by Scott Sabreen, president, The Sabreen Group Inc.
The demand for digital inkjet printing on 3D plastic products and thin films is increasing exponentially. Application challenges to achieve robust operations include optimal ink chemistry-printhead design, compatibility between the ink and polymeric substrate and curing. Fortunately, new technology advancements solve traditional inkjet problems for low surface energy and heat-sensitive substrates, including stretchable elastomers/rubbers, urethanes, nylons, polyolefins and more.
Inkjet printing is far more complex and delicate than analog printing. Inkjet requires the printhead nozzles to fire precisely sized drops with exact accuracy. High-quality inkjet printing systems must simultaneously integrate printheads, fluids, electronic controllers, pretreatment and cure. All of these items must work together to produce the intended results. Most companies investing in inkjet technology desire to decorate multiple substrates. Since there often are substantial chemical and physical differences between plastics, even within the same polymer family, it becomes challenging to print on all materials. Chemical primers and surface oxidation pretreatments often are necessary.
For chemical compatibility and adhesion three main input process parameters exist: 1) polymer substrate, 2) ink and surface pretreatment and 3) inkjet printer. All elements must be stable to achieve excellent print quality. Printing on contoured plastic geometries by inkjet is further complicated as there are relatively few OEM printer/printhead choices. Each OEM limits certified inks developed for a limited range. Thorough understanding of the intricacies of inkjet printing and surface science enables unprecedented capabilities and results.
UV-curable inkjet inks
Ultraviolet-curable inkjet ink has been widely adopted for printing 3D plastic products. UV-curable inks dry instantly, bond directly to a limited number of plastics, do not emit solvents and are available for some OEM platforms. Extending the range to “tough-to-bond” polymer substrates requires custom formulating expertise to meet the stringent requirements for decorating and printing. Formulators need to know the unique chemistries that provide fluids able to be micro-jetted and the curing mechanisms that ensure rapid hardening, as well as the physical and chemical properties of the resultant ink, so the job is more complicated than for any other ink type.
UV ink printing differs from other types of printing inks because the polymer is formed during curing by chain reaction of monomers and oligomers. Monomers are low-viscosity liquids, so they also function as the liquid ink carrier and eliminate the need for water or solvent – that is why UV cure inks are 100 percent solids and ideal solvent ink alternatives. Oligomers (larger reactive molecules) have multiple chemical functionalities and are critical to properly building the binder.
Polymerization is initiated by a short exposure to UV light, which is electromagnetic radiation with a shorter wavelength and higher energy than visible light. Near UV (390-200 nanometer wavelength) is used for most UV curing. It is further refined into UV-A (390-320 nanometers) or long wave; UV-B (320-280 nanometers) or medium wave; and UV-C (280-200 nanometers) or short wave. The UV-reactive components are photoinitiators compounds that absorb the energy and split produce highly reactive chemical species, free-radical or cationic depending on type. Each can bond to one part of a monomer or oligomer, which transfers the activity to another part, which in turn can react with another monomer, and so on to build the polymer.
Pigments and color palettes
UV inks use only pigment colorants, but the range is broad and not a serious limitation. Formulating and ink-making developments enable manufacturers to offer white (W), which is especially useful as a base on dark-colored substrates or as a background on clear. Inkjet primary colors – cyan, magenta, yellow and black (CMYK) together – offer a large color gamut because ink layers can be thick. However, being based on pigments, they are at least partially opaque and can mask colors below. The intensity does not fully translate into the secondary colors, which excludes a number of Pantone colors popular with major brands. Analog printing uses specifically formulated spot colors. They could be available in this market, especially with 6-, 8- and 10-printhead machines, but they are not popular because it is far more difficult to change out a spot color in an inkjet printer than in flexographic or screen printing. This also conflicts with the ability of digital to switch jobs with consecutive images. An alternative is to employ extended gamut ink sets with intense inks of hue intermediate between CM and Y. Red, green and blue (RGB) or orange, green and violet (OGV) are used in label and advertising printing to extend the gamut and cover most bright, specified, intense brand colors. These colors also can be custom-formulated for 3D plastic products.
Reproducing light colors, especially flesh tones, is a consistent difficulty for UV-cure inkjet printing. Even the smallest droplets of CMK, now as little as four picoliter, are too large for photo-realism. However, if a secondary set of light magenta (lm) and light cyan (lc) is included, the artifacts of individual dots are eliminated. The option of CMYKlclmW is available to this market from the printer OEMs.
Fading in sunlight can be a major print durability concern. As UV inks use pigments, not dyes, they are inherently less sensitive to sunlight than desktop inks. Very good fade resistance is obtained when the inks are made with pigments from the automotive paint industry. While more costly, they are used in premium inks. Sunlight also can degrade the polymer, producing chalking and brittleness.
Polymer stability under UV light was made necessary early in the life of UV inkjet to support the outdoor advertising industry. It is obtained from monomer and oligomer selection, incorporation of stabilizers and, in the most durable situations, from over-varnishes. Five-year outdoor light exposure is a commonly met expectation.
Additives are essential in all ink formulations. Inks for printing plastics commonly contain adhesion promoters. Jetting and droplet formation, obviously central to inkjet, require careful balances of viscosity modifiers and surfactants. If not correct, the ink can mist or wet out on the head, neither of which is satisfactory. These same compounds also control ink wetting and flow on the substrate – i.e., adhesion and dot gain. They generally are optimized for a particular surface chemistry, but there are limitations. For example, an ink that will wet a low surface energy substrate will leak from the printhead. For that reason, many surfaces must be treated to put them in the range of ink functionality.
Surface pretreatments – ink/plastic substrate compatibility
Inkjet inks have low viscosity and low surface tension, which create adhesion bonding challenges on many polymeric substrates such as acetals, polyolefins and polyurethanes. These types of chemically inert plastics are hydrophobic and not naturally wettable. Consider a single liquid fluid droplet on a flat solid surface at rest (equilibrium). The angle formed by the solid surface and the tangent line to the upper surface at the end point is called the contact angle; it is the angle (x) between the tangent line at the contact point and the horizontal line of the solid surface. (See Figure 1.)
The bubble/droplet shape is due to the molecular forces by which all liquids, through contraction of the surface, tend to form the contained volume into a shape having the least surface area. The intermolecular forces that contract the surface are termed surface tension. Surface tension, a measurement of surface energy, is expressed in dynes/cm. The higher the surface energy of the solid substrate relative to the surface tension of a liquid (water, printing inks, adhesives/encapsulation, coatings, etc.), the better will be its “wettability” and the smaller will be the contact angle (Figure 1). As a general rule, acceptable bonding adhesion is achieved when the surface energy of a substrate is approximately 8 to 10 dynes/cm greater than the surface tension of the liquid.
In reality, fluids and contact angles are dynamic, not static. See Figure 2. The dynamic contact angle (DCA) is most important. When a droplet is attached to a solid surface and the solid surface is tilted, the droplet will lunge forward and slide downward. The angles formed are termed, respectively, the advancing angle (θa) and the receding angle (θr).
Contact angles generally are considered to be affected by both changes in surface chemistry and changes in surface topography. The advancing contact angle is most sensitive to the low-energy (unmodified) components of the substrate surface, while the receding angle is more sensitive to the high-energy, oxidized groups introduced by surface pretreatments. Thus, the receding angle actually is the measurement most characteristic of the modified component of the surface following pretreatments, as measured using dyne solutions. Therefore, it is important to measure both the advancing and receding contact angles on all surface-modified materials.
Automation printing speed
The speed of the printing press also can impact inks effective surface tension. An ink that statically measures 25 dynes of surface tension could behave dynamically like an ink with 40 dynes of surface tension on a high speed press. The actions of inkjet print heads and print systems on the fluids they dispense can significantly impact the way fluid components realign during dispensing and interaction with the print surface and other ink or coating layers. As the frequency at which inkjet print heads can eject drops increases to higher levels and as the speed of inkjet printers and presses increases with single pass full-width arrays, surface tension interactions of inks, coatings and substrates present additional challenges.
There is a strong tendency for manufacturers to focus only on contact angle measurements as the sole predictor for determining bonding problems and conducting routine surface testing. Equally important is chemical surface functionality, by which hydrophobic surfaces are activated into bondable hydrophilic surfaces. Gas-phase “glow-discharge” surface oxidation pretreatment processes are used for chemical surface activation. Surface pretreatments will resolve most ink adhesion problems by increasing the surface energy of the substrate and creating oxidative chemical functionality.
Gas-phase pretreatment methods
Gas-phase surface oxidation process methods include electrical corona discharge, flame treatment, cold gas plasma and ultraviolet irradiation. Each method is application-specific and possesses unique advantages and potential limitations.
The basic chemical and physical reaction that occurs in free electrons, ions, metastables, radicals and UV, when generated in the plasma, can impact a surface with energies sufficient to break the molecular bonds on the surfaces of most polymeric substrates. This creates very reactive free radicals on the polymer surface, which in turn can form, crosslink or, in the presence of oxygen, react rapidly to form various chemical functional groups on the substrate surface.
Polar functional groups that can form and enhance bondability include carbonyl (C=O), carboxyl (HOOC), hydroperoxide (HOO-) and hydroxyl (HO-) groups. Even small amounts of reactive functional groups incorporated into polymers can be highly beneficial for improving surface chemical functionality and wettability. In the science of physics, the mechanisms in which these plasmas are generated are different, but their effects on surface wettability are similar.
Classical electrical corona discharge is obtained using a generator and electrode(s) connected to a high voltage source, a counter electrode at potential zero, and a dielectric used as a barrier. That is, high frequency-high voltage discharge (step up transformer) creating a potential difference between two points requiring earth ground 35+kV and 20-25kHz. Custom electrode configurations allow for treating many different surface geometries, e.g., flat, contoured, recessed, isolated, etc. One application example is a corona discharge treating system for electrical connectors in which a combination of pin and ball electrodes concomitantly treats 3D small diameter holes and flat exterior surfaces, US Patent US5051586 (1991). Ozone is produced in the plasma region as a result of the electrical discharge. Corona discharge has virtually no cleaning capabilities.
Atmospheric plasma or electrical blown ion plasma (also termed focused corona plasma). This treatment utilizes a single, narrow nozzle electrode, powered by an electrical generator and step-up transformer and high pressurized air in which intense focused plasma is generated within the treatment head and streams outward. This pretreatment process can clean dirt, debris and some hydrocarbons from the substrate but not most silicones and slip agents. New research indicates that fine etching of the surface can create new topographies for increased mechanical bonding. Ozone is not a byproduct, but nitrogen oxides (NOx) are produced which may have deceivingly similar odor. There are performance differences between equipment manufacturers.
Electrical “air plasma” is corona discharge spot treatment (also termed blown air plasma/forced air corona/blown arc). This treatment head consists of two hook electrodes in close proximity to each other connected to a high voltage transformer generating an electric arc of approximately 7-12kV, lower frequency 50-60 cycles/sec (relative to electrical corona discharge). Then using forced air, a continuous electric arc produces a corona discharge, “plasma.” No positive ground needed. This pretreatment process has virtually no cleaning capabilities. Ozone is produced.
Flame plasma treatment uses the highly reactive species present in the combustion of air and hydrocarbon gas (to create the plasma). While flame treatment is exothermic, heat does not create the chemical functionality and improved surface wetting. Flaming will clean dirt, debris and some hydrocarbons from the substrate. Flaming will not remove silicones, mold releases and slip agents. Flame treatment can impart higher wetting, oxidation and shelf-life than electrical pretreatments due to its relative shallower depth of treatment from the surface, 5-10nm. Ozone is not produced. When procuring flame treatment burners, compare ribbon vs. drilled port and the benefits of zero balanced regulators.
Cold gas plasma, also termed “low pressure cold gas plasma,” is conducted in an enclosed evacuated chamber, in comparison to atmospheric (air) surface pretreatment methods. Industrial-grade 100 percent oxygen gas (O2) is commonly used. Gas is released into the chamber under a partial vacuum and subjected to an RF electrical field. It is the response of the highly reactive species generated with the polymers placed in the plasma field, on inner conductive electrode aluminum shelves or cage, breaking molecular bonds that result in cleaning and chemical/physical modifications (including an increase in surface roughness which improves mechanical bonding). A significant benefit of cold gas plasma processes is the removal of hydrocarbons, thereby eliminating solvent cleaning. Atmospheric pretreatments do not remove/clean all poly-aromatic hydrocarbons, so solvent cleaning (prior to pretreatment) may be necessary. Cold gas plasma treated parts tend to demonstrate the highest quality treatment and longer shelf-life. Criticism arises from competitive equipment manufacturers.
Selecting a pretreatment method
Recognize that each method is application-specific and possesses unique advantages and potential limitations. Consider the following factors:
- Polymers react differently to oxidation processes. The type of polymeric substrate and its end use performance requirements are critical in determining the selection of pretreatment method.
- Is the substrate (product to be treated) conductive? For example, unassembled plastic electronic connector bodies, without metal contact pins, can be treated electrically. Whereas assembled connectors may experience electrical arcing problems.
- Part geometry. Flat surfaces are easily treated compared to deep recesses, extreme tapers and other irregularities. Wetting tests are difficult to conduct in small areas and on heavily textured surfaces.
- Material handling automation. Conductive belts and chains may cause electrical arcing with classical electrical corona discharge and spot treaters. Alternatively, consider using flame, cold gas plasma or low potential atmospheric plasma.
- Avoid overtreatment. Excessive plasma-oxidized surfaces may deleteriously affect downstream assembly processes, such as poor heat sealing/welding.
- All pretreatment equipment is NOT created equal! Examine the quality of constructed systems in action. For electrical treatment processes, observe the uniformity of the plasma discharge; for flame treatment, consider the differences between ribbon versus drilled port burners and combustion system components; for cold gas plasma, examine the quality of the chamber construction, electrode shelves and particularly the manufacturer of the vacuum pump.
Plasma-treated surfaces age at different rates and to varying extents relative to factors with the surrounding environment. Variation in temperature and humidity can affect the quality and uniformity of treatment. The degree or quality of treatment is affected by the cleanliness of the plastic surface. The surface must be clean to achieve optimal pretreatment and subsequent ink adhesion. Surface contamination – such as silicone mold release, dirt, dust, grease, oils and fingerprints – inhibits treatment.
Material purity also is an important factor. The shelf life of treated plastics depends on the type of resin, formulation and the ambient environment of the storage area. Shelf life of treated products is limited by the presence of materials such as antioxidants, plasticizers, slip and antistatic agents, colorants and pigments, stabilizers, etc. Exposure of treated surfaces to elevated temperatures increases molecular chain mobility – the higher the chain mobility, the faster the aging of the treatment. Polymer chain mobility in treated materials causes the bonding sites created by the treatment to move away from the surface. These components eventually may migrate to the polymer surface. Therefore, it is recommended to bond, coat, paint, print or decorate the product as soon as possible after pretreatment.
Digital UV inkjet is revolutionizing printing and decorating of 3D plastic products. Inkjet is a complex, multi-variable process. There are significant challenges for printing on tough-to-bond polymers, which extend beyond OEM standard inks and printers. Low-viscosity inks jetted on low surface energy plastics are chemically and physically incompatible and frequently require pretreatment to solve adhesion problems. While UV curing technology has significant benefits, improperly cured inks are hazardous. Understanding the intricacies of inkjet printing and material surface science enables custom solutions to meet the demands of new applications.
Scott R. Sabreen is founder and president of The Sabreen Group, Inc., an engineering company specializing in secondary plastics manufacturing processes – laser marking, surface pretreatments, bonding, decorating and finishing, and product security. Sabreen has been developing pioneering technologies and solving manufacturing problems for more than 30 years. He can be contacted at 972.820.6777 or by visiting www.sabreen.com or www.plasticslasermarking.com.