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Additives: how surface treatment affects processing of TiO2 pigments in color concentrates.

Although equipment and Ti|O.sub.2~ concentration are major influences, the ease with which the pigment disperses can be selectively controlled by the type and amount of surface treatment.
TABLE 1. Surface Treatment of the Ti|O.sub.2~ Pigments Used.

 Surface Treatment
Pigment Inorganic Organic

Grade 1 |Al.sub.2~|O.sub.3~ Polyalcohol
Grade 2 |Al.sub.2~|O.sub.3~ Polyalcohol
Grade 3 |Al.sub.2~|O.sub.3~(*) Polysiloxane A

* Same amount as in Grade 2.

Color concentrates with titanium dioxide (Ti|O.sub.2~)-based pigments are typically used for coloring polyolefins. When processing such concentrates, a resin compounder would expect excellent dispersibility of the Ti|O.sub.2~ and no excessive gas evolution or plate-out on molds. Frequently, concentrates containing no additives other than the Ti|O.sub.2~ pigment and the polyolefin are required. A recent study shows that clear variations, depending on surface treatment of the Ti|O.sub.2~, can be observed in the processability of Ti|O.sub.2~ pigments in color concentrates. Although the specific degree of processability also varies with Ti|O.sub.2~ concentration and equipment used, it is clear that surface treatments have their own effect.
TABLE 2. Properties of Ti|O.sub.2~ Concentrates.

Pigment C L S P SR MFI t E

Grade 2 50 0 75 1 17 14.8 130 936
 50 1 75 1 22 23.8 200 1040
 50 0 50 1 17 15.2 280 1456

Grade 3 50 0 75 1 18 14.7 100 800
 50 1 75 1 16 24.0 230 1288
 50 0 50 1 21 15.5 220 1408

Grade 1 50 0 75 1 22 14.4 130 884
 50 1 75 1 21 24.5 300 1920
 50 0 50 1 26 14.4 250 1200

C = pigment concentration, %; L = lubricant, %, S = blade
speed, |min.sup.-1~; P = plunger pressure, bar; SR = sieve
residue, mg/kg Ti|O.sub.2~; MFI = melt flow index, g/10 min; t
= melting time, s; E = energy needed for dispersion, kW X s.

The results of this study illustrate possible ways to influence--by choice of organic surface treatment of the Ti|O.sub.2~--the processability of color concentrates.

The processability of pigments--how readily they can be dispersed to a required fineness while optimizing extruder throughput--plays a decisive role in the manufacture of color concentrates in polyethylene. The goal, of course, is to embed the Ti|O.sub.2~ pigment particles in the polymer matrix quickly and easily.

Dispersibility of a Ti|O.sub.2~ pigment in a resin is essentially determined by the pigment's surface chemistry and physical characteristics. To selectively influence dispersibility, Ti|O.sub.2~ suppliers have long treated pigment surfaces with inorganic substances, such as hydrated aluminum oxide and hydrated silicon oxide, and with organic compounds, such as polyalcohols or polysiloxanes. Other steps in the process of pigment production, such as type and intensity of grinding, also affect dispersibility of Ti|O.sub.2~ pigments in color concentrates.

This study involved extensive laboratory work as it investigated the influence of organic surface treatment on the processability of Ti|O.sub.2~ pigments, in the formulation of concentrates in LDPE. The Ti|O.sub.2~ pigments used in the formulation of the color concentrates are presented in Table 1. Two different laboratory devices--an internal mixer and a twin-screw extruder--were employed.

Manufacture of the Ti|O.sub.2~ Concentrates

Internal Mixer. The device used was a non-heatable internal mixer with a volume of 1100 |cm.sup.3~. Because the heat required to melt the PE must be generated by friction, nonpigmented LDPE granules were kneaded before each experiment until the chamber reached a temperature of 100 |degrees~ C. The chamber was then emptied. Next, half the required amount of LDPE (together with calcium stearate when appropriate), all the titanium pigment, and the remaining LDPE were put into the preheated chamber. The amounts were gaged in such a way that, taking into account the densities of all raw materials, the required pigment concentration could be achieved and the useful volume of 1100 |cm.sup.3~ fully utilized. Blade speed was then set to the prescribed value, the floating plunger lowered onto the kneading stock, and the input power recorded as a function of time.

After melting had occurred, the kneader plunger was raised briefly to scrape into the chamber any pigment still adhering to the feedhopper. The purpose was to avoid distortion of dispersibility test results caused by the falling of nondispersed pigment residues into the pigment concentrate during emptying of the kneader. Plunger pressure and blade speed were the variable machine parameters.

Twin Screw Extruder. The shafts of the twin screw extruder rotate in the same direction and are equipped with three kneading zones. The machine has five independently regulated heating zones.

The premix, consisting of Ti|O.sub.2.~ pigment and LDPE granules, was loaded into the extruder feed zone via a hopper with metering screws. Extrudate was passed through a cooling zone and subsequently granulated; a torque load of 90% to 100% was used in all tests. The variable machine parameters were shaft speed and extrusion temperature.

Parameters for Assessment

The following parameters were used to assess dispersibility of the various Ti|O.sub.2~ concentrates tested: melt flow index (MFI), sieve residue (SR), throughput, melting time, and energy needed for dispersion.

Melt flow index (MFI) refers to the mass of molten polymer that can be forced through a standardized nozzle in a given time under a defined pressure and temperature; it is used to assess the influence of Ti|O.sub.2~ pigments on the viscosity of molten color concentrates. The MFI testing device Type 592 was used to determine MFI by ASTM method D1238. The device was operated at a melt temperature of 190 |degrees~ C, and with a floating weight of 2.16 kg.

The amount of molten mass, m(g), that was forced through the nozzle in a particular time, t(s), was measured. The equation MFI 190/2.16 = 600 m/t (g/10 min)

is used to extrapolate the 10-min value, which is then stated as MFI 190/2.16 (g/10 min). Two readings were taken for each sample and the average of the two individual measurements was calculated.

Sieve residue is also used to assess dispersibility of a Ti|O.sub.2~ pigment in a color concentrate. The lower the sieve residue, the better the pigment dispersion.

The equipment used was a measuring extruder, 25:1 L/D, fitted with an extrusion nozzle and electronic pressure measuring device. The first of the three heating zones was heated to 150 |degrees~ C and the remaining two to 190 |degrees~ C. At a shaft speed of 50 rpm, a quantity of Ti|O.sub.2~ concentrate containing 100 g of pigment was forced through a steel sieve with a mesh size of 40 microns. The machine was subsequently flushed with unpigmented polyethylene until the extrudate leaving the nozzle was totally clear.

The residue can be visually assessed under a stereo-microscope when the sieve is removed. For quantitative determination of the amount of Ti|O.sub.2~, the sieve was heated for 30 min at a temperature of 800 |degrees~ C and then reweighed. Sieve residue was calculated in mg/kg Ti|O.sub.2~ pigment by multiplying the difference in weight by a factor of 10.

Throughput of the twin screw extruder was determined by measuring the time taken for 1 kg of the premix of Ti|O.sub.2~ pigment and PE granules to be fed into the machine using a torque load of 90% to 100%. The equation T = 3600/t is used to determine throughput, T (kg/h).

Melting time refers to the time between the initial contact between the kneader plunger and kneading stock, and the point at which melting occurs when working with the internal mixer. The value can either be obtained directly from the time/throughput ratio or determined by measuring the time elapsing before characteristic noises from the kneading chamber are heard.

Energy needed for dispersion. The energy needed to achieve dispersion of Ti|O.sub.2~ pigments in the internal mixer can be roughly estimated by multiplying the melting time by the mean power input.


Internal Mixer. Titanium dioxide concentrates were formulated with pigment contents of 50%, 60%, and 75% in LDPE with an MFI of 25. Blade speed was varied (170, 75, and 50 rpm), as was plunger pressure (1 bar and 2 bar). Furthermore, each variation was carried out with and without adding calcium stearate (1% relative to the amount of Ti|O.sub.2~ pigment) as a lubricant.

Three types of Ti|O.sub.2~ pigment grades with different surface treatments were selected: Grade 1 for general-purpose plastics, Grade 2 for blue-tone fine particle plastics, and Grade 3 for siloxane-treated fine particle plastics.

The sieve residues vary dramatically with different pigments. When a maximum sieve residue of 30 mg/kg Ti|O.sub.2~ is used as the dispersibility criterion and no stearate is present, only Grade 2 and Grade 3 show satisfactory results in a 50% concentrate. However, when calcium stearate is added, all pigments satisfy this criterion at concentrates of 50%, 60%, and 75%.

In comparison with the starting material, a marked reduction in MFI was noted as pigment concentration was increased. Pigment type, too, had a pronounced effect on the values recorded. The addition of calcium stearate reduced the decline in MFI with increasing pigment concentrations. Similarly, the differences between concentrates with different pigments declined.

The effect of increasing pigment concentration on lengthening the melting time was detected only in the 75% concentrates. Use of calcium stearate increased melting time only slightly.

Likewise, an effect on the energy needed for dispersion is apparent only with a loading of 75% Ti|O.sub.2~ in the concentrate, with marked differences also occurring for different pigments. The addition of calcium stearate causes a slight increase in the energy needed for dispersion.

Table 2 shows that, when plunger pressure was reduced to 1 bar and the rotor speed to 75 rpm or 50 rpm, favorable dispersion results were obtained for almost all Ti|O.sub.2~ pigments. However, unacceptably high melting times and an increase in energy needed for dispersion were also among the results.

All in all, the studies show that the effect of process parameters (such as amount of pigment in the concentrate and the addition of a lubricant) on the processability of Ti|O.sub.2~ pigments is much more significant than that of different organic surface treatments on the pigments.

Twin Screw Extruder. Fifty-percent pigment concentrates were produced with a constant melt temperature of 140 |degrees~ C and variable screw speed, and with a constant screw speed of 150 rpm and variable melt temperature. The pigments used were Grades 1, 2, and 3.

The results are presented in Figs. 1 through 9. As expected, throughput increases as screw speed rises. More pronounced are the differences in sieve residue due to organic surface treatment of the pigments.

As screw speed is increased, degree of dispersion improves and differences in dispersion behavior of the pigments disappear. With respect to MFI, the pigment concentrates perform similarly at all screw speeds.

The processability of the pigments is thus improved by an increase in screw speed if extrusion temperature is kept constant. If extrusion temperature is varied while screw speed remains constant at 150 rpm, throughput rises with increasing temperature. This is expected, because viscosity of the molten material is reduced. The differences between tested concentrates are observed at all extrusion temperatures.

Furthermore, the organic surface treatments markedly affect the dispersibility of the pigments, as shown in Fig. 5. If a sieve residue of SR = 30 mg/kg pigment is set as the limit value for good dispersibility, only Grade 3 satisfies the criterion at all extrusion temperatures. Of the three products tested at constant screw speed, this pigment alone displays improved processability with increasing extrusion temperature.

As shown in Fig. 6, results for MFI reveal small differences in the effect of the pigments, but no dependence on extrusion temperature. Addition of calcium stearate equalizes the pigments' effect on throughput and improves dispersibility. When larger lubricant amounts are added, differences between the concentrates disappear. Figure 9 shows that with increasing amounts of calcium stearate, MFI does not fall so much. Thus, addition of calcium stearate has a positive effect on the processability of all Ti|O.sub.2~ pigments tested.

Results show that in the manufacture of color concentrates in LDPE, the processability of Ti|O.sub.2~ pigments is, on the one hand, clearly dependent on the type of machine used and the dispersion conditions. On the other hand, it is closely connected with the composition of the Ti|O.sub.2~ concentrates. Beyond this, however, concentrate manufacturers can selectively control processability of Ti|O.sub.2~ pigments in their products through the type and amount of surface treatment used on the pigment.
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Author:Buttler, Russel
Publication:Plastics Engineering
Date:Jul 1, 1993
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