It’s not the plastic. It’s the thickness of the top bar. ON plastic it is practically nonexistent.
A quick search of 50 years among the bees turns it up on page 46.
“When attending that same convention that very practical Canadian bee-keeper, J.B. Hall, showed me his thick top-bars, and told me that they prevented the building up of so much burr-comb between the top-bars and the sections. Although I made no immediate practical use of this knowledge, it had no little to do with my using thick top-bars afterwards. i was at that time using the Heddon slat honey-board (Fig. 6) and the use of it with the frames I then had was a boon. It kept the bottoms of the sections clean, but when it was necessary to open the brood-chamber there was found a solid mass of honey between the honey-board and the top bars. It was something of a nuisance, too, to have this extra part in the way, and I am very glad that at the present day it can be dispensed with by having top-bars 1-1/8 inch wide and 7/8 inch thick, with a space of 1/4 inch between top-bar and section. Not that there is an entire absence of burr-combs, but near enough to it so that one can get along much more comfortably than with the slat honey-board. At any rate there is no longer the killing of bees that there was every day the dauby honey-board was replaced.”
–C.C. Miller, Fifty Years Among the Bees.
"Q. Do you believe that a half-inch thick brood-frame top-bar will tend to prevent the bees building burr-comb on such frames, as well as the three-quarter inch top-bar? Which kind do you use?
A. I do not believe that the one-half inch will prevent burr-
combs quite as well as the three-quarter. Mine are seven-eighths."–C.C. Miller, A Thousand Answers to Beekeeping Questions
"In the early 1890’s the thick-top frame was introduced to the public but some years prior to that time J.C. Hall, then of Woodstock Ontario, Canada, had been using frames with top bars 1 inch wide by 7/8 inch thick. Soon after he began using them he discovered that the tops of these frames were free from burr combs. Likewise there were no brace combs between the frames. He made his top bars thick, he said, not because of the burr or brace comb nuisance, but because he had desired to prevent their sagging. Dr. C.C. Miller soon called the attention of the beekeeping world to Hall’s discovery and in a very few years the thick-top frame came to be almost universal. After the top bars were made stronger and heavier the end bars as well as the bottom bars were made thicker and wider. The natural result of all this was a stronger and more serviceable frame."–ABC XYZ of Bee Culture, page 314 35th Edition (1974)