Reduction In Shrinkage Stress Research Articles

Mitigating Shrinkage Stress in UV-Curable Resin: Shape Memory Microcapsules Enable Shrinkage Stress Reduction via Photopolymerization-Induced Shape Recovery

Mitigating Shrinkage Stress in UV-Curable Resin: Shape Memory Microcapsules Enable Shrinkage Stress Reduction via Photopolymerization-Induced Shape Recovery

Shrinkage stress evolution during photopolymerization: Theory and experiments

Photopolymerized materials are widely used for a broad range of industrial applications including coatings, three-dimensional (3D) printing, micro/nano-scale fabrications, restorative dentistry, etc. However, the service quality and life of these materials are dramatically impaired by polymerization shrinkage stress evolved during the photopolymerization process in practices. A theoretical model for mechanism exploration and accurate prediction of the shrinkage stress evolution is thus-far inaccessible and strategies for alleviating the stress based on existing studies are quite controversial. Herein, we develop a comprehensive theoretical model that can accurately capture the evolution of the shrinkage stress under various working conditions. The model is established by a time-incremental method based on 3D viscoelasticity theory coupling the evolutions of reaction kinetics and material properties during photopolymerization. Theoretical predictions of the shrinkage stress are then validated by groups of experimental measurements using a series of photopolymers and their composites under various testing conditions. We show that our model is able to correctly predict both the individual and combined effects of instrumental compliance, specimen geometry, material composition, and photocuring protocol on the shrinkage stress evolution, results of which well clarify the diverse conflicts existed in the literature. The magnitudes of the predicted stresses based on our model agree quantitatively with those of the experimental ones, far exceeding the prediction accuracy of all existing models. In addition, our model also successfully predicts the results measured in the literature using various testing devices, indicating that our model could be applied to existing setups for shrinkage stress measurement of bulk materials. Our theory-experiment-combined study not only provides a theoretical approach to accurately predict and thoroughly uncover the shrinkage stress evolution during photopolymerization, it will also guide further explorations on shrinkage stress reduction and material property optimization for photopolymers with improved service life. Additionally, the presented model on the shrinkage stress evolution paves a potentially new way for the derivation of material mechanical property evolutions and thus revealing the mechanical-chemical-coupled mechanisms during photopolymerization.

Simultaneously Enhanced Chain Flexibility, Three Dimensional Printability, and Reduction of Shrinkage Stress in Biodegradable and Photocurable Multiblock Copolymers

A library of biodegradable and photocurable poly(propylene fumarate)-co-poly(trimethylene carbonate) (PPF-co-PTMC) multiblock copolymers encompassing the entire composition range and various block lengths has been designed and synthesized. By combining experiments and atomistic molecular dynamics simulations, we reveal the effects of chain architectures such as composition and block lengths on the thermal, rheological, and printing properties of these copolymers. The introduction of flexible PTMC blocks, which are found to be highly miscible with PPF blocks, into the unentangled copolymer backbone dramatically expedites chain relaxation, lowers the activation energy, and enhances the solubility in toluene. The chain flexibility of the copolymers is positively correlated with the composition of PTMC, while the block length of either PPF or PTMC affects little. Thus, lower values in glass transition temperature Tg, zero-shear viscosity η0, printing speed, and shrinkage stress in photocuring are consistently seen at higher compositions of PTMC. Photocurable PPF-co-PTMCs can be printed into high-precision 3D objects. This study provides a series of novel biopolymers and also a guideline to design photocurable polymers used in biomedical implants.

Shrinkage stress of thermal cured epoxy resin reduced by addition of functional hollow microspheres

The shrinkage stress of thermal cured epoxy resin developed during curing process can cause low film adhesion strength, low dimensional accuracy and even coating films warping which severely reduce service lifetime of the coating. Therefore, there is growing interest in developing effective means to reduce the shrinkage stress. In this paper, we report the use of hollow microspheres prepared from Pickering emulsion droplets in the thermal cured epoxy resin formulations and the shrinkage stress due to resin solidification is monitored in real time to reveal the effect of hollow microspheres upon shrinkage stress of the resin. Epoxy group functionalized silica nanoparticles from the Stöber method are used as Pickering emulsion stabilizer and the emulsion droplets are polymerized to obtain hollow microspheres with controllable mechanical properties and surface morphologies. Due to the reactivity of the hollow microspheres, they can be incorporated into the epoxy resin coatings and the shrinkage stress is thus reduced with the addition of hollow microspheres. It is revealed that the shrinkage stress is reduced from 195.79 kPa to 140. 40 kPa with the addition of 12 wt% hollow microspheres and polydisperse hollow microspheres are more effective in shrinkage stress reduction. As the modulus of the hollow microspheres is reduced and elasticity is increased, the shrinkage stress of the coating film is also reduced from 181. 37 kPa to 157.42 kPa which is not achieved by hollow glass microspheres. The hollow microspheres are well dispersed in the resin matrices which doesn't undermine the basic mechanical properties of the coating such as adhesion and surface hardness.

Evaluation of allyl sulfides bearing methacrylate groups as addition-fragmentation chain transfer agents for low shrinkage dental composites

Polymerization shrinkage and the consequential shrinkage stress in methacrylate-based dental composites are crucial drawbacks for restorative dentistry. An effective way for the reduction of shrinkage stress is the incorporation of addition fragmentation chain transfer agents (AFCT agents). In this contribution, four novel polymerizable allyl sulfides were synthesized and evaluated in dental composites. Photo-DSC and DMTA measurements were performed to evaluate their influence on the polymerization rate and on the network homogeneity. Good to excellent shrinkage force values and mechanical properties were obtained for all AFCT based composites. Especially, the incorporation of AFCT agents containing multiple urethane and methacrylate moieties improved the mechanical properties. Hence, the incorporation of polymerizable and urethane-based allyl sulfides in dental composite is a promising strategy to obtain low shrinkage materials exhibiting high flexural strength and modulus.

A study on shrinkage residual stresses, microstructure and mechanical properties of ASS thick pipe welded by GMAW process

In this investigation, AISI 304LN austenitic stainless steel pipes having 25 mm thickness and 300 mm inner diameter with conventional and narrow groove were welded by continuous current gas metal arc welding (GMAW) and pulse current gas metal arc welding (P-GMAW) process. A gas tungsten arc welding (GTAW) process was used for the welding of the root pass. Microstructural study was carried out in fusion zone (FZ) and heat-affected zone (HAZ). A favourable microstructure characteristic was observed in the P-GMAW process. The study of transverse shrinkage and shrinkage stresses were done on the conventional groove with GMAW and P-GMAW process and narrow groove by P-GMAW process. It was found that narrow groove weld design with controlled welding parameters can reduce 45% shrinkage as compare to conventional groove, similarly 30% reduction in shrinkage stress can be achieved. The tensile properties were observed to increased, such as yield strength was improved from 260 MPa to 310 MPa and ultimate strength was improved from 510 MPa to 550 MPa in the case of narrow groove weld by the P-GMAW process.

Photopolymerization shrinkage-stress reduction in polymer-based dental restoratives by surface modification of fillers

ObjectivesThis research explores the use of polymer brushes for surface treatment of fillers used in polymer-based dental restoratives with focus on shrinkage stress reduction. The influence of interfacial reactive groups on shrinkage stress is explored. MethodsOligomers of varying lengths and with varying number of reactive groups along the length were synthesized by modifying commercial oligomers. Surface of silica fillers (OX50) was treated with methylaminopropyltrimethoxysilane and this was further reacted with the synthesized oligomers to obtain a series of polymer brushes on the surface. Fillers modified with γ-methacryloxypropyltrimethoxysilane were used as a control. Filler surface treatment was confirmed using diffuse reflectance spectroscopy and thermogravimetric analysis. Fillers were added at 30 wt % to a resin made of BisGMA/TEGDMA and polymerization kinetics, shrinkage stress, volumetric shrinkage, flexural strength and modulus, viscosity were measured. ResultsComposites with polymer brush functionalized fillers showed up to a 30 % reduction in shrinkage stress as compared to the control, with no reduction in flexural strength and modulus. Shrinkage stress reduced with increasing length of the polymer brush and increased with increase in number of reactive groups along the length of the polymer brush. SignificanceThe interface between inorganic fillers and an organic polymer matrix has been utilized to reduce shrinkage stress in a composite with no compromise in mechanical properties. This study gives insights into the stress development mechanism at the interface.

Vickers Hardness and Shrinkage Stress Evaluation of Low and High Viscosity Bulk-Fill Resin Composite.

The aim of this in vitro study was to evaluate the hardness and shrinkage stress (SS) of six bulk-fill resin composites. To evaluate microhardness (MH), ten 6 mm specimens were prepared using a metal mold for each selected bulk-fill resin composite and irradiated from the top side for 40 s using an LED light. After 24 h of storage, Vickers MH was evaluated on the upper, lower and lateral sides of the specimens. SS evaluation was then performed with a universal machine, which evaluated the contraction force generated by a bulk-fill composite specimen placed between two metal cylinders during and after light curing. The results were evaluated with a one-way ANOVA test with a post-hoc Bonferroni test and linear regression analysis (p < 0.05). All materials showed a significant MH decrease between the top and bottom surfaces. However, the bulk-fill materials tested performed differently when considering lateral depth progression. ANOVA tests for SS evaluation showed that both SDR and Venus Bulk Fill had significantly lower stress during irradiation than other tested materials. Further, MH decrease became significantly lower from the top surface at different depths in each tested group. Among the different resins, Venus Bulk Fill and SDR showed not only inferior hardness, but also a significant reduction in SS.

Dental Restorative Materials Based on Thiol-Michael Photopolymerization

Step-growth thiol-Michael photopolymerizable resins, constituting an alternative chemistry to the current methacrylate-based chain-growth polymerizations, were developed and evaluated for use as dental restorative materials. The beneficial features inherent to anion-mediated thiol-Michael polymerizations were explored, such as rapid photocuring, low stress generation, ester content tunability, and improved mechanical performance in a moist environment. An ester-free tetrafunctional thiol and a ultraviolet-sensitive photobase generator were implemented to facilitate thiol-Michael photopolymerization. Thiol-Michael resins of varied ester content were fabricated under suitable light activation. Polymerization kinetics and shrinkage stress were determined with Fourier-transform infrared spectroscopy coupled with tensometery measurements. Thermomechanical properties of new materials were evaluated by dynamic mechanical analysis and in 3-point bending stress-strain experiments. Photopolymerization kinetics, polymerization shrinkage stress, glass transition temperature, flexural modulus, flexural toughness, and water sorption/solubility were compared between different thiol-Michael systems and the BisGMA/TEGDMA control. Furthermore, the mechanical performance of 2 thiol-Michael composites and a control composite were compared before and after extensive conditioning in water. All photobase-catalyzed thiol-Michael polymerization matrices achieved >90% conversion with a dramatic reduction in shrinkage stress as compared with the unfilled dimethacrylate control. One prototype of ester-free thiol-Michael formulations had significantly better water uptake properties than the BisGMA/TEGDMA control system. Although exhibiting relatively lower Young’s modulus and glass transition temperatures, highly uniform thiol-Michael materials achieved much higher toughness than the BisGMA/TEGDMA control. Moreover, low-ester thiol-Michael composite systems show stable mechanical performance even after extensive water treatment. Although further resin/curing methodology optimization is required, the photopolymerized thiol-Michael prototype resins can now be recognized as promising candidates for implementation in composite dental restorative materials.

Application of an Addition-Fragmentation-Chain Transfer Monomer in Di(meth)acrylate Network Formation to Reduce Polymerization Shrinkage Stress.

A new addition-fragmentation chain transfer (AFT) capable moiety was incorporated into a dimethacrylate monomer that participated readily in network formation by copolymerizing with multifunctional methacrylates or acrylates. The process of AFT occurred simultaneously with photopolymerization of the AFT monomer (AFM) and other (meth)acrylate monomers leading to polymer stress relaxation via network reconfiguration. At low loading levels of the AFM, a significant reduction in shrinkage stress, especially for acrylate monomers, was observed with nominal effects on conversion. At higher loading levels of the AFM, the photopolymerization reaction kinetics and final double bond conversion were significantly lowered along with a delay in the gel-point conversion. Electron paramagnetic resonance studies during polymerization revealed the presence of a distinct radical species that was present in proportional quantities to the AFM content in the system. The lifetime and the character of the persistent radicals were altered due to the presence of the distinctive radical, in turn affecting the polymerization kinetics. With polymerization conducted at higher irradiance, the differential conversion between the control resin and samples with moderate AFM content was minimal, especially for the methacrylate-based formulations.

Shrinkage stress reduction in photopolymerization by polysiloxane-modified nanogel

Shrinkage stress reduction in photopolymerization by polysiloxane-modified nanogel

Influence of storage time in water on the integrity of adhesive interface in resin composite restorations

When immersed in oral fluids, water absorption by the restorative resin composite material can occur, which is identified by some researchers as one of the causes of loss of aesthetic features and reduction of mechanical properties over time. On the other hand, some authors have suggested that the fluids sorption may contribute to the reduction of shrinkage stress generated at the adhesive interface and reduce the width of gaps. The aim of this study was verifying if the storage time in water of restorations carried out with different filling techniques could influence on the integrity of tooth-restoration adhesive interface. Eighteen cavities were built in bovine incisors and they were divided into 3 groups after the adhesive procedure: group B (“Bulk”) received one single increment of light-cured resin composite; group I (“Increments”) received the same composite in three oblique increments; and the group B+S (“Bulk + Self-cured resin composite”). The last one firstly received a flowable, self-cured resin composite, and then, it was inserted one single increment of light-cured composite. After 48 h of storage, the restorations were sliced, the first measurement was accomplished, and the analysis of the adhesive interface was made each 30 days over nine months of immersion in water. The results were subjected to a split-plot analysis of variance and Tukey’s test. It was not verified significant influence of immersion time in water on the gap width, or regular increase or decrease of percentage interface free of gaps over time for any of three filling techniques. Some hypothesis could explain this occurrence, such as gain of mass without significant increase in the volume; the expansion of restoration in directions that did not contributed to the gaps closure; and the simultaneous occurrence of hygroscopic expansion and hydrolytic degradation of the resin processes.

A study of shrinkage stress reduction and mechanical properties of nanogel-modified resin systems

A series of nanogel compositions were prepared from urethane dimethacrylate (UDMA) and isobornyl methacrylate (IBMA) in the presence of a thiol chain transfer agent. The linear oligomer of IBMA was synthesized by a similar solution polymerization technique. The nanogels were prepared with different crosslinker concentrations to achieve varied branching densities and molecular weights. The prepolymers were dispersed in triethylene glycol dimethacrylate at loading levels ranging from 10 wt.% to 50 wt.%. Photopolymerization reaction kinetics of all prepolymer modified systems were enhanced relative to the nanogel-free control during early stage polymerization while limiting conversion was similar for most samples. Volumetric polymerization shrinkage was reduced proportionally with the prepolymer content while the corresponding decrease in polymerization stress was potentially greater than an additive linear behavior. Flexural strength for inert linear polymer-modified systems decreased significantly with the increase in the prepolymer content; however, with an increase in the crosslinker concentration within the nanogel additives, and an increase in the concentration of residual pendant reactive sites, flexural strength was maintained or improved regardless of the nanogel loading level. This demonstrates that covalent attachment rather than just physical entanglement with the polymer matrix is important for effective polymer mechanical reinforcement by nanogel additives. Reactive nanogel additives can be considered as a practical, generic means to achieve substantial reductions in polymerization shrinkage and shrinkage stress in common polymers.

Using hyperbranched oligomer functionalized glass fillers to reduce shrinkage stress

ObjectiveFillers are widely utilized to enhance the mechanical properties of polymer resins. However, polymerization stress has the potential to increase due to the higher elastic modulus achieved upon filler addition. Here, we demonstrate a hyperbranched oligomer functionalized glass filler UV curable resin composite which is able to reduce the shrinkage stress without sacrificing mechanical properties. MethodsA 16-functional alkene-terminated hyperbranched oligomer is synthesized by thiol-acrylate and thiol-yne reactions and the product structure is analyzed by 1H NMR, mass spectroscopy, and gel permeation chromatography. Surface functionalization of the glass filler is measured by thermogravimetric analysis. Reaction kinetics, mechanical properties and shrinkage stress are studied via Fourier transform infrared spectroscopy, dynamic mechanical analysis and a tensometer, respectively. ResultsSilica nanoparticles are functionalized with a flexible 16-functional alkene-terminated hyperbranched oligomer which is synthesized by multistage thiol-ene/yne reactions. 93% of the particle surface was covered by this oligomer and an interfacial layer ranging from 0.7nm to 4.5nm thickness is generated. A composite system with these functionalized silica nanoparticles incorporated into the thiol–yne–methacrylate resin demonstrates 30% reduction of shrinkage stress (from 0.9MPa to 0.6MPa) without sacrificing the modulus (3100±300MPa) or glass transition temperature (62±3°C). Moreover, the shrinkage stress of the composite system builds up at much later stages of the polymerization as compared to the control system. SignificanceDue to the capability of reducing shrinkage stress without sacrificing mechanical properties, this composite system will be a great candidate for dental composite applications.

Covalent adaptable networks as dental restorative resins: Stress relaxation by addition–fragmentation chain transfer in allyl sulfide-containing resins

Objectives The aim is to demonstrate significant polymerization shrinkage stress reduction in model resins through incorporation of addition–fragmentation chain transfer moieties that promote network stress accommodation by molecular rearrangement. Monomers containing allyl sulfide linkages are incorporated to affect the shrinkage stress that arises during photopolymerization of model resins that contain an initiator and dimethacrylates. Radical-mediated allyl sulfide addition–fragmentation is enabled during polymerization. We hypothesize that allyl sulfide incorporation into methacrylate polymerizations promotes stress relaxation by enabling network adaptation. Methods A 1:2 mixture of tetrathiol and allyl sulfide-containing divinyl ethers is formulated with glass-forming dimethacrylates and compared to controls where the allyl sulfide is replaced with a propyl sulfide that is incapable of undergoing addition–fragmentation. Simultaneous shrinkage stress and functional group conversion measurements are performed. The T g is determined by DMA. Results Increasing allyl sulfide concentration reduces the relative stress by up to 75% in the resins containing the maximum amount of allyl sulfide. In glassy systems, at much lower allyl sulfide concentrations, the stress is reduced by up to 20% as compared to propyl sulfide-containing systems incapable of undergoing addition–fragmentation chain transfer. Significance Shrinkage stress reduction, typically accompanying free-radical polymerization, is a primary focus in dental materials research and new product development. Allyl sulfide addition–fragmentation chain transfer is utilized as a novel approach to reduce stress in ternary thiol-ene-methacrylate polymerizations. The stress reduction effect depends directly on the allyl sulfide concentration in the given ternary systems, with stress reduction observed even in systems possessing super-ambient T gs and low allyl sulfide concentrations.

Properties of methacrylate–thiol–ene formulations as dental restorative materials

Objectives The objective of this study was to evaluate ternary methacrylate–thiol–ene systems, with varying thiol–ene content and thiol:ene stoichiometry, as dental restorative resin materials. It was hypothesized that an off-stoichiometric thiol–ene component would enhance interactions between the methacrylate and thiol–ene processes to reduce shrinkage stress while maintaining equivalent mechanical properties. Methods Polymerization kinetics and functional group conversions were determined by Fourier transform infrared spectroscopy (FTIR). Cured resin mechanical properties were evaluated using a three-point flexural test, carried out with a hydraulic universal test system. Polymerization shrinkage stress was measured with a tensometer coupled with simultaneous real-time conversion monitoring. Results The incorporation of thiol–ene mixtures as reactive diluents into conventional dimethacrylate resins previously was shown to combine synergistically advantageous methacrylate mechanical properties with the improved polymerization kinetics and reduced shrinkage stress of thiol–ene systems. In these systems, due to thiol consumption resultant from both the thiol–ene reaction and chain transfer involving the methacrylate polymerization, the optimum thiol:ene stoichiometry deviates from the traditional 1:1 ratio. Increasing the thiol:ene stoichiometry up to 3:1 results in systems with equivalent flexural modulus, 6–20% reduced flexural strength, and 5–33% reduced shrinkage stress relative to 1:1 stoichiometric thiol:ene systems. Significance Due to their improved overall functional group conversion, and shrinkage stress reduction while maintaining equivalent flexural modulus, methacrylate–thiol–ene resins, particularly those with excess thiol, beyond the conventional 1:1 thiol:ene molar ratio, yield superior dental restorative materials compared with purely dimethacrylate resins.

Influence of the shape of the layers in photo -cured dental restorations on the shrinkage stress peaks—FEM study

Objective The aim of the paper is to analyse an influence of the shape of the layers in photo-cured dental restorations of Class I on distribution of shrinkage stresses along the tooth-restoration interface. The study is a continuation of the previous considerations (Kowalczyk and Gambin (2008) [1]), where techniques, which reduce stress concentration at the top of the tooth-restoration interface, were considered. The analysis leads to proposition of new layer forming techniques, which diminish the stress peaks at the interface and prevent the crack propagation process. Methods To find the stress distributions in the dental restoration layers and the tooth tissues the finite element method implemented in the ABAQUS (Simulia, Providence, USA) software is used. For Class I restoration of the premolar tooth, the axisymmetrical model is assumed. The restoration is made of four layers of a photo-cured composite. Between the tooth tissues and the restoration, a layer of bonding agent 0.01 mm thick is placed and modeled by FEM with help of the cohesive elements. The assumed model takes into account an influence of changes of elastic properties and viscous effects. For each case of the restoration layers system, the Huber–Mises stresses are analysed. Results The investigations show that the stresses near the restoration-tooth tissue interface are reduced due to viscous flow of the cured material and due to existence of a thin layer of the bonding agent. However, the stress distribution both, in the restoration and in the tooth tissues, is strongly dependent on a shape of the filling layers. Numerical simulations disclose that stress peaks are located at the top corners of each layer. The top corners of the last layer are the places where microleakage may occur. Stress concentrations at the corners of the preceding layers may lead to a growth of uprising crack. It will be shown that the flat layers in the restoration create relatively high values of the stress peaks. The rounded layers, with shapes close to those used in dental practice, reduce maximal stresses about 40%. According to a common opinion of dentists, the wedge-shaped layers give the best result. In the present paper, another way of the shrinkage stress reduction is proposed. Before the layering, one can cover the surface of the tooth cavity with a thin “pre-layer”. Next, the remainder cavity may be filled with flat, rounded or wedged layers. It will be shown, that in the fillings with the pre-layers, stress peaks are reduced up to 75%, with respect to the fillings composed of the rounded layers only. The proposed method considerably reduces the shrinkage stress, both in the tooth restoration, as well as, in the tooth tissues. Significance The fillings with the pre-layer are easy in application and its analysis gives promising results. The pre-layer may be applied with other layers of different shapes, and its thickness may vary. The method is recommended for cavities with a great loss of the tooth tissue.

Techniques of shrinkage stress reduction in dental restorations

Shrinkage stresses are one of the major problems in dental restoration practice. New techniques of stresses reduction at the restoration-tooth interface are proposed. The propositions are tested for bonded inlays restorations using finite element analysis based on the ABAQUS program. Mechanical properties of photo-cured material, as well as, dentin, enamel and pulp are based on data given in the literature. Two kinds of restorations are taken into account: class I and class II. The paper consists of two parts. At first, polymerization shrinkage for restoration made by classical incremental technique is investigated. Here, a corresponding Huber-Mises’ stress distributions in the whole tooth and two classes of restorations are presented. Next, two methods of stress reduction based on a selective light curing of restoration are investigated. Both methods provide significant reduction of the Huber-Mises stresses in restoration and tooth tissue, especially in the class I restorations.

Geometric Factors Affecting Dentin Bonding in Root Canals: A Theoretical Modeling Approach

Cavity configuration factor (C-factor) is the ratio of the bonded surface area in a cavity to the unbonded surface area. In a box-like class I cavity, there may be five times more bonded surface area than the unbonded surface area. During polymerization, the volume of monomers is reduced, which creates sufficient shrinkage stresses to debond the material from dentin, thereby decreasing retention and increasing leakage. The important variables influencing bonding adhesive root-filling materials to canals was examined using a truncated inverted cone model. C-factors in bonded root canals exhibit a negative correlation with sealer thickness. For a 20 mm-long canal prepared with a size 25 file, calculated C-factors ranged from 46 to 23,461 with decreasing sealer thickness (500–1 μm), compared to a C-factor of 32 when the canal was filled only with sealer. As the thickness of the adhesive is reduced, the volummetric shrinkage is reduced, which results in a reduction in shrinkage stress (S-factor). C-factors above 954 calculated with sealer thickness smaller than 25 μm are partially compensated by increases in bonding area and decreases in shrinkage volume. However, the interaction of these two geometrically related factors (C- and S-factors) predicts that bonding of adhesive root-filling materials to root canals is highly unfavorable when compared with indirect intracoronal restorations with a similar resin film thickness.

Shrinkage Control in Fibre Reinforced Polymers II: Epoxy Resins with Expanding Monomers

Experiments have been carried out with dinorbornene spiro ortho carbonate (DNSOC) to determine the extent to which this expanding monomer can be used to reduce the shrinkage stress in a moderately high Tg epoxy resin. Although the copolymers had good mechanical properties and past experience has shown that they can be used to make excellent fibre composites, the effect of the expanding monomer on shrinkage stress was small. Only about 7% of the monomer could normally be dissolved in the epoxy at 100°C, and this contributed only a very small shrinkage stress reduction. At the same time the Tg was reduced. The lack of useful effect is because high Tg polymers can normally only be produced by curing at high temperatures. During cure, most of the beneficial effect of the expansion disappears due to stress relaxation at the curing temperature. Hence, expanding monomers are not recommended for high performance composites for aerospace use.