Lexan vs. Acrylic

This post originally appeared on Windborne in Puget Sound

---

If you are replacing fixed ports on your boat, you will be faced (or should be faced) with the choice between Lexan (a trade name for polycarbonate) and Plexiglass (a trade name for polymethylmethacrylate, aka acrylic).  Here are some features of each which might help you decide which to use:

• Plexiglass is transparent to UV radiation.  That means that anything inside the boat will be subject to UV degradation if the sun shines thru the window.  That also means that UV radiation passes harmlessly thru Plexiglass without having any effect on it.
• Lexan is opaque to UV radiation.  This means that it protects the boat interior from the ravages of UV.  But because the UV radiation is stopped by the Lexan, that means the Lexan is subject to the damage that it is preventing on the interior.  UV damage to Lexan causes it to turn yellowish brown and craze (millions of tiny surface cracks).  The effect is that your view eventually is destroyed:

  

  Lexan window after 7 years

• Plexiglass eventually crazes too...  But after a much longer time period.  However it does not turn brown or discolor.
  

  

  This Plexiglass port is 38 years old.

• Lexan is often touted as the "bullet-proof plastic":
  
  

  Property	Unit	Polycarbonate	Acrylic
  Tensile strength σΜ at 23°C	MPa	60-70	80
  Flexural strength σbB	MPa	90	115
  Impact strength acU (Charpy)	kJ/m2	35	15

  Sources:
  
  • Lexan 9030 Sheet Product Datasheet
  • Plexiglas GS Product Description
  
  In tensile strength and flexural strength Plexiglass is stronger than Lexan.  Plexiglass is weaker than Lexan only in impact strength (resistance to penetration by a quickly moving sharp object). 
  
  These comparisons are made on virgin material in both cases.  I have no data, but all that surface crazing has to act as stress risers and therefore crack starters - much earlier for Lexan than for Plexiglass.
• Lexan is two to three times more expensive than Plexiglass.
• Lexan is less scratch-resistant than Plexiglass

So, as in many things in life, the choice is not as clear (pun unintentional) as it might seem at first blush.  As the midway carny says, "You pays your money and you takes your chances."

I will say tho, that for Eolian, we have chosen Plexiglass whenever it was available.