wood adhesives for glulam components

[fastline12]

We are building some glulam components for a personal building. I am finding it difficult to source a product in the USA. We are primarily targeting a single part adhesive that will air dry without heating. I am open to 2 part adhesives but the air dry will be a stiff requirement.

We will be gluing CDX plywoods to dimensional SYP with a target shear strength to meet or exceed 1000psi. That seems to be about all plywood can hope to handle so I don't see a need to target glues outside the ability of the substrate.

One product we have stumbled across is MDI. I have yet to find a good source of information to select the right glue. Right now I would like to use a type I glue but I have not see how prices look on these products and this is a cost sensitive project. We intend to obtain samples and do in-house testing to ensure our procedures are good.

 

[Brad805]

Sounds a lot like you are making your own plywood box joists. Are you sure it is cost effective for a one-off personal building? I have went down this path before only to find it ends up being a colossal make work project.

Have you checked the bond stress for your design case? Normally the home handyman does not have the equipment to make a proper I section, so you either end up with two layers of plywood sandwiched over the flanges or four wood flanges with plywood in the middle. If that were the case I would not expect the bond stress to be as critical. Also, your contact area tends to be many magnitudes greater than what manufacturers offer. West makes some very good epoxies and the benefit with those is you do not need the clamping force that many other adhesives require. I like their West 610 system, but the cost is very high per unit volume. The glulam manufacturers use a resorcinol adhesive (casein years ago), and that requires in excess of 100psi to generate the bond stress it is rated for. I have heard stories of some that have used it in similar cases without the clamping force the glulam guys generate, but I never have. Another option may be a polyurethane like PL Premium. It is rated as a structural adhesive. I made some flooring inlays (5mm Tigerwood over baltic birch plywood made in a vacuum press) a few years ago and I tested several different adhesives by soaking them. This test is not all that appropriate for your case, but the PVA bonds failed, so did the Unibond, but neither the epoxy nor the PL Premium failed after the finish wood was fully saturated.

Be careful about the moisture content of your flange sections. If you are buying from a typical supplier this could be a challenge.

Good luck.

 

[fastline12]

We are basically doing a glulam roof truss. It will sandwich plywood with the dimensional lumber cords on each side of it. Mathematically it works great and most test data indicates plywood failure before glue bond but that assumes a reasonable glue bond. The clamp force will be provided with either nails or screws. There is a manufacturer that builds a similar truss and they nail them, then roll them to get the nails deeper but ultimately the nails cannot really provide much clamping force so I feel we can find a way to make this work. The screws were in an effort to forego any pre-clamping before screws but we would still need a procedure to ensure the screws pull things together properly.


Who might I contact for adhesives?

 

[Brad805]

PL Premium is available at hardware stores and might be worth considering. The most common use for West Epoxy is the marine industry. I buy from a marine repair shop. The 610 system can be used with a typical caulking gun, but the cost in Canada per tube is around $15-20 (PL Premium is about $8 here) if I recall correctly. Due to the mixing arrangement the 610 tubes do not have the same volume as the normal tube. We have bought two part West epoxy in 5gal pales before too for school repair projects, but those are not as easy to find and it is more work to mix the correct amounts before your work time expires. For MDI or industrial type adhesives you will need to beg an industrial supplier. I found a wood wholesaler that supplies adhesives and sells to individuals, but many require proof that you are a business. Sometimes I used my company name, and that can make a difference. Most do not want to fiddle with home handymen, so it can be a challenge. I lucked out one time and had one send me a 5gal pale for free, but that is rare. I would try to keep it simple. Getting the products once is possible from an industrial supplier, but if you run out or become a nuisance they will stop talking to you. You are buying pales, and they are used to selling pallets or truck loads.

I would do some tests with a polyurethane or epoxy. The clamping force generated by screws or nails is quite variable, so I would aim for an adhesive that requires the minimum clamp force. Comparing to what others might do is tricky unless you know the equipment they have.

Measure the moisture content of your lumber. I still don't quite follow what you are building, but glulam production requires strict controls on wood moisture contents. The moisture content of each piece must comply with certain limits and the difference between various pieces is also measured. Large shrinkage stresses will destroy many glue bonds.

 

[Sawbux]

There are problems gluing to SYP. The reason that SYP laminations are 1-3/8" instead of 1-1/2" as in D. Fir is that the SYP is planed just before gluing. The sap in SYP comes to the surface and seals the pores in the wood, interfering with glue bonding. You may want to run some tests with the glues you are using.

I did a popsicle stick bridge project with my daughter when she was in high school. We sanded the sticks before gluing to improve bond. Her bridge ended up breaking the test weight apparatus used to judge the contest!

 

[fastline12]

SYP seems to be the lumber of choice for some LVL applications but I am left wondering if something else could be used that would provide a more ideal glue bond? I know lots of people cuss SYP because it can start to twist up over time. SPF lumber still has reasonable properties but I might assume that it is not commonly used because of the variability of species? Glue bonds and structure kind of puts you in unknown waters?

 

[Brad805]

The wood types depend a lot on your location. The mill near us uses a lot of SPF and Doug fir. They machine stress rate the material, so they know its properties quite well. Without knowing the options in your area there is not much for us to add. A sketch of what you are building would be helpful.