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Discussion on Influencing Factors of Floor Coating Adhesion (Part 2)

Jul. 22, 2020

Discussion on Influencing Factors of Floor Coating Adhesion (Part 2)2. Reasons for poor adhesion between UV coating layers and countermeasures:

1) Improper curing degree of primer/topcoat. When UV primer is under UV curing, if it is over-cured in a process that does not require sanding and the surface is too dry, it will cause poor interlayer bonding with the subsequent UV coating. This is because, at this time, after the previous UV primer is over-cured, the reactive groups on the surface of the paint film are basically gone, and the subsequent UV coating cannot produce a chemical bonding reaction with the previous UV primer, and its interlayer bonding force Only the intermolecular force is much smaller than the chemical bonding force inside the coating film, which is prone to poor interlayer adhesion. In the process that requires sanding, if the primer is not properly dried, after the sanding belt sands a certain square floor, the sand will be covered by the paint particles that are not completely dried. These paint particles will cover the floor when the sanding belt sands the floor behind. On the surface, these paint particles will become poor points of interlayer adhesion, resulting in poor interlayer adhesion. In the process that requires sanding, if the primer is over-cured, the paint film is too hard, and the abrasive belt wears quickly. After a period of time, the abrasive belt basically loses its sanding effect on the primer, resulting in poor interlayer adhesion.

Responses:

In view of these factors, it is recommended that the primer should be sufficiently viscous in the non-sanding process; while in the sanding process, the drying degree of the primer should be non-sticky to the touch and white marks on the nails. Of course, these recommendations are Generally speaking, specific products and processes need to be determined according to the actual situation of the coating site.

2) Improper sanding or dust removal. After the sanding primer is cured, a primer sanding machine is required to sand the surface of the primer to remove particles and obtain a smoother surface effect. At the same time, the surface of the paint film maintains sufficient micro-roughness to facilitate the next UV coating. Of penetration. However, when the sanding machine is improperly controlled or the sanding belt is not replaced in time, the floor is not sufficiently sanded or the dust generated by sanding is not removed in time and effectively, poor interlayer adhesion will easily occur.

Responses:

Regularly check the effect of the sanding surface, and correct the parameters of the sanding machine in time to ensure good sanding; at the same time, a 3-axis dust removal device can be installed after the sanding machine, and it is equipped with manual air blowing to remove dust to avoid the surface after sanding. Powder particles affect the adhesion of the posterior channel.

3) The amount of UV primer coating after the wear-resistant primer is insufficient.

UV wear-resistant primer is a product developed to improve the long-term durability of floor products. The product also has a large number of large-diameter inorganic particles. The uniform distribution of these particles on the floor is the guarantee of the floor's wear resistance. These wear-resistant particles need to be completely covered by the subsequent primer before they can be sanded. Otherwise, when the wear-resistant particles are not covered by the subsequent UV primer resin, these inorganic particles will be completely exposed after sanding. On the surface of the paint film, at these exposed inorganic particle points, the subsequent UV coating cannot chemically bond with it. As a result, at these points, the adhesion level of the paint film is greatly reduced, which eventually leads to poor adhesion.

Responses:

After the wear-resistant primer, it is best to ensure that there is 2 to UV sanding primer coating, and the coating amount ratio should be controlled at ≥ 1:1.8, that is, if the wear-resistant primer coating amount is 20g/m2, Then the coating amount of the next two UV sanding primers is best controlled at (36-40) g/m2, otherwise, the risk of poor interlayer adhesion will increase greatly.

4) The UV base/top paint is color-added, and the color essence is added in an inappropriate proportion.

UV color repair refers to the process by which some floor manufacturers add color concentrates to the UV primer or topcoat to achieve a better color effect for the effect of floor color. Generally, UV coloring uses color concentrates, and the addition ratio of color concentrates varies from 5% to 30% or even more. This kind of craft was most popular in the 1990s, and it is gradually becoming less. Color concentrate particles are very easy to absorb UV light. For photoinitiators that need to absorb UV light to generate free radicals, these color concentrate particles are huge competitors. When the content of color concentrate added in UV primer or topcoat exceeds a certain level For example, when the ratio is 10%, the initiator molecules of the bottom layer cannot absorb enough UV light, and the concentration of active free radicals that can initiate the UV cross-linking reaction in the bottom coating is much lower than the surface of the coating, which will cause the additive UV coating. The surface layer of the layer has dried on the surface, but the bottom layer has only a very low cross-linking density, so the cross-linking strength of the base paint and the previous primer is much lower than the cross-linking strength of the surface layer. The internal stress is ultimately manifested as poor adhesion of the additive coating and the underlying primer layer.

Responses:

Avoid heavy color addition at one time. The addition ratio of each color additive cannot easily exceed 8%; in the case of color addition and high color addition ratio, 3-5% deep curing agent can be added to the paint, auxiliary With the drying method of gallium lamp + mercury lamp, the penetration ability of long-wave UV is used to improve the bottom curing degree of the additive primer/topcoat, thereby reducing the risk of internal stress and avoiding poor interlayer adhesion.

5) Rework boards or overnight boards are back online.

In the production process of floor coating, there will be a certain proportion of primer and topcoat bottom line boards. Most of these floors have certain surface defects. After repair, they will be re-on-line and finished. There are also some floor products that need to be removed midway. Line, after a period of time, re-on-line coating. Generally, the surface drying effect of the primer off-line board is weaker than that of the ordinary primer. Because UV curing requires a photoinitiator to absorb ultraviolet light to generate free radicals, and then start the UV crosslinking reaction, the UV curing system quickly changes from a low viscosity state to a gel state, and the chain free radicals generated during the polymerization process are surrounded by a high viscosity environment. , The "bi-base coupling termination process" that is easy to occur is suppressed, and the chain free radicals become active long-lasting free radicals. Even within at least 1 hour after the floor leaves the UV lightbox, the surface temperature is still high and the coating Due to its small size and high kinetic activity, the remaining small-molecule active monomers may still slowly travel through the "grid" of the UV coating, diffuse to the active free radical appendages and lengthen it, and continue to slowly crosslink. Although the cross-linking speed is obviously not as fast as the reaction speed under UV lamp irradiation, the research of many experts have confirmed the existence of "dark reaction" after UV coating leaves the UV lamp. The degree of crosslinking completed by the dark reaction is related to the degree of crosslinking completed when the UV light is initially irradiated and can be increased by up to 20-30%. This means that after these off-line UV primers are off-line, cross-linking reactions will still occur in the paint film, resulting in the remaining active groups on the surface of the paint film that can react with the subsequent paint layer due to the "dark reaction". Significantly reduced. As a result, it will be difficult for the downstream UV coating to produce effective chemical bonding with the UV primer after the offline, and it will eventually show up as poor interlayer adhesion.

Responses:

1. Carry out sanding treatment on the floor that is on the second line after off-line, so that the paint film remains rough, which is good for the penetration and adhesion of the subsequent UV coating;

2. For some off-line boards that have been left for too long, it is recommended that after sanding, apply a UV repair agent 621331 (available from Shanghai Jianfu Coatings) to add more active anchoring groups on the surface of the paint film. Road UV adhesion provides a good foundation.

6) The temperature control of the UV curing box fails.

There is a temperature control probe above all UV curing boxes. When the temperature of the box body exceeds a certain set temperature value, the operating speed of the exhaust motor above the UV curing box will be significantly increased to reduce the temperature of the lightbox. When this temperature control probe fails, causing the temperature inside the curing box to remain high for a long time, the primer cured of this UV lightbox is likely to have poor interlayer adhesion with the next primer. Why is this? The speed and final conversion rate of UV curing are affected by temperature. In the later stage of UV curing, the ambient temperature rises, which is beneficial to the movement of the chain segments and residual monomers in the cross-linking system, and promotes further polymerization. It is equivalent to raising the temperature of the system to above its glass transition temperature for reaction, overcoming the hindering effect of the glass state on the movement of chain segments and residual monomers. As a result, the conversion rate of the final system is significantly increased. In the case of the temperature control failure of the UV curing box, the actual conversion rate of the UV primer is much higher than the ideal value, so that all the active groups that should have remained on the surface of the paint film are all reacted, and the subsequent UV and the paint film It is difficult to achieve chemical bonding, resulting in poor interlayer adhesion.

Responses:

Check the coating line equipment regularly. When it is found that the UV light box body is overheated or suddenly rusted, it may be a temperature control failure, which needs to be eliminated in time and replaced.

7) The temperature of the UV primer and infrared drying tunnel is over-temperature.

In many UV floor coating lines, after the first UV transparent primer or sanding primer coating after the UV wear-resistant primer, an infrared heating drying tunnel will be added before UV curing to make the UV sanding primer or transparent The primer can level better and get a smoother effect. Sometimes the infrared heating temperature is too high, which can easily lead to poor adhesion between layers. This is because when the temperature of the infrared drying tunnel exceeds the temperature, the temperature of the UV coating passing through the drying tunnel will be higher than the set ideal value, and the higher the temperature of the UV coating itself, under the condition of the same UV light conditions, there will be Facilitate the movement of the chain segments and residual monomers in the cross-linking system, the higher the curing speed and conversion rate will be, resulting in that the actual surface drying effect of the coating is much higher than the ideal setting value, and its surface residual active groups The number of clusters decreased sharply, and the final manifestation was poor interlayer adhesion.

Responses:

Check the temperature of the drying tunnel frequently, and it is advisable not to exceed 40-50°C.

3. Reasons and countermeasures of poor adhesion between the water-based colored layer and a wooden substrate:

Poor adhesion of the water-based coloring layer to the wood substrate is common in the following situations: the addition ratio of the color concentrate in the water-based UV colorant is too high; the pure white or off-white water-based hiding color itself is not highly cross-linked. The following is a description of these situations:

1) The proportion of the colorant in the water-based UV colorant is too large

Water-based colorants for floor coloration generally include two categories: water-based UV colorants and water-based PU colorants. The drying of water-based UV colorants requires infrared drying to remove moisture, and then UV curing and drying; water-based PU colorants only need to rely on infrared drying. Generally speaking, when the additive color concentration is less than 20%, the adhesion performance of the water-based UV coloring system will be significantly better than that of the water-based PU system; and when the colorant addition ratio in the water-based UV colorant exceeds 30% or higher When UV irradiation is performed, too many pigment particles in the system will absorb most of the UV light, and the photoinitiator at the bottom of the UV Coating Floor will not absorb enough UV energy. As a result, the degree of cross-linking at the bottom is very low, and the bonding force between the colored layer and the floor wood substrate is very poor.

Responses:

When it is really necessary to add a high proportion of color concentrate or colorant to the water-based coloring system to obtain the desired coloring effect, the methods are as follows:

1. The water-based PU colorant is used instead of the water-based UV colorant. Because the water-based PU colorant only needs infrared drying to achieve consistent drying effect, in the heavy coloring system, the water-based PU colorant can get better adhesion more easily performance;

2. Add a certain amount of water-based cross-linking agent to the water-based UV colorant system to increase the degree of cross-linking at the bottom of the colorant, thereby enhancing the adhesion to the substrate.

2) Pure white or off-white covering color has poor self-crosslinking strength

In recent years, gray or pure white cover-color flooring products have accounted for an increasing proportion. The importance of the adhesion and boiling resistance of such cover-color flooring products has gradually been recognized by the majority of flooring processing companies. Because this kind of white or gray floor products need to use a large amount of white color paste or white covering primer, the addition ratio is often more than 60%, this kind of pure covering floor products due to the introduction of a large amount of inorganic titanium dioxide, resulting in a sharp increase in coating PVC High, the cohesion of the coating itself is significantly reduced, resulting in a decrease in adhesion to the substrate and the subsequent UV coating.

Response measures of UV Coating Floor supplier:

1. For fully covered floor products, try to use water-based white covering primer, because this white covering primer has its own cross-linking performance; of course, the specific variety of water-based white covering primer needs to choose the main resin with a strong affinity to the substrate, Products with good water resistance. The method of adding white colorant to the water-based colorant is not suitable for the coloring effect of full coverage, because this method causes the cohesion of this water-based coating to deteriorate.

2. For products that require high adhesion and boiling resistance,

About 1% of the water-based crosslinking agent can be added to the water-based white base to increase the cross-linking strength of the covering white base itself and the covering white base and the substrate, and eliminate the hidden danger of adhesion.


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