Sourcing Plastic Material

What is the difference between Material Distributors & Material Compounders?

Material Distributors provide an ‘off-the-shelf’ solution, usually supplying branded products manufactured by a range of leading feedstock manufacturers. They can offer a wide range of polymer types, (e.g., Nylon 6,6, Acetal, ABS) but each of these will have a fairly standard formulation and be predominantly uncoloured (Natural). It is possible to buy some polymer grades with a limited range of added components such as flame retardants and glass fillers at a set percentage.

Visit here to contact the leading UK companies that can provide you with advice and quotes for your polymer requirements.

Material Compounders can formulate your material to an exact ‘recipe’. They take a standard feedstock and run it through an extrusion line along with any selected additives. Coloured pigments are the most common addition, but a huge range of additives are available that can improve the processing characteristics, appearance or properties of the final product.

The extrusion (compounding) line ensures that all components are thoroughly blended before being pelletised.

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How do I specify the right plastic material?

When designing a product, you should give as much consideration to the properties the product will need to have as you give to its physical form. Selecting the correct material can make a major contribution to meeting these requirements.

When selecting a material, considerations should include:

• The application and what associated physical attributes needed. For example, is weight critical? Will it need to be rigid or flexible? Will it be subjected to high pressure or temperature fluctuations?
• How long will the component need to remain functional and what environmental factors (for example UV light) might it be subjected to?
• Is the part purely structural, or is physical appearance a consideration? As well as colour, surface textures such as a grained effect can be introduced, or post-moulding finishes such as painting, chrome plating, metalising or pad printing can be added.
• Are there any binding regulatory requirements that need to be considered? For example, is product traceability critical, does it need to be food-safe or suitable for young children to handle?

What are thermoplastic materials?

Thermoplastics are more widely used within the moulding sector than thermosets, being used to produce everything from washing up bowls to automotive components. They are characterised by how they are processed, meaning they are heated until molten and then formed and solidified within a mould through cooling.

There are many types of thermoplastic materials, each grade offering differing properties to a finished component. They can be roughly grouped into ‘commodity’ and ‘engineering’ grades.

Common commodity grades are polypropylene (PP), polystyrene (PS) and polyethylene (PE, also known as polythene). These grades are called commodity grades because they are generally used for mass-produced price-sensitive products (they are considerably cheaper than engineering grades), are easily processed and can be readily recycled.

Here are some examples of materials and some typical applications:

Engineering grades typically offer a particular characteristic, and as their name suggests, are often used where a component needs specific properties. For example, nylon grades (Polyamides) are readily machined and are resistant to impact or shock. Polycarbonate is ideal for the production of lenses due to its strength and clarity, while Acetal is used for its structural rigidity, making it ideal for metal replacement.

Some thermoplastic grades can substitute thermoset materials that are harder to process, such as rubber. Examples are TPE and TPU, which are processed on standard injection moulding machines but have the flexibility associated with an elastomer.

Here are some other examples of commonly used plastic grades and typical applications:

What are thermosetting materials?

Thermosetting polymers rely on a chemical reaction that permanently solidifies them within the mould tool. Once this reaction has taken place, they cannot be reprocessed, but offer unique characteristics such as resistance to high heat levels or chemical attack (saucepan handles are a typical application that benefits from heat resistance). Some grades are also excellent electrical insulators, so can are frequently used for manufacturing household switches and sockets.

What are colour masterbatches and plastic additives?

Once the right material is identified, you can add further refinement by considering additives that can be introduced.

In the case of Colour Masterbatches, these can be used to provide an exact colour match (RAL, Pantone reference etc.) by adding to natural raw material at a small percentage rate. This can be a better solution (and more economical) if a product has to be manufactured in a wide range of colours, as the natural base material can be purchased in bulk, rather than having to buy multiple batches of compounded material.

There are two forms of masterbatch – universal and polymer specific. This relates to the masterbatch's composition, which will be a concentrated colour pigment and a 'carrier' material that helps to dilute (to allow dosing accuracy) and disperse the colour during processing.

Universal grades are designed to work with a wide range of base polymers. In some cases, however, they won't be suitable. For example, there might be an impact on the final product's strength due to differing optimal processing temperatures for the base material and the carrier.

Polymer Specific Masterbatch is formulated, so the carrier is a similar polymer type to the primary raw material. This will minimise changes to the properties of the product and probably aid with dispersion during processing. The negative is that the masterbatch probably won't be an 'off the shelf' solution and will cost more to buy.

Plastic Additives can enhance structural properties, or add characteristics such as resistance to UV light, antistatic or antimicrobial properties, or even fluorescence. There are now also additives available that can allow standard polymers to become biodegradable.

Unlike compounded materials, colour masterbatches and plastic additives are directly dosed into the processing machines throat at a set percentage. The plasticising unit (screw and barrel) of the machine combine the base material and additives during the standard machine screw-back (recovery) phase.

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To dose the masterbatch or additives at the required percentage, specialist additive feeding or blending equipment is used.

What are purging compounds?

Purging compounds are designed to efficiently remove a polymer from a processing machine’s plasticising unit. For example, before a tool change on an injection moulding machine, or material change on an extrusion line, the machine is first run dry to remove as much molten material as possible.

Empty machine hopper > Mould or extrude as much material as possible from the plasticising unit > Use a purging compound to remove any ‘hung-up’ material left in the system > Move on to the next material to be processed.

The purging compound is designed to scrub away or dissolve residual material, shortening the time taken and amount of material that would otherwise need to run through the system before a new production run can start. Colour changes are a good example of how a change-over can be sped up by using a purging compound.

Both liquid and solid (granular) purging compounds are available, with grades designed to work best with particular material grades. In the case of compounds designed for injection moulding, some are also ‘mouldable’, meaning they can also be used to clean a tool’s hot runner system.

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What are biodegradable polymers?

Biodegradable polymers should not be confused with recyclable grades. They are designed to break down into basic and harmless chemical compounds at the end of their serviceable life. To be classed as fully biodegradable, a polymer must be reduced to carbon dioxide, water and biomass within a defined period. The standard quoted (EN 13432:2000) is most often 90% breakdown within 6-months.

The environmental conditions under which each material will degrade varies greatly. The necessary conditions determine the designation of the materials, e.g.:

• Water Soluble – The material will readily degrade when becoming wet.
• Home Compostable – These materials will degrade in the conditions typically found in a home environment.
• Landfill Compostable – These extensive facilities will allow the breakdown of material for more extended periods and at higher pressures and temperatures than domestic environments.
• Industrial Compostable - Materials in this category require a rigid set of environmental conditions to degrade.

The most popular grade currently in general use is PLA, an Aliphatic Polyester derived from fermenting biomass products such as sugar cane or beet, corn etc. This material will fully degrade, but only by using an industrial composting process with specific processing parameters and duration. Other products are water-soluble and will readily degrade in a household composter. An example would be milk protein-derived products.

Typical applications for biodegradable polymers involve single-use products, particularly ones that can enter the environment, e.g. through littering. Water-soluble grades are commonly used for their functionality, e.g. washing pouches that need to quickly release their contents in a washing machine or dishwasher.

Downsides of these polymers include:

• Not all are suitable for recycling and can cause problems if they enter the system.
• Some materials have a relatively short shelf life, so are not suited to extended periods of storage prior to use unless degradation is triggered by environmental conditions such as UV light or moisture.

When specifying a biodegradable material, consideration should be given to both the products application and its end of life outcome. For example, a single-use plastic cup will only need a relatively short life-span once used, and because it may end up as litter, should ideally degrade with extended exposure to and moisture.

Using additives to make standard polymer grades biodegradable

There are also specialist additives that can be formulated into standard polymer grades such as petrochemical-derived PP or PE. The main reason most plastics aren't readily degraded relates to their closed structure and solid form, meaning that water and/or microbes cannot penetrate. These additives are designed to remain dormant for a period of time, but then open up the structure of the plastic at the appropriate time in a product's life-cycle. This is typically achieved through a specific type of microbial attack, for example, the high concentrations of bacteria found at landfill sites. Once the additive has broken down, pores are left in the plastic material, allowing microbes to penetrate and attack the structure.

In everyday use, the additive is not subjected to high concentrations of these microbes, and can often be readily recycled. Only when entering the waste disposal system will biodegradation commence.