PARAPET: A wall or top portion of a wall extending above an attached horizontal surface such as a roof, terrace, or deck; often used to provide a safety barrier at a roof edge. 

PASS: The volume of coating or polyurethane foam applied by moving the gun from side to side and moving away from fresh material in a single continuous application. A pass is defined by its width, length and thickness. Foam passes sprayed and tied together along their adjacent edges to cover a larger area are called a lift of foam.  (See LIFT)

PASS LINES: Pass lines are created when the end of a pass of foam or coating ties into an adjacent pass. The overlapping of the polyurethane foam or coating can be seen typically as a darker color than the middle of the pass. Foam at the pass lines typically contain thinner lifts than the middle of the foam pass as the applicator tapers the foam to uniformly tie the foam passes together.

PCF: Pounds per cubic foot or lb/ft3. A measure of density.

PEEL STRENGTH: The average force (or force per unit width) required to peel a membrane or other material from the substrate to which it has been bonded. 

PEELING: Top-coating film inadequately bonded with undercoats resulting in partial delamination or detachment of final coat. 

PENETRATION: (1) Any object, such as vent pipes, electrical conduit, ducts, structural supports elements, etc., passing through a roof, wall, floor, or other building assembly. (2) Any hole or opening in a building assembly that would allow the passage of air, water, or other fluids.

PERM: A unit of water vapor permeance defined as 1 grain of water vapor per square foot per hour per inch of mercury water vapor pressure difference (1 in. of mercury = 0.49 psi). Perm = 1 grain/ft2•hr•in. Hg. The SI unit for permeance is ng/s•m2•Pa (1 perm = 57.4 ng/s•m2•Pa).

PERM RATING: The permeance of a material. Breather materials have relatively high perm ratings, vapor retarders have relatively low perm ratings, and vapor barriers have essentially zero (negligible) perm ratings. (See also VAPOR RETARDERS.)

PERMEABILITY, AIR: The rate at which air will diffuse through a unit area of material induced by a unit differential in air pressure. Air permeability is generally used as a descriptive term and specific values are not usually reported. (See PERMEANCE, AIR.)

PERMEABILITY, WATER VAPOR: The rate at which water vapor will diffuse through a unit thickness and area of material, induced by a unit differential in water vapor pressure. Permeability values are related to a common thickness and can be used to compare various materials. Units are usually grains•in./ft2•hr•in Hg or perm•in. SI unit for permeability is ng/s•m•Pa (1 perm•in = 1.46 ng/s•m•Pa).

PERMEANCE, AIR: The rate at which air can diffuse or leak through a unit area of material induced by a specific pressure differential, for example, 75 Pa or 1.57 lb/ft2. ASTM E 2178 and ASTM E 283 are the test methods used to measure air permeance. Units are L/s•m2 (ft3/min•ft2) with pressure reported as Pa (lb/ft2). (See also AIR IMPERMEABLE; AIR PERMEABLE.)

PERMEANCE, WATER VAPOR: The rate at which water vapor will diffuse through a unit area of material induced by a unit differential in water vapor pressure. Permeance values are reported for specific thicknesses (usually recommended application thickness). Units are usually grains/ft2•hr•in Hg or perm. SI unit for permeance is ng/s•m2•Pa (1 perm = 57.4 ng/s•m2•Pa).

PERSONAL PROTECTIVE EQUIPMENT (PPE): Includes all protective equipment and supplies designed to protect employees from serious workplace injuries or illnesses resulting from contact with chemical, radiological, physical, electrical, mechanical, or other workplace hazards. Besides face shields, safety glasses, hard hats, and safety shoes, PPE includes a variety of devices and garments, such as goggles, coveralls, gloves, vests, earplugs, and respirators.

pH: A measure of acidity/alkalinity of aqueous mixtures. A measure of pH 7 is neutral, lower is more acidic, and higher is more alkaline. 

PICTURE FRAMING: Outlining the perimeter of a stud wall cavity with SPF prior to filling the center. 

PIGMENT: Finely ground insoluble dispersed particles (powder) that when dispersed in a coating, may provide resistance to UV radiation, better resistance to weathering, hardness, durability, reflectivity, and other properties in addition to color.

PINHOLE: A surface defect in polyurethane foam and/or coating that appears as a small hole as if made by a pin. The term is generally applied to holes caused by expanding gases from solvents, moisture, or other volatile compounds or SPF cells open to the surface. (See BLOWHOLE.)

PLASTICIZER: A substance added to a plastic or coating to increase its flexibility or elongation. 


POLYMER: A substance consisting of high-molecular-weight chemical compounds characterized by chains of repeating simpler units. 

POLYMERIC MDI (p-MDI): Isocyanate compounds wherein the molecules contain more than two functional -NCO groups. (See also METHYLENE DIPHENYL DIISOCYANTE.) 

POLYOL: A high-weight molecule that contains hydroxyl groups (-OH), typically at the terminal position on the molecular chain. Polyol is a primary ingredient in the B-side or resin of the two-component polyurethane system. After reaction with MDI (A-side), the polyol becomes part of the polyurethane polymer.

POLYURETHANES: Polyol and MDI react to form polyurethane. When this reaction occurs multiple times, a polyurethane molecule is created. Many end-use products are created using polyurethane chemistry, including rigid and flexible foams, rigid or flexible coatings, elastomers, structural materials, etc. (See also AROMATIC; ALIPHATIC; ISOCYANATE; POLYOL; RESIN.)

POLYURETHANE COATINGS: A one- or two-part coating that contains polyisocyanate monomer and a hydroxyl containing resin, which react during cure to form a polyurethane elastomer. 

PONDING: The accumulation of water in low-lying areas that exceeds the manufacturer’s specification and/or contract documents. (See POSITIVE DRAINAGE.) 

POPCORN SURFACE TEXTURE: The surface exhibits texture of SPF where valleys form sharp angles. This surface is unacceptable for coating application. Also termed “treebark surface texture.” (See SPFA-145, “Surface Texture of Spray Polyurethane Foam.”)

POSITIVE DRAINAGE: The condition in which there is no consequential standing water on the roof 48 hours after a rain. (See PONDING.) 

POST EXPANSION: A characteristic of some single-component SPFs wherein additional expansion occurs after the initial application and froth expansion. Post expansion is due mainly to the chemical curing process.

POT LIFE: The period of time during which a multi-component or catalyzed material remains suitable for application after being mixed. 

POUR FOAM: A polyurethane foam system, with a slower reactivity profile (relative to SPF), designed for pouring or injection into confined spaces such as a mold, panel, or concrete blocks.

POWERED-AIR PURIFYING RESPIRATOR: A type of air purifying respirator which consists of a powered fan that forces incoming air through one or more filters for delivery to the user for breathing. The fan and filters may be carried by the user or, with some units, the air is fed to the user via tubing while the fan and filters are remotely mounted. (See RESPIRATOR.)


PRIMER: The first layer of coating applied to a surface to improve the adhesion of subsequently applied materials or to inhibit corrosion. 

PROPELLANT: Liquified or compressed gas formulated into single-component SPF or two-component FROTH PACKS used to expel the SPF ingredients from its containers. The propellant also functions as a BLOWING AGENT. (See also BLOWING AGENT; FROTH PACKS.)

PROPORTIONER: The basic pumping unit for spraying polyurethane foam or two-component coating systems. Consists of two positive displacement pumps designed to dispense two components at a precisely controlled ratio. 

PSI: Pounds per square inch (lb/in2). 

PSYCHROMETER: (1) A device for measuring ambient humidity by employing a dry-bulb thermometer and a wet-bulb thermometer. (2) An electronic device for measuring ambient temperature and humidity.

PSYCHROMETRIC CHART: A diagram relating the properties of humid air with temperature. 

PURGE: To cleanse or remove liquid materials from equipment or hoses.

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