Simplicity and ease of use are key to the increasing use of technical tapes
John McGann outlines the advantages of using technical tape over other fastening methods and updates on the latest demands made on these tapes. He also gives examples of applications which use these tapes in printing, building and construction, automotive assembly and aerospace industries.
Sales of technical adhesive tapes to industry are growing between 5 and 7 per cent per year. Two key attractions of using tape over other fastening methods - riveting, screwing, welding or even liquid adhesive - are their simplicity whatever the user's skill level and, because of new materials technology, their efficiency. Their ease of use belies their functionality, however: tapes are increasingly being used even in industries where safety, predictable performance, resistance to chemicals, heat and shear and high ultimate bond strength are critical.
Proof of their theoretical performance is piloting tapes' use in a huge range of industrial applications from automotive to medical, in many cases replacing traditional fastening methods. The myriad applications in which adhesive tapes are used means that the demands on them vary significantly according to their specific use.
Flame retardation, UV resistance, or microbiological issues may be as important as specific performance characteristics, such as initial tack, ultimate adhesion or ease of removal. Forces applied to the tape, temperature, moisture and expected lifetime need to be considered, too. While standard industry tests are carried out before the tape leaves the factory, specific tests for certain applications are conducted too. The predictable performance of the tape in any given set of circumstances is key.
To guarantee performance, the tapes industry uses a number of standard tests. Some of the most common areas of testing are explained here:
n Tack - There are a number of tests to measure tack of the adhesive tape. Tack is a measure of initial grab or 'stickiness' that the tape exhibits without application under pressure. Here are brief descriptions of a few tests used to measure tack.
n Rolling Ball Tack - A piece of adhesive tape is laid on a flat surface at the base of an incline. A steel ball is then rolled down the incline and allowed to roll onto the tape. The tack of the tape will stop the ball and the distance the ball rolls on the tape is measured and recorded. The shorter the distance the ball rolls, the tackier the tape.
n Polyken Probe Tack - A specially designed probe is put in contact with the adhesive surface of a tape. Immediately after contact is achieved the probe is lifted from the tape surface. The force required to separate the probe from the adhesive surface is measured and recorded. The higher the force required, the tackier the adhesive tape.
n Loop Tack - A loop is made of the adhesive tape so that the adhesive surface is on the exterior of the loop. This loop is then brought into contact with a stainless steel surface until a 25mm x 25mm area of the tape is in contact with the stainless steel surface. The adhesive tape loop is then immediately pulled from the panel and the force required is measured and recorded. The higher the force required, the tackier the adhesive tape.
n Adhesive Shear Strength - This property can be measured two ways, either dynamically or statically. The two tests yield very different results and measure different characteristics of the adhesive tape bond. Dynamic shear provides a result that is a mixture of internal adhesive strength and adhesivity while the goal of static shear is to measure the cohesive strength (internal strength) of the adhesive layer.
n Dynamic or Lap Shear - Two steel laps (25mm x 100mm) are bonded together to produce a 25mm x 25mm bonded area. The bonded material is allowed a certain amount of time for the bond to develop (usually 24 hours) and then the laps are dynamically pulled apart to produce a shearing force on the bond line. The force required to break the bond is measured and recorded, usually in N/cm2.
n Static Shear - In this test, a piece of adhesive tape is bonded to a stainless steel panel and then a weight is suspended from the tape in a way to produce a shear force on the bond line. A timer is started when the weight is suspended and the test is finished when the weight falls. The result is measured in time (usually minutes or hours). The test parameters are broad enough to allow the tester to obtain the right type of failure. The tester can alter the surface area being bonded (anywhere from 12mm x 12mm to 25mm x 25mm) and the weight (anywhere from 100g to 2Kg) used in order to achieve a cohesive failure. The parameters used must be given with the test results in order for them to have meaning, i.e. 250 hours using a 1Kg weight on a 25mm x 25mm area.
n Peel Adhesion - A measurement of adhesive characteristics of the tape. This test determines how well an adhesive tape adheres to a particular substrate. The tape is applied under pressure to a flat surface and allowed to rest for a given time. The tape is peeled back either at a 900 or 1800 angle and the result expressed in N/cm width.
n Strength and Elongation at Break - This is a tensile test where a material is pulled and stretched until the material breaks. The breaking strength is recorded as the maximum force required to cause a rupture (usually in N/cm width) and the elongation is recorded as a percentage. This percentage represents the length of stretch the material underwent before rupture as compared to the original length of the sample tested.
n Tear Resistance - This is measured by initially putting a small nick on the edge of the tape and then pulling the material in opposite directions. The force required to cause the nick to propagate and travel across the material to produce a full tear is measured. This determines the resistance offered by a tape when either nicked or cut at the edge.
n Percentage Recovery and Recovery Time - This test is more pertinent for foam tapes where the tape is being used to not only bond, but produce a seal. The tape is subjected to a certain compressive force for a given period of time. The compressive force is then removed and the foam allowed to try to return to its original dimension. Two things can be measured in this test. If the material returns to its original thickness, the recovery time can be measured. In other words, how long did it take to return to its original dimension. Alternatively, the per centage by which a tape recovers to its original dimension can be measured after a certain time has passed after removal of the compressive force.
n Ageing - Bonded materials or tape samples can be subjected to numerous types of ageing, humidity resistance, temperature resistance, a combination of temperature and humidity, temperature and/or humidity cycling, UV resistance, fluid resistance, etc. Usually the way to determine the resistance of tapes to these types of ageing methods is to measure certain important properties of the tape or the bond before ageing and then compare these results to the same tests made after ageing. The change in the tape or bond characteristics over the ageing process is usually used to gain confidence in the long-term viability of using a tape in an application. The characteristics tested and the ageing parameters chosen are therefore going to be different for each type of application and require a great deal of thought to be sure they provide the right information.
n Special Characteristics - There are many characteristics of tapes that these standard tests do not take into considerations but may be important for specific markets or applications. Tests can be done to measure characteristics such as electrical resistance, electrical conductivity, dielectric strength, thermal conductivity, flame resistance, optical clarity, etc. The list is long and these types of tests usually only done as required by the specific market or application.
Understanding the application
`
Suppliers who understand adhesive technology will want to carry out a thorough audit of the specific application. Once we have narrowed down the parameters and established a range of tapes which could fulfil the criteria, we recommend testing in situ to assess how the tape really performs in a given situation.
According to the application, tape will be tested for different key characteristics. Flexographic printers, for example, use flexo mounting tape such as Scapa 4355 to compensate for the gaps between the print cylinder and the substrate. Here, 'rebounce' or 'recoverability' of the foam backing is central to final print success. For a tape such as 4355, per centage recovery is an important measurement. The test measures the resilience of foam and how much it will 'spring back' to its original thickness after being compressed. The tape should also be easily removable after use without leaving adhesive residue - so peel adhesion is another important measurement.
Tape manufactured for fixing skirting boards in the construction industry, on the other hand, requires a very high ultimate bond strength so a predictable final adhesion level is critical. Any tape used in exterior applications will require testing to determine UV resistance. Industry-specific tests are carried out in all kinds of applications and may vary from manufacturer to manufacturer.
Fit for the purpose?
Relatively scant attention is often paid to fastening methods at the design stage - yet failure of even the smallest component in any large assembly can have catastrophic results: witness the well-documented case of an aircraft windscreen shattering as the result of the wrong size screw being used during routine repairs. While at their basic level, any of a number of adhesive tapes could be selected to hold substrates together, they also offer a flexible construction and can be adapted to be fire resistant, withstand UV exposure or extreme heat/shear and other variables.
Supported by ever more stringent legislation, resistance to burning is a key consideration in many industries. Furniture manufacture, automotive and construction all demand materials with flame retardant properties and which will not give off toxic fumes in case of fire.
The challenge for tape manufacturers lies in producing a tape which meets its primary purpose eg as a fastening method or as a sealant but, in the event of a fire, that doubles as a barrier to help prevent flames from spreading.
Recent developments in adhesive tape technology have seen the introduction of flame resistant tapes and sealants which can be used safely in fire doors and walls.
Whilst some materials - PVC is a good example - are naturally flame resistant, this may not be the best product to use for sealing gaps. On the other hand, additives may be needed to render a good sealant fire resistant. Understanding chemical reactions is necessary because additives designed to improve one property could potentially cancel out a different property. Specialist manufacturers such as Scapa understand the behaviour of different chemicals and can develop products which fulfil all criteria. Scapa 0217, for example, is an emulsion acrylic sealant which has been specially formulated to comply with fire safety standards. It can be used as a sealant around pipes and cables where they pass through walls or to seal low movement joints against smoke and fire.
Scapa 0217 has been tested to BS476 part 20 for confirmation of its performance. Exposed to fire, the sealant will swell and develop a fire resistant char on its surface. Combined with a correctly designed joint, it is possible for the seal to maintain its integrity for up to four hours in heat despite the degradation of surrounding materials.
Flame retardation is a key factor in the selection of materials for aerospace applications. Here, manufacturers generally set their own stringent tests to ensure compliance. Clearly, textiles and materials used within an aircraft cabin must be resistant to burning but so, too, must any adhesives used. Cabin carpet, for example, is changed relatively frequently due to high wear and tear; residue-free adhesive tape facilitates its simple replacement.
The tape must be as flame resistant as the carpet itself. Tested to Boeing specifications, Scapa 4927 is a double-sided tape on a cloth substrate which provides the flexibility to adhere to uneven surfaces. A rubber-based adhesive enables the tape to bond to a variety of surfaces. Exposed to flammable sources, the tape extinguishes in less than 15 seconds, after which only 95mm of tape has charred or burnt.
See the light
Extreme temperatures or exposure to UV light can have a severe detrimental effect over time. Many examples of apparent adhesive failure are caused not because it was wrong to use a tape but because a different tape would have performed better in the circumstances.
Laboratory testing and careful consideration of key performance parameters such as repositionability or recoverability are essential in selecting the right adhesive tape. Just as crucial, however, are the conditions in which the tape will be used.
Badges, bumpers and other exterior trim are often attached to cars using double-sided tapes. The damage to automotive manufacturers' reputations should name badges be easily removable was seen during the craze for collecting automotive emblems.
Forces of high speed, moisture, UV light and heat must also be survived by the adhesive used. Cars are resprayed at 120ūC so any exterior trim must be resistant to high temperatures as well as accidental oil or petrol spills. Scapa 5589, for example, is an automotive approved double-sided foam which is UV light resistant and can be used in temperatures ranging from -40ūC to 120ūC.
In aerospace or automotive applications where the stakes are high, components are tested in real life situations before use. 'Florida' or 'Arizona' weathering can be conducted on small sample components to assess resistance to UV light, ageing and corrosion. Specialist laboratories in Florida and Arizona, USA, where high UV and temperatures combined with an abundance of moisture provide worst possible case weather conditions test samples to assess weather effects over time.
Likely operating temperatures can be determined by climatic conditions and are an important clue in determining which tape to use because substrates which support the adhesive and the adhesive itself are known to withstand certain temperature ranges. Polyester, for example, is very heat stable but polyethylene is not. Silicone adhesives withstand temperatures in the range -70ūC to 380ūC whereas rubber resins are stable only from -5ūC to 70ūC.
In arctic climates, where temperatures remain below zero for several months of the year, components are tested for their ability to withstand extreme cold. Scapa 5464 is a polyethylene double-sided tape used for mounting glass into wing mirrors in cars that may be used in cold climates. It has been independently tested to -40ūC for use in automotive applications in northern Scandinavia.
Board of approvals?
Many industries are regulated by their own official bodies which set rigorous standards and approve all suppliers. The automotive industry has established its own quality system - QS9000 - to ensure potential suppliers reach minimum standards before they can even be considered. Other industries insist on specific product testing and guarantees to safeguard the end user against potential danger. Suppliers such as Scapa ensure products are manufactured in accordance with these requirements so that they can be used with confidence.
In the construction industry, external approvals are paramount in gaining end user trust. The British Board of Agrement (BBA) is part of a European body which issues product approvals to encourage architects to try new materials with confidence.
Rigorous testing by the BBA ensures products meet expectation; accreditation is recognised as a sign of quality, reliability and fitness for purpose.
In glass and glazing, tapes can be used to protect panes during transit. Scapa 5179 has recently been BBA approved. Designed to bond glass in externally beaded windows, 5179 is a closed cell PVC foam with a paper release liner. It is coated on both sides with a high tack pressure-sensitive acrylic adhesive and is available in black at thicknesses of 1.5, 2.0, 3.0 and 4.5mm.
Where technical adhesive tapes are used in association with food or water, there are particularly sensitive issues to be dealt with. Butyl tape can be used as a watertight seal between flanges on sectional water tanks, for example. Such tapes must not support the growth of micro-organisms or bacteria or cause any tainting of the water. The Water Regulations Advisory Scheme (WRAS) regulates all products in the UK which may come into contact with water.
A potential problem occurs when bolts securing the flanges are tightened with a torque wrench. Under compression sealant can be squeezed out of the joint causing a leak. To counteract this, a tape has been developed by Scapa: a strip of foam laminated onto the sealant cushions the sealant and maintains the torque on the bolts. Scapa 0311 Black Coex offers a watertight seal, is repositionable during application and conforms to all WRC (Water Research Council) requirements including contact with drinking water. It is easy to use with little waste or mess and has been tested to a tank height of four metres.
Summary
Selecting an adhesive tape is as complex as specifying any other element of a project and it should be accorded appropriate importance. There are many occasions when a technical adhesive tape could offer several advantages compared with other fastening methods - provided the right tape is selected for the application.
The starting point in selecting a tape should be a thorough assessment of the exact requirements including the substrates to be adhered together, the conditions in which the joint will operate, temperature, duration and whether the joint will ever need to be separated.
Armed with this information, the right tape can be selected. Through thorough testing of all finished products and subsequent end user testing suppliers such as Scapa are able to guarantee that their technical tapes meet the demands set.
John McGann is with Scapa, Denton, Manchester, UK. Fax: 00 44 (0) 161 335 0104.
