Waterproof Hot Melt Composition for Stitched Seams

This invention relates to hot melt adhesive compositions useful for waterproofing stitched seams and a method of using the same. This invention particularly relates to certain hot melt adhesives which can be directly applied to a stitched seam such that the resulting seam is resistant to water penetration.

In the manufacture of waterproof articles such as rainwear, workwear, tents and shoes; several methods are known in the art for waterproofing stitched seams.

GB 2 215 660 A teaches covering a stitched seam with a tape of thermoplastic material fused by hot air or pressure. A tape is made by solvent casting a 20 to 40 percent by weight polyurethane solution onto a release sheet. The coated sheet is then slit and wound onto a reel. The tape can be applied to the garment using machinery known for applying tapes having heat-activated adhesives.

Liukko U.S. Pat. No. 4,604,152 and Emrich et al., U.S. Pat. No. 4,605,578 also pertain to methods of making a waterproof stitched seam using a heat-activated thermoadhesive material.

In general, and particularly in the shoe industry, the use of solvent cements or waterbased emulsions to seal the stitched seams in shoes and boots are widely used. This is very time consuming and labor intensive as multiple coating and drying steps are employed. Seams sealed in this manner are not generally durable and a high percentage of shoes and boots manufactured are sometimes returned due to water leakage complaints.

It is therefore desirable to employ a hot melt type adhesive, that can be readily applied to form a good reliable water-proofed seam. It is especially desirable to use a hot melt adhesive that is not heat-activated.

This invention relates to novel hot melt adhesive compositions and methods of using the same to manufacture waterproof stitched seams in the manufacturing of water-proof articles. The adhesives of the present invention can be applied directly to a stitched seam with any suitable known hot melt application equipment without first premanufacturing a tape. Preferably, the hot melt adhesive of the present invention is applied as a continuous bead to a stitched seam. Such application may be done by hand, with the aid of robotics, or with any other known automated hot melt application equipment. Upon cooling, the adhesive forms a non-tacky flexible seal over the stitched seam that exhibits excellent resistance to water penetration and remains flexible over a wide temperature range. The use of a hot melt has the distinct advantage of eliminating the need for solvents, which are necessarily undesirable because of environmental concerns and health risks to workers. Furthermore, directly coating a hot melt adhesive onto a stitched seam is more economical, less time consuming, and can actually result in superior seam performance relative to prior application methods that employed solvent types of adhesives.

The hot melt adhesive composition useful in the present invention comprises:

a) from about 5 to about 40% percent by weight of a substantially linear A-B-A block copolymer, wherein A is polystyrene and B is a substantially saturated rubbery midblock comprising ethylene/butylene, ethylene/propylene and mixtures thereof;

b) an amount up to about 15% by weight of a substantially A-B diblock copolymer wherein A is polystyrene and B is a rubbery block comprising isoprene, ethylene/butylene, ethylene/propylene, butadiene and mixtures thereof;

c) an amount up to about 20% by weight of a tackifying resin;

d) an amount up to about 20% by weight of a waxy material; and

e) from about 10 to about 95% by weight of a plasticizer.

The adhesive compositions of the present invention may also be useful as a waterproof coating which could be coated onto any fabric or nonwoven surface that is intended to be resistant to water penetration.

The hot melt adhesive composition of the present invention comprises a substantially linear A-B-A block copolymer wherein A is polystyrene and B is a substantially saturated rubbery midblock comprising ethylene/propylene, ethylene/butylene and mixtures thereof. This copolymer can be present in an amount from about 5 to about 40% by weight, preferably in an amount from about 20 to about 30% by weight. Examples of such polymers include those commercially available under the Kraton G.RTM. tradename from Shell Chemical Company. Branched or grafted versions of the Kraton G.RTM. polymers such as TKG-101 and RP-6912 are also suitable for use, either alone or in combination with other ungrafted Kraton G.RTM. polymers. The high cohesive strength and high plasticizer loading capacity of this copolymer is critical to the success of the present invention. It enables the resulting adhesive composition to exhibit a sufficiently low viscosity and low surface tension in the molten state so that it readily flows along the stitch line and wicks into the seam and stitch holes when applied. Upon cooling, the adhesive also maintains its flexibility over a wide temperature range. This is especially important because the seam thus formed would be required to endure extensive flexing. The copolymer also exhibits oxidative stability and good UV resistance, thus insuring good durability and longevity of the resulting waterproof seam.

Preferably, the adhesive composition comprises a second polymer, present in an amount up to about 15% by weight, more preferably in an amount of about 5 to about 10% by weight of a substantially A-B diblock copolymer. This polymer can be a solid or liquid at ambient temperature. Representative examples include those that are available from Shell Chemical Company under the tradename Kraton.RTM. G-1726 or LVSI-101.

The adhesive composition of the present invention preferably comprises a tackifying resin. This can be present in amounts up to about 20% by weight. Ideally, the tackifier is aromatic in nature, so it will preferably associate with the polystyrene end blocks. Examples of such resins are available from Hercules Inc. under the tradename Kristalex.RTM. and typically have softening points between 70.degree. C. and 160.degree. C. Midblock tackifiers may also be useful in the adhesives of the present invention so long as the resulting adhesive is not sufficiently tacky upon cooling. Midblock tackifiers are known in the art, and representative examples include polyterpene resins, coumarone-indene resins, hydrogenated rosins and rosin ester, as well as aliphatic and aromatic petroleum hydrocarbon resins.

A waxy material can also be present in the adhesive composition of the present invention. Preferably, the waxy material is present in amounts up to about 20% by weight, more preferably, in amounts ranging from about 1 to about 10% by weight. The waxy material is added to modify the viscosity, reduce the tack, and enhance the water barrier properties. The preferred waxy material includes any petroleum derived paraffin wax. Other useful waxes include microcrystalline waxes, Fischer-Tropsch, polyethylene and by-products of polyethylene.

Plasticizing agents, especially oils are also useful in the adhesive compositions of the present invention. Preferably, the plasticizing agent is a liquid at ambient temperature, and these include naphthenic or paraffinic hydrocarbon oils. Other useful plasticizers include polybutene, liquid tackifying resins, and liquid elastomers so long as the resulting adhesive is sufficiently tack free upon cooling. Others plasticizers include olefinic oligomers, low molecular weight polymers, vegetable oils and their derivatives thereof.

Various other components can also be added to modify the tack, color, odor, etc., of the hot melt adhesive compositions of the instant invention. These can include antioxidants and other stabilizing ingredients which are added to protect the adhesives from various heat and light induced degradation.

Examples 1-3 were made in the following manner in accordance with known hot melt manufacturing methods. The A-B-A block copolymer, antioxidant, resin, and about one half the oil was added to a sigma blade mixer with a hot oil temperature of about 350.degree. F. (.about. 177.degree. C.). The ingredients are shown in Table 1. These were sheared for about 45 minutes until smooth. The diblock A-B block copolymer, wax and the remainder of the oil were then added sequentially.

A bead of hot melt adhesive was applied by hand at a temperature of about 350.degree. F. (.about. 177.degree. C.) with Slautterback Mini Squirt II hot melt application equipment. An appropriate nozzle size and application rate was selected depending on the seam width, seam thickness, and thread size. Typically, 0.3" (.about. 7.6 mm) and 0.7" (.about. 18 mm) slot die spreader nozzles as well as the standard 0.7" diameter nozzle were used to produce an adhesive bead that could sufficiently cover the seam and wick into the thread and stitch holes. The mass of adhesive applied ranged from about 0.5 grams/linear inch to 3 grams/linear inch depending on the nozzle size and application rate. The sealed stitched seam was then conditioned for at least 24 hours at ambient temperature prior to flex testing.

A waterproof stitched seam was formed as described above and then tested using a Dynamic Water Penetration Tester available from Koehler Instrument Company, Inc. in accordance with ASTM D2099, Dynamic Water Resistance of Shoe Upper Leather by the Maeser Water Penetration Tester. During testing, the waterproofed stitched seam was mounted in a flex tester, immersed in a salt water bath, and mechanically flexed. Water leakage was detected by electrical conductance. The test results are also depicted in Table 1. All three samples exceeded the minimum requirement of 20,000 flexes.