Film abrasion resistance test
(1) Erosion resistance of paint film
Urethane coatings generally have both superior abrasion resistance and solvent resistance. This combinatorial property comes from hydrogen and covalent bonds between the segments. Under low stress, hydrogen bonds behave like crosslinks and reduce swelling by solvents. When subjected to higher stress, the hydrogen bonds are disengaged and the molecules elongate without breaking the covalent bonds. When the stress is released, the molecules relax and new hydrogen bonds form. Urethane coatings can be used in the abrasion-resistant layer of floors as well as in the topcoat of spacecraft because of the properties that require this combination.
The paint film on the outer wall of beer cans is easy to wear during transportation. A small amount of immiscible wax or fluorosurfactant can be added to the paint to reduce the surface tension of the paint film, thereby reducing the coefficient of friction. When two cans of paint film rub against each other wear is reduced. A small amount of fine-grained siO2 is added to the silicone coating for plastic lenses to improve the wear resistance of the coating. The fine-grained SiO 2 reduces the contact area with another surface, making it easier to slide over. The addition of rubber latex to fabric printing pastes has been used for many years. The rubber latex particles are insoluble in the resin and are raised together with the pigment particles, significantly improving abrasion resistance. Softer rubber particles can dissipate stress in the paint film and reduce the cracking of the paint film caused by stress concentration. Adding glass beads can also improve the abrasion resistance of the epoxy paint film.
Usually, solid lubricants are added to the paint to reduce wear, and are used in occasions where liquid lubricants are not suitable, such as lubrication under high vacuum and heavy loads. Solid lubricants include inorganic substances (graphite, molybdenum disulfide, LaF 3 and CeF 3 for high temperature), organic substances (PTFE, nylon, cyanine compounds), soft metals and their compounds (such as Ag, Au, Al, etc.). They are used either to increase wear resistance or to drastically reduce the coefficient of friction. The paint base types of coatings include organic matter (such as polyurethane, epoxy, acrylic acid, etc.), inorganic matter (such as silicate, borate, phosphate, etc.) and metal base materials (such as Cu, Ag, Pb, Ni, Sn, etc. ). When the temperature is above 200°C, the organic matter should use high temperature resistant resin (such as PTFE, polyimide, etc.). Lubricating coatings are also divided into metal type, organic type and inorganic type (high temperature resistance of 800~1000°C), which can be applied to many fields such as aircraft, instruments, watches, vehicles, ships, etc.
The common feature of thermosetting polymer anti-corrosion coatings is highly cross-linked, and the thermal curing of such coatings takes a long time. UV-cured acrylic coatings do not require prolonged heat curing.
① The abrasion resistance of melamine formaldehyde resin or melamine formaldehyde resin cross-linked polyacrylate coating is average.
② Fluorosilicic acid coating is composed of a fluorine-containing high polymer and polysilicic acid-containing polymer binder. When the silicon content is 25% to 40%, the coating has good abrasion resistance and resistance to the substrate. Adhesion ability. This coating was primarily developed for the treatment of polyacrylate surfaces and is also used on eyeglass lenses. The fluorosilicic acid coating is applied in solution, firstly dried for 10~60min, then cured at 60°C for 15h, and then cured at 170°C for 45min. The coating can also be cured at room temperature, but it will take several days. The surface of the polymethacrylic sheet can be treated with acetic acid to improve the adhesion of the coating to the substrate.
(2) Test of abrasion resistance of paint film
The Taber wear Tester is commonly used in the world to measure the abrasion resistance. The instrument has two rubber grinding wheels, which are fixed by the sample plate on the rotating disc under the grinding wheel. One grinding wheel wears the sample plate from the center outward, and the other grinds the sample plate from the outside to the center. Various types of rubber can be applied to the wheel according to the test requirements. load. Trials can be milled dry or wet. The grinding wheel rotates on the sample plate to form a circular wear mark until it wears through. It is represented by the number of revolutions required to wear through 25 µm, and is expressed by the number of grinding revolutions required for the paint film to be ground through or the paint film after a certain number of grinding revolutions. Loss of weight is used to represent the abrasion resistance of the paint film, but the results are often inconsistent with the actual application.
The very hard epoxy paint film in floor coatings has the worst abrasion resistance in use, but it is very good in the Taber test. In general, the Taber test results of the softer paint film are poorer, probably because the grinding wheel rotates at a constant speed, and more energy is transferred to the softer paint film. But a paint film that is too soft can embed the wheel and give false results.
my country's national standard GB1768-79 (88) stipulates that JM-1 paint film wear Tester is used, and after a certain number of grinding cycles, the paint film's weight loss is used to express its wear resistance. Because the weight loss method is not affected by the thickness of the paint film. For the same load and rotation speed, the smaller the weight loss, the better the wear resistance. This method is most suitable for road marking paint and floor paint that are mainly rubbed by heavy loads. There is a good relationship with the field wear results.
The falling sand test is to put the sand from the funnel through the pipe at a 45° angle on the sample plate, and it is expressed by the volume (L) of the sand required to wear through the paint film of a unit thickness. When using a gas blasting erosion device, when the particle flow is perpendicular to the paint film to form a circular scar, the radius of the scar can represent wear, and the particle flow rate and impact angle of the device can be varied. Another method of measuring wear is to use a fine ball-vine abrasive slurry to rotate on the paint film to form a small area of wear, and the paint film above 30µm has good reproducibility.
(3) Scratch
Urethane paints have superior abrasion resistance, but poor mar resistance. MF crosslinked acrylic varnishes had better scratch resistance than isocyanate crosslinked varnishes, but the former had poorer environmental acid etch resistance. MF cross-linked polyurethane has both acid and scratch resistance. Acrylic polyurethane varnish film has a thin deformable plastic layer, while MF acrylic varnish film is an elastic layer.
There are two ways to improve the mar resistance of a coating: it can be made hard enough so that the marring cannot penetrate too deep into the surface, or it can be made elastic enough to rebound after the marring stress is relieved. Among the acrylic resin paints crosslinked with MF, the scratch resistance of the acrylic resin modified with silicone was further improved.
On harder varnishes, the edge of the metal rubs across the varnish, sometimes leaving a black line on the varnish. Reduce the surface tension of the paint film to reduce the friction coefficient, such as adding additives such as modified polysiloxane can increase the smoothness of the paint film, which can avoid this phenomenon.
