Color Masterbatch Compounding
Conducted a detailed investigation to demonstrate how Vitrolite® improves masterbatch compound processing, downstream production of articles and potential cost savings from masterbatch processing gains as well as material cost reduction. Results reported here are based on a specific extruder, screw configuration, polymer composition and additives; however, comparable, but not necessarily identical, results have been achieved with different masterbatch formulations and extruder characteristics.
The masterbatch compounds were produced on a 27-mm twin-screw compounding extruder with an L/D of 40. The aggressive screw configuration contained 4 kneading blocks of which two, one before and one after the vent, were of left-hand turn. The additive package, consisting of various dry ingredients, pigment and Vitrolite® (if any), were dry blended in a Henschel-type blender. The pigment was a standard commercial product consisting of a pre-dispersed black pigment in a wax binder. The dry-blended additives were metered into the extruder for compounding with a proprietary carrier resin via a Brabender weigh feeder. The ratio of dry ingredients to carrier resin in masterbatch formulas was nearly constant except for the masterbatch containing 17% Vitrolite®. In this latter instance, the amount of carrier resin was reduced so as to keep the color masterbatch letdown into the final polymer formulation at a constant 3 wt %.
The additives were compounded into the carrier resin using processing parameters preferred by the company. Optimization procedures can involve many processing variables; however, for this protocol and similar other masterbatch protocols, optimization was based on adjustments to screw RPM, temperature and feed rate. Initially, the screw rpm was set at a company recommended 805 RPM and temperature profile of:
The temperature profile was not altered during the current investigation. Typically, VitroCo optimization procedures include variation in screw RPM and/or lowering of melt temperature. In this instance, only the screw RPM was varied because the melt temperature was relatively cool and it was deemed less likely that temperature changes would result in a substantial increase in extruder output. A more effective screw RPM of 550 RPM was determined by a detailed optimization in test #2 that included 17 wt % Vitrolite®. This screw RPM was used for all subsequent tests, even though it may not be ideal for the particular formulation. By setting fixed RPM’s and temperature, the only variable was feed rate.
The test objectives were established prior to starting of compounding. The test protocol was established to meet these objectives while at the same time making most efficient use of the limited time available on the extruder. Therefore, the runs were made in a sequence most consistent with efficiency rather than achievement of individual objectives; however, the following discussion is organized according to these objectives:
- Effect of low concentrations of Vitrolite® on compounding of masterbatch but no anticipated improvement to downstream polymer processing
- Effect of high concentrations of Vitrolite® on compounding of masterbatch and a anticipated improvement in downstream polymer processing
- Effect of Vitrolite® on dispersion and distribution of pigment and potential for pigment reduction
- Effect of Vitrolite® on dry-blending of additives and pigment
- Effect of Vitrolite® on compounding of color masterbatch into a TPE.
SECTION 1: EFFECT OF LOW VITROLITE® CONCENTRATIONS
The first objective was to determine the efficacy of using concentrations of 0.5-1.0 wt % Vitrolite® in preparation of color masterbatch compounds. A concentration of about 0.5 wt % Vitrolite® has proven to be very effective in reducing the melt viscosity of every polymer composition tested. This low concentration has also been very effective in reducing the viscosity of polymer blends with up to 50 wt % solid additive (filler, pigment, flame retardants, etc.). Furthermore, a low concentration of Vitrolite® has also been demonstrated to improve the dispersion of additives and pigments, thus reducing the concentration of the additive required to achieve a given objective. At a low concentration of 0.5-1.0 wt % Vitrolite® in the masterbatch, it is presumed that Vitrolite® will have little downstream effect on processing of polymer when the masterbatch is let down at 3 wt % (final Vitrolite® concentration of 0.015-0.03 wt %).
The effects of low concentrations of Vitrolite® on masterbatch preparation are summarized in Table 1. Within a run, the last column labeled with an “F” represents the maximum extruder output as limited by highest acceptable feed rate or maximum ampere draw.
a) Unstable output attributed to poor dispersion and distribution of dry ingredients. See Section 4.
b) Optimized baseline output rate used for calculation of output-rate improvements in subsequent tests.
Discussion: The maximum output achieved for masterbatch with no added Vitrolite® was 89.6 lb/hr following the VitroCo protocol. An optimal screw RPM of 550 was used as compared to the company’s standard of 805. An increase in output at the lower screw RPM is consistent with previous investigations which determined that the maximum effect of Vitrolite® is achieved when both temperature and stress (screw speed in this application) are within the effective operating range of Vitrolite®.
Although Vitrolite® concentrations of 0.5 and 1.0 wt% both resulted in increased output rates, a 0.5% concentration provided the greatest increase. This result is consistent with numerous other investigations in a wide variety of resins. With 0.5 wt % Vitrolite®, a higher feed rate was achievable and an output increase of about 30% was achieved. Although the amperage fluctuated at the higher output rate, this variation was attributed to visible agglomerates of additives rather than instability in the compounding. Very significantly, the die pressure decreased from 19 to 16 bars at this higher output rate, a clear indication that Vitrolite® reduced the viscosity of the masterbatch compound.
SECTION 2: EFFECT OF HIGH VITROLITE® CONCENTRATION
The second objective was to determine the effect of high Vitrolite® concentration on masterbatch production. The color masterbatch was formulated so that it would be let down at a 3 wt % concentration into the final polymer composition. At a 3 wt % let down, a concentration of 17 wt % Vitrolite® is required to achieve a Vitrolite® concentration of 0.5 wt % in the final polymer. A Vitrolite® concentration of 0.5 wt % in the final polymer assures that the full effect of Vitrolite® is realized during processing of the polymer into a finished article.
a) Output rate for incompletely optimized process.
Discussion: The output rate for the masterbatch with 17 wt % Vitrolite® was not fully optimized because there was not enough material. In spite of incomplete optimization, the output rate was 47% greater than for a masterbatch without Vitrolite®. Not only was the output rate substantially higher, but the amperage draw was reduced by 39%–a clear indication that at a high concentration of Vitrolite® resulted in significantly lower masterbatch viscosity.
Although this result may seem contrary to expectations, it is consistent with results obtained by VitroCo on other polymer formulations with high solid and Vitrolite® concentrations. The results presumably have important ramifications for masterbatch formulator—decreased electrical cost and time to produce the masterbatch while at the same time affording the end user full benefits of Vitrolite®.
SECTION 3: EFFECT OF VITROLITE® ON PIGMENT DISPERSION
Vitrolite® is known to improve the dispersion and distribution of pigments as well as many other additives, often allowing reduction of those ingredients. In the current investigation, the company expressed an interest in compounding a series of samples with normal and reduced pigment concentrations to determine whether color matches could be achieved with reduced pigment concentrations. Materials prepared for color-matching are shown in tables 3 and 4. The company’s standard masterbatch with 100% pigment (2F) was used as reference for masterbatches with 0.5 wt % and 17 wt % Vitrolite® and pigment concentrations of 100, 90 and 80%.
Discussion: Each of the color masterbatch compounds were molded into chips and analyzed for color development. Most significantly, all Vitrolite®-bearing masterbatch compounds, even those with only 80% pigment concentration, had color development equal to that of the control compounds with 100% pigment concentration but no Vitrolite®. Vitrolite® is a nearly colorless material and can not contribute color. Equivalent color development in chips with only 80% pigment and Vitrolite® as compared to standard 100% pigment chips with no Vitrolite® can only be reasonably ascribed to better dispersion and distribution of pigment in the Vitrolite®-bearing formulations.
SECTION 4: EFFECT OF VITROLITE® ON DRY BLENDING OF PIGMENTS AND ADDITIVES
The company’s standard procedure was to mix dry additives and pre-dispersed pigment at 1695 RPM in a Henschel-type mixer for about 10 minutes or until a temperature of 150oF was achieved prior to compounding the blended dry ingredients and resin into a masterbatch. Although mixing of the dry ingredients was not complete, compounding dry ingredients and resin at rates typical of formulations without Vitrolite® allowed enough residence time in the extruder to adequately disperse and distribute the ingredients. Higher output rates with Vitrolite® did not provide sufficient residence time to disperse and distribute the additives and pigment with consequent fluctuation in extruder amperage draw and presumed variability in compounded product.
Based on prior experience with dry ingredient mixing such as in preparation of powder coatings and benefits afforded by addition of Vitrolite®, a new dry-blending procedure was established. The mixer RPM was reduced to 1500 RPM and the blended mixture monitored for homogeneity and temperature at shorter mix times. The results are:
Discussion: The 1500 RPM mixer speed achieved visually good mixing of dry ingredients within 6 minutes as compared to 10 minutes in the prior protocol. Addition of either a low or high concentration of Vitrolite® allowed the mixing time to be reduced even further to 5 minutes. The formulations with Vitrolite® prepared by the new process were later compounded with no indication of inhomogeneity. High concentrations of Vitrolite® appear to have an additional benefit because the dry mixture was 11-12oF lower than the dry mixes with 0.0-0.5 wt % Vitrolite®. This lower temperature may effectively preclude softening of the wax binder and formation of agglomerates in the mixer.
SECTION 5: COMPOUNDING OF COLOR MASTERBATCH WITH TPE
Company personnel asked for a demonstration that color masterbatch with Vitrolite® would compound as readily into a TPE as would color masterbatch without Vitrolite® and that Vitrolite® delivered via the color masterbatch afforded processing benefits to TPE compounding.
Time constraints precluded optimization of process parameters; therefore, processing parameters were not varied from those for compounds without Vitrolite®. In spite of no processing parameter adjustments, Vitrolite®-bearing compounds were readily compounded into TPE and the output rate increased by 10%. Substantially greater benefits undoubtedly would accrue with process optimization.
Color masterbatch compounding companies have a potential to realize multiple benefits from the use of Vitrolite®. As demonstrated in the described examination, these benefits may include some or all of the following:
- Lower masterbatch processing costs related to increased extruder output and lower electrical demand
- Lower masterbatch formulation costs because dispersion and distribution of functional additives and pigments are improved in the masterbatch and concentrations may be reduced without affecting final polymer characteristics
- Improved blending of dry ingredients
- Flexibility in adding Vitrolite® at about 0.5 wt % to accrue benefits only at the masterbatch compounding level or at high concentrations to allow benefits in both masterbatch compounding and in downstream final molding.