FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10; rule 13)
TITLE OF THE INVENTION
A Method For Quantitative Measurement Of Adhesion Strength Of Coating
APPLICANTS
TATA MOTORS LIMITED, an Indian company
having its registered office at Bombay House,
24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India
INVENTORS
Asmita Sathaye and Jayant Haridas
Both Indian Nationals
of TATA MOTORS LIMITED an Indian company
having its registered office at Bombay House,
24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India
PREAMBLE TO THE DESCRIPTION
The following Complete specification particularly describes the invention and
the manner in which it is to be performed.

FIELD OF THE INVENTION
This present invention relates to preparation of test coupons or specimens for measuring adhesion strength of coating on the substrate and more particularly Cathodic Electro deposition (CED) primer coating on sheet metal substrate by lap shear test method.
BACKGROUND OF THE INVENTION
Many of the components especially in the automobile industry are coated. Coating is provided basically to protect the substrate from environment, weathering, corrosion, abrasion, stone chipping, water etc. Variety of coatings are used for the automobile industry like paint, Electro deposition primer, underbody coatings, water repellant coatings, electroplating etc. Apart from the basic requirements of the coating like coating should be smooth, uniformly deposited and should have good corrosion resistance, solvent resistance etc, the important requirement of the coating is that it should have good adhesion to the basic substrate.
The quality of coating adhesion to the substrate is generally checked by cross cut adhesion test, knife test and peel test. However these do not give quantitative picture.
For measuring the adhesion strength of the coating on the substrate, use of a device comprising a dolly, which is stuck to the coated substrate with the help of adhesive and then applying a perpendicular tensile force to the coating till dolly separates from the substrate, is commonly used. This method and further modifications in the device to have more accuracy in the measurement have been described in many prior arts. The standards based on these prior arts are ASTM D4541 and ISO 4624.
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Drawbacks associated with conventional method are:
To measure the adhesion strength of coating special machine and device is required.
Based on the expected adhesion strength of coating choice of the proper dolly is
required.
OBJECTS OF INVENTION
The main object of this invention is to provide a method of making test coupons for measuring the adhesion strength of the Cathodic Electro deposition (CED) primer coating on sheet metal substrate.
Another object of this invention is to provide a method for measuring adhesion strength of CED primer coating on sheet metal substrate, which does not require special machine or any special device.
Yet another object of this invention is to provide a method for measuring adhesion strength of CED primer coating on sheet metal substrate, which is very simple and quick.
BRIEF DESCRIPTION OF INVENTION
The invention describes the test coupons designed for measurement of adhesion of CED primer coating on sheet metal substrate using single lap shear test method. Test coupons for measuring adhesion strength of coating in accordance with this invention mainly comprises of two sheet metal substrates. One sheet metal substrate is applied with Cathodic Electro deposition (CED) Primer coating which is coating of interest, used for corrosion resistance. Another sheet metal substrate is without having any coating on it. Two sheet metal substrates are then joined together using structural adhesive as per predefined overlap length and with predefined thickness
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of structural adhesive. After complete curing of the structural adhesive, test coupons are mounted on conventional tensile loading machine. Tensile force is applied to both the substrates in shear direction until failure of CED primer coating from the sheet metal substrate which is coated with CED primer occurs.
The test coupons made in accordance with the invention had failure between coated sheet metal substrate and CED primer coating. The structural adhesive is selected in such a way that, inherent strength of the adhesive is very high so that cohesive failure of structural adhesive would not occur. During the test, at the time of failure if structural adhesive transfers on both the sides of the substrates, then this is a cohesive failure of the structural adhesive and is due to fact that structural adhesive is weaker than adhesion of CED primer on the sheet metal substrate. In this case adhesion strength of CED primer on the sheet metal can not be evaluated. However it means that adhesion of CED primer on sheet metal substrate is at least as high as high as that basic strength of the structural adhesive. In such a case, in order to find out exact value for adhesion between CED primer coating and sheet metal substrate, structural adhesive with higher strength should be selected and used. It is also ensured that adhesion between structural adhesive and bare metal is always higher than adhesion of CED primer coating on the sheet metal. Since creep of sheet metal is much higher than that of adhesion strength of CED coating on sheet metal substrate, which is the coating of interest, plastic elongation of the sheet metal substrates giving inaccurate measurements cannot occur.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 shows test coupon samples made for testing adhesive strength of the CED primer coated sheet metal substrate.
Fig 2 shows the typical failure of Cathodic Electro Deposition (CED), primer coating after carrying out tensile shear test.
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DETAILED DESCRIPTION OF INVENTION
Referring now to the drawings wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only, and not for the purpose of limiting the same,
Sample coupon as per Fig. 1 consists of two sheet metal substrates (A, B). Both the substrates are of the same material and their dimensions like length, width and thickness are same. Substrate (A) is coated with corrosion resistance coating i.e Cathodic Electro deposition primer (C). Sheet metal substrate (B) is bare substrate, without any coating applied on it. These two sheet metal substrates are then joined together by using the structural adhesive (D) as per pre-defined overlap length and predefined thickness of structural adhesive.
Samples coupons in accordance with Fig 1 were made and Tensile force in the shear direction was applied to this assembly. Load is applied until CED primer (C) is separated from sheet metal substrate (A) ,which is coated with CED primer coat. For making test coupons in accordance with fig 1 following parameters were considered.
a) Details of first substrate (A) which is coated with CED primer (C):
Test substrate (A) for making test coupon in accordance with the invention, is of sheet metal and is coated with Cathodic Electro Deposition (CED) primer (C). Prior to CED primer (C) coating, panels are degreased and zinc phosphated to have coating weight of 2-4 gms/ m2. All the panels are then CED primer (C) coated. For CED primer (C) coating, water based Epoxy Polyamide resin system was used having pigment to resin ration of 1:3 and surface roughness value of 0.22 Ra. Three types of CED primer (C) coated substrates were made by keeping all the process
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parameters same and by varying CED primer (C) baking cycle. For the first type of samples (Type 1), CED primer (C) baking cycle was 150°C temperature for 10 minutes. For second type of samples (Type 2), CED primer (C) baking cycle was 160°C for 10 minutes. For third type of samples (Type 3), CED primer ( C), baking cycle was 170°C for 10 minutes.
b) Details of second substrate (B): Test substrates (B) for making test coupons in accordance with the invention is of bare sheet metal and is without having any coating treatment on it. It is ensured that these substrates are clean and free from rust.
c) Details of for structural adhesive (D): The acrylic based structural adhesive having high inherent strength and good adhesion on the bare sheet metal substrate, was used for making the test samples. The acrylic based adhesive had adhesion strength of minimum 16 MPa on the bare sheet metal substrates.
d) Making of test the coupons: Three sets of the test coupons were made in accordance with the invention as per following details:
i) Set 1: Substrate (A) of type 1 and substrate (B) joined together using structural adhesive (D). Length of overlap used is 25 mm.
ii) Set 2: Substrate (A) of type 2 and substrate (B) joined together using structural adhesive (D). Length of overlap used is 25 mm.
iii) Set 3: Substrate (A) of type 3 and substrate (B) joined together using structural adhesive (D). Length of overlap used is 25 mm.
All these three sets of samples were then tested on Tensile testing machine. The strength at which CED primer (C) coating separates from the sheet metal substrate (A) is measured.
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The typical CED primer failure is as shown figure 2. In all the cases the failure is separation of CED primer (C) from the sheet metal substrate (A). Table 1 mentions shear strength values giving adhesion strength of CED primer coating on the sheet metal.
Table 1

Set / sample Description Lap shear value
Set 1 6.73 Mpa
Set 2 6.32 Mpa
Set 3 6.90 Mpa
The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.
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WE CLAIM
1. A method for quantitative measurement of adhesion strength of coating
comprising the steps of;
(a) preparing test specimen of
- first substrate having coating applied on its surface,
- second substrate in bare material
- and lap joining said first and second substrate by a structural adhesive

(b) applying load in parallel to the adhesive surface of said test specimen
(c) measuring the load at failure of said test specimen

2. A method for quantitative measurement of adhesion strength of coating as claimed in claim 1 wherein tensile load is applied on free ends of the test specimen.
3. A method for quantitative measurement of adhesion strength of coating as claimed in claim 1 wherein longitudinal axis of test specimen is inline with direction of applied force.
4. A method for quantitative measurement of adhesion strength of coating as claimed in claim 1 wherein said first substrate is a sheet metal having cathodic electro deposition primer coating and said second substrate is a bare sheet metal.
5. A method for quantitative measurement of adhesion strength of coating as claimed in claim 1 wherein adhesion strength between bare substrate and structural adhesive is higher than the adhesion strength between Cathodic electro deposition coating and sheet metal substrate.
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6. A method for quantitative measurement of adhesion strength of coating as claimed in claim 1 wherein said first and second substrate having equal width, length and thickness.
7. A method for quantitative measurement of adhesion strength of coating as claimed in claim 1 wherein said lap join having width equal to the width of the said substrate.
8. A method for quantitative measurement of adhesion strength of coating substantially as herein described with reference to accompanying drawings.
Dated this 28th day of November 2007.

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ABSTRACT
A Method For Quantitative Measurement Of Adhesion Strength Of Coating
The invention describes the test coupons designed for measurement of adhesion strength of CED primer coating to sheet metal substrate using single lap shear test method. Test coupons for measuring adhesion strength of coating in accordance with this invention mainly comprises of two sheet metal substrates. One sheet metal substrate is applied with Cathodic Electro deposition (CED) Primer coating which is coating of interest used for corrosion resistance. Another sheet metal substrate is without having any coating on it. Two sheet metal substrates are then joined together using structural adhesive as per predefined overlap length and with predefined thickness of structural adhesive. After complete curing of the structural adhesive, test coupons are mounted on conventional tensile loading machine. Tensile force is applied to both the substrates in shear direction until failure of CED primer coating from the sheet metal substrate which is coated with CED primer occurs.
Fig1.