Development and characterization of bio-based …...1 Development and characterization of bio-based fibre-reinforced polymer composites for structural applications in Civil Engineering

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Development and characterization of bio-based fibre-reinforced polymer

composites for structural applications in Civil Engineering

Tiago Nuno Barbosa de Azevedo Barros Zão

[email protected]

IST, Technical University of Lisbon, Portugal

Abstract

Fibre reinforced polymer composites (FRP), frequently used in civil engineering applications, can have a

significant environmental impact stemming from the use of raw materials that are generally derived from

petroleum. In this context, FRP bio-composites, particularly comprising resin systems derived from renewable

resources, are a potential response to the problem of conventional composites’ sustainability. The main goal

of the present paper was to study and characterize a bio-resin system and its application in a bio-composite

of carbon fibre reinforced polymer (CFRP) by means of experimental tests, which aimed at assessing its

rheological and mechanical properties. Tests were performed to determine the rheological and mechanical

properties of (i) a bio-based epoxy resin and (ii) of a comparable conventional (oil-based) resin, both available

in the market. The viscosity of the bio-resin was found to be around 35% higher than the viscosity of the

conventional resin and both resins showed a very similar glass transition temperature (of approximately 57 °C

determined through the storage modulus onset method). On the other hand, the tensile strength of the bio-

resin (around 60 MPa) has been found to be about 10% higher than that of the conventional resin. It was also

found that the shear strengths were equivalent for both resins (around 40 MPa) and the bio-resin had a

compressive strength (of around 100 MPa), about 10% lower than that of the conventional resin. The two

resins were used to produce CFRP composites with identical fibre layups, using a vacuum assisted hand layup

method, and the produced composites were tested regarding their thermophysical and mechanical properties.

The glass transition temperatures of both CFRP composites were very similar (around 50 °C, determined

through the onset of the storage modulus). In what concerns mechanical properties, it was found that the bio-

based CFRP composite had a tensile strength of around 830 MPa, approximately 15% higher than that of the

CFRP composite with conventional resin; it was also observed that the bio-composite presented an in-plane

intralaminar shear strength of around 25 MPa, about 10% lower than that of the conventional CFRP. Overall,

both CFRP composites exhibited very similar thermophysical and mechanical behaviour. This points out the

viability of using the studied bio-based resin in the production of CFRP bio-composites as a viable alternative

to conventional epoxy resin (fully derivate from petroleum). In particular, it seems possible to use the CFRP

bio-composite for the strengthening of structural elements, where conventional CFRP systems are currently

being used.

Keywords

CFRP composites, conventional epoxy resin, CFRP bio-composites, bio-resin, sustainability, experimental

tests, thermophysical behaviour, mechanical properties.

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1. Introduction

Composite materials are formed by two or more constituents, which are combined to form a material with

specific properties, taking advantage of the properties of each individual constituent material [1]. In this context,

the concept of fibre reinforced polymers (FRP) arises with the incorporation of reinforcing fibres into a polymer

matrix. FRP composites have their own mechanical, physical and chemical characteristics, which result from

the combination and interaction between the reinforcement (fibres), which is responsible for a large part of the

strength and stiffness of the material, and the matrix (resin), which guarantees the transfer of loads between

the fibres and between the applied loads and the composite itself. The most often used fibres in FRP

composites are synthetic, made of glass, carbon or aramid [1, 2]. The most commonly used resins in FRP

composites are also synthetic and can be divided into two categories: thermosetting and thermoplastic; for the

first case (the most usual for structural applications in Civil Engineering) the most commonly used resins are

epoxies, phenolics, polyesters and vinylesters [1,3]

FRP composites are manufactured through many methods, such as pultrusion, hand layup, vacuum assisted

hand layup, resin transfer moulding, vacuum assisted resin transfer moulding, among others [1, 3]. In general,

four main areas of application of FRP composites in construction can be distinguished, namely: (i) the interior

reinforcement of concrete (rebars); (ii) the exterior repair and strengthening of structures (strips and sheets);

(iii) new hybrid structures, where FRP composites are combined with traditional materials; and (iv) fully FRP

structures [1, 4]. FRP composites have several advantages in the construction industry, such as reduced self-

weight, high strength and stiffness, high corrosion resistance and ease of application/installation [1]. However,

with the growing use of FRP composites, it is important to guarantee that their environmental impact is low

and that raw materials are used in a responsible manner in their production. In fact, the great majority of the

resins used in these materials are currently derived from petroleum. This is the background and the motivation

for the development of FRP bio-composites, i.e. composite materials in which at least one of its constituents

(fibres or matrix) is biologically derived [1, 5].

Resins from biological origin are now being developed based on the synthetisation of renewable raw materials,

replacing resins derived from petroleum. This change allows the reduction of the environmental impacts of

FRP composites, increasing the eco-efficiency of already used composites and, mainly, to give a significant

answer about the sustainability of the construction industry. The general acceptance of FRP bio-composites

is conditioned by the fact that bio-resins present a higher production cost compared to conventional petroleum-

derived resins and the fact that there is still few information about their thermophysical and mechanical

properties [3, 6].

Through a comprehensive experimental characterization of a bio-based epoxy resin system, the present study

aims to contribute to the analysis of the mechanical and thermophysical behaviour of FRP bio-composites,

comparing it to a conventional epoxy resin, both resins being available in the market. In particular, one intended

to evaluate the feasibility and potential of using partially bio-derived epoxy resins as a replacement product for

conventional resins (fully petroleum-derivate) in the production and use of CFRP composites; the goal was to

assess the potential of CFRP bio-composites (at medium-to-long term) to replace conventional CFRP

composites, currently used in large-scale in civil engineering. One possible application is the use of sheets

(laminated in situ) to confine and strengthen reinforced concrete structural elements.

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2. Experimental tests of the resin systems

2.1. Characterization and production of resin specimens

For this experimental study, an epoxy bio-resin (SR GreenPoxy 56 / Hardener SD 7561) and a conventional

epoxy resin (S&P Resin 55) were used.

The epoxy bio-resin has two components and it is produced with a high carbon content (56%) derived from

plant oils. Table 1 shows the main properties of this resin, according to the manufacturer [7].

Table 1 – Main properties of the bio-resin SR GreenPoxy 56 / Hardener SD 7561 [7]

The conventional epoxy resin also has two components and its density, at 20 °C and at 50% relative humidity,

is 1,11 g/cm3. Table 2 shows the main properties of this resin, according to the manufacturer [8].

Table 2 – Main properties of the conventional epoxy resin S&P Resin 55 [8]

Curing cycle Properties

24h at 20°C (after 14 days)

Modulus of elasticity [MPa] 2515

Tensile strength [MPa] 35.8

Elongation at maximum load [%] 2.3

Tg – Glass transition temperature [°C] 44

Ten specimens of both resins were produced, prepared and cut for all the tests that were performed (tensile,

compression, in-plane shear and DMA), according to the standards and geometry presented in Table 3.

Table 3 – Standards and geometry of the tested resins specimens

l – length; b – width; h – thickness;

The production process of resin specimens consisted in the production of a 50 cm by 50 cm resin plate with a

thickness of 5 mm; to that purpose, a "mould" consisting of two tempered glass plates and a cylindrical rubber

cord (O-ring cord with 5 mm diameter) was used. This production process consisted in placing the rubber cord

about 20 mm from the edges of the glass plate, the second glass plate being placed on top of it, ensuring a

distance between the plates of approximately 5 mm, corresponding to the diameter of the cord. Subsequently,

uniform pressure was applied around the edges of the mould, using metal clamps acting on rigid plates (to

protect the glass and better distribute the clamping force) and thus ensuring the mould tightness. An opening

was left between the glass plates in the upper mould area, through which the resin was slowly transferred into

the mould (Figure 1a). After a 24h period that allowed the hardening of the resin, the resin plate was removed

from the mould and showed no damage or defects, ensuring a virtually uniform thickness (5 mm) across the

Curing cycle Properties

24h at 23°C + 24h at 40°C

24h at 23°C + 16h at 60°C

24h at 23°C + 8h at 80°C

Modulus of elasticity [MPa] 3290 3160 2980

Tensile strength [MPa] 71 71 68

Elongation at maximum load [%] 3,6 4,3 5

Shear strength [MPa] 46 53 47

Compression strength [MPa] 100 100 96

Tg – Glass transition temperature [°C] 67 79 78

Tests Standards No. of

specimens l (mm) b (mm) h (mm)

Tensile ISO 527 (Parts 1 & 2) 10 170 10 5

In-plane shear ASTM D5379 / D5379M 10 75 20 5

Compressive ASTM D6641 / D6641M 10 140 12 5

DMA ASTM E1640 10 50 10 5

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plate. Finally, the cutting of the resin specimens was performed with the assistance of a CNC milling machine

(Figure 1b and Figure 1c).

Figure 1 – Production process of the resin specimens: a) resin transfer into the mould; b) cutting of the resin specimens; c) resin specimens

2.2. Laboratory tests performed on the resins

Firstly, the liquid resin was tested regarding its gel time on a Gelnorm – Gel Timer instrument and its viscosity

was determined using a Cone & Plate Viscometer instrument. This was followed by Dynamic Mechanical

Analysis (DMA) tests on the hardened resin, using a TA Instruments Q800 equipment (from LNEC), as well as

tensile, in-plane shear (Iosipescu) and compression tests, which were carried out using a universal hydraulic

test machine (Instron 8800D), with and adjustable hydraulic clamping system.

The deformations of the tensile, in-plane shear and compression test specimens were monitored through the

position of target points marked on the specimens using a video-extensometer, consisting of a Sony XCG-

5005E high definition camera and a Fujinon – Fujifilm HF50SA-1 lens.

According to the standards presented in Table 3, the following tensile properties were analysed: tensile

strength, strain at failure and elasticity modulus. The following in-plane shear properties were analysed: in-

plane shear strength, in-plane shear distortion at failure and shear modulus. The following compressive

properties were analysed: compressive strength, strain at maximum compressive load and compressive

modulus. All tests were performed at a speed rate of 2 mm/min.

2.3. Analysis and discussion of the results

Through the gel time tests, it was found that the conventional epoxy resin passed from the liquid state to the

gel state in 4 hours and 27 minutes and the bio-resin in 3 hours and 13 minutes (at 25 °C). According to the

results obtained from the viscosity tests, an initial viscosity of 650 cP was determined for the bio-resin. This

value is very similar when compared to the information collected in the data sheet of this resin. It was also

concluded that the conventional epoxy resin has a lower viscosity throughout the test period. The viscosity of

the resins was also measured after 50 min without any agitation, and it was observed that the viscosity of the

bio-resin is significantly higher when compared to the viscosity of the conventional epoxy resin. Comparing the

two resins under analysis, it was concluded that, although the two resins are comparable, the bio-resin has a

higher viscosity, less workability, and less gel time compared to the conventional resin tested - these

characteristics must be duly considered during the production of composites using that bio-resin.

The glass transition temperatures for both resins under analysis were experimentally determined based on the

onset of the storage modulus (E') as a function of temperature (Figure 2a and Figure 2b). The three test

specimens (from both resins) showed very consistent results and were identified by an alphanumeric

a) b) c)

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representation, being "D" – DMA, "conv." – conventional epoxy resin and "bio" – bio-resin. The value of the

glass transition temperature (Tg) determined through the above mentioned curves has an important meaning

in structural applications, because when this value is reached the material begins to significantly lose the

capacity to absorb loads, with significant reduction of strength and stiffness.

a) b)

Figure 2 – Storage modulus curves obtained in DMA tests for the: a) conventional epoxy resin; b) bio-resin

The glass transition temperature (in °C) of the conventional epoxy resin (52.7 ± 0.4 °C) virtually matched that

of the bio-resin (52.7 ± 0.1 °C).

The tensile properties (of both resins) were obtained according to the tensile stress-strain curves plotted in

Figure 3a and Figure 3b. Both resins exhibited approximately linear elastic behaviour, with a progressive

stiffness reduction prior to failure. This was especially noticeable for the bio-based resin, for which most

specimens exhibited a short yield plateau and a slight stress reduction preceding failure, which are indicative

of necking.

a) b)

Figure 3 – Tensile stress-strain curves: a) conventional epoxy resin; b) bio-resin

Based on the results shown, it was found that the tensile strength of the conventional epoxy resin is

56.4 ± 3.7 MPa, the strain at maximum tensile strength is 2.09 ± 0.31% and the tensile modulus of elasticity

is 3366.9 ± 79.7 MPa. Ten bio-resin specimens were tested; however, the "T9-bio" specimen had a very

premature rupture and, for this reason (it should have a critical defect), this specimen not included in the

analysis of results. Based on the results of the remaining sample of bio-resin specimens, it was found that the

tensile strength of the bio-resin is 62.5 ± 3.3 MPa, the strain at maximum tensile strength is 2.61 ± 0.3% and

the tensile modulus of elasticity is 3445.3 ± 73.1 MPa. It should be noted that the results shown for tensile

properties of the bio-resin are slightly higher (about 10%), compared to the results obtained for the

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conventional epoxy resin. These results point out the technical feasibility of using the bio-resin as a substitute

for the conventional resin.

The in-plane shear properties (of both resins) were determined based on the shear stress-distortion curves

(Figure 4a and Figure 4b). Both resins exhibited approximately linear elastic behaviour up to the maximum

stress value, with a slight reduction of stiffness, which was followed by a post-peak response with decreasing

load/stress and quite extensive distortion before final failure due to the formation of a yield line between the

notches of the test specimens.

a) b)

Figure 4 – Shear in-plane stress-strain curves: a) conventional epoxy resin; b) bio-resin

Based on the results shown, it was found that the in-plane shear strength of the conventional epoxy resin is

40.3 ± 1.0 MPa, the maximum in-plane shear distortion is 6.27 ± 0.20% and the shear modulus is

1123.0 ± 102.5 MPa. For the bio-resin, an in-plane shear strength of 39.6 ± 0.6 MPa was determined, along

with maximum in-plane shear distortion of 5.72 ± 0.12% and shear modulus of 1147.5 ± 97.9 MPa. It should

be noted that the in-plane shear properties of the bio-resin are very similar to the ones of the conventional

epoxy resin.

The compressive properties (of both resins) were obtained based on the compressive stress-strain curves

(Figure 5a and Figure 5b). Of the 10 tests carried out for each resin, 4 of each type ("C1-conv.", "C2-conv.",

"C9-conv." and "C10-conv." specimens of conventional epoxy resin and "C1- bio"," C2-bio"," C5-bio"and" C6-

bio" specimens of bio-resin) were not considered valid because of excessive slipping that occurred in the

recessed area of the test device, where load transfer by friction is carried out. The remaining test specimens

exhibited a very consistent behaviour, which was typically linear elastic up to the maximum stress value (a

slight reduction in stiffness occurred before the peak stress was reached), followed by a post-peak stage with

stress reduction and significant deformations for non-null stresses.

Based on the results shown, it was found that the compressive strength of the conventional epoxy resin is

105.2 ± 2.1 MPa, the strain at maximum compressive strength is of 4.93 ± 0.28% and the compressive

modulus of elasticity is of 4194.0 ± 432.4 MPa. For the bio-resin, the compressive strength is 96.5 ± 4.8 MPa,

the strain at maximum compressive stress is 3.98 ± 0.44% and the compressive modulus of elasticity is

3670.0 ± 301.5 MPa. In this case, the compressive properties of the bio-resin were slightly lower (~10%) than

those of the conventional epoxy resin.

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a) b)

Figure 5 – Compressive stress-strain curves: a) conventional epoxy resin; b) bio-resin

3. Experimental tests of the CFRP composites

3.1. Production and characterization of CFRP composite specimens

In the present experimental campaign, a CFRP bio-composite and a CFRP composite were produced with

respectively the epoxy bio-resin and the conventional epoxy resin (the two resins previously studied), and

analysed. Both composites were produced with a unidirectional carbon fibre layup comprising four laminas.

The process adopted to produce the CFRP composite specimens was vacuum assisted hand layup, which

consists of the successive application of carbon fibre reinforcement sheets and their impregnation with the

resins (Figure 6a). To improve the quality of the CFRP composites produced, vacuum was used during 24 h

until the resin was fully hardened (Figure 6b). Finally, the cutting of the CFRP composite specimens was

performed using a CNC milling machine (Figure 6c).

a) b) c)

Figure 6 - Vacuum assisted hand layup: a) carbon fibre reinforcement sheets and their impregnation with resin; b) vacuum used during 24 h; c) cutting of the CFRP composite specimens with the assistance of a CNC milling machine

3.2. Laboratory tests performed on CFRP composites

For each type of CFRP composite, six DMA specimens, nine tensile specimens and five tensile specimens

with 10° fibre inclination were produced with the geometry indicated in the standards for each type of test, as

presented in Table 4.

Table 4 – Standards and geometry of the tested resins specimens

Tests Standards No. of specimens l

(mm) b

(mm) h

(mm)

Tensile ISO 527 (Parts 1 & 5) 9 300 25 3

Tensile with 10° fibre inclination

ASTM D3038 / D3039M 5 300 40 3

DMA ISO 6721 (Parts 1 & 11) 6 60 10 3

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DMA tests were carried out (at LNEC) for both types of CFRP composites (produced with conventional resin

and with bio-resin), in order to determine the glass transition temperature (Tg) of these materials. Tensile tests

at 0º and 10° fibre inclination were performed according to the procedure described in section 2.2 and

according to the standards presented in Table 4 - the following tensile properties were analysed: tensile

strength, strain at maximum tensile stress and tensile elasticity modulus. According to Chamis and Sinclair [9],

the tensile tests at 10° fibre inclination allow the determination of the following intralaminar shear properties of

the CFRP composites: intralaminar shear strength, intralaminar shear distortion and in-plane shear modulus.

3.3. Analysis and discussion of the results

The glass transition temperatures of both CFRP composites under analysis were determined according to the

onset of the storage modulus (E') as a function of temperature (Figure 7a and Figure 7b). The three test

specimens were identified by alphanumeric representation, being "D" – DMA, “C” – CFRP, "conv." –

conventional epoxy resin and "bio" – bio-resin. However, the "D2-C-conv." specimen was removed from the

analysis of results because its thickness was not constant, presenting large elevations in areas with higher

amount of carbon fibre, and one of the support/load points was located precisely on one of these elevations,

making the respective test not valid.

a) b)

Figure 7 – Storage modulus curves of the CFRP composites: a) conventional epoxy resin; b) bio-resin

Based on the results shown above, the glass transition temperature of the CFRP composite with conventional

epoxy is 54.0 ± 0.6 °C and the glass transition temperature of the CFRP composite with bio-resin is

51.6 ± 0.9 °C. Both types of CFRP composites have very similar glass transition temperature values, so these

results point out again to the technical feasibility of using the bio-resin as a substitute for the conventional resin

in its typical applications.

The tensile properties (for both CFRP composites) were obtained according to the tensile stress-strain curves

(Figure 8a and Figure 8b), which exhibited the typical linear elastic behaviour up to failure of FRP composites.

Based on the results shown, it was found that the tensile strength of the CFRP composite with conventional

epoxy resin is 728,0 ± 33,7 MPa, the strain at maximum tensile strength is 1,16 ± 0,04% and the tensile

modulus of elasticity is 62,1 ± 2,8 GPa. In what concerns the CFRP composite with bio-resin, it was found that

the tensile strength is 829,5 ± 55,3 MPa, the strain at maximum tensile strength is 1,09 ± 0,09% and the tensile

modulus of elasticity is 74,1 ± 3,4 GPa. It should be noted that the results obtained regarding tensile properties

of the CFRP composite with bio-resin are slightly higher (about 15%), compared to the results obtained for the

CFRP composite with conventional resin. This difference in the tensile properties may be associated with

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potential differences in the manufacturing, for example in the way the fibres were stretched (manually) during

production. However, once again these results indicate the technical feasibility of using the bio-resin as a

substitute for the conventional resin in CFRP composites.

a) b)

Figure 8 – Tensile stress-strain curves of the CFRP composites: a) conventional epoxy resin; b) bio-resin

The intralaminar shear properties (of both CFRP composites) were determined based on the intralaminar shear

stress-strain curves (Figure 9a and Figure 9b). The five test specimens were identified by alphanumeric

representation, being "T10" – Tensile with 10° fibre inclination, “C” – CFRP, "conv." – conventional epoxy resin

and "bio" – bio-resin. However, of the five tests carried out for each CFRP composite, 2 of each type (“T10-1-

C-conv.” and “T10-3-C-conv.” of CFRP composites specimens with conventional epoxy resin and “T10-2-C-

bio” and “T10-3-C-bio” of CFRP composites specimens with bio-resin) were not considered as valid, because

failure did not occur within the gauge length of the test specimens; instead, it occurred in the clamping zone

(in the grips of the test fixtures), leading to the premature failure of the CFRP composites.

a) b)

Figure 9 – Intralaminar shear stress-strain curves of the CFRP composites: a) conventional epoxy resin; b) bio-resin

Based on the results shown, it was found that the intralaminar shear strength of the CFRP composite with

conventional epoxy resin is 29.8 ± 3.21 MPa, the intralaminar shear distortion is 2.28 ± 0, 36% and the shear

modulus is 2.87 ± 0.52 GPa. For the CFRP composite with bio-resin, the intralaminar shear strength is

26.63 ± 2.56 MPa, the intralaminar shear distortion is 2.50 ± 0.22% and the shear modulus is 2.36 ± 0.08 GPa.

It should be noted that, unlike the tensile properties, the results obtained in the 10° tensile tests for the bio-

based CFRP composite are slightly lower (about 10%) than the results obtained for the CFRP composite with

conventional epoxy resin. However, the obtained properties are of the same order of magnitude and similar

enough to confirm the technical feasibility of using the bio-resin as a substitute for the conventional resin in

CFRP composites.

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4. Conclusions

The experimental study presented in the present paper provided a thorough characterization of the mechanical

properties of an epoxy bio-resin. This resin has a tensile strength of 62.5 MPa, about 10% higher than the

conventional epoxy resin (56.4 MPa) used a reference. The bio-resin and the conventional epoxy resin show

also very similar values of in-plane shear strength (~40 MPa) and glass transition temperature (53 ºC).

Although the results regarding compressive properties of the bio-resin are slightly lower (about 10%) than the

conventional resin, in general, the experimental results obtained allow to conclude that both resins have very

similar thermophysical and mechanical behaviour.

The CFRP composite with the studied bio-resin showed a tensile strength of 830 MPa, about 15% higher than

the CFRP composite with conventional epoxy resin (728 MPa). Based on 10° fibre inclination tensile tests, the

CFRP composite with bio-resin exhibited an intralaminar shear strength of 26.6 MPa, slightly lower (about

10%) than the CFRP composite with conventional epoxy resin (29,8 MPa). In other words, the mechanical

properties of both types of CFRP composites were fairly equivalent.

In summary, the objectives proposed for the present study were reached. The results obtained show that the

studied bio-resin is suitable for use in the production of CFRP bio-composites for civil engineering applications.

One may also infer that CFRP bio-composites have potential, in the medium-to-long term, to replace conventional

CFRP composites currently used in the strengthening of existing structures, provided that the cost gap between

bio-based and oil-based products is reduced over time with the further development of bio-based resin systems.

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