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Threadlocking: coping with chrome

Threadlocking: coping with chrome
Ged McGurk of Henkel, maker of Loctite brand adhesives, reveals that recent health and safety regulations involving chrome have resulted in new adhesive technology.

Chromates provide a crucial function to a wide range of industries.  In particular, Hexavalent chromates have been employed to coat metal components in order to afford protection from corrosion and, therefore, extend their working life.   The automotive industry has been a particularly heavy consumer - with an estimated four to seven grams of Hexavalent coatings used on each new vehicle.

And while there is no doubt they did their job very efficiently, the fact remains that Hexavalent chromates are highly toxic.  In addition, it has been shown they create ecological problems during the disposal of spent chromium compositions.  More significantly, it has come to light that these Chromates could introduce the possibility of cancer among users in close contact with them. All of which meant there were very good reasons for finding another method of coating. 

The search for the alternative process formed part of two European Union projects known as the End of Life of Vehicle (ELV) and Restriction of Hazardous Substances (RoHS) initiatives. Based on the findings of the projects, the ELV Directive stated that from 2007 the Hexavalent chromium content of coatings that were employed to avoid corrosion must be reduced. Specifically, the maximum concentration must be 0.1% or 1000ppm by weight of homogeneous material. An extension was granted concerning the use of Hexavalent chromium in relation to bolt and nut assemblies for chassis applications. This came into effect in July 2008.

With this new directive in force, those companies that supplied such materials needed to find an alternative method of providing corrosion protection. Nowhere was this more pressing than in the automotive industry. The extensive research paid off - with the solution being found in the form of Trivalent chrome. Not only were the health and safety concerns answered, but a number of other benefits from using this new material came to light. For example, these chromates created a harder scratch-resistant coating with lower water content - and this led to better heat resistance.  

But although this solution solved many difficulties, it also resulted in a significant challenge for adhesives manufacturers. That's because trials revealed that most existing anaerobic grades simply did not adhere very well to Trivalent chromes. Indeed, there was no guarantee that joints and materials coated with Trivalent chrome would stay locked under all conditions. It rapidly became apparent that developments in adhesives' formulas were necessary.

Rising to the challenge
At Henkel, technology specialists were quickly at work to overcome the difficulties. This development included close cooperation with customers, especially a supplier to the automotive industry whose speciality was the precision machining and assembly of components. Working together, several trials were carried out involving substrates that were coated with Trivalent chromates.  The eventual outcome was a new product that is labelled as Loctite 2701 threadlocking adhesive. This maximum strength anaerobic has been formulated for use on all metal fasteners - including stainless steels and those with protective coatings such as zinc. Although not exclusively designed for the automotive industry, the product is especially effective on the permanent locking of studs on engine blocks and pump housings. The automotive supplier involved with the trials has utilised the threadlocker on screws used on timing covers for a major engine manufacturer - without any problems. 

That same product is used by a manufacturer who had designed an anodised aluminium fixture which enables seats to be securely firmly to the floor of minibuses with a corrosion-resistant fastener. However, the concept also permits the easy removal of the seats if, say, a wheelchair needs to be secured in the vehicle. The design necessitates steel feet being locked into an aluminium bar and the new high strength adhesive provided the solution.

Although the new adhesive answered many problems, further research has lead to the introduction of additional adhesives that could handle 'difficult' coatings. It is well known that active substrates, such as copper alloys, are very good in initiating the rapid cure process of anaerobic adhesive products. Conversely, substrates like nickel or chrome plated parts, are passive - which means traditional anaerobic adhesives achieve a much slower cure. Two anaerobic threadlockers - Loctite 243 and 270 - have recently been reformulated to have the same cure characteristics as 2701, enabling them to accomplish fast fixture on all metal substrates, including nickel and other plated surfaces.   

In addition to effectively working on plated substrates, they are designed to provide good sealing performance, with first-rate thermal and shock resistance, and because they are more 'oil-tolerant'  than previous grades, enabling them to be used more readily on 'as received' parts - including those contaminated with engine oils, corrosion prevention oils and cutting fluids. The formula not only cures at low temperatures, but can also withstand applications up to +180°C.  

Obviously, industry cannot - and does not - stand still.  There will always be new materials, updated regulations and improved production techniques. Alongside those considerations, it is right that the condition of the environment must always be uppermost. All these challenges - and more - will continue to confront industry in general. And suppliers of products - such as adhesives - must remain alert to the challenges. Clearly, customers have an expectation that the effective bonding, sealing, retaining and locking of joints will be maintained, whatever the substrates, conditions, regulations and environment. That means adhesive manufactures have a duty to remain proactive in searching out what developments may be needed in the future - but beyond that, to ensure the solutions are there at the appropriate time.
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