Substrate Wetting and Methods to Evaluate Surface Tension
Substrate Wetting and Methods to Evaluate Surface Tension
The additive market proposes many variations of each chemistry, but there is no miracle product. Many parameters must be taken into consideration while selecting the right wetting agent.
One of the parameters is substrate wetting. The substrate wetting of a liquid on a solid substrate is defined by the surface tension and function of the contact angle. The contact angle θ is the angle between the intersection of the liquid-solid interface and the liquid-vapor interface at the three-phase contact line.
The liquid coating must wet the substrate well during application to ensure optimal paint quality in the end. Incorrect substrate wetting may result in premature degradation of coating film and substrate. A liquid coating with poor substrate wetting can provoke various defects, such as:
- Apparition of craters
- Ghosting
- Bubbling due to entrapment of air during the application, and
- Weak adhesion and crawling/de-wetting
These issues not only degrade the aesthetic of the film but also its physical properties. This happens due to the creation of many weak points, corrosion, and peeling. Understanding the wetting concept and selecting the correct wetting agents can improve your paint quality.
Substrate wetting is a surface tension theory. Hence, it is ideal to have a good knowledge of surface tension before moving further.
How to measure surface tension?
Static surface tension
Du Noüy Ring method tensiometer is one of the easiest techniques for measuring the surface tension of liquids. In this method:
- A platinum-iridium ring is dipped into the liquid
- It is then slowly pulled out so that a lamella is formed at the air interface
The force needed to pull this lamella is a direct measure of the surface tension of the liquid. The Wilhelmy Plate method is similar, using a plate.
Du Noüy Ring Method Tensiometer (Source: ResearchGate)
This method compares the surface tensions of surfactants in waterborne and solvent-borne coatings. However, it is not suitable for pigmented systems. This is because of the presence of pigments that hinder lamella stability.
Dynamic surface tension
The wetting must be fast enough to obtain the required film quality while using a dynamic application process. For example, printing. Some surfactants can offer the required results using the Du Noüy Ring method but fail during the application.
- During static conditions — The surfactant has time to equilibrate at the interfaces
- During dynamic conditions — The surfactant becomes mobile. It must orientate rapidly to be efficient
The “Maximum Bubble Pressure Method” measures the ability of the surfactant to adsorb rapidly at the air/liquid interface. Pressurized air flows continuously through the capillary. When the radius of the capillary tip and the radius of curvature of the bubble are equal, the pressure becomes maximum. At that point, the surface tension is calculated. After the maximum pressure, the pressure decreases rapidly by expansion of the bubble. This happens until the bubble is detached from the capillary.
Influence of Surfactants on the Dynamic Surface Tension
Using these tests, we have prepared a summary table regrouping the surface tension of different materials.
Surface tension of various materials (mN/m at 20°C)
|
Liquids - Solvents
|
Hexamethyl disiloxane
|
16
|
Isopropanol
|
22
|
n-Butyl Acetate
|
28
|
Butyl Glycol
|
30
|
Xylene
|
32
|
Ethylene Glycol
|
48
|
Water
|
72
|
Resins
|
Long Oil Alkyd
|
26
|
Polyacrylate
|
35
|
Polyester
|
41
|
Epoxy
|
47
|
Melamine (HMMM)
|
58
|
Substrates
|
PTFE
|
19
|
Paraffin wax
|
27
|
Polypropylene
|
30
|
Polyethylene
|
36
|
PVC
|
40
|
Steel, pre-treated
|
45
|
Glass
|
70
|
Solvent-borne coatings have surface tensions similar to or lower than the common substrates. They are less exposed to the substrate-wetting problem. However, severe application defects can occur leading to ghosting, cratering, or crawling. This can be due to oil contamination and non-homogenous substrates with strong differences in surface tension. A correct wetting agent can overcome these issues.
Waterborne coatings have quite high surface tensions. The surface tension of the water itself (72 mN/m) is even higher than most of the substrates. Here, a wetting agent is a real need. Selecting the right one can also have some positive effects such as leveling improvement or crater prevention.
How to Improve Substrate Wetting?
How to Improve Substrate Wetting?
The substrate wetting is a crucial aspect of waterborne coatings. The surface tension of the water is higher than other substrates. As a consequence, a wetting agent becomes an obligation.
In solvent-borne coatings, surface tension of paints/solvents are closer to the one of the substrate. Thus, the phenomenon is not so strong. However, here the wetting agent can overcome many other problems. For example, the craters caused by oil-contaminated areas.
There are 2 solutions to improve the substrate wetting:
- Increase the surface tension of the substrate — This can be done by cleaning, removal of dust, oils, contaminants, and surface treatment (corona pre-treatment, flaming, acid, or basic treatment). This is sometimes hard and complex.
- Decrease the surface tension of the coating — The most common method is to add a component, substrate-wetting agents, into the liquid paint.
Exploring the chemistries of wetting agents
Simple organic-based agents
Organic-based agents are primarily hydrocarbon-based. They offer acceptable surface tension reduction. However, they are not as efficient as silicone or fluoro-based agents.
Silicone-based agents
Silicone-based agents enable good static and dynamic surface tension reduction. They have a wide range of uses including excellent wetting penetration on wood substrates. They are very efficient on difficult substrates. The silicone chemistry is well-known and is very flexible.
Silicone-free agents
Silicone-free agents are acetylenic and alkoxylate derivatives. Their key properties are:
- excellent reduction in dynamic surface tension
- no foam tendency
They are a good alternative to formulations where silicone structures are not accepted. Green products and environmentally friendly alternatives are emerging in this category.
Polymeric silicone-free agents
Polymeric silicone-free agents are mainly acrylate or maleate derivatives. This chemistry offers a good alternative to silicone-based products. However, their surface tension reduction is not as high as silicones or fluoro surfactants. They can also be used to fine-tune the formulation.
Fluoro-based agents
Fluoro-based agents offer excellent static surface tension reduction and poor spreading performances. They can prevent craters on contaminated substrates. They are more expensive than silicone-based agents.
Select the commercial grades of wetting agents based on their chemistry:
Role of wetting agents in substrate wetting
A wetting agent is a surfactant, a hydrophilic and a hydrophobic part. This specific structure self-orientates the additive at the surface. Thus, it reduces the surface tension of the liquid paint.
The polar parts stay in the aqueous phase while the non-polar parts orientate at the interface. Hydrophobic parts are often based on hydrocarbon. Usually, polar molecules are based on polyether chains. Polymer surfactants with fluorinated or polysiloxane backbone can attain specific properties.
Surfactant Molecules can be Polymerized to Form Polymer Surfactants (Example: Polisiloxanes Surfactants)
Chemical Model of a Silicone-based Wetting Agent (Rx are Hydrophilic Polyether Chains)
Resolving coating defects with suitable wetting agents
Like other coatings additives, wetting agents must be effective at the lowest dosage. They must not have any negative effects. While working at the interface, such an additive must not increase foam or trouble the intercoat adhesion.
Crawling in Waterborne Acrylic Coating Can be Cured by Silicone-based Agents |
Craters in 2K Epoxy Can be Cured by Silicone-free Agent |
Advantages of wetting agents
Other than substrate wetting improvement, these agents can have various benefits:
→ Prevention of surface defects (such as craters)
→ Improvement in flow and leveling
→ Improvement in droplets spreading in spray application
→ Possessing defoaming ability
→ Reduction in bubbling in the backing application
→ Improvement in polymer film formation
Selecting the Right Wetting Agent
Selecting the Right Wetting Agent
Essential pre-tests for choosing the right wetting agent
The use of wetting agents is more crucial for waterborne coatings than solvent-borne coatings. Each coatings market uses specific evaluations and focuses on different properties. Thus, it is necessary to lead some pre-tests in the laboratory to select the correct substrate wetting agent. These tests are:
- Compatibility of the wetting agent with the liquid coating
- Wetting of the substrate
- Foam formation during the coating manufacturing process
- Foam formation during the application
- Haze and turbidity of the applied film
- Substrate adhesion of the dry film
- Coatings film defects (craters, leveling, intercoat adhesion)
Assessment of properties for the right selection
A formulator has a large role to play in the selection of the correct wetting agent. Various chemistries can be tested. The table below lists the types of wetting agents available in market. Let's have a look at their properties:
In some cases, a silicone base agent can be compared with a silicone-free one, and proceed with all the tests to select the right one. Some examples are:
- Silicone-free agents — Used to achieve dynamic surface tension reduction. For example, in the printing field.
- Silicone-based agents — Used to achieve strong surface tension reduction in wood coatings or industrial fields.
- Fluoro-surfactants — Solves strong cratering issue.