Whenever using a trademarked names, you will need to capitalize it. Among the common ones that writers tend not to capitalize are Styrofoam, Xerox and Kleenix.
You do not need to include the trademark symbol (which is a capital R inside a circle) with a trademarked name, however. Some corporations want you to do that, but it usually makes text less readable. The capital letter alone suffices in telling readers that you’re using a proper noun.
If the capital letter looks odd, you can opt to use a generic common name for the product, such as foam cup for Styrofoam or facial tissue for Kleenix.
Still, sometimes such terms can be difficult to read and sound unnatural to readers. In other cases, what that common name might be isn’t even evident. What, for example, would be a generic name for Mountain Dew? Your option then is to change the type of soda that appears in the story.
My name is Rob Bignell. I’m an affordable, professional editor who runs Inventing Reality Editing Service, which meets the manuscript needs of writers both new and published. I also offer a variety of self-publishing services. During the past decade, I’ve helped more than 300 novelists and nonfiction authors obtain their publishing dreams at reasonable prices. I’m also the author of the 7 Minutes a Day… writing guidebooks, four nonfiction hiking guidebook series, and the literary novel Windmill. Several of my short stories in the literary and science fiction genres also have been published.