A look backstage to show you how a conventional interior paint earns MPI approval
Each year, paint manufacturers send MPI hundreds of products to be tested to one of our 240+ different paint standards. MPI’s test laboratory performs a defined regimen of tests on the product and Approves those that pass. The product is then published on the corresponding MPI Approved Products List, and a specifier may be confident that any product listed will perform to the expectations aligned with that standard.
Architects and specifiers need know only this to effectively use MPI’s Specification Manuals and Approved Products Lists to choose products for their projects.
But those who wish to elevate their knowledge and understanding of paints and coatings (or perhaps the cynics among you) might ask: why does one product meet MPI #138 Latex, Interior, High Performance Architectural, Gloss Level 2 —but another product you’ve used with great success in the past does not? And how does the performance for a paint that passes high-performance MPI #138 differ from that of a conventional latex like MPI #44?
This article series takes you backstage to explain how we test a paint before we declare it Approved, starting with performance requirements for conventional interior latex paints. We’ll explain what tests we run; what aspect of performance that particular test validates; and how a paint has to ‘score’ on that test in order to pass.
“Why isn’t this paint on an MPI Approved Products List?”
Listed Products versus Non-listed Products
If you don’t see a particular paint product on an MPI Approved Products List, there are three possible reasons:
- The manufacturer hasn’t submitted that product to MPI. MPI testing and approval is a voluntary process for paint suppliers – and not all paint manufacturers choose to send products to MPI for approval. Furthermore, even paint suppliers who work with MPI regularly don’t always send every product in their line for approval.
- The product is approved by MPI, but under a different standard # (aka Category). This is especially common with latex paints: depending on its VOC and durability, a low-sheen (MPI Gloss Level 2) interior latex paint could be listed under either MPI #44; MPI #138; or MPI #144. A paint could also be listed under multiple standards if it meets the requirements of each of those standards.
- The product was submitted for approval but didn’t pass. But we’ll never tell. MPI maintains a staunch standard of privacy around test results as part of our agreement with the paint manufacturers we work with.
So, what does “good” look like for an approved product?
Requirements for Conventional Interior Paints
While MPI standards for conventional interior latex paints have some requirements around expected paint material characteristics –consistency, dry time, appearance and applicability – most tests are around performance requirements like flexibility, hiding power, alkali resistance, gloss, and scrubbability.
Most MPI standards require this test that measures how well the coating remains adhered to the surface after being bent or deformed. We apply a sample to a tin-plated panel, bend the panel over a ¼ inch mandrel, and examine the film for any evidence of loss of adhesion, cracking, peeling.
Hiding power — also referred to as opacity or contrast ratio — measures the product’s ability to obscure a background of contrasting color. To measure hiding power, we apply a stripe of the sample to a standard black-and-white opacity chart. We then use a spectrophotometer to measure the white and black section and compare the ratio. As you might expect, the hiding power required for a primer is less than that required for a topcoat; products that pass MPI #50 Primer Sealer, Latex, Interior must have a contrast ratio of 95 or higher; a topcoat like MPI #54 requires 98 or higher.
Sample with insufficient hiding power
Sample with good hiding power
Since latex paints are frequently specified for masonry – and masonry can be a high-alkali surface — MPI latex paint standards call for application of a 2% sodium hydroxide solution to the coated panel to verify no signs of breakdown or deterioration.
In addition to helping define the appearance of the finish, gloss also influences performance characteristics like how well the surface hides imperfections (lower gloss), and cleansability and durability (typical of higher gloss). But specifiers can’t rely on how the manufacturer names their product to know what they’re getting: the terms “eggshell” “velvet” or “satin” are purely subjective and loosely refer to a paint with a fairly low gloss.
So MPI has quantitative standards that define seven different gloss levels, ranging from Gloss Level 1 up to Gloss Level 7 High Gloss. Every MPI standard that’s defined by a gloss level requires that the sample be tested with a standard glossmeter to validate it fits within the range of the standard; regardless of how the manufacturer names it.
We measure gloss at 60 degrees (like looking at a wall almost head-on). And for lower gloss standards, we also measure sheen at 85 degrees (similar to looking down a long wall at an oblique angle).
So for a product to meet a standard labeled MPI Gloss Level 5 (Semi-Gloss), the product’s gloss measured at 60 degrees must fall between 35 and 70. For MPI Gloss Level 2, the product’s gloss measured at 60 degrees must be a maximum of 10 and its sheen at 85 degrees must fall between 10 and 35.
It is noteworthy that gloss is less a performance measure and more a tool to standardize the specifier’s expectation of what the finished surface will look like. When a product “fails” because it fell out of range for MPI’s gloss requirement for that particular standard, it’s less an indication of performance and more a sign that the product belongs in a different MPI category with a different MPI gloss level.
This is a signature test of durability for interior service paints. The goal is to test how well the coating withstands abrasion either from incidental contact or repeated scuff. The test is performed by placing the coated test panel in a “scrubbability tester,” where a nylon brush in a set track runs back and forth across the panel while being continuously wetted with a detergent solution. The sample passes if after the required number of scrubs there is no breakthrough along the entire length of the panel or significant loss in gloss. The number of scrubs required to pass goes up based on these factors:
- the gloss requirement of the standard. Higher gloss products should have greater scrub resistance, so while MPI #44 Interior Latex Conventional Gloss Level 2 calls for 1500 scrub cycles, MPI #54 Interior Latex Conventional, Gloss Level 5 (Semi-Gloss) requires 2000 scrub cycles
- the performance expected from the standard. MPI High Performance Interior Latex standard #138 Gloss Level 2 requires 4000 scrub cycles, versus MPI #44 Gloss Level 2’s requirement for 1500.
Laboratory Scrub Tester
A sample that failed scrub testing: note the breakthrough
and total removal of the paint film
Burnish Resistance For lower gloss level products, the burnish test validates the coating’s resistance to appearing shinier after rubbing – like when employees sitting in the board room grind the back of their chair against the wall. We use a tester similar to that used for scrubbability, but with a chamois cloth instead of a nylon brush, and the test is done dry (so no liquid/detergent added). “Fail” is a change in sheen that exceeds the limit of the standard: for MPI 44, that’s 5 units; for a high-performance standard like MPI 144, it’s 3 units. NEXT MONTH: Testing high performance interior latex, ‘green’, and exterior paints
Perform no exterior painting work unless environmental conditions are within MPI and paint manufacturer’s requirements or until adequate weather protection is provided. Where required, suitable weatherproof covering and sufficient heating facilities shall be in place to maintain minimum ambient air and substrate temperatures for 24 hours before, during and after paint application.