This is tricky. There is one sense of "spare time" which concerns output - you aren't always expected to be working. But salaried knowledge workers are commonly considered to have no "spare time" in an intellectual sense, especially when it comes to overlap with the company's core competencies.

This is obviously controversial for individual contributors, but it's well established in IP precedent. It's why you can't work at Google and develop a competing product in your off hours, even if it's "on your own time" and on your own equipment.