CREATING THE PERFECT COATING

Every year, 150 million tonnes of plastic packaging is made and then disposed of. Plastic is hard to recycle, so most of it is burned, buried or remains in the environment. This causes significant problems. 

Some companies, such as Nestlé and L’Oréal, are leading the transition to primarily paper packaging. However, paper is not waterproof. This is where Archipelago’s Powerdrop coating technology and process can help. Powerdrop can apply coatings to 3D paper shapes, in order to replace plastic packaging and maintain recyclability.

Archipelago’s aim is to eliminate plastic. The company is recognised for its ethnic diversity. As a team, Archipelago aims to improve the Powerdrop coating process. The business’s combined knowledge and skills development, together with enhancement of different techniques, are all key to meeting customer needs.

Archipelago’s pioneering Powerdrop coating can be used on flat surfaces and – importantly for primary packaging – 3D objects. 

At the core of the Powerdrop process is the drum – a collection of precise nozzles built into a hollow cylinder. The coating material is loaded into the nozzles from the outside of the drum, using the filler. 

The Powerdrop drum rotates and the coating material is jetted out as drops using the blower component. The jetted drops of coating material are applied to the surface and remain there. This means that waste, drift and overspray are minimal.

At the start of every coating project, Archipelago measures the physical properties of the coating liquid and the target object.

In order to achieve the best coating, the viscosity at low shear stress (<10Pa) should be less than 200mPas. In addition, the surface contact angle of the coating liquid – on the material to be coated – should be ≤90˚.

The ability of a liquid to stay in contact with a solid surface, when brought together, is known as wetting. This is the consequence of interactions between molecules. Both the liquid’s surface tension and roughness of the substrate affect wetting. 

The term ‘wettability’ describes a liquid’s propensity to disperse over a surface. The liquid’s wettability can be determined by performing contact angle measurements. Generally speaking, surfaces with high wettability – or good wetting – are those where the liquid prefers to spread out (contact angle <90˚) and produces flatter droplets, making surface wetting particularly favourable. In contrast, certain surfaces are known as low-wettability surfaces (contact angle >90˚) and are defined as those where liquid tends to ‘ball up’ and wetting of the surface is unfavourable. 

The ability of a liquid surface to behave in the same way as a stretched elastic sheet is known as surface tension. An example of this phenomenon is the roughly spherical shape of soap bubbles and tiny liquid drops. Another example is the ability of some insects to stand on the surface of water.

The primary factors influencing surface tension are the forces of attraction between the particles in the liquid and the gas, solid or liquid that is in contact with it. 

The liquid particles are drawn together by a variety of intermolecular forces, including Van der Waals forces¹. The ratio of the surface force ‘F’ to the length ‘d’– along which the force acts – is known as surface tension. It is represented by the Greek variable gamma.

The unit of measurement for the wetting of a solid by a liquid is the contact angle or q. Geometrically speaking, it is the angle that a liquid forms at the intersection of a liquid, vapour and solid at the three-phase boundary point. A famous equation – the Young’s Equation – characterises the equilibrium at the solid-liquid-vapour, three-phase interface² 

Contact angles are measured as a way of achieving accuracy. Measuring the contact angle of the coating liquid provides an understanding of the liquid’s behaviour and how it spreads on the substrate

The ACA can be measured using a needle and syringe. A liquid droplet is placed on the surface, gradually increasing the volume of the liquid. The baseline of the drop will start to advance on the surface as more liquid is injected into the droplet. At the point where the contact angle stops growing and stabilises, the ACA can be measured. 

When liquid is extracted from a static droplet, the RCA is made in the opposite way. The baseline will not change at first, but the drop volume and contact angle will. The contact angle will eventually stop decreasing and stabilise. When the baseline length starts to recede or lowers – and the contact angle stabilises – the RCA can then be measured. 

Archipelago’s Powerdrop is a non-contact, low-waste coating technology. The company’s expertise is in defining and implementing end-to-end, non-contact, low-waste industrial coating. Today, Archipelago’s focus is on applying waterproof coating layers onto 3D paper-based containers. Its aim is to enable plastic food packaging to be replaced with sustainable, recyclable paper.

Powerdrop is a Trademark of Archipelago

References

1. A.Z. Jones, Surface Tension – Definition and Experiments, 2 February, 2020 (online). www.thoughtco.com/surface-tension-definition-and-experiments-2699204
2. Tommi Huhtamaki, Surface-wetting characterisation using contact-angle measurements, Nature Protocols, volume 13(7), p.1521–1538, 2018