Modification of epoxy adhesives to enhance glue ductility in relation to wood adherends Jan Vanerek Anna Benesova, Ámos Dufka, Nikol Zizkova and Rostislav Drochytka FACULTY OF CIVIL ENGINEERING BRNO UNIVERSITY OF TECHNOLOGY CZECH REPUBLIC
Experiment Objective To increase the durability of epoxy adhesives Possible usage for load-bearing structures (classification type I - EN 301, requiring durability during external exposure and stability at temperatures above 50 C) Fulfillment of standard requirements sufficient shear tensile strengths after hygrothermal exposure (exposures A1-5 defined in EN 302-1) the ability to withstand the strain arising from volume changes of wood adherends (delamination test after accelerated aging test according to EN 302-2 comparable to ASTM D 2559). Currently there are non-uniform results from epoxy/wood durability testing, in spite of the huge development of the FRP branch.
Introduction - Types of Epoxy Modification 1. Reinforcing process of epoxy adhesives: Epoxy adhesives are optimized for delamination resistance a) with reinforcements (suitable aspect ratio) to ensure the flexible behavior of the adhesives a) nanocellulose fibers (optimal disintegration process to avoid agglomeration of cellulose) fracture delamination improved significantly with the addition of NFC (fracture toughness increased about 61% for 1wt.% of NFC to CF/epoxy material) b) with fillers, mostly in nano-scale a) Nano-clay particles only 0.5 wt.% could improve the fracture toughness by 20%, 3.0 wt.% by 50% higher than the neat epoxy. b) Carbon-black 2.0 wt.% of CB content leads to improving the fracture toughness by 18% (ductile fracture mode) 2. Modification of application process: a) Positive results with proper wood penetration processes (wood cell stabilization theory) using different types of primer (MME, HMR) increase the ability of the bondline to withstand the wood volume changes.
Material - adhesives Using a two-step application penetration to stabilize the interfacial wood cell walls and distribute the expansion /concentration differences more into the wood structure 120 g/m 2 ), adhesive layer (200 g/m 2 ), both-side application, pressure clamping 1.0 MPa for 10 hours, Primer - epoxy resin CHS 474 and curing agent Telalit 0492 (SPOLCHEMIE, Czech Rep.) σ t = 59.2 N/mm 2, CHS 474: 0.3 0.6 Pa.s; 0492: 0.015-0.030 Pa.s Epoxy resin CHS 531 and curing agent Telalit 0492 (SPOLCHEMIE, Czech Rep.) σ t = 50.1 N/mm 2, CHS 531: 1.5 2.3 Pa.s; Telalit 0492: 0.015-0.03 Pa.s
Materials Wood Spruce (Picea abies) f m = 81.1 N/mm 2, E m = 10.5.10 3 N/mm 2, ρ 12 = 465 kg.m 3 flatsawn timber of 25 mm thickness, surface treatment - planed Reinforcing agent Carbon black (Rubber gas black, ORION Engineered Carbons, GERMANY) CK3 achieved by gas black method, pour density 380 g/dm 3 ; Different amount 0.5; 1.0; 2.0 and 3.0 wt.% of epoxy resin.
Modification of epoxy primer Epoxy basis 1.CB was first dispersed in a solvent agent (chloroform CHCl 3 ) using a magnetic heat stirrer plate at 45 C (150 rpm) for 15 minutes; 2.epoxy resin was mixed with the suspension and subjected to shear mixing (150 rpm) for 15 min at 45 C; 3.sonication of suspension for 10 minutes in water bath of constant 20 C temperature using Bandelin 3200 ultrasound apparatus at 70% amplitude; 4.nanofiller chloroform mixture was degassed in s vacuum dryer at 75 C for 12 h to eliminate the remaining solvent; 5.stoichiometric amount of curing agent was added into the mixture before application onto the wood surface.
Durability testing Hygrothermal exposure - short term durability test according to EN 302-1 (A1, A4, A5) was carried out to observed wood/epoxy bond behavior Table 1: Types of exposition prior to tensile shear testing Treatment Climatic treatment Specification A1 Standard climate 7 days 20 ºC/65% A2 Immersion in cold water 7 days 20 ºC/65% 4 days in water (15±5 ºC) Tested in wet state A3 Immersion in cold water 7 days 20 ºC/65% 4 days immersion in water (15±5 ºC), conditioning at 20 ºC/65% Tested in dry state A4 Immersion in boiling water 7 days 20 ºC/65% 6 hours in boiling water 2 hours immersion in water (15±5 ºC), Tested in wet state A5 Immersion in boiling water 7 days 20 ºC/65% 6 hours in boiling water 2 hours immersion in water (15±5 ºC) 7 days conditioning at 20 ºC/65% Tested in dry state Treatment Min. mean value of tensile shear strength (MPa) Type I A1 10,0 A2 6,0 A3 8,0 A4 6,0 A5 8,0
Testing Tensile shear strength modified single lap joint test specimens with dimension of 150 20 50 mm (EN 302-1), with thin bondline (<0,1mm), single lap shear area of 20 20 mm (notch of 4 mm width), 12 pcs per each exposure treatment, speed of tensile testing 0,1 mm/min; extensometr with gauge length of 50 mm, Tensile strength and MOE of neat and modified epoxy resin, EN 527-1 Wood failure mode 10% percentage stage graded of shear area, wood failure (Cohesive Failure), DSC complementary assessment for T g evaluation SEM, AFM micrograph complementary for dispergation of nano particles in resin, AFM for detailed bondline evaluation.
Tensile shear strength [N.mm -2 ] Tensile shear strength [N.mm -2 ] Results tensile shear strengths 7.00 6.00 A1 exposure A4 exposure A5 exposure 7.00 6.00 A1 exposure A4 exposure A5 exposure 5.00 5.00 4.00 4.00 3.00 3.00 2.00 2.00 1.00 1.00 0.00 ref 1.0 MCB 3.0 MCB 5.0 MCB 10.0 MCB content of micro carbon black in epoxy primer [wt.%] 0.00 ref 0.5 NCB 1.0 NCB 2.0 NCB 3.0 NCB content of nano carbon black in epoxy primer [wt.%] micro carbon black fillers in epoxy primer nano carbon black fillers in epoxy primer significant aspect could be seen in exposure of boiling test (A4 treatment), where specimens with micro carbon black fillers decrease strength of ca. 75 %; huge conglomeration of carbon black cause local huge interfacial strain; contrary, no significant drop was observed in A4 treatment when nano carbon black reinforcing agent was applied in primer with 0,5 wt.% of nano carbon black; additionally decrease of 40% in primer with 3,0 wt.% of nano carbon black.
Cohesive failure [%] Cohesive failure [%] Results wood failure (cohesive) criterion 100 90 80 70 60 50 40 30 20 10 A1 exposure A4 exposure A5 exposure 0 ref 1.0 MCB 3.0 MCB 5.0 MCB 10.0 MCB content of micro carbon black in epoxy primer [wt.%] 100 90 80 70 60 50 40 30 20 10 0 ref 0.5 NCB 1.0 NCB 2.0 NCB 3.0 NCB content of nano carbon black in epoxy primer [wt.%] A1 exposure A4 exposure A5 exposure micro carbon black fillers in epoxy primer nano carbon black fillers in epoxy primer Tensile shear strength specimen with 0.5 NCB fillers in epoxy primer resin
Results wood failure (cohesive) criterion cohesive failure in softwood structure mostly observed in early wood of annual ring, where better penetration into the cell wood structure adhesion failure wood imperfection (knots), insufficient penetration into the wood structure (could be confirmed by EPI-fluorescence microscopy), agglomeration of MCB
Results influence of NCB for epoxy characteristics Sample Tensile strength N.mm -2 Mean value S.D. Modulus of Elasticity N.mm -2 Mean value S.D. Relative strain % Mean value Neat resin 51.6 1.14 2,536.0 96.9 2.67 CHS 474 0.5NCB 45.3 1.52 2,578.2 81.4 2.11
Results SEM analyze for dispersion of MCB and NCB in epoxy Surface of fracture area for MFC/epoxy adhesive (left, 10,000x magn.), and 0.5NFC/epoxy adhesive (middle, 10,000x magn.), agglomerates of 0.5NFC in epoxy (right, 100,000x magn.)
Concluding remarks epoxy modification using micro and nano carbon blacks were evaluated for the primer layer. The micro carbon black caused local stress concentrations on the bondline resulting in total adhesion failures after hygrothermal exposures. In contrast, a significant aspect of carbon black deagglomerization using the ultrasonication treatments was found to enhance the stabilization of wood adherend by such modified epoxy primer. The optimum content, fulfilling criteria of strength and cohesive failure, was 0.5wt.% for nano carbon black reinforcing agents. This improvement was found despite the fact that better resin characteristics (ultimate strength, modulus of elasticity, strain) were not confirmed for 0.5NCB/epoxy primer.
The optimum penetration of modified primer into the wood structure plays the most important role to the stabilize wood surface to better interfere with internal forces arising in the glued bondline. In the spruce test samples, cohesive failures were primarily detected in the annual rings of the early wood, confirming easier adhesive penetration. According to SEM observation of fracture MFC/epoxy and NFC/epoxy surfaces, the agglomeration of reinforcing agents even in both scales influence the occurrences of cracks or other imperfections.
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