Expanded polystyrene, or EPS, is an essential material in industries that range from construction to packaging. Depending on the purpose it is used for, EPS may need special characteristics such as anti-static or flame-retardant properties or a specific colour. This can be achieved by introducing additives, often in very small quantities. Horizontal Ploughshare® Mixers have proved to be ideal for this complex process engineering task.
EPS is manufactured using a polymerisation process which involves making styrene monomer in liquid form into polymers. The blowing agent pentane is then added to the polystyrene beads that are produced in this process. The result is EPS granulate which can be turned into foam. The hard-plastic beads, which are filled with pentane, are transformed into foam by applying water vapour at a temperature of around 100°C. During this process, the blowing agent evaporates, and the compact EPS beads expand to up to 50 times their original volume. They can then be manufactured into panels, blocks and also more complex shapes.
Processing in the Ploughshare® Mixer
EPS comes in many different forms. After the polymerisation phase, additives can be used to give it specific properties. Horizontal Ploughshare® Mixers have proved to be ideal for this complex process engineering task. These mixers operate on the basis of the mixing and fluid process developed by Lödige Process Technology, which produces excellent mixing results.
The central feature of the mixer is a special arrangement of Ploughshare® Shovels on a horizontal shaft. They rotate in a cylindrical mixing vessel which is positioned horizontally. The size, number, position, shape and circumferential speed of the shovels are carefully coordinated to create a three-dimensional movement of the materials during the mixing process. This means that the materials are continuously mixed by the mixing tools to prevent dead spots or areas of low movement.
To achieve the desired product characteristics, EPS beads are coated with additives after fractioning. Additives are applied to EPS for reasons that include:
- Producing special flow characteristics
- Creating flame-retardant properties
- Adding colour
- Producing anti-static properties
Processing the plastic beads presents particular challenges. Only very small quantities (less than one percent) of additives in fine powder or liquid form are introduced. Nevertheless, it is essential to achieve a homogeneous mixture and a consistently high product quality.
Another process engineering problem is the use of pentane as a blowing agent in the production process. Firstly, the gas atmosphere during the design and construction of the system means that the requirements of ATEX directive 2014/34/EU must be met. Secondly, the mixing process, which is intensive but causes no harm to the product, must be performed without any energy input. Even temperatures above 40°C can result in the blowing agent expanding the beads.
Introducing additives
Lödige Mixers can process EPS beads in batches or continuously.
The powdered and/or liquid additives can be introduced into the moving product in several different locations. In both mixing systems, tools with a specific design ensure that even the smooth surfaces of the EPS beads come into full contact with the additives. This mixing process achieves excellent material adhesion. The high mixing quality produced by Lödige mixing machines makes it possible to achieve extremely uniform distribution even if the additives make up less than one percent of the total mixture. At the same time, the mixing process achieves high throughput rates with low retention times and short cycle times, which ensures that production is cost-effective.
The process can be performed both in the mixer for batch or continuous operation under inert conditions or with suitable measures to ensure the required level of explosion protection. Lödige mixers have the EU type examination certificate for operation in gas and dust atmospheres zone 0/20 indoors and are approved for operation in potentially explosive atmospheres.
Future-proof systems
The use of Lödige mixing systems prevents the risk of costly interruptions to production, particularly in the field of bulk plastics, for example caused by inconsistent product properties or fluctuations in quality. As a solution and service provider, Lödige offers not only mixing machines but also accompanying systems for supplying and discharging products to create a complete solution. In addition to the mixer, this includes, for example, weighing, dosing and transport systems and discharge containers. The complete solution is designed to meet the customer’s individual requirements and to operate reliably for many years. Lödige has extensive experience in this area. It has already installed 90 mixers and systems for processing EPS in 17 countries.