Method of making an ultra soft, high basis weight tissue

ES 2 187 724 T3

DESCRIPTION

A method of producing an ultra-soft, heavy-weight tissue.

Field of the invention

The present invention relates to a method for preparing an ultra-soft multi-sheet tissue, having a high basis weight, using wet press technology.

Background of the invention

In the field of hygienic tissue, softness, absorbency and resistance are key attributes considered by consumers. It is highly desirable for the tissue product to have a smooth sensation perceived by consumers. This smoothness plays a key role in consumer preference. The smoothness is related to the specific volume and the surface characteristics of the product. In addition to softness, the consumer wants a product that is strong and absorbent to minimize the amount of product that must be used to function effectively.

The method of the present invention uses wet press technology to prepare an ultra-soft, tough tissue having a high grammage. The tissue produced by the method of the present invention exhibits good strength and good absorbency, while being extremely soft. Properties such as those exhibited by the tissue produced by the present invention have only been previously seen in products produced by very expensive hot air circulation drying (TAD) technology. The tissue produced by the method of the present invention has properties similar to those of the tissue produced by TAD, but can be formed using more efficient and less expensive humid press technology.

In a conventional process and apparatus (10) with humid presses, as represented in figure 1, a manufacturing composition is fed by pipes (40, 41) from a tub (50) to chambers (20, 20 ') of the input box. On a conventional mesh (12) supported by rollers (18, 19) a continuous band (W) is formed from a liquid suspension of pasta, water and other chemicals. The materials separated from the continuous band by the Uhle box (29) and through the forming mesh when it is pressed against the forming roller (15) are returned to the tub through a pipe (24) from the fiber recuperator ( 22). The continuous web is then transferred to a felt or cloth (14), supported by a roller (11), for pressing and drying. The materials separated from the web during pressing and drying are collected in the fiber reclaimer (44) and fed to the white water pipe (45). The web is then pressed by the suction press roll (16) against the surface of a heated rotating Yankee dryer cylinder (26) to substantially dry the paper on the surface of the cylinder. The moisture contained in the web when it is on the surface of the Yankee dryer causes the web to transfer to the surface. A liquid adhesive can be applied to the surface of the dryer to provide substantial adhesion of the web to the creping surface. The web is creped on the surface with a creping blade (27). Usually the creped web passes between the rollers of a calender (not shown) and is wound into a coil (28) before subsequent processing operations, such as embossing. It is known that the action of the creping blade on the paper causes a portion of the bonds between the fibers of the paper to be broken by the mechanical action of crushing the blade against the web when it is carried on the blade. However, during the drying of the moisture contained in the web, quite strong bonds are formed between the fibers of the wood pulp. The strength of these bonds in prior art tissues is such that, even after creping, the web retains a perceived feel of hardness, fairly high density, low specific volume, and low water absorbency.

To reduce the strength of the bonds between the fibers, which inevitably form when a continuous web formed from a suspension is pressed and dried in humid presses, various processes have been used. One such process is to pass hot air through the wet fibrous web after it has been formed on the mesh and transferred to a permeable carrier [a process called hot air circulation drying (TAD)], therefore that the web is not compacted before being dried. The lack of compaction, which will occur when the continuous web is pressed when it is on a felt or cloth and against the drying cylinder when it is transferred to it, reduces the possibility of bonds forming between the fibers and allows the finished product to have greater specific volume than the one that would be achieved in a process with humid presses. Due to the softness of these products perceived by consumers, and their capacity to absorb liquids greater than that of continuous bands formed in processes with humid presses, the products formed by the

ES 2 187 724 T3 Newer processes enjoy advantageous acceptance by consumers.

Wet felt press processes are significantly more energy efficient than processes such as hot air circulating drying because they do not require heating or moving large amounts of air as required by the TAD process. In wet press operations, excess moisture is removed from the web by mechanical pressing and final drying of the web is primarily achieved in the heated Yankee dryer cylinder, which is maintained at the proper drying temperature.

EP-A-0 495 637 describes a foam-forming process, in which a tissue of high strength and high softness is produced by forming a continuous web from a foamed composition of fibers. EP-A-0 481 745 describes a foam-forming process, in which the foam is formed by directing a high-pressure jet of a foamable liquid onto a forming web and generating the foam in situ on the web. in training.

The present invention provides a method for manufacturing a tissue product that has a specific volume, absorbency and softness superior to existing tissues manufactured by conventional smoke press technology, approaching or achieving the levels achieved using hot air circulation drying and using the most advanced process. efficient use of wet presses and without requiring the use of a foaming process.

Summary of the invention

Other advantages of the invention will be set forth in part in the description that follows and in part will be apparent from the description.

To achieve the aforementioned advantages and in accordance with the purpose of the invention, made and generally described herein, the methods set forth in the claims are provided.

Brief description of the drawings

Figure 1 is a schematic representation of the preferred apparatus for the wet press process.

Figure 2 illustrates a preferred embossing template for use in the present invention.

Figure 3 illustrates the relationship between GM tensile stiffness and GM MMD friction.

Detailed description

The present invention relates to the production of an ultra-soft, multi-ply, heavyweight tissue. As used herein, "heavy weight" refers to a product (of one or more sheets) having a weight of 36 grams or more per square meter. As used herein, "ultrasoft" refers to a product located below the straight line ranging from GMTS = 19 and GM MMD = 0.140 to GMTS = 10 and GMM MD = 0.210 (GMTS = GM tensile stiffness; GM MMD = friction GM MMD). Above this line, a product should be classified as soft, but not as ultra-soft. Figure 3 shows the relationship between GMTS and GM MMD and the aforementioned line.

The pulp used to produce the web of the present invention comprises between 35 and 90% of fibers of a first type and between 65 and 10% of fibers of a second type. Fibers of the first type are characterized as short fibers, with the fibers having an average length of 2.0 mm or less and a unit weight of less than about 12 mg / 100 meters. Fibers of the second type are characterized as long fibers, with the fibers having an average length greater than 2.0 mm and a unit weight less than about 35 mg / 100 meters. Fibers of the second type preferably have a unit weight of less than 20 mg / 100 meters and most preferably have a unit weight of less than 18 mg / 100 meters.

Fibers of the first type are preferably selected from substantially unrefined, low unit weight, short fibers. In the present invention, fibers of low unit weight, less than 12 mg / 100 meters, are preferred. A full discussion of unit weight can be found, for example, in "The Significance of Coarseness in Papermaking", Kajaani Electronics Ltd., Kajaani (Finland), 1986.

Low unit weight fibers are preferably selected from eucalyptus or other hardwood fibers;

mixtures of eucalyptus fibers and fibers from other hardwoods; blends of eucalyptus or other hardwood fibers with other low unit weight fibers, more preferably with fibers of a lower unit weight than eucalyptus fibers; and mixtures of the above. Preferred fibers for use as fibers of the first type are eucalyptus fibers and northern hardwood kraft fibers. Other low unit weight fibers that can be used in combination with eucalyptus fibers include non-woody plant fibers, such as those described in US-A-5,320,710 and 3,620,911.

Preferred fibers of the first type are selected from dried northern hardwood fibers and eucalyptus fibers.

Fibers of the second type are preferably selected from substantially unrefined, high strength, long fibers. As used herein, long fibers are those that have an average length of at least 2.0 mm. Strength is usually expressed as the length to which a strip of paper breaks under its own weight. Papers made from high strength fibers typically have a break length of at least 12 km, more preferably at least 14 km. These fibers are preferably selected from softwood kraft fibers, preferably northern softwood kraft fibers; blends containing as the main portion kraft fibers from northern softwoods; Northern softwood kraft fibers of low unit weight, which have a unit weight comparable to or less than that of northern softwood kraft fibers, which is typically between 14 and 20 mg / 100 meters; and mixtures of the above.

A product produced by the method of the invention can be prepared as a layered or non-layered product. In one embodiment, the product is prepared as a non-layered product which, however, exhibits the characteristics of high smoothness and high bulk. A layered product can be produced by the present invention, but if a layered product is produced, the amount of hardwood kraft fibers should be reduced.

The paste is mixed with resistance regulating agents, such as starches, debonding agents and mixtures thereof. The paste contains up to about 2.5 kg / t, preferably 1.0 to 1.5 kg / t, of one or more 303161.0 strength regulating agents. Strength regulating agents are preferably selected from Solvitose N® starch, Reilly-Whiteman DR-170® debonding agent, Westcat PG and Redibond; and Quasoft 202 JR¹® and 218® and Varisoft 475®, from Quaker Chemicals.

In many cases, particularly when using a layered machine, starches and debonding agents can advantageously be used simultaneously. In other cases, the starches, debonders, or mixtures thereof can be delivered to the wet part while the debonders can be spray applied.

However, the proper debonders will be readily apparent to experts. Suitable debonding agents are widely described in the literature. An extensive but not exhaustive list includes US-A-4,795,530, 5,225,047, 5,399,241, 3,844,880, 3,554,863, 3,554,862, 4,795,530, 4,720,383, 5,223,096, 5,262 .007, 5,312,522 and 5,354,425 and EP-A-0 675 225.

The web can also include temporary resistance agents. Temporary resistance regulatory topical agents are well known to those of skill and the method and amounts for their effective use are also known to those of skill. Temporary wet strength agents that can be used in the present invention include, but are not limited to, glyoxal and modified starches.

Preferably an amount of resistance regulating agent is provided to control the resistance of the web of several sheets in the longitudinal direction to a value, after embossing, of 71.1 to 84.4 g / 76 mm width kg grammage. The strength regulating agent is added in an amount of up to 2.5 kg / t, preferably less than 2 kg / t, more preferably between 1.5 and 2.0 kg / t.

The tensile strength in the longitudinal direction is about 60.0 to about 84.4, preferably about 66.7 to 77.8 g / 76 mm width.kg grammage. The tensile strength in the transverse direction is preferably about 22.2 to about 35.5, more preferably about 26.7 to about 31.1 g / 76 mm / kg of grammage. In this application, "grammage" means weight of the continuous sheet per unit area and is expressed in grams per square meter (g / m²). Many of the values ​​given in this specification have been normalized.

ES 2 187 724 T3

First a continuous nascent band is formed from the pasta. The web can be formed using any of the standard wet press configurations known to those skilled in the art, for example, crescent former, head sucker cylinder, two mesh former, etc. Once the web is formed, it has a grammage (according to TAPPI laboratory conditions) of at least about 18 g / m², preferably at least about 21 g / m², more preferably at least 20-23 g / m². TAPPI laboratory conditions refer to the TAPPI T-402 test method that specifies time, temperature and humidity for a sequence of conditioning steps.

According to the method of claim 21, after the web has been formed, it can be sprayed with an amount of at least 0.5 to about 1.75 kg / t, more preferably about 0.75 to about 1.75 kg / t, of a fabric softener. Alternatively, in accordance with the method of claim 1, a softener can be incorporated into the wet part of the process to produce a continuous web that includes at least about 0.5 kg / t of a softener. It should be understood by those skilled in the art that softener spraying can take place after two continuous bands have been joined to form a two-sheet product.

The softener is a cationic nitrogen softener. The softener is preferably selected from trivalent and tetravalent cationic nitrogenous organic compounds incorporating long chain fatty acids, imidazolines, amino acid salts, linear aminoamides, tetravalent or quaternary ammonium salts and mixtures of the above compounds. More particularly, the softener can be Quasoft 202 JR¹®, 218®, 209® and 219® and Varisoft 475®, from Quaker Chemicals.

The web is dewatered afterwards, preferably by a general compaction process. The web is then adhered to a Yankee dryer. Any suitable technically admitted adhesive can be used in the Yankee dryer. Preferred adhesives include Houghton 8290 adhesive (H8290), Houghton 82176 adhesive (H82176), Quacoat A-252 (QA252), Betz creplus 97 (Betz + 97), and Calgon 675 B. Suitable adhesives are widely described in the patent literature. An extensive but not exhaustive list includes US-A-5,246,544, 4,304,625, 4,064,213, 4,501,640, 4,528,316, 4,883,564, 4,684,439, 4,886,579, 5,374,334, 5,382 .323, 4,094,718 and 5,281,307. In accordance with the present invention, typical parting agents can be used ; however, the amount of release agent, if used, will often be below traditional levels.

The web is then creped in the Yankee dryer and calendered. The adhesion between the web and the Yankee dryer is controlled to a level such that a creping level in the coil of at least about 20%, more preferably 24%, and most preferably 25% is achieved. Creping is performed with a creping angle of from about 70 to about 88 °, preferably from about 73 to about 85 °, and more preferably from about 73 to about 80 °. The bevel angle of the creping blade is preferably from about 0 to about 15^°, more preferably about 10^°.

The product produced by the method of the present invention includes a soft bulky tissue that can be manufactured on a non-layered machine. It should be understood by those of skill that a layered product can be formed using lower ratios of hardwood kraft pulp than a non-layered product. The product produced by the method of the present invention is a multi-sheet product. Tissue sheets are adhered between two doses preferably by embossing and perforating the two sheets together. Embossing and perforations explain the adhesion. The two sheets can be adhered using an adhesive alone or in conjunction with an embossing template. Suitable adhesives are well known and will be readily apparent to those of skill. Preferably the two sheets are embossed together, the adhesive being applied alone to the tips of the embossing elements. The multi-sheet product includes a two-sheet product, in which a second sheet is formed in the same way as the first sheet.

Preferably the calendering and embossing of the webs are combined to form a multi-sheet web, the multi-sheet web having a specific thickness of at least about 0.17 mm / kg of grammage, more preferably about 0.10. 17 to 0.24 mm / kg of grammage and most preferably 0.18 to 0.21 mm / kg of grammage. The GM MMD of the multi-sheet web is less than about 0.190, most preferably less than about 0.180, and most preferably about 0.150 to about 0.175. The tensile stiffness of the web is less than 1,600, most preferably less than about 1.289 and most preferably less than about 1.133 g /% elongation.kg of grammage.

ES 2 187 724 T3

The web can be embossed with any art-admitted embossing template. A preferred embossing stencil is made up of a wavy stencil, for example, a lattice of dot-shaped protrusions having hearts, flowers, or both patterns in the gapping areas of the stencil. Figure 2 is a representation of a preferred embossing template for use in the present invention. It is also preferred that the embossing template of the present invention is formed of serrated embossing elements. An embossing toothed element is one having a wide base with smaller spaced flat areas at the tip, resembling for example the top of a castle wall. The heights of the embossing element are preferably less than 2.3mm, more preferably less than 1.8mm and most preferably 1.3 to 1.8mm.

The following examples should not be considered limiting of the invention described herein.

Example 1

A nascent continuous web will be formed from a combination of 65% eucalyptus fibers and 35% kraft fibers from northern softwoods. The paste used to form the web also contained, as a strength regulating agent, 0.5 kg of Quasoft 218 per ton of pulp. The treated web is then dewatered by general compaction and adhered to the Yankee dryer. Houghton 82176 and 564 were used as adhesive and release agent in the Yankee dryer. The web was creped in the Yankee dryer and calendered. The winding speed was 1,061 meters per minute. During creping a coil creping of 24.3% will be achieved.

A second web was formed in the same manner and combined with the first web by embossing against a cross-linked pattern of hearts / flowers as depicted in Figure 2. The resulting web of multiple sheets had the following properties:

A set of 170 sheets will be formed from the multilayer product described above. The set of 170 sheets had the following characteristics:

Tensile strength in the longitudinal direction Tensile strength in the transverse direction Weight

Specific thickness

GM MMD

Traction stiffness

74.9 g / 76mm.kg

28.9 g / 76 mm.kg 43.1 g / m²

0.098mm / 8 sheets

0,159

1,111g /%. Kg

Product properties differ slightly when forming a 340 sheet product. These differences are generally due to a decrease in the amount of embossing used because, in a 340 sheet product, the need for added specific volume is reduced. The thickness of a 340 sheet topical product was about 0.081mm / 8 sheets, while the specific tensile stiffness was about 1.511 and the GM MMD was about 0.170.

Examples 2-16

Examples 2-16 were carried out in the same manner as Example 1, with the process conditions and product properties indicated in the following table.

ES 2 187 724 T3

<td> 5</td><td>&lt;L&gt; ε ε s or</td><td> 43,3</td><td>r-4 on TT</td><td> 43,0</td><td>^ 4 on*</td><td> 43,9</td><td> 43,0</td><td> 45,2</td><td> 38,6</td>

<td> 10</td><td>«« &lt;3 Ό 53 T3 33 «2 S, or '' 8 - O Ό g «S &amp;</td><td>r — 4</td><td>CM</td><td>it is</td><td>CM</td><td>r-&gt;</td><td></td><td>t</td><td>00 i — H</td>

<td> 15</td><td>Angle from crepe OR</td><td> 75</td><td> 78</td><td> 78</td><td> 78</td><td> 73</td><td> 73</td><td>on r-</td><td> 82</td>

<td> 20</td><td>Regulatory agent of the resistance and quantity (kg / t)</td><td>Q218 0.5</td><td>OR is Λ &amp; S</td><td>Ό Ό 2 &amp; S-</td><td>or cm 2 o °</td><td> 1</td><td> 1</td><td> 1</td><td>Q202JR 3.5</td>

<td>25 ""4 what • 8</td><td>Softener Y amount (kg / t)</td><td>00 OR\ &lt;S ¿-Γ α °</td><td>Q218 1.25</td><td>2 VI σ ~</td><td>Q218 1.25</td><td> 00 »—¹ O CM Τ ' σ</td><td>00 .i O CM Τ ' α</td><td>Q218 1.25</td><td>is α ~</td>

<td> 30 35</td><td>Adhesive used in the dryer Yankee</td><td>H82176</td><td>H82176 1</td><td>H82176</td><td>H82176</td><td>Betz + 97</td><td>Betz + 97</td><td>Betz + 97</td><td>H8290</td>

<td> 40</td><td>Crepe (%)</td><td> 24,3</td><td> 25,0</td><td> 25,0</td><td> 25,0</td><td> 20,6</td><td> 22,0</td><td>&lt;or on' it is</td><td> 25,0</td>

<td> 45</td><td>Homogeneous 0 stratified</td><td></td><td></td><td>ffi</td><td>C</td><td></td><td>W</td><td>M</td><td>ω</td>

<td> 50</td><td>Type and quantity of pasta</td><td>65% Euc. 35% SWK</td><td>50% Euc. 50% SWK</td><td>65% Euc 35% SWK</td><td>65% Euc. 35% SWK</td><td>65% Euc. 35% SWK</td><td>65% Euc. 35% SWK</td><td>65% Euc. 35% SWK</td><td>35% Euc. 65% SWK</td>

<td> 55</td><td>Example number</td><td> -</td><td>CM</td><td>on</td><td></td><td></td><td>Ό</td><td></td><td> 00</td>

ES 2 187 724 T3

<td> 5</td><td> 4&gt; !&lt; 2 » 0</td><td>WO I HEARD tT</td><td>I heard tT</td><td>t T “4 TT</td><td> 42,8</td><td> 43,6</td><td> 43,6</td><td> 42,3</td><td> 44,1</td>

<td> 10</td><td>«E» ° Ό * 2 13 «* IS &lt;2 S, ffl 3 fc</td><td>wo</td><td>WO</td><td>wo</td><td>wo</td><td>wo</td><td>WO</td><td>WO</td><td> 1</td>

<td> 15</td><td>Angle from crepe OR</td><td> 80</td><td> 08</td><td> 08</td><td> 75</td><td> 75 1</td><td> 75</td><td> 75</td><td> (</td>

<td> 20</td><td>Regulatory agent of the resistance and quantity (kg / t)</td><td>Q206 0.75</td><td>O wo O | cf σ °</td><td>Q206 0.5</td><td>WestcatPG 1.25</td><td>WestcatPG 2.75</td><td>WestcatPG 1.75</td><td>OO V — ι WO OI θ '°</td><td> •</td>

<td>tea on continuation)</td><td>Softener Y amount (kg / t)</td><td>00 r * I HEARD σ</td><td>m O σ -</td><td>Q218 1.2</td><td>Q218 0.75</td><td>Q218 0.75</td><td>22 ν&gt; o- -</td><td>00 ~ or r \ í θ '</td><td> 1</td>

<td>30 r-&gt; co •3</td><td>Adhesive used in the dryer Yankee</td><td>H82176</td><td>H82176</td><td>H82176</td><td>QA252</td><td>QA252</td><td>QA252</td><td>H82176</td><td></td>

<td> 35 40</td><td>Crepe (%)</td><td>w? ΟΪ</td><td> 25,0</td><td> 25,0</td><td> 25,0</td><td>Ο_ΛV? I HEARD</td><td> 25,0</td><td> 24,3</td><td></td>

<td> 45</td><td>Homogeneous 0 stratified</td><td>X</td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>W</td><td> 1</td>

<td> 50</td><td>Type and quantity of pasta</td><td>69% HWK. 31% SWK</td><td>70% Euc. 30% SWK</td><td>68% Euc 32% SWK</td><td>65% Euc. 35% SWK</td><td>60% Euc. 40% Tsuga</td><td>60% Euc. 40% Tsuga</td><td>Or g WM .o βΟ WO WO kO to</td><td>80% Euc. 20% SWK</td>

<td> 55</td><td>Example number</td><td>Ch</td><td>OR</td><td></td><td>I HEARD</td><td>PO í — H</td><td></td><td>wo</td><td>kO r * —4</td>

Table 1 (continued)

ES 2 187 724 T3

Table 1 (continued)

<td>Example</td><td>TO</td><td>B</td><td>C</td><td>D</td><td>F</td><td>G</td><td>H</td><td>I</td>

<td> 1</td><td> 0</td><td> 0,2184</td><td> 75,6</td><td> 28,9</td><td> 22,2</td><td> 344</td><td> 0,159</td><td> 1,111</td>

<td> 2</td><td> 0</td><td> 0,2016</td><td> 71,1</td><td> 24,4</td><td> 20,0</td><td> 325</td><td> 0,158</td><td> 1,244</td>

<td> 3</td><td> 0</td><td> 0,2072</td><td> 66,7</td><td> 26,7</td><td> 19,2</td><td> 334</td><td> 0,166</td><td> 1,200</td>

<td> 4</td><td> 0</td><td> 0,1848</td><td> 62,2</td><td> 24,4</td><td> 20,4</td><td> 370</td><td> 0,155</td><td> 1,289</td>

<td> 5</td><td> 0</td><td> 0,2128</td><td> 80,0</td><td> 28,9</td><td> 19,7</td><td> 365</td><td> 0,176</td><td> 1,267</td>

<td> 6</td><td> 0</td><td> 0,2128</td><td> 75,6</td><td> 26,7</td><td> 16,8</td><td> 315</td><td> 0,170</td><td> 1,156</td>

<td> 7</td><td> 0</td><td> 0,2016</td><td> 73,3</td><td> 31,1</td><td> 18,9</td><td> 359</td><td> 0,165</td><td> 1,333</td>

<td> 8</td><td> 10</td><td> 0,2016</td><td> 75,6</td><td> 51,1</td><td> 24,0</td><td> 315</td><td> 0,144</td><td> 1,600</td>

<td> 9</td><td> 5</td><td> 0,2072</td><td> 66,7</td><td> 26,7</td><td> 19,3</td><td> 350</td><td> 0,178</td><td> 0,956</td>

<td> 10</td><td> 5</td><td> 0,2016</td><td> 84,4</td><td> 31,1</td><td> -</td><td> 362</td><td> 0,174</td><td> 1,111</td>

<td> 11</td><td> 5</td><td> 0,2072</td><td> 64,4</td><td> 24,4</td><td> 21,4</td><td> 304</td><td> 0,170</td><td> 0,911</td>

<td> 12</td><td> 0</td><td> 0,2072</td><td> 73,3</td><td> 28,9</td><td> 19,3</td><td> 323</td><td> 0,169</td><td> 1,156</td>

<td> 13</td><td> 0</td><td> 0,2016</td><td> 73,3</td><td> 33,3</td><td> 19,5</td><td> 388</td><td> 0,179</td><td> 1,289</td>

<td> 14</td><td> 0</td><td> 0,2016</td><td> 73,3</td><td> 33,3</td><td> 18,8</td><td> 358</td><td> 0,172</td><td> 1,267</td>

<td> 15</td><td> 0</td><td> 0,2128</td><td> 75,6</td><td> 33,3</td><td> 18,8</td><td> 358</td><td> 0,172</td><td> 1,267</td>

<td> 16</td><td> -</td><td> 0,2184</td><td> 66,7</td><td> 31,1</td><td> -</td><td> 383</td><td> 0,174</td><td> 1,133</td>

A: Creping blade bevel (°)

B: Thickness (mm / kg)

C: Tensile strength in the longitudinal direction (g / 76 mm.kg) D: Tensile strength in the transverse direction (g / 76 mm.kg) E: Elongation in the longitudinal direction (%)

F: Perf. Traction

H: Friction GM MMD

Tensile stiffness (g /% elongation.kg)