makrolon

[jambaman]

Makrolon questions,
Our boat is a steel JNF and I recently took the windows off to deal with the rust under the pains. After replacing the pains a number of cracks appeared, which after the amount of work involved caused considerable heartache. I have written away to a number of Makrolon suppliers and Bayer for answers to questions on the Makrolon, but still await answers.
Ouma is moored in Gouvia marina, Corfu and whilst I removed the windows in May, with the weather much to my dismay rather wet with an average temp of 25 deg. During the subsequent weeks when the weather improved and temperatures reached 40+, these cracks appeared. On inspection of the pains when they were out, all screw holes had spider web cracks around them. The major cracks which appeared after placing the pains back in place seem to come from screw holes. My questions, if anyone can help, are,
1. Should the retaining bolts be screwed tight tight, or, just finger tight?
2. Does Makrolon expand and retract with the changing temp and weather pressure (i'm unable to find this out on any of the tech data sheets available)?
3. I would rather not drill holes in the plastic at all, instead have the windows encased when I replace them. There would be 20mm of pain available between the frame and the inside of the cockpit frame. Would this be enough of an overlap if hit by a sledgehammer wave, and or expanding / retracting heat/pressure?
4. Can anyone recommend a supplier who they have dealt with in the past.

Many thanks
Julian


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[Anchorite]

Are you sure it is Makrolon (polycarbonate, another trade name is Lexan). I've used it for my windows: before choosing it I did the sledgehammer test and drill test: I have never seen a spider crack appear. My method, for what it's worth: all holes drilled one mm oversize (steel and makrolon): windows on inside of hull with a full frame (wood) inside overlapping the steel and pane by 25 mm. Assemble with s/s bolts and lots of sealant: tighten finger tight first (needs someone inside and out). Next day tighten up when the sealant has gone off (no need to sweat it home). It should have oozed everywhere and will collect dirt nicely. Clean up. I did mine over 14 years ago and although not in the sea, they undergo temperature extremes between 0° and 35°+ (like today...). There are no cracks and I don't expect any. Yes, there is a differential expansion rate between steel and polycarbonate, but this material is not brittle (as perspex is) which is why I chose it. Makrolon is Bayer's trademark: mine is Makrolon, made by ROHM Gmbh.

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[Freebee]

I dont think you have makrolon. I have used it to make safety guards for machines, ive and drilled and even bent it without failures. I think you some other inferior produc.t

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[jerryat]

Hi jambaman!

Agree with the others, it doesn't sound like Makrolon to me either, but I'm not an expert. The one thing I remember having pounded into me when I was building my first boat many years ago, was the need (mentioned in other posts) for the fixing holes to be oversized to allow for both boat movement and differential expansion.

The other 'must do', was that the screws should NOT be countersunk type, but pan-head. Countersunk screws would, it seems, still 'lock' the window into position, whereas panheads allow the window to move under the head. Right or wrong, it worked for me, and we never had a single drop of water in the boat.

Hope this helps

Jerry

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[richardandtracy]

Sounds like it's behaving like acrylic (eg Perspex), which is very much more brittle and not as strong as polycarbonate (Makrolon, Lexan).

Addressing points 1 & 2.
I agree with other poster's. Prefer not to countersink as the expansion rate of PC is at least an order of magnitude greater than steel. As for how tight to do up the screws.. Just enough to prevent them coming undone. If you use thread lock and a good sealant, then finger tight should be OK.

Addressing point 3.
It is possible to encase the edge of the pane, but it would prevent you from exploting the full tensile strength of the material if your 'sledgehammer wave' appears. If screwed all round the edges, the whole window can be in tension, rather than parts remaining in compression when the sheet deflects. (means it can withstand a bigger wave with less deflection & stress). The other problem is that the edge of PC could be chemically damaged by an adhesive strong enough to allow the tension field to be set up - adhesives that don't damage PC tend to be of the weaker variety, but you may be lucky.

Point 4.
Try a local plastics supplier. My local one is very helpful, and the prices are those aimed at builders not at yotties!

Regards

Richard.


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[jambaman]

Thanks Anchorite. All the holes were 6mm diam and we used 5mm flat head screws with nylocs. Each pain is secured with one countersunk screw in two corners, then a 5mm 4 peice frame secured ontop and around the pain. I used Sikoflex 295 throughout and a darned lot of it as well. From the posts replied, i would agree that it sounds like an acrilic has been used, but when we purchesed the boat we were told that all windows were makrolon, in addition, we have a large piece left over with the makrolon backing paper still on. In addition, the actual pains which are in direct sunlight have not cracked in any shape or form like our hatches (which are acrilic). The builder, who was the prevoius owner, cut the pains himself, and i noticed that on the the curved corners there seemed to be what i can best describe as layers of plastic where the cut was not perfect. Could the spiders web of cracks around the screw holes have come about by incorrectly drilling the holes?
thanks for the advice.
Julian

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[silverseal]

One of the problems with certain materials is stress cracking, and this can be caused by solvents. I wonder if there is a solvent in the sealant which you used which could have caused the problem. My wife used superglue to reaffix the drivers mirror onto the windscreen of our Ford Fiesta, and yes it caused stress cracking of the glass, with a crack more than 8 inches long!

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[Anchorite]

The only cause of cracking mentioned in the technical literature concerns damage caused by aromatic solvents like xylene and toluene...which you may find in resin and various high spec paints. They leach out very slowly. What did you treat your rust problem with? You mention corners showing "several layers of plastic": do you mean it was delaminating ? Makrolon is very easy to cut to shape, corners are no problem with a slow-speed jigsaw. Your sikaflex choice is the recommended one.
My sledgehammer tests were: (A) put a piece on an anvil and smash it with a 5 kg
hammer. (B): bridge 2 bricks with a strip and smash as before. (C) put a piece in a vice and try to break it with hammer blows. Results were all similar: no loss of integrity, the material stretches and loses transperency. A sharp point will puncture it: the ultimate wave will probably stretch the bolt holes which may leak, but you'll be in the liferaft by then

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[jambaman]

i took the entire area back to bare steel and ground out 99% of the rust. Then two layers of Owatrol CIP, two coats of International Interprotect, then 3 coats of Perfection. We put a good beeding of 295 on the prefection prior to the pain going on, so unless there is something in the Perfection paint that could have caused the cracks, then i'm not sure what i can do untill i get back to Ouma. From what everyone has kindly said, i think my only option is to get some new Makrolon or Lexon and do a sledgehammer test with the old windows, just to satisfy my curiosity.

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