Evaluation of technologies for extruding barrier films

The effect of crystallinity within coextruded films

Harinder Tamber, PhD.

Coextruded films play a vital role in packaging thanks to their inherent cost-effective versatility; they can be manipulated to provide a wide range of functionality such as strength, barrier and shrink, and provide light-weight packaging solutions. As flexible polymer packaging continues to gain market share from paper, metal and glass, the need for new plastics structures and added production continues to grow. Along with this growth come new challenges as packaging industry standards are becoming increasingly more demanding. The requirement to protect the product from microbial degradation, physical damage, and chemical changes, combined with the need to offer attractive shelf appeal presents a challenging combination. Film producers must have a solid foundation of how films are made and which processes influence the final properties of films in order to fully reap the benefits of coextrusion technology. After all, multilayer film structures allow flexibility to create and fine-tune packaging film designs; exploring the technical and economical limitations of new film structures can lead to profitable innovations.

The manner in which multilayer films are coextruded, be it by blown, cast or biaxially-oriented (commonly referred to as bi-oriented, or biax) extrusion methods, impact the mechanical properties of the films they produce along with the intermolecular nature of the polymer chains within the films. The focus of this article is crystallinity within the film, how it is formed and the influence that it has on the final film.

When polymer melt exits the extrusion die head, whether it be a flat die utilised by the cast process or an annular die used in the blown process, it is cooled to produce a structure comprised of crystalline and amorphous phases and are often referred to as ‘semi-crystalline’ materials. The ratio of each phase can be very different depending on polymer characteristics, the method of extrusion and processing parameters, such as extrusion temperatures and film forming rates. The crystalline phase can also consist of a range of crystal sizes and crystal density, all of which are used to characterise the “crystalline morphology” of the film.

For semi-crystalline polymers, there are three important temperature characteristics: Melting Temperature (Tm), Crystallisation Temperature (Tc) and Glass Transition Temperature (Tg).

They are often represented as Tg < Tc < Tm.

If the semi-crystalline polymer has a Tg that is below ambient temperature, the amorphous phase will be rubbery and the polymer is classified as semi-crystalline rubbery polymer.

Example semi-crystalline rubbery polymers – Tg of polymer below ambient temperature (25°C):

LDPE: Tg ~–50°C; Tc ~95 °C; Tm ~114°C

LLDPE: Tg ~-95°C; Tc~ 110°C; Tm ~ 124°C

If the polymer has a Tg that is above ambient temperature, the amorphous phase will be glassy and the polymer is classified as semi-crystalline glassy polymer.

Example semi-crystalline glassy polymers – Tg of polymer above ambient temperature (25°C):

Nylon 6: Tg ~55°C; Tc ~130-160 °C; Tm ~220°C.

EVOH: Tg ~50°C; Tc ~140-160 °C; Tm ~160°C.

Taking a closer look at the two phases that comprise the semi-crystalline film structure, the amorphous phase and the crystalline phase, reveals how their presence affects film properties.

Amorphous phase

The amorphous phase is comprised of randomly oriented polymer chains in the cooled film. The nature of this phase is closely linked to the polymer’s glass transition temperature (Tg). When the ambient temperature is below the polymer’s Tg, the polymer in the amorphous phase behaves like a glassy material and has a very low chain mobility. When the ambient temperature is above the polymer’s Tg, the polymer behaves like a rubbery material and polymer chains have excellent mobility.

Films that consist primarily of an amorphous phase tend to be very transparent due to the limited number of crystals present to scatter light. The polymer chains are frozen in a random configuration and thus have less intermolecular interactions, making the film weaker in terms of lower tensile strength and modulus. Also due to the random orientation, these films have excellent elongation properties and, depending on orientation, resist tearing. Since the polymer chains in amorphous structures are held together with weak forces, the resulting films have restricted dart and puncture resistance. Films with higher amorphous content also have lower heat resistance.

Crystalline phase

Polymer chains within a crystalline phase are packed in ordered configurations typically referred to as a crystal lattice. The nature of this phase is linked to the polymer’s crystallisation temperature (Tc). If the polymer melt is cooled slowly when creating a film, crystals will form at Tc. If this plastic film consisting of crystals is reheated, the crystals will melt at the crystalline melting temperature (Tm) of polymer.

It is very important to note that two different films may have the same percentage of crystallinity, but one film may have a low number of large size crystals and the second film may have a large number of small size crystals. This distinction can create significant differences between the properties of the film.

Films with high crystalline content tend to be hazy, especially in films with large crystals, which scatter light more effectively than smaller ones. In the case where the crystalline structure dominates with a large number of very small crystals the film’s gloss may also be improved as a result of the surface morphology. Due to the close vicinity of polymer chains in the crystal lattice, there are increased intermolecular forces (dipole –dipole type for PE film or hydrogen bonding in polar polymers like PA/EVOH) giving the film high tensile strength and modulus. When the crystalline phase is characterised by a large number of small crystals, additional intermolecular forces will exist to make the film even stronger. These small crystals are less fragile than large crystals and result in a film with improved dart and puncture resistance properties. However, this configuration of a high density of small crystals produces films that have low machine-direction/transverse-direction (MD/TD) tear strength because of the increased amount of interfaces that exist between the amorphous and crystalline phases, which serve as weak areas for tear propagation.