I would say something along the lines of this. Others might not agree and would advise you not to rock the boat but I've always been a risk taker. I recently formed an email along these lines and got myself a 25% raise (but I was confident that I was an asset to this company and knew I had a leg to stand on.) I would suggest fighting for what you believe in. Two years is a long to be a part time employee. I would phrase the email in a nice enough way so if they decline, it won't be uncomfortable for you.
If you were to send an email like this, you might want to consider doing it around the time the company is doing their budget for the next fiscal year. That is the time my company considers adding full time positions.