Die Angewandte Makromolekulare Chemie 158/159 (1988) 221 -231 (2678)

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

MACROMOLECULAR STABILIZERS FOR POLYMERS

Jan PospiSil

Summary :

Inherent chemical activity, physical persistence and good compati- bility are factors determining the efficiency of a stabilizer ob- served during the degradation of amorphous and crystalline syn- thetic polymers. Synthesis of macromolecular stabilizers is one of the ways used to solve the problem of physical persistency of stabilizers under severe aggressive environmental attacks on polymers. General types of macromolecular stabilizers, routes to their synthesis and characteristic examples of macromolecular anti- oxidants, UV absorbers, hindered amine light stabilizers, flame retarders and biostabilizers, as well as of polyfunctional systems are given. Problems connected with the use of macromolecular stabilizers are mentioned.

SIGNIFICANCE OF THE PHYSICAL PERSISTENCE OF STABILIZERS

A long-term stability of technical polyolefins, polystyrene, rubber modified plastics or diene based elastomers during indoor and specifically outdoor applications has been achieved only by the use of efficient stabilizers. The proper choice of the latter for practical stabilization is based on the inherent sensitivity of

Paper presented at the 18th Colloquium of Danubian Countries for "Natural and artificial ageing of plastics" in Villach, Austria, June 24-26, 1987

D 1988 Huthig & Wepf Verlag. Bawl wO3-3146/88/$03.W

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a particular polymer to deteriorating agents and by the intensity of the attacking deteriogens. At the present time, efficient stabilizers with molecular weights in the range 300-500 (H.M.W. stabilizers) are commercialized and able to ensure sufficient service stability of polymers under moderate conditions. These stabilizers possess a high intrinsic chemical efficiency deter- mined by their molecular structure and the content of functional groups. Moreover, they mostly have a good compatibility with the host polymer.

Domestic and engineering applications of polymers in very severe and aggressive environments have increased in number in the last decade. This involves an intensive attack of environmental tempera- ture, atmospheric pollutants, solar radiation, continuous or cycli- cal- leaching by hot water, aqueous solutions of detergents or acids and various organic solvents, hot oils, gasoline, and volatiliz- ation by streaming hot gases or due to a l o w pressure. A more intensive degradation of polymers due to the limited physical resistance of stabilizers is a consequence. The problem has been met in particular in articles having high surface to mass ratios (thin films, fibres) and articles applied under extremely severe conditions. This involves, for example, rubbers used in engine seals, heavy duty truck tires, automotive belts, plastics applied as outdoor coatings or textiles exposed to repeated washing or dry cleaning. Some specific problems arise from the use of plastics as wrapping materials for foods and pharmaceuticals: the mentioned substrates may not be contaminated by the stabilizers extracted from plastics.

The practical requirement for polymers operating without failure under aggressive environments calls for stabilizers which in addition to the high intrinsic chemical efficiency also have appro- priate physical properties: persistence under aggressive environ- mental effects (i.e. low volatility and extractability) and proper- ties reflecting physico-chemical aspects of the molecular structure of the polymer-stabilizer system (i.e. good solubility and mi- gration of stabilizers and compatibility of all components of the system) . The optimum exploitation of stabilization efficiency requires a proper balance between chemical and physical properties. The permanency of stabilizers is related to their molecular weight and structure and to physico-chemical interactions dependent on dif- ferences of the polarity of components of the system. The polarity

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influences also compatibility, solubility and diffusion in the host p~lyrner~-~. Moreover, the stabilizer should be evenly dis- tributed in places of a concentrated attack of deteriogens. This involves, for example, localization of antiozonants or light stabil- izers in the surface layers of a polymer, or localization of anti- oxidants in amorphous phases of semicrystalline polymers. An even distribution of a stabilizer in the whole mass of a host polymer may be reached only in amorphous polymers with well compatible stabilizers and at concentrations of the latter not exceeding saturation at ambient temperatures . Blooming of a stabilizer into the surface layer of a polymer is a consequence of the decreased stabilizer solubility in the host polymer and of its migration. The bloomed stabilizer is easily removed physically by mechanical rubbing off, volatilization, or leaching . Physical losses by extraction are more severe than that by evaporation , in particular under cyclical alteration of extrac- tion and drying periods: the stabilizer lost by extraction has been replaced during the drying period by diffusion of new portion of the stabilizer from the polymer bulk into the surface and the stabilizer is repeatedly extracted.

The increase of the physical persistence of stabilizers is there- fore an important practical demand. It may be reached by a chemi- cal modification of the architecture of the stabilizer molecule: however, this modification may not be accompanied by a significant deterioration of the inherent chemical stabilizing activity or of some important functional properties of the host polymer caused by the damage to its overmolecular structure or by a poorer compati- bility of the modified stabilizer with a host polymer.

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SOLUTION OF THE PROBLEM: FUNCTIONALIZED POLYMERS

No fundamentallynew chemical types of stabilizers have been com- mercializedinthe last decade. The producers and users of stabil- izers are however aware of practical difficulties arising from the unsatisfactory physical properties of current stabilizers. Techni- cally and economically feasible ways to the solution to the problem have been examined to optimize the practical utility of stabil- izers, i.e. to increase their permanency in the host polymer . 6

There has been an attempt types of stabilizers in a 1500. These compounds may ecular. The experience of

to synthesize all conventional effective form having molecular weights overpassing be classified as oligomeric or macromol- synthetic organic and macromolecular

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chemistry and the knowledge of chemical factors governing inherent chemical efficiency have been successfully exploited. It has been generally accepted that macromolecular stabilizers act by the same chemical or physical protective mechanism as conventional H . M . W .

stabilizers bearing relevant functional groups. Attention has been concentrated on systems having an optimum relative content and distribution of stabilizing units. The development of macromol- ecular stabilizers having practical importance in selected appli- cation areas of rubbers and plastics is one of the most important directions in the innovation programme in polymer stabilization.

Macromolecular stabilizers belong generally to the comprehensive group of "functionalized" polymers. The latter attained importance in the last decade of the developments of polymers'. Numerous macromolecules with attached or built-in functional groups with properties of antioxidants, antiozonants, metal deactivators, light stabilizers, flame retardants or biostabilizers have been syn- thesized' '*-lo.

The data dealing with macromolecular stabilizers have been scat- tered in many hundreds of patents and articles. A system of typical classes of macromolecules bearing a stabilizing moiety @ has been arranged and ways to their synthesis have been compiled". The stabilizing moiety @ is either a part of a repeating unit built into the polymer main backbone (Type A ) , or it is attached as a pendent group to the polymer backbone directly (Type B) or by means of a spacer @ be attached to every repeating unit (homopolymers) or may alternate

(Type C). The moiety (9 in Types A to C may

A B

E F

C D

G

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with units arising from conventional monomers (copolymers, terpoly- mers, polycondensates) . The moiety @ may be part of a side chain attached to the polymer backbone (Type D, graft copolymers) or form a linkage between two polymer chains (Type E). End-chain stabil- izers of Types F and G are based on end-reactive "living" polymers.

SYNTHETIC METHODS AND EXAMPLES OF MACROMOLECULAR STABILIZERS

Macromolecular stabilizers of structural Types A-G may be obtained 10 by diverse synthetic approaches:

(i) Polymerization or copolymerization of functionalized monomers: free- radical (including grafting onto conventional polymers and exploitation of chain transfer), ring opening and coordination polymerizations, respectively

(ii) Polycondensation of functionalized monomers with compounds having carbonyl functions, sulfur chlorides and different bifunc- tional reactants, respectively

(iii) Polymer-analogous reactions: chemical binding of low-molecu- lar weight compounds carrying a stabilizing moiety onto current saturated and unsaturated hydrocarbon polymers or onto function- alized polymers and molecular rearrangements of polymer-bound ester moieties, respectively

Using variations of these methods and different functionalized monomers, macromolecular stabilizers containing combinations of moieties @ acting by diverse mechanisms may be synthesized. A s typical examples, different macromolecular phenolic chain-break- king antioxidants 1-5, aminophenolic antioxidant 1, aromatic amine antidegradants i-fi, sulfur (12) and phosphorus (GI containing hydroperoxide decomposing antioxidants, hindered piperidine light stabilizers 2, 2, UV absorbers having structures of hydroxy- benzophenone (16) or hydroxyphenylbenzotriazole (c), flame retard- ants 2, 2 and biostabilizer 0 are given. Some of these macromol- ecular stabilizers, i.e. 2, 5 , 1 and 2, respectively, are bifunc- tional, having properties of CB and HD antioxidants and acting by intramolecular cooperations. Systems acting by pseudo-intermol- ecular (i.e. inter-moiety) cooperation are also available by means of macromolecular synthesis. A system WA-phenol (2) and HALS- phenol (22) are two typical examples.

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PRACTICAL APPLICABILITY OF MACROMOLECULAR STABILIZERS AND UNSOLVED PROBLEMS

All chemical types of stabilizers have been synthesized on a lab- oratory scale in the form of functionalized macromolecules. The synthesized systems are used as masterbatches and blended with unstabilized polymers so that the molar concentration of stabil- izing moieties in obtained blends is sufficient to protect thehost polymer. Up to now, however, only some macromolecular stabilizers

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have been commercialized and some other may be commercialized in the near future, due to their excellent properties and technically acceptable mode of synthesis. (On the contrary, there are indi- cations that some macromolecular stabilizers, marked with an aster- isk in the following text, were withdrawn in the last months from the market.) Commercially available macromolecular antioxidants are: POLY-A0 TM-79 @ '* ( 2 , Dynapol) , designed for the stabiliz- ation of food; Modanox 2000 @ '* (5, Monsanto Corp.) , a processing antioxidant for polyolefins; Chemigum HR @ (8, Goodyear) having different contents of active amine units, an excellent antide- gradant for NBR goods used in contact with hot oil; oligomeric trimethyldihydroquinoline 11 commercialized under a variety of trade names Agerite Resin (Monsanto) , Good Rite 3140@ (B.F.Goodrich) or Nonox TQ@ (ICI) is an important antidegradant for rubbers; POLY TDP-2000 @ * (12, Eastman Chemical Products), a synergistic antioxidant for polyolefins. Excellent Lightstabilizers for polyolefins are: Tinuvin 622@ (14, Ciba-Geigy AG) , Chimassorb 944 @ (*, Ciba- Geigy AG) , Cyasorb UV 3346@ (E, American Cyanamid Corp.) and Spinuvex A-36@ (B, Borg Warner). The contemporary limited practical use of macromolecular stabil- izers, and antioxidants of particular, is a consequence of un- favourable price/performance relations. The high price of raw materials and some technological difficulties, like necessary modi- fications in the current polymerization technique, expensive syn- thesis of reactive polymers used as raw-materials for polymer- analogous reactions, are the main factors contributing to the un- favourable situation. Moreover, some reactants have hazardous properties. The use of macromolecular stabilizers is therefore above all focussed on the stabilization of polymers applied under extreme conditions where trouble-free performance is required, like machinery, automotive industry, solar energetics, or space vehicles. This involves materials working in contact with hot oils, under intensive long-term solar radiation, high-energy radiation or in vaccuum, and materials attacked repeatedly with organic solvents and detergents.

The extent of the consumption of macromolecular stabilizers, and predominantly of light stabilizers, is however steadily increasing, although some serious technical problems accompany the practical use of macromolecular stabilizers as blends. The difficulties arise from the limited solubility and compatibility of macromolecular

(R.T.Vanderbilt) , Flectol H @

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stabilizers with the host polymers. This results in a great hetero- geneity in the distribution of macromolecular stabilizers and makes the protecting effect in the mass of the host polymer questionable. A large part of advantages imparted by the excellent physical persistence is thus lost. The problems are more serious in crystal- line than in amorphous polymers. To reach the most flexible use of a macromolecular stabilizer in polymers differing in their polarity and overmolecular structure, the molecular weight of a macromol- ecular stabilizer must be relatively very low in comparison with the host polymer: it has been found that the number of polymers compatible with macromolecular stabilizers drops rapidly with the increase in the molecular weight of the latter. This limits the universal application of macromolecular stabilizers. Tailor-made stabilizers fitting a particular host polymer must be used. This is a very unfavourable difference from modern very efficient H.M.W. stabilizers.

At present, the knowledge of relations between the chemical struc- ture of a macromolecular stabilizer and the physical behaviour in the host polymer is not sufficient. There exist different contra- dictory data, and a lower stabilizing effect of macromolecular antioxidants and light stabilizers in comparison with their low- molecular weight analogs bearing comparable functional moieties has been found in some measurements. This may be due to unfavourable physical relations in the stabilized system. The incompatibility problem is less expressed in amorphous microheterogeneous systems, like rubbers, and with stabilizers synthesized either by copolym- erization of functionalized monomers with conventional monomers, by polycondensation and by polymer-analogous reactions on polymers with a properly chosen structure or with the same structure as the polymer to be protected. Much effort has been devoted to the eluci- dation of synthetic procedures performed during processing oper- ations and using current technique and conventional polymers. Active moieties molecular stabilizers and inherent chemical efficiency may be well balanced with physical persistency and compatibility with the host polymer. It is obvious that more serious complications arise during blending with crystalline host polymers. Stabilizers having mol- ecular weights in the range from 3,000 to 20,000 are considered to be the most acceptable .

@ are distributed statistically in these macro-

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CONCLUSIONS

Most important current macromolecular additives have properties of light stabilizers with structures of hindered piperidines. They are widely used in the stabilization of polyolefins. W absorbers con- taining @-moieties with structures of hydroxyphenylbenzotriazoles and hydroxybenzophenones are of potential importance, similarly to macromolecular fire retardants and biostabilizers. The practical importance of most macromolecular antioxidants will probably be limited even in the near future only to amorphous polymers.

REFERENCES

D. Bailey, 0. Vogl, J.Macromo1.Sci.-Rev.Macromo1.Chm.G (1976) 267 N.C. Billingham, P.D. Calvert, The Physical Chemistry of Oxidation and Stabilization of Polyolefins, In: G.Scott (Ed.), Developments in Polymer Stabilization, Applied Science Publishers Ltd, London 1980, Vo1.3, p.139 M. Moisan, Effect of Oxygen Permeation and Stabilizers Migration on Polymer Degradation, In: J. Comyn (Ed.), Polymer Permeability, Elsevier Applied Science Publishers Ltd, London 1985, p.119 P.L. Dubin, W.J. Leonard, Plast.Eng. 2 (1977) Nolo, 29 R.P. Weimer, W.P. Conner, Text. Res.J. 2 (1969) 1150 B.N. Leyland, T. J. Mayrick, Rev.Gen.Caout.Plast. 53, No562 (1976) 61 0. Vogl, J.Macromol.Sci.-Chem. A22 (1985) 541 J.A. Kuczkowski, J.G. Gillick, Rubber Chem.Techno1. 57 (1984) 621 Ch. Potin, A. Pleurdeau, C.M. Bruneau, Double Liaison-Chin. Peint. 2 (1982) No322, 15, No324, 35

P.P. Klemchuk (Eds), Oxidation Inhibition in Organic Materials, CRC Press, Boca Raton, in press

lo J. PospiZil, Macromolecular Stabilizers, In: J. Pospisil,

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