With the growth of the invisible car cover market in the past two years, more and more stores and cross-border players have begun to pay attention to the products of invisible car cover. However, how should we judge and choose a transparent car cover? What kind of car cover can withstand long-term outdoor testing?
Speaking of invisible car cover, we must start with the TPU substrate, which accounts for the largest part of the invisible car cover structure.
1. What is TPU? What are its applications?
TPU is the most basic material that determines the quality of car cover.
TPU (Thermoplastic polyurethanes) is a thermoplastic polyurethane elastomer, which is a type of polymer material between rubber and plastic. It was first successfully developed by Bayer in Germany in 1958. TPU has excellent physical and mechanical properties, can be plasticized by heating, and can be solidified by cooling.
TPU has both the high elasticity of rubber materials and the high strength of engineering plastics. It has high strength, high toughness, high elasticity, high modulus, as well as excellent comprehensive properties such as chemical corrosion resistance, wear resistance, oil resistance, and strong shock absorption. It is widely used in footwear, cables, films, pipes, automobiles, medical and other industries.
From the perspective of TPU market demand, at present, shoe materials account for the largest proportion, close to 30% of the entire TPU market demand. The second is the machinery industry, accounting for about 28%, while pipes, automobiles, and construction fields each account for about 10%.
2. Can all TPUs be used on car covers? From the classification of TPU, it can be seen that the answer is of course no.
3. Aliphatic polycaprolactone TPU is the best choice for good car covers. Why do you say that?
Let's first understand some common problems that occur during the use of inferior TPU car covers-yellowing + hydrolysis. Yellowing and hydrolysis are the two most common problems in the current PPF market, which harm the interests of dealers and end consumers. This shows that there are still some problems in the application of TPU base film. From the production level, both the selection of base film and the coordination of base film and coating play a decisive role.
1. What are the reasons for yellowing?
① PVC base film pretends to be TPU base film
PVC is a common material for early invisible car covers. Since the PVC base film contains plasticizers and stabilizers, if these chemical reagents are not weather-resistant, they will automatically decompose and discolor after long-term exposure, causing the entire film to turn yellow. However, because its material cost is very low, there are car films on the market that use PVC to impersonate TPU. End consumers often cannot distinguish and are prone to step on thunder.
② Aromatic TPU is used.
In terms of composition, TPU is a multiphase block copolymer composed of diisocyanate, chain extender, and polyol.
TPU is mainly divided into aromatic TPU with weak yellowing resistance and aliphatic TPU with strong yellowing resistance according to the type of diisocyanate.
Yellowing resistance changes of aromatic (left) and aliphatic (right) after 168 hours
③ Why aromatics are not yellowing-resistant From a structural point of view
because there is an easily oxidized methylene bridge between the two benzene rings of aromatic TPU, a similar structure will be generated during processing and use, and this compound is brown, so it is easy to turn yellow under thermal oxidation and ultraviolet radiation. Therefore, if aromatic TPU is used, even if anti-UV agent is added, long-term light exposure will cause discoloration and yellowing.
④ Why does aliphatic not turn yellow?
Aliphatic TPU does not contain benzene rings, so the structure is more stable and not easily affected by ultraviolet rays. Therefore, the car covers of international mainstream brands all use aliphatic TPU as the base material of the car cover to achieve the purpose of durability, anti-fouling and anti-yellowing.
The cost of aliphatic TPU is higher than that of aromatic TPU, which is one of the reasons why the prices of invisible car covers made of TPU material on the market vary greatly.
So is it okay to just use aliphatic TPU?
Careful analysis is required in actual operation.There are some low molecular weight impurities and stabilizers in the synthesis process of TPU. If these trace substances are not well controlled, they will also cause yellowing of the TPU base film. In addition, the wrong type of aliphatic TPU base film selected will also cause the film to crack.
2. The reason for hydrolysis of car covers
Polyester TPU
As mentioned above, in addition to diisocyanate, another important component of TPU is polyol.
According to the type of polyol, TPU can be divided into polyester, polyether, polycaprolactone and polycarbonate.
Polyester TPU materials are prone to hydrolysis. Acid and alcohol condense to form esters, and polyester comes from this.
However, this reaction is reversible. Polyester will hydrolyze under the action of acid or alkali, causing the degradation of polyester TPU, which is easy to break when degraded to a certain extent.
Polyether TPU is not easy to hydrolyze, but will undergo thermal oxidation degradation.
Polycaprolactone TPU products have good high temperature resistance and hydrolysis resistance.
In summary, aliphatic polycaprolactone type TPU is the best choice for good car cover.
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