Thermoplastics do lose some strength and resistance to pressure as the water temperature increases. For hot water and radiant heating applications, it is important to be able to calculate the strength of the piping being used. A derating table is provided to show the factor used to rate the strength of PEX (cross-linked polyethylene) pipe and PB (polybutylene) pipe at increasing temperatures in Farenhieght.
To understand derating, the factor is used to provide the true pressure resistance with an increase in temperature. For example, PEX tubing rated at 160 psi at 70º F will withstand approximately 96 psi at 180º F.
160 (psi) x 0.6 (180º F) = 96 psi
As you can see from this table, PEX remains stronger than PB piping at temperatures over 160º F. In general, temperatures above 180º F are not recommended for either pipe.
Source: Engineering ToolBox - Thermoplastic Pipes - Temperature and Strength Derating