A homogenous and continuous surface film formation is desired for any paint and coating system. Good film formation has many benefits. Some of them are as follows:
- Improved dirt pick-up resistance (DPUR)
- Improved block resistance
- Better aesthetics
- Extended adhesion and durability
- Improved scrub resistance and washability
Get answers to all your questions related to film formation:
- What is film formation?
- What forces in the paint system lead to film formation?
- What are the different types of film formation?
- What factors affect film formation?
- How to measure film formation?
What is film formation?
Film formation is the process of the formation of a thin film on the top of the surface of any painted substrate. It occurs due to the presence of film-forming polymers in the paint or ink system.
What forces in the paint system lead to film formation?
Surface film formation is a complex phenomenon where many physio-chemical interactions play roles. There are many forces acting in the paint system for film formation to occur. These include:
- The emulsion polymer and air surface tension interface
- The polymer and water interfacial tension
- Capillary forces and osmotic pressure
What are the different types of film formation?
There are different types of film formation. They are based on the nature of the curing or drying of the paint system. This relates to the mechanism of film formation. The film formation process is different for different types of coatings, inks, and other polymeric compositions like adhesives. Let's see some of the kinds of film formation.
A. Based on evaporation and coalescence
This applies to solvent or dispersion-based (binders dissolved or dispersed) in a system. Film formation takes place through the evaporation of the solvent/water. This occurs at different temperatures & humidity. This is based on the polymer emulsion type. The evaporation or drying can be either:
- forced using driers in solvent paints or
-
natural in water-based paints.
But, a
film-forming aid may be required, like the presence of coalescent in water-based paints. Examples include cellulose nitrate, chlorinated rubber paints, emulsion paints, and inks.
Process of Film Formation
Credits - ResearchGate
- In water-based systems the coalescence or film formation is abrupt.
- But, in the case of solvent-based systems, the film formation is a continuous and slow process.
Film Formation Rate in Paints
Credits - ResearchGate
B. Based on chemical curing or cross-linking materials
This type of film formation occurs in chemically active polymers.
-
In 2-pack systems, one agent is for cross-linking. In such systems, external conditions affect the rate of crosslinking or film formation. For example, temperature or humidity. Other crosslinking aids are also available. For example, in UV-curable coatings, UV light initiates cross-linking.
- In 1 pack systems, curing is done partially by crosslinking (self-initiated). It can also be done by physical drying. The crosslinking curing or way of film formation results in some of the highest-performance coating films. This is due to the irreversible 3-dimensional crosslinking or reaction of two or more components.
View All Grades of Crosslinking/Curing Agents »
Other film formation methods
C. Physical drying and chemical curing (in automotive and industrial coatings)
D. Oxidative or moisture curing (oxidative PU paint or alkyd paint) etc.
What factors affect film formation?
1. Glass transition temperature (Tg) of binder/polymer
The
glass transition temperature is a parameter of polymeric emulsion, latex, or binder. Here, the latex starts to convert into a solid, hard, and
brittle state.
Polymer Physical State with Temperature
- At low Tg, the polymer will be soft and rubbery at room temperature. It forms a film more easily.
- At high Tg, the polymer will be hard and brittle. This is due to limited mobility. It requires more effort to form the film. This means higher co-solvent levels or higher temperatures if baking or the use of a plasticizer is required.
| Polymer |
Tg (°C) |
| Polyethylene (LDPE) |
-125 |
| Polypropylene (atactic) |
-20 |
| Poly(vinyl acetate) (PVAc) |
28 |
| Poly(ethyleneterephthalate) (PET) |
69 |
| Poly(vinyl alcohol) (PVA) |
85 |
| Poly(vinyl chloride) (PVC) |
81 |
| Polypropylene (isotactic) |
100 |
| Polystyrene |
100 |
| Poly(methylmethacrylate) (atactic) |
105 |
Tg of Different Polymers
2. Minimum film formation temperature (MFFT)
The
MFFT is the minimum temperature at which the polymer/binder, or latex emulsion starts to coalesce or its particles start to fuse with each other. This leads to film formation. MFFT is one of the most critical factors which affects surface film formation.
If the temperature of application or curing is less than MFFT, then the continuous film formation will not occur and there will be cracks in the film. Polymer chain mobility which controls coalescence is dependent on MFFT.
3. Application conditions - Moisture & temperature
Application conditions have a major impact on surface film formation. Generally, a good, continuous surface film formation takes place at:
- higher temperatures and
- moderate humidity.
An increase in humidity can affect the film formation in a negative way. There can be
wrinkling or scaling seen on film surfaces at higher humidity levels.
4. Optimization of formulation
A balanced formulation is the key to good film formation. The balance between the PVC, binder/polymer or emulsion content, surfactant,
solvent or co-solvents, etc., plays a crucial role in film formation.
a. Binder chemistry & dosage
The nature of the emulsion and its content or dosage in formulation impacts the film formation.
- A lower Tg binder will have better film formation, but, it might result in a softer film.
- A higher Tg binder will need more effort to form film and it may lead to hard film. It will also need higher co-solvent/coalescent levels or higher temperatures.
Binder or polymer particle size also impacts the film formation. Higher particle size may result in non-continuous or less intact film formation, as compared to the fine particle size of emulsion polymers.
b. Presence of coalescent or co-solvent
Film formation especially in aqueous systems needs coalescence. This is due to the presence of moderate to high Tg emulsion polymer. Coalescent aid film formation in high humidity or low-temperature area or weathers. Examples of coalescent are glycol ethers, esters, or alcohol esters. Their primary role is to initiate the film formation by:
- fusing or
- deforming the polymer particles.
They also play a role in temporarily lowering the MFFT. This makes the film formation quicker.
Different grades of coalescents or co-solvents are available in the market for different purposes. For example, lower VOC, alcohol-free, APEO/NPEO-free, etc.
Proper (Coalesced) vs Improver (Uncoalesced) Film Formation
c. Effect of Surfactants
Surfactants aid in film formation by giving a better application to the paint system. It also provides the
leveling effect. A paint or coating system without an appropriate dosage of surfactant may suffer abrupt film formation. This can lead to adhesive failures. There is a
wide range of surfactants that aid in superior wetting and film formation properties.
How to measure film formation?
Analytical techniques to evaluate film formation
- Differential Scanning Calorimetry (DSC) is adopted to calculate the MFFT of a polymer or emulsion system. It can also be used to calculate the Tg of the polymers as per ASTM E 1356 or ISO 16805. ASTM D 2354 and ISO 2115 test methods correspond to the DSC.
- Dynamic Mechanical Thermal Analysis (DMTA) including Dynamic Mechanical Analysis (DMA) is used to calculate Tg. These results can be correlated with DSC results.
Test methods to evaluate film formation
The test methods used to evaluate film formation are as follows:
- ASTM D 2354 — It determines the MFFT of emulsion vehicles
- ISO 2115 — It determines the white point temperature and MFFT of polymer dispersions
MFFT Test ASTM D 2354 and ISO 2115
Credits - Spektrochem Paint Technical Center
References
- MFFT 90 Minimum Film Forming Temperature Instrument
- www.mcpolymers.com/library/film-formation-of-latex-binders
- researchgate.net/figure/Film-formation-of-a-water-borne-polymer-dispersion-and-a-solvent-based-alkyd_fig
- https://slideplayer.com
- https://www.paint.org/coatingstech-magazine/articles