EP0723044A2 - Method for manufacturing a nonwoven based on lactic acid and nonwoven obtained - Google Patents

Abstract

Process for prodn. of a non-woven fabric from molten polymer comprises: (a) melt spinning the polymer(s), (b) cooling, (c) drawing the filaments, (d) laying up the fibres on a mat, (e) heat bonding the fibres by calendering to form a web, and (f) fixation treating the prod. with adjustment of the degree of crystallinity and of the internal stress of the fibres making up the non-woven fabrics. Also claimed are: (i) the nonwoven polymeric fabric; and (ii) appts. for prodn. of the nonwoven polymeric fabric.

Description

The invention relates to a method for manufacturing a nonwoven web based on polylactides.

Nonwovens are often produced by a manufacturing process called "spund-bond" (SB) from fibers of non-biodegradable polymers because the use of biodegradable compounds such as lactic acids leads to non-wovens whose qualities mechanical and stability are currently poorly controlled.

Nowadays, all over the world, solid waste dumping sites are quickly saturated. This waste mainly includes non-woven products from diapers (for babies and adults), feminine hygiene products (period towels, etc.), disposable protective clothing, non-disposable products. woven fabrics used in agriculture and many other products.
a In recent years, the trend has been to favor a reduction in the flow of waste to landfills by opting for composting. However, all the non-woven products mentioned above are traditionally made from polyolefins, PE (polyethylene), PP (polypropylene) and their blends or other polymers which do not allow composting. The solution lies in the production of biodegradable polymers, the degradation of which is carried out by the municipalities in their solid waste composting systems.

There are several biodegradable polymers on the market, for example copolymers based on polyhydroxybutyrate / valerate (PHB / V), (Zeneca Bio Products: BIOPOL), polycaprolactones (PCL), (Union Carbide: TONE, Interox Chemicals: CAPA) , several polymers based on starch or starch derivatives, (Warner-Lambert: NOVON), polymers based on polyglygolide acid (PGA), polymers based on polylactides (PLA), (Boehringer Ingelheim: RESOMER ) and other biodegradable polyesters.

In Europe patent application No. 93303009.9, 04/19/1993, the inventor of which is Showa Shenko K.K., relates to biodegradable aliphatic polyesters used as material for disposable diapers (also non-woven parts).

The polyactide (called PLA) or its derivatives (type L and D or copolymers) is potentially one of the most degradable polymers, because it has good mechanical properties, that it is completely degradable, that the degradable products are natural materials, that the duration of degradation is modular, that the raw material comes from renewable sources such as beet sugar or whey and that it can be incinerated without problem. It can be extruded in the form of a film (European patent application No. 92304269.1, 12.05.1992, MITSUI TOATSU CHEMICALS, Inc.) or a bulky product and it can be injection molded. The addition of heat stabilizer makes it recyclable and finally it can be melted and extruded, and therefore it is suitable for making non-woven fabrics intended for hygiene, as described in the patents French 9309649, 08/02/1993, and European 944,700,186, FIBERWEB SODOCA and Japanese patent application 134425 dated 04.06.1993, MITSUI TOATSU CHEMICALS Inc.

The properties of polymers derived from polylactides vary depending on the type of polymer (type L or D), the residual amount of monomer (lactide) and, in the case of L / D copolymers, the ratio of L and D units.

The most widely used process for manufacturing nonwovens is the so-called "spun-bond" process, abbreviated as SB for the remainder of the text. In this process, the polymer is melted and extruded using a single or twin screw extruder, then routed to the spinning pump (s), which are usually gear pumps. Frequently, a filter and a static mixer are placed before the pumps.

At the outlet of the pumps, the flow of molten polymer is conveyed through the filter to the die, which comprises a series of small dies (0 0.2 to 2.0 mm), normally of the order of several thousand. The polymer is spun through the die and delivered to the cooling and drawing sections. The cooling can be carried out by supplying cooled air and the drawing is carried out by suction of air or air forced through the drawing section.

The stretching section may have a wide slot or several smaller slots or nozzles. In the stretching section, the fibers have a decreasing diameter and assume an oriented structure. The stretch ratio is generally from 1.1 to 20 times. In the SB process, the titer of the fibers is of the order of 0.5 to 20 dtex.

The spinning section is followed by a deposition section where the fibers are deposited randomly on a carpet. The carpet transports the fibers to the grille. The weight / m2 can be adjusted according to the speed of the mat.

FIG. 1 schematically represents an installation for implementing a known SB process (for example S-Tex) mainly comprising: (1) a hopper, (2) an extruder, (2 ') a screw, (3') a die, (4) a carpet, (5) a welding calender, (6) a means of guiding the sheet and of winding tension adjustment, (7) a winding means, (9) a unit fiber cooling, (11) a drawing nozzle, (11 ') a drawing suction.

Spinning in the SB process generates PLA fibers with a very oriented structure (high drawing, rapid cooling).

This means that the amorphous phase is well oriented and has a high internal tension, and that the fibers tend to shrink using temperatures above the Tg (glass transition temperature), (Ahamad YA Khan et al. "Meltprocessing of poly (lactide) resin into monwowvens ", TANDEC, University of Tennessee).

The crystallinity and the state of the amorphous phase have a considerable effect on the properties of the sheet. If the crystallinity is too high, the sheet becomes fragile and if the amorphous phase has an internal tension (high orientation rate) it will shrink at high temperatures.

Traditional welding processes (for example calendering between a smooth roller and a heated engraved roller having an external pressure control, so that the welded surface is 7 to 25%) at temperatures between 70 ° C and 100 ° C (depending on the grade and type of polymer) cannot be achieved due to shrinkage and, with lower temperatures, welding is not optimal. In addition, if satisfactory welding is carried out at a low temperature, there are problems of product stability. The shrinkage of the sheet takes place in a very humid environment at a temperature below 40 ° C.

PLA tends to stick at temperatures between 70 and 100 ° C. It is difficult to remove the PLA deposited on the calender rolls when this bonding is associated with simultaneous shrinking. Calendering at high temperatures (> 100 ° C) increases crystallinity considerably (very slow cooling), which results in less elongation.

The main object of the invention is to provide a process for the manufacture of a spun-bond nonwoven (called SB) based on polylactides, which is biodegradable and which has characteristics identical to those of nonwovens based on polyolefins.

More particularly, the method according to the invention is intended to improve the mechanical properties of the nonwoven based on polylactides and to stabilize it to avoid the shrinkage caused by high temperatures.

To this end, the method according to the invention makes it possible to fix or adjust the criticality rate and the internal tension of the fiber making up the PLA-based nonwoven web.

A method according to the invention applies to the manufacture by "spun-bond" of a nonwoven, exclusively composed of polylactides, that is to say that all the filaments which compose it are made entirely of a polymer derived from lactic acid, or a mixture of polymers derived from lactic acid, or a copolymer derived from lactic acid.

Preferably, the polymer is derived from a lactic acid L or D.

Preferably, the mixture of polymers is a mixture of polymers derived from L acid and derived from D acid.

Preferably, the filaments of the nonwoven are derived from L and D lactic acids (copolymers).

More particularly, a method according to the invention is characterized in that it comprises a fixing-adjustment treatment of the degree of crystallinity and of the internal tension of the fibers making up the nonwoven web.

According to a first variant, the fixing-adjustment treatment consists of a biaxial fixing following the calendering, then heating at low temperature followed by cooling, said heating being able to be carried out by any suitable means, for example in an oven or by infrared radiation.

According to a second variant, the fixing-adjustment treatment consists of rapid cooling, immediately following calendering at high temperature.

• figure 1 : schéma d'une installation de mise en oeuvre d'un procédé "spun-bond" ou SB de l'état de la technique,
• figure 2 : schéma d'un ensemble de traitement de fixation-ajustement selon l'invention pouvant être associé à une installation de la figure 1,
• figure 3 : schéma d'un autre ensemble de traitement de fixation-ajustement selon l'invention pouvant être associé à une installation de la figure 1.

The invention will be better understood using the following description made with reference to the following appended figures:

• FIG. 1 : diagram of an installation for implementing a “spun-bond” or SB process of the prior art,
• FIG. 2 : diagram of a set of fixing-adjustment treatment according to the invention which can be associated with an installation of FIG. 1,
• FIG. 3 : diagram of another set of fixing-adjustment treatment according to the invention which can be associated with an installation of FIG. 1.

The originality of the process according to the invention is that it comprises at least one treatment for fixing or adjusting the degree of crystallinity and the internal tension of the fiber composing the nonwoven web based on PLA.

• 1) Après calandrage dans une calandre (16) et fixation en (12) à des températures basses (voir figure 2), la nappe soudée (15) (à tension biaxiale) est soumise à un contrôle de température dans des moyens de chauffage (13) puis refroidie dans des moyens de refroidissement (14).
  Si le calandrage est effectué à des températures basses ( 70°C) et à une pression élevée et raisonnable, le soudage est satisfaisant mais le niveau des allongements et résistances est bas et la nappe a tendance à rétrécir par la suite lorsque soumise à des températures plus élevées.
  Si le calandrage est effectué à des températures basses ( 70°C) et à une pression élevée et raisonnable, le soudage est satisfaisant mais le niveau des allongements et résistances est bas et la nappe a tendance à rétrécir par la suite lorsque soumise à des températures plus élevées.
  Afin d'éliminer cette tendance et d'améliorer les propriétés mécaniques, la nappe est fixée biaxialement après calandrage et chauffée dans un four pendant 10 à 60 secondes à une température variant de 80°C à 150°C, ou chauffée pendant quelques secondes (0.5 à 10 s) par un générateur d'IR à une température variant de 80°C à 150°C. Ces traitements peuvent être effectués en ligne continue ou en post-traitement.
  Le contrôle de température selon l'une ou l'autre des variantes de chauffage (3) a pour effet de relacher la tension intérieure et d'augmenter le taux de cristallinité. On constate de ce fait un allongement et une résistance plus élevés et la nappe ne rétrécit plus.
  Le temps de chauffage et la température doivent être choisis avec précision, pour éviter la fragilisation de la nappe par suite d'une température trop élevée.
• 2) Une nappe (17) est soudée à une température de calandrage élevée dans une calandre (18) et immédiatement refroidie rapidement par des moyens de refoidissement (19).
  Des bonnes propriétés mécaniques, l'absence de collage sur la calandre et une nappe ayant des propriétés stables à des températures élevées peuvent être obtenues en utilisant des températures de calandrage très élevées (de 100 à 150°C) et en refroidissant la nappe immédiatement après calandrage en soufflant de l'air. Ce traitement permet d'obtenir un soudage très satisfaisant et un taux de cristallinité qui n'est pas trop élevé. La nappe présente un allongement et une résistance satisfaisante et, est stable à des températures élevées. La température idéale dépend du poids/m2 du non-tissé, du type de polymère, de la vitesse de la ligne et de propriétés requises.

This fixing-adjustment step can be carried out in the following two ways (which are not limiting):

• 1) After calendering in a calender (16) and fixing in (12) at low temperatures (see FIG. 2), the welded sheet (15) (at biaxial tension) is subjected to a temperature control in heating means ( 13) then cooled in cooling means (14).
  If calendering is carried out at low temperatures (70 ° C) and at a high and reasonable pressure, the welding is satisfactory but the level of elongations and resistances is low and the web tends to shrink afterwards when subjected to temperatures higher.
  If calendering is carried out at low temperatures (70 ° C) and at a high and reasonable pressure, the welding is satisfactory but the level of elongations and resistances is low and the web tends to shrink afterwards when subjected to temperatures higher.
  In order to eliminate this tendency and improve the mechanical properties, the sheet is fixed biaxially after calendering and heated in an oven for 10 to 60 seconds at a temperature varying from 80 ° C to 150 ° C, or heated for a few seconds ( 0.5 to 10 s) by an IR generator at a temperature varying from 80 ° C to 150 ° C. These treatments can be carried out online or in post-processing.
  The temperature control according to one or other of the heating variants (3) has the effect of releasing the internal tension and increasing the rate of crystallinity. There is therefore a higher elongation and resistance and the web no longer shrinks.
  The heating time and the temperature must be chosen with precision, to avoid embrittlement of the sheet as a result of too high a temperature.
• 2) A sheet (17) is welded at a high calendering temperature in a calender (18) and immediately cooled rapidly by cooling means (19).
  Good mechanical properties, the absence of bonding on the calender and a sheet having stable properties at high temperatures can be obtained by using very high calendering temperatures (100 to 150 ° C) and by cooling the sheet immediately after calendering by blowing air. This treatment makes it possible to obtain a very satisfactory welding and a degree of crystallinity which is not too high. The ply has an elongation and a satisfactory resistance and is stable at high temperatures. The ideal temperature depends on the weight / m2 of the nonwoven, the type of polymer, the speed of the line and the properties required.

According to this method, elongations are obtained 10 times higher as well as resistance twice as high as the usual values.

The invention will be illustrated by the following nonlimiting examples:

Procédé :

S-Tex

Matière première :

PLLA
Poids moyen moléculaire : 130 000 - 140 000
Polydispersité : 1.9
Point de fusion : 160 - 165°C

Température d'extrusion :

190° C - 210°C

Filature

   air refroidi : 0.3 - 1.0 m/s, 10-20°C
   étirage : 30 - 90 mm/Ce

Vitesse tapis :

15 - 30 m/s

Température calandre :

50 - 70°C (la plus élevée possible sans rétrécissement de la nappe

The nonwoven fabrics used in these examples are manufactured under the following conditions:

Process :

S-Tex

Raw material :

PLLA
Average molecular weight: 130,000 - 140,000
Polydispersity: 1.9
Melting point: 160 - 165 ° C

Extrusion temperature:

190 ° C - 210 ° C

Spinning

cooled air: 0.3 - 1.0 m / s, 10-20 ° C
stretching: 30 - 90 mm / Ce

Belt speed:

15 - 30 m / s

Calender temperature:

50 - 70 ° C (as high as possible without shrinking the web

EXAMPLE 1

Nonwoven tablecloth

Poids/m2 :

25 g/m2

Denier :

2.5 dtex

Résistance MD :

20 N/5 cm

Allongement MD :

5 %

Starting values

Weight / m2:

25 g / m2

Denier:

2.5 dtex

MD resistance:

20 N / 5 cm

MD elongation:

5%

Méthode :

Biaxialité fixée et four chauffé

Température :

100°C

Durée :

2 min

Heat treatment

Method:

Fixed biaxiality and heated oven

Temperature :

100 ° C

Duration:

2 min

Résistance MD :

100 %

Elongation MD :

1000 % (10 fois)

Rétrécissement à 100°C sans fixation : néantImproved properties (%)

MD resistance:

100%

MD elongation:

1000% (10 times)

Shrinking at 100 ° C without fixation: none

EXAMPLE 2

Nonwoven tablecloth

Poids/m2 :

65 g/m2

Denier :

2.5 dtex

Résistance MD :

80 N/5 cm

Allongement MD :

26 %

Starting values

Weight / m2:

65 g / m2

Denier:

2.5 dtex

MD resistance:

80 N / 5 cm

MD elongation:

26%

Méthode :

Biaxialité fixée et four chauffé

Température :

100°C

Durée :

2 min

Heat treatment

Method:

Fixed biaxiality and heated oven

Temperature :

100 ° C

Duration:

2 min

Résistance MD :

20 %

Elongation MD :

400 %

Rétrécissement à 100°C sans fixation : néantImproved properties (%)

MD resistance:

20%

MD elongation:

400%

Shrinking at 100 ° C without fixation: none

EXAMPLE 3

Nonwoven tablecloth

Poids/m2 :

26 g/m2

Denier :

1.8 dtex

Résistance MD :

27 N/5 cm

Allongement MD :

10 %

Starting values

Weight / m2:

26 gsm

Denier:

1.8 dtex

MD resistance:

27 N / 5 cm

MD elongation:

10%

Méthode :

Biaxialité fixée et chauffée sur ligne S-Tex avec radiateur IR

Température :

environ 120°C (puissance maximum 9 KW)

Durée :

2 s

Heat treatment

Method:

Biaxiality fixed and heated on S-Tex line with IR radiator

Temperature :

about 120 ° C (maximum power 9 KW)

Duration:

2s

Résistance MD :

40 %

Elongation MD :

400 % (4 fois)

Rétrécissement à 100°C sans fixation : 4 - 6 %Improved properties (%)

MD resistance:

40%

MD elongation:

400% (4 times)

Shrinkage at 100 ° C without fixation: 4 - 6%

EXAMPLE 4

In this example, the same process parameters are used, except for the higher calendering temperature, from 120 to 150 ° C and immediate cooling after calendering, which reduced the temperature of the web to 20 - 60 ° C. Effective cooling after calendering prevents shrinkage of the web.

Nonwoven tablecloth

Poids/m2 :

60 g/m2

Denier :

2.5 dtex

Résistance MD :

65 N/5 cm

Allongement MD :

30 %

Starting values

Weight / m2:

60 g / m2

Denier:

2.5 dtex

MD resistance:

65 N / 5 cm

MD elongation:

30 %

Méthode :

calandrage à chaud et refroidissement immédiat par soufflage d'air à une température de 15-30°

Température :

120-150°C

Heat treatment

Method:

hot calendering and immediate cooling by blowing air at a temperature of 15-30 °

Temperature :

120-150 ° C

Résistance MD :

40 %

Allongement MD :

50 %

Rétrécissement à 100°C sans fixation : 5 -10 %Improvement of properties (%) (if calendering is carried out at the temperatures mentioned in examples 1 to 3).

MD resistance:

40%

MD elongation:

50%

Shrinkage at 100 ° C without fixation: 5 -10%

For the implementation of the method according to the invention, an installation is produced for the manufacture of a nonwoven web from polymers of the type comprising means for spinning the polymer (s), for cooling, for stretching, depositing fibers on a carpet, welding said fibers by calendering to form a sheet (15, 17), characterized in that it further comprises means for fixing-adjusting treatment of the crystallinity rate and the internal tension of fibers making up the sheet (15, 17).

More particularly, the fixing-adjustment processing means consist of biaxial fixing means (15) and heating means (13) taken from the group (oven-infrared radiation) or consist of cooling means ( 19) rapids arranged just after calendering means (18) heated to high temperature.

Claims (14)

Process for the production of a nonwoven web from filaments of molten polymers, of the type comprising the steps of spinning the polymer (s), cooling, stretching, depositing fibers on a carpet and welding said fibers fibers by calendering to form the sheet (15, 17), characterized in that it also comprises a fixing-adjustment treatment of the degree of crystallinity and of the internal tension of the fibers making up the nonwoven sheet. Method according to the preceding claim, characterized in that the fixing-adjustment treatment consists of a biaxial fixing following the calendering, then in heating at low temperature followed by cooling. Method according to the preceding claim, characterized in that the heating is heating in the oven for 10 to 60 s at a temperature varying from 80 ° C to 150 ° C. Method according to claim 2, characterized in that the heating is heating for a few seconds by infrared rays at a temperature varying from 80 ° C to 150 ° C. Method according to claim 1, characterized in that the fixing-adjustment treatment consists of rapid cooling, immediately following calendering at high temperature. Method according to claim 5, characterized in that the calendering is carried out at a temperature of 100 ° to 150 ° C. Nonwoven formed from polymer fibers, characterized in that it is obtained by one of claims 1 to 6. Nonwoven according to the preceding claim, carcatized in that all of the fibers are polymer fibers, or copolymers, or mixtures of polymers all derived from lactic acid. Nonwoven according to the preceding claim, characterized in that the polymer is a derivative of lactic acid L or D. Nonwoven according to claim 8, characterized in that the blend of polymers is a blend of polymers derived from lactic acid L and lactic acid D. Nonwoven according to claim 8, characterized in that the fibers are copolymers derived from lactic acids L and D. Installation for the implementation of a method according to one of claims 1 to 6, for the manufacture of a nonwoven web from polymers of the type comprising means for spinning the polymer or polymers, for cooling, drawing, depositing fibers on a carpet, welding said fibers by calendering to form a sheet (15, 17), characterized in that it further comprises means for fixing-adjusting the crystallinity rate treatment and of the internal tension of fibers making up the sheet (15, 17). Installation according to the preceding claim, characterized in that the fixing-adjustment processing means consist of biaxial fixing means (15) and heating means (13) taken from the group (oven-infrared radiation). Installation according to claim 12, characterized in that the fixing-adjustment processing means consist of rapid cooling means (19) arranged immediately after calendering means (18) heated to high temperature.

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