When you find a company, or group of companies who can take your product at the end of its useful life as a raw material and use it to produce a product you can use as raw material , you have a closed loop partner.
A plastic manufacturer provides unrecoverable plastic to a company like New Hope Energy, that reverts the plastic to oil the manufacturer uses to power it's production machinery.
You design a product using resins that are non-toxic and recoverable, you can find a recycling partner who can mechanically regrind it into a product that replaces a portion to the resin you need for your product. (regrinds)
Find or form a network of companies looking for closed loop partners, like the Industrial Waste Reduction Network.
Many companies have sustainability officers who identify opportunities for closed loop partners.