BAE3201 IN LIGHT TO MODERATE ICING WITH MARGINAL DEICE CAPABILITY DIVERTS TO IAD.

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FLT DEPARTED GATE AT IAD AT XA21 LCL; WAS DELAYED ON TXWY FOR 1 HR IN LIGHT RAIN BEFORE TKOF. DEICE BOOTS TESTED PROPERLY ON GND BEFORE DEP. NORMAL TKOF AND CLBOUT; CEILING 9500 FT OVCST; ANTI-ICE EQUIP ON. WE FLEW IN IMC FOR 15 MINS WITH NO ICING; THEN LIGHT RIME. UPON REACHING 13000 FT; APPROX 20 MI E OF BAL AND WITH 1/2 INCH; THE CAPT TRIED TO BLOW THE DEICE BOOTS TO NO AVAIL. WE COULD SEE THE MOON THROUGH A LITTLE MIST SO WE OPTED TO TRY TO CLB OUT OF THE IMC TO 15000 FT. THE SAME CONDITIONS EXISTED THERE; STILL MISTY; STILL LIGHT ICING. MEANWHILE; THE CAPT WAS ON THE RADIO WITH DISPATCH AND MAINT; AND CONTINUALLY TRIED TO GET THE BOOTS TO OPERATE. WE COULD STILL SEE THE MOON SO AFTER 1 MIN AT 15000 FT; WE ASKED FOR 17000 FT. THE ICING THEN GOT HEAVIER (MODERATE MIXED) AND WE HAD PROBABLY ACCUMULATED 1 1/2 OR 2 INCHES BY THEN. AFTER 1 MIN AT 17000 FT; HAVING LOST 24 KTS AIRSPD AND WITH PIREPS OF LIGHT TO MODERATE ICE AHEAD; WE ASKED FOR A RAPID DSCNT. THE CTLR GAVE US 11000 FT. PASSING 12000 FT; I ASKED FOR 9000 FT (CAPT OFF #1 COM) BECAUSE I KNEW THAT 9000 FT WOULD LIKELY GET US OUT OF VISIBLE MOISTURE AND MY WORKLOAD WAS TOO HVY TO SPEND MUCH TIME LOOKING OUT THE WINDOW. THE CTLR WANTED ME TO STOP AT 11000 FT 'FOR TFC.' BY NOW; DUE TO THE DSCNT; WE LIKELY HAD 3 1/2 - 4 INCHES OF RIME ON THE LEADING EDGE WITH CLR GLAZED OVER IT AND ALL THE WAY TO THE BACK OF THE BOOTS ON THE TOP OF THE AIRFOIL. THE CTLS WERE HVY AND MUSHY AND I FIGURED THAT THE CTLR DIDN'T UNDERSTAND THE URGENCY OF THE SIT; SO I ASKED FOR AN 'EMER DSCNT' TO 9000 FT. THE ACFT BEGAN TO VIBRATE (PROP IMBAL). WE DIVERTED BACK TO DULLES. AT 8000 FT WITH BAL APCH IN VMC; THE ICE BEGAN TO CHUNK OFF THE PLANE. BEFORE TURNING US OVER TO APCH; THE CTR CTLR VERIFIED THAT WE HAD NOT 'DECLARED' AN EMER; BUT THAT WE HAD JUST NEEDED A PRIORITY DSCNT. THE DEICE BOOTS OPERATED INTERMITTENTLY ON THE APCH (VMC) AND AGAIN INTERMITTENTLY ON THE GND. IN IMC; WE DID NOT GET A SINGLE POSITIVE OPERATING INDICATION IN THE COCKPIT. I FEEL THAT WE PERFORMED WELL AS A CREW IN HANDLING THE SIT. WE HAD FLOWN TOGETHER IN THE PAST. I THINK THAT AS A CAPT; I MAY TEST THE BOOTS IN THE AIR JUST BEFORE ENTERING IMC AS WELL AS ON THE GND. ULTIMATELY; SOME OF OUR DECISIONS MAY HAVE BEEN BASED ON THE FACT THAT WE KNEW WE COULD GET BELOW THE FREEZING LEVEL.