Some high-performance plastics pose a problem during printing — their low surface energies make high-quality, reliable printing challenging. So, how can manufacturers address this challenge, and what can we learn from real life applications?
Intertronics
Plasma discharge on PTFE
Here, Kevin Brownsill, head of Technical: Learning and Development at adhesives specialist Intertronics, discusses the use of plasma surface treatment for printing onto low surface energy plastics.
Wetting is a prerequisite of adhesion, so a good understanding of the factors that influence wettability is important for a printing project to be successful. In particular, the substrate’s surface energy must be greater than that of the ink for the ink to wet to the substrate — the same applies to adhesives and coatings. Plastics such as polyethylene, polypropylene, and polytetrafluoroethylene (PTFE) have low surface energies (typically between 20 and 40 mN/m), making them difficult to bond.
When a substrate’s surface energy is too low for an ink to sufficiently wet to it, plasma can be applied to alter the chemical groups on the surface, increase wettability, and create bonding anchors for the ink. Plasma pre-treatment avoids the use of chemicals (and the associated health & safety and environmental concerns), and can be installed in-line with no appreciable additional production time.
How does plasma surface treatment work?
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Print image before and after plasma treatment RGB
Atmospheric plasma technology involves turning a small number of gas molecules into energetic electrons and ions, which can accelerate to high energy and collide with gas molecules to produce short lived chemically active species. Plasma can initiate several physical and chemical processes, which will alter chemical groups on the surface, increase wettability, and creating bonding anchors for the material.
In addition, plasma can remove organic, inorganic, and microbial surface contaminants that form due to exposure to air — such contaminants can reduce the quality of ink adhesion by forming a weak intermediate layer. Due to both surface modification and cleaning, inks, paints, or adhesives will wet the surface better.
Case study: achieving reliable inkjet printing onto PTFE
PTFE, commonly known as Teflon, is a chemically inert material with an extremely low surface energy. As a result, it is difficult for materials to wet, and almost no inks, coatings, or adhesives adhere to it without some form of surface pre-treatment.
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One interesting example of printing onto PTFE comes from Relyon Plasma’s work with a customer in Germany. To begin with, the PTFE’s low surface energy made it difficult for the customer to achieve a clear, abrasion-resistant marking of a Data Matrix code. The company trialled the piezobrush PZ3-i Plasma Surface Treatment device, aiming to print readable markings with good resistance to temperature and friction onto PTFE.
Five different fabrics were tested to validate the effectiveness of plasma pre-treatment with the PZ3-i. For all the tested fabrics, the surface energy was less than 30 mN/m without pre-treatment and between 38 – 42 mN/m after plasma treatment.
Following surface treatment, the fabric was marked with an individual, numerically ascending, 12 mm wide Data Matrix code using a KEYENCE MK-G1000SA continuous inkjet printer and KEYENCE MK-13 test ink.
Reliably marking PTFE
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Effective range of plasma treatment RGB
Plasma pre-treatment was found to be essential to reliably mark the fabric. Below a surface energy of 38 mN/m the printed image appeared faded, was not abrasion resistant, and the DMC could not be read. With plasma pre-treatment, the readability of the DMC was good, while resistance to temperature and friction was satisfactory. The customer achieved a reliable process with good traceability, and found that the PZ3-i could easily be integrated into the process.
This is a great example of where surface pre-treatment enables a bond where it would otherwise be impossible. The small-scale nature of the PZ3-i meant the company could achieve a processing performance level that would otherwise only be possible using larger and more expensive equipment, or the use of hazardous chemicals.
While low surface energy plastics like polyethylene, polypropylene, and PTFE are typically difficult to print onto, plasma surface treatment may be an option to trial to improve reliability and quality.