Fit for Use

Hard and strong or soft and weak, picking the right substrate depends on your application.

In the manufacture of labels, packaging materials, and supports, thin, flexible substrates are modified and combined to produce a product with enhanced properties that could not be obtained from any single substrate.

In general the substrate must supply several key features required by the final product: These principle attributes are strength, thermal stability, permeability, specific optical properties, large surface area per pound, and special surface properties. The factors that control the desirability of the substrate for a particular end use are the base material, the method of substrate manufacture, and additional surface modifications made at the time of manufacture.

There are two broad categories for substrates: those that are valued for their inherent surface quality (smooth, high gloss, and defect-free) and those that supply a surface area with special attributes (high yield, sealability, printability, barrier properties, and the ability to be laminated).

With the exception of metal foils, the majority of substrates used today are made from natural polymers (cellulose, rubber) or synthesized polymers [polyester, polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), etc.], and the method of manufacture or conversion of the polymer into a thin substrate will control the specific property values developed.

As an example, in the case of cellulose, the substrate may be opaque and highly permeable by matting fibers together, as in paper, or clear and semi-permeable by dissolving and precipitating the polymer (solution casting) as a sheet of cellophane. Both substrates retain the high thermal stability of the base polymer and may be used in many of the same end uses, packaging and metallization, but are very different in the properties that make them desirable for particular applications.

PE also can be made to have the same range of properties as cellulose as demonstrated in the melt spinning of fiber sheets to make Tyvek or by extrusion to make blown, cast, and oriented films. This range of substrate forms and properties generally is found for other natural and synthetic polymers as well.

Polymer Behavior
In general, a polymer will display a range of tensile properties that can be characterized by five general behaviors as shown in Figure 1.

Every polymer can show all of the behaviors by changing the temperature at which the measurements are made or the time frame (strain rate) at which the polymer is stressed or deformed. Time and temperature can be interchanged (superposed) to move between the tensile curves. Thus a polymer will seem rigid and brittle if it is impacted (high strain rate or low temperature) or can seem soft and compliant (high temperature or low strain rate).

What controls the behavior is the time available for the polymer chains to slip by each other when placed under a load. Clamp a film sample and hang a weight from it and it may not stretch right now, but wait long enough and the film will stretch and elongate (low strain rate).

If you need to have it stretch faster, then place it in a hot environment, and the additional thermal energy will permit the chains to move past each other faster. Or hit the polymer with the same force as the weight and it may shatter like glass, because the polymer chains do not have enough time to slip past each other (if the force exceeds the strength of the polymer chain bonds, then the film will break).

If you want to avoid stretching, you have to lower the forces to prevent the yielding of the polymer or lower the temperature to resist the chain slippage.

Which curve the film is on in your process will determine the success of your converting process, and your principle methods of control are film temperature and tension level. The molecular differences of various polymers, due to the molecular structure and intermolecular forces between polymer chains (melting point and glass transition temperature differences), are the source of the tensile differences between polymers that appear at a given temperature and load. How we categorize the polymer films from hard and strong to soft and weak will be determined by which of the five curves they are on at room temperature.

The substrate manufacturing process also can be used to vary which curve the polymer is on based upon the level of molecular orientation in the film (Figure 2). Thus cast films with low orientation will appear to be softer and weaker than the same polymer that has been stretched in manufacture — for example, the difference in elongation and yield strength in cast (soft and weak) and oriented (hard and strong) PP — while the difference between oriented PET and PP will cause us to say oriented PP is weak in comparison to oriented PET.

Manufacturing methods typically used for polymer film substrates are melt blowing, melt casting, and uniaxially or biaxially stretching (orienting) the polymers. Blown and cast films have low levels of molecular orientation in the polymer and, therefore, are relatively weak in tension and are very extensible but with excellent tear and impact resistance.

When stretching produces films, the molecular orientation is increased significantly, and the physical properties are enhanced dramatically. The tensile strength and stiffness are increased dramatically while the elongation, tear resistance, and gas and moisture permeability are reduced.

In many cases the optical properties also will be changed dramatically, but this may be controlled by many different factors to give a range of films with improved clarity to opaque cavitated films.

Substrate thermal stability also is very dependent on the mechanical and thermal processing history the polymer undergoes in the manufacturing process, and films of high and low shrinkage levels may be made by all methods.

Polymer selection also is important here, as the melting behavior of the polymers will determine the upper temperature range where a film is usable. Thus polyethylene terephthalate (PET) with a melting point of 266 deg C is usable at temperatures that are higher than for PP, which melts at 166 deg C.

In some cases, the films are made by solvent or solution casting, but these processes are becoming less common and generally are used for polymers that are unmeltable in conventional extrusion equipment or in cases where extremely low levels of residual orientation are desired or where exceptional film clarity or surface smoothness is required.

Consequently, the process that is employed has a tremendous impact on the strength, ductility, and permeability of the polymer film.

For a complete review of manufacturing methods for polymer-based substrates see the Encyclopedia of Polymer Science and Engineering.1

Key Properties
So, if most — if not all — polymers can be manufactured into a range of substrate forms, how do you determine which substrate material is suitable — fit for use — in your application?

The key physical, thermal, and surface properties will determine the suitability of the film for any process. First the film must be strong enough and stiff enough to be transported through the process without permanently stretching or breaking at the temperature and tension level of the process.

Thus the film's tensile strength, yield point, and modulus are very important and must be adequate to maintain the elongation of the film below the point of permanent deformation (yield point) at the tension levels needed (Figure 3) to pull the film through the converting equipment. Or if a specific material is required for a product, the tension levels in the equipment must be lowered to accommodate the substrate's tensile properties.

In general, to prevent permanent deformation, you do not want to exceed about 10%-25% of the yield point load during film transport and you should have a lower tensile load when winding to prevent the formation of tight winds and the formation of stretched (baggy) film.

If the film motion in the equipment is cyclical with start and stop motion, then the substrate must have impact properties to prevent breakage during sudden accelerations and be stiff enough to resist buckling when pushed.

In some cases these substrate/machine interactions also can be affected by film surface properties such as coefficient of friction (COF), static charges, or surface roughness. Lowering the COF or increasing the surface roughness of the film or equipment surfaces are methods used to lower the forces required to move the substrate through the machine during the conversion process.

Static, if generated, must be eliminated. If the substrate is to be exposed to heat during the converting process, such as for drying print or coatings, for curing adhesives, or for metallization, then the thermal stability and the tensile properties at the elevated temperatures the substrate will experience must be considered.

Both shrinkage and tensile strength must be measured at the conversion temperature to ensure the substrate can be processed at the required tension levels of the equipment without an unacceptable loss in width, permanent film stretching, or film relaxation giving tight winding.

While the strength and temperature stability of a substrate are important, in the final analysis, the surface properties of the substrate determine the ultimate suitability for conversion of any substrate. In metallization the substrate surface controls the barrier properties developed.1,2 If the surface cannot by printed, metallized, or laminated, then all the other attributes of the substrate are of little value, as there are few applications for plain film. The key surface properties to consider are the wettability of the surface (free energy or treatment level), the chemistry of and the interfacial strength of the surface to control adhesion, the gloss of the surface (a measure of film smoothness), and the surface roughness or COF to control machining forces. These properties often are controlled by selective layering of formulated polymers on the surface of the substrate during the manufacturing process, as few substrates today are single-layer products. The multilayer films are produced by coextrusion or perhaps in-line solution coating (subbing) or extrusion coating.

To be successful when choosing a substrate for coating, laminating, or metallizing, take into account the polymer, the manufacturing methods employed, the substrate surface modifications, and the converting and end-use conditions.


References
1Park, H.C. and E.M Mount III. 1987. “Films, Manufacture,” Encyclopedia of Polymer Science and Engineering, Vol. 7, 2nd ed., ed. J.I. Kroschwitz. John Wiley & Sons Inc., 88-106.

2Mount III, Eldridge, M. and John R. Wagner. 2001. “Aroma, Oxygen and Moisture Barrier Behavior of Coated and Vacuum Coated OPP Films for Packaging,” Journal of Plastic Film & Sheeting, 17 (3): 221-239.

3Chap. 6 in AIMCAL Metallizing Technical Reference, ed. E.M. Mount III. 2001. Charlotte, NC: Assn. of Coaters, Laminators and Metallizers.


Eldridge M. Mount III is president of EMMOUNT Technologies, a consulting and training company located in Fairport, NY. Mount received an M.E. and Ph.D. in chemical engineering from Rensselaer Polytechnic Inst. and has 24 years of experience in extrusion processes and the manufacture, design, and metallization of oriented propylene and polyester films. He is the metallizing consultant for AIMCAL (Association of Industrial Metallizers, Coaters and Laminators) and can be reached at 585/223-3996 or via email at emmount@earthlink.net. Visit Emmount Technologies at emmount-technologies.com


This article, along with future articles by other authors, is provided as a cooperative effort between PFFC and AIMCAL. Authors contribute to AIMCAL's technical and education offerings, which include the association's Fall Technical Conference, Summer School, and Ask AIMCAL.



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