White powder appearing on a rubber seal is easy to dismiss as dust or surface contamination. However, when the deposit develops after vulcanization and returns after cleaning, it may indicate a deeper formulation or processing problem.

In this case, a rubber product manufacturer found white, powdery deposits on the surface of newly produced nitrile rubber (NBR) seals. The defect affected the appearance of the finished parts and was associated with reduced sealing performance.

Xinbodi Laboratories was asked to identify the surface deposit, determine its source, and recommend corrective actions.

The investigation combined SEM-EDS, XRD, and TGA to distinguish external contamination from rubber blooming and connect the analytical findings with the compound formulation and vulcanization process.

The Problem: White Powder After Vulcanization

The customer observed irregular white deposits on the surface of molded NBR sealing rings shortly after production.

The powder was not evenly distributed. Some areas showed visible white particles, while other parts of the seal appeared normal. Cleaning improved the appearance temporarily, but did not eliminate the underlying cause.

Several possible explanations were initially considered:

• External contamination during handling
• Mold-release or cleaning-agent residue
• Migration of rubber additives
• Unreacted inorganic ingredients
• By-products generated during vulcanization
• Poor dispersion of the curing or activation system

Visual inspection alone could not determine which explanation was correct. A rubber surface deposit analysis was therefore required.

SEM-EDS Analysis of the White Deposit

Scanning electron microscopy with energy-dispersive X-ray spectroscopy was first used to examine the morphology and elemental composition of the white material.

The SEM images showed that the deposit consisted of irregular particles distributed across the NBR surface.

EDS analysis detected three dominant elements:

• Zinc
• Sulfur
• Oxygen

This result ruled out ordinary dust and suggested that the white powder was related to the rubber compound’s zinc-containing curing system.

However, SEM-EDS identifies elements rather than complete chemical phases. The presence of zinc, sulfur, and oxygen did not by itself prove whether the deposit was zinc oxide, zinc sulfide, zinc sulfate, or a mixture of compounds.

X-ray diffraction was therefore used for phase identification.

XRD Identified ZnS and ZnO

XRD analysis confirmed that the white deposit contained a mixture of:

• Zinc sulfide (ZnS)
• Zinc oxide (ZnO)

The ZnO finding indicated that part of the zinc oxide used in the compound had remained unreacted or poorly incorporated into the rubber matrix.

The presence of ZnS suggested that zinc-containing ingredients had also reacted with sulfur-containing components during vulcanization. This interpretation is consistent with the reported zinc-mediated sulfur vulcanization mechanism, in which zinc-containing intermediates participate in accelerated rubber curing and ZnS formation.

Together, the results supported a rubber blooming mechanism rather than external contamination.

The white powder was generated within the rubber formulation and subsequently accumulated at the surface.

TGA Linked the Deposit to the Rubber Formulation

Thermogravimetric analysis was used to compare the deposit with the inorganic fraction of the NBR compound.

The results indicated that the amount of high-temperature residue was associated with the zinc oxide level in the formulation.

This finding was important because zinc oxide is normally used as part of the activation system in sulfur-cured rubber. Its presence was not itself abnormal. The problem was the combination of:

• Excessive zinc oxide loading
• Inadequate dispersion
• An unsuitable balance within the curing system
• Process conditions that encouraged surface migration or precipitation

The rubber compound analysis therefore showed that the defect could not be solved simply by cleaning the finished seals.

The formulation and vulcanization process needed to be corrected.

Root Cause of the NBR Seal Blooming

Based on the combined analytical evidence and discussions with the manufacturer, the rubber failure analysis identified several contributing factors.

Excess Zinc Oxide

The zinc oxide level was higher than necessary for the formulation. Excess material increased the amount of zinc-containing species available to remain in the compound or migrate toward the surface.

Poor ZnO Dispersion

The zinc oxide was not uniformly dispersed throughout the rubber matrix. Localized concentrations made it easier for zinc-containing particles and reaction products to accumulate near the surface.

Interaction with the Sulfur-Curing System

During vulcanization, zinc oxide participated in reactions involving sulfur and accelerator DM.

Under the customer’s formulation and processing conditions, part of the system produced ZnS, while some ZnO remained unreacted. These zinc-containing compounds contributed to the visible white bloom.

Vulcanization Process Problems

The review also identified process conditions that could promote the defect:

• Excessively high curing temperature
• Insufficient compound degassing
• Inadequate mixing and dispersion
• Limited control of the two-stage curing process

The final conclusion was that the white deposit resulted from a combination of formulation imbalance, poor dispersion, and unsuitable vulcanization conditions.

Corrective Actions

The laboratory findings were translated into several practical recommendations.

Reduce the Zinc Oxide Level

The customer was advised to reduce the ZnO dosage while maintaining sufficient activation for the sulfur-curing system.

The correct amount should be confirmed through formulation trials rather than changed without validation.

Improve Zinc Oxide Dispersion

A surface-treated nanoscale zinc oxide was recommended for evaluation to improve compatibility and dispersion within the rubber compound.

The mixing process was also reviewed to reduce local agglomeration and create a more uniform distribution of inorganic ingredients.

Optimize the Activation System

The stearic-acid and zinc-oxide system was adjusted to improve zinc distribution and reduce the likelihood of poorly incorporated zinc species remaining in the compound.

Because the optimum ratio depends on the complete rubber formulation, the revised activation package required controlled production trials.

Use a Two-Stage Vulcanization Process

A more controlled two-stage curing process was recommended to improve cure uniformity and reduce the formation or migration of surface deposits.

The process was designed to balance:

• Compound flow
• Degassing
• Cure development
• Reaction uniformity
• Surface cleanliness

Strengthen Mixing and Degassing

Additional mixing and improved degassing were recommended to reduce trapped gases, ingredient agglomeration, and local formulation differences.

These changes were intended to address the source of the bloom rather than only its appearance.

Project Outcome

After the customer adjusted the zinc oxide level, improved dispersion, optimized the curing system, and strengthened process control, the white powder problem was eliminated.

The finished NBR seals showed:

• Cleaner and more uniform surfaces
• No recurring zinc salt deposits
• Improved appearance consistency
• A significant increase in sealing-performance pass rates
• Better production stability

The outcome demonstrated that rubber blooming is not always a cosmetic issue. In this case, it was evidence of a formulation and process imbalance that also affected product quality.

Why Multi-Technique Rubber Bloom Analysis Matters

White deposits on rubber products can originate from many sources, including waxes, sulfur, accelerators, lubricants, plasticizers, inorganic fillers, cleaning residues, or environmental contamination.

A deposit cannot be reliably identified from its color or texture alone.

In this investigation:

• SEM-EDS identified zinc, sulfur, and oxygen.
• XRD confirmed ZnS and ZnO phases.
• TGA connected the inorganic residue with the rubber formulation.
• Process review linked the analytical findings to ZnO loading, dispersion, and vulcanization.

Each method answered a different question. Together, they provided a defensible root-cause conclusion and a practical correction plan.

When Should You Request Rubber Failure Analysis?

A rubber failure analysis may be appropriate when a component develops:

• White powder or surface bloom
• Cracking or hardening
• Discoloration
• Tackiness or oily deposits
• Loss of elasticity
• Excessive wear
• Swelling
• Poor adhesion
• Unexpected odor
• Reduced sealing performance

Where possible, provide both the defective component and a known-good reference sample. Information about the compound, curing conditions, production history, storage, and service environment can also improve the investigation.

Need Rubber Bloom or Surface Deposit Analysis?

Xinbodi Laboratories provides rubber failure analysis, rubber compound analysis, surface deposit identification, and formulation troubleshooting for seals, gaskets, molded rubber components, and elastomer products.

Tell us:

• What defect appeared
• When it developed
• Whether it returns after cleaning
• Which formulation or process changes were made
• Whether a normal reference sample is available
• What production decision the results must support

Our technical team will review the samples and recommend an appropriate rubber bloom analysis plan.