I ndian Journal of Fibre & Text i le Research Vol. 3 1 , June 2006, pp. 335-338

Flexural properties of rice straw reinforced polyester composites

A V Raina Prasad'\ K Mural i Mohan Rao & M Anil Kumar Department of Mechanical Engineering, V R S iddhartha Engi neering Col lege, V ijayawada 520 007, I ndia

and

K Mohan Rao PVP S iddhartha I nstitute of Technology, V ijayawada 520 007, I ndia

Received 14 Septelllber 2004; revised received and accepted 5 April 2005

R ice straw fibres have been extracted and i ncorporated i n polyester resi n matrix to prepare rice straw reinforced polyester composites and the flexural properties of resultant composites studied. The composites wi th a mean flexural strength of 66.3 Mpa, which is greater than that of plain polyester (55 .08 M Pa), can be rormulated with an opt imum fibre volume of about 40%. The flexural modulus or composite is found to be 2630 M Pa which is about 1 .5 t i mes greater than that of plain polyester. The specific flexural modulus is nearly 2 t i mes greater than that of polyester res in . Straw-based composites are sui table as core material for structural board products.

Keywords: Flexural modul us, Flexural strength, Natural fibre composite, Polyester, R ice straw

IPC Code : In t . CI 8 DO l O

1 Introduction Natural fibres added to synthetic polymers act as

reinforcement and i mprove the mechanical properties of the polymer matrix . Timber byproducts, such as wood flour, saw dust and wood chips, are commonly used as fi l lers in composite board products. 1 .2 Wood flour incorporated as fil ler i n thermosetting resin improves strength and impact resistance.}

Annual crop fibres are a cheaper and more rapidly renewable source of cel lulose-rich fibre with a renewal t ime of one year as against at least thirty years for softwoods, and their ful l potential as a polymer reinforcement has yet to be achieved. Annual crop fibres, such as jute4.5 , sugarcane bagasse6, wheat straw7-9, have been used as fibrous reinforcement in composites. The performance of these fibres depends on their cellulose content.

Wood fibre and jute contain more cellulose than straw. Several investigations have been carried out on rice byproduct composites, such as rice husk reinforced boards. I O. 1 1 But in view of the 80-85 mi l l ion tones production of rice every year in India, an investigation of rice straw fibre for composites application is justified. 1 2 Accordingly various percentage volumes of rice straw have been used with an unsaturated polyester res in to produce rice straw

"To whom all the correspondence should be addressed. E-mai l : rp_atluri @yahoo.co . in

rei nforced polyester composites. Present paper reports the extraction of straw fibres and thei r i ncorporation in a polyester resi n matrix to prepare rice straw reinforced polyester composites along with the flexural properties of resulting composi tes.

2 Materials and Methods 2.1 Extraction and Preparation of Straw Fibre

The rice straw MTU 2077, which IS widely cultivated and recognized as high yielding variety, was used for the study . Each straw stalk is built up of sections of stem joi ned at nodes, which are hard bulbous areas where leaves are attached to the stem. After removing leaves the stems were cut at nodes. Straw fibres are identified in the form of a cylindrical pipe of negligible wall thickness and appear in l ight yellow colour. In order to avoid an extra chemical processing step i n fibre preparation, the straw was l ightly compressed i n a hand press at 0.05 MPa to i mprove the fibre to resin bond. This treatment reduces the porosity. To allow greater resin penetration to the i nner porous l ayers, thereby i mproving res in adhesion, approximately 4 mm diameter straws were rolled flat in a mill with a 0.25 mm clearance between the rol lers . The fibres were split longitudinally and deformed in shear, rupturing the hard epidermis . Fibre-to-resin adhesion is significantly i mproved and porosity reduces in the composi te .

336 INDIAN 1 . FIBRE TEXT. RES., JUNE 2006

2.2 Calculation of Volume Fraction and Density of Fibl'e

The density and volume fraction of fibre in a cured polyester resin matrix were calculated by a method, which enables the rule of mixtures analysis of measured composite properties .

The method involves measuring the densi ty of the composite ( Pc ) of mass M c at a given mass fraction

of resin M R . The picnometric procedure was adopted

for measuring the density of the composite. Volume fraction of resin ( VR) was calculated using the fol lowing relationsh ip :

. . . ( I )

where P R i s the resin density ( 1 2 .32 kN/m') . Then,

the volume fraction ( VF ) and density ( PF ) of straw fibre wi l l be

. . . (2)

. . . (3)

The density of fibre was also measured taking a known weight of fibre separately using picnometric method. Both the methods produced s imi lar results and an average value of 5 . 1 8 kN/m3 was taken as fibre density.

2.3 Fabrication and Testing of Composites 2.3.1 Fabrication

Rolled flat straw contain ing fibres of 1 00 mm length was accurately weighed and moulded with a mixer of unsaturated polyester resin, catalyst and accelerator ( 1 . 5% each by volume of resin) . Layers of fibres were placed in the mould alternatively with layers of resi n unti l al l the fibres were used, starting and ending with layers of res in . The fibres align in mould in parallel such that they are oriented at 0°

along the axial direction of the specimen. A pressure of 0.05 MPa was applied on the mould and left for 24 h to cure. The composites were also post-cured for 2 h at 80°C after removing from the mould.

2.3.2 Testing

Three-point bend tests were performed in accordance with ASTM D 790M test method I (procedure A) to measure flexural properties. The

samples were 1 00 mm long, 25 mm wide and 3 mm thick. Five identical spec imens were tested for each composition. I n three-point bending test, the outer rollers were 64 mm apart and the samples were tested at a strain rate of 0.2 mm/min. A three-point bend test was chosen because it requires less material for each test and el iminates the need to accurately determine center point deflections with test equipment. Flexural modulus (E8) and maximum stress (5) i n the composite were calculated using the fol lowing relationships:

. . . (4)

. . . (5)

where L i s the support span (64 mm); h, the width ; [, the thickness; P, the maximum load; and Ill, the slope of the in itial straight l i ne portion of the load­deflection curve .

3 Results and Discussion Results for all percentage volumes of rice straw

fibre are given i n Table I . A typical load-deflection curve for a straw volume of 40% (2 1 .5 wt %) composite i s presented in Fig. 1 . The curve shows a l inearly increasing trend up to a certain value of load and suddenly drops due to failure of specimen. Arrest points correspond to breakage and pul l out of indiv idual fibres from the res in matrix . The flexural modulus is calculated from the in i tial close to l inear portion of the curve and the maximum composite stress at the point of maximum load. It is observed that at low percentage volumes of fibre the load-

Table I - Mean flexural properties of rice straw composites as a function of fibre content

Weight of Volume of fibre fibre

( WI), % ( Vd, %

0 0 4.60 1 0.29 6.78 1 4.75 8.96 20.00 1 1 .40 23.43 1 4.23 28.30 1 5 .57 3 1 .30 1 8 . 1 6 34.56 2 1 .50 40.00

Density of composite

( Po ), kN/m

1 2.32 1 1 .59 1 1 .27 1 0.97 1 0.65 1 0.30 1 0.24 9.96 9.62

Flexural strength

3 ( ac )' MPa

55.08 52.00 49.80 47.40 46.20 49.50 56.50 59.60 66.30

Flexural modulus (E), MPa

1 709 1 630 1 620 1 6 1 0 1 650 1 733 1 950 2262 2630

RATNA PRASAD el (/1.: FLEXURAL PROPERTIES OF RICE STRA W R E I N FORCED POLYESTER COMPOSITES 337

250

200

� ISO -0 03 a

1 00 ....:I

50

0

0 5 1 0 1 5 20 25

Deflection (mm)

Fig. I-Load-deflection curve for 40% straw tibre volume composite tested ill flexure to failure

deflection curves are smooth to failure, suggesting that the straw fibres act more as impurities than as reinforcement.

The flexural strength of composi tes is plotted against percentage volume of fibre and the results are shown i n Fig. 2. It is observed that up to a fibre volume of 23 .43%, the mean flexural strength decreases and then starts i ncreasi ng l inearly . The result can be fitted to two theoretical curves, which help to explain the development of strength in the composites.

Assuming that the elasti c strains in the composite, matrix and fibre are all equal, at low percentage

volumes of fibre the strength of the composite ( a c ) i s calculated by the following equation :

ac = aR (l - VF ) . . . (6)

where a R i s the strength of the res1l1; and VF, the

volume fraction of the fibre. The matrix failure strain is assumed to be h igher

than the fibre fai lure strain. As the volume fraction of fibre increases, a modified rule of mixtures expression predicts the following:

. . . (7)

where a F is the fibre strength ; and a � , the stress i n

resin at fibre breaking strain . At a volume fraction of fibre VII/ill where the

composite strength is m inimum, the following relationship is obtained:

a F Villin + a � (l - Villin ) = a R (1 - Vmin ) . . . (8)

VII/ill i s calculated using Eqs (6) , (7) and (8) . Values

,--.. 03

1 60 1 40

� 1 20 £ 1 00 OIl � � 80

'"

-;;; 60 .... ::l X � c;:: � 03 20 � � 0

0

(Jc = (J}.-VF + (J� (l - VF ) '"

(JI VOlin 20 40 60 80

% Volume of fibre

1 00 1 20

Fig. 2-Mean flexural strength of rice straw reinforced polyester composite against percentage volu me of fibre, including theoretical curves for composite strength

3 000

(;' 0... � 2500 �

Vl 2 ::l 2000 I -0 a E

� ::l 1 500 _ >< cu �

1 000

0 1 0 20 30 40

% Volume o f fibre

Fig. 3--Flexural modulus of rice straw reinforced polyester composite against percentage volume of fibre

for a � and a F can be obtained by extrapolating the

experimental l i ne, which passes through the a c

values at percentage VF � 22.5 in both directions. The

stress in the res in at fibre breaki ng strain a� i s

graphically determined to be 1 1 .39 MPa. By calculation, the VII/ill i s found to be 22.5%.

The second critical volume fraction Vcril i s defined as the fibre volume fraction, where composite strength is equal to the pure matrix strength and represents the minimum volume fraction for a useful composi te . At this point,

aR = aFVF + a� (1 - VF ) . . . (9)

On substituting Vcril i n place of VF, the value of Vcril

is found to be 3 1 .4 1 % . From the curve drawn between percentage volume

of fibre and flexural modulus (Fig. 3), a s light fall i n

338 INDIAN J . FIBRE TEXT. RES. , JUNE 2006

3 +--------.-----------------,--.-----, o 1 0 20 30 40

% Volume of fibre

Fig. 4--Specific flexural strength of rice straw reinforced polyester composite against percentage volume of fibre

composite flexural modulus is observed up to 20% VF.

The i n- plane shear stress and compressive stress may possibly buckle fibres and account for thi s unexpected i ni tial fall in modulus at low fibre volume. The specific flexural strength and modulus are calculated as the rati o of flexural strength and modulus to the density of the composite respectively . The curves drawn between percentage volume of fibre and specific values of strength and modulus are shown i n Figs 4 and S . The plots exhibit the s imilar trend observed for flexural strength and modulus and same cause is attributed as stated above.

4 Conclusions

A useful composite material can be manufactured from rice straw and polyester res in . Rice straw being the low density fibre, the composi te can be regarded as a successful l ightweight engineering material . For a volume of fibre up to 20%, the flexural modulus and specific flexural modulus decrease by a small magnitude and start increasing l i nearly up to VF of 40%. The mean flexural strength and specifi c flexural strength also decrease by a small amount up to a VF of 23.43% and start increasi ng l inearly up to VF of 40%. The min imum percentage volume of fibre Vlllill is found to be 22.S and the critical percentage volume of fibre Vcril i s 3 1 .4 1 . Thus, the composites with mean flexural strength and flexural modulus greater than

0.30 l ,......,

E ! "0 � 0.25 I '-' en ;:J ;:J

"0 0 E 0.20 � � .... I ;:J

J � Q) t;:: C,) 0 . 1 5 t;:::

'(3 Q) 0. en

0 . 1 0

o 1 0 20 30 40

% Volume of fibre Fig. 5--Specific flexural modulus of rice straw reinforced polyester composite against percentage volume of fibre

that of plain polyester can be formulated which are suitable as core materials for structural board products.

References 1 David N S Han & Wayne Y Chao, J Appl PO/YIIl Sci, 50

( 1 993) 7- 1 1 . 2 Raj R G, Kokta B V & Daneault C, J App/ PolYIIl Sci, 40

( 1990) 645-655. 3 Raj R G, Kokta B V, Grouleau G & Daneaiit C. PO/YIIl P/ast

Technol Eng, 29( 1 990) 339-353. 4 Mi tra B C, Basak R K & SarkaI' M , J Appl PolYIIl Sci, 67

( 1 998) 1 093- 1 1 00. 5 Dash B N, Rana A K, Mishra H K, Nayak 5 K, Mishra S C

& Tripathy 5 S, J PolYIIl COIIIPOS, 20 ( 1 999) 62-7 1 . 6 Zarate C N, Aranguren M I & Reboredo M M, J Appl PolYln

Sci, 77 (2000) 1 832- 1 840. 7 White N M & Ansell M P, J Mater Sci, 1 8 ( 1983) 1 549-

1 556. 8 Hornsby P R, H inrichsen E & Tarverdi K, J Mater Sci, 32

( 1 997) 443-45 1 . 9 Hornsby P R, H inrichesn E & Tarverdi K, J Mater Sci, 32

( 1997) 1 009- 1 023. 1 0 Prasad Krishna K S & Joseph G , Rice husk board - A new

panel product, Plant Fibres News, 1 ( 199 1 ) 28-32. 1 1 Zoolagud S 5 & Rangaraju T 5, Plant Fibres News, 3 ( 1 997)

1 5 . 1 2 Rangarajan 5 , The Survey of Indian Agriculture (Mis Kasturi

and Sons Ltd, Chennai), 2000, 39-44.