Task and Function of the Bonding Material

The bonding materials have the task of firmly attaching the strain gauge to the measured object’s surface and transferring the object’s deformation without loss to the strain gauge. The various conditions, influences, and application options, require different bonding materials and installation methods. Bonding is the most important factor.

The particular advantages of this connection method, with regard to strain gauge installations, are as follows:

The possibility of connecting different materials, even dissimilar ones. The connection is implemented at room temperature or higher temperatures, depending on the type of adhesive.
It has no influence on the materials which are to be connected (however, some restrictions may apply for plastics).
Chemically-hardening adhesives (these are only still used in strain gauge technology) are characterized by low moisture absorption rates.
Control of working speed through the selection of different adhesive types or curing conditions (hot or cold curing).
Higher specific electrical resistance contributes to higher insulation resistance between the strain gauge and the component.

Types of Bonding Materials

Both the working conditions at the installation site, and the various requirements for the bonding material performance, particularly in regard to the operating temperature, have led to the availability of various types of bonding materials. Bonding materials can be differentiated as follows regarding the application technology.

1. Cold-curing Adhesive

These can be easily applied and do not require much effort. There are single-component adhesives that start curing with appropriate humidity as well as two-component adhesives that must be mixed before application. Adhesive with very short reaction times are also called “superglues”. They are applied preferably in experimental tests.

2. Hot-curing Adhesives

These adhesives can only be used if the test object can be brought to the required curing temperature. This is generally possible in transducers manufacturing, but also where strain gauges can be installed before machine assembly, or where parts can be dismounted for strain gauge installation. In contrast to a cold-curing adhesive, the hot-curing adhesive offers a wider application range at higher temperatures, and is suitable for meeting the generally higher accuracy requirements in transducer production.

Select the Correct Glue for Your Strain Gauge

Important Information

It is essential not to use any adhesive other than the recommended one. Strain gauge adhesives must fulfill different requirements than general adhesives. This is why they are generally based on special developments or modifications of commercial adhesives. The simple adherence of a strain gauge to an object is not a sufficient criterion to evaluate the adhesive’s suitability for measurement purposes, it must also ensure the object’s strain’s faultless transmission. This requires more in-depth investigations (strain gauge tests according to VDI/VDE 2635 or comparable standards automatically include the adhesive).

Useful Temperature Range for Bonding Materials

Bonding material (adhesive)	Main application areas	Material	Useful temperature range (approx.) for zero-point related¹⁾ and for non zero-point²⁾ related measurements³⁾		No. of components
X60, cold-curing (superglue)	Experimental test	Methacrylate	¹⁾	-200°C …+60°C [-328°F …+140°F]	2
			²⁾	-200°C …+60°C [-328°F …+140°F]
Z70, cold-curing (superglue)	Experimental test, transducer manufacturing with lower accuracy requirements	Cyanoacrylate	¹⁾	-55°C …+100°C [-67°F …+212°F]	1
			²⁾	-70°C …+120°C [-94°F …+248°F]
X280, cold-curing	Experimental test	Epoxy resin	¹⁾	-200°C …+200°C [-328°F …+392°F]	2
			²⁾	-200°C …+280°C [-328°F …+536°F]
EP70, hot-curing	Experimental tests at elevated temperature range	Epoxy resin	¹⁾	-40°C …+70°C [-104°F …+158°F]	2
			²⁾	-40°C …+70°C [-104°F …+158°F]
EP310N, hot-curing	Experimental tests at elevated temperature range, transducer manufacturing	Epoxy resin	¹⁾	-269°C …+260°C [-454°F …+500°F]	2
			²⁾	-269°C …+310°C [-454°F …+590°F]
P250/P250-R, hot-curing	Transducer manufacturing	Phenolic resin	¹⁾	-196°C …+250°C [-320°F …+482°F]	1
			²⁾	-196°C …+250°C [-320°F …+482°F]
P250 Stick-on, hot-curing	Transducer manufacturing	Phenolic resin	¹⁾	-196°C …+250°C [-320°F …+482°F]	-
			²⁾	-196°C …+250°C [-320°F …+482°F]
DP490, cold-curing	Experimental Test	Epoxy resin	¹⁾	-55°C …+120°C [-131°F …+248°F]	2
			²⁾	-55°C …+120°C [-131°F …+248°F]

¹⁾^{With zero-point related measurement, the measured values are referenced to the zero point (usually static measurements).}

^{2) With non zero-point related measurement, the zero point can fluctuate, only the dynamic part is important (dynamic measurement).}

^{3) The specified temperature limits are fluid and depend on strain gauges being used, on the expected measurement accuracy and on the curing process.}

Bonding material (adhesive)	Pot Life	Curing conditions			Suitable for strain gauges in series
Bonding material (adhesive)	Pot Life	Temp.	Time	Contact pressure	Y, C, M, D	G	A, U	E
X60, cold-curing (superglue)	3 … 5 min	0°C [32°F] 20°C [68°F] 35°C [95°F]	60 min 10 min 2 min	Thumb pressure	o	o	o	–
Z70, cold-curing (superglue)	-	5°C [41°F] 20°C [68°F] 30°C [86°F]	10 min 1 min 0.5 min	Thumb pressure	o	o	o	o
X280, cold-curing	30 min	20°C [68°F] 65°C [149°F] 95°C [203°F]	8 h 2 h 1 h	0.05 … 2 N/mm²	o	o	o	o
EP70, hot-curing	2 hours	60°C [320°F]	3 h	contact pressure	B Series
EP310N, hot-curing	4 weeks	150°C [302°F] 180°C [356°F] 200°C [392°F]	3 h 1 h 0.5 h	0.1 … 0.5 N/mm²	•	•	•	•
P250/P250-R, hot-curing	Unlimited	140°C [284°F] 160°C [320°F] 190°C [374°F]	4 h 3 h 1 h	10 – 50 N /cm²	•	•	•	–
P250 Stick-on, hot-curing	-	140°C [284°F] 160°C [320°F] 190°C [374°F]	4 h 3 h 1 h	10 – 50 N /cm²	•	•	•	–
DP490, hot-curing	12 months	20°C [68°F] 30°C [86°F] 40°C [104°F]	10 h 20 h 40 h	contact pressure	•	•	•	•