Effects of Coatings on the Fire Performance of Plastics - American

Chapter 20

Effects of Coatings on the Fire Performance of Plastics Gordon L. Nelson

Downloaded by AUBURN UNIV on January 10, 2018 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0425.ch020

College of Science and Liberal Arts, Florida Institute of Technology, Melbourne, FL 32901-6988

Plastics used in electrical/electronic applications are frequently found painted in the final application. Data are reported from a study of the effects of EMI/RFI coatings on the fire test results of engineering plastics used for business machine housings. Tests included ignitability, flame spread, heat release, ease of extinction and smoke. Most EMI coatings decrease ignitability test results. Coatings tend to level diverse flame spread and ease of extinction performance. Coated samples have a tendency to show an increase in smoke formation under non-flaming conditions but are unremarkable under flaming conditions. Coatings can also positively interact with the substrate as shown with zinc on modified polyphyenylene oxide structural foam. 2 to 5 mil coatings, both metal and metal-filled organic, can significantly alter fire performance characteristics of engineering plastics. Plastics find extensive use for business machine housing, applications which require significant fire retardancy. Previous work on plastic enclosures has shown excellent correlation between small scale and large scale fire test performance when a range of small scale tests are utilized (1-4). However, plastics used in a variety of applications and particularly for business machine housings are increasingly found painted in the final application. Both decorative and functional coatings are used. Interiors of business machine enclosures are routinely coated with an EMI (Electromagnetic Interference) coating, such coatings to provide shielding for critical electronic components. Little data have been published on effects of the presence of these coatings on the fire performance of the finished enclosure composite. This paper reports comprehensive data from a study of the effects of EMI coatings on fire test results of engineering plastics used for business machine housings (5-6). 0097-6156/90/0425-0288$06.75/0 © 1990 American Chemical Society

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

20. NELSON

Effects of Coatings on the Fire Performance ofPlastics 289


Flammability Test Methods and Test Specimens In previous papers, fire test results on plastics using the Ohio State University Heat Release Calorimeter (ASTM E-906) and the ASTM E-162 Radiant Panel Test have been discussed. Results were presented on test reproducibility and on the material factors affecting results, including thickness, moisture, aging, and pigments. Test results have been presented for a variety of engineering plastics (7-8). A comparative study was also presented on OSU Heat Release Test results and Radiant Panel Test results versus large scale room corner tests and box tests (9). This latter study showed that while neither OSU Heat Release Calorimetry nor Radiant Panel testing gave quantitative correlation with Room Corner Test or Box Test fire performance, Radiant Panel \ values provided better qualitative correlation with Room Corner Test data and with Box Test data, and properly placed wood and plywood versus the plastic materials tested. Data also revealed a correlation of Maximum Rate of Heat Release (OSU) and Q (E-162), which suggested that flame spread test data in addition to heat release data are required to properly position the full scale fire performance of materials. Data also suggested that in evaluation of small scale test versus large scale or full scale performance of materials, it is essential that a significant number of materials in several forms be used and that tests representing several large scale scenarios be evaluated in order to insure the rigorousness and scope of correlation (9). Given the above observations it was essential in the present study that multiple test methods be used, representing evaluation of the effects of coatings on ignitability, flame spread, heat release, ease of extinction, and smoke. Samples should be commercially prepared and representative of materials commonly used in business machine applications. Commercial coated samples were obtained. Coating thickness was nominal 2 mils for organic-metal filled coatings (approximately 50% metal filler). Zinc metal coatings were zinc arc spray and were thicker, 5 mils, but normal for that process. Test results for each fire parameter are given as follows: * Ignitability - (IEC 695) IEC Needle Flame and Glow Wire Tests * Flame Spread - E162 Radiant Panel * Heat Release - E162 Radiant Panel * Ease of Extinction - D2863 Oxygen Index * Smoke - E662 NBS Smoke Chamber (optical) - D4100 - Arapahoe Smoke Chamber (gravimetric) All structural foam substrates were 1/4 inch in thickness except for RIM polyurethane, which was 1/2 inch in thickness. These comments hold for samples throughout the study. For solid plaques, samples were 3/32 inch thick. Needle Flame Test The Needle Flame Test is designed to simulate a small flame as might be encountered from a small electrical malfunction in an appliance or a piece of office equipment. The test apparatus consists of a hypodermic needle 35 mm in length with a bore of 0.5 mm and an outer diameter not exceeding 0.9 mm. The hypodermic needle has the tapered end cut off to avoid any interference with the flame. The gas used is butane. With the axis of the burner vertical,


290

FIRE AND POLYMERS


the gas supply is adjusted so that the length of the flame is 12 ± 1 mm. With the needle at 45° to a vertical test specimen, the sample is subjected to the flame for 30 seconds. The needle is kept 5 mm from the test sample. After the required time, the flame is removed and the after-burn time and the height of the after-burn flame recorded. Upon extinguishment the damaged area is recorded. Results are presented in Table I. While several coatings show longer burn times than their uncoated counterparts, most coated samples showed significantly smaller areas of damage than the corresponding uncoated sample. Zinc showed smaller damage areas on all substrates. Other coatings which showed small damage areas included nickel/polyurethane on polycarbonate and on modified polyphenylene oxide structural foam substrates, and copper/epoxy on polyurethane and on modified-polyphenylene oxide structural foam. Glow Wire Test The Glow-wire test is designed to give qualitative information describing the response of a sample to a glowing wire such as might be encountered in the malfunction of the wiring in a household appliance or a piece of office equipment. A glow wire apparatus was constructed per IEC 695-2-1, using 4mm Nickel/Chromium (80/20) wire. The apparatus used for the Glow-wire test, shown in Figure 1, consists of a carriage moving on a platform. The carriage holds the 3" χ 3" polymer sample which is moved horizontally towards the glowing Nickel/Chromium wire which can be heated to a temperature of 660 C and 96CPC. The polymer sample is subjected to the wire for thirty seconds with a force of 1.8 to 2.0 Newtons. In order to heat the wire to a temperature of 66CPC and 96CP C, the apparatus had to be connected to a Tieg welder. Using the Tieg welder, very high amperage at low voltages can be obtained. The amperage output can be increased until the desired temperature of the wire is obtained. The calibration of the heated wire at 66CPC is carried out by using a foil of 99.9 percent pure aluminum placed on the tip of the wire. For a temperature of 96CP C 99.8 percent pure silver foil is used. The foil of 99.8 percent pure silver melts at 96CPC. After the desired temperature is obtained the sample is then placed on the carriage and is subjected to the glowing wire for thirty seconds. When the required thirty second time increment is completed, the sample is pulled away from the heat source. At this time the height of the after burn, the after-burn time, and the penetration are recorded. The average damage area is then determined by measuring the height and width of the badly damaged area. Each type of sample is tested three times. Glow wire data are presented in Table II. Glow wire tests are particularly sensitive to coatings, with copper/epoxy and zinc coatings, for example, showing significant reduction in damage areas and after burn time for modifiedpolyphenylene oxide. Copper/acrylic on modified-polyphenylene oxide showed a large increase in both after-burn time and damage area in Glow Wire tests at 66CPC. Table III permits a cross comparison of Needle Flame and Glow Wire data for modified-polyphenylene oxide. Nickel/polyurethane, copper/epoxy and


20. NELSON

Effects of Coatings on the Fire Performance of Plastics

291

Table I. Needle-Flame Tests on Engineering Structural Foam Samples with EMI Coatings Substrate

Coating


Polycarbonate (white)

Polyurethane (RIM)

ModifiedPolyphenylene Oxide

Damage (mnr^)

After-Burn Time (s)

uncoated

0

380

Ni/polyurethane Cu/epoxy zinc

2 0 0

240 310 290

uncoated

1

500

Ni/polyurethane Cu/epoxy zinc

1 0 1

440 120 230

uncoated

7

680

14 1 1 2 0 18

110 290 120 380 590 710

Ni/polyurethane Cu/epoxy zinc nickel/acrylic graphite/acrylic copper/acrylic Averaee of determinations bv two workers.

Flame Height

Sample

G

l

o

w

.

W

i

r

e

Carriage

ι

Tensioning Cord

M-

Î-!—r —ι—r. !M » 1

Base Plate Weight

Figure 1.

I E C Glow Wire Test Apparatus


292

FIRE AND POLYMERS


Table II. Glow Wire Tests on Engineering Structural Foam Samples with EMI Coatings (30 second exposure) 960°C

660°C T (s) b

Damage (mm )

T (S) b

Polvcarbonate (white) Uncoated Ni/polyurethane Cu/epoxy Zinc thane (RIM) Uncoated Ni/polyurethane Zinc ModifiedPPO Uncoated Ni/polyurethane Cu/epoxy Zinc Ni/acrylic Graphite/ acrylic Cu/acrvilc

Damage (mm ) 2

2

0 0

170 120

0 0

1040 980

1 0

210 70

2 0

360 280

7 0

610 130

3 7

910 600

1

160

3

480

2 4

500 190

7 7

2070 660

0 0 0 0

70 20 300 100

4 0

260 140

13

1200

-

—

13

1100

8


1560

20. NELSON

Effects ojCoatings on theFirePerformance ofPlastics

zinc coatings show significantly smaller damage areas under all three ignitability conditions, while copper acrylic showed generally higher damage areas and after burn times. In a third ignitability test, UI..-94, all substrates were rated V-1/5V or better by UI^-94.


Radiant Panel Test The flammability of solids may be considered to be a function of the heat release rate and critical ignition energy of the material being studied. Flammability is an inverse function of the actual ignition energy of the material in question, and it is directly related to the rate of heat liberated after ignition of the sample. The Radiant Panel Test method is designed to measure both of these properties in a single test (8). The test specimen faces the heat source but at a 30P angle to it such that the upper portions of the specimen are severely exposed (Figure 2). Since irradiance varies along the length of the specimen, the time progress of ignition down the specimen serves to measure critical ignition energy down the sample (this is the F , or Flame Spread Factor). The stack and the associated thermocouples placed above the specimen serve as a heat-flux meter for measuring the rate of heat release. This is the Heat Evolution Factor, Q (Q correlates with Maximum Rate of Heat Release from E906). Thus, measurements are made of the position of the flame front on the exposed surface of the specimen as a function of time, and the maximum temperature rise of the stack thermocouples. These two measurements are combined to give the flame spread index \ (F χ Q = \ ) . In practice flammability behavior is influenced by size, geometry, orientation, and other sample parameters. Rather than a precise determination of physical fire parameters, an empirical index is chosen for the reporting of Radiant Panel data which employs asbestos hardboard with an index of 0 and the mathematics adjusted to give the ζ value for red oak at 100, this latter to provide some measure of agreement with ASTM E84. The instrument is calibrated in terms of heatfluxmeasurements, however, rather than continual use of red oak as the reference material. Table IV shows Radiant Panel Test results. The substrates tested alone have substantially different \ values. Polycarbonate (1/4 inch) structural foam has an \ of 27.5, modifiedpolyphenylene oxide (1/4 inch), 84.4, and RIM polyurethane (1/2 inch), 173.3. These \ values compare with 164.4 for 1/4 inch hardboard and 139.1 for 1/4 inch plywood. A comparison of graphite, nickel, and copper/acrylic coatings on polycarbonate and modified-polyphenylene oxide substrates illustrate a dramatic result. Despite a factor of 3 difference in substrate performance, the \ , Q and F values for the coated samples are very similar. The Q for the modifiedpolyphenylene oxide samples are 0.7 to 0.5 that of the uncoated sample. One would expect a similarity in F for the coated sample, but such a reduction in Q is dramatic. Both Q and F are determined by the 2 mil surface. Two of the zinc surface samples show results that might intuitively be expected for metal coated surfaces. The polycarbonate and RIM-polyurethane substrates show much lower F values with the zinc coating as expected, but Q values are very similar to that of the uncoated substrates. For modifiedpolyphenylene oxide, however, a very low Q value is obtained suggesting flame 8

8

s

s

s

s


293

294

FIRE AND POLYMERS

Table III. Needle-Flame vs Glow Wire Results Modified-PPO Structural Foam (Damage-mm ) 2


Coating

Needle Rame

Uncoated Ni/Pu Cu/Epoxy Ni/Acrylic Graphite/Acrylic Cu/Acrylic Zinc

690 110 290 380 590 710 120

Glow-Wire 660°C

Glow-Wire 9 6 0 ° C 2070 660 260

500 190 70 300 100 1200 20

Stack

—

1560 1100 140

Thermocouple

Temperature Rise dQ

Specimen

g?/

| Radiant —Heat Source

Figure 2. The Radiant Panel Test was designed to measure both critical ignition energy and rate of heat release. A sample is mounted facing a controlled heat flux but at a 3CP angle to it such that the upper part of the specimen is more severely exposed. Since irradiance decreases down the specimen, the time progress of ignition down the specimen serves to measure central ignition energy. Thermocouples in the stack above the specimen serve as a measure of heat release rate.


20. NELSON

Effects of Coatings on the FirePerformance oj'Plastics 295

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


Materials* Ρ ο I ν cartypngte Uncoated (white) Uncoated (grey) graphite/ acrylic (grey) nickel/ acrylic (grey) copper/ acrylic (grey) zinc (white) ModifiedPolvphenvlene Oxide uncoated graphite/ acrylic nickel/ acrylic copper/ acrylic zinc RIM-Polyurethane uncoated copper/ epoxy nickel/ urethane zinc

Replicates

l

s

Q

F,

4

27.5

14.8

1.87

1

31.5

16.8

1.88

4

59.8

19.6

3.08

5

48.0

15.9

3.33

1

85.1

26.0

3.27

4

14.4

14.3

1.35

6 4

84.4 64.3

30.6 15.6

2.77 3.99

4

68.1

20.0

3.36

4

63.0

22.1

2.84

4

2.9

2.9

1.00

4 3

173.3 78.3

28.5 28.3

6.07 2.76

3

17.7

8.9

1.97

4

43.4

24.0

1.81

51.7 33.8

3.21 4.12

Hardboard 39 164.4 Plywood 3 139.1 Instrument Constant β, was 23.7 to 25.4, Average 24.7

^Samples conditioned to ambient room temperature and humidity.


296

FIRE AND POLYMERS

retardant synergism of substrate and coating for this system. A low Q value is also obtained for the nickel/urethane coating on the RIM-polyurethane substrate. Coatings can significantly alter the fire performance properties of plastics. A 2-mil coating can reduce the \ value for a more flammable substrate by an order of magnitude while more flame retardant substrates can see a doubling of the \ value.


Oxygen Index Oxygen Index measures the ease of extinction of materials, the minimum percent of oxygen in an oxygen/nitrogen atmosphere that will just sustain combustion of a top ignited vertical test specimen (10). Three substrates were available-modified-polyphenylene oxide, polycarbonate and polystyrene. All materials were 1/4" by 1/2" by 5" coated on all sides. The oxygen indices of the base substrates varied from 23.6 to 32.1. Data are shown in Table V. Oxygen Index values are leveled for different substrates with the same coating. The oxygen indices of more flame retardant materials are reduced and that of less flame retardant materials elevated by the presence of a surface coating. Table VI presents data comparing Oxygen Index and Needle Flame test data. A qualitative correlation of OI and damage area is seen. Smoke Chamber Results Smoke evolution was measured using the National Bureau of Standards Smoke Chamber (ASTM E-662). In this test 3" by 3" samples are placed vertically in front of a radiant heat source (2.5 W/cm2). Non-flaming samples are tested using the heater alone. For flaming conditions, a six flamelet burner is positioned at the base of the sample and used in conjunction with the radiant heat source. Test results are obtained by measuring the percent transmission of a light beam which travels from the bottom of the chamber, through the accumulating smoke to the photomultiplier tube at the top of the chamber versus time. The resulting percent transmittance is then converted to specific optical density, D , which is calculated by using Equation 1, where D is the specific optical density, D = V(log(100/T))/LA (1) s

s

s

V is the chamber volume, L is the light beam path length, A is the sample area, and Τ is the transmittance. The EMI coatings that were tested were copper/acrylic, nickel/acrylic, graphite/acrylic, zinc, nickel/polyurethane and copper/epoxy. The coatings tested and their corresponding substrates are shown in Table VII. Uncoated samples were tested for references. All samples were conditioned in a humidity chamber for at least 24 hours at 7(f C and 50% R H prior to testing and were tested in both flaming and non-flaming modes. Test averages were based on three replicates. The tabulated results for all samples tested can be found in Tables VIII and IX, with Figures 3 and 4 depicting the results graphically. Non-flaming conditions produce the most dramatic results. For the polycarbonate substrates all coatings except zinc increase the amount of smoke produced. Zinc shows


20. NELSON

Effects of Coatings on the Fire Performance ofPlastics Table V. Oxygen Index Data for Structural Foam Samples Substrate

Coatings uncoated nickel/acrylic graphite/acrylic copper/acrylic nickel/urethane copper/epoxy Downloaded by AUBURN UNIV on January 10, 2018 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0425.ch020

Polycarbonate

Modified PPO

Polystyrene 23.6

32.1 31.2 32.4 27.5 30.5 31.0

26.5 28.3 23.9 24.6 29.0 30.6

— — — 27.7 29.3

Table VI. Comparison of Needle-Flame Test Results and Oxygen Indices for Modified Polyphenylene Oxide Structural Foam Samples Coating

Needle Rame Area (mm )

Ol

Damage

2

uncoated Ni/pdyurethane Cu/epoxy nickel/acrylic graphite/acrylic copper/acrylic zinc

684 108 286 379 592 708 120

26.5 29.0 30.6 28.3 23.9 24.6

—

Table VII. NBS Smoke Chamber Tests EMI Coating and Corresponding Substrate Coatina

Substrate Grey Polycarbonate

Copper/ acrylic graphite/ acrylic nickel/ acrylic zinc nickel/ urethane copper/ epoxy uncoated

White Polycarbonate

Polyurethane

Modified Polypheny! ene Oxide

-

-

-

X

X

-

-

X

X

-

-

X

-

X X

X X

X X

-

X

X

X

X

X

X

X


297

298

FIRE AND POLYMERS

Table VIII. NBS Smoke Chamber Data for EMI Coated Structural Foam Samples Average NonFlaming Data* Coating

D,

D /wt s

Sample Weight (g)

Weight burned (g)


Polycarbonate uncoated (white) nickel/ urethane copper/ epoxy zinc uncoated (grey) graphite/ acrylic nickel/ acrylic

63

1.80

34.5

11.1

175

4.90

35.9

10.7

132

3.70

35.4

10.7

67

1.86

36.1

9.3

84 187

2.50 6.30

33.5 33.9

0.7 3.6

216

6.60

32.7

1.7

45.8 55.5

10.7 13.3

RIM-Polyurethane uncoated nickel/ urethane copper/ epoxy zinc

>660 >660

—

>660

-

46.5

6.5

440

9.9

44.2

9.2

Modified-Polyphenylene uncoated nickel/ urethane copper/ epoxy zinc nickel/ acrylic graphite/ acrylic copper/ acrylic

Oxide

399 323

16.5 11.0

24.4 29.2

3.0 4.2

478

18.1

26.4

3.8

35 344

1.2 10.7

29.6 31.7

0.3 2.0

374

11.7

33.4

4.5

>660

-

33.2

3.9

* Average of three replicates


20. NELSON

Effects ofCoatings on the Fire Performance ofPlastics 299

Table IX. NBS Smoke Chamber Data EMI Coated Structural Foam Samples (Average Flaming Data*)

Coating

D

a

D /wt a

Sample weight (g)

Weight burned (g)


Polycarbonate uncoated (white) nickel/ urethane copper/ epoxy zinc uncoated (grey) graphite/ acrylic nickel/ acrylic

419

12.3

33.9

10.4

455

12.9

35.4

16.3

457

12.8

35.8

17.8

377

10.4

36.5

24.2

306 240

9.2 7.2

33.4 33.3

19.0 19.1

384

11.5

33.6

19.4

45.1 52.2

6.8 6.6

49.5

5.8

42.5

4.7

RIM-Polyurethane uncoated nickel/ urethane copper/ epoxy zinc

>660 >660 >660 >660

— — — Modified-Polyphenylene

uncoated nickel/ urethane copper/ epoxy zinc nickel/ acrylic copper/ acrylic graphite/ acrylic

>660 >660

Oxide 26.9 28.6

5.3 4.2

>660

—

27.0

3.7

>660 >660

— —

38.7 31.6

4.1 5.3

>660

—

35.2

5.7

>660

—

33.6

6.2

*Average of three replicates


300

FIRE A N D POLYMERS

cvj 6.00

£ 5.00 CO

g4.00|.
660 >660

— —

>660

— —

uncoated copper/ acrylic II

>660 >660

s

Modified-Polyphenylene

>660

— —

Sample Weight (g) Oxide 15.7 17.2

Weight Burned (g) 10.8 6.3

17.1

5.6

15.8

9.7

17.3 17.4

4.6 4.8

ABS

*Average of three replicates


306

CM

£

FIRE AND POLYMERS

6.00

É δ.οομ

ζ

CO

8

4.00

NICKEL/ACRYLIC I

660 @ seconds >660 @ seconds >606 @ seconds >606 @ seconds

2.4

X

10

2

3.4

X

10

2

2.0

X

10

2

1.0

X

10

2


20.

NELSON

Effects of Coatings on the Fire Performance ofPlastics


Conclusion These results show that thin coatings 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 for a more flammable substrate by an order of magnitude, while more flame retardant substrates can see a tripling of the \ value. Coatings can interact with the substrate as shown with zinc on modifiedpolyphenylene oxide structural foam. Examination of zinc on modifiedpolyphenylene oxide has been undertaken to ascertain the origin of the synergism and is reported separately. The presence of zinc shows an increase in char, and a decrease in low molecular weight volatiles at pyrolysis temperatures versus the uncoated substrate. In general, 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 unremarkable and specific coating dependent. Clearly 2 to 5 mil coatings can significantly alter fire performance. Proper evaluation and choice of coating forfireperformance is an important, and often missed, opportunity. Acknowledgments The work of the following students is gratefully acknowledged: Undergraduate Students W. Ronald Rose Kenneth L. Weaver Manual A. Bosarge Anthony N. Morris

Graduate Students J. Kent Newman Post-doctoral Fellow Eddie K.M. Chan

References 1.

2. 3. 4. 5.

6.

R.D. Bieniarz, Fire Experiments on Structural Foam Plastic Equipment Enclosures. Study for the Society of the Plastics Industry, Underwriters Laboratories, 1981. Bernie Miller, Small-Scale Tests are Respectable-Foam Flammability, Plastics World, 1981 (9), 78-81. G.L. Nelson, Combustibility of Structural Foam Plastics, J. Cellular Plastics. 18, 36 (1982). G.L. Nelson, Flame Tests for Structural Foam Parts, Plastics Technology, 23 No. 12, 88-92 (1977). G.L. Nelson, M.L. Bosarge, and K.M. Weaver, Proceedings of the Twelfth International Conference on Fire Safety, Part I, (Jan. 12-16,1987) 12, 271282 (1987). G.L. Nelson, M.L. Bosarge, K.M. Weaver, Effect of EMI Coatings on the Fire Performance of Plastics, Part II, Proceedings of the Thirteenth International Conference on Fire Safety, January 11-15, 1988, 13, 367-378 (1988).


309

310 7. 8.

9.

10.

FIRE AND POLYMERS A.L. Bridgman and G.L. Nelson, Heat Release Rate Calorimetry and Engineering Plastics, J. Fire and Flammability. 13 114 (1982). A.L. Bridgman and G.L. Nelson, Radiant Panel Tests on Plastics, Proceedings of the International Conference on Fire Safety, Jan. 17-20, 1983, 8, 191-226 (1983). A.L. Bridgman and G.L. Nelson, Heat Release Calorimetry and Radiant Panel Testing: A Comparative Study, Proceedings of the International Conference on Fire Safety (Jan. 13-17, 1986), 11, 128-139 (1986). G.L. Nelson and J.L. Webb, Oxygen Index, Flammability and Materials, in Advances in Fire Retardant Textiles. V.M. Bhatnagar, ed., (Technomic, Westport,CT) 271-370. November 20, 1989


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Effects of Coatings on the Fire Performance of Plastics - American

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