Topics Covered
The initial work was on intumescent systems for helicopter Door & Hatches and coatings for automotive use.
This lead to work on: -
This PowerPoint document will be presented at Polymers in Defence -
Wednesday 10th & Thursday 11th February 2010
With over 40 years experience within the industrial surface coatings industry, Graham Armstrong is
eminently qualified to discuss most aspects of high performance surface finishing. Over the past 10
years he has been involved in surface coatings applications in aerospace and defence, a large proportion
of this work involving specialist performance finishes on composites.
A fellow of the Institute of Metal Finishing, and the chair of the Institutes’ organic finishing section, he regularly is asked to comment on technical aspects of paints and surface coatings.
The use of composites in aerospace and defence applications is increasing at a fast rate. The use of these new materials is presenting major challenges to the surface coatings industries, as these substrates require a totally different approach to that used for coating metals.
This paper will look at some current projects where specialist coating systems have been developed to meet the performance, protection and decorative needs of the defence and aerospace industries on composites. It will also take a look forward into new coating systems and areas where the coatings industry needs to work with the manufacturers of composites and the manufacturing primes to enable composites to become the material of choice in many more areas. This will include many specialist systems that will not have needed to be considered on metals.
Indestructible Paint are specialist manufacturers of engineered coatings for the aerospace and defence industries. Established in 1978 in Birmingham, UK, the company has seen a continued growth in all areas of high performance surface coatings across both industry areas.
The company hold approvals to AS 9100; ISO 9001 and ISO 14001, together with multiple company approvals from the primes, tier 1 and tier 2 suppliers in both aerospace and defence manufacturing.
From the base in Birmingham, the company exports coatings to over 40 countries throughout the world, and has established stockists and distributors in America; France; India and Singapore. In addition the company has representatives in Germany/Switzerland/Benelux; Eastern Europe; Israel; the Middle East and Russia.
At our Birmingham base we have full research and development teams, full manufacturing capability for our complete product range, and can offer full commercial, technical, environmental and legislative backup for our clients.
A major aspect of our activities has always been working with clients to develop, or engineer, coatings to meet specific high performance criteria to exacting specification requirements. We believe product development to be critical to our success, and to ensure this continues, our development team is the major department within the company, and is currently being expanded with additional highly qualified chemists.
Our client base covers most of the primes within aerospace and defence industries, and includes
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Our product range comprises the majority of high performance organic coating systems, including epoxy, silicone, polyurethane, polyimide, and specific resin type blends, and inorganic metallic-ceramic and pure ceramic slurry coatings.
Typical performance criteria will include temperature resistance up to 500-600oC, corrosion resistance in excess of 1000 hours salt spray to ASTM B-117, resistance to fluids and chemicals at elevated temperatures and erosion and UV resistance.
Our coatings for composites fulfil many technical and decorative functions, and this paper will examine several of these, highlighting the background behind the project, the benefits of using coatings and the technical performance of the composite-coating system.
We first started examining coatings for composites over 10 years ago, working on systems for components used within the airframe of helicopters. This initial work looked into an intumescent system for use on hatches and doors, where there was a requirement for a minimum 5 minute protection of the substrate in an avgas fire, where a temperature up to 1100°C would be expected. About the same time we first started work on erosion resistant coatings for aero engine nose spinners, but, as will be seen later on in this paper, this has been an on-going project.
Let us now examine some recent projects which illustrate our commitment to solving client problems and issues by either use or modification of existing coating systems of by engineering new materials and/or processes.
We have worked closely with the designers and engineers of Hindustan Aeronautics in the development of the HAL DHRUV advanced light helicopter.
This was conceived around 8-10 years ago, as a fully composite air framed vehicle, the first such helicopter to fly. In addition to the airframe, the rotor blades were also to be manufactured from composite. The vehicle was designed from the start for both commercial and military use.
The airframe is manufactured from carbon fibre re-enforced epoxy composite, the
components being laid up by hand in moulds before auto-claving. The rotor blades are manufactured
from a carbon fibre epoxy pre-preg.
The condition of both components out of the mould can be varied, with both resin rich and resin weak areas at the surface. Initially it was the thought that much hand filling and sanding would be necessary, an obviously time consuming and expensive process.
Working closely with the HAL engineers on site in Bangalore, India, Indestructible technical personnel, aided by the local Indestructible distributor, developed a system for the spray application of low VOC, high solids epoxy primer-filler and primer-surfacer that would provide a smooth surface for further decoration/finishing with minimal sanding and localised hand filling.
The primer-filler was of especial interest, as, as well as being an excellent deep hole filler, gave an added benefit of very low weight (the specific gravity of the coating at application is below 1.00) and capable of high film builds in a single coat without sagging. In addition, it was found that the lightweight filler used, in this case glass micro balloons, resulted in a degree of thermal barrier performance, reducing throughput of heat into the composite panel surface. This made the coating ideal for use of areas where heat transfer into the composite was to be avoided, in particular around the engine exhausts and on the firewall between the engine compartment and the passenger cabin.
Both these products have been tested, and are released to, the relevant British defence standard (Def-Stan 80-216), as required by the manufacturer.
As a supplementary coating, there was a need for a finishing system that would meet the needs of both commercial and military applications. Reviewing the technical needs of both applications, it was decided that the IP6 range of low VOC polyurethane finishes would be the ideal base.
For commercial applications, the majority of products supplied have been high gloss finishes in safety or livery colours, for example a bright red gloss to RAL 3020 for use on an air ambulance.
For military vehicles, there is a requirement for dead matt finishes with the necessary IRR properties,
in a range of earth colours as camouflage for the army, and grey / blue colours for navy / airforce.
The use of these products within India was a challenge in its own right. Typical ambient temperatures in Bangalore average 25-35°C, allied to relative humidity’s from very dry to very high, and it was thus necessary to formulate both the primers and finishes to work in these conditions.
However, as the project progressed, and these coating systems were fully adopted, the need for re-furbishment of early vehicles became necessary. A decision by the Indian army to re-furbish the helicopters on the operational base caused further application problems. The coatings that had been formulated to work in temperatures never below 25°C were now required to be applied in northern India during their winter, where ambient temperatures more akin to northern Europe were experienced.
By careful selection of thinners and diluents, both the primers and finishes were adjusted for application at an average temperature of 5°C, with the ground surrounding the (unheated) hangar covered in snow!
This is an area where coatings on composites have been used for a period of time, although the initial coatings selected were based on resin technology used for mouldings rather than paints.
One of the main benefits of elastomeric polyurethanes is their erosion resistance, to both particle and rain, and as such they are now used widely on applications where this is of importance.
A recent series of failures on a specific engine spinner brought about a research project into the coatings used.
The major difficulties encountered with elastomeric polyurethanes is with the application and use of the products as a sprayable coating, and the adhesion to the base composites, invariably carbon fibre re-enforced bisphenol A epoxies.
Our development teams looked into several areas of concern; the use of a suitable primer-basecoat system; the manufacture of an “application friendly” elastomeric coating and the use of UV absorbers to prevent degradation of the substrate and basecoat where the coating is the be used in areas of high UV radiation (as would be seen on an aircraft engine nose spinner).
After much discussion, involving the engineers of the engine manufacturer, a series of 14 coating combinations were examined, and tested for exposure to UV light (QUV testing); direct adhesion pull off and bell peel adhesion testing. In all a total of 336 test panels were produced.
The following table illustrates the work conducted and the results achieved:
| Paint System | PU Colours | PU Systems | Epoxy
Paint Condition |
Sample Ref | Bell Peel Result | Pull of Result | |||
| Average | SD | Average | SD | ||||||
| Clear | Clear | Standard | Part Cure | 4T1 | UV Exposed | 159.50 | 9.26 | 66.67 | 20.82 |
| As Received | 171.50 | 5.00 | 54.00 | 8.94 | |||||
| Full Cure | 4T2 | UV Exposed | 166.73 | 23.71 | 43.33 | 11.55 | |||
| As Received | 176.67 | 32.93 | 59.00 | 14.32 | |||||
| UV Standard | Part Cure | 3T3 | UV Exposed | 180.00 | 50.19 | 43.33 | 5.77 | ||
| As Received | 216.67 | 19.47 | 57.50 | 9.87 | |||||
| Full Cure | 3T4 | UV Exposed | 202.43 | 9.12 | 43.33 | 5.77 | |||
| As Received | 224.17 | 11.25 | 56.67 | 16.33 | |||||
| Black | Clear | Standard | Part Cure | 4T5 | UV Exposed | 191.00 | 14.03 | 66.67 | 14.43 |
| As Received | 195.83 | 17.15 | 68.33 | 29.44 | |||||
| Full Cure | 4T6 | UV Exposed | 156.33 | 13.19 | 70.00 | 17.32 | |||
| As Received | 164.92 | 26.66 | 60.00 | 15.49 | |||||
| UV Standard | Part Cure | 3T7 | UV Exposed | 220.83 | 7.64 | 73.33 | 20.82 | ||
| As Received | 212.50 | 11.83 | 63.00 | 13.96 | |||||
| Full Cure | 3T8 | UV Exposed | 209.17 | 32.63 | 70.00 | 15.28 | |||
| As Received | 224.58 | 24.26 | 70.00 | 17.89 | |||||
| Aluminium | Clear | Standard | Part Cure | 4T9 | UV Exposed | 178.33 | 12.58 | 67.22 | 14.39 |
| As Received | 197.08 | 21.99 | 68.00 | 13.78 | |||||
| Full Cure | 4T10 | UV Exposed | 158.33 | 2.89 | 68.33 | 16.07 | |||
| As Received | 170.55 | 27.05 | 60.83 | 17.44 | |||||
| UV Standard | Part Cure | 3T11 | UV Exposed | 187.50 | 27.04 | 78.33 | 20.21 | ||
| As Received | 207.08 | 14.27 | 77.50 | 17.25 | |||||
| Full Cure | 3T12 | UV Exposed | 220.00 | 9.01 | 63.33 | 5.77 | |||
| As Received | 231.67 | 27.55 | 69.17 | 22.45 | |||||
As a conclusion to this work, it was established that a correctly formulated elastomeric coating, including UV absorbers, applied over a suitably prepared composite surface base-coated with a 2 component epoxy coating (clear or pigmented) gave the required adhesion, exterior durability and life expectancy.
In a similar vein, we have been working closely with a manufacturer of propellers used on commercial and military aircraft and hovercraft.
Here we were approached to solve a problem of intercoat adhesion of the elastomeric polyurethane to the epoxy basecoat.
As can happen in the development of coating systems, not just specific to composites, the various components used in the
system were being sourced from varying suppliers. Whilst all the individual components met their required specification, when
blended and used on the epoxy composite propeller blades, spasmodic adhesion failures occurred.
By utilising low VOC epoxy basecoats and a matched 2 component elastomeric polyurethane, a simpler, single sourced, coating system was devised that fully met the technical specification for the applied coating whilst achieving 100% adhesion.
As an added benefit, by changing to this system, the manufacturer concerned showed a reduction in VOC emissions of over half a tonne per annum, which went a way to helping to meet their additional required goal in reduction of VOC emissions.
In military aerospace applications, coatings on radomes are subjected to extreme conditions, particularly erosion, both particle and rain. In addition, there are other very specific requirements for coatings used on radomes, not least the requirement for the coating not to interfere with electrical signals; what we describe as “electrical transparency”. The coating system for the composite body of the radome is required to match, in colour and gloss, the camouflage system applied to the remainder of the air frame.
However conventional pigmentation systems cannot be considered as typical pigments used in coatings formulation and manufacture (Titanium dioxide; carbon black etc) will affect the di-electric constant and therefore the ability to allow transmission of the electrical signals.
After much research, a system of non pigment tinted epoxy sealer, and a specially formulated matt coloured 2 component elastomeric polyurethane was tested, and passed the requirements for “electrical transparency”, and rain erosion resistance (to SAE-AMS-C-83231).
On the basis of these successful projects, we foresee a growing interest in this type of system in any areas where composites require exterior durability and erosion resistance.
The expanding use of composites in all types of vehicle manufacture gives rise to major problems with resistance to flame and fire. Most epoxy resin based composites will burn readily and give off toxic smoke and fumes. This is of particular importance in aerospace applications, including aircraft interior components, control boxes and recorders, and composite airframe components.
If we consider interior components, there is a controlled specification for non-burn and non-smoke emission coatings for all coated units. This will include bulkheads; overhead bins and washroom units amongst others.
A system of lightweight, thermal barrier effective, 2 component low VOC epoxy primer, over-coated with a specially formulated non burn, non smoke emission polyurethane topcoat has been proven to meet the requirements of the FAR 25853 specification. This is in use on commercial aircraft components and within both military and commercial helicopters.
As more airframe prime manufacturers consider and introduce composite constructed airframes, the use of composite construction of control boxes and recorders is being introduced across a range of aircraft, both commercial and military.
There is a major concern to protect the complex and delicate electronic instrumentation and controls within these boxes in the case of fire or high temperature exposure.
We have been approached by a major airframe manufacturer to work with them on a thermal intumescent system for use on such control boxes for installation within the airframe, but not the passenger compartment, of a newly developed composite structured aircraft.
The brief was to develop a coating system that would be of minimal additional weight, but that would protect the composite bodied control boxes in case of fire for up to five minutes. There was a requirement to keep the temperature inside the control box to below 350°C (662°F) for this five minute period, with minimal damage to the composite structure.
In the evaluation carried out, two alternative thermal intumescent coatings (IP1265; thermal intumescent and IP9189 full intumescent) were investigated, together and separately, to provide three systems, with intumescent layers at a nominal film thickness of 350µm. This film thickness is considered low for the purpose, at less than 60% of the normal recommendation, and would provide a significant weight saving.
These three systems all featured a low VOC 2 component epoxy seal coat at a nominal thickness of 16µm, followed by individual coating of either IP1265 or IP9189, or a combination system of both products to 350µm, and over-coated with a low flame spread, low VOC 2 component epoxy finish at 30µm.
The test procedure involved applying the three systems to composite panels (Cytec prepeg Cycom 5215 T650 6K-135 5HS). The coatings were force cured at 80°C (175°F), then allowed 7 days stabilisation before test.
A propane fuelled Rothenburger Superfire 2 torch provided the heat source. The flame, measured at a temperature of 1060°C (1940°F), was applied directly to the coating for two and five minutes. (The test broadly follows the set up as required by BSX37).
All three systems intumesced, and kept the substrate temperature below the required 350°C, with minimal damage to the substrate. Without coating under the same test conditions, the substrate reached 520°C, and showed substantial damage.
The following table illustrates the height of the protective char, and the degree of intumescence, as illustrated in the pictures below:
Side View |
Process 1 |
Process 2 |
Process 3 |
2 minute burn |
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 |
 |
5 minute burn |
 |
 |
 |
Front View |
Process 1 |
Process 2 |
Process 3 |
2 minute burn |
 |
 |
 |
5 minute burn |
 |
 |
 |
The level of smoke was slightly worse with the IP1265 thermal intumescent, but still substantially less than with the uncoated sample.
Although all three systems meet the original requirements, the system utilising the IP 9189 full intumescent was selected for use.
These are just some examples of current technology being used to solve surface coatings issues on composites.
In all these cases, the surface coating has been regarded as just this; a coating applied to the surface of the composite.
Whilst this is a perfectly acceptable route to achieving a technical coating on the surface, where decorative aspects are also important, we have found that the varying aspects of the surface of composite parts, with resin rich and/or resin weak areas, can result in much work being required to achieve a smooth finish.
A few years ago we were invited to become involved with a coatings system for a very expensive sports car, where the body structure was to be a carbon fibre re-enforced epoxy resin composite. The panels were to be laid up by hand, and the resin injected into the mould. This resulted in most panels having a varying surface with a large number of resin weak areas and unsealed fibres on the surface.
This had two different but equally important effects.
To achieve the high quality surface finish required, an excessive amount of hand filling and sanding of these areas was needed, and it was calculated that for a complete body structure, over 24 man hours were required.
In addition, on testing it was found that these incorrectly sealed carbon fibres were able to “move about” within the finally finished panels. This resulted in “fibre telegraphing”, to such an extent that with dark body colours, in sunlight the carbon fibre blanket pattern became visible through the final finish.
Both these problems were unacceptable.
In conjunction with the moulding division of the manufacturer, we investigated the use of in-mould priming systems.
This involved coating the inside surface of the panel mould with a coat of low VOC 2 component epoxy primer. The resin system used to formulate the primer was matched to that used in panel production. The mould surface had already been treated with an epoxy compatible, silicone free, release agent.
After curing of the primer, the composite panel was laid up and injected as per the normal system. After panel curing, when removed from the mould, the surface of the “already primed” panel exhibited a very smooth even surface, which mirrored the smooth internal surface of the mould itself.
Utilising this method, the number of man hours for further filling operations was drastically reduced. There were concerns regarding resin weak areas under the in-mould primer, but non destructive testing methods were examined, and a system devised to fully quality control panels manufactured this way.
This process has been discussed in several other composite manufacturing areas, and ongoing testing in both aerospace and defence industries is in progress.
We believe that as composites become more widely specified, new coatings systems to meet new, previously un-thought of challenges will need to be developed. The use of new advanced materials, certainly to include nano particles, will be involved in these new coatings.
The following examples cover areas where we have been involved, but which involve much further, detailed research to bring to fruition.
Although this has an application in all aspects of military and commercial aircraft,
the initial interest in
this project came from manufacturers of composite wing structures looking at novel ways to enable de-icing, and will,
we believe become more important as composites become more widely used.
We have worked very closely on a KTM with the Institute of Materials into the incorporation of piezo-electric cells into an exterior durable, chemical resistant coating.
The idea would be to excite these cells by the passing of an electric current through the coating, which would have the effect of both heating and vibrating ice deposits so as to loosen and remove.
We believe there are alternatives to using these types of cells, and the consideration of nano particles, particularly carbon nano tubes will be of great interest.
Although the project has been initially set up around the requirements of aerospace, and particularly commercial, we believe these types of conductive coatings will be of great interest across many fields.
Damage to components manufactured from metals is usually obvious, as some form of mechanical damage: dents, scratches etc.
Damage to composite components is sometimes less obvious, especially if the component has a technical or decorative surface coating applied.
There is currently concern with the manufacturers of all types of vehicle, land, sea or air, that components, sometimes structure critical, can be damaged by impact from various sources, but that the damage can be masked, even hidden by the surface coating.
This subject has been discussed in great detail with airframe manufacturers where fully composite airframes are to be introduced.
The use of nano additives in the coatings for application to structure critical composites, which will result in a visual change of the surface of the coating if it is subjected to impact or other forms of mechanical damage.
We like to describe these innovative developments as "Bruisable Coatings".
At this stage, these systems are very much in the early stages of development, but this is we believe a major area of concern in all areas of composite manufacture of structural critical components.
In line with developments in the surface coatings industries, there is an ongoing requirement for improvements
in surface hardness, heat resistance and thermal barrier effects and chemical resistance for coatings on composites.
We are aware of developments within the gas turbine industries for the development of composite blades for use in the fan section and the first stages of the compressor.
These specific areas will be subjected to potential damage from erosion, specifically particle, aggressive chemicals and fluids, and high temperatures.
Traditionally when these types of components are produced from metal alloys, high temperature cure coatings can be used which give the performance criteria demanded.
With composites, it is not always possible to utilise high temperature cure coatings, so alternative methods of achieving the required toughness of the coating must be investigated.
This can be achieved in more than one way.
Alternative cure methods, particularly radiation curing, can be investigated to cure existing formulation types, and there is some knowledge already in existence on this. However, the geometry of the component may not lend itself to radiation systems, and capital costs can be high.
There is therefore a move to formulate new coatings than can be low temperature/cold cured, and achieve the high performance properties required. We have an ongoing programme of research into the benefits of nano on the performance of surface coatings, and have certainly found with initial work that increased performances can be achieved. It should be remembered however that nano doesn’t come cheap, and there will be cost implications on these new families of coatings.
With all new developments of surface coatings, the ever changing environmental and legislative issues have to be to the forefront. This is true across the whole of the surface coatings industry, but can have more of an immediate impact for coatings for composites, as these tend to be new developments rather than existing, and need to be formulated to the latest regulations.
Reductions in VOC contents, and the emissions of VOC’s from factories are a major issue throughout the world. It should be borne in mind that local regulations and conditions can apply, and we are regularly asked to work with customers to help them meet their local requirements.
Earlier in this paper we discussed the application of coatings to propeller blades. In this case, the manufacturer had been charged with reducing VOC emissions to below 5 tonne per annum, a reduction of over 30% from the original level. This was in addition to a required increase in production, and a consequent increase in use of surface coatings.
The only way forward was to change to low VOC products, and at the time of writing, 2/3rds of their coatings are now low VOC, and they are well on the way to meeting the new target.
Current low VOC, but solvent based coatings, we believe will largely disappear over the next 5-10 years to be replaced by water based/reducible materials. This is already happening in certain market areas and will spread across the whole organic coatings industries.
New regulations on packaging, labelling and shipping will accelerate this process.
The REACH regulations will without doubt affect the formulation and use of surface coatings. It is expected that chrome and chromium compounds will not be available within the next 3-5 years. . Although not normally used in coatings for composites, chromium containing pigments are the mainstay of anti-corrosive primers, and these will have to be changed. Work is already ongoing, and it is fair to say that all our systems for composites are totally chrome free.
I hope this paper gives an insight to the complex world of surface coatings for composites.
We are constantly working on new developments for this major growth substrate, and will no doubt continue to engineer coatings to meet ever changing, ever more stringent technical and performance requirements.
Please contact our sales department if you require any further help or more information, they would be delighted to help.
Please contact our technical team if you require any further help or more information, they would be delighted to help.
