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New sealant formulations hold their own.

Few materials are called upon to fulfill the wide variety of tasks handled by industrial sealants. Some must be inert enough to protect medicines or analytical samples from contamination, others rugged and heat-resistant enough to hold ingot molds together during the steel-making process. Some sealing agents harden within minutes of exposure to the air, while others remain pliable for years.

Today, sealants are being employed in an ever-widening array of uses, from high-pressure applications, such as steam turbine joints where extremely tight seals are required, to construction, where sealants must be flexible enough to permit a building to sway in strong winds. Recent advances are taking sealants into new territory. New formulations are being developed that combine the strengths of curable sealants with hot-melt adhesives. Researchers have even developed materials that can create a seal between different materials without the need for a separate bonding agent such as silicone.

Sealant manufacturers investigating new products and processes include Bostik Inc. (Middleton, Mass.), Dow Corning Corp. Mich.), Essex Chemical, Corp. (Clifton, N.J.), General Elelctric Co. (Fairfield, Conn.), Lauren Manufacturing Co. (New Philadelphia, Ohio), Norton Performance Plastics (Wayne, N.J.), Schlegel Corp. (Rochester, N.Y.), Standard Products Co. (Dearborn, Mich.), 3M Co. (St. Paul), and Tremco Inc. (Cleveland), among others.

Heated Hybrid

A leading-edge application is the use of sealants as formed-in-place gaskets that can replace die-cut parts. One such sealant was recently developed by Norton Performance Plastics. Dynafoam was introduced in the United States last year, following its successful European debut in 1988. This curable thermoplastic elastomer creates the formed-in-place gaskets that are formed by pouring the sealant into the desired shape.

The material replaces die-cut gaskets used in automobiles to, for example, seal taillight assemblies. Dynafoam is also seeing service as a gasket around sunroofs, sealing out moisture, wind, and dust.

In addition to providing a less expensive alternative to die-cut parts, Dynafoam was designed to fill a niche between traditional hot-melt adhesives and gaskets, such as butyl rubbers or ethyl vinyl acetates, and curable silicone and urethane sealants. One difficulty when using butyl as a hot-melt material is its vulnerability to high temperatures--butyl softens at temperatures above about 160 [degrees] F. A car parked in the sun with the windows closed can easily get that hot, causing the butyl seal to weaken or fail.

Another drawback for a hot-melt butyl sealant is the tendency of its typical formulation to react with other chemicals, such as gasoline in automotive applications or cleaning solutions in appliances. Substituting curable urethanes or silicones for hot melts solves the temperature problems, because these materials can be formulated to have performance ranges beyond 250 [degrees] and 400 [degrees] F, respectively. In addition, urethanes and silicones are typically more chemically resistant than butyl. However, the cure rate for these substitute sealants suffers by comparison. "You are doing well with silicones or urethanes if they cure in an hour, as opposed to a minute or less, which is typical for hot-melt cure rates," said Norton Performance Plastics marketing manager Samuel Sher.

Dynafoam will cure within a few minutes of its application and resists softening up to temperatures of 280 [degrees] F continuous exposure and 400 [degrees] F for periods of an hour or less. These heat-resistance properties are more than adequate for automotive gasket applications. Dynafoam also has good resistance to damage from exposure to chemicals.

Dynafoam can be applied vertically, unlike most other materials, which have a tendency to creep downward. In addition, the way in which Dynafoam cures enhances its compression recovery properties; that is, it returns smoothly to its shape after being squeezed between two surfaces.

In order to prevent contact with the ambient moisture in the air which cures it, Dynafoam is vacuum-sealed, in a state similar to rubber, inside metallized foil packages. Norton Performance Plastics sells Dynafoam in 5-gallon pails or 55-gallon

A major advantage of Dynafoam is that it can be applied by robots, reducing labor costs and time. Third-party manufacturers, such as Asea Brown Boveri (Zurich, Switzerland), make the applicator robots for Dynafoam. In operation, the container of Dynafoam is hooked up to a hot-melt pump where a heated platen, or lid, melts the top layer of Dynafoam to 180 [degrees] F. The sealant is then pumped through a hose that is also heated to 180 [degrees] F, into a FoamMix apparatus manufactured by Nordson Corp. (Amherst, Ohio). In the FoamMix device, Dynafoam is mixed under pressure with a dry gas, usually nitrogen, and pumped through a heated hose and nozzle. When the sealant leaves the nozzle it immediately expands to bead size as the nitrogen in the Dynafoam expands. This worm-like bead is then applied directly to the part to form a gasket. Dynafoam begins curing upon contact with the air. The cure rate varies, depending on both the amount of gas used to form a part and the size of the part itself. The more gas or the smaller the bead, the quicker it cures.

Old Softy

While most sealants dry within minutes or hours of being laid down, a number of common applications need sealants that never harden. A notable example is automotive engines, where some joints require seals that must periodically be broken and removed for maintenance or repair and other joints need seals flexible enough to absorb potentially damaging vibrations.

Hylomar, made by W. Canning Plc (London), is a sealant that does not harden or set. "I've got a metal joint in my desk sealed with Hylomar that is 10 years old, and it has not hardened yet," said Donald Schlater, executive vice president of W. Canning's American subsidiary Marston Bentley Inc. (Houston).

Hylomar is a putty-like spreadable material made from polyurethane (PU), silica particles, and a solvent (usually methylene chloride or 111-trichlorethane). The solvent evaporates quickly on contact with the air and is nonflammable. In Germany, where chlorinated solvents are prohibited, flammable acetone is used as Hylomar's spreading agent.

The sealant was developed in the 1950s by Rolls Royce scientists to seal the oil and fuel passages on aircraft turbine engines. Hylomar's temperature range of -50 [degrees] to +250 [degrees] C and its ability to join extremely small widths down to 0.0001 inch have also made it ideal for sealing the oil, fuel, and radiator systems in motor vehicles.

Hylomar's nonhardening and nonsetting qualities are valuable in joints that must be broken for readjustment after sealing. "Hylomar is strong in shear, not in tension," said Schlater. This also makes Hylomar more vibration-resistant than hardened sealants. "Take the oil pan underneath a car. When the car drives over a bump, the vibration could crack a hardened seal such as silicone. On the other hand, Hylomar reseals when the disturbance passes," he said. Vibration resistance has helped Hylomar's U.S. distributors sell the sealant to motorcycle owners.

One cost advantage of Hylomar is its tendency to flow out of the joint, rather than compressing when the metal interfaces are joined. This means that the minimum amount of sealant is used.

Hylomar that has been manufactured with a larger solvent content is thin enough to be spread by brush or sprayed from an aerosol can. In one application, a manufacturer coats the threads of bolts with liquid Hylomar. In another use, Ford New Holland Ltd. (Basildon, England) sprays the sealant onto the head gaskets of its tractors.

Although automotive and aircraft applications remain its primary markets, Hylomar is being used in appliances, for example, sealing the cooling systems of refrigerators. "One company that makes electrical switches ships a seven-ounce tube of Hylomar to replace an O-ring that used to waterproof the switch box," said Schlater. Because it stays sticky but breaks apart easily, Hylomar is also being used as a positioning agent to hold parts, such as auto transmission covers, in place while they are being assembled.

Hylomar is sold to industrial customers in caulking cartridges or in bulk for use with machine-driven applicators.

Sealing Steam

Heat and pressure are a bane to sealants. Industrial Gasket and Shim Co. Inc. (Meadow Lands, Pa.) uses its heat- and pressure-resistant properties to sell its Temp-Tite, Turbo, and Silver Seal brand sealants. Because they can withstand up to 2700 [degrees] F and 2000 psi, they are used to create formed-in-place industrial gaskets. They are also used in repair or manufacturing applications on steam turbines, race car engines, boilers, and heat exchangers.

Temp-Tite is an extrude plastic cord available in diameters ranging from 1/16 to 1/2 inch. (Larger sizes are available on special order.) It is sold in spools and is laid in place as is. A popular application is sealing a steam turbine on a split case and framed bonnet joint.

Turbo is sold in a viscous liquid state and is applied by brushing or injection as a joint dressing to seal microscopic imperfections on a joint face. Such seals are important because tiny leaks in, for example, a high-pressure steam turbine could create a situation where escaping steam could eventually cut through the turbine wall.

Silver Seal, which is a paste, is pumped or caulked onto rough or damaged joint surfaces. The sealant has good heat transfer characteristics, allowing heat to dissipate across the joint, preventing hot spots from developing that could weaken the metal.

Though Temp-Tite, Turbo, and Silver Seal do not act as adhesives, because they attempt to expand 30 percent when heated they provide a tight seal between the metal surfaces they join. The three sealants are inert, impervious to most alkalis, and have been free of asbestos since 1987.

Steel Savings

Sealants are also a significant item in steel-manufacturing operations. For example, Mill Pac, which was recently developed by Industrial Gasket and Shim, is a paste-like substance that is used to seal the two halves of steel ingot molds, reducing the amount of molten steel that seeps into the joint. This seepage causes more steel to be poured than needed, wasting metal. (Foundries also have to foot the additional expense of grinding off the fins this seepage forms on the ingot.)

According to USX Corp. (Pittsburgh), ingot molds sealed with Mill Pac reduced the amount of steel lost through leakage from 5 percent to 0.2 percent. "One Texas steel foundry reported annual savings of $30,000 in reduced metal leakage, not including the costs that would have been incurred by grinding fins down," said Industrial Gasket and Shim vice president of production Kinsley Desch.

The most popular use for Mill Pac in the United States, Japan, and Western Europe is in continuous steel casting operations to seal in nitrogen and seal out oxygen. Using Mill Pac to seal parts such as cone nozzle gaskets prevents oxidation and contamination of semisolid steel.

Dynamic Trio

One of the most difficult sealing applications is in bonding the stems of valves used to control corrosive liquids under pressure. A case in point is a corrosion-resistant valve manufactured by the Duriron Co. Inc. (Durco) in Dayton, Ohio. The most recent model introduced by Durco's Valve Division is the BTV-2000 butterfly valve designed for handling harsh chemicals such as chlorinated brine, electrogalvanizing solutions, and bleaches.

BTV-2000 valves are equipped with a triple-stem seal. The primary seal is formed at the disk hub by a spherical ball and socket disk/liner arrangement that provides a 360-degree contact scat seal for bubble-tight shutoff. The disk itself is typically made of a nickel-plated ductile iron substrate encapsulated by a polytetrafluoroethylene (PTFE) perfluoroalkoxy (PFA) plastic for applications where corrosive liquids will be used or ultrahigh molecular weight polyethylene (UHMWPE) to prevent damage when abrasive liquids are to be used. Other disk materials in a variety of steel, nickel-bearing, and reactive alloys are available, including 316 stainless steel and palladium-stabilized titanium.

Convolutions built into the disk stem help create a better seal by eliminating a leak path. A tertiary seal is provided by O-rings made of Viton plastic from E.I. du Pont de Nemours & Co. (Wilmington, Del.) in the valve's alloy gland follower; this provides added protection from external corrosion and prevents the valve's contents from leaking into the atmosphere.

Sensitive Duo

Protecting liquids from contamination is another application where sealants are crucial. Plytrax laminates, commercially introduced by Norton Performance Plastics in 1987, are used to protect the integrity of liquids such as biomedical and environmental samples.

All Plytrax laminates are made of two components; one is a fluoropolymer such as PTFE and, depending on the application, the other is silicone, polyurethane, polyethylene (PE) foam, or medical-grade butyl rubber. During manufacture, molten silicone or polyurethane is cast onto a fluoropolymer base, while the PE foam and medical-grade butyl rubber are attached by an adhesive. In each case, a single sheet is formed, with only the inert fluoropolymer contacting the liquid being sealed.

Silicone Plytrax laminate is used to make septas, or cap liners, across the mouths of bottles that are punctured repeatedly by a hypodermic syringe, such as insulin bottles, because silicone reseals. Silicone Plytrax septas are being used to seal containers holding groundwater samples collected during environmental monitoring, as well as samples of materials undergoing gas chromatography analysis. "The key to these applications is keeping the sample unaltered despite the ongoing punctures and extraction," said Arlene Pavliv, product manager of tapes and laminates at Norton Performance Plastics.

For applications where a one-time septa is sufficient, such as for containers used to hold a blood sample, a medical-grade rubber Plytrax septa would typically be used. Like its silicone counterpart, the rubber-based laminate has sufficient heat and moisture resistance to withstand being heated in an autoclave.

A PE-foam Plytrax laminate is often used as a cap liner. The foam acts as a cushion, providing a tight seal for bottles and containers holding chemicals used in laboratory analysis, cosmetics manufacture, and pharmaceutical production.

Zitex, a predecessor to Plytrax that was developed in the 1960s and is still used today, serves to protect a container from a liquid and shields the liquid from contamination. Zitex is made of a microporous fluoropolymer and a perforated PE foam that permits gas to vent while keeping the liquid in place. Flexible Buildings

Sealants also play an important role in materials used to help ensure the structural integrity of buildings. Dynatrol II, a polyurethane architectural sealant made by Pecora Corp. (Harleysville, Pa.), is a paste applied by injection gun to precast construction panels, as well as to the control joints that connect a building's interior structure to its exterior skin. "The seal can absorb as much as 50 percent of the movement of a building caused by wind or temperature expansion, while providing a waterproof airtight seal," said Pecora marketing coordinator Tom Pennell.

Dynatrol II is made up of a base and activator element that are mixed prior to use. Dynatrol II is a nonsag PU sealant that can be applied to vertical and horizontal surfaces.

Concrete foundations also benefit from the use of sealants, because water that seeps in concrete pores and freezes in winter expands, weakening the concrete. Duramem is a coal-tar polyurethane sealant that forms a waterproof membrane when applied on an existing or newly poured concrete foundation. It comes in different grades that can be applied by brush, roller, or trowel, depending on the thickness desired.

Shrink Seal

Some manufacturers rely on the surfaces of materials themselves to form seals without using an adhesive. EG&G Mound Applied Technology (Miamisburg, Ohio) uses the different heat contraction rates of glass and metals to seal its electrical components. These components are used in pyrotechnic devices such as the ignitors used to activate automobile air bags.

According to senior research specialist William Moddeman, the component seals consist of a shell made of a high-strength nickel-based superalloy such as Inconel 718. Pins made of another nickel alloy, usually Hastelloy C-276, are inserted in the shell. These carry the electric charge sent to them via bridgewire to ignite the pyrotechnic material. However, the pins are electrically isolated from the shell by use of an insulating glass-ceramic. Because the shell, glass-ceramic, and pins have different heat contraction rates, during cooling the shell shrinks around the glass, which in turn contracts around the pins, forming a strong compression seal.

Moddeman believes the strength of this seal can be increased even further by modifying the surface of the metal to improve bond strength. "We are examining the feasibility of growing an oxide on the metal parts, which would be more compatible with the molten glass," he said. "This would provide a pore-free interface that would improve the overall quality of the seal."

New Tricks

Future trends in industrial sealing are likely to depend on both adapting veteran products for new applications and developing totally new sealing agents. Silicone-based 732 Multi-Purpose Sealant has been the workhorse industrial sealant of Dow Corning Corp. for the past 25 years. According to Dow Corning technical service representative Al Freiberg, the original formulation is still used in appliances and heavy-factory and marine equipment, The sealant offers the traditional advantages of silicones: temperature and chemical stability and resistance to weather and ultraviolet exposure.

Over the years, Dow Corning researchers have developed modifications of the original 732 formula in an effort to open up new niche markets. The polymer structure of 732 was altered to create Dow Corning's 730 Solvent Resistant Sealant. The material is aimed at uses in valves, lines, hoses, and tank seams in chemical plants.

To create Dow Corning's 786 Mildew Resistant Sealant, a fungicide was added to a modified 732 formulation. As a result, 786 can be used in high-humidity locations, ranging from bathrooms and gymnasiums to housing in Florida. A proprietary additive was blended into Dow Corning's 736 industrial Sealant to boost its heat resistance up to 500 [degrees] F for continuous exposure and 600 [degrees] F for intermittent exposure. Such heat-resistant properties make the sealant useful for oven gaskets, sealing heating ducts, and other high-temperature applications.

Dow Corning developed its alcohol-cured 738 Electrical Sealant for insulation applications on electronic switches and circuit boards. For applications where the sealant must adhere to a plastic surface, Dow Corning added an adhesion promoter to create the 739 Plastic Adhesive Scalant. The 739 sealant is also alcohol-cured and is used to seal plastics ranging from polypropylene to acrylics. As plastics continue to replace traditional materials in many areas, sealants comprise a will continue.
COPYRIGHT 1991 American Society of Mechanical Engineers
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Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Valenti, Michael
Publication:Mechanical Engineering-CIME
Date:Apr 1, 1991
Words:3046
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