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post #16 of Old 03-08-2013
Maine Sail
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Re: WTH!!!!!!! Silicone problem..

Originally Posted by deltaten View Post
I specified Lexan due to the strength and longevity in harsh environs. Acrylic sheet as found in hardware sources and even the vendor I got the Lexan from will not hold up as well. The Lexan is not as scratch resistant; but it would take a misplaced foot on the hatch or a dropped shackle a lot better
Actually it won't. Polycarbonate is very impact resistant but far more fleixible than CAST acrylic, what is mostly used in hatches. Unless your hatch was specifically designed for polycarbonate, it would have support bars if it was, then polycarbonate is the wrong product for a human stepping on it or green water pounding it. When stepped on it foreshortens/flexes and breaks the seal and crew mates can even wind up in the cabin.

Cast acrylics like Acrylite GP made by Cyro is used by most all hatch manufacturers. The only hatch I know of using polycarbonate is the Bomar 100 series and it has cross bars to brace and support the polycarbonate.

Originally Posted by deltaten View Post
While possibly not the best product for bedding, sealing or making most boat parts watertight; silicone is the recommended product for Lexan installation in this instance. There *may* be other sealant/adhesives that have been used by boat pros; but I am not one of those; nor do I have a chandler handy

Yes but the silicones recommended are specifically engineered for "glazing" applications. There are HUGE differences between the many products that all call themselves "silicone caulk". They are not all created equal.. Products like Dow 795 or GE Sil Pruf SG-4000 are the preferred products but Sikaflex 295UV with the special primer can also be used.. Boat life Life Seal is a silicone /polyurethane hybrid and really should be avoided on cast acrylics or polycarbonates. Boat Life silicone is NOT a structural glazing silicone it is a generic silicone.

Many of the marine sealants like Life Seal are moisture cure. In a dry environment they can take a long time to cure.

Originally Posted by deltaten View Post
Now 24 hrs later and warmed for half that time; the stuff seems ta be a bit firmer; but still sticky and smeary under finger pressure, once the very slight (but sticky) "skin" is broken.
Looks like it's coming off and being re-done Once there.. I'll look for some Lexel. I know I've seen it before and read up on it; but don;t know if it's available common to me
If you want the installation done right then you can't go wrong with Dow 795 or GE SG-4000... Dow 795 is available from Mcmaster Carr for $8.82, a less than Boat Life and a LOT better for the intended use....

Tony D'andrea is the marine plastics guru. He advises ABYC and works hand in hand with Lewmar on many projects. He owns Select Plastics in CT USA. For years he worked for Cyro Plastics, makers of ACRYLITE GP, and now owns one of, if not the largest hatch warranty & repair facility in the world. In short he knows marine plastics and is not shy about giving his opinions on issues such as screwing/through bolting, acrylic vs. polycarbonate and what sealants to use..

I have had lengthy discussions with Tony on all things marine plastic and he is a walking encyclopedia of plastics knowledge. I will simply default to him as he can say what needs to be said better than I....

Here are some quotes from Tony D'andrea.

Tony on Acrylic vs. Polycarbonate:

"Consider the Following:

  • All major hatch, portlight and window manufacturers use Acrylic in offshore / bluewater marine products.
  • Acrylic is more scratch resistant than standard (9034) polycarbonate.
  • Acrylic is significantly more durable when exposed to Ultra Violet radiation (sunlight).
  • Acrylic is less expensive than Polycarbonate.
Don't misunderstand my preference for acrylic. I buy, use and sell a significant amount of both products and each has its application. In my humble opinion Acrylic is more durable, versatile and cost effective in the hands of a skilled craftsman than polycarbonate.

Additional considerations may include polycarbonate with UV and scratch resistant coatings. While these products are heavily promoted by several manufacturers and carry 5, 10 even 15 year warranties the following information has been reported in "real life"applications:
  • Polycarbonate is impact resistant. When its new it is almost impossible to break.
  • Small quantities (less than a 4 by 8) in gauges over 1/8th inch are difficult to to find in the uv/scratch resistant grades.
  • Colors are limited. Only two standards (gray and bronze). Try and find anything thicker than 1/4 in UV/ scratch resistant!
  • Polycarbonate foreshortens when subject to static or dynamic loads. What this means is if you replace your hatch lens with polycarb, seal it and then step on it the ductile material will deflect (bow) in the center. One of two things may happen. 1st you will surely break the watertight seal, 2nd you may end up with a leg in your galley.
  • As for the warranty: The original owner is warranted against failure subject to the material being submitted to the distributor for evaluation with the original invoice subject to actual replacement cost at the time of purchase. I guess this means they sell you a new square of material and apply the old payment to the new cost. How about the labor to fabricate the part, install it and sealant? Why take the chance?
Polycarbonate is a great material, The US Air force uses it for fighter canopies! I sell Polycarb to the USCG and US Navy. Remember they don't mind using it because we are paying to replace it every three years.
Both Acrylic and Polycarbonate have specific uses and installation requirements.

Cast Acrylic (of a specific thickness) is in accordance with CE and ABYC guidelines, and installed on virtually all of the big blue water sail boats produced on both sides of the pond. Polycarbonate is commonly used as a replacement due to its ease of fabrication and incredible initial strength. The USCG and USN require Polycarbonate on their vessels but they also have a PM cycle of 36 to 42 months for change out. My Tax dollars at work...

Due to its ductility Polycarbonate it is more challenging to install. I have seen Sika Flex 295UV with primer and Dow 795 both mentioned. I use and recommend both. Dont go over 4 ft continuous length with a fixed portlight. Remember the coefficiant of thermal expansion for Acrylic and Polycarbonate is in the neiborhood of .000039 per inch per degree F. That means an 8ft plastic port will expand and contract up to 1/2 of an inch from the coldest day in Feb to the hottest day in summer. WOW!! Compartmentalize the job. It will be easier to install and less prone to leaks.

Tony on Screwing or Through Bolting:

Never ever bolt a plastic portlight in place. Screws are fine to hold a lens till the adhesive cures. Take them out asap and fill the holes with the afformentioned products. Both of these products are rated at 700 + percent elongation before tear.
Strong flexible and UV resistant. Been to a boat show lately? Seen any screws?

Tony on Sealants:

"I hear the question as to which sealant to use when bonding Acrylic, or Polycarbonate to aluminum, stainless or FRP over and over and over.....

Well here goes... The only three adhesives I would consider using are Sika Flex 295 UV with the primer, GE SG-4000, and Dow 795. Using the correct adhesive is only 1/2 the battle. Do not apply the sealants below 50 degrees F. The temperature must maintain at least 50F during the entire 21 day cure cycle. Cut this corner and your finished before you start. Preparation of the bond area is also very/ very important. DO NOT TOUCH THE BOND AREA WITH YOUR BARE HANDS! Contamination from the dirt and skin oils will make a solid cure impossible. You may clean the FRP and metal with acetone to prep the area but if you touch the Lexan or Plexi with harsh solvents you will ruin the portlight. A 50/50 mix of isopropal alcohol and distilled water will work well to clean the plastic if needed. Remember that clean enough is not clean enough."

Select Plastics (LINK)

-Maine Sail / CS-36T

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Last edited by Maine Sail; 03-08-2013 at 09:39 PM.
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