the tensile properties in which a slight negative effect was obtained.The void created by the warp and the weft yarn of the fabric was a great indicator of the density of the com?posite and the important places for the resin to stack.It was found that the density and the void content in the composite had significant influence on the me?chanical properties of the composite.Within research scope，the ［60°/0°-90°］LT was the most suitable condition to fabricate woven jute fabric reinforced vinyl ester composites.Another factor affecting me?chanical properties of the composite was the chemical treatment of jute fibers.The future work would focus on exploring the effect of fiber modification on the me?chanical properties of the composite.