Radiation Cured Coatings: Types, Features & Composition

What are Radiation Cured Coatings?

One of the prominent ways, since the 1960’s, has been the use of Ultra-Violet (UV) radiation for the curing of coatings.

In the preparation of UV cured coatings (also called RadTech – radiation technology):

A formulation of several ingredients is hardened by means of radiation.
The UV curing light source used, generates energy which will form free radicals which in turn will start the polymerization process of the reactive double bonds in the coating formulation.

The curing reaction of an acrylic formulation is described in the figure below.

Curing of Acrylic Formulation by UV

The market applications for these types of coatings are still expanding but the main markets of application are:

Wood and paper (For instance: furniture, wood flooring (parquet), laminates)
Overprint varnishes for packaging applications
Inks
Opto and electronics
Plastics and others

Market Application for Radiation Curing Coatings

Trends on the market lay in the development of new raw materials and applications, such as:

Water-based UV systems
LED curing
Low gloss applications
Resins derived from renewable raw materials (or bio preferred and recycled content resins),
Photoinitiator replacement with self-curing resins
Special effects and 3D printing solutions

Advantages and Disadvantages

With many advantages of radtech coatings, there are also some disadvantages discussed below.

Advantages	Disadvantages
Fast curing Solvent-free formulations Good tunable properties for various applications and substrates	Expensive raw materials Adhesion problems due to fast and high coating shrinkage Hazardous materials potentially causing skin irritation Difficulties with curing 3-dimensional objects Difficulties with obtaining matte finish

Let's explore the chemistry, composition and raw materials used in the radtech coatings.

Composition of Radiation Cured Coatings

The general formulation of radtech coatings contains the following ingredients:

Oligomers (prepolymers) are reactive substances of relatively low molecular weight (in-between monomer and polymer), which are capable of further polymerization.

Monomers are low molecular weight substances which reduce the viscosity of the formulation. Monomers act similar to solvents in solvent-borne paints. When lowering viscosity is the only function of a monomer it is called monofunctional monomer (e.g., isobornyl acrylate). Multifunctional (di-, tri-, tetra-, etc.) monomers are involved in the formation of polymer network participating in cross-linking of oligomers during radiation curing.

Photoinitiators absorb light (UV or visible) and initiate cross-linking reactions. The mechanism of the initiation is triggered by production of either free radicals (in free radicals polymerized systems) or cations (in a cationic photoinitiated systems). Photoinitiators are added in concentrations of 1-20%.

Additives are used to modify and improve various functions. Pigments, fillers for viscosity control, wetting agents, defoamers, and other additives for paints are some examples.

» View All Commercially Available Raw Materials Suitable for UV/Radiation Curing Coatings!

Oligomers

As the oligomers are the main component in the formulation, they will determine the major properties, such as:

The function of oligomers is similar to that of binders in solvent-borne paints and water-borne paints. The following oligomers are used in radiation curable coatings:

Epoxy acrylate
Urethane acrylate
Polyester acrylate

Polyether acrylate
Acrylic acrylate
Silicone acrylate

For the choice and design of the final coating formulation, a balance of several properties should be taken into account to come to the best performance of the system. Below the different variables which play a role in formulating are given:

Different Variables in Radiation Cured Coatings

When comparing the different oligomers, we can take a look at the main properties to choose from. Most important is to first determine which type of chemistry is necessary for your application. Then the selection of your substrate will further play a role and how the coating will adhere to the substrate.

Listed below are some important properties compared for the oligomers:

Types of Oligomers	Outdoor Durability	Chemical Resistance	Flexibility	Water/Hydrolysis Resistance	Specific Properties
Epoxy acrylates	+	+	++	++	Good hardness High gloss
Aliphatic urethane acrylates	++	++	+	+	Non-yellowing Good pigment wetting
Aromatic urethane acrylates	--	++	+	++	High reactivity High hardness/mechanical resistance
Polyester acrylates	-	++	++	--	Good adhesion Pigment wetting
Polyether acrylates	-	+/-	+	+	Good adhesion Non-yellowing
Silicone (urethane) acrylates	++	++	+	+	Ideal for soft touch Low shrinkage

Let's discuss the chemistry and properties of different oligomer types used in radiation cured coatings.

Epoxy Acrylate

The most commonly known epoxy acrylate is Bisphenol A diglycidyl ether diacrylate with the general structure given below.

General Structure of Bisphenol A Diglycidyl Ether Diacrylate

The advantages of epoxy acrylate are fast cure whereas the disadvantages are yellowing in time and high viscosity. Therefore, new high-performance epoxy acrylates have been created with customizable properties.

Also, the increasing legislation surrounding the use of Bisphenol A (BPA), the focus is on developing BPA-free replacements. Other important epoxy acrylates include:

Epoxidized fatty acid oils, such as soya or linseed oil
Acrylates of epoxy novolacs

Urethane Acrylate

The general structure of a urethane acrylate is given below.

General Structure of Polyurethane Acrylate

The main properties of urethane acrylates are:

Elongation and flexibility
Excellent outdoor resistance
High chemical resistance
Superior mechanical properties
Very good pigment wetting

The property differences between the urethane acrylates, and thus the selection for the most appropriate one for the final application, is determined by the combination of:

The polyol (soft part)
The isocyanate (hard part)

The choice for the polyol part (being the main component) will greatly determine the properties. The polyol is in most cases a polyester, polyether, polycarbonate, polybutadiene or polycaprolactone.

The polyol can be aromatic or aliphatic as well for the isocyanate part. The choice will depend on various factors. The polyol is formed from the reaction of a diol, triol (or other molecule with hydroxyl groups giving the desired end functionality of the polyol), and a diacid (or other multifunctional acid) by a condensation or step-growth polymerization.

Next to the composition of the polyol, the molecular weight and crystallinity play a big role in the final properties.

Polyester Acrylate

Polyester acrylates have a good balance between cost and performance. Their weakness lays in the hydrolyzable nature of the ester bonds in the backbone. However, they are the most versatile in their performance and can provide good performance between:

The epoxy acrylates – Mainly for fast cure and hard coatings, and
The urethane acrylates which excel in toughness and outdoor durability.

General Structure of Polyester Acrylate

They offer good adhesion to various substrates, such as metals, plastic and paper. They have very good wetting properties, making them very suitable for pigmented systems.

Polyether Acrylate

As compared to the polyester acrylates, the polyether acrylates backbone is less prone to hydrolysis and have good solvent resistance. They provide:

Good adhesion
Good flexibility, and
High gloss

Most common products are modified with amines to provide high reactivity. For this reason, they are mostly used as very efficient co-initiators.

Acrylic Acrylate

Acrylic acrylates are known for their:

Good adhesion properties to difficult substrates, and
High flexibility to the coating formulations

Due to their high viscosity they mostly are mixed with reactive monomers.

Silicone (Urethane) Acrylate

The silicone acrylate provides good wetting and levelling to the coating formulation upon application and increase the overall hydrophobicity of the coating. Furthermore, they increase the slip and anti-blocking properties.

Example of Silicone Acrylate Structure

Monomers and Reactive Diluents

In order to finetune the viscosity of the formulation, monomers and reactive diluents are used.

Monomers have one (meth) acrylic reactive group and are mainly used to dilute the formulation and do not add to the network formation.
Reactive diluents have a functionality of minimal 2 and therefore also play a role in the network formation by the creation of extra crosslinking.

Name	Abbreviation	Functionality	Chemical Structure	Features
1,6-hexanediol diacrylate	HDDA	2		High solvency High reactivity Good weathering properties
Dipropylene Glycol Diacrylate	DPGDA	2		High solvency High reactivity Low viscosity
Trimethylolpropane triacrylate	TMPTA	3		High reactivity Low viscosity
Isobornyl acrylate	IBOA	1		High Tg High flexibility
Pentaerythritol tetraacrylate	PETTA	4		Good adhesion High reactivity

Overview of the Most Used Monomers and Diluents

Features of UV-LED Curing Technology

Further process improvement of the UV curing technology has led to the further development of systems using UV-LED. The use of LED curing technology gives several benefits compared to the standard UV curing systems, being:

Very long lifetime and consistent energy output
No infrared emission
No heat generated
No Warm-up time required
Impact and flexible lamp design
No generation of ozone
Mercury-free system

The operating lifetime of the LED diodes, at maximum performance over their operating lifetime, is greater than 20,000 hours. And due to the lack of infrared heat, thin and sensitive substrates can be used and there is no requirement for cooling after curing.

Another gain when using LED, is that the systems cure at speeds 20–30% faster using up to 50% less energy and are more compact than traditional UV systems. As the LED’s do not generate UV-C, ozone, heat and noise (no cooling systems/ fans needed) these systems provide greater safety.

LED-curing lamps have, contrary to conventional (mercury) UV lights, a very narrow emission range at a specific wavelength ± 10 nm. Most used wavelengths are 365nm, 385nm, 395nm or 405nm. This near-monochromatic distribution requires new chemical formulations to ensure proper curing.

Specific photoinitiators have been developed which have matching absorption optimum at the emission wavelength of the LED lamp.

Photoinitiators are available in the following types:

Type I	Type II
Phosphine oxides Benzil ketal Aminoacetohpenones	Benzophenones Thioxanthones

These can be used in combination, optimizing efficiency, surface and deep cure. Furthermore, amine synergists can be used to optimize curing speed and reduce oxygen inhibition.

In the table below an overview is given of the most commonly used photoinitiators.

Name	Abbreviation	Chemical Structure	Description
Ethyl (2,4,6-trimethylbenzoyl) phenyl phosphinate (CAS no. 84434-11-7)	TPO-L		Liquid Norrish Type I photoinitiator from the phosphine oxide family, with absorption maxima at 274, 290 and 370 nm.
Phenyl bis(2,4,6-trimethylbenzoyl)-phosphine oxide (CAS no. 162881-26-7)	BPO		Norrish Type I photoinitiator from the phosphine oxide family, with absorption maxima at 281, 365 and 395 nm.
2-hydroxy-2-methyl-1-phenylpropanone (CAS no. 7473-98-5)			Liquid Norrish Type I photoinitiator from the hydroxy-acetophenone family, with an absorption maximum at 244 nm.
2-Isopropylthioxanthone (CAS no. 5495-84-1)	2-ITX		Norrish Type II photoinitiator of the thioxanthone family, with absorption maxima at 259 and 383 nm. 2-ITX provides long wavelength sensitization of appropriate photoinitiators and depth cure.

Overview of Most Used Photoinitiators

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Features of Electron Beam Curing (EB) Technology

Next to UV which remains the leading technology, another curing source used is Electron Beam (EB) which makes use of an electron gun accelerating and directing electrons emitted from the cathode surface in high vacuum. The electrons are directed towards the coating surface forming free radicals required for the polymerization reaction.

There are several advantages of EB over UV and UV-LED:

No photoinitiators needed
Low sensitivity for pigments
Curing of higher layer thickness
High speed of curing (up to 400 m/min)

Besides the lack of need for the use of photoinitiators the formulation of the radtech coating is the same as for UV or LED cured systems.

Additives, Pigments and Polymers for Radiation Cured Coatings

View a wide range of raw materials available today used for formulating radiation cured coatings, analyze technical data of each product, get technical assistance or request samples.

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