Hybrid Cure Process for Coatings: Achieve Desired Finish Efficiently

TAGS:   Plastic Coatings       Automotive Coatings      UV/EB Technology      Sustainability / Natural Coatings    

Today, cars are more than just a mode of transportation. From raw material manufacturers & formulators to automotive OEMs, everyone is regularly searching for ways to make things faster, better and cheaper. Constant advancements in materials such as lightweight plastics, specialized coatings, etc. can help develop desired surface finish as well as meet other requirements, like:

• Robust in design 
  
• Elite in performance
  
• Compliant to environmental challenges and regulations at the same time
  

Printing/coating on plastic is a growing and challenging market. It has been successfully used for many years, but new opportunities arise daily that remind formulators how difficult achieving adhesion can be.

Among several coatings used today, radiation cured coatings are gaining momentum meeting the:

• Stringent environmental legislations, 
• Durability requirements, 
• Tougher OEM standards etc. 

We would like to acknowledge Ray Baird from Heraeus for providing technical information needed to develop this article.

Let’s discuss about developments in these coatings in detail…

Curing Processes – Then and Now

Coaters and decorators used to paint, or coat plastics parts using water or solvent-based coatings and then dry them all thermally in large curing ovens. Though effective, yet:

• The water-based coatings required very high energy input to drive off the water content
• The VOC emissions from drying solvent-based formulations required expensive containment or treatment equipment 
• The drying ovens simply took up a huge amount of space 

Curing processes for coatings such as Ultraviolet (UV), electron beam (EB), IR (infrared), dual/hybrid cure etc. are gaining attention of manufacturers thanks to their benefits such as lower VOC, fast processing speed, cost-effectiveness, flexibility in applications etc. High scratch resistance, abrasion as well as chemical resistance are among the key benefits offered by these coatings making them suitable for plastics and polymer substrates widely used today in vehicles parts, such as:

• Exterior body 
• Interior coatings
• Under-the-hood components
• Lighting reflector housing
• Headlamps, door handles, mirrors, 
• Side molding, body panels
• Tail lamps 
• And many more…

Let’s check out these processes in detail and how they help meet today’s requirements:

Curing Processes for Automotive / Plastic Coatings

Curing process of paints corresponds to both drying (Evaporation of solvents and diluents) and hardening (resin or polymer which binds all the ingredients and forms a film when dried) creating a solid and adherent coating.

The curing process (physical or chemical) of paints is accelerated or activated by applying techniques such as:

• Thermal/Heat curing
• Energy/Radiation curing (UV, EB, IR…)
• Curing by electrical process
• Dual curing process (thermal + UV, IR + UV…)

Depending on the process and the coating used, the selection of curing process – whether a heat curing or UV curing or hybrid cure – is used for drying and achieve a perfect surface.

Infrared Process and Plastic Substrates

Infrared energy is a form of radiation, which falls between visible light and microwaves in the electromagnetic spectrum. Like light, infrared radiation is electromagnetic radiation and is transmitted just as quickly and at virtually unlimited power.


IR systems are usually described as high, medium or low intensity. Medium energy IR is most widely used for curing on plastics, composites and other heat sensitive substrates because energy is absorbed directly by the coating (unlike High – energy is transmitted through the coating to be absorbed by the substrate OR Low – inefficient for cure leading to surface defects).

IR technology transmits EM waves which generates heat within the product. Just like visible light:

• A part of the broad infrared radiation spectrum is reflected from the surface of the material
• While the rest is either absorbed within the material or passes through the material

Every material consists of molecules and molecular structures that absorb specific radiation wave-lengths. The radiation wavelength that is absorbed by a material coincides with the wavelength of the molecular oscillation in that material. According to their material properties, plastics:

• Absorb some of the infrared radiation,
• Reflect some at the surface and
• Allow some of the radiation to pass through

By carefully selecting emitters to match their spectra to the material being processed, most of the radiation is absorbed and converted to heat creating an efficient operation. For instance, plastics generally absorb infrared radiation in the wavelength range above 2 µm.

IR Curing Process for Coatings

Coatings such as water-based varnishes can be dried, powder coatings are gelled & cured using IR heat technology. Further radiation adjustments w.r.t wavelength, shape and power output to the product properties to be heated can further help enhance the results. During the process, radiation is rapidly converted into heat inside the paint leading to water and other solvents evaporation, while the material and the surroundings remain cooler. In this way, infrared drying speeds up production, improves quality and at the same time saves energy and costs.

The main characteristics of IR curing process are:

• Heating requires no contact
• High heat transmission capacity
• Fast transfer with high power
• Wavelength adjustment capability to the substrate and the paint system
• No energy wasted to a transmission medium
• Saving of space and energy

Benefits include:

• Dries the paint film from the inside out – by penetrating the material more and less deep
• Prevents skin or bubble formation
• Fast drying of paint
• Brilliant surface finish

By matching the infrared spectrum to a specific application, the process flow rate, heating time and energy used are optimized. However, there are certain disadvantages associated with the process such as long oven dwell time can lead substrate damage as certain plastics and composites are heat sensitive materials.

Apart from the material and coating properties, the right selection of IR emitter can further help save energy, heat faster and increase productivity. The emitter’s rates of heating and cooling are important for control, cycling process stages and process safety. Different types of commercially available infrared emitters include:

• InfraLight Halogen Emitters
• Short Wave, Twin Tube Emitters
• Fast Response, Medium Wave Emitters
• Carbon Infrared Emitters CIR
• Medium Wave Emitters
• Emitters for Targeted Heat

UV Curing: Considerations for Plastic Substrate

Push towards ‘green’ technology, increased productivity as well as availability of new raw materials are some of the key factors allowing formulators to produce UV coatings with:

• Improved physical properties 
• Better product performance

…which opens new capabilities for users.

The principle of UV curing is based on initiating a chemical polymerization inside a liquid coating using direct UV irradiation to create a cross-linked, dry, solid coating.

UV curing coatings and UV curing lamps – are the two essential elements for a successful UV curing system. The initial composition of the coating & its match with UV lamp’s spectral output, intensity and dose along with optimum physical sitting of lamps play an important role in the finish properties of UV cured coatings. These finished properties include:

• Hardness
• Scuff and scratch resistance
• Gloss or matt finish
• UV Stability
• Chemical resistance etc. 

UV coatings are typically 100% solids, but also can be made with 5-50% solvent or water. They comprise monomers, oligomers, pigments, additives and photoinitiators. When exposed to ultraviolet light, free radical-curable film formers crosslink into a paint film within seconds.

Benefits of UV Curing Process

• Low Capital Investment - Several factors contribute to this such as no drying oven required; less overhead conveyor length; smaller area for unloading painted parts; and smaller system footprint. A UV-curing system will typically require only one tenth the floor space of a conventional thermal oven.


• Suited for Sensitive Substrates/ Low heat generation - Since the exposure to UV light is typically a matter of seconds, this system is well suited for substrates that cannot tolerate high temperatures. The increase in substrate temperature is minimal, with the ability to handle the part immediately after curing.


• Lower Operating Costs - A major advantage of a liquid UV coating is the elimination of a costly thermal dryer or oven. In a liquid UV system, the oven is replaced with a small cluster of UV curing lamps. Costs to install these lamps are typically about half the cost to install a large capacity thermal dryer or oven. Annual operating costs of the oven versus the UV lamps are dramatically reduced as well.

• Improved Productivity - UV coating materials typically cure in seconds versus up to several hours in a conventional thermal oven, so the cycle time for each coated part can be cut dramatically. This allows quicker response to your customers' requests and better delivery performance. Faster cycle time also leads directly to a reduction of Work in Process (WIP), lowering inventories and freeing up capital for other investments.
  
  Since UV coatings do not dry until they are exposed to high-intensity UV light, reclaiming overspray is much easier, making for higher application efficiency.


• Reduced Environmental Impact - A 100%-solids UV formulation contains zero VOCs and no HAPs. This will have a positive impact on the allowances in your emissions permit. If required, non-VOC solvents, such as acetone, can be used for less than 100%-solids formulations. In addition, paint filters can be dried and disposed in the trash, as opposed to a costly hazardous waste hauler.


• Easy handling due to modular design

Dual Curing or Hybrid Cure Process – Adds Higher Value to the Product

During polymerization in a UV curing process, photoinitiators are activated by intense UV light and cross-linked within seconds. This quickly cures the material making the surface dry, abrasion-proof and immediately ready for further processing.

Infrared transfers energy to materials without contact, generates heat where it is required and consequently is particularly efficient in drying varnishes or melting powder coatings.

For some applications, curing can be optimized by a combination of infrared heat with UV radiation – the Hybrid Cure Process. These applications include high quality surface coatings and surfaces that must be scratch resistant. The Hybrid Cure Process solution is devised to solve the following types of issues in industry:

With UV Curing	With IR heating/drying
Process development for new product  Optimization of current UV curing process  Optimization of curing speed	Efficient utilization of heating / drying process  Optimization of overall cost of process

By overcoming aforementioned challenges, Hybrid Cure Process further helps improve the physical properties of the cured product, adding to its overall quality and value.

Higher scratch resistance on the surface of cured film is observed  Improvement of line speed by using IR and UV irradiation Reducing the concentration of photoinitiators by implementing the hybrid cure process Higher double bond conversion and higher surface reflectance are provided Growth of cross-link density and refractive index enhances surface reflectance of cured coating		Hybrid Cure Process withstands the features of the UV lamp system, which transforms high UV efficiency against energy input  It collects high power of light by reflector due to the small diameter characteristic of lamp bulb High reaction rate as well as high cross-linking density can be obtained, as the reactive species concentration is high in unit time		Film curl generated by cross-linking reaction is relaxed  Adhesion of coating, flexibility, and curling are improved  Improvement of production efficiency and stability of process of cationic curing system are obtained  Degradation odor of photoinitiator can be expected

Advantages of using infrared heat and UV radiation hybrid curing process include:

• Short response time: Some plastic components cannot endure heat for a long time. Infrared heat supplies much energy in a short space of time precisely to the place where it is needed. The machine, the surroundings and the rest of the product stay cool.


• Energy Saving: Infrared systems are energy-saving. In contrast to conventional heat sources, they need to be switched on only when heat is needed.


• Reliable and constant quality: Intensive UV light of a wavelength optimal for the photoinitiators of the paint system guarantees efficient cross-linking. This increases production speed and improves process reliability. At the end of a curing process, the surface is scratch-proof and durable.


• Optimal adjustment to the process: Activation by UV light and deburring with infrared heat prepares surfaces optimally for coating. This gives the coating a brilliant finish.

No one technology is the perfect solution for all applications and coating requirements, but hybrid curing process certainly, has many important features and benefits. The high durability of the coatings coupled with the significant improvements in production speeds and energy saving, cost-efficient certainly drives implementation of hybrid curing process in in many plastic coating operations.

Development work continues to increase the range of benefits to achieve coatings with soft touch feel, low matt and one-coat high gloss, anti-glare, anti-fingerprint, easy-to-clean display coatings and many more for unending growing plastics use in cellphones, car interior and exterior components, food packaging, E&E etc.