Adhesives and Sealants I Bonding of Commercial Vehicles

Structural Adhesives withCustomized Fast Curing
Commercial transportation industry is constantly challenged by increasing production efficiency
and reducing costs, while introducing new design solutions based on lightweight materials. Twoinnovative adhesive technologies, 'Powerflex' and 'Curing by Design', have been developed tomaximize the potential offered by modern lightweight design as well as to improve productivity
by immediate curing at precisely defined time.

Claudio Di Fratta, Antonio Corsaro, David Hofstetter, Michael Schlumpf, Bernhard Bosshard, Marko Zivaljic, Fabien Choffat

For instance, a lighter bus can accommodate more passenger seats; a caravan withthinner walls has more internal space thatcan be organized for a more livable environment for travelers. Although lightweight solutions bring various advantages in engineering and design phase, theirimplementation may be not successful if it
disregards the needs of production departments, which often struggle with efficien-© Sika AG

The development of commercial and special vehicles (e.g., buses, caravans, trucks,trailers, etc.) is progressively more characterized by lightweight design and production efficiency. The reduction of vehicles’ weight by introducing lighter materials or multi-material design concepts
leads not only to the achievement of less
CO2 emissions, but also to the incrementof goods capacity and passenger comfort.

Figure 1 > Comparison of DMA between “Powerflex” technology and standard structural
adhesive in commercial transportation

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cy problems. While production managers
aim at speeding up fabrication and assembly of vehicle components, they must reduce manufacturing costs. Moreover, theyhave often to cope with order backlogs,
which make harder to increase output atlower costs.
Among the diverse methods available forassembling and joining vehicle components, adhesive bonding keeps up withmanufacturers’ challenging demands,
as well as can open new possibilities forlightweight and multi-material design.
In order to achieve the full potential offered by assembly bonding, however,structural adhesives are required to show
higher performance and an enhanced curing behavior, compared to standard bonding products used in commercial transportation industry. As an example, combining aluminum with steel alloys or withmodern composite materials poses the issue of the accommodating differences inthermal expansion [1]. This is particularly problematic when external temperatures are low, because standard structural adhesives employed by commercial vehicle manufacturers typically undergo asignificant embrittlement in cold conditions. Consequently, they are not able tocompensate for thermal stresses and expose the joints to failure, even if they appear flexible and compliant at room tem-Material property

Typical value range

Tensile strength

10 – 20 MPa

Elongation at break

100 – 400 %Elastic modulus

20 – 800 MPa

Lap-shear strength (1 mm thick)

10 – 20 MPa

Impact peel strength> 50 N/mm

Shore A hardness

80 – 95

Table 1 > Typical properties achievable with 'Powerflex' technology

build-up with sufficiently long open times
to apply adhesives and assemble parts. In
this way, waiting times during component
bonding can be largely reduced, withoutcompromising on the assembly working
times. The following examples describe
the features of the new technologies withreference to practical advantages for commercial transportation industry.

Features of the 'Powerflex'technology
Sika’s 'Powerflex' technology [2],[3],[4] enables the formulation of 2C polyurethanebased adhesives with remarkably low
glass transition temperature Tg (around-45 °C) and, at the same time, high mechanical strength and modulus in rubbery
state above Tg. This feature allows enhanced toughness and impact resistanceof the bonded assemblies over the wholeservice temperature range and life cycle.
Moreover, the flexibility and the structural performance of cured adhesives result very stable between -40 °C to 100 °C.
Compared to traditional structural adhe-© Sika AG

perature, like regular 2C polyurethanes orMMA adhesives. On the other hand, industrial adhesives that are flexible enoughat low temperatures generally become
very soft in warm or hot climatic conditions. Thus, they are not likely to provide adequate joint strength and stiffness,risking the structural integrity of the vehicles.
Under the names 'Powerflex' and 'Curing
by Design', Sika has developed two innovative adhesive technologies that address
the specific issues of commercial vehicle
manufactures. These proprietary technologies can be coupled to formulate uniquebonding products, which provide several
benefits in terms of greater freedom of design, higher safety and durability, moretime and cost-effective production. Besides the ability to bond dissimilar materials and optimize stress distribution incomplex geometrical joints, the new adhesives allow for more stable properties
over the whole service temperature range,
keeping good flexibility at low temperatures and stiffness at high temperatures.
In addition, they combine a fast strength© Sika AGFigure 2 > Cohesive failures of 'Powerflex' adhesive on e-coated steel in 'Thick Adherent Shear
Tests' (TAST) and butt joint tensile tests at 23 °C and -40 °C

19© HSR – Hochschule für Technik Rapperswil

Adhesives and Sealants I Bonding of Commercial Vehicles

Figure 3 > Impact tests of T-joints bonded with a standard toughened adhesive (red curves)and a 'Powerflex' adhesive (blue curves): the larger area beneath the blue curves indicate agreater toughness and energy absorption.

sives, the modulus does not show big variations over temperature (Figure 1).
Typical ranges of properties, which characterize adhesives with 'Powerflex' technology, are reported in Table 1. The highlevel of strength achievable at large elongations derives from a patented molecular chemistry that incorporates organic hard segments into an elastic matrix
of long polymeric chains. Such hard segments, which are formed during the reaction of the two adhesive components
at ambient temperature (neither external
heating nor humidity is needed), boost thestructural performance of the flexible polymer matrix. Their content is designed in
the adhesive formulation to obtain a defined modulus: in this way, the adhesives
are tailored to match the specific mechanical properties of the substrates and maximize the load transfer through the joint.
This technology permits, for example, anexcellent bonding of high-grade steels thatare painted or e-coated, without damaging paints or coating layers, even at very
low temperature (Figure 2). Furthermore,compared to common toughened structural adhesives, the inherent elasticity given by the 'Powerflex' technology allows
for greater resilience and energy absorption in the case of an impact, as shownin Figure 3.

© Sika AG

Combination with 'Curing by Design'technology© Sika AG

Figure 4 > Comparison of curing behavior between 'Curing by Design' technology and astandard structural adhesive with same working time

Figure 5 > 'Curing by Design' working mechanism (schematically)20

adhesion 3 I 20

The advantages of 'Powerflex' can be additionally combined with 'Curing by Design' [3],[4][5], which indicates a uniqueproprietary technology that Sika has developed to accelerate curing reaction of2C polyurethane adhesives while keeping
long workability (Figure 4). In conventional 2C adhesives, the reaction starts as soonthe two components are mixed, thus the© VDL Containersystemen B.V.

Figure 6 > Application of 'Powerflex' and 'Curing by Design' adhesive for hook arm bonding on
a truck chassis

It is worth highlighting that heating up
the bond line can further speed up thecuring reaction. But external heat is notstrictly needed for fast curing, because
this is already achievable at ambient temperature. Consequently, the cost of heating devices is unnecessary and potential

quality issues related to cold spots in heated tools are avoided. Finally, it should bementioned that the 'Curing by Design'technology is based on 2C polyurethanes,
which release lower exothermic energy
and lower smell than standard MMA adhesives.

© Van Hool

working or open time is directly proportional to the curing time: The longer the
open time is, the slower the curing is. In'Curing by Design' adhesives, a catalyst
is designed to obtain a tailored delay before the fast reaction starts. This permits
to modulate adhesive working time andcuring speed separately and, therefore,
to reduce unproductive fixing or holding
time until the assembly can be handled.
Immediate curing performance, combined
to a long enough open time, is of high interest for all bonding applications and increases throughput of industrial processes significantly.
The working mechanism of the technology is schematized in Figure 5 and consists
in a two-step reaction [6]. During the firststep, the mixing of the two components (bymeans of a dynamic or a simple static mixer, as in cartridge application) does not initiate the curing reaction directly, but rather
induces the catalyst activation. This leaves
the adhesive wet and soft, as if it were freshapplied, for a sufficiently long time, matching the open time of the assembly. As amatter of fact, the duration of the first reaction step can be customized by adjusting
the adhesive formulation to the specific application requirement. In the second step,
the actual curing reaction of the two-component adhesive kicks off and progresses
very fast, thanks to the presence of the now
activated catalyst. This allows the system
to rapidly reach the strength required formoving the bonded assembly to the next
production stage.

Figure 7 > Application of 'Powerflex' and 'Curing by Design' adhesive for bus roof bonding21

© 3A Composites Mobility

Adhesives and Sealants I Bonding of Commercial Vehicles© Sika AG

Figure 8 > Extended designconcepts and modular bonding oflightweight bus roof

Figure 9 > Sika’s winning team of Swiss Technology Award 2019

Benefits for manufacturers
Two-component adhesives with 'Powerflex' and 'Curing by Design' technologies
are optimized solutions for primerlessbonding of composites (e.g., SMC, CFRP,etc.) and/or coated metals (Figure 6). Having low viscosity and a standard mixing
ratio of 1:1, these adhesives can be appliedout of cartridges or by regular dispensing
equipment. As above mentioned, the curing takes place at ambient temperature, but
heating devices can also be used, as in thecase of heated presses for SMC bonding.
22adhesion 3 I 20

An exemplary application is given in­Figure 7, which shows the bonding of an
integral lightweight composite roof to thesteel sidewalls of a bus [3]. In this case,the 'Powerflex' technology enables theconstruction of strong and flexible joints
in both hot and cold climates, i.e., independently of the traveling conditionsof the bus. Unlike traditional mechanical fixations, bonding the vehicle roof
with a 'Powerflex' adhesive avoids stressconcentrations and improves crashworthiness of the bus superstructure, withstanding dynamic rollover tests accord-ing to UN-ECE Regulation 66 [7] without
adding extra reinforcements and weight.
A weight reduction of up to 500 kg is actually achievable in comparison withconventional bus construction [3]. This
opens up new opportunities for installing and upgrading the design of diverse
modules on the roof, such as air conditioning units, luggage compartments andbatteries or fuel cells for e-mobility vehicles (Figure 8).
The bus roof bonding in Figure 7 benefits
from the advantages of 'Curing by Design', in addition to the 'Powerflex' technology, provided by the same adhesive
product. As a matter of fact, the practical assembly of a 12 m long bus roofneeds about 30-45 minutes to be completed in production. A regular polyurethane or epoxy adhesive with such longopen time typically requires a minimumof 8 hours at room temperature, in order to reach an adequate degree of curing that permits to move the assembly
forward on the production line. Conversely, an accelerated adhesive system could increase production efficiency with a faster curing, but at the costof a drastic shortening of the open time.
Therefore, the bus manufacturer would
be forced to invest in extra dispensing
equipment and employ additional workforce on the line, so to keep up with theshortened open time. Using a structural
adhesive with 'Curing by Design' technology solves the problem effectively,
giving enough open time for workers to
comfortably close the assembly (at least
45 minutes) and, concurrently, reducing the waiting time to reach handlingstrength to less than 1.5 hours after theadhesive bead application.

Conclusions
The innovative adhesive technologies'Powerflex' and 'Curing by Design' enhance the performance of assemblybonding in commercial transportationindustry. They combine in single bonding products contradictory features: highelasticity and structural strength, longopen time and fast curing behavior. For
the development and market introductionof adhesives with these unique characteristics, Sika has won the Swiss Technology Award 2019 [8] in the category 'Innovation Leaders' (Figure 9). //References[1] B. Burchardt, K. Diggelmann, S. Koch, B.
Lanzendörfer, R. Wappmann, J. Wolf: ElastischesKleben. Verlag Moderne Industrie (1st edition),
Landsberg am Lech (1998), ISBN: 3-478-93192-4[2] C. Di Fratta, D. Vogt: Key Features of TwoComponent Adhesives for Structural Composite Bonding. SAMPE Europe Conference 2018,
Southampton (UK)[3] C. Di Fratta, S. von Manitius: Enhancing
Lightweight and Production Efficiency in Commercial Vehicles with New Generation Structural Adhesives and Modular Composite Components. LIGTHer International Conference 2019,
Gothenburg (Sweden)[4] C. Di Fratta, M. Zivaljic, F. Choffat, B. Bosshard: Industrial Applications of 2C Structural Adhesives with Curing-by-Design Technology forComposites and Mixed Material Bonding. 1st International Conference on Industrial Applications of Adhesives (IAA 2020), Funchal (Portugal)[5] C. Di Fratta, S. Kostic, D. Hofstetter, A. Corsaro, F. Choffat: Enhanced Curing and Structural
Performance of Polyurethane Adhesives for Applications in Commercial Transportation. 5th International Conference on Structural AdhesiveBonding (AB 2019), Porto (Portugal)[6] U. Burckhardt, A. Corsaro, M. Schlumpf:
Zweikomponentige Polyurethanzusammensetzung mit einstellbarer Topfzeit. WO

2019/002538, 2018[7] Regulation No. 66 of UN/ECE: Uniform Technical Prescriptions Concerning the Approval ofLarge Passenger Vehicles with Regard to theStrength of their Superstructure, 2006[8] https://www.swiss-innovation.com/news/?_
nsid=45596

The authors
Dr. Claudio Di Fratta
Marko Zivaljic
Sika Services AG, Zürich
Antonio Corsaro
David Hofstetter
Michael Schlumpf
Bernhard Bosshard
Dr. Fabien Choffat– corresponding author –(choffat.fabien@ch.sika.com)
Sika Technology AG, Zürich

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