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Discussion on Influencing Factors of Floor Coating Adhesion (Part 1)

Jul. 21, 2020

For floor coating production, adhesion quality control is a primary task of floor coating quality control. Because no matter how well the floor coating is done, as long as there is an adhesion problem, all other work will lose value for the flooring product. How to do a good job in the adhesion control of the floor coating process, the UV Coating Floor supplier will learn from the principle of floor adhesion and the various factors that affect the floor adhesion to effectively control the floor coating Feasible solutions for adhesion.

The principle of UV coating and substrate adhesion:

The adhesion of primer and substrate is one of the key issues of UV wood floor coatings. Generally, wood substrates have a rich pore structure. UV coatings are mainly ester and ether structures, which have sufficient affinity for wood substrates. When the primer viscosity is not high, it can penetrate into the porous substrate and form an anchor structure after cross-linking and curing, which is very beneficial to improve adhesion.

From the perspective of surface tension, the overall surface tension of UV Coating Floor is often lower than the surface tension of more polar and hydrophilic wood, so that UV coatings can easily wet the wooden substrate. And under the capillary action of wood fiber and micropores, it spreads out quickly. This physical penetration is very beneficial to the adhesion promotion. On the other hand, the wood structure contains a large number of hydroxyl and ether bonds. The ester and ether structures of UV coatings may form a "hydrogen bond" force with the hydroxyl groups of wood fibers to promote adhesion. The hydrogen bond energy is between the chemical bond energy and the van der Waals force between molecules. The reason why traditional PU coatings have good adhesion on many substrates is largely due to the hydrogen bond.

From the above two aspects, there should be no difficulty in the adhesion of UV coatings on wooden floor materials. But why do poor adhesion problems often occur in the actual production process? This is to talk about the shrinkage stress that occurs during the curing of UV coatings.

Before curing, UV is mainly active monomers and oligomers, and the relative density is between 0.8-0.9. When it is irradiated with ultraviolet light and cross-linked and cured, the film density generally increases and the film volume shrinks. Because the curing shrinkage in the film does not occur uniformly, there is both shrinkage in the thickness direction of the film and transverse shrinkage. The specific shrinkage process is shown in Figure 1-1: In fact, the shrinkage of the UV coating during the curing process will not necessarily cause problems with the adhesion between the UV coating and the wood substrate. UV coatings begin to undergo free radical polymerization after receiving ultraviolet light irradiation. After the initial induction period of about 0.2 seconds, the internal oligomers and monomers of the coating begin to crosslink quickly. This crosslinking is a three-dimensional network crosslinking, because all At the beginning of the coating film, the reactive chain segments can swim freely, just like a "fish in the water", freely contacting the new cross-linkable chain segments, and the molecular weight continues to increase. In this initial stage, due to the low viscosity of the coating, the chain, The movement of the section is not hindered, so the reaction speed is very fast; to the middle of the cross-linking reaction, the viscosity of the system begins to increase, and some grid-like nodes begin to form in the coating film. At these nodes, the coating film begins to preliminary Hardening, when the chain segment moves here, it will be hindered. During the UV curing process, the volume increase of the paint film will be limited at these points. A large amount of "internal stress" will gradually accumulate inside the coating film. If the "internal stress" is not released during the curing process, the excess internal stress will continue to be released after the curing is completed. The later release of this "internal stress" is very harmful to the adhesion of the UV coating to the wood substrate. It is also reflected by many early customers of flooring, "It is possible to test the adhesion of the UV coating to the wooden substrate for the floor that has just been finished. Why does the adhesion deteriorate after being left overnight? "The root cause.

UV Coating SPC Flooring

UV Coating SPC Flooring

How to reduce the negative impact of internal stress on the adhesion of UV coatings during curing:

So, how to prevent or prevent the negative influence of internal stress on adhesion caused by the UV coating curing process?

First of all, from the perspective of the formulation of coating products, from the formulation design stage of UV coatings, according to the curing shrinkage rate of different UV oligomers and reactive monomers, the rigid group and the flexible group can be reasonably matched; at the same time, the monomer can be selected reasonably. Functional monomers, bifunctional monomers, and multifunctional monomers balance the system viscosity, paint film hardness and other physical and chemical index requirements so that UV coatings can have more crosslinking freedom and stress release space in more time In this way, in the reaction stage, too much internal stress will not accumulate and lead to poor adhesion.

Influencing factors and countermeasures of adhesion during floor UV coating process:

From the current UV coating SPC flooring process, the coating line configuration of most flooring manufacturers has reached the standard configuration of nine bottoms and three sides. This coating configuration can cater to most flooring products (regardless of Whether it is a flat product, an antique product, or a drawing product developed by the floor), we will take this nine-bottom and three-sided production process as an example. For floor UV coating, it has the greatest impact on the adhesion of the final floor product, and the most frequent problems are mainly three types of problems:

1) Poor adhesion of UV coating to the substrate;

2) Poor adhesion between UV coatings;

3) Poor adhesion between the water-based colored layer and the wooden substrate.

Below, we analyze these three issues separately:

① Reasons for poor adhesion of UV coating to wood substrate and countermeasures:

Generally speaking, poor adhesion of UV coating to the wood substrate is caused by the following factors:

· Wood species contains oil, high moisture content, excessive surface polishing of DuPont wire, etc. In this case, water-based colorants and UV coatings are not easy to penetrate and adhere to the wood surface, resulting in poor adhesion of the UV coating to the wood substrate. In view of this situation, it is generally advisable to control the moisture content of wood at (8-12)% in the front-end process, and the specific moisture content of the customer's area to be sent to the floor product shall prevail; for oily substrates, special UV can be used. Adhesion enhancement primer should be used to solve the problem; the front end of the floor should also be polished properly, not over-polishing. Over-polishing will lead to a substantial increase in the concentration of pigments in the subsequent water-based coloring, resulting in increased costs and adhesion difficulties.

· In the water-based coloring agent, the added concentration of the color material (including water-based color concentrate and color paste) is too large, resulting in poor adhesion. When the wood is lightly colored, after the incident ultraviolet light can pass through the UV coating, after removing the part absorbed by the photoinitiator, about 50% of the UV light can still be reflected back by the substrate. In this way, UV The adhesion primer can receive about 150% of the incident UV light energy, so that the UV light energy received by the bottom layer and the surface layer of the adhesion primer will not be very different, and the bottom can also achieve a better degree of crosslinking; When the concentration of the colorant in the water-based colorant is too large, the wood will absorb more color, and the color of the wood and UV contact part is close to black. At this time, the incident UV, the light energy passes through the UV coating, as shown in Figure 2. Excluding the light energy consumed by the photoinitiator molecules, there is still about 50% of the light energy that can reach the dark xylem, but because the wood part is very dark at this time, it absorbs most of the UV light energy, which should be reflected back The UV light energy of the UV coating is gone, which directly affects that the bottom of the UV adhesion primer cannot absorb enough UV light energy, the degree of crosslinking is severely weakened, and the bottom curing is insufficient, so that it will eventually perform well on light-colored substrates. The UV adhesion primer of high quality, on the dark substrate, the adhesion is significantly worse.

· Improper choice of primer. When choosing the first UV primer, only hardness support or wear-resistant support is the only consideration. Then the primer is too brittle and fails to provide considerable flexibility to adapt to the possibility of wood flooring entering the end consumer's home The deformation is very likely to bring the risk of peeling or cracking of the paint from the floor substrate.

In view of the above three factors, when the floor is produced, it is necessary to start the management and control from the front-end process, and comprehensively consider the aspects including the moisture content, the coloring process, and the choice of adhesion primer. The moisture content of wood is controlled within 8-12%; when producing dark floors, choose UV adhesion primer with better adhesion and deep curing; for tree species with heavier oil, you can choose to correspond to this kind of wood that can effectively crosslink For reactive adhesion primer, if necessary, 1-2% UV hardener can be added to the UV adhesion primer to improve the deep curing effect and the degree of cross-linking, and to obtain a better and more stable floor product.

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