SURFACE PROPERTIES OF MDF COATED WITH CALCITE/CLAY AND EFFECTS OF FIRE RETARDANTS ON THESE PROPERTIES

Th e coating of wood and wood panel surfaces basically serves for surface protection and its surface can be improved at various user areas. Diff erent application methods are used for the coating of MDF surface. Th ese are methods such as curtain coating, spraying, rolling, knife, etc. In this study, suitability of pigment coating method instead of the traditional surface coating methods used for coating MDF panels and the eff ects of fi re retardants on surface properties of MDF coated by the best appropriate coating mixture was investigated. Calcite and clay were used as coating pigment and latex, urea formaldehyde, and melamine formaldehyde were used as adhesive. Coating/adhesive mixture obtained was applied to MDF panels with knives. Before the analysis, test samples were put to condition room and they were kept in there for 1 week. According to the results obtained, the best appropriate coating material was found as calcite. 22% concentrated melamine formaldehyde as adhesive and 0.25 mm as coating thickness was determined better than other combinations. Borax, boric acid and zinc borate as fi re retardant were added to calcite /melamine formaldehyde with 22% mixture. So, the eff ects of fi re retardant on surface characterization were determined. Surface properties of coated MDF panels tested were found lower than standard requirements except for abrasion resistance.


INTRODUCTION
Wood based panels may be exposed to many harmful factors such as fungus, insects, termites, and humidity, low and high temperatures at their areas of use.In order to reducing the damage caused by these factors, wood based panels are coated by using di erent materials and di erent coating applications according to using conditions.In this industry, two di erent coating materials are used in solid or liquid form.Panels to be laminated can be coated with polyester lms, phenol kra papers, PVC or PVA/Urea based decorated papers, papers impregnated with di erent resins, papers with ammoniumsulphamate, thin papers, foil papers, wooden veneers as solid coating materials and lacquer paint as liquid coating materials (Bozkurt and Göker 1986).
Performance of panel surface during coating process is evaluated by characteristics of wood species and manufacturing method (Cassens andFeist 1991, Richter et al. 1995).Basic requirement for durable coating performance on the surface of wood-based panels is a good adhesion between the panel surface and the coating material.Manufacturing parameters such as the mixture of wood particle, the using ratio of adhesives, surface treatment, storage and conditioning of the boards appeared to be closely linked with the surface properties and their performance during coating (Barbu et al. 2000).e di erences in thickness of the coating lm performance of particleboards during coating process may be due to a many reasons, e.g., the di erence in the moisture content of the boards, hygroscopic e ect of resins, di erences in the curing conditions and homogenous application of the paint (Rolleri and Ro ael 2010).
Coating process is widely used in paper industry as well.Coating process can provide extra smoothness, quality, strength, brightness, etc. to paper products (Eroğlu and Usta 2004).
is process can increase surface quality of panels and improve its surface and color properties (Nemli 2000).In coating process, which is a commonly used technique, coating material/ adhesive mixture is applied on paper.Basic materials of the mixture include coating material (clay, calcite, etc.) and adhesive.Nowadays, calcite and clay are commonly used in coating process.Calcite and clay are important coating materials which can provide extra strength and good surface quality to paper and paper products.Calcite and clay can also be used as lling materials.Generally, there are three stages followed in coating applications.e rst stage is the preparation of the coating material, choosing the right adhesive for bonding the coating material and the last stage is the application of the mixture properly.All coating materials are added to water and all of them are mixed by blender until this mixture is homogenous.Clay, calcite, TiO 2 are commonly used as coating materials on paper industry; and starch, casein or soy protein and synthetic resins are also used as bonding materials (Casey 1961).
Panel board manufacturers try to obtain a decorative surface with coating process on particleboard (PB) and medium density berboard (MDF).ese panels are produced as homogenous and at surface.e panels have excellent surfaces with application of coating materials.ese coated panels are utilized in the construction of cabinets, furniture, paneling, kitchen worktops, laboratories, and other industrial product applications.e performance of the coated panels is dependent on the quality of wood based panel and the type of the coating material (Sparkes 1993, Hoag 1992).Niazi and Gertjojensen (1979) found that coating of panel surfaces improved the dimensional stability of wood based panels.Groah et al. (1984) and Grigoriou (1987) also stated that formaldehyde emission decreased due to coating of wood based panel surface with vinyl lm and melamine impregnated paper.Nemli (2003) researched the e ects of melamine-impregnated papers on the surface of particleboard and found that coated surface of particleboard improved the physical, mechanical properties and decreased the formaldehyde emission of particleboard.MDF panels have smother surface and more homogenous and tighter skin than other panels.ese properties are advantageous in coating of surface (Romer 2002).
e calcite and clay are important coating materials since they increase strength and improve physical and surface properties.At the same time; borax, boric acid or zinc borate can be added to coating material/adhesive mixture as a re retardant depending on materials or the area of use (Casey 1961).Chemicals used as re retardants generally may decrease many properties of wood and wood panels.It was found that re retardant with boron decreased mechanical properties and surface quality of wood panels (Baysal et al. 2006, Ustaomer 2008).erefore, the objectives of this study were to evaluate surface characterization of medium density berboard (MDF) coated by calcite and clay, and to choose the suitable surface coating material type, mixture ratio (%) and adhesives at areas of use.e e ect of re retardants on surface quality was also researched because studies done on this subject are not enough and this paper can give new information to this eld.

MATERIAL AND METHODS
MDF panels and adhesives (Latex, melamine formaldehyde (MF) and urea formaldehyde (UF) used in this study were obtained from Kastamonu Integrated Forest Industry Inc. (Turkey).
Calcite and clay were provided as solid from Farma Lab Inc. dimensions and thickness of MDF panels used were selected as 250 x 400 x 12 mm.e surfaces of all panels were sanded to obtain a smooth surface and a good bonding.Preparing conditions and using ratios of coating materials were given in table 1. Ratios of materials used were determined according to dry mass of calcite and clay.Materials used in this study were added to water and all of them were mixed by blender until it became homogenous.Coating material was applied with knife to the panels.Color of all panels is white and their surfaces are smooth.Figure 1 shows how to apply coating process.Coating process was applied to the surface of all samples.ree replicates were done for each coating material/adhesive mixture.Coating material/adhesive mixture was applied on all samples with knife.icknesses of coating/adhesive mixture were 0.15, 0.20 and 0.25 mm.A er coating process, di erent properties of coated panels were determined.Tests investigated, tests performed and standards tracked were given in table 2. According to the results obtained, borax, boric acid and zinc borate as re retardant were added to the best mixture.Ratio of re retardants were 1, 2, and 3%.Resistance to water steam, the resistance of the laminated particleboard to hot steam is measured by steam test. is test can be regarded as an endurance test to simulate the e ect of the in uence of steam.Resistance to stain is a test of the sensitivity of a surface to the e ect of foreign substance.Evaluation table for stain and steam test was given here; 5 degree: No observable changes, 4 degree: little change in color and brightness, 3 degree: little change in color and/or brightness, 2 very large change in color and/or brightness, 1 degree: delaminating or/and blistering.

Surface Characterization of MDF coated by mixture of calcite/clay
e results were showed that adhesives used and coating materials have an important e ect on surface properties.E ect of melamine formaldehyde on surface quality was determined to be higher as compared with other adhesives.It was also found that all surface properties (expect for abrasion test) were lower than the related standards.Abrasion values were determined to be higher when melamine formaldehyde was used as coating material compare with the other adhesives.It was also found that while the amount of melamine formaldehyde and coating thickness increased, the surface quality of panels promoted as well.e best surface quality of panels was obtained at 22% melamine formaldehyde and 0.25 mm coating thickness.It was found that when coating thickness was increased from 0.15 mm to 0.25 mm, abrasion and scratch increased.At the same time, it was determined that calcite and clay have not important e ect on surface quality.All data obtained were given table 3-12.Surface roughness of coated MDF samples was determined with Bendtsen device according to ISO 8791/2 standard.Surface roughness was measured as milliliters/sec.and this device uses air for measuring surface roughness.e device sends air to sample and surface roughness is found as owing amount.If the owing amount is high, the surface of panel is not smooth.If the owing amount is low, the panel has a smooth surface.As seen in table 3-12, Adhesive type, using ratio and coating materials were determined to have an important e ect on surface roughness.Datum obtained showed that there is a linear relationship between abrasion and resistance to scratch.According to results, the best appropriate adhesive, coating material and coating thickness were found as melamine formaldehyde (22%), calcite and 0.25 mm, respectively.Ozdemir et al. (2009) researched the e ect of humidity on surface roughness.e results appear that surface quality of the panels adversely in uenced from increasing relative humidity.When relative humidity served as a variable, the surface quality of coated MDF panels revealed that values of such samples increased as compared to those of samples at initial condition (Hiziroglu et al. 2008).Istek et al. (2010) evaluated the e ect of decor paper and resin type on surface quality properties of particleboards coated with decor papers impregnated by using di erent resins.e results revealed that specimens coated with melamine formaldehyde-impregnated papers showed better performance than those coated with urea formaldehyde and urea + melamine formaldehyde-impregnated papers.It was found that resin types and paper pattern a ected surface properties, such as impact, scratch, and abrasion, resistance to staining and cigarette burn.However, it did not change the cracking and steaming properties of the coated samples.
According to table 13, it was found that surface properties of MDF coated by 70% clay-30% clay, 50% clay-50% calcite mixture, and 30% clay-70% calcite at 0.25 mm thickness were lower than 100% calcite and 100% clay mixtures.Resistance to water steam was found to change between 1 and 2 degree, but resistance to stain was not changed to all formulations.It is 1 degree, which are the lowest values for all formulations.Consequently, resistances to stain and water steam were found to be low.Scratch was determined to be ≥1.5 N for all coated panels.Surface roughness for all samples ranged from 1315 ml/min to 1370 ml/min.e best abrasion strength was found to 100% calcite.As a result, using clay-calcite mixture as coating materials didn't cause an important di erence on surface properties of all samples.Using only calcite or only clay was seen to be better than using calcite-clay mixture.erefore, 100% calcite was used to determine the e ects of re retardants on surface characterization of MDF.Borax, boric acid and zinc borate were added to mixture as 1%, 2% and 3% ratios of re retardants.Melamine formaldehyde was also added to have to bonding between coating formulation and panel surfaces.e mixture obtained was applied to panels at 0.25 mm thickness.e e ects of added re retardants such as borax, boric acid and zinc borate on surface properties were given table 14, 15 and 16.According to table 14, adding of borax did not change cracking and stain as compare with control samples.However; abrasion, scratch were decreased and surface roughness decreased to 1% and 2%.But; it did not change to 3% adding of borax.As seen as table 15, 1% boric acid decreased abrasion but, it increased surface roughness.According to control samples, adding of 2% boric acid had a lightly increasing e ect on abrasion and scratch.Boric acids for all adding ratios (1%, 2%, and 3%) also have an increasing e ect on surface roughness.Increasing of surface roughness value indicates deterioration of the surface.Adding of boric acid did not cause an important e ect on resistance to water steam, cracking and stain.In table 16, abrasion was found to be lower as comparison with control panels.Zinc borate was a ected all surface roughness except for resistance to cracking.All ratios of zinc borate improved scratch and resistance to stain.However; it decreased abrasion, resistance to water steam and surface roughness according to control samples.As a result, surface roughness had generally a change for worse by adding of re retardants.Whereas, zinc borate and boric acid were found to increased scratch.Ayrilmis et al. (2006) investigated the e ect of various re retardants such as borax, boric acid, monoammonium phosphate and diammoniumphosphate on the surface roughness of plywood.Signi cant di erence was determined between surface roughness parameters (Ra; Roughness Average, Rz; Average Maximum Height of the Pro le, Rmax; Maximum Roughness Depth) for four treatments and two retentions of re retardants.Samples treated with 3%concentration of borax had the smoothest surface with 11:09 mm Ra while the roughest surface was found in samples treated with 6% boric acid having Ra value of 12:44 mm.Results revealed that the surface quality of the panel decreased as the ratio of re retardants increased.Previous work on FRT blockboard plywood found that increased chemical concentrations gave higher retention values which in turn resulted in better re resistance characteristics of the samples (Laufenberg et al. 2006).However, this study indicates that surface roughness of the panels can be adversely in uenced by increased concentrations of re retardants.
Rolleri and Ro ael (2010) studied on coating and surface properties of particleboards.e results obtained shows that, in general, higher values of roughness of uncoated particleboards lead to thinner coating lms on the surface.Particleboards made with fresh and recycled particles showed higher values of average roughness (Ra) and higher adherence strength to the coating compared to particleboards with cork particles, which had a relatively smooth surface.Not only the roughness of the surface but also interaction between the primer and surface seems to be a very important factor determining the strength of adhesion between primer and the surface.

CONCLUSION
Coating of panel surfaces is an important factor for end-use application of wood based panels.In this study, properties of panels coated by Latex, MF and UF and e ects of re retardants on these properties were investigated.According to the results, all coating materials used caused an important di erence on panel surface properties and the best results for all formulation used was obtained for melamine formaldehyde as adhesive and 0.25 mm as coating thickness.Calcite (100%), clay (100%) or mixture of clay/calcite (30%-70%, 50%-50% and 70%-30%) did not cause a signi cant di erence on surface quality.Surface quality of panels coated with only 100% clay or only 100% calcite was found to be similar when compared to panels coated by mixture of clay/calcite.But calcite was found to be better than the other formulations.erefore; calcite was used for determining of e ects of re retardants on surface properties.Borax, boric acid, and zinc borate were added to 100% calcite.e lowest value of abrasion was determined for 1% zinc borate and boric acid.Nevertheless, adding of borax (1%) was decreased more than the other ratios of re retardants as comparison with control samples.As seen as scratch properties, it increased by adding of zinc borate and boric acid.Surface roughness was determined to a ect negatively for all formulations except for 2% and 3% adding of boric acid.As results, the adding of re retardants were not improved the properties of the panel surfaces.

Figure 1 .
Figure 1.Coating Part and panels obtained a er coating process.

Table 1 .
Preparing conditions and amount of coating material.

Table 2 .
Test done a er coating process.

Table 13 .
Surface characterization of MDF coated by clay/calcite mixture

Table 14 .
Surface characterization of MDF coated by Coating Material/Borax.

Table 15 .
Surface characterization of MDF coated by Coating Material/Boric Acid.

Table 16 .
Surface characterization of MDF coated by Coating Material/Zinc Borate.Surface properties of MDF coated...Istek et al.