studies on flame retardant intumescent char: part i

Fire Safety Journal 19 (1992) 103-117 ~

Studies on Flame Retardant Intumescent Char: Part I

D. Scharf, R. Nalepa, R. Hefl in & T. Wusu

Hoechst Celanese Corporation, SCG-Research and Development, 500 Washington Street, Coventry, Rhode Island 02816, USA

A B S T R A C T

In recent years the concept of intumescent ( insulative foamed) char has received renewed attention as a means to flame retard materials. In particular, breakthrough enhancements in thermal stability and com- patibility have provided the necessary capability to perform in thermoplastics, e.g. polypropylene, polyethylene (Refs 1 and 2). As part of our efforts to develop optimized intumescent flame retardant (IFR) product technology, a study of the influence on effective char formation by metal oxide pigments has been started. Initial results show the existence of both synergistic and antagonistic behavior. Chemical and physical interactions are proposed to explain these results.

INTRODUCTION

Intumescent char formation is a highly effective flame retardant process. Generically, three main ingredients are necessary to this process: catalyst, carbonific (char-former), and spumific (blowing agent). The latter expands (intumesces) the initially formed char. Fundamental chemistry is represented in Fig. 1.

Ideally, the polymer substrate (containing an intumescent FR) under a burn condition is protected from catastrophic destruction by an insulative cellular char. The greater the substrate contribution to the char matrix, the greater the effectiveness of the char-forming additive

103 Fire Safety Journal 0379-7112/92/$05.00 (~ 1992 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland

104 D. Scharf, R. Nalepa, R. Heflin, T. Wusu

(NH4P03) n • H3PO 4 + NH 3 "Cetalyst"

CH20H ~ I

~3PO 4 4- HOCH 2 -~I-CH20H ~- (-iC-)n+ H20 CH20H

"Chef" 2 "'Carbonific" ~

N N ,~,~N ~L • NH 3 .4- N 2

H2 N NH 2 ~1¢

"'Spumific"

"'Intumescent char" Fig. 1. Intumescent chemistry.

initiator system. Benefits of this latter characteristic are lower additive requirements, lower cost, and better overall mechanical performance.

Various pigments and mineral fillers have been shown ~5 to influence flame retardant efficiency in char-forming systems. The nature of these influences are not well understood and have been noted mainly as empirical observations. Speculations on physical nucleating behavior on the part of these pigment fillers have been made. 6 By way of this behavior, char structure and tenacity variations have been rationalized.

Studies initiated at Hoechst Celanese attempt to provide insight into the nature of the above behavior. Our approach is to compare varying pigment (metal oxide) effects on a proprietary intumescent additive system (Exolit R IFR) using the following screening techniques:

--flame retardant efficiency; q p i g m e n t properties (size/shape); ---char character (structure/volume); --char composition.

The substrate polymer used in this study is polypropylene (homopolymer). In this first phase, the effects of TiO2 (synergist) and SnO2 (antagonist) are reported.

EXPERIMENTAL

The following materials and sample preparation were used in this work.

1. Polypropylene (homopolymer)--Himont profax ~ 6323 (MFI = 9- aO).

Studies on FR intumescent char 105

2. Exolit R IFR--proprietary intumescent flame retardant additive system from Hoechst Celanese. IFR is composed of the three (3) essential generic ingredients for effective intumescent FR:

~Catalyst: ammonium polyphosphate; --Char-Former: proprietary polyol (thermally stable

aliphatic polyalcohol); --Blowing agent: melamine derivative.

3. Titanium dioxide (TiO2)--pigment grade R-101 from DuPont. 4. Stannic oxide (SnO2)--pigment grade (99.99%) from Aldrich

Chemical Company.

Formulated sample composites (flame retardant polypropylene) were prepared by dispersing flame retardant additive and pigment into the PP matrix by Banbury mixing (190-195 °C/374-383°F) followed by granulation, compression molding (176-193 °C/350-380 °C) and sizing for testing.

Flame retardant effectiveness was determined as a function of minimal weight % loading (in polypropylene) to achieve UL94 V - O @ ~6 in. and by oxygen index (minimum oxygen concentration to maintain burning). 7,8

Pigment particle size and geometry, as well as char characteristics, were determined by scanning electron microscopy (JOEL SEM 1200). Further to char character, elemental composition was investigated by Energy Dispersive X-ray (EDAX Amray 1820 I), infrared spectroscopy (Perkin Elmer 1750 F/ ' - IR) and combustion analysis.

Char weight was measured as a percentage of starting compound (polypropylene plus flame retardant additives) with flame retardant additive held at a constant 25 wt % concentration. To ensure reliable char weight comparisons, all systems investigated were subjected to an identical condition of forced burning for 10 min at oxygen concentration of 2% above the LOI. This technique assured optimum combustion efficiency of the materials exposed.

RESULTS

Flame retardant efficiency

Flame retardant behavior was studied by holding Exolit IFR at a constant 25 wt % loading and measuring the influence of pigment and pigment mix as a function of its independent loadings of 1, 2 and 4%. Table 1 lists flame retardant efficiency comparisons based on UL-94 and oxygen index testing.


TABLE 1 Metal Oxide Influence on Flame Retardance

Material UL-94 (@ 16 in.) LOI

Polypropylene Burns 18.0 FR-PP (25% IFR) V-0 28.3

w/TiO2 1% V-0 32.4 2% V-0 33.8 4% V-0 34.1

w/SnO2 1% V-2 26.8 2% Burns 23.2 4% Burns 23.1

w/1 : 1 TiO2/SnO2 1% V-0 32.3 2% V-0 32.5 4% V-0 29.2

As evidenced by OI, t i tanium dioxide (TiO2) is seen to have a significant synergistic effect and stannic oxide (SnO2) a significant antagonistic effect. Fur ther , the mixed pigments show dominance by TiO~ at low concentrat ions (1, 2%), but weakens in favor of the SnO: effect at a higher concentrat ion (4%). This is rather surprising when considering physical proper ty profiles for both pigment systems. Table 2 lists several descriptive properties relevant to this study. Fur ther , particle size and geometry comparisons, de te rmined by SEM, are shown in Fig. 2. Nothing unusual can be ascribed based on this information. If anything, it would be expected that the two pigments would render the same effect to the burn behavior based on the thermal and physical similarities displayed. Therefore , dissimilarities in reac-

T A B L E 2

Property Ti02 S n O 2

Molecular weight 80-0 151.0 Crystal form Colorless, tetrah. White, tetrah. Specific gravity (g/cc) 4.26 6.95 Melting point (°C) 1850-0 1630.0 Particle size (/u) 0.2 0.3 Boiling point 2500.0 1900.0

(subl.) Reactivity - - Sol. conc. alkali


Fig. 2. SEM micrographs of TiO2 and SnO2.

tivity were speculated to be present and this conclusion is supported in the sections to follow.

Char structure

Intimate structure variations of char obtained from the forced burning of the flame retardant PP compositions were investigated by SEM. Following the method of G. Camino et al. 9 SEM micrographs of outer

surfaces and interior sect ions were obtained. Figures 3, 4 and 5 illustrate these comparisons for unmodified, TiO2 (2%) modified, and SnO2 (2%) modified IFR-based flame retardant PPs, respectively.

The outer surface of the unmodified IFR char exhibits a consistently smooth uniform appearance. The TiO2 modified char, although uniform and continuous, has a compact rugged outer surface due to the formation of agglomerates. By contrast, the SnO2 modified char displays a fine, flaky surface structure with a high degree of porosity and cracks.


(a)

(b)

Fig. 3. SEM micrographs of FR-PP char (unmodified) × 1000. (a) IFR (virgin)-outer surface. (b) IFR (virgin)-interior structure.

Interior regions of the individual chars also show significant differences. Unmodified and TiO2 modified chars demonstrate an extensive cellular network, not seen for thie SnO2 modified char. Further, the TiO2 modified char contains highly defined, rigid cellular compartments relative to the unmodified char. As with the surface characteristics, the SnO2 modified char interior has an irregular, porous, granular structure.


(a)

(b)

SEM micrographs of FR-PP char (TiO2-modified)× 1000. (a) IFR (2% TiO2)~outer surface. (b) IFR (2% TiO~)~interior structure.

Chemical composition

Elemental analysis (combustion) of individual chars from the systems investigated reveals some interesting anomalies. Table 3 lists raw data for surface (outer) and internal (int) samples of the char residues. Table 4 represents this data in a form meaningful to an attempted interpretation.


(a)

(b)

SEM micrographs of FR-PP char (SnO~-modified)x 1000. (a) IFR (2% SnO2)--outer surface. (b) IFR (2% SnO2)--interior structure.

Inspection of Table 4 shows a fairly equal distribution of phos- phorous and (where relevant) metal between outer external surface and interior regions of the char mass (refer to columns 2 and 3 of Table 4). Based on P/metal (column 4, Table 4) results of the individual TiO2 modified and SnO2 modified systems, similar P/metal ratios for the

Studies on FR intumescent char

TABLE 3 Composition Analysis of Flame Retardant Char

25% IFR-Based Flame Retardant Polypropylene

IFR P ( % ) Ti(%) Sn (%)

Unmodified ---outer 12.1 - - - - --interior 10-4 - - - -

TiO2 modified a ---outer 14.4 2-07 - - --interior 12.3 2-38 - -

SnO2 modified a ---outer 18.6 ~ 3-17 --interior 16.8 ~ 2.91

TiO2/SnO2 modified ° ---outer 12-7 0.63 3-69 --interior 13-3 0-70 6.56

° % Modifier=2% of total mass of FR polypropylene.

111

mixed 1 :1 T iO2/SnO2 modi f ied sys tem w o u l d be expec ted . In fact , a

2 - 3 - f o l d r e t en t ion o f meta l is o b s e r v e d for the mixed sys tem versus the

single meta l oxide systems. F u r t h e r , c o l u m n 5 d e m o n s t r a t e s a sig-

nificant d i s tor t ion in the T i / S n ra t io e x p e c t e d (viz. 0 . 7 5 - 1 . 0 ) . T h e s e

anomal ies suggest in te rac t ive effects tha t t end to re in fo rce (synergize)

the d o m i n a n c e o f TiO2 in w h a t e v e r influential role it (TiO2) plays.

TABLE 4 Composition Analysis of Flame Retardant Char 25% IFR-Based Flame Retardant

Polypropylene

p Metal b P/metal Ti/Sn

IFR out/ int out/ int out int out int

Unmodified 1.2 . . . . TiO2 modified c 1-2 (Ti) 0.9 6.9 5.2 SnO2 modified c 1.1 (Sn) 1.1 5.9 5.8 TiO2/SnO~2

modified ~ 1.0 (Ti/Sn) d 0.6 2.8 1.8 (expected) (6-0) (6-0)

m

B

6.5 9.3 (0-75) (0.75)

° out/int Indicates the ratio of outer surface to inner region of char mass obtained. b Metal refers to Ti, Sn, and Ti/Sn. c % Modifier = 2% of total mass of FR polypropylene. d Sum of Ti and Sn.

112 D. Scharf, R. Nalepa, R. Heflin, T. Wu~u

Additional work exploring a range of the mixed metal oxide ratios and loading level is in progress to address this question.

Returning to the differences observed in flame retardant performance (Table1) for the TiO2 modified versus SnO2 modified formulations, the data presented above (Table 4) indicate different modes of interaction. If a pure physical effect (nucleation) existed for both metal oxides, then the nearly equivalent results shown for the P/metal analyses would suggest similar effects for both systems. Since this is not the case, chemical interactions were suspected.

Further to this interpretation, relevance of particle size and shape differences of the pigment modifiers would be expected. Data ascribed in Table 2 and SEM micrographs (Fig. 2) suggest little or no difference in the size/shape parameters of the subject modifiers. The conclusion is drawn that this consideration has no bearing on the observations at hand.

To gain further insight on this question, infrared analyses on the char masses were conducted. As Illustrated in Fig. 6 (b)-(e) , different characteristics are noted for each of the modified (pigmented) chars compared with unmodified char. Interesting similarities are seen for

100

L 50

25

0 ~ ~ I I ~ ~ I

4000 3500 3000 2500 2000 1500 1000 450

cm -1

lOO (a)

~ 75 ~

~ ~0 1-

25

0 ~ 4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1500 1 0 0 0 4 5 0

c m - 1

(b) ~ . ~. I ~ e d spe~ra ( ~ ) ~ m p ~ n g H~PO~ ~t~ IFR char residues. (a) H~PO~. (b) ~ ~ th ~o pi~ent . (c) Cg~ ~ th TiO~. (d) Char ~ S~O~. (e) C ~ ~th 1 : 1

TiO~/SnO~.

Studies on F R intumescent char 113

100

~ 50 1 -

25

I 0 ~ 4000 3500 3000 2500 ~)000 1500 1000 450

cm-1

100 (~)

75

.~, "~. 50 I--

25 !

0 4000 3500 3000 2500 2000 1500 1000 450

Cm-1

100 (d)

~ 75 ~

p 5 0

2 5

0 ~ ~ ~ 4000 3500 3000 2500 2000 1500 10OO 450

¢m-1

(e) Fig. 6. (Continued)

SnO2 and unmodified cha~ in the ~ - 1 7 ~ c m fingerprint ~egion. Abso~tion bands in this ~egion compare [avo~ably with a simila~ ~atte~ found ~o~ ~PO~ (~ol~hosphoric acid) ~e[e~enc~d in Fig. 6(a). TiO~, by contrast is found to significantly alte~ these abso~tion bands, indicating an unusual interactive ~ffect. ~n t~e case ~ the mixed TiO~/SnO~-based formulation, a mixed ~esult with th~ TiO~ effect p~dominating is obse~ed.

As a [u~the~ guide, metal analysi~ of th~ cha~s (Table 3), discussed above, show simila~ levels ( ~ Ti, Sn) not unexpected ~o~ the two independent TiO~ and SnO~ systems. ~ mixed metal oxide system again show~ ~etention ~[ both ~etals, but not in ex~ect~d ~atio o~ total


TABLE 5 Char Formation versus Metal Oxide Effect on Intumes-

cent Flame Retardant Polypropylene

IFR (25 wt %) wt % Char L O I (% 02)

Unmodified 15 28.3 TiO2 modified a 25 33.8 SnO2 modified ~ 17 23.2 TiO2/SnO2 modified b 24 32.5

a 2% Metal oxide. b 1 : 1 Metal oxide ratio.

metal content. TiO2 far exceeds the expected % concentration relative to the system based on TiO2 alone.

Coupled with FR efficiency and SEM results, the above observations support a theory consistent with:

--SnO~ behaving as a reactive agent that becomes part of the intumescent chemistry and is disruptive to it; --TiO2 behaving as a physical, non-reactive reinforcing agent that strengthens the char matrix.

Char promotability

As a last consideration on factors influencing intumescent char effectiveness, the quantity of char produced from each system was measured. Theory predicts 1° that as char increases, the more the polymer substrate becomes involved in char formation, and the greater the flame retardant efficiency should become. An attempt at checking our IFR systems against this logic was conducted. Flame retardant PPs (25% IFR) were subjected to a forced burning condition at an oxygen concentration 2 units above measured LOI for 10 min. Char from this exposure as a percentage of starting weight is reported in Table 5.

A good correlation for the systems tested is shown. Even in the SnO2 modified case, where the char shows a moderate increase versus a decreasing OI, it is apparent that SnO2 is not a strong promoter of char compared with TiO2.

DISCUSSION

Studies begun here support historical findings on the complexity of intumescent flame retardant processes. The significant influence ob-


served by metal oxides on this process, demonstrate the sensitivities of the chemistry involved. Seemingly similar ostensibly inert additives have surprisingly different effects.

From the flame retardant flammability tests and SEM observations, one can define optimum char characteristics as follows:

~ d e n s e compact outer crust with structural continuity and no porosity; -- interior sections containing a highly ordered cellular network that serves as the thermal insulation barrier; ~ g r e a t e r the mass the better.

Chemical consequences that upset these characteristics are disastrous. Although a general relationship cannot yet be drawn, work reported here suggests that reactive interactive agents (SnO2) are antagonistic to effective char, , whereas physical bridging type agents (TiO2) are reinforcing or synergistic.

SnO2-based char has a highly porous, flaky structure with little to no interior cellular network that is essential for thermal insulation.

[ N H 4 P O 3 ] n

o o

(Physical)TIO 2 m

0 0 II II ;

m p - - O - - P - - O -

I I 0 0 %H %H

~ =Ti=O

/ H / H 0 0 ! I

-P--O --P--O- Il II O O

"Bridged"

Char Infrastructure

m

SnO 2 (Chemical)

I I I II ~ _ O O -

OHI Sn-O--P--O-

O O ~ O I II II ~ II /

- P - o - P - O H|H O-P- O4 i I ~ ~ / OH O--~n~---O OH /

_ O H j. "Fragmented"

Fig. 7. Metal oxide chemistry.


Consistent with the above observations, SnO2 is thought to react with the intumescent system in a manner that results in hydrolytic break- down of the polyphosphoric acid (produced on initial heating of ammonium polyphosphate) that serves as the char infrastructure. Water is sourced from oxidative combustion of the hydrocarbon substrate and dehydration of the polyol char-forming component. SnO2 is either reactively incorporated (Fig. 7) or at least catalyzes the hydrolysis.

By contrast, the data reported suggests a physical, reinforcement influence of TiO2. Rather than reacting with the polyphosphoric acid network (infrastructure), TiO2 is portrayed as offering structural support through hydrogen bonding or bridging (Fig. 7).

CONCLUSIONS

1. Effective flame retardant intumescent char must have a cellular interior structure with a compact non-porous surface crust.

2. IFR effectiveness increases as the ability to promote char (increased mass) increases.

3. Pigment oxides (TiO2) that physically interact with IFR chemistry appear synergistic to effective flame retardance.

4. Pigment oxides (SnO2) that chemically interact with IFR chemistry appear antagonistic to effective.flame retardance.

Note: Flammability information is based on laboratory flammability tests and is not intended to be used to predict performance in actual fire conditions.

REFERENCES

1. Scharf, D., Intumescent flame retardants. In Proceedings of FRCA Meeting, San Antonio, TX, 1989, pp. 183-202.

2. Mount, R. A. Non-halogen flame retarded polypropylene. In Proceedings of 14th International Conference on Fire Safety, San Francisco, CA, 1989.

3. Camino, G. & Costa, L., Polym. Degrad. & Stab., 20 (1988) 271. 4. Bertelli, G., Camino, G., Marchetti, E., Costa, L., Casorati, E. &

Locatelli, R., Polym. Degrad. & Stab., 25 (1989) 277. 5. Bertelli, G., Marchetti, E., Camino, G., Costa, L. & Locatelli, R.,

Intumescent fire retardant systems. Effect of fillers on char structure. Angew, Makromol. Chem., 172 (1989) 153.

6. Vandersal, H. L., J. Fire & Flare., 2 (1971) 97. 7. UL94, September 1973 edition w/supplements of May 1975, July 1976,

May 1978: Tests for Flammability of Plastic Materials for Parts in Devices and Applications.


8. ANSI/ASTM D2863-77. Standard method for measuring the minimum oxygen concentration to support candle-like combustion of plastics (oxygen index).

9. Bertelli, G., Camino, G., Marchetti, E., Costa, L. & Locatelli, R., Angew. Makromol. Chem., 169 (1989) 137-42.

10. Camino, G. & Costa, L. Revs. Inorg. Chem., 8 (1986) 90.

studies on flame retardant intumescent char: part i

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