voltage determining solenoid force?
Power delivered is equal to:

\$P=V^2/R\$

or

\$P=I * R\$

Now you are delivering 

\$28 * 0.12 = 3.36 W\$ of power

and coil resistance is

\$28 / 0.12 = 233 Ohm\$

at 40V you'll be delivering 

\$40^2/233 = 6.87W\$

So the answer is yes - you will definately increase the force by increasing voltage.