There are Acrylics...and there are acrylics.......
HSA uses a very special acrylic adhesive, but to understand why it is special we need to understand a little technical information. The term "acrylic" covers a wide range of adhesives with widely varying performance. Chemically, acrylics are polymers or copolymers of the esters of acrylic and methacrylic acid. Some of these polymers are hard and crystalline ("acrylic", "Perspex") but certain acrylic polymers have the unusual property that they are rubbery and intrinsically adhesive. Unlike true rubbers, they do not need the addition of tackifying resins to make them perform as pressure-sensitive adhesives.This is the group of polymers used for acrylic adhesives, including our HSA.
Polymers are long molecules made up of the same repeating chemical block, and there are generally thousands of units in a very long chain. These very long chains are physically tangled up, and this tangling gives the polymer much of its strength. If we stress a long chain polymer, the chains have to disentangle or break in order to flow, which takes a large force. Short chains of the same material would behave as liquids and will easily flow under the same conditions .It is like a saucepan of spaghetti compared to macaroni. The spaghetti will tend to act as one solid lump, whereas macaroni will easily separate. The longer the chain length (higher molecular weight) the more cohesive the polymer. A very long chain length is the key to high shear acrylic adhesives.
Be aware, however, that many adhesives simply described as "acrylics" are not wholly composed of acrylic polymers. Although acrylics do not need tackifying, many (probably most) acrylic adhesives do contain tackifying resins. This addition often improves adhesion, particularly to difficult materials. It also significantly reduces the cost, as resins are much cheaper than acrylic polymers. These tackifying resins are short chain polymers, so this addition does make the adhesive much more liquid, particularly if too much of the cheaper ingredient is used. It reduces the cohesion (shear strength) and the temperature resistance. Technibond do use these "modified" acrylics in our tape range (our HTA for example), but we always specify whether our adhesive is "pure" acrylic or "modified" acrylic. Many suppliers do not, and if not specified they are probably using the cheaper modified acrylics. Fine in their own right, but not comparable.
Actually, HSA is a solvent based, cross-linked pure acrylic adhesive. What does this mean?
Firstly, to explain cross-linking. We said earlier that the cohesion of the adhesive results from the very long, tangled polymer chains. The longer they are, the greater the cohesion or ultimate strength of the adhesive. There is a practical limit, however, because the longer the chains, the more difficult they are to get into a liquid form to allow them to be coated. Cross-linking gets around that by forming chemical bonds (links) between the chains, after the adhesive has been coated. The effect is to give very long and branched chains, that provide even more entanglement and therefore significantly higher cohesion. This gives the strongest possible adhesive.
Modified acrylics can also be cross-linked, but the cross-linking only affects the acrylic polymer, not the resin. The resin is a very short chain polymer which in effect acts as a lubricant. A cross-linked modified acrylic is stronger than a non cross-linked modified adhesive, but nothing like as strong as a cross-linked pure acrylic.
Cross-linking is most effective and best developed in solvent based adhesives. These adhesives are the only ones currently able to pass the most stringent tests in the automotive industry, which demand high strength, high temperature resistance, and excellent solvent resistance. The acrylic is dissolved in a solvent mixture and coated at the required thickness. The coated material then goes through a drying oven where the solvent is evaporated leaving the dry adhesive film. During and after this coating process, the cross-linking takes place.
There are two other coating techniques for acrylic adhesives; water based and radiation cured, both very different. Water based adhesives (also known as emulsion, dispersion or latex adhesives) use water instead of a solvent to liquefy the adhesive for coating. Acrylic polymers are not soluble in water, so they are made in the form of a suspension of tiny particles. Like other suspensions or emulsions, the solids would naturally settle out, so the dispersion is kept stable by using surfactants, which are forms of soap. This surfactant is still present in the dried adhesive film, which does reduce its performance. It also makes the adhesive rather moisture sensitive. In addition, water based acrylics cannot effectively be cross-linked, so the shear strength is limited. They are cheaper than solvent adhesives and certainly have a place in most suppliers' tape ranges, including our own. They do not have comparable performance. Again, not all suppliers readily disclose whether their acrylics are solvent or water based.
Radiation cured acrylics are coated as a liquid pre-polymer. This liquid is 100% polymer so requires no drying, but it does require curing (cross-linking) to become solid. These adhesives are closer in performance to solvent acrylics but the technology is not as flexible or controllable. Despite being around for 30 years or so, these adhesives have only a small presence in the tape market.
So HSA is a solvent based cross-linked pure acrylic adhesive. The best possible adhesive for the applications that demand it.
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