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E5 Tack Measuring Equipment for the Footwear Industry
Background
In the footwear industry, the degree of tack is the stickiness that controls the strength of a joint between two objects. If there is too much tackiness problems occur with ‘spotting’ the sole to the shoe and if there is too little tack then the sole will not be sufficiently bonded before final pressing. NPL and SATRA (the footwear technology centre) are concerned with the lack of standardisation for tack measurement. Previously, instrumental techniques to measure tack have been unsuccessful because of difficulties in reproducing in-service conditions. Other tests such as the mandrel test are available but are over simplified and have a number of disadvantages in measuring tack.
What was Done
The National Physical Laboratory has managed a DTI sponsored programme on developing materials measurement methods for adhesives and as a result of collaboration with SATRA, a tack measurement device has been developed for measuring ‘spotting tack ’. This is known as the NPL/ SATRA Tack Test.
The device developed provides a numerical value for the strength of the tack and brings two adhesively coated specimens together under a required force. The device then reverses the moving specimen-specimen holder while simultaneously recording the tensile force needed to separate the specimens.
The tack measuring device consists of a motorised test stand which carries a force gauge mounted on a base plate. The specimen holder is programmed to reverse the direction of travel when the chosen force is attained.
Joint Details
Adherends
Rubber / Rubber
Adhesive
Not specified
One-way heat reactivation
New environmentally friendly non-solvent adhesives assessed
Pre Treatment
Heat reactivation under radiant heater
Cure Cycle
None
Bonding
The device represents the industrial practice of one-way heat reactivation and consists of a short wave radiant heater to heat the test specimen. The specimen runs on a slider so that it can move under the radiant heater. The test specimen is moved under the upper material after heating which brings the two adherends together and then they are subsequently separated.
Location
Laboratory
Results
A simple tack classification system has been developed from use of the test method and this will assist those using adhesives in selection and also assist manufacturers in sales of adhesives. In the classification system, a strength of less than 50kPa is classed as low tack. Between 50kPa and 150kPa is a moderate tack with over 150 kPa classed as a high tack. Shoemakers will benefit from the tack measuring device by being able to evaluate both different adhesives and also optimising the manufacturing process the obtain the best possible bond strength.
Discussion
Under the recently introduced environmental protection laws, operators of adhesive coating processes are subject to regulatory controls which are devised to reduce the use of solvents. Manufacturers must use less than 5 tonnes of solvent per year, or less than 20 grammes on each pair of shoes. Alternatively, they must meet stringent new levels for solvent emissions from factories. In most cases, a change to a solvent-free adhesive is the most cost effective route.
There is therefore a drive for manufacturers to accurately evaluate new ‘environmentally friendly’ adhesives.
Lessons learnt
The tack measurement device has been shown to represent the footwear manufacturing process well for trials conducted at SATRA on four water base adhesives.
A further advantage is that the tack test device provides a quantifiable method for companies who are required to change from solvent to a water based adhesive.
The tack measuring device is still being developed further with the possibility of allowing measurements on adherends which are both activated with adhesive.
Source
DTI MTS programme adhesives – Dissemination Guides ADH5CS5.DOC
NPL / SATRA/ ESR Technology Limited
Shoe image courtesy http://www.shoeinfonet.com/
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