Plastic materials are electrical insulators by their own nature. This feature is usually a positive in most applications but it can generate problems when the final product undergoes rubbing stress.
In such conditions the material accumulates electrostatic charges which are responsible for the ‘shock’ received when touching an electrified object. The problem of the accumulation of electrostatic charges has increasingly become an aspect of legislative relevance so it is regulated by various sector directives.
Many TPE compounds are based on styrene-butadiene-styrene block copolymer (SBS) and styrene ethylbutylene styrene (SEBS) are excellent insulators as well as all plastic materials so they tend to accumulate electrostatic charge naturally. The tendency to accumulate these charges is inversely proportional to the specific resistivity of the material: higher resistivity means the material is more insulating ie. it does not conduct electrically and, therefore, it does not allow the electrostatic charges to dissipate/discharge to ground. Electrical resistivity is the inverse of electrical conductivity.
Electrical conduction mechanisms can be influenced by surface interface effects (eg. relative humidity of the environment) causing electrical conduction essentially localised on the surface of the material; in this case we speak of surface resistivity that must be distinguished from the specific resistivity or volume resistivity.
• specific resistivity (or volume resistivity) is measured in Ohms*cm.
• surface resistivity is measured in Ohms but the unit of measurement Ohm/sq is usually preferred.
Nevertheless, it is possible to exhaust the build-up of electrostatic charge. The conductive properties of compounds depend strongly on the fillers choice and how they are dispersed within the thermoplastic matrix. The main methods for modifying electrical conductivity in a thermoplastic elastomer can be achieved through the addition of:
Unmixable particles of conductive materials
The electrical conductivity occurs thanks to the contact of conductive particles that create a network within the thermoplastic material. Some examples of such particles are: conductive carbon black, metal microfibres, carbon nanotubes... etc. There are three conditions that must be met to achieve an effective and long-lasting conduction mechanism:
- The concentration of conductive particles must necessarily be high;
- The dispersion of these particles in the thermoplastic matrix must be homogeneous;
- Thermoplastic matrix itself must be dimensionally stable to guarantee the electrical contact between the conductive particles.
One of the drawbacks is that the high concentration of such particles heavily alters the characteristics of the base material, the other refers to flexible materials and in particular to elastomers which, by their nature, cannot satisfy the third requirement of dimensional stability; this type of electrical conductivity in the elastomers and destined to be variable over time as a function of the operating conditions of the elastomer article.
Migrant ionic and/or hydrophilic additives
These additives migrate on the surface of the material, activating an electrical surface conductivity through time. For this reason, they are easily removed so the dissipative effect is destined to decrease over time. These additives are commonly referred to as ‘non-permanent antistatic additives.’
Intrinsically conductive polymers (ICP)
The conduction mechanism is activated by a homogeneously dispersed polymer component in the thermoplastic matrix; in this case the conduction mechanism is activated by a homogeneously dispersed polymer component in the thermoplastic matrix; this mechanism guarantees a much more stable electrical conductivity over time (permanent) and independent of mechanical and/or environmental stresses applied to the material. The main drawback of this mechanism is that to activate an appreciable electrical conductivity the concentration of ICP must be elevated with repercussions on the final characteristics of the thermoplastic material.
The conductive properties of the compound strongly depend on the choice of fillers and how they are dispersed inside the thermoplastic matrix but only if ICP are used, the resistivity of the object is permanent.
Francesco Franceschetti Elastomeri has developed three new families of anti-static TPE compounds: Marfran E CD. These compounds are based on technologies that make the anti-static material both non-permanent (ionic additives) and permanent (ICP), combine the easy processability, lightness and versatility of TPE with the dissipation properties of the electrical charges.