Filling polyurethane resin with used tea or coffee

I’ve just written some outline notes explaining the uses of fillers with resins and posted them in the ‘Materials/-casting’ section entitled Common fillers for resin casting. Here you will find the various reasons for using fillers listed, i.e. to save on cost, to thicken the resin, to alter the weight or strength of casts or to achieve a particular surface appearance. I’ve also detailed the fillers I know about, how much they cost and where to get them.

At the moment I’m making some tests filling polyurethane resin with used tea-bag tea and coffee grounds and here are the first results. The tea or coffee must be thoroughly dry, and I usually just spread them out on absorbent tissue where I leave them for at least a week but drying them out in an oven on very low heat would work just as well if they’re needed sooner. For these tests I’ve used Biresin G26 polyurethane resin from Tiranti, which is my favourite for its reliability. In each case Parts A and B of the resin were mixed thoroughly but quicky first before mixing in an amount of coffee or tea and pouring (or spreading) into the oval mould. This is essential when filling resin with anything even slightly absorbent. In the first place, if the material is mixed with either part of the resin first it will start absorbing it meaning that some of it won’t be combined properly with the other resin part when added. In the second place the mix will quickly become too thick (whichever part is used first) to distribute the two resin parts evenly anyway.

resin filled with coffee or tea

Working from left to right, for the first form above I mixed 25g resin (12.5g of each part) and could add 15g coffee grounds before I felt the mixture was becoming too thick to do anything with. It was certainly not pourable but I could paste it into the mould fairly easily, making sure that it was properly packed down evenly. As the resin began to set the mix reached a ‘dough-like’ state fairly quickly where I could continue to press or even stretch it with my fingers. Certainly more press casting than anything else. When set (c.15mins) the resin had filled the surface completely smooth but the coffee had coloured it a rich brown and the coffee grains were visible underneath as an even speckle.

25g resin, 10g coffee

For the second form, next to it, I mixed tea and resin in equal weight (in this case 10g each). Tea-bag tea differs from loose tea in that the particles are smaller, more like small grains plus dust. But these expand immediately on contact with the resin and very quickly the mix became like damp soil. But it was workable enough to trowel and press into the mould and like before after a minute or so I could press it down further with my fingers. The result was equally attractive, but different. Because the tea is much more absorbent than the coffee and I had used less resin anyway there was not enough of it to fill the surface and the resultant effect is very much like a peaty soil pressed into a shape. Unlike real soil though, the resin had bonded the tea into a very strong, hard solid.

coffee and tea compared

10g resin, 10g tea

For the next sample I used less tea (half as much in fact, 20g resin to 10g tea this time). The mixture was still thick i.e. more of a paste, but much ‘wetter’ than before and could be more easily introduced into the mould. When set the form surface was smoother (albeit broken in parts) because more resin could coat the mould surface. The tea didn’t stain the resin itself like the coffee had done because the used tea-bags had been rinsed after use.

tea unsanded or 'cut back'

20g resin, 10g tea

For the final sample in this small series I added even less tea ( 20g resin, 7g tea) because I wanted the resin to coat the surface of the cast uniformally but after full curing of the form I cut back the surface, a term used to describe using an abrasive on the surface of a filled cast to expose the filler better. The result of sanding with coarse sandpaper, below, was a surface which looks very like sandstone but is much easier to carve.

20g resin, 7g tea, sanded

5 thoughts on “Filling polyurethane resin with used tea or coffee

  1. I’ve tried to do this for a recycling project with coffee grounds and the same resin (biresin G26) you’ve used. I used a plaster mould, with vaseline to demould. I’ve tried twice now but every time the resin foams and expands drastically, creating more of a pumice type material😦 I moved inside, after thinking the workshop I was working in had too much atmospheric moisture, but it did exactly the same thing – what am I doing wrong?

    • You’re not really doing anything wrong .. the tea/coffee will always cause the resin to expand to a certain extent. There will be some moisture in the used tea/coffee even if they seem totally dried out. When I first tried this I used tea/coffee powder which I’d left to dry for a couple of weeks, then stored in a tight container .. I think it was about a year old .. but there was still some expansion. I think the only solution is to properly dry the powder in an oven i.e. laying it on a flat metal tray in the oven on the lowest heat for a few hours. See if there’s an improvement. You should consider that there may be advantages to this expansion .. often the surface remains whole, and the structure is still pretty strong, but the cured material is much easier to cut or carve. Have a look at my related article ‘Filling PU with plaster’ in the ‘Fillers’ section.

  2. Pingback: Recycling Coffee Dregs | Mediterranean Bonsai

    • I’m hoping you mean unused! I don’t see why not but it may be, by nature, too absorbent! Also, there are many different types of cat litter. You’ll just have to test it yourself.

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