SELF-HEALING OF GFR/ EPOXY COMPOSITE WITH BINARY
VASCULAR SYSTEM
S. Vidinejevs, A. Aniskevich
Institute of Polymer Mechanics University of Latvia, 23 Aizkraukles str., Riga, LV-1006, Latvia – e-mail: sergejs.vidinejevs@pmi.lu.lv; andrey.aniskevich@pmi.lu.lv
Keywords: composite, epoxy matrix, self-healing, vascular approach ABSTRACT
Creation of an ideal self-healing polymer composite material with durable performance could become an alternative to traditional improvement of lightweight constructions. A vascular strategy of the self-healing of the epoxy materials is based on healing agent (HA) delivery to the damage via system of channels formed in the material. The aim of research was to apply the strategy to glass fiber reinforced epoxy composites using binary healing agent and to evaluate its efficiency.
We used a vacuum infusion to make unidirectional laminate specimens laid up from two layers of glass yarns with embedded channels codirectional to reinforcement. The specimens had average size 250×23×1.2 mm. Polytetrafluorethylene tubing (diameter 0.9 mm) was used as preform for channels. Each specimen had 5 channels filled with healing agent — epoxy resin alternated with hardener, and then sealed. These specimens demonstrated the flexural modulus 25.2±2.0 GPa similar within the error to material without channels.
Multiple “local” three-point bending of a specimen was used to cause damage to the matrix and channels across fiber direction. It was performed on the span 20 mm through about each linear 12 mm along specimen. The bending had been terminated when the load decreased by 30 % after reaching a maximum. Released HA components infiltrated into cracks and mixed partially. The triggered self-healing continued 24 h at 50 °C.
Flexural modulus of a specimen was measured in three-point bending on the span 100 mm at a small deflection and was selected as a characteristic of mechanical integrity. Flexural modulus of virgin, damaged, and self-healed specimens was designated as Ev, Ed, Eh, correspondingly. The healing efficiency was evaluated as
η=(Eh-Ed)/(Ev-Ed) and experimentally ascertained as η30 %.
The research results demonstrate the capacity self-healing approach for composite materials.
1. INTRODUCTION
Vascular approach is an important idea for realization of self-healing in epoxy polymers [1, 2]. Self-healing of composites with a vascular system is based on healing agent (HA) delivery to the damage via system of channels formed in the material. The binary system means that resin and hardener are separate components of HA. These components alternate within the channels. The aim of research was to apply the approach to glass fiber reinforced (GFR) epoxy composites and to evaluate its efficiency. In this connection, it was necessary to solve the following tasks:
to fabricate the self-healing channels which are collinear GFR,
to evaluate healing efficiency of the composites in consideration of the influence of both retention period of HA and ageing of self-healed specimens. 2. MATERIALS
Series of basic of GFR/ Epoxy Composite plates (250×300 mm) without channels, marked A, and series of the same plates with channels (B, C) were manufactured from Havel Composites CZ s.r.o. commercial products using a vacuum assisted transfer molding. Room temperature curable resin LH289 with hardener H289 (ratio 100:33) was a binder. Two layers of UD glass yarns (500 g/m2) between two
restrictive outer layers of plain glass fabric AEROGLASS (110 g/m2) were manually
stacked. In composites B and C, the set of polytetrafluorethylene tubing (external diameter 0.9 mm, TFT20028 Alpha Wire) was located codirectionally to the reinforcement in the middle of the stack. The tubing was used as a removable after curing preform for channels. Specimens, cut out from the plate, had average size 250×23×1.2 mm. In B- and C-specimens, five channels were at a distance of 4 mm from each other. Another epoxy from Nils Malmgren AB, Sweden was used as HA to syringe up the cannels. The recommended stoichiometric proportion of resin 275A to hardener 275B is equal to 100:55 by mass. This ratio is nearer to ideal for alternating channels with HA. The resin with addition of 20 % wt. 4-nonylphenol (Acros Organics) filled up the first channel alternated with hardener filled up the second channel and so on in self-healing C-specimens. Only resin component of HA filled up the channels of reference B-specimens. The filled channels were sealed with epoxy mastic. The C-specimens were kept at room conditions 1- 3 weeks, 2- 3 months, and 5- 6 months before self-healing triggering.
3. METHODS
Method of measuring the mechanical characteristics of the material. Flexural
modulus E was selected as a characteristic of mechanical integrity of the material. Load vs. deflection data were measured in three-point bending test at a small deflection (<5 %). All bending experiments were conducted on a Zwick 2.5 universal testing machine with a constant crosshead speed 1.2 mm/min. We calculated E:
3 34
l
P
E
bh w
(1)Where l=100 mm was the used span, ∆P, b, h, and ∆w were the increment of load, the width, the thickness of a specimen, and the increment of deflection, respectively. Specimens had Evirgin in the initial state.
Method of damaging of the specimens. The selection of a procedure of doing
damage to the C-specimens was essential to start up self-healing and then evaluate healing efficiency. The single local damage [3] triggered effectively self-healing after matrix cracking if a large number of microscopic tubes with HA were presented in a composite. This type of damage could not be acceptable for specimens containing five macroscopic channels. To fracture the matrix across channels direction and release the agents from channels multiple breakages were made through 12 mm along specimen. The breakages were realized using three-point bending loading on the span 20 mm. Each loading had been terminated when the load reached maximum and then dropped for ca. 30 % of this value. The same breakages were
used for reference A- and B-specimens too. Freshly damaged specimens tested on the long span to get Edamaged.
Method of self-healing. Released HA components infiltrated into cracks then mixed
and cured partially thus healing cracks and recovering the stiffness of the C-specimens. These specimens and the reference A- and B-specimens tested two times on the long span to evaluate Ehealed after repairing in accelerated regime during
24 h at 50 °C and after more than 3 weeks of further ageing at room conditions. A healing efficiency η was defined as a ratio of changes of the modulus [1]
healed damaged virgin damaged E E E E (2)
The efficiency value η was determined for both regimes of repairing. 4. RESULTS
The developed procedure for fabrication of the channels using PTFE tubing was approved earlier [4] for epoxy matrix TDCB specimens. The tubing easy combined with vacuum infusion installation to form the channels in GFR/ epoxy composites. Positioned parallel to the unidirectional reinforcement, it was easily and reliably extracted after curing of epoxy resin, without rejects at creation channels. Structural inhomogeneity of a specimen with channels practically did not change its flexural modulus Evirgin (Table 1).
Multiple fractures were used to guarantee the matrix cracking. However, both inhomogeneity of damaged specimen and extent of the breakages affected the statistical error of evaluated Edamaged, Ehealed, and η. Thus relative error of
determination of η for C-specimens reached 25 %. We suppose that the studied self-healing may be attended with 1) the relaxation process which reduces the stresses in the prefracture region and changes the configuration of the crack on the microscopic and submicroscopic levels [5]; 2) continuation of the crosslinking of the macromolecules of matrix. The observed η>0 of basic A-specimens and filled with resin B-specimens was due to these processes which were stimulated with elevated temperature at first 24 h of ageing. We believe that HA retained substantially its activity during the storage. Volatility of the value η of C-specimens was of the range equal to η observed for reference A- and B-specimens. We revealed experimentally (Table 1):
1) The filled up with HA C-specimens could self-repair to a noticeable degree the matrix thus restore flexural properties of the composite.
2) The composite recovery process was continued in time. The efficiency η increased with a further ageing of self-healed specimens.
3) There was some reduction of η and further volatility of this value vs. time of storage C-specimens.
Table 1: Flexural modulus and healing efficiency with standard deviations of different series of specimens.
Series of specimens Number of specimens
Evirgin
(GPa) accelerated regime η (%)
24h, 50°C >3 weeks, room further ageing
A Without channels 9 24.4±0.6 9±7 10±4
B Resin in channels 3 25.5±1.6 14±7 15±5
C HA, kept 1- 3 weeks 13 25.2±2.0 36±6 52±7
HA, kept 2-3 month 12 22±6 26±7
HA, kept 5-6 month 6 26±6 32±10
5. CONCLUSIONS
The healing efficiency ca. 30 % was performed experimentally in unidirectional GFR/ epoxy composite using the embedded system of channels with sealed binary healing agent, capable of maintaining the capacity for self-healing for at least six months. The sequential procedure of flexural damage of the specimen started up partial recovering of the integrity of the composite. The results can be the basis for the use of existing systems of macro-channels in certain composite products for the purposes of self-healing.
ACKNOWLEDGEMENT
The manuscript was prepared within the Project ERDF
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