Revvy, as much as I hate to make my first post here a negative one, I feel the need to clarify a few points that your chemist friend failed to recognize. Allow me to start by informing you that I worked for many years in the plastics industry.
HDPE, or high density polyethylene is an injected polymer plastic. By FDA regulations, to be stipulated as a "food grade" plastic, the object must be made
entirely from virgin material. The "Homer" buckets sold at your local Home Depot are not made with 100% virgin material. Their "recipe" for the bucket allows for up to 15% recycled material. This material, while shredded, washed and sanitized may or may not be of originally food grade plastics.
Most plastics manufacturers will try to use as much recycled material as they can, but there are times, due to delivery issues, that raw or virgin material is the used for 100% of the product. It could very well be that some of the buckets from Home Depot are technically of a food grade because there wasn't any recycled material to be used. Generally speaking though, one should always consider the fact that they are allowed to use up to 15% according to their own formula.
Another thing to consider is mold release. Mold release is a spray that is used to coat the inside of the injection mold before the plastic is injected into it. To be of food grade, a company is required to use an FDA approved lubricant for contact with food, and the product must then be washed after being formed and released from the mold. This normally isn't an issue, since we wash and sanitize everything, but since the buckets in question are potentially 15% non-HDPE, food grade material, the chance exists that mold release will be retained within the plastic and degass into your beer, or other beverage, during fermentation. Most non-food grade manufacturers use a petroleum based mold release, since it is much less expensive vs. the FDA approved mold release.
Regardless of size, I would highly recommend against using the "Homer" bucket for any kind of fermentation.....sorry.
BH
Edit: I did some research to verify that my information was still correct, and it is NOT. Food grade plastics may contain recycled material, but the conditions are very stringent. You can see what the FDA has to say about it
here. I also discovered that Home Depot does not require that their supplier of the "Homer" bucket follow the food grade requirements. This does not mean that they aren't (although the FDA does not allow for dyes in food grade plastics), only that their creation, and the materials used (as mentioned above in my original post) are not necessarily of food grade.