Barrier Coatings of Bio Films.
Robin Cooper, Michelman Inc
ABSTRACT
It is possible to use conventional water based barrier coatings on both the new and more traditional bio films to improve their barrier properties and allow them to compete with conventional flexible packaging films. This could allow a potentially greater use of Bio films to replace conventional polymer films
INTRODUCTION
Flexible packaging is a fast growing market. Global demand for flexible packaging is forecast to increase by 5% annually through to 2010 (1.), and offers several advantages over rigid packaging, being light and easier to transport. Bio packaging is an even faster growing market and it is estimated that it will show a compound annual growth rate of 22% from 2006 to 2011(1.).
One of the major benefits of flexible packaging, especially for food applications, is that it can be developed and used for high barrier, shelf life extending or niche applications. This can be done using co extruded and laminated film structures, or by taking advantage of advances in modified atmosphere packaging (MAP) and oxygen scavenging packaging.
Bio plastics, however, generally have lower barrier to gas and moisture than oil-based plastics. This is not a problem when bio packaging is used with fresh produce such as fruit, vegetable and bread, but it can be a problem for foods that require a longer shelf life.
Fresh produce doesn’t require high barrier properties or a long shelf-life and in fact the properties of bio packaging can be an advantage for some products. For example bio packaging is ideal for use with fresh bread because its high moisture transfer properties mean that the bread can be placed inside the package while still hot, and the package will not steam up, meaning that the bread does not become soggy. But if bio packaging is going to extend its range in the food market, permeability will have to be reduced.
The purpose of this work was to investigate the use of coatings on sustainable bio films and bio degradable substrates to see what level of improvement to the barrier properties of the various substrates could be obtained. For this study three Bio Film substrates were chosen o Polylactic acid (PLA) film, a naturally smooth and glossy material o Cellulose film derived from wood pulp, having a natural barrier to gases but readily permeable to water vapour o And paper
Independent coating routes were taken with each of the substrates to try and deliver a material that would give an overall barrier performance comparable to conventional barrier film. In figure 1 a typical barrier mapping is given comparing uncoated cellulose and PLA film to other flexible packaging films. Ideally sustainable chemistries should be used for the coating but current technology does not permit this level of barrier to be readily achieved. So we took the step-wise approach to use small amounts of water based polymers as coatings on sustainable substrates to see if it was possible to match the performance of the conventional packaging films. We did not want to use solvent based or halogen containing chemistries but decided that petroleum based chemistry would be an option so long as it was from a water based solution or emulsion and that the coating should not interfere with the recyclability of the original substrate(2.).
1000 Cellulose Paper
100 PA PET PLA EVOH 10 PP-0 PVDC PE 24h 24h 2. 1 Metallised PET MetallisedPP-O Moisture Vapour Transmission Vapour Transmission Moisture g/m Rate
1 10 100 1000 10000
Oxygen Permeability cc/m2.24 h
Figure 1 Barrier Mapping of Typical Substrates
METHOD
PLA Film A two coat system was applied to the base film. This was a pilot scale film approximately 25 microns thick, which had been corona treated to aid wet out of the water based coatings. Firstly a primer layer possessing good oxygen barrier based on a hot water soluble EVOH with adhesion promoting additives was applied directly to the corona treated film,and this was then subsequently overcoated with a styrene acrylic coating possessing excellent moisture vapour barrier. The MVTR and oxygen permeability were then measured on the coated film.
Cellulose film Here the substrate possessed good oxygen barrier so a single coating was applied to the film in order to improve its moisture vapour barrier. Two different coating systems were utilised. One is based on the same coating used to provide the moisture barrier to the PLA film - a hydrophobic stryrene acylic coating and the other was a coating designed to give not only moisture vapour barrier but ink receptivity with higher surface energy - based on an EAA copolymer emulsion. The MVTR and oxygen permeability were then measured on the coated material.
. Paper Here the challenge is much greater, so a variety of different techniques were investigated and used to achieve the final result. A paper was initially coated with a product resembling a conventional clay coating with an EAA binder and mineral filler combination to give a barrier coating employing a tortuous path technique. A hot water soluble high hydrolysis high molecular weight PVOH based oxygen barrier coating was applied directly to this. This coated material was then calendared with heat and pressure to achieve consolidation of the coatings and a smooth substrate.
RESULTS
It was possible in all cases to achieve significant improvement in barrier properties and the results are summarised in figure 2 and table 1 below.
Table 1. Barrier Properties after Coating Substrate MVTR gsm/day Oxygen permeability cc/day 90%RH, 38°C 0%RH, 25°C Paper 5 to 15 1 PLA film 6 to 7 7 Cellulose film 6 to 10 3
1000 Cellulose Paper
100 PA PET PLA EVOH 10 PP-0 PVDC PE 24h 24h 2. 1 Metallised PET MetallisedPP-O Moisture Vapour Transmission Vapour Transmission Moisture g/m Rate
1 10 100 1000 10000
Oxygen Permeability cc/m2.24 h
Figure 2. Barrier Mapping of Samples compared to typical substrates.
DISCUSSION
Each of the substrates presented its own challenge in applying and optimising the barrier performance.
PLA Film We found that we needed corona treatment to get wet out using water based coatings. The film is also sensitive to drying and relatively low temperature drying is needed to avoid dimensional instability. This limited our choice of products to those that would film form at relatively low temperatures. We utilised a product based on a styrene acrylate copolymer emulsion that had been formulated to give a hydrophobic surface and low surface energy (<31dynes/cm) and low moisture vapour permeability (VaporCoat® 2200R) as the topcoat and a modified EVOH giving high crystallinity as the gas barrier and primer (Michem®Flex B500). We found total dry coating weights of between 5 to 6gsm gave the optimum results.
Cellulose Film Cellulose is capable of absorbing something like 3 times its own weight of water and can be a challenge coating in the lab with water based products. We succeeded by having the film under tension throughout the coating and drying operations and flat even coated films were obtained in this way. With this product we tried two different approaches, one was to use the same styrene acrylate copolymer emulsion used above. The optimum dry coat weight for this product was found to be less than 3gsm dry. The other method was also to use a higher surface energy EAA coating (Michem®Prime 4983R) that would allow subsequent overcoating with water based materials (surface energy 45dynes/cm) for example inks and glues that would allow easier conversion without using solvent based technology. The optimum coat weight for this was found to be 2.5gsm dry.
Paper The challenge with paper is that it is fibrous in nature and this can lead to difficulties in obtaining a continuous coating without flaws, pinholes or other coating imperfections. To help achieve this, a calendaring operation was used, after coating with the initial barrier coatings. The barrier coatings in this case used a combination of a highly crystalline PVOH coating followed by a filled EAA based coating to give a tortuous path gas barrier. A significant decrease in both MVTR and oxygen permeability were obtained. We also found that putting these coatings through a vacuum deposition process had a synergistic effect. We obtained something approaching an order of magnitude reduction in both oxygen barrier and MVTR compared to either treatment on their own. This type of effect has been noted by others authors working with different substrates and coating chemistries (3.). No definitive explanation of this affect is offered here. With this multi layer coating technique a total dry coat weight of 5gsm was found to produce the barrier performance reported here.
CONCLUSIONS
Coatings can be a useful way to extend the capability of packaging films whether it is to improve barrier, offer heat sealability, alter surface properties or alter the appearance of the film or package. In this presentation we have shown that it is possible to use conventional water based barrier coatings on both the new and more traditional bio films including paper to improve their barrier properties and allow them to compete with conventional flexible packaging. Other possibilities and combination of properties are also attainable and can offer the film producer and the packaging converter further opportunities to meet their customer needs.
ACKNOWLEDGMENTS
The author is grateful to many of his colleagues for their help with this work. In particular thanks are due to Dr Talia Collins, Dr Michelle Nilsen and Antonio D’Addio.
References
1. Flexible Vol 5, Issue 6 page 23 2. John Homoelle,Sustainability-An Evolutionary Journey in Packaging, Intertech-Pira ‘Sustainability in Packaging’ conference March 6-8, Orlando. 3. Oliver Vetter, Latest Innovations in Inorganic Barrier Coatings, PIRA ‘Flexible Packaging Europe’ Conference 19-21 October 2005, Brussels.
Barrier Coatings of Bio-Films
September 16-20, 2007 By: Robin Cooper St. Louis, MO Bio-Films
PLA Cellulose Film Paper Bio-Films
MVTR Oxygen PLA 90%RH, 38°C transmission Cellulose Films gm-2d-1 0%RH,25°c cm3m-2d-1 Paper PLA 60 155
Cellulose 1450 3
Paper 1450 5000 Barrier Mapping
1000 Cellulose Paper
100 PA PET PLA EVOH MVTR 10 PP-0 g/m2.24h PVDC PE
1
Metallised PET Metallised PP-O
1 10 100 1000 10000
Oxygen Permeability cc/m2.24 h PLA
PLA Substrate PLA
Corona treatment
PLA Substrate PLA
Application Of water-based Barrier Primer
PLA Substrate PLA
Application of Hydrophobic water-based coating
Maximum 5.5 µm Barrier Primer
PLA Substrate PLA
MVTR Oxygen transmission 90%RH, 38°C 0%RH,25°c gm-2d-1 cm3m-2d-1
PLA film with Barrier 6 7 Primer and Hydrophobic overcoat 5.5µm total coating thickness Cellulose Film
Case 1
Cellulose Film Substrate Cellulose Film
Case 1
Application of Hydrophobic water-based coating
3.5µm
Cellulose Film Substrate Cellulose Film
Case 2
Application of EAA water-based Coating
2.5µm
Cellulose Film Substrate Cellulose Film
Case MVTR Oxygen transmission 90%RH, 38°C 0%RH,25°c gm-2d-1 cm3m-2d-1
1 Hydrophobic 11 3 Coating 3.5µm
2 Print receptive 16 3 EAA Coating 2.5µm Paper
Difficulties are more complicated by having a substrate which is not a continuous film but is a fibre mat The coating solution brings into play several techniques to overcome this problem Coating has to be able to resist moisture and vapour from both sides. Paper Gas Barrier
To achieve Gas barriers there are two common techniques employed – Highly crystalline polymers – Tortuous path techniques Gas Barrier by tortuous path technique Paper
Application of filled EAA Barrier Coating
3µm Paper
Application of water-based oxygen barrier coating
1µm
Filled EAA Barrier Coating
Paper substrate Paper
oxygen barrier coating
Filled EAA Barrier Coating
Paper substrate Gas Barrier by tortuous path technique
But with higher filler ratio to achieve longer path lengths a large fraction of the polymer matrix will be near a filler surface. – To help prevent losing performance we found it advantageous to incorporate heating under pressure above the Tg of the polymer matrix. (calendaring) Paper
Consolidation of the coatings by calendering
oxygen barrier coating
Filled EAA Barrier Coating
Paper substrate Effect of Metallisation
Uncoated Paper plus Paper plus Paper plus Paper metallisation Barrier Barrier Coating Coating Plus Metallisation MVTR 1500 40 50 5 90%RH, 38°C gm-2d-1
Oxygen 2000 500 100 1 transmission 0%RH,25°c cm3m-2d-1 Paper
Vacuum Deposition of Aluminium Coating
oxygen barrier coating
Filled EAA Barrier Coating
Paper substrate Paper
TopCoating of water-based EAA coating to protect Aluminium
0.5-1.0µm
oxygen barrier coating
Filled EAA Barrier Coating
Paper substrate Summary
Improvement in MVTR Barrier – Possible using low energy hydrophobic water- based surface coatings – Possible using water-based EAA coatings to give higher energy print receptive coatings Summary
Oxygen Barrier – Possible using water-soluble highly crystalline polymers – Possible using tortuous path techniques incorporating fillers Summary
Combined moisture vapour and gas barrier – Possible using correct combinations of coatings – Metallisation seems to have a synergistic effect in combination with barrier coatings Barrier Mapping
1000 Cellulose Paper
100 PA PET PLA EVOH MVTR 10 PP-0 g/m2.24h PVDC PE
1
Metallised PET Metallised PP-O
1 10 100 1000 10000
Oxygen Permeability cc/m2.24 h Barrier Mapping of Coated Products
1000 Cellulose Paper
100 PA PET PLA EVOH MVTR 10 PP-0 g/m2.24h PVDC PE
1
Metallised PET Metallised PP-O
1 10 100 1000 10000
Oxygen Permeability cc/m2.24 h Conclusion
It is possible using thin surface coatings on Bio-Films to give improved barrier to both oxygen and moisture vapour so that they can compete with more traditional barrier polymers and composite structures. Thank You Robin Cooper New Business Development Manager [email protected]
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