A Novel Technology to Improve Strength and Service Life of

Natural Fiber Composites Seminar by

RANJITH

Introduction Natural Fiber Composites (NFCs) are materials in which at least the

reinforcing fibers are derived from renewable and carbon dioxide

neutral resources such as wood or plants.

Automotive & Construction: largest segments among natural fiber

composite applications. The market size for natural fiber composites

is projected to reach $5.83 billion by the year 2019.

Major challenges for natural fibers composite industry is natural

fiber quality, sensitive to humidity, not suitable for applications

requiring optimal strength.

Performance improvement in materials will drive growth for natural

fiber composites in new application areas.

Door Panel Package Tray

Seat Back

Roof sheet Fence

Corrugated Panel

Z-Axis Reinforcement ‘‘Wet-Flocking” Technology

All laminated composite materials suffer from service life limiting

delamination failure.

This poor inter-laminar shear strength stems from lack of Z-

direction inter-ply fiber reinforcement.

Traditional methods to impart the Z-direction inter-ply fiber

reinforcement (multi-directional knitting, 3-D weaving or stitching

and Z pinning) are not cost-effective.

University of Massachusetts at Dartmouth (UMD) developed a

novel Z-axis fiber reinforced resin laminar composite materials

processing technique.

Working Principle This technique involves the electro-static pre-placement of

perpendicularly oriented short fibers between the plies during

a composite’s fabrication/lay-up stage.

Short fiber placement is accomplished by electro-statically

propelling short fiber elements at an uncured polymer (fluid)

matrix coated fibrous ply layer of the composite.

This electrostatic fiber coating process is called ‘‘flocking”.

Electro-static Flocking unit

Experimental Work

To characterize the influence of

•Surface treatments

• Reinforcement architecture

• Applied through-thickness microfiber reinforcement

on the delamination resistance of jute fiber/epoxy laminated

composite materials.

Tensile and in-plane shear tests were also conducted to gain a

more holistic understanding of the effect of through-thickness

reinforcement on mechanical properties.

Materials Reinforcement

• Plain weave jute fabric with areal weight of 330 g/m2

• Unidirectional jute yarn pre-forms with a linear density of 1440 tex

Matrix material

• Two part Epoxy resin system 2000/2120 series amine-cure with hardener to resin

mass ratio of 27:100

Materials for Fiber Surface Treatment

• Sodium Hydroxide pellets (NaOH), acetic acid,

3-Triethoxysilylpropylamine (APTES), phenolphthalein indicator

• Dow Corning Z-6173 silane coupling agent

Flock fiber for Z-axis reinforcement

• Nylon fibers 3-denier, 1.3 mm length

Fiber Surface TreatmentAlkali Treatment: Jute fabrics and preforms were soaked for 2 hours in a 5% (w/w)

aqueous NaOH alkali solution then rinsed with distilled water and soaked in a 2% aqueous acetic acid solution for an additional 1 hour.

Neutrality of the fabrics and pre-forms was verified using a phenolphthalein solution followed by thorough rinsing with distilled water and drying at 80˚C overnight.

This treatment swells the amorphous regions of the fibers and removes approximately 41% of the hemicellulose.

This has the effect of “fibrillating” the jute structure & increases its effective surface area giving more area for the epoxy matrix to penetrate and adhere to.

Silane Treatment:

A solution of 1% Dow Corning Z-6173 silane agent in methanol was prepared.

Alkali treated, jute fabrics and preforms were soaked in the prepared silane treatment solution for 1 h. Two silane agents were used and compared: APTES and Dow Corning Z-6173.

This treatment aims to increase the affinity of the cellulosic parts of the jute fibers with the epoxy matrix by grafting coupling agents onto the cellulose chain.

Efficacy of treatments was confirmed using Fourier Transform Infrared Spectroscopy (FTIR) performed on a Digilab Excalibur FTS 3000MX spectrometer.

Natural Fiber Composite Panel Fabrication

The desired number of 260 mm × 260 mm plies cut from the fabric roll were laid up in a [0/90]n sequence and pre-compacted in a hydraulic press.

The vacuum infusion procedure was adopted for NFC panel fabrication.

For Z-axis inter-ply reinforcement, nylon flock fibers were applied to the individual laminate plies using ‘‘wet-flocking” technology.

After which the laminate were laid up and infused with resin following the standard vacuum infusion procedure.

Double Cantilever Beam Testing To introduce the initial crack, a thin Teflon film is inserted in the

mid-plane of the laminar panel stack during the fabrication of the composite.

The test coupons were cut to the desired dimensions according to ASTM D5528.

Piano hinges were adhered to the ends of the delaminated ‘‘arms” using a quick-set epoxy.

In The final step sample preparation was to apply measured tick marks on the sample sides by first applying correction fluid and then marking every 1 mm for the first 25 mm past the initial delamination with black ink.

Standard Double Cantilever Beam (DCB) Specimen with Piano hinges

Double Cantilever Beam (DCB) Test samples

Reinforcement Z-axis flock fibers Thickness (mm)

Silane-treated plain weaveNo 11.0 (±0.08)

Yes 15.0 (±0.09)

Silane-treated unidirectionalNo 7.08 (±0.07)

Yes 11.6 (±0.13)

Table 1: Details of the fabricated NFC panels

Results and Discussion• Surface Treatment The purpose was to remove impurity and hemicellulose on the

fiber surface and to impart chemical accessibility to hydroxyl group of cellulose to promote silane coupling agent reactivity.

The average percentage weight loss for alkali treated jute yarns, was found to be (11 ± 1.7)%

FTIR analysis of the alkali treated yarns showed a loss of peaks at 1735 and 1240 cm-1 implies that a portion of the hemicelluloses was removed by this treatment.

This finding supports the successful partial removal of hemicellulose and other impurities.

FTIR analysis of the silane treated yarns showed that a similar

spectral peak near 1100 cm-1 for yarns treated with APTES and Z-

6173.

Peaks near 1100 cm-1 wavenumber indicate that Si–O–C bonds are

formed, which suggests that the silane coupling agent reacted to

cellulose hydroxyl groups.

This implies that the agent has chemically coupled itself to the

fiber structure rather than simply being physically adhered to the

surface.

FTIR spectra of raw and alkali-treated jute fiber

FTIR spectra of APTES and Dow Corning Z- 6173 treated jute fiber.

Mechanical Performance Properties For both types of reinforcement architecture, fiber surface

treatment increased fracture toughness.

Z-axis inter-laminar reinforcement fibers had a profound positive effect on the fracture toughness of laminated composites for both pre-form architectures tested in this study.

The addition of this reinforcement increased fracture initiation toughness by 81% for plain weave reinforcement, and 65% for unidirectional reinforcement.

These dramatic increases in fracture toughness are as a result of energy consumed by the flock fiber pull-out and fiber breaking.This required an additional amount of energy to overcome for crack growth, thereby increasing fracture toughness.

Change in fracture toughness by flock based Z-axis fiber inter-ply reinforcement

Scanning electron microscope (SEM) images of DCB fracture surfaces

It is important to examine its effect on other in-plane mechanical properties to obtain a more holistic view of the effect of this type of reinforcement.

For both plain weave and unidirectional reinforcement architectures, tensile strength and modulus experience losses with the addition of Z-axis reinforcement.

Effect of Z-axis reinforcement on (a) Young’s modulus and (b) Tensile strength

A comparison plot of the ILSS of silane-treated composites with and without Z-axis flock reinforcement.

Interestingly, ILSS is seen to increase by 13% and 18% for plain weave and unidirectional reinforcement, respectively.

This increase is a result of flocking fibers crossing the plane of shearing, thus providing additional resistance to that loading.

Effect of ‘‘flock” reinforcement on ILSS.

Specific Modulus Comparison One of the major advantages of natural plant-based fibers is their

low density, and high specific properties.

To examine how these materials would compete in today’s composite material market, it is important to evaluate the strength properties of natural fiber composites on a per unit mass basis.

Therefore to explore the potential of structural application of

NFCs, the specific moduli of jute fiber/epoxy composites in this study were compared with that of glass/epoxy composites.

The values for specific modulus of glass fiber/epoxy composites were taken from literature available.

Surface treated plain weave jute/epoxy composites have a 28% greater specific modulus than plain weave glass/epoxy composites.

Specific modulus of surface treated UD jute/epoxy composites is only 10% lower than that of UD glass/epoxy composites.

Specific modulus of jute fiber and glass fiber reinforced epoxy composites.

ConclusionsThis study explored the influence of surface treatments, reinforcement architecture, and applied through-thickness microfiber reinforcement on the interlaminar fracture toughness of jute fiber/epoxy laminated composite materials.During this process, the following conclusions were made:

A combination of alkali and silane grafting surface treatments increased delamination resistance of jute/epoxy composites by 10% due to an increased interfacial bond strength.

The use of University of Massachusetts Dartmouth’s patented Z-axis reinforcement ‘‘wet-flocking” technology was successfully extended to the vacuum infusion method, thereby increasing its applicability.

Through-thickness microfiber reinforcement greatly improved the delamination resistance of these composites (81% plain weave, 65% unidirectional), but degraded in-plane tensile properties by a small value due to the introduction of resin rich layers with non-aligned reinforcing fibers.

Through-thickness reinforcement also improved ILSS by 13% and 16% for plain weave and unidirectional reinforcement, respectively, as the reinforcing fibers bridged the plane under shear loading.

This shows that on stiffness per unit mass basis, the composites produced in this study can compete with more traditional glass-fiber reinforced composites. Coupling this with the increased sustainability and low energy of production of the jute fibers makes them a very attractive choice for polymer composite reinforcement.

REFERENCES

• Pinto M, Chalivendra V, Kim Y, Lewis A, “Evaluation of surface treatment and fabrication methods for jute fiber/epoxy laminar composites”, Polym Compos- 2014

• M. Pinto, V.B. Chalivendra, Y.K. Kim, A.F. Lewis, “Improving the strength and service life of jute/epoxy laminar composites for structural applications”, College of Engineering, University of Massachusetts Dartmouth, Massachusetts, USA-2015.