I just called a plastic company to price half inch Lexan skylight
replacement. They said that, although Lexan is stronger, acrylic will last
longer in the sunlight and is more scratch resistant. They had no idea what
"longer" means.

If Lexan lasted 10 years instead of 20 I'd be happy. If it was 5 instead of
10, I wouldn't.

Anyone here have an idea what the real numbers might be for clear skylights
on a sailboat in the Northeast? Any other pros or cons for one plastic vs
the other?


-- Roger Long

Roger Long wrote:
I just called a plastic company to price half inch Lexan skylight
replacement. They said that, although Lexan is stronger, acrylic will
last longer in the sunlight and is more scratch resistant. They had no
idea what "longer" means.

If Lexan lasted 10 years instead of 20 I'd be happy. If it was 5
instead of 10, I wouldn't.

Anyone here have an idea what the real numbers might be for clear
skylights on a sailboat in the Northeast? Any other pros or cons for
one plastic vs the other?


-- Roger Long

I always understood that Lexan was more likely to yellow after exposure
to UV. Acrylic was used in WW2 aircraft windshields and those
windshields are as clear today as when they were installed. But all
Lexan is not equal with respect to UV resistance. You might try a
different supplier who can provide better information.

It would be very surprising if a Lexan skylight did not last 10 years.

Chuck

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Rodger,
I don't know where you have received your info, but it is a bit flawed.
First Lexan is a brand of Polycarbonate just as Plexiglass is a brand of
acrylic, but there is no comparison for strength. Polycarbonate is MUCH
stronger than acrylic. Yes, cast acylic is stronger and more optically pure
than extruded acrylic, but they both scratch easily unless the surface is
hardened. Polycarbonate is sold unhardened, hardened on one surface and
hardened on both surfaces. Eyeglasses and Harley windshields are made from
polycarbonate not acrylic. I also think large steps have been taken in the
recent past to make both products more UV resistant. I do not believe
yellowing is an issue anymore.
Steve

"Roger Long" wrote in message
...
I just called a plastic company to price half inch Lexan skylight
replacement. They said that, although Lexan is stronger, acrylic will last
longer in the sunlight and is more scratch resistant. They had no idea
what "longer" means.

If Lexan lasted 10 years instead of 20 I'd be happy. If it was 5 instead
of 10, I wouldn't.

Anyone here have an idea what the real numbers might be for clear
skylights on a sailboat in the Northeast? Any other pros or cons for one
plastic vs the other?


-- Roger Long

Steve Lusardi wrote:
Rodger,
I don't know where you have received your info, but it is a bit
flawed.

I called the largest plastic supplier and fabricator in the phone book near
Portland, Maine. I'm always a bit suspicious though when I ask the girl who
answers the phone if I can talk to someone about something and she goes on
to answer the questions herself.

I take it you would go with Lexan?

Over 30 years of being involved with materials choices like this, I've often
found that "better", when applied to something like scratch resistance,
means you could detect a small difference in two alternatives used side by
side under identical circumstances but wouldn't be aware of any significant
difference otherwise. I'm trying to figure out if the scratch resistance of
these two materials falls into that catagory.

"Roger Long" wrote in message
...
I just called a plastic company to price half inch Lexan skylight
replacement. They said that, although Lexan is stronger, acrylic will
last
longer in the sunlight and is more scratch resistant. They had no idea
what
"longer" means.

If Lexan lasted 10 years instead of 20 I'd be happy. If it was 5 instead
of
10, I wouldn't.

Anyone here have an idea what the real numbers might be for clear
skylights
on a sailboat in the Northeast? Any other pros or cons for one plastic vs
the other?


-- Roger Long

One data point - Lexan is currently approved for huriccane shutters in
Florida. The manuafacuter provides a 10 year warranty against yellowing
(even under our hot subtropical sun.)

On Wed, 06 Dec 2006 06:37:53 -0500, Charlie Morgan wrote:

On Mon, 04 Dec 2006 20:49:18 GMT, "Roger Long" wrote:

Steve Lusardi wrote:
Rodger,
I don't know where you have received your info, but it is a bit
flawed.

I called the largest plastic supplier and fabricator in the phone book near
Portland, Maine. I'm always a bit suspicious though when I ask the girl who
answers the phone if I can talk to someone about something and she goes on
to answer the questions herself.

I take it you would go with Lexan?

Over 30 years of being involved with materials choices like this, I've often
found that "better", when applied to something like scratch resistance,
means you could detect a small difference in two alternatives used side by
side under identical circumstances but wouldn't be aware of any significant
difference otherwise. I'm trying to figure out if the scratch resistance of
these two materials falls into that catagory.

I am about to order some polycast myself. It now comes in a version called
Polycast SAR. You might want to google that term.

CWM


Always used Polycarbonate for ports and hatches and am not familiar
with Polycast SAR. Can you comment on its ability to resist
differential expansion cracking and crazing? For instance, if you use
it in an application where a portion of it (border for instance) sees
less temperature change than the balance, does it tend to crack at the
boundry between the two temperatures from cyclic differential
expansion.

Frank

Frank Boettcher inscribed in red ink for all to know:
On Wed, 06 Dec 2006 06:37:53 -0500, Charlie Morgan wrote:


On Mon, 04 Dec 2006 20:49:18 GMT, "Roger Long" wrote:


Steve Lusardi wrote:

Rodger,
I don't know where you have received your info, but it is a bit
flawed.

I called the largest plastic supplier and fabricator in the phone book near
Portland, Maine. I'm always a bit suspicious though when I ask the girl who
answers the phone if I can talk to someone about something and she goes on
to answer the questions herself.

I take it you would go with Lexan?

Over 30 years of being involved with materials choices like this, I've often
found that "better", when applied to something like scratch resistance,
means you could detect a small difference in two alternatives used side by
side under identical circumstances but wouldn't be aware of any significant
difference otherwise. I'm trying to figure out if the scratch resistance of
these two materials falls into that catagory.

I am about to order some polycast myself. It now comes in a version called
Polycast SAR. You might want to google that term.

CWM



Always used Polycarbonate for ports and hatches and am not familiar
with Polycast SAR. Can you comment on its ability to resist
differential expansion cracking and crazing? For instance, if you use
it in an application where a portion of it (border for instance) sees
less temperature change than the balance, does it tend to crack at the
boundry between the two temperatures from cyclic differential
expansion.

Frank


At least with acrylic (Plexiglas), the process of flame polishing the
edges used by many fabricators can impart serious stress in the edges,
resulting in premature crazing approximately 1/2" out from the edge.

I do not know how polycarbonates are polished.

bob
s/v Eolian
Seattle

Frank Boettcher wrote:

Can you comment on its ability to resist
differential expansion cracking and crazing? For instance, if you use
it in an application where a portion of it (border for instance) sees
less temperature change than the balance, does it tend to crack at the
boundry between the two temperatures from cyclic differential
expansion.

I'm pretty sure it would be fine. It is just Acrylic with the surface
layers treated to harden them. The 3/8 Acrylic in my boat lasted 26 years.
It developed a consistent pattern of small carzing cracks that were a visual
obstruction only but nothing structural. I'm going to go with the Polycast
SAR and increase the thickness to 1/2 inch. The 3/8 x 21" x 21" panels
bowed when anyone stepped on them (I can often yell quickly enough) which
may have also been a factor in the crazing. the 33% increase in thickness
should nearly double the strength and stiffness.

The hatch frame spigott is exactly 1/2 inch so the thicker glazing will
stick up above the frame by the amount of the bedding. I'm going to put a
slight bevel on the edge and use minimum bedding on the flange with careful
filling in around the edge. I may make the new lens slightly smaller to
make it easier to get a good, bubble free, application of compound between
the edge of the plastic and the aluminum. An applicator spout small enough
to reach the bottom of the slot so the compound is working in from the
bottom up is essential to a solid seal.

--
Roger Long