Polymer-Plastics Technology and Engineering, 45: 1295–1300, 2006 Copyright # Taylor & Francis Group, LLC ISSN: 0360-2559 print/1525-6111 online DOI: 10.1080/03602550600948913
Enhancement of Physico-mechanical Properties of Plywood Surface with Urethane Acrylate (M-1200) by UV Curing Method A. M. Sarwaruddin Chowdhuryy , M. Mizanur Rahmany , and Haran C. Das Department of Applied Chemistry and Chemical Technology, University of Dhaka, Dhaka, Bangladesh
Md. Nurul Islam Department of Chemistry, Dhaka University of Engineering and Technology (DUET), Gazipur, Bangladesh
Mubarak A. Khan Radiation and Polymer Chemistry Laboratory, Institute of Nuclear Science and Technology, Bangladesh Atomic Energy Commission, Dhaka, Bangladesh
Several UV-curable formulations containing urethane acrylate (M-1200) oligomer with a trifunctional monomer, trimethylol propane triacrylate (TMPTA), and photoinitiator Irgcure-369 were developed to improve the surface of plywood. Thin polymer films were prepared on glass plate with these formulated solutions and finally applied on polished plywood surface to modify the surface, and both were cured under UV-radiation. The properties of UV-cured thin films were studied as a function of UV-radiation intensities. Pendulum hardness (PH) and gel content were found to increase with the increase of TMPTA concentration up to a certain level and with further increase of monomer concentration the physical properties were decreased. The polymer-coated surface yielded enhanced physico-mechanical properties compared to that of unmodified surface. Keywords Physico-mechanical properties; Plywood; Polymer coating; Surface mutation; Urethane acrylate; UV curing
1. INTRODUCTION Natural polymer and their products are degradable and decomposable, but not very durable and stable, and as such cannot be suitably used in diversified applications in the same way as man-made materials. Natural materials are, therefore, being improved through genetic metamorphosis, to modify some characteristic properties as well through Address correspondence to M. Mizanur Rahman, Department of Chemistry and Chemical Technology, University of Dhaka, Dhaka-1000, Bangladesh. Tel.: þ 880-2-8126522; E-mail:
[email protected] y Authors have similar contribution.
thermoplastic formations[1–4]. There is a fast growth of the radiation processing technology in the field of coating in recent years, to improve different materials such as wood, partex, leather Hessian cloth (jute), paperboard paper, etc.[5–8]. At present, this field has drawn much attention to the development of the surfaces of plain board, plywood and low quality wood. Previously some work was done[9–12] to improve the quality of low-grade materials such as wood, partex, etc. by UV-cured epoxy coating. Moreover, UV-radiation induced cross-linked polymerization of photosensitive oligomers, important for the surface shield of materials, because of the fast cure and therefore high quality coating was obtained[13]. Thus, the steady development of the UV curing technology in the past few decades is a direct consequence of this unexpected behavior and it has opened the way for an ever increasing member and end uses[14–15]. As a result of its distinct advantages, this technology has found a large number of industrial applications, in particular, for the surface protection of different materials by polymeric formulations. The recent surge in applications of UV-induced photopolymer ions is motivated by at least two factors such as environmental concerns about the production of volatile organic emissions and the need for a high-speed reaction to enhance production rates. Recently, most of the work on UV-initiated photo polymerization was focused on a freeradical system based primarily on acrylate monomers. These monomers polymerize rapidly and are easily modified at the enter functionality, allowing materials with a variety of properties to be obtained. In Bangladesh, as in
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TABLE 1 Composition of different polymeric formulations (% w=w) Formulation Oligomer Monomer Photoinitiator name (M-1200) (TMPTA) (Irgacure 369) MEK A1 A2 A3 A4 A5 A6
50 30 20 10 10 5
48 68 78 88 87 93
2 2 2 2 2 2
– – – – 1 –
many other countries, plywood has always been a construction material. It is a kind of board made by sticking thin layers of wood on top of each other. Its properties such as its higher strength relative to its weight and low heat conductivity made it advantageous for use in construction and decorative purposes. It is generally used in doors, panels, windows, and furnitures in Bangladesh. However, plywood has some drawbacks; all the inherent properties of plywood are not favorable for diversive uses; and some of them restrict its versatile and potential applications. Plywood is hygroscopic, and is dimensionally instable with variation of moisture content. This eventually results in weeping and uneven distortion. The nonabrasive character and the resistance to weather conditions of plywood are relatively low. Under some favorable conditions, termites, bacteria, insects, microbes, and fungi can destroy plywood structure in a short period of time, and plywood is susceptible to combustion as well. This study is related to the modification and characterization of plywood surface coated with urethane acrylate (M-1200) mixed with trimethylol propane triacrylate (TMPTA) monomer and cured under UV-radiation.
2. EXPERIMENTAL 2.1. Materials Urethane acrylate (M-1200), an Aronix product, was procured from Toagosei Chemical Industry Co. Ltd., Japan. This oligomer is almost solid and trimethylol propane triacrylate (TMPTA), a trifunctional monomer, were procured from Radcure (Drogenbos, Belgium). Methyl Ethyl Ketone (MEK) was used as an antibubbling agent, and was obtained from Merck, Germany. Irgacure-369 (IRG-369) has an absorption band of 200–450 nm, and was used as photoinitiator during the UV-radiation process to initiate photochemical reactions. It was procured form Ciba-Geigy (Switzerland). Plywood was collected from the local market of Bangladesh.
2.2. Methods Different formulations were developed with urethane acrylate (M-1200) in combination with reactive monomer TMPTA, antibubbling agent ethyl methyl ketone (MEK) and photoinitiator IRG-369 in the proportions, as mentioned in Table 1. Thin polymer films were prepared under UV-radiation by coating a glass plate with these formulated solutions using a bar coater (model no. 0.0018 obtained from Abbey Chemicals Co., Australia). This produced films of 36 mm thickness on the plate. A UV manicure machine (IST Tecknik Germany, model Me-200-UV) was used to cure the film on the plate (as well as on the plywood surface) using a UV lamp (254–313 nm, 2 kW). The substrate was run under the lamp at a speed of 4 m min–1 with the help of a conveyor belt interlocked with the UV-radiation system. The mini cure has the efficiency of 61%. 2.3. Polymer Film Characterization Polymer films prepared from different formulating solutions were characterized after 24 h of curing under UVradiation. Film hardness, an index of cross-linking density at the surface of the film, was measured by using a pendulum hardness (PH) tester (model 5458, Byke Labotron, Germany). Gel content, amount of cross-link through the entire cured film, was determined by wrapping a known weight of the film in a soxhlet, for extraction with hot acetone for 48 h. The difference of weight of the film before and after extraction determines the gel content. The extracted film was soaked in acetone for 24 h at room temperature (approximately 25C). The difference of weight of the swollen film with that of the dry extracted film yields the swelling ratio. The cured films were carefully peeled off from the glass plates and these were cut with the help of a dumbbell cutter for tensile property measurement. The tensile properties of films and leathers were measured by using tensile strength machine (Instron, model-1011, UK) maintaining a gauge length of 0.50 in. at a crosshead speed of 0.01 in. min–1. Tensile strength (TS) and elongation at break (Eb) were directly obtained with the help of a personal computer integrated with the machine. 2.4. Application on Plywood Surface Plywood samples were cut into small pieces (size 10 5 cm) and dried in an oven at 105C to remove the free moisture until a constant weight is achieved by each sample. The surfaces of the plywood samples were polished smoothly with the help of a suitable sand paper. Then it was coated first with a base coat using a drawn down bar (no. 0.0028) and cured by a single pass under the UV lamp. It was polished again and coated with a top coat using the bar no. 0.0018, and finally the coating was cured under UV light at different intensities (number of passes). Surface
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polymers cross-linking at higher doses[16]. However, the mechanism of this degradation is not clearly known, and it could be the cleavage of the polymer chain caused by the overdose of UV-radiation. For all the formulations, the maximum PH values are obtained in between 12th and 14th passes of radiation doses. We have observed that formulation A4 containing 88% TMPTA yielded the highest PH (93.45%) after 12 passes of radiation followed by A5. TMPTA has trifunctional acrylated groups and thus has a branchlike behavior that creates more cross-linking among the ingredients of the formulation[17]. Although formulation A6 contained 93% TMPTA, it even revealed lower hardness (PH ¼ 84.8%) than A5 (PH ¼ containing 89.7%). This means that TMPTA increases surface hardness of cured polymer films to a certain level and an excess amount of TMPTA (in this case more than 88% in the formulated solutions) diminished the surface hardness.
FIG. 1. Pendulum hardness of UV-cured films on glass plate against radiation doses.
hardness was measured by pendulum method as described above. The glossiness of the modified or muted surface was measured at two angles 60 and 20 using a Micro Gloss Meter (Sheen-155) from Sheen Co., UK. Abrasion wear of the coated plywood surface was measured by the Taber abrasion method using a Taber abraser (model 5130 of Richness Co., Germany). The lower Taber index indicates better resistance to abrasion by the cured film. The adhesion ability of the UV-cured surface was determined by measuring the force that was applied to peel a certain portion of the cured film with the help of an adhesion tester (model 525) from Erichsen Co., Germany.
3.2. Gel Content The gel content is a representation of cross-linking density through the entire films. Figure 2 shows the gel content of polymer films against the number of passes. The maximum gel content value (97.65%) is achieved by formulation A4 with similar PH after 14 passes of radiation doses, because it contains more TMPTA (88%) (Table 1), and TMPTA has trifunctional acrylated groups that have a branching effect[18] and can yield more cross-linking in the cured film. A multifunctional vinyl monomer promotes rapid free radical propagation reaction[19] leading to network (cross-linking) structures. When TMPTA
3. RESULTS AND DISCUSSION It is necessary to characterize UV-cured polymer films before applying on the surface of plywood. Most of the data presented in this report are averaged values of at least six different results. 3.1. Surface Hardness of the Polymer Film (Pendulum Hardness) Surface hardness, an index of cross-linking density at the surface of UV-cured thin polymer films prepared on a glass plate was determined by the pendulum method. Results obtained for the PH of the films prepared at different UV doses represented by the number of passes are graphically shown in Fig. 1. The PH increases with the UV dose, attains a maxima, and then decreases. The decrease of the PH could be caused by the degradation of
FIG. 2. Gel content (% cross-linking) of UV-cured urethane acrylate films on glass plate at different radiation intensities.
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concentration is increased, the amount of residual unsaturation is also increased as a consequence of faster rate of formation of the three-dimensional network, causing restricted mobility at an early stage. The cross-linking rate, especially during the early stage of radiation, is proportional to the TMPTA concentration[17]. The second highest gel content is obtained by the formulation A3 (95.7%) followed by A2 and A5. The maximum gel content varies between 97.7 and 91.2%. This is a sign of very good cross-linking phenomena among the various constituents present in different formulations. 3.3. Tensile Strength (TS) Tensile property is very important in selecting diverse applications of polymer and polymer-coated flexible substrates. Results of tensile strength (kPSi) of UV-cured polymer films are graphically shown in Fig. 3 plotted against the intensity of UV-radiation. It is found that TS increases with radiation intensities and after the maximum strength is achieved, the TS then decreases. The maximum TS is obtained between 12th and 14th passes and after those TS reduces. This may be caused by radiation deterioration of the polymer at higher doses[10]. However, the highest TS (8.5 kPSi) is given by the formulation A3 followed by A4 and lowest TS at 14 passes is shown by A1. 3.4. Elongation at Break (Eb) Flexibility of a polymer is an essential parameter that is very actively considered during the application of a polymer in a certain area. The extent of elongation of these UV-cured films at the breaking point is shown in Fig. 4.
FIG. 4. Elongation at break (Eb) of UV-cured polymer films against number of passes.
It is obvious that the film that showed higher TS does not necessarily show higher elongation at break. For instance, the highest elongation is achieved by formulation A1 while the lowest elongation is observed for A4, which gave higher PH and most cross-linking (gel content). The elongation property (Eb) increases with UV-radiation and attains maximum mostly after 14 passes; after that Eb decreases gradually with UV-radiation. 3.5. Application of Formulated Solutions on Plywood Surface After characterization of the UV-cured polymer films, different formulations were applied on the plywood surface. Formulation A5 was arbitrarily chosen as top coat because of its low viscosity and presence of antibubbling agents which makes the final coated surface smooth. The other formulations A1–A4 were used as base coats while the formulation A6 was neither used as base coat or top coat because of its poor physical properties on glass plates. The base coats were applied on polished plywood surface and partially cured with UV-radiation. Then the substrates were polished with suitable sandpaper. Top coat was applied and cured with UV-radiation at different number of passes between 4 and 20. Various physical and mechanical parameters were determined as a function with the number of passes.
FIG. 3. Tensile strength (TS) (kPSi) of cured polymer films against number of passes.
3.5.1. Pendulum Hardness of the Coated Plywood Surface The surface hardness of the film coatings on plywood as measured by the pendulum technique and containing are
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graphically represented in Fig. 5 plotted against the intensities of the UV-radiation. The PH increases with an increase in the UV-radiation doses; this reflects that there is an increase of the cross-linking density in the polymer with the increase of the radiation intensities. The figure also shows that with an increase in the monomer (TMPTA) concentration in the base coat, PH also increases to a certain value. Similarly, the highest PH (68.7%) is achieved by the samples treated with the A4 þ A5 (base coat A4 and top coat A5) formulation after 14 passes of UV-radiation. This is followed by the A2 þ A5 (63.8%) formulation and the lowest PH value is obtained from the samples coated with A1 having lowest amount of TMPTA in the formulated solutions. 3.5.2. Micro Gloss Property The micro gloss property determines the attractiveness of the surface of the substrates and if the micro gloss values are higher, it appears good to the human eyes. The gloss is generally measured at two angles (60 and 20) of the plane of the modified surface. Results of surface gloss at 20 and 60 angles of the coated plain board substrates are graphically shown in Figs. 6 and 7 plotted against the doses of UV-radiation. Gloss determined at 60 angle is generally higher than at 20 and the micro gloss properties is directly related to the extent of cross-linking and the surface gloss increases with an increase of cross-linking density[10]. We observe from the figure that high surface gloss at both 20 and 60 are achieved with A4 þ A5 formulation and this also proves that high cross-link has occurred on the
FIG. 5. Surface hardness (%) of modified plywood after applying base coat and top coat against radiation intensities.
FIG. 6. Micro gloss at 20 of UV-cured plywood surface at different radiation intensities.
plywood surface that was muted with base coat A4 and top coat A5. 3.5.3. Taber Abrasion The coated (modified) plywood surface was abraded between two abradent revolving wheels, applying a load
FIG. 7. Micro gloss at 60 of UV-cured plywood surface at different radiation intensities.
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TABLE 2 Adhesion and abrasion resistance properties of UV-cured polymer coatings on plywood Formulations Physical properties Taber index Adhesion strength (N mm2)
UV doses (no. of passes)
A1 þ A5
A2 þ A5
A3 þ A5
A4 þ A5
14 14
586 1.3
578 1.7
544 1.9
412 2.3
of 100 g=cycle. The weight loss obtained by the sample, caused by such abrasion between the wheels in 100 cycles, is related to the Taber wear index. This demonstrates that the abrasion resistance is high when the Taber wear index is low. Results of Taber wear index are shown in Table 2. It is observed that abrasion resistance increases as the concentration of TMPTA increases and the coating of formulation A4 þ A5 shows the minimum Taber index (412), while A1 þ A5 yields the maximum Taber index (586) after 14 passes of UV-radiation. This reveals that the coating A4 þ A5 has the highest resistance pertained by 100 cycles in the revolving wheels compared to other coatings on the plywood surface. 3.5.4. Adhesion Strength Adhesion property (the force, N mm2) that is required to pull out the coatings of the polymer from the plywood surface is adhesion strength. If a coating is loosely bound with the substrate surface, then it can be easily peeled off by applying a small peeling force and vice versa. Results of adhesion strength (N mm2) are shown in Table 2. It is clear from the table that the coating A4 þ A5 containing 88%TMPTA with base coat has good adhesion (2.3 N mm2) and there is no chipping off area during crosscut technique. 4. CONCLUSION UV cured coatings on plain board surface as well as the polymer films were attained with formulations made with M-1200 (urethane acrylate) and TMPTA at different proportions. Various characterization tests of the coated plywood substrates revealed that the physico-mechanical properties of such formulation-coated surfaces had extremely increased. The raw plywood has a PH of 20% but the samples treated with the coating E showed a PH of 68%. This treatment thus increases the PH by 240% and the surface gloss was similarly increased by 2500% at 20 and 2750% at 60 angles. The other properties were also enhanced to similar degrees. The coated substrates also showed extreme resistance towards abrasion and the
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