n d o tli e r in i c chemical I) 1 owing agcn ts may si m u I t an eou SI y rep- E resent the past and t l ic future of

thermoplastic s t ruc tura l foain. Re- member that endotliermics, which ab- sorb heat rather than release i t when producing gas, were tlierc. at tlie be- ginning. Simple mixes of sodium bi- carbonate and citric acid originally prevailed as tlie chemical blowing agent (CBA) oi choice for structural foam. Azod icar b o 11 a in i d e, however , ea si 1 y supplanted these early, less controllable attempts at nkking foam tlie baking- soda way. “Azo” was simply a better mousetrap. Yet since tlie early to mid- dle 1980s, an increasing number of foam makers have started to sing the

=f ll~o&yi;, ~ l l lp -Gvcc j

bonate blowing agents. So hi-gct tli( k a- S e 1 t z e r coin p a r i so ti s , 11 e c a u s e endotlierinics have evolvctl and may de- serve another look.

In fact, if you talk to endothermic CBA suppliers today, you would liear strong claims to statc-of-thc-art status You’d also heal about performance anti

-control advantagc~s over the ig a~o-based exotliei mic t e c h

nology Of late, thcsc aI-gum(~nts I1avc

even taken on environmental strains b e cause endotliermics leave no ammo- nia or cyanuric acid bypi-oducts And several suppliers have recently gone af- ter new applications, coining out with endothermic products for high- teni- perature, moistui-e-sensitive resins like polycarbonate and its alloys. “There’s no question that azos were a big leap forward,” says J.M. Huber Corp. plas- t ics t e c 11 n o 1 o g y man a g e r Ger ry Mooney. “Rut we feel we’re on the cut- ting edge now with endothermics.’’

Whether the endotliermics can ac- tu a 11 y outperform their e xo th e r in i c cousins remains a matter shrouded in co t i t en t ion, however.

Endotliermics have staked out an

though by all accounts azos still control the lion’s share of the total CBA mar- ket. According to Uniroyal Cheniical Co. marketing manager Casey Leone, azos still represent 80% of a 10-million- Ib/yr CBA market, with the rest going to other exothermics and various CII- dotliermics. I n st I uctural foam alone, 1 x 0 ne says o nd o t 11 e rm ics re p i ese n t

M o o n c y also pegs entlotlicrmic~’ sharc

.... ....., 1 1 - -:,. uiidciiidvjic iiiche if i s t i ~ ~ t t ~ i ~ l foam,

abou t 25‘1, of t h e 111:il.ltet. H u I I ~ I - ’ ~

at 25%, thanks in part to some recent forays into large-volume recycling ap- plications.

‘ I k growing number of endother- mic suppliers also suggests increasing popularity. Since Gelinany’s Boehringer Ingelheini AG-distributed by Henley Chemicals 1nc.-first refined the citric acid-sodium bicarbonate technology, several relative newcomers have en- tered the domestic market. The most re- cent, EPI Inc., opened in Texas this year with a new Werner & Pleiderer twin- screw compounding line and a full range of pred ispersed and powdered en- dothermic products. And J.M. Huber has come out with a whole family of syn- thetic endothermics over the past few y&ii-.S.

NOT LIKE THE OLD DAYS Suppliers say there’s almost no

comparison between modern acid-car- bonate endothermics and t h e early ones. “It’s just not that simple anymore,” says Michael Reedy of Reedy Interna- tional Corp., another major endother- mic supplier. Nowadays, the companies making endothermic CBAs rely on im- proved encapsulation technology and an -

arsenal of carrier resins. In addition, all suppliers offer 100%-active pow- ders and have ex- tended their range of masterbatch dis- pers ions . T h e y have also worked to inc rease t h e CBA-lemperature activation range as well.

For instance, Henley Chemicals recent ly broad- ened i ts range of inasterbatch con- cent ra t ions . Ac- cording to product manager Dennis Keane, the compa- ny now offers 5%- active dispersions

1 117 152 4.47 15.85 N 2 152 266 420 ’ 14.90

” 70% concentrates

88 N 1 119 270 2.19 7.77 I

n HK70 69 N 1 174 255 5.16 18.3 25% concentrates J.M. Huber Artivex535 29 N 1 103 256 8.10* 28.73

Adivex235 36 N 1 101 165 4.83 17.13 N 1 165 144 44R 1 5 8 9

:=Exothermic. N=Endolhermic. i=lnitial. f=Final. Speed=Foam Ti”0.282 (slowness of readion compared lo AZ130) =Average (4 determinaliml4.40. Source: J.M. Huber Coy.

for any g r a d e of Boehringer Ingel- h e i in’s Hydro c e r o 1. He i l l e y m a k e s these less concciitrated dispersions es- pecially to satisfy the ncctls of some cx- trusion processors ’Ihe uppcr limit for its concentrates remains at 70%.

T h e company has also adopted styrenic carriers i n some cases, sup- plementing the e x t 4 i i r a l d i n i c carri- ers. “In t h e past the problem was to pelletize polystyreile without kicking oii 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 ex- tending the endothermics’ temperature range, newer Hydrocerol BIH works in the 325 F range while grade HK han- dles temperatures around 500 F.

Reedy International, which sells 16 products gnde!- the SaFoz;;; !r;!jcl, lias worked in a similar direction. I n addi- tion to 100’%-active powders, the com- pany sells masterbatches with both styrenic and olefinic carriers. Reedy recommends the s t y enic carriers for niaterials like ABS. €IS, I’VC, ant1 ’TPEs He emphasizes the benefits of choos- ing a carrier that can improve melt strength and flow OiIct-1 that choice is a relatively subtle onc For c>xainple, a branched 1J)I’I; and a lincar I , I , D I T __- __- -~

5 8 t’LASllCC> l l f I )(,y ALJ(, I ) ‘ I I IO(+

may seem similar and even share the same melt-flow index, but experience shows that the branched one outruns the linear eveiy time as a carrier resin. Reedy says.

Over t h e pas t couple of years . J.M. Huber has introduced a family of endothermics that takes a diffei-ent tack from others on the market 71ie Activex products pair polycarbonic acid with synthetic carbonates, which the com- pany 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 Es- chewing 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 C d I I icr io be a iunctionai part o i the resin system,” in that i t can enhance processing variables like melt flow ot strength.

J.M. Huber also differs from othet endothermic suppliers i n its empliasis on gas production rate over volunzc Mooney acknowledges that Activex en- dothermics produce less gas per gi ani o l blowing agent than do azos-rough- ly 100-140 cc/g for Activex ve rws 300 cc /g for son1e azos I3ul I1

- -~ ~

“large volumes of gas result in large lost volumes tha t don’t do anything.” F a r m o r e impor- tant, he says, is a controlled ra te of gas re lease , with Activex powders ex- hibiting the slowest of any CBA (see Table 1).

FOR POLYCAR- BONATE, TOO

Having solved the problems asso- c ia ted with ear ly a c i d - c a r b o n a t e p roduc t s , en - dothermic suppli- ers la te ly have turned to a m o r e difficult challenge. Mooney explains that in the past, en-

dothertnics did not work well with high- tetnperalure resins like PC because of premature blowing. T h e water pro- duced from the citric acid-bicarbonate reaction was another thorny problem when working with PC, he notes.

Recently, however, several com- panies have come out with products de- signed to overcome these two sticking points. Huber recently introduced Ac- tivex 736, a 25%active dispersion specif- ically for I’C 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 exotliertnics, according to Mooney. Henley now sells a pelletized master- batch for PC called CF 211. Reedy of- fers SaFoam RPC, a mixture based on magnesium and an acid. And EPI pres- ident Rod Garcia says his company al- so sells a product specifically for PC. “We’re confident even in polycarbon- ate,” he says.

MORE THAN SKIN DEEP I< nd o thermic CRAs may indeed

have changed since the early days. R u t d o they actually o u t s h i n e the exothermic competition? CBA suppli- ers of both types hotly contend this sub- ject Endothermic advocates all point primal ily to better surface skins and _-

shorter cycle times with their products. Reedy explains that endothermics simply s top 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 su r faces , improving skin thickness and smoothness. Cycle times decline too be- c a u s e n o ex t r a cool ing is needed to offset t h e h e a t generated by the thermal de- composition of an exothemiic agent, he says.

Huber’s Mooney adds that cell structure improves because endothermics ex- hibit more controlled bubble formation due to their slower g a s 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 s y s t e m s be- cause of the endothermics’ ability to produce a uniform cell structure without voids.

Another e n dot h e r in ic benefit comes from faster gas diffusion rates. Acid-carbon- ate CBAs produce COz gas, which can leave a polymer matrix more rapidly than the nitrogen gas generated by azo decom- position. When a part must be painted, there’s no two-day wait for nitrogen out- gassing, according to Reedy.

Suppliers also point to ammonia and cyanuric acid, two of the decompo- sition byproducts of azos, as a potential problem. “From an environmental point of view, endothermics a re certainly safer,” asserts Keane, noting that the Food and Drug Administration classi- fies 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 packag- ing applications, he asserts, because the cyanuric acid can yield a toxic decom- position 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 a s pre- sent 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

1 emphasizes the crucial flexibility issue.

dow with variable decomposition points. Decomposition typically starts around 325 F but can be pushed lower with ac- tivation. Uniroyal, for example, offers various azos for temperatures between 220 F and 750 F. By cont ras t , en- dothermic foaming agents normally kick off at 340 F.

T e m p e r a t u r e flexibility as ide, Uniroyal cites azos’ higher gas yields as an important plus to economically cut

&os bo hz<c 2 wide: p ~ ~ c e s s i f i g $vis-

the weight of a foamed prod- uc t “If density reduction is the primary goal, exothermics are definitely the way to go,” he says.

Leone acknowledges a s l igh t advantage for en- dothermics in skinquality and cycle-time, but says process- ing conditions can make up the difference. “If the ther- modynamics are in balance you won’t notice any differ- ence,” h e says. He disputes any advantage for exother- mics in cell structure, howev- er. H e says s o m e acid- bicarbonate types have a bi- modal gas yield. “They go off, t h e n nothing, t h e n g o off again,” possibly creating un- even 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 ad- mit azos have an edge. Reedy, for one, says exothermic azos work best on vinyl calendered foam because an endothermic would act like insulation, ab- sorbing too much heat from the rolls.

Huber notes some c u s t o m e r s 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 p r e s sure or a processor may want to extend the temperature range. “The borders between the two types are disappear- ing,” he says. no

LASTICS 1-ECHNOIOGY i ? AUGUST 1092 59