Polyurethane Adhesive

What exactly are you trying to improve? Cure time? Adhesion?

There was a thread last month (started Feb 10) entitled "Non-foaming polyurethane help". Many of the same points that were discussed there would be just as relevant to this problem.

John Aspen Research, -

"Turning Questions into Answers"

Opinions expressed herein are my own and may not represent those of my employer.

Dear John,

When I said "improve cure" I meant "cure time and adhesion over metallic substracte". I read the thread you sugested above but here I'm talking about solventless adhesive and not about foam or rigid PU. Catalyst as DABCO cannot be used in this case due to low pot life problems. I made some trials using Triethilamine and I got a improved cure response but not good enough. Solventless adhesives show low green tack ( rounded to 10 kgf/in ) and I need 50 kgf/in of bond strenght.

Rodrigo

What metals? Pretreatment of the metals is not an option?

How is you adhesive applied? If you use a 2K system and some sort of static mixer for application to the adherend, pot life shouldn't be a problem. Long pot life and fast cure exclude each other.

Regards, Oliver

Look deeper into the thoughts behind the thread. Molecular mobility in a solvent is much greater than in a solventless system - in the same way that mobility is greater in a foam system than a nonfoamed system. The same ideas of decreased diffusion of the reactive ends apply in both cases. Go back and look at the other thread again. It really does apply.

John Aspen Research, -

"Turning Questions into Answers"

Opinions expressed herein are my own and may not represent those of my employer.

Is this adhesive for food suitable flexible packaging? If so you will be severely restricted in the catalysts you can use due to regulatory requirements - most notably CFR Chapter 21 175.105. In any case the pot life and total cure time will be your primary considerations rather than green strength. If you are doing the usual mixing head onto transfer rollers for this application then you may need a pot-life of 20 - 30 minutes to allow for machine stoppages and shift breaks. This window is never going to allow you to add enough catalyst to greatly affect the green strength. I recommend for overall curing that you look at an acid blocked DABCO, or triethanolamine as a reactive catalyst.

Colin

Dr Colin Reed Business Development Manager - Adhesives Rosehill Polymers Ltd

Dear Oliver,

Sorry my late to answer your comments.

Alufoil. They use Corona Discharge as an option of preteatment.

of static mixer for application to the adherend, pot life shouldn't be a problem. Long pot life and fast cure exclude each other.

It's a 2K system and I'm using MMDC ( Mixer and Dispenser Machine ). The problem I'm facing it's indeed the fact that long pot life and fast cure exclude each other and I need 50 gf/25mm as green tack to make a double lamination ( The first lamination using solventless adhesive and the second lamination using solvent adhesive )

Thak you very much

Dear Dr. Colin,

This adhesive is for indirect contact with food but some application does not require any FDA status. Yes!I'm doing the usual mixing head onto tranfers rollers and I really need a pot life of 20-30 minutes but even when I use triethanolamine I don't get the green strenght I'm looking for. I made some trials with a solventless adhesive with 13% of free NCO groups mixed 100:15 parts with TEA and the pot life I got was rounded 6 minutes but even under this sereve conditions I didn't see any diference regarding green strenght. Do you think I'm on the correct way ? My doubt is if TEA or DABCO can give me the strenght I'm looking for even with a short pot life.

regards,

Rodrigo

Do you get the required green strength on any substrates? You won't solve this problem with catalysis. You require the adhesive to be essentially uncured for at least 30 minutes, so you can't increase the initial strength through curing. You also have viscosity limits on the material to be able to apply the correct coat weight by transfer roller, and so essentially the maximum strength of the adhesive is being determined by application factors. If you are getting good green strength on other substrates then it may be a wetting factor - common with metallised films. Try adding a wetting agent - sulfosuccinates are particularly effective in this case. Additionally make sure you are working at the maximum application temperature that local regulations will allow. The higher the application temperature you can use, the higher molecular weight you can make your adhesive and thus increase the green strength.

Colin

I didn't get the required green strenght yet. Indeed, the highest value I got was only 6gf/25mm and I need at least 50 gf/25mm. I produced a 2K solventless PU adhesive with 9% of free NCO (40000 cps). So, this polymer has high molecular weight and to be applied should be heated (60 degrees Celsius). I can produce a polymer with low NCO, i mean 3 to

5% and use propylene carbonate as diluent but I"m not sure if I'll reach the target: 50gf/25mm.

I know this an innovation, this kind of adhesive don't exist yet but I'm trying to find out how increase the green strenght when working with solventless systems.

I'm now trying to develop a catalyst based on DBTL, Voranol and Triethanolamine and react this catalyst with a polymer of high molecular weight. I agree that mobility in solventless system is a big problem. To improve adhesion on metallised films or alufoil I'll try to use Silane.

In this issue, at least for a while, don't mind the pot life or temperature. I just wanna get 50gf/25mm.

what do you suggest ?

Tk u.

rodrigo

Is there a possibility you can heat and cool between laminations?