Fibres: materials and their physics
An extension is a short length of polyester, tapered, curled with heat and coloured all the way through. Whatever it is called on a menu, three numbers decide what it does to the lash beneath it: its diameter, its length and its shape.
The material
A polyester called PBT
The standard extension fibre is polybutylene terephthalate, PBT, a polyester and a close relative of the PET in drinks bottles. Extension makers' own patents name it as the resin they use.234
Its chain repeats a rigid ring joined by a flexible run of four carbons. It is semi-crystalline, about a third ordered and the rest disordered, and springy: under load its crystals stretch into a second form and return when the load is removed.10
The numbers explain its behaviour. It weighs 1.30 grams per cubic centimetre, softens between 40 and 60 degrees, melts at 223, and takes up almost no water: half a percent when soaked.12
At the atelier
Because the fibre takes up almost no water, an extension keeps its curl in the shower while your own lash does not. Heat is a different matter, as the last section explains.
PBT on a temperature ruler, in degrees Celsius
From material datasheets and a maker's patent.
The names
Mink, silk, cashmere: trade names
A lash company's own patent draws the line plainly: natural fibres are silk or real mink hair, and synthetic mink is polyester. Its fibre lists put the synthetic names beside the plastics, not beside the animal fibres.78
Suppliers say the same in their training material: silk usually means PBT with a glossier finish, faux mink means PBT with a matte one. Real mink fur exists, and a maker's patent explains why it is not preferred: a heat-set curl in natural hair relaxes when the hair takes up water.17183
No laboratory has published an analysis of retail mink or silk extensions. The statement rests on the trade's own patents and suppliers, and the text should say so.
At the atelier
Ask about diameter, length and shape rather than the name. Those are what change the weight on your lashes.
Fibre names, in the libraryWhat the names mean, in the trade's own documents
- Synthetic mink
- polyester, in a lash company's patent
- Silk
- usually PBT with a glossier finish
- Faux mink
- PBT with a matte or semi-matte finish
- Cashmere
- a supplier name for a matte flat fibre
- Real mink
- animal fur, whose curl relaxes in water
Making
Extruded, tapered, curled with heat
A maker's patent sets out the whole line. The resin is extruded into fibre and cut to working lengths of 25 to 50 mm. The tips are dipped, in bundles, into a chemical bath that dissolves them to a point; brush makers use hot alkali for the same job.356
The fibres are then aligned by hand to within 0.2 mm, because machines cannot handle anything this thin and light, and cut at the root in 1 mm steps.3
The curl is heat-set: the fibre is wound on a metal or glass rod, heated, and cooled. A thinner rod gives a tighter curl. The curl letters are each maker's own labels; no standard defines their radii. Colour is mixed into the resin before extrusion, so it goes all the way through.3
C curl. The everyday curl: open without drama.
Curl shapes drawn from the lift each curl gives. Makers' curves differ: no standard fixes them.
Weight
Weight goes with the square of the diameter
The mass of a fibre is its density times its cross-section times its length. Because the cross-section grows with the square of the diameter, halving the diameter quarters the weight.119
One 0.15 mm fibre weighs as much as 4.6 fibres of 0.07 mm, 9 of 0.05 mm, or 25 of 0.03 mm, at the same length. That is our arithmetic, and it is the whole logic of volume fans.19
Taper takes off a quarter to a half of the weight, all of it at the far end, where it would weigh most on the root. Length costs twice: a longer fibre is heavier and its weight sits further out, so the load on the root grows with the square of the length.19
At the atelier
Diameter is the number we choose most carefully. Going from 0.15 to 0.10 mm cuts the weight by more than half; to 0.07 mm, by almost four fifths.
Thickness and weight, in the libraryWeight of a 10 mm fibre, in thousandths of a milligram
Our arithmetic for a plain cylinder of PBT; tapered fibres weigh less. The natural lash is worked out from measured lash sizes.
Halve the diameter and the fibre weighs a quarter.
That one rule is the whole logic of volume fans.
Shape
Flat, grooved and round
Makers lighten a fibre two ways: thinner, or a cross-section that is not round. Patents claim roots with grooves or star shapes, a half-moon that cups the natural lash, and flattened roots that give the glue a wider seat.349
The geometry is simple. A flat fibre 0.15 mm wide and 0.07 mm thick has 47 percent of the cross-section of a round 0.15 mm fibre, so it weighs about half as much. The common claim that a 0.15 flat weighs like a 0.10 round is geometrically reasonable.19
Shape changes stiffness even more. Bent across its thin side, that flat fibre is only about a tenth as stiff as the round one, which is why flat lashes feel soft and why they can twist.19
Cut across, to scale with each other. Choose one.
The plain cylinder: all the stiffness, all the weight.
Often said
What the evidence shows
Silk lashes are made of silk, and mink lashes of mink.
For the synthetic versions, the trade's own patents say polyester. Real fur and silk products exist; their market share is unknown.78
A 0.15 flat weighs the same as a 0.07 classic.
Geometry supports about half the weight of a round 0.15. Claims beyond that come without measurements.19
Faux mink tapers over two thirds of its length, silk over one third.
Supplier blogs only. No patent or measurement was found.
Against the lash
What an extension adds to a natural lash
A natural lash is not round. Imaged in 36 women, lashes were elliptical, on average 78 by 63 thousandths of a millimetre, with a cuticle 2 thousandths thick. In 179 women, the average upper lash was 8 mm long in the twenties and 6.4 mm at 50 to 65.1112
Keratin weighs about the same as PBT, 1.3 grams per cubic centimetre. That puts a natural lash at roughly 15 to 40 millionths of a gram, depending on its taper.1519
As a material, a natural lash is stiffer than PBT, by measured values about 1.5 to 3.5 times. It also changes with water, swelling about a tenth in width when wet, while PBT barely notices.13141
At the atelier
A thin natural lash is far less stiff than it looks beside a thick one, because stiffness goes with the fourth power of the diameter. The same fibre is not equally suited to every lash, which is why we map the weight lash by lash.
Mapping, in the libraryWhat one extension or fan weighs, in natural lashes
Our arithmetic, for untapered fibres against a natural lash of about 40 millionths of a gram. Real fibres taper, so these are upper limits.
The beam
Dead weight cannot break a lash. So what can?
A lash is a beam fixed at the follicle. Its resistance to bending goes with the fourth power of its diameter, so a lash 20 percent thinner is only 41 percent as stiff.1419
Put numbers in and something surprising appears. A heavy 0.15 mm fibre raises the resting load on the root about twelve times, a 0.07 mm fibre about twice. Yet the lash sags only 3 to 6 hundredths of a millimetre, and the stress at its root is about 0.56 million pascals against a strength of 150 to 270 million. Dead weight alone cannot snap a lash.1913
If heavy sets harm lashes, the route must be something else: thousands of blinks, rubbing and catching, glue bridging two lashes, or effects at the follicle. None of these has been measured. A bench rig found retention fell from 92.5 to 85.3 percent between 15 and 35 degrees. Heat matters to the fibre too: PBT starts to give under load at 55 degrees, so force and heat together could reshape a curl. That is reasoning from the material, not a measurement on lashes.161
At the atelier
Because no one has measured where the safe limit lies, we stay well inside what the lash can obviously carry, and we keep hot tools away from a set. Pre-made fans are fused or glued at the base; we choose and make fans by the weight they put on the lash.
Fans, in the libraryA heavy 0.15 mm fibre on a 0.07 mm lash, worked out
- Load on the root
- about 12 times the lash's own
- How far the lash sags
- 3 to 6 hundredths of a millimetre
- Stress at the root
- about 0.56 million pascals
- Strength of hair
- 150 to 270 million pascals
Our beam arithmetic with measured inputs. It shows what weight cannot do, not what is safe.
Often said
What the evidence shows
An extension should weigh no more than the natural lash, or no more than twice.
A training convention with no measured basis. By the arithmetic, a single 0.10 mm fibre already exceeds twice.
At the atelier
The three numbers we choose for you
A free 30-minute consultation comes first. We look at your natural lashes under magnification and choose the fibre from the lash that has to carry it.
- Diameter
- the biggest lever on weight
- Length
- costs twice: weight and leverage
- Shape
- flat or round, by how your lash behaves
Sources
Every numbered line above comes from one of these. Where a claim is still a hypothesis, the text says so.
- 1Ultradur B 4520, unfilled PBT: datasheetBASF. Material Data Center.Manufacturer datasheet: material constants
- 2PBT: polybutylene terephthalateNETZSCH Analyzing and Testing. Polymer reference page.Instrument maker's reference, with a measurement
- 3Artificial eyelash manufacturing method, artificial eyelash and installation methodMatsukaze Co.. Japanese patent application JP2018162548A, 2017.Patent
- 4Artificial eyelash and method for attaching the sameSanbonmatsu, Matsukaze Co.. US patent 8,967,158.Patent
- 5Tapered toothbrush bristleKweon, CJ Corp. US patent 6,090,488.Patent, same polymer
- 6Brush bristle materialNakamura and Nakamura. US patent 7,752,702.Patent
- 7Artificial lash extensionsLotti, Lashify. US patent 11,219,260.Patent
- 8Lash extensions and methods of manufacture and use thereofLotti, Lashify. US patent 11,832,669, 2023.Patent
- 9Artificial eyelashes having a curved cross-section; having a flattened rootDinh. US applications 2017/0112215 and 2018/0295916.Patent applications
- 10Supermolecular structure of poly(butylene terephthalate) fibers formed with the addition of reduced graphene oxideSlusarczyk, Sieradzka, Fabia and Fryczkowski. Polymers, 2020.Measured: PBT fibres
- 11Internal structure changes of eyelash induced by eye makeupFukami, Inoue, Kawai, Takeuchi, Uesugi and Suzuki. Journal of Cosmetic Science, 2014.Measured: lashes from 36 women
- 12Epidemiologic analysis of change in eyelash characteristics with increasing age in a population of healthy womenGlaser, Jones, Carruthers and colleagues. Dermatologic Surgery, 2014.Measured: 179 women
- 13Structure and mechanical behavior of human hairYu, Yang, Wang and Meyers. Materials Science and Engineering C, 2017.Measured: scalp hair
- 14Rapid water drainage on human eyelashes of a hydrophobic Brachistochrone fiber arrayZhou, Chen, Cheng and colleagues. Science Advances, 2024.Measured; beam theory written out
- 15Adaptive cooling strategy via human hairPal, Jeong, Otoufat, Bae and Kim. Proceedings of the National Academy of Sciences, 2024.Constant used in a peer-reviewed paper
- 16AIoT-based eyelash extension durability evaluation using LabVIEW data analysisChiang, Chang, Yao, Kuo and Hsu. Sensors, 2025.Bench study: a rig, not people
- 17What are eyelash extensions made of?Lilac St. Supplier education page.Industry: what the trade calls things
- 18Mink vs. faux minkLash Tavern Education. Supplier education page.Industry: what the trade calls things
- 19Mass of a fibre, effect of taper and flat shapes, and a lash as a loaded beamOur own arithmetic. Worked from the cited densities, sizes and stiffnesses.Standard physics, applied by us
Lash science
The natural lash
Extensions
- Origin and historyRead
- Purpose and psychologyRead
- Fibres: materials and their physicsThis chapter
- Adhesive: the bond and the eyeRead
- Application: method, mechanics and retentionRead
- The art: mapping, styles and texturesRead