HDPE or LDPE

I spent some time scouting around the net yesterday looking for a “Sit On Kayak”, something that I could lift on and off the car myself and launch from the pebble beaches around Eastbourne and Pevensey bay. There are many different option  available from brand new direct from a brand company like Fatyak or Ocean Kayaks, new or second hand from eBay or GumTree.

One thing that kept coming up, was the claims about the type of polyethylene that the body was made from; LDPE lighter and more flexible than HDPE, HDPE tougher but heavier…. etc, etc….

I decided to find out about the differences and what that means about Kayak manufacture.

It polyethylene comes in four distinct types, LLDPE, LDPE, MDPE and HDPE.

Linear Low Density Polyethylene (LLDPE)

is the most flexible of the plastic sheeting films. LLDPE is blended form of LDPE where the film has much more flexibility, tensile strength, and more conformability. It is more pliable and softer than LDPE and it’s most commonly used to make supermarket carrier bags toys and cable coverings, it’s also is used for films that need a tremendous amount of strength to absorb impacts while not tearing or puncturing. 

Low Density Polyethylene (LDPE) 

is the most common type of plastic sheeting. It is very flexible, most often from 0.5 mil thick to about 40 mil in flexible sheeting forms. Due to its flexibility is conforms well to a variety of surfaces. The downside is that this LDPE is not as strong or dense as some other types of plastic sheeting. It is not nearly as puncture resistant either. It’s commonly used in the manufacture of  washing up bowls, food containers,  plastic cups and is used quite widely in construction, agriculture, surface protection applications along with covers of all sorts, tarps and much more. UV filtering additives need to be added to increase the UV resistance of the material.

Medium Density Polyethylene (MDPE)

is the least commonly used form of polyethylene for flexible plastic sheeting. Its positive attributes is that it is stronger than LDPE and has a little more chemical resistant. It has a tighter cell structure making it more tear and puncture resistant. When pond liners are manufactured they are often made with MDPE and blended with LDPE or LLDPE to achieve a strong yet flexible pond liner for larger areas.

High Density Polyethylene (HDPE, aka HD) 

has the distinction of being the strongest, toughest, most chemical resistant and least flexible of the four types of flexible plastic sheeting. It is also the most UV resistant- holding up to the harsh rays of the sun without needing UV additives. The strength of HDPE comes from its tight molecular structure that makes it very difficult for other molecules to pass through the material on a microscopic level. In the household HDPE can be found in Rubbish bins, detergent bottle and water pipes. When applications call for very large liners such as lake liners,  HDPE is used as it is the most easily seamed or “sewn” together. HDPE is also used as secondary containment liners for oil tanks where chemical resistance is needed.

Conclusion

My conclusion is that while kayaks made from LDPE are invariably cheaper and lighter than their HDPE counterparts, it maybe a false economy as a HDPE Kayak should last longer and more be able to withstand pebble beach launching and landing without catastrophic damage . So I’ll more likely to be buying a brand name HDPE based kayak second hand rather than a new LDPE kayak thats the same price.