I've flown from the US to Europe (Austria) with powdered chemicals more than once with no problems. I usually fly through London, Heathrow. I routinely bring a PMK kit from the Formulary back with me (expensive to get in Europe, but not impossible). I've also brought sodium metaborate back with me in a plastic bag. All with no problems and never a question, even though the last time there was a TSA inspection slip inside the suitcase along with a PMK kit.

I would recommend that you 1) make sure that none of the chemicals are restricted in any way (ORM-D, etc.), 2) leave the chemicals in their factory-sealed, unopened packaging, and 3) bring along the receipt and packing slip from the purchase of the chemicals and be prepared to show them if you are questioned. That will likely allay any fears.

The worst-case scenario is that your chemicals will be confiscated and you may be delayed in your flight.