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The antioxidant effect in controlling thermal degradation of a low density polyethylene blown film
Polymer Degradation and Stability 85 (2004) 1003e1007 www.elsevier.com/locate/polydegstab
The antioxidant effect in controlling thermal degradation of a low density polyethylene blown film* P. Mariania, G. Cariannia, F.P. La Mantiab,) a Centro Ricerche Polimeri Europa, via Jannozzi, 1 20097 San Donato Milanese, Italy Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy
b
Received 24 October 2002; accepted 30 April 2003
Abstract It is well known that antioxidants are widely used to prevent thermal degradation of high density and linear low density polyethylene. Antioxidants are not always present in low density polyethylene and only small amounts are usually added to these resins. In this work the effect of an antioxidant system on a low density resin having MFI (190 (C/2.16 kg)=2 g/10# and density 0.9230 g/cm3 has been studied. Its effect on melt viscosity has been studied by means of a batch mixer and the torque vs time behaviour has been analysed. The results show that a maximum in the torque vs time curve can be seen for the material containing antioxidant. In contrast, the material without antioxidant does not show any maximum in the torque vs time curve and, after a certain time, directly undergoes chain scission. The presence of the antioxidant in a low density polyethylene seems to change the kinetics of two competitive phenomena: long chain branching formation/crosslinking and chain scissions. The antioxidant works essentially by stopping the peroxides formation. This effect slows the molecular weight decrease but does not influence long chain branching formation or crosslinking. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Low density polyethylene; Long chain branching; Chain scission; Antioxidant
1. Introduction The nature of the physico-chemical processes occurring on processing polyethylene has been extensively investigated [1e6]. Experiments have been carried out under static as well as under dynamic conditions. The main objective of experiments performed under static conditions is the understanding of the processes involved in thermal oxidation of polyolefin melts. The objective of experiments carried out under dynamic conditions is the simulation of what happens inside industrial processing equipment. This last type of experiment involves processing in open or closed mixers. The material subjected to processing undergoes degradation linked to two main mechanisms. Starting from * Based on a paper presented at the 2nd international Conference on Modification, Degradation and Stabilisation of Polymers (MoDeSt), Budapest, 30 Junee4 July 2002. ) Corresponding author. Tel.: C39-91-656-7111; fax: C39-91-599766. E-mail address: [email protected] (F.P. La Mantia).
0141-3910/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2003.04.007
the scission of a macromolecule due to shear stress or temperature, free radicals may be formed. These free radicals may either recombine, react with oxygen or form secondary radicals by abstraction of an hydrogen atom. All of these reactions can lead to formation of long chain branching (LCB), crosslinking or chain scission phenomena, two different and competitive mechanisms of polymer modification. The hydroperoxides and the other oxidation products formed may be stopped by adding a certain amount of stabilising agent. Usually, antioxidants are not present in low density polyethylene (LDPE) because their presence may lead to a worsening of some final product properties. In this work the effect of an antioxidant system on a low density resin has been studied.
2. Experimental The material used was a LDPE having MFI (190 (C/ 2.16 kg)=2 g/10# and density 0.923 g/cm3. The same
P. Mariani et al. / Polymer Degradation and Stability 85 (2004) 1003e1007
material was dry blended by adding 550 ppm of a primary antioxidants (Irganox 1076). The effect of this concentration in preventing thermal-oxidative and thermo-mechanical degradation was investigated. All the experimental tests were carried out by using a Haake Rheocord 90 batch mixer, equipped with a 45 g mixing chamber. The test temperatures were 160, 195 and 230 (C; different processing times ( from 15 to 60 min) were studied and several rotor speeds (20, 100 and 150 rpm) were applied. The tests were carried out in the presence of air to facilitate oxygen adsorption inside the macromolecular structure; the torque behaviour vs time was registered. FTIR analysis (FTIR Nicolet 20SX; range 4000e400 cmÿ1; resolution 4 cmÿ1) on samples taken every 15 min and GPC measurements (GPC Waters 150; four columns TSK gel GM-H6 mixed; solvent: 1,2,4 trichlorobenzene at 135 (C; flow: 1 ml/min) on samples taken every 30 min were also performed to verify the presence of any crosslinks or long chain branching due to thermomechanical stress during processing. A rheological characterisation in terms of melt flow index, flow curves and Newtonian viscosity determination on samples taken every 15 min from the discontinuous batch mixer was used to confirm the results obtained from the above mentioned analysis. Melt flow index measurements were performed by means of an automatic grader by Ceast. Flow curves and Newtonian viscosity determination were obtained on a Rheometric RDS II rotational rheometer at 195 (C under nitrogen atmosphere in parallel plate and cone and plate geometry.
3. Results and discussion The results of the tests have been divided into different sections, each one regarding the influence of a different parameter. In particular the effects due to temperature, to the rotor speed and to the presence of the antioxidant are discussed.
Rotor speed = 100 rpm 1000 900
Torque (cN · cm)
1004
800 700 600 500 400 300
T=160°C T=195°C T=230°C 100
1000
104
Mixing Time (s) - log scale Fig. 1. Temperature effect on the torque vs mixing time for the base resin.
Similar results can be obtained on the resin containing antioxidant. In Fig. 2 the effect of the temperature has been reported considering only 195 and 230 (C curves. LCB formation/crosslinking seems to be more important on the resin containing antioxidant, resulting in a more pronounced maximum in the torque vs mixing time curves at short times. 3.2. Thermo-mechanical stress effect In Fig. 3 the effect of the rotor speed on the base resin torque vs mixing time curve has been reported. The temperature in these tests was kept at 195 (C. Increasing rotor speed accelerates the degradation since the torque falls at shorter processing times. In contrast, if the rotor speed decreases, thermal stability is more evident as chain-scission occurs at longer times. If the rotor speed is very low (i.e. 20 rpm) LCB formation/ crosslinking mechanism prevails over chain scission Rotor speed = 100 rpm
1000 900
The behaviour of the polymer during mixing has been investigated by means of a discontinuous batch mixer. In Fig. 1 torque vs mixing time curves for the base resin mixed at 100 rpm, are reported for three different temperatures. When the temperature is low the polymer remains stable (constant torque) until chain scission causes a sharp decreases in the torque values; a temperature increase brings an acceleration in chain scission phenomena and an earlier decrease of torque values; at the same time with increasing temperature the mechanism of LCB formation/crosslinking becomes more important and causes a slight maximum at short times in the torque curve, clearly evident at 230 (C (Fig. 1).
800
Torque (cN · cm)
3.1. Temperature effect
700 600 500 400 300
T=230°C T=195°C 100
1000
104
Mixing Time (s) - log scale Fig. 2. Temperature effect on the torque vs mixing time for the resin containing additive.
1005
P. Mariani et al. / Polymer Degradation and Stability 85 (2004) 1003e1007
T=195°C
1000
20 rpm 100 rpm 150 rpm
800 700 600 500 400
800 700 600 500
100
400 10
1000
Mixing Time (s) - log scale
phenomena and a slight, but constant, torque increase is obtained at least for the longest time investigated (1 h). Similar behaviour, slightly shifted toward longer times can be seen also on the resin containing antioxidant (Fig. 4). The main difference in comparison to the base resin can be focused on a more important effect of LCB formation/crosslinking that causes more pronounced maximum on the 20 and 100 rpm curves. As already observed regarding the temperature effect, the thermomechanical stress has the same effect as temperature. As the rotor speed is lowered the polymer is stable for a longer time for both the base resin and the resin containing antioxidant. 3.3. Primary antioxidant effect The effect of the presence of the antioxidant on the torque vs mixing time has been extensively studied for tests carried out at 195 (C and at a rotor speed of 100 20 rpm 100 rpm 150 rpm
1000
T=195°C
900 800 700 600 500 400 300 100
100
1000
104
Mixing Time (s) - log scale
Fig. 3. Rotor speed effect on the torque vs mixing time for the base resin.
Torque (cN·cm)
Base resin Additivated resin
900
Torque (cN·cm)
Torque (cN · cm)
900
300
1000
1000
Mixing Time (s) - log scale Fig. 4. Rotor speed effect on the torque vs mixing time for the resin containing additive.
Fig. 5. Primary antioxidant effect on the torque vs mixing time at 195 (C and 100 rpm.
rpm. In Fig. 5 the effect of the antioxidant in the base resin is self evident. A comparison between the base resin and the resin containing antioxidant one shows that the presence of primary antioxidant leads to a maximum in the torque vs time curve, thus leading to a competition between LCB formation/crosslinking (i.e. torque increase) and chain-scission phenomena (i.e. torque reduction). The base resin seems more thermally stable than the resin containing antioxidant one, until chain scission occurs. It is worth noting that in a certain range of processing times also other primary antioxidants have a similar behaviour: they show a maximum in the torque vs time curve at the same time. The sample with Irganox 1010 shows a further viscosity increase probably due to free radical recombination following chain-scission. We can conclude that primary antioxidant leads to a shift of thermal-oxidative degradation, in terms of LCB formation/crosslinking, towards shorter times. The GPC measurements, carried out on the samples drawn from the mixer every 30 min confirm this behaviour (Figs. 6 and 7). We observe the broadest MWD curve and the highest Mw after 30 min processing resins for polymers with and without the additive. The 60 min processing time samples show a Mw reduction and a narrower MWD, close to that of the reference sample. Analysis of the vinyl group content via infrared spectroscopy also confirms the same kind of structure modification both for the base resin and the additivecontaining polymer (Fig. 8). For the base resin, at short times, when LCB formation/crosslinking prevails, the vinyl group content decreases, while at longer times the vinyl group content remains constant. The resin containing antioxidant shows a constant decrease of the vinyl group content as the mixing time in the batch mixer increases. The same kind of conclusions may be argued also from a rheological analysis of the two resins.
1006
P. Mariani et al. / Polymer Degradation and Stability 85 (2004) 1003e1007
0.36
Molecular Weight Distribution
Vinyl group content /103C
dw/d(log-1M) x 10
8.00 tq 30 min 60 min
6.00
4.00
2.00
0.00 2.00
3.00
4.00
5.00
6.00
7.00
8.00
Molecular Weight Distribution
dw/d(log-1M) x 10
8.00 tq 30 min 60 min
6.00
4.00
0.3 0.28
0
10
20
30
40
50
60
Mixing Time (min) Fig. 8. Vinyl group content via IR spectroscopy vs mixing time. Dashed lineZbase resin; continuous lineZresin containing additive. Mixing conditions: TZ195 (C; 100 rpm.
are reported. As already seen for the melt flow index values, the viscosity increases until 30 min of mixing and decreases from 30 to 60 min. The increase is much more pronounced for the resin containing antioxidant than for the base resin and is localised mainly in the lower frequency range where the contribution of the high molecular weights is more important. The values of the Newtonian viscosity, obtained by means of a rotational rheometer in cone and plate geometry at the same temperature, confirm this indication (Fig. 11). Newtonian viscosity increases till 30 min of mixing and then decreases from 30 to 60 min both for the base resin and the resin containing antioxidant. The increase for the resin containing antioxidant is more evident than for the base resin. In conclusion, there is strong evidence, both structural and rheological, that two different competitive mechanisms of molecular reaction can cause modification of
3
M.F.I. (190°C/2.16kg) (g/10')
In Fig. 9 melt flow index values on the resins taken from the batch mixer every 15 min are reported. As expected, at short times (15 and 30 min) melt flow index decreases due to LCB formation/crosslinking. It is worth noting that the decrease is much more important for the resin containing antioxidant than for the base resin. After 45 and 60 min mixing time, melt flow indexes begin to increase for the base resin and remain rather constant for the resin containing antioxidant. In fact, at these mixing times chain scission phenomena become more and more important and, especially on the base resin, prevail over LCB formation/crosslinking. The shorter chains so created cause a more fluid behaviour of the melt and higher values of the melt flow indexes. A more complete rheological approach can be given by flow curves obtained by means of a rotational rheometer in parallel plate geometry. Also in this kind of characterisation the competition between LCB formation/crosslinking and chain scission phenomena is evident. In Fig. 10 data regarding viscosity curves at 195 (C as a function of frequency for the base resin and the resin containing antioxidant, sampled every 15 min,
0.32
0.26
Log(Molecular Weight) Fig. 6. Molecular weight distribution from GPC analysis for the base resin.
0.34
2.5
2
1.5
1
2.00
0.5 0.00 2.00
3.00
4.00
5.00
6.00
7.00
8.00
Log(Molecular Weight) Fig. 7. Molecular weight distribution from GPC analysis for the resin containing additive.
0
10
20
30
40
50
60
Mixing Time (min) Fig. 9. Melt flow index vs mixing time. Dashed lineZbase resin; continuous lineZresin containing additive. Mixing conditions: TZ 195 (C; 100 rpm.
1007
P. Mariani et al. / Polymer Degradation and Stability 85 (2004) 1003e1007
η (Pa · s) - log scale
7000 6000 5000 4000
2 104
η0(Pa ·s) - log scale
No mixing time 15 min 30 min 45 min 60 min
104 9000 8000
104 9000 8000 7000 6000 5000 4000
3000 3000 0.2
0.3
0.4
0.5
0
0.6 0.7 0.8 0.9 1
10
20
30
40
50
60
Mixing Time (min)
Angular frequency (rad/s) - log scale Fig. 10. Viscosity vs angular frequency. Mixing times as in the legend. Dashed lineZbase resin; continuous lineZresin containing additive. Mixing conditions: TZ195 (C; 100 rpm.
low density polyethylene subjected to thermo-mechanical stress. Each of the two mechanisms, LCB formation/ crosslinking and chain scission, contribute to the polymer degradation by changing the melt viscosity and modifying the molecular weight distribution of the polymer. The presence of a primary antioxidant shifts degradation occurring by molecular weight reduction towards longer times but does not influence LCB formation/crosslinking.
Fig. 11. Newtonian viscosity vs mixing time. Dashed lineZbase resin; continuous lineZresin containing additive. Mixing conditions: TZ195 (C; 100 rpm.
temperature, low rotor speed) the radical mobility lowers, hence LCB formation/crosslinking is promoted, especially if double bonds are present. In contrast, high chain mobility (i.e. low viscosities) leads to vinyl end formation and then to a molecular weight reduction (chain-scission). The presence of antioxidant shifts degradation occurring by molecular weight reduction towards longer times. The antioxidant works by stopping the peroxide groups but does not influence LCB formation/crosslinking in any way.
4. Conclusions References Low density polyethylene may undergoes degradation either by LCB formation/crosslinking or by chain scission. The degradation mechanism is strictly linked to the bulk viscosity. Once radicals are produced by oxygen or thermo-mechanical stress, the probability of having LCB formation/crosslinking depends on the radical mobility. If the viscosity is high (i.e. low
[1] [2] [3] [4] [5]
Adams JH, Goodrich JE. J Polym Sci 1970;8:1269. Chakraborty VB, Scott G. Eur Polym J 1977;13:731. Rideal GR, Padget JC. J Polym Sci Polym Symposia 1976;57:1. Gol’dberg VM, Zaikov GE. Polym Degrad Stab 1987;19:221. Hinsken H, Moss S, Pauquet JR, Zweifel H. Polym Degrad Stab 1991;34:279. [6] Gugumus F. Polym Degrad Stab 1999;66:161.
The antioxidant effect in controlling thermal degradation of a low density polyethylene blown film* P. Mariania, G. Cariannia, F.P. La Mantiab,) a Centro Ricerche Polimeri Europa, via Jannozzi, 1 20097 San Donato Milanese, Italy Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy
b
Received 24 October 2002; accepted 30 April 2003
Abstract It is well known that antioxidants are widely used to prevent thermal degradation of high density and linear low density polyethylene. Antioxidants are not always present in low density polyethylene and only small amounts are usually added to these resins. In this work the effect of an antioxidant system on a low density resin having MFI (190 (C/2.16 kg)=2 g/10# and density 0.9230 g/cm3 has been studied. Its effect on melt viscosity has been studied by means of a batch mixer and the torque vs time behaviour has been analysed. The results show that a maximum in the torque vs time curve can be seen for the material containing antioxidant. In contrast, the material without antioxidant does not show any maximum in the torque vs time curve and, after a certain time, directly undergoes chain scission. The presence of the antioxidant in a low density polyethylene seems to change the kinetics of two competitive phenomena: long chain branching formation/crosslinking and chain scissions. The antioxidant works essentially by stopping the peroxides formation. This effect slows the molecular weight decrease but does not influence long chain branching formation or crosslinking. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Low density polyethylene; Long chain branching; Chain scission; Antioxidant
1. Introduction The nature of the physico-chemical processes occurring on processing polyethylene has been extensively investigated [1e6]. Experiments have been carried out under static as well as under dynamic conditions. The main objective of experiments performed under static conditions is the understanding of the processes involved in thermal oxidation of polyolefin melts. The objective of experiments carried out under dynamic conditions is the simulation of what happens inside industrial processing equipment. This last type of experiment involves processing in open or closed mixers. The material subjected to processing undergoes degradation linked to two main mechanisms. Starting from * Based on a paper presented at the 2nd international Conference on Modification, Degradation and Stabilisation of Polymers (MoDeSt), Budapest, 30 Junee4 July 2002. ) Corresponding author. Tel.: C39-91-656-7111; fax: C39-91-599766. E-mail address: [email protected] (F.P. La Mantia).
0141-3910/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2003.04.007
the scission of a macromolecule due to shear stress or temperature, free radicals may be formed. These free radicals may either recombine, react with oxygen or form secondary radicals by abstraction of an hydrogen atom. All of these reactions can lead to formation of long chain branching (LCB), crosslinking or chain scission phenomena, two different and competitive mechanisms of polymer modification. The hydroperoxides and the other oxidation products formed may be stopped by adding a certain amount of stabilising agent. Usually, antioxidants are not present in low density polyethylene (LDPE) because their presence may lead to a worsening of some final product properties. In this work the effect of an antioxidant system on a low density resin has been studied.
2. Experimental The material used was a LDPE having MFI (190 (C/ 2.16 kg)=2 g/10# and density 0.923 g/cm3. The same
P. Mariani et al. / Polymer Degradation and Stability 85 (2004) 1003e1007
material was dry blended by adding 550 ppm of a primary antioxidants (Irganox 1076). The effect of this concentration in preventing thermal-oxidative and thermo-mechanical degradation was investigated. All the experimental tests were carried out by using a Haake Rheocord 90 batch mixer, equipped with a 45 g mixing chamber. The test temperatures were 160, 195 and 230 (C; different processing times ( from 15 to 60 min) were studied and several rotor speeds (20, 100 and 150 rpm) were applied. The tests were carried out in the presence of air to facilitate oxygen adsorption inside the macromolecular structure; the torque behaviour vs time was registered. FTIR analysis (FTIR Nicolet 20SX; range 4000e400 cmÿ1; resolution 4 cmÿ1) on samples taken every 15 min and GPC measurements (GPC Waters 150; four columns TSK gel GM-H6 mixed; solvent: 1,2,4 trichlorobenzene at 135 (C; flow: 1 ml/min) on samples taken every 30 min were also performed to verify the presence of any crosslinks or long chain branching due to thermomechanical stress during processing. A rheological characterisation in terms of melt flow index, flow curves and Newtonian viscosity determination on samples taken every 15 min from the discontinuous batch mixer was used to confirm the results obtained from the above mentioned analysis. Melt flow index measurements were performed by means of an automatic grader by Ceast. Flow curves and Newtonian viscosity determination were obtained on a Rheometric RDS II rotational rheometer at 195 (C under nitrogen atmosphere in parallel plate and cone and plate geometry.
3. Results and discussion The results of the tests have been divided into different sections, each one regarding the influence of a different parameter. In particular the effects due to temperature, to the rotor speed and to the presence of the antioxidant are discussed.
Rotor speed = 100 rpm 1000 900
Torque (cN · cm)
1004
800 700 600 500 400 300
T=160°C T=195°C T=230°C 100
1000
104
Mixing Time (s) - log scale Fig. 1. Temperature effect on the torque vs mixing time for the base resin.
Similar results can be obtained on the resin containing antioxidant. In Fig. 2 the effect of the temperature has been reported considering only 195 and 230 (C curves. LCB formation/crosslinking seems to be more important on the resin containing antioxidant, resulting in a more pronounced maximum in the torque vs mixing time curves at short times. 3.2. Thermo-mechanical stress effect In Fig. 3 the effect of the rotor speed on the base resin torque vs mixing time curve has been reported. The temperature in these tests was kept at 195 (C. Increasing rotor speed accelerates the degradation since the torque falls at shorter processing times. In contrast, if the rotor speed decreases, thermal stability is more evident as chain-scission occurs at longer times. If the rotor speed is very low (i.e. 20 rpm) LCB formation/ crosslinking mechanism prevails over chain scission Rotor speed = 100 rpm
1000 900
The behaviour of the polymer during mixing has been investigated by means of a discontinuous batch mixer. In Fig. 1 torque vs mixing time curves for the base resin mixed at 100 rpm, are reported for three different temperatures. When the temperature is low the polymer remains stable (constant torque) until chain scission causes a sharp decreases in the torque values; a temperature increase brings an acceleration in chain scission phenomena and an earlier decrease of torque values; at the same time with increasing temperature the mechanism of LCB formation/crosslinking becomes more important and causes a slight maximum at short times in the torque curve, clearly evident at 230 (C (Fig. 1).
800
Torque (cN · cm)
3.1. Temperature effect
700 600 500 400 300
T=230°C T=195°C 100
1000
104
Mixing Time (s) - log scale Fig. 2. Temperature effect on the torque vs mixing time for the resin containing additive.
1005
P. Mariani et al. / Polymer Degradation and Stability 85 (2004) 1003e1007
T=195°C
1000
20 rpm 100 rpm 150 rpm
800 700 600 500 400
800 700 600 500
100
400 10
1000
Mixing Time (s) - log scale
phenomena and a slight, but constant, torque increase is obtained at least for the longest time investigated (1 h). Similar behaviour, slightly shifted toward longer times can be seen also on the resin containing antioxidant (Fig. 4). The main difference in comparison to the base resin can be focused on a more important effect of LCB formation/crosslinking that causes more pronounced maximum on the 20 and 100 rpm curves. As already observed regarding the temperature effect, the thermomechanical stress has the same effect as temperature. As the rotor speed is lowered the polymer is stable for a longer time for both the base resin and the resin containing antioxidant. 3.3. Primary antioxidant effect The effect of the presence of the antioxidant on the torque vs mixing time has been extensively studied for tests carried out at 195 (C and at a rotor speed of 100 20 rpm 100 rpm 150 rpm
1000
T=195°C
900 800 700 600 500 400 300 100
100
1000
104
Mixing Time (s) - log scale
Fig. 3. Rotor speed effect on the torque vs mixing time for the base resin.
Torque (cN·cm)
Base resin Additivated resin
900
Torque (cN·cm)
Torque (cN · cm)
900
300
1000
1000
Mixing Time (s) - log scale Fig. 4. Rotor speed effect on the torque vs mixing time for the resin containing additive.
Fig. 5. Primary antioxidant effect on the torque vs mixing time at 195 (C and 100 rpm.
rpm. In Fig. 5 the effect of the antioxidant in the base resin is self evident. A comparison between the base resin and the resin containing antioxidant one shows that the presence of primary antioxidant leads to a maximum in the torque vs time curve, thus leading to a competition between LCB formation/crosslinking (i.e. torque increase) and chain-scission phenomena (i.e. torque reduction). The base resin seems more thermally stable than the resin containing antioxidant one, until chain scission occurs. It is worth noting that in a certain range of processing times also other primary antioxidants have a similar behaviour: they show a maximum in the torque vs time curve at the same time. The sample with Irganox 1010 shows a further viscosity increase probably due to free radical recombination following chain-scission. We can conclude that primary antioxidant leads to a shift of thermal-oxidative degradation, in terms of LCB formation/crosslinking, towards shorter times. The GPC measurements, carried out on the samples drawn from the mixer every 30 min confirm this behaviour (Figs. 6 and 7). We observe the broadest MWD curve and the highest Mw after 30 min processing resins for polymers with and without the additive. The 60 min processing time samples show a Mw reduction and a narrower MWD, close to that of the reference sample. Analysis of the vinyl group content via infrared spectroscopy also confirms the same kind of structure modification both for the base resin and the additivecontaining polymer (Fig. 8). For the base resin, at short times, when LCB formation/crosslinking prevails, the vinyl group content decreases, while at longer times the vinyl group content remains constant. The resin containing antioxidant shows a constant decrease of the vinyl group content as the mixing time in the batch mixer increases. The same kind of conclusions may be argued also from a rheological analysis of the two resins.
1006
P. Mariani et al. / Polymer Degradation and Stability 85 (2004) 1003e1007
0.36
Molecular Weight Distribution
Vinyl group content /103C
dw/d(log-1M) x 10
8.00 tq 30 min 60 min
6.00
4.00
2.00
0.00 2.00
3.00
4.00
5.00
6.00
7.00
8.00
Molecular Weight Distribution
dw/d(log-1M) x 10
8.00 tq 30 min 60 min
6.00
4.00
0.3 0.28
0
10
20
30
40
50
60
Mixing Time (min) Fig. 8. Vinyl group content via IR spectroscopy vs mixing time. Dashed lineZbase resin; continuous lineZresin containing additive. Mixing conditions: TZ195 (C; 100 rpm.
are reported. As already seen for the melt flow index values, the viscosity increases until 30 min of mixing and decreases from 30 to 60 min. The increase is much more pronounced for the resin containing antioxidant than for the base resin and is localised mainly in the lower frequency range where the contribution of the high molecular weights is more important. The values of the Newtonian viscosity, obtained by means of a rotational rheometer in cone and plate geometry at the same temperature, confirm this indication (Fig. 11). Newtonian viscosity increases till 30 min of mixing and then decreases from 30 to 60 min both for the base resin and the resin containing antioxidant. The increase for the resin containing antioxidant is more evident than for the base resin. In conclusion, there is strong evidence, both structural and rheological, that two different competitive mechanisms of molecular reaction can cause modification of
3
M.F.I. (190°C/2.16kg) (g/10')
In Fig. 9 melt flow index values on the resins taken from the batch mixer every 15 min are reported. As expected, at short times (15 and 30 min) melt flow index decreases due to LCB formation/crosslinking. It is worth noting that the decrease is much more important for the resin containing antioxidant than for the base resin. After 45 and 60 min mixing time, melt flow indexes begin to increase for the base resin and remain rather constant for the resin containing antioxidant. In fact, at these mixing times chain scission phenomena become more and more important and, especially on the base resin, prevail over LCB formation/crosslinking. The shorter chains so created cause a more fluid behaviour of the melt and higher values of the melt flow indexes. A more complete rheological approach can be given by flow curves obtained by means of a rotational rheometer in parallel plate geometry. Also in this kind of characterisation the competition between LCB formation/crosslinking and chain scission phenomena is evident. In Fig. 10 data regarding viscosity curves at 195 (C as a function of frequency for the base resin and the resin containing antioxidant, sampled every 15 min,
0.32
0.26
Log(Molecular Weight) Fig. 6. Molecular weight distribution from GPC analysis for the base resin.
0.34
2.5
2
1.5
1
2.00
0.5 0.00 2.00
3.00
4.00
5.00
6.00
7.00
8.00
Log(Molecular Weight) Fig. 7. Molecular weight distribution from GPC analysis for the resin containing additive.
0
10
20
30
40
50
60
Mixing Time (min) Fig. 9. Melt flow index vs mixing time. Dashed lineZbase resin; continuous lineZresin containing additive. Mixing conditions: TZ 195 (C; 100 rpm.
1007
P. Mariani et al. / Polymer Degradation and Stability 85 (2004) 1003e1007
η (Pa · s) - log scale
7000 6000 5000 4000
2 104
η0(Pa ·s) - log scale
No mixing time 15 min 30 min 45 min 60 min
104 9000 8000
104 9000 8000 7000 6000 5000 4000
3000 3000 0.2
0.3
0.4
0.5
0
0.6 0.7 0.8 0.9 1
10
20
30
40
50
60
Mixing Time (min)
Angular frequency (rad/s) - log scale Fig. 10. Viscosity vs angular frequency. Mixing times as in the legend. Dashed lineZbase resin; continuous lineZresin containing additive. Mixing conditions: TZ195 (C; 100 rpm.
low density polyethylene subjected to thermo-mechanical stress. Each of the two mechanisms, LCB formation/ crosslinking and chain scission, contribute to the polymer degradation by changing the melt viscosity and modifying the molecular weight distribution of the polymer. The presence of a primary antioxidant shifts degradation occurring by molecular weight reduction towards longer times but does not influence LCB formation/crosslinking.
Fig. 11. Newtonian viscosity vs mixing time. Dashed lineZbase resin; continuous lineZresin containing additive. Mixing conditions: TZ195 (C; 100 rpm.
temperature, low rotor speed) the radical mobility lowers, hence LCB formation/crosslinking is promoted, especially if double bonds are present. In contrast, high chain mobility (i.e. low viscosities) leads to vinyl end formation and then to a molecular weight reduction (chain-scission). The presence of antioxidant shifts degradation occurring by molecular weight reduction towards longer times. The antioxidant works by stopping the peroxide groups but does not influence LCB formation/crosslinking in any way.
4. Conclusions References Low density polyethylene may undergoes degradation either by LCB formation/crosslinking or by chain scission. The degradation mechanism is strictly linked to the bulk viscosity. Once radicals are produced by oxygen or thermo-mechanical stress, the probability of having LCB formation/crosslinking depends on the radical mobility. If the viscosity is high (i.e. low
[1] [2] [3] [4] [5]
Adams JH, Goodrich JE. J Polym Sci 1970;8:1269. Chakraborty VB, Scott G. Eur Polym J 1977;13:731. Rideal GR, Padget JC. J Polym Sci Polym Symposia 1976;57:1. Gol’dberg VM, Zaikov GE. Polym Degrad Stab 1987;19:221. Hinsken H, Moss S, Pauquet JR, Zweifel H. Polym Degrad Stab 1991;34:279. [6] Gugumus F. Polym Degrad Stab 1999;66:161.