Study on the spraying process of powder coating on the surface of bamboo flooring.

Abstract:
this paper uses transparent epoxy polyester powder coating, in the sanding, preheated bamboo floor substrate on electrostatization spraying, curing and coating polishing and other processes.
the coating performance was measured by instruments such as paint film multi-use detector, and the super-depth 3D microscope was used to observe the episcopic effect of the coating layer and the characteristics of the internal curing structure from a micro angle.
results show that: (1) the preheat temperature and surface moisture content of the bamboo floor substrate surface resistance there is a synergy, it is placed in a constant temperature drying box of 90 degrees C preheat 15min, its surface moisture rate of 6.5%, the surface resistance of 6.94×108 s .
the powder electrostectrulation spray, cured under a 160c/6min infrared lamp, and made a transparent painted bamboo floor sample. The appearance of the coating film quality is good, to maintain the original texture and texture, the hardness of the coating film is 2H, adhesion level is l grade, no scratches, wear resistance 225 rv, resistant to coffee stains pollution, in line with China's wood coating related standards.
(2) ultra-field depth microscope observed the sample coating surface flat, uniform, no blisters, pinholes produced, the film interior also reacts completely, no powder particles, cured coating structure tight, tough texture, coating surface average height is only 34.85um.
Bamboo floor perishable, mildew, moisture absorption and cracking and other shortcomings, the current simpler and more effective treatment method is to use solvent-based coatings for coating, and solvent-based coatings easy to release organic volatiles (VOCs), both waste of resources and pollution of the environment, is to be restricted development and application of coatings.
most thermoo-solid powder coatings have a high curing temperature (≥180 degrees C) and a relatively long time (≥15min), mainly used in metal materials and their products.
In recent years, with the emergence of ≤3min) powder coatings at low temperatures (120 degrees C), powder coatings have been continuously used in non-metallic materials such as medium density fiberboard (MDF), plywood, particleboard, wood flooring, etc.
compared with liquid coatings, powder coatings have the advantages of environmental protection in processing, recycling and re-use of coatings, high production efficiency and good coating performance.
But bamboo floor surface has a natural texture, need transparent low temperature fast curing powder coating, in order to maintain the original natural texture of bamboo flooring, after transparent powder coating bamboo flooring can not only improve the surface gloss, hardness, wear resistance and other chemical and chemical properties, but also waterproof, anti-corrosion and anti-cracking deformation, extended service life.
this paper mainly carried out transparent epoxy polyester low temperature fast-curing coating on the bamboo floor surface powder spraying process and film performance research, in order to obtain suitable for bamboo floor surface transparent powder spraying process.
1, test materials and equipment
1.1 test materials bamboo flooring (provided by Zhejiang Yongyu Bamboo Industry Co., Ltd.), size 1000×100×16mm, water content of 9×.5% 10×.5%;
TS01 epoxy polyester transparent powder coating (provided by Zhejiang Tiansong New Materials Co., Ltd.), particle size ≤74 mm, pale yellow transparent, no mechanical impurities, softening point temperature of 105 degrees C, theoretical curing conditions of 150 degrees C / 5 min, 40 degrees C after storage of 28d performance remains unchanged.
other materials: sandpaper, cigarettes, coffee, pencils, acetone, etc.
1.2 Test equipment . VHX-1000 Ultra-Depth 3D Microscope (Keens (China) Co., Ltd.);
GUYI Rotary Wing Electrostectric Powder Spray Room (Suzhou Solid Art Coating Equipment Co., Ltd.);
EL-10KA Temperature Adjustment Moisturizing box (Guangzhou Espeke Environmental Instruments Co., Ltd.);
Dl (2008 Wood Moisture Rate Tester (Hangzhou Taik Drying Equipment Co., Ltd.);
PC40B Digital Insulation Resistance Tester (Shanghai Standard Electronics Co.,
Ltd.); <8>101-5A type electric thermostat blowing drying box (Shanghai Espek Environmental Equipment Co., Ltd.);
ZNLD 1800w homemade infrared lamp curing device;
QDX paint film multi-use detector (Tianjin Semperology Materials Tester Factory);
JM-IV Rolling Wear TestEr (Jinan Times Gold Instrument Co., Ltd.);
QFH-HV600 Paint Film ScratchIng Instrument (Huizhou Yupson Instrument Co., Ltd.);
. JA5003 electronic balance (0.0001g), etc.
2, test content and method
2.1 process sample preparation→ surface sanding→ surface moisture content control→ substrate preheat treatment→ powder electrostitive spraying→ coating curing→ coating film performance testing→ samples.
2.1.1 The effect of different water content on the surface at room temperature
according to the bamboo flooring national GB/T20240-2006 standard, its moisture content is 6.0% to 15.0%, therefore, this test in the thermostat temperature and humidity tank for the sand of the bamboo floor surface moisture content control test.
humidity box temperature is set at 25 degrees C, the temperature difference between the balance water content of the wet and dry ball and the balance moisture content control table adjust the humidity, so that the water content of the plate reached 6.0% to 7.0%, 7.0% to 8.0%, 8.0% to 9.0%, 9.0% to 10.0%, respectively. 0%, 10.0% to 11.0%, 11.0% to 12.0%, 12.0% to 13.0%, 13.0% to 14.0%, 14.0% to 15.0%, and the corresponding resistance value is measured using a digital insulation resistance tester.
18 200×100×16mm bamboo floor samples were taken, recorded as A, B, C, D, E, F, G, H, I, repeated 2 times, and the test results were averaged. Then the electrostation powder is sprayed and cured, and the coating sample is made and then the performance test is carried out.
2.1.2 substrate preheat treatment
controls the warm-up temperature in the drying box at 80 degrees C, 90 degrees C, 100 degrees C and 110 degrees C, and adjusts the surface moisture content of the bamboo flooring test to 11.0% by constant temperature and humidity.
With a wood moisture rate meter with a special probe (second gear) online measurement distance from the surface of bamboo floor 2mm, 4mm, 8mm depth of water content corresponding changes, repeat 2 tests, test results take average.
2.1.3 Powder electrostectrecturized spraying
powder electrostectrecturized spraying is one of the important processes, this test sets the electrostectrecturized voltage of 80kV, the powder pressure is 0.05MPa, the distance between the nozzle and the sheet 15 to 20cm. The bamboo floor sample is suspended vertically on the electrostation generator shaft of the rotary wing electrostectrecture powder spray room for spraying test.
2.1.4 Coating curing process
at ZNLD-800w homemade infrared lamp curing equipment, by controlling the temperature and time of the medium infrared lamp to the surface of the plate, according to the curry 1 epoxy polyester transparent powder coating The curing temperature and time parameters of 150 degrees C/5min were selected, and single-factor experiments were carried out at 140 degrees C, 150 degrees C, 160 degrees C and time 4 min, 5 min, 6 min and 7 min respectively.
2.1.5 Coating Performance Test
This test is based on the substrate treatment and coating performance, coating adhesion, coating wear resistance, pencil hardness, coating thickness, acetone resistance, water vapor resistance, pollution resistance The test method is tested by reference to GB/T17657-2013 "Artificial plate and finish artificial plate physical and chemical performance test method";
2.2 Microstructure analysis of the coating . A transparent finish bamboo floor sample was selected to observe the microscopic structure of the coating film under a three-dimensional microscope of ultra-depth of view. On the sample, five more representative points were randomly selected as observation points, four of which a. B. C. d Is used to observe the appearance quality of the coating film, and another point is used to observe the curing of the film interior.
3, results and analysis
3.1 sample surface appearance quality and coating performance and moisture content, resistance and powder content of the relationship
from Table 1, it can be seen that at room temperature for different water content of bamboo floor corresponding to different resistance values, resistance with the increase in moisture content and decrease.
When spraying bamboo floors with different moisture content, the powder content increases with the increase in moisture content, and the powder content tends to change slowly when the powder content reaches 4.00g (i.e., about 100g m-2 per unit area), and the corresponding moisture content is between 7.0% and 8.0%, at which time the resistance is about 9.8×109.
When the moisture content exceeds 7.0% to 8.0%, the resistance decreases rapidly, the powder content gradually increases, the moisture content increases to 11.0% to 12.0%, the resistance is 4.8×107 s, the amount of powder per unit area is 111g m-2, but at this time the coating quality is poor, the needle hole is more;
water content is 12.0% At 15.0%, the phenomenon of film pinholes and shrink holes is more serious, when the water content is less than 7.0 to 8.0%, the resistance gradually increases, the powder content drops sharply, when the moisture content is 5.0% to At 6.0%, the average resistance is 2.6×1011 s, the amount of powder is particularly small, only 60g m-2, the phenomenon of leakage, after curing the film quality is very poor.
as the thickness of the coating increases, the adhesion level decreases. At room temperature suitable for spraying bamboo floor moisture content of 7.0% to 10.0%, resistance in about 109 s, powder content in 100g m-2 or so.
and research by Christopher Jocham et al. has shown that preheat can improve the powder rate and powder coating adhesion of the sheet surface powder coating by preheating, so this experiment will be followed by a study of bamboo floor preheat treatment.
3.2 The relationship between moisture content and resistance pairs under bamboo floor warm-up conditions
As shown in Figure 1, the following conclusion can be drawn from the characteristics of the curves:
(1) Overall, the water content of the plates at each temperature drops to about 6.5% when the corresponding resistance is minimal. When the moisture content is greater than 6.5%, the resistance of the surface of the bamboo plate under the action of temperature shows a downward trend, the higher the temperature, the more obvious the decrease, and in the range of 4% and 6.5% of the water content of
, the surface resistance of the bamboo plate at all temperatures increases with the decrease of its moisture content.
(2) For a particular moisture content, the corresponding 4 sample resistors decrease with the temperature, and the difference increases with the temperature, and the resistivity of the warm-up temperature decreases significantly. It shows that bamboo resistivity p becomes smaller with the increase of temperature, which is the opposite of good conductors such as metal.
Because bamboo is the same as wood ion conduction, in a certain moisture content range (w<10%) temperature effect can also show that bamboo conduction is by means of ionization process, the temperature increase can increase the concentration and migration rate of free ions in bamboo, and the increase in moisture on the increase in the number of free ions also has a helpful effect.
(3) In the effect of different moisture content on spraying on bamboo floor surface at room temperature, it is concluded that the relatively suitable spray moisture content range of 7% to 8% at room temperature, and the corresponding surface resistance is 109 s order of magnitude.
In this experiment, when the temperature is 80 degrees C and 90 degrees C, the surface moisture content is still less than 109 degrees C, and at temperatures of 100 degrees C and 110 degrees C, the surface water content is still less than 109 degrees Order of magnitude.
indicates that in the case of warm-up temperature, the water content of the surface of the bamboo plate is reduced while the resistance does not decrease with the increase, the effect of the temperature makes the resistance of the surface of the plate increase greater than the resistance caused by the decrease in the moisture content of the surface.
, the surface resistance of the sheet will not exceed 109 ohm orders of magnitude and can be used for electrostitive spraying as long as the respective warm-up time is guaranteed not to exceed the respective warm-up time. Among them, when the water content of the surface of bamboo sheet is about 6.5%, the resistance value at all temperatures is reduced to the lowest value, the lower the resistance, the more suitable for electrostectrecturic powder spraying.
3.3 The relationship between water content and time change in each layer of bamboo flooring at different warm-up temperatures . The water content change curve of bamboo floors at 4 different temperatures is shown in Figure 2, Figure 3, Figure 4, Figure 5.
As can be known from the figure above:
(1) at different temperatures of bamboo plate 3 levels of water content change law is not the same, 4 charts of the trend of change is relatively close, over time, bamboo sheet moisture content decreased;
warm-up temperature of 100 degrees C and 110 degrees C, the water content change from the surface of the plate 4mm and 8mm is relatively flat, 2 mm water content decreases rapidly over time. It is explained that when the temperature is relatively low, the evaporation rate of water on the inside and outside surface of bamboo plate is slow and uniform, while the evaporation rate of water on the surface of bamboo sheet is faster than that of the interior when the temperature is higher.
(2) by tables 1 and figure l can be known, bamboo plate surface resistance ≤109 s order of magnitude, in warm-up situation moisture content of about 6.5%, more suitable for electrostitive powder spraying.
water content below the state of 80 degrees C and 90 degrees C reached 6. 5% of the time used was 44min, 15min, core and quarter layer water content of lO.0% and 8.7%, 10.2% and 8.95%, respectively, and the warm-up time was shorter at 90C.
water content of 10min below the 100C and 110C states dropped to 6.5 per cent. The moisture content of the core layer and a quarter layer under the conditions of 100 degrees C and 110 degrees C is much higher than that of the surface layer, mainly because the warm-up time is not long enough and not enough heat is transmitted to the core layer of the plate, so the water content within the plate does not change much.
Because the surface moisture content is relatively high, not suitable for curing, in 140-160 degrees C, 4 to 7min curing conditions, the internal moisture of the plate easily jumped to the surface, thus affecting the curing quality of the coating film.
Therefore, as shown in the figure above, the bamboo floor is heated at 15min in a drying box with a preheat temperature of 90 degrees C, at which point the water content is 6.5%, and its surface resistance is 6.943×108 s (less than 109 s), suitable for powder electrostitectrective spraying.
3.4 Bamboo floor surface powder.