WO2011027210A1

WO2011027210A1 - A general purpose solvent-based adhesive

Info

Publication number: WO2011027210A1
Application number: PCT/IB2010/002195
Authority: WO
Inventors: Ronald Douglas Sanderson; Abubaker Alzrouge Alshuiref
Original assignee: Stellenbosch University
Current assignee: Stellenbosch University
Priority date: 2009-09-04
Filing date: 2010-09-03
Publication date: 2011-03-10
Anticipated expiration: 2012-03-04

Abstract

A solvent-based adhesive in which a continuous phase includes either a solvent for an acrylic based adhesive or a solvent for a urethane based adhesive and wherein adhesive molecules are dissolved in the solvent to form said continuous phase, the adhesive being characterised in that the adhesive molecules include graft polymer or block polymer molecules, or both, wherein a major part of the molecule is either acrylic based or urethane based such that the major part is of sufficient size to provide the adhesive molecule with a required solubility in said solvent and wherein a minor part of the molecule is based on the other of said acrylic or urethane thereby providing said molecule with attributes of both acrylic based and urethane based adhesives. A solvent based adhesive is also provided in which there is present in admixture with the solvent and graft polymer, either an acrylic polymer or a short chain urethane polymer, as may be dictated by the nature of the solvent.

Description

A GENERAL PURPOSE SOLVENT-BASED ADHESIVE

FIELD OF THE INVENTION

This invention relates to a general purpose solvent-based adhesive formulation that has an enhanced range of application and types of surfaces to which it will adhere. The invention also relates to adhesive compounds for use in such formulations.

In this specification the terms adhesive and adhesive formulation are intended to include binders having adhesive properties. BACKGROUND TO THE INVENTION

General-purpose adhesives that are presently commercially available are usually selected from monomeric acrylates such as the well-known superglue that cures from monomer to polymer; acrylic adhesives in a solvent; polyurethane adhesives in a solvent; moisture curing grades of polyurethane adhesives; two component epoxy resins; and emulsion-based polyvinyl acetates that are commonly known as "wood" or "paper" glue. Each of these has its own relatively limited range of application insofar as the types of materials are concerned to which they effectively bond, in use.

Although they exhibit bonding characteristics with a relatively limited range of materials, solvent-based acrylic adhesives are probably the most commonly used primarily because they are substantially less costly than polyurethane adhesives. This is so in spite of the often superior properties of the polyurethane adhesives and is based primarily on the cost of the adhesive and the cost of the solvent. Nevertheless, polyurethane adhesives are finding increasing application and are used in many industries due to their advantageous properties, such as a wide range of flexibility combined with toughness; high abrasion resistance; high chemical resistance; high acid etch resistance; excellent weatherability; and a very low cure temperature.

Typically a solvent-based acrylic adhesive employs a low temperature solvent such as tetrahydrofuran or methylethylketone which is also of relatively low-cost, whereas a solvent-based urethane adhesive generally requires a solvent having a higher boiling point and that may be as exotic as N-methylpyrrolidone that is substantially more expensive.

OBJECT OF THE INVENTION It is an object of this invention to provide an adhesive that exhibits at least some properties that are superior to those of present commercially available acrylic adhesives and that may be cost effectively produced.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided a solvent-based adhesive in which a continuous phase includes either a solvent for an acrylic based adhesive or a solvent for a urethane based adhesive and wherein adhesive molecules are dissolved in the solvent to form said continuous phase, the adhesive being characterised in that the adhesive molecules include graft polymer or block polymer molecules, or both, wherein a major part of the molecule is either acrylic based or urethane based such that the major part is of sufficient size to provide the adhesive molecule with a required solubility in said solvent and wherein a minor part of the molecule is based on the other of said acrylic or urethane thereby providing said molecule with attributes of both acrylic based and urethane based adhesives. Further features of the invention provide for the continuous phase to be that of an acrylic based phase comprising a solvent for acrylic based adhesives with graft polymer or block polymer adhesive molecules dissolved therein wherein the adhesive molecules have a major part that is acrylic based and a minor part that is urethane based; for the adhesive molecules to be acrylic based adhesive molecules with nano inclusions of urethane therein (generally considered to be less that about 100nm); for the acrylic based adhesive molecules to include at least one of butyl acrylate, methyl methacrylate and styrene (vinyl benzene); for the graft or block polymer to be made as a polymerisation product of a macromer (macromonomer) of a urethane and at least one acrylic or vinyl monomer; and for the solvent to be selected from suitable ethers and ketones as well as mixtures thereof.

The invention also provides, as an intermediate product, a predominannly monofunctional urethane macromonomer.

As an optional variation the solvent based adhesive may be one in which there is present, in admixture with the solvent and graft or block polymer, either an acrylic polymer or a short chain urethane polymer, as may be dictated by the nature of the solvent.

In order that the invention may be more fully understood, a further more detailed discussion thereof follows. DETAILED DESCRIPTION AND EXAMPLES OF THE INVENTION

Whilst various methods of preparing graft or block polymers are available, the route of firstly preparing polyurethane macromers and then copolymerising them with acrylic monomers was chosen as being the preferred method.

The first stage is the production of polyurethane macromers and the process conditions are selected such that a large percentage of the macromer molecules have a group at one end that is reactive towards acrylic and a non-reactive group at the other end. Of course, two or more reactive groups on a macromer molecule could lead to cross-linking or gel formation in the subsequent polymerisation process that would be undesirable. On the other hand, there will also be a small proportion of macromer molecules having no reactive groups.

A urethane macromer molecule preferably contains from 4 to 32 units and most preferably from 16 to 32 units. It is, however, quite possible for there to be up to 100 units in a macromer molecule.

In tests that have been conducted to date the polyurethane macromers were produced from the isocyanates toluenediisocyanate and methylene diphenyl diisocyanate. The alcohols used were dimethyl diglycol and ethylene glycol. The preferred combination at the present stage of development is methylene diphenyl diisocyanate and dimethyl diglycol. The reason for this is that this particular combination tended to give better phase separation and better crystallisation of the urethane domains. The copolymerisation of the polyurethane macromers has been carried out with acrylic monomers selected from butyl acrylate, methyl methacrylate and styrene in ratios calculated to provide adhesive molecules having a major part that is acrylic based and a minor part that is urethane based, preferably acrylic based adhesive molecules with nano inclusions of urethane therein. At this stage, other monomer additives may be added to provide tailor-made properties for special purposes, such as for use as ceramic tile adhesives.

After the copolymerisation has taken place, unreacted macromer can be removed by a simple solvent process to yield a more technical glue. Alternatively, in the instance of at least some general-purpose adhesives, the unreacted macromer may be allowed to remain in the product graft polymer without significant detriment to the properties of the adhesive since the polyurethanes co-crystallise, in use.

The product urethane acrylic graft polymer is dissolved in a suitable ether or ketone solvent, typically a mixture of both, with the solvent quantity and selection being made to provide the required general-purpose or other adhesive properties.

In one case, an adhesive comprised a urethane graft polybutyl acrylate or polybutyl methacrylate in a quantity of from 16 to 25% in a typical volatile solvent such as an ester or ether blend with additives such as alcohols. Such solvents are commercially available for the particular purpose of producing adhesives. This adhesive would typically be classified as a soft adhesive.

Additives that are typical include, amongst other substances, a resin in an amount of about 1% for enhancing adhesiveness; from 0.1 to 0.3% of antioxidant, preferably of a non-yellowing grade; a suitable quantity of a bianco fill as a non-yellowing optical brightener; a nano silica for toughening the adhesive; and an aerosol for thickening it.

An adhesive having a medium hardness could be produced by adding methyl methacrylate, or styrene, or both, to the butyl acrylic phase when conducting the polymerisation process.

A hard adhesive could be produced by adding only methyl methacrylate, or styrene, or both as the acrylic phase when conducting the polymerisation process. It will be understood that the fact that the polyurethane parts of the graft polymer molecules can often wet a surface better than can the acrylic parts of the molecules means that they can both assemble at the surface for better adhesion or the urethane can crystallise out as a second phase which is highly reinforcing for the glue, especially because the phase is a crystalline nanophase. Such a glue therefore expands the suitability of the basically acrylic glue that is useful on paper, cardboard, leather, wood, metal etc into the fields normally covered by polyurethane adhesives and such a glue is therefore also useful on soft and hard vinyl, for example. In instances in which it is desired to lower costs further, it is possible to add to the mix of graft polymer, either additional urethane or additional main chain polymer.

Reverting to the production of the adhesive molecules, graft copolymers with a backbone of one polymer and branches of other polymer exhibit material properties that are a combination of both homopolymer constituents. The presence of long chain branching is reported to have a dramatic effect on the dynamic and rheological behaviour of well-entangled polymers. The macromonomer technique is considered by applicant to be the simplest way to prepare the graft copolymers. The macromonomers are polymers end- capped with a polymerizable end group able to copolymerize with low molecular weight monomers, so the macromonomers can either homopolymerize to give a regular comb polymer or copolymerize with a suitable monomer to give a graft copolymer. These end-functional polymers can be prepared by modifying polymer end groups or, very conveniently, by using functional initiators in living/controlled polymerizations.

In this instance, urethane macromonomers were synthesized to be predominantly monofunctional, and then the urethane macromonomers were used as grafts in solution free-radical copolymerization with methacrylate monomers. The synthesis of urethane macromonomers requires that all monomer reagents, solvents and intermediates be substantially free from moisture. Any moisture present during urethane macromonomer synthesis may react with the isocyanate, resulting in a number of competitive secondary side reactions that can lead to the formation of undesired cross-linked material. Such side reactions include the formation of dimers and trimers, acylurea, allophanate, and amide and carbamic acid derivatives. These side reactions can also be suppressed by using a low reaction temperature. In the pre-polymer process the monomers are added in intervals. The prepolymer process of forming the macromers holds advantages over the alternative one-shot process as it offers more control over the reaction, thereby resulting in polymers with smaller molecular mass distributions and better polymer morphology. Polyaddition polymerization was used in this instance to introduce a polymerizable functional group into the urethane polymer chain end. The experimental procedures used to synthesize the urethane macromonomer are described below.

The macromonomer according to formula 1 was synthesized

2-HEA-(MDI-EG) n-MDI-MeOH (1)

where 2-HEA is 2-hydroxyethyl acrylate, EG is ethylene glycol, MDI is 4,4'- diphenylmethane diisocyanate, n is the chain length, and MeOH is methanol, which was used to form the chain end. 100% conversion was assumed with an average chain length of n = 5. The reactants employed were 15.32g MDI; 3.17g EG; 1.18g 2-HEA; and 0.33g MeOH. The excess MeOH used was 0.16g and the [OH]/[NCO] ration was 1.04. An excess amount of MeOH was needed to ensure that all reactive NCO group were fully reacted. If not, then secondary reactions could take place, to ultimately form cross-linked structures. The excess thus helps to minimize difunctional acrylates. The synthesis of the urethane macromonomer was carried out under a nitrogen atmosphere. MDI and dimethylformamide were added to the flask and the reaction mixture cooled to below 10° C. The reactor was then purged with nitrogen gas and sealed. This was followed by addition of the EG, under stirring (400-500 rpm). The temperature was increased to 15-18° C and held there for 40-60 min.

In a second step, the reaction temperature was increased to 20-40° C and 2- HEA was added. The reaction was allowed to run for 60 min.

In a final step, the reaction temperature was first increased to 40-45° C, excess methanol was added, and then the reaction temperature was increased to 55° C to ensure that any previously unreacted isocyanate reacted. The degree of the reaction was verified by measuring the isocyanate group that disappears as result of reaction with hydroxyl groups. The urethane macromonomer obtained was dried in a vacuum oven at 45° C for 24 hours and then stored in a desiccator until required for use in the further polymerization stage. The reaction can be represented by the following:-

OCN - -NCO , n-l[no OH,

H

+ MeOH

H — O " CH₃

Besides the desired urethane macromonomer with only one polymerizable end group, i.e. 2-HEA on one chain end and MeOH on the other, undesirable products can also be present. One such undesirable product occurs when 2- HEA reacts on both ends of the urethane pre-polymer (urethane chain with excess isocyanate). The other possible structure that could form when methanol reacts on both chain ends of the urethane pre-polymer is also undesirable, as it renders the urethane macromonomer unreactive for further copolymerization with acrylic monomers due to no double bond being present on either of the chain ends. Thus it is very important to optimize reaction conditions in an attempt to maximize the yield of the desired product that has 2-HEA on one chain end and MeOH on the other. The urethane macromonomer was grafted with methyl methacrylate (MMA) and with normal butyl methacrylate (n-BMA) as the main chains respectively. The success of the grafting reactions was determined by characterizing the products. Various quantities of urethane macromonomer were copolymerized with various quantities of MMA, and with various quantities of n-BMA, respectively, using solution free radical copolymerization. The choice of a good solvent for the acrylate and urethane macromonomer was done by trail and error. Many different solvents were tried, such as benzene, toluene, acetone, acetonitrile, methanol, ethanol, dimethylformamide (DMF) and dimethylsulfoxide (DMSO). Complete solubilization of the urethane macromonomer and acrylate was achieved by using DMF or DMSO. However, DMSO could not readily be used because it crystallizes at room temperature and needs to be heated before use. Therefore DMF was chosen as the solvent for all the copolymerization reactions of methacrylate and urethane macromonomers. n-BMA (99%) and MMA (99.9%) were washed with a 0.4 M potassium hydroxide solution, followed by distillation under reduced pressure to remove inhibitor. The monomers were stored for 24 hours at 0° C over a molecular sieve (4 A). The following materials were also used; potassium persulphate (99%), methanol (99.8%), dimethylformamide (DMF 99.5%), distilled and deionized water and silicon oil. 2,2'-Azobis(isobutyronithle) (AIBN 98%) was recrystallized from methanol. The urethane macromonomers

MMA and n-BMA monomers were first washed with 0.4 M KOH followed by distillation under reduced pressure to remove any other impurities using potassium persulfite. The monomers were first washed with 0.4 M KOH solution to remove hydroquinone inhibitor. The distillation was carried out under reduced pressure and low heat (about 45° C) to avoid self polymerization of the monomers. The distilled fractions were collected and dried over anhydrous magnesium sulfate to ensure a completely dry monomer. The monomers were stored at - 8° C prior to use.

Graft copolymers were prepared by solution free radical copolymerization carried out in a 250-ml three-neck reactor with magnetic stirring, under a nitrogen atmosphere. DMF was first introduced into the reactor. MMA or n- BMA, and AIBN (1% wt relative to the monomer), were then charged into the reactor, followed by the urethane macromonomer. Various concentrations of urethane macromonomer were used: 0, 0, 25 and 55 wt % relative to MMA or n-BMA. The polymerization temperature was 75° C and the reaction time was 24 h. The copolymers were precipitated in MeOH, and then separated by filtration and dried under vacuum at room temperature overnight. The unreacted urethane macromonomer was removed by precipitation using DMF as solvent and THF (tetrahydrofuran) and MeOH as nonsolvent.

The reaction may be represented as follows:-

CH, CH,

H C =c

I

c=o c=o

I I

o o

I I

Ri R₂

M A or BMA urethane macromonomer

DMF

AIBN

methacrylic /urethane macromonomer graft polymer where

R1 = CH3 for MMA; or C4H9 for n-BMA

R2 = urethane macromonomer unit

n, m, w, f, = repeat unit In the event that there is any doubt as to the manner in which adhesive according to the invention may be produced, reference may be had to a thesis on the subject by Abubaker ALSHUIREF that is available from the library of the Stellenbosch University and currently available online at the internet address: http://scholar.sun. ac.za/handle/10019.1/4022. Reference is made especially to Chapter 4 thereof.

It will be appreciated that the invention provides for the production of adhesives that are more versatile in application to a variety of different surfaces and that extend the use of a single adhesive to use in relation to an extended range of materials. This is achieved basically by providing the graft or block polymer with large enough portions of the acrylic in order to render the entire polymer molecule soluble in an acrylic type of solvent with the urethane inclusions being pulled into the solution. It is equally possible, of course, if desired, to provide the graft or block polymer with large enough portions of urethane and utilise a urethane type of solvent with the acrylic, in that instance, being pulled into solution.

Numerous variations may be made to the invention as described above without departing from the scope hereof. In particular, the method of production of the graft polymers can be varied widely as can their composition and formulation.

Claims

CLAIMS:

1. A solvent-based adhesive in which a continuous phase includes either a solvent for an acrylic based adhesive or a solvent for a urethane based adhesive and wherein adhesive molecules are dissolved in the solvent to form said continuous phase, the adhesive being characterised in that the adhesive molecules include graft polymer or block polymer molecules, or both, wherein a major part of the molecule is either acrylic based or urethane based such that the major part is of sufficient size to provide the adhesive molecule with a required solubility in said solvent and wherein a minor part of the molecule is based on the other of said acrylic or urethane thereby providing said molecule with attributes of both acrylic based and urethane based adhesives.

2. A solvent-based adhesive as claimed in claim 1 in which the continuous phase is that of an acrylic based phase comprising a solvent for acrylic based adhesives with graft polymer or block polymer adhesive molecules dissolved therein wherein the graft polymer or block polymer adhesive molecules have a major part that is acrylic based and a minor part that is urethane based.

3. A solvent-based adhesive as claimed in claim 2 in which the graft polymer or block polymer adhesive molecules are acrylic based adhesive molecules with nano inclusions of urethane therein.

4. A solvent-based adhesive as claimed in any one of the preceding claims in which the acrylic based graft polymer or block polymer adhesive molecules include at least one of butyl acrylate, methyl methacrylate and styrene (vinyl benzene).

5. A solvent-based adhesive as claimed in any one of the preceding claims in which the graft or block polymer is made as a polymerisation product of a macromer (macromonomer) of a urethane and at least one acrylic monomer.

6. A solvent-based adhesive as claimed in claim 5 in which the macromer of urethane has an acrylic or vinyl functionality and a urethane length of from 4 to 100 units.

7. A solvent-based adhesive as claimed in claim 6 in which the macromer of urethane has a urethane length of from 16 to 32 units.

8. A solvent-based adhesive as claimed in any one of the preceding claims in which the solvent is selected from suitable ethers and ketones as well as mixtures thereof.

9. A solvent-based adhesive as claimed in any one of the preceding claims in which there is present in admixture with the solvent and graft or block polymer, either an acrylic polymer or a short chain urethane polymer, as may be dictated by the nature of the solvent.

10. As an intermediate product, a predominantly monofunctional urethane macromonomer.