Fire Retardancy of Polypropylene- Metal Hydroxide Nanocomposites

Chapter 6

Fire Retardancy of Polypropylene- Metal Hydroxide Nanocomposites

Downloaded by COLUMBIA UNIV on March 14, 2013 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch006

Jinguo Zhang and Charles A. Wilkie Department of Chemistry, Marquette University, PO Box 1881, Milwaukee, WI 53201

The combination of metal hydroxides (aluminum or magnesium) with an organically-modified clay has been studied as a potential fire retardant system for polypropylene. The combination of polypropylene with 5% inorganic clay and 20% of the metal hydroxide gives an 80% reduction in the peak heat release rate, which is the same as what is obtained when 40% of the hydroxide is used alone. This means that more polymer can be used, which could be an advantage in some situations.

Introduction Aluminium trihydrate (ΑΤΗ ) and magnesium hydroxide ( M D H ) are wellknown fire retardants for polypropylene (1); they are attractive because of their low price and good performance. The limitation of ΑΤΗ and M D H is that high loadings are required to achieve good fire retardant performance; the normal loading is at least 40%, and the typical loading is 60% (2). Such a high loading will cause significant degradation in mechanical properties. On the other hand, much has been heard recently about polymer-clay nanocomposites, which show greatly enhanced mechanical as well as fire and barrier properties (3). Beyer (4) © 2006 American Chemical Society

In Fire and Polymers IV; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

61

62


showed that with ethylene vinyl acetate copolymers, EVA, one could replace 1820% ATH with 5% organically-modified clay and maintain the same peak heat release rate, PHRR. In other words, the amount of polymer that is present can be increased by 13 to 15%. Normally one must use maleic anhydride as a compatabilizer for polypropylene, which could again degrade the mechanical properties. Recent work from this laboratory has shown that a new oligomerically-modified clay can be melt blended with polypropylene to give intercalated and delaminated nanocomposites (5,6). This work is based on this discovery and herein we examine the combination of polypropylene with ATH and MDH and an oligomerically-modified clay.

Experimental Materials. Aluminum trihydrate, A1 0 3H 0, OL-107/LE, and magnesium hydroxide, Mg(OH) , H7, which are uncoated materials, were obtained from Martinswerke Gmbh, a company of Albemarle Corporation. The majority of the other chemicals used in this study, including isotactic polypropylene (melt index 230°C/2.16 Kg 4g/10min) and solvents, were obtained from the Aldrich Chemical Company. The oligomerically-modified clay, COPS, was synthesized according to the published procedure (5). The inorganic clay content in COPS clay is 30%; it contains about 70% of the surfactant. Throughout the paper, the loading of inorganic clay is referenced; the loadings of oligomerically-modified clay that have been used are 3%, 10% and 17%, which corresponds to 1%, 3% and 5% inorganic clay loading. Polypropylene, COPS clay and ATH or MDH were pre- mixed in a beaker, then blended in Brabender Plasticorder at 180 °C for 10 min at 60 rpm. The mixture then was removed from the mixer and cut into pieces. A Leistriz 18 mm co-rotating twin screw extruder, L:D ratio = 40:1, was used at a feed rate of 2 Kg/hr and a screw speed of400 rpm. The utilization of ATH with polypropylene is limited industrially because the usual mixing temperature is above the temperature at which ATH will undergo thermal degradation. Instrumentation. X-ray diffraction was performed on a Rigaku Geiger Flex two -circle powder diffractometer; generator tension was 50 kV at a current of 20 mA. Scans were taken at 20 = 1.0 - 10 at a 0.1 step. Cone calorimetry was performed on an Atlas CONE-2 according to ASTM E 1354-92 at an incident flux of 50 kW/m using a cone shaped heater; exhaust flow was set at 24 L/s. Cone samples were prepared by compression molding the sample into 100 x 100 x 3mm square plaques. Typical results from cone calorimetry are considered to be reproducible to ±10% (7). Thermogravimetric analysis (TGA) was performed on a SDT 2000 machine at 15mg scale under a flowing nitrogen 2

3

2

2

2


63 atmosphere at a scan rate of 20 °C/min. Temperature are reproducible to ± 3°C, while the error on the fraction of non- volatile materials is ± 2%. Tensile properties are obtained on Reliance RT/5 ( MTS ) at 5mm/min crosshead speed; the reported values are based on the average of 5 determinations.

Results and Discussion


Characterization of nanocomposite formation by X-ray diffraction The formation of a nanocomposite is accompanied by an increase in the gallery spacing, which can be evaluated using X-ray diffraction, XRD. The compositions that have been studied as well as the 20 values and the corresponding d-spacing are recorded in Table I while the actual XRD traces are shown in Figure 1. It is obvious that COPS clay itself has a large d-spacing and this is maintained in the polypropylene nanocomposites, but there is no increase in the d-spacing. The same results have been previously reported for COPSpolypropylene nanocomposites and the TEM data suggests that the clay is welldispersed and that the system is partially delaminated (5).

Table I. XRD data for COPS clay and its PP nanocomposites

pp

ATH

MDH

0 63 70 77 63

0 20 20 20

— — — —

—

20

COPS

26

d-spacing, nm

100 17 10 3 17

2.1 2.2 2.0

4.2 4.0 4.4

2.1

4.2

—

—

Themrogravimetric analysis Both a surface treated and an untreated form of ATH and MDH have been studied; the TGA curves, which show no difference between the treated and untreated samples, are shown in Figure 2 while the data is reported in Table II. The data consists of the temperatures at which 10%, T .i, which is considered to be the onset of the degradation, and 20%, T .2, which is another measure of 0

0



Two Theta

Figure 1. X-ray diffraction pattern of COPS clay and its PP nanocomposites


65


100

200

300

400

500

600

Temperature (°C)

Figure 2. TGA curves of different metal hydroxides

Table II. TGA results for metal hydroxides

ATH MDH

To.i(°CJ

To. (°C)

282 404

301 425

2

Char at600°C (%) 67 70

thermal stability, of the mass has been lost as well as the fraction of non-volatile which remains at 600 °C. (8) It is clear that MDH has a much higher thermal stability than does ATH. Thermogravimetric analysis of the combinations of polypropylene with the metal hydroxides, without the addition of clay, have been studied and the TGA curves are shown in Figures 3 and 4 while these results are tabulated in Table III. It is clear that ATH is less stable than the polymer so the degradation of these blends commences at lower temperature, while the higher stability of MDH means that these degrade at higher temperature. The fraction of non-volatiles that remains at 600 °C is what is expected based upon the amount of metal hydroxide that is present.



66


Table III. TGA data of PP and its metal hydroxides composites pp

ATH

MDH

—

— — —


100 80 60 80 60

20 40

— —

20 40

To., (°Q 436 384 320 446 442

To.5 (°Q All 460 455 490 499

Char at 600 °C (%) 0 13 27 15 28

Upon the addition of clay to the polypropylene-ATH system, the onset temperature of the degradation increases by 40 °C while the temperature at which 50% degradation occurs is either slightly increased or unaffected and the fraction of non-volatile residue is unchanged. Apparently the addition of clay has a very positive effect on the degradation process; the TGA curves are shown in Figures 5 and 6 and the results are tabulated in Table IV.

120!

200

300

400

500

600

Temperature (°C) Figure 5. TGA curves of PP -ATH nanocomposites



68

200

300

400

500

600

TemperaturefC)

Figure 6. TGA curve ofPP- MDH nanocomposites

Table IV. TGA data of PP - metal hydroxides nanocomposites PP 100 77 70 63 63

ATH

MDH

COPS

—

— — — —

—

20 20 20 20

17

3 10 17

—

To., (°Q 436 428 432 425 435

Tas

472 472 462 464 476

Char at 600 °C (%) 0 14 16 18 18

Cone calorimetry The fire properties of these nanocomposites were evaluated using the cone calorimeter, which enables the measurement of the time to ignition (t ), the heat release rate curve and especially its peak value (PHRR), the amount of smoke evolved, known as the specific extinction area (SEA), the mass loss rate (MLR) and the total heat released (THR). The usual observations for nanocomposites are that there is a significant reduction in PHRR and in time to ignition v.hiie the total heat released is unchanged. This means that the nanocomposites are actually easier to burn than the virgin polymer and the shape of the heat release curve is changed but all of the polymer is eventually burned. The purpose of this work was to see if a combination of a metal hydroxide with a clay would have ign


69


the same effect in polypropylene that Beyer found in EVA, significant reduction in PHRR at lower loading of the metal hydroxide when clay is present (4). The results for the polypropylene-ATH system are presented in Table V and the heat release rate curves are shown in Figure 6. The combination of 63% polypropylene, 20% ATH and 17% COPS clay (5% inorganic clay) gives about the same value for PHRR and THR that is obtained when 60% polypropylene and 40% ATH are combined. The advantage of the clay-containing composition is that there is approximately 15% more polymer present in the composition, which would be expected to improve the mechanical properties.

Table V. Cone calorimeter data for polypropylene and ATH and their nanocomposites. 0

PP ATH COPS t °, s ig

100 80 60 77 70 63

— — — 20 — 40 20 20 20

3 10 17

26±4 27±3 28±2 21±1 20±2 18±1

2

PHRR Kw/m (% reduction) 1967 ±50 817±40 (59) 467+5(76) 677±38(66) 592±18(70) 536±16 (73)

SEA" 2

(m /kz) 584±20 681±32 6771147 839±17 1037±39 1143±18

MLR" (z/sm ) 29.7±0.3 15.4+1.6 8. 6±0.1 16.4±0.7 14.2±0.1 12.6±0.6 2

THR" (MJ/m ) 112±9 90±2 70±6 84±6 77±4 74±1 2

a

t , time to ignition; PHRR, peak heat release rate; SEA, specific extinction area, a measure of smoke; MLR, mass loss rate; THR, total heat released. ign

For the MDH system, only 5% inorganic clay loading was used in combination with MDH as a fire retardant in polypropylene and the results are quite similar to those seen with ATH. The combination of 20% MDH, 17% oligomerically-modified clay (5% inorganic content) and 63% polypropylene gives a very similar value for PHRR and THR but there is a substantial increase in smoke, no doubt due to the presence of styrene in the clay. This system, like that with ATH, has the advantage of an increased organic content which may lead to enhanced mechanical. It is not possible to melt blend samples that contain more than 40% MDH in the Brabender mixer so these samples were mixed in a twin screw extruder and the results are shown in Table 7 while the heat release rate curves are shown in Figure 9. The best reduction in PHRR, 86%, is obtained for a system that contains 40% PP and 60% MDH and this also shows the best reduction in the total heat released and in mass loss rate. If one replaces 10% of the MDH with COPS clay, the results are almost as good. This corresponds to about 3% inorganic clay, which is a reasonable amount, so it is surprising that the results are not more encouraging.


1000 PP/ATH 80/20 PP/ATH/COPS 77/20/3 PP/ATH/COPS 70/20/10 PP/ATH/COPS 63/20/17 PP/ATH 60/40

800 600 X


400 200 0 0

50

100

150

200

250

300

350

Time (s)

Figure 7. Comparison of the heat release rate (HRR) plots for virgin polypropylene and polypropylene with A TH and their nanocomposites at SOKW/m heat flux. 2

Table VI. Cone calorimeter data for PP and its MDH composites 2

PP MDH COPS t " s tgm

100 80 60 63

— 20 40 20

— ~ — 17

26±4 31±1 34±1 24±1

PHRR,"Kw/m (% reduction) 1967 ±50 1000±50(49) 433121(78) 476±20(76)

SEA" (m /kg) 584±20 664±39 668±33 1123±30 2

MLR* THR* (g/sm ) (MJ/m ) 29.7±0.3 112±9 19.4±1.1 98±1 8.9±1.8 75±5 13.5±0.4 70±3 2

a

2




71

Time (s)

Figure 8. Comparison of the heat release rate (HRR) plots for virgin polypropylene and combinations with MDH and oligomerically-modified clay at 50KW/m heat flux. 2



72

Tlme(s)

Figure 9. Heat release rate curves for PP-MDH-COPS clay combinations.

Table VII. Cone calorimeter data for PP and its MDH composites 2

PP MDH COPS t

a lgn

100 60 40 60 50 40

0 40 60 30 40 50

0 0 0 10 10 10

s

30±5 3±1 29±3 24±3 23±2 22±4

PHRR," Kw/m (% reduction) 16841155 377116 (78) 228112(86) 471115(72) 385±9(77) 304±16

SEA" (m /kg) 427±36 529±91 529±91 764±27 757±14 765±58 2

MLR° THR° (g/sm ) (MJ/m ) 34.5±3.5 89±6 11.5±0.2 71±1 51±1 8.4±0.7 13.6±0.3 80±1 11.7±0.3 69±2 10.0±0.4 59±3 2

2

a


Tensile properties In addition to fire properties, tensile properties were also evaluated. The peak stress, modulus and strain at break are listed in Tables IX and X for various systems. The substitution of 20% of the metal hydroxides with 17% COPS causes little change in modulus and there is a slight increase in the strain at break for the ATH system which is not present for MDH.


73 Table VIII. Comparison of the tensile properties of PP -ATH composites


DD

ATTJ

Peak stress

Modulus

Strain at break

PP

ATH

COPS

80

20

—

30.3

4.2

18.5

60

40

—

25.9

6.0

0.8

63

20

17

25.9

4.8

1.3

(

M

p

Q

)

(

Q

p

a

)

(

%

)

Table IX. Comparison of the tensile troperties of PP -MDH composites dd

i / n u i^^no

Peak stress (Mpa)

Modulus (Gpa)

Strain at break (%)

80

20

0

26.4

4.8

5.9

60

40

0

22.6

5.8

1.8

63

20

17

26.9

5.7

0.8

Conclusion Polypropylene nanocomposites can be formed by melt blending the polymer with metal hydroxides and COPS clay. The combination of 20% Al(OH) or Mg(OH) with 5% inorganic clay in polypropylene gives an 80% reduction in PHRR, which is the same reduction that is obtained when 40% Al(OH) or Mg(OH) is used. In the absence of clay, the ATH-containing polymer undergoes degradation at a lower temperature than in the presence of the clay. There is some interaction between the components. Further work is required to opimize the system, but it appears likely that one can devise a fire retardant polypropylene system by the use of metal hydroxides and nanocomposite formation. 3

3

2

2


74

References 1. 2. 3.


4. 5. 6. 7.

8.

Walter, M. D. Recent Advances in Flame Retardancy of Polymeric Materials 1998, 9, 274-285 Snyder, C. A. Plastics Compounding 1985, 8, 41-3, 45,4 Zhu, J; Morgan, A. B.; Lamelas, F. J.; Wilkie, C. A. Chem Mater 2001, 13, 3774-3780. Beyer, G. Fire Mater., 2002, 25, 193-197. Su, S.; Jiang, D. D.; Wilkie, C. A. Polym. Deg Stab 2004, 83, 321-331. Su, S.; Jiang, D. D.; Wilkie, C. A. Polym. Deg. Stab 2004, 83, 333-346 Gilman, J. W.; Kashiwagi, T.; Nyden, M.; Brown, J. E. T.; Jackson, C.L.; Lomakin, S.; Gianellis, E.P.; Manias, E. in Chemistry and Technology of Polymer Additives; Al-Maliaka, S.; Golovoy, A.; Wilkie, C.A., Eds.; Blackwell Scientific: London, 1999; pp. 249-265. J. Zhu, J.; Start, P.; Mauritz, K.A.; Wilkie, C.A.; J. Polym. Sci., Part A: Polym. Chem., 2002, 40, 1498-1503.


Fire Retardancy of Polypropylene- Metal Hydroxide Nanocomposites

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