Research on the application of anti-stone-hitting coatings to the underfest of high-foaming automobiles

1, foreword
car in the fast driving process, the road splashing sand directly on the soleplate, wheel cover, lower beams and other parts of the strong impact, so that the car electrophoresis coating was damaged, once the coating is destroyed, it will produce rust phenomenon. At the same time, the noise from the impact of sand and rock on the under the car at high speed directly affects the comfort inside the vehicle. In order to make the car have good rust resistance, sound insulation noise reduction, sealing, it is necessary to carry out special protection in the underbody, wheel cover, lower beam and other parts. At present, the domestic and foreign automobile mainframe factory is widely used in the under-car spray 1.5mm to 2.0mm thick PVC anti-stone paint to protect the under-the-car, in order to cushion the noise generated by the impact of sandstone on the under-body coating.2, anti-stone coating introduction
PVC anti-stone strike bottom coating is PVC resin, plasticizers, fillers, as well as adhesion promoters, stabilizers and other additives mixed into a high solid, solvent-free coating, is a solid up to 96% of the viscous paste-like substances. PVC anti-stone under-car paint curing needs to be at a certain baking temperature, after a certain period of time melting plasticization (140 degrees C, 30min), melting plasticized coating with good flexibility, strong adhesion, wear resistance, impact resistance, low temperature, corrosion resistance, sound insulation and other properties. PVC under-body anti-stone paint was introduced into China by Volkswagen in the 1980s, and with its excellent performance, operation, low-cost rapid popularity, almost all of the domestic automotive anti-stone paint market. With the continuous progress of the automobile industry, consumers have put forward higher requirements for the comfort of the vehicle. Ordinary anti-stone coatings can no longer meet consumer demand for automotive NVH performance, so high foam-resistant anti-stone coatings have come into being.3, high foaming anti-stone blow coating
high foaming anti-stone blow coating is in PVC-type anti-stone blow coating to add foaming agent, compared with ordinary type, high foam type construction after baking solidification volume will expand, its single car use will be greatly reduced, while curing after the dry film density is lower, so to achieve the goal of reducing the car weight. The loose porous structure after the curing of high foam-type anti-foaming coating is more conducive to sound insulation and noise reduction, and the foaming multiple of high foam-resistant anti-stone-hitting coatings is generally designed to be between 150% and 250%. The basic formula for high foaming anti-stone coatings is found in Table 1.
3.1 foaming mechanism:
coating curing process, foaming agent heating to play a role in making the coating into a loose porous structure. There are many kinds of foaming agent, from the process of producing foaming gas, there are chemical foaming agent and physical foaming agent two categories, chemical foaming agent refers to foaming agent in the heat when the heat frees the gas, so that the substrate foaming;
The foaming agent in the high foaming anti-stone blow coating is generally a physical foaming agent, foaming agent after heating the intermediate gas expansion makes the substrate form a well-elastic closed sphere, the foaming agent in the product described in this paper is microsphere foaming agent, belongs to the physical foaming agent, microsphere foaming agent by the thermoplastic shell and shell wrapped hydrocarbon composition. Foaming agent is heated to produce gas expansion, so that the cured coating has a certain expansion rate, so that the coating expansion into a micropore loose membrane.
3.2 Soundproofing Noise Reduction Principle:
noise waves often encounter obstacles in their propagation, which are transmitted from one medium (air) to another. Because of the different acoustic properties of the two media, some of the sound waves are reflected back from the surface of the obstacle, while the other part of the sound waves are projected into the obstacle. When the sand shock wave received by the car in the course of driving through the elastic uniform porous high foam anti-stone blow coating, sand shock wave in the high foaming anti-stone blow coating in the micropores to create friction and sticky resistance, sound waves into heat energy is absorbed, so that the car in the course of driving sand impact on the driver's impact to minimize.4, the main performance research:
high foam anti-stone coating applied to the car body, whether it can meet the construction process on site, whether it can achieve its due role, all the performance indicators need to meet the standard requirements of the automobile factory, the main performance of the coating research is as follows:
4.1 viscosity performance:
viscosity is an important construction parameter. At present, the main use is the rotational viscosity meter, is measured at a fixed shear rate of viscosity, for the surface viscosity. Although it is only a test result at a point in the viscosity curve of the coating and does not fully reflect the fluidity of the coating, this method is simple to measure and can be used as a reference viscosity for field construction. After fumbling in production and use, we think that the viscosity control of high foam-type anti-stone coating is more appropriate at 40 to 75Pa.s.
4.2 Soundproofing Noise Reduction Performance Study
High foaming anti-stone blow coating using the cross-linking between polymers, can improve its performance of sound insulation noise reduction, during the car driving, when the sound waves through the polymer coating, sound waves in the high foaming anti-stone blow coating molecular chain friction and sticky resistance, sound waves into thermal energy is absorbed, so that the car in the process of driving noise to minimize the impact on the driver.
(1) apply the sample to an electrophoresis board × size of 190mm to 190mm and harden it according to standard conditions. Put the steel plate in the outside size of 300mm×300mm, the internal size of 200mm×200mm, thickness of 20mm in the middle of the ordinary steel plate, with 14.7N.m of force according to the M8-50L standard screws, gaskets, nuts to secure the steel plate.
(2) secure the test plate at an angle of 45 degrees, place the hammer (2.08g) 2m above and drop one after the other. Place the receiver 150mm straight from the ground and 40mm horizontally in front of the center, resulting in an average of 8 times.
(3) High foam type and ordinary type bottom coating sound insulation results: after test research can be found that the same type of paint (high foam type or ordinary type), the thicker the coating sound insulation, the better the impedance effect. Under the same coating film thickness, the higher the foaming rate, the greater the sound insulation, the better the sound insulation effect. At frequencies of 500Hz-5000Hz, the interstable volume of non-foaming coatings is 7.04db-16.52db, the soundproofing of 150% foaming coatings is 14.32db-25.63db, and the insulation of 200% foaming coatings is 20.11db-31.26db.
4.3 stone-resistant
the product is evenly sprayed on a 150mm×70mm×0.8mm electrophoresis board, the dry film thickness is maintained at about 2mm, and then placed 24h in standard conditions for experiments. Use a multi-function stone hitter to blow the pressure (0.4±0.03) MPa with 9mm to 15mm of gravel 500g impact test five times. Take out the test film to observe if the film floats, disengages and so on. Then put the test piece into the salt mist experimental machine 72h, the experiment finally observed the number of rust on the test board, not good to judge whether the place of corrosion, with a knife cut and applied after confirmation. After experiments, the number of rust points on the surface of the test plate is 0, and the anti-stone strike performance is good.5, application research
5.1 construction process:
high foam anti-stone blow coating using high-pressure pump spraying method, high-pressure pump for the inlet pressure plate pneumatic pump, compression ratio of 55:1, air pressure control at 0.3 to 0.6M Pa, by adjusting the air pressure of the high-pressure pump to obtain the best atomization effect, spray gun for high-pressure airless spray gun, gun distance control in 600 to 1000mm, paint before use need to use a special blender stirring about 30min, to ensure that the paint mixing uniform, has a lower viscosity, suitable for spraying.
5.2 Curing process: the curing process of
high foaming anti-stone blow coating is a physical change, in the car coating workshop after the paint oven and paint oven after heating curing, the paint in 40-80 degrees C conditions began Produces a slow reaction (plasticizing PVC resin particles, producing puffing), puffing rapidly after more than 80 degrees C, and eventually the plasticizer and PVC fully melt into one, while at high temperature baking under the foam will expand, complete the curing process. A full curing of the coating requires 140 degrees C ×30min. Therefore, the coating film needs to be baked at more than 140 degrees C, lasted 30min to make it fully plasticized, in order to achieve the best performance, curing process as shown in Figure 6:
5.3 application position:
high foam anti-foam undercarry paint mainly used in the car chassis, wheel cover and other positions, play stone strike, sealing, corrosion and sound shock and other functions. High foaming anti-stone coating in packaging, storage, transshipment, construction and other aspects of the ordinary type is no difference, the main difference before and after baking. The thickness of wet film before and after ordinary baking is not very different from the thickness of dry film, and the thickness of dry film will increase to varying degrees according to the foaming rate after high foaming, so the application of high foaming anti-stone coating needs to pay attention to the effect on the lower order according to other components.
5.4 failure analysis:
(1) spraying is not smooth: the construction process will appear spray gun fan face can not open the phenomenon, the reason analysis is as follows: First, anti-stone paint viscosity is too large, poor operation, resulting in the fan surface can not be opened; The solution is as follows: The first step is to check whether the equipment pressure and heating parameters are in a stable state, adjust the parameters within the process range, and the second step: adjusting the equipment parameters can not solve the problem can replace the product batch or inform the manufacturer to reduce the viscosity of anti-stone paint.
(2) overflow: during the construction process, there will be a gap between the pressure plate and the barrel coating overflow phenomenon, the reason analysis is as follows: First, anti-stone paint is too thin, viscosity is too small; The solution is as follows: First, check whether the construction site with anti-stone paint flow phenomenon, eliminate the problem caused by the small viscosity of anti-stone-hitting paint;
(3) shedding: after the completion of the construction of the oven to complete the curing easy to fall off the phenomenon, the reason analysis is as follows: First, anti-stone paint adhesion does not meet the standard requirements lead to poor bonding with steel plate; Solution measures: the first step: laboratory test adhesion, as a basis for determining whether the adhesion is qualified, check whether the baking temperature is low, the second step: if the adhesion test is not qualified, the baking temperature is not abnormal, need to inform the manufacturer to adjust the anti-stone paint binder
(4) bubbles: after construction to complete the baking room curing prone to air bubbles Phenomenon, the reason analysis is as follows: First, the paint itself mixed with bubbles; The solution is as follows: the first step: check whether the problem car is a line car, prevent the paint in high temperature and humidity air long-term exposure, while notifying manufacturers to adjust the formula, the use of not easy to absorb moisture calcium carbonate or reduce its proportion, etc. ; 6, conclusion
high foaming anti-stone paint products have a good comprehensive performance, the current foreign car host factories such as Toyota, Honda has been applied, domestic host plants such as Great Wall Motor, Dongfeng Liuzhou has also been using high foam-type under-body anti-stone paint. With the improvement of consumers' requirements for automotive comfort, NVH performance, lightweighting, etc., the application of anti-stone coatings for high foaming vehicles will become more and more extensive.