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On demand NIR activated photopolyaddition reactionsHaifaa Mokbel, Frederic Dumur, Jacques Lalevee

To cite this version:Haifaa Mokbel, Frederic Dumur, Jacques Lalevee. On demand NIR activated photopolyadditionreactions. Polymer Chemistry, Royal Society of Chemistry - RSC, 2020, 11 (26), pp.4250-4259.�10.1039/d0py00639d�. �hal-02895855�

1

On demand NIR activated photopolyaddition reactions

Haifaa Mokbel1,2, Frédéric Dumur3, Jacques Lalevée1,2*

1Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France 2Université de Strasbourg, France

3Aix Marseille Univ, CNRS, ICR UMR 7273, F-13397 Marseille, France

E-mail: Jacques.lalevee@uha.fr

Abstract

A new approach based on a photoactivation process is proposed for the polyaddition

reaction of epoxy-based systems (epoxy/amine and epoxy/anhydride). Outstandingly, for the

first time, near-infrared (NIR) absorbing dyes are used here for this photoactivated approach

upon NIR light as safe irradiation conditions. Indeed, the use of NIR light remains a huge

challenge i.e. the photon energy is extremely low to initiate photochemical processes. But a

main advantage of the NIR light for polymer synthesis is a deeper light penetration into the

material. A photoinitiating system is proposed comprising an NIR dye combined with an

Iodonium salt for the Epoxy/Amine photopolyaddition. The new proposed approach is highly

robust and can be used for a broad selection of amines or epoxides. In the case of

Epoxy/Anhydride photopolyaddition, the presence of an accelerator is required to improve the

reaction. Additionally, the use of NIR light has a huge effect on the kinetics of reaction: the

polymerization time is greatly improved compared to the classical reaction (e.g. in 3 hours

without light vs. few seconds or minutes upon NIR light irradiation, at room temperature). Two

irradiation wavelengths are presented here: 785 nm and 1064 nm with two selected NIR dyes.

The different systems presented in this paper exhibit fast polymerizations associated with full

conversions. Real-time Fourier transform infrared spectroscopy is used to follow the

polymerization kinetics. The applications in adhesives as well as composites is presented to

highlight the interest of these NIR photoactivated processes.

Keywords: Epoxy/Amine polyaddition, Epoxy/Anhydride polyaddition, Near-infrared (NIR)

light, NIR dyes.

1/ Introduction

2

Epoxy-based polymer materials have diverse applications as adhesives, paints, coatings,

wind energy, composites, construction, and electronics [1-3]. These materials have a set of

unique properties such as excellent mechanical strength; outstanding chemical, moisture and

corrosion resistance; good thermal, adhesive, and electrical properties; and low shrinkage upon

curing [4-7]. Epoxy resins are mostly used in industry for thermosetting application, a process

in which an epoxy resin reacts with a curing cross-linking agent known as a hardener. The

largest class of hardeners (approximately 50%) uses primary and secondary amines. The second

one involves carboxylic acids and anhydrides (approximately 36%). And, the remaining class

is for phenols and other agents [8]. Sometimes, additional agents like catalysts or accelerators

are required to speed up the polymerization process or modify the final properties of the

polymers. The reaction of amines or anhydrides with epoxy monomers is a step-growth

polymerization and is commonly referred as Epoxy/Amine and Epoxy/Anhydride polyaddition.

Due to the extensive use of the polyaddition reactions, many researches have been done.

For the classical polyaddition reaction, when the reactions are carried at room temperature and

without light, the kinetics of polymerization are very slow and the systems need few hours to

reach a complete conversion. The general epoxy/amine reaction occurs via a nucleophilic attack

of the amine on the epoxy function (Scheme 1) [8-9]. In this mechanism involving a ring-

opening reaction, a primary amine can react twice with two epoxy groups, while a secondary

amine can react only once [10].

Nowadays, polyaddition reactions are facing a major issue i.e. a slow polymerization

speed so that a significant improvement would consist in the possibility to speed up the

polymerization process on demand i.e. the system could be activated by mean of an adapted

stimulus upon request. Thus, these systems should be very efficiently polymerized within a few

seconds/minutes at room temperature (RT). This is only possible if the reaction is activated by

an external source i.e. light. The development of safer systems that polymerizes at long

wavelength is crucial (UV can be toxic). However, up to now, the photoactivation of

Epoxy/Amine or Epoxy/Anhydride polyaddition reaction upon near-infrared irradiation has

never been reported from a fundamental standpoint. Photopolyaddition reaction under NIR light

exposure is challenging due to the low energy of the photon, but if successful, presents several

advantages. For example, this low energy wavelength can be much safer than the conventional

(near) UV light. Secondly, NIR allows a deeper light penetration inside the resins and therefore

a more complete polymerization in depth can be obtained, enabling the access to highly filled

materials. Additionally, the NIR dye can act as a very efficient heat generator (heater) upon

irradiation with a NIR light [11]. In this way, we propose to use the photoinitiating system PIS

3

based on the NIR absorbing dye/oxidant agent couple to photoactivate the Epoxy/Amine

reaction upon excitation at 785 nm and at 1064 nm and the Epoxy/Anhydride reaction upon

exposure to a light at 785 nm. The mechanism of Epoxy/Amine photopolyaddition reaction is

illustrated in Scheme 1. Very recently, Garra et al. proposed an Epoxy/Amine

photopolyaddition reaction by using a near UV light and a photoinitiating system comprising a

thioxanthone derivative (CPTX) combined with an iodonium salt (Iod) [12]. Upon irradiation

at 405 nm, the CPTX/Iod couple generates strong acid and/or cationic species (Scheme 1) [13].

It has been shown that the acid or cationic species can strongly activate the epoxide monomer

facilitating the amine addition onto the epoxide ring leading to much faster photopolymerization

processes. Only one example was given for the photopolyaddition reaction of Epox 1/m-XDA

showing that the use of a photochemical activation outstandingly enhances the reaction kinetic

(2.5 minutes vs. 3 hours without irradiation). Due to the main advantages of the NIR light

compared to the near UV light, this article corresponds to an original approach compared to our

previous work [12].

In the case of the Epoxy/Anhydride reaction, an imidazole accelerator is often used to

initiate the opening of the epoxide ring [14]. To the best of our knowledge, such polyaddition

reaction has never been performed upon NIR photoactivation, which highlights the originality

of this paper.

In this work, the versatility of the NIR approach for both the Epoxy/Amine and the

Epoxy/Anhydride photopolyaddition reactions is demonstrated. Two NIR dyes (IR 813 and IR

1064) are proposed for the Epoxy/Amine reactions upon exposure to laser diodes (LD)@785

nm and 1064 nm. A selection of different amines as hardeners is also presented. The possibility

to use different oxidation agents to generate the acid or cationic species that facilitate the

epoxide ring-opening is also checked. The kinetics of polyaddition reactions is followed by

Real-time Fourier transform infrared spectroscopy (RT-FTIR). The series of Epoxy/Amine (or

Anhydride) polymerizations were prepared using a two-component mixing procedure

(described in the experimental part). The proportion of Epoxy/hardener is given for each

experiment.

4

Scheme 1. Simplified chemical mechanisms for classical Epoxy/Amine polyaddition vs.

photopolyaddition approaches. The approach 2 is extracted from Ref. [12].

2/ Experimental part:

2.1/ Chemical compounds for Epoxy/Amine photopolyaddition reaction

Photoinitiating system: The NIR dyes (IR 813 and IR 1064) were purchased from

Lotchem Ltd. (China) and used as NIR absorbing dyes in combination with an oxidation agent

(Scheme 2). Iodonium salt “Iod” (bis-(4-tert-butylphenyl)iodonium hexafluorophosphate),

Sulfonium 1 named (sulfanediyldibenzene-4,1-diyl) bis(diphenylsulfonium)

bis(hexafluoroantimonate)) and Sulfonium 2 named ((4-{[4-

(diphenylsulfanylium)phenyl]sulfanyl}phenyl)diphenylsulfonium bis(hexafluorophosphate) in

propylene carbonate) were purchased from Lambson Ltd (UK). Thianthrenium salt “TH” (9-

(4-hydroxyethoxyphenyl)thiantrenium hexafluorophosphate; 9-[4-(2 hydroxyethoxy)

phenyl]thianthrenium hexafluorophosphate) were purchased from Lamberti (now IGM resins).

Iod, Sulfonium 1, Sulfonium 2 and TH have been used as oxidation agents. All chemicals were

used as received.

R1R2NH + Slow ~ 3H, @RTO

R3 R3

OH

R1R

2N

n

Amine + Epoxy

CPTX405nm

1,3CPTXIod H+, PF6

- or

cationic species

R1R2NH + Fast (~2.5 min), @ RTO

R3 R3

OH

R1R

2N

n

Amine + Epoxy H+, PF6- or cationic species

3) Photopolyaddition

Proposed approach upon NIR light (This work)

NIR dye785 nm

& 1064 nm1,3NIR dye

Iod

R1R2NH + Very Fast (~1 min), @ RTO

R3 R3

OH

R1R

2N

n

Amine + Epoxy

1) Classical polyaddition

2) Photopolyaddition

Near UV approach

H+, PF6- or cationic species

H+, PF6- or

cationic species

5

Scheme 2. Photoinitiating system used for Epoxy/Amine polyaddition NIR dye/oxidation agent (0.1/2

%wt).

Epox component: The blend of bisphenol A/F and trifunctionnal crosslinking epoxide (Epox

1), has been prepared from 70 wt% Araldite (49 wt% Araldite GY 210 – 21 wt% Araldite GY

282) and 30 wt% trimethylpropane triglycidyl ether; Poly (bisphenol A-co-epichlorydrin),

glycidyl end-capped (M = 377 g/mol) (DGEBA) were purchased from Sigma-Aldrich. 1,4

butanediol diglycidylether (Epox 2), N,N-diglycidyl-4-glycidyloxyaniline (Epox 3) and 2,2-

bis(4-glycidyloxyphenyl)propane (Epox 4) were purchased from TCI. Epox 1, Epox 2, Epox

3, Epox 4 and DGEBA have been used as benchmark monomers. Chemical structure of the

different epoxy monomers is presented in the Scheme 3.

Iod

IR 813

Sulfonium 1 Sulfonium 2 TH

Oxidant agent

NIR absorbing dye

IR 1064

6

Scheme 3. Epoxy monomers structures used in this study.

Amine component: The chemical structures of commercial amines (aliphatic, cycloaliphatic or

aromatic) purchased from TCI are gathered in the Scheme 4.

Scheme 4. Chemical structures of the different amines.

2.2/ Chemical compounds for Epoxy/Anhydride photopolyaddition reaction

The photoinitiating systems (PIS) used for this reaction are mainly based on the IR

813/Iod (0.1/2 wt%) combination. The Epox (bisphenol A diglycidyl ether), anhydride (4-

methyl cyclohex-4-ene-1,2-dicarboxylic anhydride) and the accelerator (1-methyl-1H-

imidazole) were purchased from Sigma Aldrich. All chemicals were used as received. The

chemical structures are presented in Scheme 5.

7

Scheme 5. Chemical structures of representative Epoxy/Anhydride/Accelerator mixture.

2.3/ Two-component mixing procedure

All formulations were prepared from the bulk monomers at room temperature (RT)

(21−25 °C). An epoxy cartridge was mixed with an amine or an anhydride cartridge during

about 45 s before each experiment. The photoinitiating systems were first dissolved in the amine

cartridge for the Epoxy/Amine reaction and in the Anhydride/Accelerator cartridge for the

Epoxy/Anhydride reaction and then mixed to the epoxy cartridge before analysis. Their weight

contents are given as a percentage of the total Epoxy/Amine mixture (e.g. 0.1 wt% IR 813

corresponds to 2 mg IR 813 in 2.00 g of Epoxy/Amine mixture without additives). Ratio close

to the stoichiometry between epoxy and hardener contents were investigated (see the Figures

below). The contents of hardeners used (amine or anhydride) ensure a stoichiometric reaction

with the epoxy functions. In order to have a complete Epoxy/hardeners photopolyaddition, the

hardener content should be in slight excess compared to the epoxy monomer.

2.4/ Irradiation devices

Two NIR laser diodes (LD) purchased from Changchun New Industries (CNIs) were

used for the photopolyaddition reaction: Laser diode@785 nm with a selectable irradiance from

0 W to 2.55 W/cm2, and a laser diode@1064 nm with selectable irradiance from 0 to 3 W/cm2.

A high-power LED@810 nm purchased from Phoseon with a selectable irradiance from 0 to

12 W/cm2 was used for preparing composite materials and adhesion applications.

2.5/ RT-FTIR spectroscopy

A Jasco 6600 Real-Time Fourier Transform Infrared Spectrometer (RT-FTIR) was used

to follow the reactive function conversion versus time upon irradiation. The conversions of

epoxy, primary amine (NH2) and primary and secondary amine (NH) functions were followed

from their respective peaks in the near infrared: 4470 to 4568 cm-1, from 4848 to 5050 cm-1 and

from 6390 to 6635 cm-1, respectively. Polymerization is performed in a mold (thickness = 1.4

mm), under air and at RT upon irradiation with a NIR laser diode. Light is turned on 10 s after

the first spectrum measurement. Details concerning the experimental conditions used for

photopolymerization experiments are given in the figure captions.

8

2.6/ Dynanometer experiments

A Dynamometer INSTRON 4505 modernized Zwick (speed = 10 mm.min-1, 100 kN

sensor) has been used to calculate the shear stress (MPa) of material as a function of the

elongation (mm). The materials are prepared by bonding two epoxy substrates (bonded surface

= 10 mm).

3/ Results and discussion

3.1/ Epoxy/amine system: the DGEBA/TETA example

The DGEBA/TETA system is one of the most importantly used two-component systems

in epoxy/amine polyaddition and it has been selected as a benchmark system. In the industry,

the polymerization time of two-component systems is important for a cost-effective

manufacturing but also for a full control of the work time. No on-demand systems are currently

reported. When the process is too slow (> 1 hour), it leads to a long process time and higher

process costs, which is the case of the classical polyaddition reaction at RT (Figure 1, curve 1).

Remarkably, the photoactivation of the DGEBA/TETA polyaddition reaction using the IR

813/Iod (0.1/2 %wt) photoinitiating system upon exposure to LD@785nm (I = 2.5 W/cm2)

leads to full final epoxide conversion within a few seconds of irradiation and tack free polymers

are obtained i.e. for the first time an “on demand” system can be proposed (the process is fully

controlled by light). Light drastically improves the kinetics of polymerization for the

DGEBA/TETA polyaddition reaction: epoxide functions were fully converted in less than 1

minute of irradiation, whereas only 85% of conversion is reached after ~3 hours without light

(Figure 1, curve 2 vs. curve 1). Remarkably, only the photoactivated system allowed to reach a

full final conversion (Figure 1).

0.0 0.5 1.0 1.5 2.0 2.5

0

20

40

60

80

100

(2)

(1)

Time (h)

Ep

oxid

e c

on

ve

rsio

n (

%)

9

Figure 1. Photoactivation process for the DGEBA/TETA (83/17 wt%) system followed by Real-time

Fourier transform infrared spectroscopy (RT-FTIR): epoxide function conversion vs. time, using IR

813/Iod (0.1/2 wt%) (1) without irradiation and (2) under exposure to LD@785 nm (I = 2.5 W/cm²),

thickness = 1.4 mm, under air and at RT. The irradiation starts at t = 10 s.

By Real-Time Fourier Transform InfraRed spectroscopy (RT-FTIR); the epoxy, the

primary amines (NH2) and both primary and secondary amine (NH) final conversions can be

followed simultaneously and high final conversions are obtained under photoactivation (Figure

2A). The primary amine and epoxide conversions (Figure 2A, curves 1 and 2) follow the same

polymerization profiles in agreement with a polyaddition process. In an epoxy/amine process,

primary amines are converted into secondary amines, that can also react with the epoxy ring

(See Figure 2A, curve 3) (roughly as fast as the primary ones [15-18]). Remarkably, full final

conversions are obtained for both primary amine and epoxides i.e. in full agreement with the

IR spectra recorded before and after irradiation (Figure 2B). An almost full conversion of the

secondary amine is also found (See Figure 1A, curve 3).

Figure 2. (A) RT-FTIR monitoring of the DGEBA/TETA (83/17 wt%) photopolyaddition reaction

(conversion vs. time) for (1) epoxide, (2) (primary amine (NH2) and (3) primary and secondary amine

(NH) using the initiating system IR 813/Iod (0.1/2 wt%) (B) RT-FTIR spectra between 4500 and 6500

cm−1 recorded before polymerization and after polymerization. The peaks representative of epoxide and

NH2 functions are indicated, under exposure to LD@785 nm (I= 2.5 W/cm²), thickness = 1.4 mm, under

air and at RT. The irradiation starts at t = 10 s.

Optimization of the Iodonium salt content:

As the photoinitiating system IR 813/Iod (0.1/2 wt%) was very reactive for the

photoactivation of the polyaddition reactions, impact of the Iod content has been checked. With

a low content of Iod (1 wt%) (Figure 3, curve 2), the photopolyaddition reaction is only slightly

delayed (+20 sec, curve 2 vs. curve 1), but the polymerization is still highly efficient compared

0 100 200 300 400 500

0

20

40

60

80

100

(3)

(2) (1)

Time (s)

Co

nvers

ion

(%

)

(A) (B)

4400 4600 4800 5000 5200

0

1

2

3

epoxide

NH2

O.D

.

Wavenumber (cm-1)

Before irradiation

After irradiation

10

to the profile obtained without irradiation (Figure 3, curve 2 vs. curve 3). Full final epoxy

conversions can be reached within a few seconds of irradiation: The Iod content can be thus

reduced while maintaining a good reactivity.

Figure 3. RT-FTIR monitoring of the DGEBA/TETA (83/17 wt%) photopolyaddition reaction

(conversion vs. time) for (1) IR 813/Iod (0.1/2 wt%), (2) IR 813/Iod (0.1/1 wt%), upon exposure to a

LD@785 nm (I = 2.5 W/cm²) and (3) without irradiation, thickness = 1.4 mm, under air and at RT. The

irradiation starts at t = 10 s.

Influence of the NIR dye and the irradiation device:

In this section, other NIR dyes/NIR sources combinations were investigated for the

DGEBA/TETA photopolyaddition reaction. IR 1064 is characterized by a maximal absorption

wavelength at 1064 nm (the absorption spectrum of IR 1064 is given in Supporting

Information). Absorptions of NIR dyes are typically intense and highly localized. Therefore, in

this context, IR 1064 (Scheme 2) showed a better efficiency upon exposure at 1064 nm than at

785 nm (Figure 4, curve 2 vs. curve 1) due to the weakness of its absorption at this wavelength.

In conclusion, different NIR light/NIR dyes combinations can be used “on demand” to

photoactivate the Epoxy/Amine polyaddition.

0 50 100 150 200

0

20

40

60

80

100

(3)

(2)(1)

Ep

oxid

e c

on

vers

ion

(%

)

Time (sec)

11

Figure 4. RT-FTIR monitoring of the DGEBA/TETA (83/17 wt%) photopolyaddition reaction

(conversion vs. time) for the IR 1064/Iod (0.1/2 wt%) combination upon exposure to (1) LD@785 nm

(I = 0.9 W/cm²) and (2) LD@1064 nm (I = 1.6 W/cm²); thickness = 1.4 mm, under air and at RT. The

irradiation starts at t = 10 s.

3.2/ Versatility of the NIR photopolyaddition reaction: Effect of the amine

structure

In order to achieve the desired final properties, a careful selection of the epoxy

monomer, the proper hardener, and the right epoxy/hardener curing proportions in a formulation

process must be made. Aminated compounds are among the earliest and the most broadly used

epoxy curing agents [8]. Figure 5 shows different combinations for the Epoxy 1/Amine reaction,

where different amines can be excellent candidates for the photopolyaddition reaction. It has to

be noticed that the set of amines examined for this study has been specifically selected due to

their widespread use in industrial applications, evidencing the approach reported in this work

can be easily transferred in industry. For all the investigated amines, a strong and obvious

photoactivation of this reaction was observed, reducing the polymerization time (from a few

hours for a classical polyaddition to a few seconds or a few minutes for the photoactivated

approach, Table 1) (Figure 5, curves 1-7 upon light irradiation vs. curve 8 without NIR light).

All the amines proposed in this study are primary amines characterized by two active

hydrogens that are each capable to react with an epoxy group. The efficiency of primary amines

followed this order: m-XDA ~ AEAE ~ TETA > EDAA > IPDA > DTA. Tack-free polymers

are obtained in all cases. In some studies, it has been reported that the reactivity of amines

depends on the chemical structures and decreased as the number of phenyl groups increased in

the chemical structure [19-20]. In other words, this can be explained by the low nucleophilicity

of the amino group in aromatic amines resulting from the presence of the electron withdrawing

phenyl groups. Furthermore, in comparison to the steric effects, electronic effects are noticeably

0 50 100 150 200 250 300 350 400

0

20

40

60

80

100

(2) (1)

Co

nv

ers

ion

(%

)

Time (s)

12

larger [21]. The NIR photoactivated approach is remarkably robust, extremely efficient and

highly tolerant to the substitution pattern for the primary amines.

Figure 5. RT-FTIR monitoring of the Epoxy 1/Amines photopolyaddition reaction (conversion vs. time)

for IR 813/Iod (0.1/2 wt%) using (1) m-XDA, (2) N-AEP, (3) DTA, (4) IPDA, (5) AEAE, (6) EDAA,

(7) TETA under exposure to LD@785 nm (I= 2.5 W/cm²) and (8) m-XDA without irradiation, thickness

= 1.4 mm, under air and at RT. The irradiation starts at t = 10 s.

Table 1. Comparison of the classical Epoxy/Amine polyaddition vs. the photopolyaddition approach

and quantitative estimations of polymerization times / reactivity.

Classical polyaddition NIR Photopolyaddition approach

Amine Temperature

(°C) Polymerizati

on time Reactivity

% weight Epoxy1/Amine

bPolymerization time

cReactivity

Aliphatic

DTA 100°C 30 min + (83/17 %wt) 2.7 min +

TETA 100°C 30 min + (83/17 %wt) 80 s ++

AEAE an.d. n.d. n.d. (75/25 %wt) 85 s ++

EDAA n.d. n.d. n.d. (88/12 %wt) 1.8 min ++

Aromatic m-XDA 60°C 1h + (75/25 %wt) 80 s ++

Cyclo -aliphatic

N-AEP 200°C 30 min + (75/25 %wt) 1.2 min ++

IPDA 80 to 150°C 4 h + (75/25 %wt) 2.2 min ++ a n.d. Not determined b Polymerization time: time required to reach a full conversion of epoxide functions c (+): GOOD reactivity, (++): EXCELLENT reactivity

The IR 813/Iod photoinitiating system was the most efficient one for the

photopolyaddition reaction of Epox 1/m-XDA upon exposure to the LD@785 nm (I = 2.5

W/cm2). In attempt to highlight the reactivity of this system, other oxidation agents have been

examined. This is a great advantage to have the possibility to use sulfonium or thianthrenium

salts (Scheme 2) which can be photosensitized in the presence of IR 813 and then lead to the

0 50 100 150 200 250 300 350 400

0

20

40

60

80

100

Ep

oxyd

e c

on

vers

ion

(%

)

Time (s)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

13

formation of acid or cationic species to activate the photopolyaddition reaction. Sulfonium salts

are cheaper than the iodonium ones [22]. Figure 6 clearly shows the high efficiency of

sulfoniums (Figure 6, curves 2-4 vs. curve 5 without irradiation) which is relatively similar to

that of iodonium (Figure 6, curve 1). The slight difference in polymerization time can be

ascribed to the most favorable reduction potential (Ered) of Iod (higher than sulfonium and TH).

In this case, it is easier to sensitize Iod vs. sulfonium or TH: Ered (Iod) = - 0.2 V > Ered (TH) = -

1.4 V > Ered (Sulfonium salt) = - 2 V [22]. However, in all cases, the photopolyaddition reaction

is still possible and tack-free polymers can be obtained.

Figure 6. RT-FTIR monitoring of the Epox 1/m-XDA (75/25 wt%) photopolyaddition reaction

(conversion vs. time) using IR 813/oxidation agent (0.1/2 wt%) with: (1) Iod, (2) TH, (3) Sulfonium 1,

(4) Sulfonium 2 under exposure to a LD@785 nm (I = 2.5 W/cm²) and (5) Iod without irradiation,

thickness = 1.4 mm, under air and at RT. The irradiation starts at t = 10 s.

3.3/ Effect of the epoxy structures on the NIR photopolyaddition efficiency

Different structures of epoxy monomers are proposed for the NIR photopolyaddition

reaction in the presence of IR 813/Iod. Table 2 summarizes the reaction times necessary to

reach a full epoxy function conversion for different Epoxy/Amine combinations. Remarkably,

these systems are always activable “on demand” and triggered by light (from a few

minutes/seconds for a full polymerization upon NIR light vs. > 3h without light @RT). This

suggests again the broad versatility of the proposed NIR systems to photoactivate multiple

Epoxy/Amine mixtures.

0 100 200 300 400 500 600 700 800

0

20

40

60

80

100

Time (sec)

Ep

ox

ide

co

nv

ers

ion

(%

)

(1)

(2)

(3)

(4)

(5)

14

Table 2. Quantitative estimations of polymerization times for Epoxy/Amine (~78/22 wt%)

photopolyaddition reaction using IR 813/Iod (0.1/2 wt%) upon exposure to a LD@785 nm (I = 2.5

W/cm²), thickness = 1.4 mm, under air and at RT. The irradiation starts at t = 10 s.

Epox 2 Epox 3 Epox 4

m-XDA n.d. 30s 80s

N-AEP 100s 65s 86s

AEAE n.d. n.d. 50s

n.d. Not determined Polymerization time: time required to reach a full FC of epoxide function

3.4/ Access to adhesives using NIR Epoxy/Amine reaction

Good adhesion properties are difficult to obtain by the traditional UV photochemical

approaches due to the very low penetration of light into the substrates. NIR light can offer a

good alternative to solve this issue. Partially opaque epoxy substrates have been used to

investigate the adhesion properties using the IR 813/Iod NIR initiating system in a

DGEBA/TETA blend upon exposure to a high-power LED (LED@810 nm, I = 12 W/cm2) as

shown in Scheme 6. A successful bonding of two epoxy substrates has been observed upon NIR

light irradiation; this bonding is not possible using a near UV light. As the method of bonding

has a significant effect, the frontal polymerization process has been checked (See Scheme 6).

This latter has been proposed by Zhang et al. [23-24]. In the polymerizable media, the light is

switched on for a few minutes of irradiation. Reactive species are generated which lead to an

exothermic polymerization of the area close to the surface. Then, a frontal propagation

associated with a Dark Polymerization process “DPP” can be observed when the light is

switched off, leading to a polymerization in depth.

15

Scheme 6. Representative procedure used in this study for adhesive application.

Using the frontal polymerization, the substrates have been successfully bonded after

only a few minutes of irradiation. The mechanical properties determined by the dynanometer

experiments also showed that the resulting materials have a good resistance (High Shear stress,

See Table 3). Alcohol additives (See Scheme 7) have been added in order to obtained good

adhesion properties with the substrates.

Scheme 5. Chemical structures of the different alcohol additives used in this study.

Table 3. Representative results obtained by dynanometer experiments for DGEBA/TETA (83/17

wt%) photopolyaddition reaction using the IR 813/Iod/alcohol (0.1/2/2 wt%) three-component system. Time for bonding Shear stress (MPa)

IR 813 / Iod /BPC (0.1/2/2 wt%)

1 min 30 sec (No need for DPP, totally polymerized)

13 MPa

Distance between the LED

and the sample = 6 cm

Bonding method: Frontal polymerization

10mm

(1)

Deposition of the

second substrate

(3)

Irradiate the sample from the top (FP)

for few minutes → Dark polymerization

process “DPP”

Organic mixture

(2)

Epoxy substrates

16

IR 813 / Iod /CARET

(0.1/2/2 wt%) 1 min then 8min DPP 6.3 MPa

3.5/ Epoxy/Anhydride photopolyaddition reaction

Anhydrides are the second largest class of hardeners for epoxides [25-26].

Epoxy/Anhydride systems usually exhibit a rather low viscosity, long pot life, low

exothermicity of reaction, and little shrinkage when polymerized at elevated temperatures. Due

to the low exothermicity of polyaddition reactions, these Epoxy/Anhydride combinations can

be used in large scale applications [8]. In most of the case, this reaction can be catalyzed by an

imidazole derivative used as an accelerator [14,27]. This is in full agreement with the results

obtained in this paper for the photopolyaddition reactions of epoxy/anhydride in the presence

of an accelerator (Imidazole). Especially, the photoactivation using the NIR approach of

Epoxy/Anhydride system is reported for the first time in this work.

The IR 813/Iod photoinitiating system has also been used here. Remarkably, this system

showed a complete polyaddition reaction in the presence of the accelerator: a full epoxide

conversion was obtained within a few seconds of irradiation (Figure 7A, curve 1) which is also

highlighted by the decrease of the characteristic epoxide peak (Figure 7B). Obviously, no

polymerization occurs without accelerator (Figure 7A, curve 2), as well as no polymerization

takes place without light (Figure 7A, curve 3). Therefore, NIR light can clearly triggered on-

demand the polymerization process that is extremely slow without light (not polymerized at RT

after 3 hours).

Figure 7. (A) RT-FTIR monitoring of the EPOX/Anhydride photopolyaddition reaction (conversion vs.

time) using IR 813/Iod (0.1/2 wt%) in the presence of (1) Accelerator, (2) without Accelerator under

exposure to LD@785 nm (I = 2.5 W/cm²) and (3) without light, (B) RT-FTIR spectra recorded before

and after polymerization for the representative peak of epoxide used to calculate the

(A) (B)

4300 4400 4500 4600 4700 4800

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Epoxide

O.D

.

Wavenumber (cm-1)

Before irradiation

After irradiation

0 40 80 120 160 200 240 280 320 360

0

20

40

60

80

100

(3)

(2)

(1)

Ep

oxid

e C

on

vers

ion

(%

)

Time (sec)

17

photopolymerization profile, thickness = 1.4 mm, under air and at RT, Epoxy/Anhydride/Accelerator

(100/9/2 wt%). The irradiation starts at t = 10 s.

3.6/ Access to glass fibers composites using the NIR Epoxy/Anhydride reaction

Composite materials present many appealing advantages: high strength, relatively low

weight and good corrosion resistance. As the IR 813/Iod (0.1/2 wt%) combination has shown a

high efficiency in Epoxy/Anhydride photopolyaddition reaction, the following section focuses

on the use of this system in impregnated resins which are well-known for their improved

mechanical properties. Glass fibers (one and two layers) are impregnated by the organic resin

(50%/50% of organic resin/glass fibers) and irradiated using a high-power LED (LED@810

nm, I = 12 W/cm2, Scheme 6). As shown in the Table 4, an effective polymerization was

observed, where a few seconds to 1 minute of irradiation are enough to reach tack-free surfaces

(for the surface and for the bottom of the sample using one or two layers of glass fibers).

Without NIR light, no curing is observed even after 3 hours, demonstrating the huge effect of

the NIR irradiation.

Table 4. Curing time to reach fully cured composites (impregnated glass fibers with the organic resin,

IR 813/Iod (0.1/2 wt%) in Epoxy/Anhydride/Accelerator (100/9/2 wt%) using a LED@810 nm, I = 12

W/cm2).

Experimental Conditions Irradiation time to reach fully cured composites

(surface and bottom are tack-free)

One layer of glass fibers (GF)

Thickness = 0.66 mm

1 min

Two layers of glass fibers (GF)

Thickness = 1.9 mm 1 min 30 sec

4/ Conclusion:

New NIR photoactivation processes for both Epoxy/Amine and Epoxy/Anhydride

reactions are proposed. Outstandingly, for the first time here, we propose to use the photoacidic

catalysis in order to enhance the reaction kinetics (seconds/minutes vs. hours) upon safe

irradiation conditions. The activation of the epoxy ring by the strong acid or cationic species

released allows the development of on-demand systems. The main advantages of these reactions

18

are the versatility (different NIR dyes, epoxy monomers, hardeners, irradiation wavelengths can

be used) and their high reactivity i.e. the polymerization occurs in the seconds-minutes time

scale vs. hours without light. To evidence the crucial interest of the NIR photoactivation concept

developed in this work, this latter has been tested with the most common formulations under

use in industry, evidencing the breakthrough herein achieved for polyaddition reactions. Other

NIR photoactivated polymerization processes are currently under development in our group.

Acknowledgements:

The authors would like to thank the SATT Conectus for the funding of this work and the

stimulated discussions (Dr. Lucie Schiavo, Dr. Jérémie Fournier and Miss Gwendoline

Lejeune).

19

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21

TOC

0.0 0.5 1.0 1.5 2.0 2.5

0

20

40

60

80

100

(2)

(1)

Time (h)

Ep

oxid

e c

on

ve

rsio

n (

%)

R1R2NH

Amine O

R3Epoxy Anhydride

Adhesives

Composites

22

Supporting information

Figure 1. UV-visible spectrum of IR 1064 in acetonitrile (ACN)

600 800 1000

0,0

0,5

1,0 IR 1064

O.D

.

(nm)