Dirt pickup resistance in paints and coatings

The dirt pickup in paints and coatings is a complex process that includes multiple types of physical and physiochemical activities. There can be multiple constituents in the dirt or dust on walls. These include:
 

• fine particle solids (from a few nanometers to a micrometer range),
• soot particles,
• ash particles,
• organic or inorganic particles, etc.

The three main phases in the dirt pickup process of paints and coatings are explained below:

 

Stage 1 — Deposition of dirt/dust on the surface

Deposition is a stage where the particles get on the surface. The deposition can occur due to multiple factors, such as:
 

• gravitational forces,
• Van der Waals forces,
• electrostatic charges,
• size and charges of particles, etc.

Van der Waals forces and electrostatic forces act remotely. However, the capillary forces act when the particles are sitting on the surface (contact).

 

Stage 2 — Adhesion or bonding of dirt/dust on the surface

The second stage is the adhesion or bonding of the dirt particles to the surface with time. DPUR is a time-dependent process of particles coming together. Thus, they get further bound strongly due to capillary action and concentration up to a certain degree. The adhesion can be irreversible and reversible in different cases, which can affect the performance of coatings.

Several factors that influence strong adhesion or entrenchment are:
 

• reactivity of the surface and dust particles,
• encrustation or solubilization of dirt particles (especially in aerosols) on the painted surface, and
• creep phenomenon where the paint film softens and traps the dirt or dust particles on it
   

Figure 1: Euler diagram showing strong adhesion forces and their factors for DPUR²

Stage 3 — Release or particle shedding of dirt/dust

The process where the attached particles debond and release themselves from the surface is known as shedding/release. It is important for a DPUR paint or coating to release or shed the dirt particles on it.

The release can also be understood with an equation that relates to the particle behavior under inertial and gravitational forces. This leads to the removal or bonding nature of dirt/dust on surfaces, regarded as the Péclet number. 
 

Here,
 

• Δρ = effective difference in density between the particle and the continuous phase
• g = acceleration due to gravity
• a = spherical equivalent particle radius
• k = Boltzmann's constant
• T = temperature

The particle Péclet number provides a useful gauge of where a particle (or cluster of particles) lies on this spectrum as the balance between Stoke's sedimentation and Brownian motion. Particles dominated by gravitational interactions (e.g., Pe > 10) are likely to settle rapidly and are not as likely to be available to surfaces.
 

Figure 2: Process and factors responsible for dirt pickup and release²

The self-cleaning effect or property of a paint or coating system refers to the ability of paints to remove dirt or dust from its surface by its hydrophobicity and photochemical dynamics. Self-cleaning can occur in multiple ways. It can be the surface morphology of the paint or coating where nanoparticles come together and give the surface a dirt or water-repelling effect.

Major types of self-cleaning paints work on three properties:
 

• hydrophobic nature
• hydrophilic nature
• photocatalytic nature

Self-cleaning can also occur by photochemical reactions where UV rays interact with the paint matrix and cleanse the dirt and dust by debonding. It can also remove air pollutants on the surface and in close proximity. Self-cleaning paints also come in a class of dirt-pickup resistance paints. They bear the property of cleaning dirt or dust themselves. A schematic diagram of dirt removal or self-cleaning paint is shown in Figure 3.

 

Stages of self-cleaning paints

There are three stages in self-cleaning paints to clean dirt or dust accumulated on the surface which are as follows:
 

1. Photoactivation of coating by UV light: This occurs on the surface where dirt or dust is accumulated. This can occur in one to two weeks of daylight exposure. The activation can take longer in areas where UV light is not sufficiently available.
    
2. Breakdown of dust and dirt (organic pollutants) by photochemical reactions: This also results in the debonding of inorganic particles on the surface. Here the cleaning process starts and the dirt particles which are strongly bound to the surface, start to release from the surface.
    
3. Washing away of dirt/dust by rain: In this stage, the rain showers wash away the loosely bound dirt and dust particles. It is due to the lotus effect (water repellency effect/water beading on surface paint).

This leaves a clean surface behind without any human intervention and is hence called the self-cleaning effect of paints and coatings. Additionally, UV activation breaks down the VOCs and harmful SO_x and NO_x gases and acts as anti-pollution paint.
 

Figure 3: Schematic representation of the stages of self-cleaning paints and coatings³

Some of the important factors which contribute in dirt pickup up of paints are discussed as follows:
 

Van der Waals forces

Van der Waals forces are the forces of attraction between two objects. In this case, they are dirt particles and painted surfaces. These forces are not very strong but when dirt particles of size ranging from a few nanometres to a few microns come in proximity of painted surfaces they attract it towards surfaces.

 

Capillary action

These are the forces acting on dirt or dust particles already settled on the walls. With capillary action, the bonding of dirt to the walls gets stronger.

 

Electrostatic forces

Electrostatic forces arise due to the ionic charges on the dirt particles and the surface energy of walls/surfaces. This way the charged particles bond with surfaces leading to the dirt picking up effect.

 

Thermal cycles

Generally with hot temperatures the tendency of encrustation (solubilization of dirt particles on the surface) and creep (fluid nature of surfaces especially at elevated temperatures) increases. There also occurs a strong deposition or bonding with dirt. It further strengthens with the precipitation during nights or colder temperatures and humidity.

 

PVC or sheen of paints

Low PVC paints tend to repel dirt and dust more as compared to high PVC paints due to the increased slip, lower surface energy, and a low porous nature. The right product selection is equally important for DPUR paint.

 

Presence and characterization of dirt particles/pollutants

The DPUR performance of a paint or coating also depends on the nature of dirt or dust particles present in the proximity of surfaces. A fine particle suspension of dirt or dust particles includes soot, organic pollutants, and inorganic particles.

The fine particle distribution tends to get more deposition and strong bindings as compared to higher particle sizes. For instance, a particle with a nanoscale size will stick with more bonding energy as compared to a few micrometer-sized particles. Also, the organic dirt can be more susceptible to attach and hard to shed off by the surfaces.

 

Environmental conditions

Various environmental and chemical factors are involved in the shedding of particles. These include surfactant blooming, excess accumulation of dirt on specific regions of coated substrates, chalking, cracking, wind speed, and thermal cycles.

 

There are multiple approaches to formulating a paint or coating for improved dirt pickup resistance. A few of them are as follows:

 

Selection of the right polymer system
 

The right polymer system for paints and coatings must be selected especially considering the weather conditions.
 

• Conventional polymer systems tend to have low to moderate DPUR properties.
• Modified polymers have shown excellent DPUR properties. These polymers may have one or more alterations in the chemical structure and nature. For example, crosslinking polymers, benzophenone-modified emulsions, core and shell-modified polymers, etc.

Adopting high glass transition temperature (Tg) of polymers

One of the most important factors to consider is the Tg of polymer or binder system in paints.
 

• High Tg polymers are more rigid and show less susceptibility to dirt or dust accumulation.
• Low Tg emulsions or polymers are prone to dirt accumulation due to their softer nature.

Lowering the PVC of paint

Managing a low PVC of paint can significantly improve its dirt pickup resistance. This is because as the porosity reduces, the capillary action of dirt pickup reduces significantly.

 

Increasing slip or sheen levels of paints

A compact paint film (with low porosity or roughness) will have reduced surface area which will make it difficult for dirt particles to stick in the cavities. A low porosity surface that can be smoothened can significantly improve the DPUR of paints.

Similarly, a higher slip will result in the release of dirt on the paint surface. Surface slip can be increased by adding:
 

• fluorosurfactants or modified silicones,
• modified silanes,
• submicron-size powders

Apart from this, multiple other factors can be considered for getting better dirt pickup resistance. One of the considerations is the nature of the surface/substrate.

 

Natural exposure - ASTM D3274
 

The most basic yet impactful testing for dirt pickup performance is done by naturally exposing the painted samples at selected sites. This can give very relatable results. These sites are selected based on their extreme or diverse weather conditions like:
 

• dust intensity,
• particle size distribution of the pollutants or dirt/dust,
• rainfall,
• heat,
• humidity, etc.

The exposures can be done at different inclinations from horizontal to vertical surfaces and at certain angles.

 

Dirt pickup UNI 10792

Italian standard for dirt accumulation on the painted surface with the help of a liquid slurry on painted panels in the laboratory.