Following on from our post on Velcro Adhesives, I though it pertinent to go on and explain in more detail the issues surrounding bonding of substrates, namely plastics, in our work supplying Exhibition Displays we are often required to stick a wide variety of substrates together to fit a clients requirements.
To the layman, adhesives should be a simple process … 1. go buy some glue 2. apply and stick … unfortunatley not, this is a complex technical area prone to failure, unless there is a proper understanding of each substrates makeup, and then applying the suitable adhesive, the bond between surfaces will simply fail, proper knowledge and testing are crucial. Advice across the trade is sometimes patchy and not awlays reliable, and often going higher up the technology ‘food chain’ is required to gain a better understanding.
Plastics are a completely man made material, the term ‘Plastics’ covers a range of materials, each with it own distinct qualities.
Plastics start as a ‘Monomer’, this is extracted from a process of refining petroleum, coal also can be used to extract monomers. A chemical reaction called polymerisation turns the monomers into a type of plastic resin, this can be in the form of pellets, granules, powder or flakes, which when processed using heat and pressure are forced into a desired shape, and allowed to cure into a finished plastic product.
Adhesive bonding is assisted in 3 ways:
- Surface Roughness (surface energy)
- Chemical Reaction
- Electrostatic Forces
The process of ‘wetting out’ and keying of the adhesive into the substrate (flow of adhesive into the substrate) are important factors.
Some polymer based substrates can be described as possessing a ‘low surface energy’, one such, Polypropelene (used in the production of our ‘Seamless Semi Rigid Rollable Shell Scheme Panels‘) which has a hard surface, and can require a softer adhesive that will flow better.
Surface Energy can be tested with a ‘Dyne Pen’, Dyne Technology© manufacture a simple system for testing the surface energy of most substrates, using a Dyne Pen a line is drawn across the substrate, the liquid will either form a continuous film on the surface or pull back into small droplets depending on the value of the Dyne Pen, buy using different pens a ‘Dyne’ level, expressed in mN/m (Dynes), the exact surface energy can be determined.
Surface preparation greatly assists adhesion, treatment of the surface using various methods:
- Corona Treatment, removal of static field by high voltage, high frequency electrical discharge or corona discharge, this forms a strong chemical attraction to inks, paint, coatings, adhesives etc
- Plasma Treatment, the surface of the substrate is altered using special energy that changes its molecular structure – allowing better ‘wetting’ and improves adhesion or other applications (such as printing ink) to the products surface.
- Flame (Plasma) Treatment, the surface of the substrates molecular structure is changed by ingredients within the gas used, heat is actually an unwanted by product of the process.
- Surface Abrasion
The two most common adhesives found are Acrylic and Rubber Based, these are compared:
Acrylic adhesives come in 2 forms:
- Water based
Good for most clean smooth surfaces.
- Solvent based
Some are a 2 part adhesive with a surface activator/catalyst (Methyl acrylate)
This etches the surface and provides a key for the adhesive to then bond.
Acrylic is a hard adhesive, with strong cohesion properties, although has a lower wetting out, and so is suitable for plastics with a higher ‘surface energy’.
It has good longevity, and is better designed for strong loading.
As a general rule an Acrylic based adhesive will work fine with products containing a plasticiser, although it has been known for these to migrate out of the plastic and into the adhesive causing failure.
Plasticisers are added to allow the plastic to be flexible, take PVC as an example, your double glazed PVC windows have no plasticers, but PVC vinyl does (hence its flexibility), in essence any plastic on a roll will contain plasticers. Occasionally products on a roll maybe produced with a back coating of silicon or plasticers, which increase the difficulty in bonding of adhesives.
Foam PVC board whilst flexible, though, does not contain plasticers, it is a rigid board.
With a pressure sensitive Acrylic adhesive work should be left for 24 hours to bond, once applied.
A rubber based adhesive is a softer adhesive, with a lower cohesive strength, it will flow better (good wetting), so good on plastics with a ‘low surface energy’, such as Polypropylene (PP), it is also better on wood and paper, bonding within just half an hour. But its use is more temporary than acrylic, with lower loading capability, it will degrade quicker with continued exposure to heat (dries out) and cold (less sticky).
To conclude, the number of factors determining which adhesive type to use on what substrate – is best proved by testing on a bespoke basis, with correct surface treatment. The length of use, environment, load, all have an influence, as does production time, often jobs are last minute, and testing time available is limited.
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