Co-Extrusion Technology for Plastics | Materials | Industrial Engineering

In this article we will discuss about:- 1. Co-Extrusion Technology 2. Applications of Co-Extrusion Technology 3. Raw Meterials Property 4. The Semi-Finished Products 5. Criteria.

Co-Extrusion Technology:

In recent years the co-extrusion technique has become a dominating position for the production of plastic packing materials. It offers a unique way to produce new and different products from different polymers into multilayer composite.

As no polymer enables by just itself a sophisticated packaging it is necessary to combine the inherent properties of various polymers in order to achieve the specific application requirements. Even more “tailor made” qualities can be achieved by the extrusion of polymer blends and alloys or fillers into the layers of the structure.

The semi-finished products produced by the co-extrusion technique are flexible film or rigid sheets, tubes or pipes.

Coextruded film or sheets can be made to have structures with specific mechanical properties, and structures with barrier properties.

The first step in the design of coextruded plastic products is to define the structural and compositional requirements of the multilayer film or sheet.

Applications of Co-Extrusion Technology:

For example, a multilayer structure for a new package (refer Fig. 4.17) may be required to fulfill the following application requirements e.g.:

i. Be competitive with glass, metal and other conven­tional materials.

ii. Be consumer acceptable.

iii. Have a defined shelf life.

iv. Trouble-free co-extrusion of various plastic grades with differing rheological properties, e.g. melt temperatures and viscosities.

v. Flexibility of the co-extrusion line to allow for the production of different layer thicknesses with dif­ferent resins by merely changing processing condi­tions.

vi. Extrusion of recycling.

Choice and combination of thermoplastics for the extrusion of multilayer films is not only dependent on requirements for the finished product, but in some cases, also on extrusion behaviour, or the extrusion criteria, e.g. permissible melt temperature for avoiding a so-called heat degradation and thus the possible loss of the original raw materials properties.

On the other hand, melt viscosity and the layer-thickness which at least can be achieved, have to be considered, in any case. This is particularly true for the so-called glues which must be employed for connecting e.g. polyethylene with polyamide because these heterogeneous resins do not adhere to each other.

Raw Meterials Property for Co-Extursion Technology:

The raw materials, apart from properties like melt index density etc. must possess following properties also:

(а) The highest possible melt temperature dependent on the dwell time, avoiding polymer degradation.

(b) Dependence of the melt viscosity on the melt ve­locity (shear rate) and the melt temperature.

Thermoplastics like linear lower density polyethylene (LLDPE), PVDC-grades and EVOH-polymers have excellent barrier properties, particularly with respect to the impermeability to gases, like oxygen and water vapor.

These materials enable the production of packaging’s whose contents e.g. food-stuff remain fresh for several months without affecting on taste, odor or colour.

Though co-extrusion of various polymers provides optimum film properties it is a common experience to extend the properties by blending.

This has especially become true for stretch film co-extrusion. Various LLDPE-grades are blended for the extrusion of the middle layer of the three layer film structure in order to influence toughness.

The Semi-Finished Products by Extrusion Process:

Multilayer films (flexible) are manufactured either by using the blown film extrusion process or the cast film process.

Multilayer sheets (rigids) are produced by the sheet extrusion method using 3 rolls polishing-stacks.

Criteria for Choosing Co-Extrusion Technology:

The choice of the process mainly depends on following criteria:

1. Film Extrusion:

(а) The film qualities necessary for further convert­ing into the finished products. One crucial factor is that cer­tain properties such as stable values of strength and shrink­age in the cross and length direction of a film can only be achieved by the film blowing process. High degrees of trans­parency or stiffness can be achieved by the cast film extru­sion process.

(b) The combination of raw materials, the number of layers and types of thermoplastics which are to be extruded.

(c) The film dimensions, particularly the width and total thickness of the films.

(d) The necessary production capacity.

(e) The investment and operating expenses for the plant.

2. Sheet Extrusion:

For sheet extrusion it is important to choose the vertical or horizontal roll arrangement of the polishing stack. It is advantageous to use the horizontal stack when extruding low viscous resins in one structure, in order to simplify the melt web guiding of the sheet and pulling off from the die slot.