A Basic Introduction to Formulating Thermoplastic Compounds.
In order to achieve the right aesthetics and mechanical performance for any plastic article it’s important to design the most cost- effective formula.
The first task is to choose the most suitable polymer for the application. The properties of Thermoplastics can be very diverse, and pricing can vary tremendously, especially for some of the more exotic high-performance polymers.
Thermoplastics can generally be classified into three groups, Commodity grades, Engineering grades and High-Performance grades.
Commodity grades- Are thermoplastics made in vast quantities for applications where exceptional mechanical properties are not generally required. E.g., Milk bottles, Food Packaging, Carrier bags, Household products etc. Costs for these types of polymers are generally cheaper when compared to engineering plastics and high-performance grades.
Polyethylene, Polypropylene, PVC, Polyethylene Terephthalate(PET) and Polystyrene are some good examples of commodity grades.
Engineering grades- The mechanical properties of these types of thermoplastics are generally superior to commodity grades. Certain grades can handle heat and stress well, when in use, and because of their high mechanical strength can often be used as metal replacements. The Automotive, Aerospace, Building Construction and Electronics industries all benefit from the use of these types of polymers.
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Acrylonitrile Butadiene Styrene (ABS), Polycarbonate, Polyamides (Nylon) are some of the more common engineering grades.
High-Performance grades- Thermoplastics that fall into this group have a variety of exceptional properties. Their ability to withstand high temperatures, chemical attack and their excellent mechanical strength make them ideal candidates for use within the Aerospace, Automotive and Electronics industries. Some good examples of high- performance grades are Poly Ether Ketone (PEEK), Polysulfone (PSU), and Polyphenylene Sulfide (PPS)
When the polymer with the most suitable properties has been selected for the end article, further modifications can be made during a compounding process. This often involves the addition of colourants and or additives and fillers to achieve the desired look and finished product properties.
When it comes to colouring thermoplastics compounds, care must be taken to ensure the colourants selected for the formula, are compatible with the chosen polymer, and that their properties such as heat stability, light and weather fastness are all at the desired level to suit the end application.
Performance enhancing additives can also be compounded into the polymer of choice to further enhance their outdoor UV performance, Antistatic properties, Conductivity, Surface Frictional properties, Antimicrobial, Flame Retardancy, Heat Stability, etc.
Fillers are often added to thermoplastic compounds to drastically modify the mechanical/performance properties or aesthetics required for the end article. The most commonly used fillers are; Magnesium Silicate (Talc) Calcium Carbonate (Chalk) Mica, Glass fiber or beads, Aluminum Trihydrate, Magnesium Hydroxide, these last two fillers are specifically used to improve flame retardancy. The other fillers are mainly used to improve stiffness, mechanical strength, surface hardness, or change visual appearance.
Fully formulated compounds tend to be used for applications where the technical properties are paramount. They just need to be processed as supplied under the recommended processing conditions. Flame retardant compounds, Talc/Chalk/Glass filled compound and Conductive compounds are some of the most common grades supplied to the end converters.
There is another process whereby natural thermoplastic polymers can be modified and that’s by using a masterbatch. A masterbatch is a concentration of colourants, additives or a combination of both that’s added into the natural polymer during processing at a specified addition rate. Whilst this method is often cheaper than the fully compounded route, the end converter must ensure that he uses the recommended addition rate, and that their equipment is fully capable of achieving a homogenous mix. Failure to achieve this can lead to mechanical failures and inconsistencies in the colour appearance.
Polymer chemists, Plastic engineers and experienced Colour Chemists should always be consulted when designing plastic applications in order for the right solution to be reached.
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Author:
Pravin S Mistry - Global CEO of PREA Ltd and International Polymer Consultancy - Plastics, Composites, Rubber, Adhesives, Polyurethane... focuses globally on:
He personally has worked for over 40 years in the industry - UK, USA, Mainland Europe, Asia... in the polymer other manufacturing industries as Divisional Managing Director and CEO for multinational companies. Early career roles include Operations Director, Technical Director, Technical Quality Manager , Chemical Engineer, Laboratory Technician.