Intumescent Systems for Flame Retarding of Polypropylene

Chapter 7

Intumescent Systems for Flame Retarding of Polypropylene 1

Downloaded by PENNSYLVANIA STATE UNIV on January 23, 2016 | http://pubs.acs.org Publication Date: July 21, 1995 | doi: 10.1021/bk-1995-0599.ch007

Menachem Lewin and Makoto Endo

Polymer Research Institute, Polytechnic University, Brooklyn, NY 11201

Six ammonium polyphosphate (APP) based i n tumescent FR systems (IFR) for injectionmolding grade PP were investigated and compared for OI, FR effectivity (EFF), thermal decomposition and char structure. Synergistic E f f e c t i v i t y (SE) is defined and found for the systems investigated to be in the range of 5.5-11.3, compared to the SE of other phosphorus-nitrogen sys tems (1.75), of aliphatic and aromatic bromine-antimony oxide formulations (4.3 and 2.2), and of bromine-phosphorus based systems (1.4-1.6). Correlations were ob tained between the % Ρ and the TGA r e s i due-after-transitions (RAT). Significant linear relationships were found between OI and RAT for all cases. Cone calorimeter results for several IFR-treated PP samples are reported and compared; a correlation with OI and EFF values is noted. SEM scans of char obtained from the combustion of APP— containing intumescent PP samples were ex amined and differences in cellular struc ture discussed. Ammonium polyphosphate (APP) w i t h c o - a d d i t i v e s is ex t e n s i v e l y used f o r flame r e t a r d i n g polypropylene (PP) ill. The co-additives usually consist of char-forming and b l o w i n g a g e n t s which are synergists with APP. The co-additives differ among the various commercially available formulations. For this work, it was o f interest to compare the flame retardant behavior of v a r i o u s f o r m u l a t i o n s b y 0 1 , T G A , Cone calorimetry and Current address: Showa Denko Co., Kawasaki Plastics Laboratory, Kawasaki, Japan

0097-6156/95/0599-0091$13.50/0 © 1995 American Chemical Society

In Fire and Polymers II; Nelson, Gordon L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

92

FIRE AND POLYMERS II

T a b l e I. Phosphorus-Based A d d i t i v e s Results

UL-94

FR

FR Additives

Additives

-

0

0

17.8

2

20

3.9

3

25

No


i n P o l y p r o p y l e n e F o r m u l a t i o n and

%P

01

0I/%P 1/16-

1/8"

-

NR

NR

26.0

2.1

NR

NR

4.8

27.8

2.1

NR

NR

30

5.9

29.8

2.0

V-2

V-0

5

35

6.9

32.3

2.1

V-2

V-l

6

40

7.8

34.1

2.1

V-2

V-0

7

15

4.7

19.3

0.32

NR

NR

20

6.2

19.7

0.31

NR

NR

25

7.8

20.2

0.31

NR

NR

15

3.0

26.2

2.8

NR

NR

20

4.0

30.7

3.2

NR

V-0

25

5.1

33.0

3.0

V-2

V-0

15

3.0

21.4

1.2

NR

NR V-2

1

4

8

EDAP

APP

9 10 11 12 13 14 15 16 17 18

APP + Spinflam MF 8 2 APP + PETOL

APP + THEIC

19 20

Exolit IFR 23P

21 22 23 24

APP + PETOL t-benzoate

Wt%

20

4.0

23.6

1.5

V-2

25

5.1

26.4

1.7

V-2

V-2

15

3.0

24.6

2.3

NR

NR

20

4.0

27.6

2.5

V-2

V-2

25

5.1

32.2

2.8

V-2

V-0

15

3.6

29.9

3.4

NR

NR

20

4.8

34.8

3.5

V-2

V-0

25

6.0

38.8

3.5

V-2

V-0

15

3.0

19.4

0.44

NR

NR

20

4.0

19.6

0.38

NR

NR

25

5.1

19.9

0.35

NR

NR


7.

Intumescent Systems for Flame Retarding of PP

LEWIN & ENDO

char morphology as well a s t o a t t e m p t t o d e f i n e amd compare the synergisms involved with other known synergisms, including bromine, n i t r o g e n , antimony and phosphorus. Experimental. T h e P P u s e d was a n i n j e c t i o n - g r a d e p o w d e r (MFR = 15g/ 10 m i n . , 2 3 0 C ) . The s i x intumescent formulations used were: 1 . AMGARD E D A P , b e l i e v e d t o c o n t a i n APP reacted with ethylene diamine [ 2 , 3 D ; 2. APP w i t h o u t co-addi t i v e s ; 3 . A P P + S p i n f l a m MF82 ( a s s u m e d t o c o n t a i n p o l y triazine-piperazine) [3D; 4. APP + pentaerythiritol (petol); 5. APP + trishydroxyethyl isocyanurate (THEIC); 6. Exolit IFR 23P C 1 3 . The r a t i o of APP to c o - a d d i t i v e s was 2 : 1 i n a l l c a s e s . The components were mixed i n the molten s t a t e i n a Brabender blender of 240 m l a t 2 0 0 C a n d 40 r p m f o r 12 minutes. The samples were compression-molded at 230 C, cooled to room t e m p e r a t u r e and cut to t e s t p i e c e s . Flame retardancy testing i n c l u d e d 01 a n d U L - 9 4 t e s t s . Formulations are given i n T a b l e I.


e

e

e

Cone c a l o r i m e t e r measurements were c a r r i e d out at the N a t i o n a l I n s t i t u t e of S t a n d a r d s and T e c h n o l o g y on 4 s a m p l e s : p u r e P P , P P + 30 w t . % o f E D A P , P P + 20 w t . % of A P P + S p i n f l a m a n d P P + 20 w t . % o f A P P + p e n t a e r y thritol (2:1). The samples were compression-molded to 1/6" a n d c u t t o 10 χ 10 cm t e s t p i e c e s . The heat flux a p p l i e d was 3 5 . 0 K W / m in a l l cases. Spark ignition and grid and frame were u s e d . The sample orientation was h o r i z o n t a l . The m o r p h o l o g y o f s u r f a c e s and cross-sections of char specimens was investigated by s c a n n i n g electron microscopy at enlargements of 250-1500. The char specimens were taken from the s u r f a c e s of 5 samples of PP, c o m p o u n d e d w i t h 25 w t . % o f 5 FR formulations as described above, a f t e r c o m b u s t i o n i n t h e 01 a p p a r a t u s at the oxygen c o n c e n t r a t i o n of t h e i r 01. 2

Results

and

Discussion

I t a p p e a r s t o be g e n e r a l l y a c c e p t e d t h a t , for intumes cent systems, three basic ingredients are required: a catalyst, i.e. APP, a char former i . e . pentaerythritoi (petol) and a blowing agent, i . e . melamine or another n i t r o g e n - d e r i v a t i v e forming incombustible gases during pyrolysis and combustion. [13. F o r the formulations u s e d 01 a n d U L - 9 4 d a t a a r e p r e s e n t e d i n T a b l e I. T h e c h a n g e o f 01 versus weight percent of the flame retardant additives calculated as percent p h o s p h o r u s i s s h o w n i n F i g u r e 1 . T h e 01 v a l u e s increase linearily w i t h t h e amount o f a d d i t i v e s . The slopes of the s t r a i g h t l i n e s differ markedly for the various additives. The lowest slope i s f o r APP w i t h o u t co-


93

94



60

50H

Δ

EDAP

x

APP

Ο

APP+MF82

•

APP+PETOL

•

APP+THEIC

•

Exollt IFR 23P

10

2

4

6

Phosphorus Content [wt%] Figure

1. 01 v s phosphorus

content.



7.

LEWIN & ENDO


95

additives, showing t h e low a c t i v i t y o f APP i n s p i t e of the high (7.8%) Ρ c o n t e n t (see T a b l e I ) . Substitution of 1/3 o f t h e APP b y p e t o l i n c r e a s e d t h e F R p e r f o r m a n c e although, at t h e l e v e l o f 25% a d d i t i v e , o n l y a n 01 o f 25 a n d U L - 9 4 V - 2 t e s t p e r f o r m a n c e achieved. A h i g h e r F R a c t i v i t y i s s h o w n b y AMGARD E D A P C3J. At 25% additive, an 01 of 27.6 and UL-94 V - 2 t e s t p e r f o r m a n c e was n o t o b t a i n e d , w h e r e a s at a 30% EDAP l e v e l , U L - 9 4 V - 2 f o r t e s t p e r f o r m a n c e 1/16" and V - 0 f o r 1/8" samples were a c h i e v e d . The r e s u l t s a r e c o n s i d e r a b l y i m p r o v e d - when THEIC, Spinflam MF82 and Exolit IFR are used. 20% o f the a d d i t i v e i n t h e l a s t t w o c a s e s r e s u l t e d i n 01 v a l u e s of 30 and 35, respectively, and a UL-94 V-2 test p e r f o r m a n c e f o r 1/16" a n d V - 0 f o r 1/8" samples, similar t o t h e s a m p l e s w i t h 30% E D A P . In F i g u r e 1, t h e i n f l u e n c e o f t h e c o - a d d i t i v e s on the APP, which are c o n s i d e r e d s y n e r g i s t s , is evident. The system a p p e a r s t o be v e r y s e n s i t i v e t o t h e n a t u r e of the c o - a d d i t i v e . In F i g u r e 2, t h e FR effectivity, e.g. 01/% Ρ C43 i s c o m p a r e d f o r t h e v a r i o u s samples; i t v a r i e s f r o m 0.31 f o r APP t o 2.1 f o r EDAP and 3 . 5 for Exolit IFR. These values are much h i g h e r t h a n the g e n e r a l v a l u e o f 1 . 3 g i v e n b y V a n K r e v e l e n C43 f o r Ρ i n PP, which does not r e f e r to intumescent systems. Synergistic Effectivity. In T a b l e I I , the FR e f f e c t i v i t y o f APP + p e t o l and APP + melamine a r e g i v e n as 1.7 and 0.92, respectively. When d i v i d i n g t h e s e effectivit i e s by the FR effectivity of APP, a "synergistic effectivity" (SE) is obtained, y i e l d i n g values of 5.5 for petol and 3.0 for melamine. The SE for the combined synergists is 7.7. The values of FR effectivity and S E are considerably higher for the other intumescent systems i n t h i s s t u d y and a r e , i n the case of E x o l i t IFR, 3.5 and 1 1 . 3 , respectively.

T a b l e 11. FR E f f e c t i v i t y ( E F F ) and S y n e r g i s t i c E f f e c t i v i t y ( S E ) of A P P - b a s e d Systems on PP

FR

SYNERGIST

APP Petol Melamine Petol + Melamine EDAP APP EXOLIT IFR 23P

Spinflam MF82

EFF

SE

0.31 1.7 0.92

5.5 3.0

2.4 2.1

7.7 6.8

3.0

9.7

3.5

11.3




4

ι ι,ι ι Ijljiiljliiill

2H

h -ιϋΐϋϋϋϋϋϋ

« :ti|ii>s.'|1

11

t

ι»

EDAP

APP

t :

I» Ί

APP+

APP-f

APP*

MF82

PETOL

THEIC

Exolit

FR Additives Figure

2.

FR e f f e c t i v i t y

of

phosphorus

FR

additives.


7.

LEWIN & E N D O


97

S y n e r g i s t i c s y s t e m s a r e w e l l known i n t h e litera ture d e a l i n g with flame retardancy of polymers [5,6,73. It i s of i n t e r e s t to compare several of the known synergisms with the PP-APP intumescent s y s t e m shown i n Table II.


Comparing Synergisms. In T a b l e III a r e p r e s e n t e d data computed from the literature on a number of FR synergistic systems. The FR E F F and the SE are calculated for c o t t o n to w h i c h P y r o v a t e x PC ( m e t h y l o l d e r i v a t i v e of d i a l k y l phosphonic propionamide) together

Table

III.

POLYMER Cotton

PP

PS

PC/PET) 2: 1

PAN

FR E f f e c t i v i t y and S y n e r g i s t i c Effectivity of B r - S b , B r - P amd O t h e r S y s t e m s SYNERG.

FR Pyrovatex Pyrovatex

Aliph.. Aliph..

SE

REF.

1.75

[83

SbtzO-s

0.45 1.0 0.6 2.6

4.3

S02O3

0.5 1.1

2.2

Sb 0 2

CI CI

TPP BrPC BrPC/TPP BrP:70:3

4.0 7.0

TMM

Arom. Br Arom. Br A l i p h ., Br A l i p h .. Br

EFF.

3

2.2 [43

[43

13.3 1.7 Blend

APP HBCD APP + HBCD

1.38 1.57

70:3

1 .02 1 .21

[103

[93 1 .55

with TMM ( t r i m e t h y l o l m e l a m i n e ) w e r e a p p l i e d a t a 2% Ρ and 5% Ν level [83. Whereas relatively high FR e f f e c t i v i t i e s are noted, the SE i s o n l y 1.75. Although the i n g r e d i e n t s of t h i s system phosphorus, nitrogen and a polyhydric alcohol (cotton cellulose) resemble an i n t u m e s c e n t system, the SE is much lower than observed in this study. Data on the S E of a r o m a t i c and a l i p h a t i c bromine d e r i v a t i v e s with antimony t r i o x i d e , another convention al synergistic system, a r e computed from r e f e r e n c e 4, showing S E v a l u e s of 2.2 and 4 . 3 , respectively. Sim ilarly, for aliphatic chlorine derivatives with a n t i mony t r i o x i d e , a S E v a l u e o f 2 . 2 i s computed f o r poly styrene [4 3. Of particular interest a r e the d a t a on b r o m i n e phosphorus synergism. In the case of p o l y a c r y l o n i t r i l e



98


(PAN) t r e a t e d w i t h v a r y i n g r a t i o s o f APP and h e x a b r o m o cyclododecane C93, a S E value of 1.55 is obtained. This low SE value is remarkable since the bromine c o m p o u n d was shown t o a c t a s a b l o w i n g a g e n t with the gaseous hydrogen bromide, released i n the pyrolysis, s e r v i n g to foam c h a r . S i m i l a r S E v a l u e s a r e computed from d a t a i n r e f e r ence 10 f o r a p o l y c a r b o n a t e - p o l y e t h y l e n e terephthalate 2:1 b l e n d , t r e a t e d w i t h varying ratios of triphenyl phosp h a t e (TPP) and b r o m i n a t e d p o l y c a r b o n a t e . An SE v a l u e o f 1.38 is found. When a brominated phosphate with the ratio of bromine to phosphorus of 70:3 is a d d e d t o t h e same b l e n d , i t y i e l d s a value of 1.58. There are some i n d i c a t i o n s , though no c l e a r evidence, t h a t t h e b r o m i n e compound may, in these cases, also serve, at least p a r t l y , as b l o w i n g a g e n t s and not as gas-phase flame r e t a r d a n t s . The bromine-phosphorus SE v a l u e s are c o n s i d e r a b l y lower than the bromine-antimony ones, i n d i c a t i n g the possibility of different flame retardant mechanisms. FÇJlaUvg Importance of C o - a d d i t i v e s . The synergistic effectivity of the PP-APP system as d e s c r i b e d in this paper is very much higher than the other known synergisms, which emphasizes the sensitivity of the intumescent system to the nature of the c o - a d d i t i v e s on one hand, and the vast possibilities for further improvement, on t h e o t h e r h a n d . It i s , however, s t i l l u n c l e a r w h a t t h e r e l a t i v e i m p o r t a n c e i s o f t h e two last co-additives. Results of a s e r i e s of experiments in w h i c h the wt. % of PP and APP were kept c o n s t a n t at 75 and 16.6 wt. % respectively, w h i l e the p r o p o r t i o n of melamine i n the 8.4 wt. % of petol + melamine was varied, i s shown i n F i g u r e 3. B o t h p e t o l and melamine a r e c l e a r l y s e e n t o be s y n e r g i s t s for APP, the effect being much more pronounced for petol. When used together, they are, i n f a c t , co-synergists. However, it is of interest to note that compositions of p e t o l / m e l a m i n e 2 0 : 8 0 t o 8 0 : 2 0 % show v i r t u a l l y t h e same 01 values of ca 30. This indicates that, at l e a s t in this range, melamine and petol are equivalent and interchangable. Since the petol shows a higher effectivity, t h e r o l e o f t h e b l o w i n g a g e n t seems t o be minor and n e c e s s a r y o n l y i n s m a l l amounts. Mechanistic Considerations The strong synergistic effect in intumescent systems is not suprising, considering the h i g h l y complex n a t u r e of the c h e m i c a l and p h y s i c a l i n t e r a c t i o n s between m a t e r i a l s i n v o l v e d C113. The " c a t a l y t i c " a c t i o n of APP i s b e l i e v e d to c o n s i s t of a series of processes occurring during the comb u s t i o n : d e c o m p o s t i o n and r e l e a s e o f ammonia and w a t e r , phosphorylation of petol and possibly also of the oxidized PP, dephosphorylation and double-bond




LEWIN & E N D O

Figure

3.

01 v s m e l a m i n e / m e l a m i n e polypropylene.

+ petol

for

FR



100


f o r m a t i o n [123, p a r t i c i p a t i o n as one of the building blocks of char. The petol is dehydrated and cross-linked via etheric linkages; the PP is oxidized and d e h y d r o g e n a t e d d u r i n g c o m b u s t i o n , f o r m s cross-links and is a dominant c o n s t i t u e n t i n the char. The blowing a g e n t decomposes to i n c o m b u s t i b l e g a s e s , w h i c h foam the char s t r u c t u r e . A l l the above p r o c e s s e s occur i n a v e r y short time, with various reaction rates. The r a t i o of these rates which determines the sequence of the r e a c t i o n s and t h e i r t i m i n g has a dominant e f f e c t on the p r o p e r t i e s o f the f i n a l c h a r and on the FR behavior. These rates might be influenced by catalysts and p o s s i b l y engineered to b e t t e r flame retardancy. The d e h y d r a t i o n r e a c t i o n a p p e a r s to occur by a p h o s p h o r y l a t i o n - d e p h o s p h o r y l a t i o n mechanism [ 5 , 6 , 7 ] , in which water molecules are r e l e a s e d . The rate of the phosphorylation reaction is strongly influenced by n i t r o g e n - c o n t a i n i n g m o i e t i e s [63. It has been shown that phosphorylation of cellulose by phosphorus t r i amide c a n be c a r r i e d o u t at low temperatures (60 C) even in aqueous mediums [12 3. Urea is known to accelerate phosphorylation [53. e

A d d i t i o n of p e t o l , which is not a flame retardant but r a t h e r an a d d i t i o n a l f u e l , to APP i s e f f e c t i v e (see T a b l e I and F i g u r e s 1-3), albeit only partially. The dehydration c h a r - f o r m i n g mechanism appears, therefore, to be o p e r a t i v e e v e n without the additional blowing agent. The application of petol-tetra-benzoate as a co-additive instead of petol in the intumescent formulation yields negative results, showing that the mechanism indeed proceeds v i a e s t e r i f i c a t i o n . In o r d e r for the petol-t-benzoate to be a c t i v e , a transesterf i c a t i o n would have to take p l a c e , forming a phosphate during combustion. This t r a n s e s t e r f i c a t i o n is too slow for an efficient dehydration to occur under the c o n d i t i o n s o f t h e 01 a n d U L - 9 4 tests. Thermogravimetric Analysis. The results of the thermogravimetric analysis (TGA) i n a i r are shown in Table IV and Figures 4-7. In F i g u r e 4, s e v e r a l TGA d i a g r a m s of PP w i t h a d d i t i v e s a r e shown. The PP b e g i n s to d e c o m p o s e a t 2 7 0 - 2 8 0 ° C a n d d e c o m p o s e s i n o n e o r two stages. The slope of the t r a n s i t i o n s of a l l additivecontaining samples i s s m a l l e r t h a n t h a t o f P P . In t h e c a s e of APP w i t h S p i n f l a m , the slope of the second t r a n s i t i o n i n c r e a s e d w i t h t h e amount o f f l a m e retardant agent present. The s l o p e s of other samples a r e nearly unaffected by t h e amount o f t h e f l a m e r e t a r d a n t addit i v e . The f i n a l r e s i d u e at 650°C does not seem to c o r r e l a t e w i t h t h e amount o f a d d i t i v e o r w i t h % P . The h i g h e s t amounts of r e s i d u e a r e o b t a i n e d f o r E x o l i t , APP w i t h MF82 a n d E D A P . A significant linear correlation, however, found between the r e s i d u e - a f t e r - t r a n s i t i o n s (RAT) the amount of the flame r l e t a r d a n t a d d i t i v e ( F i g .


was and 5).


7.

101

Intumescent Systems for Flame Retarding of PR

LEWIN & E N D O

160

140 - Polypropylene

Polypropylene + EDAP

27l.8 C l Î « , . e

, 0 0

l σ> 80 α>

V 1 J 1 '\ 96 68% (IQ51 mg) 305.4 cl «

^ 60

7 40 _ 1 20 0

e

331 5 C T679$%(6.90mg)

»Τ»

e

/ \\ 340.5 cl 1 1 u=fc=-H e

1

Rttlduts 2 44% (0.285mg) Η 1 i_l

9.137. (0.939mg) RAT,

160

Polypropylene + APP

140 -

\ ^36.5^^73%(I OOmq)

Polypropylene + APP + Spinflam MF82

120100-

2903-C i\ 4.76% (0.54 mg) 3399TTV. » i

806071.65% ( ai4mg)

4020 200 400 600 Temperature (°C) Figure

800 4.

TGA

200 400 600 Temperature ( °C ) diagrams.


800

102



60

Δ

EDAP

X APP

50H

40 0

Ο

APP+MF82

•

APP+PETOL

•

APP+THEIC

•

ExoM IFR 23P

s

30 Β

QC fl χ 2

20H

10H

—ι

10

•

1

20

•

ι

30

·-

40

50

FR Additives wt% Figure

5.

TGAr e s i d u e - a f t e r - t r a n s i t i o n s wt. % of FR a d d i t i v e s .

(RAT) vs


7.

LEWIN&ENDO

Table


IV. TGA R e s u l t s o f Phosphorus-Based 1st T r a n s i t i o n

No

FR

FRwt% Ts['C]

S l o p e Tp[-C]

wt%

Slope

[wt%]

Residue at 650Ό (vt%l

1.9

2.4

RAT

[vtt/daq]

272

305

96.7

-1.52

20

281

363

74.2

-0.60

434

3.1

22.7

9.2

25

261

332

67.9

-0.60

430

9.1

23.0

10.7

30

257

336

59.1

-0.49

434

13.2

27.7

9.0

5

35

273

351

59.3

-0.55

440

11.7

29.0

8.6

6

40

263

338

62.2

-0.58

431

11.8

26.0

14.7

15

260

293

33.4

-0.36

387

45.9

-1.15 20.7

7.5

PP

2 3 4


Additives i n Polypropylene

2nd T r a n s i t i o n

0

1

PP + EDAP

7 8

PP + APP

9

20

251

282

27.1 -0.42

25

253

285

13.8

-0.25

380

47.8

-0.54

25.1

7.7

384

57.4

-1.39 28.8

7.4

15

307

322

9.2

407

72.7

-0.88

18.1

4.5·

11

PP + APP

20

297

324

6.7

425

71.4

-1.09 21.9

9.6

12

+Spiiifl«n MF 8 2

25

290

340

4.8

432

71.7

-1.40

15

269

307

77.4

20

261

300

25

259

301

15

269

3 1 0

20

266

25

10

13 14 15 16 17 !

Tpc-C] wt%

103

1

8

19

PP + APP + PETOL PP + APP + THEIC

PP + 20 Exolit 21 IFR 23P

23.5

11.3

-0.80

22.6

5.9

74.3

-0.81

25.7

6.5

71.0

-0.70

29.0

5.9

78.1 -0.90

21.9

8.2

306

76.3

-0.88

23.7

6.7

262

307

73.1

-0.78

26.9

6.6

15

264

360

83.1 -0.54

16.9

5.8

20

260

362

75.4

-0.49

24.6

14.7

25

258

298

72.1 -0.58

27.9

13.5

.

The RAT i n c r e a s e s w i t h the % a d d i t i v e in all cases. This behavior appears c o m p a t i b l e w i t h an intumescent mechanism. It i s r e a s o n a b l e to assume t h a t the actual com b u s t i o n and f l a m i n g r e a c t i o n o c c u r r e d d u r i n g the trans itions. The char remaining d i r e c t l y a f t e r the transi t i o n s has j u s t been formed i n t o a foaming s t r u c t u r e and d i d n o t y e t have t i m e t o be further oxidized at the higher TGA t e m p e r a t u r e s and d e t e r i o r a t e and p o s s i b l y collapse. T h e amount o f t h i s "primary" char clearly d e p e n d s on t h e amount and n a t u r e o f t h e f l a m e retardant a d d i t i v e i n the sample. I t i s t o be e x p e c t e d that this primary char is linked to the flammability of the samples. T h e r e f o r e , i t is of particular interest and not surprising to note the h i g h l y s i g n i f i c a n t linear


104



r e l a t i o n s h i p b e t w e e n t h e 01 and % RAT ( r = 0 . 9 9 ; see Figure 6 ) . T h i s r e l a t i o n s h i p a p p e a r s t o be t y p i c a l f o r t h e i n t u m e s c e n t m e c h a n i s m a n d may s e r v e a s a clue for recognizing and c h a r a c t e r i z i n g the n a t u r e of the flame retardant activity. It was obtained for all six formulations investigated. The slopes, 01/RAT, of the l i n e s of F i g u r e 6 are given i n F i g u r e 7 f o r the v a r i o u s formulations. When c o m p a r i n g t h i s d a t a to the d a t a on the flame r e t a r d a n t e f f e c t i v i t y , some similar trends can be d i s c e r n e d . Cone Calorimetry. The r e s u l t s of the cone c a l o r i m e t e r t e s t i n g of four samples are shown in Table V and Figures 8, 9 and 10. Uncompounded polypropylene y i e l d e d the lowest r e s i d u e and the h i g h e s t total heat release. T h e p e a k r a t e o f h e a t r e l e a s e was t h e highest and i t was r e a c h e d i n t h e s h o r t e s t t i m e ; c o m b u s t i o n was rapid. The amounts of heat r e l e a s e d d e c r e a s e d w i t h the d e g r e e of flame r e t a r d a n c y , p o s s i b l y due to a slower and lower p o l y m e r - f u e l s u p p l y to the flame and to the f a c t that the char d i d not combust. It i s t y p i c a l for the flame r e t a r d a n t samples that the peak r a t e of heat r e l e a s e is delayed by the f o r m a t i o n of char. The f a c t t h a t t h e amounts o f c a r b o n monoxide were higher for the flame r e t a r d a n t samples w h i l e the C 0 amounts were lower i n d i c a t e s i n c o m p l e t e combustion in the flame retardant samples, which c o r r e l a t e s with the high char residue. 2

Two i n t e r e s t i n g observations can be made from Figures 8 and 10: (1) the time of peak r a t e of heat r e l e a s e is delayed p r o p o r t i o n a t e l y to the effectivity of the flame retardant formulation; (2) the heat release occurs in 2 stages. The f i r s t stage occurs at about the same t i m e f o r a l l 3 f o r m u l a t i o n s . Its peak i s smaller than the main second peak. It decreases with the i n c r e a s e i n flame r e t a r d a n t e f f e c t i v i t y . The mass l o s s r a t e a n d t h e s p e c i f i c smoke extinction area behave i n a s i m i l a r manner. The reason for the two peaks resides i n the dynamics of the combustion and the char f o r m a t i o n . It is b e l i e v e d t h a t a r a p i d l a y e r of c e l l u l a r foamed c h a r is formed at f i r s t . The pressure of the combustion gases produces a char s t r u c t u r e which is impermeable to the gases as w e l l as to the m o l t e n polymer. With an increase in pressure, the char surface barrier is p u s h e d away f r o m t h e m a i n b u l k o f t h e p o l y m e r and a gas bubble is formed which separates and i n s u l a t e s the b a r r i e r from the rest of the polymer. The continued heating b r e a k s t h e b a r r i e r a f t e r some t i m e ; b u r n i n g is resumed and a n o t h e r l a y e r o f char i s p r o d u c e d C83. Figure between most

9 of

illustrates a remarkable the cone c a l o r i m e t e r v a l u e s

correlation and the 01.


Intumescent Systems for Flame Retarding of FF


LEWIN & ENDO


106

FIRE AND P O L Y M E R S II


2

110: .!ililf!i|lH!.| !

I I J I J I

1 H ι

M i !

t : : : :

M l

:;:|:::|:|:!ΐ|:| ι 111 :

t

,: : ::t iti « : t

:

t

ι :

t Mi t>i : 11 : ι ι » ι ι ι ι ι

•iVt.il l

EDAP

APP

! !

ι t ι : ι » ti

: ι : t s t

'ί1"!!ΐ!!·ί!'!" ι :»t t » ι t*i :«t t J ι tjî; 11 W s ' t t « »

l|M«t,t{jj|

1η

l i a i

APP+

APP+

APP+

MF82

PETOL

THEIC

Exolit

FR Additives Figure

7.

01/RAT

for

FR

additives.



*Rate

of

Heat

2

568.26 2.11012 0.03184 29.8 V-2

615.52 3.17844 0.03008 17.8 NR

m /kg kg/kg kg/kg %

3

30.35

38.94

MJ/kg

9 9 . 17 3.27

2

246.76

28. 1 9.4 33.5 47 330 143.8 58.85

0.06174 23.6 V-2

2.39999

1.95889 0.04183 30.7 NR

735.82

32.62

4.63

151.15

27.4 5.6 20.4 41 245 205.4 69.09

PP + APP+PETOL 20%

215.34

27.71

1 .46

40.38

28.3 12.2 43. 1 47 460 136.9 51 . 2 1

PP + APP+MF82 20%

Sample

Calorimeter

PP + EDAP 30%

Cone

6.34

2

of

26.7 2.5 9.4 62 165 575.3 91 . 3 2

PP

Results

g/s*m

kw/m

3 5

g g wt.% sec . sec . kw/m MJ/m

V.

Release

Initial Mass Final Mass Residue Ignition Time T i m e o f P e a k RHR* P e a k RHR T o t a l Heat Release Heat Release a t 300 s e c . Mass Loss Rate a t 300 s e c . Heat of Combustion a t 300 s e c . Specific E x t . Area a t 300 s e c . Carbon Dioxide a t 300 s e c . Carbon Monoxide a t 300 s e c . 01 UL-94 1/16"

DATA

Table



108

FIRE A N D P O L Y M E R S II

6 0O. 5M.

3QQ. 200.

Minutes Polypropylene Α-β 3KM roiypropsiivni ne B-3 £ ο Ι y p r o p y l e nne e Ç-Ç lypi»opylene " " Τ5868 P l a s t i c P o l y p r o p y l e n i -T3863 t ιi ο c i^oDj P πl«aχs τ "

Σ|§§2 £ϊ**ί*°

Figure

8.

Rate o f heat release (RHR) v s t i m e f o r PP and P P t r e a t e d w i t h i n t u m e s c e n t FR f ormulat ions.


7.


LEWIN & ENDO

500 ^t l,l,:,t,l,l,l|

Φ

t

*400

ce I ce ^300 Λ Φ ο.

mm i

-200

i • 1*11 M

rii.i

È

iis,t:

F7500 î i i l i ε £ 400

!»»««: :, j

...Λ

S ο η **

§300 !·

t i «{ι >

§200 α 100

"IllSl !i t>t>: : tj Jt,i,:,t t :

:

« : .» I

EDAP

It,!, I,t,t • t

i!

MF82

CM

8 EDAP

Peak RHR

MF82

C02 at 300sec

FTtOO

PP

EDAP

MF82

PETOL

Total Heat Release Figure

9.

EDAP

MF82

PETOL

01

Cone c a l o r i m e t e r and LOI v a l u e s f o r PP and 3 PP s a m p l e s t r e a t e d w i t h IFR f ormulations.


109


110


Figure

10.

Time of (TPRHR)

peak of r a t e of heat release vs FR e f f e c t i v i t y (EFF).


7.

LEWIN & ENDO

Intumescent Systems for Marne Retarding of PP

111


T h e s e q u e n c e o f t h e 01 v a l u e s i n t h e t e s t e d s a m p l e s is rather c l o s e to the i n v e r s e sequence of the t o t a l heat released, o f m a s s l o s s r a t e , o f t h e p e a k RHR a n d t o the sequence of the t i m e o f peak RHR. It a p p e a r s , therefore, that the 01 measurements allow a qualitative prediction as to the b e h a v i o r of an intumescent flame r e t a r d a n t - t r e a t e d polymer i n the Cone c a l o r i m e t e r . Char Morphology. SEM s c a n s o f c h a r f r o m the surfaces of f i v e PP samples, t r e a t e d by the intumescent systems applied in this study, are presented in Figures 11-13. The c e l l s t r u c t u r e of the c h a r i s c l e a r l y v i s i b l e , both on t h e s u r f a c e as w e l l as on t h e cross-section scans. The c e l l s a p p e a r t o be c l o s e d and t h e i r d i a m e t e r s range from 7-40 microns in the surface scans and 15-45 microns i n the c r o s s - s e c t i n scans. Differences can be seen i n the g e n e r a l appearance and in the ranges of cell diameters of the various scans. An attempt to define these difference is made in Table VI. The widest range for c e l l diameters i s found i n the PP + APP + p e t o l f o r m u l a t i o n , i n w h i c h no b l o w i n g a g e n t was present. Although a c e l l structure is developed, only a p a r t of the s u r f a c e is foamed. In the c a s e o f PP + EDAP and PP + E x o l i t , the whole surface of the char scans i s foamed and the cell dimensions are in a r e l a t i v e l y narrow range. Table

VI.

SEM O b s e r v a t i o n s

Diameter of c e l l microns

of

the

Chars

Observation

Sample

Surface

Cross section

EDAP

20-27

20-40

MF82

7-20

30-45

Foaming forms membrane

Partial foaming

PETOL

27-40

15-45

Wide d i a m e t e r . Clear foaming.

Partial f oaming Disturbed structure

EXOLIT

10-20

15-30

Rigid foam structure. Clear foaming

Nearly complete f oaming

THEIC

23-37

27-40

Surface

Cross

Section

Nearly uniform Nearly cell diameter complete f oaming

unclear foaming

Unclear foaming Expanded as whole



Figure

11.

SEM s c a n s o f c h a r s u r f a c e s of 4 PP samples t r e a t e d w i t h 4 IFR f o r m u l a t i o n s ; 25 w t . %, χ 300.


Ο

•

50


Figure

12.

SEM scans o f c r o s s - s e c t i o n s samples of F i g u r e 11.

of

char

from



Figure

13.

SEM s c a n s o f c h a r 25 w t . % o f E D A P .

from

PP

treated

with


H g

Ο

>

7.

LEWIN&ENDO


115

In s e v e r a l c a s e s , s u c h as i n the sample treated with Spinflam M F 8 2 , a b a l l o o n i n g o r v o i d was observed below the c r u s t o f the foamed c h a r . This appears to correspond to the first p e a k i n RHR o b s e r v e d i n the Cone c a l o r i m e t e r t e s t i n g , and seems t o c o r r o b o r a t e the above consideration on the two-stage intumescent process i n APP—treated samples [133.


Conclusions The flammability behavior of injection-grade poly propylene treated with six phosphorus-based flame r e t a r d i n g a d d i t i v e f o r m u l a t i o n s was i n v e s t i g a t e d by 01, TGA, Cone c a l o r i m e t r y , SEM and U L - 9 4 m e t h o d s . Linear correlations between the weight percent of the additives and of percent phosphorus and 01 were established in all cases. The flame retardant effectivity (EFF), 01/% Ρ i n c r e a s e d f r o m 0.31 to 3.5 i n the f o l l o w i n g o r d e r : APP, APP + p e t o l , EDAP, APP + T H E I C , APP + S p i n f l a m MF82, E x o l i t IFR. The Synergistic Effectivity ( S E ) was d e f i n e d as the r a t i o of E F F of the flame r e t a r d a n t additive with the synergist co-additive to the E F F of the flame retardant additive alone. The SE was found for the formulations investigated to be in the range of 5 . 5 - 1 1 . 3 and compared to the S E of other phosphorusnitrogen systems (1.75), of aliphatic and a r o m a t i c b r o m i n e - a n t i m o n y o x i d e f o r m u l a t i o n s (4.3 and 2.2), of chlorine-antimony oxide (2.2) and of bromine-phos phorus-based systems (1.4-1.6). S i g n i f i c a n t l i n e a r c o r r e l a t i o n s were found f o r a l l formulations between the 01 v a l u e s a n d t h e v a l u e s of the r e s i d u e - a f t e r - t r a n s i t i o n s (RAT) obtained i n TGA measurements c a r r i e d out i n the presence of a i r . Cone c a l o r i m e t r y measurements showed t h a t t h e peak rate of heat r e l e a s e is delayed p r o p o r t i o n a t e l y to the EFF of the flame r e t a r d a n t f o r m u l a t i o n s . Heat release o c c u r s i n two s t a g e s , indicating a stepwise formation of char layers during the combustion in the Cone calorimeter. An inverse correlation was found between the s e q u e n c e o f t h e 01 v a l u e s o f t h e t e s t e d s a m p l e s a n d t h e s e q u e n c e s o f the t o t a l h e a t r e l e a s e , o f mass l o s s rate and of the peak r a t e of h e a t r e l e a s e , indicating that the 01 measurements allow a prediction of the combustion behavior of an intumescent flame retardant t r e a t e d - p o l y m e r i n the Cone c a l o r i m e t e r . The SEM s c a n s of the chars obtained in the combustion of polypropylene samples t r e a t e d w i t h the intumescent flame retardants reveal a well-defined foamed c e l l s t r u c t u r e i n a l l c a s e s , b o t h on the surface as w e l l as on t h e c r o s s - s e c t i o n of the c h a r s . The cell dimensions were in the range o f 7-40 m i c r o n s i n the s u r f a c e scans and 15-45 m i c r o n s i n the c r o s s sections, d e p e n d i n g on the formulations.


116



Literature Cited 1. Sharf, D . , Nalepa, R . , Heflin R. and Wus, T. Fire Safety J. 19, 103 (1992). 2. Weil, E . in Rec. Adv. in FR of Polym. Mat., V o l . 3, M. Lewin, E d . , BCC, p. 1 (1992). 3. Goin, C.L and Huggard, M . T . , i b i d , V o l . 2, p. 94 (1991). 4. Van Krevelen, D.W., in App. Poly. Symp. J . App. Polym. S c i . 31, M. Lewin, Ed. p. 269 (1977). 5. Lewin, Μ., in Chemical Processing of Fibers and Fabrics, Part 2 - Functional Finishing, M. Lewin and S. B. Sello, Eds., Marcel Dekker, 1983, pp. 1-141. 6. Khanna, Y.P. and Pearce, E . M . , in Flame Retardant Polymeric Materials, V o l . 2, M. Lewin, S.M. Atlas and E.M. Pearce, Eds., Plenum, 1978, pp. 43-61. 7. Lewin, M. and Sello, S . B . , i b i d , V o l . 1, (1975), pp. 19-136. 8. Willard, J . and Wondra, A.E., Tex. Res. J. 40, 203 (1970). 9. B a l l i s t e r i , Α., Montaudo, G., Puglisi, C., Scamporrino, E . and Vitallini, D . , J . Appl. Polym. Sci., 28, 1743 (1983). 10. Green, J., Rec. Adv. in FR of Polym. Mat., M. Lewin, E d . , V o l . 4, BCC, p. 8 (1993). 11. Camino, G . , Costa, L . and T r o s s a r e l l i , L . , Polym. Degr. Stab. 12, 213 (1985). 12. Basch, A. and Lewin, Μ., Tex. Res. J., 45, 245 (1975). 13. P a g l i a r i , Α . , C i c h e t t i , Ο . , Bevilacqua, A. and Van Hees, P . , Proc. of Flame Retardants '92 Conf., London, 22-23 J a n . , 1992, Elsevier Applied Science, pp. 41-52. RECEIVED January 4, 1995


Intumescent Systems for Flame Retarding of Polypropylene

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