Preprocessing and Coating on the Skin Surface of General Aircraft

You ming Wu

Bazhong city Bazhou District army retired cadres Management Institute Bazhong 636000

Keywords: Aircraft skin; pretreatment; epoxy polyamide primer; polyester putty; polyurethane finish; coating.

Abstract: Aircraft coating is the highest requirement for aeronautical coatings. The electrochemical oxidation,

chemical oxidation and phosphating primer used for aircraft skin pretreatment are described. The aluminum

alloy coating system and composite skin system are also discussed.

1 INTRODUCTION

The aircraft skin preprocessing and coating is an

important part of the aviation industry. Aerospace

coatings comprise only a small share of the market

in the coating industry, but it is highly demanded

dedication, because Aerospace coatings has been

under severe load external environment, compared

with other varieties of paint, the technical

requirement is very special, this is because it must

meet the extreme conditions of use. Aerospace

coatings must withstand temperature, air pressure

change, to withstand different air turbulence. No

matter how the flight conditions, aerospace coatings

need to withstand the temperature fluctuation of high

intensity ultraviolet irradiation, humid environment,

chemical invasion (such as fuel, hydraulic oil,

cleaning chemicals) and corrosion test, in addition,

airlines must have the coating thickness and quality

as low as possible, so as to reduce energy

consumption. Therefore, the aviation coating must

be a very high performance coating, light quality,

high performance and environment friendly is the

development direction of the aviation coatings.

2 PREPROCESSING OF THE

SURFACE OF AN AIRPLANE

Because the base material of the aircraft is

mainly aluminum alloy and composite material, it

should be treated with proper surface before coating.

Why should the airplane surface pretreatment,

summed up the

[1~2]

for two reasons: first, the main

material for aircraft Aluminum Alloy base, although

its sturdy, but not in the adhesion of coating on the

metal substrate on other metals so strong,

pretreatment can increase the adhesion between

coating and substrate second, oxygen reaction;

surface to form a layer of oxide film of pure

aluminum and the air, the antioxidant protection

effect, but can not effectively prevent Aluminum

Alloy corrosion in a changing climate and complex

flight environment. During the maintenance of the

skin of the aircraft, the surface of the aluminum

alloy plane must be pretreated artificially to increase

the surface oxide film and strengthen the

anticorrosion ability of the metal

[3]

. The compact

oxide film produced by surface pretreatment not

only has certain corrosion resistance, but also has

uniform and porous film, and the porous structure

increases the contact area with the coating, so as to

improve the adhesion of the surface coating and

improve the corrosion resistance of the aluminum

plate. The function of substrate surface pretreatment

is mainly to improve the corrosion resistance of

substrate and the binding force between substrate

and primer, and the quality of surface treatment will

directly affect the quality and performance of

coating.

2.1 Surface Treatment of Aluminum

Alloy in Aircraft

Aluminum is widely used in the aviation industry.

For the aircraft coating system, most of the surface

of the coating is aluminum skin. The treatment of

the metal surface mainly refers to the treatment of

the aluminum plate, [4~5]. Electrochemical

oxidation (anodizing), chemical oxidation and

phosphating primer 3 [6] are used for the surface

treatment of aluminum skin on aircraft. Aluminum

Alloy aircraft material has the advantages of light

weight, high speed, large payload and strong

corrosion resistance and other advantages, especially

in the coastal sea perennial pressure

Inevitably, there are various forms of corrosion

for aircraft used in the ocean environment, which

may cause partial failure of aircraft, increase

maintenance cost and reduce product life. Therefore,

measures must be taken to reduce aircraft corrosion.

Surface coating is one of the most economical and

effective anticorrosion methods, and the

construction of coating is simple and easy to

maintain. Because of its strong plasticity, negative

potential and easy oxidation, aluminum alloy has

special requirements for surface treatment, coating

and coating, [8]. 

The surface treatment of aircraft aluminum alloy

has its special requirements. The quality of surface

treatment is strict, which has an important influence

on the quality of subsequent coating. The surface of

aluminum alloy has special requirements for coating.

Choosing the right coating and making the proper

matching is very important for the anticorrosion

effect of aircraft. With the continuous development

of material science and corrosion science, the new

surface treatment methods and antiseptic antifouling

coatings will continue to mature, and more

convenient, economic, efficient and environmentally

friendly technologies and products will be applied to

this field.

In recent years, water-based technology has been

applied to the surface protection of aluminum alloys.

The waterborne epoxy functional anticorrosive

coatings with pre emulsification or post

emulsification have been developing rapidly, but

there are still many problems, such as the

contradiction between convenience and

performance. The emulsion or water soluble resin

coating for emulsifier, average relative molecular

mass distribution and molecular structure, water

resistance etc. it is difficult to fully meet the needs of

performance, resistance of special medium is also a

lack of practical use and the use of data to support;

after the emulsion process can satisfy the using

performance the requirements, but the process is

needed before mechanical emulsification

construction is not easy to be accepted by the user,

and the future aircraft repair inconvenience.

Waterborne coatings also have certain requirements

for the construction environment, such as the change

of ambient temperature and humidity will seriously

affect the curing cycle, the effect and final

performance of coatings. Therefore, the large-scale

promotion of waterborne coatings in aviation field

also needs the attention of manufacturers and the

necessary transformation of the construction

environment, otherwise, it will eventually affect the

promotion of coating Waterborne in the aviation

field.

After repair, assembly and pre flight work

process, the surface of the aircraft is inevitably

adsorbed by dust and oil stains after being

overhauled. It must be cleaned before the whole

machine is painted. Aluminum alloys are usually

anodized to form compact oxide films of several

microns to tens of microns. The surface is smooth,

and the coating is not adherent. The anodic film on

the aluminum surface must be removed before

painting. Therefore, the purpose of aircraft surface

treatment is to clean the oil, dust and anodized film,

and have a certain roughness suitable for the surface

of the coating. The procedures for surface treatment

usually include solvent scrubbing, cleaning agent

cleaning, and so on. In many surface treatment

methods, at present, the widely used technology in

China is anodization and chemical oxidation. The

two methods are relatively mature. The oxide film

produced is good for aluminum plate adhesion and

corrosion resistance. However, these two methods

are to use a lot of electrolytes, such as sulfate,

chromate and phosphate, causing serious pollution to

the environment, especially the C production has a

serious harm to the human body, although in recent

years have developed low chromium treatment,

closed system technology, but still can not to solve

the serious pollution caused by surface treatment on

the environment fundamentally. Polymer membrane

treatment and sol gel hybrid membrane method can

replace chemical oxidation and anodization, so as to

solve the problem of C pollution to environment and

human body. However, the two methods need

further research in terms of process control and sex.

The main coatings used in magnesium alloy

kernel are organic coating, silica sol and polymer

coating technology. Silica sol and polymer coating

technique for the corrosion protection effect is good,

but the construction of protective resin can not meet

the complex conditions of the requirements for the

aircraft engine magnesium alloy shell are: silica sol,

silicon resin, epoxy resin, modified epoxy resin, the

epoxy resin coating because of its strong adhesion

and infiltration of water, high strength are widely

used. In addition, there are also ten organic coatings

which are treated by organic coating or chemical

oxidation. A r high temperature, scour resistant and

anticorrosive coatings related research has been

carried out, and some progress has been made.

2.1.1 Electrochemical Oxidation (Anodic

Oxidation) Surface Treatment

Anodizing is a three oxidation two aluminum oxide

film on the surface of aluminum by electrochemical

oxidation. The thickness of the film is generally

5~15 u m. The chromic acid anodizing most used as

an example, the membrane structure is divided into

two layers; the outer layer of oxide film (Al

, H

to the electrolyte side, large volume, low hardness,

with loose hole, tapered capillary tube; inner film

near the substrate is of high purity aluminum side,

three thick two aluminum oxide. The anodized oxide

film has a larger volume, better durability, good

corrosion resistance and good adsorption capacity to

the primer. But because of its hard, flexibility and

impact resistance, the oxide film produced by

Alodin is not as good as it is.

1.1.2Chemical oxidation method [7] produces a

thin layer of oxide film on the surface by chemical

reaction. Modern aircraft aluminum is used widely

for chemical oxidation of Allodin (alodine) method,

a Tu Rodin generated oxidefilm generated than

anodized film, the film thickness is about 0.5 ~ 4 u

m. , and the film is soft, flexibility is good, but poor

resistance to abrasion, corrosion and serious when

subjected to touch that film will be destroyed

quickly, so it should not be used alone, and

supporting the use of skin coating, can make up for

deficiencies in this regard.

The general Aluminum Alloy substrate after

cleaning the surface, with anoding or chromate

oxide surface treatment, and titanium alloy plate and

stainless steel plate material, the alkali wash oil

removal, and then polished. The treated surface

should be clean and pollution-free so as to ensure

that the 30s water film does not bifurcate

continuously. It is recommended that the bottom

coat be applied within 24h, otherwise the cleaning

and surface oxidation processes should be

reconsidered. The quality of the surface treatment

directly affects the adhesion of the coating and the

quality of the coating.

2.1.2 Surface Treatment of Phosphating

Primer

Phosphating primer means the phosphating primer

on the surface of aluminum alloy skin is formed by

phosphating at the same time, and the phosphating

primer itself can not play the role of lacquer alone. It

is a surface pretreatment method. The phosphating

primer is sprayed on the surface of the skin

aluminum plate. A part of phosphoric acid is

combined with metal aluminum to make the metal

surface integrated with the coating film, and it can

provide a good base surface with good combination

of primers in the coating system. After coating the

phosphating primer, it can directly spray the primer

without flushing. The phosphating primer is

commonly used for the whole aircraft surface

pretreatment layer with the advantages of low cost,

short construction time and good environmental

protection performance.

2.2 Surface Pretreatment of Aircraft

Composite Material

The aircraft surface material in addition to all kinds

of metal materials and carbon fiber, aramid fiber and

ceramic non-metallic materials such as

[11]

, including

carbon fiber, aramid fiber and other advanced

material consumption is an important indicator of

the level of technological progress of the aircraft, but

these advanced composite materials is limited to the

processing of complex processes, usually appears as

a large area, a single monolithic structure form. Such

materials have very high tensile strength and good

wave transmission properties, but there are also

prominent shortcomings. For example, harsh

environmental conditions will seriously affect the

service life. Improper use, such as penetration of

acidic medium, will destroy the mechanical

properties of materials.

Advanced composite material prepreg, curing

molding process has been used a lot of adhesive

agent in weaving, Hector, mainly epoxy, silicone

and silicon sol / gel and poly phenol imine, forming

the porosity of the material is large, and the part of

the material bibulous rate is higher, not only on the

internal equipment later in the process of using

"three" (anti heat, anti salt, anti mildew) caused by

adverse effects, even if the long-term use of their

own also have a negative impact, so the surface

sealing and protection of the important. For different

prepreg, the similar coating system is more

appropriate. Choosing proper pigments and fillers

can not only improve the sealing ability of the

coatings, but also be beneficial to the strength of the

coating itself. With the improvement of the

temperature resistance of the aircraft, the

improvement of the temperature resistance of the

surface coating and the rubber Bob has become the

key point for the next stage of development. The

long-term service temperature of the coatings

prepared by phosphate series gum / resin is

increased to above 650 C, and there is no

carbonization residue. Compared with the traditional

resin, the coating has a rare advantage.

In recent years, with the energy saving weight of

aerospace materials, high-performance composite

materials have been widely used in aircraft, such as

radome, radio antenna cover, aileron, part of the

wing, horizontal and vertical tails, and the most part

of the fuselage skin is made of composite material.

The surface treatment of composite materials is

mainly aimed at removing all kinds of release agents

on the surface when forming, and cleaning the

surfaces at the same time, forming a good paint

surface. It is important to clean up the release agent,

especially the silicon containing release agent, which

is related to the bonding strength of the coating on

the surface of the composite material, and whether it

is an important step to get the good coating system.

The surface treatment of the composite material is to

remove the demoulding agent by mechanical

grinding or manual grinding, and cleaning the

surface with solvent. A certain fineness of water

grinding paper should be used to gently grind the

surface of the composite in order to avoid the

damage to the surface state of the composite. After

finishing the surface of the whole composite

material, it is cleaned with solvent or special

cleaning agent. Water film continuous method can

be used to test the degree of surface treatment. The

water film is not broken in 30 s, indicating that the

surface has been treated clean. Otherwise, repeated

grinding and cleaning work is still needed.

3 COATING SYSTEM AND

COATING OF ALUMINUM

ALLOY SKIN AND

COMPOSITE MATERIAL

In view of the special requirements of aircraft

protection, the coatings provided must have

comprehensive anticorrosive capability, strong

chemical resistance, water penetration resistance,

enough abrasion resistance, flexibility, weather

resistance, strong adhesion, good leveling, and full

color. However, only one coating or single coating is

difficult to meet this requirement. Therefore, a

variety of coatings need to be made to cooperate

effectively to form an effective aircraft skin coating

system.

3.1 Aircraft Aluminum Alloy Coating

System

For aeronautical coatings, it is necessary to have

high corrosion resistance to ensure the safe flight of

the aircraft. The coatings for Aircraft Skins at home

and abroad have gone through several developing

stages: nitro / alkyd coatings to thermoplastic acrylic

coatings, and then to normal temperature curing two

component polyurethane coatings. With the

development and replacement of air coating

products, the primers and topcoat on the surface of

the aircraft must meet the American military

standards and Boeing standards. The primers used

for aircraft outer skin coating system conform to the

US military standard MIL-P-23377. Currently, the

primers that meet this standard are mainly epoxy

polyamide primers and polyurethane primers. Epoxy

polyamide primers have good resistance to various

liquid media on aircraft, and the corrosion factors of

environmental conditions, such as moisture and salt

fog, are also good.

It has good mechanical properties and good

adhesion to different treated oxide films. Two

component polyurethane primer on liquid medium

machine has good resistance; has good resistance to

corrosion factors under atmospheric conditions;

good mechanical properties; good adhesion on the

surface treated by various Aluminum Alloy;

polyurethane paint and surface adhesion between the

layers is better than that of epoxy polyamide primer.

High gloss double component polyurethane used at

present for civil aviation aircraft

The top coat is cured by aliphatic isocyanate, and

the top coat for military aircraft is based on

thermoplastic acrylic resin single component system,

but it is being replaced by polyurethane finish. Two

component aliphatic polyurethane coatings are

usually used in the surface coating of the outer

surface coating system of the outer surface of the

aircraft. The performance of this polyurethane

coating needs to meet the requirements of US

military standard MIL-PRF-85285 and the standard

BMS10-72 of Boeing company in the United States.

Although the products produced by many aviation

coatings suppliers at home and abroad can meet the

above two criteria, there are slight differences in the

type of curing agents and the selection of hydroxyl

resins. The choice of curing agents is mainly HDI

(six methylene diisocyanate) trimer and HDI two

urea. Most of them are polyester resin and hydroxyl

containing acrylic resin. With the development and

application of domestic and foreign fluorocarbon

resin, fluorocarbon resin super weather ability,

corrosion resistance, oxidation resistance, high

temperature resistance and wear resistance has

aroused great concern of the people, and through the

introduction of fluorine resin modified acrylic resin,

fluorine resin coating is introduced into the air in the

paint. Aircraft manufacturing companies from all

over the world choose the coating system on the

outer surface of the aircraft according to their own

experience and design requirements.

Outside the aircraft with paint for dope, the

primer (corresponding to the middle of the primer)

the main varieties of epoxy ester primer, epoxy

primer, epoxy primer, polyurethane silicone amine

adduct zinc chromate primer and topcoat, is acrylic,

alkyd modified silicone, silicone polyurethane,

aliphatic polyurethane (polyester polyurethane,

acrylic polyurethane etc.). But for some aircraft parts,

such as the wing, and the level below stable region

and under repair is not easy, need high requirements

of corrosion protection, adopts polysulfide with

aliphatic polyurethane topcoat is still; and some

military aircraft such as the seaplane carrier aircraft

and the surface also has excellent resistance to water

and salt fog resistance the needs of coatings,

corrosion resistant high like epoxy polyamide epoxy

and ammonia adduct primer and acrylic

polyurethane topcoat, in body shape material and

non metal (glass steel, carbon fiber composite

materials), used for aircraft wing skin structure and

the front fuselage skeleton, flap dome etc., in order

to prevent these non metal material surface water

and resin aging, and prevent the occurrence of

electrostatic interaction, so the use of polyamide

epoxy varnish and primer primer, and supplemented

by epoxy polyamide or polyester putty The external

use polyurethane topcoat, which contains anti - static

conductive pigment, such as carbon black,

conductive oxide, etc.

In order to reduce the overall weight of the

aircraft, increase the distance, the aircraft skin

coating system of thin film, lightweight has become

more and more important, to meet the premise of

constant protection performance of aircraft skin

coating under the requirements of the coating

thickness decreased by 20%~ 30%. The new lacquer

system developed by PPG company, compared with

the traditional coating system, can greatly reduce the

weight of the coating without losing the protective

performance of the coating. Today, with the

increasing international fuel prices, the promotion

and successful application of the technology will

greatly reduce the fuel cost of aircraft and create

objective economic benefits for airlines. In high

performance aircraft coating, the internationally

renowned aviation paint manufacturers to carry out

the research and application of the work in the last

century in 80s, mainly analyze and study the aircraft

during the flight system of the coating corrosion

resistance and service life, because the flight

environment of the complex diversity, improve the

performance of tW coating skin the plane in extreme

conditions of coating, such as heat resistance, oil

resistance, scratch resistance, character painting,

impact resistance [9], 90s launched related products

and applications, such as PPG Polyure company,

anePrimer 8WKD89, Polyurethane Primer PAC33,

Conventional Solods Primer823's etc. the film series

products, high and low temperature resistance,

impact resistance, soft performance, excellent

adhesion and anti vibration performance and the

metal substrate, can ensure the process of flying

violent In this case, the coating and substrate of the

skin will not cause cracking due to the change of

stress. Aeroflex series coating product development

Akzo Nobel company in the aircraft skin protection

has been in use for more than 10 years of P01, in the

process of using Months and years pass by. flight,

the film still can keep the excellent performance, the

film gloss decreased no more than 20%, especially

in the erosion tolerance of hydraulic oil flow

containing vinegar, cash very good. With the

development of science and technology, aircraft

flying faster and the heat resistance of external skin

Aerospace coatings products also put forward higher

technical requirements, modern aircraft flying at

high speed in the process of production and the air

friction heat can reach 220 CW, which can resist

high temperature dope requirements and

performance changes do not occur P1'221. However,

the traditional aircraft skin coating temperature can

not exceed 15 (TC), during the accelerated aging

process of hot air, it will greatly affect the service

life of coatings, and seriously affect the safety of

flight. The new high temperature skin coating

developed by the Aeronautical Materials Laboratory

has been successfully applied to XB-70 aircraft,

which solves the defect of the traditional aircraft

skin coating with insufficient temperature resistance.

PPG and The Nobel company product Akzo

Aerospace coatings with excellent corrosion

resistance, salt fog resistance and its product

performance can reach 300 ^ heat, outer surface of

skin coating is widely used in large aircraft

manufacturers, but due to the characteristics of the

marine environment of high humidity, high salinity,

Xu Cengnai skin salt fog performance index used in

the the U.S. aircraft carriers is as high as 5000h.

3.1.1 Aircraft Composite Coating System

The surface protection coating system of composite

materials includes surface treatment layer, primer,

putty and face coat. Primer Aluminum Alloy skin

surface for the general use epoxy or polyurethane

primer, to prevent metal corrosion, rust primer added

fillers, but not on the surface of the composite metal

component, there is no problem of metal corrosion,

so it does not need to add anti rust pigment of

chromates. Because composite material is a water

sensitive material, the strength and electrical

properties of composite material will be significantly

reduced after absorbing water. Therefore, the primer

layer on composite surface should be highly dense,

which can prevent water penetration. In addition,

because the flight will be affected by solar radiation,

air temperature and alternating, high-speed flight, air

dust impact, therefore should be selected with

weatherability, adhesion, flexibility, high and low

temperature resistance, wear resistance and high

corrosion resistance of the coating is excellent.

When the primer is selected, the weatherability is

better, and the polyurethane primer with certain

elastic deformation, excellent adhesion and high

crosslinking density is used. The composite surface

has a porous appearance. After degreasing and

grinding, the surface will still have a hole

appearance. After priming, the surface pores can not

be completely covered. At this time, we need to

scrape the elastic polyester putty on the surface of

the primer and fill the surface pores. Elastic

polyester putty is made from elastic polyester resin

and inorganic filler. The putty has good adhesion to

polyurethane primer and topcoat, with excellent

flexibility and elastic deformation, and has good

filling on the surface of the composite. Lightly

polish the putty layer after scraping, wipe the surface

with the cleaning solvent, then spray a polyurethane

primer, and the primer can be sprayed at 6 h above

normal temperature, then the polyurethane finish can

be sprayed.

At present, the main material for aircraft

Aluminum Alloy and high performance composite

materials, the pretreatment of Aluminum Alloy

surface mainly by anodic oxidation, chemical

oxidation and phosphating primer method, the 3

methods have their own characteristics, both at

home and abroad, the aircraft manufacturing

company has been applied. On the surface of

composite material, the cleaning agent should be

used to wipe, remove the release agent and polish it.

After polishing and cleaning, continuous water film

test will be carried out. The water film within 30 s

will not break, indicating that the surface of the

composite material has been cleaned. The surface

treatment of epoxy polyamide primer or primer

polyurethane and polyurethane paint is coated on the

surface of Aluminum Alloy; surface treated surface

composite spraying polyurethane primer, primer

after drying, also need scraping elastic polyester

putty, used to fill the composite surface pore elastic

putty, drying after minor polishing, and again spray

polyurethane primer primer sprayed polyurethane

topcoat. After the surface treatment of the aircraft,

the coating is not only decorative, but also has

excellent weatherability, medium resistance and

other protective properties.

4 CONCLUSIONS

The progress of aviation technology is an important

symbol of a country's industrial system perfection,

industrial technology development level and R & D

capability construction. [10] is the important branch

of material science. The progress of functional

coatings technology even influences the

development level of large aircraft. With the

development of Chinese aviation industry, high

speed, high altitude and far space development

direction will put forward higher requirements for

the paint industry, because of China's coating

technical level there is a great development space,

the United States Air force is the first development

of the air force system, at present in the world still

occupy a dominant advantage of the air force, so

there is a close relationship between, to improve the

performance of protective material -- Application of

aerospace coatings very seriously, has formed a

relatively perfect system of aviation standard paint,

paint the military standard, Airlines Boeing standard,

and standard application of PPG company, AKZO

NOBEL company related products [11] Aerospace

coatings.

Most of China's aviation coatings standards are

based on foreign aviation coatings standards or

conversion, and lack of a perfect standard system for

aviation coatings. Widely recognized in the aviation

coatings is the American Standard and Boeing

standard. Besides the general enterprise standards

and the military standards and industry standards of

various countries, these standards are implemented

within a certain range and are generally not public.

The establishment of domestic aviation coating

standards will promote the development of

Aeronautical coatings.

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