REPULPABLE SILICONE RELEASE LINER PAPER WITH DISSOLVABLE LAYER

Abstract

A method of producing and repulping paper having a silicone release liner includes producing paper having a silicone release liner that is initially secured to the paper by a dissolvable material. During production of the paper, a water-soluble coating may be applied to a paper surface, and a silicone release coating is then applied to the water-soluble coating. Upon rewetting of the paper during repulping, the water-soluble coating dissolves, thereby enabling separation of the silicone release coating from the paper. The repulpable silicone release liner permits recovery of paper fibers using conventional repulping and recycling processes.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Application No. PCT/US2017/051999, filed on Sep. 18, 2017, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 62/396,926, filed on Sep. 20, 2016, entitled “REPULPABLE SILICONE RELEASE LINER PAPER WITH DISSOLVABLE LAYER,” the entire disclosures of which are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

Paper is a porous material composed of cellulose fibers that can be readily penetrated by silicone coatings. For this reason, dense papers, such as glassine, parchment, and greaseproof papers, are used in order to minimize the amount of silicone that must be applied to obtain the required release characteristics of paper. Other low-porosity and smooth papers used as the base stock for release papers are: coated publishing papers, label face papers, non-coated supercalendered (SC) papers, and machine glazed (MG) papers. Due to the high cost of silicone, papers requiring minimal additions of silicone to their surface for release are typically most desired. Porosity and smoothness may greatly impact the amount of silicone required.

Due to the large amount of paper that is used, a considerable amount of waste paper is collected and recycled. This waste paper contains reusable fiber if the fibers can be separated from their non-cellulosic impurities during the recycling process. During known recycling processes, collected wastepaper and water is added to a pulper, which breaks apart (disintegrates) the paper into individual fibers by mechanical action. This is followed by various stages of cleaning and screening to obtain fibers that are as pure as possible to prevent or reduce problems that may otherwise occur during re-use in a papermaking process.

Release papers play a vital role in the fabrication of pressure-sensitive adhesive (PSA) labels and tags. Release papers serve as a disposable base layer to carry the labels through conversion and dispensing. Release papers are also used in the manufacturing of self-adhesive materials and components for tapes, industrial and graphic applications. Methods for obtaining release papers for adhesives are typically based on at least one of three known chemistries: silicone, chrome complexes, and extruded PE (polyethylene). However, backing paper that has been made utilizing these known chemistries may create difficulties during recycling of paper.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present disclosure is a method making repulpable paper. The method includes coating at least a portion of a sheet of paper with a water-soluble material. At least a portion of the water-soluble material is coated with a silicone material to form a silicone release paper. The method may include providing label stock having adhesive on at least one side thereof, and releasably adhering the label stock to the silicone release paper by bringing the adhesive into contact with the silicone material. The silicone material may initially comprise a silicone polymer, a crosslinker, and a catalyst. The silicone material may be selected from a group consisting of solvent-based silicone, water-based silicone, and solvent-less silicone. The water-soluble material may comprise sodium alginate and partially hydrolyzed polyvinyl alcohol. The water soluble material may comprise a water soluble polymer. The water soluble polymer may be selected from the group consisting of carboxymethylated cellulose, ethylated starch, and carboxylated soy protein. The water soluble material may form a coating having a coat weight of about 2.0-6.0 gsm. The silicone material may form a coating having a coat weight of about 2.0-100.0 gsm. The method may include repulping the paper after coating the paper with water-soluble material and silicone material. Repulping the paper may include bringing the water-soluble material into contact with water. The paper may be soaked in water for at least about 30 minutes or at least about 60 minutes. A hydrapulper machine may be utilized to repulp the paper. The paper may be soaked in water prior to placing the paper into a hydropulper machine. Repulping the paper may result in less than about 15% rejects according to predefined criteria. The percent rejects may comprise oven dried rejects divided by the product of the consistency of the paper and the total mass of the paper.

Another aspect of the present disclosure is a method of making repulpable paper. The method includes coating at least a portion of a sheet of paper with a layer of dissolvable sacrificial material. At least a portion of the dissolvable sacrificial material is coated with a release material having non-stick properties. The dissolvable sacrificial layer may be dissolved in water, and the release material may comprise silicone.

These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic cross sectional view showing the layers of a typical silicone release paper;

FIG. 2 shows a condensation reaction between a polymer and a crosslinker to form a cross linked silicone network;

FIG. 3 is a chart showing % rejects for various materials;

FIG. 4 is a chart showing tensile load for repulped samples utilizing different materials;

FIG. 5 is a chart showing tensile extension for different materials; and

FIG. 6 is a graph showing Oxford Twin X XRF Silicone Content Test Results.

DETAILED DESCRIPTION

With reference to FIG. 1, a typical silicone release paper 1 includes label stock 2 that is adhered to a silicone layer 6 by adhesive 4. Silicone 6 is disposed on surface size 8 of a base sheet of paper 10.

With reference to FIG. 2, a typical silicone release coating includes a silicone polymer, a crosslinker, and a catalyst. A condensation reaction occurs between the catalyst and silicone polymer liberating hydrogen inducing the formation of a cross-linked silicone network.

Silicone may be used to produce release papers due to its very low surface tension. The silicone molecule also has a low molecular polarity, which facilitates binding or crosslinking of the silicone to the cellulose fibers in paper. Three different types of silicones used for the manufacture of release papers are: solvent-based, water-based, and solvent-less. While the use of these materials provides performance for a broad range of adhesives, release liners produced from these materials are not repulpable or recyclable. Chrome complexes represent a potential environmental hazard due to their chromium content and the resulting problems of heavy metals. PE extruded papers are also not recyclable because the PE polymer cannot be disintegrated in a typical repulper.

Non-recyclable release liners result in significant cost because non-recyclable release liners must be sent to a landfill or incinerated. Silicone coatings are typically not recyclable because the silicone cannot be completely separated from the fibers. Even trace amounts of silicone in a recycle stream may cause spots on the resulting paper and/or build up on a paper machine, resulting in expensive down time for cleaning.

The present disclosure provides for the application of a water-soluble sacrificial coating layer to a low porosity, smooth base paper, such as an MG paper, to provide repulpability of such papers. The water-soluble coating layer may be of sufficient thickness to completely cover the surface of the paper to prevent penetration of the silicone into the network of paper fibers. It will be understood that providing sufficient thickness does not necessarily imply a specific coverage. Completely covering the surface of the fibers with a sacrificial coating layer permits the removal of a silicone release coating applied over the sacrificial coating upon immersion in water. Specifically, the underlying sacrificial layer is re-solubilized when the coated fibers are immersed in water, thereby permitting removal of the silicone release coating.

As discussed below, a sacrificial layer of sodium alginate and partially hydrolyzed polyvinyl alcohol may be utilized. However, other water soluble polymers such as carboxymethylated cellulose, ethylated starch, and carboxylated soy protein could also be used. An advantage of the method/technology of the present disclosure is the ability to produce silicone release papers that can be repulped using conventional repulping processing equipment without chemical additives, hence making recovery of the fibers economically feasible.

EXAMPLE

A commercial grade Duncote Ultra, Lightweight Coated, paper was used as the base paper to fabricate test samples. For the experiment, first and second samples of the paper were coated with first and second different water-soluble polymers, respectively, using different Myer rods to obtain approximately the same coat weight (3.0-4.0 gsm) on the first and second test samples. The first and second water soluble polymers used were sodium alginate, S-15-C (“alginate”) (available from SNP Inc.) and PVA, Selvol 2035 (available from Sekisui America Corporation, Secaucus, N.J.), respectively. The alginate was formulated to an 8.0% solution (by weight) by slowly adding the alginate to water and allowing it to mix for about 30 minutes. The PVA was formulated to a 28% solution (by weight) by adding the dry powder into room temperature water under a mixer. The contents were then heated on a steam table under agitation until the PVA was completely dissolved. Use of #30 and #6 rod for the alginate and PVA, respectively, enabled the desired target coat weight to be obtained for the first and second test samples. The actual coat weights applied were 3.84 (+0.19) gsm for the alginate and 4.05 (+0.14) gsm for the PVA. Coat weights were measured gravimetrically using a Labwave CEM solids analyzer.

The first and second test samples were then coated with two different commercially available silicones using a 1.5 mil Byrd applicator. The first test sample was coated with Sylgard®-184 (Dow Corning) silicone mixed in the recommended 10:1 ratio of silicone to cross-linker, and the second test sample was coated with Wacker 944 Dehesive® 944 silicone release coating (available from Wacker Chemie AG) using a low SiH formulation consisting of 60.35% Dehesive® 944, 37.59% toluene, 0.3% Crosslinker V90, and 1.76% Catalyst C 05. To cure the silicone coatings, the first and second test samples were dried in a thermal oven at 257° F. for 20 minutes. After drying, a silicone cure test was performed by measuring the tape adhesion properties using Scotch® brand cellophane tape 610 to ensure a complete cure had been achieved. All test samples showed no degree of detackification. After being dried, the first and second test samples were conditioned in a TAPPI standard control room (50% RH and 85° F.) for 24 hours and the coat weights of the applied silicone was measured. The high coat weight of the Sylgard® in comparison to the Wacker product is likely due to the extremely high viscosity of the Sylgard®.

The first and second test samples were then separately repulped using a Maelstrom laboratory hyrapulper (available from Maelstrom Advanced Process Technologies LTD, Derbyshire, UK). As a control sample, a commercial silicone release paper (label stock removed) was also repulped. The repulping was performed at a 6% consistency by tearing a known weight of each test sample into an appropriate amount of water contained within the Maelstrom hydrapulper. During addition of the test samples, the conditions of the hydrapulper were initially maintained at 300 rpm and 140° F. For each test sample, the mixing speed was increased to 400 rpm, approximately half way through the addition of samples (i.e., after about 15 minutes) to the hydrapulper. Upon completion of each test sample addition, the speed of the hydrapulper was increased to 700 rpm and held at this speed for 20 minutes. After 20 minutes the hydrapulper was discharged and the consistency of the repulped material was measured.

A portion of each test sample was then taken and weighed for screening. Screening was performed using a Johnson® 6-cut screen (available from Johnson Screens®) to separate and collect the acceptable and non-acceptable waste materials (streams) for weighing. The first and second test samples were allowed to run in the 6-cut screen until no apparent fibers were present in the accepts stream. To determine the weight of rejects, the collected reject test samples were poured into a Buchner funnel containing a pre-weighed oven dried filter Whatman filter paper. The reject test sample and filter paper were then dried on a hot plate until bone dry, at which time the combined weight of the reject sample and filter was measured. The % rejects was then calculated by dividing the weight of the oven dried rejects (ODR) of each test sample by the consistency of the stock multiplied times the total mass of the test stock fed into the 6-cut. It will be understood that the % rejects is found using an equation that divides the weight of over dried fiber in reject stream by the total mass of oven dried fiber in the input stream. The consistency is the weight of oven dried fiber/oven dried fiber+water. All terms are defined by industry standards and the 15% is set by the RPTA. Next, the consistencies of the accepts were measured to enable the mass required to produce a 1.2 g oven dried TAPPI standard hand sheet to be determined.

TAPPI standard hand sheets were then made for each test sample set following TAPPI standard method T-205 procedures. After allowing the hand sheets to condition for a minimum of 24 hours the weight of each test sample was measured using a Mettler (AE 260) 3-point scale to enable the calculation of tensile index values. Tensile tests were performed according to TAPPI standard T-494 using an Instron Tensile Tester. Peak load and extension were tested. Ten tests were performed for each sample. The water absorptivity of each sample was also measured using a FTA dynamic contact angle apparatus. Both sides of the samples were tested utilizing an Oxford Instruments Lab-X 3000 EDRF (X-Ray) unit calibrated for silicone coat weights.

For this testing, each set of test samples was corrected using the Duncote base paper as the control to determine baseline of Si present, from clay, prior to silicone coating. The roughness and permeability of the samples prior to application of the silicone were measured using a Parker Print-Surf, Model No. ME-90 instrument. For roughness readings, a soft-backing plate was used at both 500 and 1000 kPa.

Test Results and Discussion

Contact angle measurements of the hand sheets made from the accepts stream failed due to water penetrating the substrate too quickly to enable image capture, and hence, analysis of contact angle changes from sample to sample. Results of the base sheet properties are shown in Table 1.

TABLE 2
Paper Properties (Base Sheet Duncote Paper)
	Surface Energy	Roughness	Caliper	Permeability
	(dyne/cm)	(μm)	(μm)	(mL/min)
	49.3 ± 0.5	0.83 ± 0.03	74.0 ± 1.5	2.11 ± 0.08

The results of the reject percentages collected from each repulping study are shown in FIG. 3.

The results show a decrease in the rejects for the Wacker silicone coated papers with sacrificial layer present. The % rejects was lower for the PVOH pre-coated papers than the alginate pre-coated papers. This could be due to the higher roughness of the alginate samples caused by greater shrinkage during drying do to its lower solids. The results for the Sylgard were completely different. As shown, the % rejects increased for the samples pre-coated with alginate and PVOH. Upon further investigation of these samples, it was observed that the majority of the rejects was silicone. The high coat weight of Sylgard in comparison to that of the Wacker may be the cause of the differences obtained.

To better understand these findings, an additional repulping study was performed. In this study a soaking step (1 hour) was added prior to adding the material into the hydrapulper. After soaking, any materials floating on the surface were skimmed off, subsequently, the same pulping procedures were followed as previously performed. The significant reduction in rejects for the Sylgard pre-coated samples confirm that the majority of the rejects were from the higher amount of Sylgard addition level, in comparison to the Wacker. By pre-soaking the samples, the sacrificial layer performed as desired, enabling the release of the silicone. The lower level of rejects obtained brought the percent rejects to a level to meet the Recycled Paperboard Technical Association (RPTA) requirements (less than 15%) to be considered a repulpable product.

The results of the tensile tests are shown in FIGS. 4 and 5, which show the tensile load and extension results, respectively. The results show a decrease in load strength of the samples relative to the commercial silicone release paper. However, only the Sylgard sample shows a significant difference prior to the introduction of the sacrificial layer. With the introduction of the sacrificial layer all samples, with the exception of the alginate Sylgard sample, showed a significant decrease in load strength versus that of the commercial paper. The load strength of the pre-soaked and Duncote control samples shows that the addition of the pre-soaking step was effective in removing a majority of the silicone. The lowest load strength was found for the Wacker coated samples. These low strength values are attributed to the low coat weight and insolubility of the Wacker silicone, enabling the largest amount of silicone to make it through the accept stream and into the handsheets.

The tensile extension results (FIG. 5), show similar trends as the tensile load results. Here, however, all extensional values are lower in comparison to the commercial silicone paper. These results show the Sylgard coated samples having larger extensional values as compared to that of the Wacker samples, and the alginate/Sylgard samples having the highest average values of all the coated samples.

From FIG. 6 it may be seen that the Commercial silicone release paper has the highest silicone content present. The handsheets having the lowest silicone content are those of the Duncote/Wacker and Duncote/Alginate/Sylgard. A decrease in silicone content may be seen between the Duncote/Sylgard and Duncote/Alginate/Sylgard samples, showing the alginate sacrificial layer decreased the amount of silicone carried over into the accepts stream.

The large standard deviation seen with the Duncote/Polyvinyl Alcohol/ Wacker sample could be due to sampling error. It is believed that without this error the average value for this sample would align with that of the Duncote/Polyvinly Alcohol/Sylgard test sample. The two lowest test sample averages are that of the Duncote/Wacker, and Duncote/Alginate/Wacker. These test samples may have the lowest averages due to the lower coat weight and ability to flow through the waste stream presented from the Wacker coating. With the Duncote/Alginate/Sylgard sample the sacrificial coating may be completing separation of the PDMS from the paper fibers, thereby allowing the (PDMS) film to be removed during the recycling process.

In the example discussed above, the application of a water-soluble sacrificial pre-coat prior to the application of a silicone release coating resulted in the majority of the silicone being separated and removed during the repulping process. The lower level of rejects obtained by first applying the sacrificial layer decreased the percent rejects to a level that meets the Recycled Paperboard Technical Association (RPTA) requirements (less than 15%) to be achieved. Thus, the application of a water soluble layer successfully enabled a silicone release paper product to be repulped by use of conventional repulping and recycling processing methods.

APPENDIX

                                 Tensile Index Data
                                 Load/Grammage
Commercial Silicone Release      0.05
Duncote                          0.04
Duncote_Sylgard                  0.04
Duncote_Wacker                   0.03
Duncote_Alginate_Sylgard         0.04
Duncote_PVOH_Sylgard             0.03
Duncote_Alginate_Wacker          0.03
Duncote_PVOH_Wacker              0.02
Duncote_Alginate_Sylgard (Pre Soaked) 0.03

Claims

1. A method of making repulpable paper, the method comprising:

coating at least a portion of a sheet of paper with a water-soluble material;

coating at least a portion of the water-soluble material with a silicone material to form a silicone release paper.

2. The method of claim 1, including:

providing label stock having adhesive on at least one side thereof; and

releasably adhering the label stock to the silicone release paper by bringing the adhesive into contact with the silicone material.

3. The method of claim 1, wherein:

the silicone material initially comprises a silicone polymer, a crosslinker, and a catalyst.

4. The method of claim 1, wherein:

the silicone material is selected from the group consisting of solvent-based silicone, water-based silicone, and solvent-less silicone.

5. The method of claim 1, wherein:

the water-soluble material comprises at least one of sodium alginate and partially hydrolyzed polyvinyl alcohol.

6. The method of claim 1, wherein:

the water-soluble material comprises a water soluble polymer.

7. The method of claim 6, wherein:

the water soluble polymer is selected from the group consisting of carboxymethylated cellulose, ethylated starch, and carboxylated soy protein.

8. The method of claim 1, wherein:

the water soluble material forms a coating having a coat weight of about 2.0-6.0 gsm.

9. The method of claim 1, wherein:

the silicone material forms a coating having a coat weight of about 2.0-100.0 gsm.

10. The method of claim 1, including:

repulping the paper after coating the paper with water-soluble material and silicone material.

11. The method of claim 10, wherein:

repulping the paper includes bringing the water-soluble material into contact with water.

12. The method of claim 11, wherein:

the paper is soaked in water for at least about 30 minutes.

13. The method of claim 12, wherein:

the paper is soaked in water for at least about 60 minutes.

14. The method of claim 10, wherein:

a hydrapulper machine is utilized to repulp the paper.

15. The method of claim 14, including:

soaking the paper in water prior to placing the paper into a hydrapulper machine.

16. The method of claim 10, wherein:

repulping the paper results in less than 15 percent rejects according to predefined criteria.

17. The method of claim 15, including:

skimming off material floating on the surface after soaking the paper.

18. A method of making repulpable paper, the method comprising:

coating at least a portion of a sheet of paper with a layer of dissolvable sacrificial material; and

coating at least a portion of the dissolvable sacrificial material with a release material having non-stick properties.

19. The method of claim 18, wherein:

the dissolvable sacrificial layer dissolves in water.

20. The method of claim 18, wherein:

the release material comprises silicone.