all-aromatic liquid crystalline thermosets as high temperatures adhesives
TRANSCRIPT
International Journal of Adhesion & Adhesives 30 (2010) 682–688
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives
0143-74
doi:10.1
n Corr
E-m
journal homepage: www.elsevier.com/locate/ijadhadh
All-aromatic liquid crystalline thermosets as high temperatures adhesives
Mazhar Iqbal a, Alwin Knijnenberg a, Hans Poulis a,b, Theo J. Dingemans a,n
a Delft University of Technology, Faculty of Aerospace Engineering, Kluyverweg 1, 2629 HS Delft, The Netherlandsb Delft University of Technology, Adhesion Institute, Faculty of Aerospace Engineering, Kluyverweg 1, 2629 HS Delft, The Netherlands
a r t i c l e i n f o
Article history:
Accepted 2 June 2010In this paper we will describe the synthesis and characterization of a novel series all-aromatic liquid
crystal thermosets (LCTs) for high temperature (4100 1C) adhesive applications. Our ester-based
Available online 24 July 2010Keywords:
High temperature adhesives
Novel adhesives
Liquid crystal thermosets
96/$ - see front matter & 2010 Elsevier Ltd. A
016/j.ijadhadh.2010.06.006
esponding author. Tel./fax: +31 15 2784520.
ail address: [email protected] (T.J. Ding
a b s t r a c t
oligomers, end-capped with reactive phenylethynyl functionalities, were synthesized using a standard
one-pot melt condensation technique. The obtained reactive oligomers exhibit low melting
temperatures and low melt viscosities, which resulted in improved flow and surface wetting. The
adhesive properties of our LCTs based on 4-hydroxybenzoic acid, hydroquinone and isophthalic acid
were investigated using standard (ASTM) lap-shear experiments conducted at room temperature
(25 1C) and at elevated temperatures, i.e. 150, 200 and 250 1C. Our LCTs were used on Al2024, MS and
Ti6Al4V substrates and we found lap-shear values of 11, 12 and 16 MPa for Al, MS and Ti, respectively.
For the Al2024 substrate the lap-shear strength remained constant up to 150 1C and dropped to 4 MPa
at 200 1C. A post-mortem inspection of the fracture surface, using high-resolution scanning electron
microscopy (HRSEM), showed that there was no adhesive failure at the metal substrate at room
temperature or at elevated temperatures. The lap-shear values of our polymers are significantly higher
as reported for other commercial available LCPs such as VectraTM.
& 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Structural adhesives are frequently used in the aerospaceindustry for bonding aluminum, titanium and steel alloys.Polymer-based adhesives offer many advantages over conven-tional mechanical techniques such as welding, riveting andbolting. However, rivet- and bolt-holes act as stress concentrators,which can ultimately lead to premature failure. The advantages ofpolymer-based adhesives includes weight reduction, an increasein fatigue life, slow crack growth rate, ability to join differentsubstrates and reduced production costs [1,2]. Current genera-tions of adhesives, as used in the aerospace industry today, arebased on reactive esters, phenolics and epoxies [3]. The advantageof epoxies is that they can be cured without the evolution ofvolatiles. Other high-performance polymer systems, such aspolyimides, are available but their use is limited due to problemsassociated with processing and outgassing. Fully imidizedphenylethynyl-based poly(etherimide)s such as PETI-5, on theother hand, appear to be excellent adhesives but they are ratherexpensive [4].
A potential route towards obtaining high temperature adhe-sives is based on combining a low temperature and hightemperature adhesive in one formulation. However, adhesives
ll rights reserved.
emans).
used for high temperature applications are typically quite brittleat room temperature, while adhesives used for low temperatureapplications degrade when they are exposed to high tempera-tures. This mixed modulus approach was therefore met withlimited success [5].
The use of liquid crystalline polymers (LCPs), such as VectraTM (I),an all-aromatic polyester prepared from 4-hydroxybenzoic acid(HBA) and 6-hydroxy-2-naphthoic acid (HNA), was investigated byEconomy et al. [6,7] and it was found that this polymer could beused as a high temperature adhesive (T range of �100–150 1C).
O CO
OCO0.73
0.27 n
VectraTM (I)The obtained lap-shear values, however, were low and the lap-
shear strength dropped considerably at temperatures above150 1C. Frich et al. [8] reported the use of liquid crystal thermosetsas well but during the curing process volatiles were expelled andvery long curing times at high temperatures were required. Inorder to provide novel and affordable high-temperature adhe-sives, we have designed and synthesized reactive liquid crystalthermoset resins. This new class of high temperature polymerresins exhibits several desirable properties such as high glass-transition temperatures (Tg4150 1C), low moisture uptake,improved chemical resistance and excellent mechanical proper-ties, which makes them ideal candidates for high-temperature
M. Iqbal et al. / International Journal of Adhesion & Adhesives 30 (2010) 682–688 683
adhesion applications. These adhesives have the ability to bond tomany different kind of substrates and no mixing is required sincethese are single component adhesives.
Herein we describe the adhesive properties of two new seriesof thermosetting liquid crystal thermosets (LCTs). In the firstseries, the effect of oligomer molecular weight and crosslinkdensity on the adhesive performance was investigated, using a
VectraTM-type reactive oligomer based on 4-hydroxybenzoic acid
(HBA) and 6-hydroxy-2-naphthoic acid (HNA) terminated withphenylethynyl end-groups. The structure of this reactive oligomeris shown below.
N
O
O
CN
O
O
OO C
OO
CO
O0.73
0.27 nHBA HNA
We prepared 3 reactive oligomers, i.e. Mn¼1000, 5000,9000 g mol�1 and one high-molecular weight reference polymerwithout reactive end-groups. The synthesis of this series isdescribed elsewhere [9]. Curing of the phenylethynyl terminatedoligomers proceeds via chain-extension and crosslink chemistrywithout the evolution of volatiles. Since our polymers are cured athigh temperature (Tcure4300 1C) they can be stored at roomtemperature for an infinite amount of time. It was demonstratedby us that all-aromatic reactive liquid crystal oligomers exhibitexcellent thermal (Tg’s4200 1C) and mechanical properties atboth room temperature (E0 �4 GPa) and elevated temperatures(E0 �2 GPa at 200 1C) [10].
Since the synthesis of VectraTM requires expensive monomersand the final polymers exhibit low glass-transition temperatures(Tg�100 1C), which limits their final use temperature, weexplored a second series LCTs with high glass-transition tem-peratures. In this series, shown below, cheap and readily availablemonomers such as 4-hydroxybenzoic acid (HBA), hydroquinone(HQ) and isophthalic acid (IA) were utilized.
N
O
O
CO
O CO
O
x n
N
O
O
OO CO
CO
y z
HBA HQ IA
The physical and thermal properties of our new LCTs will bepresented and the adhesive performance on aluminum, titanium andstainless steel substrates at different temperatures will be discussed.
2. Experimental section
2.1. Materials
All chemicals were obtained from the indicated sourcesand used as received. hydroquinone (HQ), isophthalic acid (IA),and 4-hydroxybenzoic acid (HBA) were obtained from Aldrich.4-Phenylethynylphthalic anhydride (PEPA) was purchased fromDaychem. The synthesis of the para-substituted phenylethynylreactive end-groups, i.e. N-(4-carboxyphenyl)-4-phenylethy-nylphthalimide (PE-COOH) and N-(4-acetoxyphenyl)-4-pheny-lethynylphthalimide (PE-OAc) and the HBA/HNA-based LCT-serieswas reported elsewhere [9].
As metal substrates we selected Al2024, titanium and steel.Although aluminum is not an ideal substrate for these high curingpolymers, since mechanical properties such as ductility will becompromised, it allows us to investigate the adhesive properties ofour resins at room temperature and elevated temperatures. Wehave also included Ti6Al4V and martinsitic stainless steel sub-strates, which are more suited for high temperature applications.The thickness of the substrates was 1.57, 0.53 and 1.58 mm forAl2024, MS and Ti6Al4V, respectively. The ultimate tensilestrength, as provided by the supplier, of Al2024 was 469 MPa,tensile strength at yield was 324 MPa and an elongation at break of
20%. For Ti6Al4V, the ultimate tensile strength was 950 MPa, thetensile strength at yield 880 MPa and the elongation at break was14%. The ultimate tensile strength, tensile strength and elongationat break for MS were 655 MPa, 413 MPa and 25%, respectively.
2.2. Preparation of lap-shear specimen
The substrates were cleaned with ethanol and grit blasted toget the required surface for metal-to-metal bonding, and wasused without further treatment. The polymer was applied as apowder between the adherends and the thickness of the joint wascontrolled by glass beads (f¼0.17) mm or using a glass scrimcloth with a thickness of 0.25 mm. The glass beads were used insmall quantities only, i.e. 2 wt% percent and evenly distributed inthe powdered matrix. Impregnation of the glass cloth was carriedout by applying the powder on the cloth followed by consolidat-ing the polymer between KaptonTM films in a hot press at 330 1Cfor 5 min. The lap shear specimens were prepared by placing the
adhesive between the metal substrates creating an overlap of12.7 mm�25.4 mm. This assembly was placed in a hot press andheated to 370 1C at a heating rate of 9 oC min�1. The sampleswere cured at this temperature for 45 min and were cooledafterwards to 40 1C at 2 1C min�1. All these specimens wereprepared in 5-fold and were cut according to standard dimen-sions, ASTM (1002-05) prior to testing.
2.3. Characterization
The mesophase behavior of our oligomers was investigated witha Leica DMLM polarizing optical microscope (POM) equipped witha Linkham hot-stage. Samples were investigated using untreatedglass slides in air. The thermal properties of the reactive oligomersand cured polymers were determined using a Perkin-ElmerSapphire differential scanning calorimetry (DSC) with a heatingrate of 10 1C min�1. All measurements were conducted under
M. Iqbal et al. / International Journal of Adhesion & Adhesives 30 (2010) 682–688684
a nitrogen atmosphere. The thermal stability of the cured polymerswas investigated by thermogravimetric analysis (TGA) using aPerkinElmer Pyris Diamond TG/DTA. Samples were investigatedusing a heating rate of 10 1C min�1, in either nitrogen or airatmosphere. The dynamic mechanical analyses were performed ona PerkinElmer Diamond dynamic mechanical thermal analysis(DMTA), using thin films (10 mm�5 mm�0.25 mm) under anitrogen atmosphere and at a heating rate of 2 1C min�1. Thin filmswere prepared using standard melt pressing techniques. Thereactive oligomers were consolidated for 1 h at 370 1C to allowthe oligomers to polymerize completely. All these samples wereprepared using a computer controlled hot press.
A Zwick tensile test machine equipped with a heating chamberwas used for performing the lap-shear experiments. With the use ofthe heating chamber we were able to measure the lap-shearstrength at room temperature, 150, 200 and 250 1C. The testspecimens were mounted at the required test temperature for5 min before the test commenced. The actual temperature wasmeasured using a thermocouple attached to the specimen joint. Alltests were performed according to the ASTM (1002-05), using acrosshead speed of 1.3 mm min�1. The data were recorded as anaverage of 5 samples. High resolution scanning electron microscopy(HRSEM) was used to examine the fracture surface of the joints.
2.4. Synthesis of the phenylethynyl end-capped oligomers
The reactive oligomers were synthesized using standard meltcondensation techniques [11–13]. Two reactive oligomers based on4-hydroxybenzoic acid (HBA), isophthalic acid (IA) and hydroquinone(HQ) were prepared with a target Mn of 5000 and 9000 g mol�1. Thesamples were labeled, HBA/IA/HQ(25)-5K and HBA/IA/HQ(25)-9K,respectively, where HBA/HQ/IA refers to the polymer backbonecomposition and the integers refer to the molar ratio of HQ used(25 mol%) and the polymer molecular weight, i.e. 5 K¼5000 g mol�1.The synthesis is represented in Scheme 1 and these oligomers couldbe synthesized in high yield without difficulty.
2.5. Synthesis of HBA/HQ/IA(25)-5K
As a representative example we describe the synthesis of a5000 g mol�1 reactive oligomer with a HBA/HQ/IA(25)-5K back-bone composition. Hydroxybenzoic acid (HBA) (0.25 mol, 34.53 g),hydroquinone (HQ) (0.125 mol, 13.77 g), isophthalic acid (IA)(0.125 mol, 20.77 g), N-(4-acetoxyphenyl)-4-phenylethynylphtha-limide (PE-OAc) (0.012 mol, 4.68 g), N-(4-carboxyphenyl)-4-phe-nylethynylphthalimide (PE-COOH) (0.012 mol, 4.50 g) and
N
O
O
CO
O CO
O
x
N
O
O
X
X = -OAc or COOH
+ HO COOH
HBA
Acetic acid
Scheme 1. Synthesis of our phenyethynyl terminated HBA/HQ/IA-based oligom
potassium acetate (0.1 mmol, 10 mg) were charged to a 250 mlthree necked round bottom flask. The flask was equipped with anitrogen gas inlet, an overhead mechanical stirrer and a refluxcondenser. The reactor was purged with nitrogen for 0.5 h prior tothe start of the reaction and a slow nitrogen flow was maintainedthroughout the duration of the polymerization. Acetic anhydride(51 mL, 0.5 mol), was added for the in-situ acetylation of HQ and 4-HBA. The reaction mixture was slowly stirred under a nitrogenatmosphere and heated to 140 1C to allow acetylation to take place.After 1 h isothermal hold, the temperature of the reaction mixturewas slowly increased to 310 1C using a heating rate of 1 1C min�1.During this process acetic acid was collected as a condensation by-product. At 300 1C the nitrogen flow was stopped and vacuum wasapplied to remove the remaining acetic acid. This was continueduntil the melt could no longer be stirred. The reaction flask wasallowed to cool down overnight under a nitrogen flow and the finalproduct was removed from the flask and processed into a powder.The product was placed in a vacuum oven at 250 1C for 48 h inorder to complete the polymerization and remove traces of aceticacid and side products. Yields for these syntheses were generallyabove 95%.
In order to confirm the molecular weight of our oligomers weattempted to find suitable solvents or solvent mixtures, e.g.trifluoroacetic acid, pentafluorophenol or phenol/1,1,2,2-tetra-chloroethane (70:30). All oligomers, however, appeared insolubleat room temperature and elevated temperature, which precludessize exclusion chromatography (SEC) and inherent viscositymeasurements of these oligomers.
3. Results and discussion
3.1. Thermal analysis
Differential scanning calorimetry (DSC) was used to study themelt behavior of our new reactive oligomers using a heating rateof 10 1C min�1. The molecular weight of our oligomers wascontrolled by adding a pre-calculated concentration of reactiveend-groups (Carothers equation). The HBA/HQ/IA(25) basedoligomer with molecular weight of a 5000 g mol�1 melts at328 1C. Thus, a reduction of �50 1C in melting temperature wasachieved when a low molecular weight reactive oligomer wasused. The high molecular weight (Mn�20,000 g mol�1) version ofthis polymer melts at 380 1C, which is a temperature too high formost processing techniques [14]. A representative DSC thermo-gram of HBA/HQ/IA(25)-5K is shown in Fig. 1.
n
N
O
O
OO CO
CO
y z
HOOC COOH+ HO OH +
HQ IA
1- Ac2O, 1h. hold at 140 oC2- 140 oC to 310 oC in 3 h.3- 30 min. hold at 310 oC/vacuum
ers. All oligomers can be made via a one-step melt condensation method.
M. Iqbal et al. / International Journal of Adhesion & Adhesives 30 (2010) 682–688 685
The onset temperature to decomposition Td, defined as thetemperature at 5% weight loss, was investigated by thermogravimetric analysis (TGA) using a heating rate of 10 1C min�1.Together with the char yields, the Td values are summarized inTable 1. All reactive oligomers were cured at 370 1C for 1 h undera nitrogen atmosphere and cooled to room temperature beforemeasuring the thermal stability. All samples showed excellentthermal stabilities, i.e. 5% weight loss at 435 1C (air) and 450 1C(nitrogen). For reference purposes, the thermal stability of a hightemperature epoxy-based adhesive ranges from 380 to 425 1C,depending on the curing agent used [14]. We observed that thereis virtually no relation between oligomer molecular weight on thethermal stability of the final LC thermoset.
3.2. Dynamic mechanical thermal analysis
The storage modulus (E0) and loss modulus (E00) of our fullycured LCTs were measured using DMTA in the temperature rangeof �100 to 400 1C at a frequency of 1 Hz. These films show highstorage moduli up to 150 1C, i.e. 1.0 GPa for HBA/HQ/IA(25)-5Kand this demonstrates that the high degree of crosslinking in the5000 g mol�1 LCT has a beneficial effect on retaining a highstorage modulus at high temperatures. Furthermore, crosslinkingresults in a significant increase of the glass-transition tempera-ture (Tg) when compared to their high molecular weight counter-parts. Final Tg values of 164–181 1C for our fully crosslinked 5 and9 K oligomers denote an increase of 40–60 1C [15]. The storagemoduli (E0) and glass-transition temperatures for the HBA/IA/HQ(25) series are reported in Table 1. The storage modulus (E0)and Tan d as a function of temperature for a fully cured HBA/HQ/IA (25)-5K film is shown in Fig. 2.
Table 1Thermal and mechanical properties of the HBA/HQ/IA(25) based liquid crystal thermos
Sample Tm (1C) Td (1C) N2/Aira Char yield (wt%)
HBA/HQ/IA(25)-5K 328 450/435 54
HBA/HQ/IA(25)-9K 346 456/434 52
a TGA experiments performed at a heating rate of 10 1C min�1.b The storage modulus (E0) and Tg data were obtained from DMTA experiments usin
and a frequency of 1 Hz.
Fig. 1. DSC thermogram of our HBA/HQ/IA(25)-5K reactive oligomer. First heat
(10 1C min�1) under nitrogen. The K-N transition (Tm) can be seen at �330 1C.
3.3. Polarized optical microscopy
Polarizing optical microscopy was used to investigate thenematic to isotropic (N–I) transition temperatures and nematicphase stability of our reactive oligomers. All oligomers showednematic textures, but nematic-to-isotropic (N–I) transition couldnot be observed. Upon rapid heating above 340 1C, the chainextension- and crosslink chemistry results in a fixed nematictexture, which becomes difficult to shear. The nematic texture of a5000 g mol�1 oligomer, i.e. HBA/HQ/IA(25)-5K before and aftercure, is shown in Fig. 3 and these textures are representative forall our reactive oligomers.
3.4. Adhesive properties
In order to find the optimum processing conditions forpreparing our lap-shear specimen, some initial experiments wereperformed using our HBA/HNA-9K reactive oligomer. Differentlap-shear consolidation pressures were applied during curing andthe lap-shear test values were evaluated. The data seems toindicate that the cure pressure does not significantly affect thefinal lap-shear strength, as can be seen in Fig. 4. However, thelap-shear values decrease somewhat at high consolidationpressures, i.e. 9 bar, because of the low viscous nature of ourliquid crystalline oligomers. The resin was easily expelled fromthe joint and this resulted in insufficient material for adhesionand a reduction in adhesive strength. The latter was confirmed bythe post-mortem inspection of the lap-shear joints, where resinpoor areas were clearly visible. Based on these results we useda consolidation pressure of 1.5 bar for all experiments.
To observe the effects of molecular weight and crosslinkdensity on the adhesive properties of our liquid crystal thermo-sets, HBA/HNA oligomers with various molecular weights, i.e.
Mn¼1000, 5000 and 9000 g mol�1 were investigated. A high
ets.
E0 (GPa) (25 1C)b E0 (GPa) (100 1C) E0 (GPa) (150 1C) Tg (1C)
2.6 2.1 1.03 181
2.9 2.3 0.25 164
g fully cured films. Measurements were conducted at a heating rate of 2 1C min�1
Fig. 2. The storage modulus (’) and tan d (K) of a fully cured HBA/HQ/IA(25)-5K
film as measured by DMTA using a heating rate of 2 1C min�1 and measured at
1 Hz (nitrogen atmosphere).
Fig. 3. Microphotographs of HBA/HQ/IA(25)-5K between crossed polarizers: (A) nematic texture before crosslinking and (B) nematic texture locked in after crosslinking for
1 h at 370 1C.
Fig. 4. The lap-shear strength as a function of different consolidation pressures
during isothermal curing at T¼370 1C (SD¼0.25–0.52). The reactive oligomer used
was a 9000 g mol�1 random co-polyester based on HBA/HNA (HBA/HNA-9K) [9].
Al2024 was used as the substrate.
Fig. 5. Lapshear values for our HBA/HNA-based reactive oligomer series on Al2024
substrates (‘‘ne’’ refers to a high molecular weight polymer without reactive end-
groups). An optimum between lap-shear strength and end-use temperature (Tg) is
found to be around 5000–9000 g mol�1 (SD¼0.25–0.35).
Fig. 6. Lapshear values for our HBA/HNA(27)-9K and HBA/HQ/IA(25)-9K based
oligomers. GB¼glass beads, GC¼glass impregnated cloth and Al¼Al2024
(SD¼0.25–0.42), MS¼martinsitic stainless steel (SD¼0.25–0.42) and Ti¼Ti6Al4V
(SD¼1.8).
M. Iqbal et al. / International Journal of Adhesion & Adhesives 30 (2010) 682–688686
molecular weight HBA/HNA polymer without reactive end-groupswas used for reference purposes and the results are presented inFig. 5. In order to control the bond-line we blended glass beads
with a diameter of 0.17 mm with our reactive oligomer powder.Aluminum (Al2024) was used as the metal substrate.
From the data presented in Fig. 5 it is evident that our1000 g mol�1 oligomer displays rather poor adhesive properties,i.e. a lap-shear value of �5 MPa was obtained. When themolecular weight of the precursor oligomer was increased to5000 or 9000 g mol�1 a significant increase in lap-shear strengthwas observed. Increasing the molecular weight at this point doesnot appear to be useful since this results in a reduction in Tg andhence a reduction in final use temperature. In all lap-shearspecimens we found that resin had been expelled from the bond-line, a consequence of the low viscous nature of our LC oligomers;therefore, we anticipate that the reported values can be improvedupon when this process is optimized.
3.5. Lap-shear strength of HBA/IA/HQ(25)-based polymers
Based on the encouraging results obtained for the first series ofHBA/HNA-based oligomers, we decided to synthesize and evalu-ate the properties of HBA/HQ/IA(25)-based oligomers with targetmolecular weights of 5000 and 9000 g mol�1. The glass transitiontemperatures (Tg) for the HBA/HNA-based thermosets were in therange of 100–138 1C whereas these new thermosets have higherTg values in the range of 164–181 1C. Since the cured thermosets
M. Iqbal et al. / International Journal of Adhesion & Adhesives 30 (2010) 682–688 687
have higher Tg values it is expected that these polymers can beused at higher maximum operating temperatures.
To study the effect of the bond line thickness on the lapshearstrength we performed several lap-shear experiments usingAl2024 where we used glass beads (f¼0.17 mm) or a glass scrimcloth with a thickness of 0.25 mm to control the thickness of thebond line. We observed that there was no significant difference inthe lap-shear values when glass beads (Al-GB) or resin impreg-nated glass cloth (Al-GC) was used. The results are shown in Fig. 6.
In order to explore the adhesive behavior of our new HBA/HQ/IA(25)-9K resin different metal substrates were used, i.e. alumi-num 2024, titanium 6Al4V and martinsitic stainless steel. The lap-shear values found for this series are summarized in Fig. 6. Weobserved almost similar lap-shear values for martinsitic stainlesssteel (MS-GC) and aluminum (Al-GB or Al-GC), i.e. �12 MPa.Titanium 6Al4V seems to be a suitable substrate as well althoughthe reproducibility in lap-shear values is somewhat less, i.e. lap-shear values between 11 and 16 MPa were obtained. We observeda considerable increase in the lap-shear strength as compared toHBA/HNA-based oligomers but these values are still somewhatlower than that of commercial epoxies [16]. It has to be noted atthis point that no primers were used in this study, which suggeststhat the reported values can even be improved upon when asuitable high temperature primer is applied.
Fig. 7. Lap-shear strength measured at different temperatures using (’) HBA/HQ/
IA(25)-5K and (K) HBA/HQ/IA(25)-9K reactive oligomers on Al2024 substrates.
The Tg of each polymer is included for reference purposes.
Fig. 8. Scanning electron micrographs (HRSEM): (A) showing cohesive failure of the ad
substrate.
To investigate whether our LCTs show potential for hightemperature applications a series of lap-shear tests at elevatedtemperatures were performed, i.e. 150, 200 and 250 1C usingAl2024 substrates. The results are summarized in Fig. 7. Weobserved that HBA/HQ/IA(25)-9K not only showed high lap-shearvalues at room temperature but this polymer retained itsproperties up to 150 1C where we observed a lap-shear strengthof �11.5 MPa. It was also shown by our measurements that atelevated temperatures these adhesives fail in a ductile fashion. Theelongation at break was 9% at room temperature and this valueincreased to 15% at 150 1C (Standard deviation (SD)¼0.39 at150 1C). When the temperature was increased to 200 1C, i.e. thetemperature is above the Tg of the polymer, the lap-shear strengthdropped sharply to �4 MPa. This decreasing trend was continuedupon further heating to 250 1C. We performed control experimentsusing our Al2024 substrates and found that the tensile strength ofAl2024 was 4250 MPa, after the same treatment as we used forcuring. Also, the dimensional change of Al2024 at roomtemperature appears to be negligible up to 50 MPa, a value farabove the max lap-shear strengths measured for our samples.
When a 5000 g mol�1 reactive oligomer was used, i.e. HBA/HQ/IA(25)-5K, the lap-shear values at room temperature and150 1C drop to �7 MPa. This effect could be attributed to thehigher crosslink density. However, a higher degree of crosslinkingis advantageous above the polymer Tg, as can be seen in Fig. 7.HBA/HQ/IA(25)-5K retains its lap-shear strength somewhat betterabove its glass-transition temperature (Tg¼181 1C) as comparedto fully cured HBA/HQ/IA(25)-9K, which has a lower degree ofcrosslinking. Our experiments demonstrate that we are able totune the adhesive properties of our new polymers by controllingthe oligomer length and hence the crosslink density.
Finally, high resolution scanning electron microscopy (HRSEM)was employed to inspect the lap-shear joints after failure. Represen-tative microphotographs are shown in Fig. 8 and they reveal thatfailure takes place mainly in the polymer adhesive with some signs ofadhesive failures at the polymer/glass–fabric interface. There were nosigns of adhesive failure on the polymer–metal interface for bothroom temperature and elevated temperature (150 1C) experiments.
4. Conclusions
The use of all-aromatic ester-based liquid crystal thermosets(LCTs) as adhesives for metallic substrates at high temperatures(4100 1C) was successfully demonstrated. Our LCTs exhibit excellentthermal and mechanical properties, i.e. Tg’s up to 181 1C, storagemoduli of 2.9 GPa at room temperature and 1.0 GPa at 150 1C after
hesive joint at room temperature and (B) at 150 1C. HBA/HQ/IA(25)-9K on Al2024
M. Iqbal et al. / International Journal of Adhesion & Adhesives 30 (2010) 682–688688
a 45 min cure at 370 1C. The adhesive properties of our LCTs wereinvestigated using standard lap-shear experiments conducted atroom temperature and at elevated temperatures, i.e. 150, 200, and250 1C. The best lap-shear test results were obtained for our HBA/HQ/IA(25)-9K oligomer, i.e. 11, 12, and 16 MPa for Al2024, MS-GC and Ti6Al4V, respectively. For the Al2024 substrate the lap-shear strengthremained constant up to 150 1C and dropped to �4 MPa at 200 1C. Apost-mortem inspection of the fracture surface, using high-resolutionscanning electron microscopy (HRSEM), showed that for all experi-ments the failure occurred cohesively, thus inside the polymer, andnot at the polymer-metal interface. The lap-shear values of our LCTsare significantly higher as reported for other commercial availableLCPs such as VectraTM, which makes them potential candidates forhigh temperature adhesive applications. New ester–amide and ester–imide LCT chemistries are currently under investigations in our groupand it is our aim to provide all-aromatic liquid crystal thermosets foradhesive applications with continuous use temperatures in excess of300 1C.
Acknowledgements
We would to thank Mr. Frans Oostrum for his help with theHRSEM experiments. This work was sponsored in part by the
NIVR (Netherlands Agency for Aerospace Programs), Ticona GmbHand Ten Cate.
References
[1] Pizzi A, Mittal KL. Handbook of adhesive technology. 2nd ed. New York:Marcel Decker, Inc.; 2003. (Chapter 3), p. 53–7.
[2] Cognard P. Adhesives and sealants: basic concepts and high tech bonding.Oxford: Elsevier Ltd.; 2005. (Chapter 5), p. 237–41.
[3] Higgins A. Int J Adhes Adhes 2000;20:367.[4] Hergenrother PM. High Perform Polym 2003;15:3.[5] da Silva LFM, Adams RD. Int J Adhes Adhes 2007;27:362.[6] Economy J, Gogeva T, Habbu V. J Adhes 1992;37:215.[7] Economy J, Andreopoulos AG. J Adhes 1993;40:115.[8] Frich D, Economy J, Goranov K. Polym Eng Sci 1997;37:541.[9] Knijnenberg A, Weiser ES, St. Clair TL, Mendes E, Dingemans TJ. Macro-
molecules 2006;39:6936.[10] Iqbal M, Norder B, Mendes E, Dingemans TJ. Polym Sci Part A: Polym Chem
2009;47:1368.[11] Dingemans TJ, Knijnenberg A, Iqbal M, Weiser ES, St. Clair TL. Liq Cryst Today
2006;15:19.[12] Dingemans TJ, Weiser ES, St. Clair TL. U.S. Patent. 6,939,940, 2005 (to the
National Aeronautic and Space Administration).[13] Calundann GW U.S. Patent 4,161,470, 1979 (to the Celanese Corporation).[14] Bansal RK, Agarwal R. Die Angew. Makromol. Chem. 1984;127:43.[15] Erdemir AB, Johnson DJ, Tomka JG. Polymer 1986;27:441.[16] http://www.hexcel.com/NR/rdonlyres/6BB90127-DAE
B-4664-A7E5-23B1D3906FFC/0/Redux322_eu.pdf.