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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
Joining Bioplastics Figure 2 Global production capacities of bioplastics 2014 (by market segment) in 1,000 tonnes 800 600 400 359 790 Biodegradable PLA & PLA-blends Starch blends Other 1 (biodegradable) Biobased/non-biodegradable Bio-PET30 2 Bio-PE Other 3 (biobased/non-biodegradable) 1 Contains regenerated cellulose and biodegradable cellulose ester; 2 Biobased content amounts to 30 %; 3 Contains durable starch blends, Bio-PC, Bio-TPE, Bio-PUR 200 0 6.7 7.6 Electrics & electronics Others 20 Building & construction 94 Automotive & transport 107 Agriculture & horticulture 126 Consumer goods 186 Textiles Flexible packaging Rigid packaging (except thermosets), Bio-PA, PTT Source: European Bioplastics, Institute for Bioplastics and Biocomposites, nova-Institute (2015) More information: www.bio-based.eu/markets and www.downloads.ifbb-hannover.de bonds generally do not resist to peeling stresses. In principle, it should be kept in mind that bond strengths are depending on the technique with which they are tested [4]. Therefore, a transfer to real conditions is difficult and should be tested separately. Despite that, in an automobile non-degradable bioplastics are already used. From polyamide exterior parts such as engine hood or trunk lid can be manufactured. For the interior and trunk trim polyurethane foams and polyolefin are applied. Since polyolefins poorly adhere to other surfaces they must be pre-treated first. For this corona treatment has been proven successful. This process has a high degree of automation and all plastics can be pre-treated that way. Through the corona treatment, the upper atomic layers of the plastic surface are functionalised, so that wetting of the adhesive is improved. This technique is also suitable for reinforced plastics such as wood fibre plastics since the fibres are embedded in the polymeric matrix and do not stick out of the surface. Summary Bioplastics are equally well suited for adhesive bonding as conventional plastics. However, when it comes to high-strength bonds, most plastics as well as bioplastics adhere poorly to other substances. The surface, however, can be pretreated with existing methods. Whether and to what extent this is necessary, also depends on the respective type of bioplastics, the scope of application and type of loading. Since the demand for sustainable materials is constantly rising, bioplastics come to the fore. Many manufacturers already use plastics on a large scale for their products. For more complex implementations and in terms of lightweight applications adhesive bonding becomes more and more important in the field of bioplastics. It is expected that this further increases in the future. References [1] european bioplastics. European Bioplastics e.V. Available at: http://www.european-bioplastics.org/. (Accessed: 12 th May 2016) [2] Kim, S. in Biopolymers (ed. Elnashar, M.) (Sciyo, 2010). [3] Clark, J. H. Green Materials from Plant Oils. (The Royal Society of Chemistry). [4] Rasche, M. Handbuch Klebtechnik. (Carl Hanser Verlag, 2012). Tiplock ® the world’s first compostable ziploc packaging Tiplock is a joint development of and Bio4Pack. NEW Bio4Pack GmbH • PO Box 5007 • D-48419 Rheine • Germany T +49 (0) 5975 955 94 57 • F +49 (0) 5975 955 94 58 info@bio4pack.com • www.bio4pack.com bioplastics MAGAZINE [03/16] Vol. 11 35 Bio4pak-adv-BioPlastick-Magazine105x148_5.indd 1 18-05-16 11:04
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