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The thermal degradation of polypropylene promoted by organic halogen compounds
The ThermalDegradationof Polypropylene Promoted by OrganicHalogen Compounds A. R. CAVERHILL and G. W. TAYLOR The thermal degradation of polypropylene is promoted by certain halogenated compounds at temperatures above 220°C. In the chlorinated methanes, the activity increases with degree of chlorination. It is suggested that these compounds take part in a chain transfer process, involving attack of radicals derived from the polypropylene on the halogenated compound, followed by further reactions of the radicals produced from the halogenated compound, the fate of the~e radicals being dependent on the temperature.
IN A recent plant operation, it was found that traces of perchloroethylene markedly reduced extrusion pressures in the spinning of isotactic poly, propylene filaments. The present paper describes laboratory experiments designed to explain this observation. Further semi-technical work showed that the chloro- and bromomethanes, and a variety of other alkyl halides, reduced the extrusion pressure. By contrast, chloro- and bromo-benzene were inactive, though benzyl chloride was very active. Accordingly, the chloromethanes were selected for laboratory study. The polymer was 'Propathene' powder, of intrinsic viscosity 3-0dl/g. Samples of polymer (5 g) were heated in evacuated sealed tubes in the presence of the halogenated compounds. In general, the dose of halogenated compound was measured in a gas burette. The extent of degradation was measured by determination of melt flow index, and these values were converted into intrinsic viscosities. E V A L U A T I O N OF THE C H L O R O M E T H A N E S These were used at a concentration of 10 -3 mole per mole of polymer repeat unit; this is about 0-3 wt per cent for carbon tetrachloride. The samples were heated for 20 min in a vapour bath at 282°C. The compounds used, and final polymer viscosities, are shown in Table 1. Table 1. Effectof chloromethanes on degradation of polypropylene Compound
I.V. (dl / g)
None Methylene chloride Chloroform
1"9 1"8
0.4
Compound
Carbon tetrachloride Dichlorodifluorometh~ine
I.V. (dl / g)
0.4 2"0
The data of Table 1 showed that a high degree of chlorination was necessary for promotion of degradation. Other work had suggested that bromo compounds were more effective than chloro compounds; the result for the dichlorodifluoromethane suggests that, in turn, chlorine compounds are more effective than fluoro compounds. 193
A. R. CAVERHILL and G. W. TAYLOR By heating at a lower temperature (259°C), it was found that carbon tetrachloride was more effective than chloroform. EFFECT
OF C O N C E N T R A T I O N OF C A R B O N TETRACHLORIDE Again, the samples were heated for 20 min at 282°C. The additive concentrations and polymer viscosities are shown in Table 2. Table 2. Effect of concentration of carbon tetrachloride on degradation of
polypropylene
Ca14 c o n c n
CC14 conch (mole~polymer repeat unit) × 10~
(dl/g)
None 0-8 1-0
1"9 1.0 0'8
I.V.
(mote~polymer repeat unit)
I.V.
(dl/g)
×10a 1-8 3"5 10
,
0"7 0.4 Too low to measure
These results show that the extent of polymer degradation in a given time depends on the concentration of carbon tetrachloride. E F F E C T OF T I M E In this experiment, the temperature was 244°C, and the carbon tetrachloride concentration 4 x 10 -4 mole per mole of polymer repeat unit. The data, given in Table 3, show that, during the early part of the heating period, there is a sharp drop in intrinsic viscosity. This is followed by a decreased rate of degradation. Table 3. Effectof time on degradant action of carbon tetrachloride Heating time (rain)
0
15
30
45
I.V.
3-0
1"64
1'56
1.3
(el/g)
E F F E C T OF T E M P E R A T U R E Various concentrations of carbon tetrachloride were used over a range of temperatures. The data are shown in Table 4. T h e heating time was 20 rain. Table 4. Effectof temperature on activity of carbon tetrachlofide Additive
Additive Temp. (°C)
197 197 197 223 223
COrtcn
(mole/polymer repeat unit) × 104 0 8
220 0 8
I.V.
(dl/g)
2"93 2"74 2"67 2-92 1 "98 194
conch
Temp. (°~
(mole/polymer repeat unit) X 104
244 244 259 259
0 8
0 4
I.V.
(dl/g)
2.82 1.13 2-87 0-48
THE THERMAL DEGRADATION OF POLYPROPYLENE The above results show that (a) carbon tetrachloride is active as low as 223°C, but essentially inactive at 197°C, and (b) the activity markedly increases as the temperature is raised. FATE OF c A R B o N T E T R A C H L O R I D E In this experiment, the volatile reaction products were analysed. Polypropylene (5 g) and carbon tetrachloride (0' 1 g) were heated in a break-seal tube at 282°C for 30 min. The volatile products, analysed by gas chromatography and mass spectrometry, consisted mainly of equal parts of chloroform and methylene chloride, with traces of carbon tetrachloride and possibly methyl chloride. In addition, water, carbon dioxide and traces of acetic acid were detected. In another experiment, it was shown that carbon tetrachloride alone was stable under these conditions. DISCUSSION It has been found that a variety of halogenated hydrocarbons are active in reducing the molecular weight of polypropylene heated in the absence of air in the temperature range 240 ° to 280°C. In the chlorinated methanes, the activity ranking is as follows : CC14 > CHCI~ > CH2CI~ ( > CF2C12) This is also the ranking found when these compounds are used as chain transfer agents in vinyl polymerization reactions1. The activity does not, however, originate in the thermal decomposition of the chlorinated compound; carbon tetrachloride has been shown to be stable in the temperature region under consideration. In the presence of polypropylene carbon tetrachloride is successfully dechlorinated to chloroform, methylene chloride, and probably methyl chloride. These products are most easily explained in terms of radical attack on the tetrachloride, the radical originating from the thermal decomposition of a 'weak link' in the polymer chain: Polymer ~ Ri Ri + CC14 > R~CI+ CC13 In this context, a 'weak link' is some intrusive element in a polymer structure which is generally considered to be the source of initiation of thermal degradation~. At temperatures in the region of 280°C, the CC13 radical undergoes further reactions as follows CC13 + RH > CHCI~ + R etc. (polymer) At lower temperatures, the rate of this reaction falls, and presumably the chlorinated radicals undergo other reactions, for example recombination. Even in the absence of these further reactions of CCI~, the above mechanism leads to a fall in molecular weight of the polymer. In the absence of chlorinated compound, the radicals derived from the scission of 'weak links' usually recombine Polymer, " nRt 195
A. R. CAVERHILL and G. W. TAYLOR The effect of the halogenated compound is to upset this equilibrium, stabilizing Rt by converting it to RiC1. This is a form of chain transfer. At very low temperatures (e.g. 200°C) the low rate of chlorine abstraction by R , coupled with the high melt viscosity favouring geminate recombination of Ri (these are probably formed in pairs), results in the halogenated compound having no effect on the change in molecular weight of the polymer. At a given temperature, the molecular weight initially falls rapidly. This is followed by a much slower rate of fall. This behaviour is to be expected if the driving force of the process, that is, the generation of radicals from the scission of 'weak links' in the polymer, is rapidly exhausted.
I.C.I. Fibres Ltd, Research Department, Hookstone Road, Harrogate
(Received August 1964)
REFERENCES 1 BAMFORD,C. H., BARB,W. G., JENKINS,A. D. and ONYON,P. F. Kinetics of Vinyl Polymerization by Radical Mechanisms, p 239. Butterworths: London, 1958 2 GRASSm,N. Chemistry of High Polymer Degradation Processes, p 73. Butterworths : London, 1956
196
IN A recent plant operation, it was found that traces of perchloroethylene markedly reduced extrusion pressures in the spinning of isotactic poly, propylene filaments. The present paper describes laboratory experiments designed to explain this observation. Further semi-technical work showed that the chloro- and bromomethanes, and a variety of other alkyl halides, reduced the extrusion pressure. By contrast, chloro- and bromo-benzene were inactive, though benzyl chloride was very active. Accordingly, the chloromethanes were selected for laboratory study. The polymer was 'Propathene' powder, of intrinsic viscosity 3-0dl/g. Samples of polymer (5 g) were heated in evacuated sealed tubes in the presence of the halogenated compounds. In general, the dose of halogenated compound was measured in a gas burette. The extent of degradation was measured by determination of melt flow index, and these values were converted into intrinsic viscosities. E V A L U A T I O N OF THE C H L O R O M E T H A N E S These were used at a concentration of 10 -3 mole per mole of polymer repeat unit; this is about 0-3 wt per cent for carbon tetrachloride. The samples were heated for 20 min in a vapour bath at 282°C. The compounds used, and final polymer viscosities, are shown in Table 1. Table 1. Effectof chloromethanes on degradation of polypropylene Compound
I.V. (dl / g)
None Methylene chloride Chloroform
1"9 1"8
0.4
Compound
Carbon tetrachloride Dichlorodifluorometh~ine
I.V. (dl / g)
0.4 2"0
The data of Table 1 showed that a high degree of chlorination was necessary for promotion of degradation. Other work had suggested that bromo compounds were more effective than chloro compounds; the result for the dichlorodifluoromethane suggests that, in turn, chlorine compounds are more effective than fluoro compounds. 193
A. R. CAVERHILL and G. W. TAYLOR By heating at a lower temperature (259°C), it was found that carbon tetrachloride was more effective than chloroform. EFFECT
OF C O N C E N T R A T I O N OF C A R B O N TETRACHLORIDE Again, the samples were heated for 20 min at 282°C. The additive concentrations and polymer viscosities are shown in Table 2. Table 2. Effect of concentration of carbon tetrachloride on degradation of
polypropylene
Ca14 c o n c n
CC14 conch (mole~polymer repeat unit) × 10~
(dl/g)
None 0-8 1-0
1"9 1.0 0'8
I.V.
(mote~polymer repeat unit)
I.V.
(dl/g)
×10a 1-8 3"5 10
,
0"7 0.4 Too low to measure
These results show that the extent of polymer degradation in a given time depends on the concentration of carbon tetrachloride. E F F E C T OF T I M E In this experiment, the temperature was 244°C, and the carbon tetrachloride concentration 4 x 10 -4 mole per mole of polymer repeat unit. The data, given in Table 3, show that, during the early part of the heating period, there is a sharp drop in intrinsic viscosity. This is followed by a decreased rate of degradation. Table 3. Effectof time on degradant action of carbon tetrachloride Heating time (rain)
0
15
30
45
I.V.
3-0
1"64
1'56
1.3
(el/g)
E F F E C T OF T E M P E R A T U R E Various concentrations of carbon tetrachloride were used over a range of temperatures. The data are shown in Table 4. T h e heating time was 20 rain. Table 4. Effectof temperature on activity of carbon tetrachlofide Additive
Additive Temp. (°C)
197 197 197 223 223
COrtcn
(mole/polymer repeat unit) × 104 0 8
220 0 8
I.V.
(dl/g)
2"93 2"74 2"67 2-92 1 "98 194
conch
Temp. (°~
(mole/polymer repeat unit) X 104
244 244 259 259
0 8
0 4
I.V.
(dl/g)
2.82 1.13 2-87 0-48
THE THERMAL DEGRADATION OF POLYPROPYLENE The above results show that (a) carbon tetrachloride is active as low as 223°C, but essentially inactive at 197°C, and (b) the activity markedly increases as the temperature is raised. FATE OF c A R B o N T E T R A C H L O R I D E In this experiment, the volatile reaction products were analysed. Polypropylene (5 g) and carbon tetrachloride (0' 1 g) were heated in a break-seal tube at 282°C for 30 min. The volatile products, analysed by gas chromatography and mass spectrometry, consisted mainly of equal parts of chloroform and methylene chloride, with traces of carbon tetrachloride and possibly methyl chloride. In addition, water, carbon dioxide and traces of acetic acid were detected. In another experiment, it was shown that carbon tetrachloride alone was stable under these conditions. DISCUSSION It has been found that a variety of halogenated hydrocarbons are active in reducing the molecular weight of polypropylene heated in the absence of air in the temperature range 240 ° to 280°C. In the chlorinated methanes, the activity ranking is as follows : CC14 > CHCI~ > CH2CI~ ( > CF2C12) This is also the ranking found when these compounds are used as chain transfer agents in vinyl polymerization reactions1. The activity does not, however, originate in the thermal decomposition of the chlorinated compound; carbon tetrachloride has been shown to be stable in the temperature region under consideration. In the presence of polypropylene carbon tetrachloride is successfully dechlorinated to chloroform, methylene chloride, and probably methyl chloride. These products are most easily explained in terms of radical attack on the tetrachloride, the radical originating from the thermal decomposition of a 'weak link' in the polymer chain: Polymer ~ Ri Ri + CC14 > R~CI+ CC13 In this context, a 'weak link' is some intrusive element in a polymer structure which is generally considered to be the source of initiation of thermal degradation~. At temperatures in the region of 280°C, the CC13 radical undergoes further reactions as follows CC13 + RH > CHCI~ + R etc. (polymer) At lower temperatures, the rate of this reaction falls, and presumably the chlorinated radicals undergo other reactions, for example recombination. Even in the absence of these further reactions of CCI~, the above mechanism leads to a fall in molecular weight of the polymer. In the absence of chlorinated compound, the radicals derived from the scission of 'weak links' usually recombine Polymer, " nRt 195
A. R. CAVERHILL and G. W. TAYLOR The effect of the halogenated compound is to upset this equilibrium, stabilizing Rt by converting it to RiC1. This is a form of chain transfer. At very low temperatures (e.g. 200°C) the low rate of chlorine abstraction by R , coupled with the high melt viscosity favouring geminate recombination of Ri (these are probably formed in pairs), results in the halogenated compound having no effect on the change in molecular weight of the polymer. At a given temperature, the molecular weight initially falls rapidly. This is followed by a much slower rate of fall. This behaviour is to be expected if the driving force of the process, that is, the generation of radicals from the scission of 'weak links' in the polymer, is rapidly exhausted.
I.C.I. Fibres Ltd, Research Department, Hookstone Road, Harrogate
(Received August 1964)
REFERENCES 1 BAMFORD,C. H., BARB,W. G., JENKINS,A. D. and ONYON,P. F. Kinetics of Vinyl Polymerization by Radical Mechanisms, p 239. Butterworths: London, 1958 2 GRASSm,N. Chemistry of High Polymer Degradation Processes, p 73. Butterworths : London, 1956
196