New Developments at ERICHSEN Scratch Testing

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1 New Developments at ERICHSEN Scratch Testing ERICHSEN development has been continuous for almost 100 years: their test and measuring instruments have played a pivotal part in the development of modern coating materials. We are in the age of quality Certification in industry: Even as recently as 20 years ago many users still considered it to be adequate just to assess significant properties on the basis of simple judgement based on experiences, but today overwhelmingly this kind of judgement is no longer acceptable and measurement based quality assurance is the rule. From our vantage point, what our customers are looking for is to be enabled to guarantee quality at minimum cost without applying more coating than required and even, in more extreme cases, avoiding the necessity for the removal of subsequent replacement of coatings. Tight quality control is fundamental for a company s reputation. Quality Assurance cannot be based on luck. Defence against unjustified complaints requires documented evidence that quality standards are achieved. The evidence must show that the process that is used complied with the laid down standards. Thus, today, testing is necessary at every stage; when materials are received, during the production process and again on the finished products before despatch. Testing of course has a long history: the demand today is that the equipment is certified to comply with standards and is regularly re-calibrated. Furthermore, in the past test methods were frequently specially developed by the user but the trend is to make use of established standards and tests which are characterised by good repeatability. Luck in Quality Control belongs to the day before yesterday To the traditional measured parameters in coating technology, namely thickness coating, adhesion and gloss, the testing of the resistance of the coated surface to mechanical damage, namely the measurement of scratch or graze resistance, and abrasion resistance have also become essential for many applications. Test methods and test equipment development has kept pace with the demand for higher levels of quality assurance. In the following we describe 3 examples of how the required levels of quality management in resistance to mechanical damage is achieved. It has long been realised that the first step for a sensible quality management policy is to fully indentify the parameters that need to be tested.

2 A made up list of parameters that it might be possible to test would not necessarily coincide with a list of the parameters that it would be sensible to test. Erichsen therefore concentrate on the provision of test equipment for the surfaces of high quality products on the basis of a sensible and logical selection from these parameters. To put it another way, by working in partnership with users and research establishments over a period of many years we have developed a range of test instruments to measure the qualityrelevant properties of a very wide range of products in accordance with established standards, designed to take account of the needed precision and ease of operation. Under the particular heading of Surface Resistance to Physical Damage Testing we now have a range of three instruments to cover a wide range of possible damage. 1.) SCRATCH HARDNESS TESTER 413 (The top of the range instrument) Most scratch hardness tests involve penetration into the material or coating to be tested until contact is made with the base material. For many versions of this type of test the sequence to do just that is all that is required. However, very often it turns out to be very useful to find out what it takes to overcome first the resistance of the outer surface, before any more damage is done. In addition to establishing what would cause the start of the full destruction of the coating resulting from damage to the outer surface the measurement also identifies first stage of harm to the appearance of the surface (for example in the coating on vehicles). Damage to the top surface of a coating may also have a harmful effect on the functioning of the product for example in the case of high-tech coatings on glass. Because of the unique facility for adjustment the ERICHSEN SCRATCH HARDNESS TESTER 413 is ideally suited for identifying the small differences in scratch resistance. The instrument offers the means for representing the surface quality numerically with good repeatability The scratch resistance of many products is now at such a high level that great differences in quality are not in general to be expected but even the small differences are of significant importance. A higher level of sensitivity of the test method, which can differentiate between these, is therefore required. For the test using the SCRATCH HARDNESS TESTER 413 a series of parallel scratch marks is applied to the test surface. The depth of the scratch marks is reduced from one line to the next by means of fine adjustment, up to the point where no scratch mark remains visible or is noticeable under an accepted definition. Development translation Jan10A 2

3 Various types of scratch tools, ranging from sharp pointed to rounded and flat with sharpedges are available so that the test can mimic the scratch effects to be allowed for under practical conditions to represent the product properties over as wide a range of conditions as possible. The SCRATCH HARDNESS TEST ER 413 in this way covers a wide variety of scratch resistance as required for different materials: vehicle or general coatings or plastic surfaces of light switches and push buttons, mobile phone displays, handles, housing components, general displays, vehicle components, vehicle cockpit switches, plastic products in general use, small plastic components (with or without coating), glass, glass coatings, high tech glass coatings and coatings for spectacle lenses (made of glass or plastic) and many other examples. In today s world we are surrounded by a very wide range of materials, most being produced industrially. The surfaces are intended to achieve specific surface qualities but often the manufacturers of many products for the home, car, train, tram, bus, office, etc often do not yet monitor the scratch resistance quality of these surfaces. The application of the Erichsen Model 413 can in all such cases provide a basis for assurance of the surface quality of these products. 2.) SCRATCH HARDNESS TESTER 430 P (for the widely established test method) This equipment was originally developed for the cross cutting test. Today it is used by 10 of the best known motor (automobile) manufacturers and their sub-suppliers for testing coated metal and plastic surfaces in this industry. The test is even recommended by the VDA. Very recently the capabilities of the model 430 P have been extended by an additional function based on wishes expressed by a considerable numbers of users. In order to apply a series of parallel test marks using test forces varied in prescribed steps, the current version of Model 430P in the cross cut mode can be stopped after producing an initial test mark so that the perpendicular test force can be altered as desired for the next test stage, up to a maximum of 50N. Using one selected cross cutter it is theoretically possible to apply up to 8 lines of test marks at a pitch of 0.5 mm using different values of test force. In order to be able to assess the result more clearly it is better to at least increase the distance between marks to 1 mm producing 40 test marks side by side. In most cases experience has shown that a set of 20 test marks with a distance between the marks of 2 mm is fully sufficient, resulting in a convenient process giving good visual differentiation between results. Development translation Jan10A 3

4 The cross cutting test, which is probably the most widely used test for adhesion will, if conducted manually, occasionally result in a pattern of variation in test results that depends on the users of the equipment. The SCRATCH HARDNESS TESTER 430P eliminates this inconsistency since the test procedure uses a pre-programmed electrically motorised drive. The substrate is fixed in a specimen holder. After setting the test force and starting the instrument the depth of the individual cuts is automatically reduced in a sequence of stages in accordance with a pre-selected pattern. Plastic components with an imitation leather surface, in most cases in the form of moulded thermo plastics, have long been employed for internal motor vehicle furnishing. For testing the surfaces of these plastic parts tests similar to those described above, using a cross cutter tool (20 lines at 2 mm pitch) is employed. A relatively low test force of 5 N is applied. For this test, in place of the sharp-edged tool for the cross cutting test, a tungsten carbide test tool with hemispherical tip made of 1 mm diameter is used. Ideally, if the material tested is satisfactory the process should leave no marking on the product. On a plastic surface with imitation embossed leather marking the application of the test can result in the breaking out of surface particles, or in deformation of the edges of the product. Experience shows that the breaking out of small particles of material results in a diffusion effect of light: the object appears lighter depending on the severity of the marks applied. Plastic deformation generally leaves a much less obvious change in appearance. The differences in brightness, which for good quality products should be as low as possible, is a quality criteria and can be measured with a colour measuring instrument operating in the brightness mode. Measuring may result in judging a plastic material unsuitable for the application. A further plastic test employs a test disc of defined design with a rounded edge similar to that of a coin, to produce a tight cross cut pattern (each with 80 lines at a 0.5 mm pitch). Again, in this case, a relatively low perpendicular test force of 7 N is used. In the test the surface to be assessed is covered with over lapping test marks based on the geometry of the disc. If the gloss surface resembled glossy silk before the test it may after the test become a bright glossy surface. This is the result of levelling out or flattening of the micro structure which prior to the test had a defusing effect on reflected light. Flat polishing the micro structures may result in a similar definite glossy surface. The marking of surfaces that are intended to Development translation Jan10A 4

5 resemble silk gloss (be more matt) in this way is of course undesirable. In everyday used such surfaces will show uneven wear after regularly repeated touching at the same place. The before and after differences can be measured with a gloss measuring instrument with 60⁰ measuring geometry (e.g. ERICHSEN Pico Gloss 560 MC) and provides another quality criteria for the evaluation of the plastic material involved. 3.) LINEAR TESTER 249 (The newest and most versatile in the range) The optimum scratch hardness test instrument LINEAR TESTER 249 is, in addition to its original purpose of measuring scratch resistance on surfaces, also applicable for the following tests: Scratch hardness testing Reciprocating abrasion testing Crockmeter test MEK tests, or solvent resistance tests in general, or wipe test Although the instrument is in the first instance for testing coating materials it can also be used to test the surface of solid materials provided an adequate flat specimen, which can be mounted on the test slide, is available. The perpendicular test force on LINEAR TESTER 249 is adjustable in the range from 0 20 N in 0.5 N steps and in the range from N in 1 N steps. Three fixed speed programmes are available as well as a programme in which the speed can be selected by the user. For the scratch tests a large selection of different scratch tool types is available. The specimen slide initially moves to the start position with the scratch tip raised and then returns to the end position during which the scratch tip is lowered, leaving a test mark if the surface resistance is insufficient to prevent this. Just before the end position is reached the scratch point is raised again. The specimen can now be moved sideways and further scratch movements can be carried out, parallel, one after another. By reducing the test force after each scratch movement it is possible to establish the test force which will either result in no visible mark or one which is judged as insignificant. Continuously Reciprocating Test Abrasion test: the surface is subjected to a pre-selected number of reciprocating movements of the test head which remains in contact with the surface continuously. Two types of test Development translation Jan10A 5

6 head are available (with round test surface 25 mm diameter or square test surface with 25 mm length of size). The abrasive material (e.g. emery paper) is attached to the test head surfaces by means of double sided adhesive tape or other adhesive method. Crockmeter test It is of great importance that materials which are used for the internal furnishing of vehicles do not stain passenger clothing. For a wide range of ways in which this can happen the effect is investigated by means of the Crockmeter test. In this a piece of relatively coarse cotton is stretched over a Perspex cylinder 16 mm diameter and is then moved to and fro across the surface whilst applying a defined perpendicular test force, resulting in a rubbing action. After this procedure the specimen is examined to establish if any colour has been transferred onto the cotton.. The test is used both for plain plastic and coated surfaces including vehicle seats of textile or leather material, i.e. everything with which the passenger might come into contact. MEK Test Methyl-ethyl-ketone (Butanone) is a relatively strong organic solvent. The MEK adapter is fitted to hold a test body of high density special felt. The felt is soaked with MEK and is moved across the specimen in a reciprocating motion whilst a perpendicular force is applied, to perform a rubbing action on the surface. After the test it is possible to examine the felt for any possible colour transfer and also to check if any mark such as a change to a matt surface is left on the specimen after it has dried. The ideal result is if there is no mark left to change the appearance of the tested surface. The method can of course also be used with other solvent materials such as, for example synthetic perspiration which, with frequent contact, may also damage coatings. The MEK adapter can also be employed, either dry or soaked in any desired substance, to conduct a wipe test to see if any surface printing or writing is damaged. By using these various test methods in combination and to supplement one another, research and development involving the use of only three test instruments will provide wide ranging data on the properties involved and, subsequently, the basis for quality assurance testing. It is highly probable that additional possibilities will present themselves once the user has become familiar with the capabilities of the equipment and devise test variations to resolve problems and meet the needs of individual applications which may range from the general to the exceptional. Development translation Jan10A 6