The use of various types of plastics as structural and decorative material has been growing for many years in a variety of applications. This includes end uses in the container and packaging industry, in the automotive industry in both the interior and exterior of cars, and also in consumer and industrial applications. The articles being produced require printing and/or coating, but the sensitivity of plastics to heat (e.g., all thermoplastics, by definition) or to solvent attack puts constraints on the type of coating or ink that can be used. Because radiation-curable coatings or inks are natural contenders for use on such substrates, they are growing in use and popularity. Nevertheless, the adhesion of radiation-curable coatings to certain plastics remains a challenge not only because of their low surface energy, but also due to wide variation in the surface properties for a given category of plastic. The low-surface-energy plastic substrates, e.g., thermoplastic olefins (TPOs), have a very high growth rate in a variety of applications, as a result of their superior flexibility, durability and lower cost.
After reviewing the main parameters or factors that can influence or affect adhesion of radiation-curing coatings to plastic substrates, this paper presents our proposal regarding possible solutions to obtain adhesion to difficult plastic substrates. To support this proposal, the performance of recently developed adhesion promoters is presented, as well as the methodology we recommend for the optimum selection of components for a radiation-curable formulation.
Within the UV/EB-curing coating market, only approximately 10% are coatings for plastic substrates, and that is pretty much dominated by coatings for PVC flooring. Although still limited, it is growing fast in certain sectors, mainly automotive and consumer electronic applications. According to marketing consultants, the UV/EB plastic coating market shows higher growth rate and value than the overall UV/EB coatings market.
The current biggest challenges for formulators could be summarized as follows:
The reasons for the poor adhesion of UV/EB-curing coatings onto plastics can come from both the properties of the radiation curing system, as well as from the characteristics of the substrate.
Main Properties Affecting AdhesionShrinkage
It is well known that upon exposure to radiation, the transformation of the coating from liquid to solid results in volume shrinkage. The latter is mostly coming from the reactive diluents used in the formulation because they have a high double bond per mole ratio and are present at high concentration, as they are responsible for reducing the viscosity of the (usually) high-molecular-weight oligomers. Table 1 presents the typical film shrinkage of various low-viscous acrylates, i.e., the change in the density of the formulation based on the acrylate and a photoinitiator (4% Darocur 1173, Ciba Specialties) during curing under UV light (120 W/cm).
As it is a very fast process, the radiation-induced polymerization of acrylates leads to the development of internal stress in the coating. Since it is a much faster curing mechanism, this stress is developed to a much higher extent than in conventional thermal-curing coatings.
The presence of unreacted materials at the interface between the coating and the substrate is generally a cause for adhesion failure. This is obviously more an issue with UV-curing systems than with EB-curing systems. Therefore, the type of UV lamp, the type and level of photoinitiators, as well as the choice of oligomers and monomers are very important.
Substrate Characteristics Affecting AdhesionSubstrate Wetting
The surface tension for both the substrate and the wet (uncured) coating should be considered with regard to substrate wetting. Table 2 presents the typical value (or range) for the surface tension of common plastic substrates.
The surface tension of several low-viscous acrylate diluents, which is an inherent characteristic of the material, is shown in Table 3. As a general rule, coatings must have a lower surface energy than the substrate in order to wet the substrate well.
If every individual component of a formulation has its own surface tension, which cannot be changed, the surface tension of a coating formulation can be decreased by means of surfactants. But the selection of the surfactants can be very limited depending on the end use, especially if the coating is used as a primer or an intermediate coating layer.
On the other hand, the surface energy of a substrate, especially plastic substrates, can be increased by means of both chemical and physical treatments. The chemical treatments include mainly the following two treatments:
Various types of physical treatment can be used to increase the surface energy of plastics, mostly through oxidation of the superficial layer. These physical treatments include:
Similarly to what is observed with solvents, although often to a lower degree, some monomers and oligomers can attack or swell certain plastic substrates, e.g., PVC, PC and PS in particular. This generally results in improved adhesion of the coating to the substrate because an interpenetrating link may be established between the substrate and the coating. Due to their lower molecular weight, monofunctional and difunctional acrylate diluents tend to swell more plastic substrates than oligomers or higher-functional acrylate diluents. In our investigation, we have observed that the acrylate monomers and oligomers that are listed in Table 4 have the ability to swell plastics.
The surface topography of a substrate can also have an influence on the adhesion of a coating. Compared to a smooth surface, a rough surface offers a higher area of contact, thus a greater and more-effective bonding area, and offers sites for mechanical interlocking for the polymer chains of the coating. Certain treatments, e.g., sanding or acid treatment, can be used to create or increase surface roughness.
"Hooked on Plastics" SolutionResults & Discussion
The "hooked on plastics" solution is the "3S approach". Taking into consideration all the characteristics and parameters described above for both the substrates and the radiation-curable coatings, the present study was designed to probe the following concept: In order to obtain a good adhesion to plastic substrates, the selection of the coating ingredients is critical and depends mainly on the following three properties of the monomers and oligomers: Surface tension, Shrinkage and Swelling capability. This is the reason for calling this product selection: the "3S approach".
Although the study has focused primarily on the adhesion of the radiation-cured coatings to plastic substrates, attention should obviously also be given to other properties that are important for the application of the coating and the end-use requirements. This may include application viscosity, scratch and abrasion resistance, chemical resistance, flexibility, non-yellowing, etc.
The materials used in this study are either fully commercial or only developmental at this stage, but they are all registered and can be sampled. In addition to the selection made within the commercial products on the basis of the so-called "3S approach", some products were developed as adhesion promoters specifically for this study. The selected products have been evaluated in the two simple formulations presented in Table 5.
Coatings were applied (draw-down technique) at a thickness of approximately 6 mm, using a wire-wound rod, onto various plastic substrates (see Table 6) obtained from various sources.
In order to limit as much as possible the influence of the curing state, all coatings were cured extensively by means of a very high UV dose achieved by passing the coated substrate two times at a belt speed of 5 m/min under 1 mercury lamp operating at 120 W/cm (UV-curing equipment from F.D.S.).
Successful Adhesion Promoters to PlasticsThe adhesion to the substrates presented in Table 6 was assessed by means of the conventional crosshatch adhesion tape test ("Crystal Scotch" cellotape, 3M). The products that have been found to provide excellent adhesion of at least one of the coating formulas given in Table 5 are shown in Table 7, with their relative performance on all the substrates. In this Table, an "x" means that the product provided 100% adhesion.
Formula Suggestion for Practical Cases
On the basis of the results that are summarized in Table 7 and in application of the so-called "3S approach," we have developed coatings formulations exhibiting excellent adhesion (100% adhesion at the crosshatch tape test) to substrates used in some specific end uses. These formulations are described as follows.
Coating for PVC FlooringThe substrate used in this application is plasticized PVC, which is characterized by the following primary features: it is highly flexible and can undergo swelling rather easily.
The details of the coating formula that was developed and met the requirements are given in Table 8. In addition, this coating has been designed to exhibit high flexibility, as assessed by the bend test carried out on the coated substrate.
Coating for Polycarbonate Display Rack
The substrate used in this application is a hard and thick extruded polycarbonate; the main feature to take into consideration is that it can swell or be attacked very easily. The details of the coating formula that was developed to meet these requirements are given in Table 9.
Primer for TPO (Car Bumper)The substrate used in this application is TPO, which is characterized by the following features: low surface energy, non-porous and high chemical resistance. The details of the primer formula that were developed and exhibited excellent adhesion to this difficult plastic substrate are given in Table 10.
Laminating Adhesive for PP/Paper Laminate
The plastic substrate used in this application is a polypropylene (PP) film, which is characterized by the following features low surface energy, is non-porous and very flexible. As this is the case for making PP/paper laminate currently, the plastic film is Corona treated prior to the application. Table 11 reports the details of the laminating adhesive formula that was developed and provided excellent adhesion to PP, as assessed by the delaminating test, where the paper could not be detached from the PP film without tearing.
The solution that we have probed in the present study and propose to make the optimum selection of monomers and oligomers to formulate coatings that exhibit good adhesion to plastic substrates is the so-called "3S approach". It consists in making the selection on the basis of the following characteristics:
1. SHRINKAGE: should be as low as possible; low functionality diluents are preferred.
2. Capacity to SWELL the substrate: certain monomers and oligomers have been identified.
3. SURFACE TENSION: as low as possible to help wetting of the plastic substrate.
The following factors are also worth considering when formulating radiation-curing coatings for plastic substrates:
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