Troubleshooting Adhesives & Sealants (Part 1)
In the adhesive and sealant business, one often deals with problems involving product performance and failures. Getting to the root cause of adhesive and sealant failures requires taking a closer look into the product's components and formulation.
CAS-MI Laboratories uses analytical chemistry to investigate common problems with adhesives and sealants. Part 1 of this article discusses six causes of adhesive failures, including case studies. Part 2 will introduce the art of deformulation and formulation of adhesive and sealant materials.
The causes of adhesive and sealant product failures fall into six main categories:
1. Poor Material Selection
It is important to choose an adhesive that is appropriate for the type of materials being used. Otherwise, serious quality problems can occur.
Case Study: A client needed to test a concrete block sealer for below grade applications. The ASTM Test Method D7088-04, "Standard Practice for Resistance to Hydrostatic Pressure for Coatings Used in Below Grade Applications Applied to Masonry", specified using 8 inch by 8 inch by 8 inch concrete blocks, metal plates, and rubber gaskets to make a box construction for testing the ability of water to penetrate a coating applied to the masonry block under low pressure (4 psi). A construction adhesive was used to bond the coated block to the vinyl clad medium density fiberboard. However, this poor material selection led to water leaking easily through the seal at the bottom of the block. Therefore, the construction adhesive was replaced with silicone adhesive. It was then possible to pressurize the construction and perform the testing without the adhesive failing.
2. Inadequate Amounts of Adhesive
An adhesive may work well in a particular application, but the correct amount of material needs to be used in order for it to adhere properly.
Case Study: A UV curable adhesive was used to assemble two plasticized PVC parts. Some parts were found to fail randomly in a quality test that involved forcing water under pressure through the assembly. As a result, water leaks occurred. An investigation of good and suspect assembled parts was conducted using Scanning Electron Microscopy (SEM). It was found that there were voids present between the two mating surfaces in the failing parts. The voids were found to be due to an inadequate amount of adhesive present. Small changes were made in the automated applicator for applying the adhesive, solving the problem.
3. Incomplete Cure
An adhesive cannot be expected to perform if it has not been completely cured. The low molecular weight will lower the strength of the adhesive.
Case Study: A new formulation for an epoxy sealant was found to delaminate from a steel can after thermal cure. A comparative study was conducted between a control sample and the can with the delaminating sealant. A number of potential failure mechanisms were postulated including incorrect epoxy application, contamination, and insufficient epoxy curing. A cross-sectional analysis was conducted to address the issue of incorrect epoxy application. The film thickness met specifications. Contamination was addressed by conducting solvent extractions which were analyzed by Gas Chromatography/Mass Spectrometry (GC/MS) and Fourier Transform Infrared Spectroscopy (FTIR). No contaminants were found. Also, the sealant surface was examined using Scanning Electron Microscopy/Energy Dispersive X-Ray Analysis (SEM/EDXA). There was no difference in surface elements present between the control sample and the delaminating sample. Modulated Differential Scanning Calorimetry (MDSC) then found that there was residual heat in the delaminated sample compared to the control. This residual heat was evidence that the epoxy sealant had not fully cured in the delaminating sample. A recommended process change prevented further delamination issues.
4. Incorrect Mixing
Incorrect mixing can lead to failures since the full potential strength of the adhesive system cannot be achieved.
Case Study: In a large commercial office building project, a two component urethane adhesive/sealant system was used to form a seal around windows to hold the glass in place. Soon after the windows were installed, leaks were noticed around some windows, leading to the damage of interior walls. Gaps had formed at the edges of the windows due to thermal contraction during cold weather. It was suspected that a mistake had been made in mixing the material prior to application. The ratios of the two components were compared using Fourier Transform Infrared Spectroscopy (FTIR). The FTIR data from the failed material did not match that of the material mixed at the correct ratio, proving that there had been a mistake in the mixing of the two component system.
5. Contamination
Contamination of an adhesive or sealant can occur when something migrates into or out of the material. There are many chemical ingredients in the adhesive and its environment that may have a better affinity to adjacent materials and hence want to migrate.
Case Study: A client was using a pressure sensitive adhesive applied to a film carrier to provide a moisture barrier around exterior doors and windows. The tape was applied above and around the windows, but the tape lost adhesion after only a few days of exposure. Analysis by Liquid Chromatography/Mass Spectrometry (LC/MS) indicated that a plasticizer from the film had migrated into the adhesive layer, causing loss of adhesion to the substrate.
6. Physical Changes
Physical changes in an adhesive can prevent the adhesive from achieving its full potential. Changes in the adhesive such as UV degradation and thermal degradation can lead to molecular weight loss and hence performance failures. Additionally, abrupt temperature changes can cause an adhesive to fail due to excessive stresses within the system.
Case Study: A client noticed problems when trying to hermetically seal a freezer pack using a new adhesive formulation. After only a few days, the seal released. They suspected that the new adhesive's formulation, a polyurethane-acrylic hot melt was the issue. A comparative deformulation was conducted on the old and new hot melt adhesive formulations. Although many analytical techniques were used, no significant differences could be found in the new and old formulations. Finally, Gel Permeation Chromatography (GPC) analysis of the two samples indicated that the new formulation had a lower molecular weight, indicating thermal degradation had occurred. A formulation adjustment was recommended which solved the problem.
