阅读说明：本技术 一种树脂复合型绝缘云母纸及其制备方法 (Resin composite insulation mica paper and preparation method thereof ) 是由 吴海峰 赵建虎 张永平 陈战胜 姜志 于 2020-04-10 设计创作，主要内容包括：本发明公开了一种树脂复合型绝缘云母纸,包括组合云母纸层,其组合云母纸层中包括层叠的两层细粒径云母单片层和一层粗粒径云母单片层,其作为中间层的第二细粒径云母单片层上开点阵通孔,并在两面利用硅烷偶联剂改性并施胶,其胶层中通过芳纶纤维和纳米高导热无机颗粒增强,并在表层的第一细粒径云母单片层表面成型通过纳米高导热无机颗粒填充的补强树脂膜层。其制备时先分别制备三层云母单片层,然后对第二细粒径云母单片层进行处理,并依次叠层预成型,并在预成型后通过热压定型后收卷、裁边得到成品。本发明制得的云母纸在保持云母纸介电性能的同时能有效提高其机械强度和韧性,并具有高导热与高透气性能。(The invention discloses resin composite insulating mica paper, which comprises a composite mica paper layer, wherein the composite mica paper layer comprises two fine-grain-size mica single-sheet layers and one coarse-grain-size mica single-sheet layer which are stacked, a second fine-grain-size mica single-sheet layer serving as a middle layer is provided with dot-array through holes, the two surfaces of the second fine-grain-size mica single-sheet layer are modified and glued by a silane coupling agent, an adhesive layer is reinforced by aramid fibers and nano high-thermal-conductivity inorganic particles, and a reinforcing resin film layer filled with the nano high-thermal-conductivity inorganic particles is formed on the surface of the first fine-grain-size mica single-sheet layer of the surface. The preparation method comprises the steps of respectively preparing three mica single-sheet layers, processing the second fine-particle-size mica single-sheet layer, sequentially laminating and preforming, and rolling and cutting edges after hot-press forming to obtain the finished product. The mica paper prepared by the method can effectively improve the mechanical strength and toughness while keeping the dielectric property of the mica paper, and has high heat conduction and high air permeability.)

1. The resin composite insulating mica paper is characterized by comprising a combined mica paper layer, wherein the combined mica paper layer comprises a first fine-particle-size mica single-sheet layer, a second fine-particle-size mica single-sheet layer and a coarse-particle-size mica single-sheet layer which are sequentially stacked, two sides of the second fine-particle-size mica single-sheet layer are modified by using a silane coupling agent, and resin adhesive layers are formed on the surfaces of two sides; the surface of the second fine-grain-size mica single-sheet layer is provided with a lattice through hole, and the surface of the first fine-grain-size mica single-sheet layer is formed with a reinforcing resin film layer;

the resin adhesive layer is reinforced by 7-9 wt% of aramid fiber and is filled with 0.5-0.8 wt% of nano high-thermal-conductivity inorganic particles; the reinforced resin film layer is a layer of epoxy resin film material, is formed on the surface of the first fine-particle-size mica single-sheet layer in a hot pressing mode, and is filled with 0.5-3 wt% of nano high-thermal-conductivity inorganic particles.

2. The resin composite type insulating mica paper according to claim 1, wherein the first fine particle size mica single sheet layer and the second fine particle size mica single sheet areThe mica flake particle size in the layer is-80 to +200 meshes, the mica flake particle size in the coarse-particle-size mica single-sheet layer is-60 to +80 meshes, and the mica ration in the coarse-particle-size mica single-sheet layer is 70 to 80g/m²。

3. The resin composite insulating mica paper according to claim 1, wherein each mica single-sheet layer in the combined mica paper layer is a non-calcined mica paper made of a mica scale separated by water peeling.

4. The resin composite insulating mica paper according to claim 1, wherein the sum of the sizing amounts of the resin adhesive layers on both sides of the second fine particle size mica single sheet layer is 15 to 20 wt% of the mica content per unit area.

5. The resin composite type insulating mica paper according to claim 1, wherein the thickness of the reinforcing resin film layer is 0.05 to 0.07 mm.

6. The resin composite insulation mica paper according to claim 1, wherein the aramid fiber is a short-cut meta-aramid fiber having a monofilament length of 4 to 7mm and a fineness of 1 to 1.5D.

7. The resin composite insulating mica paper according to claim 1, wherein the surface of the second fine particle size mica single sheet layer is provided with lattice through holes having a single hole diameter of 3 to 5mm, and the sum of unit areas of the lattice through holes is 25 to 35% of the unit area of the corresponding fine particle size mica single sheet layer.

8. The resin composite insulating mica paper according to claim 1, wherein the resin base material used in the resin adhesive layer and the reinforcing resin film layer is one or a combination of bisphenol a type epoxy resins E44, E51, E20 and phenol type epoxy resins F44, F51.

9. According to the rightThe resin composite type insulating mica paper according to claim 1, wherein the nano high thermal conductive inorganic particles are BeO and Al with a particle size of 50-200 nm₂O₃NiO, AlN, BN, SiC and BC or a combination thereof.

10. A method for producing a resin composite insulating mica paper having the technical features of claim 1, which comprises the following steps:

s1, selecting natural muscovite and natural phlogopite as raw materials, washing and drying the raw materials in sequence to obtain dry mica sheets, crushing and peeling the dry mica sheets into mica flakes by utilizing water power in a high-pressure hydraulic crusher, washing, filtering and screening the crushed mica flakes to screen out two groups of mica flakes meeting the particle size condition, and drying the mica flakes to obtain non-calcined mica raw powder;

s2, taking mica raw powder as a raw material, and using a cylinder mould mica paper machine to make a first fine-grain-size mica single-sheet layer, a second fine-grain-size mica single-sheet layer and a coarse-grain-size mica single-sheet layer, wherein the thickness of the corresponding mica single-sheet layer is controlled by controlling the concentration of slurry in the making process;

s3, punching the second fine-particle-size mica single-sheet layer on the surface of the paper by using a plane punching machine to obtain a second fine-particle-size mica single-sheet layer with lattice through holes, and performing double-sided modification treatment on the punched second fine-particle-size mica single-sheet layer by using a silane coupling agent;

s4, blending the nano high-thermal-conductivity inorganic particles with a resin base material, extruding by a double-screw extruder, granulating, crushing and sieving to obtain powdery nano modified resin adhesive powder; blending nano high-thermal-conductivity inorganic particles with a resin base material, adding an auxiliary agent and water, uniformly mixing, and then carrying out heat treatment to prepare an epoxy resin film material, wherein the auxiliary agent is a toughening agent and/or a curing additive, the toughening agent is one or a combination of terpolymers of methyl methacrylate, butadiene and styrene, and the using amount is 0-3 wt%; the curing additive is water glass and/or sodium hydroxide, and the using amount is 0-0.05 wt%;

s5, respectively reeling a first fine-particle-size mica single sheet layer, a second fine-particle-size mica single sheet layer and a coarse-particle-size mica single sheet layer, uniformly spraying resin adhesive powder and aramid fibers on the surface of the coarse-particle-size mica single sheet layer serving as a base material layer, stacking the second fine-particle-size mica single sheet layer after spraying is completed, uniformly spraying the resin adhesive powder and the aramid fibers on the surface of the second fine-particle-size mica single sheet layer again, controlling the spraying amount and the proportion relation during spraying, and stacking the first fine-particle-size mica single sheet layer on the surface of the second fine-particle-size mica single sheet;

s6, preheating the laminated mica paper layer prepared in the step S5 to melt resin adhesive powder, then stacking an epoxy resin film on the surface of the first fine-grain mica monolithic layer, and performing hot-pressing treatment for 30-45 min by using a hot-pressing roller under the conditions that the hot-pressing temperature is 120-130 ℃ and the hot-pressing pressure is 12-16 MPa to obtain a mica paper blank;

and S7, rolling and trimming the resin composite type insulating mica paper blank to obtain the finished product of resin composite type insulating mica paper.

Technical Field

The invention relates to the field of insulating mica products, in particular to resin composite insulating mica paper with better physical properties and a preparation method thereof.

Background

Mica paper products are one of the most extensive application approaches of the current mica, the mica paper made from mica sheet raw materials through crushing, pulping and papermaking usually has most properties corresponding to the types of mica sheet products, and the mica paper is widely used for insulating materials in cables and large-scale electromechanical equipment in industry, has high electrical strength, low dielectric loss, high surface resistance and volume resistance, and simultaneously has excellent physicochemical properties of high insulation, corona resistance, acid and alkali resistance, radiation resistance, chemical stability, high elasticity, high stripping property, high shearing force resistance, high tensile strength and the like.

With the integration development of large motors, particularly wind power, nuclear power, high-speed rail traction motors and other equipment, the motors are required to be small in size, high in insulation strength, long in service life, safe and reliable, so that higher requirements are provided for the size or certain physical and chemical properties of mica paper, one of the main performance indexes is physical strength which is externally expressed as toughness, tensile and compression resistance, wear resistance, bending resistance and impact resistance, and the mica paper directly made of mica sheets is poor in mechanical strength and toughness and cannot meet the increasing application requirements of the field industry.

In the prior art, mica paper is also subjected to strengthening treatment, and the main common strengthening treatment method is to improve the physical structure of the mica paper by a sizing structure, but the structural design is not reasonable, the physical properties of the mica paper are obtained by sacrificing part of electrical properties, and the manufacturability requirement of high insulation conditions is difficult to meet; however, the glue-sprayed layer of the mica paper reinforced by spraying glue is usually formed on the surface of the mica paper, and the mica paper reinforced by spraying glue is only reinforced on one side or two sides of the mica paper, so that the internal strength of the mica paper cannot be improved.

Disclosure of Invention

The technical problem to be solved by the invention is to provide a resin composite type insulating mica paper and a preparation method thereof, so as to solve the defects in the background technology.

The technical problem solved by the invention is realized by adopting the following technical scheme:

a resin composite type insulating mica paper comprises a combined mica paper layer, wherein the combined mica paper layer comprises a first fine-particle-size mica single-sheet layer, a second fine-particle-size mica single-sheet layer and a coarse-particle-size mica single-sheet layer which are sequentially stacked, two surfaces of the second fine-particle-size mica single-sheet layer are modified by a silane coupling agent, and resin adhesive layers are formed on the surfaces of the two sides; the surface of the second fine-grain-size mica single-sheet layer is provided with a lattice through hole, and the surface of the first fine-grain-size mica single-sheet layer is formed with a reinforcing resin film layer;

the resin adhesive layer is reinforced by 7-9 wt% of aramid fiber and is filled with 0.5-0.8 wt% of nano high-thermal-conductivity inorganic particles; the reinforced resin film layer is a layer of epoxy resin film material, is formed on the surface of the first fine-particle-size mica single-sheet layer in a hot pressing mode, and is filled with 0.5-3 wt% of nano high-thermal-conductivity inorganic particles.

Further, the mica flake particle size of the first fine particle size mica single sheet layer and the second fine particle size mica single sheet layer is-80 mesh to +200 mesh, the mica flake particle size of the coarse particle size mica single sheet layer is-60 to +80 mesh, and the mica ration of the coarse particle size mica single sheet layer is 70 to 80g/m²。

By way of further limitation, each mica monolithic layer in the combined mica paper layer is non-calcined mica paper made from water-stripped mica scales.

By way of further limitation, the sum of the sizing amounts of the resin adhesive layers on both sides of the second fine-particle-size mica single sheet layer is 15-20 wt% of the mica content in a unit area.

The thickness of the reinforced resin film layer is 0.03-0.12 mm, and the preferable thickness is 0.05-0.07 mm.

The aramid fiber is further limited to be short-cut meta-aramid fiber with the monofilament length of 4-7 mm and the fineness of 1-1.5D.

The surface of the second fine-particle-size mica single-sheet layer is provided with lattice through holes, the single-hole diameter of each lattice through hole is 3-5 mm, and the sum of unit areas of the lattice through holes is 25-35% of the unit area of the corresponding fine-particle-size mica single-sheet layer.

By way of further limitation, the resin base materials used in the resin adhesive layer and the reinforcing resin film layer are one or a combination of bisphenol A type epoxy resins E44, E51 and E20 and phenolic type epoxy resins F44 and F51.

The nano high-thermal-conductivity inorganic particles are BeO and Al with the particle size of 50-200 nm₂O₃NiO, AlN, BN, SiC and BC or a combination thereof.

A preparation method of resin composite insulating mica paper specifically comprises the following operation steps:

s1, selecting natural muscovite and natural phlogopite as raw materials, washing and drying the raw materials in sequence to obtain dry mica sheets, crushing and peeling the dry mica sheets into mica flakes by utilizing water power in a high-pressure hydraulic crusher, washing, filtering and screening the crushed mica flakes to screen out two groups of mica flakes meeting the particle size condition, and drying the mica flakes to obtain non-calcined mica raw powder;

s2, taking mica raw powder as a raw material, and using a cylinder mould mica paper machine to make a first fine-grain-size mica single-sheet layer, a second fine-grain-size mica single-sheet layer and a coarse-grain-size mica single-sheet layer, wherein the thickness of the corresponding mica single-sheet layer is controlled by controlling the concentration of slurry in the making process;

s3, punching the second fine-particle-size mica single-sheet layer on the surface of the paper by using a plane punching machine to obtain a second fine-particle-size mica single-sheet layer with lattice through holes, and performing double-sided modification treatment on the punched second fine-particle-size mica single-sheet layer by using a silane coupling agent;

s4, blending the nano high-thermal-conductivity inorganic particles with a resin base material, extruding by a double-screw extruder, granulating, crushing and sieving to obtain powdery nano modified resin adhesive powder; blending the nano high-thermal-conductivity inorganic particles with a resin base material, adding an auxiliary agent and water, uniformly mixing, and then carrying out heat treatment to prepare an epoxy resin film material;

s5, respectively reeling a first fine-particle-size mica single sheet layer, a second fine-particle-size mica single sheet layer and a coarse-particle-size mica single sheet layer, uniformly spraying resin adhesive powder and aramid fibers on the surface of the coarse-particle-size mica single sheet layer serving as a base material layer, stacking the second fine-particle-size mica single sheet layer after spraying is completed, uniformly spraying the resin adhesive powder and the aramid fibers on the surface of the second fine-particle-size mica single sheet layer again, controlling the spraying amount and the proportion relation during spraying, and stacking the first fine-particle-size mica single sheet layer on the surface of the second fine-particle-size mica single sheet;

s6, preheating the laminated mica paper layer prepared in the step S5 to melt resin adhesive powder, then stacking an epoxy resin film on the surface of the first fine-grain mica monolithic layer, and compounding under the hot-pressing action of a hot-pressing roller to form a mica paper blank;

and S7, rolling and trimming the resin composite type insulating mica paper blank to obtain the finished product of resin composite type insulating mica paper.

As a further limitation, the auxiliary agent is a toughening agent and/or a curing additive, the toughening agent is one or a combination of terpolymers of methyl methacrylate, butadiene and styrene, and the using amount is 0-3 wt%; the curing additive is water glass and/or sodium hydroxide, and the using amount is 0-0.05 wt%.

Further, when the hot-pressing is performed by the hot-pressing roller in step S7, the hot-pressing temperature is 120 to 130 ℃, the hot-pressing pressure is 12 to 16MPa, and the hot-pressing time is 30 to 45 min.

Has the advantages that: the resin composite type insulating mica paper is combined by mica single-sheet layers with different particle sizes, and a resin layer structure with nano high-thermal-conductivity inorganic particles is matched, so that a large number of pore channels can be formed in the mica paper, and the resin composite type insulating mica paper has excellent high temperature resistance, air permeability and permeability, is favorable for improving the treatment time of a VPI insulating treatment process and improves the insulating compactness; the intermediate mica paper layer structure with the lattice through holes and modified by the silane coupling agent can be favorable for compounding the resin layer and the mica paper layer, and the filling glue layer is optimized, so that the aramid fiber reinforced structure can be lapped with three mica single-sheet layers, and the finished mica paper has good flexibility, high strength, high wear resistance and tear resistance.

Drawings

Fig. 1 is a schematic structural diagram of a preferred embodiment of the present invention.

Wherein: 1. a reinforcing resin film layer; 2. a first fine particle size mica veneer layer; 3. a second fine particle size mica veneer layer; 4. a mica single-layer with a coarse particle size; 5. and (6) lattice through holes.

Detailed Description

In order to make the technical means, the creation characteristics, the achievement purposes and the effects of the invention easy to understand, the invention is further explained below by combining the specific drawings.

In the following examples, it will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Referring to the embodiment of the resin composite type insulating mica paper of fig. 1, in this embodiment, the mica paper comprises a reinforcing resin film layer 1, a first fine particle size mica single sheet layer 2, a second fine particle size mica single sheet layer 3 and a coarse particle size mica single sheet layer 4 from top to bottom, wherein the first fine particle size mica single sheet layer 2, the second fine particle size mica single sheet layer 3 and the coarse particle size mica single sheet layer 4 are non-calcined mica paper made of water-peeled and separated mica flakes, and the mica weight of the coarse particle size mica single sheet layer 4 is 75g/m²The mica basis weight of the first fine particle size mica single sheet layer 2 and the second fine particle size mica single sheet layer 3 at the time of molding was 45g/m²。

The coarse-particle-size mica single-sheet layer 4 is made by taking mica scales with the particle size of-60 to +80 meshes as a raw material, the first fine-particle-size mica single-sheet layer 2 and the second fine-particle-size mica single-sheet layer 3 are made by taking the mica scales with the particle size of-80 to +200 meshes as the raw material, and the second fine-particle-size mica single-sheet layer 3 is also formed with array-shaped lattice through holes 5, the single hole diameter of the lattice through holes 5 is 4mm, and the arrangement density of the lattice through holes 5 is controlled, so that the sum of the areas of the lattice through holes 5 in unit length is 30% of the area corresponding to the unit length of the second fine-particle-size mica single-sheet layer 3.

A resin adhesive is filled between the first fine-particle-size mica single-sheet layer 2 and the coarse-particle-size mica single-sheet layer 4, the resin adhesive is uniformly filled in gaps between the first fine-particle-size mica single-sheet layer 2 and the second fine-particle-size mica single-sheet layer 3 and between the second fine-particle-size mica single-sheet layer 3 and the coarse-particle-size mica single-sheet layer 4 and lattice through holes 5 between the second fine-particle-size mica single-sheet layers 3, the resin adhesive comprises 7-9 wt% of aramid fiber with the monofilament length of 4-7 mm and the fineness of 1D and 0.5-0.8 wt% of nano high-thermal-conductivity inorganic particles, and the sum of the glue application amount of the resin adhesive is 15-20 wt% of the mica content in a unit area; meanwhile, 0.5-3 wt% of nano high-thermal-conductivity inorganic particles are filled in the reinforcing resin film layer 1 formed on the surface of the first fine-particle-size mica single-sheet layer 2, and the thickness of the reinforcing resin film layer 1 is 0.03-0.12 mm.

In this embodiment, the resin base materials used in the reinforcing resin film layer 1 and the resin adhesive are the mixture resin of bisphenol a epoxy resin E44 and E51 in a mass ratio of 2:1, and the nano high thermal conductive inorganic particles are Al with a particle size of 50-70 nm₂O₃AlN and SiC in a mass ratio of 3:1: 1.

In other embodiments, the resin base materials used in the resin adhesive layer and the reinforcing resin film layer 1 are made of one or a combination of bisphenol a type epoxy resins E44, E51, E20 and novolac type epoxy resins F44, F51, and the performance difference is only based on the difference of the physical and chemical properties of the basic resin performance, and the corresponding performance difference can be predicted by the corresponding resin performance and proportion; the nanometer inorganic particles with high thermal conductivity can also be adoptedBeO and Al with particle size of 50-200 nm₂O₃One or the combination of NiO, AlN, BN, SiC and BC has the difference of the cost and the heat conductivity of the raw materials based on the nano high heat conductivity inorganic particles with different particle sizes and types, and the corresponding difference of the properties can be deduced by the prior art.

The resin composite insulating mica paper having the structure of the above example was prepared by the following method:

selecting natural muscovite and natural phlogopite as raw materials, washing and drying the raw materials in sequence to obtain dry mica sheets, crushing the dry mica sheets by utilizing water power in a high-pressure hydraulic crusher to be peeled into mica flakes, washing, filtering and screening the crushed mica flakes to obtain coarse-grain-size mica flakes with the grain size of-60 to +80 meshes and fine-grain-size mica flakes with the grain size of-80 to +200 meshes, classifying and drying the coarse-grain-size mica flakes to obtain non-calcined mica raw powder;

using the raw mica powder as a raw material, respectively pulping by a cylinder mould paper machine to form a first fine-particle-size mica single-sheet layer 2, a second fine-particle-size mica single-sheet layer 3 and a coarse-particle-size mica single-sheet layer 4, and controlling the mica ration of the first fine-particle-size mica single-sheet layer 2 and the second fine-particle-size mica single-sheet layer 3 to be 45g/m²And the mica basis weight of the coarse-particle-size mica single-sheet layer 4 was 75g/m²Pressing holes on the paper surface of the second fine-particle-size mica single-sheet layer 3 by using a plane hole pressing machine, and adjusting the hole pressing aperture and the hole pressing interval of the plane hole pressing machine to ensure that the single-hole diameter of the lattice through holes 5 on the second fine-particle-size mica single-sheet layer 3 is 4mm, and the sum of the areas of the lattice through holes 5 in unit length is 30% of the area corresponding to the unit length of the second fine-particle-size mica single-sheet layer 3; and then performing double-sided modification treatment on the second fine-particle-size mica monolithic layer after punching by using a silane coupling agent.

Blending nano high-thermal-conductivity inorganic particles with different proportions with a resin base material, extruding the mixture by a double-screw extruder, granulating, crushing and sieving to obtain powdery nano modified resin adhesive powder, simultaneously blending the high-thermal-conductivity inorganic particles with different proportions with the resin base material, adding 2 wt% of methyl methacrylate as a toughening agent to toughen the resin base material, and adding 0.03 wt% of water glass as a curing additive to improve the curing rate.

Respectively rolling up a first fine-particle-size mica single-sheet layer 2, a second fine-particle-size mica single-sheet layer 3 and a coarse-particle-size mica single-sheet layer 4, uniformly spraying resin adhesive powder and aramid fiber on the surface of the coarse-particle-size mica single-sheet layer 4 serving as a base material layer, stacking the second fine-particle-size mica single-sheet layer 3 after spraying is finished, uniformly spraying the resin adhesive powder and the aramid fiber on the surface of the second fine-particle-size mica single-sheet layer 3 again, controlling the spraying amount during spraying, controlling the gluing amount of the resin adhesive, adjusting the application proportion relation between the aramid fiber and the resin adhesive, stacking the first fine-particle-size mica single-sheet layer 2 on the surface of the second fine-particle-size mica single-sheet layer 3 to form a stacked mica paper layer, conveying the stacked mica paper layer into a tunnel furnace, preheating and melting the resin adhesive powder, stacking epoxy resin film materials 1 with different thicknesses, hot-pressing for 30min by using a hot-pressing roller under the conditions that the hot-pressing temperature is 125 ℃ and the hot-pressing pressure is 14.5MPa to obtain a mica paper blank, and rolling and cutting edges to obtain the finished resin composite insulating mica paper.

The resin composite type insulating mica paper with the structural characteristics can be prepared by the method, eight groups of resin composite type insulating mica paper are prepared by taking the glue application amount of the resin adhesive, the blending amount of the aramid fiber and the nano high-thermal-conductivity inorganic particles in the resin adhesive and the change value of the thickness of the reinforcing resin film layer 1 within a range as variables under the process conditions, the eight groups of resin composite type insulating mica paper and the common epoxy mica paper which is sold in the market and has the same mica quantitative amount, the glue application amount of the resin adhesive and the similar proportion of aramid fiber reinforcement are used as comparison groups for technical parameter comparison, and the results are shown in the following table:

as can be seen from the data in the above table, compared with the commercially available ordinary epoxy mica paper having the same mica basis weight and resin binder sizing amount and similar reinforcing aramid fiber ratio, the resin composite insulating mica paper prepared under the conditions of the technical solutions of the embodiments has the advantages that the physical strength represented by the tensile property is slightly improved and the volatility is reduced while the dielectric property is kept similar and slightly increased, and the air permeability, the stiffness and the thermal conductivity are all significantly better than those of the commercially available ordinary epoxy mica paper.

The foregoing shows and describes the general principles, essential features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.