A process for creating polyolefin blends from waste streams with controlled rheological properties can include processing a waste stream to make a mixture comprising a mixture comprising polypropylene and polyethylene and compounding the mixture with one or more peroxides or nitroxides to produce a polyolefin blend. For example, a process can include: determining the required melt flow rate of the end product; determining the ratio of polyethylene to polypropylene to achieve the required melt flow rate; effecting a separation of polypropylene from polyethylene to achieve the desired composition; determining the amount of peroxide or peroxide concentrate required to increase the melt flow rate to the required level; melt compounding the mixture; evaluating the melt flow rate of the product; and adjusting the composition of polyethyelene and polypropylene or the amount of peroxide added if necessary to achieve the required melt flow rate.