How items such as dust, oligomers & slip agents can affect the polymer film surface

quality and be potential problems in high-tech roll-to-roll vacuum deposition

applications.

Charles A. Bishop C.A.Bishop Consulting Ltd.

Shepshed, Nr. Loughborough, Leicestershire, LE12 9RL, UK

Gregory Tullo SE Associates, Inc.,

9209 Baileywick Rd., Suite 103, Raleigh, NC 27615. USA

Key Words: Polymer substrates Film quality

Contamination Polymer surface

ABSTRACT
These newer applications take the polymer surface
There is often a contradiction in the requirements asked of
requirements to a new level. Many of the requirements
polymer films. Often they are required to be perfectly flat
want a degree of perfection of the coatings that need the
& smooth which could be achieved but to the detriment of
polymer substrate surface to be as smooth, flat and as
ease of winding. It is no use producing a very smooth & contamination free as possible. In earlier papers I have
flat surface if the film cannot be wound up without referred to the surface contamination on the polymer and
blocking &/or surface damage occurring. Methods to
the possible cleaning techniques such as ultrasonic pulsed
reduce this problem are to add fillers that provide bumps
gas cleaning and tacky roll cleaning (1). In both of these
on the surface to encourage air to be entrapped & reduce techniques the best that can be achieved is to reduce size
the contact area or alternatively to coat one surface with
of the debris that remains on the surface down to 0.3
slip agents. The slip-agents by their nature often can be microns. Debris of this size is still as large as the
transferred from one surface to another or even migrate thickness of the active coatings for OLEDs and is many
through the polymer web thickness. These can be a
times the line widths of the circuitry that is envisaged.
source of poor adhesion to subsequent coatings.
Barrier coatings, irrespective of whether they are
Oligomers are a by-product of the polymerisation process.
metallized and used for food packaging or transparent
Oligomers are a low molecular weight residue that will be barriers used in displays, all are adversely affected by
present on the surface & through the bulk. It is possible pinholes in the coating that are directly attributed to the
to clean the surface but over time these oligomers can
levels of surface debris present at the start of coating (2).
diffuse from the bulk back to the surface.
Additional problems can arise if the coatings have poor
adhesion. A lack of adhesion prevents any stresses being
This paper will elaborate on these issues & what might be
transferred from the coating to the substrate. If the stress
done to manage the films to make a useable substrate. cannot be transferred the coating will be prone to cracking
at lower stress levels and cracks will be additional route
INTRODUCTION for gases to pass through reducing the barrier
performance.
There are many reasons for interest in the surface of
polymer webs. The traditional users for metallization Another area of ongoing concern with all polymers relates
continue to have problems with quality and adhesion. to any potential adverse affect on human health
Many of these problems can be related back to the particularly where there is food contact. Thus there is
polymer composition, residual oligomers, migration of interest in the chemistry of the polymer surface and the
low molecular weight organics as well as dust and debris rate of migration of any chemicals to the surface. In fact
on the surface. Newer uses for polymer webs are in the there is much information on the migration of oligomers
area of flexible electronics such as flexible displays of available from work done testing of polymers for food
which the organic light emitting displays is a current contact approval (3,4).
favourite. The other area of developing interest is in
nanotechnology.

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49th Annual Technical Conference Proceedings (2005) ISSN 0737-5921
Thus we can see there is great interest in improved refractive index for PE & PP and used to make
surfaces for most applications. highly transparent films.

TERMINOLOGY Natural silica – a different mixture of shapes and

sizes and often contains impurities that the
Blocking. synthetic silica does not. Cheaper than synthetic
This is where two polymer webs once brought together silica.
cannot slip against each other and in some instances may
not be easily separated. This can occur during processing Talc – Magnesium hydrosilicate, a lamella type
and/or during storage of the film. This can be made worse of soft rock that has a refractive index that is also
by processing at elevated temperatures and/or increased a good match to PE & PP.
pressure. Thus tightly wound warm films are much more
likely to block than lower hardness ambient temperature Limestone – A low cost filler. Calcium carbonate
rolls. Blocking can be controlled by the use of anti- and sometimes a mixture with magnesium
blocking additives and/or slip agents. carbonate these are used in lower quality film
applications.
The tendency to block can be measured in accordance
with ASTM 1893 – 67 and ASTM 3354. The first of HDPE, paraffin and carnauba waxes are typically used in
these tests measures blocking & the second re-blocking. coatings to counteract blocking. The use of anti-blocking
fillers can reduce the Coefficient of Friction (CoF) down
Blocking is worse for smoother film surfaces, for films to 0.3–0.4 which may still be higher than required and so
with high electrostatic charge and for films with a high slip additives may be used in conjunction with the fillers.
surface energy such as those corona treated.
Unfilled webs have little or no
Anti-blocking additives. surface roughness to allow air
to be trapped between the
These are fillers that are added to the polymer that will layers and act as a lubricant.
These surfaces can stick together
increase the surface roughness. The effect of this is that known as blocking.

the two surfaces are kept apart by the fillers that protrude
Film blocking can be caused by surface being
from the surface. This in turn reduces the contact surface too smooth as well as by too hot or wound too
area and when winding in air guarantees an amount of hard or with too much surface charge
Filled films have a rougher
entrained air between the polymer surfaces. The surface. The surfaces touch
at the high spots and the air
effectiveness of the filler is dependent upon the number of trapped in the pockets acts
as a lubricant helping the
fillers and the size and hence height that the fillers surfaces slip over each other.
protrude from the surface. Anti-blocking fillers do not
migrate after the polymer has solidified and so only a Figure 1. A schematic of the cause of blocking
limited amount of the filler added will be present at the
surface. As they increase the surface roughness they
increase the haze and reduce the specular reflectance.
Typically the haze increases by 0.4-1.0% per 1000ppm of
silica filler used. Thus there is a balance between the ease Upper layer – unfilled polymer

of handling and the optical performance. It is also

common for a combination of anti-block filler and slip
agent to be used in order to minimize the impact on the
optical performance. Also it is common to find co- Lower layer – filled polymer – filler near surface causes
extruded films with a thin outer layer including the filler protuberances that aid winding in vacuum & prevent blocking
to provide the handling performance but because the bulk
of the thickness does not contain filler the optical quality Figure 2. A schematic of a filled co-extruded
is kept high. polymer film that provided improved handling
with a thin filled co-extruded layer with a thicker
Filler types can vary in shape, size and hardness. Some unfilled front surface that gives a smoother surface.
examples are as follows.

Synthetic silica – high surface area, hydroxylated

and microporous surface. A good match of

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49th Annual Technical Conference Proceedings (2005) ISSN 0737-5921
 slip agent will migrate through bulk polymer. The slip
agents can be tuned by increasing the molecule chain
length by adding or subtracting carbon atoms. Adding
more atoms increases the chain length and slows down
the speed of migration it also increases the compatibility
Figure 3a) Melinex ‘S’ b) Melinex ‘442’ c) Melinex ‘O’ with the host polymer.
Differential Interference Contrast (DIC) optical Slip agents in bulk polymer
microscopy micrographs.

Melinex ‘S’ contains more fillers than ‘442’. The ‘O’

grade is the optical grade and contains minimum fillers.
Slip agents after some have migrated
The ‘O’ grade also shows a line of damage where to the polymer surface
something has contacted the surface and caused a
plucking type defect.

Slip effect.
This is the term applied to polymer films where Figure 5. A schematic of the effect of slip agents.
there is the sliding of parallel polymer web surfaces over Initially the slip agent is distributed throughout the
each other or over of one polymer surface over some bulk of the polymer. This is shown in the upper
other surface. schematic. The lower schematic shows what happens
after some time or with an increased temperature
Coefficient of Friction (CoF) and some time. The slip agent has migrated through
The resistance to slip is measured as the the polymer and a significant proportion resides on
Coefficient of Friction. both the surfaces.
The measurement of this gives two measures, the static Amorphous

CoF, which is the force, required to initiate slip and the matrix

kinetic CoF, which is the force, required to keep the

movement going at a pre-determined speed. The
measurement of CoF of polymer films is described in Migration route
tension

ASTM D 1894 – 73 Where polymer chains run parallel crystallites can form
The three shown here are in random orientation
In pulling tension on the polymer more of the polymer chains
are brought in close proximity to each other and near to parallel.
A single polymer chain may be part of several crystallites If heated these will form crystallites that are oriented in the
with intermediate parts of the chain being in amorphous regions direction of the tension.

CoF of LDPE can be up as high as 0.7 but with 0.1% slip

additive it can be reduced to below 0.2 where 0.16 – 0.2 Figure 6. A schematic showing how polymer films
CoF is regarded as acceptable for easy processing. change with orientation and how migration can be
restricted with increasing crystallinity.
Load
Cell
Film The speed of migration is also affected by the bulk
polymer structure. Polypropylene (PP) is more crystalline
than polyethylene (PE) and so the migration is slower in
PP than in PE. The slip agents are solublized into the
polymer when it is molten but as the polymer cools it is
Figure 4 Method for measuring coefficient of friction. precipitated out of the polymer that crystallizes and acts
Two layers of film are in contact with each other with as a lubricant. Figure 6 shows that if there is a proportion
an applied load and the force used to pull one film of amorphous polymer there will be a migration route
over the surface of the other is measured. through the polymer.

Slip agents. Slip agents not only reduce the CoF but also are used to
These are the materials added to reduce the CoF. reduce adhesion between the mould and injection
They are frequently added during the mixing and moulded polymer. The slip agents have exactly the same
extrusion stage and operate by migrating through the effect on metallisation and reduce adhesion. It is worth
polymer bulk to the surface. The slip agents have a noting that there is no control over which surface the slip
designed incompatibility with the bulk polymer so that agents appear on. They will migrate to both surfaces thus
they are not bonded into the bulk but are free to migrate. if preparing a surface for vacuum coating it is no use
The size of the molecule determines the ease of which the treating one side only before putting the roll into the
vacuum system. After an atmospheric treatment the side

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49th Annual Technical Conference Proceedings (2005) ISSN 0737-5921
treated will be brought back into contact with the other The list of additives has increased with time and now can
surface as the roll is wound up and hence will become re- include the following.
contaminated. In vacuum treatment will allow the surface
to be coated before it is recontaminated however the Nucleation promoters
freshly deposited coating will become contaminated as Anti-oxidation
the roll is wound up. Thus there may not be an adhesion Radical scavengers
problem with the vacuum coating but there may be on the UV absorbers
next coating applied to the vacuum deposited coating. Cross-linking agents
Gloss improvers
Some slip agents are produced by the amidisation of long Surface active agents
chain fatty acids. Examples of the most commonly used Anti-static
of these are Steramides, Erucamides & Oleamides. Anti-mist
Slip or lubricants
Erucamides are longer chain, more heat stable and more Anti-blocking fillers
oxidation resistant than Oleamides and with a lower Colour (pigments or dyes) and/or whitening
vapour pressure create fewer volatiles during high agents (optical brighteners)
temperature processing. The Oleamides migrate through Voiding agents
to the surface more quickly and are sometimes referred to Viscosity modifiers
as ‘fast blooming’. Dispersion stabilisers
The aim is to provide sufficient slip additive so that with a Flame retardants
suitable time for migration no more than a monolayer will Foaming agents
have accumulated at the surface. More than a monolayer
does not make any further improvements on the slip In PP the crystallisation process, which controls stiffness
performance. and clarity, is helped by the addition of a nucleation
additive such as benzoate salts (sodium, aluminium,
Stearamide is prepared by amidation of stearic acid potassium) or sorbitols are used at levels of up to 0.5%.
(C18:0). Mpt. 98-104 oC
Oleamide is prepared by amidation of oleic acid PP is prone to oxidation and so phenolic or phosphite type
(C18:1). Mpt. 66-76 oC anti-oxidants are added at levels between 0.01% to 0.5%.
Erucamide is prepared by amidation of erucic acid
(C22:1). Mpt 79-85 oC Anti-static additives such as glycerol monoesters or
ethoxylated secondary amines are used at concentrations
O between 0.1% and 1.0%. These additives are hydrophilic
Stearamide C18:0
and also have limited solubility in the polymer and so
NH2 they too migrate to the surface where they are most
Oleamide C18:1
O effective.
NH2
The anti-mist agents are similarly hydrophilic and may be
O added as an additive or applied after as a coating.
Erucamide C22:1
NH2
Colorants & whitening agents are too diverse to list and
Figure 7. A schematic of the three basic slip agent may or may not be stable in the polymer
structures.

Waxes can also be used as slip agents. Waxes are similar Oligomers
to oils except they are solid at ambient temperature and If we consider the polymer film polyethylene
generally have a melting point in excess of 40oC. A very terephthalate (PET), it has many inherent properties such
interesting evaluation study with microcrystalline waxes that it does not need some of the additives that are used to
clearly demonstrated that the harder the wax, the better enhance the performance of polypropylene (PP) or
the slip properties. polyethylene (PE) films.

Additives. PET does have a problem with the production of residual

There are other chemicals added to the polymer oligomers during polymerisation and subsequent
melt in order to assist producing the precise film processing. Unpolymerised monomer can volatilise from
performance that is expected of modern polymer webs. the surface of the polymer film and may condense and fall
back to the polymer film surface as a white powder. There

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49th Annual Technical Conference Proceedings (2005) ISSN 0737-5921
will be much more unpolymerised or incompletely
polymerised monomer within the bulk of the film. As O O
with slip agents this low molecular weight material can HO C C OH
migrate through the amorphous regions of the PET and

HO
OH
accumulate on the surface. Usually this will be at low

C
C
levels and small enough to be invisible to the eye.

O
O

C
O

O
HO

OH
Figure 9. Cyclic tris(ethylene terephthalate)
‘cyclic trimer’ – oligomer, 2 dimensional
representation.

Extraction refers to the method of determining the

quantity of oligomer in the polymer. A simple process of
washing the polymer in a suitable solvent, e.g. xylene,
will remove the surface oligomers. Evaporating off the
Figure 8. A micrograph of Melinex ‘442’ that has solvent will leave the oligomer that can be weighed or
been subjected to prolonged heating at 140oC. alternatively the weight reduction of the film can be taken
Scanning electron microscope Magnification 2000x and the oligomer percentage calculated. Variations of this
Showing the growth of ‘mer’ (dimers, trimers….. are to repeat the test after a period of time to give a
hexamer) units into substantial crystals measure of the rate of migration of the oligomers from the
bulk to the surface. Other variations are to use different
Figure 8 is of a film that has been heated to encourage this time scales and temperatures. Using mass spectrometry
migration and the oligomer has grown into the and chromatography also enables additional information
characteristic angular shapes shown in the micrograph. to be gathered on the type of oligomer present in the
These short chain monomer units are not well bound into extracts.
the surface and so, if not removed, form a weak boundary
layer that once coated exhibits poor adhesion. The MHET; Monohydroxyethyl terephthalate,
oligomers can have a variety of forms and different BHET: Bishydroxyethyl terephthalate
molecular weights, some of which have a low melting B-2: Linear dimer,
point (5-9). C-2: Cyclic dimer,
C-3: Cyclic trimer,
It is worth noting that PET is often referred to as being TA: Terephthalic acid
amorphous or crystalline. This can be a misleading
statement. Amorphous PET in reality is a mixture of The volatility of the main monomers and oligomers that
approximately 60% amorphous and 40% crystalline PET. are generated from PET resin in descending order is:
Similarly crystalline PET has approximately 60% TA>BHET>MHET>C-3.
crystalline and 40% amorphous PET. Hence both have On the other hand, the amount of each in PET resin in
plenty of amorphous regions such that migration can take decreasing order is: C-3>C-2>B-2>BHET>TA>MHET
place although it is slower with the crystalline PET where
the migration path is likely to be more tortuous. The MHET amount being much smaller than BHET.
BHET converts to MHET in air or when in contact with
These can be present in the bulk around 1% - 3% absorbed oxygen. BHET & TA are the main volatile
depending on the process and conditions used. Usually substances in PET resins
there is a cost versus performance balance that has to be Melting Point oC
traded so that PET film can be obtained with extractable ---------------------------------------------------------------------------
Cyclic dimer 175 224 229
oligomer levels <0.5% but these are sold at premium. ---------------------------------------------------------------------------
Cyclic trimer (B type crystal) 319
Approximately 90% of the extracted material is cyclic 317 – 320
321
trimer, a specific PET oligomer common in all PET films. crystalline transition temperature 199
( A type - B type ) 195
---------------------------------------------------------------------------
Cyclic tetramer 326
---------------------------------------------------------------------------
Cyclic pentamer 256

Figure 10 Typical cyclic oligomers found on PET

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49th Annual Technical Conference Proceedings (2005) ISSN 0737-5921
 Structure Molecular Melting Where surface cleaning or treatments are done before the
Weight Point oC
---------------------------------------------------------------------------------------------
H[AB]1OH 210.2 178
polymer enters the vacuum it is worth noting that
O O
H[AB]2OH 402.4 200 - 205 contamination and exudates will be present on both sides
H[AB]3OH 594.6 219 - 223
A =C C of the web. A common fault is to treat only the side to be
H[AB]1-B-H 254.2 109 - 110
H[AB]2-B-H 446.4 173 - 174 coated and following treatment to immediately re-
H[AB]3-B-H 638.6 200 - 205
H[AB]4-B-H 830.8 213 - 216 contaminate the surface from the contact made during
B = O CH2 CH2 O H[AB]5-B-H 1023.0 218 - 220
rewinding with the still contaminated back surface.
HO-A-[AB]1OH 358.3 >360
HO-A-[AB]2OH 550.5 280 - 281
HO-A-[AB]3OH 742.7 268 - 270
HO-A-[AB]4OH 934.9 252 - 255 Ideally specifying a substrate material with limitations on
HO-A-[AB]5OH 1127.1 233 - 236
the quantity of extractables as a way of helping to better
define the incoming film quality.
Figure 11 Typical acyclic oligomers found on PET

It can be seen in the Figures relating to oligomers that the REFERENCES

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49th Annual Technical Conference Proceedings (2005) ISSN 0737-5921