Lu Hui, Stansbury Jeffery W, Bowman Christopher N
Department of Chemical and Biological Engineering, Engineering Center, ECCH 111, University of Colorado at Boulder, Boulder, CO 80309-0424, USA.
Dent Mater. 2004 Dec;20(10):979-86. doi: 10.1016/j.dental.2004.05.002.
Although various photo-curing techniques aiming to partially relieve the shrinkage stress by flow have been proposed and investigated, the direct monitoring and observation of stress relaxation behavior received little attention. In this study, the elucidation of stress relaxation behavior and its impact on the overall stress development were investigated.
A novel experimental set-up capable of real-time, simultaneous shrinkage stress and double bond conversion measurements on the same sample was utilized. The coupled shrinkage stress and conversion for specimens of a barium glass filled Bis-GMA/TEGDMA composite were continuously monitored during and after irradiation for various intervals (2, 3, 6, 10 and 60s).
Specimens irradiated for 60s reached the highest conversion (67.9+/-1.2%) and shrinkage stress (2.9+/-0.1 MPa). Shrinkage stress relaxation phenomena were only observed prior to vitrification, which only applied to specimens partially cured for 2 or 3 s; specimens irradiated for 2 s exhibit the largest capability to relieve stress (approximately 40%). However, to achieve this a much longer post-cure stress relaxation time is required as compared to clinically practical time scales. For specimens irradiated for 6, 10 or 60 s, the majority of the shrinkage stress is developed during and after the vitrification stage, and no appreciable stress relaxation can be observed.
Although stress relaxation prior to vitrification stage has been observed, this study revealed that it did not provide a significant benefit towards the reduction of overall shrinkage stress since, to achieve clinically relevant conversion, the majority of the shrinkage stress is developed during and after the vitrification stage, which does not permit stress relaxation on the time scales used in this study.
尽管已经提出并研究了各种旨在通过流动部分缓解收缩应力的光固化技术，但对应力松弛行为的直接监测和观察却很少受到关注。在本研究中，对应力松弛行为及其对整体应力发展的影响进行了阐明。
使用了一种新颖的实验装置，能够在同一样品上实时、同时测量收缩应力和双键转化率。在不同照射时间间隔（2、3、6、10和60秒）期间及之后，连续监测钡玻璃填充的Bis-GMA/TEGDMA复合材料样品的耦合收缩应力和转化率。
照射60秒的样品达到最高转化率（67.9±1.2%）和收缩应力（2.9±0.1兆帕）。仅在玻璃化之前观察到收缩应力松弛现象，这仅适用于部分固化2或3秒的样品；照射2秒的样品表现出最大的应力缓解能力（约40%）。然而，与临床实际时间尺度相比，要实现这一点需要长得多的后固化应力松弛时间。对于照射6、10或60秒的样品，大部分收缩应力在玻璃化阶段期间及之后产生，并且未观察到明显的应力松弛。
尽管在玻璃化阶段之前观察到了应力松弛，但本研究表明，由于要实现临床相关的转化率，大部分收缩应力在玻璃化阶段期间及之后产生，而在本研究使用的时间尺度上不允许应力松弛，因此它对降低整体收缩应力没有显著益处。
