Glue, Adhesive, Glue Sticks, White Glue, Polyurethane.

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I get so many questions asked of me about the use of adhesives in general

I just wanted to put pen to paper and make a couple of points which might help you understand the way an adhesive works.

Firstly let me state the obvious, and that is, if your using a new adhesive or a new substrate (surface to be bonded) you should always glue a test sample first and leave it for a minimum of 24 hours to allow the adhesive to fully cure before testing the bond strength.

Why leave it for 24 hours ?

All types of adhesive work differently but generally speaking they have completed a chemical or mechanical process after the elapse of this period.

I have seen good results after 30 minutes which turn out to be a poor bond after 24 hours. Likewise, I've experienced poor results in a short period which when left to allow the adhesive to get into the molecular structure of the surface, exhibit excellent bond strengths. Sometimes you can get plasticiser migration on a surface which will show itself after 24 hours and have a detrimental effect on the bond.

How much Glue do I use ?

Normally a lot less than you think, it's a fallacy to think the more glue you use the better chance you'll have of getting a good bond. The opposite is true the less you put on, the faster the bond developes, there is less of a chance of creating a pocket of wet adhesive which could remain in that state indefinately weakening the final bond.

How much pressure should I apply ?

This varies considerably but as a general rule if your bonding two pourous surfaces, wood, paper, board etc then a reasonable amount of pressure should be applied. This is a mechanical bond 

When bonding a non-pourous surface to a pourous surface or non-pourous to non-pourous you are trying to achieve a chemical bond so don't apply too much pressure and squeeze the adhesive away from the area you are trying to bond, you have to leave a film thick enough to allow interlacing to take place.

 Polyurethane Adhesives

Commonly called 'Moisture Curing Adhesives' but actually they are Hydrogen curing. These adhesives cure by extracting hydrogen as a accelerant from their surroundings be that air or water. These products tend to foam/expand whilst curing and require considerable pressure if you don't want an open bond. It is very easy to apply way too much of this product resulting in ugly joints.

Hot Melt and Low Melt Adhesive

Again forgive me for stating the obvious, these product work by turning from a liquid to a solid. This is not all that is happening within though, The resins and polymers used within the formulae can and will effect the ability to bond a wide variety of surfaces. However, lets assume you've bought one that will bond most difficult surfaces. These adhesives have what is called an 'open time' at a given temperature. The industry norm is 160oC, the open time (time the adhesive remains in a fluid state and suitable for use) can vary from as little as 5 seconds to as long as 45 seconds and beyond if using a pressure sensitive product. So you have to bring the two surfaces together within that specified time. The 'setting time' is also an important element to the adhesive, this is the amount of time it takes for the bond to develope once the two surfaces have been joined together. As a general rule of thumb, don't apply too much glue, bring the surfaces together as quickly as possible and leave them to develope a bond for as long as you can. Incidentally, a cold surface will have a shock effect on these types of adhesives, try to ensure the surfaces to be bonded are at a reasonable temperature.

I hope you've gleaned something from this text, if not your already an adhesives genious and took a tour to see how you could add to the content to give further assistance to the user. Feel free to comment.

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