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Curing mechanism of alkoxysilyl-functionalized epoxy(II): Effect of catalyst on the epoxy chemistry
Accepted Manuscript Curing mechanism of alkoxysilyl-functionalized epoxy(II): Effect of catalyst on the epoxy chemistry Hyunaee Chun, Yun-Ju Kim, Sook-Yeon Park, Su-Jin Park PII:
S0032-3861(19)30319-2
DOI:
https://doi.org/10.1016/j.polymer.2019.04.002
Reference:
JPOL 21385
To appear in:
Polymer
Received Date: 30 January 2019 Revised Date:
28 March 2019
Accepted Date: 1 April 2019
Please cite this article as: Chun H, Kim Y-J, Park S-Y, Park S-J, Curing mechanism of alkoxysilylfunctionalized epoxy(II): Effect of catalyst on the epoxy chemistry, Polymer (2019), doi: https:// doi.org/10.1016/j.polymer.2019.04.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
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Curing mechanism of Alkoxysilyl-functionalized epoxy [Imidazole system]
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EP
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[TPP system]
ACCEPTED MANUSCRIPT
Curing mechanism of alkoxysilyl-functionalized epoxy(II): Effect
Hyunaee Chun*, Yun-Ju Kim, Sook-Yeon Park, Su-Jin Park
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of catalyst on the epoxy chemistry.
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Gyeonggi Regional Division, Korea Institute of Industrial Technology, Ansan 15588, South Korea
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ABSTRACT
Alkoxysilyl-functionalized epoxy composites were recently reported to offer the ultra-low thermal expansion properties as low as 3 ~ 4 ppm/℃ at 85wt% of silica, not normally achievable with conventional epoxy systems.
Understanding the curing mechanism of
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alkoxysilyl-functionalized epoxies is necessary for future applications, especially in semiconductor packaging.
For this purpose, the distinctive chemistry of the alkoxysilyl-
functionalized epoxy using imidazole and triphenylphosphine catalysts was studied.
The
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chemistry of alkoxysilyl-functionalized epoxies was investigated using model compounds
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such as monofunctional epoxy, phenol and monofunctional alkoxysilanes. reaction and reaction products were determined by NMR spectrometry.
The extent of
It was observed that
the alkoxysilyl-functionalized epoxy system shows the unique curing characteristics, different from both (1) ordinary epoxy system and (2) hydrolysable alkoxysilanes.
The unique curing
characteristics for each catalyst system are explained by suggested mechanisms. 1. Introduction *
Corresponding author. E-mail address: [email protected] (H. Chun). 1
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The thermal expansion characteristics of a material significantly influences the dimensional stability of the devices made using them [1-3].
Especially, in the
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semiconductor packaging, high coefficients of thermal expansion (CTEs) of epoxy composites frequently cause the serious problems such as cracking, warpage, peelings of the packages [2-7].
In achieving reliable semiconductor packaging, the processing low-CTE
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epoxy composites is necessary [8-11].
The preparation of the high filled silica composites is most frequently used to decrease the Since the thermal expansion of the epoxy composites
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CTEs of epoxy materials [8-13].
depend on the filler loading, many researches have focused on how to increase the fill ratio of the inorganic components as much as possible.
At 80 ~ 85 wt % silica added, conventional
epoxy systems show CTE 1 (CTE at TTg) of
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35~45 ppm/℃, which are still too high to solve the problems associated with the high CTE of epoxy materials [14, 15].
In achieving the reliable semiconductor packaging, the CTE-
mismatch between the epoxy materials and silicon chip need to be solved.
However, since
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the CTE (2.8 ppm/℃) of silicon chip is much lower than that of epoxy material, the problems
[2].
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associated with CTE-mismatch cannot be avoided if the conventional epoxy systems are used The CTE values of the conventional epoxy composites are still too high for the reliable
semiconductor packaging.
Very recently, our group attempted to solve the high thermal expansion of epoxy composites using a different approach [14, 15].
Rather than focusing on how to increase
filling ratios, new epoxy resins, i.e., the alkoxysilyl-functionalized epoxy resins were designed. The new resins were synthesized by the (1) hydrosilylation of the diallyl-bisphenol A epoxy or (2) reaction of the isocyanato-silane coupling agent with the hydroxylated novolac epoxy, 2
ACCEPTED MANUSCRIPT as shown in Fig. 1.
These alkoxysilyl-functionalized epoxy systems in Fig. 1 showed the
excellent thermal expansion properties, which cannot be achieved with the ordinary commercial epoxy systems.
That is, Tthe composite of silica 85 wt% prepared with the The CTE1 (T
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alkoxysilyl-bisphenol A epoxy (Fig. 1(a)) showed ultra-low CTEs:
CTE 2 (T>Tg) were 3.2 ppm/℃ and 6.0 ppm/℃, respectively, one of the lowest thermal expansion value reported to date [14].
From these studies, we were motivated to
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high thermal expansion of the epoxy composite systems.
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demonstrate that our alkoxysilyl-functionalized epoxy can be strong candidates to solve the
(b)
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(a)
Fig. 1. Chemical structures of (a) alkoxysilyl-functionalized bisphenol A epoxy (Si(OEt)3-
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BPA) and (b) alkoxysilyl-functionalized ortho-cresol novolac epoxy (Si(OEt)3-EOCN).
Alkoxysilane compounds have been used extensively as silane coupling agents, sol-gel precursors, the formation of organic-inorganic hybrid and so on [16-18]. As shown in Fig. 2(a), the typical reaction chemistry of alkoxysilane compounds is hydrolysis and condensation [17, 18]: The trialkoxysilane group is easily hydrolyzed to form silanol and subsequently reacts with the hydroxyl groups on filler surfaces or other silanols. 3
The
ACCEPTED MANUSCRIPT application of alkoxysilane compounds in various fields is based on this chemistry.
In our
previous studies, however, we found that alkoxysilyl group within epoxy-curing matrix
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showed different chemistry from that of typical hydrolysable alkoxysilanes [14, 15].
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(a)
(b)
Fig. 2. Chemistry of alkoxysily group in (a) typical hydrolysable alkoxysilane system and (b) alkoxysilyl-functionalized epoxy system cured by phenol/imidazole
4
ACCEPTED MANUSCRIPT The alkoxysilyl group was observed to participate in epoxy-phenol curing reactions, as shown in Fig. 2(b).
During epoxy curing reactions catalyzed by imidazole, we obtained
structures that formed by reactions of the alkoxysilyl unit.
That is, in addition to reaction
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product of epoxy and phenol, the structures formed by the reactions of (1) alkoxysilyl group with epoxide and (2) alkoxysilyl group with phenol were also observed.
This observation
indicates that the curing mechanism for alkoxysilyl-functionalized epoxies is no longer
It is quite interesting to understand the
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similar with that of the conventional epoxy system.
distinct curing chemistry of the newly-designed epoxy system.
However, we still do not
of the epoxy curing reaction is.
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know how the alkoxysilyl-functionalized epoxy system works and what the controlling factor
In this paper, therefore, the curing mechanism of alkoxysily-functionalized epoxy systems The phenol curing systems catalyzed by imidazole and
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was investigated more extensively.
triphenyl phosphine (TPP) were chosen in this study, since they are the most frequently used hardener/catalyst systems for the semiconductor packaging encapsulation [3, 19-21].
The
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effects of catalyst on the curing mechanisms were studied for the first time.
2. Experimental
2.1. Materials Glycidylphenylether, phenol and bisphenol A diglycidyl ether (DGEBA) with a molecular weight of 340 were purchased from Sigma-Aldrich Chemical Co., Inc. and used as received. 2-Phenyl imidazole (Shikoku Chem. Co., Curezol® 2PZ) and triphenylphosphine (Sigma5
ACCEPTED MANUSCRIPT Aldrich Chemical Co., Inc., TPP) were used as catalysts for the epoxy reactions. Triethoxy(3-phenoxypropyl)silane, trimethoxy(3-phenoxypropyl) silane and triethoxysilylfunctionalized bisphenol A epoxy (Si(OEt)3-BPA) were synthesized, using the method Phenol novolac resin (Meiwa Plastic Ind. Ltd, HF-
1M®, OH equivalent of 107 g/eq) was used as a curing agent.
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described in our previous study [14, 15].
Acrylic polymer (Nagasei
ChemteX Corp., SG-80H®) was added to assist the formation of composite film.
anhydrous grade and were purchased from Aldrich Chemical Co.
The chemical structures
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of the compounds used in this study are given in Fig. 3.
All solvents used were
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(Denka fused silica, 20 µm-cut) was used as a low-CTE filler.
The silica
Fig. 3. Chemical structures of (a) 2-phenyl imidazole (2PZ), (b) triphenylphosphine (TPP), (c) glycidyl phenyl ether, (d) trialkoxy(3-phenoxypropyl)silane, (e) diglycidyl ether of bisphenol A (DGEBA) and (f) phenol novolac.
6
ACCEPTED MANUSCRIPT 2.2. Model reactions for mechanism study The monofunctional epoxy, phenol, monofunctional alkoxysilyl compound were used as model compounds for the ‘epoxy’, ‘hardener’ and ‘alkoxysilyl-moiety’ of alkoxysilyl-
(TPP) were used as catalysts.
2-Phenyl imidazole (2PZ) and triphenylphosphine
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functionalized epoxy resin, respectively.
The model reactions were carried out at 100℃ under the
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conditions described in Tables 1 and 2.
Table 1 a
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The model reaction conditions for the conventional epoxy curing.
All reactions were carried
out at 100℃ under the 1 phr of imidazole or TPP catalyst. Catalystb
Reaction code Ref-1
2PZ
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Ref-2 Ref-3 Ref-4 Ref-5
TPP
1.0
1.0
2PZ: 2-Phenyl imidazole, TPP: Triphenylphosphine.
c
Glycidyl phenyl ether
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0.6 1.0
0.6
b
7
Phenol (Eq)
2.5
Ref-6 phr: part per hundred epoxy resin.
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a
Epoxyc (Eq)
1.0 2.5
ACCEPTED MANUSCRIPT Table 2 The model reaction conditions for the alkoxysilyl-functionalized epoxy curing.
All
a
b
Catalyst
Epoxyc (Eq)
Phenol (Eq)
Ethoxysilaned (Eq)
Methoxysilanee (Eq)
0.6
1.0
-
1.0
1.0
1.0
E-P-Si-1 E-P-Si-2
2PZ
1.0
E-P-Si-3
1.0 TPP
1.0
E-P-Si-5 P-Si-1
TPP
-
-
1.0
1.0
-
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E-P-Si-4
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Reaction code
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reactions were carried out at 100℃ under the 1 phr of imidazole or TPP catalyst.
1.0
-
1.0
1.0
-
1.0
phr : part per hundred epoxy resin.
b
2PZ: 2-Phenyl imidazole, TPP: Triphenylphosphine.
c
Glycidyl phenyl ether.
d
Triethoxy(3-phenoxypropyl)silane.
e
Trimethoxy(3-phenoxypropyl)silane.
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a
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The chemistry of the alkoxysilyl-functionalized epoxies was investigated, as described in our previous studies [14, 15].
The extent of reaction was characterized by the variation of
the characteristic NMR peaks of model compounds.
The epoxide peak at δ=2.83 ppm and
the aromatic hydrogen peaks at δ= 6.77 ppm and at δ=7.17 ppm are used to characterize the chemistry of epoxy and phenol, respectively.
With the progress of the reaction, the
epoxide peak finally disappears as a result of the epoxide-opening reaction and the phenol peaks gradually merge into the neighboring aromatic peaks at δ= 6.92 ppm and δ=7.27 ppm. The chemical shifts of trialkoxysilane, more specifically the peak of α-hydrogen (-CH2-CH28
ACCEPTED MANUSCRIPT CH2-Si(OEt)3) were δ = 0.68 ppm and δ = 0.73 ppm for triethoxysilane and trimethoxysilane, respectively.
When the trialkoxysilyl compound reacts with epoxide or phenol, the intensity
of α-hydrogen peak (-CH2-CH2-CH2-Si(OEt)3) decreases with reaction time and at the same For the case
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time, two new peaks (more exactly, shifted α-hydrogen peaks) appear [14, 15].
of triethoxysilane, new peaks appear at δ=0.79 ppm and δ=0.89 ppm, which are considered to relate with the reaction of triethoxysilane with the epoxide and the phenol, respectively.
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Thus, in this study, the extent of reaction of the trialkoxysilyl component was monitored by the shift (decrease) in the α-hydrogen peak (-CH2-CH2-CH2-Si(OR)3), as reported previously
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[14, 15].
2.3. Preparation of silica composite film
Into the MEK, the predetermined amount of epoxy, acrylic polymer (20 wt% in MEK) The MEK solution (solid content of 70 wt%) was mixed for 20 min
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and silica were added.
using a planetary centrifugal mixer (Thinky Co., Thinky Mixer ARE-310®).
Then the
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phenol novolac hardener was added to the mixture and then mixed further for 10min. Finally, the 2-phenyl imidazole or TPP was dispersed in sonication bath for 10 min.
The
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homogeneously mixed solution was casted onto the copper film using doctor blade and then the solvent was removed in the convection oven at 60℃ for 30 min.
The dried composite
sheet was cut into the 4 mm wide strips and then cured in the oven at 120℃ for 2h, 180℃ for 2h and 230℃ for 2h.
Cu layer with etching.
The thickness of the cured epoxy film was 70 µm, after removing the The formulations of composites with silica 80 wt% were given in
Table 3.
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ACCEPTED MANUSCRIPT Table 3 Formulations of epoxy composites used in this study.
epoxy system (Si(OEt)3-BPA)
(BPA)
TPP (g)
Si(OEt)3-BPA epoxy
1.00
1.00
BPA epoxy(DGEBA)
-
-
Phenol novolac
0.29
0.29
2-phenyl imidazole
0.01
-
TPP
-
Acrylic polymer
0.25
Silica (80 wt%)
6.19
Imidazole (g) -
1.00
0.57 0.01
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Imidazole (g)
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Reference system
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component
Alkoxysilyl-functionalized BPA
-
0.25
0.25
6.18
7.31
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0.01
2.4. Measurement of thermal expansion property
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The coefficient of thermal expansion and glass transition temperature were measured
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using a thermomechanical analyzer (TMA Q400, TA Instruments). specimen was 16 mm long, 4 mm wide and 0.1 mm thick. measurement, 0.05N of the applied tension force was used.
Tension mode was used.
10
For
The temperature was varied
from room temperature to 270℃ at a heating rate of 10℃/min.
3. Results and Discussion
The dimension of
ACCEPTED MANUSCRIPT 3.1. Chemistry of the conventional epoxy/phenol curing system for imidazole or triphenylphosphine The model reactions of monofunctional epoxy and the phenol were carried out using When 1 Eq of epoxide reacts
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imidazole and TPP catalysts using Table 1 reaction conditions.
with 1 Eq of phenol (reaction code in Table 1: Ref-2 & Ref-5), both catalyst systems exhibit almost the same curing characteristics.
Plotting the extent of reaction for epoxide group vs All epoxide groups disappear at t >
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time, both catalyst systems show similar curing rates: 3h at 100℃, by the reaction with phenol, Fig. 4(a).
When the reacted epoxy concentration is
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plotted against the reacted phenol concentration, both systems show the virtually same stoichiometry (1:1 for epoxide and phenol), as shown in Fig. 4(b).
1.2
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extent of reaction(Phenol, Eq)
1.0
0.8
EP
0.6
0.4
TPP 2PZ
0.2
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extent of reaction(Epoxide, Eq)
1.2
1
2
3
0.8
0.6
0.4
0.2
0.0
0.0
0
TPP 2PZ
1.0
4
0.0
5
0.2
0.4
0.6
0.8
1.0
1.2
extent of reaction(Epoxide, Eq)
time (hr)
(a)
(b)
Fig. 4. Curing characteristics of the conventional epoxy/phenol systems under the imidazole and TPP-catalysts. The concentration (Eq) of the reacted epoxide group (a) with time and (b) as a function of reacted phenol (Eq). The reactions of the monofunctional epoxy(1Eq) and phenol (1Eq) were carried out at 100℃.
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ACCEPTED MANUSCRIPT Although the curing characteristics of two catalyst systems are almost indistinguishable as above, it should be mentioned that their curing mechanisms are quite different [19, 20, 22-25].
In the TPP-catalyzed system, a betaine intermediate forms by attack of TPP on the
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(a)
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epoxide group and is protonated by phenol to produce the phenolate, Fig. 5(a) [25].
(b)
Fig. 5. Curing mechanism of conventional epoxy/phenol system catalyzed by (a) triphenylphosphine (TPP) and (b) the imidazole (2PZ) [19, 20, 22-25]. 12
ACCEPTED MANUSCRIPT Since the curing reaction proceeds via reaction of the phenolate with ring-opened epoxide group, it inevitably results in the epoxy reacting stoichiometrically with the phenol.
In
contrast, in the imidazole-catalyzed system, two epoxide groups react with imidazole and The highly-reactive sec-
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produce the sec-alkoxide ion, as shown in Fig. 5(b) [22-24].
alkoxide ion subsequently reacts with phenol and produces another reaction intermediate, i.e., phenolate.
Hence, the epoxy curing reaction can proceed via two reaction pathways.
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Path (i) is via the reaction of phenolate with the epoxide and the other path (ii) is via the reaction of the sec-alkoxide ion with the epoxy (i.e., self-polymerization).
However, the
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stoichiometric relationship observed in Fig. 4(b) indicates that the epoxy curing reaction proceeds exclusively via the phenolate when the same concentrations of epoxy and phenol are used.
If some epoxides carry out the reaction via sec-alkoxide ion path (self-
polymerization), the 1:1 stoichiometry between epoxy and phenol cannot be observed.
If
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so, the epoxide group must be consumed more than phenol, differently from our observation. The 1:1 stoichiometry between epoxy and phenol for both catalyst systems is observed to be maintained even at higher concentrations of phenol than epoxide, as found in Ref-3 and However, when the concentration of phenol is less than
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Ref-6 systems described in Table 1.
the concentration of epoxide (Ref-1 and Ref-4 in Table 1), both catalyst systems show For the TPP-catalyzed system, when 0.6 Eq of phenol is used
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different stoichiometries.
(reaction code: Ref-4 in Table 1), the epoxy is not consumed completely. epoxide remains unreacted.
About 0.4 Eq of
In contrast, in the imidazole-catalyzed system, residual epoxy
was not detected, even when the less than 1 Eq of phenol is used (reaction code: Ref-1 in Table 1).
All epoxy groups react, despite of the lack of the phenol.
Fig. 6 shows that for
the system of Ref-1, the 1:1 stoichiometric ratio is maintained under the condition where the phenol is still available. react.
But, after the all phenol are consumed, the epoxide continues to
This observation suggests that the imidazole-catalyzed epoxy reaction proceeds 13
ACCEPTED MANUSCRIPT through two different mechanisms depending on the phenol concentration.
To sum up, the
epoxy reaction proceeds via phenolate-epoxide reaction mechanism exclusively while the phenol is available and, after the phenol is used up, the residual epoxide groups keep on
1.0
0.8
0.6
0.4 2PZ_Phenol 0.6Eq 2PZ_Phenol 1.0Eq 0.2
0.8
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1.0
2PZ_Phenol 0.6Eq 2PZ_Phenol 1.0Eq
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1.2
extent of reaction(Phenol, Eq)
1.2
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extent of reaction(Epoxide, Eq)
reaction via a self-polymerization route, i.e., sec-alkoxide ion reaction with epoxide.
0.6
0.4
0.2
0.0
0.0
1
2
3
4
5
6
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0
EP
time (hr)
0.2
0.4
0.6
0.8
1.0
extent of reaction(Epoxide, Eq)
(b)
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(a)
0.0
Fig. 6. Curing characteristics of the conventional epoxy/phenol systems. The concentration (Eq) of the reacted epoxide group (a) with time and (b) as a function of reacted phenol (Eq). The monofunctional epoxy (1Eq) reacts with phenol (1Eq, 0.6Eq) at 100℃under the imidazole (2PZ).
14
1.2
ACCEPTED MANUSCRIPT Before going to the following section, we’d like to mention the stability of TPP catalyst during epoxy reaction. We observed experimentally that the TPP loses its catalytic activity by transforming to the triphenylphosphine oxide (O=PPh3), when the phenol is no longer As shown in Fig. 7(a), when 1 Eq of phenol is
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available, as reported in the literature [25].
used, as in the system of Ref-5, the TPP peaks gradually decrease and finally disappear at t > Simultaneously new peaks corresponding to triphenylphosphine oxide (O=PPh3)
gradually increase with time.
At lower concentrations (0.6 Eq) of phenol, the TPP decay
time becomes shorter (tdecay < 2h).
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3 hours.
However, when the excess phenol is used, as in system Thus, throughout
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Ref-6, significant amounts of TPP remain even after 24 hours, Fig.7(b).
this study, we carefully took the experimental results of the TPP system by considering the
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catalyst stability.
(a)
(b)
Fig. 7. The variation of TPP concentration with reaction time during epoxy/phenol reaction. The monofunctional epoxy (1Eq) reacts with phenol at 100℃ and the phenol concentrations are (a) 1.0 Eq and (b) 2.5 Eq.
The reaction codes are Ref-5 & Ref-6 in Table 1. 15
ACCEPTED MANUSCRIPT 3.2. Chemistry of alkoxysilyl-functionalized epoxy during phenol curing (1): The alkoxysilyl groups react exclusively with phenol in the TPP system, whereas it prefers to react with the
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epoxy in the imidazole system.
The reactions of the monofunctional epoxy (1 Eq), the phenol (1 Eq) and triethoxysilane (1 Eq) were carried out as model reactions.
The reaction conditions employed are recorded The NMR results of imidazole and TPP-
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in Table 2 (reaction code: E-P-Si-2 & E-P-Si-4).
catalyzed systems are compared in Fig. 8.
+
H2 R C Si(OR)
R
Si C O H2
Both catalyst systems show that the α-hydrogen
O-
+
H2 R C Si(OR)
R
C H2
Si O
O
peak at 0.79ppm
peak at 0.89ppm
Fig. 8. NMR spectra for the reaction of monofunctional epoxy, phenol and triethoxysilyl compounds (blue box) under the triphenylphosphine (TPP) and imidazole (2PZ) catalyst at 16
ACCEPTED MANUSCRIPT 100℃ for (a) t = 0 h; (b) TPP, t = 3 h; (c) 2PZ, t=7h.
The reaction codes are E-P-Si-2 & E-
P-Si-4 in Table 2. peak of the alkoxysilyl compound at δ=0.68 ppm decreases with time, arising from participation of the alkoxysilane in the epoxy reaction.
However, the newly-formed peaks The NMR spectrum of TPP-
catalyzed reaction shows only one peak at δ=0.89 ppm.
For the imidazole-catalyzed system,
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of both systems are different, depending on the catalyst.
two newly-formed peaks are observed at δ=0.79 ppm and δ=0.89 ppm.
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former peak shows the higher.
Of the two, the
in Fig. 5.
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The above observations cannot be explained by conventional epoxy mechanisms, given The modified mechanism for the alkoxysilyl-functionalized epoxy only should be
suggested in order to include the experimental observations.
That is, for TPP-catalyzed
epoxy-phenol reaction, the active reaction intermediate is phenolate only.
It is highly
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probable that the alkoxysily group also participates in the epoxy reaction via the phenolate. Thus, it is not difficult to anticipate one shifted peak of α-hydrogen (-CH2-CH2-CH2-Si(OR)3, δ=0.89 ppm) from the reaction of alkoxysilane and phenolate.
In contrast, for the
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imidazole-catalyzed reaction, two active species, i.e., phenolate and the sec-alkoxide ion are likely involved in alkoxysilyl group reaction.
Since both ions can react with the alkoxysilyl
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group, two newly-formed peaks must appear at δ=0.79 ppm and δ=0.89 ppm.
Based on the
higher NMR intensity of peak at δ=0.79 ppm, the alkoxysilane seems to be consumed more favorably with the sec-alkoxide ion (i.e., epoxy) than phenolate (i.e., phenol), in accord with the reactivity difference between reaction intermediates.
By considering the above
observations, the mechanisms of the alkoxysilyl-functionalized epoxy under the both catalysts are suggested, as schematically drawn in Fig. 9.
17
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ACCEPTED MANUSCRIPT
AC C
EP
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(a)
(b)
Fig. 9. Modified curing mechanism for the alkoxysilyl-functionalized epoxy system under the (a) TPP and (b) imidazole catalysts.
18
ACCEPTED MANUSCRIPT 3.3. Chemistry of alkoxysilyl-functionalized epoxy during phenol curing (2): The stoichiometry of epoxy reaction is different from that of the conventional epoxy system and
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depends on the catalyst type
The stoichiometry of alkoxysilyl-functionalized epoxy for both catalyst systems were investigated using the model reaction systems (reaction code: E-P-Si-2 & E-P-Si-4).
In Fig.
systems.
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10, the extents of reaction for phenol and epoxy are plotted against time for both catalyst For the imidazole system, the epoxy is consumed more than phenol, whereas for
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the TPP system, the extent of reaction for epoxide is more or less similar with that of phenol.
(a)
(b)
Fig 10. The extent of reaction of epoxide and phenol for alkoxysilyl-functionalized epoxy with time under the (a) imidazole and (b) TPP-catalysts. E-P-Si-4 in Table 2. 19
The reaction codes are E-P-Si-2 &
ACCEPTED MANUSCRIPT The stoichiometry ([epoxide] vs [phenol]) of the alkoxysilyl-functionalized system must be different from that of the conventional epoxy, since the alkoxysilyl group participates in epoxy curing reaction, depending on the type of catalyst.
It reacts more favorably with the
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epoxide in the imidazole-catalyzed epoxy reaction, which probably result in the observation that the epoxide group is consumed more than the phenol, as observed in Fig. 10(a).
In
contrast, the alkoxysilyl compound exclusively reacts with the phenol in the TPP-catalyzed It is reasonable that the phenol may be consumed more in TPP system than in
imidazole system, as observed in Fig. 10(b).
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system.
Consequently, for the most of case, the
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stoichiometry between epoxy and phenol of the alkoxysilyl-functionalized epoxy cannot be 1: 1, differently from the conventional epoxy system
It is worthy noticing that the stoichiometry of the alkoxysilyl-functionalized epoxy system cannot be as simple as that of the ordinary epoxy system.
The stoichiometry is determined
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by the how much the alkoxysilyl group participates in the epoxy reaction, which in turn depends on the reaction catalyst, type of alkoxysilane, the molecular structure of epoxy and so on.
Hence, the prediction of the stoichiometry of the alkoxysilyl-functionalized epoxy At this point, the only thing we can assure is that the stoichiometric
EP
reaction is not simple.
ratio of epoxy to phenol may not be 1:1 for the most of alkoxysilyl-functionalized epoxy
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reactions.
3.4. Chemistry of alkoxysilyl-functionalized epoxy during phenol curing (3): The alkoxysilyl group shows the different chemistry, being compared with the that of the typical hydrolysable alkoxysilane compound.
20
ACCEPTED MANUSCRIPT The alkoxysilane reactivity of the alkoxysilyl-functionalized epoxy was investigated using the model reactions.
The triethoxysilane (OEt) and trimethoxy silane (OMe) were
chosen for this study, as given in Table 2 (reaction code: E-P-Si-2, E-P-Si-3, E-P-Si-4, E-PFig. 11(a) shows that, in imidazole-catalyzed system, the reactivity of
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Si-5, P-Si-1).
trimethoxysilane (OMe) is somewhat higher than that of triethoxysilane (OEt). speaking, both silanes show the rather similar reactivity.
But, roughly
During the reaction for 7 hr at
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100℃, the peak intensity of α-hydrogen (-CH2-Si-) is decreased to the 51 mol % of initial value for triethoxysilyl (OEt) compound, whereas the residual peak intensity of
Im-OEt (E-P-Si) Im-OMe (E-P-Si)
0.2
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0.0 0
2
4
6
extent of reaction (-CH2-Si-, Eq)
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0.4
TPP-OEt (E-P-Si) TPP-OMe (E-P-Si) TPP-OMe (P-Si, no Epoxy)
0.6
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extent of reaction (-CH2-Si-, Eq)
0.6
For the case of TPP system, the reactivity
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trimethoxysilyl (OMe) compound is 46 mol %.
0.4
0.2
0.0 8
0
1
2
3
4
time (hr)
time (hr)
(a)
(b)
Fig 11. The extent of reaction of alkoxysilyl-functionalized epoxy with time under the (a) imidazole and (b) TPP-catalysts. The substituents of alkoxysilane used were trimethoxy (OMe) and triethoxy (OEt).
The reaction codes are E-P-Si-2, E-P-Si-3, E-P-Si-4, E-P-Si-5,
and P-Si-1 in Table 2. 21
ACCEPTED MANUSCRIPT
difference between the substituents of alkoxysilane is apparently observed.
The extent of
reaction for trimethoxysilane (OMe) is observed to be much higher than that of However, in spite of the low reactivity of
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triethoxysilane (OEt), as shown in Fig. 11(b).
triethoxysilane (OEt) under the absence of the acid or base catalysts, its appreciable reactivity under the TPP catalyst is worthy of notice [17, 26].
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If we consider the well-known chemistry of the alkoxysilane, the similar reactivity of trimethoxy- and triethoxy-silane observed in the imidazole-catalyzed system is somewhat For the typical hydrolysable alkoxysilane reaction (i.e., sol-gel reaction and silane
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unusual.
coupling agent reaction), the trimethoxysilane (OMe) generally shows the much higher reactivity than the triethoxysilane (OEt) [26, 27].
Furthermore, the high reactivity of
alkoxysilyl compound observed under the TPP catalyst at 100 ℃ is also very unique.
If the
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reaction is carried out in the absence of epoxy just like the typical alkoxysilane reaction (reaction code: P-Si-1), even the trimethoxysilane (OMe) compound hardly reacts with phenol under the TPP catalyst, as shown in Fig. 11(b).
The results for both catalyst systems
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clearly show that the alkoxysilyl group in the alkoxysilyl-functionalized epoxy possesses the
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quite different chemistry from that of the typical hydrolysable alkoxysilane compound. For the case of the typical alkoxysilane compounds, alkoxysilyl group are mostly hydrolyzed to silanol before or during applications, as depicted schematically in Fig. 2(a) [25].
The subsequent process, i.e., the application of alkoxysilane compounds for surface
treatment, can be done via the highly reactive silanol.
As a result, the reaction of the
alkoxysilane compound depends on the reactivity toward the hydrolysis.
In contrast, when
we think about the reactivity of alkoxysilyl group in the alkoxysilyl-functionalized epoxy systems, the effect of epoxide group must be taken into account: In the presence of the epoxy, 22
ACCEPTED MANUSCRIPT each catalyst system has its own characteristic reaction intermediate, which finally determines the chemistry of alkoxysilyl group.
As suggested in Fig. 9, the key species for
imidazole-catalyzed epoxy reaction is the sec-alkoxide ion.
Due to the high reactivity of the Thus, in the
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sec-alkoxide ion, the reactivity of alkoxysilyl group must be less significant.
imidazole system, both systems show the similar reactivity regardless of the type of substituents.
In TPP-catalyzed system, the key reaction intermediate is the phenolate
Thus, the alkoxysilyl group in the
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formed via the interaction of TPP, epoxy and phenol.
presence of epoxy can show the significantly improved reactivity, being compared with the However, the phenolate is not reactive as much as the
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system without the epoxide group.
sec-alkoxide ion and thus the alkoxysilane reactivity of TPP system may depend on whether the substituent structure is methoxy or not, as observed in Fig. 11(b).
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3.5. Chemistry of alkoxysilyl-functionalized epoxy during phenol curing (4): After the phenol is used up for imidazole system, epoxy stops the further reaction, different from the
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conventional epoxy-phenol reaction.
The imidazole-catalyzed epoxy reaction was carried out using the lower concentration of
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phenol, as found in the system of E-P-Si-1.
When 0.6 Eq of phenol is used, the epoxy
reaction does not proceed further after the phenol is used up after 3h, as shown in Fig. 12. That is, for the alkoxysilyl-functionalized epoxy system catalyzed by imidazole, the epoxide group is not consumed completely if the phenol concentration is not sufficient, differently from the conventional epoxy curing reaction.
23
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1.0
0.8
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0.6
0.4
0.2
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extent of reaction(Phenol, Eq)
1.2
Im-Phenol 0.6Eq (No silyl) Im-Phenol 0.6Eq (with silyl)
0.0 0
2
4
6
8
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time (hr)
Fig 12. The extent of reaction of the epoxide group with time under the imidazole(2PZ) catalyst.
The phenol concentrations used are 0.6 Eq and 1.0 Eq.
The reaction codes are
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Ref-1 in Table 1 and E-P-Si-1 in Table 2.
In the conventional epoxy-phenol reaction, epoxy can keep the reaction by choosing another reaction pathway (i.e., self-polymerization) when the phenol is used up, as described in the section 3-1.
As a result, the epoxide group can be consumed completely.
However,
in the presence of the alkoxysilyl group, the additional route of self-polymerization seems to be no longer available.
In the early stage of the imidazole-catalyzed reaction, the key
reaction intermediate (sec-alkoxide ion) can be shared with phenol and alkoxysilyl group together.
It is speculated that after the phenol is used up, the sec-alkoxide ion can be still 24
ACCEPTED MANUSCRIPT taken by the residual alkoxysilyl group.
The reaction intermediate may not need to take
another self-polymerization path, as found in the ordinary epoxy reaction.
Consequently, it
is suggested that the residual epoxy probably does not progress the further reaction in the
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presence of the alkoxysilyl group, differently from the conventional epoxy curing.
Finally, we’d like to speculate the role of RO- released from siloxane.
Whenever the
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epoxy reaction intermediates (i.e., phenolate and sec-alkoxide ion) react with the alkoxysilane, We think that the released alkoxide ion (RO-)
the alkoxide ions are released from siloxane.
imidazole-catalyzed epoxy reaction.
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may show the similar reactivity to the sec-alkoxide ion, i.e., epoxy reaction intermediate in Just like the sec-alkoxide ion shown in Fig. 9(b), we
think that the released alkoxide ion can abstract the proton from phenol or react with the alkoxysilane.
The reactivity of the deprotonation is considered to be higher, due to the large However, we think
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pKa difference between phenol and alcohol (phenol≒10, Alcohol≒16).
that the reaction of alkoxide with epoxy is not likely to occur significantly, as found in
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alkoxysilyl-functionalized epoxy reaction catalyzed by imidazole.
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3.6. Thermal expansion property of silica composite In order to investigate the effect of catalyst on thermal expansion property, the composite films with silica 80wt% were prepared and the results are given in Fig. 13.
Irrespective of
the catalyst types, the silica composites of alkoxysilyl-functionalized epoxy (Si(OEt)3-BPA) systems show the excellent thermal expansion properties.
After the alkoxysilyl modification,
the thermal expansion properties of silica composites (Si(OEt)3-BPA) are significantly improved, being compared with that of non-functionalized BPA epoxy composite. catalyst systems give the similar CTE results. 25
Both
Within the measured temperature range of
ACCEPTED MANUSCRIPT 50℃~270℃, the CTEtotal of Si(OEt)3-BPA composite is 6~7 ppm/℃ for the both catalyst
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systems, which is about 20 ~23% of CTE total (=29.7 ppm/℃) of BPA composite.
140
BPA epoxy -Imidazole Si(OEt)3-BPA-Imidazole
120
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80 60 40 20 0 -20
100
150
200
250
300
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50
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Dimension Change(um)
Si(OEt)3-BPA-TPP 100
Temperature(℃)
Fig. 13. Thermal expansion properties of (1) conventional BPA composite (BPA) and (2)
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triethoxysilyl-functionalized BPA composite (Si(OEt)3-BPA).
Two kinds of reaction
catalysts, i.e., imidazole (2PZ) and triphenylphosphine (TPP), were used.
The 80 wt% of
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silica was filled for the preparation of composites.
The network structure of the cured epoxy system is definitely influenced by the catalyst type, as found in the mechanism study.
In spite of the fact, the similar thermal expansion
properties of both catalyst systems are observed as in Fig. 13.
This observation indicates
that the change of a network structure does not give the any noticeable effect for the CTE of alkoxysilyl-functionalized epoxy composite systems, which is in good accordance with the 26
ACCEPTED MANUSCRIPT previous work [14, 15].
By comparing with the thermal expansion properties of the
conventional epoxy system with the alkoxysilyl-functionalized epoxy system, we suggested that the excellent thermal expansion property of alkoxysilyl-functionalized composite results
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mainly from the improved interfacial attraction via the alkoxysilyl moiety with silica surface,
4.
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rather than the network structure change associated with the alkoxysilyl-functionalization.
Conclusions
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The effects of catalyst on the curing chemistry were studied in order to understand the curing characteristics of the alkoxysilyl-functionalized epoxy system.
The curing reaction
of the alkoxysilyl-functionalized epoxy system shows the different chemistry from (1) epoxide reaction of the ordinary epoxy system and (2) alkoxysilyl reaction of the typical hydrolysable silane compound.
First of all, the alkoxysilyl group reacts preferably with the
catalyzed system.
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epoxide in the imidazole-catalyzed system while it reacts exclusively with phenol in the TPPAs a result, the stoichiometry (epoxy vs phenol) of alkoxysilyl-
functionalized systems is observed to be dependent on the type of catalyst.
Differently from
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the typical chemistry of the alkoxysilane, trimethoxysilane unit (OMe) shows the similar For the case of
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reactivity with triethoxysilane unit (OEt) in imidazole-catalyzed system.
TPP-catalyzed system, the reactivity of the alkoxysilane unit is significantly increased in the presence of epoxy via the formation of the phenolate under the TPP-catalyzed epoxy reaction. It is speculated that all observations at various curing conditions are controlled by the key reaction intermediates, that is, the sec-alkoxide ion in the imidazole system and the phenolate in the TPP system.
The unique chemistry of alkoxysilyl-functionalized system for both
catalyst systems can be explained by the reaction of the key reaction intermediates with the neighboring epoxy, phenol and alkoxysilyl group. 27
ACCEPTED MANUSCRIPT References [1] P.A. Tipler, G. Mosca, Physics for scientists and engineers, Volume 1 Mechanics/oscillations and waves/thermodynamics, Worth Publishers, New York, 2008,
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pp. 666-670. [2] M.G. Pecht, L.T. Nguyen, Plastic packaging, in: R.R. Tummala, E.J. Rymaszewski, A.G. Klopfenstein (Eds.), Microelectronics packaging handbook, semiconductor packaging
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part II, Chapman & Hall, New York, 1997, pp. 407-436.
[3]. N. Kinjo, M. Ogata, K. Nishi, A. Kaneda, Epoxy molding compounds as encapsulation
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materials for microelectronics devices, Adv. Polym. Sci. 88 (1989) 1-59. [4] S. Sulaiman, C.M. Brick, C.M. De Sana, J.M. Katzenstein, R.M. Laine, R.A. Basheer, Tailoring the global properties of nanocomposites. Epoxy resins with very low coefficients of thermal expansion, Macromolecules 39 (2006) 5167-5169.
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[5] Y. Sun, Z. Zhang, C.P. Wong, Study on mono-dispersed nano-size silica by surface modification for underfill applications, J. Colloid Interface Sci. 292 (2005) 436–444. [6] D.G. Lee, B.C. Kim, Investigation of coating failure on the surface of a water ballast tank
EP
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AC C
[7] J. de Vreugd, K.M.B. Jansen, L.J. Ernst, C. Bohm, Prediction of cure induced warpage of micro-electronic products, Microelectron. Reliab. 50 (2010) 910–916. [8] J.H. Okura, S. Shetty, B. Ramakrishnan, A. Dasgupta, J.F.J.M. Caers, T. Reinikainen, Guidelines to select underfills for flip chip on board assemblies and compliant interposers for chip scale package assemblies, Microelectron. Reliab. 40 (2000) 11731180. [9] K.C. Yung, J. Wu, T.M. Yue, C.S. Xie, Size effect of AlN on the performance of printed 28
ACCEPTED MANUSCRIPT circuit board (PCB) material-brominated epoxy Resin, J. Compos. Mater. 40 (2006) 567581. [10] K.C. Yung, J. Wang, T.M. Yue, Fabrication of epoxy-montmorillonite hybrid
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composites used for printed circuit boards via in-situ polymerization, Adv. Compos. Mater. 15 (2006) 371-384.
[11] Q. Wei, A. Lazzeri, F. Di Cuia, M. Scalari, E. Galoppini, New epoxy resins cured with
SC
tetraaminophenyladamantane (TAPA), Macromol. Chem. Phys. 205 (2004) 2089-2096. [12] P.J. Yoon, T.D. Fornes, D.R. Paul, Thermal expansion behavior of nylon 6
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nanocomposites, Polymer 43 (2002) 6727–6741.
[13] K.Y. Lee, K.H. Kim, S.K. Jeoung, S.I. Ju, J.H. Shim, N.H. Kim, S.G. Lee, S.M. Lee, J.K. Lee, D.R. Paul, Thermal expansion behavior of composites based on axisymmetric ellipsoidal particles, Polymer 48 (2007) 4174-4183.
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[14] H. Chun, Y.-J. Kim, S.-Y. Tak, S.-Y. Park, S.-J. Park, Preparation of ultra-low CTE epoxy composite using the new alkoxysilyl-functionalized bisphenol A epoxy resin, Polymer 135 (2018) 241-250.
EP
[15] Y.-J. Kim, H. Chun, S.-Y. Park, S.-J. Park, Preparation and curing chemistry of ultra-low CTE epoxy composite based on the newly-designed triethoxysilyl-functionalized ortho-
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cresol novolac epoxy, Polymer 147 (2018) 81-94. [16] S. Sakka, The outline of applications of the sol-gel method, in: L. Klein, M. Aparicio, A. Jitianu (Eds.), Handbook of sol-gel science and technology, Springer, Gewerbestrasse, 2018, pp. 1906-1937. [17] C.J. Brinker, G.W. Scherer, Sol-Gel Science: The physics and chemistry of sol-gel processing, Academic Press Inc., San Diego, 1990, pp. 1-18.
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ACCEPTED MANUSCRIPT [18] G.L. Witucki, A silane primer: Chemistry and applications of alkoxy silane, J. coating Technol. 65 (1993) 57-60. [19] W.R. Ashcroft, Curing agents for epoxy resins, in: B. Ellis (Ed.), Chemistry and
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technology of epoxy resins, Chapman & Hall, London, 1993, pp. 37-70. [20] H.Q. Pham, M.J. Marks, Epoxy resins, Encycl. Polym. Sci. Technol. 9 (2004) 678-804. [21] S. Han, W.G. Kim, H.G. Yoon, T.J. Moon, Kinetic study of the effect of catalysts on the
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curing of biphenyl epoxy resin, J. Appl. Polym. Sci. 68 (1998) 1125-1137.
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[22] Y.R. Hama, S.H. Kim, Y.J. Shin, D.H. Lee, M. Yang, J.H. Min, J.S. Shin, A comparison of some imidazoles in the curing of epoxy resin, J. Ind. Eng. Chem. 16 (2010) 556–559. [23] V. Jisova, Curing mechanism of epoxides by imidazole, J. Appl. Polym. Sci. 34 (1987) 2547-2558.
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[24] Y.-C. Chen, W.-Y. Chiu, The structural properties of imidazole cured epoxy-phenol resins, Polymer 42 (2001) 5439~5448.
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[25] W.A. Romanchick, J.E. Sohn, J.F. Geibel, Synthesis, morphology, and thermal stability of elastomer-modified epoxy resins, in: R. Bauer (Ed.), Epoxy Resin Chemistry II, ACS
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Symposium Series, American Chemical Society, Washington DC, 1983, pp. 86-89. [26] B. Arkles, J.R. Steinmetz, J. Zazyczny, P. Mehta, Factors contributing to the stability of alkoxysilanes in aqueous solution, in: K.L. Mittal (Ed.), Silanes and other coupling agents vol 3, CRC press, London, 2004, pp. 91-104. [27] D.A. Loy, B.M. Baugher, C.R. Baugher, D.A. Schneider, K. Rahimian, Substituent effects on the sol-gel chemistry of organotrialkoxysilanes, Chem. Mater. 12 (2000) 36243632.
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ACCEPTED MANUSCRIPT
Research highlights ► CTE of epoxy composite film is significantly by the alkoxysilyl modification. ► New alkoxysilyl-functionalized epoxy shows the unique curing mechanisms.
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► Epoxide reaction of new epoxy system is different from that of the ordinary epoxy system. ► Alkoxysilyl moiety within epoxy shows the different chemistry from typical hydrolysable
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EP
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alkoxysilane.
S0032-3861(19)30319-2
DOI:
https://doi.org/10.1016/j.polymer.2019.04.002
Reference:
JPOL 21385
To appear in:
Polymer
Received Date: 30 January 2019 Revised Date:
28 March 2019
Accepted Date: 1 April 2019
Please cite this article as: Chun H, Kim Y-J, Park S-Y, Park S-J, Curing mechanism of alkoxysilylfunctionalized epoxy(II): Effect of catalyst on the epoxy chemistry, Polymer (2019), doi: https:// doi.org/10.1016/j.polymer.2019.04.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Curing mechanism of Alkoxysilyl-functionalized epoxy [Imidazole system]
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EP
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[TPP system]
ACCEPTED MANUSCRIPT
Curing mechanism of alkoxysilyl-functionalized epoxy(II): Effect
Hyunaee Chun*, Yun-Ju Kim, Sook-Yeon Park, Su-Jin Park
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of catalyst on the epoxy chemistry.
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Gyeonggi Regional Division, Korea Institute of Industrial Technology, Ansan 15588, South Korea
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ABSTRACT
Alkoxysilyl-functionalized epoxy composites were recently reported to offer the ultra-low thermal expansion properties as low as 3 ~ 4 ppm/℃ at 85wt% of silica, not normally achievable with conventional epoxy systems.
Understanding the curing mechanism of
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alkoxysilyl-functionalized epoxies is necessary for future applications, especially in semiconductor packaging.
For this purpose, the distinctive chemistry of the alkoxysilyl-
functionalized epoxy using imidazole and triphenylphosphine catalysts was studied.
The
EP
chemistry of alkoxysilyl-functionalized epoxies was investigated using model compounds
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such as monofunctional epoxy, phenol and monofunctional alkoxysilanes. reaction and reaction products were determined by NMR spectrometry.
The extent of
It was observed that
the alkoxysilyl-functionalized epoxy system shows the unique curing characteristics, different from both (1) ordinary epoxy system and (2) hydrolysable alkoxysilanes.
The unique curing
characteristics for each catalyst system are explained by suggested mechanisms. 1. Introduction *
Corresponding author. E-mail address: [email protected] (H. Chun). 1
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The thermal expansion characteristics of a material significantly influences the dimensional stability of the devices made using them [1-3].
Especially, in the
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semiconductor packaging, high coefficients of thermal expansion (CTEs) of epoxy composites frequently cause the serious problems such as cracking, warpage, peelings of the packages [2-7].
In achieving reliable semiconductor packaging, the processing low-CTE
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epoxy composites is necessary [8-11].
The preparation of the high filled silica composites is most frequently used to decrease the Since the thermal expansion of the epoxy composites
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CTEs of epoxy materials [8-13].
depend on the filler loading, many researches have focused on how to increase the fill ratio of the inorganic components as much as possible.
At 80 ~ 85 wt % silica added, conventional
epoxy systems show CTE 1 (CTE at T
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35~45 ppm/℃, which are still too high to solve the problems associated with the high CTE of epoxy materials [14, 15].
In achieving the reliable semiconductor packaging, the CTE-
mismatch between the epoxy materials and silicon chip need to be solved.
However, since
EP
the CTE (2.8 ppm/℃) of silicon chip is much lower than that of epoxy material, the problems
[2].
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associated with CTE-mismatch cannot be avoided if the conventional epoxy systems are used The CTE values of the conventional epoxy composites are still too high for the reliable
semiconductor packaging.
Very recently, our group attempted to solve the high thermal expansion of epoxy composites using a different approach [14, 15].
Rather than focusing on how to increase
filling ratios, new epoxy resins, i.e., the alkoxysilyl-functionalized epoxy resins were designed. The new resins were synthesized by the (1) hydrosilylation of the diallyl-bisphenol A epoxy or (2) reaction of the isocyanato-silane coupling agent with the hydroxylated novolac epoxy, 2
ACCEPTED MANUSCRIPT as shown in Fig. 1.
These alkoxysilyl-functionalized epoxy systems in Fig. 1 showed the
excellent thermal expansion properties, which cannot be achieved with the ordinary commercial epoxy systems.
That is, Tthe composite of silica 85 wt% prepared with the The CTE1 (T
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alkoxysilyl-bisphenol A epoxy (Fig. 1(a)) showed ultra-low CTEs:
CTE 2 (T>Tg) were 3.2 ppm/℃ and 6.0 ppm/℃, respectively, one of the lowest thermal expansion value reported to date [14].
From these studies, we were motivated to
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high thermal expansion of the epoxy composite systems.
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demonstrate that our alkoxysilyl-functionalized epoxy can be strong candidates to solve the
(b)
EP
(a)
Fig. 1. Chemical structures of (a) alkoxysilyl-functionalized bisphenol A epoxy (Si(OEt)3-
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BPA) and (b) alkoxysilyl-functionalized ortho-cresol novolac epoxy (Si(OEt)3-EOCN).
Alkoxysilane compounds have been used extensively as silane coupling agents, sol-gel precursors, the formation of organic-inorganic hybrid and so on [16-18]. As shown in Fig. 2(a), the typical reaction chemistry of alkoxysilane compounds is hydrolysis and condensation [17, 18]: The trialkoxysilane group is easily hydrolyzed to form silanol and subsequently reacts with the hydroxyl groups on filler surfaces or other silanols. 3
The
ACCEPTED MANUSCRIPT application of alkoxysilane compounds in various fields is based on this chemistry.
In our
previous studies, however, we found that alkoxysilyl group within epoxy-curing matrix
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showed different chemistry from that of typical hydrolysable alkoxysilanes [14, 15].
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(a)
(b)
Fig. 2. Chemistry of alkoxysily group in (a) typical hydrolysable alkoxysilane system and (b) alkoxysilyl-functionalized epoxy system cured by phenol/imidazole
4
ACCEPTED MANUSCRIPT The alkoxysilyl group was observed to participate in epoxy-phenol curing reactions, as shown in Fig. 2(b).
During epoxy curing reactions catalyzed by imidazole, we obtained
structures that formed by reactions of the alkoxysilyl unit.
That is, in addition to reaction
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product of epoxy and phenol, the structures formed by the reactions of (1) alkoxysilyl group with epoxide and (2) alkoxysilyl group with phenol were also observed.
This observation
indicates that the curing mechanism for alkoxysilyl-functionalized epoxies is no longer
It is quite interesting to understand the
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similar with that of the conventional epoxy system.
distinct curing chemistry of the newly-designed epoxy system.
However, we still do not
of the epoxy curing reaction is.
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know how the alkoxysilyl-functionalized epoxy system works and what the controlling factor
In this paper, therefore, the curing mechanism of alkoxysily-functionalized epoxy systems The phenol curing systems catalyzed by imidazole and
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was investigated more extensively.
triphenyl phosphine (TPP) were chosen in this study, since they are the most frequently used hardener/catalyst systems for the semiconductor packaging encapsulation [3, 19-21].
The
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effects of catalyst on the curing mechanisms were studied for the first time.
2. Experimental
2.1. Materials Glycidylphenylether, phenol and bisphenol A diglycidyl ether (DGEBA) with a molecular weight of 340 were purchased from Sigma-Aldrich Chemical Co., Inc. and used as received. 2-Phenyl imidazole (Shikoku Chem. Co., Curezol® 2PZ) and triphenylphosphine (Sigma5
ACCEPTED MANUSCRIPT Aldrich Chemical Co., Inc., TPP) were used as catalysts for the epoxy reactions. Triethoxy(3-phenoxypropyl)silane, trimethoxy(3-phenoxypropyl) silane and triethoxysilylfunctionalized bisphenol A epoxy (Si(OEt)3-BPA) were synthesized, using the method Phenol novolac resin (Meiwa Plastic Ind. Ltd, HF-
1M®, OH equivalent of 107 g/eq) was used as a curing agent.
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described in our previous study [14, 15].
Acrylic polymer (Nagasei
ChemteX Corp., SG-80H®) was added to assist the formation of composite film.
anhydrous grade and were purchased from Aldrich Chemical Co.
The chemical structures
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of the compounds used in this study are given in Fig. 3.
All solvents used were
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(Denka fused silica, 20 µm-cut) was used as a low-CTE filler.
The silica
Fig. 3. Chemical structures of (a) 2-phenyl imidazole (2PZ), (b) triphenylphosphine (TPP), (c) glycidyl phenyl ether, (d) trialkoxy(3-phenoxypropyl)silane, (e) diglycidyl ether of bisphenol A (DGEBA) and (f) phenol novolac.
6
ACCEPTED MANUSCRIPT 2.2. Model reactions for mechanism study The monofunctional epoxy, phenol, monofunctional alkoxysilyl compound were used as model compounds for the ‘epoxy’, ‘hardener’ and ‘alkoxysilyl-moiety’ of alkoxysilyl-
(TPP) were used as catalysts.
2-Phenyl imidazole (2PZ) and triphenylphosphine
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functionalized epoxy resin, respectively.
The model reactions were carried out at 100℃ under the
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conditions described in Tables 1 and 2.
Table 1 a
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The model reaction conditions for the conventional epoxy curing.
All reactions were carried
out at 100℃ under the 1 phr of imidazole or TPP catalyst. Catalystb
Reaction code Ref-1
2PZ
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Ref-2 Ref-3 Ref-4 Ref-5
TPP
1.0
1.0
2PZ: 2-Phenyl imidazole, TPP: Triphenylphosphine.
c
Glycidyl phenyl ether
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0.6 1.0
0.6
b
7
Phenol (Eq)
2.5
Ref-6 phr: part per hundred epoxy resin.
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a
Epoxyc (Eq)
1.0 2.5
ACCEPTED MANUSCRIPT Table 2 The model reaction conditions for the alkoxysilyl-functionalized epoxy curing.
All
a
b
Catalyst
Epoxyc (Eq)
Phenol (Eq)
Ethoxysilaned (Eq)
Methoxysilanee (Eq)
0.6
1.0
-
1.0
1.0
1.0
E-P-Si-1 E-P-Si-2
2PZ
1.0
E-P-Si-3
1.0 TPP
1.0
E-P-Si-5 P-Si-1
TPP
-
-
1.0
1.0
-
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E-P-Si-4
RI PT
Reaction code
SC
reactions were carried out at 100℃ under the 1 phr of imidazole or TPP catalyst.
1.0
-
1.0
1.0
-
1.0
phr : part per hundred epoxy resin.
b
2PZ: 2-Phenyl imidazole, TPP: Triphenylphosphine.
c
Glycidyl phenyl ether.
d
Triethoxy(3-phenoxypropyl)silane.
e
Trimethoxy(3-phenoxypropyl)silane.
EP
TE D
a
AC C
The chemistry of the alkoxysilyl-functionalized epoxies was investigated, as described in our previous studies [14, 15].
The extent of reaction was characterized by the variation of
the characteristic NMR peaks of model compounds.
The epoxide peak at δ=2.83 ppm and
the aromatic hydrogen peaks at δ= 6.77 ppm and at δ=7.17 ppm are used to characterize the chemistry of epoxy and phenol, respectively.
With the progress of the reaction, the
epoxide peak finally disappears as a result of the epoxide-opening reaction and the phenol peaks gradually merge into the neighboring aromatic peaks at δ= 6.92 ppm and δ=7.27 ppm. The chemical shifts of trialkoxysilane, more specifically the peak of α-hydrogen (-CH2-CH28
ACCEPTED MANUSCRIPT CH2-Si(OEt)3) were δ = 0.68 ppm and δ = 0.73 ppm for triethoxysilane and trimethoxysilane, respectively.
When the trialkoxysilyl compound reacts with epoxide or phenol, the intensity
of α-hydrogen peak (-CH2-CH2-CH2-Si(OEt)3) decreases with reaction time and at the same For the case
RI PT
time, two new peaks (more exactly, shifted α-hydrogen peaks) appear [14, 15].
of triethoxysilane, new peaks appear at δ=0.79 ppm and δ=0.89 ppm, which are considered to relate with the reaction of triethoxysilane with the epoxide and the phenol, respectively.
SC
Thus, in this study, the extent of reaction of the trialkoxysilyl component was monitored by the shift (decrease) in the α-hydrogen peak (-CH2-CH2-CH2-Si(OR)3), as reported previously
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[14, 15].
2.3. Preparation of silica composite film
Into the MEK, the predetermined amount of epoxy, acrylic polymer (20 wt% in MEK) The MEK solution (solid content of 70 wt%) was mixed for 20 min
TE D
and silica were added.
using a planetary centrifugal mixer (Thinky Co., Thinky Mixer ARE-310®).
Then the
EP
phenol novolac hardener was added to the mixture and then mixed further for 10min. Finally, the 2-phenyl imidazole or TPP was dispersed in sonication bath for 10 min.
The
AC C
homogeneously mixed solution was casted onto the copper film using doctor blade and then the solvent was removed in the convection oven at 60℃ for 30 min.
The dried composite
sheet was cut into the 4 mm wide strips and then cured in the oven at 120℃ for 2h, 180℃ for 2h and 230℃ for 2h.
Cu layer with etching.
The thickness of the cured epoxy film was 70 µm, after removing the The formulations of composites with silica 80 wt% were given in
Table 3.
9
ACCEPTED MANUSCRIPT Table 3 Formulations of epoxy composites used in this study.
epoxy system (Si(OEt)3-BPA)
(BPA)
TPP (g)
Si(OEt)3-BPA epoxy
1.00
1.00
BPA epoxy(DGEBA)
-
-
Phenol novolac
0.29
0.29
2-phenyl imidazole
0.01
-
TPP
-
Acrylic polymer
0.25
Silica (80 wt%)
6.19
Imidazole (g) -
1.00
0.57 0.01
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Imidazole (g)
RI PT
Reference system
SC
component
Alkoxysilyl-functionalized BPA
-
0.25
0.25
6.18
7.31
TE D
0.01
2.4. Measurement of thermal expansion property
EP
The coefficient of thermal expansion and glass transition temperature were measured
AC C
using a thermomechanical analyzer (TMA Q400, TA Instruments). specimen was 16 mm long, 4 mm wide and 0.1 mm thick. measurement, 0.05N of the applied tension force was used.
Tension mode was used.
10
For
The temperature was varied
from room temperature to 270℃ at a heating rate of 10℃/min.
3. Results and Discussion
The dimension of
ACCEPTED MANUSCRIPT 3.1. Chemistry of the conventional epoxy/phenol curing system for imidazole or triphenylphosphine The model reactions of monofunctional epoxy and the phenol were carried out using When 1 Eq of epoxide reacts
RI PT
imidazole and TPP catalysts using Table 1 reaction conditions.
with 1 Eq of phenol (reaction code in Table 1: Ref-2 & Ref-5), both catalyst systems exhibit almost the same curing characteristics.
Plotting the extent of reaction for epoxide group vs All epoxide groups disappear at t >
SC
time, both catalyst systems show similar curing rates: 3h at 100℃, by the reaction with phenol, Fig. 4(a).
When the reacted epoxy concentration is
M AN U
plotted against the reacted phenol concentration, both systems show the virtually same stoichiometry (1:1 for epoxide and phenol), as shown in Fig. 4(b).
1.2
TE D
extent of reaction(Phenol, Eq)
1.0
0.8
EP
0.6
0.4
TPP 2PZ
0.2
AC C
extent of reaction(Epoxide, Eq)
1.2
1
2
3
0.8
0.6
0.4
0.2
0.0
0.0
0
TPP 2PZ
1.0
4
0.0
5
0.2
0.4
0.6
0.8
1.0
1.2
extent of reaction(Epoxide, Eq)
time (hr)
(a)
(b)
Fig. 4. Curing characteristics of the conventional epoxy/phenol systems under the imidazole and TPP-catalysts. The concentration (Eq) of the reacted epoxide group (a) with time and (b) as a function of reacted phenol (Eq). The reactions of the monofunctional epoxy(1Eq) and phenol (1Eq) were carried out at 100℃.
11
ACCEPTED MANUSCRIPT Although the curing characteristics of two catalyst systems are almost indistinguishable as above, it should be mentioned that their curing mechanisms are quite different [19, 20, 22-25].
In the TPP-catalyzed system, a betaine intermediate forms by attack of TPP on the
AC C
EP
TE D
(a)
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SC
RI PT
epoxide group and is protonated by phenol to produce the phenolate, Fig. 5(a) [25].
(b)
Fig. 5. Curing mechanism of conventional epoxy/phenol system catalyzed by (a) triphenylphosphine (TPP) and (b) the imidazole (2PZ) [19, 20, 22-25]. 12
ACCEPTED MANUSCRIPT Since the curing reaction proceeds via reaction of the phenolate with ring-opened epoxide group, it inevitably results in the epoxy reacting stoichiometrically with the phenol.
In
contrast, in the imidazole-catalyzed system, two epoxide groups react with imidazole and The highly-reactive sec-
RI PT
produce the sec-alkoxide ion, as shown in Fig. 5(b) [22-24].
alkoxide ion subsequently reacts with phenol and produces another reaction intermediate, i.e., phenolate.
Hence, the epoxy curing reaction can proceed via two reaction pathways.
SC
Path (i) is via the reaction of phenolate with the epoxide and the other path (ii) is via the reaction of the sec-alkoxide ion with the epoxy (i.e., self-polymerization).
However, the
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stoichiometric relationship observed in Fig. 4(b) indicates that the epoxy curing reaction proceeds exclusively via the phenolate when the same concentrations of epoxy and phenol are used.
If some epoxides carry out the reaction via sec-alkoxide ion path (self-
polymerization), the 1:1 stoichiometry between epoxy and phenol cannot be observed.
If
TE D
so, the epoxide group must be consumed more than phenol, differently from our observation. The 1:1 stoichiometry between epoxy and phenol for both catalyst systems is observed to be maintained even at higher concentrations of phenol than epoxide, as found in Ref-3 and However, when the concentration of phenol is less than
EP
Ref-6 systems described in Table 1.
the concentration of epoxide (Ref-1 and Ref-4 in Table 1), both catalyst systems show For the TPP-catalyzed system, when 0.6 Eq of phenol is used
AC C
different stoichiometries.
(reaction code: Ref-4 in Table 1), the epoxy is not consumed completely. epoxide remains unreacted.
About 0.4 Eq of
In contrast, in the imidazole-catalyzed system, residual epoxy
was not detected, even when the less than 1 Eq of phenol is used (reaction code: Ref-1 in Table 1).
All epoxy groups react, despite of the lack of the phenol.
Fig. 6 shows that for
the system of Ref-1, the 1:1 stoichiometric ratio is maintained under the condition where the phenol is still available. react.
But, after the all phenol are consumed, the epoxide continues to
This observation suggests that the imidazole-catalyzed epoxy reaction proceeds 13
ACCEPTED MANUSCRIPT through two different mechanisms depending on the phenol concentration.
To sum up, the
epoxy reaction proceeds via phenolate-epoxide reaction mechanism exclusively while the phenol is available and, after the phenol is used up, the residual epoxide groups keep on
1.0
0.8
0.6
0.4 2PZ_Phenol 0.6Eq 2PZ_Phenol 1.0Eq 0.2
0.8
RI PT
1.0
2PZ_Phenol 0.6Eq 2PZ_Phenol 1.0Eq
SC
1.2
extent of reaction(Phenol, Eq)
1.2
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extent of reaction(Epoxide, Eq)
reaction via a self-polymerization route, i.e., sec-alkoxide ion reaction with epoxide.
0.6
0.4
0.2
0.0
0.0
1
2
3
4
5
6
TE D
0
EP
time (hr)
0.2
0.4
0.6
0.8
1.0
extent of reaction(Epoxide, Eq)
(b)
AC C
(a)
0.0
Fig. 6. Curing characteristics of the conventional epoxy/phenol systems. The concentration (Eq) of the reacted epoxide group (a) with time and (b) as a function of reacted phenol (Eq). The monofunctional epoxy (1Eq) reacts with phenol (1Eq, 0.6Eq) at 100℃under the imidazole (2PZ).
14
1.2
ACCEPTED MANUSCRIPT Before going to the following section, we’d like to mention the stability of TPP catalyst during epoxy reaction. We observed experimentally that the TPP loses its catalytic activity by transforming to the triphenylphosphine oxide (O=PPh3), when the phenol is no longer As shown in Fig. 7(a), when 1 Eq of phenol is
RI PT
available, as reported in the literature [25].
used, as in the system of Ref-5, the TPP peaks gradually decrease and finally disappear at t > Simultaneously new peaks corresponding to triphenylphosphine oxide (O=PPh3)
gradually increase with time.
At lower concentrations (0.6 Eq) of phenol, the TPP decay
time becomes shorter (tdecay < 2h).
SC
3 hours.
However, when the excess phenol is used, as in system Thus, throughout
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Ref-6, significant amounts of TPP remain even after 24 hours, Fig.7(b).
this study, we carefully took the experimental results of the TPP system by considering the
AC C
EP
TE D
catalyst stability.
(a)
(b)
Fig. 7. The variation of TPP concentration with reaction time during epoxy/phenol reaction. The monofunctional epoxy (1Eq) reacts with phenol at 100℃ and the phenol concentrations are (a) 1.0 Eq and (b) 2.5 Eq.
The reaction codes are Ref-5 & Ref-6 in Table 1. 15
ACCEPTED MANUSCRIPT 3.2. Chemistry of alkoxysilyl-functionalized epoxy during phenol curing (1): The alkoxysilyl groups react exclusively with phenol in the TPP system, whereas it prefers to react with the
RI PT
epoxy in the imidazole system.
The reactions of the monofunctional epoxy (1 Eq), the phenol (1 Eq) and triethoxysilane (1 Eq) were carried out as model reactions.
The reaction conditions employed are recorded The NMR results of imidazole and TPP-
AC C
EP
TE D
M AN U
SC
in Table 2 (reaction code: E-P-Si-2 & E-P-Si-4).
catalyzed systems are compared in Fig. 8.
+
H2 R C Si(OR)
R
Si C O H2
Both catalyst systems show that the α-hydrogen
O-
+
H2 R C Si(OR)
R
C H2
Si O
O
peak at 0.79ppm
peak at 0.89ppm
Fig. 8. NMR spectra for the reaction of monofunctional epoxy, phenol and triethoxysilyl compounds (blue box) under the triphenylphosphine (TPP) and imidazole (2PZ) catalyst at 16
ACCEPTED MANUSCRIPT 100℃ for (a) t = 0 h; (b) TPP, t = 3 h; (c) 2PZ, t=7h.
The reaction codes are E-P-Si-2 & E-
P-Si-4 in Table 2. peak of the alkoxysilyl compound at δ=0.68 ppm decreases with time, arising from participation of the alkoxysilane in the epoxy reaction.
However, the newly-formed peaks The NMR spectrum of TPP-
catalyzed reaction shows only one peak at δ=0.89 ppm.
For the imidazole-catalyzed system,
RI PT
of both systems are different, depending on the catalyst.
two newly-formed peaks are observed at δ=0.79 ppm and δ=0.89 ppm.
SC
former peak shows the higher.
Of the two, the
in Fig. 5.
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The above observations cannot be explained by conventional epoxy mechanisms, given The modified mechanism for the alkoxysilyl-functionalized epoxy only should be
suggested in order to include the experimental observations.
That is, for TPP-catalyzed
epoxy-phenol reaction, the active reaction intermediate is phenolate only.
It is highly
TE D
probable that the alkoxysily group also participates in the epoxy reaction via the phenolate. Thus, it is not difficult to anticipate one shifted peak of α-hydrogen (-CH2-CH2-CH2-Si(OR)3, δ=0.89 ppm) from the reaction of alkoxysilane and phenolate.
In contrast, for the
EP
imidazole-catalyzed reaction, two active species, i.e., phenolate and the sec-alkoxide ion are likely involved in alkoxysilyl group reaction.
Since both ions can react with the alkoxysilyl
AC C
group, two newly-formed peaks must appear at δ=0.79 ppm and δ=0.89 ppm.
Based on the
higher NMR intensity of peak at δ=0.79 ppm, the alkoxysilane seems to be consumed more favorably with the sec-alkoxide ion (i.e., epoxy) than phenolate (i.e., phenol), in accord with the reactivity difference between reaction intermediates.
By considering the above
observations, the mechanisms of the alkoxysilyl-functionalized epoxy under the both catalysts are suggested, as schematically drawn in Fig. 9.
17
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
(a)
(b)
Fig. 9. Modified curing mechanism for the alkoxysilyl-functionalized epoxy system under the (a) TPP and (b) imidazole catalysts.
18
ACCEPTED MANUSCRIPT 3.3. Chemistry of alkoxysilyl-functionalized epoxy during phenol curing (2): The stoichiometry of epoxy reaction is different from that of the conventional epoxy system and
RI PT
depends on the catalyst type
The stoichiometry of alkoxysilyl-functionalized epoxy for both catalyst systems were investigated using the model reaction systems (reaction code: E-P-Si-2 & E-P-Si-4).
In Fig.
systems.
SC
10, the extents of reaction for phenol and epoxy are plotted against time for both catalyst For the imidazole system, the epoxy is consumed more than phenol, whereas for
AC C
EP
TE D
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the TPP system, the extent of reaction for epoxide is more or less similar with that of phenol.
(a)
(b)
Fig 10. The extent of reaction of epoxide and phenol for alkoxysilyl-functionalized epoxy with time under the (a) imidazole and (b) TPP-catalysts. E-P-Si-4 in Table 2. 19
The reaction codes are E-P-Si-2 &
ACCEPTED MANUSCRIPT The stoichiometry ([epoxide] vs [phenol]) of the alkoxysilyl-functionalized system must be different from that of the conventional epoxy, since the alkoxysilyl group participates in epoxy curing reaction, depending on the type of catalyst.
It reacts more favorably with the
RI PT
epoxide in the imidazole-catalyzed epoxy reaction, which probably result in the observation that the epoxide group is consumed more than the phenol, as observed in Fig. 10(a).
In
contrast, the alkoxysilyl compound exclusively reacts with the phenol in the TPP-catalyzed It is reasonable that the phenol may be consumed more in TPP system than in
imidazole system, as observed in Fig. 10(b).
SC
system.
Consequently, for the most of case, the
M AN U
stoichiometry between epoxy and phenol of the alkoxysilyl-functionalized epoxy cannot be 1: 1, differently from the conventional epoxy system
It is worthy noticing that the stoichiometry of the alkoxysilyl-functionalized epoxy system cannot be as simple as that of the ordinary epoxy system.
The stoichiometry is determined
TE D
by the how much the alkoxysilyl group participates in the epoxy reaction, which in turn depends on the reaction catalyst, type of alkoxysilane, the molecular structure of epoxy and so on.
Hence, the prediction of the stoichiometry of the alkoxysilyl-functionalized epoxy At this point, the only thing we can assure is that the stoichiometric
EP
reaction is not simple.
ratio of epoxy to phenol may not be 1:1 for the most of alkoxysilyl-functionalized epoxy
AC C
reactions.
3.4. Chemistry of alkoxysilyl-functionalized epoxy during phenol curing (3): The alkoxysilyl group shows the different chemistry, being compared with the that of the typical hydrolysable alkoxysilane compound.
20
ACCEPTED MANUSCRIPT The alkoxysilane reactivity of the alkoxysilyl-functionalized epoxy was investigated using the model reactions.
The triethoxysilane (OEt) and trimethoxy silane (OMe) were
chosen for this study, as given in Table 2 (reaction code: E-P-Si-2, E-P-Si-3, E-P-Si-4, E-PFig. 11(a) shows that, in imidazole-catalyzed system, the reactivity of
RI PT
Si-5, P-Si-1).
trimethoxysilane (OMe) is somewhat higher than that of triethoxysilane (OEt). speaking, both silanes show the rather similar reactivity.
But, roughly
During the reaction for 7 hr at
SC
100℃, the peak intensity of α-hydrogen (-CH2-Si-) is decreased to the 51 mol % of initial value for triethoxysilyl (OEt) compound, whereas the residual peak intensity of
Im-OEt (E-P-Si) Im-OMe (E-P-Si)
0.2
AC C
0.0 0
2
4
6
extent of reaction (-CH2-Si-, Eq)
TE D
0.4
TPP-OEt (E-P-Si) TPP-OMe (E-P-Si) TPP-OMe (P-Si, no Epoxy)
0.6
EP
extent of reaction (-CH2-Si-, Eq)
0.6
For the case of TPP system, the reactivity
M AN U
trimethoxysilyl (OMe) compound is 46 mol %.
0.4
0.2
0.0 8
0
1
2
3
4
time (hr)
time (hr)
(a)
(b)
Fig 11. The extent of reaction of alkoxysilyl-functionalized epoxy with time under the (a) imidazole and (b) TPP-catalysts. The substituents of alkoxysilane used were trimethoxy (OMe) and triethoxy (OEt).
The reaction codes are E-P-Si-2, E-P-Si-3, E-P-Si-4, E-P-Si-5,
and P-Si-1 in Table 2. 21
ACCEPTED MANUSCRIPT
difference between the substituents of alkoxysilane is apparently observed.
The extent of
reaction for trimethoxysilane (OMe) is observed to be much higher than that of However, in spite of the low reactivity of
RI PT
triethoxysilane (OEt), as shown in Fig. 11(b).
triethoxysilane (OEt) under the absence of the acid or base catalysts, its appreciable reactivity under the TPP catalyst is worthy of notice [17, 26].
SC
If we consider the well-known chemistry of the alkoxysilane, the similar reactivity of trimethoxy- and triethoxy-silane observed in the imidazole-catalyzed system is somewhat For the typical hydrolysable alkoxysilane reaction (i.e., sol-gel reaction and silane
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unusual.
coupling agent reaction), the trimethoxysilane (OMe) generally shows the much higher reactivity than the triethoxysilane (OEt) [26, 27].
Furthermore, the high reactivity of
alkoxysilyl compound observed under the TPP catalyst at 100 ℃ is also very unique.
If the
TE D
reaction is carried out in the absence of epoxy just like the typical alkoxysilane reaction (reaction code: P-Si-1), even the trimethoxysilane (OMe) compound hardly reacts with phenol under the TPP catalyst, as shown in Fig. 11(b).
The results for both catalyst systems
EP
clearly show that the alkoxysilyl group in the alkoxysilyl-functionalized epoxy possesses the
AC C
quite different chemistry from that of the typical hydrolysable alkoxysilane compound. For the case of the typical alkoxysilane compounds, alkoxysilyl group are mostly hydrolyzed to silanol before or during applications, as depicted schematically in Fig. 2(a) [25].
The subsequent process, i.e., the application of alkoxysilane compounds for surface
treatment, can be done via the highly reactive silanol.
As a result, the reaction of the
alkoxysilane compound depends on the reactivity toward the hydrolysis.
In contrast, when
we think about the reactivity of alkoxysilyl group in the alkoxysilyl-functionalized epoxy systems, the effect of epoxide group must be taken into account: In the presence of the epoxy, 22
ACCEPTED MANUSCRIPT each catalyst system has its own characteristic reaction intermediate, which finally determines the chemistry of alkoxysilyl group.
As suggested in Fig. 9, the key species for
imidazole-catalyzed epoxy reaction is the sec-alkoxide ion.
Due to the high reactivity of the Thus, in the
RI PT
sec-alkoxide ion, the reactivity of alkoxysilyl group must be less significant.
imidazole system, both systems show the similar reactivity regardless of the type of substituents.
In TPP-catalyzed system, the key reaction intermediate is the phenolate
Thus, the alkoxysilyl group in the
SC
formed via the interaction of TPP, epoxy and phenol.
presence of epoxy can show the significantly improved reactivity, being compared with the However, the phenolate is not reactive as much as the
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system without the epoxide group.
sec-alkoxide ion and thus the alkoxysilane reactivity of TPP system may depend on whether the substituent structure is methoxy or not, as observed in Fig. 11(b).
TE D
3.5. Chemistry of alkoxysilyl-functionalized epoxy during phenol curing (4): After the phenol is used up for imidazole system, epoxy stops the further reaction, different from the
EP
conventional epoxy-phenol reaction.
The imidazole-catalyzed epoxy reaction was carried out using the lower concentration of
AC C
phenol, as found in the system of E-P-Si-1.
When 0.6 Eq of phenol is used, the epoxy
reaction does not proceed further after the phenol is used up after 3h, as shown in Fig. 12. That is, for the alkoxysilyl-functionalized epoxy system catalyzed by imidazole, the epoxide group is not consumed completely if the phenol concentration is not sufficient, differently from the conventional epoxy curing reaction.
23
ACCEPTED MANUSCRIPT
RI PT
1.0
0.8
SC
0.6
0.4
0.2
M AN U
extent of reaction(Phenol, Eq)
1.2
Im-Phenol 0.6Eq (No silyl) Im-Phenol 0.6Eq (with silyl)
0.0 0
2
4
6
8
TE D
time (hr)
Fig 12. The extent of reaction of the epoxide group with time under the imidazole(2PZ) catalyst.
The phenol concentrations used are 0.6 Eq and 1.0 Eq.
The reaction codes are
AC C
EP
Ref-1 in Table 1 and E-P-Si-1 in Table 2.
In the conventional epoxy-phenol reaction, epoxy can keep the reaction by choosing another reaction pathway (i.e., self-polymerization) when the phenol is used up, as described in the section 3-1.
As a result, the epoxide group can be consumed completely.
However,
in the presence of the alkoxysilyl group, the additional route of self-polymerization seems to be no longer available.
In the early stage of the imidazole-catalyzed reaction, the key
reaction intermediate (sec-alkoxide ion) can be shared with phenol and alkoxysilyl group together.
It is speculated that after the phenol is used up, the sec-alkoxide ion can be still 24
ACCEPTED MANUSCRIPT taken by the residual alkoxysilyl group.
The reaction intermediate may not need to take
another self-polymerization path, as found in the ordinary epoxy reaction.
Consequently, it
is suggested that the residual epoxy probably does not progress the further reaction in the
RI PT
presence of the alkoxysilyl group, differently from the conventional epoxy curing.
Finally, we’d like to speculate the role of RO- released from siloxane.
Whenever the
SC
epoxy reaction intermediates (i.e., phenolate and sec-alkoxide ion) react with the alkoxysilane, We think that the released alkoxide ion (RO-)
the alkoxide ions are released from siloxane.
imidazole-catalyzed epoxy reaction.
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may show the similar reactivity to the sec-alkoxide ion, i.e., epoxy reaction intermediate in Just like the sec-alkoxide ion shown in Fig. 9(b), we
think that the released alkoxide ion can abstract the proton from phenol or react with the alkoxysilane.
The reactivity of the deprotonation is considered to be higher, due to the large However, we think
TE D
pKa difference between phenol and alcohol (phenol≒10, Alcohol≒16).
that the reaction of alkoxide with epoxy is not likely to occur significantly, as found in
EP
alkoxysilyl-functionalized epoxy reaction catalyzed by imidazole.
AC C
3.6. Thermal expansion property of silica composite In order to investigate the effect of catalyst on thermal expansion property, the composite films with silica 80wt% were prepared and the results are given in Fig. 13.
Irrespective of
the catalyst types, the silica composites of alkoxysilyl-functionalized epoxy (Si(OEt)3-BPA) systems show the excellent thermal expansion properties.
After the alkoxysilyl modification,
the thermal expansion properties of silica composites (Si(OEt)3-BPA) are significantly improved, being compared with that of non-functionalized BPA epoxy composite. catalyst systems give the similar CTE results. 25
Both
Within the measured temperature range of
ACCEPTED MANUSCRIPT 50℃~270℃, the CTEtotal of Si(OEt)3-BPA composite is 6~7 ppm/℃ for the both catalyst
RI PT
systems, which is about 20 ~23% of CTE total (=29.7 ppm/℃) of BPA composite.
140
BPA epoxy -Imidazole Si(OEt)3-BPA-Imidazole
120
SC
80 60 40 20 0 -20
100
150
200
250
300
TE D
50
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Dimension Change(um)
Si(OEt)3-BPA-TPP 100
Temperature(℃)
Fig. 13. Thermal expansion properties of (1) conventional BPA composite (BPA) and (2)
EP
triethoxysilyl-functionalized BPA composite (Si(OEt)3-BPA).
Two kinds of reaction
catalysts, i.e., imidazole (2PZ) and triphenylphosphine (TPP), were used.
The 80 wt% of
AC C
silica was filled for the preparation of composites.
The network structure of the cured epoxy system is definitely influenced by the catalyst type, as found in the mechanism study.
In spite of the fact, the similar thermal expansion
properties of both catalyst systems are observed as in Fig. 13.
This observation indicates
that the change of a network structure does not give the any noticeable effect for the CTE of alkoxysilyl-functionalized epoxy composite systems, which is in good accordance with the 26
ACCEPTED MANUSCRIPT previous work [14, 15].
By comparing with the thermal expansion properties of the
conventional epoxy system with the alkoxysilyl-functionalized epoxy system, we suggested that the excellent thermal expansion property of alkoxysilyl-functionalized composite results
RI PT
mainly from the improved interfacial attraction via the alkoxysilyl moiety with silica surface,
4.
SC
rather than the network structure change associated with the alkoxysilyl-functionalization.
Conclusions
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The effects of catalyst on the curing chemistry were studied in order to understand the curing characteristics of the alkoxysilyl-functionalized epoxy system.
The curing reaction
of the alkoxysilyl-functionalized epoxy system shows the different chemistry from (1) epoxide reaction of the ordinary epoxy system and (2) alkoxysilyl reaction of the typical hydrolysable silane compound.
First of all, the alkoxysilyl group reacts preferably with the
catalyzed system.
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epoxide in the imidazole-catalyzed system while it reacts exclusively with phenol in the TPPAs a result, the stoichiometry (epoxy vs phenol) of alkoxysilyl-
functionalized systems is observed to be dependent on the type of catalyst.
Differently from
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the typical chemistry of the alkoxysilane, trimethoxysilane unit (OMe) shows the similar For the case of
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reactivity with triethoxysilane unit (OEt) in imidazole-catalyzed system.
TPP-catalyzed system, the reactivity of the alkoxysilane unit is significantly increased in the presence of epoxy via the formation of the phenolate under the TPP-catalyzed epoxy reaction. It is speculated that all observations at various curing conditions are controlled by the key reaction intermediates, that is, the sec-alkoxide ion in the imidazole system and the phenolate in the TPP system.
The unique chemistry of alkoxysilyl-functionalized system for both
catalyst systems can be explained by the reaction of the key reaction intermediates with the neighboring epoxy, phenol and alkoxysilyl group. 27
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Research highlights ► CTE of epoxy composite film is significantly by the alkoxysilyl modification. ► New alkoxysilyl-functionalized epoxy shows the unique curing mechanisms.
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► Epoxide reaction of new epoxy system is different from that of the ordinary epoxy system. ► Alkoxysilyl moiety within epoxy shows the different chemistry from typical hydrolysable
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alkoxysilane.