Chemical Etching Method

A. Introduction 

DestructiveTo separate or dissolves overlaying materials to expose a sealant layer by chemicals for visual inspection of seal contamination.Not applicableOff-lineNot applicable.

B. Operation 

Multilaminate and composite packaging materials may be etched to remove overlying layers, revealing the hermetic seal of packages that have polyolefin heat seals. The outer layers of a package are removed by tearing, abrasion, and chemical action to expose the sealant layer intact. By photographing the package before etching, the etched seal can be compared with the photograph to determine the significance of visually discernible defects.

Composite paperboard packages

  • Water bath and heater with thermostat
  • Three l-L Pyrex glass beakers
  • Running tap water
  • Graduated cylinder
  • Automatic stirring device (heated is preferred)
  • Drying oven equilibrated to 65oC (150oF)
  • Paper towels
  • Rubber gloves, protective goggles, apron, tongs
  • Fume hood with chemical-resistant surface

Retortable pouches 

  • Two l-L Pyrex beakers
  • Running tap water
  • Paper towels
  • Rubber gloves, apron
  • Protective goggles, tongs
  • Fume hood with chemical-resistant surface
  • Chemicals for etching retortable pouches
  • 6 N HCl solution, commercial grade
  • Tetrahydrofurant (THF), commercial grade, stabilized

CAUTION: Always pour acid into water; never pour water into acid. Pour 0.5 L of concentrated HCl into 1 L of cold distilled water. Pour slowly, as heat will be produced when acid and water mix. Stir until mixed completely. Cover to prevent evaporation. Solution will be 3.7 N HCl. Pour 0.5 L of concentrated HCl into 1.5 L of cold distilled water. Add 10 g of CuCl2. Stir until completely mixed. Cover beaker and let warm to room temperature before using. Pour enough Na2CO3 into a container to make a saturated solution at room temperature. Some undissolved Na2CO3 should remain on bottom of beaker after stirring.

Composite paperboard packages 

  1. Cut transversal seal from package approximately 1 inch from end.
  2. Identify multiple samples by notching cut edge with scissors.
  3. Manually strip paper from sample to be etched.
  4. Place sample in hot HCl solution (65oC) for 5 min.
  5. Remove sample with tongs and immerse it in Na2CO3 solution to neutralize the acid.
  6. Remove sample from the Na2CO3 solution with tongs and rinse it in running tap water.
  7. Pull off polyethylene layer that lies between paperboard layer and aluminum foil.
  8. Using a glass stirring rod to manipulate the sample, drop it into the CuC12 solution so that it is completely immersed.
  9. Observe closely while stirring to ensure that the heat of the reaction does not damage the polyethylene sealant layer as the foil is dissolved. Remove from solution.
  10. Dip sample in Na2CO3 solution to neutralize it, and then rinse it with water.
  11. Press sample gently between soft absorbent paper towels and place in oven at 65oC (150oF) until dry.
  12. Apply alcohol-based dye solution to inner and outer seal edges. (See fluorescein dye solution formula, described above).
  13. Observe pattern of ink dispersion and check for leaks and channels within fused seal area. Use overhead projector to enlarge seal samples and provide a more accurate visual inspection.

Retortable pouches 

  1. Cut off end of pouch and remove contents.
  2. Wash inside of pouch.
  3. Dry the pouch.
  4. Cut all but suspected area away from area of interest, leaving about 1 inch adjacent to seal.
  5. Soak sample in THF to remove outer polyester layer by softening adhesive and/or inks.
  6. Do this in a fume hood; wear protective gloves resistant to THF. (If separation cannot be obtained, proceed to next step.)
  7. Remove most of the ink and adhesive from aluminum foil with THF and paper towels.
  8. Soak remaining structure in 6 N HCl in a fume hood to remove aluminum foil by etching.
  9. Rinse sealant layers with water and dry with paper.

C. Application 


DEFECTS Flexible Pouch Semi-rigid and
Rigid plastic container
Plastic Can (Double-seam Metal End) Paperboard
Corner Dent   
Cut (Fracture)
Double Seam Defects*   
Foreign Matter Inclusion  
Label Foldover   
Leaker (Channel) 
Leaker (Corner)   
Leaker (Notch)   
Leaker (Perforation)   
Leaker (Pulltab)   
Leaker (Seal)   
Loose Flap or Ear   
Puncture (Pinhole)
Seal Defects (Blister)   
Seal Defects (Blocked)   
Seal Defects (Burnt)   
Seal Defects (Compressed)   
Seal Defects (Contaminated) 
Seal Defects (Convolution/Embossing)   
Seal Defects (Creep)   
Seal Defects (Crooked)   
Seal Defects (Incomplete)   
Seal Defects (Misaligned/Deformed)  
Seal Defects (Nonbonding/Weak)   
Seal Defects (Plastic Lumps)   
Seal Defects (Seal-width Variation)   
Seal Defects (Stringy)   
Seal Defects (Uneven Impression)   
Seal Defects (Uneven Juncture)   
Seal Defects (Wrinkle)  
Swell (Swollen Package)

D. Source 

ARNDT., G.W. JR. 1998. Chapter 22C Examination of Flexible and Semirigid Food Containers for Integrity. FDA Bacteriological Analytical Manual (8th Ed)