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How to improve the barrier performance of plastic film

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How to improve the barrier performance of plastic film

By Bruce Dou

Plastic films are widely used in packaging, such as food, electronic items, medicine, ect, that often require excellent barrier performance for the plastic films, so the barrier performance imposes significant meaning for the film itself. Then let’s talk more about how to improve the barrier performance.

1. Lamination

Lamination is one of the best methods to improve the barrier performance. Two or three layers or even more, can be combined together into one integration. The more the layers, the better the barrier performance. In case the compatibility of the two plastic material is good, two layers can be laminated directly. Otherwise, adhesive has to be needed, of which polyurethane adhesives are most common used.

The barrier index of the multilayer lamination film can be calculated as below:

 

 

 

Among which P——The overall transmission rate of multilayer lamination film

T1,T2——thickness of each single layer

P1,P2——transmission rate of each single layer

Sum up from the above formula, barrier performance of multilayer lamination film improve along with the barrier performance and thickness of each single layer. Plastic materials that can be applied in lamination are LDPE,LLDPE, HDPE, PP, EVA, PA, PET, EVOH, PVDC, Aluminum foil, paper, etc.

  • Lamination between plastic resins

Even though the barrier performance of some single plastic film is not good, however, it can be well improved by multilayer lamination. Multilayer lamination can make up the defects or pin holes of the plastic film, as the possibility of pinholes in the same position for the different layers is very very low, so the transmission route becomes more difficult for the small molecular suchas water vapor, oexygen, Co2, that comes the better barrier performance. This is mainly achieved by hot melting coextrusion, which suits LDPE,HDPE,LLDPE,PP and PVC, the main lamination structures are LDPE/LDPE,LDPE/HDPE, LDPE/HDPE/LDPE,LDPE/LLDPE/LDPE and LDPE/HDPE/LLDPE/LDPE.

  • Lamination between common barrier resin and middle barrier resin

The outside layers are common barrier plastic resin, with middle barrier resin in between. As the compatibility of the two plastic resins is not good, so adhesive is needed, that can achieve three layer or five layer. The common barrier plastic resins are LDPE, LLDPE, HDPE, PP and PVC, and middle barrier plastic resins are PA, PET, EVA. In this case, the common barrier resin can also act as inner heat sealing layer when PP or PE is applied during bag making.

  • Lamination between common barrier resin and high barrier resin

Common barrier resins are applied in outside layer, with PVDC or EVOH in between, this can great improve the barrier performance. PP or PE resin is applied as inner layer, acting as heat sealing layer. The compatibility for these two resins is bad, so adhesive is needed. The most common items are HDPE/Adhesive/EVOH/Adhesive/HDPE, PP/Adhesive/EVOH/Adhesive/PP, PP/Adhesive/PVDC/Adhesive/HDPE, etc

  • Lamination between plastic film and aluminum foil

In this case, the excellent barrier ability of aluminum foil is preferred, so this lamination can greatly improve the barrier ability. This lamination film is widely welcomed in cases when anti-moisture, fragrance protection are required. The most common plastic films applied in this case are BOPP, BOPET, PE, and CPP.

Adhesive is applied in this lamination case, that differs when the applications of the lamination film are different. When the lamination film has to fight against high temperature to kill the bacteria, anti-retort adhesive shall be applied, otherwise common adhesive is preferred as that will help reduce the cost. The lamination methods can be coextrusion or dry lamination.

2Additives added

Additives are added into the plastic resins, however the common plastic additives will not do much effects for the barrier performance. Among these additives, isinglass, French chalk, active argil, clay, aluminium hydroxide, quartz powder, bentonite, graphite are commonly used.

With these additives added, it lowers the dependency for the barrier performance of plastic film on temperature. With different additives added, different films are achieved, that can be widely applied

in packing fruit, etc.

3. Surface coating

Surface coating means depositing a layer of high barrier resin or inorganic materias on the surface of plastic film to achieve the aim. Now, it is widely applied.

(1)Vacuum metallization

By depositing a layer of aluminum atom on the plastic film, the advantages of aluminum foil is combined, however this will cause the plastic film not transparent. The craftsmanship is the plastic film put on a roll in vacuum metallization machine. When the aluminum wire is heated under 1200-1400 celsius, the aluminum atoms will deposit on the film surface when the plastic pass through. The thickness of aluminum layer is up to the temperature. The width of finished film achieved in this case is 0.6-2.1m.

The finished product, is usually called vacuum metallized film, abbreviated as VM film. It combines the advantages of plastic film and aluminum foil. With the flexibility of plastic film, it can also solve the problem of cracks for the aluminum foil when bended for several times. This metallized film is the lowest cost when applied in high barrier application, so it is widely applied in packaging. In many cases, metallized films has replaced aluminum foils.

What concerns producer for the metallized film is whether the plastic film is able to hold the potential heat for the high temperature aluminum atom. When drawn into a complete roll, the potential heat in the roll inside can reach up to 80-100 celsius. So, the most common substrate film for metallzied items are BOPP, CPP, BOPET, PA, PVC. As to LDPE film, it can also be aluminum metallzied when the technician modifies the machine and impose strict control. With the aluminum layer on metallized film, it shows great reflective effects, so it can also be further processed into hot stamping foils or decoration materials.

(2)SiOx or other oxides vacuum depositing

Like vacuum metallization, SiOx or some other oxides can also be deposited on film surfaces to improve barrier performance. The substrate film can be BOPA, BOPP, PET, etc. The most advantage of SiOx deposited film is transparency, no reflective effects for the microwave, so it is more suitable to be used as packaging materials for products that need to be heated directly in microwave ovens.

The depositing process is much alike vacuum metallized film in concept. The thickness of SiOx depositing layer is usual 10-100nm, with each Si atom surrounded by 1-2 atoms. When SiOx deposited on PET//PP lamination film, the barrier ability is highly improved, with OTR reducing from112cm3/(m2d)to 0.47 cm3/(m2d),WVTR reducging from 1.1 g/(m2d)to 0.47g/(m2d)。

Mixture of Al2O3,TiO2and MgO can also be deposited on film surface to achieve transparent barrying film. When Al2O3 is deposited on film, it will not help much for barrier performance against oxygen, however will do much for water vapor, about 3-10 times than original plastic film.

(3)High barrier resin coating

High barrier resins include EVOH,PVDC and PAN, of which PVDC is the most used. This coating works similar with lamination process. Spray coating can be adapted.

PVDC latex can be coated on retort film, such as BOPET, BOPA, and then heated up to evaporate the solvents, finally a thin layer of PVDC resin is combined on the plastic film. This finished film is called K-film, K-PET film means PVDC coated PET film, K-PA film means PVDC coated PA film. When K-film is further processed to laminate with retort CPP film, then the final packaging film with high barrier performance and transparency is realized, designed as BOPET-PVDC/CPP or BOPA-PVDC/CPP.

The usual thickness of PVDC layer is about 3 microns, so K-film performs worse than aluminum foil laminted film. Also, the PVDC layer is much cristile, easy to crack or fall off when damaged with outside pressure, so the performance will lower when the PVDC layer is damaged.

When K-film heated up to 100 celsius, the PVDC layer will easily fall off from the substrate film, so K-film is not suitable to pack retort food.

 

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