Specific adhesion model for bonding hot-melt polyamides to vinyl

International Journal of Adhesion & Adhesives 24 (2004) 415-422 Specific adhesion model for bonding hot-melt polyamides to vinyl USDA Forest Service, Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53726, USA Abstract
H o t - m e l t polyamides a r e an i m p o r t a n t m a r k e t f o r t h e dimer acid m a d e f r o m t h e tall oil fatty acids liberated d u r i n g t h e K r a f t pulping process. T h e s e polyamides b o n d well t o m a n y substrates, b u t n o t t o polyvinyl chloride (PVC), c o m m o n l y called vinyl. Dimer- based polyamides m a d e f r o m secondary amines such as piperazine b o n d well t o vinyl. No m o d e l f o r this unique adhesion h a s been proposed even t h o u g h m a n y commercial adhesives a n d o t h e r p a t e n t s have utilized this feature o f dimer-based polyamides. T h i s p a p e r examines t h e validity of models based on mechanical interlock, rheological properties, solubility parameters, a n d acid- base interactions. Available d a t a a r e consistent w i t h t h e acid- base model, which depends on t h e availability o f tertiary amides m a d e f r o m secondary a m i n e t o b o n d t o t h e vinyl hydrogen i n c o n t r a s t t o t h e s t r o n g internal polyamide hydrogen bonding o f secondary amides m a d e f r o m primary amines. Poly(ester-amides) with a similar p r o t o n acceptor ability also b o n d t o vinyl. Keywords: Polyamide; Hot-melt; Adhesive; Polyvinyl chloride; Model; Adhesion; Rheology; Solubility parameter: Acid-base; Poly(ester-amide); Hydrogen bond: Piperazine 1. Background
manufacture of durable goods. Examples are shoe assembly, kitchen and bathroom cabinets, telecommu- One of the highest value products from trees is the nication cable repair sleeves, and window assembly [3]. dimer-based polyamide. To make this product, the fatty The original polyamides were made using linear acid is liberated as a salt during the pulping of trees, diamines [1,2]. A very novel patent was claimed for a especially pines from the Southeastern United States. specific type of polyamide that was made from cyclic The fatty salt is collected in soap skimmings during diamines, such as piperazine [4]. Compared to other recycling of pulping chemicals in the Kraft process. dimer-based polyamides, these resins were declared to These skimmings are acidified and distilled to yield a give unique adhesion to polyvinyl chloride (PVC), fatty acid fraction and a rosin acid fraction, both of termed vinyl in the remainder of this paper. Although which are utilized in adhesives. The fatty acid is heated more than 35 years have passed since this discovery [4], with a clay catalyst to yield a "dimer acid" by reaction and there have been over 40 patents that involve a t the olefinic portion of the fatty acid. This dimer acid piperazine-dimer acid polyamides, no model has been is then reacted with diamines to form dimer-based proposed to explain this specific adhesion to vinyl. In polyamide resins [1,2], which are used as binders in addition, the only polyamide hot-melt adhesives that printing inks, epoxy-curing agents, and hot-melt adhe- have been reported to bond to vinyl were made from sives. The market for hot-melt adhesives is mainly in the these secondary diamines. Although these polyamides bond well to plastics, some other dimer-based poly- amides bond equally or better to plastics such as The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by US Government employees on official time, and it is therefore in the public domain and not subject to copyright. The use of trade or firm names in this publication is for reader information and does not imply 2. Adhesion data for polyamides
endorsement by the US Department of Agriculture of any product or Dimer-based polyamides are used to form strong E-mail address: cfrihart@fs.fed.us (C.R. Frihart). bonds to a wide variety of materials. Original substrates 0143-7496/$ - see front matter Published by Elsevier Ltd doi:10.1016/j.ijadhadh.2003.11.008 C.R. Frihart / International Journal of Adhesion & Adhesives 24 (2004) 415-422 Table 1 Adhesion data from US patent 3,377,303 Ethylenediamine, 1,3-di (4-piperidyl)propane Ethylenediamine, 2,5-dimethyl-piperazine Ethylenediamine, N-aminoethyl-piperazine Ethylenediamine, 1,4-bis(dminoethyl)benzene Ethylenediamine, 1,5-cyclohexyl-bis(methylamine) aPatina to standard cement liner in accordance with the Standard Cement Test Method for Elastomer Soles and Heels proscribed by Rubber Manufacturer’s Association, Inc., adopted May 20, 1960. Values for vinyl adhesion are converted from pounds per square inch to newton per centimeter. included wood, metal, and leather. For bonding to these substantial quantities of linear coadiacid raises the substrates, most dimer-based polyamides give suitable softening point too much for the polyamide to be useful adhesion. Their amorphous character, their ability to as a hot-melt adhesive. Thus, one cannot make similar form hydrogen bonds in the amide region, and the polymers with and without piperazine and expect to presence of low-polarity domains in the fatty region play understand the effect on adhesion at the interface. Those a definite role in good adhesion. Dimer-based poly- polymers would have such different physical properties, amides have served well in the shoe industry, but the such as softening point and strength, that the compar- introduction of imitation leather made from vinyl led to ison would not be meaningful in an investigation of the the need for new polyamides since the dimer-based polyamides at that time did not bond well to vinyl. A The original patent by Peerman and Vertnik [4] only patent by Peerman and Vertnik [4] provided examples of described which polyamides gave good bonding to vinyl, polyamides that d o and do not bond well to vinyl. The but it did not propose any theory as to why these data in this patent showed that piperazine-derived polyamides were unique in their performance. Some polyamides provide good bonds to vinyl, while those properties of piperazine-derived polyamides were de- derived from dimer diamine, 1,4-(bisaminoethyl) ben- scribed in a recent paper [5], but the unique bonding zene, and 1,4-cyclohexyl bismethylamine do not (Table character of these polymers was not discussed. All the 1). Dimer diamine is a primary diamine made from hot-melt polyamides described in the literature that dimer acid by reaction with ammonia, followed by bond well to vinyl are made from piperazine or similar dehydration and reduction to the dimer diamine. In type compounds (Fig. 1). Hot-melt poly(ester-amides) addition, polyamides that are derived from linear that bond to vinyl [6] will be discussed later in this diamines such as ethylenediamine and hexamethylene- The Peerman and Vertnik patent [4] showed that polyamides made from cyclic disubstituted diamines 3. Adhesion models
provide the characteristic good bonding to vinyl. The two amines that were used extensively in this patent The standard adhesion models can be classified by were piperazine and 1,3-di-(4-piperidyl)propane (dipip). mechanism as either mainly mechanical or chemical in Note that those polyamides derived from piperazine and nature. Mechanical interlock usually depends on the dipip have much higher levels of linear codiacid, such as roughness of the surface. Chemical models depend on sebacic acid, azelaic acid, and terephthalic acid, than do the type of interaction between the atoms present in most other dimer-based polyamides. With most dimer- both the substrate and adhesive. Both types of bonding based polyamides, the hydrogen bond between the depend on good wetting of the surface, which, in turn, amide groups makes the materials solid and gives them requires that the surface energy of the adhesive be near strength. When hydrogen bonds are absent, as in the or, preferably, less than that of the substrate. In case of piperazine- and diol-based homopolymers, the addition, dynamic wetting is critical for mechanical resulting polymers are liquid or semi-solid. Thus, for interlock during the application of a hot-melt adhesive useful adhesives, piperazine-derived polymers need a significant amount of monosubtituted diamines and Dynamic wetting involves viscosity as well as a linear codiacids to give the product strength. In typical surface energy term [7]. As expected, the lower the polyamides, with a dimer acid and linear diamine, the viscosity the better the wetting. Because the Peerman softening point is near 100°C, and the addition of and Vertnik patent [4] does not show a correlation C. R. Frihart / International Journal of Adhesion & Adhesives 24 (2004) 415-422 Fig. 1. Hydrogen bonding in polyamides and to vinyl between adhesive strength to vinyl and viscosity of based polyamide is based on the dimer fraction, which polyamides, we can assume that dynamic wetting is not often constitutes 60% to 80% of the weight of the final likely the most important factor in these experiments. product. The dimer acid is composed of a large number Moreover, polyamides bond to smooth as well as to of components, whose structures include a multitude of rough vinyl substrates. Thus, because a mechanical branched acyclic, unsaturated cyclic, aromatic, and interlocking model does not explain adhesion very well, bicyclic species. Thus, the dimer portion contributes a we need to consider a chemical interaction model. very amorphous and non-polar character to the poly- A number of models can be proposed to explain the mer. Moreover, dimer-based polyamides usually involve specific adhesion related to the chemistry and physical a 9- to 12-carbon diacid feed to raise the softening point properties of polyamides. These models are divided into and impart the strength of a more crystalline nylon. three classes: rheology, solubility parameters, and acid- Polyamides are often made from two or more diamines base interaction. Under rheology, the concepts involve to impart strength, flexibility, and sufficient open time. the flow of the hot melt or its rate of setting to allow Linear aliphatic diamines, such as ethylenediamine and better wetting of the vinyl surface or adhesive with a hexamethylenediamine, impart hardness and strength. modulus more similar to that of vinyl to better distribute Longer chain diamines, such as dimmer diamine, the applied stress. The solubility parameter concept is provide flexibility as a result of fewer amide groups in that the polarity of vinyl-bonding polyamides is closer the resulting polyamide. Certain diamines, such as to that of vinyl than is the polarity of non-vinyl-bonding polyether diamines, can provide good low temperature polyamides. The third model embraces an acid-base properties. Where one diamine is piperazine or a similar concept, which involves hydrogen bonding. The validity diamine, the polyamide is softer as a result of fewer of each model is examined on the basis of the available First, we need to better understand what makes up a typical hot-melt polyamide. Because of the commercial 4. Rheology
importance of these resins, most technology is in the patent literature. Thus, for many of these formulations, Rheological factors play a role in many stages of the there is compositional information, but data on proper- bonding process. Since an adhesive needs to be liquid for ties are limited. Most of the mass of a typical dimer- application, its viscosity/temperature profile needs to be C. R. Frihart / International Journal of Adhesion & Adhesives 24 (2004) 415-422 proper for good wetting of the substrate. The proposals available facts do not agree with the dynamic wetting are that piperazine-derived polyamides give good specific adhesion by controlling the flow of the molten The other rheological factor is related to properties of polymer or by giving the adhesive a modulus similar to the solid adhesive rather than the molten adhesive. that of vinyl for minimizing stress localization. For these Researchers have observed that bonds are often stronger explanations to be valid, non-vinyl bonding polyamides if the modulus of the adhesive is similar to that of the substrate. The rationale is that less stress is concentrated The wetting starts as the molten adhesive is applied to at the interface if the adhesive and substrate are not the surface of the vinyl or as the adhesive film between highly different in modulus. A stiffer adhesive would two vinyl layers is melted by heating of the vinyl cause greater stress at its interface with the substrate as substrate. In wetting the substrate, it is important to fill the substrate is flexed, and the greater stress could the irregularities that occur on almost all surfaces [8]. A exceed the bond strength of the adhesive to the number of equations have been developed for filling the substrate. Since piperazine incorporation reduces the pores on the surface, but all of these contain both internal hydrogen bonding of polyamides compared to viscosity and surface tension parameters. The first the use of mono-substituted diamines, these polyamides parameter involves the viscosity of the molten adhesive. generally have lower moduli. The hypothesis is that the It is not surprising that the lower the viscosity, the lower moduli of piperazine-derived polyamides are greater the flow into the pores, especially as the cross- closer to that of vinyl. Although most piperazine- sectional area of the pore decreases. The data in the derived polyamides have lower moduli than many other Peerman and Vertnik patent [4] do not show any polyamides, some non-piperazine-derived polyamide correlation between vinyl bond strength and viscosity formulations have lower moduli and do not bond to for polyamides that do and do not bond vinyl. In vinyl [9]. In fact, polyamides from dimer diamine have addition, many commercial polyamides have low moduli and elongations similar to those of piperazine viscosities but do not bond vinyl [9]. Surface tension is polyamides, but they show very low vinyl bond strength. a bit harder to evaluate directly. However, surface In addition, flexibility (modulus) of vinyl is dependent tension is related to the solubility parameter. In the on the amount of plasticizer added. The flexibility of following section, the data show that solubility para- vinyl does not influence which polyamides bond to vinyl meters do not relate well to vinyl-bonding ability. Thus, and which do not. Thus, low stiffness of the polyamide neither viscosity nor surface tension properties are does not appear to play a major role in this specific different enough between vinyl- and non-vinyl-bonding polyamides to be a reasonable explanation for the difference in performance. A second stage of wetting occurs as the molten adhesive is cooling into a solid. Since the initial temperature of the adhesive is high for only a short Solubility parameters have found wide use since they time, there is insufficient time for the adhesive to flow explain the solubility of the polymers and oligomers in into all the pores as could be expected under equilibrium solvents, polymer miscibility, and wetting of substrates temperature conditions. It is known that not all dimer- for inks, coatings, and adhesives [12]. These parameters based polyamides have the same open time, i.e., the time measure cohesive energy density and therefore are an for the adhesive to become solid [7,10]. The longer the indication of the interaction of a material with itself and open time the more likely the polyamide is to wet the other materials [13]. Solubility parameters have also surface prior to solidification. The proposal is that the been related to bond strength [14]. In this case, the greater open time of piperazine-derived polyamides solubility parameter model predicts that the lower allows better wetting of the vinyl prior to setting. polarity of piperazine-derived polyamides is closer to However, plastics are less sensitive to open time than are that of vinyl than is the polarity of non-piperazine- metals due to the lower rate of heat transfer in plastics derived polyamides, which results in good bond [ 11]. Both piperazine- and non-piperazine-derived poly- amides have good adhesion to metals, while only Acetamide and N,N -dimethylactamide could be mod- piperazine-derived polyamides have good adhesion to el compounds for piperazine- and non-piperazine- vinyl. In addition, the Peerman and Vertnik patent derived polyamides. As shown in Table 2, the lower states that vinyl bonding occurs independent of whether cohesive energy data for dimethylacetamide can be due the polyamide is applied as a molten liquid to unheated to reduced hydrogen bonding, since dimethylacetamide vinyl or as a film with both the adhesive and vinyl is only a hydrogen bond acceptor and not both an being heated to form a bond. The latter method should acceptor and donor like acetamide, or to the more not be dependent on the open time of the adhesive since aliphatic character of dimethylacetamide. By using the the substrate is not cooling the adhesive. Thus, the data for N -ethylacetamide, we can sort out the C. R. Frihart / International Journal of Adhesion & Adhesives 24 (2004) 415-422 Table 2 Solubility parameters for model acetamides aHildebrand values from Chapter 8, and 6 of Ref. [12] bHoy solubility parameters calculated according to Ref. [15]. Calculated solubility parameters of polaymides Ethylenediamine, 1,3-di (4-piperidyl) propane Ethylenediamine, N -aminoethyl-piperazine Ethylenediamine, 1,3-propanediamine, 1,4-cyclohexanedimethanol aHoy solubility parameters calculated according to Ref. [15]. contributions of both the aliphatic nature and hydrogen that the dimer used to make polyamides consists of a bonding. The less polar nature of dimethylacetamide large number of individual structures. For our calcula- compared to ethylacetamide shows that less hydrogen tions, we used the weight percentage of branched bonding lowers the Hildebrand and hydrogen bonding acyclic, unsaturated cyclic, aromatic, and bicyclic parameters. These data give credence to the solubility species as previously determined [16]. Although this parameters as a reasonable model for better wetting of assumption could throw off the absolute value, we used the values to compare on a relative basis the difference The solubility parameter (6) for liquids can be between piperazine- and non-piperazine-derived poly- measured directly by one of several methods. However, amides. Thus, we mainly looked for the difference in the the main methods are indirect for solids. They consist of contribution of the amide functional groups. contact angles with a variety of liquids of different Differences in solubility data for several representa- surface tensions, solubility data in different solvents, or tive polyamides calculated by the Hoy method were the use of calculation methods. The calculation methods generally small (Table 3). A main reason for the small involve group contribution methods. One widely used difference between different polyamides is that the dimer method involves the factors derived by Hoy [15]. As is a dominant part of the molecule on a weight basis so shown in Table 2, these calculations indicate that the that it dilutes the effect of the functional groups. transition from N -ethylacetamide to N,N -dimethylaceta- Piperazine-derived polyamides are similar or in some mide lowers total (&), hydrogen bonding (&), and polar cases actually more polar by these calculations than are ( d P) solubility parameters but has a smaller influence on polyamides made with dimer diamine and ethylenedia- the non-polar solubility parameter ( d NP). This is in mine or just ethylenediamine, because polyamides need agreement with the measured data for these compounds, more sebacic or azelaic acid to have good strength and giving credence to the utility of solubility parameters for creep resistance. The use of more of the shorter chain rationalizing the difference in adhesion of polyamides. diacids with piperazine- or dipip-derived polyamides There are a number of difficulties in applying the raises the number of amide groups and the solubility group contribution methods to polyamides. The first is parameters. In addition, the solubility parameter of the C.R. Frihart / International Journal of Adhesion & Adhesives 24 (2004) 415-422 polyamide made from just ethylenediamine and dimer (Fig. 1, H-2 hydrogen bond). However, at high enough acid is close to that of vinyl. Thus, the solubility levels of piperazine, the piperazine-derived amides will parameter data do not provide any support to the seek external hydrogen bond donors. In this case, the theory that piperazine-derived polyamides have solubi- hydrogen next to the chloride in vinyl can be the donor lity parameters closer to that of vinyl. Some of the [19], although there are other explanations for the polyamides not derived from piperazine bond low interaction of carbonyls with polyvinyl chloride (PVC). energy plastics, such as polyethylene and polypropylene, The use of sufficient secondary amines leads to a as well as-and sometimes better than-those made polyamide that is seeking proton donors, and substrates, such as vinyl, can be the source of these donating Note that there are a number of different systems for groups, as in the H-3 hydrogen bond in Thus, breaking down total solubility into individual compo- this model is consistent with the data showing that only nents. One of these alternative systems could explain certain polyamides bond to vinyl because of a specific specific interactions, but most do not contain all the functional groups necessary for doing this type of The question can be raised as to why polyamides calculation. Our solubility study did not show any made from primary amines that form secondary amides dramatic difference in the solubility of polyamides that with a proton do not participate in hydrogen bonds with bond to vinyl and those that d o not [unpublished data]. vinyl. These polyamides probably do participate in The bulk of the data do not support the use of solubility hydrogen bonds with vinyl to a small degree, but the parameters to explain specific adhesion results. secondary amide would rather form strong bonds to other amide groups (see H-1 hydrogen bond in Fig. 1) than form weaker bonds to vinyl. Amides from primary 6. Acid-base and hydrogen bonding interaction
amines can bond as well to the esters that are used as plasticizers for vinyl, which would not form a strong The third chemical model involves acid-base and bond between polyamide and vinyl. On the other hand, hydrogen bond concepts. The use of acid-base concepts amides from secondary amines can bond only to vinyl. in relation to adhesion has been investigated [17]. As a result of the strong internal hydrogen bond, Stronger interactions are expected if the substrate is an polyamides from primary amines are energetically electron donor and the adhesive an electron acceptor or unfavorable for forming a miscible phase with vinyl. vice versa. In this case, adhesive attraction should be On the other hand, tertiary polyamide chains from greater than cohesive attraction. However, if the polar secondary amines do not have the strong internal groups are tied up in cohesive attraction when the hydrogen bonds; thus, they are more free to form a adhesive is both an electron donor and acceptor, then miscible phase with vinyl. Since polyamides made from the polar groups are less free to participate in adhesive greater piperazine contents bond to vinyl strongly, it is most likely that a greater number of the bonds bear the The properties of polyamides strongly depend on internal hydrogen bonding [18]. This is especially The definition of an adhesive is the ability of a important for dimer-based polyamides since the large material to hold two substrates together under some number of structures for the dimer portion lead to an force. This ability is obviously dependent on the amorphous polymer that should have little of the adhesive coming into close contact with the substrate. crystallinity that controls many properties of structural If the surfaces are held together by chemical attraction, nylons. On the other hand, dimer-based polyamides then the strength and number of the individual chemical have primary amide hydrogens and carbonyl groups bonds will determine the overall strength at the that should be quite free for hydrogen bonding. The two interface. A normal polyamide forms few hydrogen fatty acids are tied together at about the 9-12 carbon bonds with vinyl because the amide tends to bond atoms and have little or no branches or cyclic structures internally within the polyamide adhesive. On the near the amide groups. Consequently, the amide groups other hand, if the polyamide has significant tertiary are free for hydrogen bonding, but the branching and amide groups, a number of amides cannot bond cyclic structures hinder the normal stacking of poly- internally and will therefore be involved in external chemical bonds with vinyl. A large number of these On the other hand, polyamides that are made from weaker hydrogen bonds can result in a strong bond piperazine amide groups can only be hydrogen bond overall. There is a limit to this approach for strengthen- acceptors since there is no proton for donation on the ing the bond. As the amount of internal hydrogen tertiary amides. If vinyl-bonding, piperazine-derived bonding in the polyamide decreases, stronger bonds may polyamides have many more moles of ethylenediamine form to vinyl but the polyamide has less cohesive than of piperazine, the piperazine amide can generally strength, so the failure mode changes from adhesive to form hydrogen bonds with the ethylenediamine amide C.R. Frihart / International Journal of Adhesion & Adhesives 24 (2004) 415-422 Polyamides made from the less polar 2,5-dimethylpi- interdiffusion occurs on the molecular level. Chemical perazine showed lower strength than polyamides from theories depend on the atoms and functional groups that piperazine or dipip (Table 1). This supports the interact between adhesive and substrate. For all of these interaction of the amide with vinyl as being the site of theories of adhesion the adhesive is assumed to have adhesion. It would be expected that adding methyl sufficient wetting properties to develop contact at the groups would hinder the amide by weakening its bond molecular level contact, at least in spots. Hot-melt to vinyl. The use of N -aminoethylpiperazine also polyamides made from primary diamines do not form reduced bond strength because some amide groups strong bonds. The data do not fit for explanations from secondary amines were replaced by those from involving mechanical bonding, rheology, and solubility primary amines. This is again consistent with the ratio parameters. The mechanical interlock mechanism is not of internal versus external hydrogen bonding for likely on the bases that adhesion does not depend on the roughness of the vinyl surface and that hot-melt In summary, polyamides made from secondary adhesives do not penetrate the roughness well because diamines have several advantages over those from of their high molecular weight and fast solidification. primary amines for bonding to vinyl. First, surface Vinyl- and non-vinyl-bonding polyamides are not amide groups without hydrogen can bond only to vinyl. distinct based upon either their flow properties or their On the other hand, amides from primary amines can moduli, as would be required by two rheological models. bond both to vinyl and to the ester plasticizer, with the Calculated solubility parameters are not able to latter mode not contributing to adhesive strength. distinguish polyamides on the basis of whether or not Second, piperazine-derived polyamides have longer they bond to vinyl. However, a model based on acid- segments without hydrogen bonds than do ethylenedia- base interaction between the vinyl and adhesive fits with mine polyamides, which are free to interdiffuse with the available data. This model relies on the reduced vinyl to form a type of mechanical interlock. Thus, this internal bonding of the polyamide for the amides made model is consistent with the available data. from secondary diamines compared to those made from primary diamines. Those tertiary amide groups that are not tied up in internal bonds are free to be proton 7. Poly(ester-amides)
acceptor groups for bonding to the proton-donating vinyl resins. This model, in which free proton acceptors As discussed in the previous text, polyamides that are required for bonding to vinyl, not only explains the bond to vinyl are made from secondary diamines, such adhesion of polyamides made from secondary diamines as piperazine and dipip. In the literature, the only other but also fits with the adhesion to vinyl for poly(ester- vinyl-bonding, hot-melt, dimer acid-based polymers are poly(ester-amides). These polymers are made from diols The acid-base interaction model explains the here- and diamines reacted at the same time with the dimer tofore unexplained specific adhesion to vinyl of acid and codacid. This generates polymers consisting of polyamides made from secondary diamines compared random ester groups and amide groups that may or may to the lack of adhesion of polyamides made from not contain a proton, depending on the diamine used. primary diamines. This is not meant to imply that As discussed, for adhesion to vinyl, the most suitable rheological and cohesive energy density factors are model involves the bonding of the tertiary amide groups not important for good adhesion. Rather, these factors to the hydrogen alpha to chlorine on vinyl. The ester are unsatisfactory explanations of the improved adhe- group can bond in a similar manner with the ester sion observed with polyamides that are made from group, replacing the piperazine-derived portion of the polyamide. In the literature, the poly(ester-amides) derived from a primary diamine and a diol gave values of 4.6-24.2N/cm for peel strength to vinyl [6]. These data support our model for the relationship of vinyl References
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