By Barry Cowles, EVC Rigid Film
This article is intended to be informative and offer considerations
that one may want to understand in determining if material specifications
are important to your production process in the manufacturing of plastic
cards.
Industry changes are abundant over the past few years - in equipment,
card shapes, and in available materials with the introduction of ABS,
PVC/ABS, styrene, polyester and polycarbonate for card production. Each
one of these brings new processing problems and challenges.
Traditional PVC and PETG continue to be the materials of choice for
a variety of applications that cover our industry.
The intention of this article is to offer typical specifications that
one might want to consider, the test procedures, along with the test
report requirements from your suppliers.
Important Mechanical Properties
These are covered by specific ASTM documents and it is important that
both manufacturer and supplier utilize the same ASTM and test method.
Important Physical Properties
These are generally related to the limits of the material producers'
equipment and processes utilized in manufacturing. They can also be
more speculative due to individual interpretations or perceptions.
Surface
This is expressed in terms of roughness Ra or microinches (um). "Mean
Roughness" as detailed by DIN 4768 & ISO/DIS 42871 is the arithmetic
average of all departures of the roughness profile from the centerline
within the evaluation length of 1 meter.
This value is expressed as Ra: 1.4 to 2.1. To determine the (um) micro
inches of the Ra value you divide the number by .000254 (example: Ra
of 1.98 divided by .000254=7795 or 78um). Conversely, if you know the
um one would multiply by .000254 (78 x .000254=.0198) or 1.98 Ra.
Gloss
The capacity for a surface to reflect more light in some directions
than in others. Measured ratings are obtained by comparing the specular
reflectance from the specimen to that from a black glass standard. Generally,
the higher the gloss value, the smoother the surface in terms of Ra.
Color & Opacity
These must be linked since opacity is related to the content of TIO2
(titanium dioxide) which directly effects the color. The common way
to express the value is by L, a, b (L= 93.89; a= + 0,20; b= - 4.80)
readings associated with color spectrum. The standard acceptable limit
is 1 unit Delta E. The opacity of material is the transmission of light
or the density which is measured in terms of various nanometer levels.
Dimensions
Two concerns related to this aspect, size tolerance and squareness,
are equally important especially when utilizing split core. The dimensional
tolerance for size is expressed in terms of plus/minus 1mm or .5mm,
determined by the cutting equipment and the size of the lift being cut.
Squareness is expressed in terms of maximum angle of deviation plus/minus
1.2mm/m (millimeters per meter of length).
Gauge
The acceptable limits are related to the material manufacturing process
but is generally within these levels: .002-.004 @ +/-15% .0041-.0069
@ +/- 10% .007 > @ +/- 5%. It is common today to have material >.0118
to be +/- 3%.
Surface Tension-Dyne Level
The force acting on the surface of a liquid, tending to minimize the
area of surface; quantitatively, the force that appears to act across
a line of unit length on the surface. Also known as surface energy.
The key is matching the inks to the substrate for good adhesion. Expressed
in terms of dynes/cm for both front and back side, where there is generally
a slight difference.
Dimensional Stability
The linear dimensional changes of thermoplastic film at elevated temperatures
for a specific period of time. Material that is calendared has both
a MD (machine direction) and TD (transverse direction). Specifically,
PVC will shrink in the MD and expand in the TD directions. Both formulation
and thickness have a direct relationship to the results achieved in
testing.
Vicat
The softening point of rigid PVC and is determined by the base resin
and K value either in copolymer or homopolymer formulations. Generally,
low vicat (72 to 77) is an indication of copolymer and high vicat (78-83)
indicates homopolymer. It is also possible to have a blend formulation
with both copolymer and homopolymer materials, which has some distinct
advantages. Stabilizers, impact modifiers, pigments and fillers also
have an impact on the final vicat.
Test Methods for Consideration
Surface
Roughness is measured using a profilometer which utilizes a needle to
inspect a specified length, following the irregularities. The deviation
from a straight parallel line to the material is recorded. This is done
in both the cross and grain direction under the following parameters:
Covered in detail by DIN 4774 and ISO/DIS 4287/1.
Gloss
Generally measured with a glossmeter of specific design so that light
is directed at the surface under the specified angle from perpendicular,
which in most cases is 85 degrees. This measures the percentage of light
reflected in the mirror direction which is then compared with measurements
taken in the standards supplied with the glossmeter. It is important
to note that the sample being measured must lay flat. Gardner is a common
unit of measure. The readings are typically reported in terms of max
4.5. The print side and back side of sheets will have different readings.
Fully detailed in ASTM D-523 & DIN 2457.
Color
The establishment of a color standard is critical to the finished effect
of the card product, since both inks and overlay have an effect. This
is the determination of the "shade" of white, understood to
be towards the "bluish" or "red" tone as applied
to the color spectrum and related in terms of L, a, b. The indications
of +L being white and -L being black; +a being red and -a being green;
+b being yellow and -b being blue. The Delta E for white is a maximum
of 1 unit-the sum of L, a, b. One good way to establish a standard with
vendors is to take five readings from different locations of the sheet,
using a 1-2 inch circle and submit this for comparison readings. In
this manner, due to variables with the equipment, a comparative limit
can be easily set.
Opacity
The ability of a substrate to either transmit or block a certain percentage
of light, and is a function of its surface reflectivity-the refractive
index of the pigment relative to its light scattering power. This "hiding
power" can be measured in several ways. The simplest is to measure
the contrast ratio or the film's reflectance over a black substrate
divided by its reflectance over a white substrate. It is TiO2 (titanium
dioxide) that gives substrates the hiding power. The higher the percentage
of TiO2, the higher the opacity of the material. Measurements can be
made with a Transmission Densitometer and/or a Scanning Spectrophotometer.
ISO 10373 provides complete testing methods.
Dimensions
There are two considerations for width and length as follows: (1) is
the +/- tolerance associated with the cutting equipment and is generally
+/- 0.5mm. (2) is the angle of deviation per meter of length and generally
is +/- 1mm/m as the maximum. Steel rule is the common method of measurement.
There are also size measuring tables that have steel rules attached
at perfect 90 degrees for verification.
Gauge
This is established by the production controls within the manufacturing
process of calendared PVC. It is common to utilize a traversing Beta
gauge as well as fixed Beta gauges along the edges in this process.
The industry tolerances are based on film thickness and the end application
of the finished card. The micrometer is the instrument of choice when
measuring.
Surface Tension-Dyne Level
Commonly referred to as the dyne level or wetting tension that indicates
how well the ink will adhere to the substrate. A given level of dyne
can be accomplished by corona treatment that alters film surface with
electrical discharge to enhance adhesion of printing inks. Most producers
of substrates can verify the range of the dyne level to be expected.
The most accurate method of testing is with the Contact Angle Meter
which utilizes a more analytical and repeatable water contact angle
method of measuring wetting tension. The procedure is fully covered
is ASTM 2578.
Simple tests can be accomplished using cotton swabs and dyne solutions
of various concentrations where the appropriate solution will give an
indication by remaining wetted out on the surface. Another common practice
is the dyne-pen method, these pens are usually employed by an inspector;
they are similar to felt tip pens. There is also a draw-down method
where a substrate sample 8 ? x 11 is clamped to a clipboard. At the
top, a drop of three different solutions that bracket the desired level
are placed horizontally. A wire wrapped metering rod starting above
the droplets, is drawn down. When the film of liquid breaks within two
seconds, the tester knows that the dyne level is below the indicated
solution.
Dimensional Stability
The change of dimensions can have a serious consequence and create process
concerns. The conditioning of the test samples, which are 10 x 10, should
be at 74 degrees F and relative humidity of 50% for not less than 40
hours. What one needs to understand, realizing the excellent memory
characteristics of PVC, is what is the % of change in terms of shrinkage
or expansion. It is important to note that the MD (machine direction
or calendaring direction) will always have a - (negative) value, while
the TD (transverse calendaring direction) will always have a + (positive)
value. The procedures are clearly outlined in ASTM D-1204. It should
be noted that two sets of oven conditions are possible: one at 105 C
for 5 minutes and one at 140 C for 20 minutes, so be sure both user
and vendor are testing under the same conditions.
Vicat
This is one area that is often overlooked and commonly not understood.
It is important to know this parameter due to the conditions under which
lamination will occur. When lamination is being accomplished, the goal
is to return the substrate to a soft state and obtain a "homogeneous"
bonding of four sheets. In addition, some applications of cards require
a high vicat substrate, due to the conditions under which the card may
be used. When establishing your test method, it is important to understand,
completely, ASTM D-1525 which will describe the two best procedures.
The goal again remains to have user and vendor test in the same manner
and conditions, otherwise there will never be a correlation of results.
Summary
In the event you do not have any material specifications established,
perhaps this information will be of some assistance in the future should
you decide to accomplish this task. For sure, it is one of the ways
that lot-to-lot consistency can be established from your suppliers.
If different types of materials are used, such as core, overlay uncoated
and coated, then it is generally required to have a material specification
for each product.