[CHAPTER 3]
[Test for Finished Film]
2K17
LE 4213
Md. Enamul Hasan Zahin, Lecturer, LE, KUET
Finish Film
A separate film strongly adhered to the leather surface.
It is formed by heterogeneous mixture of different compatible ingredients such as binders, pigments, dyes, preservatives, hardener, softener, wax etc.
The compatibility, holding power, bondage, heat resistivity, cold crack resistivity, light fastness, rub fastness, color migration, water fastness, elasticity, resistance to solvents, washing, cleaning, dressing etc. should be under consideration for the test of finish film.
1. Bond Strength Test
The anchorage between the finish film and the leather surface and between the individual finish coats can be tested in two ways:
Adhesive tape method
Amos and Thompson method
The widely used method is the Amos and Thompson method.
a) Adhesive tape method
A strip of adhesive tape of approximately 50mm length is pasted to the finished leather surface in such a way that 10mm length of the tape remains outside the leather.
The 40 mm length of the tape is rubbed several times with finger on the leather surface so that the tape is nicely pasted to the leather surface without any air bubble inside.
After 15 seconds, the tape is stripped off holding the 10mm long free end.
Assessment: No finish should adhere to the tape if the bondage between the leather surface and finish film is strong and satisfactory.
b) Amos and Thompson method
Materials: Plastic plate/metal plate (80/40×10mm). Standard adhesive (PU) or Epoxy resin (araldite: hardener =1:1).
Procedure:
A strip of 50 or 100mm long and 10mm wide is cut from the finished leather to be tested.
The finished surface of the strip is then degreased by wiping with a cotton pad, soaked in petroleum ether.
A hole (about 2mm dia) is made to one end of this strip for hanging with pan.
A thin layer of standard adhesive is applied (e.g. epoxy resin) on the plastic plate and finish film of leather side. Allow to dry for 30 mins. Apply 2nd coat adhesive on leather surface and again dry for 30 mins.
Leather sample and plastic plate are joined together and composite specimen is then pressed to remove any trapped air inside.
1 kg of load is placed on the composite specimen and allowed to leave for 24 hours.
Next day, the load is removed and excess adhesive in the edge is removed by a sharp razor blade.
The free end of the leather strip is slightly pulled to open a gap of approximately 1mm (3mm is enough) in the bond so that misleading test results are avoided.
The slide is then fastened to a suitable stand and the leather specimen is stripped off by pulling the free end at an angle of 90°.
To do this a pan is hanged to the loose end of the specimen through the hole and weights are put on the pan till the detachment between the specimen and the slide occurs.
These weights including the weight of the pan are taken as the measure of adhesive value.
To carry wet test, the specimen is soaked in water for 15 minutes or more to get the wet sample before attaching.
Observations
Separation may occur in the following way:
Total leather can be separated from plastic plate with adhesive.
Finish film can be separated from leather.
Corium minor can be separated from leather to plastic.
Standard value 200gm/cm.
Factors influencing leather-finish film bondage
Bondage mainly depends on the following factors – a) Bond strength of the binder used.
Solution: Change and select the proper binder.
b) Condition of the leather surface during application of season on it.
This may be caused for any one of the following surface conditions of the leather surface:
Poor absorbency of the leather surface.
Solution: Generally corrected by lightly snuffing the leather surface before application of finish.
Too much oiliness of the leather surface.
Solution: Oiliness of the grain is corrected by cleaning the leather surface, before season application, with a dilute solution of ammonia.
Shortage of electric charge to the leather surface.
Solution: To recover electrical charges the leather surface should be cleaned with a dilute solution of oxalic or acetic acid.
c) Use of excess amount of pigment, plasticizer and stabilizer.
d) Use of insufficient amount of binder in the season.
e) Method followed to apply the season on the leather surface.
Usual method of finish application (e.g. automatic spraying and curtain coating) provide poor bondage between leather surface and the finish film.
Hand brushing or mechanical brushing machine for bottom coat improve bondage between leather surface and finish film.
Spraying of season without air always gives better bondage with leather than spraying with air.
2. Heat Resistance Test
Effects of heat on finish film
Heat may cause migration of free fats and oils, used in leather making, from one layer to another layer changing the character of the leather completely.
Many dyes and organic pigments change their color under the influence of heat e.g.
Red shade may become slightly violet or blue.
Yellow may become slightly orange and so on.
The intensity and brilliancy of dyes and pigments may also change due to heat treatments.
The casein, nitrocellulose etc. sometime become slightly brown due to such heat treatments and create serious defects on white and pale pastel shade leathers.
Low melting point thermoplastics used in finishing become soft and tacky due to heat.
Significance
For judging the compatibility of finish film to hot surface, as shoe and leather goods with upper leather undergoes through different types of heat treatments like hot ironing, hot pressing, steaming, pouring of hot liquid plastic for molding.
The shoes and leather articles may be used in hot countries, sometimes dried by blowing hot air that may cause migration of free fats and oils.
Amount of visible damage to a materials by hot surface is evaluated by this test.
Procedure
Heating surface is cleaned with organic solvents (Acetone).
Leather surface to be tested is covered with a piece of Aluminium foil to prevent contamination of heating surface.
Temperature is set according to type of upper materials and specimen requirements.
Temperature range is usually 100-250°C for upper materials and 100-300°C for soling and safety items.
The specimen is brought in contact with the hot surface under slight pressure for few (05 sec for upper and 1 min for soling materials) seconds only either in static or dynamic conditions and visual observation is done.
The change in color (5-3 at grey scale is acceptable), smearing and other visible changes are then assessed generally by eye estimation or by grey scale (Burn, melt, shrink are not acceptable).
3. Other Heat Resistance Test
3.1. Hot air blowing
The leather specimen is suspended in a small chamber through which hot air at 150°C is blown for 1 minute.
The hot specimen is then cooled to room temperature and finish is examined for shift in shade and other visible changes.
The specimen is then tested for percentage elongation at break, tensile strength, grain crack strength and flexing and compared with those properties of the original leather.
3.2.Steam test
The leather specimen is suspended in a small chamber which is heated to 65°C and made saturated with steam and kept for five minutes.
The specimen is then examined for migration or bleeding of dyes and swelling of the finish.
How to improve heat resistance?
If the heat resistance of finish film of a particular lot of leathers is found poor, the finisher must trace out the ingredients for whom this poor heat resistance resulted-
It may be the binder, the pigment or dye or oil or any other material of poor heat resistance.
Silicone top coat at the top of the leather finish may improve this heat resistance of the finish film.
4. Cold Crack Resistance Test
Significance of Cold Crack Resistance Test
Since the majority of exportable leather items are exported to cold countries of Europe, America, Australia etc. Tg of the finishing binder must be very low i.e. within the range - 5°C to -30°C.
Thermoplastic binders used for leather finishing become soft at elevated temperature and hard at low temperature.
Above glass transition temperature (Tg) the binder is flexible and forms film whereas below Tg it becomes hard, brittle and cracky.
Method-1
A leather strip is cooled to a specified temperature and sharply bent with the grain side out only once.
The finish film should not crack due to such bending.
Limitation
The strip may become hard and stiff but it cannot be touched by hand at such low temperature.
Bending problem becomes more if the strip is cooled to a very low temperature in the flat condition.
Method-2
A leather strip is cooled to a refrigerator or deep freezer to a low specified temperature in the U shape with the grain side out for one hour.
The cold and stiff U shaped leather specimen is then pressed once with a precooled wooden stick very near the fold so that the two hands of the U touches each other.
If there is any crack to the finish film due to such bending in the cold condition, the cold crack resistance of the specimen should be considered poor.
How to improve cold crack resistance
Use of binders of very low cold crack temperatures.
Applying the season on the leather surface as thinly as possible.
5. Light Fastness Test
Significance
Leather articles are not sold immediately and remain in show cases, exposed to sun and artificial lights for months. So the color may fade, become paler and duller when it is exposed to light for a long period.
A strip of leather is taken and covered half portion with standard materials and exposed to sunlight for a considerable period of time (72 hours).
Then the specimen is visually assessed for variation in shades by grey scale. For a good leather variation should be minimum (acceptable range is 5-3).
Light fastness test by artificial light
In this test, Xenon light is used. A powerful Xenon lamp is hanged through a voltage stabilizer at the center of a rotating black colored metallic vertical cylinder.
Air is blown from the bottom open end of the cylinder with an electric to remove the heat produced by the lamp.
The leather specimens are hanged and remains on the inside periphery of the rotating cylinder with their grain sides exposed to the light of Xenon lamp.
A standard blue-wool textile fading test card also exposed to the Xenon lamp along with the specimen for a specific period of 72 hrs.
Then the blue scale and the leather specimen are taken out and finally compared the contrast between the exposed and unexposed leather with the contrast of exposed blue cloth strips with their corresponding unexposed cloths.
The no. of cloth whose contrast found equal to the contrast of the leather specimen is taken as the light fastness value.
How to Improve Light Fastness?
The poor light fastness of a finish film is due to poor light fastness of any material used in the season.
The dyes, pigments, plasticizers etc. may be of poor light fastness.
Nitrocellulose lacquer film may become yellowish if its light fastness value is not high enough.
So the material of poor light fastness value should be replaced by a similar material of higher light fastness value.
6. Color Rub Fastness Test
To measure the transfer of color from the test specimen to a piece of white/black cloth or cotton felt.
Leather such as upholstery, lining, garments, glove and shoe upper etc. need to be tested for color fastness.
Dry rub fastness test Test Method
A dry felt pad (dia 25.4mm and thickness 6.35mm) is fitted to the base of the spindle.
Leather sample of 60mm×60mm or 60 mm dia is placed on the platform as such that area to be rubbed is uppermost and directly below the spindle.
Spindle guard is lowered and a weight of 2.5 kg is fitted to apply force.
Number of rub is set (4,8,16,32,64,128,256,512,1024…..) and observed after 64, 128, 256, 512 and 1024 cycles. Change in color is noticed for both leather sample and cotton felt.
Rating is done by grey scale and the acceptable range is 5-3 in gray scale.
The change of leather surface due to dry rubbing-
Production of high gloss on leather surface.
Leather surface becomes tacky.
The cotton pad is stained.
Rubbed portion of leather becomes oily.
Wet rub fastness test
A felt pad is immersed in cool distilled water and boiled for 1-3 minutes and allowed to cool to room temperature.
If the pad seems to be excessively swollen or soft then it should be rejected. Pad should not be kept in water for more than 24 h.
Liquid in pad is adjusted by gently squeezing and fixed in spindle.
Then test is carried out by placing 0.73 kg load on the top and 4,8,16,32,64,128,256,512, and 1024 cycle is observed.
Change of color in felt and leather are assessed by grey scale. After wet test both the pad and leather are dried before contrast measurement with grey scale.
7. Resistance to Water Stain
A drop of water is put on the finish surface of leather and allowed to dry overnight.
If any stain is developed then the stained area is rubbed with a soft cloth.
If the stained is not removed small quantity of colorless shoe polish is applied there and again rubbed with a soft cloth.
If the stain does not go out this time also, the leather should be considered of poor water resistance.
8. Test for Bleeding
A strip of leather specimen of dimension 10×2cm is dipped into water or perspiration solution upto 5cm length in vertical direction.
Wait until the leather sample is wetted upto 15 to 50mm (look from the flesh side).
The leather sample is raised from the solution and left it to dry for 24 hours at room temperature.
Then visual assessment is done by grey scale (acceptable range is 5-3).
If stains or dark edges are formed along the upper soaked up water line shows bleeding defect.
How to improve color bleeding properties?
Proper selection of dyestuff.
Use of required quantity of dye.
The dye should be completely dissolved in warm water before adding to the dyeing drum.
Sufficient float of water should be provided with proper revolution.
Dyeing should be started at the isoelectric point of Cr tanned leather for getting uniform distribution of dye and proper fixation is done by formic acid.
9. Resistance to solvent
During manufacturing of products e.g. shoe, bag etc. adhesive/solvents are to applied to the unfinished flesh side of the leather.
Sometimes cap stiffener are dipped in solvents to soften or render moulding.
The common solvents are: esters, ketones, alcohols, toluene, chlorinated hydrocarbons, benzene and its substituted products.
Test method:
1cc solvent is applied on the flesh side of the leather specimen.
The leather is then kept in that condition for 1 min at room temperature.
Finally dry rub fastness test is carried out on the grain side of the leather.
Both the pad and leather (after drying) are compared with grey scale.
10. Elasticity Test of Finish Film
Significance of elasticity test
All types of leather elongate when pulled either by hand or by machine.
The extensibility of shoe upper varies from 10-40 of original length.
As for highly stretchy leathers like glove, garment, chamois etc. it is 40-80%.
If the elongation of the finish film is less than the elongation of grain layer, the finish may crack during lasting.
Through tensile tester
The linear elongation in one direction is determined with tensile testing machine at the point of finish film crack (not grain crack).
Through ball burst tester
Extensibility of the finish film is determined by lastometer upto the point of finish crack.
Bally Flexometer test
To get an idea whether creases will form on leather in shoe during walking.
Key test
To get an idea about the elasticity or elongation of finish film of the upper leather.
Control of elasticity of finish film
Casein, shellac, nitrocellulose, albumin etc. can not hold plasticizers for a long time and so their films gradually become less extensible and ultimately crack.
Internally plasticized resin film of resin emulsion through copolymerization posses high degree of polymerization and show varying degree of elongation.
Not only the elongation but should have some elasticity so that creases do not form on the shoes during walking.
Pure plastic film should not be used because they do not pose any elasticity, so use some amount of non plastic binder like casein in the emulsion.
Resistance to washing and cleaning
To determine the resistance of finish film to cleaning agents the wet rub fastness test are carried out with the following reagents one after another.
1% of aqueous solution of neutral detergent (e.g. fatty alkyl sulphate).
A weak alkali solution (e.g. dilute caustic soda solution with water).
Petroleum solvent (e.g. white sprit having a boiling range of 130-200˚C).
Trichlorethylene.
Resistance to dressing agent
To carry out this test the emulsions (a. water free solvent, b. nonionic wax containing solvent and water, c. solvent free, aqueous, nonionic wax emulsion) is applied on the finish sides of two pieces of leather with the finger.
The surface of one piece is then immediately rubbed with a dry linen cloth either by hand or in fastness tester (ten times).
Surface of the second piece is rubbed in the same manner after 30 minutes when the solvent of the emulsion has evaporated out.
The linen clothes for both the leather pieces are then examined for stain development and compared with the grey scale.
Resistance to buffing of suede leather
Emery paper of specified grit is fixed to the pad holder of rub fastness tester and the leather specimen is rubbed back and forth for 10, 20 and 50 times under a load of 500gms.
Assessment is made with the grey scale for change of leather color. Variation of color should be minimum. Acceptable range is 5-3.
Moisture fastness
Two samples are taken. One is preserved as original in the atmospheric condition and the other one is kept in a desiccator containing water at the bottom to ensure 100% relative humidity inside the desiccator for 72 hours.
Dry color rub fastness test is carried out for original and two pads for two pieces are compared with a grey scale.
Resistance to ageing
The tensile strength, elongation, stitch tear, lastometer and flexing test are carried out first.
Conditioning of the rest of the leather sample at 50-70˚C for 72 hours is done.
Then conditioning is done for room temperature at 100% RH for 24 h.
Again conditioning of the sample is done at room temperature at 65% RH for 48 hrs.
The specimen is then tested for tensile strength, elongation, stitch tear, lastometer and flexing test.
Assessment is done by comparing the value. Variation should not be more than 10%.
Resistance to plasticizer
A filter paper moistened with plasticizer (e.g. Dibutyl phthalate) is placed on the finished side of the leather.
The leather with the filter paper is placed in between two glass plate and weighted down for 4 hours at 60˚C.
The filter paper then examined for staining or compared with a grey scale. Staining is not allowed.
Prepared by Al-Mizan, Lecturer, LE, KUET
