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Polymer Degradation and Stability 96 (2011) 277e284

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

Characterisation of soot particulates from fire retarded and nanocompositematerials, and their toxicological impact

Jennifer Rhodes, Cameron Smith, Anna A. Stec*

Centre for Fire and Hazards Science, University of Central Lancashire, Preston PR1 2HE, UK

a r t i c l e i n f o

Article history:Received 2 February 2010Received in revised form7 June 2010Accepted 2 July 2010Available online 16 July 2010

Keywords:FireToxicityNanocompositesRetardantSootCascade impactor

* Corresponding author.E-mail address: aastec@uclan.ac.uk (A.A. Stec).

0141-3910/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2010.07.002

a b s t r a c t

Polyamide 6 (PA 6) and polypropylene (PP) containing fire retardants, nanofillers or a combination of bothadditives have been investigated using the steady state tube furnace (ISO TS 19700). The samples weretested under three different fire conditions, to determine the effect of additives on the soot production ortoxic product yields. The particle size distribution of the soot was investigated with a cascade impactor,and the separated soot fractions examined by SEM. The predicted deposition based on aerodynamic size ofparticulates in the human respiratory tract shows clear differences between the pure polymer and itsadditive counterparts. In all ventilation conditions the virgin polymer produces the least amount of soot,both the additives used (fire retardant and nanoclay) increase the amount of soot, mainly within 0.5e1.0 mm range, for each fire condition. A large contribution to the total soot mass originated particlessmaller than 0.5 mm.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Synthetic andman-made polymers are becoming ever prevalentas the product of choice for consumer and construction materials.Coupled with most synthetic polymers being easier to ignite andhaving a higher rate of heat release than their natural counterparts,attention has to be given to their flammability in order to ensuretheir fire safety. Fire retardants, including nanofillers, can easily beincorporated into synthetic polymers, such as plastics, during themanufacturing process to inhibit this increase. The use of nano-scopic additives, which impart fire retardant properties to theresultant polymer nanocomposites, shows greater activity due tothe increased surface in the polymer matrix, enhanced physicalproperties and thus higher loadings of conventional fire retardantswithout compromise the physical properties. Despite increasedusage, little is understood about what impact these additives willhave on environmental and health and safety aspects should a firetake hold.

While it is widely believed that carbonmonoxide is themajor, orindeed the only significant toxicant in fire effluents, other reportsindicate the importance other toxicants [1], particularly particulatematerial, [2] can cause incapacitation and prevent escape. Somestudies have been carried out on particulate matter of a physical

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diameter less than 100 nm, particles known as atmospheric ultra-fine particles (UFP), which originate from combustion sources andgas-to-particle formation processes in the atmosphere. A reductionof the particle size gives nanoparticles unique properties. Firstly,a substantial increase in the specific surface area and the surfaceGibbs free energy reflects the fact that chemical reactivity increasesrapidly as particle size diminishes [3]. Secondly, since a greaternumber of nano-scale particles will have a greater surface area perunit mass than that of larger particles, their accumulation in thelung increases the potential for biological interaction [4e6].New studies have also shown that the inflammation caused due tothe inhalation of such particles causes vascular dysfunction andimpaired endogenous fibrinolysis [7] leading to reduced blood flow.Dust deposition in the pulmonary system varies considerablyaccording to the granulometry of ultrafine dusts and their airbornebehaviour [3] (Fig. 1). It is found that particles with an aerodynamicdiameter of less than 10 mm can be inhaled directly into the lung byhumans [8], and fires in enclosures, containing polymer nano-composites would pose a greater risk to health if nanoparticleswere released. This underlines the importance of understandingthe composition, size and structure of the soot and particulatesproduced during combustion in order to understand the distribu-tion and toxicity of the soot once inside the respiratory tract.

In this study the effluents from two burning virgin polymerswere compared to samples containing fire retardant and nano-composite additives. The most important factor determining soot

Fig. 1. Potential distribution of inhaled particles within the human respiratory tract.

Table 2Loadings of the samples tested.

Material Polymer % Fire Retardant % Nanoclay %

AP OP

Polypropylene (PP) (all samplescontain 5% PPgMA)

100

PP þ FR 70 30PP þ NC 95 5PP þ FR þ NC 65 30 5

Polyamide 6 (PA 6) 100PA 6 þ FR 82 18PA 6 þ NC 95 5PA 6 þ FR þ NC 77 18 5

J. Rhodes et al. / Polymer Degradation and Stability 96 (2011) 277e284278

distribution, and the focus of this paper, is the particle size distri-bution, quantified by mass [2]. This data can be then correlated tothe dose response effect, which is both time and concentrationdependent [9].

1.1. Terminology

In this developing field, clear and consistent definitions andterminology for nanotechnology are still missing [10]. Therefore forthe purposes of this paper the term nanocomposite will be used todescribe the additives with one dimension of 100 nm or lessused within polymers to improve their fire retardant properties.The term nanoparticles refer to the particles formed from thecombustion of polymers which are present in the soot and fireeffluent. This is an important distinction to make as the polymernanocomposite may not be at all toxic in the solid form; howeveronce it is burnt the resultant nanoparticles may be respirable andhence potentially harmful.

1.2. Fire conditions

Most fires progress through several different stages (detailedin Table 1) from ignition to decay. The generalised stages in thedevelopment of a fire are used to classify fire growth [12] in termsof equivalence ratio, f [11], Table 1, and have been successfullyreplicated by using the steady state tube furnace, ISO TS 19700[12,13]. This is probably the best method for replicating individualfire stages on a bench scale [14].

Table 1ISO Classification of fire stages (Adapted) [15].

Fire Stage Max Temp/�C Equivalenceratio f

CO/CO2

ratioFuel Smoke

Non-flaming1a. Self sustained smouldering 450e800 25e85 e 0.1e1

Well-ventilated flaming2. Well-ventilated flaming 350e650 50e500 <0.75 <0.05

Under-ventilated flaming3a. Low vent room fire 300e600 50e500 >1.50 0.2e0.43b. Post flashover 350e650 >600 >1.50 0.1e0.4

2. Experimental

2.1. Materials

The compositions of the samples tested are shown in Table 2.The virgin polymers used for the preparation of nanocompositeswere commercial polypropylene (PP) grafted with maleic anhy-dride used as a compatibiliser (Moplen HP500N-Basell blendedwith 5% PPgMA Polybond 3200 by Crompton as amasterbatch), andpolyamide 6 (PA6 S27 from Radici Plastics). The fire retardant (FR)for PP was Exolit AP 760 (based on ammonium polyphosphate), forPA6 was OP 1311 an organic aluminum phosphinate combinedwithmelamine polyphosphate, all supplied by Clariant. The nanoclay(NC) was Cloisite 20A for PP and Cloisite 30B for PA6 supplied bySouthern Clay Products. The preparation and characterisation ofthese materials have been reported elsewhere [16,17].

2.2. Steady state tube furnace (ISO TS 19700)

The steady state tube furnace (ISO TS 19700) [12,18,19] shown inFig. 2, is a small-scale fire model designed to replicate the differentstages of fire [20,21]. Fuel and air are introduced into the furnace atfixed rates enabling the equivalence ratio, f, to be controlled, basedon knowledge of the material composition or its stoichiometricoxygen requirement [22].

f ¼ Actual Fuel=Air RatioStoichiometric Fuel=Air Ratio

A sample of material in the form of granules or pellets, spreadevenly along the furnace boat is introduced into a tube furnace ata constant rate. A current of air is passed through the furnace over thespecimentosupportcombustion.Theeffluent isexpelled fromthetubefurnace into a mixing and measurement chamber, where it is diluted

Fig. 2. Steady state tube furnace.

Fig. 3. Cascade impactor filters.

J. Rhodes et al. / Polymer Degradation and Stability 96 (2011) 277e284 279

withsecondaryair. Thedecompositionconditions in the furnaceare setusing different combinations of temperature and air flow in separateruns, to model the decomposition conditions for a range of stages andtypes of fires as required. For these experiments a specimen mass of25e50mg/mmwasusedandprimaryair-flowswerevariedbetween2and 13 L min�1 order to cover the range of combustion conditions(equivalence ratios) required. Fire conditions were well-ventilated(f < 0.75, 650 �C), small under-ventilated (f > 1.5, 650 �C) and largeunder-ventilated or fully developed fire (f> 1.5, 825 �C).

Fire Effluent

2.3. Cascade impactor

A cascade impactor is one of the only techniques which providesa particle size distribution expressed in terms of mass (rather thannumber) of particles in each size range. The comparisons have beenmade between cascade impactor and other methods [23e25].

Cascade impactors measure aerodynamic particle size directly,as physiological effects are a function of size distribution based onmass. This is the most relevant parameter for predicting particletransport and depositionwithin the respiratory tract [26]. Airborneparticles pass through the apparatus and no impaction occurswhen streamlines (straight arrows in Fig. 3) bend as air flowsbypass a solid object (i.e., a collection plate). Particles larger thanthe cut off-size (Table 3) of each impactor plate will slip across thestreamlines and impact upon the filter while smaller particles willbe carried by the streamlines and pass through the impactor stageto be separated on subsequent filters [27].

In this study the cascade impactor was attached to the mixingchamber of the Purser Furnace. Fire effluent was drawn through ata flow rate of 2.0 Lmin�1 for a period of 5min during the steady statestage and aerosol mass distributions were determined and collectedfor further analysis (i.e., the chemical composition). Each test, andthe cascade impactor analysis was carried out in triplicates (Fig. 4).

3. Results and discussion

3.1. POLYAMIDE 6 (PA 6) burnt in well-ventilated conditions

Fig. 5 shows themass of soot collected on each stage of the filter,expressed in terms of the particle size (or strictly the aerodynamicdiameter) using a log scale on the x-axis. Particulates around 1 mm

Table 3Filter size and corresponding deposition point within the human respiratory tract[28e30].

Filter cut off Approximate maximumaerodynamic diameter

Position in the humanrespiratory tract

21.3 mm 21.5 mm Nasal cavity14.8 mm 15 mm Oral cavity9.8 mm 10 mm Larynx6.0 mm 6.5 mm Trachea3.5 mm 4 mm Bronchi1.55 mm 2 mm Bronchioles0.93 mm 1 mm Bronchioles0.52 mm 0.5 mm Alveoli

in size have the potential to be deposited deep in the lung tissue atthe bronchial level. Fig. 5 demonstrates that the greatest amount ofsoot is produced from the burning of the PA6 þ FR þ NC andPA6 þ FR samples; 1.4 and 1.2 mg respectively. The virgin polymershows the least amount of soot produced (0.4 mg) of size range1e4 mm indicating that deposits in the trachea, lung and bronchialtissues are likely to be the most prevalent. The PA6 þ NC samplefollows the same trend as the unaltered polymer, with particlesof size range 2e4 mm the most prevalent (0.5 mg). Very fewparticles of larger sizes were observed during burning of thesesamples within these conditions. In all cases the peak mass of sootproduction was less than 1.5 mg showing the least soot productionfor all fire conditions due to the more complete combustion.

3.2. PA 6 burnt in small under-ventilated conditions

Tests carried out in small under-ventilated conditions yieldeda far greater amount of soot when compared to the well-ventilated

Fig. 4. Cascade impactor.

0.00.20.40.60.81.01.21.41.6

0.1 1 10 100Particle Size (microns)

Mas

s of

Par

ticle

s (m

g) PA6PA6+FRPA6+NCPA6+FR+NC

Fig. 5. Soot distribution of PA6 samples from well-ventilated condition tests.

0.00.51.01.52.02.53.03.54.0

0.1 1 10 100Particle Size (microns)

Mas

s of

Par

ticle

s (m

g) PA6PA6+FRPA6+NCPA6+FR+NC

Fig. 7. Soot distribution of PA6 samples from large under-ventilated conditions.

J. Rhodes et al. / Polymer Degradation and Stability 96 (2011) 277e284280

conditions; due to the oxygen deficient atmosphere preventingcomplete combustion, which in turn produces more soot andparticulate matter. All samples in this condition showed the sametrend with the greatest amount of soot deposited on the 1 mmstage (Fig. 6) indicating that these particles would be likely to traveldown the respiratory tract as far as the bronchioles. PA6 þ FR þ NCyielded the most amount of soot, 3.5 mg at its peak, the unchangedpolymer the least (1.6 mg) and PA6 þ FR and PA6 þ NC yieldingsimilar amounts at 2.3 mg and 2 mg, respectively.

3.3. PA 6 burnt in large under-ventilated conditions

Fig. 7 shows that PA6 þ FR þ NC samples yield the largestamount of soot, 3.7 mg of size 1 mm. All other samples had similaryields of soot with peaks around 1.4e1.7 mg, more in line withthose obtained under well-ventilated conditions. The particulatesof the PA6 þ NC samples were slightly larger (1.55 mm) and so areunlikely to travel as far down the respiratory tract as the unchangedpolymer or PA6 þ FR particles (0.93 mm).

3.4. POLYPROPYLENE (PP) burnt in well-ventilated conditions

Fig. 8 shows that the well-ventilated condition yields less sootthan the under-ventilated condition for all PP samples tested,similarly to PA6. The unchanged polymer and PP þ NC followa similar patternwith size ranges from 1 to 10 mmand peak amountsat 0.9 mg and 1.1 mg, respectively. PP þ FR þ NC and PP þ FR alsoshow similar trends in distributionwith the greatest amount of soot

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.1 1 10 100Particle Size (microns)

Mas

s of

Par

ticle

s (m

g)

PA6PA6+FRPA6+NCPA6+FR+NC

Fig. 6. Soot distribution of PA6 samples from small under-ventilated condition tests.

deposited at 1 mm. PP þ FR þ NC had a greater amount of soot,1.3 mg compared to PP þ FR 0.8 mg.

3.5. PP burnt in small under-ventilated conditions

The unchanged polymer has a wide distribution range oversizes 0.5e6.0 mm, with approximately 1.2 mg of soot produced at1.55 mm, as shown in Fig. 9. PPþNC and PPþ FRþNC follow similartrends with a peak at 0.93 mm and produce similar amounts of soot2.6 mg and 2.3 mg, respectively. PPþ FR produces the least amountof soot of all the samples with a first peak at 0.93 mm (0.8 mg) andsecondary peak of 0.25 mg at 3.5 mm.

3.6. PP burnt in large under-ventilated conditions

PP þ NC produced the overall largest amount of soot (peak massofw3.4 mg) of size 1.55 mm. Very similar trends are observed for allother sampleswhere PPþ FR, PPþ FRþNCandPPhavemass peak at0.93 mm and masses 2.1 mg, 1.8 mg, and 1.4 mg respectively, Fig. 10.There is a decrease in amount of soot produced as well as a decreasein the size of themost abundant particle produced, from the PPþNCthrough PP þ FR and PP þ FR þ NC to virgin polymer.

3.7. Cascade impactor and total soot

Total soot yield data was compared to the distribution of sootcollected and separated by the cascade impactor. The cascadeimpactor total soot estimation was the sum of the soot collected ateach stage and included the soot collected by a final porous paperfilter at the bottom of the cascade impactor which reflects particles

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.1 1 10 100

Mas

s of

Par

ticle

s (m

g)

Particle Size (microns)

PPPP+FRPP+NCPP+FR+NC

Fig. 8. Soot distribution of PP samples from well-ventilated condition tests.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.1 1 10 100

Mas

s of

Par

ticle

s (m

g)

Particle Size (microns)

PPPP+FRPP+NCPP+FR+NC

Fig. 9. Soot distribution of PP samples from small under-ventilated condition tests.

0

2

4

6

8

10

12

14

PA 6

PA6

+FR

PA6+

NC

PA6+

FR+N

C

PA 6

PA6

+FR

PA6+

NC

PA6+

FR+N

C

PA 6

PA6

+FR

PA6+

NC

PA6+

FR+N

C

Well-Ventilated Small Under-Ventilated Large Under-Ventilated

Mas

s / m

g

Cascade Impactor -Final FilterCascade Impactor- Filters 1-8Gravimetry-Filter

Fig. 11. Total mass of soot collected in the cascade impactor and on the filter for PA6polymers.

J. Rhodes et al. / Polymer Degradation and Stability 96 (2011) 277e284 281

too small to be impacted onto any cascade impactor levels (smallerthan 0.5 mm). The gravimetric determination of the total amount ofsoot on the filter paper taken from the mixing chamber with a flowrate of 2 L min�1 for 5 min during steady state burning.

The mass from either filter can be converted into a yield ingrams of soot per gram of polymer material. The feed rates forthe different materials were 1 g min�1. As sampling was taken over5 min at 2 L min�1 from a total effluent flow of 50 L min�1, 10 mg onthe filter corresponds to 0.05 g/g of polymer for 1 g min�1 feed rate.Results are compared for PA6 and PP samples in Figs. 11 and 12.

The first observation is that the mass determined gravimetri-cally is greater compared to the mass determined in the cascadeimpactor for all PA6 þ FR and PA6 þ FR þ NC samples in all fireconditions (triplicate tests). For all other samples similar masseswere obtained for the cascade impactor and gravimetric measure-ments. In the case of PA6 þ FR, there is a consistent increase in sooton the single filter compared to the cascade impactor. It may be thatlarge soot particles were collected on the filter, but were caught inthe sampling tube leading to the cascade impactor.

For well-ventilated conditions, Fig. 11, the highest mass wasobtained from PA6 þ FR þ NC, followed by PA6 þ FR, PA6 þ NC andpure polymer samples.

In the case of PA6 þ FR and PA6 þ FR þ NC there are almost thesame masses observed for the gravimetric measurements and forthe 8 cascade impactor stages. The higher mass for PA6 þ FR þ NCand NC comes from final filter (0.5 mm) placed at the base of thecascade impactor where all particles larger than 0.5 mmwould havealready been removed.

00.5

11.5

22.5

33.5

4

0.1 1 10 100

Mas

s of

Par

ticle

s (m

g)

Particle Size (microns)

PPPP+FRPP+NCPP+FR+NC

Fig. 10. Soot distribution of PP samples from large under-ventilated condition tests.

For the small under-ventilated condition, the least particulatemass is for PA6, similar for PA6 þ NC and PA6 þ FR and the greatestis for the PA6 þ FR þ NC sample. A similar trend is observed for thecascade impactor samples except PA6þ FR which produce a similaramount of soot to the other samples (PA6 and PA6 þ NC).

In large under-ventilated conditions bigger differences areobserved between the two sampling systems. Samples ofPA6þ FRþNC, PA6, and PA6þ FR all produce similar masses of sootof size 1 mmPA6þNC produces a similar quantity of soot comparedto all other samples except PA6 þ FR þ NC where the highest massis produced. The total soot is greatest for PA6 þ FR andPA6 þ FR þ NC and the least for PA6 þ NC and the pure polymer.

For polypropylene samples, a very similar trend and amounts areobserved for the two sampling systems. Of the soot collected inwell-ventilated conditions, the greatest amount was from thePP þ NC sample. PP þ FR, and PP þ FR þ NC produced similaramounts, but lower compared to the pure polymer.

The small under-ventilated condition produced similar amountsof soot for PP, and PP þ NC. The least soot was attributed toPP þ FR þ NC and PP þ FR samples.

Large under-ventilated conditions produced the highest amountof soot. The greatest amount of soot comes from the PP þ NCsample, followed by PP and PP þ FR. The smallest contribution isattributed to PP þ FR þ NC samples where bigger discrepancybetween cascade impactor and simple filter is observed.

0

2

4

6

8

10

PP

PP +

FR

PP+N

C

PP+F

R+N

C

PP

PP +

FR

PP+N

C

PP+F

R+N

C

PP

PP +

FR

PP+N

C

PP+F

R+N

C

Well-Ventilated Small Under-Ventilated Large Under-Ventilated

Mas

s / m

g

Cascade Impactor -Final FilterCascade Impactor- Filters 1-8Gravimetry-Filter

Fig. 12. Total mass of soot collected in the cascade impactor and on the filter for PPpolymers.

Table 4Peak deposition zones in lung.

Samples Human respiratorytract and Aerodynamicdiameter

Bronchioles2 mm

Alveoli <1 mm

PP Well-Ventilated PP, PP þ NC PP þ FR, PP þ FR þ NCSmall Under-Ventilated PP PP þ FR, PP þ NC,

PP þ FR þ NCLarge Under-Ventilated PP þ NC PP, PP þ FR, PP þ FR þ NC

PA6 Well-Ventilated PA6, PA6 þ NC PA6 þ FR, PA6 þ FR þ NCSmall Under-Ventilated PA6, PA6 þ FR, PA6 þ NC,

PA6 þ FR þ NCLarge Under-Ventilated PA6 þ NC PA6, PA6 þ FR,

PA6 þ FR þ NC

J. Rhodes et al. / Polymer Degradation and Stability 96 (2011) 277e284282

3.8. Comparison between PA6 and PP

The ventilation condition determines the amount of sootproduced from the burning of the polymer. The large under-ventilated condition produces the most, and well-ventilated theleast, as would be expected due to the reduced combustion effi-ciencies in under-ventilated conditions.

For all fire conditions the highest particle concentration isobserved for PA6þ FRþNC. In addition, a very highmass is observedfor PA6 þ FR samples in well-ventilated conditions, however theseconcentrations decrease and are comparable with other PA6combinations. In all ventilation conditions the unchanged polymer(PA6)produces the least amount of soot suggesting that thepresenceof the additives increases the amount of soot produced duringcombustion.

Fig. 13. SEM images from various fire

Similarly to PA6 samples, the highest particle concentration isobserved for samples with fire retardant and nanoclay additives.PP þ FR þ NC for well-ventilated fires and PP þ NC for both under-ventilated conditions yield the most soot. PPþ FR samples have thesmallest mass of particles except for the large under-ventilated fireswhere PP þ FR þ NC and pure polymer are much lower in quantity.

The results observed for all polymer samples show the points ofmaximum soot deposition are the deeper locations within lungtissue, Table 4, suggesting that the size of the majority of particlesproduced is in the range of 0.5e2 mm. The toxic effects of suchdeposits are summarised elsewhere [5,7].

3.9. Scanning Electron Microscope

Scanning Electron Microscopy (SEM) was utilised to analyse themorphology of the separated soot particulates and enable determi-nation of the extent of coagulation and agglomeration for each typeof polymer in each fire condition (Fig. 13). This could provide usefuldata giving an indication of the nanoclay or fire retardant changes inthe samples under different fire conditions. However, unambiguousinterpretation of the micrographs was not achieved in this case.

Particles generated in combustion processes and commercialnanoparticle products are often in the form of aggregates [31] in thesize range of 5e50 nm. SEM analysis was carried out on PP þ NCand PP þ FR þ NC samples for all fire conditions and for all cascadefilters. The structure of the particulates and degree of agglomera-tion is presented in Table 5.

The SEM results show that agglomeration is most likely to occuron the smaller range of filter sizes, except PP þ NC sample in well-ventilated conditions where bigger sizes are observed. This suggests

conditions for the size <0.5 mm.

Table 5Agglomeration of primary particles on the cascade filters.

Sample size/mm 21.5 15.0 10.0 6.5 4.0 2.0 1.0 0.5 <0.5

Well V PP þ NC e þ þ e e e þ þ e

PP þ FR þ NC e e e þ þ þ þþ þ þSmall UV PP þ NC e e e e e e þ þ þ

PP þ FR þ NC e e e e e e þ þ þLarge UV PP þ NC e e e e e þ þ þ þþ

PP þ FR þ NC e e e e e e þ þ þþWell V e Well-Ventilated, Small UV e Small Under-Ventilated, Large UV e LargeUnder-Ventilated.e no agglomeration.þ slight agglomeration.þþ agglomeration.

J. Rhodes et al. / Polymer Degradation and Stability 96 (2011) 277e284 283

that ageing and transport may reduce the acute toxicity of smokeparticulates and may explain observations of high fire deaths withlow carboxyhaemoglobin levels in the room of fire origin. There islittle to distinguish between the small and large under-ventilatedconditions. Additional tests would be required to establish if this isthe case for all materials in all ventilation conditions.

4. Conclusions

The results appear to show that the particle size and distributionis unaffected by the ventilation condition and the sample composi-tion for PA6 materials. Experiments on PA6 samples show that inall ventilation conditions the unaltered polymer produces the leastamount of soot, indicating that the additives used increase theamount of soot produced. In addition, in all ventilation conditions,PA6 þ FR þ NC with both additives present, produces the mostamount of soot, closely followed by the samples of PA6 þ FR. Themajority of the soot produced from the PA6 experimentswas around1 mm aerodynamic diameter, this trend was most noticeable inthe small under-ventilated condition, but also mirrored in the largeunder-ventilated condition. The well-ventilated condition sup-ported the production of a range of particle sizes for the unchangedpolymer and the PA6 þ NC sample, but again the PA6 þ FR andPA6 þ FR þ NC followed the trend of the other conditions.

Similarly to PA6, the ventilation condition does not appear toconsistently influence the size distribution of particles, and the addi-tives have higher impact for the soot production. For the large under-ventilated condition, the particle size showed an increase with thelargest mass being around 0.93e1.55 mm diameter for PP þ NCsamples and PPþ FRþNC for well-ventilated conditions. The cascadeimpactor data suggests that the incorporated additives determine theparticle size of the soot produced and not the basic polymer.

The studies on particle mass distribution show that there issignificant particulate mass above the lower limit for particle mass,belowwhich there is no health danger. Toxicological studies tend toshow that particles become more toxic per unit mass as their sizedecreases as smaller particles show more vigorous surface activity.Recently concerns have been raised about insoluble particles in thefine, ultrafine, and nanoparticle size range as it has been shown thatparticles which are non-toxic in the mm size range may be toxic innm range. These studies showed that additives compared to purepolymers increased the soot formation with particle sizes lowerthan 0.5 mm (with some exceptions).

Total soot from the cascade impactor and from the single filterare very similar, except for FR additives in PA6 samples. This may bedue to deposition of large particles in the sampling tubes. There isa significant contribution from the particulate matter with smallersize than 0.52 mmwhich suggesting themost harmful particles mayalso be the most abundant.

According to SEM analysis further testing is required forother materials to establish whether the trends are the same and todetermine if the composition of the material burned changes thesusceptibility of the soot to agglomerate.

Acknowledgements

We thank the European Commission for funding Predfire-Nanoa STREP Framework 6 Project, and the University of CentralLancashire for funding two internships.

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