Passive fire protection (PFP) coatings are used to protect marine offshore and other industrial structures from fire. They differ from active methods of protection in that they require no additional action, such as activation of a sprinkler system, to function. Several varieties of PFP are applied across different markets and applications. Epoxy-based PFP coatings are used where light weight, exterior durability and corrosion resistance are required, such as exterior structural steel in the offshore and petrochemical industries.
The chemistry of epoxy PFP coatings, their application and their typical end uses, mean unusual properties and operational requirements. Among these is the requirement for good adhesion well below room temperature – and well below normal service temperatures for other coatings. Hydrocarbon PFPs used in the petrochemical and offshore industries are often applied in extreme weather regions where temperatures reach as low as minus 40°C.
Until recently, there had been little published research available on the reactions of these coatings to extreme changes in temperature. As a result, PPG Protective and Marine Coatings undertook to examine the low temperature adhesion and tensile strength properties of epoxy based intumescent coatings.
Understanding Low Temperature Operations.
Intumescent epoxy PFP coatings are usually mastic-based, meaning that they are applied at high film thickness. The thickness required can be 100 to 200 times that of a typical coating. Like most crosslinking coating systems, there is a net reduction in the volume of the coating on curing or drying that increases with crosslink density.
Because epoxies have a ring opening in the cure mechanism, they tend to have lower but still significant volume reduction compared with other coating chemistries. This shrinkage introduces residual stress within the coating, which increases with coating thickness and crosslink density. The net shrinkage in a system will also fluctuate with temperature – at higher temperatures, cross-linked coatings may continue to cure leading to a moderate increase in the internal stress of a coating. Organic materials have a larger temperature-related length fluctuation than metals, which becomes important when organic coatings are applied to metal substrates that are then subject to heating and cooling.
Because epoxy PFP coatings can be used to protect exterior structural steel it is important to understand a coating’s response to temperature variations.
It is typical to accelerate test results by exposing samples to higher than normal temperatures to take advantage of the increase in reaction rate with increased temperature. For example, salt fog testing, using ASTM B-1171 is run at 35°C, and accelerated UV exposure testing using ASTM G 154 can subject the test specimen to temperatures as high as 80°C. Because of this, high temperature behaviour of coatings is fairly well understood. However, in real-world applications, most exterior structures experience temperature well below 25°C and the behaviour of coatings at low temperatures is less well understood.
Low Temperature Performance Testing.
At their expected service temperatures, the amount of shrinkage in epoxy PFPs will be high, increasing the amount of internal stress. If the internal stress exceeds the adhesion force to the substrate, the coating may relieve this internal stress by dis-bonding from the substrate or cracking. Neither result is desirable and can compromise the integrity, durability, corrosion and fire protection properties of the PFP. But the question remains: do flexible PFP coatings maintain those flexible properties at low temperatures, and what happens to hard, brittle coatings as the temperature drops?
In order to test the low temperature properties of these coatings, two standard test methods were employed: tensile testing and lap shear testing.
Tensile testing determines the strength of materials by measuring the amount of force necessary to pull them until they break. Test methods such as ASTM D6383 use a sample shaped like a dog bone, with wide areas at the end for grip and a narrow test area with a known cross-sectional area. During the test, a sample of the coating is pulled at a constant rate, measuring the percentage elongation and maximum stress required to break the sample. Elongation is generally expected to decrease with decreasing temperature, while maximum stress typically increases.
At some temperatures, the polymer binder in the coating will pass its glass transition temperature, below which it will behave more like a glassy solid than an elastic solid. This means that below the glass transition, a coating will have more of a tendency to snap than to tear, and will often take more stress to break. However, the individual performance will vary based on the coating according to factors such as the architecture of the resin and the nature of the fillers used. It is therefore necessary to consider more absolute measurements like elongation at expected service temperature rather than simply noting whether a coating will be used below its glass transition.
Typical reporting properties for tensile tests are strain, maximum stress, and modulus. Strain, or percentage elongation, is simply a measure of how far a material can be pulled before it breaks, and is typically reported as a percent increase in length. Maximum stress or tensile strength has units of pressure, and is the amount of force used to break the sample divided by the cross sectional area in which the break happens. Modulus is often interpreted as a measure of the stiffness of a material. As the slope of the stress/strain curve, it also has units of pressure.
Lap shear testing, described in ASTM D10024, is typically used for measuring the bond strength of adhesives. Two pieces of a stiff substrate, steel in the present study, are glued together using the subject material, and then pulled apart using the same equipment as a tensile test. The area of contact is either known or measured so that the force used to pull the sample apart can be normalized, similar to the known cross-sectional area used in tensile testing.
After testing, the failure is characterised as either adhesive (that is, the material pulling from the substrate) or cohesive (failure within the bulk of the subject material). The percentage of cohesive failure is typically reported along with the maximum stress, as in tensile testing. As with tensile testing, polymeric coatings or adhesives are expected to behave differently above and below their glass transition temperature.
To provide some conclusive data that could be used by industry when selecting appropriate systems, PPG undertook studies to explore the low temperature mechanical properties of PFP coatings.
Conclusions and Areas for Future Study.
Low temperature lap shear and tensile testing has shown differences in PFP coatings that may prove to be useful tools in predicting long term durability, adhesion, and resistance to stress induced cracking in thick film intumescent coatings.
The coatings selected for the PPG research were chosen deliberately to reflect extremes of possible real-world performance and reflected the expected behaviours. At lower temperatures, the elongation of hard, brittle coatings approached zero whereas the flexible coatings retained significant elongation. The flexible system increased substantially in tensile strength as the temperature decreased, surpassing the maximum strength of more brittle material at any temperature. The more flexible coatings tended to show less adhesive failure to the substrate in lap shear testing, but in all cases, adhesive failure (as opposed cohesive splitting within the coating) was most pronounced at the lowest temperature. These results match general expectations of coatings at low temperature.
Due to their chemical nature, epoxy coatings continue to cure throughout their service life. Coatings are typically tested once they have achieved an initial set after several days. This degree of cure is seldom fully measured or understood. A possible avenue of future research would be to determine cure by end group analysis and compare mechanical properties to measured degree of cure as the epoxy PFP is exposed to the standard NORSOK M-501 test method for durability and to measure low temperature properties.
In addition, it would be of interest to extend the low temperature lap shear study to blasted and primed substrates, in order to understand whether a surface profile might improve the stress results. Use of primers, especially low shear strength zinc rich primers, may show more telling differences at low temperatures. The results also indicate that PFP properties for temperatures above room temperature should also be explored since glass transition temperatures for some of the commercial coatings tested seem to occur around room temperature or slightly above.
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