Today's purging agents work 'just-in-time.'

In today's short-run, JIT environment, quick materials changeovers are a growingnecessity. And purging agents are becoming more specialized for particular jobs. Purging the barrels of extruders and molding machines has always been a necessary housekeeping task whenever changing colors or resin formulations. But as manufacturers move increasingly to "just-in-time" schedules--shorter runs andmore frequented job changeovers--the need for more frequent and efficient purging has become paramount.

Processors are less and less willing or able to tolerate the vast amounts of scrap and wasted hours consumed in traditional purging. "People are realizing the importance of purging and don't accept it as a necessary evil. Today there'sa more sophisticated outlook, where efficient purging is viewed as a strategic, competitive advantage that translates to increased production," explains KennethShuman, v.p. of Shuman Plastics Inc., a purging compound supplier. While it appears that the large majority of processors have stuck with the most basic of conventional purging materials, such as wide-spec or regrind material--chiefly low-melt-index PP, HDPE and PS--today's quick-change mentalityis attracting a growing number of adherents to materials developed specifically to enhance purging efficiency. These products are geared to provide more thorough cleaning while considerably reducing scrap, time and labor--including the frequency with which the equipment must be disassembled and hand cleaned. Asindicated in the following review of new

developments, today's purging agents   are becoming ever more specialized for
particular types of equipment--e.g.,     blown film lines and hot-runner
injection systems--and for the processing       temperature of the material to

be purged. TWO CLASSES OF COMPOUNDS

While there are a number of ways of describing the action of different purging agents, the simplest approach is to divide them into two general categories: those that work by a physical action and those that work by chemical action. Physical-type purging compounds are typically high-viscosity thermoplastics thatmechanically push the residue out. Generally, the purging agent must be of higher viscosity than the residue it is trying to displace. Many of these compounds also contain fillers or other additives to provide some degree of scouring and cleaning action.

Chemical-type purging compounds typically employ a heat-activated chemical reaction to break down the polymer residue in the barrel, thereby lowering its viscosity. At the same time, or with a slightly delayed action, a chemical blowing agent in the purge compound causes foaming to raise its viscosity and aid in mechanical displacement of residue.

PHYSICAL PURGING AGENTS

For many years, the two main types of purging agents available were in the physical category--ground or "cracked" cast-acrylic sheet scrap and polyethylene-based products. Cast acrylic is a partially crosslinked material with high melt viscosity. While reportedly providing the greatest scrubbing action of any purging compound, critics maintain that over time it can damage processing machinery. Also, because it does not melt completely, cast acrylic will not readily pass through nozzles, dies, screen packs and breaker plates, thus requiring their removal before purging. In addition, acrylic typically leaves residue that must be removed, an odor that some processors find objectionable, and off-gassing that may irritate the eyes and respiratory tract if ventilation is inadequate. Cast acrylic supplies, meanwhile, are said to be shrinking as extrudable pellets take ever greater share of the acrylic sheet

market. As a result, one major supplier, Claude P. Bamberger

International, has "diluted" the cast-acrylic content of its Bamberco purge compound. New Bamberco Hi-Flow is a blend of cast acrylic with another, proprietary acrylic material. Meanwhile, the PE-based purging compounds, most of which contain an abrasive component, have been geared mainly to purging polyolefins from injection machines or extruders. However, new thermoplastic blends of polyolefins or styrenics with mildly abrasive fillers and other additives, can be used on a much wider range of equipment and processing temperatures. Besides their mild scouring action, they have a tackiness in the melt state that helps pull residueoff metal surfaces.

Shuman Plastics has marketed its DynaPurge polyolefin compounds for 12 years. They include four "workhorse" pelletized products, formulated for different temperature ranges and types of processing equipment. They're heat-stable and nonabrasive, says division manager Joie Komarmi.

Two new polyolefinic additions to the line have been developed for critical applications in extrusion and coextrusion, as well as hot-runner injection molding because it exposes resins to more heat and residence time. Says Komarmi,"We saw a need for a product with smaller, more uniform pellet size

that will   feed better through smaller openings, but has the same stability
as the other   DynaPurge products." Designed to avoid the need to dismantle

complex equipment, the new purging agents contain a nonabrasive, scrubbing agent that reportedly will not cause blockage or hang-up.

One of the new products, DynaPurge Excel, is designed for extruders and coextruders of sheet, custom profiles, pipe and tubing, as well as blow molders with accumulator heads. It can purge materials with processing temperatures of 320-525 F, including PVC, PVDC, PS, PP, and polycarbonate.

Brand-new DynaPurge Pulsar is for purging hot-runner systems and small injectionmachines of resins processed at 320-575 F. Also in the works is a developmental product that is a sister to workhorse-grade DynaPurge Elite; it will be aimed specifically at hot-runner systems processing engineering resins. Bellex, Inc. has been nationally distributing Asaclean purging compounds from Japan's Asahi Chemicals since 1990. These modified polystyrene compounds are forinjection, extrusion and blow molding machines and any thermoplastic material processed between 350 and 680 F. They provide cleaning performance equal to thatof semi-meltable or chemical purges but without the drawbacks

(odor,            abrasiveness, hold-up time etc.), according to marketing
coordinator Jack       Spielberg. Asaclean has high affinity for other

polymers and a very low affinityfor metal, he explains, so it powerfully grips the residue and quickly removes it. "Asaclean can save 75% of the time typically required to make changeovers and greatly reduces material waste,"

Spielberg claims. Unitemp offers proprietary Pekutherm granular products that are described as "thermoelastic" or rubber-like and semi-solid at elevated temperature, rather than truly thermoplastic. The company has offered an acrylic since the mid-'80s,and has just introduced a new purge compound suitable for both higher and lower temperatures than the acrylic. Manager of R&D and marketing Bernd Krebs says thenew compound is based on a thermoelastic polyolefin, with a temperature range of350-750 F. It produces no fumes, Krebs says, and is highly effective in average purging times of about 7 min for smaller machines and about 15-20 min for largermachines.

LIQUID PURGES

There is a newer category of physical purging agents consisting of liquid additives that are mixed with a carrier purging resin or may be predispersed in thermoplastic pellets. The liquids have a lubricating nature and often contain amild abrasive filler.

Slide PDQ, introduced by Percy Harms Corp. (now called Slide Products, Inc.) about three years ago, is a liquid that contains both a surfactant, which acts as the wetting agent, and a mildly abrasive cleaner--"fine particles that do nothurt the barrel or screw," according to company chemist Jan Vandergrass. "The key feature is that it allows you to reduce the amount of purging resin used by as much as half," he says.

Last year, Ferro Corp.'s Liquid Coatings Div. introduced SpectraPurge, its firstpurging compound. Said to be compatible with all resin systems, this

nontoxic   liquid has a temperature range of 350-550 F. It's pumped directly
into the      machine feed throat (similar to liquid colorant) at a 10-12%
ratio with the     carrier resin. SpectraPurge is mildly abrasive and has
lubricating qualities.   Bill Meyers, technical support, says average purge

time is 10-15 min. The purge material can be reground and reused in some cases, as it is non-caustic and emits no noxious odors.

For processors unable to handle liquids conveniently, Ferro now also has pellet concentrate form of SpectraPurge that is mixed at a 10:1 ratio with resin in themachine hopper.

A newcomer to plastics is Blackbrook Chemical, which has developed what president Tom Barrow says is a "user-friendly and earth-friendly" purging material. Enviropurge is a proprietary liquid product that is mixed with a smallamount of scrap resin for purging injection machines and extruders at 150-700 F.Barrow says the product has both a chemical and physical action. "There's one component that breaks down the molecular chain of the polymer, and another that pushes the material off the screw," he explains. The product

contains no        aromatic solvents, VOCs, or free ammonia, and is odorless,
nontoxic,            noncorrosive, and ground-water safe, according to Barrow.

CHEMICAL PURGING AGENTS

Chemical purging compounds, which emerged a little more than a decade ago, are now available as pure powders or pellet concentrates to mix with a purge resin and as ready-to-use premixes with resin. Most of these agents use a PE or styrenic carrier. They are typically used in smaller amounts than physical types--normally a volume sufficient to fill up the barrel. After charging the barrel, a "heat-soak" cycle of 10-15 min allows the heat-activated chemical reaction to partially depolymerize the residue resin. Suppliers note that the depolymerizing action is only partial, so that no dangerous gases are released. They also claim that a single barrel charging is usually sufficient to purge themachine.

Responding to criticisms of the wait for heat soaking, suppliers of chemical purging agents claim that it's a lot less time than may be required for other methods of purging or cleanout. Says sales manager Barbara Giaquinto of RapidPurge Corp., "Our products are primarily used where it takes someone longerthan 30 min to make color or material changes or where there are problems with black specking." She also says her chemical purging product can

usually be      heat-soaked at normal barrel temperature, but "you may have to
raise the        temperature if it's a difficult purge. The higher the heat,
the better the      product will go after hard-to-remove carbonized material."

RapidPurge, a pioneer in the chemical purging sphere, now offers three grades ofits RapidPurge premix in pellet form. These are chemically the same but have different PE carriers and are geared to purging a wide variety of resins processed at up to 800 F on all types of processing equipment, including those with screens, mixers, gear pumps, and hot runners. The company also offers a powder designed to be mixed with a user's carrier resin.

A typical purge ranges from 30 min for injection machines to about an hour for extrusion and blow molding machines. The company says its product cuts color andmaterial changeover time by 50-90%. Giaquinto says several of the company's customers have FDA approval for use of RapidPurge in their facilities. RapidPurge reportedly emits only harmless gases such as nitrogen, carbon dioxide, water vapor, and a very small trace of ammonia. The purged material is said to be environmentally safe and has been approved for disposal by incineration or in landfills.

Novachem, founded in 1990, started with a Novapurge chemical purging compound that is very similar to RapidPurge. Novapurge is still available, but the company has a new chemical purging compound called SuperNova, a pellet concentrate in HDPE, which is said to be ammonia-free and compliant with all FDAregulations. It comes in two grades. The basic product is designed for purging at up to 550 F. SuperNova HT is designed for even higher temperatures

and all   types of processing equipment. The company claims that regular use
of the       product can, in most cases, eliminate teardown of the machine for

hand cleaning. Company v.p. of sales & marketing Frank Winner says a SuperNova grade optimized for processing clear polycarbonate and acrylic will be launched early this year. Omni-Purge chemical purging compound from Plastics Process Equipment is available as powder, premix in various PE carriers, or pellet concentrate to be blended with the user's carrier resin. Sales manager Gary Croy says the powder and premixes are designed to work on resins processed at up to 550 F.

A new concentrate can be blended with a high-temperature carrier resin for use at over550 F.

Over the last few years, Chisso America has marketed a Japanese family of heat-activated purging compounds called Z Clean. These pellet compounds (small-diam. for easy feeding) are based on PP, PE, or PS resins, require no heat-soak residence time, and can be used with most types of processing machinery to purge all major types of resins. Although aimed mainly at polyolefins and engineering resins, they have been used successfully with flexible and rigid PVC, according to marketing manager Richard Tuttle.

Three product series are designed to purge resins processed at different temperature ranges. For example, the S Series is designed for engineering resinsprocessed at up to 750 F. Different Z Clean grades use different technologies--foaming and nonfoaming, "soft" filled and nonfilled. Most involve some foaming or swelling to aid in pushing residue from the barrel. Nonfoaming grades like the E Series are designed for hot-runner systems and blown film

lines. In the latter case, the purging compound can actually be used to blow a bubble in order to avoid downtime. Other grades are designed for tough-to-purge fluorescent colors and flame-retardant systems.

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