PBDS

Polybutadiene. The many forms that can result from the polymerisation of butadiene, depending on the catalysts used, include high cis, medium cis, low cis, and high vinyl polybutadiene (PBD) (see Elastomers, synthetic-polybutadiene). 

Ethylene—Propylene Rubber. Ethylene and propjiene copolymerize to produce a wide range of elastomeric and thermoplastic products. Often a third monomer such dicyclopentadiene, hexadiene, or ethylene norbomene is incorporated at 2—12% into the polymer backbone and leads to the designation ethylene—propylene—diene monomer (EPDM) mbber (see Elastomers, synthetic-ethylene-propylene-diene rubber). The third monomer introduces sites of unsaturation that allow vulcanization by conventional sulfur cures. At high levels of third monomer it is possible to achieve cure rates that are equivalent to conventional mbbers such as SBR and PBD. Ethylene—propylene mbber (EPR) requires peroxide vulcanization. 

Transitional Bmsh (author s term for energetic non-PBD discharge from nonconductive layer) s 10-100 mJ... 

FIQURE 2-6.2.3. Transition from brush discharge to PBD on 80-/.m polycarbonate film. 

PBDs were not observed for layers less than 23 /rm thick and the PBD film potential threshold varied from 4-5 kV at 23/rm to 11 kVatl75/rm [228]. A later study, possibly using a less valid method for measuring film potential, reported smaller threshold values [ 29 ]. The later study concluded that if film breakdown voltage is less than about 4 kV at 20/rm and about 8 kV at 200/rm, PBDs cannot be produced. This is shown schematically in Figure 2-6.5. 

PLATE 5. Propagating brush discharge (PBD) on charged layer initiated by grounded electrode. 

Figure 2-6.S. Schematic conditions for PBDs (adapted from [29] see text)...

PBDs may occur on plastic surfaces with no metal substrate, for example the wall of a plastic pipe conveying charged material, in this case the double layer forms between the inner charged wall of the pipe and a countercharge which accumulates on the outer wall via conduction or via corona discharge. in the latter case both layers of charge reside on nonconductive sur-... 

PLATE 6. Approximately l-m-long spontaneous PBD on plastic pipe. 

For road tankers with nonconductive coatings or linings the possibility of static accumulation on the inner surface causing pinhole damage or even a PBD should be considered (2-6.5, 5-4.1.3, and 5-4.4.1). However, no case history of a PBD is available for lined liquid tanks. 

As in the case of road tankers there are no available case histories of PBDs in lined rail cars for liquids. However, owing to the large charge densities developed in some powder transfers the effect of linings may need serious consideration for rail cars in powder service (6-4.3). 

Since static discharges frequently occur at the liquid interface as liquid drains from the wetted wall, a vapor ignition hazard may also exist [ 157,159]. These discharges may be either brush types or PBDs (2-6.2 and 2-6.5). In 1997 it was concluded that Manufacturers have so far not paid attention to requests by the operators of enamel apparatus for a coating which is completely or partially capable of dissipation [159]. However, this situation is believed to be changing and it might now be possible to specify special static dissipative coatings for the vessel and stirrer. 

Nonconductive plastic pipe should not be used for transfer of ignitable powder or for any powder transfer through electrically classified areas. Hazards comprise brush discharges, PBDs and external sparks due to induction. Charge accumulation is discussed in 5-3.2.1. 

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