Robert E. Lee, Donald Hallenbeck, and Jane Likens Great Lakes Chemical Corporation, West Lafayette, Indiana
4.1 BACKGROUND
Ultraviolet (UV) stabilization of polypropylene (PP) systems containing flame retardants proves to be a difficult technical challenge. The reason for this diffi- culty is expanded on later. However, generation of acidic products from bromine- based flame retardants during processing or exposure can cause a catastrophic deactivation of the hindered amine light stabilizer (HALS). An understanding of the mechanism for generation of acidic products from aliphatic and aromatic- based flame retardants has led to formulation approaches based on flame-retardant structure.
As polypropylene fiber continues to expand its share of the textile market (1), new application areas are identified. One such critical area is flame-retardant fiber for wall coverings, upholstery, commercial carpeting, and automotive uses. For flame-retardant PP fiber, the key issues are processing, UV stability, and economics. Recent advances in flame-retardant technology significantly im- proved processing, but UV stability and formulation of economically acceptable additive packages are difficult to achieve.
In addition to fiber, molded applications require UV-durable flame-retarded features from polypropylene. Perhaps better put, if these features were available, polypropylene could expand its market share rapidly. The following mechanisms for degradation, stabilization, and flame retardation present information required
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to develop improved systems. Methods of performance evaluation are key to proper formulation and are covered in some detail. It will be of value to see the commercial formulations included and the insights to next-generation systems.
4.2 MECHANISMS
Those close to the end-use applications of UV/flame retardant (FR) PP generally ‘‘formulate.’’ Formulate means the use of components as ingredients as when cooking. As such, additives and their interactions are not considered in a chemical or mechanistic way. The combination of additives for UV/FR PP requires a greater level of scrutiny because the interaction of additives and reaction by- products can lead to catastrophic results. Specifically, the wrong combination of independently effective additives can hurt performance in one or more criteria. The mechanisms of degradation, stabilization, and flame retardation indicate what interactions are possible.
4.2.1 Degradation
As outlined in a simplified mechanisms in Fig. 4.1, degradation proceeds through a radical chain mechanism (2,3). Initiation typically occurs through exposure to heat generated during production. Trace metal impurities such as copper or iron accelerates radical formation. Reactive hydroperoxides are formed after reaction of the carbon-centered radical with oxygen. Thermally induced homolytic cleav- age of hydroperoxides leads to additional reactive radical formation and subse- quent polymer chain scission.
Degradation of polymeric material with heat and oxygen initially involves breaking a bond between carbon and hydrogen atoms to make uncharged species called free radicals, as shown (Fig. 4.2) in the chain initiation reaction (1). These react quickly with oxygen to form peroxide radicals in a chain propagation reac- tion (2). These peroxy radicals in turn lead to many other reactive species, includ- ing peroxides as shown in reaction (3). The problem becomes the exponential increase in these reactive species through branching reactions (4).
4.2.2 Stabilization
An effective method of thermal stabilization is through the use of a radical termi- nating primary antioxidant. The most common class of antioxidant for radical termination is a hindered phenol (4). Phenolic antioxidants are highly effective
at relatively low concentrations (i.e., ⬍0.5 wt%) in inhibiting decomposition.
The mechanism involves a chain-breaking donation of a hydrogen atom from the antioxidant to the reactive peroxy-radical (Fig. 4.3). This produces a less-reactive
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Figure 4.1 Auto-oxidation of polymers.
resonance-stabilized phenolic radical. Peroxycyclohexadienones can then be formed after reaction with a second peroxy-radical (5). Each phenolic moiety is capable of trapping a total of two radicals before it is completely consumed.
Preventative or secondary antioxidants act at the initiation stage of the radical chain mechanism to prevent the formation of radical products. Their mechanism involves the decomposition of hydroperoxides to form stable nonradical products. In the absence of peroxide scavengers, hydroperoxides thermally or photolyti- cally decompose to radical products and accelerate decomposition. The most common secondary antioxidants are sulfur-based ‘‘thiosynergist’’ or phosphorus- based ‘‘phosphites.’’
Thiosynergists have been shown to decompose several moles of hydroperox- ide per mole of stabilizer (6). The hydroperoxide is typically reduced to an alcohol and the thiosynergist is transformed into a variety of oxidized sulfur products, including sulfenic and sulfonic acids. Synergistic combinations with phenolic antioxidants are often used to enhanced thermal stability in polyolefins at elevated
temperatures (⬎100°C).
Phosphites are also commonly used in combination with phenolic antioxi- dants to inhibit polymer degradation and to improve color (7). As in the case of thiosynergists, phosphites reduce hydroperoxides to the corresponding alcohols
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Figure 4.2 Radical reactions (1) through (4).
and are transformed into phosphates at temperatures above 180°C. The tempera-
ture limits for secondary antioxidants can be linked to both reaction kinetics and diffusion control characteristic.
Figure 4.4 shows thermogravimetric analysis (TGA) data for selected pheno- lic antioxidants. Volatility is an important criterion because of the potential for loss during manufacturing. For example, butylated hydroxytoluene (BHT) (Fig.
4.5) is the most volatile of the group, with a 5 wt% loss at about 90°C and a
90 wt% loss at about 142°C. These temperatures are below normal processing
temperatures, so BHT can be seen fuming from the extruder. However, dispersion and solubility of additives in the polypropylene prevent total loss. Dibutylnonyl
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Figure 4.4 TGA of four specific phenolic antioxidants, BHT, Lowinox DBNP,
Anox PP18, and Anox 20.
phenol (DBNP) (Fig. 4.6) is structurally similar to BHT except for the nine carbon group in the four position. Its resistance to volatility is improved in that it has
a range for the 5 and 90 wt% losses of 120–182°C, respectively. Anox PP18
(Fig. 4.7) and Anox 20 (Fig. 4.8) have TGA volatility values that are predomi- nantly above the peak processing temperature of polypropylene. They would be expected to evaporate significantly less during process if incorporated than BHT. The decreased volatility is achieved by increasing the molecular weight of the antioxidant. This is accomplished through the addition of long hydrocarbon
Figure 4.5 Lowinox BHT, CAS no. 128-37-6, 2,6-di-tert-butyl-4-methyl phenol
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Figure 4.6 Lowinox DBNP, CAS no. 4306-88-1, 2,6-di-tert-butyl-4-nonyl phenol.
Figure 4.7 Anox PP18, CAS no. 2082-79-3, octadyl-3-(3′,5′-di-tert-butyl-4′-hy-
droxyphenyl) propionate.
Figure 4.8 Anox 20, CAS no. 6683-19-8, tetrakismethylene (3,5-di-tert-butyl-4-
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chains in the case of Anox PP18 and DBNP. Anox 20 achieves a high molecular weight without dilution of the active content by coupling four phenolic moieties. DBNP is higher in molecular weight than BHT and also a liquid. Liquid antioxi- dants are generally easier to handle and meter into liquid color concentrate pro- duction than solids like BHT, Anox PP18, and Anox 20.
Volatility and migration are also two issues affecting performance. Although some degree of volatility is generally considered desirable, excessive migration and volatility can have a deleterious impact. It has been demonstrated that large- scale migration of volatile BHT can occur during normal production (8). This migration results in a dramatic decrease in antioxidant concentration. Migration of stabilizers to the surface leads to loss during rain, laundering, or other wash and clean events. Thus, higher molecular weight additives having a greatly re- duced rate of migration and loss are an advantage.
Primary antioxidants, like phenolics, scavenge radical species to prevent fur- ther reaction. Reactions (5) and (6), shown in Fig. 4.9, illustrate the fact that a phenolic antioxidant can scavenge two radical species. Therefore, the efficiency of a phenolic antioxidant will be related to its ratio of active phenolic weight to its total weight. For example BHT scavenges two radicals and has a molecular weight of 220 g/mol. However, 2,6-di-tert-butyl-4-nonyl-phenol scavenges two radicals and has a molecular weight of 332 g/mol. Therefore, the DBNP is over 50% greater in molecular weight without an increase in activity. The trade-off is that DBNP is a liquid with low volatility compared with BHT, which is a volatile solid.
The advantage of effective polyphenolic antioxidants like Anox 20 (Fig. 4.8) is that they contain more than one active group. Here the molecule traps up to
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eight radicals and has a molecular weight of 1178 g/mol. This is 147 molecular weight units per radical, which is between BHT and DBNP, but it has a much greater advantage in volatility and other attributes.
Unlike primary antioxidants, all secondary antioxidants work by decompos- ing reactive species like peroxides as shown in Fig. 4.9 for reactions (7) and (8). They do not have the ability to trap radicals initially (9). However, one of the most popular phosphite secondary antioxidants is made from three phenolic groups (Alkanox 240, Fig. 4.10). As those groups are made available, they have primary activity. Unfortunately, most phosphites typically do not become effective until
180–200°C. Therefore, they cannot work well by themselves. Primary and sec-
ondary antioxidants work by different mechanisms and often are synergistic. The stabilization methods above are predominately involved during pro- cessing temperatures or molten conditions. However, they cannot be neglected when considering long-term thermal stabilization or light stabilization. This is because of the residual stabilizers and reaction byproducts carried into the next phase of the product life. A clear example of this is the loss of UV durability experienced as a function of multiple extrusion of the same material. Multiple extrusion creates color in PP and increases in melt flow. Figure 4.11 shows the
reduction in UV durability of a single system processed at 230 and 300°C.
The material formulated for Fig. 4.11 had a melt flow of 14 g/10 min at
230°C with a mass of 2160 g. as pelletized. The fiber processed at 230°C had a
subsequent melt flow of 20 (g/10 min at 230°C with a mass of 2160 g). However,
the fiber processed at 300°C had a melt flow of 39 (g/10 min at 230°C with a
mass of 2160 g). The change from 14 to 20 is typical because of a second pro- cessing step that causes degradation and reduction of residual stabilizers that protect the polymer during the measurement of melt flow (third heat history).
However, the increase to a melt flow of 39 in the material processed at 300°C
Figure 4.10 Alkanox 240, CAS no. 31570-04-4, tris(2,4-di-tert-butyl-phenyl)
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Figure 4.11 Loss of UV durability (kJ/M exposure to reach 50% tensile strength)
with changes in processing temperatures for polypropylene homo polymer, 1296 denier, 72 filament partially oriented yarn for carpet fiber. Fiber was stabilized with 0.2%wt Chimassorb 944 and 0.15%wt Alkanox P24.
clearly indicates additional degradation has occurred in the polymer that also had the shortest lifetime for UV exposure. Thus, the degradation from processing contributes to a reduction of UV durability.
To stop degradation of polypropylene by light, there are several common and effective methods. The simplest may be coating the polymer surface with either a clearcoat or a color coat of paint that can provide UV durability. This is done for better color matching in automotive applications. However, lack of UV absorbers in these coatings may allow damaging light penetration to the poly- mer surface where coating adhesion is lost. Additives to the polypropylene to prevent light damage directly may work by competing for the light or interfering with degradation chemistry like the antioxidants previously discussed.
A loading of 2.5 wt% carbon black will allow many systems to retain their physical integrity during exposure to UV light. In this system, the light is ab- sorbed at the surface and damage is reduced and contained to the surface layers. For flame-retarded polypropylene, this is seldom a viable option because it limits the color options and highlights a common problem with blooming of white coad- ditives.
A UV light absorber for polypropylene can be of a number of different chem- ical classes. Benzophenones and benzotriazoles are the most commercial. Their mechanism involves absorption of the light and subsequent dissipation of the energy as heat. Figure 4.12 indicates the mechanism for Lowilite 22, a common benzophenone-type absorber. The benzotriazoles work by a similar mechanism. Figure 4.13 shows Lowilite 28, a common benzotriazole for polypropylene.
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Figure 4.12 Nonconsuming mechanism for a benzophenone-type UV light ab-
sorber, Lowilite 22.
The particular advantage of absorbers is the protection they afford coaddi- tives and the polypropylene. Pigments and brominated flame retardants are coad- ditives that undergo light damage independently of the polypropylene. Figure 4.14 shows the UV reflectance spectra for two brominated flame retardants. The lack of reflectance for DE83R, decabromodiphenyloxide, at damaging UV light wavelengths shows that damage from light can occur directly with the flame retardant. Thus, a costabilizing UV absorber would be specifically helpful with aromatic flame retardants like DE83R.
Most polypropylene uses a radical trapping stabilizer of the hindered amine type. These hindered amine light stabilizers are often referred to as HALS. Figure
Figure 4.13 Lowilite 28, CAS no. 25973-55-1, 2-(2′hydroxy-3′,5′-di-tert-amylphe-
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Figure 4.14 UV spectra of flame-retardant particles and the reflectance of dam-
aging light for CD 75 (——), hexabromocyclododecane and absorption for DR83 (– – –), decabromodiphenyloxide.
4.15 shows a common example sold as Lowilite 77. Figure 4.16 shows the cyclic mechanism of HALS that attributes to their good performance to weight ratio.
The fact is that HALS are so effective in polypropylene they often preclude the uses of absorbers in many applications. However, a significant limitation is the alkaline nature of the common secondary HALS. If coadditives are acidic or produce acidic degradation byproducts, these can coordinate with the HALS to both interfere with stabilization and contribute to color variations. The alkylation of secondary HALS to form tertiary HALS like Lowilite 76, shown in Fig. 4.17, greatly reduces these harmful coordinations.
Another way to reduce the alkalinity of a HALS is to form a polymeric molecule through the active amine groups as Lowilite 62, shown in Fig. 4.18. With flame-retarded polypropylene, a combination of stabilizers is often found to be synergistic. Later, common commercial systems and some next-generation systems are discussed.
Figure 4.15 Lowilite 77, CAS no. 52829-07-9, bis-(2,2,6,6-tetrametyl-4-pip-
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Figure 4.16 Cyclic mechanism for HALS stabilization through radical trapping.
Alcohol and carbonyl species are also produced, which can have degrading ef- fects.
4.2.3 Flame Mechanisms
Combustion is a very complex combination of physical and chemical phenomena that must interact in balance for combustion to occur. The three components necessary to support combustion are fuel, heat, and oxygen, which form the clas- sic fire triangle (Fig. 4.19).
Fire suppression is accomplished by means that affect one or more of the legs of the fire triangle. Additives that liberate water of hydration upon heating cool the substrate and dilute the combustible gases. Although these additives are inexpensive, this is a relatively inefficient approach to flame retardation because
high load levels,⬃30 wt%, are required.
Figure 4.17 Lowilite 76, CAS no. 41556-26-7,bis-(1,2,2,6,6-pentamethyl-4-pip-
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Figure 4.18 Lowilite 62, CAS no. 65447-77-0, dimethyl succinate polymer of 4-
hydroxy-2,2,6,6-tetramethyl 1-piperidine ethanol, a tertiary HALS that is polymeric in nature.
Figure 4.19 Classic fire triangle shows the three components necessary to sup-
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Intumescent additives functioning in the condensed phase form a thermal barrier that protects the substrate and limits diffusion of combustible gases out of the substrate and oxygen into the substrate. Significant effort in developing these systems is underway, as they are characterized by relatively low heat re- lease.
Additives functioning in the vapor phase exhibit the highest efficiency be- cause they interfere with the combustion chemistry. Total load levels as low as 3 wt% are sufficient for certain flammability performance requirements. The halogen-containing systems that function in the vapor phase are characterized by higher heat releases than systems operating in the condensed phase. The steps involved in the combustion of polymers as described by Troitzsch (10), shown in Fig. 4.20, are summarized below.
The condensed phase is heated by an ignition source or by thermal feedback of radiant heat from the gas phase oxidation reactions. Thermolytic cleavage of the polymer supplies combustible and noncombustible gaseous products to the gas phase combustion zone. These products react with oxygen and release heat during the production of carbon dioxide, carbon monoxide, water, and soot.
Oxidative degradation of polymers (Fig. 4.21) leads to the formation of highly reactive H and OH radicals as described by Thiery (11). Thermal feedback reinforces pyrolysis to further fuel the flame. A simple model of growth and branching is shown in Fig. 4.22.
The free radicals of H and OH, which are proliferated by the chain— branching reactions, confer a high velocity to the flame front. They attack the
Figure 4.20 General steps of polymer combustion. The flame processes differ
from light and low temperature thermal degradation processes in that tempera- tures are hundreds of degrees centigrade and vapor phase.
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Figure 4.21 Thermal oxidation radical products of combustion.
hydrocarbon species and participate in reactions that yield the various terminal combustion products.
4.2.4 Mechanisms of Flame Retardancy
Combustion is prevented or stopped by affecting one or more of the three compo- nents necessary to support combustion (heat, fuel, and oxygen). Flame-retardant mechanisms cluster into four general classes.
Heat Sink Mechanisms
Endothermic processes inherent to additives such as aluminum trihydrate
(Al2O3 ⋅3H2O, also known as aluminum hydroxide, Al(OH)3) and magnesium
hydroxide, Mg(OH)2, cool the substrate to temperatures below those required to
sustain combustion. Water vapor evolved in their endothermic decomposition
Figure 4.22 Growth and branching model for pyrolysis that further fuels the
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Figure 4.23 Flame-retardancy mechanisms occurring by heat sink.
dilutes the combustible fuel in the gaseous phase, whereas the alkaline inorganic residues afford a level of thermal barrier protection (Fig. 4.23).
Thermogravimetric analysis shows that aluminum trihydrate (ATH) yields
its waters of hydration in the range of 205–225°C. Because this temperature is
often achieved during the compounding of thermoplastics, ATH is not generally used in these systems. Magnesium hydroxide dehydrates in the range of 300–
320°C and can be safely processed in polypropylene, with load levels on the
order of 60 wt% being used to achieve desired flammability performance. Figure
4.24 shows the TGA thermograms of polypropylene, Mg(OH)2, and a formulation
containing polypropylene and⬃30 wt% Mg(OH)2. Observe the degradation pro-
file of the formulation shifting toward lower temperatures and its increased resi-
dues content due to the presence of Mg(OH)2.
Figure 4.24 TGA thermograms, % mass retention vs. temperature, of (䊐) poly-
propylene, (◊) Mg(OH)2, and (䉭) polypropylene with 30% Mg(OH)2, under ni-
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Flame retardants that inhibit combustion by the physical actions of cooling, diluting, and insulating are less effective on a weight basis than those that chemi- cally inhibit the flame chemistry.
Condensed-Phase Mechanisms
Condensed-phase systems decompose upon heating to form a large amount of thermally stable residue, or char. This char acts as a thermal shield for radiant heat transfer from the flame to the polymer and as a physical barrier to limit diffusion of flammable gases from the polymer to the combustion zone.
Intumescent chars result from a combination of charring and foaming of the polymer surface, resulting in a thick protective barrier. Because of their low environmental impact and their relatively low heat release, there is considerable interest in the development of intumescent systems. These systems typically con-