GB2445748A

GB2445748A - Coating composition

Info

Publication number: GB2445748A
Application number: GB0700970A
Authority: GB
Inventors: John Victor Shepherd
Original assignee: Arjobex Ltd
Current assignee: Arjobex Ltd
Priority date: 2007-01-18
Filing date: 2007-01-18
Publication date: 2008-07-23
Also published as: GB0700970D0

Abstract

A coating composition for use on plastic sheet materials, such as polyolefins, for example high density polyethylene (HDPE) and polypropylene, comprises an aqueous dispersion that includes 10-95% by dry weight of a polymer binder, especially an acrylic polymer. having a minimum film forming temperature (MFFT) of at least 40{C and at least 1% by dry weight of a crosslinking agent, especially methylolmelamine condensates, aziridine, polycarbodiimide and urethane crosslinkers, capable of reacting with groups of the polymer binder. Also disclosed is a plastic sheet material comprising the coating composition and a method of treating a plastic sheet material, comprising applying the coating composition.

Description

PLASTIC SHEET MATERIAL

The present invention relates to a plastic sheet material and a coating composition for plastic sheet materials. The invention also relates to a method of manufacturing plastic sheet materials and a method of treating plastic sheet materials. The term "plastic sheet materials" as used herein refers to sheet materials made of plastics materials such as polyolefins, for example high density polyethylene (HDPE) and polypropylene.

One particular application of the invention (but not the only application) is in the production and treatment of synthetic paper: that is, plastic sheet material having the feel and printability characteristics of cellulose paper. Examples of such materials and methods of producing them are described for example in GB 1490512, GB 1470372, EP0470760, :... EP0703071 and EP0863 177, the contents of which are incorporated by reference herein. * * ****

Synthetic paper is sometimes used as a replacement for conventional cellulose-based paper, *. . for example in products such as maps, labels and leaflets, where its strength and the fact that it is waterproof are particularly valuable. Typically, synthetic paper is made by *::: : 15 stretching a web of the plastics material either uniaxially or biaxially. The stretching * : * process orientates the polymer molecules and increases the stifthess of the plastic material so that it is close to that of conventional cellulose-based paper.

The synthetic paper material may also include various surface treatments or coatings to modify or improve its properties. For example, coatings may be applied to improve the printability of the material or to prevent the build-up of static electric charge. These coatings are preferably deposited as water-based formulations, as these tend to be environmentally more desirable than formulations based on other solvents. Water-based formulations also allow access to compounds that are soluble in water and to those that can be dispersed in water. To improve the water-resistance of the coating, cross-linking agents can be included, typically at 1.5% or less of the total solids of the mixture, for example as described in GB2 177413 and EPlO5S7l 1. The resulting synthetic paper material can be used in all normal paper printing, binding, stitching, die cutting and other finishing processes.

Although orientation greatly increases the stiffness of plastic sheet material, one disadvantage of most synthetic papers is that they are significantly less stiff than conventional cellulose-based paper, particularly over the long term. This is because plastics materials are not perfectly elastic and exhibit viscoelastic properties, which result in the phenomenon of creep when the material is subjected to a continuous stress over a long period, for example a few days or weeks. One effect of this is that products made of synthetic paper, such as maps, menus, information displays or leaflets, may eventually sag or bend if stood upright without support, resulting in an unattractive and unsatisfactory display.

There is therefore a need for a plastics sheet material with improved stiffness and in particular good long-term stiffness (i.e. resistance to creep). * *

Stiffness is normally measured over a relatively short time span, typically a minute or less.

*. .: Various test methods are used, including ultrasound tests that operate at frequencies of 20kHz or higher, the Kodak vibration test that is performed at a frequency of 25Hz, two, three and four point bending tests that are usually completed in less than a minute and : 20 classical bending tests such as the Clark, Taber and Gurley tests, which are to some extent *..*.* * operator-dependent but are usually completed in less than a minute. Stiffness can also be measured by reference to a bending angle, for example as set out in ISO 9073-7. This method is often used for measuring the stiffness of textiles.

In order to measure long term stiffness and the effect of creep, we used the test method illustrated in the accompanying drawing (figure 1). A strip of test material 2 with a width of 15mm is secured with weights or adhesive tape to the horizontal upper surface of a support 4, so that it overhangs the edge of the support by a length of 100mm. Gravity causes the strip 2 to bend dowrwards, and the bending angle 0 is obtained by measuring with a protractor the contained angle 0 between a line 6 that extends from the corner 8 of the support 4 to the free end 10 of the strip, and a horizontal reference line 12.

We have found that plastic sheet material is generally satisfactory (that is, it has adequate stiffness and resistance to creep for most applications) if the bending angIe 0 is initially less than 3 0 and after three weeks is less than 550 However, for improved performance in demanding applications it is preferred that the bending angle 0 is initially less than 25 and after three weeks is less than 45 .

With conventional plastic sheet materials based on polyolefin such as stretch-orientated HDPE (coated or uncoated), it is necessary for the material to be quite thick, for example having a thickness of approximately 170-220 microns, in order to meet the preferred bending angle requirements. Although this may be acceptable for certain products, for other products such as maps, leaflets or menus it may be preferred to use a thinner, lower grammage material that can be folded more easily.

:. According to the present invention there is provided a coating composition for use on :..::: 15 plastic sheet materials, wherein the coating composition comprises an aqueous dispersion that includes 10% to 95% by dry weight of a polymer binder having a minimum film * * forming temperature (MFFT) of at least 40 C, and at least 1% by dry weight of a cross- *, linking agent capable of reacting with groups of the polymer binder.

* : We have found that when this coating composition is applied to a plastic sheet material, the * : 20 stiffness and resistance to creep of the material is greatly improved. Products made from the material do not therefore tend to sag, even after standing vertically for prolonged periods. The increased stiffness of the material also allows its thickness to be reduced, thereby reducing raw material costs and allowing products to be made that can be folded more easily.

We have also found that the coating composition of the present invention provides the sheet with good printing characteristics. The plastic sheet can therefore be coated in a single step, as no additional coating is required to provide to ensure printability.

The polymer binder is insoluble in water and is applied as an aqueous dispersion. We have found that it is preferable to apply the polymer binder as a dispersion rather than as a solution. Water-soluble polymers are likely to have high viscosities for a given solids concentration and they remain water-soluble after coating. Although this water solubility can be reduced with cross-linking there is still a tendency for the polymer to remain re-wettable. It is therefore easier to obtain good water resistance by using polymer dispersions, which are intrinsically water insoluble.

These polymer dispersions form a film by coalescence of the individual polymer particles in the dispersion. Unlike water-soluble polymers, they require an elevated temperature to form a film. The minimum film forming temperature (MFFT) is generally at least 40 C and preferably at least 70 C. This implies a glass transition temperature Tg in the same region.

By comparison, the water soluble coatings described in GB2 177413 and EP 1055711 have a MFFT of less than 30 C. This allows a low coating drying temperature of less than 60 C, which is desirable to prevent distortion of the resulting film.

15 The tendency of orientated polyolefin materials to deform and shrink under the action of heat generally limits the types of coating that can be applied to this substrate to those that can be dried at relatively low temperatures. Surprisingly, we have found that it is practical : to coat polyolefin materials with the coating composition of the present invention, despite * the fact that the coating requires a relatively high temperature in order to form a film. We * : 20 believe that this is because the coating helps to protect the substrate from distortion even * : * at high drying temperatures.

The polymer binder is preferably an acrylic polymer. For example, we have used Texacryl 13-809 from Scott Bader Company Limited, which has a solids content of 44%, a MFFT of 76 C and a Tg of 82 C.

The cross-linking agent preferably has three or more sites of reactivity per molecule. The molecules should either be single molecules or trimers or oligomers. The cross-linking agent may be selected from a group consisting of methylolmelamines, aziridine, polycarbodiimide and urethane crosslinkers, and is preferably aziridine. We have used CX100 aziridine cross-linker from DSM. This aziridine is notable in that it does not require an additional coalescing agent. The other crosslinkers do require an additional coalescing agent.

By comparison, the polyamide epichlorohydrin crosslinkers used in the formulation disclosed in EP105571 I are not useful to enhance stiffness and do not fall within the scope of this invention. The ionic insolubilising agents described in EP1055711 could however he incornorated if required to reduce static charge build up on the sheet.

The coating composition preferably includes a filler material, comprising 20-85% of the composition by dry weight. The filler material may be a powder selected from a group consisting of ground calcium carbonate, kaolin, alumina trihydrate, titanium dioxide and barium sulphate, and is preferably calcium carbonate. The filler material may be mixed with a dispersant. We have used 50-80% ground calcium carbonate (Hydrocarb 60 OG from Omya) in water mixed with 1% acrylic dispersant (Dispex N40 from CIBA).

* The coating composition preferably includes a coalescing agent, comprising 0.5-10% of the composition by total weight. Suitable coalescing agents include Dipropylene Glycol Methyl Ether (Dowanol DPM from DOW) and Dipropylene Glycol n-Butyl Ether *:*. (Dowanol DPnB from DOW).

* The coating composition preferably includes one or more processing aids, for example * : dispersing agents and wetting agents. These processing aids may improve the behaviour * : 20 of the coating composition while being applied to a substrate using coating machinery.

According to a further aspect of the invention there is provided a plastic sheet material including a substrate comprising a sheet of a polyolefin material and a coating on at least one surface of the substrate, wherein the coating includes 10-95% by dry weight of a polymer binder having a MFFT of at least 40 C, and at least 1% by dry weight of a cross-linking agent that has reacted with groups of the polymer binder.

The (or each) coating preferably has a substance in the rangeS to 20 gsm. The coating may have a thickness in the range 2 to 40 microns, preferably 2 to 20 microns.

Advantageously, the substrate and the coating (or coatings) have a total thickness in the range 50 to 200 microns.

The substrate preferably comprises a stretch-orientated sheet of a polyolefin material. The substrate may be at least partially voided. Preferably, the substrate comprises a base layer and at least one co-extruded surface layer. Preferably, the base layer is voided. The substrate is preferably based on HDPE.

According to a further aspect of the invention there is provided a iiicthod of treating a plastic sheet material, comprising applying a coating composition according to any one of the preceding statements of invention to at least one surface of the sheet material as an aqueous dispersion, and heating the coated plastic sheet material to dry the coating composition and form a film containing the polymer binder and the cross-linking agent.

The coating composition is preferably applied at a coat weight of 5 to 20 gsm.

The coating composition may be applied simultaneously to both surfaces of the substrate and then dried, for example by heating to a surface temperature of 50-100 C, before being *:* 15 cooled to ambient temperature. We have found that the coating helps to prevent distortion of the substrate even at these relatively high drying temperatures. Alternatively, the coating composition may be applied to a first surface of the substrate and dried by heating to a first * ** temperature, and subsequently applied to a second surface of the substrate and dried by heating to a second temperature. The first temperature is preferably lower than the second temperature. This technique may help to prevent curling in the final material.

According to a further aspect of the invention there is provided a method of manufacturing a plastic sheet material, the method comprising forming a substrate comprising a sheet of a polyolefin material, and treating the plastic sheet material by a method according to any

one of the preceding statements of invention.

Advantageously, the substrate is formed by stretch-orientating a sheet of a polyolefin material. The substrate is preferably at least partially voided. The substrate may be formed by co-extruding a base layer and at least one surface layer. Preferably, the base layer is voided. The substrate is preferably based on HDPE.

The coating composition may be applied by any suitable coating technique, including coating with a wire wound bar, roller coating with air-knife metering or print-coating.

Drying of the coating may be by any means whereby the temperature may be adequately controlled to keep the plastic sheet substantially undistorted. For example, in the case of coatings on synthetic papers, air drying temperatures in the range 50-70 C may advantageously be employed to achieve rapid drying while preserving a uniform dried coating.

It will be appreciated that the plastic substrate may be comprised of any suitable plastics material. However, particularly where the surface of such plastics material is strongly hydrophobic, modification of the surface by known chemical or corona discharge treatment may be desirable prior to coating to assist wetting by the coating composition and/or to assist in achieving a good bond between the dried coating and the substrate. Preferred plastic substrates are synthetic papers as described and claimed in GB1470372 and EP105571 1, incorporated herein by reference.

An embodiment of the invention will now be described by way of the following illustrative and comparative examples.

*:r,. Example!

The following formulation was made up: * S S

Table 1

* 20 Component Material Parts by % dry __________ _________________________________________ weight weight Filler 50% ground calcium carbonate (Hydrocarb 60 OG) 30 58.1 in water with I % acrylic dispersant (Dispex N40) ________ ______ Polymer Acrylic polymer dispersion in water (Texacryl 13-20 34.1 binder 809) ________ _______ Cross-linker Aziridine (CX100) 2 7.8 The quantities shown as parts by weight relate to the wet weight of the ingredients at addition.

The coating composition was coated onto a stretch-orientated HDPE sheet made as described in EP0863 177, the content of which is incorporated by reference herein. The HDPE sheet had a substance of 6Ogsm, a thickness of 85 microns and comprised a relatively thick voided core layer between two thin co-extruded, unvoided surface layers.

The coating was applied to one side of the sheet using a wire wound bar made with 15 thou (0.3 8mm) wire and was then dried for 2 minutes at 60 C, giving a dry coat weight of l5gsm. This was then repeated for the other side of the sheet. The resulting coated sheet had a substance of 9Ogsm and a thickness of 105 microns.

Example 2

The following formulation was made up:

Table 2

Components Material Supplier Parts by % dry ___________ _______________________________ __________ weight weight Water 90 -Filler 78% ground calcium carbonate Omya 650 75.4 (Hydrocarb 60 OG) in water with 1% . : ____________ acrylic dispersant (Dispex N40) __________ _______ _______ * 15 Surfactant Surfinol Air 0.6 -

IS

Products Polymer Acrylic polymer dispersion in water Scott Bader 289 18.9 binder (Texacryl 13809) __________ _______ *S*.*.

* Alkali pH 20% ammonia generic 5.3 -adjustment hexameta-phosphate ________________________________ __________ Cross-linker Aziridine (CX100) DSM 30 4.5 The quantities shown as parts by weight relate to the wet weight of the ingredients at addition.

The materials were added to the water in the above order and dispersed, allowing about 5 minutes after each addition. An amount of the coating composition sufficient to fill both tanks of an industrial coating machine was manufactured and introduced to the coater. A reel I.4m wide of HDPE sheet with a substance of 6Ogsm and a thickness of 85 microns comprising a relatively thick voided core layer between two thin co-extruded, unvoided surface layers was loaded onto the coater. The coater applied the coating composition to one side of the substrate, using air knife doctoring to apply the coating, and dried the coating. It then applied and dried a coating on the other side of the substrate. The surface temperature of the substrate was measured as ii emerged from each drying oven, this being the maximum temperature of the web surface. The coating was applied to give a dry weight of I 5gsm to each side of the substrate and the web was run through the coater at 80mlmin.

The drying ovens were adjusted such that the web had a surface temperature of 47 C after the first side had been coated and dried and a surface temperature of 85 C after the second side had been coated and dried. * S

The material was cut into sheets with dimensions of 700mm x 1000mm, which were litho S...

printed at 7000 sheets per hour. There was no set off between the sheets. Fully oxidizing * .: inks were used, which dried in about 4 hours. The sheets were then folded into maps.

S. There was no cracking at the folds and the maps held upright without support. S..

: Comparative exampie 1 * *...

* 20 The procedure of example 1 was repeated using a similar stretch-orientated HDPE sheet and a coating composition as described in EP105571 1, comprising a filler (Martifin OL- 107), a polymeric binder (Arconal 728 and Arconal 504 in aqueous dispersions), and cross-linking agents (Bacote and Kymene SLX2), and various processing aids. The coated sheet had a similar substance and thickness to that of Example 1.

Comparative example 2 The procedure of example I was repeated using a similar stretch-orientated HDPE sheet and a coating composition made up as follows:

Table 3

Component Material Parts by % dry __________ __________________________________________ weight weight Filler 50% ground calcium carbonate (Hydrocarb 60 00) 30 63 in water with 1% acrylic dispersant (Dispex N40) _______ _______ Polymer Acrylic polymer dispersion in water (Texacryl 13-20 37 binder 809) ________ ________ It may be noted that the coating composition used in comparative example 2 is identical to that of example 1, except for the omission of the aziridine cross-linker. The resulting coated sheet had a similar substance and thickness to that of example 1.

Test results Strips with a width of 15mm were cut from each of the sheets and tested using the method : *** described above to determine the bending angle, both immediately and after three weeks.

The results were as follows: * * S

* *. Table4 * .

* ** 15 Coating composition Bending angle * : Immediately After three weeks

Example 1 25 50

Example 2 20 43

Comparative example 1 45 70 Comparative example 2 >50 -It can be seen that the inclusion of the aziridine crosslinker in examples I and 2, which increases the molecular weight of the polymer binder, greatly improves the coated sheet's stiffness and resistance to creep, even at low drying temperatures (less than 100 C). The resulting coated plastic sheet material of example 1 provided adequate performance for most applications, whereas the coated plastic sheet material of example 2 provided

II

excellent performance. With both comparative examples the performance was unsatisfactory. It was also noted that the coating in comparative example 2 was friable and easy to remove, and tended to rub off when the sheet was folded.

If improved performance is required for more demanding applications, it is expected that this can be achieved by slightly increasing the thickness of the plastic substrate sheet and br the coatings. S. *. * *0S S... * S S... *. S * .. * S.

S S * S..

S SS S.. * . . 5

S

S.....

S S

Claims

1. A coating composition for use on plastic sheet materials, wherein the coating composition comprises an aqueous dispersion that includes: a. 10% to 95% by dry weight of a polymer binder having a MFFT of at least 40 C, and b. at least 1% by dry weight of a cross-linking agent capable of reacting with groups of the polymer binder.

2. A coating composition according to claim 1, wherein the polymer binder has a MFFT of at least 70 C.

*

3. A coating composition according to claim 1 or claim 2, wherein the polymer binder * .** is an acrylic polymer. **.. * . *

4. A coating composition according to any one of the preceding claims, wherein the ::. cross-linking agent has three or more sites of reactivity per molecule.

5. A coating composition according to any one of the preceding claims, wherein the *** : cross-linking agent is selected from a group consisting of methylolmelamine * condensates, aziridine, polycarbodiimide and urethane crosslinkers.

6. A coating composition according to any one of the preceding claims, including a filler material, comprising 20-85% of the composition by dry weight.

7. A coating composition according to claim 6, wherein the filler material is selected from a group consisting of ground calcium carbonate, kaolin, alumina trihydrate, titanium dioxide and barium sulphate.

8. A coating composition according to any one of the preceding claims, including a coalescing agent, comprising 0.5-10% of the composition by total weight.

9. A coating composition according to any one of the preceding claims, including one or more processing aids, selected from a group consisting of dispersing agents and wetting agents.

10. A plastic sheet material including a substrate comprising a sheet of a polyolefin material and a coating on at least one surface of the substrate, wherein the coating includes 10-95% by dry weight of a polymer binder having a MFFT of at least 40 C, and at least 1% by dry weight of a cross-linking agent that has reacted with groups of the polymer binder.

11. A plastic sheet material according to claim 10, wherein the polymer binder has a MFFT of at least 70 C.

12. A plastic sheet material according to claim 10 or claim 11, wherein the polymer binder is an acrylic polymer.

: ...

13. A plastic sheet material according to any one of claims 10 to 12, wherein the cross-linking agent has three or more sites of reactivity per molecule.

* *

14. A plastic sheet material according to any one of claims 10 to 13, wherein the cross-linking agent is selected from a group consisting of methylolmelamine, aziridine, polycarbodiimide and urethane crosslinkers.

15. A plastic sheet material according to any one of claims 10 to 14, wherein the coating includes a filler material, comprising 20-85% of the coating by dry weight.

16. A plastic sheet material according to claim 15, wherein the filler material is selected from a group consisting of ground calcium carbonate, kaolin, alumina trihydrate, titanium dioxide and barium sulphate.

17. A plastic sheet material according to any one of claims 10 to 16, wherein the coating includes a coalescing agent, comprising 0.5- 10% of the coating by total weight.

18. A plastic sheet material according to any one of claims 10 to 17, wherein the coating has a substance in the range 5 to 20 gsm.

19. A plastic sheet material according to any one of claims 10 to 18, wherein the coating has a thickness in the range 2 to 40 microns, preferably 2 to 20 microns.

20. A plastic sheet material according to any one of claims 10 to 19, wherein the substrate and the coating have a total thickness in the range 50 to 200 microns.

21. A plastic sheet material according to any one of claims 10 to 20, wherein the substrate comprises a stretch-orientated sheet of a polyolefin material.

22. A plastic sheet material according to any one of claims 10 to 21, wherein the substrate is at least partially voided.

23. A plastic sheet material according to any one of claims 10 to 22, wherein the :*. substrate comprises a base layer and at least one co-extruded surface layer.

24. A plastic sheet material according to claim 23, wherein the base layer is voided.

25. A plastic sheet material according to any one of claims 10 to 24, wherein the * * * substrate is based on HDPE.

*.

26. A method of treating a plastic sheet material, comprising applying a coating : * composition according to any one of claims 1 to 9 to at least one surface of the sheet material as an aqueous dispersion, and heating the coated plastic sheet material to thy the coating composition and form a film containing the polymer binder and the cross-linking agent.

27. A method according to claim 26, wherein coating composition is applied at a coat weight of 5 to 20 gsm.

28. A method according to claim 26 or claim 27, wherein the coating composition is applied simultaneously to both surfaces of the substrate.

29. A method according to claim 26 or claim 28, wherein the coating composition is applied to a first surface of the substrate and is heated to a first temperature, and subsequently the coating composition is applied to a second surface of the substrate and is heated to a second temperature.

30. A method according to claim 29, wherein the first temperature is lower than the second temperature.

31. A method of manufacturing a plastic sheet material, the method comprising forming a substrate comprising a sheet of a polyolefin material, and treating the plastic sheet material by a method according to any one of claims 26 to 30.

32. A method according to claim 31, wherein the substrate is formed by stretch-orientating a sheet of a polyolefin material.

33. A method according to claim 32, wherein the substrate is at least partially voided.

34. A method according to any one of claims 31 to 33, wherein the substrate is formed by co-extruding a base layer and at least one surface layer.

*

35. A method according to claim 34, wherein the base layer is voided.

:

36. A method according to any one of claims 29 to 35, wherein the substrate is based * on HDPE.

*.**.. * *