Wool Felt Technical Specifications & Comparative Property Chart — Density, NRC, Tensile Strength, Oil Absorption
How to read this specification chart
The table below gives the full comparative property set for our four main pressed wool felt hardness classes: Extra Hard, Hard, Medium Hard and Soft. These map directly onto SAE J314 grades F-1 to F-55 (see our SAE grade chart) and to equivalents in DIN 61200, ASTM D461, BSI and IS 282. Use it to confirm whether a single grade meets all of your application's requirements — or to pick the right trade-off when properties pull in opposite directions.
Technical Specifications
We manufacture to international standards like SAE, DIN, BSI and IS
Comparative Wool Felt Specifications
| COMPARATIVE WOOL FELT | EXTRA HARD | HARD | MEDIUM HARD | SOFT |
|---|---|---|---|---|
| General Properties | ||||
| Wool Contents, % (Min. Fibre basis) | 100 | 100 | 80-100 | 50-100 |
| Density Range, gms/cm³ | 0.45-0.75 | 0.31-0.44 | 0.24-0.30 | 0.15-0.23 |
| Standard Thickness Range, mm | 2-75 | 2-75 | 2-75 | 2-75 |
| Physical Properties | ||||
| Air Permeability, cm/sq.ft. | 5-10 | 10-35 | 25-50 | 75-200 |
| Oil Absorption % By Weight (1.0 specific Gravity Oil) | >150 | >175 | >225 | >150 |
| Oil Absorption % By Volume | 70 | 74 | 80 | 88 |
| Capillary (Oil Wicking Height 575 SSU, 25°C) | 125 | 100 | 75 | 60 |
| Coefficient of Friction | 0.42 | 0.42 | 0.42 | 0.42 |
| Vibration Absorption | High | High | Medium | Low |
| Noise Reduction Coefficient (25mm) | 0.45 | 0.50 | 0.50 | 0.64 |
| Mechanical Properties | ||||
| Tensile Strength kPa/m², min. | 6000 | 5200 | 3800 | 1500 |
| Elasticity - Compressibility | Low | Low | Medium | High |
| Elasticity - Recovery | High | High | Medium | Low |
| Tear Strength kPa/m², min. | 90 | 87 | 70 | 35 |
| Abrasion Resistance | Excellent | Excellent | Good | Fair |
| Chemical Properties | ||||
| Solvent Resistance and stability in Oil | Excellent | Excellent | Excellent | Excellent |
| Temperature °C (Max.) | 120 | 120 | 70-120 | 70-120 |
What each property means in practice
Density (g/cm³)
The single most important spec. Higher density = harder felt, lower air permeability, higher tensile strength, lower compressibility, finer polishing finish. Lower density = better sound absorption, higher oil-wicking capillary height, more cushioning. SAE grade is shorthand for density.
Air Permeability (cm/sq.ft.)
How freely air passes through felt at a defined pressure drop. Falls predictably as density rises — F-1 Extra Hard is 5–10, F-55 Soft is 75–200. Critical for dust seals, breather elements and filtration where you need controlled airflow.
Oil Absorption (% by weight / volume)
Maximum oil mass a felt can hold relative to its own mass. Wool felt absorbs more than 150% by weight (1.0 SG oil) and 70–88% by volume. This is the property that makes wool the natural choice for oil-wicking lubricators, transformer oil-wicks and journal-bearing wicks.
Capillary (Oil Wicking) Height
How high felt can draw oil vertically against gravity (mm). Higher density = higher capillary height — F-1 reaches 125, F-55 only 60. Critical for lubrication-wick design where the oil reservoir sits below the bearing surface.
Coefficient of Friction (μ)
Resistance to sliding against a mating surface. Wool felt sits at μ ≈ 0.42 across all hardness classes — high enough to maintain seal contact, low enough to allow controlled sliding. Used in design of anti-slip pads and motion-damping interfaces.
Vibration Absorption
Internal hysteresis dampens kinetic energy. Hard wool felt (F-1 to F-13) excels at low-frequency, high-displacement vibration (5–60 Hz machine bases, fans, compressors). Soft felt (F-50–F-55) is less effective for vibration but better for high-frequency acoustic damping.
Noise Reduction Coefficient (NRC, 25 mm)
Average sound absorption across 250, 500, 1000 and 2000 Hz octave bands at 25 mm thickness. Counter-intuitive trend: softer felt has higher NRC (0.64 at F-55 Soft) because more porosity dissipates sound energy. Critical for acoustic-panel and headliner design.
Tensile Strength (kPa/m²)
Force per unit area required to pull felt apart in straight tension. Range: 1,500 (Soft) to 6,000 (Extra Hard) kPa/m². Predicts how the part will survive over-pressurisation in a gasket or repeated loading in a wick.
Elasticity — Compressibility vs Recovery
Soft felt is highly compressible but slow to recover; hard felt is barely compressible but recovers instantly. Choose by load profile: cyclic compression = need recovery (hard); static load distribution = need compressibility (soft).
Tear Strength (kPa/m²)
Resistance to propagating a tear once initiated. Drops sharply at lower densities (35 for Soft vs 90 for Extra Hard). Important when felt is used near sharp edges or in flexed assemblies.
Abrasion Resistance
How well the felt holds up under repeated rubbing contact. Extra Hard and Hard rated Excellent; Medium Good; Soft Fair. Critical for polishing-wheel service life and sliding-seal applications.
Solvent Resistance & Oil Stability
Wool felt is chemically stable in mineral oils, motor oils, hydraulic fluids, transformer oil and most non-polar solvents (rated Excellent across the range). Avoid strong alkalis (>pH 10), bleach and concentrated acids.
Maximum Temperature (°C)
Untreated wool felt is safe to 120 °C continuous (150 °C intermittent). For higher temperatures we offer aramid (Nomex) blends to 250 °C, or needle-punch polyester / PPS for filtration to 180 °C. Always specify operating temperature on enquiry.
Technical questions, answered
How do I convert between SAE, DIN, ASTM and IS grades?
Use our cross-reference table on the SAE Grades page. Standards aren't perfectly identical, so for safety-critical applications we always test against the originating standard. Premier Felt's material certificates state compliance against the standards you specify.
What's the difference between density and hardness?
In wool felt they are essentially the same measurement — hardness is graded by density. SAE J314 uses density (g/cm³) and minimum wool content as its only classifying variables, and from those, all other mechanical properties (tensile strength, NRC, air permeability, capillary height) are predicted.
Can you provide a material test certificate?
Yes — every order can include a material certificate stating measured density, tensile strength, oil absorption and compliance against the standards we agreed at quotation. ISO 9001-controlled documentation is included in our export paperwork at no extra cost.
Do you offer felts with FR, oil-resistant or hydrophobic treatments?
Yes — fire-retardant finishes (FMVSS 302, UL 94 V-0, EN 45545), oil-resistant binders, hydrophobic / hydrophilic finishes, anti-static treatments and silicone impregnations are all available on request. Specify the target standard at quotation.