Plasticstrends

Polymer products are metallized by many different means for many different reasons ranging from decoration, light reflection, light barrier, to supply protection from light and gasses, to lower surface resistance, the storage of electrical charge, for control of energy dissipation during microwave cooking and many other applications. Metallized plastics are all around us in our everyday lives. In general the polymer surface is completely covered with the metal but pattern metallized surfaces are becoming more popular for both decorative and technical reasons such as antenna for security devices and control of browning reactions during microwave cooking. In general the metal most widely used in commercial applications is aluminum but large amounts of copper, silver and stainless steel alloys are also used commercially for the production of flexible circuits, reflective mirrors and susceptors for microwave pop corn. The metal is generally deposited onto a treated plastic surface by the physical vapor deposition from an evaporation source where the metal is melted in a high vacuum chamber and allowed to condense on the polymer surface which is moving above the evaporation source. In some instances the metal is sputtered off a solid metal target by impacting it with high energy ions formed with (often an argon gas) plasma generated above the metal target. Sputtering is slower than evaporation for many materials but is important for deposition of alloys or for better quality metal layers than may be achieved by evaporation. Metallized plastics are everywhere and can be found in snack packages, window films, capacitors, mirrors, Holograms, Automobile headlights, wrapping paper, bottle labels, decorative metal replacement, CD's etc.

Perhaps the most widely used metallized products today are the metallized films used in flexible snack packaging where volume has been growing since the early 1980's to replace clear barrier packaging such as Saran coated films. One of the most important reasons for using metallized films over the existing clear barrier films is the excellent light barrier which is added to the packaging material by the metal layer. Metallized films are produced such that the typical light transmission rate is less than 1% of the ambient light (Optical density of 2.0 or higher) which has been determined to be important for snack packaging. In fact, many of the first snacks were packaged in cans to supply a light barrier and then managed with distribution to minimize stale and rancid products. In the absence of the light barrier, at the level of metallized films, many of our snack foods would be quickly ruined not by the moisture gain or loss (staling) or oxygen gain (oil oxidation giving rancidity) but because the visible and UV light would attack the oils and give an accelerated rancidity reaction. So without the light barrier the need for improved moisture and oxygen barrier are not needed.

In general the level of light barrier for snacks is about 1% transmission (an optical density [OD] of 2.0) or perhaps slightly higher¹ depending on additional moisture and oxygen barrier requirements. In comparison, a bright reflective, or decorative, layer would generally be formed at about 2.5% light transmission or an optical density of 1.6. Optical density (OD) is defined as in equation 1 and is used as an indirect measure of the aluminum layer thickness² Figure 1 plots the relationship between optical density and aluminum layer thickness for a polyester film³. Similar, but different curves will exist for other substrates and Figure 1 should not be considered a universal relationship for the substrate surface energy and evaporation process conditions will determine the relative optical density for a given evaporation rate for the aluminum. From figure 1, for the polyester film, an OD of 2.0 the aluminum layer is approximately 150 angstroms thick.

OD = Log₁₀(1/T) ……………. Equation 1

where: T is the transmittance defined as P/P₀
P is the unabsorbed energy remaining in the light beam passing through the sample, and P₀ is the energy of the incident light beam

Figure 1: Plot of Optical Density verses Aluminum layer thickness for metallized Polyester film, Data of Scharr Industries figure 2.3a of reference 2

While the permeation barrier properties of a given metallized film will generally be a function of optical density, Figure 2, the barrier properties are a stronger function of the surface on which the metal is deposited⁴. This is especially true for oxygen barrier but also holds for moisture barrier as well and is displayed in Figure 3. In the early days of metallization of polypropylene films it was widely expected that the barrier properties of metallized PET and metallized OPP would be the same due to the deposited metal layer. However, this was found not to be the case as shown in Figure 3. Ignoring the SiO2 barrier samples, there are two main branches for the barrier properties which display the relationship of oxygen barrier on metallizing surface. The upper branch consists of 70MET/60MAC, Al/APET/OPP, Al/amPA/OPP & Met PET. The first three films are coextruded biaxially oriented polypropylene (OPP) films where the surface polymer metallized are 3.5% ethylene propylene copolymer, amorphous polyester and amorphous polyamide respectively and the last is metallized polyethylene terephthalate. What these four samples show that while the moisture barrier remains about the same the oxygen barrier changes over many orders of magnitude and the more polar the surface layer the better the oxygen barrier. This allowed metallized OPP to replace metallized PET in many applications where the moisture barrier was more important than the oxygen barrier because the product failure was to go stale before rancid. If the product is stale first then there is no real value to the oxygen barrier and the less expensive OPP is a better choice than metallized PET.

Figure 2. Moisture and Oxygen barriers for 48 gauge metallized polyester film as a function of optical density. Data courtesy Camvac International as figure 3.2 of reference 2 (WTR unit is g/m² 24 hrs)

In the second branch are two films MET-UHB and MET-HB which show an order of magnitude better moisture barrier than our first four example films and a range of oxygen barrier. These two films are EVOH/OPP and HDPE/OPP coextrusions metallized on the EVOH and HPDE surfaces. Based upon the work to show the barrier results of the first films and the importance of the film surface free energy the metallized EVOH coextrusion was discovered to have excellent oxygen and moisture barrier⁵. As EVOH is essentially partially oxidized polyethylene, in that it contains hydroxyl groups (-O-H) attached to carbon atoms along the polymer backbone.

WVTR: ASTM test method F1249-90, 37.8 °C (100 °F) and 90% relative humidity (RH).
OTR: ASTM test method D 3985-81, 23 °C (73.4 °F) and 0% RH.

Figure 3: Plot of moisture barrier as a function of oxygen barrier showing the variation in oxygen barrier at constant moisture barrier from reference 4

This invention lead to the invention of flame treated HDPE surfaces⁶ which makes an EVOH like surface by the incorporation of hydroxyl groups on the HDPE film surface. It is hypothesized from these results that it is the hydroxyl groups on polyethylene (EVOH and flame treated HDPE) which give the optimum metallized barrier properties⁷. It turns out that the improved moisture barrier level of the MET-HB creates a situation where the snacks no longer become stale faster than they become rancid if the films oxygen permeation is greater than approximately 0.39 cc/m²/day/atmmosphere (6 cc/100in²/day/atm) and benefit from gas flushing to extend shelf life and improved packaging seals. After this discovery, work in metallized packaging films focused on improving MET-HB oxygen barrier as the most cost effective barrier film for snacks in the North American market. The success of the MET-HB then drove research to improve other films moisture and oxygen barrier by surface functionalization⁸ and by using improved surface treatment technologies such as plasma treatment in the metallizing chamber⁹ which has had good success for many films. Much of the new film product design and surface modification technology is finding acceptance and wide spread use in Europe and South America.

While metallized films have seen wide spread growth in packaging due to the excellent cost benefit compared to other barrier packages, growth continues as PVDC coated films are replaced by metallized films where the combination of light, moisture and oxygen barrier create a unique combinations of effective product protection. Films such as the MET-UHB are expensive to produce due to material costs but are capable of replacing foil and can compete effectively where packaging weight is taxed, where chemical and flavor barrier are needed or where distribution cycles are long giving a technical need for the more expensive product.

Because of the unique combination of barrier properties (light, moisture & oxygen) and the excellent product protection achievable with a single film component, metallized films will enjoy more growth as the more complex laminated structures are replace with simpler, less expensive combinations containing metallized films and as new high barrier packaging applications are generated.

Eldridge M. Mount III, EMMOUNT Technologies, LLC,88 Country Downs Circle,Fairport, NY 14450, USA

Eldridge's career in Plastics began in the summer of 1970, at General Electric, as a summer engineer working with Epoxy/glass filament winding. He worked two years as a synthetic Chemist for Sterling Drug and then went to Rennselear Polytecnic to perform an experimental and theoretical study of the melting and extrusion behavior of solid polymers to earn his advanced degrees. From 1978 to 2000 he has worked for ICI Americas and Mobil Chemical Films Division in the area of Extrusion, Coextrusion, orientation technology and product development for biaxially oriented films.

Eldridge has been a member of Society of Plastics Engineers (SPE) since 1975, when he joined as a student member. He presented the results of his Masters thesis at the 1976 ANTEC and his Ph.D. thesis at the 1979 ANTEC. Since then he has presented Papers at ANTEC in 1981, 1987, 1992, 2000 as well as at several RETEC's and TOPCON.

He was elected to the Extrusion Division Board of Directors in 1980. While a member of the Board he helped index the Consultants Corner book, developed a data base of Extrusion Division ANTEC papers, oversaw the Fellows process, worked to develop the TAPPI paper exchange as Films Focus Chairman, and served as 1988 ANTEC Program Chairman, Division Chairman 1990-91, and Division Councilor for two 3 year terms ending in 1998. Currently he is overseeing the Fellows Process and is the Chairman of the Packaging Focus Group. At the 2000 ANTEC he became a Fellow of the Society. From 2001 to 2004 he served as a Vice President and Executive Board Member of the Society of Plastics Engineers.

Dr. Mount is now an independent consultant in polymer extrusion, film converting and intellectual property. Currently, he holds six US and two European Patents in the field of Metallized Films.