An Investigation on Fire and Flexural Mechanical Behaviors of Nano and Micro Polyester Composites Filled with SiO2 and Al2O3 Particles

Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 2 (2015) 8 – 19

5th International conference on Advanced Nano Materials

An investigation on fire and flexural mechanical behaviors of nano and micro polyester composites filled with SiO2 and Al2O3 particles M.C.S. Ribeiro*a,b, S.P.B. Sousaa and P.R.O. Nóvoab a

INEGI, Institute of Mechanical Engineering and Industrial Management, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal b FEUP, Faculty of Engineering of University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal

Abstract

This study is aimed at developing a new type of unsaturated polyester based composite material with enhanced fire retardancy through polymer matrix modification with nano/micro oxide particles, such as silicon dioxide, carboncoated silicon dioxide and aluminium oxide, in combination with common flame retardants systems. For this purpose, the design of experiments based on Taguchi methodology and analyses of variance were applied. Samples with different material and processing parameters resultant from L9 Taguchi orthogonal array were produced, and their fire and mechanical properties were assessed and quantified by vertical flammability tests (UL-94), flexural and Charpy impact tests. The effect of the sonication time of mixtures on final properties was also assessed. It was found that both material and processing parameters have different effects on the various analysed properties. The results revealed that addition of hybrid flame retardant systems introduced reasonable improvements in at least one fire reaction property; however, filler addition also led to decreases in some mechanical properties, most likely due to poor matrix-filler adhesion. Future studies are anticipated in order to improve mix design formulations towards further fire retardancy enhancement without significantly compromising mechanical properties. © 2014 The Authors. Elsevier Ltd. All rights reserved. © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation.

Keywords: Metal oxide nano/micro particles, flame retardant systems, unsaturated polyester composites, fire behaviour, mechanical properties.

* Corresponding author. Tel.:+351 22 9578710; fax:+351 22 9537352. E-mail address: [email protected]

2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation. doi:10.1016/j.matpr.2015.04.002

M.C.S. Ribeiro et al. / Materials Today: Proceedings 2 (2015) 8 – 19

1. Introduction Unsaturated polyester composites (UPC), due to their singular engineering properties, are increasingly replacing traditional materials like wood, glass and metal, in building, furniture, machinery and vehicle components, as well as in other consumer products [1]. Although there are great advantages of UPC over conventional materials, these composites have great sensibility to high temperatures and, in most cases, present poor behaviour in fire situations [1-4]. The increasing use UPC and other polymer based composite materials have led to a rise of the fire load in buildings and hence, to increasing fire hazards. According to statistical data, these materials have contributed to an exponential decrease of ‘escape-time’ from 17 minutes, in 1977, to 3 minutes, in 2007 [5]. One common approach to improve fire reaction behaviour of composite materials is the use of fire-resistant resins, which usually present poorer mechanical properties, or more often, the incorporation of flame retardant (FR) systems, such as halogenated, phosphate, melanine or metal hydroxide based compounds. However, fire retardancy by means of FR systems, either additives or reactive compounds, is far from being an easy and pacific issue. Halogenated based FR compounds may well lead to toxicological impacts in the environment and generate a great amount of smoke [1,6]. On the other hand, traditional FR filler additives, requiring large loadings for effective levels of fire retardancy, generally lead to significant decreases in the mechanical properties of resulting microcomposite, and most of the times, have a deleterious effect on its processability [1,4,7]. One emerging solution to overcome these undesirable features, induced by large filler loadings, is the replacement of conventional FR fillers by nanosized particles. The reduction from micro to nano-scale reduces the mobility of polymer chains and significantly enhances the surface area of the particles, leading to considerable differences in the amount of filler that is required, as well as in the degradation pathways and processes [8]. Unusual property combinations can be achieved, resulting in a new class of material for almost every application. Ongoing research revealed that polymer nanocomposites, with homogeneous nanoparticles dispersion, present improved characteristics over unfilled or micro-filled polymers. Depending on polymer and nanoparticle nature and content, the main improvements found are in mechanical properties such as toughness and stiffness, and barrier properties such as permeability and solvent resistance [9]. Other interesting properties include increased thermal stability and, foremost, improved ability to promote flame retardancy at very low filling levels [1, 9-13]. An exhaustive review of the state of the art in this field can be found in the book by Morgan and Wilkie [14]. The main findings, summarized therein, as well as ongoing research [7,11,15-17], clearly show that nanoparticles are in fact the basis for improved FR systems. The majority of research work on the subject of thermal stability and fire retardancy of polymers, by means of nanophase FR systems, has been carried out on polymer-clay nanocomposites, specially based on montmorillonite [18-20]. The use of nanoclays brings, however, some drawbacks. Nanoclays generally present a hydrophilic nature and this feature requires functionalization with hydrophobic end groups, which implies previous steps on manufacturing process [18]. On the other hand, due to the particular layered structure, homogeneous dispersion of nanoclays is hardly achieved. Most of the times, only conventional filled composites are attained, as the clay nanolayers feature is retained and no intercalation of the polymer into the clay structure occurs. Intercalated, or even more, exfoliated clay-nanocomposites, requires complex mixing procedures and are not easily achievable [21]. Lastly, the chemical composition of each type of clay is very diverse, enclosing many and diversified elements and compounds: being natural product, the guarantee of the same composition from batch to batch, with the same weight proportion of constituent elements, is not always verified. Taking into account the aforementioned reasons, there is a consensual feeling in the polymer community that more research work must be done using nanomaterials other than clay [1,22]. Other emerging nanoparticles that have already shown promising effects on polymer thermal degradation are metal oxide nanoparticles such as alumina, hydrated aluminium, titanium and magnesium hydroxides. The few studies focusing on this subject show promising results attesting that nano-oxides incorporation can improve thermal stability and other relevant properties of final product [7,11,12,16,21,23-26]. Fumed silica and silicon carbide at nanosize scale have also shown good results, conferring to polymers higher thermal stability, hardness, corrosion resistance and strength [27]. Synergistic effects between metal nanoparticles, including nano-oxides, and conventional FR systems have also been reported [28-32], and this approach seems very promising towards the efficient flame retardancy of polymers. Under this framework, in the present study an effort is undertaken to develop a new UPC material with improved fire reaction behaviour, by matrix modification with hybrid flame retardant systems based on nano/micro oxide

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particles (aluminium oxide, silicon dioxide and carbon-coated silicon dioxide), and phosphinate based compounds. Experiments were planned and conducted according to Taguchi methodology with basis on a L9 Taguchi orthogonal array, with 4 material/processing parameters at three variation levels. Fire reaction behaviour was analysed and quantified by vertical flammability tests (UL-94), and mechanical properties were assessed by flexural and Charpy impact tests.

2. Experimental Program 2.1. Raw materials and manufacturing process of UPC A commercially available unsaturated polyester resin (Aropol® FS3992, Ashland Chemical Hispania S.L., Spain), with a styrene content of 42%, was used as matrix. Polymerization process was induced by cobalt octoate (0.5 phr), as promoter, and 50% methyl ethyl ketone peroxide solution (1.0 phr), as initiator. Physical and mechanical properties of cured resin, as supplied by the manufacturer, are displayed in Table 1. Table 1. Physical and mechanical properties of cured resin (Aropol® FS3992). Property

Standard Method

Values

Heat Distortion Temperature (ºC)

ASTM D 648

90-100

Water Absorption (%)

ASTM D 570

0.2

Tensile Strength (MPa)

ASTM D 638

50-70

Ultimate Elongation (%)

ASTM D 638

3.0

Flexural Strength (MPa)

ASTM D 790

90-110

Barcol Hardness (-)

ASTM D 2583

45

The phosphinate based flame retardant was Exolit® OP1240 (Clariant-Químicos, Lda., Portugal) which consists of a fine white grain-based organic phosphonate powder with high phosphorus content (> 23%). The aluminum oxide nanoparticles, with the trade name of NanoDur ® (Al2O3, 99.5% purity, Alfa Aesar®), were purchased from Cymit Quimica S.L. (Spain), and both uncoated and carbon-coated silicon dioxide sub-microparticles (SiO2 and SiO2©, 98% purity) were provided by Innovnano (Portugal). All fillers have spherical shape, and some of their physical properties are specified in Table 2. Table 2. Physical characteristics of fillers. Property

Al2O3

SiO2

SiO2©

Exolit OP1240

Average Particle Size (ηm)

45

437

172

25000-50000

Specific Surface Area (m2/g))

36

64

57

-

Bulk Density (g/cm )

0.27

0.05

0.06

0.4-0.6

True Density (g/cm3)

3.60

2.69

2.57

1.35

Crystal Phase

70:30 δ:γ

Amorphous

Amorphous

-

Appearance/Color

White powder

White powder

Grey powder

White powder

3

A well determined quantity of filler was manually pre-mixed with the unsaturated polyester resin and promoter for a few minutes at room temperature. The mixing process was then further supported by sonication in an ultrasonic processor device (Hielscher UP200H), in intermittent mode (0.5s on/0.5s off), for several minutes. The initiator was then added and thoroughly mixed. All the mixtures were poured into preheated silicon moulds (1h/80ºC) and the samples were then allowed to cure for 6h at 80ºC. After demoulding they were subject to an additional thermal treatment for 3h at 80ºC.

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2.2. Mix design formulations The design of experiments (DoE) based on a Taguchi methodology was applied for the planning of the experiments. DoE is an approach for systematically varying the controllable input factors and observing the effects of these factors on the output product parameters, and it has been widely applied for product and material development [33]. Using Taguchi methodology, the same information provided by an experience plan based on full factorial method can be assessed but with the advantage of requiring a smaller number of experiments. This method also allows identifying the factors and the possible interactions between factors that most influence the response, using analyses of variance (ANOVA) [34]. The main objective of this study is to determine the optimal formulations of UPCs modified with nano/micro oxides and phosphinates that result in the best fire reaction and mechanical properties. The selected control material and processing factors were: type of oxide particle (A) and content (B), phosphinate FR content (C) and total sonication time (D). Each of these factors was run at three variation levels as shown in Table 3. The 34 full factorial design (4 factors at 3 variations levels) leads to 81 different experiments; however, using a suitable Taguchi orthogonal array, the dimension of experimental plan can be significantly reduced. In the present case the L9 orthogonal array was selected which conducts to 9 different experiments. The resultant trial formulations are specified in Table 4. No interactions between factors will be analysed taking into account that all the columns of L9 array are locked for the 4 factors in analysis. Table 3. Mix design factors and correspondent variation levels. Levels

Factor A

Factor B

1

SiO2©

0.0 wt%

Factor C 0.0 wt%

Factor D 3 minutes

2

Al2O3

2.5 wt%

10.0 wt%

15 minutes

3

SiO2

5.0 wt%

15.0 wt%

9 minutes

Table 4. Trial formulations resultant from Taguchi L9 orthogonal array. Trials

Factor A

Factor B

Factor C

Factor D

F1

SiO2©

0.0

0.0

3’

F2

SiO2©

2,5

10.0

15´

F3

SiO2©

5,0

15.0

9’

F4

Al2O3

0.0

10.0

9’

F5

Al2O3

2,5

15.0

3’

F6

Al2O3

5,0

0.0

15’

F7

SiO2

0.0

15.0

15’

F8

SiO2

2,5

0.0

9’

F9

SiO2

5,0

10.0

3’

2.3. Test procedures After UPCs preparation according to trial formulations defined in Table 4, the fire and mechanical properties were assessed by the UL-94 vertical flammability, flexural and Charpy impact tests. The UL94 test (Underwriters Laboratories, USA) allows the determination of plastics flammability characteristics [35]. This standard classifies plastics according to their burning behaviour in different orientations horizontal or vertical - and thicknesses. The vertical method was applied in this study. Accordingly, the specimens were supported vertically and a flame was applied to the bottom of the specimen (Fig. 1). The flame was applied for 10 seconds and then removed until flaming stopped, at which time the flame was reapplied for another 10 seconds and then again removed. Materials can be classified as V-0, V-1, V-2 or unrated (U), on the basis of overall results (time to ignition, 1st and 2nd afterflame times) for a set of five specimens with 125x13x5 mm3 dimensions.

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c)

b)

a)

d)

Fig. 1. Test specimens (a) and testing set-ups for UL-94 vertical flammability (b), flexural (c) and Charpy impact (d) tests.

Flexural mechanical properties were obtained according to a procedure based on the method described in standard ISO 178 [36]. Beam shaped specimens (100x10x5 mm3) of UPC formulations were tested in three-point bending at a constant loading rate of 1.15 mm.min-1 over a 60 mm span (Fig. 1). For each trial formulation five specimens were tested, and for each experiment, a load-deflection curve was recorded, from which the strength and elastic modulus were obtained. The Charpy unnotched impact strength was evaluated according to a superseded version of ISO 179 [37], using type 2 specimens (50x6x4 mm3) and a pendulum impact tester (Hounsfield Tensometer Ltd., U.K.) (Fig. 1). Five specimens of each formulation were prepared for impact strength testing. All specimens were pre-conditioned for 48 hours at 23°C/50% RH before being tested for fire reaction and mechanical properties.

3. Results and Discussion 3.1. Data test results Data test results corresponding to vertical flammability tests (i.e., ignition time - TIG -, after flame times -T1, T2-, and corresponding UL 94 classification), bending tests (i.e., flexural strength - σF - and elasticity modulus – EF -) and Charpy impact tests (-IS-) are summarized in Table 5. Presented values represent the average values obtained for five test specimens, and correspondent standard deviations. The appearance of test specimens after UL-94 flammability test is presented in Figure 2. Table 5. Data test results of UL-94, flexural and Charpy impact tests. Test Trials F1: Si©-0-0-3

UL-94 vertical flammability test

Flexural test

Impact test

T2 (s)

Class

σF (MPa)

EF (GPa)

IS (kJ/m2)

281.0 ± 24.4

(-)

U

84.92 ± 15.04

2.91 ± 0.54

4.23 ± 0.35

TIG (s)

T1 (s)

2.0 ± 0.0

F2: Si©-2.5-10-15

3.4 ± 0.3

18.6 ± 4.6

39.2 ± 23.8

U

62.38 ± 4.96

3.70 ± 0.40

1.94 ± 0.27

F3: Si©-5-15-9

2.6 ± 0.4

43.0 ± 4.2

35.5 ± 27.5

U

54.36 ± 9.11

3.07 ± 0.97

1.18 ± 0.30

F4: Al-0-10-9

2.2 ± 0.4

51.0 ± 30.4

23.0 ± 27.7

U

62.76 ± 3.54

3.80 ± 0.24

1.84 ± 0.41

F5: Al-2.5-15-3

2.0 ± 0.0

2.2 ± 2.6

29.5 ± 18.0

V-1

43.87 ± 4.43

3.07 ± 0.15

1.91 ± 0.21

F6: Al-5-0-15

2.5 ± 0.9

333.2 ± 46.3

(-)

U

47.50 ± 3.98

2.89 ± 0.63

3.41 ± 0.90

F7: Si-0-15-15

2.0 ± 0.7

2.8 ± 2.2

15.0 ± 21.5

V-1

62.45 ± 3.53

3.11 ± 0.24

1.72 ± 0.16

F8: Si-2,5-0-9

1.8 ± 0.1

229.0 ± 123.2

(-)

U

78.20 ± 3.37

4.14 ± 0.36

2.46 ± 0.74

F9: Si-5-10-3

2.5 ± 0.2

56.6 ± 29.7

120.0 ± 20.0

U

65.80 ± 3.56

3.70 ± 0.40

1.02 ± 0.46

nd

st

(-) Test specimens were not submitted to 2 flame application as they totally burned during and after the 1 flame application.

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F3: Si©-5-15-9

F1: Si©-0-0-3

F2: Si©-2,5-10-15

F4: Al-0-10-9

F5: Al-2,5-15-3

F6: Al-5-0-15

F7: Si-0-15-15

F8: Si-2,5-0-9

F9: Si-5-10-3

Fig. 2. Test specimens of each trial formulation after UL-94 flammability test.

Globally, as shown in Table 5, filler addition improved at least one fire response parameter when compared to unfilled formulation F1, either by increasing the time to ignition (TIG), and/or decreasing afterflame times (T1 and T2). Trial formulations F5 and F7 showed particularly significant improvements, with relative very low combustion times, which led to a V-1 classification for the vertical UL-94 test. The filler containing materials exhibited lower flexural strength and worse impact energy; however, they show in general a significant higher elasticity modulus. 3.2. Analyses of variance ANOVA and response graphs The Taguchi method uses ANOVA to analyse data obtained from implementation of the experimental design. The ANOVA performs the overall variance analysis of a sample set, identifying its origins and assessing the contribution of each factor to the global dispersion. The various parameters obtained from both the fire and mechanical tests can be submitted to statistical treatment with one single exception: the 2nd after flame parameter obtained from UL-94 test. During this test, some specimens were not submitted to the 2nd flame application because they have totally burned after removal of the 1 st flame (Fig. 2). Hence, only the other five parameters (TIG, T1, σF, EF and IS) were submitted to statistical treatment. The results of the analyses of variance for the five aforementioned parameters are presented in Table 6. The results are detailed in terms of sum of squares (SQ), degrees of freedom (dF) quadratic mean deviation (QMD), Fratio (Frat) and percent contribution to global variation (P). All analyses were performed for a significance level (α) of 5% (or a confidence level of 95%). For this level of significance, the F-test ratio critical value (Fcrit) is 3.26, for all 4 factors under study. The response graphics are a graphical presentation of the main affects and allow a much more perceptible evaluation of the relative importance of each factor, compared to the numerical values of the effects. The response graphics for the main effects of each factor are presented in Figure 3.

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dF

QMD (s2)

Frat

P (%)

Factor A Factor B Factor C Factor D Error Total Variation source

2.27 1.30 3.05 2.81 3.70 13.12 SQ (s2)

2 2 2 2 36 44 dF

1.13 0.65 1.53 1.40 0.10

11.03 6.32 14.87 13.78

15.7 8.3 21.7 19.8

QMD (s2)

Frat

P (%)

Factor A Factor B Factor C Factor D Error Total Variation source

8178.18 27162.84 638560.84 927.24 107719.20 782548.31

2 2 2 2 36 44 dF

4089.09 13581.42 319280.42 463.62 2992.20

1.37 4.54 106.70 0.15

(*) 2.7 80.8 (*)

QMD (MPa2)

Frat

P (%)

1394.58 762.63 1054.36 284.60 46.50

29.99 16.40 22.68 6.12

31.11 16.53 23.26 5.50

QMD (GPa2)

Frat

P (%)

0.72 0.08 3.25 0.02 0.25

2.90 0.32 13.13 0.09

(*) (*) 6.0 (*)

Frat

P (%)

10.07 8.92 67.38 6.35

8.19 7.15 59.96 4.84

1st Afterflame Time

Time to Ignition

Variation source

Flexural Strength

Factor A Factor B Factor C Factor D Error Total Variation source

SQ (MPa2) 2789.16 1525.27 2108.71 569.21 1673.85 8666.19 SQ (GPa2) 1.43 0.16 6.50 0.05 8.91 17.05

2 2 2 2 36 44 dF

SQ ((kJ/m2)2)

2 2 2 2 36 44 dF

Factor A Factor B Factor C Factor D Error Total

4.67 4.13 31.23 2.95 8.34 51.32

2 2 2 2 36 44

Impact Strength

Elasticity Modulus

Factor A Factor B Factor C Factor D Error Total Variation source

QMD ((kJ/m2)2) 2.33 2.07 15.62 1.47 0.23

(*) – The null hypothesis was not rejected for a significance level of 5% (i.e., Frat < Fcrit)

With basis on response graphs, the best combinations of factors’ levels that lead to improved response on each analysed property are shown in Table 7. For each optimal formulation, the estimated response values (RVEST), predicted by Taguchi method and determined according to Equation 1, are also shown in Table 7. RV EST = A + Σ (Xi – A)

(1)

where A is the global average of all formulations, and Xi represents the marginal mean corresponding to all responses involving factor X with the optimum level. It must be pointed out that only the material factors with a statistical significant effect on global response were considered for the computation of estimated response value.

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1st Afterflame Time (s)

SiO2C

Time to Ignition (s)

Factor A

Factor B

Factor C

Factor A

Factor D

Flexural Strength (MPa)

Factor A

Factor B

Factor C

Factor B

Factor C Factor D

Flexural Elasticity Modulus (GPa)

Factor A

Factor D

Factor B

Impact Strength (kJ/m2)

Factor A

Factor B

Factor C

Factor D

Fig. 3. Response graphics: Main effects of each factor on response parameters.

Factor C

Factor D

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Table 7. Optimal formulations and estimated response values. Response Property

Optimal Formulation

Estimated Value

Best Experimental Value

Time to Ignition (s)

Si©-5-10-15

3.57 (Not tested)

3.40

F2: Si©-2,5-10-15

1st Afterfame Time (s)

Si-2,5-15-9

16.60 (Not tested)

2.80

F5: Al-2,5-15-3

Flexural Strength

Si-0-0-9

84.12 (Not tested)

84.92

F1: Si©-0-0-3

Elasticity Modulus

Si-0-10-15

3.88 (Not tested)

4.14

F9: Si-5-10-3

Impact Strength

Si©-0-0-3

3.37 (Tested)

4.23

F1:

Si©-0-0-3

3.3. Effect of material factors on mechanical responses Filler addition, regardless of its nature (i.e., alumina nanoparticles, neat and carbon-coated silica submicroparticles, or phosphinate based FR) results in minor to large reductions in flexural and impact resistances and a global increase in elasticity moduli of the resulting micro/nano composites. With respect to neat or control formulation (F1), averages decreases of 29% and 54% were found, respectively, on flexural and impact strengths of modified formulations; though, an average increase of 19% was also observed on elasticity modulus. This initial trend is confirmed by the analyses of variance performed for each analysed property, which clearly show that the factors with greater influence in mechanical resistance are the FR content (Factor C) or the oxide type (Factor A), followed by the oxide content (Factor B). Sonication time (Factor D) is the factor with minor influence on global variance of all analysed mechanical properties. Among the modified formulations, the best results with regard to flexural resistance are achieved with the single additions of neat silica (F8), phosphinate FR (F4), or the hybrid combination of FR and silica (F9). These formulations are also the ones exhibiting the higher values of elasticity modulus, with increases between 27% up to 42% over the control formulation. However, no clear trend was found for the effect of the different material factors on elasticity modulus, since none of them showed to have significant influence on this parameter. For a significance level of 5%, the null hypothesis was not rejected for Factors A, B and D; and the single factor that showed significant statistical influence on the elasticity modulus (Factor C -the content of FR), only contributes with 6.6% to global variation. With respect to impact resistance, it seems that FR is the filler with the most deleterious effect on this property. The worst results were obtained for trial formulations incorporating 10% or 15% of this compound, and the contribution of this factor to global variance of impact energy is almost 60%. In fact, the lowest decreases on impact resistance were found for trial formulations with single addition of nanolumina or sub-micro silica (0% of FR). The functionalization of silica sub-micro particles with carbon coating was not efficient at improving mechanical properties. Indeed, trial formulations with carbon-coated silica particles presented worst mechanical results than those with neat silica particles. No definitive conclusion was reached regarding the influence of the ‘Sonication time’ factor on mechanical behaviour of the nano/micro composites. This factor showed modest contributions to global variation of flexural and impact strengths (5.5% and 4.8%, respectively), and it was considered not influential with regard to elasticity modulus. However, with basis on previous results of the authors [38], it is believed that longer sonication times, by promoting more effective particle dispersion within the matrix, lead in general to an improved mechanical behaviour of resultant nano/micro composites. 3.4. Effect of material factors on fire reaction response Filler incorporation leads to a general improvement of fire properties as evaluated by the UL-94 test, either by increasing time to ignition, or by decreasing combustion time after flame removal, or by both features. The use of fillers leads, on average, to 18% increase in time to ignition and 67% decrease in combustion time. The most influential factor on fire properties is the FR content, contributing with 22% and 81% to global variance of ignition and afterflame times, respectively.

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With respect to ignition time, the greater improvements were observed with formulations containing double additions of FR and silica (SiO2 and SiO2©) – with increases between 25% and 75%. The single additions of nanoalumina also lead to satisfying results (25% increase). Although there is a relatively significant contribution of ´Sonication time’ to global variance of this fire response (it is the second most influential factor, contributing with almost 20%), no clear trends were observed with basis only on data results. Nevertheless, response graphic clearly shows that higher ignition times are achieved with higher sonication times, and in fact, the best experimental result was obtained for a trial formulation with 15´sonication time. The best results for 1st afterflame time were obtained for trial formulations containing the double addition of FR and nanoalumina (F5) or single FR addition (F7), with reductions in combustion time reaching 99%. These improvements resulted in those formulations being classified as V-1 in the UL-94 test. ANOVA results indicate, with certainty, FR content to be the most (and almost the single) influential factor on the variance of combustion time, with a percent contribution of more than 80%. The single FR additions, as well as the double additions of FR and silica also stood out with respect to this response parameter. A relatively high difference was obtained between the estimated response value predicted by Taguchi method (Eq. 1) for the optimal formulation, with regard this parameter, and the best value experimentally obtained. This last one was in fact considerably lower and corresponds to a trial formulation with equal contents of oxide and FR of optimal formulation, but with distinct variation levels for Factors A and D. These factors were considered with no significative statistical influence on the 1st afterflame time, and consequently, their contributions were not considered in the computation of the estimated response value. The same feature also explains the differences found between the estimated response value of optimal formulation for elasticity modulus, and the best value obtained experimentally. In this case, only the contribution of one factor was considered (Factor C). Despite the factor ‘Sonication time’ not having been considered a statistically influencing factor on 1st afterflame time, trial formulations with the maximum sonication time (15´) tend to present the worst results for the combustion time as confirmed by the response graphics; though, maximum sonication time also produces higher values of time to ignition. With increasing sonication time a higher content of air bubbles is produced within the composite. The void content increase may be the cause of lowering the material fire behaviour, creating “pathways” to flame propagation. Finally, the functionalization of silica sub-micro particles with carbon coating was proficient, at least, at improving both fire response properties: trial formulations with carbon-coated silica particles present improved fire behaviour than neat silica filled UPCs.

4. Conclusions The Taguchi methodology was used, as an alternative approach to the full factorial experimental plan, in order to determine material factors’ effects on fire and mechanical properties of polyester composites modified with nano/micro oxides and phosphinates. The following conclusions can be drawn from the investigation: x The introduction of nano/micro oxide particles, in combination or not with phosphinate based FR, leads to quite considerable decreases in flexural strength and impact energy values, regardless of the particle average size, content and chemical nature. The overall decrease in these mechanical properties, more noticeable in the case of alumina nanoparticles addition, may be associated to a weak filler-matrix interface and/or non-homogeneous filler dispersion. x The incorporation of the different combination of fillers has, in general, a significant improvement effect on flexural elasticity modulus, especially in the case of the single addition of silica sub-micro particles (2,5%). x The addition of FR, alone or in combination with alumina nanoparticles, produces significant improvements on the fire behaviour of the resulting nano/micro composites, resulting in a V-1 classification for the UL-94 test. The hybrid filling consisting of FR and carbon-coated silica sub-micro particles also results on improved fire reaction properties. While these last formulations are not the ones with better mechanical properties, they clearly show the best compromise between both mechanical and fire performances. This fact confirms the synergetic effect between FR and nanoxides, already found in previous studies [29, 31, 38].

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x The material factors have different effects on fire behaviour of resultant UPC formulations; though, it can be stated that 0 wt.% to 2.5% wt.% of nanoalumina or carbon-coated micro silica particles, in combination with 10 wt,% to 15 wt,% of phosphinate based FR, result in the best fire properties of final UPC. For each analysed property, the optimized formulations are divergent and the factors had different effects on final results of formulations; though, it is clear from the results that the optimal formulations for mechanical properties, with the exception of flexural elasticity modulus, do not contain fillers. Composites with micro and/or nano-oxide fillers may show better fire performance than unmodified traditional composites, but more studies are required. The filler content value is a fundamental factor regarding composite final properties. Additional studies are thus foreseen in order to explore the synergistic effect between nano-oxides and FR, and more thoroughly study the use of fillers (e.g., content, treatments), so that products with better fire performance and without significant loss in mechanical properties can be obtained.

Acknowledgements The financial support of FCT, COMPETE and FEDER (under PTDC/ECM/110162/2009 project) is gratefully acknowledged. Acknowledgements are also due to Innovnano and Clariant, for providing the SiO2 microparticles and the fire retardant Exolit® OP 1240, respectively, and to J. Rodrigues, for his help and valuable assistance.

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