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1.
This study focused on the removal of cured urea–formaldehyde (UF) resins by hydrolysis of medium-density fiberboard (MDF) at different temperatures and times for MDF recycling. Five aqueous solutions, i.e., two acids, two alkalis, and water, were used for MDF hydrolysis to obtain extract solutions and solid residues for the analysis of mass balance, nitrogen content, and resin removal. As expected, acidic solutions removed the greatest amount of cured resins, followed by water and alkaline. Moreover, temperature had a greater impact on hydrolysis than time. Fourier transform infrared spectra of MDF fibers before and after hydrolysis clearly showed the change in intensity of the amide group in UF resins. Reduction of the amide group was greater with increase in hydrolysis temperature than in time. Statistical analysis results suggested that the hydrolysis of MDF at 80°C for 2 h using oxalic acid solution was optimum condition for the removal of cured UF resins. However, water could be used as hydrolysis agent for the practice of MDF hydrolysis in the future. 相似文献
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Hydrolytic stability and crystallinity of cured urea-formaldehyde resin adhesives with different formaldehyde/urea mole ratios 总被引:1,自引:0,他引:1
This study investigated the relationship between the hydrolytic stability and the crystalline regions of cured UF resins with different formaldehyde/urea (F/U) mole ratios to better understand the hydrolysis of cured urea-formaldehyde (UF) resin adhesives responsible for its formaldehyde emission in service. As the F/U mole ratio decreased, the hydrolytic stability of cured UF resins improved, but decreased when the particle size of the resin was reduced. To further understand the improved hydrolytic stability of cured UF resin with lower F/U mole ratios, X-ray diffraction (XRD) was extensively used to examine the crystalline part of cured UF resins, depending on F/U mole ratios, cure temperature and time, hardener type and level. Cured UF resins with higher F/U mole ratios (1.6 and 1.4) showed amorphous structure, while those with lower F/U mole ratios (1.2 and 1.0) showed crystalline regions, which could partially explain the improved hydrolytic stability of the cured UF resin. The crystalline part intensity increased as cure temperature, cure time and hardener content increased. But the 2θ angles of these crystalline regions did not change, depending on cure temperature and time, hardener type and level, suggesting that the crystalline regions of the cured UF resin were inherent. This study indicates that the crystalline regions of cured UF resins with lower F/U mole ratio contribute partially to the improved hydrolytic stability of the cured resin. 相似文献
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低游离甲醛脲醛树脂合成工艺研究 总被引:1,自引:0,他引:1
脲醛(UF)树脂是一种广泛应用于木材加工业的胶黏剂,文章采用碱-酸-碱合成工艺,探讨了甲醛与尿素的摩尔比、pH、反应温度及尿素加料方式等因素对UF树脂黏度及游离甲醛含量的影响,发展了制备低毒(游离甲醛含量0.13%)UF树脂的新工艺,并用IR对UF树脂进行了确证。 相似文献
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低毒耐水脲醛树脂胶的研制 总被引:6,自引:2,他引:6
本文采用降低甲醛和尿素摩尔比、同时加入改性剂聚乙烯醇和分批加尿素的方法合成脲醛树脂。可降低脲醛树脂中游离甲醛的含量.提高树脂的耐水性。 相似文献
5.
Thomas J. Pratt William E. Johns Richard M. Rammon Walter L. Plagemann 《The Journal of Adhesion》1985,17(4):275-295
Condensation of an undefined mixture of water-soluble methylolurea reactants is proposed to proceed through the transient formation of a lyophobic colloidal sol intermediate species that subsequently coalesces to produce gelation during UF resin “cure”. An unprotected UF sol dispersion is very unstable, and its brief lifetime may explain why the intermediate sol phase has eluded detection for many years.
Unique characteristics of UF resin behavior observed during preparation, storage and cure are difficult to explain by a conventional mechanism of condensation polymerization. These abnormalities are more logically interpreted by analogy with established colloidal behaviour. Additional support for our coalesced dispersion concept is provided by (i) the known discontinuity in the viscosity of UF resin during cure; (ii) the observed requirement of a minimum threshold concentration of UF resin necessary for gelation to occur and (iii) the SEM of fracture surfaces of cured UF resin which exhibit distinct features that are characteristic of a coalesced sol structure. 相似文献
Unique characteristics of UF resin behavior observed during preparation, storage and cure are difficult to explain by a conventional mechanism of condensation polymerization. These abnormalities are more logically interpreted by analogy with established colloidal behaviour. Additional support for our coalesced dispersion concept is provided by (i) the known discontinuity in the viscosity of UF resin during cure; (ii) the observed requirement of a minimum threshold concentration of UF resin necessary for gelation to occur and (iii) the SEM of fracture surfaces of cured UF resin which exhibit distinct features that are characteristic of a coalesced sol structure. 相似文献
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环保低毒脲醛树脂的合成 总被引:1,自引:0,他引:1
研究了三聚氰胺和聚乙烯醇改性脲醛树脂胶粘剂的制备工艺和配方。甲醛/尿素物质的量比为13:1,在反应温度为90℃,pH为5.5,三聚氰胺和聚乙烯醇添加量为尿素质量的2%,尿素按70:25:5比例分三次加入,制备出环保型脲醛树脂。结果表明,uF树脂的剪切强度可达278MPa,游离甲醛质量分数为0.030%,耐沸水时间为95min。 相似文献
7.
Effects of resin formulation, catalyst, and curing temperature were studied for particleboard binder‐type urea‐formaldehyde (UF) and 6 ~ 12% melamine‐modified urea‐melamine‐formaldehyde (UMF) resins using the dynamic mechanical analysis method at 125 ~ 160°C. In general, the UF and UMF resins gelled and, after a relatively long low modulus period, rapidly vitrified. The gel times shortened as the catalyst level and resin mix time increased. The cure slope of the vitrification stage decreased as the catalyst mix time increased, perhaps because of the deleterious effects of polymer advancements incurred before curing. For UMF resins, the higher extent of polymerization effected for UF base resin in resin synthesis increased the cure slope of vitrification. The cure times taken to reach the vitrification were longer for UMF resins than UF resins and increased with increased melamine levels. The thermal stability and rigidity of cured UMF resins were higher than those of UF resins and also higher for resins with higher melamine levels, to indicate the possibility of bonding particleboard with improved bond strength and lower formaldehyde emission. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 377–389, 2005 相似文献
8.
Jacek Dutkiewicz 《应用聚合物科学杂志》1983,28(11):3313-3320
The degradation of cured urea–formaldehyde (UF) resin in aqueous suspension was investigated by gravimetric analysis of the changes in the content of nonextractable low-molecular components. In acid conditions (pH 4.0) at 47°C the consecutive processes of post-curing and of polymer break-down (activation energy 90 kJ/mol) are detectable whereas at 80°C and 97°C only the formation of the extractable hydrolysates is observed. The degraded polymer contains less carbonyl groups than does the original resin substrate as shown by means of infrared (IR) analysis. In contrast to the results of the tests carried out in acid conditions, in neutral and basic aqueous media the hydrolytic decomposition of UF macromolecular network is less significant. During the hydrolysis of UF polymer at 30°C–45°C the concentration of formaldehyde released from the resin to the aqueous phase increases initially (2 days) at a relatively high rate both at acid and alkaline pH. Then its growth slows down but is still detectable in acid conditions, whereas in basic medium no further liberation of HCHO is observed. 相似文献
9.
木质素在脲醛树脂胶粘剂中的应用 总被引:3,自引:3,他引:0
为解决脲醛树脂(UF)胶粘剂中游离甲醛含量偏高等问题,首先确定了n[甲醛(F)]∶n[尿素(U)]的合适比例,然后以羟甲基化木质素作为UF的改性剂,制备木质素改性UF胶粘剂。研究了木质素的种类及用量对UF各项性能的影响。结果表明:UF中游离甲醛含量随n(F)∶n(U)比例增加而增大;在UF改性过程中,硝酸木质素对游离甲醛的捕捉能力优于硫酸木质素,当w(硝酸木质素)=30%(相对于尿素总质量而言)时,游离甲醛含量(0.126%)相对最低;木质素加入的同时还有助于提高改性UF胶粘剂的胶接强度和耐水性。 相似文献
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This work examined micro-morphological features responsible for the crystallinity of cured urea-formaldehyde (UF) adhesives, using transmission electron microscopy (TEM) to identify and characterize distinctive crystalline structures in resins obtained with different formaldehyde to urea (F/U) mole ratios and hardener levels. The TEM examination of cured UF resin adhesives impregnated into wood cell lumen revealed the presence of spherical particles with variable diameter and number per unit area. The diameter and number/area of the spherical particles increase for decreasing F/U mole ratio and decrease with an increase in the hardener levels, an effect which is closely related to their crystallinity. Therefore, the present findings suggest that the spherical particles are responsible for the crystallinity of cured UF resin adhesives. The results also indicate that crystalline structures represent an inherent feature of cured UF resin adhesives, particularly for low F/U mole ratios, even though these resins are usually classified as amorphous and cross-linked thermosetting polymers. 相似文献
12.
The hot pressing operation is the final stage in MDF (medium density fiberboard) manufacture; the fiber mat is compressed and heated up to promote the cure of the resin. The aim of the investigations is to study the curing reactions of UF (Urea–Formaldehyde) resins as commonly used in the production of MDF, and to develop a simplified kinetic model. This investigation has combined Raman spectroscopy to study the reaction cure and 13C‐NMR for the quantitative and qualitative characterization of the liquid and still uncured resin. Raman spectroscopy was found very interesting for the study of the resin cure and permitted to obtain kinetic data as the basis for a simple empirical model, considering a homogeneous irreversible reaction of a single kind of methylol group and ureas with rate constants depending on their degree of substitution. Although these results can provide a better understanding of the composition and the cure of an UF resin, several issues remain open, such as the influence of the reversibility of the reactions taking place during the curing process as well as the possible formation of cyclic groups in the resin. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5977–5987, 2006 相似文献
13.
采用碱处理、偶联剂处理、醋酸处理等方法,对剑麻柄进行处理,再与自行合成的脲醛树脂(UF)进行捏合、模压,制成剑麻纤维/脲醛树脂共混复合材料。研究了碱处理最佳工艺以及不同处理方法对复合材料的力学性能、耐磨性、耐水性、电性能和热性能的影响,并与木粉/UF复合材料的性能进行了对比。结果表明:剑麻柄的处理方法对复合材料的电性能、热性能和吸水性影响不大;采用乙酰化处理剑麻柄时,复合材料强度较高。耐磨性好,此时剑麻纤维/UF复合材料的各项性能接近木粉/UF复合材料的性能 相似文献
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为从源头上降低UF树脂及其胶合板材的甲醛释放对环境和人体健康所造成的危害,选用乙二醛(G)取代甲醛(F)与尿素(U)反应,制备乙二醛-尿素(GU)树脂。用差示扫描量热分析(DSC)方法研究了原料物质的量比、反应pH、反应时间、反应温度、pH调节剂对所合成GU树脂固化性能的影响规律;并用不同原料物质的量比的GU树脂制备刨花板并测定了其各项性能。结果表明,GU树脂的较优合成条件为:在弱酸性条件下,乙二醛与尿素物质的量比(G/U)=1.2∶0~1.4∶1.0,反应温度70~80℃,反应时间3h;此条件下合成的GU树脂胶合的刨花板内结合强度IB达到0.44MPa、弹性模量MOE达2298MPa、静曲强度MOR为10.5 MPa,且无甲醛释放。 相似文献
18.
The thermal behavior of pre-curing urea–formaldehyde (UF) resin with different solid content was investigated by different scanning calorimetry (DSC), and the activation energies (Ea) in different pre-curing stage of UF resin were also analyzed by Kissinger method. The results indicated that with pre-curing degree increasing, the DSC curves of pre-curing UF resin shifted to lower temperature, and both the onset and peak temperature decreased. The pre-curing process of UF resin included two stages: In the first stage, the Ea and Z value decreased obviously due to the activity of component increased with water evaporation, and then, these two values increased in the second stage due to pre-curing degree increased even partial resin was cured. 相似文献
19.
探讨了改性剂对降低游离甲醛含量、改善胶粘剂性能的作用机理,研究了三聚氰胺、聚乙烯醇(PVA)改性脲醛树脂胶粘剂(UF胶)的制备工艺和配方,实验得出:采用弱碱-弱酸-弱碱的工艺,添加适量三聚氰胺和聚乙烯醇,反应最高温度控制在90℃,反应时间2~3 h,n(尿素)∶n(甲醛)=1∶1.4,尿素按7.0∶2.0∶1.0的质量比分3次投料,可制得低毒脲醛树脂胶粘剂。 相似文献
20.
以有机蒙脱土(OMMT)作为脲醛树脂(UF)胶粘剂的改性剂,制备UF/OMMT纳米复合材料;然后将其用于刨花板的制备,并采用红外光谱(FT-IR)法和X-射线衍射(XRD)法对纳米复合材料的结构进行了表征。结果表明:刨花板的力学性能随OMMT含量增加呈先升后降(或先升后降再升)态势,并且在w(OMMT)=3%时相对较好;当w(OMMT)=1%时,经硅烷偶联剂(KH-550)处理后的OMMT在UF反应终了阶段加入时,刨花板的综合力学性能相对最好;在纳米复合材料体系中,OMMT的层状纳米片层已被剥离,并且OMMT和UF之间发生了一定程度的化学反应,而偶联剂和OMMT之间并没有形成化学键。 相似文献