Drop-On vs Premix Glass Beads: The Technical Difference and How to Spec Both

When a buyer asks for “glass beads for road marking”, the honest answer is: which of two? Drop-on and premix beads end up in the same stripe, but they are different products with different specifications, different sieve tables and different jobs. Buying one as a substitute for the other is one of the more common and more expensive mistakes in this trade.

The division of labour

Drop-on beads (also called surface-applied or broadcast beads) are scattered onto the marking while the binder is still hot or wet. They sit partly embedded, partly exposed. Because the exposed portion is right at the surface, headlights hit them immediately and the line reads bright the night it is laid.

Premix beads (also called internal or intermix beads) are stirred into the coating before it is applied — blended into thermoplastic at the factory, or into paint at the mixing stage. For the first months they contribute little visible reflection because the drop-on layer above them is doing the work. Their function is deferred: as traffic polishes the surface down and the top beads wear away, the premix beads are progressively exposed and take over.

A line with only drop-on beads goes dark once that surface layer erodes. On a busy road that can happen within a year. A line with only premix beads starts off weak. The specification that works is both, in defined proportions.

GB/T 24722-2020 defines the two types separately

China’s national standard assigns the two roles explicitly:

Bead type	Intended use per GB/T 24722-2020
Type 1	Drop-on beads for thermoplastic, two-component and waterborne pavement marking coatings
Type 2	Premix beads for thermoplastic and two-component pavement marking coatings
Type 3	Drop-on beads for solvent-based pavement marking coatings
Type 4	Wet-night beads, used in combination with non-wet-night beads as drop-on for thermoplastic and two-component coatings

Note that Type 3 exists at all. Solvent-based coatings take their own drop-on grade — another reason a single “beads” line item on a purchase order is a bad idea.

The sieve tables are not interchangeable

This is the part that catches people out. The two types are graded differently, and the difference is not cosmetic.

Type 1 (drop-on) particle size distribution:

Size band	Mass percentage
S > 850 µm	0
850 µm ≥ S > 600 µm	15% – 30%
600 µm ≥ S > 300 µm	30% – 75%
300 µm ≥ S > 106 µm	10% – 40%
106 µm ≥ S	0 – 5%

Type 2 (premix) particle size distribution:

Size band	Mass percentage
S > 600 µm	0
600 µm ≥ S > 300 µm	50% – 90%
300 µm ≥ S > 150 µm	5% – 50%
150 µm ≥ S	0 – 5%

Drop-on beads run coarser, with a tail up to 850 µm. Premix beads are cut off above 600 µm and cluster in the 300–600 µm band. That makes physical sense: a premix bead has to be small enough to sit inside the coating film without poking through it prematurely.

When the 2020 edition replaced the 2009 version, it shifted mass toward the larger fractions in both types. For Type 1, the 600–850 µm band moved from 5%–30% up to 15%–30%, and the 300–600 µm band from 30%–80% to 30%–75%. For Type 2, the 300–600 µm band moved from 40%–90% to 50%–90%. The direction of travel is deliberate — coarser beads deliver better retroreflectivity, so the standard loosened the top end.

How much of each

Premix (internal) content

The numbers here vary with the marking grade, and it is worth being precise about which figure applies to which situation.

GB/T 16311-2024 requires thermoplastic reflective markings to contain at least 30% premix glass beads by mass.
A Chinese public-procurement technical specification for a municipal striping project set the minimum internal bead addition at not less than 20%.
A Ganzhou highway administration technical scheme required bead usage of not less than 30% of the coating.
A standard-grade expressway design document specified premix content of no less than 30%, with roundness no less than 80%.
A high-performance, long-life expressway design went much further: premix content no less than 48%, split as 212–850 µm at 30% (roundness ≥80%) and 850–1700 µm at 18% (roundness ≥90%).
The commonly cited practical range for thermoplastic coatings is 20%–30% by weight, with premix types typically blended in at 18%–25%.

The defensible summary: the standard floor is 30%, and real projects run from about 20% up to 48% depending on the marking grade and the target service life. If a project specifies a long-life design, expect the premix fraction to climb well above the minimum.

Drop-on dosage

A Ganzhou highway technical scheme and a public-procurement specification both required a total drop-on dosage of not less than 0.4 kg/m², applied using a double-drop method — coarse beads first, then fine.
Industry practice commonly recommends 0.3–0.4 kg/m².
One manufacturer’s guidance puts flat-line markings at 280–320 g/m².
The peer-reviewed Chinese review cites an optimum spread rate of 0.37–0.45 kg/m².
A high-performance long-life design called for not less than 600 g/m² — an outlier reflecting an unusually demanding specification, not a general rule.
For reference, US practice as documented by TxDOT runs bead drop rates of roughly 6–12 lb per 100 ft² for thermoplastics, higher for paints and epoxies — which converts to about 0.29–0.59 kg/m².

Consensus band: 0.3–0.45 kg/m². More is not better. The Chinese review is explicit that over-application causes beads to pile up and overlap, which reduces their embedment in the coating, forces incoming light through repeated internal refractions, and slashes the effective reflection rate. Over-beaded lines also trap dust and go grey.

The variable that decides whether either type works: embedment

Neither bead type performs if it is buried or floating. Embedment depth is the single most controllable field variable, and the requirements are consistent across sources.

A Ganzhou highway administration scheme states it as a mandatory requirement: the portion of a drop-on bead embedded in the coating should be 40%–60% of the bead diameter, and if it is not, the coating temperature at the time of application must be adjusted and a trial application passed before full production begins.
An expressway design document specifies a settling depth of 55%–60% of bead diameter.
Industry guidance converges on 50%–60% embedded, with the rest exposed to catch headlights.
TxDOT states the optimum as 60% of the bead diameter for maximum retroreflective performance.

The failure modes are asymmetric. Beads that are under-embedded reflect light in many directions instead of back to the driver, and they are worn away quickly by traffic. Beads that are embedded too deeply still reflect, just less. TxDOT’s guidance is blunt about which error is preferable: if you are going to get it wrong, err on the side of too deep.

Checking depth in the field

Embedment is not something you measure in a lab after the fact — it is set during application, by coating temperature, application timing and coating thickness. Three practical controls follow:

Timing. Beads must land while the coating is semi-solid. A Ganzhou technical scheme requires the drop-on operation to be completed within 10 to 30 seconds of coating application. Practical guidance points to the moment just after the striping truck’s rear wheel passes, and recommends a trial section to calibrate. Drop too early and beads sink entirely; drop too late and they sit on top and fall off.

Temperature. Coating temperature governs viscosity, and viscosity governs how fast beads sink. Guidance for thermoplastic ranges from about 180–220°C, with another source giving 200–205°C in spring and autumn, around 200°C in summer and 210–215°C in winter. An expressway design document specifies 200°C ± 10°C. Above roughly 230°C the coating becomes too thin and beads sink to the bottom; below roughly 180°C it is too viscous for beads to settle at all.

Thickness. Beads need depth to sink into. Expressway designs typically call for 2.0–2.5 mm, and a Ganzhou scheme specified 2.0 mm for general markings. A thin overlay line has no settling space, so coarse beads cannot embed and will shed — even though the initial reading may look fine.

A note on why 0.4 kg/m² is applied in two passes

Both the government technical scheme and the procurement specification call for a double-drop, coarse-then-fine sequence. The reasoning is straightforward once you see it: coarse beads give a higher initial retroreflective reading and sit proud enough to catch light; fine beads fill the gaps between them. Single-drop application, or dropping in the wrong order, leaves the coarse beads unsupported and easier to dislodge.

If you are sourcing, tell us four things

Drop-on or premix, the RI class, the required sieve band, and the destination market’s standard. With those four, a supplier can quote against something checkable instead of a generic “glass beads” line. Send the marking method and target RL and we will come back with a specific grade and a sample bag if you want to run a trial section first.