Dripping and decomposition under fire: Melamine cyanurate vs. glass fibres in polyamide 6

Journal Pre-proof Dripping and decomposition under fire: Melamine cyanurate vs. glass fibres in polyamide 6 Analice Turski Silva Diniz, Christian Huth, Bernhard Schartel PII:

S0141-3910(19)30376-3

DOI:

https://doi.org/10.1016/j.polymdegradstab.2019.109048

Reference:

PDST 109048

To appear in:

Polymer Degradation and Stability

Received Date: 7 August 2019 Revised Date:

6 December 2019

Accepted Date: 7 December 2019

Please cite this article as: Turski Silva Diniz A, Huth C, Schartel B, Dripping and decomposition under fire: Melamine cyanurate vs. glass fibres in polyamide 6, Polymer Degradation and Stability (2020), doi: https://doi.org/10.1016/j.polymdegradstab.2019.109048. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Dripping and decomposition under fire: Melamine cyanurate vs. glass fibres in polyamide 6 Analice Turski Silva Diniz, Christian Huth, Bernhard Schartel* Bundesanstalt für Materialforschung und –prüfung (BAM) Unter den Eichen 87, 12205 Berlin, Germany, [email protected]

* corresponding author

ABSTRACT Manipulating the melt dripping of thermoplastics makes a fire scenario more or less dangerous. Yet, a detailed understanding of this phenomenon has remained a question mark in studies of the flammability of plastics. In this work, the individual and collective impacts of additives on the dripping behaviour of polyamide 6 (PA6) were studied. A set of materials compounded with melamine cyanurate (MCA) and glass fibre (GF) was investigated. Under UL 94 vertical test conditions, the dripping during first and second ignition was quantified and investigated in detail. The number, size and temperature of the drops were addressed, and the materials and their drops evaluated with respect to such aspects as their averaged molecular weight, thermal decomposition and rheological properties. PA6 with V-2 classification improved to V-0 with the addition of MCA, and achieved HB in the presence of GF. PA6/GF/MCA achieved V-2. Non-flaming drops of PA6/MCA consisted of oligomeric fragments. Flaming drops of PA6/GF showed a more pronounced decomposition of PA6 and an increased GF content. The dripping behaviour of PA6/GF/MCA can be understood as a combination of the influence of both additives. The results showed nicely that dripping under fire is neither a straightforward material property nor a simple additive influence, but the complex response of the material influenced by the interaction and competition of different phenomena.

Keywords: Dripping; UL 94; Polyamide 6; Melamine cyanurate; Glass fibre; Flame retardant 1

1.

Introduction

Under fire, some polymers exhibit considerable physicochemical deformation due to melt flow: the melt dripping effect. Its importance lies in its capacity to change the fire properties of the specimen, such as its flammability, and to change the fire scenario through relocation of a crucial amount of fuel. An advantage of this behaviour is that melt dripping can remove fuel and heat from the pyrolysis zone, contributing to fire extinguishment. The disadvantage is that burning dripping and melt flow can lead to additional sources of ignition or form pool fires [1]. Understanding the balance between these antagonistic aspects opens up a crucial and promising approach to achieving fire retardancy in polymers. A small-scale flammability test that observes and accounts for the occurrence of melt dripping is the UL 94 vertical burning test [2]. A single test consists of igniting a standard specimen bar by a defined pilot flame for 10 seconds. After removing the burner, the burning time is recorded and dripping is registered. Roughly simplified, if burning stops within 30 seconds, the aspect that determines the material’s rating as V-2 or V-1 is the dripping: flaming drops lead to V-2, and no dripping or non-flaming drops lead to V-1. The highest classification of V-0 means extinction within 10 seconds and allows only no dripping or non-flaming drops. A second ignition is applied only when the sample extinguishes before the criteria limits are achieved. The possibility of improving the UL 94 classification by working only on the influence of dripping has increased the interest in such behaviour, such that this test has become a useful tool to detect and evaluate dripping. Melt flow and dripping are distinct for each polymer. Hence, the first scientific question posed was: which factors govern dripping behaviour? Some works on this aspect have been published [3,4]. Temperature distribution has been identified to deliver a better understanding of UL 94 classification, when dripping occurs [3]. Obviously, critical temperatures like Tg (glass transition temperature), Tm (melting temperature) and Td (decomposition temperature) were found to be crucial to controlling dripping [5-7]. The way a material softens – i.e. in bulk or on the surface – was revealed to be dependent on the typical decomposition mechanism of the polymer. Thus, this mechanism (e.g., depolymerization and end-chain or random-chain scission) determines the kind of drops generated (large and irregular, or small and uniform, respectively) [6,7]. For materials that 2

produce large drops, in particular, the dripping time and the mass of the drops generated were found to be proportional to specimen thickness [6]. Studies on the decomposition of drops by means of thermogravimetric and rheological measurements demonstrated that they are formed of partially decomposed polymer [8-11]. More elaborated dripping quantification methods revealed the dripping yields of different polymers exposed to fire or heat [9-11]. The size (diameter), mass, viscosity and temperature of drops were assessed, as well as the dripping time and rate, among others. Drop and specimen temperature evolution during the UL 94 test have been illuminated the in detail [12]. Relevant differences between polymer drops and flame-retarded polymer drops were found [11, 12]. Likewise, reinforced materials [7,13] and blends [8,14] showed modified dripping behaviour when compared to the polymers. Competition and interaction between different phenomena occur during burning, controlling the dripping behaviour, i.e. pyrolysis and gasification, melt flow and dripping, as well as flame-retardant modes of action such as fuel dilution, charring, and flame inhibition. According to these results, the main characteristics influencing dripping under fire were highlighted as temperature, decomposition and viscosity. So how can we control the phenomenon of dripping? Many pieces of this puzzle have yet to be put together. In our previous work [14], a selected group of materials based on polycarbonate/acrylonitrile-butadiene-styrene copolymer (PC/ABS) blend, including a flame retardant working mainly as a flame inhibitor in the gas phase and an ‘anti-dripping’ agent, was investigated in the UL 94 vertical test. This approach enabled a better understanding of the impact of the various components on dripping in this system. Here, an experimental investigation of typical polyamide 6 (PA6)-based compositions is presented. As an engineering thermoplastic, PA6 is used in a wide range of applications such as electrical and computer parts [15]. For this, improvement in its flame retardancy is crucial. Melamine cyanurate (MCA) is a flame retardant active in the gas and condensed phases, producing mainly fuel dilution. This nitrogeneous substance (around 67 wt.-% of N) is halogen-free and its efficiency in PA6 has been established [16,17]. Usually, it is referred as a ‘dripping promoter’ in PA6. On the other hand, glass fibre (GF) is a reinforcement filler used extensively in polyamides to improve their mechanical properties, and is commonly reported to show ‘anti-dripping’ effects. The aim of this work was to investigate the action of these two additives on the dripping behaviour of PA6. Dripping was quantified under UL 94 vertical burning 3

test conditions and distinguished for the two ignitions. The time to dripping, mass, temperature, morphology, decomposition, and rheology of the drops were evaluated. It is the aim of this paper to contribute to our understanding of how dripping under fire can be tailored to commercial needs.

2.

Experimental

2.1.

Materials and Preparation

Two commercial grades of PA6 were used, one with and the other without GF (Technyl® C216 V30 and Technyl® S27 BL, respectively, from Solvay). The flame retardant was MCA (Melapur MC15, BASF), with an average particle size of 1.1–1.4 µm and a density of 1.7 g/cm³. These components were compounded and generously supplied by École Nationale Supérieure de Chimie de Lille (ENSCL, France). The compositions investigated are presented in Table 1. The specimens were prepared as follows. First, pellets (dried overnight in a vacuum oven at 80 °C) were moulded in sheet form (147 mm x 147 mm x 3 mm) by compression (BUCHER KHL 100, hydraulic press). Each moulding was made in a three-step sequence: compressed for 20 minutes under 3 MPa at 245 °C, then for 10 minutes under 7 MPa, and finally cooled for 15 minutes. Afterwards, specimens 120 mm x 13 mm x 3 mm in size were cut out of these sheets to carry out fire testing. The molecular weight of the PA 6 was well preserved for the composites during compounding and specimen preparation (Table 1); the number average molecular weight Mn indicated some deterioration in molecular weight for higher filling grades such as for PA6/10MCA/20GF, PA6/20MCA/20GF and PA6/30GF, but nevertheless the reduction in weight average molecular weight Mw was below 10% for all composites.

Table 1 Tested materials; Polyamide 6 (PA6), melamine cyanurate (MCA), glass fibre (GF), number average molecular weight (Mn), and weight average molecular weight (Mw). Composition / wt.-%

PA 6 molecular weight (Mn; Mw) 4

Acronym

2.2.

after processing / g mol-1

PA6

MCA

GF

100

-

-

17600; 74100

PA6

90 87.5 80 70 70 60

10 12.5 10 20

20 30 20 20

15400; 66800 16660; 67100 15700; 68200 14010; 73600 15300; 70100 15100; 71700

PA6/10MCA PA6/12.5MCA PA6/20GF PA6/30GF PA6/10MCA/20GF PA6/20MCA/20GF

Methods

Fire Testing and Temperature Measurements: All materials had their reaction to small-scale fire testing assessed by the vertical flammability test according to UL 94 (Underwriters Laboratories Inc.). In the case of materials not rated in vertical position, the UL 94 horizontal test was performed. Additional investigations were conducted to collect drops during the test. For this, an aluminium foil roll was extended below the specimen (instead of the standard cotton). The roll was linked to a lever that allowed the manual rotation of the roll to wind out the foil during dripping. Thus, it was possible to collect well-separated drops, as illustrated in Fig. 1 a. The drops were separated into two groups: originating from the first ignition (FIG) and from the second ignition (SIG). Each of these ignitions includes the 10 seconds of flame application and the subsequent time needed for the specimen to extinguish. Before the test, each specimen’s mass was measured. After the test, the mass of the drops was weighed. The volatilized portion was determined based on these data. Another set of specimens (5 of each material) underwent temperature evaluation during the vertical test. A thermocouple system and two cameras were used. The temperature when dripping starts was measured by applying a thermocouple (type K, NiCr/NiAl, detection range 0–1100 °C) into a centred drilled cavity in the bottom end of the specimen. This hole of about 1.0 mm diameter and 1.5 mm thickness was placed 3.0 ± 0.5 mm from the tip, as shown in Fig. 1 b. The measured temperature was assumed to be the approximate temperature of the material’s pyrolysis (~Tp) [18]. Simultaneously, the temperature of the fallen drops was measured (Td) using a wooden box with thermocouples attached beneath the specimen. This device is described in detail elsewhere [10,11]. 5

The distance between the box and the tip of the specimen was 8.0 ± 0.5 cm. The Td thus obtained was called Td from Method A. Furthermore, experiments were recorded with an infrared camera (Therma CAM S65 FLR) as well as a conventional camera (Panasonic HDC-SD600). These recordings detected the dripping time. Both cameras were placed at a distance of 0.5 m ± 0.1 m from the specimen. The spectral recording range of the IR camera was between 7.5–13 µm, with 241 images captured in 10 s. The input parameter emissivity was adjusted in IR camera software to match the ~Tp measured by the thermocouple. Assuming such an apparent constant emissivity is a crucial oversimplification compared to knowing the emissivity as function of temperature and decomposition. Furthermore, the impairment of the IR temperature measurement due to the flame was not ruled out. Nevertheless, it yields reasonable temperature to some extent, thus the IR camera was also used for temperature determination of the drops (Td), when assisted by thermocouple results at the bottom of the specimen (Tp). The Td monitored by the IR camera is called Method B. However, the IR camera was used mainly to characterize the temperature distribution over the specimen and drops during the test, rather than to determine a single temperature value.

Fig. 1. Dripping measurements in UL 94 test chamber: a) scheme of the foil roll system for collecting drops; b) cross section of the specimen during the temperature evaluation test

Thermal Analysis: Thermal conductivity (κ) was measured using the hot disc method (Hot Disk TPS 1500 Basic) according to ISO 22007-2. Fire test specimen was used and a sensor with a radius of 3.189 mm, which is hardly the best set-up limiting somewhat the precision of the results. Differential scanning calorimetry (DSC) was performed on the materials in a NETZSCH DSC 204F1 Phoenix. Samples in pellet form (8.0 ± 0.1 mg) were submitted to a first heating from 25 °C 6

to 240 °C, followed by cooling down to -50°C and then to a second heating up to 240 °C. The heating rate was 10 °C/min, under a nitrogen atmosphere (50 mL/min). Materials and their respective drops had their decomposition characteristics investigated through thermogravimetry (TGA). Powder and small pieces were used, respectively (10.0 ± 0.1 mg). Every measurement was repeated, and no significant deviation was observed. Measurements were carried out in a NETZSCH TG 209F1 under 30 ml min-1 nitrogen flow, heating from 25 °C to 900 °C at a heating rate of 10 °C/min. Structural Analysis: The molecular weight of the composites after processing and the molecular weight of the drops collected in the flammability tests was checked by the means of gel permeation chromatography (GPC). The GPC consisted of an isocratic pump (Agilent 1200) and a refractive index detector (Agilent 1260). Hexafluoroisopropanol (HFIP) + 0.005 m potassium trifluoroacetate (KTFAc) were used as eluent with polymer weights of 2.5 mg ml-1. One PL-HFIP gel 30 x 0.8 cm and two PL-HFIP gel 25 x 0.46 cm 9 µm columns were used, the flow was 0.5 ml min-1 and the measurements were performed at 35°C. The molecular weight calibration was done using 8 polymethylmethacrylate (PMMA) standards between 505 g mol-1 and 833 000 g mol-1. The contribution of the additives (flame retardant and filler) to each material’s properties was investigated with respect to density and hardness. Density was calculated by geometry and mass. Hardness Shore D (Digi test II, Bareiss Prüfungerätebau GmbH, Germany) was performed according DIN ISO 7619-1:2010. The rheological properties of the materials and their respective molten drops were measured by a plate-plate rheometer (MCR 501, Anton Paar), in oscillation mode at 233 °C, with an angular frequency of 100-0.1 rad.s-1 and a deformation amplitude of 0.5 %. Samples were previously conditioned at 23 °C at 50 % relative humidity. The surface morphology of the drops was analysed by scanning electron microscopy (SEM, Zeiss EVO MA10). Samples were sputtered with gold before measurement, and the imaging was performed with a voltage of 10 kV.

3.

Results and discussion

3.1.

Density, hardness and DSC measurements 7

As shown in Table 1, the molecular weight of the PA 6 was well preserved for the composites during processing; the reduction in Mw was below 10% for all composites. Hardness is a mechanical property strictly related to the density, defined as the resistance of the polymer to indentation. From both density and hardness, the viscoelastic tendency of the materials can be assessed at room temperature. Additionally, the thermal properties of the materials were characterized in DSC [19], the thermal conductivity (κ) measured with the hot disc method. The average results are shown in Table 2. Since all of the materials had the same polymeric matrix – PA6 – any variations between them in density and hardness were attributed to the additives. PA6 had the lowest density (1.109 g/cm³). A comparison of the compositions by pairs showed an obvious pattern. The higher the amount of additives, the higher the density, so that PA6/12.5MCA > PA6/10MCA, PA6/30GF > PA6/20GF, and PA6/20GF/20MCA > PA6/20GF/10MCA. The increase in the density of all PA6 composites is explained well by considering the typically higher densities of the fillers GF (density ca. 2.46 g/cm³) and MCA. As to the results for hardness, all compositions presented average values above those of PA6 (72.4 Shore D). For PA6/10MCA, PA6/12.5MCA, PA6/20GF/10MCA, and PA6/20GF/10MCA, the hardnesses achieved were between ~ 75.9–76.6 Shore D. Despite the significant difference in reinforcement filler content, PA6/20GF and PA6/30GF showed very similar increases in hardness (79.9–80.9 Shore D). Comparing the results with the thermal conductivities published in the data sheets of comparable commercial PA 6 products, the thermal conductivity of all the composites were within the expected range. The impact of adding MCA is not clear (PA6 < PA6/MCA, but PA6/GF ≈ PA6/GF/MCA), whereas the comparison of the compositions by pairs (PA6 < PA6/20GF; PA6 < PA6/30GF, PA6/10MCA < PA6/20GF/10MCA) yielded an obvious pattern. The thermal conductivity is crucially increased by adding glass fibres. Apart from PA6/10MCA, which might have a slightly reduced glass transition temperature (Tg), the Tg registered for each composition does not differ significantly from the Tg of PA6 (55 °C). This result indicates that no major decomposition of PA6 occurred during processing, nor any interaction between the fillers and stress relaxation in PA6 in the amorphous phase. As regards the melting temperature (Tm), Fig. 2 a shows the samples’ DSC curves. Materials containing MCA 8

displayed only one well-defined peak of melting (around 223 °C). Materials without flame retardant (PA6, PA6/20GF, and PA6/30GF) exhibited a shoulder around 217 °C in addition to the main peak. The shoulder and peak, respectively, describe two crystalline forms of PA6: γ, which melts earlier, and α, the most thermodynamically stable. The γ form has a twisted, less extended conformation, while the α form has a zigzagged planar conformation [20]. Thus, MCA seems to work as a selective nucleation agent for the α crystalline form of PA6.

Table 2 Density, hardness, thermal conductivity (κ), and DSC results; Tg: glass transition temperature; Tm: melting temperature; ∆Hm: melting enthalpy; Tc: crystallization temperature; ∆Hc: crystallization enthalpy; Xc: the polymer’s degree of crystallization ∆Hm J/g

Tc °C

∆Hc J/g

PA6 PA6/10MCA

1.11 ±0.04 72.4 ±1.9 0.27 ±0.03 55 216/223 56.4 1.13 ±0.02 76.5 ±2.3 0.32±0.01 50 222 49.4

187 194

-65.0 30.0 -52.1 29.2

PA6/12.5MCA PA6/20GF PA6/30GF PA6/20GF/10MCA

1.13 ±0.02 1.20 ±0.03 1.27 ±0.03 1.19 ±0.05

194 189 191 195

-42.2 -48.9 -42.3 -30.9

Sample

Density g/cm³

Hardness Shore D

76.6 ±1.0 79.9 ±1.1 80.9 ±2.3 76.5 ±3.3

κ W/mK

0.32±0.01 0.39±0.02 0.36±0.03 0.36±0.01

Tg °C

Tm °C

54 222 51.0 52 217/222 43.1 54 217/221 32.9 52 221 40.8

PA6/20GF/20MCA 1.23 ±0.07 75.9 ±7.6 0.40±0.01 56

221

Xc %

31.0 28.6 25.0 31.0

32.0 195/204 -31.2 28.4

Materials containing MCA had a crystallization temperature (Tc) of around 195 °C, slightly higher than PA6 (187 °C). Furthermore, PA6/20GF/20MCA presented a shoulder at 204 °C, as shown in Fig. 2 b. Clearly, MCA plays a crucial role in PA6 crystallization, acting as a nucleation agent incorporated in the polymeric matrix. However, the Xc was very similar for all samples, with the exception of PA6/30GF with 25 %. The crystallizations might be hindered in PA6/30GF by the high proportion of glass fibres.

9

Fig. 2. DSC curves: a) melting temperatures, b) crystallization temperatures.

3.2.

UL 94 vertical test and dripping behaviour

Table 3 describes the UL 94 classifications achieved by the materials investigated. PA6 had V-2 classification. The materials with only MCA as an additive both achieved V-0 classification. Therefore, a convincing flame-retardant efficiency occurred for both MCA concentrations tested (10 and 12.5 wt.-%). In contrast, the HB classification of PA6/20GF and PA6/30GF demonstrated a crucial

influence

of

GF,

yielding

increasing

flammability.

PA6/20GF/10MCA

and

PA6/20GF/20MCA both achieved V-2 classification. When analysing the total after-flame time (total ta = t1 + t2) and dripping behaviour, the two materials classified as V-2 proved to be very different from each other. The total ta is the entire period necessary for the specimen to extinguish after the two flame applications. The ‘dripping behaviour’ listed in Table 3 comprises a set of observations: the number of drops released, the kind of drops (flaming or non-flaming) and when 10

they were observed (FIG – during first ignition, or SIG – during second ignition). Evidently, standard criteria led materials to the same UL 94 classification and concealed some of the differences between them in terms of dripping. Specimens of PA6 always released 1–2 flaming drops at FIG and 2–4 flaming drops at SIG. Specimens of PA6/MCA compositions generated no drops at FIG, and only non-flaming drops at SIG. PA6/10MCA tended to release a few more drops than PA6/12.5MCA. Otherwise no self-sustained flame was observed in these specimens, yielding an after-flame time of zero.

Table 3 Fire testing results. Dripping behaviour (1)

UL 94 Test

Material

Vertical Horizontal Total ta (s)

FIG

SIG -

PA6

V-2

-

89.9

PA6/10MCA

V-0

-

0.0

None

PA6/12.5MCA

V-0

-

0.0

None

PA6/20GF

Not rated

HB

>400.0

>

Not applicable

PA6/30GF

Not rated

HB

>400.0

>

Not applicable

PA6/20GF/10MCA

V-2

-

56.5

PA6/20GF/20MCA

V-2

-

25.5

(1)

None

: flaming drop, : non-flaming drop, FIG: first ignition, SIG: second ignition

PA6 materials reinforced with GF (PA6/20GF and PA6/30GF) showed pronounced sustained ignition in UL 94 testing. A single flame application was enough to effect consumption of the entire specimen, with many flaming drops produced during burning. For this reason, no SIG was applied to these materials. Compositions of PA6 including GF and MCA presented moderate flammability. They clearly merged individual characteristics of burning and dripping observed for PA6/MCA and PA6/GF. Ignition occurred after the first flame application, and few flaming drops were released. The total decrease in ta was similar not only to PA6/GF, but also to PA6. Further, the total ta decreased by more than half from PA6/20GF/10MCA to PA6/20GF/10MCA, and the production of drops was reduced analogously. In particular, PA6/20GF/20MCA had no drops at 11

FIG in 50 % of the times tested. Pictures taken from recorded tests are shown in Fig. 3 and illustrate the common steps for drop formation of PA6/20GF/10MCA and PA6/20GF/20MCA. Initially, fire remained on the specimen surface after the flame ignition; thus, a flaming pre-drop was formed (Fig. 3 a). After some seconds, the drop detached from specimen bar (Fig. 3 b). This flaming drop fell, removing the flame, and thus the heat, from the specimen surface (Fig. 3 c), leading to extinction.

Fig. 3. PA6/20GF/20MCA dripping behaviour: a) pre-drop, b) drop, c) specimen extinguished.

Other phenomena observed during fire testing are discussed in the following section. Fig. 4 shows the residual specimens after extinction. PA6 presented a high melt flow, characterized by the formation of a filament along which the drops flowed (Fig. 4.a). PA6/10MCA and PA6/12.5MCA (Fig. 4 b and Fig. 4 c) both had the same characteristic: the specimen tip bubbled away during flame application and shortly thereafter. PA6/20GF and PA6/30GF exhibited a continuous flaming melt flow and dripping accompanied by smoke and soot production (Fig. 4 d and Fig. 4 e). PA6/20GF/10MCA and PA6/20GF/20MCA (Fig. 4 f and Fig. 4 g) showed a combination of effects caused by MCA and GF: bubbling specimen tips and the generation of black soot during the test.

12

Fig. 4. Specimens after fire testing: a) PA6, b) PA6/10MCA, c) PA6/12.5MCA, d) PA6/20GF: manual (I) and natural (II) extinguishment, e) PA6/30GF: manual (I) and natural (II) extinguishment, f) PA6/20GF/10MCA, g) PA6/20GF/20MCA.

Infrared recordings enabled the thermal reaction of the material to be evaluated during the flammability test. The many shots taken are compiled in Fig. 5. The start of flame application to the specimen, the specimen after flame removal, the moment when a pre-drop or a neck formed, and the first drop obtained or the melt flow are shown where applicable.

13

Fig. 5. Infrared images of UL 94 vertical test grouped by first (FIG) and/or second (SIG) ignition.

14

PA6, PA6/10MCA and PA6/12.5MCA (Fig. 5 a, Fig. b and Fig. c) show limited heat spread on the specimen bars at both ignitions. The high temperatures were very localized at the tip of the specimens. In contrast, the PA6/GF compositions (Fig. 5 d and Fig. 5 e) were characterized by crucial heat conduction upwards along the specimen bars, due to the increase in thermal conductivity through the addition of GF [21], which tend to form entanglements with each other and thus a network [22] enhancing heat transfer. Further, much more heat is necessary to make these more structured and reinforced materials flow. By the time this happened, PA6/GF accumulated more heat in the pyrolysis zone [23]. PA6/20GF/10MCA had a higher heat spread (Fig. 5 f) than PA6/20GF/20MCA (Fig. 5 g).

3.3

Mass loss by dripping Table 4 shows the average total mass loss for each material. It was obtained by the

difference between the original specimen’s mass and the mass of the residue after the test. The portion of mass loss due to dripping, as measured by the mass of the drops generated during test, is shown in the second column (total mass loss by dripping). The portion remaining was attributed to volatilization, the third column (total mass loss by volatilization). No significant difference between PA6 and PA6/MCA compositions was registered in terms of total mass losses and mass losses by dripping. However, PA6/10MCA presented a lower mass loss by volatilization (0.7 %). Comparing PA6/20GF and PA6/30GF, even though, as mentioned above, they were both consumed completely, PA6/30GF generated more volatiles than PA6/20GF (around +16 %). This is explained by the more pronounced retainment of polymeric material in the pyrolysis zone with increasing GF content, consequently leading to increased consumption of the matrix due to decomposition. PA6/20GF/10MCA and PA6/20GF/20MCA showed similar mass losses due to dripping, whereas volatilization nearly vanished for PA6/20GF/20MCA.

Table 4 Total mass loss of specimens. Specimen

Total Mass Loss

Total Mass Loss by

15

Total Mass Loss by

PA6 PA6/10MCA PA6/12.5MCA PA6/20GF PA6/30GF PA6/20GF/10MCA PA6/20GF/20MCA

3.4.

(wt.-%)

Dripping (wt.-%)

Volatilization (wt.-%)

2.6 2.3 2.6 82.0 82.0 3.2 1.3

1.4 1.6 1.3 57.7 41.9 1.3 1.2

1.2 0.7 1.3 24.3 40.1 1.9 0.1

Characteristic dripping times and drop appearance

The times to dripping of each material were extracted from recordings. They correspond to the time point when a pre-drop or a neck detached itself from the specimen bar. Averages of the times to dripping are shown in Fig. 6. All drops generated in FIG arose after flame removal (after 10 seconds). On the other hand, all drops from SIG arose during flame application (less than 10 seconds). This points to a strong influence of the first ignition on second ignition behaviour. Comparing times to first drop from FIG, an influence of additives was observed: PA6 (12s), PA6/20GF (21s), PA6/30GF (55s), PA6/20GF/10MCA (17s), and PA6/20GF/20MCA (19s). GF significantly increases the time to dripping in PA6. The higher the GF content was, the longer the time to first drop. MCA slightly reduced the time to dripping of reinforced PA6, in comparison to PA6/20GF. PA6 with a lower amount of MCA tended to start dripping a bit earlier than systems with higher MCA content. Comparing times to first drop from SIG, this destabilization promoted by MCA was observed again: PA6 (6.5s), PA6/10MCA (2s), PA6/12.5MCA (9s), PA6/20GF/10MCA (6s), and PA6/20GF/20MCA (8s). However, in the case of PA6 flame-retarded with only MCA, the high MCA content yielded a time to dripping greater than that for PA6.

16

60

1st drop FIG 2nd drop FIG rd 3 drop FIG

50

1st drop SIG 2nd drop SIG rd 3 drop SIG

Time (s)

40

30

20

10

PA6/20GF/20MCA

PA6/20GF/10MCA

PA6/30GF

PA6/20GF

PA6/12.5MCA

PA6/10MCA

PA6

0

Fig. 6. Materials’ time to dripping at each ignition.

Due to the set-up for collecting drops of material, it was shown that the drops generated had distinct visual characteristics in each test. Great similarities and thus good repeatability between distinct tests was observed, suggesting characteristic patterns. Evidently, the drops’ characteristics convey some representative information about the burning process of the original materials. Fig. 7 shows images of drops collected during one fire test. Identified characteristics are described next. Drops from PA 6 are shown in Fig. 7 a. The drop placed before the dash was from FIG, and the drops after it were from SIG. While one was very thin, transparent and yellow, the others were thicker and amber-coloured, indicating slight and major damage, respectively. In addition, the first drop from SIG was thicker than the next ones. This phenomenon is a result of the previous accumulation of melted, partially decomposed material on the tip of the specimen during FIG. Drops originating from the SIG of PA6/10MCA (Fig. 7 b) and PA6/12.5MCA (Fig. 7 c) had nearly 17

the same appearance. They kept the colour of PA6 drops from SIG, which pointed to a similar degree of decomposition. A closer look at the PA6/10MCA drops revealed some black points not present in PA6/12.5MCA. All of the drops of PA6/20GF (Fig. 7 d) and PA6/30GF (Fig. 7 e) formed at FIG through an intense burning process. However, they demonstrated quite distinct dripping. PA6/20GF produced drops similar to those from PA6/MCA compositions, but with more pronounced black points. a)

b)

c)

d)

e)

f)

g)

Fig. 7. Collected drops from: a) PA6, b) PA6/10MCA, c) PA6/12.5MCA, d) PA6/20GF, e) PA6/30GF, f) PA6/20GF/10MCA, g) PA6/20GF/20MCA

18

PA6/30GF produced drops with the most distinctive characteristics observed. In general, its drop diameter was always over 13 mm, while the drops of all other materials had a diameter of around 9 mm. Unlike PA6/20GF, which presented flaming drops that were spontaneously extinguished in contact with the aluminium foil, PA6/30GF had robust flaming drops that burned for a long while after falling. Hence, these last drops appeared to consist of consumed PA6; they were irregular in shape, black, and structured mainly by GF. PA6/20GF/10MCA (Fig. 7 f) and PA6/20GF/20MCA (Fig. 7 g) both had no apparent changes between the drops from FIG and from SIG, respectively. Not even the effect of the increased mass for the first SIG drop, as noted in PA6, occurred. Drops were very thin but not fragile as PA6’s first drop from FIG, probably due to GF content. Drops of PA6/20GF/20MCA were brittle, filled with bubbles on the surface and blacker than drops of PA6/20GF/10MCA.

3.5.

Drop structure: morphology, mass and thermal decomposition

Morphological characterization of the drops’ surface was performed by SEM. Micrographs of drops from first ignition (dFIG) and second ignition (dSIG) are shown in Fig. 8. Fig. 8 a shows the surface of PA6 drops. While the first ignition (dFIG) presented sparse, large round holes inside quite a uniform matrix, the other (dSIG) showed huge craters surrounded by somewhat damaged material. This was attributed to the lack of burning protection, which probably volatilized more of the polymer. The incorporation of MCA promoted slight interference in the uniformity of the drops’ surface. In PA6/10MCA drops (Fig. 8 b) an undermined structure without holes was observed. And in PA6/12.5MCA drops (Fig. 8 c) this effect is attenuated, still without holes. Materials filled with GF (Fig. 8 d and Fig. 8 e) produced drops formed mostly by GF. However, only in PA6/30GF drops were some agglomerated formations surrounding the fibres detected, indicating char. Compositions of PA6/20GF/MCA (Fig. 8 f and Fig. 8 g) produced dFIG with a surface very similar to that of dFIG from PA6/20GF. On dSIG, however, the difference was clear. In the case of PA6/20GF/10MCA, dSIG presented a dispersed amount of GF with the presence of small particles like powder. The dSIG from PA6/20GF/20MCA had a structure of fibres covered by a very fragile polymeric layer. Some small holes were identified near the GF structures. 19

a)

dFIG

b)

dSIG

c)

dSIG

d)

dSIG

e)

dFIG

dFIG

dFIG

dSIG

dFIG

dSIG

f)

g)

Fig. 8. SEM images of the drops from: a) PA6, b) PA6/10MCA, c) PA6/12.5MCA, d) PA6/20GF, e) PA6/30GF, f) PA6/20GF/10MCA, g) PA6/20GF/20MCA.

Fig. 9 shows the mass of the individual drops presented for each material, and for the first and second ignitions, respectively. The tendency found for both ignitions was that the mass of the first drop was nearly matched by the following ones. PA6 generated drops with the same range of mass in both FIG and SIG, between 16–24 mg. Drops from PA6/10MCA (SIG) had a mass of 23– 33 mg, and those from PA6/12.5MCA were between 18–37 mg. The more MCA was added to PA6, 20

the larger the drop mass range. The addition of 20 wt.-% GF in PA6/20GF stabilized the mass of drops to a very narrow range, between 15–19 mg. On the other hand, drops from PA6/30GF had the most variable range of mass, between 28 mg and 5 g. The flame-retarded composites had identical range of drop mass in FIG and in SIG, as also observed for PA6. Drops from PA6/20GF/10MCA had 19–23 mg, and PA6/20GF/20MCA 22–25 mg. That result demonstrated that these materials do not comply with the relation reported in [7], which corresponded greater drop masses to longer drip times. Furthermore, these mass results indicate that the melt strength forces compete with the melt gravity, breaking the equilibrium [24] at a very close level for each drop produced from the same material. The exception was PA6/30GF, in which the diverse drop masses described a complex mechanism of flow.

SIG

FIG

18

PA6/20GF/20MCA B

PA6/20GF/20MCA C

PA6

PA6/20GF/20MCA A

PA6/20GF/ 20MCA

PA6/20GF/10MCA B

PA6/20GF/20MCA B

PA6/20GF/20MCA C

PA6/20GF PA6/20GF/ 10MCA

PA6/20GF/20MCA A

PA6/20GF/10MCA B

PA6/20GF/10MCA C

PA6/20GF/10MCA A

PA6/20GF B

PA6/20GF C

PA6/20GF A

PA6 B

PA6

PA6 C

PA6 A

1st drop (mg)

15 25 24 23 22 21 20 19 18 17 16 15

PA6/20GF/10MCA C

16

PA6/20GF/10MCA A

17

PA6/12.5MCA B

18

PA6/12.5MCA C

19

PA6/12.5MCA A

20

PA6/10MCA B

21

PA6/10MCA C

38 36 34 32 30 28 26 24 22 20 18 16 34 32 30 28 26 24 22 20 18 16 14

PA6/10MCA A

24

23

2nd drop (mg)

2nd drop (mg)

26

15

22

1st drop (mg)

28

PA6 B

16

30

PA6 C

17

PA6 A

3rd drop (mg)

3rd drop (mg)

32

PA6/ PA6/ PA6/20GF/ PA6/20GF/ 20MCA 10MCA 12.5MCA 10MCA

Fig. 9. Mass of 1st, 2nd and 3rd drops of three samples of materials (A, B, C) in first and second ignition (FIG and SIG).

The mass curves of materials during decomposition (Fig. 10) and of their drops (Fig. 11) clarified their nature. The results are summarized in Table 5.

21

Fig. 10. Mass curves of materials during decomposition with a) one decomposition step, and b) two decomposition steps (compared to PA6 curve).

Two distinct thermal decomposition characteristics were identified among the materials: some had only one step of decomposition (Fig. 10 a) and others two (Fig. 10 b). The first case included PA6, PA6/20GF and PA6/30GF. This unique step represents PA6 decomposition, starting around at 420 °C (Tonset). Its size was proportional to the polymeric fraction of each material. Only PA6/30GF demonstrated a tiny decrease in Tonset, from 420 to 416 °C, while PA6/20GF kept roughly the same temperature of PA6 starting decomposition. Residual masses from PA6/GF materials amounted to the GF content. The second kind of materials included all of those containing MCA. A previous decomposition step occurred due to its decomposition around 315 °C. Of these materials, PA6/12.5MCA exhibited the lowest Tonset at 313.4 °C. Materials with 10 wt.-% flame retardant had a mass loss of about 9 wt.-%, while PA6/20GF/20MCA had a mass loss of about 16 wt.-%. The Tonset of the main decomposition step was decreased only for PA6/20GF/MCA samples. However, PA6/20GF/20MCA had a slight increase in expected polymeric mass loss (+ 1.4 wt.-%) and in expected residual mass (+ 2.0 wt.-%). 22

As to the results for the drops, the polymeric thermal stability was reduced when compared to the original materials; except for PA6/20GF/20MCA drops, where it improved slightly. However, the gradual decrease of Tonset PA6 > dFIG > dSIG was not observed for PA6/20GF/10MCA drops, which had Tonset dFIG = dSIG. Moreover, the drops from all the flame-retarded materials no longer presented a previous decomposition step. This change confirmed that no unreacted MCA remained inside the drops. The residual masses of PA6 showed the sequence dFIGrm > PA6rm > dSIGrm. For PA6/MCA materials, the drops’ residue decreased slightly. The drops of PA6/GF material increased by about 15 wt.-% as compared to the residual masses of PA6/GF. When the PA6/GF drops exhibited a smaller loss of their polymeric fraction, that increase in residual mass was attributed to an increase of GF content inside these drops. In the meantime, for PA6/20GF/MCA materials, which had dFIG with about 10 wt-% less polymeric lost than expected, the pattern was dFIGrm > dSIGrm > PA6/20GF/MCArm. This indicated a higher GF content in dFIG than in dSIG.

Table 5 Thermal decomposition data from thermogravimetric measurements; MLLMW = Mass loss of low molecular weight molecules; ML = mass loss. Sample

MLLMW Drops

PA6 dFIG dSIG PA6/10MCA dSIG PA6/12.5MCA dSIG PA6/20GF dFIG PA6/30GF dFIG PA6/20GF/10MCA dFIG dSIG PA6/20GF/20MCA dFIG dSIG

Previous Decomposition

/ wt.-%

Tonset / °C

2.25 2.21 2.54 2.94 1.62 2.04 1.07 1.06 1.98 1.60

317 313 318 317 -

Tendset ML / °C / wt.-%

323 337 322 345 -

8.65 12.84 8.78 15.8 23

Main Decomposition

Tmax / °C

Tonset / °C

Tendset / °C)

ML / wt.-%

Tmax / °C

324 327 326 335 -

421 415 411 418 417 419 418 419 411 416 412 412 408 408 406 410 408

469 467 466 464 466 459 465 464 468 463 467 464 465 464 460 465 467

97.7 95.6 96.5 90.0 96.4 86.7 94.9 79.9 62.4 69.7 52.7 69.9 58.2 70.1 61.4 51.0 60.7

449 449 445 446 447 445 445 447 448 447 447 446 447 443 442 445 448

Residue at 900 °C / wt.-%

0.9 1. 5 0.7 0.6 0.5 0.4 0.3 19.3 35.1 29.4 44.9 20.4 38.9 27.3 22.0 46.2 37.2

a)

100 0

PA6 drop FIG drop SIG

PA6 drop FIG drop SIG

60

dm/dt

Mass (%)

80

40

20

0 200

400

600

800

200

Temperature (°C)

b)

400

600

800

Temperature (°C)

100 0

60

dm/dt

Mass (%)

PA6/10MCA drop SIG

PA6/10MCA drop SIG

80

40

20

0 200

400

600

800

200

Temperature (°C)

c)

400

600

800

Temperature (°C)

100 0

PA6/12.5MCA drop SIG

60

PA6/12.5MCA drop SIG dm/dt

Mass (%)

80

40

20

0 200

400

600

800

200

Temperature (°C)

d)

400

600

800

Temperature (°C)

100 0

PA6/20GF drop FIG

PA6/20GF drop FIG

60

dm/dt

Mass (%)

80

40

20

0 200

400

600

800

200

Temperature (°C)

e)

400

600

800

Temperature (°C)

100 0

PA6/30GF drop FIG

PA6/30GF drop FIG

60

dm/dt

Mass (%)

80

40

20

0 200

400

600

800

200

Temperature (°C)

f)

400

600

800

Temperature (°C)

100 0

PA6/20GF/10MCA drop FIG drop SIG

60

PA6/20GF/10MCA drop FIG drop SIG

dm/dt

Mass (%)

80

40

20

0 200

400

600

800

200

Temperature (°C)

g)

400

600

800

Temperature (°C)

100 0

PA6/20GF/20MCA drop FIG drop SIG

60

dm/dt

Mass (%)

80

PA6/20GF/20MCA drop FIG drop SIG

40

20

0 200

400

600

800

200

Temperature (°C)

400

600

800

Temperature (°C)

Fig. 11. TGA: Mass curves of materials and their respective drops. 24

3.6.

Drop temperature

The average drop temperatures in the first (TdFIG) and second (TdSIG) ignitions are grouped in Table 6. In addition, Method B recorded the approximate pyrolysis temperature during first ignition (~Tp1) and second ignition (~Tp2).

Table 6 Averages of measured temperatures (in °C). Method A

Method B

Sample TdFIG PA6 PA6/10MCA PA6/12.5MCA PA6/20GF PA6/30GF PA6/20GF/10MCA PA6/20GF/20MCA

TdSIG

160 ± 29 208 ± 2 183 ± 22 308 ± 43 426 ± 91 178 ± 45 291 ± 83

~Tp1 = TdFIG ~Tp2 = TdSig 415 ± 55

435 ± 21 507 ± 44 418 ± 19 410 ± 13

339 ± 40 396 ± 112 479 ± 81 481 ± 113 462 ± 64

In all cases, Method A registered lower drop temperatures than Method B. This was attributed to the conditional cooling of the drops, as the thermocouples were a distance of 8 cm below the specimen in Method A. For Method A, when materials displayed dripping in both ignitions (PA6, PA6/20GF/10MCA and PA6/20GF/20MCA) the registered value of TdFIG was equal to TdSIG. MCA reduced the melt temperature during dripping in PA6/20GF/10MCA and PA6/20GF/20MCA, the effect was not observed in PA6/10MCA to PA6/12.5MCA. For PA6/20GF to PA6/30GF, the temperatures of drops increased, as they did for PA6/20GF/10MCA to PA6/20GF/20MCA as well. In general, the temperatures of the drops were a bit below Tm for PA6, PA6/10MCA, PA6/12.5MCA and PA6/20GF/10MCA. For the other materials, the dripping temperatures were above Tm. Only for PA6/30GF was the temperature of the drops found to exceed the Tonset of polymer decomposition. These results agree with the relation between drops’ temperature and the characteristic temperatures of the polymer reported in [11]. The exposure of some PA6 (HB, V-2 25

and V-0) to low furnace temperatures produced drops with temperatures near the Tm. When the furnace temperature was increased, the materials with lower classificationshowed drop temperatures near the temperature of starting decomposition (by TGA). Here, under fire, materials tended to this behaviour despite the much more intense heat than in a furnace. The drop temperatures estimated with Method B were higher than the temperatures obtained with Method A, and also higher than Tp. Since IR methods of temperature measurement are very sensitive to variation, flaming drops could mask the real value of polymer melted temperature. Nonetheless, most tendencies registered by Method A were also evident in Method B. The exception was PA6/MCA materials, where the TdSIG of PA6/12.5MCA > TdSIG of PA6/10MCA in method B. Since PA6/MCA materials released only non-flaming drops, this difference may not be evident in Method A due to the distance between drop formation and the temperature measurement. When ~Tp1 and ~Tp2 are compared with thermogravimetry data, it becomes clear that they are close to the Tonset of the polymer decomposition step, with some exceptions: PA6 ~Tp2, the lowest value, and PA6/30GF ~Tp1, the highest value. For PA6/30GF, the flaming specimens probably interfere with the thermocouple measurement. The ~Tp1 results are very consistent with [9], where 415 °C was found to be the temperature at which PA6 starts dripping in furnace tests.

3.7.

Rheological properties and molecular weight of the materials and drops

Complex viscosity (|η*|) results were plotted as a function of frequency (ω) in Fig. 12. The region between 10-1 and 10° rad s-1 was delimited as of most interest to study dripping, as dripping is associated with low shear rates. PA6 has a characteristic increase in viscosity towards lower frequencies due to its chemical structure. Hydrogen from amide groups and oxygen from carbonyls form intermolecular hydrogen bonds that contribute to solid-like behaviour at low shear rates [25]. At greater yield stresses, the viscosity curve tends to a Newtonian plateau around 300 Pa.s., where the loss modulus curve (G´´) remains above the storage modulus curve (G´), expressing the typical viscoelastic behaviour dominated by viscous liquid behaviour.

26

Fig. 12. Complex viscosities and PA6’s storage and loss moduli as function of angular frequency.

Materials containing only the flame-retardant MCA increased complex viscosity in the lowest frequency through enhanced particle-particle interactions. Extended yield stresses were observed, followed by plateaus around 150 Pa.s., which revealed a decrease in PA6/MCA viscosities by around 0.6 rads-1 to below the PA6 reference. Adding GF increased the viscosities over the entire range of angular frequencies. For the lowest frequency, GF strongly enhanced the complex viscosity of materials – much more than MCA particles – to a value of ~105 Pas. MCA enhanced the viscosities as its content increased: PA6/20GF < PA6/20GF/10MCA < PA6/20GF/20MCA. And PA6/30GF had the highest viscosity observed. After 1.0 rads-1, only PA6/20GF/20MCA showed a strong dependence on frequency. The comparisons between the viscosity of the materials and their respective drops collected in the UL 94 set-up are shown in Fig. 13. The drops of burning PA6 displayed curves different to PA6 (Fig. 13 a), suggesting severe damage to the molecular structure. Higher complex viscosities than the original material at low shear rates, and what is more, a pronounced decrease in viscosity for increasing shear rates, were interpreted as features of decomposed PA6. Comparing the viscosity of dFIG and dSIG, a further reduction in molecular weight or reduced interaction, such as hydrogen bonds, was proposed for dSIG. Table 7 shows the change in molecular weight for the drops collected after first and second ignition, respectively. The reduction in molecular weight is drastic from values of Mn = 14.000 up – 18.000 g/mol for the slab materials down to 2 700 – 5 700 g/mol for the drops. The number averaged molecular weight Mn decreased by 2/3 up to 4/5. The degradation of the weight averaged 27

molecular weight Mw was even more pronounced. The molecular weight distributions were crucially broadened (supplementary Figure S1). Adding MCA yielded an additional characteristic peak at low molecular weight; this peak was reduced and even vanished completely for the drops (supplementary Figure S2). MCA enhances the decomposition of PA6, larger reduction in the molecular weights were observed accompanied by somewhat less broadening. This enhanced molecular weight reduction with less broadening corresponds with the increased dripping without increasing the release of volatile PA6 pyrolysis products in thermogravimetry. The drops of the glass fibre reinforced materials showed the smallest molecular weights. It is concluded that longer residence times and higher drop temperatures resulted in pronounced degrees of decomposition.

28

a)

105

PA6 dFIG dSIG

Viscosity (Pa.s)

104

103

102

101

100

10-1 10-3

10-2

10-1

100

Shear Rate (1/s)

b)

c)

105

105

PA6/10MCA dSIG

4

10

Viscosity (Pa.s)

10

Viscosity (Pa.s)

PA6/12.5MCA dSIG

4

3

10

2

10

3

10

2

10

1

10

1

0

10

10

0

10

10-1

10-1 10

-3

-2

-1

10

10

0

10

10

-3

Shear rate (1/s)

d)

106 10

104 103 2

10

PA6/30GF dFIG

104 103 102 10

1

0

10

0

10

-1

10 10

-3

-2

-1

10

10

-1

0

10

10

-3

Shear rate (1/s)

f)

106

-2

-1

10

10

0

10

Shear rate (1/s)

g)

PA6/20GF/10MCA dFIG dSIG

105

106

PA6/20GF/20MCA dFIG dSIG

105

104

Viscosity (Pa.s)

Viscosity (Pa.s)

0

10

106

1

10

10

-1

10

105

Viscosity (Pa.s)

Viscosity (Pa.s)

e)

PA6/20GF dFIG

5

-2

10

Shear rate (1/s)

103 102

104 103 102 1

101

10

0

10

0

10

10-1

10-1 10

-3

-2

10

-1

10

0

10

10

Shear rate (1/s)

-3

-2

10

-1

10

0

10

Shear rate (1/s)

Fig. 13. Complex viscosity as a function of shear rate of each sample and its respective drops. 29

Table 7 Molecular weights, Mn = number average molecular weight, Mw = weight average molecular weight, and Mp = Peak molecular weight (GPC) of the drops collect after the first ignition (dFIG ) and drops after the second ignition (dSIG).

PA6 dFIG dSIG PA6/10MCA dSIG PA6/12.5MCA dSIG PA6/20GF dFIG PA6/30GF dFIG PA6/20GF/10MCA dFIG dSIG PA6/20GF/20MCA dFIG dSIG

Mn / g mol-1

Mw / g mol-1

Mp / g mol-1

17600 4580 5490 15400 5630 16660 5570 15700 3290 14010 4170 15300 3050 3070 15100 2860 2760

74100 16900 20700 66800 18300 67100 17600 68200 14000 73600 19400 70100 12000 10600 71700 9450 7540

59880 9900 12500 58600 12700 56800 11800 56700 5900 66000 8160 61400 5480 56660 61700 4970 4820

Both flame-retarded PA6/MCA specimens presented drops with lower viscosities, but still plateau behaviour for higher shear rates and thus a distinct decrease in molecular weight (Fig. 13 b and Fig. 13 c). The drops displayed a reasonable level of preservation of the polymeric characteristics, indicating the oligomeric character or due to the broad molecular weight distribution. The drops of PA6/GF showed higher complex viscosity at low shear rates (Fig. 13 d and Fig. 13 e). This feature is attributed to the increase in GF content inside the drops as well as some presence of char in PA6/30GF. For higher shear rates, the viscosity constantly decreased, which was interpreted as a loss of polymeric character due to pronounced decomposition of the PA6. 30

Drops from PA6/20GF/10MCA (Fig. 13 f) demonstrated the competition of effects due to flame retardant and reinforced filler. The dFIG had the same rheological behaviour as the drops from PA6/20GF. And the dSIG had approximately the same behaviour as PA6/MCA drops. In short, the first drops are more viscous than pure material, while the second drops are more fluid than pure material. As to PA6/20GF/20MCA (Fig. 13 g), the huge amount of MCA clearly reduced the complex viscosity of all drops to below the original reference. The dFIG and dSIG had almost identical curves.

4.

General discussion

The decomposition pathway of PA 6 leads through intra-aminolysis/acidolysis and randomchain scission [7,26,27]. The main volatile decomposition product is ε-caprolactam, produced by complete depolymerization. Other products like cyclopentanone, lower hydrocarbon, and nitrile compounds are released during burning. Molten PA6 and involatile oligomers through incomplete depolymerization contribute to the dripping material [28]. The expected dripping behaviour for PA6 is caused by surface melting, producing small drops [7]. Here, the PA6 was classified V-2, produced filaments (slight melt flow) and many small flaming drops. The thermogravimetric and rheological results pointed out that PA6 drops are formed by involatile hydrocarbon decomposition products. The addition of GF and/or MCA to PA6 intensified the polymer decomposition, according to their influences in physical and chemical interactions, respectively. Consistent with that, the melt dripping of PA6 was changed. MCA is a crystalline complex that sublimates at around 350 °C by an endothermic process that releases melamine and cyanuric acid [28]. In other words, the hydrogen bonds between the two are broken. The consumption of energy required for this reaction cools down the burning material, an effect commonly called ‘heat sink’. The melamine mostly dilutes the fuel in the gas phase, but it also facilitates the decomposition of PA6 through interference with its hydrogen bonding [29]. Cyanuric acid induces chain breakage to promote dripping, removing heat and mass from the pyrolysis zone (the ‘run-away’ effect) [17,30]. The effectiveness of MCA depends on the number of

31

possible reactions between polymer decomposition products and NH3/other decomposition products of MCA [31]. Properties related to dripping measured in the original materials demonstrated that the influence of additives started before combustion processes and dripping development. The arrangement of polymeric chains and the uniformity of the material were modified in the presence of additives, especially in compositions with low polymeric content (PA6/30GF and PA6/20GF/20MCA). MCA acts as a plasticizer in small amounts (PA6/10MCA and PA6/20GF/10MCA) and as a reinforcement/nucleating additive in greater amounts (PA6/12.5MCA and PA6/20GF/20MCA). The influence of smaller additive particles in polymer crystallization was already reported in a study of carbon nanotubes in PA6 [32]. Even so, the lower MCA addition always generated lower dripping rates than the references PA6 and PA6/20GF. In contrast to what was found in [33], MCA had no tendency to increase dripping with increasing additive content. More MCA led to less dripping. In all cases, the addition of MCA reduced the temperature of starting decomposition, as was also reported before [11,16]. It is likely that cyanuric acid enhanced the PA6 natural acidolysis, producing more involatile oligomers than caprolactam. GF, despite its chemical inertness, exhibited physical interference that aggravated the flammability of the PA6. GF entanglements spread heat during burning efficiently due to their high isotropic thermal conductivity. PA6 dripping time was prolonged. The more GF, the more the pyrolysis zone was amplified and dripping delayed. The burning became self-sustained. PA6/30GF changed PA6 dripping behaviour to a bulk softening process (great melt flow), producing large, flaming drops. The same conversion was reported in [7] for LDPE loaded with 20 wt.-% talc. The more GF inside the material, the higher the drop temperature and its decomposition. Thermogravimetric results showed that PA6/GF compositions had increased GF content in the residual mass. For PA6/GF/MCA compositions, a competition between the individual actions of the additives was noted. MCA could not promote non-flaming drops and GF could not spread so much heat along the specimen. Even so, both had some effect in reducing the dripping behaviour. Through MCA protection and GF preventing the flow, FIG drops were formed mostly by GF. And the drops released on SIG were formed by GF and highly decomposed polymer. This degree of 32

decomposition of SIG drops was related to the increase in MCA amount. Enhanced loading (GF+MCA) did not increase the drop mass, but instead restricted it into a narrow range. Colour, as pointed out in [26], and other evaluated aspects of drops’ structure, were considered to offer good clues about the involatile decomposition products of each material. For instance, black points inside the drops pointed to high viscosities, while holes pointed to a serious loss of polymeric characteristics. IR recordings confirmed the individual and conflicting effects of GF and MCA inside PA6. Rheological measurements demonstrated that reduced dripping is related to high complex viscosities at low shear rates. They also indicated the drops’ loss of polymeric properties. However, in the case of PA6/20GF/20MCA, complex viscosity decreased for higher shear rates.

5.

Conclusion

The dripping behaviour of PA6-based materials was studied under fire. The competing effects of the flame-retardant MCA promoting non-flaming dripping, and GF as reinforcing fillers reducing dripping, were discussed. Dripping was modified according to the additive(s) used and their respective amounts. Differences in drops from first and second ignition in the UL 94 set-up were reported, as a result of the competition of the complex interactions. Exploring the dripping phenomena and the detailed action of even these well-known players is a suitable way to enhance our understanding and thus enable their potential to reduce dripping in future materials.

Acknowledgements The authors thank the National Council of Technological and Scientific Development from Brazil (CNPq) for its financial support (205385/2014-1). Thanks to ENSC Lille for providing the materials. We thank T. Raspe for her help with the IR-based temperature measurements, M. Morys for performing SEM, J. Falkenhagen for measuring the molecular weight distributions, and M. Matzen for her support investigating the dripping phenomena.

References 33

[1] J. Sherrat, The effect of thermoplastic melt flow behaviour on the dynamics of fire growth, PhD Thesis, University of Edinburg, UK, 2001. [2] UL 94 Standard. IEC 60695-11-10. 50W (20mm) Vertical Burning Test. [3] G. Bertelli, R. Benassi, R. Marchini, G. Camino, L. Costa, M.P. Luda, Evaluation of polymer combustion and fire retardance by using thermography, Fire Mater. 17 (1993) 125-129. [4] P. Joseph, S. Tretsiakova-McNally, Melt-flow behaviours of thermoplastic materials under fire conditions: recent experimental studies and some theoretical approaches, Materials 8 (2015) 8793– 8803. [4] J. Zhang, T.J. Shields, G.W.H. Silcock, Effect of melting behaviour on upward flame spread of thermoplastics, Fire Mater. 21 (1997) 1–6. [5] Y. Wang, F. Zhang, X. Chen, Y. Jin, J. Zhang, Burning and dripping behaviors of polymers under the UL 94 vertical burning conditions, Fire Mater. 34 (2010) 203–215. [6] Y. Wang, J. Jow, K. Su, J. Zhang, Dripping behavior of burning polymers under UL 94 vertical test conditions, J. Fire Sci. 30 (2012) 477–501. [7] Y. Wang, J. Zhang, Thermal stabilities of drops of burning thermoplastics under the UL 94 vertical test conditions, J. Hazard. Mater. 246–247 (2013) 103–109. [8] B.K. Kandola, D. Price, G.J. Milnes, A. Da Silva, Development of a novel experimental technique for quantitative study of melt dripping of thermoplastic polymers, Polym. Degrad. Stab. 98 (2013) 52–63. [9] B.K. Kandola, N. Ndiaye, D. Price, Quantification of polymer degradation during melt dripping of thermoplastic polymers, Polym. Degrad. Stab. 106 (2014) 16–25. [10] M. Matzen, B.K. Kandola, C. Huth, B. Schartel, Influence of flame retardants on the melt dripping behaviour of thermoplastic polymers, Materials 8 (2015) 5621–5646. [11] C. Hu, G. Fontaine, P. Tranchard, T. Dlaunay, M. Collinet, S. Marcille, S. Bourbigot, In-situ investigation of temperature evolution of drippings via an optimized UL-94 instrumentation: Application to flame retarded polybutylene succinate, Polym. Degrad. Stab. 155 (2018) 145–152. [12] R. Dupretz, G. Fontaine, S. Duquesne, S. Bourbigot, Instrumentation of UL-94 test: understanding of mechanisms involved in fire retardancy of polymers, Polym. Adv. Technol. 26 (2015) 865–873. 34

[13] F. Kempel, B. Schartel, J.M. Marti, K.M. Butler, R. Rossi, S.R. Idelsohn, E. Onate, A. Hofmann, Modelling the vertical UL 94 test: competition and collaboration between melt dripping, gasification and combustion, Fire Mater. 39 (2015) 570–584. [14] E.D. Weil, S. Levchik, Current practice and recent commercial developments in flame retardancy of polyamides, J, Fire Sci. 22 (2004) 251–264. [15] A. Casu, G. Camino, M. De Giorgi, D. Flath, V. Morone, R. Zenoni, Fire-retardant mechanistic aspects of melamine cyanurate in polyamide copolymer, Polym. Degrad. Stab. 58 (1997) 297–302. [16] S.V. Levchik, E.D. Weil, Combustion and fire retardancy of aliphatic nylons, Polym. Int. 49 (2000) 1033–1073. [17] B. Schartel, A. Weiß, Temperature inside burning polymer specimens: pyrolysis zone and shielding, Fire Mater. 34 (2010) 217–235. [18] J.E. Mark, Physical Properties of Polymers Handbook, Springer, 2007. [19] E. Piorkowska, Handbook of Polymer Crystallization, Wiley, 2013. [20] A.P. Mouritz, A.G. Gibson, Chapter 2 - Thermal decomposition of composites in fire, In: Fire Properties of Polymer Composite Materials, Springer, 2006. [21] B.K. Kandola, R. Toqueer-Ul-Haq, The effect of fibre content on the thermal and fire performance of polypropylene–glass composites, Fire Mater. 36 (2012) 603–613. [22] S. Brehme, T. Köppl, B. Schartel, V. Altstädt, Competition in aluminium phosphinate-based halogen-free flame retardancy of poly(butylene terephthalate) and its glass-fibre composites. ePolymers 14 (2014) 193–208. [23] K. Yangkyun, A. Hossain, Y. Nakamura, Numerical modeling of melting and dripping process of polymeric material subjected to moving heat flux: Prediction of drop time, Proceed. Combust. Inst. 35 (2015) 2555–2562. [24] L.R. Schroeder, S.L. Cooper, Hydrogen bonding in polyamides, J. Appl. Phys. 47 (1976) 4310–4317. [25] B.J. Holland, J.N. Hay, Thermal degradation of nylon polymers, Polym. Int. 49 (2000) 943– 948.

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[26] B.N. Jang, M. Costache, C.A. Wilkie, The relationship between thermal degradation behavior of polymer and the fire retardancy of polymer/clay nanocomposites, Polymer 46 (2005) 10678– 10687. [27] Y. Liu, Q. Wang, The investigation on the flame retardancy mechanism of nitrogen flame retardant melamine cyanurate in polyamide 6, J. Polym. Res.16 (2009) 583–589. [28] J. Zhang, M. Lewin, E. Pearce, M. Zammarano, J.W. Gilman, Flame retarding polyamide 6 with melamine cyanurate and layered silicates, Polym. Adv. Technol. 19 (2008) 928–936. [29] M. Klatt, Chapter 4 – Nitrogen-based flame retardants, In: The Non-halogenated Flame Retardant Handbook, A.B. Morgan, C.A. Wilkie, (Eds.), John Wiley & Sons, 2014. [30] P. Gijsman, R. Steenbakkers, C. Fürst, J. Kersjes, Differences in the flame retardant mechanism of melamine cyanurate in polyamide 6 and polyamide 66, Poly. Degrad. Stab. 78 (2002) 219–224. [31] B. Schartel, P. Pötschke, U. Knoll, M. Abdel-Goad, Fire behaviour of polyamide 6/multiwall carbon nanotube nanocomposites, Eur. Polym. J. 41 (2005) 1061–1070. [32] S.V, Levchik, A.I. Balabanovich, G.F. Levchik, L. Costa, Effect of melamine and its salts on combustion and thermal decomposition of polyamide 6, Fire Mater. 21 (1997) 75–83.

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Highlights

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Dripping during flammability testing of PA 6 materials is investigated in detail Mechanisms behind dripping in fire are explored using a set of PA6 materials Complex competition between the melamine cyanurate and glass fibres is discussed.

Conceptualization

B. Schartel, A. Turski Silva Diniz

Methodology

B. Schartel, A. Turski Silva Diniz, C. Huth

Investigation

A. Turski Silva Diniz, C. Huth

Resources

B. Schartel

Data Curation A. Turski Silva Diniz, C. Huth Writing - Original Draft A. Turski Silva Diniz Writing - Review & Editing

B. Schartel, A. Turski Silva Diniz

Visualization

A. Turski Silva Diniz

Supervision

B. Schartel

Project administration B. Schartel Funding acquisition

A. Turski Silva Diniz, B. Schartel

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: