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Infrared studies of thermal oxidative degradation of polystyrene-block polybutadiene-block-polystyrene thermoplastic elastomers
Infrared studies of thermal oxidative degradation of polystyrene-blockpolybutadiene-block-polystyrene thermoplastic elastomers Shang-Ming Wang, Jen-Ray Chang & Raymond Chien-Chao Tsiangq Depurtnlenr
o,f Chemicul
(Received
Engineering,
18 September
N~~tiorurl Chung
Cheng University,
IYYS: accepted 26 October
Chiqi,
7iriwn
1995)
Thermal oxidative degradation of a polystyrene-hlock-polybutadiene-hlockpolystyrene thermoplastic elastomer (SBS rubber) has been conducted in an in-situ Infrared cell. By monitoring the disappearance of trctns-I .4 and vinyl- I ,2 double bonds and the appearance of the hydroxyl and the carboxyl/ carboxylate groups in the FTIR spectra, the temperature. air, antioxidant. and molecular microstructure dependence of the polymer degradation was studied. The experimental results indicate that the l.4-polybutadiene portion of the SBS polymer is easier to degrade than the 1.2-polybutadiene portion and the hydroxyl group appears concomitantly with the disappearance of the polybutadiene. Based on data from the temperature-programmed desorption (TPD) of HZO, it is concluded that the formation of hydroxyl group makes the polymer hydrophilic and promotes the H,O adsorption on it. The amount of H1O adsorption varies with the temperature and the process appears reversible. At low temperatures, the moisture adsorbed onto a degraded polymer sample amounts to approximately 7-10% of total hydroxyls. I wt% of lrganox 1076 (antioxidant) would effectively thwart the thermal oxidation process even at severe conditions as high as 225°C for 12 h. No degradation occur\ in ;IH air-free environment. 0 lYY6 Elsevier Science Limircd
1 INTRODUCTION The polystyrene-hlock-polybutadiene-block-polystyrene thermoplastic elastomers (SBS rubbers) tend to degrade by exposure to heat and UV light causing discoloration and surface embrittlement.‘.’ While the basic SBS rubber has two glass transition temperatures-an upper one of -93°C for the rigid polystyrene domain and a lower one of -65°C for the soft polybutadiene domain-various additives such as resins and plasticizers which modify the domains will alter these glass transition temperatures. In particular, resins which associate with the rigid polystyrene domain frequently increase the upper glass transition temperature. Therefore, SBS rubbers are often processed at temperatures higher than :::To whom correspondence
should be addressed.
120°C. At temperatures like these, degradation is likely to occur in the presence of air. Since SBS rubbers contain an unsaturated rubber midblock, the degradation mechanism is similar to SBR and polybutadiene rubbers.3.’ Its degradation arises from the generation of free radicals causing undesirable reactions of the double bonds in the polybutadiene segment. While various antioxidants are available to reduce the degradation,’ such as the use of hindered phenols to scavenge the free radicals and the use of phosphites to decompose the hydroperoxides, fundamental studies of the SBS degradation process and its associated phenomenon are seldom reported. Thus the purpose of this work is to study the temperature and air effects of SBS thermal oxidative degradation and its rate dependence on the molecular microstructure. FTIR (Fourier transform infrared spectroscopy) has been
extensively used to enable qualitative and quantitative analyses of the appearance/ disappearance of functional groups of the polymers. All polymer samples were thermally tested within a temperature range of 70-250°C.
ifold allowing an in-situ analysis. Depending upon reaction requirement, the analyses can be done in vacuum, under air, or in the presence of nitrogen only. 2.2.2 Temperature-programmed (TPD)
2 EXPERIMENTAL 2.1 Materials
The SBS polymer containing no antioxidants was anionically synthesized in cyclohexane using n-butyllithium as the initiator in the presence of polar structure modifier THF. The amount of THF was such that the polybutadiene segment of the resulting polymer should contain significant amounts of 1,4- and 1,2-microstructures. Our synthesized polymer has a molecular weight of 68 000, 30/70 styrene to butadiene ratio, 16 mol% c&-1,4, 29mol% trans-1,4, and 55 mol% 1,2polybutadiene microstructures. The amounts of 1,2- and 1,4-polybutadiene are approximately equal so as to facilitate the comparison of their degradation rates. Irganox 1076 (octadecyl 3-(3,5-di-rerj-butyl-4hydroxyphenyl) proprionate) was the antioxidant used in this work for the antioxidation studies. its molecular structure is shown in Fig. 1. 2.2 Methods 2.2. I FT/R characterization of SBS degradation
Fourier transform infrared spectroscopy characterization of the degradation of SBS was performed with a Shimadzu SSU-8101M instrument having a spectral resolution of 4cm ‘. SBS test samples were prepared by first dissolving an appropriate amount of polymer in cyclohexane to make a 30% solution, and then casting and drying this solution into a polymeric film of -0.5 mm thickness. The resulting film was then loaded into a specially designed IR cell which was connected to a vacuum/gas-handling man-
0
HO
ii
CH2CH2COC,fiHq7
+
Fig. 1. Molecular
structure
of antioxidant
Irpanox
1076.
of H,O
desorption for degraded SBS rubber
Prior to the TPD experiment, an SBS sample was degraded at 250°C under atmosphere for 2 h and then cooled to room temperature. A small amount (0.5 g) of this degraded sample was packed in a quartz tube. When the instrument equilibrated (i.e. at 30°C and 20 ml/min He flow rate), the desorption experiment was started by increasing the catalyst bed temperature from 40 to 250°C at a rate of lO”C/min. Evolved H,O was monitored by a thermal conductivity detector (TCD). For comparison, TPD was also carried out on a non-degraded SBS sample.
3 RESULTS
AND
DISCUSSION
3.1 FAIR characterization SBS rubber
of the degradation
of
The thermal degradation at 131°C of SBS polymer containing no antioxidants is shown by FTIR spectra in Fig. 2. Three major species have been generated during the degradation process. The hydroxyl group appears as a broad absorption band’.’ between 3200 and 3600cm ’ as indicated by peak A. No hydroxyl groups were observed prior to the thermal degradation, and their formation began approximately one hour into the experiment. The amount of hydroxyl groups increases with the degradation time. Peak B may well be assigned to the peroxide, carbonyl, or carboxylate group, since it does not exist before the degradation but only appears afterwards. As shown in the spectra, the intensities of both hydroxyl and carboxylate groups tend to reach asymptotic maximum values after 46 h of thermal degradation. The increasing intensity of the broad absorption band around 1300cm ’ (indicated by peak C) reflects the gel product due which frequently occurs for to crosslinking, polybutadiene polymers? In addition to these three generated species, the degradation can be quantified by monitoring the disappearance of two absorption peaks, i.e. the rrans-1,4 double bond at 967 cm ’ (peak D) and the vinyl-l,2 double bond at 911 cm ’ (peak E). The
53
Thermal oxidative degradation of SBS rubber
/
-0.500
I 4000.0
I
I
3600.0
2000.0
2800.0
I 1000.0
400.0
cm-1
Fig. 2. FTIR spectra of SBS polymer containing no antioxidants oxidatively degraded at 131°C (from bottom to top: 0 h. 4 h. I I h. 46 h): A-hydroxyls. B-carbonyls or carboxylates. C-crosslinked gels, D-l .4 double bonds, E-l 2 double bonds.
intensities of these two peaks decrease with the degradation time. 3.2 Mechanism
of thermal oxidation
The mechanism for the thermal oxidation of hydrocarbon polymers has been previously described as: ‘.-I heat or UV
cc CC.
+ 0,
cc00
l
CCOOH cc0
HO.
l
,CC.
+H.
+
CCOO.
+ CC ___) +
CCOOH + CC*
CCO.
CC +
+ CC +
+ HO. CCOH + CCe
H,O
+ CC.
further react with polymers to generate hydroperoxides (CCOOH). The hydroperoxides then decompose to form two new free radicals (CC00 and HO.) and the cycle repeats. The free radical reactions can be terminated via the combination of radical sites on adjacent polymer molecules forming carbon-carbon, peroxide or carbonyl crosslinks.‘.J Our experimental results as observed from FTIR were consistent with the proposed mechanism. The I,4 and 1.2 double bonds were oxidized by oxygen and peroxide. and carbonyl or carboxylate groups were formed. Although the exact microstructures of the degraded SBS cannot be determined by FTIR alone without resort to other characterization techniques, the formation of gel and hydroxyl groups was clearly shown in the FTIR spectrum (Fig. 2). 3.3 The reactivity
A polymeric free radical (CC.) is formed initially by exposure to heat or light. Rapid reaction of these radicals with oxygen follows forming peroxy
radicals
(CCOO*).
These
peroxy
radicals
of 1,4 and 1,2 double bonds
The extent of degradation was analyzed for different microstructures of the polybutadiene segments. As shown in Fig. 3, the content of
S.-M. Wmg et al.
54
.
.
I 20
1 (I
.
JO
Time
I x0
ho
(hr)
Fig. 3. Reduction of 1,2-polybutadiene during the thermal oxidative degradation of SBS polymer at 131°C.
vinyl-l,2 double bonds of the polymer degrading at 131°C decreases and reaches asymptotically 55% degradation. The 1,4 double bonds, shown achieve 82% degradation. in Fig. 4, however, Since our polymer contains approximately equal amounts of 1,4- and 1,2-microstructures initially, these data indicate that the 1,4-microstructure is easier to thermally degrade than the 1,2microstructure, suggesting that the 1,4-micro1,2in air than active structure is more microstructure. 3.4 Implication To better hydroxyl
of the hydroxyl groups
understand the formation and origin of the sample film, after being groups,
kept at 13 1°C for 72 h, was cooled down to a series of specified temperatures. At each of these lower specitied temperatures, the temperature was maintained for 15 min and was followed by FTIR inspection. After the specified temperature reached room temperature, the sample film was reheated again to 98°C. Again, the FTIR spectra were taken during the reheating process. The amount of hydroxyl groups calculated from FTIR spectra are shown in Fig. 5. It has been found that the amount of hydroxyl groups varies with the surrounding temperature and decreases with an increase of temperature. In addition. the variation of the amount of hydroxyl groups due to the temperature change appears reversible. Thus, it is conceivable that the hydroxyl groups come from two sources: (1) those from the termination of peroxy or alkoxy radicals by proton donation: and (2) those from H,O physically adsorbed onto the polymeric film. The latter accounts for the negative effect of temperature and the reversibility of the process and amounts to approximately 7-10% of total hydroxyls as shown in Fig. 5. The physical adsorption of H,O may be attributed to the various polar sites in the sample generated during the degradation, such as hydroxyl, carbonyl, and carboxylate functional groups. The comparison of TPD of H,O for the degraded SBS and non-degraded SBS samples is consistent with the inference drawn from FTIR results. As shown in Figs 6 and 7, the desorption of H,O for the degraded SBS sample starts at about 1 lO”C, and peaks at about 150, 180. and
. I 10
1
10
Time
,
,
60
x0
20
#I
I hll
I x0
Temperature
I Oil
I ?I)
I-II)
( ‘0
(hr)
Fig. 4. Reduction of 1.4-polybutadiene during the thermal oxidative degradation of SBS polymer at 131°C.
Fig. 5. Reversible changes function of temperature:
in the amount of hydroxyls as a 0, cooling process: 0. heating process.
Fig. 6. Temperature
programmed dqrnded
desorption
of H,O
for the
SBS polymer.
240”C, while no peaks were observed for the non-degraded SBS sample. These results indicate that the degraded SBS is hydrophilic, in contrast to the hydrophobic nature of non-degraded SBS. Aside from the hydrophilicity, those different desorption peaks characterizing the degraded SBS suggest that the adsorption of H,O occurs at different polar sites (groups). 3.5 Function
of antioxidant
For comparison, 1076 antioxidant
Fig. 7. Temperature
ma1 testing. Our results in Fig. 8 show that no evidence of thermal degradation was discerned even when the polymer was held at severe conditions such as 225°C for 12 h. The small peak at point A is from the hydroxyls in the antioxidant rather than those generated from polymer degradation. since raising the amount of lrganox to 3% does not help reduce the peak intensity. The polymeric film began melting when heated to and held at 235°C for one hour. and therefore no further temperature rise was attempted. These experimental results show clearly that the antioxidant inhibits the oxidation and thereby prevents the degradation of SBS rubber. The thermal degradation was also conducted in the absence of air. After putting the polymeric film into the IR cell tilled with nitrogen and carrying out the experiment at 131°C for 4X h, no polymer degradation was ever found, as shown in Fig. 9. Thus, this result shows that the propagation reaction during the thermal oxidative degradation cannot be activated without oxygen. Furthermore. the result corroborates the fact that the thermal oxidation is the main reason for the degradation of SBS rubber, and heat and UV light only prompt the formation of free radicals to initiate the chain reaction.
in SBS
the polymer with 1% Irganox added underwent similar ther-
programmed desorption non-degraded SBS polymer.
of H.0
i’or the
4 CONCLUSIONS
Thermal oxidative degradation of the polystyrene - block - polybutadicne - /dock - polystyrcnc thermoplastic elastomer (SBS rubber) hits been monitored in silt{ in a specially made inl’rarccl cell. The FTIR spectra enable us to study 111~ effects of various paramctcrs. IlillllCl~ tctllpcr~lture. air. antioxidant. and n~(~lcculilr microhtruclure of the polymer. on polvmcr tlcgr~~d;~lion. The 1.-Lpolybutadienc port& 01‘ the SW polymer is easier to degrade thiln the 1.7. polybutadiene portion. The degradation imparts polarity to the SBS polymer and converts its hydrophobicity to hydrophilicity. At low tcnperatures. moisture was adxorbccl onto Ihc polymer sample in an amount approximatcl~ 01’ 7-I()%, of total hydroxyls. This amount 01 physical adsorption varies with the tcmpcrature and the adsorption process ilppCill3 rcvcrsiblc. I \vt’% of Irganox 1076 (antioxidant) would IX
56
S.-M. Wmg et al. I
I
I
I
I
1
/ I
3.000,
A
c
2.000.
3660.0
4000.0
io.0
28bO.O
1000.0
201
400.0
cm -1
Fig. 8. FTIR
spectra of SBS polymer
containing
1% lrganox
1076 antioxidant
held at 225°C (from
bottom
to top: 0 h, 4 h. 12 h).
4.500
4.000
3.000
A
2.000
/ ._:...._..;_.,.__ ____
‘7 -T
-~__l_____________~_~,~.,“,-,-~,
I
I 3600.0
I
2800.0
I 1000.0
2000.0
400.0
cm-I
Fig. 9. FTIR
spectra of SBS polymer
held at 131°C and air free conditions
(from
bottom
to top: 0 h, 4 h, 12 h. 48 h).
Thermal oxidative degradation of SBS rubber
sufficient to thwart the thermal oxidation process even at as severe conditions as 225°C for 12 h. Thermal oxidative degradation does not occur without oxygen.
ACKNOWLEDGEMENT Financial support provided by the National Science Co&;1 of the Republid of China under the program NSC83-0405-E-194-001-T is greatly appreciated.
57
REFERENCES ,.
Jellinek, H. H. G. (ed.), Dqradution and Stuhilizrrtion of Po/ymrr.s, Vol. 1. Elsevier Science Publishers Inc.. New York. 1987. Klemchuk, P. P. (ed.), Polymer Strthilizutim md Dqyudation. Vol. 2X0, ACS Symposium Series. American Chemical Society, Washington DC, lY8S. .3. Patel. A.. Antioxidants: Modrrn Plmticr EmyVopdirr . McGraw-Hill. New York. 1084. 4. Pospisil. J., Drvrlopnwnt.s in Polwwr Stohilizcrtiorl-/ . ed. G. Scott. Applied Science Publishers, London. 1979. p. 1. 5. Klemchuk, P., Ullnmn’.s Emyloprdin of Im~ustritrl C’hrmistry. 5th edn, Vol. A3. VCH Publishers. Deerticld Beach, FL, 198.5, p. 91.
o,f Chemicul
(Received
Engineering,
18 September
N~~tiorurl Chung
Cheng University,
IYYS: accepted 26 October
Chiqi,
7iriwn
1995)
Thermal oxidative degradation of a polystyrene-hlock-polybutadiene-hlockpolystyrene thermoplastic elastomer (SBS rubber) has been conducted in an in-situ Infrared cell. By monitoring the disappearance of trctns-I .4 and vinyl- I ,2 double bonds and the appearance of the hydroxyl and the carboxyl/ carboxylate groups in the FTIR spectra, the temperature. air, antioxidant. and molecular microstructure dependence of the polymer degradation was studied. The experimental results indicate that the l.4-polybutadiene portion of the SBS polymer is easier to degrade than the 1.2-polybutadiene portion and the hydroxyl group appears concomitantly with the disappearance of the polybutadiene. Based on data from the temperature-programmed desorption (TPD) of HZO, it is concluded that the formation of hydroxyl group makes the polymer hydrophilic and promotes the H,O adsorption on it. The amount of H1O adsorption varies with the temperature and the process appears reversible. At low temperatures, the moisture adsorbed onto a degraded polymer sample amounts to approximately 7-10% of total hydroxyls. I wt% of lrganox 1076 (antioxidant) would effectively thwart the thermal oxidation process even at severe conditions as high as 225°C for 12 h. No degradation occur\ in ;IH air-free environment. 0 lYY6 Elsevier Science Limircd
1 INTRODUCTION The polystyrene-hlock-polybutadiene-block-polystyrene thermoplastic elastomers (SBS rubbers) tend to degrade by exposure to heat and UV light causing discoloration and surface embrittlement.‘.’ While the basic SBS rubber has two glass transition temperatures-an upper one of -93°C for the rigid polystyrene domain and a lower one of -65°C for the soft polybutadiene domain-various additives such as resins and plasticizers which modify the domains will alter these glass transition temperatures. In particular, resins which associate with the rigid polystyrene domain frequently increase the upper glass transition temperature. Therefore, SBS rubbers are often processed at temperatures higher than :::To whom correspondence
should be addressed.
120°C. At temperatures like these, degradation is likely to occur in the presence of air. Since SBS rubbers contain an unsaturated rubber midblock, the degradation mechanism is similar to SBR and polybutadiene rubbers.3.’ Its degradation arises from the generation of free radicals causing undesirable reactions of the double bonds in the polybutadiene segment. While various antioxidants are available to reduce the degradation,’ such as the use of hindered phenols to scavenge the free radicals and the use of phosphites to decompose the hydroperoxides, fundamental studies of the SBS degradation process and its associated phenomenon are seldom reported. Thus the purpose of this work is to study the temperature and air effects of SBS thermal oxidative degradation and its rate dependence on the molecular microstructure. FTIR (Fourier transform infrared spectroscopy) has been
extensively used to enable qualitative and quantitative analyses of the appearance/ disappearance of functional groups of the polymers. All polymer samples were thermally tested within a temperature range of 70-250°C.
ifold allowing an in-situ analysis. Depending upon reaction requirement, the analyses can be done in vacuum, under air, or in the presence of nitrogen only. 2.2.2 Temperature-programmed (TPD)
2 EXPERIMENTAL 2.1 Materials
The SBS polymer containing no antioxidants was anionically synthesized in cyclohexane using n-butyllithium as the initiator in the presence of polar structure modifier THF. The amount of THF was such that the polybutadiene segment of the resulting polymer should contain significant amounts of 1,4- and 1,2-microstructures. Our synthesized polymer has a molecular weight of 68 000, 30/70 styrene to butadiene ratio, 16 mol% c&-1,4, 29mol% trans-1,4, and 55 mol% 1,2polybutadiene microstructures. The amounts of 1,2- and 1,4-polybutadiene are approximately equal so as to facilitate the comparison of their degradation rates. Irganox 1076 (octadecyl 3-(3,5-di-rerj-butyl-4hydroxyphenyl) proprionate) was the antioxidant used in this work for the antioxidation studies. its molecular structure is shown in Fig. 1. 2.2 Methods 2.2. I FT/R characterization of SBS degradation
Fourier transform infrared spectroscopy characterization of the degradation of SBS was performed with a Shimadzu SSU-8101M instrument having a spectral resolution of 4cm ‘. SBS test samples were prepared by first dissolving an appropriate amount of polymer in cyclohexane to make a 30% solution, and then casting and drying this solution into a polymeric film of -0.5 mm thickness. The resulting film was then loaded into a specially designed IR cell which was connected to a vacuum/gas-handling man-
0
HO
ii
CH2CH2COC,fiHq7
+
Fig. 1. Molecular
structure
of antioxidant
Irpanox
1076.
of H,O
desorption for degraded SBS rubber
Prior to the TPD experiment, an SBS sample was degraded at 250°C under atmosphere for 2 h and then cooled to room temperature. A small amount (0.5 g) of this degraded sample was packed in a quartz tube. When the instrument equilibrated (i.e. at 30°C and 20 ml/min He flow rate), the desorption experiment was started by increasing the catalyst bed temperature from 40 to 250°C at a rate of lO”C/min. Evolved H,O was monitored by a thermal conductivity detector (TCD). For comparison, TPD was also carried out on a non-degraded SBS sample.
3 RESULTS
AND
DISCUSSION
3.1 FAIR characterization SBS rubber
of the degradation
of
The thermal degradation at 131°C of SBS polymer containing no antioxidants is shown by FTIR spectra in Fig. 2. Three major species have been generated during the degradation process. The hydroxyl group appears as a broad absorption band’.’ between 3200 and 3600cm ’ as indicated by peak A. No hydroxyl groups were observed prior to the thermal degradation, and their formation began approximately one hour into the experiment. The amount of hydroxyl groups increases with the degradation time. Peak B may well be assigned to the peroxide, carbonyl, or carboxylate group, since it does not exist before the degradation but only appears afterwards. As shown in the spectra, the intensities of both hydroxyl and carboxylate groups tend to reach asymptotic maximum values after 46 h of thermal degradation. The increasing intensity of the broad absorption band around 1300cm ’ (indicated by peak C) reflects the gel product due which frequently occurs for to crosslinking, polybutadiene polymers? In addition to these three generated species, the degradation can be quantified by monitoring the disappearance of two absorption peaks, i.e. the rrans-1,4 double bond at 967 cm ’ (peak D) and the vinyl-l,2 double bond at 911 cm ’ (peak E). The
53
Thermal oxidative degradation of SBS rubber
/
-0.500
I 4000.0
I
I
3600.0
2000.0
2800.0
I 1000.0
400.0
cm-1
Fig. 2. FTIR spectra of SBS polymer containing no antioxidants oxidatively degraded at 131°C (from bottom to top: 0 h. 4 h. I I h. 46 h): A-hydroxyls. B-carbonyls or carboxylates. C-crosslinked gels, D-l .4 double bonds, E-l 2 double bonds.
intensities of these two peaks decrease with the degradation time. 3.2 Mechanism
of thermal oxidation
The mechanism for the thermal oxidation of hydrocarbon polymers has been previously described as: ‘.-I heat or UV
cc CC.
+ 0,
cc00
l
CCOOH cc0
HO.
l
,CC.
+H.
+
CCOO.
+ CC ___) +
CCOOH + CC*
CCO.
CC +
+ CC +
+ HO. CCOH + CCe
H,O
+ CC.
further react with polymers to generate hydroperoxides (CCOOH). The hydroperoxides then decompose to form two new free radicals (CC00 and HO.) and the cycle repeats. The free radical reactions can be terminated via the combination of radical sites on adjacent polymer molecules forming carbon-carbon, peroxide or carbonyl crosslinks.‘.J Our experimental results as observed from FTIR were consistent with the proposed mechanism. The I,4 and 1.2 double bonds were oxidized by oxygen and peroxide. and carbonyl or carboxylate groups were formed. Although the exact microstructures of the degraded SBS cannot be determined by FTIR alone without resort to other characterization techniques, the formation of gel and hydroxyl groups was clearly shown in the FTIR spectrum (Fig. 2). 3.3 The reactivity
A polymeric free radical (CC.) is formed initially by exposure to heat or light. Rapid reaction of these radicals with oxygen follows forming peroxy
radicals
(CCOO*).
These
peroxy
radicals
of 1,4 and 1,2 double bonds
The extent of degradation was analyzed for different microstructures of the polybutadiene segments. As shown in Fig. 3, the content of
S.-M. Wmg et al.
54
.
.
I 20
1 (I
.
JO
Time
I x0
ho
(hr)
Fig. 3. Reduction of 1,2-polybutadiene during the thermal oxidative degradation of SBS polymer at 131°C.
vinyl-l,2 double bonds of the polymer degrading at 131°C decreases and reaches asymptotically 55% degradation. The 1,4 double bonds, shown achieve 82% degradation. in Fig. 4, however, Since our polymer contains approximately equal amounts of 1,4- and 1,2-microstructures initially, these data indicate that the 1,4-microstructure is easier to thermally degrade than the 1,2microstructure, suggesting that the 1,4-micro1,2in air than active structure is more microstructure. 3.4 Implication To better hydroxyl
of the hydroxyl groups
understand the formation and origin of the sample film, after being groups,
kept at 13 1°C for 72 h, was cooled down to a series of specified temperatures. At each of these lower specitied temperatures, the temperature was maintained for 15 min and was followed by FTIR inspection. After the specified temperature reached room temperature, the sample film was reheated again to 98°C. Again, the FTIR spectra were taken during the reheating process. The amount of hydroxyl groups calculated from FTIR spectra are shown in Fig. 5. It has been found that the amount of hydroxyl groups varies with the surrounding temperature and decreases with an increase of temperature. In addition. the variation of the amount of hydroxyl groups due to the temperature change appears reversible. Thus, it is conceivable that the hydroxyl groups come from two sources: (1) those from the termination of peroxy or alkoxy radicals by proton donation: and (2) those from H,O physically adsorbed onto the polymeric film. The latter accounts for the negative effect of temperature and the reversibility of the process and amounts to approximately 7-10% of total hydroxyls as shown in Fig. 5. The physical adsorption of H,O may be attributed to the various polar sites in the sample generated during the degradation, such as hydroxyl, carbonyl, and carboxylate functional groups. The comparison of TPD of H,O for the degraded SBS and non-degraded SBS samples is consistent with the inference drawn from FTIR results. As shown in Figs 6 and 7, the desorption of H,O for the degraded SBS sample starts at about 1 lO”C, and peaks at about 150, 180. and
. I 10
1
10
Time
,
,
60
x0
20
#I
I hll
I x0
Temperature
I Oil
I ?I)
I-II)
( ‘0
(hr)
Fig. 4. Reduction of 1.4-polybutadiene during the thermal oxidative degradation of SBS polymer at 131°C.
Fig. 5. Reversible changes function of temperature:
in the amount of hydroxyls as a 0, cooling process: 0. heating process.
Fig. 6. Temperature
programmed dqrnded
desorption
of H,O
for the
SBS polymer.
240”C, while no peaks were observed for the non-degraded SBS sample. These results indicate that the degraded SBS is hydrophilic, in contrast to the hydrophobic nature of non-degraded SBS. Aside from the hydrophilicity, those different desorption peaks characterizing the degraded SBS suggest that the adsorption of H,O occurs at different polar sites (groups). 3.5 Function
of antioxidant
For comparison, 1076 antioxidant
Fig. 7. Temperature
ma1 testing. Our results in Fig. 8 show that no evidence of thermal degradation was discerned even when the polymer was held at severe conditions such as 225°C for 12 h. The small peak at point A is from the hydroxyls in the antioxidant rather than those generated from polymer degradation. since raising the amount of lrganox to 3% does not help reduce the peak intensity. The polymeric film began melting when heated to and held at 235°C for one hour. and therefore no further temperature rise was attempted. These experimental results show clearly that the antioxidant inhibits the oxidation and thereby prevents the degradation of SBS rubber. The thermal degradation was also conducted in the absence of air. After putting the polymeric film into the IR cell tilled with nitrogen and carrying out the experiment at 131°C for 4X h, no polymer degradation was ever found, as shown in Fig. 9. Thus, this result shows that the propagation reaction during the thermal oxidative degradation cannot be activated without oxygen. Furthermore. the result corroborates the fact that the thermal oxidation is the main reason for the degradation of SBS rubber, and heat and UV light only prompt the formation of free radicals to initiate the chain reaction.
in SBS
the polymer with 1% Irganox added underwent similar ther-
programmed desorption non-degraded SBS polymer.
of H.0
i’or the
4 CONCLUSIONS
Thermal oxidative degradation of the polystyrene - block - polybutadicne - /dock - polystyrcnc thermoplastic elastomer (SBS rubber) hits been monitored in silt{ in a specially made inl’rarccl cell. The FTIR spectra enable us to study 111~ effects of various paramctcrs. IlillllCl~ tctllpcr~lture. air. antioxidant. and n~(~lcculilr microhtruclure of the polymer. on polvmcr tlcgr~~d;~lion. The 1.-Lpolybutadienc port& 01‘ the SW polymer is easier to degrade thiln the 1.7. polybutadiene portion. The degradation imparts polarity to the SBS polymer and converts its hydrophobicity to hydrophilicity. At low tcnperatures. moisture was adxorbccl onto Ihc polymer sample in an amount approximatcl~ 01’ 7-I()%, of total hydroxyls. This amount 01 physical adsorption varies with the tcmpcrature and the adsorption process ilppCill3 rcvcrsiblc. I \vt’% of Irganox 1076 (antioxidant) would IX
56
S.-M. Wmg et al. I
I
I
I
I
1
/ I
3.000,
A
c
2.000.
3660.0
4000.0
io.0
28bO.O
1000.0
201
400.0
cm -1
Fig. 8. FTIR
spectra of SBS polymer
containing
1% lrganox
1076 antioxidant
held at 225°C (from
bottom
to top: 0 h, 4 h. 12 h).
4.500
4.000
3.000
A
2.000
/ ._:...._..;_.,.__ ____
‘7 -T
-~__l_____________~_~,~.,“,-,-~,
I
I 3600.0
I
2800.0
I 1000.0
2000.0
400.0
cm-I
Fig. 9. FTIR
spectra of SBS polymer
held at 131°C and air free conditions
(from
bottom
to top: 0 h, 4 h, 12 h. 48 h).
Thermal oxidative degradation of SBS rubber
sufficient to thwart the thermal oxidation process even at as severe conditions as 225°C for 12 h. Thermal oxidative degradation does not occur without oxygen.
ACKNOWLEDGEMENT Financial support provided by the National Science Co&;1 of the Republid of China under the program NSC83-0405-E-194-001-T is greatly appreciated.
57
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