A complete guide to the use of defoamers for shoe coatings: scientific operation to solve the
problem of foaming
In the process of shoe manufacturing, paint foaming is an "invisible killer" that affects product quality - bubbles can
cause defects such as pinholes, shrinkage holes, and orange peel in the coating, reduce wear resistance and gloss,
and even cause rework and material waste. As the core auxiliary material to solve this problem, the effect of the
defoamer depends on scientific selection and precise operation.
I. Selection principle: matching the coating system and process requirements
1). Select defoamers according to the type of coating
Water-based coatings: polyether or mineral oil modified silicone defoamers are preferred. This type of defoamer has
low surface tension, can quickly destroy the bubble film in the water-based coating, and has strong shear resistance,
which is suitable for high-speed stirring and spraying processes.
Oil-based coatings: fluorine-modified silicone defoamers or mineral oil defoamers are recommended. Fluorine-modified
silicone has a temperature resistance of over 200°C, which is suitable for high-temperature curing coatings such as PU
and epoxy; mineral oils are low-cost and suitable for cost-sensitive scenarios such as sole polyurethane coatings.
Special process coatings: UV curing coatings need to choose non-migrating silicone defoamers to avoid affecting the
curing efficiency; high-gloss coatings need to choose transparent defoamers to prevent the coating from fogging.
2). Pay attention to the durability and compatibility of defoamers
Durability: High-quality defoamers should still maintain defoaming ability after the coating is stored for 6 months. This can
be verified by accelerated aging tests: After the sample with defoamer is sealed, it is placed in a 50°C oven for 30 days. If the
residual amount of bubbles increases by ≤20%, it means that the durability meets the standard.
Compatibility: There may be conflicts between defoamers and additives such as wetting and dispersing agents and leveling
agents in coatings. For example, when silicone defoamers are mixed with polyacrylate leveling agents, if the HLB value
difference is too large, it is easy to cause shrinkage holes in the coating. Compatibility can be verified through the "pilot-pilot"
process: first test the defoaming effect at 0.1%, then gradually increase to 0.5% to observe whether shrinkage or stratification
occurs. When a shoe material factory was developing a new water-based coating, it used this method to screen out a defoamer
that is fully compatible with the system, avoiding quality problems in large-scale production.
2. Timing of addition: staged control to improve defoaming efficiency
1). Add during the grinding stage: inhibit pigment dispersion and foaming
During the pigment grinding process, high-speed shearing will involve a large amount of air to form micron-sized bubbles.
At this time, an anti-foaming defoamer should be added. Its mechanism of action is to pierce the bubble membrane through
hydrophobic particles to prevent bubbles from merging and growing. The specific operation suggestions are as follows:
Divide the total amount of defoamer into two parts, 50% of which is added before grinding, and the grinder is started after
pre-mixing with the pigment and resin;
The remaining 50% is added in the middle of grinding (when the pigment fineness reaches 80% and passes through the 325
mesh sieve) to ensure full-process foam suppression.
When a shoe material factory produced white PU coating, it adopted the staged addition method, and the amount of bubbles
in the grinding stage was reduced by 70%. After the coating was stored for 3 months, there was still no bubble precipitation,
which significantly improved the product quality stability.
2). Addition in the paint mixing stage: Eliminate construction bubbles
During the paint mixing stage (that is, after adding emulsion and additives), the viscosity of the coating decreases, and bubbles
are more likely to float to the surface. At this time, a defoaming agent should be added, whose molecules can quickly spread on
the bubble film, reduce the surface tension to below 15mN/m, and achieve "one touch and break". The operation suggestions
are as follows:
First stir the paint at a low speed of 300rpm in the paint mixing tank, and then slowly pour the defoamer (avoid directly impacting
the bottom of the tank);
After stirring for 5 minutes, increase to 800rpm for high-speed dispersion for 10 minutes to ensure that the defoamer is evenly
dispersed.
Experiments show that after spraying, the pinhole rate of the coating using this method is reduced from 3.2% to 0.5%, and the
glossiness is increased by 18%, which significantly improves the appearance of the product.
3. Dosage control: accurate measurement to avoid side effects
The amount of defoamer added needs to be adjusted according to the type of coating, construction method and defoaming
requirements. Excessive addition may cause shrinkage, turbidity or decreased leveling of the coating, while insufficient
addition will not effectively defoam. The following is a reference for the dosage of common coating types:
Water-based acrylic coating: The addition amount is usually 0.05%-0.3% of the total coating amount.
Oil-based polyurethane coating: The addition amount is 0.1%-0.5%. A shoe material factory added 0.3% of Digo 810 defoamer
to the sole wear-resistant layer coating, achieving the best balance between defoaming effect and cost.
Special process coatings: It is recommended to add 0.05%-0.2% to UV-curing coatings to avoid affecting the curing efficiency;
high-gloss coatings need to strictly control the addition amount below 0.1% to prevent the coating from fogging.
In actual operation, it is recommended to determine the best dosage through "gradient test": first test the defoaming effect by
adding 0.1%. If the effect is insufficient, increase 0.05% each time until the ideal state is reached. At the same time, record the
changes in coating performance to avoid excessive addition.
4. Dispersion method: uniform mixing is the key
The dispersion effect of the defoamer directly affects its defoaming efficiency. If the dispersion is uneven, the local defoamer
concentration is too high, which may cause shrinkage holes in the coating, while the concentration is too low and the defoaming
cannot be effectively defoamed. The following are two commonly used dispersion methods:
High-speed stirring method: suitable for water-based coatings and low-viscosity oil-based coatings. After adding the defoamer
to the paint mixing tank, stir at a high speed of 800-1200rpm for 10-15 minutes to ensure that the defoamer is evenly dispersed.
Pre-dispersion method: suitable for high-viscosity oil-based coatings or special process coatings. First, mix the defoamer with a
small amount of solvent (such as xylene) in a ratio of 1:5, stir until completely dissolved, and then slowly add it to the coating
and stir at a low speed for 30 minutes.
When a shoe material factory produced high-viscosity PU coatings, the pre-dispersion method was used, and the defoamer
dispersion uniformity was improved by 40%, and the coating shrinkage rate was reduced from 1.8% to 0.1%.
5. Environmental adaptation: the influence of temperature and pH value
1. Temperature control
The defoaming efficiency of the defoamer is closely related to the temperature. Generally speaking, the increase in temperature
will accelerate the migration and spreading of the defoamer, but too high a temperature may cause the defoamer to decompose
and fail. For example, when the temperature exceeds 80°C, the defoaming efficiency of the silicone defoamer will drop by more
than 30%. Therefore, in high-temperature curing coatings (such as PU sole coatings), it is necessary to select a defoamer with
excellent temperature resistance (such as fluorine-modified silicone) and strictly control the curing temperature.
2. pH value adaptation
The pH value of water-based coatings will affect the stability of the defoamer. For example, polyether defoamers are easily
hydrolyzed and failed in a strong acidic (pH<4) or alkaline (pH>10) environment. Therefore, the pH value of the coating needs
to be tested before use, and adjusted to the optimal application range of the defoamer (usually pH 6-8) by adding a buffer
(such as ammonia or phosphoric acid).
Conclusion
The scientific use of defoamers for shoe coatings is the key to improving product quality and reducing production costs.
From selection to timing of addition, from dosage control to dispersion method, every link requires precise operation.
By matching the coating system, adding in stages, accurately measuring the dosage, uniformly dispersing and mixing,
and adapting to environmental conditions, the effectiveness of the defoamer can be maximized to create bubble-free,
high-gloss, wear-resistant high-quality shoe coatings. In the future, with the advancement of materials science, defoamers
will develop in a more efficient, environmentally friendly, and intelligent direction, injecting more innovative vitality into
shoe manufacturing.
