TACKIFIER FOR LACTIC ACID BASED HOT-MELT ADHESIVE

Abstract

A lactic acid based hot-melt adhesive with an amorphous lactic acid oligomer tackifier. A hot-melt adhesive (HMA) includes: a lactic acid and caprolactone copolymer resin, a crystalline lactic acid oligomer wax, and said amorphous lactic acid oligomer tackifier, characterized in that the amorphous lactic acid oligomer tackifier includes an amorphous polymerisation product of a) lactic acid monomer and b) a multifunctional polymerization initiator containing three or more hydroxy and/or amino groups. The use of the amorphous lactic acid oligomer tackifier in a lactic acid-based hot-melt adhesive, to a method for adhering substrates together using a hot-melt adhesive including said amorphous lactic acid oligomer tackifier, and to a specific amorphous lactic acid oligomer tackifier and a method of its preparation.

Claims

1. A hot-melt adhesive (HMA) comprising: a lactic acid and caprolactone copolymer resin, a crystalline lactic acid oligomer wax, and an amorphous lactic acid oligomer tackifier, wherein the amorphous lactic acid oligomer tackifier has a crystallinity as defined by its enthalpy of melting of at most 2.5 J/g and comprises an amorphous polymerisation product of a) lactic acid monomer and b) a multifunctional polymerization initiator containing three or more hydroxy and/or amino groups, wherein the amorphous polymerisation product of the amorphous lactic acid oligomer tackifier has a number average molecular weight (M.sub.n) of 1000 to 10000 g/mol.

2. The hot-melt adhesive according to claim 1, wherein the multifunctional polymerization initiator of the amorphous lactic acid oligomer tackifier contains three or more hydroxy groups.

3. The hot-melt adhesive according to claim 1, wherein the amorphous polymerisation product of the amorphous lactic acid oligomer tackifier has a number average molecular weight (M.sub.n) of 2000-4000 g/mol.

4. The hot-melt adhesive according to claim 2, wherein the multifunctional polymerization initiator of the amorphous lactic acid oligomer tackifier is selected from D-sorbitol, pentaerythritol and dipentaerythritol.

5. The hot-melt adhesive according to claim 1, wherein the lactic acid monomer of the amorphous lactic acid oligomer tackifier is derived from one or more of L-lactic acid, D-lactic acid, L-lactide, D-lactide and meso-lactide.

6. The hot-melt adhesive according to claim 1, wherein the ratio of lactic acid monomer per hydroxy or amino group of the multifunctional polymerization initiator in the amorphous polymerization product is 2-25 moles of lactic acid per moles of hydroxy group or amino group.

7. The hot-melt adhesive according to claim 1 comprising 30-80 wt. % of the copolymer resin, 0.5-30 wt. % of the oligomer wax, and 5-45 wt. % of the amorphous tackifier, based on the total weight of hot-melt adhesive.

8. The hot-melt adhesive according to claim 1, further comprising one or more additional components selected from a nucleator, an anti-oxidant and a stabilizer.

9. The hot-melt adhesive according to claim 1, wherein the copolymer resin is a block copolymer comprising a first block and a second block, wherein the first block is an amorphous copolymer of lactic acid and caprolactone and the second block is a crystalline polymer of lactic acid.

10. An amorphous polylactic acid tackifier for use in a hot-melt adhesive (HMA) according to claim 2, comprising an amorphous polymerisation product of a) lactic acid monomer and b) a multifunctional polymerization initiator containing four or more hydroxy groups, wherein the amorphous polymerisation product has a number average molecular weight (M.sub.n) of 1000 to 10000 g/mol, wherein the amorphous polylactic acid tackifier has a crystallinity as defined by its enthalpy of melting of at most 2.5 J/g and a glass transition temperature of at least 30° C.

11. Method for preparing an amorphous polylactic acid tackifier according to claim 10, comprising forming an amorphous polymerisation product by reacting a) lactic acid and/or lactide and b) a multifunctional polymerization initiator containing four or more hydroxy groups, until a polymerisation product with a number average molecular weight (M.sub.n) of 1000 to 10000 g/mol is obtained.

12. A method comprising adding an amorphous polylactic acid tackifier to a hot-melt adhesive comprising a lactic acid and caprolactone copolymer resin and a crystalline lactic acid oligomer wax, wherein the tackifier and the hot-melt adhesive are as defined in claim 1.

13. Method for arranging substrates in a fixed position with respect to each other, comprising the steps of applying a an amount of a hot-melt adhesive according to claim 1 in liquid form onto a surface of a first substrate, applying a surface of a second substrate onto the amount of hot-melt adhesive composition, and cooling the assembly of substrates and hot-melt adhesive composition to a temperature below the melting point of the hot-melt adhesive composition.

14. Method for arranging substrates in a fixed position with respect to each other according to claim 13, wherein the substrates are elements of a cardboard box.

Description

EXAMPLES

Preparation of Tackifiers

[0109] All oligomer or polymer tackifiers were prepared from monomers as indicated in each of the examples below using a 1 L scale four-necked glass setup reactor with stirring from an overhead stirrer, stannous(II) ethylhexanoate as catalyst, Irgafos® 126 as anti-oxidant and monomeric carbodiimide (Stabaxol® 1) as stabilizer. The polymerization was performed at 180° C. and was stopped after maximum of 3 hours reaction time when the target molecular weight (Mn) was reached.

[0110] The number average molecular weight (Mn) was determined by gel permeation chromatography, using a liquid chromatographer with a C18 column, which discriminates on molecular weight, chloroform as solvent and running phase, polystyrene as a reference and detection via Refractive Index.

Example 1: Sorbitol and Lactic Acid Oligomer, Derived from 100 wt. % L-Lactide with an Mn of 3000 g/Mol

[0111] 751.4 g of L-lactide (Puralact® B3 from Corbion) was polymerized with 48.6 g of D-sorbitol (from Sigma-Aldrich) in the presence of stannous(II) ethylhexanoate, according to the general procedure above. The resulting lactic acid oligomer had an Mn of 3000 g/mol, a crystallinity of below 0.5 J/g, and a Tg of 40.3° C.

Example 2: Sorbitol and Lactic Acid Oligomer, Derived from 88 Wt. % L-Lactide and 12 wt. % D-Lactide and with an Mn of 5000 g/Mol

[0112] 678.4 g of L-lactide (PURALACT® B3 from Corbion) and 92.5 g of D-lactide (PURALACT® D from Corbion) were polymerized with 29.2 g of sorbitol (from Sigma-Aldrich) in the presence of stannous(II) ethylhexanoate, according to the general procedure above. The resulting lactic acid oligomer had an Mn of 5000 g/mol, a crystallinity of below 0.5 J/g, and a Tg of 38.2° C.

Example 3: Melamine and Lactic Acid Oligomer, Derived from L-Lactide and with an Mn of 3000 g/Mol

[0113] 719.1 g of L-lactide (PURALACT® B3 from Corbion) was polymerized with 31.8 g of melamine (from Alfa Aesar) in the presence of stannous(II) ethylhexanoate, according to the general procedure above. The resulting lactic acid oligomer had an Mn of 3000 g/mol, a crystallinity of below 0.5 J/g, and a Tg of 45.4° C.

Comparative Example 1: Neopentyl Glycol and Lactic Acid Oligomer, Derived from 88 wt. % L-Lactide and 12 wt. % D-Lactide and with an Mn of 5000 g/Mol

[0114] 646.3 g of L-lactide (PURALACT® B3 from Corbion) and 88.1 g of D-lactide (PURALACT® D from Corbion) were polymerized with 15.6 g of neopentyl glycol (from Perstorp AB) in the presence of stannous(II) ethylhexanoate, according to the general procedure above. The resulting lactic acid oligomer had an Mn of 5000 g/mol, a crystallinity of less than 0.5 J/g, and a Tg of 36.0° C.

Preparation of Hot-Melt Adhesive Formulations

[0115] Hot-melt adhesives (HMA) were prepared using different tackifiers prepared in Examples 1-3 and Comparative Example 1, as detailed in Table 1. Further a reference formulation without tackifier was preferred and HMA formulations were prepared using commercially available tackifiers including rosin resins (e.g. Permalyn™ 5110 from Eastman and Pineclear™ 2498E from Lawter) and hydrocarbon resins (Eastotac™ H-130W, Picco™ A-100, Kristalex™ F-100 and Regalite™ R1100CG from Eastman). These however were not be successfully prepared given the incompatibility of the tackifiers with the copolymer resin and oligomer wax and therefore were not evaluated further.

[0116] The hot-melt adhesive formulations all contained 48.5 wt. % of copolymer resin, 18.5 wt. % of oligomer wax, 3 wt. % of a nucleator and 30 wt. % of tackifier. The reference formulation without tackifier contained 48.5 wt. % of oligomer wax (instead of 18.5 wt. % of oligomer wax and 30 wt. % of tackifier). The copolymer resin was a lactic acid caprolactone block copolymer prepared according to Example 1 of WO2017/149019 having an Mn of 15000 g/mol and a lactic acid to caprolactone ratio of 78/22.

[0117] The oligomer wax was a crystalline lactic acid oligomer obtained from polymerizing L-lactide (Puralact® B3 with enantiomeric purity of over 99%) as described above for the tackifiers, having an Mn of 2250 g/mol and a crystallinity of greater than 25 J/g.

[0118] The nucleator was a crystalline lactic acid oligomer obtained from polymerizing D-lactide (Puralact® D with enantiomeric purity of over 99%) as described above for the tackifiers, having an Mn of 3500 g/mol and a crystallinity greater than 25 J/g.

[0119] The HMA formulations were prepared in a glass flask with an overhead stirrer wherein the tackifier was molten at a temperature of 163° C. under stirring, the nucleator was added to the molten tackifier and mixed until molten and a uniform mixture was obtained. The oligomer wax was added and mixed until molten and a uniform mixture was obtained. Finally, the copolymer resin was added in small portions and mixed until molten and a uniform mixture was obtained. The mixture was kept at 163° C. under stirring until applied.

Physical Characterization of the HMA Formulations

[0120] The HMA formulations were evaluated by DSC & rheometer measurements.

[0121] DSC was determined using a heat-cool-heat cycle starting at −50° C., heating at 10° C./min up to 220° C., then cooling at the same rate and repeating the first heating step.

[0122] TA Instrument Dynamic hybrid rheometer 2 was used to determine the complex viscosity of the HMA formulations using an angular frequency 10 rad/s and strain of 10% at 163° C.

[0123] The results are displayed in Table 1.

TABLE-US-00001 TABLE 1 DSC Viscosity Sample Tackifier Tg (° C.) Tm(° C.) Tc(° C.) Pa .Math. s Reference — −14.1 130.1 69.5  2.03 HMA 1 Example 1 6.94 134.11 — 1.64 HMA 2 Example 2 8.08 — — 2.13 HMA 3 Example 3 13.34 133.53 76.77 3.42 Compara- Comparative 5.31 132 — 1.10 tive HMA 1 Example 1

[0124] Inclusion of a tackifier was found to have significant effects on the thermal properties of the HMA formulations. The glass transition temperature (Tg) of the formulations were significantly higher compared to the reference HMA formulation.

[0125] In all four cases the Tc enthalpy appeared to decrease; this might be caused by the reduced amounts of crystalline components and the influence of the tackifier on crystallization rate.

[0126] Inclusion of a tackifier was found to significantly reduce the viscosity or to remain significantly the same.

Adhesion Properties of the HMA Formulations

[0127] For the adhesion tests three types of cardboard were evaluated. A general single waved cardboard (WP 20/20) was obtained via Smurfit Kappa and two types of thin cardboard (Frovi) from BillerudKorsnäs. A bead of HMA (molten at 163° C.) was applied manually with a glass rod to a cardboard substrates, providing a bead of 1.5 to 2.5 mm. A second cardboard substrate was applied onto the bead to form a bond. 1 and 4 days after making the bond, the bond was pulled apart at an angle of almost 180° to destroy the bond. The percentage fiber tear, was evaluated for each sample by recording the percentage of adhesive covered by fibers.

[0128] The results of the adhesion tests is shown in Table 2.

TABLE-US-00002 TABLE 2 Percentage fiber tear transfer WP 20/20 Frovi 250 Frovi 290 Sample 1 day 4 days 1 day 4 days 1 day 4 days Reference 100%  90% 75% 20% 20%  0% HMA 1 95% 100%  90% 90% 90% 90% HMA 2 100%  100%  90% 90% 100%  95% HMA 3 95% 95% Na Na 70% 70% Comparative 95% 95% 60% 10% 60%  0% HMA1

[0129] Results of Table 2 show that HMA formulations with both sorbitol- and melamine-initiated lactic acid oligomer tackifier resins (HMA 1, HMA 2 and HMA 3) have improved adhesion to almost all substrates compared to the reference and comparative example. The reference and Comparative HMA 1 formulation showed to perform significantly worse after 4 days, showing that the addition of the sorbitol lactic acid oligomer tackifier in HMA 1, HMA 2 and HMA 3 improved long term adhesion.

Peel Adhesion Failure Temperature

[0130] The peel adhesion failure temperature (PAFT) was determined for most of the above HMA formulations.

[0131] The PAFT test was performed as follows:

[0132] A bead of HMA (molten at 163° C.) was applied manually with a glass rod to a cardboard substrate, providing a bead of 1.5 to 2.5 mm. A second cardboard substrate was applied onto the bead to form a bond.

[0133] 24 hours after making the bonds they were suspended with a 100 gram weight attached to the substrate. When either the substrate or the adhesive fails under temperature, the weight drops and the temperature was recorded.

[0134] The reference HMA showed a PAFT of over 90° C.

[0135] The PAFT of HMA 1 was over 90° C., the PAFT of HMA 2 was 52 and the PAFT of Comparative HMA 1 was over 90° C.

[0136] In view of the DSC and rheometry properties of the HMA formulations and the results of the adhesion and PAFT tests HMA 1 was found to have a better overall performance than HMA 2, but both HMA formulations were found to outperform the reference and comparative formulations in most instances.

Influence of Multifunctional Polymerization Initiator on the Properties of the Oligomer Tackifiers

[0137] Several tackifiers were prepared with the same Mn and lactic acid content but with initiators of varying functionalities to investigate the influence of the multifunctional polymerization initiator on the properties of the tackifier.

[0138] The preparation of the tackifier was performed as described above for Example 1 with the lactic acid monomer derived from 100% L-lactide and using different initiators as indicated in Table 3.

[0139] The number average molecular weight (Mn) was determined by GPC as described above. The enthalpy of fusion (J/g), the glass transition temperature (Tg) and the melting temperature (Tm) were of determined by differential scanning calorimetry (DSC) as described above.

[0140] The results are provided in Table 3.

TABLE-US-00003 TABLE 3 Mn Tg Tm enthalpy Sample Initiator (g/mol) (° C.) (° C.) (J/g) Comparative neopentyl 5600 43.3 148.7 22.7 Example 2 glycol Example 4 trimetylol 5600 40.47 138.7 2.5 propane Example 5 pentaerythriol 5600 42.98 Na 0 Example 6 Sorbitol 5600 36.9 Na 0
Adhesion Properties of HMA with Varying Amount of Tackifier

[0141] HMA formulations were prepared with varying amounts of tackifier of Example 1 as detailed in Table 4, to evaluate the influence of the tackifier amounts on the adhesion properties of HMA formulations.

TABLE-US-00004 TABLE 4 Tackifier Copolymer resin Oligomer wax nucleator Sample wt. % wt. % wt. % wt. % HMA - 0% 0 90 9 1 HMA - 5% 5 85 9 1 HMA - 15% 15 75 9 1 HMA - 25% 25 65 9 1 HMA - 35% 35 55 9 1 HMA - 45% 45 45 9 1

[0142] The HMA formulations further comprised the same copolymer resin, oligomer wax and nucleator as described above for the HMA 1 and HMA 2 formulations, and were prepared as follows. In a glass flask with an overhead stirrer the tackifier was molten at a temperature of 163° C. under stirring, the nucleator was added to the molten tackifier and mixed until molten and a uniform mixture was obtained. The oligomer wax was added and mixed until molten and a uniform mixture was obtained. Finally the copolymer resin was added in small portions and mixed until molten and a uniform mixture was obtained. The mixture was kept at 163° C. under stirring until applied. The adhesion of the different HMA formulations were tested on a Frovi 250 cardboard. A bead of HMA (molten at 163° C.) was applied manually with a glass rod to a cardboard substrate, providing a bead of 1.5 to 2.5 mm. A second cardboard substrate was applied onto the bead to form a bond. Subsequently all test substrates were stored at −20° C., 20° C. and 52° C. in duplicate and tested on adhesion. The bond was pulled apart at an angle of almost 180° to destroy the bond. The percentage fiber tear, was evaluated for each sample by recording the percentage of adhesive covered by fibers. The adhesion value is expressed in the perceptual amount of fibers transferred on the applied adhesive after pulling of the cardboard strip (% Fiber tear).

[0143] The results are presented in tables 5, 6 and 7.

TABLE-US-00005 TABLE 5 Percentage fiber tear after storage at room temperature (% at 20° C.) Percentage of tackifier in HMA Hours of storage formulation (wt. %) after application 0 5 15 25 35 45 0.5 50 85 90 100 75 5 24 5 50 70 90 75 5 168 5 5 30 80 75 5

TABLE-US-00006 TABLE 6 Percentage fiber tear after storage at deep-freezer temperature (% at −20° C.) Percentage of tackifier in HMA Hours of storage formulation (wt. %) after application 0 5 15 25 35 45 0.5 5 40 90 90 5 0 24 5 25 90 85 0 0 168 5 10 20 40 0 0

TABLE-US-00007 TABLE 7 Percentage fiber tear after storage at high temperature (% at 52° C.) Percentage of tackifier in HMA Hours of storage formulation (wt. %) after application 0 5 15 25 35 45 0.5 5 85 100 90 90 5 24 1 15 95 90 90 90 168 1 70 90 90 90 90

[0144] The data shows at different temperatures the retention of adhesion is significantly increased by the use of a tackifier resin. The optimal amount of tackifier may depend on the temperature of the final application.

[0145] An optimum amount of tackifier which would provide good adhesion performance over a broad range of temperatures (e.g. from—20° C. to 52° C.) both at the start and over time was found to be of, e.g., 15-35 wt %.