The Influence of Flame Retardant Structure on UV Stabilization Approaches in Polypropylene

ROBERT L. GRAY, ROBERT E. LEE, and BRENT M. SANDERS

Great Lakes Chemical Corporation West Lafayette, Indiana 47906

UV stabilization of polypropylene fiber containing brominated flame retardants has been the focus of intense technical efforts with only limited success. Acid generated by the flame retardants deactivates HALS, thus severely reducing the HALS’ effectiveness. Because of this interaction, flame retarded polypropylene fiber has been generally restricted to applications requiring little or no UV stability. Evaluation of structure-performance characteristics of both flame retardants and stabilizers allow development of packages which optimize processability, flame retardancy, and UV stability. This paper reviews progress in the stabilization of flame retarded PP fiber and presents new advances in this field.

INTRODUCTION

olypropylene fiber continues to expand its share of P the textile market (1). As polypropylene (PP) fiber enters into new applications, additional performance criteria must be met. One such critical area is flame retardant fiber. Key issues for flame retardant PP fiber are processability, co-additive interactions and eco- nomics. The flame retardant structure will affect both optimal spinning conditions and selection of stabilizer type and concentrations.

Limited success has resulted from intense technical efforts (2.3) to produce PP fiber containing brominated flame retardants. The flame retardants deactivate HALS by acid generation, thus severely limiting HALS effectiveness (4-6). Because of this interaction, only applications with little to no W stability requirements could be considered for flame retarded PP fiber. Progress in the stabilization of flame retarded PP fiber is reviewed in this paper, along with new advances in this field.

EXPERIMENTAL.

Compounding was accomplished using a Berstorff ZE-25 mm twin-screw extruder. Temperature profiles nominally were 200°C except a s noted below. Fiber extrusion was performed with a pilot fiber line (Hills Research & Development-West Melbourne, Fla.) with temperature profiles matched to those used in com- pounding. A 72 round filament spinneret was used to achieve 18 dpf POY fiber with a 3 to 1 draw ratio fiber.

Accelerated W exposure testing was done with xe- non arc under method SAE 5-1885 (interior automo- tive) and ASTM D-4459 (interior dry xenon). Sample fibers were wrapped around 6 cm X 15 cm cards and clamped into standard specimen holders.

Tensile strengths were measured using a Instron model 1123 with a 200 lb (90.7 kg) load cell, 8 cm sample length, 12.5 cm/min pull rate and a 5 lb (2.268 kg) calibration weight.

The corrosion evaluations were carried out as fol- lows. The polypropylene resin to be tested is first pro- cessed into pellets using a twin screw extruder. A piece of shim steel is polished and compression molded at 215°C with light compression for 5 min in order to bring the FR resin to mold temperature. Then full compression was applied for 1 hr and cooled in the water cooled platens. After being molded together and cooled, the plastic is stripped away and the shim steel piece is weighed. The shim steel is exposed to water vapor from boiling de-ionized water for one hour and then conditioned at 50% relative humidity and 23°C for 1 hr. It is then reweighed and the change in weight reported.

RESULTS AND DISCUSSION

Bromine Radical Generation Mechanisms

The mechanism by which brominated flame retar- dants function is believed to involve a thermal-in- duced release of bromine radicals a t or near the site of the flame. The ease in which bromine radicals are released can often be directly related to flame retar- dant performance. It is well documented that the rel- ative effectiveness of a flame retardant increases with decreasing thermal stability (7).

A comparison of bond dissociation energies of aro- matic C,H,-Br bonds and aliphatic CH,CH,-Br bonds shows a significant difference in thermal stability of aliphatic and aromatic flame retardants (Table 1 ) . Thermogravametric analysis (TGA) data confirm that

JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, MARCH 1996, Vol. 2, No. 1 63

Robert L. Gray, Robert E. Lee, and Brent M. Sanders

Table 1. Dissociation Energies of Carbon-Bromine Bonds (8).

Bromine Bond Dissociation Type Energies (KcaVMole)

C,H,-Br 65

C,H,-Br 71 i-C,H,-Br 59

aliphatic brominated flame retardants are much less thermally stable than brominated aromatics (Table 2). The result of this is that aliphatic flame retardants are typically more efficient flame retardants but may be limited by temperature in the processing and molding conditions used in the end-use construction.

Thermally induced HBr formation can be demon- strated through a corrosion test protocol. A corrosion test used for polypropylene was chosen to examine the corrosion potentials of aliphatic and aromatic FR com- pounds. The testing was done at the upper end of the normal use level of six percent bromine loading for each of the three FR compounds. These were com- pared to a control sample containing 100% polypro- pylene resin.

The data in Fig. 1 demonstrate that the level of HBr generated (as measured by corrosion) is directly re- lated to the concentration of aliphatic bromine con- tained in the flame retardant. The purely brominated aliphatic FR, such as FR- 1, does have the potential to cause corrosion as a result of thermally induced HBr generation. Brominated polystyrene, which contains a low level of aliphatic halogen along the polymer back- bone, still does exhibit a degree of corrosion potential. An analogous flame retardant, poly(dibromostyrene), which would not be expected to have any aliphatic bromine, exhibits significantly less potential for cor- rosion or HBr generation. It should be noted that cor- rosion would not be expected with any of these flame retardants once the processing temperature was re- duced below the critical carbon-bromine bond break- ing level.

One of the consequences of premature bromine re- lease is loss of UV stability. Bromine radicals can either react directly with hindered amine light stabi- lizers (HALS) or abstract a hydrogen from the polymer matrix and deactivate the HALS through an acid-base reaction. The result is loss of light stabilizer and rapid UV degradation of the unprotected polymer.

To understand these results, it is useful to consider the proposed mechanism for bromine radical genera- tion from aromatic flame retardants. In polypropylene fiber applications, it appears that the liberation of bromine radicals from aromatic flame retardants is

Table 2. TGA Weight Loss of Selected Flame Retardants.

Flame 5% Weight 10% Weight 50% Weight Retardant Loss Loss Loss

~

Aliphatic, FR-I 265°C 271 "C 283°C Aromatic, FR-2 357°C 373°C 41 8°C Aromatic, FR-3 383°C 398°C 41 6°C

Control (PP Alone)

Poly-dibromostyrene

Brominated Polystyrene

CD-75 (Aliphatic)

0 2 4 6 8 1 0 1 2 1 4

Weight Gain after Exposure (mg)

Polypropylene + 6% OBr

Fig. I . Corrosion evaluation offlarne retardants.

primarily the result of photo-activation rather than thermal-activation. Solid state UV spectra (Fig. 2) show that aromatic C,H,-Br bonds are more suscep- tible to photo-initiated bond cleavage. While FR1, the aliphatic flame retardant, absorbs relatively little UV radiation above 290 nm, the aromatic flame retardant (FR2) is strongly absorbing through 360 nm. This re- sult suggests that the primary mechanism for bro- mine liberation from aromatic flame retardants is photolytic cleavage.

The mechanism for bromine radical generation by flame retardants is quite structure dependent. Ali- phatic brominated flame retardants are primarily de- composed thermally, which may occur during the ex- trusion process. Alternatively, aromatic brominated flame retardants are relatively stable through the pro- cessing step but may generate bromine radicals dur- ing UV exposure.

As one would expect, the stabilization approach of polyolefins containing halogenated flame retardants is highly dependent upon the structure of the flame re- tardant. Previous work (9) has demonstrated that bro- mine generation from aromatic flame retardants can be partially inhibited through the use of UV absorbers. This approach is not as effective with aliphatic flame retardants since the mechanism for bromine genera- tion is clearly thermal induction.

% Reflectance

loo 80 I 60

40

FR-1

0 ' 200 220 240 260 280

UV cutoff for Sunlight

FR-2

300 320 340 360 380 400

Wavelength (nrn) 1 FR-1 FR-21

Fig. 2. W spectra offlarne retardant particles.

64 JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, MARCH 1996, Vol. 2, No. 1

Influence of Flame Retardant Structure

MELT”G RANGE

(“C)

Physical Effects

Another important factor in stabilization is compat- ibility. High melting flame retardants are often insol- uble in polypropylene and therefore act as a filler in the polypropylene fiber. Polypropylene fiber has a very thin cross section (sub-denier fiber is now common) and can easily lose physical integrity with the pres- ence of high levels of insoluble flame retardant, which are needed to achieve the proper degree of flame re- tardancy. Additionally, inherent incompatibility can lead to bloom or exudation of the flame retardant.

In this study three flame retardants were evaluated: a brominated aliphatic (FR- 1 -hexabromocyclodode- cane), a brominated aromatic (FR-2-decabromodiphe- nyl oxide), and a novel polymer-bound brominated aromatic (FR-3-polypropylene bound dibromosty- rene). Figure 3 compares structures and properties.

Table 3 compares the initial color after processing, maximum fiber take-up speed, and level of bloom. FR-1 shows a significant level of color development upon fiber spinning. This is consistent with the ther- mally induced bromine generation expected for ali- phatic flame retardants. The aromatic flame retar- dants showed relatively little color development upon processing.

The maximum take-up speed is indicative of the degree of loss of physical integrity resulting from the inclusion of high levels of dispersed particles. FR-2 has a large loss in spinnability relative to the polypro- pylene graft technology (FR-3). A qualitative ranking of bloom (1 = best & 5 = worst) shows no bloom for FR-3. Addition of this compatible polymer to polypropylene has clear advantages over the traditional approach of dispersing a relatively high level of insoluble flame retardant .

W Stability

Series I. In the first series of experiments ( lo), each formulation was exposed in a C165 Xenon Weatherom-

POLYMER INTEGRATION

Fig. 3. Flame retardant structures and properties.

300-315

TRADE NAME

FRI

FR2

FR3

DISPERSIBLE

Table 3. Flame Retardant Physical Effects in Polypropylene Fiber.

150-1 70

Initial Maximum Bloom, Flame Color, Take-up (1 = best &

Retardant delta E Speed, m/min 5 = worst)

GRAFT

~~~ ~

“Additive,” FR-1 30.5 N/A 4 “Additive,” FR-2 4.8 1000 4 Graft, FR-3 4.2 1500 1

eter @ 89°C (SAE 51885). Time to failure was noted when the knitted fabric catastrophically failed when scratched.

In the absence of light stabilizer all formulations showed poor stability. Figure 4 shows both FR-2 and FR- 1 have a negative impact on UV stability. The FR-3 does not appear to affect the polypropylene UV stabil- ity. The FR-2 does not melt a t typical spinning tem- peratures (melt point = 300-315°C). The insolubility of the FR-2 in combination with the high concentra- tions used made spinning these formulations difficult and required frequent screen pack changes. Both the FR- 1 and the FR-3 melted during the extrusion and no problems were encountered. The relatively poor FR-2 results are likely due to degradation induced by bro- mine formation during the extrusion. The FR-3 flame retardant, which is polymer bound and not an “addi- tive,” may provide important advantages in polypro- pylene fiber as it can be used at relatively high levels without exudation or negatively impacting physical properties.

Figure 5 compares the performance of HALSI (N-H HALS), HALS2 (NOR HALS), and the UVA1 with each of the flame retardants evaluated. As previously re- ported (1 I ) , the lower basicity of the NOR W S re- duces the negative impact of the flame retardant, thus resulting in a performance advantage over the tradi- tional N-H HALS. The W A is most effective only in the case of the aromatic flame retardants (FR-2 and FR-3) where it may serve the dual role of providing protec-

STRUCTURE

BrQBr 3RAFT COPOLYMER

’FPP‘DBS & B r2

DBS

Br TYPE

ALIPHATIC

AROMATIC

AROMATIC

185-195 BLENDABLE I MELT

I

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, MARCH 1996, Vol. 2, No. 1 65

Robert L. Gray, Robert E. Lee, and Brent M. Sanders

ALL FORMULATIONS CONTAIN 4.1% Br.

Fig. 5. Comparison of HALS and W A effectiveness.

tion to both the polymer and the flame retardant. As shown previously, the primary pathway for degrada- tion of the aliphatic flame retardant most probably involves a thermal initiation. A W A therefore would not be expected to provide significant stabilization.

Combinations of HALS and UVA show superior per- formance over HALS alone (Fig. 6). A 1:l combination of HALS1 and W A l provides a performance advan- tage over that achieved by HALS2 alone. This combi- nation would have a much lower sensitivity to pro- cessing temperature than HALS2 as well as a significant cost advantage. A synergistic combination of HALS2 and WA1 provides the best overall W sta- bility. It appears that the LNA significantly reduces the extent of acid generation, thus allowing even a basic HALS such as HALS1 to retain a portion of its effectiveness. The liberation of acid has not been com- pletely inhibited as the NOR HAL5 still shows a per- formance advantage over the N-H HALS. The aliphatic flame retardant has not been discussed as it was ob- viously degraded during fiber spinning.

The influence of HALS structure on stabilization in these systems is examined in Fig. 7. The non-basic NOR HALS (HALS2 - pKa = 4.2) has a large perfor- mance advantage over the basic 2" HALS1. Surpris- ingly, W S 3 achieved comparable stability to the NOR HALS (HALSB) despite its basic nature (pKa = 9.8).

Fig. 6. Effectiveness of HALS plus W A .

ALL FORMULATIONS CONTAIN FR-3' (6% Er) + 0.5% HALS + 1.5% UVAl

Fig. 7. Comparison of HALS performance.

This performance attribute may be related to ihe unusual compatibility of HALS3 in polypropylene. A simplified rationalization of the results can be pro- posed. The HALS3 molecules are "content" to remain well dispersed throughout the bulk of the polypro- pylene matrix. Conversely, HALS 1 tends to migrate towards the polar regions of sample that contain the brominated flame retardant. This, unfortunately, places the HALS in direct proximity to the sites of bromine generation.

Activity of this siloxane based HALS3 can be further enhanced using a methylated analog of the product.

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Influence of Flame Retardant Structure

HALS3-Me demonstrates superior performance over that achieved by the less basic HALS2.

CONCLUSIONS Aromatic flame retardants are generally less prone

to thermally induced degradation but rather generate bromine radicals through a photo-activated process. Combinations of W absorbers (WAIand HALS have been shown to provide outstanding UV stability to polypropylene fiber containing aromatic flame retar- dants. It appears that the primary benefit of the UVA is its role in providing a UV screen for the flame retardant, thus inhibiting the generation of bromine radi-cals and hydrobromic acid. With the level of

acid minimized, even relatively basic HALS can be successfully incorporated into these formulations. The effective-ness of the HALS is also greatly influ- enced by the compatibility between HALS and polypropylene. The highly compatible siloxane based HALS have been shown to out-perform the less basic NOR HALS.

It is clear that an understanding of the relationship between flame retardant structure and bromine radical generation will be key to developing stabilization sys- tems with minimal co-additive interactions. Future suc- cess will be dependent on developing total systems in which the stabilized formulation is custom de-signed for use with a specific flame retardant struc-ture. Modifica-

APPENDIX

H H

HALSl

4 2 H

HALS3

C H3

HALS3-Me

UVAl

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, MARCH 1996, Vol. 2, No. 1 67

Robert L. Gray, Robert E. Lee, and Brent M. Sanders

tions in process approaches will also likely play an im- portant role in future developments.

ACKNOWLEDGMENTS The contributions and research effort of Olga Ku-

vshinnikova and Bill Fielding are gratefully acknowl- edged. Special appreciation is also extended to Great Lakes Chemical Corporation for permission to use the data presented.

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3. J. K. Ghosh and P. R. Patel, “UV-Stabilization of Flame

Retardant Polypropylene Fiber,” Polypropylene Technol- ogy Conference, Clemson, S.C. (September 1993).

4. E. G. Rozantsev, Free Nitroxyl Radicals, Plenum Press, New York (1970).

5. L. Dulog and H. Bleher, Makromol. Chem., 187. 2357 (1986).

6. S. Chemela, D. J. Carlsson, and D. M. Wiles, Polym. Deg. Stab., 26, 185 (1986).

7. W. G. Kuryla and A. J. Pappa, Flame Retardancy of Polymeric Materials Wol. 2), p. 109, Marcel Dekker. New York (1973).

8. R. T. Morrison and R. N. Boyd, Organic Chemistry - Third Edition, Allyn and Bacon, Inc., New York (1973).

9. R. L. Gray and R. E. Lee, SPE ANTEC Tech. Papers, 41 (1995).

10. W. R. Fielding, “New Technology for Effective Control of Flammability in Polyolefin Fiber/Textiles,” SPO ’93, 587 (1993).

1 1. R. L. Gray, “Recent Advances in Polyolefin Stabilization Technology--A Novel Nonreactive Hindered Amine Light Stabilizer,” Society of Plastic Engineers’ Polyolefins VII International Conference (199 1).

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