The NFL players' union has appealed the league's suspension of Adrian Peterson.

In a letter to its player representatives and executive committee obtained by ESPN and other media outlets Thursday, the NFLPA calls commissioner Roger Goodell's punishment "unprecedented, arbitrary and unlawful" and demands an independent, neutral arbitrator hears Peterson's case.

NFLPA Appeals Peterson's Suspension The NFL Players Association has appealed Adrian Peterson's suspension for the rest of the season without pay and is demanding that an independent arbitrator, not Roger Goodell, hears the Minnesota Vikings running back's case. PDF of appeal

"Moreover, given the intense criticism that you and the League office have faced from NFL business partners, Congress, the media, NFL fans and the public at large, and the public pronouncements and commitments you have personally made in response to this criticism, it is clear that you have, by your actions, rendered yourself evidently partial and biased in this matter," the union writes to Goodell in its appeal of Peterson's suspension, which is dated Nov. 19.

Goodell on Tuesday suspended Peterson without pay for the rest of the season and told him he will not be considered for reinstatement before April 15 for his violation of the NFL's personal conduct policy. Peterson pleaded no contest Nov. 4 to misdemeanor reckless assault in Texas for injuries to his 4-year-old son with a wooden switch.

The union says Peterson was told the games he missed while on a special exempt list would count as time served toward a suspension. The 2012 NFL MVP played in just one game for the Minnesota Vikings this season and was placed on the commissioner's exempt list on Sept. 18.

NFL executive vice president Troy Vincent told USA Today Sports that he was the person who told Peterson that he would get credit for his time on the exempt list. But because Peterson refused to attend a meeting with the league last Friday, that promise couldn't be kept.

"But Adrian, you've got to come talk," Vincent said to Peterson, he told USA Today Sports. "You, your team, your counselors, whoever. Just come and tell us where you are."

Peterson filed a grievance over his status on the exempt list, arguing that he should have been able to play as soon as his criminal case was over. Arbitrator Shyam Das ruled in favor of the NFL on Tuesday, however, meaning Peterson will remain on the exempt list despite his appeal, effectively ending his 2014-15 season.

Peterson has said he intended no harm to his son, only discipline. The 2012 NFL MVP was paid part of his $11.75 million during the leave. He will keep the money accrued while on the exempt list. But the NFL's punishment now amounts to a 14-game ban, with six unpaid weeks. That's the equivalent of a fine of more than $4.1 million.

Goodell announced Aug. 28 tougher punishment for players involved with domestic violence. That action stemmed from a torrent of criticism for the initial leniency toward former Baltimore Ravens running back Ray Rice, who was caught on camera hitting his then-fiancee and knocking her unconscious on an elevator in an Atlantic City casino. Rice was later suspended indefinitely and recently had his appeal heard by an arbitrator.

According to the enhanced policy, first offenses of assault, battery or domestic violence bring a six-game suspension.

But the union is arguing that the new policy was enacted after the incident had already occurred and shouldn't be retroactively applied to Peterson's conduct in May 2014.

The union claims in the appeal to Goodell that the "NFL is making up the process and punishment as it goes -- a blatant violation of the CBA, Mr. Peterson's industrial due process rights, and any notion of fundamental fairness.''

ESPN's Chris Mortensen and The Associated Press contributed to this report.