UV Coatings

Basics, Recent Developments and New Applications

Reinhold Schwalm

• Publisher: Elsevier Science
• Year: 2006
• ISBN:
  9780080466897
• Topic: Technik & Maschinenbau
• Subtopic: Chemie- & Biochemietechnik

Since UV curing (light induced polymerisation of multifunctional oligomers) is a very ecoefficient and energy saving curing method, the growth rates of UV curable coatings are in the range of 10% per year. The typical UV coatings are solvent free (100% solids), thus helping the industry and the environment to reduce significantly VOC (volatile organic compounds). Recently, the automotive industry has discovered that UV cured coatings are very scratch resistant, which stimulated very extensive work into the development of UV coatings for automotive applications. Since UV curing is very universal, also other systems besides the 100% solid (typical) UV coatings are developed, like waterbased UV-, UV powder and Dual cure (UV and thermal) systems.
UV Coatings contains an overview of the technology, the curing process including the equipment necessary, the raw materials (resins, diluents, photoinitiators) used, the advantages and drawbacks of this fast emerging technology, as well as proposed technical solutions to tackle the disadvantages. Structure-property relationships will be given, especially regarding the mechanical properties of coatings as well as scratch resistance, mainly dealing with automotive performance criteria.
The main part of the book will deal with new developments, like water-based UV coatings, UV powder coatings and dual cure systems, cured by UV and thermal energy, which have been developed to cure the coating on three dimensional substrates in shadow areas. The main applications of UV Coatings will be described, starting with the classical ones on temperature sensitive substrates, like wood, paper and plastics, where the UV curable coatings are already well established.* Looking at UV curing as a key to scratch resistant automotive clear coats
* Ecoefficiency of UV Coatings
* Comprehensive overview of the technology, materials and markets

CHAPTER 1

Introduction to Coatings Technology

Publisher Summary

The chapter discusses ultraviolet (UV) coatings technology. The main purposes of a coating are to ensure the desired appearance (color or gloss), necessary protection against corrosion, stone chipping, scratches, abrasion, or chemical attacks like red wine, coffee, or mustard on furniture coatings, or acid rain, tree resins, or bird excrements on automotive coatings. The usage of UV coatings technology is different in the three major regions—NAFTA, Europe, and Far East. In Europe, the industrial wood coatings dominate, whereas the major applications in the United States are graphic arts (inks and overprint varnishes) and in Far East, the area of electronic and display applications (others). The graphics area of inks and overprint varnishes also has a considerable share in all regions and has, by far, the highest market share of UV coatings on a worldwide basis. UV curing has been established as an alternative curing mechanism to thermal hardening, contrary to the past, where it was only considered for the curing on temperature sensitive substrates like wood, paper, and plastics. This alternative curing technology uses the energy of photons of radiation sources in the short wavelength region of the electromagnetic spectrum, to form reactive species that trigger a fast chain growth curing reaction.

Coatings are found almost anywhere in daily life, the most prominent examples are architectural wall coatings and automotive paints. They are applied in order to provide:

• decorative appearance, and/or

• protective barrier.

The main functions of a coating are thus on the one hand to ensure the desired appearance (colour, gloss) and on the other hand the necessary protection, against corrosion, stone chipping, scratches, abrasion or chemical attack, like red wine, coffee or mustard on furniture coatings or acid rain, tree resins or bird excrements on automotive coatings, as shown in Figure 1.1.

FIG. 1.1 General function of a coating (e.g., automotive).

Whereas the do-it-yourself architectural coatings are almost all water-based, the vast majority of industrially used coatings, applied in factories on various substrates, like vehicles, furniture, metal cans, paperboards, etc., still contain solvents.

The coatings and application spectrum discussed in this book are predominantly based on the industrial coatings sector, which had a share of about 40% of the whole worldwide coatings market (60% architectural).

1.1 COATING MARKETS AND MARKET PROSPECTS

The market prospects of future coating technologies in the industrial paint sector are reflecting the environmental concerns about the use of solvents, and hence governed by VOC (volatile organic carbon) regulations. According to these regulations, the market share of solvent-based coatings is declining significantly and the share of alternative, environmentally friendly systems, especially water-based, powder, and radiation curable (UV/electronbeam) coatings is steadily increasing, as depicted in the chart in Figure 1.2 (Paulus, reported at RadTech Conference, Barcelona 2005).¹ Further information about market developments in the specific sectors of the coatings industry is available by numerous market research institutes.² As can be seen from the technology split in Figure 1.2, the predictions of the total amount of solvent-based systems for 2015 are only slightly lower than the amounts in 2003, however, a considerable part of the classical solvent-based systems containing from 50–70% solvents are shifted to “higher solids” systems with up to 80% solids. The reluctance to switch away from solvent-based systems is often related to the excellent properties of such coatings, as well as the ease of handling and the high comfort factor gained over the years of working with solvent-based coatings.

FIG. 1.2 Market development of industrial coatings by technology and region.

A comparison of the most suitable future coating technologies reveals that all alternatives to the classical solvent-borne coatings have specific advantages and drawbacks. “High solids” systems are closest to conventional solvent-based coatings and hence most easily adopted by manufacturers of solvent-borne coatings. However, they still consist of up to 30% solvents and have to be replaced in the long run. Water-based systems are well developed, however, they still lack performance when directly exposed to the environment, mainly due to their sensitivity to humidity, which is a consequence of the use of water compatible groups for solubilizing or dispersing the systems in water. Furthermore drying of water-based systems requires more energy and specially designed drying units. The most environmentally friendly coatings are powder and radiation curable (UV/EB) systems, which are based on 100% solid or liquid formulations. The dra...