With prices going up and up, and supplies getting snugger, it's time to re-examine the available extender pigments that can make TiO2 go further.

The 10-year run of cheap and plentiful titanium dioxide is over. While prices are still some distance below the levels of 1989, and no actual shortages are in sight, there is every indication that TiO2 prices will climb farther and delivery lead times will continue to grow longer. All this adds up to a revival of interest in white mineral fillers that can be used as partial TiO2 replacements to extend supplies and minimize costs.

While nothing can match the opacity of prime rutile TiO2, some white, bright mineral fillers and alternative grades of TiO2 can offer some relief from rising prices. Other ways to get more value out of white pigments include new TiO2 grades that offer improved dispersion and durability.

What's sending prices up?

Some readers may recall the late 1980s, when supply shortages pushed the price of TiO2 to over $1.20/lb and forced users to explore various extenders. Then, by 1996, TiO2 prices dropped to the low-to-mid 80¢/lb range, dragged down by a slowed economy and a glut in the market. Pigment suppliers reacted by shelving expansion plans and building no more "greenfield" capacity since 1995. Gradually, market demand caught up with stagnant supply. TiO2 is now selling at 95¢ to $1.15/lb, a 10-20% increase since the mid-90s. Another 3-5¢/lb hike is probable before the end of the year, according to consultant James Fisher, president and CEO of International Business Management Associates (IBMA), Princeton Junction, N.J.

Although today's TiO2 prices are 9-14% below those of 1989 (and 20-24% lower than '89 in constant dollars), producers say that is still below the level required to support new investment in future capacity.

Though worldwide supply is still ample, there are signs of reviving market demand in Asia, and supply in North America is becoming a bit more snug. Market demand has been particularly strong in the last few years. For example, while TiO2 consumption in 1998 was down by about 3-5% globally over the previous year, it was up about 1% in North America following growth of 7-9% in 1997. Domestic consumption this year will grow by 2-3%, according to IBMA's Fisher. Producers' plant utilization rates are in the 90% range.

TiO2 suppliers expect their industry to become less cyclical and less subject to price volatility as a result of the consolidation and globalization that has been taking place among both pigment suppliers and buyers. Still, with global demand projected to keep rising over the next couple of years and no major capacity additions planned, other than debottlenecking, supply is expected to get tighter.

Opacity and color

Two properties appear paramount when considering alternatives to standard TiO2: opacity and color (or whiteness). Not all candidates for TiO2 extension are pure white, and even those that have good whiteness cannot match TiO2 for opacity or hiding power. Opacity or light-scattering ability of a pigment is related to two characteristics. One is refractive index (R.I.). Rutile grades of TiO2, which are produced by the chloride process, have the highest R.I. (2.75) of any white pigment. However, particle size and particle-size distribution also affect opacity, notes plastics marketing manager Peter Krinsky of Millennium Inorganic Chemicals. He says the optimal light-scattering particle size is half the wavelength of visible light, or about 0.2 micron. This is the average particle size of TiO2 pigments used in plastics. In addition, advances in particle-size control have yielded very narrow particle-size distributions for the latest rutile pigment grades.

One area of disagreement among plastics-color experts is how to describe the mechanism of TiO2 extension. Bill Kearns, technical specialist at M.A. Hanna Color, expresses one school of thought when he says TiO2 can be extended by separating the typical 0.2-micron particles of the pigment with larger particles of a less expensive material. This "spacer" approach reportedly has been effective in coatings, but is a topic of debate in plastics. The critical difference apparently is that in thin coatings, the volume fraction of TiO2 pigment can be above 50%, but it is much lower in plastics.

Victor Mimeault, technical v.p. of Ampacet Corp., notes that a white concentrate containing 50% by weight TiO2 has a pigment volume fraction of only 12-14%. "At such low volume fractions, the TiO2 particles are already apart, so the spacers cannot and do not contribute to opacity."

Seconding that view is Bill Lashway, plastics services manager at Millennium. "Due to the low final loading--2-4% by weight--in most film applications, adding sufficient extender to physically space out the already widely scattered TiO2 would compromise the film's physical properties and add cost." He adds that extenders may be more beneficial in heavier films and molded parts, where product thickness adds to opacity.

The top contenders

Most of the interest in TiO2 extenders has focused on fine-ground calcium carbonate and calcined kaolin clays because of their low price--as much as 60-90% lower than TiO2. Jack Goldhammer, v.p. of sales at mineral supplier Omya Inc., says a typical white film masterbatch of 45% TiO2, 30% calcium carbonate, and 25% polyolefin can yield a cost saving of 15%. He notes that most users opt for ultra-fine calcium carbonate grades with average particle size of 0.5-1.0 micron. Coarser grades may cost even less, but they can adversely affect film properties.

Engelhard Corp. has done extensive work in evaluating two of its specialty clays as TiO2 extenders in color concentrates. ASP 170 is a fine-particle (0.4-micron) hydrous kaolin said to offer effective spacing of TiO2 particles for excellent color and opacity retention. Ultrex 96 is a special calcined kaolin with 0.9-micron median particle size. It reportedly has the highest brightness (96 GE brightness) of any kaolin-based extender. These two pigments cost about 20% and 40% as much as TiO2, respectively.

Interesting results were observed when these pigments were evaluated as partial replacements in a typical 50%-TiO2 white LLDPE concentrate. In injection molded parts, replacement of 10% TiO2 with Ultrex 96 retains both good color and opacity. Even at a 20% replacement, very good color is achieved, with only a light drop in opacity. ASP 170 provides good color at lower levels of substitution (5%), but does not provide as much opacity as Ultrex 96.

In blown film, 5% and 10% replacements of TiO2 with ASP 170 retain color and opacity equal to those of the all-TiO2 control. At 20% replacement, the resulting film shows excellent color with only a slight drop in opacity.

Engelhard also reports that Ultrex 96 can replace up to 20% of the 7 phr TiO2 used in white PVC siding. Initial whiteness is reportedly equal or slightly better than with all TiO2. Initial yellowness is equal to the TiO2 control at a 10% replacement level, while both accelerated and actual outdoor weathering tests show that Ultrex 96 at 10% replacement performs even better than the TiO2 control.

One caution is offered by David Skelhorn, technical director of mineral supplier ECC International. He says formulators must remember that "kaolin clays and calcium carbonate do not provide TiO2's inherent uv stability."

Alternative TiO2 grades

Not all TiO2 is created equal. Rutile TiO2 has an R.I. of 2.75, compared with 2.55 for anatase TiO2, which is produced by the older sulfate process. While anatase generally costs a little less than rutile grade, it is about 20% less efficient in opacifying plastics.

Blending rutile and anatase TiO2 grades to reduce cost is not widely recommended. Besides lower opacity, anatase can result in color problems, according to John Prichard, manager of plastics at DuPont. On the other hand, anatase TiO2 is actually necessary in some formulations, he notes. For instance, anatase grades do not quench fluorescent whitening agents as much as rutile grades. Another example of where anatase pigments are used is in polyolefin masterbatches for extrusion coating of photographic film.

Use of lower-priced, "wide-spec" rutile TiO2 grades that are not pure white are also an option for partial TiO2 replacement. Such a product is being imported from Ukraine by K&T Export/Import. Called Titan 2, this 0.3-micron grade is being evaluated by some color masterbatch suppliers and compounders. It sells for 78-82¢/lb. K&T says it can provide good whiteness and acceptable opacity when substituted for 15% (or more for non-critical applications) of the standard TiO2 in a formulation.

Another "alternative" TiO2 is a synthetic rutile TiO2 from Hitox Corp. Buff-colored Hitox pigment has slightly larger and more irregularly shaped particles than typical rutile TiO2. It sells for 68¢/lb, and can substitute for up to 45% of standard TiO2 in products that require opacity but do not have to be pure white.

More white candidates

Barium sulfate is used to some degree as a TiO2 extender, but its price differential is not as advantageous as those of kaolin and calcium carbonate. J.M. Huber has introduced a new surface-treated barium sulfate that is said to perform well at up to 50% replacement of TiO2. Called Huberbrite Interlok, it has been used by one extruder of flexible PVC to replace all the TiO2 in his formulation. The product has an average particle size of 1.0 micron and appears to be promising for polyolefin uses as well.

Other alternative white pigments include lithopone (a co-precipitate of barium sulfate and zinc oxide) and technical-grade zinc oxide. Lithopone is not much used in the U.S. because it tends to react with some grades of TiO2, resulting in a noxious odor. Zinc oxide has been used during past TiO2 shortages, but degradation of plastics such as polypropylene has severely limited its use.

What compounders say

TiO2 extension is hardly a new practice, and although few pigment users readily admit it, everybody does it, or so suppliers claim. Some compounders seem wary of talking openly about TiO2 extension for fear that if customers hear about it, they will demand a price cut.

While cautious in sharing information on TiO2 extenders they may use, major color-concentrate manufacturers, such as Ampacet, Clariant's Masterbatches Div., M.A. Hanna Color, Teknor Color, and WedTech indicate they are always on the lookout for better alternatives, particularly if they offer moderate opacity.

Kearns of M.A. Hanna Color says a specialty fine-ground calcined kaolin with a particle size of 0.9 micron can typically replace TiO2 at levels of up to 15% while retaining acceptable brightness (96% GE Brightness) and only a slight loss of opacity. Fine-ground barium sulfate with a particle size of 8.5 microns can also be used at about the same TiO2 replacement level, but it will have lower brightness (93%) and will cost more, he notes.

Ampacet Corp., says Mimeault, approaches extenders as a way to cut cost by minimizing resin use rather than TiO2. For example, as an alternative to the usual 50% TiO2/50% PE white masterbatch, Ampacet offers one containing 50% TiO2, 25% calcium carbonate, and 25% PE. Another formulation contains 40% TiO2, 30% lithopone, and 30% resin. Mimeault says it offers almost the same opacity as a 50% TiO2 masterbatch and is a "gentler, softer white" because it is less abrasive. These two masterbatches sell for around 7-12% less than the standard 50% TiO2 product. (A different way to get more for your money and save resin cost is to use a more highly loaded concentrate--e.g., 80% TiO2, advises Ampacet's director of marketing Frank Iannotti.)

WedTech business manager Rick Tizzard says, "The extended white most commonly seen in the film industry contains 50% TiO2, 20% calcium carbonate, and 30% carrier resin." WedTech's version, grade 1022, is aimed at grocery sacks and typically costs 1-2¢/lb less than standard 50%-TiO2 masterbatch. Although calcium carbonate does little to improve opacity, it can add bubble stability in blown film, he notes. The trade-off, he says, is that TiO2 extenders add density to the finished product, which can offset some of the savings.

Wedtech has also developed a new white masterbatch that uses a proprietary extender. WT-164 masterbatch is geared for white t-shirt bags and is said to give good opacity. The company recommends it as an alternative to a 50%-TiO2 masterbatch for a saving of about 2¢/lb.