TR 18001 A literature review of naturalfdcb17f/UQfdcb...In flammability research of natural fibre...

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unless permission has been obtained in writing from the author.

TR 18001 – A literature review of natural fibre composite fire properties

Asanka Basnayake, Juan Hidalgo, Luigi Vandi, and Michael Heitzmann

April 2018

CIC Fibre-Resin Interface Project: 16-020-004

Commercial-in-Confidence

TR 18001 Commercial-in-Confidence Revision 0

Summary

The past decade has seen an increase in natural fibre composite research, with a focus towards

improving mechanical properties to compete with traditional composites. Despite this

academic interest, the commercial interest and uptake is comparatively small. Furthermore, the

commercial applications are also not driven by mechanical properties, but other features.

One is the fire performance of natural fibre composite, and integrating with a commercial light

resin transfer moulding (RTM) process.

In flammability research of natural fibre composites, there is a general formula that researchers

follow: select fibre/matrix combination(s), select retardant additives or processes, manufacture

sample, test samples, and discuss findings. This formula has been pivotal in establishing the

type of retardants and processes that are capable of reducing the flammability of bio-

composites.

The following report provides a review of current literature on fire properties. It reviews the

behaviour of natural fibre composites during a fire, including a common testing method, and

the results obtained from this process. The report also reviews common fire retardant additives

which can be used with natural fibre composites. Finally, it identifies areas of future research

that will contribute to the fire properties body of knowledge.


© 2018 UQ Composites Page 1 of 41

Table of Contents

1 Fire properties of natural fibre composites ........................................................ 4

1.1 Plant natural fibres for composites .............................................................................. 4

1.1.1 Plant fibre structure ................................................................................................ 4

1.1.2 Plant fibre chemistry .............................................................................................. 5

1.2 Combustion, decomposition, and thermal stability ..................................................... 6

1.2.1 Stages of combustion ............................................................................................. 6

1.2.2 Thermogravimetric Analysis ................................................................................. 7

1.2.3 Heat release rate ..................................................................................................... 9

1.2.4 Interaction between cellulose, hemicellulose and lignin ..................................... 10

1.3 Understanding the behaviour of composites in a fire ................................................ 12

1.3.1 Stages of combustion ........................................................................................... 12

1.3.2 Composite decomposition overview .................................................................... 12

1.3.3 Decomposition of common polymers .................................................................. 14

1.4 Fire property testing – cone calorimeter ................................................................... 18

1.4.1 Cone calorimeter .................................................................................................. 18

1.4.2 Combustion properties ......................................................................................... 20

1.4.3 Material properties, test parameters and combustion behaviour ......................... 21

1.5 A review of fire retardants ........................................................................................ 27

1.5.1 Retardant classifications ...................................................................................... 28

1.5.2 Examples of reactive retardants ........................................................................... 29

1.5.3 Examples of active fillers ..................................................................................... 30

1.5.4 Examples of inert fillers ....................................................................................... 32

1.5.5 Discussion of findings.......................................................................................... 32

1.6 Further research ......................................................................................................... 34

1.6.1 Limited focus on composite manufacturing processes ........................................ 34

1.6.2 Influence of mat architecture and the effects of fire retardants ........................... 34

1.6.3 Permeability of resins with retardant additive fillers ........................................... 35

1.6.4 Effect of viscosity on permeability ...................................................................... 35

1.6.5 Retardant mixing and dispersion ......................................................................... 35

References .................................................................................................................... 36



List of Figures

Figure 1 – Structure of plant fibre [9] ........................................................................................ 4

Figure 2 – Common plant fibre chemical composition, adapted from [12] ............................... 5

Figure 3 - Cellulose pyrolysis [13] ............................................................................................ 6

Figure 4 - TGA curves for cellulose, hemicellulose and lignin [23]. ........................................ 8

Figure 5 - TGA curves for natural fibres [21] ............................................................................ 9

Figure 6 - Heat release during combustion of cellulose and lignin [21, 22] .............................. 9

Figure 7 - Heat release during combustion of natural fibres, adapted from [22] ..................... 10

Figure 8 – Selected literature on natural fibre pyrolysis and combustion ............................... 11

Figure 9 - Stages of fire growth [25]. ...................................................................................... 12

Figure 10 – Composite decomposition mechanism, adapted from [25] .................................. 13

Figure 11 - TGA curves for hemp/polyester composite [31] ................................................... 15

Figure 12 - Heat release of polyester composite, adapted from [32] ....................................... 15

Figure 13 - TGA curves for hemp/polyester composite [33] ................................................... 16

Figure 14 - Heat release during the combustion of epoxy [33] ............................................... 16

Figure 15 - TGA curves for kenaf/polypropylene composite, adapted from [34] ................... 17

Figure 16 - Heat release during the combustion of kenaf/PP composite, adapted from [34] .. 17

Figure 17 – Range of specimen sizes of different fire test methods, adapted from [25] ......... 18

Figure 18 – Basic cone calorimeter layout [36] ....................................................................... 18

Figure 19 - Petrella Plot of various composites at 50kW/m2[54] ............................................ 21

Figure 20 - Burning behaviour characteristics [28] ................................................................. 22

Figure 21 - Ignition times of woven and chopped strand mats [32] ........................................ 23

Figure 22 - Heat release curves for chopped strand mats and woven mats [25] ..................... 23

Figure 23 - Fabric weave types tested by Chai et al [56] ........................................................ 24

Figure 24 - Combustion behaviour of different fabric weaves, adapted from [56] ................. 24

Figure 25 – HRR curves of fabric different weaves [56] ........................................................ 24

Figure 26 - Effect of thickness on time to ignition [25] .......................................................... 25

Figure 27 - Effect of thickness on HRR for PMMA [28] ........................................................ 25

Figure 28 - Ignition time variation with heat flux [57] ............................................................ 26

Figure 29 – Combustion property variations with heat flux [58] ............................................ 27

Figure 30 - Fire retardant classifications ................................................................................. 28

Figure 31 – Effect of APP on the HRR, adapted from [34] .................................................... 30

Figure 32 – Effect of aluminium trihydroxide on the HRR, adapted from [61] ...................... 31

Figure 33 – Effect of ZnB on the HRR, adapted from [41] ..................................................... 31

Figure 34 – Effect of nano-SiO2 on the HRR, adapted from [42] ........................................... 32



List of Tables

Table 1 - Sources of plant fibres, adapted from [7] ................................................................... 4

Table 2 - Combustion properties of PMMA, adapted from [28] ............................................. 26

Table 3 - Summary of retardants used with natural fibre composites ..................................... 33

Table 4 - Comparison of composite fabrication processes [71] .............................................. 34



1 Fire properties of natural fibre composites When assessing the fire properties of natural fibre composite, consideration must be given to

the matrix, reinforcing fibre, and the interaction between them. There are also combustion

characteristics that must considered, especially in comparing the flammability of two or more

composites (these will be discussed in more detail in later sections). Some researchers suggest

that the heat release rate is the most significant combustion characteristic as it is the best

indicator of a fire hazard of a combustible material. Research into combustion characteristic

has found methods of improving fire performance, but these methods also pose other issues

such as effects on material properties, processability, and impact to the environment.

1.1 Plant natural fibres for composites

Plant natural fibres, also known as lignocellulose fibres, originate from different parts of the

plant. Table 1 , shows the most common sources of plant natural fibres.

Stalk Grass Stem Fruit Leaf Root Seed

Straw

(cereal)

Bagasse,

Bamboo

Flax, Hemp,

Isora, Jute,

Kenaf,

Kudzu,

Nettle,

Roselle,

Ramie,

Cadillo,

Wood,

China Jute,

Sun hemp

Coir,

Kapok,

Oil palm,

Sponge,

Gourd

Abaca, Banana,

Cantala, Caroa,

Curaua, Date

palm,

Henequen, Istle,

Mauritius hemp,

Piassava,

Pineapple,

Phormium,

Sansevieria,

Sisal

Broom

root

Cotton

Table 1 - Sources of plant fibres, adapted from [1]

Bast fibres (collected from plant stems) are commonly used in applications in Australia, and

will be the focus of this review.

1.1.1 Plant fibre structure

The structure of a plant natural fibre is a complex layered construction which contains a

primary cell wall, covering three secondary cell walls (S1, S2, and S3), around the lumen which

transports water to the fibre (refer to Figure 1). The middle layer (S2), which is thicker than

the others walls (making up 80% of the total thickness, governs the mechanical properties of

the fibre [2]).

Figure 1 – Structure of plant fibre [3]

Cellulose, hemicellulose, and lignin are the major constituents of the cell walls, with small

amounts of pectins, oils and waxes. The secondary walls are made up of a crystalline



microfibrils of cellulose, arranged in a helical manner to create long chains of cellulose

molecules. The microfibrils cellulose reinforcements are bound together by a matrix of lignin

and hemicellulose. This bound structure forms cellulose fibres. The helical arrangement of

cellulose in the secondary walls results in an angle which can be measured from along the

length (or axis) of the fibre to the microfibril. Known as the microfibril angle, it is the reason

for the strength and stiffness of fibres – a smaller angle lead to a higher strength, stiffer fibre.

The thicker S2 layer has smaller microfibril angles, while the S1 and S3 have larger ones [3,

4].

1.1.2 Plant fibre chemistry

As mentioned previously, the major constituents of natural fibres are cellulose, hemicellulose,

and lignin. With the exception of cotton, the cell walls of plant fibres are made up of these

three components[5]. Plant fibres also contain small amounts of pectin, wax and ash.

There are also various sources that provide a breakdown on the chemical composition for

different fibre types, the below shows some common fibres and typical values for the three

constituents [6].

Fibre Cellulose Hemicellulose Lignin

Abaca 56 – 63 20 – 25 7 – 9

Pineapple 70 – 82 0 5 – 12

Sisal 67 – 78 10 – 14.2 8 – 11

Coir 36 – 43 0.15 – 0.25 41 – 45

Cotton 82.7 5.7 0

Flax 71 18.6 – 20.6 2.2

Hemp 70.2 – 74.4 17.9 – 22.4 3.7 – 5.7

Jute 61 – 71.5 13.6 – 20.4 12 – 13

Kenaf 31 – 39 21.5 15 – 19

Ramie 68.6 – 76.2 13.1 – 16.7 0.6 – 0.7

Bagasse 55.2 16.8 25.3

Bamboo 26 – 43 30 21 – 31 Figure 2 – Common plant fibre chemical composition, adapted from [6]

Cellulose

Cellulose is a considered to be a linear macromolecule with a degree of polymerisation ranging

up to 10,000. It forms a chain of repeating units of the sugar group D-anhydroglucose

(C6H11O5) connected by β-1,4-glycosidic linkages, with each unit containing three hydroxyl

(OH) groups. The crystalline structure and physical properties are controlled by the hydroxyl

groups and their hydrogen bonding ability [6].

The type of cellulose, with its individual cell geometry, and geometrical condition, controls a

given natural fibres mechanical properties. The microcrystalline structure of solid cellulose

contain both crystalline and amorphous regions [5].

Hemicellulose

Hemicellulose comprises of a group of polysaccharides (a polymeric carbohydrate molecule),

containing glucans, mannans, galactans, arabians, and xylans [6]. It has a random, amorphous,

and weak structure that is susceptible to hydrolysis – as opposed to the crystalline structure of

cellulose which resists hydrolysis [7]. Also unlike the linear structure of cellulose,

hemicellulose shows a substantial degree of chain branching. Hemicellulose also has a degree

of polymerisation to a maximum of 100 time less than cellulose, and varies is different plant

species [5].



Lignin

Lignins are made up of complex aromatic and non-aromatic hydrocarbon polymers [6, 8]. It is

amorphous, hydrophobic, and is the component that gives rigidity to plants. Generally

considered to be a thermoplastic polymer, it has a glass transition temperature of approximately

90oC, with a melting temperature of approximately 170oC [5].

1.2 Combustion, decomposition, and thermal stability

Unlike synthetic fibres used in composite material, natural fibres (due to their make-up)

contribute significantly to the decomposition of a natural fibre composite during a fire. The

combustion of natural fibres results in heat release, however for combustion to occur, certain

conditions have to be met. These include an interaction with air and a physical, chemical or

microbiological stimulus associated with heat release [9]. The time take for ignition, is

influenced by the intensity of the heat source, concentration of oxygen, and the movement of

gas in the area [9].

1.2.1 Stages of combustion

Combustion can be divided into three stages as summarised by Kozlowski and Wladyka-

Przybylak [9]. The following sections reviews these stages.

Preliminary stage

This stage can be classified as a flameless stage, where heating above 105oC causes moisture

removal, with predominantly slow endothermic reactions occurring Above 105oC (between

150 - 200oC), slow thermal decomposition occurs, resulting in the release of gaseous product,

bonds in the structure weakening, and discolouring of the fibres [9].

Lignin begins decomposition approximately at 160oC, and contributes to char formation [9,

10]. The majority of the char production during natural fibre combustion occurs from the

thermal degradation of lignin, due to the formation of phenols from ether and carbon-carbon

links being separated [9].

Hemicellulose decomposes between 180oC and 260oC, with less tar production but higher non-

combustible gases than cellulose [9, 10]. The released gases contain about 70% of

incombustible CO2 and about 30% of combustible CO [9]. Also in a similar temperature range,

between 200oC and 260oC, the reactions become exothermic, and are evident by the increase

of gaseous substances and tar produced, along with local ignition areas of low boiling point

hydrocarbons [9]. Although spontaneous ignition is not expected to occur at this stage, it is

possible for a pilot flame to ignite the fibres [9].

Cellulose decomposes between 260oC and 360oC, forming flammable volatiles and gases, non-

combustible gases, tars and char [9, 10]. This is a two-step process, and can be represented in

Figure 3.

Figure 3 - Cellulose pyrolysis [7]

Cellulose

200-280oC (slightly

endothermic)Dehydrocellulose Exothermic

Chargaseous products

280-340oC (exothermic)

Tar (mainly laevoglucosan)

Exothermic decomposition

Flammable volatiles

Endothermic dehydration

Char



In the slightly exothermic step, gradual degradation including polymerisation, dehydration and

decarboxylation, resulting in dehydrocellulose. This decomposes exothermically to form char

and gaseous products [7].

In the exothermic step, volatilisation occurs rapidly, forming laevoglucosan. Further

exothermic pyrolysis results in flammable volatile, leaving a char residue [7].

Main stage

This is also classified as the flame stage of combustion, and is an active process of

decomposition. During this stage, products of thermal decomposition ignite and the amount of

heat released and mass lost increases [9]. It occurs in the range of 260oC and 450oC. Early in

the stage, around 275oC and 280oC, high amounts of heat, gaseous and liquid products such as

methanol are created. CO and CO2 production reduces, and ignition occurs [9]. From this point

onwards, the reactions are exothermic, with secondary pyrolysis reactions driving the

decomposition and increasing the ability of the gases formed to ignite (provided there is enough

oxygen) [9].

Final stage

Occurring above 500oC, this is a flameless stage, which involves slow burning of residue and

ashing of the remnants [9]. The amount of combustible material formed is small, but the

charcoal formed increases.

1.2.2 Thermogravimetric Analysis

One way to investigate the thermal stability of natural fibres is by thermogravimetric analysis

(TGA). TGA is a technique in which the mass of a material is measured as a function of

temperature (or time), while the material is being exposed to a controlled temperature profile

in a controlled environment [11]. Inert, oxidizing, or reducing gases can be used to purge the

gas flowing through the balance. Inert purge gases include nitrogen, argon, or helium; oxidising

gases such as air or oxygen; and reducing gases such as forming gas (8-10% hydrogen in

nitrogen) [11]. The results are depicted with a thermogravimetric or TG curve, which is the

mass loss over temperature (or time), and a differential thermogravimetric or DTG, which is

the rate of mass loss. The TG ad DTG curves provide a visualisation of how different material

decomposes with increasing temperature. In the case of natural fibres, both TG curve indicates

the amount, and temperature range of decomposition, while the DTG curve indicates the rate,

and peak temperature of decomposition.

TGA of natural fibres is typically conducted under inert gases such as nitrogen or argon [12-

16], with common heating rates between 5 and 10oC/minute [12, 13, 15, 16].

The TGA results of the three major constituents can be reviewed to understand the thermal

stability of natural fibre. Figure 4 shows TGA curves for cellulose, hemicellulose, and lignin

conducted by Zhao et al, and provides a commentary on the results [17]. These results

corresponded to TGA profiles obtained by Yang et al [18] and Dorez et al [15] (although the

earlier studies used a heating rate of 10oC/minute, compared to the 5oC/minute used by Zhao

et al).



Cellulose

Decomposed at a narrower range,

between 286-426oC – maximum

loss rate of around 23%/minute,

occurred at 335oC. At the

completion of the test, at 900oC,

the residual mass was 4% of the

original.

Hemicellulose

Readily decomposed, with the

main weight loss occurring

between 203-386oC – maximum

mass loss rate of around

6%/minute, occurred at 295oC. At

the completion of the test, at

900oC, the residual mass was 13%

of the original.

Lignin

Decomposed at a wider

temperature rage, between 215-

585oC – maximum loss rate of

around 2.2%/minute, occurred at

385oC. At the completion of the

test, at 900oC, the residual mass

was 34% of the original.

Figure 4 - TGA curves for cellulose, hemicellulose and lignin [17].

Figure 5 shows the TG and DTG curves for fibres tested by Dorez et al.



TG Curve for natural fibres DTG for natural fibres Figure 5 - TGA curves for natural fibres [15]

All fibres decomposed at different temperature ranges and peak decomposition temperatures.

The interaction of cellulose, hemicellulose, and lignin is evident as the curves appear to contain

combined aspects of the individual curves.

1.2.3 Heat release rate

Section 1.4.2 will discuss heat release rate (HRR) in more detail, but the concept will be briefly

described here to further investigate the combustion of natural fibres during a fire. HRR is a

quantitative measure of the amount of thermal energy released per unit area of a material, when

exposed to a fire [19]. During a fire, the heat release rates changes as the material is consumed,

so the heat release can be described in terms of the highest or peak heat release, and the average

heat release. These values need to be examined to determine a materials fire risk, so that

development and extent of fire is minimised. [19].

In their studies, Dorez et al looked at the heat release rates as well as TGA. Figure 6 shows the

heat release curves for cellulose and lignin in their 2013 and 2014 investigations. The earlier

onset of heat release of lignin corresponds to the thermal degradation can be seen in a TGA

curve. For both, after the initial peak is reached, the heat release continues to increase – in the

case of lignin, it increases significantly. This is due to the residual char formed constantly

releasing heat. The studies did not contain a heat release curve for hemicellulose.

Figure 6 - Heat release during combustion of cellulose and lignin [15, 16]



Looking at the heat release curves for natural fibres that Dorez et al investigated (Figure 7), it

is evident, that there is a relationship between the constituents. A closer look at the heat release

curves during combustion of some natural fibres in Figure 7 shows that all expect hemp resulted

in charring – this is evident by the shallower slope after reaching the peak heat release. The

authors concluded that hemp and flax had a low peak heat release due to the low lignin content,

but the hemp extinguished much sooner due to the low compactness of the fibres after

compression moulding [15, 16].

Figure 7 - Heat release during combustion of natural fibres, adapted from [16]

1.2.4 Interaction between cellulose, hemicellulose and lignin

The interaction between the concentrations of cellulose, hemicellulose, and lignin, and the

resulting thermal stability and combustion characteristics, are interesting points for discussion.

The literature on this topic presents differing conclusions which are worth mentioning. Figure

8 examines some available literature that the author initially reviewed, and looks at similarities

and difference in their conclusions.

There are some discussions in literature regarding the contribution of lignin to the flammability

and thermal stability of natural fibres. Manfredi et al concluded that a low lignin content in flax

resulted in a higher decomposition temperature [12], while Chapple et al (reviewing work done

by other researchers) surmised that high lignin and hemicellulose, low cellulose should result

in lower flammability [10], and Kozlowski et al concluded that lower lignin of hemp and flax

resulted in lower heat release rates [20]. However, when reviewing thermal stability and

combustion test results from literature, the conclusions with respect to the three constituents,

are not clear.

In a recent investigation completed by Galaska et al, it was found that the peak heat release

rates of a selected group of natural fibres resulted the following ordering:

Jute > Flax > Sisal > Hemp > Kapok [21]

Considering the theory that low lignin results in a lower peak heat release, the order should be:

Flax > Hemp > Sisal > Jut > Kapok

They concluded that further investigations, which include chemical analysis, is required to

determine the actual cause of peak heat release rate variation [21].

It is also important to note that while this analysis focuses on composition, when considering

fibre types, there is still significant variability. Natural fibres differ in dimensions, which results

in variability of the preforms or test specimens. Even within the test specimens used by Dorez

et al, there was variability in mass, density, and compaction [15, 16]



Selected summaries from literature Comments

Yang et al 2006

Investigated the pyrolysis characteristics using thermogravimetric analysis (TGA) of cellulose,

hemicellulose, and lignin at different ratios.

Concluded that there is no significant interaction between the three components, but rather a superposition

of the three components.

Similar TGA profiles for

cellulose, hemicellulose,

and lignin

Char yield form lignin is

highest

Yang et al 2007

TGA of cellulose, hemicellulose, and lignin as well as online gas monitoring.

Lignin had the highest CO/CO2 ratio and CH4 due to methoxyl-O-CH3.

Cellulose had highest CO – due to C=O.

Hemicellulose had highest CO2 – due to OH and CO.

Combustion characteristics

for flax and hemp are

similar

Manfredi et al 2007

Investigated the combustion characteristics of jute, flax, and sisal with MODAR matrix.

Sisal showed highest fire risk – combination of high total heat released and time to ignition. However peak

heat release rate was low, so conclusion is not complete.

Similar overall findings for

flax

Kozlowski & Wladyka-Prybylak 2008

Presented heat release rate versus time graphs for abacca, hemp, cabuya and flax.

Differing opinions on

interactions of cellulose,

hemicellulose, and lignin.

Pasangulapti et al 2012

Conducted TGA of switch grass, wheat straw, eastern red cedar, and dried and distilled grains with

solubles.

Weight loss profiles were not significantly different, even with different chemical compositions.

Switch grass and wheat straw produced highest CO and CO2.

Eastern red cedar (with highest lignin) produced highest CH4

Dorez at al 2014

Investigated the correlation between the chemical composition of natural fibres with pyrolysis and

combustion characteristics, especially char yield, effective heat of combustion, activation energy, and

CO/CO2.

Characteristics were closely correlated to lignin content. Char yield and activation energy increased with

increasing lignin, while CO/CO2 reduced. This was balanced out by an increased effective heat of

combustion.

For flax, hemp, and cotton linter, an interaction between cellulose and lignin was found, Low lignin

resulted in a low peak heat release rate (and due to a low effective heat of combustion of cellulose), and a

long burn time due to char induced by lignin/cellulose interactions.

Similar findings with

regards to CO, CO2, and

CH4

Figure 8 – Selected literature on natural fibre pyrolysis and combustion



1.3 Understanding the behaviour of composites in a fire

The behaviour of composites in a fire scenario depends on factors such as composition, fibre

content, component dimensions, intensity of the fire source, and ventilation. A detailed

understanding of fire engineering is required to describe the thermodynamics,

thermochemistry, and combustions mechanism during a fire. However, some elementary

concepts help describe the behaviour of composites during a fire.

1.3.1 Stages of combustion

Mouritz and Gibson describe fire growth in five stages:

1. Ignition – The point at which the fuel sources is ignited to and sustained combustion

commences

2. Growth – The fire grows by consumption of the fuel source. Temperatures exceeding

350-500oC will ignite composite material

3. Flashover – Occurs when the temperature of the fire exceeds 600oC. At this stage, the

all the combustible items fuel the fire

4. Fully developed fire – The temperature and heat released by the fire are at the highest

points. The fire temperature reaches 900-1000oC

5. Decay – All the fuel is consumed by the fire

These stages are graphically shown in Figure 9.

Figure 9 - Stages of fire growth [19].

Schartel et al identifies this as a three-stage process (focusing from ignition to a fully developed

fire), and highlight some of the combustion properties that occur in these stages – some of the

important combustion properties being [22]:

Time to ignition – ignition stage

Heat release rate – changes through the progression of all stages

Peak heat release – fully developed fire

Total heat release – increases as the fire progresses

1.3.2 Composite decomposition overview

The decomposition of a composite during a fire results in volatile gases – consisting of

flammable (e.g. CO, methane, low molecular organics) and non-flammable (CO2 and water)

vapours and gases, solid carbonaceous char and smoke [19]. Figure 10 provides a basic outline



of the overall process. Given that the chemical make-up of each matrix system used in

composite material varies, the resulting ignition temperatures also vary.

Composite- Matrix resin decomposition

- Fibre decompositionFire (Heat flux)

Volatiles

Combustion

CO CO2 Other gases Smoke Heat release

Possible feedback loops

Laminate causes resistance to the penetration of oxygen and heat

Figure 10 – Composite decomposition mechanism, adapted from [19]

Both the matrix and reinforcement of a typical fibre composite are flammable, but the extent

to which degradation occurs depends on the thermal stabilities of the individual components.

Decomposition reactions

There are various decomposition reactions that occur when a polymer is introduced to a high

heat source. The primary ones that result in molecular weight loss are:

Random chain scission

Chain-end scission - ‘unzipping’

Chain stripping – removal of side groups

Two other process that increase the molecular weight, which are induced thermally are:

Cross-linking

Condensation

Polymer decomposition typically involves more than one of the above reactions, but the

dominant one in most cases is random chain scission, occurring at the location with the weakest

bond in the chain [19]. This location in the molecular structure is often where an ‘irregularity’

occurs, for example a relatively unstable linkage with a low dissociation energy [19]. As the

temperature increases, bonds of higher dissociation energies separate, resulting in the break-

down into oligomer, monomers, and other low molecular weight compounds [19, 23]. For the

generation of compounds that are volatile in a fire, approximately 10% of bonds have to rupture

[19].

Competing with random chain scission, another important decomposition reaction is chain-end

scission. Volatile chain fragments or individual monomer units are detached from the chain

end until complete depolymerisation of the polymer [19].

Oxygen can accelerate the decomposition process, but the production of volatiles on the surface

prevents diffusion of oxygen past the outer layer [19, 23]. As decomposition progresses into

the composite thickness, the influence of oxygen is minor in comparison to heat [19, 23].

The significance of char

Char is a porous material that contains crystalline and amorphous regions, and more carbon

than its underlying material, such as a polymer [9, 19]. The formation of char is dependent on

the chemical nature of the fibre (as discussed in section 1.2) and matrix, as well as the

combustion atmosphere and combustion temperature. The purpose of char formation during

combustion is to protect the underlying material from further decomposition from fire or heat



[9]. The properties of char that ensures the protection of the underlying material are, effective

insulation, resistance to combustion, structural soundness and inability to be damaged, and

stability during combustion [9].

1.3.3 Decomposition of common polymers

Section 1.2 looked at different fibre types, so the following sections will overview the thermal

decomposition and combustion of polyester, epoxy, and polypropylene composites.

The CIC Agri-Innovation project and AMIC Biosphere project focus mainly on polyester and

epoxy matrix systems respectively. While these projects concentrate on thermoset matrix

systems, thermoplastic matrix systems such as polypropylene are extensively used in natural

fibre composite manufacture. Therefore, it is prudent to consider polypropylene for

comparison.

Polyester

Polyesters are a common group of thermoset resin, which have advantages including curing

over large temperature ranges under moderate pressures, low viscosity resulting in better

compatibility with fibres, and their ability to be modified by other resins [9]. They contain an

unsaturated polyester chain of relatively low molecular weight, dissolved in styrene. During

the cure process, the styrene polymerises, creates permanent cross-links between the

unsaturated polyester chains. High exothermic curing of polyester influences processing rates,

due to high heat generated.

Thermal decomposition and fire behaviour of polyester resins are controlled initially by

scission of highly strained portion of the polystyrene cross-link this is a two-stage process. The

scission results in the formation of free radicals that promote further decomposition – including

accompanying scission of the polyester backbone [19]. This results in a variety of low weight

flammable volatiles such as CO, CO2 methane, ethylene, propylene, butadiene, naphthalene,

benzene and toluene, which contributes to the increased flammability of polyester [19, 24].

During decomposition, the polyester tends to go through a ‘liquid’ or low viscosity stage

resulting in flaming droplets, which is reduced by the presence of the reinforcing fibre or fillers

[19].

TGA curves (see Figure 11).comparisons for hemp/polyester with neat polyester, shows that

about 98% of the original mass is decomposed into these volatiles rather than char – the main

reason being the high heat release rate of polyester [19]. The DTG curve shows three peaks,

one small peak at around 200oC, another around 350oC, and the final at 390oC. As per the

observation from Figure 4, these three peaks may correspond to lignin, hemicellulose, and

cellulose decomposition.

An example of heat release of polyester during combustion is shown in Figure 12. When

reinforced with fibres, the amount of polymer is reduced, and heat released is effected.



0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500

WeightLoss(%)

Temperature(oC)

UP

Hemp/UP

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500

Deriv.W

eight(%/oC)

Temperature(oC)

UP

Hemp/UP

TG Curve for hemp/unsaturated polyester composite DTG Curve for hemp/unsaturated polyester

composite

Figure 11 - TGA curves for hemp/polyester composite [25]

The effect of the reinforcing fibres can be seen in the curve of heat release over time, Figure

12. This is an example of how the heat release can be reduced when polyester is combined with

a reinforcing fibre.

0

100

200

300

400

500

600

700

0 100 200 300 400 500

HeatR

eleaaseRate(kW/m

2)

Time(s)

Neatresin

Reinforcedwthglass

Figure 12 - Heat release of polyester composite, adapted from [26]

Epoxy

Widely used in structural composite applications, these consist of a resin and a curing agent or

hardener, and can range from low-viscosity liquids to high melting point solids [9]. Epoxies

can have glass transition temperatures up to 220oC, meaning that they can be used safely up to

these temperatures [9]. Epoxy resins can be kept in an uncured state for long durations, which

allow the manufacture of pre-pregs, which are resin impregnated fibres that are partially cured.

Similar to polyester, a large number of flammable volatiles are produced during the

decomposition reaction, which increase the flammability of epoxy resins. Of the original

weight of polymer, around 10 to 20% is converted into porous char, and above temperatures

of 550oC, in an oxygen environment, this char will oxidise [19].

For a majority of epoxies, the decomposition happens over a range of 380 to 450oC, through

random chain scission reactions. Where amides or amines are used for curing, the scissions



occur at the carbon-nitrogen bonds, due to lower dissociation energies of nitrogen linkages.

Hydroxyl groups in epoxies are also susceptible to high temperature degradation [19].

The TGA curves in Figure 14 compare epoxy with flax/epoxy composite. For the TG curves,

it is evident that as expected, there is a higher residual mass at 500oC than polyester, and an

even higher residual mass of the composite. The same peaks seen with the hemp/polyester

composite is not visible, however the decomposition range corresponds approximately with

that of flax, between 270oC and 400oC, also seen earlier in Figure 5.

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500

WeightLoss(%)

Temperature(oC)

Epoxy

Flax/Epoxy

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 100 200 300 400 500

Deriv.W

eight(%/oC)

Temperature(oC)

Epoxy

Flax/Epoxy

TG Curve for epoxy composite DTG Curve for epoxy composite

Figure 13 - TGA curves for hemp/polyester composite [27]

Figure 14 shows the heat release comparison of epoxy, and a glass/epoxy composite. As with

the hemp/polyester composite, addition of a reinforcing fibre significantly reduces the peak

heat released during combustion.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 20 40 60 80 100 120 140 160

HeatReleaaseRate(kW

/m2)

Time(s)

Epoxy

Glass/Epoxy

Figure 14 - Heat release during the combustion of epoxy [27]

Polypropylene (PP)

Thermoplastics differ from thermosets mainly in that the former do not undergo the permanent

cross-linking reactions, but rather melt and flow when heat and pressure are applied, then

resolidify upon cooling [9].



A weak point in the polypropylene backbone chain is created by the presence of a ‘tertiary’

carbon. During thermal decomposition, main scission occurs at this tertiary carbon which forms

two free radicals, which randomly attacks the chain at various points, forming a variety of

olefinic components [19]. The formation of free radicals is also increased by the presence of

oxygen, which in turn reduces the decomposition temperature. No char remains after the

decomposition of these olefins as they completely decompose into volatiles [19]. Figure 15 and

Figure 16 shows the comparison of TGA and heat release curves for polypropylene and

kenaf/polypropylene composite.

Observing the TGA curves for polypropylene and a kenaf/polypropylene composite, some of

the expected outcomes when heating to high temperatures can be seen. The TG curve of

polypropylene shows no residual mass after the final temperature is reached. As seen with the

hemp/polyester composite, the DTG curve shows the three peaks which could be attributed to

the decomposition of the kenaf fibres.

TG Curve for kenaf/polypropylene composite DTG Curve for kenaf/polypropylene composite

Figure 15 - TGA curves for kenaf/polypropylene composite, adapted from [28]

Figure 16 shows polypropylene reinforced with kenaf. Unlike the two previous heat release

curves in Figure 12 and Figure 14, the reduction of the heat release due to the addition of kenaf,

does not significantly reduce the heat release. This is attributed to the high flammability (low

thermal stability) of kenaf fibres.

Figure 16 - Heat release during the combustion of kenaf/PP composite, adapted from [28]



1.4 Fire property testing – cone calorimeter

Fire property testing allows engineers and designers to determine how a specific material

behaves in a fire. Furthermore, it allows comparisons of materials for a given task, and helps

determine the impact on health and safety of personnel in the vicinity of a burning object or

structure.

There are various techniques for conducting flammability testing of materials, but there is no

single test that is able to provide all the combustion characteristics of a composite material

during a fire [19]. The tests also vary depending on the size of the specimen, as shown in the

Figure 17 below.

100m2

Room fire test

Room corner test

10m2

US Navy quarter scale room

Furnace test

Single item burning test

1m2

DTRC burn-through test 0.1m2

Smoke toxicity (NFPA269)

Flame spread test

NASA flame propagation

0.01m2

Cone calorimeter

NBS smoke chamber

LOI test

0.001m2

Figure 17 – Range of specimen sizes of different fire test methods, adapted from [19]

Of these tests, the cone calorimeter is considered to be very versatile bench-scale test, because

it is able to extract a large number of combustion characteristics from a relatively small

specimen, with one test. Additionally, the combustion environment of a cone calorimeter is

considered to effectively represent most fire scenarios [19, 26].

1.4.1 Cone calorimeter

ISO5660 [29] provides a comprehensive guide to the design and set-up of the cone calorimeter.

A basic layout is shown in Figure 18.

Figure 18 – Basic cone calorimeter layout [30]



The cone calorimeter consists of the following main components:

A sample holder, which allows the sample to be placed in the horizontal of vertical

position

Load cell

Cone heater

Spark ignitor

Exhaust hood

Exhaust extraction pump

Laser exhaust analyser

Operating system (PC), and software

Prior to conducting a test, calibrations of the equipment are completed. Some of the key

calibrations are:

Exhaust flow rate

Laser smoke measuring

Gas analyser (Oxygen, CO2 and CO)

Mass

Heat flux

During a test, the sample is placed on the holder below a cone heater. It can be ignited by a

spark ignitor, or spontaneously via the radiant heat flux of the cone heater. As combustion

occurs, the produced exhaust gases are extracted through an exhaust extraction pump at a flow

rate of 24l/s [29]. This is a fixed value that is required to ensure that all combustion gases are

captured by the exhaust system for analysis. A laser exhaust analyser measures the exhaust

products.

The cone calorimeter uses the principal of oxygen consumption calorimetry observed by

Huggett – the premise is that for most organic material, the heat produced per kilogram of

oxygen consumed is approximately 13.1MJ [31].

The data collected via the gas analyser is inputted into the operating system, which calculates

the combustion properties.

Heat flux selection

The selection of the heat flux is at the discretion of the researcher, based on the fire scenario

being investigated, and dependant on the sample being tested. ISO 5660 recommend using a

heat flux of 35kW/m2 in the absence of further information on heat flux requirements [29]. The

selection of literature reviewed by the author has found that for natural fibre composites, a

range of 25 - 50 kW/m2 is commonly used [28, 32-46].

As a further guide, Mouritz and Gibson provide the following guide for heat fluxes based on

certain fire scenarios [19]:

Small smouldering fire: 2 – 10 kW/m2

Trash can fire: 10 – 50 kW/m2

Room fire: 50 – 100 kW/m2

Post flashover fire: >100 kW/m2

Gas-jet fire: 150 – 300 kW/m2

Spontaneous ignition vs. piloted ignition

Tests can be conducted either by spontaneous ignition or piloted ignition. Spontaneous ignition

occurs due to the flammable vapour/air mixture on a hot composite surface. For pilot ignition,

an electric spark or flame initiates combustion in the flammable vapour/air mixture near the

surface of the composite surface [19]. ISO 5660 – 1 recommends the use of a spark ignitor for



exploratory testing, but tests can be conducted with spontaneous ignition. It is important to note

that the results of the two methods vary.

Sample dimensions

The dimensions of sample that can be tested is 100mm x 100mm with a thickness less that

50mm. Earlier, it was mentioned that a unique aspect of the cone calorimeter is the ability to

obtain several combustion characteristics from one test. There is a disadvantage with small

sample size; combustion only penetrates the thickness, i.e. it is one dimensional, and no useful

flame spread measurements can be obtained [22].

1.4.2 Combustion properties

The cone calorimeter simulation extends up to the point of flashover, as shown in the stages of

combustion (Section 1.3.1), so it does not look at the post flashover or fully developed

conditions. The results obtained from a cone calorimeter test can be entered into simulations

for predictions of large-scale fire behaviour [22].

This section will review some of the critical combustion properties obtained during a cone

calorimeter test. Individually they provide a metric

TTI - Time to ignition (s)

This is the point at which flaming combustion commences. It defines the ease of ignition of a

material when an external heat flux (and ignition source) is applied. In a cone calorimeter test,

this is the first event that is observed [19]. The requirement for ignition is sufficient production

volatiles by mass loss, which at the air flow of the cone calorimeter test set-up, can be ignited

by a spark [22]. This mass loss rate measured at ignition has been found to be approximately 1

– 6gs-1m-2 [22].

HRR - Heat release rate (kW/m2)

When a material exposed to a fire or external heat flux, the amount of thermal energy produced

per unit surface area, is the heat release of a material. The heat released during combustion of

a material provides further thermal energy to allow the fire to develop and propagate, so the

HRR is an important property to identify [19]. When plotted against elapsed time, this forms

the heat release curve (for example Figure 12, Figure 14, and Figure 16).

pkHRR - Peak heat release rate (kW/m2)

The peak heat release rate is the highest heat release rate over the course of combustion. It can

be used as a measure to compare the flammability of different materials. On the heat release

curve, the pHRR is the highest point.

THR/THE - Total heat release/Total heat evolved (MJ/m2)

The THR is the total heat output to a specific point of combustion, and is the integral of the

heat release rate. At the end of a test, the THR becomes the total heat evolved (THE), which is

the fire load of the tested specimen. THE is another suitable measure for comparing materials

as it indicates the input of heat to a sustained fire [19].

Mass loss rate (g/s)

The heat flux, decomposition temperatures, and heat transfer all effect pyrolysis, and the mass

loss rate is a result of pyrolysis [22]. HRR is controlled by the mass loss rate, when the materials

being tested has a constant effective heat of combustion – which is the case for most materials

[22]. THR is controlled by the total mass loss, which is the difference between residual mass

and specimen mass.



SEA - Specific extinction area (m2/kg)

The specific extinction area is a means of determining how effective a specified mass of

flammable volatiles, which is produced by a combustible material, is converted into smoke.

Smoke density can be defined using SEA. It is often used to quantitatively define the smoke

density [19].

Petrella Plot

The Petrella Plot is two-dimensional visualisation of the fire hazard of different materials tested

to the same conditions. Petrella suggested this plot since a cone calorimeter test does not

consider for flame spread [47].

The x-axis of the plot is the pkHRR/TTI, and the y-axis is the THR (THE). Figure 19 shows a

Petrella plot for various glass fibre composites.

Figure 19 - Petrella Plot of various composites at 50kW/m2[48]

As a material moves right along the x-axis, the fire growth is fast, while it moves up the y-axis,

the duration is long. Therefore, materials that are positioned closer the origin of the plot have

a lower fire risk.

Other indices

In simplifying results obtained from cone calorimeter testing, researchers have created other

indices. Three such indices are:

FPI - Fire performance index (TTI/pkHRR = sm2/kW). A higher values corresponds

with better fire retardant performance [49]

MARHE – Maximum average rate of heat emission

FIGRA – Fire growth rate (HRR/TTI = kW/s)

While these are mostly used for regulatory purposes, where certain fire properties are isolated

to a single metric so comparisons of different materials can be made, the risk is that they

simplify the complex behaviours of materials in a fire scenario.

1.4.3 Material properties, test parameters and combustion behaviour

While there are several factors that interact with combustion properties, there are some notable

ones to mention. Not every interaction has been researched, however the below examples

provide some insight, and allow the prediction of these behaviours.



Burning behaviour characteristics

Certain characteristic heat release curves are expected from specific burning behaviours, see

Figure 20.

Heat release curve characteristics:

Initial increase in heat release,

followed by steady heat release

Peak heat release is reached towards

the end of combustion, followed by

rapid decline in heat release

Material behaviour is thermally thick.


Initial increase in heat release,

followed by a shallower heat release




Material behaviour displays properties of

both thermally thin and thermally thick.


Initial increase in heat release until

peak is reached, and char layer is

formed

As char thickens, heat release

gradually declines




Material behaviour is thermally thick.


Initial increase in heat release until

peak is reached, and char layer is

formed

As combustion continues, char layer

cracks or is damaged

Second peak is reached as the

material underneath the damaged

char combusts

Material behaviour is thermally thick. Figure 20 - Burning behaviour characteristics [22]



Mat architecture

Brown and Mathys observed that chopped strand and woven mats (both of glass fibres), had

differing combustion properties [26], see Figure 21.

Figure 21 - Ignition times of woven and chopped strand mats [26]

Mouritz and Gibson suggest that a heat release curve for composites with chopped strand mats

typically begin with a broad peak, followed by a stead decomposition. While woven mats

decompose with fluctuating peaks and troughs [19], see Figure 22. The authors explain that the

distribution of resin between layers is more even in chopped strand mats, as opposed to woven

mats, which have identifiable layers of resin and fabric. During decomposition, the chopped

strand mat composites will combust evenly, while the woven mat composites will combust

unsteadily as each layer decomposes [19].

Figure 22 - Heat release curves for chopped strand mats and woven mats [19]

Chai et al compared unidirectional, plain weave, and twill weave fabrics (both glass and flax),

with an epoxy matrix [50]. They also observed variations in combustion properties, see Figure

23, Figure 25, and Figure 25. The weaves are:

a) Unidirectional glass, b) Plain woven glass c) Twill woven glass d) Unidirectional flax

e) Light twill woven flax f) Heavy twill woven flax



Figure 23 - Fabric weave types tested by Chai et al [50]

Figure 24 - Combustion behaviour of different fabric weaves, adapted from [50]

Figure 25 – HRR curves of fabric different weaves [50]



The heat release curves (Figure 25) correspond to the suggestions made by Mouritz and

Gibson, however other than the HRR, other combustion characteristics are not significantly

influenced by the type of weave. It would be interesting to compare these results with chopped

strand mats of both glass and natural fibres.

The shape of these curves correlates to the combustion behaviour of a thick charring composite

(as per Figure 20), which is expected from an epoxy.

Sample thickness

The thickness of the sample can also effect combustion properties. A graph of the time to

ignition against thickness shows how the increase in thickness increases the ignition time [19],

Figure 26 - Effect of thickness on time to ignition [19]

In another example, the thickness does not impact the time to ignition, but impacts the peak

heat release and total heat release, see Figure 27 and Table 2.

0

100

200

300

400

500

600

700

800

900

1000

0 50 100 150 200 250 300 350

Heatreleaserate(kW/m

2)

Time(s)

3mm

6mm

10mm

12.5mm

20mm

25mm

Figure 27 - Effect of thickness on HRR for PMMA [22]

There is a gradual reduction of the peak heat release rates, however as the thicker samples

combust for a longer duration, they produce more heat. The samples greater than 6mm also

display the behaviours of intermediate to thick non-charring materials.



Sample thickness pHRR (kW/m2) THR (MJ/m2)

3mm 999 29

6mm 899 55

10mm 898 91

12.5mm 897 121

20mm 859 190

25mm 678 211

Table 2 - Combustion properties of PMMA, adapted from [22]

Sample thickness variation is critical when attempting to benchmark fire performance of

thermally thin materials. A thermally thin material absorbs the thermal wave rapidly with no

thermal gradient through the thickness of the material. Thermally thick material on the other

hand, will have a temperature gradient through the thickness. This difference between the faces

of the sample will influence the combustion characteristics [19].

Heat flux

In addition to showing the effect of thickness on ignition time, Figure 26 shows how increasing

the incident heat flux reduces the ignition time.

Dao et al investigated this same behaviour for carbon fibre/epoxy composites with different

fibre volume fractions, see Figure 28 and Figure 29.

Figure 28 - Ignition time variation with heat flux [51]

By conducting multiple tests using material of identical properties, they were able to determine

the critical heat flux (CHF) – the heat flux below which ignition does not occur.

Their results also show how the peak heat release rate, total heat release, and mass loss rate all

varied with increasing heat flux see Figure 29.



Figure 29 – Combustion property variations with heat flux [52]

(a) 56vol% carbon fibre (b) 59vol% carbon fibre

Fibre volume fraction

In the above research done by Dao e t al (Figure 28 and Figure 29), composite materials with

fibre volume fractions of 56% and 59% were compared. With a 3% difference, they were able

to observes significant variations in the results. El-sabbagh et al also looked at flammability

and thermal stability with 30% and 50% fibre volume fractions. There findings showed that

50% had better fire performance than 30% [43].

When using inert synthetic fibres, it is logical to increase the fibre volume fraction to improve

fire performance. However, the same case in not so trivial when using natural fibre, as both

natural fibres and matrix resin are combustible.

This is a brief review of some of the material properties, test parameters, and characteristics

that influence the combustion characteristics of materials while testing using a cone

calorimeter. There are other properties that can be developed in future work.

The properties described in the above sections have not been thoroughly investigated for bio-

composite. Therefore, contributions to these areas will enable better understanding of bio-

composite fire performance.

1.5 A review of fire retardants

The use of fire retardants is not a recent development. Ancient Egyptian soaked reed and grass

with sea-water prior to using them as roof materials – fire retardant properties were obtained

by the mineral salts that were deposited once the fibres dried [9]. Chinese people mixed alum

and vinegar, which they applied to wood and covered with clay to delay and minimise fire

spread [9]. Alum was also used in ancient Rome to soak wood. Studies on the use of fire

retardant properties began in the 17th century, some attributed to Gay-Lussac, who in 1821,

identified the scientific principals relating to fire retardants. Gay-Lussac developed retardants

from ammonium phosphate and borax for cellulosic material, which can still be used as

effective retardants [9].

As natural fibre composites are made up of a matrix system, and a reinforcing fibre, the three

main methods of achieving fire retardancy is by adding a retardant to the matrix, treating the

fibres, or by surface treatment of the composite [53].



1.5.1 Retardant classifications

Researchers [9, 19, 53] who have studied fire retardants, have similar classifications for

retardants – reactive or additive. Reactive compounds are incorporated into the molecular

structure of the polymer during processing [9, 19, 53]. Reactive compounds may also interact

with the three constituents of natural fibres, but is dependent on how the retardant is used [9].

Some common reactive retardants are based on halogen, phosphorus, inorganic compounds,

and melamine compounds.

Additives are closely mixed with the polymer during processing, however they do not react

with the polymer. Additives contain elements such as phosphorus, aluminium, boron, bromine,

and chlorine [19]. They are also used as fillers, and can be incorporated in the final stage of

processing, for example, mixed with the matrix polymer prior to hand layup during composite

manufacturing. The minimum required filler loading is typically high – at approximately 20%

of the volume, with values below this being ineffective. Additive fillers can further be classified

as inert and active. Inert and active fillers both retard a fire by diluting the mass fraction of the

organic material in a composite, and acting as a heat sink [19]. The difference with an active

filler is that the decomposition reaction of the filler further assists with reducing a fire. Figure

30 categorises these classifications.

Figure 30 - Fire retardant classifications

Gas phase vs condensed phase

An additional classification for fire retardants is based on whether they act during the gas phase

or condensed phase.

The gas phase retardants act by affecting the combustion reaction, hence preventing flame

spread and reducing the amount of heat back from the combusting material to the fire. A

common method of flame retarding in the gas phase is releasing radicals based on bromine,

chlorine or phosphorous [9, 19]. These retardants stop the exothermic combustion reactions by

removing H and OH radicals from the flame [9, 19]. Alternatively, the gas phase retardants can

release non-combustible vapours that dilute the concentrations of H and OH gases in the flame,

subsequently lowering the flame temperature [9, 19].

Condensed phase refers to the polymer in a molten or solid state. There are several ways that a

condensed phase retardant can function. Inert fillers operate in the condensed phase by diluting

the combustible organic material content, and acting as a heat sink [9, 19]. Active fillers

undergo endothermic decomposition in a fire, producing water or other non-combustible

Fire retardants

Reactive

Interact with matrix chemicstry

Interact with fibre constituents

Additive

Active filler

Dilute mass fraction of organi material

Act as heat sink

Decomposition reaction reduces fire

Inter filler

Dilute mass fraction of organic material

Act as heat sink



products [9, 19]. Reactive retardants can interact with the polymer so that during

decomposition, an insulating char layer is created on the surface of the substrate, which reduces

the conduction of heat, and decreasing the flammable gas emissions [9, 19].

Many flame retardants function in one phase only, but a majority of the effective ones operate

in both.

The trend in recent literature on using fire retardants with natural fibre composite avoids

halogented retardants given the assumed toxic effects and environmental concerns that the use

of them present. As a result, this review will not look focus on these.

In literature, reactive retardants are more common than additive retardants. While, within the

category of additive retardants, inert fillers are often used with either reactive retardants or

active fillers. This can be seen in the summary provided in Table 3. The order of preference of

these findings suggests:

Reactive retardants > Active (additive) Fillers > Inert (additive) Fillers

Some examples of these retardants will be discussed in the following sections.

1.5.2 Examples of reactive retardants

Ammonium polyphosphate (APP, [NH4PO3]n)

Of all the nitrogen containing retardants, APP is the most significant one. It is an inorganic salt

of polyphosphoric acid and ammonia comprising linear and branched chains [23]. APP

compounds are divided into two families: Crystal Phase I APP (APP I) and Crystal Phase II

APP (APP II) [23]. The differences are:

APPI – variable linear chain length, lower decomposition temperature ~150degC,

higher water solubility, number of phosphate units (n) is usually lower than 100

APPII – branched or cross linked, higher molecular weight than APP I, n>1000,

decomposition at ~300degC, lower water solubility [23].

During combustion of an APP containing material, as the retardant begins to decompose,

ammonia and polymeric phosphoric acid are produced. The decomposition reaction is

endothermic which removes heat, and combustible gases are dilute by the water and ammonia

produced. On the surface of the material, a foamed char layer (or intumescent layer) is created,

which insulates and averts further decomposition. The condensed phase charring of APP results

in the reduction of smoke density [23].

For the majority of polymers, the reaction occurs in the condensed phase, during which acid

catalysed dehydration reactions occur and cross-linking promotes char formation. In other

polymers, the reaction may also occur in the gas phase. Below the matrix decomposition

temperature, phosphorous radicals are discharged from the matrix and extinguish the

combustion by reacting with H* and OH* radicals in the flame. Furthermore, oxygen at the

surface of the material can be constrained by a vapour-rich phase created by heavy phosphorous

containing volatiles [10].

APP can be used as either a reactive or additive retardant. Figure 31 shows the effect of APP

on the heat release curve. The shape of the curve corresponds to a thick-charring material

identified in Figure 20. It is also interesting to note that while peak heat release rate and total

heat evolved are reduced, the material burns for a longer duration.



Figure 31 – Effect of APP on the HRR, adapted from [28]

1.5.3 Examples of active fillers

Metal hydroxides and oxides

This group of active fillers gain attention due to their low toxicity, ability to supress

combustion, and reduce the temperature of the substrate [54]. They decompose

endothermically, releasing water during burning, and thus diluting the fuel or flammable gases

fed to the fire [19, 54]. The ability of these to be effective depends on the concentration used,

which can be between 20 – 60% [19].

Some common metal hydroxide retardants discussed in this section are:

Aluminium trihydroxide (ATH, Al(OH)3)

Magnesium hydroxide Mg(OH)2

Zinc borates (ZnB, xZnO.yB2O3.zH2O)

Other metal oxides, containing antimony (Sb2O3, Sb2O5) and tin (SnO2), are also occasionally

used [19, 54].

Aluminium trihydroxide (ATH, Al(OH)3)

ATH is a commonly used active retardant filler, due to its low cost, good flame retardant

properties, and non-toxic smoke. During a fire, the following endothermic reaction occurs:

2Al(OH)3 Al2O3 + 3H2O

Per gram of ATH, this reaction absorbs approximately 1kJ of heat [19]. The water created from

the hydroxyl groups dilutes the flammable gas concentration and temperature (in the gas phase)

deterring combustion.

The endothermic reaction also limits access of oxygen to the combustion surface, and also

functions in the condensed phase, acting as a heat sink which delays the composite in reaching

decomposition temperature. Al2O3 produced during decomposition can also act as refractory

layer, however very high loading of ATH is required [19, 54].

There are some disadvantages. Due to the high loadings required, it has been found to effect

mechanical properties. ATH also decomposes between 220 and 400oC, so is not suitable for

polymers that have a high processing temperatures above 220oC [19, 54].

Figure 32 shows the effect of aluminium trihydroxide on the heat release rate.



Figure 32 – Effect of aluminium trihydroxide on the HRR, adapted from [55]

Magnesium hydroxide (MDH, Mg(OH)2)

MDH acts in a similar way to ATH, with the following reaction:

Mg(OH)2 MgO + H2O

Unlike ATH, MDH is thermally stable from 330 to 400oC, and the MgO (magnesia) that is

produced in this reaction act as good insulator [19, 54].

Zinc borates (ZnB, xZnO.yB2O3.zH2O)

There are at least 25 compounds in this inorganic group of compounds, typically represented

as a combination of oxides and water – (x)ZnO•(y)B2O3•(z)H2O. However only two,

2ZnO•2B2O3•3H2O and 4ZnO•6B2O3•7H2O have ideal flame retardant properties [56].

The combustion reaction during a fire is endothermic, but unlike ATH and MDH, per gram of

zinc borates, they absorb approximately 0.5kJ of heat. The reaction releases water which dilutes

the flammable volatiles in the gas phase. Its primary function is in the condensed phase, where

the flame is suppressed by the formation of char activated by the zinc in the compound.

Furthermore, the production of boron oxide (B2O3) during decomposition protects the

substrate. It is often used with ATH and MDH, which during a fire can create a porous ceramic

layer to thermally insulate the underlying material [56].

Figure 33 shows the effect of zinc borates on the heat release curve. The shape of the curve

corresponds to a thick-charring material as identified in Figure 20.

Figure 33 – Effect of ZnB on the HRR, adapted from [35]

Melamine based retardants

Melamine based active fillers are another widely used group of retardants. Comprised

predominantly of nitrogen, melamine decomposition is also endothermic, resulting in the

cooling of the substrate and releasing of and non–combustible gases such as CO2 and ammonia,

and water [23]. During its endothermic decomposition, melamine requires approximately

470kcal/mole. It undergoes sublimation around 200oC, requiring of 29kcal/mole during the

process [23].



1.5.4 Examples of inert fillers

Silica based (Silicon dioxide, SiO2) and Calcium Carbonate (CaCO3)

Both these fillers work using a physical process rather than a chemical reaction. The efficacy

of these fillers depend on factors such as loading, particle size, pore size, surface area, density

and viscosity [57]. The volatilisation of thermally degrading products during a fire can be

trapped or reduced by the presence of silica which increases the melt viscosity during pyrolysis

[57].

An equilibrium achieved between density and surface area of the filler, along with the polymer

melt viscosity, governs whether the filler moves towards the sample surface or sinks through

the polymer melt layer. The filler may act as a thermal insulation layer if they move towards

the surface. Additionally, the presence of the filler on the surface also dilutes the polymer

concentration that would be in contact with the flame [57].

It is common to use both fillers with other retardant retardants such as ATH, MDH or APP –

some findings will be discussed later.

Figure 34 shows the effect of nano-SiO2 on the heat release curve. In this instance, the addition

of nano-SiO2, has reduced the peak heat release, and total heat evolved.

Figure 34 – Effect of nano-SiO2 on the HRR, adapted from [36]

1.5.5 Discussion of findings

There are some variations in the findings, however the overall results suggest that retardants

can be used with natural fibre composites. And as expected, and discussed in section 1.3, the

retardant loadings were high. Table 3 summarises selected recent literature on the use of

retardants with natural fibre composites.

In the authors survey of recent literature, APP was found to be a commonly used retardant [28,

34, 35, 39-41, 44, 45]. There are several additive manufacturers such as Clariant, who make

several grades APP fire retardants that are suitable for various application [28, 39-41].



Reference Retardants

used

Comments

Boccarusso et

al 2016 [45] APP

16.32w% of APP was used without significantly affecting the mechanical properties,

other than strain to failure, of hemp epoxy composites.

Reduction in peak heat release rate, total heat released, and total smoke released.

Increased residual mass.

Subasinghe et

al 2016 [28] APP

Used Taguchi and design of experiments for determining the optimum parameters for

a polypropylene kenaf composites.

Found the optimal concentration to be 27wt% using analysis of variance (ANOVA)

Peak heat released rate and total heat released was reduced.

Branda et al

2016 [44]

APP

Silica

Investigated the effect of treating hemp fibres with waterglass (a silica based coating),

and adding 15wt% APP, prior to manufacturing an epoxy composite

The combined treatment improved combustion properties, and formed stable char

Low interfacial adhesion between fibres and matrix resulted in brittle behaviour which

showed lower stiffness

Fracture energy absorbed was higher

Nikolaeva et

al 2015 [35]

APP

ZnB

Melamine

Comparison of polypropylene conifer wood flour composite

Best improvement in HRR and residual mass was achieved with 30% ZnB

20-30% APP had similar reductions in pHRR

Best improvement in THR with 20% APP

Both concentrations of APP used had a detrimental effect on TSR

Both melamine concentrations showed low CO and SEA

20% ZnB showed no benefit

No synergetic effects were investigated

Szolnooki et

al 2015 [32]

Phosphorus

based

Investigated twill woven hemp/epoxy composites, comparing difference fire-retardant

and fibre treatments

Considering only the fibres - a combination of fibre treatments was the best

With the matrix, combination resulted in higher residual mass, but also high THR

Combination with matrix had no benefit to SEA, but CO and CO2 were lower

The mechanical propertied for the matrix and fibres individually were poorer, but the

composite was comparable to the reference with no retardants– increased adhesion

between treated fabric and matrix, and easier wetting interaction in the more polar P-

containing matrix.

Alongi et al

2015 [46]

Silica,

sodium

closite,

carbonate

hydrotalcite

Various nano-particle treatment of cotton fibres – no matrix

Synergetic effect observed with silica and carbonate hydrotalcite (which contains Mg

and Al)

Katancic et al

2013 [40]

APP, silica,

CaCO3,

Al(OH)3

Investigated the effects of nano-particle and retardant combinations in pine wood

flour/HDPE composite

CaCO3 nanofiller reduced the stiffness showing lower storage modulus

SiO2 induced crystallinity in polymer matrix and increased the elastic modulus SiO2

and APP showed best combined fire performance.

Mingzhu et al

2013 [36] Silica

Tested various concentrations of silica with poplar wood flour/HDPE composite

Considering all combustion characteristics, 8% performed the best

Mechanical properties were adversely affected

El-sabbagh et

al 2013 [43] Mg(OH)2

20 and 30% Mg(OH)2 with varying PP in flax /PP composite

30% Mg(OH)2 showed best results

Stiffness increased with Mg(OH)2 addition – 30%

Strength, elongation to break, and impact reduced.

Suppakarn et

al 2009 [33]

Mg(OH)2,

ZnB

Compared sisal/PP composite with zinc borate and Mg(OH)2.

Fibres were treated with NaOH prior to composite fabrication Mg(OH)2 reduced the

burning rate better than ZnB

No synergetic effects were observed

Viscosity at the shear stress rates indicate that flame retardant had no influence on the

processability

Morphology suggests good distribution

Comparable tensile and flexural properties with neat composite, but PP composite had

lower impact strength

Hapuarachchi

et al 2007 [42]

CaCO3,

Al(OH)3

Hemp/UP with 40% CaCO3/Al(OH)3

20 and 50kW/m2 was used

Al(OH)3 improved pHRR and TTI

CaCO3 had no significant benefit

Table 3 - Summary of retardants used with natural fibre composites



1.6 Further research

The literature of fire performance of natural fibre composites focuses mostly on the scientific

aspects of using fire retardants. Based on general readings and overall use of natural fibre

composites, the following gaps where identified:

Limited focus on composite manufacturing processes

Influence of mat architecture and the effects of fire retardants

Permeability of resins with retardants

Effects of viscosity on permeability

Retardant mixing and dispersion

These topics are discussed in the following sections. It should be noted that there are overlaps

in these topics, therefore further research can encompass one or more of these areas in one

given study.

1.6.1 Limited focus on composite manufacturing processes

Where researchers have investigated the effects of retardants on natural fibre composites, there

tends to be a preference for certain manufacturing processes. Surveying a selection of the

literature where retardants were used with natural fibre composites, it was found that when

using thermoplastics, extrusion (following by hot press moulding) was a common

manufacturing process [28, 35, 36, 40, 43, 58-61].

When using thermosets, hand lay-up was more common [25, 27, 32, 37, 44, 62-64], than liquid

composite moulding [39, 45, 50, 62].

Liquid composite moulding (LCM) techniques such as resin transfer moulding (RTM) and

vacuum assisted resin transfer moulding (VARTM) are extensively used as they are cost

effective and efficient manufacturing processes. Some other benefits are shown in Table 4.

Therefore, while some research has considered retardant addition and manufacturing processes,

limitations of liquid moulding with retardant addition has not been adequately considered.

Process Skill

required Productivity

Fibre

volume

fraction

Cost Strength Reinforcement

arrangement Shape

Common

materials

Hand lay-up Low Low Medium Low Medium

to high 2D

Simple to

complex

Prepreg

fabric with

polyester

vinyl ester

Autoclave

processing High Medium High

Medium

to High High 2D

Simple to

complex Prepreg

RTM/VARTM Medium High Medium

to High Low

Medium

to high 2d and 3D

Simple to

complex

Preform

and fabric

with

polyester,

vinyl ester,

and epoxy

Table 4 - Comparison of composite fabrication processes [65]

1.6.2 Influence of mat architecture and the effects of fire retardants

When using thermoset matrices in liquid composite moulding methods, it was also observed

that researcher preferred to use woven fabrics. From the articles, mentioned above, only two

used random fibre [39] or chopped strand [25] mats.

It is understandable that researchers would prefer to focus on woven mats as the final composite

has better mechanical properties than random or chopped fibre mats. However, there are

applications where mechanical properties are not critical, so non-woven mats would be suitable

for these applications. Further contribution will not only fill gaps in the current knowledge, but



will also allow for comparisons, and possibly increase the deployment of natural fibres for

composite applications.

1.6.3 Permeability of resins with retardant additive fillers

The study of permeability is important when considering liquid composite moulding such as

RTM and VARTM. Permeability indicates the comparative ease with which resin is

transported through the pores of a porous medium [65]. Therefore, when considering RTM and

VARTM, factors such as fabric architecture, geometry of mould or part, resin viscosity, resin

surface tension, and contact angle influences permeability [65]. While some researchers have

studied permeability of natural fibre composites [66-69], there is limited research that considers

retardants. The investigation by Bensadoun et al, is one of the few comprehensive studies.

Research into fire retardants has found various retardants and investigated their effectiveness.

Therefore, incorporating retardants permeability studies would assist in the understanding of

how retardants impact the feasibility of processing.

1.6.4 Effect of viscosity on permeability

There are some studies that suggest that viscosity is not a factor that impacts permeability [70,

71], but it is also suggested that as viscosity decreases, so does the permeability [65]. In

analysing processability of composites via LCM methods, Bensadoun et al suggested that fibre

impregnation and impregnation duration, are closely connected to viscosity [72]. They also

specified a processability limit of 10Pas for RTM, which limited the retardants that could be

used with their composite. Bensadoun et al investigated bi-directional fabrics, with unsaturated

polyester and nanoclay retardants, so further research using other retardants will also contribute

to this field.

1.6.5 Retardant mixing and dispersion

Ensuring that the retardants are thoroughly mixed is an important part of ensuring improved

fire retardancy. Some researchers, such as Khalili et and al and Bensadoun et al both used high-

frequency ultrasonication baths for mixing the retardants they tested [39, 72], while other used

homogenisers [73], and high shear mixers [62]. A comparison of these techniques would be

beneficial, as it would provide industry users of these materials guidance on practical aspects

of using retardants.

Dispersion of retardants between the resin inlet, and vacuum outlet ports (in other words,

filtration caused by the reinforcing fabric) is another interesting topic for consideration.

Karimzadeh et al examined the dispersion at the inlet and outlet using energy dispersive X-ray

(EDXA) analysis and mapping. The micrographs that were generated in the process confirmed

good dispersion at the resin inlet port, but less dispersion at the vacuum post, suggesting a

filtering affect caused by the reinforcing fabric [73]. They estimated a maximum of 21% of the

retardant being filtered through the fabric [73].

This is also another useful topic that will contribute to the field of fire retardants.



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