Versaperm Vapour Permeability measurement

Glossary of Vapour Permeability Terms



ABS: Acrylonitrile Butadiene Styrene: a polymer. 

Accelerant / Accelerator: A chemical substance that accelerates chemical, photochemical, biochemical, etc., reactions or processes.  Also called promoter or­ catalyst. 

Acetal Resins: Thermoplastics prepared by polymerization of formaldehyde or its trioxane trimer.   

Acetone:  A volatile, colourless, highly flammable liquid (autoignition temperature 537°C) - CH3COCH3. Often used as nail varnish remover.

Acrylic Resins: A range of clear thermoplastic polymers of alkyl acrylates such as methyl methacrylates that show good, weatherability, surface hardness, chemical resistance, rigidity, impact strength, and dimensional stability but poor solvent resistance, resistance to stress cracking, flexibility, or thermal stability.  

Adaptive Clothing: Clothing that adapts to the ambient conditions and the physical state of the wearer. Many functional textiles also have these properties.

Adsorption:  The adhesion of atoms, ions or molecules to a surface. The process creates a film of the adsorbate on the surface of the adsorbent. In absorption, the atoms enter into the bulk phase of a liquid or solid whereas adsorption substances adhere to the surface of the molecule.

Alcohols: A class of hydroxy compounds in which a hydroxy group(s) is attached to a carbon chain or ring.   Alcohols divide into groups: mono­ hydric, dihydric, trihydric and polyhydric.  

Amorphous Nylon: Transparent aromatic polyamide thermoplastics created by condensation of hexamethylene diamine, isophthalic and terephthalic acid. 

Annulus Test: An ozone resistance test for rubber based on a flat-ring specimen that is stretched and subjected to ozone.   The result is compared to a calibrated template to determine the minimum elongation at which cracking occurred. 

Antimicrobial: Antimicrobials are compounds which inhibit or even prevent the growth of micro-organisms such as bacteria.

Area Factor: The ratio between the total area of pore openings on the surface of a membrane, and the area of this surface that is in contact with an incoming flow of a penetrant.
Aroma Barrier:  Often a plastic film or its component that prevents or inhibits the escape of aromatic volatiles from foods or cosmetics.

ASTM 096: An American Society for Testing of Materials (ASTM) standard test method for determining the water vapour transmission of materials such as paper, plastic film and sheeting, fibre boards, wood products, etc., that are less than 31 mm in thickness.   There are two methods - the desiccant method and the water method are used.   Specimens have either one side wetted or one side exposed to high humidity and another to low humidity. This leads to in a change in weight over time. 

ASTM D1434: An official Standard.  A method for measuring gas transmission rate, permeance and permeability for homogeneous materials of plastic film, sheeting, laminates, and plastic-coated papers or fabrics under steady-state conditions.  The sample becomes a barrier between two chambers.   One with the test gas under pressure, the other receives gas that permeates through the sample.

ASTM F2476-05An official Standard.  Test method for the determination of carbon dioxide gas (CO2) transmission rate through barrier materials using an infrared detector

ASTM D3985: An official Standard.  A test method to determine the steady-state transmission rate of oxygen gas through a plastic film, sheeting, laminates, co-extrusions, or plastic-coated paper or fabric. 

ASTM D471: An official Standard.  A method for determining the resistance of nonporous rubber to hydrocarbon oils, fuels, service fluids, and water involving immersion in fluids and measuring the changes in mass, volume, tensile strength, elongation, and hardness for solid specimens and the changes in breaking strength, burst strength, tear strength, and adhesion for rubber­ coated fabrics. 

ASTM D570: An official Standard.  A method to measure the relative rate of water absorption of immersed plastics.   The absorption is calculated as a percentage of weight gain. 

ASTM E398: An official Standard.  A technique to determine WVTR rate of sheet materials with (at least) one side using hydrophobic film such as plastic.   The specimen is again placed between two chambers, one of known relative humidity and the other of dry air.   The build-up is measured by electrical sensor, for example using Faraday‘s law.  Also referred to ASTM E398-70. 

ASTM E398·70: See ASTM E398

ASTM E96An official Standard.  Methods for measuring the water vapour transmission of materials.

ASTM F1249An official Standard.    For measuring water vapour transmission rate through plastic films and sheeting using a modulated infrared sensor.

ASTM F-1307An official Standard.  For testing oxygen permeability through dry packages using a coulometric sensor.

ASTM F-1927An official Standard.  Oxygen permeability and permanence at controlled relative humidity through barrier materials using a coulometric detector

ASTM F372An official Standard.  Used for measuring the WVTR of a flexible barrier film and thin sheeting consisting of single or multilayer polymers and metal foils, also including coated materials.  The result is based on calculation on the different measurements using two infrared bandwidths, one where water molecules absorb and the other where they do not.  

ASTM Fl249-20: An official Standard.  A method for measuring the WVTR through plastic film and sheeting up to 3 mm thick using a pressure-modulated infrared sensor. This Standard also covers measuring the permeance of the film to water vapour and the water vapour permeability coefficient.  

Atm /Atmosphere: This is the air pressure measured at mean sea level under specific standard conditions.  It is equal to 101,325 Pascals (Pa) (1,013.25 hPa or 1,013.25 mbar) and has a dimension of a unit of force per unit of area.   It is commonly used for gases, vapours and liquids.   Also called Atm, Standard Atmosphere, and Std Atm. 


Bar:  A metric (but not SI) unit of pressure that is defined as exactly equal to 100,000 Pa (100 kPa).  It is slightly less than one Atmosphere of pressure at sea level (which is approximately 1.013 bar). It has the dimensions of units of force per unit of area.  

Barrier Material: Materials such as plastic films, sheeting, wood laminates, particle board, paper, fabrics, etc., with low permeability to gases and vapours.   Used in construction as water vapour insulation, food packaging, protective clothing, etc. 

Blown Film: Blown film, (also known as the tubular film). Extrusion is a process involving the extrusion of a plastic through a circular die, followed by "bubble-like" expansion.  The thickness of the film is controlled by air pressure and the rate of extrusion. 

Blow-up Ratio: In blown film this is the ratio of the extrusion die diameter to the diameter of the tubular film.   In blow moulding, it is the ratio between the diameter of a parison and the maximum diameter of the mould cavity. 

Boyle's law:  Boyles law states that the volume of a gas at constant temperature varies inversely with the pressure exerted on it.  It is expressed as PV/T=R, where R is the Boltzmann constant, Pressure (P), volume (V), and temperature (T) relationship.  It can also be expressed as the volume of a gas multiplied by its pressure is constant at a given temperature. 

Breathability: The word breathability is often used in relation to clothing as being virtually synonymous with water vapour permeability of a the material.  It refers to letting moisture (evaporating sweat) escape while acting as a barrier to liquid water (such as. rain).  The Ret value of a material is an indication of its breathability. In buildings it sometimes carries a meaning of water and/or air permeability.

BS 2782: An official Standard. 820AAn official Standard.  Methods of testing plastics.. Determination of water vapour transmission rate (dish method) – gravimetric.

BS 3177An official Standard.  A method for determining the permeability to water vapour of flexible sheet materials used for packaging.

BS 7406 method A: An official Standard.  Water vapour permeability of sheet materials (paper and board) by dynamic sweep and static gas methods.

ISO 11092: An official Standard.  Textiles – Physiological effects – Measurement of thermal and water-vapour resistance under steady-state conditions (sweating guarded-hotplate test).

ISO 15106An official Standard.  Specifies an instrumental method for determining the water vapour transmission rate of plastic film, plastic sheeting and multi-layer structures including plastics, using an humidity detection sensor.

Bubbling: This is the presence of trapped bubbles of air or volatile vapours in non-metallic coating of a plastic or article.   It is frequently caused by improper application, over mixing paints or degassing. 


Carbon Footprint: The CO2 footprint or balance, is an indication of the total carbon dioxide emissions caused either directly or indirectly by a specific activity,or that produced over the life cycle of a product.

Cast Film: Film produced by pouring or spreading resin solution or melt over a temporary substrate, followed by curing via solvent evaporation or cooling before removing the cured film from the temporary substrate.
Centimetre of Mercury: See cm Hg. 

Chain Scission: Breaking of the chainlike molecule of a polymer as a result of chemical, photochemical reaction such as heat degradation or photolysis.
Chalking: Formation of a dry, chalk-like, loose powder on or just beneath the surface of paint film or plastic.  It is caused by the exudation of compounding ingredients such as pigments.  It is often a result of ingredients that move to the surface or surface degradation. 

Chemical Saturation: This is the absence of double or triple bonds in a chain organic molecule such as that of most polymers, (usually carbon atoms).   Saturation makes the molecule less reactive and polymers less susceptible to degradation and cross-linking. 

Chemical Unsaturation: Presence of double or triple bonds in a chain organic molecule such as that of some polymers, usually between carbon atoms.   Unsaturation makes the molecule more reactive, especially in free­ radical addition reactions such as addition polymerization, it makes polymers more susceptible to degradation, cross-linking, and chemical modification.   Also called Polymer Chain Unsaturation. 

cm Hg: 1 centimetre of mercury at zero degrees Celsius (equals 1333.22 Pascals).  It is a small unit of pressure equivalent to that caused by a centimetre high volume of liquid mercury Also called a Centimetre of Mercury. 

Coating: Coating commonly means the application of plastics to base materials, for example to textile fabrics.  Other materials can also be used.

Cocatalyst: See Accelerant. 

Co-extruded Film: A film made by the co-extrusion of two or more different or similar plastics through a single die with two or more orifices which weld together into a laminar film before cooling.   Each of the plastics adds a desired property, such as a specific vapour permeability or resistance to chemicals or heat.  

Colour: The wavelength of light, specifically of the light reflected or emitted by a material causing its visual appearance (red, blue, etc.).   Also called Hue, Tint, Colouration.   

Compact Membrane: In contrast to microporous membranes, compact membranes have a poreless structure.

Compatibilizer: A chemical compound used to increase the compatibility or miscibility and to prevent the separation of the components in a plastic composition. 

Concentration Units: The units for measuring the content of a substance in a medium in a different substance, such as a solvent.   Usually expressed as the mass or volume of substance per unit of mass or volume of the medium.   Sometimes noted in per centage terms.

Conditioning: Process of bringing a material or apparatus to a certain state, e.g., moisture content or temperature, before additional processing.   Sometimes called conditioning cycle.   

Corona Discharge Treatment: A process to increase the affinity between inks, adhesives, or coatings to a surface by treating it with a corona discharge.   The resultant surface oxidization makes it more receptive to finishing.   It is sometimes called corona treatment. 

Covulcanization: This involves the vulcanization of a blend of two or more different rubbers to enhance individual properties such as ozone resistance.   Rubber compounds can be modified to improve the covulcanization. 

Cracking: Cracking is the appearance of stress cracks in a material.   It can be caused by structural defects, impact, aging, corrosion.   See also processing defects. 

Crazing:  Crazing is the appearance of thin surface cracks or minute frost-like internal cracks.  They are caused by stresses that exceed the strength of the material, Stresses include thermal, impact, degradation, etc.   They are also known as Crazes. 

Cross-linking: This is a bond that links one polymer chain to another. They can may take the form of covalent or ionic bonds and the polymers can be either synthetic or natural polymers.  It may turn polymers hard and infusible.   It is caused by heat, UV or electron-beam radiation, oxidation, etc.  It can also be catalysed by the presence of transition metal complexes, thiols, or other compounds. 

Crystalline Melting Point: This is the melting temperature of the crystallite phase of a crystalline polymer and is higher than the melting point of the surrounding amorphous phase. 

Cycle Time: See processing time.


Decolouration: This is a partial or complete loss of colour of a material.   It is also called Decolouring. 

Degradation: Is the loss or undesirable change in properties of a material as a result of aging, chemical reaction, wear, exposure, etc. 

Denier or den: A measure of the linear density of a fibre measured in Grams per 9,000 metres.
Diffusion Coefficient: The diffusion coefficient weight of a substance is the amount of diffusing through a unit area in a unit time per unit concentration gradient.   Also known as Diffusivity.

Diffusion: Diffusion is the intermingling of substances by the natural movement of their particles.

Diffusivity:  See diffusion coefficient. 

DIN 53122An official Standard.  For determination of water vapour transmission (density of moisture flow rate) of plastic films, elastomer films, paper, board and other sheet materials; electrolysis method.

DIN 53380:  An official Standard.  A test method for measuring the gas permeability of flat materials such as plastic film and sheeting. 

DIN 53380-1: An official Standard.  A manometric method for measuring the vapour permeability of plastic films.

Discolouration: A change in colour due to chemical or physical changes in the material.   Also called Colour Change. 

Dtex (deci-tex): Dtex is a measure of linear density measured in grams per 10,000 metres of yarn.


Elasticity: Elasticity is the property of a body or material that allows it to change when subjected to force but return to its original form when the force is removed.

Embrittlement: This is the reduction or loss of ductility / toughness of materials resulting from chemical or physical damage. 

Ethers: Ethers are a class of organic compounds, including aliphatic, cyclic, and polymeric ethers, where an oxygen atom is interposed between two carbon atoms in a chain or a ring.   The lower molecular weight ethers can be dangerous fire or explosion hazards. 

EVOH: Ethylene Vinyl Alcohol. 

Extenders: These are relatively low-cost resins, plasticizers, or fillers such as carbonate, they are used to reduce cost or improve the processing of plastics, rubbers and non-metallic coatings. 

Extrusion Coating: This involves coating a substrate by extruding a layer of molten resin onto it with sufficient pressure to make it bond.   It is used on paper and fabrics with polyolefins. 

Extrusion Temperature: The temperature at which molten thermoplastic are extruded.


Fick's First Law: A physics law that states that the volume (V) of a fluid, such as gas, that penetrates a barrier, is directly proportional to the area (A) of the barrier, the partial pressure differential (p) of the penetrant, and the time (t).  And it is inversely proportional to the barrier thickness (s), (if the barrier is homogeneous in the direction of penetration).   The coefficient P in the equation representing Fick's first law,
V = P ·(A ·p ·t)/s, is the permeability coefficient. 

Flame Retardant: A substance that reduce the flammability of materials, such as plastics or textiles, in which they are incorporated.  

Fleece: Fleece is a textile, often made of polyester, with a fleecy surface. It is typically used in clothing as a heat insulator. It is elastic, soft on the skin and can have water-repellent properties.

Fluorocarbon-Free Clothing: Fluorocarbons can be used in the production of water-repellent textile coatings.

Fourier-transform: A mathematical process that converts a function based on space or time into one depending on spatial or temporal frequency. It makes many classes of mathematical problem easier, or possible, to solve.

Fractional Melt Index Resin: Thermoplastics having a low melt index of less than 1 (or less than a gram / 10 minutes).   These resins have higher molecular weights and are harder to extrude because of greater force requirements compared to the lower molecular weight resins.  Resins with high viscosity (resistance to flow) are inherently strong.

FTIR Analysis: Fourier-transform infrared spectrometry. 

Functional Clothing / Function Textiles: Functional textiles are materials which can be waterproof, windproof and breathable, hard wearing and offer heat management abilities. These textiles are designed to offer a good level of comfort across in changing ambient conditions. They are frequently used in sport, leisure, work and safety clothing or footwear.


Gas Permeability Coefficient: A measure of the gas permeability of a material.   It can be expressed as the product of the diffusion coefficient and the solubility coefficient
Gravimetric Technique:  The Gravimetric method for measuring vapour permeability (also known as the cup method) measures the change in weight of a sealed cup, filled with desiccant, after it has been exposed to a vapour source over time (usually days or weeks). The accuracy of the test is a function of time and the care taken in preparing the samples. 


Haze: Haze is the percentage of transmitted light which, in passing through a specimen, deviates from the incident beam via forward scattering of more than 2.5° on average (ASTM 0883). 

HDP: High Density Polyethylene. Also called HDPE. 

Heat Seal Temperature: Temperature of a thermoplastic film or sheet needed to join (or fuse) two or more films or sheets. 

Henry's Law: A law that states that, under equilibrium conditions, the weight of the gas that dissolves in a given quantity of liquid is proportional to the pressure of the gas above the liquid.  

Hot Tack Strength: The force required to separate a seal between heat-sealable thermoplastic films.   Also called Ultimate Hot Tack Strength. 

Hydrophilic: water-attracting and moisture-directing. The opposite to hydrophobic.

Hydrophobic: water-repellent and not moisture-directing. The opposite to hydrophilic.


Ignition Resistant Chemical Additives: A flame retardant. 

Impregnation: Generally, impregnation means saturating or coating textiles with substances which add properties, for example water or dirt-repellence or flame-proofing.

Inch of Mercury / in Hg: A unit of measurement of pressure equal to 3.3864 x 249.089 pascals or 1e+04 dynes/cm2 at 0°C (32°F).   One inch of mercury is the pressure that would support an inch high column of mercury with a density of 12,595 kg/m3.  Also called Inch of Mercury. 
Ionomer: a polymer of electrically neutral repeating units and ionized units covalently bonded to the polymer backbone as pendant moieties.

Initial Tear Resistance: The force required to start a tear in a flexible plastic film or thin sheeting at very low rates of loading.  It is measured as maximum stress usually at the onset of tearing.   Also called Tear Resistance. 

ISO 7229: An official Standard. 1997 An official Standard.  Based on the measurement of gas permeability through fabrics coated with rubber or plastic.

ISO 9932An official Standard.  Methods for determination of water vapour transmission rate (WVTR) of sheet materials (paper and board) by dynamic sweep and static gas methods.


JIS K 7126An official Standard.   For oxygen permeability testing of plastic films.

JIS Z0208: A Japanese Standards Association (Nippon Kikaku Kyokai) standard test method for determining water vapour transmission of flat materials such as plastic film and sheeting. 


Kinetic Coefficient of Friction: The ratio of the force needed between two surfaces that is needed to sustain motion without acceleration, to the force which presses the surfaces together.   Also called Coefficient of Friction, and Coefficient of Friction, kinetic. 


Lamellar Injection Moulding: The injection moulding of individual thermoplastics or their blends, e.g., with liquid-crystal polymers that produces a lamellar (platelike crystallite) skin texture of the moulding to enhance surface properties or appearance. 

Laminate: A laminate is a material made up of several layers to improve the materials performance. For example, wear, strength, permeability etc..

LCP: Liquid Crystal Polymers. 

LDPE: Low Density Polyethylene. 

lzod Impact Energy: The kinetic energy needed to initiate a fracture and then continue the fracture until the specimen is broken.

lzod Impact Strength Test: The Izod impact strength test is an ASTM standard method of determining the impact resistance of materials.


Mass Spectrometry: Is an analytical technique for identifying materials by measuring the mass-to-charge ratio of its ions. Results are characteristically presented as a spectrum, plotting intensity against the mass-to-charge ratio.

MBT: 2-Mercaptobenzothiazole. 

Mechanical Properties: The way a material responds to stress and strain. 

Membrane: A membrane is a thin layer of a material. Typically, it allows some things, such as molecules, ions, or other small particles, to pass through but stops others.

Microfibre: These are any fibres which, if they are 10,000 m long, weigh one gram or less.  This corresponds to 1 Dtex. Microfibres are finer than natural fibres such as silk and are usually supple, soft, lightweight and absorb moisture easily.

Micron: A SI unit of length equal to a millionth (1×10- ) of a metre.  Also called a micrometre or micrometer.  Its symbol is a Greek small letter mu (μ) or μm. 

Microporous Membrane:  A membrane (or coating) is a membrane with a network of pores which allow, for example water vapour, to pass through. It is a barrier to liquid water as the pores are too small to allow liquid droplets to pass.
Moisture Vapour Transmission Rate (MVTR): Also known as water vapour transmission rate. A measure of the permeability of a material to the passage of moisture in the gaseous state, not as a liquid. Expressed in many types of units (see Versaperm Conversion Units )

Molecular Weight Distribution: The relative amounts of polymeric molecules of different weights in a specimen.  Also called Polydispersity, MWD, and Molecular Weight Ratio. 

Molecular Weight Ratio: See Molecular Weight Distribution. 

Molecular Weight: The total sum of the atomic weights of the atoms in a molecule.   Also known as MW. 

Moulding Defects: Structural and other defects in material caused during moulding.  Also called moulding flaws.   Such flaws are usually preventable. 

Moulding Flaw: See Moulding Defects. 

Mulch Film: A film, usually of dark coloured PVC, used instead of a mulch for agriculture to prevent rot, runners, and weed growth. 

Multilayer Film: A thermoplastic film consisting of two or more different or similar films jointed together, usually by co-extrusion or lamination, to exhibit specific properties such as increased strength or resistance.

MWD: Molecular Weight Distribution. 


Non elastomeric: Rigid thermoplastic polyurethanes. 

Notch effect: The effect of a notch on the outcome of a specimen test such as an impact strength test.   Notching produces local stresses and accelerates failure in both static and cycling testing (mechanical, ozone cracking, etc.).


Oeko-Tex® Standard 100: The Oeko-Tex® Standard 100 is an independent certification system for textiles showing that they are free from harmful levels of more than 100 substances.

Oriented Polypropylene: A type of polypropylene with the polymer molecules oriented by stretching.  The process is designed to offer enhanced mechanical properties, i.e. it will shrink in the direction of stretching when reheated.  Also called OPP. 

Ozone: An allotrope of oxygen, O3.  Ozone is an active oxidizing agent and accelerates the deterioration of rubber. 

Ozone Cracking: Cracks formed in different elastomers caused by ozone attack.


Pa: See Pascal

Paraffinic Plasticizer: These are plasticizers for plastics and are based on liquid or solid long-chain alkanes or paraffins.  

Partial Pressure: The pressure that would be exerted by a gas in a gas mixture if it, alone, were present. Also called vapour pressure. 

Parts Per Hundred Million (pphm): A relative unit of concentration – the ratio of the parts of one substance per 100 million parts of another.   Parts can be measured by weight, volume, count, or any other suitable unit of measure.  

Parts Per Hundred (pph or phr): A relative unit of concentration – the ration of the parts of one substance per 100 parts of another.   Parts can be measured by weight, volume, count, or any other suitable unit of measure.  

Pascal: A Pascal, the SI unit of pressure.  It is one kilogram per metre per second squared (or one Newton per square metre). The kilopascal (kPa) is 1,000 Newtons per square metre and is more common usage.

Pendant Aromatic Rings: Aromatic rings (conjugated unsaturated rings such as benzene C6H6) attached to the main chain of a polymer molecule. 

Penetrant: A substance such as gas that penetrates another substance, usually a solid barrier such as plastic film.  Also called Permeant. 

Perm:  A perm is a unit of permeance or vapor transmission. The U.S. perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury. The equivalent SI measure is the nanogram per second per square meter per pascal (see Versaperm Conversion Units

Permanent Gas: Gases that only become liquid at pressures and temperatures far from normal environmental conditions of 1 atm and 0 C).   These gases include air, oxygen, argon, and carbon dioxide. 

Permeability: Vapour permeability is the rate at which vapour passes through a material.  To be more precise it is a measure of how much vapour is transmitted through a material (or compound object) under a given set of conditions. The higher the permeability of the material, the more rapidly vapour can pass through it.

Permeance: The rate of water vapour transmission through a unit area of a flat material or construction.
Phase Transition Point: The temperature at which a phase transition occurs in a material or system.  Also called Phase Transition Temperature, Transition Point, and Transition Temperature. 

Phase Transition Properties: these are the properties of physical systems, such as materials, associated with their transition from one phase to another, e.g., from liquid to solid phase.
Phase Transition Temperature: See Phase Transition Point. 

Photolysis: The process where molecules are broken down into smaller units by the absorption of light.

phr: Parts Per Hundred. 

Plasticiser: A substance incorporated into a material to increase its softness, processability and flexibility.  Plasticisers can lower viscosity, improve flow and increase low-temperature resilience of material.   Some play a dual role as stabilizers or cross-linkers. 

Polyester: Polyester is the name given to a class of polymers.

Polymer Chain Unsaturation: See Chemical Unsaturation. 

Polymers: High-molecular-weight organic or inorganic compounds that consist of linear, branched, cross-linked, or shaped chains of repeating molecular groups.   Also called resins, plastics. 

pphm: Parts Per Hundred Million.  

ppm: Parts Per Million

Pressure Differential: The rate of decrease of pressure over time or space.   Also called Pressure Differential.  Also known as pressure gradient. 

Pressure: Stress exerted equally in all directions.  

Processing Agents: These are substances used in the manufacture or treatment of a material to improve its processing or properties.   Also called process media, processing aids or processing additives.    
Processing Aids: Processing agents. 

Processing Defects: Defects in a material caused inadvertently during manufacturing.  They are often preventable and are also called processing flaws or defects. 

Processing Pressure: This is the pressure maintained in an apparatus during manufacture or treatment.  

Processing Procedures: See processing methods. 

Processing Rate: The speed of a process.
Promoter: Accelerator. 

PTFE: Polytetrafluoroethylene 

PVT Relationship: Boyle's law, or the interaction of pressure (P), volume (V) and temperature (T). The law states that the product of the volume of a gas times its pressure is a constant at a given temperature, PV/T=R, where R is Boltzmann constant. 


Ra: Roughness average.  ASME B46.1 describes Ra as the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length.

Reaction Injection Moulding System: This is a relatively new way to produce structural plastic foams where highly reactive plastic liquids are mixed under high pressure and then injected into a mould to react, polymerize, and turn into foamed plastics.  Also known as RIM. 
Recursive:  see recursive.

Relative Humidity Gradient: The rate of change of relative humidity either in space at a fixed time or just the magnitude of the change.   The relative humidity gradient changes the transmission rate of vapour through a barrier and should be recorded for the rate values to be meaningful. 

Relative Humidity: The ratio of the actual vapour pressure of the air to the saturation vapour pressure.   Also called RH. 

Relative Viscosity: The ratio of a solution’s viscosity to the viscosity of the solvent. 

Resins: Polymers. 

Ret Value: Ret is the resistance that a material has to a vapour passing through it. The lower the value, the less resistance and the higher the breathability. The Hohenstein Institute, considers textile breathability as very good if it has a value of less than 6, good for values of 6 – 13, satisfactory for 13 – 20 and unsatisfactory for values of over 20.

RH: Relative humidity.  It is the amount of water vapour present in air expressed as a percentage of the amount needed for saturation at the same temperature.

RIM: See reaction injection moulding system. 

RMS:  ASME B46  defines RMS as the root mean square average of the profile height deviations from the mean line, recorded within the evaluation length.


Seal Initiation Temperature: The lower limit to temperature at which a thermoplastic material such as a film begins to fuse and adhere to itself or another material. 

Service Life: The period of time required for specific properties of a material to deteriorate, under normal use conditions, to the minimum acceptable level. 

Shelf Life: The time during which a physical system, such as a material, food, pharmaceutical or cosmetic, retains its storage stability under specified conditions.   Also called Storage Life. 

Siloxane: See Silicone. 

Slip Factor: A property that characterizes the lubricity of a material – it is the reciprocal of the friction coefficient. 

Softening Point: The temperature where a material alters from rigid to soft or where it shows a sudden and substantial decrease in hardness.   Also known as softening temperature and softening range. 

Solubility Coefficient: The volume of a gas that can be dissolved by a unit volume of solvent at a fixed pressure and temperature. 

Solubility: The capacity of a substance to be fully dissolved in another without any phase separation.   It is usually expressed as a percentage of dissolved substance. 

Stability: The ability of a system or material to resist change or degradation when exposed to external forces.

Standard Atmosphere (Std atm): Atmosphere. 

Storage Life: Shelf life. 

Storage Stability: The resistance of a system or material to a deterioration in its properties. 

STP: An official Standard temperature and pressure of one atmosphere and 0°C, respectively.   Permeability varies with temperature and pressure and these should always be stated.  

Stretchability: The word stretchability comes from strength theory where it means the ability of a material to change its form when subjected to force. Stretchability indicates how much a material can be stretched without breaking or tearing.


TAPPI T557: An official Standard.  For measuring water vapour transmission rate through plastic films or sheeting using a modulated infrared sensor.

Tear Propagation Resistance: The force required to propagate a slit in a flexible film or sheet under a constant rate of loading. It is the average between the initial and maximum tear-propagation forces. 

Temperature: This is the degree or intensity of heat present in a substance and is expressed according to a comparative scale.  Heat flows naturally from objects with a higher temperature to lower. 

Tensile strength: Tensile strength specifies the point where a material passes from elastic to plastic deformation.  It is the greatest tensile stress that a material can be subjected to before it fails (e.g. breaks or is permeant deformed).

Tex: Tex is a direct measure of linear density, it is measured in grams per 1,000 metres of yarn.

Toughness: Property of a material indicating its ability to absorb energy by plastic deformation rather than crack or fracture. 

Transition Point: Phase Transition Point. 

Transition Temperature: the temperature at which a phase transition point occurs. 

Tubular Film:  Tubular film is also known as blown film and is produced by the extrusion of a plastic through a circular die, followed by "bubble-like" expansion.  The thickness of the film is controlled by air pressure and rate of extrusion. 

Turbidity: The cloudiness in a liquid caused by a suspension of colloidal liquid droplets, or fine solids. 


USP 671An official Standard.  This standard covers a series of test methods specific to the functionality and performance of containers and package systems used for solid and liquid dosage forms

UTS: See tensile strength.


Vapour Barrier: A material, typically a plastic or foil sheet, that resists the diffusion of a vapour, such as moisture, through a barrier.

Vapour Check: A building related term where the vapour check restricts the transfer of the warm, possibly moisture-laden air into or out of an internal area.. Different classes of membrane are defined by BS5350 as a series of ‘Classes with different abilities to limit the flow of water vapour (moisture) under different conditions.   

Vapour Control Layer (VCL):  VCLs are designed to control moisture from penetrating to prevent condensation forming within the structure of the building where it can cause rot, fungus and other problems.

Vapour Permeability: is the rate at which vapour passes through a material. It is a measure of how much vapour is transmitted through a material under predefined conditions. The higher the permeability value, the more rapidly vapour can pass through it. 

Vapour Permeable Membrane / Membrane:  materials that limit the flow of a vapour, such as water, from passing through them. In the construction industry it refers to a barrier which allows moisture to enter of leave a building, but prevents the flow of liquid..

Vapour Phase Inhibitor: These are chemicals applied to materials such as paper orplastics and metals to prevent rust and corrosion on adjacent metals.
Veneer:  A thin film applied on a rubber or other material often to protect it against, damage such as ozone attack.  It may also be as a migration barrier or for decoration. 

Vicat Softening Point (or Temperature): The temperature at which a prescribed flat-ended needle can penetrate a specimen to a certain depth under a specified load.  it can be determined under the ASTM d1525 standard.

Volatile organic compounds (VOCs): These are organic chemicals that vaporise quickly at room temperature. Some VOCs can may have adverse short- and long-term health effects.


Warpage: Is the distortion or deviation from an intended shape, that arises from a non-uniform internal stress such as heat.   It is also known as warping.

Warping: See warpage. 

Water Column:  Relates to the maximum pressure that a column of water column creates on a material without the being able to pass through.  It is often usually measured in mm of water.

Water Repellency: This is the property of a material that prevents the immediate absorption of water.

Water Swell: Is the expansion of material volume that results from the absorption of water. 

Water Vapor: also called water vapour, or aqueous vapor is the gaseous phase of water.

Water Vapour Migration or transferor transmission:  Is the transmission of water vapour through a material caused by a difference or partial pressures between the two sides.

Water Vapour Permeability: A material’s ability to allow water vapour to pass through it. It can vary under different environments of temperature and vapour pressure but follows Flick’s law for stable conditions.

Water Vapour Resistivity: Vapour resistivity is a material’s resistance to vapour flow.  It is the precise reciprocal of vapour permeability.

Water Vapour Retarder:  This is a compound, material of structure that impedes the transmission of water vapour under specified conditions. Retarders have a permeance of less than 1.0 perm when tested under ASTM E 96.

Water Vapour Transmission Rate (WVTR): Sometimes known as moisture vapour transmission rate (MVTR). It is a rate of a steady-state water vapour flow per unit time through a unit area of a body under specified temperature and partial pressure conditions. It is a measure of the permeability of a material and is expressed in many types of units (see Versaperm Conversion Units).

Water-Formed Deposit: any accumulation of insoluble material created by the reaction with water and any surfaces in contact with it.

Water-Holding Capacity: The ability of a substance to retain water.

Waterproofing: Waterproofing is the property of materials which prevents the ingress or passage of water.

Waterside Economizer Cooling: This is a term used by the heating and ventilating industry.  It is an  process that uses cooling tower-water directly or indirectly in cooling coils to allow a chiller to be turned off when the outside wet-bulb temperature is sufficiently low.

Waterside Economizer: This is a term used by the heating and ventilating industry when a heat exchanger uses the condenser water side of a system for cooling without using the chiller. It is also a coil on entrance air side of a heat pump that uses condenser water flow to precondition the air.
Water-tight: That quality of a container or package that prevents liquid water flowing into or out of the package.

Watertightness: Theability to resist water penetration.

Weather Protection Clothing: Offers the wearer protection against water, wind, low temperatures or dampness.

Weight: The force of the Earth’s gravity on any mass. 

Wettability: The degree to which something absorbs moisture. 

Whiting: Used as a filler in plastics and rubbers it is finely milled form of limestone, calcium carbonate (CaCO3).

Wind Chill Effect: The amount by which a perceived temperature differs from the air temperature as the wind increases.