Antioxidative activity of 3-aryl-benzofuran-2-one stabilizers(Irganox1HP-136) in polypropylene

Alexander Mar’ina, Lucedio Grecib,*, Paul Dubsc

aDepartment of Bioengineering, University of Utah, 2480 MEB, Salt Lake City, UT 84112, USAbDipartimento di Scienze dei Materiali e della Terra, Universita degli Studi di Ancona, Via Brecce Bianche, I-60131, Ancona, Italy

cCiba Specialty Chemicals Inc.,CH-4002, Basel, Switzerland

Received 29 November 2001; received in revised form 18 January 2002; accepted 27 January 2002

Abstract

Irganox1HP-136(lactone), which is a mixture of 90% of 5,7-di-tert-butyl-3-(3,4-dimethylphenyl)3H-benzofuran-2-one and 10%of 5,7-di-tert-butyl-3-(2,3-dimethylphenyl)3H-benzofuran-2-one behaves as a medium strength chain-breaking antioxidant during

polypropylene oxidation at 180 and 200 �C. The critical concentration (vide infra) required to inhibit oxidation is higher than thatof the phenolic antioxidant 2246. The lactone is slowly consumed during the induction period and then much faster when the cri-tical concentration is reached. Phosphites, sulphides and phenols increase the efficiency of the lactone during polymer oxidation.The use of lactone allows the amount of phenolic and phosphorous containing stabilizers to decrease without decreasing the ther-

mooxidative stability of the polymer. # 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Irganox1HP-136; Lactone; Polypropylene; Oxidation; Stabilization; Synergism

1. Introduction

Recently, Irganox1HP-136 (a mixture of new 3-aryl-benzofuran-2-ones) has been used as an excellent stabi-lizer during polymer processing at high temperaturesunder oxygen deficient extrusion conditions. In practice,only small concentrations of lactone (0.005–0.01%) arerequired for stabilization when used in combinationwith phenols and phosphites [1–6]. Lactones gave goodresults during ‘‘controlled’’ degradations of poly-propylene, yielding polymers with a higher melt flowrate and a narrow molecular weight distribution. Sys-tems based on benzofuranones provide equivalent meltprocessing performances at significantly reduced con-centrations compared to those based on combinationsof phenols and phosphites alone [3]. Their stabilizingeffect is explained by the formation of stable benzofur-anonyl radicals, which can react with alkyl radicals ter-minating chain reactions [1–3]. Unfortunately, very little isknown on the formal kinetics of oxidation of polyolefinsin the presence of lactones at elevated temperatures.

In this work, the antioxidant activity of the isomericmixture Irganox1HP-136 and its combination withother stabilizers during polypropylene oxidation at 180and 200 �C were studied.

2. Experimental

2.1. Materials

Isotactic PP powder (Moplen FL F20) was purified bywashing with boiled ethanol and then dried undervacuum. Irganox1HP-136, which is a mixture of about90% of 5,7-di-tert-butyl-3-(3,4-dimethylphenyl)3H-ben-zofuran-2-one and about 10% of 5,7-di-tert-butyl-3-(2,3-dimethylphenyl)3H-benzofuran-2-one (lactone), 2,4,6-tri-tert-butylphenol (TTP), octadecyl-3-(3,5-di-tert-butyl-4-hydroxy-phenyl) propionate (Irganox11076), dilaurylthiodipropionate (DLTP) were supplied from CIBASpecialty Chemicals Inc.

2.2. Oxygen consumption measurements

The polymer powder was mixed with the appropriatequantity of antioxidants dissolved in ethanol or benzene,

0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(02 )00053-8

Polymer Degradation and Stability 76 (2002) 489–494

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.; +39-071-2204408; fax: +39-071-

2204714.

E-mail address: greci@mta01.unian.it (L. Greci).

and the mixture was kept in air until dryness. Since thepreparation took few minutes and the samples wereprepared in the same way, a partial oxidation of thelactone occurring during this step was neglected. Oxy-gen consumption was monitored measuring the pressuredecrease during heating at 180 and 200 �C using a glassequipment of 12 ml connected to an oil manometer[7,8]. Volatile products were removed by solid KOH.Each experiment was performed on 50 mg samples.

2.3. Lactone consumption during oxidation

Samples of 50 mg of PP powder containing always thesame amount of lactone were introduced in glass tubes(volume 5 ml). The air was removed from the tubes,replaced with oxygen and the tubes were then sealedunder 300 mm Hg pressure. The samples were heated at200 �C for different periods of time, then the tubes wereopened and the content dissolved in hot decane andprecipitated with methanol. The resulting solution wasthen analysed by GC–MS to measure the concentrationof lactone in solution. Samples with known concentra-tions of lactone were used as a reference.

3. Results and discussion

The rates of oxygen consumption at 200 �C by PPcontaining different concentrations of lactone are shownin Fig.1. The induction period (the time correspondingto slow oxidation) in the presence of small concentra-tions of lactone (0.002, 0.0057 mol/kg) is practically the

same as that of unstabilized PP; at higher lactone con-centrations, the induction period starts to increase.Fig. 2 shows the dependence of the induction periodversus the initial lactone concentration in the polymer at180 and 200 �C: the efficiency of the lactone at 180 ishigher than at 200 �C because of the instability of thepolymer at higher temperatures.It is well known that the efficiency of an antioxidant isreflected by its critical concentration, which is the lowestconcentration required for the inhibition of peroxida-tion [8]. The critical concentration of the lactone at200 �C is about 4–6 � 10�3 mol/kg and at 180 �C it is0.7 � 10�3 mol/kg.The consumption of lactone during polymer oxida-tion follows a first-order kinetics (Fig. 3a and b); devia-tion from first-order kinetics occurs when theconcentration of lactone decreases to 1–2 � 10�3 mol/kg, i.e. below the critical value (results not shown). The

490 A. Mar’in et al. / Polymer Degradation and Stability 76 (2002) 489–494

effective rate constant (k) of lactone consumption cal-culated from the slope of the curve in Fig. 3b is 4.3 �

10�4 s�1; whereas k for the phenolic antioxidant 2246 inthe same conditions is 2 � 10�4 s�1 [8].The stabilizing effect of lactone increases in the pre-sence of DLTP (Fig. 2): in fact, the induction periodsignificantly increases and the critical concentration ofthe lactone decreases. DLTP itself possesses a slightstabilizing effect: the induction period of PP oxidationin the presence of 0.01 mol/kg of DLTP is only 30 minwhich is a little higher than that of the unstabilizedpolymer. In order to better understand the behaviour ofthe lactone during PP oxidation, its efficiency was com-pared with other stabilizers (Table 1). The data pre-sented in Table 1 correspond to the experimentsperformed under the same experimental conditions,starting from the same molar concentrations and

measuring oxygen consumption in the same way. Themost efficient antioxidant among those listed in Table 1is 2246 (see the low critical concentration). The activityof lactone is close to that of 2246 when the lactone isused in the presence of DLTP. This behaviour is similarto that of the alkylated monophenol Irganox 1076 andthe 9,10-dihydro-9,9-diphenylacridine which do notexhibit a good stabilizing effect in the absence of thesulphide. On the other hand, the phenothiazine con-taining a sulphur atom in its structure, behaves like themonophenol and 9,10-dihydro-9,9-diphenylacridine inthe presence of DLTP.It is noteworthy that the lactone activity increases inthe presence of phenols as shown in Fig. 4. A mixture oflactone and TTP gives practically the same stabilizingeffect as the mixture of lactone and DLTP at the samemolar concentrations (Table 2).The activity of lactone at 180 and 200 �C was alsoevaluated in the presence of other stabilizers such asIrganox 1010, Irganox B215, Irgafos 168 and Tinuvin770. The results are reported in Tables 3 and 4. Theconcentrations of used stabilizers are 0.05 and 0.1%.

Fig. 1. Oxygen consumption during PP oxidation at 200 �C and

PO2=300 mm Hg in the presence of different concentrations of lac-

tone.

Fig. 2. The induction period of PP oxidation as a function of initial

concentration of lactone at 180 and 200 �C and PO2=300 mm Hg in

the absence and in the presence of 0.01 mol/kg DLTP.

Fig. 3. Consumption of lactone during PP oxidation at 200 �C and

PO2=300 mm Hg in coordinates (a) io vs t and (b) logio vs t.

A. Mar’in et al. / Polymer Degradation and Stability 76 (2002) 489–494 491

Table 3 shows that the efficacy of lactone and IrganoxB215 when used separately is low: the induction periodof PP oxidation is close to the induction period ofunstabilized polymer. This is because the concentrationof stabilizers was below or close to their critical con-centrations (i.e. 0.05% of lactone corresponds to1.4�10�3 mol/kg while the critical concentration is 4–6� 10�3 mol/kg). When both stabilizers are used toge-ther, the induction period is considerably higher. How-ever, addition of Tinuvin 770 to the mixture of IrganoxB215 and lactone resulted in an unexpected smalldecrease in the induction period. At the moment, thisresult is unclear and needs further investigation but it isin agreement with the findings of Kohler et al. whichobserved shortening of induction periods as measuredby chemiluminescence, using sterically hindered amines[9].

Table 4 shows that lactone increases the stability ofPP during oxidation at 180 �C when used in combina-tion with other stabilizers, such as Irganox 1010 andIrgafos 168. From the data reported in Table 4, it is notclear whether there actually is synergism between lac-tone and phenol or phosphite. In order to obtain thebest performance for lactone/Irganox 1010 and lactone/Irgafos 168, the thermal oxidation of PP was studiedagainst the ratio of the couple of the mentioned stabi-lizers. The results of the experiments are given in Figs. 5and 6: the mixture containing 25% of lactone and 75%of Irganox 1010 (Fig. 5) and the mixture containing atleast 50% of lactone in a mixture with Irgafos 168(Fig. 6) gave the best stabilizing effects. These resultsshow that lactone allows to decrease the concentrationof phenolic and phosphorous containing stabilizerswithout affecting the polymer stability.

Table 1

Parameters of PP oxidation in the presence of different stabilizers at T=200 �C

Additive i cr �103 mol/kg Induction time (min)

at io=0.01 mol/kg

Reference

9,10-Dihydro-9,9-diphenylacridine No 10–12 7

9,10-Dihydro-9,9-diphenylacridine

+DLTP (0.01 mol/kg)

2–3 290 7

Phenothiazine 2–3 145 7

Irganox 1076 – 20 This work

Irganox 1076

+DLTP (0.01 mol/kg)

– 140 This work

Lactone

Lactone+DLTP (0.01 mol/kg)

(4–6) 2.5 35 205 This work

Cyanox12246 1.2 230 8

2,20-Thiobis(4-methyl-6-tert-butylphenol) 3.0 258 8

492 A. Mar’in et al. / Polymer Degradation and Stability 76 (2002) 489–494

The results described above show that the kinetics ofpolymer oxidation in the presence of lactone are similarto the kinetics observed in the presence of typical chainbreaking antioxidants like, e.g. hindered phenols andaromatic amines. As mentioned above, lactone caneasily form benzofuranonyl radicals by cleavage of theweakly bonded benzylic hydrogen atom [4]. These radi-cals can reversibly form dimers or react with alkyl (R.),peroxy (RO2.) or alkoxy (RO.) radicals [1–3]. Becausethe concentration of oxygen is high in the experimentshere described, the RO2. radicals concentration is alsohigh and, therefore, these radicals are the responsiblespecies for the chain propagation reactions of polymeroxidation, and the benzofuranonyl radicals behave asRO2. radicals scavengers. The combination of lactonewith species decomposing hydroperoxides such asphosphites and sulphides, results in a synergistic effect,similar to that observed in the case of the mixture ofphenols or amines with phosphites and/or sulphides.Due to new indications that the mixture of lactone andphenols shows a synergistic effect which has alreadybeen discussed[1,2], it may be admitted that the lactonecould regenerate the phenol from the correspondingphenoxy radical formed during peroxidation.

Fig. 5. The induction period of PP oxidation in the presence of lac-

tone and Irganox 1010 at 180 �C and PO2=300 mm Hg as a function

of lactone/Irganox 1010 ratio (wt.%) in the mixture; total mixture

concentration is 0.2 wt .%.

Fig. 4. Oxygen consumption during PP oxidation at 200 �C and

PO2=300 mm Hg in the presence of lactone, or TTP, or their mixture.

Table 3

Oxidation of PP in the presence of lactone Irganox 215 and Tinuvin

770. T=200 �C

Stabilizer Concentration (%) Induction time (min)

No stabilizer – 17

Irganox B215 0.1 30

Lactone 0.05 20

Irganox B215 0.1 103

Lactone 0.05

Irganox B215 0.1

Lactone 0.05 80

Tinuvin 770 0.05

Table 2

Effect of 2,4,6-tri-tert-butylphenol (TTP), DLTP and lactone on PP

oxidation at 200 �C

Stabilizer Concentration (mol/kg) Induction time, min

No stabilizer – 12

Lactone 0.02 70

Lactone 0.02 200

TTP 0.01

TTP 0.01 20

Lactone 0.02 270

DLTP 0.01

DLTP 0.01 30

Table 4

Oxidation of PP in the presence of lactone and different stabilizers.

T=180 �C

Additives Concentration (wt.%) Induction time (min)

Lactone 0.1 35

DLTP 0.1 30

Irganox 1010 0.1 25

Irgafos 168 0.1 35

Lactone 0.1

Irganox 1010 0.1 100

Lactone 0.1

DLTP 0.05 53

Lactone 0.1

DLTP 0.1 65

Lactone

Irgafos 168 0.05 62

Lactone 0.1

Irgafos 168 0.1 71

Lactone 0.1

Tinuvin 770 0.05 48

A. Mar’in et al. / Polymer Degradation and Stability 76 (2002) 489–494 493

It is well known that the activity of an antioxidant,which works as a chain-breaking donor, depends mainlyon its hydrogen donating power and, therefore, in thecase of phenols, on the bond dissociation energy (BDE)of the O–H bond. The results described above demon-strate that Irganox 1010 and TTP at the same con-centration show very close antioxidant activities. This isin agreement with their O–H BDE, which is for bothcompounds, 81 kcal/mol [10,11]. Because the lactoneshows a slightly better performance than Irganox 1010and TTP (vide infra), it may be assumed that the lactoneC-H BDE is one or two kcal/mol lower than that of thetwo considered phenolic antioxidants.

4. Conclusions

The kinetics of oxygen consumption during PP oxi-dation show that lactone itself behaves as a mediumstrength chain-breaking antioxidant by reacting withperoxyl radicals. The efficiency of lactone is con-siderably higher in the presence of hindered phenols,

phosphites and sulphides. This approach allows todecrease the concentration of phenolic and phosphor-ous containing stabilizers in the polymer when usedtogether with lactone or even to eliminate the phenolicstabilizer and in this way the colour development duringoxidation may be reduced or cancelled.

Acknowledgements

The authors thank Ciba Specialty Chemicals Inc.(Basel) for financial support.

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Fig. 6. The induction period of PP oxidation in the presence of lac-

tone and Irgafos 168 at 180 �C and PO2=300 mm Hg as a function

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494 A. Mar’in et al. / Polymer Degradation and Stability 76 (2002) 489–494