The effect of paper properties on the strength of starch gluing for Kraft sack papers has been investigated. We analyzed the effect of surface roughness, wettability and glue penetration. Surface roughness was found not to be related to gluing strength, also surface wetting measured by contact angle showed only a weak relation. Liquid penetration measured by ultrasound (ULP) was found to have a substantial correlation to gluing strength. Comparing ULP liquid penetration speed with actual glue uptake during the gluing process we found that they are only moderately connected. We are attributing this to the fact that the penetration and spreading of the glue on the paper is driven by applying an external pressure during the gluing process, which is not the case for the liquid penetration measurement. Investigating how asymmetrical glue penetration affects gluing strength we found that the relationship was low. The best indication for gluing strength turned out to be the surface wetting/substrate swelling parameter from the ultrasonic liquid penetration measurement. We conclude that the main parameter capturing gluing strength combines the influence of fiber wetting and penetration of the glue into the fibers.
Adhesive bonding is a process that joins two surfaces permanently together by application of an adhesive material. While this process is well understood for compact materials it gets more complicated for porous fibre-based materials like paper. The paper packaging and board industry is the major consumer of natural product adhesives (Ashley et al. 1995) like starch, which finds a still increasing market due to its high performance, cost effectiveness and biodegradability (Pelton et al. 2001).
adhesive failure: failure of the bond between adhesive and one of the adherends
cohesive failure: failure within the adhesive
substrate failure: failure within one of the two adherends
The strength of an adhesive bonding depends on various properties of both, paper and glue.
The most frequently mentioned mechanism for successful bonding is a good wetting of the adhesive on the substrate (Williams et al. 1977, Kolegov 2017, Ebnesajjad and Landrock 2015, Korin et al. 2007, Zisman 1964, Habenicht 2006, Paris 2000, Edin et al. 2002). In other words the surface energy of paper has to be higher than the surface tension of the glue (Borch 1991). A corresponding measure for the wettability is the contact angle between liquid and paper (Bristow 1961, Huntsberger 1964). While most of the literature agrees that the relation between surface energy of the paper and surface tension of the liquid is decisive for a good adhesive bond (Habenicht 2006, Vähä-Nissi et al. 2009, Kolegov 2017, Zisman 1964) the direct correlation to the contact angle on the rough and porous paper surface has been questioned (Borch 1991). Daub and Göttsching (Daub and Göttsching 1988) for instance found that predicting adhesion to paper by measuring the contact angle did not provide useful results. It is also stated (Bristow 1961) that although the contact angle gives good prediction of adhesion in wet state the strength after glue setting is not directly related to it.
Also glue penetration is mentioned to be important for the adhesive bonding process. The penetration matters at two stages: First good penetration provides a strong bond since the surface area of bonding is increased (Bristow 1961, Borch 1991, Daub and Göttsching 1988), second the fiber structure of the paper is reinforced by the penetrated starch (Johnson and Popil 2015). Also influenced by glue penetration is the drying process where water is removed and the remaining starch bonds to the cellulose fibers (Kroeschell 1990, Daub and Göttsching 1988, Paris 2000). Fast penetration provides good green bond strength (initial wet bond strength) which is necessary in corrugated board production. Kroeschell (Kroeschell 1990) claims that even if the penetration is an important factor for adhesive bond formation it is not responsible for adhesive bond failure. It seems that in most cases good penetration is desired, but too high glue uptake may also cause problems. If penetration is so strong that little or no adhesive is left at the interface no bonding can occur (Shires 1988). For instance gluing problems due to one extremely absorbing paper can be resolved by applying the glue first to the paper with less penetration (Shires 1988, Bristow 1961).
One paper property that influences gluing is the amount of sizing during the production process. With increasing sizing the paper becomes increasingly hydrophobic, which affects the paper-adhesive interactions. While studies with hot melt adhesives (Edin et al. 2002, Vähä-Nissi et al. 2009) showed no influence on gluing strength with increased AKD sizing, others find that the appearance of adhesive failure increases with sizing intensity (Kroeschell 1990, Borch 1991, Johnson and Popil 2015).
In basic literature on gluing (Ebnesajjad and Landrock 2015, Kolegov 2017, Kücükpinar and Langowski 2012, Korin et al. 2012) it is emphasized that a rough surface increases the adhesive bonding area and promotes mechanical interlocking with the glue, which both reinforces the bond. Accordingly, it is common practise in converting to apply the glue on the rougher side of the paper. Nevertheless, also no difference in gluing strength was reported upon a smoothness increase from fiber beating or paper calendering (Edin et al. 2002). Most of the literature dealing specifically with gluing of paper finds that a rough surface promotes substrate failure (Pelton et al. 2001, Hoffman et al. 1971, Borch 1991, Ebnesajjad and Landrock 2015). In summary substrate roughness is mostly found to be beneficial for gluing.
Analysis in industrial practice
In industrial practice often the paper-water interaction is analyzed instead of using glue, as it also correlates well with the gluing performance. Standard measurements to analyze water absorption are Cobb test (ISO standard 535), Hercules size test (TAPPI standard T530) and the water drop absorption test (TAPPI standard 835). For surface wetting the water contact angle is analyzed. Another approach is to analyze the rate of contact angle decay. Contact angle rate is a combined measure of surface wetting and liquid penetration, it strongly depends on the drop size (Krainer and Hirn 2021).
Aim of the work
The aim of the present work is to quantify the relevance of the individual mechanisms that are claimed to be related to gluing strength: contact angle, amount of glue penetrated, evenness of glue penetration on both sides on the paper, initial wetting and mechanical interlocking between glue and paper. Research was focused on comparing the gluing strength to the penetration behaviour of glue into paper, which was evaluated with ultrasonic liquid penetration measurement (ULP) as well as gravimetrically after the gluing process. Also the problem of gluing differently absorbing papers is addressed.
Materials and methods
Eight different commercial sack paper grades from industrial production (four bleached and four unbleached of each grade) were tested on top and on wire side. Those papers showed a large variation in their water penetration behaviour due to different degrees of internal sizing. All the tested papers are grades used for powder filled sacks and thus have a high air permeability between 14 s and 25 s Gurley (ISO 5636-5). We used the following grades:
Two unsized papers, which are highly water absorbing.
Two sized papers with medium water absorption, i. e. a Cobb value of 25–31 g/m2.
Two sized and clupaked papers, also with medium water absorption, i. e. a Cobb value of 26–32 g/m2. Clupaked means that it is a semi-extensible sack paper that has been crepped on the paper machine in a Clupak process.
Two hard sized papers which are water repellant, i. e. a Cobb value below 1 g/m2.
In this study the dispersion glue Collabond 8017 from Agrana Stärke GmbH was used. This adhesive is based on chemically modified corn starch, which is treated for solubility in cold water. The product is delivered as a light yellowish powder to be mixed with water in a gravimetric ratio of 1 part starch and 5 parts of distilled water, which corresponds to a suspension solids content of 17 %. The glue was analyzed and applied to the paper at a temperature of 23 °C. At this temperature the glue had a viscosity of 98 s with Ford 6 cup, which corresponds to a Brookfield viscosity of about 2000 mPas (measured at 100 rpm with spindle 5).
For the determination of adhesive bond strength a T-peel test was used similar to (Ebnesajjad and Landrock 2015), compare Figure 2. The glue was always applied on the paper with lower penetration, further designated as bottom paper, while the so called lid paper with better penetration was put on top. Ten stripes of bottom paper and ten stripes of lid paper were cut into pieces of in cross direction (CD) of the paper, which imitates the gluing situation in the longitudinal seam of a paper sack. On the bottom paper a glue spot of 5 µl volume was applied using a dispenser for highly viscous liquids. The lid paper was put on the bottom paper, then the sandwich was compressed by rolling over a roll of weight (diameter 7 cm, height 25 cm). 10 pairs of glued paper stripes were air dried at 23 °C and 50 % RH before they were separated in a tensile test according to ISO 1924-3.
The contact angle measurement was performed using a Fibro DAT 1100 instrument. All measurements were carried out with 3 µl drop size. Once a drop is released onto the substrate’s surface, the instrument starts taking pictures of the drop, as shown in Figure 3. A tangent was fitted to the drop’s outline at the contact point between paper and liquid using digital image analysis, and the contact angle (Figure 3, red line) was calculated. The initial contact angle was defined as the angle measured at after the drop had been put on the surface. A high contact angle is indicating bad wetting of the liquid on the surface.
Ultrasonic liquid penetration measurement
The Emtec Penetration Dynamics Analyser 2.0 was used for all ultrasonic measurements (Hernádi et al. 2003), Figure 5. As a testing liquid we used starch glue as described in the Materials and methods section. The paper samples (8) were cut to a rectangle of and fastened to the sample holder (6) with a two sided adhesive tape (7). In the measurement cell an ultrasonic emitter (2) and receiver (3) are placed opposite to each other. When the sample is released into the testing cell (4) filled with liquid, the transmitter instantly starts to send ultrasonic waves through the sample. Measurement frequency was set to . The receiver measures the intensity of the ultrasonic signal. Sensor area is a circle with a diameter of . The ultrasonic waves are reflected, scattered or absorbed during the process of liquid penetration, Figure 4. As penetration of the liquid in the substrate proceeds, the receiver records the changes in the signal. The result is ultrasound intensity over time (Krainer and Hirn 2018).
A typical measurement result is shown in Figure 6, it is from sized unbleached paper measured with starch glue as testing liquid. The curve is the mean value of 5 paper samples. The time until maximum intensity is usually interpreted as the surface wetting phase, however comparison with contact angle measurements did not confirm this (Krainer and Hirn 2018). According to the instrument manufacturer (N.N. 2002) and a physical interpretation of the resulting measurement curves (Waldner and Hirn 2020) also represents the start of fiber swelling within the paper structure. The penetration speed PS is calculated as the slope of the intensity curve between the point of maximum intensity and later, Figure 6. The faster the liquid penetrates into the paper, the higher is the change in ultrasound intensity and the steeper is the slope, which has been confirmed by direct, volumetric reference measurements of liquid penetration into paper using a Bristow-wheel type instrument (Krainer and Hirn 2018).
For evaluating the penetration amount of glue into the paper a gravimetric method was used. 10 bottom and 10 lid paper samples were cut into pieces of . Each piece was weighed and the initial paper weight was recorded. A glue drop of 130 µl, corresponding to a dry amount of starch, was placed on the bottom paper. The lid paper was placed on top and a cylinder of 1 kg weight was rolled over the sandwich, simulating the nip pressure on the gluing spot during sack production. Please note that this pressure is forcing the glue to spread on the paper surface and penetrate into the paper. After the penetration of the glue was assumed to be finished and the stripes were separated from each other and dried for . After those the paper stripes with dried glue on them were weighed again () and the dried glue amount on each paper sides was calculated as -.
The ratio of the glue amount in the two papers was taken as a measure how equally the glue is distributed between each side. A penetration amount ratio of 1.0 indicates equal amounts of glue in bottom and lid paper. The more the number differs from 1.0 the less equal the glue was distributed.
For the main part of the work the same paper and side were glued together, in order to show the influence of the paper properties roughness, contact angle and penetration behaviour on the gluing strength. Another part was investigating the effect of uneven glue distribution on gluing strength by pairing papers with different glue absorption. The color coding and symbols consistently used throughout the manuscript are explained in Figure 15 in the Appendix.
For all except one paper all gluing spots showed clear adhesive failure, i. e. the gluing spots were failing at the interface between glue and paper. From the unsized paper in several cases individual fibers were ripped out of the papers which indicates the initial onset of substrate failure. Thus we can conclude that the results in this work are reflecting differences in adhesive failure at the interface between glue and paper. A more detailed analysis can be found in (Dohr 2019).
Roughness and contact angle
The first paper parameter investigated is the roughness of the papers surface, which was between and . Figure 7 shows clearly that the surface roughness, as captured with the Bentsen instrument, does not contribute to the overall gluing strength of an adhesive bond. For the investigated sack papers mechanical interlocking on the size scale of paper surface roughness was apparently playing a minor role.
The contact angle characterizes the wetting behavior of a liquid on a surface, which is essential for the paper-glue interaction. Due to the fact that the contact angle after gave almost the same value for all papers (see Figure 8), here the angle after was chosen where the wetting is fully completed.
As expected for a lower contact angle (better wetting) higher gluing strength can be observed, however the correlation is moderate, (see Figure 9). One reason for this could be the pressure applied on the paper sandwich during the gluing. This external pressure reduces the contact angle as a driver for wetting and penetration as it forces the liquid to spread on the surface and to penetrate into the pores.
The penetration speed measured with the ultrasonic measurement method showed a clear relation to the gluing strength with (see Figure 10), fast penetration (left) lead to higher gluing strength. Still prediction of gluing strength only on the basis of this parameter is not possible. While the penetration speed parameter spread between and for unsized, sized and sized & clupaked papers (red, green, blue) the gluing strength showed similar values. In the case of the hard sized paper the big difference in gluing strength between the bleached (black unfilled symbols) and unbleached (back filled symbols) paper can also not be explained by this measurement parameter.
We also measured the ultrasonic liquid penetration curves for all the papers using distilled water as testing liquid, the resulting curves can be found in the electronic supplementary information. For all sized papers the curves showed equivalent trends for water and glue. For the unsized paper, however, the penetration of water was very much faster than glue, leading to quite different curves. We conclude that for evaluation of the gluing performance of unsized papers we recommend to use glue (and not water) as a testing liquid for ultrasonic testing. For sized papers ultrasonic measurements with water as a testing liquid gave quite similar results like measurements with glue.
Gluing papers having different liquid absorption
Gluing a highly absorbing paper to a non-absorbing paper can be difficult (Shires 1988, Bristow 1961). In this work we are also analyzing how the distribution of the glue between the individual papers is influencing the gluing strength. For that we selected paper pairings with an increasing difference in ULP liquid penetration speed.
In Figure 11 six combinations of papers are shown, with a continuously increasing ratio of ULP penetration speed. An equal paper paring is represented by a ratio of 1 meaning that bottom and lid paper had the same penetration speed. The poorest pairing consisted of the paper with slowest penetration speed for the bottom paper and the fastest one (18 times faster) for the lid paper. Figure 12 shows the glue penetration amount ratio for these six papers. The penetration amount ratio quantifies out how equally the glue is distributed between the two paper sides after the paper gluing. A value of 1 means equal distribution, the more the value differs from 1 the more uneven the glue had penetrated into the two papers glued together.
When comparing Figure 12 with Figure 11 it can be seen that the ratio of liquid penetration speed does not coincide with the ratio of glue penetrated to the different sides in the gluing process. First the continuously increasing difference in penetration speed in Figure 11 is not reflected in a continuous increase in Figure 12. Second the values for the liquid penetration speed ratios are higher (between 1 and 18) compared to the glue amount ratio (between 1 and 1.4). Both mismatches can be explained by the difference in measurement methods. While the glue liquid penetrated freely into the paper during the ULP measurement an external pressure was applied in the gluing trials forcing the glue to penetrate into the paper.
This leads to the conclusion that a higher penetration speed of the glue in the paper (ULP measurement) does not correspond to a bigger amount of glue uptake by the paper in the gluing process. Furthermore it demonstrates that, when studying glue penetration in packaging applications, it is highly relevant to choose a measurement technique that correctly reflects the external pressure on the gluing spot.
In Figure 13 the ratio of glue penetration amount is plotted over the gluing strength for the 6 paper pairings. The unequal pairing which showed the worst ratio (around 1.4) had a better gluing strength than the equal pairing with a value of 1, i. e. symmetrical glue penetration.
The fact that this parameter shows a worse correlation than the penetration speed parameter from ULP measurement is surprising because the penetration in the gluing process for the subsequent T-peel strength test is forced by an external pressure while the ULP method measures free penetration of the glue into the paper. Anyway, the results suggest that an uneven distribution of glue, within the range of the industrial papers tested in this work, does not affect gluing strength.
Ultrasonic measurement of fiber swelling/wetting time
Time of maximum (see Figure 6) is a parameter measured with ULP measurement indicating wetting (Krainer and Hirn 2018) and/or the start point of fibre swelling in contact with the liquid (Waldner and Hirn 2020). When it is plotted over the gluing strength a of 0.93 indicates as a good predictor for gluing strength. Only with this parameter it was possible to see a difference between bleached (black unfilled symbols) and unbleached (black filled symbols) hard sized papers matching the big difference in gluing strength. This is consistent with the conclusion of (Wichmann 2016), stating that the gluing of high wet-strength paper is good when fibre swelling and thus strong interaction between paper and glue is achieved. Anyway, the correlation was substantially carried by the two papers with the lowest gluing strength. Thus, even if the is high, experiments with more data points should be carried out in order to confirm the time to maximum from ultrasonic liquid penetration as the key parameter describing the strength of starch gluing of packaging papers.
Multiple regression analysis was carried out in order to quantify the combined influence of the investigated variables. Considering the high value for time of maximum from the ultrasonic liquid penetration it is not surprising that each one of the other variables was not able to increase the correlation significantly (p > 0.13). The combination of ultrasonic penetration speed and contact angle gave nearly the same predictive power () like penetration speed alone, indicating that the contact angle measurement was not adding new information, that had not already been contained in the ultrasonic penetration speed measurement.
In this study the impact of paper properties like roughness, penetration behaviour, wetting and glue uptake on the gluing strength was evaluated. We tested 8 commercial sack paper grades representing the different levels of water repellence available on the market.
Paper roughness did not shown any relation to the strength of the gluing spot, which demonstrates that mechanical interlocking at the size scale of paper surface roughness does not contribute to the adhesive bond strength, a contradiction to other findings in the literature (Ebnesajjad and Landrock 2015, Kolegov 2017, Kücükpinar and Langowski 2012). Surface wetting of the glue evaluated by contact angle measurements revealed the expected relationship between lower contact angle and higher gluing strength. However the correlation was moderate, , indicating that surface wetting is only a weak predictor for gluing strength. Thus, surface wetting of the starch glue measured by the contact angle is not suitable as a reliable predictor for gluing strength.
Glue penetration speed measured with the ULP measurement showed a significant relationship with adhesive bond strength, however the correlation can only partly predict the gluing strength, . Pairs of papers with increasing difference in ULP glue penetration speed were glued together to study the relation between free liquid penetration (like in the ULP measurement) and actual glue penetration in the gluing process which involves an external pressure on the gluing spot. It has been found that the liquid penetration speed is only very weakly correlated to the actual glue penetration in the gluing process. We conclude that for studying glue penetration in paper it is very relevant to choose a testing method that includes a pressing step in the gluing procedure. Finally we investigated if asymmetric penetration (more glue uptake on one paper than the other) leads to a decrease in gluing strength. For a pair of highly absorbing paper glued to a water repellant paper, a 1800 % difference in ULP glue penetration speed lead only to a difference of 40 % in glue uptake in the gluing process. No relation between asymmetry in glue penetration and gluing bond strength was found. In contradiction to our expectation the symmetry of glue penetration does not seem to play a relevant role in gluing strength, at least not for the industrial bag papers we investigated.
The best indication for gluing strength turned out to be the parameter of the ULP measurement, which is related to surface wetting (Krainer and Hirn 2018) and the swelling of the fibers (Waldner and Hirn 2020). This supports the literature (Wichmann 2016) stating that fibre swelling is significant for a good gluing strength. The key conclusion we are drawing is that the main condition for gluing strength apparently is to form good adhesion between glue and fibers by strong glue uptake of the liquid glue into the fibers. It seems that the parameter from ultrasonic liquid penetration testing is capturing a combination of two key factors governing the gluing, namely good surface wetting and good penetration of the glue into the paper fibers.
Funding source: Bundesministerium für Wissenschaft, Forschung und Wirtschaft
Funding source: Österreichische Nationalstiftung für Forschung, Technologie und Entwicklung
Funding statement: The financial support by the Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development is gratefully acknowledged. Finally we want to mention the financial contributions of the industrial partners Mondi, Kelheim and SIG Combibloc.
The authors wish to thank Gilles Berger, Reinhard Kainz and Uwe Vogelskamp for interesting discussions.
Conflict of interest: The authors declare no conflicts of interest.
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The online version of this article offers supplementary material (https://doi.org/10.1515/npprj-2021-0039).
© 2021 Dohr and Hirn, published by De Gruyter
This work is licensed under the Creative Commons Attribution 4.0 International License.