Uncured Material Research Articles

High Thermal Conductivity Silicone Elastomer Doped with Graphene Nanoplatelets and Eutectic GaIn Liquid Metal Alloy

This paper reports the thermal conductivity and mechanical properties of Sylgard 184 polydimethylsiloxane (PDMS) elastomer loaded with graphene nano-platelets (GnPs) and eutectic Ga-In (EGaIn) liquid metal droplets. We fabricated samples with different GnP and EGaIn concentrations and measured their thermal conductivity using the steady-state absolute technique. The results show that the thermal conductivity of the elastomer can be enhanced up to 5.6X when both GNP and EGaIn are included in the elastomer. Without EGaIn, the enhancement is limited to 4.4X. The results suggest that EGaIn inclusion did not change the viscosity of the uncured material significantly at any GnP loading level. We also observed that addition of just EGaIn to PDMS did not have a significant impact on the material’s stiffness while lowering its ultimate tensile strength by a factor of 2X and the maximum elongation at the break point by a factor of 1.6X. On the other hand, it was demonstrated that GnP addition to pure PDMS or EGaIn doped PDMS made the elastomer stiffer and less tear resistant with lower elongation at the break point.

Evaluation of Polymerization Shrinkage and Depth of Cure of Silorane Based Composite Resin and Methacrylate Based Composite Resin (Comparative Study)

Background and objectives: composite constitutes the majority of the direct tooth colored restorations that replace the biological tissue. The major drawbacks of composite resins are related to polymerization shrinkage. This study aimed to compare the polymerization shrinkage and depth of cure between different types of composite resins. Materials and methods: fifteen premolar teeth for each group were collected and cavities prepared on the buccal surface to the depth of 1.5 mm with 3 mm using a diamond cylindrical bur with water coolant. Then each fifteen cavities were filled with one type of composite resin according to the manufacturing instructions. After that specimens were stored in deionized water for one week. Subsequently, the marginal gaps were observed and measured by using a light microscope with a reticular measuring ocular. For the depth of cure Fifteen samples for each group were prepared using metal molds with 6 mm thickness and 4 mm in diameter The composite materials were light cured, then the specimen removed from the mold and the uncured material gently removed with the plastic spatula, the height of cured material was measured with a micrometer and the values were divided by 2, this value was recorded as a depth of cure. Results: The descriptive statistics for the degree of polymerization shrinkage among three groups cleared that the P90 showed the lowest value for polymerization shrinkage with significant difference among three groups. For the depth of cure, it was clear that the Sigma methacrylate based composite resin showed the highest value with significant difference among three groups. Conclusion: Silorane based composite resin produced less polymerization shrinkage in comparison with methacrylate composites resin. The Depth of cure of Sigma methacrylate based composite resin is more in comparison silorane based composite resin.

Predicting Formation of Manufacturing Defects in Contoured Composites

Composite helicopter rotor components are typically thick and often have areas with a tight radius of curvature, which make them especially prone to process-induced defects, including wrinkles and voids at ply interfaces. Such flaws cause high rejection rates in production of flight-critical components and structures. This work seeks to fill the gaps in understanding generation of the noted defects in contoured polymer–matrix composite laminates. In particular, understanding and modeling defect formation at the early stages of the manufacturing process might be the missing link to enable the development of practical engineering solutions allowing for better control of the manufacturing process of contoured composite parts. In this work, an approach based on a continuum description of the uncured prepreg material, including the initial bulk or void content, and finite element modeling (FEM) is used to simulate the consolidation process at the early stages of manufacturing of contoured laminates. The simulation predicts instabilities leading to formation of both wrinkles and voids at ply interfaces during laminate debulking or vacuum consolidation. Applicability of the method to consolidation in both closed-cavity and open-face tooling is also demonstrated. FEM results show good correlation with X-ray computed tomography data. This work also introduces a new simulation concept based on finite element and discrete modeling of voids at ply interfaces to improve the accuracy of predicting their evolution during the debulking operations.

The effect of fiber undulation on the strain field for pinned composite/titanium joints under tension

The way fiber undulation around pins affects pinned composite/titanium joints’ stiffness and strength properties is illustrated for double-lap-shear joints under quasi-static tensile loading. 3D digital-image-correlation techniques were employed to measure in-plane strain distribution on the surface of the composite joint member. Pinned joints with formed holes and three different composite stacking sequences were manufactured and compared to joints with drilled holes. Whereas in the case of joints with formed holes, the process of inserting the pin into uncured prepreg material created undulations, joints with drilled holes are characterized by a uniform fiber direction but partly broken fibers. Due to homogenization of the strain field within the load-carrying 0° layers, pinned composite/metal joints with formed holes proved to significantly outperform joints with drilled holes in terms of tensile stiffness and strength.

Suspended Liquid Subtractive Lithography: One-step generation of 3D channel geometries in viscous curable polymer matrices

The miniaturization of synthesis, analysis and screening experiments is an important step towards more environmentally friendly chemistry, statistically significant biology and fast and cost-effective medicinal assays. The facile generation of arbitrary 3D channel structures in polymers is pivotal to these techniques. Here we present a method for printing microchannels directly into viscous curable polymer matrices by injecting a surfactant into the uncured material via a steel capillary attached to a 3D printer. We demonstrate this technique using polydimethylsiloxane (PDMS) one of the most widely used polymers for the fabrication of, e. g. microfluidic chips. We show that this technique which we term Suspended Liquid Subtractive Lithography (SLSL) is well suited for printing actuators, T-junctions and complex three dimensional structures. The formation of truly arbitrary channels in 3D could revolutionize the fabrication of miniaturized chips and will find broad application in biology, chemistry and medicine.

Understanding and predicting defect formation in automated fibre placement pre-preg laminates

Fibre path defects are detrimental to the structural integrity of composite components and need to be minimised through process optimization. This requires understanding of the uncured pre-preg material, which is influenced by multiple process parameters, and sophisticated multi-scale modelling tools. Even though the capabilities of process modelling techniques have been improved over the past decades, the occurrence of localised wrinkles remains challenging to predict. One of the processes known to influence the formation of fibre path defects is the consolidation of laminates manufactured by automated fibre placement. The particular focus of this paper is to understand how out-of-plane wrinkles form during debulking and autoclave curing of laminates with embedded gaps and overlaps between the deposited tapes. Predictions are made using a novel modelling framework and validated against micro-scale geometry characterisation of artificially manufactured samples. The paper demonstrates the model’s ability to predict consolidation defects for the latest generation of toughened pre-pregs.

Numerical characterisation of uncured elastomers by a neural network based approach

This research paper contributes to the model-free characterisation of elastic and inelastic materials. The introduced material description is based on neural networks for the constitutive stress-strain-relationship and is an alternative to a classical constitutive material description. This approach is an efficient method to represent the material behaviour numerically. The performance of the novel model-free formulation is exemplary shown for uncured natural rubber. One major advantage of the presented approach is the capability to use it for the representation of a wide range of materials.The novel model-free characterisation consists of a neural network that is coupled to the so-called micro-sphere approach. This formulation was developed for the characterisation of rubber like materials and takes the micro-structure of the material into account. As a benefit of this coupling, the neural networks, representing the stress-stretch-dependency, have to be exploited analysed only in onedimensional direction. In the first instance, the derivation is introduced for an Artificial Neural Network to yield a pure elastic description. Subsequently, the model-free approach is expanded in order to represent inelastic material behaviour as well. This extended formulation is obtained via a Recurrent Neural Network.Finally, uncured natural rubber material is described by the derived numerical approach. The model-free characterisation is validated by the finite element simulation of material tests. A complex forming of a rubber block into a mould is basis for a final validation of the model-free description.

Exploiting the oxygen inhibitory effect on UV curing in microfabrication: a modified lithography technique.

Rapid prototyping (RP) of microfluidic channels in liquid photopolymers using standard lithography (SL) involves multiple deposition steps and curing by ultraviolet (UV) light for the construction of a microstructure layer. In this work, the conflicting effect of oxygen diffusion and UV curing of liquid polyurethane methacrylate (PUMA) is investigated in microfabrication and utilized to reduce the deposition steps and to obtain a monolithic product. The conventional fabrication process is altered to control for the best use of the oxygen presence in polymerization. A novel and modified lithography technique is introduced in which a single step of PUMA coating and two steps of UV exposure are used to create a microchannel. The first exposure is maskless and incorporates oxygen diffusion into PUMA for inhibition of the polymerization of a thin layer from the top surface while the UV rays penetrate the photopolymer. The second exposure is for transferring the patterns of the microfluidic channels from the contact photomask onto the uncured material. The UV curing of PUMA as the main substrate in the presence of oxygen is characterized analytically and experimentally. A few typical elastomeric microstructures are manufactured. It is demonstrated that the obtained heights of the fabricated structures in PUMA are associated with the oxygen concentration and the UV dose. The proposed technique is promising for the RP of molds and microfluidic channels in terms of shorter processing time, fewer fabrication steps and creation of microstructure layers with higher integrity.

Dynamic characterization of viscoelastic materials used in composite structures

The main focus of this paper is the experimental comparative analysis of the viscoelastic properties of acrylic- and silicon-based viscoelastic materials. These materials are widely used in the aeronautic industry for structural and/or damping applications. It is therefore required to determine their viscoelastic properties such as shear modulus and loss factor following their integration to aircraft composite structures. The influence of material thickness, bonding quality, curing and cocuring with composite material was evaluated using dynamic mechanical analysis machine in plan shear configuration. This comparative analysis in the 0–600 Hz frequency range provides useful information that should be taken into consideration when designing bonded joints for structural and/or damping applications. The small variation of the acrylic-based material loss factor over a wide range of frequencies suggests that this material is a good candidate for damping applications at room temperature in metallic structures where no curing is required. However, cocuring of acrylic-based material with composite laminae increases its shear modulus up to six times with respect to the uncured material whereas its loss factor is reduced by about 20%. On the other hand, the silicon-based material remains stable after thermal treatment (curing and cocuring) suggesting that it is well suited for in situ damping treatment involving laminate composite structure. However, at room temperature its shear modulus after cocuring is lowered by almost 25% due to poor bonding quality, which depends on the nature of the substrate.

In-plane deformation of multi-layered unidirectional thermoset prepreg – Modelling and experimental verification

Sheet forming of unidirectional prepregs is gaining increased interest as a cost efficient alternative manufacturing method. Its potential lies within the use of automatically and efficiently stacked flat prepregs, which in a second step can be formed. A successful forming requires understanding of the properties of the uncured material. Here, the in-plane deformation behaviour of two different unidirectional thermoset prepregs is investigated. Experimental measurements are performed, showing the importance of stacking sequence and its effect on the forming behaviour of stacked prepreg. Finite element models are developed, using material models calibrated from bias extension tests and interlaminar friction tests. The method developed can be used to predict the reaction force and fibre reorientation during in-plane forming of thermoset prepreg, for one of the considered material systems. Further, it enables prediction of the effect of stacking sequence, which is promising for future full-scale forming simulations.

Investigation of the curing behaviour of carbon fibre epoxy prepreg by Dynamic Mechanical Analysis DMA

Carbon fibre prepregs have found widespread application in lightweight constructions. They are based on a carbon fibre fabric impregnated with reactive epoxy resin. Measurements were carried out using commercially available prepreg material. For Dynamic Mechanical Analysis (DMA), a single cantilever measuring device was applied. The DMA results were refined by additional DSC measurements. The measurements were carried out with dynamic heating in the temperature range −90 to 280 °C. The heating rates were 1 and 2 K/min, respectively. A glass transition of the uncured material (Tg0) near 1 °C, and crosslinking-induced vitrification and devitrification at the maximal glass transition temperature of the cured material (Tgmax) in the temperature range 220 to 230 °C were found. The activation energies for the glass transitions were determined using an Arrhenius plot. By detailed consideration of the influence of the frequency on the DMA data, indications for gelation were deduced.

Self-alignment method for solution-processable dielectric structures via joule heating

We present a versatile method to create self-aligned patterns of polymer dielectric on metal by the use of Joule heating. In contrast to global thermal or light based curing, the method self-aligns the dielectric to a metal pattern on the substrate. A current-induced temperature rise along a metal line cures the insulator locally; uncured material is then removed by rinsing with solvent. We have obtained very promising results for aligning thermally cross-linkable and selectively baked dielectrics. Alignment of cross-linkable polymers is less sensitive to the rinsing step than selectively baked dielectrics. Finite-element simulations were used to determine the range of suitable curing current. After optimization of the curing parameters high yield and low leakage dielectrics were obtained.

Devulcanization of ethylene‐propylene‐diene polymer residues by microwave—Influence of the presence of paraffinic oil

AbstractVulcanized rubbers are materials commonly used in various industrial applications. In this study, scraps of ethylene‐propylene‐diene rubber (EPDM‐r) from the automotive industry were submitted to different microwave exposure times (2–5 min). Samples of recycled rubber with (as received) and without (after extraction) paraffinic oil were analyzed. The devulcanized EPDM‐r was characterized by gel content, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The gel content indicated that the presence of paraffin oil in EPDM‐r affects the devulcanization process. The DSC analysis showed significant changes in the glass transition temperature (Tg). The Tg values for EPDM‐r decreased with an increase in the microwave exposure time. Furthermore, the presence of a thermal phenomenon characteristic of uncured material was observed. Sample degradation was studied through TGA, and the values for the activation energy (Ea) of the degradation process were determined using the Flynn‐Wall‐Ozawa method. For conversions up to 0.10, the Ea values of the EPDM‐r samples without oil decreased with an increase in the microwave exposure time. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers

Time–temperature–transformation cure diagrams of phenol–formaldehyde and lignin–phenol–formaldehyde novolac resins

AbstractIn this study, the time–temperature– transformation (TTT) cure diagrams of the curing processes of several novolac resins were determined. Each diagram corresponded to a mixture of commercial phenol–formaldehyde novolac, lignin–phenol–formaldehyde novolac, and methylolated lignin–phenol–formaldehyde novolac resins with hexamethylenetetramine as a curing agent. Thermomechanical analysis and differential scanning calorimetry techniques were applied to study the resin gelation and the kinetics of the curing process to obtain the isoconversional curves. The temperature at which the material gelled and vitrified [the glass‐transition temperature at the gel point (gelTg)], the glass‐transition temperature of the uncured material (without crosslinking; Tg0), and the glass‐transition temperature with full crosslinking were also obtained. On the basis of the measured of conversion degree at gelation, the approximate glass‐transition temperature/conversion relationship, and the thermokinetic results of the curing process of the resins, TTT cure diagrams of the novolac samples were constructed. The TTT diagrams showed that the lignin–novolac and methylolated lignin–novolac resins presented lower Tg0 and gelTg values than the commercial resin. The TTT diagram is a suitable tool for understanding novolac resin behavior during the isothermal curing process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Characterization of melded carbon fibre/epoxy laminates

‘Melding’ is a novel in situ method for joining thermosetting composite structures, without the need of adhesives. Laminate joining is achieved using uncrosslinked resin matrix of the pre-preg. This study used Hexply914C pre-preg material to characterize melded CFRP structures produced using the melding method. A designated area of a laminate was maintained at temperatures below 40 °C retaining uncured (B-staged) material, while the remainder of the laminate was cured at 175 °C. After a 2.5 h cure cycle, the cured region showed a high degree of cure (0.88) and glass transition temperature (176 °C). The uncured area of the same laminate was cured in a second stage, simulating an in situ melded joint. By controlling the temperature and duration of the intermediate dwell and affecting minimum viscosity values prior to final cure, low values of porosity (<0.5%) were achieved. The mechanical properties of the resulting joint were consistent throughout the melded laminate. Flexural strength (1600 MPa), flexural modulus (100–105 MPa) and short beam strength (105–115 MPa) values observed where equivalent or greater than those found in the recommended autoclave cured control specimens. After the entire laminate was post cured, glass transition temperatures of 230 °C (peak tan δ ) were observed in all areas of the laminate.

Carbon Fiber Reinforced Rigidizable Space Structures

ABSTRACTIn attempts to provide an active solution for the rigidization of flexible space structures, internal resistive heating is applied to a novel thermosetting resin. Carbon-fiber tow coated in U-Nyte Set 201A, which cures at ∼150°C, was heated by passing electric current through the reinforcing material. Using a proportional-integral (PI) controller, precise temperature control of the heating process was established. Samples cured via controlled internal resistive heating were heated to 160°C and underwent material consolidation in less than 7 minutes. A change in material stiffness was measured to be almost two orders of magnitude greater than that of an uncured material.

Evaluation of depth of cure and Knoop hardness in a dental composite photo-activated using different methods

The aim of this study was to evaluate the depth of cure and Knoop hardness in the P60 composite resin photo-activated using different methods. A bipartite brass matrix (3 mm in diameter X 11 mm in height) was filled with the composite and photo-activation was performed using continuous light, exponential light, intermittent light, plasma arc curing (PAC) or light-emitting diodes (LED). After opening the matrix, the uncured material was removed with a steel spatula and the polymerized composite was measured using a pachymeter. The specimens were then included in self-curing acrylic resin and worn longitudinally and the hardness was measured on the surface and at depths of 1, 2, 3, 4 and 5 mm. The data were analyzed by ANOVA and Tukey's test (5%). The results showed that the depth of cure was higher with the intermittent light, followed by continuous light, exponential light, PAC and LED methods. Up to a depth of 2 mm, all methods revealed similar hardness values, but there were differences between them at other depths, at which LED demonstrated the lowest values followed by PAC.

Curing-light intensity and depth of cure of resin-based composites tested according to international standards

Several factors control the light curing of a resin-based composite: the composition of the composite, the shade of the composite, the wavelength and bandwidth of the curing light, the distance of the light from the composite, the intensity of the curing light and the irradiation time. The authors investigated the depth of cure of several shades of five brands of resin-based composites when irradiated via light in the 400- to 515-nanometer wavelength bandwidth at the International Organization for Standardization, or ISO, recommended intensity of 300 milliwatts per square centimeter. The resin-based composites were irradiated for the times recommended by the products' manufacturers. The authors used a curing light adjusted to emit 300 mW/cm2 in the 400-nm to 515-nm wavelength bandwidth to polymerize five samples of each composite brand type and shade. They measured depth of cure using a scraping method described in the ISO standard for resin-based composites. Depth of cure was defined as 50 percent of the length of the composite specimen after uncured material was removed by manual scraping. The authors determined a mean from the five samples of each composite brand and shade. Thirteen (62 percent) of 21 composite materials met the ISO standard depth-of-cure requirement of 1.5 millimeters. Six of the eight remaining materials met the depth-of-cure requirement when the authors doubled the irradiation time recommended by the product manufacturers. Curing lights with an intensity of 300 mW/cm2 appear to effectively cure most resin-based composite materials when appropriate curing times are used, which, in some cases, are longer than those recommended by the manufacturers. Dentists should verify the depth of cure of a composite material as a baseline measure, and then check depth of cure periodically to confirm light and material performance. The ISO depth-of-cure measurement method can be used for this purpose.

Absorption of Dimethylacetamide (DMAC) Following Application of a Polymer Film to the Skin of Rabbits

Kapton ® Film is a polymer with high strength and thermal resistance finding use in a wide variety of applications. In the preparation of this film, the uncured material could contain up to 30% dimethylacetamide (DMAC). Note that the final product Kapton Film has been thermally cured with the DMAC removed. During processing, dermal contact with the film is anticipated, and the possibility exists of DMAC transfer from the uncured film, to and through the skin. In this study, 2 2 2-inch pieces of film were applied to the skin of a group of rabbits and secured in place for a single 4-hour contact time. An amount of liquid DMAC corresponding to the amount contained in the 2-inch square was applied to the skin of a separate group of rabbits for 4 hours. Urine samples were collected over the intervals of application to 4 hours post application (8 hours total) or 4 to 20 hours post application (16 hours). The amount of the urinary metabolite monomethylacetamide (MMAC) was determined analytically in these samples. Urine from 2 untreated rabbits was collected at the same time to serve as controls. The amount of urinary MMAC found in the rabbit urine from animals exposed to uncured Kapton Film at both collection intervals was similar to the amount seen in the controls (background). Rabbits treated with DMAC liquid had measurable urinary MMAC levels that were approximately 100 times background. It is concluded that, under the conditions of this study, very little, if any, DMAC from the uncured Kapton Film was absorbed through the skin (and excreted in the urine as MMAC).

Techniques used by a group of United Kingdom-based dental practitioners during the provision of indirect tooth-coloured restorations.

A number of techniques involving the use of various instruments, brushes and devices have been described for the removal of excess luting material and subsequent finishing around the margins of indirect tooth-coloured restorations. There is little information available on the clinical techniques employed by UK dental practitioners in the placement of these restorations, or on the effectiveness of these techniques. A questionnaire was designed to elicit such information. This was distributed to 500 UK-based dental practitioners. A total of 301 practitioners returned completed questionnaires, giving a response rate of 60%. The results indicated that 88% of the respondents routinely provided porcelain veneers for their patients, with 94% of these respondents indicating that they routinely prepared teeth during the provision of such restorations. A light-cured resin composite luting material was used by 29% of the respondents in placement of veneers, while 69% used a dual cured composite. Excess uncured luting material was removed by a number of methods and a variety of methods were used by respondents to finish the margins of the veneers after placement. Of the respondents 68% indicated that they routinely provided tooth-coloured inlays and onlays, with 89% indicating that they routinely used a dentine bonding system when luting their indirect tooth-coloured restorations. It is concluded that a high proportion of the practitioners surveyed were providing porcelain veneers and tooth-coloured inlays and that a wide variety of materials and finishing techniques were being used.