POLYMER FOR INCREASING THE ELECTRICAL RESISTANCE OF A POLYURETHANE COMPOSITION

Abstract

A poly(meth)acrylate urethane polymer, obtained from the reaction of at least one monomeric diisocyanate and a poly(meth)acrylate polyol having an OH number in the range from 50 to 200 mg KOH/g in an NCO/OH ratio of at least 3/1, followed by removal of a majority of the monomeric diisocyanate by means of a suitable separation method, wherein it has an NCO content in the range from 2.5% to 8% by weight, based on the poly(meth)acrylate urethane polymer, and a residual content of unconverted monomeric diisocyanates of not more than 0.5% by weight, and to the use thereof as additive and to moisture-curing polyurethane compositions.

Claims

1. A poly(meth)acrylate urethane polymer, obtained from the reaction of at least one monomeric diisocyanate and a poly(meth)acrylate polyol having an OH number in the range from 50 to 200 mg KOH/g in an NCO/OH ratio of at least 3/1, followed by removal of a majority of the monomeric diisocyanate by means of a suitable separation method, wherein it has an NCO content in the range from 2.5% to 8% by weight, based on the poly(meth)acrylate urethane polymer, and a residual content of unconverted monomeric diisocyanates of not more than 0.5% by weight.

2. The poly(meth)acrylate urethane polymer as claimed in claim 1, wherein the monomeric diisocyanate is diphenylmethane 4,4′-diisocyanate.

3. The poly(meth)acrylate urethane polymer as claimed in claim 1, wherein the isocyanate groups of the poly(meth)acrylate urethane polymer have been partly or wholly depleted by reaction with a monoalcohol or a monoamine in an additional subsequent reaction step.

4. The poly(meth)acrylate urethane polymer as claimed in claim 1, wherein the poly(meth)acrylate polyol has an OH number in the range from 100 to 180 mg KOH/g.

5. The poly(meth)acrylate urethane polymer as claimed in claim 1, wherein the NCO/OH ratio is in the range from 3/1 to 10/1.

6. The use of the poly(meth)acrylate urethane polymer as claimed in claim 1 as additive for increasing volume resistivity and/or for lowering impedance in a moisture-curing polyurethane composition.

7. The use as claimed in claim 6, wherein the poly(meth)acrylate urethane polymer is used in an amount in the range from 0.5% to 15.0% by weight, based on the overall polyurethane composition.

8. A moisture-curing polyurethane composition suitable as elastic adhesive and/or sealant, comprising at least one polyether urethane polymer containing isocyanate groups, and the poly(meth)acrylate urethane polymer containing isocyanate groups as claimed in claim 1.

9. The moisture-curing polyurethane composition as claimed in claim 8, wherein the polyether segments in at least one polyether urethane polymer consist of at least 80% 1,2-propyleneoxy units.

10. The moisture-curing polyurethane composition as claimed in claim 8, wherein the polyether urethane polymer has an NCO content in the range from 1% to 5% by weight.

11. The moisture-curing polyurethane composition as claimed in claim 8, wherein at least one further constituent selected from carbon black, blocked amines, further fillers, plasticizers, diisocyanate oligomers, organosilanes, catalysts and stabilizers is present.

12. The moisture-curing polyurethane composition as claimed in claim 8, wherein it contains 15% to 60% by weight of polyether urethane polymer containing isocyanate groups, 0.5% to 10% by weight of poly(meth)acrylate urethane polymer, 10% to 25% by weight of carbon black, 10% to 60% by weight of fillers, 0% to 35% by weight of plasticizers, and optionally further constituents.

13. The moisture-curing polyurethane composition as claimed in claim 8, wherein a total of less than 0.3% by weight, of monomeric diisocyanates is present.

14. A method of bonding or sealing, comprising the steps of (i) applying the polyurethane composition as claimed in claim 8 to a first substrate and contacting the composition with a second substrate within the open time of the composition, or to a first and to a second substrate and joining the two substrates within the open time of the composition, or between two substrates, (ii) curing the composition by contact with moisture.

15. An article obtained from the method as claimed in claim 14.

Description

EXAMPLES

[0186] Working examples are adduced hereinafter, which are intended to further elucidate the invention described. The invention is of course not limited to these described working examples.

[0187] “Standard climatic conditions” (“SCC”) refer to a temperature of 23±1° C. and a relative air humidity of 50±5%.

[0188] Unless otherwise stated, the chemicals used were from Sigma-Aldrich Chemie GmbH.

[0189] Preparation of Polymers Containing Isocyanate Groups:

[0190] Viscosity was measured with a thermostated Rheotec RC30 cone-plate viscometer (cone diameter 25 mm, cone angle 1°, cone tip-plate distance 0.05 mm, shear rate 50 s.sup.−1).

[0191] Monomeric diisocyanate content was determined by means of HPLC (detection via photodiode array; 0.04 M sodium acetate/acetonitrile as mobile phase) after prior derivatization by means of N-propyl-4-nitrobenzylamine.

[0192] Polymer PE-1: Polyether Urethane Polymer.

[0193] 727.0 g of Acclaim® 4200 polyether diol and 273.0 g of diphenylmethane 4,4′-diisocyanate (Desmodur® 44 MC L, from Covestro) were reacted by a known method at 80° C. to give a polyether urethane polymer having an NCO content of 7.6% by weight, a viscosity of 5.2 Pa.Math.s at 20° C. and a monomeric diphenylmethane 4,4′-diisocyanate content of about 18% by weight.

[0194] Subsequently, the volatile constituents, especially a majority of the monomeric diphenylmethane 4,4′-diisocyanate, were removed by distillation in a short-path evaporator (jacket temperature 180° C., pressure 0.1 to 0.005 mbar, condensation temperature 47° C.). The polyether urethane polymer thus obtained had an NCO content of 1.8% by weight, a viscosity of 15.2 Pas at 20° C. and a monomeric diphenylmethane 4,4′-diisocyanate content of 0.08% by weight.

[0195] Polymer PE-2: Polyether Urethane Polymer.

[0196] 390.0 g of polyTHF diol (polytetrahydrofuran diol) Terathane®PTMEG 650 and 610.0 g of diphenylmethane 4,4′-diisocyanate (Desmodur® 44 MC L, from Covestro) were reacted by a known method at 80° C. to give a polyether urethane polymer having an NCO content of 15.1% by weight, a viscosity of 3.8 Pas at 20° C. and a monomeric diphenylmethane 4,4′-diisocyanate content of about 34% by weight.

[0197] Subsequently, the volatile constituents, especially a majority of the monomeric diphenylmethane 4,4′-diisocyanate, were removed by distillation in a short-path evaporator (jacket temperature 180° C., pressure 0.1 to 0.005 mbar, condensation temperature 47° C.). The polyether urethane polymer thus obtained had an NCO content of 6.0% by weight, a viscosity of 7.5 Pas at 60° C. and a monomeric diphenylmethane 4,4′-diisocyanate content of 0.08% by weight.

[0198] Polymer PE-3: Polyether Urethane Polymer.

[0199] 725 g of Desmophen® 5031 BT (glycerol-started ethylene oxide-terminated polyoxypropylene triol, OH number 28.0 mg KOH/g, OH functionality about 2.3; from Covestro) and 275 g of diphenylmethane 4,4′-diisocyanate (Desmodur® 44 MC L, from Covestro) were converted by a known method at 80° C. to a polyether urethane polymer having an NCO content of 7.6% by weight, a viscosity of 6.5 Pas at 20° C. and a monomeric diphenylmethane 4,4′-diisocyanate content of about 20% by weight.

[0200] Subsequently, the volatile constituents, especially the majority of the monomeric diphenylmethane 4,4′-diisocyanate, were removed by distillation in a short-path evaporator (jacket temperature 180° C., pressure 0.1 to 0.005 mbar, condensation temperature 47° C.). The polyether urethane polymer thus obtained had an NCO content of 1.7% by weight, a viscosity of 19 Pas at 20° C. and a monomeric diphenylmethane 4,4′-diisocyanate content of 0.04% by weight.

[0201] Polymer PP-1: Polyester Urethane Polymer.

[0202] 597.5 g of Priplast® 1838 (dimer fatty acid-based polyester polymer, Croda) and 402.5 g of diphenylmethane 4,4′-diisocyanate (Desmodur® 44 MC L, from Covestro) were reacted at 80° C. to give a polyester urethane polymer having an NCO content of 11.0% by weight, a viscosity of 36 Pas at 20° C. and a diphenylmethane 4,4′-diisocyanate content of about 26% by weight.

[0203] Subsequently, the volatile constituents, especially a majority of the diphenylmethane 4,4′-diisocyanate, were removed by distillation in a short-path evaporator (jacket temperature 180° C., pressure 0.1 to 0.005 mbar, condensation temperature 47° C.). The polyester urethane polymer thus obtained was slightly cloudy and was of fluid, viscous consistency at room temperature. It had an NCO content of 2.8% by weight, a viscosity of 312 Pas at 20° C. or 11.5 Pas at 60° C. and a diphenylmethane 4,4′-diisocyanate content of 0.09% by weight.

[0204] Polymer PA-1: Inventive poly(meth)acrylate urethane polymer. 460 g of Joncryl® 963 (polyacrylate polyol from BASF, OH number 130 mg KOH/g, equivalent weight 432 g) and 540 g of diphenylmethane 4,4′-diisocyanate (Desmodur® 44 MC L, from Covestro) were converted by a known method at 80° C. to a poly(meth)acrylate urethane polymer having an NCO content of 14.0% by weight, a viscosity of 14.7 Pas at 20° C. and a monomeric diphenylmethane 4,4′-diisocyanate content of about 33% by weight.

[0205] Subsequently, the volatile constituents, especially the majority of the monomeric diphenylmethane 4,4′-diisocyanate, were removed by distillation in a short-path evaporator (jacket temperature 180° C., pressure 0.1 to 0.005 mbar, condensation temperature 47° C.). The poly(meth)acrylate urethane polymer thus obtained had an NCO content of 4.8% by weight, a viscosity of 50.7 Pas at 60° C. and a monomeric diphenylmethane 4,4′-diisocyanate content of 0.12% by weight.

[0206] Polymer PA-1b: Polymer PA-1 (NCO Groups Blocked with Monoalcohol).

[0207] 473.4 g of the poly(meth)acrylate urethane polymer PA-1 was dissolved in 493.6 g of diisodecyl phthalate (DIDP) at 23° C. (RT). Subsequently, 32.9 g of methanol was added to the mixture together with 0.2 g of tin catalyst solution. The amount of methanol corresponded to twice the amount which is required to react with all the isocyanate groups in the polymer PA-1. The mixture was stirred at RT for 3 hours. After this time, an NCO content of 0.07% was measured. Subsequently, the reaction mixture was heated to 110° C., and the excess methanol was removed at a reduced pressure of 50 mbar.

[0208] Polymer PA-La: Polymer PA-1 (NCO Groups Blocked with Monoamine).

[0209] 471.3 g of the poly(meth)acrylate urethane polymer PA-1 was dissolved in 491.4 g of DIDP at 60° C. Subsequently, 37.4 g of butylamine was added. This is the stoichiometric amount to react with all isocyanate groups. Owing to evaporation losses, however, an additional 20 g of butylamine had to be added in order to attain a final NCO content of 0.12%.

[0210] Polymer PA-2: Nonfunctional poly(meth)acrylate Polymer.

[0211] Arufon® UP1020 (Toagosei, Japan). Commercially available liquid, nonfunctional (especially having no OH and NCO groups) acrylate polymer with low Tg (−80° C.), molecular weight 2000 g/mol, viscosity 500 mPa.Math.s at 25° C.

[0212] Polymer PA-3: Blocked poly(meth)acrylate Polymer.

[0213] 328.9 g of Joncryl® 963 was dissolved in 500.0 g of DIDP. Subsequently, 171.1 g of p-toluenesulfonyl isocyanate (tosyl isocyanate, Sigma-Aldrich) was added in portions at RT. On conclusion of the addition, the reaction mixture was stirred for another 30 min until a stable NCO content of 0.3% had been attained.

[0214] Polymer PA-4: Blocked poly(meth)acrylate Polymer.

[0215] 1100.00 g of Joncryl® 963 (OH number about 130 mg KOH/g, about 2.54 mol of OH) and 285.35 g (2.80 mol) of acetic anhydride were weighed into a 200 ml round-bottom flask equipped with Vigreux column, T-piece and collecting flask. The mixture was stirred at 130° C. under a gentle nitrogen stream for 3 hours, after which no OH bands were visible any longer in the IR. The volatile components were removed from a rotary evaporator at 80° C. and 5 mbar over the course of 30 minutes, followed by aftertreatment under high vacuum to 30 minutes. The product had a viscosity of 3.53 Pa.Math.s at 20° C.

[0216] Polymer PC-1: Polycarbonate Urethane Polymer.

[0217] 498.0 g of Eternacoll® PH-50 and 502.0 g of diphenylmethane 4,4′-diisocyanate (Desmodur® 44 MC L, from Covestro) were reacted at 60° C. to give a polycarbonate urethane polymer having an NCO content of 8.4% by weight and a diphenylmethane 4,4′-diisocyanate content of about 15% by weight.

[0218] Polymer PC-2: Polycarbonate Polyol.

[0219] Eternacoll® PH200D (UBE Chemical, Spain). Commercially available liquid copolycarbonate polyol (OH number about 56 mg KOH/g) with molecular weight 2000 g/mol, viscosity 2500 mPa.Math.s at 75° C.

[0220] Polymer PC-3: Polycarbonate Polyol.

[0221] Eternacoll® UT-200 (UBE Chemical, Spain). Commercially available liquid linear polycarbonate/polyether diol (OH number about 56 mg KOH/g) with molecular weight 2000 g/mol, viscosity 8500 mPa.Math.s at 25° C.

[0222] Polymer PC-4: Blocked Polycarbonate Urethane Polymer.

[0223] 1000.00 g of Eternacoll® PH-50 (OH number about 220 mg KOH/g, about 3.92 mol of OH) and 440.21 g (4.31 mol) of acetic anhydride were weighed into a 200 ml round-bottom flask equipped with Vigreux column, T-piece and collecting flask. The mixture was stirred at 130° C. under a gentle nitrogen stream for 3 hours, after which no OH bands were visible any longer in the IR.

[0224] The volatile components were removed from a rotary evaporator at 80° C. and 5 mbar over the course of 30 minutes, followed by aftertreatment under high vacuum for 30 minutes. The product had a viscosity of 0.60 Pa.Math.s at 20° C.

[0225] Moisture-Curing Polyurethane Compositions:

[0226] Compositions Z1 to Z16:

[0227] Each composition, using the ingredients specified in tables 1 to 4, was mixed well in the amounts specified (in parts by weight) by means of a planetary mixer under reduced pressure and with exclusion of moisture, and the composition was dispensed into an aluminum cartridge with an airtight seal and stored at room temperature.

[0228] “DIDP” denotes diisodecyl phthalate, a plasticizer. The carbon black used, Monarch® 570 (from Cabot), is a highly structured, nonoxidized carbon black having very good reinforcing properties. The tin catalyst is a dibutyltin dilaurate (10% by weight in DIDP). Desmodur® N3300 (from Covestro) is a hexamethylene diisocyanate trimer.

[0229] Each composition was tested as follows:

[0230] For determination of mechanical properties, each composition was pressed between two silicone-coated release papers to give a film of thickness 2 mm and stored under standard climatic conditions for 14 days. After removing the release papers, some test specimens were punched out and tested as described as follows:

[0231] For determination of tensile strength (TS), elongation at break (EaB) and modulus of elasticity at 0.5-5% elongation, dumbbells having a length of 75 mm with a bar length of 30 mm and a bar width of 4 mm were punched out of the film, and these were tested to DIN EN 53504 at a strain rate of 200 mm/min.

[0232] Impedance was determined via the dielectric constant (product of resistance and capacitance) at radiofrequencies of 1 MHz, 6 MHz and 100 MHz by means of an Agilent/Hewlett Packard HP 4291A impedance meter. For this purpose, samples were produced by applying the adhesive to be examined between two copper plates (Rocholl, Germany) and compressing to a thickness (adhesive) of 2 mm. Once the adhesive had cured (7 days at 23° C., 50% r.h.), the cured adhesive fraction that had been squeezed out between the plates was cut off with a sharp knife, and hence a sandwich-like test specimen was obtained, in which 2 mm of the cured adhesive was trapped between the copper plates.

[0233] This test specimen was analyzed for impedance in the abovementioned instrument at the frequencies specified.

[0234] Electrical resistance was measured at a voltage of 1 V, 10 V and 100 V. The experiments were conducted according to standard DIN IEC 60167 VDE 0303-31:1993-12 using a Chrome Insulation Tester 11200 (Chroma 11200 Capacitor Leakage Current/IR Meter, Chroma USA) on the samples from the impedance measurement.

[0235] Compositions labeled “(Ref.)” are comparative examples.

TABLE-US-00001 TABLE 1 Composition (in parts by weight) and properties of Z1 to Z6. Z1 Z6 Composition (Ref.) Z2 Z3 Z4 Z5 (Ref.) Polymer PE-1 39.4 35.6 37.4 39.4 40.4 41.4 Polymer PA-1 — 5.25 2.0 2.0 1.0 — Polymer PC-1 2.0 2.0 2.0 — — — DIDP 20.16 20.16 20.16 20.16 20.16 20.16 Monarch 570 15.0 15.0 15.0 15.0 15.0 15.0 (Carbon black) 21.0 21.0 21.0 21.0 21.0 21.0 Tin catalyst 0.94 0.94 0.94 0.94 0.94 0.94 Desmodur N3300 1.5 1.5 1.5 1.5 1.5 1.5 Tensile strength 8.4 7.4 7.9 7.4 7.7 7.5 [MPa] Elongation at break 730 418 594 662 671 687 [%] MoE 0.5-5% [MPa] 4.9 5.8 5.4 4.1 4.3 4.1 Resistivity [Ωcm] 1 × 10.sup.9 4 × 10.sup.10 3 × 10.sup.9  3 × 10.sup.10 2 × 10.sup.9  4 × 10.sup.8 100 V Resistivity [Ωcm] 3 × 10.sup.9 6 × 10.sup.13 5 × 10.sup.10 9 × 10.sup.13 3 × 10.sup.10 2 × 10.sup.9 10 V Resistivity [Ωcm] .sup. 2 × 10.sup.12 4 × 10.sup.13 1 × 10.sup.13 3 × 10.sup.16 2 × 10.sup.13 .sup. 1 × 10.sup.12 1 V Impedance [εr′/εr″] 18.7/2.8 16.0/1.7 16.1/1.8 15.1/1.8 17.0/2.7 20.2/4.0 1 MHz Impedance [εr′/εr″] 16.2/2.2 14.4/1.5 14.4/1.6 13.3/1.6 14.4/2.1 16.6/3.0 6 MHz Impedance [εr′/εr″] 12.2/1.9 11.4/1.5 11.3/1.5 10.2/1.4 10.8/1.7 11.9/2.1 100 MHz Impedance [εr′/εr″] 11.0/1.2 10.5/0.9 10.3/1.0  9.3/1.0  9.7/1.1 10.5/1.4 500 MHz

TABLE-US-00002 TABLE 2 Composition (in parts by weight) and properties of Z7 to Z12. Z7 Z8 Z10 Z11 Z12 Composition (Ref.) (Ref.) Z9 (Ref.) (Ref.) (Ref.) Polymer PE-1 36.4 36.4 36.4 36.4 36.4 36.4 Polymer PE-2 5.0 5.0 5.0 5.0 5.0 5.0 Polymer PA-1 — — — — — — Polymer PA-1b — — 5.0 — — — Polymer PA-2 5.0 — — — — — Polymer PA-3 — 5.0 — — — — Polymer PC-1 — — — — — 5.0 Polymer PC-2 — — — 5.0 — — Polymer PC-3 — — — — 5.0 — DIDP 15.16 15.16 15.16 15.16 15.16 15.16 Monarch 570 15.0 15.0 15.0 15.0 15.0 15.0 (Carbon black) 21.0 21.0 21.0 21.0 21.0 21.0 Tin catalyst 0.94 0.94 0.94 0.94 0.94 0.94 Desmodur N3300 1.5 1.5 1.5 1.5 1.5 1.5 Tensile strength 10.5 11.0 10.1 8.6 11.0 12.3 [MPa] Elongation at break 687 800 691 526 690 586 [%] MoE 0.5-5% [MPa] 7.8 7.4 7.7 5.3 6.3 17.9 Resistivity [Ωcm] 2 × 10.sup.7 7 × 10.sup.7 8 × 10.sup.13 2 × 10.sup.9 3 × 10.sup.8 1 × 10.sup.9 100 V Resistivity [Ωcm] 2 × 10.sup.7 9 × 10.sup.7 6 × 10.sup.14 3 × 10.sup.9 3 × 10.sup.8 2 × 10.sup.9 10 V Resistivity [Ωcm] 6 × 10.sup.7 1 × 10.sup.8 2 × 10.sup.15 .sup. 5 × 10.sup.12 9 × 10.sup.8 6 × 10.sup.9 1 V Impedance [εr′/εr″] 22.2/9.3 17.9/3.3 15.2/1.7 27.1/7.5 20.3/5.8 18.8/4.1 1 MHz Impedance [εr′/εr″] 16.6/4.5 14.9/2.5 13.4/1.6 20.4/5.1 15.5/3.7 15.4/2.7 6 MHz Impedance [εr′/εr″] 11.6/2.1 11.2/1.7 10.6/1.3 13.4/2.9 10.7/1.9 11.6/1.6 100 MHz Impedance [εr′/εr″] 10.3/1.3 10.1/1.1  9.8/0.9 11.5/1.5  9.5/0.8 10.9/0.6 500 MHz

TABLE-US-00003 TABLE 3 Composition (in parts by weight) and properties of Z13 to Z16. Z14 Z15 Composition Z13 (Ref.) (Ref.) Z16 Polymer PE-1 36.4 36.4 36.4 36.4 Polymer PE-2 5.0 5.0 5.0 — Polymer PA-1 5.0 — — — Polymer PA-1b — — — 5.0 Polymer PA-4 — 5.0 — — Polymer PC-4 — — 5.0 — DIDP 15.16 15.16 15.16 15.16 Monarch 570 15.0 15.0 15.0 20.0 (Carbon black) 21.0 21.0 21.0 16.0 Tin catalyst 0.94 0.94 0.94 0.94 Desmodur N3300 1.5 1.5 1.5 1.5 Tensile strength 9.8 9.8 9.1 11.9 [MPa] Elongation at break 461 710 675 703 [%] MoE 0.5-5% [MPa] 9.4 7.2 6.9 9.3 Resistivity [Ωcm] 9 × 10.sup.9  9 × 10.sup.7 5 × 10.sup.7 3 × 10.sup.10 100 V Resistivity [Ωcm] 5 × 10.sup.10 5 × 10.sup.7 5 × 10.sup.7 4 × 10.sup.13 10 V Resistivity [Ωcm] 1 × 10.sup.13 5 × 10.sup.8 2 × 10.sup.8 8 × 10.sup.13 1 V Impedance [εr′/εr″] 18.5/3.4 20.0/4.8 22.1/7.5 20.4/2.7 1 MHz Impedance [εr′/εr″] 15.8/2.5 16.1/3.1 17.0/4.0 17.6/2.4 6 MHz Impedance [εr′/εr″] 11.8/1.5 11.9/1.9 12.2/2.1 13.7/1.6 100 MHz Impedance [εr′/εr″] 11.1/0.4 10.8/1.3 11.0/1.5 12.9/0.4 500 MHz

TABLE-US-00004 TABLE 4 Composition (in parts by weight) and properties of Z17 to Z22. Z17 Z20 Composition (Ref.) Z18 Z19 (Ref.) Z21 Z22 Polymer PE-1 27.4 27.4 27.4 39.4 39.4 40.4 Polymer PE-2 4.5 4.5 4.5 6.0 6.0 6.0 Polymer PE-3 10.0 10.0 10.0 — — — Polymer PP-1 5.0 5.0 5.0 — — — Polymer PA-1b — — — — 3.0 1.5 Polymer PA-1a — 2.0 4.0 — — — DIDP 13.16 9.16 5.16 15.66 9.66 12.66 Monarch 570 18.0 18.0 18.0 18.0 18.0 18.0 (Carbon black) 20.0 20.0 20.0 17.5 17.5 17.5 Tin catalyst 0.94 0.94 0.94 0.94 0.94 0.94 Desmodur N3300 1.0 1.0 1.0 2.5 2.5 2.5 Tensile strength 10.5 10.1 10.2 11.3 11.9 12.7 [MPa] Elongation at break 609 633 637 583 594 586 [%] MoE 0.5-5% [MPa] 9.5 9.7 10.0 10.9 10.8 10.0 Resistivity [Ωcm] 5 × 10.sup.7 6 × 10.sup.8 3 × 10.sup.9 1 × 10.sup.7 2 × 10.sup.9 1 × 10.sup.8 100 V Resistivity [Ωcm] 5 × 10.sup.7 2 × 10.sup.8 8 × 10.sup.8 2 × 10.sup.7 3 × 10.sup.9 1 × 10.sup.8 10 V Resistivity [Ωcm] 1 × 10.sup.8 2 × 10.sup.8 8 × 10.sup.8 2 × 10.sup.7 8 × 10.sup.9 2 × 10.sup.8 1 V Impedance [εr′/εr″] n/m n/m n/m  36.0/23.6 24.1/5.7 25.3/7.8 1 MHz Impedance [εr′/εr″] n/m n/m n/m 24.1/9.7 19.3/3.8 19.4/4.6 6 MHz Impedance [εr′/εr″] n/m n/m n/m 15.1/3.5 14.1/2.2 13.5/2.5 100 MHz “n/m” means that the value was not measured.