The Process:
Extrusion Lamination Overview
Adhesive Lamination
- Resin polymer enters hopper
- Resin is carried through a heated barrel by means of a screw
- Screw/barrel system is broken into 3-5 segments
- Feed zone: feeds the resin into the extruder; channel depth is usually the same throughout the zone.
- Melting zone: most of the polymer is melted in this section; channel depth gets progressively smaller.
- Metering zone: melts the last particles and mixes to a uniform temperature and composition; channel depth is constant throughout this zone.
- Melt passes through screen pack/breaker plate to be filtered
- Melt enters die and is shaped into desired extrudate film
- Extrudate leaves die, being exposed to air where it is oxidized
- Extrudate reaches the nip where it is pressed together with substrates to yield laminated product
Why Coextrude?
- Improves line speed
- Improves bonding strength
- Potentially no need for oxidation treatment
Typical Co-Extrusion Line
Costs?
- Running more than one extrusion line increases cost
- Cost is offset by superior product, improved line speed leads to faster manufacturing thus reduction of labor costs
- If copolymers are used, coextrusion becomes very profitable
Choosing a Resin Polymer:
- LDPE, PET, PLA most common in industry (PLA biodegradable)
- Copolymers thereof are now manufactured
- can choose one that has been optimized for specific project to improve adhesion to desired substrate
- EVOH often added as coextrudate to improve oxygen barrier or system
- Oriented metalized PET resins are fantastic barriers (more expensive)
- HDPE better moisture/oxygen barrier than LDPE (more expensive)
- Nylon excellent for preventing pinholes
- Aluminum very common in industry due to its protection against light and its ability to suppress the transport of almost any matter through stages of production and shelf life
- Tie layers of "pre-extruded" resins, hardened into new pellets for later extrusion are becoming more common
- Ester-maleic anhydride additives are common
- Chlorinated PE is a fantastic, polar additive
Study: Scratch Behavior of Extrusion and Adhesive Laminated Multilayer Food Packaging Films
Brian A. Hare, Hung-Jue Sue, et al.
- BOTH FILMS HAVE:
- extruded polyolefin sealant layer
- metalized oriented PET layer
- same proprietary sealant layer
- THE ONLY DIFFERENCE:
- how the layers were adhered; one by adhesion, one by extrusion
Results of Scratch Damage Analysis:
EL System
AL System
vs.
Common Problems & Solutions
Important Factors to Consider
Common Problems & Solutions
Important Factors to Consider
- Surface tensions are very important!
- Extrudate melt surface tension should be similar or slightly below that of the substrate to be laminated
- Controlling the melt flow index is imperative to yield proper adhesion
- Poor wetting or wetting degradation is routinely caused by over-oxidation
- Primers, such as small amounts of dilute PE imine, can promote covalent bonding within the lamination, increasing bond strength dramatically
- Priming has been shown to yield consistent performance of a given system
- Number 3: Worming
- Unbalanced physical tensions as melt leaves die causes worming (physical defect)
- Maintaining uniform viscosity and melt temperature (can be higher in the middle) as extrudate leaves die
- Number 4: Bubbles
- Air or gas trapped in the melt can lead to bubbles in the lamination when the process is complete
- Choosing the correct resin polymer, copolymers, tie resins, as well as ensuring the resins are dry and pure before melting down
- Number 1: Poor Adhesion
- Over-oxidation and improper surface tension are biggest cause
- Controlled oxidation techniques, use of copolymers, tie resins, and co-extrusion as well as consistent monitoring of melt flow index
- Number 2: Pinholes in Lamination
- Curtain stability issues or contaminants (ex/ moisture) are biggest cause
- Introduction of nylon, ensuring resin pellets are completely dry before melting down (especially with PET or PLA)
- Bonding strength is affected by melt temperature, line speed, thickness of melt, and level of oxidation
- Melt Temperature must be:
- Hot enough to wet
- Yield the correct viscosity
- If co-extruding, all extrudates must have similar viscosity
- Line Speed must be:
- Fast enough to sustain profitable manufacturing
- Fast enough not to over-oxidize melt
- Fast enough not to cool below critical adhesion temperature when melt reaches the nip
- Slow enough to oxidize enough
- Thickness of Melt must be:
- Thick enough to provide good adhesion
- Thin enough to be profitable (choice of resin/copolymer combination is paramount)
- Level of Oxidation must be:
- Sufficient enough to allow bonding to occur
- Over-oxidation leads to chain fission; a break in the polymer chain, thereby reducing molecular weight and ruining bond strength
Types of Surface Treatment:
- Corona
- Flame
- APT (atmospheric plasma treatment)
- Ozone
Why Surface Treatment:
- Improves oxidation of substrate
- Improved oxidation leads to better hydrogen bonding
- Corona, Flame, and APT used to treat substrates
- Ozone used to treat molten extrudate
- getting away from ozone due to increased use of copolymers and tie resins
What's the difference?