Outline Chapter 2 will provide background and discuss the current state-of-the-art of the composition and manufacture of sandwich panels, as well as of the theories of adhesion in current literature. The instrumentation used in this work is examined and described in Chapter 3. From there on the results of the analytical work are presented in Chapters Chapter 4 details the analysis of the substrates used in the sandwich panels and the effects of each of the standard treatments to achieve the finished coated coils. In Chapter 5 the effects of common surface treatments are reported. Chapter 6 reveals the properties of the foams used in this product. In Chapter 7 analysis of the buried interfaces of the finished product are introduced. Chapter 8 examines the diffusion of components of the foam into the coated coil stock surfaces. Finally in Chapter 9, a study of a panel that displays blistering is presented. The last part of the thesis then concludes the work and makes suggestions with the aim of improving the performance of the final product. Literature Review Introduction As the aim of this investigation to increase the understanding of the nature of adhesion in the sandwich panel and help predict and make suggestions towards an improved product, this chapter provides a background relevant to the current investigation at the time of its initiation. As such it reviews some of the literature relevant to this project. The manufacturing process of sandwich panels is described and the typical components introduced, including a view of the expected chemistry, starting with the steel used in the substrate, the effects of treatment processes from cleaning to coil-coatings and the
be important in understanding how inadequate performance might occur, are detailed below. The metal substrate used in this application is usually galvanised steel for standard building sandwich panels, although other substrates can be used for specialised use, such as stainless steel for refrigerated buildings. These materials are developed to provide the structural and cost benefits of steel but with the increased corrosion resistance provided by the zinc coating which acts as both a protective barrier and also a sacrificial (anodic) component, corroding preferentially to the steel. There are different ways to apply a zinc coating but the type used in this application is a continuous galvanising hot-dip process, as shown in Figure 22 where the large coil is being fed into the series of baths containing the galvanising solutions. Figure Continuous galvanising plant As can be seen schematically in Figure 23 a coil of suitable cold rolled steel, of controlled standardised composition with respect to its alloying components, is initially cleaned using either an acid or caustic degreasing bath to remove dirt, oil and grease from the surface. Next is a pickling process in hydrochloric acid, which removes mill scale and other oxides. This is followed by fluxing, where any final oxidation products formed since the pickling is removed and the surface is coated with a temporarily protective layer which also facilitates a good bond in the galvanising process. The coil is then annealed at a variety of temperatures and is now ready to be immersed in a bath of molten zinc at 438-460°C (for 0.5% aluminium alloy). Generally many variants of these zinc coatings are often (erroneously) referred to as HDGS. Exact bath composition is proprietary to the steel manufacturer although these must be within guidelines established by relevant standards. Certainly it is known that the surface analysis of HDGS often shows significant variation in its surface chemistry. The types of zinc coated steel substrates, which are to be considered of particular importance commercially, are listed below in order of decreasing manufactured quantities, where the HDGS product forms the vast majority of product. The ArcelorMittal recognised designation is shown in brackets:
Oxygenated species functionality are being introduced to the surface as a result of CDT, leading to changes in surface energy On CDT of Types 1-3 coatings extraneous surface species removed revealing core organic components used in the bulk of the binder. CDT on Type 4 material removes organic materials revealing inorganic components It is evident that there are further topographical, morphological and chemical changes on CDT which will become relevant in understanding how the polyurethane foam precursor components interact, which will be relevant in later Chapters. In the next Chapter the polyurethane foams which are applied to the coated substrates are characterised. Polyurethane Introduction In the preceding Chapters the substrates, coatings and treatment processes were examined, in this Chapter a comprehensive understanding of the filling of the sandwich panel, the polyurethane foams, is provided. This includes analysis of the individual components. As reported in the literature review the findings of Shimizu demonstrated that polyurethane foam catalysts can migrate through to an interface when polyurethane foam is applied to a number of different metals. Thus the characterisation of the principal components used in the manufacture of the polyurethane foam as well the typical formulation used in the Arcelor product was carried out. The full formulation is made up of many chemicals; catalysts are required in the isocyanate PUR/PIR processes as well as for the blowing process. The addition of surfactants is included to help control the cell size and structure. These tend to silicone based and are known to show a characteristic peaks at 147 u (Si2C5H15O+). Flame retardants are included in the formulations to comply with fire standard within the construction industry. Blowing
intended, others not, sometimes giving multiple peaks at almost every nominal mass unit. This is demonstrated in Figure 711 which shows the assignment of the peak used to demonstrate the polyester in the backcoat, C8H5O3+. In this example the data is integrated over the whole 500µm square, 128 x 128 pixel dataset instead of the 32 x 128 used to construct the linescan. The information presented starts at region 6 (6x500µm) this is where the backcoat was most evident. It finishes at region 9 (9x500µm), which was obviously into the cellular region of the panel. This gives information from 2501 – 4500µm displacement on the plane of the cut surface, from an arbitrarily taken point in the zinc of the hot dipped galvanised steel. In region 6 an obvious shoulder is seen and is highlighted in red, this is reminiscent of either a charging effect or the tail often seen on a silicone molecule. By region 7 and becoming more evident as the scanned area moves further into the polyurethane a second peak becomes apparent. This second peak is identified as SiC5H13O3, a fragment from the PDMS used as a surfactant. Accuracies for both peaks are correct to within 100ppm deviation from the exact mass of fragment ion under consideration indicating an excellent level of certainty of the identity of the ions. It is evident that these peaks will influence the information gained through the imaging. Figure 711 High resolution 149u mass peak assignment; sample 3 regions 6 to 9 The ability of using the SIMS data to investigate regions of interest was further employed to investigate further the region where the backcoat to polyurethane interface should be found. Spectra from different areas of the sample surface were compared in order to monitor ions present. A region of interest was identified in the 500µm square where both ions could be seen and thus likely reveal useful information; this is shown in Figure 712, indicated by the blue arrow. In this region the line scans indicated approximately median intensities for the polyester, MDI and Catalyst ions over their area integrated 125 x 500µm RPI section.
failure. The substrate was determined to be stainless steel, indicating that it came from a refrigerated store or similar location. Although little time remained to embark on a thorough experimental programme revolving around this panel specifically, with the gained understanding of the nature of the adhesion now established for the sandwich panels it is possible to have an understanding of the causes of these blisters. This type of failure is the most problematic for the manufacturer as it occurs with products already in use. As noted in the literature temperature variations in use can cause stresses in the materials which have been linked to the appearance of the blister failure. Although it is unlikely to cause an issue regarding safety or the structural integrity of the building, or even affect the thermal insulating properties, customers for this product consider the unsightly appearance of these blisters, which are very apparent on the outside surface of the building, unacceptable. Experimental method Introduction Shown in Figure 91 is the panel as received, it was approximately 2.5 x 1m. It demonstrates the area and appearance of the blistered region. It is important to note that this failure occurs on the exterior face of the building as this implies that this is the blister has occurred to the side which the foaming polyurethane is applied. Figure 91 Received blister failed panel Sample preparation When this panel was cut by hacksaw there was an obvious stress relieving effect in the substrate as it straightened (from being bent to form the blister) and delaminated from the foam bulk. This can be seen in Figure 92. Also demonstrated is the detailing of the formed substrate to which the foam was applied. As can be seen it is complex with a design that obstructs the flow of a fluid.