Significance of Common Sizing Test Methods

Transcription

1 Significance of Common Sizing Test Methods M.Daivapu, T.Nemasasi and A.Lind University of Munich R.Beltz, Global Paper Consulting, Leipzig 1

2 Structure Sizing, applicability and functionality of paper Common sizing test methods and Cobb-value Objectives Experimental design for sheet formation Interpretation of data of the experimental design Ultrasound-transmission measurement and paper physics Presentation of results Conclusion 2 2

3 What is sizing? Sizing describes the reduction of the naturally existent water absorbency of paper and fibres through the use of chemicals originally sizing agent used in the production of paper. Sizing affects: - the wettability of the paper surface, - the hydrophilicity of the paper fibres and - the capillarity of the paper structure Sizing defines: - the durability and climate stability, - the suitability of paper for its use and - the functionality.. 3 3

4 Applicability and functionality of paper In further processing, the paper has to meet certain requirements that are relative to technologies and the ambient conditions (temperature, humidity, pressure). It has to be functioning in terms of processing and subsequent use. The three decisive features for its functionality are the dynamics of: absorbency capillarity dimensional change in X-Y-Z in case of moisture change in the fibre wall. The concept of functionality (introduced by Robert Beltz at the IMPS in 1993) helps to formulate requirements concerning a paper product, which can be implemented by paper manufacturers with chemical and technological practices into process relevant properties. 4 4

5 Common sizing test methods All test methods of the previous century concerned with the proof of paper sizing have in common: They are conducive to the practical proof of applicability when using resin sizing, they use a test liquid, mainly standardised liquids like distilled water and dilute test ink, they expose the paper to the liquid, after a fixed exposure time, they gather a measured value that is defined by the measuring principle and thus, are unable to distinguish between surface sizing and internal sizing as well as the use of starch and additives. 5 5

6 Test method: Cobb-value In terms of sizing and applicability, the measuring method according to Cobb gained acceptance, referred to as Cobb-value (DIN EN ISO 535). The Cobb-value describes the water absorption capability of paper after a fixed period of time and with a water column of 10 mm. The water absorption capability is affected by: capillary volume, cavities, combined water. The storage capacity changes dynamically through fibre swelling. Therefore, the Cobb-value is strongly influenced by: fibre type, fibre morphology and fibrillation, filler and thickness. 6 6

7 Objective of the study Production of sheets with the exact same amount of fibrous material of 2.4 g oven-dry per sheet Testing the relevance of the Cobb-value with regard to paper sizing and water absorption Usage of the dynamic ultrasound-transmission measurement DPM in order to detect sizing, applicability and functionality of paper 7 7

8 Experimental design materials (1) Experimental design for sheet formation by means of a sheet former Rapid-Köthen Fibrous material 80 % eucalyptus, 20 % spruce without refining (SR: 17) and refined in the Holländer Hollander beater (SR: 36) the amount of fibrous material was kept constant, even after filler was added (2.4 g oven-dry) Starch AKD cationic, average degree of substitution 8 8

9 Experimental design materials (2) Filler: GCC with 60 % < 2µm Filler-retention was measured in advance tests as a result, twice as much filler was added than initially indicated (30 % were added per 15 % desired filler content). Retention medium: 0.06 % are contained in all experiments 9 9

10 DoE: Experimental design (Software: Modde, Company: Umetrics) Test SR Filler [%] AKD [%] Starch [%]

11 Ash content of the papers Filler Starch Refining degree AKD 11 11

12 Problems in interpreting data according to experimental design (1) Tests with 0.04 % sizing agent plus filler as well as tests without sizing agent are contained in the experimental design. During these tests a measurement of the Cobbvalue is not possible since the water would penetrate through the paper immediately. To interpret these values, an anticipated value of 100 g/m² was given

13 Problems in interpreting data according to experimental design (2) The differing filler substance makes it difficult to interpret the results. The impact of starch on the sizing is hardly verifiable since both retention and filler content rise as the starch content rises

14 Significance of the influencing variables Cobb-values (filler) emco 7-value Refining degree starch Refining degree starch filler AKD AKD 14 14

15 Cobb-values using the example of Cobb 30 Based on this model, the filler has little effect on the Cobbvalue. However, the data suggest that the filler content has an effect on the Cobb-value

16 Cobb30 [g/m²] Cobb-values of different papers Cobb30 Cobb60 In addition, starch seems to have a (positive) effect on the Cobb-value T3 SR: 36 FS: 0 AKD: 0,12 Stärke: 0,7 T12 SR: 17 FS: 0 AKD: 0,04 Stärke: 0 T9 SR: 36 FS: 13 (15) AKD: 0,12 Stärke: 3 T5/7 SR: 17 FS: 7 (15) AKD: 0,12 Stärke: 0 By means of this test, it is not possible to clarify whether this is caused by the existence of starch or the reduced specific capacity

17 Ultrasound-transmission measurement The sample is directly penetrated by the sonar wave. A recipient converts the arriving sonar wave into a digital signal. At a time interval of milliseconds, an ultrasound-signal-time chart emerges. During the measurement the hydrostatic pressure of 2 and 4 millibar works permanently. For paper structures 1 and 2 MHz are significant. 17 The correlation between paper sizing and the ultrasound attenuation at 2 MHz has been verified for the first time by Dr. Y. Pan at the University of Tokyo in The first sizing tester was available in

18 emco DPM 66 emco DPM66 fourth-generation system ultrasound - transmission and reflection Absolute, calibrated ultrasound electronics with certified sound transducers ensure the comparability in historical periods of time

19 Signal interpretation - physical basics (1) Both the fibre structure and the compressible ingredients have an effect on the sound transmission (Rayleighscattering). A change of the signal is a proof of these changes. The ultrasound transmission through dry paper is exclusively effected by means of the fibre structure. Ultrasound transmission through the air is not possible. Through the absorption of water through the fibre wall, the paper s density, modulus of elasticity, stiffness and speed of sound changes. The attenuation increases depending on the water-absorbing amount of fibrous material by ten to thousand times (60 decibel) and generates the maximum measuring effect. 19 The application of ultrasound for the examination of materials, composite materials, buildings, etc. is known for a long time. For 100 years, ultrasound physics is investigated and well-known. 19

20 Signal interpretation - physical basics (2) By reducing the fibre absorption of water it is possible to prove sizing. The development of hydration shells around the fibres through starch reduces the attenuation. Surface starch even results in signal rise. The hydration shells around the fibres prevent or impede the absorption of the fibre wall which is being detected sensitively by the use of ultrasound. 27 th IMPS - International Munich Paper Symposium 7-9 March

21 Signal interpretation - physical basics (3) Throughout the attenuation measurements in water the sample undergoes the transition from a solid to the suspension of microfibres in a number of stages. The highest attenuation of paper in water is achieved when the absorption of the fibre wall is complete. The loosening of the fibre bonding due to the swelling of fibres results in suspension (pulp) and involves a signal rise. 27 th IMPS - International Munich Paper Symposium 7-9 March

22 Transmission [db] Basic curve water (ultrasound signal) The basic curve represents the change of paper structure in water if the stages are consecutive. I. Wetting phase of surface 0 Water Paper better know! emcodpm - Base curve II. Capillary penetration III. Fibre absorption I Pulp II III IV V IV. Fibre swelling and elimination of fibre structures V. Pulp (water-fibre-mix) Time [s] 22 22

23 Transmission [db] Key figures for surface sizing and internal sizing emco2 Wetting static Papier besser kennen! emcodpm - Kennzahlen Leimung emco3 Wetting dynamic 5 S 6 emco4 Hydrophobic number emco5 Stiffness index emco6 Begin of fibre absorption emco7 Dynamic fibre absorption emco8 Fibre absorption static I II t t 8 III Zeit [s] Sizing and stiffness Surface sizing and using starch Internal sizing, also with starch phases I, II and III emco2 to emco6 emco7 and emco

24 Results: average curves 1 MHz db 150 s 24 An average curve of 1 and 2 MHz is the summary of as many as 32 individual measurements using at least three different sheets of paper. During each measurement, a sample with the size of a circle and a diameter of 10 mm is analysed. All measurements were carried out with a special sample holder, so that no sample holder plate or double-sided adhesive tape affects the absolute value in decibel. The ultrasound field only contained the medium of water and the paper sample. 24

25 db Sizing and Cobb-value T3 T9 Apparently, the Cobb-value does not correspond to the hydrophobicity of the fibres that were obtained by sizing T12 T5 T7 30 s 60 s 150 s Test SR Filler [%] AKD [%] Starch [%] Cobb 30 Cobb 60 T T (15) T (15) T (8) T The above graphics demonstrate that T9 and T5 have the same Cobbvalues at Cobb 30 and Cobb 60 but are different in sizing. This becomes apparent in the absorption of the fibre wall at T5. The comparison of the Cobb-values of T3 and T12 is a striking example of the fact that the Cobb-value does not show sizing. Using the example of T3, the ultrasound-transmission shows full sizing while one can witness a strong absorption of the fibre wall at T12 in the first 30 seconds. 25

26 Transmission [db] Influence of starch on the hydrophobicity and paper statics db T9 3 % T3 0.7 % T5 0 % Test SR Filler [%] AKD [%] Starch [%] emco5 [db] emco8 [db] T T (15) T (8) Papier besser kennen! emcodpm - Kennzahlen Leimung 5 S s 60 s 150 s I II t t 8 III Zeit [s] 26 The development of hydration shells of starch on the surface at T9 and T3 leads to a signal rise in the very first seconds. The emco5-value tells you how more starch results in greater solidity. For the first time it can be examined as to how and by what means the effect occurs. The hydration shells protect the fibres against water and the higher affinity of starch causes the reduction of water in the fibres which in turn increase their moisture-dependent solidity. Please note: 6 db difference implies a doubling or else halving. 26

27 db Stability through starch and sizing SR remains the same T3 reached full sizing ensures stability against dampness Test SR Filler [%] AKD [%] Starch [%] T (8) 0 0 T T (15) T4 T2 starch-fibre-bonding prevents a swelling fibre swelling loosens bonding 120 s 27 The signal curves in the graph are not standardised. The value of the y-axis in decibel describes the absolute physical state of the paper. 27

28 Fibre bonds wet db Test SR Filler [%] AKD [%] Starch [%] emco9 [s] emco10 [db] T (8) T T (8) T6 emco 10 T10 T1 emco9 10 s 28 All papers are highly absorbent. The index emco9 is the point in time (expressed in seconds) when the minimum is reached. The index emco10 (expressed in decibel) describes the absolute condition of the fully soaked paper and can be interpreted as wet strength. As you move toward 0, the strength increases, provided that the amount of fibrous material is the same as can be seen in the approach. 28

29 Sizing and process relevant functionality db T5 internal sizing 0.12 % AKD In the first second, the course of the curve T6 is caused by the absorption of the fibre wall. Sizing is evident from the reduction of the absorption (T5). T6 unsized 5 s 29 The unsized papert6 stops the absorption of the fibre wall in the very first second and reveals the loosening of the fibre bonding as a result of the fibre swelling immediately afterwards. 29

30 Time ratio within the printing machine -4 5 mm ~ 10 sec µm sec Example: Screen width 70Lpc, 40 % coverage Results in matrix dot diameter: 102 µm 5 mm Druck-Nip RW Raster- Punkt ~ 100 µm Trockner / KWS Überbau / Falz 10 m/s 2 m 2 m 2 m 15 m 19 m V=10 m/s 0,2 sec 0,2 sec 0,2 sec 1,5 sec 2 sec 4 sec V=15 m/s 0,13 sec 0,13 sec. 0,13 sec 1 sec 1,3 sec 2,6 sec Water must be vanished in the paper surface before the paper reaches the next Niparea! > 1 sec duration In the dryer 30 The requirements concerning the functionality of paper can be deduced from the model of processing technology and the assumption of a certain speed. The picture shows the functional diagram of a heatset printing press (Source: Lecture Dr. Meder IMPS 2007). 30

31 Functionality: telephone book complaint db The paper produced on a gap former shows an extreme twosidedness at the capillarity. The printed telephone books could not be delivered due to board-like hardening. Reason: The colour structure on the upper side led to the ink adhesion with the back side in the telephone book s 31 As can be seen from the diagram, it is about the dynamics of 130 to 200 milliseconds amongst two printed works. 31

32 Example of functionality: rotogravure paper db capillarity S1 und S2 absorbency S1 und S2 32 The typical gravure print functionality of SC-papers, with kaolin used as filler, does not tolerate water. When it comes to offset printing, this functionality would have serious consequences: dry-running rubber blankets, fan-out effects and an extreme tear rate. 32

33 Conclusion (1) The equation of the sizing degree as fibre hydrophobicity and Cobb-value is common practice in paper manufacturing but it is not applicable. The equation of Cobb-value and functionality of paper for all practical purposes for converting and the suitability for use is common practice among paper manufacturers and paper converters but it is not conclusive

34 Conclusion (2) The ultrasound-transmission measurement makes a major contribution to the technological description of - sizing, - the functionality with processual relevant dynamics, - applicability and durability of paper. With the help of the sheet formation practised in the project, every paper-manufacturing company will be able to optimise its paper technology and chemistry and provide an ideal functionality for its customers