STRUCTURE REINFORCED LATEX PARTICLES AND PROCESS FOR THE PREPARATION THEREOF

Abstract: A process for preparing a structure rein­ forced latex particle whose film has improved tensile and elongation properties prepared by an emulsion polymerization comprising a three-stage monomer feed addition. The second monomer feed is an amount from 5 to 30 parts per hundred parts monomer in said latex particle and is a polymer of which would have a T g greater than either the first or third monomer feed. The subject structure reinforced latex particle pro­ vides a film having improved tensile properties without a corresponding reduction in elongation. A pressure sensitive adhesive formulated with the structure rein­ forced latex particle is also provided.

Description: The present invention is directed towards the preparation of a latex having high tensile strength without a loss in elongation. This improvement in physical properties results in a latex suitable as an adhesive having high shear adhesion without a loss of peel adhesion. In the adhesive industry there is a great desire for adhesives having a balance of shear and peel adhesion. Unfortunately, shear adhesion is contributed by the tensile property of the latex film and peel adhesion is contributed by the elongation property of the latex film. Tensile and elongation are antagonistic properties and; therefore, increasing one tends to decrease the other. In order to cope with this problem, various methods for preparing latexes have been attemp­ ted. One method, and perhaps most common, is to copolymerize various monomers to obtain the benefit of the individual monomer components. Another polymer­ ization method is to stage the addition of monomers to form structured latex particle having diverse proper­ ties. While either method has produced latexes having properties acceptable for adhesives, especially pressure sensitive adhesives, it is still desirable to increase either shear or peel adhesion without sacrificing the other. The present invention is a process for pre­ paring a structure reinforced latex particle whose film has improved tensile and elongation properties prepared by an emulsion polymerization wherein a first monomer feed (a) is polymerized to form a core region, a second monomer feed (b) is polymerized in the presence of polymerized feed (a) and a third monomer feed (c) is polymerized in the presence of polymerized feed (a) and polymerized feed (b) to form a shell region, the T g of the polymer of (b) is greater than the T g of either of the polymer of (a) or (c), characterized in that the first monomer feed (a) comprises from 10 to 30 parts per 100 parts total monomer, the second monomer feed (b) comprises from 5 to 30 parts per 100 parts total monomer, and the third monomer (c) comprises from 40 to 70 parts per 100 parts total monomer of the latex particle, and further characterized in that the polymerization is conducted in the absence of a crosslinking agent. The T g of the polymer formed from the (b) monomer feed is generally at least 25°C. In one aspect, the polymers formed from the monomer feeds (a) and (c) have a similar T g or may, in fact, comprise the same composition. A typical monomer feed for (a) and (c) is a mixture of styrene and butadiene. In yet another aspect of the process, the monomer feed (b) can be an amount from 15 to 25 parts per hundred parts monomer in the latex particle. The monomer feed (b) can be styrene or a mixture thereof whose polymer has a T g greater than 25°C. The addition of the monomer feeds (a), (b) and (c) can be made in a continuous feed manner. EP-A- 136 552 describes a multi-layer graft copolymer ( 1) having at least 3 layers (A), (B) and (C)- which correspond to parts (c), (b) and (a) of the present invention. This copolymer (1) is not intended to be used as a latex. It is intended to be used as an impact modifier for plastics, after it has been blended with another copolymer (2), coagulated, washed, dried and then melt-blended into plastics. The copolymer (1) is not indicated to be film-forming. Example 1 describes the preparation of a multilayer graft copolymer (1) : 20 parts (C) (n-BA/triallyl isocyanurate)/50 parts (B) (MMA/triallyl isocyanurate)/30 parts (A) (n-BA/triallyl isocyanurate), that cannot be a good film-forming latex. Thus, the copolymer described in this document cannot produce a film having good tensile and elongation properties. FR-A- 2 239 488 describes in Example II a latex particle composed of an agglomerated butadiene/n-butyl acrylate copolymer core (50 parts) grafted with three layers of methyl methacrylate (12.5 parts), styrene (25 parts), and methyl methacrylate (12.5 parts). This latex is not a film former. Even if this latex is dried at high temperatures to form a film, the resulting film would not have any elongation and would be extremely brittle. Unlike the copolymers of the subject invention, the copolymers described in the 1st document are not good film-formers and those described in the 2nd document are not film-formers at all. In addition, both of the cited documents relate to multilayer copolymers wherein the layers are grafted to each other by means of a small amount of a crosslinking agent, such as the triallyl isocyanurate employed in the above-mentioned Example 1 of EP-A-′ 552. On the other hand, the structured latex produced by the process of the subject invention is not a grafted copolymer; it does not require the presence of a crosslinking agent to graft the layers of the multi-layer structure to one another. The present invention also provides for a structure reinforced latex particle whose film has improved tensile and elongation properties prepared by the process described above. In yet another aspect, the present invention provides for a pressure sensitive adhesive comprising structure reinforced latex particles having improved tensile and elongation properties prepared by an emul­ sion polymerization process, as described above, and a tackifier. Generally, the tackifier comprises from 1 to 90 percent by weight on a solids basis in the pres­ sure sensitive adhesive formulation. The process of the present invention allows for the preparation of a latex particle having increased tensile properties without a proportional decrease in the elongation properties. Advantageously, the tensile and elongation characteristics of the subject structure reinforced latex particle allows for the preparation of a pressure sensitive adhesive having desirable shear and peel adhesion strengths. Figure 1 depicts the stress-strain curve of a structure reinforced latex film and a comparative homogeneous latex film. The latexes of the subject invention are prepared by a three-stage polymerization process which provides good tensile properties without a corresponding reduction in elongation properties. In the area of adhesives, this characteristic provides excellent shear and peel strengths. The three-stage latex is also suitable for applications as a pressure sensitive adhesive. Three-stage process is meant to describe a polymerization process where a first monomer feed whose polymer has one transition temperature (T g ) is fed into the reactor followed by a second monomer feed whose polymer has higher T g and, finally, a third monomer feed, similar to said first monomer feed, is added to the polymeriza­ tion system. The polymerization process can be either staged, i.e., each monomer feed individually added and polymerized prior to addition of the subsequent monomer feed or a continuous feed process. The continuous feed process involves the incremental addition of each monomer feed such that as one monomer feed is reduced the subsequent monomer feed is increased to provide a smooth transition between various monomer feeds. The reinforcing portion of the subject reinforced structured latex particles is contributed by the second monomer feed in the present three-stage polymerization. The second monomer feed, whose polymer has a T g higher than the polymers formed by the first and third monomer feeds, is generally referred to as the hard monomer. Generally, the polymer of the hard monomer has a T g in excess of that temperature for which the latex is to be employed. Therefore, in a pressure sensitive adhesive (hereinafter, PSA) the polymer of the hard monomer feed would have a T g greater than 25°C. The second monomer feed, or hard monomer portion of the latex particle, reinforces the first and third monomer portions to provide the novel balance of tensile and elongation properties. The reinforcement of the first and third stage monomer feeds is observed when the second monomer feed comprises from 5 to 30 parts per hundred parts total monomer (hereinafter, phr) of the latex particle. More preferably, the second mono­ mer feed or hard monomer comprises from 10 to 25 phr. Typical candidates for the second monomer feed, or hard monomer, are vinyl aromatic monomers whose polymers have a T g greater than 25°C, such as styrene and copolymers thereof. However, homopolymers or copolymers having a T g higher than those of the polymers of the first third stage monomer feeds and higher than the tempera­ ture at which the latex is to be employed are suitable. Other suitable hard polymers are styrene/butadiene copolymers, styrene/isoprene copolymers, methylmeth­ acrylate copolymers, copolymers or terpolymers of methylmeth­ acrylate with ethylacrylate, butylacrylate, 2-ethylhexylacrylate, sec-butylacrylate or butadiene, copolymers or terpolymers of acrylonitrile with ethyl­ acrylate, butylacrylate or butadiene. The first and third stage monomer feeds, or soft monomers, are generally of the same composition; however, variation in monomer content and monomer type is suitable provided the T g of their respective polymers does not exceed that of the polymer of the second monomer feed. The respective polymers of the first and third monomer feeds, or soft monomers, have a T g less than that of the polymer of said second monomer feed or hard monomer. Typically, the polymer of the soft monomer has a T g less than 25°C. Suitable soft polymers can include styrene/butadiene copolymers having a T g of less than 25°C, copolymers or terpolymers of styrene with isoprene, butylacrylate or 2-ethylhexyl­ acrylate, copolymers or terpolymers of acrylonitrile with butadiene, isoprene, butylacrylate, 2-ethylhexyl­ acrylate, sec-butylacrylate, ethylacrylate and other soft polymers having a T g less than the polymer of the second stage monomer feed. The first and third monomer feeds comprise the remainder of the latex particle, i.e., that por­ tion not formed by the reinforcing second monomer feed. Generally, the first monomer feed comprises from 10 to 30 parts per one hundred parts of total monomers in the latex particle and the third monomer feed comprises from 40 to 70 parts per one hundred parts of total monomers of the latex particle. The polymerization of the three monomer feeds is conducted pursuant to conventional emulsion polymer­ ization techniques. Thus, for example, the monomers to be employed for the particular stage involved are typically dispersed with agitation sufficient to emulsify the mixture in an aqueous medium which may contain known emulsifying agents (i.e., surfactants) as well as other ingredients conventionally employed in the art as polymerization aids (e.g., conventional chain transfer agents, etc.). Such monomers are then subjected to polymerization with the aid of a conventional source for generating free radicals, such as conventional free radical polymerization catalyst, activating radiation, etc. Optionally, conventional seeding procedures can be employed in the first stage polymerization to aid in the control of such polymerization and to achieve the desired average particle size and particle size distribution for the dispersed first stage copoly­ mer particles. Also, as has been noted, conventional chain transfer agents can be employed in the practice of the present invention and, indeed, in polymerization stages employing aliphatic conjugated dienes it is preferable to do so. Examples of such conventional chain transfer agents include bromoform, carbon tetra- chloride, long chain mercaptans (e.g., dodecylmercap­ tans, etc.), or other known chain transfer agents. Conventional amounts in general, from 0.1 to 10 weight ­ percent based on total monomer charge of such chain transfer agents are typically employed in such pre­ ferred embodiments. In addition, it is sometimes desirable to have small amounts of certain known additives incorporated into the latex. Typical examples of such additives are surfactants, bac­ teriocides, neutralents, anti-foamers, etc. Such additives can be incorporated into the latex of the invention in a conventional manner and at any con­ venient point in the preparation of such latexes. The reinforced latexes of the present invention are suitable for use in a variety of applications such as in carpet backsizing, as binders in paper coating compositions, as binders in other compositions, as adhesives for binding together various types of substrates as free films, as film-forming components for protective or decorative coatings, in general paints and the like. The subject reinforced latexes are particularly desirable for use as pressure sensi­ tive adhesives. The pressure sensitive adhesives (PSA) are adhesive materials, which when dried, possess a lasting aggressive tack which enables then to tena­ ciously adhere to a wide variety of substrates when applied with only a light pressure. These adhesives are useful in a variety of applications including tapes, labels, wall coverings, floor tiles, and wood veneers. The subject reinforced latex composition is particularly suitable for use as a pressure sensitive adhesive in view of the excellent shear and peel adhesion properties. Generally, when employing the subject latex composition as a pressure sensitive adhesive, it is desirable to include a tackifying resin emulsion or tackifier. Tackifying resins useful in the preparation of pressure sensitive adhesives are com­ mercially available and are generally disclosed in U.S. Patent No. 4,189,419. Typically, the resins comprise emulsified resin, partially decarboxylated rosin, glycerol ester of polymerized rosin, partially dimerized rosin, natural resin, hydrogenated wood rosin, plasticized hydrogen­ ated rosin, aliphatic hydrocarbon resins derived from petroleum, aromatic resins derived from petroleum, terpene resins, coal tar, polyindene resins, ethylene vinylacetate copolymer resins, terpene phenolics, coumarone-indenes, rosin esters, pentaerythritol esters, polydicyclobutadiene resins. The properties of the PSA composition can be varied for a particular application by the selection of an appropriate tacki­ fying resin. Naturally, the particular tackifiers employed can contain conventional additives, such as softeners, plasticizers, antioxidants and the like which can be emulsified along with the tackifying resin or emulsi­ fied separately and mixed with the tackifying resin emulsion. Tackifiers are desirably added in an amount such that the latex is given additional tack (quick stick and peel adhesion) without detracting unduly from the shear adhesion. Suitable formulations will typ­ ically have from 1 to 90 percent, preferably 20 to 80 percent, and more preferably, from 30 to 50 percent by weight on a solids basis tackifier in the formulation. Typically, the PSA composition can be pre­ pared by blending the desired amounts of reinforced latex and tackifier in any conventional manner. It is understood that no requirement or limitation to the scope of the invention is intended as to how the latex and tackifier components are combined. The following examples are provided to illus­ trate specific embodiments of the present invention. Example 1 A three-staged structure reinforced latex was prepared as follows. All measurements are in parts per one hundred parts total monomer unless otherwise indi­ cated. The monomeric charges were added to an initial mixture consisting of 55.99 parts deionized water, 3.5 parts of a polystyrene seed latex to produce a latex of 1000 Å (0.1µm). The initial mixture was agitated and the temperature was maintained at 90°C. To this ini­ tial mixture an aqueous charge was added at time zero and continued until time 270 minutes consisting of 200 parts deionized water, 0.5 part of 45 percent solution of Dowfax®-2A1 surfactant (trademark of The Dow Chemical Company for sodium dodecyl diphenyl oxide disulfonate) and 0.7 parts of sodium persulfate. From time zero to 54 minutes, the first monomer feed consisting of 5.6 parts styrene, 14 parts butadiene, 0.45 part t-dodecyl mercaptan and 0.6 part carbon tetrachloride was added. The second monomer feed consisting of 20 parts styrene was commenced at 54 minutes and continued until time 107 minutes. The third monomer feed was commenced at time 107 minutes and continued until time 270 minutes. The third monomer feed consisted of 16.4 parts styrene, 42 parts butadiene, 0.35 part t-dodecyl mercaptan and 1.8 part carbon tetrachloride. In addition, at time 15 minutes and continu­ ing until time 210 minutes, a second aqueous charge consisting of 20.15 parts deionized water, 0.14 part of a 10 percent solution of sodium hydroxide and 2.0 parts itaconic acid was continuously added to the reaction mixture. At time 270 minutes all additions were com­ pleted and the mixture was cooked down for 60 minutes. The solids were collected, latex films prepared and the physical properties measured. The results of these measurements are shown in Table I. A conventional homogeneous styrene/butadiene latex was also prepared as a comparison. The com­ parative homogeneous latex was prepared by a continuous feed process using 70 parts butadiene, 28 parts styrene, 2 parts itaconic acid, 2.25 parts t-dodecyl mercaptan and 3 parts carbon tetrachloride with all other ingre­ dients and conditions being essentially equal to the three-stage latex preparation. Naturally, the struc­ ture reinforced latex formulation contains more styrene than the homogeneous latex due to the intermediate charge of reinforcing styrene. Essentially, the structure reinforced latex as prepared above consists of a first monomer charge equivalent to 20 percent of the comparative homogeneous latex monomer feed, fol­ lowed by 20 percent reinforcing styrene monomer charge, and finally a third monomer charge equivalent to 60 percent of the comparative homogeneous latex monomer feed. The physical properties for the film prepared from the homogenous latex are included in Table I as the comparative homogeneous latex. The adhesive properties of the latexes were measured according to Pressure Sensitive Tape Council (PSTC) Procedure No. 1, 5, and 7. The peel adhesion test involves peeling the tape off at a 180 degree angle after application under relatively heavy pressure. The quick stick test involves peeling the tape off at a 90 degree angle after application under a relatively light pressure. Both tests are reported as the force required to remove the tape. Shear adhesion is measured according PSTC No. 7 using a 500 gram (g) mass at a 120°F (49°C). In this test, a shear force 2 degree angle is applied in an oven. The results are reported as the time required for the bond to fail. The physical data above shows an increase in tensile properties without a corresponding loss in elongation for the structure reinforced latex. The gain in tensile is demonstrated by the excellent improve­ ment in shear adhesion with no loss in peel adhesion when compared to the homogeneous latex. The stress-strain curves for films formed from the structure reinforced latex and comparative homogeneous latex of Example 1 are depicted in Figure 1. Figure 1 indicates the improved stress properties of the subject structure reinforced latex while maintaining good elongation comparable to the homogeneous latex. Generally, one would expect an increase in tensile stress, i.e., to cause an increase in the slope of a line graphing stress versus elongation. Table I shows the peel adhesion (a function of elongation) and shear adhesion (a function of tensile) for the structure reinforced latex of Example 1 versus the comparative homogeneous latex. The latexes are shown neat and with varying tackifier levels. Pressure Sensitive Tape Council Test Nos. 1 and 7 were employed for conducting the peel adhesion and shear adhesion tests, respectively. The data indicate that the structure rein­ forced latex particle maintained good peel adhesion while dramatically increasing the tensile related property of static shean. Example 2 A three-staged latex, structure reinforced latex particle, was prepared following the procedure outlined in Example 1. All measurements are in parts per one hundred parts total monomer unless otherwise indicated. The first and third monomer feeds, or soft monomer feeds, were identical and consisted of 56 parts butadiene, 22 parts styrene, 2.2 parts t-dodecyl mer­ captan and 2.4 parts carbon tetrachloride. The monomer feed was added from time zero to 54 minutes and from 107 to 270 minutes to form the soft regions of the structured latex particle. The second monomer feed, or hard monomer feed, consisted of 20 parts styrene and was added from 54 to 107 minutes to form the hard reinforcing region of the structured latex particle. Thus, the total monomer feed for the structure reinforced latex con­ sisted of 20 percent soft monomer followed by 20 per­ cent reinforcing styrene and finally 60 percent soft monomer feed. All other polymerization conditions were similar to Example 1. For comparison purposes a homogeneous latex was prepared by a continuous feed process as described in Example 1 but with increased chain transfer agent, i.e., 2.75 parts t-dodecyl mercaptan. Each latex was tested neat and with varying tackifier levels as in Example 1 for peel adhesion and shear adhesion. The values are reported in Table II. The values indicate that the structure rein­ forced latex particle had markedly improved tensile related property of static shear while maintaining approximately equal elongation related property of peel adhesion. It is also noted that the increase in chain transfer agent between Examples 1 and 2 has softened the latex particle which has accordingly resulted in lower tensile related values. Example 3 A three-staged latex, structure reinforced latex particle, was prepared as in Example 2 except that the amount of chain transfer agent present in the first and third monomer feeds, or soft monomer feeds, was increased from 2.2 parts t-dodecyl mercaptan to 2.6 parts. All other additions and monomer feed sequences were similar to Example 2. A comparative homogeneous latex particle was also prepared having the increased chain transfer agent, i.e., 3.25 parts t-dodecyl mercaptan. Each latex was tested neat and with varying tackifier levels as in Examples 1 and 2 for peel adhesion and shear adhesion. The values are reported in Table III. The values again indicate as in Examples 1 and 2 that the structure reinforced latex has improved tensile related properties of shear adhesion while maintaining the elongation related property of peel adhesion. Also Examples 2 and 3 demonstrate the effect of increasing amount of chain transfer agent which has a tendency to soften the latex particles thus reducing tensile properties.

Claims: 1. A process for preparing a structured reinforced latex particle by emulsion polymerization wherein a first monomer feed (a) is polymerized to form a core region, a second monomer feed (b) is polymerized in the presence of polymerized feed (a) and a third monomer feed (c) is polymerized in the presence of polymerized feed (a) and polymerized feed (b) to form a shell region, the T g of the polymer of (b) is greater than the T g of either of the polymer of (a) or (c), characterized in that the first monomer feed (a) comprises from 10 to 30 parts per 100 parts total monomer, the second monomer feed (b) comprises from 5 to 30 parts per 100 parts total monomer, and the third monomer (c) comprises from 40 to 70 parts per 100 parts total monomer of the latex particle, and further characterized in that the polymerization is conducted in the absence of a crosslinking agent. 2. The process of Claim 1 where a polymer formed from said (b) has a T g of at least 25°C. 3. The process of Claim 1 or 2 where a polymer formed from said monomer feeds (a) and (c) has the same T g . 4. The process of Claim 3 where said monomer feeds (a) and (c) are the same composition. 5. The process of Claim 1 or 2 where said monomer feeds (a) and (c) are a mixture of styrene and butadiene. 6. The process of Claim 1 where said second monomer feed (b) is in an amount from 15 to 25 parts per hundred parts total monomer in said latex particle. 7. The process of Claim 1 where said monomer feed (b) is styrene or a mixture thereof whose polymer has a T g greater than 25°C. 8. A structure reinforced latex particle whose film has improved tensile and elongation properties obtainable by the process of any Claims 1 to 7. 9. A pressure sensitive adhesive comprising: (i) a structure reinforced latex particle having improved tensile and elongation properties prepared by the process of any of Claims 1 to 7; (ii) a tackifier. 10. The pressure sensitive adhesive of Claim 9 where said tackifier comprises from 1 to 90 percent by weight on a solids basis in the pres­ sure sensitive adhesive formulation.

Inventors: ASH, MEZZIE L., LEE, DO IK, MAKATI, ASHOK C.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1986
Main classification: C08F285
IPC codes: C09J151, C08F285, C08F279, C08F2, C08F291