A new method for shear bond strength measurement

Transcription

1 A new method for hear bond trength meaurement KAUSTUBH JOSHI 1, WARREN BATCHELOR, IAN PARKER, LOI NGUYEN 4 1 RESEARCH STUDENT, SENIOR LECTURER, ASSOCIATE PROFESSOR, 4 SENIOR LECTURER Autralian Pulp and Paper Intitute, Department of Chemical Engineering, Monah Univerity, Clayton, Victoria 800 ABSTRACT Modeling the tenile trength of paper ha been a topic of interet over the lat few decade. Fibre propertie and fibre network propertie are the two main et of input required for any uch analytical model. Out of thee, the mot difficult quantity to meaure i the Fibre-Fibre Shear Bond Strength. No reliable method exit for the meaurement of thi quantity. Thi paper report a imple method baed on the idea of weakening the fibre independently of the bond in a heet of paper, uing acid hydrolyi until all the fibre break acro the fracture line. The bond trength i then calculated from the fibre trength, a meaured by the zero pan tet at the point where the fibre firt are weakened uch that they all break. The method wa ued to calculate the average bond trength of handheet made out of never dried, 60% yield unbleached Radiata Pine fibre and i found to be 6.9 ± 0.5 MPa. Thi i coniderably higher than mot previou etimate for the hear bond trength. The limitation of the method, which have been ued previouly ued to meaure the fibre-fibre hear bond trength, will be dicued in thi paper in detail. INTRODUCTION Reearcher have previouly taken five different approache to deigning experiment that meaure the hear bond trength of wood pulp fibre. Mot of the reearch ha focued on uing ingle fibre joint prepared by variou technique. The pioneering work in thi field wa that by McIntoh and Leopold (1). They glued a ingle fibre (Unextracted and Alkali extracted Loblolly pine holocelluloe) acro a few hive, holding thi aembly in place with cellophane. Thi aembly wa put under heat and preure and mounted on gla lide and teted. The value of bond trength obtained ranged from 4 to 7 MPa. Thorpe et al. () laid a ingle fibre (Scotch Pine fibre, thermo mechanical pulp) at right angle on a hive of 4-6 fibre, andwiched between gla microcope lide (previouly treated to prevent adheion of fibre to gla) and clamped at an approximate preure of 1500 pi. The andwiched tructure wa heated at 115 C in a preure cooker and tranferred for overnight treatment to an oven at 105 C. The fibre/hive croing wa then glued onto manila folder tock and thi aembly wa ued for further teting. The average bond trength meaured wa 6.9 MPa. Schniewind et al. () took a different approach to the deign of ingle fibre joint and mounted the fibre (white fir fibre from an experimental nitric acid pulp) on Teflon block and bonded them firmly with the help of heat and preure. A drop of water wa placed on the joint to prevent premature drying of the fibre. The meaured bond trength value ranged from.4 to 4 MPa. Stratton and Colon (4) modified thi aembly lightly by uing Teflon-rubber dic and cementing the fibre (earlywood and latewood Loblolly pine, 47.5% yield) to the dic uing pecial Mylar mount to reduce the high percentage of mounting lot in handling in the previou arrangement by Schniewind et al. (). The meaured average bond trength value ranged from 1.5 to 4 MPa. Torgnydotter and Wagberg (5) modified the aembly further and ued a dilute fibre upenion (Spruce wood, target Kappa number 110, 80 and 16) placed on a Teflon dic allowing the fibre to ink lowly and form a web. Thi i a better approach a it cloely mimic the way a heet i formed. Average bond trength value ranged from 8 to MPa. Ruell et al. (6) focued more on the effect of wet trength additive and heet trength (St.Regi beaten and unbeaten, unbleached biulfite oftwood pulp), uing pecially deigned clamp. Handheet were prepared uing TAPPI T 05 and after tandard drying cycle were oven dried at 10 C for ten minute. They obtained average bond trength value of.6 ± 1 MPa for untreated ample and.4 ±

2 MPa and.5 ± 1.5 MPa for the ample containing melamine and polyamide rein repectively. Button (7) came up with a novel method of treating fibre like joint uing theorie in mechanical engineering. He ued Loblolly pine trachied (58% yield) for hi tudie of bond trength. Fibre were andwiched between three layer of membrane and placed under a preure of 100 pi for 4 hour. Average bond trength value of 1.9 MPa and 8.9 MPa were obtained for the latewood and earlywood fibre repectively. It can be een that previou meaurement have produced a wide range of etimate of hear bond trength. The reearcher all ued different approache to deign the joint and there are many difficultie with the deign. Mot of the joint are prepared uing heat and preure which mean that there i going to be a change in the fibre propertie during joint preparation itelf. The method alo fail to take into account the random nature of fibre tacking during heet forming. Single fibre joint preparation and repreentivity of uch bond for a heet a whole are major iue. For example, Ruell et al (6) prepared 07 individual joint but jut 5% which could be ued for teting a the other were damaged during handling. It can be quetioned whether o few bond are repreentative of a heet a a whole. Another approach to the meaurement of bond trength ha been to try and meaure the bond trength indirectly from meaurement on the heet. The firt attempt in thi area wa the work of Nordman (8) whoe approach wa to ue irreverible work done during training of a paper heet a a meaure of bond trength. Bond trength wa then given a the energy per unit area of bond failure. Seth (9) modified Nordman equation and obtained a bond trength value of 8 J/kg (pruce pulp beaten in PFI mill for 5000 revolution). Gurnagul et al. (10) applied Page theory (11) for indirect heet meaurement and obtained a bond trength value of MPa (black pruce unbleached Kraft pulp, 47.% yield, Kappa number 0.5). Thi etimate cannot be conidered fully reliable becaue it i baed on Page theory (11) which ha never been completely verified. The core idea of the new method for bond trength meaurement preented here i to adjut the fibre trength independently of the bond trength by weakening the fibre. The fibre are weakened uing acid vapour expoure for a erie of time (1). Sheet are expoed to acid vapour, teted for tenile trength and the expoure time at which all of the fibre appear broken, except thoe that are not bonded on one ide of the fracture line a oberved under a confocal microcope i determined. The optimum point i etimated uing the confocal microcopy technique decribed further. Thi point i where the fibre trength i equal to the bond trength of the mot heavily loaded bond. The fibre trength i obtained at thi point for heet of varying grammage and extrapolated to calculate the hear bond trength. The approach calculate bond trength from the heet directly, thu eliminating the problem of forming repreentative individual bond for teting a well a meauring. EXPERIMENTAL METHOD The moving belt heet former (1, 14) (MBSF) wa ued for making handheet from 60% yield, never dried Radiata Pine pulp. The pulp wa beaten in a Valley beater for 1 minute. The initial freene of the pulp wa 70 CSF before beating and 610 CSF after beating. After refining, a 0.mm lotted creen for hive and 00 meh creen for fine were ued to remove the hive and the fine repectively Sheet of three grammage 0 gm, 45 gm and 60 gm, were made at 0.05% conitency. Zero pan trength how an upward trend a grammage i reduced and the true trength i obtained by extrapolating the meaured value to 0 gm (15). The heet were preed on a dynamic roller pre which ue two roller operated with the help of pneumatic ytem. Two pae at 0 preing preure and two pae at 0.75 MPa were ued. The heet were retrain-dried and conditioned a per ISO 187:1990. Sample were cut and teted a per ISO 194- for tenile trength meaurement and ISO 1561 for the Dry Zero pan tet. A chamber wa deigned to provide uniform acid expoure to the paper heet and to alo have

3 more control over the acid expoure proce. The rapid de-aeration ability of the chamber i one of the major advantage. The acid expoure i compoed of a two chamber aembly (Figure 1) made from platic (PVC) wa connected by a platic valve. The chamber on the left hold the acid (HCl %) and the chamber on the right hold the paper heet. The mall metal protruion from the right chamber i a pipe connected to the vacuum apparatu for tarting a flow of acid ga from the left chamber into the right. During the rapid de-aeration tage the valve i cloed and the right chamber i evacuated. Dimenion of each chamber i 0 mm x 0 mm x 40 mm. The paper were expoed to acid ga for time ranging from 50 to 10 minute. The rapid de-aeration time i not included in the expoure time reported in thi paper but wa generally le than 5 minute. Ceation of bubbling indicated that the oil had penetrated through the paper pore and impregnated the ample completely. Thee dyed and oil impregnated ample were examined under the confocal microcope. For each level of acid expoure, 150 fibre croing the fracture line were meaured and aigned a either broken or pulled out. The fraction of broken fibre wa then calculated. Thi wa later ued to determine the optimum expoure time. Image analyi wa done uing IMAGE PRO 5 oftware. Fibre width wa meaured from the image uing the built in drawing and ditance option for converting pixel into ditance. Figure : Pulled out fibre (Expoure time 0 minute) Figure 1: Acid Expoure Chamber Aembly Confocal Laer Scanning Microcopy (confocal microcopy) wa ued to meaure the fraction of broken fibre after the tenile tet were conducted on the acid expoed ample. Confocal Microcopy wa the preferred method becaue it allowed u to can into the interior of the ample to image all the fibre. An Optican F900e BH confocal microcope operating in fluorecence mode with a 0X oil immerion len wa ued. After the heet were acid expoed and tenile teted, they were dyed uing a fluorecent acridine orange dye and then allowing them to air dry under a fume hood a per afety procedure. The ample were then put into a Petri dih filled with immerion oil and placed in a vacuum chamber. Vacuum wa created in the chamber and the ample wa kept there until gentle bubbling could be een. Figure : Pulled out fibre and Broken Fibre (Expoure time 105 minute)

4 be een to increae with increaing acid expoure until it reache a plateau of The fraction of broken fibre never reache 100% becaue there are alway a mall fraction of fibre that are bonded only on one ide of the fracture line. The optimum point where the fraction of pulled out fibre firt reache the plateau value wa determined by fitting traight line to the data from 80 to 100 minute expoure and to the data in the plateau region and determining the interection point. Figure 4: Broken Fibre (Expoure time 10 minute) Figure, Figure, Figure 4 how how the fibre appear when they are pulled out (0 and 105 minute acid expoure) and when they are broken (105 and 10 minute). The Zero pan trength meaurement were done after the ample were expoed to acid vapour for the optimum time. The clamping preure ued to hold the ample during the zero pan tet wa 0.41 MPa (60 pi). Thi particular preure wa choen becaue it i the clamping preure at which the ample i held firmly without cruhing the fibre. The clamping preure of 0.48 MPa (70 pi) and 0.55 MPa (80 pi) were oberved to lightly cruh the fibre. RESULTS Figure 5 how that the heet tenile trength fall approximately linearly with acid expoure time. Tenile Index (Nm/g) Acid Expoure Time (Minute) Figure 5: Tenile Index v. Acid Expoure Time Figure 6 how the fraction of broken fibre croing the fracture line a a function of acid expoure time. The fraction of broken fibre can Fraction of Broken Fibre Acid Expoure Time (Minute) Figure 6: Fraction of Broken fibre v. Acid Expoure Time For the data hown in Figure 6, the optimum point wa found to be 115 minute. The Zero pan trength value obtained for the varying grammage heet at the optimum expoure time of 115 minute can be een in Figure 7. Zero Span Tenile Index (Nm/g) Grammage (gm) Figure 7: Zero Span Tenile Index v. Grammage A expected we ee a light increae in the value of zero pan trength a the grammage decreae. It ha been argued that the hear bond trength at zero grammage can be calculated by extrapolating thi graph in the direction of the y- axi (15). The value of the zero pan trength obtained from the extrapolated graph (Figure 7)

5 i 67.6 MPa. Thi value of the zero pan trength can then be ued to calculate the hear bond trength. The hear bond trength i given a: C σ = σ A...Equation 1 σ = Shear Bond trength σ = Fibre trength C = Cro ectional area of one fibre wall A = Area of the bond The fibre trength i calculated a: F total σ =...Equation A c F total = Force on the ample A c = Total cro ectional area of all fibre The breaking force on the ample in the zero pan tet i given by: F total = F w...equation F = The breaking force for a random heet per unit width w = width of the ample heet Van den Akker (16) found that the zero pan breaking force for a random heet i /8 the zero pan breaking force for a heet in which all the fibre are oriented in the direction of the tre. 8 F = Fo F o = breaking force for an oriented heet...equation 4 There fore the breaking force for a random heet i given by: 8 F = Z M...Equation 5 Z = Zero pan tenile index M = Ma per unit area of the heet (i.e.: grammage) aumed to be 1500 kg/m The ma per unit are of the heet, i.e.: grammage i given by: M = ρ t...equation 6 ρ = Fibre wall denity t = thickne of heet if it wa compreed to a olid ma of fibre Multiplying both ide of Equation 6 by w, we get: w M = ρ t w...equation 7 The cro ectional area of fibre wall material in a heet of paper i A c = t w...equation 8 M A c = w...equation 9 ρ Equation,, 5 and 9 can be ued to calculate the fibre trength a: 8 σ = Z ρ...equation 10 It i worth noting that uing the aumed fibre wall denity of 1500 kg/m, Equation 10 implifie to: σ (MPa) = 4 Z (knm/kg)...equation 11 The area of one bond i given by: D w A = R...Equation 1 in θ avg A = Area of one bond R = Correction factor to account for the preading of the fibre when it i bonded; found to be 1.5 (17)

6 θ avg = Average croing angle of fibre, which ha been derived to be.7 (17) D w = Width of the fibre (40 micron) Subtituting Equation 10, 11, 1 and the value of θ avg and R into Equation 1 we get the complete expreion for the hear bond trength a: σ 8 Z ρ C =...Equation 1 D w R in θ Thi implifie to σ (MPa) = a R in θ avg σ (MPa) = avg 4Z C = D w 8Z C D w...equation 14...Equation 15 The average tenile index obtained from the zero pan tet wa 67.6Nm/g after an optimum expoure time of 10 minute and when extrapolated back to zero gm. When all the other variable were ubtituted into Equation 1, we obtained the average bond trength of 6.9 ± 0.5 MPa. DISCUSSION The value of the bond trength eem ignificantly higher than the publihed previou meaurement. The only imilar value are thoe by Torgnydotter and Wagberg (5), ranged from MPa to MPa. The method ued previouly didn t concentrate on teting the bond trength in the heet itelf and mot of them focued on deigning ingle fibre bond which would be repreentative of the whole heet. Unfortunately no number of ingle fibre bond can correctly repreent what happen in a heet. Alo the teting procedure become complicated and there are ignificant difficultie with joint creation, poitioning etc. Mot of the previou tet method alo ue heat and preure a a mean of bond formation and thi change the nature of the joint and affect the propertie of fibre in individual joint. Glue and adheive alo do not help much becaue they may interfere with the actual bonding proce in itelf and ubequently in the teting procedure. The ame criticim may apply to thi new method a well becaue the zero pan trength and other parameter are alo average. The new method doe have ome potential weaknee. One of the quetion ariing i what if the bond are weakened due to HCl expoure? HCl expoure eem to caue polymer chain ciion and no report have been found which indicate that HCl affect urface bonding. Hence we aume that the HCl doen t affect the bond. In addition, the bonded area of the paper are likely to be amongt the leat acceible area, thu leat prone to being affected by HCl. However if we aume that HCl doe affect the bond then the meaured value would be an underetimate and thu even further away from mot previouly meaured value. Paper i a variable tructure due to the non uniform nature of the raw material and the non uniform pulping proce. One may quetion the utility of a technique which give a ingle value of bond trength. Mechanical propertie of paper are directly related to the bond trength and the Shear bond trength wa the lat unmeaured variable required to validate and develop model and theorie for paper mechanical propertie. Shear bond trength ha been meaured in term of in-plane tenile trength and zero pan tenile trength a a network parameter. Other factor that influence tenile trength are formation, preing preure, type of fibre ued, pulping technique, quantity of fine etc. The experiment performed here illutrate that if all the other variable are known then thi new technique can be ued to meaure the hear bond trength directly. The value of hear bond trength can be plugged into the different model to predict the tenile and other mechanical propertie of paper. Experiment have alo been conducted to meaure the hear bond trength of recycled fibre and heat treated fibre, but thoe reult will be preented in later work. CONCLUSION The average hear bond trength value of heet made of never dried unbleached Radiata pine

7 fibre (60% yield) i 6.9 ± 0.5 MPa. Thi value i much higher than mot previouly publihed value and take into account the entire fibre network intead of jut ingle fibre joint. REFERENCES (16) Van Den Akker, J.A., Lathorp, A.L., Voelker, M.H., and Dearth, L.R. - Tappi, 41(8):416 (1958) (17) Batchelor, W., He, J., and Johnton, R.E. - International paper Phyic Conference:77 (00) (1) Mcintoh, D.C. and Leopold, B., - The formation and tructure of paper: Tranaction of the Sympoium held at Oxford in 1961 BPBMA, London (196) () Thorpe, J.L., Mark, R.E., Euufzai, A.R.K., and Perkin, R.W. - Tappi, 59(5):96 (1976) () Schniewind, A.P., Nemeth, L.J., and Brink, D.L. - Tappi, 47(4):44 (1964) (4) Stratton, R.A. and Colon, N.L. - IPST Technical paper erie:1 (1990) (5) Torgnydotter, A. and Wagberg, L. - Nordic Pulp and Paper Reearch, 1():411 (006) (6) Ruell, J., Kallme, O.J., and Mayhood, C.H. - Tappi, 47(1): (1964) (7) Button, A., - Fiber-fiber bond trength: A tudy of the linear elatic model Intitute of Paper Chemitry, Lawrence univerity,appleton, Wiconin (1975) (8) Nordman, L.S. - Fundamental of papermaking fibre: Tranaction of Cambridge Sympoium, Bolam, F., Ed., : B.P.B.M.A, Cambridge (1958) (9) Seth, R.S. - Paperi ja Puu-paper and timber, 84(5):8 (00) (10) Gurnagul, N., Ju, S., and Page, D.H. - Journal of Pulp and Paper Science, 7():88 (001) (11) Page, D.H. - Tappi, 5(4):674 (1969) (1) Wathen, R., Joutimo,O ; Tamminen,T, - Effect of different degradation mechanim on the axial and z- directional fibre trength, 1th fundamental reearch ympoium: cambridge (005) (1) Xu, L. and Parker, I. - APPITA, 5(4):8 (000) (14) Mitchell, C., Parker, I., and Johnton, R.E. - APPITA:87 (00) (15) Batchelor, W., Weterlind, B., Hagglund, R., and Gradin, P. - Tappi, 5(10): (006)