# R940 Unsaturated Acrylic-Polyurethane Reactive Flame Retardant: Once the Flame Retardant Is Integrated into the Resin Network, Migration and Blooming No Longer Occur

Source: 锎锔-功能助剂1.10.pdf (The page number refers to the physical page of the PDF). All quotations and data in this article are reproduced verbatim from the original, with no inferential additions.

In flame-retardant applications for UV-curing and polyurethane systems, the first problem that often arises is usually not an insufficient flame-retardant rating, but the failure to retain the flame retardant.

Additive flame retardants enter the formulation through physical blending, and the dosage required is not small. This approach has two unavoidable drawbacks: flame retardant molecules “float” within the resin and, during long-term use or under specific temperature and humidity conditions, migrate to the surface and precipitate as bloom. After prolonged use or exposure to heat and light, a layer of hazy deposit appears on the surface, reducing transparency and deteriorating the tactile feel. R940 takes a different approach.

The first product feature listed for R940 in the manual is:

“Reactive flame retardancy: Contains a methacrylate double bond, can participate in UV/free-radical polymerization, and is chemically bonded into the resin network, with no migration, no precipitation, and no blooming. Compared with additive flame retardants, it has less impact on transparency and mechanical properties.” (Source: 锎锔-功能助剂1.10.pdf p61)

As a result, the flame-retardant component is no longer free-floating. It does not migrate, precipitate, or bloom, and its impact on transparency and mechanical properties is also smaller than that of additive flame retardants.

The R940 model and product type are reproduced below:

“R940 Unsaturated Acrylic-Polyurethane Reactive Flame Retardant Technical Data Sheet Model: R940 Product type: Halogen-free reactive liquid flame retardant, suitable for UV-curable unsaturated acrylic and polyurethane systems” (Source: p61)

The Product Composition section states:

“Total phosphorus content approximately 10.5%; halogen-free and compliant with RoHS and REACH requirements.” (Source: p61)

I. Reactive vs. Additive: The Difference Lies in "Whether It Is in the Network"

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The mechanism of additive flame retardants relies on physical shielding and endothermic cooling: the flame retardant decomposes to absorb heat when heated, or forms a protective layer on the surface. It works by "staying in the formulation," and it does indeed merely stay in the formulation—the molecules are not immobilized, and over long periods of exposure to heat, light, and humidity, they will find their way out on their own.

Reactive flame retardants generally carry one additional reactive double bond. R940's double bond is a methacrylate double bond, which participates in polymerization under UV or free-radical initiation and is incorporated directly into the resin network as it cures. Once incorporated, it becomes part of the resin structure rather than an independent particle surrounded by resin.

This difference brings three direct results:

• No migration, no precipitation, and no blooming;
• Less impact on the transparency and mechanical performance of the product;
• Flame-retardant components are less likely to be extracted or lost through water, oils, or solvents.

For transparent products, the third point is especially important. For transparent adhesives, transparent coatings, 3D-printed photocurable resins, and similar products, appearance is a selling point and flame retardancy is a threshold requirement; the two must not hold each other back.

II. Phosphorus–Phosphorus–Nitrogen Synergy: Three Components Each Handle One Stage

7Data figure 7

DOPO49%HEMA40%HPCTP10%TPP0.8%MEHQ0.2%R940 Component Content (Mass Fraction)1.0%2.0%3.0%4.0%1.5%3.0%System Total Phosphorus Window (Formulation Target)Total phosphorus content of the formulation %Component Content 0–100%

• R940 Component Content (Mass Fraction)0–100%
• System Total Phosphorus Window (Formulation Target)1.5%–3.0%

The component table for R940 is clearly laid out.

(Source: 锎锔-功能助剂1.10.pdf p61)

DOPO is the main component; flame retardancy works through two simultaneous pathways: gas-phase flame retardancy and condensed-phase flame retardancy. Hexaphenoxycyclotriphosphazene adds phosphorus–nitrogen synergy, strengthening char formation in the condensed phase and reducing smoke density, delivering both flame retardancy and smoke suppression. HEMA provides a reactive double bond, enabling the entire molecule to participate in copolymerization while improving compatibility. Triphenylphosphine serves as the catalyst, while MEHQ serves as the polymerization inhibitor—the latter is key to storage stability.

The manual gives the following figure in the recommended process section:

“It is recommended to control the total phosphorus content of the system at 1.5-3.0%, which can achieve the UL94 V-0 flame-retardancy rating.” (Source: p62)

This figure is a formulation target, not a product specification; the two should not be conflated.

III. Technical Specifications

III. Technical…

Item	Specification
Model	R940
Product type	Halogen-free reactive liquid flame retardant
Applicable systems	UV-curable unsaturated acrylic and polyurethane systems
Total phosphorus content	Approximately 10.5%
Halogen	Halogen-free; compliant with RoHS and REACH
Appearance	Colorless or pale yellow opaque liquid
Viscosity (25℃)	800-2000 mPa·s
Acid value	≤15 mgKOH/g
Flash point (closed cup)	＞120℃
Solubility	Miscible with acrylic monomers, polyurethane polyols, esters, and ketones; insoluble in water
Storage stability	Store in a cool place away from light; avoid high temperatures and prevent self-polymerization
Packaging	25kg plastic drum
Shelf life	6 months under sealed conditions

(Source: 锎锔-功能助剂1.10.pdf p61–62)

Its insolubility in water determines how it is used: it is introduced into organic systems, not the aqueous phase. If the wrong system is selected, no matter how good the specifications are, they will be of no use.

IV. Formulation and Process

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The manual underlines the first sentence of the recommended application process:

“For a reactive flame retardant, it must participate in polymerization and crosslinking; simple physical blending without participating in the reaction is not recommended.” (Source: p62)

The recommended dosage for the UV acrylic system (UV curing) is 15-30% of the total resin mass. The method is straightforward: mix thoroughly with acrylic monomers, prepolymers, and photoinitiators at room temperature, degas, and then perform UV curing. Keep the total phosphorus content at 1.5-3.0% to achieve UL94 V-0.

The recommended dosage for the polyurethane system (PU/PUA) is 12-25% of the polyol component. It can be mixed directly with the polyol and then reacted with the isocyanate. There is one point that must be factored in advance: R940 contains hydroxyl groups, which consume -NCO groups, so the amount of isocyanate in the formulation must be adjusted accordingly. If this is not accounted for, the NCO/OH ratio of the system will be off.

The manual lists three points in the process notes:

“①The temperature should not exceed 120℃ for prolonged heating, as this may cause self-polymerization; ②strongly alkaline substances should be avoided in the formulation as much as possible, as this may cause the polymerization inhibitor to fail and gelation to occur; ③if a higher flame-retardancy grade is required, the product can be compounded with a small amount of aluminum hydroxide and phosphate ester.” (Source: p62)

V. Applicable Applications

(Source: 锎锔-功能助剂1.10.pdf p62)

The manual adds the following information in the section on applicable systems: It can be used in transparent systems and does not noticeably affect the light transmittance of the products; it can be used together with photoinitiators and reactive diluents.

VI. Common Questions

Q: Should I choose a reactive flame retardant or an additive flame retardant?

Consider the product’s requirements for long-term stability and transparency. For applications requiring no precipitation over the long term, high transparency, and no loss of the flame-retardant components through extraction, the reactive route is more suitable; where cost is sensitive or the system is not amenable to participating in polymerization, additive flame retardants still have their place. The prerequisite for R940 is that the formulation genuinely contains a stage that can participate in polymerization.

Q: Why is the recommended dosage given as a range?

Because the flame-retardant rating is not determined by the dosage alone. The total phosphorus content of the system is the key indicator; only at 1.5-3.0% can UL94 V-0 be achieved. With different resins and different proportions of reactive diluent, the same dosage corresponds to different total phosphorus contents, so a range is given rather than a single value.

Q: Can R940 be used directly in all polyurethanes?

No. R940 contains hydroxyl groups, which react with isocyanates and consume -NCO groups, so the amount of isocyanate must be adjusted accordingly. The manual also lists an explicit prohibition:

“Note: R940 cannot be used in addition-cured silicone (platinum-catalyzed) systems due to the risk of toxicity.” (Source: p62)

Q: Are there any special storage requirements?

Store in a cool, well-ventilated, dry warehouse at a temperature of 5℃-30℃, away from heat sources and direct sunlight, and strictly avoid exposure to high temperatures. The shelf life is 6 months under sealed conditions; if the shelf life has been exceeded, a small-scale trial is recommended first to confirm that there is no gelation and that the viscosity is normal before use. Use the contents as soon as possible after opening, and prevent contact with the air, which may cause autopolymerization.

Q: What should I do if I need a higher flame-retardant rating?

It can be compounded with small amounts of aluminum hydroxide and phosphate ester. This is a formulation-level matter, and this article does not prescribe a compounding ratio for the customer.

VII. Statement of Facts and Basis

All data in this article comes solely from 锎锔-功能助剂1.10.pdf, with source page numbers indicated alongside the text for item-by-item verification. Key original sentences cited from the product materials identify their sources in the main text and have not been rewritten. The manual lists a total phosphorus content of 1.5-3.0% in the recommended process section associated with UL94 V-0. This article treats it as a formulation target and does not present it as a product factory-release specification.

What was not done: fabricating specific specimen data for flame-retardancy testing or finalizing formulations on behalf of customers. The areas requiring formulation adjustments have already been specified—hydroxyl consumption of -NCO, avoidance of strong alkalis, and temperature control. Cost accounting and equipment compatibility, which are not covered by the manual, require separate confirmation.