Recent Advances in Flame Retardant Compositions

Recent Advances in Flame Retardant Compositions

UV Stable, Antimony Free Flame Retardant Systems for Polypropylene Molding

By

N. Kaprinidis, R. E. King III, P. Shields, J. Zingg* and G. Leslie*

Society of Plastics Engineers 2002 Polyolefins RETEC

Houston, Texas

Ciba Specialty Chemicals Corporation

Polymer Additives

Tarrytown, New York

Basel, Switzerland*

Abstract

Hindered amines are known for their ability to provide UV stability as well as long term thermal stability. Recently, there have been new classes of hindered amines, based on the novel NOR functionalization, that afford additional benefits beyond UV or long term thermal stability. For example, over the last several years we have clearly established the utility of nove l NOR type hindered amines, such as NOR 1, either as a flame retardant, or flame retardant synergist in combination with conventional halogenated flame retardants in polyolefin thin section substrates, e.g., polypropylene fibers. In this paper, we will provide an update on the recent advances in thick section substrates including both flame retardant efficacy and UV stability. Overall, we have found that these novel NOR type hindered amines not only allow for substantial reduction in the overall concentration of the bromine compounds, mineral fillers, or/and the elimination of antimony trioxide flame retardant synergist but also, they provide superb UV stability in the presence of halogenated flame retardant. Antimony free, UV stable flame retardant systems for polypropylene homopolymer molding with UL 94 V-0 and V-2 ratings respectively, have been achieved.

Introduction

Currently, there is an increasing need for flame retardant polyolefin applications.1 Typical polyolefin flame retardant systems contain either bromine or phosphorous in combination with fillers such talc or inorganic synergists such antimony trioxide. Although these particular systems are very effective in providing flame retardant efficacy at relatively low cost, they raise a lot of concerns from a practical and environmental point of view. Also, halogenated flame retardants, especially the aromatic ones, detrimentally affect the polymer light stability.

Moreover, there are processing problems such as odor, toxicity, and significant negative impact on the physical and mechanical properties of the polymers associated with the high levels of use of the flame retardant additives.2 In some European countries there are proposals to ban the use of halogenated and heavy-metal containing flame retardants.3,4Accordingly, a market need to replace these conventional flame

retardants has been developed. In this report, the efficacy and advantages of the non-halogenated NOR 1 (Appendix 1) as a flame retardant synergist in halogenated and non- halogenated systems are discussed.

Historic Background

N-alkoxy hindered amines (NOR) were first introduced as non-interacting light stabilizers for automotive coatings, flame retarded fiber and agricultural film applications.5 NOR hindered amines have low basicity and are in a more active oxidation state than conventional hindered amines.

In agricultural film applications, pesticides generate acidic species and deactivate conventional hindered amines. Similarly, hindered amines are deactivated by the thermal or photo generation of HBr from flame retardants resulting in inferior light stability. Light stabilization studies which were conducted with brominated flame retardants, conventional hindered amines (HALS), and NOR HALS confirmed that NORs perform significantly better than conventional light stabilizers in the presence of brominated flame retardants. Similar observations were made in agricultural films in the presence of pesticides.

Papers have been already presented demonstrating the e fficacy of N-alkoxy hindered amines with brominated flame retardants and UV absorbers.6 These papers however, only describe the light stabilization performance and do not show any flame retardant efficacy data. It was believed at that time that the non-interactive nature of NOR HALS with the halogen in the brominated flame retardant positively influenced the performance. Subsequently, when NORs were tested for flame retardancy without the presence of brominated flame retardants, it was observed that they provide flame retardant efficacy to polypropylene fibers. The efficacy of Flamestab NOR 116 as a flame retardant in passing flame retardant industry standard tests such as NFPA 701 at surprisingly low concentrations was discussed in a previous paper.7 The present report discusses the flame retardant efficacy of systems containing Flamestab NOR 116 and halogenated and non halogenated flame retardants in molded articles.

Experimental Details

The flame retardant efficacy studies were performed in injection molded polypropylene plaques. Each of the formulations was compounded initially, and plaques were then shaped from these formulations. The plaques (3.2 mm thickness) were injection molded using a laboratory Boy Injection Molder. The detailed conditions for compounding and injection molding of the plaques are summarized in Apendix 2.

The artificial weathering (WOM, wet) was contacted at 0.35 W/m2 at 340 nm.

The UL-94 testing was performed on the molded materials according to the UL protocol.8 Various factors such as homogeneous dispersion of the flame retardant in the polymer, presence of pigments and other additives, material construction, variability in the samples may play a role on the measured flame retardant efficacy.9

Results and Discussion

Part 1. Flame Retardant Efficacy

Based on the initial range finding work we have carried out in our labs, we were unable to achieve a UL rating with NOR 1 alone in polypropylene molded plaques. Following up the promising results in polypropylene fiber, combinations of halogenated and non halogenated flame retardants with NOR 1 were evaluated in polypropylene. Results obtained from four systems A, B, C, D and E are presented herein. System A is a UV, stable antimony free, halogenated flame retardant system Systems B, C and D are antimony free halogenated flame retardants while D is antinony free, non halogenated flame retardant system. The results obtained from systems B, C, D and E are summarized in Table 1 and are related to flame retardant efficacy. The results obtained from system A are presented in Part 2 and are related to both UV stability and flame retardant efficacy.

The results suggest that NOR 1 acts as a synergist with halogenated flame retardants by replacing antimony trioxide and achieving the flame retardant efficacy that traditional halogenated/antimony trioxide systems have provided. Noteworthy, a V-0 rating is achieved by the antimony free system B, which is a three component flame retardant blend. The loading (weight percent) of syst em B in the flame retardant homopolymer polypropylene is referring to the total composition (see Table1). The V-0 rating is achieved with significantly lower concentrations of the halogenated flame retardant and the absence of antimony trioxide. The flame retardant polymer show less than one second after flame time (self extinguished) and although it is dripping upon application of the flame, the drops do not ignite the cotton underneath the plaque.

Antimony free V-2 ratings are also achieved with remarkably low concentrations of systems C and D. The loading (weight percent) for each system is referring to the total composition of the flame retarded polypropylene. The systems C and D are two component mixtures based on NOR 1 and halogenated flame retardants. The flame retardant polymer in both cases, show less than one second after flame time (self extinguished) and the drops ignite the cotton underneath the plaques.

There are numerous advantages to benefit from the new, antimony free, systems. It is observed that antimony trioxide free flame retardant systems result in substantially decreased smoke density and improved physical and mechanical properties of the thick section substrates, such as tensile and impact strength. It is also known that antimony trioxide in combination with halogenated (brominated) flame retardants produces toxic byproducts which are hazardous to the environment and health. The elimination of such hazardous materials eventually casts the antimony free systems B, C, D and E as effective flame retardants whichdo not pose any threat to the environment and health. In addition, the substantially lower effective concentrations of the halogenated flame retardant results in flame retardant polymers with better melt processability, lower density, better physical and mechanical properties.

The non-halogenated , antimony free product E which results in a V-2 performance rating in flame retardant polypropylene molding, is also very exiting. The system is based on NOR 1 and it is a two component mixture with a non halogenated flame retardant. The loading (weight percent of the system) is referring to the total composition

of the homopolymer polypropylene. It is anticipated that further optimization of the system will result in a totally non halogenated V-0 system with substantially improved properties. At present, there are ongoing studies focusing on identifying these totally non-halogenated systems.

The plaques were injection molded and were conditioned prior testing (48 h, room temperature at 50% humidity ). The burning test was performed according to the UL specifications. The appropriate controls were also tested: neither NOR 1 nor the halogenated or non halogenated flame retrardant used in B, C, D and E achieve the respective UL rating (listed in Table 1) alone.

Table 1. UL-94 Vertical Burn Test Results in Polypropylene Molding

Antimony Free Systems UL-94 Rating

System B 14.5 % V-0 (after flame time <1sec)

System C 3.5 % V-2 (after flame time <1sec)

System D 5.5 % V-2 (after flame time <1sec)

System E 17.0% V-2 (after flame time < 1sec)

Part 2. UV Light stability

The UV light stability of molded polypropylene items and other polymers containing halogenated flame retardants is a growing concern in many outdoor applications. Today hindered amines (HALS) are the UV stabilization technology of choice for polyolefins, styrenics and several engineering polymers. HALS are not UV absorbers but act primarily as radical traps. On occasion HALS are used in conjunction with UV absorbers to maximize UV stability in selected systems and applications.

As effective as these UV stabilizers have been, their use in conjunction with many flame retardants has proven problematic. The decomposition of halogenated flame retardant materials and subsequent hydrogen abstraction from the polymer by halogen radicals leads to the formation of mineral acids. Acids can also be formed from the decomposition or the hydrolysis of phosphate flame retardants. These acids and the HALS react to form quaternary ammonium salts that are inactive as UV stabilizers. Bromine radicals are also capable of reacting directly with the hindered amine to similarly form an inactive salt. The combination of the loss of UV stabilizer protection and the presence of significant radical concentrations leads to rapid degradation and loss of polymer physical properties.

As a general rule, aliphatic halogenated flame retardants are more UV stable than aromatic halogenated flame retardant compounds. Aromatic flame retardant materials are strongly UV absorbing. Exposure to UV radiation will cause the flame retardant to undergo photolytic cleavage, releasing halogen radicals. The halogen radicals abstract hydrogen from the polymer. This inevitably results in chain scission or cross-linking with the consequent rapid loss of polymer physical properties. As earlier mentioned these flame retardants deactivate HALS. UV absorbers can be employed to somewhat slow the rate of halogen evolution but performance improvements are minimal.

The N-alkoxy hindered amines (NOR HALS) are far less basic than N-H hindered amines. With their pKa’s in the range of 4.2 – 4.7, they evidence enormously lower levels of interaction compared to traditional HALS. In flame retardant compositions, NORs are not prone to the acid base reactions and are free to act as UV stabilizers. In non flame retardant formulations they demonstrate a level of performance consistent with traditional HALS.

Weathering of unstable systems of polymer, containing flame retardants and UV absorbers will lead to premature discoloration and surface cracking. Therefore, the selection of the best combination of flame retardant and UV absorber becomes critical. The ideal UV stable system is defined by the following criteria:

? To be effective at low concentrations (no effect on mechanical properties)

? To be thermally stable (not decomposed during compounding/molding)

? To be compatible with the polymere matrix (easy to disperse)

? To be UV stable and not pro-degradative to the polymer when exposed to light ? The flame retardant has minimal interaction with UV absorber,

? the UV absorber must be highly effective in the chosen matrix

Figure 1 : Change of the Surface Gloss during Artificial Weathering

Figure 1 shows the change in gloss as a function of accelerated weathering (WOM, wet) of a copolymer grade of polypropylene. A complete loss of gloss will lead to a polymer surface that is chalked or cracked and radically degraded in appearance.

The resin used in this particular experiment is blown molding grade polypropylene with MFI 0.3 and containing 0.2% Cromopthal Blue 4GNP. The same light stabilizer (LS) was used in all formulations (at 0.5% loading). The flame retardant was used in a concentration to achieve a V-2 UL rating. Flame retardant 1 is aromatic while flame retardant 2 is aliphatic. Figure 2 shows the actual photographs taken from these polymer formulations after several weathering intervals.

Figure 2: Influence of Flame Retardant on UV Stability of Polypropylene

.

Figure 3: Color Change During Artificial Weathering

Traditional aliphatic and aromatic flame retardants combined with traditional HALS are not capable of maintaining surface gloss for significant periods of time. System A is a completely formulated system of flame retardant and UV absorber targeted at achieving a V-2 level of flame retardant performance and a high level of UV stability. Gloss retention reaches to several thousand hours, which allows for extended periods of outdoor use for these polymers.

Figure 3 demonstrates that some non-halogen flame retardants can also have deleterious effects on UV stability. In this example the UV absorber is the same in all formulations and it is the flame retardant that changes the level of UV stability. Work continues to develop the synergies of these new classes of absorbers and flame retardants in connection with both UV stability as well as flame retardant efficacy.

Part 3. Mechanistic Considerations

It is known from earlier research work that thermolysis of NOR hindered amines may follow two seemingly similar, but distinctly different reaction pathways. The two cleavage processes are as follows:

>NO-R →>NO? + ?R

(aminoether) (nitroxyl radical) (alkyl radical)

>N-OR →>N? + ?OR

(aminoether) (aminyl radical) (alkoxy radical)

It can be seen that the thermal ly induced breakdown of NOR hindered amines may lead to the formation of either alkyl and nitroxyl radicals, or alkoxy and aminyl radicals. The relative extent of these two reactions depends on the chemical structure of the starting NOR aminoether. Aminyl and alkoxy radicals are very reactive and may be involved in the free radical chemical reactions during the combustion process. The effect of aminyl radicals on polyolefins at higher temperatures are under investigation. On the other hand, effect of alkoxy radicals on polyolefins has been widely studied. Alkoxy radicals may promote chain scission or crosslinking reactions in polyolefins depending on the

polymer molecular architecture. Alkoxy radicals promote chain scission of polypropylene, while in most cases promote crosslinking or chain enlargement of polyethylene.

Ignition of a polymer results in the formation of volatile combustion products. When these combustion products burn, they release heat. This heat energy from the flame is then fed back to the polymer to sustain the burning process. It is believed that the thermolysis of NOR hindered amines and the consequent generation of the radical species from the NOR significantly reduces the amount of thermal feedback from the flame thereby providing flame retardant efficacy.

Extensive research has been done in the area of using various radical generators as flame retardants and flame retardant synergists10. A possible explanation of the synergy in flame retardant performance obtained by using the combination of NOR 1 and conventional brominated and phosphorus flame retardants is the following: generation of aminyl, alkoxy and nitroxyl radicals from the NOR compounds can interact with the brominated compounds and facilitate the release of bromine, thereby improving the flame retardant performance. In the case of the phosphorus based flame retardants, radical reactions between the NOR, phosphorus flame retardant, and the polymer may lead to more efficient condensed phase reactions, resulting in improved flame retardant efficacy.

Concluding Remarks

The conclusions derived from this investigation can be summarized as follows:

? Improved flame retardant efficacy and UV stability can be achieved in polypropylene molded plaques through systems containing NOR 1 and halogenated and non halogenated flame retardants.

? NOR 1 synergize s with halogenated and non halogenated flame retardants in molded items to achieve V-O and V-2 UL ratings. Using this technology, it possible to design flame retardant polypropylene compositions with lower levels

of halogenated or non halogenated flame retardants and eliminate the presence of antimony trioxide.

? The lower levels of the additives and the absence of antimony trioxide, provide better prossecability, lower density, lower smoke density, improved physical and mechanical properties. In addition, they are in compliance with environmental regulations and provide safer use and recycling.

Acknowledgments

The development of new stabilization chemistries and commercial products is the result of a lot of hard work by many people. The authors wish to thank their colleagues for their direct and indirect contributions, from the Ciba Specialty Chemical sites around the world. The authors would also like to thank worldwide management for their continued support and commitment as well as permission to publish this paper. Very special thanks go to Guy Chandrika and Paul Nugent for all of the Tarrytown lab work. Sincere appreciation goes to Mary Beth Ryan for editorial assistance.

References

1. Cullis C. F. and Hirschler M. M., “The Combustion of Organic Polymers," Calendron Press,

Oxford (1981)

2. Marchall A., Delobel R., Le Bras M., Leroy J. and Price D., Polymer Degradation and Stability, 44 (1994) 263-272

3. Hardy M. L., Conference Proceedings, Fire Retardant Chemicals Association, (1994) 123-128

4. Van Riel, Conference Proceedings, Fire Retardant Chemicals Association, (1994) 167-174

5. Guo M., Horsey D., Lelli N. and Bonora M., Conference Proceedings, Polyolefins X,

Society

of Plastics Engineers (1997) 439

6. Davis L., “Recent Developments in UV Stabilization of Polypropylene Fiber Containing Melt

Processing Halogenated Flame Retardants” Additives (1996)

7. Srinivasan R., Gupta A. and Horsey D., Conference Proceedings, Additives XI, Society

of Plastics Engineers (1998)

8. Tests for Flammability of Plastic Materials, Underwriters Laboratories Standard (1996)

9. Green J., Flame Retardant Polymeric Materials, 3 (1982)

10. Eichorn J., Journal of Applied Polymer Science, 8, (1964) 2497

Appendix 1. Description of Stabilization Chemistries.

Appendix 2. Experimental Design

Polymer ID#: Profax 6501; Mondell

Compounding Conditions:Dry mixing of all components for 30 min; Leistritz 27mm

Twin Screw Extruder; Extruded under nitrogen; 200 0C;

300 RPM

Injection Molding:400 o F at Nozzle; 375 o F at Zone 1,2 and 3; 75 o C Molding

Temperature; 60 Screw Speed

UL-94 Burn Test: Plaques were made, conditioned and tested according to

the UL-94 protocol

Weathering: Xenon weatherometer (WOM Ci 65); wet; 0.35 W/m2; 340 nm

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