Measuring BOPP Films
Biaxially-orientated film is used to manufacture shrink wrap and barrier type films. It is essentially stretched in two directions—machine and transverse direction. The stretching allows for the alignment of the polymer molecules to improve physical properties. When manufactured, bi-oriented films have typical shrinkage in the 20 to 60 percent range dependent on formulation, label wraps, tapes, capacitance films and many other applications.
Specific films of this nature are Biaxially Orientated Polypropylene (BOPP), Biaxially Orientated Polyethylene Terephthalate (BOPET/polyester) and Biaxially Orientated Polyamide (BOPA/nylon). BOPP film is used in snack, pasta and other packaging applications including the overwrap for multi-packs. BOPP is typically 15 to 30 microns thick, but can be as thick as 50 microns. Water-based acrylic and polyvinylidene dichloride (PVDC) coatings are often applied to increase moisture and odor barrier levels, as well as sealing properties.
Summary of Process
The film is coated with acrylic, PVDC, or other coatings for moisture, odor and other film barriers that can be measured with Near Infrared (NIR) technology on the wet end to optimize coat weight across the web profile.
Quality Parameters and Measuring Points
Cost, regulation, standardization and maintenance are moving the measurement preference from nuclear to NIR. The wax is a strong NIR absorber. Essentially, the NIR transmitter measures the water-based carrier solution based upon a constant liquid to solid ratio per product and supplies an engineering output in #/ream, mils, GSM or other.
The MCT series of NIR stand-alone transmitters are usually mounted in the center of the web or in series on the left, right and center of the web. This is of great value to the converter. The Guardian Cross Direction Web Profile System supplies a cross direction zoned web profile and machine direction trend for system operation. The color-coded CD profile visually displays in-spec product as green zones, alerts in yellow and red alarms. This is often connected to a color-coded Stack Light with annunciator to alert operator when product is in the alarm condition. Machine Direction Trends allows troublesome zones to be diagnosed and corrected.
The Profile System or fixed-point transmitter is usually mounted on the wet end at an angle of 15–20 degrees off the normal, 6–16 inches from the web. The system is calibrated in engineering units such as GSM or mils thickness based on the fixed liquid solids ratio of the coating.
Value and Quality
Implementation of NIR technology to measure coatings on BOPP improves speeds of startup, increases production, reduces scrap and allows for data archiving by roll or run. Without an on-line measurement system, the line must be started up, the sample collected for laboratory analysis, system optimized, and started up again with another sample collected to confirm the optimization. With an on-line system, this process is streamlined, resulting in much greater efficiency. The data archiving, whether from fixed or scanning transmitters via Ethernet communications, protects against customer complaints and allows data histories to be associated with modifications to unit operations or work shifts.
