Filled vs Unfilled TPE: What Specific Gravity Tells You

Specific gravity is the quickest way to tell whether a TPE compound carries mineral filler, and roughly how much. Unfilled SEBS compounds in this catalogue sit tightly between 0.880 and 0.900. Filled compounds run from 0.940 to 1.270. That single figure has direct consequences for cost per part, colour reproduction and dimensional stability.

Key takeaways

• Unfilled compounds here span 0.880 - 0.900; filled compounds span 0.940 - 1.270.
• Specific gravity is inversely related to parts per kilogram: a 1.27 compound yields about 30% fewer parts per kilo than a 0.88 one at the same part volume.
• Unfilled grades are supplied translucent and give the cleanest colour match. Filled grades are supplied natural.
• In the FARRPRENE code the second letter states it directly: U for unfilled, F for filled.
• Filler buys dimensional stability and cost position; it costs some clarity and colourability.

What filler does in a TPE compound

A thermoplastic elastomer compound is not just polymer. It typically contains the base block copolymer, a plasticising oil, a polyolefin, stabilisers, and often a mineral filler such as calcium carbonate or talc. Filler is denser than the polymer matrix, so adding it raises the compound's specific gravity in a way that is directly readable off the datasheet.

Filler is not a defect or a shortcut. It improves dimensional stability, reduces shrinkage variability, stiffens the compound at a given hardness and lowers the cost per kilogram. What it costs you is optical clarity and some latitude in colour matching.

Reading the number

Specific gravity	What it indicates	Example grades
0.88 - 0.90	Unfilled. Neat polymer plus oil. Translucent.	FARRPRENE IEU150A, FARRPRENE IU450A, FARRPRENE EU502A, FARRPRENE IU601A
0.94 - 1.05	Lightly filled	FARRPRENE EF902AV, FARRPRENE IF458A, FARRPRENE EF753AV, FARRPRENE IF453A
1.06 - 1.15	Moderately filled	FARRPRENE EF651A, FARRPRENE IF801A, FARRPRENE IF850A, FARRPRENE IF900A
1.16 - 1.27	Highly filled	FARRPRENE IF350A, FARRPRENE EF501AU, FARRPRENE EF551AU, FARRPRENE EF600AU

The most heavily filled grade in the catalogue is FARRPRENE EF600AU at 1.270; the lightest are the unfilled translucent grades at 0.880.

Why it matters commercially

Polymer is bought by weight and parts are made by volume. A part moulded in a compound at 1.27 specific gravity consumes about 44% more material by weight than the same part in a 0.88 compound. That does not automatically make the filled grade more expensive per part - filled compounds usually cost less per kilogram - but it does mean the comparison has to be made on cost per part, not cost per kilo.

The arithmetic is straightforward: divide the price per kilogram by the specific gravity to compare grades on a volume basis.

Colour and finish

Unfilled grades are supplied translucent, which makes them the natural base for colour matching: pigment goes into a clear medium and reproduces predictably, including for tinted and translucent effects. Filled grades are supplied natural and will accept pigment, but the mineral content mutes the result and rules out clear or lightly tinted finishes.

If a customer has supplied a colour reference to match, start from an unfilled grade such as FARRPRENE IU550A, FARRPRENE IU250A or FARRPRENE EU400A.

Choosing between them

Choose unfilled when colour fidelity, translucency or the maximum number of parts per kilogram matters most. Choose filled when dimensional stability under load, stiffness at a given hardness, or cost per part governs - which describes most sealing profiles, structural components and high-volume moulded parts. Of the 49 grades in this catalogue, 38 are filled and 11 are unfilled, which reflects how most industrial applications land.

All values quoted are typical average values from a limited sample set, not a formal specification. Request the signed datasheet for current values.

Scope note. The guidance on this page is based on published mechanical properties - hardness, tensile strength, elongation, melt flow, specific gravity, base polymer and stabilisation. Food contact, skin contact, biocompatibility, flame retardancy, halogen-free formulation and electrical performance are not characterised in the public catalogue and require a separately qualified compound and testing. Contact us to qualify a grade against any of those.

Frequently Asked Questions

Related guides

• How to Choose a TPE Grade: A Specifier's Guide
• SBS vs SEBS: Which TPE Base Polymer Your Part Needs
• Why Hardness Does Not Predict TPE Strength
• TPE Melt Flow Index: Why Injection and Extrusion Grades Are Not Interchangeable

About the FARRPRENE range

FARRPRENE is the thermoplastic elastomer (TPE) compound range of Farr Polychem FZCO, a polymer compounder based in Jebel Ali Free Zone, Dubai, United Arab Emirates, supplying the UAE, the wider MENA region and Europe. The range covers injection moulding and extrusion compounds from soft gel-like Shore A grades through rigid Shore D grades, in unfilled translucent and filled natural variants, colour-matchable to a customer reference, with free samples and technical datasheets available on request. Browse the full grade catalogue, see every grade side by side on the comparison table, or request a sample.