OPTICAL DENSITY MEASUREMENT IN METALLISED SUBSTRATES

Indian metallised market is growing at a rate of 15 – 20% across the major market sectors – packaging & decorative . Most metallized materials go into the packaging sector where demand is driven by the good barrier properties, aesthetics and appeal .

The amount of metal deposited on the surface of the film will govern the barrier properties and optics of the metallised substrate . Since it is not viable and also impractical to directly measure the amount of metal deposition but an indirect means to measure this coating were developed on the basis of surface resistance and a USA standard which provides the basis for the concept of diffuse optical density.

All the producers and users of metallized packaging substrates like Metallized BOPP, PET, PVC ,CPP et. al are using transmission densitometers for optical density measurement for their quality control measures . Optical density is a measure of the light blocking ability of the material and data on optical density is provided by the transmission densitometers that meet standard conditions defined by "ANSI" ( American National Standards Institute - Standard reference ANSI / NAPM IT 2.19 -1994 : Density measurements Part 2 : Geometric conditions for transmission density) or any other related national or international standard .  Most of the Indian industries are using the equipments which are easily available at very low price in the local markets but do not meet the standard technical conditions of the optical densitometers . Data provided by such equipments is not accurate and causes difference in quality standards and issues among the users and hence the loss to the organisations and to the nation . These equipments show readings on negative side by 0.3 to 0.4 units against the precision equipments available from the quality manufacturers.     All users must have a common basis for the technical specifications and should use the optical densitometers that are compliant to the national or international standards .

DEFINITION

Dictionary meaning for the optical density is “ The degree to which a refractive medium retards transmitted rays of light” .

Optical density which is measured with the transmission densitometers is a representation of the materials light blocking ability and is calculated as the Log10 ( 1 / T) .

Where : T is the transmittance defined as P / Po P is the unabsorbed energy remaining in the light beam passing through the sample Po is the energy of the incident light beam

DENSITOMETERS

Densitometer is a device that measures the density or the degree of darkening of a photographic film or plate by recording photometrically its transparency ( Fraction of incident light transmitted *)

* Light transmission : % of incident light that passes through a film sample.

OTHER METHODS OF MEASUREMENT

Other methods employ photoelectric cells to measure the intensity of the same beam with and without film or plate inserted in the path, the difference in intensity being a measure of density.

RELEVANCE TO PERFORMANCE

Transmission densitometers are traditionally used to evaluate the light transmission properties of a photographic film and same test is now used also to give an idea on the thickness of the aluminium layer of vacuum metallised films. Layer thickness affects important performance related properties like film barrier * , light transmission & appearance, hence a means to measure this coating is needed so that one can determine the amount of metal that will meet the particular requirement . Optical density provides estimation of the aluminium deposition ( see figure 1) on the substrate and hence the critical data for process control of the metallising process**. Depending upon the product design, metallised films have optical density values ranging from 2.0 to 3.0, which is equivalent to light transmission values of 0.1% to 1.0%

*Moisture and gas barrier properties.** Coating of aluminium on the moving web under vacuum conditions by evaporation method.

Figure 1. Aluminium thickness Vs Optical density

Relationship of Aluminium thickness to Optical density

Amount of metal deposition can also be calculated by measuring the surface resistance of the metallised film . Figure 2 shows the relation between the metal thickness and surface resistance for various metals.

Figure 2. Metal thickness Vs Surface resistance

Metal thickness = Resistivity x 100 /( Surface resistance)**

The values of the bulk resistivities in micro ohm cm (moc) used in the calculation of Figure 2 are given below:

Ag Al Gold Cu Ni Tin Zn

1.59 2.65 2.35 1.67 6.84 11 5.91

The surface resistance is measured in ohm / square.

Correlations of resistivity or optical density with metal thickness should be measured for each new base sheet to obtain the most accurate results.

WHAT AFFECTS OPTICAL DENSITY OF METALLISED FILMS

Metallising process parameters and base film characteristics control the aluminium layer thickness and uniformity , and therefore the optical density.

New technologies for the treatment of films in the metallizing chamber are also showing that the film optical density at a fixed aluminium deposition rate will depend upon the surface treatment level of the film surface. Thus for a particular metallized film product the process used to make it will determine the ultimate properties of the aluminium layer deposited based upon the thickness and density of the aluminium and any other interfacial effects which might be important.

Optical density is a measure of the percent light transmission of the film and serves as a key measure of the light barrier of the product. However, as discussed above the optical density obtained for a given aluminium deposition is determined by the surface free energy of the polymer film surface. From this it would be expected that the aluminium thickness vs optical density curve would be different for PET and OPP or any other polymer film used for metallisation.

TEST PRINCIPLES

Optical density data is provided by commercial densitometers that meet standard conditions defined by “ANSI” ( American National Standards Institute ) standard reference number ANSI/NAPM IT 2.19 – 1994 : Density measurements – Part 2 Geometric conditions for Transmission density.

A unidirectional , perpendicular light beam is directed on the film sample . The light that is transmitted through the film is collected, measured and logarithmically amplified.

The densitometer calculates and displays the optical density value which represtnt the following calculation and relationship to % light transmission :

Optical density ( unitless ) = log10 ( Incident light / Transmitted light) = log 10 ( 100 / light transmission %)

Optical density values are reported to 2 decimal places . Following chart compares optical density values to the light transmission values :

OPTICAL DENSITY VS BARRIER PROPERTIES

Barrier properties improve with increase in optical density but there is no direct correlation between the two because of other factors which determines barrier properties as well.

Barrier properties are affected by :

a) Base film quality – Raw material composition and surface characteristics.b) Quality of Metallisation – Aluminium layer uniformity and adhesion, pinholing, surface scratches and

vacuum level during metallisation .

Metallised film barrier properties are controlled by the surface chemistry of the film on which the metal is deposited. This mean that the polymer surface controls the deposition of the aluminium so that it forms a better layer. This is also hinted at by examining the optical changes in metal layers with treatment or surface polymer changes. If you join an untreated OPP film to the same film with a higher level of treatment and then metallize the composite roll, the optical density of the metallised film would change after the joint. Higher surface energy levels give higher optical densities ( less light transmission) indicating a difference in the metal layer. This difference also results in better barrier properties.

A study was conducted on “How does vacuum chamber pressure effect metallised film properties ? “ and to highlight some of the effects on optical density uniformity and gas barrier, designed experiments were performed with OPP and PET films .

The responses measured were : Average optical density (OD), OD uniformity across transverse direction , film roll temperature after metallisation, metal adhesion, moisture barrier and oxygen barrier .

Focussing on the pressure changes, direct affects were 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 average OD dropped. This is due to the width of the aluminium vapor plume from each boat norrowing from approximately from 22 in. of coverage , with a peak OD of 2.5 at 1 x 10-3 torr, to approximately 13 in. with a peak OD of 1.8 at 1 x 10-1 torr. This is certainly due to the decrease in aluminium vapor mean free path length at the higher pressure.

There are 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 films optical properties are all within acceptable limits. For OPP ,at 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/d at

1x10-4 torr , 3.0 OD to 14.5 cc/100in2/d at 1x10-3 torr and 2.4 OD – three times the standard value of 4.0 cc/100in2/d expected for an OD of 2.4 at standard evaporation conditions.

From this it was concluded that OD vs oxygen barrier curve was not a fixed relationship but was also dependent upon evaporation pressure . Chamber pressure is a critical parameter in determining important barrier properties of metallised 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.

Care must be exercised in handling the metallized films to ensure that the barrier properties are not compromised during the many times the film is wound and unwound as it is processed into final products. All rolls must be clean and speed is matched with the web speed to minimize any scratching of the metal surface. Also, film tension must be at a level to prevent stretching of the films such that the metal layer is not cracked.

METALLISED FILMS

The last ten years have seen an impressive growth in the use of aluminium metallised materials. The driving reasons behind it are of multiple nature and can be summarized as below :

Energy and metal saving

The metallisation with its 0.05 micron of aluminium coating improves the barrier properties of the substrates mainly the oxygen , water and light barrier comparable to the aluminium foil.

Technical

The lower weight and stiffness of metallised materials structure result in better machinability for all converting operations.

Environmental

Aluminium metallising meets the environmental and source reduction requirements; infact , since the aluminium coating is a molecular layer, metallised substrates are considered monomaterials. Therefore , they can be easily disposed off and are more suitable for recycling and incineration.

Versatility

Aluminium metallised flexible substrates are nowadays used in larger varieties of decoration and barrier packaging applications that cannot be obtained by other means.

METALLISED PRODUCTS, A BROADENING RANGE OF APPLICATIONS :

Flexible packaging for the food industry

Snacks, Chips Biscuits and Bakery products Candies and confectionary Coffee, Tea Chocolate bars Liquid packaging Soups Cereals Microweavable food

Functional – Various industrial and market sectors

Metallic yarn for the textile industry Solar control film for architectural and automotive

industry Capacitors Security and anti-counterfeiting (holograms) Road signs Shielding for electrical cable / shielded packaging

for electronic.

METALLISED FILMS AND PAPER : VERSATILE AND COST EFFECTIVE MATERIALS FOR CONVERTING

Any product requirement can be virtually met by combining the properties of different materials :

Polyester , BOPP, Polyethylene, Nylon, PVC, Cellophane, paper are the most commonly used substrates for the metallisation. Chart on the barrier properties ( as below ) of various packaging material can be referred as a guide for selection of materials for the specific end use .

Barrier improvement, aesthetics, color enhancement, weight reduction, flexibility , heatsealability , recycleability are some of the functional characteristics which can be achieved by making the structures incorporating metallised products.

Compiled by : Anil Bamby

Max Speciality Products

References :

AIMCAL technical referenceAsk AIMCAL ANSI PH 2.19

Decoration

Gift wrapping Labels Hot stamping foils Holograms Garland festoons etc.