Blowing agents for thermoplastics: endothermics on the rise.

The action in chemical blowing agents is shifting to endothermics in place of traditional azos. New endo/exothermic blends combine the best of both.

There's plenty of fizz left in chemical blowing-agent (CBA) development for thermoplastic foams, judging from the number of new products introduced recently. Although there's little new in the way of chemistry, suppliers continue to refine and combine existing CBA types to optimize performance for particular applications.

And the range of applications is expanding in all directions. In extrusion, low-density foam sheet is now being made from resins such as PET, PETG, and PP. Industry observers also report a resurgence of activity in rigid foam extrusion, particularly PVC profiles and both PVC and ABS foam-core pipe.

And in molding, there are new opportunities for weight and material savings and sink reduction. "People making large components such as HIPS tv cabinets and PP furniture are looking for cost savings by going to structural foams," explains Bill Maisel, marketing manager of J.M. Huber's Chemicals Div., Havre de Grace, Md. "Chemical blowing agents also help correct such major processing problems as sink marks and warpage due to stress introduced in the injection molding process." There's even the brand-new field of structural-foam blow molding, though it's commercial only in Europe (see PT, Dec. '95, p. 43).

Physical blowing agents for direct gas injection are the main choice for low-density foams - those with over 60% density reduction. But even here, chemical blowing agents (CBAs) often serve as nucleating agents to improve processing and yield a finer cell structure.

[TABULAR DATA OMITTED]

In medium- and high-density thermoplastic foams (up to 60% density reduction), the major story continues to be the growing success of endothermic (heat-absorbing) CBAs as alternatives to exothermic (heat-releasing) types. Most endothermics are of the acid/carbonate family, whereby inorganic carbonates react with acidic species to evolve carbon dioxide. Until recently, endothermics have been used mainly in molding, but several products now are tailored for extrusion. Traditional azo-type exothermics have dominated in extrusion because of the higher gas pressure they generate. A new trend appears to be emerging in which endothermics and exothermics are combined to achieve optimum performance.

WHY ENDOTHERMICS ARE HOT

Suppliers of endothermic CBAs say these additives produce finer cell structures and less discoloration than their exothermic counterparts, so that smoother part surfaces and pure-white foams can be produced. Endothermics reportedly are better behaved in coextrusion processes - they tend to stay more within the foam layers. These attributes are helping them gain ground in areas such as injection molding of rigid PVC, where up to 20% faster cycles, finer cell structures, and thicker skins reportedly can be obtained.

Another advantage of endothermics is that the C[O.sub.2] they evolve readily diffuses from the foamed product, reducing or eliminating the need for aging between molding and painting. By contrast, painting azo-foamed parts right after production can lead to blisters as the retained nitrogen slowly outgasses.

Suppliers are working to eliminate two disadvantages of endothermics. One is their tendency to rust steel molds because of water released during the foaming reaction. Environmental Products Inc. (EPI) of Conroe, Texas, has launched three new endothermics that do not release water. Epicor 882, 456, and 753 all perform well in injection molding and extruded sheet and profiles, says v.p. of sales and marketing John Kosin. Low moisture generation is also claimed for a new liquid endothermic concentrate to be released this month by Ferro Corp.'s Liquid Coatings and Dispersions Div. in Edison, N.J., for extruding low-density foams of PP and HDPE.

Another disadvantage is endothermics' higher cost. While they are becoming ever more price competitive, they still sell in the $3-5/lb range, while typical azos cost $3-3.50/lb in volume quantities. Typical use levels for both types is 1-2%.

There is progress on this front as well. For example, EPI's new Epicor 882 is a lower-cost ($2.50-3.00/lb), 55%-active concentrate for polyolefins. And although new Epicor 456 for PC and PC/ABS foams costs more ($4.50-6.00/lb) than the workhorse azo 5-phenyl tetrazole that it's designed to replace, Epicor 456 is used at only 11.5% vs. 5-10% for 5-PT.

B.I. Chemicals' Henley Div. in Montvale, N.J., has also reduced the cost of its latest endothermic. New Hydrocerol CT-1004 is a 55% concentrate that's priced lower ($2-3/lb) than other Hydrocerol products and can provide good density reduction at levels of 0.75-1.5%.

Suppliers are expanding the performance range of endothermics. EPI's new Epicor 882 reportedly has a higher gas yield and higher decomposition temperature (410-420 F) than other endothermics. J.M. Huber Chemicals has added a new grade (Activex 447) to its synthetic carbonate line that has double the gas yield of previous grades and more controllable yield as well.

Meanwhile, Reedy International Corp., Keyport, N.J., is adding extra functionality to several of its new endothermic concentrates. They include a styrenic block copolymer resin modifier to boost impact strength of the foam.

ENDO/EXO COMBINATIONS

Combination endothermic/exothermic CBAs combine the fine, uniform cell structure of endothermics with higher gas pressure from the exothermic component. Priced competitively with azos, the blends appear to be finding a niche in commingled recycled resins by offering the right balance of gas pressure for foaming mixtures of low- and high-temperature materials.

B.I. Chemicals now offers a whole line of these products. Marketing manager Dennis Keane notes that these products cause discoloration - even more than azos alone. However, in some dark-colored injection molding and extrusion applications, these blends are said to perform much better than just an exothermic or endothermic CBA.

A target for endo/exo blends is PVC or ABS foam-core pipe. New Exocerol 232 from B.I. Chemicals is one example. Another is EPI's Polycor 50, which reportedly gives 10-15% higher extrusion rates and finer cell structures than exothermics in ABS foam-core pipe. And Reedy International says field tests of its new Safoam AP-40 blend have shown lower densities and higher output rates than azos alone in foam-core pipe. Compared with a lower cost liquid azo, AP-40 reportedly offers potential cost savings of 5.5[cents]/ft of pipe.

EXOTHERMICS GROW, TOO

A growing market for exothermic CBAs is foaming crosslinked polyolefins that replace rubber insulation of pipe and telecommunications wire, as well as padding for auto interiors, sports equipment, and artificial turf. Uniroyal Chemical Co., Middlebury, Conn., has added two azo products to its Celogen line for achieving densities of 2 pcf with XLPE. Celogen AZ 1901-C and R-9370 allow precise matching of reaction rates for foaming and crosslinking.

For growing applications in rigid PVC profiles, Witco Corp.'s Polymer Additives Group, Greenwich, Conn., has come out with Ficel EPE. That product and Uniroyal's Celogen AZRV are recommended for the emerging application of foamed PVC sheet as backing boards for artwork or highway speed-limit signs.

Another emerging market for azos is metallocene-catalyzed polyolefin foams for packaging and other sheet. Uniroyal marketing manager Casey Leone says Celogen 780 and 765 look promising for this use: "They work especially well with the lower melt indices of these resins in injection molded and extruded foam." Both are blends of azos and activators formulated to decompose at lower temperatures and at faster rates.

CBAs IN LOW-DENSITY SHEET

New resins and nucleating/blowing-agent packages are contributing to the emergence of low-density foam-sheet extrusion technology for PET and PP packaging. B.I. Chemicals has introduced Hydrocerol HK-40BF blowing and/or nucleating agent for foaming PET, APET, and CPET.

Eastman Chemical Co., Kingsport, Tenn., has used this product and other CBAs to produce very fine-celled sheet of PCTA and PETG, achieving 30-40% weight reductions and density of 0.75 g/cc using standard flat-die extrusion equipment. Thermoformed dual-ovenable trays can be made from Thermx PCTA sheet, while the PETG foam can be used in yogurt and margarine cups.

Advanced development chemist Gary Boone says Eastman is focusing most heavily on developing foamed CPET sheet for food packaging and industrial uses in insulation, automotive, and construction. Unlike PETG, CPET is hydrolytically sensitive to water emitted by typical endothermics. Achieving lower densities of CPET foam (0.50-0.75 g/cc) require use of direct gas injection with nitrogen or C[O.sub.2] in combination with an endothermic nucleant such as Hydrocerol HK40BF.

Because CPET has very low melt strength, Eastman developed a soon-to-be-commercial melt-strength enhancer and nucleator concentrate. Boone says a reaction takes place in the extruder that raises the I.V. 15-20% over that of CPET.

Reedy International is also developing a foam system for PET, says president Michael Reedy. For low- and medium-density microcellular foams, the technology employs direct gassing with C[O.sub.2] or nitrogen, a surfactant, and a new Safoam nucleating agent that creates a branched network within the crystalline polymer, thereby increasing melt strength and reducing brittleness.

Foamed PP could be the next microwavable food packaging, says Reedy. He points to potential cost savings of 20-60%. New foamable PP grades are emerging from several major resin suppliers, and work on foamable PE is also under way. Examples are PP grade HMS814 from Montell Polyolefins, Wilmington, Del.; Rexflex "soft" PP grades from Rexene Products Co., Dallas; and a foamable HDPE in development at Mobil Polymers, Edison, N.J.

New CBAs for PP, HDPE, and LDPE include Reedy's Safoam FPE50, and Ferro is launching this month a liquid endothermic concentrate for use alone or with other blowing agents in low-density PP and HDPE foams.

There's also new technology for extruding low-density PS foam sheet for food packaging. Dow Plastics, Midland, Mich., already has seven licensees for its patented all-C[O.sub.2] blowing technology. But Reedy International thinks it can achieve even better results with technology developed in partnership with Genpak Corp., Glens Falls, N.Y., a major producer of thermoformed PS food containers. The partners offer for license a foam-sheet extrusion process using standard equipment and dies along with a blend of C[O.sub.2] and another gas (i.e., isobutane, isopentane, HFC 134A, or HCFC 152a). The process also uses one of three new Safoam endothermic concentrates (P50, FP40, or P20) as a nucleating agent and processing aid, plus a styrenic block copolymer as an impact modifier. According to Reedy, this system duplicates the processing characteristics of the old CFC methods in providing small bubbles, good post-expansion, high output rates, and non-brittle, completely closed-cell foams. Reedy says the key to this technology is that it can be customized by changing the gas ratios and concentrations of the Safoam nucleating agent and the impact modifier.

In addition, B.I Chemicals has a new endothermic nucleating agent (Hydrocerol KN50S) for use with C[O.sub.2] or other gas injection in PS insulation board.

RELATED ARTICLE: A Different Kind of Endothermic Blowing Agent

A relatively new type of endothermic blowing agent isn't a CBA at all. In 1994, Cylatec Corp., Canton, Ohio, introduced CylaCell, an inorganic physical blowing agent, which acts like an endothermic CBA (PT, March '94, p. 13). Bound water in this sodium silicate product causes the particles to expand into closed-cell spheres at temperatures above 300 F.

Initially aimed at rubber and thermoset polyester BMC, this product has found use in PP and other thermoplastics, according to marketing manager Kim Brechbuhlet. In unfilled PP, it functions both as a foaming and nucleating agent, while in highly filled thermoplastics, it functions as a compatibilizer and coupling agent. It reportedly allows incorporation of as much as 20% higher filler levels than were previously possible. Higher processing speed, better mixing, and improved product physicals are also claimed. Although priced at $5-7/lb, use levels are much lower (0.2%-0.4%) than for typical CBAs.