Antioxidants: the hunt for BHT alternatives.

With the workhorse BHT under suspicion of being a cancer-causing agent, polyolefin suppliers are taking the initiative to find alternatives.

While there's still some debate over whether the popular antioxidant BHT is carcinogenic, it has become quite clear that polyolefin producers and users are not standing idle awaiting the FDA's verdict. Responding to growing customer demand for BHT-free materials, polypropylene and polyethylene makers are actively seeking new antioxidants to stabilize their resins. And, in turn, additive suppliers are helping provide those alternatives.

"Our customers are concerned with BHT; and if they're concerned then we're concerned," says Daryl Stein, applied research specialist at Quantum Chemical Corp. "We've had more than one customer who has said, 'We don't want BHT in our resin.'" Quantum says it has eliminated BHT from about half its resin grades.

Mobil Chemical Co. says it dropped BHT from all its resins six years ago, choosing alternative hindered phenolic antioxidants. The consequence, a Mobil spokesman says, is about 0.5|cents~/lb higher cost of the compounded resin.

Exxon Chemical Co. began eliminating BHT from all of its LDPE resins, replacing it with BHEB (butylated hydroxy ethyl benzene--sold as Prodox 500 by PMC 

Specialties, for example). Exxon says it never used BHT in LLDPE and HDPE, and the company is currently removing BHT from food-packaging grades of polypropylene on a customer-by-customer basis. All new PP development is being done with BHT alternatives--which Exxon declined to identify.

Warning flags have been raised periodically about BHT for at least 16 years (see PT, July '76, p. 56). Since 1977, the FDA has been considering banning the substance from use in direct and indirect food packaging. Last year, the agency considered canceling the proposed ban but was convinced to keep the matter under consideration because of continued concern over BHT's carcinogenicity.

For the present, however, the official word from the FDA is that BHT is "generally recognized as safe (GRAS)" and has been cleared for use as an antioxidant in a variety of food-contact and other applications, including adhesives, polymeric coatings and EVA copolymers.

Despite continuing doubts about BHT, the very scrutiny it has undergone is a mark in its favor. "Any substance to replace BHT is going to have to undergo the same type of scrutiny as BHT has," says Dr. John Lawrence, a researcher at Schenectady Chemicals. Attaining FDA clearances, Lawrence points out, is a long and costly process and few antioxidants have received the across-the-board approvals BHT has. A typical example is Schenectady's Isonox 132. A high-molecular-weight hindered phenol similar in chemical structure to many of the most common BHT alternatives, Isonox 132 has FDA approval for use only in adhesives.

One alternative antioxidant that has gained the same sort of wide approval as BHT is Ethanox 330 from Ethyl Corp. "But while this product has the same level of approval as BHT, it is also much more expensive," concedes John McChesney, Ethyl's manager of marketing communications. That drawback is common to other BHT alternatives and helps explain the reluctance of many processors and compounders to use them.

BHT will continue to enjoy widespread popularity as the most cost-effective stabilizers until its use is forbidden by government regulators, say spokesmen for PMC Specialties and Uniroyal Chemical, the world's largest BHT producers. Sources at PMC Specialties doubt that a ban will ever occur. "I don't think the negative pressure on BHT is that heavy right now," says Kim Hryniewicz, PMC's BHT market manager. "We look forward to a very strong life for BHT. The more people look at it they will see it's safe and maybe even beneficial."

The beneficial aspects Hryniewicz refers to come from a rat feeding study done over the past two years by the American Health Foundation, Valhalla, N.Y., suggesting that BHT and its sister compound, BHA, actually act as anticarcinogens when ingested in small doses, countering the effects of known cancer-causing substances.

In the study, 350 rats were fed a potent liver carcinogen together with between 0 and 6000 ppm BHT. Rats eating all but the highest dosages of BHT developed less cancer than those who were denied the antioxidant. Rats fed between 3000-6000 ppm of BHT, though, did develop either pre-cancerous conditions or actual cancers. Health Foundation researchers concluded that BHT can provide "significant" protection in humans in doses of 100 ppm or less.

ALTERNATIVES HAVE ADVANTAGES

Periodic BHT health scares and worldwide shortages (see PT, July '79, p. 77; July '80, p. 89) have given BHT users ample opportunities to examine alternatives. More costly HMW hindered phenols are available from numerous suppliers. These antioxidants provide more permanent protection than BHT and are usually less volatile at high process temperatures. They also require lower doses to achieve the same effect and don't promote yellowing or color washout like BHT can. And most of these other phenolics are liquids, which don't present the processing problems experienced with the solid BHT. In order to circumvent the problems of incorporating very small amounts of a solid, some compounders have begun using molten BHT kept in a heated storage tank and delivered to the compounding line via heated and insulated piping.

Easier handling and improved long-term antioxidant protection is said to help balance the expense of alternative products that range from $2 to $10/lb, versus $1.50/lb for BHT.

While most resin and additive suppliers still recommend HMW hindered phenols as the best alternatives to BHT, some exploration into other chemistries has recently come to light.

Earlier this year, Quantum released data on the stabilizing effectiveness of ATP (alphatocopherol), more commonly known as vitamin E (see PT, May '92, p. 14). According to senior research engineer Mark McManus, using a vitamin E/phosphite antioxidant combination in PP has proven to be more effective than hindered phenols in controlling melt-flow and color stability. Because of ATP's high molecular weight (twice that of BHT), about one-fifth as much ATP is needed in HDPE to provide the same gel suppression as BHT. Thus, overall additive cost can be lower with ATP, Quantum says, even though it costs more per pound than BHT. And because it has a lower volatility than BHT, ATP can be used at higher process temperatures.

In addition, ATP is reportedly heat stable to 572 F, promotes easier flow than resins compounded with BHT (as measured by changes in die pressure) and does not cause yellowing or pinking reactions with Ti|O.sub.2~. All told, McManus concludes that "ATP is a viable alternative to BHT." Quantum offers a 2% masterbatch of ATP in HDPE, called Spectratech KM 11988.

More New Products

Convenient handling: Ciba-Geigy Corp. has developed a process-consumable bag for its Irganox 1076 FF and 1010 FF antioxidants for polystyrene. The butadiene-styrene copolymer bag can be tossed right into the mixer unopened, Ciba-Geigy says.

New blends: Ciba-Geigy also introduced an all-liquid blend of its CGA 1135 and Irganox 5057 hindered phenolics. The system is designed to replace amine stabilizers in polyurethane.

From GE Specialty Chemicals comes four new free-flowing solid blends of the company's Ultranox 210 or 276 phenolic primary antioxidants with Ultranox 626 phosphite secondary antioxidant for a wide range of polymers. The blends are Ultranox 815A (1:1 626 and 210), Ultranox 817A (2:1 626 and 210), Ultranox 875A (1:1 626 and 276), and Ultranox 877A (2:1 626 and 276).

Safe for PP pouches: Witco's Argus Div. has introduced Mark 2180 sodium bisphosphate for use in polypropylene. It's said to be one of the first to receive FDA clearance for boil-in-bag and other high-temperature packaging applications.