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Recently, there are more and more discussions about recyclable flexible packaging. Mono-material PE is an indispensable part of the recyclable flexible packaging options; and MDO-PE film is an important technical direction of Mono-material PE packaging.
Today we will talk about MDO-PE related technologies, applications and formula.
It’s an in-depth article that takes 15 minutes to read.
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MDO is the abbreviation of Machine Direction Orientation. This is a film making technology.
In MDO process, the film is extruded then stretched several times in machine direction, which gives it a shrink index. This shrink value determines the maximum final retraction and sets during the plastic material cooling process.
MDO (Machine Direction Orientation) is a post extrusion process used to enhance film properties such as resistance, optics or even rigidity.
MDO-PE means the PE is made by MDO technology.
MDO-PE films are the type films that are extruded by heating the PE resin at a temperature slightly below the melting point and then the PE is stretched several times in a machine direction. After that the PE film’s new properties are locked during the annealing stage; and finally cool down. Now MDO-PE film is finished production & ready for use.
MDO technology used to manufacture MDO-PE films requires less material which saves resources and it improve the film properties.
The following model diagram illustrates the change of crystal structure with film stretching before the critical stretching ratio. The unstretched film has lamella of different sizes and is freely oriented (Picture A); The stretched film formed smaller crystals and the molecular segments were oriented in the MD direction, which made the crystallinity in the MD direction higher (Picture B).
After the critical stretching ratio, the molecular segments are further stretched in the MD direction, and the lamellar structure is elongated. Because of the change of molecular morphology during the stretching process, the macroscopic properties of the film have been improved.
The WVTR value is related to the path length of water vapor molecules through the film; Generally speaking, water vapor molecules pass through the amorphous region and bypass the crystal region. Before the critical stretching ratio, the crystal arrangement is more regular than before stretching, which makes the diffusion path longer, thus improving the WVTR value. After the critical stretching ratio, the molecules are in a string structure, and the crystal arrangement is parallel to the MD direction, which shortens the passage path of water vapor molecules, resulting in the decrease of WVTR value.
It should be noted that the tensile orientation cannot completely change the absolute structure of the film; Even at the maximum stretching ratio, the influence on the structure of the film is not more than 5%, so the WVTR value will not change in magnitude.
The optical properties of the film include haze, gloss and transparency. The optical properties are closely related to the surface flatness and the internal crystal size of the film. During MDO stretching, the film softens, stretching and then cooling, which will improve the surface flatness; Secondly, smaller crystals will be formed in the process of tensile orientation; These two factors make the optical properties of the films increase with the increase of the film stretching ratio until reaching the critical value.
The stiffness and Tensile strength will become stronger too.
Because the stretched PE film has the advantages of high stiffness, high heat resistance, high tensile strength and good optical performance, MDO stretched PE film is applied in mono material packaging to replace the former pet or PA surface layer.
It should be noted that the MDO-PE film still needs to be laminated with the heat sealing PE film. Because the heat resistance of MDO-PE film is greatly reduced compared with that of PET or PA, the formulation of the heat sealing layer PE film also needs to be redesigned to reduce the heat sealing temperature of the overall film.
The selection of MDO-PE raw materials is mainly considered from three factors such as stretchability, heat resistance and crystal point. Unlike BOPE, MDO-PE only needs unidirectional stretching, and the stretching ratio is about 5-7. Most linear products (LLDPE, HDPE) on the market can meet this requirement, and the raw material manufacturers will not redesign the molecular structure and develop new brands for this process; The heat resistance is closely related to the initial density of raw materials. HDPE and MDPE are the main brands recommended by raw material manufacturers; The crystal point control is related to the raw material production process control, and the majority of users can distinguish by themselves during use.
The mainstream of MDO film formulation design is three-layer. With the popularity of five layer online MDO machines, there will be more and more five layer formulations. The design idea should be combined with the machining situation and the final use scenario of the film
For light-weight flexible packaging, HDPE can be used to maximize the stiffness and heat resistance of the film under the conditions allowed by the machine In case of uneven stretching, HDPE + MDPE can be used to solve the problem.
If the formulation is to balance stiffness and toughness, the combination of HDPE + MC6 / C8 / MC8 LLDPE is required. It is possible to greatly improve the toughness of the film, especially the falling impact strength and puncture resistance.
The selection of LLDPE is very particular. If C8LLDPE is selected, the stretched film will eventually show the effect of longitudinal easy tearing and transverse difficult tearing; If C8LLDPE is selected, the tear strength of the film is generally improved, but the longitudinal tear strength is much greater than the transverse tear strength.
You can decide the film formula combination according to the final use scenario of the film.
The design idea of the heat sealing layer PE film is mainly to reduce the heat starting temperature and enlarge the temperature difference with MDO PE film as much as possible to ensure a wide heat sealing window and good heat sealing effect during heat sealing.
The recommended raw materials are VLDPE (density 0.905-0.915g/cm3) or pop (density 0.880-0.905g/cm3). It should be noted that the density of VLDPE and pop is low. During the actual film blowing process, it may be difficult to open the film due to too large viscosity. At this time, the inner film needs to be blown out or an additional opening smoothing agent needs to be added. The smoothing agent will migrate, and if it migrates to the composite surface, it will affect the composite strength of the film as a whole, which is also a factor that reflects the low composite strength of MDO films in the market.
It is necessary to find a balance point between the selection of raw materials and the amount of smoothing agent to optimize the formula.
Longdapac is the leading flexible packaging manufacturer from China. We focus on sustainability packaging as you can see below:
Longdapac is the first high barrier MDO-PE film supplier in China.
Longdapac is the first Chinese factory who can make high barrier mono-material PE pouch.
Longdapac recyclable high barrier mono-material PE pouches can meet the recyclability of
Longdapac is a BRC and ISO certified factory with annual capacity of more than 25,000 tons.
Basically Longdapac can satisfy all your recyclable packaging needs, both standard and high barrie
With our professional packaging team on artwork design, bag production, and recyclable material development, we are the first high barrier mono-material pouch manufacturer in China who can make
Recyclable Coffee Bag
Recyclable Tea Bag
Recyclable Dog Food Bag
Recyclable Cat Food Bag
Recyaclable Laminating Pouches