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Photo-oxidation of dope-dyed polypropylene fibres
Polymer Degradation and Stability 13 (1985) 201-210
Photo-oxidation of Dope-dyed Polypropylene Fibres O. 15ur6ov/t, T. Rzyman, Z. Samuhelov/t & B. Benovi6 Research Institute of Man-Made Fibres, 059 21 Svit, okr. Poprad, Czechoslovakia (Received: 23 April, 1985)
ABSTRACT The efficiencies of several light stabilizers have been evaluated in commercially manufactured polypropylene fibres. For this purpose the rate of photo-oxidation has been followed using the measurement of infra-red specular reflection absorption spectra and mechanical properties, as well as structural transformation induced by photo-oxidation. It has been found that benzophenone light stabilizers have a very low efficiency in the applied combination of antioxidant and pigment in polypropylene fibres. Significantly better effects were observed in the case of sterically hindered piperidine compounds. Correlations between the rate of photo-oxidation of fibres and the decrease in their tenacity were studied. It was found that the loss of tenacity in the oxidized fibres does not depend only on the quantity of non-volatile degradation products formed, but also on the character of morphological photo-oxidation transformations.
INTRODUCTION
Different types of uv stabilizer are employed to improve the light stability of polypropylene (PP) fibres. 1,2 After uv absorbers and nickel quenchers the trend has been towards hindered amine compounds (HALS). They were found to be highly efficient for improving the light stability of polyolefin-based products with a high surface/mass ratio, and there have 201
Polymer Degradation and Stability 0141-3910/85/$03-30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
202
O. Durgov6, T. Rzyrnan, Z. Samuhelov&, B. Benovi~
been many intensive studies of the mechanism of the photo-stabilizing activity of HALS. 3- 6 The first information on the stabilizing activity of HALS in PP fibres was presented ten years ago. 7 However, most information has been obtained from the study of the photo-oxidation of polymer films, s'9 Gugumus recently demonstrated significant differences between the photo-oxidation of PP films and fibres. 1° While photo-oxidation of surface layers does not affect the tenacity of fibres, it reduces this property considerably in the case of films. The application of HALS compounds also improves the light stability of textured dope-dyed PP fibres. This fact has stimulated our more intensive study of their photo-oxidation, with special attention to the factors which are important in affecting the light stability. In this paper we report upon the kinetics of photo-oxidation of flat and textured PP fibres studied by ir specular reflection absorption spectra (ir SRA). For PP fibres containing different light stabilizers correlations between the kinetics of photo-oxidation and loss of tenacity were investigated. At the same time, morphological structural transformations caused by photo-oxidation were evaluated. The results obtained have confirmed the importance of tenacity analysis for the evaluation of the activity of light stabilizers in PP fibres as distinct from PP films. In the latter case, chemical changes, particularly the induction period, are preferably utilized for degradation studies.
EXPERIMENTAL Virgin polypropylene powder (Tatren HPF, Slovnaft, Bratislava) was used for the preparation of samples after dry mixing with antioxidants and light stabilizers followed by granulation at 230°C. The following compounds were used as antioxidants: 2,6-di(tertbutyl-4-methylphenol) (AO-4K, CHZJD Bratislava), tris(3,5-di(tertbutyl-4-hydroxybenzyl) isocyanurate) (B. F. Goodrich Chemical Co.) and distearyl-thiodipropionate (Irganox PS 802, Ciba-Geigy). The light stabilizers were: 2hydroxy-4-n-octyloxybenzophenone(Cyassorb U V 531, Cyanamid Co.), di(1,2,6,6-pentamethyl-4-piperidinyl)-butyl(3,5-ditertbutyl-4-hydroxybenzyl)malonate (Tinuvin 144, Ciba-Geigy), polyester of succinic acid with N-fl-hydroxyethyl-2,2,6,6-tetramethyl-4-hydroxypiperidine(Tinuvin 622, Ciba-Geigy) and poly-2-N,N-di(2,2,6,6-tetramethyl-4-piperidinyl)
Photo-oxidation of dope-dyed polypropylene fibres
203
hexane-diamino-4- l-amino- 1,1,3,3-tetramethyl symtriazine (Chimassorb 944, Ciba-Geigy). The concentration of light stabilizers was 0.3 ~o by weight. The uv absorber UV 531 was used in combination with antioxidant AO-4K and Irganox PS-802. HALS compounds were used as mixtures with antioxidant AO-4K and Goodrite 3114. The additives were dry blended with PP powder and the resulting blends extruded and pelletized. Multifilament yarns were prepared from these pellets under laboratory spinning conditions. A Masterbatch of Cromophtal Rot BR (Ciba-Geigy) was used after blending with PP pellets and the resulting mixture fed directly to the extruder hopper. The spun fibres were drawn at 125 °C to a draw ratio of 1 : 4 and simultaneously textured. The textured fibres were set in a hot-air dryer at 130 °C for 15 rain. The average filament fineness was 6 dtex.
RESULTS A N D DISCUSSION A contactless non-destructive ir SRA technique was used for the determination of carbonyi index and photo-oxidation rate. In our previous papers we have discussed the advantages of this method for the evaluation of PP fibres and foils. ~1'12 Figure 1 shows typical spectral changes observed in the ir SRA spectra of PP fibres during photooxidation. As the irradiation time is increased the broad absorption band in the carbonyl region, with a maximum at 1715 c m - 1, increases. The presence of the red pigment is confirmed by the band at 1665 cm - 1, which does not interfere with carbonyl absorption. The photo-protective activity of the combination of antioxidant with benzophenone type of stabilizer for red pigmented PP fibres is illustrated in Fig. 2. The photo-oxidation curve was observed to have a typical autocatalytic shape, without any induction period. Non-volatile degradation products are formed rather quickly. A very short time to 50 ~ retained strength after exposure in a Xenotest corresponds to a high photo-oxidation rate (tso = 400 and 300h). Figure 2 shows that the corresponding values of carbonyl indices were 1.2 for flat and 1-3 for textured fibres. Qualitatively different photo-oxidation curves were obtained for red PP fibres containing piperidine compounds (Figs 3, 4 and 5). The most
204
O. bur?ov6, T. Rzyman, Z. Samuhelova, B. Benovi6
1715
/" 3w, m 3w D
HRS
q.a
rub tad
HRS
~ . ~
J
I
I
HRS
I
1700 1900 WAVENUMBER , cm"1 Fig. I.
Infra-red SRA spectra of photo-oxidized PP fibres.
noticeable effect is different photo-oxidation rates at different stages of irradiation as well as general retardation. The results show greater piperidine stabilizing efficiency in the case of red pigmented PP fibres. It is of interest that in the initial stages of irradiation an increase in the carbonyl degradation products was observed in all PP fibres stabilized by piperidine compounds. After about 200h of irradiation, however, retardation takes place, which is most obvious in the case of the Chimassorb 944-containing fibres. There is good cause to believe that the photo-activity of stabilizers begins at this stage due to a generally accepted mechanism, namely the regeneration of nitroxyl radicals. 3 - 5 It
Photo-oxidation of dope-dyed polypropylene fibres
/
205
/
'/ /
2,0
/
/ /
/
/
x
_= 1,0 /¢
7. z o I
ir
,/
r-"50
i 200
i
IRRADIATION
Fig. 2.
I 600 TIME~
HRS
Photo-oxidation rate of PP fibres containing Cyasorb UV 531 light stabilizer: O, fiat fibres; O, textured fibres.
1,5 a
1,0 Z 0
.-" O,S 41
I 200
1
IRRADIATION
Fig. 3.
I 600
I
TIME I
HRS
Photo-oxidation rate of P P fibres containing Tinuvin 144 light stabilizer: O, fiat fibres; O, textured fibres.
O. Dur(ova, T. Rzyman, Z. Samuhelov&, B. Benovi?
206
/ 9/ _=
1,0
z o
3 0,s I
I
200 IRRADIATION
Fig. 4.
I 600 TIME
I ,
I
1000
HRS
Photo-oxidation rate of polypropylene fibres containing Tinuvin 622 light stabilizer: 0 , flat fibres; ©, textured fibres.
is understandable that a longer period of observed 'balance' between oxidation and stabilization, characterized by retarded carbonyl formation, is to be associated with a higher efficiency of the stabilizer/antioxidant combination used (Figs 3, 4 and 5). It follows that the highest stabilizing efficiency in the red pigmented PP fibres is that of Chimassorb 944. Evaluation of the tensile strength loss agrees with this. In the chosen irradiation time (1000h) 50% tensile strength loss was achieved only in
-
1,0
m
o,s~
I 200 IRRADIATION
Fig. 5.
t!°
I
t!s
600 TIME~
I 1000
HRS
Photo-oxidation rate of polypropylene fibres containing Chimassorb 944 light stabilizer: 0 , flat fibres; O, textured fibres.
Photo-oxidation of dope-dyed polypropylene fibres
207
the case of textured fibres. For the flat fibres values of only 45, 40 and 35 % were reached. Figures 3 to 5 show that photo-oxidation of textured and flat fibres follows similar courses. However, for the former the photo-oxidation rate is higher and the exposure times to similar tensile strength loss are shorter. This is connected with increased diffusion of oxygen into the fibres due to accidental damage to the compact mass of fibres in the texturizing process. Comparison of the exposure time to tensile strength retention (t, ~) and corresponding values of carbonyl indices for all the PP fibres measured are shown in Table 1. It can be concluded that there is no direct dependence between fibre tendering, expressed in terms of exposure times to 50 ~o tensile strength loss, and corresponding degree of oxidation. TABLE I Carbonyl Indices (ACI) Corresponding to Exposure Time to n ~o Tensile Strength Loss (t.,~) for PP Fibres in Xenotest
Sample
PP (UV531)
PP ( Tinuvin 144)
PP ( Tinuvin 622)
PP (Chimassorb 944)
Flat
Text.
Flat
Text.
Flat
Text.
Flat
Text.
tso (h)
400
300
520
1-30
890 t35 0"75
700
1"20
890 t45 0'95
450
ACI
890 t40 1'20
0"95
0"55
0"60
A more detailed view of the course of photo-oxidation of PP fibres was obtained by studying changes in morphological structure induced by photo-oxidation. The results obtained made possible an explanation of some of the fluctuations in the relationship between tensile strength and the quantity of carbonyl species in PP fibres containing different light stabilizers (Table 1). A photo-micrograph of the flat fibre containing benzophenone light stabilizer after 580 h of irradiation is shown in Fig. 6. A clear, smooth fibre surface without cracks is clearly illustrated. The inner structure represents a 'fossil' of the original fibrillar morphology. It can be assumed that the quick tensile strength loss of the fibre is due to fast tendering of the whole volume without changes in fibrillar morphology. Different morphological transformations during photo-oxidation were observed
Fig. 6. Photo-micrographof photo-oxidized polypropylene fibre containing Cyasorb UV 531 after 580h irradiation in Xenotest. (Magnification 1:1000.)
Fig. 7. Photo-micrograph of inner morphological structure of photo-oxidized polypropylene fibre containing Chimassorb 944 light stabilizer after 980 h irradiation in Xenotest. (Magnification I : 1600.)
N
oo
to
Photo-oxidation of dope-dyed polypropylene fibres
209
Fig. 8. Photo-micrograph of the surface morphological structure of photo-oxidized polypropylene fibre containing Chimassorb 944 light stabilizer after 1800 h irradiation in Xenotest. (Magnification l "940.)
for piperidine stabilized fibres. Photo-micrographs of the surface and inner structure of fibre containing Chimassorb 944 at different stages of irradiation are shown in Figs 7 and 8. In this case photo-oxidation is manifested by characteristic cracks on the fibre surface. In the inner layers fibrils remain which can be regarded as tensile strength carriers. On prolonged irradiation tendering occurs accompanied by large, regular, transverse cracks spreading within the whole fibre diameter. The original fibrillar fibre morphology transforms to a lamellar one (Fig. 8). At that stage of irradiation spontaneous fibre disintegration takes place. The longer an undamaged inner fibre structure exists and morphological transformations induced by photo-oxidation concentrate on the fibre surface, the more resistant is the fibre against uv ageing. Thus, the effect of different light stabilizers on super molecular and morphological structure of fibres is important as well as the chemical structure of the light stabilizers themselves when considering the light stability of PP fibres, especially dope-dyed fibres.
210
o. bur(ovit, T. Rzyman, Z. SamuhelovLt, B. Benovi(
CONCLUSIONS Data have been presented which make it possible to draw the following conclusions: (1) A stabilizing system containing benzophenone light stabilizer, Cyasorb UV-531, has been found to be of low performance for light protection of Cromophtal Rot BR pigmented PP fibres. (2) The light stability has been improved considerably by hindered piperidine compounds combined with a low molecular phenolic antioxidant. The highest stabilizing efficiency has been observed for a stabilizing system containing Chimassorb 944. (3) There has been no proportional relationship observed between the quantity of carbonyl species and tendering of oriented PP fibres. The tensile strength retention as a measure of the light protecting efficiency of stabilizers is decisively influenced by the character of morphological transformations induced by photo-oxidation.
REFERENCES 1. B. Ranby and J. F. Rabek, Singlet oxygen, John Wiley Int., New York (1979). 2. G. Scott (Ed.), Developments in polymer stabilization, 1-5, Applied Science Publishers Ltd, London (1979). 3. D. Wiles, in: Processing and long-term stabilities oj'hydrocarbon polymers, Prague (1983). 4. D. Carlsson, in: Modified polymers, Bratislava (1984). 5. N. S. Allen and A. Parkinson, Polym. Deg. and Stab., 4, 161 (1982). 6. D. J. Carlsson, K. H. Khan, J. Durmis and D. M. Wiles, J. Polym. Sci., Polym. Chem. Ed., 20, 575 (1982). 7. F. Gugumus, in: 4th European Confi, rence on Plastics and Rubbers, Paris (1974). 8. N. S. Allen, A. Parkinson, F. F. Loffelman and P. V. Susi, Polym. Deg. and Stab., 5, 241 (1983). 9. N. S. Allen, A. Parkinson, J. L. Gardette and J. Lemaire, Polym. Deg. and Stab., 5, 135 (1983). 10. F. Gugumus, in: Polypropylenefibres and textiles, !!i, York, UK (1983). I 1. O. ISur~ov'h and M. Mitterpachova, Apl. a sprac, plasto~,, 5, 9 (1984). 12. (~SP 233465.
Photo-oxidation of Dope-dyed Polypropylene Fibres O. 15ur6ov/t, T. Rzyman, Z. Samuhelov/t & B. Benovi6 Research Institute of Man-Made Fibres, 059 21 Svit, okr. Poprad, Czechoslovakia (Received: 23 April, 1985)
ABSTRACT The efficiencies of several light stabilizers have been evaluated in commercially manufactured polypropylene fibres. For this purpose the rate of photo-oxidation has been followed using the measurement of infra-red specular reflection absorption spectra and mechanical properties, as well as structural transformation induced by photo-oxidation. It has been found that benzophenone light stabilizers have a very low efficiency in the applied combination of antioxidant and pigment in polypropylene fibres. Significantly better effects were observed in the case of sterically hindered piperidine compounds. Correlations between the rate of photo-oxidation of fibres and the decrease in their tenacity were studied. It was found that the loss of tenacity in the oxidized fibres does not depend only on the quantity of non-volatile degradation products formed, but also on the character of morphological photo-oxidation transformations.
INTRODUCTION
Different types of uv stabilizer are employed to improve the light stability of polypropylene (PP) fibres. 1,2 After uv absorbers and nickel quenchers the trend has been towards hindered amine compounds (HALS). They were found to be highly efficient for improving the light stability of polyolefin-based products with a high surface/mass ratio, and there have 201
Polymer Degradation and Stability 0141-3910/85/$03-30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
202
O. Durgov6, T. Rzyrnan, Z. Samuhelov&, B. Benovi~
been many intensive studies of the mechanism of the photo-stabilizing activity of HALS. 3- 6 The first information on the stabilizing activity of HALS in PP fibres was presented ten years ago. 7 However, most information has been obtained from the study of the photo-oxidation of polymer films, s'9 Gugumus recently demonstrated significant differences between the photo-oxidation of PP films and fibres. 1° While photo-oxidation of surface layers does not affect the tenacity of fibres, it reduces this property considerably in the case of films. The application of HALS compounds also improves the light stability of textured dope-dyed PP fibres. This fact has stimulated our more intensive study of their photo-oxidation, with special attention to the factors which are important in affecting the light stability. In this paper we report upon the kinetics of photo-oxidation of flat and textured PP fibres studied by ir specular reflection absorption spectra (ir SRA). For PP fibres containing different light stabilizers correlations between the kinetics of photo-oxidation and loss of tenacity were investigated. At the same time, morphological structural transformations caused by photo-oxidation were evaluated. The results obtained have confirmed the importance of tenacity analysis for the evaluation of the activity of light stabilizers in PP fibres as distinct from PP films. In the latter case, chemical changes, particularly the induction period, are preferably utilized for degradation studies.
EXPERIMENTAL Virgin polypropylene powder (Tatren HPF, Slovnaft, Bratislava) was used for the preparation of samples after dry mixing with antioxidants and light stabilizers followed by granulation at 230°C. The following compounds were used as antioxidants: 2,6-di(tertbutyl-4-methylphenol) (AO-4K, CHZJD Bratislava), tris(3,5-di(tertbutyl-4-hydroxybenzyl) isocyanurate) (B. F. Goodrich Chemical Co.) and distearyl-thiodipropionate (Irganox PS 802, Ciba-Geigy). The light stabilizers were: 2hydroxy-4-n-octyloxybenzophenone(Cyassorb U V 531, Cyanamid Co.), di(1,2,6,6-pentamethyl-4-piperidinyl)-butyl(3,5-ditertbutyl-4-hydroxybenzyl)malonate (Tinuvin 144, Ciba-Geigy), polyester of succinic acid with N-fl-hydroxyethyl-2,2,6,6-tetramethyl-4-hydroxypiperidine(Tinuvin 622, Ciba-Geigy) and poly-2-N,N-di(2,2,6,6-tetramethyl-4-piperidinyl)
Photo-oxidation of dope-dyed polypropylene fibres
203
hexane-diamino-4- l-amino- 1,1,3,3-tetramethyl symtriazine (Chimassorb 944, Ciba-Geigy). The concentration of light stabilizers was 0.3 ~o by weight. The uv absorber UV 531 was used in combination with antioxidant AO-4K and Irganox PS-802. HALS compounds were used as mixtures with antioxidant AO-4K and Goodrite 3114. The additives were dry blended with PP powder and the resulting blends extruded and pelletized. Multifilament yarns were prepared from these pellets under laboratory spinning conditions. A Masterbatch of Cromophtal Rot BR (Ciba-Geigy) was used after blending with PP pellets and the resulting mixture fed directly to the extruder hopper. The spun fibres were drawn at 125 °C to a draw ratio of 1 : 4 and simultaneously textured. The textured fibres were set in a hot-air dryer at 130 °C for 15 rain. The average filament fineness was 6 dtex.
RESULTS A N D DISCUSSION A contactless non-destructive ir SRA technique was used for the determination of carbonyi index and photo-oxidation rate. In our previous papers we have discussed the advantages of this method for the evaluation of PP fibres and foils. ~1'12 Figure 1 shows typical spectral changes observed in the ir SRA spectra of PP fibres during photooxidation. As the irradiation time is increased the broad absorption band in the carbonyl region, with a maximum at 1715 c m - 1, increases. The presence of the red pigment is confirmed by the band at 1665 cm - 1, which does not interfere with carbonyl absorption. The photo-protective activity of the combination of antioxidant with benzophenone type of stabilizer for red pigmented PP fibres is illustrated in Fig. 2. The photo-oxidation curve was observed to have a typical autocatalytic shape, without any induction period. Non-volatile degradation products are formed rather quickly. A very short time to 50 ~ retained strength after exposure in a Xenotest corresponds to a high photo-oxidation rate (tso = 400 and 300h). Figure 2 shows that the corresponding values of carbonyl indices were 1.2 for flat and 1-3 for textured fibres. Qualitatively different photo-oxidation curves were obtained for red PP fibres containing piperidine compounds (Figs 3, 4 and 5). The most
204
O. bur?ov6, T. Rzyman, Z. Samuhelova, B. Benovi6
1715
/" 3w, m 3w D
HRS
q.a
rub tad
HRS
~ . ~
J
I
I
HRS
I
1700 1900 WAVENUMBER , cm"1 Fig. I.
Infra-red SRA spectra of photo-oxidized PP fibres.
noticeable effect is different photo-oxidation rates at different stages of irradiation as well as general retardation. The results show greater piperidine stabilizing efficiency in the case of red pigmented PP fibres. It is of interest that in the initial stages of irradiation an increase in the carbonyl degradation products was observed in all PP fibres stabilized by piperidine compounds. After about 200h of irradiation, however, retardation takes place, which is most obvious in the case of the Chimassorb 944-containing fibres. There is good cause to believe that the photo-activity of stabilizers begins at this stage due to a generally accepted mechanism, namely the regeneration of nitroxyl radicals. 3 - 5 It
Photo-oxidation of dope-dyed polypropylene fibres
/
205
/
'/ /
2,0
/
/ /
/
/
x
_= 1,0 /¢
7. z o I
ir
,/
r-"50
i 200
i
IRRADIATION
Fig. 2.
I 600 TIME~
HRS
Photo-oxidation rate of PP fibres containing Cyasorb UV 531 light stabilizer: O, fiat fibres; O, textured fibres.
1,5 a
1,0 Z 0
.-" O,S 41
I 200
1
IRRADIATION
Fig. 3.
I 600
I
TIME I
HRS
Photo-oxidation rate of P P fibres containing Tinuvin 144 light stabilizer: O, fiat fibres; O, textured fibres.
O. Dur(ova, T. Rzyman, Z. Samuhelov&, B. Benovi?
206
/ 9/ _=
1,0
z o
3 0,s I
I
200 IRRADIATION
Fig. 4.
I 600 TIME
I ,
I
1000
HRS
Photo-oxidation rate of polypropylene fibres containing Tinuvin 622 light stabilizer: 0 , flat fibres; ©, textured fibres.
is understandable that a longer period of observed 'balance' between oxidation and stabilization, characterized by retarded carbonyl formation, is to be associated with a higher efficiency of the stabilizer/antioxidant combination used (Figs 3, 4 and 5). It follows that the highest stabilizing efficiency in the red pigmented PP fibres is that of Chimassorb 944. Evaluation of the tensile strength loss agrees with this. In the chosen irradiation time (1000h) 50% tensile strength loss was achieved only in
-
1,0
m
o,s~
I 200 IRRADIATION
Fig. 5.
t!°
I
t!s
600 TIME~
I 1000
HRS
Photo-oxidation rate of polypropylene fibres containing Chimassorb 944 light stabilizer: 0 , flat fibres; O, textured fibres.
Photo-oxidation of dope-dyed polypropylene fibres
207
the case of textured fibres. For the flat fibres values of only 45, 40 and 35 % were reached. Figures 3 to 5 show that photo-oxidation of textured and flat fibres follows similar courses. However, for the former the photo-oxidation rate is higher and the exposure times to similar tensile strength loss are shorter. This is connected with increased diffusion of oxygen into the fibres due to accidental damage to the compact mass of fibres in the texturizing process. Comparison of the exposure time to tensile strength retention (t, ~) and corresponding values of carbonyl indices for all the PP fibres measured are shown in Table 1. It can be concluded that there is no direct dependence between fibre tendering, expressed in terms of exposure times to 50 ~o tensile strength loss, and corresponding degree of oxidation. TABLE I Carbonyl Indices (ACI) Corresponding to Exposure Time to n ~o Tensile Strength Loss (t.,~) for PP Fibres in Xenotest
Sample
PP (UV531)
PP ( Tinuvin 144)
PP ( Tinuvin 622)
PP (Chimassorb 944)
Flat
Text.
Flat
Text.
Flat
Text.
Flat
Text.
tso (h)
400
300
520
1-30
890 t35 0"75
700
1"20
890 t45 0'95
450
ACI
890 t40 1'20
0"95
0"55
0"60
A more detailed view of the course of photo-oxidation of PP fibres was obtained by studying changes in morphological structure induced by photo-oxidation. The results obtained made possible an explanation of some of the fluctuations in the relationship between tensile strength and the quantity of carbonyl species in PP fibres containing different light stabilizers (Table 1). A photo-micrograph of the flat fibre containing benzophenone light stabilizer after 580 h of irradiation is shown in Fig. 6. A clear, smooth fibre surface without cracks is clearly illustrated. The inner structure represents a 'fossil' of the original fibrillar morphology. It can be assumed that the quick tensile strength loss of the fibre is due to fast tendering of the whole volume without changes in fibrillar morphology. Different morphological transformations during photo-oxidation were observed
Fig. 6. Photo-micrographof photo-oxidized polypropylene fibre containing Cyasorb UV 531 after 580h irradiation in Xenotest. (Magnification 1:1000.)
Fig. 7. Photo-micrograph of inner morphological structure of photo-oxidized polypropylene fibre containing Chimassorb 944 light stabilizer after 980 h irradiation in Xenotest. (Magnification I : 1600.)
N
oo
to
Photo-oxidation of dope-dyed polypropylene fibres
209
Fig. 8. Photo-micrograph of the surface morphological structure of photo-oxidized polypropylene fibre containing Chimassorb 944 light stabilizer after 1800 h irradiation in Xenotest. (Magnification l "940.)
for piperidine stabilized fibres. Photo-micrographs of the surface and inner structure of fibre containing Chimassorb 944 at different stages of irradiation are shown in Figs 7 and 8. In this case photo-oxidation is manifested by characteristic cracks on the fibre surface. In the inner layers fibrils remain which can be regarded as tensile strength carriers. On prolonged irradiation tendering occurs accompanied by large, regular, transverse cracks spreading within the whole fibre diameter. The original fibrillar fibre morphology transforms to a lamellar one (Fig. 8). At that stage of irradiation spontaneous fibre disintegration takes place. The longer an undamaged inner fibre structure exists and morphological transformations induced by photo-oxidation concentrate on the fibre surface, the more resistant is the fibre against uv ageing. Thus, the effect of different light stabilizers on super molecular and morphological structure of fibres is important as well as the chemical structure of the light stabilizers themselves when considering the light stability of PP fibres, especially dope-dyed fibres.
210
o. bur(ovit, T. Rzyman, Z. SamuhelovLt, B. Benovi(
CONCLUSIONS Data have been presented which make it possible to draw the following conclusions: (1) A stabilizing system containing benzophenone light stabilizer, Cyasorb UV-531, has been found to be of low performance for light protection of Cromophtal Rot BR pigmented PP fibres. (2) The light stability has been improved considerably by hindered piperidine compounds combined with a low molecular phenolic antioxidant. The highest stabilizing efficiency has been observed for a stabilizing system containing Chimassorb 944. (3) There has been no proportional relationship observed between the quantity of carbonyl species and tendering of oriented PP fibres. The tensile strength retention as a measure of the light protecting efficiency of stabilizers is decisively influenced by the character of morphological transformations induced by photo-oxidation.
REFERENCES 1. B. Ranby and J. F. Rabek, Singlet oxygen, John Wiley Int., New York (1979). 2. G. Scott (Ed.), Developments in polymer stabilization, 1-5, Applied Science Publishers Ltd, London (1979). 3. D. Wiles, in: Processing and long-term stabilities oj'hydrocarbon polymers, Prague (1983). 4. D. Carlsson, in: Modified polymers, Bratislava (1984). 5. N. S. Allen and A. Parkinson, Polym. Deg. and Stab., 4, 161 (1982). 6. D. J. Carlsson, K. H. Khan, J. Durmis and D. M. Wiles, J. Polym. Sci., Polym. Chem. Ed., 20, 575 (1982). 7. F. Gugumus, in: 4th European Confi, rence on Plastics and Rubbers, Paris (1974). 8. N. S. Allen, A. Parkinson, F. F. Loffelman and P. V. Susi, Polym. Deg. and Stab., 5, 241 (1983). 9. N. S. Allen, A. Parkinson, J. L. Gardette and J. Lemaire, Polym. Deg. and Stab., 5, 135 (1983). 10. F. Gugumus, in: Polypropylenefibres and textiles, !!i, York, UK (1983). I 1. O. ISur~ov'h and M. Mitterpachova, Apl. a sprac, plasto~,, 5, 9 (1984). 12. (~SP 233465.