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Flame Retardant Smoke Suppressant Protection for Poly Vinylchloride



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Spread of fire along electrical cables may be alleviated by using flame-retardant smoke suppressant (FRSS) additives-based PVC compositions designed for use as insulation and sheathing. These compositions offer improved resistance to ignition, flame
  Fire Technology, 40, 385–398, 2004 C   2004 Kluwer Academic Publishers. Manufactured in The United States. Flame Retardant Smoke SuppressantProtection for Poly Vinylchloride Sunil K. Sharma and N. K. Saxena, Scientists, Fire Research Laboratory,Central Building Research Institute, Roorkee, 247667   Abstract.  Spread of fire along electrical cables may be alleviated by using flame-retardantsmoke suppressant (FRSS) additives-based PVC compositions designed for use as insulationand sheathing. These compositions offer improved resistance to ignition, flame spread andsmoke generation. FRSS protection may also be achieved by the application of intumescentcoatings as fire-protective layers and barriers for electrical cables and cable installations.Metal-based organic (MBO) complexes were synthesized for use as flame retardant smokesuppressantsinPVCformulations.PlasticizedPVCcompositionswerepreparedandevaluatedfor their fire performance. Excellent smoke suppression was observed when either of the twoMBOs was used. The limiting oxygen index was also found to be fairly high, particularlywhen the PVC samples were plasticized with a phosphate plasticizer. Flame-retardant smoke-suppressant intumescent coatings were developed for PVC-sheathed electrical cables. Thecoatings were applied on cables, and their fire performance was evaluated by using nationaland international standard procedures. The coated cables did not show any surface spread of flame on exposure and their generation of smoke was found to be very low. The coatings werefound to be quite effective in reducing the burning behaviour of power cables. Significantimprovement in circuit failure time was noted for coated cable specimens. Keywords:  flameretardant,smokesuppressant,metalbasedorganiccomplex,intumescent,cablecoating,circuitfailure time, smoke generation Introduction Fire is an unexpected event which is often frightening as well as dramatic. It can lead toloss of life and expensive damage to property and equipment. PVC has long been the basematerial of choice for wires and cables. Large quantities of cables are being used becauseof the modern trends in construction and the growing need for electronic communication aswell as for industrial applications. Modern production facilities demand long runs of powerandcontrolcablesallovertheplantarea.Anyfireincidenctsinthevicinityofthecablesmayresult in the initiation of fire at the cable surface and flame spread along it. Fortunately ithas been shown that PVC is among the safer materials in a fire scenario. This is so basicallybecause the chlorine content of the polymer renders it self-extinguishing. When exposed toheat, PVC emits hydrogen chloride into the gaseous phase and forms a cross-linked char inthesolidphase.HClreactswiththeenergy-richoxygen-basedchainbranchingfreeradicalsin the gaseous phase which propagate the flame, and thus it acts as a chain terminator. Thechar, on the other hand, insulates parts of the solid phase, more remote from the fire source,from heat spread and reduces oxygen penetration. International attention was drawn toward  386 Fire Technology Fourth Quarter 2004 the problem of fire performance of cables after a major conflagration that burned down theLa Spezia power station in Italy in 1967. One lesson learned from this event was that highcable densities with their larger fuel load of polymeric insulation and sheathing requiredmuch more protection than mere PVC.PVC has been combined with a variety of additives to improve upon the performance of severalpropertiesnecessaryforwireandcablesheathings.However,manyoftheseadditivesresult in a compromise on fire performance of PVC. Rapid and unrestrained propagationof fire can be checked either by the addition of flame-retardant smoke-suppressant (FRSS)additives during the preparation of PVC sheets or by the application of an effective fire-retardantcoatingonPVCsheathedcables.ManyFRSSsystemsforPVChavebeenreportedin the literature [1–3]. These can be classified as fillers, additives and intumescent coatings.For an effective FRSS formulation only small amounts of additives are required. They aregenerally non polymeric compounding ingredients, typically used at levels below 20% of the polymer mass. Essentially all of the reported smoke-suppressant additives for PVC areeither metals or metal complexes. Fillers, on the other hand, are used in large quantities andmay result in unfavorable mechanical properties. While FRSS additives can be used onlyduring polymer processing, coatings can be applied to the existing cable systems.Basically there are two types of coatings that retard the spread of fire. One, the fire retar-dant non intumescent coating, uses additives such as borax, boric acid, antimony trioxide,zinc oxide and chlorinated compounds that do not support combustion [1, 4–6]. The othertype is called a fire-retardant intumescent coating, which on heating produces residues thatare swelled by escaping gases. A combustion residue is expanded up in order to produce atough insulating foam over the surfaces to protect the materials [4, 7–11]. These coatingsperform better than the non intumescent ones. Various fire retardant coatings have beenstudied by different workers in order to find their efficacy for retardation of the burning of cellulosic materials such as wood and wood-based products [1, 4–10]. However, sufficientattention has not been paid toward the use of suitable coatings to impart fire protection toelectric cables.This paper deals with the development and evaluation of (a) fire-retardant smoke-suppressant metal based organic (MBO) complexes used as FRSS additives for PVC sheetsand (b) FRSS intumescent coatings for PVC-sheathed electrical cables. Experimental Procedure  Preparation of PVC Specimens VirginPVCresinhavinglessthan0.1ppmofvinylchloridemonomerwasused.Specimenswere prepared by mixing virgin resin with different ingredients, including heat stabilizersand lubricants such as Ba-Cd stabilizer, dibutyl tin dilaurate and stearic acid. Pentanedionecomplexes of molybdenum and chromium (FRSS additives) were synthesized using 2,4pentanedione (acetylacetone) [12]. Here the hydrogen of the hydroxyl group of the enolform of the 1,3 diketone was replaced by metal, resulting in the formation of a chelate ring.Ingredients were used in different quantities depending on the design of the experiments(Table1).Drychemicalsweremixedthoroughlyinadryblender.Liquidssuchaslubricants,stabilizers and plasticizers were then added slowly and the mixing continued till a uniformconsistency of the mix was obtained in the form of a free flowing powder. The dry mix  Flame Retardant Smoke Suppressant Protection 387  TABLE 1 Recipe and Processing Conditions for Plasticized Poly(Vinyl Chloride) Samples Ingredients Control Sample Modified Sample Poly (vinyl chloride) resin 100 phr 100 phrPlasticizer 30–90 phr 30–90 phrProcessing aids 2 phr 2 phrFRSS 0 phr 0–3 phr Processing Conditions Dry blending at room temperature for 20 minutesMastication on two-roll mill at 165 ◦ C for 10 minutesCompression molding at 175 ◦ C ∗ Phr: parts per hundred parts of resin. thus obtained was masticated on an electrically heated two-roll mill at 140–150 ◦ C for 5 to10 minutes depending on the formulation. Masticated mix was molded into sheets of 300 × 150 mm using a compression molding machine at 175 ◦ C, followed by slow cooling.  Preparation of FRSS Coatings Afewformulationsofflameretardantintumescentcoatingswerepreparedbyusingdifferenttypes of phosphate amides, polyols, propanediono complex of molybdenum and binder indifferentweightratios.TheircompositionsaregiveninTable2.Thecoatingswerepreparedby mixing fire retardant smoke suppressant ingredients of 325–400 mesh size with 2–5percent solution of thickener along with appropriate quantity of antisettling, wetting andanti-foaming agents. Vinyl acetate and vinyl versatate copolymer emulsion (binder) wasmodified by reacting with dihydroxydimethylol ethylene urea (DHDMEU) and polymericplasticizer to increase the adhesion to the PVC as well as to improve flexibility of the TABLE 2 Compositions of Fire Retardant Intumescent Coatings Parts by Weight for Formulation NumbersIngredients 1 2 3 4 Phosphate 14.0 12.0 15.5 17.0Amide 12.0 10.0 9.0 11.0Polyol 10.0 12.0 14.0 8.0Pigment 5.0 3.0 6.0 4.0Thickener 0.30 0.30 0.24 0.24Filler 8.0 9.0 6.0 10.0Co-polymer Emulsion 33.0 35.5 33.0 33.0Molybdenum Complex 2.0 2.0 2.0 2.0Antifoaming, Wetting & 0.20 0.20 0.26 0.16Antisettling agentsWater 15.5 16.0 14.0 14.6  388 Fire Technology Fourth Quarter 2004 coating. The required quantity of this modified binder was added to the above contents tomake homogenous mixture in order to obtain brush consistency by adding enough water. Itwasstirredvigorouslywithaheavy-dutystirrerforonehour.DifferentthicknessesofFRSSintumescent coating thus prepared were applied with brush on PVC insulated electricalcables. Performance Evaluation  Evaluation of PVC Sheet TheeffectofFRSSadditivesonthecombustionofpolymerwasstudiedintermsofflamma-bility, smoke generation and char formation and generation of combustion gases.  Limiting Oxygen Index.  Flammability was studied using the Limiting Oxygen Index (LOI)test as per ASTM D 2863 [13]. LOI is the value at which the material just supports flamingcombustion. The minimum concentration of oxygen in a mixture of oxygen and nitrogenflowing upward in a test column that will just support combustion is determined underequilibrium conditions of candle like burning. The limiting oxygen index of a material isdetermined as:LOI = 100 × [O 2 ][O 2 ] + [N 2 ](1)where [] is the volume concentration of each gas in the air stream. A Stanton Redcroft FTAflammability unit was used to determine the oxygen index of different compositions. PVCsheet of size 52 × 140 × 5 mm was used to determine the limiting oxygen index. Specimenwasclampedinthespecimenholderandmountedverticallyinthecenterofthetestcolumn.A tentative initial oxygen concentration was selected to start the set of experiments. Theentire top of the specimen was ignited with the help of propane gas. Ignition flame was thenremoved and the timer started to record the duration of specimen burning. The modifiedand the control specimen of the PVC sheet were thus evaluated under the same conditionsusing the same set up.The objective of the test was to determine the minimum oxygen concentration that justallowed the specimen to continue burning either for three minutes or for at least 75 mmlength of the material. The oxygen concentration may be on the higher side and needs to bereduced if the specimen continues to burn beyond 75 mm or three minutes. Also it mightbe on the lower side and needs to be raised if the burning of specimen extinguished beforemeeting either of the criteria. The optimum value of oxygen concentration in the gaseousmixture, flowing through the column is termed as limiting oxygen concentration. SmokeSuppressionIndex.  ThesmokegenerationandhencetheeffectofFRSSadditivesonsmoke suppression was estimated in terms of a smoke suppression index (SSI—a dimen-sionless quantity) using a dynamic method employing LOI apparatus for the combustion of test specimen [14]. Specimen was burnt for three minutes at an oxygen concentration thatwas2%higherthantheLOIvalueofthatspecimen.Thesmokeevolvedwasallowedtoflowthrough a duct. A He-Ne laser source and receiver were employed to measure the density  Flame Retardant Smoke Suppressant Protection 389 of smoke flowing through the duct. Efficacy of smoke suppressant systems was calculatedin terms of the Smoke Suppression Index (SSI) as follows:SSI = S  − S  t  S  × 100 (2)where  S   = area under the time vs. smoke density curve for the untreated sample, and  S  t   = area under the time vs. smoke density curve for the treated sample.Since the initial weight as well as the weight loss for each sample may not be identical,  A  (Area under the time vs. smoke density curve for the untreated sample/weight loss of thesample) and  A t   (Area under the time vs. smoke density curve for the treated sample/weightloss of the sample) may be used in place of   S   and  S  t  . Positive values of SSI (i.e.  S   >  S  t  or  A  >  A t  ) indicate reduction in amount of smoke generated whereas a negative valuerepresents an increase. CharFormation.  Thecharformationwasassessedusingtheoxygenindexapparatus.Sam-plesmodifiedbytheFRSSadditiveswhichgavethebestresultsintermsofflameretardanceand smoke suppression were exposed to burning conditions; to produce char; in an LOIapparatus. The duration of exposure was three minutes and the oxygen concentration em-ployedwas2%higherthantheLOIvalueforthatspecimen.Awiregaugewasplacedabout15 mm below the specimen for collection of char. The specimen and char were carefullyweighed before and after burning. The backbone char percentage (BC%) was computed asdescribed by Kroenke [15]. CombustionGases.  Combustiongasesi.e.carbonmonoxideandcarbondioxideweremea-sured by using infra red spectroscopic technique. Hydrocarbons were measured by em-ploying a flame ionization detector. The concentrations reported in Tables 3 and 4 are peak concentrations over a three-minute period (test duration) and are over and above the baseline i.e. the concentrations observed in case of control samples.Since the maximum effectiveness of MBO additives in suppressing smoke emission isobtained at relatively small concentrations it was decided to use pentanediono complexesof molybdenum (VI) and chromium (III) up to 3 parts per hundred parts of resin (phr)loading. Samples were prepared as described earlier, using both the phthalate (dioctylphthalate—DOP) and the phosphate (tri cresyl phosphate—TCP) plasticizer at 50 phr. Asample containing 0.0 phr of the additives was used as the control or the reference samplefor computation of smoke suppression index (SSI).  Evaluation of Coating The fire retardant intumescent coatings prepared as above were applied on PVC insulatedelectriccablesofdifferentdiameters.Inordertoobtaineffectivefireretardancytheeffectof coatingthicknessonfireperformancewasstudiedbyemployingdifferentamountofcoatingtoachievedifferentcoatingthickness.Standardprocedureswerefollowedfordeterminationofphysicalpropertiessuchascolour,specificgravity,flexibilityofcoatingswhicharegivenin Table 5. Fire performance of the coatings was also studied by evaluating circuit failuretime and smoke generation characteristics using different standard procedures.
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