epoxy composites

Polymer Degradation and Stability 97 (2012) 2418e2427

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A comparative study on the efficacy of varied surface coatings in fireproofing glass/epoxy composites Baljinder K. Kandola a, *, Waqas Bhatti a, Everson Kandare a, b a b

Institute for Materials Research and Innovation, University of Bolton, Bolton BL3 5AB, UK School of Aerospace, Mechanical & Manufacturing Engineering, RMIT University, GPO Box 2476, Melbourne, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 April 2012 Received in revised form 5 July 2012 Accepted 7 July 2012 Available online 20 July 2012

This paper investigates the efficacy of varied (non-intumescent and intumescent) polymer-based surface coatings in providing fire protection to glass fibre-reinforced (GFR) epoxy composites exposed to onesided radiant heating. In addition to an intumescent surface coating, two other non-intumescent surface coatings are considered e one that is active in the condensed phase and promotes surface char formation and another that is active in the gaseous phase and inhibits flaming combustion. The fire resistance of surface-coated GFR epoxy composite laminates is evaluated using the cone calorimeter at incident heat fluxes of 25, 50 and 65 kW/m2. For all tests conditions considered, there is a significant improvement in the fire performance of surface-protected GFR epoxy laminates relative to their unprotected counterparts. The intumescent surface-coated laminate showed the most significant variations in fire reaction properties with changes in irradiance. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Glass/epoxy composites Fire retardation Char formation Flame inhibition Intumescence

1. Introduction Since their inception in the 1960s, fibre-reinforced polymer (FRP) composites have increasingly found usage in aerospace, marine, automotive and construction industries because of their favorable structural properties, ease of processibility, lightness and cost effectiveness compared to traditional monolithic materials such as wood or metals. However, despite their desirable structural properties and affordability, FRP composites have to pass rigorous fire standards [1] before they can be deployed in fire-prone engineering structures. One of the methods commonly used to improve the fire resistance of FRP composites is the modification of the resin matrix using fire retardant additives [2]. However, while capable of improving the flaming behavior of FRP composites, the inclusion of fire retardant additives has little or no effect on the thermal softening temperature of typical structural composite matrices [3,4]. Low cure-temperature thermosets typically used in engineering structures have low (<150
C) glass transition temperatures (Tg). Therefore, at elevated temperatures (60 < T < 150
C), even the modified resin matrices will tend to soften thereby causing the FRP composite structures to distort and collapse well before ignition and combustion. Further, the inclusion of particulate additives

* Corresponding author. Tel.: þ44 1204 903517; fax: þ44 1204 399074. E-mail address: [email protected] (B.K. Kandola). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.07.023

within the resin matrices has been shown to have adverse effects on the ambient-temperature mechanics of FRP composites [3]. A preferred and efficient method of protecting FRP composites against heat/fire without adversely altering their inherent mechanical properties is the application of fire retardant surface coatings or fibrous (ceramic or phenolic) mats on the heat-exposed surfaces. These are some of the oldest and most commonly used methods for protecting flammable substrates from reaching ignition temperatures or preventing non-flammable materials from reaching their softening temperatures [2]. The application of “passive” fireproofing materials on the heat-exposed surfaces of FRP composites serves to decrease the rate and extent of heat conduction into the composite structure. In some cases, the insulative surface layer may reflect the heat back to the radiant source. Additionally, in the event that the composite ignites and flaming combustion commences, the physical barrier at the heat-exposed surface may inhibit mass transport of oxygen and combustible volatiles into the pyrolysis zone. The retardation or prevention of heat transfer and mass transport of combustible volatiles and/or oxygen into the pyrolysis zone effectively reduces flame propagation. Typical fireproofing materials are ceramic- or intumescentbased formulations exhibiting high heat resistances [2,5e9]. The use of mineral and ceramic cladding is quite popular in naval applications for fireproofing conventional composite hulls, decks and bulkhead structures [2]. These materials are thermal insulators

B.K. Kandola et al. / Polymer Degradation and Stability 97 (2012) 2418e2427

and in some cases they reflect the radiant heat back towards the heat source thereby delaying temperature increase across the composite thickness [8]. Intumescent coatings, on the other hand, are chemical formulations, which by the action of heat; melt, form bubbles and then rapidly expand to form a multi-cellular, carbonaceous char layer [9]. The resultant carbonaceous char layer acts as a thermal barrier effectively protecting the substrate against rapid increases in temperature [9,10]. Non-intumescent surface coatings containing char-forming or flame-inhibiting additives are also used in fireproofing FRP composites [2]. Char-promoting agents such as phosphate derivates or gas-phase-active halogenated additives such as chlorinated paraffin are usually the active ingredients in most non-intumescent fire retardant formulations. Because of inherent differences in their fire retardation mechanisms, different types of fireproof coatings described above demonstrate varying degrees of fire protection under the same principal test conditions. Also, the same surface coating can have distinct physical and chemical reactions to varied heat intensities. In order to evaluate the fire retardation efficacy of different types of surface coatings under low, moderate and extreme thermal environments, three different types of coatings were considered in this study: a phosphorus-based non-intumescent and charring (NIC) formulation; a non-intumescent and halogenated (NIH) coating, and an intumescing and charring (IC) system. The radiant heatexposed surfaces of E-glass fibre-reinforced (GFR) epoxy composite laminates were independently surface-coated using formulations NIC, NIH and IC. The flammability behaviors of resultant composites were then studied under varied cone calorimeter conditions (e.g., external heat fluxes of 25, 50 and 65 kW/ m2 in the presence of a spark-assisted ignition source). The fire performances (e.g., flame retardation characteristics) of distinctively surface-coated GFR epoxy laminates was then analyzed in the context of the chemical composition of the coatings as well as their thermalephysical response to heat impingement. This study was performed in order to ascertain fundamental mechanisms responsible for the observed fire retardation in surface-insulated fibre/polymer composites. The dissimilarity in fire retardation mechanisms exhibited by the three distinct thermal protection coatings investigated in this study suggests that the underlying (fire-protected) composite laminates will display variant thermal responses and flammability behaviors. The reaction of the composite laminates to thermal loading is predominantly the factor that is dictating residual mechanical properties. For example, underlying composite laminates protected by char-forming coatings may exhibit different post-fire microstructures (hence structural integrity) to their counterparts protected by non-charring fire retardant formulations. In addition to discussing fundamental fire retardation mechanisms, this paper is also a prelude to a later study [11] in which the authors investigated the post-fire flexural properties of GFR epoxy laminates thermally-insulated by the same fireproofing formulations. That is, without clearly understanding the fire retardation mechanisms for each of the different systems, it would not be possible to draw a plausible correlation between the post-fire flexural behavior of composite laminates and the efficacy of the fireproofing formulations. This correlation is critical in the design of engineering structures with acceptable fire safety levels especially for applications in fire-prone environments. 2. Experimental 2.1. Preparation of glass fibre/epoxy composite laminates

GFR epoxy composite laminates. The fibre reinforcement (E-glass) was supplied by Glasplies, UK in the form of a woven roving fabric with a nominal area density of between 280 and 300 g/m2. GFR epoxy composite laminates were fabricated via hand lay-up as described in our previous publications [3,4]. Master laminates (300 mm  300 mm) were fabricated by stacking eight layers of woven glass fabric impregnated with epoxy resin followed by vacuum bagging, curing (25
C; 24 h) and post-curing (80
C; 24 h). A 1:1 resin: reinforcement w/w ratio was aimed for e however, slight deviations were unavoidable (see Table 1). The master laminates were then cut into plaques (75 mm  75 mm) weighing approximately 22 g each for cone calorimeter tests. Each specimen was weighed individually as the hand lay-up composite fabrication process and sample cutting could cause variability from specimen to specimen. The weight measured for the square plaques was rather lower than theoretically-calculated. This was probably due to the loss of solvents contained in the resin mixture during postcuring. It is also noteworthy; that in practice, the areal density of E-glass varies between 280 and 300 g/m2 due to batch variability. The plaques were then independently-coated on the surface intended for direct heat exposure using three different commercial formulations. GFR epoxy laminates coated with the three formulations NIC, NIH and IC are identified as EP/F-NIC, EP/F-NIH and EP/ F-IC, respectively. The control laminate (e.g., laminate without fire protection) is referred as EP/F. A consistent dry coating thickness of 0.5  0.1 mm was the target in all test specimens according to the application instructions supplied with the two non-intumescent coatings. This meant that the thickness of the intumescent coating had to be lower than usually expected (>3 mm) for this type of fireproofing material in order to afford a fair comparison. The chemical composition and thermalephysical characteristics of the three surface coating are: The non-intumescent and char-forming, NIC, surface coating is a co-polymer emulsion tape seal adhesive containing tricresyl phosphate in propan-2-ol supplied by Bostik Findley Ltd under the trade name Idenden Tape Seal Adhesive 10-63. This coating is designed for bonding woven glass cloth tape over mineral wool marine board joints and does not intumesce when exposed to heat [12]. The solid content of NIC is 55% by volume and is made-up mainly of tricresyl phosphate, a condensed phase-active phosphate-based fire retardant additive [13e18] The non-intumescent and halogenated, NIH, surface coating is a co-polymer emulsion of sprayable vapor barrier coating containing 1e5% chlorinated paraffin. It is supplied by Bostik Findley Ltd under the trade name Idenden Sprayable Vapor Barrier Coating ET-150. This coating is suitable for insulating piping and, air-conditioning ducting [19]. The active ingredient is a halogenated paraffin which is a gaseous phase-active fire retardant; and,

Table 1 Physical properties and mass composition of the GFR epoxy laminates with/without surface coatings. Sample

Fire protective surface coating

EP/F EP/F-NIC

No coating Non-intumescent, char-forming Non-intumescent, flame-inhibiting Intumescent, char-forming

EP/F-NIH EP/F-IC

A low viscosity and room temperature-curing epoxy resin (LY5052) and an amine-based hardener (HY5052) supplied by Huntsman, UK were used as the matrix during the fabrication of

2419

Weight composition (%)

Coating thickness (mm)

a Weight of dry coating (g)

Fibre

Epoxy

51 53

49 47

e 0.5

e 2.4

50

50

0.5

4.0

52

48

0.6

4.1

a The weight of dry coating required to achieve approximately 0.5 mm-thick surface layers on a 75 mm  75 mm square plaque.

2420

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The intumescing and charring, IC, coating is an epoxybased coating, supplied by Leighs Paints as FireTex M90. It contains epoxy resin (10e25%), bisphenol-F-epichlorohydrin (10e25%), ethyl hexyl glycidyl ether (2.5e10%), 2,4,6-tris(dimethylaminomethyl) phenol (2.5e10%) and triethylenetetramine (2.5e10%) [20].

the surfaces using Araldite epoxy resin in order to maintain contact for as long as possible during thermal exposure. Three temperatureetime profiles were recorded for each test conditions with the average data reported herein. For each test condition, the individual profiles are within 5
C of the average temperatureetime data.

2.2. Thermal analysis of epoxy composites and/or surface coatings 3. Results and discussion The thermal stability of the commercial coatings and dip-coated epoxy (cured) were independently investigated via thermogravimetric analysis (TGA) using an SDT 2960 simultaneous DTGeTGA instrument. Pieces of cured epoxy resin (EP) were weighed and then immersed in the coating formulation for 5 min. The coated epoxy specimens were retrieved and allowed to dry at ambient conditions to yield the following samples: NIC-coated epoxy (EPNIC); NIH-coated epoxy (EP-NIH) and IC-coated epoxy (EP-IC). Specimens with/without surface coatings and weighing 15  1 mg were analyzed between 20 and 900
C at a constant ramp rate (10
C/min) in flowing air (100  5 mL/min). The experiments were performed in triplicates and showed good reproducibility e average data is reported. 2.3. Flammability behavior of GFR epoxy composite laminates with/ without surface coatings Square (75 mm  75 mm) plaques with/without surface coatings were tested under cone calorimetry conditions at incident heat fluxes of 25, 50 or 65 kW/m2 in the presence of a spark-assisted ignition source in accordance with ISO 5660. The test specimens were horizontally-mounted at a distance of 25 mm from the cone heater. The unexposed surfaces as well as the free edges of the test specimens were insulated using a ceramic blanket. The determined/derived fire reaction properties from cone calorimeter data are given in Table 2. They include critical fire performance parameters such as the time-to-ignition (TTI), the heat release rate (HRR) and its peak value (PHRR), the time-to-PHRR (tPHRR), the total heat release (THR), the maximum average heat rate emission (MAHRE) and the total smoke release (TSR). While the test specimens used in this study have relatively shorter dimensions (75 mm  75 mm) than those recommended in ISO 5660 standard (100 mm  100 mm), the results are discussed in comparative terms. Furthermore, previous research in our laboratories [15] showed that the reduction in the surface area of the test specimens does not significantly affect fire behavior. Reverse surface temperature profiles of some of the specimens were recorded using K-type thermocouples. Thermocouples were spot-attached onto

The combustion behaviors of the GFR epoxy composites with/ without surface insulation were investigated under varied irradiance conditions (heat fluxes of 25, 50 or 65 kW/m2) using a cone calorimeter. All other test characteristics such as the ignition source, specimen geometry and ventilation were kept the same. This paper discusses experimentally-measured fire reaction properties of GFR epoxy composite laminates independently surfacecoated using non-intumescing and intumescing fire retardant formulations. Specifically, this study explains the inevitable variations in the fire performance of GFR epoxy composites resulting from dissimilar fire retardation mechanisms exhibited by these distinct surface coatings. The paper also compares the fire retardation efficacies of three different types of insulative surface coatings with varied mechanisms under variant irradiance conditions. The fire reaction properties of the GFR epoxy laminates with/without surface coatings are evaluated against irradiance conditions simulating typical fire scenarios that FRP composites could be subjected to when deployed in fire-prone environments. Under these varied test conditions, the thermal insulation effectiveness of each and every surface coating is largely dictated by its chemical, physical and thermal response to radiant heat sources of varying intensities. This work was followed-up with another study which investigated the post-fire flexural behaviors of laminates independently fire-protected by the three formulations [11]. The thermogravimetric massetemperature data for dry surface coatings are graphically presented in Fig. 1. When the massetemperature data over the entire temperature range (20e900
C) is considered, NIH is the most thermally-stable followed by IC and then NIC. If the residual char yields at 700
C are used as a basis for comparative analysis, thermal stability follows the order NIH (45%) > IC (28%) > NIC (9%). However, since the fire retardation mechanisms are not solely dictated by the residual yields, it would be misleading to conclude the fire retardation efficacy of these three formulations based on these data. Fig. 2 shows the experimental and theoretical massetemperature profiles of cured epoxy with/without surface coatings. Over the

Table 2 Experimentally-measured cone calorimetry data collected at different incident heat fluxes. Sample

Heat flux (kW/m2)

TTI (s)

PHRR (kW/m2)

tPHRR (s)

THR (MJ/kg)

TSR (L)

MAHRE (kW/m2)

TZRM (s)

EP/F

25 50 65 25 50 65 25 50 65 25 50 65

72 52 17 102 [þ42] 59 [þ13] 20 [þ18] 171 [þ138] 70 [þ35] 27 [þ59] 204 [þ183] 81 [þ56] 32 [þ88]

563 733 750 494 [12] 584 [20] 631 [16] 515 [9] 544 [26] 548 [27] 194 [66] 400 [45] 438 [42]

105 95 75 215 110 115 255 125 100 400 195 140

27 38 32 56 [þ107] 54 [þ42] 46 [þ44] 54 [þ100] 50 [þ32] 42 [þ31] 28 [þ4] 55 [þ45] 48 [þ50]

397 212 120 1104 248 203 1242 298 195 1471 363 227

196 277 298 190 311 320 150 257 263 52 192 189

64 54 45 125 81 79 140 93 96 203 122 124

EP/F-NIC

EP/F-NIH

EP/F-IC

TTI is the time-to-ignition; PHRR is the peak heat release rate; tPHRR is the time to reach the peak value of the HRR; THR is the total heat release; TSR is the total smoke released and MAHRE is the measure of the propensity for fire spread to nearby objects; TZRM is the time taken by the reverse side surface of the laminate to reach 150
C, which is the temperature at which the laminate has zero residual flexural modulus in-situ [3]. The data in brackets [], represent the percent increase (þ) or reduction () in a given fire reaction parameter for fire-protected glass-reinforced epoxy composites relative to the control.

B.K. Kandola et al. / Polymer Degradation and Stability 97 (2012) 2418e2427

2421

a

100

100

80 NIC NIH IC

60

40

Mass (%)

Mass (%)

80

60

40 EP-NIC (Measured) EP-NIC (Calculated) EP

20

20

0

0 0

150

300

450

600

750

900 0

o

Temperature ( C)

450

600

750

900

Temperature ( C)

b 100

80

Mass (%)

temperature range investigated herein, the thermal degradation of the unmodified epoxy in air is characterized by two major physical stages. Between 300 and 450
C, the cured epoxy resin decomposes to form a primary char. As the temperature is raised further, the primary char degrades to yield negligible solid residues at 900
C [3]. Theoretical massetemperature profiles of surface-coated epoxy samples were calculated using the rule-of-mixtures. Since the weight ratio of the cured epoxy to the surface coating is known, the theoretical massetemperature data of surface-coated epoxy samples was calculated using [(mass fraction  measured mass of cured epoxy) þ (mass fraction  measured mass of dry surface coating)]. The calculated massetemperature profiles of EP-NIC, EPNIH and EP-IC closely match the experimental data. Thermogravimetric data collected for EP-NIC, EP-NIH and EP-IC revealed little or no chemical interactions in the thermal degradation behaviors of constituent elements. This observation may be attributed to the fact that the surface coatings degrade first since they are on the exterior. By the time, the cured epoxy begin to decompose, the by-products from the thermal degradation and/or decomposition of the surface coating are no longer in the vicinity to facilitate chemical interactions. It is likely that there are little/no chemical interactions between the decomposition products of the laminate and the surface coating. Therefore, the mechanisms responsible for fire protection of GFR epoxy composite laminates may only be restricted to the formation of an insulative surface barrier and/or the prohibition of flaming combustion. In general, GFR polymer composites subjected to one-sided radiant heat (e.g., cone calorimeter tests) are characterized by a rapid increase in the temperature at the exposed surface. The exposed surface temperature continues to increase until it reaches a value corresponding to the onset temperature of decomposition of the polymer resin matrix. Above this temperature, the polymer matrix begins to decompose, releasing gaseous volatiles some of which are combustible. The volatiles progressively accumulate in the space between the laminate and cone heater. Upon reaching a critical mass flux and in the presence of a spark-assisted ignition source, these combustible volatiles ignite. Immediately following ignition, the surface temperature of the laminate rapidly increases to reach the pyrolysis temperature. The pyrolysis temperature is dictated by the nature and burning behavior of the combustible volatiles occupying the space between the decomposing laminate and the cone heater. If the net heat flux of the system exceeds

300

o

60 EP-NIH (Measured) EP-NIH (Calculated) EP

40

20

0 0

150

300

450

600

750

900

o

Temperature ( C)

c 100

80

Mass (%)

Fig. 1. TGA massetemperature data for dry surface coatings (NIC, NIH and IC). The data was collected between 20 and 900
C at a constant ramp rate (10
C/min) in flowing air (100  5 mL/min).

150

60 EP-IC (Measured) EP-IC (Calculated) EP

40

20

0 0

150

300

450

600

750

900

o

Temperature ( C) Fig. 2. TGA massetemperature data for cured epoxy (EP) and surface-coated cured epoxy composites; (a) EP-NIC, (b) EP-NIH and (c) EP-IC, respectively.

a critical value above which specimens cannot self-extinguish, an unremitting flame is observed and the composite is classified as being flammable. The morphological evolution of the surface coating on the heat-exposed surface and the ensuing thermalephysical property changes dictate the overall thermal response and combustion behavior of the protected laminate. In

B.K. Kandola et al. / Polymer Degradation and Stability 97 (2012) 2418e2427

a

600

2

Heat Release Rate (kW/m )

500

400

300 EP/F EP/F-NIC EP/F-NIH EP/F-IC

200

100

0

0

100

200

300

400

500

600

Time (s)

b 26 24

Mass (g)

22 20 18 16 14 12 0

100

200

300

400

500

400

500

600

Time (s)

c

200

160

2

this paper, the burning characteristics of GFR epoxy laminates with/ without surface coatings are discussed in relation to these thermalephysical changes. The evolution of the HRRetime profiles during cone calorimeter tests is the most important and commonly interpreted result in fire science literature of polymer-based materials [21e30]. Features of the HRRetime profiles directly correspond to the flaming combustion reactions of the burning laminate. In an attempt to replicate the fire behavior of GFR epoxy composites exposed to a developing fire, cone calorimeter tests were conducted at a relatively low incident heat flux of 25 kW/m2. Flammability tests were also conducted under moderate cone calorimeter conditions (50 kW/m2) simulating a well-developed fire. Furthermore, variations in the fire retardation mechanisms of surface coatings with increasing radiant heat flux were investigated at a higher heat flux of 65 kW/m2. It is noteworthy; however, that an applied heat flux of 65 kW/m2 is still lower than typical test conditions used for fire qualification of polymer composites for naval applications (w100 kW/m2). In this study, the heat flux was restricted by the operating capacity of the cone calorimeter at the time these experiments were conducted. The HRRetime histories of all GFR epoxy composite laminates subjected to a constant applied radiant heat flux of 25 kW/m2 are graphically presented in Fig. 3(a). The control laminate, EP/F, ignited after 72 s of continuous radiant heat exposure. The HRRetime profile of EP/F is described by a Gaussian curve e a rapid increase in the heat release rate reaching a peak value of 563 kW/m2 at 105 s mirrored by a rapid reduction in the HRR which signifies the conclusion of the flaming combustion process. The symmetrical HRRetime profile observed for EP/F at an incident heat flux of 25 kW/m2 is typical of non-charring polymers. At the conclusion of the cone calorimeter test, the char residue was found to be primarily constituted of the glass reinforcement together with thin layers of carbonaceous material. The char yield was calculated to be 61% of the original mass of the laminate, Fig. 3(b). Since E-glass is thermally inert up to temperatures as high as 600
C, it is therefore, not surprising that the reinforcement fabric maintained its woven architecture at the conclusion of the test. Another important parameter that can be derived from cone calorimeter data is the maximum average heat rate emission, MAHRE. This value is calculated by dividing the cumulative heat release/emission by exposure time with the peak value of the profile designated MAHRE. MAHRE can be best thought of as an ignition modified rate of heat emission parameter, which can be useful to rank materials in terms of ability to support flame spread to other objects. A MAHRE value of 196 kW/m2 was calculated for the control laminate subjected to an incident radiant heat flux of 25 kW/m2, Fig. 3(c). Later, we will see that the propensity for flame spread to other objects in relation to a burning control is significantly higher than calculated for surface-protected laminates under the same test conditions. GFR epoxy laminates which are protected by non-intumescing fire retardant surface coatings (EP/F-NIC and EP/F-NIH) revealed significantly different combustion behaviors relative to the control laminate. In relation to EP/F, the time to self-sustained ignition is significantly delayed e by 30 s for EP/F-NIC and 99 s for EP/F-NIH. Consequently, the time to reach the PHRR is significantly prolonged e by 110 and 150 s for EP/F-NIC and EP/F-NIH, respectively. In addition to delayed ignition and peak heat release events, the PHRR values of EP/F-NIC and EP/F-NIH are considerably lower than observed for the control, Table 2. However, while the HRRetime profiles for the surface-insulated laminates are similar to that of the control, their half-heightewidths are twice as long (20 versus 10 s); e.g., the combustion flaming time for surface-protected laminates increased considerably. A two-fold increase in the halfheightewidth of HRRetime curves for surface-insulated laminates,

AHRE (kW/m )

2422

120

80

40

0 0

100

200

300

600

Time (s) Fig. 3. Plots of (a) Heat release rate, (b) Mass and (c) AHRE versus exposure time for GFR epoxy composites (EP/F, EP/F-NIC, EP/F-NIH and EP/F-IC), respectively. Data was measured at an external heat flux of 25 kW/m2.

EP/F-NIC and EP/F-NIH, point to an increase in the integrated area under their HRRetime curves. This is clearly reflected by a two-fold increase in the THR following the flaming combustion process, Table 2. Fire retardant coatings used in this study contain considerable amounts of combustible constituents, the combustion of which may contribute to the increase in the THR. The massetime

B.K. Kandola et al. / Polymer Degradation and Stability 97 (2012) 2418e2427

data in Fig. 3(b) suggest that mass loss from surface-protected laminates (EP/F-NIC and EP/F-NIH) is delayed and also occurs over a prolonged period of time when compared against the control laminate. In comparison to EP/F, the propensity for fire to spread to nearby objects, the MAHRE, is considerably diminished (150 kW/m2) for EP/F-NIH but remains unchanged for EP/F-NIC (190 kW/m2). In EP/F-NIC, the presence of a phosphate-based and condensed phase-active fire retardant coating at the exposed surface leads to the formation of an insulative physical barrier during the initial stages of heat exposure. The formation of this physical barrier slows down and/or inhibits heat transfer into the laminate as well as the mass transport of combustible volatiles into the pyrolysis zone. If the concentration of combustible volatiles in the space between the laminate and the cone heater does not exceed the critical mass flux necessary to achieve self-sustained ignition, the GFR epoxy laminate will continue to decompose without flaming. In this study, while the amount and quality of char formed at the exposed surface of EP/F-NIC is sufficient to drastically delay self-sustained ignition, it is however, not adequate to prevent this event from occurring. Following ignition, a sustained flaming combustion stage is initiated which has detrimental effects on the fire protection efficacy of the formed char layer. The initially consolidated surface char network is progressively thermally-oxidized as a result of prolonged direct exposure to heat. The incremental loss in the structural integrity of the surface char with prolonged heat exposure eventually leads to inadequate protection of the underlying laminate from heat. In practical terms, the post-fire structural properties of an engineering composite structure would be degraded leading to catastrophic collapse. The flame retardation efficacy exhibited by the char-forming surface coating NIC is demonstrated by the increase in the time it takes for the reverse side of EP/F-NIC to reach the temperature at which the laminate no longer possess any (in-situ) residual flexural stiffness relative to the control. For the epoxy resin used in this study, this value is 150
C [3]. Relative to the control laminate, there is a 61 s delay in the reverse side temperature of EP/F-NIC reaching the temperature-to-zero residual modulus (TZRM) as given in Table 2. Halogenated fire retardants such as chlorinated paraffin follow significantly different fire retardation mechanisms to those exhibited by char-forming fire retardants. This class of fire retardant additives is active in the vapor phase. In other words, these fire retardants do not rely on the formation of a physical barrier at the exposed surface; instead they ‘scavenge’ for free radicals during the flaming combustion process thereby terminating chain reaction that would otherwise support flame propagation. If any char were to be realized from the flaming combustion of EP/F-NIH, it could only be a result of incomplete combustion of volatile products from various decomposition pathways. However, despite the absence of a highly integrated surface char, there are significant improvements in the fire resistance of composite laminates insulated by NIH. This is primarily due to halogen radicals ‘snuffing’ out the flames thereby reducing the net heat input into the composite laminate substrate. The reduction in the PHRR and the time to its occurrence may be attributed to the gaseous phase flame retardation action of this chlorinated paraffin. The TZRM for EP/F-NIH is 15 s longer than observed for EP/F-NIC. This observation is partly attributed to delays in self-sustained ignition and occurrence of the PHRR events for EP/F-NIH in comparison to EP/F-NIC. The fire reaction properties of the composite laminate protected by an intumescing coating (EP/F-IC) significantly differ from those measured for its counterparts protected by non-intumescing coatings (e.g., EP/F-NIC and EP/F-NIH). Crucially, the time-toignition is drastically increased (204 s) for EP/F-IC relative to 102 and 171 s measured for EP/F-NIC and EP/F-NIH, respectively. While the HRRetime profiles for EP/F-NIC and EP/F-NIH feature

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a shoulder just before the peak HRR event, EP/F-IC’s HRRetime profile exhibits two distinct but overlapping peaks. The first peak occurring after 260 s of exposure may be attributed to the heat generated from the activation of the intumescent coating. In the initial stages of thermal exposure, the constituent elements of the intumescent coating decompose releasing large volumes of volatiles which trigger rapid physical expansions of the charred coating towards the cone heater. Combustible volatiles evolved from this activation process such as melamine derivatives could undergo flaming combustion releasing a significant amount of heat. This heat release corresponds to the first of the two observed HRR peaks. This first peak is then followed by a relative larger peak that is centered at 405 s, presumably resulting from the combustion of the resin contained in the underlying laminate due to increasingly higher temperatures as a result of uninterrupted thermal exposure. The superior fire retardation efficacy demonstrated by the intumescing surface coating relative to its non-intumescing counterparts, NIC and NIH, is also evident from significant improvements in other cone calorimeter parameters. In addition to a considerably delay in self-sustained ignition, the PHRR measured for EP/F-IC (194 kW/m2 and corresponding to a 66% reduction relative to the control) is remarkably lower than those measured for EP/F-NIC and EP/F-NIH. In addition, EP/F-IC has a higher residual char yield than EP/F-NIH, Fig. 3(b). At an applied heat flux of 25 kW/ m2, EP/F-IC has the most consolidated char of all the laminates tested as revealed in Fig. 4. While the THR measured for EP/F-IC is similar to that of the control laminate, the TZRM for the former is more than three times that of the latter (Table 2). The consolidated surface char in EP/F-IC acts to slow down heat conduction from the cone calorimeter and/or flaming combustion processes into the laminate while also inhibiting the mass of transport of combustible volatiles from the decomposing laminate into the pyrolysis zone. The amount of combustible fuel is thus effectively reduced hence the THR at the completion of the flaming combustion stage. A 73% reduction in the propensity for fire to spread to nearby objects, otherwise referred to as MAHRE, is calculated for EP/F-IC relative to the control. It is therefore; apparent, the effectiveness of the intumescent surface coating in improving the fire performance GFR epoxy composite structures. Under an external heat flux of 25 kW/m2, the intumescing surface coating outperforms its nonintumescent counterparts with respect to all measured fire reaction parameters with the exception of the total smoke produced. EP/F-IC produced the largest volume of smoke (1471 L) compared to EP/F (397 L), EP/F-NIC (1104 L) and EP/F-NIH (1242 L). The dramatic increase in the volume of smoke produced from all the insulated laminate under relatively low irradiance conditions may be attributed to incomplete combustion processes. The presence of physical barriers in the form of char layers at the exposed surface or the scavenging of free radicals by chlorinated paraffin radicals may slow down the flaming combustion process leading to partially incomplete flaming combustion reactions. This leads to the production of large volumes of carbon soot or smoke. Since most flame retardation mechanisms including those presented in this study are known to have a strong dependency on the fire scenario, it is inevitable that significant changes in the fire behavior of GFR epoxy composite laminates with/without fire protection will be observed with changes in the applied heat flux. GRF epoxy laminates with/without surface coatings were evaluated under external radiant heat fluxes of 50 and 65 kW/m2. The HRRetime profiles collected at these heat fluxes are graphically presented in Fig. 5(a) and 6(a), respectively. In general, the ignition event for all GFR epoxy composites with/without surface protection is significantly accelerated when the applied heat flux is increased. The net heat flux on the exposed laminate surface considerably increases with increased incident heat flux. This leads to accelerated

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Fig. 4. Digital images of pristine/undamaged and charred GFR epoxy composite laminates.

incremental changes in the exposed surface temperature in the initial stages of heat exposure thereby considerably increasing the rate of matrix decomposition. Thus, the critical volatile mass flux is quickly achieved thereby providing ideal thermodynamical conditions that can sustain ignition much earlier. The control laminate showed significant increases in the flaming intensity (e.g., PHRR) with increasing applied heat flux e 30 and 33% for 50 and 65 kW/m2, respectively. The notable increments in PHRR with the elevation of the applied heat flux may be attributed to a measurable increment in the net heat flux of the system. The increase in the flaming intensity of GFR epoxy laminates under elevated incident heat flux conditions results in significant reductions in TZRM values, Table 2. Further, the total heat release from EP/F considerably increased with increasing applied heat fluxes. An increase in THR suggests a more complete and efficient flaming combustion process which does also explain a considerable reduction in the total smoke release e 47 and 70% reductions at 50 and 65 kW/m2, respectively. The fire retardation efficacy of fire retardant additives that are active in the condensed phase is generally enhanced by increased char formation at the heat-exposed surface. Polymer composite formulations containing uniformly dispersed non-intumescent but char-forming fire retardant additives have been shown to have improved fire retardancy with respect to the PHRR and char yields

[31]. In such polymer composite systems, increasing the applied heat fluxes continuously promotes char formation within the pyrolysis zone as it recedes away from the heater. However, in this study, a non-intumescent coating was applied at the exposed surface of the laminate only; therefore the fire reaction behavior of EP/F-NIC should be different to those observed in fire-retarded polymer composite systems. In the initial stages of heat exposure, a surface char is formed which is effective in delaying selfsustained ignition and the PHRR event. However, once this char layer decomposes due to increasing temperatures, there is a reduction in the fire retardation efficacy. The decomposition of the char formed during this stage of exposure occurs at a faster rate but more or less to a similar extent when relatively higher heat fluxes are considered (see images in Fig. 4 and massetemperature data in Fig. 5(b) and 6(b)). The degradation of the physical and/or thermal barrier in the form of a surface char with elevated heat fluxes leads to increased flame intensities. Higher flaming intensities (e.g., PHRR) at elevated heat fluxes are evidence of the reduction in the fire retardation efficacy of the surface coatings. The THR for EP/F-NIC decreased with the increasing applied heat flux primarily due to significant reductions in the flaming combustion times as a result of increased net heat flux in the system. As the applied heat flux is ramped up,

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the increase in the PHRR does not fully compensate the reduction in the burn times; i.e., the area under the HRRetime curves otherwise the THR progressively decreases. The half-heightewidths of the HRRetime profiles for EP/F-NIC were measured to be 103, 85 and 64 s for cone calorimeter tests conducted under applied heat fluxes of 25, 50 and 65 kW/m2, respectively. At elevated incident heat fluxes, increased pyrolysis temperatures cause rapid depletion of

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the protective surface char. The degradation of the surface char can improve flaming combustion efficiency thereby promoting complete combustion which would lead to lower volumes of smoke. This is evident from the four-fold drop in the TSR measured at 50 and 65 kW/m2 relative to the value measured at 25 kW/m2. The propensity for fire spread to nearby objects, MAHRE, increases with the increase in the applied heat flux. This is not necessarily due to increased THR values but rather shorter combustion periods.

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It is logical that the tendency for fire spread should increase with increased net heat flux in the system e which is the case when the applied heat flux is ramped up. The effect of varied applied heat flux on the fire retardation effectiveness of a non-intumescent but gas-phase-active surface coating was investigated using laminate EP/F-NIH. Halogenated fire retardants are effective in the gaseous phase through a flame inhibition mechanism [32]. Unless the flame inhibition mechanism is temperature-dependent, increases in the incident heat flux would not be expected to significantly alter the flaming behavior with respect to the PHRR or the THR. The PHRR values measured for EP/F-NIH at 50 and 65 kW/m2 are substantially higher than those measured at 25 kW/m2. However, relative to the control laminate there is a 26 and 27% reduction in the PHRR at the applied heat fluxes of 50 and 65 kW/m2, respectively. These data suggest that the use of a non-intumescent but gaseous phase-active surface coating reduces the flaming intensity of GFR epoxy composites to a similar extent as the char-forming coating, NIC. The proposed flame inhibition mechanism for EP/F-NIH is further augmented by consistently lower THR values relative to EP/F-NIC in all test conditions. Relative to EP/F-NIC, the propensity for fire spreading from a burning EP/F-NIH to neighboring objects is reduced in all test conditions investigated, Table 2. Regardless of the flame inhibition mechanism exhibited by NIH, interestingly, TSR values for composites protected by this formulation are similar to those measured for EP/F-NIC. Intumescent fire retardant formulations perform by forming an expanded, porous and carbonaceous or ceramic char network during the initial stages of heat exposure. This physical barrier (char) then acts to insulate the underlying composite laminate. At relatively low applied heat fluxes (25 kW/m2), the intumescent char formed at the beginning of the test is preserved for longer exposure periods since the net heat flux in the system is less severe to cause rapid degradation. However, as the external heat flux is increased (e.g., 50 or 65 kW/m2), it is inevitable that the expanded surface char will exist in its most consolidated form for a reduced period of time. It is therefore not surprising that the PHRR of EP/F-IC drastically increased with the increase in the incident heat flux. Relative to data collected at an external heat flux of 25 kW/m2, the PHRR values measured for EP/F-IC increased by 106 and 126% at applied heat fluxes of 50 and 65 kW/m2, respectively. The depletion in the amount of surface char and the degradation in its quality (see Fig. 4) are the primary factors that may be implicated over the reduced fire retardation efficacy of coating IC as the incident heat flux increases. At elevated heat fluxes, the accelerated degradation of the surface char leads to the loss of the physical and/or thermal barrier which would have otherwise prevented heat transfer and mass transport of combustible volatiles into the pyrolysis zone. The degradation of the intumescent char at elevated heat fluxes improves the flaming combustion efficiency thereby promoting the decomposition of the resin matrix in the underlying composite laminate. The increase in the flaming combustion efficiency subsequently leads to an increase in the THR. Thus, relative to data measured at 25 kW/m2, the THR values measured for EP/F-IC increased by 27 and 20 MJ/kg at incident heat flux to 50 and 65 kW/m2, respectively. Compared to all other composite systems considered in this study, EP/F-IC has the lowest tendency for fire spread to nearby objects as revealed by its relatively low MAHRE values, Table 2. EP/ F-IC’s improved flaming combustion efficiency at elevated incident heat fluxes is also evident from the reductions in the TSR values as given in Table 2. The degradation of surface char with increasing applied heat flux is evident from the reduction in the TZRM. There is a 40% reduction in the TZRM of EP/F-IC as the incident heat flux is increased from 25 to 50 or 65 kW/m2. Of all the composite systems investigated in this work, EP/F-IC has the largest reduction in TZRM

values with increasing heat flux (25e50 or 65 kW/m2). This observation suggests that, relative to EP/F-NIC and EP/F-NIH, heat conduction into EP/F-IC increases with applied heat flux. The intumescing surface coating demonstrated the most superior fire retardation efficacy of all composite systems investigated herein. However, the fire retardation effectiveness of the intumescent coating has also been shown to have a strong dependency on the fire scenario (e.g., irradiance). Variations in the simulated fire intensity are bound to result in significant changes in the fire reaction of composite systems protected by intumescent formulations. Variations in fire reaction properties of intumescent-coated FRP composites provide practical challenges to fire engineers by making it harder for them to predict the fire behaviors in varied fire scenarios. Therefore, all GFR intumescent-coated polymer composite structures have to be experimentally-assessed for each and every conceivable fire scenarios before they can be deployed in fire-prone structures. These experimentally-based qualification exercises are labor-intensive, time-consuming and very expensive. 4. Summary of flammability results This study has demonstrated increased fire protection of GFR epoxy composite laminates using surface coatings exhibiting distinct fire retardation mechanisms. The improvements in fire reaction properties of GFR epoxy composite systems were achieved through; (1) delayed ignition, (2) retarded heat transfer into the specimen, (3) retarded mass transport of combustible volatiles and oxygen into pyrolysis zone and, (4) inhibition of flame propagation. The order of ignition hence the occurrence of the PHRR event was found to be: EP/F-IC > EP/F-NIH > EP/F-NIC > EP/F. When the PHRR is used as the measure of fire retardation, the general order of fire safety under all cone calorimeter test conditions considered is EP/FIC > EP/F-NIH z EP/F-NIC > EP/F. Increasing the external heat flux has been demonstrated to have detrimental effects on the fire retardation efficacy of coating NIC or IC while no significant changes were observed for the halogenated coating, NIH. The flame retardation mechanisms exhibited by coatings NIC or IC rely on the formation of a consolidated char network at the exposed surface which serves to inhibit heat transfer into the laminate. A physical barrier in the form of surface char also slows down the mass transport of combustible volatiles and oxygen into the pyrolysis zone. As the external heat flux is increased, the amount and quality of the surface char is degraded thereby reducing the fire retardation efficacy. On the other hand, surface coating NIH does not rely on char formation but flame inhibition. When the THR is considered, EP/F and EP/F-IC showed an increase in the total heat output with increased applied heat flux perhaps as a result of increased net energy input hence combustion efficiencies. For EP/F, the increase in the net heat flux of the system enhances the PHRR which in turn increases the amount of total heat released as the applied heat flux is ramped up. In the case of EP/F-IC, the intumescent char formed in the initial stages of thermal exposure degrades with prolonged exposure until the underlying laminate no longer has sufficient thermal protection. At this stage, the flaming combustion efficiency is boosted leading to a larger amount of heat being generated from the system otherwise higher THR values. For the laminates EP/F-NIC and EP/F-NIH, the opposite is true e the THR values decreased with increasing applied heat flux. The loss of surface char at elevated heat fluxes increase the combustion efficiency (e.g., notable increments in the PHRR) of EP/F-NIC but reduce its flaming time. For EP/F-NIC, the integrated area under the HRRetime curve (e.g., the THR) is reduced with increasing applied heat flux. Generally, the TSR values measured for the fire-protected laminates were higher than those measured for the control laminate especially at low irradiance levels. Fire

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retardation mechanisms considered in this study reduce the flaming efficiency leading to incomplete combustion which may be implicated for the significant increase in smoke production. The time taken for the insulated/reverse surface of the exposed GFR epoxy laminates to reach 150
C (otherwise referred to as the TZRM) decreases with increasing applied heat flux for all composite systems. The intumescent surface-coated GFR epoxy laminates had the longest TZRM. This is due to the formation of an efficient thermal and physical barrier in the form of a consolidated char at the exposed surface. The char-forming (NIC) and flame-inhibiting (NIH) surface coatings improved the heat resistance of the GFR epoxy laminates albeit not to the same extent as the intumescent coating, IC. If the TZRM was used as an assessment/evaluation tool, the order of fire protection efficacy of the surface coatings would be IC > NIH > NIC. The information gathered in this study advances our knowledge in understanding the fire reaction properties of FRP composite systems protected by surface coatings with varied fire retardation mechanisms. This information is valuable in the design of fire-safe engineering structures that could potentially be exposed to low, moderate or extreme thermal environments. Understanding the fire reaction behaviors of thermally-insulated FRP composite systems in varied thermal environments offers engineers the ability to design new structures with adequate fire safety. Without the knowledge collected in this study, there is high probability of under or over-protecting engineering structures in fire-prone environments. Further, fire reaction properties and thermal responses including the evolution of laminate structural integrity are some of the critical variables in understanding the structural performance of load-bearing elements during or after fire. This paper presents critical data that is essential in explaining variations in the structural performance of elements thermally-protected with coatings exhibiting distinct fire retardation mechanisms. 5. Conclusions The fire reaction properties of GFR epoxy laminates with/ without fire protective surface coatings were successfully evaluated under varied cone calorimeter conditions. In general, the time-toignition and the occurrence of the peak heat release rate event were delayed for fire-protected laminates relative to control. Increasing the applied heat flux significantly reduced these parameters as a result of increased net heat flux in the system. Relative to the control, the flaming intensities (e.g., PHRR values) of surface-protected laminates were significantly reduced. However, this parameter variably changed with increasing irradiance. In comparison to the control laminate, the time for the reverse surface of the test specimens to reach 150
C is significantly higher for fireproofed composite systems. The total heat release erratically changed with applied heat flux for all composite systems. At a relatively low incident heat flux of 25 kW/m2, surface-protected laminates produced four times more smoke than the control laminate. However, as the incident heat flux is increased, the smoke produced by fire-protected laminate systems was only 50% higher. This observation may be attributed to enhanced flaming combustion efficiencies due to increased heat energy in the systems in elevated heat flux test conditions. The propensity for fire spread to nearby structures show tendency to increase with increasing applied heat flux for all composite systems. References [1] Zhang H. Fire-safe polymers and polymer composites. Federal aviation administration DOT/FAA/AR-04/11; 2004.

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