This paper investigates volatile organic compounds (VOC) emissions during the process of polyethylene extrusion. Two kinds of polymers such as linear low density polyethylene and high density polyethylene were tested. Blowing film extrusion in an experimental technological line was done. VOCs were collected on sorbent tubes.

Gas chromatography coupled with mass spectrometry (GC/MS) was applied for the identification of volatile degradation products. PE-LLD (low density polyethylene) material emits a significantly larger amount of hydrocarbons than PE-HD (high-density polyethylene). Its emission contains mainly C18 and C20 hydrocarbons (alkanes, 1-alkenes, and α,ω-alkadienes). In case of the PE-HD polymer, a lower degree of degradation was observed. The main emission products are C18 and C23.

Introduction

Degradation of extruding polymers may lead to changes in the properties and structure of a polymer film, influencing the consumer products. Degradation factors such as physical and chemical can result in structural damage, which decreases average molecular weight as well as chemical composition.

Mechanical degradation occurs in processing operations of polymers due to shearing forces (stress). Thermal and oxidising degradation occurs under the influence of heat and environment conditions. In current extruders’ design, the heat stream is generated by the friction phenomenon of polymers in a system. In most cases this action is sufficient for extrusion.

There are always problems with even heat distribution along the plasticising system and the extrusion head. It may cause local overheating of the polymer resulting in degradation, which usually lowers the quality of the final product.

The degradation phenomena of commercial polyethylene (PE) resin in the industrial extrusion process of lamination packaging materials were extensively investigated. PE resin subjected to extrusion at temperatures of 250–325° C flows out of the extrusion head for the material to be covered.

Such an intensively plasticised polymer undergoes considerable degradation, including the oxidising phenomenon. As a result of the degradation, the polymer product acquires enhanced adhesion to the covered material. Moreover, gaseous products of the degradation process arising at the lip of the extrusion smoke have also been subjected to investigation. They are mostly low-molecular compounds resulting from the reaction of chain radicals with oxygen. They evaporate from the plasticised, hot PE creating the so called ‘extrusion smoke’ of a specific smell, which depends on the extrusion conditions.

Investigation of the extrusion blow molding process of a PE film for food packaging purposes showed that in extreme conditions the processed polymers underwent considerable degradation, characterised by changes in the rheological and mechanical properties of the film. Comparing the extrusion tests in extreme conditions, in which the processing temperatures reach up to 275° C for linear low density polyethylene (PE-LLD) and 280° C for high density polyethylene (PE-HD), the decrease in molecular mass was 25% and 28%, respectively.

Among numerous methods of instrumental analysis applied for the investigation of polymers’ degradation, the most common are infrared spectroscopy (IR), gas chromatography coupled with mass spectrometry (GC/MS), and nuclear magnetic resonance (NMR). GC/MS seems to be the most useful method for the identification of volatile degradation products resulting from extrusion and blow molding, however.

The objective of this work was the identification of gaseous products formed under critical extrusion blow molding conditions. Extreme film extrusion conditions were achieved using intensive shearing tips of the screw (groove gap clearing of 0.3 mm) and starve-feeding of the plasticising system, which resulted in high shearing speed of the polymer and its elongated residence time in the plasticising system. The extreme extrusion conditions of the film are given by Stasiek.

Experiment

Materials Two different extrusion grade polymers which are intended for food contact were applied in granulated form. The films were extruded using an experimental technological line described elsewhere. Samples of the following materials were used: • PE-HD MFR (melt flow rate), (190° C, 2.16 kg) = 0.15 g per 10 min, density 0.951 Mg m–3, copolymer • PE-LLD (1-butane co-monomer), MFR (190° C, 2.16 kg) = 1.0 g per 10 min, density 0.918 Mg m–3, heat stabilised Main differences between the polymers PE-HD and PE-LLD concern: • Degree of crystallinity: PE-HD – from 15 % to 23 %, PE-LLD – above 55% • Molecular weight: PE-HD – 80 000, PE-LLD – 150 000 • Polydispersity (Mw/Mn): PE-HD – about 6, PE-LLD – about 36 • Oxidation induction time: PE-HD – at temperatures below 190° C – 10 min, PE-LLD – at the temperature of 200° C – above 30 min

High resistance of PE-HD to oxidation at high temperatures compared to PE-LLD shows that PE-HD is used with effectively acting antioxidants.

Acetonitrile ultra grade and chromatographic standards of linear alkanes (C8-C23) were purchased.

Sample taking and preparation Gaseous products of thermo-mechanical degradation of PE were gathered by two methods: either employing or not employing iris blend. When the first method was used, the volatile products were gathered at the lip of the extrusion smoke without using iris blend. Gaseous products were diluted by atmospheric air, the presence of which can considerably influence the measurement results – intensity of peaks was lower. Moreover, this method can cause the occurrence of artifacts.

Employing the second method, gaseous samples were gathered directly at the lip of the extrusion smoke-head using iris blend, which can minimise the influence of atmospheric air. In this case, the chromatogram peaks were more intense. Therefore, in further investigation the method using iris blend was applied.

Gaseous products were adsorbed on a glass tube containing two layers (100 mg and 50 mg) of a porous polymer resin. The air flow rate was set to 0.2 L min–1. Samples were taken in 50 min and 100 min at this flow rate. Air was passed through an adsorption tube using a pump. Volatile compounds adsorbed in the trap tube were extracted in acetonitrile (2 mL) by ultrasonication (5 min). Afterwards, acetonitrile was separated by centrifugation at 3500 min–1 for 10 min. The decanted solution was transferred to another vial. The aliquot of 2 µL was injected to GC/MS.

Apparatus All instrument with a split–splitless injector was used for chromatographic separation of the sample component and identification. The temperature of the split–splitless injector was 220° C. The splitless time was 0.5 min, while the split ratio was 1 : 25. Helium with linear velocity of 40 cm s–1 was used as a carrier gas. Mass spectrometer work in full scan mode with the mass range of 35–600 amu and the scan rate of 4 scans per second.

Electron impact ionisation (EI) at energy 70 eV was used. The ion source and transfer line were set to 210° C and 200° C, respectively. The capillary column at dimesions 30 m × 0.25 mm × 0.25 µm was used. Oven temperature programme was as follows: initial temperature of 50° C was held for 1 min, then it was increased at 15° C min–1 to 110° C, then the temperature was increased at the heating rate of 5° C min–1 to 240° C and held for 5 min.

Results and discussion

Volatile degradation products resulting from extrusion and blow molding of polyethylene present a complex mixture. GC/MS analyses show series of triplets consisting of hydrocarbons having the same carbon number. The elution order of these species on the capillary column is as follows: α,ω-alkadiene, 1-alkenes, and alkane. A similar order was confirmed by other authors (Soják et al, 2007; Williams & Williams, 1999).

Decomposition of PE-LLD in comparison with PE-HD gives different distribution of compounds. In case of PE-LLD, more degradation products were emitted. The most abundant degradation components are C18 and C20 with 25.8 % and 20.1 % of total ion chromatogram (TIC), respectively.

In the retention time range tR = 16.80–20.10 min, broad peaks of unresolved compounds were observed. These are probably a mixture of oxygenated compounds and branched alkene isomers. Identification of these species based on libraries (NIST and Wiley) was not sufficient, however. Most abundant compounds were found at tR = 17.57 min and 20.01 min. The use of a longer capillary column (150 m instead 30 m) allowed more efficient identification.

Considering polymer degradation it must be remembered that the quantity of formed hydrocarbons is connected with its structure. In the screw extrusion process in extreme conditions, polymer of branched or unordered structure is more susceptible to degradation.

In several cases, thermal stability of the polymer considerably depends on the content of additives enhancing this parameter. In such a case, the analysis of gaseous degradation products can be considered to be more difficult. Moreover, the quality of formed hydrocarbons can be strongly influenced by isomeric configuration, sequence of mers in the chain, tacticity, poly-dispersion of molecular weight, as well as by crosslinking degree.

Chromatograms obtained for the PE-HD sample present fewer degradation products due to the less branched structure of the polymer. When comparing the sampling times of 50 min and 100 min, C12–C20 hydrocarbons were mainly observed for the shorter period. Heavier molecular weight hydrocarbons (C20–C24) were found during 100 min aspiration period. The most abundant triplet (C23) represents 43.8% and (C18) 24.3% of TIC, for the 50 min and 100 min aspiration period, respectively.

Analysing both chromatograms, slightly inferior degradation was observed for PE-HD rather than for PE-LLD. Less intense signals are in this case connected with the macromolecule structure with fewer side carbon chains (on average three times less compared to PE-LLD), which is therefore less susceptible to degradation. As a result of the PE-HD macromolecule structure there are more intense intermolecular interactions responsible for higher resistance of this polymeric material against degradation.

Besides the hydrocarbons, formation of carbonyl species as well as alcohols under oxidative conditions (presence of oxygen existing in ambient air) is quite possible. Relatively low content of these compounds in comparison to the hydrocarbons amount and poor chromatographic separation cause identification problems, however. In addition, the analysis of aldehydes and ketones at low concentrations often requires the use of sorbents impregnated with 2,4-dinitrophenylhydrazine (DNPH).

Conclusions

During the extrusion blow molding process of PE films, degradation of polymer with emission of hydrocarbons occurs. The main products of polymer degradation are linear alkanes, 1-alkenes, and α,ω-alkadienes with 11–24 carbon atoms in the molecule. Volatile degradation products were identified by means of GC/MS.

Among decomposition products of PE-LLD, the most abundant hydrocarbons are C16 (12 %), C18 (26 %), and C20 (18 %). For PE-HD material, the main hydrocarbons are: C20 (12 %), C22 (20 %), C23 (40 %), and C16 (23 %), C18 (33 %), C20 (15 %) for 100 min and 50 min sample aspiration time, respectively.

A comparison of obtained results with those presented by Andersson et al shows that in our extrusion process, degradation products of higher molecular mass are formed. They are stated as the presence of gaseous products of the chain length from C2 to C12.

In conclusion it can be stated that of the qualitative characteristics of gaseous products, plasticising temperature as well as shearing forces are the most crucial.

Published: 07th Mar 2013 in AWE International