Acrylic or polycarbonate?

snowleopard

snowleopard

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Last season I had a nasty accident which, more by luck, didn't result in a serious accident. A gust of wind from a different direction caused an accidental gybe resulting in the shackle securing the mainsheet to the deck parting. The resultant forces caused the block to fly into one of the doghouse windows causing it to shatter. The flying shards fortunately missed SWMBO who was standing nearby.

I am now replacing all the doghouse windows which have all started to crack after 15 years. The current ones are 6mm acrylic and I am considering switching to polycarbonate for safety. I've always used acrylic in the past and have heard polycarbonate fogs.

So - is polycarbonate stronger in such an impact? How well does it last? If it starts to fog, can it be polished out? Is the lower scratch resistance a problem?
Anything else I need to know about it?
 
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If it has not improved since 1980 it will become yellow and opaque. I have used acrylic in a shed and after three years it seems very good. From Penryn Plastics. Online catalogue may help you.
Edit: oh, and brittle after the sun gets to it.
 
I did the big front window on my doghouse with 6mm polycarbonate UV stabalized and it has a slightly milky transparency after only 1 season. It also shows some refractions where it has a little stress from the screw fixings so does not look dead flat. It is already showing small scratch marks. My side windows are acrylic and are totally transparent and look great and are scratch free. The polycarbonate is much more flexible in the thickness I used and I could not imagine that shattering as your window did. A freak accident and I am pleased that you did avoid all harm.
 
Polycarbonate will not shatter like Acrylic but

Polycarbonate surface scratches easier than acrylic

Polycarbonate is less rigid than acrylic so large areas flex and could pop out if not retained by screws in holes

Polycarbonate tends to crack easier than acrylic around and bolted fixing.

Polycarbonate goes milky when exposed to UV light.

On my big wheelhouse windows I used toughened/ laminated glass for the above reasons as I used all Polycarbonate on my previous boat.

I have use Polycarbonate in my see through wash boards bolted with stainless steel backing strips
 
There is a glass merchant on Commercial road penryn, they can cut toughened glass to order, and in the past I've had laminated cut as well. An odd shape about 18in square was £35. Tallack Windscreens.
 
fisherman said:
There is a glass merchant on Commercial road penryn, they can cut toughened glass to order, and in the past I've had laminated cut as well. An odd shape about 18in square was £35. Tallack Windscreens.
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I would be surprised if they can cut toughened glass as it would shatter into little pieces. Laminated glass can be cut but not toughened.
 
Back in the day, the armoured windows in the Snatch Landrovers in NI were made of polycarbonate. They had two problems. It was very easy to scratch them, although the scratches could be polished out with patience. The second one was the effect of UV on both the strength and opacity of the materiel. The windscreens in particular had a fairly short service life, which had to be carefully monitored so as to ensure they were replaced on time. Now, that's not because the things were going to fall apart, rather that after the prescribed time their ballistic resistance had dropped below acceptable levels. I'd therefore tend to go for acrylic rather than polycarbonate as I understand it is more UV resistant.
 
Rogershaw said:
I would be surprised if they can cut toughened glass as it would shatter into little pieces. Laminated glass can be cut but not toughened.
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He probably means he can cut the glass to shape and have it toughened, which is the normal method. Once toughened, as you say, it can not be cut without fracturing into a thousand pieces.
 
pmagowan said:
He probably means he can cut the glass to shape and have it toughened, which is the normal method. Once toughened, as you say, it can not be cut without fracturing into a thousand pieces.
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I agree with that as my glass supplier found when they supplied my first batch oversize to my patterns and had to remake the whole lot at their cost. Even some of the second batch was a little oversize but did manage to grind a few millimeters off the edge without shattering the glass. It was try that or remake the oversize pains again.

So it is possible to grind the edges a little but not to cut toughened glass.
 
pmagowan said:
He probably means he can cut the glass to shape and have it toughened, which is the normal method. Once toughened, as you say, it can not be cut without fracturing into a thousand pieces.
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Yes of course, made to order, in my case a new end for a chill counter in 6mm, basically a quadrant shape.
Laminated can crack of course.
 
Polycarbonate (PC) will pick up scratches more easily than acrylic (PMMA) and it is not easily polished. they are very similar in terms of rigidity but PC is far easier to cut and will not chip as easily. PMMA stresses far more than PC and unless you anneal it after cutting you may see cracks develop around cut edges and drilled holes as it will stress during any machining operation. If drilling you will need to remove the leading edge of the drill bit so it scrapes the material rather than cutting it, this will allow you to drill clean holes without chipping. Acetic acid and other additives used in some sealants can cause crazing when used with PMMA. PMMA is UV resistant, and will not transmit UV light. PC is not inherently UV stable, specialist UV grades are available but be aware tbat often only one side is protected, so you need to make sure it is fitted the right way round, doubled sided protection is also available at a price. You can also obtain IM (Impact modified) grades of PMMA although the clarity is not quite as good. It is a lot tougher than standard acrylic. It will be very difficult to break PC panels it's impact strength is significantly higher than PMMA. PETG is not a good option although it has 90% the impact strength of PC it is much softer and scratches easily, it cannot be polished easily and abrasive salt water would quickly destroy its surface, standard material is not UV stable. I hope that helps!
 
Referring back to the OP, I have considered glass as it is less prone to scratch with windscreen wipers. My thought is that with the risk of impact, I would prefer laminated as it won't shatter and spray us with broken glass. I know that was the thinking on car windscreens up to about 30 years ago.

Any thoughts?
 
snowleopard said:
Referring back to the OP, I have considered glass as it is less prone to scratch with windscreen wipers. My thought is that with the risk of impact, I would prefer laminated as it won't shatter and spray us with broken glass. I know that was the thinking on car windscreens up to about 30 years ago.

Any thoughts?
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That's why I went for 2 sheets of toughened glass laminated together so that in the unlikely event that the toughened glass shattered it would not spray broken glass every where. Also only one of the sheets of toughened glass will shatter leaving the window still intact.

I need to replace some of the acrylic in some of my deck hatches due to crazing and considering replacing the acrylic with laminated/ toughened glass same as my large wheel house windows.
 
It is most unlikely that toughened glass would shatter. I have a toughened glass 'hearth' under my wood stove. The salesman invited me to hit it as hard as I could with a hammer - nothing happened.
Come to think of it, my boat has toughened glass windows in the deck saloon. These are double glazed and have withstood many knocks from spinnaker poles and other things with not a scratch.
 
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FWIW, the owner of this beast....
2014_04_09_0353-1000x560.jpg


Specified toughened glass for it. The total weight was 3/4 ton!

images
 
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