are separately subjected to the humid heat treatment and then blended it is surprising that the degree of foaming depends on the melt viscosity and falls to such a low degree for both high and low viscosity copolymers that each can be subjected to the humid heat treatment without substantial foaming and then combined to obtain a blend of copolymers having a melt viscosity that is within the range of that of a badly foaming copolymer the blend of copolymers can contain one high and one low melt viscosity component or it may contain more than one high melt viscosity component more than one low melt viscosity component and may also contain one or more components having the middle melt viscosity range e g 5 u00d7 10 4 to 13 u00d7 10 4 i e the badly foaming range the calculation of how much of each blend component to use in a preferred embodiment to obtain a desired melt viscosity mv at 372 u00b0 c of a final blend z can be found by mixing n components a b n according to the following relationship equ1 this calculation provides the melt viscosity of a final blend mv z assuming no viscosity change occurs during the extrusion process actually melt viscosity will change during such process and the calculation must be adjusted in practice in accordance with the viscosity change for example if the melt viscosity has been found to be decreasing during such process by about 4 u00d7 10 4 poises and the desired viscosity of the blend is 8 u00d7 10 4 poise the blend would.