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Influence of moisture absorption on the interfacial strength of bamboo/vinyl

ester composites

Hongyan Chen a,b, Menghe Miao a,*, Xin Ding b

a CSIRO Materials Science & Engineering, PO Box 21, Belmont, Victoria 3216, Australiab College of Textiles, Donghua University, Shanghai 201620, China

a r t i c l e i n f o

Article history:

Received 12 June 2009

Received in revised form 30 July 2009

Accepted 6 September 2009

Keywords:

A. Bamboo

A. Polymermatrix composites (PMCs)

B. Interface

E. Cure

a b s t r a c t

Moisture absorption is a major concern for natural fibers used as reinforcement in structural composites.

This paper reports a detailed study on the moisture sorption characteristics of bamboo strips and their

influence on the interfacial shear strength (IFSS) of bamboo/vinyl ester composite. The IFSS determined

by pull-out test decreased dramatically as the fabrication humidity increased. The bamboo strips provide

a reservoir of moisture which diffuses into the interfacial region and inhibits the hardening of vinyl ester

matrix. The interface of thebamboo/vinyl ester composite canalso be damaged due to moisture exposure

after fabrication. Post-fabrication exposure of composites to moisture was found to be less damaging

than the moisture exposure during the composite fabrication. The IFSS of the composite decreased by

nearly 40% in the first 9 d of water immersion. Further immersion up to 100 d did not cause any further

reduction in interfacial shear strength.

Crown Copyright 2009 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Natural bamboo itself is a unidirectional fiber reinforced com-

posite consisting of long and parallel cellulose fibers (vascular bun-

dles) embedded in a ligneous matrix (ground tissues). The fibers

have a specific density of 1.16 g/cm3 while the ground tissues,

which are honeycomb foams, have a much lower density of

0.67 g/cm3 [1]. The density distribution of bamboo fibers in the

bamboo plant varies according to the radial position in the cross-

section and along the length of the plant. The fiber density is much

higher in the outer skin region than in the inner region [2], and for

this reason bamboo is often referred to as a functionally gradient

material [3]. The compressive strength of bamboo is closely related

to the mean fiber density in the section [4].

Bamboo can be used as reinforcement in different forms, includ-

ing whole bamboo [5], sections of bamboo [6], bamboo strips [7,8]

and bamboo fibers [9,10]. This study focused on bamboo strips,

which can be produced by manual or machine splitting of the bam-

boo culm.

Bamboo strip reinforced polymeric composites have been stud-

ied by various research groups. Shin et al. [11] found that the ten-

sile strength of bamboo strip reinforced epoxy composites was

superior to that of glass fiber reinforced epoxy composites. The

bamboo composite was found to be more durable than the bamboo

itself[12]. Bashar et al. [13] observed improvement of mechanical

properties when the bamboo strips were made into composites

with poly(methyl methacrylate) in the presence of additives. Das

et al. [8] studied the effect of mercerization of bamboo strips on

the mechanical properties of unidirectional bamboo strip rein-

forced novolac composites.

Because of its structure and composition, bamboo absorbs

moisture when it is exposed to humid conditions or immersed in

water. The mechanical properties of bamboo may change signifi-

cantly due to moisture absorption depending on the bamboo spe-

cies and treatment conditions. Chung and Yu [14] investigated the

influence of moisture on the mechanical properties of two species

of bamboo, Kao Jue (Bambusa pervaribilis) and Mao Jue (that is

Moso, Phyllostachys pubescens). When Kao Jue absorbed 20% mois-

ture (by soaking in water), its bending strength was reduced by

nearly a half and its bending modulus was reduced by about

one-third. On the other hand, when moso absorbed 30% moisture,

the bending strength did not change significantly, but its bending

modulus was reduced by about one-third. Godbole and Lakkad

[15], soaked a bamboo of unspecified species in distilled water

for 144 h, which allowed the bamboo to absorb 81.2% moisture

by its dry weight. Consequently, the tensile strength of the bamboo

was reduced by 37% and the tensile modulus was reduced by

nearly a half. The transverse dimension of the bamboo swelled

up to 6% due to the soaking.

The mechanical properties of natural fiber (such as flax, hemp

and sisal.) reinforced composites can decrease considerably when

they are exposed to moisture [1625]. Besides affecting the prop-

erties of the polymer matrix and the natural fibers themselves,

1359-835X/$ - see front matter Crown Copyright 2009 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.compositesa.2009.09.003

* Corresponding author. Tel.: +61 3 5246 4000; fax: +61 3 5246 4057.

E-mail address: [email protected] (M. Miao).

Composites: Part A 40 (2009) 20132019

Contents lists available at ScienceDirect

Composites: Part A

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m p o s i t e s a
http://dx.doi.org/10.1016/j.compositesa.2009.09.003mailto:[email protected]://www.sciencedirect.com/science/journal/1359835Xhttp://www.elsevier.com/locate/compositesahttp://www.elsevier.com/locate/compositesahttp://www.sciencedirect.com/science/journal/1359835Xmailto:[email protected]://dx.doi.org/10.1016/j.compositesa.2009.09.003

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moisture can seriously damage the fibermatrix interface, leading

to poor stress transfer efficiencies from matrix to reinforcement

[26,27]. The degradation process starts with the swelling of cellu-

lose fibers that develops stress at the interface and causes micro-

cracking of the matrix around the swollen fibers [19,28]. The cracks

exacerbate water absorption and its attack on the interface. The

absorbed water starts to establish intermolecular hydrogen bond-

ing with the fibers and thereby reduces the interfacial adhesion be-

tween the fiber and the matrix, and water soluble substances start

leaching from the fibers. These eventually lead to the ultimate deb-

onding between the fiber and the matrix. For long periods of water

exposure, biological activities (growth of fungi) can play an impor-

tant role in the degradation of the natural fibers. The damaging ef-

fects of moisture on the natural fiberresin interface could be

reduced to some extent by different fiber treatments [2934].

Compared to polymeric composites reinforced with glass fibers

and carbon fibers, the natural fiber composites generally have low

interfacial strength. A major cause of the poor interface is the

incompatibility between the hydrophilic natural fibers and the

hydrophobic polymer matrix. The moisture present during manu-

facture can further exacerbate this problem, leading to poor pro-

cessability and low mechanical performance of composite [17].

For example, when methyl ethyl ketone peroxide (MEKP) is used

as the catalyst, vinyl ester resin cures poorly or does not cure under

excessive moisture conditions. Although the problem is known to

the industry [35,36], quantitative relationship between the mois-

ture exposure in manufacture and the interfacial bonding strength

of final composites has not been found in scientific literature.

In this study, bamboo strips and vinyl ester resin were used to

investigate the influence of moisture on interfacial shear strength

(IFSS). The large size and regular sectional shape make the bamboo

strips a material particularly convenient for pull-out testing. The

study was aimed to clarify how pre-fabrication moisture exposure

affects the matrix hardening process and to establish the relative

importance between pre-fabrication moisture exposure and post-

fabrication moisture exposure.

2. Materials

The middle section of a mature moso bamboo plant (P. pubes-

cens) grown near Jinhua, Zhejiang Province, China, was used in this

investigation. The bamboo section was manually split into strips by

a bamboo craftsman. All the strips used in this investigation were

from the second and third layers below the epidermis to minimize

variability in material properties. The strips used in pull-out spec-

imens were 0.71.0 mm thick (radial direction in the plant), 2.0

3.0 mm wide and approximately 130 mm long. Longer strips

(approximately 250 mm) were used for tensile testing.

The matrix used in this study was a commercial vinyl ester resin

SPV 6017 supplied by Nuplex Industries (Australia) Pty Ltd. (vis-

cosity of 120230 MPas, tensile strength of 7085 MPa and elonga-

tion at break of 35%). A Norox 925H methyl ethyl ketone peroxide

(MEKP) hardener was used for curing. Vinyl ester is commonly

used with glass fibers in the marine industry due to its good corro-

sion resistance and ability to withstand water degradation [35].

The resin is also known for its sensitivity to ambient conditions

during manufacturing, especially when used together with MEKP.

3. Experimental

3.1. Moisture content and dimensional expansion

The bamboo strips were dried in a vacuum oven dryer at 105 C

for 5 h followed by cooling in a desiccator, as recommended inASTM D2495-07. The dry weight (W0) was measured using an elec-

tronic balance to the accuracy of 0.0001 g. The predried sample

was then conditioned in a humidity chamber set to a chosen rela-

tive humidity at the constant temperature of 20 0.1 C. The mois-

ture content of the sample at any given time (Mt) was calculated

from:

Mt WtW0

W0 100% 1

The dimensional changes of the bamboo strips in width, thick-

ness and length at each humidity condition were determined by

using a digital micrometer that applies a standard compression

force during measurement.

3.2. Tensile test of bamboo strips

An Instron 5567 testing machine was used to determine the

tensile properties of the bamboo strips according to ASTM D-

3039. The length of the tensile test samples was 250 mm. The bam-

boo strip specimens were predried in the vacuum oven and then

conditioned to moisture equilibrium at the relative humidity level

chosen to investigate. Again, the temperature was kept to a con-

stant (20 0.1 C) in all humidity treatments. To minimize loss ofmoisture, the specimens were taken out from the conditioning

chamber immediately before the tensile testing. The cross sec-

tional dimension of each bamboo strip was measured by using

the digital micrometer mentioned earlier. The tensile tests were

conducted at a gauge length of 150 mm and a cross-head speed

of 5 mm/min.

3.3. Pull-out test of bamboo strips embedded in vinyl ester resin

The regular shape and large cross-sectional size of the bamboo

strips are ideal for pull-out testing. Vinyl ester resin containing

1.5% MEKP hardener was filled into a cylindrical mold of 20 mm

in diameter while a bamboo strip was kept in an upright position

by a bracket. The embedded length of the strip was kept to10 mm or less to ensure that the bamboo strips could be pulled

out from the resin block instead of breaking.

Predried bamboo strips were first conditioned in the humidity

chamber to reach their moisture equilibrium at the chosen relative

humidity levels (the temperature was kept constant at 20 0.1 C

in all cases). After the resin was filled into the pull-out test mold,

the mold was immediately placed in the humidity chamber to cure

under the chosen relative humidity. The cured specimens were

stored in the humidity chamber under the same humidity and

temperature conditions. To maintain the moisture content of the

specimens, the specimens taken out from the humidity chamber

were kept in a sealed plastic bag during transport to the pull-out

testing room, and they were taken out from the sealed bag imme-

diately before the pull-out test.The bamboo strip pull-out tests were also carried out on the In-

stron 5567 tensile tester. The resin block was placed on top of the

upper clamp that was kept open and the exposed bamboo strip

was gripped by the lower clamp. The gauge length was set to

20 mm, which gave a 50 mm distance between the resin surface

and the gripping point. The pull-out test was carried out at a

cross-head speed of 2 mm/min. The average interfacial shear

strength (IFSS) of each specimen was calculated using the Kelly

Tyson equation [37]:

s Fm

SL2

where Fm is the maximum load recorded when debonding occurs, S

is the perimeter of the cross section of bamboo strip, and L is theembedded length of the bamboo strip in the matrix. Twenty speci-

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mens were used for each evaluation, and the average value and

standard deviation were reported.

4. Results and discussion

4.1. Moisture sorption behavior of bamboo strips

When cellulose fibers are exposed to moisture, the first watermolecules are absorbed directly onto the hydrophilic groups of

the fiber material and after that, the other water molecules are at-

tracted either to other hydrophilic groups or they may form further

layers on top of the water molecules already absorbed [38,39]. At

high humidity levels, liquid water may be held by the forces of sur-

face tension in capillary spaces available in the material.

To obtain the moisture absorptiondesorption curve, the bam-

boo samples were subjected to a step-by-step moisture cycle while

the temperature was kept to a constant 20 0.1 C. Due to the lim-

itation of the conditioning chamber, the cycle started from 40% RH

and increased in 10% steps up to 90% RH, then decreased stepwise

down to 40% RH. Sample weight at each step was measured at a

series of time intervals until the moisture absorption reached its

equilibrium level (Me). Moisture equilibrium was considered tobe reached when the instantaneous weight Wt of the sample varied

less than 0.01% within 1 h, as recommended in ASTM D2495-07.

The instantaneous moisture absorption by bamboo strips at

each RH level initially increased with the square root of time and

then slowed down as the moisture content approached equilib-

rium (Fig. 1a). This is similar to the behavior of other natural fibers

[12,34] and natural fiber composites [13]. The equilibrium mois-

ture contents for different relative humidity levels increased

approximately in proportion to the relative humidity up till about

60% RH (Fig. 1b). The equilibrium moisture content then took off

sharply at 70% relative humidity, and it nearly doubled as the rel-

ative humidity increased from 80% to 90%. This trend is very sim-

ilar to that demonstrated by other cellulose fibers, such as cotton

[39]. In bamboo, the ground tissues that surround the cellulose fi-bers occupy a large proportion of the volume in the bamboo strip

(refer to the microphotographs in Fig. 7). These ground tissues

are of a honeycomb structure [1] with numerous capillary spaces

that are ideal sites for holding liquid water. As shown in Fig. 1b,

the experimental relationship between relative humidity and equi-

librium moisture content in bamboo strips fitted quite well with

the model proposed by Peirce [39].

Fig. 2 shows the change of equilibrium moisture content of the

bamboo strips when the relative humidity was varied in a stepwise

manner as indicated by the dotted lines. This equilibrium absorp-

tiondesorption cycling curve demonstrated distinctive character-istics of hysteresis. In other words, the equilibrium moisture

content is determined not only by the storage environment (rela-

tive humidity and temperature) but also by the previous humidity

history of the material.

0 2 4 6 8 10 12

0

5

10

15

20

25

30

RH 60%

RH 70%RH 80%

RH 90%

RH 50%Moisturecontent(%)

Square root of time (Hour1/2

)

RH 40%

0

5

10

15

20

25

30

0 20 40 60 80 100

Relative Humidity (%)

EquilibriumMoistureContent(%)

Fit to Peirce's model

(a) (b)

Fig. 1. Moisture absorption curves from oven dry to equilibrium at different levels of relative humidity (temperature was kept at 20 C): (a) absorption curves at differentrelative humidity levels and (b) relationship between relative humidity and equilibrium moisture content.

0 200 400 600 8000

5

10

15

20

25

30

0

20

40

60

80

100

MoistureContent(%)

Time (Hour)

Moisture Content

------ Relative Humidity

absorption desorption

RelativeHumidity(%)

Fig. 2. Moisture hysteresis of bamboo strips: change of equilibrium moisture

content of bamboo strips in a cycle of absorption and desorption.

0 20 40 60 80 1000

1

2

3

4

5

6

7

8

9

10

DimensionalExpansion(%)


Width

Thickness

Length

Fig. 3. Dimensional expansion of bamboo strips in a cycle of absorption and

desorption.

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Bamboo strips undergo an anisotropic dimensional expansion

as they absorb moisture. This is shown in Fig. 3. The expansion

in the width direction (i.e., along the circumference of the bamboo

plant) was the largest, followed by the expansion in the thickness

direction. The dimensional expansion along the length direction

was 100 times smaller than that along the width. Like moisture

content, the dimensional expansion of bamboo strips demon-

strated hysteresis as the samples went through a moisture absorp-

tiondesorption cycle.

4.2. Effect of moisture absorption on tensile properties of bamboo

strips

Moisture absorption has a softening effect on the bamboo strip

as indicated by the consistent increase in breaking strain (Fig. 4a)

and decrease in elastic modulus (Fig. 4b). Moisture absorption also

caused a small increase to the tensile strength of the bamboo

strips, as shown in Fig. 4b. These are in broad agreement with

the bending test results on moso bamboo by Chung and Yu [14].

They found that the moisture absorption had no effect on flextural

strength. It should be noted that their bending test was conducted

on bamboo trunks while our tensile test was done on bamboo

strips taken from the high fiber density region of the bamboo

trunk. Improved tensile strength of cellulose fibers due to humid-

ification has been reported by other workers. For example, Stam-

boulis et al. [22] reported that after being humidified at 90% RH,

greenflax fibers increased their tensile strength by about 20%. They

suggested that the availability of some free water molecules had a

plasticizing effect which was advantageous to the strength of cel-

lulose fibers.

4.3. Interfacial shear strength (IFSS) of bamboo/vinyl ester composites

Fig. 5 shows two loaddisplacement curves obtained from the

bamboo strip pull-out tests. Fig. 5a is a typical curve of specimens

manufactured at dry and low RH conditions, and Fig. 5b is a typicalcurve of specimens manufactured at high RH conditions (>80% RH).

For the specimens produced at lower RH, the loaddisplacement

curves exhibited typical linear-elastic behavior up until it reached

its maximum load, followed by a precipitous load drop when the

interface failed completely and then frictional pull-out. The pull-

out force of samples produced at high RH conditions was of much

lower magnitude, coupled with high frequency fluctuations. In this

case, the maximum load spike in the loaddisplacement curve was

used to calculate the IFSS. The force fluctuations appeared to be the

result of a high frequency stick-slip frictional force between two

surfaces sliding apart. There was no sudden load drop after the

maximum load point, indicating a gradual transition from debond-

ingto pull-out, which continued until the total embedded length of

the bamboo strip was extracted. This behavior is typical of a poor

interfacial bonding between fiber and matrix [40].

Fig. 6a shows how the interfacial shear strength of the bamboo/

vinyl ester composite was affected by the humidity condition dur-

ing manufacture. The IFSS decreased steadily with the increase of

relative humidity at manufacture. As compared with the compos-

ites manufactured at dry condition, the IFSS value was reduced

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 10 20 30 40 50 60 70 80 90 100

Relative humidity (%)

Breakingstrain(%)

(a)

0

100

200

300

400

0

10000

20000

30000

40000

50000

TensileStrengt

h(MPa)


Tensile StrengthTensile Modulus

TensileModulus(MPa)

(b)

0 10 20 30 40 50 60 70 80 90 100

Fig. 4. Effect of humidity on the tensile properties of bamboo strips: (a) breaking strain and (b) tensile strength and modulus.

0.0 0.5 1.0 1.5 2.0 2.5 3.00

100

200

300

400

500

600

Load(N)

Displacement (mm)

0.0 0.5 1.0 1.5 2.00

1

2

3

4

Load(N)

Displacement (mm)

(a) (b)

Fig. 5. Typical loaddisplacement curves from pull-out tests of bamboo strip/vinyl ester composites: (a) low RH and (b) high RH (>80%).


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by over a half as RH increased to 70%, by more than 90% at 80% RH,

and to almost zero value at 90% RH. This drop of IFSS was mainly

attributed to the poor interfacial bonding formed at the high rela-

tive humidity conditions. The bamboo dimensional expansion due

to moisture absorption had little to do with the IFSS deterioration.

To illustrate this, one set of specimens manufactured at dry condi-tion were conditioned to moisture equilibrium at 80% RH and then

tested for IFSS. Despite the swelling of bamboo strips inside the re-

sin block, which should increase the mechanical locking between

the bamboo strip and the matrix, the average IFSS suffered a small

loss, from 10.82 MPa to 9.58 MPa. Damage caused by post-fabrica-

tion moisture exposure will be discussed in Section 4.4. Another

set of specimens manufactured at 80% RH were dried in a desicca-

tor for 48 h before subjecting to pull-out test. The drying of the

composite caused a reduction of IFSS, from 0.8 MPa to 0.25 MPa.

The relationship between IFSS and the equilibrium moisture

content of bamboo strip in composite manufacture is plotted in

Fig. 6b. The IFSS decreased almost linearly with the increase of

bamboo moisture content from 0% to about 12%. The rate of de-

crease suddenly accelerated after this point. A further increase of

2% in bamboo moisture content caused a reduction of more than

80% in IFSS.

The influence of moisture during manufacture on the quality of

interface formed betweenthe bamboo strip and the vinyl ester ma-

trix may be further demonstrated by the optical microphotographs

in Fig. 7. Small bamboo/vinyl ester composite samples were pre-

pared and cured at dry condition (0% RH) and at 80% RH, respec-

tively. After being conditioned in the laboratory (20 C and 65%

RH), the composite samples were sectioned by using an ultra-

microtome and then observed under an optical microscope. The

specimen produced at dry condition showed no visible void at

the interface between the bamboo and the matrix (Fig. 7a) while

the sample produced at 80% RH exhibited thin void lines along

the fibrous region and wide void spaces along the ground matrix

regions (Fig. 7b). The capillary water accumulated in the honey-comb cells in the ground tissue regions became a physical barrier

between the bamboo and the vinyl ester matrix, leading to the for-

mation of these wide voids in the final composite.

4.4. Influence of water immersion on composite interface

The bamboo strips were dried at 105 C for 5 h followed by cool-

ing in the desiccator. To further eliminate the effect of moisture

absorption, the impregnated specimens were placed in the desicca-

tor for curing. The cured specimens were then immersed in 2 L

beakers of tap water at room temperature (20C) for different time

intervals. Ten specimens were taken out from the water beakers at

the end of each predetermined time interval. Immediately before

subjecting the specimens to pull-out test, surface water was re-moved from the specimens using absorbent tissue papers.

The interfacial shear strength is plotted with water immersion

time in Fig. 8. The IFSS of the specimens increased slightly after

being immersed for one day. In the following 8 d, the interfacial

strength decreased linearly with immersion time. The accumulated

loss of interfacial strength on the ninth day amounted to about

38%. Thereafter, the interfacial shear strength remained very stable

as the immersion timeextended up to 100 d. Kim and Seo [28] sub-

jected sisal fiber in vinyl ester and epoxy matrix composites to cy-

0

2

4

6

8

10

12

0 20 40 60 80 100

Relative humidity (%) during manufacture

IFSS(MPa)

0

2

4

6

8

10

12

0 5 10 15 20 25 30

Bamboo moisture content (%)

IFSS(MPa)

(a) (b)

Fig. 6. Influence of relative humidity (a) and moisture content (b) during manufacture on interfacial shear strength of resultant composites.

Wide void

space

Wide void

space

Thin void

line

Wide void

space

Wide void

space

Thin void

line

(a) (b)

Fig. 7. Optical microphotographs of sectioned bamboo vinyl ester composites prepared at dry condition (a) and 80% RH (b).


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cles of wetting and drying (each cycle consisted of 9 d of wetting at

21 C and one day of drying at 50 C) and tested for change of

mechanical properties. They found a similar decreasing pattern of

breaking stress of the composites, i.e., the breaking stress suffereda significant decrease after one cycle and then maintained its value

up until five cycles of wetting and drying.

There are three possible scenarios for the reduction of the IFSS

of bamboo/vinyl ester composite caused by water immersion:

water damage on the bamboo strip, on the vinyl ester matrix, or

on the interfacial bonding. The weakest of the three elements

(bamboo, matrix and interface) determines the actual composite

failure mechanism. Vinyl ester matrix is known for its good water

resistance [35]. The flexural strength of cured neat vinyl ester resin

was not significantly affected after being immersed in water for

1300 h (i.e., 54 d) [41]. We tested the tensile strength of the bam-

boo strips without water immersion, after immersion in water for

50 d and 100 d. As shown in Fig. 9, the bamboo strips maintained

their tensile strength up until 50 d of immersion and the tensilestrength started to deteriorate sometime between 50 d and

100 d. At 100 d, the bamboo strips lost 46% of their tensile strength.

This change of bamboo strip strength was not reflected in the IFSS

results in Fig. 8. The reduction of IFSS in the initial nine-day period

can thus be attributed to water damage on the interface. The hy-

droxyl groups in the bamboo structure allow a large number of

hydrogen bonds to be formed between the macromolecules of

the cellulose and polymer. With the absorption of water, hydrogen

bonds between water molecules and the cellulose fibers are

formed, causing loss of compatibilization between the cellulose fi-

bers and the matrix, which results in debonding and weakening of

the interface adhesion [23].

5. Conclusions

Bamboo is a hydrophilic material that absorbs a significant

amount of moisture at standard humidity conditions. Bamboo

strips demonstrate moisture sorption hysteresis similar to other

natural fibers such as flax and hemp. They also experience aniso-

tropic expansion with the absorption of moisture. The largest

expansion took place along the circumference of the bamboo culmand this was twice as much as that in the radial direction. The lon-

gitudinal expansion was negligibly small. Moisture absorption

softened the bamboo strips, causing an increase in extensibility

and a reduction in elastic modulus while the tensile strength was

not significantly affected.

The influence of moisture absorption on the interfacial shear

strength (IFSS) of bamboo/vinyl ester composite was studied using

the pull-out test. The relative humidity in composite manufacture

had a severe impact on the IFSS of the resulting composites. The

IFSS achieved at normal room conditions (20 C, 60% RH) was only

a half of what is achieved in the dry condition. Composites pro-

duced at high relative humidity conditions (80% and 90%) had neg-

ligible interfacial strength.

Exposure of the bamboo/vinyl ester composite to water caused

significant damage to the interfacial shear strength. The damage

took place in the first 9 d, causing a 38% reduction in IFSS. Pro-

longed water immersion up until 100 d did not result in further

reduction in IFSS. Therefore, exposure to high humidity during

material storage and composite manufacture can be much more

damaging to the interfacial strength of the final composites than

post-fabrication moisture exposure.

Acknowledgements

The authors would like to acknowledge the China Scholarship

Council for granting a scholarship that enabled Honyan Chen to

pursue this work at CSIRO and the Cooperative Research Centre

for Advanced Composite Structures Limited (CRCACS) for its sup-

port of their work, which has been carried out as part of the CRC

ACS research program. The assistance of Margret Pate on micros-

copy is highly appreciated.

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0

2

4

6

8

10

12

14

16

0 20 40 60 80 100 120

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ResidualIFSS(MPa)

Fig. 8. Residual interfacial shear strength of bamboo strip/vinyl ester composite

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0

50

100

150

200

250

300

350

0 20 40 60 80 100 120

Time immersed in water(day)

Tensilestrength(MPa)

Fig. 9. Effect of water immersion time on the tensile strength of bamboo strips.


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