Effects of Chemical Treatments on Mechanical and ...

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Science and Engineering of Composite Materials 15, 43- 57 (2008)

Effects of Chemical Treatments on Mechanical and Physical Properties of Flax Fiber-reinforced Composites Bei Wang', Lope Tabil, Satya Panigrahi

Department of Agricultural and Bioresource Engineering University of Saskatchewan, Saskatoon, SK, Canada, S7N 5A 9 Tel: 647-588-3369; Fax: 416-978-3834

ABSTRACT Flax fibers can be used as ecological alternatives to conventional reinforcing fibers (e.g., glass) in composites. Flax fibers have some advantages over glass fiber, because they are less dense, renewable, combustible and are relatively low in price. This excellent price-performance ratio at low weight, in combination with the environmentally friendly character is very important for the acceptance of natural fibers in large volume engineering markets. A major restriction to the successful use of natural fibers in durable composite applications is their high moisture absorption and poor dimensional stability. In order to improve the above qualities, various surface treatments of fibers including silane treatment, benzoylation, and peroxide treatment were carried out, to improve mechanical performance of fiber composites. Also, composites consisting of high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE) or HDPE/LLDPE, chemically treated fibers and additives were prepared by extrusion process. The extruded samples were then ground and test samples were prepared by rotational molding. The chemical analysis showed that selective chemical treatments increased the α-cellulose content of flax fibers from 73% to 95%, but caused a decline in hemicellulose and lignin content. Derivative thermogravimetry (DTG) curves indicated that chemically treated fibers were thermally stable in the region below 250 °C and chemcial treatments increased the onset thermal decomposition temperature of flax fibers. The mechanical properties demonstrated an increase in tensile strength from 17.56 MPa of untreated fiber (20 wt%) reinforced LLDPE to 25.86 MPa of peroxide treated fiber (20 wt%) reinforced LLDPE. The increased hardness of flax fiber-reinforced composites was also very promising; it was 22.1 of untreated fiber (20 wt%) reinforced HDPE compared to 25.1 of silane treated fiber (20%) reinforced HDPE. This increase in fiber content has a positive effect on the mechanical properties of composites. The water absorption of the chemically treated flax fiberbased composites was lower than that of the untreated fiber-based composites. Key Words: Silane treatment, benzoylation, peroxide treatment, rotational molding, tensile strength, chemial charaterization, water absorption, derivative thermogravimetry

* Author to whom correspondence should be addressed. E-mail: [email protected]

Brought to you by | University of Saskatchewan Authenticated | [email protected] Download Date | 5/23/13 6:59 PM

43

Effects of Chemical Treatment on Properties of Flax Fiber-reinforced Composites

Vol. 15, No. 1, 2008

INTRODUCTION Saskatchewan's economy is largely dependant on agriculture. Diversification of the industry is crucial for encouraging economic stability and growth. Value-added processing of various agricultural products is one example of such diversification. The goal of value-added processing is to find new uses for agricultural materials which are currently processed elsewhere, or disposed of post-harvest. The development of biocomposite materials has accelerated rapidly, primarily due to improvements in process technology and economic factors. Research and development of biorenewable fibers for use in the plastic industry could provide attractive new value-added markets for agricultural materials. Our biocomposite material contains polymers reinforced with flax fiber. There are a number of reasons we want to do this, one being the fact that flax is an oilseed crop grown throughout Saskatchewan. Also, flax fiber will make the material partially biodegradable; and glass is relatively expensive to make. Flax is currently disposed of by burning; flax has extremely high tensile strength; this will allow for agricultural diversification within Saskatchewan. Natural flax fiber-reinforced composites

are

better than

synthetic

fiber-reinforced composites

since they

are

more

biodegradable, light in weight, non-corrosive, temperature resistant, less polluting to the environment and ease to recycle. The fiber-based composites industry relies on the premise that the addition of lower cost materials (fillers and reinforcements) to plastic matrix will decrease overall materials manufacturing costs III. These advantages give the natural fiber based composites high performance composites having economic and environmental advantages. In the field of technical utilization of plant fibers, flax fiber-reinforced composites represent one of the most important areas. Natural

fiber-reinforced

composites form a new class of materials which seem to have good future potential as a

substitute for wood-based material in many applications 111. Biocomposites containing thermoplastics and pre-treated flax fiber have mechanical properties comparable with those of glass fiber-based thermoplastic composites /3/. Typically, blending thermoplastic matrix with fibers or other additives in an extruder produces

fiber-based

composites. Blended materials may be post-processed by rotational

molding or other manufacturing techniques to form final products or parts. The major drawbacks associated with the use of natural fibers as reinforcements in thermoplastic matrix are the poor compatibility and weak interfacial bonding between fiber and matrix as well as the dispersability of the hydrophilic cellulose fibers with the hydrophobic thermoplastic. Chemical treatment of fibers can help stop the moisture absorption process, clean and chemically modify fiber surfaces and increase the surface roughness in order to increase the interfacial adhesion between fiber and matrix. Several studies reported on the influence of various types of chemical modification on the mechanical properties of natural fiber and

fiber-reinforced

thermoplastic composites. Karnani et al. /4/ have extensively studied the effect of

different chemical modifications such as silane treatment on the mechanical properties and dimensional stability of cellulosic

fiber-thermoplastic

composites.

They

reported

that

chemically

modified

cellulosic

fiber-reinforced

composites offered superior physical and mechanical properties under extreme conditions even after recycling. Sreekala et al. 151 performed a pioneering study on the mechanical performance of treated oil palm

fiber-reinforced

composites.

In their study, they looked at the tensile strength of composites having 4 0 % by weight fiber loading. Isocyanate-, silane, acrylated-, latex coated and peroxide treated fiber-based composites could withstand the tensile stress to a higher strain level. Alkali-treated fiber composites showed about a 20% increase in tensile strength 161. Weyenberg et al. ΠΙ observed that a remarkable positive effect on composite properties with a simple acetone treatment of fibers. Compared to the untreated fiber-based composite, flexural properties were improved by these treatments (tests were done with alkali,

44


Science and Engineering of Composite

Β. Wang, L. Tabil, and S. Panigrahi

Materials

dilute epoxy, acetone and silane). Herrera-Franco et al. /8/ deposited a silane-coupling agent to henequen fibers and have shown that adhesion between the natural hard fibers and matrix plays an important role on the final mechanical properties of the composites. The objective of this study was to determine the effects of pre-treated flax fibers on the mechanical and physical properties of fiber-reinforced LLDPE, HDPE and HDPE/LLDPE composites. This work also evaluated the effects of fiber loading on the composite products. The ultimate goal was to find which chemically treated fiber-based composites demonstrated high tensile strength and hardness, while maintaining a low water absorption level. In this study, the changes of the chemical composition of fibers after different treatments were studied. Derivative thermogravimetry was carried out in order to investigate the onset thermal decomposition temperature of fibers. The effects of different chemical treatments on tensile properties, durometer hardness and water absorption of the composite were also reported.

EXPERIMENTAL DETAILS

Materials Flax fibers were derived from linseed flax grown in Saskatchewan and decorticated on a standard scutching mill at Durafibre in Canora, SK, Canada. The fibers were first washed thoroughly with 2% detergent water and dried in an air oven at 70 °C for 24 h. The dried fibers were designated as untreated fibers. Reagent-grade chemicals, namely, sodium hydroxide (NaOH), benzoyl chloride, ethanol, dicumyl peroxide, acetone, alcohol, sulfuric acid, hydrochloric acid and the coupling agent, triethoxyvinylsilane (Aldrich Chemical Co. Ltd.) were used for fiber surface modifications. In this series of experiments, high-density polyethylene (HDPE 8761.27), linear low-density polyethylene (LLDPE 8460.29, Exxon Mobil, Toronto, ON) and LLDPE/HDPE 25087 (NOVA Chemicals Ltd., Calgary, AB) were used as polymer matrix.

Fiber Surface Treatments The first step is the mercerization process (pre-treatment process) for all fiber surface treatments. This causes changes in the crystal structure of cellulose. Fibers were soaked in 5-18% NaOH (silane treatment: 5% NaOH, benzoylation: 18%, peroxide treatment: 10%) for about half an hour in order to activate the OH groups of the cellulose and lignin in the fibers. •

Silane treatment: The pre-treated fibers were dipped in an alcohol water mixture (60:40 v/v), containing 5% triethoxyvinylsilane coupling agent by weight for 1 h. The pH of the solution was maintained between 3.5 and 4, using both METREPAK Phydrion buffers and pH indicator strips. Fibers were washed in double distilled water and dried in the oven at 80 °C for 24 h.

•

Benzoylation: The treated fibers were suspended in a 10% NaOH solution and agitated with benzoyl chloride (5% w/w). The mixture was left for 15 min, filtered, washed thoroughly with water and dried between filter papers. The isolated fibers were then soaked in ethanol for 1 h to remove the untreated benzoyl chloride and finally were washed with water and dried in the oven at 80 °C for 24 h.

•

Peroxide treatment: Fibers were coated with 5% (w/w) dicumyl peroxide in saturated acetone solution after alkali pre-treatments. The soaking of the fibers in the solution was conducted at a temperature of 70 °C for 30 min. High temperatures were favored to increase decomposition with the peroxide. The chemically treated fibers were then washed with distilled water and placed in an oven at 80 °C for 24 h.


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Vol. 15, No. 1, 2008

Pretreatment

Pretreatment

Pretreatment

(5 % w/w NaOH, 0.5h)

( 1 8 % w/w NaOH, 0.5h)

( 1 0 % w/w N a O H , 0.5h)

1r

ιr

1r

Si lane treatment

Benzoylation

Peroxide treatment

(5% w/w

(5% w/w benzoyl chloride in

(5% w/w dicumyl peroxide

triethoxyvinylsilane coupling

10% NaOH, 15 min + in

in in saturated acetone

agent in alcohol water

ethanol, l h )

solution, 70°C, 0.5h)

mixture (60:40 v/v), pH: 3.54,lh)

Drying at 80°C, 24h

Fiber grinding + LLDPE, HDPE or HDPE/LLDPE

Blending

Rotational molding (250°C, 0.5h) Fig. 1. The whole process of flax fiber-reinforced composites manufacturing.

46




Materials

Alkalis, acids and other chemicals are very difficult to remove from the fiber surface after diffirent types of treatment due to their porous structure. Additionally, the presence of trace chemicals can have a significant impact on the final composites. The cleaning process is a critical procedure to be addressed in this study. After each chemical treatment, the cellulosic residue was dispersed in distilled water and washed for several hours, until constant neutral pH was achieved.

Composite Preparation and Manufacturing by Rotational Molding Pre-treated and untreated fibers were ground in the grinding mill (Falling Number, Huddinge, Sweden) and oven dried at 80 °C for 24 h to reduce the moisture content to less than 2%. Mixtures of thermoplastic powder and 10% or 20% by weight of flax fibers were prepared using a food blender (Waring Products Corporation, New York, NY). Silane coupling agents were added at a rate of 5% by weight as "resin additive". The blend was fed into the twin-screw extruder (Werner & Pfleiderer, Ramsey, NJ) at the Centre for Agri-Industrial Technology (CAIT) in Edmonton, Alberta using a barrel to die temperature profile of 175 °C, a screw speed of 125 rpm and feed rate to the extruder of 20 kg/hr. Blends prepared in this manner were extruded using a strand die. Extruded strands were then pelletized. The pellets were ground using a grinding mill (Retsch GmbH 5657 HAAN, West Germany) and the ground product was used in rotational molding. The powder of fiber/matrix was dried in an air-circulating oven for 24 h at 70 °C before rotational molding. Test samples were prepared from ground extruded strands using a rotational molding machine, carousel-type molding machine with four separate arms that can each rotate at two separate axes, while completely enclosed in an oven at 250 °C for 30 min located at Norwesco Canada Ltd. in Saskatoon, SK. Figure 1 shows the whole process of flax fiberreinforced composites manufacturing.

Experimental Design The study was divided into two phases. The first phase focused on the effect of chemical modifications on the flax fibers (chemical characterization and thermal property test), while the second phase focused on the performance of the fiber-reinforced composites. Mechanical and physical properties of composites were tested. The experimental design is a factorial arrangement of treatments conducted using a randomized design. Table 1 shows the outline of the experimental design for three types of treated fibers. Main treatment: T1 (Untreated); T2 (Silane treatment); T3 (Benzoylation); T4 (Peroxide treatment) = 4 Sub-treatment: SI (LLDPE); S2 (HDPE); S3 (HDPE/LLDPE) = 3 Treatment combination = 4 types of fiber * 3 types of thermoplastic matrix = 12 (T1S1, T1S2, T1S3, T2S1, T2S2, T3S3, T3S1, T3S2, T3S3, T4S1, T4S2, T4S3) Total tests conducted = 1 2 * 5 (replicates) = 60 tests Table 2 shows both the property tests performed on each fiber and composite sample and the number of replicates. According to the appropriate ASTM standard, five samples were replicated for tensile strength at yield test of composites.


47


Vol. 15, No. 1, 2008

Table 1 Experimental design for three types of treated fibers. Percentage of Silane Plastic Matrix

Fiber

Percentage of Fiber

Coupling Agent (additive)

LLDPE

10

20

5

HDPE

10

20

5

HDPE/LLDPE

10

20

5

LLDPE

10

20

5

(triethoxyvinylsilane coupling

HDPE

10

20

5

agent)

HDPE/LLDPE

10

20

5

LLDPE

10

20

5

HDPE

10

20

5

HDPE/LLDPE

10

20

5

LLDPE

10

20

5

HDPE

10

20

5

HDPE/LLDPE

10

20

5

Untreated (2% detergent water) Silane treated

Benzoylation treated (benzoyl chloride) Peroxide treated (dicumyl peroxide)

Table 2 Property tests conducted on the fibers and composite samples. Property

Fibers

Composites

Test

Replicates

Chemical Characterization (TAPPI Τ 222 om-02.2002 and TAPPI Useful

j•j

Method UM250) Thermogravimetric Analysis (TGA)

3

Tensile strength at yield of composites (Instron) ASTM: D638

5

Durometer hardness of composites (ASTM: D2240-97)

10

Water absorption of composites (ASTM: D570-99)

3

Chemical Characterization of Fibers In addition to the different types of chemical treatment, untreated flax fibers, silane-, benzoylation- and peroxidetreated fibers were chemically analyzed for hemicellulose, lignin and cellulose contents. The procedure used here for cellulose determination was given by Zobel et al. 191. Lignin content was determined based on TAPPI Τ 222 om02.2002 (acid-insoluble lignin in wood and pulp) and TAPPI Useful Method UM250 (raw material and pulpdetermination of acid-soluble lignin).

Derivative Thermogravimetry The thermal stability of both untreated and chemically treated fibers was investigated using a TA Instruments TGA Q500. The Samples were heated from room temperature up to 500 °C with a heating rate of 10 °C/min and a nitrogen flow of 100 ml/min. Three samples were used to characterize each material. A D T G curve is the differential curve automatically obtained from the TGA.



Science and Engineering of Composite Materials

Mechanical Properties Test Composites having 10% and 2 0 % fiber b y weight loading were prepared and properties were evaluated through mechanical tests. At least five replicate specimens were tested and the results were presented as an average of tested specimens.

Tensile

test

The familiar dog-bone shape o f the rotationally molded sample was utilized in the testing procedure. An Instron Universal testing machine ( S A T E C Systems, Inc., Grove City, P A ) was used to p e r f o r m the tensile strength test at a crosshead speed of 5 m m / m i n as described in A S T M procedure D638-99 ( A S T M 1999), and each test was performed until tensile failure occurred. T h e m a x i m u m (peak) load value (force) (F m a x ) w a s recorded by the instrument, which can be recalled after the completion of the test. T h e tensile strength at yield (aty) is calculated from the following: _

F _ ' max ty=—

ι, \ 0)

CT

where A is the cross sectional area.

Durometer

hardness

Durometer readings were performed according to A S T M D2240-97 ( A S T M 1998). T h e Durometer hardness tester (Shore Instrument and M F G Co., Freeport, N Y ) consists of a pressure foot, an indenter, and an indicating device. Due to the slightly harder sample being e x a m i n e d ; the T y p e D gauge was used. E a c h specimen was subjected to ten Durometer hardness readings at designated positions on the sample bases.

Water Absorption of Composites Rectangular specimens were cut f r o m each sample with dimensions of 25.4 m m χ 76.2 m m . T h e samples were dried in an oven at 50 °C for 24 h, cooled in a desiccator, and immediately weighed to the nearest 0.001 g. In order to measure the water absorption of composites, all samples were immersed in water for about 24 h at room temperature as described in A S T M procedure D 5 7 0 - 9 9 ( A S T M 1999). The percentage increase in weight during immersion was calculated to the nearest 0 . 0 1 % as follows:

. , „ , Wet weight - Initial weight increase in weight (percent) = χ 100 Initial weight

(2)

RESULTS AND DISCUSSION Chemical Characterization of Fibers Natural fibres consist of cellulose, hemicellulose, pectin, lignin and w a x e s ; t h e r e f o r e chemical treatment is necessary to solubalize hemicelluloses, lignin and pectins to obtain purer cellulose content. Chemical treatments have been extensively used for the removal of lignin/pectins surrounding cellulose and destroying its crystalline structure.


49


Vol. 15, No. 1. 2008

The fibers obtained after the chemical treatment contained mainly alpha-cellulose with some hemi-cellulose and lignin. As shown in Table 3, the α-cellulose content in the silane-treated fibers was 95% as compared to the original 73% and the hemicellulose content was reduced to 3%. The same trend was observed in the benzoylation- and peroxide-treated fibers. Chemical analysis of these fibers over different treatments showed a drastic increase in cellulose content and a decline in hemicellulose and lignin content. Lignocellulosic flax fibers contain a trace amount of hemicellulose, which is a hetero polysaccharide consisting mainly of pentoses and hexoses. The treatment of cellulosic, starch, or hemicellulosic materials using different type of chemicals to break down the polysaccharides to simple sugars allows the solubilization of both pectins and hemicelluloses. Dilute sodium hydroxide treatment of lignocellulosic fibers causes separation of structural linkages between lignin and carbohydrates and disruption of lignin structure /10/. The removal of lignin and hemicellulose benefits the interaction between the matrix and the fiber at their interface. Table 3 Chemical analysis of flax fibers after selective chemical treatments.

Other Compounds

α-cellulose (%)

Hemicellulose (%)

Total Lignin (%)

Untreated fibers

73 (±3)

13 (±2)

5(±1)

±9

Silane treated

95 (±1)

1(±1)

3(±1)

±1

94 (±1)

1(±0)

4(±1)

± 1

94 (±1)

2(±0)

3(±1)

± 1

Benzoylation treated Peroxide treated

(%)

Chemical treatments lead to almost pure cellulose fibers, which ensure high stiffness and strength. Although cellulose possesses excellent strength and good stability, it can be degraded by resorting to a variety of chemical and physical processes under certain conditions. Therefore the concentrations of the chemicals used in chemical treatments are very important. The alkali extraction is expected to hydrolyze pectin by a ß-elimination process and solubilize it /1112/. The appropriate concentration of NaOH solution used in mercerization before each type of chemical treatment was completed in the initial work. Sreekala et al. 151 indicated that a 10-30% sodium hydroxide solution produced the best effects on natural fiber properties. Flax fibers were soaked into 2.5, 5, 10, 13, 15, 18, 20, 25, or 30% NaOH solutions before the chemical treatment. It was found that 5%, 18% or 10% of sodium hydroxide solution were the appropriate concentrations for mercerization

before silane, benzoylation or peroxide treatment, respectively. The precise

concentration of soda solution used was low enough to avoid this ß-elimination phenomenon on the cellulose molecules.

Derivative Thermogravimetry of Fibers Cellulose materials are very sensitive to temperature. When cellulose is heated, it undergoes physical and chemical changes, at elevated temperatures around 300 °C, decomposition /13/. As flax fibers were processed by rotational molding at a high temperature (250 °C), finding the degradation temperature of fibers in the composites was essential. In this study, TGA (Thermogravimetric Analysis) was carried out in order to determine the thermal properties of the fibers. However, TGA is not a good tool to characterize the degradation or decomposition temperature of samples.

50




Materials

Rather, in order to address the decomposition of fibers during processing, the onset temperature of the TGA curve is more meaningful. The DTG is useful in those cases where thermograms contain changes so closely spaced that temperature assignments are difficult.

250

300

350

400

Temperature (°C) Untreated - - - - S i l a n e treated

Benzoylation

Peroxide treated

Fig. 2. DTG curve of untreated and chemically treated fibers. The differential curves obtained from the TGA are presented in Figure 2, which shows derivative weight vs. temperature for both untreated and chemically treated fibers. The decomposition peaks were endothermic. Figure 2 concludes that chemically treated fibers were thermally stable in the region below 250 °C. It can be seen that the thermal decomposition of untreated fibers occurs at 240 °C. The silane treated fibers showed a slight shift in decomposition temperature, 15 °C, indicating slightly improved thermal stability of the treated fibers. The onset decomposition temperatures of benzoylation and peroxide treated fibers are 260 °C and 257 °C, respectively. The chemically treated fibers were influenced by changes in chemical composition and the surface porous structure and seemed to degrade more slowly compared to untreated fibers. The materials were not exposed as long to the high operating temperature (250 °C) during the rotational molding process. No colour change was observed in the final composites, although untreated fiber started decomposition below 250 °C. The color change and dark spots on the composites were normally related to degradation of the cellulose fibers caused by a high operating temperature. On the whole, DTG showed that there were no chemical reactions taking place and thermal degrading in chemically treated fibers resulting in large weight reduction in the region below 250 °C.


51


Vol. 15, No. 1, 2008

Tensile Test Tensile testing was p e r f o r m e d along with varied methods of chemical pre-treatment of flax fibers in order to develop a sense of h o w these pre-treatments affected the tensile strength of biocomposites. T h e tensile strength test was also intended to determine which chemical treatments of flax fiber have a positive effect on the mechanical properties of composites w h e n the a m o u n t o f fiber w a s increased ( f r o m 10% to 20%). T a b l e s 4-6 s h o w the tensile strength at yield of 10% flax fibers compared to 2 0 % flax fibers based composites with different types o f thermoplastic matrix. Table 4 The mechanical properties of 10% flax fibers compared to 2 0 % flax fibers based composites with L L D P E .

Durometer Hardness (Tape D

Tensile Strength at Yield S.D.

Materials

S.D. Gauge)

(MPa) Untreated fiber (10%) + LLDPE

15.25

0.6

11.2

0.5

Untreated fiber (20%) + LLDPE

17.56

0.4

20.3

0.3

Silane treated fiber (10%) + LLDPE

15.80

0.3

17.0

0.3

Silane treated fiber (20%) + LLDPE

22.12

0.2

21.7

0.4

Benzoylation treated fiber (10%) + LLDPE

16.13

0.5

16.8

0.1

Benzoylation treated fiber (20%) + LLDPE

17.57

0.7

22.2

0.6

Peroxide treated fiber (10%) + LLDPE

15.62

0.1

20.6

0.5

Peroxide treated fiber (20%) + LLDPE

25.86

0.6

22.4

1.0

Table 5 T h e mechanical properties of 10% flax fibers compared to 2 0 % flax fibers based composites with H D P E .

Tensile Strength at Yield Materials

Durometer Hardness S.D.

(MPa)

52

S.D. (Tape D Gauge)

Untreated fiber (10%) + HDPE

16.82

0.5

18.7

0.3

Untreated fiber (20%) + HDPE

15.40

0.9

22.1

0.2

Silane treated fiber (10%) + HDPE

17.48

0.8

16.2

0.4

Silane treated fiber (20%) + HDPE

21.33

0.1

25.1

0.9

Benzoylation treated fiber (10%) + HDPE

16.82

0.5

16.2

0.5

Benzoylation treated fiber (20%) + HDPE

19.09

0.6

21.4

0.6

Peroxide treated fiber (10%) + HDPE

16.88

0.8

17.1

0.4

Peroxide treated fiber (20%) + HDPE

24.74

0.3

22.7

1.0




Materials

Table 6 The mechanical properties of 10% flax fibers compared to 2 0 % flax fibers based composites with H D P E / L L D P E .

Materials

Tensile Strength at Yield (MPa)

S.D.

Durometer Hardness (Tape D Gauge)

S.D.

Untreated fiber (10%) + HDPE/LLDPE

16.47

0.1

16.9

0.9

Untreated fiber (20%) + HDPE/LLDPE

17.59

0.7

16.3

1.0

Silane treated fiber (10%) + HDPE/LLDPE

16.78

0.2

12.9

0.8

Silane treated fiber (20%) + HDPE/LLDPE

19.89

0.1

20.6

0.2

16.72

0.3

13.1

0.5

28.43

0.5

16.9

0.6

Peroxide treated fiber (10%) + HDPE/LLDPE

17.12

0.7

16.2

0.6

Peroxide treated fiber (20%) + HDPE/LLDPE

29.51

0.4

17.9

0.4

Benzoylation treated fiber (10%) + HDPE/LLDPE Benzoylation treated fiber (20%) + HDPE/LLDPE

N o significant difference in tensile strength at yield was observed between composite specimens having 10% by weight fiber loading, while the tensile strength value of 2 0 % fiber-based composites was improved significantly by the three treatments. This is probably due to the increased fiber-matrix adhesion and a m o u n t of flax, which had high tensile strength when acting as reinforcing material. When compared across chemical treatments the composites containing peroxide-treated flax fibers had c o m p a r a b l e or higher tensile strength than composites containing other chemically treated fibers or untreated fibers. T h e variation in tensile properties could be explained on the basis of the changes in chemical interactions at the fiber-matrix interface on various treatments. T h e tensile strength of flax fiber-reinforced composites is determined both by the tensile strength of the fiber and by the presence of weak lateral fiber bonds /14/. The variations in the tensile strength at yield of the composites on different modifications were attributed to the changes in the chemical structure and bondability of the fiber. The alkali treatment increased the surface roughness that resulted in better mechanical interlocking and it increased the amount of cellulose exposed on the fiber surface. T h e fiber surface silanization resulted in a better interfacial load transfer efficiency. Benzoylation on the fiber improved the fiber matrix adhesion and thereby increased the strength considerably. Peroxide treated composites showed an enhancement in tensile properties due to the peroxide induced grafting. Also, the tensile properties of natural fiber-reinforced composites could be improved by the use of a silane coupling agent. Coupling agents usually improve the degree of cross-linking in the interface region and a perfect bond results.


53

Vol. 15, No. I, 2008


Durometer Hardness The data and numerical results from each durometer hardness test are presented in this section. Tables 4-6 show the hardness of 10% flax fibers compared to 20% flax fibers based composites with different types of thermoplastic matrix. The hardness of plastics was measured by the Shore (Durometer) test. This method measures the resistance of plastics to indentation and provides an empirical hardness value that does not correlate well to other properties or fundamental characteristics. The hardness value is determined by the penetration of the Durometer indenter foot into the sample. The results obtained from this test are a useful measure of relative resistance to indentation of various grades of polymers. Ten readings were taken for each specimen, as material properties were expected to vary with location on the sample. For the 10% fiber-based composites, chemically treated flax fibers did not increase the hardness of specimens. However, for the 20% fiber based-composites, all samples performed well. When compared across fiber pre-treatment types, composites containing chemically treated fibers had higher hardness than the composites containing untreated fibers. As the amount of fiber was increased (from 10% to 20%), hardness increased. The fiber impregnation allows a better fiber-matrix adhesion, which enhances the mechanical interlocking between fiber and matrix. In addition, the modification of the interfacial bond between the reinforcing fiber and matrix by grafting the fibers with polyethylene shows further improvement in mechanical properties.

Water Absorption of Composites Natural fibers are amenable to modifications as they bear hydroxyl groups from the cellulose and lignin. The hydroxyl groups may be involved in the hydrogen bonding within the cellulose molecules thereby reducing the activity towards the matrix. Chemical treatments may activate these groups or can introduce new moieties that can effectively interlock with the matrix. During chemical treatment of the flax fiber, the hemicellulose and lignin were separated and cellulose was used for the biocomposite. After chemical pre-treatment, it would be expected that the water absorption of composites would be lower in flax fiber. Water absorption was measured on a mass percentage basis before and after 24 h of immersion. Figures 3-4 show the water absorption of 10% fiber-based composites and 20% fiber-based composites at room temperature, respectively. In both cases, the water absorption of the chemically treated flax fiber-based composites was lower than that of the untreated fiber-based composites. Chemically treated flax fibers have an additional benefit as a supplementary material for water absorption compared to the untreated flax fibers which do not react this way. Strong intermolecular fibermatrix bonding decreased the rate of moisture absorption in biocomposites. It shows that chemical treatments of flax fiber can decrease the water absorption of the biocomposites. As the amount of fiber increased from 10% to 20%, the water absorption of composites increased, for all types of composites. Physical properties, however, deteriorated due to an increased amount of flax fibers used in the composites. In both groups of 10% and 20% flax fiber-based composites, peroxide treatment performed quite well and showed promise with respect to water absorption. The formulation which showed the best results in the water absorption test contained (by mass) was the 20% peroxide-treated flax fiber, with 75% HDPE/LLDPE and 5% silane-coupling agent.

54




Materials

0.15 t,

α ο "Έ α.

ΙΟ ΕΛ

0.1 -

Λ
Λ £

1-

0.5 -

LLDPE

HDPE

HDPE/LLDPE

Untreated Β Silane treatment Ε3 Benzoylation 0 Peroxide treatment Fig. 4. Water absorption of 20% fiber-based composites.


55


Vol. 15, No. 1, 2008

CONCLUSIONS

Natural fibers have good potential as reinforcements in polymer (thermoplastics) composites. Due to the low density and high specific properties of natural fibers, composites based on these fibers may have very good implications in the automotive and transportation industry. This project focused on determining the ideal chemical pre-treatment and the fiber content of flax fiber-based biocomposite material. The results showed that the fiber content influenced the tensile, hardness and water absorption properties. The 20% flax fiber-based composites had higher tensile strength at yield and Durometer hardness than the 10% flax fiber-based composites. Compared to the untreated fiber-based composites, tensile strength and hardness were improved with a suitable fiber surface treatment. Silane, benzoylation, and peroxide treated fiber-based composites offered superior physical and mechanical properties. It has been demonstrated that the additon of a small amount of silane coupling agent during processing significantly improves the mechanical properties of natural fiber-reinforced plastic composites. Chemical analysis of the cellulose fiber under the different chemical treatments showed an increase in cellulose content and a decrease in lignin and hemicellulose content. Derivative thermogravimetry (DTG) curves indicated that chemically treated fibers were thermally stable in the region below 250 °C and chemcial treatments increased the onset thermal decomposition temperature of flax fibers. The hydrophilic nature of biofibers leads to biocomposites having high water absorption characteristics that can be overcome by treating these fibers with suitable chemicals to decrease the hydroxyl groups in fiber molecules. The water absorption and swelling of the treated flax fiber composites was lower than that of composites based on untreated flax fibers. In the present study, chemically treated flax fiber is used as filler and reinforcing material in thermoplastic matrix through a rotational molding process. The addition of fibers can result in significant material cost savings, as the fibers are available at a cheaper price than more expensive glass fibers or other inorganic additives. At the same time, an improvement in mechanical properties is also achieved. The use of natural fibers as a source of raw material in the plastic industry not only provides a renewable resource, but could also generate a non-food source of economic development for farming and rural areas. Natural fiber reinforced polymer composites and the subsequent applications could be very attractive from the economic point of view. It is worth mentioning that these composites can be used as a substitute for wood products. There will be a high potential and bright future for natural fiber-reinforced composites.

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