Does your LSZH cable compound fail the smoke density test? Start your investigation with volatiles from the compatibilizer.

Against the backdrop of increasingly stringent fire safety standards, low‑smoke zero‑halogen (LSZH) cable compounds have become the material of choice for high‑occupancy spaces such as rail transit, buildings, and ships. The national standard GB/T 32129‑2015 Halogen‑free low‑smoke flame‑retardant cable compounds for wires and cables sets clear requirements for smoke density — flaming mode ≤100, non‑flaming mode ≤350. However, many cable manufacturers, after completing their flame‑retardant formulation design, frequently encounter failures in the smoke density test, and even experience significant batch‑to‑batch fluctuations with the same formulation. After extensive investigation, the problem may lie in an unassuming additive — the compatibilizer.

1. Residual volatiles in compatibilizers: the hidden driver of excessive smoke density

It is widely believed in the industry that smoke generation in LSZH compounds mainly comes from the release of water vapor from inorganic flame‑retardant fillers such as aluminum hydroxide (ATH) and magnesium hydroxide (MDH) at high temperatures, as well as from thermal pyrolysis of the base resin. However, one fact has long been overlooked: small‑molecule residues in the compatibilizer can contribute a considerable proportion of the smoke measured in the test.

During the production of compatibilizers (e.g., maleic anhydride‑grafted POE/PE/EVA), organic peroxide initiators (such as dicumyl peroxide, DCP) are introduced into the system to facilitate the grafting reaction. After the grafting reaction is completed, unreacted initiator fragments, ungrafted small‑monomer molecules, and reaction by‑products remain in the compatibilizer. Studies have confirmed that in the preparation of POE‑g‑MAH via twin‑screw reactive extrusion, the amounts of MAH and DCP added directly affect the grafting degree and product performance. When cable compounds containing these residues are extruded, small volatile molecules are released in the die zone; these volatile organic compounds undergo thermal cracking and incomplete combustion under flame or high‑temperature conditions, generating smoke that directly raises the smoke density value. This is why many companies, after optimizing flame retardants and adjusting the base resin, still cannot meet the smoke density requirement — the smoke contribution from the compatibilizer may have been overlooked.

2. Main sources of small‑molecule residues in conventional compatibilizers

The root cause lies in the inherent shortcomings of the production process for traditional compatibilizers. Three factors叠加 together make the residue problem prominent:

First, initiator residues and side reactions. To achieve a high grafting degree, a certain amount of peroxide initiator must be added to the formulation. Studies show that the addition of MAH and DCP in the grafting reaction reduces the melt index (MI) and increases the grafting degree; however, when the MAH/DCP ratio is too low, the extrudate surface becomes rough and melt fracture occurs. After the reaction, unreacted initiator fragments may remain in the system, and these fragments volatilize upon heating during subsequent processing, contributing to smoke density.

Second, the balance between grafting degree and gel content. Research confirms that as the initiator dosage increases, the grafting degree rises to a certain level and then slowly declines, while the gel content continues to increase until complete gelation. When the reaction temperature is raised, the grafting degree gradually increases, then slightly decreases and levels off, while the gel content can be maintained within a relatively stable range. This indicates that process control of the grafting reaction directly affects the purity of the product — excessive pursuit of a high grafting degree may lead to increased by‑products.

Third, lack of a devolatilization step. Most compatibilizer products are simply pelletized after reactive extrusion and packaged directly. Without an effective post‑treatment such as vacuum devolatilization or stripping, the residual small molecules are “sealed” inside the compatibilizer pellets and are released intensively during cable processing, acting as a “time bomb” for smoke density. Industry discussions have noted that the melt flow index of a grafted material can decrease by nearly 80% after being stored for nearly one month, possibly due to the slow reaction of residual monomers and initiators over time.

3. Controlling small‑molecule residues at the source: technical approaches for low‑volatility compatibilizers

To solve this problem, measures should be taken at both the synthesis and purification ends of compatibilizer production, focusing on three directions:

Direction 1: Optimize the initiator system and MAH content ratio. Studies show that as MAH content increases, the melt index decreases and the grafting degree increases; however, when MAH content reaches a certain threshold, the grafting degree may drop due to volatilization of excited‑state MAH. Properly controlling the ratio of initiator to monomer can ensure a high grafting degree while reducing by‑product formation.

Direction 2: Control reaction temperature and process conditions. Research confirms that when the reaction temperature is raised, the grafting degree first increases, then decreases and levels off, while the gel content can be maintained within a stable range; extending the reaction time causes the grafting degree to increase rapidly at first and then more slowly. Precise control of reaction temperature and residence time helps to minimize side reactions.

Direction 3: Add a vacuum devolatilization post‑treatment step. After pelletizing, the compatibilizer can be treated in vacuum devolatilization equipment under high temperature and negative pressure. Leveraging the high vapor pressure of small volatile molecules under vacuum, these residues are forcibly removed from the compatibilizer melt, ensuring that residual monomers and initiator fragments are thoroughly eliminated and the volatile content is reduced to a low level.

4. Troubleshooting approach for formulation engineers

For formulation engineers currently troubled by smoke density issues, the following path is recommended:

Starting point of investigation: If the flame retardants and base resin have not been changed, yet smoke density suddenly becomes high, shift your focus to the additive system. Prepare cable compound samples using different batches of compatibilizer, and conduct smoke density comparison tests under the same conditions. Observe the data differences. If the differences are significant, the problem is likely related to volatile control in the compatibilizer.

Verification method: Pay attention to the thermogravimetric loss data of the compatibilizer. This is the most direct way to assess the level of small‑molecule residues — the lower the thermal weight loss, the fewer volatile small molecules and the higher the cleanliness. Also monitor batch‑to‑batch consistency, as industry practice shows that fluctuations in grafting degree can lead to unstable product performance.

JP‑M13 cable compound compatibilizer from Shanghai Jiuju Polymer Materials Co., Ltd. is a POE‑based maleic anhydride grafted compatibilizer developed along the “low volatility, low exudation” technical route.

At the synthesis stage, it optimizes the initiator‑to‑monomer ratio and controls reaction temperature and process conditions; at the purification stage, it undergoes deep treatment via vacuum devolatilization to ensure thorough removal of residual small molecules. For cable compound formulations that aim to meet low‑smoke and low‑toxicity standards, JP‑M13 can reduce the negative impact of compatibilizer‑derived volatile small molecules on smoke density testing without compromising the filling efficiency of flame retardants. During processing, it exhibits low die‑build‑up and no surface exudates on finished products, and has been stably serving the LSZH compound systems of multiple cable manufacturers.

If you are also struggling with smoke density issues in your LSZH cable compounds, feel free to contact us. We can provide free samples and direct technical support from our engineers.