Why do we use mat? And do you use it with epoxy?

[Rachel]

Okay, this is going to be long and wordy. Please don't feel you have to read it! And I've been arguing with myself about whether to post it, because in a way it sounds like some of the same old saws about epoxy, binders, and etc.

Eye-candy, so Dave can keep reading ;)
(Sunrise at sea)

But.... do you ever have a moment where someone asks you to explain something that you thought you knew, and then you find that you aren't so sure you DO know? Or that what you "assumed" made sense, but when examined.... maybe doesn't? it's like when someone asks you to define a word you "know" and you think "Oh sure, that means... erm...."

I'm having such a moment now about two things, inter-related:

1) Epoxy and mat together (how the question arose).

2) Why we use mat at all (the question it led to and which is the main one).

Someone new to boats and fiberglass repairs was asking about resins (that discussion went fine, and none of my foundations were shaken ;). He decided to use epoxy, and then was asking about how to fiberglass some cleats and bulkheads. I recommended a couple of the good and free epoxy manuals, and then gave a few suggestions, such as how to take advantage of the green stage, why fillets are good on inside corners, etc.

Then, because, he had asked about cloth weights, I explained why I like to use a biaxial cloth for things like bulkheads with one long "parallel" stress edge, and mentioned that I often use something like biaxmat 1708 or similar, lighter reinforcements.

(An island shoreline in the San Blas, Panama)

Well, a fellow who is a boatbuilder, came back and said that first of all, one shouldn't use mat with epoxy because of the binders that will only dissolve in styrene. Fair enough, I said, but the stuff I use it stitched together. Well okay, but when you think about it, the mat itself is not stitched together, I guess, is it. Just the biaxial layer is stitched to the mat layer. Now I know it works, as I have used it, but is it wrong or undesirable?

And, the main thing that made me post here (sorry this is so long, but I don't want to make you all feel like you have to "pull" information out of me in subsequent posts). Is mat outdated? Does it just add useless weight? Should I be using biaxial cloth without mat? As I thought about it it started to really bother me, and I hate the idea of giving someone wrong advice and basically spreading it around. Ugh!

(Local boat in Colombia)

I suppose I had the idea that mat somehow helps "fill" the pockets between "real" cloth layers, in order to avoid resin pockets, or starved places. But then, I do have some biaxial tape that doesn't have mat on it. So why use mat? I'm pretty sure I have seen it recommended as an "every other" layer on deck re-coring jobs, but then, like I say, I found I really couldn't state my case properly.

So that's why I'm here asking.

I haven't used polyester resin in so long that I don't really remember how it behaves (and I'm not that curious about it). But I do want to know that I'm using (and recommending) the proper cloth(s) with epoxy, for things like tabbing bulkheads, deck re-coring, filling in disused holes, and the like. Here is an excerpt of a boat-builder's comments when I suggested the stitched biaxmat for use with epoxy for things like tabbing in bulkheads. I think I can see some obviously wrong things in here, but.... maybe not all of it?


Mat doesn't work with epoxy.

#1,,,he binder doesn't dissolve so it never lays down. And.. #2 The primary job of mat is as a binder between layers of roving or cloth. Without it, when using poly & hand layup, the weak link is the resin between the cloth layers. This will fail pretty easily and the two will come apart. (Delamination) Mat goes a long way in solving this.

Mat adds very little strength itself, just bulk and weight...

Remember, poly resin shrinks when it kicks so it tends to pull up the edge of your tape tabbing. The mat layer stops this. Epoxy shrinks very little so this is really not necessary. Or even appropriate.

Lastly, mat is designed to be used with styrene based resin. Because it uses the styrene to dissolve the binder, letting the material drape over shaped surfaces.

So, epoxy enters the mix..

Epoxy is a much better bonder, so it doesn't need a binder to help between cloth layers.

Epoxy shrinks very little, so it doesn't need something to hold down taped edges in hand tabbing.

Epoxy doesn't use styrene, so it makes using mat much more difficult than with poly.

( Extra credit: Styrene is why you should never put poly over epoxy. The styrene in the poly attacks the epoxy melting/softening it. )

Is it a bad thing to use mat with epoxy? No, its just added weight and work with little benefit for the part.

So you haven't ruined anything, you just took an unnecessary longer rout to where you were headed. I see mat with epoxy kinda' like buggy whips with cars. A traditional throw back to the old days.

Thanks to anyone who read through (I feel as though I should serve snacks and beverages for intermission). I know that nothing I have fiberglassed has failed - at least yet - but in addition to that I would like to know I'm doing things the "reasonably best" way, and that I understand why. I certainly don't want to be bothering to wet out and install mat layers if they are useless excess.

Rachel
(A Norseman 44 at anchor)

[Chris Campbell]

Nice post! Great idea to intersperse it with eye candy - hopefully you've started a trend. I'll try to follow suit myself...

The only part I can respond to is the binder in the mat part - all of the biax with mat that I've bought doesn't have a binder in it - i.e. it's been made specifically for use with epoxy. As to whether or not it serves a purpose when combined with biax sewn cloth I can't tell you - hopefully someone else can, since I'm using it willy-nilly everywhere as well!

Carter's Beach, Queen's County, NS

[Rachel]

Thanks, Chris. I really agonized over this post, as I know it's really wordy, and there is more than one question in it. I'm glad you are curious too.

I, too, have used the biax that is stitched to mat, so then I think the biax and mat are not "bindered" together. But then.... what holds the mat itself together, is one of the things I got to wondering. I mean, why don't all those loose shards of fiberglass just fall apart? Kind of like wouldn't waferboard just be a pile of wood chips if it did not have glue?

Nice photo! I like how the rounded rocks contrast with the terraced sand. Plus, just gorgeous in general.

Rachel

[mitiempo]

I have always understood that the mat was used to even things out between layers of woven roving. Stitched biaxial is smoother than woven roving and I don't think it is as necessary here. I have used the biaxial with mat on one side and it is epoxy compatible, as is some mat by itself. Most mat isn't compatible with epoxy though because of the binders as posted above. I never use mat by itself. And I always use epoxy.

In keeping with the trend here's a pic of a seal I took in Victoria harbor.

[Tim]

Fiberglass material with mat secured to the back, like the common stuff we mostly all use, holds it shape far better when wet out than the plain-backed version. Regular 45° biaxial and similar materials distort significantly and easily when wetted out, which isn't ideal for many reasons.

With care, it's possible to avoid this distortion, but generally the slim layer of mat stitched to the back of the material simply adds stability that is helpful to the user. Whether or not the minimal bulking qualities of this very thin mat are desirable depends on the use and application.

I have a piece of mat-backed 45° biaxial cloth (commonly called 1708, stitch mat, Knytex, and other names) in my hand here right now, and I really don't think there's any binder in the mat. I just made a mess all over my desk by crumbling and folding and pulling my sample, and the mat fibers come right off individually, with no sense of anything holding them together. Once they're released from the stitching, they fall apart. The mat layer is extremely thin, and the extensive stitching holding everything together is closely-spaced (1/8" or so apart) and appears to be securing the mat to the cloth above. But since I don't manufacture fiberglass cloth, I'll have to stop short of pretending whether or not I know for sure if there's some sort of adhesive involved. I don't know. My observations suggest no. I'm comfortable with that.

I never use stand-alone mat, and would never suggest that anyone do. It serves no purpose in modern construction with modern materials, and most of the quotes you listed above apply accurately to stand-alone mat. But let's not pretend that product really has much or anything to do with mat backing on stitched materials. I think much of the information you quoted above, while basically accurate in and of itself, lacks the context necessary and therefore is misleading, and doesn't apply to the mat backing that we're really talking about.

[Tim]

Here's a photo of the partially-dismantled biax as described above. I didn't want to disrupt the flow of "nice" pictures with one that was all business.

[Paulus]

From the web:

It is important to remember to use the correct type of glass matting depending upon which type of resin system you are working with.

Each tiny strand of glass that is used to weave the matting has a 'finish' applied to it in production that is designed to facilitate maximum 'wetting out' - it just means that the resin attaches itself to the glass with maximum adhesion.

For polyester resins, specify a glass matting with a 'volene' finish.

For epoxy resins, specify a glass matting with a 'silane' finish.

http://answers.yahoo.com/question/index ... 401AAOKFyt

[Rachel]

Thanks Tim. What you are saying fits with what I was thinking, and elaborates plus states it more clearly. Too, I was starting to wonder if I had just not noticed that the world had gone on without me, in terms of good practice.

I appreciate the practical photo (what is it about photos in a thread --- they're ALL nice).

Rachel

[Zach]

My theory... We as a people and human race use mat with epoxy because the biaxial glass with a mat backer is compatible with polyester resin. That is why everyone has exposure to it, as the shops and such would have to stock two rolls... People tend to use and recommend what they know.

With epoxy there is no mechanical reason to use biaxial glass with a mat backer, the epoxy can hold the distance between the bundles of glass and the surface.

Polyester doesn't have the strength to span the gap between strands of glass and the surface, which is the reason for the resin rich mat that squashes down to the surface. It also squashes in between the layers of roving, or bundles (for uni, biax, triax... et al) An over catalyzed polyester is brittle enough that you can still pull the top layer of bundles right off of the bottom layer, which surprisingly is why roving was used. Of course, this seldom happens when a boat is floating about being a boat, but polyesters strength is only 800-900psi when there is a sufficient gap and you are destroying a past repair. Polyester is closer to 15,000-17,000 when in a well made laminate or repair. Epoxy is 20,000-26,000 psi when put together with a few layers of bi- or tri like the hull sides on a big custom.

Strength wise, mat is isotropic. This means that a lump of mat is equal strength in all directions no matter which way you pull or push it. This is why a chopper gun boat works... It is heavier than a hand laid custom, but once its thickness is made it has strength. Mat is as strong as steel in some ways, and quite weak in others. Withstands well to being a boat, not so good at rough docking or a sledge hammer to the hull.

I haven't come across any new mat, thats binder won't dissolve in epoxy new off the shelf. The old stuff had a starch that was dissolved by the styrene in polyester resin. Doesn't mean its not out there, just means you have to test it first. Mat is great for doing decorative work and covering seams and curves that would otherwise be impossible with biax, or would take 12 layers of finish cloth to make any strength. If you have a corner that you want square, with no fillet and the seam never to show, you can rebate both sides of the seam or corner (make it a low spot from both directions) put in mat, and work it with a paint brush until it fits the corner to as close to a sharp corner as you'll get out of fabric. The mat relaxes and sticks to everything, but works well.

I use both the 1708, and the 1700 (which is 17 ounce biax with no mat...) Mat is great to make something water resistant, as it is 30/70 Cloth to Resin. It is also expensive to wet out. I use the stuff with no mat on cabin sides, and to tie across seams that I don't want working under paint. (Sometimes 9 or 12 ounce, if there is a harsh curve or bend...)

I wouldn't use 1700 in a shower... I wouldn't use 1708 on the top cabin of a fly bridge.

Math:

One yard of 1708... 50 inches wide, is 27 inches long. It weighs in between 23.75 and 24.2 ounces. Some 17's are 18 ounce cloth branded 17... Some .08's are really .08, and others are .075's rounded up. The 17, is the weight of the cloth, biax, it is measured by the yard. The .08 is the weight of the mat, by the square foot.

Your resin in a hand layup is roughly 50/50 by weight. So you'll end up with around 24 ounces of resin to wet out one yard of cloth with biax.

17 ounce cloth with no backer weighs 17 ounces. It takes 17 ounces of resin to wet out. Every yard you use is 7 ounces of resin saved over the mat backer. You get a gallon back every 24 yards. One roll of cloth is generally 200 - 210 pounds.

Glass by the roll is sold by the pound. At 200 pounds, it weighs 3,200 ounces. You get 188 yards of 1700 with no mat, you get 133 yards of 1708. The price difference is around a 25 cents a pound, the stuff with no mat is more expensive per pound. At the end of the roll, it takes 19 gallons to wet out the cloth no matter which you choose...

Build thickness. It takes two layers of 1708 to make an 1/8th inch. It takes 3 layers of 1700 to make a fat 1/8th. If you are trying to build some thickness, the mat is nice.

The fudge factor for resin is 2 ounces per square foot if you are glassing on to plywood. The wood takes some of the resin. I almost always do a wet layup, where I pre-wet the glass either folded back in place or on a wet out bench... and wet the substrate before the wet glass goes into place. The other camp, is the dry method which involves laying the glass in place and wetting out through the cloth... Works well for finish cloth, not so much for the heavier stuff... At least with west system in the cold.

Foam takes a bit more resin unless you resin coat it and let it gel before going on. Ultralight stuff you'll resin coat, and flip it upside down so the excess drips off. This is similar to how balsawood is pre-coated with resin to minimize the weight and absorption while you are wetting out. Really porous substrate steals the resin from the cloth and you end up with dry spots.

Almost everyone uses much more resin than is required... One of the best things you can do for your finish is to air roll and then come back with a squeedgee and pick up the excess. When air rolling, actually mash the glass cloth down, and push the excess resin from under the cloth. The wider the gap between the glass fibers and the surface, and between the glass fibers in the laminate the lower the strength. The strength of a composite structure comes from the fibers being locked in place, the glass doesn't stretch but the glue does.

To much resin under the cloth reduces strength and makes things quite difficult to fair out. Once it is down, you can either fill the weave with resin, or mix up some microballons with the excess and squeedgee it out if it is to be finished.

Cheers,

Zach

[mitiempo]

Zach
Is that 27" yard the American version? Ours up here is 36".

[Rachel]

I'm guessing that's because it's a square yard of actual cloth, and since it is 50" wide and not 36" wide, some is taken off the length to make up for it.

36 x 36 = 1296
50 x 27 = 1350

Very interesting Zach, and the part about the mat being "weighted" by the square foot answered a question for me. The fellow who was talking in italics in my post above had said the "08" in biaxmat 1708 was .08, and since I had heard it was about 3/4 oz. I was thinking it should be .8

But of course there are 9 square feet in 1 square yard, so the .08 becomes .72 and it all makes sense :)

Rachel

PS: Do we have to move this to "Boat Nerdery" now? ;)

[mitiempo]

Rachel
I quickly checked that (too quick I guess) and must have hit the wrong number so I didn't think that was what he was trying to show.

[Quetzalsailor]

I understand that mat is used to fill space between roving and tighter weave material. Also to provide a lamina under the gelcoat that reduces the print-through of fabric. Since its lots less dense than 'glass you get a much poorer glass to resin ratio so it's not desirable for strength to weight and you waste lots of resin for the amount of 'glass.

I think there are coatings and binders. 'Glass doesn't really stick to anything so each fiber is coated. It's this coating that's important to be correct for the resin. The binders keep a mass of 'glass in order so that you can convey the sheet to the part without wearing it or having it flutter to the floor.

Industrial chic...

An abandoned Link-Belt crane next to the St Francisville LA ferry landing.

[Hirilondë]

I don't have anything to add to this discussion, but I must say that I like the idea of random pictures being added to threads when no specific picture is required or available to accompany the words/descriptions.

Good one Rachel, I hope this trend continues!

[Rachel]

Ooh, nice! (is total sucker for Dutch boats) (which are normally a bit difficult to work into a thread topic ;)

Is that in your home port? I love it!

Rachel

(Hmm, might have to start a ramblings thread on Dutch boats.... )

[Idon84]

Love those Dutch boats. Had a friend backpack all around Europe and sent me a bunch of photos of them several years back. His reasoning was because the "looked really strange" and had never seen anythning like that before.

P.S. Great post! I used both the 1708(with mat) and the 1700(W/O mat) after my supplier said that they indeed work with epoxy. But, as Tim mentioned above, the mat keeps the fabric from getting all distorted and that's the reason why I bought more of the 1708 vs 1700.

[Zach]

Rachel, you've got the measurements right.

Took some noodling when I was trying to come up with some figures for what certain jobs cost and the materials they require, when everything is measured differently by weight and a yard... isn't.

Grin.

I like that little dutch boat.

Zach

[seasailor55]

Wow. Beautiful little dutchman. That cabin roof camber (concave to convex) is a work of art.