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A Brief Overview of the Development of Light-Curing Waterborne Polyurethane Resins

Release time:

2015-12-01 14:57

A Brief Overview of the Development of Light-Curing Waterborne Polyurethane Resins

I. A Brief Overview of Waterborne UV Coatings

1. Comparison between conventional UV-curable coatings and waterborne UV coatings

	Regular UV	Water-based UV
Dispersing medium	Use a solvent or active diluent.	Using inexpensive water as a solvent
VOC emissions	Large VOC emissions and minimal residual amounts	Near-zero VOC, water-based viscosity adjustment or cleaning.
Usability	High shrinkage rate (affecting adhesion), relatively high cost.	Low shrinkage, low solids content, low viscosity (suitable for thin-film spray coating processes), low cost.
Safety performance	Toxicity, irritancy, odor	Low toxicity, non-flammable, high safety

A common challenge: Oxygen inhibition or colored systems can lead to incomplete curing.

2. Challenges in the use of water-based UV coatings

(1) Water has a high heat of vaporization (40.6 kJ/mol).

—Pre-drying consumes a lot of energy, takes a long time, and reduces efficiency. (Jiapuli Company’s “microwave-infrared coupled drying technology,” launched at the beginning of the year, has successfully addressed the drying challenges faced by water-based paints.)

—Prone to shrinkage and deformation in absorbent substrates (such as paper, laminates, etc.)

(2) Water has a high surface tension (72.8 mN/m).

— It does not easily wet the substrate and can easily lead to uneven coating (causing defects such as shrinkage cavities and bubbles).

— Poor pigment wetting performance affects dispersion.

(3) Certain additives need to be added to generate a certain amount of VOCs.

— Cosolvent (increases solubility, enhances stability; reduces viscosity, improves leveling)

— Antifreeze (water has a higher freezing point—0℃—preventing it from solidifying at low temperatures)

— Antifungal agent (prevents the easy growth of mold in water-based systems)

(4) The system has poor stability.

— Highly sensitive to pH; prone to precipitation during prolonged storage.

(5) Performance defects such as lower gloss and hardness, as well as poorer water resistance and washability.

3. Development History of Waterborne UV Coatings

Waterborne UV coatings typically consist of water-based oligomers, photoinitiators, additives, and water.

In terms of emulsification methods, the development of waterborne UV coatings can be divided into three stages:

(1) First-generation external emulsification type

Non-ionic self-emulsifying type	Basic process	A certain amount of hydrophilic segments, such as polyethylene glycol, are introduced into the polymer.
	Product Features	The resin itself is hydrophilic and exhibits good stability.
	Application Progress	The long-chain polyether segments result in a softer film, and due to the presence of hydrophilic structures in the polymer backbone, the product exhibits poor water resistance and chemical resistance.

(2) Second-generation non-ionic self-emulsifying type

External emulsification type	Basic process	Traditional light-curable resins are emulsified by adding a surfactant (emulsifier) and applying high shear force.
	Product Features	The production process is simple, and the emulsion particle size is relatively large (0.7–3 μm).
	Application Progress	Poor stability, affected by various factors (such as acidity/alkalinity, electrolytes, excessive shear stress, low temperatures, etc.); residual emulsifiers can compromise the quality of the dry film.

(3) Third-generation ion-based self-emulsifying type

Ion-based self-emulsifying	Basic process	By introducing hydrophilic groups into the polymer and then neutralizing them to ionize, the hydrophilicity is enhanced, thereby imparting water solubility to the entire polymer.
	Product Features	Self-emulsifying, excellent wettability, good stability, and superior performance.
	Application Progress	There are many varieties, and this is the main direction for future development.

II. Waterborne UV Polyurethane

1. Classification

Depending on the introduced ionogenic groups (hydrophilic chain extenders), they can be classified into three major categories:

(1) Anionic (most common)

a. Carboxylic acid type (-COOH): Dihydroxymethylpropionic acid, tartaric acid, and other dicarboxylic acid chain extenders; or dihydroxy hemiesters, typically obtained by reacting low-molecular-weight triols or oligomeric triols (such as glycerol or low-molecular-weight polyether triols) with dicarboxylic anhydrides to yield dihydroxy compounds containing carboxyl groups. The anhydrides include maleic anhydride, phthalic anhydride, succinic anhydride, and glutaric anhydride, among others.

b. Sulfonic acid groups (-SO3H): such as 1,4-butanediol-2-sodium sulfonate and sodium ethylenediamine sulfonate. Due to the strong ionic characteristics of sulfonates, the synthesized PUA can be easily dispersed in water without the need for volatile amine neutralizers or organic co-solvents. Neutralizing agents (salting agents) typically include aqueous ammonia or tertiary amines, such as triethylamine, triethanolamine, dimethylaminoethanol, and butyl ethanolamine.

(2) Cationic

There are a limited range of ionizable groups available, such as amino and quaternary ammonium groups. Typically, tertiary amine groups are introduced into the main chain and then quaternized to achieve stable dispersion—or even water solubility—of polyurethane acrylates in water. For instance, chain extenders containing tertiary amine groups, such as N-methyldiethanolamine and triethanolamine, are commonly used, and the neutralizing agents (salting agents) employed are usually hydrochloric acid, acetic acid, or alkylating agents like epichlorohydrin.

(3) Non-ionic

When the molecular chain contains hydrophilic groups such as ether groups/hydroxyl groups/carboxyl esters composed of oxygen-containing functional groups, block polyethers (such as polyethylene glycol segments), and aldehyde groups, water solubility will also improve to some extent, even reaching full water solubility. However, these materials are typically used in combination with ionic components to form hybrid waterborne polyurethane systems.

They can also be categorized into three types based on their appearance:

(1) Aqueous solution type, (2) Dispersion type, (3) Emulsion type

Name	Aqueous solution type solutions	Dispersed liquid type dispersions	Emulsion type emulsions
Status	Dissolution-Colloid	Disperse	Disperse
Appearance	Completely transparent	Translucent, milky white	White turbidity
Particle size	<1nm	1-100nm	>100nm
Molecular weight	1000-10000	Thousands to 200,000	>5000

2. Use of solvents in synthesis

During the synthesis of polyurethane prepolymers, it is sometimes necessary to add a small amount of organic solvent to reduce viscosity, facilitate the dispersion of the prepolymer, and enable the subsequent emulsification to yield a stable emulsion with fine particle size. This method is commonly referred to as the acetone process.

Suitable solvents include water-soluble (hydrophilic) organic solvents such as acetone, methyl ethyl ketone, dioxane, N,N-dimethylformamide, and N-methylpyrrolidone (which is genotoxic), as well as hydrophobic solvents like toluene. Taking into account factors such as cost and ease of operation, acetone and methyl ethyl ketone are the most commonly used.

Generally, after preparing a stable emulsion, low- and medium-boiling-point solvents can be removed by vacuum distillation to minimize solvent residues and odors in the emulsion. If the amount of solvent used is very small, it may not be necessary to remove it. During the drying of waterborne polyurethanes, the evaporation of residual small amounts of low-boiling-point solvents can accelerate the drying time of the coating film, while the presence of small amounts of high-boiling-point solvents can help produce a smoother coating film—this practice is commonly employed in the coatings industry.

3. Main Synthesis Route

4. Polyurethane-acrylate grafted acrylate hybrid system

In recent years, graft modification using acrylate systems combined with polyurethane systems has yielded chemically hybridized systems that exhibit synergistic and complementary performance characteristics, demonstrating promising application prospects.

This type of resin is mostly obtained by copolymerizing HEMA or GMA with other acrylate monomers to yield acrylate oligomers containing hydroxyl or epoxy groups. These oligomers are then grafted and modified with polyurethane semi-adducts, or reacted with unneutralized carboxylic acid-type polyurethane oligomers, ultimately introducing acryloyl groups. The reaction schemes are shown below:

(1) Hybrid System 1

(2) Hybrid System 2

In the context of green and environmentally friendly development, the coatings industry has been facing increasingly stringent VOC restrictions year by year. Meanwhile, driven by multiple factors—including maintaining high coating performance and market competitiveness—high-performance, cost-effective waterborne UV coatings are gradually entering the market and are bound to gain widespread adoption.

In the future, waterborne UV resins are expected to experience unprecedented growth in fields such as furniture and wood coatings, automotive interior coatings, leather finishing, plastic clear coats, and printing inks.

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Introduction to Light-Curable Monomers and Their Applications

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