Zinc Borate as a Flame Retardant, Smoke Suppressant, and Afterglow

Chapter 12

Zinc Borate as a Flame Retardant, Smoke Suppressant, and Afterglow Suppressant in Polymers Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 4, 2016 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0425.ch012

1

2

Kelvin K. Shen and T. Scott Griffin 1

United States Borax and Chemical Corporation, Los Angeles, CA 90010 United States Borax Research Corporation, Anaheim, CA 92801 2

Zinc Borate (2ZnO·3B O ∙3.5H O) is a unique multifuntional fire retardant. It can function as a flame retardant, smoke suppressant, afterglow suppressant, as well as an anti-tracking agent in polymers. It has been used extnesively as a fire retardant in PVC, polyolefin, nylon, polyester, epoxy, acrylic, urethane, rubbers, etc. 2

3

2

The use of bromine or chlorine halogen sources to impart f i r e retardancy in polymers i s well known in the p l a s t i c s industry. The halogen source can be physically blended into the resin (additive approach) or chemically b u i l t into the polymer backbone (reactive approach). In order to enhance f i r e retardancy, antimony oxide i s usually used as a synergistic additive. In recent years, considerable e f f o r t has been expended to find either p a r t i a l or complete substitutes f o r antimony oxide. This e f f o r t has been spurred by the desire to achieve smoke reduction, better cost/performance balance, to obtain a product with low t i n t i n g strength, and by the concern f o r the possible t o x i c i t y of antimony oxide. One of these substitutes i s a special form of zinc borate with a molecular formula of 2Zn0-3B 0 -3.5H 0, known in the trade as FIREBRAKE ZB (J., 2). Zinc borates with d i f f e r e n t mole ratios of Zn0:B 0 :H 0 can be readily prepared by reacting zinc oxide with boric acid ( F i g . 1). Among a l l these zinc borates, the compound with a molecular formula of 2Zn0-3B 0 -3.5H 0, i s the most commonly used f i r e retardant. This a r t i c l e w i l l review recent research results on the use of this p a r t i c u l a r form of zinc borate as a multifunctional f i r e retardant. 2

3

2

2

2

3

3

2

0097-6156/90/0425-00157$06.25/0 © 1990 American Chemical Society

Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

2

158

FIRE AND POLYMERS

DISCUSSION The m a j o r a d v a n t a g e s as f o l l o w s :

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 4, 2016 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0425.ch012

1.

2.

3. 4.

5. 6.

of

using

the

zinc

borate

can

be

summarized

High Dehydration Temperature In contrast to other forms of z i n c b o r a t e s , the water of hydration of the z i n c borate is retained up to 290°C, thus allowing it t o be u s e d in polymers r e q u i r i n g high processing temperatures. The p r o p o s e d molecular s t r u c t u r e f o r the z i n c borate is depicted in F i g . 2. The high dehydration temperature can be e x p l a i n e d by the absence of i n t e r s t i t i a l water i n the c r y s t a l l a t t i c e . Completely Reacted Material The z i n c borate contains no free zinc oxide. T h u s , i t does n o t have s e r i o u s d e t r i m e n t a l e f f e c t s on t h e t h e r m a l s t a b i l i t y o f PVC o r c h l o r i n a t e d p a r a f f i n as f r e e z i n c o x i d e d o e s . A Multifunctional Smoke Suppressant, Flame Retardant, and Afterglow Suppressant. Low T i n t i n g S t r e n g t h - The z i n c b o r a t e has a r e f r a c t i v e i n d e x similar to most organic polymers, which results in the retention of considerable translucency. This provides for easy v i s u a l i n s p e c t i o n of the f i n i s h e d products and allows t h e use o f l o w e r pigment loadings in formulations requiring deep s h a d e s . Improving Electrical Properties - The z i n c b o r a t e has good a n t i t r a c k i n g and a n t i a r c i n g p r o p e r t i e s . Low T o x i c i t y Unlike antimony oxide, the zinc borate is considered non-toxic.

The p e r f o r m a n c e o f t h e summarized i n the f o l l o w i n g :

zinc

borate

as

a

fire

retardant

is

ZINC BORATE IN HALOGEN-CONTAINING POLYMERS. AS A FLAME RETARDANT. The zinc borate is an e f f i c i e n t synergist of organic halogen sources. In certain halogen-containing systems s u c h as unsaturated polyester, epoxy (_3), a n d r i g i d P V C , t h e z i n c b o r a t e a l o n e c a n o u t p e r f o r m a n t i m o n y o x i d e as shown by t h e O x y g e n I n d e x a n d U L - 9 4 t e s t s ( F i g . 3, 4, and 5 ) . In other halogen-containing systems, s u c h as flexible PVC and polyolefins, it is preferable to use the zinc borate in conjunction with antimony oxide for maximum performance. In flexible PVC, f o r example, the z i n c borate alone is not very e f f e c t i v e i n t h e Oxygen Index t e s t ( F i g . 6 ) , but a combination of the z i n c b o r a t e and a n t i m o n y oxide (1:1 ratio) outperforms equal weight of antimony oxide at a t o t a l l o a d i n g o f more t h a n 10 p h r (4_). In the presence of alumina t r i h y d r a t e (ATH), the b e n e f i c i a l e f f e c t o f u s i n g a c o m b i n a t i o n o f t h e z i n c b o r a t e and antimony oxide i s d r a m a t i c a l l y increased ( F i g . 7). AS A SMOKE SUPPRESSANT. Antimony oxide performs well as a f l a m e r e t a r d a n t i n f l e x i b l e PVC, h o w e v e r , as a gas phase f l a m e r e t a r d a n t , i t s u s e c a n d r a s t i c a l l y i n c r e a s e t h e smoke production d u r i n g PVC c o m b u s t i o n a s i l l u s t r a t e d i n F i g . 8 . The z i n c b o r a t e ,


12.

SHEN & GRIFFIN

Zinc Borate in Polymers

10 C/min.

ZB 2:3:3.5

10


ZB 1:1:2 CO CO

O ZB 2:3:7.5 20h

ZB 2:3:9

30h -L. 300

-L.

100

200

JL 500

400

600

TEMPERATURE (°C) Fig.

1.

Thermogravimetric

Analysis

2(2ZnO-3B 0 -3.5H 0) 2

3

0^

2

o

0 H0 I

/

V

B

v

x B x

of Zinc

Borates

H

o

0 I

O-B^o^C " 0

H F i g . 2. Proposed Molecular (2Zn0-3B 0 -3.5H 0). 2

3

Structure

f o r Zinc

Borate

2



5/5

2

2/8 3

WT. RATIO ( 2B/Sb 0 )

8/2

(total additive 10phr)

Fig. 3. Oxygen Index Test of P o l y l i t e 33-442 Containing Dibromoneopentyl G l y c o l , ZB - Zinc Borate, Sb - Antimony Oxide. (Reproduced with permission from Ref. 2. Copyright P l a s t i c s Compounding 1985).

ADDITIVE (phr)

25 10/0

30

35 -

40


0/10


SHEN & GRIFFIN


31

EPON 828 (1/8" CASTING) DECHLORANE PLUS 25 (25 PHR) TRIETHYLENETETRAMINE ( 1 3 PHR) (-11.8% CL) -

29

-

^2B:SB 0 (1:1) 2

3

^

• V-0

x LU

V-0

(JD X o

>-

27

25 0

2

4

6

10

ADDITIVE (PHR) Fig. 4. Oxygen Index and UL-94 Tests on Epoxy (NR - not (Reproduced with ratable, sample completely consumed), permission from Ref. 2. Copyright P l a s t i c s Compounding 1985).



FIRE AND POLYMERS

PVC (100 PARTS)

V

42

0

I

1

I

2

I

3

I

4

*

5

ADDITIVE (PHR) Fig. 5. Rigid PVC Conduit permission from Ref. 2. 1985).

Formulation. (Reproduced with Copyright P l a s t i c s Compounding



12.

SHEN & GRIFFIN

163


PVC 3 2 |- DOP ( 5 0 PHR) BA/CD/ZN STABILIZER ( 3 PHR)

—

ZB: S B 0 ( 1 : 1 ) 2

3

EPOXY SOYA ( 5 PHR)

SB 03 2

—

ZB ATH

10

15

20

25

TOTAL ADDITIVE (PHR) PVC Formulations. Fig. 6. Oxygen Index of Flexible Copyright P l a s t i c s (Reproduced with permission from Ref. 2. Compounding 1985).


30


164

FIRE AND POLYMERS

0

5

10

15

20

25

TOTAL ADDITIVE ( R ) P H

Fig. 7. Oxygen Index of Flexible PVC Formulations Containing Alumina Trihydrate (30 phr). ZB - Zinc Borate. (Reproduced with permission from Ref. 2. Copyright P l a s t i c s Compounding 1985).


SHEN & GRIFFIN


165


12.

Fig.

8.

NBS Smoke Test of Flexible PVC.

ZB - Zinc Borate.


166

FIRE AND POLYMERS

on t h e o t h e r h a n d , i s an e f f e c t i v e smoke s u p p r e s s a n t . Used w i t h antimony oxide, formulations with high ratios of zinc borate t o a n t i m o n y o x i d e y i e l d t h e g r e a t e s t smoke r e d u c t i o n ( F i g . 9 ) . In the presence of ATH, the zinc borate reduces smoke e f f e c t i v e l y , e v e n when u s e d w i t h a n t i m o n y o x i d e a t a 1:1 ratio. F i g . 10 shows t h i s d r a m a t i c smoke r e d u c t i o n . AS AN AFTERGLOW SUPPRESSANT. Due t o i t s u n i q u e glass-forming ability ( i . e . , from the B 0 m o i e t y ) , t h e z i n c b o r a t e can also function as an afterglow suppressant as i l l u s t r a t e d in Table I (5J. In addition, the zinc borate is also an effective antitracking and antiarcing agent(^). In a 1978 U.S. Patent, ICI disclosed the use of the z i n c borate in combination with a n t i m o n y o x i d e and D e c h l o r a n e Plus (an o r g a n i c chlorine source) to give a V-0 reinforced nylon-6,6 (28% f i b e r g l a s s ) with high tracking resistance. While antimony oxide can decrease the tracking resistance, the zinc borate can improve the tracking resistance. (Table II). The w a t e r of hydration of the zinc b o r a t e , when r e l e a s e d a t h i g h t e m p e r a t u r e s c a n d i f f u s e a n d c o o l the arcs. The resulting anhydrous zinc borate undergoes v i t r i f i c a t i o n and d i s p l a y s good e l e c t r i c a l i n s u l a t i o n p r o p e r t i e s . MODE OF ACTIONS IN HALOGEN-CONTAINING POLYMERS. In halogenated polymers, such as flexible PVC, the zinc borate markedly increases t h e amount o f c h a r f o r m e d d u r i n g combustion; whereas the addition of antimony oxide, a vapor phase flame retardant, has little e f f e c t on char formation. Analysis of t h e c h a r shows t h a t a b o u t 8 0 - 9 5 % o f a n t i m o n y v o l a t i l i z e d , whereas the m a j o r i t y of boron and z i n c from the z i n c borate remained in char (86% and 61%), respectively. (Table III). The fact t h a t the m a j o r i t y o f the boron remains i n the condensed phase i s i n agreement w i t h t h e f a c t t h a t b o r i c o x i d e i s a good a f t e r g l o w suppressant. The z i n c s p e c i e s remaining i n the condensed phase can alter the pyrolysis chemistry by catalyzing the dehydro-halogenation and promoting cross-linking, resulting in the increased char formation and a decrease in both smoke p r o d u c t i o n and f l a m i n g c o m b u s t i o n . The B 0 moiety of the z i n c b o r a t e can form a g l a s s y l a y e r i n h i b i t i n g f u r t h e r o x i d a t i o n of the char.


2

3

2

3

ZINC BORATE IN HALOGEN-FREE POLYMERS. AS A FLAME RETARDANT AND SMOKE SUPPRESSANT. In r e c e n t y e a r s , t h e d e v e l o p m e n t o f h a l o g e n - f r e e f i r e r e t a r d a n t p o l y m e r s has b e e n a subject of considerable i n t e r e s t , because these polymers will n o t p r o d u c e any c o r r o s i v e and t o x i c h y d r o g e n - h a l i d e g a s e s d u r i n g combustion. One of the common approaches to flame-retard a halogen-free polymer has been the addition of high loadings ( 1 0 0 - 2 0 0 p h r ) o f ATH. ATH c a n u n d e r g o an e n d o t h e r m i c dehydration by r e l e a s i n g 34% by w e i g h t of water in the temperature range of 220°-450°C. Recently, it was found that there are major advantages i n p a r t i a l l y r e p l a c i n g ATH w i t h t h e z i n c b o r a t e as a fire retardant in a variety of halogen-free polymer systems Fig. 11 illustrates the smoke suppression e f f e c t of the z i n c b o r a t e when u s e d i n c o n j u n c t i o n w i t h ATH i n a halogen-free ethylene-vinyl acetate (EVA) system. Sumitomo E l e c t r i c , in a



SHEN & GRIFFIN


PVC DOP

250 ' 0

• 2

• 4

100 PARTS 50

EPOXY

5.

STAB.

3!

6

8

• 10

ADDITIVE (PHR) Fig. 9. NBS Smoke Test ( ) of Zinc Borate in Flexible PVC.

and Oxygen

Index Test


(-

FIRE AND POLYMERS


168

TOTAL ADDITIVE (PHR) Fig. 1 0 . NBS Smoke Test of Flexible Trihydrate. ZB - Zinc Borate.

PVC Containing

Alumina



12.

SHEN & GRIFFIN

Table

I.

F i r e Retardant

Compositions

1

Exampi e 3 2

70

70

70

70

35

35

35

-

-

-

-

40

12

12

12

14

-

20

40

10

0

0

0

Ferrite DBDPO

(% by w t . )

b

c

Brominated P o l y s t y r e n e Sb 0 2

169


3

Z i n c Borate Afterflame Afterglow

(sec.) (sec.)

UL-94 (1/16 i n . )

Magnetic

Polypropylene

3

4

0-2

115-127

22-28

10-15

3-5

HB

V-0

V-0

V-0

a

Yokoda, K., e t . a l . , U. K. Patent a p p l i c a t i o n GB 2,115,822A (1983, t o Kanegafuchi Kagaku), U. S. Patent 4,490,498.

D

A11 the o t h e r a d d i t i v e s are p a r t s per hundred by wt.

c

Decabromodipheny1 o x i d e .

parts resin


170

FIRE AND POLYMERS

Table II.

Thermoplastic Nylon-6,6

UL-94 (1.6 Components

50%RH


Zinc Borate

(15 wt-%)

mm)

70C

b

Compositions

Tensile strength (MN/m ) 2

C

CTR

d

Fail

Fail

141

>600

Dechlorane 515 (10 wt-%)

Fail

Fail

147

300

Dechlorane 515 (10 wt-%) plus S b 0 (5 wt-%)

V-0

V-0

140

200

Dechlorane 515 (10 wt-%) plus Zinc Borate (15 wt- %) plus S b 0 (0.1 wt-%)

V-l

V-l

138

500

Dechlorane 515 (10 wt-%) plus Zinc Borate (15 wt-•%) plus S b 0 (lwt-%)

V-0

V-l

137

475

Dechlorane 515 (10 wt-%) plus Zinc Borate (15 wt-%) plus S b 0 (2 wt-%)

V-0

V-0

-

Zinc Borate as a Flame Retardant, Smoke Suppressant, and Afterglow

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