Qin Y. Micromanufacturing Engineering and Technology

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Polymer Thin Films –

Processes, Parameters

and Property Control

Bertrand Fillon

INTRODUCTION

Over the last 10 years, the use of polymer thin films

has increased significantly in various domains of

application: in traditional industries (e.g. papers

and metal strips), in the automotive industry, in

packaging, etc. Moreover, emerging markets such

as consumer electronics and photovoltaic energy

use polymer thin films also. This growth has been

driven mainly by important advances in the prop-

erties of polymer thin films (better barrier proper-

ties (water, oxygen, etc.), wear resistance, optical

properties, etc.) and a much more extensive range

of applications (solvent-based coatings, water-

based coatings, plasma deposition, emulsions,

etc.). It would indeed be very ambitious to attempt

to cover all the properties of polymer thin films in

the space of a few pages. Nevertheless, there are

two major families of parameters which influence

the properties of these thin films:

Type of the polym er deposited;

Implementation conditions.

Clearly, the properties of these thin films

depend mainly on the type of polymer and its

thickness. Certain polymers are better for improv-

ing barrier properties, while others offer very

good sealing functions. As for the implementation

conditions, three processes are used to produce

most of these thin films: extrusion coating, depo-

sition in solution with radiation cross-linking

(thermal, UV, electron beam), and physical depo-

sition involving plasma. The ranges of polymer

thickness generally deposited by these three pro-

cess types are respectively: >5 mm, 1 < t < 5 mm

and <1 mm. Note that extrusion coating is at the

upper limit of the range defined for the thin films.

Generally speaking, a key technological req-

uirement is to provide all the desired functions

of the coating (optical, barrier, adhesive, electri-

cal, catalytic, etc.) while controlling the interfaces

and micro-structures (film, charge dispersion,

orientation, crystallinity, porosity, etc.).

This chapter is broken down into three major

sections. Each section will focus on one of the

three implementation processes mentioned above.

The objective is to briefly describe these processes,

indicating the most wid ely used polymers, and to

show how thin film properties can be improved by

the type of polymer deposited, but also through

control of the interactions between process para-

meters and material parameters. Each of these

sections will be illustrat ed by examples of appli-

cations requiring specific properties, e.g. water or

oxygen barrier properties, optical properties, seal-

ing properties or surface properties.

CHAPTER

241

THIN FILMS OBTAINED

BY EXTRUSION COATING

The process of extrusion coating (Fig. 15-1)enables

covering a flexible substrate with a very thin poly-

mer film. In industry, polyethylene is generally used

to cover commercial substrates such as paper, card-

board, polyester and other polymeric films, metal

foils including aluminum foil, textiles, etc. The

polymer deposited is often a semi-crystalline poly-

mer such as polyolefin (polyethylene, copolymers

such as EMA, EBA, etc.), polyesters, PVC, etc.

The objective is to coat a substrate with a poly-

mer, combining the best properties of the two

materials. The advantages of this process are a

heat-sealable surface, improved resistance to tears

and wrinkling, an excellent barrier against mois-

ture, oxygen and odors, enhanced optical proper-

ties (dullness, gloss) and an imprintable surface.

Principle of Extrusion Coating

Polymer granules are extruded using a slot die to

obtain a thin film of molten polymer. The role of

this coating die is to present the molten polymer in

a form similar to its final form (coating film), to

keep it at a co nstant temperature, to obtain a

stable flow of polymer and to coat the desired

width of the substrate with it. The molten poly-

mer travels over the distance between the slot die

and the substrate: this distance is often called the

air gap. The air gap value may vary depending on

the desired extrusion conditions.

The film of molten polymer is pressed through

two rollers positioned directly below the die. The

first of these, coated with neoprene, is the pressure

roll. It draws the substrate to be coated and

presses it against the molten polymer film. This

is the precise moment at which the extruded poly-

mer adheres to the substrate. The pressure roll is

cooled by internal water circulation with adjust-

able temperature. This prevents its temperature

from rising too rapidly.

The second roller is chrome plated and called

the chill roll. It is also cooled by internal water

circulation with adjustable temperature. Its role is

threefold:

It must cool and solidify the polymer in a frac-

tion of one rotation.

Its rotational speed controls the quantity of

polymer deposited on the substrate (coating

thickness or grammage).

Its surface (polished, matte, etc.) determines the

surface appearance of the polymer coating.

The resulting complex is then directed towards

the system’s draw roll and winder.

FIGURE 15-1 The process of extrusion coating.

242 CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control

Process Parameters and Influences

Polymer suppliers and converters must deal with a

growing market demand for low cost ‘deposited

film/substrate’ complexes which still offer high

performance properties. Manufacturers, polymer

and substrate suppliers, and converters must

work together to reach these objectives. Various

studies have enabled improving polymer perfor-

mance while at the same time optimizing process

parameters. For example, in the packaging indus-

try, several complexes such as aluminum/polyeth-

ylene, paper/PE, etc., are used with low density

polyethylene (LDPE) without a binder layer.

These various structures are produced using

extrusion coating. Good adhesion between the

LDPE and the substrate is clearly vital. However,

the adhesive strength is generally very weak

between the substrate and the LDPE, due to the

lack of polar or reactive functions in LDPEs. Like-

wise, following the extrusion coating process, the

complexes alu/LDPE, paper/LDPE, etc., maintain

the sealing properties of polyethylene.

Example: Adhesive Properties of an Aluminum/

LDPE Structure. For adhe sion to occur between

LDPE and the aluminum foil, the LDPE has to

oxid ize during the extrusion process . This func-

tionaliz ation of the LDPE primarily occurs dur-

ing the molten polymer’s dwell time between the

die’s opening and the point of contact with the

substrate, i.e. in the air gap. Adhesion between

the LDPE and the substrate is affected by several

factors, whi ch can be categorized into three

groups: extrusion parameters (melt temperature,

air gap, etc.), polymer parameters (melt index,

density, etc.) and substrate parameters (type, sur-

face roughness, etc.). Over the last 20 years, the

practical problem of ensuring good adhesion

between the LDPE and various substrates during

extrusion coating has been the subject of much

interest, both from a technical and a scientific

poin t of view [1–7]. Whereas some parameters

(corona treatment, ozone, post-treatment, etc .)

generally increase adhesion, it is difficult to draw

a single conclusion for process parameters overall,

but most authors agree that interactions do exist

between these parameters. The main process

parameters with the greatest effect on adhesion

between the polymer and the substrate are: tem-

perature of the molten polymer, thickness of the

film deposited, pressure of the rollers, extrusion

speed, and air gap.

It is very difficult to separate the last two para-

meters: increasing the substrate speed effectively

reduces the dwel l time (d

) in the air gap and LDPE

oxidation becomes difficult, resulting in poor

adhesion. If the air gap is increased, the dwell time

(or freeze time t

) and LDPE oxidation will also

increase, but the LDPE will cool and its viscosity

will increase, reducing its wettability relative to

the substrate’s roughness. This in turn reduces

mechanical adhesion. These two phenomena con-

flict with one another; hence the need for a com-

promise. According to most authors, line speed

and air gap can be expressed as a single parameter,

that of freeze time t

 V

 Ln

where V

= line speed, V

= material through P

and H = air gap.

Figure 15-2 illustrates the interactions between

the process parameters and the polymer, which

lead to changes in adhesive strength. At low tem-

peratures (T =285



C), the adhesive strength (F

)

of the LDPE increases with dwell time, attaining a

maximum value then falling. This increased adhe-

sion is correlated with an increase in the LDPE’s

carboxylic functions and, as a result, in its surface

energy (Y

). The drop in adhesion at high t

values

is related to cooling of the LDPE, which becomes

more viscous on contact with the substrate; as

a result, the LDPE does not mold as well to

the substrate’s surface. At high temperatures

(T =315



C), adhesion of the LDPE is very strong

despite short dwell times. This adhesion

decreases with high t

values. Oxidation of the

LDPE becomes excessive and initiates its degra-

dation, resulting in a high number of ruptured

chains and a drop in its surface energy ( Y

). This

leads to a layer with poor cohesion at the s urface

of the LDPE film, in turn provoking the drop

in adhesion.

CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control 243

Achieved through co ntrolling process para-

meters, LDPE oxidation is necessary to attain

good LDPE/aluminum adhesive strength.

Example: Sealing Properties of an Aluminum/

LDPE Structure. Besides the adhesion between

the aluminum foil and the polyethylene, the

LDPE film should have good sealing properties.

Figure 15-3 illustrates the changes in the seal

strength of one alu/LDPE complex compared to

another alu/LDPE complex, both obtained by

extrusion coating. The sealing capacity of a poly-

mer is most often linked to its melt index (MI). A

high MI results in a high degree of fluidity and

improves interfacial diffusion during sealing,

allowing the polymer to attain a high seal strength

at low temperatures. This is illustrated by Fig.15-3;

the LDPE2 sealing curve (MI = 7) starts at a lower

temperature than that of LDPE1 (MI = 3).

Extruded at low pressure and a low rate (45 kg/h),

LDPE1 attained a seal strength of 300 g/15 mm at

a temperature of 100



C. Under the same condi-

tions, LDPE2 attained a seal strength of 700 g/

15 mm.

Process parameters clearly have an influence

on sealing properties as well as on adhesion.

Figure 15-3 illustrates the effect of extrusion rate

on seal strength. Within the operating range for

the selected process parameters, when LDPE2

is deposited on aluminum foil at a lower rate

(45 kg/h), seal strength goes from 300 g/15 mm

FIGURE 15-3 Changes in seal strength of two LDPEs (LDPE1: MI = 3; LDPE2: MI = 7) according to sealing temperature. The

alu/LDPE complexes were obtained by extrusion coating at two different rates (45 and 140 kg/h) and at two different

pressures (90 and 150 bar). The low pressure curves are represented by dotted lines.

FIGURE 15-2 Freeze time (t

) dependent changes in the oxidation rate, polar function (Y

) and peel strength (F

) of a low

density polyethylene extruded on aluminum foil at two temperatures (285



C and 315



C). The thickness of the LDPE film and

the pressure between the rollers were constant (e =10mm; P = 6 bars).

244 CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control

to 700 g/15 mm. Likewise the air gap has an effect

on seal strength, which is weaker when the air gap

is reduced (Fig. 15-4). Sometimes these process/

material interactions do not exist; this is the case

for pressure during the extrusion process [3].

Summary on Extrusion Coating

Variation in process parameters can have a signif-

icant impact on the final properties of film com-

plexes such as alu/PE, paper/PE, alu/EVA, etc.

Final products with excellent performance can

be obtained at low cost through polymer selection

and adjustment of the process parameters. In the

section above, the examples presented for adhe-

sive and sealing properties clearly illustrate the

interactions between material and process para-

meters. Whatever the target property – barrier,

optical, etc. – these interactions between pro-

cess/material parameters will always play an

important role in determining the final properties

of the product [8].

ORGANIC COATINGS DEPOSITED

IN SOLUTION

Organic coatings deposited in solution (i.e. using

wet methods) are found in very diverse areas.

They are commonly produced by coating sub-

strates with thin layers of a liquid or suspension,

which are then transformed into solids by gela-

tion, drying or cross-linking. Such structures are

vital elements for an extremely broad range of

industrial products. They are developed for pa per,

steel, aluminum, polymer films, printed materials,

selective membranes, photographic film, photo-

sensitive coatings, adhesives, microelectronics,

integrated circuits, etc. However, deposition tech-

nologies may differ greatly depending on the final

application. For example, the manufacture of

microelectronic components uses spin coating,

whereas varnish is generally deposited on steel foil

using roll or spray coating.

This chapter focuses exclusively on organic

coatings deposited with the roll coating tech-

nique; this covers a large majority of the polymer

thin films produced industrially. Most of the

deposition processes based on this technique

involve a series of rollers producing polymer thin

films which can have thicknesse s below one

micron. These varnishes offer an effective protec-

tive barrier for metal substrates (aluminum, steel),

paper substrates or polymer films. These coatings

may also be used for decorative or aesthetic pur-

poses (glossy or matte effects, colors, etc.), or act

as sealants, etc. They must also offer specific char-

acteristics of durability and resistance to temper-

ature, solvents or UV radiation depending on the

final application of the coated product:

Exterior applications (pre-finished steel and

aluminum for construction);

Decorative interior elements, household appli-

ances, etc.;

Packaging for food products, cosmetics, etc.

FIGURE 15-4 Changes in seal strength according to temperature for alu/LDPE complexes extruded with various air gaps.

CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control 245

The composition of the varnishes or paints, the

choice of substrate, and the conditions under

which organic coatings are applied, dried and

cured are inextricably lin ked in determining the

properties of the end product [8].

Main Polymers used in Roll Coating

As in extrusion coating, the polymer used to coat

the substrate will strongly influence the quality of

deposition and the final properties. The most

commonly used coatings are indicated in

Table 15-1. Note that the molecular weights of

polymers deposited in solution are lower than

those of polymers deposited by extrusion.

In general, six categories of basic polymers

(Fig. 15-5) are used in roll coating [9]. The differ-

ent categories are based on:

Solid state (amorphous or crystalline);

Solubility in common solvents;

Swellability of crystalline or cross-linked

polymers.

Amorphous and soluble polymers (A), for

example polyesters, epoxies or acrylics, produce

liquid varnishes and paints which are 30% to

70% non-volatile. Amorphous and insoluble

polymers (B), which are primarily polyesters,

can be used in powder coating technolo gy. Semi-

crystalline polymers (C), which form the basis of

the extrusion coating films listed in the previous

chapter, can also be used in roll coating after a

process of precipitation or grinding to produce

fine powders.

Dispersing these fine powders in other basic

polymer solutions produces organosols (E), which

are 40% to 60% non-volatile. Examples of orga-

nosols include PVDF films and polyester polyure-

thane systems modified by polyamide fine

powders.

Organosols can also be formulated with cross-

linked polymers (D) in the form of micro-gels or

ground into fine powders. Certain applications use

these substances, specifically cross-linked amine

polymers, acrylics and unsaturated polyesters.

A plastisol (F) is obtained if a fine and crystal-

line powder polymer is dispersed in a plasticizer

that homogeneously dissolves it at temperatures

above the crystallite melting point, forming a gel

after cooling. Plastisols are more than 90% non-

volatile. Little solvent is needed.

Note that in addition to the specific character-

istics for roll application and ‘flash’ form ing or

curing of the film, the varnishes or paints for the

pre-finishing of flat steel or aluminum must offer

excellent flexibility, e.g. meeting the post-forming

requirements for drawing food cans.

TABLE 15-1

Performance of Main Coatings used in Solution

Polymer

Family

Polyester/

Amine

Polyester/

Polyurethane

Epoxy PVDF PVC

Flexibility Good Very good Poor Good Very good

Hardness Good Average Very good Average Poor

Adhesion to

metal

Good Good Very good Poor Poor

Corrosion

protection

Good Good Very good Average Very good

Weather

resistance

Good Very good Poor Very good Average

Temperature

resistance

Good Good Good Good Poor

Recyclability Very good Good Good Poor Poor

246

CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control

Principle of Organic Coating

Deposition in Solution

Applying organic coatings to reels of flat substrate

offers major technical and financial advantages

due to the follow ing:

Increased productivity and yield of finishing sys-

tems (with linear speeds of up to 1000 m/mn), re-

ducing the cost of varnish and paint application.

Flexibility of roll coating; rapid changes can be

made without stopping the application process.

Various curing temperature levels (80–250



C),

enabling a wide range of binder systems to be

used via various film formation processes (flash

cross-linking, gelation or fusion). Cross-linking

increasingly involves the environment-friendly

techniques of UV radiation or electro n beams.

Efficient effluent treatment systems (e.g. incin-

eration of solvent vapors).

Generally, the process of roll coating includes

three phases:

Surface preparation and treatment: corona

surface treatments are widely used on various

substrates, e.g. chromate treatment of steel (no-

rinse process) which helps to control film

weight and eliminate releases.

Roll coating of the polymer: can be performed

on both sides of the reel simultaneously.

FIGURE 15-5 Morphology of basic resins [13].

CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control 247

Various approaches can be used for this step

(Fig. 15-6). More than 35 systems are available

on the market to coat a variety of substrates

[10]. The varnish properties, surface appear-

ance and thickness ranges differ according to

the process used.

Solvent evaporation and flash curing: necessary

for forming the binding network.

FIGURE 15-6 Various systems for varnish deposition [10]. Diagrams from Coatema sales documentation.

TABLE 15-2

Three Varnish Deposition Processes most Commonly Encountered

Type of Process Characteristics Main Comments on the Process

Knife coating Thickness range: 5–500 mm

Viscosity: 5000–50,000 mPas

Maximum speed: 100 m/mn

Very dependent on varnish and

substrate. A very smooth film can be

obtained with 4% precision over the full

width of the substrate. For low viscosity

varnishes, there may be problems with

deposition uniformity. Solvent

evaporation during the implementation

process may lead to the appearance of

agglomerates.

Slot die Thickness range: <1–200 mm

Viscosity: <1000 mPas

Maximum speed: 1000 m/mn

Possible to work with minimum contact.

Substrate does not have a big influence.

Precision is on the order of 1%. The low

viscosity (<200 mPas) often results in

very good precision.

Roll coating Thickness range: 2–100 mm

Viscosity: <2000 mPas

Maximum speed: 1000 m/mn

Very dependent on the surface of the

deposition roller (very smooth with very

good precision, or gravure roller). A very

smooth film can be obtained. The

precision of deposition depends on the

parameters of the application roller and

the roller opposite to it. For film

thickness, precision is on the order

of 2%.

248

CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control

The choice of technology is based on the appli-

cation, but there are some country-specific trends

emerging in the technologies chosen [11].With

the process of varnish deposition, it is possible

to influence precision, coating weight and also

surface appearance (Table 15-2).

Most often, the coating is applied to the sub-

strate using a series of application rollers. The

reasons cited for using such a system are as fol-

lows: the ability to transmit shear to a liquid, to

smooth the coating before its deposition, to attain

an acceleration between the slow movement of

the rollers and the speed at which the substrate

passes through them (i.e. line speed) and to pro-

duce thin films by multiplying the separation

phases at each roller. The configurations are

selected based on the deposition thickness and

precision. In the two-roller configuration , there

is only one gap for adjusting and controlling the

thickness deposited. In a three-rol ler configura-

tion, the additional gap allows this thickness to

be optimized. Each roller has two functions. First,

rollers act as a divider by reducing the thickness

and uniformly spreading the varnish between all

the rollers. Second, they create the necessary shear

to provoke Newtonian behavior in the varnish.

There are few publications [12–15] explaining

how these deposits form, how a thin film is cre-

ated, how good the sensitivity is and how to per-

fectly control coating weight based on roller

speed, gap, and varnish properties.

But obviously there will always be interactions

between the varnish type, its viscosity and the

additives (size of charges, chemical nature, form,

etc.). Note that for certain production processes,

the central chemical treatment and coating sec-

tion is isolated from the unwinding and winding

sections by accumulators, allowing for uninter-

rupted operation during reel changing.

Process Parameters and Influences

Example: Optimizing Optical Properties.

There is a great deal of demand for coatings with

specific visual or optical propert ies (transparency,

gloss, matte finish). By adjusting the parameters

of the implementation process, it is possible to

influence a coating’s surface appearance as well

as its thickness and precision.

As the varnish is applied to the substrate, the

shear rate changes abruptly, potentially attaining

very high values (Fig. 15-7), but this intense shear

strain lasts only a short time (<0.01 s). Use of an

elastomer roller can be estimated to increase the

dwell time and reduce shear by a factor of around

10 compared to a rigid roller. The viscosity of the

varnish will thus have a non-negligible effect on

its thickness and visual qualities, and it will

FIGURE 15-7 Timescale and typical values of process parameters for wet deposition.

CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control 249

interact with the system of application. Eleven

types of flow between two rollers have been

described. Experimental and theoretical studies

have examined some of them [16] and have

enabled correlating the quantity deposited with

the flow characteristics. Likewise, a strong inter-

action between the implement ation parameters

and the optical properties has been demonstrated

by the number of capillaries:

where h

= viscosity, V = line speed and s =

surface energy.

Whatever the mode of varnish transfer (co-

rotating or reverse), the optical characteristics of

the film vary strongly according to Ca. When Ca is

high, an ‘orange peel’ effect or bubbles appear

(Figs. 15-8 and 15-9). This is caused by a reso-

nance phenomenon provoking a wave effect in the

liquid solution when it is transferred by the appli-

cation roller to the substrate. This defect can mea-

sure several microns [17] and increases in size

when the film is thin an d the line speed is high.

It is generally agreed that competition exists

between the viscosity of the varnish and the sur-

face energy of the surface. This phenomenon is

also observed for Newtonian liquids [18].

Optical properties can be improved by increas-

ing substrate surface energy or by decreasing the

viscosity of the varnish as much as possible. How-

ever, one of the conditions essential to forming a

continuous film of varnish at the surface of the

substrate is that the latter have a surface energy

higher than the surface tension of the liquid var-

nish, to ensure good wetti ng of the substrate.

Figure 15-10 clearly illustrates that a critical sur-

face energy is needed to eliminate these defects.

Coating quality also plays a very important

role in determining its final properties, but several

types of defects are possible, depending on the im-

plementation conditions. They can include lines,

pores, irregular thickness, etc. Extensive cracking

of thin films may be linked to the internal strain in

these fine organic coatings. This strain can also

develop over time, either due to age or UV radia-

tion. In addition to the loss of optical properties,

ageing often provokes delamination between the

polymer thin film and the substrate [19–20].

There is also consensus that the film’s glass tran-

sition temperature (T

) plays a role in this mech-

anism, given that polymer chain mobility may

facilitate the penetration of various species in

the polymer matrix, leading to a loss of adhesion

at the polymer/substrate interface. Figure 15-11

illustrates changes in glossiness for two families

FIGURE 15-8 Typical optical defect with a ‘bulge’

appearance.

FIGURE 15-9 Diagram of the two main appearance

defects (bulges and bubbles).

FIGURE 15-10 Changes in appearance defects according

to substrate surface energy for an acrylic varnish on a poly-

ethylene film.

250 CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control