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epoxy based adhesives for aluminIum joints
Author’s Accepted Manuscript Structural METHACRYLATE/EPoXY BASED adhesives for AluminIum JOINTS Karina Kerber Hennemann, Denise Maria Lenz
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S0143-7496(18)30259-8 https://doi.org/10.1016/j.ijadhadh.2018.11.006 JAAD2294
To appear in: International Journal of Adhesion and Adhesives Accepted date: 4 November 2018 Cite this article as: Karina Kerber Hennemann and Denise Maria Lenz, Structural METHACRYLATE/EPoXY BASED adhesives for AluminIum JOINTS, International Journal of Adhesion and Adhesives, https://doi.org/10.1016/j.ijadhadh.2018.11.006 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 galley proof before it is published in its final citable 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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STRUCTURAL METHACRYLATE/EPOXY BASED ADHESIVES FOR ALUMINIUM JOINTS
Karina Kerber Hennemann1,2, Denise Maria Lenz2* 1
Artecola Química Programa de Pós-graduação em Engenharia de Materiais e Processos Sustentáveis, Universidade Luterana do Brasil, Avenida Farroupilha 8001, Prédio 14, sala 215, Canoas - RS, Brazil. 2
ABSTRACT
The aim of this work was to investigate the influence of elastomer and monomer concentration on the properties of methacrylate/epoxy based adhesive formulations using a Plackett-Burman experimental design. The adhesive formulations were developed in order to bond non-treated aluminium panels. Their chemical structure was analysed by Fourier transformed infrared spectroscopy (FTIR) and the thermal behaviour by thermogravimetry (TGA) and differential scanning calorimetry (DSC) in comparison to a commercial adhesive. After room temperature mixing, time and temperature of the curing reaction were measured. Further, non-treated aluminium single-lap joints were adhesively bonded and their shear strengths were evaluated. The formulations showed chemical structure similar to a commercial adhesive but the thermal behaviour was quite distinct. The commercial adhesive cured slower than the developed adhesive formulations which can directly affect the bonding quality. However, some formulations showed higher shear strengths than the commercial one, but all with adhesive failure mode. The adhesive formulations with elastomers in methacrylate part showed the highest shear strengths. Keywords: epoxides (A), structural acrylics (A), aluminium and alloys (B), lap-shear (C). ______________________________________________________________________ 1. Introduction Adhesive bonded joints have been designed for automotive, aerospace and civil industries due to their advantages over the traditional joining technologies as corrosion resistance, weight reduction and elimination of stress concentration due to the fastener mounting hole [1]. These industries have also focused on aluminium alloys and composites bondings which can be used in aerospace components, satellite components, heat sinks, cryostat, LED assemblies and lasers. Structural adhesives are considered the
Corresponding author. E-mail address: [email protected]
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evolution of the conventional joining systems being able to be used in weather demanding and in high tensile demands since they must be capable of transmitting stress from one element of the joint to another without losing their integrity. Thus, the stresses are distributed more uniformly than in mechanical methods of joining [2]. However, the performance of structural adhesives is often limited by their poor mechanical properties, i.e., the adhesives are usually much weaker than the adherends they join, thus studies on adhesive formulations must be performed in order to develop reliable knowledge for adhesive technologies [3]. Most of the studies regarding structural adhesives are mainly for epoxy or acrylate-based adhesives. Epoxy-based adhesives are used due to their hardness and excellent weatherability characteristics despite their slow cure speed and high brittleness. The properties of cured epoxy adhesives can be tailored by the curing agent and the curing process as well [4]. The use of nanoparticles in adhesives can improve the mechanical strength of structural adhesive joints [3, 5]. Acrylate-based adhesives are known for their fast curing process and excellent bonding to a wide variety of materials. The main components of (meth)acrylic adhesives are (meth)acrylic monomers or polymers, initiating systems, hardeners (curing agents), free radical stabilizers and modifiers (accelerators) [6]. Methyl methacrylate (MMA) is often used as the main monomer since it provides high adhesive strength and is a good solvent for a number of polymers. When combined with oils, MMA makes possible to glue untreated surfaces [7]. Acrylate-based adhesives have been modified by many methods including crosslinking polyacrylates, copolymerizing acrylates with other monomers and blending polyacrylates with other polymers in order to reduce the brittleness and poor film-forming properties [8,9].
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Current approaches towards structural acrylate/epoxy adhesives are mostly restricted to patents [10-14] which combine the properties of both fully cured ingredients, although some works have already studied this blend system. Blends of epoxy resin up to 50 vol% of poly(methyl-methacrylate) (PMMA) show different levels of apparent miscibility due to variations in phase separation-morphology of PMMA and in polymerization rate [15]. Curing time and curing temperature have also influence on phase miscibility and consequently on glass transition temperature of these systems [16]. A significant increase in impact strength without changes in the tensile strength, modulus and glass transition temperature with addition of 10 phr of an acrylic copolymer in epoxy resin matrix was already reported [17]. The improvement of adhesive toughness can be explained by the nanostructures formed upon polymerization [9]. A blending system with 90 wt% of acrylic resin and 10 wt% of epoxy resin showed also improved impact resistance properties and good adhesion on mild steel substrate [18]. Thus, an effective polymer blend based on epoxy and PMMA can be formed, showing improved toughness compared to the rigid, brittle epoxy and acrylic homopolymers [19]. This work has developed two-ingredient structural adhesive formulations with methacrylate and epoxy adhesives for joining difficult-to-anchor substrates such as aluminium without pretreatment. The influence of elastomers and acrylic monomers of the adhesive formulations on time and temperature of cure, thermal behaviour and shear strength were evaluated. 2. Materials and Methods 2.1 Materials
4
Adhesive formulations with two-ingredients (A-B) were developed. Table 1 presents the formulation of the ingredients A and B of the adhesive.
Table 1: Formulation of the ingredients A and B of the structural adhesive. Abbreviation/ Chemicals
Commercial
Function
Supplier
name methyl methacrylate
MMA
Monomer
Rudnik
methacrylic acid
MA
Comonomer
Rudnik
isobornyl methacrylate
IBOMA
Comonomer
HYPRO RLP polybutadiene
VTB 2000 X168LC
Ingredient A
styrene and isoprene-based triblock copolymer
Kraton D1161
Elastomer (toughener) Elastomer (toughener)
SigmaAldrich
Emerald
Kraton
N, N-dimethyl p-toluidine
DMPT
Reaction initiator
Rudnik
p-methoxy-phenol
MEH
Reaction inhibitor
Rudnik
zinc carboxylate
®
Catalyst
Shepherd
BiCAT Z 1365
Inhibitor of evaporation paraffin wax
Bareco PX 105
of volatile reactive
Megh
monomers ethylene diamine tetraacetic acid zinc dimethacrylates mono- and di-phosphate esters of 2-Hydroxyethylmethacrylate N-dimethyl p-toluidine DMPT
Ingredient B
diglycidyl ether of bisphenol A poly (methyl methacrylate-bbutyl acrylate- b- methyl methacrylate)
EDTA
Emulsifier
AkzoNobel
Dymalink® 708
Adhesion promoter
Total
Harcryl® 1228
Adhesion promoter
Harcros
Dissolvine E-39
Vestamin® TMD DGEBA EponTM Resin 828 Nanostrength® M52N
Ingredient B Curing agent
Evonik
Epoxy resin
Hexion
Elastomer (toughener)
Arkema
a combination of benzoate esters
BenzoflexTM 2088
Plasticizer
Eastman
benzoyl peroxide
BPO
Ingredient A curing
DiproFiber
5 agent
For ingredient A (methacrylate-based), the monomers were: methyl methacrylate MMA as primary monomer and methacrylic acid MA and isobornyl methacrylate IBOMA comonomers, all industrial grades. Since methacrylate-based polymers are brittle materials, two elastomers were added to increase their toughness: a vinyl terminated polybutadiene HYPRO and a styrene and isoprene-based triblock copolymer Kraton. The free-radical polymerization of ingredient A was promoted by a curing system consisted of N, N-dimethyl p-toluidine DMPT as initiator and benzoyl peroxide BPO (oxidizing agent). The role of the tertiary amine DMPT is to carry out the redox (reduction–oxidation) initiation, together with BPO in a short period of time at ambient temperature. Additives used were: zinc carboxylate (BiCAT®) as catalyst that mediate the redox equilibrium process, p-methoxy-phenol MEH as reaction inhibitor (radicaltrapping agents), paraffin wax and the EDTA emulsifier. Also, dimethacrylates (Dymalink® and Harcryl®) were used as adhesion promoters. A simplified polymerization reaction mechanism that provides several crosslinking sites is described by Equations 1, 2 and 3, adapted from Garra et al. [20]. Initiation Step (after mixing): DMPT + BPO → Radical∙
(1)
Propagation Step: ∙
∙
Radical + Monomer and Comonomers (M) → R(M) n
(2)
Inhibition Step: ∙
R(M) n + MEH → Inactive species
(3)
For ingredient B, (epoxy-based), the diglycidyl ether of bisphenol A (DGEBA) was used. This resin has an equivalent weight of epoxy resin (EEW) of 185-192 g.eq-1 and a viscosity of 110-150 cPs at 25ºC. Epoxy resins are inherently brittle and thus acrylic tri-block MAM copolymers were used for epoxy formulations. MAM
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copolymers known as Nanostrength® have a poly (butyl acrylate) centre block which is a soft rubber phase for toughening and two poly (methyl methacrylate) side blocks (melting point of 160ºC and particle size range of 10~100nm). As curing agent for epoxy resin, trimethylhexamethylenediamine TMD was used. According to the suppliers, EEW for DGEBA resin is in range of 185-192g.eq-1 and the AHEW (Amine Hydrogen Equivalent Weight) of TMD is approximately 79.2 g.eq-1. Equation 4 was used to define the concentration of the curing agent for each formulation. Also, as plasticizer, a combination of benzoate esters named BenzoflexTM was used in ingredient B. AHEW X 100 = mass g of curing agent per 100g of resin EEW
(4)
A hydrophilic fumed silica named Cab-o-sil® TS-720 was added (0.05 g per 100g of the formulation) as thixotropic agent only to cured adhesive formulations subjected to shear experiments. Silica was used as a thickening and to reduce flow of the adhesives since the formulations were in liquid state before curing. The Cab-o-sil® TS-720 has an average particle size of 0.2–0.3 microns.
2.2 Plackett-Burman (PB) experimental design The PB12 Plackett-Burman experimental design was used to evaluate the effect of five variables (factors) related to the concentration of elastomers in ingredients A and B and the IBOMA and MA monomers in ingredient A on thermal and mechanical properties of two-ingredient adhesive. The five factors were varied simultaneously in three levels: −1 for low level, 0 for the medium level and +1 for high level based on Plackett–Burman matrix design. The values of three levels were set according to our previous preliminary experimental results and the aforementioned patents. Table 2 shows the quantities (in mass) for the five variables investigated considering the
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production of 200g of ingredient A and 100g of ingredient B. After that, the two ingredients of adhesive were mixed in a mass ratio of 10:1 (A:B). In order to maintain the same ratio between monomers and elastomers concentration in all formulations, some adjustments were made in the levels of the experiments. Thus, only a qualitative evaluation of the significant effects can be performed. The PB12 Plackett-Burman experimental design is shown in Table 3 and it is composed by 13 adhesive formulations which include a central point formulation (HKN27 formulation) made in triplicate for evaluation of experimental reproducibility. The codes of the formulations in Table 3 are assigned to: H – with HYPRO elastomer in ingredient A, K – with KRATON elastomer in ingredient A, N – with Nanostrength® elastomer in ingredient B, the first number makes reference to MA monomer amount for 200g of ingredient A and the second number represents the IBOMA monomer amount for 200g of ingredient A, according to the bold numbers in Table 2.
Table 2. Adhesive formulation variables and their quantities for the three-leveled (-1, 0 and 1) PB-12 experimental design. Mass (g) Variables
Raw materials
x1
-1
0
1
HYPRO RLP VTB 2000X168 LC Elastomer*
0
23.13
18.60
x2
Kraton D 1161 Elastomer*
0
18.60
23.13
x3
Nanostrength® M52N Elastomer**
0
7.00
9.72
x4
Methacrylic acid (MA) Monomer
10.00
26.21
54.40
x5
Isobornyl methacrylate (IBOMA) Monomer
10.00
27.20
13.10
* - elastomer added in 200g of ingredient A
** - elastomer added in 100 g of ingredient B
Table 3. The PB12 Plackett-Burman experimental design. Experimental Run 1
Formulation Code HN11
x1
x2
x3
x4
x5
1
-1
1
-1
-1
8
2 3 4 5 6 7 8 9 10 11 12 13-A 13- B 13-C
HK51 KN13 HN51 HK53 HKN13 KN51 N53 53 H13 K11 11 HKN27-A HKN27-B HKN27-C
1 -1 1 1 1 -1 -1 -1 1 -1 -1 0 0 0
1 1 -1 1 1 1 -1 -1 -1 1 -1 0 0 0
-1 1 1 -1 1 1 1 -1 -1 -1 -1 0 0 0
1 -1 1 1 -1 1 1 1 -1 -1 -1 0 0 0
-1 1 -1 1 1 -1 1 1 1 -1 -1 0 0 0
2.3 Adhesive preparation Ingredients A and B were prepared separately. A glass reactor equipped with an anchor type mechanical stirrer, condenser and monomer doser was used for preparation of 200 g of ingredient A (Figure 1). Initially MMA and MA monomers were introduced in the reactor with the aid of a monomer doser and after 2 g of EDTA emulsifier, 0.02 g of p-methoxy-phenol MEH, 2 g of adhesion promoter Dymalink® and the HYPRO elastomer were added under stirring (1000 rpm) for 30 minutes at 70ºC ± 2. Paraffin wax (9 g) was added further with stirring for two hours at 110 °C ± 2. In the next step, the Kraton elastomer was added with stirring for one more hour at 115ºC. After that, the system was cooled to 80°C and IBOMA monomer was slowly added for 30 minutes with further stirring for ten minutes and cooling up to 45 °C for addition of the curing agents DMPT (1.25 g) and TMD (1 g), the adhesion promoter Harcryl® (2 g) and the BiCat Z catalyst (2 g) with stirring of twenty minutes. Ingredient A formulations were stored in polypropylene flasks.
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Fig 1. Reactor system for preparation of ingredient A formulations.
For preparation of 100g of ingredient B, an aluminium container was used with a mechanical stirrer with dispersing disc type blades for addition of 45-50 g of Benzoflex plasticizer and 23-25 g of BPO cure agent for one hour at 1000 rpm. After, Nanostrength® elastomer was added with a 30 minutes-stirring and right after the DGEBA resin was added and stirred for more 30 minutes. BPO is ingredient A cure agent which was added to ingredient B as well as TMD is ingredient B cure agent which was added to ingredient A. This procedure was used so that the curing reaction occurs only when both ingredients were mixed in a mass ratio of 10:1 (PMMA:DGBEA).
2.3 Adhesive characterization Ingredients A and B of the adhesive were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) using a Shimadzu Affinity spectrophotometer. Thermogravimetric analysis (TGA) was performed in a TGA Q50 (TA Instruments) thermogravimetric analyser under N2 flow from 25 to 600ºC at 10°C/min. Differential scanning calorimetry measurements (DSC) were carried out in a Jade DSC instrument (Perkin Elmer) at 10°C/min over the temperature range of -70 to 70°C (uncured ingredient B) and 0 to 300ºC (cured adhesive) under N2
10
flow. The cured adhesives were kept in a desiccator at room temperature and analysed after seven days of cure reaction. Ingredient A was not analysed by DSC since the acrylic monomer can damage the instrument. Time and maximum temperature reached in the cure reaction were measured according to ASTM D 2471-99 standard. All formulations were cured in triplicate at room temperature (25 ° C ± 2) and relative humidity of 55%. As the crosslinking speed of the adhesive formulations was high, ingredients were stirred for five seconds and the temperature of the bulk specimens was measured using a pyrometer. Lap Shear strengths (ASTM D-1002 standard) of the cured adhesives were measured on 1" wide x 1" single overlap aluminium specimens (6.25 cm2). Unclad 2024-T3 aluminium substrates with aluminium fraction of 90.90 – 99.0% and cooper fraction of 3.80 – 4.90 % were only cleaned using a towel paper carefully. After, 2.000g of adhesive formulations were applied on aluminium specimens using a mixer (Loctite) and then joined in a single overlap without pressure application. After 72 hours of adhesive application, specimens were subjected to shear stress at a shear rate of 5 mm/min. ASTM D-1002 is capable of providing relative joint strengths, when comparing different adhesive systems. All experiments were carried out in triplicate at room temperature and the characterization tests with adhesive formulations were compared to a two-ingredient commercial adhesive (SAF®150 (Alphakem) assigned as adhesive C) prepared in a mass ratio of 10:1 (PMMA:DGBEA).
3. Results and Discussion 3.1 Chemical structure of adhesive formulations
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FTIR spectra of the ingredient A (methacrylate-based) formulations showed almost the same bands, identified according to Smith [21] and Skarma [22], although some differences were found in band intensities as well as in wavenumber shifts (Table 4). Figure 2 shows the spectra for ingredient A formulations with elastomers. The bonds found at higher intensities are directly related to methyl methacrylate, methacrylic acid, dimethyl-p-toluidine and trimethylhexamethylenediamine. Figure 3 shows the spectra for some formulations with ingredient B of the adhesive since all spectra are also similar. Characteristic bands of the epoxy group, based on Smith [12], can be found at: 1246 and 1222 cm-1 related to C−O−C asymmetrical stretching, at 1718 cm-1 assigned to ester bondings due to BenzoflexTM plasticizer presence as well as at 1273 cm-1 corresponding to C−O−C bonded to aromatic ring and at 1109 cm-1 due to O−C−C bonding from aromatic ester. Significant differences were observed at 1718 cm-1 and 1510 cm-1 (bondings from aromatic rings) with the highest intensity in HKN27 formulation and at 1602 cm-1 with the lowest intensity in HK51 formulation which is attributed to C=C bondings from aromatic rings. FTIR spectra of ingredients A (methacrylate-based) and B (epoxy-based) of a commercial adhesive (adhesive C) showed the same bands with some shifts. Only for adhesive C, a band with significant intensity was observed at 1157 cm-1 which can be attributed to Si−O−Si from silica that can be related to mineral fillers or thixotropic agent presence [23]. Table 4. FTIR band wavenumbers for formulations with ingredient A and related vibrations. Wavenumbers (cm-1) for formulations with ingredient A and Adhesive C Vibrational assignments H2 angular bending of amine
Without Elastomer 1640
Kraton HYPRO Elastomer Elastomer 1635
1632
HYPRO + Kraton
Adhesive C (methacrylate ingredient)
1638
1637
12
C=O out-of-plane stretching of esters from methacrylate
1675
1696
1696
1694
1685, 1718
O−CH3 stretching from esters
1462
1452
1453
1463
1454
C−CH3 asymmetrical stretching
1428
1427
1450
1428
1440
1375
1376
1375
1375
1376
C−N−C bonding near to aromatic ring C−C−O bonding of aromatic ester asymmetrical stretching of carbons bonding to ester
1294
1298
1298
1296
1298
1206, 1055
1200, 1054
1202, 1081
1204, 1068
1062
C=C bonding out-ofplane bending
930
944
944
942
940
Out-of-plane bending of vinyl double bonds C=CH2
-
-
915
915
-
silicon–oxygen covalent bonds
-
-
-
-
1100,1157,1270
Si−O in-plane stretching vibrations of the Si−OH groups
-
-
-
-
949
13
Transmitance (arbitrary units)
O−CH3
C=O 1793
1696
C=C
NH2 1600
1504
1407
1311
Wavenumber
1214
1118
1021
925
(cm-1)
Fig 2. FTIR spectra for formulations with ingredient A using HYPRO and Kraton elastomers. Spectra listed from top to bottom: HK51, HK53, HKN13, HKN27-A,
Transmitance (arbitrary units)
HKN27-B and HKN27-C formulations.
C−O−C
C=O
C=C O−C−C
1954
1809
1665
1520
1375
1231
1086
941
Wavenumber (cm-1)
Fig 3. FTIR spectra for formulations with ingredient B. Spectra listed from top to bottom: HN11, HK51, HKN27-A formulations.
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3.2 Thermal properties The thermal degradation profile of all formulations is presented in Figure 4 with HKN27 formulation after cure of the two-ingredient adhesive. This thermogram shows three thermal events: (1) a continuous weight loss due to release of volatiles such as water, solvents and unreacted monomers from room temperature to around 180 ºC; (2) a thermal event possibly associated to decomposition of oily compounds and/or oligomers from 180 ºC to 320 ºC and (3) polymer (resin and elastomers) degradation from around 325 ºC to 500 ºC. The residue of around 5 wt% is related to inorganic compounds.
Temperature (℃)
Fig 4. TGA thermogram of the cured two-ingredient adhesive (HKN27 formulation). Figure 5 shows DSC thermograms for uncured formulations with ingredient B. In all adhesive formulations, the plasticizer effect on glass transition temperatures (Tg) was identified since it has decreased about 30°C with respect to neat epoxy resin (-15°C). The higher the plasticizer concentration, the lower Tg and heat capacity, as shown in Table 5 and also reported by KU et al. [24]. Additionally, the elastomer showed influence on the mobility of the polymer chain since a melt temperature of 0,1°C was observed for HK51 formulation which has no elastomer (endothermic peak
15
in Figure 5). The Nanostrength® M52N elastomer used only in ingredient B is composed by nanometric material which can avoid the nucleation of some crystals formed on epoxy formulation cooling. The same was reported by Zaman et al. [25] who introduced nanoclay in epoxy resin and reported an increase in Tg which was attributed to formation of an interpenetrating network morphology that could occur in HN11 and HKN27 formulations (Table 5). -18
Heat flow (mW) exothermic→
-19 -20 -21 -22 -23 -24 -25 -26 -27 -28 -72 -64 -56 -48 -39 -31 -23 -14 -6
2 11 19 27 36 44 52 61
Temperature (°C)
Fig 5. DSC heating thermograms of uncured epoxy resin (ingredient B) of the adhesive. From top to bottom: HN11, HK51 and HKN27 formulations. Table 5. Glass transition temperatures and heat capacities of uncured ingredient B adhesive formulations from DSC measurements. Raw material / Formulation Neat epoxy resin
Elastomer Plasticizer Tg ∆Cp (weight %) (weight %) (°C) (J.g-1) 0.0 0.0 -15.1 0.60
HN11
9.2
45.14
-47.4
0.43
HK51
0.0
49.99
-48.1
0.40
HKN27
7.0
46.49
-44.5
0.82
Through DSC analysis of the two-ingredient adhesive after seven days of cure reaction, some thermal events were observed, depending on the adhesive formulation.
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In the first scan at around 60°C, a little and broad exothermic peak was observed for all formulations (excepting the HKN27 formulation) that could be attributed to a post-cure event with release of volatiles and an endothermic event at 223 and 231°C attributed to decomposition of some compounds as found at the same temperature range by thermogravimetric analysis. On cooling scanning, some Tg values were observed from 100 to 110°C for all formulations. The presence of just one Tg value after the first scan suggests a relative miscibility of the two polymer phases. Adhesive C showed a distinct thermal behaviour with two Tg values: around 120°C and at 200°C, probably due to methacrylate ingredient and epoxy ingredient, respectively. Figure 6 shows the DSC thermograms for HKN27 formulation and adhesive C.
Fig 6. DSC heating thermograms of the commercial adhesive C (above) and HKN27 adhesive formulation (below). Remiro et al. [16] found Tg values of around 180°C for cured neat epoxy resin (DGEBA) and 113°C for cured neat PMMA. Mixing and curing DGEBA and 15 wt. % of PMMA, Tg values have depended on the curing time. After five hours of curing at
17
80°C, only one Tg value was found, around 150°C, but below this time two Tg values were found: around 116 and 160°C, the higher Tg can be attributed to an epoxy-rich phase, and the lower one is reasonably ascribed to a PMMA-rich phase.
3.3 Time and temperature for cure reaction Table 6 shows the parameters of cure reaction of adhesive formulations. The mass loss after mixing is related to acrylic vapors released by an exothermic reaction (boiling point at 101°C) and the mass gain after seven days is attributed to water absorption, according to Parker et al. [26]. The commercial adhesive C showed the longest gel time for and the lowest maximum temperature achieved for cure reaction, followed by HKN27 formulations. Figure 7 shows the cure behaviour for all adhesive formulations (HKN27-A, -B and -C formulations are overlapping). The elastomer and IBOMA monomer added to HKN27 formulation in higher concentration can be affected the cure time since it has polymer chains with greater both steric hindrance and molar mass with respect to other monomers. The formulations 11, K11 and KN13 showed the fastest cure reactions. The presence of methyl methacrylate monomer in higher concentration than in other formulations and the absence of elastomer probably have contributed to higher rates of cure reaction. The elastomer and IBOMA monomer added to HKN27 formulation in higher concentration can be affected the cure time since it has polymer chains with greater both steric hindrance and molar mass with respect to other monomers. The formulations 11, K11 and KN13 showed the fastest cure reactions. The presence of methyl methacrylate monomer in higher concentration than in other formulations and the absence of elastomer probably have contributed to higher rates of cure reaction.
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Table 6. Maximum temperature, time, mass loss after mixing and mass gain after seven days for cure of the two-ingredient adhesive formulations and Adhesive C.
Formulation
Maximum Temperature (°C)
Gelatinization Time (s)
Mass loss after mixing (%)
Mass gain after seven days (%)
HN11
143
157
6
2
HK51
144
73
3
1
KN13
133
93
5
2
HN51
118
140
7
1
HK53
144
107
5
1
HKN13
113
117
8
1
KN51
115
125
6
2
N53
137
135
6
1
53
140
115
6
1
H13
136
105
8
2
K11
139
75
6
2
11
134
80
5
1
HKN27-A
133
197
7
2
HKN27-B
133
197
7
2
HKN27-C
133
197
7
1
Adhesive C
120
250
6
1
19 160 HN11 HK51
140
KN13
Temperature (°C)
120
HN51 HK53
100
HKN13 KN51
80
N53 53
60
H13 K11
40
11 20
HKN27-A HKN27-B
0 30
50
70
90
110
130
150
170
190
210
230
HKN27-C
Time (s)
Fig 7. Temperature and time behaviour for cure reactions of two-ingredient adhesives. Curves listed from left to right: K11, 11, KN13, HK53, H13, HKN13, 53, KN51. HK51, N53, HN11, HN51, HKN27-A, HKN27-B, HKN27-C formulations (the latter three are overlapping).
3.4 Lap shear strength experiments Figure 8 shows the maximum shear force that can be applied to single-lap joints bonded with cured adhesive formulations and Adhesive C.
Considering the same
bonding area (6.25 cm2) for the specimens, the adhesive formulations that used both elastomers in ingredient A (called HK formulations) showed the greater shear strength average as in HK51, HK53, HKN13 and HKN27 formulations, the latter one showing an average of 730 N. With respect to acrylic monomers, higher MA and IBOMA concentration in the formulation, higher the shear strength. Besides, the presence of elastomer in ingredient B has also contributed to higher strength values.
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Thus, the introduction of MA and IBOMA comonomers in methacrylate ingredient and elastomers affects positively the shear strength of the adhesive. The introduction of them have influence on physicochemical factors on the polymerization process [7]. The increase in toughness due to the elastomer is obtained either by plasticization, increasing the ductility of the adhesive or by generation of a two-phase system where the structural adhesive forms a matrix in which the elastomeric particles are imbedded and act as stress concentrators [27]. The introduction of comonomers with polarity (as MA comonomer) into the formulation leads to an increase in the strength of the adhesive compound of poly(methyl-methacrylate) [7]. The –COOH groups generated from MA comonomer introduction in ingredient A formulation would be present at the polymer surface and the variation in the initial monomer concentration of methyl methacrylate (MMA) and methacrylic acid (MA) would yield plastic material with varying MA segments in the polymer chain [28]. The isobornyl side group of IBOMA comonomer have difficulty to free rotation that result in reduced polymer chain flexibility. Besides, IBOMA comonomer has higher molar mass than methyl methacrylate. Both IBOMA characteristics change thermalmechanical properties of the adhesive [29], as observed for HKN27 adhesive formulation which has the highest concentration of IBOMA. Among the adhesive formulations, HKN27 formulation showed a proper adjustment of the amount of each type of monomer into adhesive that yielded adhesive of balanced hardness or flexibility. The failure mode between aluminium-adhesive was adhesive for all formulations and Adhesive C. In this failure, only one side of the substrate was adhered to the adhesive, leaving the other side partially or completely clean (Figure 9). It is important to emphasize that as the open time (time after adhesive is applied during which a
21
serviceable bond can be made) of the adhesive formulations and adhesive C was too short, a small part of the bonding area cannot be covered (about 5% of area in all adhesives). The adhesive failure is a consequence of little or no interfacial bonding between the adhesive and the substrate. However, not only the type of failure should be considered for adhesive quality evaluation since the adhesive-substrate interface may fail adhesively but exhibiting higher strength than a similar joint bonded with a weaker adhesive that shows a cohesive failure [30].
Average maximum shear force (N)
800 700 600 500 400 300 200 100 0
Adhesive formulations and Adhesive C
Fig 8. Average maximum force for formulations and commercial adhesive C obtained by shear strength experiments. The arrows indicate the formulations with higher shear forces. Bonding area: 6.25 cm2.
22
(a)
(b) Fig 9. Specimens after shear strength experiments: (a) HKN27 formulation and (b) commercial adhesive – adhesive C.
Conclusion Adhesive formulations with methacrylate and epoxy-based ingredients were developed and showed remarkable similarities to the chemical structure of a commercial adhesive. Shear strength, measured on aluminium substrates, can be up to 30% higher for formulations with addition of elastomers in acrylate ingredient (as in HKN27 formulation) when compared to the results obtained with a commercial adhesive for the same application. However, adhesive failures were observed for all adhesives. Temperature and time of curing reaction of HKN27 formulation were close to those of the commercial adhesive. Thermal analyses showed thermal events for all formulations developed which can be assigned to uncured monomers or additives degradation that were not detectable in thermograms of the commercial adhesive. Thus, HKN27 formulation has demonstrated a good potential to be used as structural adhesive.
23
Acknowledgements Authors are grateful to Artecola Química S.A. for the supply of raw materials for the adhesive formulations.
References 1. Barnes TA, Pashby IR. Joining techniques for aluminium space frames used in
automobiles. Part II - adhesive bonding and mechanical fasteners. J Mater Process Technol. 2000; 99:72–79. 2. Bartczak B, Mucha J, Trzepieciński T. Stress distribution in adhesively-bonded joints and the loading capacity of hybrid joints of car body steels for the automotive industry. International Journal of Adhesion & Adhesives. 2013; 45: 42–52. 3. Tutunchi A, Kamali R, Kianvash A. Effect of Al2O3 nanoparticles on the steelglass/epoxy composite joint bonded by a two-component structural acrylic adhesive. Soft Materials. 2016; 14(1): 1-8. 4. Pradhan S, Pandey P, Mohanty S, Nayak SK An Insight on the chemistry of epoxy and its curing for coating applications: A Detailed Investigation and Future Perspectives. Polymer-Plastics Technology and Engineerin. 2015; 55(8): 862-877. 5. Quan D, Murphy N, Ivankovic A. Fracture behaviour of epoxy adhesive joints modified with core-shell rubber nanoparticles. Engineering Fracture Mechanics. 2017; 182: 566576. 6. Allen KW. Acrylates as reactive adhesives. International Journal of Adhesion and Adhesives. 1989; 9(2): 103-106. 7. Khromiak U, Levytskyi V, Stepova K, Tarnawsky A. Synthesis and Properties of Adhesive Polymer-Methylmethacrylate Materials. International Journal of Polymer Science. 2018; Article ID 4905304, 9 pages. 8. Li M, Zheng Z, Liu S, Us Y, Wei W, Wang X. Synthesis and properties of poly(acrylatesco-urethane) adhesives for low surface energy materials. International Journal of Adhesion & Adhesives. 2011; 31: 565–570. 9. Kishi H, Kunimitsu Y, Nakashima Y, Imade J, Oshita S, Morishita Y, Asada M. Relationship between the mechanical properties of epoxy/PMMA-b-PnBA-b-PMMA block copolymer blends and their three-dimensional nanostructures. Express Polymer Letters. 2017; 11(10): 765–777. 10. Cheng CM, Murray J, Schuft CF. Adhesive compositions. US 2014/0235758 A1, 01 may. 2014, 21 aug. 2014. 11. Curet A. Composition for structural adhesive. WO2009115610 A1, 20 mar, 2009, 24 sept. 2009. 12. Czaplicki M. Structural adhesives. US 2011/0098382 A1, 7 apr. 2009, 28 apr. 2011. 13. Czaplicki M. Oriented structural adhesives. US 2013/0206333 A1, 26 jul. 2011, 15 aug. 2013. 14. Valloir N, Fradin O, Bret C, Vaique E, Chiron G, Roux M. Structural Adhesives, Process for the Preparation Thereof and Application Thereof. US 2014/0053976 A1, 26 mar. 2012, 27 feb. 2014. 15. Phua J-L, Teh P-L, Ghani SA, Yeoh C-K. The influence of blending methods in epoxy/PMMA polymer blend. Materials Science Forum. 2016; 857: 174-178.
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16. Remiro PM, Riccardi CC, Corcuera MA, Mondragon I. Design of morphology in PMMA‐ modified epoxy resins by control of curing conditions. I. Phase behavior. Journal of Applied Polymer Science. 1999; 74(24):772-780. 17. Zaioncz S, Silva A A, Sirqueira AS, Soares, BG. Toughening of Epoxy Resin by Methyl Methacrylate/2‐ Ethylhexyl Acrylate Copolymers: The Effect of Copolymer Composition. Macromolecular Materials Engineering. 2007; 292(12): 1263-1270. 18. Rau SR, Vengadaesvaran B, Naziron NN and Arof AK. Strength and adhesion properties of acrylic polyol-epoxy polyol resin protective coating on mild steel substrate. Pigment and Resin Technology. 2013; 42(2): 111-116. 19. Das B, Chakraborty D, Hajra AK, Sinha S. Epoxy/poly(methyl methacrylate) interpenetrating polymer networks — morphology, mechanical and thermal properties. Journal of Applied Polymer Science. 1994; 53(11-12): 1491-1496. 20. Garra P, Morlet-Savary F, Dietlin C, Fouassier J-P and Lalevée J. Charge-Transfer Complexes as New Inhibitors/Photoinitiators for On-Demand Amine/Peroxide Redox Polymerization. American Chemical Society Omega. 2018; 3 (6): 6827–6832. 21. Smith BC. Infrared spectral interpretation: a systematic approach. 1th Edition. Boca Raton: CRC Press, 1999. ISBN 978-0-8493-2463-5. 22. Sharma BK. Instrumental methods of chemical analysis, In; Introduction to Analytical chemistry: 23rd edition. Goel Publishing House, Meerut, India, 2004. 23. Chen W-C, Lee S-J. Synthesis and characterization of poly(methyl methacrylate)-silica hybrid optical thin films. Polymer Journal. 2000; 32: 67-72. 24. Ku H, Cardona F, Rogers D, Vanderbroucke A. Effects of EPON on mechanical and thermal properties of epoxy resins. Advanced Materials Research. 2008; 47-50: 536539. 25. Zaman I, Nor FM, Manshoor B, Khalid A, Araby S. Influence of interface on epoxy/clay nanocomposites: 2. Mechanical and Thermal Dynamic Properties. Procedia Manufacturing. 2015; 2: 23–27. 26. Parker S, Riggs P, Kalachandra S, Taylor D, Braden M. Effect of composition on the mechanical properties and water sorption of a butadiene/styrene copolymermethacrylate monomer soft lining material. Journal of Materials Science: Materials in Medicine. 1996; 7(4): 245–250. 27. Pocius AV. Elastomer Modification of Structural Adhesives. Rubber Chemistry and Technology. 1985; 58(3): 622-636. 28. Hosseini S, Ibrahima F, Djordjevic I, Koole LH. Polymethyl methacrylate-comethacrylic acid coatings with controllable concentration of surface carboxyl groups: A novel approach in fabrication of polymeric platforms for potential bio-diagnostic devices Applied Surface Science. 2014; 300: 43–50. 29. Alvarez F, Colmenero J, Wang CH, Xia JL, Fytas G. Segmental Dynamics in Bulk Poly(isobornyl methacrylate) and Its Random Copolymer with Poly(methyl methacrylate) near Tg. Macromolecules. 1995; 28(19): 6488–6493. 30. Silva, L F M Da, Öchsner A, Adams RD. Introduction to Adhesive Bonding Technology Handbook of adhesion technology. Heidelberg: Springer, 2011. Chapter 1, pages 1-7, ISBN 978-3-642-01168-9.
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PII: DOI: Reference:
S0143-7496(18)30259-8 https://doi.org/10.1016/j.ijadhadh.2018.11.006 JAAD2294
To appear in: International Journal of Adhesion and Adhesives Accepted date: 4 November 2018 Cite this article as: Karina Kerber Hennemann and Denise Maria Lenz, Structural METHACRYLATE/EPoXY BASED adhesives for AluminIum JOINTS, International Journal of Adhesion and Adhesives, https://doi.org/10.1016/j.ijadhadh.2018.11.006 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 galley proof before it is published in its final citable 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.
1
STRUCTURAL METHACRYLATE/EPOXY BASED ADHESIVES FOR ALUMINIUM JOINTS
Karina Kerber Hennemann1,2, Denise Maria Lenz2* 1
Artecola Química Programa de Pós-graduação em Engenharia de Materiais e Processos Sustentáveis, Universidade Luterana do Brasil, Avenida Farroupilha 8001, Prédio 14, sala 215, Canoas - RS, Brazil. 2
ABSTRACT
The aim of this work was to investigate the influence of elastomer and monomer concentration on the properties of methacrylate/epoxy based adhesive formulations using a Plackett-Burman experimental design. The adhesive formulations were developed in order to bond non-treated aluminium panels. Their chemical structure was analysed by Fourier transformed infrared spectroscopy (FTIR) and the thermal behaviour by thermogravimetry (TGA) and differential scanning calorimetry (DSC) in comparison to a commercial adhesive. After room temperature mixing, time and temperature of the curing reaction were measured. Further, non-treated aluminium single-lap joints were adhesively bonded and their shear strengths were evaluated. The formulations showed chemical structure similar to a commercial adhesive but the thermal behaviour was quite distinct. The commercial adhesive cured slower than the developed adhesive formulations which can directly affect the bonding quality. However, some formulations showed higher shear strengths than the commercial one, but all with adhesive failure mode. The adhesive formulations with elastomers in methacrylate part showed the highest shear strengths. Keywords: epoxides (A), structural acrylics (A), aluminium and alloys (B), lap-shear (C). ______________________________________________________________________ 1. Introduction Adhesive bonded joints have been designed for automotive, aerospace and civil industries due to their advantages over the traditional joining technologies as corrosion resistance, weight reduction and elimination of stress concentration due to the fastener mounting hole [1]. These industries have also focused on aluminium alloys and composites bondings which can be used in aerospace components, satellite components, heat sinks, cryostat, LED assemblies and lasers. Structural adhesives are considered the
Corresponding author. E-mail address: [email protected]
2
evolution of the conventional joining systems being able to be used in weather demanding and in high tensile demands since they must be capable of transmitting stress from one element of the joint to another without losing their integrity. Thus, the stresses are distributed more uniformly than in mechanical methods of joining [2]. However, the performance of structural adhesives is often limited by their poor mechanical properties, i.e., the adhesives are usually much weaker than the adherends they join, thus studies on adhesive formulations must be performed in order to develop reliable knowledge for adhesive technologies [3]. Most of the studies regarding structural adhesives are mainly for epoxy or acrylate-based adhesives. Epoxy-based adhesives are used due to their hardness and excellent weatherability characteristics despite their slow cure speed and high brittleness. The properties of cured epoxy adhesives can be tailored by the curing agent and the curing process as well [4]. The use of nanoparticles in adhesives can improve the mechanical strength of structural adhesive joints [3, 5]. Acrylate-based adhesives are known for their fast curing process and excellent bonding to a wide variety of materials. The main components of (meth)acrylic adhesives are (meth)acrylic monomers or polymers, initiating systems, hardeners (curing agents), free radical stabilizers and modifiers (accelerators) [6]. Methyl methacrylate (MMA) is often used as the main monomer since it provides high adhesive strength and is a good solvent for a number of polymers. When combined with oils, MMA makes possible to glue untreated surfaces [7]. Acrylate-based adhesives have been modified by many methods including crosslinking polyacrylates, copolymerizing acrylates with other monomers and blending polyacrylates with other polymers in order to reduce the brittleness and poor film-forming properties [8,9].
3
Current approaches towards structural acrylate/epoxy adhesives are mostly restricted to patents [10-14] which combine the properties of both fully cured ingredients, although some works have already studied this blend system. Blends of epoxy resin up to 50 vol% of poly(methyl-methacrylate) (PMMA) show different levels of apparent miscibility due to variations in phase separation-morphology of PMMA and in polymerization rate [15]. Curing time and curing temperature have also influence on phase miscibility and consequently on glass transition temperature of these systems [16]. A significant increase in impact strength without changes in the tensile strength, modulus and glass transition temperature with addition of 10 phr of an acrylic copolymer in epoxy resin matrix was already reported [17]. The improvement of adhesive toughness can be explained by the nanostructures formed upon polymerization [9]. A blending system with 90 wt% of acrylic resin and 10 wt% of epoxy resin showed also improved impact resistance properties and good adhesion on mild steel substrate [18]. Thus, an effective polymer blend based on epoxy and PMMA can be formed, showing improved toughness compared to the rigid, brittle epoxy and acrylic homopolymers [19]. This work has developed two-ingredient structural adhesive formulations with methacrylate and epoxy adhesives for joining difficult-to-anchor substrates such as aluminium without pretreatment. The influence of elastomers and acrylic monomers of the adhesive formulations on time and temperature of cure, thermal behaviour and shear strength were evaluated. 2. Materials and Methods 2.1 Materials
4
Adhesive formulations with two-ingredients (A-B) were developed. Table 1 presents the formulation of the ingredients A and B of the adhesive.
Table 1: Formulation of the ingredients A and B of the structural adhesive. Abbreviation/ Chemicals
Commercial
Function
Supplier
name methyl methacrylate
MMA
Monomer
Rudnik
methacrylic acid
MA
Comonomer
Rudnik
isobornyl methacrylate
IBOMA
Comonomer
HYPRO RLP polybutadiene
VTB 2000 X168LC
Ingredient A
styrene and isoprene-based triblock copolymer
Kraton D1161
Elastomer (toughener) Elastomer (toughener)
SigmaAldrich
Emerald
Kraton
N, N-dimethyl p-toluidine
DMPT
Reaction initiator
Rudnik
p-methoxy-phenol
MEH
Reaction inhibitor
Rudnik
zinc carboxylate
®
Catalyst
Shepherd
BiCAT Z 1365
Inhibitor of evaporation paraffin wax
Bareco PX 105
of volatile reactive
Megh
monomers ethylene diamine tetraacetic acid zinc dimethacrylates mono- and di-phosphate esters of 2-Hydroxyethylmethacrylate N-dimethyl p-toluidine DMPT
Ingredient B
diglycidyl ether of bisphenol A poly (methyl methacrylate-bbutyl acrylate- b- methyl methacrylate)
EDTA
Emulsifier
AkzoNobel
Dymalink® 708
Adhesion promoter
Total
Harcryl® 1228
Adhesion promoter
Harcros
Dissolvine E-39
Vestamin® TMD DGEBA EponTM Resin 828 Nanostrength® M52N
Ingredient B Curing agent
Evonik
Epoxy resin
Hexion
Elastomer (toughener)
Arkema
a combination of benzoate esters
BenzoflexTM 2088
Plasticizer
Eastman
benzoyl peroxide
BPO
Ingredient A curing
DiproFiber
5 agent
For ingredient A (methacrylate-based), the monomers were: methyl methacrylate MMA as primary monomer and methacrylic acid MA and isobornyl methacrylate IBOMA comonomers, all industrial grades. Since methacrylate-based polymers are brittle materials, two elastomers were added to increase their toughness: a vinyl terminated polybutadiene HYPRO and a styrene and isoprene-based triblock copolymer Kraton. The free-radical polymerization of ingredient A was promoted by a curing system consisted of N, N-dimethyl p-toluidine DMPT as initiator and benzoyl peroxide BPO (oxidizing agent). The role of the tertiary amine DMPT is to carry out the redox (reduction–oxidation) initiation, together with BPO in a short period of time at ambient temperature. Additives used were: zinc carboxylate (BiCAT®) as catalyst that mediate the redox equilibrium process, p-methoxy-phenol MEH as reaction inhibitor (radicaltrapping agents), paraffin wax and the EDTA emulsifier. Also, dimethacrylates (Dymalink® and Harcryl®) were used as adhesion promoters. A simplified polymerization reaction mechanism that provides several crosslinking sites is described by Equations 1, 2 and 3, adapted from Garra et al. [20]. Initiation Step (after mixing): DMPT + BPO → Radical∙
(1)
Propagation Step: ∙
∙
Radical + Monomer and Comonomers (M) → R(M) n
(2)
Inhibition Step: ∙
R(M) n + MEH → Inactive species
(3)
For ingredient B, (epoxy-based), the diglycidyl ether of bisphenol A (DGEBA) was used. This resin has an equivalent weight of epoxy resin (EEW) of 185-192 g.eq-1 and a viscosity of 110-150 cPs at 25ºC. Epoxy resins are inherently brittle and thus acrylic tri-block MAM copolymers were used for epoxy formulations. MAM
6
copolymers known as Nanostrength® have a poly (butyl acrylate) centre block which is a soft rubber phase for toughening and two poly (methyl methacrylate) side blocks (melting point of 160ºC and particle size range of 10~100nm). As curing agent for epoxy resin, trimethylhexamethylenediamine TMD was used. According to the suppliers, EEW for DGEBA resin is in range of 185-192g.eq-1 and the AHEW (Amine Hydrogen Equivalent Weight) of TMD is approximately 79.2 g.eq-1. Equation 4 was used to define the concentration of the curing agent for each formulation. Also, as plasticizer, a combination of benzoate esters named BenzoflexTM was used in ingredient B. AHEW X 100 = mass g of curing agent per 100g of resin EEW
(4)
A hydrophilic fumed silica named Cab-o-sil® TS-720 was added (0.05 g per 100g of the formulation) as thixotropic agent only to cured adhesive formulations subjected to shear experiments. Silica was used as a thickening and to reduce flow of the adhesives since the formulations were in liquid state before curing. The Cab-o-sil® TS-720 has an average particle size of 0.2–0.3 microns.
2.2 Plackett-Burman (PB) experimental design The PB12 Plackett-Burman experimental design was used to evaluate the effect of five variables (factors) related to the concentration of elastomers in ingredients A and B and the IBOMA and MA monomers in ingredient A on thermal and mechanical properties of two-ingredient adhesive. The five factors were varied simultaneously in three levels: −1 for low level, 0 for the medium level and +1 for high level based on Plackett–Burman matrix design. The values of three levels were set according to our previous preliminary experimental results and the aforementioned patents. Table 2 shows the quantities (in mass) for the five variables investigated considering the
7
production of 200g of ingredient A and 100g of ingredient B. After that, the two ingredients of adhesive were mixed in a mass ratio of 10:1 (A:B). In order to maintain the same ratio between monomers and elastomers concentration in all formulations, some adjustments were made in the levels of the experiments. Thus, only a qualitative evaluation of the significant effects can be performed. The PB12 Plackett-Burman experimental design is shown in Table 3 and it is composed by 13 adhesive formulations which include a central point formulation (HKN27 formulation) made in triplicate for evaluation of experimental reproducibility. The codes of the formulations in Table 3 are assigned to: H – with HYPRO elastomer in ingredient A, K – with KRATON elastomer in ingredient A, N – with Nanostrength® elastomer in ingredient B, the first number makes reference to MA monomer amount for 200g of ingredient A and the second number represents the IBOMA monomer amount for 200g of ingredient A, according to the bold numbers in Table 2.
Table 2. Adhesive formulation variables and their quantities for the three-leveled (-1, 0 and 1) PB-12 experimental design. Mass (g) Variables
Raw materials
x1
-1
0
1
HYPRO RLP VTB 2000X168 LC Elastomer*
0
23.13
18.60
x2
Kraton D 1161 Elastomer*
0
18.60
23.13
x3
Nanostrength® M52N Elastomer**
0
7.00
9.72
x4
Methacrylic acid (MA) Monomer
10.00
26.21
54.40
x5
Isobornyl methacrylate (IBOMA) Monomer
10.00
27.20
13.10
* - elastomer added in 200g of ingredient A
** - elastomer added in 100 g of ingredient B
Table 3. The PB12 Plackett-Burman experimental design. Experimental Run 1
Formulation Code HN11
x1
x2
x3
x4
x5
1
-1
1
-1
-1
8
2 3 4 5 6 7 8 9 10 11 12 13-A 13- B 13-C
HK51 KN13 HN51 HK53 HKN13 KN51 N53 53 H13 K11 11 HKN27-A HKN27-B HKN27-C
1 -1 1 1 1 -1 -1 -1 1 -1 -1 0 0 0
1 1 -1 1 1 1 -1 -1 -1 1 -1 0 0 0
-1 1 1 -1 1 1 1 -1 -1 -1 -1 0 0 0
1 -1 1 1 -1 1 1 1 -1 -1 -1 0 0 0
-1 1 -1 1 1 -1 1 1 1 -1 -1 0 0 0
2.3 Adhesive preparation Ingredients A and B were prepared separately. A glass reactor equipped with an anchor type mechanical stirrer, condenser and monomer doser was used for preparation of 200 g of ingredient A (Figure 1). Initially MMA and MA monomers were introduced in the reactor with the aid of a monomer doser and after 2 g of EDTA emulsifier, 0.02 g of p-methoxy-phenol MEH, 2 g of adhesion promoter Dymalink® and the HYPRO elastomer were added under stirring (1000 rpm) for 30 minutes at 70ºC ± 2. Paraffin wax (9 g) was added further with stirring for two hours at 110 °C ± 2. In the next step, the Kraton elastomer was added with stirring for one more hour at 115ºC. After that, the system was cooled to 80°C and IBOMA monomer was slowly added for 30 minutes with further stirring for ten minutes and cooling up to 45 °C for addition of the curing agents DMPT (1.25 g) and TMD (1 g), the adhesion promoter Harcryl® (2 g) and the BiCat Z catalyst (2 g) with stirring of twenty minutes. Ingredient A formulations were stored in polypropylene flasks.
9
Fig 1. Reactor system for preparation of ingredient A formulations.
For preparation of 100g of ingredient B, an aluminium container was used with a mechanical stirrer with dispersing disc type blades for addition of 45-50 g of Benzoflex plasticizer and 23-25 g of BPO cure agent for one hour at 1000 rpm. After, Nanostrength® elastomer was added with a 30 minutes-stirring and right after the DGEBA resin was added and stirred for more 30 minutes. BPO is ingredient A cure agent which was added to ingredient B as well as TMD is ingredient B cure agent which was added to ingredient A. This procedure was used so that the curing reaction occurs only when both ingredients were mixed in a mass ratio of 10:1 (PMMA:DGBEA).
2.3 Adhesive characterization Ingredients A and B of the adhesive were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) using a Shimadzu Affinity spectrophotometer. Thermogravimetric analysis (TGA) was performed in a TGA Q50 (TA Instruments) thermogravimetric analyser under N2 flow from 25 to 600ºC at 10°C/min. Differential scanning calorimetry measurements (DSC) were carried out in a Jade DSC instrument (Perkin Elmer) at 10°C/min over the temperature range of -70 to 70°C (uncured ingredient B) and 0 to 300ºC (cured adhesive) under N2
10
flow. The cured adhesives were kept in a desiccator at room temperature and analysed after seven days of cure reaction. Ingredient A was not analysed by DSC since the acrylic monomer can damage the instrument. Time and maximum temperature reached in the cure reaction were measured according to ASTM D 2471-99 standard. All formulations were cured in triplicate at room temperature (25 ° C ± 2) and relative humidity of 55%. As the crosslinking speed of the adhesive formulations was high, ingredients were stirred for five seconds and the temperature of the bulk specimens was measured using a pyrometer. Lap Shear strengths (ASTM D-1002 standard) of the cured adhesives were measured on 1" wide x 1" single overlap aluminium specimens (6.25 cm2). Unclad 2024-T3 aluminium substrates with aluminium fraction of 90.90 – 99.0% and cooper fraction of 3.80 – 4.90 % were only cleaned using a towel paper carefully. After, 2.000g of adhesive formulations were applied on aluminium specimens using a mixer (Loctite) and then joined in a single overlap without pressure application. After 72 hours of adhesive application, specimens were subjected to shear stress at a shear rate of 5 mm/min. ASTM D-1002 is capable of providing relative joint strengths, when comparing different adhesive systems. All experiments were carried out in triplicate at room temperature and the characterization tests with adhesive formulations were compared to a two-ingredient commercial adhesive (SAF®150 (Alphakem) assigned as adhesive C) prepared in a mass ratio of 10:1 (PMMA:DGBEA).
3. Results and Discussion 3.1 Chemical structure of adhesive formulations
11
FTIR spectra of the ingredient A (methacrylate-based) formulations showed almost the same bands, identified according to Smith [21] and Skarma [22], although some differences were found in band intensities as well as in wavenumber shifts (Table 4). Figure 2 shows the spectra for ingredient A formulations with elastomers. The bonds found at higher intensities are directly related to methyl methacrylate, methacrylic acid, dimethyl-p-toluidine and trimethylhexamethylenediamine. Figure 3 shows the spectra for some formulations with ingredient B of the adhesive since all spectra are also similar. Characteristic bands of the epoxy group, based on Smith [12], can be found at: 1246 and 1222 cm-1 related to C−O−C asymmetrical stretching, at 1718 cm-1 assigned to ester bondings due to BenzoflexTM plasticizer presence as well as at 1273 cm-1 corresponding to C−O−C bonded to aromatic ring and at 1109 cm-1 due to O−C−C bonding from aromatic ester. Significant differences were observed at 1718 cm-1 and 1510 cm-1 (bondings from aromatic rings) with the highest intensity in HKN27 formulation and at 1602 cm-1 with the lowest intensity in HK51 formulation which is attributed to C=C bondings from aromatic rings. FTIR spectra of ingredients A (methacrylate-based) and B (epoxy-based) of a commercial adhesive (adhesive C) showed the same bands with some shifts. Only for adhesive C, a band with significant intensity was observed at 1157 cm-1 which can be attributed to Si−O−Si from silica that can be related to mineral fillers or thixotropic agent presence [23]. Table 4. FTIR band wavenumbers for formulations with ingredient A and related vibrations. Wavenumbers (cm-1) for formulations with ingredient A and Adhesive C Vibrational assignments H2 angular bending of amine
Without Elastomer 1640
Kraton HYPRO Elastomer Elastomer 1635
1632
HYPRO + Kraton
Adhesive C (methacrylate ingredient)
1638
1637
12
C=O out-of-plane stretching of esters from methacrylate
1675
1696
1696
1694
1685, 1718
O−CH3 stretching from esters
1462
1452
1453
1463
1454
C−CH3 asymmetrical stretching
1428
1427
1450
1428
1440
1375
1376
1375
1375
1376
C−N−C bonding near to aromatic ring C−C−O bonding of aromatic ester asymmetrical stretching of carbons bonding to ester
1294
1298
1298
1296
1298
1206, 1055
1200, 1054
1202, 1081
1204, 1068
1062
C=C bonding out-ofplane bending
930
944
944
942
940
Out-of-plane bending of vinyl double bonds C=CH2
-
-
915
915
-
silicon–oxygen covalent bonds
-
-
-
-
1100,1157,1270
Si−O in-plane stretching vibrations of the Si−OH groups
-
-
-
-
949
13
Transmitance (arbitrary units)
O−CH3
C=O 1793
1696
C=C
NH2 1600
1504
1407
1311
Wavenumber
1214
1118
1021
925
(cm-1)
Fig 2. FTIR spectra for formulations with ingredient A using HYPRO and Kraton elastomers. Spectra listed from top to bottom: HK51, HK53, HKN13, HKN27-A,
Transmitance (arbitrary units)
HKN27-B and HKN27-C formulations.
C−O−C
C=O
C=C O−C−C
1954
1809
1665
1520
1375
1231
1086
941
Wavenumber (cm-1)
Fig 3. FTIR spectra for formulations with ingredient B. Spectra listed from top to bottom: HN11, HK51, HKN27-A formulations.
14
3.2 Thermal properties The thermal degradation profile of all formulations is presented in Figure 4 with HKN27 formulation after cure of the two-ingredient adhesive. This thermogram shows three thermal events: (1) a continuous weight loss due to release of volatiles such as water, solvents and unreacted monomers from room temperature to around 180 ºC; (2) a thermal event possibly associated to decomposition of oily compounds and/or oligomers from 180 ºC to 320 ºC and (3) polymer (resin and elastomers) degradation from around 325 ºC to 500 ºC. The residue of around 5 wt% is related to inorganic compounds.
Temperature (℃)
Fig 4. TGA thermogram of the cured two-ingredient adhesive (HKN27 formulation). Figure 5 shows DSC thermograms for uncured formulations with ingredient B. In all adhesive formulations, the plasticizer effect on glass transition temperatures (Tg) was identified since it has decreased about 30°C with respect to neat epoxy resin (-15°C). The higher the plasticizer concentration, the lower Tg and heat capacity, as shown in Table 5 and also reported by KU et al. [24]. Additionally, the elastomer showed influence on the mobility of the polymer chain since a melt temperature of 0,1°C was observed for HK51 formulation which has no elastomer (endothermic peak
15
in Figure 5). The Nanostrength® M52N elastomer used only in ingredient B is composed by nanometric material which can avoid the nucleation of some crystals formed on epoxy formulation cooling. The same was reported by Zaman et al. [25] who introduced nanoclay in epoxy resin and reported an increase in Tg which was attributed to formation of an interpenetrating network morphology that could occur in HN11 and HKN27 formulations (Table 5). -18
Heat flow (mW) exothermic→
-19 -20 -21 -22 -23 -24 -25 -26 -27 -28 -72 -64 -56 -48 -39 -31 -23 -14 -6
2 11 19 27 36 44 52 61
Temperature (°C)
Fig 5. DSC heating thermograms of uncured epoxy resin (ingredient B) of the adhesive. From top to bottom: HN11, HK51 and HKN27 formulations. Table 5. Glass transition temperatures and heat capacities of uncured ingredient B adhesive formulations from DSC measurements. Raw material / Formulation Neat epoxy resin
Elastomer Plasticizer Tg ∆Cp (weight %) (weight %) (°C) (J.g-1) 0.0 0.0 -15.1 0.60
HN11
9.2
45.14
-47.4
0.43
HK51
0.0
49.99
-48.1
0.40
HKN27
7.0
46.49
-44.5
0.82
Through DSC analysis of the two-ingredient adhesive after seven days of cure reaction, some thermal events were observed, depending on the adhesive formulation.
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In the first scan at around 60°C, a little and broad exothermic peak was observed for all formulations (excepting the HKN27 formulation) that could be attributed to a post-cure event with release of volatiles and an endothermic event at 223 and 231°C attributed to decomposition of some compounds as found at the same temperature range by thermogravimetric analysis. On cooling scanning, some Tg values were observed from 100 to 110°C for all formulations. The presence of just one Tg value after the first scan suggests a relative miscibility of the two polymer phases. Adhesive C showed a distinct thermal behaviour with two Tg values: around 120°C and at 200°C, probably due to methacrylate ingredient and epoxy ingredient, respectively. Figure 6 shows the DSC thermograms for HKN27 formulation and adhesive C.
Fig 6. DSC heating thermograms of the commercial adhesive C (above) and HKN27 adhesive formulation (below). Remiro et al. [16] found Tg values of around 180°C for cured neat epoxy resin (DGEBA) and 113°C for cured neat PMMA. Mixing and curing DGEBA and 15 wt. % of PMMA, Tg values have depended on the curing time. After five hours of curing at
17
80°C, only one Tg value was found, around 150°C, but below this time two Tg values were found: around 116 and 160°C, the higher Tg can be attributed to an epoxy-rich phase, and the lower one is reasonably ascribed to a PMMA-rich phase.
3.3 Time and temperature for cure reaction Table 6 shows the parameters of cure reaction of adhesive formulations. The mass loss after mixing is related to acrylic vapors released by an exothermic reaction (boiling point at 101°C) and the mass gain after seven days is attributed to water absorption, according to Parker et al. [26]. The commercial adhesive C showed the longest gel time for and the lowest maximum temperature achieved for cure reaction, followed by HKN27 formulations. Figure 7 shows the cure behaviour for all adhesive formulations (HKN27-A, -B and -C formulations are overlapping). The elastomer and IBOMA monomer added to HKN27 formulation in higher concentration can be affected the cure time since it has polymer chains with greater both steric hindrance and molar mass with respect to other monomers. The formulations 11, K11 and KN13 showed the fastest cure reactions. The presence of methyl methacrylate monomer in higher concentration than in other formulations and the absence of elastomer probably have contributed to higher rates of cure reaction. The elastomer and IBOMA monomer added to HKN27 formulation in higher concentration can be affected the cure time since it has polymer chains with greater both steric hindrance and molar mass with respect to other monomers. The formulations 11, K11 and KN13 showed the fastest cure reactions. The presence of methyl methacrylate monomer in higher concentration than in other formulations and the absence of elastomer probably have contributed to higher rates of cure reaction.
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Table 6. Maximum temperature, time, mass loss after mixing and mass gain after seven days for cure of the two-ingredient adhesive formulations and Adhesive C.
Formulation
Maximum Temperature (°C)
Gelatinization Time (s)
Mass loss after mixing (%)
Mass gain after seven days (%)
HN11
143
157
6
2
HK51
144
73
3
1
KN13
133
93
5
2
HN51
118
140
7
1
HK53
144
107
5
1
HKN13
113
117
8
1
KN51
115
125
6
2
N53
137
135
6
1
53
140
115
6
1
H13
136
105
8
2
K11
139
75
6
2
11
134
80
5
1
HKN27-A
133
197
7
2
HKN27-B
133
197
7
2
HKN27-C
133
197
7
1
Adhesive C
120
250
6
1
19 160 HN11 HK51
140
KN13
Temperature (°C)
120
HN51 HK53
100
HKN13 KN51
80
N53 53
60
H13 K11
40
11 20
HKN27-A HKN27-B
0 30
50
70
90
110
130
150
170
190
210
230
HKN27-C
Time (s)
Fig 7. Temperature and time behaviour for cure reactions of two-ingredient adhesives. Curves listed from left to right: K11, 11, KN13, HK53, H13, HKN13, 53, KN51. HK51, N53, HN11, HN51, HKN27-A, HKN27-B, HKN27-C formulations (the latter three are overlapping).
3.4 Lap shear strength experiments Figure 8 shows the maximum shear force that can be applied to single-lap joints bonded with cured adhesive formulations and Adhesive C.
Considering the same
bonding area (6.25 cm2) for the specimens, the adhesive formulations that used both elastomers in ingredient A (called HK formulations) showed the greater shear strength average as in HK51, HK53, HKN13 and HKN27 formulations, the latter one showing an average of 730 N. With respect to acrylic monomers, higher MA and IBOMA concentration in the formulation, higher the shear strength. Besides, the presence of elastomer in ingredient B has also contributed to higher strength values.
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Thus, the introduction of MA and IBOMA comonomers in methacrylate ingredient and elastomers affects positively the shear strength of the adhesive. The introduction of them have influence on physicochemical factors on the polymerization process [7]. The increase in toughness due to the elastomer is obtained either by plasticization, increasing the ductility of the adhesive or by generation of a two-phase system where the structural adhesive forms a matrix in which the elastomeric particles are imbedded and act as stress concentrators [27]. The introduction of comonomers with polarity (as MA comonomer) into the formulation leads to an increase in the strength of the adhesive compound of poly(methyl-methacrylate) [7]. The –COOH groups generated from MA comonomer introduction in ingredient A formulation would be present at the polymer surface and the variation in the initial monomer concentration of methyl methacrylate (MMA) and methacrylic acid (MA) would yield plastic material with varying MA segments in the polymer chain [28]. The isobornyl side group of IBOMA comonomer have difficulty to free rotation that result in reduced polymer chain flexibility. Besides, IBOMA comonomer has higher molar mass than methyl methacrylate. Both IBOMA characteristics change thermalmechanical properties of the adhesive [29], as observed for HKN27 adhesive formulation which has the highest concentration of IBOMA. Among the adhesive formulations, HKN27 formulation showed a proper adjustment of the amount of each type of monomer into adhesive that yielded adhesive of balanced hardness or flexibility. The failure mode between aluminium-adhesive was adhesive for all formulations and Adhesive C. In this failure, only one side of the substrate was adhered to the adhesive, leaving the other side partially or completely clean (Figure 9). It is important to emphasize that as the open time (time after adhesive is applied during which a
21
serviceable bond can be made) of the adhesive formulations and adhesive C was too short, a small part of the bonding area cannot be covered (about 5% of area in all adhesives). The adhesive failure is a consequence of little or no interfacial bonding between the adhesive and the substrate. However, not only the type of failure should be considered for adhesive quality evaluation since the adhesive-substrate interface may fail adhesively but exhibiting higher strength than a similar joint bonded with a weaker adhesive that shows a cohesive failure [30].
Average maximum shear force (N)
800 700 600 500 400 300 200 100 0
Adhesive formulations and Adhesive C
Fig 8. Average maximum force for formulations and commercial adhesive C obtained by shear strength experiments. The arrows indicate the formulations with higher shear forces. Bonding area: 6.25 cm2.
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(a)
(b) Fig 9. Specimens after shear strength experiments: (a) HKN27 formulation and (b) commercial adhesive – adhesive C.
Conclusion Adhesive formulations with methacrylate and epoxy-based ingredients were developed and showed remarkable similarities to the chemical structure of a commercial adhesive. Shear strength, measured on aluminium substrates, can be up to 30% higher for formulations with addition of elastomers in acrylate ingredient (as in HKN27 formulation) when compared to the results obtained with a commercial adhesive for the same application. However, adhesive failures were observed for all adhesives. Temperature and time of curing reaction of HKN27 formulation were close to those of the commercial adhesive. Thermal analyses showed thermal events for all formulations developed which can be assigned to uncured monomers or additives degradation that were not detectable in thermograms of the commercial adhesive. Thus, HKN27 formulation has demonstrated a good potential to be used as structural adhesive.
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Acknowledgements Authors are grateful to Artecola Química S.A. for the supply of raw materials for the adhesive formulations.
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