聚甲醛的光稳定化研究

采用了二苯甲酮类和苯并三唑类紫外线吸收剂(UVA)、位阻胺类光稳定剂(HALS)以及光屏蔽剂对聚甲醛进行光稳定化改性,对比了改性前后聚甲醛光降解后的物理力学性能及表面层分子量保持率。结果表明,0 5%的UVA可以显著提高POM的耐紫外光性能,苯并三唑类效果优于二苯甲酮类UVA。在酚类抗氧剂存在下,低含量的HALS(<0 25%)对聚甲醛没有光稳定化作用。     

Photostabilization of cationic UV-cured coatings in the presence of nanoTiO2

The effect of TiO₂ nanoparticles for sun-weathering protection of UV-cured coatings is investigated. TiO₂ is either introduced in the form of nanoparticles in the photocurable formulations or generated in situ via sol–gel process. Cured films containing comparable amounts of TiO₂ were weathered for 800 h under UV irradiation and compared with free-TiO₂ coating. The TiO₂ presence induces a clear lower mass loss decrease during weathering as well as a lower gel content decrease. The TiO₂ screen effect is also confirmed by a lower alkyl-band reduction monitored by FT-IR during weathering. The TiO₂ generated in situ via sol–gel gives rise to transparent coatings without interfering with photopolymerization process and therefore without compromising UV-cured film properties.     

Enhancing exterior performance of clear coatings through photostabilization of wood. Part 2: coating and weathering performance

Clear-coated boards have not been recommended for use in exterior conditions since irradiation with visible and UV radiation darkens them and photodegrades the lignin in the wooden surface beneath the coating, leading to delamination and subsequent catastrophic coating failure due to the continued action of sun, rain, and biological factors. Many approaches to rectify this problem have been explored. Chemical modification of the surface with hexavalent chromium, reaction with various anhydrides, grafting of UV absorbers, and esterification are among the methods attempted. A second approach has been via the clear coating itself where UV absorbers, antioxidants, and ultrafine titanium and iron oxides have been added. However, these have had limited or no success in stopping photodegradation processes. Since the main cause of photodegradation is photooxidation of lignin in the wooden surface as a consequence of free radical reactions initiated by UV irradiation, the approach taken in the present study, in an attempt to enhance the weathering performance of clear-coated boards outdoors, was to delignify the surfaces of wooden boards and then apply clear coatings to try and retard possible photodegradation. Two different pretreatments were used. Firstly, chemical surface delignification with a peracetic acid treatment created a partial delignification to a depth of 2–3 mm while still retaining the structural integrity of the surface. Secondly, a preweathering treatment, which resulted in a 100-μm-deep delignification zone, was compared. The coatings applied to the exposure surface of the pretreated boards were either polyurethane or an acrylic varnish. The clear-coated boards were exposed to exterior and accelerated weathering regimes for 3 years or 3000 h, respectively. Pretreated coated boards did not darken and yellow on exposure but untreated coated boards did. However, despite apparently arresting photodegradative processes on board surfaces, there were no significant gains in the performance ratings of coated pretreated boards over those of coated untreated control boards. Explanations for this involve the effectiveness of design factors incorporated into boards for exposure trials. These factors were the fungicidal dipping of boards before coating, precoating the exposure surface with a reactive primer, and applying a full polyurethane system to the back side and edges of boards. Both pretreatments resulted in clear-coated board surfaces that performed very similarly on exposure outperforming systems reported previously. It was surprising to observe that the preweathering treatment, which resulted in a 100-μm-deep delignification zone, performed as effectively as the chemically pretreated boards with 2- to 3-mm treatment zone. However, preweathered surfaces had lost all lignin in the middle lamella and there was cell separation, whereas in peracetic acid-treated boards, there was more or less complete lignin removal from the cell corner middle lamella only and partial lignin removal from other cell wall regions. Furthermore, it is anticipated that refinements in treatment methods and coating formulations will bring desired benefits and future work should focus in this area.     

Parison Inflation in Extrusion Blow Molding: A Theoretical Analysis for Identifying Critical Process Parameters

Abstract

An analysis for describing parison (cylindrical) inflation behavior in the extrusion blow molding process is presented. A general growth equation is developed starting from the basic conservation principles. Assuming the polymer melt constituting the parison to behave as a purely viscous Generalized Newtonian Fluid, the effect of different process and material parameters on the inflation process is investigated. From the numerical results, it is inferred that the growth behavior for inelastic liquid exhibits a general tendency of approaching exponential (constant stretch rate) growth as elapsed time progresses. Besides, the initial parison dimensions are determined to play a very significant role in governing the inflation process. Moreover, the inertial contribution owing to fluid motion is found to exert an appreciable influence on the growth dynamics, and hence cannot be neglected without introducing severe approximations in the analytical development.     

Organo-clay formulations of pesticides: reduced leaching and photodegradation

Adsorption of organic cations on several clay minerals is reviewed with an emphasis on the effect of ionic strength and modeling. The clay exchanged with suitable organic cations forms a basis for ecologically acceptable formulations of herbicides with reduced leaching, ground water contamination and enhanced weed control efficacy. Incomplete neutralization of the clay surface charge by an organic cation may be advantageous in achieving maximal adsorption of hydrophobic herbicides. One conclusion from these studies is that optimization of clay-based herbicide formulations requires a selection of structurally compatible organic cations preadsorbed on the clay at optimal coverage. New experimental results are presented for alachlor formulations, which significantly reduce herbicide leaching under conditions of heavy irrigation. We were able to demonstrate that organo-clay formulations of alachlor and metolachlor can increase crop yields in a 1-year field experiment. The photostabilization of pesticides is reviewed and improved organo-clay formulations of the herbicides trifluralin and norflurazon are described. A pillared clay, nanocomposite micro- and/or meso porous material, was effective in reducing leaching and in conferring photostabilization, without added organic cations.     

聚烯烃光稳定化技术进展

本文介绍了聚烯烃光稳定化的研究进展,并就受阻胺光稳定剂新的发展趋势及受阻胺光稳定剂与其它稳定剂之间的相互作用进行了较详细的讨论。     

Enhancing exterior performance of clear coatings through photostabilization of wooden surfaces. Part 1: Treatment and characterization

The aim of this work was to produce a less photodegradation-prone substrate for clear coating by partially delignifying the surface cells of Pinus radiata boards to a depth of 2–3 mm while maintaining the integrity of the wood surface tissues in the delignified zone. To achieve this, several surface oxidative delignification treatments were trialled in the method development process and peracetic acid was chosen as the method for refinement and deployment. The treatment method was optimized to yield a significant degree of delignification compatible with the aim of producing a photostabilized yet intact wooden surface. A preweathering technique was also used as a second delignification method for producing photostabilized boards. Microscopic and chemical techniques were used to characterize the effects of chemical and preweathering treatments, which produced delignified surface envelopes 2–3 mm and 100 μm deep, respectively. Acetyl bromide lignin analyses, infrared spectroscopic analyses, and density changes of 30-μm-thick sections of peracetic-acid-treated samples, as a function of depth from the wood surface, suggested that a partial delignification had occurred that diminished with depth. Light and transmission electron microscopy provided evidence of delignification at the cellular level. In the surface layers of peracetic-acid-treated boards, all cell wall regions were delignified, with the middle lamella being the most severely affected. Lignin appeared to be completely removed from the cell corner middle lamella regions, but tracheids were still joined in other parts of the middle lamella. The S₁, S₂, and S₃ walls were also delignified. In subsurface layers, cell walls were only partially delignified and the tissues held their integrity. In contrast, in the preweathered boards, cell walls in the outermost layers were completely separated at the middle lamella from photodegradation. Preferential lignin removal over that of hemicelluloses was achieved via oxidative treatment of solid wood. The outcome of this was the successful fulfillment of our aim to produce partially delignified wooden surfaces that retained sufficient strength and aesthetic appearance and were suitable for application in wooden structures, provided that the necessary protection measures were carried out. Due to the narrow surface zone produced, machining of such surfaces would not be recommended as the most delignified zone would be removed first. In Part 2 of this series, the photostability of treated boards, which had been clear coated and weathered, will be considered.