Effect of Reactive Monomer on
Polymer structure and abrasion
resistance of UV cured thin films
Prof Jeng Ywan Jeng
Mechanical Engineering Dept
Taiwan Tech
PHOTOPOLYMER
A photopolymer or light-activated resin is a polymer that
changes its properties when exposed to light, often in
the ultraviolet or visible region of the electromagnetic
spectrum.
A mixture of Monomers, Oligomer and Photo initiator that
conform into a hardened polymeric material through a
process called curing.
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Monomer is a molecule that can undergo polymerization
thereby contributing constitutional units to the essential
structure of a macromolecule. Large numbers of monomers
combine to form polymers in a process called polymerization.
Oligomer is a molecular complex of chemicals that consists
of a few monomer units, in contrast to a polymer, where the
number of monomers is in principle unlimited.
Dimers, trimers, and tetramers are for instance oligomers
composed of two three and four monomers respectively.
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Effect of Reactive Monomer
UV-curable coatings are usually designed for substrate
protection.
In UV-curing technology, low molecular weight monomers
are widely used as reactive diluents instead of solvents in
order to adjust the viscosity to the application process.
Monomers incorporation into the polymer network modifies
the structure of the 3D polymer network by affecting the
crosslink density and consequently the glass transition
temperature, the Young’s modulus and the elongation and
stress at break.
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Final properties of light-cured coatings not only result from
the chemical constitution of the oligomer backbone but also
on the reactive diluent functionality and structure.
Modifying the structure of the polymer network will have an
impact on abrasion or scratch resistance.
Understanding the influence of reactive diluents with
different structures and functionalities both on polymer
network structure and the abrasion resistance is important.
Most important parameters governing the abrasion resistance
is concluded in result.
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Materials for Experiment
Commercially popular UV coatings urethane acrylates are
selected because they have high abrasion resistance,
toughness and tear strength.
Two aliphatic Urethane Diacrylates have been selected
(Table1) for this study and different reactive diluents from
Sartomer were investigated.
(Table1) Characteristics of Oligomers
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Characteristics Of Monomers
(functionality, glass transition temperature Tg, Viscosity and
molecular weight)
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Photoinitiator is 2-Hydroxy-2-methyl-1-phenylpropan-1one (D1173).
Methylethyl ketone (MEK, ≥99.0 %) and a wetting agent (BYK 3505)
were also used in the formulations
Sample Preparation
UV-curable coatings
30 wt.% of reactive diluent with
70 wt.% of oligomer,
3 wt.% of the photo initiator and
5 wt.% of BYK 3505 as adhesion promoter.
Some formulations displayed high viscosity so
MEK was added at 10 parts for 100 parts of formulation containing
either TMP9EOTA, TMPTA, PETTA or 4EOPETTA.
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Sample Characterization
1. Confocal Raman Microscopy (CRM)
CRM enables the record of Raman spectra at different depths
into a sample by focusing the exciting laser beam at different
linear positions through the sample.
2. Dynamic Mechanical Analysis (DMA)
DMA were performed on the UV cured thin sample to
investigate viscoelastic behavior of the coating.
The Full Width at Half Maximum (FWHM) of the loss
modulus is measured as an indication of the network
heterogeneity.
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3. Abrasion resistance–ASTM D-1003 and D-6037
The coatings cured on a glass substrate is damaged with
a rotative taber 5135, a circular abrasion simulator.
To discriminate more precisely the influence of the
monomer structure, the number of cycles were 10 and
50.
The abrasion was evaluated with a Varian CARY 4000
double beam UV–vis spectrophotometer. A double beam
light was emitted from 400 to 700 nm going through the
sample placed at the entrance port of the integration
sphere.
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The scattered light is measured at different transmittance
Modes as described in ASTM D-1003 standard in order to
calculate the haze.
This is an optical phenomenon due to surface irregularities and/or
presence of contaminants in the coating inducing light diffusion. Haze
can create undesired halos and/or lead to an optical impression of
cloudy appearance (loss of transparency).
ASTM D-1003 standard defines the haze as a percentage of
transmitted light which deviates from the incident beam by
forward scattering, when passing through a specimen. Only
light flux deviating more than 2.5° on the average was
attributed to haze
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Results And Discussion
Different formulations based on two oligomers A and B were
characterized. The conversion, thermomechanical properties and
abrasion resistance is in Table 3
Table 3: γR is the crosslinking density AR10C and AR50C are the abrasion at 10 or 50 cycles,
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Conclusion From Table 3
1) Oligomer A gives lower Tα, crosslink density and a larger FWHM.
2) 3D network of A is flexible and more heterogeneous. It also
exhibits a lower abrasion resistance performance compared to B.
3) These values are considered as references to study the effect of the
reactive diluents on the final mechanical properties.
4) Thermomechanical characterization confirms that the addition of
reactive diluent increases the crosslinking density (except for
ODA-1B) and consequently Tα,
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Abrasion Resistance
Abrasion resistance after 10 and 50
cycles for formulations containing
reactive diluents with increased
functionality and oligomer B.
(diluent/oligomer ratio: 30/70 wt
%).
The gradual increase in crosslink density and consequently in network
heterogeneity seems to work against abrasion resistance for mono- to
trifunctional diluents
So there is influence of reactive diluents on abrasion resistance.
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Influence of Chemical Structure of
Reactive Diluents on Mechanical Properties.
Oligomer B allows a better handling due to its lower viscosity at room temperature.
1. Reactive diluents with functionality ≤3
Table 4: Conversion, thermomechanical data and abrasion resistance obtained for photocured
films with mono, di and trifunctional reactive diluents
Influence of an aliphatic or ether skeleton for difunctional diluents
HDDA & TIEGDA (Table 4), Both photopolymerized films show
similar conversion and Tα.
Aliphatic structure promotes a tighter but more heterogeneous network
whereas ethylene glycol chain affords flexibility and homogeneity
which account for better abrasion resistance
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Table 4 continued
Preserving the same backbone structure but increasing the functionality
results in films with excellent abrasion resistance (TMP9EOTA-3B).
Nevertheless, a decrease in the chain length (TMP3EOTA-3B) is
strongly linked to higher crosslinking density and heterogeneity that
are detrimental for the abrasion resistance.
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Table 4 continued
Influence of methyl group on the monomer backbone structure on the
abrasion resistance
For this purpose (TIEGDA, TPGDA and DPGDA) have been compared
with one other and with hexanediol- and 3 methyl 1,5 pentanediol
diacryalte (HDDA and MPDA).
The methyl group systematically leads to increased crosslink density
and Tα. The beneficial effect of the ether chains observed previously is
drastically reduced due to presence of labile hydrogen on the methyl
group.
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Table 4 continued
Influence of hydroxyl group on the backbone structure
(PETIA-3B) and (TMPTA-3B) were compared, Tα is similar for both
monomers but the FWHM is significantly narrower for PETIA based
film that also exhibits an important crosslinking density.
Hydroxyl groups seem to strongly affect final properties. They can be
involved in the polymerization process by chain transfer with the
hydrogen atom, located in α position from the OH group. They can
also strengthen the network by formation of hydrogen bond. As a
result, an extremely tighter network with better abrasion resistance.
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These results illustrate the long-lasting effect of ethylene
glycol chains on wear resistance, even with trifunctional
monomers. That is, the chemical structure of the monomer in
this case has a greater effect on the abrasion resistance than
the increase in the functional group.
Polymer networks can be seen in three-dimensional space as
elastic soft chains are linked together by small, hard
units. By adding monomers with ether bonds, the chemical
structure of these linking units can be changed, thereby
increasing the flexibility of the network. These soft phase
units can elastically disperse the stress caused by mechanical
failure, thereby improving wear resistance.
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2. Reactive diluents with functionality > 3
Table 5
Behavior of tetra & penta
functional monomers was first
investigated in combination
with oligomer B (PETTA–4B
and DiPEPA-5B).
Both diluents lead to high
crosslink density compared to
di-functional ones
The presence of the hydroxyl
group on DiPEPA and the central
ether group give rise to increased
network homogeneity.
However the beneficial effect on abrasion resistance is not observed.
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Comparing the influence of two tetra functional diluents (PETTA–4B
and 4EOPETTA-4B), we can noticed that for similar Tα, the
ethoxylated monomer does not result in increased abrasion resistance.
Contrary to the previous results (f≤3), the significant drop in crosslink
density and the increased homogeneity lead to detrimental
performances.
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Conclusion
when different reactive monomers and urethane acrylates are combined
into a formulation, the functionality of the monomers and the chemical
structure of the main chain all affect the wear resistance. The effect of
functionality on wear resistance presents a V-shaped trend.
For functionalities less than or equal to 3, the triethylene glycol main
chain structure gives better wear resistance because it can lead to lower
crosslink density and a more uniform network structure, providing a
more flexible structure .
The cured coating can have better elastic deformation and disperse the
stress shock generated when breaking.
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The presence of aliphatic backbones or methyl groups can result in a
loss of moderate performance and abrasion resistance.
Increasing the functional group from 2 to 3 will result in a loss of
mechanical properties due to a more compact network structure.
However, this effect can be balanced by the choice of monomers
containing ether chain long chains (such as TMP9EOTA-3B).
For monomers with a functionality greater than 3, the influence of
crosslink density exceeds that of the chemical structure. A tighter
network will have better hardness and cohesion to resist impact and
thus improve wear resistance
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Thank You
References:
Liza Marasinghea, Céline Croutxé-Barghorna, Xavier Allonasa, Adrien
Criquib, Effect of reactive monomers on polymer structure and abrasion
resistance of UV cured thin films, Progress in Organic Coatings, 118 (2018)
22–29
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