Halogen-Free Multicomponent Flame Retardant Thermoplastic

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Halogen-free Multi-component Flame Retardant Thermoplastic Styrene-Ethylene-Butylene-Styrene Elastomers Based on Ammonium Polyphosphate – Expandable Graphite Synergy Antje Wilke, Kirsten Langfeld, Bernhard Ulmer, Vlad Andrievici, Andreas Hörold, Patrick Limbach, Martin Bastian, and Bernhard Schartel Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01177 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Industrial & Engineering Chemistry Research

Halogen-free Multi-component Flame Retardant Thermoplastic Styrene-Ethylene-Butylene-Styrene Elastomers Based on Ammonium Polyphosphate – Expandable Graphite Synergy Antje Wilke,† Kirsten Langfeld,† Bernhard Ulmer,‡ Vlad Andrievici,‡ Andreas Hörold,† Patrick Limbach,‡ Martin Bastian,‡ Bernhard Schartel *,†

†

Bundesanstalt für Materialforschung und –prüfung (BAM), Unter den Eichen 87, 12205

Berlin, Germany, [email protected] ‡

SKZ German Plastic Center, Friedrich-Bergius-Ring 22, 97076 Würzburg, Germany

Corresponding Author Bernhard Schartel, Bundesanstalt für Materialforschung und –prüfung (BAM), Unter den Eichen 87, 12205 Berlin, Germany. *E-mail: [email protected]

Keywords: Thermoplastic elastomers, flammability, expandable graphite, ammonium polyphosphate

ABSTRACT: Flame retarded thermoplastic elastomers (TPE-S) based on styrene-ethylenebutylene-styrene, polypropylene, and mineral oil are a challenging task, due to their very high fire loads and flammability. A promising approach is the synergistic combination of expandable graphite (EG) and ammonium polyphosphate (APP). Cone calorimeter, oxygen index, and UL 94 classification were applied. The optimal EG:APP ratio is 3:1, due to the most effective fire residue morphology. Exchanging APP with melamine-coated APPm yielded crucial improvement in fire properties, whereas replacing EG/APP with melamine 1 ACS Paragon Plus Environment


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polyphosphate did not. Adjuvants, such as aluminum diethyl phosphinate (AlPi), zinc borate, melamine cyanurate, titanium dioxide, dipentaerylthritol, diphenyl-2-ethyl phosphate, boehmite, SiO2, chalk, and talcum, were tested. All flame retardants reinforced the TPE-S. The combination with AlPi is proposed, since with 30 wt% flame retardants a MARHE below 200 kWm-2 and aV-0 rating was achieved. Multicomponent EG/APP/adjuvants systems are proposed as a suitable route to achieve efficient halogen-free flame retarded TPE-S.

1. INTRODUCTION

In recent years, thermoplastic elastomers based on styrene (TPE-S), and especially styreneethylene-butylene-styrene copolymers (SEBS), have attracted much attention due to their advantageous combination of properties: easy processability, low cost, elasticity, and recyclability. As TPE-S is applied in daily usage, e.g. in electronic devices and cable jackets, their flammability must not be neglected. The TPE-S used in this work consists of a rubber component, which is SEBS, and a thermoplastic component, namely polypropylene (PP). Mineral oil is added as a plasticizer. On account of those ingredients, TPE-S does not show any charring and has an extremely high effective heat of combustion, and thus very high fire loads (e.g. total heat evolved, THE), high peak heat release rate (PHRR) and short ignition times (tig). The development of suitable flame retardants is a very challenging task, because highly efficient systems are demanded, and because the polymeric ingredients exhibit a pronounced lack of charring ability. At first glance, only flame retardants containing halogen are known to possess efficiencies high enough to flame retard polyolefines like TPE-S at a reasonable additive load. However, the discussion on possible and probable disadvantages of halogenous flame retardants, concerning environmental issues and the production of corrosive smoke, has shifted market demand and thus current research activities towards halogen-free flame retardants.1-3 Apart from systems with high metal hydrate content, there are only few examples in the literature that propose halogen-free flame retarded TPE-S systems.4-7 2 ACS Paragon Plus Environment

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Comparing the starting points for TPE-S: char yield = 0 wt%, effective heat of combustion of the volatiles = ca. 42 MJ/kg,7 with the characteristics reported for thermoplastic polymeric materials with V-0 classification in UL 94: 20-30 wt%/20-22 MJ/kg (flame retarded polycarbonate (PC)/acrylonitrile butadiene styrene blends),8,9 25 wt%/20 MJ/kg (flame retarded PC/silicone rubber),10 20 wt%/22 MJ/kg (flame retarded high-impact polystyrene),11 11-15 wt%/13-17 MJ/kg (flame retarded polybutylene terephthalate),12-14 indicates that neither flame inhibition (= reducing the combustion efficiency in the flame) nor charring (= storing fuel in the condensed phase as residue) will ever do the job alone when a reasonable amount of halogen-free filler (≤ 30 wt%) is used. The flame retardancy efficiency for flame inhibition based on phosphorus released into the gas phase usually levels off at higher concentrations.14-16 This nonlinear behavior limits the potential flame retardancy effect to a certain extent, so that the demand for flame inhibition of around 66% for TPE-S cannot be achieved.7 The low charring tendency of the main components of TPE-S, PP, mineral oil, and SEBS, rules out a char yield as high as 60%. Consequently, the most promising approaches among environmentally friendly flame retardants for TPE-S are intumescent systems17-20 that combine charring with an efficient protective residual layer as the main flame retardancy mode of action. Intumescent flame retardant systems are composed mainly of compounds containing phosphorus and/or nitrogen, which lead to the formation of a multicellular foamlike protective char layer when exposed to heat. Apart from ammonium polyphosphate (APP) systems, a promising candidate for an effective additive to reduce heat transfer is expandable graphite (EG).21-23 Consisting of carbon sheets with sulfuric acid inserted, gases like SO2, CO2 and H2O are released upon heating. As a result the graphite sheets expand and a thick insulation layer is formed. The combination of APP and EG to flame retard polymeric materials has been proposed several times,24-28 also as the basis of multicomponent systems.29 Ge et al. investigated the influence of APP/EG in acrylonitrile-butadiene-styrene;30 Xie et al. in polyolefin blends.31

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The presented study discusses the performance of the combination of EG and APP in TPE-S. The study was part of a larger research project questioning different routes of halogen-free multicomponent flame retardant TPE-S and was performed in accord with a committee of industrial partners. Instead of proposing novel flame retardants, the goal was to compare the fire behavior of comprehensive sets of multicomponent materials delivering scientific understanding and quantitative results illustrating what is going on and what routes makes sense to pursue. The concentration dependencies are analyzed to find the optimal ratio of EG to APP, and the synergism between EG and APP is quantified. Temperature measurements were performed during burning, and the morphology of the fire residue was characterized with scanning electron microscopy (SEM) to illuminate the origin of the synergism. TPE-S/EG/APP with melamine polyphosphate (MPP) replacing part of the EG/APP, and replacing APP with melamine-coated APPm, have been investigated to improve flame retardancy. TPE-S/EG/APPm systems using several different adjuvants were examined to optimize performance. The comprehensive study shows that multicomponent systems based on EG/APPm are promising routes towards efficient halogen-free flame retarded TPES.

2. EXPERIMENTAL

2.1. Materials. The SEBS (Calprene H 6174) was purchased from Dynasol Gestion, Spain. Mineral oil (Shell Ondina Oil 941, CnH2n+2 (n = 15-40)) was purchased from Maagtechnic, Switzerland.

The

PP

was

SABIC

PP

PCGH10

(SABIC

Sales,

Netherlands).

Pentaerythrittetrakis(3-(3,5-di-tert-butyl-4-hyydroxyphenyl)propionate) (Irganox 1010, BASF SE) was used as antioxidant. APP and melamine-coated APP (APPm), Exolit AP 422 and Exolit AP 462, respectively, were purchased from Clariant Produkte (Deutschland) GmbH, Germany. The EG used was NORD-min 35 (NRC Nordmann, Rassmann GmbH, Germany) with a expansion volume of 35 ml/g and a particles size of minimum 80% < 0.15 mm. This graphite is modified by sulfuric acid and starts expansion at 180 °C +/- 10 °C. Further 4 ACS Paragon Plus Environment

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adjuvants are listed in Table 1. All raw materials were commercially available and used as received.

Table 1. Trade names, vendors, chemical names and acronyms for flame retardants and adjuvants Trade name

Chemical name

Acronym Vendors

Exolit AP 422

Ammonium polyphosphate

APP

Clariant

Exolit AP 462

Melamine-coated APP

APPm

Clariant

NORD-min 35

Expandable graphite

EG

Nordmann, Rassmann

Exolit OP 1230

Diethyl-aluminum phosphinate

AlPi

Clariant

Charmor DP15

Dipentaerythritol

DiPer

Perstorp

Melapur 200

Melamine polyphosphate

MPP

BASF

Melapur MC25

Melamine cyuanurat

MC

BASF

Sidistar

Spherically shaped amorphous

SiO2

Elkem

silicon dioxide Disflamoll DPO

Diphenyl-2-ethylhexyl phosphate DPO

Lanxess

Firebrake ZB

Zinc borate

ZB

NRC

Kronos 2220

Titanium dioxide (Rutile)

TiO2

Kronos International

Actilox B60

Boehmite AlO(OH)

Nabaltec

Omyacarb 5 GU

Chalk

C

Omya

Jetfine 8 CF

Talcum

Ta

Imerys Talc

2.2. Sample Preparation. The composition of the basic polymeric material is 38.4 wt% SEBS, 15.4 wt% PP, 46.1 wt% mineral oil, and 0.1 wt% antioxidant. First, SEBS granulate, PP, plasticizer, and antioxidant were mixed in a Mixaco CM80-h mixer. The compounds were prepared using a corotating twin screw extruder (ZSE 27 Maxx, Leistritz) with a screw speed of 400 min-1, a throughput of 15 kg h-1 and a barrel temperature of 170 – 180 °C. The premix was mixed with the flame retardant by means of a gravimetric unit from Brabender 5 ACS Paragon Plus Environment


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Technology. The pellets obtained were dried for 3 h at 80 °C. Injection molding was performed using a Wittman-Battenfeld HM 800 at a constant melt temperature of 210 °C to prepare the test pieces. The analyzed materials’ compositions are summarized in Table 2 and Table 3.

Table 2. Composition of TPE-S/EG/APP and TPE-S/EG/APP/MPP with 30 wt% and 50 wt% additive content TPE-S

EG

APP

MPP

Material Abbreviation

/ wt%

/ wt%

/ wt%

/ wt%

0.00EG-30

70.0

0

30.0

0.25EG-30

70.0

7.5

22.5

0.33EG-30

70.0

10.0

20.0

0.50EG-30

70.0

15.0

15.0

0.66EG-30

70.0

20.0

10.0

0.75EG-30

70.0

22.5

7.5

1.00EG-30

70.0

30.0

0

0.00EG-50

50.0

0

50.0

0.25EG-50

50.0

12.5

37.5

0.33EG-50

50.0

16.7

33.3

0.50EG-50

50.0

25.0

25.0

0.66EG-50

50.0

33.3

16.7

0.75EG-50

50.0

37.5

12.5

1.00EG-50

50.0

50.0

0

EG/APP-2.5M

70.0

20.6

6.9

2.5

EG/APP-5.0M

70.0

18.8

6.2

5.0

EG/APP-7.5M

70.0

16.9

5.6

7.5


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Table 3. Compositions of TPE-S/EG/APPm + adjuvant (overall 30 wt% flame retardants), TPE-S/EG/APPm-A with different amounts of flame retardant, and TPES/EG/APPm-A with 20 wt% additional incombustible filler TPE-S Composition Material Abbreviation

/ wt%

EG/APPm

70.0

22.5 wt% EG + 7.5 wt% APPm

EG/APPm-A

70.0

18.7 wt% EG + 6.2 wt% APPm + 5 wt% AlPi

EG/APPm-ZB

70.0

20.6 wt% EG + 6.9 wt% APPm + 2.5 wt% ZB

70.0

16.9 wt% EG + 5.6 wt% APPm + 5 wt% MPP

EG/APPm-ZB-M

+ 2.5 wt% ZB 70.0

10 wt% EG + 7.5 wt% APPm + 7.5 wt% AlPi

EG/APPm-A-MC

+ 5 wt% MC 70.0

15 wt% EG + 5 wt% APPm + 7.5 wt% AlPi

EG/APPm-A-T

+ 2.5 wt% TiO2

EG/APPm-A-M

70.0

15 wt% EG + 5 wt% APPm + 5 wt% AlPi + 5 wt% MPP

EG/APPm-A-DP-MC

70.0

5 wt% EG + 7.5 wt% APPm + 7.5 wt% AlPi + 5 wt% DiPer + 5 wt% MC

EG/APPm-D

70.0

18 wt% EG + 7 wt% APPm + 5 wt% DPO

EG/APPm-B

70.0

18 wt% EG + 6 wt% APPm + 6 wt% Boehmite

EG/APPm-S

70.0

18 wt% EG + 6 wt% APPm + 6 wt% SiO2

EG/APPm-D-B

70.0

12 wt% EG + 6 wt% APPm + 6 wt% DPO + 6 wt% Boehmite

EG/APPm-A-40 wt%

60.0

24.9 wt% EG + 8.3 wt% APPm + 6.7 wt% AlPi

EG/APPm-A-35 wt%

65.0


EG/APPm-A-27.5 wt%

72.5


EG/APPm-A-S20

50.0

18.7 wt% EG + 6.2 wt% APPm + 5 wt% AlPi + 20 % SiO2

EG/APPm-A-Ta20

50.0

18.7 wt% EG + 6.2 wt% APPm + 5 wt% AlPi + 20 % Talcum

EG/APPm-A-C20

50.0

18.7 wt% EG + 6.2 wt% APPm + 5 wt% AlPi + 20 % Chalk



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2.3. Fire Behavior. The flammability of the materials (reaction to small flame) was analyzed in UL 94 vertical (V) and horizontal (HB) tests using the relevant burning chamber (Fire Testing Technology, UK) in accordance with IEC 60695-11-10. The specimen size was 126 mm x 12.7 mm x 3.2 mm. The oxygen index (OI) was measured according to ISO 4589 using specimens 126 mm x 6.5 mm x 3.2 mm in size in an oxygen index apparatus (Fire Testing Technology, UK). The burning behavior under forced flaming conditions was investigated with a cone calorimeter (Fire Testing Technology, UK) according to ISO 5660 with specimens 100 mm x 100 mm x 3 mm in size. The plates were put into a tray of aluminum foil and measured at a heat flux of 50 kW·m-2. Due to the expected expansion of the samples, the distance between the surface of the specimen and the cone heater was set to 35 mm. This moderate increase in distance delivered advantages for measuring the intumescent specimen without significantly disturbing the homogeneity of the heat flux with respect to the surface area.32,33 Before measurement the samples were conditioned for 48 h at 23 °C and 50% rel. humidity. The quality of material and specimen preparation resulted in a good repeatability of the cone calorimeter test, so that first two specimens were tested for each material and only when any characteristic result differs more than 10% a third specimen was measured. The morphology of fire residues was characterized by SEM using a Zeiss Evo MA 10 (Oberkochen. Germany) with an accelerating voltage at 10 kV. To perform repeatable and reliable temperature measurements on the back of the specimen, a slightly modified setup with a special frame was used (Figure 1). A type K thermocouple soldered to a copper disk (diameter: 12 mm, thickness: 0.2 mm) was glued to the back of the sample using high-temperature glue and an insulating blanket. During the cone calorimeter test run, the temperature of the back surface was recorded every 2 seconds by a Keithley 2700 multimeter connected to a computer.


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Figure 1. Temperature measurement setup and attachment of the thermocouple onto the back of the specimen.

2.4. Mechanical Testing. A tension testing machine Z010 (Zwick/Roell, Germany) was used with a preload force of 1.5 N and a testing speed of 200 mm min-1. Tests were performed until breakage according to DIN 53504 using 5 dumbbell specimen of typ S2. The longitudinal axle of the test specimen coincided with the flow direction of the injection moulding. Before measurement the samples were conditioned for 24 h at 23 °C and 50% rel. humidity. The Shore A hardness was determined according ISO 868 using 3 plate specimens with 6 mm or 2 mm thickness and testing 3 different positions for each plate.

3. RESULTS AND DISCUSSION

3.1. Morphology of the TPE-S/EG/APP Fire Residues. The unprotected TPE-S is a highly combustible material and burned completely without leaving any char. The photographs of the residues of the samples containing 30 wt% of EG/APP combinations after the cone calorimeter measurements are depicted in Figure 2. From Figure 2a to Figure 2e the partial fraction of EG in the EG/APP combination increased from 0 to 0.25, 0.50, 0.75 and 1, respectively. Obviously, the concentration of EG strongly influenced the fire residue. Without EG, that is, with just APP in 0.00EG-30 (Figure 2a), the fire residue consisted of a thin layer of inorganic residue, somewhat glassy polyphosphate ash, on the aluminum foil. Without a proper charring agent present, APP alone in TPE-S did not induce any relevant charring or intumescence at all. As the concentration of EG increased and APP decreased, the residual



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char became more voluminous and even fluffy (Figure 2b-2d). No APP in 1.00EG-30 led to complete disaggregation, loose residue, and loss of the sample shape (Figure 2e).

Figure 2. Fire residues after the cone calorimeter test of a) 0.00EG-30, b) 0.25EG-30, c) 0.50EG-30, d) 0.75EG-30, and e) 1.00EG-30.

The macroscale photographs showed the effect of different EG/APP ratios on the volume increase and the mechanical stability of the residues. To explain these observations on the macroscale, the impact of APP on the morphology of the residue was analyzed on the microscale using SEM. The samples with the single FR, namely 0.00EG-30 and 1.00EG-30, and the combination of both, namely 0.25EG-30 and 0.75EG-30, are shown in Figure 3.

Figure 3. SEM images (two different resolutions) of a) 0.00EG-30, b) 0.25EG-30, c) 0.75EG30, and d) 1.00EG-30.

The increase in EG and the decrease in APP obviously influenced the morphology on the microscale. In Figure 3a, the residue of the 0.00EG-30 reveals a dense and glassy thin 10 ACS Paragon Plus Environment

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layer. The other materials showed the typical features for materials containing expandable graphite; the fire residues were dominated by expanded sheets. In Figure 3b (0.25EG-30) the graphite sheets are covered with a sticky coating, although this feature does not appear in Figure 3c and Figure 3d, where the APP content was lower. A more open and expanded structure was observed for 0.75EG-30 and 1.00EG-30 (Figure 3c and Figure 3d). In Figure 3d the flakes of the sample contain only EG, yield large cavities and are fully expanded. This strong expansion of the sheets was expressed in a high volume increase on the macroscopic scale, as confirmed by the residue photographs. Thus one factor controlling the different volume expansion is the content of EG in the material. However, the SEM images clearly revealed another important effect: the sticky coating, which arose due to the presence of APP. During pyrolysis APP decomposes to polyphosphoric acid and ultimately to glassy polyphosphate. Based on the SEM images, this glassy polyphosphate was concluded to increase the adhesion effect of the EG stacks as well as between the expanded worms of the residues containing EG. That means that, at high concentrations of APP, as in 0.25EG-30, the expansion of the graphite layers was hindered to some extent and a sticky coating was left, which glued the graphite particles together. This observation is in agreement with studies by other groups,31 and explains well the limited volume increase observed for the residues, as well as the mechanical stability of the fire residue. To achieve the most effective protective layer, a high content of expandable graphite is demanded to ensure the insulating increase in volume, and some APP to avoid loose particles and to provide some mechanical stability.

3.2. Flammability of TPE-S/EG/APP Combinations: The Influence of EG Content. The flammability (reaction to small flame) of the materials was determined using the OI and the classification according to UL 94. The results are shown in Table 4.



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Table 4. OI and UL 94 tests results of EG/APP combinations Sample name

OI / vol%

UL 94

Horizontal flame speed / mm min-1

TPE-S

17.2

NR

0.00EG-30

18.7

NR

0.25EG-30

19.6

HB

27.5

0.33EG-30

20.1

HB

26.2

0.50EG-30

22.4

HB

22.2

0.66EG-30

23.3

HB

18.4

0.75EG-30

24.0

HB

24.8

1.00EG-30

21.4

HB

30.2

0.00EG-50

21.1

HB

23.6

0.25EG-50

23.2

HB

14.9

0.33EG-50

23.2

HB

0

0.50EG-50

28.6

V-0

0.66EG-50

32.2

V-0

0.75EG-50

39.0

V-0

1.00EG-50

26.5

HB

0

TPE-S showed an OI of 17.2 vol% and did not receive any rating in UL 94. When discussing the materials with 30 wt% EG/APP loading, starting from 0.00EG-30 with an OI of 18.7 vol%, OI increased up to 24 vol% for 0.75EG-30 (EG:APP ratio = 3:1) with increasing EG content, whereas the material without APP, 1.00EG-30, showed an OI of 21.4 vol%. An analogous dependency was observed for the OI of the materials containing 50% of the various EG/APP mixtures. The material without EG containing only 50% APP, 0.00EG50, had an OI of 21.1 vol%. The OI increase with increasing EG content up to an OI as high as 39 vol%, when the EG/APP ratio was 3:1. TPE-S with 50% EG, 1.00EG-50, had an OI of 26.5 vol%. EG/APP combinations exhibit a clear synergistic behavior in TPE-S with respect to OI. In the UL 94 test, all samples containing 30 wt% EG/APP and EG as flame retardants achieved a HB classification. 0.00EG-30, which contains only 30 wt% APP, was not rated in the UL 94 as TPE-S. The horizontal flame speed decelerated for the materials based on the 12 ACS Paragon Plus Environment

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EG/APP combination, which implies improvement; the best result was observed for 0.66EG50 (18.4 mm min-1). Adding EG alone resulted in a significantly higher flame speed (30.2 mm min-1). Compared to the samples with 30 wt% flame retardants, the UL 94 classifications were crucially improved for the materials containing 50 wt% EG/APP. 0.00EG-50 was classified HB with a flame speed of 23.6 mm min-1. With increasing amounts of EG the flame speed decreased; even 0.33EG-50 already showed the same flame extinction in the horizontal test as 1.00EG-50. The materials 0.50EG-50, 0.66EG-50, and 0.75EG-50, containing EG ratios of 0.50, 0.60 and 0.75, respectively, achieved V-0 classification. The UL 94 results of the EG/APP combination were improved (V-0) compared to the results for TPE-S/EG (HB) and TPE-S/APP (no rating), respectively, when using the same overall amount of flame retardants. A valuable synergism is concluded for combining EG/APP in TPE-S, particularly for the EG/APP 3:1 mixtures. Figure 4 illustrates the OI results graphically as well as the results of quantifying synergism by means of the synergy index (SE) according to Equation 1.34,35 The plot reveals not only better performance than expected assuming an ordinary mixing rule, but even a pronounced maximum for both the series of TPE-S containing 30 wt% and the one with 50 wt% EG/APP. An optimal composition, EG:APP = 3:1, is suggested. The SE was calculated and plotted in Figure 4b. As all EG/APP combinations had an SE above 1, synergism was proven for these materials. In both series, the materials containing 30 wt%and 50 wt%, very strong synergism of up to SE(OI) = 1.9 and 2.8 was established for 0.75EG30 and 0.75EG-50, respectively.

ܵ‫= )ܫܱ(ܧ‬

∆ܱ‫்ܫ‬௉ாିௌା௫ாீା௬஺௉௉ ‫ݔ‬ ‫ݕ‬ ቀ‫ ݔ‬+ ‫ݕ‬ቁ ൫∆ܱ‫்ܫ‬௉ாିௌା(௫ା௬)ாீ ൯ + ቀ‫ ݔ‬+ ‫ݕ‬ቁ ൫∆ܱ‫்ܫ‬௉ாିௌା(௫ା௬)஺௉௉ ൯

(1)



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Figure 4. a) OI results for the TPE-S/EG/APP materials containing 30 wt% and 50 wt%, b) corresponding synergy indices.

3.3. Fire Behavior of TPE-S/EG/APP Combinations under Forced Flaming Conditions. The results obtained in the cone calorimeter investigation of TPE-S/APP/EG materials are illustrated in Figure 5. The change in the HRR pattern was most obvious, and accompanied by a dramatic decrease in the PHRR by up to a factor of 10. The fire behavior of TPE-S, which resembles a pool fire with an extremely high PHRR at the end of burning, was replaced by the pattern of a residue forming material.33 The PHRR moved to the beginning of burning, when an efficient protection layer was formed, particularly when the EG/APP mixtures were added to TPE-S (Figure 5a). When APP was added to TPE-S, 0.00EG-30 did not induce any 14 ACS Paragon Plus Environment

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charring; the reduction in PHRR was large compared to TPE-S, but disappointing in absolute terms. The HRR pattern of 1.00EG-30 showed a reduction of ca. 90% to a small PHRR close to 200 kW/m2 at the beginning of burning, but a second PHHR twice as high as the loose structure of the largely expanded residue robbed it of some of its protective properties. 0.25EG-30, 0.33EG-30, 0.50EG-30, 0.66EG-30, and 0.75EG-30 showed better performance than 1.00EG-30. The HRR curves of 0.25EG-30 and 0.33EG-30 reveal a considerable step after 300 s (Figure 5b). This might be due to delamination during the heating process. The flattest HRR curve, which indicates the lowest PHRR (258 kW·m-2), was measured for 0.75EG-30, which is consistent with the OI results. As proposed above in the discussion of the microscopic appearance of the fire residue, the combination of EG/APP in TPE-S exploited the thermal insulation of expanded graphite thanks to an improvement in the fire stability of the fire residue due to polyphosphate. Figure 5b and Figure 5c show the HRR curves for all of the TPE-S/EG/APP mixtures. In Table 5, the THE, the maximum averaged rate of heat emission (MARHE), and fire residue are summarized. 0.00EG-30 and 1.00EG-30 reduced the fire load of TPE-S, most probably due merely to the replacement of 15 vol% TPE-S in the test specimen. The reduction in THE was somewhat larger when EG/APP was added, indicating that the thermal insulation and stability of the fire residue is good enough to prevent the materials from burning completely. The residue increased accordingly. 0.75EG– 30 showed the highest residue (26.8 wt%). Comparing to the unprotected material, the THE was reduced by up to 22%, when EG/APP mixtures were added. The results for the TPES/EG/APP mixtures with 50 w% EG/APP supported this interpretation, although the quality of the data was diminished by artifacts such as touching the heating cone and the loss of material. Actually, the samples containing 50% of FR show additional peaks at roughly 200 s, which arose due to contact with the heating element of the cone calorimeter, and thus must not be evaluated in all details. Using the MARHE as the index for fire growth or flame spread, respectively, 0.75EG–30 and 0.75EG–50 performed the best in accordance with the results of the OI measurement.



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Figure 5. Comparison of a) the HRR of TPE-S, 0.00EG-30, 1.00EG-30 and 0.75EG-30, b) HRR of EG/APP samples with 30% and c) 50% flame retardant content (0.00EG samples are left out for the sake of clarity), d) Petrella plot and Petrella-like plot using the MARHE instead of PHRR/tig of samples containing 30 wt% EG/APP.

Table 5. Results from cone calorimeter measurement of EG/APP combinations Material

THE

MARHE

Residue

/ MJ·m-2

/ kW·m-2

/ wt%

+/- 0.6-1.5 +/- 2.5-6.5 +/- 0.2-1.5 TPE-S

203.8

903.1

0

0.00EG-30

173.2

599.8

22.4

0.25EG-30

162.2

312.9

24.6

0.33EG-30

162.7

310.7

24.6

0.50EG-30

159.5

240.5

26.6

0.66EG-30

168.2

233.2

26.6

0.75EG-30

163.9

215.3

26.8

1.00EG-30

174.8

260.5

16.8

0.00EG-50

147.0

460

39.7 16

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0.25EG-50

141.1

227.1

42.0

0.33EG-50

138.8

222.2

41.5

0.50EG-50

123.5*

221.3*

36.7*

0.66EG-50

106.4*

195.4*

42.0*

0.75EG-50

112.5*

172.3*

50.1*

1.00EG-50

134.6*

285.8*

16.2*

* Values are not completely reliable due to contact with the heating element

A possibility for assessing the fire behavior graphically, the Petrella plot, and thus the flame retardancy effect of a material, was suggested in 1994.36 The Petrella plot addresses the fire load (total heat evolved) and PHRR/tig as an index for fire growth, and is shown in Figure 5d together with a Petrella-like plot using MARHE as the index for fire growth instead of PHRR/tig. Excellent flame retardancy is indicated by the lower left corner, that is, low MARHE, low PHRR/tig and low THE. In the Petrella plot depicted in Figure 5d it is evident how much better the combination of EG/APP is compared to the samples containing only APP or EG, respectively. Besides the 0.75EG sample, the sample 0.50EG is also located in the lower left corner and is therefore a promising candidate for good flame retardancy performance in the cone calorimeter fire scenario. All of the improvements in MARHE, PHRR, THE, and residue were not only better than can be expected for simple rule of mixture, but significantly better than the performance of 1.00EG-30 and 0.00EG-30, respectively. The EG/APP functions synergistically in forcedflaming fire behavior. The SEs were calculated and plotted in Figure 6. Equation 1 is used again, replacing ∆OI with ∆MARHE, ∆PHRR, ∆THE, and ∆amount of residue, respectively. All synergy indices of the combinations were above 1, up to 1.5, which indicates significant synergy.



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Figure 6. Synergy indices for MARHE, PHRR, THE, and residue.

3.4.

TPE-S/EG/APP Combinations: Temperature Measurements during Burning.

Temperature measurements at the back of the specimen delivered detailed insight into the insulating mode of action of the intumescent layer during the cone calorimeter test. In contrast to the cone calorimeter investigation of the fire behavior, the temperature measurements were performed using a retainer frame to fix the thermocouple on the backside of the sample. In Figure 7a the HRR results during temperature measurement are compared to the HRR results obtained without using the frame. Except for the initial peak, of course, measuring with or without retainer frame has a significant influence on the HRR.37-39 In particular, the height of the second peak HRR, the time to the second peak HRR, and the burning time differ from each other. Nevertheless, the values correspond to each other; the HRR curve pattern is the same. Thus the interpretation as well as the deduced conclusions are analogous.


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Figure 7. a) Comparison of HRR without and with the retainer frame setup, b) temperature curves of TPE-S and TPE-S containing EG/APP, c) plot of the temperature curve and the corresponding HRR curve of 0.75EG-30.

Figure 7b compares the temperature curves and HRR curves of 0.25EG-30 and 0.75EG-30. Four stages are concluded. After the initial peak in HRR (stage I), the HRR drops 19 ACS Paragon Plus Environment


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to low HRR values (stage II). The initial peak in HRR is due to the burning out of combustible material in the top layer, and during the first two stages a protective char layer is formed. This fire residual layer protects the material underneath due to increasing thermal insulation, as indicated by the flattened temperature curve between 100 and 250 s during stage II. Afterwards, the temperature rises with a steeper increase before levelling off to a moderate temperature increase (stage III). The HRR show a broad maximum (Figure 7b). Expansion of the graphite flakes was observed during the first three stages. After the maximum, the HRR decrease and the specimens extinguish, but not abruptly (stage IV). The HRR may show a shoulder in the declining HRR curve. Decomposition of the fire residue occurs, accompanied by a pronounced loss of insulation properties in the case of 0.25EG-30. Another step occurs in the temperature curve (Figure 7b). Figure 7c compares the temperature curves of TPE-S, 0.00EG-30, 0.25EG-30, 0.33EG-30, 0.50EG-30, 0.66EG-30, and 0.75EG-30. In TPE-S and 0.00EG-30 no protective fire residue was formed; their temperature curve pattern differs from the other materials containing EG/APP, and high temperatures are reached after a short time. Despite the dramatic change in EG content, the temperature curves on the back of the specimen for 0.25EG-30, 0.33EG-30, 0.50EG-30, 0.66EG-30, and 0.75EG-30 were quite similar in stages I, II, and III. The difference in thermal properties such as conductivity, density and heat capacity are concluded to be of minor importance, and the intumescence based on the expansion of different amounts of EG seems to play a minor role as well. As the amount of graphite increases, only the temperature curves in stage IV show a clear difference. The increase in temperature, due to deterioration of the insulating properties towards the end of testing, shifts in terms of both time and the size of the effect. The greater the EG content, the later deterioration occurs, and the smaller the effect. The best insulation properties, and thus the lowest temperature at the end of the test, were measured for 0.75EG-30; only 0.66EG-30 showed similarly strong performance. The results for the temperature curves, and thus the thermal insulation, match perfectly with the reduction in the second peak HRR, MARHE and the synergy index for the residue (Table 5 and Figure 6). Therefore, the


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temperature measurements underline the proposed explanation that the stabilization of fire residue is the synergistic mechanism when EG and APP are combined.

3.5.

TPE-S/EG/APP Combined with MPP and Melamine Coated APPm. Ultimately,

exploiting the synergy between EG/APP in TPE-S was a large step in the right direction but not sufficient to achieve V-0 classification at 30 wt% load. Only 0.50EG-50, 0.66EG-50, and 0.75EG-50 have shown self-extinction in the vertical UL 94 test so far. Thus, the effect of adding different adjuvants to replace EG/APP in 30 wt% mixtures was investigated and is described in the following sections. Some of the best established adjuvants in intumescent systems are based on melamine. Two melamine-based approaches, adding MPP and using melamine-coated APPm, were addressed and compared. Different amounts of MPP were investigated to replace 2.5 wt%, 5 wt%, and 7.5 wt% of EG/APP in TPE-S. Thus the overall amount of flame retardant used was still 30 wt.-%. The HRR curves of these materials had roughly the same shape as 0.75EG-30 (Figure 8a). After the initial sharp peak within the first 100 s, a main broad peak appeared in HRR between 100 and 500 s. Above 500s a third process occurred, which appears as a shoulder in 0.75EG-30, or as a third peak in EG/APP-2.5M, EG/APP-5.0M, and EG/APP-7.5M. The initial peak was caused by the pyrolysis of the top layer. The HRR dropped when the protective layer on the surface was formed, followed by the development of a thick intumescent layer. It protects the sample for a while (decrease of HRR at 100s). Due to the huge volume expansion, the sample was moved vertically in the aluminum pan, exposing the polymeric material underneath on the free sample edges. The HRR increased until the polymeric material from the sides of the sample was burned (at 220 s) and an efficient thick protection layer was formed. As a result of the complete formation of an insulation layer and the consumption of fuel, the HRR finally decreased. This burning behavior (i.e. the HRR curve shape) was not influenced by MPP. However, when MPP was added the PHRR at 220 s increased considerably. In terms of THE and residue, the best values were achieved by the sample containing 2.5 wt% EG/APP-2.5M (Table 6). Although this sample had the highest PHRR, it had the lowest initial peak HRR. 21 ACS Paragon Plus Environment


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Figure 8. a) HRR of TPE-S/EG/APP and MPP to show the effect of replacing EG/APP with different concentrations of MPP, b) comparing the HRR of EG/APPm and 0.75EG-30 to show the influence of coating APP with melamine; the overall flame retardant content was kept at 30 wt%.

MPP introduced an additional charring action, indicated by the increase in residue by 4.8 wt% to 31.6 wt%. At high MPP concentration (7.5 wt%), the load of the main flame retardant combination EG/APP became too low (22.5 wt%) for effective protection, and the positive influence of MPP vanished, leading to high THE and MARHE (163 MJ m-2 and 217 kW m-2). The results in OI and UL 94 are analogous to the cone calorimeter results. Compared to EG/APP, the OI of the EG/APP-2.5M sample increased from 24 to 26.3 vol% (Table 6). At higher MPP concentration the OI was the same as without MPP. The UL 94 classification (HB) was unaffected by additional MPP. Thus in sum, replacing some of 22 ACS Paragon Plus Environment

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EG/APP mixture with MPP was not convincing with respect to overall performance. Nevertheless, applying small amounts effected a crucial improvement in one of the key features of the burning behavior of intumescent systems: the reduction in initial peak HRR.

Table 6. OI and UL 94 tests results of 0.75EG-30, EG/APP/MPP, EG/APPm, and EG/APPm/adjuvant combinations Material

OI

UL 94

/ vol%

THE / MJ·m

+/- 0.3-0.7

MARHE -2

-2

/ kW·m

+/- 0.6-1.5 +/- 2.5-6.5

Residue / wt% +/- 0.2-1.5

0.75EG-30

24.0

HB

163.9

215.3

26.8

EG/APP-2.5M

26.3

HB

153.7

209.4

31.6

EG/APP-5.0M

24.5

HB

160.8

209

28.8

EG/APP-7.5M

24.2

HB

162.5

217.3

24.5

EG/APPm

26.4

HB

162.5

195.7

30.3

EG/APPm-A

26.0

V-0

170

199.3

23.8

EG/APPm-ZB

25.1

HB

166.3

199.6

26.8

EG/APPm-ZB-M

22.8

HB

163.5

243.2

24.6

EG/APPm-D

24.4

HB

165.1

217.6

24.5

EG/APPm-B

23.7

HB

172.7

275.1

26.1

EG/APPm-S

23.7

HB

169.8

265.4

28.3

EG/APPm-D-B

22.7

HB

172.4

275.1

20.9

Since it is not based on adding another additive, a very elegant approach to adding small amounts of melamine is to replace APP with melamine-coated APPm. Coated APP is usually proposed to improve resistance against hydrolysis and stability during processing,2,40 but is also reported to harbor the potential to improve flame retardancy.41-43 As shown in Figure 8b, except for the reduction in the height of the initial peak HRR, there is no relevant difference in the curve shapes. The PHRR increased only slightly and the fire residue seems to be improved. Using APPm yielded a reduction in HRR after the main maximum and a somewhat prolonged burning time. Therefore EG/APPm outperforms EG/APP-2.5M and 0.75EG-30. The charring and the formation of the protection layer was more effective than for 23 ACS Paragon Plus Environment


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Page 24 of 40

the EG/APP combinations. Furthermore, the sample containing melamine-coated APPm exhibited slight improvements regarding OI, MARHE, and residue (Table 6).

3.6.

TPE-S/EG/APPm + Adjuvants. Several possible adjuvants to replace some of the

EG/APPm were checked in order to optimize the fire properties without increasing the overall amount of additive. Thus in all of these systems the amount of EG/APPm was reduced by 2.5, 5, 6, and 7.5 wt% (Table 3). Their fire behavior is depicted in Table 6 and Figure 9.

Figure 9. HRR of TPE-S/EG/APPm + adjuvant systems; overall flame retardant content was 30 wt%.

Regarding MARHE, the combination of EG/APPm with AlPi and ZB, respectively, achieved good results below 200 kW·m-2 (Table 6). The highest fire residues were achieved by the samples with rather inert fillers, e.g. SiO2. As to PHRR, the best result was achieved by 24 ACS Paragon Plus Environment

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EG/APPm-A, which showed a reduction in PHRR of 21 kW·m-2 to 245 kW·m-2 (Figure 9). What is more, this material, which contained the second phosphorous flame retardant AlPi, improved the UL 94 classification from HB to V-0 (Table 6). Compared to EG/APPm, the OI of EG/APPm-A remained almost constant at 26.0 vol%, whereas the other materials showed a lower OI when EG/APP was replaced. The improvement is most likely due to the flame inhibition effect caused by diethyl phosphinic acid molecules released by AlPi. The UL 94 classification and the PHRR were particularly improved, whereas other fire properties were similar to or even slightly worse than EG/APPm. Due to discontinuities and inhomogeneities in the materials and the burning, the growth of the chars was partially uneven, leading to unbalanced expansion (Figure 10). With the additives discussed in this paragraph the residues were fluffy and partially decompose, due to the huge gain in volume. The EG/APPm-D-B sample had low expandable graphite content, so that only slight expansion occurred (Figure 10f). Except for EG/APPm-D-B, the residues had almost the same appearance after the cone calorimeter test. There is no way to correlate the difference in performance with any clear difference in fire residue appearance. Figure 11 compares the temperature on the back surface for EG/APPm-A to some of the temperatures measured for the TPE-S/EG/APP systems. The temperature curve EG/APPm-A fits extremely well in the series of curves, showing the same characteristics and somewhat better insulation than 0.75EG-30.



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Figure 10. Fire residues of a) EG/APPm-A, b) EG/APPm-ZB, c) EG/APPm-D, d) EG/APPmB e) EG/APPm-S, and f) EG/APPm-D-B.

Figure 11. Temperature on the back of the specimen during the cone calorimeter test for EG/APPm-A compared to the results for the TPE-S/EG/APP materials.

In terms of the flammability test and the cone calorimeter measurements, the EG/APPm-A provided the best fire performance so far. However, apart from the important V26 ACS Paragon Plus Environment

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0 classification, the differences between EG/APPm-A and EG/APPm are negligible, indicating that the system may be sensitive to small modifications. Therefore the influence of the overall amount of EG/APPm-A on the fire performance was analyzed. It is known that low concentrations of intumescent flame retardant behave similar to a charring agent; their efficiency is limited until a concentration is used for which the thermal insulation of the fire residue is strong enough to protect the underlying material from reaching the pyrolysis temperature.16,44 Then the specimens extinguish before complete pyrolysis occurs. In this range of concentration, where the main flame retardancy mode of action switches from charring plus some protective layer, to an efficiently thermally insulating protective layer plus incomplete pyrolysis, small changes in the flame retardant concentration resulted in a great increase in flame retardancy efficiency. This strongly nonlinear behavior is also indicated when comparing the OI for TPE-S, 0.75EG-30, and 0.75EG-50 (Table 4), best explained by such a strong increase in efficiency starting somewhere between 40 wt% and 50 wt%. Figure 12 illustrates the nonlinearity of the data presented in Table 7 for the UL 94 rating, OI, fire residue after cone calorimetry measurement and the THE of EG/APPm-A samples with 27.5 wt%, 30 wt%, 35 wt%, and 40 wt% flame retardant content. At the lowest loading of 27.5 wt% the sample achieved an UL 94 rating of HB. Increasing the amount of flame retardant led to an improvement in the UL 94 rating to V-0. Furthermore, with increasing amounts of flame retardants the OI and residue increased, whereas the THE decreased. Nevertheless, the strong nonlinear increase in flame retardancy efficiency started at 35 wt%, but not below 30 wt% flame retardant. Even though V-0 was achieved, the system clearly demands further improvement to exploit a more incomplete pyrolysis. A series of four component systems was investigated, EG/APPm-A-MC, EG/APPm-A-T, EG/APPm-A-M, and EG/APPm-A-DP-MC, but all of these materials showed deteriorated fire performance, probably because the EG/APPm content was no longer large enough to function as the foundation of efficient intumescent flame retardancy. Thus EG/APPm-A showed a promising route toward V-0 TPE-S products, but identifying multicomponent systems for which the



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Page 28 of 40

threshold flame retardant concentration for incomplete pyrolysis is below 30 wt% remains an open task for the present and future.

Figure 12. UL 94, THE, OI and residue of EG/APPm-AlPi as related to the overall flame retardant content.

Table 7. OI and UL 94 tests results of 0.75EG-30, 0.75EG-50, EG/APPm-A, and EG/APPm-A-mineral filler combinations Material

OI

UL 94

THE

MARHE

Residue

/ vol%

/ MJ·m-2

/ kW·m-2

/ wt%

+/- 0.3-0.7

+/- 0.6-1.5

+/- 2.5-6.5

+/- 0.2-1.5

0.75EG-30

24.0

HB

163.9

215.3

26.8

EG/APPm

26.4

HB

162.5

195.7

30.3

0.75EG-50

39.0

V-0

112.5

172.3

50.1

EG/APPm-A 27.5 wt%

24.4

HB

171.5

223.6

20.9

EG/APPm-A

26.0

V-0

170

199.3

23.8

EG/APPm-A 35 wt%

26.7

V-0

168.1

214

26.8

EG/APPm-A 40 wt%

29.9

V-0

151.3

178.2

44.1 28


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EG/APPm-A-S20

30.6

V-0

130.1

203

49.2

EG/APPm-A-Ta20

33.0

V-0

142.7

196.4

55.4

EG/APPm-A-C20

29.1

HB

144.9

175.4

39.6

Instead of searching for a better adjuvant than adding a second phosphorous flame retardant such as AlPi, of course, one possibility is to reduce the fire load in general by changing from a TPE-S to a TPE-S/inert filler system. Thus combinations with additional 20 wt% mineral fillers were checked, for which 20 wt% of TPE-s was replaced and the overall filling percentage was 50 wt%. SiO2, talcum and chalk were used. The residue increased by 16 to 28 wt%, the THE decreased by 15% to 23%. Reasons for the deviation from 20% are that wt% is unequal to vol%, influencing the cone calorimeter results, and that the inert fillers are not perfectly inert with respect to the complex pyrolysis of these multicomponent systems. Talcum and SiO2 behaved quite similarly, whereas chalk showed the somewhat lower residue and lower reduction in THE. The OI increased to over 30 vol% and a V-0 classification was achieved using SiO2 and talcum. Despite the improvements in OI and fire load, the impact on the HRR curve is limited (Figure 13). Adding mineral filler decreases the initial peak HRR, but deteriorates the following PHRR. The reduction in THE does not reduce the HRR at all stages of burning, but results mainly from a reduction in HRR towards the end of burning. Therefore, the reduction in MARHE is limited, if it occurs at all.

Figure 13. HRR of EG/APPm-A samples with 20 wt% non-combustible fillers. Overall FR content: 50 wt%. 29 ACS Paragon Plus Environment


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3.7.

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Mechanical Properties. The basic material showed typical mechanical properties for

an unfilled TPE-S in this hardness range (63 Shore A). The mechanical properties of all compounds are listed in Table 8. The general trend for the flame retarded samples is a decrease in the tensile strength and elongation at break, and an increase in the hardness. The flame retardant concentration (i.e. 30 wt% or 50 wt%) has a strong impact on the hardness. At 50 wt% EG/APP content, the maximum hardness value was 83 Shore A, and it did not fall below 76 Shore A. As the tensile strength and elongation at break strongly decreased as well, it is beneficial for potential applications to keep the maximum content of flame retardants at 30 wt%. Further additives have no relevant influence on the mechanical properties. In the case of the addition of boehmite, boehmite/DPO, SiO2 and ZB/MPP, the hardness fell below 66 Shore A, probably because of the overall reduction of EG in the mixture. As expected, the hardness increased to 80 – 83 Shore A when additional 20 wt% inert fillers were added. As the main focus in this study is the flame retardancy performance, no compatibilizers were added or formulations changed to compensate for the rise in hardness.

Table 8. Mechanical properties of the compounds

σ

ε

Hardness

MPa

%

Shore A

TPE-S

6.6 +/-0.3

677 +/-13

63.4 +/-0.5

0.00EG-30

6.2 +/-0.3

695 +/-15

70.6 +/-0.5

0.25EG-30

3.4 +/-0.4

564 +/-52

71.8 +/-0.4

0.33EG-30

4.1 +/-0.3

612 +/-50

67.2 +/-0.9

0.50EG-30

3.7 +/-0.2

571 +/-34

69.5 +/-0.6

0.66EG-30

3.9 +/-0.3

553 +/-31

72.7 +/-0.9

0.75EG-30

3.6 +/-0.2

542 +/-29

76.3 +/-0.4

1.00EG-30

2.9 +/-0.1

403 +/-28

76.8 +/-0.4

0.00EG-50

3.3 +/-0.2

539 +/-44

74.6 +/-0.6

0.25EG-50

1.8 +/-0.2

330 +/-39

77.8 +/-0.4

0.33EG-50

1.8 +/-0.1

326 +/-49

76.3 +/-0.3

Material


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0.50EG-50

2.1 +/-0.1

179 +/-30

80.8 +/-0.2

0.66EG-50

1.6 +/-0.1

100 +/-13

81.5 +/-0.8

0.75EG-50

1.6 +/-0.2

75 +/-16

80.7 +/-0.4

1.00EG-50

2.0 +/-0.6

49 +/-7

82.8 +/-0.4

EG/APP-2.5M

2.8 +/-0.1

405 +/-41

73.2 +/-0.5

EG/APP-5.0M

3.3 +/-0.2

497 +/-33

70.6 +/-0.6

EG/APP-7.5M

3.6 +/-0.2

515 +/-30

72.1 +/-0.5

EG/APPm

2.8 +/-0.1

328 +/-25

72.7 +/-0.9

EG/APPm–A

3.0 +/-0.2

430 +/-22

72.8 +/-1.5

EG/APPm–ZB

2.8 +/-0.1

364 +/-13

71.9 +/-1.6

EG/APPm–ZB–M

n.m.

n.m.

61.6 +/-0.5

EG/APPm–D

2.9 +/-0.2

474 +/-37

69.0 +/-0.8

EG/APPm–B

3.0 +/-0.3

488 +/-51

64.5 +/-0.6

EG/APPm–S

3.1 +/-0.2

471 +/-40

65.2 +/-1.0

EG/APPm–D–B

3.1 +/-0.2

515 +/-71

65.8 +/-1.8

EG/APPm-A-40 wt%

2.4 +/-0.1

170 +/-33

78.8 +/-1.9

EG/APPm-A-35 wt%

2.8 +/-0.1

241 +/-23

76.6 +/-0.1

EG/APPm-A-27.5 wt%

3.3 +/-0.2

288 +/-35

74.3 +/-0.6

EG/APPm–A–MC

3.5 +/-0.3

516 +/-39

71.3 +/-1.2

EG/APPm–A–T

3.5 +/-0.3

502 +/-53

73.7 +/-1.1

EG/APPm–A–M

3.7 +/-0.3

490 +/-58

75.7 +/-0.7

EG/APPm–A–DP–MC

4.8 +/-0.3

585 +/-26

75.3 +/-0.7

EG/APPm-A-S20

2.6 +/-0.2

164 +/-12

79.5 +/-1.5

EG/APPm-A-Ta20

2.5 +/-0.1

89 +/-18

83.2 +/-1.4

EG/APPm-A-C20

2.1 +/-0.1

123 +/-24

80.5 +/-0.8

4. CONCLUSION

In this study the flame retardancy of TPE-S using a combination of EG and APP was examined by means of cone calorimetry, OI and UL 94 test and by analyzing the morphology of the residues. A comprehensive set of multicomponent materials was investigated to assess the potential and limits of EG/APP combinations in terms of flame retardancy as well as to 31 ACS Paragon Plus Environment


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Page 32 of 40

depict ways for optimization. The influence on performance of the ratio of EG to APP as well as the flame retardant concentration was studied. Strong synergism was found for EGP and APP, with the best ratio at EG/APP = 3:1 showing the highest synergy index. A high OI of 39 vol% and V-0 rating in UL 94 was achieved for the EG/APP combinations with 50 wt% of the flame retardants. At this concentration the volume expansion was huge, even leading to contact between the sample and the cone heater during cone calorimetry measurements. At 30 wt% 0.75EG-30 has an OI of 24 vol% and a MARHE of 215 kW m-2, as well as a HB rating in UL 94. The influence of the EG content was apparent in the photographs of the residue and the corresponding SEM images, as well as the stabilizing synergistic interaction when combined with APP. Temperature measurements were performed to analyze the heat development on the back surface of the samples during the cone calorimetry test. The results confirm the increasing thermal insulation with increasing EG content. Using melamine-coated APP improved the performance of the material with a 3:1 EG/APP ratio. Furthermore, several additives were tested as adjuvants to the EG/APPm mixture, maintaining the 30 wt% overall flame retardant content. Due to the additional flame inhibition, the combination of EG/APP with AlPi improved the flame retardancy performance; the UL 94 rating was improved to V-0. The addition of noncombustible fillers revealed promising results for fire load reduction. Using chalk reduced the MARHE by 12 % to 175 kW m-2. The best OI of 33 vol% and a V-0 rating were achieved by the addition of talcum. Adding 30 wt.-% and 50 wt.-% flame retardant significantly reinforces the TPE-S, e.g. the Shore A hardness increase by 4 up to 20. The largest effect was observed for layered fillers, e.g. EG, talcum and chalk. The results point out the need for limiting the flame retardancy amount and the demand for balancing the mechanical properties via adjusting the TPE-S composition. To sum up, cleverly composed halogen-free multicomponent systems, like EG/APPm plus AlPi, are suitable combinations to effectively flame retard TPE-S while keeping the range of mechanical properties needed for potential applications. 32 ACS Paragon Plus Environment

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ACKNOWLEDGMENTS

The IGF Project (438 ZN) of the Research Association (Fördergemeinschaft für das Süddeutsche Kunststoff-Zentrum e.V.) was supported by the AiF within the framework of the program “Förderung der Industriellen Gemeinschaftsforschung (IGF)” of the German Federal Ministry for Economic Affairs and Energy based on a decision of the Deutschen Bundestag. Special thanks go to H. Bahr, M. Morys and P. Klack for their support in performing the experiments.

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Legends of the Figures

Figure 1. Temperature measurement setup and attachment of the thermocouple onto the back of the specimen.

Figure 2. Fire residues after the cone calorimeter test of a) 0.00EG-30, b) 0.25EG-30, c) 0.50EG-30, d) 0.75EG-30, and e) 1.00EG-30.

Figure 3. SEM images (two different resolutions) of a) 0.00EG-30, b) 0.25EG-30, c) 0.75EG30, and d) 1.00EG-30.

Figure 4. a) OI results for the TPE-S/EG/APP materials containing 30 wt% and 50 wt%, b) corresponding synergy indices.

Figure 5. Comparison of a) the HRR of TPE-S, 0.00EG-30, 1.00EG-30 and 0.75EG-30, b) HRR of EG/APP samples with 30% and c) 50% flame retardant content (0.00EG samples are left out for the sake of clarity), d) Petrella plot and Petrella-like plot using the MARHE instead of PHRR/tig of samples containing 30 wt% EG/APP.

Figure 6. Synergy indices for MARHE, PHRR, THE, and residue.

Figure 7. a) Comparison of HRR without and with the retainer frame setup, b) temperature curves of TPE-S and TPE-S containing EG/APP, c) plot of the temperature curve and the corresponding HRR curve of 0.75EG-30.


Page 39 of 40

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Figure 8. a) HRR of TPE-S/EG/APP and MPP to show the effect of replacing EG/APP with different concentrations of MPP, b) comparing the HRR of EG/APPm and 0.75EG-30 to show the influence of coating APP with melamine; the overall flame retardant content was kept at 30 wt%.

Figure 9. HRR of TPE-S/EG/APPm + adjuvant systems; overall flame retardant content was 30 wt%.

Figure 10. Fire residues of a) EG/APPm-A, b) EG/APPm-ZB, c) EG/APPm-D, d) EG/APPmB e) EG/APPm-S, and f) EG/APPm-D-B.

Figure 11. Temperature on the back of the specimen during the cone calorimeter test for EG/APPm-A compared to the results for the TPE-S/EG/APP materials.

Figure 12. UL 94, THE, OI and residue of EG/APPm-AlPi as related to the overall flame retardant content.

Figure 13. HRR of EG/APPm-A samples with 20 wt% non-combustible fillers. Overall FR content: 50 wt%.



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Halogen-Free Multicomponent Flame Retardant Thermoplastic

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