Bonding Foils in the Smart Card Sector

By Andreas Mehlhaff, DELO Industrieklebstoffe

The smart card branch of the card industry is currently in a rapid phase of development. In the future, the technical capabilities and areas of application of the chip card will expand immensely and also be integrated in many areas of our lives. Soon chip cards with displays, keyboards, sensors and other elements ("System on Card") will be established. Because the high pressure and temperature used during heat-lamination are dangerous for these sensible components, it is necessary to develop more careful processing procedures. Cold-lamination is an interesting alternative to the currently dominating heat-lamination procedure used to manufacture smart cards. This means that the single smart card foil layers are bonded to a foil compound by using adhesives which cure at room temperature.

As part of a dissertation, conventional smart card plastic foils such as PVC, PETG, PC and ABS were bonded with light-activated cationic-curing epoxy resin adhesives (DELO-KATIOBOND 4552, DELO-KATIOBOND 4557 and DELO-KATIOBOND 4571). After establishing the necessary processing parameters, the bond strength of the laminated test samples was to be determined. In order to fulfill today's smart card requirements, the properties of the bonded test samples were to be examined after simulated ageing tests as well as a mechanical test, the so-called "dynamic flexion test."

The photo-initiated epoxy resin adhesives manufactured by DELO Industrieklebstoffe contain photinitiators for UVA radiation and visible light (VS). Therefore, it was possible to use the preactivation procedure for the preparation of these test samples: after application on the foil, the adhesive is briefly radiated with visible light which then starts the activation (radiation-initiated polymerization). Afterwards the second foil is joined. The preactivation principle of DELO-KATIOBOND adhesives is explained by the following processing steps:

When working with photoinitiated systems, the UV-curing device has to be optimally adjusted to the light-activated adhesives. After the processing parameters, radiation intensity and activation time were determined, 25 mm wide compound strips were prepared according to DIN EN 1895. The T-peel test was applied to examine the peel resistance or peeling strength of the bond.

According to the DIN ISO 7810 "Identification cards" standard, a minimum bond strength of 6N/cm is required during the T-peel test. The peel test was successful if during the test procedures it is obvious that the bond strength is higher than the tearing resistance of the used plastic foils (join part break).

The following became clear during the test procedures:

• test samples made of PVC and ABS which were bonded with
  epoxy resin adhesives supplied very good results
• a low peel resistance was determined by bonds with PETG and PC.

To improve the bond strength of PETG and PC test samples, both plastic foil types received a surface pretreatment. It was determined that PETG-foils pretreated with HF-Corona and PC-foils pretreated with atmospheric plasma resulted in improved peel resistance values with an adhesive system (DELOKATIOBOND 4571).

The table below displays that the established peel values requested by DIN ISO 7810 "Identification cards" highly exceed 6N/cm.

Peel resistance of PVC-, ABS-, PETG- and PC-bonds with light-activated epoxy resin adhesives.

Plastic foil	Surface pretreatment	Adhesive DELO-KATIOBOND	average in [N/cm]	Break behaviour
PVC	none	4552	-	join part break
PVC	none	4557	20-26	cohesion break
PVC	none	4571	14-16	cohesion break
ABS	none	4552	-	join part break
ABS	none	4557	-	join part break
ABS	none	4571	13	combination break
PETG	HF-Corona	4571	-	join part break/ brittleness of foil
PC	atmospheric plasma	4571	12	cohesion break/ join part break

Processing parameters: thickness of adhesive layers: 50µm
radiation intensity: 50 mW/cm2
activation time: 1.3 s

Testing the bond strength after temperature storage ("aging test")

Although smart cards are subjected to a multitude of physical stress in everyday life, their reliable use must be maintained. According to ISO 7816-1, a smart card should be fully functionable by 0°C to 50°C. Part of this examination was to subject the test samples to a one-month "ageing test" and to evaluate the peeling strength of the bonded test samples thereafter. During the temperature storage the bonds were constantly checked for delamination and other visible changes. To determine changes in the bond strength of the test samples, the peeling strength of "aged" as well as "non-aged" test strips was examined and compared at different surrounding temperatures (room temperature and 50°C).

Results:

The values of 40N/cm represent a join part break and not the peeling strength of 40N/cm. Outstanding results were achieved by bonds made of PVC and ABS. The PC test samples bonded with the adhesive DELO-KATIOBOND 4571 barely reached the norm. It was not possible to measure the average peel resistance of the PETG bonds after the "ageing test" because the foil was brittle. After the PETG foil was subjected to a temperature of 50°C for a longer period of time, the material was brittle and crumbly even in places which were not bonded.

Dynamic flexion test

Aside from environmental influences such as temperature and humidity, the mechanical requirements of a chip card also play a big role. One of the most frequent performed mechanical tests is the "dynamic flexion test." Since these are bonded foil compounds, this test is mainly for checking out signs of delamination on the test samples. ISO 7816-1 requests that a Smart Card withstands a flexion cycle of 1000 flexions. The test samples, which had chip card format, were subjected to even longer flexion cycles at different surrounding temperatures:

• 8000 flexions at room temperature (ISO 7816-1)
• 4000 flexions at 50°C (ISO 7816-1)
• Flexion until mechanical failure or delamination of cards (DIN 32753).

Results:

Plastic foil	Surface pretreatment	Adhesive DELO-KATIOBOND	Test IAW ISO 7816-1 at room temperature	Test IAW ISO 7816-1 at 50°C	Test IAW DIN 32753 at room temperature
PVC	none	4552	pos.	pos.	10500
PVC	none	4557	pos.	pos.	5600
PVC	none	4571	pos.	pos.	7100
ABS	none	4552	pos.	pos.	8400
ABS	none	4557	pos.	pos.	3600
ABS	none	4571	pos.	pos.	6000
PETG	HF-Corona (dose 6-9)	4571	pos.	pos.	19300
PC	atmospheric plasma	4571	pos.	pos.	19300

The results demonstrate in accordance with ISO 7816-1 that all examinations at room temperature as well as at 50°C were always successful. In accordance with DIN 32753, bonds of PETG/KATIOBOND 4571 and PC/KATIOBOND 4571 could be stressed the longest with 19300 flexions until the foil broke. The highest number of flexions were determined when the test samples PVC and ABS were bonded with DELO-KATIOBOND 4552, the second highest with DELO-KATIONBOND 4571 and the lowest with DELO-KATIOBOND 4551. Decisive is that
no delamination was discovered by any of the examined test samples.

Summary

In the future, the cold-lamination technique (bonding of single card layers) will succeed for manufacturing smart cards with integrated elements (display, battery, sensor, etc). As part of a dissertation, conventional smart card plastic foils were bonded with light-activated, cationic-curing epoxy resin adhesives from DELO Industrieklebstoffe and the bond strength examined.
In summary, one can say that bonds made of PVC and ABS fulfilled the required norms with all of the three adhesive systems. Bonds with PC and the adhesive DELO-KATIOBOND 4571 fulfilled the desired requirements. PETG is not suited for bonding in the smart card sector because the foil turns brittle. See the chart for product parameters to help you better understand the adhesive properties of the foils discussed in this article.

Product parameters DELO-KATIOBOND		4552	4557	4571
Areas of application		bonding, sealing, coating	bonding, sealing, coating	bonding, sealing, coating
viscosity [mPa*s] IAW Brookfield		1200	3500	11000
colour in cured condition	in 0.1 mm layer thickness	brown, clear	brown, clear	brown, milky
	in 1.0 mm layer thickness	brown, clear	brown, clear	brown, milky
Wavelength range for curing [nm]		400-550	400-550	400-550
shore strength		D 49	D 38	D 40
tensile strength [MPa]		14	10	10
elongation at tear [%]		3	18	40
modulus of elasticity [MPa]		730	120	not measurable
water absorption [weight %]		1.3	1	0.6
average linear expansion		100	90	130
coefficient [ppm/K]
Glass transition temperature [°C]		140	75	45
Temperature range of use [°C]	permanent	-30 to +150	-30 to +150	-30 to +150
	short-term	+180	+180	+180
product specialties		good flow behaviour strong bond, fast curing	good flow behaviour tough-hard	medium-viscous tough-elastici equalizing