The main causes of static electricity are:

1. Contact and Separation between two materials (including friction, travelling over rollers etc)
2. Rapid heat change (e.g. material going through an oven)
3. High energy radiation, UV, x-ray, intense electric fields (not very common in industry)
4. Cutting action (e.g a slitter or sheet cutter)
5. Induction (standing in the electric field generated by a static charge)

Contact and separation is probably the most common cause of static in industry where film and sheet are being processed. It happens when material unwinds or passes over a roller. This process is not fully understood, but the clearest explanation of how the static is generated here is an analogy with a plate capacitor where mechanical energy to separate the plates of a capacitor is converted into electrical energy:

Resultant Voltage = Starting Voltage X (End distance between plates / Start distance between plates).

When the material touches the roller a small charge flows from the material to the roller causing an imbalance. As the material leaves the roller the voltage is magnified like the separating plates of a capacitor. The size of the resultant voltage is limited in practice by the breakdown strength of the surrounding materials, surface conduction etc. You often hear small cracks, or static discharges, as the material leaves the roller. This is where the static has reached the breakdown strength of the surrounding air.
The plastic film is neutral before the roller, but as the film separates from the roller electrons flow onto the film giving it a negative charge. The positive charge on the roller will disappear quickly if it is an earthed metal roller.

On most machines there will be many rollers and the level of charge and polarity of the charge may change often. The important place to investigate the static charge is immediately before the problem area. If the charge is neutralised too early it may regenerate before it reaches the problem area.

In theoretical terms, a static charge can be represented by a simple electrical circuit:

C is the capacitor function which stores the charge, like a battery. It is usually the surface of the material / product.
R is the charge relaxation ability of the material / system - usually a small current flow. If the material is conductive the charge will escape to earth and so will not become a problem. If the material is non-conductive, the charge cannot escape and so can become a problem.

The Spark Gap is the limit to the voltage build-up.

The charging current is the charge generated by the action of the product over a roller etc.

The charging current charges up the capacitor (i.e the product), increasing its voltage V. As the voltage increases, current flows through the resistor R. An equilibrium will be reached where the charging current is equal to the current flowing through the resistor. (Ohms Law applies here: voltage = current x resistance)

A static problem arises where the product has the ability to store a sizeable charge and a high voltage is present.

The static problem will show itself in the form of a spark, electrostatic repulsion/attraction or shocks to operators.

Polarity of the Charge
The static charge may be positive or negative. For AC static eliminators and passive dischargers (brushes) the polarity of the charge is not usually important.