Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal 24-28 July 2016

Editors J.F. Silva Gomes and S.A. Meguid

Publ. INEGI/FEUP (2016)

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PAPER REF: 6239

MECHANICAL ANALYSIS OF RECYCLED DENIM FIBRE

COMPOSITES

Josh Illingworth, Yu Zhang(*), Nick Tucker

School of Engineering, University of Lincoln, Lincoln, LN6 7TS, U.K. (*)Email: yzhang@lincoln.ac.uk

ABSTRACT

Cotton is one of the oldest polymer composite reinforcements. There is considerable interest

in denim twill composites for artisanal materials such as knife handles and guitar scratch

plates, but as yet there has been little work to quantify the mechanical performance of the

materials with a view to extending the areas of application. The focus in this research is to

determine whether recycled denim cotton twill coupled with current commercial

manufacturing techniques can be used to make practical composite materials, offering a

cheaper replacement to glass or virgin natural fibre (flax) reinforcements in semi-structural

applications. A wet lay-up vacuum consolidation process with epoxy resin is used to

manufacture composite laminates. Glass was shown to provide the highest tensile and flexural

strength with recycled denim showing the lowest. However recycled denim showed

comparable values of tensile and flexural strength to that of other fibre reinforced polymer

composites. This allows denim to offer a suitable replacement to glass fibre composites in

application with a lower load bearing. Recycled denim also showed a reduction in cost when

material weight is compared for the price paid.

Keywords: Natural fibres, recycled Denim, tensile, flexural.

INTRODUCTION

The first mass application of composite materials was in the manufacture of aeroplane

propellers at the beginning of the last century (Caldwell and Clay, 1924), and cotton

(Gossypium sp.) reinforcement was used to improve impact strength. Crop origin fibres offer

a sustainable source for composite reinforcements which approach material carbon neutrality,

making them a viable option to replace synthetic fibres. However natural fibres cannot

directly compete with the mechanical properties of carbon fibre (Cook, 1984), but have been

seen to rival the specific mechanical properties of glass fibre (Pickering, 2008; Bongarde and

Shinde, 2014). Recycled denim is widely in use as fibre reinforcement for the artisanal

manufacture of semi-structural articles such as knife handles and guitar scratch plates, but

little has been done to examine the mechanical properties and cost of recycled denim

composites. To encourage commercial uptake of this material, this paper examines the cost of

recycled denim composites against their mechanical performance compared to glass and flax

reinforcement.

The production of fibres for composites can impact on the environment, with natural fibres

providing a more environmentally friendly option to that of synthetic. One difference between

the two categories of fibres is a reduced carbon footprint. As already mentioned natural fibres

are in some cases carbon neutral as they take in more carbon dioxide than is used in

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cultivation. Natural fibres also consume less fossil energy per kg than synthetic fibres, for

example S.V. Joshi et al. found that glass fibre took 54.7 MJ/kg to produce whereas flax

(Linum usitatissimum) fibre only took 9.55 MJ/kg, and China reed (Miscanthus giganteus)

consumed less with a value of 3.64 MJ/kg. This significant reduction in energy consumption

when using natural fibres over synthetic fibres will help to reduce industrial production costs.

Using a recycled natural fibre will further reduce the impacts made to the environment, which

includes a reduction in new raw material and reduced carbon emissions when choosing

recycling over disposal (Craighill and Powell, 1996). Not only will it make the material a

more sustainable choice but it will also reduce the cost of product, as it will be purchased as a

waste product. The use of denim also provides a marketing edge that has previously been

exploited by General Motors and Volkswagen who have both offered models trimmed with

denim.

EXPERIMENTAL

Materials

The materials used inside this experiment were glass fibre (woven, 100 gsm), flax fibre (twill

woven, 100gsm) and recycled denim fibre (twill woven). The glass and flax were supplied by Easy

Composites in 10m x 1m mats, the recycled denim was sourced from charity shops and donations in

the form of jeans. The matrix used was a mix of EL2 epoxy casting resin with either a slow or fast

hardener supplied by Easy Composites.

Composite Fabrication

All composite samples were produced using a wet lay-up technique with vacuum bagging

finish. All samples were produced to possess a similar weight content of fibre, this meant flax

and glass samples contained 6 plys with denim containing 2 plys. All samples used a constant

ply orientation. Glass/epoxy samples were fabricated using a fibre to resin wt% of 50:50, with

flax/epoxy samples using a 40:60 fibre to resin mix, both glass and flax samples used a slow

hardener. When manufacturing the recycled denim composite samples a wt% of 30:70 fibre to

resin mix was used. Samples of recycled denim were produced using both slow and fast

hardeners. The epoxy resin and hardener was mixed using a ratio of 100:30, the resin

hardener mix was degassed inside a degassing chamber using an ECVP425 vacuum pump,

both provided by Easy Composites. After the fibres were laid up with resin, they were placed

into a vacuum bag set to draw 20% vacuum and left to cure for 24h. The composites were

produced into samples for tensile and flexural tests through the use of a Warco 58” foot

operated treadle shear. The samples were produced into 15mm wide strips, with the sample

length being determined by the testing machine.

Tensile/Flexural Testing

Tensile tests were carried out using an Instron 3345 single column tester using a 5kN load cell

fitted with screw action grips. All specimens were measured before testing using dial gauge

callipers to ensure the correct dimensions were entered into the software. The specimens were

then tested at a rate of 10mm/min with a fixed length of 90mm for denim A and 110mm for

the remaining materials. Flexural Tests were carried out with the same machine using a 3

point bending attachment with a 5kN load cell. The flexural specimens were tested at a rate of

20mm/min with a support span of 50mm.

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SEM Imaging

SEM imaging of each composite type was carried out using a JEOL NeoScope5000, images

were taken looking at the fracture faces directly. Samples were sputter coated in EMITECH

K550X before being placed into the SEM machine.

Tensile Testing

Fig. 1 - Maximum Tensile Strength of Fibre Reinforced Epoxy Composites

Fig. 2 - Tensile Modulus of Fibre Reinforced Epoxy Composites

0

50

100

150

200

250

300

Ma

xim

um

Te

nsi

le S

tre

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MP

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Composite Samples

Maximum Tensile Strength of Denim, Flax and Glass Epoxy

composites

Denim A

Denim B

Flax

Glass

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3

4

5

6

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Composite Sample

Tensile Modulus of Recycled Denim, Flax. Glass Epoxy

Composites

Denim A

Denim B

Flax

Glass

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Flexural Testing

Fig. 3 - Maximum Flexure Stress of Fibre Reinforced Epoxy Composites

Fig. 4 - Flexural Modulus of Fibre Reinforced Epoxy Composites

0

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100

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Ma

xim

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Composite Sample

Maximum Flexure Stress of Recycled Denim, Flax and Glass

Epoxy Composites

Denim A

Denim b

Flax

Glass

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4

6

8

10

12

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Fle

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Composite Sample

Flexural Modulus of Recycled Denim, Flax and Glass Epoxy

Composites

Denim A

Denim B

Flax

Glass

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SEM Imaging Results

Fig. 5 - Denim A SEM Image Fig. 6 - Denim B SEM Image

Fig.. 7 - Flax SEM Image Fig. 8 - Glass SEM Image

DISCUSSION

Tensile/Flexural Strength

Figure 1 shows the maximum tensile strength achieved by the fibre reinforced epoxy

composites. Glass fibre composites demonstrated the highest tensile strength (229 MPa).

Recycled denim B which used a fast hardener displayed the lowest value of tensile strength

(41 MPa) which is comparable but slightly lower to the values achieved by Denim A which

used a slow hardener (42 MPa). This shows that the use of slow hardener inside the epoxy

resin will provide a fractionally higher tensile strength. Flax composite samples provided a

tensile strength of around 84 MPa which lies in between the denim and glass. The values

given by the recycled denim are comparable to other natural fibre composites which have

previously been tested, such as the wood/PP and cotton/PP samples produced by Sam – Jung

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Kim et al (41.2 MPa and 49MPa respectively) (Kim, 2008). Flexural results have shown have

again shown the same pattern with Denim A & B achieving similar ultimate flexure stresses

of approximately 79 MPa. Glass again demonstrated the highest flexural stress of 186 MPa

with flax giving a value of 97MPa. The result of ultimate flexural stress for recycled denim

has shown to be comparable to that of other natural fibre reinforced polymer composites, such

as the samples produced by Janis Kajaks et al, who produced timber waste/HDPE and timber

waste/Recycled HDPE providing flexural strengths of 40.18MPa and 50.2MPa respectively

(Kajaks, 2015).

Tensile Modulus/Flexural Modulus

The tensile modulus results has shown glass to have the young’s modulus of 6.7GPa whereas

as recycled denim showed the lowest of 2.8GPa and 2.9GPa for denim A and B respectively.

All samples have shown a similar value of strain at yield point, however from the stress

results we can see the glass has shown the highest resistance to change in length.

The flexural modulus results show the glass composites were more flexible requiring a higher

stress and deflection to be fractured achieving a flexural modulus of 9.6GPa. The denim has

shown to be the least resistant to bending achieving the lowest flexural modulus of 4.6GPa

and 3.6GPa for denim A and denim B respectively. Differences between values for tensile and

flexural modulus can be seen as tensile testing will indicate the material going into plastic

deformation when the weakest fibres in the material start to elongate. Flexural testing differs

to this, as they test the strength of the extreme fibres (the fibres at the top and bottom edge of

the test sample) and if these fibres possess little or no defects a higher stress will be seen

before plastic deformation.

The results for tensile and flexural tests can referred to as the equivalent strength properties as

the values given have been achieved assuming the material is homogenous and isotropic. In

reality the samples produced will not have constant properties throughout leading to the

tensile and flexural data to differ from the true value.

The ultimate tensile/flexural strength of the composite will depend on the mechanical

properties of the matrix and reinforcement as well as the fibres internal structure. This why

the glass samples achieved a much higher tensile and flexural strength, as its mechanical

properties are much higher than that of cotton (Cook, 1984). The glass is also made up of long

continuous monofilament whereas the cotton and flax are made from shorter fibres wound

tightly together, this is supported by the SEM results where the different types of fibre are

shown. This difference in fibre structure leads to the glass having better tensile properties.

(Cook, 1984) Other factors for the denim achieving a much lower result may be due to the

denim being recycled. Because the denim was sourced from used jeans the fibres will have

been pre damaged by being worn and washed regularly, leading to lower mechanical strength.

Other inaccuracies could have been caused by the method in which the composites were

prepared. Using the SEM results it can be seen the samples contain dry spots indicating

proper resin impregnation was not achieved which could lead to a lower quality composite.

When the composites were prepared into sample specimens using the Warco 58” foot

operated treadle shear, micro fractures could be seen in the matrix along the cut edge. These

micro fractures could lead to the test samples providing a lower strength due to the structural

integrity of the matrix being damaged.

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Cost Analysis

The Flax and glass fiber used in this research were purchased from Easy Composites, the flax

cost £5.70 per m2 whereas the glass fibres were purchased for £2.34 per m

2. The recycled

denim used was acquired from charity shops, each pair of jeans gave approximately 1m2 of

useful material, with £5 being the approximate price. However 1m2 of denim weighs

approximately triple that of flax and glass fibers, making denim cheaper by weight. The

denim will also become cheaper when purchased in bulk like the flax and glass fibre.

CONCLUSION

The mechanical properties of recycled denim, flax and glass epoxy composites have been

cross-examined. Considering all the samples tested, recycled denim showed the lowest tensile

and flexural strength whereas glass has shown the highest. This has shown the mechanical

properties of recycled denim cannot directly compete with that of glass fibers. However

recycled denim may offer a suitable replacement to glass fibers where the application of high

bearing loads is not needed, as it still offers a competitive strength in relation to other natural

fibre reinforced polymer composites. Recycled denim has also shown to be a cheaper material

when the weight of materials bought are compared.

ACKNOWLEDGMENTS

The authors acknowledge the work done by Mr Daniel Stones on the composite samples, and

Mr Philip Staton in producing the scanning electron microscope pictures.

REFERENCES

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[2]-FW Caldwell and NS Clay “Micarta Propellers - 1 Materials” Technical Note No. 198,

National Advisory Committee for Aeronautics, August 1924.

[3]-J. G. Cook, Handbook of Textile Fibres Vol 1: Natural Fibres; Vol. 2: Man-Made Fibres.

5th ed. Durham: Merrow Publishing Co, 1984.

[4]-K. L. Pickering, Ed., Properties and performance of natural-fibre composites. Cambridge:

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[6]-A. L. Craighill and J. C. Powell, “Lifecycle assessment and economic evaluation of

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