Plastic Compounding and its Process Value
Plastic compounding is the process of adding functionality and value to polymers or polymer systems through melt blending or mixing of additives into a polymer matrix or melt mixing two or more polymers together to make an “alloy”. Melt processing is commonly used in thermoplastic polymers.
Why is it necessary for a plastic material to go through a compounding step? Many applications need enhanced or tailored properties to meet specific requirements…a compounding step where additions can be made helps to satisfy this need.
These additions are made in such a way that they are properly dispersed throughout the material in order to maximize their benefits. This process changes the physical, electrical, thermal and/or aesthetic characteristics of plastic. There are two types of mixing that need to occur during the compounding process. The first is distributive mixing which evenly distributes or spreads the ingredients throughout the batch to create a homologous blend. The second is dispersive mixing, which makes sure the ingredients are well incorporated into the batch.
Think of compounding in terms of baking. A classic cookie recipe calls for salt to be mixed with flour and other dry ingredients before blending with butter, eggs, etc. If the salt is added according to the recipe, it does what it is designed to do – contribute to the overall flavor. The salt will need to be evenly distributed and mixed throughout the batch of cookies creating a homologous batter. However, if that small amount of salt is instead added at the end, it won’t get blended into the dough well, resulting in quite a bit of taste variation between cookies – and even between bites.
Similar to how the ingredient salt plays a part in the overall taste of cookies so do the additives used during the compounding process. By incorporating a variety of additives, fillers and reinforcers, a range of properties can be achieved to fit your performance needs. Consider how an experienced compounder of styrenic thermoplastic elastomers (TPEs) can enhance various features of the material:
- heat/light stabilization
- surface tack
- flame retardance
- electrical/thermal properties
- vibration damping
- as well as many others
Technique and a strong knowledge base are important in achieving skillfully compounded plastics, bringing to life a robust matrix of properties and features. Star Thermoplastics has been a player in the compounding of thermoplastics for over 20 years.
We’ve included a few other terms you should be familiar with when discussing plastic compounding:
- Shear – is the force of material and energy resulting in the material moving against itself. Higher shear is found in highly filled alloys and alloys with a lot of rubber. High shear products can build up quite a bit of energy in the extruder. They can also affect the quality if not properly managed.
- Residence time – known as the RPM or the rate the screw turns. By adjusting the residence time the operator can control the heat exposure, the mixing area, and the shear of the material
- Volatiles – in some material and on some machines, as the polymer matrix is heated certain ingredients and residual moisture will begin to vaporize creating what is called “volatiles.” These vapors escape from the matrix through open vents located about 80% of the way through the extruder. The vents allow the volatiles to be removed from the mix. Despite having the name “volatiles,” they are actually quite safe and stable.
- Volume – the amount of free space in the screw and barrel in an extruder will dictate the amount polymer that can be produced per hour.
Stay tuned for our next blog posts where we will dig deeper into the anatomy of a compounding line, and details of how twin screw extruders are used in the compounding process.
Questions or comments on plastic compounding, leave us a comment.
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