WHY BLENDING OF REWORK IN COOLER MIXER IS ADVANTAGEOUS IN PVC MIXING PROCESS

Introduction:

Compounding mixing system for PVC consists of two mixers:

1.      Hot mixer, and

2.      Cooler mixer

The role of hot mixer is:

 1.       To get PVC grains to temperature above its Tg for fast diffusion of liquid stabilizers, internal lubricants and plasticizers into the PVC pores.

2.      To disperse and distribute other solid ingredients.

3.      To melt waxes and other ingredients and coat PVC grains uniformly.

4.      To get relaxation of the grain shape to reduce porosity and create a higher apparent bulk density.

Generally, U-PVC compound is dropped from the hot mixer around 120 C. At this juncture, the air in the cooler mixer is displaced and vented out through the breather bag.

At drop point, compound particles from hot mixer are coated with molten waxes and are sticky.

To get a free flowing dry blend, it is necessary to quench the hot compound in the cooler mixer. It is therefore necessary to use cold/chilled water in the cooler mixer jackets. Impeller RPM is kept very low in order to avoid generation of heat due to friction, the blade radius is larger. In other words, the Froude number for cooler mixer is very low. This also provides more contact time with the surface to effectively cool compound.

The cooling mixer should therefore be of higher capacity (should accommodate 2-3 batches from the hot mixer) with more surface area. Horizontal cooler mixer designs are advantageous.

Dry blend is cooled around 45 deg C. More temperature will make the compound sticky and will pose problems for pneumatically transportation to silos. If compound is cooled below 40 deg C, it will absorb moisture and pose problem during processing, especially if it contains carbon black that absorbs moisture.

Addition of rework in crushed or pulverized form:

Many a times reprocessed material in the crush form or pulverized form is added to the compound in constant proportion. It should be added in the cooler mixer and not in hot mixer.

The advantages of adding it to the cooler mixer are:

1.      The batch size in hot mixer will be more if rework is added to hot mixer and dry blend will not be formed properly.

2.      Rework does not have porosity; only PVC grains have it and absorbs liquid ingredients.

3.      All the additives are meant for PVC, part of the additives would go to the rework mixed.

4.      Rework is already a compound so when it is added in cooler mixer, a dry blend compound is blended with the crushed compound.

5.      In cooler mixer, dry blend at 120 deg C will exchange heat with rework at room temperature and cooling will be faster.

6.      Blowing agents, glass spheres, Nitrile rubber etc are preferably added to the cooler mixer where a gentle mixing takes place and they are distributed evenly without breakage or coagulation.

7.      Some manufacturers of process aids suggest adding process aid in cooler mixer at 80 deg. C.

8.      Some advice to add more abrasive materials like TiO2 in cooler mixer.

If hot mixing is carried out properly, formation of beads in the compound is almost nil and with crushed / pulverized material in the compound, the vibrator screen can be bypassed.