Paint delamination: causes and remedies of adhesion defect in industrial painting

Technically speaking, the phenomenon can manifest itself in two main forms:

• detachment from the substrate, when the paint film separates completely from the substrate (adhesion failure);
• detachment between sequentially applied layers, when loss of adhesion occurs between primer, basecoat and topcoat in multilayer systems (intercoat delamination).

Delamination is thus identified as “paint peeling off,” “paint flaking off iron,” or “paint skipping.” In some cases it can be equated or confused with cracking or crazing phenomena in paintwork, which, however, have different origins and do not always involve a loss of adhesion at the interface.

In operational language, delamination can thus be associated with surface defects that have similar manifestations but different underlying mechanisms, following are some examples.

In order to understand the defect of delamination, identify the phenomenon with certainty, define intervention procedures and thus solve the root causes of the problem, it is good to assume three key concepts of industrial painting.

• Adhesion: force that binds the film to the substrate.
• Cohesion: internal strength of the paint film.
• Interface: contact area between two materials (substrate-paint or paint-paint).

In fact, paint delamination occurs when adhesion forces at the interface fall short of mechanical stresses, environmental stresses or internal stresses generated in the film due to inconsistency or instability of process parameters. In multi-coat systems or coating with multilayer cycles, delamination takes on an even more complex, multifactorial dimension, generally attributable to process errors such as:

• Application out of time window (overcoating time);
• Use of unsuitable products;
• Poor chemical compatibility between products;
• Contamination between applied layers;
• Failure of surface activation of the previous film.

Paint adhesion to the substrate is therefore the result of a series of conditions built upstream. For this reason, delamination must be addressed as a systemic problem, as an expression of an improperly prepared and poorly controlled interface throughout the coating process.

This phenomenon can also manifest itself in the form of cracks in the paintwork or localized detachments, which are often misinterpreted as surface defects.

The pretreatment stage plays a crucial role in preventing major paint defects-as seen so far for: pinholes or pinholes, dripping, pitting, orange peel

Indeed, pretreatment has the task of transforming a raw textural substrate into a functional surface suitable for coating adhesion. This occurs through the two key principles or steps of substrate preparation, namely the removal of surface contaminants and the subsequent chemical stabilization of the surface.

Regardless of the type of coating, powder or liquid, the risk factors to be monitored in the execution of the process include:

In liquid coating, delamination is built up at the microscopic level, as it is closely related to the chemistry of the pretreatments and the proper management of the evaporation, application, and crosslinking phases of the film.

In summary, the defect is related to:

• cycle management, thus process steps and timing, sequence and type of applications;
• chemistry of the system, i.e., use of solvents and compatibility between products.

In powder coating, pretreatment can be either chemical or mechanical (sandblasting or shotblasting), depending on the material and performance required. Film formation occurs by melting and thermal polymerization; mass, geometry, and raw material greatly influence the process.

Therefore, the delamination defect is related to:

• Substrate conditions, i.e., quality and effectiveness of mechanical pretreatment;
• thermal cycle, thus curing oven temperature and baking parameters.

From an engineering point of view, the distinction between delamination, cracking, and peeling requires timely analysis of the film breaking point (interface vs. internal cohesion), supported by mechanical testing and microscopic observations.

Correct identification of the mechanism behind paint flaking and peeling is the first step in structural defect prevention. To prevent delamination, it is also good practice to proceed with measurement of coating adhesion through adhesion tests.

Among them:

• cross-cut test (cross-cut test);
• pull-off test (pull-off test);
• impact test (impact test);
• bend test (bend test).

Thus, prevention comes through coordinated control of the following variables:

Solvent evaporation and chemical stability of the surface

Trapped solvents or uncontrolled evaporation prevent the formation of a stable interface between film and substrate or between multiple layers in multiple-coat applications. For the film to stabilize before the next steps, it is necessary to:

• Correctly define the flash-off times;
• Ensure adequate ventilation in the cabin.

Application sequence and overcoating

In multilayer cycles, adhesion is a progressive construction, and proper management of process timing is essential. In addition to complying with paint application specifications (paint windows), it is necessary to:

• Avoid contamination between hands;
• Provide for intermediate surface activations if necessary.

Mixing control and application parameters in 2K systems

To avoid incomplete crosslinking and loss of coating cohesion and adhesion in two-component systems, it is recommended:

• Use automatic and controlled dosing systems;
• Check the mixing ratios regularly;
• Avoid using out-of-specification product.

Compatibility between paint cycles in multilayer applications

Using non-compatible products or cycles that are not designed as a system can cause detachments between primer and finish. Among prevention strategies, we suggest:

• Validate cycles as integrated systems;
• Avoid uncertified combinations;
• Test the adhesion in the industrialization stage of the process.

Variables to be monitored and intervention procedures concern:

Electrical conductivity and surface state of the substrate

The control of conductivity and surface condition accounts for the transfer efficiency of the coating to the substrate and is the prerequisite for preventing delamination phenomena. To promote electrostatic deposition of powder particles, it is necessary to:

• Ensure perfectly clean surfaces with effective pretreatments;
• Optimize electrostatic application parameters.

Temperature curve and polymerization

Improper management of temperature and time can generate the phenomena of: undercooking, with production of weak films; overcooking, with thermal stress on the surface; and failure to degassing, with violent and sudden release of gas. The parameters under control are:

• furnace thermal curve, to be validated on actual production;
• Thermal uniformity of the firing chamber;
• Dwell time of the part at the target temperature.

Addressing the problem effectively therefore requires a systems approach, including: materials analysis, pretreatment design, control of application parameters and validation of process curves.

In this scenario, system design plays a crucial role. Only integrated systems that can guarantee stability, repeatability and traceability of critical parameters make it possible to structurally prevent adhesion defects. It is in this ability to govern the process as a whole that the competitive advantage of manufacturing is measured…