The third installment in a series of articles on the processing and use of Premix’s electrically conductive plastics concentrates on sheet – one of the biggest end-use applications for conductive plastics. Electrically conductive sheet will be further thermoformed and processed into electrostatic protective packaging for electronic components. In most cases, the base polymer for carbon black compounds is polystyrene. For special applications, other base polymers such as ABS or polycarbonate can also be used.
Electrical properties without sacrificing mechanical properties
When carbon black is compounded to the polymer matrix, it has a significant impact on the polymer’s mechanical properties. The introduction of carbon black increases both hardness and flexural modulus, while at the same time decreasing elongation and tensile strength. This has to be taken into account in material development, since the goal is not to secure electrical properties at the expense of mechanical properties. That’s why the compound formulation must always contain special modifying additives that will recover or exceed the original properties of the base polymer. The level of modification depends on the application and the requirements set for the end product.
Table 1. Comparison of mechanical properties between a carbon black filled HIPS compound and natural HIPS.
Product design – choose the right approach
Mono- or co-extrusion? The end product play a critical role in choosing the right sheet construction approach. The required resistance levels for ESD packaging are described in the EU standard EN 61340-5-3:2010 “Protection of electronic devices from electrostatic phenomena – Properties and requirements classifications for packaging intended for electrostatic discharge sensitive devices”. The below table illustrates how packaging can be classified according to the resistance range of different packaging materials and structures.
Table 2. Material classification according to the resistance range of different packaging materials and structures.
Monolayer construction is usually the preferred approach, as the sheet will possess both surface and volume conductivity. For example, low volume resistance is essential in applications where electrostatic field shielding is required.
Co-extrusion is an effective choice if you are looking to reduce materials costs. However, you need to pay careful attention to the thickness of the conductive layer in order to prevent the carbon black network from breaking during the thermoforming process.
The thickness of the sheet determines the choice of conductive compound grade. The most delicate compounds are used in thinner sheets (up to 1 mm). In coextruded HIPS-based carrier tape sheets, total sheet thickness is 300-350 microns at its lowest. Conductive layer thickness, on the other hand, can be as low as 35 microns. In thicker sheets (up from 1 mm), you can choose between compounds and concentrates. In most cases, compounds are used as such or with small polymer dilution (5-20%), whereas concentrates can be diluted from 40% up to 70% depending on the thickness of the sheet.
Sheet extrusion – keep track of carbon black
The carbon black network is sensitive and prone to shear forces. That’s why you need to avoid high shear conditions and keep the extrusion speed lower than normal. For this same reason, stretching should also be avoided after extrusion; the most appropriate die gap is the same as the thickness of the sheet. Carbon black is also a hydroscopic material, so it absorbs moisture easily. Using a vacuum vent at the end of the extruder is therefore highly recommended. High moisture content in the compound can result in poor surface quality, voids or die lip deposits.
When processing carbon black based compounds, a good starting point is to use the same processing parameters as with normal polymer. To minimize shear forces, the die and cylinder temperatures can be slightly higher than normal. The below table shows the typical processing temperatures for the standard polystyrene based electrically conductive PRE-ELEC® grade:
Table 3. Typical processing temperatures for the standard polystyrene based PRE-ELEC® compound.
The die temperature peaks at the end of the die. This is because the material flow distance is longer from the die ends compared to middle of the die.
It is important to monitor the sheet’s electrical properties during processing – especially if you are working with diluted compounds or concentrate dry blends.
Thermoforming process – avoid losing resistivity
Even if the sheet’s electrical properties are within the specified limits after extrusion, the end product may not exhibit the same electrical properties as the sheet. Why? The material’s carbon black network can be damaged during thermoforming. The more the material is stretched, the weaker the carbon black network becomes. This also increases resistivity significantly. The critical points are sharp edges and corners. Although thermoforming is usually seen as the final hurdle or challenge in the process, you should never overlook this process stage in the bigger process picture. Simply put, the thermoforming tool – in particular stretch ratio and pocket depth – largely determines the sheet construction and material composition. Getting it right from the beginning can make all the difference.