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New ways to incorporate additives without extrusion compounding.

Last month (P.15), we reported briefly on two new technologies for incorporating additives into resins without going through a conventional extrusion compounding process. One is a concentrate technology that uses a proprietary "reactor process" to incorporate high levels of liquid or low-melting additives within a polypropylene particle. The other is a do-it-yourself method for coating additives from an aqueous dispersion onto the outside of granules or pellets of a variety of polymers. In both cases, the benefits of bypassing the extrusion compounding step are said to include significant cost savings and the avoidance of a processing beat history that can affect the properties of both additives and matrix resin. Here are further details on both new technologies.

HIGH LOADINGS OF LIQUIDS

The new Xantrix Additive Delivery System from Himont Incorporated, Wilmington, Del., is intended to overcome a particular limitation of single-screw extrusion compounding. According to Dr. Michael Goldin, business manager of Himont's new Specialty Chemicals business unit, single screws usually cannot produce concentrates containing more than 5-7% of a liquid additive without encountering screw slippage, resulting in variable quality. By contrast, Xantrix ADS reportedly can incorporate up to 30% by weight of liquid or low-melting (up to 248 F) additives, as well a higher-melting solids. These additives are said to be uniformly distributed throughout a spherical PP particle very similar in size to those produced by Himont's Spheripol process.

Whereas twin-screw extruders can handle higher levels of liquid additives, the absence of an extra heat history provides an advantage similar to that obtained with Himont's Valtec product technology for coating additives onto the outside of reactor spheres. Himont says the PP retains its as-polymerized low crystallinity, which makes for faster melting and thus easier dispersion in subsequent processing operations. Also, heat-sensitive additives are protected from degradation: for example, peroxide catalysts for so-called "controlled rheology" viscosity modification suffer no loss of activity, Dr. Goldin reports.

Besides peroxides, Xantrix ADS can incorporate acid neutralizers, antifogging agents, antimicrobials, antioxidants, antistats, fragrances, nucleants, oils, slip agents, and uv stabilizers. Himont is also working on inclusion of liquid flame retardants. Combinations of such additives from any supplier can be provided at total loadings up to 30%. Resin producers, compounders, and other processors need not change their existing formulations: Himont intends to supply the concentrates on a custom basis in quantities as small as 75 lb.

Xantrix ADS uses a 20 MFR PP homopolymer as a carrier, which Himont says is suitable for the majority of applications. A different carrier can be used if market demand justifies it. Trials in single-screw sheet extrusion reportedly show that at 50:1 to 100:1 letdowns, Xantrix ADS distributes evenly in final products at least as well as conventionally compounded concentrates. This has already allowed compounders to run liquid-containing products on single screws that formerly required twin screws.

Dr. Goldin says that Xantrix ADS causes no deterioration of optical clarity in PP copolymers. He also thinks it should work in TPOs. Ongoing research is investigating a similar product for LLDPE.

LIQUID ADDITIVE COATINGS

A totally different approach is represented by the new Aquastab technology from Eastman Chemical Co., Kingsport, Tenn. This is not a substitute for conventional concentrates; rather, it's a method for coating polymer particles-whether reactor granule, flake or extruded pellet-with the proper amounts of all the additives required for final molding or extrusion. Eastman has been quietly selling Aquastab "water-based topical additive delivery systems" to at least one major polyolefins producer for as long as four years, and is just now going public.

Aquastab can utilize any of a wide variety of solid or liquid commercial additives-uv stabilizers, antioxidants, acid acceptors, antistats, lubricants, nucleants, clarifiers, metal deactivators, blowing agents, antiblocks, optical whiteners, and inorganic fillers-at loadings of 25-50%. Virtually any additive that's not hydrolysis sensitive can be used, says Aquastab manager Paul J. Palermo, Jr. However, the coating technique limits the amount of additive that can be incorporated in the final product. The maximum achieved so far is 0.6% by weight, though in the lab Eastman has gone up to 1.2%. Company spokesmen do not yet know what the ultimate limit may be. Aquastab systems are applicable to PE, PP, PS, ABS, polycarbonate, and possibly other resins.

The way it works is that the additives and a low-molecular-weight polyolefin wax are emulsified in water along with surfactants, antifoaming agent and antimicrobials to ensure storage stability. Active ingredient level in the final emulsion is 25-50%. All carrier components are FDA approved for food contact, says Eastman. The emulsion is sprayed onto the polymer pellets or granules in a batch or continuous process, and then the water is evaporated. With the wax the dried coating reportedly adheres firmly to the resin particle during airveying, shipping and storage.

This nondusting method can coat particles of very high- or very low-viscosity resins that may be difficult to extrusion compound. Users gain the flexibility of producing a wide range of end-use tailored compounds from either reactor flake or granule or from a basic compounded pellet containing only antioxidant and acid acceptor. According to Eastman's calculations, a 220-million-lb/yr resin plant could spend $1.5 million for equipment to apply the Aquastab coatings, using six operators. Still, the cost savings relative to extrusion compounding would pay for the system in eight months.

Aquastab systems cost up to 2.50/lb over the basic cost of the active ingredients, supplied in drums or in bulk. Each Aquastab formulation contains a single active ingredient, though several can be premixed by the user prior to application, or Eastman can supply preblends at extra cost.

An 84%-active powder form that can be dispersed in water at the customer's plant is in advanced development. This will save the expense of shipping water.
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Title Annotation:plastics additives
Author:Naitove, Matthew H.
Publication:Plastics Technology
Date:Jul 1, 1991
Words:958
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