GB2147225A

GB2147225A - Method of manufacturing coated resin bonded glass fibre rods

Info

Publication number: GB2147225A
Application number: GB08325998A
Authority: GB
Inventors: Richard Hall Clucas
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-09-28
Filing date: 1983-09-28
Publication date: 1985-05-09
Also published as: GB2147225B; GB8325998D0

Abstract

It is known to include a coupling agent (silane) in a filled cycloaliphatic amino hardened resin system which is employed in coating resin bonded glass fibre rods used as electrical insulators. It has been found that the method of introducing the coupling agent in the filled cycloaliphatic amine hardened resin system is critical to the success of the subsequent coating process. The invention lies in a method of preparing and treating a substrate of resin coated glassfibre material prior to coating with the filled cycloaliphatic amine hardened resin system which comprises the following steps: a) the surface of the substrate is abraded or ground to remove the thin resin-rich surface layer which is formed during preparation of the material, thus exposing the composite structure of the rod and providing a roughened surface to promote mechanical adhesion, b) the exposed roughened surface is painted overall with the silane coupling agent. The coating of the substrate with the filled resin system is preferably carried out in a mould whilst the substrate is in a state of stress e.g. under an applied tension, and the coating is preferably also cured whilst the stress is maintained in the substrate. Thus when the substrate is released from the mould the applied coating/interface is in a state of compression shear.

Description

SPECIFICATION Method of manufacturing coated resin bonded glass fibre rods This invention relates to a method of manufacturing coated resin bonded glass fibre rods.

It is known from UK patent application 2077271Ato include a coupling agent (silane) in a filled cycloaliphatic amino hardened resin system which is employed in coating resin bonded glass fibre rods.

Such coated rods may be used for example as electrical insulators.

I have now found that the method of introducing the coupling agent in the filled cycloaliphatic amine hardened resin system is critical to the success of the subsequent coating process.

According to my invention a method of preparing and treating a substrate of resin coated glass fibre material prior to coating with the filled cycloaliphatic amine hardened resin system, comprises the following steps: a) the surface of the substrate is abraded or ground to remove the thin resin-rich surface layer which is formed during preparation of the material, thus exposing the composite structure of the rod and providing a roughened surface to promote mechanical adhesion, b) the exposed roughened surface is painted overall with the silane coupling agent.

Preferably the coating of the substrate with the filled resin system is carried out in a mould whilst the substrate is in a state of stress e.g. under an applied tension.

The coating is preferably also cured whilst the stress is maintained in the substrate.

Thus when the substrate is released from the mould the applied coating/interface is in a state of compression shear.

In accordance with another aspect of my invention in a method of manufacturing coated resin bonded glass fibre rods using a coating mixture comprising a filled cycloaliphatic amine hardened resin system, the coating mixture is prepared by the following series of steps taken in order: firstly, drying powdered alumina and powdered clay to remove any water; secondly, introducing the powders into the resin and mixing; thirdly, adding a silane coupling agent slowly to ensure complete dispersion whilst blending the components using a high shear mixer.

Preferably fillers are also added in the second step i.e. at the same time as the alumina and clay powders.

Preferably the fillers are of particle size less than 50 microns. A preferred filler is a low friction fluorocarbon polymer such as polytetrafluoroethylene.

Another aspect of my invention concerns the moulding technique and preparation of the mould surface. I have found that preparation of the mould surface before each coating operation is essential to ensure clean release of the casting.

In accordance with this aspect of my invention the mould (which may be for example a mild steel) is preheated to 70 to 90 C and a release agent applied to the mould surface. The release agent may be QZ13 manufactured by CIBA-GEIGY.

I have discovered that the method of coating is also very important and, from a further aspect, my invention includes a method of coating involving a) gravity casting b) vacuum degassing of the coating mixture.

Preferably the required amounts of coating mixture and hardener which may be warmed to about 65 C, are blended and immediately placed in a vacuum enclosure which allows agitation of the mixture so that the resin mixture expands under the reduced pressure and foams.

After initial collapse of the foam more gentle agitation continues with the vacuum still being maintained until a second collapse occurs when the vacuum is released quickly.

The mixture is then preferably poured directly into the mould which has been preheated to 60 to 900C with the insulator in place and set at an inclination to the horizontal. When filling is complete the mould is laid horizontally.

The mould is then allowed to cool to room temperature.

One application of the invention is to provide an improved coating of an electrically insulating material capable of withstanding high voltage, having improved resistance to breakdown (tracking) under adverse conditions, having high abrasion resistance (wear) and better impact resistance to currently available insulators made from ceramics. Among other properties of an insulating material made by my method are the following: The coating has good resistance to ultra-violet radiation. It has high resistance to water penetration and absorption and high chemical resistance to environmentally polluted atmospheres such as may occur in the vicinity of railway tracks. Its coefficient of expansion. It has a coefficient of expansion similar to that of the resin bonded glass fibre to which it must adhere strongly.It has the ability to undergo self-cleaning i.e. it does not allow the build-up of extraneous materials on the exposed surface which may result in electrical breakdown. The coating is specifically suitable to the coating of resin bonded glass fibre (RBGF) insulator bodies, although other materials may also benefit by such a coating. The use of RBGF insulators has been practised for some time, but in service the material is found to be unsatisfactory in the long term because of environmental factors, namely by absorption of water and by contamination with atmospheric and degradation products as a result of arcing, eventually resulting in breakdown of the insulation and in disintegration of the insulator.In order to overcome these difficulties a coating material was sought which would be electrically insulating and have adequate mechanical properties and durability, to protect these RBGF substrate. The preferred electrical insulation material comprises an admixture of a cycloaliphatic resin consisting of a diglycidyl ester of hexahydrophthalic acid with or without additions of low polymers of that ester, one or more filler materials; at least one hardener material selected from fully saturated cycloaliphatic polyamines and polyoxypropylene amines; dispersion grade polytetrafluoroethylene (PTFE); and, an adhesion promotor or coupling agent (silane).

A typical formulation of the insulation material, i.e. as an insulating coating material is: Maker's reference Constituent Parts by weight CY 184 Cycloaliphatic resin 100 XD 927 Amine Hardener 15-35 DT 079 Trihydrated Alumina 45 - 200 Supreme China Clay 30 - 60 L160 or L170 Dispersion grade PTFE 0 - 15 A187 Silane adhesion promoter 1-5 The cycloaliphatic resin (CY184) employed in the example is of the type consisting of a diglycidyl ester of hexahydrophthalic acid viz:

The hardener used in the example is a fully saturated cycloaliphatic polyamine (XD 927) and may be used with a minor addition (about 5%) of an accelerator containing phenolic hydroxy (OH) groups.

The invention is applicable to the production of electrical insulators and more particularly to insulators which are suitable for use as insulators forming the whole or part of insulators used in overhead conductors used in electric traction system and in particular they are suitable for in-line running such that they are in direct contact with the pick-up shoe of an electric traction vehicle.

Hitherto such insulators have been manufactured by vacuum-casting and a variety of other techniques, none of which has proved to be satisfactory.

The technique is basically a casting process in which the insulator body is placed in a mould allowing clearance for the required thickness of resin coating to be applied.

A number of important steps in the process are collectively responsible for the successful application of the coating material to the insulator body.

The technique specifically relates to the coating of resin bonded glass fibre rods (RBGF) with a filled cycloaliphatic amine hardened resin system.

In carrying the invention into practice as applied to the coating of RBGF insulator bodies with the above mentioned resin system the following steps are taken: 1. Preparation of coating material An important aspect of the invention is the dispersion (mixing) of the filler materials with the resin system.

The coating mixture is prepared in two steps in a batch basis. The powders are dried by heating in a ventilated oven at 105do to remove any water which could affect the curing of the resin system. Appropriate quantities of the powders are then introduced into the resin and the silane coupling agent added. The mixture is then blended using a high shear mixer to obtain the necessary dispersion and homogeneity in the mixture. The order of addition of the powders is considered to be important to obtain the best mixing.

It has been found that it is often beneficial to include additives to the resin. The particle size of the fillers is preferably not greater than 50 microns, but should cover a higher proportion of fines. The type of filler depends on a number of factors the more important of which are the abrasion resistance and tracking properties.

In general the abrasion resistance will be dependent on relative hardness of the fillers to that of the collection shoe which should be similar. If the filler is too soft the coating will show excessive wear and if too hard is in danger of picking up carbon or other material on the running surface which will cause loss of insulation. Thermal and environmental degradation processes, as a result of exposure to atmospheric conditions, are responsible for the breakdown in the chemical structure and the dielectric losses.

Additives may increase losses either because of their intrinsic ionic and/or polar nature or because they may absorb moisture which further increases the dipolar and ionic nature of the system. In order to reduce surface friction and reduce wear low friction fluorocarbon polymers i.e. polytetrafluoroethylene (PTFE lubricant powders) are especially useful.

The clay, PTFE and alumina are added in turn to the resin and mixed in by hand using a wooden spatula.

The mixture is then placed in a high shear mixer and the silane coupling agent added slowly to ensure complete dispersion. As a consequence the coating mixture contains a large amount of air which is not desirable in the finalised coating. This air, along with some low volatile material in the resin system is removed at the coating stage.

2. Preparation of substrate It is known that for a given filler-resin system the exotherm depression is proportional to the surface area of the filler. Clay and alumina can both retard and inhibit the cure particularly in highly filled systems. The inclusion of a silane coupling agent in the coating mix restores some of the lost exotherm attributed to mineral surfaces, Silane treatment of the resin coating mixture does not necessarily ensure that this will promote satisfactory adhesion between the coating and the substrate (RBGF) and indeed certain fillers may not restore sufficient reactivity at the resin interface to allow higher filler loadings of the resin in practical systems. In this case additional means must be used.

An important aspect of the invention is the interfacial bond between the substrate and the coating material. The optimum mechanical performance of a mineral-filled organic resin coating composition imposes contradictory requirements on the interface between the coating and the substrate.

(i) optimum transfer between substrate and coating requires an interface of intermediate modulus.

(ii) toughness and ability to withstand differential thermal shrinkage and expansion and mechanical shock, between the substrate and the coating requires a more flexible boundary region to relieve local stresses.

The success of the invention is evidenced by the retention of an interfacial reaction zone even following cracking of the coating material, which prevents ingress and interaction of moisture and othercontaminents with the substrate which would otherwise be detrimental to the performance of the insulator.

In addition the coupling agent is involved in coreaction with the resin to promote the interfacial bond between resin and mineral filler. The mechanical and electrical properties, particularly under humid condition are thereby improved.

The substrate of RBGF material is first prepared by grinding to remove the smooth thin resin-rich surface layer that has a tendency to form during the preparation of the material. The effect is two-fold, to provide a roughened surface which will promote mechanical adhesion and to cause the composite structure of RBGF rod to be exposed. The substrate is then painted overall with the silane coupling agent before the coating is applied. The priming of the surface in this way results in the formation of an inter-facial layer which simultaneously reacts with both the substrate (glass fibre and resin) and the coating material (resin and mineral filler). As a result of the complex reactions at the interface, normally carried out at an elevated temperature a deformable resin layer is produced.The formation of this restrained layer gives rise to the better morphology at the substrate - coating interface, resulting in better mechanical properties and moisture resistance of the composite structure.

3. Tensioning of substrate The coating material of this invention is characteristically brittle by nature. It is anticipated that the coated insulators of the invention will be used in situations where they will invariably be subject to tensile stresses or a combination of both tensile, shear or even flexural stresses which may be constant or fluctuating.

It is well known that most brittle materials are strong in compression and weak in tension. In order to overcome their tensile weaknesses a number of techniques have been adopted which have been well documented and applied to a wide range of materials.

The coating of the substrate is preferably carried out whilst the substrate is in a state of stress (tension) that it is likely to experience in service. The coating is applied and cured whilst maintaining the stress on the substrate throughout. The cured coated substrate is then released from the mould placing the applied coating/interface in a state of compression/shear. When the insulator is installed this compression/shear force must first be overcome before the coating is subject to real stresses. As a consequence the mechanical performance of the coating is greatly improved.

The mould is normally made of mild steel although almost any material could be suitable (medium and high carbon steels, low and high alloy steels, nickel based alloys, aluminium and aluminium alloys, copper, bronze, brasses, polymeric materials and reinforced materials).

Preparation of the mould surface is essential in order to release the casting from the mould. On mild steel the best method of treatment has been achieved using a propriety release agent QZ13 (manufactured by Ciba-Geigy) which is applied to the preheated (70-900C) mould before each casting operation. Over a number of casting operations a thin coating of material builds on the mould surfaces which has to be removed periodically before it can affect the dimensional tolerances set on the coating thicknesses.

The RBGF substrate is supported in the mould through openings in the end plates of the mould and is tensioned using pins and small metal wedges. The arrangement is normally in the form of a double cavity with an open top, although multiple cavity moulds could be used. The arrangement has two advantages in that it allows the substrate to be accurately aligned in the mould prior to the coating and improves the final mechanical properties as already indicated.

4. Coating procedure Coating of the substrate by the coating material is achieved by a gravity casting technique following vacuum degassing of the coating mixture.

The requisite amount of coating mixture and hardener, which may be warmed ( < 65 C) to reduce the viscosity and reaction time, are blended together in a suitable container and immediately placed in a vacuum enclosure which allows agitation of the mixture. During this procedure the air in the resin mixture expands under the reduced pressure and causes the mixture to foam. Agitation causes the foam to collapse with a loss of the air and some low volatile material to the vacuum system. Following the initial collapse more gentle agitation continues with the vacuum still being maintained until the mixture commences to foam for a second time. At this point the vacuum is released as quickly as possible but avoiding the reintroduction of air into the mixture, which is then ready to be used for coating the RBGF rods.The whole process of mixing and degassing is usually completed in no more than five minutes, although the temperature of the mixture will dictate the time available. Preheating the mixture assists the subsequent vacuum degassing of the mixture but also accelerates the curing process which limits the time available to introduce the mixture into the mould since viscosity will increase rapidly as the curing process proceeds.

The mixture is then poured directly into the mould containing the pretensioned insulator. The mould is preheated (60 - 80 C) with the insulator in place and inclined to help the resin mixture to infiltrate the mould.

After filling, the mould is placed on a horizontal plane and the open exposed surface skimmed to remove the excess of resin mixture. The mould is allowed to cool to room temperature during which time the coating undergoes curing, until such time as it may be released from the mould. Any excess material can be removed later by sanding or grinding. The substrate is normally over-sized when the coating is cast and may be cut to size before encapsulation of the cut ends following their pretreatment. Should surface air bubbles be present in the coating these can be removed by filling with fresh resin mixture after pretreatment and further cured without detrimental effects to the coating.

Where a RBGF substrate is coated in accordance with the invention and its use is intended as a section insulator in overhead electric traction rail systems they are required not only to be effectively resistant to high voltage current tracking and erosion but also to be abrasion resistant and not pick up carbon when in contact with the pantograph of an electric locomotive. Practical tests carried out to establish the suitability of the coating for this purpose include: (i) tracking comparitortests (ii) arcing when sprayed with a saline solution (iii) arcing in salt/fog chamber (iv) simulation wear testing The results all indicate the coating is most effective in every respect. Other applications for the coating material in accordance with this invention are for the insulators of high voltage switch gear and of high voltage power lines, tension beam insulators and steady arms in overhead electric traction systems which operate in conditions where environmental attack is a major factor for concern. As already indicated and particularly in the case of small insulator elements, the insulator can be made mainly or entirely of the insulation material according to this invention. It is to be understood that the invention includes not only the electrical insulation as herein defined and described but also insulators made therefrom or insulator bodies or substrates when coated therewith.

Claims

1. A method of preparing and treating a substrate of resin coated glass fibre material prior to coating with a filled cycloaliphatic amine hardened resin system, in which: a) the surface of the substrate is abraded or ground to remove the thin resin-rich surface layer which is formed during preparation of the material, thus exposing the composite structure of the rod and providing a roughened surface to promote mechanical adhesion; and b) the exposed roughened surface is painted overall with the silane coupling agent.

2. A method of coating a substrate prepared by the method of claim 1 in which the coating is carried out whilst the substrate is in a state of stress e.g. under an applied tension.

3. A method according to claim 2 and in which the coating is also cured whilst the stress is maintained in the substrate, so that when the substrate is released from the mould the applied coating/interface is in a state of compression shear.

4. In a method of manufacturing coated resin bonded glass fibre rods using a coating mixture comprising a filled cycloaliphatic amine hardened resin system, the coating mixture is prepared by the following series of steps taken in order: firstly, drying powdered alumina and powdered clay to remove any water; secondly, introducing the powders into the resin and mixing; thirdly, adding a silane coupling agent slowly to ensure complete dispersion whilst blending the components using a high shear mixer.

5. A method according to claim 4 in which a filler is added in the second step i.e. at the same time as the alumina and clay powders.

6. A method according to claim 5 and in which the filler is of particle size less than 50 microns.

7. A method according to claim 5 or claim 3 and in which the filler is a low friction fluorocarbon polymer.

8. A method according to claim 7 and in which the filler is polytetrafluoroethylene.

9. A method of coating a substrate with a filled resin system in which, prior to the coating a mould is preheated to 70 to 900C and a release agent applied to the mould surface after which the substrate is placed in the mould and coated with the filled resin system.

10. A method of coating a substrate with a filled resin system involving gravity casting of the coating material around the substrate and vacuum degassing of the coating mixture.

11. A method of coating according to claim 9 or claim 10 and in which the required amounts of coating mixture and hardener are blended and immediately placed in a vacuum enclosure which allows agitation of the mixture so that the resin mixture expands under the reduced pressure and foams, and after initial collapse of the foam more gentle agitation continues with the vacuum still being maintained until a second collapse occurs when the vacuum is released quickly.

12. A method according to claim 11 and in which the mould is preheated to 60 to 80 C and the mixture is then poured directly into the mould, with the insulator in place and set at an inclination to the horizontal.

13. A method according to claim 12 and in which the mould is then allowed to cool to room temperature.

14. A method according to any of claims 7 to 13 and in which the substrate is a resin bonded glass fibre rod.

15. A method according to any of claims 7 to 14 and in which the coating is a filled cycloaliphatic amine hardened resin system.

16. An electrical insulator produced by a method according to any of the preceding claims.

17. Method of producing an electrical insulator substantially as hereinbefore particularly described.