The past, present and future of UV coatings-what are the reasons for the rapid growth for 35 consecutive years?

Paint composition

Resin

Liquid solvent or water reduces the viscosity of the base material to a workable level, and then volatilizes to form a coating film.
The measured values ​​of solvents are VOCs and are subject to EPA control.
Some solvents, such as acetone, are not covered by the EPA, and are not allowed to be emitted as VOCs in most states in the United States.

pigment

Pigments are solid particles that provide color and gloss control, and are insoluble in the base material.
Commonly used pigments include: inorganic pigments, organic pigments and metallic pigments.

Additives

The coating formulation contains chemical additives to provide unique performance and improve the performance of the coating.
Commonly used additives include catalysts, anti-blocking agents, defoamers, flow control agents, gloss modifiers (increase or decrease), storage stabilizers, UV shielding agents, dispersants and rust inhibitors.

Thermal curing and radiation curing

In UV curing coating formulations, resins, solvents and catalysts can be replaced with oligomers, monomers and photoinitiators.

Oligomers provide coatings with adhesion, pigment encapsulation, flow and physical properties.
Like solvents, monomers also reduce viscosity, but also provide some other properties similar to oligomers, namely flexibility and crosslinking.

In most cases, the addition of monomer can reduce the viscosity to a certain level, which can be applied to high-efficiency coating methods such as roller coating or rotary atomizing spraying.
For easier construction or better fluidity, solvent or water can be added when preparing UV coatings to make the solid content less than 100%.
Thus, traditional methods or HVLP spray guns can be used for painting.

Photoinitiators act as catalysts to initiate chemical reactions.
During this free radical reaction, monomers and oligomers are transformed into polymers and crosslinked.

Curing

The main difference between UV curable coatings and solvent/waterborne coatings is that they use different types of polymers and different curing methods.
The difference between solvents and water-based polymers lies in their film-forming methods.

·Volatile paint-is a synthetic thermoplastic film-forming polymer based on soluble in organic solvents, which forms a coating film through the volatilization of the solvent.

·Self-oxidation-the polymer contains carbon-carbon double bonds, which can introduce oxygen molecules into the double bonds to form a cross-linked coating film.
Alkyd is a typical species of this type of reaction.

·Chemical crosslinking-In this case, the system contains more than one polymer.
One is a backbone polymer such as alkyd or acrylic, and the other is a cross-linked resin such as melamine.
Heating and adding a catalyst (autocatalytic or external catalyst) can promote the reaction, causing the backbone polymer and the crosslinking agent to react, crosslinking to form a coating film.

· Latex-type film-forming substance-similar to volatile paint, this type of coating is through the volatilization of water (and possibly existing film-forming aids) to aggregate polymer particles to form a coating film.
The emulsion is usually acrylic, but polyester, polyurethane, alkyd, and epoxy can also be used.

All these polymers and their curing methods are the same in one thing.
Through curing, the solvent or water volatilizes and the resin, additives and pigments remain, and the solid content of the coating can be considered as 100%.
This process is illustrated in Figure 1.

Figure 1 Schematic diagram of curing process

In contrast, UV-curable coatings use a different way of forming a cured coating film.
When exposed to ultraviolet light, the free-radical cured film cross-links to form a film within a few seconds.
These compositions are usually 100% solid content, but 5-50% solvent or water can also be added (as shown in Figure 2).

Figure 2 Free radical curing

Radiation curing

Radiation-curable coatings contain various accelerators or catalysts, which do not initiate reactions under normal conditions, but will initiate reactions when exposed to UV light.
After absorbing UV light, it will trigger a free radical reaction in the chemical group within a few seconds to form a crosslink (curing).

Specifically, this chemical process (chain reaction) is the formation of polymers from monomers and oligomers.
UV curing can be performed in a closed chamber with UV light.
A closed cell must be used because the presence of oxygen will inhibit free radicals from initiating the curing of the system.
The oxygen must be replaced with nitrogen or other inert gas.
Due to the high cost of airtight chambers, specific formulas are often used to overcome the effect of oxygen during the curing process.
Now, it is rarely necessary to use a closed cell for UV systems.

Regarding coating shrinkage, thermosetting coatings are cross-linked with polyurethane and/or melamine resins, and their co-reaction produces only about 1% shrinkage.
In contrast, free radical polymerization in UV coatings produces about 20% shrinkage.
In formula design, it is necessary to consider compensation for shrinkage.

In UV coatings, accelerators or catalysts are called photoinitiators.
In order for the curing to proceed completely, the UV light must be able to activate or "see" all the photoinitiators.
As a result, this leads to a limitation in the dry film thickness of UV coatings (about 1 mil), especially when pigments are contained, which can affect UV curing.
The pigment molecules will absorb and reflect or shield UV light, making it impossible to contact a part of the light initiator.

As expected, black and dark paints are the most difficult to cure because of their ability to absorb light waves.
Lighter, brighter pigments are usually easier to cure.

The decrease in UV light energy is inversely proportional to the square of the distance between the light source and the light-receiving surface.
Therefore, the UV light source should be kept as close as possible to the painted part.
For this reason, UV curing is easier to perform on flat surfaces.
However, the use of highly polished polished surface reflectors, through the reasonable placement of the light source and/or the rotation of the substrate (object), can successfully UV coating specific three-dimensional objects, such as bicycle racks and hand tools.

UV curing takes place in a few seconds to form a dense coating film.
Fast curing also reduces the adhesion of foreign particles on the surface, and can reduce the heating of the substrate as much as possible, which is very beneficial for heat-sensitive substrates.

Energy source

wavelength

The wavelength range of UV light used for curing is 240-450nm.
In this wavelength range, a photoinitiator is needed to initiate polymerization.
Generally, three different light sources can be used to obtain three different wavelength ranges.
240-320nm (UVB and UVC) is obtained by the "H" lamp, which is a mercury (Hg) doped lamp, which emits a shorter wavelength and mainly promotes surface curing.
These lamps are usually used for transparent UV coatings that do not contain pigments.

320-400nm (UVA) is obtained using "D" lamp.
This is an iron (Fe) doped lamp that emits light of medium wavelength.
400-450nm is obtained by the "V" lamp, which is a gallium (Ga) doped lamp, which emits a longer wavelength and can penetrate below the surface of the coating film to the substrate.
It can provide excellent adhesion and must be used for complete curing of paints containing pigments.

UV lamp

The UV lamp is a controllable emission device that generates electromagnetic energy and releases infrared and visible light and ultraviolet radiation.
The main types of UV lamps are (a) electrodes (medium pressure mercury arc lamps) and (b) non-electrodes (microwave energy).
Other applicable UV lamps include low-pressure mercury arc lamps (fluorescent) and high-pressure xenon arc lamps (two-electrode type) and lasers, which are non-electrode type.

Parameter measurement

Each UV curable coating specification should include some parameters to determine the amount of radiation and the appropriate time when applied to the surface2.
The key parameters are as follows.

1.
The amount of radiation, watts/cm2.
This parameter is used to indicate the required light or UV lamp output intensity; the greater the amount of radiation, the greater the curing depth.

2.
2.
Radiation wavelength, nm range (or UVB, UVC, UVA and UVV bands).
This parameter helps determine the best curing lamp type (H, D or V lamp).
This wavelength also affects the choice and absorption of the photoinitiator in the composition.

3.
Time, s or ft/min.
The time required for the composition to cure will determine the composition of the lamp.
The static curing time is defined in seconds, and the dynamic curing in the form of part transport is defined as ft/min.

4.
Energy, J/cm2.
Energy is used to measure the amount of radiation and time.
Logically speaking, the intensity and time of light exposure to the coating will have a great impact on curing.
Usually, the operator of the painting line uses a device called a radiometer to measure and monitor the production process.

Radiometer

In order to effectively cure UV systems, three factors are important: radiation, energy and wavelength.
These can be measured with a radiometer, and their size is about twice the size of mobile phones currently in use.
This kind of device can be installed on a conveyor belt or fixed on an object to transmit the UV light and measure the UV light exposed to the component (coating).
The value can usually be read from the LED display.
After determining the satisfactory UV conditions, this instrument can be used to quantify the optimal exposure conditions to ensure repeatability and reproducibility.

    Advantages of UV coating

Regarding investing in UV coating lines, when collecting information and data, the following advantages need to be considered.

Low investment

This is because of the following factors:

·No need for curing oven;

·The transmission length that needs to be elevated is shorter;

·The loading and unloading of painted parts occupies a smaller area;

·The system occupies a smaller area.

UV curing systems usually require only 1/10 of the area of ​​a traditional thermal oven.
Traditional thermal ovens generally need to be 100 feet long by 10 feet wide and cover an area of ​​more than 1,000 square feet.
Assuming a conservative estimate of US$0.
50/square foot/month, the annual rental cost of a 1,000-square-foot thermal oven is US$6,000.
In contrast, the UV system only costs US$600.

It is strongly recommended to design and install an infrared (IR) lamp area after the painting area and in front of the curing lamp, covering an area of ​​10×20 feet.
In some cases, heating and flashing time are conducive to the flow of 100% solids paint, and it is also necessary when using (water-based or solvent-based) UV paints with a solid content of less than 100%.
Redesigning and installing IR lights after the system is set up will increase costs.

Suitable for heat-sensitive substrate

Since exposure to UV lamps usually takes only a few seconds, this system is very suitable for substrates that cannot withstand high temperatures.
It is possible to reduce the heating requirements of the substrate as much as possible, and the coated parts can be used quickly after curing.

Reduced operating costs

The main advantage of liquid UV coatings is the elimination of costly thermal curing devices or ovens.
In the liquid UV system, the oven is replaced by a small cluster of UV curing lamps.
The cost of installing these lamps is usually half the cost of installing a large-capacity heat curing device or oven.
At the same time, compared with the thermal oven, the annual operating cost of the UV lamp can be significantly reduced.

For example, a large air oven consumes about 1MBTU/hr and requires a larger fan to achieve the specified production capacity.
To achieve the same capacity, the total consumption of four UV lamps is only 72kW/hr.

Figure 3 shows a typical annual operating cost.
The fastest curing UV process reduces the time that the wet film is exposed to the external environment.
This reduces the possibility of particles contaminating the coating and reduces the generation of non-conforming products and waste.

Figure 3 Operating cost/year

In addition, some defects may not appear until the coating is fully cured.
Due to the fast curing speed of the UV system, defects can be quickly observed and corrected, which improves the efficiency of the coating system.

The length of the conveyor system is directly proportional to its cost.
Conveyor systems passing through traditional hot ovens will cause accelerated wear due to rapid loss of lubricant, resulting in excessive load and device wear.
Global Coatings Network learned that, taking a 100-foot thermal oven as an example, the average cost of conveying and maintaining the curing part of the coating line is US$8,000 per year.
When using 4 lights of 10 feet, the cost of transmission and maintenance is about US$400/year.
Figure 4 illustrates the cost of the conveyor chain after replacement.

Figure 4 Comparison of alternative transmission links

Productivity increase

The curing time can be up to several hours in the traditional thermal oven, while the UV coating is usually several seconds, so that the coating time of each workpiece can be greatly shortened.
This can be adjusted in time to meet customer requirements and provide better delivery product performance.
A faster cycle can also directly lead to a reduction in workload (WIP) during operation, reducing inventory and improving capital flow for other investments.

Since UV coatings can only be cured when exposed to high-intensity UV lamps, it is more convenient to recover oversprayed materials, which improves the utilization rate.

Reduced the impact on the environment

The 100% solids UV formula does not contain VOCs and HAPs.
This is very beneficial for adapting to emission requirements.
If necessary, non-VOC solvents such as acetone can be used in formulations with a solid content of less than 100%.
In addition, the paint filter can be cured and disposed of as garbage without the need for costly transportation and disposal of hazardous waste.
Figure 5 analyzes the cost comparison in this regard.

Figure 5 List of cost comparisons

Construction method and end use

The UV curing formula can be applied in various ways.
Including roller coating, curtain coating and curtain coating, dip coating and spraying, traditional methods or HVLP spray guns can be used.

Generally, UV curing materials can be used as coatings, inks, adhesives and sealants for metals, plastics, wood, glass and composite materials.
The following describes some of the main industries and products that use UV-curable coatings.

·Painting art-sometimes use EB curing; products include screen printing, lithography, inkjet and overprint varnish;

·Wood industry-products include scaffolding, cabinets, furniture and flooring, etc.
;

·Optical fiber-Two UV protective coatings are applied to prevent the optical fiber from bending and damage during use.
One is a hard outer layer (buffer layer) coating, and the other is a soft coating (air cushion layer) between the outer layer and the fiber to achieve the required protective properties.

· Optical discs-also used for the protective coating of CDs and DVDs;

·Adhesive-used to bond various substrates;

·Quick and accurate proofing-this is a newly developed application that enables the 3-dimensional model created in the CAD system to be directly transferred into a 3-dimensional functional model.
Products include molded castings, functional product models (engine manifolds), damaged bone structures (for surgery) and teeth (for dentistry);

·Dental supplies-dental resins used for tooth repair, sealing, imprinting materials and rapid and accurate proofing;

·Composite materials/plastic parts-used in auto parts, vacuum metallization, aircraft parts, bottles, bicycles, cosmetics, glass fiber and carbon fiber;

·Industrial-products include hand tools, metal cans, propane cans and electronic circuit boards.

    The future of UV coatings

It is predicted that the annual growth of UV coatings in North America will continue to maintain 5-10%.
The driving force is:

·Adapt to environmental requirements (increasing VOC limits);

· Higher speed/production capacity;

·Energy saving (reducing costs);

· Performance improvement;

·It can cure the pigment-containing formula;

·Improve adhesion to metals and plastics;

·Automotive industry (plastic parts);

·New technology (LED curing).

The painting art, woodware, medical/dental, automotive, fast and accurate proofing and LED curing markets are expected to see the fastest growth in the near future.

LED curing is defined as longer wavelength (greater than 400nm, but narrower range), emitted by LED lights.
Some of its advantages include the use of lower energy, and the lamp is easy to carry.
Due to the longer wavelength used, safety is improved and the cost is reduced compared with the UV lamps currently used.
Global Coatings Network understands that the biggest challenge it faces is to find a suitable photoinitiator that can initiate free radical reactions in this longer wavelength range and still provide the required properties of the coating.
Research on this technology is still in progress.

    in conclusion

It is hopeful that some of the mysteries about UV coatings have been eliminated, so that people can realize some potential advantages that UV systems can bring to coatings and coating products.
Some advantages have been realized, including: reducing operating costs by improving efficiency, adapting to new environmental control requirements and being able to enter some new markets.

With the substantial increase in the application of UV coatings in the past 15 years, a new type of product/technology has been formed.
Recall that in the early 1990s, water-based liquid coatings also faced this similar situation.
UV coatings have made great technological progress, and performance improvements have become possible; more and more markets can take advantage of this breakthrough achievement, mainly due to stricter regulations.
This is why the title of this article is set as "UV coatings based on the status quo and look forward to the future.
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