In this study we examined the inﬂuence of laurolactam on the thermo-oxidative degradation of a copolymer of poly(butylene
terephthalate) and polybutylene glycol and of di(ethyleneglycol) dibenzoate, used as model compound. The degradation was
followed by measurement of the oxygen uptake, FTIR and 1H-NMR spectroscopy and measurement of the intrinsic viscosity
changes during degradation. A stabilizing effect of laurolactam on the thermo-oxidative degradation could be observed, as an
induction time was deﬁned in which hydroperoxides were not formed and molecular weight was kept constant. After this period the oxygen uptake, hydroperoxide formation and polymer conversion increased more slowly

Botelho, Gabriela

Gijsman, P.

Machado, A. V.

Queirós, Arlete

Polymer Degradation and Stability

Universidade do Minho: RepositoriUM

Polymer Degradation and Stability 85 (2004) 991e995www.elsevier.com/locate/polydegstabLaurolactam as a stabilizer against the thermo-oxidativedegradation of poly(ether-esters)*G. Botelhoa,), P. Gijsmanb, A.V. Machadoc, A. Queiro´saaDepartamento de Quı´mica, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, PortugalbDSM Research BV, PO Box 18, 6160 MD Geleen, The NetherlandscDepartamento de Engenharia de Polı´meros, Universidade do Minho, Campus de Azure´m, 4800-058 Guimara˜es, PortugalReceived 28 October 2002; accepted 25 April 2003AbstractIn this study we examined the inﬂuence of laurolactam on the thermo-oxidative degradation of a copolymer of poly(butyleneterephthalate) and polybutylene glycol and of di(ethyleneglycol) dibenzoate, used as model compound. The degradation wasfollowed by measurement of the oxygen uptake, FTIR and 1H-NMR spectroscopy and measurement of the intrinsic viscositychanges during degradation. A stabilizing eﬀect of laurolactam on the thermo-oxidative degradation could be observed, as aninduction time was deﬁned in which hydroperoxides were not formed and molecular weight was kept constant. After this period theoxygen uptake, hydroperoxide formation and polymer conversion increased more slowly. 2004 Elsevier Ltd. All rights reserved.Keywords: Poly(ether-esters); Thermo-oxidative degradation; Stabilization; Laurolactam1. IntroductionPolymers are very diﬀerent as far as their inherentsensitivity to oxidation is concerned. Generally, diﬀer-ences in stability arise from a variety of chemicalstructures, manufacturing processes and polymer mor-phology [1]. Poly(ether-esters) are very sensitive tooxidative degradation due to the ether soft segment.The mechanism of the thermo-oxidative degradation ofthese polymers involves a radical chain process withformation of hydroperoxides at the carbon adjacent tothe ether oxygen. The hydroperoxides undergo furtherreactions leading to new esters and to oxidized productsformed after chain scission, such as formates, acids andalcohols [2,3].* Based on a paper presented at the 2nd international Conferenceon Modiﬁcation, Degradation and Stabilization of Polymers(MoDeSt), Budapest, 30 Junee4 July 2002.) Corresponding author. Tel.: C351-253-604-058; fax: C351-253-678-983.E-mail address: gbotelho@quimica.uminho.pt (G. Botelho).0141-3910/$ - see front matter  2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.polymdegradstab.2003.04.006By the addition of radical scavengers, such asphenolic antioxidants and aromatic amines the longterm heat stability of poly(ether-esters) can be in-creased [4]. Recently, the interaction of a phenolic anti-oxidant and an aromatic amine, was studied usingpoly(tetramethylene oxide) as a mimicking solvent[5,6]. Besides, with well known antioxidants it is alsopossible to increase the long term heat stability ofpoly(ether-esters) with compounds containing an amidefunctionality [7,8]. It is suggested that these amidesfunction by scavenging formaldehyde or formic acidand, in this way, preventing acidolysis [7] or by de-activation of antioxidants [8]. To get a better under-standing of the mechanism of action of amides inpoly(ether-esters), in this paper results are presented onthe degradation of this polymer and one of its modelcompounds (DEGDB) in the presence of laurolactam,an aliphatic amide. As degradation methods oxygenuptake, hydroperoxide formation, intrinsic viscositymeasurements and changes in the FTIR and NMRspectra during ageing were used [9].992 G. Botelho et al. / Polymer Degradation and Stability 85 (2004) 991e9952. ExperimentalLaurolactam (C12H23NO) was obtained from FLU-KA chemicals and used as received. Diethyleneglycoldibenzoate (DEGDB) was supplied by Aldrich (struc-ture in Fig. 1). The polymer studied was a blockcopolymer of poly(butylene terephthalate) and poly-butylene glycol (PBT/PBO) supplied by DSM. Laur-olactam was mixed with DEGDB at a 4% (w/w)concentration. The polymer ﬁlm was immersed ina saturated solution of laurolactam in pure methanolfor 24 h. All samples were vacuum dried for 24 h beforedegradation.Degradation experiments were carried out in an oilbath at 100 (C in a closed vessel ﬁlled with oxygen, asdescribed elsewhere [10]. Two experiments have beencarried out independently and the results obtained inboth cases were reproducible. The oxygen uptake wascalculated from the pressure drop and thus, notcorrected for the evolution of gases, a procedure thatproved to be unnecessary for oxygen uptake valuesbelow 1800 mmol/kg [11]. At various degradation timessamples were taken and characterized using the analyt-ical techniques indicated below.1H-NMR spectra were recorded on a Varian 300MHz spectrometer using deuterated chloroform assolvent and tetramethylsilane as internal standard.FTIR spectra were recorded on a Bomen spectropho-tometer. The absorptions were corrected for baselinevariations by subtracting the absorption at 2500 cm1.The intrinsic viscosity was determined using an Ubbe-lohde capillary viscometer with 5 mg ml1 solutions inchloroform at 25.0G 0.5 (C.3. Results and discussion3.1. Model compoundInterruption of the experiment for sampling had noeﬀect on the course of the oxidation, in agreement withthe results obtained by Gijsman et al. [11]. Oxygenuptake results for the model compound are shown inFig. 2. An induction period of ca. 24 h was observed forthe stabilized model compound, while for the unstabi-lized model compound oxygen is consumed right fromthe beginning. After the induction period a signiﬁcantFig. 1. Structure of the model compound DEGDB.diﬀerence in the oxygen uptake rates for the unstabilized(2.12 mmol kg1 h1) and stabilized model compound(1.53 mmol kg1 h1) was observed.In spite of the eﬀectiveness of laurolactam inreduction of the oxygen consumed, yellowing of thestabilized samples was observed after 266 h at 100 (C.This can be explained in terms of the formation of anactive chromophore after oxidation of laurolactam.The molecular structure of yellowing products couldnot be assessed on the basis of NMR and FTIR spec-troscopy due to the lack of sensitivity of these tech-niques [12].The 1H-NMR spectra of stabilized and unstabilizedDEGDB degraded for 338 h are shown in Figs. 3a,b,respectively. The signals at d 3.90 and 4.52 ppmcorrespond to the H1 and H2 protons, respectively.The observed decrease in the intensity of both peaks ishigher for the unstabilized sample, being more evidentfor the H1 protons in both cases. Besides, for thestabilized sample, new peaks associated with oxidationproducts show lower intensities. This is true for thesignals at d 4.56 and 4.91 ppm corresponding to themain oxidation products, previously identiﬁed asa formate and a new ester, respectively [10]. Theseresults are consistent with the previously postulatedmechanism for the thermo-oxidative degradation ofpoly(ether-esters).Fig. 4 shows the conversion of the model compound,calculated from the ratio of the intensities of the H1signals of degraded and non-degraded samples, atd 3.90 ppm. After the induction period, the conversionfor the stabilized compound was always lower than thatof the unstabilized compound, in agreement with theoxygen uptake curve (Fig. 2). Moreover, for the sameoxygen uptake, the stabilized compound shows lowerconversions.3.2. Polymer ﬁlmThe oxygen uptake curves for the polymer ﬁlm areshown in Fig. 5. For the stabilized polymer ﬁlm anFig. 2. Oxygen uptake of diethyleneglycol dibenzoate thermo-oxidizedat 100 (C vs. time: (C) stabilized; and (¤) unstabilized.993G. Botelho et al. / Polymer Degradation and Stability 85 (2004) 991e995Fig. 3. 1H-NMR in CDCl3 of diethyleneglycol dibenzoate thermo-oxidized during 338 h at 100 (C: (a) stabilized; and (b) unstabilized.994 G. Botelho et al. / Polymer Degradation and Stability 85 (2004) 991e995induction period of ca. 1.25 h was observed and theoxygen uptake rate was much lower than that of theunstabilized polymer ﬁlm. The same was previouslyobserved for the model compound (Fig. 2).It is known that, degradation of poly(ester-ethers)leads to the formation of secondary hydroperoxides[10,13] that are intermediates in the mechanism leadingFig. 4. Conversion of diethyleneglycol dibenzoate thermo-oxidized at100 (C vs. time: (C) stabilized; and (¤) unstabilized.to other oxidation products, such as aromatic carboxylicacids, formates and alcohols [2]. Figs. 6a,b show theFTIR spectra, in the 4000e2000 cm1 region, of thestabilized and unstabilized polymer ﬁlms, respectively.The formation of hydroperoxides is demonstrated bythe increasing absorptions observed in the 3600e3100cm1 region and previously conﬁrmed by the nitricFig. 5. Oxygen uptake of the polymer ﬁlm thermo-oxidized at 100 (Cvs. time: (C) stabilized; and (¤) unstabilized.Fig. 6. FTIR spectra of the polymer ﬁlm: (a) stabilized; and (b) unstabilized.995G. Botelho et al. / Polymer Degradation and Stability 85 (2004) 991e995oxide treatment [14]. Fig. 6a shows that no changes inthe spectra were observed up to 1.25 h and that theabsorbance after 3 h oxidation was ca. 0.613. In Fig. 6ban increase in the absorption is observed from thebeginning reaching an absorbance of 0.928 after 3 h. So,when laurolactam is present an induction period isobserved and less hydroperoxide is formed in the sametime period.The occurrence of chain scission or cross-linkingduring degradation was evaluated by measurements ofthe intrinsic viscosity of the stabilized and unstabilizedpolymer ﬁlms with diﬀerent degradation times. Theresults are shown in Fig. 7. For the unstabilizedpolymer ﬁlm the observed decrease in intrinsic viscosityindicates that chain scissions occur from the beginningof the degradation. For the stabilized polymer ﬁlma constant value of the intrinsic viscosity could beobserved, for 1.25 h, prior to a signiﬁcant decrease.However, the value obtained after 3 h is higher thanthat obtained for the unstabilized polymer ﬁlm. Ascould be expected, the change of the intrinsic viscosityalong degradation follows that of the oxygen uptake.No chain breaking is observed before hydroperoxidescould be formed.In this study no other stabilizers were present, whichmeans that the mechanism postulated by Hoeschele [8](deactivation of antioxidants) cannot explain the mech-anism of action of laurolactam in this study. Preventingacidolysis can also not explain the present resultsbecause this should only be visible for the viscositymeasurements and not for the oxygen uptake andhydroperoxide data. From this study it is clear thatamides function by preventing the polymer fromoxidation.Fig. 7. Intrinsic viscosity of the polymer ﬁlm thermo-oxidized at100 (C vs. time: (C) stabilized; and (¤) unstabilized.4. ConclusionsThe results described herein show clearly that laur-olactam has a stabilising eﬀect on the thermo-oxidativedegradation of poly(ether-esters). Amides in poly(ether-esters) do not function by preventing acidolysis ordeactivation of stabilizers. In the presence of laurolactaman induction period could be observed. Concomitantly,hydroperoxides are not formed and no changes in themolecular weight are observed. After that period theoxidation rate, as well as hydroperoxide formation arereduced. The results reveal that laurolactam is able toprevent the polymer from oxidation.References[1] Hamid SH. Handbook of Polymer Degradation, Revised andExpanded. 2nd ed. Berlin: Marcel Dekker, Inc; 2000.[2] Botelho G, Gijsman P, Queiro´s A. A comparative study on thethermo-oxidative degradation of poly(ether-esters). Polym De-grad Stab 2001;73:431e5.[3] Botelho G, Gijsman P, Queiro´s A. Thermo-oxidative studies ofpoly(ether-esters) 2. Copolymer of poly(butylene terephthalate)and polybutylene oxide. Polym Degrad Stab 2000;68:35e42.[4] Zeilstra JJ. Heat stabilization of segmented copoly(ether-esters).Die Angew Mak Chem 1985;137:83e92.[5] Barret J, Gijsman P, Swagten J, Lange R. The interaction ofa phenolic anti-oxidant and an aromatic amine in a thermo-oxidative ageing process. Polym Degrad Stab 2002;75:367e74.[6] Barret J, Gijsman P, Swagten J, Lange R. A molecular studytowards the interaction of phenolic anti-oxidants, aromaticamines and HALS stabilisers in a thermo-oxidative ageingprocess. Polym Degrad Stab 2002;76:441e8.[7] Gugumus F. In: Ga¨chter R, Mu¨ller H, editors. Polymer Additives,3rd ed. Munich, Vienna, New York: Hanser Publisher; 1990.p. 1e104.[8] Hoeschele GK. Die Angew Mak Chemie 1977;58/59:299e319.[9] Zweifel H. Stabilization of Polymeric Materials. Berlin: SpringerVerlag; 1998.[10] Botelho G, Gijsman P, Queiro´s A. Thermo-oxidative studies ofpoly(ether-esters) 1. Copolymer of poly(butylene terephthalate)and polyethylene oxide. Polym Degrad Stab 2000;67:13e20.[11] Gijsman P, Verdun F. The inﬂuence of polymer type, stabilizersand sample geometry on the relationship between chemilumines-cence and oxygen uptake. Polym Degrad Stab 2001;74:533e42.[12] Lemaire J, Arnaud R, Gardette JL. Low temperature thermo-oxidation of thermoplastics in the solid state. Polym Degrad Stab1991;33:277e94.[13] Tabankia MH, Philippart JL, Gardette JL. Photo-oxidation ofblock copoly(ether-ester) thermoplastic elastomers. Polym DegradStab 1985;12:349e62.[14] Carlsson DJ, Lacoste J. A critical comparison of methods forhydroperoxide measurement in oxidized polyoleﬁns. Polym De-grad Stab 1991;32:377e86.

Laurolactam as a stabilizer against the thermo-oxidative degradation of poly(ether-ester)

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Laurolactam as a stabilizer against the thermo-oxidative degradation of poly(ether-ester)

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