Лако-красочные материалы — производство

Specific gravities (SG) have historically been reported on pigment data sheets along with two derived values, solid weight and bulking value. Solid weight is sim­ply the SG measured in different units (lbs/gal). Bulking value is simply the recip­rocal of solid weight. These values are for completely densified materials and have no relation to the powder properties of the pigments. The significance of SG prob­ably dates back to its requirement to measure particle sizes on the Fisher Sub­Sieve Sizer, and should be of little use today.

There are two good techniques for measuring SGs of fine powders: helium pyc — nometry and XRD. Accurate and reproducible values have been obtained on a Micromeritic’s 1305 Multivolume Pycnometer. However, given the submicron size of most of the powders, careful calibration, sealing, and extensive purging cycles (>50) are needed before the true values are asymptotically approached.

Given a known chemical formula, lattice constants measured by XRD can be used to calculate the SG. This technique should be more accurate and can give verification of the pycnometer values.

The DR pigments have higher SGs than pure rutile, with SGs ranging from 4.40 to 4.85 g/cc, compared to 4.25 g/cm3 for rutile TiO2. The SGs followed the trend MNbTi<MSbTi<MWTi, as expected, since the high-valency dopant changes significantly in density.

The approximate SGs for the common MSbTi pigments are given in Table 6.7.

The newer MnSbTi pigments have higher dopant levels than the older versions, and have therefore a higher SG.

6.5.2.2 Oil Absorption and Specific Surface Area

Oil absorptions are traditionally reported on pigment data sheets. However, this measurement is sensitive to the oil used and is somewhat operator dependent. At best, measurements are accurate to +/-5% of the value with a single operator. In addition, pigments are controlled for color, so there will be some lot-to-lot varia­tion.

ASTM D281-84 is the commonly accepted standard test. This test method stan­dardizes the oil and much of the technique. The measurements are reported as % oil based on pigment weight.

A review of data for the DR pigments from several companies shows a range of oil absorptions from 10 to 27%, i. e. lower than for organic pigments. It appears that the higher values correlate with older pigments, so it is likely that some remeasurements are needed, especially for any pigments reported to have values >20%. These higher values may have been obtained using the ASTM 1483 test, which gives higher values. A data range of 10-20% appears more reasonable, assuming the absence of highly absorptive additives or surface treatments. Like­wise, data from different companies are not necessarily directly comparable.

As expected, good correlations are seen between mean particle size and oil absorption. However, significant changes in particle size have only a mild effect on the oil absorptions, as seen in Table 6.8.

Specific surface area (SSA) measurements are becoming more popular for pig­ments. Although this technique will measure surface areas of pores (that will be inaccessible to oils or polymer) in addition to the external surface area, these inor­ganic pigments have negligible internal porosity. Therefore, SSA measurements

give good correlations to particle size and should give trends that correlate to oil absorptions.

SSAs are also subject to limited accuracy. The technique is quite sensitive to the degassing conditions, as well as calibration. Calibration standards are only certi­fied to +/-10%.

The range of SSAs for DR pigments is approximately 2-6 m2/g for unadulter­ated pigments. These values are much more sensitive to particle size than oil absorption and may be more useful for those using the pigments. Some SSA data are given in Table 6.8 along with oil absorptions.