Peering at a Paint Film Cross-section

What Happened?

A company based in the United States of America uses a timber product known as Medium Density Fiberboards (MDF) to manufacture interior furnishings, such as tables, skirting boards and window sills. The MDF is sourced from many different locations, so the quality of the product they receive can vary. One particular batch of boards was causing significant issues during post-processing, as the final painted boards had a rough, gritty surface texture. The supplier of this batch was otherwise reputable and trusted, with a good track record of customer service, so the cause of the issue was not clear. I was asked to investigate the cause of this MDF painted surface defect.

I received two samples of the MDF boards which were showing issues. Unfortunately, no unpainted samples from that batch were available for comparative examination. The painted surface did have intermittent surface defects, in the form of small, hard particulate matter embedded beneath the painted surface. I began my examination.

“ I had to take a sideways approach using metallographic techniques....The particulate contamination must have originated from the board itself as supplied.”

How did we analyse it?

Because the small particles had been painted over, I had to take a sideways approach using metallographic techniques. My examination had to be on a cross-section through the paint film, to expose the particulate matter. I cut three specimens from each of the MDF boards, which I mounted (encased) in epoxy resin. Using a suite of metallography equipment, I was able to grind horizontally through the MDF and paint film, until some of the hard particulate matter was revealed. The exposed cross-section was polished to a fine, smooth finish for the next section: microscopy examination.

The microscopy examination used a combination of SEM (scanning electron microscopy) and EDS (energy dispersive x-ray spectroscopy) to view and photograph the sample in high resolution, and determine the elements present within the sample. One of the hard particles was found in the paint film cross-section, examined in closer detail, and chemically analysed.

The elemental analysis identified this particle to silicon carbide, a material which is often used as an abrasive for sanding process. This particle was found right in the middle of paint layer, and therefore must have either been within the paint as it was painted on, or upon the surface of the board before the paint was applied. Luckily, there was a retained sample of the batch of paint that had been applied to the board sample I received. I was able to filter the paint sample for any particulate contamination within the paint, but none was found. The particulate contamination must have originated from the board itself as supplied.

What was the resolution?

I prepared a technical report on my findings and conclusions which was issued to my client, and ultimately on to their supplier. My scientifically-backed findings prompted the supplier to conduct a thorough investigation of their own processes, particularly their abrasion steps. A misalignment in their equipment under certain conditions was found, which resulted in the abrasive materials being pressed into the board surface with excessive force, thereby embedding silicon carbide particles in the board surface.

The issue was remedied, further repeat incidents were prevented, and my client was reimbursed for their difficulties.

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