New priming technology adhesion performance of UV-curable coatings on plastic.

Plastic substrates often offer an unfriendly surface for coating applications, resulting in insufficient adhesion of applied coatings, inks, and adhesives. This paper presents a new priming process which overcomes the problem of poor adhesion of ultraviolet (UV)-curable coatings on plastic surfaces. In this process, the plastic surface is activated by a plasma or corona treatment and then a special primer composition is applied as a thin layer to form a covalently attached primer. The thermally stable primer layer contains latent groups, which can react with the UV-curable coating under UV light exposure. This novel process produces significantly enhanced adhesion to plastic substrates. Experimental results, applications examples, and perspectives are discussed further.

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INTRODUCTION

The decoration of synthetic substrates by coatings and inks is almost impossible without a pretreatment of the surface. (1) Nonpolar and low surface tension of common grades of polyethylene (PE), polypropylene (PP), polyester (PET), polyvinylchloride (PVC), and other synthetic substrates lead to very poor adhesion of the coated film, unacceptable for the final applications, such as OEM, graphic arts, etc.

When UV-curable coatings and inks are considered, the adhesion problem is even more severe than conventional solventborne systems since swelling at the surface of polymers does not occur with UV-curable formulations. The film shrinkage resulting from the free radical polymerization of the film further weakens the adhesion of the coated film. Various formulation strategies are possible (2) but are often restricted to certain substrate types and lead to additional drawbacks such as irritancy of the preparation for the manipulators, residual odor, or other environmental, health, and safety (EH & S) concerns.

To overcome adhesion failure of inks and coatings on plastic substrates, a surface treatment is usually carried out (3,4):

* A conventional solvent-based primer is applied in-line or in a preparative step off-line

* A corona treatment or flame treatment leads to an oxidation of the surface

* A plasma treatment, eventually coupled with a surface modification by selected chemical reactants.

Solvent-based primers are commonly used today, (5) not only as a preparative coating step, but also for finishing of the plastic films or pieces during the manufacturing process. However, they do not allow the possibility of limiting VOC emissions, in particular when subsequent coatings and inks are solvent-free systems. In addition, they are usually not as effective for adhesion improvement when radiation-curable coatings are considered.

Flame, and in particular corona treatments, are widespread techniques for improving plastic substrates coating ability. However, they suffer from a lack of versatility versus polymer types (only almost universal on PE surface when coated in-line) and also from an uncontrolled loss of properties upon aging. (6) Moreover, the physical boundaries between such treated polymeric surfaces and radiation-curable coatings are often too weak to reach an acceptable level of adhesion.

Finally, plasma treatment (7,8) is an alternative which has been unsuccessful due to the complexity of the technique (e.g., vacuum conditions) and the limitation of a batch process.

The new priming technology described in this paper is aimed at enhancing the adhesion of UV coatings and inks on plastic surfaces by offering:

* A higher versatility on various polymer surfaces types and grades

* A stable treatment against aging

* An industrially applicable technique

* A cost-effective surface modification.

PRINCIPLE OF THE NEW PRIMING TECHNOLOGY

This new priming technology (9) is based on a sequential process (10) that leads to a permanent modification of the polymer surface.

In the first step, the surface is activated by a plasma, flame, or corona treatment. Subsequently, an unsaturated photoinitiator is deposited on the activated polymer surface and grafted by the formation of covalent bonds (Figure 1a). Due to the polar groups of the photoinitiator, the surface energy of the treated surface is enhanced and wetting is improved. In addition, the steric hindrance and the crosslinking of the photoinitiator molecules prevent the diffusion of the polar groups into the polymer. By anchoring the photoinitiator at the surface, a practically unlimited storage stability of the surface is achieved, as long as exposure to short wavelength light is prevented. For this reason, a polymer surface which was treated as described above can be stored, shipped, and treated further by the customer without an additional surface treatment.

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In this way, a very homogeneous surface modification is achieved. Another advantage is that only the polymer surface is changed, whereas the bulk remains unchanged. For example, an acrylate derivative of an [alpha]-hydroxy-ketone photoinitiator (AHK-1) can be used to modify the surface of a polymer according to this new process.

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Grafting occurs at the surface of the activated polymer and is supported by a subsequent UV lamp irradiation in the case of corona or flame treatment. (10) Chemical bonding through radical or energy transfer from the photoinitiator to the surface, hydrogen abstraction (11) at the surface, or UV activation of the oxidized surface are possibly participating to the grafting process.

As shown schematically in Figure 1b, latent functionalities are introduced at the surface of the polymer, allowing further modifications and in particular subsequent crosslinking with a UV coating or ink.

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A solution containing 5% of AHK-1, and using ethanol as a carrier, was applied to a transparent oriented polypropylene (OPP) film at one microns thickness following this new sequential priming technology.

The produced samples were cut in two pieces and half of the substrates were either immersed in ethanol or dimethylsulfoxide (DMSO) for 12 hr.

Through all the substrate pieces, we measured the UV spectrum by transmission between 200 and 400 nm. For all the samples (not immersed in solvents and immersed in solvents), the typical curves of AHK-1, with its maximum absorption peak at 274 nm, were monitored. In addition, no significant difference of coefficient of extension (between 0.3 and 0.5 at 274 nm) was found between the non-immersed samples and the samples immersed overnight in solvents.

This simple experiment showed that it was possible to permanently functionalize a polymeric surface using this new priming technology.

The presence of functional groups, covalently bonded to the surface, will allow further modifications. This is primarily due to the specific properties of the light sensitive unit. The photoinitiator groups can absorb light and the absorbed energy results in the cleavage of the molecule into two radicals, whereby one of these radicals remains linked to the surface. One of the most important reactions of these radicals is the start of a radical chain reaction in the presence of reactive double bonds. if a UV-curable printing ink is applied onto the treated surface and subsequently irradiated with UV light, reactive radicals are formed which add to double bonds in the formulation and start the radical polymerization reaction. The outstanding property of the new method is that, due to the addition of the initiator radical onto the double bond, a covalent bond between the surface and the cured ink is formed (Figure 2). A superior adhesion on different and difficult substrates can be achieved.

RESULTS

Laboratory Examples

In the laboratory small pieces of foils were treated according to the new priming process and then hand-printed using standard UV inks. An ink layer of about 1-2 [micro]m was applied with a rubber roller and subsequently cured at a speed of 30 m/min with a UV-processor (Fusion Type H, 120 W/cm). Crosscuts were made; a tape was applied and jerkily removed. In the case of weak adhesion, the ink was totally or partially removed. In the case of good adhesion, the ink remained entirely on the polymer. Instead of a quantitative evaluation, the results are shown as photographs. A visual representation provides a more realistic basis for judgement. In the following examples, untreated, only plasma, or corona treated samples are compared with those that were treated according to the new priming process.

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EXAMPLE 1: TRANSPARENT POLYPROPYLENE -- In the first example, a transparent polypropylene foil was pretreated differently (see Figure 3) before printing with a blue UV offset ink (UV-Temp from Hostmann und Steinberg) and irradiated. From Figure 3, it can be seen that the ink obviously does not adhere at all on the untreated foil (left side). After the plasma treatment, the adhesion is far from optimal (center). However, when the new priming process is used (right side), the ink completely adheres in the crosscut areas. Similar results were achieved when a corona treatment was applied instead of a plasma treatment.

With polypropylene, the problem of poor storage stability of surface treatments is severe, which is due to special structure and properties. After a few hours or days the surface tension of corona-treated foils is markedly reduced. (12) This results in a drastic reduction in wetting and adhesion. Yet, samples modified by the new priming process were stored for days and weeks without a reduction in surface tension or adhesion.

EXAMPLE 2: BOPP -- A tough challenge is the printing on bi-axial oriented polypropylene (BOPP) with black or dark inks. Also in this case the new priming process can prove its special effectiveness. In Figure 4, prints with a black ink (Sicura Typo from Sicpa) on a transparent 40-[micro]m thick BOPP foil are displayed. The adhesion on the foil, which was treated with the new priming process, is perfect (right side), whereas on the two other samples (untreated on the left and corona-treated in the center), no adhesion was achieved. In the case of the untreated foil, the wetting was also poor, which is shown by the formation of pinholes at the left side of the image.

EXAMPLE 3: HARD PVC -- Hard PVC sheets are very often used for the manufacturing of packaging material and advertisement panels. Even though the adhesion problems on this polymer are different than in the case of polyolefines, the adhesion of UV inks could be improved significantly by the application of the new priming process. Figure 5 shows the results, which were obtained on a transparent hard PVC sheet.

Again, no adhesion of the ink could be achieved on the untreated material. Even after a corona treatment, the adhesion was poor and the ink completely adhered to the tape. With the new priming treatment, outstanding adhesion was achieved, using identical application and irradiation conditions.

EXAMPLE 4: REDUCTION OF CORONA TREATMENT -- A transparent PP foil from Klockner Pentaplast was used as substrate to print a UV offset ink, designed for plastic substrates, from Sicpa. However, on this difficult substrate (surface tension < 34 mN/m), good adhesion was not achieved without treatment. Then, the performance of corona treatment alone was compared with the new priming technology. The coating C1 associated with the new priming technology contained an experimental photoinitiator (1%) bearing a non-acrylate insaturation and isopropanol as a carrier.

The following results were obtained:

Corona
Nominal     Speed Through  New Priming  Adhesion
Power       Corona         Process      Results*

1 X 1000 W  50 m/min       Not used     Poor-1B
2 X 1000 W  50 m/min       Not used     Good-3B
1 X 1000 W  50 m/min       With C1      Good-4B

*Results are reported according to ASTM D 3359 Tape Test (Six parallel
cuts).

In this experiment, it was demonstrated that the use of the new priming technology can give access to substrates for which enormous amounts of corona treatment would have been needed to get a correct adhesion performance when used alone. Such a high level of corona treatment is normally not available in industrial conditions and is eventually detrimental to line speed and also substrates properties such as sealing ability.

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Another goal of this set of experiments was to demonstrate the possibility of using alternative unsaturations than acrylates (with high toxicological hazard potential) as linker to introduce a latent function at the surface of a treated plastic substrate. In this case, the use of a vinyl-ether functionality exhibited a very satisfactory adhesion improvement.

Pilot Production

To print sheets formatted at 50 X 35 centimeters, a roll of white-pigmented polypropylene was treated on a standard pilot coating unit according to the new priming process. The treated roll was then cut into the right format. Printing of different motives was performed on a Komori Lithrone 5/20C five-color offset printing machine. The machine was equipped with two UV lamps from Adnos, which were run at 160W/cm. The irradiation was done after all five colors were applied. The speed was adjusted to 5500 sheets per hour (about 40 m/min). The inks used were from Hostmann Steinberg. A systematic testing of the sheets was performed about one half-hour after the printing. Tapes (Tesa 4104) were applied in different areas, jerkily removed, and then evaluated.

In Figures 6-8, the results of these tape tests on the sheets, treated according to the new priming process, are compared to sheets treated only with corona or untreated sheets.

In Figure 6, a simple two-colored motive is displayed and the removal of ink on untreated (left) or only corona treated (center) sheets is clearly visible. However from the material treated with the new priming process (right), no ink was removed.

Figure 7 shows a motive with bright colors and low hiding. Compared to the untreated sheet an improved adhesion was achieved by a corona treatment, but the result is not yet acceptable. However with the new priming process, the adhesion is excellent.

A motive with higher hiding and dark colors is compared in Figure 8. Poor adhesion becomes most clearly visible due to the high contrast with the white background. Here also, the new priming process provides outstanding adhesion properties.

CONCLUSION

This new priming process can be used for many different polymers in various ways and in many applications. When adhesion is a critical feature, it can be improved very significantly so that outstanding resistance to tape test can be obtained.

One of the most important application areas is certainly printing on plastic packaging materials, where UV-curable inks are often applied and would probably penetrate further the application with better adhesion performances. In addition, the process could be used for other printing areas such as labels, smart cards, banknotes, etc.

This new process is not restricted to substrates using a roller coating application. The use of an application by spray has been found suitable in term of adhesion performance and consequently, industrial and decorative coatings on non-flat treated areas can be considered.

This new priming technology should support the development of UV-curable coatings and inks. These liquid or powder coatings offer high productivity at low energy consumption and can use clean and solvent-free chemistry. (13)

Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 12-14, 2003, in Philadelphia, PA.

References

(1) Baghdachi, J.A., "Adhesion Aspects of Polymeric Coatings," Federation Series on Coatings Technology, Federation of Societies for Coatings Technology, 19-23, 1996.

(2) Adhesion of Radiation Cured Coatings to Plastics, Sartomer, Application Bulletin.

(3) Ryntz, R.A., Prog. Org. Coat., 25, 73 (1994).

(4) Kunststoffe mit beschrankter Haftung, M. Bauer, and M. Kunz, inprotec AG, 27th Munich Adhesive and Finishing Symposium 2002.

(5) Hintze-Bruning, H., and Borgholte, H., Prog. Coat., 40, 49 (2000).

(6) Hyun, J., et al., J. Appl. Polymer Sci., 77, 1679 (2000).

(7) Yashuda, H., Plasma Polymerization, Academic Press, New York, 1985.

(8) Bauer, M., Kohler, M., Kunz, M., and Misev, L., Ciba Specialty Chemicals, WO 002/4527.

(9) Prime IT, www.cibasc.com/primeit.

(10) Kunz, M., Bauer, M., Baranyai, A., and Macor, G., Ciba Specialty Chemicals, WO 03/064061.

(11) Ranby, B., "Surface Modification and Lamination of Polymers by Photografting," Intl. J. of Adhesion & Adhesives, 19, 337-343 (1999).

(12) Muhlan, C. and Nowack, H., Surf. Coat. Tech., 98, 1107 (1998).

(13) "UV Drying Inks and Lacquers, Forum UV-Technology," Drupa 2000, Dusseldorf, Brochure published by Berufsgenossenschaft Druck und Papierverarbeitung, Wiesbaden, Germany.

by Sebastien Villeneuve, Jean-Pierre Wolf, Thomas Bolle, and Giorgio Macor--Ciba Specialty Chemicals Inc.*

Gabriele Stipan, Martin Kunz, and Michael Bauer--inprotec AG ([dagger])

* Klybeckstrasse 141, CH-4002 Basel, Switzerland.

([dagger]) Neuer Weg 1, D-79423, Heitersheim, Germany.