Investigation of a Composite Material Painting Method: Assessment of the Mixture Curing of Organic Coatings

Abstract

The present investigation highlights the importance of evaluating the painting process on a composite material, namely the Kevlar validation process. Kevlar, a synthetic fabric, is well known for its remarkable strength and durability. Kevlar is used in the construction of spaceships and airplanes because it is lightweight and five times stronger than steel. This paper will present the methods for measuring paint layer thickness in accordance with EN ISO 2808:2019, confirming that organic coatings have fully cured, and coating thickness will be measured using magnetic currents. This study will also address the topic of determining liquid resistance. The protocols for manufacturing the Kevlar specimen are in accordance with ISO 2812-2:2018 using the water immersion method and structural testing. The investigation also demonstrates the progress of the framing test following immersion in Airbus PTP metal test tubes.

1. Introduction

Given its superior mechanical qualities and a high strength-to-weight ratio, Kevlar, a high-performance aramid fiber, is frequently utilized in composite materials. However, because of the surface properties of the fiber and the requirement for high interfacial adhesion between the paint and the composite material, painting Kevlar composites poses special difficulties [1].

The American scientist Stephanie Kwolek created an incredibly lightweight and strong synthetic material known as Kevlar in 1965 while she was employed at DuPont [2]. With its special qualities, this aramid fiber is perfect for a variety of uses.

Challenges with Kevlar Composite Painting

• Porosity: A common feature of the Kevlar composites, porosity can be challenging to remove and has an impact on the final surface finish [1].
• Surface condition: The presence of reinforcement fibers, such as Kevlar, may result in an uneven surface that affects the paint’s adhesion and appearance [1].
• Adhesive quality: For longevity and functionality, it is essential that the paint layers and the Kevlar composite adhere strongly [1].

Composite materials are a combination of two or more chemically different materials with an interface between them. If, at the macromolecular level, the constituent materials maintain their separate identity, their combination generates properties and characteristics different from those of the component materials in part. The phase of the composite that is found in a greater proportion and that serves as a binding medium between the elements of the other phases is called the matrix. The other main element is called reinforcement and is added to the matrix to improve or modify its properties. If the matrix is defined as forming the continuous phase, the reinforcement forms the discontinuous phase, evenly distributed throughout the entire volume of the matrix [3]. Since this paper gives as a practical example the validation of the Kevlar painting process, it is important to define some characteristics of this material. Among the synthetic fibers used in the industry, aramid fiber, also known as Kevlar, possesses unique properties. It can also be seen as a nylon with extra benzene rings in the polymer chain to increase its rigidity. Kevlar is mainly popular for its growing applications in industrial and advanced technologies, such as ballistic armor, helicopter blades, various pneumatic hardening structures, thermal radiation protection panels, sporting goods, etc. Compared to other synthetic fibers, the elongation of the fibers under effort is significantly reduced and has a high resistance to deformation. Kevlar also has a great resistance to temperature, up to about 360 °C, and does not melt like nylon [4].

Properties:

• High tensile strength: Kevlar has five times the weight-to-tear strength ratio of steel.
• Light weight: It is very lightweight, making it ideal for applications where weight is important.
• High temperature resistance: Kevlar can withstand temperatures up to 360 °C without losing its mechanical properties.
• Chemical resistance: It is resistant to many chemicals, such as acids and other corrosive substances.

Uses:

• Personal protective equipment: Kevlar is used in the production of bulletproof vests, helmets, gloves, and protective footwear.
• Aeronautics: Kevlar is used in the construction of airplanes and helicopters to reduce weight and increase safety.
• Shipbuilding: It is used to produce anchor cables and ropes.
• Sports: Kevlar is used in the production of sports equipment, such as tennis rackets or diving equipment, to increase endurance and reduce weight.
• Construction: Kevlar is used to increase the earthquake resistance of building and bridge structures.

The mechanical characteristics of Kevlar composite materials can be greatly impacted by the painting process.

The proposed topic of this paper is to obtain a composite material painting method through an assessment of the mixture curing of organic coatings.

The ongoing necessity to improve the performance, durability, and adhesion of coatings on Kevlar composites underscores the persistent interest in surface treatment techniques.

This topic includes multiple methods to enhance the mechanical and thermal properties of Kevlar composites, including chemical treatments, plasma treatments, and the incorporation of nanoparticles.

2. Method for Validating the Complete Curing of Organic Coatings

In the aeronautical industry, the first test of the painting process on both structural and non-structural parts is the test of complete polymerization of the coating. Airbus regulates this test by rule AITM1_0024_issue 3_2020.

Kevlar is a strong and lightweight material commonly used in the aerospace industry. In aeronautics, Kevlar is used for a variety of applications, including aircraft panels, structural components, and even aircraft engine construction. One of the main advantages of Kevlar is its high strength-to-weight ratio. Kevlar is also highly resistant to impact and abrasion, making it an excellent choice for aircraft parts that are exposed to harsh environments [5,6]. In addition to its use in aircraft panels and structural components, Kevlar is also used in aircraft engine construction. Kevlar can be used to create lightweight, high-strength components that can withstand the high temperatures and stresses present in aircraft engines (Figure 1).

Overall, Kevlar is an important material in aeronautics due to its strength, lightweight properties, and resistance to damage. Its use in aircraft components can help improve performance, reduce weight, and increase safety.

Before the use of Kevlar on aircraft, it must be validated using several safety tests:

• Tensile tests: These tests measure Kevlar’s ability to withstand tensile forces and stretch without breaking. These tests are typically performed by tensing a sample of Kevlar material into a test fixture and measuring the force required to break the material (Figure 2).

2.

Compression tests: These tests measure Kevlar’s ability to withstand compressive forces and maintain its shape. These tests are typically performed by compressing a sample of Kevlar material into a test device and measuring the force required to compress the material (Figure 3).

3.

Fire tests: These tests measure Kevlar’s ability to withstand high temperatures and prevent flames from spreading. These tests are typically performed by exposing a sample of Kevlar material to a flame and measuring the time it takes for the flame to propagate.

4.

Corrosion tests: These tests measure Kevlar’s ability to resist corrosion caused by chemicals and moisture. These tests are typically performed by exposing a sample of Kevlar material to chemicals or wet environments and measuring the changes in weight and physical properties.

5.

Impact resistance tests: These tests measure Kevlar’s ability to withstand impact with other objects. These tests are usually performed by hitting a sample of Kevlar material with an object and measuring the level of damage.

Due to the special properties of composite materials—tensile strength, rigidity, thermal conductivity, low weight, etc.—their industry has experienced accelerated development since the 1970s. The global market for the consumption of composite materials is about 180,000 tons/year and has a financial value of about USD 1.2 billion.

The cost of composite materials has also decreased dramatically, from USD 12–15/kg to USD 10/kg, because of advancements in composite manufacturing and processing techniques. As a result, they are used more frequently in a wider range of fields [3,6]. Mainly due to the particularly high weight/strength ratio, the aeronautical industry is and will remain the pioneer in the development of composite materials, with both giants Boeing and Airbus using them [7].

Phenolic matrix fiberglass is generally used for non-structural parts located within the pressurized area. This is due to the low costs but also the very high resistance to high temperatures, which leads to a low risk of flammability.

Carbon-fiber-reinforced composite materials, due to their special strength but also the high production and processing costs, are mainly used in the construction of the fuselage.

To solve issues like porosity and surface imperfections, painting Kevlar composites requires a careful consideration of surface preparation and treatment. The painting process can be optimized and assessed with the help of methods like reflectometry measurement, Fourier transforms, and profilometry. The adherence and look of painted Kevlar composites can be greatly enhanced by surface treatments and cutting-edge painting techniques, such as IMC, powder coatings, and UV-curable primers.

Normally, the drying time of the coating is provided in the material data sheets from the supplier. For the two coatings used in the painting process presented as an example, Apret 5425/6407 Light Blue and Pyroflex Black Paint, the parboiling time at 600 °C is 60 min. In certain situations, the layer may not be fully cured. This problem can be solved by reinserting the parts/specimens into the thermal enclosure.

Causes for incomplete curing of the covering layer:

• Failure to comply with the drying time specified in the material data sheet.
  Drying temperature lower than that specified in the material data sheet.
  Failure to comply with the destocking time prior to the steaming process [3,8,9].
• The presence of moisture above the limits specified in the material data sheet in the storage, preparation, or application area.
• Failure to comply with the volumetric or mass ratio when preparing the mixture.
• The presence of water vapor on hoses or paint guns.
• Non-compliant chemical commotion of raw materials.
• Non-compliant applied layer thickness.

Airbus recommends this test on all paint parts and test tubes.

The test involves rubbing a clean cotton cloth of neutral color soaked with about 2–3 mL of methyl-ethyl-ketone (MEC) over an area of about 40 mm, making 50 back-and-forth passes. To simplify the process, a 40 × 40 mm square cut-out template can be used. Also, the 50 passes can be converted into test time. For these, three consecutive timings can be performed:

• 28 s for 50 passes
• 24 s for 50 passes
• 32 s for 50 passes

The determined values show that to comply with the AITM1_0024 requirement, it is sufficient to rub the cotton cloth back and forth over an area of about 40 mm for 30 s.

Interpretation of the results:

Any detachment of the coating leads to non-compliance with the painting process.

A slight staining of the impregnated cloth due to the transfer of pigments is allowed. In this situation, the duration of steaming or drying in the environment will be extended. The test will be repeated after this new time frame until the results are compliant (Figure 4).

The test report must include the following elements:

• In the case of series production, testing is performed on a minimum of three pieces from the batch. In the case of test tubes, it is mandatory that this test be carried out on each individual test tube before any other paint validation test is carried out.
• The solvent agent used: recommended Methyl-Ethyl-Ketone (MEC)
• Type of coating, reference, or name of the substance/paint.
• Description of the curing cycle, ambient drying time, or steaming time.
• Test results
• Any pertinent comments related to the result, the way of testing, or other details that could influence the result
• Name and signature of the person who performed the test [5,10].

2.1. Measurement of Paint Layer Thickness, According to EN ISO 2808:2019 [11]

Painting process of the test tube: The painting of the test specimen must reflect as much as possible the painting process of the part. In the case of manual dyeing, for the result to be relevant, a minimum of one test tube per lot of painting is required. To achieve a stable, controllable process that is integrated into the quality management system, it is necessary to establish the process parameters in advance using a Turtle process diagram (Figure 5).

The metallic specimen’s painting procedure can be modified for each project if it conforms to the supplies, procedures, tools, and working hours used for the part’s painting. In the next part of the paper, the test tube painting process will be exemplified for a structural part of the Airbus A320 aircraft, safety class 2, located outside the pressurized area (Figure 6 and Figure 7).

All chemical materials must be stored in the original packaging at a temperature between 5 and 30 °C (according to the material data sheet and supplier recommendation). They will be removed from the storage area at least 12 h before the preparation of the mixture. The piece is prepared in a 2:1:1 volume ratio, i.e., 2 basic measures to 1 measure of hardener to 1 measure of diluent. If working with masses, according to the specific density of the materials, the ratio is transformed into 100 g base: 32 g hardener: 32 g thinner. The quantities can be adjusted according to the result of the viscosity test without exceeding the tolerances: ±6 g for the base, ±1 g for the hardener, and +5 g for the thinner. Before preparation, the compounds must be mixed individually for 2 min using a mixing ruler. The shelf life of the dish is 3 h in the environment. The viscosity measurement is performed with an ISO 4 cup (

Table 1. Material resources necessary for the painting process.

Nr.crt.	Material Resource	Resource Type
1	Aluminium test tube	Raw materials
2	Apret 5014 PU Base Blue	Raw materials
3	Apret 5014 PU Blue-Hardener	Raw materials
4	Pyroflex Paint 7D713 Black-Base	Raw materials
5	Pyroflex 7D713 Black-Hardening Paint	Raw materials
6	Thinner 0651	Raw materials
7	Paper Scotch tape	Consumables
8	Paint filter, 190 μm	Consumables
9	Lavete antistatic	Consumables
10	Alcool izopropilic	Consumables
11	Volumetric mixing vessel	Tools
12	Mixing ruler	Tools
13	ISO 4 viscosity cup	Tools
14	Timer	Tools
15	Paint Gun WS400	Equipment
16	Scale	Tools
17	Temperature- and humidity-controlled Paint preparation area	Facilities
18	Paint booth	Facilities
19	Sandpaper P500	Consumables

), and according to the material data sheet, the flow time must be between 15 and 20 s at an ambient temperature of 23 °C. Before use, the test tube is degreased with isopropyl alcohol and a lint-free cloth. With the help of the Scotch tape, about 1 cm of the metal test tube is protected; this can be observed in Figure 8. This area will remain free for calibrating the measuring device [4,12,13].

Before application, the paint is passed through a 190 μm paint filter. With the help of a WS 400 paint gun with a diameter of 1.5, a uniform layer of primer is applied to the entire surface of the specimen at a pressure between 2.5 and 4 bar (Figure 9).

After the completion of the parboiling time, 60 min at 65 °C, the test tube is removed from the paint booth, and the Scotch tape is removed.

Pyroflex 7D13 paint is prepared in a 3:1:1 volume ratio, i.e., 3 basic measures to 1 measure hardener to 1 measure thinner. If working with masses, according to the specific density of the materials, the ratio is transformed into 100 g base: 32 g hardener: 32 g thinner. The quantities can be adjusted according to the result of the viscosity test without exceeding the tolerances: ±6 g for the base, ±1 g for the hardener, and +10 g for the thinner. Before preparation, the compounds must be mixed individually for 2 min using a mixing ruler. The shelf life of the dish is 8 h in an ambient environment. The viscosity measurement is performed with an ISO 4 cup, and according to the material data sheet, the flow time must be between 15 and 30 s.

Before applying the paint, the test tube is covered with Scotch tape on about two-thirds of the surface. On the area left free, it is lightly finished with P500 abrasive paper to reactivate the pressure, after which it is degreased with isopropyl alcohol. As with the price, Pyroflex paint must be passed through a 190 μm filter to prevent any impurities from contaminating the painting process. Apply the paint in an even, crosswise coat with the WS 400 gun at a pressure between 2.5 and 4 bar.

After the end of the steaming period, 60 min at 65 °C, the Scotch tape is removed, and three distinct areas will remain on the test tube: a free area for calibrating the measuring device, an area covered with blue PU coating, and an area covered with Pyroflex paint [13,14] (Figure 10).

The time required to paint a metal test tube is detailed in

Table 2. Dyeing times of the metal specimen.

Crt. No.	Operation	Timed Time (s)
1	Masking with Scotch tape, 1 cm	120
2	Preparation of Apret mixture	600
3	Viscosity measurement	480
4	Filter apron mixture	480
5	Application of apron mixture	180
6	Prepared stew	3600
7	Masking with Scotch tape 2/3 of the surface	180
8	Reactivate apron	300
9	Preparation of paint mixture	600
10	Paint viscosity measurement	480
11	Paint mixture filtration	480
12	Paint application	180
13	Paint steaming	3600
14	Specimen unmasking	120
Timp total de lucru = 12,480 s

.

Under normal working conditions, the specimen is produced at the same time as painting the batch of parts. So, the production time of the specimen mostly corresponds to the painting time of the parts. The thickness of the coating is measured by calculating the thickness difference using the outdoor micrometer or the comparator. Although less precise, this method is widely used due to its simplicity. The principle of the method is the calculation of the difference in thickness between the coating layer and the free specimen. The measuring instruments used are the classic ones: the outdoor micrometer with an accuracy of at least 5 μm or various types of micrometric comparators. Regardless of the type of measuring instrument used, it must be calibrated according to the rules in force [15,16,17].

2.2. Measurement of the Thickness of the Coating Using Magnetic Currents

Principle of the method: The most common method used is that of magnetic induction devices. This method is used to measure a magnetic neutral layer on a magnetic substrate. It enables non-destructive testing of layer or material thickness, e.g., galvanic layers of chromium, zinc, copper, or aluminum on steel and iron. In addition, the test method is also used for measuring paint and varnish layers on steel and iron. The magnetic induction method is a contact measurement method. An excitatory current generates a low-frequency magnetic field, the strength of which depends on the distance between the measuring probe and the base material, i.e., the thickness of the non-magnetic layer. The resulting magnetic field is recorded via a measuring coil. The received measurement signal is then converted into the exact layer thickness value in a special measuring device via the probe characteristics—the functional interrelationship between the measuring signal and the layer thickness (Figure 11).

Another method of measuring the thickness of the coating layer is that of eddy currents. The probes used for eddy current measurement are amplitude-sensitive and have a ferrite core. A coil is wound around this core, and a high-frequency alternating current passes through it. This creates an alternating high-frequency magnetic field around the coil. When the pole of the probe approaches a metal, an alternating current or “eddy current” is induced in this metal. This, in turn, generates another alternating magnetic field. Since this second magnetic field is the opposite of the first, the initial magnetic field is attenuated (weakened). The degree of attenuation depends on the distance between the probe pole and the metal. For coated parts, this distance corresponds exactly to the thickness of the layer.

The factors that can Influence the measurement results, according to ISO2178:2016, are as follows [18]:

Coating thickness: For thin layers, the accuracy is constant, while for thicker layers, the accuracy is an approximately constant fraction of the thickness.

Magnetic properties of the base layer: Magnetic variations of the base layer can affect the measurement of the thickness of the coating. To avoid this, it is recommended to calibrate the device on a base metal with the same properties as the test specimen or, if possible, even on the test specimen.

Base material thickness: For each instrument, there is a critical thickness above which measurements will no longer be accurate. Since this thickness depends on the type of probe of the apparatus and the nature of the base material, this thickness must be determined experimentally, unless specified by the manufacturer of the apparatus.

Influence of extremities: This method is sensitive to sudden changes in the contour of the surface. For this reason, measurements taken close to the edge or in corners will not be considered valid unless instrumentation is calibrated for this type of extreme situation. This effect can extend up to 20 mm from the edge, depending on the type of tool used.

Curvature: Measurements can be affected by surface curvature. The influence of curvature varies considerably depending on the type of device, but it always becomes more pronounced when the radius or curve is decreasing.

Surface roughness: On a rough surface, the repeatability of measurements decreases significantly, so it is recommended to increase the number of measurements to 5.

Direction of mechanical processes on the base material: The measurements can be affected by the direction followed by various mechanical processes, such as grinding the base material; the readings change depending on the orientation of the probe on the surface.

Strong magnetic fields, such as those produced by various electrical equipment, can strongly interfere with the probe reading.

Foreign particles: Because the probe of the device must be in intimate contact with the coating, any foreign particles can prevent this contact.

Pressure applied to the sample: The poles of the probe must be pressed with a constant but sufficient pressure so that no deformation of the coating occurs, even if it is soft.

Probe orientation: Measurements with a probe positioned horizontally or vice versa may require a different type of calibration or are even impossible for some devices [19,20].

The process of measuring paint layer thickness with the Elcometer 456B with removable probe: Elcometer’s range of digital coating thickness gauges has been specifically designed to provide highly accurate, reliable, and repeatable coating thickness measurements on almost any substrate, whether ferrous or non-ferrous. Dry film thickness can be measured on either magnetic steel surfaces or non-magnetic metal surfaces, such as stainless steel or aluminum, using a digital coating thickness indicator. The principle of electromagnetic induction is used for non-magnetic coatings on magnetic substrates, such as steel. The eddy current principle is used for non-conductive coatings on non-ferrous metal substrates. The eddy current principle has been used in this case (Figure 12).

Before starting a measurement, the operator must ensure that the entire surface of the specimen, including the uncovered area, is free of impurities. Otherwise, the probe may be affected, and the accuracy of the measurements is lost.

The calibration of the device consists of two stages. Calibration to the target value: the one that has a thickness value as close as possible to the actual measured value is chosen from the track set. This hold is placed on the remaining unpainted portion of the test tube. From the device menu, select the option “Calibration step 1” (Figure 13).

A series of tests are taken in the hold positioned on the free area of the specimen until the value stabilizes. After the calibration is performed, the measurements are repeated both with the hold and at the zero point. If the displayed values exceed the tolerance of ±1 μm, the calibration of the machine is repeated because it was not adequate. Once the calibration has been successfully completed, measurements can begin. As with calibration, the probe must be placed at an angle of 900 to the measuring surface. In the standard version, the device displays the thickness of the coating directly in microns.

For greater accuracy, five measured values were decided for each area, respecting the minimum distance of 20 mm to the edge of the specimen.

The results of the measurements are entered into a control report (Figure 14). For the thickness of the paint layer to be compliant, the average measured value for Apret 5425/6407 Light Blue must be between 10 and 65 μm, and for Pyroflex Black paint, between 25 and 35 μm. If the average thickness of the coating does not fall within these tolerances, the entire painting process is considered non-compliant, and all parts in that batch are repainted (if allowed by the customer’s specification).

Paints and varnishes—Adhesion test according to ISO2409:2020

Adhesion testing is a popular and reliable technique to confirm if organic coatings on materials like Kevlar have completely cured.

Crosshatch adhesion test: This is used to measure the amount of coating that is removed when cut and tugged using adhesive tape to evaluate how well the cured coating sticks to the Kevlar substrate.

Method:

• A sharp blade is used to cut a grid through the coating, producing tiny squares.
• Although the incisions should pass through the coating, the Kevlar substrate should not sustain significant damage.
• The sliced area is covered with regular adhesive tape, which is then swiftly removed.
• The quantity of coating that separates with the tape is examined.

ISO 2409, adopted in this form on 16 August 2020, presents a test method for evaluating the resistance of paint layers to separation from the base layer by making incisions. The properties determined by this empirical method depend, among other things, on the adhesion of a layer to the previous layer or to the specimen. However, this method is not considered to measure adhesion. However, it can be useful either as an “all or nothing” test or as a test to fit into one of the six classes described by the norm [13].

Principle of the method: Six parallel incisions are made on the coating layer, followed by another six incisions perpendicular to the first ones. All the paint that comes off is removed. The cut area is subject to a visual examination and is classified in one of six classes.

The necessary tools are a test tube, a cutting machine, and a magnifying glass.

The cutting machine can be

• Single-blade hand cutting tool.
• Single-blade cutting tool fixed in a motorized device.
• Cutter with rigid blade and “V”-shaped blade. In this situation, the thickness of the blade is not specified, as long as the condition that the cut is in the shape of a “V” over the entire thickness of the layer is observed.
• Multi-blade hand-cutting tool.
• Multi-blade cutting tool clamped in a motorized device

In the case of using a single-blade tool, the use of a guide is also mandatory to ensure equal spacing between the blades. The handheld magnifier must have a magnification factor of ×2 or ×3.

The specimen must be flat and free of deformation. Its size must allow at least three tests to be carried out in different places with a minimum distance of 5 mm both between the test areas and up to the edge. If the test tube is made of a soft material (wood), the minimum thickness must be 10 mm. In the case of a hard material test tube (Kevlar in our case), the minimum thickness is 0.25 mm. The ambient conditions in the work area must be 23 ± 20 C temperature and 50 ± 5% relative humidity. The distance between the incisions is established both according to the thickness of the coating layer and according to the material of the specimen, as shown in

Table 3. Choosing the distance between incisions.

	60 μm	61–120 μm	121–250 μm
Soft material test tube	2 mm	2 mm	3 mm
Hard material test tube	1 mm	2 mm	3 mm

.

The framing test is not suitable for specimens with a coating thickness greater than 250 μm. In this case, the “St. Andrew’s Cross” method described in rule ISO16276-2 is recommended. To remove the paint detached after making the incisions, the following can be used [21]:

• Light brushing with a soft brush along each diagonal of the frame.
• Pressure-sensitive adhesive tape.
• Compressed air or nitrogen.

The results are classified using a grading scale ranging from 0 to 5, according to

Table 4. Rating of results.

Classification	Description	Framing Layout
0	The edges of the incisions are perfectly smooth. The paint did not come off.
1	Detachment of small pieces at the intersection of the incisions. Less than 5% of the squared area is affected.
2	The paint peels off along the lines and/or at the intersection of the incisions. Between 5% and 15% of the tiled area is affected.
3	The paint peels off along with the incisions in large pieces, and/or certain squares are partially or totally detached. Between 15% and 35% of the checkered area is affected.
4	The paint peels off along with the incisions in large pieces, and/or certain squares are partially or totally detached. Between 35% and 65% of the tiled area is affected.
5	All large detachments that cannot be classified in class 4.

.

Production of the Kevlar specimen for framing testing.

The process of producing and painting a Kevlar specimen follows approximately the same steps as the final part (for example, a structural part located outside the pressurized area for the Airbus A32 aircraft).

The Draping: On the worktable, place a well-stretched piece of loosening film without creases. Using a plastic or composite wedge to smooth out the creases after each layer, seven folds of material that have been previously cut to the appropriate size are placed on top of one another. Another piece of release film is positioned over the material folds (Figure 15).

The last layer is represented by the drainage tissue, a film with small holes for the exhaust of air between the folds. The part is transferred to the compaction table (Figure 16) for complete removal of air between the folds. This compaction is carried out at a pressure of −0.7 bar for 15 min.

Polymerization is carried out in the Italmatic press (Figure 17). The curing cycle (Figure 18) follows the same parameters as standard parts compatible with the recommendations in the material data sheet. Before transferring the material to the platters of the workpiece, it is important to check that there are no impurities on either the lower or upper plates, as they can cause indentations on the surface of the specimen [12].

Stripping of parts—The three sheets are removed manually or with the help of a cutter (but carefully, so as not to scratch the surface of the piece). If there is an excess of resin on the edges, it is easily removed with the help of the cutter.

Cropping—From the flat panel, specimens with a size of 150 × 100 mm are cut on a three-axis BIESSE CNC milling machine (Figure 19), using specific tools for Kevlar processing.

Painting of specimens for the framing test

The surface preparation is carried out according to the same technical instruction as the finished parts—using the same material, the same heat treatment, and the same machining by cutting. The surfaces of the part/specimen are moistened with water; then, the edges are sanded first perpendicularly and then along them using a sanding path provided with 180 μm sanding paper (Figure 20). After preparing the contour—and in the case of parts with holes, following the same process—the part/specimen is degreased with isopropyl alcohol and sanded over the entire surface with P240 sandpaper (Figure 21). Finally, the part/specimen is blown with compressed air and degreased once again with isopropyl alcohol.

Before inserting the specimen into the painting area, it is recommended to perform a wetting test to ensure that the release agent used during the draping process has been completely removed from the surface. To do this, deionized water is poured over the test tube in an upright position. If the area is clean, the water will flow smoothly and evenly, without forming beads. In contaminated areas, the surface will become shiny, and pearls will be formed by mixing deionized water with the release agent; in this way, it will be easy to distinguish it from the clean surface.

Painting the test tube, like surface preparation, follows the process of painting the final part.

All materials used during the dyeing process must be stored at a temperature between 5 °C and 30 °C, according to the supplier. The substances are brought for acclimatization to the paint preparation area at least 12 h before the start of the painting process. On the vessel in which the mixture is prepared, the references of the substances contained, the batch number, and the date and time when the mixture was prepared must be noted to enable the traceability of the life of the mixture later. The Apret mixture is made according to the volume and mass ratio presented in

Table 5. Ratios for the Apret mixture [22].

Apret Report and Mixture
Product Name	Reference	Volume Ratio	Mass Ratio
Baza Apret 5425/6407	MP00846	2	100 g (±6 g)
Hardener Apret 0707/9000	MP00848	1	32 g (±1 g)
Diluent 0491/9000	MP00050	1	20 g (0/+5 g)

. The components are mixed individually for two minutes with a spatula. Pour the base, hardener, and thinner into a bowl, and continue mixing for at least 2 min [12,22].

The viscosity of the mixture is measured with the AFNOR 4 viscosity cup. Pour the mixture into the cup until it is filled and a meniscus that exceeds the top level is formed. This meniscus is removed with a spatula, after which the lower orifice is released, and the flow time is timed (Figure 22). According to the technical data sheet of the material (supplier), it should be between 15 and 20 s at an ambient temperature of 230 °C. If the flow time is too long, the amount of diluent can be adjusted within the permissible tolerances.

After preparation, the mixture has a three-hour shelf life. To prevent contaminants, the solution is transferred into the spray cannon via a 190 μm paint filter. A 1.5-diameter gun is used to apply a cross-tightened veil (Figure 9). A 30 min room degradation period is then followed by a 60 min oven drying period at 600 ± 50 °C.

The Pyroflex paint mixture is made according to the volume and mass ratio shown in

Table 6. Ratios for the Pyroflex paint mixture [22].

Ratio and Mixture Pyroflex 7D13 Black
Product Name	Reference	Volume Ratio	Mass Ratio
Base Pyroflex 7D13 Black	MP01647-AA	3	100 g (±6 g)
Hardner pyroflex 0651	MP01648-AA	1	32 g (±1 g)
Thinner C25/90S	MP00363-AA	1	32 g (0/+10 g)

. Mix the components individually for two minutes with a spatula. For the base, pour the hardener and thinner into a bowl, and continue mixing for at least 2 min.

The viscosity measurement is performed similarly to the apron, the only difference being that an ISO 4 viscosity cup is used (Figure 23). The flow speed according to the material data sheet is between 15 and 30 s.

The shelf life of the Pyroflex paint mixture is 8 h after preparation. The paint is transferred to the gun through a 190 μm paint filter so that no impurities remain. With the help of the gun with a diameter of 1.5, at a pressure of 3–4 bar, a layer of cross paint is applied (Figure 24). It is left to dry at ambient temperature for 30 min and then dried in the oven for 60 min at 600 ± 50 °C.

Immediately following drying, it is recommended to perform the competent polymerization test described in this paper’s second section at the conclusion of the drying period. Framing test on the composite specimen—The framing test is performed according to ISO2409 specifications. For the exemplified part, the test frequency is once a month if no more than 1000 parts are produced per month. A manual cutting tool with multiple, interchangeable blades is used to carry out the framing (Figure 25). The peel-off paint is removed with the help of TESA 4657 adhesive tape (Figure 26).

First, check whether the cutting edges of the scratching tool are in good condition. The marks made should scratch the substrate down to the base material, but they should be as weak as possible. For each series of tests, the same scratching tool should be used. Using the claw with a 2 mm distance between the blades, a series of three horizontal incisions is made, followed by another three perpendicular to the first ones, resulting in three squares in the upper part of the specimen (Figure 27).

To remove the detached paint particles, apply the TESA adhesive to the three squares, wait five minutes, and then remove with a single, strong motion at an angle of around 60⁰. Using a magnifying glass, the results are inspected and categorized in accordance with the ISO 2409 guidelines (Figure 28, Figure 29 and Figure 30) examples of notations written on Airbus PTP metal test tubes are shown below [23,24,25].

The results of the test shall be noted in a control report, which must contain the following:

Information required for specimen identification: internal reference and production order number (supplier).

Tests related to ISO 2409:2020—Elements related to the preparation of the specimen: base material, thickness, surface preparation, method of applying the paint, and thickness of the paint measured according to ISO2808.

• Temperature and humidity during the test
• Type of cutting tool used and working mode
• The method used to remove peeling paint off
• Test result
• Any deviation from the way of working
• Any abnormal features observed during the test
• Test date
• Identification of the inspector who performed the test [12,22,26]

3. Determination of Liquid Resistance: Water Immersion Method, According to ISO 2812-2:2018

This method allows the evaluation of the action of water on paints and varnishes and, if necessary, the evaluation of the deterioration of the test tube.

The principle of the method consists of immersing the test tube in water and analyzing the effects based on criteria previously agreed upon between the interested parties, these criteria being usually subjective in nature.

The necessary equipment for this test is

• A tank of the right size, equipped with a heating system and thermostat
• Water recirculation system
• Specimen holder, allowing specimens to be placed at an angle between 0° and 20° in vertical input with a minimum spacing of 30 mm between specimens and from the tank’s edge or bottom.

According to the Airbus specifications for the part presented in the previous chapters, the validation of the painting process by the checkering test, according to ISO2409, is carried out both before and after immersion. For this, the Kevlar specimen produced, painted, and tested according to what was previously described in this paper is taken. Using the method described in the previous chapter, a new series of three incisions is made (Figure 31) [27,28].

The water in the tank should be brought to a temperature between 15 and 25 °C at least 12 h before the start of the test. Before immersing the test tubes, the water temperature is checked with the help of a liquid thermometer with a calibrated probe.

The samples must be immersed for 336 h (14 full days) in a tank filled with demineralized water, the temperature of which must be maintained between 15° and 25 °C throughout this period. The tank should be covered as much as possible to prevent evaporation. The samples will be placed on the appropriate support at the bottom of the tank. After the specified number of days has elapsed, the specimens are removed from the water and wiped with a dry cloth, and TESA tape is applied to remove any detached paint particles, according to the working method described in ISO2409 and in the previous chapter. The results are examined with a magnifying glass and classified according to ISO2409. The classification shall be noted in the previously prepared control report.

According to Airbus specifications, the results of the framing test for paint validation must be

• Class 0 before immersion
• Class 1 maximum after immersion

Below is an example of the evolution of the framing test after immersion in Airbus PTP metal test tubes (Figure 32). The test tube was classified as Class 2 before immersion, with about 10% of the surface affected. After immersion, it was reclassified as Class 3, with about 25% of the surface affected [13]; considering the results, further studies had to be carried out to reach the immersion Class 1 required by Airbus [29,30].

4. Discussion

The tests regarding ISO 2812-2:2018 were repeated until Class 1 was obtained, as required by the beneficiary, so that a detachment of small pieces was obtained at the intersection of the incisions. Less than 5% of the squared area is affected. A well-executed painting process ensures the durability, appeal, and functionality of Kevlar composite surfaces. Improvements such as careful adhesion testing, controlled curing conditions, and ideal primer selection boost the procedure’s performance. Continuous testing and monitoring are necessary to guarantee quality and consistency.

Assessing the quality of the coating has been described in Section 3 of the paper, and it can be observed in Figure 28, Figure 29, Figure 30, Figure 31 and Figure 32. Regarding the Apret layer, the average determined result for Apret 5425/6407 Light Blue must be between 10 and 65 μm, and for Pyroflex Black paint, it must be between 25 and 35 μm for the paint layer thickness to be compliant. The entire painting procedure is considered non-compliant, and all items in that lot are repainted (if allowed by the customer’s specifications) if the average coating thickness does not fall within these limits. Future research is needed to extend the study [31]; future studies will focus on enhanced microscopic inspection to evaluate the surface quality and laboratory analysis to quantify the disparity in surface energy between composites and coatings.

5. Conclusions

Composites’ special properties, such as tensile strength, stiffness, thermal conductivity, low weight, etc., have led to an accelerated development of their industry.

Mainly due to its particularly high strength-to-weight ratio, the aeronautics industry is and will remain the pioneer in the development of composite materials, with both giants Boeing and Airbus using them.

The stresses to which an aircraft is subjected to fly at high altitudes are extreme, making the importance of these tests overwhelming. This is why they must be carried out with utmost care and responsibility, in strict compliance with the specified rules.

The test tube classified as Class 2 before immersion (with about 10% of the affected surface) passes to Class 3 after immersion, with about 25% of the affected surface; the tests have been optimized to an immersion Class 1 required by the beneficiary, so that a detachment of small pieces was obtained at the intersection of the incisions; less than 5% of the squared area is affected. Future research is needed to extend this study.

Surface preparation, adhesion, durability, aesthetics, and mechanical testing are all used to evaluate the painting process on Kevlar. A thorough assessment guarantees that the painted Kevlar satisfies the visual and functional specifications of the application for which it is designed. Depending on the application, the procedure needs to be modified. For example, it might call for more layers, a different kind of paint, or more thorough testing, given the conditions to which the material would be exposed.

The longevity, appearance, and performance of Kevlar composite surfaces are guaranteed by a properly conducted painting procedure. The procedure’s performance is increased by enhancements, including meticulous adhesion testing, regulated curing conditions, and optimal primer selection. To ensure quality and consistency, testing and monitoring should be performed continuously.

Strong mechanical and adhesive qualities are provided by the fully cross-linked polymer in the organic coating, which is ensured by adequate curing.

The tests studied in this paper rapidly show that under-cured coatings are softer, less cohesive, and have poor adherence.

Future developments and research will focus on some advanced microscopic analysis to assess the surface quality and laboratory analysis to measure the differences in surface energy between the composite and coating.