Tape that

Adhesive tape can now do a lot more than help you make spaceships out of toilet rolls. Roger Brownlie explains.

As technology becomes more sophisticated and materials evolve, adhesive tape is fast becoming the preferred fixing method across a range of manufacturing processes.

‘The range and quality of adhesive tapes currently being created is phenomenal,’ says Scapa’s John McGann. ‘Almost anything you put your hands on – mobile phones, car mirrors, carpets, even greenhouses – use adhesive tape in its construction’.

Easy to use, clean and safe, adhesive tapes don’t need to be blended for each application, and, because they’re pressure sensitive they are ideal for applications where the tape needs to be left in-situ until needed. Car parts such as replacement wing mirrors can be held in stock with adhesive tape in place. When required, the release liner is removed, exposing the adhesive surface, and the mirror is pushed into position.

One reason for the increasing choice of technical tapes over other fastening methods is that tape can be easily adapted for different applications.

Pads and gaskets in various shapes and sizes can be made from a roll or sheet of adhesive tape. The name badge on many cars is secured using tape manufactured to just the right shape. Adhesive creep is avoided, wastage is minimised and assembly is speedy. In the construction industry, items are often supplied with die-cut adhesive tapes so that they can be held in place pending firm fixing with screws.

A recent innovation involves airbags secured in cars using specially engineered technical tape with exactly the right shear strength to allow the airbag to deploy at the critical moment.

In packaging, too, tapes can offer more than just a method of closing or sealing. For example, a security tape available from Scapa enables a barcode visible only under UV light to be incorporated into the tape.

The variety of adhesive forms, liners and carriers available ensures tapes can be manufactured to offer just the right properties for a specific application.

Polyisobutylene (PIB) self-amalgamating tape, for example, is a construction with negligible initial tack. Stretched to two to three times its own length, however, wrapped upon itself and left for five minutes, it forms a bond with itself, resulting in a tight waterproof seal. Meanwhile, Scapa 2501 is ideal for cable jointing, seals on polyethylene sleeve joints both above and below ground. Self-amalgamating seals are also maintenance free and ideal for use in areas that are ultimately inaccessible.

With the diverse characteristics of the synthetic polymers and resins used in different tapes, there’s a product available to meet most applications. PVC and polyethylene tapes, waterproof cloth tapes, specialist films, single- and double-sided foams and metallic tapes each have their own particular advantages and limitations in a given situation.

How is tape constructed?

Understanding how a typical tape is made helps the selection process.

An adhesive tape consists of a relatively thin, flexible backing or carrier combined with a pressure-sensitive adhesive and in some cases a release liner, and can be single or double sided (see above right). It will adhere to most clean and dry surfaces, is often permanently tacky at room temperature, does not need activation by water, solvents or heat and does not cure, dry or change its physical properties under recommended conditions.

The four main criteria which characterise an adhesive are its tack – the initial ‘grab’ of a tape; its shear – its cohesive strength; its adhesion – its holding power strength; and its cleavage – its resistance to being pulled apart on one side.

The major groups of adhesive used in tapes are:

Rubber resins. The largest group of adhesives, these can be natural or synthetic in origin. They have high tack and a particularly good adhesion to plastics which have a low surface energy but a comparatively low resistance to temperature, ageing and ultra violet light. Good general-purpose tapes, these can be used in temperatures of -5 degrees C to 70 degrees C.

Acrylics. Acrylics can be designated pure, modified or cross-linked, and have a wide degree of surface acceptance. Although pure acrylics have low/medium tack and adhesion, modified acrylics have improved tack and shear, and cross-linked acrylics extremely high adhesion and shear. All (but especially cross-linked) acrylics have good resistance to temperature, ultra violet, ageing and chemicals and high final adhesion. They are ideal for high-strength ‘magic’ tape and overlaminating on signs, and can be used between -40 degrees C and 150 degrees C.

Silicones. Silicone adhesives, a blend of silicone rubbers and special resins, are effective between -60 degrees C and 260 degrees C, making them ideal for use in high-temperature situations. They also have very high resistance to chemicals and high shear strength (being used in brake pads) and are completely removable. Silicone pressure sensitive adhesives are ideal for bonding to other silicone treated surfaces, such as splicing silicone coated release liners.

Urethane/phenolics. These are heat activated adhesives which show excellent resistance to chemicals, ultra violet light and ageing. Bond strength is particularly high after curing, essentially providing a structural bond. Phenolics are often used to bind metals together.

Specials/differentials. Sometimes special combinations of the above are possible, such as a combined rubber resin/silicone adhesive. The resulting hybrid adhesive provides much of the performance of a silicone pressure sensitive adhesive and is more cost effective as well. Differentials are used in double-sided tapes where different adhesion performance is required on each side of a tape. Carriers

The first backing to be used in an adhesive tape was cloth – commonly as sticking plaster in medical applications. Now there are over 400 carriers in everyday use, which can be grouped together as follows:

Papers. These can be flat, crimped, impregnated or coated, and are used, for example, in masking tape.

Non-woven or tissues. Although it breaks easily, tissue gives more body to a transfer tape, has high shear strength and is easy to handle. Felts are a commonly used example of this type of carrier.

Cloth. The most commonly used types are cotton and rayon. These cloths can be used as is or they can be coated for waterproofing or lacquered. Glass cloths are also used for speciality electro-technical tapes.

Plastic films. These include plasticised and unplasticised PVC which can be stretchy and stable, are easy to handle and are commonly used for packing tape; polyethylene, which makes good all-weather tape and is commonly used for insulation and repairs; polypropylene, which can be either mono- or bi-oriented, and is used in food packaging and nappy production; and polyesters, which have very high temperature and chemical resistance and are found in masking tapes used in harsh conditions. Sometimes very specific and technically advanced films are required like polyimide (Kapton), which has a very high resistance to temperature and is used to mask gold contacts in the production of PCBs.

Celluloses. Used in high-volume office tapes which can also be written on and colour coded.

Foils. Usually aluminium foil, which has a low water vapour transmission rate and can be used in tapes to prevent the ingress of moisture, vapour or gases. They are often used to seal food products and in the construction industry on HVAC ductwork.When selecting a tape, manufacturers need to look at the exact requirements including temperature range, how the tape will be applied, duration of the application, whether clean removability or repositioning are issues and any forces the joint must withstand.

Adhesive tapes are now used across all the major industrial segments: aerospace, electrical and electronics, building and construction, printing and graphics, automotive, industrial assembly, manufacturing and sports and medicine. With continuing advances in tape technology it seems likely that manufacturers will increasingly opt to use tapes over other fixing methods.

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