Recent Developments of New Components for Dental Adhesives and Composites

The currently used commercial self‐etching enamel‐dentin adhesives and restoratives composites are mainly based on a mixture of various monofunctional and cross‐linking dimethacrylates. New developments of enamel‐dentin adhesives concern the improvement of technique insensitivity and storage stability. Improvements of restorative composites are focused on the reduction of the polymerisation shrinkage, as well as the improvement of wear resistance, biocompatibility, and processing properties. In the past five years, many research efforts have been carried out to develop new monomers and tailor‐made components for filling materials, such as fillers or initiators. New phosphonic acid ether acrylates and cross‐linking bis(acrylamide)s enable the preparation of self‐etching enamel‐dentin adhesives with improved storage stability. With free‐radically polymerisable cyclic monomers, such as bicyclic cyclopropyl acrylates or cyclic allyl sulfides, low‐shrinkage storage‐stable restorative composites could be prepared. In case of the cationic polymerisable cyclic monomers, like siloxane‐based cycloaliphatic epoxides, the lower curing rate, stronger exothermic effect and lower curing depth compared to dimethacrylate‐based composites presently prevent their dental application. Designed methacrylates with tailor‐made properties and sol‐gel polycondensates can also contribute to the improvement of the currently used restorative composites. Radical polymerisable dental materials are initiated by light curing and by redox initiator systems. Under acidic conditions amine containing initiator systems are deactivated by acid‐base reaction. Non amine‐based initiators have to be developed for acidic monomer containing dental materials. The use of discrete nano‐filler particles mainly concerns the reinforcement and, in some cases, the increase of X‐ray opacity of composite filling materials.

     

Elaboration of Ultra‐High Molecular Weight Polyethylene/Carbon Nanotubes Electrospun Composite Fibers

Micron‐sized fibers of UHMWPE reinforced with CNT were fabricated by the electrospinning process. Conditions for a metastable mutual solution of UHMWPE and CNTs were found at elevated temperature. These solutions were used for electrospining using a device having controlled temperature and gaseous environment around the electrospun liquid jet. The fabricated micron‐sized fibers exhibited the reinforcing CNTs as self‐organized nano‐ropes embedded within them. A post‐spinning drawing process enhanced the mechanical properties of the composite fibers to the level of 6.6 GPa strength and elongation at break of 6%. The CNT nano‐ropes form spontaneously in the liquid jet during electrospinning, and provide the reinforcement framework which is amenable for post‐drawing of the fibers for subsequent utilization as composite nanofibers. The experimental results exhibit the highest strength value reported to date for electrospun fibers.

     

CuxS/PAN 3D Nanofiber Mats as Ultra‐Lightweight and Flexible Electromagnetic Interference Shielding Materials

Shielding materials are becoming increasingly important, but present materials suffer from either insufficient mechanical stability or limited shielding properties. In this study, 3D flexible copper sulfide (Cu_xS)/polyacrylonitrile (PAN) nanofiber mats are developed via air spinning followed by chemical reaction with copper salt. The Cu_xS/PAN nanofiber mats exhibit an ultra‐lightweight density of 0.044 g cm^?3 and a thickness of 0.423 mm. Stable electromagnetic interference (EMI) shielding effectiveness (SE) (29–31 dB) of the Cu_xS/PAN composite is achieved in the frequency range of 500–3000 MHz. EMI SE per unit surface density of 16 655.92 dB cm² g^?1 is several orders of magnitude higher than most copper sulfide containing EMI shielding materials reported in literature. In addition, the introduction of the Cu_xS improves the thermal stability and launderability of the PAN mats giving the mats thermal, mechanical, and aqueous stability. Finally, the shielding mechanism of the Cu_xS/PAN nanofiber mats for electromagnetic waves is proposed     

Adhesion and Cellular Compatibility of Silicone‐Based Skin Adhesives

Pressure‐sensitive adhesives based on silicone materials have emerging potential as adhesives in healthcare products, in particular for gentle skin adhesives. To this end, adhesion to rough skin and biocompatibility are crucial factors for a successful implementation. In this study, the mechanical, adhesive, and biological properties of the two‐component poly(dimethylsiloxane) Soft Skin Adhesive MG 7‐9800 (SSA, Dow Corning) have been investigated and compared to Sylgard 184. Different mixing ratios of SSA's components allow for tuning of the shear modulus, thereby modifying the adhesive properties of the polymer. To give a comprehensive insight, the authors have analyzed the interplay between pull‐off stress, adhesion energy, and stretch of the adhesive films on smooth and rough surfaces. The focus is placed on the effects of substrate roughness and on low pressure oxygen plasma treatment of the adhesive films. SSA shows superior biocompatibility in in vitro cell culture experiments. High pull‐off stresses in the range of 3 N cm⁻² on a rough surface are achieved, promising broad application spectra for SSA‐based healthcare products.

     

Joining of C/SiC composites by spark plasma sintering technique

CVD–SiC coated C/SiC composites (C/SiC) were joined by spark plasma sintering (SPS) by direct bonding with and without the aid of joining materials. A calcia-alumina based glass–ceramic (CA), a SiC + 5 wt% B₄C mixture and pure Ti foils were used as joining materials in the non-direct bonding processes. Morphological and compositional analyses were performed on each joined sample. The shear strength of joined C/SiC was measured by a single lap test and found comparable to that of C/SiC.     

基于核应用下碳化硅陶瓷及其复合材料的连接研究进展

SiC陶瓷及其复合材料凭借其自身固有的核辐射下的稳定性而有望成为新一代核裂变以及未来核聚变反应堆中重要的结构材料.能否满足核应用环境下各种苛刻条件而实现完美连接是其能够得到最终应用的关键.本文综述了目前国际上基于核应用上SiC陶瓷及其复合材料的几种连接工艺的发展情况.     

Pressure‐less joining of C/SiC and SiC/SiC by a MoSi2/Si composite

A MoSi₂/Si composite obtained in situ by reaction of silicon and molybdenum at 1450°C in Ar flow is proposed as pressure‐less joining material for C/SiC and SiC/SiC composites. A new “Mo‐wrap” technique was developed to form the joining material and to control silicon infiltration in porous composites. MoSi₂/Si composite joining material infiltration inside coated and uncoated C/SiC and SiC/SiC composites, as well as its microstructure and interfacial reactions were studied. Preliminary mechanical strength of joints was tested at room temperature and after aging at service temperatures, resulting in interlaminar failure of the composites in most cases.     

Design and Synthesis of Hydrolytically Stable N‐Nitrourea Explosives

A series of high energy density compounds (HEDCs) based on N‐nitrourea were designed and their theoretical performances were calculated using the Gaussian programs. The predicted values of the energy density of these compounds are in the range 1.848–1.93 g cm⁻³, and their calculated VODs are in the range 6700–8305 m s⁻¹. Tetranitrodiglycoluril (TNDGU) ( 1 ) and tetranitrotriglycoluril (TNTGU) ( 2 ) were synthesized and characterized by NMR (¹H, ¹³C) and IR spectroscopy as well as elemental analyses. The structure of the aminolysis product ( 7 ) of TNDGU was further confirmed by single‐crystal X‐ray diffraction, which indicated that the structure belongs to P2₁/c space group in the monoclinic system.     

Electrical properties of C/C and C/C-SiC ceramic fibre composites

The electrical properties of carbon/carbon (C/C) and carbon/carbon-silicon carbide (C/C-SiC) ceramic composites were measured. The results show that the capacitance decreases rapidly with an increase in frequency and it becomes constant above a frequency of 500 kHz, whereas the dissipation factor increases with increasing frequency. C/C-SiC composites give higher value than C/C composites due to the presence of microcracks.     

A Facile Method for the Synthesis of a MoS2/g‐C3N4 Photocatalyst

A MoS₂/g‐C₃N₄ composite was synthesized by a significantly simple method using Na₂MoO₄ and thiourea as precursors, without the need for a hydrothermal or an ultrasound step. The photocatalytic activity of the synthesized material was evaluated by the degradation of rhodamine B (RhB) in aqueous solution under blue light. The composites with low content of MoS₂ performed better than pure g‐C₃N₄. Using a low‐power light‐emitting diode light source, an improvement of two orders of magnitude in the photochemical space‐time yield was achieved, proving enhanced energy efficiency and productivity compared to earlier studies. The degradation pathway of organic pollutants was confirmed by the effects of selective scavengers.     

Harnessing the Melting Peculiarities of Ultra‐High Molecular Weight Polyethylene Fibers for the Processing of Compacted Fiber Composites

Summary: Compacted fiber composites offer unique properties due to their lack of an extraneous matrix. The conditions of processing ultra‐high molecular weight polyethylene (UHMWPE) fibers were simulated in a heated pressure cell. In situ X‐ray diffraction measurements were used to follow the relevant transitions and the changes in the degree of crystallinity during melting and crystallization. The results strongly support the suggestion that the hexagonal crystal phase, in which the chain conformation is extremely mobile on the segmental level, constitutes the physical basis of compaction technologies for processing UHMWPE fibers into a single‐polymer composite. This report suggests that using a pseudo‐phase diagram outlining the occurrence of different phases during slow heating and the degree of crystallinity can provide valuable insight into the technological parameters relevant for optimal processing conditions.

Degree of crystallinity as a function of pressure and temperature in a region relevant to compaction processes.     

Preparation and Characterization of Organic/Inorganic Polymer Composites Based on Mg‐Silicates

Summary: An organic‐inorganic hybrid material consisting of a 3‐(methacryloxy)propyl functionalized SiO₂/MgO framework was synthesized. This hybrid was successfully reacted with styrene, butyl acrylate and butyl methacrylate via a free radical emulsion polymerization to form polymer composites. The polymer composites were investigated by means of FT‐IR spectroscopy, TGA, DSC and rheometry. It is shown that the polymer is linked covalently to the organic/inorganic hybrid. Although the polymer content is rather low, the composites exhibit a polymer‐like character and enhanced mechanical properties compared to the corresponding homopolymers.

     

Design,Preparation and Characterization of Self‐Reinforced Starch Films through Chemical Modification

A new technique for design and preparation of self‐reinforced starch films is introduced. The films were based on a high‐amylose corn starch that was chemically modified in different ways. Hydroxypropylation was used to decrease gelatinization temperature and improve processability. The reinforcing component consisted of cross‐linked starch granules, where the crosslinking increased granule thermal stability and moisture resistance. Distribution of the cross‐linked starch was imaged by CLSM, and the matrix/particle interface was studied by SEM. Modulus and tensile properties of the starch film were increased by about 30 and 20%, respectively, after addition of rigid cross‐linked starch particles. A perfect interface between matrix and reinforce agent was obtained.

     

炭/炭复合材料用浸渍剂残炭率研究

研究了两种炭/炭复合材料液相致密用高效、安全的浸渍剂.一种为混合树脂体系,即氨酚醛树脂与树脂B混合且不添加其他任何固化剂;一种为利用新型固化剂C取代磷酸作为糠酮树脂的固化剂.结果表明,混合树脂与纯氨酚醛树脂浸渍剂相比较,其残炭率提高约10％;当混合树脂体系中树脂B的质量分数为30％～50％时,具有较好的残炭率;新型固化剂C作为糠酮树脂固化剂时,其残炭率与磷酸作为固化剂时相当,且安全性更高,具有取代磷酸作为糠酮树脂固化剂的可能性.     

针刺C/C复合材料拉伸性能和韧性的研究进展

综述了近年来国内外针刺C／C复合材料拉伸性能和韧性研究进展,归纳了影响针刺C／C复合材料拉伸性能和韧性的主要因素,即炭纤维、炭基体、界面和针刺工艺参数等,初步得到了提高材料拉伸性能和韧性的方法：提高针刺C／C材料拉伸性能的方法主要是提高纤维体积分数;提高针刺C／C材料韧性的方法主要是改善致密工艺。在此基础上,对未来的进一步研究进行展望。     

Nanosized (Zr,Hf)O2 coating reinforced by AlN whiskers for the ablation protection of SiC coated C/C composites

In this work, AlN whisker-reinforced (Zr, Hf)O₂ coating was prepared on SiC coated carbon/carbon composites by sol-gel and plasma spraying methods. Its cyclic ablation resistance was evaluated using an oxyacetylene torch with a heat flux of 2.38 MW/m². After ablation, the coating showed apparently decreased crack sizes and quantity as compared with the one without AlN whisker addition, pointing out its better crack tolerance. Moreover, the coating had a thinner oxidized region based on the linear ablation rate (−0.042 µm/s), greatly lower than that of pure oxide coating (−0.922 µm/s). After detailed observation and characterization, probable protection mechanism was proposed.     

Preparation of C/C‐SiC Composites from All‐Cellulose Precursors

All‐cellulose composites (ACCs) are manufactured from high‐performance cellulose fibers and a cellulose‐containing ionic liquid (IL) as matrix‐forming dope via wet‐winding processes, using different concentrations of cellulose in the IL. ACCs are carbonized at 1650 °C and then infiltrated with liquid silicon. Application of a carbonization aid (ammonium dihydrogenphosphate, ADHP) substantially improves the carbon yield after carbonization but also results in the depletion of the mechanical properties of the final carbon/carbon silicon carbide (C/C‐SiC) material. The microstructure of the porous carbon/carbon preforms strongly depends on both the concentration of cellulose in the IL and the concentration of ADHP. A C/C‐SiC composite manufactured from 6 wt% cellulose in the matrix‐forming dope, in the absence of ADHP, has a maximum flexural strength of 60 MPa. New C/C‐SiC composites with different shapes including Z‐profiles and tubes are successfully manufactured from pre‐shaped ACC precursors. These composites keep their shape during carbonization and the final siliconization process step.