WO2020001399A1

WO2020001399A1 - Multi-layered, food-safe and non-stick coating

Info

Publication number: WO2020001399A1
Application number: PCT/CN2019/092541
Authority: WO
Inventors: Wenjun MENG; Deryck Hinyeung LI; Michael Kwunfung LO; Sheungyin LI; Yueyue ZHAO; Connie Saukuen KWOK
Original assignee: Nano And Advanced Materials Institute Limited
Priority date: 2018-06-24
Filing date: 2019-06-24
Publication date: 2020-01-02

Abstract

A multi-layered, food-safe, and non-stick coating comprising at least one silica gel layer(202) and at least one polydimethylsiloxane-based layer being coated on a substrate(201) is provided, wherein the silica gel layer(202) is superhydrophobic and prepared by adding hexamethyldisilazane into an aged silica sol formed by hydrolysis and condensation of tetraethyl orthosilicate(203b) in a mixture of ethanol, ammonia and water, and the polydimethylsiloxane-based layer is hydrophobic and prepared by partially cross-linking silane moieties of polydimethylsiloxane(203a) with tetraethyl orthosilicate(203b) such that polydimethylsiloxane(203a) is silanol-terminated and covalently grafted onto said silica gel layer(202), and the substrate(201) coated with the multi-layered, food-safe, and non-stick coating has a contact angle of at least 50° with respect to a viscous food substance.

Description

MULTI-LAYERED, FOOD-SAFE AND NON-STICK COATING

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional patent application serial number 62/689,243 filed on June 24, 2018, and the disclosure of which are incorporated herein by reference in their entirety.

FIELD OF INVENTION

The present invention relates to formation of a multi-layered, food-safe, non-stick coating on a substrate for enhancing fluid flow on a surface of the substrate. In particular, the present coating comprises at least two layers and the substrate can be made of glass, metal, silicone, or thermoplastic polymer which are in contact with viscous food substances such as water, ketchup, oyster sauce, syrup, chili sauce, etc.

BACKGROUND

When bottles of ketchup or oyster sauces are nearly empty, it takes great efforts to remove the remaining materials from the walls of the bottle.

Amorphous grade poly (ethylene terephthalate) (PET) sheets for food contact application have been used in food industries. Several compounds were known for their omniphobic performance, such as fluorinated compounds and polydimethylsiloxane (PDMS) . But due to the potential health hazard of fluorinated compounds leaching out from coatings, chemical industry is now phasing out the use of perfluorooctanoic acid (PFOA) .

Although PDMS is widely used as antifouling coating due to its low surface free energy (19.8 mN/m) , fully crosslinked PDMS rubber, such as Sylgard 184, does not show any slippery performance. Crosslinking reduces the flexibility of polymer chain, leading to reduction in repellent performance. Several research groups have worked on this issue by grafting low molecular weight PDMS to form a PDMS brush to reduce the friction against different liquid such as blood. However, this approach mainly worked on inorganic substrates but seldom on plastics due to their chemical inertness.

SUMMARY OF INVENTION

To solve the problems in the existing technologies, a coating comprising at least one silica gel layer and at least one polydimethylsiloxane (PDMS) -based layer is provided. A method for preparing the present coating on a substrate is also provided.

In one embodiment, the silica gel layer is prepared by hydrolysis and condensation with tetraethyl orthosilicate in an alcohol with an acid or a base catalyst to form a silica gel solution. The weight percentage of the tetraethyl orthosilicate used in preparation of the silica gel solution can range from 5 w/v%to 40w/v%. The weight percentage of the acid or the base catalyst used in preparation of the silica gel solution can range from 0.0001 w/v%to 5 w/v%. The alcohol used in preparation of the silica gel solution comprises methanol, ethanol and isopropanol, or any combination thereof. The acid catalyst used in preparation of the silica gel solution comprises hydrochloric acid, sulfuric acid, nitric acid, acetic acid, trifluroacetic acid and p-toluenesulfonic acid, or any combination thereof. The base catalyst could be sodium hydroxide, potassium hydroxide, ammonium hydroxide, tetramethylammonium hydroxide, hexamethyldisilazane, pyridine, caffeine, purines, or pyrimidines. The duration for preparation of the silica gel solution can range from 1 week to 3 months. The temperature used in preparation of the silica gel can range from -20 degrees Celsius to 40 degrees Celsius.

In another embodiment, the coating of the silica gel solution applies onto the substrate to obtain a silica gel coated substrate. The application of silica gel solution can be carried out by spray coating, spin coating, dip coating and painting, or any combination thereof. The temperature used for coating the silica gel layer can range from -20 degrees Celsius to 40 degrees Celsius.

In other embodiment, the grafting of polydimethylsiloxane on the silica gel coated substrate comprises immersing the silica gel coated substrate into a mixture of silanol-terminated polydimethylsiloxane and tetraethyl orthosilicate under an elevated temperature. Molecular weight of the silanol terminated polydimethylsiloxane can range from 500 to 20, 000 Da. The weight ratio between silanol terminated polydimethylsiloxane and tetraethyl orthosilicate can range from 100: 1 to 1: 10, more specifically, 10-4: 1-6. The temperature used in the grafting process can range from 40 degrees Celsius to 250 degrees Celsius, more specifically, from 120 degrees Celsius to 180 degrees Celsius. The duration of the grafting process can range from 30 minutes to 48 hours, more specifically, from 30 minutes to 22 hours.

Optionally, an additional primer layer of poly (dopamine) (PDA) can be deposited on the substrate prior to the deposition of the silica gel and PDMS-based layers. In one embodiment, the primer coating comprises a crosslinked polymer with catechol derivatives on polymer side chains or backbones. Monomer (s) of the crosslinked polymer comprises dopamine hydrochloride and L-3, 4-dihydroxyphenylalanine, or any combination thereof. The primer coating is prepared by immersing the substrate in dopamine solution with alkaline buffer. In said preparation of primer coating, the weight percentage of dopamine in the dopamine solution can range from 0.05 w/v%to 2 w/v%. The pH value of the alkaline buffer used in the preparation of the primer coating can range from 8 to 11. The duration for preparation of the primer coating can range from 30 minutes to 48 hours. The temperature used in preparation of the primer coating can range from 5 degrees Celsius to 60 degrees Celsius.

This Summary is intended to provide an overview of the present invention and is not intended to provide an exclusive or exhaustive explanation.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described in more detail hereinafter with reference to the drawings, in which:

FIG. 1 schematically illustrates formation of the present coating on a substrate according to an embodiment of the present invention;

FIG. 2A is a schematic diagram showing a structure of the present coating on a substrate;

FIG. 2B is a schematic diagram showing modification of polydimethysiloxane to form a silicone layer which is rich of silanol groups according to an embodiment of the present invention;

FIG. 3 is an image of different glass slide samples subject to water droplet test;

FIGs. 4A and 4B are images of glass substrate with and without the present coating taken during a water contact angle measurement, respectively;

FIGs. 5A and 5B are images of glass substrate with and without the present coating taken during a honey contact angle measurement, respectively;

FIGs. 6A and 6B are images of glass substrate with and without the present coating taken during a cooking oil contact angle measurement, respectively;

FIGs. 7A and 7B are images of glass substrate with and without the present coating taken during a soy sauce contact angle measurement, respectively;

FIGs. 8A and 8B are respective images of glass bottles from perspective view and bottom view, which are filled with some ketchup and where their inner surface is coated with single, double and triple silica gel layers;

FIGs. 9A and 9B are images of glass bottles from bottom view which are filled with some ketchup and soy sauce, respectively, where the inner surface is uncoated, coated with one layer of silica gel layer and one layer of PDMS-based layer, and coated with one layer of silica gel layer and two PDMS-based layers;

FIG. 10 illustrates coating of an additional primer layer on a PET substrate prior to the present coating according to an embodiment of the present invention;

FIG. 11 is an image showing the morphology of a water droplet on a silicone substrate with and without the present coating.

DETAILED DESCRIPTION OF INVENTION

The present invention is not to be limited in scope by any of the following descriptions. The following examples or embodiments are presented for exemplification only.

References in the specification to “one embodiment” , “an embodiment” , “an example embodiment” , etc., indicate that the embodiment described can include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

Values expressed in a range format should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a concentration range of “about 0.1%to about 5%” should be interpreted to include not only the explicitly recited concentration of about 0.1 wt. %to about 5 wt. %, but also the individual concentrations (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.1%to 0.5%, 1.1%to 2.2%, and 3.3%to 4.4%) within the indicated range.

In this document, the terms “a” or “an” are used to include one or more than one and the term “or” is used to refer to a nonexclusive “or” unless otherwise indicated. In addition, it is to be understood that the phraseology or terminology employed herein, and not otherwise defined, is for the purpose of description only and not of limitation. Furthermore, all publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.

In the methods of preparation described herein, the steps can be carried out in any order without departing from the principles of the invention, except when a temporal or operational sequence is explicitly recited. Recitation in a claim to the effect that first a step is performed, and then several other steps are subsequently performed, shall be taken to mean that the first step is performed before any of the other steps, but the other steps can be performed in any suitable sequence, unless a sequence is further recited within the other steps. For example, claim elements that recite “Step A, Step B, Step C, Step D, and Step E” shall be construed to mean step A is carried out first, step E is carried out last, and steps B, C, and D can be carried out in any sequence between steps A and E, and that the sequence still falls within the literal scope of the claimed process. A given step or sub-set of steps can also be repeated.

Furthermore, specified steps can be carried out concurrently unless explicit claim language recites that they be carried out separately. For example, a claimed step of doing X and a claimed step of doing Y can be conducted simultaneously within a single operation, and the resulting process will fall within the literal scope of the claimed process.

Definitions

The singular forms “a” , “an” and “the” can include plural referents unless the context clearly dictates otherwise.

The term "about" can allow for a degree of variability in a value or range, for example, within 10%, or within 5%of a stated value or of a stated limit of a range.

The term "independently selected from" refers to referenced groups being the same, different, or a mixture thereof, unless the context clearly indicates otherwise. Thus, under this definition, the phrase "X1, X2, and X3 are independently selected from noble gases" would include the scenario where, for example, X1, X2, and X3 are all the same, where X1, X2, and X3 are all different, where X1 and X2 are the same but X3 is different, and other analogous permutations.

The following examples accompanied with drawings will illustrate the present invention in more detail.

EXAMPLES

The embodiments of the present invention can be better understood by reference to the following examples which are offered by way of illustration. The present invention is not limited to the examples given herein.

1. Preparation of silica gel layer

As shown in FIG. 1, the superhydrophobic silica coating can be derived from a hexamethyldisilazane (HMDS) treated silica sol, which was synthesized by the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol (EtOH) solvent. The mole ratio was TEOS: EtOH: NH ₃: H ₂O = 1: 38: 0.54: 1.53 (All the water was from the 28%ammonia solution) . First half of EtOH was mixed with TEOS and the other half was with ammonia and water, and the two solutions were stirred for about 10 minutes or more. Afterwards, the solution with NH ₃ was added into the TEOS solution with stirring for about 10 minutes or more. The mixed solution was then aged at room temperature for 5-7 days to become a silica sol, then a certain quantity of HMDS (0.3-0.6 molar ratio of TEOS) was added into the sol with stirring for about 10 minutes or more; after that, the sol needed to settle for at least 1 day and was ready for use. Glass bottles were coated with this silica sol. It was not necessary to heat the coating, and just two minutes drying in air was enough to obtain the superhydrophobic surface. The resulting surface with CH ₃ groups were from HMDS; unhydrolyzed –OC ₂H ₅ groups were from TEOS; and unmodified –OH groups were on the silica particles. Deposition of this layer can be done multiple times to improve lubricity.

2. Preparation of PDMS-based Layer

After forming the first layer of silica gel, the object (e.g., glass bottles) were then coated with the mixture of PDMS and TEOS in a mole ratio of 4: 6 and cured at 120 ℃ for 22 hours (or 180 ℃ for 30 minutes) . Again, multiple depositions of this layer can be done to improve lubricity of the final coated surface.

3. Washing by detergent

After forming the second layer of PDMS-based material, the surface was washed by detergent and under sonication for 30 minutes or until the surface is free of loose lubricant.

As shown in FIG. 2A, the present coating formed on a glass substrate (201) includes at least a silica gel layer (202) and a layer of PDMS-based material (not shown in FIG. 2) , where the PDMS-based layer is composed of silanol-terminated PDMS by partially crosslinking silanol moieties of PDMS (203a) with TEOS (203b) such that the silanol-terminated PDMS is covalently grafted on the silica gel layer or substrate to form an omniphobic coating. The added HMDS to the silica sol turns the silica gel layer into superhydrophobic. As shown in FIG. 2B, the coated glass substrate (201a) with the silanol-terminated PDMS layer provides a flexible, stable and non-leaching structure. TEOS in the present coating has two main roles: as a precursor of silica particles in the silica gel layer; as a cross-linking agent for PDMS to form the PDMS-based layer. PDMS is selected as the polymeric material of the non-stick surface coating in the present invention because it can be hydrophobic after cross-linking, is not wetted by polar solvents, provides a clear and shiny surface, and is a FDA-approved food-safe polymer.

Apart from applying on a glass substrate, the present coating is also applicable on other substrates such as metal (e.g. aluminum, stainless steel) , silicone, and thermoplastic elastomer (TPE) , etc.

4. Droplet Sliding Test

To evaluate the slippery performance of the present coating on a substrate (e.g., glass substrate) and the effect of different curing temperatures and durations on such performance, water droplet test was performed on glass slides coated with the present coating cured at 120℃ for 22 hours, 180℃ for 30 minutes, and uncoated glass slides. As shown in FIG. 3, two water droplets were applied onto a flat surface of each of different glass slide samples and the samples were tilted at the same angle at the same time. It was observed that after tilting, the droplets on the glass slide with the present coating cured at 120℃ for 22 hours slided off first, followed by the slide with the coating cured at 180℃ for 30 minutes, the last was the uncoated glass slide.

5. Contact Angle Measurement

Different viscous food substances with different viscosities may have different contact angles on a substrate with and without the present coating. To verify, different food substances including water, honey, cooking oil, and soy sauce were used to drop on a flat surface of glass and silicone substrates with and without the present coating. The photos showing the morphology of the food substance droplet on the surface of the glass substrate with and without the present coating are shown in FIG. 4, and the measured contact angle of each food substance on each glass substrate sample is summarized in Table 1:

Table 1

A surface with a lower contact angle represents a more hydrophilic surface to a substance, whereas a surface with higher contact angle represents a more hydrophobic surface to the same substance. In this test, it was shown that the glass substrate coated with the present coating has a contact angle with all tested food substances more than double the contact angle of the uncoated glass substrate with those substances FIGs. 4A and 4B, 5A and 5B, 6A and 6B, and 7A and 7B are images of different food substances on glass substrates with and without the present coating, respectively. FIG. 11 further shows a water droplet on a silicone substrate with and without the present coating.

6. Effect of Deposition of Multiple Layers of the Present Coating On Sliding (Non- stick) Performance

As shown in FIGs. 8A and 8B, the inner surface of the glass bottle coated with three (triple) layers of silica gel prior to the deposition of the PDMS-based layer had the most non-stick surface (the highest sliding rate) with respect to the tested food substance, ketchup, as compared to those coated with two (double) layers and one (single) layer of silica gel prior to deposition of the PDMS-based layer.

As shown in FIGs. 9A and 9B, the inner surface of the glass bottles coated with two PDMS-based layers on top of the silica gel layer had the most non-stick surface (the highest sliding rate) with respect to the two tested food substances, ketchup and soy sauce, respectively, as compared to that coated with one PDMS-based layer and without the present coating.

Accordingly, it can be seen that the present coating comprising two or more silica gel layers followed by depositing two or more PDMS-based layers coated on a substrate will give the best sliding or slippery (non-stick) performance in terms of the residue of the tested food substances left on the surface over a period of time.

7. Optional Primer Layer Deposited on Substrate Prior to Deposition of Silica Gel Layer

In this embodiment, a primer layer comprising a crosslinked polymer with catechol derivatives on polymer side chains or backbones is deposited on the substrate prior to the deposition of the silica gel layer and PDMS-based layer of the present coating. Monomer (s) of the crosslinked polymer comprises dopamine hydrochloride and L-3, 4-dihydroxyphenylalanine, or any combination thereof. The primer layer is prepared by immersing the substrate in dopamine solution with alkaline buffer. The weight percentage of dopamine in the dopamine solution can range from 0.05 w/v%to 2 w/v%. The pH value of the alkaline buffer used in the preparation of the primer coating can range from 8 to 11. The duration for preparation of the primer coating can range from 30 minutes to 48 hours. The temperature used in preparation of the primer coating can range from 5 degrees Celsius to 60 degrees Celsius. FIG. 10 shows an example of how to prepare a primer layer on a PET substrate.

A 4 cm x 5 cm PET sheet was immersed into TRIS buffer with pH at 8.5. Dopamine hydrochloride was then added into the buffer to give a 0.2 wt%dopamine solution. It should be noted that the auto-oxidation of dopamine starts instantly under alkaline condition, followed by the polymerization. Obvious colour change could be noted during the polymerization process. The reaction mixture turned from colourless to yellow during auto-oxidation and then turned into black during the polymerization. After soaking in 0.2 wt%dopamine solution for 2 hours, the PET sheet was rinsed by deionized water to remove any excess reagents on surface, and then dried in oven at 50 ℃ for 4 hours to remove any moisture content. The PDA coated PET sheet (PET/PDA) turned slightly grey comparing to the virgin material. Another observation was its excellent wetting performance, which suggested the presence of hydrophilic PDA coating on top of PET.

Poly (dimethylsiloxane) glycidyl monoether (PDMS epoxide) with an average molecular weight of 5, 000 Da was then added on the PDA coated PET at 50 ℃ for 4 hours. The glycidyl monoether (epoxide) reacted with any one of the available functional groups of the catechol on PDA through ring opening reaction. Excess silicone oil on PET was removed by successive cleaning in detergent solution with sonication for three times, followed by deionized water to remove detergent left on surface. This coating formulation was named as PET/PDA-E5000.

A sliding performance test was carried out on this PDMS grafted PET, and the result was shown in Table 2. 0.5 mL of viscous sauce was added on the sample sheet. The sample sheet was then held nearly vertical to observe the sliding behaviour of the viscous sauce. The PDMS grafted PET had an excellent performance comparing to the uncoated PET in the sliding experiment. The sliding speed increased more than 86%for ketchup and 22%for oyster sauce.

Table 2

Sliding speed/mm s ^-1	Ketchup	Oyster sauce
PET	0.36	0.32
PET/PDA-E5000	0.67 (+86%)	0.39 (+22%)

The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

INDUSTRIAL APPLICABILITY

The present invention is useful in forming non-stick surface on various substrates which require food-safe grade materials. Ease of scale-up by forming multi-layered structure of the present coating on various substrates increases the applicability of the present invention on various food containers and food processing industries.

Claims

A multi-layered, food-safe, and non-stick coating comprising at least one silica gel layer and at least one polydimethylsiloxane-based layer being coated on a substrate, said at least one silica gel layer being superhydrophobic and prepared by adding hexamethyldisilazane into an aged silica sol formed by hydrolysis and condensation of tetraethyl orthosilicate in a mixture of ethanol, ammonia and water, and said at least one polydimethylsiloxane-based layer being hydrophobic and prepared by partially cross-linking silane moieties of polydimethylsiloxane with tetraethyl orthosilicate such that polydimethylsiloxane is silanol-terminated and covalently grafted onto said silica gel layer, and the substrate coated with the multi-layered, food-safe, and non-stick coating having a contact angle of at least 50° with respect to a viscous food substance.
The coating of claim 1, wherein said tetraethyl orthosilicate to ethanol, ammonia and water, respectively, in the mixture of ethanol, ammonia and water is in a mole ratio of 1: 38: 0.54: 1.53, and said hexamethyldisilazne is added in a mole ratio of 0.3 to 0.6 to said tetraethyl orthosilicate.
The coating of claim 1, wherein said polydimethylsiloxane to said tetraethyl orthosilicate is in a mole ratio of 10-4: 1-6.
The coating of claim 3, wherein said polydimethylsiloxane to said tetraethyl orthosilicate is in a mole ratio of 4: 6, and a curing temperature of at least 120 degree Celsius and duration from 30 minutes to 24 hours are used in said cross-linking.
The coating of claim 4, wherein said curing temperature is 120 degree Celsius and said duration is 22 hours.
The coating of claim 4, wherein said curing temperature is 180 degree Celsius and said duration is 30 minutes.
The coating of claim 1, wherein said substrate is made of glass, metal, silicone, or thermoplastic polymer.
The coating of claim 1, wherein said coating comprises three of said silica gel layers and three of said polydimethylsiloxane-based layers coated on said substrate.
The coating of claim 1, wherein said food substance comprises water, honey, cooking oil, soy sauce, and chili sauce.
A method for forming the coating of any one of claims 1 to 9, comprising:

mixing tetraethyl orthosilicate with ethanol, ammonia and water in a mole ratio of 1: 38: 0.54: 1.53 to ethanol, ammonia and water to form a mixture and aging the mixture for at least 5 days until an aged silica sol is formed;

adding hexamethyldisilazne into the aged silica sol in a mole ratio of 0.3 to 0.6 to said tetraethyl orthosilicate and stirring for about 10 minutes or more until a silica gel is formed;

depositing said silica gel on said substrate to form the silica gel layer;

mixing polydimethylsiloxane and tetraethyl orthosilicate in a mole ratio of 4: 6 to form a mixture and curing the mixture at 120℃ for 22 hours or at 180℃ for 30 minutes such that said polydimethylsiloxane is partially silanol-terminated; and

depositing the cured mixture of silanol-terminated polydimethylsiloxane on the at least one silica gel layer to form the at least one polydimethylsiloxane-based layer.
The method of claim 10, wherein said substrate is made of glass, metal, silicone or thermoplastic polymer.
The method of claim 10, wherein said aging is performed at room temperature for 5-7 days.
The method of claim 10, wherein said stirring of hexamethyldisilazne in the aged silica sol is followed by settling for about one day in order to form the silica gel.
The method of claim 10, wherein said depositing said silica gel on said substrate and said depositing the cured mixture of silanol-terminated polydimethylsiloxane on the at least one silica gel layer are repeated for more than two times in order to form two or more of said silica gel layers on the substrate and two or more of said polydimethylsiloxane-based layers on said two or more silica gel layers.