Letter 17052

Coating high density polyethylene with a light weight metal? (N. Ireland)  

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I am currently doing a project based on the process of rotational moulding. I am re-designing a kayak (canoe). The kayak will be made from high density polyethylene, if I were to coat certain parts with a light weight metal, what type would be best suited? Would the kayak still be able to float successfully?

Jo neely
student - L,derry, N.Ireland

Hi Jo,

You'd have to double check with a manufacturer of rotational (Engel process) ... but methinks you are up a gum tree.

There's no, no WAY of bonding ANYTHING to Polyethylene except for Pe to itself !!! What in the heck for? And were you to do this, then the 'weight' of the metal should be LESS than the floation (full of water!) of the Kayak otherwise you'd have an Irish Titanic Coracle !!!

As you know (I hope), the s.g. of Pe is around, depending on type (h.d., m.d., l.d) is 0.900 and any metal is damnably higher. I suggest that you go to a rotational moulder and watch how they make 'things' ... there's a metal mould ... you pour in so much Pe powder ... you rotate as you heat the mould so that you get even distribution ...you let the mould cool down ... but the processes max. max. thickness is l/2" and in your case, probably l/4" is OK. If you want to do something 'different', you could ask the moulder to use two colours of Pe. The first colour would be the outside colour and the 2nd one the inside colour ... also, unlike pure white Pe., colours help in UV attack somewhat.

Cheers!

Freeman Newton
- White Rock, B.C. Canada .... (thermo)plastics, mainly, background

I beg to differ with Freeman Newton about the feasibility of metal coating polyethylene; - metal can be deposited onto it if it is activated correctly. The only way I know is to use a plasma discharge and then use an electroless metal deposition process with a suitable activator such as Pd/Sn. HOWEVER, the adhesion is not brilliant and it is definitely not suitable for thermal cycling because of the differences in the coefficients of expansion. Freeman is correct about the problems of density and floatation, but if you were to coat the bottom of the kayak, perhaps it would make it more stable by lowering the centre of gravity?!? Nevertheless, this doesn't answer the question as to WHY you want to do it - just to make it look different?

Trevor Crichton R&D practical scientist - UK

Hi again Joe,

Sure seems that I was wrong about coating Polyethylene ... but as Trevor C pointed out, Pe has an awfully bad coefficient of thermal expansion, so a PRACTICAL solution of metallizing is not on.

Moreover to lower the centre of gravity you'd need a keel. And if you were to make a Kayak with a keel (to which lead etc weights could be bolted to) then you'd probably be the first in the world to do so.... which should make life very interesting if you are traversing any rocky rapids !!!

And Trevor wondered, too, WHY you were even contemplating 'metallizing' the kayak. To be different? I guess so ... but at your age and with an inquiring mind, why not ask these questions.

Re that suggestion of two colours, ah, maybe you have something there because any kayaks I've seen were just a mono colour. So if, for example, the moulder had to use 50 lbs of Pe powder for the Kayak, if he dropped in 5 lbs of a coloured Pe ... and then later dropped into the mould the balance of 45 lbs, you'd at least achieve a fairly economical boat ... but, at a guess, if you metallized it, you'd better get a big student loan!

Freeman Newton
- White Rock, B.C. Canada

Although the metal MAY be more resistant to damage from rocks, it would seriously reduce the performance of the kayak because of waterflow and weight. Another point - High Density Polyethylene is not rotational moulded, it is blow-moulded. If you were going to rotomould a kayak you would use Low Denisity Polyethylene.

You really should take a trip to a kayak manufacturer and ask them to show you how it is done.

You are doing a Skool project right, do you really NEED to go to the complicated process of coating polyethylene with metal? From what I've experienced, you should get your qualifications the easiest way possible and go for 'revolutionary' processes later once you are established in a job/career.

G. Auld
- Auckland, NZ

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