Long-Term Properties of Polyethylene Films – Efficiency of a Natural Antioxidant Clara Strandberg Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan framlägges till offentlig granskning för avläggande av teknisk doktorsexamen fredagen den 2 juni 2006 kl.13.00 i sal K2, Kungliga Tekniska Högskolan, Teknikringen 28, Stockholm. Avhandlingen försvaras på engelska. Abstract There is a growing awareness of the risks of pollution in biological systems and one potential problem is the synthetic antioxidants, used for e.g. the stabilisation of polymeric materials. Natural antioxidants are an interesting alternative, if the high efficiency and thermal stability of the synthetic compounds can be reached. In the work described in this thesis, vitamin E (α-tocopherol) was studied as a natural antioxidant for the stabilisation of one of the major plastics, polyethylene (PE). The dependence of the surrounding environment for the efficiency of α-tocopherol in polyethylene (PE), throughout thermal aging, was characterised by sensitive techniques. Two techniques which have shown a high sensitivity in oxidation detection of polymers; chemiluminescence (CL) and gas chromatographic analysis, were compared with the commonly used methods, infrared spectroscopy (FT-IR) and thermal analysis. Three different additive systems were selected as active domains for α-tocopherol in PE. Two of these contained carboxylic acid groups, poly (ethylene-co-acrylic acid) (EAA) and polyTRIM/PAA core-shell particles (Core), and the third, oat starch, had no such groups. The additives containing carboxylic groups improved the long-term efficiency of α-tocopherol in PE, according to carbonyl index measurements made by FT-IR, while the additive without carboxylic acid groups gave no improvement. The amount of carboxylic acids emitted from the materials after thermal aging, assessed by head-space solid-phase microextraction (HS-SPME) and gas chromatography-mass spectroscopy (GC-MS), also showed that EAA increased the antioxidant efficiency of α-tocopherol, whereas the Core system showed lower antioxidant efficiency. Reference systems containing the synthetic antioxidant Irganox 1076 and EAA or oat starch had the same performance as the materials stabilised with only the antioxidants. CL measurements in an inert atmosphere (TLI) have earlier been shown to give earlier oxidation detection than carbonyl index measurements in unstabilised PE. In this work, the TLI analysis and the carbonyl index measurements had the same sensitivity in the detection of oxidation in the stabilised materials. Assessment of low-molecular weight carboxylic acids in PE during the aging was made by gas chromatographic analysis together with solid-phase extraction. Propanoic acid showed the best correlation with the carbonyl index measurements, even if the carbonyl index showed earlier detection of oxidation. It was also found that TLI and CL in an oxidative atmosphere (CL-OIT) had the same sensitivity and were in accordance for all of the materials, with exception of the materials containing EAA and α-tocopherol or Irganox 1076. CL-OIT was also compared to the oxygen induction time determined by thermal analysis. Keywords: α-tocopherol, Polyethylene (PE), Thermal aging, Infrared Spectroscopy (FT- IR), Chemiluminescence (CL), Solid-phase microextraction (SPME) Svensk sammanfattning I ett hållbart samhälle krävs inte bara förnyelsebara resurser utan även biologiska system fria från föroreningar. Ett potentiellt problem är syntetiska antioxidanter, vilka används för att stabilisera polymera material. Naturliga antioxidanter är ett intressant alternativ, ifall de syntesiska antioxidanternas höga effektivitet och goda termiska stabilitet kan uppnås. Denna avhandling är fokuserad på sätt att förbättra effektiviteten hos den naturliga antioxidanten vitamin E (α-tokoferol) i ett av de mest använda plastmaterialen, polyeten (PE). Den långsiktiga effektiviteten hos α-tokoferol och dess beroende av omgivningen i PE karakteriserades med känsliga tekniker. Kemiluminescens (CL) och gaskromatografisk analys jämfördes med de vanligtvis använda teknikerna, infraröd spektroskopi och termisk analys. Tre olika tillsatssystem valdes till aktiva domäner för α-tokoferol i PE. Två av dessa innehöll karboxylsyragrupper, poly(eten-co-akrylsyra) (EAA) och polyTRIM/PAA core- shell partiklar (Core), och den tredje, havrestärkelse, hade inga sådana grupper. Tillsatserna som innehöll karboxylsyragrupper förbättrade α-tokoferols långtids- egenskaper, enligt mätningar av karbonylindex, medan tillsatsen utan karboxylsyragrupper inte gav någon förbättring. Mängden av korta karboxylsyror emitterade från materialen efter termisk åldring, analyserade med headspace mikroextraktion (HS-SPME) och gaskromatografi-masspektrometri (GC-MS), visade också på att EAA ökade α-tokoferols effektivitet, medan Core-systemet hade lägre effektivitet av antioxidanten. CL i en inert atmosfär (TLI) hade likvärdig känslighet som karbonylindex för oxidationsdetektion i stabiliserad PE, då antioxidanternas verkan i de stabiliserade materialen minskade både ökningen av hydroperoxider och karbonyler i dessa material. I icke-stabiliserad PE och i två andra, svagt stabiliserad material, så visade emellertid TLI tidigare detektion av oxidationen än karbonylindex mätningarna. Enligt den gaskromatografiska analysen så var ökningen i mängd av karboxylsyrorna relaterad till långtidsegenskaperna i materialen och propansyra hade den bästa överensstämmelsen med den uppmätta karbonylökningen, med avseende på alla materialen. TLI resultaten jämfördes även med CL i en oxidativ atmosfär (CL-OIT). CL-OIT överensstämde med TLI för alla utom de två material som innehöll EAA. CL-OIT jämfördes även med OIT utförd med termisk analys. List of Papers This thesis is a summary of the following papers; referred to in the text by their Roman numerals. I Process Efficiency and Long-Term Performance of α-tocopherol in Film-Blown Linear Low-Density Polyethylene Clara Strandberg, Ann-Christine Albertsson Journal of Applied Polymer Chemistry, Vol. 98, 2427-2439, 2005. II Improvement of α-tocopherol’s Long-Term Efficiency by Modeling Its Heterogeneous Natural Environment in Polyethylene Clara Strandberg, Ann-Christine Albertsson Journal of Polymer Science Part A: Polymer Chemistry, Vol 44, 1660-1666, 2005. III Total Luminescence Intensity (TLI) Offers Superior Early Oxidation Detection in Unstabilised Polyethylene but is no better than FT-IR for Stabilised Polyolefins Clara Strandberg, Lina Burman, Ann-Christine Albertsson European Polymer Journal, In press (available online 29 March, 2006). IV Assessment of Carboxylic Acids from Water-Aged and Air-Aged Polyethylene by Head-Space Solid-Phase Microextraction (HS-SPME) and their correlation to Off- Flavours and Carbonyl Index Clara Strandberg, Ann-Christine Albertsson Submitted. The contributions of the author: Paper I: All experiments and all writing. Paper II: All experiments and all writing. Paper III: All experiments and all writing, with exception of film-blowing and TLI measurements of the polypropylene materials (made by Lina Burman (KTH)). Paper IV: All experiments and all writing, with exception of processing of the extruded polyethylene materials and the determination of the off-flavours (made by Thorbjörn Andersson (Tetra Pak Research & Development)). Table of Contents 1 PURPOSE OF THE STUDY 9 2 INTRODUCTION 11 2.1 POLYOLEFINS 11 2.2 DEGRADATION AND OXIDATION IN POLYOLEFINS 11 2.2.1 Thermal oxidation 12 2.2.2 Degradation products 13 2.3 STABILISATION OF POLYOLEFINS 14 2.3.1 Natural antioxidants 15 2.3.1.1 α-tocopherol as antioxidant in polyolefins 16 2.3.1.2 The natural environment of α-tocopherol 17 2.3.1.2.1 Modelling the natural environment of α-tocopherol 17 2.4 EXTRACTION TECHNIQUES 19 2.5 CHARACTERISATION OF POLYOLEFINS 20 2.5.1 Detection of oxidation in polyolefins 20 2.5.1.1 Chemiluminescence (CL) of polymers 21 2.5.1.1.1 Correlation between CL and other techniques 21 2.5.2 Detection of low-molecular weight products from polyolefins 21 2.5.2.1 Correlations between chromatographic analysis and other techniques 22 3 EXPERIMENTAL 23 3.1 MATERIALS 23 3.1.1 Film-blown polyethylene 23 3.1.2 Compression-moulded polyethylene 24 3.2 EXTRACTION TECHNIQUES 24 3.2.1 Microwave assisted extraction (MAE) 24 3.2.2 Head space-solid phase microextraction (HS-SPME) 25 3.2.2.1 Selection of extraction conditions 25 3.2.2.2 HS-SPME of carboxylic acids emitted from PE samples 25 3.3 ANALYTICAL TECHNIQUES 25 3.3.1 Fourier transform-infrared spectroscopy (FT-IR) 25 3.3.2 Differential scanning calorimetry (DSC) 25 3.3.2.1 Oxygen induction time (OIT) measurements 25 3.3.2.2 Crystallinity measurements 26 3.3.3 High-performance liquid chromatography (HPLC) 26 3.3.4 Chemiluminescence (CL) 26 3.3.4.1 Inert measurements (TLI) 26 3.3.4.2 Oxidative measurements (OIT) 26 3.3.5 Gas chromatography-mass spectrometry (GC-MS) 27 4 RESULTS AND DISCUSSION 29 4.1 EFFECT OF ADDITIVES ON THE EFFICIENCY OF α-TOCOPHEROL AS AN ANTIOXIDANT IN POLYETHYLENE FILMS 29 4.1.1 Carbonyl index and total luminescence intensity measurements 29 4.1.2 Assessment of carboxylic acids from PE by gas chromatography-mass spectrometry (GC-MS) and head-space solid-phase extraction (HS-SPME) 40 4.1.2.1 Development of a HS-SPME method for extracting carboxylic acids emitted from PE 40 4.1.2.2 Method validation 40 4.1.2.3 HS-SPME and GC-MS identification of carboxylic acids emitted from the PE samples 41 4.2 SENSITIVITY IN DETECTION OF OXIDATION IN STABILISED POLYETHYLENE FILMS 46 4.2.1 Relation between emitted amount of carboxylic acids and carbonyl index 46 4.2.2 Early oxidation detection by total luminescence intensity and carbonyl index 48 4.2.3 Comparison of OIT’s measured by CL and DSC 51 4.2.4 CL techniques: Comparison of oxidation detection by TLI and OIT 51 5 CONCLUSIONS 55 6 FUTURE WORK 57 7 ACKNOWLEDGEMENTS 59 8 REFERENCES 61 Purpose of the Study 1 Purpose of the study In a sustainable society, not only must resources be used in a manner that allows them to be replenished by natural systems, but pollution in biological systems must also be avoided. One potential problem is the use of synthetic antioxidants for e.g. the stabilisation of polymeric materials. A possible replacement of these synthetic antioxidants in plastics with natural antioxidants has been discussed for many years. Unfortunately, problems arise in attempts to use natural antioxidants for the protection of plastics. Most of them have lower molecular weight and are less thermally stable than synthetic antioxidants. The aims with the work described in this thesis were to replace synthetic antioxidants in polymers with an efficient natural antioxidant, to optimise the antioxidant efficiency and to characterise the antioxidant efficiency by suitable characterisation techniques. The main issues addressed in this work were: • The effect of the environment surrounding the natural antioxidant vitamin E (α- tocopherol) on its long-term efficiency in polyethylene (PE). α-tocopherol and PE were chosen as a model system since PE is one of the major plastics and α- tocopherol has been shown to give good process stabilisation of polymers, but varying results in long-term stabilisation. • The use of sensitive methods for oxidation detection in stabilised materials. Besides the commonly used methods, infrared red measurements and thermal analysis, chemiluminescence and chromatographic techniques were selected, since they earlier have shown a high sensitivity in detection of oxidation. - 9 - - 10 -