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Combining environmental and productivity benefits, UV/EB technology is attractive for hardcoat applications, as high abrasion resistance can be achieved. Wood coatings, CD/DVD, LCD displays for mobile phones, touch-panel displays, and polycarbonate headlamp applications are examples where Radcure technology is well established. However, new applications like flexible displays, the new generation of electronic storage devices (HD-DVD, Blu-Ray) or in-mould decorations also mean new requirements like improved dimensional stability, higher flexibility, lower viscosity and better adhesion.

In this paper, we present a new precursor for the preparation of acrylic esters based on hyperbranched aliphatic polymer polyols. The unique molecular structure of the hyperbranched polymers provides high functionality at low viscosity and allows further improvements in all above-mentioned properties, compared to state-of-the-art high-functionality acrylates typically used in hardcoats like di-pentaerythritol hexaacrylate. A unique combination of properties such as high hardness, scratch resistance and flexible hardcoats with low viscosity and low shrinkage can be achieved.

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Since the first synthesis of a dendritic polymer in the late 70´s, there has been growing interest for this new family of polymers because of their unique and specific properties compared to their conventional linear and branched homologues (see Hult1 and Fréchet2 for a review). They are obtained by reacting a poly-functional core with ABx monomers, typically AB2 monomers, yielding a "tree-like" amorphous structure (dendron means tree in ancient Greek). The obtained macromolecule is thus characterized by an exponential growth in both molecular weight and end group functionality.

Dendritic polymers (Figure 1) have traditionally been classified into two categories: dendrimers and hyperbranched polymers. A dendrimer is characterized by a perfectly symmetrical globular shape that results from a step-wise controlled process giving a monodispersed molecular weight distribution. The second category, the hyperbranched polymers, are attractive because they resemble dendrimers (their difference lies in their polydispersity and the less perfect globular shape) but they can be produced more easily on a larger scale and at a reasonable cost, thus making them commercially available in large quantities today.

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Unlike conventional polymers, the high number of end groups and their nature participate actively in the physical properties (solubility, glass transition temperature and viscosity) in combination with the backbone structure. This characteristic is exceptional since it leads to the possibility of designing the macromolecule with the combination of many different end groups, thus defining the type of reactive chemistry, properties and applications. The lack of entanglement results in a Newtonian behaviour with lower viscosity than the linear homologues (i.e., same nature and molecular weight). The solution viscosity is furthermore only slightly dependent on the molecular weight.3

The applications are numerous, and some important polymer fields can be mentioned, such as:

  • polymer additives (i.e., processing aids, compatibilizers for thermoplastics, toughener agents for thermosets4 and polyurethane foams5); and
  • polymer building blocks for coatings6,7,8 (i.e., high solid alkyds, powder coatings and radiation-curable coatings).

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Dendritic Polymers for Radiation-Curing Technology

Dendritic Polymers for Radiation-Curing Technology Radiation-curable technology is an ideal application to explore all the potential of such a macromolecule used as a building block. The combination of its characteristics makes the dendritic polymer a unique and promising candidate to today's stringent coatings quality requirements. The fundamental specific properties of dendritic polymers and their potential impact on coating performances are summarized in Figure 2.

Furthermore, the versatility of the technology (the type of core, the nature of the backbone, the number of generations and the shell structure) provides a wide range of different properties (viscosity, reactivity and flexibility/hardness). An appropriate design can lead to specific desired properties for any particular application.

A low-viscosity dendritic polymer has already been developed especially for radiation-curing applications10 exhibiting the mentioned performances like low viscosity at high molecular weight, high wear resistance and low extractables.

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Properties of the Hyperbranched Polyol

The hyperbranched polymer used to prepare the acrylic ester used in this study is a new, clear, low-viscosity hydroxyl-functional aliphatic hyperbranched polyester/ether blend called Boltorn® P500 having characteristics given in Table 1.

The characteristics of the acrylated hyperbranched polyol synthesized according to a conventional esterification process with acrylic acid are presented in Table 2. A simple comparison of viscosity as a function of molecular weight for five types of conventional oligomers used in radiation curing is given in Figure 3.

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Coating Characterization

Basic Comparison

The new dendritic acrylate was compared with two other high-functionality acrylates, di-pentaerythritol penta/hexaacrylate (DPHA, acrylic ester of di-pentaerythritol, polyol obtained from Perstorp Specialty Chemicals AB, Sweden) and an ethoxylated pentaerythritol tetraacrylate (acrylic ester of Polyol R4631, a tetrafunctional polyether polyol from Perstorp Specialty Chemicals AB, Sweden). All tests were done with 3% Irgacure 500 obtained from CIBA, Switzerland. The UV-curing unit was a Fusion F600 equipped with one H bulb.

As noted in Table 3, the new dendritic acrylate exhibited the following characteristics:

  • a significantly lower viscosity than DPHA;
  • as good surface cure as DPHA and better than Polyol R4631 acrylate;
  • as good flexibility as Polyol R4631 acrylate;
  • almost as good hardness as DPHA; and
  • almost as good water and ethanol resistance as DPHA.

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Scrub Scratch Resistance

Scratch resistance was evaluated by Scotch-Brite (green) rubs (constant weight of 500 g) and by measuring gloss retention (20º and 85º) as a function of cycles (50, 100, 150 and 200 rubs) and a UV dose of 500 mJ/cm2, 2 pass at 12 m/min for 40 µm thick films on black panels). The results are reported in Figure 4.

The new dendritic acrylate and DPHA demonstrated a very high scratch resistance as seen by gloss retention. It was significantly better than the tetrafunctional Polyol R4631 acrylate and similar to the DPHA.

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Hardcoats on Polycarbonate (PC)

The influence of the substrate is important for pencil hardness, especially at low film thickness (as compared to hard substrates like glass). A basic comparison was performed on polycarbonate sheet (Lexan 8010C, 250 µm, from GE) for the pure acrylates cured with 3% Irgacure 500 under a 160 W/cm H bulb in air. The coatings were conditioned at 25 °C and 50% RH for 24 hours before testing.

The new dendritic acrylate yielded similar pencil hardness as DPHA at high film thickness as shown in Table 4. The new dendritic acrylate exhibited significantly better adhesion than DPHA on PC (Table 5) as well as better flexibility as shown in Table 6.

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Topcoat Scratch Resistance and Accelerating Weathering

The scratch resistance and accelerating weathering performance of a UV topcoat based on Boltorn P500 acrylate was compared with a 2K isocyanate coating based on acrylic polyol and HDI trimer. All coatings were formulated with UV absorber and hindered amine light stabilizers (HALS) and a proper choice of photoinitiators was used for the UV formulation.

The UV topcoat formulations based on Boltorn P500 acrylate were cured with a total UV dose of 2000 mJ/cm2 under an H bulb from Fusion (F600). The UV coating exhibited largely superior scratch resistance (pencil hardness and scrub test) compared to the 2K isocyanate coating as shown by the results given in Table 7 and Figure 5.

The accelerated weathering performances of the topcoats were investigated on blue metallic waterborne basecoats. The panels were subjected to the alternate effects of condensation (4 hours at 50 °C) and the damaging effects of sunlight simulated by the fluorescent UVA lamps (4 hours at 60 °C).

Gloss (60°) and delta E were measured as a function of exposure time. The results (Figure 6) show that the Boltorn P500 formulation performs more or less as good as the reference 2K isocyanate based on HDI trimer and polyacrylic polyols with a gloss retention superior to 90% and a delta E inferior to 2 after 2000 hours of accelerated weathering exposure. Long-term exterior durability tests are currently running.

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The dendritic acrylate based on the new hyperbranched polyester/ether polyol blend Boltorn P500 is an excellent precursor for the preparation of hard, yet flexible acrylic esters providing the following benefits compared to other high-functionality acrylates:

  • a low-viscosity, high-functionality acrylate, much lower viscosity than DPHA, allowing easy formulating of low viscosity formulations with a high degree of latitude;

  • very good pencil hardness, almost as good as other high-functionality acrylates such as di-pentaerythritol penta/hexaacrylate (DPHA);

  • excellent scratch resistance, as good as DPHA and largely superior to a 2K isocyanate coating;

  • excellent water and ethanol resistance;

  • good flexibility, much better than DPHA;

  • very good adhesion to plastics, for example polycarbonate; and

  • good accelerating weathering performance.

The authors would like to thank Dogan Nazire from Akzo Nobel Car Refinishes, NL for supplying the primer and basecoat panels.