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 DOI: 10.1177/073490418500300604

1985 3: 432Journal of Fire SciencesRonald E. Myers, E. Douglas Dickens, Jr, Eugene Licursi and Robert E. Evans

Thermoplastic PolyurethanesAmmonium Pentaborate: an Intumescent Flame Retardant for

  

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AMMONIUM PENTABORATE: ANINTUMESCENT FLAMERETARDANT FORTHERMOPLASTICPOLYURETHANESRonald E. Myers, E. Douglas Dickens, Jr., Eugene Licursi andRobert E. Evans

BFGoodrich Research and Development Center9921 Brecksville RoadBrecksville, Ohio 44141

(Received July 8, 1985)(Revised December 6, 1985)

ABSTRACT

Ammonium pentaborate (APB) is shown to be an effective intumescent, char-forming additive for thermoplastic polyurethanes (TPU). As little as 5 to 10parts of APB, added to a flame retarded TPU, provides a 7 to 10-fold improve-ment in burn-through resistance. The APB/TPU char is characterized by itsglassy, multicellular structure which provides virtually instantaneous thermalprotection to heat-sensitive substrates and is able to resist thermo-oxidativedegradation.

In contrast to its behavior in polyurethanes, APB is found to be somewhatless effective as an intumescent additive for several non-urethane polymers suchas natural rubber, polyester resin, polyamide and polyvinyl chloride. Amechanism is proposed to account for the unique nature of the APB/TPUsystem. It is suggested that, via a series of chemical reactions, APB redirectsthe thermal decomposition of the polyurethane thereby producing less volatilesand increased char.

INTRODUCTION

INTUMESCENT TECHNOLOGY [1], ALTHOUGH NOT A NEW AREA OFscience, has only recently been utilized in commercial flame retardantformulations for polymers [2,3]. Typically consisting of multi-component formulations (spumific, catalyst, carbonific), many in-tumescent systems contain one or more organic-based components; a

JOURNAL OF FIRE SCIENCES, VOL. 3-NOVEMBER/DECEMBER 1985

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detrimental aspect in that carbonaceous chars oftentimes lack struc-tural integrity and may easily be destroyed by mechanical and/or ther-mal stress encountered during a fully developed fire.

In a previous paper [4], we described several inorganic-based, non-halogenated, intumescent char-forming additives. Low meltingphosphate-sulfate glasses, borate-carbonate mixtures and ammoniumpentaborate were shown to provide enhanced thermal protection (burn-through resistance) for various organic polymers. One of these ad-ditives, ammonium pentaborate (APB) was shown to be particularly ef-fective in Estane thermoplastic polyurethanes. APB, when added to anon-flame retardant Estane at levels > 33 weight percent, generates in-tumescent chars capable of withstanding high temperature exposure forprolonged periods; thermal protection times of 7-10 minutes weremeasured for a 0.18 cm thick Estane/APB coating exposed to &dquo;-i800°C.As an intumescent flame retardant additive, APB is of interest for

several reasons. It is a readily available, low density, non-halogenatedinorganic compound. APB has recently been the subject of increased in-terest due to suggested improvements relating to its manufacture [5].Upon thermal decomposition, APB evolves substantial quantities of

water vapor and ammonia.

In the process, a boric oxide glass is formed. In other words, APB func-tions as both an inorganic blowing agent and a glass-forming com-pound. Collectively, these features contribute to the overall effec-tiveness of APB. Figure 1 schematically depicts the intumescentbehavior of APB, showing how it can be used to protect a heat-sensitivesubstrate.

Figure 1. Intumescent activity of APB.

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In spite of its excellent intumescent activity, APB does not functionas a primary flame retardant for thermoplastic polyurethanes [4]. Self-extinguishing formulations are possible, but only when APB is used atrather high levels (~ 50 parts APB per 100 parts of polymer). At suchhigh loadings, APB is incompatible with conventional, polyurethaneprocessing techniques. In the present study, we describe the thermalbarrier performance of conventional, flame retarded, thermoplastic poly-urethanes containing 5-25 parts of APB. The intumescent activity ofAPB in several non-urethane polymers is also reported.

EXPERIMENTAL

Materials

Ammonium pentaborate tetrahydrate (powdered grade, U.S. Borax)was heated to 150 °C in an air circulating oven for 1 hour (typical weightloss of 10-15% due to partial dehydration of the pentaborate) in order toimprove its processability.A low melting sulfate glass (25 mole % K2S04, 25 mole % Na2S04, 50

mole % ZnS04) was prepared according to Kroenke’s procedure [6]. Thephosphate-sulfate glass has been described elsewhere [4].Melamine orthophosphate, C3N6H6 . H3P04 . H20, was prepared by

mixing equimolar proportions of melamine and H3P04[7].All other materials were used as received, to include: Al203 . 3H20

(Solem Industries), CaC03 (Georgia Marble), BaS04 (C. K. Williams Co.),HAF carbon black (J. M. Huber Corp.) and ammonium polyphosphate(Phos-Chek@ P-30, Monsanto). Zinc borate was either Firebrake@ ZB(U.S. Borax) or ZBX511 (Humphrey Chemical).

Polymer Formulations and Processing Conditions

Simple polymer recipes were used in this study in order to test the per-formance of potential flame retardant additives and as such, the for-mulations described herein are not suited for most commercial applica-tions.

Thermoplastic Polyurethane

A UL 94 V-O thermoplastic polyurethane resin (Estane° 58202 black,BFGoodrich Chemical Co.) was dried at 70 °C, four (4) hours in vacuoprior to processing. Processing conditions were similar to those de-scribed elsewhere [4].

Rigid Poly(vinyl chloride)

100 parts Geon® 103EP Resin (The BFGoodrich Company)

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2 parts Microthene 510, polyethylene lubricant2 parts dibutyltin thioglycolate stabilizer

Plasticized Poly(vinyl chloride)

100 parts Geon° 103EP Resin40 parts DOP plasticizer, (di-2-ethylhexylphthalate)2 parts Microthene 5102 parts barium/cadmium liquid stabilizer

Processing conditions for both types of PVC recipes have been de-scribed elsewhere [4].

Polyamide

Nylon 6/6 pellets (density 1.09, Aldrich Chemical Co.) were cryo-genically ground at -196 °C using a SPEX freezer/mill. Flame retardantadditives were then mixed into the powdered nylon and the mixture wasagain cryogenically ground. The resulting composition was weighed outin 0.3 gram batches and pellets were pressed using a plunger mold ( m35MPa). These pellets (typical dimensions, 0.6 cm diameter x 0.6 cmheight) were then used for Smoke-Char testing.

Natural Rubber

100 parts natural rubber (sp. gr. 0.93)40 parts HAF carbon black5 parts zinc oxide3 parts stearic acid1 part antioxidant

2.5 parts sulfur0.6 parts accelerator

The rubber masterbatch (all components except the accelerator, sulfurand flame retardant additives) was Banbury mixed at N65 °C, 4minutes. The accelerator, sulfur and flame retardant additives were in-corporated during cold milling (-37 OC) and the resulting samples werecured in the press (140 °C/1380 MPa/30 minutes).

Polyester Resin

26.0 ml AROPOLTM WEP® 662-P (Ashland Chemicals)0.25 ml Lupersol DSW (Pennwalt-Lucidol)

The flame retardant additive was hand mixed with the water ex-tendible polyester (WEP° ) resin. The methyl ethyl ketone peroxide

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catalyst (Lupersol DSW) was then added to the mixture, hand stirredfor 15 seconds and poured into a Teflon@ mold (dimensions, 5.1 cm x 5.1cm x 0.19 cm). Samples were cured for 24 hours at 22 °C, followed by 2hours at 80 °C in an air circulating oven.

Flammability Testing

The Torch Test (described in detail in the Appendix) is a thermalpenetration test which provides information relating to the thermalstability and insulative value of the char which forms when a verticallypositioned sample is exposed to a propane torch flame. Time to failure,tf, is defined as the time required for the thermocouple at the rear of thesample to reach 250 °C. In this study sample thickness was a nominal0.18 cm (except as otherwise noted).Oxygen Index (01) was measured by using a modified version of

. ASTM D2863-70. The modification did not significantly increase testerror (±0.3 01 units) and substantially reduced testing time [8].The BFGoodrich Smoke-Char Test is a small scale laboratory test

which is useful for quickly evaluating the smoke-forming and char-forming characteristics of polymer samples. Samples were tested in theform of pellets (Experimental Section, Polyamides). This test has beendescribed in detail by Kroenke [9].The results of small scale laboratory tests will not necessarily reflect

the performance of materials in either large scale test or real fire situa-tions. The test results and all related data in this article are based onsmall scale tests that do not necessarily relate to the hazards en-countered under real fire conditions.

Figure 2. Torch Test evaluation of APB in a flame retarded thermoplastic poly-urethane.

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--.--..-.. ,....- ’1’-&dquo;’’’’’’’’’’’’’’’’’

Figure 3. Torch Test evaluation of selected additives in a flame retarded thermoplasticpolyurethane.

RESULTS AND DISCUSSION

Evaluation of APB in a Flame Retarded Estane

The Torch Test was used to evaluate the performance of APB in a con-ventional flame retarded Estane; the Estane control is a polyether-based, thermoplastic polyurethane having a UL94 V-O rating. Asshown in Figure 2, as little as 5 phr of APB (phr = parts of additive perhundred parts of polyurethane resin) provides N7 minutes of thermalprotection for the polyurethane. Protection times in excess of 10minutes have been achieved by using 10 phr of APB. The V-O ratedEstane control survives the Torch Test for a period of N60 seconds. Onthe basis of our previous results [4] and the data shown in Figure 2, it isapparent that 5-10 phr of APB contained in a UL94 V-O Estane pro-vides thermal protection at least equivalent to that of a non-flameretarded Estane containing 50-200 phr of APB.Zinc borate (Firebrakee ZB) also provides substantial thermal in-

sulative value ( tf ~ 5 minutes) especially when used at the 10 phr levelin this Estane formulation. However, the overall Torch Test perfor-mance of APB exceeds that of zinc borate by as much as 300 percent.Our previous study [4] had shown zinc borate to be essentially ineffec-tive as a char-forming additive for a simple, non-flame retarded Estane.Figure 3 compares the Torch Test performance of APB with that of

several inorganic-based additives which are known to possess intumes-cent and/or flame retardant activity in various polymers. Each additivewas evaluated at the 25 phr level in a flame retarded (UL 94 V-O) Estanethermoplastic polyurethane.

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The results of the Torch Test indicate that the APB-containingsamples outperform all other additives tested. The combination of APBand melamine phosphate (2:1 wt/wt mixture), with a tj of ~665 seconds,displays synergistic behavior. Melamine phosphate is reported in theliterature as a component of intumescent paints and coatings [10].The performance of the low melting glasses (a sulfate and a

phosphate-sulfate) is, in each case, substantially inferior to that of APB.It is of interest to note that the same glasses, when added to rigidpolyvinyl chloride, afford excellent thermal barrier protection [4,6]. Suchvariation in test performance is quite likely due to differences in the in-trinsic flammability characteristics and thermal degradation patterns ofthe individual polymers.Although Figure 3 shows the performance of APB to be superior to

that of either ammonium polyphosphate or zinc borate, the latter ad-ditives are more readily processable (as compared to APB) in thethermoplastic polyurethane. Ammonium polyphosphate is the subjectof several patents relating to its use as a moisture-insensitive, flameretardant for polyurethane foams [11]. The phosphate is also one of thecomponents in a recently introduced intumescent flame retardantsystem designed especially for use in polyolefins [3].

It is apparent that the dual mode behavior of APB i.e., intumescenceaccompanied by glass formation, is quite important in determining theextent to which this compound provides thermal barrier protection.Also note that, at least for the case of the V-O rated Estane, there is noobvious advantage in using APB in excess of 5-10 phr. Comparing tfvalues (Figures 2 and 3), it is found that equivalent (indeed, if not better)Torch Test performance is realized for those samples containing thelower loadings of APB. The literature [1] describes such behavior as

Table 1. Char evaluation in nylon 616a.

aAll samples tested as pellets using Goodnch Smoke Char Testb25 phr loading of additivecglass compositions descnbed m Reference [4]dPercent char is expressed on the basis of the original sample weight

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being rather characteristic of many intumescent systems i.e., intumes-cent additives, when used above an optimum loading, may not allow forproper expansion of the polymeric resin. The overall thermal protectiveeffect is thereby diminished.

Evaluation of APB in Nylon 66

The Smoke-Char Test [9] was used to evaluate APB in Nylon 66. Thisis a small scale, rapid screening test which simultaneously measureschar formation and smoke generation of polymer compounds. In termsof evaluating potential intumescent and/or thermal barrier activity, theextent of char formation is a parameter of importance. In addition, thetest operator may assess intumescent activity by simply observingwhether or not significant volume expansion occurs during sample com-bustion.The test results, shown in Table 1, indicate that substantial char

formation occurs in the Nylon 66/APB sample. Nylon 66, itself, is com-pletely consumed during the test. On the other hand, Nylon 66 contain-ing 25 phr of APB generates a 40.7% char yield accompanied by signifi-cant volume expansion i.e., intumescence. Zinc borate at a similarloading generates only 16% char yield with no evidence of intumescence.When APB is combined with a low melting phosphate-sulfate glass,

synergistic char formation is observed (APB/glass, 2:1 wt/wt mixture).The glass itself is relatively ineffective as a char forming agent forNylon 66. APB/glass mixtures containing higher proportions of theglass component (1:2 wt/wt, APB/glass) exhibit marked dripping effects

Figure 4. Char formation in the Nylon 66/APB system. Percent char is expressed onthe basis of the original sample weight.

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Figure 5. Torch Test evaluation of APB in rigid poly(vinyl chloride). Zinc borate isZBX511 (Humphrey Chemicals).

(much like the sample containing only the glass) and generate non-intumescent char yields of ~20%.Figure 4 further illustrates the char-promoting nature of the Nylon

66/APB system by comparing it with theoretical char yields of a ther-mally inert additive. It is also important to note that APB, itself,undergoes a 35% weight loss when heated to 450°C; a temperaturelower than that at which the Smoke-Char Tests operates. This furtheremphasizes the fact that APB, at the 25 phr level, is converting a

Figure 6. Torch Test evaluation of APB in plasticized poly(vinyl chloride). PVC controlcontains 40 parts of di-2-ethylhexylphthalate plasticizer. Zinc borate is ZBX511(Humphrey Chemicals).

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significant amount, as much as 28%, of the Nylon 66 into char. (TheAPB used in the nylon char study was used in its as-received form.)

Evaluation of APB in Rigid Poly(vinyl chloride), PVC

The results of the Torch Test, shown in Figure 5, indicate that APBimproves the thermal barrier protection of a simple, rigid PVC formula-tion. For example, 10 phr of APB provides a 5-fold increase in thermalprotection relative to the PVC control (sample thickness, 0.18 cm). Theimportance of intumescent char formation is evidenced by comparingthe Torch Test results of the APB-containing samples with those of thenon-intumescent additives, barium sulfate ( tf N 30 seconds, 10 phr) andzinc borate ( tf N 75 seconds, 10 phr).

Evaluation of APB in Plasticized PVC

As shown in Figure 6, 25 phr of APB when added to a plasticized PVC(40 phr of dioctyl phthalate plasticizer, DOP) provides a significantcharring effect with a tf of N295 seconds versus a tf of N92 seconds forthe control sample. However, APB is not effective at the 10 phr level.Zinc borate (ZB-X511, Humphrey Chemicals) is ineffective at all levelstested.The samples containing zinc borate generate almost no char and

exhibit profuse dripping during the Torch Test. The APB samples(except for the 10 phr loading) generate rapid charring which, in turn,retards dripping. However, the overall intumescent effect of the APB/plasticized PVC samples is minimal. That is, the resulting char lacks thehigh volume, cellular structure which is characteristic of a true intumes-cent system. Nevertheless, APB provides a thermal barrier effect bypromoting the rapid formation of a uniform, coherent, glassy char layer.

In a qualitative sense, APB provides a greater degree of intumescentchar formation in rigid PVC than it does in plasticized PVC. In a quan-titative sense, this effect is best observed by comparing the PVCsamples containing 10 parts of APB. In the case of rigid PVC (Figure 5),10 parts of APB provides N154 seconds of thermal protection. The com-parably loaded plasticized sample (Figure 6) fails the test after N64seconds, actually performing worse than the control. This difference inbehavior may be due to the different melt viscosities shown by rigid andplasticized PVC. The plasticized recipe simply becomes too fluid as it isheated. As the APB releases gaseous decomposition products, thehighly fluid melt is unable to provide sustained volume expansion i.e.,the melt is very permeable with respect to the evolving gases. The morehighly viscous melt associated with rigid PVC, however, allows somedegree of char expansion to occur. In neither case, however, is theoverall degree of intumescence comparable to the marked effect shownby the APB/polyurethane system.

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a25 phr loading of flame retardant additiveTorch test time to failure, Initial sample thickness was 0 18 cmcglass compositions descnbed m Reference [4]

Evaluation of APB in Natural Rubber

Table 2 summarizes Oxygen Index and Torch Test data for 25 phrloadings of various flame retardants contained inBoil natural rubber for-mulation. Natural rubber ignites easily, evolving large quantities ofvolatile, highly flammable hydrocarbons which, in turn, generatesubstantial heat as they burn [12]. This behavior is reflected in therather low oxygen index of the control sample (O.I. = 17.5). APB, at the25 phr level, raises the O.I. of the rubber formulation to 19. However,this is only comparable to the performance of alumina trihydrate. And,in fact, the alumina with a tf N 120 seconds affords a better thermal bar-rier effect than does APB ( tf ~ 84 seconds).

Table 3. Torch Test evaluation of APB in polyester resin.

al5 phr loading of additivebglass composition descnbed m Reference f4l

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In general, APB is not an effective intumescent char-forming additivein the natural rubber formulation. Intumescence occurs but it is short-lived and not sustained. It is possible that the carbon black in thisrubber formulation (the recipe contains 40 parts of black) is interferingwith the intumescent process; an effect which has been noted with otherintumescent systems [3].

Evaluation of APB in a Polyester Resin

A water extendible polyester (WEP® 662-P, Ashland Chemicals) wasused for the Torch Test evaluation of APB and several other flame retar-dant additives. Time to failure, tf, is a function of sample thickness(refer to Appendix). The variable sample thicknesses shown in Table 3must, therefore, be taken into consideration. Nevertheless, the relativeperformance of these samples is readily apparent, particularly withregard to the APB formulations.As shown in Table 3, APB provides a substantial thermal barrier ef-

fect in this polyester formulation. Although the polyester control failsthe Torch Test after only 30 seconds, 15 phr of APB provides more than4 minutes of thermal protection. During testing the APB sample ex-hibits only a moderate intumescent effect, however, APB does promoterapid charring of the polyester. This retards any dripping tendency anda uniform char layer is established. Even more effective is the mixture ofAPB and melamine phosphate (tf - 618 seconds). Melamine phosphate,itself, provides only 149 seconds of thermal protection. As noted earlierin this study, the APB/melamine phosphate system displays synergisticbehavior in a thermoplastic polyurethane.

Several other flame retardant chemicals such as zinc borate (Fire-brake° ZB), ammonium polyphosphate (Phos-Cheke P-30) and aluminatrihydrate are rather ineffective in terms of increasing the burn-throughresistance of this polyester. (Zinc borate and alumina trihydrate are notintumescent agents per se. However, like APB, they are inorganic-based, nonhalogenated compounds which undergo endothermic decom-position and have the potential to enhance the formation and/or thequality of char.) Various low melting glasses also exhibit relatively poorperformance in the Torch Test; as in the case of the polyester control,these samples show a marked dripping effect which inhibits char forma-tion.

APB as an Intumescent Charring Agent for Polyurethanes:Mechanistic Implications

On the basis of the information presented thus far, it is apparent thatAPB functions as an effective thermal barrier system for a variety ofpolymers. In general terms, APB is most effective when used as an ad-ditive for polyamides (Nylon 66), polyesters and thermoplastic poly-

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urethanes. With regard to the formation of actual intumescent charring,APB is most active in thermoplastic polyurethanes [4]. It is, therefore,quite likely that the pentaborate is interacting with the pyrolysis prod-ucts of the polyurethane. An examination of borate and urethanechemistries tends to support this hypothesis.Most thermoplastic polyurethanes are prepared from three types of

monomers; a diisocyanate, a macroglycol and a glycol chain extender.Specific structural components and properties of these polymers havebeen discussed in the literature [13]. In this and our previous study [4],we have used polyether-based thermoplastic polyurethanes which aregenerally derived from monomers such as:

diphenylmethane-4,4’-diisocyanate (MDI)

poly(tetramethylene oxide) glycol

1,4-butanediol

The thermal behavior of Estane thermoplastic polyurethanes hasbeen studied [14]. A series of endothermic transitions, associated withthe melting of hard segment domains, occur within the range of~ 180-250 °C. At temperatures in excess of ~300 °C, the polyurethanedecomposes.Thermal volatilization analysis of a simple, model polyurethane

derived from 1,4-butanediol and MDI indicates that volatile degra-dation products reach a maximum rate of evolution at N310°C [15].Initially chain scission occurs to yield diol and diisocyanate. Carbondioxide, water, tetrahydrofuran and hydrogen cyanide are also formed.Carbodiimide (-N=C=N-) formation occurs due to thermally inducedcondensation of isocyanate groups; CO2 is released in the process.In order to see how APB may be involved in the thermal degradation

of polyurethanes, it is necessary to examine the chemistry of borates. Inparticular it is of interest to note that both boric acid, B(OH)3, and thepentaborate anion, B,O,(OH)-4, contain the characteristic B-OH group.The reactivity of the B-OH group is well documented [16,17]. For ex-

ample, boric acid reacts with alcohols or polyols to produce borate estersi.e.,

A modified procedure utilizes boric oxide,

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In addition, boric acid reacts with isocyanates i.e.,

Heat stable polymers are formed in a similar reaction with diiso-cyanates.The initial thermal decomposition of APB occurs within the range of

100-230 °C with the maximum weight-loss rate occurring at 160 °C [18].This temperature range encompasses that in which the polyurethaneundergoes melting, thereby providing the basis for the initial intumes-cent activity.At higher temperatures (230-450 °C), APB undergoes further dehy-

dration, releases ammonia, forms pentaboric acid and is eventually con-verted to a boric oxide glass [18]. It is also within this temperaturerange that the polyurethane decomposes. APB and/or pentaboric acidcould react directly with the diisocyanate and/or diol fragments whichare produced during the thermal degradation of the polyurethane.Furthermore, B,O, may also react with the diol fragments to produceborate esters. Reactions such as these would tend to:

1. promote rapid charring of the polyurethane due to the formation of ahighly crosslinked borate ester and/or boron-nitrogen polymer. Thegaseous products (H20, COZ) evolved during such reactions would ex-pand the developing inorganic-organic char layer, thereby producingthe multicellular, intumescent char which is so characteristic of theAPB/polyurethane system.

2. reduce the quantity and/or rate of formation of volatile (flammable)pyrolysis products. In other words, via a series of chemical reactions,APB redirects the thermal decomposition of the polyurethane pro-ducing more char and less volatiles.A somewhat similar mechanism has been proposed to account for the

char-promoting effect of ammonium polyphosphate in polyurethanes[15]. This may explain the synergistic activity shown by the APB/melamine phosphate system (Figure 3).The mechanism suggested in this study may also explain the poor

char-forming behavior shown by the APB/natural rubber system. Thevolatile pyrolysis products of rubber include, among others, isopreneand dipentene [12]. It is reasonable to assume that such products wouldbe unreactive towards APB.

If this proposed mechanism is correct, then it immediately suggeststhe possibility of designing a highly effective, intumescent, flame retar-dant system for polyurethanes. For example, borate isocyanates or diolscould be used to prepare a modified polyurethane, thereby incorporatingthe flame retardant system directly within the urethane structure. A

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similar approach utilizing reactive phosphorus-containing moieties hasrecently been reported [19].

Concluding Remarks

We have shown that ammonium pentaborate functions as an intumes-cent charring additive for thermoplastic polyurethanes. The thermalbarrier performance of APB is significantly enhanced by adding as littleas 5-10 phr to a conventional flame retarded polyurethane. APB alsofunctions as an effective thermal barrier additive for a variety ofpolymers. The advantages of APB are numerous. Basically, the penta-borate

· is a non-halogenated, inorganic compound· is a readily available chemical· is a low density material (sp. gr. 1.58). promotes rapid char formation· retards flaming drip. generates a thermally insulating, glassy char layer. suppresses char afterglowOn the negative side, APB is water soluble and lacks high tem-

perature stability. Both factors will obviously restrict the utility ofAPB as a polymer additive. As previously reported [4], we have beensuccessful in processing APB into thermoplastic polyurethanes only byusing lab-scale equipment and specialized techniques which would notbe compatible with conventional, industrial processing operations.

ACKNOWLEDGEMENTS

We express our thanks to the BFGoodrich Company for permission topublish this work.

REFERENCES

1. Vandersall, H. L., J. Fire and Flammability, 2, p. 97 (1971).2. Naitove, M. H., ed., Plastics Technology, 28, p. 69 ( July 1982).3. Naitove, M. H., ed., Plastics Technology, p. 37 (May 1981).4. Myers, R. E. and Licursi, E., J. Fire Sciences, submitted for publication.5. Heylin, M., ed., Chem. Eng. News, 62, p. 36 (February 1984).6. Kroenke, W. J., J. Materials Science, in press (1986).7. Vol’fkovich, S. I., Zuzzer, E. E. and Remen, R. E., Bull. Akad. Sci. U.S.S.R.

Klasse Sci. Khim., 571 (1946); Chem. Abs., 43, 7781h (1948).8. Chambers, W. E., Internal Report, BFGoodrich R&D Center Report, un-

published (July 1980).9. Kroenke, W. J., J. Appl. Polym. Sci., 26, p. 1167 (1981).

10. Kay, M., Price, A. F. and Lavery, I., J. Fire Retard. Chem., 6, p. 69 (1979)and references contained therein.

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11. Barnett, J. C., Fr. Pat. 1,462,604 (1966 to Monsanto Co.); J. C. Barnett, U.S.Pat. 3,423,343 (1969 to Monsanto Co.).

12. Fabris, H. J. and Sommer, J. G., "Flame Retardation of Natural and Syn-thetic Rubbers," Flame Retardancy of Polymer Materials, Vol. 2, pp.135-199, Marcel Dekker, Inc., New York (1973).

13. Schollenberger, C. S. and Stewart, F. D., "Thermoplastic Urethane Struc-ture and Ultraviolet Stability," Advances in Urethane Science and

Technology, Vol. 2, pp. 71-108 (1973).14. Hewitt, L. E., Soc. Plastics Eng. Proceedings, SPE 38th Annual Technical

Conference, pp. 497-499 (May 1980).15. Grassie, N. and Zulfiquar, M., "The Effect of the Fire Retardant, Am-

monium Polyphosphate, on the Thermal Degradation of a Polyurethane,"Developments in Polymer Stabilization, Vol. 1, pp. 197-217 (1979).

16. Brotherton, R. J., "Boric Acid Esters," Kirk-Othmer Encylcopedia ofChemical Technology, Vol. 4, 3rd ed., pp. 111-123, John Wiley & Sons, NewYork (1978).

17. Aries, R. S., U.S. Pat. 2,931,831 (1960); Aries, R. S., U.S. Pat. 2,945,841(1960).

18. Erdey, L., Gal, S. and Liptay, G., Talanta, 11, p. 913 (1964).19. Grassie, N. and Mackerron, D. H., "Synthesis and Degradation of Poly-

urethanes Containing Phosphorus—Part III," Polymer Degradation andStability, Vol. 5, pp. 89-103 (1983).

20. Dickens, E. D., Jr., "ILS Sheet: A Decorative Sheet for Reducing the FlameSpread of Flammable Substrates," Paper given at the 31st Annual Meetingof the Forest Product Research Society, Denver, Colo. (July 3-8, 1977).

21. Dickens, E. D., Jr. and Smith, G. F., "New Low Smoke Thermoplastics toMeet New Needs in the Marketplace," 8th International Conference on FireSafety, p. 227 (January, 1983).

APPENDIX

A Simple Torch Test for Intumescence

We describe a simple test for intumescence which we developed andused as a rapid screening test in our earlier work on char-forming PVCformulations [20,21] and by Myers in more recent work on low meltingglasses in polymers [4].The concept is shown in Figures 7a and 7b. We use a 3.8 cm x 3.8 cm

(1.5 in. x 1.5 in.) sample of variable thickness and center it in the holderon the Marionite block over an exposed Chromel-Alumel thermocouplebead. The sample back and sides are wrapped in aluminum foil to aid inremoving and handling the sample once the test is over. To assure con-tact with the thermocouple a 1.0 cm hole is cut in the center of the backof the foil. The heat source is a pencil-tip burner for a propane torchbeing fed propane at a regulated pressure (30 psi). Because of the flameshape the thermal impact on the sample is very rapid and concentratedand is accompanied by strong ablative action at the point of contact. It

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Figure 7. Test used for intumescent behavlor testing.

only requires a few seconds to burrow through noncharring polymersand expose the thermocouple to the heat of the flame. As a test criteriawe chose the time for the bare thermocouple to reach 250 °C ( tf~.In developing and characterizing the test we have found the test’s

operational simplicity and rapid sample throughput is offset by muchmore complex heat transfer and chemical degradation than expected.The sample is mounted in the holder and the temperature recorderstarted. The flame is rotated into place exposing the sample to the fullheat flux. Because the heat flux is extremely non-uniform, the center ofthe sample is hit by a large thermal shock which in extreme cases cancause the sample to crack and expose the thermocouple, leading to in-stant failure. If it does not crack and the sample is capable of char for-mation, then the higher temperatures in the center of the sample lead torapid char formation and outgassing in the center first and the forma-tion of a characteristic volcano-shaped protrusion under the flame. For-mation of a stable intumescence char structure under these conditionswill depend (as in all classical intumescent systems) on the relative ratesof gas generation versus char formation. The time to failure valuesrecorded in the test then reflect on how stable the char was, the type ofcell structure formed and the amount of material available to slow downthe effects of ablation by the flame.The range of sample thickness allowed in the test depend on how effec-

tive they form stable char structures. Films below 0.1 mm are too thinand the amount of sample mass too small for an effect to be seenbecause heat transport time to the thermocouple and char formationtime are comparable and any shrinkage of the film to form char willdestroy the film. At the other extreme, samples that form char and arequite thick (3-5 mm or more) can form such a stable char that heat isconducted across the surface of the char and into the holder block which,although much larger than the sample, can still be slowly heated and thethermocouple will mistake the source of heat it is sensing. This case iseasily detected by the nature of the temperature trace recorded from thethermocouple and the length of time involved in testing.

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Within the range of sample thicknesses accessible by the test we havefound the relationship between time to failure ( tf) and sample thicknessto be of the power-law type:

where tf is the time to failure from the Torch test, T is the samplethickness, and A and x are constants different for each material tested.Figure 8 shows a log-log plot of tf versus sample thickness for somechar-forming PVC materials tested. The difficulty with a power-law rela-tionship such as described above is that comparing samples of equalthickness may not be adequate in all cases and data at two or morethickness should be obtained for comparison.Our experience to date suggests that materials that show good perfor-

mance in a vigorous test of this type for intumescence also show goodperformance in larger scale fire tests that we have run [21]. Part of thisis due to the thermal conditions used in our test and part is due to the in-clusion of features such as char integrity which is very important in pro-tecting virgin substrates from the heat.

Figure 8. Relationship between t, and sample thickness.

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