Development and analysis of novel electroluminescent yarns and fabrics for localized automotive interior illumination

Driver section

The electro-conductive yarn is delivered from a package, which is held by the yarn package holder. The yarn unwinds from the top end of the package into a yarn feeding unit. This unit consists of a yarn guide and a roller of 50 mm diameter (Figure 3), which is driven by a stepper motor. OPEN IN VIEWER

The yarn dispensed from the cone package is guided through the built-on yarn guide to the roller. For proper feeder operation, the yarn is wound once around the roller.

The yarn is then sent through the dispensing and curing sections and guided by a stepper-motor-driven yarn take-up unit, which has a similar construction to the yarn feeder unit (Figure 3). The yarn is then wound as a cheese package by a stepper-motor-driven yarn winder unit (Figure 4). OPEN IN VIEWER

Coating system

The yarn delivered from the yarn feeder unit is sent through three dispensing and curing sections of the coating system. The first section applies a single layer of the dielectric paste, the second section applies a layer of phosphor ink, and the third section applies a single layer of the transparent encapsulant. The operation of the dispensing sub unit and the curing sub unit is described in the following sections.

Control system

A Labview-based software was created to control the yarn coating system. The program generates signals to control the dispense and curing units, which is converted to TTL pulses by a NI PCI 7334 stepper motor controller unit and sent to the respective equipment. Moreover, the stepper motors of the system are also activated by the same software program with an NI MID 7664 stepper motor controller.

System operation

The system is set up on a metric base plate of 1500 mm × 300 mm. The syringes of the dispenser units, the UV liquid light guides of the curing units, and the associated parabolic mirrors, are mounted on this base plate using posts, bar type lens filter holders, cross clamps, and optical mounts. ¹³ Moreover, the yarn feeder unit, the yarn take-up unit, and the yarn winder are also assembled on this base. The assembled system is shown in Figure 8. As can be seen from Figure 8, the yarn is taken from the yarn package through a ceramic yarn guide on to the yarn feeder unit. Next the yarn goes through the dielectric paste dispenser section and the UV curing sections. Afterwards the coated and dry yarn goes to the phosphor ink-dispensing section via a yarn guide. The phosphor-coated (wet) yarn is then cured by the UV section. This phosphor-coated and dry yarn then enters the transparent fluid dispensing section via a ceramic yarn guide. The UV section cures this (wet) yarn, which is then taken up, via the yarn take-up unit and a ceramic yarn guide, by the yarn winder, which winds the yarn evenly on to a cheese package. OPEN IN VIEWER

For a proper coating operation and locomotion of the system, it has been observed that the yarn requires tension devoid of slack. This can be achieved with all three stepper motors operating in unison. Moreover, to obtain tension devoid of slack in the flow of yarn through the system, the yarn winder can be operated while the yarn feeder and yarn take-up units are switched off prior to the coating operation. Furthermore, to avoid bulging due to gravity of the uncured fluid due to its weight under the yarn at the phosphor coating stages, the yarn is set at an inclination. This causes the uncured fluid to trickle away from the curing section and drop to the metal base plate, resulting in yarn to free from gnarls after the curing stage.

Structure of the EL yarn

The multifilament electro-conductive yarn, after passing through the bath holder of the dielectric paste coating stage, conforms to a monofilament yarn due to the paste adhering to the yarn surface by means of its surface tension and viscosity (Figure 9). Moreover, the interstices between filaments may be filled with the paste due to wicking. OPEN IN VIEWER

Thus, after the curing stage there is a reduction in the flexibility of the yarn. An image of the cured yarn of coated with 8153L dielectric paste, taken from an optical microscope, is shown in Figure 10 where the compacted individual filaments are evident. OPEN IN VIEWER

The dielectric paste holds the compacted filaments together even when there is zero tension on the yarn. As such the application of the phosphor ink layer is in effect coating a monofilament. The phosphor coating adheres on to the outer surface of the dielectric paste coated yarn due to surface tension (Figure 11). OPEN IN VIEWER

An optical microscope image of the yarn coated with two layers of 8152L blue green phosphor ink is shown in Figure 12, where it can be seen that the phosphor coating on the yarn is virtually devoid of any gnarled surfaces and the yarn is now effectively a monofilament yarn. OPEN IN VIEWER

Similarly, the coating of the DYMAX E-9001 V3.5 transparent flexible layer in effect is coating a monofilament yarn. Therefore, in this case, the fluid also adheres to the outermost surface of the phosphor coating of the yarn due to surface tension.

Finally, a 0.174 mm diameter copper strand was wound around the yarn in a helical manner using an AGTEKS Direct Twistcone cone-to-yarn twisting machine. An optical microscope image of the completed yarn is shown in Figure 13. Images of the EL yarn in the undriven and driven states are given in Figure 14. OPEN IN VIEWER

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Figure 14.

Images of electroluminescent (EL) yarn in undriven and driven states. (a) EL yarn in the unexcited state. (b) EL yarn when excited by an alternating current power source.

Material content of the respective layers

In order to determine the material content of the respective layers, the weight of the uncoated yarn was measured, being the average weight of 10 samples of the uncoated yarn each 30 cm in length (w_yarn). Secondly, the weights of 10 samples each of 30 cm length of this yarn coated with a single layer of the dielectric paste were measured and their average taken (w_dielectric). Similarly 10 samples of 30 cm length of single layers of dielectric paste and phosphor ink coated yarn were measured, their average being (w_phosdie).

Finally the weights were measured of 10 samples 30 cm of length of the complete inner EL yarn, their average being w_EL.

The material content of the dielectric paste (m_die), the phosphor ink (m_p), and the transparent encapsulant (m_e) for a unit length of the yarn at the cured state are obtained from the following equations:

(1)

(2)

(3)

The volume of material of a particular layer in the uncured state can be obtained by multiplying its material content in the cured state with the solid content of that particular material. The solid contents of the dielectric paste, the phosphor ink, and the transparent flexible encapsulant are 67%, 61%, and 100%, respectively.^7,9,11 The measured data and the calculated values of the material content in the cured and uncured state of a particular single layer for a unit length of the yarn are shown in Table 1 and Table 2, respectively.

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Table 1.

Measured data of a single layer of electroluminescent (EL) yarn in the cured state

Material type	Weight (g/m)
Uncoated yarn (wyarn)	0.083
Dielectric paste coated yarn (wdie)	0.14
Phosphor ink and dielectric paste coated yarn(wphosdie)	0.27
Complete EL yarn (wEL)	0.48

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Table 2.

The material content of single layers of the respective coating both in the cured and uncured state of the electroluminescent yarn

State	Material content of dielectric paste (g/m)	Material content of phosphor ink (g/m)	Material content of transparent encapsulant (g/m)
Cured	0.14	0.13	0.21
Uncured	0.21	0.21	0.21

The volume of the material content is effectively measured in terms of weight, as these are commercially available in weight rather than volume. It can be seen from Table 2 that in the cured state the material content of the transparent encapsulant is nearly the same as the combined weight of the dielectric and phosphor layers.

Coating thickness of individual layers of the EL yarn

To measure the thickness of each individual layer, the diameter of the uncoated yarn was first measured (d_y), followed by the diameters of the dielectric paste coated yarn (d_dy), the phosphor and dielectric paste coated yarn (d_dpy), and the complete EL yarn (d_EL). A Projectina optical microscope with associated Projectina 4000 image acquisition software was used for this purpose, measurements being taken at 50 different locations on the yarn samples described in the previous section.

Considering the coated layers and the yarn as concentric cylinders, the thicknesses of the dielectric layer (t_die), phosphor layer (t_p), and the encapsulation layer (t_enc) can be determined as follows:

(4)

(5)

(6)

The measured and calculated data is shown in Table 3.

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Table 3.

Thickness of individual layers of the electroluminescent yarn

dy (mm)	ddy (mm)	ddpy (mm)	dEL (mm)	tdie (mm)	tp (mm)	tenc (mm)
0.296	0.38	0.5	0.72	0.04	0.06	0.1

From Table 3 the thickness of the transparent encapsulant is seen to be higher than the thickness of the other layers, as there is more material content of this layer on the yarn in comparison to the material content of the other layers. Moreover, it is the same as the combined thickness of both the dielectric and the phosphor coating layers.

Therefore, as the thickness and the material content of the transparent encapsulant is more or less the same as the coating composed of both the dielectric and the phosphor coating layers, the flexible properties of this layer may offset the brittle nature of the combined phosphor and dielectric layers. Thus the yarn has the appearance of a flexible plastic strip.

Analytical prediction of luminance of EL yarn

Both the dielectric and transparent insulation layers of the EL coating of the yarn act as capacitors, with capacitances per unit area of C_d and C_t, respectively. When the applied AC voltage (which is a square wave form) is increased from 0 volts the phosphor coating acts as a leaky capacitor beyond a certain threshold voltage (V_th), which can be best described as a capacitor in parallel with a non-linear resistor of resistance R_EL. This phenomenon can be depicted as the electrical circuit shown in Figure 15, based on the corresponding electrical circuit derived by Ono ¹⁵ for thin-film AC EL devices. OPEN IN VIEWER

The luminance (L) of the yarn can be described from the derivation given by Ono ¹⁵ for thin-film AC EL devices as

(7)

The inner conductive yarn of the EL yarn is assumed to be a cylinder and the coating layers around it are considered as concentric cylinders. Moreover, the fine copper wire wrapped as a helix about the EL yarn can be assumed as composed of circular loops separated by the pitch (p) of the helix, considering the methodology used in analyzing the radiation field of helical antennas. ¹⁶ The cross section of the copper wire is assumed to be a rectangle with its side in contact with the EL coating equal to its actual diameter (d_c). Thus, EL yarn can be depicted as in Figure 16 based on these assumptions. OPEN IN VIEWER

The nomenclature for the inner yarn and the coating layers in Figure 9 is as described in the Coating thickness of individual layers of the EL yarn section. The capacitances of the dielectric layer (C_d) can be given as follows, upon considering the concentric cylinder of the dielectric layer and the inner conductive yarn: ¹⁷

(8)

This can be expressed in terms of the coating thickness of the dielectric (t_die) layer:

(9)

Similarly, by considering the concentric cylinders of the complete EL yarn, the capacitance of the transparent layer can be expressed in terms of the thickness of the transparent encapsulation (t_enc), the phosphor (t_p), and the dielectric (t_die) layers:

(10)

The series capacitance (C_it) of the dielectric and transparent encapsulation layers can be given as

(11)

According to Ono, ¹⁵ the threshold voltage of the EL phosphor is governed by the applied electric field through the phosphor layer and its thickness, as shown below:

(12)

In order to calculate the luminance L, Equations (9) and, (10) are substituted into Equation (11), and the resultant is in turn substituted into Equation (7). Equation (12) is substituted into Equation (7). The simplification of the equations as explained above was carried out by using Mathematica, and the result is given below:

(13)

Equation (13) gives the luminance of the EL yarn in terms of the thickness of the dielectric, the phosphor and the encapsulation layers of the EL coating, the applied voltage, and the frequency.

EL yarn driver and illuminance measurement system

Effect of twists per meter of the outer conductive yarn on the luminescence of the EL yarn

To analyze this effect, three samples of EL yarn of 1 meter length were prepared with the electro-conductive yarn described in the Electroluminescent yarn construction section, wound with a 0.174 mm, diameter 40 SWG copper wire. The samples were wound with 400, 200, and 100 turns per meter (TPM); they were denoted as EL400, EL200 and EL100, respectively.

The EL yarns were driven with a square waveform from the driver system explained in the section titled The EL yarn driver. The applied rms voltage (V_rms) was measured as 370 V and its frequency was varied from 100 to 2000 Hz. The luminescence was detected using the illuminance detector explained in the section titled The illuminance measurement system. The measurements were taken on five random locations of a particular sample. Three sets of readings were taken on a particular location. All the readings were then averaged to obtain the illuminance with respect to the driven frequency of theparticular sample. The measured illuminances of the yarns are given in Figure 19. OPEN IN VIEWER

The luminance of the yarns was simulated using Equation (13). The applied peak voltage (V_a) required for Equation (13) was obtained as 523.18 V from the following relationship:

(14)

The rms threshold voltage of the EL yarn at which illumination can be detected was measured as 60 V using an Agilent 34401A Digital Multimeter. Thus, the peak threshold voltage (V_th) of the EL yarn was obtained as 90 V from Equation (14). The permittivity of the transparent encapsulant (ε_t) and the dielectric paste (ε_d) were obtained from their manufacturers as 3.27 and 40, respectively. The electric permittivity of free space was taken as 8.85419 · 10^{- 12} F/m. ¹⁷ The threshold electric field is taken as 1.5 Mvolts/cm [19]. The thickness of the transparent encapsulant (t_enc), the phosphor layer (t_p), and the dielectric paste (t_die) of the yarn were obtained from Table 3. The simulated luminances of the yarn samples are given in Figure 20. OPEN IN VIEWER

It can be seen from Figures 19 and 20 that the luminescence of the EL yarn increases more or less linearly with the frequency of the applied voltage. Moreover, the figures indicate a general trend in the increase of luminescence with the increase of the TPM of the EL yarn.

The illuminance of a single strand of the EL yarn is poor, as it is less than 1 lux; however, the illuminance canbe improved by integrating more EL yarns into a fabric.