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Influence of additives on some physical properties of high density polyethylene-II. commercial u.v. stabilizers
Eur. Polym. J. Vol. 23, No. 9, pp. 729-732, 1987 Printed in Great Britain. All rights reserved
0014-3057/87 $3.00+ 0.00 Copyright © 1987 PergamonJournals Ltd
INFLUENCE OF ADDITIVES ON SOME PHYSICAL PROPERTIES OF HIGH DENSITY POLYETHYLENE--II. COMMERCIAL u.v. STABILIZERS ALFONSO J. CHIRINOS PADR•N,* ZIMBUL RUBINZTAINand MARIA A. COLMENARES Laboratorio de Polimeros, Centro de Quimica, IVIC, Apartado 21827, Caracas 1020-A, Venezuela (Received 19 August 1986)
Abstract--The changes in mechanical properties of processed high density polyethylene, when "u.v. stabilizers" are added, are examined using tensile and melt flow measurements. The results show that, at low additive concentrations, antioxidant effects predominate; at high concentrations, a reinforcement effect is observed. Degradation measurements show that the u.v. stabilizers behave as excellent oxidation inhibitors in the dark; this finding has implications regarding their mechanistic action.
in this parameter at higher loadings. The results can be explained as follows. (a) At low concentrations, the "u.v. stabilizers" inhibit the degradation reactions which give rise to a drop in modulus and which, as explained previously [1], cannot be observed through e.g. carbonyl development. The best inhibitor seems to be Methasan, and the weakest Cyasorb UV 531. This is obviously an indication of their ability to act through an antioxidant mechanism in the dark. (b) At higher concentrations (0.5-2.0% w/w), there is a drop in efficiency probably due to the increased importance of transformation products which would tend to lower the efficiency of the inhibitor through side reactions. However, the modulus never attains a value lower than the unstabilized
INTRODUCTION
In Part I [1] the effects of commercial antioxidants on the modulus of elasticity, melt flow index (MFI) and degradation temperature (To) of high density polyethylene (HDPE) were investigated. In this paper, the effect of "u.v. stabilizers" has been studied in the absence of other additives to determine whether these compounds can be effective inhibitors of oxidative degradation (which leads to a loss in mechanical properties) under "dark" conditions. The stabilizers studied were chosen on the basis of difference in structure and reported mode of action.
EXPERIMENTAL Materials
Unstabilized HDPE (Altaven 1300-J) was obtained from commercial sources and was used as supplied. The stabilizers, nickel dimethyl dithiocarbamate (ROBAC Ni D.D.), and zinc dimethyl dithiocarbamate (Methasan) were kindly supplied by Robinson Brothers Ltd, U.K. and the Monsanto Co., U.S.A., respectively. Nickel (II) 2,2'-thiobis(4-tert-octyl phenolato) n-butylamine (Cyasorb UV 1084) and 2-hydroxyl-4-n-octoxy-benzophenone (Cyasorb UV 531) were kindly supplied by the American Cyanamid Company, U.S.A. For experimental details see Part I [1].
12.0 o
lO.O
u'l =
8.0
E
6.0
=
4.0
2.0 16.0 F
RESULTS AND DISCUSSION
-,00 I-
Figures 1-4 show the effects of stabilizer concentration on the tensile and flow properties of HDPE containing Cyasorb UV 531, Cyasorb UV 1084, Robac Ni D.D., and Methasan respectively. It is evident that, with the exception of Methasan, the u.v. stabilizers produce the same effects on HDPE, i.e. retention of tensile properties at ~ 13 MPa at very low concentrations (as was observed for the antioxidants [1]), a drop in modulus at concentrations between 0.5 and 2% w/w, and a subsequent increase *To whom all correspondence should be addressed.
- -
o 14.0 I_
/
"./4/.
~, 13.0~
~ lZ.Ot ~ ' ~ 11.0 L 0.0
I
I
l
I
1.0
2.0
3.0
4.0
Concentrotion / w t
I 5.0
%
Fig. 1. Variations in the tensile modulus and MF! of HDPE (processed for 10min at 150°C) with concentration, for Cyasorb UV 531. 729
730
ALFONSO J. CHIR1NOSPADRON et al. 14.0
--
o 12.0 ~ A n (.9 "~ 10.0
= o
8.0
~
6.0
E
2.0 15.0 A c 14.0
E
0 vo~
13.0 t2.0
~ 11.0 10.0
0.0
I 1.0
I 2.O
I 3.0
I 4.0
J 5.0
Concentration / wt %
Fig. 2. Variations in the tensile modulus and MFI of HDPE (processed for 10m in at 150°C) with concentration, for Cyasorb UV 1084.
sample (4 MPa) and thus inhibition is still observed. The only exception is Methasan, which actually shows a pro-oxidant effect at ~0.5% w/w. This is obviously due to the mechanistic action of this compound which is believed to function as a peroxidolytic antioxidant [2]. Its mode of action involves a balance
between antioxidant and pro-oxidant stages either of which can predominate depending on the conditions involved [2]. It is interesting to note that this compound was chosen since it is the zinc analogue of Robac Ni D.D. and, in previous studies by other workers [2], it has been found to be a better antioxidant than the nickel complex. The present work shows that the effect is obviously concentration dependent, since the nickel complex shows a stabilizing effect at all concentrations (Fig. 3). (c) At high concentrations (~>2% w/w), the stabilizers produce a further increase in modulus. This is probably due to a "stiffening" effect similar to that observed for carbon black Ill. Table 1 shows that the u.v. stabilizers tend to be more soluble than the antioxidants in low molecular weight organic solvents. In addition, previous work by Frank and Frenzel [3] has shown that in polypropylene the solubilities of additives are much lower than in low molecular weight organic solvents. The very low solubilities of the antioxidants would cause them to be exuded from the matrix, and this is indeed observed [1]. In the case of u.v. stabilizers, the high crystallinity of HDPE (typically ,-, 75% [4]) indicates that only a small region of the structure ( ~ 15%) is available for the additive which tends to aggregate in the amorphous sections between the crystallites [5]. The overall result is one of stiffening of the polymer, similar to that observed for carbon black [1]. Although, a high degree of dispersion is observed in the MFI results, these tend in general to follow the opposite pattern to the modulus values, as expected (i.e. a drop in MFI is observed with a rise in modulus and vice versa). It can also be observed that, again, Methasan shows a decrease in modulus at high concentrations. This follows the same pattern as observed for corn-
14.0 14.0 ~r o 12.0 0.
12.0
"~" 10.0
i,.0 /
"~ o 8.0 E
~ e.01
~
¢~ 4.0 p-
g~ 2.o~ 40
2.0 15.0 --
15.0
14.0 E 0T-- 13.0
6.0
-
,-" 14.0 E 0 13.0 .Do-o'-~-e---------.. e ~
°
12.0
12.0 11.0
~:S 11.0 lO.vo. r, 0
ol
1.0
2.0
3.0
4.0
5.0
Concentration / wt %
Fig. 3. Variations in the tensile modulus and MFI and HDPE (processed for I0 rain at 150°C) with concentration, for Robac Ni D.D.
10.C 0.0
I
1.0
~
2.0
~
3.0
I
4.0
I
5.0
Concentration / wt %
Fig. 4. Variations in the tensile modulus and MFI of HDPE (processed for lOmin at 150°C) with concentration, for Methasan.
Commercial u.v. stabilizers
731
Table 1. Solubility o f additives in organic solvents Solvent Additive*
Acetone
Chloroform
n-Heptane
n-Hexane
>70 < 1 ~2 ~75 --
~2 ~ 15 ~40 ~58 --
< 1 ~7 ~ 12 ~ 16 51
< 1 ~7 ~ 12 ~40 --
Hostanox 03t H o s t a n o x SE 10"t H o s t a n o x P a r 241" C y a s o r b U V 531 C y a s o r b U V 1084
*g/100 ml solvent. Obtained f r o m manufacturers' leaflets, t F o r chemical n a m e s see Part I [1].
300
280
260
?
24O
2~0 I "
~
~
l
'
•
2oo !
180 0.0
I 1.0
I 2.0
I 3.0
Concentration/wt
I 4.0
I 5.0
%
Fig. 5. Variations in the degradation temperature (To) with stabilizer concentration in HDPE (processed for 10 min at 150°C) containing: (--O--): Cyasorb UV 1084; (--Q--): Methasan; (--I-q--): Robac Ni D.D., and ( - - 1 - - ) : Cyasorb UV 531.
mercial antioxidants [1], and reflects the difference in the mechanistic behaviour and properties of this compound. Another feature of the results is the antioxidant behaviour exhibited by the nickel complex Cyasorb UV 1084 (Fig. 2). This compound has previously been found to operate though antioxidant mechanisms that include radical scavenging [6] and hydroperoxide decomposition [7]. All these observations are supported by the results shown in Fig. 5, which displays the variation in To with concentration. It is evident that the "u.v. stabilizers" have a very pronounced effect on TD. In some cases an increase of ~ 70°C is observed indicating the powerful "antioxidant" behaviour of the so-called "u.v. stabilizers", as no irradiation is involved here. The only exception is Cyasorb UV 531, which has little or no effect on TD. This observation supports recent findings by Vink [8, 9], Scott [10] and Allen [11] who have postulated a small antioxidant contribution to the overall mechanism of this compound which has generally been referred to as a "u.v. absorber". Another feature from Fig. 5 is the "anomalous" behaviour displayed by Robac Ni D.D. and Methasan in the concentration range 1.0-4.0% w/w.
It can be seen that a drop in To is obtained here. This again can be correlated with the modulus values which drop in this concentration range, probably because of an increased importance of the transformation products as highlighted previously. At higher concentrations, the stabilizing effect again increases. Finally, it must be pointed out that no changes in the fracture behaviour of the samples was observed here. This is in direct contrast with the results obtained for the phenolic antioxidant Hostanox 03 (see Part I [1]). CONCLUSIONS
The results show the powerful antioxidant behaviour observed with so-called "u.v. stabilizers", providing further evidence to support modem theories of u.v. stabilization activity which are based on antioxidant mechanisms. Traditional mechanisms, such as u.v. screening/absorption and excited state quenching, seem to lose popularity each day, particularly as new evidence is gathered. The effect observed at high concentrations is interesting since there
732
ALFONSOJ. CHIRINOSPADR6N et aL
are no published reports on it in the literature. The conclusion that it is probably due to reinforcement is obviously a direct result of the higher solubility of the u.v. stabilisers in the matrix in comparison with "traditional antioxidants" such as short and long chain phenols, coupled with the results obtained for carbon black. Acknowledgements--The authors are grateful to Robinson Brothers Ltd, U.K. for a sample of Robac Ni D.D., the Monsanto Co., U.S.A. for a sample of Methasan; Mr Frank G. Pinto of Cyanamid U.S.A. (Latin American Group) for a sample of Cyasorb UV 1084 and Mr Humberto Rubio Q., of Cyanamid de Venezuela, for a sample of Cyasorb UV 531. The authors are also grateful to the Venezuelan Council for Scientific and Technological Research (CONICIT) for financial support of this project.
REFERENCES
1. A. Chirinos Padr6n, M. A. Colmenares, Z. Rubinztain and L. A. Albornoz. Eur. Polym. J. 23(9), 723 (1987).
2. S. Al-Malaika, K. B. Chakraborty and G. Scott. In Developments in Polymer Stabilization---6 (Edited by G. Scott), p. 73. Applied Science, London (1983). 3. H. P. Frank and R. Frenzel. Fur. Polym, J. 16, 647 (1980). 4. N. C. Billingham and P. D. Calvert. In Degradation and Stabilization of Polyolefins (Edited by N. S. Allen), p. I. _Applied Science, London (1983). 5. N. S. Allen, A. Chirinos-Padr6n and T. J. Henman. Polym. Deg. Stab. 13, 31 (1985) and references therein. 6. N. S. Allen, A. Chirinos Padr6n and J. H. Appleyard. Polym. Deg. Stab. 4, 223 (1982). 7. N. S. Allen, A. J. Chirinos Padr6n and J. H. Appleyard. Eur. Polym. J. 21, 101 (1985). 8. P. Vink and Th. J. Van Veen. Eur. Polym. J. 14, 533 (1978). 9. P. Vink. In Degradation and Stabilization of Polyolefins (Edited by N. S. Allen), p. 213. Applied Science, London (1983). 10. G. Scott. Pure and AppL Chem. 52, 368 (1980). 11. N. S. Allen. In New Trends in the Photochemistry of Polymers (Edited by N. S. Allen), p. 209. Applied Science, London (1985).
0014-3057/87 $3.00+ 0.00 Copyright © 1987 PergamonJournals Ltd
INFLUENCE OF ADDITIVES ON SOME PHYSICAL PROPERTIES OF HIGH DENSITY POLYETHYLENE--II. COMMERCIAL u.v. STABILIZERS ALFONSO J. CHIRINOS PADR•N,* ZIMBUL RUBINZTAINand MARIA A. COLMENARES Laboratorio de Polimeros, Centro de Quimica, IVIC, Apartado 21827, Caracas 1020-A, Venezuela (Received 19 August 1986)
Abstract--The changes in mechanical properties of processed high density polyethylene, when "u.v. stabilizers" are added, are examined using tensile and melt flow measurements. The results show that, at low additive concentrations, antioxidant effects predominate; at high concentrations, a reinforcement effect is observed. Degradation measurements show that the u.v. stabilizers behave as excellent oxidation inhibitors in the dark; this finding has implications regarding their mechanistic action.
in this parameter at higher loadings. The results can be explained as follows. (a) At low concentrations, the "u.v. stabilizers" inhibit the degradation reactions which give rise to a drop in modulus and which, as explained previously [1], cannot be observed through e.g. carbonyl development. The best inhibitor seems to be Methasan, and the weakest Cyasorb UV 531. This is obviously an indication of their ability to act through an antioxidant mechanism in the dark. (b) At higher concentrations (0.5-2.0% w/w), there is a drop in efficiency probably due to the increased importance of transformation products which would tend to lower the efficiency of the inhibitor through side reactions. However, the modulus never attains a value lower than the unstabilized
INTRODUCTION
In Part I [1] the effects of commercial antioxidants on the modulus of elasticity, melt flow index (MFI) and degradation temperature (To) of high density polyethylene (HDPE) were investigated. In this paper, the effect of "u.v. stabilizers" has been studied in the absence of other additives to determine whether these compounds can be effective inhibitors of oxidative degradation (which leads to a loss in mechanical properties) under "dark" conditions. The stabilizers studied were chosen on the basis of difference in structure and reported mode of action.
EXPERIMENTAL Materials
Unstabilized HDPE (Altaven 1300-J) was obtained from commercial sources and was used as supplied. The stabilizers, nickel dimethyl dithiocarbamate (ROBAC Ni D.D.), and zinc dimethyl dithiocarbamate (Methasan) were kindly supplied by Robinson Brothers Ltd, U.K. and the Monsanto Co., U.S.A., respectively. Nickel (II) 2,2'-thiobis(4-tert-octyl phenolato) n-butylamine (Cyasorb UV 1084) and 2-hydroxyl-4-n-octoxy-benzophenone (Cyasorb UV 531) were kindly supplied by the American Cyanamid Company, U.S.A. For experimental details see Part I [1].
12.0 o
lO.O
u'l =
8.0
E
6.0
=
4.0
2.0 16.0 F
RESULTS AND DISCUSSION
-,00 I-
Figures 1-4 show the effects of stabilizer concentration on the tensile and flow properties of HDPE containing Cyasorb UV 531, Cyasorb UV 1084, Robac Ni D.D., and Methasan respectively. It is evident that, with the exception of Methasan, the u.v. stabilizers produce the same effects on HDPE, i.e. retention of tensile properties at ~ 13 MPa at very low concentrations (as was observed for the antioxidants [1]), a drop in modulus at concentrations between 0.5 and 2% w/w, and a subsequent increase *To whom all correspondence should be addressed.
- -
o 14.0 I_
/
"./4/.
~, 13.0~
~ lZ.Ot ~ ' ~ 11.0 L 0.0
I
I
l
I
1.0
2.0
3.0
4.0
Concentrotion / w t
I 5.0
%
Fig. 1. Variations in the tensile modulus and MF! of HDPE (processed for 10min at 150°C) with concentration, for Cyasorb UV 531. 729
730
ALFONSO J. CHIR1NOSPADRON et al. 14.0
--
o 12.0 ~ A n (.9 "~ 10.0
= o
8.0
~
6.0
E
2.0 15.0 A c 14.0
E
0 vo~
13.0 t2.0
~ 11.0 10.0
0.0
I 1.0
I 2.O
I 3.0
I 4.0
J 5.0
Concentration / wt %
Fig. 2. Variations in the tensile modulus and MFI of HDPE (processed for 10m in at 150°C) with concentration, for Cyasorb UV 1084.
sample (4 MPa) and thus inhibition is still observed. The only exception is Methasan, which actually shows a pro-oxidant effect at ~0.5% w/w. This is obviously due to the mechanistic action of this compound which is believed to function as a peroxidolytic antioxidant [2]. Its mode of action involves a balance
between antioxidant and pro-oxidant stages either of which can predominate depending on the conditions involved [2]. It is interesting to note that this compound was chosen since it is the zinc analogue of Robac Ni D.D. and, in previous studies by other workers [2], it has been found to be a better antioxidant than the nickel complex. The present work shows that the effect is obviously concentration dependent, since the nickel complex shows a stabilizing effect at all concentrations (Fig. 3). (c) At high concentrations (~>2% w/w), the stabilizers produce a further increase in modulus. This is probably due to a "stiffening" effect similar to that observed for carbon black Ill. Table 1 shows that the u.v. stabilizers tend to be more soluble than the antioxidants in low molecular weight organic solvents. In addition, previous work by Frank and Frenzel [3] has shown that in polypropylene the solubilities of additives are much lower than in low molecular weight organic solvents. The very low solubilities of the antioxidants would cause them to be exuded from the matrix, and this is indeed observed [1]. In the case of u.v. stabilizers, the high crystallinity of HDPE (typically ,-, 75% [4]) indicates that only a small region of the structure ( ~ 15%) is available for the additive which tends to aggregate in the amorphous sections between the crystallites [5]. The overall result is one of stiffening of the polymer, similar to that observed for carbon black [1]. Although, a high degree of dispersion is observed in the MFI results, these tend in general to follow the opposite pattern to the modulus values, as expected (i.e. a drop in MFI is observed with a rise in modulus and vice versa). It can also be observed that, again, Methasan shows a decrease in modulus at high concentrations. This follows the same pattern as observed for corn-
14.0 14.0 ~r o 12.0 0.
12.0
"~" 10.0
i,.0 /
"~ o 8.0 E
~ e.01
~
¢~ 4.0 p-
g~ 2.o~ 40
2.0 15.0 --
15.0
14.0 E 0T-- 13.0
6.0
-
,-" 14.0 E 0 13.0 .Do-o'-~-e---------.. e ~
°
12.0
12.0 11.0
~:S 11.0 lO.vo. r, 0
ol
1.0
2.0
3.0
4.0
5.0
Concentration / wt %
Fig. 3. Variations in the tensile modulus and MFI and HDPE (processed for I0 rain at 150°C) with concentration, for Robac Ni D.D.
10.C 0.0
I
1.0
~
2.0
~
3.0
I
4.0
I
5.0
Concentration / wt %
Fig. 4. Variations in the tensile modulus and MFI of HDPE (processed for lOmin at 150°C) with concentration, for Methasan.
Commercial u.v. stabilizers
731
Table 1. Solubility o f additives in organic solvents Solvent Additive*
Acetone
Chloroform
n-Heptane
n-Hexane
>70 < 1 ~2 ~75 --
~2 ~ 15 ~40 ~58 --
< 1 ~7 ~ 12 ~ 16 51
< 1 ~7 ~ 12 ~40 --
Hostanox 03t H o s t a n o x SE 10"t H o s t a n o x P a r 241" C y a s o r b U V 531 C y a s o r b U V 1084
*g/100 ml solvent. Obtained f r o m manufacturers' leaflets, t F o r chemical n a m e s see Part I [1].
300
280
260
?
24O
2~0 I "
~
~
l
'
•
2oo !
180 0.0
I 1.0
I 2.0
I 3.0
Concentration/wt
I 4.0
I 5.0
%
Fig. 5. Variations in the degradation temperature (To) with stabilizer concentration in HDPE (processed for 10 min at 150°C) containing: (--O--): Cyasorb UV 1084; (--Q--): Methasan; (--I-q--): Robac Ni D.D., and ( - - 1 - - ) : Cyasorb UV 531.
mercial antioxidants [1], and reflects the difference in the mechanistic behaviour and properties of this compound. Another feature of the results is the antioxidant behaviour exhibited by the nickel complex Cyasorb UV 1084 (Fig. 2). This compound has previously been found to operate though antioxidant mechanisms that include radical scavenging [6] and hydroperoxide decomposition [7]. All these observations are supported by the results shown in Fig. 5, which displays the variation in To with concentration. It is evident that the "u.v. stabilizers" have a very pronounced effect on TD. In some cases an increase of ~ 70°C is observed indicating the powerful "antioxidant" behaviour of the so-called "u.v. stabilizers", as no irradiation is involved here. The only exception is Cyasorb UV 531, which has little or no effect on TD. This observation supports recent findings by Vink [8, 9], Scott [10] and Allen [11] who have postulated a small antioxidant contribution to the overall mechanism of this compound which has generally been referred to as a "u.v. absorber". Another feature from Fig. 5 is the "anomalous" behaviour displayed by Robac Ni D.D. and Methasan in the concentration range 1.0-4.0% w/w.
It can be seen that a drop in To is obtained here. This again can be correlated with the modulus values which drop in this concentration range, probably because of an increased importance of the transformation products as highlighted previously. At higher concentrations, the stabilizing effect again increases. Finally, it must be pointed out that no changes in the fracture behaviour of the samples was observed here. This is in direct contrast with the results obtained for the phenolic antioxidant Hostanox 03 (see Part I [1]). CONCLUSIONS
The results show the powerful antioxidant behaviour observed with so-called "u.v. stabilizers", providing further evidence to support modem theories of u.v. stabilization activity which are based on antioxidant mechanisms. Traditional mechanisms, such as u.v. screening/absorption and excited state quenching, seem to lose popularity each day, particularly as new evidence is gathered. The effect observed at high concentrations is interesting since there
732
ALFONSOJ. CHIRINOSPADR6N et aL
are no published reports on it in the literature. The conclusion that it is probably due to reinforcement is obviously a direct result of the higher solubility of the u.v. stabilisers in the matrix in comparison with "traditional antioxidants" such as short and long chain phenols, coupled with the results obtained for carbon black. Acknowledgements--The authors are grateful to Robinson Brothers Ltd, U.K. for a sample of Robac Ni D.D., the Monsanto Co., U.S.A. for a sample of Methasan; Mr Frank G. Pinto of Cyanamid U.S.A. (Latin American Group) for a sample of Cyasorb UV 1084 and Mr Humberto Rubio Q., of Cyanamid de Venezuela, for a sample of Cyasorb UV 531. The authors are also grateful to the Venezuelan Council for Scientific and Technological Research (CONICIT) for financial support of this project.
REFERENCES
1. A. Chirinos Padr6n, M. A. Colmenares, Z. Rubinztain and L. A. Albornoz. Eur. Polym. J. 23(9), 723 (1987).
2. S. Al-Malaika, K. B. Chakraborty and G. Scott. In Developments in Polymer Stabilization---6 (Edited by G. Scott), p. 73. Applied Science, London (1983). 3. H. P. Frank and R. Frenzel. Fur. Polym, J. 16, 647 (1980). 4. N. C. Billingham and P. D. Calvert. In Degradation and Stabilization of Polyolefins (Edited by N. S. Allen), p. I. _Applied Science, London (1983). 5. N. S. Allen, A. Chirinos-Padr6n and T. J. Henman. Polym. Deg. Stab. 13, 31 (1985) and references therein. 6. N. S. Allen, A. Chirinos Padr6n and J. H. Appleyard. Polym. Deg. Stab. 4, 223 (1982). 7. N. S. Allen, A. J. Chirinos Padr6n and J. H. Appleyard. Eur. Polym. J. 21, 101 (1985). 8. P. Vink and Th. J. Van Veen. Eur. Polym. J. 14, 533 (1978). 9. P. Vink. In Degradation and Stabilization of Polyolefins (Edited by N. S. Allen), p. 213. Applied Science, London (1983). 10. G. Scott. Pure and AppL Chem. 52, 368 (1980). 11. N. S. Allen. In New Trends in the Photochemistry of Polymers (Edited by N. S. Allen), p. 209. Applied Science, London (1985).