The process of Electrostatic discharge involves two processes: Charging and Discharge.

One of the most vital problems in the Electronic Industry and specifically the Semi-conductor Industry today is how to protect sensitive micro-electronic devices from the damaging effects of Electrostatic Discharges. This problem is becoming more acute with the advancement of Technology. In order to choose and employ proper protective methods, a deep understanding of the process of Static Charge generation becomes essential. This understanding is vital in the case of Polymers consisting of Polymerized Olefins like Polyethylene, Polypropylene, etc.,which have a propensity to generate Static charges when they undergo very simple processes like Friction, Seperation and being placed in Electrostatic Fields. These materials are being extensively used all over including Clean Room areas. An attempt is being made here to understand the ease of Charge generation in these materials using Models based on generally accepted Chemical Principles.

It is a generally accepted Chemical concept that in Olefins like Ethylene, Propylene, Acetylene and materials like Isoprene, and Butadiene their molecular Structures can be written in Mesomeric forms of their Double or Triple Bonds for ex:

The Carbon chain of Ethylene Molecule can be written as:

The Carbon chain of Propylene Molecule can be written as:

The Carbon chain of Acetylene Molecule can be written as:

The Carbon chain of Butadiene Molecule can be written as:

The Carbon Chain of Isoprene Molecule can be written as:

The corresponding Mesomeric Structures are:

And

The intriguing observation however is that these Monomers when they are polymerized show electrical properties which are different from what can be expected from their Monomers , For eg:

1. Commercial Polymers like Polyethylene (LLDPE/ HDPE/LDPE), Polypropylene (PP), etc are generally considered insulators. However, it is not commonly appreciated that the Resistences of these materials are measurable and are indeed commonly measured in most Electronic Labs using Surface Resistivity Meters. In these cases the measuring voltage is 100 Volts instead of the usual 10 Volts for Conductive material.

2. Still more amazing is the fact that these Polymers are prone to generate Static Charges Even under very mild conditions such as Slight Friction, Separation of two Polymers which are in intimate contact. Generation of Static Charge would mean that electrons have escaped out of the material into the external surface. The energy required for generating Static thus appears to be extremely low.

3. Both Polyethylene and Polypropylene can be easily modified in their Static generating Capacities either during their polymerization stage or more generally by intimate blending with suitable Chemicals in their molten condition. In fact quite commonly Anti static character is produced in Polypropylene using for instance Quaternary Ammonium Compounds. Polypropylene can be rendered Conductive by loading the material with conductive Carbon in sufficient amounts. Both these types of modifications imply a considerable increase in the electrical conductivity of the Polymers after modification.

4. Relative Humidity plays a very important role in Static Charging. High Relative humidity in the environment reduces the Static Potentials generated.

The generally accepted molecular structure for the different Polymers (loc.cit) is given below:

1. In Polyethylene the Carbon skeleton is indicated as:

where the repeating unit is in Brackets

2. In Polypropylene the Carbon skeleton can be indicated as:

the repeating units are in brackets

3. In Poly Butylene the Carbon skeleton can be indicated as:

It is suggested that the propensity of static generation, that is the capacity for an Electron in the Material to escape to the outside of the material is most probably due to the presence of double bonds in the molecule. It is suggested that the p-bond electrons are sufficiently rich in energy so that they are enabled to escape to the outside if some more energy becomes available to the molecule. Friction or seperation may provide the necessary extra energy. It should be pointed out that p-Bonds are specifically oriented in space and this spatial orientation may facilitate the escape of an electron to the outside in a linear chain polymer. The fact such topologically distributed charge orientation exists is substantiated through almost two decades of work by Organic chemists in the fields of 1H and 13C NMR Spectroscopy.

1. Butyl Rubber is a high Static Generator . It consists of Polybutene as a repeat unit. Polybutene contains a Double bond at the end and is generally written as:

The repeat unit is in Brackets and carries a double bond

2. Natural Rubber again is also a high Static Generator and it is also made up mostly of repeat units of 1,4- Polyisoprene containing double bonds.The Carbon chain can be indicated as:

the structure in brackets indicates the repeat unit.

3. PolyButadiene is a Static Generator and contains repeat units of 1,3-Butadiene

The brackets indicate the repeat chain. This material is also a Static Generator

5. In the case of Polyethylene, Polypropylene, Polybutylene the increase In electronic charge on the Carbon atoms containing the Methyl substituents

Is explained by the +I (Inductive) effect very well known in Organic Chemistry In fact the charge contribution should increase in the order: Polyethylene < Polypropylene < Polybutylene the effect would be that Polyethylene Should be higher in the Negative side of the Triboelectric series than Polypropylene Which should be higher in the Triboelectric series on the Negative side than Polybutylene. The point is all of them should act as Negative charge generators.

The author wishes to thank Dr.Alber Kow ,chairman ,Tako Technologies pvt.Ltd.,
Malaysia and Mr.K.V.Keshava Murthy, Managing Director, Cir-q-Tech Tako
Technologies pvt.Ltd., Bangalore who has been a constant source of encouragement.