8. The adhesion principle and key element of fusible interlining

The adhesion of fusible interlining and fabric is a physical change process. It is mainly to bond fabric and linings together through the various modalities of fusible interlining. The whole change process is affected by many elements and it is difficult to control. So it is a very important technical link that relates to adhesion directly.

1).The mechanism of pressing adhesion

Thermosol is a kind of non-crystalline high molecular compound. Without outside power, it takes on three states: glassy state, high-elastic state and viscous flow state. At atmospheric temperature, it is in solid glassy state. At this time, the change outer power arouses is very small. When the temperature is below melting point, the thermosol become very pliable and elastic, and for the action of outer force, it tends to straighten along the forced direction. It will restitute when the outer force is eliminated and at this time it in high-elastic state. When the temperature rises continuously to reach or exceed melting point---viscous temperature, the thermosol is in the flowing viscous liquid state-viscous flow state. The texturing of viscous flow state could not resume and it is a plastic strain. The three states of thermosol are reversible to each other. The two changing temperatures are the important reference of establishing the production technique and application technique of thermosol. The adhesion of fusible interlining and fabric together is completed by making use of the reversibility of thermosol's three states and it is realized by the forming adhesion force resulting from thermosol's entering into the two kinds of fabric. See the state change of thermosol in graph-2.

2). Pressing adhesion equipment

Iron ---- the constant temperature iron with convenient operation and low cost is applicable for small-area lining with low melting point and provisional processing. It is also very suitable for domestic use. But the time and pressure of the pressing with iron are difficult to control and the adhesion effect is difficult to guarantee.

Flat-plate pressing machine---it is composed by top and bottom press tables. The top press table is heated by electrical strip. It throws pressure by compressing air or by hydraulic pump. The bottom press table is matted with permeable non-woven cloth and cover cloth to guarantee the even pressure. Its working principle is: put fabric and lining between two press tables, the top press table pins the bonded fabric with the action, heat the fabric with heater and do weight relieving and cooling after pressing the fabric for some time. The strong points of this kind of equipment are: simple structure, even temperature of heating face, little damage to fabric. Its weak points are: only applicable for small-area fabric and lining, low production efficiency.
Continuous pressing machine-----it is made up of operating platform, table, conveyer band, etc. There are two suits of heaters and two suits of pressure rollers in the machine storeroom which has pre-adhesion area and main adhesion area. Fabric and lining are sent into the pre-adhesion area of the machine storeroom through operating platform, then compress them to the smallest degree by slow heating, and heat and press them in the main adhesion area to achieve the best adhesion effect of fabric and lining. The strong points of continuous pressing machine are: applicable for various kinds of fabric or lining parts, various heat sensitive fabrics and the pressing and adhesion of the fabric that is difficult to bond; high production efficiency.

3). Pressing adhesion condition

Adhesive fabric-heating and pressing-cooling-adhesion (some time)

Adopting different pressing equipments, the adhesion process will be a little different.

（1）Flat-plate pressing machine:

Adhesive fabric-pressing and heating-natural cooling-adhesion (some time)

（2）Continuous pressing machine:

Whether heating and pressing are at the same time is the difference of the two pressing machine's pressing process. While the essence of adhesion---the change of thermosol is the same.

Thermosol solid-softening-melting-adhesion--wetting ( heating time)

permeation-softening-solidification--adhesion (cooling time)

In this process, thermosol changes from glassy state to high-elastic state and then to viscous flow state, then it changes from viscous flow state to high-elasticity state and back to glassy state. In this going there and back process, thermosol will go through glass-transition temperature and viscous temperature and its change is just physical change. What make actions to this process are temperature, time and pressure which are the three key elements of pressing adhesion.

4). The three key elements of pressing adhesion

（1）Pressing temperature

The concept of temperature----it denotes the necessary condition that solid thermosol becomes liquid after being heated and melted. It is the chief element of pressing adhesion. In order to achieve the best matching of fabric and lining, the temperature condition must be controlled well. To achieve this aim, we must firstly be clear and definite about the different concepts of temperature.


A. Gauge temperature: it denotes the temperature that the gauge on the pressing machine indicates.
B. Storeroom inside temperature: it denotes the factual temperature between the top press band and bottom press band inside pressing machine. It can be measured by the thermocouple on the surface.
C. Adhesive face temperature: it denotes the temperature of the interface of fabric and fusible interlining. It can be measured by thermopaper.
D. Adhesive temperature: it denotes the best temperature after thermosol's melting.


The determination of pressing temperature

Pressing temperature is the adjusting comparative temperature of pressing machine determined according to the temperature difference between adhesive temperature and pressing equipments. It is also called gauge temperature. The reading of the gauge temperature of Pressing machine (continuous) does not equal to storeroom inside temperature completely. The gauge temperature is only the temperature of the point that thermo-sensor contacts but not the factual temperature inside the storeroom. Especially when the heat quantity of the heaters of pressing machine is not even, the temperature difference will be bigger. Because of the heat conductivity difference of fabrics, the effective temperature of adhesive face is variable. Generally it is lower than storeroom inside temperature by about 5℃. Adhesive temperature is the needed temperature during adhesion. Usually adhesive temperature is higher than thermosol melting point by about 30℃. So we should pay attention to these temperature difference elements when determining pressing temperature. The determining formula of pressing temperature is:
Thermosol melting temperature + adhesive temperature difference + adhesive face temperature difference = pressing temperature

After getting the pressing temperature by above-mentioned formula, do adhesion testing to test its peeling strength, washing effect and then confirm the best pressing adhesion temperature. For example, the melting scope of PA fusible interlining is 110-125℃, the calculating process is 110℃+30℃+5℃=145℃, the result is the lowest temperature; 125℃+30℃+5℃=160℃, the result is the highest temperature. The best temperature is confirmed between 145℃--160℃. After getting the best temperature by calculating, we should have a good command of gauge temperature, the temperature difference of left, middle, right and the temperature and humidity change of weather to determine the adjusting temperature of pressing machine.

Pressing temperature plays a decisive role for adhesion quality. Improper temperature will not create the best peeling strength and excellent property. Both over-high and over-low temperature will lead to pseudo-adhesion. When temperature is very low, the melting of thermosol will be insufficient, the adhesion force will be low and the bonding of lining and fabric will be unstable. In serious condition, there would appear degumming and frothing phenomena. When the temperature is very high, thermosol will go through aging, melting, its viscosity will become smaller, it will penetrate into fabric in excess and make the strength degrade, and the adhesion fastness of lining and fabric will decline. When the temperature is over high, the fabric would present superheating shrinking, frothing, discolorization, hardening and seriously affect pressing quality.

To determine pressing temperature scientifically, we must have a good common of lining's property and the major technical parameters and know fabric's gram-weight, heat shrinking, color sublimation or other fabric's properties.

The influence that pressing temperature makes on adhesion effect
Pressing temperature has direct influence on the peeling strength, elasticity and stiffness of adhesive fabric.

A. The influence on peeling strengh

Pressing temperature directly influences peeling strength, see graph-3. When pressing temperature is lower than the melting point scope of thermosol, there would be no adhesion effect. When pressing temperature reaches the melting point scope, adhesion starts. With the increasing of pressing temperature, peeling strength improves quickly. When the temperature reaches adhesive temperature, peeling strength reaches the highest value. During the adhesive temperature scope, peeling strength will not improve. And when the temperature increases again, thermosol will engender thermolysis and lose its intrinsic property completely.

B. The influence that pressing temperature makes on elasticity, see graph-4.

The curve in the graph indicates that the higher the temperature is, the lower the elasticity is. Calculating with the adding of the warp-wise and filling-wise slow elasticity, measuring from 140-175℃, if the temperature increases by 5℃, the elasticity will decline by 1.14%, and during this temperature, the elasticity decline by 8% accumulatively.

C. The influence on stiffness

See the influence that pressing temperature makes on stiffness in graph-5. The smallest value of stiffness is 5.7cm when the temperature is at 160℃. Take this data as 100%, with the rising of temperature, the stiffness improves accordingly. During the temperature of 160-180℃, if the temperature increases by 5℃, the stiffness will increase by 5℃ by degrees. During this temperature phase, the stiffness increases by 8% accumulatively.

（2）Pressing time

Pressing time denotes the needed time that makes thermosol reach effective adhesion by heating and it is the assistant condition of temperature. During pressing adhesion, pressing temperature is not isolated. It is affected by pressing time to great extent.

The determination of pressing time

The whole pressing process falls into three stages, and the time each stage needs falls into heating time, adhesion time and fixation time. Because fixation is carried through after eliminating pressure, pressing time is often considered as heating time and adhesion time. Heating time has something to do with fabric's thickness and heating conducting coefficient. It also has something to do with the heat conducting way of pressing machine. Adhesion time depends on the inbibition time and diffusion rate of thermosol. It shortens with the increasing of the melting index of thermosol.

The control of pressing time

Pressing time and pressing temperature are two different concepts. But in adhesion process, they are two important and closely associated parameters. During pressing adhesion, under the precondition of satisfying fabric's thermal stability, needed adhesion fastness and satisfying adhesive-bonded fabric's requirements, the temperature and time can adjust with each other and compensate each other. See graph-6. Temperature rises, time shortens; temperature falls, time lengthens. Within the temperature fabric allows, we can appropriately heighten temperature and shorten time to improve labor efficiency. For the fabric that has bad heat resistance, we can lengthen time and lower temperature to guarantee adhesion quality. Of course, the proper choice of time and temperature has to be obtained through test.

The influence that pressing time makes on adhesion strength

From the melting index of thermosol we can see that thermosol's thermal flow time is very short-only from several seconds to dozens of seconds. So imbibition time is a very important problem. Proper pressing time can make thermosol fully imbibe fabric and diffuse and penetrate into its inside. This is very helpful for the improvement of peeling strength. But overlong pressing time would lead to strike-though and the peeling strength will reduce instead.

(3) Heating pressure

The pressure is for implanting the liquid-state thermosol in the structure space of bonded fabric. The choice for pressure lies on different kinds of thermosols and needs a integrative consideration in experience and effects. To choose the exact pressure, it should realize the functions of pressure and its influence on the bonding effect.

Functions of pressure:
Heating pressure has three functions. The first function is to stick backing fabric tightly to face fabric, which is convenient for heat conduction. The second is to impulse the thermosol flowing to permeate the fabric. Especially for the HDPE thermosol, which has low melting index and poor theric fluid, it should put a big pressure (generally 3-4kg/cm2). The third is to reduce the space between thermosol and fabric, which will be convenient for implanting the thermosol in the fabric and diffusing the thermosol molecule lines with the fiber molecule lines to improve the bonding strength.

Influence of pressure on the bonding effects:
Pressure is to improve the bonding strength, but an improper pressure will directly make an influence on the bonding effects. A smaller pressure couldn't make a strengthening impact, while an excess pressure will destroy the integrative performance after bonded. That's because with an excess pressure, the thermosol is pressed to extend with a thin form and exude the fabric, which will cause the fabric hardened and reduce verticality, handle, air-permeability and wearability.

Relations between pressure and peeling strength:
Pressure has positive or negative correlations to the peeling strength. Before thermosol is implanted in the fabric and reaches to a certain standard needed by the fiber interface, the peeling strength rises with the rising of pressure. And when it reaches to the standard, the peeling strength will decrease with the rising of pressure. That's because the best peeling strength destroys the cohesion of thermosol. Before reaching to the standard needed by the fiber interface, the cohesion hasn't achieved its maximum, so the peeling strength has a positive correlation to the pressure. And when the thermosol reaches to the standard, the rising of the pressure will make the thermosol exude fabric, so the thermosol in the fiber interface can't meet the need standard and the cohesion decreases. At the moment, the peeling strength has a negative correlation to the pressure.