SUMMARY
TPU is a new type of environmentally friendly material, which has developed rapidly in recent years. With the continuous development of new products by TPU elastomer manufacturers, some new types of TPU elastomers are gradually being put into the market. Waterproof and moisture-permeable products are one of them. It has greatly promoted the development of waterproof and moisture-permeable TPU film. This film has a waterproof and moisture-permeable function and is widely used in outdoor sportswear, medical accessories, surgical gowns, etc. In order to improve the moisture permeability of the film, various film manufacturers are also constantly carrying out all aspects of optimization, adjustment, and improvement.
BRIEF INTRODUCTION OF TPU AND TPU FILM
Market conditions
TPU is one of the fastest-growing new elastomer materials, with an annual growth rate of about 15% in recent years. With the restrictions on the use of PVC materials in certain areas in Europe, America, Japan, and South Korea, some of the enterprises have gradually begun to use TPU materials to replace PVC. In addition, it has many excellent characteristics such as environmental protection, high strength, high elongation, tear resistance, water resistance, oil resistance, and abrasion resistance. It can be used for injection molding, extrusion, flow traditional processes such as film extension and blown film processing have accelerated the development of TPU.
TPU film is an application of TPU elastomer. It is mainly used in clothing, fabrics, shoe materials, life jackets, military tents, medical accessories, and other fields. There are many varieties of TPU film, according to the type of material, it can be divided into ordinary polyester type, waterproof and moisture-permeable polyether type, hydrolysis-resistant polyether type, polycarbonate type, polycaprolactone type, polyolefin type, etc., and each type of film has matte and transparent, dyeing, matte and others.
Waterproof and moisture-permeable products are one of the fastest-growing varieties in the market in the past years. The commonly used specifications are 0.01mm, 0.012mm, and 0.015mm, which account for about 10%-15% of the TPU film market share.
PREPARATION OF TPU BREATHABLE FILM
Raw material drying
Since the waterproof and moisture-permeable TPU absorbs moisture in the air very quickly, the drying of raw materials needs to be carried out with a dehumidifying dryer. The dew point is set at -40℃～-30℃, and the drying temperature is adjusted at 90～110℃. To dry the material in time, it generally takes 4-6 hours, and the moisture content is controlled below 300ppm before it can be produced. Under normal circumstances, the raw materials dried by the dehumidifier dryer are left open in the air for 3 to 5 minutes. The raw materials cannot be used because of moisture absorption. Therefore, the raw materials need to be transported in closed pipelines.
Mixing and requirements of raw materials
It is necessary to select a waterproof and moisture-permeable TPU elastomer, and then mix it with various processing aids such as color masterbatches, slip agents, matte additives, antistatic additives, etc., according to the actual needs of different products, adjust the appropriate ratio for mixing, weighing the raw materials are taken, mixed evenly by a mixer, and then added to the desiccant dryer for drying. Because of the different melting points of various additives, you need to pay attention to the setting of the drying temperature. Some manufacturers have multiple automatic machines through mechanical equipment. Weighing equipment, drying and weighing various drying aids individually, then adding them to the mixer of the extruder for mixing, and then feeding them into the extruder through the screw of the extruder.
Since the breathable film is very thin, most of them on the market is between 0.01mm and 0.025mm thick, so the crystal point or glue point in the raw material must be within a controllable range, otherwise, there will be small uneven spots in the produced film, which will cause a series of problems such as film breakage or low water pressure test of the film when it is combined with the fabric in the later stage. Furthermore, the TPU material itself has high processing viscosity, relatively poor film-forming properties, a narrow processing temperature range, and high requirements for the stability of the melt index of the raw materials.
Film processing method
There are many methods for processing waterproof and moisture-permeable membranes, which should be selected according to actual needs.
1) At present, the most commonly used method in the market is the double-layer co-extrusion blown film method. This method has high production efficiency, and the quality of the products produced is relatively stable, and the thickness deviation is smaller. There are also very few manufacturers that adopt the single-blown method. But the single blowing method has high requirements on the precision of the equipment, the temperature of the processing environment, and the air pressure in the bubble. The production speed is relatively 30% to 50% lower, but it can save the cost of raw materials.
2) There are fewer manufacturers produce TPU breathable film by the casting method, mainly due to the low production efficiency, more scraps produced during production, and deviation of thickness control ability.
3) There are very few manufacturers that dissolve TPU in solvents and then coat them to make some special products. However, in the products made by this method, there will be solvent residues in many products, which are costly and not environmentally friendly, can only be used in some special areas.
Processing equipment
There will be some differences in different processing equipment according to the processing method. The commonly used processing equipment is film blowing machine, casting machine, and coating machine. Due to some characteristics of TPU itself, the equipment required by the blown film method is different from the equipment for processing PE film on the market in many details, mainly including blown film die runner and film line position, extruder screw, rotating device The main aspects such as winding device, cooling device, motor capacity, etc., should be adjusted appropriately according to the characteristics of the TPU raw material. The casting method mainly needs to be optimized and adjusted in the extruder screw and casting film die, coating the method usually uses DMF, EAC, TOL, MEK, and other solvents to dissolve the raw materials individually or in the mixture, and then choose knife coating, roll coating and other methods for coating.
Process control
The coating method is mainly to control the viscosity of the product after dissolution, the pressure of the knife and the coating surface, the weight of the coating, the production speed, the setting of the oven temperature and other main aspects. If the control is not good, it will cause coating. Various problems such as uneven layer thickness and bubbles in the coating.
Product testing
At present, there are many test methods for breathable films on the market, mainly including the positive cup method, the inverted cup method, the moisture absorption method, the sweat hot plate method, etc., and the Japanese standard, the American standard, the international standard, the European standard and so on, there is a slight difference among them. The data tested by different versions of the same test standard and different test conditions are also very different. If you blindly pursue high index values without understanding the difference between different test methods for the same product, it will have a great impact on the choice of breathable membrane products.
FACTORS AFFECTING BREATHABILITY
TPU raw materials
Raw materials are the main factor affecting breathability. Most TPU products currently on the market are not breathable products. Even if a film can be made, the air and moisture permeability is very low. It is usually called an air-tight membrane. TPU raw materials with waterproof and moisture-permeable properties, the film made out of it has waterproof and moisture-permeable properties. The amount of hygroscopic components in TPU will have a greater impact on moisture permeability. This aspect has a great effect on the formula when TPU is synthesized. relationship.
Film thickness
The thickness of the film also has a great influence on waterproof and moisture permeability, but the thickness of the product is not directly proportional to the size of the moisture permeability.
Additives
In the production process of TPU, due to its high viscosity, various anti-adhesive additives are usually added. Sometimes in order to produce film products of different colors, it is necessary to add some additives such as color masterbatches. These additives do not have waterproof and moisture permeability themselves. After adding, the moisture permeability of the film will be more or less reduced. In addition, the carrier of such additives also has a great influence on the moisture permeability, it is best to choose to use waterproof and moisture-permeable TPU as a carrier product, which can reduce the impact on moisture permeability.
Hot melt adhesive
If the fabric is compounded with the film, the area occupied by the hot melt adhesive per unit area will have a great influence on the moisture permeability of the compounded product. You can reduce the gluing point area per unit area by reducing the weight of glue during compounding. Replace moisture-permeable hot-melt adhesive, use point-sticking glue compounding methods, etc., to reduce the impact on moisture permeability.
CONCLUSION
TPU is a new type of material, and the TPU breathable film is a new type of functional film. It has only been widely used in recent years, and related new products and processing equipment are gradually being explored and adjusted. In the process of improvement, the annual growth rate of the entire market is more than 20%, which has led to the rapid growth of the related apparel fabrics, medical-surgical gowns, and surgical gowns.
