Synergistic Fire Performance between a Zinc Coating and a Modified

Chapter 21

Synergistic Fire Performance between a Zinc Coating and a Modified Poly(phenylene oxide) Substrate

Downloaded by CORNELL UNIV on September 19, 2016 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0425.ch021

Gordon L. Nelson and Eddie K. Chan College of Science and Liberal Arts, Florida Institute of Technology, Melbourne, FL 32901-6988

EMI coatings affect thefireperformance properties of engineering plastics. Zinc arc spray on modified-polyphenylene oxide is particularly effective. The enhanced fire retardancy has its origins in several factors assisted by the fact that zinc melts (420ºC) just at the early stage of the decomposition of m-PPO, allowing intimate contact with the charring substrate. As in pure polystyrene, char formation is enhanced in air in m-PPO, and this is further enhanced by the presence of zinc. As seen in SEM, zinc oxide which is part of the thermal stabilization package of mPPO is present at the charring surface enhanced by the zinc coating. It is observed that volatilization of small molecules is reduced for m-PPO with zinc present at temperatures under 700ºC, with preference for volatilization of the triaryl phosphate flame retardant, styrene trimer, and PPO dimers. The flame retardant and larger entities formed preferentially lead to enhanced char formation. Plastics find extensive use in business machine housings. Business machine housings are increasingly found painted in the final application. Both decorative and functional coatings are used. Interiors are routinely coated with an EMI (Electromagnetic Interference) coating, such coatings to provide shielding of critical electronic components. Comprehensive data have been provided in a separate report on the effects of EMI coatings on the fire performance of engineering plastics (1-3). Test results show that thin coatings (2-5 mils) can significantly effect the fire performance of plastic substrates. Most EMI coatings decrease ignitability test results. Coatings tend to level diverse flame spread and ease of extinction performance. A 2-mil coating can reduce the \ value in ASTM E162 Radiant 0097-6156/90/0425-0311$06.25/0 © 1990 American Chemical Society

Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

312

FIRE AND POLYMERS

Panel tests for a more flammable substrate by an order of magnitude, while more flame retardant substrates can see a tripling of the \ value. For NBS Smoke Chamber data, coated samples have a tendency to show an increase in smoke formation under non-flaming conditions. Smoke results under flaming conditions are coating specific and unremarkable. Of particular interest, coatings can interact with the substrate as shown with zinc arc spray on modified-polyphenylene oxide structural foam. While this synergism is evident in ignitability, flame spread, heat release, and smoke tests, the ASTM E162 Radiant Panel test results are of most interest. One would expect a metal coating (as opposed to an organic coating) to affect the F (flame spread) component of Radiant Panel data, but not the heat release portion. Since the substrate is the same, so should be the maximum rate of heat release. As shown in Table I, that is observed for both 5 mil zinc arc spray on RIM polyurethane and polycarbonate structural foam. On modifiedpolyphenylene oxide structural foam, however, a dramatic reduction of both heat release (Q) and flame spread (F ) are seen resulting in an \ l/30th of the uncoated substrate. The purpose of this paper is to discuss the origin of the synergism of zinc on modified-polyphenylene oxide. Modified-polyphenylene oxide (or ether) is a blend of high impact polystyrene (PS) and polyphenylene oxide (PPO), plus thermal stabilizers and a triarylphosphate flame retardant. Studies of the mechanism of the flame retardant in modified-polyphenylene oxide have shown some evidence for both solid phase and vapor phase inhibition (4). Indeed, one is always interested to know whether flame retardant action is on the solid or vapor phase. The charring process plays an important part in the combustion of many polymers; certainly so for modified-polyphenylene-oxide. Martel has shown that for PPO/PS alloys, T G A experiments yield 33% char in air versus only 1012% in nitrogen. PPO yields 40% char in nitrogen (5). It is known that polystyrene alone does not char under nitrogen, yielding monomer, benzene, toluene, and dimer and trimer (6). Polystyrene containing ammonium phosphate chars in T G A experiments carried out under oxygen but volatilizes without carbonization under nitrogen (7). When polystyrene is subjected to a small hydrogen flame a layer of carbon is formed on the polymer in contact with the flame (8). In detailed T G A / D T A studies of polystyrene, Martel (5) found a single endothermic peak corresponding to carbon-carbon scission at about 41CPC. In air, two exothermic peaks were observed at about 41CP C and 53CPC, corresponding to the charring process. If the D T A experiment was interrupted at 400-42CPC and cooled, the carbonaceous residue (10% of original) did not volatilize under nitrogen, but yielded a strong exothermic oxidation peak at 53CP C. A DTA experiment of char obtained in an oxygen index experiment gives the same exothermic peak at 500-55CP C. Therefore the peak at 53CP C is characteristic of the oxidation of char formed in earlier oxidative degradation. The charring process was strongly dependent on the oxygen concentration in the atmosphere down to 10% oxygen. The oxygen concentration, however, had little effect on the DTA exothermic peak at 41(f C. It was concluded that the charring process is dependent upon the competition between two reactions-the endothermic scission of carbon-carbon bonds with formation of monomer and volatilization, and an exothermic process which leads to the formation of a precursor to char. As the heating rate is reduced, volatilization is reduced in favor of the charring process. Char results from


s

s


21. NELSON & CHAN

313

Synergistic Fire Performance

formation of and subsequent transformation of olefinic bonds in the polymer chain, first to crosslinked or polycyclic structures, then to char.


Results and Di$çu$$iç>n In the present study DSC and T G A data were run on DuPont 910 and DuPont 951 instruments, respectively. Arapahoe Smoke Chamber results were obtained on a commercial apparatus. Coated samples used were commercially prepared zinc arc spray samples on Noryl® FN 215 Structural Foam. An initial experiment involved determination of Arapahoe Smoke Chamber results for samples with and without the zinc coating present. Data are presented in Table II. Depending upon orientation of the sample, an increase in char occurred for some samples with zinc present, while no change in smoke formation was seen. Initial pyrolysis GC/mass spectroscopy results at 90CPC in helium showed no difference in volatiles formed with or without zinc. These results suggested enhanced char formation as the origin of the Radiant Panel results for zinc on modified-polyphenylene oxide (m-PPO). Zinc oxide is a known, effective thermal stabilizer in the alloy. The next work then focused on DSC/TGA studies. DSC. DSC was used to study the thermal behavior of the decomposition of mPPO with and without a zinc coating, when heated in an air or nitrogen atmosphere. Different heating rates were used and varied from 2.5 C/min to 2CPC/min. In an inert atmosphere (nitrogen), the DSC trace at different heating rates for m-PPO are shown in Figure 1. The decomposition temperature (Td) increases with higher heating rate. Figure 2 shows the results of Td both in nitrogen and in air. The Td in air is a few degrees higher than in nitrogen. In air, m-PPO is slightly more stable to decomposition than in nitrogen due to the formation of char, which will be discussed later. Figure 3 shows the decomposition behavior of m-PPO in air and in nitrogen. m-PPO resin, when heated in air will absorb energy (endothermic) from 38CP C to 48CP C and evolve a variety of decomposition products. The shoulder between 450P C to 48CP C is char formation in the presence of oxygen. In air, above 48CPC, heat is evolved (exothermic) as char is decomposed. Figure 4 shows the heating curves of m-PPO with and a without zinc coating in nitrogen. As is shown, the Zn melted at about 42CP C (melting point of zinc by DSC is 420.7° C, during the early stages of m-PPO decomposition). The decomposition temperature of mPPO is almost the same with or without the zinc coating. Figure 5a and 5b show the m-PPO coated with Zn heated under different rates in nitrogen and in air, respectively. )

TGA. Thermogravimetric analysis measurements of m-PPO are shown in Figure 6. In air char and its decomposition is shown between 450 and 650PC. Figure 7a shows the T G A of m-PPO under different heating rates in nitrogen. In all cases, the m-PPO resin started to lose weight at about 19CPC and stopped at -35CPC, the weight loss is approximately 5% which is due to the loss of the triaryl phosphate. Decomposition temperatures were measured at the point of maximum by the derivative of the weight loss curve. Td at 5°C/min is 448° C, which increases to 471°C at a rate of 2CPC/min. A residue of -17% by weight is found at all three heating rates. In air, Figure 7b, the first part of the weight


314

FIRE AND POLYMERS

Table I. Radiant Panel Test Results for Structural Foam Samples Average Values


Materials*

Replicates

«s

Q

27.5 31.5 59.8 48.0 85.1 14.4

14.8 16.8 19.6 15.9 26.0 14.3

1.87 1.88 3.08 3.33 3.27 1.35

Polvcarbonate Uncoated (white) Uncoated (grey) graphite/acrylic(grey) nickel/acrylic (grey) copper/acrylic (grey) zinc (white)

4 1 4 5 1 4

M o d i f i e d PolvDhenvlene Oxide uncoated graphite/acrylic nickel/acrylic copper/acrylic zinc

6 4 4 4 4

84.4 64.3 68.1 63.0 2.9

30.6 15.6 20.0 22.1 2.9

2.77 3.99 3.36 2.84 1.00

4 3 3 4 39 3

173.3 78.3 17.7 43.4 164.4 139.1

28.5 28.3 8.9 24.0 51.7 33.8

6.07 2.76 1.97 1.81 3.21 4.12

RIM-Polyurethane uncoated copper/epoxy nickel/urethane zinc Hardboard Plywood

Instrument Constant 0, was 23.7 to 25.4, Average 24.7 *Samples conditioned to ambient room temperature and humidity.

Table II. Arapahoe Smoke Chamber Data for Modified-PPO and Zn

Sample

% Smoke*

% Char*

2.6 ±1.0

4.3 ±1.9

2.7 ± 0.8

5.1 ± 3.1

Experiment 1 Uncoated w/Zn Experiment 2 Uncoated w/Zn Experiment 3 Uncoated w/Zn

3.5 ±1.3

5.7 ± 2.4

4.6 ±1.5

11.8 ±3.6

3.1 ± 1.3

4.2 ± 1.2

2.7 ±0.5

3.8 ±1.7

of initial sample weight



21. NELSON &CHAN


Temperature (°C)

Figure 1.

DSC on m-PPO-Different Heating Rate, In K>


315

316


FIRE AND POLYMERS

Figure 2. Rate

Decomposition Temperature by DSC with Different Heating




NELSON & CHAN

Figure 3.

DSC on m-PPO-In Air and In Ng, lCPC/min


317


318 FIRE AND POLYMERS


NELSON & CHAN



21.


319


320 FIRE A N D POLYMERS


NELSON & CHAN



21.


321

322


FIRE AND POLYMERS

Figure 7.

T G A on m-PPO-Different Heating Rate (a) In ISl>; (b) In Air



21. NELSON & CHAN


323

loss is due to triaryl phosphate as well. The Td at 5°C/min is 431°C and the second decomposition between 475° C to 570PC is the formation of char in the presence of oxygen as shown in the DSC. Char formation is also found at heating at Iff C/min, and in both cases the residue is -6%. At 2CPC/min, the Td is 460PC and char decomposition is less evident. The residue is -18%. Figure 6 compares the T G A of m-PPO heated at 1CP C/min in air and in nitrogen. The Td's of both are about the same, 455° C. In air, the m-PPO coated with zinc yielded an additional weight loss feature between 50CPC to 60CPC which is the formation of additional char as shown in Figure 8. The zinc coating enhanced char formation and protected the substrate better. After 60CPC, oxidation of zinc to ZnO yields a gain of 3% by weight. The high weight percent remaining is due to a large amount of zinc in the tested sample. In Figure 9 are shown the decomposition curves of m-PPO and several coated m-PPO's. The m-PPO coated with zinc has the highest Td and highest residue. The zinc metal coating is 5 mils compared to the other commercial coated samples which were nominal 2 mils. The zinc coated sample shows added char, however, between 500-70CPC versus the other two EMI coatings. The decomposition temperature (Td) measured at maximum rate of heat loss from T G A curves was as follows: Sample

Td(°C)

m-PPO m-PPO/Zn m-PPO/Ni-polyurethane m-PPO/Cu-epoxy

471 475 460 463

modified-Polyphenylene oxide/zinc shows a marginally higher Td(°C). Clearly the metal filled organic coatings show lower Td values. Isothermal TGA. m-PPO and several coated m-PPO's were isothermally held at 40CP C for 90 min in nitrogen and air, Figure 10. In air, the weight loss is mainly in the first 20 min. Except for the copper/epoxy coating, the weight is almost the same after 30 min. In a nitrogen atmosphere, m-PPO without coating is the most stable one for the first 15 min. m-PPO with zinc coating is more stable in air than in nitrogen as char is formed in air. In nitrogen the metal filled organic coatings seem to promote degradation. Pvrolvsis-GC/MS. Pyrolysis-GC/MS were performed for m-PPO and m-PPO coated with zinc at 30CPC, 40CPC, 45CPC, 50CPC, 70CPC and 9O0PC in helium as shown in Figure 11. The object of this study was to see the effect of the different pyrolysis temperatures. At 30CPC, m-PPO started to release volatiles such as phenol, trimethyl phenol, 1,3-diphenylpropane, triphenyl phosphate, styrene, benzene, ethylbenzene and bibenzyl. For m-PPO with zinc, lesser amounts of low molecular weight volatiles are formed at low pyrolysis temperatures. At 40CPC, the volatiles start to be released for m-PPO coated with zinc. After -70CP C, the amounts of volatiles are the same with or without the zinc coating.


324


FIRE AND POLYMERS

Figure 9.

T G A on m-PPO and Coated m-PPO's, 2CP C/min, In H,




21. NELSON & CHAN


325

326

FIRE AND POLYMERS H

m-PPO

M


"m-PPO Coated With Zinc"

—-

1

3

5 7 9 11 13 15 17 19 21 23 25 27 29

11

3

5

7 9 11 13 15 17 19 21 23 25 27 29 e

Figure 11. Pyrolysis of M-PPO and M-PPO coated with Zn at (A) 300 C , (B) 400 -C, (C) 450 -C, (D) 500 C , and (F) 900 C e

#


21.

NELSON & CHAN

327



Char Analysis. Analyses of char samples were performed on specimens prepared at 2CP C/minute and held at temperature for 30 minutes. Below 55(f C carbonaceous char is present. Above 55CrC in air and above 6O0PC in nitrogen the residue consists of zinc, zinc oxide, glass and other inorganic species as shown in Table III. Scanning Electron Microscopy. SEM photographs were taken of samples in air and nitrogen, with and without a zinc coating present. Figure 12 shows the pyrolysis product in air at 200x magnification. Figure 13 shows the pyrolysis products in air (a) and (b) nitrogen at lOOOOx magnification. Figure 14 shows the pyrolysis products of a zinc coated sample in air (a) and (b) in nitrogen. Zinc oxide nodules are clearly seen in Figure 13 and 14 for all samples, but substantially more so for the zinc coated samples. Table III. Analysis of Carbon and Hydrogen Content of Char Samples for Modified Polyphenylene Oxide With and Without Zinc Coating

% Residue on Ignition

% Carbon on Residue

% of Original Weight as Carbon

% Hydrogen on Residue

m-PPO/Zn Nitrogen

4O0°C 500°C 550°C 600°C 700°C

96.17 37.82 28.79 31.17 28.53

59.63 42.25 23.13 7.19 0.54

57.3 38.2 6.66 2.24 0.15

4.85 2.07 1.06 0.35 0.08

Air

400°C 500°C 550°C 600°C 700°C

96.33 26.77 20.37 25.15 22.57

56.24 5.39 4.56 0.23 0.25

54.2 1.44 0.93 0.06 0.06

4.72 0.31 0.41 0.05 0.06

m-PPO Nitrogen

400°C 500°C 550°C 600°C 700°C

93.62 21.57 17.09 13.05 9.57

78.87 55.81 42.90 18.57 1.16

73.8 12.1 7.3 2.4 0.11

6.27 2.74 2.10 1.00 0.13

Air

400°C 500°C 550°C 600°C 700°C

93.88 10.09 5.72 5.16 5.20

80.23 30.30 12.43 0.66 0.52

75.3 3.1 0.71 0.03 0.03

6.45 1.12 0.65 0.17 0.09



328

FIRE AND POLYMERS

Figure 12. Scanning electron micrograph of m-PPO char pyrolysis in air (mag. 200x)

Conclusion The presence of a zinc coating on modified polyphenylene oxide shows results different from that expected for a "simple" metal coating. The enhanced fire retardancy has its origins in several factors assisted by the fact that zinc melts (420P C) just at the early stage of the decomposition of m-PPO, allowing intimate contact with the charring substrate. As in pure polystyrene, char formation is enhanced in air in m-PPO, and this is further enhanced by the presence of zinc. As seen in SEM, zinc oxide which is part of the thermal stabilization package of m-PPO is present at the charring surface enhanced by the zinc coating. It is observed that volatilization of small molecules is reduced for m-PPO with zinc present at temperatures under 70CP C, with preference for volatilization of the triaryl phosphate flame retardant, styrène trimer, and PPO dimers. The flame retardant and larger entities formed preferentially lead to enhanced char formation.



329


21. NELSON & CHAN

Figure 13. Scanning electron micrographs of m-PPO char (mag. 10,000x); (a) pyrolysis in Ng and (b) pyrolysis in air Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

FIRE AND POLYMERS


330

Figure 14. Scanning electron micrograph of m-PPO coated with Zn (mag. ΙΟ,ΟΟΟχ) (a) pyrolysis in air, (b) pyrolysis in N r



21. NELSON &CIIAN


331

Figure 14c. Scanning electron micrograph of m-PPO coated with Zn (mag. ΙΟ,ΟΟΟχ) pyrolysis in N y

References 1.

2.

3. 4.

5. 6. 7. 8.

G.L. Nelson, M.L. Bosarge, and K.M. Weaver, Proceedings of the Twelfth International Conference on Fire Safety, (Jan. 12-16, 1987) 12, 271-282 (1987). G.L. Nelson, M.L. Bosarge, K.M. Weaver, Proceedings of the Thirteenth International Conference on Fire Safety, (Jan. 11-15, 1988) 13, 367-378 (1988). G.L. Nelson, Effects of Coatings on the Fire Performance of Plastics, this book. J. Carnahan, W. Haaf, G . Nelson, G . Lee, V. Abolins and P. Shank, Proceedings of the 4th International Conference on Flammability and Safety, (Jan. 15-19, 1979) 4, 312-323 (1979). B. Martel, Journal of Applied Polymer Science. 35, 1213 (1988). H.A. Mackay, Carbon, 8, 517 (1970). K. Kishore and K. Mohandas, Combust. Flame, 43 145 (1981). F.R.S. Clark, J. Polym. Sci., Polym. Chem. Ed., 22 263 (1984).

RECEIVED November 20, 1989


Synergistic Fire Performance between a Zinc Coating and a Modified

Recommend Documents