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The physical and mechanical properties of laminates can be influenced by the types, general characteristics, and pressing conditions of the papers and resins used in their production. The goal of this study was to determine the effect of layer structure on some of the physical and mechanical characteristics of high-pressure laminates (HPL). Laminates with five different layer structures were produced. Laminate types included standard laminate, laminate with a barrier layer, laminate with a barrier and an overlay, laminate with an overlay, and laminate with a barrier and an aluminum oxide overlay. Specimens chosen from these laminates were subjected to resistance to wear, scratching, staining, steam, dry heat, cigarette burns, cracking, breaking, and color change under xenon arc lamp light tests. Specimens were also subjected to immersion in boiling water tests in conformance with the EN 438 standards. Results obtained from these tests showed that the arrangement of different layers does not influence resistance to cracking, breaking, or change of color under xenon arc lamp light but does influence the other resistance characteristics. Use of an aluminum oxide overlay positively contributes to all of the resistance characteristics.
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Presently many materials are used as interior linings including imitation leather, plastic-covered textiles, plastic films, and to a large extent laminate boards. Paper-based decorative plastic laminate boards (in short, decorative laminates or laminate boards) are manufactured from paper and plastic. Plastic-impregnated papers are pressed between steel plates into a homogeneous board at an elevated temperature under particularly high pressure (i.e., 130[degrees] to 170[degrees]C and 100 kgf/ [cm.sup.2]). Decorative laminate contains two different types of plastic and five types of paper. The core consists of kraft paper and phenol-formaldehyde (PF) resin. Thickness can be considerably changed by varying the number of core papers in the laminate. The visible surface consists of decorative, printed, or unicolored paper and a completely transparent overlay. Both of these papers have been impregnated with melamine-formaldehyde (MF) resin, which is a hard, clear substance highly resistant to heat. If the decorative paper and overlay are omitted from the laminate, the product is called industrial laminate or technical laminate. This type of laminate is mainly used in machine parts and in furniture (e.g., on the reverse side of table tops to provide balanced construction). If the color of the decorative paper is light and unicolored, then the dark-colored core paper could cause the light and unicolored decorative paper to darken under high temperature and pressure. To prevent darkening, a white-colored barrier paper impregnated with MF resin can be placed between the decorative paper and the core paper. The core paper is placed in the center to provide symmetry (balance) and thus, the balancing paper impregnated with MF resin is placed on the back surface of the laminate to decrease the risk of deformation (Ahonen 1977). Laminate boards produced with this structure are used extensively, mostly in furniture, especially kitchen furniture, in passenger transport vehicles such as ships, buses, and trains, and in door and wall coverings.
The types, general characteristics, and pressing conditions of the papers and resins used in production influence the physical and mechanical properties of the laminates. In addition to the use of barriers and overlays, chemicals can be used to improve the antibacterial, antistatic, and antiradiation properties of the laminate and materials can be included between the layers to improved insulation and heat resistance (Balsells Coca and Cistero 1999, Frisk et al. 2000, Krebs et al. 2001, Yang and Trabbold 2005, Sumin and Hyun-Joong 2006). Both the chemical substances and additional materials can influence the usage properties of the laminate. If the kraft papers used in production are not homogeneously treated with resin, the surface quality of the laminate can be negatively impacted (Roberts and Evans 2005).
In practice, manufacturers are producing laminates by pressing the core and decorative layers together without using other layers in an attempt to reduce their costs and remain competitive in the global market. Therefore, whether the resistance characteristics of the laminates are sufficient and whether using barriers and overlays is necessary is being questioned. In order to address these questions, the goal of this study was to determine the influence of different layer structures on some of the physical and mechanical characteristics of the laminates.
Materials and methods
Materials
High-pressure laminate (HPL) boards with five different layer structures were manufactured to determine the effect of different layer structures on some of the physical and mechanical characteristics. Laminate types studied were standard laminate (L1), laminate with a barrier layer (L2), laminate with a barrier and an overlay (L3), laminate with an overlay (L4), and laminate with a barrier and an aluminum oxide overlay (L5). The layer structures, paper used, general characteristics of the resins, and the pressing conditions of the laminates produced are given in Table 1.
Specimens, with the characteristics specified in EN 438, were taken from the laminated boards produced in accordance with the general structure given in Table 1. Surface wear, surface scratching, immersion in boiling water, staining, steam, dry heat, cigarette bums, cracking, breaking, and color change tests were conducted. Three specimens were used for the surface wear and immersion in boiling water tests and two specimens were used for the other tests as specified in the standard. All of the tests were conducted in the laboratories of the Turkish Institute of Standards (TIS 2001a, 2001b). The dimensions of the specimens, the climatization conditions, and the method for applying each test are given in Table 2.
Resistance levels of the specimens were found by comparing surface wear and surface scratching values obtained at the conclusion of the tests with the standard values given in Table 3; by comparing the immersion in boiling water, staining, steam, dry heat, cigarette bums, cracking, and breaking values with the standard values given in Table 4; and by comparing the color change values with the standard values given in Table 5.
Findings and data analysis
The results obtained by the tests for the effect of some characteristics of the layer structures on the laminates are given in Table 6.
The Kruskal-Wallis unidirectional analysis of variance was applied for determining whether there were differences among wear, scratching, and boiling water resistance values, which are the relative resistance values of the laminates with different characteristics (Table 7). It was accepted that the differences in the "Z" table values above 1.64 with an error rate of 5 percent were significant and differences below this were insignificant (Ozdamar 2004).
Resistance to surface wear
According to the number of cycles of wear resistance connected to the types of laminates with different layer structures, the most resistant laminate type was the laminate with a barrier and an aluminum oxide overlay (L5) (12,433 cycles). This type of laminate was followed by the laminate with a barrier and an overlay (L3) (659 cycles), the laminate with a barrier layer (L2) (600 cycles), and the laminate with an overlay (L4) (488 cycles). The lowest resistance to wear was observed in the standard laminate (L1) in which a barrier and an overlay were not used.
The differences among the resistance to wear values of the L2, L3, and L4 laminates were insignificant (Table 7). But, the differences among the resistance to wear values of these laminates and the L1 and L5 laminates were significant. Accordingly, the use of a barrier and/or overlay in the production of laminates increased the resistance to wear. The highest resistance to wear was obtained in the laminate with an aluminum oxide overlay (L5).
Resistance to scratching
The surface scratching values of the different layer structures are given in Table 7. The highest scratching force values (5 N) were observed in the standard laminate (L1), laminate with a barrier layer (L2), and laminate with a barrier and an aluminum oxide overlay (L5). These three laminate types were followed by the laminate with an overlay (L4) (4 N). The laminate with an overlay was within the scope of L3 according to the scratching force and the related standard. The other laminate types were within the scope of L4.
The differences among the scratching values of the L1, L2, L4, and L5 laminates were insignificant. Whereas, the difference among the scratching values of these laminates and the scratching values of the L3 laminates were significant. If it is taken into consideration that the laminate with an overlay is the laminate having the second lowest scratching value, then these results show that the use of an overlay in the production of laminates decreases the resistance to scratching characteristic.
Resistance to immersion in boiling water at 100[degrees]C for 2 hours
The thickness and mass increase at the conclusion of immersion of laminates with different layer structures in boiling water for 2 hours are given in Table 6. The highest increase in thickness was observed in the laminate with a barrier layer (L2) at 20.82 percent. This was followed by the standard laminate (L1) at 20.53 percent, the laminate with a barrier and an overlay (L3) at 17.97 percent, and the laminate with an overlay (L4) at 17.05 percent. The lowest increase in thickness was observed in the laminate with a barrier and an aluminum oxide overlay (L5) at 15.45 percent. According to the EN 438 standards, it is necessary for the increase in thickness of the standard laminate to be 12.9 percent, the laminate with a barrier layer to be 12.8 percent, the laminate with a barrier and an overlay to be 12.1 percent, the laminate with an overlay to be 12.7 percent, and the laminate with a barrier and an aluminum oxide overlay to be 12.3 percent. According to these results, the values for the increases in thickness that occurred were high and were not in conformance with the standards. The use of an overlay decreases the increase in thickness. This decrease, however, is insufficient for conformance to the standards.
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