Polylactic acid shrink films and methods of casting same

[0001] The present invention generally relates to a heat shrink film. More particularly, the present invention relates to heat shrinkable polylactic acid films showing a temperature-dependent retraction in a transverse direction, with little or no retraction in the other direction of the machine.

BACKGROUND OF THE INVENTION [0002] Heat shrink films have been widely used for various industrial applications such as, for example, with shrink wrap films, shrink labels and lid seals, making use of their thermodependent retraction property. The films have been applied to a variety of surfaces, including plastic and glass surfaces. Retractable films have been manufactured from vinyl chloride resins, polystyrene resins or polyester resins; however, in many cases, the retraction of the film is concomitant with an undesirable recoil of the outer edges of the film (also known as the smile or frown effect). The undesirable recoil of the outer edges has limited the application of shrink films. Therefore, there is a need for shrink films that have little or no recoil.

[0003] In addition, interest in compostable polymers, that is, biopolymers, has increased tremendously, and many companies have made efforts to sell, for example, packaging materials, hygiene products, sacks and films with compostable polymers. Polylactic acid (PLA), that is, polylactide, or condensation polymers based on lactic acid, are for many reasons a very attractive group of biopolymers. Its main degradation product, lactic acid, is a common product in nature, is not toxic and is widely used in the food and pharmaceutical industries.

[0004] PLA films can be manufactured by blown or molded film techniques (eg, molding and tenner). Although each method has advantages and disadvantages, the molded film is generally more suitable for certain end-user applications, such as those that need film sleeves instead of a wrap film. In addition, a molded film has an optics generally much better than a blown film and can be produced at higher line speeds. However, PLA films that are manufactured by current molding methods show excessive retraction in the machine direction, which contributes substantially to the winding and limits its field of application. For example, EP1491585 A1 discloses a shrinkable PLA film for packaging, comprising a polymeric mixture of PLA, which stretches 4: 1 in the transverse direction and 1.08: 1 in the machine direction, and shows a 50 , 8% retraction in the transverse direction and 3.6% retraction in the machine direction after exposure to hot water at 80 ° C for 10 seconds. Thus, there is a need for retractable PLA films manufactured by molding methods that show little to no retraction in the machine direction.

SUMMARY OF THE INVENTION [0005] The aforementioned needs are met, to some extent, with the present invention, where in one aspect a retractable film is provided comprising a polymeric film of PLA oriented in a transverse direction which, after exposure at heat for 10 seconds at 60 ° C, it shows a retraction in the transverse direction greater than 25% and shows a retraction in the machine direction less than 1%, where a negative retraction indicates growth, where the polymer blend of PLA includes two or more grades of PLA polymer, and where the shrink film comprises from 60 percent by weight to 80 percent by weight of PLA polymer having 11 to 13 percent by weight of D-lactide; and from 20 percent by weight to 40 percent by weight of a PLA polymer having 1 to 2 percent by weight of D-lactide.

[0006] The retractable PLA films of the invention may further comprise one or more additives, such as a block antiforming additive, a sliding additive, a viscosity enhancer or a combination thereof. The block antiforming additive can be natural silica, synthetic silica, talc, talcum-filled magnesium, calcium carbonate and N, N'-ethylene-bis (stearamide) (EBS). The preferred block antiforming additive in some embodiments is talcum-filled magnesium. The sliding additive can be oleamide, erucamide, stearamide,

ES 2 726 751 T3 behenamide, oleylpalmitamide, stearyl rucamide, ethylene-bis-oleamide, EBS, or a combination thereof, and preferably EBS in some embodiments. Viscosity enhancers may include, for example, stabilizers or coupling agents. A preferred coupling agent is CESA®-extend.

[0007] Polymeric PLA blends of the shrink films of the invention described herein include two or more grades of PLA polymer. For example, the PLA polymer may comprise about 1 to about 2 percent by weight of D-lactide; about 3 to about 5 weight percent D-lactide; or about 11 to about 13 percent by weight D-lactide. Retractable films comprise from about 60 percent by weight to about 80 percent by weight of PLA polymer having from about 11 to about 13 percent by weight of Dlactide; and from about 20 percent by weight to about 40 percent by weight of a PLA polymer having from about 1 to about 2 percent by weight of D-lactide. In yet other embodiments, the shrink films can comprise from about 65 percent by weight to about 75 percent by weight of PLA polymer having from about 11 to about 13 percent by weight of D-lactide ; and from about 25 percent by weight to about 35 percent by weight of a PLA polymer having from about 1 to about 2 percent by weight of D-lactide.

BRIEF DESCRIPTION OF THE DRAWINGS [0008]

Figure 1 is a schematic diagram of the motion of the polymer film from a matrix through molding and MDO machines and then to a tenner machine. The path through the molding machine and the tenner machine is illustrated with arrows. A general profile of the respective machines is provided by dashed lines. The circles represent rollers.

Figure 2 is a graph describing the percentage shrinkage of a molten PLA film (TDO) of the present invention at a given temperature for ten seconds compared to a blown PLA film (MDO). Retraction is provided in both machine direction (MD) and transverse direction (TD).

Figure 3 is a graph describing the percentage TD shrinkage of the TDO PLA films of the invention in relation to non-PLA TDO films. PLA: polylactic acid; TMOPS: transverse mono-oriented polystyrene; PETG: glycol modified polyethylene terephthalate; PVC: polyvinyl chloride.

Figure 4 is a graph describing the percentage MD shrinkage of the TDO PLA films of the invention in relation to non-PLA TDO films.

DETAILED DESCRIPTION [0009] In one aspect of the present invention, a shrink film is provided which comprises a polymeric mixture film of polylactic acid (PLA) oriented in a transverse direction which, after exposure to heat for 10 seconds at 60 ° C, shows a retraction in the transverse direction greater than 25% and shows a retraction in the direction of the machine less than 1%, where a negative retraction indicates growth, where the polymer blend of PLA includes two or more grades of PLA polymer, and where the shrink film comprises from 60 percent by weight to 80 percent by weight of PLA polymer having 11 to 13 percent by weight of Dlactide; and from 20 percent by weight to 40 percent by weight of a PLA polymer having 1 to 2 percent by weight of D-lactide. Preferably, retraction in either direction occurs substantially simultaneously.

[0010] The machine address will be defined herein as the longitudinal or length direction. The transverse direction, also called cross direction or cross network address, will be defined herein as the direction perpendicular to the machine direction. The embodiments of the films described herein show reduced edge wrap characteristics and can be produced in light, matt, translucent and opaque colors. The films of the present invention may be suitable for printing on the back side and / or the front side.

[0011] The films of the present invention may comprise various polymers and degrees of polylactic acid polymers (PLA) known in the art. Preferably, in some embodiments, polymers are selected which, when used alone or in a mixture, allow memory of a described orientation process to be stored.

ES 2 726 751 T3 herein. Many of the determinants for the selection of polymers are known to a person skilled in the art and / or will be apparent from the instructions herein. For example, when high temperature processing is desired, polymers with higher softening points may be selected, such as, for example, relatively high molecular weight polymers.

[0012] Retractable films of the present invention comprise PLA, and may optionally include additives known in the art, for example, block antiforming additives, sliding additives, plasticizers and viscosity enhancers. These additives are generally used to improve the processing, performance and appearance of the final product as will be discussed later. In each of the aforementioned classes, it is understood that various grades of the respective polymers are included. Each of these classes of polymers and additives will now be discussed, in turn, as relevant to the present invention.

PLA [0013] Since both lactic acid and lactide can achieve the same repeating unit, the general term polylactic acid as used herein refers to polymers having the repeating unit of formula I without any limitation in as to how the polymer was produced (for example, from lactides, lactic acid or oligomers), and without reference to the degree of polymerization.

ch₃ o. I II

-fc-c-o-hrH [I] [0014] A high molecular weight polymer can be produced by ring-opening polymerization from a lactic acid dimer, lactide. Lactic acid is optically active, and its dimer therefore appears in four different forms: L, L-lactide; D, D-lactide; L, D-lactide (mesolactide); and a racemic mixture of L, L- and D, D-lactides. By polymerizing these dimers either as pure compounds or in different mixing ratios, polymers are obtained that have different stereochemical structures that affect their resilience and crystallinity and, consequently, also their mechanical and thermal properties. The polymers obtained are normally hard and optically transparent.

[0015] Copolymers or polymer blends can also be used in PLA films of the present invention. The weight average molecular weight (MW) of the polymers suitable for the invention is approximately 10,000 400,000, preferably 40,000-250,000.

[0016] The polylactide is in equilibrium with its monomer, lactide. This chemical property can lead to rapid hydrolysis and cause adhesion problems in polymer processing. In addition, the presence of the monomer reduces thermostability during the fusion treatment. Therefore, the residual lactide of the polymer is typically and preferably removed. The preferable monomer content is preferably less than about 2%, and more preferably less than about 1%.

[0017] Heat, during the film processing steps, can also contribute to polymer degradation. In addition to the elimination of lactide monomer, another way to delay premature hydrolysis of the polymer is to reduce the water content of the polymer below 500 ppm, and more preferably, below 200 ppm. Methods for reducing and / or maintaining the low water content are described hereinafter.

[0018] The PLA is available from multiple suppliers. NatureWorks® polymers, supplied by Cargill, Inc. are preferred in some embodiments of the present invention (eg, grades 4060D, 4042D, 4032D). Although each of grades 4060D, 4042D and 4032D has an average molecular weight of 200,000 to about 400,000, they are prepared with different percentages of D-lactide. Grade 4042D is prepared with from about 3 to about 5 percent by weight of D-lactide. The grade 4060D comprises from about 11 to about 13 percent of D-lactide; The 4032D grade polymer comprises from about 1 to about 2 percent of D-lactide. These polymers are supplied with a lactide concentration of less than about 1 percent, an amount of mesolactide of

ES 2 726 751 T3 about 10 percent to about 20 percent, and an amount of moisture less than about 500 ppm.

[0019] The concentration of D-lactide, in particular, may affect the physical properties of the resulting PLA polymer. By way of example, increasing the percentage of D-lactide in a polymer or a polymer mixture reduces the ability of the resulting polymer to crystallize, which, in turn, increases the undesirable degradation of the polymer at higher temperatures. In other words, reducing the weight percentage of D-lactide in a polymer composition increases the temperature resistance of the polymer and, therefore, also the viscosity of the resulting melt at a given temperature.

[0020] Polymers and / or polymer blends with higher levels of D-lactide can produce films that begin to shrink at lower temperatures when exposed to heat; These films also tend to show smoother retraction curves, that is, less shrinkage due to temperature rise. In contrast, films comprising polymers with a relatively low concentration of D-lactide generally require exposure to higher temperatures to shrink. It should also be noted that, typically, PLA polymers with lower concentrations of D-lactide are more expensive than other equivalent polymers that have more D-lactide. Therefore, there may be an economic incentive to maximize the use of PLA with higher levels of Dlactide; However, this incentive must and can be balanced with the desired physical properties of the films.

Block anti-forming additives [0021] Block anti-forming additives (also called non-sticks) serve to improve the processing and application of polymeric films. Specifically, this class of additives is used to reduce adhesion between films. Block antiforming agents, generally finely divided solid minerals, but also waxes, act to produce a slight surface roughness. Block antiforming agents are mainly used in the extrusion of films and include natural silica, synthetic silica, talc, calcium carbonate and N, N'-ethylene-bis (stearamide) (EBS).

[0022] Block antiforming additives are typically loaded with a carrier compound. Although it is by no means a requirement, it is preferable that the carrier polymer is similar or equivalent to one or all of the polymers in the PLA master mix. In the present invention, for example, it is preferred that the carrier polymer be a PLA polymer. Since the "active substance in a block antiforming agent comprises only a small fraction of the final composition, the addition of a carrier compound provides ease and consistency in the measurements. A person skilled in the art will know how to consider the concentration of filler when calculating the final concentration of block antiforming agent in the final product. For example, if a composition comprising 10 percent block antiforming agent consists of 10 percent of the "active substance, the final concentration of the" active substance is 1.0 percent of the total. The values ​​set forth below, including those in Tables 1 and 2, are provided as a percentage of the "active substance in the final formulation.

[0023] In the present invention, the final concentration of the block antiforming agent is less than about 3 percent by weight, preferably less than about 2 percent by weight and more preferably less than about 1 percent by weight . In one embodiment of the present invention, Cargill block antiforming agent 2100D is preferred. The 2100D comprises 10% talc-filled magnesium silicate in a 4032D grade PLA carrier and has an average particle size of less than about 1 micron.

[0024] In the selection of an appropriate block antiforming agent, the particle size thereof is a factor to be considered. In fact, the particle size of a block antiforming agent can contribute directly to the overall smoothness of the resulting film. That is, if all other factors are equal, a smaller block antiforming agent particle size will result in a softer film. Another factor to consider is the concentration of the block antiforming agent. The concentration of the block antiforming agent is preferably minimized in particular, as in some cases, because block antiforming additives can introduce turbidity to the film. Therefore, in particular in applications where turbidity is undesirable (for example, envelope windows, etc.), the concentration of block antiforming agent can be minimized.

ES 2 726 751 T3

Sliding additives / processing additives [0025] Processing additives refer to a class of agents that generally assist in downstream manufacturing (ie, post-extrusion operations) of the various film products. Sliding additives, for example, are modifiers that act as an internal lubricant to reduce the coefficient of friction (COF) between two overlapping films, for example, in films rolled after production. In fact, lower COFs are especially desirable for film applications. These additives migrate to the surface of the plastic during and immediately after processing. That is, a non-visible coating flourishes on the surface to provide a microscopic layer of air between two adjacent sheets of film. In this way, improved lubricity and sliding characteristics are provided.

[0026] Accordingly, sliding additives can be considered similar to block antiforming additives because both serve to decrease COF between two overlapping films. The films of the present invention may comprise one, both or no class of additives. Typical gliding agents are, for example, oleamide, erucamide, stearamide, behenamide, oleylpalmitamide, stearylerucamide, ethylene-bis-oleamide, EBS, including most degrees of their respective refinement. In some embodiments, EBS is a preferred sliding agent, and EBS with carrier 4032D is more preferred. These and other sliding additives and processing additives are commercially available under the following trade names: Adogen 42, Armid 18, Crodamide VRX, E, ER and BR, Danimer 9205, Freeflow Z100 and Z200, Incromold, Incroslip, Kemamides, Kemamide P- 181, MoldWiz Int-33CRS, Paraloid EXL, Uniwax 1750 and Viton. EBS is sold under the trade names Advawax, Lubrol EA and Micotomic 280.

[0027] As with block antiforming agents, the "active substance" of the sliding additives is generally supplied with a carrier. The films of the present invention comprise less than about 1 percent by weight of a sliding additive (in reference to the active substance alone), and more preferably less than about 0.5 percent by weight. It should be noted that excessive amounts of sliding additive can produce films that are excessively soft, which can compromise the ability of substances (eg, ink, adhesives, etc.) to adhere to the surface. Thus, to improve, for example, the printing properties of the shrink films of the present invention, the amount of sliding additive may require adjustment accordingly.

Plasticizers [0028] Although the practice of the present invention does not require the use of impact modifiers (also known as plasticizers), their use may be beneficial in processing. For example, some plasticizers can reduce fragility. Examples include: commonly available commercial plasticizers, for example, mono- and polycarboxylic acid esters, polymeric polyesters, polyalkyl ethers, glycerol esters and glycol (for example, glycerol triacetate and glycerol tripropionate), aliphatic and aromatic polyesters of low molecular weight, and mixtures thereof. Various polymeric plasticizers such as citrate esters, adipate esters, epoxidized soybean oils, acetylated coconut oil, flaxseed oil and mixtures thereof are also suitable. In addition, it is commonly appreciated that the addition of lactic acid, lactides (including D- and L-lactide, D, meso and racemic lactide), lactic acid oligomers, lactide oligomers, and mixtures thereof, in a mixture PLA can act as polymer plasticizers.

Viscosity Enhancers [0029] Although numerous methods are known and available to increase the viscosity of polymers during the processing of films molded and processed with tenner, the term viscosity enhancer is defined herein to encompass any chemical agent. that increases or maintains the viscosity of a polymer at a given temperature. Viscosity enhancers can be introduced into the polymer mixture at any time until the polymer enters the matrix (discussed below), however, viscosity enhancers are preferably introduced before extrusion and, more preferably, during the mixture of polymer granules.

[0030] The viscosity enhancers can improve the final properties of the films by avoiding and / or reversing the degradation found during the processing of the polymeric films. Some viscosity enhancers are stabilizers. That is, they are used in virgin plastic to (1) protect against degradation in processing and / or (2) reverse the degradation caused by recycling, and return the plastic to almost its properties of

ES 2 726 751 T3 original performance. Another class of viscosity enhancers, the coupling agents, for example, improves the processability of the extruded polymer by coupling the individual polymer chains thereby increasing the melt strength of the plastic.

[0031] Viscosity enhancers are generally known and available to a person skilled in the art and have their widest application in polyesters, polyamides (nylon) and polycarbonates. Although, in general, the chemical identity of viscosity enhancers is patented, products are available from vendors such as Johnson Polymer LLC (USA) and Clariant Intemational Ltd. (Switzerland). In fact, any chemical agent that increases or maintains the viscosity of a polymer at a given temperature may suffice.

[0032] In a preferred embodiment of the invention, shrink films are generated comprising a coupling agent to increase viscosity. One such coupling agent, CESA®-extend, is available from Clariant. These viscosity enhancers contain a patented copolymer of styrene, methyl methacrylate and glycidyl methacrylate. Without being limited by or linked to the theory, it is believed that CESA®extend viscosity enhancers repair the damage (for example, polymer breakdown) that heat and moisture can cause in PLA resins by coupling PLA polymers individual In this way, the viscosity enhancer can extend the polymer chains in the presence of some degradation and thus attenuate the overall loss of molecular weight and the viscosity of the polymer melt.

[0033] The optimum reactivity of CESA®-extend can be achieved with a preferable amount of use less than about 0.15 percent by weight, and preferably less than about 0.75 percent by weight of the active substance. Again, the "active substance" of viscosity enhancers is generally supplied with a carrier. CESA®-extend viscosity enhancers may experience a residence time in the process of about 2 to about 20 minutes, depending on the initial humidity and / or the inherent initial viscosity of the polymer. In addition, the residence times in the process are preferably minimized to attenuate the degradation of the polymer by the heat required in the process. A detailed discussion of the processing conditions is provided below. In any case, a person skilled in the art will appreciate that it may be necessary to adjust the concentration of any polymeric enhancer based on at least one of the factors listed above.

[0034] In some embodiments, film polymers can be selected from one or more of five groups (ie, PLA, block and slide antiforming additives, plasticizers, viscosity enhancers) and combined to create mixed polymeric films. . The following tables 1 and 2 provide examples of formulations that may be suitable for the preparation of the films of the present invention. For greater clarity in the description, a person skilled in the art will appreciate, from the instructions herein, that the percentage of additives calculated is only the "active substance." In other words, although it should be appreciated that the additives are generally supplied as a combination of "active substance" and carrier, the carrier percentage, if any, has not been listed.

Table 1. Polymeric formulations (O: according to the invention, X: not according to the invention)

<td rowspan="2">No.</td><td colspan="6">Composition (percentage by weight)</td>

<td>4060D</td><td>4042D</td><td>4032D</td><td>Block Antiforming</td><td>Slider</td><td>Vise enhancer.</td>

<td>1 (O)</td><td> 69,58%</td><td> 0%</td><td> 30%</td><td> 0,15%</td><td> 0,18%</td><td> 0,09%</td>

<td>2 (X)</td><td> 97,50%</td><td> 0%</td><td> 0%</td><td> 1,50%</td><td> 0,50%</td><td> 0,50%</td>

<td>3 (X)</td><td> 0%</td><td> 0%</td><td> 99,58%</td><td> 0,15%</td><td> 0,18%</td><td> 0,09%</td>

<td>4 (X)</td><td> 0%</td><td> 0%</td><td> 97,50%</td><td> 1,50%</td><td> 0,50%</td><td> 0,50%</td>

<td>5 (X)</td><td> 0%</td><td> 98,58%</td><td> 0%</td><td> 0,15%</td><td> 0,18%</td><td> 0,09%</td>

<td>6 (X)</td><td> 0%</td><td> 98%</td><td> 0%</td><td> 1,50%</td><td> 0,50%</td><td> 0,50%</td>

<td>7 (X)</td><td> 98,58%</td><td> 0%</td><td> 0%</td><td> 0,15%</td><td> 0,18%</td><td> 0,09%</td>

<td> 8(0)</td><td> 59,58%</td><td> 10%</td><td> 30%</td><td> 0,15%</td><td> 0,18%</td><td> 0,09%</td>

<td>9 (X)</td><td> 49,58%</td><td> 20%</td><td> 30%</td><td> 0,15%</td><td> 0,18%</td><td> 0,09%</td>

<td>10 (X)</td><td> 39,58%</td><td> 30%</td><td> 30%</td><td> 0,15%</td><td> 0,18%</td><td> 0,09%</td>

Ί

ES 2 726 751 T3

<td>11 (X)</td><td> 48,61%</td><td> 30%</td><td> 20%</td><td> 1,12%</td><td> 0,18%</td><td> 0,09%</td>

<td>12 (X)</td><td> 48%</td><td> 30%</td><td> 20%</td><td> 1,00%</td><td> 0,50%</td><td> 0,50%</td>

<td>13 (X)</td><td> 30%</td><td> 20%</td><td> 48%</td><td> 1,00%</td><td> 0,50%</td><td> 0,50%</td>

<td>14 (X)</td><td> 30%</td><td> 30%</td><td> 39,58%</td><td> 0,15%</td><td> 0,18%</td><td> 0,09%</td>

<td>15 (X)</td><td> 30%</td><td> 0%</td><td> 68,85%</td><td> 0,15%</td><td> 0,50%</td><td> 0,50%</td>

<td> 16(0)</td><td> 68,85%</td><td> 0%</td><td> 30%</td><td> 0,15%</td><td> 0,50%</td><td> 0,50%</td>

<td>17 (X)</td><td> 0%</td><td> 68,85%</td><td> 30%</td><td> 0,15%</td><td> 0,50%</td><td> 0,50%</td>

<td>18 (X)</td><td> 30%</td><td> 68,85%</td><td> 0%</td><td> 0,15%</td><td> 0,50%</td><td> 0,50%</td>

<td>19 (X)</td><td> 29,58%</td><td> 30%</td><td> 40%</td><td> 0,15%</td><td> 0,18%</td><td> 0,09%</td>

<td> 20 (0)</td><td> 70%</td><td> 0%</td><td> 29%</td><td> 0,50%</td><td> 0,25%</td><td> 0,25%</td>

<td>21 (X)</td><td> 29%</td><td> 0%</td><td> 70%</td><td> 0,50%</td><td> 0,25%</td><td> 0,25%</td>

<td>22 (X)</td><td> 99%</td><td> 0%</td><td> 0%</td><td> 0,50%</td><td> 0,25%</td><td> 0,25%</td>

<td>23 (X)</td><td> 0%</td><td> 99%</td><td> 0%</td><td> 0,50%</td><td> 0,25%</td><td> 0,25%</td>

<td>24 (X)</td><td> 0%</td><td> 0%</td><td> 99%</td><td> 0,50%</td><td> 0,25%</td><td> 0,25%</td>

<td>25 (X)</td><td> 100%</td><td> 0%</td><td> 0%</td><td> 0%</td><td> 0%</td><td> 0%</td>

<td>26 (X)</td><td> 0%</td><td> 100%</td><td> 0%</td><td> 0%</td><td> 0%</td><td> 0%</td>

<td>27 (X)</td><td> 0%</td><td> 0%</td><td> 100%</td><td> 0%</td><td> 0%</td><td> 0%</td>

<td>28 (X)</td><td> 0%</td><td> 0%</td><td> 0%</td><td> 100%</td><td> 0%</td><td> 0%</td>

<td>29 (X)</td><td> 99,63%</td><td> 0%</td><td> 0%</td><td> 0,10%</td><td> 0,18%</td><td> 0,09%</td>

<td>30 (X)</td><td> 0%</td><td> 99,63%</td><td> 0%</td><td> 0,10%</td><td> 0,18%</td><td> 0,09%</td>

<td>31 (X)</td><td> 0%</td><td> 0%</td><td> 99,63%</td><td> 0,10%</td><td> 0,18%</td><td> 0,09%</td>

<td>32 (X)</td><td> 99,68%</td><td> 0%</td><td> 0%</td><td> 0,05%</td><td> 0,18%</td><td> 0,09%</td>

<td>33 (X)</td><td> 0%</td><td> 99,68%</td><td> 0%</td><td> 0,05%</td><td> 0,18%</td><td> 0,09%</td>

<td>34 (X)</td><td> 0%</td><td> 0%</td><td> 99,68%</td><td> 0,05%</td><td> 0,18%</td><td> 0,09%</td>

<td>35 (X)</td><td> 30%</td><td> 30%</td><td> 39,68%</td><td> 0,05%</td><td> 0,18%</td><td> 0,09%</td>

<td>36 (X)</td><td> 39,68%</td><td> 30%</td><td> 30%</td><td> 0,05%</td><td> 0,18%</td><td> 0,09%</td>

<td>37 (X)</td><td> 0%</td><td> 39,68%</td><td> 30%</td><td> 0,05%</td><td> 0,18%</td><td> 0,09%</td>

<td> 38 (0)</td><td> 70%</td><td> 0%</td><td> 29,3%</td><td> 0,1%</td><td> 0,6%</td><td> 0%</td>

Table 2. Polymeric formulations (O: according to the invention)

<td rowspan="2">No.</td><td colspan="6">Composition (percentage by weight) </td>

<td>4060D</td><td>4042D</td><td>4032D</td><td>Block Antiforming</td><td>Slider</td><td>Plasticizer</td>

<td> 39 (0)</td><td> 70%</td><td> 0%</td><td> 24,3%</td><td> 0,10%</td><td> 0,60%</td><td> 5%</td>

<td> 40 (0)</td><td> 70%</td><td> 1%</td><td> 24,3%</td><td> 0,10%</td><td> 0,60%</td><td> 4%</td>

<td> 41 (0)</td><td> 65%</td><td> 5%</td><td> 26,3%</td><td> 0,10%</td><td> 0,60%</td><td> 3%</td>

<td> 42 (0)</td><td> 68%</td><td> 2%</td><td> 28,3%</td><td> 0,10%</td><td> 0,60%</td><td> 1%</td>

<td> 43 (0)</td><td> 70%</td><td> 5%</td><td> 24,3%</td><td> 0,10%</td><td> 0,60%</td><td> 0%</td>

<td> 44(0)</td><td> 70%</td><td> 0%</td><td> 29,3%</td><td> 0,10%</td><td> 0,60%</td><td> 0%</td>

<td> 45 (0)</td><td> 70%</td><td> 0%</td><td> 24,5%</td><td> 0,10%</td><td> 0,40%</td><td> 5%</td>

<td> 46 (0)</td><td> 70%</td><td> 0%</td><td> 24,7%</td><td> 0,10%</td><td> 0,20%</td><td> 5%</td>

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<td>47 (O)</td><td> 70%</td><td> 0%</td><td> 24,3%</td><td> 0,05%</td><td> 0,65%</td><td> 5%</td>

<td>48 (O)</td><td> 70%</td><td> 0%</td><td> 24,3%</td><td> 0,15%</td><td> 0,55%</td><td> 5%</td>

<td>49 (O)</td><td> 70%</td><td> 0%</td><td> 24,3%</td><td> 0,25%</td><td> 0,45%</td><td> 5%</td>

[0035] The mixtures of the above tables can be chosen or prepared to create the feel or flexibility of the film corresponding to an end-use application. When all other processing parameters are equal, the proportion of PLA polymers can be adjusted in accordance with the instructions of the present invention to provide shrink films with desirable physical properties. For example, reducing the concentration of D-lactide (that is, increasing the weight percentage of 4032D) produces polymers with a higher softening point and films that require a higher temperature to retract, which displaces the curve of right retraction.

[0036] Although the formulations listed in the above tables have been categorized based on the presence of a viscosity enhancer (table 1) or a plasticizer (table 2), the formulations of the present invention may include both a enhancer of the viscosity as a plasticizer. In addition, all formulations (ie, PLA polymer blends) may include one or more color correction dyes. Such dyes, known and available to those skilled in the art, are typically added to dye films (for example, from a yellow to a blue index) to make them more suitable for printing thereon. The concentration of color correction dyes generally ranges from about 0.002% -0.009%, preferably about 0.006%, by weight.

[0037] In a preferred embodiment, the films are produced with a mixture of about 94 percent by weight of PLA polymer and less than about 6 percent by weight of additives and in which the polymer component of PLA is approximately 70 percent PLA 4060D and 24 percent 4032D by weight. In another preferred embodiment, the films are produced with a polymer mixture according to formula No. 39.

Processing [0038] After the polymer composition of the film is selected, the polymer is then processed to generate a film with desirable retraction properties. Generally, polymers are obtained in granules or grains. In cases where multiple polymers must be included, the polymer granules are first mixed dry. That is, the granules are mixed together. Preferably, the granules are then processed into a film by molding and tenner technology.

[0039] Molding and tenner processing involves extrusion of molten polymers through a groove or flat die to form a thin molded sheet or film. This film is first polished by the surface of a cooling roller (typically with temperature regulation and chrome plating). The film is then laminated through a machine direction orientation machine (MDO) for temperature conditioning and stretched through a tenner frame. Finally, the edges of the film are cut before rolling.

[0040] Thus, the molding and tenner method can be characterized in four essential steps: extrusion, molding, temperature conditioning and stretching. A preliminary drying step of the polymer granules is preferable, but not required. In addition, a terminal annealing step may be preferable, but not required according to the present instructions. A set for each processing step is generally depicted in Figure 1 and will now be described in detail.

Drying [0041] PLA polymers are generally supplied in sealed bags from the manufacturer and in a relatively dry state. Typically, the moisture content of these PLA polymers as supplied is less than about 500 ppm and preferably less than about 200 ppm. When the humidity level is considered desirable, additional drying may not be necessary or required. However, the PLA easily absorbs moisture from the atmosphere and, therefore, the mixed polymer granules are optionally dried and preferably first heated in a dryer to remove surface moisture. Without being bound or limited by theory, it is believed that moisture content removal can help control viscosity loss.

ES 2 726 751 T3 relative to hydrolysis. As mentioned above, higher temperatures and the presence of even a small amount of moisture can hydrolyse the PLA in the next melting phase.

[0042] PLA is generally produced by a reversible condensation reaction, which produces water; When non-dried PLA is heated, hydrolysis can occur and key mechanical properties of PLA can be compromised. For example, the viscosity of the polymer, when molten, is inversely proportional to the percentage of free monomer in it. Therefore, in an attempt to minimize the variation in viscosity between batches, preferably, significant moisture is removed from the polymer granules. A moisture content of less than about 200 ppm is preferable, and less than about 50 ppm, more preferable (measured by the Karl Fisher method).

[0043] A dehumidification hopper with hot air can be used at a relatively low dew point; however, a variety of air dryers are known in the art and many of them may be suitable for drying. The present invention need not be limited only to air dryers, but may include other types of dryers, including cooking ovens. A dehumidification hopper in which dehumidified air passes through a bed of PLA to extract moisture from the resin may be desirable. A desiccant material, such as silica, absorbs moisture from the circulating air. Double desiccant bed systems are common, so that a bed is in operation while a bed is regenerated in standby mode. A time cycle or a predetermined reduction in the air dew point is used to change the air flow from one bed to the other. It is believed that such a methodology is effective in removing some moisture that may reside below the surface of the polymer granules in addition to the surface moisture.

[0044] Preferable dryers for drying PLA can have one or more of the following characteristics:

one. Desiccant beds capable of achieving a dew point of approximately -40 ° C in the feed air

2. A means, for example, a subsequent cooling unit, to eliminate or reduce the probability of a temperature peak in the feed air

3. A higher temperature regulation in the drying interval of the PLA [0045] The temperature and drying duration may be dependent on the total amount and condition of the polymer (s) (ie, the amount of surface moisture initial), and it may be necessary to adjust them batch by lot. Preferably, the polymers undergo little or no fusion in this step. As an example, typical drying conditions require temperatures to vary from approximately 40^or C at about 110 ° C, and preferably from about 55 ° C to about 90 ° C for varying periods of time. As a specific example, the residence time to dry the polymer with air (dew point, -40 ° C) with a flow greater than about 0.5 feet³/ min requires approximately 4 hours at around 40 ° C and approximately 2 hours at around 90 ° C. Higher drying temperatures can lead to softening and polymer block formation, while lower drying temperatures can result in prolonged drying times and / or incomplete drying.

[0046] The dew point is an absolute measure of air humidity and is independent of air temperature. The dew point can be used to control dryer performance. The air stream is another component to dry, since it heats the resin and absorbs its moisture. A sufficient air flow can maintain the resin at the appropriate temperature during its entire residence time. When dyes, additives or other additional ingredients are used, it may be preferable to minimize moisture related degradation by additional drying.

Extrusion [0047] The extrusion is whereby the granules melt into a low viscosity melt, thus combining the individual polymer beads or grains up to that point into a melt. The viscosity of the melt will depend on the temperature. Temperatures can vary from approximately the temperature at which the polymers will remain molten to approximately the temperature where the degradation of the polymers begins to occur. By way of example, extrusion melting temperatures can be maintained between about 165 ° C and about 230 ° C for certain polymer mixtures of PLA, but may ultimately depend on the different polymers that have been mixed and their respective points of fusion. A temperature of about 175 ° C is preferred.

EN 2 726 751 T3 [0048] The molten plastic is then extruded through a T-type die with a flat groove opening. The opening should be adjusted to admit the thickness of the finished film. In other words, a larger opening should be used when thicker films are desired and vice versa. Generally, the opening will vary between about 0.01 inches and about 0.10 inches and preferably between about 0.05 inches to about 0.07 inches. The stretching distance (that is, the distance from the die to the molding rollers) can also affect the thickness of the resulting film. Generally, the longer the distance, the thinner the film, and vice versa. Without being bound or bound by theory, it is believed that larger stretching distances introduce an undesirable machine direction orientation in the resulting film. Thus, while stretching distances may vary from 0.75 inches to approximately 1.5 inches, shorter distances are preferred. A stretching distance of approximately 0.750 inches is generally appropriate.

Molding [0049] The relatively hot plastic (for example, from about 175 to about 190 ° C) of the extruder is then molded between at least two rollers. Without being bound or bound by theory, the temperature of the laminating rollers should be adjusted to achieve rapid cooling on the exterior of the extrudate to form a skin, which causes the extrudate to adhere to the rollers. At the same time, the temperature of the rollers should be kept high enough so that the core of the extrudate remains flexible to thereby allow a bend around the rollers without breakage or cracking.

[0050] The molding machines typical in the art are equipped with one or two rollers, where the plastic moves from the die, between a single large roller or two rollers and then to the orientation machine in the machine direction. The present inventors have discovered, however, that while typical molding machines are suitable for preparing the films of the invention described herein, the novel use of a third roller in the molding machine is preferable in the production of films, in particular when an improved quality of the surface of the films is desired. In fact, it is believed that the use of three or more rollers has not been made in the production of PLA film, more particularly a thin gauge PLA film (for example, a film of less than about 0.002 inches).

[0051] It is believed that the use of a third or more roller (s) is a reason for a significantly improved production speed. While the film is typically molded at about 130 feet / minute, the process described herein provides a line speed of approximately 250 feet / minute. Without being bound or bound by theory, it is believed that the additional roller (s) helps in balancing the hottest core temperature with the coldest skin temperature. Therefore, these or this third roller (s) is preferably maintained at a higher temperature than the first two rollers.

[0052] By way of example, all laminating rollers can be set at relatively cold temperatures (for example, from about 20 to about 45 ° C, preferably from about 30 ° C to about 40 ° C, for the first two rollers , and from about 40 to about 65 ° C, preferably from about 55 to about 60 ° C, for the third roller). The temperature of the rollers can be maintained with liquids, such as water, oil. Water is a preferable coolant for laminating rollers due to its greater thermal capacity than most conventional liquids. Typically, water can eliminate three times more heat (BTU) than oils for this purpose.

[0053] A person skilled in the art will appreciate from the instructions presented herein, that the absolute temperature of the rollers will depend on several factors, including the initial temperature and the thickness of the extruded plastic. The temperature of the rollers can and should be adjusted depending on the thickness of the die opening, the stretching ratio and the like. The first roller is set, for example, at about 40 ° C, the second at about 32 ° C and the third at about 57 ° C.

[0054] In fact, more significant than the absolute temperature, for any rolling mill, is the temperature of the extruded film when it leaves the molding machine. The temperature of the film when leaving the molding machine is preferably between about 55 ° C and about 75 ° C, and more preferably at about 60 ° C.

[0055] In addition to adjusting the temperature of the plastic when it arrives from the die, the laminating rollers can also be used to adjust the thickness of the plastic. This adjustment is made by presetting the space between the roller to the desired thickness. For example, the space between the first two rollers is preferably 2-10 times, more preferably

ES 2 726 751 T3

5-7 times, the desired final thickness of the film. Similarly, the space between the second and third rollers is preferably 3-9 times, more preferably 4-6 times, the desired final thickness of the film.

Temperature conditioning [0056] The next step in the preparation of the films of the present invention is a temperature conditioning step. Temperature conditioning refers to the process of balancing the temperature of the plastic film so that the temperature difference between the core and the skin is minimized, if any. Although an MDO machine can be used in this process, the orientation (i.e. stretching) in the machine direction (MD) is preferably minimized because the MD stretching leads to an undesirable MD retraction.

[0057] A variety of measures can be taken to limit or prevent MD stretching when using an MDO machine. For example, the speed towards MDO rollers can be adjusted to minimize pulling the film from one roller to the next. For example, the rollers can be maintained at similar speeds. (A film is at risk of being pulled when a rear roller rotates faster than the previous one.) Alternatively, the distance between and the location of the MDO rollers can contribute to the MD stretch. For example, with a lateral movement of the film it is typically pulled less than with a horizontal movement.

[0058] It has been found that the novel use of free rollers to avoid some MDO rollers is beneficial for reducing MD stretching. The free rollers are not driven by an engine and therefore are free unless the film passes over them. Free rollers may, but not necessarily, have a controlled temperature. The number and location of the free rollers is not limiting and can be adjusted to suit specific processing parameters. For example, two free rollers are installed to avoid two central rollers in an MDO machine (Figure 1, A and B).

[0059] As with the laminating rollers, more significant than the absolute temperature of any MDO roller is the temperature of the film when it leaves the MDO machine. In fact, the temperature of the roller may have to be adjusted according to the thickness of the film, the desired stretch, etc. Preferably, the temperature of the film is equilibrated to between about 60⁰C and about 90 ⁰C, and more preferably, at about 70 ⁰C. An MDO machine with 6 motorized and two free rollers can be used as shown in Figure 1: Roller 4 is at approximately 72 ⁰C, roller 5 is at about 85 ⁰C, the free rollers 6 and 7 do not have a controlled temperature, the roller 8 is at approximately 85 ⁰C and roller 9 is at approximately 60 ⁰C.

Tenner [0060] Once the extrudate has been conditioned to the temperature, it enters the tenner machine. This machine is equipped with tenner clips that hook over the edge of the plastic film, which is generally about 20 inches wide. The tenner machine is equipped with a frame A that stretches the plastic film to approximately 4 times its input width (for example, from 20 inches to 80 inches).

[0061] The tenner machine also consists of three zones with the temperature controlled along frame A. The preheating zone is provided to uniformly raise the temperature of the polymer and ensure that the temperature of the film core has been balanced with the skin temperature The stretch zone maintains an appropriate stretch temperature. It should be appreciated, from the instructions herein, that the stretching temperature will have to be adjusted according to the particular PLA mixture, practices and experience available to a person skilled in the art. However, this temperature may be shared with the preheating zone, which is between approximately 70⁰C and about 90 ⁰C, more preferably at about 80 ⁰C. The final zone is the annealing zone.

Annealing [0062] Annealing, also called crystallization or relaxation, is typically the final step in the preparation of the films of the present invention. According to the instructions herein, an annealing step is optional. When desired, annealing is generally performed after stretching and is performed at temperatures between approximately 49⁰C and about 90 ⁰C, preferably at about 70 ⁰C. During annealing, the film relaxes to a small extent. Relaxation rates of approximately 0.5% are typical at

ES 2 726 751 T3 approximately 5%, and approximately 3% or less are preferred. Finally, the edges of the stretched film are trimmed and rolled on rollers by conventional means.

[0063] In one embodiment of the present invention, the percentage of shrinkage as a function of temperature was studied with, for example, a 200 gauge (0.002 inch) film subjected to 10-second exposures to heat (Figure 2 ). The films evaluated herein were prepared from a mixture of approximately 70% PLA grade 4060D, approximately 26% PLA grade 4032D, approximately 1% block anti-formation agent grade 2100D of which a 10% is "active substance", and approximately 3% of sliding EBS, of which 20% is active substance. (The term approximately has been used herein and throughout this specification to consider the usual variations in mass, temperature, and percentage shrinkage measurements common and expected by a person skilled in the art.)

[0064] Using the process described above, the polymer mixture, without further drying, was extruded at approximately 190 ° C, and then extruded through a T-type matrix with an opening of approximately 0.070 inches. The film was then molded between three rollers at approximately 40 ° C, 32 ° C and 57 ° C in sequence producing a film temperature of approximately 65 ° C. Then, the film was conditioned to the temperature in an MDO machine equipped with six motorized rollers and two free rollers as shown in Figure 1. This step produced a film temperature of approximately 70 ° C. Finally, the film was stretched 4x in a tenner machine, annealed at approximately 70 ° C, trimmed and rolled.

[0065] The film was placed in a hot water bath at the temperature shown during the indicated times, and the retraction in the machine direction (MD) and the transverse direction (TD) was represented as a percentage of the original dimensions . Although the shrink films studied in this example were subjected to heated water, exposure to heat in any form (for example, heated air from a hot air dryer) can induce retraction of the films of the invention. As is evident from the results, the films described show retraction in the TD and little or no retraction in the MD. At approximately 60 ° C, for example, the film of the invention retracts more than 25% in the TD and less than 1% in the MD.

[0066] The retractable properties of the films of the invention were also compared with a blown PLA film (Figure 2) and films molded / processed with tenner comprising polymers other than PLA (Figures 3 and 4). Figure 2 shows that the present films (TDO) show a higher retraction than the MDO blown films in the TD at a given temperature. Figure 3 demonstrates that the TD shrinkage curve of the present PLA films, although comparable in magnitude, occurs at a lower temperature than the non-PLA TDO films. For example, the PLA TDO can be retracted more than 60% in the TD at approximately 70 ° C while the PETG TDO must be heated to approximately 80 ° C to retract the same amount. Effective shrinkage at lower temperatures is desirable in particular in applications where heat can be detrimental to the packaged product, such as, for example, with dairy products. Thus, the TDO PLA films of the invention outperform the PETG TDOs where pre-labeled bottles (for example, milk and / or milk packaging) are preferably kept at the coolest operable temperature. Finally, Figure 4 shows that (1) the present films have a retraction in the MD concomitant with the retraction in the MD and (2) this retraction can occur at relatively low temperatures. PETG TDO, for example, retracts in the MD before retracting in the TD, resulting in undesirable retraction at the edges.

[0067] This growth in the cross direction can prevent the label from being rewound at the edges during the retraction process. Accordingly, the films of the present invention may have application in the labeling of sleeves (ie tubes) of cylinders and cones with various conformations such as, for example, batteries, cans, bottles, disposable lighters, pens, floral wraps and Other decorative items.

[0068] The films of the present invention can also be used in the printed / unprinted application to hold double packages, connections, neck bands and perforated neck bands together for decoration or use with guaranteed tampering.

ES 2 726 751 T3