Hardness is one of the important properties of the paint film, it represents the mechanical strength of the paint film, the dry paint film should have a certain hardness, to withstand the physical damage of the outside world, to protect the surface of the coating.The physical meaning of hardness can be understood as the resistance of the paint film surface to other hard objects acting on it.Through a certain mass of load, acting on a small contact area, you can determine the ability of the paint film to resist deformation (including collision, indentation, abrasion and other deformation caused by).Used to determine the damage resistance of coating under external forces, such as collision and friction.
There are many hardness measurement methods of coating film. There are 3 kinds of methods commonly used at present, namely pendulum rod damping hardness method, scratch hardness method and indentation hardness method.These three methods express different types of resistance of the film, each representing different stress-strain fire system.
Determination of paint film hardness – pendulum rod hardness, scratch hardness, indentation hardness
(1) the swinging rod damping by rocker rail hardness method Embedded beneath the coating samples of two ball contact and the swinging rod with a certain cycle oscillation, swinging rod fixed on the coating quality of oppression, and make the coating produce resistance, according to the regulations of the swing of the pendulum amplitude of the time required to determine the hardness of coating, the swing attenuation time long the hardness of coating film is high.This detection method is also called pendulum damping test.The instrument is called pendulum rod damping test instrument, general: Konig (Konig) pendulum, referred to as K pendulum;Per-soz pendulum, or P-pendulum for short.The general conversion relation between the two pendulum measurements cannot be established.In the product inspection, usually only one of the pendulum rod instrument is prescribed, and the pendulum rod test instrument’s measurement results are measured in seconds.Film thickness and substrate material have an effect on damping time, and also depend on the complex elasticity and viscoelasticity of the coating.The result of pendulum rod damping test is related to the test environment, which should be controlled under the condition of temperature and humidity.Without the influence of air flow.The pendulum damping test method has the advantage of not damaging the coating film.Now these two types of pendulum rod hardness test instrument has been China’s national standard GBT 1730-88 {paint film hardness determination pendulum rod damping test “used.
When the coating thickness is measured with the swing bar Sunny tester, the main reason of the oscillation attenuation is the absorption of mechanical energy by the coating, and the oscillation attenuation time is inversely proportional to the loss modulus, which is used to represent the ability of absorbing mechanical energy.Because from the discussion of dynamic property determination, the loss modulus in the glass state region and rubber state region is relatively low, it can be inferred that in these two regions, the good attenuation time of swing is long, in fact, the pendulum rod test of soft rubber coating its attenuation time is longer to prove this situation.Therefore, it is considered that swing bar test is the test of coating loss modulus rather than coating hardness.
(2) Scratch hardness method is used in the paint film surface with hard objects to mark or scratch the coating film method to determine the hardness of the coating film.Commonly used pencil hardness method and needle determination method.
Pencil hardness method has two methods of manual operation and instrument test, is the use of known hardness of the pencil to determine the hardness of the film, the film will not be hurt by the plough pencil hardness (manual operation).Or the next stage of the plough film pencil hardness (instrument test) as the hardness of the film.The pencils shall be made of standard high-grade drawing pencils made by the specified manufacturer, and the cores shall be sharpened according to the regulations.Different countries use different grades of pencil hardness.National Standard GB/T 6739-86 <Coating hardness pencil determination method “provides the use of pencils from 6H to 6B a total of 13 grades, 6H the hardest, 6B the softest.As arbitration test to use instrument test method, general instrument model QHQ pencil scratch hardness tester.
The needle test method uses the needle tip of the instrument to scratch the film, and the film resistance to scratch needle penetration to represent the hardness of the film.It is indicated by whether the coating is scratched through by the needle under the specified load, or the minimum load required by the needle to scratch through the coating.China’s national standard GB/T 9279-88 Seranhe-Qing hazel scratch test & GT; specifies the use of automatic instrument as the secondary test instrument, the results are more accurate.When the hardness is measured by scratch method, the film is not only affected by pressure, but also by shear force, and the adhesion of the film is also reflected. Therefore, the hardness characteristics of the film measured by scratch method are different from the swing rod damping test method.
Determination of paint film hardness – pendulum rod hardness, scratch hardness, indentation hardness
(3) indentation hardness method using a certain quality of the pressure head in addition to the film, from the length or area of the indentation to determine the hardness of the film.The national standard (CB/ T9275-88 color paint and varnish Barker belt Erz indentation test) prescribes the method of using Buchholz pressure tester to test the hardness of the coating film. The indentation length measured shows the resistance of the coating to the indentation machine of the instrument.Indentation hardness in the hard film is more obvious, the results are related to the thickness of the film, for the same – – xu material, the indentation hardness value of the film is higher than the thick film.From actual measurements, the indentation test results for elastic coatings are inaccurate.