UKDIVING MICHELLEHAYWOOD
Your flexible friend
An ideal material for drysuit seals , the chemical composition of latex makes it both flexible and fragile . Michelle Haywood has the science
Let ’ s talk all things rubber ... or more specifically , latex . Latex is found naturally in about 10 per cent of flowering plants , including rubber trees and dandelions . It ’ s a sticky mix of proteins , resins and gums that coagulates when exposed to air . Plants don ’ t make latex so that we can harvest it . Latex is produced as a defence against insect damage , a way of plugging the hole created when insects bore into the plant . As well as being sticky , latex fluid includes a number of toxins that repel herbivores . But most importantly for divers , latex is the starting material for making rubber and that ’ s what we need for latex seals .
Rubber is useful because of the presence of polymers of isoprene molecules . Isoprene is a hydrocarbon which in pure form is a colourless , volatile liquid . It was originally identified in 1860 when natural rubber was heated , causing the isoprene to boil off ( around 34ᵒC ). Although we associate
18 isoprene with plants when they are damaged , it evaporates from undamaged plants as well , making plants a significant source of hydrocarbons released into the atmosphere . Strangely , isoprene is also the most common hydrocarbon in the breath of humans ( around 20 mg per day for an adult ).
If we could view relaxed latex seals at a microscopic level , we would find a disorganised cluster of long chains of isoprene molecules . These wrinkled chains bond to each other fairly erratically . Once the latex is stretched , the chains become almost linear and more organised . The reason latex rubber returns to its nonstretched state is because the molecules prefer to be in their disorganised state . Hold a rubber band close to your face and stretch it , and you will feel the heat released as the chains all line up and pack together to form a crystal-like structure .
If latex rubber is cooled down , it will drop below the glass transition temperature and become ‘ frozen ’. At low temperatures , it becomes impossible to reorder the chains and so the elasticity is low ’. This crystallisation is fast and reversible , but any attempt to use the seal during this time is likely to tear it .
It was just this effect that contributed to the Challenger space shuttle disaster , when a flattened O-ring failed to expand to fill a widening gap , allowing gas to escape past the seal . For anyone who has had their drysuit freeze overnight , carefully warming the seals before putting on your drysuit will avoid the risk of tearing . Stiffening due to low temperatures is fully reversible . Hairdryers on low settings can help ( hand-dryers in toilet blocks are a good alternative ).
But there are other risks to latex rubber seals too . When natural latex liquid is collected , it is allowed to coagulate into blocks of rubber , which can be easily transported . Processing the raw rubber includes chopping up the blocks and use of turpentine or naphtha as a solvent . The solvent breaks the bonds between the long chains of isoprene .
Now , anyone who ’ s ever ended up dealing with a boat with fuel issues will know that fuel spills on seals can melt them into a sticky mush . The solvent breaks the bonds between the molecules , fatally softening the latex . Other organic solvents like DEET ( used in insect repellent ), perfumes , make-up and sun cream can cause similar damage .
Rubber deteriorates with oxidation and thermal damage too . The molecular changes brought about by these environmental conditions can significantly impact the mechanical properties . Synthetic rubber tends to harden as more crosslinks are formed ( called chain hardening ). Natural rubber , however , undergoes chain scission where the main polymer chains start to break apart and the rubber softens . Oxidation can be caused by exposure to ozone ( generated by motors and high voltage devices ). Keeping drysuits cool and away from ozone will prolong the shelf-life of your seals . If your seals show signs of cracking , then the material has suffered ageing which has caused it to become brittle . There isn ’ t a magic fix for this either , you will need new seals fitted . �