Over the years, many automotive parts have been made from rubber. Initially made from the latex of the rubber tree, the security of supply and the vast amounts needed during the 1920s and ’30s encouraged the development of synthetic alternatives. So while cooling system hoses may once have been made from any number of synthetic rubbers, these days they are restricted to more or less only two – ethylene propylene diene monomer (EPDM) and silicone.
EPDM is the product preferred for most automotive cooling systems currently. Capable of withstanding temperatures typically in the 110-130 C range, and sometimes up to 150 C, EPDM combines good heat and ozone (weather) resistance with better than adequate protection against oil and other under-bonnet chemicals such as ethylene or propylene glycols. To enhance its performance, especially under partial vacuum (for instance at the entry to the cooling pump), it can be reinforced with aramid braided yarn to increase stiffness, give greater burst strength and yet still maintain a level of flexibility.
Properly designed, hoses made from EPDM generally last for the full lifetime of the vehicle, which is normally reckoned to be up to at least 15 years or 150,000 miles. However, a word of warning: there have been instances where corrosion inhibitors in some organic acid-based coolants– which are designed to protect modern cylinder head/block alloys containing aluminium and/or magnesium – may have leached out certain components of the EPDM, causing corrosion in the narrow passageways of the cylinder head. While we are told that this issue has now been solved, it emphasises the need to use only the approved engine coolant anti-freeze/corrosion inhibitor in any application.
While EPDM hoses are the most likely to be found on many road transport vehicles, for applications where the requirements are more severe then silicone rubber is making headway. This is a polymerised siloxane or polysiloxane that consists of a chemical backbone of Si-O-Si-O-Si units. Unlike most other polymers, which have a carbonaceous backbone to them, polysiloxanes are inorganic and, unlike their organic counterparts, the bond angles in them are large and the bonds also vary in length. Thus, when the product is injected into the mould and then cured, the system of crosslinking between each chain of molecules gives a far less rigid product than EPDM, and it is this flexibility as well as many other attributes which is courting appeal.
Perhaps the best attribute of silicone elastomers is their incredible resistance to extremes of temperature while still maintaining their useful properties. Silicone elastomers can routinely withstand temperatures as low as -55 C and as high as +170 C, which is more than adequate for most under-bonnet applications. To counter this flexibility if used under partial vacuum – when the hose can collapse – the material can be reinforced with up to five layers of a medium-duty knitted or woven polyester fabric. A liner made of a natural rubber can also sometimes be placed inside to seal off potential leakage paths caused by any exposed fibre.
In applications where hydrocarbon fluids may be present (oil mist in engine induction systems, for example), silicone hose lined with a fluorosilicone is normally used; this prevents the migration of the fluids through the hose wall over time, causing the hose material to swell. In this way the hose will not become brittle and fail. Furthermore, the operating temperatures will be increased to up to 250 C, which makes them ideal for turbo or supercharger/intercooler hoses or on passenger vehicles that are subject to stringent emissions tests.
Above all though, silicone rubber hoses can seriously improve the appearance of your engine bay.
Fig. 1 – Seen in the paddock at the Shelsley Walsh Hillclimb, a black EPDM hose in the cooling circuit, and blue silicone for the supercharger air hose
Written by John Coxon