Effects of characteristic inhomogeneity of bamboo culm nodes on mechanical properties of bamboo f i ber reinforced composite

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SHORT COMMUNICATION

Effects of characteristic inhomogeneity of bamboo culm nodes on mechanical properties of bamboo f i ber reinforced composite

Jinqiu Qi1•Jiulong Xie1•Wenji Yu2•Simin Chen1

©Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2015

Dendrocalamus farinosusandPhyllostachys heterocyclabamboo logs were subjected to a novel treatment process for the preparation of bamboo f i ber mats (BFMs),and the obtained BFM were used to fabricate bamboo f i ber reinforced composite(BFRC).We studied the mechanical properties of the BFRCs manufactured from the mats with and without bamboo nodes.The presence of nodes in BFM greatly reduced tensile strength, compressive strength,modulus of elasticity,and modulus of rupture of the BFRCs,while the BFRCs fabricated from BFMs with nodes possessed higher horizontal shear strength.Therefore,the nodes in bamboo culms were an important factor in the uniform distribution of mechanical properties,and BFMs should be homogeneously arranged to reduce the impact of nodes on the mechanical strengths of BFRCs.

Bamboo f i ber reinforced composite⋅Culm node⋅Mechanical properties

Introduction

Bamboo is abundant in southwest China,and craftworks made of bamboo have been widely used in daily life since ancient times.Recently,bamboo has been used as a prominent non-wood resource in f l ooring,furniture manufacturing,and construction materials because of its rapid growth rate,abundance,renewable nature,and short maturity cycle(4–5 years).Previously,bamboo was mainly used to manufacture bamboo-based panel,bamboo glue-laminated timber,and in bamboo scrimber industries as a sustainable raw materials replacement for wood (Zhang et al.2013).However,the eff i ciency of use of bamboo in manufacturing these composites was low,about 30–50%,due to the limited number of processing technologies.Recently,a novel process to manufacture bamboo-based composites has been extensively promoted in China and it has improved bamboo composite panel yield to over 90%(Yu et al.2014).The fabricated composites from the novel process showed excellent mechanical performance and have been proposed for manufacturing blades of wind turbines.

For structural design using bio-based composites as the main materials,high uniformity of the raw material is required.Although the new type of bamboo-based composite has acceptable mechanical properties,uneven stress distribution severely limits its application as a commonly used structural material(Chen et al.2014).Previous studies focused on the fabrication or quality evaluation of this new composite while ignoring assessment of the factors that affect uniformity of mechanical properties.Properties of bamboo internodes are signif i cantly different from those of bamboo nodes(Bahari and Ahmad 2009).Differences in bamboo properties by species,age,location,and portion can inf l uence processing procedures and affect the performanceof end products(Hisham et al.2006).Thus,in order to estimate the inf l uence of bamboo nodes on the mechanical properties of the new composite,we prepared bamboo f i ber mat(BFM)fromDendrocalamus farinosusandPhyllostachys heterocyclabamboos via a promoted mechanical treatment process.The object of this study was to describe the effect of bamboo culm characteristics on the variation in mechanical properties of bamboo f i ber reinforced composites(BFRCs).

Materials and methods

Preparation of bamboo f i ber mat(BFM)

Dendrocalamus farinosusandP.heterocyclabamboos(4-year-old)were obtained from Sichuan Province,China.A commercial phenol formaldehyde(PF)resin obtained from Taier Corporation(Beijing,China)was used as the matrix material for composite fabrication.The parameters of the PF resin were 44.6%solid content,viscosity of 41 mPa s, pH of 11.2,and solubility in water at a ratio(v/v)of 1:7–1:8.

Bamboo logs had initial moisture content of 123 and 84%forD.farinosusandP.heterocycla,respectively,the bamboo logs measured 2,000 mm in length and were f i rst split longitudinally into two semicircular tubes.After the inner nodes were removed,the semicircular tubes were pushed into a f l uffer.With brooming and rolling,the bamboo tubes were processed into a loosely laminated reticulate sheet.The laminated sheet was cross-linked in the width direction with a series of dotted and/or linear shaped cracks along the longitudinal/f i ber direction.The netlike bamboo sheet of uniform thickness and maintaining the original bamboo f i ber arrangement was f i nally cut into pieces of 300 mm length using an electrical saw.This formed the BFM.The BFM was classif i ed into two catalogues:without nodes or only internodes(BFM-I)and with nodes in the mat middle(BFM-N)(Fig.1).The BFMs were dried to moisture content of 10%.

Preparation of bamboo f i ber reinforced composite (BFRC)

The PF resin was diluted with water to a solids content of 15%.The BFMs were immersed into the PF resin for 3 min and then placed to dry the surface for 5 min;the amount of glue was controlled to about 11–12%of the oven dry weight of the BFM.BFMs were then air-dried to a moisture content of 8–9%.BFMs were weighed and those meeting the specif i ed density(1.0 g cm-3)and were assembled in a designed mold.A 500×500 mm hot press was used to press the BFRC at a platen temperature of 150°C.The pressure was maintained at 2.5 MPa for a holding time of 1.5 min mm-1.The fabricated BFRCs measured 450 mm(length)×160 mm(width)×15 mm (thickness).Ten replicates were prepared for mechanical properties test.The BFRCs made from BFM-Ns and BFMIs were designated as BFRC-N and BFRC-I,respectively.

Mechanical properties of BFRC

Mechanical properties including modulus of elasticity (MOE),modulus of rupture(MOR),horizontal shear strength(HS),tensile strength(TS),and compressive strength(CS)were determined according to standard methods GB/T17657-1999,GB/T20241-2006,ASTM D3500-1990,and ASTM D3501-2005,respectively.

Results and discussion

Mechanical properties of the BFRCs are shown in Table 1. TS,CS,MOR and MOE of BFRC-I fabricated from bothD.farinosusandP.heterocyclabamboos were higher than those for BFRC-N.TS of BFRC-I was signif i cantly greater than that of BFRC-N,i.e.,more than 3 and 4 times greater for BFRCs manufactured fromP.heterocyclaandD. farinosus,respectively.

Shao(2004)reported that TS and CS of the original bamboo were correlated with the strength of f i ber cells. Since the BFM manufactured in this study maintained its original f i ber arrangement,it could be concluded that the TS and CS of the BFRCs also corresponded to the strength of the original bamboo f i bers.Accordingly,the f i ber cells in the bamboo internodes arranged directly and continuously,while those in the bamboo nodes were arranged discontinuously,which reduced TS and CS of bamboo nodes.Moreover,f i bers in the nodes were much shorter than those in internodes(1.15 mm for nodes and 2.94 mm for internodes),resulting in much more or larger dotted and/or linear shaped cracks on the BFMs,usually in the sheaths and separated places on the nodes(Fig.2). Therefore,the variations in f i ber morphology and f i ber arrangement between internodes and nodes might be the main factors causing the signif i cant difference in the TS and CS of the BFRCs.Since the MOR and MOE of the bamboo raw material had a relationship with the vascular bundle distribution in bamboo,the higher MOR and MOE recorded for BFRC-I might have also been due to the higher vascular bundle density in internodes(3.53 and 2.44 bundle/mm2for internodes and nodes,respectively).

HS of BFRC-I was less than that of BFRC-N.HS usually ref l ects the bonding performance of bio-based composites.In our investigation,internodes and nodes were treated via the same mechanical process,however,asdiscussed above,BFMs made from nodes were much more loose because of the special characteristics nodes(morphology and anatomy),and that resulted in a much more uniform distribution of PF resin in the BFMs.The loose BFMs also contributed to a higher PF resin content.Thus, both the uniform distribution of PF resin and its highercontent created better bonding in the BFRC,and that contributed to the greater HS of BRFC-Ns.

Fig.1 Preparation of bamboo f i ber mat(BFM)and bamboo f i ber reinforced composite(BFRC)

Table 1 Mechanical properties of bamboo f i ber reinforced composites

Fig.2 Morphology of bamboo f i ber mat(outer surface)manufactured from a internodes and b nodes

Conclusion

BFMs prepared from bamboo with nodes was characterized by many more linear cracks,and this may be attributed to the special characteristics of nodes,such as their shorter fi ber length and discontinuous f i ber arrangements.BFRC manufactured from BFMs without nodes had greater TS, CS,MOE,and MOR,while the BFRCs fabricated from BFMs with nodes had greater HS.The differences in mechanical properties of BFRCs manufactured from BFMs with and without nodes might be associated with the characteristic variation between the bamboo internodes and nodes.

AcknowledgmentsThe project was supported by‘Key Laboratory of Wood Industry and Furniture Engineering of Sichuan Provincial Colleges and Universities’and the National Forestry Public Welfare Scientif i c Research Program(201304503).The Science and Technology Innovation Foundation for College Students was also greatly appreciated.

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9 February 2014/Accepted:14 April 2014/Published online:21 July 2015

Project funding:This work was f i nancially supported by the Key Laboratory of Wood Industry and Furniture Engineering of Sichuan Provincial Colleges and Universities,the National Forestry Public Welfare Scientif i c Research Program(201304503),and the Science and Technology Innovation Foundation for College Students.

The online version is available at http://www.springerlink.com.

Corresponding editor:Yu Lei.

✉Jinqiu Qi

qijinqiu2005@aliyun.com

1College of Forestry,Sichuan Agricultural University, Ya’an 625014,China

2Research Institute of Wood Industry,Chinese Academy of Forestry,Beijing 100091,China