JP7721519B2 - Crosslinked polyolefin foam with large core cells - Google Patents

Description

The present disclosure relates generally to polyolefin foams, and more specifically to highly recyclable and splittable large core-cell polyolefin foams and methods for making the same, which foams contain numerous improved structural and mechanical properties, for example, as a single sheet of foam (before splitting), as well as after splitting into two sheets of foam.

PRIORITY CLAIM This application claims the benefit of U.S. Patent Application Nos. 16/699,062 and 16/699,063, filed November 28, 2019, the entireties of which are incorporated herein by reference.

A conventional method for producing polyolefin foam involves extruding a foamable sheet containing a thermoplastic resin, a blowing agent, and additives. Prior to foaming the foamable sheet in an oven at the activation temperature of the blowing agent, the foamable sheet may be crosslinked via irradiation, for example, by passing it through an electron beam irradiator. As practiced in the art, the electron beam irradiator is configured to deliver a sufficient dose of electrons to each side of the foamable sheet so that the electrons pass through the foamable sheet material and exit on the opposite side, providing a uniform degree of crosslinking throughout the foamable sheet. As the electrons pass through the foamable sheet, they impart their energy to the material forming crosslinks between polymer chains, thereby strengthening bonds throughout the thickness of the foamable sheet.

For example, in a conventional method, a foamable sheet may be passed through an electron beam irradiation device, which first exposes a first side to radiation. The irradiation device is configured to deliver a sufficient dose of electrons so that the electrons pass from the first side through the foamable sheet and exit the opposing second side, losing energy as they pass and interact with the polymeric material to form crosslinks. This means that the degree of crosslinking, i.e., gel content, is high in the surface region of the first side and gradually decreases through the thickness of the foamable sheet to the surface region of the second side, creating a crosslinking gradient. To compensate and create a uniform degree of crosslinking and gel content throughout the thickness of the foamable sheet, the sheet is then passed through the electron beam irradiation device again, whereby the second side is exposed to the same radiation dose, thereby balancing the dose throughout the foam thickness and achieving an overall uniform degree of crosslinking, including the intermediate region of the foamable sheet where the crosslinking degrees overlap.

A foamable sheet irradiated in this manner will have a uniform degree of crosslinking throughout the thickness of the material, including both surface regions and the intermediate region therebetween, and will also have a uniform cell size throughout after being expanded in an oven. While it is traditionally desirable to have uniform crosslinking and uniform cell size throughout the foam, failure to do so can result in the formation of weak spots within the foam that render it unusable for various applications, as well as anomalies in other properties of the foam that can affect performance.

However, Patent Publication No. US 2003/0082364 A1 discloses a foam material with variable crosslinking, in which one side of the foam is intentionally given a higher radiation dose than the other side. Such a foam would have a region on one side with a higher amount of crosslinking and smaller cell size, and a region on the second side with a lower amount of crosslinking and larger cell size. The reference also discloses that the higher crosslinking on one side allows the foam to be used in combination with a wider range of materials than a foam material with uniform crosslinking. The reference also discloses that a foam material with variable crosslinking can have two different degrees of crosslinking throughout the material, or a gradient of crosslinking throughout the material, allowing for control of various properties, such as heat resistance during molding, and allowing for the use of plastics with higher melt temperatures, improved compression distortion properties in the final product, and improved high-temperature performance in the final product. Additionally, the reference also discloses that a second crosslinking can be performed after foaming, such as by irradiation. This allows foam sheets to be produced at lower densities without sacrificing the heat resistance required during end-use processes (low-pressure molding, insert molding, compression molding, etc.).

While several useful variable crosslinking methods and advantages are disclosed in reference US 2003/0082364 A1, these only involve producing foams with two regions of different crosslinking degrees (e.g., a high side and a low side) and gradients (e.g., a gradual increase from high to low crosslinking or a gradual increase from low to high crosslinking from one side to the other). That is, only foams with two regions with two different gel contents, or foams with only a gradient of gel content, are envisioned. Notably, each of these examples produces foams with asymmetric levels of crosslinking, gel content, and cell size from one side of the foam to the other.

While these variably crosslinked foams and conventional foams (with uniform crosslinking) are known prior art, there remains a strong need for foams with additional properties that cannot be achieved by these methods alone.

For example, some end uses require dividing one foam sheet in a controlled, uniform manner to produce two foam sheets. When attempting to divide or tear a foam sheet produced by any of the prior art methods, the tear does not consistently propagate through the core or mid-region of the foam, but rather surfaces to one side or the other, resulting in only a portion or chunk of the foam being torn away. In the case of double-sided foam tape, for example, upon removing an adhered object from a wall or other substrate surface, the entire foam sheet may peel away from the wall surface, peel away from the adhered object, or, more frequently, tear randomly, leaving uneven portions of foam on both the wall and the object. In the worst case scenario, portions of the substrate or object may also break and be torn away.

Furthermore, for foams such as acoustic foams that benefit from an open-cell surface structure, a conventional method for converting one sheet of foam into two sheets involves cutting the sheet with a blade rather than physically pulling or tearing, as described in Patent Publication No. EP 0286571 B1. Cutting the foam longitudinally is generally preferred in the industry because it results in a uniform open-cell surface profile with little thickness variation, for example. For example, in EP 0286571 B1, the reference teaches cutting a closed-cell polyurethane foam panel to form two panels of identical dimensions, each having one surface with a high-density, small-cell structure and one surface with a low-density, large-cell structure, with the large-cell structure being cut to create large open cells. However, even such cut foams do not necessarily exhibit sufficient acoustic and other properties to meet industry requirements, including, but not limited to, adhesive bonding, water retention, and anti-slip properties.

Other known methods of dividing foam sheet structures include laminating each side of the foam to a structural layer and then separating them, as taught, for example, in Patent Publication GB 738494A. In that reference, two sheets of flexible, non-rubber material were adhered to either side of a layer of sponge rubber, and the thickness of the sponge rubber was longitudinally divided to create two sheets of material, each having a flexible, non-rubber backing sheet secured to one side of the sponge rubber layer, the exposed surface of which had surface pores of larger dimensions than the pores within the thickness of the sponge rubber layer. However, incorporating a backing layer laminated to each of the separated sheets of foam is not necessarily desirable, as it affects overall product properties, limits design and use flexibility, and requires additional processing steps and material costs.

Therefore, there remains a need for improved foam structures and methods for their manufacture to address many of the problems of the prior art. Such foams should have appropriate characteristics tailored to the unique specifications of the application in which the foam is to be used.

This Summary is provided to introduce a selection of concepts that are further described below in the Detailed Description. This Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

The present disclosure relates to closed-cell crosslinked polyolefin foam sheets, processes for segmenting closed-cell crosslinked polyolefin sheets, and segmented crosslinked polyolefin foams having a closed-cell skin side and an open-cell segmented side.

In one aspect, a closed-cell crosslinked polyolefin foam sheet comprises opposing first and second surface regions and an intermediate region disposed therebetween, wherein the ratio of the gel content of the intermediate region to the average gel content of the first and second surface regions is about 75% or less, and the ratio of the average cell size of the intermediate region to the average cell size of the first and second surface regions is about 125% or more.

In another aspect, a closed-cell crosslinked polyolefin foam sheet comprises opposing first and second surface regions and an intermediate region disposed therebetween, the intermediate region being configured to have a gel content lower than the average gel content of the first and second surface regions to enable controlled tear propagation within the intermediate region when a parting force is applied to the closed-cell crosslinked polyolefin foam sheet.

In another aspect, a process for producing a segmented crosslinked polyolefin foam sheet includes producing a crosslinked polyolefin foam sheet having opposing first and second surface regions and an intermediate region disposed therebetween, the intermediate region configured to have an average gel content and an average cell size greater than the average cell sizes of the first and second surface regions, and applying a splitting force to the crosslinked foam sheet such that a controlled tear propagation proceeds through the intermediate region until the first side of the crosslinked polyolefin foam sheet and the second side of the crosslinked polyolefin foam sheet separate to produce two segmented polyolefin foam sheets.

In another aspect, the split crosslinked polyolefin foam sheet includes a skin side including a closed-cell surface and a split side including an open-cell surface having a peak height of about 150 μm to about 550 μm.

FIG. 1 is a schematic perspective view of a foamable sheet according to the present disclosure. FIG. 2 is a schematic perspective view of a foam sheet after the foamable sheet of FIG. 1 is foamed. 3 is a schematic cross-sectional side view of the foam sheet of FIG. 2. 4 is a schematic side view of the foam sheet of FIGS. 2-3, one side of which is adhered to a substrate and which has been split. FIG. 4A-4C are schematic side views of the foam sheet of FIGS. 2-3 during and after division, respectively. FIG. 4 is a schematic diagram of an exemplary manufacturing portioning process for the foam sheet of FIGS. 2-3 according to the present disclosure. 1 is a graph of gel fraction and foam density for foam sheets of the present disclosure. 1 is a graph of gel fraction and foam density for foam sheets of the present disclosure. 1 is a graph of gel fraction and foam density for foam sheets of the present disclosure. 1 is a graph of cell size ratio and foam density for foam sheets of the present disclosure. 1 is a graph of cell size ratio and foam density for foam sheets of the present disclosure. 1 is a graph of cell size ratio and foam density for foam sheets of the present disclosure. 1 is a graph of 50% compression set versus foam density for foam sheets of the present disclosure. 1 is a graph of 50% compression set versus foam density for foam sheets of the present disclosure. 1 is a graph of 50% compression set versus foam density for foam sheets of the present disclosure. FIG. 1 is a schematic diagram of a splitting strength test apparatus according to the present disclosure. 1 is a graph of splitting force strength versus foam density for foam sheets according to the present disclosure. 1 is a graph of splitting force strength versus foam density for foam sheets according to the present disclosure. 1 is a graph of splitting force strength versus foam density for foam sheets according to the present disclosure. 1 is a graph of T-peel strength versus foam density for foam sheets according to the present disclosure. 1 is a graph of T-peel strength versus foam density for foam sheets according to the present disclosure. 1 is a graph of T-peel strength versus foam density for foam sheets according to the present disclosure. 1 is a graph of depression depth recovery for foam sheets according to the present disclosure. 1 is a graph of peak height versus foam density for a divided foam sheet according to the present disclosure. 1 is a graph of surface roughness and foam density of a divided foam sheet according to the present disclosure. 1 is a graph of dynamic friction coefficient and foam density for a divided foam sheet according to the present disclosure. 1 is a graph of static friction coefficient and foam density for a divided foam sheet according to the present disclosure. 1 is a graph of average sound reduction (acoustic transmission) versus foam basis weight for a divided foam sheet according to the present disclosure. 1 is a graph of water mass acquired (surface water retention) versus foam density for a divided foam sheet according to the present disclosure. 1 is a graph of peel strength versus test surface type for a divided foam sheet according to the present disclosure.

Disclosed herein are improved polyolefin foam compositions and methods for their manufacture, wherein the foams, for example, include numerous improved structural and mechanical properties, not only as a single sheet of foam, but also after being separated into two sheets of foam. In one non-limiting example, the foam includes a mid-region configured to allow controlled splitting crack propagation upon application of a threshold splitting force, such that the first and second sides of the foam separate from each other in a consistent and uniform manner only through the mid-region, without undesirable tears in the foam outside of that region. Further disclosed is a manufacturing process for splitting the foam without the need for cutting.

FIG. 1 is a perspective view of a foamable sheet 10 having a first side A, a second side B, an intermediate region 12 between sides A and B, a surface region 14A between the intermediate region 12 and side A, and a surface region 14B between the intermediate region 12 and side B. Additionally, the machine direction MD (i.e., length dimension), transverse direction TD (i.e., width dimension), and Z direction ZD (i.e., thickness dimension) of the foamable sheet 10 are depicted relative to the extrusion direction of the sheet. Foamable sheets can be made by a variety of processes common in the art, including extrusion molding, and can include one or more resins, blowing agents, and appropriate additives.

Suitable polymers or resins for use in foamable sheets include, but are not limited to, low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), ethylene vinyl acetate (EVA), polypropylene (PP), ethylene propylene diene monomer (EPDM), thermoplastic olefin (TPO), thermoplastic elastomer (TPE), and rubber. Individual resins may be selected for foamable sheets, as well as blends of two or more resins. Suitable foaming agents may include, for example, azodicarbonamide (ADCA).

In contrast to conventional methods for irradiating foamable sheets, the present disclosure utilizes a novel controlled depth cross-linking technique such that each side A and B of the foamable sheet 10 is irradiated with a low energy that is configured based on the material and effective thickness of the foamable sheet 10. In such a case, electrons entering each side from opposite directions penetrate the thickness of the overlapping foamable sheet in the intermediate region 12 before losing energy and ceasing their movement within the foamable sheet without exiting the surface of the other side. The deceleration and eventual annihilation of high-energy electrons after traveling a certain distance through a material can be referred to as the electron's effective range. Adjusting the effective range of electrons penetrating the foamable sheet 10 from each side to pass through a minimal or controlled width overlap region within the intermediate region 12 before losing energy results in the defined intermediate region 12 of the foamable sheet 10 being configured to have a lower degree of cross-linking relative to other regions of the foamable sheet 10, such as surface regions 14A and 14B. Furthermore, the location and characteristics of the intermediate region 12 (including gel content, location, and width) can be precisely controlled by adjusting variables such as material type, effective thickness, potential, dose, linear velocity, and beam width.

For example, to determine optimal irradiation conditions, the electron beam irradiation device can be adjusted to change its potential (kV) and current (mA) to affect the dose (Mrad), for example, based on the effective thickness (ET) of the foamable sheet 10 and its material type. ET is calculated by multiplying the actual thickness (mils) of the foamable sheet by the density (g/cm3) of the material. Other variables that can be adjusted by one of ordinary skill in the art are the linear velocity (m/min) of the foamable sheet 10 as it travels through the electron beam irradiation device and the scanning beam width (inches). For example, if the linear velocity is increased, the dose (Mrad) can be kept constant, but the current (mA) must be adjusted accordingly.

Referring to FIG. 2, after the foamable sheet 10 is irradiated using the novel controlled depth crosslinking technique described above so that the middle region 12 of the foamable sheet 10 has a lower degree of crosslinking relative to the surface regions 14A and 14B, the foamable sheet 10 can then be foamed above the activation temperature of the blowing agent. During the foaming process, the foamable sheet 10 expands in each of the TD, MD, and Z directions to generate a foam 10A (e.g., a foamed sheet). The foam 10A will have a gel content in the middle region 12 that is lower than the gel content of the regions 14A and 14B, and will also have a symmetrical cell size structure with larger cells in the middle region 12 and proportionally smaller cells in the surface regions 14A and 14B, as described in more detail below. Importantly, this structure is achieved within a single sheet of foam 10A and does not require lamination or coextrusion of other foams or other post-processing steps.

Figure 3 is a cross-sectional view of the foam 10A of Figure 2, showing that after utilizing the novel controlled depth cross-linking method, a larger cell structure in the intermediate region 12 resulted after foaming due to a lower degree of cross-linking (and lower gel content) in that region. Cell formation can be affected, for example, by the degree of cross-linking in the polymer surrounding the blowing agent in the material, which can limit the expansion of gas from the blowing agent during the foaming process. Uniformity in the degree of cross-linking throughout the foamable sheet, as well as the activation of the blowing agent during the foaming process, typically results in cells expanding to similar, limited sizes throughout the material, relative to the degree of cross-linking. However, when using controlled depth cross-linking as described herein, the gel content and cell size in the intermediate region 12 of the foam 10A can be tightly controlled and configured to specific specifications for the surface regions 14A and 14B, including desired performance characteristics. These specifications and desired properties can be controlled not only for a single sheet of foam 10A, but also for each sheet of foam produced after the foam is separated by the controlled tear propagation also enabled by the method of the present invention.

For example, with respect to a single-sheet foam 10A produced by the methods described herein, the foam may have a gel fraction configured to be about 90% or less, preferably about 75% or less, and more preferably about 50% or less. The gel fraction is calculated by comparing the gel content of the intermediate region 12 to the combined average gel content of the surface regions 14A and 14B, as further described in Example 1 of the present disclosure. Alternatively, the gel fraction may be from about 15% to about 90%, preferably from about 15% to about 75%, and more preferably from about 15% to about 50%. Below about 15%, the foam may begin to exhibit blistering due to cell wall rupture and the formation of larger blister cells.

The thickness of the intermediate region 12 can also be adjusted depending on the needs of the foam application, but is generally about 1/3 to about 2/3 of the total foam thickness.

The cell size ratio of a single-sheet foam 10A produced by the method herein can be calculated as the difference in cell size between the intermediate region 12 divided by the combined average of the surface regions 14A and 14B, as further described in Example 2 of the present disclosure. The cell size ratio can be configured to be about 125% or greater, preferably about 200% to about 400%, and more preferably about 250% to about 400%.

Suitable densities of foams produced by the methods of the present disclosure may be from about 1.5 pcf to about 20 pcf, preferably from about 1.5 pcf to about 15 pcf, and more preferably from about 2 pcf to about 10 pcf.

Furthermore, by configuring the gel ratio and cell size ratio as described, foam 10A, in particular, can be tailored to have desirable properties suitable for a wide range of applications. For example, foams produced by the methods of the present disclosure can have a 50% compression set of less than about 10%, preferably about 3% to about 10%, and more preferably about 4% to about 9%, which cannot be achieved with standard irradiated foams.

Foam 10A produced by the method of the present disclosure may also be configured to have improved depression depth recovery relative to standard foam, including an improvement of about 55% to about 80% over standard foam, and an improvement of about 35% to about 65% over standard foam, even after post-crosslinking the foam to improve its heat resistance, as described in more detail with reference to Example 7 of the present disclosure. Foam 10A may have a depression depth of about 35% to about 45% of its original thickness at time zero and about 4% to about 6% of its original thickness after 168 hours, according to test standard VDA 237-101-2, for example. Additionally, foam 10A may have a depression depth recovery of about 50% to about 55% of its original thickness at 0 hours and about 5% to about 10% of its original thickness after 168 hours, according to test standard VDA 237-101-2, after the polyolefin foam sheet has been post-crosslinked to have a gel content in the mid-region of about 50% or greater.

In another example, a lower degree of crosslinking or gel content in the intermediate region 12 relative to the surface regions 14A and 14B can be configured based on the condition of the irradiation and foamable sheet 10 to control fractional crack propagation along the intermediate region 12 of the foam 10A when a threshold parting force is applied to the foam sheet 10A. The threshold parting force can be determined based on the needs of the end application, for example, adjusting the degree of crosslinking to be high enough to meet the holding strength of a foam tape application, yet low enough to allow controlled, uniform fractional crack propagation along and within the boundaries of the intermediate region 12 without damaging any substrates or objects attached to the foam 10A.

Figure 4 shows an example of a foam sheet 10A bonded to a substrate S on side B, where the separating force SF is represented by several vectors pulling on side A in the Z direction or at an acute angle thereto. After the intermediate region 12 is separated, new open cell faces 22A and 22B are created, with open cell face 22A facing closed cell face A and open cell face 22B facing closed cell face B.

Additionally, a splitting tear propagation line 16 is shown that remains within the intermediate region 12 in a controlled manner during splitting of the foam 10A, thereby enabling side A of the foam 10A to be uniformly peeled from side B when a threshold splitting force SF is applied, such as by a user, ultimately producing two separate foam sheets from the halves 20A and 20B. As noted above, the relative degree of cross-linking between the surface regions 14A and 14B and the intermediate region 12 can be configured and tailored to the specific needs of an application, for example, to avoid damaging the substrate S when objects bonded to side A are pulled apart at the threshold splitting force PF. In contrast, foams produced from the foamable sheet 10 by prior art methods will not produce uniform splitting tear propagation through the material when split in the machine direction (MD) or transverse direction (TD) using, for example, a splitting force in the Z direction (ZD). Rather, tears 16 may propagate randomly through the material outside of midregion 12 and surface on either side A or B of foam 10A, tearing the mass of foam before the entire side is peeled away. This is particularly troublesome in some foam tape applications where it is desired to evenly separate one side of the foam from the other, or in processes where side A is peeled continuously from side B to produce two foam sheets from one foam sheet.

Figure 5A shows a similar concept to split foam 10A shown in Figure 4, except that neither side of foam 10A is fixed to a substrate, allowing foam 10A to be pulled apart by two separate, opposing splitting forces SF to create two separate foams 20A' and 20B'. After intermediate region 12 is split, new open-cell surfaces 22A and 22B result, with open-cell surface 22A facing closed-cell surface A in foam 20A' and open-cell surface 22B facing closed-cell surface B in foam 20B'.

The splitting strength of foam 10A, measured by the splitting strength test method as described in Example 4 of the present disclosure, can be configured to be about 5 lbf to about 35 lbf, preferably about 8 lbf to about 33 lbf, and more preferably about 9 lbf to about 30 lbf. The splitting strength of foams of the present disclosure can be significantly reduced compared to standard foams of comparable density, by about 25% to about 80%, preferably about 30% to about 70%, and more preferably about 40% to about 60%.

The T-peel strength of foam 10A, measured by the T-peel strength test method as described in Example 5 of the present disclosure, may be configured to be about 1 lbf to about 5 lbf, preferably about 1.5 lbf to about 4 lbf, and more preferably about 1.6 lbf to about 3.1 lbf. The T-peel strength of foams of the present disclosure may be significantly reduced compared to standard foams of comparable density, by about 20% to about 70%, preferably about 30% to about 60%, and more preferably about 40% to about 50%.

FIG. 6 is a schematic diagram of a manufacturing process 100 illustrating one example of a method for continuously dividing a foam 10A according to the present disclosure. A supply roll F containing foam 10A can be fed into a pair of nip rollers N1, N2 rotating in opposite directions, such as counterclockwise (N1) and clockwise (N2), as shown. By rotating in opposite directions, a dividing force SF, as shown in FIG. 5A, can be mechanically achieved from both sides A, B of foam 10A, thereby producing two sheets of foam 20A', 20B' that can be guided, if necessary, using guide rollers and ultimately wound by winders W1, W2. This method eliminates the need for or maintenance of cutting equipment, as foam 10A can be divided in half using only nip rollers N1, N2. Furthermore, the rate at which foam 10A is divided is sufficiently fast to allow it to be part of an in-line manufacturing process with the foam itself.

The controlled split crack propagation enabled by the present disclosure is highly effective and can be tailored to produce two split foams 20A' and 20B', each having an average gauge of about 30% to about 70%, preferably about 40% to about 60%, and more preferably about 45% to about 55% of the original full gauge of the unsplit foam 10A. Furthermore, the split foams 20A' and 20B' can each have a mass of about 30% to about 70%, preferably about 40% to about 60%, and more preferably about 45% to about 55% of the original total mass of the unsplit foam 10A. Segmented foams 20A' and 20B' may each also have a density of about 75% to about 125%, preferably about 85% to about 115%, and more preferably about 90% to about 110% of the original overall density of unsegmented foam 10A, as further described in Example 8 of the present disclosure.

Furthermore, when equal, symmetrical halves are desired, the method of the present disclosure may result in a maximum variation of about 20% (ideally from 50%) for gauge, about 10% (ideally from 50%) for mass, and about 16% (ideally from 100%) for density, as further described in Example 8 of the present disclosure. Furthermore, the method of the present disclosure may result in a maximum variation of about 5% (ideally from 50%) for gauge, about 2% (ideally from 50%) for mass, and about 6% (ideally from 100%) for density, as further described in Example 8 of the present disclosure.

After division, foams 20A' and 20B' can also be configured for a number of desirable properties. For example, in addition to the gel content, cell size, and open-cell structure that can be created on surfaces 22A and 22B, foams 20A' and 20B' can also include advantageous physical properties that provide additional performance benefits as described herein. For example, the peak height of surfaces 22A and 22B can range from about 150 μm to about 550 μm, preferably from about 200 μm to about 500 μm, and can include an approximately 12-fold increase in peak height compared to a standard foam skin (closed-cell surface) and an approximately 4-fold increase compared to a standard foam cutting surface (open-cell surface), as further described in Example 9 of the present disclosure. The surface roughness (Sa) of surfaces 22A and 22B may range from about 70 μm to about 150 μm, preferably from about 75 μm to about 140 μm, and more preferably from about 80 μm to about 135 μm, and may include an increase in surface roughness of about 6 times compared to a standard foam skin (closed cell surface) and an increase in surface roughness of about 1.8 times compared to a standard foam cutting surface (open cell surface), as further described in Example 10 of the present disclosure.

The divided foams 20A' and 20B' may also be configured so that the coefficient of friction (COF) between surfaces 22A and 22B is in the range of about 1.0 lbf to about 4.5 lbf, preferably about 1.5 lbf to about 4.0 lbf, and more preferably about 1.8 lbf to about 3.5 lbf for static COF, and about 1.0 lbf to about 4.0 lbf, preferably about 1.5 lbf to about 3.5 lbf, and more preferably about 1.5 lbf to about 3.0 lbf for dynamic COF. The static COF may include an increase of about 4.1 times compared to a standard foam skin (closed cell surface) and an increase of about 3.3 times compared to a standard foam cutting surface (open cell surface), and the dynamic COF may include an increase of about 4.4 times compared to a standard foam skin (closed cell surface) and an increase of about 3.7 times compared to a standard foam cutting surface (open cell surface), as further described in Example 11 of the present disclosure.

Segmented foam surfaces 22A and 22B may also be configured such that the average sound reduction achieved by surfaces 22A and 22B ranges from about 5 dB to about 25 dB, preferably from about 5 dB to about 20 dB, and more preferably from about 8 dB to about 17 dB, as measured by the acoustic transmission test method described in Example 12 of the present disclosure. Segmented foam surfaces 22A and 22B according to the present disclosure will perform better than any standard foam skin surface of comparable basis weight, and better than any standard foam machined surface of basis weight equal to or greater than about 7 g/ft2.

Segmented foams 20A' and 20B' may also be configured to have surface water retention on surfaces 22A and 22B ranging from about 0.010 grams to about 0.050 grams, preferably from about 0.015 grams to about 0.045 grams, and more preferably from about 0.017 grams to about 0.043 grams, according to the Surface Water Retention Test Method further described in Example 13 of the present disclosure, which may include an improvement of about 3.9 times compared to a standard foam cutting surface and an improvement of about 1.5 times compared to a standard foam cutting surface.

Split foams 20A' and 20B' also exhibit excellent adhesion on surfaces 22A and 22B, providing better adhesive fixation than standard foam cutting surfaces and comparable performance to standard foam skin surfaces. For example, using the adhesive testing method and results further described in Example 14 of the present disclosure, the split foam surface skin peel strength may be about 0.95 N/mm or greater, preferably about 1.00 N/mm or greater, and more preferably about 1.10 N/mm or greater.

Experimental Methods The formulations in Table 1 were used to prepare foamable sheets and foams for the experiments and examples described below.

EVA is ethylene vinyl acetate, PE is polyethylene, PP is polypropylene, LDPE is low density polyethylene, LLDPE is linear low density polyethylene, AO is antioxidant, h-PP is a homopolymer of PP, TPE is thermoplastic elastomer, and AR means "as needed" to achieve the target density of the foam, as known to those skilled in the art.

Expandable sheet samples were prepared based on the formulation in Table 1 and crosslinked according to standard methods to produce a control sample with uniform crosslinking throughout, as well as a sample with varying degrees of crosslinking within its mid-region (i.e., its "core") versus its surface region via the novel controlled depth crosslinking method of the present disclosure. The expandable sheet was then expanded at the activation temperature of the blowing agent to produce foam samples with various densities for testing according to the following procedure.

Example 1 - Gel Ratio The gel content of the middle region 12 or "core" region, Surface A (Surface Region 14A), and Surface B (Surface Region 14B) was measured for each irradiated foam sample produced by the experimental method described above. Each foam sample and control sample was divided into three layers of equal thickness using a sharp razor and shims to separate an outer (side A) specimen, a middle region/core specimen, and an outer (side B) specimen. The degree of crosslinking of the cut foam samples and control was measured by preparing a 12 mm wide sample with three equal slits cut into it, creating four 3 mm wide strips, and cutting them to the appropriate length so that the sample weight ranged from 0.047 g to 0.053 g. A weighed amount of crosslinked polyolefin foam (A in grams) was immersed in 25 mL of xylene at 120°C for 24 hours. After 24 hours, the foam was filtered through a 200-mesh wire mesh and allowed to sit in a fume hood for at least 12 hours. The insoluble matter on the wire mesh was then placed in a vacuum oven set at 100°C and 15 inHG for 4 hours to dry under vacuum. The dry weight (B (grams)) of the insoluble matter was measured, and the degree of crosslinking was calculated using the following formula: Degree of crosslinking (wt%) = 100 x (B/A).

The results for sides A and B were averaged to generate an outer gel volume value, and the core region gel volume measurements were divided by the averaged outer gel volume value to determine the core to outer ratio. The results are shown in Tables 2A, 2B, and 2C below, and in the corresponding Figures 7A, 7B, and 7C, respectively.

As shown in Figures 7A, 7B, and 7C, the gel percentages all hovered around 100%, indicating a uniform level of crosslinking and gel content throughout the samples. However, the gel percentages of the controlled depth-crosslinked samples were consistently much lower than the control. These gel percentages ranged from about 15% to about 65% using the methods of the present disclosure.

Example 2—Cell Size Ratio The cell sizes of the intermediate region 12 or “core” region and the surface A (surface region 14A) and surface B (surface region 14B) were measured for each irradiated foam sample produced by the experimental method described above. A sharp razor blade was used to make vertical cuts in the foam, ensuring that the sliced surfaces were intact. A microscope with measurement capabilities (Keyance 3D Microscope VHX-6000) was used to observe the cross-sections of each region of the foam. Specifically, five cells were selected from side A and five from side B near the surface, and their cell sizes in the Z direction were measured to determine the average cell size. Additionally, 10 cells were selected from the core region, measured in the Z direction, and the average was calculated. Finally, the difference in cell size between the core and surface regions was compared and expressed as a ratio (core divided by the overall average of the outer regions). The results are shown in Tables 3A, 3B, and 3C below, and the corresponding Figures 8A, 8B, and 8C, respectively.

As shown in Figures 8A, 8B, and 8C, the cell size ratios all hovered around 100%, indicating a uniform level of cell size throughout the samples. However, the cell size ratios of the controlled depth-crosslinked samples were consistently much higher than the control samples, meaning that the cell size in the core was much larger than the cell size in the surface region. These cell size ratios were achieved between approximately 220% and approximately 365% using the methods of the present disclosure.

Example 3 - 50% Compression Set For each irradiated sample of foam produced by the experimental method described above, the 50% compression set was measured according to ASTM D 3575. The results are shown in Tables 4A, 4B, and 4C below and in the corresponding Figures 9A, 9B, and 9C, respectively.

As shown in Figures 9A and 9B, the 50% compression set was improved (reduced) for all samples relative to the standard control foam. As shown in Figure 9C, the 50% compression set was at least equal to or better than standard PP foam, so as not to sacrifice performance.

Additionally, for each PE and EVA sample (shown in Tables 4A and 4B), the 50% compression set was calculated for each control sample of the same density based on the trend data in Figures 9A and 9B, respectively, and compared in Table 4D below. As shown, each PE and EVA sample had a reduction (improvement) in 50% compression set compared to the control sample of the given density, with the reduction ranging from about 45% to about 75%.

Example 4 - Splitting Force Strength Splitting force strength was measured for each irradiated foam sample produced by the experimental method described above. Splitting force strength was measured using the fabrication apparatus shown in Figure 10 using the splitting force strength test method as follows. Each foam sample was corona treated on both sides, and then a strong pressure-sensitive adhesive was applied to both sides of foam 10A to convert it into double-sided foam tape. The foam tape was then cut into 2-inch by 1-inch test pieces. After ensuring that the stainless steel of the fixture was clean, double-sided foam tape 10A was attached to a small stainless steel plate ("S.S. Small Plate" in Figure 10) at the end without a slit and the end without a recess. The small stainless steel and foam assembly was then placed into the 2-inch-wide groove of the positioning fixture, and the hook of the positioning fixture was then placed on the edge of the larger stainless steel plate ("S.S. Large Plate" in Figure 10). The small plate was firmly pressed against the large plate, and the positioning fixture was removed. The small stainless steel and foam assembly was positioned 1 inch from the edge of the large plate. The tape was allowed to cure for 24 hours. The large plate was then slid onto the table and the posts under the table were inserted into the Instron machine. The horizontal bar was slid under the small plate into the recessed area, and the vertical bar was raised into the slit. The Instron grips clamped the tabs on the ends of the bar and started in a stretch mode at 40 inches/minute, measuring the parting force in pounds in both the machine direction (MD) and cross-machine direction (CM) of the foam sheet sample.

The results are shown below in Tables 5A, 5B, and 5C and in the corresponding Figures 11A, 11B, and 11C, respectively.

As shown in Figure 11A, regardless of foam density, the force required to split the EVA foam samples was much less than the control samples. As shown for the PE foam samples in Figure 11B, the lowest density control sample had slightly lower split force strength than the much higher density foam samples, but a similar phenomenon was observed. Referring to Figure 11C, the PP foam samples had lower split forces than the control samples, including the control sample with a slightly lower density. These results indicate that for a given foam density, foam samples prepared by the controlled depth crosslinking method of the present disclosure will consistently have lower split force strength than the control foam.

Additionally, for each PP, PE, and EVA sample (shown in Tables 11A-11C), the splitting force strength was calculated for each control sample of the same density based on the trend data in Figures 9A and 9B, respectively, and compared in Table 5D below. As shown, each PP, PE, and EVA sample had a reduction (improvement) in splitting force strain compared to the control sample of the given density, with the % reduction ranging from about 25% to about 80%.

Example 5 - T-Peel Strength T-peel strength was measured for each irradiated foam sample produced by the experimental method described above. The T-peel strength test method was as follows: 1 inch x 6 inch specimens were cut from each foam. The samples were partially split by splitting approximately 1 inch of the sample by hand for foam samples produced with controlled depth crosslinking (which have large cell sizes in the core region) or by using a razor blade for control samples. The two partially split ends were clamped with Instron grips, and the Instron was operated at 10 inches/minute in a 3-inch stretch mode to measure the maximum force in pounds required to split the foam in half. It was observed that the foam samples produced with controlled depth crosslinking continued to split consistently and evenly throughout the entire 3 inches, while the control sample split unevenly shortly after stretching began.

The results are shown below in Tables 6A, 6B, and 6C and in the corresponding Figures 12A, 12B, and 12C, respectively.

As shown in Figure 12A, the T-peel strength of the EVA foam samples was much lower than that of the control samples, especially as the foam density increased. As shown in Figure 12B for the PE foam samples, the lowest density control sample had a slightly lower T-peel strength than the much higher density sample foams, but a similar phenomenon was observed. With reference to Figure 12C, the PP foam samples had lower T-peel strengths than the control samples, including the control sample with a slightly lower density. These results indicate that for a given foam density, foam samples prepared by the disclosed controlled depth crosslinking method will have consistently lower T-peel strengths than the control foams, as well as exhibit controlled tear propagation not achieved by the control foams.

Additionally, for each PP, PE, and EVA sample (shown in Tables 12A-12C), the T-peel strength was calculated for each control sample of the same density based on the trend data in Figures 9A and 9B, respectively, and compared in Table 6D below. As shown, each PP, PE, and EVA sample had a reduction (improvement) in T-peel strength compared to the control sample of the given density, with the % reduction ranging from about 25% to about 70%.

Example 6 - Post-Crosslinking: Heat Resistance Heat resistance was measured for each irradiated sample of foam produced by the experimental method described above. EVA foam samples were produced using a controlled depth crosslinking method, and post-crosslinked (Post-XL) samples were produced by subjecting EVA foam samples to post-foam irradiation until their core regions had a gel content of at least 50%. These samples were compared with standard EVA control foams of similar density to measure their ability to withstand extreme heat in a thermoformer for 21 seconds, such that the foam surface temperature reached approximately 225°C. The results are shown in Table 7 below.

The EVA control foam showed areas of surface degradation, indicating that the heat exceeded the foam's maximum processing temperature. The EVA foam sample (with a lower gel content core) not only exhibited surface degradation, but also severe cases of blistering across the entire surface. The blisters occurred when large core cells ruptured, causing multiple cells to fuse together into larger cells that appeared as blisters below the surface. This occurred because the EVA foam sample had less gel in the core region, making it too weak and heat-stable. However, the post-crosslinking (Post-XL) EVA foam sample received an additional irradiation dose, which strengthened the foam in the core region and increased the gel content there while maintaining the large cell structure. Surprisingly, the blistering no longer occurred, but instead exhibited surface degradation due to excessive heat, much like the standard EVA control foam. Thus, post-crosslinking can impart heat-resistant characteristics to foams while retaining the controlled tear propagation and other performance benefits of the foam sample.

Example 7 - Post-Crosslinking: Dimple Depth Recovery Dimple depth recovery was measured for each irradiated sample of foam produced by the experimental method described above. EVA foam samples were produced using a controlled depth crosslinking method, and post-crosslinked (Post-XL) samples were produced by subjecting EVA foam samples to post-foam irradiation until their core regions had a gel content of at least 50%. These samples were compared to standard EVA control foams of similar density to measure their dent depth recovery performance after 24, 48, and 168 hours according to automotive test standard VDA 237-101-2. The results are shown in Table 8A below.

Figure 13 is a graph of the 2 kg indentation depth recovery shown in Table 8. As can be seen, both EVA samples, including the post-crosslinked EVA, exhibit superior indentation depth recovery to the standard EVA control foam, while the post-crosslinked sample experiences only a slight decrease in performance compared to the non-post-crosslinked EVA sample. Thus, the mechanical and performance characteristics of foam samples produced by the methods of the present disclosure can be largely retained even after the benefits of post-crosslinking are imparted to such samples.

Additionally, the percent improvement in dent depth recovery was also compared separately between the EVA control foam vs. the EVA foam samples, and the EVA control foam vs. the EVA foam samples (+After XL), as shown in Table 8B below.

As can be seen in Table 8B, the dent depth recovery of the EVA samples was improved by 55% to approximately 80% relative to the control, and the dent depth recovery of the post-crosslinked EVA samples was improved by approximately 35% to approximately 65%.

Example 8 - Split Foam Gauge, Mass, and Density Foam samples produced by the above experimental method for controlled depth crosslinking were measured for gauge and mass before and after splitting using controlled tear propagation down the middle/core region of the foam as previously described with reference to Figures 4-6, allowing the resulting split foam samples to be evaluated for consistency of gauge, mass, and calculated density. Mass was measured using a calibrated scale and gauge was measured using a calibrated micrometer.

Ten 4" x 4" specimens were cut from the center of the foam sheet sample at 1-inch intervals in the machine direction. The gauge and mass were measured and used to calculate the density of each specimen. Each specimen was then split into an A-side and a B-side using controlled tear propagation. The gauge and mass of each split foam sample on the A-side and B-side were then measured and the corresponding density was calculated. Because any deformation of the foam specimen would be caused by the force splitting it, the gauge was measured from the undeformed area on the side of the split foam.

In the cross-machine direction, ten 4-inch x 4-inch specimens were cut at equal intervals across the width of the foam sheet sample. For sheets too narrow to accommodate ten specimens, as many specimens as the sheet dimensions permitted (seven specimens for the 8 pcf PE foam sample, eight specimens for the 6.5 pcf EVA foam sample) were collected. The first and last specimens were collected from the extreme edges of the foam sheet. Gauges and masses were collected and densities were calculated for the foams before and after splitting using the same methods as described above with reference to the machine direction.

The gauge, mass, and density were then averaged across all specimens of each foam sample, and each value was compared before and after foam splitting. A machine direction (MD) comparison is provided in Table 9A below, while a cross-machine (CM) comparison is shown in Table 9B below. Each value is expressed as a percentage of the original undivided foam sample, whereby a perfectly evenly divided foam would yield ideal A-side and B-side average gauge and mass on each side as close to 50% as possible and density as close to 100% as possible of the original undivided sample. The maximum amount of sample variance from the ideal was also determined, and is shown in Tables 9A and 9B below, as well as the average sample variance from the ideal.

In some cases, deformation of the foam sample caused some slight error in the gauge measurement, resulting in the total of Side A and Side B not adding up to 100%. Since density is a function of gauge and mass, the density calculation was similarly affected in these cases. However, based on the number of sample specimens examined, the data still has a high level of confidence.

As can be seen from Tables 9A and 9B, the gauge variance for the split foam specimens was a maximum of 16%, but averaged only a maximum of 5%. For mass, the variance for the split foam specimens was a maximum of 7%, but averaged only a maximum of 2%. For density, the variance for the split foam specimens was a maximum of 14%, but averaged only a maximum of 6%. Thus, the foam samples produced by the experimental method have very consistent tear propagation through the foam core, leading to sides A and B having little average variance in gauge, mass, and density, making this method of the present disclosure highly suitable for mass production of foams as described with reference to Figure 6.

Example 9 - Peak Height Foam samples produced by the above experimental method for controlled depth crosslinking were measured with a three-dimensional microscope to evaluate surface peak heights. In particular, split foam samples torn using controlled tear propagation below the mid/core region of the foam as previously described with reference to Figures 4-6 were measured for peak height on the new open-cell surface (such as 22A and 22B, as shown in Figures 5A and 5B), as well as measurements of the same surface of a standard foam sample cut using a blade. In addition, peak heights were also measured on the skin-side surface (such as the closed-cell surface, such as sides A and B in Figures 5A and 5B) as a control. Using a Keyence VHX-6000 three-dimensional microscope, peak heights were measured by placing each specimen under the microscope and scanning a 5 mm x 5 mm area using the built-in function. The base height was determined by averaging the overall height within the scanned area. A locally high peak was selected, and the peak height relative to the base height, determined using the microscope's function, was measured. The results are shown below in Tables 10A, 10B, and 10C for each measured foam sample density and also graphed as shown in FIG.

As can be seen from the measurements and Figure 14, the split-cell open-cell surface of each sample had a significantly higher average peak height than either the cut or skin surfaces of comparable foams. Comparing the average peak heights, the split-cell surface was approximately 11.8 times higher than the skin and approximately 4.1 times higher than the cut foam surface.

Example 10 - Surface Roughness Foam samples produced by the above experimental method for controlled depth crosslinking were measured with a three-dimensional microscope to evaluate surface roughness. In particular, split foam samples torn using controlled tear propagation below the mid/core region of the foam as previously described with reference to Figures 4-6 were measured for surface roughness on the new open-cell surfaces (such as 22A and 22B, as shown in Figures 5A and 5B) as well as measurements of the same surfaces of standard foam samples cut using a blade. In addition, surface roughness was also measured on the skin-side surfaces (such as closed-cell surfaces, such as sides A and B in Figures 5A and 5B) as a control. Using a Keyence VHX-6000 three-dimensional microscope, surface roughness was measured by placing each specimen under the microscope and using the built-in function to scan a 5 mm x 5 mm area. Surface area was measured with the microscope to calculate a roughness value, Sa, where Sa is the arithmetic mean of the surface roughness and Ra is the extension of the arithmetic mean height. It is expressed as the absolute difference in height of each point relative to the arithmetic mean of the surface. The results are shown in Tables 11A, 11B, and 11C below for each density of the foam sample measured, and also graphed as shown in Figure 15.

As can be seen from the measurement results and Figure 15, the split-cell open-cell surface of each sample had a much higher surface roughness than either the cut or skin surfaces of comparable foams. Comparing surface roughness, the split-cell foam surface was approximately 6 times higher than the skin and approximately 1.8 times higher than the cut foam surface.

Example 11 - Coefficient of Friction (COF)
Foam samples produced by the above experimental method for controlled depth crosslinking were evaluated according to ASTM D1894 to measure their coefficients of friction, both kinetic and static. In particular, split foam samples torn using controlled tear propagation below the mid/core region of the foam as previously described with reference to Figures 4-6 were measured for COF on a fresh open-cell surface relative to an opposing open-cell surface (such as 22A relative to 22B as shown in Figures 5A and 5B), as well as measurements of the same surface on a standard foam sample cut using a blade. In addition, COF was also measured on the opposite skin-side surface (such as the closed-cell surface, such as side A and side B in Figures 5A and 5B) as a control. The results are shown in Tables 12A, 12B, and 12C below for each density of the foam sample measured, and are also graphed as shown in Figures 16A and 16B.

As can be seen from the measurements and Figures 16A and 16B, the split-foam open-cell surface of each sample had both a static and dynamic COF that was significantly higher than either the cut or skin surfaces of the equivalent foam. Comparing the static COF, the split-foam surface was approximately 4.1 times higher than the skin and approximately 3.3 times higher than the cut foam surface. Comparing the dynamic COF, the split-foam surface was approximately 4.4 times higher than the skin and approximately 3.7 times higher than the cut foam surface.

Example 12 - Acoustic Transmission Foam samples produced by the above experimental method for controlled depth crosslinking were evaluated for acoustic transmission properties to measure the average sound reduction achieved by the foam. In particular, split foam samples torn using controlled tear propagation below the mid/core region of the foam as described above with reference to Figures 4-6 were measured for average sound reduction across a new open-cell surface (such as 22A or 22B, as shown in Figures 5A and 5B), as well as the same measurements on standard foam samples cut using a blade. In addition, the average sound reduction of the skin-side surface was also measured on the skin-side surface (such as the closed-cell surface, such as side A or side B in Figures 5A and 5B) as a control.

The acoustic transmission test method was performed as follows: Two pieces of PVC pipe, measuring 4 inches long and 12 inches in diameter, were prepared. One end of each pipe was sealed with a 1-inch-thick piece of wood. A ¾-inch diameter hole was drilled in the center of one of the pieces. The pipe without the drilled hole was placed on a table with the wood end facing down. A Bluetooth speaker was placed in the center of the first pipe, facing upward. A 12-inch x 12-inch foam specimen was placed over the opening of the first PVC pipe, with the test surface facing the speaker. A second PVC pipe with a drilled hole was used to sandwich the specimen, with the wood facing upward. A decibel meter was inserted through the top hole, approximately 3 inches above the foam specimen. Tones ranging from 1,000 Hz to 20,000 Hz were played in 1,000 Hz intervals using tone generator software, and the resulting decibels passing through the foam specimen were recorded. The sound volume was checked to ensure it was not too loud to avoid maxing out the decibel meter's functional range. The process was then repeated without the foam specimen to generate a baseline control measurement. Once the data was collected, a best-fit line was generated through the data to derive an equation. Using this equation, dB at 1,000 Hz and 20,000 Hz was calculated, and the specimen value was subtracted from the baseline value to generate a sound reduction value. An average sound reduction value was then calculated to generate a single value representing the level of sound reduction for the specimens tested. The calculated average sound reduction value was then graphed against the basis weight of each foam specimen.

The average sound reduction results are shown below in Tables 13A, 13B, and 13C for each basis weight of the foam samples measured, and also graphed as shown in FIG.

As can be seen from the measurements and Figure 17, the split foam open cell surface samples had superior average sound reduction performance compared to the standard foam skin surface at all foam basis weights. Compared to the cut standard foam, the split foam surface had superior average sound reduction performance at foam basis weights of approximately 7 g/ft² and above.

Example 13 - Surface Water Retention Foam samples produced by the above experimental method for controlled depth crosslinking were evaluated for surface water retention properties. In particular, split foam samples torn using controlled tear propagation below the mid/core region of the foam as previously described with reference to Figures 4-6 were measured for surface water retention on the fresh open-cell surface (such as 22A or 22B as shown in Figures 5A and 5B), as well as the same measurements on standard foam samples cut using a blade. In addition, surface water retention was also measured on the skin-side surface (such as the closed-cell surface, such as side A or side B in Figures 5A and 5B) as a control.

The surface water retention test method was performed as follows: 4-inch x 4-inch foam specimens and a shallow water tank were prepared. Each specimen was dried and weighed, then submerged in water with the target side of the specimen floating. The specimen was allowed to float for 45 seconds, during which time forceps were used to slide it across the surface of the water, allowing any trapped air underneath the specimen to escape while keeping the dry side free of water. After 45 seconds, the specimen was removed from the water and placed dry-side down on a balance, and its new mass, including any water retained by the specimen, was measured. The before and after masses were compared to calculate the amount of water retained on the surface of each specimen.

The average surface water retention results are shown below in Tables 14A, 14B, and 14C for each measured foam sample density, and also graphed as shown in FIG.

As can be seen from the measurements and the results in Figure 18, the segmented foam retained more water on its surface than either the standard skin or the machined foam surface. This result is interesting because segmented foam also exhibits an open-cell surface. However, the specific surface characteristics of the open-cell surface of the segmented foam, likely due to increased peak height and surface roughness as described with respect to previous examples, make it superior for water retention. Comparing the total average water retention, the segmented foam surface retained approximately 3.9 times more water than the skin surface and approximately 1.5 times more water than the machined foam surface.

Example 14 - Adhesion Testing Foam samples produced by the above experimental method for controlled depth crosslinking were evaluated for adhesive properties. In particular, split foam samples torn using controlled tear propagation below the mid/core region of the foam as previously described with reference to Figures 4-6 were measured for adhesive properties on the fresh open-cell surface (such as 22A or 22B as shown in Figures 5A and 5B) as well as the same measurements on standard foam samples cut using a blade. In addition, adhesive properties were also measured on the skin-side surface (such as the closed-cell surface, such as side A or side B in Figures 5A and 5B) as a control.

The adhesion test method was as follows: Three 350 mm x 50 mm test specimens were cut across the width of the foam sample in the machine direction. Both sides were then corona treated. Samples with densities lower than 3.2 pcf did not require treatment. Samples with densities higher than 12.5 pcf were not tested. Pressure-sensitive double-sided tape was prepared using a Coatema coating machine with KS900 as a release liner. 65 g/m² of Collano T2 1434 was applied at 160°C. Foam specimens were coated on both sides with the prepared double-sided tape, avoiding any air entrapment. The adhesive was rolled twice with a 5 kg roller at a uniform speed of 600 mm/min. Test specimens were die-cut to 300 mm x 25 mm. The release liner on one side was removed, and a 0.019 mm thick MYLAR PET film was placed on top. The specimen was then rolled twice with a roller, as described above. A flat chrome-nickel plate (50 mm x 210 mm) was cleaned (lengthwise only) with 600-grit paper and then any shavings, grease, etc., removed with lint-free paper soaked in benzene. The other release liner was removed approximately 11 cm from the specimen, and the open adhesive side was placed on the cleaned metal plate. Starting with the edge of the specimen resting on the cleaned edge of the metal plate, it was pressed four times using a roller, as described above. The specimen was then allowed to rest for 24 hours. A load cell was then installed on the tensile tester, and the grip distance was set to 170 mm. Using a high-density specimen, peeling was initiated by hand with a jerky movement. The free end of the metal plate was placed in the grip without the load cell, and the free portion of the specimen was placed in the grip with the load cell. The peel strength was tested at a speed of 300 mm/min while observing the peel position within the sample specimen, and recorded in N/mm.

The adhesion test results are shown below in Table 15 for each foam sample measured and also graphed as shown in FIG.

Regarding failure modes, foam tearing refers to a situation in which the foam itself is torn, while foam adhesive tearing refers to the interface between the foam and the adhesive tape being destroyed, indicating that the adhesive is not well bonded to the foam. These test results indicate that the adhesive is not well bonded to the cut surface, while the split foam split side caused foam tearing, suggesting strong adhesive bond. Furthermore, the split open surface performed similarly to the skin side surface, but better than the cut surface. Therefore, the split foam surface produced by the method of the present disclosure performs well for adhesive tape applications.

While the present invention has been described with reference to exemplary embodiments, those skilled in the art will recognize that various modifications may be made and equivalent elements may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is not intended that the invention be limited to the particular embodiments disclosed, but rather, the invention is intended to include all embodiments falling within the scope of the appended claims.

Claims (14)

1. A closed-cell crosslinked polyolefin foam sheet comprising: a first surface region and a second surface region opposite each other; and an intermediate region disposed therebetween, wherein the ratio of a gel content of the intermediate region to an average gel content of the first surface region and the second surface region is less than 50% , the gel content of the intermediate region is less than 35%, and the ratio of an average cell size of the intermediate region to an average cell size of the first surface region and the second surface region is from about 200% to about 400% . 10. The closed-cell crosslinked polyolefin foam sheet of claim 1, wherein the ratio of the gel content of the intermediate region to the average gel content of the first surface region and the second surface region is from about 15% to about 45% . The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising: the ratio of the average cell size of the intermediate region to the average cell size of the first surface region and the second surface region is from about 250% to about 400%. The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising a 50% compression set of about 10% or less. The closed-cell crosslinked polyolefin foam sheet of claim 1 further comprises a depression depth of about 35% to about 45% of the original thickness of the closed-cell crosslinked polyolefin foam sheet at 0 hours, and about 4% to about 6% of the original thickness after 168 hours, according to test standard VDA 237-101-2. 10. The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising a depression depth of about 50% to about 55% of the original thickness of the closed-cell crosslinked polyolefin foam sheet at 0 hours, and about 5% to about 10% of the original thickness after 168 hours, according to test standard VDA 237-101-2, after the closed-cell crosslinked polyolefin foam sheet has been post-crosslinked to have a gel content in the intermediate region of about 50% or greater. The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising a density of about 1.5 pcf to about 15 pcf. The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising a density of about 2 pcf to about 10 pcf. 10. The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising a splitting force strength of about 5 lbf to about 35 lbf according to the Splitting Force Strength Test Method . 10. The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising a splitting force strength of from about 8 lbf to about 33 lbf according to the Splitting Force Strength Test Method . 10. The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising a splitting force strength of about 9 lbf to about 30 lbf according to the Splitting Force Strength Test Method . The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising a T-peel strength of about 1 lbf to about 5 lbf according to a T-peel strength test method. 10. The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising a T-peel strength of about 1.5 lbf to about 4 lbf according to the T-peel strength test method . 10. The closed-cell crosslinked polyolefin foam sheet of claim 1, further comprising a T-peel strength of about 1.6 lbf to about 3.1 lbf according to the T-peel strength test method .