Q: How does vacuum-chamber pressure affect my metallized film properties?
Eldridge Mount, EMMOUNT Technologies. 716/223-3996. emmount@dellnet.com -- Converting Magazine, 1/1/2001
A: One of the principal maintenance items on a vacuum metallizer is routine preventive maintenance on the high vacuum diffusion pumps. This is a big job and is usually scheduled at long fixed intervals. The question often arises as to when to shut down and perform diffusion-pump maintenance on a metallizing chamber whose pump performance has begun to degrade.
In the daily drive to continue production, maintenance may be delayed. Meanwhile the product may seem "OK," i.e. you can obtain average optical density, and because the pump-down seems a little long, we start a roll at a higher pressure than normal to regain productivity. What's the harm? After all, the OD is OK. Depending on the end use, it may not have gotten bad enough to create a finished-product failure. However, in many of today's more sophisticated products, which rely on the development of gas barriers, the film's properties might be significantly degraded.
What happens to the evaporation process as the chamber pressure rises? In an early study by Robert Szoke, to highlight some of the effects on OD uniformity and gas barrier, designed experiments were performed with OPP and PET films. The principle parameters that were changed were:
- Wire feed rate (standard optical density)
- Evaporator roll temperature
- Evaporation chamber pressure.
The responses measured were average OD, transverse direction (TD) OD uniformity, exiting film-roll temperature, metal adhesion, moisture barrier, and oxygen barrier. Focusing on the pressure changes, several important direct effects are noted on the average OD, the OD uniformity and oxygen barrier.
As the pressure was raised in the vacuum chamber, the TD uniformity of the metal deposit deteriorated as pronounced bands appeared over each boat and the average OD dropped. This is due to the width of the aluminum vapor plume from each boat narrowing from approximately 22 in. of coverage, with a peak OD of 2.5 at 1x10-3 torr, to approximately 13 in. with a peak OD of 1.8 at 1x10-1 torr. This is certainly due to the decrease in aluminum vapor mean free path length at the higher pressure.
Another study by Taylor (see reference on p. 26) clearly shows that for a metallizer of fixed-boat geometry, the peak OD and width of aluminum coverage from each boat will directly control TD uniformity of the OD. In order to compensate for the lower aluminum coverage at higher pressure, the wire feed rate and/or the boat temperature would need to be raised to drive up the evaporation rate. This increases the heat load on the film and lowers the life of the boats. Also, there would be more waste aluminum in the chamber for disposal. As the heat load increased it could cause localized film shrinkage and wrinkles and raise the average roll temperature. This could make the film prone to blocking, and the metal adhesion of PET might decrease. Chamber pressure can indeed have a dramatic impact on film properties.
There are, however, other effects that cannot be directly observed in the film as the evaporation pressure rises. Most importantly for today's products, the film oxygen barrier has been shown to be very sensitive to chamber pressure, even when the rest of the film's optical properties are all within acceptable limits. For OPP, at a fixed wire feed rate the average film optical density decreased from 3.0 to 2.4 as the evaporation pressure rose from 1x10-4 torr to 1x10-3 torr. For the same samples, oxygen permeation jumped from 2.5 cc/100in2/day, the standard value at 1x10-4 torr and 3.0 OD, to 14.5 cc/100in2/day at 1x10-3 torr and 2.4 OD-three times the standard value of 4 cc/100in2/day expected for an OD of 2.4 at standard evaporation conditions.
From this it was concluded that the OD vs. oxygen barrier curve was not a fixed relationship but was also dependent upon evaporation pressure. However, the moisture barrier was found to be insensitive to the chamber pressure.
Chamber pressure is a critical parameter in determining important barrier properties of metallized film products. As vacuum-chamber pressure rises, the many process changes needed to compensate for it may generate other product changes, lowering film quality and overall productivity.
1. Szoke, R.L., "Vacuum Metallizing Plastic Films And Papers With Aluminum Control Parameters And Limitations A Converters Perspective", Proceedings Of the First International Conference on Vacuum Web Coating, Nov. 29 to Dec. 1, 1987, ed. R. Bakish, pp. 149-158
2. Taylor, K.A., "Design of Metallization Equipment For Web Coating", Thin Solid Films, 109(1983), pp. 295-304.
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Based in Ft. Mill, S.C., the Assn. of Industrial Metallizers, Coaters & Laminators provides a broad range of services to the converting industry. If you have inquiries concerning products, equipment or material sourcing, converting capabilities or technical issues, the group's "Ask AIMCAL" service is available at its Web site: www.aimcal.org For more information, contact AIMCAL at 803/802-7820, fax: 803/802-7821.
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