Update time:2022.05.28 Views:104
Recently, Chengde Technology Co., Ltd. held an online sharing event on "The Properties and Applications of PVA High Barrier Coatings", which mainly introduced the relevant properties and application examples of PVA when applied to packaging.
The following is the content of the exchange:
PVA high-barrier coating is a safe, environmentally friendly and cost-effective material. Most of my friends have a deep understanding of K coating, that is, PVDC coating, aluminum foil, aluminum plating and EVOH barrier materials, because these materials have been used for many years and are very familiar to them, but PVA coating, many friends have contacted Not much, so there are a lot of questions.
First of all, I will use the familiar high-barrier materials to compare and introduce what PVA coatings are and what are their characteristics.
1. Characteristics of PVA High Barrier Coatings
Common high barrier materials are PVDC, EVOH, aluminum foil, aluminum plating, silicon plating, etc. From the perspective of material properties, PVDC and EVOH belong to organic high barrier materials, while aluminum foil, aluminum plating, and silicon plating belong to inorganic high barrier materials. According to this division, PVA should be classified as organic high barrier material.
As far as organic high barrier materials are concerned, PVA is a material similar to PVDC process and similar to EVOH in barrier properties. That is to say, the PVA high barrier layer is also realized by the coating process. PVA has excellent barrier properties to gases such as oxygen and carbon dioxide. If it is used alone, PVA will not help much in the barrier of water vapor.
Compared with PVDC, PVA is different in that:
① PVA coating is suitable for a wider range of substrates
Including all kinds of common printing and heat sealing substrates, BOPET, BOPP, BOPA, various PE and CPP. In particular, the good coating adaptability on the heat-sealing substrate can better prevent the contamination of the package contents by the residual solvent after printing and compounding.
Since the bonding force of PVDC to various substrates is relatively weak, primer coating is required, so the thickness of the PVDC coating is basically between 3 and 5 μ.
The PVA coating itself has good affinity with various substrates, and the coating thickness of conventional products is less than 1μ.
The PVDC coating is prone to yellowing, which affects the appearance of the material. The transparency and appearance color of the substrate after the PVA coating is basically unchanged.
The PVDC coating contains chlorine, which is toxic to incineration, and the film containing the coating cannot be recycled.
PVA coating can be incinerated and recyclable.
Except that the water vapor barrier property is slightly lower than that of PVDC coating, the barrier property of PVA to oxygen, carbon dioxide and other gases is much better than that of PVDC coating.
Compared with EVOH, EVOH is an ethylene-vinyl alcohol copolymer. The barrier property is directly related to the ethylene content and the thickness of the EVOH layer. Therefore, if you want to obtain higher barrier properties, the cost will also increase accordingly.
PVA coating, because it can obtain pure polyvinyl alcohol film after coating, so even if the coating thickness is less than 1μ, as long as a uniform and complete coating is achieved, its barrier performance is still very good. Therefore, compared with EVOH, it is more cost-effective.
At the same time, due to the above reasons, it is not easy to achieve thin film thinning when using EVOH as a high barrier layer, while PVA coating can.
The above is a comparison of the basic characteristics of PVA coating and other organic high-barrier materials. Compared with inorganic high-barrier materials such as aluminum foil, aluminum plating, and silicon plating, theoretically, its oxygen and moisture barrier properties will be slightly inferior, but please Note that this is only a theoretical level.
However, from the perspective of practical application, the biggest problem of inorganic high-barrier materials such as aluminum foil is that they have poor flexibility, lack of ductility, and are prone to cracking, fractures, etc., and have strict requirements for use scenarios. Even aluminum foil is prone to local increase in air permeability after bag making and pinching. In these respects, PVA organic high-barrier materials are more advantageous. Of course this is not absolute.
2. Barrier properties and applications of PVA high barrier coatings
The reason why it is not absolute is that inorganic high-barrier materials can also be primed with some organic materials and treated with a protective layer to solve this problem, just like PVA coating can also be achieved by stacking with inorganic high-barrier materials. The performance of high oxygen barrier and high moisture barrier is the same. However, these performance improvements also mean an increase in cost.
The characteristics and applications of PVA coatings are briefly summarized. The PVA coating gives the material a good barrier function to gases such as oxygen, nitrogen, carbon dioxide and small molecule volatiles.
It has a good protective effect on the contents with high oil content, easy oxidative deterioration, browning and aroma escape, such as meat products, spices, small breads, small cakes, dried fruits, and tea to prolong the shelf life of products.
PVA coating is a water-based coating with better safety and environmental protection. It can be used for microwave heating. It has excellent oil resistance, organic solvent resistance and corrosion resistance. It has good ductility and flexibility.
Typical features are:
The coating thickness is less than 1um, and the typical data of various substrates that have not been compounded after simple coating are as follows:
In practical application scenarios, the typical data of the finished product after printing and compounding are as follows:
The above structure is mainly used in the packaging of bakery, dried fruit, tea, spices, dried meat products, etc.
Typical data for barrier properties applied in a single material structure are as follows:
Changes in barrier properties at different temperatures and humidity:
The ordinate is the oxygen transmission rate, and the blue line represents 6 months of storage at 25°C and 50% relative humidity. The purple line represents 7 days of storage at 38°C with a relative humidity of 80%.
3. Questions about the use cost of PVA high barrier coating film
On the premise of achieving the same level of barrier performance, the PVA coating film has a significant cost advantage compared to the EVOH barrier film. As mentioned above, the high barrier properties of EVOH are inversely proportional to the ethylene content in the material and proportional to the thickness of the material. (Maybe this expression is not rigorous, and everyone understands the meaning.) Therefore, if you want to achieve an oxygen transmission rate of less than 5, you must ensure that you cannot cut corners. This cost should be understood by everyone.
If the PVA coating wants to achieve an oxygen transmission rate of less than 5, it can be achieved with a thickness of less than 1um.
Therefore, under the same barrier property requirements, the use cost of PVA coating is definitely more advantageous than EVOH.
Compared with PVDC coating film:
Compared with the PVDC coating film, it may be slightly more complicated. Because everyone buys film is accustomed to settlement by weight. The PVDC coating is thick, the increase is heavy, and the actual coating meters are few. The PVA coating is thin, the weight gain is small, and the actual coating meters is large. Therefore, simply in terms of tonnage price, the cost of PVA coating is higher than that of PVDC coating.
But in terms of square unit price, the cost of PVA coating is not higher than that of PVDC coating. That is to say, when the average price of PVA coating film is about 15% higher than that of PVDC coating film, the actual use cost will not increase.
free exchange of content
1. Can it be printed after PVA coating? Does it affect composite performance?
PVA coating does not affect printing and lamination, provided that water ink and water glue are not recommended. Both dry compounding and solvent-free compounding are possible.
2. Can PVA be coated on PE? Is it OK to put water-based substances after painting? Can it be pasteurized if it can contain water-based substances?
It can be coated on PE, and water-based substances can also be used. For pasteurization, our current conventional products are not recommended to be used in the field of pasteurization.
3. When extruding the PVA coating film (coating process), what kind of resin material is more suitable? What are the points to pay attention to regarding parameters such as processing temperature, compounding speed, and adhesive layer thickness?
EAA resin can be used for coating surface lamination, and the specific process can refer to aluminum foil lamination lamination.
4. What is the approximate level of barrier performance of PVA coating? Is there a proportional relationship between thickness and barrier properties (OTR) of PVA? If so, what is it like?
For the barrier properties of PVA coatings, you can refer to my introduction above. Various commonly used substrate structures are basically involved.
Thickening of the PVA coating does improve the barrier performance, but the improvement is not very obvious.
Taking PET as an example, for example, the current dry weight of the coating is about 0.6gsm to achieve an oxygen transmission rate of less than 1. If the dry weight of the coating continues to increase, it is difficult to achieve a smaller oxygen transmission value, and the cost will increase a lot. , so there is a cost-effectiveness issue that needs to be considered.
Of course, for some substrates, such as PE, the oxygen permeability of about 2 is conventionally achieved, but the actual oxygen permeability data is required to be smaller, and this effect can be achieved by increasing the coating thickness. However, this will have higher requirements for equipment, technology, etc.
5. What is the difference between the application of PVA coating film and other barrier films such as PVDC, aluminum oxide coating, and silicon oxide barrier films?
In the field of application, in terms of the substrate level, PVA can be well applied to the coating of heat-sealing substrates, which is the biggest difference from the above-mentioned materials.
As far as the terminal application scenario is concerned, specific problems may need to be analyzed in detail. Generally speaking, in the field of PVDC coating film application, PVA coating film can basically be used.
If the product using aluminum oxide silicon oxide has high requirements on moisture resistance, the PVA coating film is not suitable for use alone.
What I am talking about here is that in terms of conventional products, it does not involve the field of boiling.
6. The cost of coating PVA is compared with alumina, silicon oxide, EVOH, etc. How about the application cost?
Compared with the cost comparison of EVOH and PVDC coating film, I also have a general description in the previous introduction, you can refer to it.
For alumina and silica, it is not so easy to say. Because the product grades of alumina are different, the use targets are also different. For example, some grades of alumina products are used by customers to replace conventional aluminized films. The cost is low and the barrier properties are not very high. This PVA will definitely be more expensive.
If the goal of using alumina and silica is only for higher oxygen barrier requirements, and the water vapor transmission rate is not too high, then PVA will be more affordable.
7. Is PVA a hydrophilic material? What is the barrier performance under high humidity? Do you have specific data?
This issue has also been explained in the above introduction. What needs to be added here is that PVA is a hydrophilic material. After modification, it can be improved with the appropriate coating process. For example, according to the data I gave you before, the barrier property will not change very much under high humidity, but it is not recommended. Use naked or in direct contact with water.
8. Is AC agent required between coating and substrate?
9. When the PVA used in the coating is in contact with the adhesive, how does it perform, and whether there is any phenomenon of mutual influence.
The contact with the adhesive, the dry compounding and solvent-free compounding adhesives commonly used at present, will basically not affect.
10. You have seen the application of PVA film in water transfer printing. What are the applications in packaging? Does it need to be compounded after printing with PVA? How is the barrier property of PVA products compared to K film (oxygen barrier, air permeability, alkali resistance, oil resistance and acid resistance)?
The application in packaging is mainly in the fields that require barrier properties, such as food, medicine, daily chemicals, electronic products, and metal rust prevention.
Lamination is required after printing.
In terms of barrier properties, the above introduction is involved, you can refer to it.
11. Does the absence of solvent mean that it will not affect the performance of the substrate, such as not becoming so brittle?
As for the properties of the substrate, the mechanical properties of the original film after PVA coating are basically unchanged.
12. If PVA is applied, what is the appropriate dharma factor of the substrate?
The requirements of PVA coating for the dyne value of the substrate can refer to the requirements of the dyne value of the composite substrate.
It needs to be explained that the quality of PVA coating performance is not only related to the arrival factor value, but also has a very direct relationship with the flatness of the film surface and the surface precipitates.
So sometimes the dyne value is very good, and the coating effect is not necessarily very good. The dyne value is only one of the necessary conditions.
13. Only coating 1um, how to achieve PVA coating?
This known gravure coating, reverse kiss coating can be achieved.
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