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Nano-Al (OH)3 and Mg (OH)2 as flame retardants for polypropylene used on wires and cables

  • Witold Brostow
  • Sven Lohse
  • Xinyao Lu
  • Allison T. Osmanson
Original Article
  • 44 Downloads

Abstract

While materials sustainability is mostly considered in terms of their long-time aging, fires are more important—while polymers are flammable. We have developed flame retardants (FRs) for polypropylene (PP) with varied FR concentrations since PP is a very widely used polymer. Stearic acid (SA) has been added to increase filler–matrix interactions and filler dispersion in the matrix. Effects of filler types, particle size, and their concentrations on fire resistibility of different composite samples have been determined via limiting oxygen index tests and burning times. As expected, the oxygen index increases with increasing concentration of either FR. The highest value of the index is seen for 10 wt% 50 nm Al (OH)3 sample. Mixing the two hydroxides does not provide a synergy effect. The longest burning time is seen for 5 wt% 50 nm Al (OH)3 + 7 wt% SA, longer than the time for pure PP sample by 137%. Neat PP has a relatively high dynamic friction value, while addition of 5% SA results in the lowest friction value of all. The highest tensile modulus is seen for PP with 7.5 wt% 15 nm Al (OH)3 particle addition. Thermogravimetric analysis shows high thermal stability for PP with 5% each of Al (OH)3 and Mg (OH)2. Scanning electron microscopy shows that the surfaces of Al (OH)3-containing samples are slightly rougher than those with Mg (OH)2. Energy dispersive X-ray spectroscopy shows uniform distribution of both Al and Mg atoms in the composites.

Keywords

Polymer fire resistance Polypropylene Polymer flame retardant Oxygen index Burning time Nano-Al (OH)3 Nano-Mg (OH)2 Stearic acid 

1 Introduction

The number of studies on materials sustainability is rapidly increasing—but typically, these studies deal with materials aging. Verified statistics of fires in the USA are available for the year 2015: 1,345,500 fires; 15,700 injuries; 3280 deaths; financial losses of $ 14.3 billion. Clearly, this area of Materials Science and Engineering deserves not only attention but priority. Metals and ceramics do not burn, but polymers and polymer-based composites (PBCs) do.

Polypropylene (PP) is a widely used engineering polymer, largely because of its mechanical properties and also in composites such as those containing wood flour [1, 2, 3, 4, 5]. For decades, the usefulness of PP has been limited by its high flammability. This is a problem especially in the cable and wire industry where the Joule heating (ohmic heating, resistive heating) caused by the passage of an electric current constitutes a potential danger for polymeric coatings that prevent electrocution.

To solve this problem, the main approach is to use flame retardants (FRs) blended into PP to decrease the flammability. Reducing the inherent flammability of PP is normally achieved either by incorporating FR additives such as brominated compounds in combination with antimony trioxide, or by using intumescent formulations based on chlorine- or bromine-containing materials [6], or else using aluminum diethylphosphinate–based compositions [7].

Certain hydrated inorganic compounds such as metal hydroxides, in particular aluminum hydroxide (Al (OH)3), and magnesium hydroxide (Mg (OH)2), offer an attractive alternative because they are acid and halogen-free. Aluminum hydroxide is amphoteric; hence, it can be applied independently of the pH of the main component. In acidic media, it acts as a base forming a salt. In basic media, Al (OH)3 acts as a Lewis acid. Upon heating, Al (OH)3 decomposes at ≈ 180 °C absorbing large amounts of heat releasing water vapor. Magnesium hydroxide undergoes a similar endothermic decomposition at 332 °C. Also, here, water is released—what dilutes combustible gases. In addition to inhibiting polymer combustion, those metal hydroxides function as effective smoke suppressants [8]. Therefore, they should be of significant interest as FRs. The equations for Al (OH)3 and Mg (OH)2 acting as FRs are:
$$ \to 2\mathrm{A}1{\left(\mathrm{OH}\right)}_3\mathrm{A}{1}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O} $$
(1)
$$ \to \mathrm{Mg}{\left(\mathrm{OH}\right)}_2\mathrm{MgO}+{\mathrm{H}}_2\mathrm{O} $$
(2)

These are strong endothermic reactions with heat absorption values close to 2000 J/g; hence, they provide cooling to the polymer. The water vapor generated in those decomposition actions can dilute polymerization of flammable gas, inhibit combustion spread, while newly generated highly active Al2O3 can absorb smoke particles to obtain smoke-suppression effect. However, to be effective, high filler loadings are necessary, typically in excess of 60% by weight (37% by volume) when used in PP, resulting in a significant loss in composite toughness. Therefore, in order to decrease the loading level of FRs, we have decided to use nano-FRs. Our idea was, for a given FR concentration, we expected that nanoparticles will provide stronger effects than “large” particles. In the case of Al (OH)3, we have also used two different nanoparticle sizes. While going from ordinary particles to nanosize ones was expected to enhance the flame retardance, it was not known in advance what effects changing the nanoparticle size would bring about.

By definition, nano-FR particles have the size between 1 and 100 nm. A “generic” relationship between the oxygen index and nano-FR concentration and also between the oxygen index and the nano-FR particle size is shown in Fig. 1. The oxygen index represents the minimum percentage of oxygen in the test atmosphere that is required to marginally support combustion.
Fig. 1

Relationships of loading level of nano-FR and oxygen index (top) and particle sizes of nano-FR and oxygen index (bottom) [8]

We see in Fig. 1(top) that for FR concentrations not exceeding 20 wt%, we need to keep the oxygen index below 20%. We use FR concentrations between 2.5 and 10 wt%. In turn, Fig. 2(bottom) tells us that FR particle size should be larger than 10 nm to have sufficiently low oxygen index. Accordingly, we use FR particles with the sizes of 15 and 50 nm.
Fig. 2

Oxygen index results

A useful additive to PP seems to be stearic acid (SA), a saturated fatty acid with an 18-carbon chain. It is a waxy solid and its chemical formula is C17H35CO2H, its name comes from the Greek word στέαρ “stéar,” which means tallow. SA is one of the most popular saturated fatty acids in nature, following palmitic acid. SA can be applied as a low molecular additive with long aliphatic chains and is often used for coating of fillers in the production of PP composites [9, 10, 11, 12]. In addition, fatty acids are phase-changing materials, which can improve the thermal insulation of the PP films. Introduction of SA into fibers and wrapping their drops in a polymer matrix should enable the formation of fibers with better thermal properties without the use of microcapsules [13]. Thus, adding SA into PP films with FR might increase the flame retardancy of PP products. SA can also improve elongation at break of PP [14].

One of our objectives was to evaluate the difference of effects between nano-Al (OH)3 and nano-Mg (OH)2 FRs. The questions include effects of the loading level of FRs, particle size, and the presence of SA on the flammability and other properties of PP films used on wires and cables.

2 Experimental

2.1 Materials

Polypropylene (EP315J), bought from LyondellBasell, was wire and cable grade, with melt flow rate 2.6 g/10 min, density 0.9 g/cc, tensile strength 3200 psi, elongation at break 600%, and ductile-brittle impact transition temperature − 30 °C. Fifty nanometer particle size Al (OH)3 was bought from US Research Nanomaterials, Inc., with the purity 99.9%; 15 nm Al (OH)3 and 15 nm Mg (OH)2 bought from the same source had the purity 99.8%; stearic acid bought from Sigma has the purity ≥ 98.5% determined by capillary gas chromatography.

2.2 Sample preparation

Each nano-powder was stored and mixed with PP resin in a glove box, model 5503-11&550311 (Electro-tech Systems, Inc.) to protect the composites from moisture. Mixed powders were formed with the compositions shown in Table 1. The well-mixed powders were then poured into a Brabender (C.W. Brabender Instruments Inc. 105800), and then air cooled. The cooled and stabilized PP products were then poured into a pelletizer (Greiffenberger Antriebstechnik 4EK90SBX-2), respectively, to break down the particles into granules. These granules for each of the respective mixtures were then poured into an extruder (at Encore Wire Corporation, 1329 Millwood Road, McKinney, TX, 75069) and then films were formed. The films were finally cut into the desired dog-bone shapes.
Table 1

Components of PP samples blended with different fillers

Components

Sample

Control 1

1

2

3

4

PP

40phr

40phr

40phr

40phr

40phr

15 nm Al (OH)3

N/A

2.5 wt%

5 wt%

7.5 wt%

10 wt%

15 nm Mg (OH)2

N/A

N/A

N/A

N/A

N/A

50 nm Al (OH)3

N/A

N/A

N/A

N/A

N/A

Stearic acid

N/A

N/A

N/A

N/A

N/A

 

Sample

Components

Control 1

5

6

7

8

PP

40phr

40phr

40phr

40phr

40phr

15 nm Al (OH)3

N/A

N/A

N/A

N/A

N/A

15 nm Mg (OH)2

N/A

2.5 wt%

5 wt%

7.5 wt%

10 wt%

50 nm Al (OH)3

N/A

N/A

N/A

N/A

N/A

Stearic acid

N/A

N/A

N/A

N/A

N/A

 

Sample

Components

Control 1

9

10

11

12

PP

40phr

40phr

40phr

40phr

40phr

15 nm Al (OH)3

N/A

N/A

N/A

N/A

N/A

15 nm Mg (OH)2

N/A

N/A

N/A

N/A

N/A

50 nm Al (OH)3

N/A

2.5 wt%

5 wt%

7.5 wt%

10 wt%

Stearic acid

N/A

N/A

N/A

N/A

N/A

 

Sample

Components

Control 1

13

14

15

 

PP

40phr

40phr

40phr

40phr

 

15 nm Al (OH)3

N/A

N/A

N/A

N/A

 

15 nm Mg (OH)2

N/A

7.5 wt%

5 wt%

2.5 wt%

 

50 nm Al (OH)3

N/A

2.5 wt%

5 wt%

7.5 wt%

 

Stearic acid

N/A

N/A

N/A

N/A

 
 

Sample

Components

Control 2

16

17

18

 

PP

40phr

40phr

40phr

40phr

 

15 nm Al (OH)3

 

N/A

N/A

N/A

 

15 nm Mg (OH)2

 

5 wt%

5 wt%

5 wt%

 

50 nm Al (OH)3

 

N/A

N/A

N/A

 

Stearic acid

5 wt%

5 wt%

7 wt%

9 wt%

 
 

Sample

Components

Control 2

19

20

21

 

PP

40phr

40phr

40phr

40phr

 

15 nm Al (OH)3

 

N/A

N/A

N/A

 

15 nm Mg (OH)2

 

N/A

N/A

N/A

 

50 nm Al (OH)3

 

5 wt%

5 wt%

5 wt%

 

Stearic acid

5 wt%

5 wt%

7 wt%

9 wt%

 

2.3 Film characterization

Characterization and evaluation of the efficiency of the plasticizers in stretch polymer films for packaging were accomplished by determination of tensile behavior, oxygen index, burning time, thermogravimetric analysis (TGA), dynamic friction determination, surface morphology observation by scanning electron microscopy (SEM), and energy-dispersive X-ray spectrometry (EDX). We recall that our project is related to the cable and wire industry which uses PP and other polymers as films or coatings. The tests were conducted at 23 °C ± 2 °C and at 50% ± 5% relative humidity after conditioning the samples in the same conditions for at least 48 h.

2.4 Oxygen index analysis and flammability tests

According to the ASTM D2862 standard [15], samples were cut into strips 100 × 10 × 1 mm. The limiting oxygen index tester mode CSI-178 (CSI@Custom Scientific Instruments, Inc.) was used. Samples were burned in the same oxygen index tester in fresh air at room temperature.

2.5 Tensile properties

Tensile strength (TS), tensile elongation at break (εb), and Young’s modulus (E) of the films were determined at room temperature using a Mariana Tensile machine (TestWorks@4, USA) according to the ASTM D882 standard [16]. Films were cut into dogbone-shaped strips 10 × 5 × 1 mm (testing parts) and mounted between the corrugated tensile grips of the instrument. The initial grip spacing and cross-head speed were set at 50 mm and 0.1 cm/s, respectively. The tensile strength was expressed as the maximum force at break divided by the initial cross-sectional area of the film strip.

2.6 Thermogravimetric analysis

Thermogravimetric analysis (TGA) is important in our evaluation of the effects of stabilizers used in PP films. TGA was carried out in a Micromeritics TGA apparatus (Micromeritics Instruments Corp., USA) in N2 atmosphere (50 mL/min) at a heating rate of 20 °C/min. The samples were put into platinum pans and scanned from ambient temperature to 800 °C. After temperature attaching 800 °C, the temperature was kept stable for 1 min and then the sample cooled down to room temperature.

2.7 Dynamic friction analysis

Tribology is a very broad area that includes the studies of friction, lubrication, wear, adhesion, scratch resistance, and any interactions of multiple surfaces [17, 18, 19, 20]. Dynamic friction is an important indicator and it is determined by using a tribological tester produced by Nanovea Inc. in the “pin-on-disk” mode. As the name implies, a specimen is secured on a spinning disk and it is contacted with a stationary pin which is subjected to normal 5.0 N force while the machine is running. A SS302 stainless steel ball with 3.2 mm diameter was used as the pin. During the testing, the total sliding distance is 75.36 m (6000 revolutions and a track with 2 mm radius) and the spinning speed 200 revs/min.

2.8 Morphology of film surfaces (SEM)

Each of the prepared samples which had undergone different temperature treatments was examined under SEM (TM3030 Plus Tabletop Microscope from Copyright@ Hitachi High-technologies Corp. 2014). The microstructures on the surfaces of each of the treated samples were scanned to determine the influence of temperature on the microstructure.

2.9 Energy dispersive X-ray spectroscopy

PP samples with particles of Al (OH)3 and/or Mg (OH)2 were examined by using the energy dispersive X-ray spectroscopy (EDX) machine (Oxford Instruments, Version 3.2, x-stream-2) so that element distribution on the surfaces of those samples was determined.

3 Results and discussion

Almost all PP films (with Al (OH)3, Mg (OH)2, and stearic acid) produced by extrusion show white and homogenous surfaces, except PP films with 15 nm Al (OH)3 FRs, which show a white but rough surface. The chemical modification of the FR does not show a significant influence in film thickness compared to the PP resins without FRs.

3.1 Oxygen index values

Our results in Fig. 2 confirm the expected tendency noted in [8] and displayed in Fig. 1: FRs enhance the oxygen index.

We see in Fig. 2 that largely the oxygen index increases as the concentration of FRs increases. Thus, for PP plus Mg (OH)2 compositions that index goes symbatically with the magnesium hydroxide concentration. Similarly, the oxygen index of 5 wt% 15 nm Al (OH)3 samples is higher than 2.5 but lower than 7.5 and 10 wt% samples. Compositions containing 50 nm Al (OH)3 fire-retardant particles have higher oxygen index values that those with 15 nm aluminum hydroxide at the same hydroxide concentration. The highest value of the index is seen for 10 wt% 50 nm Al (OH)3 sample. Interestingly, the second highest oxygen index value is for the mixture of 2.5 wt% Mg (OH)2 with 7.5 wt% Al (OH)3.Thus, there is a limited synergy effect between the two hydroxides.

Adding SA seems to be a two-edged sword. Adding SA to pure PP increases the oxygen index. However, when we compare PP with 5% Mg (OH)2 containing in turn 7 and 9% SA, we find that the latter has a lower oxygen index.

3.2 Flammability analysis results

Figure 3 shows the results of burning time determinations. All samples were burned in fresh air and times were recorded at which samples had burned 50 mm.
Fig. 3

Burning times

The burning times increase as the loading of FRs increases. The longest burning time is seen for 5 wt% 50 nm Al (OH)3 + 7 wt% SA, longer than the time for pure PP sample by 137%. The second longest time is seen for 7.5 wt% Mg (OH)2 + 2.5 wt% 50 nm Al (OH)3, namely 80 s, longer than that for pure PP by about 125%.

3.3 Dynamic friction results

These results are displayed in Fig. 4.
Fig. 4

Dynamic friction values

As seen in Fig. 4, neat PP has a relatively high dynamic friction value. Addition of 5% SA results in the lowest friction value of all. The highest friction is seen for the PP with 2.5% 50 nm Al (OH)3 sample.

3.4 Tensile testing results

The Young modulus values are shown in Fig. 5 and elongations at break in Fig. 6.
Fig. 5

Young’s modulus results (MPa)

Fig. 6

Elongation at break (mm)

Consider first the results in Fig. 5. The highest modulus value is seen for 7.5 wt% 15 nm Al (OH)3. The second best result is for 5% Mg (OH)2 plus 9% SA. Addition of any amount of Mg (OH)2 alone to PP enhances the modulus; the higher hydroxide concentration, the better. Interestingly, low concentrations of 50 nm Al (OH)3 cause lowering of the modulus with respect to neat PP. Five or 7% of 15 nm Al (OH)3 particles cause significant lowering of the modulus with respect to neat PP. Thus, the size of the filler is important. Fifteen nanometer Al (OH)3 particles provide a clear reinforcement. Particles of the same filler with the large diameter of 50 nm apparently first weaken the cohesion of the PP matrix. Thus, the modulus at 7.5% aluminum hydroxide is lower than that of neat PP. Only at the high load of 10% of the hydroxide the modulus E exceeds that of the PP.

We recall that the tensile elongation at break εb is inversely proportional to brittleness B, namely B = 1/(E’.εb) where E’ is the storage modulus determined by dynamic mechanical analysis at 1.0 Hz [20]. In a series with Mg (OH)2, the elongation at break values passes through a minimum at 7.5% FR, with the values much lower than for neat PP; also, in this property, we see that the filler disrupts the cohesion of PP chains. In a series with 50 nm Al (OH)3, there is similarly a minimum at 7.5% FR; however, in this series, the values are comparable to that for the neat PP.

3.5 TGA results

Figures 7, 8, 9, 10, 11, 12, and 13 show TGA diagrams for our samples.
Fig. 7

TGA of PP with 2.5 wt% 15 nm Al (OH)3, 50 nm Al (OH)3, 15 nm Mg (OH)2 and neat PP

Fig. 8

TGA of PP with 5 wt% 15 nm Al (OH)3, 50 nm Al (OH)3, 15 nm Mg (OH)2, and neat PP

Fig. 9

TGA of PP with 7.5 wt% 15 nm Al (OH)3 (red), 50 nm Al (OH)3 (green), and 15 nm Mg (OH)2(blue)

Fig. 10

TGA of PP with 10 wt% 15 nm Al (OH)3, 50 nm Al (OH)3, 15 nm Mg (OH)2, and neat PP

Fig. 11

TGA of PP with 5 wt% 15 nm Mg (OH)2 + 5% SA, 5 wt% 15 nm Mg (OH)2 + 7 wt% SA, 5 wt% 15 nm Mg (OH)2 + 9 wt% SA, and PP + 5 wt% SA

Fig. 12

TGA of PP with 5 wt% 50 nm Al (OH)3 + 5 wt% SA, 5 wt% 50 nm Al (OH)3 + 7 wt% SA, 5 wt% 50 nm Al (OH)3 + 9wt% SA, and with 5% SA

Fig. 13

TGA of PP with 2.5 wt% 50 nm Al (OH)3 + 15 nm, 7.5 wt% Mg (OH)2, 5 wt% 50 nm Al (OH)3 + 5 wt% 15 nm Mg (OH)2, and 5 wt% 50 nm Al (OH)3 + 5 wt% 15 nm Mg (OH)2

The differences in thermal stability of our compositions are not dramatic. Comparison of Figs. 7, 8, and 9 with the following ones shows that the thermal stability of all PP films containing FRs is higher than that of pure PP, except 10 wt% 50 nm Al (OH)3 samples. A high thermal stability is seen in Fig. 13 for PP with 5% each of Al (OH)3 and Mg (OH)2.

3.6 SEM results

We show scanning electron micrographs for selected compositions in Figs. 14, 15, and 16. We compare here pairwise 5%, 7.5%, and 10% of Mg and Al hydroxides.
Fig. 14

SEM surface of PP with 5 wt% 15 nm Mg (OH)2 (left) and 5 wt% 15 nm Al (OH)3 (right)

Fig. 15

SEM surface of PP with 7.5 wt% 15 nm Mg (OH)2 (left) and 7.5 wt% 15 nm Al (OH)3 (right)

Fig. 16

SEM surface of PP with 10 wt% 15 nm Mg (OH)2 (left) and 10 wt% 15 nm Al (OH)3 (right)

We see that the surfaces of the two kinds of samples do not differ significantly—while the surfaces of Al (OH)3-containing samples are slightly rougher than those with Mg (OH)2. Several holes are visible on the surfaces of Al (OH)3-containing samples, while the hole size increases along with the Al (OH)3 concentration. The surface concentration of the holes is much lower in the Mg (OH)2-containing samples.

3.7 EDX results

In the following figures, we see spatial distribution of Mg and Al elements, also here pairwise for 5, 7.5, and 10% of the hydroxides.

We see in Figs. 17, 18, and 19 that the metal atoms are distributed uniformly within the samples—an important result.
Fig. 17

Distribution of Mg in PP with 5 wt% 15 nm Mg (OH)2 samples (left) and of Al in PP with 5 wt% 15 nm Al (OH)3 (right)

Fig. 18

Distribution of Mg in PP with 7.5 wt% 15 nm Mg (OH)2 samples (left) and Ali in 7.5 wt% 15 nm Al (OH)3 (right)

Fig. 19

Distribution of Mg in PP with 10 wt% 15 nm Mg (OH)2 samples (left) and Al in 10 wt% 15 nm Al (OH)3 (right)

4 Concluding remarks

We find that the hydroxides we have used provide significant effects as flame retardants for polypropylene. Ours is not an only option; Sterzynski and coworkers have used tetrasilanolphenyl silsesquioxane to lower flammability and to increase thermal stability of PP [21]. PP has a very large range of applications: insulations for electric cables, plastic “living” hinges resistant to fatigue, piping systems including those for potable water, plastic containers, cases for car batteries, pharmacy prescription bottles, ropes, carpets, rugs, mats, foams (called “expanded polypropylene”), and fibers for concrete reinforcement [22, 23]—while this list is by no means complete. The Sterzynski group has developed also a unique method of reinforcing low melting point PP with high strength PP fibers [24]. We also note a series of books on PP edited by Karger-Kocsis [25].

Expectedly, having studied such a variety of properties, we find that there is no “magic” composition. The lowest dynamic friction is seen for PP containing 5% SA—while this composition has a low oxygen index, a short burning time, and also fairly low tensile Young modulus.

For aluminum hydroxide containing PP samples, we have studied the effect of filler particle size. Figure 2 tells us that at each FR concentration, the oxygen index values for 15 nm diameter Al (OH)3 particles are lower than for PP containing 50 nm Al (OH)3. Thus, small FR particles provide less improvement.

To provide a broader perspective, we need to note some other options. Thus, Ryszkowska and her colleagues [26] achieved lower flammability of flexible polyurethane foams by inclusion of expandable graphite and a phosphorus-containing material.

As noted in the beginning of this article, there is a large variety of actions aimed at the improvement of polymer properties. Fatigue is mitigated by inclusion of reinforcements [27]. Impact strength in amorphous polymers is enhanced [28]. Improvement of thermal and mechanical properties is achieved by nanofillers such as carbon nanotubes [19, 29]. Plasticizers are added when needed [30]. Effects of such modifications on flammability of polymers and PBCs deserve attention.

To continue with a wider perspective on the results reported above, we recall work on fire fighter gloves [31], on high temperature ablation of composites under plasma jet impact [32] and work of a Lithuanian-Spanish-US team on slabs for fire doors [33]. While high temperatures are the common denominator, means to assure sufficient service at high temperatures are different in each case.

Notes

Acknowledgements

Some financial as well as technical support for this project has been provided by the Encore Wire Corp., McKinney, TX, and are gratefully acknowledged. Several colleagues have provided useful comments on our work and on this manuscript including Ray H. Pahler and Andrew Hull. Anonymous reviewer comments have been taken into consideration and are gratefully acknowledged.

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Copyright information

© Springer Nature Switzerland AG 2018

Authors and Affiliations

  • Witold Brostow
    • 1
  • Sven Lohse
    • 1
  • Xinyao Lu
    • 1
  • Allison T. Osmanson
    • 1
  1. 1.Laboratory of Advanced Polymers and Optimized Materials (LAPOM), Department of Materials Science and EngineeringUniversity of North TexasDentonUSA

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