Multi Threading with BinaryWriter

Question

Im using TcpClient to write data to a port.
Using NewtworkStream & BinaryWriter.
All works fine. The only problem im having is that my UI freezes
when the BinaryWriter is sending allot of data from the database.
This is what I have with the binarywriter writer = new BinaryWriter(networkstream)
writer.Write(str);
I treid to add the BackgroundWorker.RunWorkerAsync(writer.Write(str);
This throws an error.
How do I make the writer use a different thread when writing ?