Foaming of clear topcoat

Concrete Floor Coatings Forensics Case History — Part 2

Concrete Floor Coatings Forensics Case History- Part 2

When examining a coating problem in the laboratory, background information gathered during discussions with the involved parties and information obtained during a field investigation help to direct the course of the laboratory investigation. The background information collected for this case was included in last month’s F-Files, “Concrete Floor Coatings Forensics Case History – Part 1, Field Investigation Phase.” The problem involves a previously painted concrete floor that was overcoated with two coats of beige epoxy and two coats of clear moisture-cured urethane. The floor exhibited blistering and delamination within the newly applied coating system.

The samples collected during the field visit were submitted to the laboratory along with the background information. After reviewing all of the information, the laboratory chose the following techniques to analyze the problem: detailed microscopic examination, infrared spectroscopy, ion chromatography, and gas chromatography-mass spectroscopy. In addition to samples from the field, the laboratory contacted the coating manufacturer for wet control samples of the materials specified for use.

Where to Begin? Detailed Microscopic Examination

Detailed microscopic examination was the first task performed during the laboratory investigation. The examination was performed to determine the number and thickness of the coating layers, as well as to determine the plane of failure and look for the presence of contamination or other objectionable properties. This gave the analyst the ability to compare and contrast the laboratory findings with the information provided from the field visit.

One interesting discovery made during the laboratory microscopic examination was the presence of foaming in the clear topcoat in a sample taken from the area near the exterior door. During the field interview, it was noted that the door was left open to increase ventilation during the application of the topcoat. However, this was done on a day when it was raining. Although the urethane requires moisture to cure, the moisture from the rain was excessive causing the coating to foam (Fig. 1).

Fig. 1: Foaming of clear topcoat. Photos courtesy of KTA-Tator, Inc.

The laboratory microscopic examination revealed the presence of five layers in samples from a non-failing area, and one-to-two layers from samples in failing areas. The five coats in the non-failing area were composed of the previously applied floor coatings, as well as the newly applied system. The one-to-two layers observed in the failing areas consisted of the clear topcoat as well as a thin layer of the beige coat. During an interview with the coating applicator, it was stated that two layers of the clear topcoat were applied. During the laboratory examination, only one clear coat was observed in the cross-section, but because the coats were the same color (clear) it was not possible to definitively state the number of coats based on the cross-section. However, the total thickness of the clear layer, determined microscopically, supports the claim that two layers of the clear coat were applied (Figs. 2-4).

The thicknesses of the coating layers were determined to be in accordance with the manufacturer’s recommended thickness ranges. Furthermore, no differences were found between the thicknesses in failing and non-failing areas. No evidence of dirt or debris was detected at the failing interface (Figs. 5, 6).

Fig. 2: Cross-section of non-failing area.

The plane of failure observed in the laboratory investigation sometimes differed from that determined by the field visit. During the field visit, the field investigator concluded that the plane of failure appeared to be between the clear coat and the beige coat with some beige discoloration noted on the bottom surface of the delaminating chips. Laboratory examination indicated that in some cases the failure was between the clear coat and the final beige coat with discoloration present, but in other cases it occurred within the surface of the final beige coat itself, leaving a measurable layer of beige coating (approximately 1 mil in thickness) on the bottom surface of the clear coat. This measurable layer indicated a cohesive break in the surface of the final beige layer as opposed to an adhesion issue between the clear and final beige coat.

Fig. 5: Discoloration on bottom delaminating chip.

The Right Stuff? Infrared Spectroscopy

By analyzing the infrared spectrum produced by a coating material, a great deal of information regarding the sample is revealed, such as generic resin type, potential problems with mixing, evidence of degradation and more. Just as your fingerprint is unique to you, the spectrum of a particular product is unique to it. While generic resins have similar bands in determined ranges, each product provides a unique spectrum. Two different epoxies will show different spectra even though the spectral bands will be similar.

Fig. 3: Cross-section of failing area.

Analysis of the coating system by infrared spectroscopy indicated that the materials used were chemically consistent with laboratory-prepared control samples of the specified materials. The clear topcoat was consistent with the specified urethane resin and the beige coats were consistent with the specified epoxy resin. These results confirm that the materials used had the same chemical composition as the specified coating system and that the specified materials were not substituted with a different coating system.

The laboratory-prepared control samples of the epoxy were mixed at the proper ratio and over- and under-catalyzed mixes were prepared to determine if mis-mixing of the components could have played a role in the failure. The analysis revealed that the samples obtained from the field were consistent with the properly mixed materials and that mis-mixing was not a concern.

Fig. 4: Cross-section of area below delamination.

Additionally, the bottom surface of the failing coating chips were rinsed with solvents, the run-off collected and dried, and the resulting residue analyzed for evidence of organic contamination such as oils or waxes. No presence of organic contamination was detected.

Contamination by Cleaning? Ion Chromatography

Ion chromatography testing was performed to see if contamination from the cleaning agent used in the field or other similar material may have contributed to the failure. Even though the cleaning agent was reportedly used during the preparation of the original coating, it could also have been used to remove any contaminants that may been deposited on the surface between coats. Ion chromatography identifies the presence of certain ions (anions) common in salts such as chloride, nitrite, bromide, nitrate, phosphate and sulfate. The safety data sheet (SDS) for the cleaning agent utilized prior to coating application indicated that the product was composed of phosphate-containing compounds. Testing revealed no detectable levels of any of the six anions associated with common salts on the submitted samples. Test results indicated that residual contamination from sources such as the cleaning agent were not the cause of the blistering.

Fig. 5: Discoloration on bottom delaminating chip.

Entrapment! Gas Chromatography-Mass Spectroscopy

The background information provided indicated that the clear coat may have been applied before the final beige coat was fully cured. Some of the samples sent to the laboratory were in sealed septum vials. These samples were placed in the vials immediately upon removal in order to trap any solvents that may be present. If any solvents were present in the vials, they could be identified using gas chromatography-mass spectroscopy (GC-MS). The gas chromatograph separates molecules at specific retention times as they travel the length of a specialized column installed in the instrument. Each molecule is fragmented by mass.

The analysis revealed the presence of solvents in the failing samples. The solvents were consistent with solvents listed in the SDS of the beige epoxy coats. The quantity of solvent materials was much higher in areas where blistering occurred as opposed to samples taken from areas that did not experience blistering.

Fig. 6: Cohesive break in bottom delaminating chip.

Conclusions

The laboratory findings were provided to the field investigator who issued a report combining the field observations with the laboratory data. The report included the following observations.

  • Preparation of the previously existing coating was satisfactory.
  • The new coating materials were mixed properly.
  • The thicknesses of the new coating materials were acceptable.
  • Moisture content of the concrete slab was low and did not create a problem.
  • The failure occurred between the clear topcoat and the final beige epoxy coat as well as within the surface of the beige epoxy itself.
  • In areas where delamination was noted between the clear coat and the final beige coat, a beige discoloration was visible on the bottom surface of the clear coat.
  • There was no contamination present at the plane of failure.
  • The failing samples from the area with a high incidence of blistering contained high concentrations of solvents from the beige epoxy.
  • The failing samples near the door exhibited foaming of the urethane.

Based on the field and laboratory observations and analysis, the cause of the failure was a combination of several application-related problems.

  • The blistering failure in the corner was due to poor ventilation that retarded the cure of the epoxy. The urethane was applied before the epoxy had cured, entrapping solvents and creating a weak plane at the top surface of the epoxy, causing both an adhesion issue between the clear topcoat and the beige epoxy, and a cohesive failure within the top surface of the epoxy.
  • The variance in the plane of failure indicated that some areas of the beige coat may have come closer to curing than other areas, but in both cases, it was evident that full cure was not reached.
  • The failure near the door was due to exposure of the clear urethane coats to moisture by applying the coating with the doors open while it was raining.

About the Author

Chrissy Stewart is a senior chemist with KTA-Tator, Inc. Employed with KTA since 2006, she is heavily involved in coating failure investigation and comparative coating testing services. Stewart has achieved SSPC Protective Coatings Specialist (PCS) certification, is a voting member of ASTM and a past president of the Pittsburgh Society for Coatings Technology (PSCT) where she currently serves on the Board of Directors. She holds a Bachelor of Science degree in chemistry from Mercyhurst University, has had several articles published in JPCL and was featured inJPCL’s 2015 annual bonus edition, “Coatings Professionals: The Next Generation.”

By Chrissy M. Stewart , PCS, 

Richard A. Burgess, PCS, Series Editor KTA-Tator, Inc.

 

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