Like others here, who have commented, I am unfamiliar with disc golf discs. However after a brief Google Search, I did learn something of the composition of the dics used for this purpose. You should have been more specific as to anything you know about the disc in question, the brand, the possible composition, etc, to better enable those of us chemists who don't know discs, but do know something about plastics to better respond. Perhaps you could add to your commentary by anything else you know about them.
From my limited on-line reading, I learned that discs are generally made of thermoplastics and are injection molded for the most part. Many dics are said to consist of virgin plastic, plus recycled plastic in some proportion. Types of plastics cited were polypropylene, nylon, Hytrel (DuPont tradename for a polyester), and urethane. There may be others.
I am assuming that the physical integrity of the disc is such that there is no visible loss of material or dimension. Is this the case? If not and if there has been a weight loss and in this case a substantial one of 5.1%, it is likely that this loss is of a material added to the plastic in the thermoforming process to improve properties. Such materials are usually low molecular weight compounds and are termed, "plasticizers."
Of the various materials, which I found listed for use in disc golf discs, here are some quick thoughts about plastic properties and plasticizers.
- Polypropylene is rather impervious to moisture and does not generally contain a plasticizer. Yet, sometimes mineral oils are used along with other non-polar materials, but this is usually in film or fiber applications. Polypropylene is generally insoluble in most organic solvents and can only be dissolved with difficulty in things like hot xylene or hot ortho-dichlorobenze, etc.
- Nylon will absorb a small amount of water which affects its mechanical properties generally making it tougher. An example is nylon used in string trimmer cord, which can be life extended by a water soak of a few days. N-butylbenzenesulfonamide and 2-ethylhexyl-4-hydroxybenzoate are the two plasticizers of various polyamides in most commercial applications.
- Polyester usually has a plasticizer of some sort, used to soften and extend the mechanical properties by altering the glass transition point. Various compounds including phthlates are used.
- Urethanes may in some circumstances be plasticized with various materials including phthlates.
You can read more about various plasticizers in this pdf document found here: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwjl5OfmtargAhUEWqwKHR8ZC60...
With the exception of maybe water, it would be difficult to impossible to reimpregnate a thermoformed plastic product with a plasticizer after the fact. Without more information, then, I would suggest that you soak a disc at room temperature, weighing it before and after a soak of a week or more to gauge if there has been any weight gain.
Alternatively, you might also try one of the automotive conditioners suggested by Barbara Bossard above weighing before and after to see if any weight is gained.
If not, then, I suggest abandoning those dics.
Hope this helps.
