Modifying LDPE for Improved Adhesion to Aluminum Foil Hariharan.K Technical Specialist, Packaging & Industrial Polymers, E I DuPont India Pvt. Ltd.
Outline Why modify LDPE? Modification Technology Factors that Influence Adhesion Benefits Example Conclusions
Extrusion Coating/Lamination to Al Foil LDPE is predominately used Low cost Good processing Typical structures: Paper / LDPE / Al / LDPE OPET/primer / LDPE / Al / LDPE Typical applications: Packaging of dry powders and mixes
Why Modify LDPE? Moderate Adhesion Inconsistent Adhesion Oxidation required High temperature processing smoke odor/taste Line speed limitations
Ways to Enhance Adhesion to Al Ozone treatment- promotes oxidation but has similar drawbacks to LDPE Acid copolymers Ethylene (meth)acrylic acid Acid groups bond to basic Al surface Provides durable bonds But too expensive for some applications Primers
Goal of this work Develop a modifier for LDPE to enhance its adhesion to foil extrusion coating/lamination process pellet blend moderate cost (between LDPE and acid copolymers)
Resin Designations Resin LDPE ACR1 ACR2 AE Characteristics 7 MI, 0.918 g/cc Acid copolymer with 14 MI, 12% acid Acid copolymer with 11 MI, 4% acid Adhesion modifier - "adhesion enhancer"
Approach: Add Acid Functionality Peel Strength, g/25mm 400 350 300 250 200 150 100 Peel Strength to Foil 50-μm Al / 35-μm resin 320 C, 63 ms TIAG Simply blending standard grades of ACR s does not improve adhesion 50 0 LDPE 80% LDPE, 20% ACR1
Discovery: Need Compatible Modifer Poor Compatibility LDPE Poor compatibility gives large domains - ACR may not be accessible to surface for adhesion Large domains increase haze ACR 700 Compatibility Study Good Compatibility Peel Strength to Foil, g/25mm 650 600 550 500 5 6 7 8 9 10 Film Haze, %
Modification Provides Mid-Level Performance 1400 Extrusion Coating Results 50-μm Al / (8-μm blend - 38-μm LDPE) 320 C, 244 m/min line speed, 127 mm air gap Peel Strength to Foil, g/25mm 1200 1000 800 600 400 200 0 100% LDPE 20% AE/80% LDPE 100% ACR2 Layer Next to Foil * Results will vary with processing conditions.
Factors That Affect Adhesion Statistically Designed Experiment Structure: 50μm Al / coating Four Variables % AE blended with LDPE (0 to 20%) Temperature (293-327 C) Thickness (20-45 μm) TIAG (60-100 ms) TIAG = air gap / line speed Kept air gap at 152 mm and varied line speed from 91 to 152 m/min
Results of DOE Analysis 700 Peel Strength, g/25mm TIAG, msec = 80.0 Thickness, um = 32.5 700 Peel Strength, g/25mm Temperature, C = 310.0 Thickness, um = 32.5 600 600 500 500 400 400 300 300 200 200 100 ECHIP 100 ECHIP 0 2 4 6 8 10 12 14 16 18 20 %AE Low Temperature, C = 295.0 Middle Temperature, C = 310.0 High Temperature, C = 325.0 0 2 4 6 8 10 12 14 16 18 20 %AE Low TIAG, msec = 60.0 Middle TIAG, msec = 80.0 High TIAG, msec = 100.0
Results of DOE Analysis 700 Peel Strength, g/25mm Temperature, C = 310.0 TIAG, msec = 80.0 600 500 400 300 200 100 ECHIP 0 2 4 6 8 10 12 14 16 18 20 %AE Low Thickness, um = 20.0 Middle Thickness, um = 32.5 High Thickness, um = 45.0
Advantages of LDPE Modification Retains processing characteristics of LDPE Reliability - higher and more consistent adhesion than LDPE Productivity - higher line speeds without sacrificing adhesion Lower temperatures for same adhesion Improved organoleptics less smoke less gel lower energy costs These depend on the structure and processing conditions
Lower Temperature (DOE Model Results) 20% AE, 80% LDPE 293 ºC 450 g/25mm 20 15 Peel Strength,g/25mm TIAG, msec = 80.0 Thickness, μm = 38 %AE 10 400 450 500 550 5 0 ECHIP 250 200 300 350 100% LDPE 325 ºC 450 g/25mm 290 305 315 325 Temperature, C
Improved Productivity (DOE Model Results) %AE Peel Strength,g/25mm 20 15 10 640 600 580 540 500 Temperature, C = 315 Thickness, μm = 38 20% AE, 80% LDPE 150 m/min 580 g/25mm 100% LDPE 90 m/min 450 g/25mm 5 0 ECHIP 460 420 380 90 120 150 100% LDPE 150 m/min 380 g/25mm Line Speed, m/min
Improved Reliability Converter Trial Foil lamination Used Six Sigma Methodology Project Y: Peel strength to Al Unit: Peel strength, 3 measurements per production lot Defect: Peel strength below 250 g/25mm
Xbar Control Charts 250 200 150 100 Subgroup 0 5 10 15 UCL=218.8 Mean=165.6 LCL=112.5 100% LDPE 325 C 85 m/min 400 300 200 UCL=383.3 Mean=308.1 LCL=232.9 20% AE, 80% LDPE 325 C 85 m/min Subgroup 0 5 10 15 450 400 350 300 UCL=448.3 Mean=384.1 LCL=320.0 20% AE, 80% LDPE 330 C 85 m/min Subgroup 0 5 10
Box Plots Peel Strength, g/25mm 400 300 200 100 20%AE, 325C 20%AE, 330C Resin, Temperature LDPE, 325C
Summary Statistics Temperature Peel Strength, g/25mm Resin deg C Mean Stdev Yield* DPMO LDPE 325 166 30 4% 963400 20% AE, 80% LDPE 325 308 43 86% 139000 20% AE, 80% LDPE 330 384 37 99.6% 4400 * Specification of 250 g/25mm or higher Statistically significant increase in peel strength by using modifier and by raising temperature Improved process by 4 sigma levels: improved yield from 4% to greater than 99%.
Conclusions Appropriate modifier can significantly improve the adhesion of LDPE to Al DOE techniques help map out the key factors that influence performance % Modifier Temperature Thickness TIAG Benefits include higher productivity, lower temperature and improved reliability Statistical analysis and modeling were found to be useful in demonstrating the benefits