APPLICATION TECHNOLOGY


Like the two little tubes of epoxy we have at home for DIY fixes? Yes. Araldite®


started selling two part epoxy for bonding in the 1940s, and it is a strong household brand in the UK. With an epoxy


adhesive, the “resin” and the “hardener” are mixed which causes the cure, the speed of which can be anything from a few minutes to some hours. In industrial applications, two part structural adhesives are usually based on epoxy, methacrylate or polyurethane chemistries - different properties like flexibility, toughness, cure speed, viscosity or adhesion to different substrates will determine the correct choice for each application. To make mixing and application easier, the two parts are often packaged in side-by-side cartridges, with the capability to mix and dispense through a special nozzle.


Some acrylic adhesives use an activator, rather than a two part mix. Typically, the adhesive as applied to one substrate, and a film of liquid activator is applied to the other. Once brought together, the adhesive and activator mix to cause cure, which again can be very fast.


Apply activator to one substrate


Apply adhesive to other surface


Assemble


Sometimes this mixing can be a nuisance. Can’t I have a single part adhesive? Sure – but you still need to make it cure somehow. Another way to do this is to add


energy. Single part epoxies require heat to cure, usually 120°C or more. This requires ovens or induction heating. Single part addition cure silicone is another example requiring heat. You can also use light energy to cure an adhesive. Light curing adhesives are single part systems, based primarily on acrylic or acrylated urethane chemistry (although there are epoxy systems). They are formulated with photo-initiators which react to UV and/or visible light to cause cross linking. The speed of cure is very fast (seconds). The limiting factor is that you must expose the adhesive to light whilst it is in the bondline, so one of the substrates must transmit light.


Adhesive cured with light


Can I cure single part adhesives without using an external energy source? This is the third way – change the conditions. For example, a thread-locking adhesive


will cure when it is placed on the threads of a bolt and the bolt is screwed into its hole. This is an example of an anaerobic adhesive. They cure in the absence of oxygen and in the presence of metal ions. Consequently, anaerobics are used for metal bonding where you can change the conditions by removing the oxygen, like bonding gears to shafts. Other single part adhesives cure when in the presence of moisture, like single part


polyurethane adhesives, which are used as woodworking glues and construction sealants. One part silicone RTV sealants and adhesives cure under the action of atmospheric humidity. These are known as “condensation” curing, as a by-product is given off in the chemical reaction. Bathroom sealants based on silicones liberate acetic acid, but for technology applications, silicone condensation cure systems have been developed which have small amounts of non-corrosive by-product, safe on electronics, etc. Cure depth is limited to about 10mm.


164 Fastener + Fixing Magazine • Issue 67 January 2011


Single part RVT Silicone adhesive


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188