Mike McCoy's Thoughts on LNVT Resins

I was re-reading the last Tug Talk [Spring 2018] and noticed that orthophthalic was misspelled in the title of the article.  It's correctly spelled in the quote from Net Composites in the body of the article. That launched a teaching script in my brain. It's a hereditary problem. Not that it's interesting to anybody but a science nerd like me but here we go…

First let's review what a polymer is.  The word has two parts poly meaning many and mer which for this tutorial will define as "the building block".  So starting with a big soup of "mers" a catalyst and a stop molecule a chemist can build polymers of specific lengths.  Dimer and trimer would be a two and three starting blocks connected. The stop molecule puts a cap on the amount of mers that can be connected. Alternatively large chains can be created and then chemically chopped up to specific lengths and the purified by separating them by apparent molecular weights.

In the resins you have described there are two starting blocks; iso and ortho phthalic acids (the ph in phthalic acid is silent). They are isomers of each other.  Iso means "the same". But wait! "They have to be different or there is no discussion. Right?" Iso an ortho phthalic acids have the same chemical formula C8H6O4  or better yet C6H4(CO2H)2.  They have the same molecular weight and share many other physical attributes like melting point but they differ in how the constituents are connected to each other. In this case we have two carboxylic acids and a benzene ring.  IUPAC (a bunch of old guys that make the rules for chemical nomenclature) explains the difference by using a naming system that tells where things are connected. So we have benzene-1,3-dicarboxylic acid and benzene-1,2-dicarboxylic acid respectively.

Figure 1 Isophthalic Acid

Figure 2 Orthophthalic Acid

It's extremely simplistic but accurate to say when two isophthalic acid mers are connected they create more linear polymers than when orthophthalic  acid mers are connected. The latter polymer takes up a different three dimensional space. When the polymers crystallize into solids their crystal lattices differ.  This might explain their differing susceptibility to permeation by water.

Lastly but maybe most importantly…  I don't believe that this difference is a significant factor in the blistering problem on some LNVT's . The greater influence on the hull integrity is the quality of the polymers as delivered by the manufacture(s). Some were not as good as others.  Batch to batch reproducibility is hard to maintain especially if many sources where used. The yard would have accepted the product as delivered and not done any in house testing. The guys working in the shop would see the difference and a good crew would adapt accordingly.

BTW…when I took my paint down last I saw clear resin. This makes me believe that Tug E. Bears bottom was possibly gel coated but no pigment was added. I 'll take a better look next time.

Mike McCoy, Tug E. Bear 37VT62