聚偏氯乙烯乳液最低成膜温度对涂层阻隔性能的影响
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2008年第15卷第4期化工生产与技术ChemicalProductionandTechnology!!!!!!"!"!!!!!!"!"研究与开发收稿日期:2008-06-07;修回日期:2008-06-20聚偏氯乙烯乳液最低成膜温度对涂层阻隔性能的影响陈繁荣韩金铭(浙江巨化股份有限公司电化厂,浙江衢州324004)摘要通过对连续乳液聚合的偏氯乙烯-丙烯酸甲酯(VDC-MA)共聚乳液最低成膜温度(MFFT)的测量和涂覆膜阻隔性能的测试,研究了VDC含量和PVDC胶乳的存放时间对乳液MFFT的影响。
结果表明,当单体中w(VDC)>92%时,MFFT剧增,阻隔性能急剧下降;当w(VDC)<92%时,MFFT随VDC的质量分数上升而缓慢下降,但表观质量下降;当w(VDC)约为92%时,MFFT最低,涂层阻隔性能最佳。
关键词聚偏氯乙烯;乳液;最低成膜温度;涂层;阻隔性能中图分类号TQ325.3文献标识码A文章编号1006-6829(2008)04-0017-02聚偏氯乙烯(PVDC)的突出优点是对气体、水蒸汽、油和异味具有极好的阻隔性。
由于PVDC制膜比较困难,其薄膜制品的价格较高,而且热封性差,因此,用PVDC做包装材料时,通常是作为其他塑料的涂覆层使用,以改善材料的阻隔性能被广泛应用于食品、医药包装行业。
PVDC涂层的阻隔性受多因素的影响,但一直以来被人们认为主要受偏氯乙烯(VDC)共聚物中VDC含量的影响,却忽视了PVDC乳液最低成膜温度(Minimumfilm-formationtemperature,MFFT)对涂层阻隔性的影响。
尤其是受此思想的影响,往往会在PVDC乳液合成中追求高阻隔而一味提高VDC共聚物中VDC的含量,最后导致PVDC涂层丧失了材料应有的良好热封性能。
国内未见关于PVDC乳液MFFT方面的研究和报道,关于PVDC阻隔性方面的研究也很少见到[1-3],本文就影响PVDC乳液MFFT的因素和PVDC乳液的MFFT对PVDC涂层阻隔性影响进行讨论。
1试验部分1.1试剂VDC单体,聚合级;共聚单体丙烯酸甲酯(MA),甲基丙烯酸甲酯(MMA),丙烯酸(AA),均为工业级;乳化剂十二烷基苯磺酸钠(SAS),工业级;过硫酸胺(APS),硫代硫酸钠,均为分析纯。
QPM型最低成膜温度测试仪。
1.2聚合方法聚合在100L高压釜中进行。
先进行种子聚合至规定压降后再加料聚合的种子半连续乳液聚合,待加料聚合结束后,经过降温、熟化和冷却后过滤出料。
聚合温度控制在(50±0.2)℃,转化率>99.5%。
1.3测试和表征乳液的MFFT是指乳液在特定环境((20±0.5)℃,(55±3)%相对湿度环境)下能形成连续膜的温度的下限。
PVDC乳液的MFFT测试按照标准GB/T9267—97进行,梯度温度范围0~45℃,测量头数目为10点,测温精度±0.2℃[4]。
PVDC乳液在涂敷过程中版辊温度为15℃左右,若乳液MFFT高于此温度,则涂敷后、红外线干燥前难以在基膜上形成稳定的连续液膜,干燥后涂层不均匀,影响外观和阻隔性能;若乳液MFFT太低,则胶乳易在高速旋转的版辊上成膜造成版辊“板结”,影响涂敷质量和产品外观。
因此,适宜的MFFT对PVDC乳液来说非常重要。
2结果与讨论2.1VDC含量对MFFT的影响为考察VDC共聚物中VDC含量对PVDC乳液MFFT的影响,实验中选用了MA作为第2共聚单体,以消除竞聚率的不同造成的共聚物组成的偏离。
表1是不同VDC含量时的PVDC乳液的MFFT。
从表1可以看出,随着混合单体中VDC的含量・17・上升,聚合物中的氯含量也随之上升,即VDC含量也随着上升;随着聚合物中VDC含量的上升,PVDC胶乳的MFFT出现了类反“马鞍”趋势,即先下降后上升,且在w1(VDC)=92%附近出现最小值。
造成上述现象的机理有人认为这是由于VDC-MA共聚物Tg受VDC含量所致[1],受测试手段的限制,未能考证。
2.2乳液存放时间对MFFT的影响实验中发现,同一批样品在不同的时间段取样进行手工涂布实验时,样品的干燥行为发生了变化,严重时样品干燥后薄膜表面呈乳白色。
经过存放测试实验后发现,PVDC乳液随着存放时间的延长,最低成膜时间发生了较大变化。
具体实验数据见表2。
表2存放时间对PVDC胶乳MFFT的影响这可能与PVDC结晶有着密切关系。
Walling和Briggs等人曾对PVDC的结晶行为进行过测试,发现PVDC虽然在常温下结晶速度非常慢,但如果PVDC一旦发生结晶将会产生诱导效应,能在常温下提高结晶速度。
这也许就是所有PVDC胶乳生产商规定PVDC胶乳货柜期为6个月的主要原因。
2.3VDC含量对涂层阻隔性能的影响为了考察VDC共聚物中VDC含量对涂层阻隔性能的影响,实验中选用了MA作为第2共聚单体,以消除由于竞聚率的不同而造成的共聚物组成的偏离。
将PVDC乳液涂布在12μm厚的聚碳酸酯(PET)上测试数据,涂布量为3g/m2,不同VDC含量时,PVDC涂层的阻隔性能见图1。
从图1可以看出,随着单体配比中VDC含量的上升,涂层的阻隔性能(氧气透过率和水蒸汽透过率)先上升后下降,且在w1(VDC)=92%附近出现最高值。
这与传统的VDC含量越高阻隔性能越高的概念并不相符合。
其原因可能是PVDC乳液的MFFT的变化引起,且阻隔性的变化趋势与PVDC乳液的MFFT的变化趋势非常一致,这并非巧合。
乳液高聚物一般由分散在水中的微小的固体聚合物粒子组成,PVDC乳液也一样,干燥开始时,体系中的水蒸发,乳胶粒子越来越接近在一起。
起初粒子间的斥力使粒子相互分离,但最后粒子聚集在一起,乳液收缩,同时在粒子间强大的作用力下,形成连续的膜。
当VDC含量上升时,PVDC乳液MFFT也出现先下降,乳液干燥时容易形成致密的连续PVDC涂层,结晶度也随之提高,阻隔性能也出现提高;但当w1(VDC)>92%时,PVDC胶乳MFFT剧增,即粒子变硬,成膜性变差,形成的膜容易龟裂成薄片或粉状,涂层的致密性下降,这时涂层的结晶度虽然高但涂层存在缺陷,阻隔性能急剧下降。
3结论通过对连续乳液聚合合成的VDC-MA共聚乳液MFFT的测量和涂覆膜阻隔性能的测试发现:(1)当单体中w1(VDC)>92%时,PVDC胶乳MFFT剧增,即粒子变硬,成膜性变差,形成的膜容易龟裂,涂层的致密性下降,涂层的结晶度虽然高但涂层存在缺陷,阻隔性能急剧下降;(2)当w1(VDC)<92%时,PVDC胶乳MFFT随VDC的含量上升缓慢下降,但涂覆膜在“熟化”后雾度大,表观质量下降;(3)当w1(VDC)约为92%时,PVDC胶乳MFFT最低,涂层阻隔性能最佳;(4)存放时间对PVDC乳液的MFFT有较大的影响,且随存放时间的延长MFFT呈缓慢增加。
因此,研究PVDC乳液的MFFT对PVDC胶乳的生产科研和产品应用具有重要的指导意义。
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