Paper Bridge Project

Question

I am an engineering student and, for my final project this semester, I have to build a paper bridge. I plan to use rubber cement to glue the tubes of my bridge, but I want to use something else to glue the joints together. Elmer's glue has been suggested but it could warp the paper. Is there anyway to stop this? Or, does anyone suggest some other type of adhesive. It needs to be strong and light weight, and I need to try and apply it in thin layers, if possible. My bridge will be graded on weight to weight held ratio, so that's why.

Answer 1

I'm inclined to think Elmer's Glue-All (PVA glue) is a good choice, even for the tubes.

I would definitely apply it with a paint brush (like a wide artist's paint brush) rather than just glob it on. It could distort a single sheet, but if you're dealing with rolled tubes I don't expect you'd have too much trouble.

Note that painting it onto, say, the surface of a rolled tube, like a "varnish", will form a composite material with much, much greater strength than the paper tube alone. This is the same principle used in fiberglass and carbon fiber composites.

I would definitely experiment with the particular members you'll be working with. Get a feel for the methods that work before depending your project's success on them :)

You can "seal" an end or even a broad cross section with a thin coat painted on and left to dry. After that, even a big glob will not distort the surface because the fibers have been stabilized.

If you really, really needed to coat a single sheet, perhaps to act as a tension member, I would try pressing it flat (while still wet) after applying the glue to both sides (absolutely as thinly as possible). You would need some sort of non-stick surface for the pressing, perhaps that paper backing that address labels come on or a silicone baking sheet. You might also need some heat to drive off the moisture, maybe a very, very low oven (100F) or just a blow dryer.

A blow dryer will speed drying for many joints, which can be handy during sub-assembly and final assembly. Again, experiment a few times to work out how hot is too hot.

The notion of applying (and letting dry) a thin, brushed-on "primer" coat to key areas before final gluing can be an important one. That way you can use glue "beads" that will provide something of a fillet that can greatly improve the strength of the joint.

Answer 2

You should look into using bookbinders glue, which is an air drying adhesive - or you could try with artist’s grade acrylic gloss mediums - which depending on the medium used can range in viscosity from milky to paste.

Here is a recipe for bookbinder glue (PVA + methyl cellulose). While preparing the methyl cellulose, be sure to use distilled water, the author on that page leaves that bit out.

No matter what you use, do not dilute with any water. That will introduce wrinkles because the water molecules are only in suspension. A further method to reduce wrinkling and warping is to coat both sides of the material with the glue.

But really, if it was about weight vs strength, if I were you I would fashion something like corrugated cardboard - or do some self-tensioning dovetail joined origami and ditch the glue altogether...