Study on the parameters of UV curing coating curing process!

Abstract:
This paper studies the relationship between UV curing process parameters and coating performance, and the experiment shows that coating thickness affects the hardness, adhesion, flexibility and wear resistance of the coating, and the UV curing light source and manufacturing environment also have a great influence on the coating material performance.
the right curing process and environmental conditions to achieve the target performance of the coating by designing a reasonable coating thickness.
1, foreword
Since Bayer introduced the first generation of unsaturated polyester system UV coatings in the 1960s, and has been initially used in the wood coatings industry, due to its low VOC emissions of environmental characteristics have been more and more extensive development .
china's radiation curing technology started late, but after more than 20 years of development of the national demand for UV coatings has reached more than 50,000 tons.
UV-cured coatings have grown rapidly since the first generation, thanks to their "4E" features: Energy (energy saving), Ecology (environmentally friendly), Economy (economy), and Excelent finishing of radiationcuring."
that by 2015, worldwide production of UV-cured coatings will reach more than 8% of the total production of the entire coating industry.
UV coating from the original mainly used in furniture, electrical materials, such as surface coating, in the past ten years has been in electronic products, display equipment and energy-saving environmental protection products and other fields have been rapid development and popularization, UV coatings by virtue of its own advantages are widely used in functional film material surface changes.
of the functional film material is obtained by applying UV coating to the substrate and curing the ultraviolet light into a film. There is a lot of literature about UV curing coatings.
But in the coating and curing process, under different process conditions, the resulting film quality and economic production of the law is less discussed, the study of the general law of coating curing process, but also to obtain high-quality, low-cost functional film key.
This paper focuses on some laws of the coating and curing process of UV curing coating, and explores and summarizes the factors that affect the curing speed and film quality of UV coating from the aspects of coating thickness, UV energy, drying temperature and environment:
different coating thickness, UV energy corresponding to the film layer performance is different, dry temperature control, the influence of ambient temperature and humidity will affect the final film layer.
the best coating, drying and curing conditions to ensure the correct environmental conditions, you can obtain the target performance of the coating cost-effectively.
2, experimental part
2.1 test raw material VI
official-based polyurethane acrylic low-polymer, dual-official energy-based active monomer (Changxing Chemical Materials Co., Ltd.), light trigger: 184 (Beijing Yingli Technology Development Co., Ltd.), leveling agent, substrate wetting agent (BYK Chemical Company), solvent: ethyl acetate.
2.2 Test instrument
thickness meter, pencil hardness meter (Shanghai Modern Environmental Engineering Technology Co., Ltd.);
2.3 Experimental Method
The coating parts according to the formula proportion, fully stirred dissolved, coated with silk rod on PET, oven baking volatile solvent, UV curing machine cured into film.
2.4 Results Test
2.4.1 Coating Adhesion
The adhesion of the coating film is determined by the method of the rowing experiment by reference to the GB9256-88 method.
2.4.2 coating hardness
the pencil hardness of the coating is tested using the coated pencil scratch hardness meter using the GB6739-86 method.
2.4.3 Solvent-resistant
immersed ethyl acetate to observe the episopha changes in the membrane layer.
2.4.4 Flexibility
GB/T1731-93. After bending the coating film with substrate 180 degrees around the shaft rod in 1-3 seconds, observe the cracking of the paint film at the bending area and record the minimum shaft rod diameter causing the cracking.
3, results and discussion
3.1 coating thickness on the performance of the film layer
the film layer thickness is the most basic element of the coating. UV curing coating curing into a film, the appropriate coating thickness, you can get a reasonable excellent film performance. Table 1 is the film adhesion and hardness of different thickness coatings.
: Coating ratio: resin 78%; UV light: 800W.
table 1 shows that: with the increase of coating thickness, the hardness of the coating increases, but the coating adhesion is significantly reduced, the thickness of about 3 m when the best performance, adhesion 0, hardness 3H, to meet the requirements of use. When the coating is thick, according to Lambeth-Bill's Law: A-lg (1/T) -Kbc.
It can be seen that, due to the light trigger itself, light trigger cracking fragments, resin and other UV light absorbent shielding effect, ultraviolet penetration coating to the substrate and coating interface of the ultraviolet light intensity will be reduced, resulting in the bottom of the coating is not easy to cure completely, affecting the adhesion of the coating.
process of simultaneously curing from liquid to solid state, the coating cohesion is enhanced and the coating shrinks, which weakens the binding strength between the film layer and the substrate. Table 1 of
shows that if the thickness of the coating is more than 4 m, the binding strength of the film layer and substrate is obviously reduced, so the thinning coating can ensure that the bottom curing is complete, while reducing the cohesion generated by the film-forming process, coating adhesion is guaranteed.
the hardness of the scratch reflects the combination of material mod, pull strength and adhesion, and when testing the hardness of the scratch, the pressure and shear force act simultaneously.
UV coating began to cure, resin cross-linking reaction, the formation of a mesh three-dimensional structure, after curing the coating reached a certain degree of viscosity elasticity, when the coating is thicker, the viscosity elasticity is better, the resistance to pressure is stronger, the performance of the hardness slightly improved.
the flexibility and wear resistance of the film coating is a key indicator of the functional film, Figure 1 shows the relationship between the coating thickness and flexibility and wear resistance of the film.
1 shows that as the thickness of the coating increases, the flexibility and wear resistance of the coating shows the opposite trend. Friction resistance requires that the coating has sufficient hardness to make the adesthat difficult to penetrate the coating surface.
the coating must be flexible enough to bounce back after frictional stress is removed. Therefore, with the increase of coating thickness, friction resistance is improved, when the thickness of the coating reached more than 6 m, the number of frictions remained at about 30 times.
is the ability of the coating film to adapt to the deformation motion of its carrier body, also known as the elasticity of the paint film. It has to do with the extension of the coating itself and the adhesion of the substrate.
the thicker the coating, the worse the adhesion, the greater the stress in the coating, the worse the spread, so as the thickness of the coating increases, the flexibility of the coating significantly decreased.
effect of 3.2 UV curing energy
. The basic principle of UV curing: UV light energy excitation triggers the production of free fundamentals, resulting in a chain polymerization reaction. The prerequisite for the reaction to occur is to provide the minimum energy for the decomposition of the polymer reaction trigger, i.e. the critical exposure.
a coating that theoretically fixes components requires a critical exposure, but the energy provided during the actual curing of the coating is much higher than this theoretical value. The condition of the UV lamp, the curing condition, the coating performance, including the influence of the environment, all make the actual exposure required increase.
the accumulation and time of irradiated energy at different lamp distances is shown in Figures 2 and 2.
visible from Figure 2, UV energy increases linearly over time, but the lamp distance is different, the degree of increase is different, reflected in the curve slope, which represents the strength of the UV lamp when it reaches the coating surface. The closer the distance, the greater the intensity, the farther the distance, the smaller the intensity.
As can be seen from Table 2, UV curing is a chain polymerization reaction that is excited in an instant, the closer the lamp is, the greater the intensity of irradiation of the UV lamp, the higher the energy per unit of time to reach the area of the coating, the greater the amount of trigger excited, the faster the polymerization speed, the corresponding reduction in the exposure time required, and the cumulative total UV energy received by the coating is lower.
at the same time, oxygen blocking is an important factor in photo-curing, and the radiation curing reaction of almost all radiation-cured materials is affected by oxygen in the air.
the UV lamp, the lower the energy required for curing. The purpose of increasing strength can be achieved by adjusting the power of the lamp and the distance of the lamp. Table 3 shows the relationship between irradiated energy and performance when coating UV curing.
table 3 shows that the adhesion, hardness and solvent resistance of the coating increase with the increase of exposure time, i.e. the increase of curing energy.
but the energy reaches a certain amount, the exposure time increases and the coating performance no longer changes significantly. UV energy is the driving force of polymerization reaction, irradiated energy reaches the coating, light trigger absorbs ultraviolet light and breaks down into free fundamentals, and polymerization reaction begins.
UV curing process, for thin-layer curing system, the effect of oxygen in the air on the surface and bottom of the membrane is not obvious.
curing speed due to the weak penetration of ultraviolet light reasons, the surface will be faster, hand contact table dry when the bottom of the curing is not sufficient, with the continuous accumulation of UV energy, the degree of curing of the bottom layer, the performance of the coating also improved accordingly, to achieve the best coating can achieve.
But flexibility is due to the difference between the lamp distance shows a slight difference, close irradiation, provides high UV strength, the resulting trigger free-form, long-chain free-form number, free-form and free-form collision rate is large, the reaction speed is fast, the resulting molecular chain is shorter, less flexible.
so to get a flexible performance of the coating, by selecting a long chain of flexible resin formulation ingredients, to provide low irradiance energy source can also play an effective role.
3.3 Drying temperature effect
UV coatings used for thin film surface coatings generally require the addition of solvents to adjust coating properties, so before exposure to UV lamps, the solvent needs to be evaporated through the drying process, and then UV curing.
it is necessary to choose the appropriate drying temperature and drying time according to the coating conditions. Figure 3 shows the curing of ethyl acetate as a solvent under different dry conditions.
: Curing energy: 335mJ/cm2, light distance: 18cm. Curing degree determination: finger contact method measurement to reach the degree of table dryness.
can be seen by Figure 3, at a fixed exposure, the coating curing to the table dry state as the standard, drying temperature from 40 degrees C to 100 degrees C, the required drying time is reduced accordingly.
show that the curing speed accelerates with the increase of drying temperature. This is because: First, the presence of a large number of solvents affect the penetration of ultraviolet light in the coating, solvents consume a certain amount of irradiated energy, reduce the intensity of light trigger absorption of ultraviolet light, trigger decomposition quantum efficiency is reduced, light polymerization reaction slows down.
the drying temperature is conducive to the evaporation of solvents, less residual solvents in the coating, curing speed is relatively fast; the higher temperature of
makes the viscosity of the liquid coating decrease, which is beneficial to the prepolymer and active monomer molecular chain creep in the photo-curing system, and the chance of unsaturated bond collision increases, which accelerates the curing reaction and makes the curing reaction thorough.
the drying temperature mainly evaporates the solvent, the drying theory proves that the drying process is generally the surface to the bottom, the temperature is too high, the surface solvent evaporates too fast, the viscosity increases rapidly, is not conducive to the bottom solvent migration to the surface.
temperature is too low to achieve the purpose of drying. Generally according to the type of paint solvent and formulation ingredients, while taking into account the requirements of the coating process to design a reasonable drying temperature and temperature gradient.
3.4 Effects of environmental conditions
Effects of oxygen in the air on curing
The curing atmosphere has a greater impact on the curing reaction, and Table 4 is the curing of coatings in air and nitrogen environments.
4 shows UV curing in an air and nitrogen atmosphere. The photo-curing coating has a very large surface/volume ratio, so the obstruction of oxygen in the air to curing is particularly obvious.
The blocking effect of oxygen on the reaction is mainly due to: the oxygen molecule of the base state is in the three-line state, is a double free agent, the active freelance produced in the trigger reaction has a strong addition activity, thus forming peroxy freelance, so that the active free agent loses the activity of triggering monomer polymerization, this process is very fast.
aerodynaxic free fundamentals competes with the addition reactions of active free fundamentals and reaction monosomes, hindering the reactions of active fundamentals and monosomes.
In addition to reducing the effects of antioxidant impedance by improving the formulation of coatings, some measures can also be taken in the coating curing process: providing an isolated oxygen environment such as: cladding curing, inert gas environment, etc., effectively reducing the impact of oxygen impedance.
3.4.2 Effects of ambient temperature and humidity
The temperature of the environment has a positive effect on curing, just as drying temperature has a positive effect on curing, appropriately raising the temperature of the environment, reducing the viscosity of the coating, and increasing the reactive activity.
The choice of ambient temperature is generally not used as a curing process adjustment measure in industrial production, mainly because of increased energy consumption and weak results, and other materials, equipment and processes need to be taken into account for temperature requirements.
humidity in the environment will definitely have a negative impact on the curing process. When the relative humidity of the air reaches more than 85%, the solvent in the coating absorbs heat and evaporates, and the surface temperature of the coating film is temporarily reduced.
of moisture in the air will quickly condense to the membrane surface, so that the coating surface white, not permeable. For the UV curing process, the relative humidity control is required between 30% and 80% to ensure that the curing and appearance requirements are met.
4, Conclusion
. UV solid.