I'm the product manager at CODi Incorporated and I noticed our materials research was referenced in this thread. I wanted to take the opportunity to shed some additional light on this subject. My email is under my profile if anyone has further questions and would like to speak to me directly.
First and foremost, Denier is a unit of measure for the linear mass density of fibers. It is defined as the mass in grams per 9,000 meters. The term denier is a literal combination of the words linear and density.
You are correct when stating the D stands for Denier, not Dupont. With that said, there are 4 main yarn sizes used to make manmade fabric.
210D;
420D;
840D;
1050D;
210D is usually only used for lining. 420, 840, and 1050, are mainly used as exterior materials.
Obviously, the larger the yarn size, the more durable the man-made fabric will be.
Determining the quality of the fabric is basically just simple math. (the four most popular man-made fabrics you'll see on the market are 1680, 1682, 840, and 1050)
1680D uses 4 strands of 420 yarn.
1682D uses 4 strands of 420 yarn, but there are 3 twists of the yarn bundle per CM in length
- (If you take a 1 foot strand of rope, then take an identical strand and twist it 3 times per CM, you will get more rope per foot of twisted vs. the non-twisted, which would make the twisted stronger.)
840D uses two strands of 840D yarn twisted, so it's "technically" a twisted 1680D, but being that the industry calls four strands of 420D 1680D, I believe they just keep it as 840D to avoid confusion. I don't create the terms used in the industry so I'm not 100% sure why they went this route.
1050D uses 2 strands of 1050D yarn twisted. So it's technically 2100D but the industry still calls it 1050D, (this is what we use in all our cases).
To respond to the original question in this thread, I'm not exactly sure about 2520 nylon. The math would tell me that 2520D is either 6 strands of 420D, or 3 strands of 840D. My intuition tells me it's 3 strands of 840D as "tfar" stated, only because it would be more difficult to weave 6 strands of 420D, and using the 420D yarn would actually be less durable since it's a smaller yarn size.
Keep in mind when purchasing and researching the different man-made fabrics, the fabric itself is only as good as the wear/tear of the yarn. And by twisting during production, there is 'more' yarn in the same area making it stronger.
Some points to consider when researching/buying cases based on material.
Twisting creates more density/mass/denier making the fabric stronger.
Twisting also creates more surface area which improves wear/tear and abrasion results
Different backings can improve fabric wear, but will not improve yarn wear.
Polyester yarns are often called ballistic nylon, which is common in lesser quality products. Wear/tear of polyester is far worse than actual Nylon.
I hope this helps and as I stated earlier, my email is under my profile so please don't hesitate to contact me directly with any questions you may have. Lastly, here's the link to our white paper again for your reference.
http://www.codi-inc.com/adv_1050d_Ballistic.pdf