Time for another look at endothermic blowing agents.

They're not your father's baking-soda systems. Today's improved endothermics are winning converts back to one of the oldest methods of blowing structural foam.

Endothermic chemical blowing agents may simultaneously represent the past and the future of thermoplastic structural foam. Remember that endothermics, which absorb heat rather than release it when producing gas, were there at the beginning. Simple mixes of sodium bicarbonate and citric acid originally prevailed as the chemical blowing agent (CBA) of choice for structural foam. Azodicarbonamide, however, easily supplanted these early, less controllable attempts at making foam the baking-soda way. "Azo" was simply a better mousetrap. Yet since the early to middle 1980s, an increasing number of foam makers have started to sing the praises of modern, improved acid-carbonate blowing agents. So forget the Alka-Seltzer comparisons, because endothermics have evolved and may deserve another look.

In fact, if you talk to endothermic CBA suppliers today, you would hear strong claims to state-of-the-art status. You'd also hear about performance and process-control advantages over the prevailing azo-based exothermic technology. Of late, these arguments have even taken on environmental strains because endothermics leave no ammonia or cyanuric acid byproducts. And several suppliers have recently gone after new applications, coming out with endothermic products for high-temperature, moisture-sensitive resins like polycarbonate and its alloys. "There's no question that azos were a big leap forward," says J.M. Huber Corp. plastics technology manager Gerry Mooney. "But we feel we're on the cutting edge now with endothermics."

Whether the endothermics can actually outperform their exothermic cousins remains a matter shrouded in contention, however.

Endothermics have staked out an undeniable niche in structural foam, though by all accounts azos still control the lion's share of the total CBA market. According to Uniroyal Chemical Co. marketing manager Casey Leone, azos still represent 80% of a 10-million-lb/yr CBA market, with the rest going to other exothermics and various endothermics. In structural foam alone, Leone says endothermics represent about 25% of the market. Huber's Mooney also pegs endothermics' share at 25%, thanks in part to some recent forays into large-volume recycling applications.

The growing number of endothermic suppliers also suggests increasing popularity. Since Germany's Boehringer Ingelheim AG--distributed by Henley Chemicals Inc.--first refined the citric acid-sodium bicarbonate technology, several relative newcomers have entered the domestic market. The most recent, EPI Inc., opened in Texas this year with a new Werner & Pleiderer twin-screw compounding line and a full range of predispersed and powdered endothermic products. And J.M. Huber has come out with a whole family of synthetic endothermics over the past few years.

NOT LIKE THE OLD DAYS

Suppliers say there's almost no comparison between modern acid-carbonate endothermics and the early ones. "It's just not that simple anymore," says Michael Reedy of Reedy International Corp., another major endothermic supplier. Nowadays, the companies making endothermic CBAs rely on improved encapsulation technology and an arsenal of carrier resins. In addition, all suppliers offer 100%-active powders and have extended their range of masterbatch dispersions. They have also worked to increase the CBA-temperature activation range as well.

For instance, Henley Chemicals recently broadened its range of masterbatch concentrations. According to product manager Dennis Keane, the company now offers 5%-active dispersions for any grade of Boehringer Ingelheim's Hydrocerol. Henley makes these less concentrated dispersions especially to satisfy the needs of some extrusion processors. The upper limit for its concentrates remains at 70%.

The company has also adopted styrenic carriers in some cases, supplementing the existing olefinic carriers. "In the past the problem was to pelletize polystyrene without kicking off the blowing agent," notes Keane. Now, advances in encapsulation techniques let the company use a styrenic carrier when needed, opening up the sodium bicarbonate-citric acid approach to a wider range of applications. For extending the endothermics' temperature range, newer Hydrocerol BIH works in the 325 F range while grade HK handles temperatures around 500 F.

Reedy International, which sells 16 products under the SaFoam label, has worked in a similar direction. In addition to 100%-active powders, the company sells masterbatches with both styrenic and olefinic carriers. Reedy recommends the styrenic carriers for materials like ABS, PS, PVC, and TPEs. He emphasizes the benefits of choosing a carrier that can improve melt strength and flow. Often that choice is a relatively subtle one. For example, a branched LDPE and a linear LLDPE TABULAR DATA OMITTED may seem similar and even share the same melt-flow index, but experience shows that the branched one outruns the linear every time as a carrier resin, Reedy says.

Over the past couple of years, J.M. Huber has introduced a family of endothermics that takes a different tack from others on the market. The Activex products pair polycarbonic acid with synthetic carbonates, which the company says can be more easily tailored for individual applications.

Huber offers 100%-active powders but has also been refining its carrier resins for masterbatch dispersions. Eschewing waxy carriers because their low melting points can allow premature foaming, Huber has instead adopted a range of styrenic and olefinic carriers. Mooney says the company "designs the carrier to be a functional part of the resin system," in that it can enhance processing variables like melt flow or strength.

J.M. Huber also differs from other endothermic suppliers in its emphasis on gas production rate over volume. Mooney acknowledges that Activex endothermics produce less gas per gram of blowing agent than do azos--roughly 100-140 cc/g for Activex versus 300 cc/g for some azos. But he asserts that "large volumes of gas result in large lost volumes that don't do anything." Far more important, he says, is a controlled rate of gas release, with Activex powders exhibiting the slowest of any CBA.

FOR POLYCARBONATE, TOO

Having solved the problems associated with early acid-carbonate products, endothermic suppliers lately have turned to a more difficult challenge. Mooney explains that in the past, endothermics did not work well with high-temperature resins like PC because of premature blowing. The water produced from the citric acid-bicarbonate reaction was another thorny problem when working with PC, he notes.

Recently, however, several companies have come out with products designed to overcome these two sticking points. Huber recently introduced Activex 736, a 25%-active dispersion specifically for PC and PC alloys. To meet the temperature requirements, it produces gas between 450 F and 700 F and allows cycle-time reductions of 25% relative to exothermics, according to Mooney. Henley now sells a pelletized masterbatch for PC called CF 211. Reedy offers SaFoam RPC, a mixture based on magnesium and an acid. And EPI president Rod Garcia says his company also sells a product specifically for PC. "We're confident even in polycarbonate," he says.

MORE THAN SKIN DEEP

Endothermic CBAs may indeed have changed since the early days. But do they actually outshine the exothermic competition? CBA suppliers of both types hotly contend this subject. Endothermic advocates all point primarily to better surface skins and shorter cycle times with their products. Reedy explains that endothermics simply stop producing gas when removed from a heat source. "Heat is the throttle," he explains. So in foam molding, gas is not generated near the cool mold surfaces, improving skin thickness and smoothness. Cycle times decline too because no extra cooling is needed to offset the heat generated by the thermal decomposition of an exothermic agent, he says.

Huber's Mooney adds that cell structure improves because endothermics exhibit more controlled bubble formation due to their slower gas release. Reedy agrees, claiming SaFoam can achieve uniform cells of 25 microns. In fact, all the endothermic makers sell their products as nucleating agents for direct gas-injection systems because of the endothermics' ability to produce a uniform cell structure without voids.

Another endothermic benefit comes from faster gas diffusion rates. Acid-carbonate CBAs produce |CO.sub.2~ gas, which can leave a polymer matrix more rapidly than the nitrogen gas generated by azo decomposition. When a part must be painted, there's no two-day wait for nitrogen outgassing, according to Reedy.

Suppliers also point to ammonia and cyanuric acid, two of the decomposition byproducts of azos, as a potential problem. "From an environmental point of view, endothermics are certainly safer," asserts Keane, noting that the Food and Drug Administration classifies both sodium bicarbonate and citric acid as "Generally Recognized as Safe (GRAS)." Reedy agrees and stresses that all the SaFoam products are GRAS, including the one for PC. Pressure is mounting against azos in food packaging applications, he asserts, because the cyanuric acid can yield a toxic decomposition product, hydrogen cyanide. Uniroyal's Leone, however, points out that azos still have FDA approval as a food additive. "Our research reveals that cyanuric acid is not hazardous as present from azo decomposition," he says.

AZO ADVANTAGES

Azo suppliers dispute the notion of endothermic superiority. "Lots of sizzle, but no steak," says Leone. Explaining the continued dominance of azos, Leone emphasizes the crucial flexibility issue. Azos do have a wider processing window with variable decomposition points. Decomposition typically starts around 325 F but can be pushed lower with activation. Uniroyal, for example, offers various azos for temperatures between 220 F and 750 F. By contrast, endothermic foaming agents normally kick off at 340 F.

Temperature flexibility aside, Uniroyal cites azos' higher gas yields as an important plus to economically cut the weight of a foamed product. "If density reduction is the primary goal, exothermics are definitely the way to go," he says.

Leone acknowledges a slight advantage for endothermics in skin-quality and cycle-time, but says processing conditions can make up the difference. "If the thermodynamics are in balance you won't notice any difference," he says. He disputes any advantage for exothermics in cell structure, however. He says some acid-bicarbonate types have a bimodal gas yield. "They go off, then nothing, then go off again," possibly creating uneven cell structure.

And then there's cost. Uniroyal sells azos in the $2.70 range. Endothermic makers quote comparable or higher prices for products that give off less gas per unit volume.

For some applications, even endothermic suppliers admit azos have an edge. Reedy, for one, says exothermic azos work best on vinyl calendered foam because an endothermic would act like insulation, absorbing too much heat from the rolls.

Huber notes some customers blend its endothermic product with azos while EPI even markets such a blend. Huber's Mooney explains that thick parts may need a little extra gas pressure or a processor may want to extend the temperature range. "The borders between the two types are disappearing," he says.

FOR DETAILS ON ITEMS IN THIS ARTICLE, USE READERS' SERVICE CARD

EPI Inc., Conroe, Texas (CIRCLE 59)

Henley Chemicals Inc., Montvale, N.J. (CIRCLE 60)

J.M. Huber Corp., Chemicals Div., Havre de Grace, Md. (CIRCLE 61)

Reedy International Corp., Keyport, N.J. (CIRCLE 62)

Uniroyal Chemical Co. Inc., Middlebury, Conn. (CIRCLE 63)