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112 Birgit Kjærside Storm rate of crosslinking the higher strength will be obtained. Opposite will a low density of reactive groups for crosslinking gives a lower strength and a higher flexibility of the adhesive. Bonding with a high strength and stiffness is necessary under a shear stress and especially if a peeling is a risk. If the load will be a dynamical load the bonding need to have some flexibility as well as strength and the adhesive need certain flexibility. The epoxy adhesives can general obtain the highest strength, because I most epoxy adhesives the density of aromatic groups is high. Most of the epoxy resins are made on basis of bis-phenol A, which contains two aromatic groups. In Fig. 6 the formation of the epoxy resion bisphenol-A-diglycidylether (DGEBA) is shown. Aliphatic groups can be added and they will change the strength and the flexibility of the cured adhesive. Adding aliphatic groups can also be used for changing the opening time of the adhesive, because a longer aliphatic chain will decrease the density of reactive epoxide groups and there fore change the reaction time. Epoxy adhesives are normal two component systems. With mixing the two reactants the curing process will start. Increasing temperature will decrease the reaction time. With changing in reaction temperature the curing process – the cross linking process – can change and the properties of the adhesive can change. Fig. 6 – Formation of DGEBA from bisphenol A and epichlorhydrin [4]. The polyurethane adhesives can as well obtain a good strength. Most of the polyurethane adhesives can obtain a good flexibility, especially if they are made on basis of an aliphatic isocyanate. The polyurethane adhesives can be one- or two component systems. Two component systems will start the reaction, when the isocyanate and the polyol – the two active components – are mixed with each other. Again increased temperature will decrease the reaction time and increased temperature can change the properties of the adhesives. One component polyurethane adhesives will start the reaction under influence of humidity in the atmosphere. The humidity will react with one of the components in the adhesive and make it active for reacting with the other component. If the humidity is low the reaction will not take place as expected.
Bul. Inst. Polit. Iaşi, t. LVI (LX), f. 2, 2010 113 The acrylic adhesives cure up with a radical reaction over a double bond. For initiating the reaction an initiator system shall be reacted. This can happen under influence of adding the initiator or under adding energy f.ex. with UV irradiation for starting the reaction. The acrylics can be one or two component adhesives. The rate of cross linking depends on the curing system. The rate of the cross linking depends also on the temperature and on the time in which the system will react. Fig. 7 – The HSP plot of DP460 from 3M. DP460 is an epoxy based adhesive. Fig. 8 – The HSP plot of Sikaflex-360HC. Sikaflex-360HC is a polyurethane based adhesive.
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BULETINUL INSTITUTULUI POLITEHNIC D
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Papers presented at THE INTERNATION
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SILVIU BERBINSCHI, VIRGIL TEODOR, N
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CONSTANTIN CHIRIȚĂ, ADRIAN HANGAN
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Bul. Inst. Polit. Iaşi, t. LVI (LX
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Stachie Marius 221 Storm Kjærside
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