DIN51mm Gray Color Breathable Cap used on plastics drums and Jerricans. 

The DIN51mm Vent Cap is a special concept designed by the company to provide its clients with simple packaging solutions. 

Cap Houses a special Membrane that allows only gases and not liquids to pass through .
Used by people for Storage and transportation of acids and other substances that create a lot of pressure on containers causing it to leak or crack .
Each of these closures are checked to make sure that of their guaranteed satisfaction.
Application Case: 
For hydrophobic (high surface tension) liquids. 
e.g. Sodium Hypochlorite and Hydrogen Peroxide with only water added.
For Household, industrial and commercial products containing bleach,peroxide or other gaseous ingredients can cause containers to swell or bloat. 
Other products containing surfactants, solvents or oils can cause containers to collapse or panel due to oxygen scavenging or product migration. 
In addition, fluctuations in temperature and altitude during shipping or storage can accelerate these undesirable container changes. 
Features:
Process:Precision injected molded for leak-tight fit.
Model No.: CFD-VCP-5150B
Dimensions:51mm,DIN Standard. 
Threaded Pitch: 5mm
Height: 29mm
Materials: vented cap body is HDPE,vented film is e-PTFE film.vented plug usually is D13,D15,D17
Gasket: Silicon rubber gasket,PE foamed liner,or O-ring gasket
Colors: Gray color,red,yellow,blue,Natural,or customer made. 
Unit Packing:800PCS/CTN
Unit Weight: 21-22 g/PC
Typical applications for Jerrycans venting include:
Bleaches
Peroxides
Household Cleaners
Industrial Cleaners
Liquid Fertilizers
Pool Chemicals
Pesticides solutions
Sterilizers
Disinfectants solutions
Pharmaceutical Dispensers
Many others! 
Related breathable caps and vented plugs,vented liners:
DIN61mm vented caps for poly drums,
DIN51mm vented covers for plastic Jerrycans,
DIN40mm vented closure for plastic Jerrycans or plastic bottles.
DIN38mm vented caps for plastic collapsible cubitainers.
vented plugs,vented liners....