The significance of elastic modulus for paint protection film
For most car owners, "elastic modulus" is a completely unfamiliar word, and a small number of invisible car clothing veteran players may have heard of it, so, what is the concept of elastic modulus in the field of TPU car coating film? What are we talking about when we talk about modulus of elasticity? What is the appropriate modulus of elasticity?
01 What is Modulus of Elasticity?
The elastic modulus is an external force exerted on the elastomer, and the shape of the elastic body will change, called "deformation", which is an important physical quantity inherent in the material, and the stress and strain of the material in the elastic deformation stage are proportional (that is, in accordance with Hooke's law), and its proportional coefficient is called the elastic modulus. The general definition of "modulus of elasticity" is: stress divided by strain.
In GB/T 13022-1991, 7.3 stipulates: to make a stress-strain curve, calculate the tensile elastic modulus from the initial straight part of the curve, expressed by E(MPa), E=δ/ξ, where δ-stress; ξ - strain.
The modulus of elasticity characterizes the difficulty of elastic deformation of the material under certain stress, and the larger the elastic modulus, the more difficult it is to deform.
02 The significance of the modulus of elasticity for the car coating film
Excellent ductility and super tensile strength are the important properties and obvious advantages of TPU car coating film, which is mainly due to the good physical properties of TPU material.
As an elastomer, TPU is a material between rubber and plastic, which can be seen from its elastic modulus.
● The elastic modulus of rubber is usually 1~10Mpa, and the elongation can reach 1000%, but the processing performance is low, and vulcanization treatment is required;
● The elastic modulus of TPU is 10~1000Mpa, the range is wider than that of rubber, the tensile strength is up to 70MPa, the elongation can be as high as 800%, no vulcanization is required, and it can be processed by ordinary thermoplastic molding machines;
● The elastic modulus of plastic (nylon, ABS, PC, POM) is 1000~10000Mpa, and the elasticity and elongation are far less than that of rubber and TPU, but it is easy to mold and process.
Therefore, TPU material has the advantages of high elasticity of rubber and the processing performance of plastic, and the car coating film made of TPU material has excellent performance in terms of elongation and tensile strength.
As can be seen above, the larger the elastic modulus, the more difficult it is to deform, which is reflected in the coating film, that is, the lower the elongation. However, the elastic modulus of TPU material has a wide range, so that the formula and performance of TPU can be arranged and combined in a variety of ways. At this time, the value of elastic modulus becomes one of the important indicators.
At the same time, the elastic modulus of TPU coating film is also the basis for the measurement and evaluation of its mechanical properties such as yield strength and fracture toughness.
03 What is the appropriate modulus of elasticity?
What is the appropriate value of elastic modulus?
Here we want to mention another mechanical property of TPU - hardness. The TPU molecular chain is composed of soft segments and hard segments, and the higher the content of hard segments, the higher the hardness of TPU and the greater the elastic modulus, so there is a certain correlation between elastic modulus and hardness.
● The lower the elastic modulus, the lower the hardness, the softer the car coating film product, although it is easy to stretch, but in the construction, the places with large arcs and corners are prone to earthworm patterns or rubber flowers, and even the rebound stress is large, and it is easy to rise;
● The higher the elastic modulus, the higher the hardness, the harder the car coating film product, and the construction stretching is difficult, especially in the winter in the north, the product does not reflect the soft elasticity, and it is difficult to construct without the assistance of steam.
How to take the appropriate value of the elastic modulus of the car coating film? It is also necessary to follow the construction of the car coating film, first of all, we must consider the normal elongation rate of the construction, in addition to the rearview mirror, the construction elongation rate of less than 25% is the most appropriate, since the elastic modulus is a judgment index of the difficulty of construction, the natural value is the maximum elongation in the construction of the data presented by the most meaningful.
According to a large number of precision tensile tests and market feedback data, its elastic modulus of 12-22MPa is more appropriate.
Why is elastic modulus a range and not a fixed value? Because the main performance of different manufacturers or different models of the same manufacturer is different, some models may emphasize more tensile performance, and some models may focus more on their hardness, so the value of elastic modulus is also different. In addition, the modulus of elasticity will also be affected by the external environment such as temperature, so the manufacturer should consider it comprehensively when taking the value.
04 What are the influencing factors of modulus of elasticity?
TPU coating film is a composite film, which is composed of TPU substrate, coating and glue, so all three have an impact on the elastic modulus of the car coating film. Among them, the TPU substrate has a great impact on the elastic modulus of the car coating film due to its large proportion.
· The hard content of TPU particles plays a decisive role, so the hardness of the particles for making TPU substrates is very critical, and the most suitable range is 90~95A.
· The decisive factors for the elastic modulus of TPU substrate also include the formulation composition, manufacturer and process conditions of TPU particles. The TPU substrate produced by Kaiyang uses TPU particles for Lubrizol's car coating film, which has stable quality, excellent performance, elastic modulus and other indicators.
· TPU is a thermoplastic polyurethane, and its tensile properties are greatly affected by temperature, and with the gradual rise of temperature, the tensile properties will gradually change from hard and brittle to viscous strength, and the tensile strength and elastic modulus will become smaller. This is also the reason why hair dryers are used for wrapping in the winter in the north.
· In addition, due to the thin thickness of the car coating film (the relevant data can be found here for the previous article), it is easy to produce a size effect, that is, the elastic modulus of the car coating film will be affected by the thickness of the film.
· The hardness of the coating is also important, the soft coating can maintain the modulus of the substrate, but it is easy to age and produce rain spots; The hard coating retains its brightness for a long time and resists rain spots, but it is extremely easy to increase the load.
· There is also a certain interaction between the TPU substrate and the coating and glue, so the affinity and matching between the three will affect the elastic modulus of the car coating film.
