BIOCOMPOSITES BASED ON REGENERATED CELLULOSE FIBER … · Fiber Treatments Alkali Treatment...

BIOCOMPOSITES BASED ON REGENERATED CELLULOSE FIBER

AND BIO-MATRIX

Sunil Kumar Ramamoorthy, Chanchal Kundu, Kayode Adekunle, Mikael Skrifvars

School of Engineering, University of Boras, Sweden

Abstract

Wood pulp based regenerated cellulose fibers like Lyocell and viscose which are from

natural origin have high and even quality; used to develop superior composites with good

properties. In this project, Lyocell and viscose fibers were reinforced in chemically modified

soybean based bio-matrix, acrylated epoxidized soybean oil (AESO) by compression molding

technique. The composites are characterized for mechanical performance by tensile, flexural

and impact tests, viscoelastic performance by dynamical mechanical thermal analysis (DMTA)

and morphological analysis by scanning electron microscopy (SEM). In general, Lyocell

composites had better tensile and flexural properties than viscose based composites. The same

goes with elastic and viscous response of the composites. Hybrid composites were formed by

fiber blending; on addition of Lyocell to viscose based composites improved the properties. The

amount of Lyocell and viscose fibers used determined the properties of hybrid composites and

the possibility of tailoring properties for specific application was seen. Hybrid composites

showed better impact strength. Morphological analysis showed that the viscose composites had

small fiber pull out whereas Lyocell composites had few pores. Hybrid composite analysis

showed that they had uneven spreading of matrix; delamination occurred on constant heating

and cooling.

To overcome the above mentioned issue and to reduce the water absorption, surface

modification of the fiber was done by alkali treatment and silane treatment. The effect of

treatment is done through swelling, water absorption and morphological analysis tests. The

properties could be increased on proper modification of the fibers. The results show the good

potential of these composites to be used in automotives and construction industries.

Background and Requirements

Researchers developed several biocomposites from natural fibers which are used in

automotives and construction. The potential of these composites is high and the possibility of

replacing synthetic fibers is more. The main drawbacks of the natural fibers being used in

composites are hydrophilic nature and quality variation which is due to plant maturity, place

dependent, harvesting method etc. Few industries like automotives need smell-free products

and this adds to disadvantages of the natural fiber reinforced composites. Several authors have

addressed the problems associated with water absorption by chemical modification of the

surface of the fiber. But the quality variation is difficult to control as various factors come into

consideration.

Wood pulp based regenerated cellulose fibers like Lyocell and viscose which are from

natural origin have high and even quality; used to develop superior composites with better

properties.2,3 These types of composites are not explored as much as natural fiber composites.

Some researchers have been trying to explore the possibilities of using these fibers as

reinforcements in structural composites. As the fibers come from natural origin; it would be

beneficial for the environment. Green composites could be formed when these fibers are

reinforced in bio-matrix.

However the processes of producing these regenerated fibers are not completely

environmentally friendly. Ongoing research activities to produce these fibers in environmentally

friendly way have many challenges. It is necessary to have closed loop cycle and minimize the

toxic chemicals.4,5 The same should be done with bio-matrix as small amounts of synthetic

materials are used in producing these resins. The amount of work done on natural fibers

composites is enormous and these fibers are the most environmentally friendly reinforcements.

On considering that these fibers are not produced globally and limited to certain parts; an

alternative is required to avoid transportation and import of these fibers. One of the main

challenges in natural fiber composites is that these fibers give irregular results due to

unevenness of the fibers.

Materials and Methods

Two types of regenerated cellulose fibers such as Lyocell and viscose were used as

reinforcements. Lyocell fibers were supplied by Lenzing AG, Austria and it has specific gravity

1.5 gm/cm3, linear density 1.7 dtex, average fiber length of 38 mm and diameter of 13.4 µm.

Non-woven Lyocell mats were formed by carding and needling as reported by Adekunle1 with a

surface weight of 525 g/m2. The viscose fiber non-woven mats were supplied by Suominen

Nonwovens Ltd, Finland, and they had a surface weight of 60 gm/m2 and a sheet thickness of

0.66 mm. Bio-based thermoset resin AESO (Acrylated Epoxidized Soybean Oil) derived from

soybean oil was used as matrix, it is commercially available as Tribest S350-01 EXP supplied

by Cognis GmbH, Germany. The cross linking initiator (tertiary-butyl peroxy benzoate) was

supplied by Aldrich Chemical Company, Wyoming, IL, USA. Sodium hydroxide pellets, 3-

aminopropyl-triethoxysilane and absolute ethanol were supplied by sigma aldrich.

Fiber Treatments

Alkali Treatment

Regenerated cellulose fibers were pre-dried at 105 ˚C for 2h before immersing in sodium

hydroxide (NaOH) solution with three different concentration, 4wt%, 5 wt% and 6 wt% and

stirred continuously at 25 ˚C. The fibers were treated for 24h, 48h and 72h. After the treatment,

fibers were washed thoroughly with distilled water for neutrality. The pH is checked periodically

using litmus paper. Then the fibers were dried in room temperature for 4h followed by oven

drying for 3h at 105 ˚C. The similar treatment was done at 50 ˚C.

Silane Treatment

APTES (3-aminopropyl-triethoxysilane) was used as silane coupling agent to treat the

regenerated cellulose fibers. APTES was added to ethanol-water mixture (8/2 volume ratio) to

make three different concentration, 2wt%, 4wt% and 6wt%. Pre-dried regenerated cellulose

fibers (105 ˚C, 2h) were immersed in three different concentration solutions and stirred

continuously at 25 ˚C. The treatment was done for three time intervals, 24h, 48h and 72h.

Fibers were washed thoroughly with distilled water after treatment and pH is checked for

neutrality. Then the fibers were dried in room temperature for 4h followed by oven drying for 3h

at 105 ˚C. The similar treatment was done at 50 ˚C.

Composite Preparation

Acrylated epoxidized soybean oil was used as a matrix to make composites. Viscosity of

resin is the main factor influencing fiber impregnation. The viscosity of AESO was reduced by

heating in oven at 60˚C for 5 minutes. AESO was then blended with initiator for high

temperature curing. Tert-butyl peroxybenzoate was used as a free radical initiator (2 wt %) and

was mixed well with AESO to give a homogeneous solution.

The treated and untreated fiber mats were cut to 20cm×20cm dimension. The fibers were

impregnated with the blended resin and the fiber-resin ratio was taken as weight fraction.

Composites were made with different weight fraction of regenerated cellulose fiber by

compression molding. This method was adopted to fabricate the composites. Curing was done

with heat and pressure for 5 min. Pressure (40 bar) was used to make composites and at a

temperature between 160oC to 170oC on hot press from Rondol Technology, Staffordshire, UK.

Specimens were cut according to ISO standard by using laser cutting technology (GCC

LaserPro Spirit).

Characterization

Mechanical performance is characterized by tensile, flexural and impact tests. Dynamic

mechanical analysis was done see the viscoelastic properties. Water absorption test was done

to see the hydrophilic nature of the composites. SEM images were made to see analyze the

morphological properties.

Tensile Test

The tensile test was carried out based on ISO 527 using universal Tinius Olsen H10KT

testing machine and QMat software. The rate of loading was 10 mm/min and the load range

was 5 kN and 10 kN. Atleast 10 specimens were analyzed for each sample. The dumb-bell

shaped specimens were cut from the laminates with laser cutting machine with overall length of

150 mm which includes parallel-sided portion (60 mm). The widths at ends and at parallel-side

are 20 mm and 10 mm respectively. Gauge length was 50 mm and initial distance between

grips was 115 mm.

Flexural Test

The three point flexural test was adopted to determine the flexural stress (σf) and flexural

modulus (Ef). The tests were performed based on ISO 14125 standard using a Tinius Olsen

H10K-T UTM (universal testing machine). At least 5 specimens were tested for every sample.

The specimen dimension was 80×15mm (length×width) while the thickness varied depending on

the sample. The load range was 5 kN and 10 kN and the rate of loading was 10 mm/min. The

outer span was 64 mm and the displacement range was 10 mm.

Impact Test

This test is performed based on ISO 179 to obtain the charpy impact strength of un-notched

specimens. Atleast 10 specimens were tested for each sample using Zwick test instrument, and

mean impact resistance was determined. The specimens were tested flatwise.

Charpy impact strength = [energy absorbed/cross-sectional area]

Dynamic Mechanical Thermal Analysis

Dynamic mechanical thermal analysis (DMTA) was done to determine the viscoelastic

properties of the composites with Q series TA instrument supplied by Waters LLC, Newcastle,

DE, USA. Dual cantilever clamp was used to mount the specimens. The specimen’s dimension

was 35 × 10 × 2 mm3 and the temperature ranges from 30˚C to 150˚C at frequency 1 Hz.

Water Absorption Test

Water absorption test was carried out on the samples to determine the dimensional stability

of the composites. Four specimens were examined for each sample and the average was taken.

The specimen dimension was approximately 36×12 mm. The specimens were dried in an oven

for 24 hr at 60oC. Then the specimens were kept in desiccators in order to cool down to room

temperature and the weights of these specimens were denoted as Wo. Then specimens were

immersed in distilled water at room temperature. The amount of water absorbed was measured

every 24 hours for 10 days. The specimen was taken out of the water each time, and the

surface wiped dry and weight recorded as W. The percentage water absorption (WA %) was

then calculated using the formula below.

WA% = [(W-Wo)/Wo] × 100

Scanning Electron Microscopy

Morphological analysis was done by studying the cross section of the fractured specimens

by environmental scanning electron microscope (ESEM), FEI Quanta 200 F. The equipment

was run at low vacuum and high voltage (5-10 Kv). This is done to see the fiber-matrix interface

and pores.

Swelling

Treated fibers were subjected to swelling measurement. Swelling of the fibers were noticed

by measuring the diameter of the fibers through microscope. Fibers were treated for 30 minutes

before measuring the diameter. Several measurements were made and mean was taken.

Weight Loss

Weight loss was checked for fibers treated for 30 minutes, 12 hours and 24 hours. Dried

fibers were weighed and noted. Then fibers were treated to respective time and dried before

checking the weight loss.

Results and Discussion

Tensile Test

Table 1 shows that the tensile strength of Lyocell fiber reinforced composites was higher

than the viscose fiber reinforced composites and the hybrid composites, which indicated that the

Lyocell reinforced composite was the toughest and strongest. Composite consisting 60 wt%

Lyocell fiber had tensile strength of 135 Mpa. Whereas for the same fiber content, the tensile

strength was approximately 96 MPa and 117 MPa in the viscose fiber reinforced composite and

hybrid composite respectively. This is because Lyocell fiber on its own has higher tensile

strength (750 MPa) compared to viscose fiber (310 Mpa). The general trend of an improvement

of tensile strength of all composites was associated on increasing Lyocell fiber. For Lyocell fiber

reinforced composite, the tensile strength increased from 113 MPa to 135 MPa with 20% fiber

increase and the same trend was observed for viscose fiber reinforced and hybrid composite.

The uniformity in the results could be due to the consistency in impregnation, curing condition,

sheets alignment (all fiber sheets were aligned in 00).

The tensile moduli of the Lyocell and viscose fiber composites increased with an increase in

fiber content from 40 wt% to 50 wt%, but surprisingly there was no increment in the modulus

when the fiber content was increased from 50 wt% to 60 wt% in the hybrid composite. An

interesting trend was observed in the hybrid composite when 20 wt% of Lyocell was hybridized

with 30 wt% viscose fiber because the tensile modulus increased, whereas the tensile strength

was lower in the hybrid composite with 25 wt% Lyocell and 25 wt% viscose.

The percentage elongation was quite good for viscose fiber reinforced composites which

was approximately 2.5 %. This was expected due to the higher percentage elongation of

viscose. For Lyocell fiber reinforced composites, it was less than 2% irrespective of fiber

content. It was expected, due to the morphology of regenerated cellulose fiber. For the hybrid

composites, the values were more or less in between 2 to 2, 5 % for all compositions.

Table 1: Tensile Properties of the Composites

Product Composition

Tensile Strength (MPa)

Tensile Modulus (GPa) Elongation %

Lyocell/Resin-40/60 113.3 14.17 1.95

Lyocell/Resin-50/50 124.15 16.77 1.74

Lyocell/Resin-60/40 135.42 16.64 1.89

Viscose/Resin-40/60 77.48 9.39 2.44

Viscose/Resin-50/50 91.6 12.68 2.56

Viscose/Resin-60/40 95.94 12.1 2.6

Lyocell/Viscose/Resin-20/20/60 94.22 11.28 2.31




Flexural Test

The hybrid composite had the highest flexural strength compared to Lyocell and viscose

fiber reinforced composites. Composite consisting 60 wt% fiber content (30 wt% Lyocell & 30

wt% viscose), the flexural strength was approximately 140 Mpa; whereas the flexural strength

was 127 MPa and 92 MPa for Lyocell and viscose fiber reinforced composites respectively with

same fiber content (Table 2). So, the effect of hybridization was quite significant in case of

flexural strength of composites. For the viscose fiber reinforced composite, the flexural strength

followed a downward trend with the increase of fiber content; it reduced from 101 MPa to 92

MPa for 20 wt% fiber increase. But for other cases, there was noticeable increase in flexural

strength with fiber content increase.

The Lyocell-reinforced composite had the highest flexural modulus of about 7 GPa for 60

wt% fiber content. Hybridized composite and viscose fiber reinforced composite had flexural

modulus of 6 GPa and 5 GPa respectively for the same fiber content. However the effect of

hybridization was negligible, increasing the viscose fiber content did not have effect on the

flexural modulus of the composite.

Table 2: Flexural Properties of the Composites

Product Composition

Flexural Strength (MPa) Flexural Modulus (GPa)

Lyocell/Resin-40/60 108.49 6.46

Lyocell/Resin-50/50 118.81 6.17

Lyocell/Resin-60/40 127.04 6.79

Viscose/Resin-40/60 101.18 5.16

Viscose/Resin-50/50 98.29 5.2

Viscose/Resin-60/40 92.92 5.28

Lyocell/Viscose/Resin-20/20/60 91.83 4.48




Impact Test

Table 3 shows the impact resistance properties of the composites which indicates the

amount of energy absorbed in the cross sectional area of a material. The Lyocell fiber

composites showed impact resistance between 40 and 50 kJ/m2. Viscose and the hybrid

composites have impact resistance between 45 and 50 kJ/m2. It was noticed that the increase in

fiber content imparted higher impact resistance in composites materials. It resulted in longer

average fiber pull-out lengths, and therefore caused higher impact strength. In contrast, higher

fiber-matrix adhesion results in shorter average pull-out lengths and make the material brittle

and that ultimately induce lower impact resistance in the material. However these results are

expected because natural fibers are quite different from regenerated cellulose fibers due to their

morphology. In previous studies the impact behavior of regenerated cellulose fiber reinforced

composites is quite different from natural fiber reinforced composites.

Table 3: Impact Properties of the Composites

Product Composition Impact Strength (kJ/m2)

Lyocell/Resin-40/60 42.35



Viscose/Resin-40/60 44.69



Lyocell/Viscose/Resin-20/20/60 35

Lyocell/Viscose/Resin-20/30/50 45.93



Dynamic Mechanical Thermal Analysis

Table 4, The storage modulus (E') is a measure of elastic response of a material, and

Lyocell fiber composite had the highest storage modulus and variation in fiber content wt% had

effect on results. Similar trend was observed for viscose fiber reinforced composites and the

hybrid composites. The hybrid composites seemed to have the least storage modulus which

could be due to delamination during constant heating and deformation for about 1 hour in the

equipment and the possibility of mismatch in the hybrid composite structure. In this case,

Lyocell fiber and viscose fiber were combined, so a micro-structural analysis of a transverse

section of the specimen could give a better explanation.

Table 4: Viscoelastic Properties of the Composites

Product Composition E' (MPa) E" (MPa) Tg tanδ

Lyocell/Resin-40/60 4805.33 233.23 82.86 0.049

Lyocell/Resin-50/50 3956.33 207.5 83.95 0.052

Lyocell/Resin-60/40 4672 237.8 82.5 0.051

Viscose/Resin-40/60 3761.67 196 82.38 0.052

Viscose/Resin-50/50 3599.33 186.97 81.95 0.052

Viscose/Resin-60/40 3542.33 176.27 81.22 0.05

Lyocell/Viscose/Resin-20/20/60 3428.33 201.17 81.32 0.059


Lyocell/Viscose/Resin-20/30/50 3252 189.3 84.87 0.058


The loss modulus indicates a materials response to a viscous behavior. Loss modulus of

Lyocell fiber composites were superior and was between 200 to 250 MPa for different fiber

content. So, from that perspective the Lyocell fiber composites exhibit the best viscoelastic

properties compared to others. For viscose fiber reinforced composites, the loss modulus was

nearly 200 Mpa.

The Tg values were measured from the tanδ curve and it was in between 800C to 850C for

Lyocell and viscose fiber reinforced composites. But it was a bit higher in hybrid composites with

50 wt% and 60 wt% fiber content. Though the value of Tg will vary depending on which

parameter used to detect the transition. It also depends on the experimental parameters such

as frequency of oscillation, temperature ramp rate and sample dimensions and it is expected

that Tg should be measured on a material which is not under mechanical stress.

The structural or material damping of a composite material could be analyzed using DMTA.

Tanδ is the ratio of the loss modulus (E") to storage modulus (E') or it could be defined as the

ratio of the energy lost to the energy retained during a loading cycle. And the values of tanδ

were measured at 350C in this study. The most significant result was obtained from the hybrid

composites with a tanδ of approximately 0.06. This result indicates that the hybridization has

optimized the good structural damping properties in the composite materials and that could be

considered for automotive application.

Scanning Electron Microscope

Alkali and silane treatments made the surface of fiber rougher. At higher concentration or

longer treatment exposure, treatment had adverse affects due to fibrillation. Fibrillation occurred

also at increased treatment temperature.

Figure 1: Untreated Fiber Figure 2: Alkali-treated

Figure 3: Fibrillated Fiber(at higher Concentration Figure 4: Fibrillated Fiber(at longer time)

Figure 5: Dispersed Fiber in Composite

Figure 1 shows the SEM image of untreated fiber which has smoother surface than alkali

treated fiber, figure 2. Rougher surface in figure 2 helps in improving the properties. Optimizing

the treatment temperature, time and concentration is important as higher treatment conditions

gives adverse effects like fibrillation. Figure 3 and 4 shows the fibrillation in fibers which has

extreme treatment conditions. Figure 5 shows the dispersion of fibers in matrix.

Figure 6b shows the pores in composites and this could affect the properties of the

composites. This should be addressed to have better composites. Uneven spreading of matrix

was seen in hybrid composites which could be due to hybrid structure.

Figure 6. SEM images ; (a) Viscose-matrix interface, (b) Lyocell-matrix interface, (c)

Viscose fiber pull-out in hybrid specimen and (d) Hybrid composite specimen

Swelling

Swelling increased on increasing the concentration of treatments and reaches highest

swelling at 13 wt% concentration. This is seen in both alkali and silane treatments, 51.8% and

9.4% were corresponding swelling %. This is quite similar with natural fibers as alkali treatment

gives higher swelling.

Table 5: Swelling and Weight Loss % of the Composites

Weight Loss

Weight loss is seen after treatment of fibers on different treatment concentration for different

treatment time at room temperature. Highest weight loss was noticed at 10 wt% for alkali and

silane treatments, table 5.

Summary and Next Step

Composites made out of regenerated cellulose fibers had good properties. Treatments

improved the surface roughness. Swelling and weight loss was noticed on various treatment

concentrations. It was noticed that treatment time, temperature and concentration affects the

fiber. It is necessary to improve the properties by reducing the pores and have optimized

treatments. It is also required to look into hybrid structure to have better properties.

References

1. Adekunle KF. Bio-based Composites from Soybean Oil Thermosets and Natural Fibers. PhD Thesis,

Department of chemical and biological engineering, Chalmers University of Technology, Sweden,

(2011).

2. George L. Handbook of composites. 2nd

ed. Chapman & Hall, (1998).

3. Allin SB. Polymer Science and Technology, 2nd Edition (Joel R. Fried). Journal of Chemical

Education (2004); 8: 809.

4. Borbély É. Lyocell, The new generation of regenerated cellulose. Acta Polytechnica Hungarica

(2008); 5: 11-18.

5. Carrillo F, Colom X and Cañavate X. Properties of regenerated cellulose lyocell fiber-reinforced

composites. Journal of Reinforced Plastics and Composites (2010); 29: 359-371.

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Transcript of BIOCOMPOSITES BASED ON REGENERATED CELLULOSE FIBER … · Fiber Treatments Alkali Treatment...