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《纸页的结构与性能》摘要

《纸页的结构与性能》摘要
《纸页的结构与性能》摘要

《纸页的结构与性能》

-------摘要

1 在造纸领域,植物纤维经不断回用后,纤维的细胞壁发生了不可逆的变化-纤维的角质化。二次纤维的角质化现象宏观表现为纤维润张能力,表面条件等纤维性能的劣化,以及再生纸页强度的衰变。

2 纤维发生角质化后,纤维润张能力下降,弹性模量增大,整根纤维挺硬,均一性差。

3 干纸是一种具有明显强度性质的非触变性结构。只有在不会引起不可逆的范围内增加负荷时,纸才会表现出高度弹性的性质。而在应力的作用下,纸页的变形由以下几种变形组成:

(1)在纸的结构中,纤维素的分子及单根纤维之间的距离发生变化为特征的弹性变形。(2)弹塑性变形,也称为弹性后效变形,产生这种变形时,组成纤维的大分子结构和纤维相互联结的分子之间的氢键发生改变,部分氢键发生破坏,纤维发生局部交错滑动。(3)塑性变形,在产生这种变形时,纤维之间产生不可逆的位移,并且伴随纤维间氢键结合的大量破坏。

4 材料的5种损伤:

4.1 延性、塑性损伤:

微孔洞和微裂纹的形成和发展,使材料产生塑性应变,最后导致塑性断裂。这类损伤的表现形式主要是微孔洞、微裂纹的萌生、成长和聚合。主要发生在塑性材料。

4.2 蠕变损伤:

在长期载荷作用或高温环境下,伴随着蠕变变形会发生蠕变损伤,其宏观表现为微裂纹、微孔洞,蠕变损伤的扩展导致材料的耐久性下降。蠕变损伤使蠕变变形增加,最后导致裂纹。

4.3 疲劳损伤:

在循环载荷作用下,材料性能逐渐劣化。在每一步载荷循环中的损伤累计起来,将

导致材料的寿命减少,导致疲劳破坏。有趣的是纸页开始受到第一轮低应力循环载荷后其抗张强度反而有一定的提高。

4 .4 动态损伤:

在动态载荷如冲击载荷作用下,材料内部会有大量的微孔纹形成并且扩展。这些微孔纹的数量很大,但一般还来不及出现很大的扩展(因为载荷时间非常短,常常是几微秒)时就导致在一截面上布满微裂纹,断裂就发生了。

4.5 高分子损伤:

上述几种损伤在高分子材料中都会发生。高分子的损伤不同于金属。纸页的损伤在某种程度上与高分子损伤相近,既有弹性损伤,又有塑性和蠕变损伤,还可能有疲劳损伤。因为纸页和高分子一样具有复杂的多组分性,多相性和不均匀性。

5 挺度:

挺度与纸张的流变性质有关,决定于纸张受弯曲时,其外层的伸长能力和里层的受压能力,挺度可衡量纸张支持自重的能力。挺度表示了纸张柔软或挺硬的性质,但不应把刚度及表观硬度混为一谈。刚度和表观硬度是以翻动纸页时的声响,触觉硬度及可压缩性的大小为依据的,而挺度仅以抗弯曲能力为标准。纸张的挺度和刚度,硬度有一定的关系,但硬度不等于挺度,硬度大的纸张也不一定挺度大。

6 湿抗张强度:

纸机干燥前的湿纸页所具有的抗张强度,又称初始抗张强度。湿纸页的纤维间尚未形成氢键,此时形成湿纸页的强度因素有两个,一是纤维间的机械交织力,它主要取决于纤维的平均强度;二是水的表面张力,湿纸页干度越高,纤维间水膜越薄,表面张力越大。一般的,干度提高,湿纸强度提高,除与表面张力有关外,还由于降低了纤维间水膜的润滑作用而使机械交织力增加的缘故。

初始湿抗张强度对纸机高速运行下的作业性能至关重要。车速越高,纸页运行时的牵引力越大,要防止断头,必须有较高的湿抗张强度。解决的办法是适当配加长纤维浆

料,提高开放引纸前的的纸页干度或调整剥离角。为了更有效地防止纸页断头,现代纸机多采用封闭引纸,真空引纸等更先进的引纸方法。

7 Z向抗张强度:

影响纤维间结合力的因素,也是影响Z向强度的因素。适当提高打浆度,提高压榨压力,控制填料的用量,合理控制干燥的温度曲线,采用干强剂等,均有利于Z向强度的提高。

8 浆料的流送:

在浆流中,纤维的速度与网速不同,剪切应力可使纤维部分地随纵向排列,如二者速度相等,此效应最小,而随着速差的增加,则对任一方的效应都将增加。

这种现象在成型期间极易发生在最靠近网面的浆料的那些纤维。随着浆层厚度在移动过程中逐渐减薄,此效应将越来越小,这形成了,纸页从网面到正面“纤维定向排列”的梯度变化。对于双向脱水的夹网纸机,其整个纸页的纤维定向排列将更为对称和均一。

9 匀度成型机理:

流体动力学滤波指的是悬浮液在脱水过程中能够改善匀度,在纤维层的最低定量区域内,通过沉积的纤维层流速最高,流动阻力最小。因此,悬浮液流向定量低的地方。这种机理就是流体动力学滤波。由于这种机理,手抄片比完全随机的行为网状结构的匀度好。

纤维絮聚是导致匀度差的主要原因。纸料悬浮液的密度直接影响絮聚,密度高会引起严重絮聚,因为密度高,纤维互相交织的可能性大,0.5%是一个界限。

匀度还取决于网前箱和网部的—动,因为,适当大小的—动可以破坏纤维的絮聚。在纸机生产的浓度下,-动对于防止纤维絮聚很有必要。在网部,调节脱水元件可以控制—动,在长网纸机上,匀度提高得很小,但是对于混合和夹网成型器,匀度提高很大。

在悬浮液中,不同的速度产生了剪切力从而直接破坏了絮聚,产生了-动,-动能产生正面的影响或负面的影响。对于夹网成型器,在浆网速差为0或浆网速相近时,匀度

最好。

对于长网纸机和混合成型器,最好的匀度发生在有较大的速度差时,值得注意的是改变网速或浆速都能够改变浆网速度差。对于给定定量和纸机产量,我们只有通过改变堰板开口或网前箱流速来改变浆速。这两种方法在网前箱内的-动程度不同。

10 纸页的憎液性能:

纸张是由纤维通过分子间的范德华力以及氢键等作用力重叠交织而成。由于纤维本身是多孔性的,加上纤维与纤维之间的空隙,在纸张表面和内部都存在着无数的“毛细管”。当这些毛细管与液滴接触时,毛细管便会对液体产生一个拉力,在一定的条件下,滴在纸张表面上的液滴首先将纤维润湿,然后引着纤维表面向四周扩散,进而浸入纸张内部,在纤维间扩散,最后纸张被润湿。

液体对纸页的浸透深度和浸透速度均取决于液体的性质(包括液体的表面张力和粘度)和纸张结构,但在纸张结构以及液体性质不变的情况下,液体对纸页的浸透仅取决于液固两相间的接触角,而液固两相间的接触角又取决于表面张力的作用。

10 AKD是烷基烯酮二聚体的简称,是一种纤维素反应型施胶剂,是由硬质酸解聚制备而成的,它既能作为内施胶剂,也能做为表面施胶剂。AKD分子中有2种基团,一是疏水基团,含有12-20个碳原子的长链烷基,赋予良好的疏水能力;二是反应基团,烯酮二聚体形成的脂环,赋予AKD分子与纤维素羟基结合的反应活性,在中性或弱碱性的条件下,其内酯环能和纤维素的羟基直接反应,生成稳定的?-一乙烯酮脂键,这种通过酯化反应形成的强共价脂键,不用硫酸铝作为沉淀剂,能把AKD分子中两个疏水长链烷基固定于纤维表面,形成疏水层,改善纤维表面的抗渗透性,从而起到施胶效果。

11 纤维在干燥过程中,纤维间会形成化学键合,纤维素与半纤维素间发生半羧醛型的自动交联,从而引起了纤维的角质化。

12 液体要把固体表面完全润湿,应满足这样的条件:即该液体的表面张力应小于固体的表面张力,因此,越是表面张力小的固体表面,越难被各种液体润湿,即易排斥液体。

如果纸张的表面张力小于水的表面张力72mN/m时,才会憎水。当考虑憎油性时,因为一般的油类(有机液体)大都具有20-40mN/m的表面张力,因此,若能把纸的表面张力处理的低于这一表面张力时,才能拒油并且赋予憎油性,这是取得憎油性能的条件。

目前,只有应用全氟烷基化合物才能使纸的表面张力处理到低于油的表面张力范围。含氟化合物一般能使纸的表面张力降至18-22mN/m,一般油类物质如食油等的表面张力为30mN/m。当含氟化合物的亲水基部分吸附或结合在纸张表面时,氟碳链基的三氟甲基朝外排列成低表面能的疏油疏水膜,在不改变纸张本身性质的基础上达到抗水抗油作用。

13 水蒸气渗透性是包装纸一种十分重要的性质。纸对水蒸气的抵抗性和对水的抵抗性是两种不同的性质,防水性是通过加进施胶物质而产生的,但施胶对水蒸气渗透没有多大作用。水蒸气是一种气体,良好的防水蒸气性能是通过加进诸如石蜡等材料而获得的。石蜡等材料把纸页的所有空隙填满,气相的水蒸气就没有通路,但是仍有一些水能透过纸张,那是由于水蒸气被暴露在纸和纸板的一面的纤维所吸收,然后以水的形式通过纤维再在纸或纸板的另一面释放出来。

14 纸张的强度是由多种因素所决定的,首先取决于纸中纤维间的结合力,和纤维本身的强度,以及纸中纤维的分布和排列等。而最重要是纤维间结合力。绝干纸的纤维过干发脆,因此机械强度(特别是耐折度)指标比较低,随着含水量增加,纤维变得柔软,纸的主要机械强度指标上升。但是,这种关系只是在水分达到某一最适宜数值之前才是正确的,超过这一数值,由于渗入到纸空隙中的水分子层的挤涨作用,纤维间的结合减弱,就会对机械强度指标带来不利的影响。最适宜的水分含量依机械强度指标而定。如与其他许多机械强度指标的最大值相比,通常耐折度指标的最大值要在更高的水分下才可得到。

15 卷曲是纸张收到湿度和温度的影响所发生的特有的一种变形现象。它是纸张不均匀伸长和收缩所产生的结果。卷曲与尺寸稳定性有关。从表现形式上分三种:纵向卷曲,

即卷曲的轴线与纸机的运行方向一致;另一种是横向卷曲,即卷曲的轴线与纸机的运行方向垂直;还有一种是对角线卷曲,即斜卷曲,这种情况比较少见。

从卷曲的原因来看,卷曲可分为机械性卷曲;结构性卷曲;和水分卷曲。机械性卷曲是由于在生产储存和加工过程中,纸幅收到机械拉伸,使纸和纸板的一个表面在受力时超过其弹性极限所引起的卷曲。很多卷曲是结构性卷曲,由于纸幅的两面具有不同的结构和组成,在干燥时两面产生不同程度的应变而产生的卷曲,称为结构性卷曲。

水分卷曲是当纸页的一面比另一面有较多的纤维交叉而发生的。一种情况是当纸张所处的湿度环境变化引起纸张的水分变化时发生的。在某种水分时,纸页可能是平衡的,但当水分变化,平衡破坏了,就产生卷曲;另一种是当纸张两面含水量变化时也会发生水分卷曲。

波状边缘是由于堆在一起的纸沿边缘吸收水分而造成的。当把存放在低温仓库里的纸运到温暖的印刷车间去后,时常出现这种情况。如果纸的温度低于室内空气的露点,水就会在纸的边缘凝结,从而引起边缘膨胀。当纸堆很干燥,进入较高湿度环境时,也会出现波状边缘,但是纸从空气中吸取水份,后果是一样的。

起皱是由于纸局部的膨胀或收缩而造成的。当纸的匀度,定量不均匀时,局部的吸水或放水造成起皱现象。

一般的讲,纸张经过水分平衡后,内部应力基本消除,纸张与环境基本上形成了一个平衡,组成纸张的纤维处于一种相对静止或极慢的变化状态。但当环境湿温度发生变化时,纸张内部的纤维也在逐渐地改变。吸收性好的与吸收性慢的纸张对于吸湿的反应不同。

纸的养生就是把纸长时间放在大气中,以消除环境变化对纸产生的不良影响。

纸的调态也是把纸放在特定的环境中,但其目的在于使纸获得使用条件下所要求的水分。

纸厂的调态与养生后,要在随后用防潮材料包装,否则没有意义。

16 半纤维素是由两种或两种以上单糖基组成的不均一聚糖,大多带有短的侧链。构成半纤维素聚糖主链的单糖主要是:木糖,葡萄糖和甘露糖;构成半纤维素短的侧链糖基有:木糖、葡萄糖、半乳糖和阿拉伯糖等。由于半纤维素在化学结构上具有支链,所以它的物理结构是无定形的,半纤维素聚合度一般在150-200,其分子链比纤维素的更短,同时,半纤维素分子结构上有许多游离的羟基,半纤维素的首位羟基和次位羟基具有强亲水性,加上半纤维素分子间结合力小,容易游离出羟基,因此半纤维素比纤维素更亲水。

17 纸张是一种多孔性的物质,其密度大约为0.5-0.6g/cm3,它的相对密度为0.5-0.8 g/cm3,比纤维素低,纸中70%是空隙,纸中的空隙是由真正的空隙,即贯通全层纸的开放细孔;凹陷,即仅与一面相通的空穴;空腔,即包在纸张内部不与任何表面相通的空气空间所组成的。其中,真正细孔为全部空隙所占体积的1.6%,其余的98.4%都是凹陷和空腔。纤维也是多孔的,再加上纸页本身的吸收性,因此纸页具有良好的吸收性能。-115

18 纸页的平衡水分的大小,首先决定于各种纸料的配比,以及填料的种类和数量。在同一周围空气湿度下,纸的平衡水分随纸料配比中磨木浆含量的增加而增加,随着矿物填料含量的增加而减少。

19 纸张的吸收性能与浆料中纤维的纯度和半纤维素含量有关。浆料中α-纤维素含量高,吸收性能大,半纤维素含量高吸收性能低,漂白影响吸收性,长纤维易吸收,短纤维吸收性小;阔叶木的吸收性优于针叶木;棉浆吸收性好于木浆;草类,阔叶木短纤易吸油;木材纤维,针叶木浆,长纤难吸油。湿浆解冻易吸收。

20 纸张对液体的吸收过程可以分为两个阶段:

20.1液滴刚刚接触到纸张,在表面张力的作用下,很容易迅速渗透到纸张纤维之间的空隙,几乎没有任何阻力,这一过程十分短促,可称为快速吸收阶段,实际上是一个渗透过程。

20.2纤维间的液体被纤维细胞壁吸收继而扩散进入胞腔。扩散过程的阻力较大,持续时间也较长,可称为慢速吸收阶段,其本质是液体的扩散过程。很显然,纸张对液体的吸收时间取决于慢速吸收阶段所需的时间。

21 回用纤维的吸收性优于未回用的纤维。回用三次后,纤维细胞壁持续明显分层,回用五次后纤维径向严重开裂。

22 BCTMP经回用后纤维形态变化很小。由于其含有较多的木素,纤维比较挺硬,纤维间的空隙较大,因此回用前后,对液体的吸收行为类似于经回用的化学浆。

23 纸张的吸收性随着打浆度的增加而降低。这是因为随着打浆度的提高,纤维润张和细纤维化增加,纤维的比表面积增大,游离出更多的羟基,促进纤维间的氢键结合,使纤维的结合力不断增加,成纸结合更为紧密,使纸张中的气孔的口径变小,数量不断减少,纸张的透气度下降,紧度上升,导致纸页的吸收性下降。游离状打浆提高纸张的吸收性;粘状打浆吸收性差。

加添的纸张,填料分散于纤维之间,减少了纤维间连接和氢键结合,使纸的结构疏松多孔,从而提高纸张的吸收性。施胶降低吸收性。

影响纸页吸收性能最重要的因素是纸页内部的疏松程度,也就是纸张内部空隙所占空间的大小。纸张的松紧程度用紧度来表示。

相同定量的纸张,其紧度大者内部空隙较小,因此,紧度只是具有相对比较的含义,并没有表明纸张内部空隙部分所占的比例。

纸张的孔隙率是空隙体积与纸的总体积之比。包括纤维间的空隙(大空隙)和纤维中的空隙(微空隙)。

孔隙率是说明纸张多孔性的一种指标,但是深入分析液体向纸张的渗透状态,仅仅知道纸张的孔隙率是不够的,还必须了解纸张中毛细管的大小及其分布,或者是毛细管气孔容积及其分布。

涂布纸的空隙平均有效半径为0.2μm,证券纸为1.2μm,吸墨纸为0.1-4μm,大多

数纸种的空隙平均半径为0.5-2μm。

纸页中的纤维排列决定了液体主导扩散方向,从而影响纸的吸收性。由于纸中的纤维主要是呈纵向排列,因此,纸面的吸收性能总是高于其内部的吸收性。表面方向上的吸收性高于厚度方向上的吸收性,纸的纵向的吸收性也高于横向的吸收性。纸的纵向吸收性最大,厚度方向最小(1-5,-30%),纤维材料横向空隙的有效半径为纵向空隙的60-90%,因此,纸张纵向的吸收性最大。

水渗入纸内可能是纤维间的渗透,也可能是纤维内的渗透,纤维间的渗透速度服从于毛细管定律,决定于影响毛细管渗透的各种因素。纤维内的扩散速度正比于纤维间结合力的大小,因为正是这些结合,保证了纤维内扩散通道的连续性。所以,由纤维间渗入纸中的水分,随着打浆度的升高而降低,而由纤维内渗入的水分将增加。对于未施胶的纸,液体的渗透到反面的时间非常短,以至于很难把这两种的渗透分开。对于施胶的纸,纤维内的扩散速度比毛细管渗透快1000倍,因为憎水粒子妨碍水渗透到毛细管中。提高干燥温度都会使纸产生较大的空隙。

液体的粘度、表面张力、液体润张纤维的能力和因此而改变纸页毛细管结构的能力,在液体中有没有溶解的或悬浮的对液体沿纸页毛细管运动的速度有影响的物质等,都影响纸张对液体的吸收。液体的粘度越高,越难被吸入到纸的空隙中。液体的表面张力越低,则越容易被纸张所吸收。此外,液体的PH值也有重要原因,因为纤维润张的难易和液体与纸相互作用的特条件,在一定程度上决定于这个指标。由于纸面和纸内部为液体润湿的条件不一样,所以在这些情况下,纸面与纸内部的吸收性也不同。

在水中加入碱,水分更容易扩散到纸内,因为碱有助于纤维渗入水分。正因为如此,在水中加入碱,也促使垂直于水分表面的和此表面接触的纸张中的水分产生毛细管现象。

环境的相对湿度对各种纸孔隙大小的分布的特点有影响,从而对纸的吸收性能产生影响。

受到纸张吸收性能的影响,不同的纸种,浸入水中为水完全浸透所需的时间以及为

水饱和时的含水量都是不一样的。

24 纸页的油墨吸收原理:

纸张对油墨的吸收同样取决于极性吸附和毛细管吸收。纸张主要吸收油墨中的联结料,油墨联结料被纤维间的空隙与纤维间毛细管所吸收。对于一般纸来讲,纸张纤维中的空隙比颜料颗粒大若干倍,因而在印刷瞬间,油墨被压入纸张空隙内,然后在纤维毛细管的作用下被吸收,毛细管越多,孔径越大,其吸墨能力越强。

25 纸张的平滑度主要取决于造纸原料和造纸工艺,特别是纸张内纤维的形态,纤维的分布状态、加添量、加添品种、表面施胶、表面涂布、纸页形成及压光方式和压光程度。

26 影响纸页平衡水分的因素

26.1相对湿度和温度对平衡水分的影响

26.2化学组分对平衡水分的影响

27 水分对纸页性质的影响

几乎所有的纸张性能都受到纸张中水分的影响。当相对湿度由0增加70-90%,耐折度增加,但不能预计增加的量。

水分对结构性能的影响一般比对强度性能的影响小,水分的增加也能引起厚度的变化,通常增加10%,当然定量也有所增加,因此纸张的紧度变化不是特别明显。增加水分还使纸张的孔隙度和渗油性减小。

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