PVC anti stone impact coating is a solvent-free coating made by blending polyvinyl chloride resin as the matrix material with plasticizers, accelerators, stabilizers, fillers, etc. It is a viscous paste like adhesive with a solid content of over 95%. PVC anti stone impact coating was introduced to the Chinese market by Volkswagen in the 1980s, and quickly occupied the domestic market with its excellent mechanical properties, good construction performance, process compatibility, low cost and other advantages. PVC anti stone impact coating is a widely used type of automotive coating adhesive, with a single vehicle usage of about 10 kg. On the automobile production line, PVC anti stone impact coating is sprayed onto the surface of the electrophoretic paint layer on the bottom of the car through a high-pressure pump, and melted and plasticized for a certain period of time at a specific baking temperature.
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In recent years, with the development of the automotive industry, vehicle manufacturers have put forward higher requirements for vehicle lightweighting and driving comfort. Reducing usage and minimizing vehicle bottom impact noise while ensuring performance has become an important topic in the research of anti stone impact coatings. At present, PVC anti stone impact coatings mainly include ordinary type, low-density type, and foaming type. Among them, low-density type and foaming type are modified based on the lightweight and functional requirements of automotive systems on the basis of ordinary type. By studying the performance differences of three types of anti stone impact coatings, this study provides reference for the selection of anti stone impact coatings for vehicle development.

Introduction to anti stone impact coatings

Low density anti stone impact coating is made by adding a certain proportion of hollow thin-walled, uniformly sized, and highly compressive spherical glass microspheres to ordinary PVC anti stone impact coating, and blending and curing them. Hollow glass microspheres are prone to breakage during spraying and external impact, leading to an increase in colloid density and a decrease in impact resistance of the coating. Therefore, the filling amount of hollow glass microspheres needs to be controlled within an appropriate range to achieve lightweight while ensuring coating performance. The dry film density of low-density anti stone impact coatings can currently reach below 1.0 g/mL, which is more than 30% lighter than the ordinary type (density 1.4-1.5 g/mL). Low density anti stone impact coatings are currently mainly maturely applied to foreign and joint venture brand car models, and domestic OEMs are gradually introducing their applications.

Foaming anti stone impact coating is a type of coating that adds plastic foaming agent to ordinary anti stone impact coatings. When the foaming agent is heated, it produces gas, which causes the volume of the baked and cured coating to expand and become a loose and porous honeycomb structure. Foaming agents are divided into two categories based on the mechanism of gas generation: chemical foaming agents and physical foaming agents. The foaming agent in foaming anti stone impact coatings is generally a physical foaming agent. After being heated, the gas expands, causing the substrate to form a tightly sealed sphere with good elasticity. Compared with ordinary anti stone impact coatings, foamed coatings can be designed with thinner wet spray film thickness due to volume expansion after curing, thereby reducing the amount of coating used per vehicle and achieving lightweight goals. At the same time, foamed coatings have good compression resilience and good buffering and energy absorption effects when subjected to local impacts [3]. However, the loose cavity structure of the foam coating weakens the strength of the anti stone impact coating, resulting in a decline in its anti stone impact performance. At present, the expansion rate of commonly used foaming anti stone impact coatings is 150% to 200%, mainly used in Japanese car models.

Introduction to the main performance of anti stone impact coatings

The application of anti stone impact coating on the bottom of automobiles requires both basic performance indicators to meet the standard requirements of the vehicle manufacturer and matching with on-site construction technology. Select representative ordinary type (density 1.45 g/mL), foamed type (foaming ratio 150%), and low-density type (density 1) from a certain factory0 g/mL).
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Rotational viscosity of anti stone impact coating

Viscosity is an important construction parameter for anti stone impact coatings, which objectively reflects the rheological properties of the coating. Therefore, anti stone impact coatings need to be adjusted to a suitable viscosity according to on-site construction to meet the continuity of spraying and a certain spraying area, avoiding failure phenomena such as sagging, obstruction, and overflow. Using the rotational viscometer method, ordinary, low-density, and foamed stone impact resistant coatings are tested according to GB/T 2794— The 2013 "Determination of Adhesive Viscosity - Single Cylinder Rotational Viscometer Method" was tested.

The glass microspheres in low-density anti stone impact coatings hinder the internal flow of PVC substrates, which can increase the viscosity of the coating and improve its anti sagging performance. However, excessive addition of glass microspheres can lead to tight packing between particles, making it difficult to disperse under high shear forces, causing friction between particles and increasing viscosity. Therefore, although the addition of glass microspheres can improve the anti sagging performance of coatings, it is not conducive to spraying. The foaming agent in foaming coatings has a high thixotropy ratio, which makes the coating rare under high shear force and conducive to construction spraying. After spraying, it forms a network structure that is not easily damaged, giving the system a high viscosity. The viscosity of ordinary type is controlled at 100-140 Pa· s,， Low density viscosity controlled at 140-180 Pa· s， Foam viscosity is controlled between 75-120 Pa· S is more suitable.

Mechanical properties of anti stone impact coating

Apply three types of anti stone impact coatings onto the electrophoresis test plate, bake under standard conditions, and place in the test environment for 24 hours. Shear strength test shall be conducted in accordance with GB/T 7124‐ The determination of tensile shear strength of adhesives (rigid materials to rigid materials) was conducted in 2008, with a distance of 40 mm from the clamping point to the overlapping end and a tensile speed of 50 mm/min; The tensile strength and elongation at break tests shall be conducted in accordance with GB/T 528-; In 2009, the determination of tensile stress-strain properties of vulcanized rubber or thermoplastic rubber was carried out [7], and dumbbell shaped specimens were prepared with a gauge length of 25 mm and a tensile speed of 200 mm/min. The shear strength, tensile strength, and elongation at break of the 1.0 g/mL low-density anti stone impact coating are superior to those of the ordinary type, mainly due to the appropriate proportion of glass microspheres evenly dispersed in the PVC matrix, forming a good bonding interface with PVC, hindering the relative movement of molecular chains, and improving the tensile and shear properties of the material. The shear strength, tensile strength, and elongation at break of the foamed anti stone impact coating are inferior to those of the ordinary type, mainly because the heating of the foaming agent causes the matrix material to expand, and the rich loose cavity structure weakens the strength of the anti stone impact coating. Therefore, while achieving lightweight effects, low-density and foaming anti stone impact coatings need to control the addition of glass microspheres and foaming agents to balance various performance requirements.

Anti stone impact coating has stone impact resistance

Apply anti stone impact coating evenly on 190 mm× 100 mm× On a 0.8 mm electrophoresis test plate (ED plate), it is baked under standard conditions and left for 24 hours for testing. Then, on the gravel impact testing machine, steel balls with a diameter of 4-5 mm were used at an impact pressure of 0.5 MPa and 54 ° C; Impact angle, impact test piece 10 times, impact duration 20 minutes. Finally, remove the fragments that have not fallen off the surface of the sample and place it in a cyclic corrosion test chamber. After 240 hours of neutral salt spray testing, observe the number of rust spots on the sample. The ED board coated with anti stone impact coating did not show any bubbles after the anti stone impact test, and the number of rust spots on the surface of the test board was 0, meeting the technical requirements. The bottom coat adhesive with excellent stone impact resistance can effectively protect the electrophoretic layer of the car bottom sheet metal from damage and improve the anti-corrosion performance.

Adhesion of anti stone impact coating

Adhesion is an important parameter for testing the matching performance between anti stone impact coatings and electrophoretic layers. Once the PVC hardened layer is peeled off from the body sheet metal, it loses its protective effect against sand and gravel impact. Therefore, good adhesion between the coating and electrophoretic sheet metal is the primary guarantee for stone impact protection. Apply three types of anti stone impact coatings to the ED board in a broken shape with a length of 80 mm, a width of 40 mm, and a thickness of 1-4 mm. After baking under standard conditions, place it under test conditions for 24 hours. Then use a knife to cut through the adhesive layer, cut it into 2 parallel lines with a 5mm interval, and peel off the cut pieces from the thick end to uate the adhesion of the coating. The adhesion test in Figure 2 shows that when the PVC coating is peeled off from the ED board, the coating body fractures, while the residual coating adheres to the ED. It is judged that all three anti stone impact coatings have undergone cohesive failure during the adhesion test. Good adhesion ensures the adhesion ability between the coating and sheet metal, avoiding peeling under complex environmental conditions and causing protection failure.
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Anti stone impact coating sound insulation performance

The anti stone impact coating on the bottom of the car utilizes the cross-linking effect between polymers to enhance its sound insulation and noise reduction capabilities. When the noise emitted by the impact of sand and gravel on the car floor passes through the elastic anti stone impact coating, the sound waves generate friction and viscous resistance inside the molecular chains, which are converted into heat energy and absorbed, reducing the impact of car noise on drivers and passengers during driving [8-9]. The sound insulation performance test method using steel balls falling is shown in Figure 3. At a size of 150 mm× 150 mm× 0Spray 100 mm× onto the. 8mm ED board respectively; 100mm, dry film thickness 250-1800μ M's anti stone impact coating is baked and hardened according to standard conditions. The steel ball fell from a height of 2000 mm and tested the sound pressure decibel emitted by the ED board due to the impact of the steel ball. The sound pressure of ED board without anti stone impact coating is recorded as P0, and the impact sound pressure fraction after applying three types of anti stone impact coatings (UBC-ED board) is recorded as Py, with a sound insulation amount of P0 to Py. The frequency range of human harshness is generally 4-10 kHz, and the experiment uses the sound insulation level at a representative frequency of 8 kHz for comparison. The dry film thickness of the anti stone impact coating is less than 500 μ m; There is no significant difference in sound insulation performance among the three types of coatings at m, but with the increase of coating film thickness, the sound insulation of low-density and foaming coatings is significantly better than that of ordinary coatings. Among them, the sound insulation effect of foaming anti stone impact coating is more prominent as the film thickness increases. Ordinary coating thickness 1800μ The sound insulation of m is 15 dB, while the low-density film thickness is 1500 μ m; m,， Foam type film thickness 1235μ M can achieve the same sound insulation effect, with film thickness reduced by 16.7% and 31% respectively compared to the ordinary type5%.

Ordinary anti stone impact coatings are mainly made by blending PVC substrates, calcium carbonate fillers, and other additives, with a dense coating cross-section. The cross-section of the low-density coating shows a closed cell hollow structure formed by the filling of glass microspheres. Due to the need to balance the mechanical properties of the coating, the proportion of glass microspheres added should be controlled within a certain range. The foamed coating presents a large porous structure with abundant honeycomb like air layers. When noise encounters the hardened layer of anti stone impact coating during propagation, a portion of the sound waves are reflected on the surface of the coating, and a portion of the sound waves generate frictional and viscous resistance when passing through the cavity structure of the coating. The sound waves are converted into heat energy and absorbed [10]. The rich porous impedance of foaming coatings converts sound waves into more thermal energy than low-density and ordinary types, resulting in a more significant sound insulation effect.

Anti stone impact coating vehicle coating performance

Low density and foaming anti stone impact coatings have no differences in packaging, storage, and spraying compared to ordinary types, and do not require adjustments to the coating and related equipment. Anti stone impact coatings usually need to be stirred for about 30 minutes before use until they are evenly mixed, and then atomized and sprayed using a high-pressure pump with a spraying pressure of 18-30 MPa. The coating is heated and cured in the intermediate coating baking room and the topcoat baking room in the painting workshop, which is a physical change. Usually, PVC anti stone impact coatings need to be baked at a temperature of 140 ℃ or above for more than 20 minutes to fully plasticize and achieve the best performance of the coating film.
Low density anti stone impact coatings involve the breakage of hollow glass microspheres in production and construction processes, such as high-speed shearing during mixing and pipeline and nozzle pressures up to 30 MPa during construction. If the hollow glass microspheres break, the coating density will increase significantly and lose its weight reducing effect. Therefore, the performance verification of low-density coatings requires the addition of density stability tests to ensure the compressive strength of hollow glass microspheres, so as to reduce the density change rate of the coating before and after construction; 3%. The foaming agent in the foaming anti stone impact coating expands under high temperature baking, causing the PVC plastic substrate to form a loose honeycomb cavity structure. According to the different foaming ratios, the dry film thickness of the coating will increase to varying degrees. Therefore, in addition to considering coating strength and stone impact resistance, the spraying thickness of foam type anti stone impact coatings also needs to pay attention to the impact of baking on the layout and installation of related parts in the next process.