Glossary of Terms:

Acrylic:
A type of synthetic polymer that is (generally) very resistant to degradation (oxidation, UV, etc.), has very good to excellent flexibility and elasticity, and has inherently high adhesion qualities. (See "Vinyl")

Adhesion:
The state in which two surfaces are held together by inter-surface forces which may consist of electronic forces or interlocking (mechanical) action, or both.

Adhesives:
Adhesives are materials that are initially fluid or semi-fluid, placed between two opposing solid materials, become solids themselves (by solvent evaporation or chemical reaction), bond to the surfaces they are applied to and prevent joint movement. (See "Sealant")

Adhesive failure:
This type of failure occurs when the adhesion of the sealant to the substrate it was applied to fails. This is the worst type of failure for a sealant to experience. This is because to repair it usually means that all of the sealant must be removed from the joint, the joint must be thoroughly cleaned, and then resealed. (See "Cohesive Failure")

Backer rod:
Backer rod is typically a foam-like material used to properly configure expansion/contraction joints and to provide a surface that either the sealant will not adhere to or will be so flexible in its own right that, even though the sealant adheres to it, does not hinder the needed movement capability of the sealant itself. Backer rod usually comes in long lengths of a circular profile and is pressed into the joint just before the sealant is applied. (See "Two-point adhesion")

Binder:
A component of an adhesive or sealant composition which is primarily responsible for the adhesive and cohesive forces which cause the adhesive/sealant to perform its function. Generally, the higher the binder level in a formulation the better the quality. (See "Filler") Oils and latex emissions are examples.

Cohesive failure:
This type of failure occurs when the sealant cracks down the middle (or somewhat off-center), while the adhesion of the sealant on both sides of the joint is maintained. If failure is unavoidable (such as when expectedly large joint movement occurs), cohesive failure is the most desirable failure mode. The reason is that usually this type of failure does not require that the failed sealant be removed; only that it be cleaned well and resealed with fresh material of the same type. (See "Adhesive Failure")

Cracking:
The severe breakdown of internal cohesive forces of a sealant either during cure or afterward, as a result of excessive stress. Two typical examples are side and center cracking in an expansion joint. Shrinkage upon curing of elongation can build up excessive stress either at the center of the joint or near the center of the joint causing severe failure. (See "Crazing")

Crazing:
Is similar to cracking, however, it is much less severe and does not destroy the basic function of the caulk. Crazing only extends a few molecules deep into the surface of the product (due again to excessive stress) and can occur during the curing phase or after being exposed to high elongation and weathering.

Cure:
The process by which a semi-liquid adhesive/sealant becomes a firm, functioning solid. 1. Latex systems cure by evaporation/coalescence; whereby, as the water evaporates from the system the particles of polymer binder come closer and closer together until they touch and coalesce together, forming a continuous film. 2. Chemically curing systems (silicone, polyurethane, polysulfide, polymercaptan, etc.) function by using highly reactive chemical components of simple chemical structure that interact to form complex polymers in place. 3. Oil based caulks rely on the slow process of air oxidation to cause vegetable oils to polymerize in place.

Elastic:
The property of a material which allows it to be stretched substantially and, upon immediate release of the stress, to return with force to its approximate original length. An excellent example is a rubber band; it can be stretched and will return to its original strength many times. If a material can be deformed under stress but will not return to its original shape or dimension it is not elastic. (See "Flexible" and "Stress Relaxation")

Elastomeric:
Materials that are "elastomeric" could also be said to be "elastic". Both terms describe what most people would refer to as "rubberiness", or behaving like rubber (which stretches and compresses, and returns to its original shape instantly after stress is removed).

Elongation:
The ratio of the change in length of a stretched sample to the original length of the sample (measured in per count), usually the ultimate value (elongation at failure). (See "Recovery")

Expansion Joint:
A type of building joint where a significant distance is left between two substrates so as to accommodate normal building expansion and contraction due primarily to temperature changes. A sealant placed in this type of joint is subject to being stretched and compressed, rather than being sheared. (See "Lap Joint")

Flexible:
The property of a material to undergo deformation under stress, but not exhibit the ability to stretch and return to its original shape when the stress is relieved. An excellent example is a sheet of paper: it can be bent but will tear if you try to stretch it. (See "Elastic")

Filler:
A relatively neutral ingredient added to a sealant / adhesive formulation to reduce its cost, improve its working properties, increase its cohesive strength or improve other qualities. Most formulations require a certain amount of filler to achieve desirable finished products, but high filler levels generally degrade performance.

Freeze-Thaw Stability:
The ability of a material to undergo cycles of freezing and thawing with no deterioration.

Lap-Joint:
A type of building joint where two substrates overlap one another and there relative motion is one of sliding past one another. Sheet metal joints are good examples of this. The stress placed on a sealant placed between two substrates of this kind is shear. The sealant has a tendency to be torn instead of being pulled apart as an expansion joint. (See "Expansion Joint")

Latex:
Several definitions apply, however, the two most important are as follows: 1. A dispersion of particles of polymeric material suspended in water by emulsifiers and surfactants and used in the manufacturer of water-based paints, adhesives, and sealants. 2. A general term describing any system using a water-based latex emulsion as its binder.

Modulus:
The force required to obtain a certain elongation. Measured in pounds per square inch of cross section of the sample. Low to moderate modulus materials make the best caulks and sealants. (See "Shore Hardness")

Oil Caulk:
This is a type of caulking of low performance, primarily in the area of poor elasticity, flexibility, yellowing, and an inability to accommodate joint movement. (Adhesion is often very good, however.) The caulk is usually heavily loaded with filler and relies on either linseed or soybean oil as its binder.

Oxidation:
Is the action of oxygen (or ozone) on other chemicals; including caulks and sealants. Oxidation causes caulks and sealants to become hard and brittle. Chalking and cracking are the most common indicators of oxidation. (See "UV")

Peel Strength:
The force required to break a bond by peeling a sealant away from a given substrate. Measured in pounds per linear inch of bond width. The angle of peel for most sealant tests is 180º

Plasticizer:
A chemical incorporated into a sealant formulation to increase its flexibility and elasticity. As the level of plasticizer is increased in a formula (generally) its modulus will decrease.

Polymer:
A complex chemical (sometimes natural, but generally synthetic) composed of repetitive units of one or more simple chemical compounds that have been strongly reacted together. The finished polymer often bears little resemblance to its chemical antecedents in physical form or physical properties.

Primer:
Primers are usually thin, fluid substances that are applied to surfaces to greatly aid the ability of a sealant or adhesive to adhere to the substrate (especially under extraordinarily difficult circumstances). In architectural applications, primers are often used in applications where the joint is likely to be subjected to prolonged standing water (such as in concrete sidewalks).

Recovery:
The ability of an elastic material to regain its shape after being deformed. It is expressed as a percent of the length regained after release from a given elongation. (See "Elongation")

Rheology:
The term generally describing the flow behavior of liquid and semi-liquid materials when subjected to applied force. The reology of sealants is very important from an application standpoint since the ease of application of a sealant is often the determining factor in a product's acceptance in the market. (See "Thixotrophy")

Sealants and Caulks:
Sealants are materials that are initially fluid or semi-fluid, placed between two opposing solid materials, become solids themselves (by solvent evaporation or chemical reaction), bond to the surfaces they are applied to and accommodate joint movement. In architectural applications, sealants and caulks use their ability to accommodate joint movement to seal out the intrusion of water, wind, pollutants, dust and insects. "Sealants" are generally considered to deliver appreciably higher performance than "Caulks" - while they both serve the same basic purpose. (See "Adhesive")

Shear Strength:
The force required to break a bond by shearing the glue line (measured in pounds per square inch of bond area).

Shore Hardness:
For sealants, the 'A' scale is usually used, whereby a hardened steel pin (which is connected to a calibrated spring meter) is pressed into the sealant and its depth of penetration is measured. Shore hardness is a strong indicator of modulus. Low modulus sealants have shore hardnesses of 20 or less. Medium modulus sealants have values from 21 to 50; while high modulus materials have shore hardnesses above 50 (See "Modulus")

Shortness:
A term used to describe a rheological property of a sealant. It indicates a low resistance to shearing action and yet the maintenance of good sag resistance. A good example would be the short peaks left in whipped cream when a finger is drawn through it. (See "Thixotrophy")

Shrinkage:
The ratio of the volatile ingredients in a sealant formula to the nonvolatile components, expressed as a percent. It can indicate a weight percentage but most importantly indicates a volume percentage.

Stress:
A force applied to a solid substance resulting in deformation.

Stress Relaxation:
The very important property of some sealants to be able to be subjected to extended periods of constant stress and internally relax to more easily accommodate the continual stress. Silicone is one notable product that does not have this ability.

Tack Free:
The state in which material will not adhere to anything as a result of curing and/or drying. A caulk is said to have "skinned" when it is tack free.

Tensile Strength:
Force required to break a test material when stretched. Measured in pounds per square inch of cross section.

Thermoplastic:
A material which will repeatedly soften when heated and harden when cooled. Virtually all sealants that are not chemically curing are of this type.

Thermoset:
A material which will undergo or has undergone a chemical reaction by the action of heat, catalysts, ultraviolet light, etc., leading to a relative insensitivity to temperature changes.

Thixotropy & Pseudo-plastic flow:
"Thixotropy" is a desirable flow property of an uncured sealant, but not as desirable as "pseudo-plastic flow". Thixotropy is the phenomenon of a fluid material (such as a caulking compound) thinning out and flowing very easily as long as force is applied to the material (i.e., when gunning the product); then, once the applied force is removed (i.e., when the product is on the wall) the caulk thickens up almost right away so it doesn't run down the wall. With pseudo-plastic flow - which is more desirable - once the applied force is removed, the caulk thickens back up instantly, with no time delay.

Two-point adhesion:
Sealants need to be applied so that they only adhere to the two opposing sides of a properly configured joint (usually created by using backer rod). If this rule is not followed (as in what is termed "three-point adhesion"), there is often a high risk that extreme and unforeseen stresses can be induced at certain points along the bond-line that can cause failure. The use of backer rod helps to insure two-point adhesion. (See "Backer rod")

UV:
Ultraviolet (UV) light is that portion of the light spectrum which is most energetic and therefore most damaging. For sealants to give good long-term life they must be formulated for adequate UV resistance.

Vinyl:
Is a term describing a type of synthetic polymer that is moderately resistant to degradation (oxidation, UV, etc.), fair to good flexibility and has inherently good adhesion qualities. It can refer to polyvinyl chloride, but in caulks it generally indicates polyvinyl acetate (PVA). (See "Acrylic")