West System Epoxy and Tufnol - Does it stick?

[MagicalArmchair]

I am rebuilding my mast step and the small bulkheads that support it fore and aft. 25mm Whale Tufnol has about three times the strength of Marine Ply of the same thickness and none of the problems associated with water ingress, and costs broadly speaking the same.

Has anyone had any experiencing gluing Tufnol with Epoxy? Some say it will 'peel right off', whilst Tufnol themselves suggest they glue it with 2 part Araldite Glue (which is epoxy based, indicating the West stuff should work just fine). EC Fibreglass say they have never tried gluing Tufnol with West System or indeed any epoxy, but suggest with a good key (say with 80 grit) it should bond just fine.

Has anyone had any experience with this?

 

[lw395]

I think you need to consider the forces involved and think about making test pieces.
Depending on how your mast step is designed, the forces on the glue joints could be huge or negligible.
Not sure tufnol/SRBP is exactly a high strength material in tension or bending?

 

[MagicalArmchair]

All the forces of the step are in compression (keel stepped mast), so all the adhesion will be required to do is to keep the bulkheads in place and the truss in column. There should be no bending or tension in the truss.

Compressive strength along Tufnols edge is 200Mpa (http://www.theplasticshop.co.uk/plastic_technical_data_sheets/tufnol_whale_technical_data_sheet.pdf), and you are quire right. The tensile strength of the sapele is 109 MPa (albiet solid, I can't find any data on Robbins plywood), the tensile strength of the Tufnol is only 68 MPa. The compressive strength of the Sapele (which in this case we are interested in), however, is only 60.4 MPa.

 

[pyrojames]

I have tried epoxying Tufnell in the past with no success. The epoxy simply didn't stick.

It might work if you sand all the surface finish of the tufnol of and expose the fibres, but I am not sure how this might affect the tufnol itself.

 

[Tranona]

Do a test piece, although as I see your design it is not critical whether it adheres or not as the purpose of the Tufnol is to take compression and the epoxy/glass is a structural lamination tieing it to the hull and bulkhead. However drilling some holes through it to make some epoxy "nails" might be a good idea.

 

[MagicalArmchair]

I don't have any Tufnol to test, and ordering a bit and spending precious time (two kids!) and money (errr, two kids!) doing testing would be a waste. The Plywood will do the job, its a tested option from another owner who has done the same job, and the truss would be able to support the rig on the wood alone three times over, and once it is reinforced with fibreglass that will go up by multiples. I'll order the plywood from Robbins and stop over thinking (and over engineering) it.

Thanks for the input all, my gut feeling from my research was that the bond would not be satisfactory between Tufnol and Epoxy, Pyrojames has confirmed that. Tranona, a good idea making holes in the Tufnol for epoxy nails, if I went that route I would certainly employ that.

 

[Quandary]

I made the extended autopilot ram to rudder shaft drive arm from Tufnol laminated with epoxy because Jefa who made the steering and rudder for the boat recommend electrical isolation, I am convinced it is as strong as the aluminium arm it replaced.
I recall that Direct Plastics sell two grades of Tufnol with different properties, might be worth looking at their site.

 

[MagicalArmchair]

Thanks Quandary, I've been all over Direct Plastics website I've spoken directly to them too, however, I think I will still go the Plywood route. I had a very helpful response from Robbins that explained the glue line has a higher strength than the wood fibres, so it should only be stronger (although they were reluctant to give a definitive answer without test data).

They commented:

The requirements of BS1088 and Lloyds certification do not call for compressive strength data. Both standards are geared towards modulus of elasticity, durability, bonding classification etc. as per the data sheet we previously supplied.

He has put the question to the mill re compressive strength and plywood vs solid strength re compression also, so we shall see what they respond with