The causes and solutions of the problem of thin film TPU crystal point of Rainbow TPU

Date:Sep 22, 2019

Thermoplastic polyurethane elastomer (TPU) is a kind of special elastic material between rubber and plastics, with high modulus, high strength, high elongation, high elasticity and excellent wear resistance, oil resistance, low temperature resistance and aging resistance.

1. TPU film forming method

TPU can be processed into different USES by extrusion, injection, blow molding, calendering, melt spinning and other methods. It is widely used in many fields.

TPU film is formed by extruding TPU particles, mainly including blow molding, flow, calendering or calendering sheet forming and extrusion coating.

Blow molding TPU film: it can be used in the fields of shoe material, waterproof and moisture-permeable clothing fabric, medical and national defense articles, weapon packaging, tent, sports and leisure equipment, sanitary towel, toy, and daily necessities packaging, etc., and it is the most widely used.

Flow TPU film: it can be used in shoes material, waterproof and moisture-permeable clothing fabrics, weapon packaging, tents, sports clothes, daily necessities and other fields.

Calendering or calendering: mainly used in the production of TPU sheet products, used in the production of transparent shoulder strap, medical packaging materials, bulletproof glass sandwich film, conveyor belt, interior materials, liner and other products;

Extrusion coating: generally used in the production of composite fabrics, TPU and non-woven fabric, industrial cloth, clothing cloth coating molding, can be used for clothing fabrics, trademarks, gloves, etc.

2. Causes of TPU crystal point

In addition to proper crystallization speed and melt viscosity, TPU also requires the control of product crystal point.Blow molding process due to the product thickness is thin, usually within 0.1mm, thinner film on the crystal point higher requirements.

In addition to the influence of film extrusion and blow molding processes, the crystal point is more important due to the influence of factors such as raw material quality, process method, machine structure, personnel control and environment in the process of TPU synthesis and manufacturing.The second is chemical hyperpolymer.

Three, TPU crystal point influence factor and solution

The influence factors and solutions of TPU on crystal point are briefly introduced from three aspects of raw material quality, process method and machinery.On the whole, the "fishbone analysis method" in figure 1 can be seen.


Figure 1 "fishbone diagram analysis"

1. Raw material quality

TPU is generally formed by reaction of polyester or polyether polyols, diisocyanate and chain extender of small molecule diol.When TPU is synthesized from polyester diol or polyether diol with diphenyl methane-4, 4' -diisocyanate (MDI) and chain extender 1, 4-butanediol (BDO), it may produce side reactions such as urea, urea formate and biuret in the presence of certain temperature, trace water or trace metal impurities or other impurities.

Therefore, raw material quality standards should be strictly required, and some raw material standards related to crystal points of TPU products are shown in table 1.


The effects of additives should not be ignored. If additives are added to polyesters or polyether polyols, their complete compatibility or melting point with polyesters or polyether polyols should be considered.

High temperature resistance and decomposition temperature should be taken into consideration for all auxiliaries. It is required that they will not decompose and carbonize to form impurity crystal points when the TPU processing temperature is up to 200 ~ 220 ℃.

2. Process method

In the process of TPU synthesis and production, process control is particularly important for product stability and product crystal point, mainly including the control of stability of raw material flow, melting index (MI) on-site detection control, raw material reactivity control, etc., which is pre-control.After control is TPU processing method, including internal cleaning of equipment, adjustment of processing temperature and screw speed, number of mesh, lubrication effect and other control points, some need to add additional lubricant to improve the surface effect and fluidity.The main influencing factors such as isocyanate index (R value) and MI value in production and processing and solutions are shown in table 2.


Table 2 main factors and solutions of TPU process and processing affecting crystal point

The R value is the molar ratio of NCO to OH in the raw material.In preparation of TPU, NCO/OH ratio plays an important role in controlling TPU relative molecular weight.

MI is a kind of value indicates the liquidity of plastic materials processing, is the material and test association (ASTM) according to the United States dupont plastic characteristics of conventional identification methods, the test method is to first make plastic grains within a certain time (10 min), a certain temperature and pressure, melt plastic fluid, and then through the outflow of a diameter of 2.1 mm pipe grams (g).The larger the value, the better the processing fluidity of the plastic material, and vice versa.

TPU is also very sensitive to the processing temperature. Tm of TPU has two kinds, namely hard segment Tmh and soft segment Tms.The usual TPU melting point is Tmh.Tmh mainly depends on the hard segment content and relative molecular weight of TPU.

There was a certain correlation between the R value of TPU and MI value. In the experiment, the weight fraction of the designed hard segment was 31.0% ~ 33.0% with polyester polyol PBA2000, MDI and BDO as raw materials, and TPU80A was synthesized.TPU95A was synthesized from polyester polyol PBA1000, MDI and BDO with a designed hard segment mass fraction of 48.0% ~ 50.0%. The corresponding relationship between R value and MI value is shown in figure 2.


The corresponding relationship between its R value and MI value is shown in figure 2

As can be seen from figure 2, when R < 1.015, MI value decreases as R value increases.After R > 1.015, MI value increased slightly as R value increased.When R= 1.015, MI reaches the lowest value, at which time theoretically TPU has the largest relative molecular mass and the slowest melt fluidity.

The effect of R value and MI value on the crystal point shows that different R value (or MI value) mixed processing will affect the crystal point of TPU thin film products.In this experiment, polyester polyol (pba-1000), MDI and BDO were used as raw materials to prepare TPU(95A) products with different R values and corresponding MI values. The results of film extrusion are shown in table 3.


Table 3 effects of different R values and MI on the crystal point of the film

As can be seen from table 3, R value is too low or too high, MI value is too large or too small, and the number of film crystal points per unit area is relatively large.This is because the difference in degree of polymerization leads to a wide distribution of relative molecular mass. Affected by the difference in melting temperature, TPU with high relative molecular mass is not easy to melt and form crystal points.

Therefore, the R value of transparent thin-film TPU is generally between 0.985 and 1.010.In addition, there is another important control point on TPU production process control, which is the flow control of three components, because the difference in flow will directly lead to the difference in R value and MI value.In order to verify the effect of different MI product mixtures on crystal points, the above mentioned TPU95A particles with different Numbers were respectively paired and combined for extrusion evaluation in this experiment. The results are shown in table 4.


Table 4 effects of TPU particles with different MI values on crystal points

As can be seen from table 4, MI value difference ≥10, R < 1 and R > 1 should be very cautious when mixing products, which may cause crystal point problems.

3. Machinery and equipment

In the process of TPU synthesis and production, the perfusion machine is a key equipment used for conveying raw materials and controlling the accuracy of raw material flow, and the measurement accuracy is generally required to be within 0.2%.

Twin - screw extruder is an ideal reaction device for TPU continuous reaction.

TPU has many advantages: reaction under high temperature and pressure, temperature is about 140 ~ 250 ℃, pressure is 4 ~ 7MPa, can ensure the side reaction to the minimum, high pressure can basically inhibit the side reaction that produces gas, product quality is stable;In the twin-screw, the speed gradient of over 2000 s-1 can be reached, and the kneading times can be up to 7 ~ 15 times/S, which can prevent the reactants from adhering to the shaft and the wall of the cylinder and producing uneven due to too long residence time.The quality of TPU produced by twin-screw continuous reaction method is higher than that of batch method.

The concomitant twin screw extruder is the core equipment in the continuous TPU reaction, and the combination of screw elements is crucial to the reaction speed and degree.

The drying equipment is not only used to dry TPU to reduce water content, but also to assist the re-curing of TPU.The moisture content of TPU should be controlled within 0.03% after drying.The influence of equipment on crystal points and solutions are shown in table 5.


Table 5 influence of equipment on crystal point and solutions


The important link of TPU film crystal point control is the TPU production and synthesis stage. Through the analysis and continuous practice of three quality factors, including raw materials, methods, machinery and equipment, the problem of crystal point on TPU film level is well improved. The product quality and stability are obviously improved, which can meet the requirements of film processing.TPU film processing is the post-processing control of crystal points. Attention should be paid to the extruder cleaning and melt filtration, as well as the selection of processing temperature and speed.

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