Journal of Sc ienti fi e & Industri al Research

Vol.6 1. October 2002, pp 805-809

Flame Retardant Smoke Suppressant Coatings for PVC Sheathed Electric Cables

N K Saxena and Sunil K Sharma· Fi re Research Laboratory, Central Bui lding Research Institu te. Roorkee 247 667

Received: 04 April 2002; accepted: 04 June 2002

Flame retardant smoke suppressant intumescent coatings. based on phosphates, amides. polyo ls, propanediono complex of molybdenum. viny l acetate and viny l versatate copolymer emulsion are developed for polyvinyl chloride (PVC) sheathed elec­tri ca l cables. Nati onal and international standard procedures are followed to evaluate the fire performance of the coatings. The coated cab les do not show any surface spread o f fl ame on exposure and generati on of smoke is found to be well w ithin the limits, in accordance w ith the ASTM standards. The coatings are found quite effec ti ve in reducing the burning behav iour of power cab les. Signi ficant improvement in circuit failure time is noted in coated cable specimens.

Introduction Electrical cables are ex tensively used in all majo r

industri es. These are mos tl y run through underground cable tunnels, trenches, overhead cable trays and ducts. As the modern produc ti on faciliti es demand lo ng runs of power and control cables a ll over the pl ant area, a fire incidence in the vic inity of the cables may result in spread of fire a long the entire length o f cables laid in trays. Igniti on may also result due to short c ircuit or overheat­ing of cable itse lf. Any fire incidence to the cables causes extensive damage to the occupancy when it spreads a long the e lectrical cables and th rough these to o ther combu s­tible material s . Generation o f smoke during burning o f e lectrical cables creates major probl ems, both for fire contro l and fire fi ghting. Fire fi ghters find it difficult to approach the fire zone, thereby resulting in hi gher num­ber o f fire causalities . In order to prevent vertical or lateral spread of fire through cables, all apertures or open­ings, which are part o f vertical or hori zonta l partiti on e lements, through which cable or cable trays pass, should there fore be segregated . It is essenti al that these are sub­di vided into smalle r zones. Thi s may be do ne by provid­ing fire barrie rs at diffe rent inte rval s by sealing all the openings with the use o f material s that may result in pro­viding adequate fire res istance . Such barrie rs may not be appli cabl e fo r the ex isting e lec tric cabl e sys te ms. Rapid and unres tra ined propagation o f fire can al so be checked by appl ying an effec ti ve fire retardant coating on e lectri c cables. Applica ti o n of coating is also pos-

* Author for correspondence

sible in the existing cable systems. Various fire retar­dant coatings have been studi ed by different workers fo r finding their effi cacy of retardatio n of the burning char­acteri stics of cellulos ic materi als such as wood and wood­based products I -s Much attentio n has not been paid

towards fire protec tio n o f electric cables using suitable coatings. Therefore, there seems to be an ample scope to develop fire and smo ke re tardant coatings fo r e lectric cabl es.

There are two types of coatings, which retard the spread o f fire. One type of coatings, ca ' · .·d fire retardant non-intumesce nt coatings , use additi\ .s · ·• .. ;· s borax, boric acid, antimony trioxide, zinc oxide and chi Jr!nated compounds which do not support co mbustio n 1'

4• The

other type is called fire retardant intumescent coatings, whi ch, on heating produce residues, which are swelled by escaping gases. A combustion residue can be effi­ciently puffed up in order to produce a tough insulating foam o ver the surfaces to protect the material s 15 -

9. These

coatings perform better than simple fire reta rdant coat­ings. Thi s paper deals with the development and evalua­tion o f fire and smo ke suppressant intumescent coatings fo r e lectric cables .

Experimental Procedure

( i) Preparation of Coating A few formul atio ns were prepared by using di fferent

types of phosphate am ides, po lyo ls, propanedio no com­plex of molybdenum and binder in different weight ra­tios. Their compos itio ns are given in Table I . The coat-

806 J SCIIND RES VOL 6 1 OCTOBER 2002

Tab le ! - Compos itions of fire retardant Intumescent coati ngs

lngred iems Parts by weight for formulati on Numbers

2 3 4

Phosphate 140 12.0 15.5 17.0 Amide 12.0 10.0 9.0 11 .0 Pol yo I 10.0 12.0 14.0 8.0 Pigment 5.0 3.0 6.0 4.0 Thi ckener 0.30 0.30 0.24 0.24 Filler 8.0 9.0 6.0 10.0 Co-polymer emulsion 33 .0 35.5 :n.o :n.o Molybdenum compkx 2.0 2.0 2.0 2.0 ami foam ing. welling and 0.20 0.20 0.26 0.16 an ti se ttling agents Water 15.5 16.0 14.0 14.6

Tab le 2- Ph ysical Properties of Coat ings

Colour Odour Specific gra vity pH Flex ibility (As per IS: 10810 ) Solid bi nder ratio Volatile conten ts Covering capacity

Off white Faint odour 1.2 - 1.36

6- 8 o cracking observed

2.6- 2.8 30 - 32 per ccm 2.25 kg/m2

ings were prepared by mi xing fire retardant smoke sup­pressant ingredients of 325-400 mesh size with 2-5 per cen t soluti on of thickener along with appropri ate quan­tity of anti settling, wetting, and antifoaming agents. Vinyl acetate and vinyl versatate copo lymer emu ls io n (b inde r) was modifi ed by reactin g with dihydroxy­dimethylol ethylene urea and po lymeric plastici zer to increase the adhes ion to the PVC as well as to improve flex ibility of the coating. The required quantity o f this modified binder was added to the above contents to make homogenous mixture in order to obtain brush consistency by add ing enough water. It was st irred vigorously with a heavyduty stirrer for I h . The fire retardant smoke suppre. sant intumescent coating thus prepared is applied with brush on PVC insul ated e lectrica l cab les.

(ii) Evaluation of Coating The standard procedures were fo ll owed for determin­

ing physical properties such as colour, spec ifi c gravity, flexibility of coatings, and are recorded in Table 2.

The fire performance and smoke generat ion charac­teri sti cs were also studied, using standard procedures. In order to obta in effec ti ve fire reta rdancy the cab les

were coated with different amounts of coating to achi evt various coating thickness. The effect o f coating thick· ness on fire performance is also studied.

Determination of Circuit Failure Time (as per IEC-331 Method)

Three- and- a- half- core alu minum conductor PVC insul ated armored cable spec imens o f length 1200 mm and having an outer diam of 3 1 mm were used to dete r­mi ne the circuit fai lure time. The cables were coated with different amoun ts of coating to achi eve 1.0 mm to

3.0 mm dry coating thickness. 100 mm sheath of outer covering of the cable was removed fro m each end of the specimens. At o ne end of the cable the conductor wires were suitably connected for e lectrical connections and at the other end the exposed cores were spread apart to avoid contact with each other. The cable speci men was held hori zonta lly by means of suitab le clamps at each end of the sheathed portion. The mi dd le porti on of cable was supported by two metal rings , placed approximately 300 mm apart and a ll the metal part of the supporting apparatus were properly earthed. The cores of the cable under test were connected to separate phases fo r obtain ­ing three sets of connections to the three phases. Adja­cent conductors were connected to different phases. The test was caiTied out in a chamber provided with proper means for disposing of gases resulting from the burning cable. A Tubol type gas burner of 610 mm long was used to ig nite the specimen. A thermocouple was fixed . par­a ll e l to the burner and 75 mm above it to measure the temperature of the flame . The liquefied pe tro leum gas was used as a fuel. The a ir suppl y and fl ame he ight were so adjusted that a temperature of 750"C was obtai ned

throughout the test. As per IEC-33 110

, 440 V, 3-phase elec tric suppl y was connected to the cable specimen through a 3 A fuse in each phase and a 5 A fuse in the neu tral phase which was earthed. The cable rema ined energized with rated vo ltage throughout the fire test. The cable was then low­ered into the positio n so that it remained para llel to the burner and its lower surface was 75 mm above the burner. The eva luati on was started after ve rifying flame tem­perature of 750"C at the he ight of 75 mm above the burner. The flame and rated vo ltage were applied con­tinuo usly till no fai lure of 3 A fuse took place. A few spec imens were ex posed at 750"C for 20 min and the n re-ene rg ized test using rated voltage (as above) was car­ri ed o ut 12 h after to check the continuity of the e lectric

SAXENA & SHARMA: PVC SHEATHED ELECTRI C CABLES 807

Compositi on No.

2 3 4

Uncoated

Tab le 3- Effect of coa ting th ickness results as Per !EC - 33 1

Cable Coating diameter thickness (mm) (mm)

3 1 1.64 3 1 1.60 3 1 1.62 31 1.6 1 3 1 1.5 - 1.8 31 2.0 - 2.2 3 1 2.5-2.6 3 1 3.0 -3.2 3 1

Fai lure o f c ircuit time (mm )

12.60 11. 80 10.80 13 .90 14.7 2 1.5 23.0 27. 1 5. 1

Tab le 4 - Fire performance of compos it io n - 4 in re-energise test as per IEC-33 1

Cable eli am Coating Ti me o f Re-energ ise thi ckness (mm) exposure test results

(mm) (min)

3 1 1.5- 1.7 20 Fail 3 1 2.0 - 2 .2 20 Fail 3 1 2.2 - 2.5 20 Pass 3 1 2.5 -3.0 20 Pass 3 1 3.0 -3.5 20 Pass

cable. Observati ons, made during the evaluati o n with

di ffere nt four compos itio ns, are recorded in Table 3. Further the effect of coating thickness on fire perfor­mance was also studied but w ith compos iti on- 4 o nly. The results for the same are recorded in Tab les 3 and 4 .

Smoke Generation Test Tests were carri ed out in the NBS (National Bureau

of Standards) Smoke Density Chamber, as described in

ASTM E- 662 11• The PVC sheet of size 76 x 76 x 3 mm

was used to dete rmine the smoke generati on charac te r­istics of the deve loped coat ings . The PVC shee t was coated with fo ur diffe rent compositi ons of abo ut 2 mm dry thi ck ness of coat ing. The coated spec imen was mounted in spec imen ho lder fo llow ing standard proce­dure. The above specimen was placed to face the e lec­

trically heated radiant energy source. whi ch is mounted within an insulated ceramic tube, and positio ned so as to produce an irradi ance level of 2.5 w/cm 2 averaged over the central 38. 1 mm di am area of the vertical ly mounted specimen. A photometric system with a vertica l light path

Table 5- Smoke density results in NBS c hamber (Smoldering test co ndi tions)

Co mpositi on Values o f No. D / 90pcrc\!nt D<)(),. SoN v

'" 11'\;1\ .

I 32.0 18.0 2.2 14.6 3.8 2 35 .0 16.8 2 .8 16 .0 4. 7 3 34 .6 17.5 2.6 15.2 4 .35 4 3 1.2 18.8 2.0 14.1 3.6 Uncoated 63 2 2.4 127 986 301

• Lower value o f D, . Doos· SoN. V,, and hi gher va lue of t,x, per cent indicate better the performance of a materia l

was used to measure the varying light tran sm ittance as

smo ke accumulated. The light transmittance measure­

ments were used to calculate the specifi c opti cal density of smoke generated during the time period to reach the

max imum value. The tests were stopped when maximum

li ght transmiss io n was reached or after 20 min . The un­coated PVC sheet spec imen of same size was a lso evalu­ated under simil ar condition s. The fo ll ow ing paramete rs

were de term ined:

D Ill

/ 90 pcr

D l)()S SoN

v lll:I X

cen t =

= =

Maximum spec ific optical density

Time where upon 90 per cent of 0111

is reached (min)

Optical density at 90 s Sum of specific optical densities at I min, 2 min, 3 min and 4 min, re­

spective ly, a measure for the rate of smoke development Maximum rate of smoke genera­

tion estimated at every 30 s and ex pressed in de ns ity of s mok e/

minute (DJ min)

The resu lts are listed in Tabl e 5.

Results and Discussion The main const ituents of fire and smoke retardant in­

tumescent coatings under study are am ides, polyo ls phos­phates , and propanediono complexes of transition met­

a ls . The comp lex used in the present st ud y was propanediono complex of mo lybdenum . When this com­

bination is exposed to fire the phosphate decomposes to produce phosphoric acid, w hich acts as a dehydrating agent. The po lyo l is dehydrated by the ac id, formin g a

808 J SCI IND RES VOL 61 OCTOBER 2002

large amount of carbonaceous char that produces a non-combustible barrier to protect the substrate. Amide givesoff non-flammable gases causing the foamable carbonto produce a honeycomb blanket resulting in highly ef-fective insulation. The binder on softening forms an ex-pandable skin over the carbonaceous char to resist theescape of gases produced by amides. Smoke retardantsacted through increase in char formation as well asthrough change in pyrolyzates. The resulting speciesare abundant in CO and CO2 instead of volatile vapours,which help in accelerating the combustion process. Thisresults in slower burning and lower smoke generation.All these reactions take place within the coating, therebyprotecting the material from heat. The above mechanismmay be expressed by Eqs (1-4) 1.9.12

1o

Heat IX (NH4h HPO. --+ HOP - 0 - H + 2xNH3 + (x-l)H20

IoH x ... (1)

H

IoI

- Pv O H

IoH

IoIr-o H+RGbGI;QI-. Rm.m.Cl'O.lH:! + HIoH

x-Ix

... (2)

MoreR.CHz.CH20PO:JH2 --+ R.CH = CH2 + H,P04 •.• (3)

Heat

... (4)

6Heat I

X H3P04 -. HO - P - 0 - H + (x-I) H20IoH x

It is evident from the fire tests, that paint formula-tions under the discussion are very effective in reducingthe burning characteristics of electric cables. Circuit fail-ure time was also increased from 3.35 min to 13.9 minin coated specimens (Table 3). The studies have alsorevealed that the fire performance is also dependent upon

thickness of the coating. It is observed from Table 4 and5 that cable of 31 mm diam required coating thicknessof above 2.2 mm to pass re-energize test after 20 minexposure as per IEC-331. The specimen was consideredas 'Pass' in which no failure of any 3 A fuse occurredduring exposure and after 12 h, in re-energize test, cablealso withstood 3 phase 440V electric supply. Those speci-mens, which could not meet above criteria, were con-sidered as 'Fail'. The cable coated with about 2.2 mmdry coating thickness failed in re-energize test as it couldnot withstand rated voltage after 12 h. However, circuitfailure time was increased significantly in cable speci-men coated with different compositions to 1.6 mm coat-ing thickness tested as per IEC-331 (Table 3). The smokeretardants (based on pentanediono complex of molyb-denum) studied were also found quite effective in re-ducing the rate and amount of smoke formed. The valueof t90 was increased from 2.4 min to 18.8 min indi-per centeating that the smoke generation time was increased sig-nificantly on using these coatings. Further, it was alsofound that during the standard exposure the amount ofsmoke was noted in the range of 31.2 to 35 D m. whereasthe value for uncoated specimen was 632 D when evalu-m.ated as per ASTM E-662 (Table 5).

ConclusionA few fire retardant and smoke suppressant in-

tumescent coatings based on indigenously availablechemicals are developed which are found to be quiteeffective in reducing the flammability as well as smokegeneration characteristic of electric cables. Circuit fail-ure time is increased significantly in the coated cables.When rated voltage was given to coated cables after ex-posing them to standard fire conditions the cables werefound in workable condition. On exposure the coatingintumesises and provide a spongy cellular insulatingfoam that acts as an effective barrier to the conductionof heat. The formulations were also found quite effec-tive in reducing the production rate and amount of smoke.

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4 Dua A C, Fire retardant paints - effects of various additivesin their formulatons, Paint India, 32 (No.7) (1982) 3-5.

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SA XENA & SHARM A :PVC SHEATHED ELECTRI C CABLES 809

Bhatnagar V M & Yergnaud J M, Fire retard ant paints, Paint India, 33 (No.7 ) ( 1983) 15- 17.

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