Highly insulating polyethylene blends for HVDC power cables

Transcription

1 Supporting Information Highly insulating polyethylene blends for HVDC power cables Mattias G. Andersson 1, Jonna Hynynen 1, Mats R. Andersson 2, Villgot Englund 3, Per-Ola Hagstrand 3, Thomas Gkourmpis 3, Christian Müller 1, * 1 Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 41296, Göteborg, Sweden 2 Future Industries Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia 3 Innovation & Technology, Borealis AB, Stenungsund, Sweden * christian.muller@chalmers.se 1 / 6

2 Table S1. Crystallinity X and peak lamellar thickness from DSC melting thermograms. X is calculated using the total enthalpy method for all samples except thermoplastic and crosslinked HDPE, for which = was used, where is the enthalpy of fusion obtained by integrating the area under DSC melting endotherms and = 290 J g -1. and were calculated by integrating the area under DSC melting endotherms from the respective temperatures onwards. The thickness of LDPE, co-crystal and HDPE lamellae (, and ) were calculated from the peak temperature of DSC melting endotherms using the Gibbs-Thomson equation. blend (%) (%) (%) (nm) (nm) (nm) LDPE LDPE LDPE HDPE XLPE XLPE XLPE XLPE XLPE XLPE XLPE XLPE XLPE / 6

3 Table S2. Long period =2 / at 30 C, 70 C and 90 C where is the peak scattering vector obtained from SAXS Kratky plots. The long period measured at 30 C is related to the lamellar thickness by =,, which yields for LDPE-0 and XLPE-0 a value of 6.7 nm, i.e. a value that is similar to extracted from DSC (cf. Table S1). Sample, (nm), (nm), (nm) LDPE LDPE XLPE XLPE Figure S1. SEM images of etched LDPE-0, LDPE-5, XLPE-0 and XLPE-5. We note that in case of XLPE-5 thicker lamellae appear to cluster. However, given that crosslinking was carried out at 180 C (where the blend exists as a homogeneous melt), which leads to a high gel content of about 88 %, we exclude that phase separation has occurred in this sample. 3 / 6

4 Experimental Materials. LDPE and HDPE were supplied by Borealis AB, Stenungsund. The molecular weight of LDPE (weight-average molecular weight ~ 117 kg mol -1 ; polydispersity index PDI ~ 9) and HDPE ( ~ 58 kg mol -1 ; PDI ~ 6) as well as the LDPE degree of long chain branching ~ 1.9 per 1000 carbons were determined with size exclusion chromatography (SEC) in 1,3,4-trichlorobenzene at 150 ºC using an Agilent PL-GPC 220 system equipped with a refractive index and viscosity detector, which allowed universal calibration with polystyrene standards (Mark-Houwink equation for LDPE: = ; and for HDPE: = ). The degree of long chain branching of LDPE was calculated according to the method of Zimm and Stockmayer using a perfectly linear HDPE standard (BRPE0 from American Polymer Standard Corporation with ~ 18.3 kg mol -1 and ~ 52 kg mol -1 ) and the intrinsic viscosity from SEC measurements. 1-3 Blends comprising 0.5 to 40 wt% HDPE were compounded with a Prism TSE 24TC extruder with a temperature gradient from 80 C to 180 C and screw speed of 225 rpm, no heat stabilizer was used. Blends with 0.5 wt% were prepared by dry mixing 1 wt% material together with neat LDPE-0. Neat LDPE-0 and HDPE were extruded in the same way in order to exclude any influence from the extrusion step. To facilitate crosslinking dicumyl peroxide (DCP) was added. Thermoplastic blends did not contain DCP. To prepare materials for DSC and SAXS, DCP was added by infusing pellets of extruded material with a DCP solution of methanol for 1 h. Then, the methanol was left to evaporate and finally the pellets were dried in vacuum oven overnight. Samples were prepared by meltpressing ~100 µm thick films at 150 ºC (LDPE), or 180 ºC (XLPE) followed by cooling to room temperature at ~10 ºC min -1 whilst maintaining the pressure. Note that DCP crosslinking occurs at 180 C. 4 / 6

5 For DC conductivity measurements films with a thickness of 1 mm were melt-pressed in two steps, first at 130 C and then 180 C, which in case of materials that contained DCP led to crosslinking. The gel content was determined by extraction in boiling decahydronaphthalene. 4 SAXS samples had an average gel content of 86 ± 4% for XLPE-0 to XLPE-40, and 67 % for XLPE Samples for DC conductivity had an average gel content of 67 ± 2% for XLPE-0 to XLPE-40. We explain the difference in gel content between SAXS and DC conductivity samples with the difference in sample thickness. Differential Scanning Calorimetry (DSC). DSC was carried out under nitrogen between -50 to 160 ºC at a scan rate of 10 ºC min -1, unless otherwise indicated, using a Mettler Toledo DSC2 calorimeter equipped with a HSS7 sensor and a TC-125MT intracooler. The sample weight was 2 to 3 mg. Small-angle X-ray scattering (SAXS). SAXS was carried out using synchrotron radiation (λ = 0.91 Å) at the I911-SAXS beamline of the MAX Laboratory, Lund, Sweden. 5 A Pilatus 1M 2D-detector placed at a distance of 1.9 m from the sample was used to record transmission SAXS patterns for 0.2 mm thick solid samples within a q range of 0.08 to 4 nm -1. Then, background-corrected 2D SAXS patterns were radially integrated and calibrated with silver behenate. Variable-temperature SAXS measurements were performed with a Linkam heat stage between room temperature and 140 ºC at a heating rate of 10 ºC min -1. Scanning Electron Microscopy (SEM). Samples for SEM were prepared by etching cryofractured surfaces for two hours using a solution of 1wt% potassium permanganate in a mixture of sulfuric acid, ortho-phosphoric acid and water, followed by cleaning in water, hydrogen peroxide and methanol. 6 Etched surfaces were gold sputtered and then imaged with a Leo Ultra 55 SEM using an acceleration voltage of 2 to 3 kv. 5 / 6

6 DC Conductivity. The test cell consisted of a temperature controlled oven with two sets of brass electrodes (measuring area of Ø = 10 cm) connected to a high voltage source with positive polarization. After insertion of films with a thickness of 1 mm between the brass electrodes the cell was left to equilibrate to the right temperature for one hour before starting the measurement. The voltage was ramped to the final field strength at 0.5 V s -1. The field strength and temperature were kept constant whilst measuring the current during the experiment, samples where run for 24 h. Table S3. Field strength E, temperature T and time t for tested specimens. Sample wt% HDPE E (kv mm -1 ) T ( C) t (h) LDPE- 0, 5, 10, XLPE- 0, 0.5, 1, 2, 5, 10, 20, 40, , 40 70, Kulin, L. I.; Meijerink, N. L.; Starck, P. Long and Short Chain Branching Frequency in Lowdensity Polyethylene (LDPE). Pure & Appl. Chem. 1988, 60, Mendelson, R. A.; Drott, E. E. On the determination of long-chain branching in low density polyethylene. J. Polym. Sci B: Polym. Lett. 1968, 6, Zimm, B. H.; Stockmayer, W. H. The dimensions of chain molecules containing branches and rings. J. Chem. Phys. 1949, 17, Andersson, L. H. U.; Hjertberg, T. The effect of different structure parameters on the crosslinking behaviour and network performance of LDPE. Polymer 2006, 47, Labrador, A.; Cerenius, Y.; Svensson, C.; Theodor, K.; Plivelic, T. The yellow mini-hutch for SAXS experiments at MAX IV Laboratory. J. Phys. Conf. Ser. 2013, 425, Shahin, M. M.; Olley, R. H.; Blissett, M. J. Refinement of Etching Techniques to Reveal Lamellar Profiles in Polyethylene Banded Spherulites. J. Polym. Sci. B: Polym. Phys. 1999, 37, / 6