Pre-Applied Anti-Loosening Adhesive: Technical Evolution from Nylon Inserts to Microcapsule Layers
Executive Summary
Among anti-loosening measures, pre-applied adhesive offers the best value: no extra parts, no structural change, and no space after assembly. From the earliest nylon insert to today's microcapsule layer, pre-applied adhesive has evolved into a precision chemical system. This article reviews the principle differences across three generations, how to interpret prevailing torque, coating process control points, and environmental limits.
Table of Contents
A Problem Surfaced by a Rework Ticket
A typical rework once occurred on an electronic-control housing line: a batch of M6 screws gradually loosened three months after assembly, and the customer demanded a full disassembly and recheck. After removal, the original pre-applied adhesive layer on the screws was found to have completely crumbled into powder lodged in the thread roots, providing no locking at all.
The problem lay in the adhesive selection: this location runs continuously above 130 °C, but the pre-applied adhesive used was rated for only 120 °C. Once over temperature, the polymer matrix of the adhesive layer degraded and the mechanical locking failed.
This case shows that pre-applied adhesive is not "put on glue and it won't loosen"; it is a functional layer with clear limits on temperature, media and number of reuses. Before selecting, you must understand the principle differences across the three generations.
Evolution of Three Technology Generations
First generation: nylon insert
The ancestral form is the nylon-insert lock nut. The principle is to embed a nylon ring at the top of the nut; as the thread is run on, the nylon is squeezed and deformed, producing continuous radial pressure that resists loosening by friction. Advantages: reusable 5–15 times, slow decay of prevailing torque, temperature range about -40 to +120 °C. Drawbacks: needs an extra part, takes space, works only on nuts, and is not resistant to high temperature or strong solvents.
Second generation: pre-applied adhesive film (dry film)
Nylon or polyamide is made into a coating liquid, applied to the threaded portion of the bolt and dried into a film. During assembly the film is sheared and deformed, filling the thread gap and creating friction resistance. This solves the "no extra part" problem and can be applied to bolts, screws and studs. But prevailing torque is relatively low and it is reusable about 3–5 times.
Third generation: microcapsule layer
Anaerobic adhesive resin and hardener are separately encapsulated in microcapsules, then applied to the thread as a dry film. During assembly the capsules rupture under the squeezing and shearing of the thread; the resin and initiator mix and polymerize, shielded from air and catalyzed by metal ions, forming a cross-linked structure. Prevailing torque can reach 2–5 times that of the second generation, it provides sealing, and temperature resistance reaches 150–200 °C.
| Generation | Locking mechanism | Prevailing torque | Reusable | Temperature range |
|---|---|---|---|---|
| Nylon insert | Elastic squeeze friction | Medium | 5–15 times | -40–+120 °C |
| Pre-applied film | Shear-fill friction | Low–medium | 3–5 times | -50–+150 °C |
| Microcapsule (low strength) | Anaerobic polymerization | Medium–higher | 1–3 times | -50–+150 °C |
| Microcapsule (high strength) | Anaerobic polymerization | High | 1 time | -50–+180 °C |
| Microcapsule (high-temp type) | Anaerobic polymerization | Higher | 1 time | -50–+200 °C |
How to Interpret Prevailing Torque
The core metric of pre-applied anti-loosening adhesive is prevailing torque; following GB/T 3098.9, three values must be distinguished.
| Metric | Definition | Typical requirement |
|---|---|---|
| First run-on torque | Maximum torque as the thread begins to engage | ≤ 30% of tightening torque |
| First breakaway torque | Maximum torque on first loosening | Core anti-loosening metric |
| Fifth breakaway torque | Breakaway torque after 5 reuses | ≥ 40%–60% of first value |
Three points for reading a report:
- Size dependence: prevailing-torque requirements rise with size; M6 and M16 are not in the same order of magnitude, so absolute values cannot be compared horizontally.
- Whether cure time is included: microcapsule types must be tested after the specified cure time; testing immediately after assembly gives a falsely low conclusion.
- Whether post-high-temperature testing is done: re-testing after holding at 120 °C/150 °C reflects real service performance. Most "adhesive layer failure" cases pass at room temperature but fail after high temperature.
Four Control Points in the Coating Process
Control point 1: adhesion and pretreatment
For the adhesive to bond firmly, pretreatment must be oil-, rust- and loose-plating-free. On low-surface-energy bases such as zinc-aluminium or PTFE coatings, adhesive adhesion drops noticeably, requiring a dedicated primer or adjusted coating formulation.
Control point 2: coating location and length
- Apply to the last 3–6 threads of the threaded portion: after assembly this part stays inside the nut and locks effectively.
- Avoid the first 1–2 threads that first engage the nut: to avoid excessive run-on torque or the layer being scraped off.
- Match coating length to engagement length: too short leaves insufficient locking area; too long makes running-on difficult.
Control point 3: film thickness and cure
Film thickness is generally 0.02–0.10 mm. Too thin means too few capsules, and the resin released after shear is insufficient to fill the gap; too thick makes run-on torque too high or even impossible to run on.
Control point 4: storage and shelf life
Microcapsule layers are sensitive to storage conditions; high temperature and humidity accelerate ageing of the capsule shells. The standard practice is storage at room temperature (< 30 °C), dry (relative humidity < 60%), away from light; shelf life is typically 12–24 months, and prevailing torque must be re-tested after expiry.
Practical experience: performance decay of pre-applied adhesive is often not the adhesive itself but failure to control storage and transport. For products shipped from a rainy southern region into a hot warehouse, falling prevailing torque is very common.
Combining with Mechanical Anti-Loosening Methods
| Anti-loosening method | Mechanism | Reusability | Temperature range | Added function |
|---|---|---|---|---|
| Pre-applied adhesive (microcapsule) | Layer fill + cure | Fair (need to clean residue) | -50–+200 °C | Sealing, corrosion protection |
| Nylon insert locking | Elastic squeeze friction | Good | -40–+120 °C | None |
| All-metal locking | Thread deformation interference | Poor | -50–+400 °C | High-temperature resistance |
| Spring washer | Elastic preload compensation | Good | Material-limited | Compensates relaxation |
| Serrated washer | Teeth dig in to resist slip | Good | Material-limited | Conductive, anti-loosening |
Putting Anti-Loosening into the Assembly Specification
The value of pre-applied adhesive is "reliability gain with zero added parts": no structural change, no extra parts, no assembly space, yet it sharply reduces loosening risk and also solves sealing and thread corrosion along the way.
The key to selection is to clarify the boundaries first: what is the operating temperature, is sealing required, must the joint be disassembled repeatedly after assembly, and can the adhesive survive subsequent cleaning and painting processes? Once these four questions are answered, then choose the matching generation—and only then does pre-applied adhesive truly do its job.
