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CHEMICAL DEGRADATION
CHAPTER 22 CHEMICAL DEGRADATION Degradation of polymers by chemical reactions is a typical constitutive property. No methods for numerical estimations exist in this field. Only qualitative prediction is possible. The rate of chemical degradation can often be measured by means of physical quantities, e.g. stress relaxation measurements.
Introduction A polymer may be degraded by chemical changes due to reaction with components in the environment. The most important of these degrading reagents is oxygen. Oxidation may be induced and accelerated by radiation (photooxidation) or by thermal energy (thermal oxidation). Besides the oxidative degradation, also other forms of chemical degradation play a part, the most important of which is the hydrolytic degradation. Degradation under the influence of light Of the electromagnetic energy emitted by the sun only a small portion reaches the earth's surface, namely, rays with a wavelength above 290 nm. X rays are absorbed in the outermost part of the atmosphere and UV rays with wavelengths up to 290 nm in the ozone atmosphere. Although the total intensity is subject to wide variations according to geographical and atmospheric conditions, the overall composition of sunlight is practically constant. In photochemical degradation the energy of activation is supplied by sunlight. Most ordinary chemical reactions involve energies of activation between 60 and 270 kJ/mol. This is energetically equivalent to radiation of wavelengths between 1900 and 440 nm. The energies required to break single covalent bonds range, with few exceptions, from 165 to 420kJ/mol, which corresponds to radiation of wavelengths from 710 to 290 nm (see fig. 22.1). This means that the radiation in the near ultraviolet region (300-400 nm) is sufficiently energetic to break most single covalent bonds, except strong bonds such as C—H and O—H. Only the part of the radiation which is actually absorbed by the material can become chemically active. Most pure, organic synthetic polymers (polyethylene, polypropylene, poly(vinyl chloride), polystyrene, etc.) do not absorb at wavelengths longer than 300 nm owing to their ideal structure, and hence should not be affected by sunlight. However, these polymers often do degrade when subjected to sunlight and this has been attributed to the presence of small amounts of impurities or structural defects, which absorb light and 655
656
Fig. 22.1. Energy equivalence of light waves. initiate the degradation. Much of the absorbed light energy is usually dissipated by either radiationless processes (rotations and vibrations) or by secondary emission (fluorescence). Although the exact nature of the impurities or structural defects responsible for the photosensitivity is not known with certainty, it is generally accepted that these impurities are various types of carbonyl groups (ketones, aldehydes) and also peroxides. The primary chain rupture or radical formation in the various photochemical processes is often followed by embrittlement due to cross-linking, but secondary reactions, especially in the presence of oxygen, cause further degradation of the polymer. Mechanical properties, such as tensile strength, elongation and impact strength, may deteriorate drastically. Coloured degradation products are often developed. Surface crazing can also be a sign of UV-induced degradation. Some polymers show discoloration as well as reduction of the mechanical properties (e.g. aromatic polyesters, aromatic polyamides, polycarbonate, polyurethanes, poly(phenylene oxide, polysulphone), others show only a deterioration of the mechanical properties (polypropylene, cotton) or mainly yellowing (wool, poly(vinyl chloride)). This degradation may be less pronounced when an ultraviolet absorber is incorporated into the polymer. The role of the UV-absorbers (usually o-hydroxybenzophenones or o-hydroxyphenylbenzotriazoles) is to absorb the radiation in the 300-400 nm region and dissipate the energy in a manner harmless to the material to be protected. A current development in the UV-protection of polymers is the use of additives (e.g. nickel chelates) which, by a transfer of excitation energy, are capable of quenching electronically excited states of impurities (e.g. carbonyl groups) present in the polymer (e.g. polypropylene).
657 Oxidative degradation At normal temperature polymers generally react so slowly with oxygen that the oxidation only becomes apparent after a long time. For instance, if polystyrene is stored in air in the dark for a few years, the UV spectrum does not change perceptibly. On the other hand, if the same polymer is irradiated by UV light under similar conditions for 12 days, there appear strong bands in the spectrum. The same applies to other polymers such as polyethylene and natural rubber. Therefore, in essence the problem is not the oxidizability as such, but the synergistic action of factors like electromagnetic radiation and thermal energy on the oxidation. By the action of these factors on the polymer free radicals are formed, which together with oxygen initiate a chain reaction. Hence most oxidation reactions are of an autocatalytic nature. If the oxidation is induced by light, the phenomenon is called photooxidation. If the oxidation is induced by purely thermal factors, the term thermal oxidation is used. Photooxidation The most thoroughly investigated oxidative degradation is that of natural rubber. In 1943 Farmer and Sundralingham found that in the photochemical oxidation of this polymer a hydroperoxide is formed, the number of double bonds in the chain remaining constant. The oxygen was found to act on an activated methylene group, not on a double bond, as had previously been assumed. Later the mechanism of the rubber oxidation was studied extensively by Bolland and coworkers (1946-1950), who mainly used model substances. In his first publication Bolland proposed the following mechanism for the propagation reaction: R· +02->ROCT
(a)
ROO * + RH-+ ROOH + R '
(b)
where RH is the olefin, R ' a radical obtained by abstraction of hydrogen in the allyl position, the R O O " the peroxy radical obtained by addition of oxygen to this radical. According to Bolland the reaction chains are terminated by the combination of allyl and peroxy radicals, and the length of the main reaction chains is of the order of 50-100. This reaction could be initiated by any type of reaction in which free radicals are formed. The autocatalytic nature of the reaction is due to the decomposition of the hydroperoxides: R O O H ^ R O ' + *OH
(c)
The hydroperoxides also give rise to secondary reactions in which coloured resinous products are formed (via carbonyl compounds). Stabilization to photooxidation can be achieved by the use of suitable UV absorbers in combination (synergistic action) with antioxidants (AH) which are capable of preventing
658 reactions (a) and (b): R ' + A H ^ R H + A'
(d)
ROO e + A H ^ ROOH + A '
(e)
A * —> inactive products.
(f)
Thermal oxidation Especially above room temperature many polymers degrade in an air atmosphere by oxidation which is not light-induced (heat ageing). A number of polymers already show a deterioration of the mechanical properties after heating for some days at about 100°C and even at lower temperatures (e.g. polyethylene, polypropylene, polyformaldehyde and poly(ethylene sulphide)). The rate of oxidation can be determined by measuring volumetrically the oxygen uptake at a certain temperature. Such measurements have shown that the oxidation at 140°C of low-density polyethylene increases exponentially after an induction period of two hours. It can be concluded from this result that the thermal oxidation, like photoxidation, is caused by autoxidation, the difference merely being that the radical formation from the hydroperoxide is now activated by heat. The primary reaction can be a direct reaction with oxygen RH + 0 2 - + R ' + Ό Ο Η .
(g)
The thermal oxidation can be inhibited by antioxidants as before (eqs. (d), (e), (f)). Effects of oxidation degradation The principal effects of oxidative degradation of polymers are the decay of good mechanical properties (strength, elongation, resilience, etc.) and discoloration (mainly yellowing). The behaviour of polymers may vary widely. A polymer may be resistant to mechanical decay but not to colour decay, or the reverse. Often the two go together. Table 22.1 gives a survey of these effects for the different polymer families in the case of photodegradation. Stabilization The oxidative degradation of a polymer can be retarded or even practically prevented by addition of stabilizers. The following types of stabilizers may be used: a. UV absorbers A good UV absorber absorbs much UV light but no visible light. It should dissipate the absorbed energy in a harmless manner by transforming the energy into heat. Other requirements are: compatibility with the polymer, nonvolatility, light fastness, heat stability and, for textiles, also resistance to washing and dry cleaning. The optimum effect of a UV absorber in a polymer film can be calculated from the absorbancy of the UV absorber and the thickness of the film. Such calculations show that the effect of UV absorbers is small in thin films and in yarns.
659 TABLE 22.1 Photodegradation of polymers Polymer
Mechanical properties
Poly(methyl methacrylate) Polyacrylonitrile Cotton
Discoloration (yellowing) 0 0
+
Rayon Polyoxymethylene Polyethylene Poly(vinyl chloride) Qiana® Terlenka Nylon 6 Polystyrene Polypropylene Polycarbonate Wool Polyurethanes Polysulphone Poly(2,6-dimethylphenylene oxide) Poly(2,6-diphenylphenylene oxide) Meaning of symbols: + = improvement 0 = no change
-
= slight = moderate f deterioration = strong = very strong
b.
Antioxidants The degradation of polymers is mostly promoted by autoxidation. The propagation of autoxidation can be inhibited by antioxidants (e.g. hindered phenols and amines). c. Quenchers A quencher induces harmless dissipation of the energy of photoexcited states. The only quenchers applied in the polymer field are nickel compounds in the case of polyolefins. A worthwhile survey of this field was given by De Jonge and Hope (1980). Hydrolytic degradation Hydrolytic degradation plays a part if hydrolysis is the potential key reaction in the breaking of bonds, as in polyesters and polycarbonates. Attack by water may be rapid if the temperature is sufficiently high; attack by acids depends on acid strength and temperature. Degradation under the influence of basic substances depends very much on the penetration of the agent; ammonia and amines may cause much grater degradation than substances like caustic soda, which mainly attack the surface. The amorphous regions are attacked first and the most rapidly; but crystalline regions are not free from attack. Stress relaxation as a measure of chemical degradation Stress relaxation occurs when a molecular chain carrying a load breaks. This occurs, e.g., during the oxidation of rubbers. When a stretched chain segment breaks, it returns to
660 a relaxed state. Only stretched chains carry the load, and a load on a broken chain in a network cannot be shifted to other chains. It may be assumed that the rate at which stretched network chains are broken is proportional to the total number of chains (n) carrying the load: (22.1)
~^=kn From the theory of rubber elasticity one can then derive in a simple way that:
(22.2)
Z = «Ü. = e -
This expression is of the same shape as that of stress relaxation of viscoelastic materials (Chapter 13). By analogy Ilk is called the "relaxation time" (Θ). Since chemical reactions normally satisfy an Arrhenius type of equation in their temperature dependence, the variation of relaxation time with temperature may be expressed as follows: 1ηβ = 1 η ^ = 1 η Α + ^
(22.3)
where Eact is the activation energy of the chemical reaction. A typical value of Eact is 125kJ/mol for the oxidative degradation of rubbers.
BIBLIOGRAPHY, CHAPTER 22
General references Conley, R.T. (Ed.), "Thermal Stability of Polymers", M. Dekker, New York, 1970. Grassie, N., "Chemistry of High Polymer Degradation Processes", Interscience, New York, 1956. Grassie, N. and Scott, G., "Polymer Degradation and Stabilisation", Cambridge University Press, Cambridge, 1985. Guillet, J. "Polymer Photophysics and Photochemistry" Cambridge University Press, Cambridge, 1985. Jellinek, H.H.T., "Degradation of Vinyl Polymers", Academic Press, New York, 1955. Neimann, M.B. (Ed.), "Aging and Stabilization of Polymers", Consultants Bureau, New York, 1965. Reich, L. and Stivala, S.S., "Autoxidation of Hydrocarbons and Polymers", M. Dekker, New York, 1969. Reich, L. and Stivala, S.S., "Elements of Polymer Degradation", McGraw-Hill, New York, 1971. Scott, G., "Atmospheric Oxidation and Antioxidants", Elsevier, Amsterdam, 1965. Special references Bergen, R.L., S.P.E. Journal 20 (1964) 630. Bolland, J.L., Proc. Roy. Soc. (London) A 186 (1946) 218. Bolland, J.L. et al., Trans. Faraday Soc. 42 (1946) 236, 244; 43 (1947) 201; 44 (1948) 669; 45 (1949) 93; 46 (1950) 358. De Jonge, I. and Hope, P., pp 21-54 in "Developments in Polymer Stabilization" - 3 , Ed. G. Scott, Applied Science Publishers (Elsevier), London, 1980. Farmer, E.H. and Sundralingham, A., J. Chem. Soc. (1943) 125.
Introduction A polymer may be degraded by chemical changes due to reaction with components in the environment. The most important of these degrading reagents is oxygen. Oxidation may be induced and accelerated by radiation (photooxidation) or by thermal energy (thermal oxidation). Besides the oxidative degradation, also other forms of chemical degradation play a part, the most important of which is the hydrolytic degradation. Degradation under the influence of light Of the electromagnetic energy emitted by the sun only a small portion reaches the earth's surface, namely, rays with a wavelength above 290 nm. X rays are absorbed in the outermost part of the atmosphere and UV rays with wavelengths up to 290 nm in the ozone atmosphere. Although the total intensity is subject to wide variations according to geographical and atmospheric conditions, the overall composition of sunlight is practically constant. In photochemical degradation the energy of activation is supplied by sunlight. Most ordinary chemical reactions involve energies of activation between 60 and 270 kJ/mol. This is energetically equivalent to radiation of wavelengths between 1900 and 440 nm. The energies required to break single covalent bonds range, with few exceptions, from 165 to 420kJ/mol, which corresponds to radiation of wavelengths from 710 to 290 nm (see fig. 22.1). This means that the radiation in the near ultraviolet region (300-400 nm) is sufficiently energetic to break most single covalent bonds, except strong bonds such as C—H and O—H. Only the part of the radiation which is actually absorbed by the material can become chemically active. Most pure, organic synthetic polymers (polyethylene, polypropylene, poly(vinyl chloride), polystyrene, etc.) do not absorb at wavelengths longer than 300 nm owing to their ideal structure, and hence should not be affected by sunlight. However, these polymers often do degrade when subjected to sunlight and this has been attributed to the presence of small amounts of impurities or structural defects, which absorb light and 655
656
Fig. 22.1. Energy equivalence of light waves. initiate the degradation. Much of the absorbed light energy is usually dissipated by either radiationless processes (rotations and vibrations) or by secondary emission (fluorescence). Although the exact nature of the impurities or structural defects responsible for the photosensitivity is not known with certainty, it is generally accepted that these impurities are various types of carbonyl groups (ketones, aldehydes) and also peroxides. The primary chain rupture or radical formation in the various photochemical processes is often followed by embrittlement due to cross-linking, but secondary reactions, especially in the presence of oxygen, cause further degradation of the polymer. Mechanical properties, such as tensile strength, elongation and impact strength, may deteriorate drastically. Coloured degradation products are often developed. Surface crazing can also be a sign of UV-induced degradation. Some polymers show discoloration as well as reduction of the mechanical properties (e.g. aromatic polyesters, aromatic polyamides, polycarbonate, polyurethanes, poly(phenylene oxide, polysulphone), others show only a deterioration of the mechanical properties (polypropylene, cotton) or mainly yellowing (wool, poly(vinyl chloride)). This degradation may be less pronounced when an ultraviolet absorber is incorporated into the polymer. The role of the UV-absorbers (usually o-hydroxybenzophenones or o-hydroxyphenylbenzotriazoles) is to absorb the radiation in the 300-400 nm region and dissipate the energy in a manner harmless to the material to be protected. A current development in the UV-protection of polymers is the use of additives (e.g. nickel chelates) which, by a transfer of excitation energy, are capable of quenching electronically excited states of impurities (e.g. carbonyl groups) present in the polymer (e.g. polypropylene).
657 Oxidative degradation At normal temperature polymers generally react so slowly with oxygen that the oxidation only becomes apparent after a long time. For instance, if polystyrene is stored in air in the dark for a few years, the UV spectrum does not change perceptibly. On the other hand, if the same polymer is irradiated by UV light under similar conditions for 12 days, there appear strong bands in the spectrum. The same applies to other polymers such as polyethylene and natural rubber. Therefore, in essence the problem is not the oxidizability as such, but the synergistic action of factors like electromagnetic radiation and thermal energy on the oxidation. By the action of these factors on the polymer free radicals are formed, which together with oxygen initiate a chain reaction. Hence most oxidation reactions are of an autocatalytic nature. If the oxidation is induced by light, the phenomenon is called photooxidation. If the oxidation is induced by purely thermal factors, the term thermal oxidation is used. Photooxidation The most thoroughly investigated oxidative degradation is that of natural rubber. In 1943 Farmer and Sundralingham found that in the photochemical oxidation of this polymer a hydroperoxide is formed, the number of double bonds in the chain remaining constant. The oxygen was found to act on an activated methylene group, not on a double bond, as had previously been assumed. Later the mechanism of the rubber oxidation was studied extensively by Bolland and coworkers (1946-1950), who mainly used model substances. In his first publication Bolland proposed the following mechanism for the propagation reaction: R· +02->ROCT
(a)
ROO * + RH-+ ROOH + R '
(b)
where RH is the olefin, R ' a radical obtained by abstraction of hydrogen in the allyl position, the R O O " the peroxy radical obtained by addition of oxygen to this radical. According to Bolland the reaction chains are terminated by the combination of allyl and peroxy radicals, and the length of the main reaction chains is of the order of 50-100. This reaction could be initiated by any type of reaction in which free radicals are formed. The autocatalytic nature of the reaction is due to the decomposition of the hydroperoxides: R O O H ^ R O ' + *OH
(c)
The hydroperoxides also give rise to secondary reactions in which coloured resinous products are formed (via carbonyl compounds). Stabilization to photooxidation can be achieved by the use of suitable UV absorbers in combination (synergistic action) with antioxidants (AH) which are capable of preventing
658 reactions (a) and (b): R ' + A H ^ R H + A'
(d)
ROO e + A H ^ ROOH + A '
(e)
A * —> inactive products.
(f)
Thermal oxidation Especially above room temperature many polymers degrade in an air atmosphere by oxidation which is not light-induced (heat ageing). A number of polymers already show a deterioration of the mechanical properties after heating for some days at about 100°C and even at lower temperatures (e.g. polyethylene, polypropylene, polyformaldehyde and poly(ethylene sulphide)). The rate of oxidation can be determined by measuring volumetrically the oxygen uptake at a certain temperature. Such measurements have shown that the oxidation at 140°C of low-density polyethylene increases exponentially after an induction period of two hours. It can be concluded from this result that the thermal oxidation, like photoxidation, is caused by autoxidation, the difference merely being that the radical formation from the hydroperoxide is now activated by heat. The primary reaction can be a direct reaction with oxygen RH + 0 2 - + R ' + Ό Ο Η .
(g)
The thermal oxidation can be inhibited by antioxidants as before (eqs. (d), (e), (f)). Effects of oxidation degradation The principal effects of oxidative degradation of polymers are the decay of good mechanical properties (strength, elongation, resilience, etc.) and discoloration (mainly yellowing). The behaviour of polymers may vary widely. A polymer may be resistant to mechanical decay but not to colour decay, or the reverse. Often the two go together. Table 22.1 gives a survey of these effects for the different polymer families in the case of photodegradation. Stabilization The oxidative degradation of a polymer can be retarded or even practically prevented by addition of stabilizers. The following types of stabilizers may be used: a. UV absorbers A good UV absorber absorbs much UV light but no visible light. It should dissipate the absorbed energy in a harmless manner by transforming the energy into heat. Other requirements are: compatibility with the polymer, nonvolatility, light fastness, heat stability and, for textiles, also resistance to washing and dry cleaning. The optimum effect of a UV absorber in a polymer film can be calculated from the absorbancy of the UV absorber and the thickness of the film. Such calculations show that the effect of UV absorbers is small in thin films and in yarns.
659 TABLE 22.1 Photodegradation of polymers Polymer
Mechanical properties
Poly(methyl methacrylate) Polyacrylonitrile Cotton
Discoloration (yellowing) 0 0
+
Rayon Polyoxymethylene Polyethylene Poly(vinyl chloride) Qiana® Terlenka Nylon 6 Polystyrene Polypropylene Polycarbonate Wool Polyurethanes Polysulphone Poly(2,6-dimethylphenylene oxide) Poly(2,6-diphenylphenylene oxide) Meaning of symbols: + = improvement 0 = no change
-
= slight = moderate f deterioration = strong = very strong
b.
Antioxidants The degradation of polymers is mostly promoted by autoxidation. The propagation of autoxidation can be inhibited by antioxidants (e.g. hindered phenols and amines). c. Quenchers A quencher induces harmless dissipation of the energy of photoexcited states. The only quenchers applied in the polymer field are nickel compounds in the case of polyolefins. A worthwhile survey of this field was given by De Jonge and Hope (1980). Hydrolytic degradation Hydrolytic degradation plays a part if hydrolysis is the potential key reaction in the breaking of bonds, as in polyesters and polycarbonates. Attack by water may be rapid if the temperature is sufficiently high; attack by acids depends on acid strength and temperature. Degradation under the influence of basic substances depends very much on the penetration of the agent; ammonia and amines may cause much grater degradation than substances like caustic soda, which mainly attack the surface. The amorphous regions are attacked first and the most rapidly; but crystalline regions are not free from attack. Stress relaxation as a measure of chemical degradation Stress relaxation occurs when a molecular chain carrying a load breaks. This occurs, e.g., during the oxidation of rubbers. When a stretched chain segment breaks, it returns to
660 a relaxed state. Only stretched chains carry the load, and a load on a broken chain in a network cannot be shifted to other chains. It may be assumed that the rate at which stretched network chains are broken is proportional to the total number of chains (n) carrying the load: (22.1)
~^=kn From the theory of rubber elasticity one can then derive in a simple way that:
(22.2)
Z = «Ü. = e -
This expression is of the same shape as that of stress relaxation of viscoelastic materials (Chapter 13). By analogy Ilk is called the "relaxation time" (Θ). Since chemical reactions normally satisfy an Arrhenius type of equation in their temperature dependence, the variation of relaxation time with temperature may be expressed as follows: 1ηβ = 1 η ^ = 1 η Α + ^
(22.3)
where Eact is the activation energy of the chemical reaction. A typical value of Eact is 125kJ/mol for the oxidative degradation of rubbers.
BIBLIOGRAPHY, CHAPTER 22
General references Conley, R.T. (Ed.), "Thermal Stability of Polymers", M. Dekker, New York, 1970. Grassie, N., "Chemistry of High Polymer Degradation Processes", Interscience, New York, 1956. Grassie, N. and Scott, G., "Polymer Degradation and Stabilisation", Cambridge University Press, Cambridge, 1985. Guillet, J. "Polymer Photophysics and Photochemistry" Cambridge University Press, Cambridge, 1985. Jellinek, H.H.T., "Degradation of Vinyl Polymers", Academic Press, New York, 1955. Neimann, M.B. (Ed.), "Aging and Stabilization of Polymers", Consultants Bureau, New York, 1965. Reich, L. and Stivala, S.S., "Autoxidation of Hydrocarbons and Polymers", M. Dekker, New York, 1969. Reich, L. and Stivala, S.S., "Elements of Polymer Degradation", McGraw-Hill, New York, 1971. Scott, G., "Atmospheric Oxidation and Antioxidants", Elsevier, Amsterdam, 1965. Special references Bergen, R.L., S.P.E. Journal 20 (1964) 630. Bolland, J.L., Proc. Roy. Soc. (London) A 186 (1946) 218. Bolland, J.L. et al., Trans. Faraday Soc. 42 (1946) 236, 244; 43 (1947) 201; 44 (1948) 669; 45 (1949) 93; 46 (1950) 358. De Jonge, I. and Hope, P., pp 21-54 in "Developments in Polymer Stabilization" - 3 , Ed. G. Scott, Applied Science Publishers (Elsevier), London, 1980. Farmer, E.H. and Sundralingham, A., J. Chem. Soc. (1943) 125.