|
ONLINE CATALOG MSDS SEARCH PROFILE PRODUCTS CUSTOM MANUFACTURING TECHNICAL PUBLICATIONS REQUEST INFORMATION WHAT'S NEW INTERNATIONAL SALES SITE SEARCH
Contents Adhesion Promotion for Polymers Improving the Scratch Resistance of Polymers |
Adhesion and Scratch Resistance of Coatings on Plastics This issue discusses three topics, two having to do with the coating of polymeric materials for optical applications; the third is related to an alternate material to ITO as the transparent electrode for solar cells. Progress on these topics is occasionally updated in Coating Materials News (CMN) issues since polymer substrates are being used with greater frequency in displays, solar panels, laminated window glazing, reflectors for illumination, and other optical applications. Injection molded or form cast polymer parts are now mass-produced as economical and light-weight replacements for glass parts. The substitution of polymeric materials has required modification of coating processes to accommodate lower abrasion resistance compared with those of glass surfaces. Processes for applying optical coatings with improved adhesion and scratch resistance at lower temperatures have been developed. Previous discussions in CMN dealing with the coating of polymer surfaces may be obtained on the Technical Publications selection at www.cerac.com. Some past discussions include: adhesion in V7, I7 (1997); types of polymers and processes in V15 I4 (2005); general coating treatment in V10 I4 (2000). Further background is provided in the article on surface cleaning: V18 I4 (2008).
Adhesion
Promotion for Polymers
The majority of
adhesion issues involving polymers are with AR and metallization
coatings. Durable coatings on commercial products such as ophthalmic
AR coatings require scratch resistance as well as good adhesion.
Therefore, such coatings are designed and processed to provide both
functions. Chemical hard coats protect the polymer lens surface, but
the optical coating on top of the hard coat must itself be durable
to handling and exposure. Metallization on plastic objects is often
a second-surface coating where the coating can be protected with a
mechanically durable overcoat, and in this case adhesion is the
dominant challenge. Adhesion between
metals and polymers is difficult because of the chemical
incompatibility of the two materials. In the case of glass
(silicate) surfaces, intermediate compounds, generally suboxides,
can be formed to promote adhesive bonding. Polymer surfaces require
not only intermediate chemical interfaces, but initial surface
conditioning to form strong chemical bonds. A recent study explored
adhesive-promoting layers, surface conditioning, for different
polymers that are often used in metallized products [1]. Injection
molded samples of Acrylonitrile-butadine-styrene (ABS),
polycarbonate (PC), polypropylene (PP), and polyethylene
terephthalate (PET) were tested. The energies required to peel the
coating from the surface and to fracture it were measured. Metal
layers were Cu, Ti, Cr, and Al, all sputter deposited to 50 nm
thickness. A 10µm layer of copper was electrodeposited on the
adhesor metals for the peel test. The results using the four metals
on ABS ranked the sequence of both failure energies as follows: Al > Cr > Ti
> Cu The aluminum bond was 1.75 x stronger than the copper bond for ABS. Process variables and internal stress levels and cohesive vs adhesive strengths can influence the results, as can surface preparation, as we shall see. The researchers tested the role of the polymer type when coated with a Cr adhesor layer. The results are ranked:
ABS > PET-G > PC > PET > PP PET-G and PET have different compositions. ABS more easily forms metal-oxide bonds due to its functional C=O bonds and benzene rings. Stronger adhesion requires modifying the surface chemistry to promote the creation of reactive groups on the metal – plastic interface. A measure of the effectiveness of this chemistry is the water contact angle (see CMN V18 I4 (2008)). A low contact angle indicates high surface free energy (wetting), and correspondingly good adhesion. Atmospheric plasma pretreatment of the surface of PP with He/N2, reduced the contact angle more than plasma pretreatment with He/O2. Sputter etching of PP surfaces with argon ions immediately before deposition produced slightly better adhesion and fracture results than plasma etching. Perhaps the reason is that there is the possibility of surface modification during installation in the coating system after the atmospheric procedure. This study showed the importance of the specific metal adhesor layer and surface pretreatments on adhesion and fracture energies for common polymers.
Improving
the Scratch Resistance of Polymers
The mechanical
hardness and wear resistance of coatings have been topics of
extensive discussion in past CMN issues [2]. The model presented in
the referenced issue illustrates the multiple and inter-related
components that contribute to and determine the hardness, strength,
and wear properties of mechanically durable thin films. Polymeric
materials are soft, and therefore require a coating to improve their
resistance to abrasive damage. Mechanical properties: modulus of
elasticity, fracture strength, and hardness of the substrate and its
coating determine the ability to resist scratching up to specific
forces. Resistance to scratching is tested by moving contact with an
indenter point at different force loads. The Rockwell indenter of
200 µm is typically used. The force at which the coated surface
develops transverse cracks is termed the critical load that induces
failure. Aside from the structural properties mentioned, the
coefficient of sliding friction of the coating affects the coupling
of the applied force into the system. Compared to glass surfaces,
plastics are many times softer, suffer larger plastic deformation,
and have a higher coefficient of sliding friction. Plastic surfaces
also must be hardened to abrasive wear imposed by cleaning and sand
erosion. Scattered light increases with exposure, and this can be
measured with the Taber abraser test, sometimes referred to as a ‘haze
meter’. Thin layers of metal oxides are applied to overcome the
properties of the bare polymer surface, by producing a new surface
with more durable mechanical properties. Window and display
applications that use polymer such as those mentioned in the
previous section are generally coated with sputter-deposited silica,
SiO2, layers. A recent study
determined the scratch and wear resistance of polycarbonate for
different thickness of AC magnetron sputtered SiO2 [3]. The critical
load at which transverse cracking (spalling) occurred was ~3 N for
thickness 0 to 1 µm. A thickness of 2 µm was required to increase
the critical load to 5-6 N. Doubling the thickness to 4 µm doubled
the critical load for the onset of cracking. Scatter (haze)
decreased from 60% for the uncoated PC surface to 40% for a
thickness of 1 um, and to 15% for 2 um SiO2 thickness, and only
slightly decreased for greater thickness. Titanium
Dioxide Films on Polycarbonate
TiO2 is a high-index
material used for optical coating, photo catalysis, gas sensor, and
electronic device applications. We have discussed these various
applications, the evaporation material processing (starting
composition), and deposition techniques in many CMN pages. Among the
lessons learned is that the crystalline and nano-structural
properties, and associated optical and mechanical characteristics,
are strongly process dependent. The type and nature of the
crystalline phase presented is dependent on the energy of growth,
and IAD or high substrate temperature are parameters employed to
achieve the desired result. Amorphous films are formed under low
energy conditions, and can be transformed to the anatase phase with
heat. Deposition on temperature sensitive polymers such as PC
requires IAD. A recent study
describes the influence of ion anode voltage and film thickness on
refractive index, porosity, and grain structure for evaporated films
[4]. What we learn from this work is that ion anode voltage in
excess of ~70 eV are required to realize an increase in index and
simultaneous decrease in porosity. Increasing voltage to 110V
produced denser and thicker layers, while maintaining the anatase
structure. An interesting bit of spectral data is presented that
illustrates the optical absorption of titania at wavelengths larger
than 700 nm, and to at least 1500 nm. This absorption is the reason
tantala or lanthanum titanate are used as the high-index layers in
NIR and SWIR coatings [5].
Improving
the Properties of Zinc Oxide as a TCO
We have discussed the
substitution of doped-ZnO for ITO in applications such as solar
cells and displays in previous CMN issues [6]. Al-doped compositions
known as AZO or ZAO have been studied. Recall that the lower cost
relative to ITO, its high visible-Near-IR transparency due to a wide
bandgap (3.3 eV), and its relatively low sheet resistance are
advantages offered by doped ZnO. We have seen that the
addition of a dopant can result in the modification of the growth
and structural properties of a host material. Associated with
nano-structure are optical and mechanical properties. We review an
interesting paper that reports the use of ZnO targets with 0-4 wt%
of Al2O3 added and R. F. magnetron sputtered [7]. The authors
suggest that the resistance of ZnO to an ‘aggressive plasma’ is
an advantage that ZnO has in the application to amorphous silicon
solar cell manufacture. In the study reported, the grain size of the
sputtered films was observed to decrease from 160 nm at 0% to 41 nm
at 4 wt% alumina concentration. The dopant apparently acts to
inhibit grain growth and crystallinity (as indicated by x-ray
diffraction). A strong influence of concentration on electrical
resistivity is observed; with higher concentrations decreasing
resistivity. A change of 5 orders of magnitude is reported over the
0-4% dopant range. Pure films of ZnO had a resistivity of 74 W-cm,
and increasing the dopant to 4% decreased the resistivity to 2.2 E-3
W-cm. A 4 wt% film that is ~130 nm thick will have a sheet R of ~170
W/sq. Higher carrier density due to defect substitution or greater
grain boundary scattering might be responsible for the resistivity
decrease. The absorption edge for ZnO is between wavelengths 400 and
300 nm. Higher doping shifts the edge to shorter wavelengths. Films
with higher alumina content are more transparent throughout the
Visible and Near IR. This work represents one example of the ability
to manipulate and improve on the optical and structural properties
of ZnO.
References
1. O. Dos Santos
Ferreira, A. Stevens, and C. Schrauwen, Thin Solid Films, V517,
3070 (2009). 2. S. F. Pellicori,
Cerac Materials News, V18 Issue 2 (2008). 3. W. Boentoro, A.
Pflug, and B. Szyszka, Thin Solid Films V517, 3121 (2009). 4. Su-Shia, Yuan-Hsun
Hung, and Shin-Chi Chen, Thin solid Films 517, 4621 (2009). 5. CERAC Coatings
Materials News V17, I1 (2007) and CMN V16 I1 (2006). 6. CERAC Coating
Materials News V15, I1 (2005) and references contained therein. 7. S. N. Bai and T.
Y. Tseng, Thin Solid Films V515, 872 (2006).
|
|
(S.F. Pellicori is available for private consulting on matters concerning optical thin films. Please contact him directly for more information) |
Editor: Principal Contributor: |
| back to top |
All printed, graphic and pictorial materials made available on this website are owned by CERAC and protected by Federal Copyright laws. None of the materials, in whole or in part, may be reprinted and distributed or otherwise made available to others for any purpose without CERAC's prior written consent. Phone: 414-289-9800 / FAX: 414-289-9805 / ceracinfo@beminc.com |
