The influence of matrix stoichiometry on interfacial adhesion in composites for wind turbine applications

It is well known that the fibre-matrix interface plays a key role in defining the mechanical properties of fibre composite materials. The ability to efficiently transfer stress between the matrix and the fibres is critical in ensuring the required performance level needed for advanced composite materials. Stress transfer across the fibre-matrix interface is often reduced to a discussion of 'adhesion'. Past discussions of thermosetting matrices have typically focussed on the chemistry of thematrix system, specifically the task of maximising the level of chemical bonding between the fibre and the matrix to produce the strongest interface. However, many authors have also commented on the potential for residual radial compressive stresses formed at the interface to be a significant contributor to the strength of the interface. There is still a significant weight of opinion that holds that even if these residual stresses at the interface can contribute to the stress transfer capability, then chemistry and chemical reactions must play an active role in defining their magnitude. As such it was the objective of this thesis to develop an understanding of how chemistry and residual stresses formed at the interface could be interrelated to influence the stress transfer capability of the interface. First an understanding was established of how the amine-to-epoxy group ratio (R) of an amine cured epoxy influences the thermomechanical properties of the matrix. Overall it was shown that the R value had a major influence over all of the thermomechanical properties studied. The glass transition temperature (Tg) was shown to change significantly depending on the R value, with a maximum of 87.3 °C observed at a value (R [approx. equal to] 1.25) slightly above the stoichiometric point. The matrix Tg decreased as the R value deviated from this value, approaching room temperature for the extreme ratios. Above Tg, the linear coefficient of thermal expansion (LCTE) was shown to reach a minimum at the stoichiometric ratio due to this ratio inducing the highest crosslink density. Below Tg, the R value appeared to have a less clear influence. The storage modulus (E') of the matrix was also shown to be affected by the R value, with the stoichiometric ratio possessing the largest magnitude of E' for temperatures 20°C above Tg. However, for temperatures 20 °C below Tg the storage modulus decreased in magnitude the closer the R value was to R [approx. equal to] 1.25.;This was the ratio measured to possess the largest Tg value and thus the highest temperature at which E' was plotted relative to the other ratios. The effect of changing the R value on the interfacial shear strength (IFSS) was investigated using the microbond test. This was done in combination with changing the surface chemistry of the glass fibre and purity of the hardener. Results showed the magnitude of IFSS to be significantly affected by the R value, independent of the fibre sizing applied. The chemistry of the fibre sizing was shown to influence the maximum IFSS achievable and the R value at which it would occur, however the magnitude differences were not as significant. From these results it was concluded that the R value of the matrix has a greater influence than the chemistry of the fibre sizing in defining the level of adhesion at the fibre-matrix interface. The changes in IFSS were shown to correlate with Tg and the decrease in the contribution of residual thermal stresses at the interface. However, this contribution only represented a portion of the total IFSS value measured. It was concluded that other mechanisms, such as cure shrinkage stresses, must provide the remaining portion of IFSS shown. To expand upon this, the influence of temperature in combination with the other variables discussed was studied using the microbond test within a thermomechanical analyser. At lower temperatures the maximum IFSS value was shown to occur at R [approx. equal to] 1.0. The magnitude of IFSS was then shown to decrease as the R value deviated further from this value, again independent of the chemistry of the fibre sizing. Above Tg it was observed that a small value of IFSS remained which appeared to possess a linear relationship with the level of amine present within the Rratio. It was hypothesized that the magnitude of IFSS being greater for excess amineratios (R > 1) resulted from a combination of an increase in the level of hydrogen bonding and a variation in the shear failure behaviour of the matrix due to the differing crosslink densities.;Using Nairn's model, a correlation was shown to exist between the IFSS values measured and the potential total contribution of residual stresses for R [approx. equal to] 1.0. However, as the ratios deviated further from R [approx. equal to] 1.0 the degree of correlation was shown to decrease. It was concluded that assumptions made by the model regarding the contribution of cure shrinkage stresses appeared to be oversimplifications once the R value deviated significantly from R [approx. equal to] 1.0. To address this a novel technique using hot-stage microscopy was used to measure the cure shrinkage of a minute epoxy droplet upon a single fibre during the curing process. The results showed that as the R value was increased, the level of cure shrinkage increased. Rheometry was then used to study the influence of the R value on the gel time of the matrix and applied to the data collected using the hot-stage method. This cure shrinkage data was then reapplied to the model where again good correlation was shown for R [approx. equal to] 1.0, yet the discrepancies regarding off-stoichiometric ratios remained. It was concluded that this may be due to a lack of understanding regarding the Tg of a microdroplet and the adhesion mechanisms of a rubbery statepolymer. Overall it was concluded that Nairn's model supports the hypothesis that residual radial compressive stresses at the interface can contribute significantly to the stress transfer capability of the interface. Since these stresses were shown to be affected by the R value it would also satisfy the need for chemistry to be involved significantly in some role.It is well known that the fibre-matrix interface plays a key role in defining the mechanical properties of fibre composite materials. The ability to efficiently transfer stress between the matrix and the fibres is critical in ensuring the required performance level needed for advanced composite materials. Stress transfer across the fibre-matrix interface is often reduced to a discussion of 'adhesion'. Past discussions of thermosetting matrices have typically focussed on the chemistry of thematrix system, specifically the task of maximising the level of chemical bonding between the fibre and the matrix to produce the strongest interface. However, many authors have also commented on the potential for residual radial compressive stresses formed at the interface to be a significant contributor to the strength of the interface. There is still a significant weight of opinion that holds that even if these residual stresses at the interface can contribute to the stress transfer capability, then chemistry and chemical reactions must play an active role in defining their magnitude. As such it was the objective of this thesis to develop an understanding of how chemistry and residual stresses formed at the interface could be interrelated to influence the stress transfer capability of the interface. First an understanding was established of how the amine-to-epoxy group ratio (R) of an amine cured epoxy influences the thermomechanical properties of the matrix. Overall it was shown that the R value had a major influence over all of the thermomechanical properties studied. The glass transition temperature (Tg) was shown to change significantly depending on the R value, with a maximum of 87.3 °C observed at a value (R [approx. equal to] 1.25) slightly above the stoichiometric point. The matrix Tg decreased as the R value deviated from this value, approaching room temperature for the extreme ratios. Above Tg, the linear coefficient of thermal expansion (LCTE) was shown to reach a minimum at the stoichiometric ratio due to this ratio inducing the highest crosslink density. Below Tg, the R value appeared to have a less clear influence. The storage modulus (E') of the matrix was also shown to be affected by the R value, with the stoichiometric ratio possessing the largest magnitude of E' for temperatures 20°C above Tg. However, for temperatures 20 °C below Tg the storage modulus decreased in magnitude the closer the R value was to R [approx. equal to] 1.25.;This was the ratio measured to possess the largest Tg value and thus the highest temperature at which E' was plotted relative to the other ratios. The effect of changing the R value on the interfacial shear strength (IFSS) was investigated using the microbond test. This was done in combination with changing the surface chemistry of the glass fibre and purity of the hardener. Results showed the magnitude of IFSS to be significantly affected by the R value, independent of the fibre sizing applied. The chemistry of the fibre sizing was shown to influence the maximum IFSS achievable and the R value at which it would occur, however the magnitude differences were not as significant. From these results it was concluded that the R value of the matrix has a greater influence than the chemistry of the fibre sizing in defining the level of adhesion at the fibre-matrix interface. The changes in IFSS were shown to correlate with Tg and the decrease in the contribution of residual thermal stresses at the interface. However, this contribution only represented a portion of the total IFSS value measured. It was concluded that other mechanisms, such as cure shrinkage stresses, must provide the remaining portion of IFSS shown. To expand upon this, the influence of temperature in combination with the other variables discussed was studied using the microbond test within a thermomechanical analyser. At lower temperatures the maximum IFSS value was shown to occur at R [approx. equal to] 1.0. The magnitude of IFSS was then shown to decrease as the R value deviated further from this value, again independent of the chemistry of the fibre sizing. Above Tg it was observed that a small value of IFSS remained which appeared to possess a linear relationship with the level of amine present within the Rratio. It was hypothesized that the magnitude of IFSS being greater for excess amineratios (R > 1) resulted from a combination of an increase in the level of hydrogen bonding and a variation in the shear failure behaviour of the matrix due to the differing crosslink densities.;Using Nairn's model, a correlation was shown to exist between the IFSS values measured and the potential total contribution of residual stresses for R [approx. equal to] 1.0. However, as the ratios deviated further from R [approx. equal to] 1.0 the degree of correlation was shown to decrease. It was concluded that assumptions made by the model regarding the contribution of cure shrinkage stresses appeared to be oversimplifications once the R value deviated significantly from R [approx. equal to] 1.0. To address this a novel technique using hot-stage microscopy was used to measure the cure shrinkage of a minute epoxy droplet upon a single fibre during the curing process. The results showed that as the R value was increased, the level of cure shrinkage increased. Rheometry was then used to study the influence of the R value on the gel time of the matrix and applied to the data collected using the hot-stage method. This cure shrinkage data was then reapplied to the model where again good correlation was shown for R [approx. equal to] 1.0, yet the discrepancies regarding off-stoichiometric ratios remained. It was concluded that this may be due to a lack of understanding regarding the Tg of a microdroplet and the adhesion mechanisms of a rubbery statepolymer. Overall it was concluded that Nairn's model supports the hypothesis that residual radial compressive stresses at the interface can contribute significantly to the stress transfer capability of the interface. Since these stresses were shown to be affected by the R value it would also satisfy the need for chemistry to be involved significantly in some role

Are silanes the primary driver of interface strength in glass fiber composites? An exploration of the relationship of chemical and physical parameters in the micromechanical characterisation of the apparent interfacial strength in glass fiber composites

It is probably not an overstatement to say that organosilanes are the most important class of chemicals used in the glass fiber, and consequently the composites, industry. One of the best-known assertions about these multifunctional silane molecules is that they promote chemical bonding across the fiber-matrix interface. However, the development of (non-reactive) thermoplastic matrix composites raises questions about the simplistic chemical bridging model of silanes at the interface. Moreover, despite the high level of attention commonly focused on the chemical influences on interfacial adhesion, a growing number of researchers have also commented on the role of residual stress contributing to the stress transfer capability at the fiber-matrix interface. We will review data on the temperature dependence of the apparent interfacial shear strength (IFSS) in (unsized) glass fiber-polypropylene, a system where there is no a priori reasoning to expect any chemical bonding at the interface. The results indicate that the apparent IFSS in thermoplastic composites can be largely explained by the residual thermal stresses. This phenomenon is characterised by a large drop in the measured IFSS when the test temperature is raised above the matrix Tg. We will also present data to show that the same phenomenon is present in the IFSS of glass fiber-epoxy composites, although the magnitude of the measured values of IFSS for epoxy systems cannot be explained by residual thermal stress alone. However, by further considering the possible contribution of the thermoset phenomenon of cure shrinkage we will demonstrate that it is also possible to explain the level of IFSS in this chemically reactive system by physical residual stresses alone. The state of the interface/interphase in epoxy systems is somewhat more complex than for (relatively) non-reactive thermoplastics. This presentation will review our results on the investigation of this complex experimental challenge. Many of the properties required in the modelling of residual stress in these systems vary with the curing agent to epoxy resin ratio near the interface. Since fibers are often coated with sizings containing reactive groups found in both curing agents and epoxy resins it is likely that the polymerised matrix near the fiber surface will have a different ratio of reactive groups than was mixed in the original liquid resin system. To fully explore this concept it is therefore necessary to characterise both the IFSS and the epoxy matrix properties (such as Tg, modulus, and thermal expansion coefficient) as a function of temperature and stoichiometry

The role of the epoxy resin : curing agent ratio in composite interfacial strength by single fibre microbond test

This paper focuses on an investigation into the role of the epoxy resin: curing agent ratio in composite interfacial shear strength of glass fibre composites. The procedure involved changing the percentage of curing agent (Triethylenetetramine [TETA]) used in the mixture with several different percentages used, ranging from 4% up to 30%, including the stoichiometric ratio. It was found by using the microbond test, that there may exist a relationship between the epoxy resin to curing agent ratio and the level of adhesion between the reinforcing fibre and the polymer matrix of the composite

The role of the epoxy resin : curing agent ratio on composite interfacial strength and thermal performance

This paper focuses on analyzing the interfacial and thermal properties of an epoxy resin glass fibre reinforced composite. The interface was studied using the microbond test to investigate interfacial shear strength values while thermo-mechanical analysis and differential scanning calorimetry were used to find variations in the glass transition temperature and coefficient of thermal expansion. For both, the role of the epoxy resin: curing agent ratio was studied to see if it influenced fibre-matrix adhesion and whether it had similar effects on thermal properties. It was found that the epoxy resin: curing agent ratio did indeed influence both interfacial and thermal properties, with maximum performance occurring around the stoichiometric point

The influence of hardener-to-epoxy ratio on the interfacial strength in glass fibre reinforced epoxy composites

This work seeks to develop a better understanding of the influence that the chemistry of an epoxy thermoset system has on the stress-transfer capability of the fibre-matrix interface. We discuss the correlation between the interfacial shear strength (IFSS) and the properties of the matrix such as glass transition temperature (Tg), storage modulus and linear coefficient of thermal expansion (LCTE). The results indicate that each is strongly dependent on the hardener-to-epoxy ratio and it was found that changes in IFSS can be related to changes in the thermomechanical properties of the matrix. From the results presented it is hypothesized that residual radial compressive stresses at the interface are influenced by the chemistry of the matrix system due to the changes in the properties of the matrix. The combination of these residual stresses with static friction may lead to a potential variation of the interfacial stress-transfer capability in glass-fibre reinforced epoxy composites

The dependence of interfacial shear strength on temperature and matrix chemistry in glass fibre epoxy composites

The present work focuses on a fundamental investigation into the influences of the chemistry of epoxy and the testing temperature on the stress-transfer capability of the fibre-matrix interface in a glass fibre reinforced epoxy composite. We discuss how the interfacial shear strength (IFSS) is influenced by the hardener-to-resin ratio, testing temperature and fibre silane coating respectively. It was observed that the IFSS showed a significant inverse dependence on testing temperature for both silanes, with IFSS values dropping as the temperature was increased, for all ratios studied. Notably, it was shown that once the testing temperature was raised above the glass transition temperature that ratios possessing excess amine hardener had larger IFSS values. From the results presented it is hypothesized that residual radial compressive stresses at the interface are influenced by the chemistry of the matrix system and then relax away at the higher testing temperatures

The influence of temperature and matrix chemistry on interfacial shear strength in glass fibre epoxy composites

The present work focuses on further investigating the influences of the chemistry of an epoxy system and the testing temperature on the stress-transfer capability of the fibre-matrix interface in a glass fibre-reinforced composite. We discuss how the apparent interfacial shear strength (IFSS) is influenced by the hardener-to-epoxy ratio and testing temperature. The results indicated that the IFSS was strongly dependent on both matrix chemistry and testing temperature. It was observed that the IFSS showed a significant inverse dependence on testing temperature, with IFSS dropping as the temperature was increased for all ratios. Notably it was shown that once the testing temperature was raised above the glass transition temperature (Tg) that ratios possessing excess hardener had larger IFSS values. From the results presented it is hypothesized that residual radial compressive stresses at the interface are influenced by the chemistry of the matrix system and relax away at the higher testing temperatures

Are silanes the primary driver of interface strength in glass fibre composites? : exploring the relationship of the chemical and physical parameters which control composite interfacial strength

It is probably not an overstatement to say that organosilanes are the most important chemicals used in the glass fibre, and consequently the composites, industry. One of the best-known assertions about silanes is that they promote chemical bonding across the fibre-matrix interface. This concept was fixed in the collective consciousness of the composites community early in its history when developments were focussed strongly on reactive matrices. Indeed, the chemical bridging mindset is strongly entrenched in the interface research community and extends to most other fibre-matrix combinations. However, the development of thermoplastic matrix composites raises questions about the simplistic chemical bridging model of silanes at the interface. A growing number of researchers have also commented on residual stress contributing to the stress transfer capability at the fibre-matrix interface. We will review experimental data on the temperature dependence of the apparent interfacial shear strength (IFSS) in glass fibre-polypropylene and of glass fibre-epoxy composites. This phenomenon is characterised by a large drop in IFSS when the test temperature is raised above the matrix glass transition temperature. These results can be shown to support the hypothesis that the apparent IFSS in composites can be largely explained by residual thermal stresses in the syste