Use of Optical Imaging/lmage Analysis System for the Quantitative Analysis of Surface Changes Induced by Outdoor Weathering on Sealants

ABSTRACT: Six sealant samples that had been weathered outdoors for 6.8 years in Japan were evaluated using an Optical Imaging/lmage Analysis System, Atlas VIEEW™. The specimens were first visually evalu­ated with aesthetic ratings assigned. These ratings were then used as the reference for the optical image analysis. Image analysis was carried out on sealant images taken under optimized diffuse illumination at a constant illuminance. Two sets of images were captured per specimen, first the weathered surface for deterioration evaluation, and second the unweathered surface as a control (reference) image. Four distinct surface defects are quantifiable in the samples. These are cracking (crazing), visual color change, spatial uniformity of deterioration (due to dirt pick-up and uneven color change, or both), and overall surface texture. Chalking and dirt pick up, as rated visually prior to this evaluation, could not be accurately as­sessed with the digital imaging technique. The analysis shows that surface cracking and crazing generally can be well characterized using the automated VIEEW™ system. Judging color changes visually is prob­lematic, since cracking and crazing interferes with color judgment. Further investigations are needed to develop an automated surface characterization method for sealants.

KEYWORDS: aesthetic appearance, sealant, cracking, crazing, color change, dirt pick-up, chalking, optical imaging, Image analysis

Introduction

The appearance of any building changes with time as a result of complex processes generally summarized under the term “aging.” While sealants can contribute to these processes, many aesthetic issues will appear on a building facade over time with or without the presence of a sealant. Aesthetic issues raised by sealants may be classified into two groups: those that affect the sealant itself and those that affect the adjacent building substrates. Aesthetic issues that fall within the first category are blooming, dirt pick-up, microbial growth, chalking, surface crazing, change of color, and change of gloss. Aesthetic issues, which affect the adjacent building substrates, are fluid migration and fluid streaking.

In 1995, the International Standardization Committee TC59/SC8 (Building Construction, Jointing Products) formed a Work Group (WG10) to assess the need for developing test standards on aesthetic issues caused by sealants. In its inauguration meeting in 1997, WG10 prioritized the development of test standards for aesthetic issues as follows:

High Priority: Fluid Migration (Substrate Staining)

Dirt Pick-up Chalking Surface Crazing

Medium Priority: Mold/Algal Growth Fluid Streaking

Low Priority: Blooming

Change of Color Change of Gloss

Good progress has been made by WG10 in the meantime on the issue of fluid migration, which had been rated as the highest priority work item, with an International Standard published: ISO 16938 Building Construction—Determination of the Staining of Porous Substrates by Sealants Used in Joints—Part 1: Test With Compression, and Part 2: Test Without Compression.

Concepts for a qualitative or semi-quantitative rating of aesthetic surface changes (dirt pick-up, chalk­ing, and crazing) were developed within WG10 based on pictorial references for general comparison to allow a classification of the severity of a sample’s defects. These methods, while admirably compensating for our visual limitations, did not meet the requirement for observer-independent ratings, since they do not fully eliminate the perceptual and psychological biases of the observer.

Therefore, the need was identified for an objective, quantitative method of rating the aesthetic surface changes occurring on weathered sealants that eliminates the bias of an inspector’s judgment. This paper describes an early attempt at investigating the feasibility of such a method based on a digital Optical Imaging/Image Analysis System that initially had been developed for the detection of defects in automo­tive topcoats.