Issue 03/2016

Joining Bioplastics
Adhesive capacity
of bioplastics
By:
Diana Syperek
University of Applied Sciences and Art Hannover
Dept. of Mechanical and Bio-Process Engineering
Hannover Germany
The aim of bioplastics is using them as an alternative
solution for conventional plastics. Therefore, to a
large extent, they should be compatible with the existing
technologies. Moreover, biobased products gain value in
terms of reducing the carbon footprint. Now, when it comes
to bonding technologies for bioplastics, the same conditions
apply bioplastics as for conventional plastics. In both, joining
of the same materials as well as in hybrid constructions the
demands on the connection shall prevail.
Classification of bioplastics
Bioplastic does not necessarily mean that it must
be biodegradable. European Bioplastics suggests the
classification of bioplastics as shown in figure 1 [1]. There are
also bio-based plastics that do not degrade biologically but are
resistant, as polyethylene produced of bioethanol or polyamide
made of castor oil. Biodegradability, in turn, is not only confined
to bio-based plastics. The biodegradability is resulting from
the chemical structure of the plastic. Also, crude-oil based
plastics can degrade such as polycaprolactone. This must be
taken into account when bioplastics are bonded together. If
the connection has a biodegrading character, the adhesive
should meet this as well. In this case, protein or plant oilbased
adhesives are suitable [2, 3]. They are non-toxic and
can be either biodegradable or non-degradable. Often, they
are obtained as by-products from other processes.
Bonding of bioplastics and surface treatment
On surface treatment and adhesive technology of bioplastics,
there is only a little literature available. The reason for this
might be that for bioplastics the same conditions apply
as for conventional plastics. It is not possible to tell
whether bioplastics or crude-oil based plastics
are more suitable for adhesive bonding since
the chemistry and the surface structure,
as well as the surface composition of
the adherends, is crucial. For highstrength
bonds, a pre-treatment
of plastics is often necessary.
For printing, there are
different requirements.
Crudeoil-based and
PLA, for example, can biodegradable
be printed quite well
plastics
without any pre-treatment. e. g.: PCL, PVA
Although adhesive bonding
of plastics is not as significant
as that of metals, in the industry
it plays a significant role, because not all parts are completely
manufactured by primary shaping. While only thermoplastics
can be bonded by welding, adhesive bonding has much larger
applications. This especially is true with regard to connecting
different plastics with different melting temperatures. Since
in the packaging industry mainly thermoplastics are used, it
is common to weld them. Through the influence of heat or
ultrasound and slight pressure, the parts or plastic films
Biobased and
biodegradable
plastics
e. g.: PLA, PHA
Bioplastics
Figure 1
are joined together. For plastics having a short life cycle and
similar melting temperatures, it is appropriate to weld them
unless it is a high-strength bond. Provided the weld is not
interrupted, high load capacities can be obtained and usually,
no welding consumables are required.
The advantage of adhesive bonding, however, is that
different types of materials can be firmly bonded. Adhesive
bonding technology is used in all industrial sectors (figure 2
[1] shows those sectors where bioplastics are already in
use). In dentistry, ceramics are bond with metal or plastic
by means of UV curing adhesives. In the automobile or
aircraft construction adhesive technology plays a more
important role concerning weight reduction and fuel saving.
Wherever high forces are acting adhesive bonding has a
decisive advantage comparing to other bonding techniques.
Besides the adherends, the adhesive itself also affects the
force transfer in the adhesive bond. Ductile adhesives like
polyurethanes are more flexible and thus, forces impacts are
distributed better over the adhesive area. Thus, higher bond
strengths are achieved comparing to those adhesives, which
have a higher inherent strength but are less flexible [4]. Biobased
polyurethanes are also already available. In addition,
curing and application temperatures must be considered. The
adhesive curing extends the process time. If the operating
temperature is low, this must be considered for the selected
adhesive as well as for the adherends. The purpose is to
consider whether the bond is dynamically loaded because
here it can come to embrittlement and thus the bond fails.
Stress peaks on brittle parts lead to failure or a lower loadbearing
capacity. In addition, a difference in stiffness of
the adherents causes a notch effect whereby the force
transfer on the adhesive area is compromised [4].
A good adhesion of the bonding parts precludes
separation of the adhesive bonds which in
turn makes it difficult to recycle. However,
this is necessary for the mechanical
recycling of different materials. At the
end of their life cycle, biodegradable
bonds can be composted. For
non-degradable bioplastics,
Biobased and
non-degradable
plastics
e. g.: PE, PA
this is not that easy. However,
they can be used to produce
energy through incineration
because the adhesive bond
cannot be separated into their
individual components.
Other issues are the creep behaviour and ageing which also
occur in bioplastics and bio-based adhesives. They do not
withstand to long-lasting stress. Ageing is caused by diffusion
of substances into and out of the plastic or the adhesive
respectively. Ambient conditions such as temperature also
have an adverse effect on the bond. The adhesion in the bond
decreases due to ageing which can cause the bond to fail,
particularly under dynamic stress. Furthermore, adhesive
34 bioplastics MAGAZINE [03/16] Vol. 11