Ann and Bernard Wilkins of Oamaru, asks :-
Because of the presence of silverfish, we placed camphor balls in a kitchen drawer which was lined with 'malamine' plastic. Some time later the plastic had become slushy. Why?
Alan Happer, an organic chemist at the University of Canterbury, responded.
Camphor is a natural substance first obtained from a tree native to Southeast Asia. It was used in China from earliest times for medicinal purposes, and eventually reached Europe courtesy of the arabs. In addition to its medicinal applications, it was later found to repel moths, and has been used for this from time to time.
However in the mid-19th century another use for camphor was discovered. It was found to be a good plasticiser for cellulose nitrate, the first man-made plastic. (A plasticiser is a material that has the ability to penetrate a rigid polymer and make it soft and flexible.) Camphor-plasticised cellulose nitrate, given the name 'celluloid', was initially used to make billiard balls, which up to then had to be carved from elephant tusks. Later it was used for photographic film ('nitrate film'), and for ping pong balls).
It seems that in your efforts to take advantage of its insect repelling property you have accidentally encountered its plasticising one. Camphor is a relatively volatile substance, and in a closed space the vapour could easily penetrate the melamine coating and soften it.
The irony is that there is little evidence that it repels insects other than moths.
This explains your problem, but doesn't solve it. My advice would be to spray the drawer with a water-based synthetic pyrethrin spray sold for pests such as ants, cockroaches, and other crawling insects.