Dr. Richard Marsden – Chief Chemist, Linx Printing Technologies Ltd
Coding onto PE & PP plastics – what do I need to know?
Polyethylene (PE) and polypropylene (PP) are the most important members of a group of polymers called polyolefins.
PE is the most widely used polymer in the world (around 80 million tonnes per year), whilst polypropylene (PP) accounts for over 50 million tonnes per year.
These polymers are used in applications spanning a multitude of industry sectors such as food packaging, automotive, pharmaceuticals, cable, wire & pipes, etc. They are used widely in industries where CIJ printers are used for coding and marking.
Their characteristics are such that effective coding and marking onto these plastics can be a challenge for manufacturers.
Polyethylene and polypropylene have chemical structures containing only carbon and hydrogen:
These structures result in a very low polarity material, which means it is difficult to achieve strong associations with other chemicals (e.g. those in a printing ink). In other words, it is hard for ink to ‘grab onto anything’ on the surface of the polymer, resulting in poor adhesion of the ink. Additionally, drops of ink may not wet the surface efficiently, giving poor code legibility or transfer of ink onto other products.
To some extent, these issues can be overcome by treatment of the surface of the PE or PP by methods such as heat, corona or plasma. The surface energy of the substrate is increased from about 30mN/m to >40mN/m by these processes. The surface tension of the ink jet ink (26-28mN/m) is sufficiently lower than this, and hence the ink is able to achieve good wetting and adhesion.
However, this requires an additional process on the production line and is often not convenient or appropriate. Coding ink manufacturers therefore need to develop inks that will adhere to both treated and untreated PE and PP.
PE/PP are highly versatile as they can be easily manipulated during manufacture to produce a wide range of forms, from thin film to rigid components, by processes such as blow moulding, injection moulding, rotational moulding, and extrusion.
PE is widely used across food packaging applications such as film, as well as non-food use such as bottles & drums, pipes and cable jackets. PE and other polyolefins are increasingly being used in place of PVC in cable applications.
PP is also widely used across multiple industry types, including rigid packaging (crates, pails, CD/DVD boxes, etc), automotive components, pipes, textiles, food packaging films, bottles, caps.
PE and PP for food packaging
It is important that food packaging has the right combination of properties; both PE and PP have properties which can fulfil the following functions:
• Easy to process into various packaging forms
• Helps to keep food fresh (barrier to air, moisture)
• Light and strong, protects food
• Convenience – easy to open and lightweight
• Resistant to acids and other chemicals
• Low intrinsic level of contaminants which could migrate into the food
• Cost effective
PE for cable
Coding onto PE cable presents a potential problem of codes transferring from one part of the cable to another when the cable is wound on the reel. If the ink does not adhere or dry effectively then this code transfer may occur, especially in high speed cable winding applications. The code may therefore become illegible and the cable either reworked or scrapped, resulting in additional costs to the manufacturer.
Additional considerations include the need for the code to stand out against the colour of the cable, so a choice of black, white or other colours is important for the cable manufacturer. Add to this the need for the code to resist chemicals during manufacture or end-use, plus regulatory drivers which affect specific chemical use in the printing inks, and the process of selecting an appropriate coding ink becomes complex.
Drivers and trends in use of PP and PE – what you need to know about coding
A myriad of pressures, both environmental and legislative, are driving the use of PP and PE in coding and packaging applications. These drivers affect both the plastic used in the product and packaging, as well as the ink used to code onto them. In addition, there are standards which affect the materials in the printers that deliver the printing ink.
There is a trend towards the use of polyolefins which are seen as having a better environmental profile than some plastics, e.g. PVC and Polycarbonate. The manufacture, use and end of life fate of polyolefins generally involve less hazardous chemical substances. PE and PP can also be widely and easily recycled; they are thermoplastic materials, which means they can be melted unlimited times and made into new products.
Neither PE nor PP are easily biodegradable (a challenge for all the commonly used plastics), although additives are now available which can accelerate compostibility. There are routes under development to manufacture PE and PP from renewable (non-petroleum based) raw materials, although this is not currently cost effective enough to be universally adopted.
Legislation / Standards
A range of standards and directives need to be considered, which can impact the plastic, the coding inks and also the machinery that delivers them.
Food packaging – a mixture of legislation and guidelines exist which restrict the use of certain chemicals in inks used for coding and marking in food packaging applications, including:
• Food Contact Materials Framework Regulation (EC) 1935/2004
• Swiss Ordinance SR 817.023.21, which has gained widespread acceptance by the food packaging industry; this includes a positive list of substances which can be used in food packaging inks
• EuPIA exclusion policy – defines substances which should be excluded from printing inks
Heavy metals – use in coding and marking is covered by:
• EU Packaging directive 94/62/EC
• EU RoHS directive 2011/65/EU – this restricts the use of certain hazardous substances in electrical and electronic equipment, such as coding and marking equipment
Electronics industry standard – this covers the contents of the ink used for coding in the electronics industry, as well as the material being coded such as cables:
• IEC 61249-2-21 standard for halogen free electronics components. In the event of a fire, a halogen-containing plastic material releases poisonous and corrosive gases such as hydrogen chloride, which also forms hydrochloric acid when it comes into contact with water. Halogen-free cables, sometimes called low smoke zero halogen (LSZH), reduce the amount of toxic and corrosive gas emitted during combustion, and so may be used in poorly ventilated areas such as in aircraft, rail carriages, and track signal wires which may run into and through underground tunnel systems. The IEC standard defines halogen free as containing <900ppm bromine & chlorine. PVC cable does not comply with this standard, but PE, PP and other polyolefins do. The coding ink must also comply with the standard.
Implications for coding and marking
So, PE and PP are becoming increasingly popular for food packaging applications and in the cabling industry. But due to their chemical makeup, coding onto them with ink jet printers can be tricky – codes may not adhere correctly or may become illegible through code transfer.
This doesn’t mean that coding with ink is inappropriate for PE and PP – in fact far from it. Both materials can be formed into almost any shape, so non-contact coding technology such as Continuous Ink Jet (CIJ) is ideal as the ink is literally ‘jetted’ onto the surface. Irregular shaped products can be clearly coded, and from any angle.
Most manufacturers of CIJ printers supply a wide range of inks that can adhere to many materials, including PE and PP. Each material will have its own set of unique properties, and it is the skill of the chemist in creating an ink that will ensure the code stays in place, as well as meeting the multitude of environmental and legislative demands on the ink formulation. With new materials being developed to better meet these directives, manufacturers are continually developing new inks to stay ahead of the game.
Dr. Richard Marsden – Chief Chemist, Linx Printing Technologies Ltd
Dr. Richard Marsden has a degree and Ph.D. in Colour Chemistry from the University of Leeds, followed by postdoctoral fellowships in Synthetic Organic Chemistry at McMaster University in Canada and Hull University, UK.
He first worked on CIJ inks for binary systems with Elmjet, before joining Linx Printing Technologies in 1990. He is a chartered chemist and member of the Royal Society of Chemistry.