oxidation and stabilization of hydrocarbon polymers: relationships, new aspects and unsolved...

Oxidation and Stabilization of Hydrocarbon Polymers: Relationships, New Aspects and Unsolved Problems

Jan PospiSil Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6 , Czechoslovakia

Oxidation processes involved in processing, atmospheric ageing and the pre-burning stages of hydrocarbon polymers are outlined. Open problems based on up-to-date knowledge of the mechanisms of oxidation and stabilization processes are considered. Stabilizers used to assist processing and long-term stability are transformed during their service-time. Some of the products thus formed, as well as the products of polymer oxidation, should be considered as inherent impurities of polymers and play an active role in the degradation. Because of safety consideration, some of the products have to be protected also against catastrophic oxidation, i.e. burning. Significant changes in the mechanisms of degradation take place in the pre-burning step in the phases of high temperature oxidation and oxidative pyrolysis. Fuller understanding of the mutual relationships in the mixtures of antioxidants and/or light stabilizers with flame-retardants is lacking.

INTRODUCTION

Increased consumption of man-made polymers, a growing demand on their technical properties and legislative claims, and at the same time the necessity to exploit expensive hydrocarbon raw materials more economically are reflected in their requirements for their improved processing and long-term stabilities, including fire-retardancy. Saturated and unsaturated hydrocarbon polymers, i .e. polyolefins, polystyrene plastics and elastomers, form a substantial part of technically available man-made polymers.

General schemes applied in the formulation of individual oxidation steps in hydrocarbon polymers are deduced from oxidation studies performed using highly purified low molecular weight liquid hydrocarbons and more or less strictly defined model experimental conditions. There are limits in the simple correlation of the data thus obtained to oxidation processes in technical polymer, however. The choice of a structure for a model hydrocarbon influences the correlation less than some trace impurities and/or physical factors operating in solid technical polymers. Knowledge of processes taking place within the environments in real-life conditions is necessary for each individual polymer or polymeric system. A complex systematic basic research of the phenomena involved and an expensive analytical evaluation of processes cannot be compensated for by ad hoc technical testing. From contemporary knowledge of the oxidative degradation of polymers, a series of problems emerges relating to their long-term stability. Queries connected with the oxidative deterioration of polyolefins are relatively simpler than those dealing with processes taking place in elastomers. The oxidative deterioration is motivated not only by the overall higher chemical reactivity of unsaturated hydrocarbons, but also the amount and nature of additives contributing to the complexity of the processes. Distinct oxidative deterioration proces-

ses are discussed, together with methods of their con- trol, stabilizers and stabilizer systems used, and factors affecting their activity.

MECHANISMS

Ageing, thermooxidative degradation, oxidative pyrolysis, and burning of hydrocarbon polymers in tropospheric conditions are affected by inherent factors (chemical nature and morphology of a particular polymer or of a polymer blend, the nature of the additive system or impurities, internal stress, shape or volurne/mass ratio of the product body) and by external factors (temperature, air flow and partial oxygen pressure, oxidizing atmospheric pollutants and humidity, solar radiation, mechanical fatigue). The external factors just mentioned are reflected in varying time sequences and relative intensities, and result in sets of radical, ionic and molecular chemical reactions or in energy transfer processes. The complexity in this mul- titude of sensitized or catalytic events is still further enhanced by the fact that they occur in an internal inhomogeneous polymeric matrix.

The thermal autocatalytic oxidation of polymers is accelerated by thermolabile compounds or by catalysts (i.e. ions of transition metals or their complexes and especially residues of polymerization catalysts). The mechanism of the chain oxidation process is greatly influenced by the reaction temperature. Pronounced changes in the participation of individual oxidation steps are observed in the temperature region of about 200 "C. The burning takes place above 450 "C and is accompanied by oxidative pyrolysis. Volatile low molecular weight hydrocarbons are released in the gas phase over the polymer surface.

Photooxidation of hydrocarbon polymers takes place in the presence of actinic solar radiation. It can be initiated by the presence of photolabile additives or impurities photolysing easily with high quantum

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42 FIRE AND MATERIALS, VOL. 6, NO. 1, 1982 0 Wiley Heyden Ltd, 1982

OXIDATION AND STABILIZATION OF HYDROCARBON POLYMERS

yields. An important role is played by chromophores: these can be introduced in the form of sensitizers or can be formed stepwise in the system during oxidative transformations of either polymers or additives. Some photosensitization is believed to be caused by the absorption of polynuclear aromatics from the tropos- phere onto the polymer surface. Actinic radiation is absorbed preferentially by chromophores. Excited states thus formed participate in subsequent physical and chemical steps. The participation of singlet oxygen in photoxidation is one of the frequently cited problems not yet entirely solved. The reactive species is '0' (lAg), the energy of which is 94 kJ mol-l; its lifetime is reasonably longer than that of '0' ('Z'g). The role of singlet oxygen in the photoxidation of unsaturated hydrocarbon polymers has been discussed in detail, for example by Ranby and Rabek;' this agent is of minor importance in the photodegradation of saturated hydrocarbon polymers. Physical and chemical quenchings are involved in reactions of '02 with the organic polymers and/or additives.' Singlet oxygen should be considered as one of the important tropospheric oxidizing pollutants, together with ozone and nitrogen dioxide. Ozone combines very easily with unsaturated hydrocarbon polymers, and ozonolytic processes take place as a consequence. There is a strong indication of the rapid formation of hydroperoxides in polyolefins and polystyrene after treatment with ozonized air.3 It seems that the ozone addition to olefinic unsaturation followed by release of singlet oxygen is involved in this p r o ~ e s s . ~ Nitrogen dioxide should be considered from the point of view of the direct initiation of oxidation processes and scission of C-C bonds in the main chain of a p01ymer.~ More unsolved problems remain among the initiation processes influenced by the atmospheric factors mentioned above.

Thermal oxidation in the temperature range below 200 "C is a free radical chain process involving initiation, propagation, branching and termination steps. It is described by the classic Bolland's scheme6 derived from experimental data obtained on low molecular weight compounds, and generally is used also in the description of processes involved in polymers. The formation of a primary oxidation product, i.e. hydroperoxide ROOH, is exemplified in propagation steps (1) and (2):

R' + 0 2 - ROz' (1) (2) R02' + RH -+ R02H + R'

Compounds having reactive C-H bonds fit this scheme. Reaction (2) requires a relatively high activation energy. For tertiary C-H bonds it is in the range 46 f 8 kJ mol-' and is even higher for secondary and primary CH-H and CH-H bonds.'

Experimental data indicate a significant change in the mechanism of the oxidation proceeding above 200 "C. The chain propagation process can be described by7

R'CHCH3 + 0 2 + RCH=CHz + H02'

H02' + RCHzCH3 -+ H02H + R'CHCH3 (3)

(4)

Important information can be obtained from a com-

parison of the thermochemistry of summarized propagation steps of oxidation below (Eqn ( 5 ) ) or above (Eqn (6)) 200 "C. The former process is exothermic, and consequently autocatalytic. The high temperature oxidation is nearly thermoneutral and is characterized by a relatively small free energy change:

RH+Oz <200 "' >RO,H+75-160 kJ ( 5 ) RCH2CH3+02 >2"""c >RCH=CH2+H,0,+8 kJ (6)

In the later stages of both low and high temperature oxidation, the mechanism of the process becomes very complicated due to the presence of oxidation products, some of which are more reactive than the original hydrocarbon (substrate).

In processes below 200 "C, hydroperoxides are the primary oxidation products. Other products such as alcohols, carbonyl compounds (aldehydes, ketones, carboxylic acids), epoxides, olefinically unsaturated compounds and other peroxidic compounds, e.g. dioxy peroxides, hydroxy hydroperoxides, hyd- roperoxyperoxides and mixed alkyl peroxides, appear in the polymer in the subsequent steps. Isomerization reactions of C-centred R' radicals also contribute to the variety of oxidation products.* Products of the oxidative transformation of hydrocarbon polymers are inherent impurities contributing to the deterioration of polymer properties. Some of them are efficient chro- mop hores.

The hydroperoxides formed initially already under- go radical induced decomposition at 100 "C and are unstable above 150 "C because of the relatively weak RO-OH bond. Hydrogen peroxide arising from the high temperature oxidation is much more stable. It does not decompose, in the gas phase below 450 "C.

Thermally induced monomolecular (7) and bimolecular decompositions (8) of hydroperoxides participate in branching steps of the oxidation process."

RO;.H+ RO' + HO' (7)

(8) 2R02H + RO' + RO2' + H20

Carbonyl compounds (efficient chromophores) are formed via the generated alkoxy radicals RO.

Redox reactions (9) and (10) are involved in systems containing ions of transition metals. " > " The sum of the products of both these redox reactions is equiva- lent to products arising from bimolecular decomposition (8):

R 0 2 H + M"' + RO' + M@+') + HO- (9)

(10) R02H + M("+l)+ + R02'+M"+ + Hf

Table 1. Dissociation energies AHo of hydroperoxidic species formed in propagation steps (1)-(4) and of alkylperoxides in kJ mol-' 9310

Species AH" Species AH'

HO-OH 213 RO-OH 180 HOO-H 377 ROO-H 377 H 0-0. 268 R-OOH 393 H-00' 197 R-00 ' 117

RO-OR 176

FIRE AND MATERIALS, VOL. 6, NO. 1, 1982 43

J. POSPfSIL

There is an important problem in the catalysis of the decomposition of hydroperoxides: the relative activity of various metal ions is unpredictable and to a great extent dependent on the character of oxidized polymer.

Hydroperoxides decompose photolytically with a high quantum yield. The overall process is formally analogous to reaction (7). The released hot radicals HO' and RO' possess a high energy excess (c. 230 kJ mol-' at 313 nm). As a consequence, a quick collisional deactivation takes place, resulting in H-abstraction from the polymeric chain. Radicals R' thus generated enter into the propagation cycle. The photolytic decomposition of hydro eroxides is often sensitized by carbonyl compounds.' This means that the disappearance of the primary oxidation product is facilitated by the secondary one.

To summarize the importance of hydroperoxides, one can say that they rank among the most important impurities in oxidized polymers because of their efficiency in the participation in the induced thermal, photolytic and catalytic processes. Alkyl eroxides are a similar reliable source of free radicals.

The data mentioned earlier indicate a characteristic temperature dependence which influences the formation and content of hydroperoxides in oxidized hydrocarbons. According to Eqn ( 5 ) this reactive product is already formed during the induction period. It decomposes to active radicals participating in the auto- accelerated phase of the process. This results in a decreased overall content of R02H in the oxidized polymer, the temperature of which is increasing. At a sufficiently high temperature level, the importance of propagation processes (1) and (2) lessens and the contribution of the pair of reactions (3) and (4) becomes more important. After reaching a temperature limit of about 350-450 "C, the hydroperoxides are practically replaced in the reaction mixture by hydrogen peroxide. The latter is more stable than R02H, however, and decomposes efficiently in the gas phase only in the temperature range above 450 "C. Hence, one can assume that an efficient source of chain transfer radicals, ascribed to R02H, in the low temperature oxidation becomes operative only at a relatively high temperature. Above 450 "C, the reaction

P,

RH + 0 2 - R'+HO2' - 167-210 kJ (11) becomes very important, and is therefore extraordi- narily efficient in the chain initiation step. Because of the importance of the high temperature processes (6) and ( l l ) , and of the major role played by R02' and H02' radicals, a more profound knowledge of the thermochemistry of these species as well as of H02H is desirable. The data possess a specific value in con- nection with pre-burning processes and involve reactions dealing with volatiles formed via pyrolysis of polymers.

The high temperature degradation described above provides information which is very important for the assessment of reactivity of the gas phase above a heated polymer surface. Valid data were obtained by means of pyrolysis performed in the absence of oxygen. According to the behaviour of polymers during thermal pyrolysis at a temperature near that of burn-

ing, it is possible to distinguish between hydrocarbon polymers that unzip easily to give flammable monomers (e.g. polystyrene), and those which are pyrolysed via random scission and release flammable gases (e.g. polyolefins). As an example data are given on the low temperature pyrolysis of isotactic and atactic polypropylene in the carrier stream of helium.'' The overall activation energy of the pyrolysis of isotactic polypropylene at 388, 414 and 438°C was A E = 209 f 20 kJ mol-l. The first order constants valid for these temperatures were 4.0 x 1.1 X and 6.2 X s-l, respectively. The same reaction products were always formed and varied only in their relative amounts. 2,4-Dimethyl-l-heptene was the major product in all cases. Propylene, 2- pentene and 2-methyl-l-pentene were formed in amounts exceeding 10%. Minority products were me- thane, ethane, isobutylene, 2,4,6-trimethyl-l-heptene, 4,6-dimethy1-2-nonene, 2,4,6-trimethyl-l -nonene and 3-methyl-3,5-hexadiene. The same hydrocarbons were formed in the pyrolysis of atactic polypropylene (AE=234 f 25 kJ mol-l). Incombustible volatiles are released during the pyrolysis of polymers containing heteroatoms (e.g. hydrogen chloride from polyvinyl chloride (PVC).

Oxidative pyrolysis is operative in the presence of oxygen. The structure of components of the mixture of volatile products formed is not yet fully resolved. In addition to monomers and low molecular weight hydrocarbons, other combustible volatiles (carbon monox- ide, hydrogen) and incombustible volatiles (carbon dioxide, water) can be formed. The problem of increased evaporation of stabilizers and other volatile additives from the surface of overheated polymer has not yet been solved. Consequences of the evaporation of additives are very grave. Even when burning did not take place, technical polymer which has passed the temperature phase of oxidative pyrolysis is mostly unable to resist weathering conditions for long due to oxidative changes in the polymer, and to chemical as well as physical losses of stabilizers.

There are more distinct oxidative stages in the degradation history of a polymer before burning. The polymer is partially oxidized at least in the surface layers (and a substantial part of the active form of stabilizers are lost too).

Processes preceding burning are outlined in an over- simplified Fig. 1. The transfer of thermal energy from an external source through thermal radiation, hot air or pyrolytic gases causes endothermic phase transi- tions of a polymer and evaporation of occluded humidity. Hence, the advanced stages of combustion are influenced by some bulk properties of polymers: glass transition, melting and decomposition temperatures. Later, the temperature of the polymer increases, and irregular surface overheating connected with pyrolysis and release of volatiles takes place in most cases. The extent of the following exothermic processes is controlled by the overall situation above the polymer surface, including the temperature and laminar flow of the mixture of air-combustible volatiles. Above 450 "C, local spontaneous ignition of gases and surface glowing of residual polymer can be expected. The overall oxidation reaction is strongly

44 FIRE AND MATERIALS, VOL. 6, NO. 1, 1982


Chemical and photochemical Physical endothermal processes processes

I

Oxidized Polymer (s)

Phase transformation Thermodegradation in polymer (s,!) Local pyrolysis

b Dehydration

302

Volatile combustible Volatile incombustible Non-volatile residue (s,l)

\ Glowing /ocal Diffusion bur flame nil: (internal

source of heat)

J I I (External sources of heat) - Transfer of thermal energy -

Combustion products u Figure 1. Oxidative and thermal chemical and physical processes preceding burning of hydrocarbon polymers.

exothermic.' For each CH2 group, thermal energy (the internal source of heating) is released

CHZ + 1.502- C02 + H20 + 653 kJ (12)

and the temperature in the polymer increases stepwise. A cycle energy flow in the high temperature oxidation intensifies, and a laminar diffusion flame along the polymer surface sets in. It is very difficult to. locate exactly the place where the primary chemical reaction was converted to burning because of the concentration gradient and laminar flow of volatile combustibles over the thermally exposed polymer surface, and of the local distribution of thermal energy between the gaseous and solid phases.

Types of stabilizer

Some typical groups of stabilizers are used to protect hydrocarbon polymers against individual deterioration events. They differ in the mechanisms of their protective action and consequently in their chemical nature. Stabilization during the high temperature processing of polymers is to be considered as the first line of protection. Chain-breaking antioxidants and decomposers of hydroperoxides have been successfully applied. Residues of original antioxidants and products of their transformation, are present in polymers after processing, together with products of their oxidation. All newly formed structures should be considered as inherent impurities in the polymer. Because of their chemistry, they are able to participate very actively in thermooxidative, and particularly in photooxidative

processes. Their role in the deterioration of polymers is under serious consideration at present; basic charac- teristics of their partici ation in the ageing of polymers

Stabilization of long-term properties is the second line of protection of polymers. Chain-breaking antioxidants, light stabilizers and deactivators of metals are applied for polyolefins. Antiozonants have specific importance in the stabilization of elastomers. Temper- ature is a factor limiting the efficiency of all additive antioxidants. In air they quickly lose their activity above 180 "C, and therefore are of a very limited value for protection against extreme high temperature oxidations involved in burning. Hence, a third line of protection has been used in some products, i.e. flame- retardancy. This stabilization is important not only from the point of view of economy, but also from that of human safety.

Most effective or widely applied chemical classes of stabilizers are summarized. Antioxidants with the pre- dominant ability to decompose ROOH (classified also as preventive antioxidants) include organic phosphites, sulphides and disulphides, dialkyldithiophosphates and dialkyldithiocarbamates. Aliphatic amines can also be included (tertiary amino groups can be attached to molecules of more complicated stabilizers). The mechanism of activit of hydroperoxide decomposers has been described.' Some of them were considered also with respect to chain-breaking efficiency and/or photostabilizing activity. l9 Queries which arose in the discussion of the formation and nature of catalytically active species appearing during the action of hydroperoxide decomposers have not yet been answered in full.

have been described.' ?

ry

FIRE A N D MATERIALS, VOL. 6, NO. 1, 1982 45

J. POSPISIL

Phenols and aromatic amines are the only chain- breaking antioxidants in practical use. The most important phenols are sterically hindered ones. They represent the most important antioxidants for polyolefins. Typical members are monohydric mono- nuclear phenols, monohydric polynuclear phenols, polyhydric phenols and phenols containing a further active functional group. Essentially, the mechanism of their activity and transformation types are known.16,20,21. The second large group of chain- breaking antioxidants comprises aromatic amines. They can be classified as derivatives of diphenyl amine or phenyl-2-naphthyl amine, N,N’-disubstituted 1,4- phenylene diamine and derivatives of 5- and 6- membered heterocyclic compounds. They are successfully applied to the stabilization of elastomers. Some of them possess pronounced antiozonant properties. Theoretical knowledge of the mechanism of activity of amines is considerably poorer than that of phenols.

Carbon-black, hydroxybenzophenones, phenyl benzoate, benzotriazols, s-triazines, metal chelates of ketimines, thiobisphenols or dithiocarbamates and steric hindered cyclic amines are used as light stabilizers for polyolefins. Many aspects of their protective mechanisms have been e ~ p l a i n e d . ~ ~ , ~ ~ , ~ ~ , ~ ~ Data has also been published describing the transformation of the phenolic part of benzotriazole light stabilizer^.^^ Some mechanistic features in the group of hindered amines, i.e. the most recent and at the present most intensively studied class of stabilizers remain unsolved, however.

Polyolefins, namely, polyethylene, must be protected during some technical applications against deterioration caused by contamination with metals, especially with copper. Efficient metal deactivators are derivatives of salicyclic acid, oximes of salicycaldehy- de, alkylenediaminotetraacetic acids or hydroxylated aliphatic and aromatic acids. Structural features which make the formation of bi- or polydentate ligands possible are also involved in some chain-breaking antioxidants, e.g. in aromatic diamines and 2,2‘-alkylidene or 2,2’-thiobisphenols. The mechanism of participation of metal chelates in the complicated process of atmospheric ageing has been reviewed recently. l9 Pre- sent day knowledge is still not exhaustive. This can be exemplified by the discovery of technically very important compounds having properties of delayed action pro oxidant^.^^ For a long time, attention has been paid to the deactivation of residues of polymerization catalysts in polyolefins, viz. titanium and chromium compounds. It was reported26 that tetraethyl-0- titanate accelerates the photooxidation of polypropylene to a certain extent, as well as its liquid model 2,4,6,8-tetramethylnonane. Deterioration is only neg- ligible if titanium compounds are present in amounts below 100 ~ p m . ~ ~

Owing to the development of polymer blends, it is necessary to mention also thermal stabilizers applied on an industrial scale for the stabilization of PVC in polyblends with ABS polymers. Because of migration processes, the hydrocarbon part of these blends can also be affected to some extent by PVC stabilizers having structures of epoxides of unsaturated oils, salts of carboxylic acids or organometallic compounds

(alkyltin-mercaptides, -sulphides or -carboxylates). Although the mechanism of activity of these stabilizers in thermal Stabilization of PVC is relatively well known,28 there is still a lack of information dealing with the participation in low as well as high temperature oxidation of hydrocarbon polymers.

FLAME RETARDANTS

Enumeration of important structural types of compounds applied in the first and second lines of stabilization of polymers shows their chemical variety. It is possible to expect that some of them can be actively involved, either in their original or transformed forms, in several steps of the complicated process of polymer ageing. Mordover, the situation is complicated by the presence of flame-retardants in some polymers.

Flame-resistant polymers are emerging as a specific class of materials. One way of simplifying the situation can be seen in the replacement of hydrocarbon polymers, the nature of which involves the ability to serve as a fuel, by thermo and flame-resistant polymers based mostly on highly aromatic s stems or systems contain-

lack of some desirable properties restrict their general large-scale application.

Therefore the technical solution to the problem is focused on the application of additives, i.e. fire inhibi- tors or retardants. These additives operate in commer- cial polymers by either physical or chemical mechanisms, or more probably by a combination of both. The polymer itself remains flammable, of course. Some additives generally used for other reasons can contribute to the protection of polymers. For example, inert fillers are moderately effective retardants. They can either dilute the combustible hydrocarbon polymer or help to dissipate the accumulated thermal energy in the condensed phase. However, traces of metallic impurities can be introduced with these fillers into the polymer bulk and cause deterioration of their long- term properties.

Inhibition of burning is also achieved by the consumption of thermal energy in endothermic chemical processes in the bulky polymer-containing flame- retardants. These processes involve thermolysis of additives in the step of oxidative pyrolysis. Incom- bustible gases (water, ammonia, hydrogen halides) thus formed dilute the combustible gaseous pyroly- sate.

Flame-retardants should protect the polymer in those temperature ranges where antioxidants lose their activity. Due to this fact, it is believed that the chemical mechanism of flame retardation consists of interfer- ence with the key free radical steps involved in high temperature oxidation, oxidative pyrolysis and burning processes. More detailed experimental elucidation is necessary. Another mechanistic possibility consists of the formation of a thermally stable barrier. This can be either a glassy or a char coating on the polymer surface. This barrier reduces the heat and gas flow between the condensed and gas phases, slows down penetration of oxygen from the atmosphere to the

ing boron or phosphorus.29, Y However, high costs and



heated surface, limits dripping of the melted polymer and contributes to the dissipation of thermal energy from locally overheated sites. Of course, these effects are greatly influenced by the nature .of the burned polymer.

Until now, knowledge of the distribution and migration of flame-retardants in the mass of polymers has been very scarce. These physical phenomena are of great importance, especially in semicrystalline polymers. New data31x3* dealing with problems in a system antioxidant-polymeric matrix confirm the necessity of understanding of physical processes, in addition to knowledge of the chemical mechanisms.

Various inorganic and organic compounds have been proposed. Some of them have been practically applied for the fire-retardation of polymers33734 and the mechanism of their activity has been e ~ t i m a t e d . ~ ~ 36 Inorganic retardants are salts, oxides, hydrates or sulphides of some metals (especially of aluminium, silicon, antimony and molybdenum), salts of phos- phorous acid, halides or oxyhalides of phosphorus, salts of boric acid and mixtures of these compounds. Organic retardants consist of halogen-rich compounds (chlorinated aliphatic and aromatic hydrocarbons, bro- minated aromatic hydrocarbons and aromatic oxygen- containing compounds, e.g. of phenols and anhydrides of polybasic acids), organic compounds of phosphorus (phosphines, phosphine oxides, phosphites, phos- phates, phosphonates, phosphonium compounds), es- ters of boric acid and aliphatic nitrogen-containing compounds (derivatives of urea and thiourea, cyanides, rhodanides). This is a rather wide variety of compounds. Fortunately, not all the flame-retarded polymers take part in catastrophic fires. On the other hand, they have been exploited for a long time in specific areas of application, and are affected by current environmental effects. Data is lacking to draw a picture of the influence of flame-retardants on the long-term properties of polymers. By experience and comparison with known facts, we can conclude that most organic flame-retardants from the group of compounds of phosphorus, boron or nitrogen do not nega- tively influence the atmospheric stability of hydrocarbon polymers. Most of these additives have structures very similar to those of inferior antioxidants or anti- zonants. It seems probable that these compounds were proposed as flame-retardants in years of an explosive search for such stabilizers. This trend was caused by legislative restrictions dealing with flammability of polymers, announced in many countries. Compounds of phosphorus, boron or nitrogen are sufficiently thermo- stable and of low volatility, and therefore we can suppose that they remain unchanged in the processed polymer as well as during atmospheric ageing until the phase of oxidative pyrolysis.

Some facts are not clear in the class of halogen-rich additives. Many of them are thermo- and photoreac- tive. Only scarce information is available about their transformation and side-effects in polymers. A part of the halogen-rich additives is destroyed during the processing of polymers. Another part can be depleted during long-term use. The release of hydrogen halides has been considered with respect to ecology and industrial toxicology. No data is available, however,

concerning their effect on lon term properties of polymers. It has been reported’’ that a mixture of perbrominated organic compounds with Sb203 very efficiently retarded the burning of the gas phase above the polymer surface of high-impact polystyrene. At the same time, the same additives lowered the thermal stability of the flame retarded polymer. However, after photooxidative treatment of high-impact polystyrene stabilized with the above-mentioned combination of flame-retardant~,~~ an improved self- extinguishing was observed.

COMBINATION OF ADDITIVES

It is evident that the way of reaching a complex protection of hydrocarbon polymers against distinct deteriorative effects consists mainly of the application of a mixture of additive stabilizers, i.e. of an additive system. There is still some empiricism in compound- ing. This can be connected with failures caused by sometimes only randomly selected components of the mixture. This fact evokes the necessity of profound knowledge of mechanistic relationships in mixtures of various stabilizers, and of stabilizers with other additives.

The system of additives offers more than one possibility. The optimum approach consists in the proper choice of individual components from a diversity of available compounds. The danger of unwanted secondary effects can be minimized in this way. An inte- grated stabilization effect is generally assumed to be reached on the principle of additivity of contributions of individual components. It is not an unusual fact, however, that by using a mixture of stabilizers, either the summary effect or a specific property are dimi- nished. This phenomenon is called antagonism. It rarely appears in mixtures of antioxidants, but has been observed in mixtures of antioxidants with light stabilizers. In this context, the question of the influence of carbon-black on phenolic antioxidants remains open.

Information dealing with mixtures of antioxidant- flame-retardant is very scarce. The importance of transformation processes has been revealed by studies of chain-breaking antioxidants2’ Owing to the ex- tremely low concentration of newly-formed structures, they are not always explicitly expressed in the deterioration of long-term properties of polymers. Flame retardants are currently used in concentrations signi- ficantly surpassing those of antioxidants and/or light stabilizers, however. It seems that unfavourable influences of some transformation products of flame- retarders formed during weathering of polymers are still overlooked. Elucidation of mutual relationships is very desirable therefore in the mixture antioxidant- flame-retardant .

An integral activity surpassing that expected from the validity of the principle of additivity of individual contributions can be reached in some stabilizer systems. This phenomenon is characterized as synergism and must be considered seriously with respect to its technical applicability. The high resulting stabilizing


J. POSPISIL

effect is an important consequence of synergism. Another advantage is the possibility of lowering the amount of stabilizers required to obtain a desirable stabilizing effect. The second advantage is of specific value, above all, to flame-retardants. Until now, they have to be applied in too high concentrations which can very unfavourably influence some mechanical properties of hydrocarbon polymers. The high concentration of flame-retardants indicates at the same time that it is the physical mode of their retardation activity that predominates over the chemical one.

On the basis of current knowledge, it is possible to classify synergistic events more precisely than has been done previously, dividing them into (i) intermolecular synergism in a physical mixture of antioxidants, light stabilizers, antiozonants and/or flame retardants, and (ii) intramolecular synergism in polyfunctional stabilizers. In each of this particular groups, one can distinguish: (a) homosynergism: it can take place in a mixture of two stabilizers participating in the stabilization process by the same mechanisms, providing that the individual components of the mixture participate to a distinct extent in single partial steps of the degradation process. This kind of synergism has been known among some chain-breaking antioxidants.'' (b) hetero- synergism: it is valid in a mixture of stabilizers participating in the process by very distinct mechanisms. This kind of synergism is of particular technical value. Many technically important available data have not been published, however, especially those on mixtures of flame-retardants.

It is surprising that systematic research studies are lacking which would clarify the relationship between antioxidants and flame-retardants. Overall synergism (or additivity) could probably be considered as a specific contribution of flame-retardants to protection against atmospheric ageing. This can be the case because of chemical, as well as some mechanistic relationships between both kinds of stabilizers. The contribution of antioxidants to flame-retardancy seems to be more questionable and consists of the possibility of an overall suppression of the formation of oxidized volatiles and non-volatiles from the polymer before inflammation. It is not surprising that data on changes in synergistic events caused by the chemical transformation of a substantial part of a particular compo- nent of the stabilizer system after a period of atmospheric ageing are completely lacking. An in-depth theoretical treatment is therefore desirable, by using model systems and technical polymers. Data thus obtained is necessary for a rational selection of suit- able stabilizer systems.

Problems arising from chemical and physical in- teractions in systems stabilizers-polymer matrix are complicated still further by the inevitable presence of other additives or their transformation products. These include processing additives (e.g. lubricants, blowing and vulcanization agents, or more probably their residues and products of thermal treatment) and additives having properties of fillers, reinforcing agents, antistatics, optical brighteners, dyestuffs and pigments. Most of these additives are present in the polymer in amounts considerably exceeding those of antioxidants and light stabilizers, sometimes perhaps

also of flame-retardants. Therefore, their influence on long-term stability can be decisive. Some specificities arising from the application of mixtures of additives are reflected in many technical observations. In most cases the nature of these effects has not been eluci- dated systematically, although a more profound knowledge of some phenomena could enable us to eliminate unsuitable components of the mixture. Results of some recent studies explaining the role of some trace impurities in mineral fillers or that of anthraquinone pigments or dyestuffs on hotoprocesses in man-made

such data. It is possible to tolerate some negative features arising from the application of systems of additives if the latter are designed to obtain optimum processing properties.

hydrocarbon polymers 39,47: underline the usefulness of

CONCLUSIONS

Knowledge of the degradation processes of polymers, of the chemical structures of applied stabilizers and mechanisms of their activity, including their transformations during stabilization processes, enable us to formulate limitations to the applications of stabilizers and resolve some unexplained facts. The following phenomena are generally operative: (1) Chemical and photochemical transformations of

stabilizers during processing, atmospheric ageing and oxidative pyrolysis

(2) Formation of products involved in initiation processes as a consequence of changed thermal and photolytic stability and/or chemical reactivity com- pared with the original stabilizer

(a) Solubility of stabilizers in the polymer and its dependence on the polymer morphology, compatibility in the system polymer-stabilizer and changes caused by chemical transformations of the stabilizer or polymeric matrix

(b) Staining of stabilized polymer by transformation products of the stabilizer

(c) Volatility and surface blooming of stabilizers (d) Extractability of stabilizers with water, solu-

tions of surface active agents and organic sol- vents

(e) Deterioration of mechanical properties or of surface appearance of polymers

(f) Deteriorative effects of some other additives on the chemical and physical properties of stabilizers causing their accelerated depletion or loss of the expected long-term stabilization effect

(3) Physical phenomena

(8) Ecological and legislative requirements All the factors mentioned above require special

attention and include some unsolved problems for each specific group of stabilizers. To exemplify the variation in possibilities of the transformations of antioxidant during the thermal oxidation and atmospheric ageing of a hydrocarbon polymer, a product scheme is given involving a phenolic antioxidant InH (Fig. 2).*l As a consequence of parallel and consecutive molecular and radical reactions, typical groups of compounds are formed: simple and polynuclear quinons methides



BQ --- ROO-CHD

RO-CHD r

HIn‘In‘ H HO-CHD

QM - (QM lx

Figure 2. Scheme of the formation of the main classes of transformation products from a phenolic antioxidant InH.

\

QM and (QM), respectively, polynuclear phenolic coupling products H1n’-In’H, phenoxycyclo- hexadienones InO-CHD , biscyclohexadienones (- CHD)*, alkylperoxycyclohexadienones ROO-CHD , alkoxycyclohexadienones RO-CHD, dioxycyclohex- adienones (O-CHD)2, hydroxycyclohexadienones HO-CHD; and quinoid compounds: dipheno- quinones DPQ and derivatives of benzoquinone BQ. Compounds having a system of conjugated double bonds participate in consecutive photochemical processes. l6 The complexity of reactions and some relationships have been shown recently. Transforma- tions of some other types of antioxidants have also been d e ~ c r i b e d . ~ ” ~ ~ However, many questions have not yet been answered using exact experimental results. Elucidation of transformations of light stabilizers is very scarce,24 and systematic research about transformations of flame-retardants during atmospheric ageing is completely lacking.

Limitations of the optimum exploitation of the chemical capacity of stabilizers caused by physical factors are specifically attributed to a particular polymer and conditions of its application. Excellent analyses of

physical relationships in the polymer-stabilizer show factors which play a determining role

in semicrystalline polymers. The relationships are yet more complex in elastomers and in polymer blends, where any generalization has been impossible so far. Available data has a rather informative character.

Problems arising from the high volatibility and extractability of antioxidants or light stabilizers from polymers can be solved by means of syntheses of high molecular , polymeric or polymer-bound ~tabi l izers .~~ Research and development in this area is very inten- sive.

Oxidative degradation processes and application of stabilizers are two opposite sites of the same problem, viz. of the long-term property of polymers. Topics under discussion are very diverse, but at the same time very attractive, both for theoretical elucidation and for technical as well as economic purposes. Much data is reported in the literature and much practical experience has been collected by workers in the field. The basis of theoretical studies has been shifted from for- mal kinetic analyses to complex chemical and physical studies on technical polymers. It includes many problems mentioned in the text. Research and rapid development in flame-retardancy are forced specifically by legislative requirements. It seems, however, that application of basically new antioxidants, light stabilizers and flame retardants can to some extent be hand- icapped by strongly or wrongly applied ecological regulations. These are intended to protect people exploit- ing products from man-made polymers which are in direct contact with foodstuffs or are used in childrens’ toys and sanitary products. Some regulations also arise as consequences of industrial toxicology: it is necessary to keep correct conditions of working hygiene during the production of stabilizers and their processing with polymers. However, producers of stabilizers and ecologists can have a very conflicting approach to the problem. It is to be hoped that generally valid and objective rules for rating of the toxicity of stabilizers under particular conditions of application will be available in the near future. The further development of stabilizers must be coincided with them.

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Received 17 February 1981


oxidation and stabilization of hydrocarbon polymers: relationships, new aspects and unsolved...

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