KR820001497B1 - Dielectric composition stabilized against water treeing with organo silane compounds - Google Patents

Abstract

Polyethylene-based dielectric compns. are described in which elec. breakdown by water treeing is inhibited by organo silanes(I; R, R1 = H, substituted hydrocarbyl, R2 = C1-8alkyl, alkoxy, C6-18 aryl, C1-8 acyloxy, C6-18aryloxy; R3,R4 = C1-8 alkoxy, C6-18 aryloxy, C1-8 acyloxy; Y = C1-8 alkylene, C6-18 arylene, -CH2CH2NH(CH2)3-). Thus, 28 lbs of polyethylene, 122.4 g of benzylaldehyde and 256g of N-β-(aminoethyl)-γ- amino propyltrimethyloxysilane were mixed until the melt temp. reached 325↿F, after which the charge was stranded and diced in a conventional extruder, and a water tree test speciment was molded at 165≰C.

Description

Dielectric composition stabilized against water treeing with organosilane compounds

1a is a cross-sectional view of a specimen according to the present invention.

FIG. 1B shows an enlarged view of the conical depression in FIG. 1A.

2 shows a typical water tree grown for 24 hours in low density polyethylene.

3 is a graph of tree growth rate plotted as a function of E ² .

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to suppressing electrical blessed phenomenon due to water treeing in dielectric materials based on ethylene polymers.

Compositions based on polyethylene are well known and they are used extensively primarily as insulation for wires and cables. For insulating materials, the properties of the composition are important, such as high dielectric strength, corona resistance and resistance to treeing.

Inherent electrical bless is a major breakdown of the dielectric accompanied by arcing or discharge through the ionized channel in the dielectric. Intrinsic dielectric strength is considered an inherent property of dielectric materials.

In power cables for transmitting relatively high voltage loads, such as 5 kV or more, corona can be a problem, because of the risk of premature failure of the cable insulation. Corona is an electrical plasma due to ionization of gaseous dielectrics in the region of high electric fields. Corona resistance is the dielectric's ability to resist the corrosive action of the electrical plasma in contact with the dielectric. When used as a high voltage power cable insulator, compositions based on olefins experience an early blessed phenomenon known as treeing. This type of damage progresses through the dielectric site under electrical stress, so if visible to the eye, the passage looks like a tree. The treeing may occur slowly and progress by periodic partial discharges, and may also occur slowly in the presence of moisture without any partial discharges, or may occur rapidly as a result of an impact voltage.

Tree-shaped crystals may be formed in areas of high electrical stress, such as contaminants or voids in the insulator, or may be formed in irregular surfaces at the insulator-semiconductor screen interface. In solid organic dielectrics, treeing does not occur abruptly, but rather is a mechanism of electrical breakdown that is believed to be the result of a longer reaction. It is desired to extend the life of the olefin insulated cable by trees starting at a voltage higher than the normal voltage or by modifying the insulation material so that the growth rate of the trees once started is reduced.

Electrical treeing is due to the internal electrical discharges that break down the dielectric. Although high voltage shocks can create electrical trees, and the presence of internal voids and contaminants is undesirable, damage caused by the application of moderate alternating voltage to the interface of the electrode and insulator that contains the defective portion is commercially available. It is a serious matter. In this case, very high local stress gradients may be present, which, after sufficient time, will lead to the generation and growth of the tree, causing breakage. An example of this case is a high voltage power cable or connector having a rough interface between the conductor or conductor protective film and the primary insulator. Failure mechanisms include the substantial destruction of the modular structure of the dielectric material, possibly by electron impact. Most of the known art relates to the suppression of electrical trees.

Water treeing is the degradation of solid dielectric materials exposed to moisture and electric fields. It is an important factor in determining the useful life of buried high voltage power cables. Water trees originate from locations of high electrical forces, such as rough interfaces, protruding conductor tips, voids, or buried contaminants, but at lower electric fields than required for electrical trees. In contrast to electrical trees, water trees have the following characteristics:

(a) The presence of water is essential in the growth of water trees.

(b) Normally no partial discharge is detected during the growth of the water trees.

(c) Water trees can grow for years before they reach breakable sizes.

(d) Water trees grow slowly, but start and grow in an electric field much lower than required for the development of electrical trees.

Therefore, the inherent electrical breakdown by corona, electrical treeing and water treeing are different, and their respective mechanisms are also different. Thus, different solutions are needed to improve each of these types of breakdown in dielectric materials.

In addition, cross-linking olefin polymers, especially polyethylene, are used for power cable insulation, whereby the cross-linking agent can act as a water trimming inhibitor. For example, when dicumyl peroxide is used as crosslinking agent in polyethylene, the peroxide residue acts as a tri-inhibitor for some time after curing. However, these residues eventually disappear at the temperature at which the cable is used. Therefore, to be an effective water trimming inhibitor, the additive must remain in the olefin composition at the temperature at which the cable is used.

Dielectric compositions based on ethylene polymers and used as insulation in high voltage wires and cables have been found to prevent rule treeing due to prolonged use under high voltage conditions if they contain one or more organic silanes as water tree suppressing agents. .

The organic silanes of the present invention have good affinity with ethylene polymers, have high shelf life under the temperature at which cables are used, and are resistant to water tree growth.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method of adopting dielectric materials as insulating material in high voltage wires and cables while preventing the water treeing of dielectric materials, and also being resistant to water treeing in high voltage wires and cables, In addition to providing a dielectric material that retains this resistance under the conditions in which the cable is used, it also provides high voltage wires and cables that are resistant to water treeing.

This object is achieved by incorporating ethylene polymers in the dielectric insulating compositions of the present invention to employ certain organosilane compounds as water tree suppressing agents.

The dielectric composition of the present invention.

The dielectric composition of the present invention comprises, by weight ratio, at least one of 100 weight parts of ethylene polymer and about 0.1-10 preferably at least 0.5-3.0 weight parts of organosilane compounds.

Ethylene polymer

Ethylene polymers used in the compositions of the present invention are solid (at 25 ° C.) materials that are homopolymers or copolymers of ethylene. Ethylene copolymers include at least 30% by weight of ethylene, up to about 70% by weight of propylene, and one or more other organic compounds capable of interpolymerizing with up to 50% by weight of ethylene. Other compounds that can be polymerized to ethylene are preferably compounds containing polymerizable unsaturated bonds, such as those present in compounds containing ethylene bonds of > C = C <. These other interpolymerizable compounds include hydrocarbon compounds such as butene-1, pentene-1, isoprene, butadiene, bicycloheptene, bicycloheptadiene, and styrene, as well as vinyl compounds such as vinyl acetate and ethyl acrylate. . Therefore, these copolymers include 0-70% by weight of propylene and 30-100% by weight of ethylene 0-50% by weight of butene-1 or vinyl acetate and 50-100% by weight of ethylene and 0-30% by weight of Propylene, 0-20 wt% butene-1 and 50-100 wt% ethylene.

Preferred copolymers include ethylene / ethyl acrylate, ethylene / propylene, ethylene / butene and the like.

The term polymer also includes mixtures of one polymer with one or more other polymers. Examples of such mixtures are polyethylene and polypropylene, low density polyethylene and high density polyethylene, and polyethylene and olefin copolymers such as those described above. Low density ethylene copolymers with α-olefins are catalysts based on carrier-supported chromium oxide modified with titanium and optionally fluorine as described in US Pat. Nos. 3,606,736 and 4,011,382, at low pressures of about 150-300 psi. Can be prepared under conditions.

The ethylene polymers described herein have a density of about 0.86-0.96 g / cm 3 (ASTM 1505 test method under the same conditions as in ASTMMD-1248-72) and a melt index of about 0.1-10 decigrams / min (test of 44 psi). ASTM D-1238 at pressure).

Organosilane

The organosilanes employed in the dielectric composition of the present invention are selected from one or more compounds having the structure

Wherein R and R ₁ are each hydrogen or alkyl of C _1-8 , substituted or unsubstituted aryl of C _6-18 , and hydroxy, halogen, nitro, alkyl of C _1-8 , C _1- Independently selected from hydrocarbyl groups substituted with substituents such as alkoxy of ₈ and R and R ₁ together with adjacent carbon atoms form a 3-7 carbon atom ring. R ₂ is C _1-8 alkyl, C _1-18 alkoxy C _6-18 aryl, C _1-8 acyloxy C _6-18 substituted or unsubstituted aryloxy and R ₃ and R ₄ are Each independently selected from alkoxy of C _1-18 , substituted or unsubstituted aryloxy of C _6-18 , or acyloxy of C ₁₈ and Y is substituted of alkylene C _6-18 of C _1-18 or Unsubstituted arylene of -CH ₂ CH ₂ NH (CH ₃ )-.

Partially condensed products of these organosilanes also inhibit water treeing when mixed with ethylene polymers. Condensation products are formed by partial hydrolysis and subsequent condensation which separates the alcohol as a byproduct.

For example, if R ₃ is ethoxy, a partial condensation reaction will occur according to the following reaction.

을 포함한다. 응축 단계는 여러번 반복되어서 더 긴 사슬을 형성할 수 있다.The condensed molecules

It includes. The condensation step can be repeated several times to form longer chains.

These organosilanes are prepared by reacting aminoalkylsilanes with either aldehydes or ketones, as described, for example, in US Pat. No. 2,942,019, which is incorporated herein by reference. Good organic silanes are derived from N-β- (aminoethyl) -γ-aminopropyltrimethoxysilane and γ-aminopropyl triethoxysilane.

Supplements

In addition to the ethylene polymers and organosilanes described above, the dielectric compositions of the present invention also include crosslinkers, wherein the dielectric composition must be used as a cured composition rather than as a thermoplastic composition and must also be chemically crosslinked. Such chemical curing agents, aging agents or crosslinkers are well known in the art and are described, for example, in the form of organic peroxides described in US Pat. Nos. 2,826,570, 2,888,424, 2,916,481, 3,079,370 and 3,296,189. Cross-linking agents. These chemical crosslinkers can be used individually or in combination with each other, and they are used in an amount effective for crosslinking. Preferred crosslinkers are di-α-cumyl peroxide.

The dielectric compositions of the present invention also advantageously comprise one or more suitable high temperature antioxidants for the ethylene polymer. Antioxidants include sterically hindered phenols or amines. Polymerized 2,2,4-trimethyl dihydroquinoline can also be used. These are used in known amounts to obtain the intended effect in the composition.

Other auxiliaries that can be employed in the compositions of the present invention include auxiliaries commonly employed in ethylene polymers based on dielectric compositions, including lubricants, oil co-agents, dyes and colorants, and metal deactivators. do.

These adjuvants will be used in the amounts planned to provide the intended effect.

The dielectric compositions of the present invention may also be expanded or filled as polymers other than ethylene polymers that are compatible with the ethylene polymer, ie, may be physically mixed or alloyed with the ethylene polymer, or may be grafted. The final compositions should contain at least about 30% by weight of interpolymerized ethylene for all polymers that may be present in the composition, based on their total weight. Other polymers that can be used include polyvinyl chloride and polypropylene, ethylene propylene rubber (EPR), ethylene, propylene, diene polymers (EPDM), styrene-butadiene-styrene block copolymers, urethane elastomers, polyester elastomers, There are natural rubber.

The total amount of adjuvant used is in the range of 0-60% by weight, preferably 0-10% by weight, based on the total weight of the dielectric composition.

Processing of the Dielectric Composition

All components of the dielectric compositions of the present invention are typically mixed or synthesized prior to being put into an extruder that extrudes onto an electrical conductor. The ethylene polymer and other components can be mixed together by any technique used in the art to mix and synthesize the thermoplastic resins homogeneously. For example, these components can be melted on a variety of devices including multi-roll mills, screw mills, continuous mixers, composite extruders, Banbury mixers, and the like. Organosilanes can be prepared by adding appropriate amounts of aldehydes, ketones and aminosilanes in advance or optionally during the synthesis process. Separate preparation steps may be used for ketones that are less reactive, and catalysts such as ammonium chloride may be used. After the various components of the compositions of the invention are all uniformly mixed, they are, according to the method of the invention, in a known extrusion apparatus, from about 120 ° to 160 ° C. for crosslinkable compositions, and about 200 for thermoplastic compositions. The process proceeds further at < RTI ID = 0.0 >

The crosslinkable compositions of the present invention are extruded onto wires, cables, or other substrates and then vulcanized at high temperatures of about 180 ° C., preferably 215-230 ° C., according to known vulcanization methods.

Evaluation of Organosilane Water Trimming Inhibitors in Dielectric Compositions

In order to determine the use and effect of the water tree stabilizing dielectric composition of the present invention, the composition is bound to its organosilane by using an accelerated water tree test method that uses a combination of frequency acceleration and standard defect samples. It was evaluated with them. This test uses a compression molded sample with 24 cylindrical depressions in the base. 1a is a cross-sectional view of a sample, where a is 152.4 mm, b is 25.4 mm, and c is 19.05 mm. The conical depression W of FIG. 1a is enlarged by FIG. 1b, where d is 3.18 mm, e is 60 °, f is 3.18 mm, g is 6.35 mm and the tip radius is about 5 μm. The calculated maximum electric field is about 250 KV / mm for a typical 15 KV power cable.

When these samples are measured, 100 ml of the electrolyte solution is poured into a dish and then transferred into a grounded precursor, which usually contains the same electrolyte solution. A 50-mm-diameter platinum wire ring is immersed in the electrolyte in the dish and then connected to a voltage source.

Preliminary experiments at 8.5 KHz show that electrical treeing and subsequent failures occur rapidly at test voltages above 10 KV. In order to limit the study on the effect of water treeing, a voltage of 5 KV was used in this embodiment. Under these conditions, a low density polyethylene sample will develop water trees at the tip of the conical depression between 120 and 240 μm in length for 24 hours when using 0.01 N Nacl dissolved in distilled water as the electrolyte solution. To facilitate review of the found water treeing, each of the twelve conical depressions closest to the center of the sample was fitted with a 12.7 mm (1/2 inch) circular die and arbor press. It is drilled using. The resulting plate is placed in a boiling solution containing 0.50 g of methylene blue and 8 ml of concentrated ammonia water (about 28%) in 250 ml of distilled water for about 30 minutes. The disc is then sliced and attached to a microscope slide for observation. Samples prepared in this way were maintained for a period of up to about two years without apparently breaking the shape of the water tree.

A typical water tree grown for 24 hours in low density polyethylene under these conditions and colored as described above is shown in FIG. Growth occurs in an approximately hemispherical region centered on the cone's correction phase, which is usually very obscure. For this reason, it has been found that the most satisfactory direction for measuring the degree of tree growth is perpendicular to the side of the cone, as shown in FIG. In the test performed by the inventors, all twelve colored trees were measured from the central group of conical depressions and the average length was calculated.

는, 이러한 성장속도가 매우 빠른, 성장개시로부터 물 트리가 크게 성장한 후의 훨씬 낮은 속도로까지 변한다. 그 데이터는 다음의 실험식과 일치한다.Growth rate of water tree

This growth rate is very rapid, ranging from the onset of growth to a much lower rate after the water tree grows significantly. The data is consistent with the following empirical formula.

Or likewise

Where L is the length of the growing tree,

t is the time at which the measurement was made (hours).

Assuming that the water trees are relatively more electrically conductive than polyethylene, the electric field E at the boundary of the growth tree can be calculated from equation (3) derived for the point-to-point geometry of this test method. .

Where V is the voltage applied to the electrolyte in the sample dish, r is the point electrode radius, which is equal to the length L of the growth tree, and d is 3.175 mm-L, the distance to the ground electrode.

A graph of experimentally measured tree growth rate dL / dt, shown as a function of E ² , calculated by equation (3) is shown in FIG. 3. The straight line through the data points is a graph of the following equation.

Where K is the rate constant for water treeing and has a value of 2.47 × 10 ⁻¹¹ (mm / hr) (V ² / mm ² ) in this experiment. Since E is a function of L from Formula (3), the following Formula (5) can be integrated numerically.

By measuring the tree lengths individually to obtain the rate constant for treeing,

Here, dL, E, V, r, d and t are as defined above. The following examples are merely to illustrate the scope of the present invention and not to limit the scope thereof.

General mixing method

The dielectric compositions used in the examples were all prepared according to Method A below.

[Way]

A polyethylene independent polymer with a melt index of 0.2 dg / min and a density of 0.92 g / cc, or an ethylene-α-olefin copolymer with an melt index of 0.8 dg / min and a density of 0.92 g / cc, an organosilane and an antioxidant, was charged in a Banbury mixer. It is synthesized at a melting temperature of about 140 ° C. The resulting composition was then plated in a two-stage roll mill at about 165 ° C., granulated and compression cast to a test sample.

Water tree growth rate (WTGR) is the ratio of rate to control rate for the test composition. Control samples were commercially available high molecular weight containing 0.10% of 4,4'-thiobis (3-methyl-6-tishbutylbutyl phenol) antioxidant at a melt index of 0.2dg / min and a density of 0.918 g / cm 3. Polyethylene (control A), or 0.10% thiodimethylene-bis (3,5-di-butyl-4-hydroxy) hydrocinnamate oxidation if the melt index is 0.8 dg / min and the density is 0.92 g / cm 3 Ethylene-butene-1 copolymer (control B) containing an inhibitor. The components of these control samples were mixed according to Method A but no organosilane was added. The water tree growth rate was obtained by measuring the length of the water trees formed within 24 hours and calculating the absolute rate of tree growth using the equations (1) to (7). Controls A and B were given relative values of 1.0 and 1.3 relative to the rate of water tree growth, respectively.

Example 1

This example illustrates a method of forming an organosilane by adding aldehyde and aminosilane to polyethylene in the synthesis step.

The following ingredients were charged to the Banbury mixer.

Continue mixing until 28 pounds of Control A polyethylene, 122.4 g of benzaldehyde and 256 g of N-β (aminoethyl) -γ-aminopropyltrimethoxysilane melt temperature reached 325 ° F. It was put into the extruder of and cut into rhythm. Water tree test samples were cast at 165 ° C. The water tree growth rate was obtained by measuring the length of the water trees formed in 24 hours. The relative rates of tree growth were calculated using the formulas (1) to (7), and the data are summarized in Table 1.

Example 2

After synthesis the method of Example 1 was repeated as it was except that the composition was prevented in a vacuum oven at 75 ° C. for 48 hours. WTGR is shown in Table 1.

Example 3-5

The following compositions were prepared by synthesizing according to Method A in a laboratory Brabender mixer. The polyethylene homopolymer, antioxidant and organosilane organosilane additives of Control A were formed as in Example 1.

The relative velocities of water tree growth were calculated using the equations (1) to (7). The reactants used to produce the organosilanes as well as their concentrations and WTGR are shown in Table 1.

Example 6

This example illustrates the process for preparing an organosilane by reacting a ketone with an organosilane and adding the reaction product to the polymer.

Organosilanes were prepared by reacting acetophenone with -γamino triethoxy silane in the presence of an ammonium chloride catalyst.

1.5% by weight of organo silane thus prepared was added to the polyethylene of Control A in a laboratory Banbury mixer according to Method A. The WTGR after setting this composition in the 75 degreeC vacuum oven for 48 hours was 0.21.

This example shows that heating under vacuum does not reduce the water tree resistance as can be seen by the low value for WTGR.

TABLE 1

A is N- (aminoethyl) -γ-aminopropyltrimethoxy silane

B is γ-aminopropyltriethoxysilane

The data in Table 1 show that the water tree growth rate is, in all cases, less than that of Control A. In these examples (2 and 6) subjected to vacuum treatment, the resistance to water treeing was maintained as it can be seen by the low value of the WTGR.

Example 7-16

The following compositions were prepared by synthesis according to Method A in a laboratory Brabender mixer or Banbury mixer. Polyethylene copolymer of Control B and antioxidant and organosilane.

Organosilane additives were prepared in the same manner as in Example 1. Relative rates of water tree growth were calculated in equations (1)-(7). The reactants used to produce the organosilane, as well as their concentrations and WTGR, are shown in Table 2.

TABLE 2

(1) Samples were left in a vacuum oven at 72 ° C. for 48 hours after preparation.

A is N-β (aminoethyl) -γ-aminopropyltrimethoxy silane.

B is γ-aminopropyltriethoxy silane.

Example 17-20

The method of Example 6 was repeated as it is except that the amounts of the reactants, polyethylene copolymers and organosilanes added to the WTGR were as shown in Table 3 for organo silane preparation.

TABLE 3

(1): The samples were left in vacuum oven at 72 degreeC for 48 hours after manufacture.

B is γ-aminopropyltriethoxy silane.

The data in Tables 2 and 3 show that the WTGR is significantly lower in these examples compared to the case of Control B with a high WTGR value. In addition, this suppression is that the low values for the compositions of Examples 7,10,17 and 20 are retained by the composition, as can be seen by the WTGR, which is processed in a vacuum oven after preparation. .

Example 21

This example shows the water tree inhibiting properties of partially condensed organosilanes.

Organosilanes were prepared by first reacting the same moles of benzaldehyde with γ-aminopropyl triethoxy silane and then allowing the byproduct water to react with the silyl ester groups to produce about 50% condensed material.

The composition was prepared according to Method A in a laboratory Brabender mixer using the ethylene homopolymer and antioxidants of Method A and 1% by weight of partially condensed organosilane. The WTGR of this composition was initially 0.04 and 0.06 after vacuum at 85 ° C. for 14 days.

Example 22

This example shows the water tree inhibiting properties of organosilanes having high molecular weight alkoxy groups.

The organosilane was prepared by first reacting benzaldehyde with γ-aminopropyl-triethoxy silane in the same molar number as in Example 21, and the side product water was removed. The resulting azomethinesilane was then reacted with dodecyl alcohol, whereby half of the ethoxy groups were replaced with dodecyloxy groups.

Again using 1% by weight of organosilane, the composition prepared as in Example 21 had a WTGR of 0.09 before being vacuumed and 0.04 after vacuuming.

Claims (1)

As described herein above, a dielectric composition composed of an ethylene polymer and an effective amount of at least one organosilane having the following structure as a water trimming inhibitor.

Wherein R and R ₁ are each selected from hydrogen, hydrocarbyl groups or substituted hydrocarbyl groups wherein the substituents are hydroxy, halogen, nitro, C _1-8 alkyl, C _1-8 alkoxy, or R and R ₁ together with adjacent carbon atoms form a ring of 3-7 carbon atoms, R ₂ is C _1-8 alkyl, C _1-8 alkoxy, C _6-18 aryl, C _1-8 acyloxy, C _6-18 substituted or unsubstituted aryloxy, R ₃ and R ₄ are each C _1-8 alkoxy, C _6-18 substituted or unsubstituted aryloxy or C _1-8 acyloxy , Y is substituted or unsubstituted arylene or -CH ₂ CH ₂ NH (CH ₂ ) ₃ -of C _1-8 alkylene C _6-18 .

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