how to export a list of png files into a list of alpha black and white png image files

Hello to everyone.

I'm trying to rotoscope a list of 187 png pictures using the tool rotobrush because I want to convert them into a list of png black and white alpha images,like these that you can see here :

https://drive.google.com/open?id=11HHgS7U8wnC_wpZMeEIpiKF2fOOfLEi-

I would like to know how to save / export all these 187 images in black and white,alpha sequence. I have also made a post here :

https://www.reddit.com/r/AfterEffects/comments/ekl550/how_to_save_convert_a_list_of_png_files_into_a/

and the user "ikiper" told me to do this :

"File - export - media Than you should select PNG,and rgb+alpha. İt will export every frame as png image with alpha"

but I'm not able to reach the result that I want. Infact this is how should be :

https://drive.google.com/open?id=1cpbzsmdc3UCL-bLs0tlGaSZYHYpuonz9

instead,this is how it is :

https://drive.google.com/open?id=1R3K4Oyrw_6-E1IyDYArOzohkBcPq_pQ7

what's wrong ? thanks.