Application Research of UV-LED Curing Stone Plastic Floor Coating

The base material of stone plastic (SPC) floor is a composite board made of stone powder and thermoplastic polymer material after being evenly mixed and then extruded at high temperature.

At present, SPC floor coatings are mainly cured by mercury lamps, and their own shortcomings such as high energy consumption, ozone generation, mercury pollution, and inconvenient use cannot be ignored

Regarding the problem of oxygen inhibition of free radicals under UV-LED light curing, a lot of research has been carried out at home and abroad.

In addition to optimizing the equipment process, in the formulation design, oxygen scavengers can be added to eliminate the surface oxygen and generate new active free radicals

In overcoming oxygen inhibition, the mobility of fluorine-modified compounds has unique advantages

The cationic light curing system is a reaction system that generates super acid or Lewis acid to initiate the polymerization of cationic oligomers and monomers through the absorption and decomposition of light acid generation.

This article mainly combines the research of many scholars in the past to study the UV-LED curing SPC floor coatings.

1 Experimental part

1.

Modified epoxy acrylate (CN2003 NS), trimethylolpropane triacrylate (TMPTA), 1,6-hexanediol diacrylate (HDDA), tripropylene glycol diacrylate (TPGDA), active amine (CN371 NS): American Sartomer Company; Amine modified polyester acrylate (DM285): Double Bond Chemical Co.

1.
2 Equipment

Mercury lamp (RW-UVA202-20GL), 395 nm LED lamp (RWLED-YT200GL): Shenzhen Runwo Electromechanical Co.
, Ltd.
; EIT UVPower Puck Ⅱ: EIT, USA; QHZ coating scratch tester: Tianjin Jingkelian Materials Testing Machine Co.
, Ltd.
; Martindale Abrasion Tester (DZ-335-4): Dongguan Dazhong Instrument Co.
, Ltd.
; JM-IV Paint Film Abrasion Tester: Shanghai Pushen Chemical Machinery Co.
, Ltd.
; WG68 gloss meter: Shenzhen Weaver Optoelectronics Technology Co.
, Ltd
.

1.
3 Test method

The energy of the light source is tested using EIT UV Power Puck Ⅱ; the surface dry test adopts the finger dry method, the lower the energy required for surface dry, the faster the curing speed of the coating; the adhesion is tested according to GB/T 9286-1998; the gloss is tested according to GB /T 9754—2007 is tested; the scratch resistance is tested in accordance with ISO 1518-1: 2019 with a scribing needle B [with a hemispherical hard metal tip with a diameter of (1.
00±0.
01) mm]; micro-scratching The rubbing is tested in accordance with BS EN 16094; the wear resistance is tested in accordance with GB/T17657-2013, the grinding wheel is CS17 (no gauze is needed), external force (4.
9±0.
2) N is applied, and the number of abrasion revolutions is 1,000r
.

2 Results and discussion

2.
1 Free radical light curing system

The traditional mercury lamp curing SPC floor coating usually adopts one-bottom and one-side roller coating construction process.
This research uses this as the basis to design a UV-LED curing formula system.
The formulas of primer and topcoat are shown in Table 1 and Table 2, respectively.
, Performance test conditions and test results are shown in Table 3
.

Table 1 SPC floor primer

Table 2 SPC floor finish

Table 3 Test results of SPC floor primer and topcoat

SPC floor primer mainly provides adhesion to the PVC wear-resistant layer of the base material and reduces the gloss.
Conventional online production will generally achieve a semi-dry state to increase the adhesion of the primer layer.
This is because the primer is half-dry In the state, the surface curing degree is relatively low, the crosslinking density is relatively low, the topcoat easily penetrates into the primer, the anchoring effect and the intermolecular interaction will improve the adhesion between the layers, and there will be some unreacted double bonds on the surface.
The cross-linking reaction on the interface can occur when the lacquer is combined
.
Therefore, it is relatively difficult to adjust the traditional mercury lamp curing formula to the UV-LED curing formula.
By replacing the photoinitiator with the long-wavelength photoinitiator TPO/ITX, appropriately increasing the content of active amine to overcome the oxygen inhibition can be satisfied.
The performance requirements of the application
.
After the SPC floor is coated with the primer, the topcoat is cured, and the topcoat usually needs to be tested for surface dryness, gloss, scratch resistance, micro-scratch, and abrasion resistance
.
Gloss can be adjusted according to different requirements of customers, but generally it is mostly matte, scratch resistance requirement ≥20N, micro-scratch ≥B2, abrasion amount ≤0.
015g
.
It can be seen from Table 3 that the traditional mercury lamp SPC floor finish can usually meet these requirements, but when it is changed to the UV-LED formula, some performance will be reduced to varying degrees
.
Topcoat ① After simply adjusting the photoinitiator to TPO/ITX/active amine, due to the monomer activity problem of the system resin, the surface dryness is poor, the surface dryness requires higher energy, and the performance is generally poor
.
This is mainly because the surface initiator content is low, and oxygen can penetrate the depth of 0.
1~10 μm within 0~5 s.
The free radical active center generated at the moment of light is easily quenched by oxygen, which leads to the surface Low cross-linking density, micro-scratch, chemical resistance marks are easy to appear, and wear resistance is reduced
.
Although the performance of the top coat ② has been improved by increasing the content of initiator, the surface drying still needs higher light energy
.
This shows that increasing the content of photoinitiator is beneficial to increase the concentration of active centers generated at the moment of light, thereby consuming oxygen and reducing the effect of oxygen inhibition, but it did not meet expectations.
This is because only when the photoinitiator is increased to a sufficient content can be satisfied The performance requirements of the application
.

After the SPC floor is coated with the primer, the topcoat is cured, and the topcoat usually needs to be tested for surface dryness, gloss, scratch resistance, micro-scratch, and abrasion resistance
.
Gloss can be adjusted according to different requirements of customers, but generally it is mostly matte, scratch resistance requirement ≥20 N, micro-scratch ≥B2, abrasion amount ≤0.
015 g
.
It can be seen from Table 3 that the traditional mercury lamp SPC floor finish can usually meet these requirements, but when it is changed to the UV-LED formula, some performance will be reduced to varying degrees
.

Topcoat ① After simply adjusting the photoinitiator to TPO/ITX/active amine, due to the monomer activity problem of the system resin, the surface dryness is poor, the surface dryness requires higher energy, and the performance is generally poor
.
This is mainly because the surface initiator content is low, and oxygen can penetrate the depth of 0.
1~10 μm within 0~5 s.
The free radical active center generated at the moment of light is easily quenched by oxygen, which leads to the surface Low cross-linking density, micro-scratch, chemical resistance marks are easy to appear, and wear resistance is reduced
.

Although the performance of the top coat ② has been improved by increasing the content of initiator, the surface drying still needs higher light energy
.
This shows that increasing the content of photoinitiator is beneficial to increase the concentration of active centers generated at the moment of light, thereby consuming oxygen and reducing the influence of oxygen inhibition, but it did not meet expectations.
This is because only when the content of photoinitiator is increased to a sufficient level.
There will be better results.
For example, studies have shown that when the TPO content reaches 6% (molar fraction), the curing uniformity of the surface and the deep layer can be consistent.
This is obviously not due to cost and yellowing in SPC floor coatings.
Realistic
.

The topcoat ③ increased the viscosity of the system by replacing the monomer HDDA with TPGDA and alleviated the effect of oxygen inhibition.
However, the results showed that the surface dryness of the topcoat did not improve, but decreased.
This may be due to the high viscosity of TPGDA and poor dilution.
The segment contains an ethoxy group, which can theoretically be used as a hydrogen donor, which is helpful to alleviate the inhibition of oxygen polymerization, but in the end it may be due to the methyl substituent in the TPGDA segment.
The induction and steric hindrance of the methyl group make the active hydrogen The extraction cannot be performed efficiently
.

Topcoat ④In the topcoat ② system, adjust the resin polyurethane acrylate to amine-modified polyester acrylate.
It can be clearly seen that the surface dryness, scratch resistance, micro-scratch or abrasion resistance have been greatly improved.
This is because, on the one hand, amine-modified polyester acrylates similar to amine hydrogen donors can greatly improve the overcoming of oxygen inhibition.
On the other hand, compared with difunctional polyurethane acrylates, tetrafunctional amine-modified poly Ester acrylate can greatly increase the double bond density in the system and increase the collision probability of double bonds, thereby increasing the activity and crosslinking density, so that the performance can basically meet the application requirements
.

In summary, by optimizing the resin, monomer, photoinitiator and their amounts in the formula of the free radical light curing system, an SPC floor coating with surface dryness and basic properties meeting the requirements can be obtained
.

2.
2 cationic light curing system

Radical light curing system is susceptible to oxygen inhibition due to its own characteristics, while cationic light curing system is not sensitive to oxygen.
This study tried to use cationic system for SPC floor finish, using free base paint to match cationic top coat to explore
.
The formulas of free primer and cationic topcoat are shown in Table 4 and Table 5, and the performance test results are shown in Table 6.

Table 4 UV-LED curing SPC floor free primer

Table 5 UV-LED curing SPC floor cationic topcoat

Table 6 Test results of supporting cationic topcoat

As shown in Table 6, when primer ① is used, the surface dryness of the cationic topcoat is poor, and the energy required for surface dryness is >5 000 mJ/cm2.
This is mainly due to the presence of active amines in the primer.
Although the content of such alkaline substances is Very few, but it will still exist on the surface of the paint film after curing, so it will inhibit the super acid generated by the cationic photoinitiator under light and the active center generated by the epoxy ring opening, which will cause the cationic curing to slow down or even fail to cure
.

When the primer ② changes the active amine to PETMP, the cationic curing problem is solved, and the scratch resistance and abrasion resistance of the final coating can meet the requirements in this way
.
When the cationic topcoat ① uses a matte powder with a content equivalent to the free radical system, the gloss is higher.
This is also because the cationic surface does not have the influence of oxygen inhibition, and the surface is dried better, and the volume shrinkage of the cationic system is smaller than that of free radicals, so the surface The matte powder cannot effectively form an uneven microstructure, and thus cannot effectively scatter light, which makes the gloss high.
At the same time, the E51 epoxy resin in the system itself contains an aromatic ring structure which is also the reason for the high gloss
.
Because of the higher gloss, under the same micro-scratch test conditions, it is easier to highlight the scratch marks.
The test result is only B3
.
The cationic topcoat ② increases the content of matte powder and reduces the content of E51 at the same time.
The gloss can reach 8, which can achieve good matting, and at the same time, the micro-scratch can also reach B2
.
The cationic system as a whole shows good scratch resistance and abrasion resistance.
This is mainly because cationic light curing is not inhibited by oxygen.
The use of cycloaliphatic epoxy and oxetane can increase the overall curing speed.
And rely on its own "post-curing" to further slow the reaction after the end of the light, thereby increasing the degree of reaction of the overall coating, and achieving excellent performance
.

In summary, the cationic light-curing system through a reasonable formula match, although it has certain weaknesses in matting and requires the corresponding cooperation of the free base paint coating, but in terms of performance, the cationic system has unique advantages over free radicals
.

3 Conclusion

Explore the application of free radical light curing and cationic light curing system UVLED curing SPC floor coatings
.
The basic requirements of SPC floor coatings can be met by rationally optimizing the resins, monomers and photoinitiators of the free radical light curing system, but there is still a certain gap compared with mercury lamp curing; the cationic light curing system is matched by a suitable free base paint The cationic topcoat can obtain performance equivalent to that of free radical system traditional mercury lamp curing, and has very good promotion and application prospects
.