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    Home > Coatings News > Paints and Coatings Market > General knowledge of paint: mechanical properties of paint film

    General knowledge of paint: mechanical properties of paint film

    • Last Update: 2020-04-03
    • Source: Internet
    • Author: User
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    Core tip: as a protective coating, the coating is often affected by various forces, such as friction, impact, stretching, etc., so it requires the necessary mechanical properties of the paint film

    1

    Mechanical properties of amorphous polymers The mechanical properties of materials mainly refer to the response of materials to external forces

    When a material is affected by an external force and its condition makes it unable to move in inertia, its geometric shape and size will change, and the difficulty of geometric size change is related to the original size of the material

    The deformation size after dividing the original size by the force is called strain

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    It is suitable for leather Paint Home News: as a protective layer of paint, it is often affected by various forces, such as friction, impact, stretching, etc., so it requires the necessary mechanical properties of the paint film

    1

    The mechanical properties of amorphous polymers mainly refer to the response of materials to external forces

    When a material is affected by an external force and its condition makes it unable to move in inertia, its geometric shape and size will change, and the difficulty of geometric size change is related to the original size of the material

    The deformation size after dividing the original size by the force is called strain

    When the material is strained, the relative positions and distances between the molecules and the atoms in the molecule will change

    Because atoms and molecules deviate from the original equilibrium position, the internal force between atoms and molecules is generated, which resists the external force and tends to return to the state before change

    When the balance is reached, the restoring internal force is equal to the external force and the direction is opposite

    The restoring internal force per unit area is defined as stress, and its value is equal to the external force per unit area

    The stress required to produce unit deformation is called modulus

    Modulus = stress / strain according to the form of external force, such as tensile force, shear force and static pressure, modulus is called Young's modulus, shear modulus and bulk modulus respectively

    From the point of view of materials, modulus is the ability of materials to resist external force deformation, which is related to the chemical structure and aggregate structure of materials, and is the most important parameter of materials

    2

    The stress-strain curve of the paint film strength ① and the strength of the polymer material stretch under the action of tensile force

    The stress-strain curve (expressed by the elongation) before stretching to the occurrence of fracture is called the tensile curve

    The end of the curve is the point of fracture of the material, which is the expression of the material strength

    ② The expansibility of paint film is used for coil steel, can and other coatings to undergo various tests during processing and forming after film forming on the metal surface

    It is required that the paint film will not be too thinned even if it is subject to large deformation and not broken during processing and forming

    In machining, there are not only tensile force, but also compression force, and different positions and forces, so it is difficult to have corresponding test methods to accurately describe

    However, there is no doubt that whether the polymer material can meet this requirement is related to its stress-strain curve, and the most important is the situation of tensile curve, in which the elongation at break is an important measure

    If the polymer film is in the hard glass state, i.e

    under the brittle fracture temperature, the fracture elongation is very low, and the paint film is hard and brittle, so it is bound to be brittle during processing

    If the film is in a high elastic state, although the film has a great elongation and a great resilience after the external force is withdrawn, the film is very soft

    The ideal situation is that the paint film is in the soft glass state, that is, above the brittle temperature TB and below the glass temperature Tg

    At this time, the paint film has a considerable elongation (forced high elastic deformation) under the action of external force, and this deformation can be retained, that is, the paint film has certain expansibility, and the paint film shows the properties of hardness and toughness

    Therefore, when choosing film-forming materials of coatings, attention should be paid not only to TG but also to TB

    Generally, the difference between TG and TB divided by the value Q of TG is used as a measure of the degree of expansibility

    The glass transition temperature of PMMA and PS is very close, but PMMA has better processing properties than PS, because TB of PMMA is much lower than TB of PS

    Although the polymer molecules between TB and TG can not have free movement of chain segments, their groups can still rotate, the phenyl rotation on PS is difficult, and the ester rotation of PMMA is easier, so PS is more brittle than PMMA

    The following table lists the TG, TB and Q values of several typical polymer materials for comparison

    TG, TB and Q polymers of several polymers TG TB Q PMMA 105 45 0.159 PS 100 90 0.061 PVC 80 10 0.198 bisphenol a polycarbonate 150-200 0.835 Homopolymers like those in the above table will not be used in coatings, because TG is too high, they are generally copolymers; in addition, coatings for metal surfaces, such as coil coatings, are generally not thermoplastic, but cross-linked

    Therefore, the influence of the composition of copolymers and the size of cross-linking degree on TG and TB, that is, the influence on brittleness and expansibility, should be paid attention to in the design of formula

    ③ The elongation of paint film and the requirements of wood restoration for paint are various, but it is very important that its elongation and restoration properties

    The paint film must be able to extend with the water absorption and expansion of wood, and recover with the drying and contraction of wood

    Generally, the insufficient elongation can cause the crack of the paint film along the direction of wood texture, so the fracture elongation is closely related to the crack

    In addition, if the elongated paint film cannot be restored with the shrinkage of the wood, wrinkles may occur

    If the paint film is in the state of soft glass, i.e

    in the state of expansibility, when the wood expands, it can have a large elongation due to the forced high elastic deformation

    Such deformation, as mentioned before, is the result of the molecular orientation caused by the movement of the chain segment

    After the external force is withdrawn, it cannot be completely restored, even if the force in the opposite direction is applied (i.e

    the force in the contraction), it cannot be restored

    If the film is in a high elastic state above Tg, it can have a high elongation

    Because the deformation occurs when the chain segment can move freely, the deformation is easy to recover when the external force is removed, especially when there is reverse shrinkage

    On the other hand, when the deformation of paint film caused by wood expansion is maintained for a long time, the stress can be gradually reduced due to mechanical relaxation

    ④ The wear resistance of the paint film is related to the friction coefficient, brittleness and elasticity of the paint

    The experimental results show that the wear resistance is closely related to the fracture work, which can be measured by the area surrounded by the stress-strain curve

    It has been discussed that the formation of stress-strain curve is related to the speed of strain

    In order to measure the wear resistance, the fracture work corresponding to the friction speed should be used

    Due to the limitation of test methods, sometimes the wear resistance data measured at room temperature is not consistent with the actual results, which may be due to the mismatch between strain rate and stress time

    Sometimes, the better results can be obtained when the sample is measured at a lower temperature

    According to the principle of temperature time equivalence, reducing the test temperature is equivalent to increasing the strain rate

    Polyurethane coating has the best wear resistance, which may be due to the formation of hydrogen bond between polyurethane molecules

    Under stress, polyurethane shows high hardness due to the effect of hydrogen bond; when the stress is high, hydrogen bond breaks and absorbs energy, thus protecting the covalent bond; once the external force is removed, hydrogen bond can be formed again

    The wear resistance is also related to the friction coefficient

    The addition of paraffin or fluorine-containing surfactant in the coating can reduce the friction coefficient and increase the friction resistance

    The inert pigment particles with large particles can also increase the friction resistance, which may be due to the reduction of the contact area of the paint film, thus reducing the transfer of force between the surface and the surface

    ⑤ The impact strength of the film is the fracture resistance under the condition of high speed impact

    In the stress-strain curve, the impact strength is also related to the fracture work, but the corresponding stress-strain curve should be the curve at high speed

    The polymer film with high impact resistance depends on the situation of energy absorption and conversion

    Because internal friction is a measure of converting mechanical energy into heat, the greater the internal friction is, the greater the impact energy is absorbed

    Therefore, internal friction is also an important measure of impact resistance

    There is a peak of internal friction of polymer in the transition zone of glass transition temperature, and the impact strength of glass state tends to be great

    Due to the extremely rapid impact, the polymer molecular chain segment often breaks before the relaxation movement and stress dispersion

    Only when the polymer with flexible molecular chain is in high elastic state, it has better impact resistance

    Therefore, it is generally believed that the glass transition temperature is closely related to impact resistance

    However, it should be noted that the glass transition temperature is not a reliable standard to measure impact resistance

    For example, the impact strength of polystyrene is much worse than that of carbonate when the Tg of polystyrene is 100 ℃ and the Tg of polycarbonate is 150 ℃

    This is because polycarbonate has a large β internal friction peak at - 60 ℃, while polystyrene has no β transition below room temperature and has no contribution to impact resistance

    Other secondary peaks are very high It's small, and it doesn't play a big role in resisting impact

    It can be seen that the glass polymer with strong secondary internal friction peak at low temperature has better impact resistance

    ⑥ From the microscopic point of view, the fracture failure of polymer materials means that the external forces destroy or overcome the chemical bond, hydrogen bond and van der Waals force

    Therefore, the strength of polymer material can be calculated theoretically according to its structure

    Because the calculation is carried out in a very ideal situation, the difference between the results and the actual strength is very large, and the actual strength is much worse than the theoretical strength

    It is of great significance to find out the reason of the difference between the theoretical calculation and the actual results for how to improve the material strength

    There are many factors that affect the actual strength of polymer, such as temperature, force speed, and the influence of pigment and polymer blending on the film strength

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