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Glossary

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A

ABRADE

To prepare a surface by roughening, sanding or other means.

ABSOLUTE SEALING

A level of sealing that requires all seams, slots, holes and fasteners passing through the SEAL PLANE to be sealed. (Absolute sealing is required for sealing of all INTEGRAL FUEL TANKS.)

ACCELERATOR

An ingredient included in the formulation to accelerate the rate of sealant cure. It is also used separately in a mixture to accelerate surface curing only of applied polysulfide sealant referred to as a TACK FREE ACCELERATOR also used occasionally as an alternate term for CURING AGENT or CATALYST.

ACCEPTANCE TESTS

Tests required by the specification to be performed prior to shipping and again as part of receipt inspection procedures. See CONFORMANCE TESTS and QUALIFICATION TESTS.

ADHESION

The physical bonding of a material to a surface — i.e., both chemically and mechanically.

ADHESION PROMOTER

A material applied to a surface to chemically enhance adhesion of sealant. The largest portion, by volume, of sealant adhesion promoters consists of a mixture of organic solvent and various coupling agents. Adhesion promoter may also refer to certain chemicals contained in the sealant formula to enhance ADHESION.

ADHESIVE SEALING

The bonding of faying surfaces to each other using a structural adhesive in the form of a tape or a film.

AGITATOR

See DASHER

AMS SPECIFICATION

Aerospace Material Specification published by the Aerospace Division of SAE.

ANAEROBIC SEALANT

Also called a “thread-locking compound” or a “retaining compound,” this is a fastener sealant in which cure is inhibited by exposure to air. The sealant can be applied to the faying surface and will not cure until the surfaces are joined.

APPLICATION TIME

Also called “work-life,” this is the time available for the sealant to be applied and wet the bond surface, immediately after mixing or thawing. Acceptability limits for the application time of Class B and Class C sealants are based on minimum extrusion rate [expressed in grams/minute or cubic centimeters (cc)/min], which is determined at, or immediately after, the required application time. (Sealant applied after the application time is exceeded tends not to wet the surface well and thus not to form a good bond.) Acceptability limits for application time of Class A sealants are based on an increase in the material’s viscosity to a specified level.

ASSEMBLY TIME

Also called “squeeze-out time” and “squeeze-out life,” assembly time refers to the amount of time available, after sealant mixing, before faying surfaces must be joined (or assembled) to obtain squeeze-out of the faying surface sealant. If the time is exceeded, the cure will have progressed too far (i.e., the material will have become too firm) to permit squeeze-out of the faying surface sealant. Note that while the application time of a faying-surface sealant may be only 8 to 48 h, the assembly time can be 20 to 336 h (or 2 weeks), depending on the formulation of the particular Class C sealant.

B

BACKUP SEAL

A seal on the dry side of the fuel tank. It is considered a secondary or redundant seal; and it never is designated as a primary seal.

BASE COMPOUND

This is the major polymeric component of a two-part fuel tank sealant that cures upon mixing with the accelerator.

BLOCK FLOW

The distance a sealant sags (or slumps) on a vertical surface within a given period of time. A two-part sealant is thoroughly mixed at standard conditions into the cylindrical recess of a vertically positioned metal test fixture. The test fixture is calibrated in tenths of an inch to measure sagging, flow (or slumping) of the sealant within the specified period of time (usually 30 min).

BUTTON SAMPLES

Small disc-shaped specimens made with sealant extruded from each mixed sealant cartridge onto cardboard or paper prior to freezing the cartridge. It is used as an indicator of the condition of the sealant in each tube — i.e., uniformity of mix (or homogeneity), presence of air, cure rate, durometer hardness upon completion of cure.

C

CARTRIDGE

A polyethylene cylinder with one end open the full width and the other end necked down to a 1/8″ IPS female thread used to attach various nozzles. May be used alone for pre-mixed and frozen sealants or with other components as a TWO-COMPONENT PLASTIC INJECTION KIT

CATALYST

The component of a two-part curing-type sealant that causes the base compound to polymerize. see ACCELERATOR, CURING AGENT

CENTERLINE GROOVE

An injection-sealing groove machined along the fastener line.

CHANNEL

(a) A groove machined in a faying surface to accept a uniform bead or section of sealant. Usually, a noncuring fluorosilicone-based fuel tank sealant is injected from the exterior of the fuel tank through the injection ports leading into the channel. (b) A passage formed by a structural discontinuity, such as the opening under a joggle or some other void.

CHANNELING LEAK

A leak that develops at a source located some distance from the leak exit point.

COHESION or COHESIVE STRENGTH

Refers to internal forces holding a sealant together. For example, when peel tests are conducted on samples of sealant bonded to a surface, a strip of canvas cloth or wire screen is embedded in the sealant. The specimen is clamped in a testing machine, secured, and pulled at 180°. Assuming the adhesive bond of the sealant to the panel does not fail, the force necessary to tear the sealant, making it fail within itself, is the cohesive strength.

CONFORMANCE TESTS

Tests required for insuring the conformance of sealant material to the specification see ACCEPTANCE TESTS

CONTAMINANT

Any solid or liquid substance that can interfere with the function of the sealant, coating, adhesion promoter, etc.

CORROSION

Degradation of structure by oxidation. Corrosion of aircraft parts often is induced by a reaction of airframe structure with the environment — which includes oxygen, moisture, and other oxidizing chemicals — or, in the case of galvanic corrosion, by joining electrochemically-dissimilar materials.

CURING AGENT

The component of a two-part curing-type sealant that causes the base compound to polymerize.

CURING

RATE, CURE TIME The time required for a sealant to polymerize and develop the end-item physical and mechanical properties specified by engineering. It also is the time required to reach a hardness called for by a specification. Curing rate, Cure time generally relates degree of a sealant’s hardness to the time required for full cure. Polysulfide fuel tank sealants are fully cured when they reach a Shore A or a REX A hardness of at least 35.

D

DASHER

The part of a two-part plastic injection kit that has the appearance of a spoked wheel. It is the dasher that mixes the material when stroked.

DASHER ROD

The part of a two-part plastic injection kit that contains the catalyst, curing agent. The dasher rod is inserted through the neck of the cartridge.

DOME NUTS

These are nutplates with a mechanical seal at the base and a cap over the top; both the seal and the cap provide a fuel tight seal. Dome nuts are commonly used for attaching screws or bolts to inaccessible areas or to access doors.

E

END CAP

The cap applied to the large end of a sealant cartridge some times referred to as an F-CAP

F

F-CAP

See END CAP

FAIRING or FEATHERING

Also called “TOOLING,” this is the shaping of applied sealant to produce a smooth transition from one angular direction to another; or the act of producing this smooth contour. Its purpose in tooling sealants is to ensure good contact with the surfaces and to minimize air entrapment.

FAYING-SURFACE SEAL

A pre-assembly seal installed between two mating (overlapping) surfaces. Faying-surface sealants are used to prevent corrosion, and, in conjunction with fillet seals, to prevent a leak path from extending through a faying surface to another area. When modified by a groove, a faying surface seal has been used as a primary seal.

FEATHERING

See FAIRING.

FILLET SEAL

A primary seal (post assembly) applied at the juncture of two adjoining parts or surfaces and along the edges of faying surfaces as a continuous bead of sealing material. It can be applied over, along the edges of, and between installed parts.

FLOW

See BLOCK FLOW.

FLUOROSILICONE SEALANT

A sealant based upon a fluorosilicone polymer. The following types of fluorosilicone sealant are used in integral fuel tanks: A one part, moisture cured polymerizing elastomer used in brush coating and filleting (extrusiontype) applications. A one-part noncuring channel or groove sealant, It can be prepacked into the grooves in the faying surface in assembly or injection though external ports.

FUEL TANK BOUNDARY

Also called the “sealing plane,” this is the fuel-tight primary structure of an integral fuel tank, including skin panels, tank-end ribs, and tank-end spars.

G

GROOVE SEAL

A post assembly seal formed by injecting a noncuring fluorosilicone based or polysulfide-based sealant into a groove machined in one faying surface of the mating or overlapping structure.

H

HARDNESS

See SHORE A and REX hardness.

I

INJECTION SEAL

A seal that is produced by injecting sealant into holes, joggles, channels, grooves, and other voids formed upon assembly of fuel tank structure. This seal is used to provide fuel tank seal continuity where fillet seals are interrupted by structure and to fill structural cavities.

ISO

International Organization for Standards formerly (International Standards Organization)

INTEGRAL FUEL TANK

A load-carrying structure of an aircraft absolutely sealed to provide for fuel containment. It exists as a major structural compartment in a wing or in the fuselage, or both.

ISOLATION SEAL

A secondary seal used to isolate potential fuel leakage paths, in order to prevent channeling of fuel along a leak path between structural members. It can be a repair seal installed to reestablish seal continuity in areas of direct contact with the fuel.

J

JOGGLE

A structural cavity formed when stepped, angled, or otherwise contoured members are mated to a flat part. Joggles are sealed by prepacking during preassembly, whenever possible; otherwise, joggles must be sealed after assembly by injection sealing.

L

LEAK PATH

The path a fuel leak follows from the leak source to its external exit.

M

MECHANICAL SEAL

A seal, as in access door structure, produced by an elastomeric O-ring or other molded or extruded shape when pressure is applied to the seal. Applied pressure causes the seal to mechanically deform, thus filling the gap between parts assembled with the seal.

N

NADCAP

National Aerospace and Defense Contractors Accreditation Program Nadcap is a manufacturer accreditation program managed by PRI

P

PARTING AGENT

(or RELEASE AGENT) A material that prevents sealant from bonding to a surface. Therefore, parting agent is used only when installing removable seals in accordance with the applicable engineering drawing.

PLUNGER

A cup like piston inserted into the big end of the cartridge it is used to force sealant material out of the cartridge. See PLUGGED WIPER PLUNGER and WIPER-PLUNGER

PRECOAT

A brushable (Class A) sealant, which provides a base coating for a fillet seals and other types of fuel tank seals.

PREPOLYMERS

High molecular weight molecules which, essentially, are precursors (or base compounds) for polymeric materials used in sealing of integral fuel tanks. When catalyzed, prepolymers undergo a chemical chain extension reaction that ultimately forms the polymeric sealing material.

PRI

Performance Review Institute Maintains QPL for SAE specifications maintains QML for SAE specifications, administers the Nadcap program.

PRIMARY SEAL

A seal which, in combination with fuel tank structure, forms a continuous, durable, and absolute seal in the SEAL PLANE and requires no additional seals for fuel containment.

Q

QUALIFICATION TESTS

Tests required by a specification to qualify a sealant material to the SPECIFICATION. These tests are performed in order to place a sealant material on the aircraft.

QUALIFIED MANUFACTURES LIST (QML)

A listing of manufactures qualified to produce certain materials. Currently the QML for AS and AMS specifications is maintained by PRI.

QUALIFIED PRODUCTS LIST (QPL)

A list of manufactures and their products that meet the requirements of a particular SPECIFICATION. See CONFORMANCE TESTS and ACCEPTANCE TESTS

R

RAM-ROD

The rod provided with two part plastic injection kits used to expel the catalyst, curing agent from the DASHER ROD

REDUNDANT SEALING

Installation of supplemental sealing, which serves as a backup for the primary seal and contributes to maximizing the potential for preventing fuel leaks and corrosion during the air vehicle’s operational lifetime.

REENTRANT EDGE

A negative contact angle between a fillet and its substrate caused by improper or insufficient tooling (fairing or feathering) of applied fillets.

REX HARDNESS

The hardness of a sealant as measured by a REX hardness gauge. The reading is valid only if the sealant thickness is at least 0.25 in (6.4 mm).

S

SAE

Society of Automotive Engineers

SAG

The slumping or drooping of a sealant after it is applied to an overhead or vertical surface. See BLOCK FLOW

SEAL

Closure of an aircraft fuel tank to make it leak proof and corrosion resistant by application of sealant to fasteners, seams, and any other possible leak paths. Seals may also be accomplished by compression of premolded seals — such as O-rings and elastomeric seals installed into grooves and counterbores machined into the fuel tank structure.

SEAL PLANE

All surfaces of a tank that establish fuel seal continuity and are in immediate contact with fuel. These surfaces are, generally, composed of structure, permanent fasteners, and fuel tank sealing materials.

SECONDARY SEAL

A seal that, by itself, does not constitute an absolute seal. It is sometimes used in conjunction with a primary seal.

SEMKIT

Copyrighted product name. See description under “TWO-COMPONENT PLASTIC INJECTION KIT.”

SHORE A HARDNESS

A measurement of sealant hardness using a Short A hardness gauge. The gauge has a dial, a blunt tip (or foot), and a pin that protrudes slightly through a hole in the face of the foot. The foot is pressed against a sealant specimen of at least 1/4 in (6.4 mm) thickness.

SKINNING

Formation of a thin, hardened layer on surface of the base compound. This is caused by oxidization of the surface of the base compound skinning will be aggravated with sealant storage at high temperatures — i.e., approximately 130 to 150 °F (54 to 66 °C).

SLUMP

See BLOCK FLOW.

SPECIFICATION

A document delineating the performance requirements of a material or the procedure to be utilized when performing a specified test. The custodian for specifications may be Industry ANSI, ASME ASTM, SAE, ISO or other consensus entity. Specifications may be Government known variously as MIL-SPEC, NAVSEA or controlled by the individual air frame builder. The specification may have a QPL or it may be a PERFORMANCE SPECIFICATION.

SPONGING

A rare phenomenon with fuel tank sealants, characterized by swelling and blistering, that may be observed after exposures to fuel soaking, thermal cycling, and other environmental conditioning. Sponging should not be confused with a layer of bubbles caused by an insufficient cure time between layers of built up sealant.

STORAGE LIFE (SHELF LIFE)

The length of time any kit of unused sealant can be stored at supplier recommended storage conditions and still retain the properties (in the unmixed and mixed state) required by the applicable engineering specification or as advertised in the sealant manufacturer’s product data sheet.

T

TACK-FREE TIME

The time required for a curing sealant to lose surface tackiness, or stickiness; this is determined by placing a small piece of polyethylene sheet on the slant’s surface, then peeling away the sheet. The sealant surface is tack free when no sealant is carried with the sheet.

THIXOTROPIC

The property of a non sagging sealing compound that permits the sealant to be moved — i.e., stirred or extruded — with less force than would be required with a Newtonian fluid. Non-Newtonian pseudoplastic materials, such as non sagging fuel tank sealants, stand like whipped cream — i.e., the sealants do not gravitate to a specific level — but they flow easily from a sealant gun under relatively low pressure.

TOOLING A FILLET

This is the shaping or fairing of a sealant fillet after the fillet has been applied to structure. The goal is to ensure formation of a contoured (or feathered) edge where sealant meets with structure to eliminate voids and entrapped air, and to minimize the weight impact produced by sealing the air vehicle.

TWO-COMPONENT PLASTIC INJECTION KIT

A kit, for two-part fuel tank sealants, Available as a 2-1/2, 6, or 12 fl oz (74, 177, or 355 ml) polyethylene cartridge fitted with a plunger at one end and a hollow dasher rod extending through the throat at the other end. The polymeric base compound is contained in the cartridge. The catalyst is packaged within the hollow dasher rod that is closed on the inside end with a “DASHER” or ”agitator.”

V

VOID

Any opening, small crack, or crevice occurring at the juncture of structural members, this includes (but is not limited to) structural cavities, holes, recesses, joggles, and gaps.